research papers on nanomaterials pdf

Nanomaterials: a review of synthesis methods, properties, recent progress, and challenges

First published on 24th February 2021

Nanomaterials have emerged as an amazing class of materials that consists of a broad spectrum of examples with at least one dimension in the range of 1 to 100 nm. Exceptionally high surface areas can be achieved through the rational design of nanomaterials. Nanomaterials can be produced with outstanding magnetic, electrical, optical, mechanical, and catalytic properties that are substantially different from their bulk counterparts. The nanomaterial properties can be tuned as desired via precisely controlling the size, shape, synthesis conditions, and appropriate functionalization. This review discusses a brief history of nanomaterials and their use throughout history to trigger advances in nanotechnology development. In particular, we describe and define various terms relating to nanomaterials. Various nanomaterial synthesis methods, including top-down and bottom-up approaches, are discussed. The unique features of nanomaterials are highlighted throughout the review. This review describes advances in nanomaterials, specifically fullerenes, carbon nanotubes, graphene, carbon quantum dots, nanodiamonds, carbon nanohorns, nanoporous materials, core–shell nanoparticles, silicene, antimonene, MXenes, 2D MOF nanosheets, boron nitride nanosheets, layered double hydroxides, and metal-based nanomaterials. Finally, we conclude by discussing challenges and future perspectives relating to nanomaterials.

1. Introduction

The term nanometer was first used in 1914 by Richard Adolf Zsigmondy. 5 The American physicist and Nobel Prize laureate Richard Feynman introduced the specific concept of nanotechnology in 1959 in his speech during the American Physical Society's annual meeting. This is considered to be the first academic talk about nanotechnology. 5 He presented a lecture that was entitled “There's Plenty of Room at the Bottom”. During this meeting, the following concept was presented: “why can’t we write the entire 24 volumes of the Encyclopedia Britannica on the head of a pin?” The vision was to develop smaller machines, down to the molecular level. 6,7 In this talk, Feynman explained that the laws of nature do not limit our ability to work at the atomic and molecular levels, but rather it is a lack of appropriate equipment and techniques that limit this. 8 Through this, the concept of modern technology was seeded. Due to this, he is often considered the father of modern nanotechnology. Norio Taniguchi might be the first person who used the term nanotechnology, in 1974. Norio Taniguchi stated: “nano-technology mainly consists of the processing of, separation, consolidation, and deformation of materials by one atom or one molecule.” 5,9 Before the 1980s, nanotechnology remained only an area for discussion, but the concept of nanotechnology was seeded in the minds of researchers with the potential for future development.

The invention of various spectroscopic techniques sped up research and innovations in the field of nanotechnology. IBM researchers developed scanning tunneling microscopy (STM) in 1982, and with STM it became feasible to attain images of single atoms on “flat” ( i.e. , not a tip) surfaces. 10 Atomic force microscopy (AFM) was invented in 1986, and it has become the most crucial scanning probe microscope technique. 11 The motivation to develop hard discs with high storage density stimulated the measurement of electrostatic and magnetic forces. This led to the development of Kelvin-probe-, electrostatic-, and magnetic-force microscopy. 12 Currently, nanotechnology is rapidly evolving and becoming part of almost every field related to materials chemistry. The field of nanotechnology is evolving every day, and now powerful characterization and synthesis tools are available for producing nanomaterials with better-controlled dimensions.

Nanotechnology is an excellent example of an emerging technology, offering engineered nanomaterials with the great potential for producing products with substantially improved performances. 13 Currently, nanomaterials find commercial roles in scratch-free paints, surface coatings, electronics, cosmetics, environmental remediation, sports equipment, sensors, and energy-storage devices. 14 This review attempts to provide information in a single platform about the basic concepts, advances, and trends relating to nanomaterials via covering the related information and discussing synthesis methods, properties, and possible opportunities relating to the broad and fascinating area of nanomaterials ( Scheme 1 ). It is not easy to cover all the literature related to nanomaterials, but important papers from history and the current literature are discussed in this review. This review provides fundamental insight for researchers, quickly capturing the advances in and properties of various classes of nanomaterials in one place.

2. Descriptions of terms associated with nanomaterials

3. Approaches for the synthesis of nanomaterials

3.1. Top-down approaches

The conditions under which arc discharge takes place play a significant role in achieving new forms of nanomaterials. The conditions under which different carbon-based nanomaterials are formed via the arc discharge method are explained in Fig. 6 . Various carbon-based nanomaterials are collected from different positions during the arc discharge method, as their growth mechanisms differ. 44 MWCNTs, high-purity polyhedral graphite particles, pyrolytic graphite, and nano-graphite particles can be collected from either anode or cathode deposits or deposits at both electrodes. 46–48 Apart from the electrodes, carbon-based nanomaterials can also be collected from the inner chamber. Different morphologies of single-wall carbon nanohorns (SWCNHs) can be obtained under different atmospheres. For example, ‘dahlia-like’ SWCNHs are produced under an ambient atmosphere, whereas ‘bud-like’ SWCNHs are generated under CO and CO 2 atmospheres. 49 The arc discharge method can be used to efficiently achieve graphene nanostructures. The conditions present during the synthesis of graphene can affect its properties. Graphene sheets prepared via a hydrogen arc discharge exfoliation method are found to be superior in terms of electrical conductivity and have good thermal stability compared to those obtained via argon arc discharge. 50

3.2. Bottom-up approaches

The hard template method is also called nano-casting. Well-designed solid materials are used as templates, and the solid template pores are filled with precursor molecules to achieve nanostructures for required applications ( Fig. 10 ). 78 The selection of the hard template is critical for developing well-ordered mesoporous materials. It is desirable that such hard templates should maintain a mesoporous structure during the precursor conversion process, and they should be easily removable without disrupting the produced nanostructure. A range of materials has been used as hard templates, not limited to carbon black, silica, carbon nanotubes, particles, colloidal crystals, and wood shells. 85 Three main steps are involved in the synthetic pathway for obtaining nanostructures via templating methods. In the first step, the appropriate original template is developed or selected. Then, a targeted precursor is filled into the template mesopores to convert them into an inorganic solid. In the final step, the original template is removed to achieve the mesoporous replica. 86 Via using mesoporous templates, unique nanostructured materials such as nanowires, nanorods, 3D nanostructured materials, nanostructured metal oxides, and many other nanoparticles can be produced. 87 From this brief discussion, it can be seen that a wide range of unique structured nanomaterials can be produced using soft and hard template methods.

4. Unique nanomaterial features

The electronic properties of semiconductors in the 1–10 nm range are controlled by quantum mechanical considerations. Thus, nanospheres with diameters in the range of 1–10 nm are known as quantum dots. The optical properties of nanomaterials such as quantum dots strongly depend upon their shape and size. 96 A photogenerated electron–hole pair has an exciton diameter on the scale of 1–10 nm. Thus, the absorption and emission of light by semiconductors could be controlled via tuning the nanoparticle size in this range. However, in the case of metals, the mean free path of electrons is ∼10–100 nm and, due to this, electronic and optical effects are expected to be observed in the range of ∼10–100 nm. The colors of aqueous solutions of metal nanoparticles can be changed via changing the aspect ratio. Aqueous solutions of Ag NPs show different colors at different aspect ratios. A red shift in the absorption band appears with an increase in the aspect ratio ( Fig. 12 ). 21

Among a range of unique properties, the following key properties can be obtained upon tuning the sizes and morphologies of nanomaterials.

4.1. Surface area

4.2. magnetism, 4.3. quantum effects, 4.4. high thermal and electrical conductivity, 4.5. excellent mechanical properties, 4.6. excellent support for catalysts, 4.7. antimicrobial activity.

Overall, these features have made nanoscale materials valuable for a wide range of applications, substantially boosting the performances of various devices and materials in a number of fields. Details of various nanomaterials, their properties, and applications in various fields will be discussed below.

5. Nanomaterials, characteristics, and applications

5.1. special carbon-based nanomaterials.

In the carbon-based nanomaterial family, fullerenes were the first symmetric material, and they provided new perspectives in the nanomaterials field. This led to the discovery of other carbon-based nanostructured materials, such as carbon nanotubes and graphene. 110 Fullerenes are present in nature and interstellar space. 111 Interestingly, fullerenes were the molecule of the year in 1991 and attracted the most research projects compared to other scientific subjects during that period. 112 Fullerenes possess several unique features that make them attractive for applications in different fields. Fullerenes display solubility to some extent in a range of solvents, and these characteristics make them unique compared to the other allotropes of carbon. 108

The chemical modification of fullerenes is an exciting subject, improving their effectiveness for applications. There are two main ways to modify fullerenes: 113 fullerene inner-space modification, and fullerene outer-surface modification.

Endohedral and exohedral doping examples are shown in Fig. 13 . 114 Fullerenes are hollow cages, and the interior acts as a robust nano-container for host target species when forming endohedral fullerene. 115 Endohedral fullerenes do not always follow the isolated pentagon rule (IPR). 116 To date, fullerene nanocages have received substantial consideration in the materials chemistry field due to their useful potential applications. Neutral and charged single atoms in free space are highly reactive and unstable. In the confined environment of fullerenes, these reactive species can be stabilized; for example, the LaC 60 + ion does not react with the NH 3 , O 2 , H 2 , or NO. Thus, reactive metals can be protected from the surrounding environment by trapping them inside fullerene cages. 117 Another emerging carbon nanomaterial is endohedral fullerene containing lithium (Li@C 60 ). 118 Lithium metal is very reactive, and extreme controlled environmental conditions are required to preserve or use it. In other words, secure structures are required for lithium storage. Li-Based endohedral fullerene shows unique solid-state properties. The encapsulation of lithium atoms in fullerene helps to protect lithium atoms from external agents. Li-Based endohedral fullerenes have the potential to open the door to nanoscale lithium batteries. 119 For the development of endohedral metallofullerenes, larger fullerenes are generally required, as they possess large cages to accommodate lanthanide and transition metal atoms more smoothly. 118 Fullerene nanocages are useful for the storage of gases. Fullerene is under consideration for hydrogen storage. 120,121

Exohedral fullerenes carry more potential for applications as outer surfaces can be more easily modified or functionalized. The exohedral doping of metals into fullerenes strongly affects the electronic properties via shifting electrons from the metal to the fullerene nanocage. 122 The practical application of fullerenes can be achieved with tailor-made fullerene derivatives via chemical functionalization. As fullerene chemistry has matured, a wide range of functionalized fullerenes has been realized through simple synthetic routes. 123 The combination of hydrogen-bonding motifs and fullerenes has allowed the modulation of 1D, 2D, and 3D fullerene-based architectures. 124 The excellent electron affinities of fullerenes have shown great potential for eliminating reactive oxygen species. The presence of excess reactive oxygen species can cause biological dysfunction or other health issues. The surfaces of fullerenes have been functionalized via mussel-inspired chemistry and Michael addition reactions for the fabrication of C 60 –PDA–GSH. The developed C 60 –PDA–GSH nanoparticles demonstrated excellent potential for scavenging reactive oxygen species. 125

Amphiphiles have great importance in industrial processes and daily life applications. Amphiphilic molecules consist of hydrophilic and hydrophobic parts, and they perform functions in water via forming two- and three-dimensional assemblies. Recently, conical fullerene amphiphiles 126 have emerged as a new class of amphiphiles, in which a nonpolar apex is supplied by fullerenes and a hydrophilic part is achieved through functionalization. The selective functionalization of the fullerene on one side helps to achieve a supramolecule due to unique interfacial behavior. The unique supramolecular structure formed via the spontaneous assembly of one-sided selectively functionalized fullerenes through strong hydrophobic interactions between the fullerene apexes and polar functionalized portions is soluble in water. Conical fullerene amphiphiles are mechanically robust. Via upholding the structural integrity, conical fullerene amphiphiles can be readily aggregated with nanomaterials and biomolecules to form multicomponent agglomerates with controllable structural features. 127 Fullerenes, after suitable surface modification, have excellent potential for use in drug delivery, but there have only been limited explorations of their drug delivery applications. 128,129 Fullerene-based nano-vesicles have been developed for the delayed release of drugs. 130 Water-soluble proteins have great potential in the field of nanomedicine. The water-soluble cationic fullerene, tetra(piperazino)[60] fullerene epoxide (TPFE), has been used for the targeted delivery of DNA and siRNA specifically to the lungs. 131 For diseases in lungs or any other organ, efficient treatment requires the targeted delivery of active agents to a targeted place in the organ. The accumulation of micrometer-sized carriers in the lung makes lung-selective delivery difficult, as this may induce embolization and inflammation in the lungs. Size-controlled blood vessel carrier vehicles have been developed using tetra(piperazino)fullerene epoxide (TPFE). TPFE and siRNA agglutinate in the bloodstream with plasma proteins and, as a result, micrometer-sized particles are formed. These particles clog the lung capillaries and release siRNA into lungs cells; after siRNA delivery, they are immediately cleared from the lungs ( Fig. 14 ). 132

The supramolecular organization of fullerene (C 60 ) is a unique approach for producing shape-controlled moieties on the nano-, micro-, and macro-scale. Nano-, micro-, and macro-scale supramolecular assemblies can be controlled via manipulating the preparation conditions to achieve unique optoelectronic properties. 133 The development of well-ordered and organized 1D, 2D, and 3D fullerene assemblies is essential for achieving advanced optical and organic-based electronic devices. 134 Fullerene-based nanostructured materials with new dimensions are being developed from zero-dimensional fullerene and tuned to achieve the desired characteristics. 1D C 60 fullerene nanowires have received substantial attention over other crystalline forms due to their excellent features of potential quantum confinement effects, low dimensionality, and large surface areas. 135

Carbon nanomaterials are also used as supports for catalysts, and the main reasons to use them are their high surface areas and electrical conductivities. Carbon supports strongly influence the properties of metal nanoparticles. In fuel cells, the carbon support strongly affects the stability, electronic conductivity, mass transport properties, and electroactive surface area of the supported catalyst. 136 In fuel cells, the degradation of some catalysts, such as platinum-based examples, and carbon is correlated and reinforced as a result of both being present. Carbon support oxidation is catalyzed by platinum and the oxidation of carbon accelerates platinum-catalyst release. Overall, this results in a loss of catalytically active surface area. 137 Fullerenes are considered suitable support materials due to their excellent electrochemical activities and stability during electrochemical reactions. 138 Due to their high stability and good conductivity, fullerenes can replace conventional carbon as catalyst support materials. Fullerenes are also used for the development of efficient solar cells. 139

Apart from the applications mentioned above, fullerenes have a broader spectrum of applications where they can be used to improve outcomes considerably. Fullerenes have the potential to be used in the development of superconductors. 140 The strong covalent bonds in fullerenes make them useful nanomaterials for improving the mechanical properties of composites. 141 The combination of fullerenes with polymers can result in good flame-retardant and thermal properties. 142 Fullerenes and their derivatives are used for the development of advanced lubricants. They are used as modifiers for greases and individual solid lubricants. 138 Fullerenes have tremendous medicinal importance due to their anticancer, antioxidant, anti-bacterial, and anti-viral activities. 104

Fullerenes are vital members of the carbon-based nanomaterial family and they certainly possess exceptional properties. This discussion further emphasizes their importance for advanced applications. However, the discovery of other carbon-based nanomaterials has put fullerenes in the shade, and the pace of their exploration has been reduced. As fullerenes are highly symmetrical molecules with unique properties, they can act as performance boosters, but more attention is needed from researchers for their practical expansion. 110

Single-walled carbon nanotubes consist of a seamless one-atom-thick graphitic layer, in which carbon atoms are connected through strong covalent bonds. 146 Double-walled carbon nanotubes consist of two single-walled carbon nanotubes. One carbon nanotube is nested in another nanotube to construct a double-walled carbon nanotube. 147 In multi-walled carbon nanotubes, multiple sheets of single-layer carbon atom are rolled up. In other words, many single-walled carbon nanotubes are nested within each other. From different types of nanotubes, it can be concluded that the nanotubes may consist of one, tens, or hundreds of concentric carbon shells, and these shells are separated from each other with a distance of ∼0.34 nm. 148 Carbon nanotubes can be synthesized via chemical vapor deposition, 149 laser ablation, 150 arc-discharge, 143 and gas-phase catalytic growth. 151

Single-walled carbon nanotubes display a diameter of 0.4 to 2 nm. The inner wall distance between double-walled carbon nanotubes was found to be in the range of 0.33 to 0.42 nm. MWCNT diameters are usually in the range of 2–100 nm, and the inner wall distance is about 0.34 nm. 147,152 However, it is essential to note that the diameters and lengths of carbon nanotubes are not well defined, and they depend on the synthesis route and many other factors. The electrical conductivities of SWCNTs and MWCNTs are about 10 2 –10 6 S cm −1 and 10 3 –10 5 S cm −1 , respectively. SWCNTs and MWCNTs also display excellent thermal conductivities of ∼6000 W m −1 K −1 and ∼2000 W m −1 K −1 , respectively. CNTs remain stable in air at temperatures higher than 600 °C. 153 These properties indicate that CNTs have obvious advantages over graphite.

Single-walled carbon nanotubes can display metallic or semiconducting behavior. Whether carbon nanotubes show metallic or semiconducting behavior depends on the diameter and helicity of the graphitic rings. 154 The rolling of graphene sheets leads to three different types of CNTs: chiral, armchair, and zigzag ( Fig. 15 ). 155

Carbon nanotubes demonstrate some amazing characteristics that make them valuable nanomaterials for possible practical applications. Theoretical and experimental studies of carbon nanotubes have revealed their extraordinary tensile properties. J. R. Xiao et al. used an analytical molecular structural mechanics model to predict SWCNT tensile strengths of 94.5 (zigzag nanotubes) and 126.2 (armchair nanotubes) GPa. 156 In another study, the Young's modulus and average tensile strength of millimeter-long multi-walled carbon nanotubes were analyzed and found to be 34.65 GPa and 0.85 GPa, respectively. 157 Carbon nanotubes possess a high aspect ratio. Due to their high tensile strength, carbon nanotubes are used to enhance the mechanical properties of composites.

Carbon nanotubes have become an important industrial material and hundreds of tonnes are produced for applications. 158 Their high tensile strength and high aspect ratio have made carbon nanotubes an ideal reinforcing agent. 159 Carbon nanotubes are lightweight in nature and are used to produce lightweight and biodegradable nanocomposite foams. 160 The structural parameters of carbon nanotubes define whether they will be semiconducting or metallic in nature. This property of carbon nanotubes is considered to be effective for their use as a central element in the design of electronic devices such as rectifying diodes, 161 single-electron transistors, 162 and field-effect transistors. 163 The chemical stability, nano-size, high electrical conductivity, and amazing structural perfection of carbon nanotubes make them suitable for electron field emitter applications. 164 The unique set of mechanical and electrochemical properties make CNTs a valuable smart candidate for use in lithium-ion batteries. 165 CNTs have the full potential to be used as a binderless free-standing electrode for active lithium-ion storage. CNT-based anodes can have reversible lithium-ion capacities exceeding 1000 mA h g −1 , and this is a substantial improvement compared with conventional graphite anodes. In short, the following factors play a role in controlling and optimizing the performances of CNT-based composites: 166 (i) the volume fraction of carbon nanotubes; (ii) the CNT orientation; (iii) the CNT matrix adhesion; (iv) the CNT aspect ratio; and (iv) the composite homogeneity.

For some applications, a proper stable aqueous dispersion of CNTs at a high concentration is pivotal to allow the system to perform its function efficiently and effectively. 167 One of the major issues associated with carbon nanotubes is their poor dispersion in aqueous media due to their hydrophobic nature. Clusters of CNTs are formed due to van der Waals attraction, π–π stacking, and hydrophobicity. The CNT clusters, due to their strong interactions, hinder solubility or dispersion in water or even organic-solvent-based systems. 168 This challenging dispersion associated with CNTs has limited their use for promising applications, such as in biomedical devices, drug delivery, cell biology, and drug delivery. 167 Carbon nanotube applications and inherent characteristics can be further tuned via suitable functionalization. The functionalization of carbon nanotubes helps scientists to manipulate the properties of carbon nanotubes and, without functionalization, some properties are not attainable. 169 The functionalization of nanotubes can be divided into two main categories: covalent functionalization and non-covalent functionalization.

The heating of CNTs under strongly acidic and oxidative conditions results in the formation of oxygen-containing functionalities. These functional groups, such as carboxylic acid, react further with other functional groups, such as amines or alcohols, to produce amide or ester linkages on the carbon nanotubes. 172 One of the main issues preventing the utilization of CNTs for biomedical applications is their toxicity. The cytotoxicity of pristine carbon nanotubes can be reduced via introducing carbonyl, –COOH, and –OH functional groups. Apart from functionalization through oxidized CNTs, the direct functionalization of CNTs is also possible. However, direct functionalization requires more reactive species to directly react with the CNTs, such as free radicals. Addition reactions to CNTs can cause a transformation from sp 2 hybridization to sp 3 hybridization at the point of addition. At the point where functionalization has taken place, the local bond geometry is changed from trigonal planar to tetrahedral geometry. Some addition reactions to the sidewalls of CNTs are shown in Fig. 16 . 155

It is important to discuss how the covalent functionalization of carbon nanotubes comes at the price of the degradation of the carbon sp 2 network. This substantially affects the electronic, thermal, and optoelectronic properties of the carbon nanotubes. 169 Efforts are being made to introduce a new method of covalent functionalization that can keep the π network of CNTs intact. Antonio Setaro et al. introduced a new [2+1] cycloaddition reaction for the non-destructive, covalent, gram-scale functionalization of single-walled carbon nanotubes. The reaction rebuilds the extended π-network, and the carbon nanotubes retained their outstanding quantum optoelectronic properties ( Fig. 17 ). 173

Polymers are frequently combined with CNTs to enhance their dispersion capabilities. Polymers interact with CNTs through CH–π and π–π interactions. 174 Hexanes and cycloalkanes are poor CNT solvents but the good solubility or dispersion of CNTs in these solvents is required for surface coating applications. Poly(dimethylsiloxane) (PDMS) macromer-grafted polymers have been prepared using PDMS macromers and pyrene-containing monomers that strongly adsorb on CNTs, thus improved the solubility of CNTs in chloroform and hexane. 176 The use of head–tail surfactants is another attractive way to achieve a fine dispersion of CNTs in an aqueous medium. In head–tail surfactants, the tail is hydrophobic and interacts with the CNT sidewalls, and the hydrophilic head groups interact with the aqueous environment to provide a fine dispersion. 177

For electrical applications, non-covalently functionalized CNTs are more preferred because the electrical properties of the CNTs are not compromised. CNTs have been non-covalently functionalized with a variety of biomolecules for the fabrication of electrochemical biosensors. 175 Non-covalently functionalized SWCNTs are used for energy applications. Single-walled carbon nanotubes (SWCNTs) have been non-covalently functionalized with 3d transition metal( II ) phthalocyanines, lowering the potential of the oxygen evolution reaction by approximately 120 mV compared with unmodified SWCNTs. 178 The toxicity of pristine CNTs toward living organisms can be lessened via using surfactant-functionalized CNTs. 170 However, in some cases, during polymer non-covalent functionalization, the polymer may wrap CNT bundles and make it difficult to separate the CNTs from each other. Polymers can develop into insulating wrapping that affects the CNT conductivity.

In the literature, several graphene-related materials have been reported, such as graphene oxide and reduced graphene oxide. 187 Among graphenoids, graphene oxide is a more reported and explored graphene-related material as a precursor for chemically modified graphene. The synthetic route to graphene oxide is straightforward, and it is synthesized from inexpensive graphite powder that is readily available. 188 Graphene oxide has many oxygen-containing functional groups, such as epoxy, hydroxyl, carboxyl, and carbonyl groups. The basal plane of graphene oxide is generally decorated with epoxide and hydroxyl groups, whereas the edges presumably contain carboxyl- and carbonyl-based functional groups. 189 The presence of active functional groups in graphene oxide allows its further functionalization with different polymers, small organic compounds, or other nanomaterials to realize several applications. 190

Graphene oxide, due to its oxygen functionality, is insulating in nature and displays poor electrochemical performance. The presence of oxygen functionalities in graphene oxide breaks the conjugated structure and localizes the π-electron network, resulting in poor carrier mobility and carrier concentration. 196 Its electrochemical performance is improved substantially after removing the oxygen-containing functional groups. 197 These functional groups can be removed or reduced via thermal, electrochemical, and chemical means. The product obtained after removing or reducing oxygen moieties is called reduced graphene oxide. The properties of reduced graphene oxide depend upon the effective removal of oxygen moieties from graphene oxide. The process used to remove oxygen-containing functionalities from graphene oxide will determine the extent to which reduced the properties of graphene oxide resemble pristine graphene. 198

Reduced graphene oxide is extensively used to improve the performances of various electrochemical devices. 199 It is essential to mention that even after reducing graphene oxide, some residual sp 3 carbon bonded to oxygen still exists, which somehow disturbs the movement of charge through the delocalized electronic cloud of the sp 2 carbon network. 200 Apart from this, the electrochemical activity of reduced graphene oxide is substantially high enough to manufacture electrochemical devices with improved performances. Recently, the demand for super-performance electrochemical devices has increased to overcome modern challenges relating to electronics and energy-storage devices. 201 Graphene-based materials are considered to be excellent electrode materials, and they can be proved to be revolutionary for use in energy-storage devices such as supercapacitors (SC) and batteries. Graphene-based electrodes improve the performances of existing batteries (lithium-ion batteries) and they are considered useful for developing next-generation batteries such as sodium-ion batteries, lithium–O 2 batteries, and lithium–sulfur batteries ( Fig. 18 ). Being flat in nature, each carbon atom of graphene is available, and ions can easily access the surface due to low diffusion resistance, which provides high electrochemical activity. 202

Graphene and its derivatives are extensively used for the development of electrochemical sensors. 203 The surfaces of bare electrodes are usually not able to sense analytes at trace levels and they cannot differentiate between analytes that have close electrooxidation properties due to their poor surface kinetics. The addition of graphene layers to the surfaces of electrodes can substantially improve the electrocatalytic activity and surface sensitivity towards analytes. 204 Graphene has definite advantages over other materials that are used as electrode materials for sensor applications. Graphene has a substantially high surface-to-volume ratio and atomic thickness, making it extremely sensitive to any changes in its local environment. This is an essential factor in developing advanced sensing tools, as all the carbon atoms are available to interact with target species.

Consequently, graphene exhibits higher sensitivity than its counterparts such as CNTs and silicon nanowires. 205 Graphene has two main advantages over CNTs for the development of electrochemical sensors. First, graphene is mostly produced from graphite, which is a cost-effective route, and second, graphene does not contain metallic impurities like CNTs can. Graphene offers many other advantages when developing sensors and biosensors, such as biocompatibility and π–π stacking interactions with biomolecules. 206 Graphene-based materials are ideal for the construction of nanostructured sensors and biosensors.

The mechanical properties of graphene are used to fabricate highly desired stretchable and flexible sensors. 207 Graphene can be utilized to develop transparent electrodes with excellent optical transmittance. It displays good piezoresistive sensitivity. Researchers are making efforts to replace conventional brittle indium tin oxide (ITO) electrodes with flexible graphene electrodes in optoelectronic devices such as liquid-crystal displays and organic light-emitting diodes. 208 For human–machine interfaces, transparent and flexible tactile sensors with high sensitivity have become essential. Graphene film (GF) and PET have been applied to develop transparent tactile sensors that exhibit outstanding cycling stability, fast response times, and excellent sensitivity ( Fig. 19 ). 209 Similarly, graphene is applied for the fabrication of pressure sensors. 210 Overall, graphene is an excellent material for developing transparent and flexible devices. 211,212

The use of graphene-based materials is an effective way to deal with a broad spectrum of pollutants. 213 There are many ways to deal with environmental pollution; among these, adsorption is an effective and cost-effective method. 214,215 Graphene-based adsorbents are found to be useful in the removal of organic, 216 inorganic, and gaseous contaminants. Graphene-based materials have some obvious advantages over CNT-based adsorbents. For example, graphene sheets offer two basal planes for contaminant adsorption, enhancing their effectiveness as an adsorbent. 192 GO contains several oxygen functional groups that impart hydrophilic features. Due to appropriate hydrophilicity, GO-based adsorbents can efficiently operate in water to remove contaminants. Moreover, graphene-oxide-based materials can be functionalized further through reactive moieties with various organic molecules to enhance their adsorption capacities. 217

In short, extensive research must continue in order to develop graphene-based materials with high performance and bring them to the market. Massive focus on graphene research is also justified due to the extraordinary features described in extensive theoretical and experimental research works.

Nanodiamonds possess a core–shell-like structure and display rich surface chemistry, and numerous functional groups are present on their surface. Several functional groups, such as amide, aldehyde, ketone, carboxylic acid, alkene, hydroperoxide, nitroso, carbonate ester, and alcohol groups, are present on nanodiamond surfaces, assisting in their further functionalization for desired applications ( Fig. 20 ). 226

Furthermore, nanodiamond surfaces can be homogenized with a single type of functional group according to the application requirements. 227 The use of nanodiamond particles as a reinforcing material in polymer composites has attracted great attention for improving the performance of polymer composite materials. The superior mechanical properties and rich surface chemistry of nanodiamonds have made them a superior material for tuning and reinforcing polymer composites. Nanodiamonds might operate via changing the interphase properties and forming a robust covalent interface with the matrix. 228 Nanodiamond (ND)-reinforced polymer composites have shown superior thermal stabilities, mechanical properties, and thermal conductivities. Nanodiamonds have shown great potential for energy storage applications. 229 Nanodiamonds and their composites are also used in sensor fabrication, environmental remediation, and wastewater treatment. 230,231 Their stable fluorescence and long fluorescence lifetimes have made nanodiamonds useful for imaging and cancer treatment. For biomedical applications, the rational engineering of nanodiamond particle surfaces has played a crucial role in the carrying of bioactive substances, target ligands, and nucleic acids, resisting their aggregation. 232,233 Nanodiamonds have a great future in nanotechnology due to their amazing surface chemistry and unique characteristics.

Carbon quantum dots can be synthesized through several chemical routes. 241–245 Some methodologies for synthesizing carbon dots are described in Fig. 21 . 246–248 Carbon itself is a black material and displays low solubility in water. In contrast, carbon quantum dots are attractive due to their excellent solubility in water. They contain a plethora of oxygen-containing functional groups on their surface, such as carboxylic acids. These functional moieties allow for further functionalization with biological, inorganic, polymeric, and organic species.

Carbon quantum dots are also called carbon nano-lights due to their strong luminescence. 248 In particular, carbon quantum dots offer enhanced chemiluminescence, 249,250 fluorescent emission, 251 two-photon luminescence under near-infrared pulsed-laser excitation, 252 and tunable excitation-dependent fluorescence. 253 The luminescence characteristics of carbon quantum dots have been used to develop highly sensitive and selective sensors. In most cases, a simple principle is involved in sensing with luminescent carbon quantum dots: their photoluminescence intensity changes upon the addition of an analyte. 254 Based on this principle, several efficient sensors have been developed using carbon quantum dots. 255–257 They can be used as sensitive and selective tools for sensing explosives such as TNT. Recognition molecules on the surfaces of carbon quantum dots can help to sense targeted analytes. Amino-group-functionalized carbon quantum dot fluorescence is quenched in the presence of TNT through a photo-induced electron-transfer effect between TNT and primary amino groups. This quenching phenomenon can help to sense the target analyte ( Fig. 22 ). 258 Chiral carbon quantum dots (cCQDs) can exhibit an enantioselective response. The PL responses of cCQDs were evaluated toward 17 amino acids and it was found that the PL intensity of the cCQDs was only substantially enhanced in the presence of L -Lys ( Fig. 22 ). 254

Carbon quantum dots have received significant interest in the fields of biological imaging and nanomedicine ( Fig. 23 ). 239 Direct images of RNA and DNA are essential for understanding cell anatomy. Due to the limitations of current imaging probes, tracking the dynamics of these biological macromolecules is not an easy job. Recently, membrane-penetrating carbon quantum dots have been developed for the imaging of nucleic acids in live organisms. 259 It is important to note that most of the carbon quantum dots utilized to attain cell imaging under UV excitation emit blue radiation. Some biological tissue also emits blue light, specifically that involving carbohydrates, and this interferes with cell imaging carried out with blue-emitting CQDs. This seriously hinders their potential in the field of biomedical imaging. Due to this reason, researchers are focusing on tuning CQDs in a way that their emission peak is red-shifted to avoid interference. 260 Carbon quantum dots with yellow and green fluorescence have been reported for bioimaging purposes. 261,262 The suitable doping of carbon quantum dots can red-shift the emission to enhance the bioimaging effectiveness. 263 Doped carbon quantum dots are capable of biological imaging and display advanced capabilities for scavenging reactive oxygen species. 264

Carbon quantum dots demonstrate photo-induced electron transfer properties 265 that make them valuable for photocatalytic, light-energy conversion, and other related applications. 266 Carbon quantum dots enhance the activities of other photocatalysts to which they are attached. Carbon quantum dots, along with photocatalysts, provide better charge separation and suppress the regeneration of photogenerated electron–hole pairs. Moreover, the proper implantation of carbon quantum dots into photocatalysts can broaden the photo-absorption region. Implanted carbon quantum dots form micro-regional heterostructures that facilitate photo-electron transport. 267 The implantation of carbon quantum dots into g-C 3 N 4 can substantially enhance charge transfer and separation efficiencies, prevent photoexcited carrier recombination, narrow the bandgap, and red shift the absorption edge. 268 The intrinsic catalytic activity of polymeric carbon nitride is improved as a result of the nano-frame heterojunctions formed with the help of CQDs. 269

Carbon quantum dots offer many advantages over conventional semiconductor-based QDs and, thus, they have attracted considerable researcher attention. 244 Due to their remarkable features, they have shown importance in recent years in the fields of light-emitting diodes, nanomedicine, solar cells, sensors, catalysis, and bioimaging. 236

The production of carbon nanohorns has some obvious advantages over carbon nanotubes, such as the ability for toxic-metal-catalyst-free synthesis and large-scale production at room temperature. Carbon nanotube synthesis involves metal particles, and harsh conditions, such as the use of strong acids, are required to remove metallic catalysts. This process introduces many defects into CNT structures and may cause a loss of carbon material. 270 Carbon nanohorns possess a wide diameter compared to CNTs. CNHs possess good absorption capabilities and their interiors are also available after partial oxidation, which provides direct access to their internal parts. Heat treatment under acidic or oxidative conditions facilitates the facile introduction of holes into carbon nanohorns. Holes in graphene sheets of single-walled carbon nanohorns can be produced with O 2 gas at high temperatures. A large quantity of material can be stored inside CNH tubes. 274 The surface area of CNHs is substantially enhanced upon opening the horns to make their interiors accessible. 275 Carbon nanohorns have great potential for energy storage, 275 electrochemiluminescence, 276 adsorption, 277 catalyst support, 278 electrochemical sensing, 279 and drug delivery system 273 uses. CNHs as catalyst supports can provide a homogeneous dispersion of Pt nanoparticles ( Fig. 25 ). The current density of Pt supported on single-walled carbon nanohorns is double compared to a fuel cell made from Pt supported on carbon black. 280 Thus, carbon nanohorns provide a better uniform dispersion that facilitates a high surface area and better catalyst performance.

5.2. Nanoporous materials

In nanoporous materials, the size distributions, volumes, and shapes of the pores directly affect the performances of porous materials for particular applications. It has become a hot area of research to develop materials with precisely controlled pores and arrangements. Recent research has focused more on the precise control of the shapes, sizes, and volumes of pores to produce nanoporous materials with high performance. Several state-of-the-art reviews are present in the literature that focus explicitly on the synthesis, properties, advances, and applications of nanoporous materials. 85,287–289 Based on the materials used, nanoporous materials can be divided into three main groups: inorganic nanoporous materials; carbonaceous nanoporous materials; and organic polymeric nanoporous materials.

Inorganic nanoporous materials include porous silicas, clays, porous metal oxides, and zeolites. The generation of pores in the material can introduce striking features into the material that are absent in non-porous materials. Nanoporous materials offer rich surface compositions with versatile characteristics. Nanoporous materials exhibit high surface-to-volume ratios. Their outstanding features and nanoporous framework structures have made these materials valuable in the fields of environmental remediation, adsorption, catalysis, sensing, energy conversion, purification, and medicine. 284,290

Porous silica is a crucial member of the inorganic nanoporous family. Over the decades, it has generated significant research interest for use in fuel cells, chemical engineering, ceramics, and biomedicine. It is essential to note that specific morphologies and pore size diameters are required for each application, and these can be achieved via tuning during the synthesis process. Nanoporous silica offers two functional surfaces: one is the cylindrical pore surfaces, and the second is the exterior surfaces of the nanoporous silica particles. The surfaces of nanoporous silica can be easily functionalized for the desired applications. The nanoporous silica surface is heavily covered with many silanol groups that act as reactive sites for functionalization ( Fig. 26 ). 291,292 For biomedical applications, mesoporous silica has emerged as a new generation of inorganic platform materials compared to other integrated nanostructured materials. Several factors make it a unique material for biomedical applications: 293,294 (a) its ordered porous structure; (b) its tunable particle size; (c) its large pore volume and surface area; (d) its biocompatibility; (e) its biodegradation, biodistribution, and excretion properties; and (f) its two functional surfaces. For instance, ordered MCM-48 nanoporous silica was used for the delivery of the poorly soluble drug indomethacin. It has been found that surface modification can control drug release. 295 Mesoporous silica-based materials have emerged as excellent materials for use in sustained drug delivery systems (SDDSs), immediate drug delivery systems (IDDSs), targeted drug delivery systems (TDDSs), and stimuli-responsive controlled drug delivery systems (CDDSs). The drug release rate from mesoporous silica can also be controlled via introducing appropriate polymers or functional groups, such as CN, SH, NH 2 , and Cl. Researchers are currently focusing on developing MSN-based (MSN = mesoporous silica nanoparticle) multifunctional drug delivery systems that can release antitumor drugs on demand in a targeted fashion via minimizing the premature release of the drug ( Fig. 27 ). 296

Hierarchically nanoporous zeolites are a vital member of the nanoporous material family. They are crystalline aluminosilicate minerals whose structures comprise uniform, regular arrays of nanopores with molecular dimensions. The microporous structures of zeolites contain pores that are usually below 1 nm in diameter. In zeolites, the micropores are uniform in shape and size, and these pores can effectively discriminate between molecules based on shape and size. 297 Currently, based on crystallography, more than 200 zeolites have been classified. 298 Zeolites have been proved to be useful materials in the field of host–guest chemistry. In solid catalysis, about 40% of the entire solid catalyst field is taken up by zeolites in chemical industry. The excellent catalysis success of zeolites is based on their framework stability, shape-selective porosity, solid acidity, and ion-exchange capacity. Oxygen tetrahedrally coordinates with the Al atoms in most zeolite crystalline silicate frameworks, resulting in charge mismatch between the oxide framework and Al. Extra-framework Na + ions compensate for this charge mismatch. The Na + ions are exchangeable for other cations such as H + and K + . 298 The zeolite crystalline networks are remarkable in that they provide high mechanical and hydrothermal stabilities. The most crucial task facing the zeolite community is to find new structures with desired functions and apply them more effectively for different applications.

Apart from these inorganic porous materials, several other metal- and metal-oxide-based nanoporous materials have been introduced that are more prominent for use in electrode material, catalyst, photodegradation, energy storage, and energy conversion applications. 299–302 Nanoporous metal-based materials are famous due to the nanosized crystalline walls, interconnected porous networks, and numerous surface metal sites that provide them with unique physical/chemical properties compared with their bulk counterparts and other nanostructured materials. 303 For example, nanoporous WO 3 films were developed via tuning the anodization conditions for photoelectrochemical water oxidation. It has been observed that the morphology of the film strongly affected the photoelectrochemical performance. 304 Nanoporous alumina is also a unique material in the inorganic nanoporous family due to several aspects. Nanoporous alumina can be prepared in a controlled fashion with any size and shape in polyprotic aqueous media via the anodic oxidation of the aluminum surface. The parallel arrangement of pores on alumina can easily be controlled from 5 nm to 300 nm, and alumina is stable in the range of 1000 °C. The anodizing time plays a significant role in controlling the pore length. Nanoporous alumina membranes offer various unique properties, such as pores of variable widths/lengths, temperature stability, and optical transparency. Nanoporous alumina pores can be filled with magnetically and optically active elements to produce the desired applications at the nanoscale level. Photoluminescent alumina membranes can be produced via introducing cadmium sulfide, gallium nitride, and siloxenes inside nanoporous alumina using appropriate precursors. 305 Porous alumina also acts as an efficient support and template for the designing of other nanomaterials. Palladium nanowires, 306 high aspect ratio cobalt nanowires, 307 and highly aligned Cu nanowires 308 were developed using porous alumina as a template. Ni–Pd as a catalyst was supported on porous alumina for hydrogenation and oxidation reactions. 309 Nanoporous anodic alumina is also considered to be an efficient material for the development of biosensors due to the ease of fabrication, tunable properties, optical/electrochemical properties, and excellent stability in aqueous environments. 310

Nanoporous carbon-based materials are a hot topic in the field of materials chemistry. Nanoporous carbon materials have become ubiquitous choices in the environmental, energy, catalysis, and sensing fields due to their unique morphologies, large pore volumes, controlled porous structures, mechanical, thermal, and chemical stabilities, and high specific surface areas ( Fig. 28A ). 311 Nanoporous materials are found to be useful in the treatment of water. The separation of spilled oil and organic pollutants from water has emerged as a significant challenge. 312–314 The design of materials that can allow the efficient separation of organic, dye, and metal contaminants from water has become a leading environmental research area. 315,316 Nanoporous carbon can be derived from different natural and synthetic sources. 317–319 Nanoporous carbon foam can be derived from natural sources, such as flour, pectin, and agar, via table-salt-assisted pyrolysis. The agar-derived nanoporous carbon foam showed high absorption capacities, a maximum of 202 times its own weight, for oil and organic solvents. Air filtration paper developed from carbon nanoporous materials and non-woven fabrics has shown a filtration efficiency of greater than 99% ( Fig. 28B ). 320 Nanoporous carbon can also be produced from other porous frameworks, such as metal–organic frameworks. MOF- and COF-based materials are promising precursors for nanoporous carbon-based materials. The direct carbonization of amino-functionalized aluminum terephthalate metal–organic frameworks has produced nitrogen-doped nanoporous carbon that shows an adequate removal capacity of 98.5% for methyl orange under the optimum conditions. 321 Fe 3 O 4 /nanoporous carbon was also produced with Fe salts as a magnetic precursor and MOF-5 as a carbon precursor for removing the organic dye methylene blue (MB) from aqueous solutions. 322 The mesoporous carbon removal efficiency could be further enhanced via modifying or functionalizing the surface with various materials. Unmodified mesoporous carbon has shown a mercury removal efficiency of 54.5%. This efficiency can be substantially improved to 81.6% and 94% upon modification with the anionic surfactant sodium dodecyl sulfate and cationic surfactant cetyltrimethyl ammonium bromide (CTAB), respectively. 323

Ordered nanoporous carbon, CNTs, and fullerenes are extensively applied for energy and environmental applications. The complicated synthesis routes required for fullerenes and CNTs have slowed down the full exploitation of their potential for highly demanding applications. In comparison, the synthesis of highly ordered nanoporous carbon is facile, and the properties of ordered nanoporous carbon are also appealing for energy and environmental applications. 311 CO 2 is a greenhouse gas, and its sustainable conversion into value-added products has become the subject of extensive research. A nitrogen-doped nanoporous-carbon/carbon-nanotube composite membrane is a high-performance gas-diffusion electrode applied for the electrocatalytic conversion of CO 2 into formate. A faradaic efficiency of 81% was found for the production of formate. 324 Nanoporous carbon materials modified with the non-precious elements P, S, N, and B have emerged as efficient electrode materials for use in the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), batteries, and fuel cells. 311,325–327

Nanoporous polymers, including nanoporous coordination polymers and crystalline nanoporous polymers, have emerged as impressive nanoporous materials. 328 Nanoporous polymers have many applications, and these materials are extensively being evaluated for gas separation and gas storage. The great interest in these applications arises from the presence of pores providing an exceptionally high Brunauer–Emmett–Teller (BET) surface area. Recently, new classes of metal organic framework and covalent organic framework porous materials have been reported that have shown exceptionally high and unprecedented surface areas. For instance, in 2010, a MOF was reported with a surface area of 6143 m 2 g −1 ; 329 in 2012, a MOF was reported with a surface area greater than 7000 m 2 g −1 ; 330 and in 2018, a MOF (DUT-60) was reported with a record surface area of 7836 m 2 g −1 . 331 Mesoporous DUT-60 has also shown a high free volume of 90.3% with a density of 0.187 g cm −3 . 331

Due to their exceptionally high surface areas and porous networks, these MOFs and COFs are ideal for gas storage. Air separation and post-combustion CO 2 capture have become integral parts of mainstream industries related to the energy sector in order to avoid substantial economic penalties. Due to the inefficiencies of available technology and the critical importance of this area, earnest efforts are being made to design gas-selective porous materials for the selective adsorption of desired gases. Nanoporous MOF- and COF-based materials can significantly capture CO 2 and help reach zero or minimum CO 2 emission levels. For instance, nanoporous fluorinated metal–organic frameworks have shown the selective adsorption of CO 2 over H 2 and CH 4 . 332 Hasmukh A. Patel et al. developed N 2 -phobic nanoporous covalent organic polymers for the selective adsorption of CO 2 over N 2 . The azo groups in the framework rejected N 2 , leading to CO 2 selectivity. 333 Nanoporous polymers that are superhydrophobic in nature can also be used for volatile organic compounds and organic contaminants. 334 Nanoporous polymers, due to the presence of a porous network, have been considered as highly suitable materials for catalyst supports. Furthermore, organocatalytic functional groups can be introduced pre-synthetically and post-synthetically into solid catalysts. 335

Nanoporous polymeric materials are amazingly heading towards being extremely lightweight with exceptionally high surface areas. These high surface areas and the fine-tuning of the nanopores has made these nanoporous materials, specifically MOFs and zeolites, ideal support materials for encapsulating ultrasmall metal nanoparticles inside void spaces to produce nanocatalysts with exceptionally high efficiencies. 336 In the coming years, more exponential growth of nanoporous materials is expected in the energy, targeted drug delivery, catalysis, and water treatment fields.

5.3. Ultrathin two-dimensional nanomaterials beyond graphene

However, from a material synthesis standpoint, a graphite-like layered form of Si does not exist in nature and there is no conventional exfoliation process that can generate 2D silicene, although single-walled 351 and multi-walled 352 silicon nanotubes and even monolayers of silicon have been synthesized via exfoliation methods. 353 Forming honeycomb Si nanostructures on substrates like Ag(001) and Ag(110) via molecular beam deposition, so-called “epitaxial growth”, was then proposed as a method for the architectural design of silicene sheets. 354–356 The successful synthesis of a silicene monolayer was first achieved on Ag(111) and ZrB 2 (0001) substrates in 2012; 357,358 later, various demonstrations were made using Ir(111), ZrB 2 (001), ZrC(111), and MoS 2 surfaces as the silicene growth substrates. 359–361 Despite various extensive studies to date involving the “epitaxial growth” of silicene on different substrates and investigations of the electronic properties, 357,362–364 the limited nanometer size, difficulties relating to substrate removal, and air stability issues have substantially impeded the practical applications of silicene. Bearing in mind all these known difficulties, Akinwande and co-workers recently reported a growth–transfer–fabrication process for novel silicene-based field-effect transistor development that involved silicene-encapsulated delamination with native electrodes. 365 An etch-back approach was used to define source/drain contacts in Ag film. Without causing any damage to the silicene, a novel potassium-iodide-based iodine-containing solution was used to etch Ag, avoiding rapid oxidation, unlike other commonly used Ag etchants. The results demonstrated that this was the first proof-of-concept study confirming the Dirac-like ambipolar charge transport predictions made about silicene devices. Comparative studies with a graphene system, the low residual carrier density, and the high gate modulation suggested the opening of a small bandgap in the experimental devices, proving that silicene can be considered a viable 2D nanomaterial beyond graphene.

Nonetheless, the synthesis of silicene on a large-scale is greatly limited, as “epitaxial growth” is the only promising method for obtaining high-quality silicene, and this presents an enduring challenge in relation to silicene research and development. Xu and co-workers recently introduced liquid oxidation and the exfoliation of CaSi 2 as a means for the first scalable preparation of high-quality silicene nanosheets. 366 This new synthetic strategy successfully induced the exfoliation of stacked silicene layers via the mild oxidation of the (Si 2 n ) 2 n layers in CaSi 2 into neutral Si 2 n layers without damage to the pristine silicene structure ( Fig. 29 ). The selective oxidation of pristine CaSi 2 into free-standing silicene sheets without any damage to the original Si framework was carried out via exfoliation in the presence of I 2 in acetonitrile solvent. Furthermore, the obtained silicene sheets yielded ultrathin monolayers or layers with few-layer thickness and exhibited excellent crystallinity. This 2D silicene nanosheet material was extensively explored as a novel anode, which was unlike previously developed silicon-based anodes for lithium-ion batteries. It displayed a theoretical capacity of 721 mA h g −1 at 0.1 A g −1 and superior cycling stability of 1800 cycles. Overall, during the last decade, silicene has been widely accepted as an ideal 2D material with many fascinating properties, suggesting great promise for a future beyond graphene.

Like other 2D materials, MXenes exhibit crystal geometry with a hexagonal close-packed structure based on the equivalent MAX-phase precursor, and the close-packed structure is formed from M atoms with X atoms occupying octahedral sites. 371 According to the formula, there are three representative structures of MXenes: M 2 XT x , M 3 X2T x , and M 4 X3T x . In these combinations, X atoms are formed with n layers, whereas M atoms have n + 1 layers ( Fig. 30 ). 372 Apart from graphene, MXenes are considered the most dynamic developing material, and they have incredible innovation potential amongst typical 2D nanomaterials because of their remarkable properties, such as hydrophilicity, conductivity, considerable adsorption abilities, and catalytic activity. These vital properties of MXenes suggest their use for various potential applications, including in the photocatalysis, electrocatalysis, 373,374 energy, 375 membrane-based separation, 376,377 and biological therapy 378 fields. In this section, we focus on describing new developments relating to MXenes that are utilized for electrocatalytic and energy storage applications, competing as alternatives to graphene materials.

Interestingly, due to the presence of abundant terminal groups, mainly –O, –OH, and –F, and their modifying nature, MXenes can exhibit outstanding hydrophilic properties and high conductivity and charge carrier mobility, making them a very attractive material for various electrocatalytic applications, such as the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, and CO 2 reduction reaction. To further increase their electrocatalytic activities, recent works involving MXenes have included incorporation with CNTs, 379 g-C 3 N 4 , 380 FeNi-LDH, 381 NiFeCo-LDH, 382 and metal–organic frameworks. 383

Cho and co-workers designed and developed MXene–TiO 2 2D nanosheets via the surface oxidation of MXene with defect-free control. These MXene–TiO 2 2D nanosheets were successfully implemented in nano-floating-gate transistor memory (NFGTM) providing a floating gate ( i.e. , multilayer MXene) and tunneling dielectric ( i.e. , the TiO 2 layer). A process of oxidation in water further represented a cost-effective and environmentally benign method, as depicted in Fig. 31 . The MXene NFGTM with an optimal oxidation process displayed exceptional nonvolatile memory features, having a great memory window, high programming/erasing current ratio, long term retention, and high durability. 384

There have been some exciting reports on 2D materials from the pnictogen family, particularly phosphorene. Recently, more attention has also been given to the remaining group 15 elements, 390 with the novel 2D materials arsenene, antimonene, and bismuthene being obtained from the key elements arsenic, antimony, and bismuth, respectively. It is reported that 2D monolayers of group 15 elements, including phosphorene allotropes, have five distinct honeycomb (α, β, γ, δ, and ε) and four distinct non-honeycomb (ζ, η, θ, and ι) structures, as depicted in Fig. 32 . Dissimilar crystal orientations were found for single-layered As, Sb, and Bi. Zeng and co-workers also reported comprehensive density functional theory (DFT) computations that proved the energetic stability and broad-range application of these materials in 2D semiconductors. 391 Previously, following theoretical predictions, Wu and co-workers successfully demonstrated that α-phosphorene showed lowest energy configurations in both honeycomb and non-honeycomb nanosheets. 392 In contrast, Zeng and co-workers proved that the buckled forms of 2D sheets of As, Sb, and Bi allotropes are the most stable structures, particularly their β phases. 391

Among monolayer group 15 family materials, 2D sheets of arsenic (As) and antimony (Sb) have gained considerable attention from researchers. 393,394 Studies have shown that As and Sb exhibit better stability than black phosphorus; they are highly stable at room temperature and less reactive to air, likely inhibiting the oxidization process. 395–398 Nevertheless, it has been demonstrated that the oxidation process is perhaps favorable for fine-tuning the electronic properties; increases in the indirect band gaps ranging from 0 to a maximum of 2.49 eV are found in free-standing arsenene and antimonene semiconductors. 399–403 Simultaneously, arsenene and antimonene can also be transformed into semiconductors with direct band gaps. These two 2D nanosheets can be used to design mechanical sensors, moving beyond common electronic and optoelectronic applications. These two extraordinary 2D nanosheets have been studied for their structural–property relationships via first-principles methods. 403–405

Continuing the characterization and structural property studies of arsenene carried out by Kamal 404 et al. and Zhang 403 et al. , Anurag Srivastava and co-workers analyzed applications of arsenene to explore the possibility of improving sensor devices that can be utilized to detect ammonia (NH 3 ) and nitrogen dioxide (NO 2 ) molecules. 406,407 They investigated the affinities of NH 3 and NO 2 molecules for pristine arsenene sheets, examining the binding energies, bonding distances, density distributions, and current–voltage features. The results showed that arsenene 2D sheets are highly durable, with significant electronic charge transfer. They also considered germanium-doped arsenene and characterized the 2D lattice based on molecular affinity relationships with respect to the dopant.

However, the incorporation of any dopants into 2D nanomaterials not only results in experimental difficulty but it also lowers the stability of 2D materials. 408 Recently, Dameng Liu and co-workers reported the electronic structures, focusing on band structures, band offsets, and intrinsic defect properties, of few-layer arsenic and antimony. 409 The spontaneous oxide passivation layer that is formed naturally on pristine antimonene provides excellent stability. 410 Very recently, Stefan Wolff and co-workers conducted DFT calculations on various single or few-layer antimony oxide structures to describe the stoichiometry and bonding type. Interestingly, the samples exhibited various structural stabilities and electronic properties with a wide range of direct and indirect band gaps. Showing band gaps between 2.0 and 4.9 eV, these 2D layers of antimonene exhibited the potential to be used as insulators or semiconductors. 411 The same group also analyzed Raman spectra and discussed identifying the predicted antimonene oxide structures experimentally. The enduring task of exploring the utility of antimonene has boosted recent research interest in 2D nanomaterials due to the broad range of potential applications, such as their use in electrochemical sensors, 412,413 stable organic solar cells, 414 and supercapacitors 415 to name a few.

The 2D MOF nanosheets are also evaluated for the development of high-performance power-storage devices. For example, Li et al. 427 recently reported two novel Mn-2D MOFs and Ni-2D MOFs as anode materials for rechargeable lithium batteries. The Mn-based ultrathin metal–organic-framework nanosheets, due to thinner nanosheets, a higher specific surface area, and smaller metal ion radius, had structural advantages over Ni-based ultrathin metal–organic-framework nanosheets. Due to these features, the Mn-based ultrathin metal–organic-framework nanosheets displayed a high reversible capacity of 1187 mA h g −1 at 100 mA g −1 for 100 cycles and a rate capability of 701 mA h g −1 even at 2 A g −1 .

The expensive metal oxides utilized in the catalytic process can be replaced in due course by 2D-MOF-based nanosheets with exposed metal sites that impart an adjustable pore structure, ultrathin thickness, a high surface-to-volume atom ratio, and high design flexibility. As a result, 2D-MOFs have extensively been explored for various electrocatalytic applications, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and carbon dioxide reduction reaction (CO 2 RR). For example, Marinescu et al. 428 combined cobalt dithiolene species with benzenehexathiol (BHT) and yielded 2D-MOFs capable of acting as electrocatalysts for the HER in water ( Fig. 34 ). In the presence of 2D-MOF sheets, a high current density of 41 mA cm −2 , at −0.8 V vs. SHE and a pH value of 1.3, is observed. Similarly, Feng et al. 429 also developed single-layer Ni-based 2D-MOF sheets that are highly effective for electrocatalytic hydrogen evolution. Later, Patra et al. 430 reported similar 2D sheets from covalent organic frameworks (2D-COFs) as metal-free catalysts for HER applications. 2D-MOFs are also being explored as active catalysts for the OER process. For example, Xu et al. 431 reported the preparation of 2D Co-MOF sheets using polyvinylpyrrolidone as a surfactant under mild solvothermal conditions. These novel 2D Co-MOFs displayed ultrathin nanosheets with many surface-based metal active sites, improving the overall OER performance.

Interestingly, experimental electrochemical measurement data showed that Co-MOF sheets offer a low overpotential ( i.e. , 263 mV at 10 mA cm −2 ). Similarly, Wang et al. 432 also reported that double-metal 2D-sheets (2D NiFe MOFs) consisting of a very ultrathin structure with a thickness of ∼10 nm further offer a low overpotential of 260 mV at 10 mA cm −2 . In other reports, Zhang et al. 433 successfully performed the OER process with ultrathin 2D-MOF sheets prepared via electrochemical and chemical exfoliation strategies.

Recent work on the catalytic activity of 2D-MOFs has also been reported in relation to the ORR and CO 2 RR because of their layered crystal structures and high-volume modifiable porous structures. For example, Dincă et al. 434 demonstrated that ultrathin layered conductive sheets of the 2D-MOF Ni 3 (HITP) 2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) could actively be utilized as a catalyst in an alkaline medium for the ORR process. These 2D-MOF sheets show high stability while retaining 88% of the initial current density over 8 h at 0.77 V vs. RHE. In another report, through fabricating Co x Zn 2− x (bim) 4 2D-sheets as precursors, Zhao et al. 435 successfully synthesized cobalt nanodots (Co-NDs) with bimetallic Co x Zn 2− x (bim) 4 nanosheets encapsulating few-layer graphene (Co@FLG). For the CO 2 RR, a cobalt–porphyrin-containing 2D-MOF was achieved for the selective electrochemical reduction of CO 2 to CO with enhanced stability by Peidong Yang and co-workers. 436 The results further proved that these thin-film catalysts have the highest selectivity for CO ( i.e. , 76%) at −0.7 V vs. RHE with the little-to-no substantial decrease in activity over 7 h at −0.7 V vs. RHE, and 16 mL of CO was produced. Besides, like many other porous materials, 2D-MOFs were also shown to be a supporting platform for catalytic nanoparticles because of their high specific surface areas and favorable porosity distributions. To this end, an example can be noted from Wang et al. 437 reporting that fine porous MOF-5 nanosheets can be utilized to immobilize Pd nanoparticles.

5.4. Metal-based nanostructured materials

As discussed, catalysis is one of the main uses of metal-based nanostructured materials. A continuous increase in the demand for energy, the rapid depletion of conventional energy reservoirs, and rising concerns over the emission of CO 2 have increased the challenges and urgency in the energy field. 460 Metal-based nanostructured materials are extensively being explored to produce alternative clean and renewable energy sources. A range of metal-based nanomaterials has been evaluated and is under consideration for developing robust electrodes that can be effectively applied to water splitting, batteries, and solar cells.

High energy demands have led to more pressure to improve the performances of existing highly demanded lithium-ion batteries. Researchers have focused on improving their lifetimes, sizes, and safety. 462 Nanostructured metal-oxide-based materials are promising electrode materials for use in high-performance charge-storage devices. A metal-based nanostructured electrode is evaluated as both the anode and cathode to overcome the challenges of conventional electrodes. 463 In a conventional LIB, LiCoO 2 was used as the cathode material. Controlled morphology plays a crucial role in determining the performance of a material. Powder composed of spherical particles of LiNi 0.8 Co 0.2 O 2 showed a higher tap density compared to irregular particles and the material substantially improved the power density of secondary lithium batteries. 464 Hierarchical nanostructures of metal-based oxides (such as 3D hierarchical ZnCo 2 O 4 nanostructures) have emerged as a new trend for the development of high-capacity electrodes for lithium-ion batteries. 465 Since their commercialization by Sony in the early 1990s, LIBs have achieved tremendous success in bringing portable electronic devices to the market. However, their sustainable development on the grid-scale is hampered due to limited Li resources in nature, and this is causing a continuous increase in cost. 466 Sodium-ion batteries are in the spotlight to replace powerful lithium-ion batteries due to the widespread availability of sodium and its lower cost compared with lithium. 467 It is essential to note that, in terms of energy densities for SIBs, it is difficult to bypass LIBs because of the low standard electrochemical potential and higher weight of Na. SIBs could be proved to be ideal for those applications where cost is a critical factor compared to energy density. 466

SIBs also operate similarly to LIBs, based on an intercalation mechanism. SIBs also consist of cathode and anode electrodes separated through an electrolyte. During the charging process, sodium ions are extracted from the cathode and inserted into the anode via the electrolyte. In the discharging process, the electrons leave the anode through an external circuit to reach the cathode, providing electricity to the load, whereas Na + moves to the cathode during this process. The radius of Na + (1.02 Å) is greater than that of Li + (0.76 Å), making it challenging to intercalate into electrode materials. 468 Thus, appropriate electrode materials are required in which fast Na-ion insertion and extraction is possible. However, SIBs are suffering from a lack of appropriate electrode materials. It is important to develop electrode materials that have enough interstitial space within their crystallographic structures and better electrochemical performance. Among the various proposed electrode materials, Na x MO 2 layered transition-metal oxides (M = V, Fe, Cu, Co, Ni, Cr, Mn, and their combinations) are considered to be promising electrode materials for SIBs. Layered metal oxides are considered to be promising electrode materials due to their facile scalable synthesis, simple structures, appropriate operating potentials, and high capacities. 469,470 Large volume expansion and poor kinetics during the charge–discharge process can severely affect the cyclability and performance of SIBs. One of the effective strategies to deal with the mechanical stress triggered by large volume changes is the design of hollow or porous structures. In response, three-dimensional network-based Sb 2 O 3 @Sb composite anode materials can help to relieve the volume-change-related stress through their uniform porous networks and provide better transportation channels for Na + . 471

The large volume expansion of electrodes can also be buffered via designing 2D metal-oxide materials with large interlayer spacing. The ultrathin nanosheets provide high reversible capacity with enhanced cycling stability and contribute to providing reaction sites for electrons/ions, decreasing the diffusion distance, providing effective diffusion channels, and facilitating fast charge/discharge for sodium and lithium. 2D SnO nanosheet anodes were evaluated for SIBs. The capacity and cyclic stability improved, as the number of atomic SnO layers is decreased in the sheets. 472 Sb is a promising anode material, but during the sodiation/desodiation processes, huge volume expansion of 390% is observed, which hinders its practical use. Nanostructured Sb in the form of nanorod arrays with large interval spacing displays the great capacity to accommodate volume changes during cycling. 473 A comparison of various nanostructured metal-based electrodes for various charge storage purposes is shown in Table 3 . Overall, well-structured metal or metal-based oxide nanomaterials have the capacity to resolve current issues relating to charge storage devices.

Recently, an immense focus of research has been to produce H 2 fuel via water-splitting to replace conventional fossil fuels. This will help to eliminate emissions from the use of carbonaceous species. 484 Electrochemical method are considered simple water splitting approaches, as these methods only require an applied voltage and water as inputs to produce hydrogen fuel. 485 The coupling of solar irradiation to electrochemical water splitting has enhanced the performance and reduced the process cost. Due to these reasons, this has become a hot area of research. 486 During water electrolysis, H 2 is produced through the hydrogen evolution reaction at the cathode and O 2 is produced through the oxygen evolution reaction at the anode. However, water splitting is not so straightforward, and it requires an efficient catalyst that can facilitate the splitting of water. Metal- and metal-oxide-based catalysts are extensively being explored for water splitting. For the HER reaction, Pt-based catalysts are found to be suitable, whereas for OER reactions, Ir-/Ru-based compounds are found to be benchmark catalysts. Scarcity and high cost have limited the widespread use of these metals. The barrier of noble-metal cost can be mitigated through developing noble-metal nanostructured surfaces that produce more active sites or via depositing monolayers of noble metals on low-cost materials. The alloying of noble metals with other metals has enhanced site-specific activity. 484 At present, more focus is being placed on developing noble-metal-free catalysts for water splitting. 485 Usually, an efficient electrocatalyst is characterized by: 487 a low overpotential; high stability; low production costs; and high electrocatalytic activity.

The nano-structuring of catalysts is an effective tool to boost their surface areas. The electrolysis of water occurs at the surface of a catalyst, and nanostructured catalysts provide more active sites and the better diffusion of ions and electrolytes. 484 Non-noble metals that are under observation for the development of HER electrocatalysts include nickel (Ni), tungsten (W), iron (Fe), molybdenum (Mo), cobalt (Co), and copper (Cu). 487 For instance, a noble metal-free catalyst, carbon-decorated Co 3 O 4 nanoarrays on carbon paper, required a small overpotential of 370 mV to reach a current density of 10 mA cm −2 . It can maintain a current density of 100 mA cm −2 for 413.8 h and 86.8 h under alkaline and acidic conditions, respectively. 488

Metal-based semiconductor materials play a crucial role in a range of applications. For photoelectrochemical water splitting, the semiconductor material plays a central role in the solar-to-hydrogen conversion efficiency. Some critical features are prerequisites when it comes to selecting the right semiconductor material for the photoelectrochemical splitting of water: 489 an extraordinary capacity to absorb visible light; an appropriate bandgap; suitable valence and conduction band positions; commercial feasibility; and chemical stability.

For an ideal semiconductor for water splitting, the valence band and conduction band edge positions must straddle the oxidation and the reduction potentials of water. Metal oxides have received significant attention among semiconductors due to their wide band gap distributions, remarkable photo-electrochemical stabilities, and favorable band edge positions. 490 Semiconductor-based photoelectrodes become excited upon light irradiation, and electrons from the valence band move to the unoccupied conduction band. Some of the generated electrons at the cathode surface reduce protons to hydrogen gas, whereas holes at the photoanode produce oxygen gas via water splitting. 490 As a result, various nanostructured metal oxides can be used as photoelectrode materials, such as WO 3 , 491 Cu 2 O, 492 TiO 2 , 493 ZnO, 494 SnO 2 , 495 BiVO 4 , 496 and α-Fe 2 O 3 , 490 for the efficient splitting of water. As discussed, the nano-structuring of semiconductors can significantly impact the electrode photoelectrochemical performance during water splitting.

Metal-based nanomaterials have been used for the development of sensitive sensors. These metal-based sensors can replace the complex and expensive instruments that are conventionally used for the sensing of analytes. Metal-oxide-based sensors have the interesting characteristics of low detection limits, low cost, high sensitivity, and facile operation. 497 Mostly, semiconducting metal-oxide-based sensors are used for the sensing of toxic, flammable, and exhaust gases. Semiconductor metal oxides with a size in the range of 1–100 nm have been significantly investigated as gas sensors due to their size-dependent properties. The geometry and size of a nanomaterial can considerably affect the hole and electron movement in semiconductors. 498 The surface-to-volume ratio and surface area are substantially enhanced at the nanoscale level, and this is amazingly beneficial for sensing. Chemiresistive semiconducting metal oxides are potential candidates for gas sensing due to the following features: 499 rapid response times; fast recovery times; low cost; simple electronic interfaces; user-friendliness and low maintenance; and abilities to sense a wide range of gases.

Electrode materials decorated with metal- or metal-oxide-based nanostructured materials have shown better responses and selectivity for determining various analytes over conventional electrode materials. The nano-sized metal structures act as an electrocatalyst and electronic wires to provide rapid electron transfer between the transducers and analyte molecules. 500 The electrochemical redox reaction of H 2 O 2 can be improved via the thermally controlled anchoring of Pt NPs on the electrode surface. 501

Currently, researchers are not just concentrating on the development of randomly shaped nanomaterials; instead, they are very focused on and interested in the rational design of materials with controlled nano-architectures for boosting their performances for specific applications. As a result, extensive research has been carried out to develop metal-based materials with controlled dimensions to achieve better catalytic responses. Particle morphology is a crucial factor in the performance of nanomaterials for specific applications. Laifa Shen et al. rationally designed an electrode architecture via growing mesoporous NiCo 2 O 4 nanowire arrays on carbon textiles, which boosted the electrode performance ( Fig. 37 ). 474

The same materials with different morphologies can produce different outcomes. For instance, MnO 2 nanoflowers have provided high initial sodium-ion storage capacity compared with MnO 2 nanorods. 481 Radha Narayanan and Mostafa A. El-Sayed have analyzed various nanoscale morphologies of Pt, such as tetrahedral, cubic, and near-spherical nanoparticles. The highest rate constant is observed with tetrahedral nanoparticles and the lowest rate constant was observed with cubic nanoparticles, whereas spherical nanoparticles exhibited an intermediate rate constant during catalysis. 502 Xiaowei Xie et al. found that Co 3 O 4 nanorods show high activity compared to conventional Co 3 O 4 nanoparticles for the low-temperature oxidation of CO. 503 The catalytic activity of metal-based nanomaterials is strongly affected by their shape. 504 Shape-defined mesoporous materials (TiO 2 ) have shown superior photoanode activities ( Fig. 38 ). 505 As a result, in the literature, several nanostructured morphologies of metal-based materials, such as nanotubes, 506,507 nanorods, 508,509 nanoflowers, 510 nanosheets, 511 nanowires, 512 nanocubes, 513 nanospheres, 514,515 nanocages, 516 and nanoboxes, 517 have been reported for a range of applications.

Hollow nanostructures have surfaced as an amazing class of nanostructured material, and they have received significant attention from researchers. Hollow nanostructures have the unique features of: 518,519 low density; abundant inner void spaces; large surface areas; and the ability to act as nanoscale containers with high loading capacity, nanoreactors, and nanocarriers.

Various metal-based hollow nanostructures, such as hollow SnO 2 , 520 hollow palladium nanocrystals, 521 Co–Mn mixed oxide double-shell hollow spheres, 521 hollow Cu 2 O nanocages, 522 three-dimensional hollow SnO 2 @TiO 2 spheres, 523 hollow ZnO/Co 3 O 4 nano-heterostructure, 524 triple-shell hollow α-Fe 2 O 3 , 525 and hierarchical hollow Mn-doped Ni(OH) 2 nanostructures, 526 have been developed for various applications. The presence of nanoscale hollow interiors and functional shells imparts them with great potential for gas sensing, catalysis, biomedicine, energy storage, and conversion. 519

From this discussion, it can be concluded that metal-based nanostructured materials have great potential compared to their bulk counterparts. The conversion of materials to the nanoscale is not enough to achieve high performance with better selectivity. Now, research is switching from conventional nanomaterials to more advanced and smartly designed nanomaterials. In modern research, nanomaterials are being designed with better-controlled morphologies and regulated features.

5.5. Core–shell nanoparticles

A spherical nanoparticle core–shell nanostructure is a practical way to introduce multiple functionalities on the nanoscopic length scale. 528 The properties arising from the core or shell can be different, and these properties can be tuned via controlling the ratio of the constituent materials. The shape, size, and composition play a critical role in tuning the core–shell nanoparticle properties. 529 The shell material can help to improve the chemical and thermal stabilities of the core material. The core–shell design has become effective where an inexpensive material cannot be used directly due to its instability or easily oxidizable nature. The core can consist of an easily oxidizable inexpensive metal, whereas the shell might consist of noble metals, oxides, polymers, or silica. 530 For instance, magnetic nanoparticles when prepared can be sensitive toward air, acids, and bases. Magnetic nanoparticles can be protected via coating with organic or inorganic shells. 528

Core–shell metal nanoparticles are an emerging nanostructured material with great potential in the fields of energy and catalysis. 531 The first report of core–shell nanoparticles (2007) for supercapacitor applications consisted of a polyaniline/multi-walled-carbon-nanotube composite (PANI/MWNTs). 532 Metal-based core–shell structured nanoparticles have shown enhanced catalytic performance due to their shape-controlled properties. 533 Ming-Yu Kuo et al. developed Au@Cu 2 O core–shell particles with controllable shell thicknesses that acted as a dual-functional catalyst. The shell thickness of Cu 2 O increased with an increasing concentration of Cu 2+ precursor. The thicknesses of the shells of Au@Cu 2 O-1.5 (12.2 ± 1.7 nm), Au@Cu 2 O-2 (13.2 ± 1.8 nm), Au@Cu 2 O-3 (18.2 ± 2.2 nm), and Au@Cu 2 O-4 (20.8 ± 2.5 nm) due to various concentrations are shown in Fig. 40 . 534 A NiO@SiO 2 core–shell catalyst provided a higher yield of acrylic acid from acetylene hydroxycarbonylation. 535 Core–shell architecture can be used to prevent active metal nanoparticles from oxidation during operation. For instance, a plasmonic photocatalyst was developed that consisted of silver nanoparticles embedded in titanium dioxide. The direct contact of Ag with TiO 2 could lead to its oxidization; this is prevented via developing core–shell architecture in which Ag is used as the core and SiO 2 is used as a shell to protect it. 536 Another excellent option is to replace an expensive core with a non-noble metal to reduce the core–shell cost while using a thin layer of a noble metal that consumes a small amount of metal as the shell. This will ensure the prolonged stability of the catalyst during operation. 533 Overall, core–shell morphologies provide better catalytic activity due to the synergistic effect of the metallic core–shell components. 152

Among the several classes of nanomaterials, core–shell nanoparticles are found to be more promising for different biomedical applications. For instance, magnetic nanoparticles are considered to be useful for biomedical applications due to the following reasons: (a) aggregation is prevented due to superparamagnetism; (b) delivery and separation can be controlled using an external magnetic field; (c) they can be appropriately dispersed; and (d) there is the possibility of functionalization. A range of magnetic nanoparticles is available, such as NiO, Ni, Co, and Mn 3 O 4 . The most famous example is iron oxide, but uncoated iron oxides are unstable under physiological conditions. This may result in controlled drug delivery failure due to improper ligand surface binding and the promotion of the formation of harmful free radicals. Therefore, the formation of shells around magnetic nanoparticles has tremendous significance for biomedical applications. 537 One of the approaches is to use gold shells on magnetic nanoparticles. Au NPs are also called surface plasmons and they substantially enhanced the absorption of light in the visible and near-infrared regions. Thus, coating magnetic nanoparticles with a Au shell can result in a core–shell nanostructure that displays both optical and magnetic functionality in combination. 529

Numerous biocompatible core–shell nanoparticles are being developed for photothermal therapy, as core–shell materials are found to be useful for photothermal therapy. Hui Wang et al. have developed bifunctional core–shell nanoparticles for dual-modal imaging-guided photothermal therapy. The core–shell nanoparticles consist of a magnetic ∼9.1 nm core of Fe 3 O 4 covered by an approximately 3.4 nm fluorescent carbon shell. The Fe 3 O 4 core leads to superparamagnetic behavior, whereas the carbon shell provides near-infrared (NIR) fluorescence properties. The bifunctional nanoparticles have shown dual-modal imaging capacity both in vivo and in vitro . The iron oxide–carbon core–shell nanoparticles absorbed and converted near-infrared light to heat, facilitating photothermal therapy. 538 Au-Based core–shell structures are also being prepared for photothermal therapy. Bulk gold is biocompatible, but Au NPs can accumulate in the spleen and liver, causing severe toxicity. Koo Chul Kwon et al. have developed Au-NP-based core–shell structures that did not result in any gross or histological lesions in the major organs of mice, which revealed that this is a potent and safe agent for photothermal cancer therapy. The core–shell nanoparticles consisted of proteinticle/gold (PGCS-NP) and were developed via proteinticle surface engineering. PGCS-NP was injected intravenously into mice with tumors, and the injected core–shell nanoparticles successfully reached the EGFR-expressing tumor cells. The tumor size was significantly reduced upon exposure to near-infrared laser irradiation ( Fig. 41 ). No accumulation of Au NPs was observed in the mice organs, which indicated that PGCS-NP disassembled into many tiny gold dots, which were easily excreted by the kidneys and liver without causing any toxicity. 539 In another example, multifunctional Au@graphene oxide nanocolloid core@shell nanoparticles were developed, in which the core and shell consisted of gold and a graphene oxide nanocolloid, respectively. The developed core–shell structure showed multifunctional properties, allowing Raman bioimaging and photothermal/photodynamic therapy with low toxicity. 540 Apart from this, numerous other core–shell nanoparticles, such as polydopamine–mesoporous silica core–shell nanoparticles, 541 AuPd@PVP core–shell nanoparticles, 542 Au@Cu 2− x S core–shell nanoparticles, 543 bismuth sulfide@mesoporous silica core–shell nanoparticles, 544 and Ag@S-nitrosothiol core–shell nanoparticles, have been used for photothermal therapy. 545

Due to their unique features and the combination of properties from the shell and core, these core–shell nanoparticles have received considerable interest in many fields, ranging from materials chemistry to the biomedical field. For electrochemical reactions, the core–shell structure conductivity can be enhanced via conducting polymers, carbon materials, and metals. Core–shell nanoparticles as electrode materials showed better performance compared to single components. Most of the core materials are prepared via hydrothermal methods, and shells can be prepared via hydrothermal or electrodeposition methods. 546 Even though significant progress has been made relating to the synthesis methods of core–shell materials, a major challenge is the high-quality production of core–shell materials in more effective ways for required applications, specifically biomedical applications.

6. Challenges and future perspectives

(a) The presence of defects in nanomaterials can affect their performance and their inherent characteristics can be compromised. For instance, carbon nanotubes are one of the strongest materials that are known. However, impurities, discontinuous tube lengths, defects, and random orientations can substantially impair the tensile strength of carbon nanotubes. 547

(b) The synthesis of nanomaterials through cost-effective routes is another major challenge. High-quality nanomaterials are generally produced using sophisticated instrumentation and harsh conditions, limiting their large-scale production. This issue is more critical for the synthesis of 2D nanomaterials. Most of the methods that have been adopted for large-scale production are low cost, and these methods generally produce materials with defects that are of poor quality. The controlled synthesis of nanomaterials is still a challenging job. For example, a crucial challenge associated with carbon nanotube synthesis is to achieve chiral selectivity, conductivity, and precisely controlled diameters. 548,549 Obtaining structurally pure nanomaterials is the only way to achieve the theoretically calculated characteristics described in the literature. More focused efforts are required to develop new synthesis methods that overcome the challenges associated with conventional methods.

(c) The agglomeration of particles at the nanoscale level is an inherent issue that substantially damages performance in relevant fields. Most nanomaterials start to agglomerate when they encounter each other. The process of agglomeration may be due to physical entanglement, electrostatic interactions, or high surface energy. 550 CNTs undergo van der Waals interactions and form bundles, making it difficult to align or properly disperse them in polymer matrices. 159 Similarly, graphene agglomeration is triggered by the basal planes of graphene sheets due to π–π interactions and van der Waals forces. Due to severe agglomeration, the high surface areas and other unique graphene features are compromised. These challenges hinder the practical application of high-throughput electrode materials or composite materials for various applications. 551

(d) The efficiency of nanomaterials can be tuned via developing 3D architectures. 3D architectures have been tried with several nanomaterials, such as graphene, to improve their inherent features. 3D architectures of 2D graphene have provided high specific surface areas and fast mass and electron transport kinetics. This has become possible due to the combination of the exceptional intrinsic properties of graphene and 3D porous structures. 194,552 The combination of graphene and CNT assemblies into 3-D architectures has emerged as the most investigated nanotechnology research area. Porous architectures of other nanomaterials can be developed to enhance their catalysis performance through providing nanomaterial interior availability.

(e) 2D ultrathin materials are an outstanding class of nanomaterial with promising theoretical properties; however, very little experimental evaluation of these materials has been done, apart from the case of graphene. The synthesis and stability of 2D ultrathin materials are some of the major challenges associated with them. In the future, more focus is anticipated to be placed on their synthesis and practical utilization.

(f) Nanomaterial utilization in industry is being increased, and there is also demand for nanoscale material production at higher rates. Moreover, nanotechnology research has vast horizons; the exploration of new nanomaterials with fascinating features will continue and, in the future, more areas will be discovered. One of the significant concerns relating to nanomaterials that cannot be overlooked is their toxicity, which is still poorly understood, and this is a serious concern relating to their environmental, domestic, and industrial use. The extent to which nanoparticle-based materials can contribute to cellular toxicity is unclear. 553 There is a need for the scientific community to put efforts into reducing the knowledge gap between the rapid development of nanomaterials and their possible in vivo toxicity. A proper and systematic understanding of the interaction of nanomaterials with cells, tissues, and proteins is critical for the safe design and commercialization of nanotechnology. 14

The future of advanced technology is linked with advancements in the field of nanotechnology. The dream of clean energy production is becoming possible with the advancement of nanomaterial-based engineering strategies. These materials have shown promising results, leading to new generations of hydrogen fuel cells and solar cells, acting as efficient catalysts for water splitting, and showing excellent capacity for hydrogen storage. Nanomaterials have a great future in the field of nanomedicine. Nanocarriers can be used for the delivery of therapeutic molecules.

7. Conclusions

Conflicts of interest, acknowledgements.

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• Published: 23 January 2023

The state of the art of nanomaterials and its applications in energy saving

• Hala. S. Hussein 1  

Bulletin of the National Research Centre volume  47 , Article number:  7 ( 2023 ) Cite this article

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Nanomaterials have emerged as a fascinating class of materials in high demand for a variety of practical applications. They are classified based on their composition, dimensions, or morphology. For the synthesis of nanomaterials, two approaches are used: top-down approaches and bottom-up approaches.

Main body of the abstract

Nanoscale materials and structures have the potential to be used in the production of newly developed devices with high efficiency, low cost, and low energy demand in a variety of applications. There are several contributions in renewable energy conversion and storage in the energy sector, such as solar photovoltaic systems, fuel cells, solar thermal systems, lithium-ion batteries, and lighting. Furthermore, nanofluid-based solar collectors are a new generation of solar collectors based on the use of nanotechnology. It has the potential to increase collector efficiency by up to 30%.

Short conclusion

Graphene and graphene derivatives are known as more efficient energy-saving materials, with the ability to maximize heat transfer efficiency and save up to 30% of energy in water desalination. Silver nanoparticles (Ag NPs) are a powerful antibacterial material that can kill a wide variety of microorganisms. They are commonly used in water treatment and are incorporated into polyethersulfone (PES) microfiltration membranes. The use of an Ag-PES membrane improved the antibiofouling performance of PES membranes. From the industrial application of nanotechnology, applications of TiO 2 -based nanocoatings that can be used as dust-repellent coatings for solar panels improve their efficiency and reduce the amount of required maintenance. Furthermore, the nanoscale dimension of these particles facilitates their movement in various body parts, resulting in serious diseases such as cancer and organ damage. As a result, it is suggested to focus in our incoming research on the disposal of nanomaterial waste and their safe application.

• Nanomaterials

Generally, any powdered materials with particle diameter ranged from 1 to 100 nm are categorized as nanosized materials (Manaktala and Singh 2016 ; Changseok et al. 2013 ). Accordingly, the nanomaterials have received much interest because of their high efficiency in many applications, such as smart coating devices (e.g., thermochromic, photochromic, and electrochromic devices), solar energy systems, and sensing. Also, they also improve efficiency and lower prices in a variety of fields, including solar photovoltaic systems, hydrogen production, and fuel cells. (Mageswari et al. 2016 ; Baig et al. 2021 ). Moreover, nanomaterials demonstrated unique chemical, physical, and biological properties that they can be applied in different fields.

Classification of nanomaterials

There are three types of nanomaterials based on their composition, including inorganic-, organic-, and carbon-based materials as shown in Table 1 . Owing to unique size, nanomaterials exhibited higher reactivity, high sensitivity, large surface area, and strength.

Additionally, the nanoparticles can be categorized according to their dimensions (Jeevanandam et al. 2018 ; Buzea et al. 2016 ) exhibiting one or more dimensions within the nanoscale. The nanomaterials made from different materials demonstrate one nanoscale dimension such as surface coating attached on a substrate. By contrast, nanomaterials with two dimensions are usually nanoparticles applied on a substrate, nanoporous alumina wires or tubes, and nanoporous thin films. Three-dimensional nanomaterials can be exhibited a small nanostructure of a substrate or nanoporous membranes on a substrate. Nanomaterials can also be classified based on their morphology, such as nanocubes and nanowires. Finally, they can be graded according to their uniformity and agglomeration as represented in Fig.  1

Classification of nanostructured materials based on their morphology, dimensionality, composition, uniformity, and agglomeration state (Baig et al. 2021 )

Synthesis of nanomaterials

As stated by Singh et al. 2020 , top-down and bottom-up methods are the main approaches used for synthesizing nanomaterials (Fig.  2 ). For top-down strategy, coarse materials are disintegrated into nanostructured particles using laser ablation, mechanical milling, etching, electroexplosion, and sputtering. Meanwhile, bottom-up strategy includes spinning, vapor deposition, and sol–gel process.

The synthesis of nanomaterials via top-down and bottom-up approaches (Baig et al. 2021 )

Top-down methods

Mechanical milling.

This method is considered as a cost-efficient approach, in which the bulky materials transformed into nanosized ones. A blend of different nanophases as well as nanocomposites, such as copper-based nanoalloys and aluminum nickel magnesium, can be effectively prepared using this method (Sharma et al. 2021 ) .

Electrospinning

It is regarded as one of the simplest top-down methods for synthesizing nano-based materials. This method is usually applied for synthesizing nanofiber-based polymers such as polyurethane nanofibrous membranes (Xiao et al. 2022 ).

Sputtering is a technique for producing nanoscale materials by blasting solid objects with high-energy particles delivered by plasma or gas. It is regarded as a viable technology for creating nanoscale thin film. Sputtering is a process in which intense gaseous ions bombard the target surface, causing the physical discharge of small-sized atom clusters based on the energy of the incident gaseous ion (Chodun et al. 2022 ).

Laser ablation

The process of laser ablation synthesis entails the formation of nanoparticles by shattering the target material with a powerful laser beam. In this method, high-energy laser irradiation caused a vaporization of the precursor or source material to yield nanosized materials. Owing to there is no need for chemical or stabilizing agent, laser ablation approach is categorized as a green method for synthesizing noble nanosized metals.

Bottom-up methods

Chemical vapor deposition (cvd).

The chemical vapor deposition approach was found to have a great importance in the formation of nanosized carbon-based materials, such as carbon nanotubes. The chemical reaction of vapor-phase materials produces a thin coating on the surfaces of substrate in CVD. The precursor should achieve the following requirements: high chemical purity, a nonhazardous nature, volatility, good stability during evaporation, a long shelf-life, and low cost. Furthermore, no residual impurities resulted from the decomposition process (Wang et al. 2022 ).

Hydrothermal methods

One of the most important and extensively utilized methods for producing nanosized materials is the hydrothermal process. In this method, nanoparticles synthesized though a heterogeneous reaction in a liquid medium at high temperature and pressure around the critical point in a sealed vessel. This approach exhibited different advantages over others. It can produce nanosized materials with no stability at high temperature. It can also generate nanosized materials with high vapor pressure with very low loss of starting materials. Hydrothermal method is beneficially used for synthesizing nanomaterials with different morphologies, such as such as nanorods, nanowires, nanosheets, and nanospheres (Gan et al. 2020 ).

The sol–gel method

The sol–gel method is a wet-chemical process for the creation of nanomaterials that has been widely employed. It was utilized to synthesize many types of nanomaterials-based metal oxide with high quality. The sol–gel method as eco-friendly approach exhibited many other advantages, including the production of nanomaterials with homogeneity, application of low processing temperature, and a facile way to produce complex nanostructures and composites (Khan et al. 2022 ).

Application of nanomaterials in energy sector

Energy has importance role in our daily life, and it is considered as the major resource for the human activity. According to the International Energy Agency (IEA), energy demand will continue to rise until 2030. Owing to increasing energy demand, there is a great need to new technology with low energy demand as a potential way to conserve energy. Lithium-ion batteries, light-emitting diode (LED), fuel cells, ultra-capacitor, and solar cell were used to conserve the energy as they improve the efficiency and application period. Nanotechnology is expected to contribute to low-cost and efficient energy generation, transmission, and storage systems in the future. The fabrication of materials and structures with nanoscale can potentially use for producing a newly developed devices with high efficiency, low cost, and low energy demand in many applications such as hydrogen production, solar photovoltaic systems, solar thermal systems, and energy saving technologies (Christian 2013 ; Yianoulis and Giannouli 2008 ). As a result, nanotechnology’s application in the field of energy is a hot topic in many scientific fields. The current trend is being hampered by the high cost of production compared to previous technologies. To obtain higher efficiency with low production cost, the priority should be given to nanotechnology in terms of energy. As shown in Fig.  3 , piezoelectric, thermoelectric, triboelectric, catalytic, and photovoltaic are the main nanomaterials, which strongly contributed to several energy applications. Inorganic nanoparticles have superior thermal and electrical conductivity, chemical stability, and a wide surface area due to their unique properties. For the application of energy generation, nanosized materials recorded two-time thermoelectric performance higher than those of conventional materials.

Different energy applications: energy generation, storage, conversion, and saving up on nanomaterials substances (Wang et al. 2020 )

As reported by International Energy Agency (IEA), the nanomaterials with high thermal insulation and energy efficiency will lead to conserve about 20% of the current energy consumption. There are three advantages, observed from the application of nanotechnology in the production of nanosized materials for renewable energy as follows:

An improvement in the efficiency of heating and lighting,

Higher capacity of electrical storage.

A significant reduction of the pollutants resulted from the use of conventional energy resources.

In energy conversion applications, the active sites of catalytic materials improved significantly by applying nanostructuring advanced strategy. The recent advances were found to have the potential to impel further the visions of alteration the hybrid properties at the nanoscale, which could lead to produce next generation materials for energy applications. Multifunctional nanomaterials research can be defined as the study on how the structures of materials govern their properties, including their fabrication and design (Wang et al. 2020 ).

Solar cell technology, as a valuable source of renewable energy, is nevertheless somewhat costly when compared to fossil fuels used to generate power. Although solar cells have a low efficiency, with a maximum of 30 percent, the most widely used kind has a 15–20 percent efficiency ( http://www.i-sis.org.uk/QDA 2011 ). Owing to losing more than a half of their efficiency through heat up, the conventional solar cells are inefficient. Recently, adding nanomaterials, such as fullerenes, carbon nanotubes (CNTs), and quantum dots, as shown in Figs. 4 and 5 to solar cell could increase its efficiency. Basically, nanotechnology technique can be beneficially used to build up high solar cells with high efficiency and low cost. Nanoparticles exhibited the following advantages in the solar power plants: -

Because of the small particle sizes, nanomaterials can easily pass through pumps and plumbing with no adverse effects.

Owing to high ability of nanofluids to absorb energy directly, they exceeded intermediate heat transfer steps.

High optical selectivity of nanofluids (i.e., low emittance in the infrared range and high absorption in the solar range).

Solar collector with nanomaterials exhibited more uniform receiver temperature, which associated with a reduction in material constraints.

The enhancement of the heat transfer after incorporating nanoparticles, as a result of thermal conductivity and higher convection may improve receiver performance.

The efficiency of absorption could be improved by tuning the nanoparticle shape and size to the appropriate application.

Zero-dimensional carbon nanomaterials, a carbon dots, and b fullerene. One-dimensional carbon nanomaterial, c carbon nanotube. Two-dimensional carbon nanomaterial, d graphene, and e graphene oxide (Han et al. 2015 )

Evolution of photovoltaic technology: from conventional (silicon-based solar cells) to nanostructured solar cells (quantum-based and dye-sensitized solar cells) (Sharma et al. 2018 ; https://www.gamry.com/application-notes/physechem/dssc-dye-sensitized-solar-cells )

Quantum-based solar cell

As shown in Fig.  5 , the addition of quantum dot crystals to the solar cell could improve the efficiency, whereas they possess high ability to absorb and convert light into electrical energy. This design is ideal for enhancing the solar cell efficacy from the small distance between the dots. Accordingly, multi release of electrons were gained via incorporation of quantum dots for solar cells. The efficiency of converting solar energy into electrical one increased by 42%.

Dye-sensitized solar cell (DSSC):

A dye-sensitized solar cells (DSSCs) are a class of low-cost solar cells belonging to the group of environmentally friendly thin-film solar cells. They have a good efficiency (nearly 20–30%) even under low flux of sunlight. The temperature sensitivity of the liquid electrolyte is considered as the main disadvantage of this type of solar cell. Many researches are carrying out to develop the electrolyte’s performance and consequently stability of solar cell. It is based on the photoelectrochemical processes. As shown in Fig.  6 , the employing electrode is fabricated by depositing a thin layer of oxide semiconducting materials such as TiO 2 (n-type) and NiO (p-type) on a transparent conductive glass plate made of indium tin oxide (ITO). These oxides have a wide band gap in the energy range from 3 to 3.2 eV. Because it is non-toxic, less expensive, and available, TiO 2 is mostly used as a semiconducting layer. The dye is covalently attached to the surface of TiO 2 . Due to the high porosity structure and large surface area of the electrode, a large number of dye molecules are attached to the surface of the TiO 2 nanoparticles, and thus, the light absorption on the surface of the semiconductor enhances. After laying an ultra-thin coating of TiO 2 on the upper surface, the efficiency of about 18% was determined, it was shown that TiO 2 was by 50% more efficient compared to the same material without coatings (Takabayashi et al. 2004 ), and they had synthesized a thin-film electrode made of a polycrystalline Si/doped TiO 2 semiconductor. The system consists on a particulate doped TiO 2 thin film supported on the surface on inexpensive polycrystalline Si, thus allowing to the absorption of short- and long-wavelength parts of the solar light, respectively. As a result, this combination can yield a high solar-to-chemical conversion efficiency of more than 10%, which results in a very promising approach to efficient and low-cost solar energy conversion.

Dye-sensitized solar cell (Serrano et al. 2009 )

Consequently, TiO 2 reduces directly the energy cost and yields cheaper solar cells.

Organic-polymer-based PV Solar cell (OPV):

In this type, particles are excited donating free electron–hole pairs via the effective field created between two dissimilar organic materials, known as the donor and accept or molecules. This type is obtained as an inexpensive renewable energy sources for the production of energy from light at very low cost (Pelemiš et al. 2013 ).

Hot carrier solar cells

In this cell, a free electron is highly bumped into the conduction band by a too-energetic photon. Therefore, its electronic temperature becomes quite hot (as high as 3000 K). The hot electron will relax to the bottom of the conduction band, typically through a few hundred femtoseconds, passing heat to the lattice. These cells have the advantages that a high-energy electron will enhance the photovoltage of the device that will result in increasing the cell efficiency. Finally, the advantages of solar cells are as follows:

Increasing device photovoltage as well as its efficiency due to using a high-energy electron.

Thin-film solar cells are the next generation of solar cells (flexible solar panels) that use less materials at low cost and are easier to produce and install.

For example, these sheets can be incorporated into a bag that charges laptop and cell phone. It can also cover buildings windows to collect solar energy from the entire building rather than just its roof (as shown in Fig. 7 ). So, it can be suitable to use as supply power to high-rise buildings.

Solve the problem of diffusion light absorption

PV system install on a Commercial Office Building ( https://www.pointloadpower.com/articles/10-common-questions-commercial-building-owners-have-about-rooftop-solar )

Accordingly, Alamri et al. 2020 presented an investigation to improve the energy efficiency of solar PV panels using hydrophobic SiO 2 nanomaterial. They concluded that the use of SiO 2 coating for PV panels results in the better performance of the PV panels. The overall efficiency of the coated panel increased by 15% and 5%, compared to the dusty panel and the uncoated panel which was manually cleaned daily, respectively, that improve the overall efficiency and producing more efficient solar photovoltaic system.

Solar thermal collector

The solar collector is a key component of water heating systems and solar energy applications. It can be elucidated as a green heat exchanger device which converts the energy of incident solar radiation or sunlight either to electrical energy directly in PV (photovoltaic) applications, or to the thermal energy in solar thermal applications.

The performance of the solar collector is known as the ratio between the rates of useful heat (Q) transferred to a fluid and the solar radiation intensity falling on the collector surface. It was expressed previously as follows (Shaffei et al. 2021a , b ):-

where η is collector efficiency, Q is the gained energy by water (W), A c is the collector area (m 2 ), and Gt is incident solar radiation W/m 2

where η is collector efficiency, m mass of water, Cp = 4180 J/Kg o C, A c area of collector, Gt W/m 2 intensity of incident light, T i inlet temperature, and T o outlet temperature.

To increase the collector efficiency, the researchers investigated several types of nanoblack coating to maximize the amount of solar energy absorbed by the black surface and converted into heat or electricity. In solar water heating systems, scientists studied different types of selective coatings such as black paint, sol-chrome, black chrome, black nickel, and black anodized aluminum to increase the collector efficiency and saving the energy consumption. However, some of these coating is high cost and also not friendly environmental such as chrome coating (Karuppiah et al. 2000 ). Moreover, the commercial black paint is strong emitters for thermal infrared radiation at high temperature which decrease the overall collector efficiency. Accordingly, Girginov et al. 2013 stated that electrodeposition of metal ions within porous alumina results in more efficient coating. The formed anodic aluminum oxide layer (AAO) is characterized by specific structure having self-organized and more-ordered nanopores as shown clearly in Fig.  8 (Shaffei et al. 2021a , b ).

A Schematic of the ideal densely packed hexagonal array of pores; B Actual cross-sectional view of a typically synthesized AAO layer (Poinern et al. 2011 )

Shaffei et al. ( 2021a , b ) studied and tested the performance of two solar collector panels in two similar heating systems. The first system comprised the nanoblack-colored anodized aluminum solar panel and the other had black aluminum solar panel colored by commercial black paint for a comparison. The results confirmed that the nanoblack coating results in increment of the efficiency of solar heating system by approximately 12% compared with the commercial black paint.

Nanofluid-based solar collector

The nanofluid-based solar collector is a new generation of solar collectors based on nanotechnology (Hussein 2016 ), in which nanoparticles in a liquid medium scatter and absorb solar radiation.

The use of a nanofluid as a working fluid in the collector improves efficiency.

On top of the collector, a nanofluid absorbs the sun’s radiation directly.

As a result, this layer may eliminate the need for the absorber plate and tubes found in traditional solar collectors, as well as materials for both conventional and nanocollectors. The main difference between the conventional and nanofluid-based collectors lies in the mode of the working fluid heating. In the conventional collector, the sunlight is absorbed by a surface and then transmitted to the fluid (water or air), while in the nanofluid collector the sunlight is immediately absorbed by the working fluid through the radiative heat transfer as shown in Fig.  9 .

Schematics and materials for both conventional and nanocollectors

Yousefi et al. 2012 studied the effect of utilizing the (Al 2 O 3 –water) nanofluid as an absorbing medium in a flat-plate solar collector. The nanoparticles weight fraction was taken as 0.2 and 0.4%, respectively. Moreover, the particles dimension was nearly 15 nm. The results showed that, on the addition of 0.2 wt% of nanofluid, the collector efficiency is increased by 28.3% . Consequently, the collector using an alumina–water nanofluid had higher efficiency than that using the water only. Regarding the data listed in Table 2 , various types of nanofluids enhance the collector efficiency. The researchers concluded that nanoparticles increased heat-collection efficiency by up to 10 percent. (Taylor et al. 2011 ; www.sciencedaily.com/releases/2011/04/110405081910.htm ).

Limitations of nanofluids in solar collectors

The benefits of using nanofluids in solar thermal collectors include increased thermal efficiency, potential reduction in collector size, and cost-effectiveness, while nanofluids are still limited in application in collectors as the nanofluids are highly unstable and their particles tend to precipitate. Meanwhile, the nanoparticles move in Brownian motion that results in aggregation and increases the fluid viscosity. As a result, increased viscosity reduces flow rate in thermosyphons or increases pump power required in forced convection systems. Both of these will eventually reduce the system’s efficiency (Wole‑osho et al. 2020 ).

Nanostructured materials are being successfully used to increase the conversion of hydrogen energy into electricity via fuel cells. Fuel cell technologies have emerged as one of the most promising approaches to various energy resources, as well as to energy sustainability and the environment (Peterson et al. 2010 ). In a fuel cell, hydrogen and oxygen combine to form water, which produces electricity and heat. As illustrated in Fig.  10 , this occurs in an environmentally beneficial manner, with no damaging carbon dioxide (CO 2 ) emissions. Despite its many advantages, the fuel cell still has a number of disadvantages, including high cost, operability, and durability difficulties. Nanotechnology can be used to overcome these disadvantages. In practice, nanomaterials can be used in the fuel cell membrane (which is responsible for separating hydrogen into protons and electrons), the contribution of carbon nanotubes (CNTs) to improving the mechanical strength and proton conductivity of polymer electrolyte membranes, as well as catalysts and electrodes. Furthermore, storing huge amounts of hydrogen fuel is either too cumbersome or too expensive. Large amounts of hydrogen can be stored inside nanomaterials such as carbon nanotubes (CNTs) and carbon nanofibers, which is another key constraint in fuel cells. Carbon nanotube fuel cells, which are regarded the most environmentally acceptable form of energy, are currently being used to store hydrogen. Carbon nanotubes have a layered graphene tubular shape that allows them to store hydrogen efficiently. In reality, hydrogen can be adsorbed by carbon nanotubes (CNTs) by a physic-sorption phenomenon in which hydrogen is trapped in the cylindrical structure of the nanotubes or in the interstitial regions between nanotubes. Consequently, hydrogen has the best energy-to-weight ratio of any fuel and is widely utilized in space vehicles (Liu et al. 2010 ).

Hydrogen fuel cell (Bishop 2014 )

Accordingly, the application of CNTs as the catalyst support in hydrogen production appears to be effective and attractive owing to their special structural morphology and characteristics. The surface of CNTs is usually modified to create the functional groups for specific needs. Nikitin et al. 2008 stated that the hydrogenation of single-walled carbon nanotubes (SWCNTs) using atomic hydrogen as the hydrogenation agent depends on the nanotube diameter, and for the diameter values around 2.0 nm. Hence, the hydrogenation (nanotube–hydrogen complexes) was close to 100% and is stable at room temperature. This results in enhancement of hydrogen storage capacity to 7 wt%. Girishkumar et al. 2005 investigated the power density of hydrogen cell based on using single-walled carbon nanotubes (SWCNTs) support and platinum catalyst. They concluded that the maximum power density of CFE/SWCNT/Pt electrodes was nearly 20% better than CFE/CB/Pt electrodes. Also, Orinakova and Orinak 2011 stated that the storage capacity of SWNTs and MWNTs for hydrogen is lower than 1 wt% at ambient temperature, but the capacity could be raised considerably between 4 and 8 wt% when decreasing the temperature of adsorption or modifying the CNTs. Some of the experimentally reported hydrogen storage capacities for different CNTs are summarized in Table 3 . The variation in data of hydrogen storage capacity in CNT is based on their characteristics. The milled MWNTs at the same temperature and applied pressure result in increasing in hydrogen storage capacity; meanwhile, utilizing of milled MWNT may result in saving energy because there is no need for additional heating or higher pressure.

In addition, Amin et al. 2014 investigated the catalytic activity of impregnation of Ni and Pd/Ni nanoparticles on Vulcan XC-72R carbon black electrode for methanol oxidation in hydrogen cell. They tested the methanol oxidation reaction at Ni/C and Pd/Ni/C electrocatalysts in (0.2 M MeOH,‏ 0.5 M KOH) solution and concluded that Pd/Ni/C is more stable than Pd/C and Ni/C electrocatalysts. Therefore, Pd/Ni/C is a suitable as a less expensive electrocatalyst for methanol oxidation that can be beneficially applied in fuel cells. Khater et al. 2022 studied the effect of bifunctional manganese oxide–silver nanocomposites anchored on graphitic mesoporous carbon to promote oxygen reduction and inhibit cathodic biofilm growth for long-term operation of microbial fuel cells, and they concluded that MnOx–Ag/GMC nanocomposites showed high antibacterial activity in MFCs, suppressing biofilm growth on the cathode. Consequently, MFCs with MnOx–Ag/GMC nanocomposites had a much higher maximum power density (160 mW m −2 ) compared to Pt/C.

Cathode-based MFCs (60 mW m −2 ) with a much lower closed-circuit potential decay during continuous operation for 5 months. Also Xie et al. 2022 presented highly efficient, economical, and environment friendly electrocatalysts for the hydrogen and oxygen evolution reactions that is necessary for economical water splitting. FeS 2 nanoparticles were anchored on the surface of MXene through a simple adsorption-growth route (FeS2@MXene). The large active surface area of FeS 2 and its robust interfacial interaction with conductive and hydrophilic MXene nanosheets and the obtained FeS2@MXene composite can accelerate the transfer of mass/charge and facilitate contact between water molecules and reactive sites of FeS 2 . They stated that the functioned hybrid bifunctional electrocatalyst requires only a cell voltage of 1.57 V to deliver a current density of 10 mA cm −2 . The high catalytic activity of the FeS2@MXene hybrid is attributed to the well-designed structure and constructed interface between FeS 2 and MXene, which results in a larger specific surface area, facilitated mass/charge transfer, and saving energy.

Lithium-Ion batteries

Nanostructured materials have recently been proposed for use in energy storage devices, particularly those with high charge/discharge current rates, such as lithium-ion batteries, which are widely used in mobile phones and laptops (as shown in Fig.  11 ).

A Li-ion battery from a Nokia mobile phone ( https://en.wikipedia.org/wiki/Lithium-ion_battery )

Furthermore, the success of electric and hybrid electric vehicles (EVs and HEVs, respectively), which are predicted to at least partially replace conventional vehicles, is dependent on the development of energy storage devices with high power and high energy density. As a result, these energy storage solutions will rely on cutting-edge materials research, namely the development of electrode materials that can charge and discharge at high current rates. In general, nanostructure active electrode materials have the ability to increase the available power from a battery while reducing the time required to recharge it.

These advantages are obtained by coating the electrode’s surface with nanoparticles, which enhances the electrode’s surface area and allows more current to flow between the electrode and the chemicals within the battery. This technology could improve hybrid vehicle efficiency by lowering the weight of the batteries required to generate appropriate power. When the battery is not being used, nanomaterials are used to separate the liquids in the battery from the solid electrodes, extending the battery’s shelf life. This separation eliminates the low-level discharge that happens in a traditional battery, extending the battery’s shelf life significantly ( https://www.understandingnano.com/batteries.htm ).

The applications of nanotechnology in batteries are discussed as follows: -

Firstly, the modification of the active substance in the electrode material (cathode or anode) by adding nanomaterials.

Secondly, the application of nanotechnology to improve the performance of electrodes by using of nanocoatings. For example, nanodimensional additives such as nanocarbons, graphene, and carbon nanotubes have better electron conduction, or the use of nanothick coatings on the active material to prevent unwanted reactions with the electrolyte resulting the electrode stability and stress modulation. Regarding LiFePO 4 cathode, the amount of electron conductivity is poor. Hence, the conductivity is enhanced by using a conductive carbon coating on its particles or applying a conductive carbon material as an additive. Also, LiCoO 2 cathode is unstable at high currents in the vicinity of the electrolyte; for stabilization, nanothick oxide coating can be utilized. Accordingly, carbon coating increases conductivity, capacity, and consequently the cell power. However, the research in this area (to create this coating) is still insufficient. Therefore, research in the field of synthesis methods is very important (Songping et al. 2015 ).

The creation and usage of energy efficient LEDs based on inorganic and organic semiconductor materials was the first nanotechnology application in the field of lighting. LED technology has already tapped huge commercial potentials in the illumination of displays, buildings, and cars due to its compact form, flexible color scheme, and high energy yield. It was created with the purpose of enhancing the energy efficiency of LEDs by using quantum dots. Furthermore, nanoscale light-emitting particles aid in the reduction of LED scattering effects, resulting in an increase in light production. To boost particle stability, the particles must be coated (Hessian 2008 ).

As shown in Fig.  12 , LEDs are predicted to account for 87% of lighting sources by 2030, with lighting controls accounting for 50% of lighting installations in commercial buildings. In 2015, about 13% of lighting installations in the commercial sector were LED lights. Up to 91% of indoor illumination energy usage occurs in the commercial and industrial sectors.

Lead lighting share in industry ( https://www.warehouse-lighting.com/blogs/lighting-blog/led-lighting-statistics )

The high efficient nanomaterials in energy saving

Graphene is one of nanomaterials having high electrical conductivity and excellent mechanical strength. It is a super-capacitive biodegradable material that is less expensive than pure silicon. Furthermore, graphene can absorb solar energy’s ultraviolet radiation, which is ignored by Si solar cells. As a result, solar cells with a wavelength of 0.345 nm can absorb more solar light. Das et al. 2017 investigated the melting of carbon-based nanocomposites in a vertically oriented shell-tube thermal energy storage system. They looked at the effect of carbonic nanomaterial structure on the thermal behavior of n-eicosane (phase change materials (PCM): as nanofillers to improve n-alkane thermal conductivity). They investigated the effects of three structures on melting time: single-walled carbon nanotubes (SWCNT), nanodiamond, and graphene nanoplatelets. Their research found that using nanodiamond had no effect on melting time when SWCNT and graphene were used at a 1% by volume ratio. It was discovered that melting duration could be reduced by up to 15% and 25%, respectively. The improved thermal conductivity caused by the nanostructures incorporation was attributed to the shorter melting time. In addition, Hussein et al. 2021 investigated the effect of nanographene dispersion on the rate of evaporation of saline water to save energy consumption during thermal desalination and concluded that graphene is one of the best nanomaterials for maximizing heat transfer efficiency because it maximized the broken hydrogen bond between water molecules and minimized the viscosity of saline water, resulting in maximum energy savings by 30% on using 10 g/l.

The energy-saving efficiency declined as the graphene increased. Meanwhile, even though graphene breaks the hydrogen bond, the increase in saline viscosity lowers the boiling point. As a result, there is a critical dose that maximizes hydrogen bond breaking while lowering saline water viscosity. The optimum dose was determined to be 10 g of nanographene/1 L of saline water based on these findings (Hussein et al. 2021 ). Furthermore, Hussein et al. 2020 investigated the use of nanographene oxide in the thermal desalination of saline water. The generated desalinated water quantity acquired utilizing nanographene oxide was more than double that obtained using the usual thermal desalination process, resulting in a 22% energy savings.

Quantum dots

The quantum dots are mainly semiconductor crystals with nanoscale, which possess high ability to absorb and convert light into electrical energy. Owing to their small size, dots can spray onto flexible surfaces like plastic. This design is ideal for enhancing the efficacy of the small distance between the dots. The efficiency of converting solar energy into electrical one increased on applying quantum dots (Sabr et al. 2022 ). Because of its superior properties, such as thermal conductivity, metallic or semiconducting electronic behavior, and surface area, CNTs strongly contributed to the enhancement of conversion of solar energy into electricity or the generation of fuels through photocatalysis (Sharma et al. 2018 ). Experiments have already shown that quantum dots (tiny nanoparticles only a few nanometers in size) are three times more efficient at converting solar energy than the best material currently used in solar cells.

The application of nanomaterials in water and wastewater treatment has grown wide attention. Nanomaterials have high reactivity and adsorption capacities due to their unique characteristics, small diameters, and large specific surface areas. Antibacterial silver nanoparticles (Ag NPs) are effective against a variety of microorganisms, including viruses, bacteria, and fungi (Borrego et al. 2016 ). As a result, Ag NP is an effective antimicrobial agent that is widely used in water decontamination (Kalhapure et al. 2015 ). Because of their high antibacterial activity and cost-effectiveness, these nanoparticles (Ag NPs) adhered to the filter materials in the water disinfection process. Chemical reduction is also used to make Ag NPs, which are then integrated into polyethersulfone (PES) microfiltration membranes.

These PES-Ag NPs membranes have excellent antibacterial properties and are widely used in water treatment. For disinfection and biofouling reduction in domestic water treatment, Ag NPs are also integrated into ceramic materials/membranes. It is possible to improve the removal effectiveness of Escherichia coli by adding Ag NPs to ceramic filters made of clay and sawdust. Furthermore, it was discovered that filters with a higher porosity removed more germs than those with a lower porosity (Krishnaraj et al 2012 ). Furthermore, colloidal Ag NPs have been coupled with clay-rich soil-based cylindrical ceramic filters. Colloidal Ag NPs increased filter performance, and the filters can remove Escherichia coli at a rate of between 97.8 and 100% (Lu et al. 2016 ; Oyanedel-Craver and Smith 2008 ). Several types of nanomaterials applications for water and wastewater treatment are listed in Table 4 .

Nanomaterial on the solar still productivity

Solar still distillation is one of the most primitive types of water treatment that contributes to the solution of the water crisis. Solar stills, as generally known, have numerous advantages, including being simple, inexpensive, pollution-free, and requiring less maintenance. However, the freshwater yield of a conventional solar still (CSS) is restricted. As a result, researchers are attempting to improve its efficiency by employing a variety of nanomaterials. The performance of a modified solar still (MSS) can be improved by adding copper oxide nanoparticles to the black paint, for example. As the concentration of nanoparticles increases, the water productivity enhanced. Meanwhile, raising the concentration of nanomaterials increases the rate of heat transmission between the basin water and the still walls. Furthermore, as compared to CSS, using CuO nanoparticles boosts freshwater productivity by around 16% and 25% at weight fraction concentrations of 10% and 40%, respectively, as shown in Fig.  13 (Kabeel et al. 2017 ).

Layout diagram of conventional solar still and modified solar still with nanoparticles (Kabeel et al. 2017 )

Industrial application of nanomaterials

Nanotechnology was applied in many industrial activities and private sectors through the optimization of products for substantially maintaining the energy consumption.

Dust-repellent coatings with TiO 2 -based nanomolecules

The conventional applications of nano-TiO 2 -modified coatings can be effectively used to control the dirt and dust after exposure of an area to ultraviolet radiation. The system of nanoparticle delivery was developed by a new company (Swift Coat), which can be beneficially used to synthesize coatings with high efficiency in dust repellent for solar panels with less needed maintenance. It is expected that the application of such nanoparticle delivery can be successfully used in preventing solar panels soiling. The accumulation of debris, dust, and dirt on the surface’s panel with increasing time can decrease its efficiency. With more exposure time to these pollutants, the efficiency of solar cells decreases by 30%. This can result in serious problems for consumers and businesses. The installed solar panels on home’s roof could not be easily reachable by a homeowner. A continues loss in the efficiency of panels for months occurs if the homeowners cannot continuously be cleaning the panels. Similar challenges can be faced by businesses with remote solar installations. The costs of labors who clean solar panels can negatively affect. The labor’s costs for cleaning solar panels can significantly cut into the savings the company may have safeguarded by adopting a source of green energy (Newton 2021 ).

Corrosion protection

Corrosion is regarded as an important issue in the industrial world. Globally, the expected annual cost of preventing corrosion, including checking and substituting parts and shielding against corrosion—is around $2.5 trillion. Based on National Association of Corrosion Engineers (NACE), this value is equivalent to 3.4% of the global gross domestic product (GDP) in 2020. As well known, the application of industrial paints and coatings on panels act as a protected layer against corrosion. Moreover, the life span of metal parts can be extended by after coating it. Alternatives that are highly effective in resisting corrosion have been developed using nanotechnology. The use of nanotechnology has resulted in corrosion-resistant alternatives.

Paints and coatings

Graphene’s anticorrosive properties have been discovered by recent study conducted by South Australian chemicals company Technology using commercially available coatings and paints. Figure  14  shows the lattice-like structure of graphene, which is made up of carbon atoms arranged in a thin sheet. Scientists exposed steel surfaces to 1344 h of salt spray to test the power of the graphene coating and a control coating. The company claims that scribe creep improved sixfold when graphene was added to the control coatings, suggesting improved corrosion resistance. These paints and coatings were applied to smooth, cold-rolled steel as part of the company’s tests. Steel is considered to be a challenging substrate for anticorrosive coatings. Coatings tend to adhere to abrasive blast-cleaned steel more readily due to its more suitable anchor profile compared with similar substrates. Graphene coatings proved very successful for the research team, combining with other corrosion management techniques like temperature control and improved air flow to make valuable assets for industrial applications.

Structure of graphene ( https://www.graphene-info.com/graphene-structure-and-shape )

Risk of Nanomaterials

Nanoparticles can get up in the environment by mistake or as a result of their synthesis, transportation, storage, usage, or disposal. The nanoscale diameter of these particles facilitates their migration in numerous bodily areas, which can result in catastrophic illnesses such as cancer and organ damage. Furthermore, nanoparticles inhaled by an organism can easily reach the heart, liver, and blood cells via the circulation. As a result, further study is needed to close the knowledge and information gap concerning nanoparticle behavior in soil, air, and water, as well as their accumulative qualities in food chains. Because nanotechnology is still in its infancy, there are concerns regarding the impact of its industrial and commercial applications on the environment and organisms (Pandey and Jain 2020 ). Table 5 lists the overall features as well as the danger of nanoparticles.

Cost of nanomaterials

On applying nanotechnology-based treatments, distinct contributions from the purchase or synthesis of nanomaterials, energy (such as electricity necessary for photocatalysis treatments using nanoparticles, for example), and labor should all be taken into account when estimating operational costs (Kamali et al. 2019 ). Nanomaterial prices are largely dependent on the material type and desired features such as purity level (wt%), surface functionalization, and particle size. Prices for TiO 2 nanoparticles range from $0.03 per gram to $1.21 per gram, with treatment expenses ranging from $0.50 to $1.00 per gram of pollution. The cost of nanozero-valent iron (nZVI) particles has fallen as the technology for their manufacture has improved, and they are now roughly $0.05–0.10/g. In terms of manufacturing, however, micro- and bulk zero-valent iron are still significantly less expensive ($0.001/g) (Crane and Scott 2012 ). The cost of magnetite nanoparticles was predicted to be around 0.0035 €/g, according to (Simeonidis et al. 2015 ). CuO on -Al 2 O 3 is also the most cost-effective material, with an estimated treatment cost of 0.07 $ per gram of metal for an 80 percent pollution load reduction. Application of visible-light active nanoparticles such as N-TiO 2 or solar energy consumption is recommended as ideal alternative in locations where power prices are high (Yoshida et al. 2014 ).

Case studies

Case study (1) Performance Improvement of Solar Water.

Distillation System Using Nano fluid Particles (El-Ghetany et al. 2021 ).

Location The experimental pilot unit was installed in Solar Energy.

Department, National Research Centre, and Giza, Egypt.

Capacity up to 6000 L /day.

Process The technology is designed to produce freshwater.

As indicated in Fig.  15 , solar energy was used in the desalination process to gain freshwater. The water desalination system is divided into two loops: the heat transfer fluid (HTF) loop and the water loop. To transport thermal energy from the heat pipe evacuated tube collector to the thermal oil storage tank, synthetic thermal heating oil was chosen as the heat transfer fluid. It offers high resistance to thermal cracking, high heat transfer efficiency, and proper properties of heat transfer, low maintenance costs, and a long life (Fig.  16 ).

Schematic diagram of the solar water desalination system

Photographic view of a solar water desalination system using evacuated tube collector (El-Ghetany et al. 2021 )

The HTF loop is made up of four parts: an evacuated tube collector with a storage tank, a hot oil stainless steel coil immersed in a thermal storage tank of hot water that acts as a heat exchanger, a backup electric heater for auxiliary heating, and a hot oil circulation pump. After passing through the backup electric heater, the HTF is heated in the solar evacuated tube collector and the obtained thermal energy is transported to the 100 L hot water storage tank. The temperature of the pumped HTF is increased in the evacuated tube collector to between 75 and 90 degrees Celsius, and then, it is heated to 200 degrees Celsius in the hot oil auxiliary heater. On a solar water distillation system, the influence of nanoparticle concentrations of TiO 2 combined with hot oil loop was investigated to evaluate the daily water productivity with varied concentrations of TiO 2 .

Results Applying nanofluid particles (TiO 2 ) to the heat transfer fluid of the solar water distillation system improves heat transfer properties and hence boosts daily water productivity. When utilizing 100 mg/l TiO 2 nanoparticle concentration, the given system may produce 5616 L per day in the reference unit (without nanoparticle), and 7128 L per day in the case of using 100 mg/l TiO 2 nanoparticle concentration. The use of TiO 2 nanoparticles at a concentration of 100 mg/l resulted in a 26.9% increase in daily distilled water productivity. This boost in performance will improve the use of nanofluids in heat transfer loops, resulting in a higher daily rate of productivity.

Case study (2) Nanosilver-enabled composite for water treatment (Adonizio et al. 2019 ).

Location Solar energy-powered water treatment technology, developed by Quest Water Solutions Inc. (Canada).

Capacity up to 20,000 L per day.

Process A nano-enabled composite is included in a point-of-use (POU) water purification system with a two-stage filtering process that produces 10 L of clean water in 1 h; on the other hand, the second stage absorbs chemical contaminants.

The first stage It had an antibacterial unit that was in charge of eliminating viruses and germs. Nanocrystalline aluminum oxyhydroxide–chitosan composite embedded with 10–20 nm silver nanoparticles makes up the unit. Over a long length of time, the device may constantly emit regulated quantities of silver ions (40 ± 10 ppb) into natural drinking water. The antibacterial composite was created using a green synthesis approach that can be used at room temperature and does not require electricity to function.

The second stage It uses an activated carbon black filter with a nominal pore size of 4 m to filter out cysts and adsorb the organic pollutants, bacterial biomass, metals, pesticides, and other contaminants.

Results For a household of five, a cartridge containing 120 g of the nanosilver-based antimicrobial compound may supply safe drinking water for one year (assuming daily drinking water consumption of 10 L). This equates to a $2 yearly cost per household, which includes cartridge packing, media, plastic assembly, and sediment pre-filter. The unit may be readily activated by submerging it in water (natural drinking water) for 3–4 h at 70–100 degrees Celsius, which reduces the cost and increases the device’s durability.

Case study (3) Surface modification of RO membrane and spacer of Seawater desalination pilot set-up in Wukan desalination plant (Yang et al. 2009 ):

Location Pilot plant in the Wukan desalination plant at Penghu, which is one of Taiwan’s main off-shore islands.

Feed Actual sea water pretreated by cross-media sand filter and 5-mm cartridge filter.

Process The pilot plant setup was simulated to the actual process of the full-scale desalination plant. The influent was collected from the full-scale desalination plant’s sand filter unit and feed water tank storage, as indicated in Fig.  17 . The seawater was injected to pass through two (in-series 5-mm cartridge) filters then flow into a buffer tank. Then, using a high-pressure pump, RO feed was continuously pushed into one commercial flat-sheet membrane filtering machine, with feed flow rates set at 2 L/min. Throughout the testing, the system was operated in a constant pressure mode with a set pressure of 55.2 bar.

Schematics of seawater desalination pilot plant and cross-flow RO membrane cell for biofouling test (T1: feed water tank; T2: RO feed tank; T3: permeate tank; T4: concentrate tank; CF 1 , CF 2 : 5-mm cartridge filter; P 1 : pump; P 2 : high-pressure pump; P: pressure meter; F: flow rate meter; S: scale; V 1 , V 2 : ball valve; and V 3 : pressure release valve)

Surface modification of RO membrane and spacer

A simple nanosilver-coating process was used as a surface modification solution to minimize membrane biofouling on site in this case study. All of the flat-sheet RO membranes came from a spiral-wound membrane module (SW30-2514, FILMTECDOW), which is the same membrane that was utilized in the full-scale desalination plant. To enhance silver ion adsorption on the membrane sheet, it was first soaked in silver nitrate solution for thirty minutes. The membrane was then immersed in formaldehyde solution for forty minutes to initiate the reduction process. The same method was used to coat the spacer, which was separated from the same spiral-bound module.

Results In terms of permeate flow and TDS rejection, both the silver-coated membrane with uncoated spacer and the silver-coated spacer with uncoated membrane excelled the unmodified membrane and spacer. The antibacterial action of the silver-coated spacer, on the other hand, lasted longer. During the whole testing time of the silver-coated spacer test, there was essentially no cell reproduction on the membrane. Furthermore, the cells that adhered to the membrane appeared to lose activity soon. Hence, the modified membrane saving the total cost required for replacing by another units and labor cost for cleaning the unit.

Case study (4) NANOTECHNOLOGY FOR WATER TREATMENT.

Location The desalination plant in Llobregat, Barcelona, Spain (Fig. 18 ).

A picture of the desalination plant in Barcelona (Adeleye et al. 2019 )

Capacity The largest reverse osmosis-based desalination plant in Europe with an output of 200,000 cubic meters per day (Adeleye et al. 2019 ).

Process They were able to build membranes that resist biofouling better than any other membranes before because they used nanoparticles in their membrane coating. Nanotechnology was employed by nano-H 2 O to improve the permeability of their desalination membranes. They were able to develop a membrane with improved permeability of 50–100% by incorporating zeolite particles into the polyamide rejection layer.

Results Spain has adopted the NAWADES project. TFN membranes from nano-H 2 O have already been installed in Los Angeles, China, with ambitions to extend to the Middle East. Carbon nanotubes have offered a new way for water desalination, in addition to these approaches. The previous reverse osmosis processes will be replaced by these new technologies. Desalination plant efficiency will improve, resulting in the production of more clean, drinkable water. Finally, advancements in desalination technology can only contribute to the system’s long-term survival.

Conclusions

Nanotechnology has a significant impact on human life because it provides cheap and clean energy. As a result, it provides a significant evolution in several renewable energy devices used for energy storage and conversion, as well as environmentally beneficial materials. It has been demonstrated that they improve efficiencies and lower costs in a several areas. Nanomaterials promote solar photovoltaic systems, lithium-ion batteries, and fuel cells that lead to conserve about 20% of the current energy consumption. Moreover, nanomaterials demonstrated unique chemical, physical, and biological properties. Considerable energy saving potentials through the optimized products and production plants are found in nearly all branches of industry via nanotechnology. Nanofluids, nanographene, nanosilver, TiO 2 , CuO, Al 2 O 3 , and nanocomposites are potentially applied in industrial field. Nanoparticles emitted into the environment cause catastrophic illnesses such as cancer and organ damage. Furthermore, nanoparticles inhaled by an organism can easily reach the heart, liver, and blood cells via the circulation. As a result, the disposal of solid nanomaterial waste must be researched in order to maintain a clean and safe environment.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Abbreviations

Silver nanoparticles

Polyethersulfone

Carbon nanotubes

Chemical vapor deposition

International Energy Agency

Light-emitting diode

Dye-sensitized solar cell

Indium tin oxide

Organic-polymer-based PV solar cell

Photovoltaic

Anodic aluminum oxide

Carbon dioxide

Electric vehicles

Hybrid electric vehicles

Phase change materials

Single-walled carbon nanotubes

Reverse osmosis membranes

National Renewable Energy Laboratory

Conventional solar still

Modified solar still

Global gross domestic product

National Association of Corrosion Engineers

Point of use

Part per billion

Heat transfer fluid

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Hussein, H.S. The state of the art of nanomaterials and its applications in energy saving. Bull Natl Res Cent 47 , 7 (2023). https://doi.org/10.1186/s42269-023-00984-4

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Organic and inorganic nanomaterials: fabrication, properties and applications

Basmah h. alshammari.

a Department of Chemistry, College of Science, University of Hail, Hail 81451 Saudi Arabia

Maha M. A. Lashin

b Department of Electrical Engineering, College of Engineering, Princess Nourah bint Abdulrahman University, P.O. Box 84428, Riyadh 11671 Saudi Arabia

Muhammad Adil Mahmood

c Department of Physics, University of Lakki Marwat, Lakki Marwat 28420 KP Pakistan, nc.ude.ujz@kapilawjar moc.liamg@304nayraalida

Fahad S. Al-Mubaddel

d Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh 11421 Saudi Arabia

e King Abdullah City for Renewable and Atomic Energy: Energy Research and Innovation Center, (ERIC), Riyadh 11451 Saudi Arabia

Nasir Ilyas

f School of Optoelectronic Science and Engineering, University of Electronic Science and Technologyof China, Chengdu 611731 P.R. China

Nasir Rahman

Mohammad sohail, aurangzeb khan.

g Department of Physics, Abdul Wali Khan University, Mardan 23200 KP Pakistan

Sherzod Shukhratovich Abdullaev

h Researcher, Faculty of Chemical Engineering, New Uzbekistan University, Tashkent Uzbekistan

i Researcher of Scientific Department, Tashkent State Pedagogical University Named After Nizami, Tashkent Uzbekistan

Rajwali Khan

j School of Physics and Optoelectronic Engineering, Shenzhen University, Nanshan, 518000 Shenzhen Guangdong China

Nanomaterials and nanoparticles are a burgeoning field of research and a rapidly expanding technology sector in a wide variety of application domains. Nanomaterials have made exponential progress due to their numerous uses in a variety of fields, particularly the advancement of engineering technology. Nanoparticles are divided into various groups based on the size, shape, and structural morphology of their bodies. The 21st century's defining feature of nanoparticles is their application in the design and production of semiconductor devices made of metals, metal oxides, carbon allotropes, and chalcogenides. For the researchers, these materials then opened a new door to a variety of applications, including energy storage, catalysis, and biosensors, as well as devices for conversion and medicinal uses. For chemical and thermal applications, ZnO is one of the most stable n-type semiconducting materials available. It is utilised in a wide range of products, from luminous materials to batteries, supercapacitors, solar cells to biomedical photocatalysis sensors, and it may be found in a number of forms, including pellets, nanoparticles, bulk crystals, and thin films. The distinctive physiochemical characteristics of semiconducting metal oxides are particularly responsible for this. ZnO nanostructures differ depending on the synthesis conditions, growth method, growth process, and substrate type. A number of distinct growth strategies for ZnO nanostructures, including chemical, physical, and biological methods, have been recorded. These nanostructures may be synthesized very simply at very low temperatures. This review focuses on and summarizes recent achievements in fabricating semiconductor devices based on nanostructured materials as 2D materials as well as rapidly developing hybrid structures. Apart from this, challenges and promising prospects in this research field are also discussed.

ZnO nanomaterials and nanoparticles are a burgeoning field of research and a rapidly expanding technological sector in a wide variety of application domains.

1. Nanomaterials

Nanomaterials (NMs) and nanoparticles (NPs) are a fast increasing technical industry and a blossoming topic of study in a wide range of application disciplines. Due to their adjustable physicochemical properties such as wettability, scattering, light absorption, thermal and electrical conductivity, catalytic activity and melting point. 1 NPs and NMs have acquired significance in technological breakthroughs. A nanometer is a SI (System international of units, SI) unit of length equal to 10 −9 metres. NMs are characterised in principle as materials with at least one dimension length of 1–1000 nm; nonetheless, they are frequently defined as having a diameter of 1–100 nm. Today, various articles of legislation in the United States of America (USA) and the European Union (EU) make specific mention to NMs. However, there is no universally accepted definition of NMs. Different organisations have divergent views on how to define NMs. 2 “NMs can display behaviour unrelated to chemical components of comparable size”, according to the Environmental Protection Agency. 3 Nanomaterials are also known as “materials with at least one dimension in the 1–100 nm range that exhibit dimension dependent behaviours”, according to the US Food and Drug Administration. 4 The International Organization for Standardization defines Nanomaterials as “any exterior nanoscale dimension or surface structure with an internal nanoscale dimension”, 5 This ISO definition has been used to define quantum dots, nanowires, nanoplates, nanofibers and other related words. 6 According to the EU Commission, a nanomaterial is “a man-made or naturally occurring substance that can be unbound, aggregated, or agglomerated and has particles whose sizes on the surface range from 1 to 100 nanometers”. 7

Since the last century, nanotechnology has been a well-known field of study. Richard P. Feynman is credited with coining the phrase “nanotechnology” in the year 1959, when he delivered the famous lecture “There is Plenty of Room at the Bottom”. 9 Nanoscale technology has been used to create a variety of materials. Nanoparticles are divided into several categories. Nanoparticles are particles with a diameter of 1 to 100 nanometers. 10 Depending on the shape, nanoparticles can be 0D, 1D, 2D, or 3D. 11

2. Classification of nanomaterials

Nanoparticles are categorized into their respective categories based on their morphology, which refers to their structure, as well as their size and shape. The nanoparticles listed below are some of the most significant types currently known.

2.1. Organic nanomaterials

Fig. 1 displays a number of different types of organic nanoparticles, some of which are micelles, dendrimers, liposomes, nanogels, polymeric NPs, and layered biopolymer. Certain organic nanoparticles, such as micelles and liposomes, have a hollow sphere, and they are non-toxic and biodegradable. Organic nanoparticles can also be broken down naturally. This name is also used to refer to nanocapsules, which are extremely sensitive to light and heat. 12 Due to the fact that organic nanoparticles exhibit these properties, they are an excellent option for the transportation of pharmaceuticals. Nanoparticles are also frequently used in the process of transporting target medications to their intended locations. Organic nanoparticles are also sometimes referred to by the label polymeric nanoparticles. The nanosphere or nanocapsule is the most famous form of polymeric or organic nanoparticles. 13 The matrix particles have a solid sphere of mass and adsorb other molecules at the outer boundary of spherical surface. Particles encapsulated the solid mass in the later case. 14 Fig. 1 displaying the organic nanoparticles.

2.2. Inorganic nanomaterials

Inorganic nanoparticles do not contain carbon. Inorganic nanoparticles have the advantages of being hydrophilic, non-toxic, and biocompatible with living systems. The stability of inorganic nanoparticles is superior to that of organic nanoparticles.

Magnetic nanoparticles (mNPs) are one of the most significant inorganic nanomaterials. 15 A magnetic core ( e.g. maghemite (g-Fe 2 O 3 ) or magnetite (Fe 3 O 4 )) is generally present. 16 Other metals, such as nickel and cobalt, are also employed, although their applications are limited due to their toxicity and oxidation vulnerability. 17 Ferritin, a type of protein, is where the vast majority of iron is kept in the human body. Iron oxide mNPs have the ability to digest excess iron and restore the supply in the human body. There is a continuous presence of these cationic mNPs in the endosomes for a considerable amount of time. This continues to be the case over and over again. 18 After that, during the postcellular absorption process that takes place in the endosome and the lysosome, elemental components like iron and oxygen are brought into the body's storage, where hydrolytic enzymes either digest them or cause their destruction. In the human body, homeostasis is the process through which iron levels are maintained and adjusted. Adsorption, excretion, and storage are all processes that contribute to this process. Iron oxide nanoparticles help the body digest any excess iron that may be present. 19 Iron is essential in almost all biological tissues, yet it has a low bioavailability. In certain circumstances, it can damage the cells when they are in the form of free iron or when it is not associated with haemoglobin. Additionally, it can be harmful to cells when it is present alone. Fig. 2 displaying the inorganic nanomaterials.

2.2.1. Metal nanomaterials

Nanoparticles of metallica can be synthesized from metals through either constructive or destructive mechanisms. In order to create pure metal nanoparticles, metal precursors are required for the production process. Metal nanoparticles have distinctive optoelectrical properties, which can be attributed to their plasma on resonance characteristics. 20 The size, shape, and surface of the metal nanoparticles all play a role in the synthesis process in their own unique ways. 21 All metal nanoparticles can be synthesised. 22 Nanoparticles of the metals cadmium, aluminium, copper, silver, lead, cobalt, zinc, gold, and iron are all examples of well-known nanoparticles of the element. Nanoparticles can be identified by their smaller size (10 to 100 nm), their surface properties such as pore size, surface charge, environmental factors, surface area to volume ratio, structure (amorphous and crystalline), shapes (irregular, rod, spherical, cylindrical, tetragonal and hexagonal), surface charge density and colour. Nanoparticles have been shown to have a variety of applications in a variety of fields (air, heat, moisture and sunlight). In the field of medicine, nanoparticles made of zinc, gold, silver, platinum, iron and copper, as well as those made of other metals, have garnered a lot of attention. Metallic nanoparticles can occur in solution, as Sathyanarayanan (2013) 23 demonstrated. Later onwards, Salas et al. (2019) 24 conducted research on the colour and shape of metallic nanoparticles. Changing the chemical groups that help antibodies bind to nanoparticles may now be done during the manufacturing process, which also allows for improvements. Noble metal nanoparticles have been utilized in a diversity of biomedical applications, including those that treat cancer, diagnose diseases, improve the effectiveness of radiotherapy, fight germs and fungi, perform thermal ablation, deliver drugs and transport genes (Au, Pt, Ag). Nanoparticles made of noble metals have a number of unique properties, all of which contribute to their increased value. In order to target a wide variety of cell types, metal nanoparticles can be functionalized with a wide variety of functional groups, including antibodies, peptides, DNA, and RNA, as well as biocompatible polymers, such as polyethylene glycol. (Prasanna et al. , 2019). 25 Fig. 3 displaying the metal nanomaterials.

2.2.2. Metal-oxide nanomaterials

The major goal in the manufacture of metal oxide nanomaterials is the alteration of the properties of the relevant metal nanomaterials. One example of this would be the transformation of iron nanoparticles into iron oxide nanoparticles. When compared to the reactivity of nanoparticles made of iron, the reactivity of nanoparticles made of iron oxide is substantially higher. Nanoparticles of metal oxide are produced when the efficiency and reactivity of metal oxide are increased. This results in the production of the nanoparticles. 26 Zinc oxide, iron oxide, silicon dioxide, magnetite, titanium oxide, aluminium oxide and cerium oxide are examples of metal oxide nanomaterials. Nanoparticles made of metal oxides have shown promising outcomes in studies conducted in the field of biomedicine. Antibacterial activity has been demonstrated for a wide variety of metal oxide nanoparticles, including but not limited to MnO 2 , FeO, Ag 2 O, ZnO, Bi 2 O 3 , CuO, CaO, Al 2 O 3 , MgO, and TiO 2 . Researchers Sigmund et al. (2006) 27 looked into the impacts of Ag 2 O nanoparticles and came to the conclusion that they could be a potential new source of antibiotics. Thomas et al. (2015) 28 also revealed that Ag 2 O nanoparticles had antibacterial capabilities against E. coli . Zinc oxide nanoparticles demonstrated strong bactericidal efficacy both Gram-negative and Gram-positive bacteria and spores. High pressure and temperature have little effect on these bacteria and spores. In addition, Fadeel et al. (2010) 29 looked at the antibacterial characteristics of ZnO at a range of different particle sizes. According to the findings, the bactericidal effectiveness of ZnO nanoparticles increased in direct proportion to the decrease in particle size. Akif et al. (2022) 30 investigated the antibacterial effects of ZnO, Fe 2 O 3 and CuO nanoparticles on Gram-negative bacteria such as P. aeruginosa , E. coli , and others, as well as Gram-positive bacteria such as Bacillus subtilis and Staphylococcus aureus . They found that all three nanoparticles inhibited the growth of the bacteria. According to these data, ZnO has a potent antimicrobial impact, whereas Fe 2 O 3 nanoparticles have the weakest effect against bacteria. Fig. 4 displaying the metal & metal oxide nanomaterials.

2.3. Ceramics nanomaterials

Ceramic nanoparticles are also referred to as nonmetallic solids in some circles. The process of synthesising ceramic nanoparticles involves periodically heating or cooling the material. Ceramic nanoparticles can have a variety of different structures, including amorphous, polycrystalline, dense, hollow or porous. 32 These nanoparticles are of interest to the researchers due to the vast range of applications that can be achieved with their utilisation, including photocatalysis, dye photodegradation, imaging and catalysis. 33

The research and development of innovative ceramic materials with potential uses in biomedicine is currently advancing at a rapid speed. In order to reduce the cytotoxicity of nanoscale ceramics such titanium oxide (TiO 2 ), hydroxyapatite (HA), alumina (Al 2 O 3 ) silica (SiO 2 ), and zirconia (ZrO 2 ) in biological systems, new synthetic techniques were utilized to optimize the physical-chemical characteristics of these nanoscale ceramics. Nonetheless, when novel ceramic materials were used, the host had negative reactions (in a number of organs, including the immune system). When it comes to the applications of ceramic nanoparticles in biomedicine, regulated drug release is one of the sectors that has received the most attention. In this field, the size and the dose are extremely significant factors. Nanoparticles are a promising technique for managing drug delivery due to a number of properties, including their load capacity and high stability, as well as their ease of absorption into both hydrophilic and hydrophobic systems. Additionally, nanoparticles can be administered via a variety of different routes (inhalation, oral, etc. ). A wide variety of organic groups that are capable of being functionalized on its surfaces also make it possible to achieve a specific effect. Titanium dioxide is a photocatalytic substance with a wide range of dielectric and optical properties due to its various crystalline structures.

Titanium dioxide nanoparticles are stable in anatase at the nanoscale; nonetheless, they also have the maximum level of cytotoxicity in the region of 3 to 10 nm, which is more than 100 times the scale in the rutile phase. Nanoparticles like this are regularly put to use as drug-eluting carriers or excipient formulations in the field of pharmacology. In point of fact, they are being used in photodynamic therapy due to the fact that they photooxidize oxygen quite well. In addition, the cytotoxic properties of nanoparticles are lessened when they are combined with other substances for example, hydroxyapatite. The pharmaceutically active mesoporous silica molecules have a number of key properties, some of the most important of which are the automatic release of prospective drugs, the ease with which they can be dissolved, and their availability in the organism.

Because even a minor shift in the conditions of the synthesis can result in variable forms, sizes, and subsequent physicochemical properties, it is challenging to develop strategies that can combine biocompatibility and minimise the harmful effects that these nanoparticles may exhibit in biological systems. This is because it is difficult to create methods that can combine biocompatible materials with nanoparticles. Because of this, it is difficult to develop techniques that can combine biocompatibility and minimise the bad effects that these nanoparticles may exhibit in biological systems. This is due to the fact that it is difficult to create methods that can combine biocompatibility with nanoparticles. This makes it difficult to find methods that can combine biocompatibility and physicochemical properties. 34 Fig. 5a–d displaying SEM & Fig. 5e–h displaying TEM images of SiO 2 nanoparticles.

2.4. Bionanomaterials

A biological or bio-nanomaterials is an assembly of atoms or molecules that is produced in a biological system and has at least one dimension that falls within the range of 1 to 100 nm. 35 Other terms for this type of particle include bio-nanomaterial and biological nanomaterial. Nanoparticles that can be found in nature are referred to as bionanoparticles. These nanoparticles can either have an extracellular or an intracellular structure, depending on their location. Magnetosomes are an example of an internal structure, whereas viruses and lipoproteins are examples of structures that are found outside of cells. Bionanoparticles include exosomes, magnetosomes, lipoproteins, viruses and ferritin. 36 Fig. 6 displaying the bionanomaterials.

2.5. Carbon based nanomaterials

Carbon-based materials have been debated as valuable treasure in recent times owing to the presence of a variety of allotropes of carbon, which range from well-known allotropic phases like amorphous diamond, graphite and carbon to recently discovered allotropes like opportune graphene quantum dots (GQDs), fullerene, graphene oxide (GO) and carbon nanotubes (CNTs). Amorphous carbon is one of the most common forms of carbon. 37 Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) are the two categories that can be used to categorise carbon nanotubes. A carbon nanotube is a hollow cylinder that is constructed of graphitic sheets. After rolling out a single graphitic sheet with a high aspect ratio, single-walled carbon nanotubes with a cylindrical nanostructure were produced. Multi-walled carbon nanotube is composed of a few graphitic layers arranged in a rolling pattern with a gap of 3.4 angstroms between each layer. 38 Graphene possesses a wide array of exceptional qualities, any one of which could make it an asset for use in bio-applications. Simple functionalization has the ability to result in a rise in the number of functional groups on the surface of the material, which then permits the precise and selective detection of a variety of biological components. In addition, it is an excellent option for the delivery of pharmaceuticals due to the exceptionally wide surface area it contains, the chemical purity it possesses, and the free electrons it possesses. This makes it a great alternative for the administration of pharmaceuticals. 39,40 Another appealing biomaterial from the carbon family that has recently been developed is graphene quantum dots, it has lateral dimensions of less than 100 nm and is comprised of a single layer or a few layers and is described as a zero-dimensional graphene sheet. 41 As a result of the quantum confinement that takes place when two-dimensional graphene sheets are converted into graphene quantum dots, the photoluminescence properties of the graphene quantum dots are enhanced to an exceptional standard. 42 Surprisingly, graphene quantum dots exhibits excellent biocompatibility and photo-bleaching durability relative to conventional fluorochromes or semiconductor quantum dots. This is because graphene quantum dots are made from natural materials. In addition, graphene quantum dots have important graphene properties, such as accessible electrons and a large surface area. These properties make graphene quantum dots a smart nanomaterial for a variety of biomedical applications, including biomolecule sensing, cancer therapy, imaging, targeted drug delivery, and so on. Graphene quantum dots have been shown to be useful in these applications. In addition, graphene quantum dots have a large surface area and accessible electrons. 43,44

Carbon-based nanomaterials are formed entirely of carbon. Carbon-based nanoparticles include carbon nanotubes, graphene, carbon nanofibers, fullerenes and carbon black. 45 Fig. 7 displaying the carbon based nanomaterials.

2.5.1. Fullerene

Fullerene has a highly symmetric cage that can range in size from C 60 , C 70 , and beyond due of its unusual structure of sp 2 carbons. In the as-synthesized formulation, C 60 is the most prevalent fullerene, and its molecular structure, which can be shown in Fig. 8 together with that of C 70 , can be seen to be very similar. 46 C 60 is composed of 60 carbon atoms bonded to one another by single C 5 –C 5 bonds, which result in the formation of 12 pentagons, and double C 5 –C 6 bonds, which result in the formation of 20 hexagons. 47 In reality, a ‘ n ’ hexagon is present in every fullerene that has 2 n + 20 carbon atoms. 48 Fullerene C 60 has the shape of a football, and according to Yadaf and colleagues, the diameter of the earth is 12.75 × 10 6 metres, the diameter of a soccer ball is 2.2 × 10 1 metres, and the diameter of a fullerene molecule is 7.0 × 10 10 metres. According to the findings of the study, the proportion of a fullerene molecule to a soccer ball is analogous to the proportion of a soccer ball to the earth. 49

In terms of crystallographic properties, the presence of symmetric elements such as 20 tripled axes, 12 fivefold axes and 30 twofold axes has made fullerene the most symmetrical molecule that is regulated by the Golden Mean rule (as was previously stated). Fullerene also possesses 12 fivefold axes and 20 tripled axes. 49,50 C 60 has a structure that is sufficiently stable that it may be described as having face-centered cubic lattices in its solid phase where fullerene cage disintegration occurs at temperatures more than 10008 °C. To analyse fullerene, many spectroscopic approaches such as FTIR, NMR, UV-vis, and Raman spectroscopy could be used. 46 Furthermore, Biomolecules, particularly those structured by the Fibonacci sequence and exhibiting Golden Mean properties, have discovered fullerene to be a promising nanomaterial. 51 C 60 is an outstanding candidate for photodynamic therapy due to the fact that, among its many properties, it possesses the capacity to generate oxygen species when it is illuminated by visible light. 52 The puzzling behaviour of fullerene in solutions is evidence of a newly known interaction between solvents and solute. Because of this interaction, the fullerene molecule has not changed conformationally or in a way that is dependent on the solvent, nor has it taken on the shape of a hexagon. Fullerenes have sp 2 hybridized carbon atoms that connect them together. Fullerenes constructed of C 70 or C 60 have diameters of 7.648 and 7.114 nm, 53 respectively. A single layer of fullerene or a multilayer of fullerene can be used. Fig. 8a–c displaying the fullerene C 60 & Fig. 8d–f displaying the fullerene C 70 .

Carbon may bond in a variety of ways to develop structures with vastly diverse properties. Carbon sp 2 hybridization produces a layered structure with strong inplane limitations and modest out-of-plane van der Waals bonding. Multi walled carbon nanotubes are produced by surrounding a standard core hollow with a few to a few tens of concentric cylinders with regular periodic interlayer spacing. An interlayer spacing range was discovered during realspace evaluation of multiwall nanotube images (0.34–0.39 nm). 56 MWCNTs can have an interal diameter anywhere from 0.4 nm to a few nanometers, while their outside diameters can range anywhere from 2 nm to 20 to 30 nm, dependent on the number of layers they are composed of. Both of the normally closed tips of the MWCNT have dome-shaped half-fullerene molecules capping them. These defects, which are also referred to as pentagonal defects, cap the normally closed points. The axial sizes of these defects range from one metre to just a few centimetres in width. The purpose of the half-fullerene molecules, which are also referred to as a pentagonal ring defect, is to make the operation of capping off both ends of the tube more easy. On the other side, single-wall carbon nanotubes (SWCNTs) have diameters that can be anywhere from 0.4 to 2 to 3 nanometers, while their lengths typically fall somewhere in the micrometre zone. In most cases, SWCNTs are able to create bundles by joining together (ropes). Within a bundle structure, the SWCNTs are organised in a hexagonal pattern to create a structure similar to a crystal. 57

Carbon nano tubes are elongated tubular structures with a diameter of 1 to 2 nm. 58 Based on diameter, a carbon nanotube can be classified as semiconducting or metallic. 59 CNT has a structure that looks like a graphite sheet rolling on itself. Fig. 9 shows how (a–c), (d–f) and (g–i) single-walled nanotubes (SWNTs), double-walled nanotubes (DWNTs) and multi-walled nanotubes (MWNTs) looks like. Which is further classified based on their rolling properties.

2.5.1.1. Single walled carbon nanotubes

Carbon has a ground state structure of 2s 2 2p 2 with a valence shell of four electrons. Both graphite and diamond are naturally occurring crystalline forms of pure carbon. Graphite is the more common of the two. Unlike graphite, which has sp 2 hybridization, diamond possesses sp 3 hybridization, which gives it its amazing rigidity. Graphite, on the other hand, has sp 2 hybridization. Along the x – y plane, each carbon atom in graphite forms C–C bonds with three more carbon atoms at an angle of 120°, while a – bond is formed along the z axis. 54 In sp 2 hybridization, the C–C bond length is 1.42 nm and the spacing between carbon layers is 3.35 nm. 55,56 Graphite is an exceptional electrical conductor because it possesses delocalized electrons that are free to flow throughout the structure of the graphite. This makes graphite a wonderful material. Carbon nanotubes, also known as CNTs, are hexagonal networks made up of carbon atoms. The diameter of a carbon nanotube can range from one to one hundred metres, and it has a diameter of one nanometer. CNTs are cylindrical structures that are made up of sheets of graphene that have been rolled up to form a continuous tube. 55 In the middle of the 1970s, Endo was able to take the very first high resolution transmission electron microcopy (HRTEM) pictures of carbon nanotubes. 57 Later, Iijima 58 found helical carbon microtubules, now known as nanotubes, using HRTEM and electron diffraction in the Arc-Discharge Fullerene Reactor. 58 Single-walled carbon nanotubes, also known as SWNTs, are cylinders with a nanometer-scale diameter that are composed of a single sheet of graphene that has been wrapped around to form a tube. Nanowires can be metallic or semiconducting based on the chirality of SWNTs. 59–62 The level of twist in the graphene sheet is the primary factor that influences the electrical conductivity of carbon nanotubes.

A single rolled sheet is used to make single-walled nanotubes. Single-walled nanotubes have a diameter of 0.7 nanometers. The length varies depending on the method used to prepare it. 63 Fig. 9a–c displaying the single walled Carbon nanotubes.

2.5.1.2. Double walled carbon nanotubes

Carbon nanotubes, also known as pure carbon polymer chains, are nanometer-sized cylindrical structures made up of single or concentric multilayers of graphene sheets. Many scientists have been interested in double walled carbon nanotubes in recent years because their intrinsic coaxial topologies give birth to novel electrical and mechanical properties that have not been previously observed. We were able to determine whether or not double-walled carbon nanotubes of a particularly high purity behave as quantum wires and whether or not there is a symmetric relationship between concentric tubes during the growing process by painstakingly preparing them. Also investigated was the shell influence on the electrical conductivity as well as the adsorption characteristics of a coaxial nanotube wire. When compared to single walled carbon nanotubes (SWCNTs), double walled carbon nanotubes (DWCNTs) and multi walled carbon nanotubes (MWCNTs) are preferred materials for bi-cables, atomic force microscopy tips, hydrogen storage materials, electrochemical electrodes, nanocomposites, field emission display sources, nanotube and various electrical devices. 64

Double-walled nanotubes are made up of two layers of rolled sheet. Fig. 9d–f displaying the double walled carbon nanotubes.

2.5.1.3. Multi walled carbon nanotubes

Multiple rolled sheets make up Multiwalled Nanotubes (MWNTs). Multi-walled nanotubes have a minimum diameter of 100 nm. A graphene nanofoil with a hexagonal carbon lattice is coiled into a cylindrical shape to make nanotubes. Carbon tubes range in length from a few micrometres to many millimetres. CNT is a strong material. 65 When CNT is bent, it returns to its original shape without becoming brittle. CNT has a variety of structures and shapes, as well as varied thicknesses, lengths, and layers, 66 but its properties are based on graphene sheets. Fig. 9g–i displaying the multi walled carbon nanotubes.

2.5.2. Graphene

The first two-dimensional atomic crystal to be synthesised in a laboratory is graphene. They are employed in a wide number of applications due to the remarkable chemical and physical qualities, such as elasticity, mechanical stiffness and strength, as well as extraordinarily high thermal and electrical conductivity, that they possess. 67,68 Graphene is a carbon allotrope. It's a planar hexagonal honeycomb carbon atom lattice with a two-dimensional hexagonal honeycomb carbon atom lattice. The graphene layer is 1 nm thick. Graphene is made up of a single layer of carbon atoms that are sp 2 -bonded to one another in a honeycomb lattice. Since Novoselov et al. initial's synthesis at Manchester University in 2004, 69 Because of its excellent physical properties, graphene has attracted considerable attention and scientific curiosity. 69 Graphene is a semiconducting material that has no effective mass and a band gap that is equal to zero. 67,70,71 At ambient conditions, it exhibits a significant ambipolar electric field effect with a high charge carrier mobility (up to 10 000 cm 2 V −1 s −1 ). 67,69 Graphene is the strongest material that has ever been examined, as evidenced by its breaking strength of 42 Nm −1 and its Young's modulus of approximately equal 1.0 TPa. 72 Due to these fascinating features, graphene has demonstrated promise in a variety of applications, including electrical and photonic devices, 73,74 sensing platforms, 75,76 and clean energy applications. 77 Fig. 10a–i displaying the different types of graphenes.

2.5.3. Carbon nanofibres

Carbon is a chemically unique element. Because of its unique electrical structure, it is able to make covalent bonds, either in the form of rings or long chains, with other chemical elements such as hydrogen, as well as with itself. Carbon nanofibers, also known as CNFs, are a type of nanomaterial that only exists in one dimension and have a more intricate structure than carbon nanotubes (CNTs). Because of their one-of-a-kind properties, CNFs are well-suited for a diverse range of applications, such as hydrogen storage, electrochemical catalysis, polymer reinforcing and selective adsorption. 78–80 The direction in which carbon layers in CNFs are arranged has an effect on their mechanical properties. Carbon nanofibres are structured as discontinuous linear filaments based on sp 2 , having an average aspect ratio that is larger than 100 : 1. 81 The identical graphene nanofoils that are used to make carbon nanotubes are transferred into carbon nanofiber; however, rather than being twisted into long cylindrical tubes, the nanofoils are formed into a cup or cone.

In the majority of carbon nanofibers, subsequent examinations revealed that the layers of graphitic planes are not adjusted along the axis of the fibre, as the name suggests. 82 Carbon nanofibers can take on a variety of morphologies, as illustrated in Fig. 11 , controlled by the angle of the graphene layers that comprise the filament. 82 In addition to platelet carbon nanofibers (shown in Fig. 11a–e ) and ribbon or tubular carbon nanofibers (also known as carbon nanotubes) (shown in Fig. 11f–i ), there is another type of carbon nanofiber known as fishbone carbon nanofibers. In fishbone carbon nanofibers, the graphene layers are arranged at an angle to the primary and perpendicular axes of the nanofiber. Fig. 11a–i displaying the different type of carbon nanofibers.

2.5.4. Carbon black

Carbon black is the brand name for a variety of produced fine-particle materials that are also sold under a wide range of other business names and have a range of different physicochemical features, but almost entirely consist of EC. These products are referred to as “carbon black” because they are sold under this brand name. CB has been produced on a commercial scale for more than a century, and with a global production that totaled around 9.8 million metric tonnes in 2008, it has been recognised as one of the top fifty industrial chemicals produced anywhere in the world. 83,84 Rubber applications account for approximately 90% of CB use in the Japan, United States and Western Europe. These applications include rubber automotive products ( e.g. , hoses, belts, and miscellaneous), tire-related automobiles applications and non-automotive industrial rubber products. 83,84 The remaining 10% is allocated to various specialty CB applications, such as UV absorber, pigment and conductor in polymers, inks, and coatings. 85,86 It is made up of carbon and is an amorphous substance. Carbon black has a spherical form. The diameter varies between 20 and 70 nanometers. Fig. 12a–i displaying the carbon black.

3. Classification of nanomaterials on the basis of dimensions

One-dimensional nanomaterials, two-dimensional nanomaterials, and three-dimensional nanomaterials are the three types of nanomaterials.

3.1. One dimension nanomaterials

The preceding ten years have witnessed a rise in interest for one-dimensional nanostructured materials (1D NSMs), which may be attributed to the significance of 1D nanomaterials in research and development as well as the breadth of their probable applications. It is generally agreed upon that one-dimensional nanomaterials are suitable systems for investigating a wide variety of one-of-a-kind nanoscale phenomena as well as the dependence of functional features on size and dimensions. In addition to this, it is anticipated that they will play an important part in the construction of nanoscale EED, electrical and optoelectronic devices by acting as interconnects and fundamental units. In the wake of the groundbreaking work done by Iijima, the field of 1D nanomaterials, which includes nanotubes, has attracted a significant amount of interest. 87 1D NSMs have significant implications for alternative energy sources, nanodevices and systems, national security, nanoelectronics, and nanocomposite materials. 88 Nanowires, nanoribbons, nanobelts, nanotubes, nanorods, and hierarchical nanostructures are some examples of 1D nanomaterials that we provide in Fig. 13 . These 1D nanomaterials have been made in laboratories operated by others. 89,89 The number 10 −9 90 appears in the word nano, it is the equivalent of one billionth of any unit and outcomes in the fabrication of one-dimensional nanomaterials just like thin films. Nanoparticles have a wide range of uses in many different scientific disciplines, including chemistry, engineering, electronics, and pharmaceutics. 91 The thickness of the monolayers or thin films might range anywhere from one to one hundred nanometers. Nanomaterials like these are utilized extensively in research and also play a role in the production of nanoscale LEDs, electronic devices and storage systems, 92 optoelectronic, chemical, and biosensing, 93 magnetooptics, 94 fibre optic systems, and optical devices. One-dimensional nanomaterials are used to make essential nanoscale materials such nanotubes, nanobelts, nanowires, 95 nano-ribbons, 96 and hierarchical nanostructures. 97,98 Fig. 13a–h displaying the one dimension nanomaterials.

3.2. Two dimension nanomaterials

Outside of the nanoscopic range of sizes, there is the possibility of two-dimensional nanostructures. In recent years, synthetic 2D nanomaterials have emerged as a primary focus of research in the field of materials science due to the various low-dimensional properties that differentiate them from the volume properties of traditional materials. This is because of the numerous advantages that these nanomaterials offer. Over the past few years, a significant amount of attention in scientific research has been directed toward the production of 2D nanomaterials in an effort to obtain 2D NSMs. Certain geometries of 2D NSMs exhibit unique shape-dependent features, which enables them to be subsequently utilized as building blocks for the construction of important parts of nanodevices. 104–106 Additionally, 2D NSMs are particularly attractive for investigating and creating new applications in nanoreactors, templates, photocatalysts, nanocontainers and sensors for 2-dimensional structures of other materials. 107,108 In Fig. 14 , we illustrate the two-dimensional nanomaterials known as nanodisks, nanosheets, nanoprisms, nanowalls, branched structures, nanoplates and junctions (continuous islands). 89 2-Dimensional nanostructures are characterised by their singular form and the presence of two dimensions that lie outside of the nanometric size range. Nanomaterials with a two-dimensional structure are utilized as the fundamental building blocks for the essential parts of nanodevices. 97 Nanocontainers, templates, nanoreactors and sensor photocatalysts for 2D structures are all examples of two-dimensional nanomaterials. Two-dimensional nanoparticles include carbon nanotubes. Fig. 14a–h displaying the two dimensional nanoparticles.

3.3. Three dimension nanomaterials

Due to the quantum size effect and other factors, 3D nanomaterials have gained a substantial amount of research attention because of their enormous specific surface area. Additionally, 3D nanomaterials have various benefits over their bulk components as a result of the quantum size effect. As a result, numerous 3D nanomaterials have been produced over the course of the past decade. 89 It is well established that the behaviours of nanomaterials are heavily dependent on their forms, sizes, morphologies, and dimensionality, all of which are critical considerations in determining their eventual performance and applications. As a consequence of this, the synthesis of three-dimensional NSMs that have a specified structure and shape is of the extreme significance. In addition, three-dimensional nanostructures are an important material because of the many different uses that can be discovered for them in the fields of magnetic materials, battery electrode materials and catalysis. 89 As a result of increase in surface area of these materials and their ability to offer sufficient absorption sites for all molecules that are in demand within a constrained area, there has been a surge in recent times of interest in the study of three-dimensional nanomaterials. This is one of the reasons why there has been a surge in interest in the study of three-dimensional nanomaterials. 112 On the other hand, such three-dimensional porous materials may facilitate the transit of molecules. 112,113 We illustrate typical 3D NMSs in Fig. 15 , including nanocoils, nanoflowers, nanopillers, nanocones, and nanoballs (dendritic structures). 89 The behaviour of nanomaterials is determined by their size, shape, morphology and dimension, which are the fundamental parameters for nanostructure performance and application. 114 Three-dimensional nanomaterials have aroused interest in research and medical science throughout the last decade. Nanoparticles like these have a wide variety of applications, including rechargeable batteries, catalysis, and the transport of reactants and products in magnetic materials. Nanoparticles with three dimensions can be represented by examples such as quantum dots, dendrimers, and fullerenes. Fig. 15a–h displaying the three dimensional nanomaterials.

4. Introduction of ZnO

The most important innovations of the 21st century are the design and fabrication of nanoscale materials made of metal oxides, metals, carbon allotropes and chalcogenides. These materials are used in a vast range of fields, such as energy storage, catalysis and biosensors, conversion devices and biomedical applications. In particular, the unique physiochemical properties of semiconducting metal oxides, such as SnO 2 , ZnO, and TiO 2 , which vary depending on size and shape, have been extensively researched and exploited. One of the most stable n-type semiconducting materials for chemical and thermal applications is ZnO, which is available in a variety of forms including pellets, bulk crystal and thin film for use in everything from luminescent materials to batteries, supercapacitors and solar cells to biomedical and photocatalysis sensors. Because of their non-toxicity, large specific area, high sensitivity, high isoelectric point and good compatibility, ZnO nanostructures (nanorods, nanowires, nanorings, nanospheres and nanotubes) have recently received attention. When compared to their macroscopic counterparts, nano-sized materials have faster dissolution rates and higher solubility. 119

In the category of semiconductor metal oxides, semiconductors in the 2–6 group at nanoscale are widely recognized for their diverse and extensive uses in a variety of fields, including solar cells, diluted magnetic semiconductors (DMS), optoelectronic devices, field effect transistors and photoluminescence appliances, to name a few. 120

In general, nanomaterials can be subdivided into one of three categories: zero-dimensional, one-dimensional, or two-dimensional. Nanostructures with zero dimensions, also known as nanoparticles with a near-unity aspect ratio or quantum dots, have found widespread application in the field of biological research. 121,122 These nanoparticles have a two-dimensional structure and find widespread use in a variety of applications, including optical coatings and corrosion prevention. Nanomaterials can be utilised in a variety of ways, one of which is the production of thin films. One-dimensional semiconductor nanomaterials, such as nanorods, nanobelts, nanowires and nanofibres, have attracted a lot of attention in both academic research and industrial applications due to the fact that they can be used as building blocks for other types of materials. This is because of the fact that one-dimensional semiconductor nanostructures can be constructed from other types of structures. 123 Materials with 1D nanostructures can be helpful for research into the interaction between thermal and electrical transport, dimensionality, mechanical characteristics and size reduction (or quantum confinement). 124 In addition to this, they are extremely important in the production of nanodevices that are electromechanical, electrical, electrochemical and optoelectronic in nature, acting as interconnects and functional units respectively. 125 It is possible to classify nano sized zinc oxide as a unique material because to the diverse structures that may be observed inside it. This material has the potential to be utilized in a broad variety of nanotechnology fields. Forms of zinc oxide can be categorised as either one-dimensional, two-dimensional, or three-dimensional. Nanorods, 126–128 needles, 129 helixes, rings and springs, 130 ribbons, 131 tubes, 132–134 belts, 135 wires 136–138 and combs 139 are among the most common one-dimensional structures. Zinc oxide can be found in the form of nanopellets, nanosheets and nanoplates, all of which are two-dimensional structures. 140,141 Zinc oxide may produce a variety of three-dimensional structures, some of which resemble dandelions, snowflakes, flowers or even urchins on coniferous trees. 142–145 Fig. 16a–h displaying the zinc oxide structure.

ZnO possesses one of the most varied ranges of particle configurations of any material that is currently known. Due to their exceptional efficiency in photonics, electronics, and optics, ZnO nanowires are promising materials for a vast range of uses, including nanogenerators, ultraviolet lasers, light-emitting diodes, solar cells, photodetectors, photocatalysts and gas sensors. ZnO nanowires, when subjected to the appropriate light irradiation, are currently being utilized as photocatalysts for the purpose of inactivating viruses and bacteria, as well as for degrading environmental contaminants such as volatile organic compounds, dyes and insecticides. 8 Furthermore, ZnO exhibits a vast morphological variation in nanomaterials such as nanobelts, nanotubes, nanowires, nanorods and other complex morphologies. These nanostructures can be fabricated quite easily at very low temperature, and a variety of different growth techniques for ZnO nanostructures have been documented, including chemical and physical techniques such as sol–gel deposition, cyclic feeding CVD, surfactant and capping agent-assisted growth, electrochemical deposition, hydrothermal and solvothermal growth, chemical vapour deposition (CVD) and thermal evaporation. Because of the growth procedures, disciplines and applications that were discussed above, ZnO has the potential to become one of the most significant candidates for use in future research and applications. 153

4.1. Structure of ZnO

ZnO is typically hexagonal in structure. Four oxygen atoms are tetrahedrally coordinated to zinc atoms. The combination of these two ZnO structures produces perfect polar symmetry with the hexagonal axis of the zinc oxide crystal structure. These crystalline structures are responsible for ZnO-based piezoelectricity and spontaneous polarization. 154 The cubic zinc blende structure and the hexagonal wurtzite structure are the two most common types of zinc oxide crystallisation. In typical conditions, the crystal structure of zinc oxide takes the form of the wurtzite, which has a hexagonal arrangement of its atoms. (JCPS card no. 36-1451) In order to evaluate whether or not ZnO is crystalline, one may examine the structure of hexagonal ZnO, which has the following dimensions: a = 0.32498 nm, b = 0.32498 nm, and c = 5.2066 nm. The value of c / a , which is approximately 1.60, is rather near to the perfect value of c / a , which is equal to 1.633 for a hexagonal cell. In Fig. 17b , the structure of ZnO can be described as a sequence of alternating planes made up of tetrahedrally connected oxygen and zinc ions that are stacked alternately along the c -axis. This sequence of alternating planes represents the structure of ZnO. These planes are arranged in a spiral pattern. As seen in Fig. 17e , the combination of O 2 and Zn 2+ results in the formation of a tetrahedral unit that lacks central symmetry. 152 The wurtzite structure of crystalline ZnO features a hexagonal unit cell, and it either belongs to the C 4 6v or P 6 3 mc space group. Its lattice parameters are a and c . Lattice parameters for hexagonal unit cells are typically in the range of 3.2475 to 3.2501 for a, and 5.2042 to 5.2075 for c. 155–157

ZnO has a density of 5.606 gram per cubic centimetre. As can be seen in Fig. 18 , a single zinc atom is tetrahedrally connected with a total of four oxygen atoms. The piezoelectric nature of the material, which is an crucial property for the creation of micro-electromechanical systems consisting of transducers, sensors and actuators, is the cause of ZnO's noncentrosymmetric structure. This structure can be attributed to the material's piezoelectric nature. 161 Due to the fact that it is noncentrosymmetric, it also possesses two polar surfaces on sides that are opposite one another. Each of these polar surfaces is terminated by a single type of ions ( Table 1 ).

Polarity is referred to as zinc polarity when the bonds along the c -direction are from cation (Zn) to anion (O), whereas polarity is referred to as oxygen polarity when the bonds are from anion (O) to cation. Zinc polarity can refer to either Zn polarity or oxygen polarity; either one can be used interchangeably (Zn). This polarity is also the cause of a number of other properties that ZnO possesses, such as spontaneous polarisation and piezoelectricity. In addition to playing an important role in the creation of crystals, the formation of defects, plasticity, etching, and other processes, this polarity is also the cause of a number of other properties that ZnO possesses. It possesses both polar and non-polar surfaces, in addition to the polar ones it already has. The c -axis is the direction in which the polar Zn-terminated (0001) and O-terminated (0001) sides of wurtzite ZnO are oriented toward. Wurtzite is composed of ZnO, which has an equal number of atoms of both Zn and O on its non-polar (2110) ( a -axis) and (0110) faces. The most common wurtzite ZnO crystals have these four faces arranged in a square. It has been demonstrated that the development of ZnO crystals into a wide variety of shapes is brought about by variations in the relative growth rates of different crystal facets as well as differences in the growth rates of various crystal planes. The years 1970 were the ones in which this discovery was made. 163,164 Polar surfaces ought to be unstable from an electrostatic point of view, unless charge configurations and, as a consequence, opposite ionic charges on the surface result in spontaneous polarization and a normal dipole moment. In addition to this, it was discovered that both the surface with the coordinates (0110) and the polar surface are stable. On the other side, it has been determined that the (2110) face is less stable than its contemporaries and that it has a higher level of surface roughness than its rivals. This was found to be the case through extensive testing. 165

4.2. Properties of ZnO

ZnO, once it has been developed, is considered to be a negative (n-type) semiconductor. Zinc oxide is a type of semiconductor that falls within groups 2–4 of the periodic table. The energy gap in zinc oxide is measured to be 3.37 eV. In addition, zinc oxide possesses a high binding energy. Zinc oxide has a binding energy of around 60 meV. 166 ZnO possesses a high exciton binding energy and is very stable at high temperatures. In addition to that, it boasts a high optical gain. 167 As a result of the characteristics that have been discussed thus far, ZnO has emerged as one of the most intriguing substances for the creation of electrical and optoelectronic devices. On the other side, due to the high binding energy of ZnO, a wide variety of photonic devices that are highly effective in their utilisation of light may be fabricated. This opens up a lot of opportunities for research and development in this area. Additionally, the large band gap of ZnO is being utilized in the research and development of short wavelength optoelectronic devices. 168 ZnO is a type of optical material that is see-through and is optimized for use with visible wavelengths. 169 Numerous research organisations have examined the ZnO's unique features. This results in an improvement of ZnO's electrical and optical characteristics. Numerous other features of ZnO enable a diverse range of uses. These include light-emitting diodes, photovoltaics, microelectromechanical systems and photodetectors. 170–173 ZnO is an incredibly versatile material with semiconducting, pyroelectric and piezoelectric characteristics. ZnO is a material that exhibits a wide variety of nanostructures, significantly more than any other nanomaterial, including carbon nanotubes. 174–176

4.2.1. Optical properties of zinc oxide

The way in which a substance reacts when illuminated by light is what establishes its optical characteristics. Based on their physical characteristics, such as their vibrational and electrical states, as well as the presence and nature of impurities and defects in the material, semiconductor substances have contributed a great deal to our understanding of a wide range of topics, providing a wealth of information along the way. This knowledge has been gleaned from the study of semiconductor materials. The extrinsic and intrinsic qualities of a material are what designate it as a semiconductor. Excitonic causes related to the Coulomb attraction, in addition to the interaction between electrons in the conduction band and holes in the valence band, are the fundamental building blocks for the intrinsic properties of semiconductors. These characteristics are known as the electronic band structure. The dopants and defects that are injected into the semiconductor are what govern its extrinsic optical properties. These dopants and imperfections form discrete electronic states between valence band and conduction band. 177 Investigations into an optical transition in a zinc oxide semiconductor have been carried out utilizing a variety of methodologies, including transmission photoluminescence, cathode luminescence, reflection, optical absorption, and others. The photoluminescence method is one of these methods that has seen widespread usage in the process of determining the optical behaviour of zinc oxide structures. The photoluminescence spectra of various zinc oxide nanostructures include UV emission in addition to one or two bands of visible emission that are brought about by vacancies, antisites, interstitials, defects, and complicated defects. 178,179 The zinc oxide material has a band gap of 3.37 eV and an exciton energy of 60 meV when it is at ambient temperature. Due to the fact that this material has a higher exciton energy value than GaN, it is capable of emitting excitons in an effective manner at room temperature and below low excitation energy (25 meV). As a direct result of this, zinc oxide is currently considered to be one of the most promising photonic materials in the blue-ultraviolet region. 180 The optical properties of zinc oxide nanorods have been studied using photoluminescence spectroscopy, which provides information about the band gap, defects, and crystal features. 180,181 Zinc oxide nanorods show a near-band edge for UV emission and a broader band emission due to deep level defects when subjected to photoluminescence study at ambient temperature. A single emission at UV emission (from 3.236 to 3.307 eV) has been seen in zinc oxide nanorods with lower impurity concentrations, and deep level emissions have been seen in these nanorods as well. 182 Near-band edge and deep level emissions band emissions in the UV and visible ranges are attributable to defects in the zinc oxide nanostructure such as oxygen vacancy, oxygen interstitial, zinc vacancy, zinc interstitial and extrinsic contamination. 180,182 The optical quality of various zinc oxide systems can be assessed by comparing the relative intensity of near-band edge and deep level emissions emission. Thus, the optical quality of zinc oxide is given by the ratio of near-band edge emission intensity to deep level emissions emission intensity ( I NBE / I DLE ). The high ( I NBE / I DLE ) number indicates a deep level defect with lesser concentration. 182 Different type of ZnO doped optical properties graphs are displayed in the Fig. 20 (a–f) .

4.2.2. Magnetic properties of zinc oxide

The orientation of electron spins in the magnetic semiconductor host lattice is used to categorise the various types of magnetic semiconductors. On the basis of this alignment, semiconductors can be divided into the following three distinct categories: (a) magnetic semiconductor, (b) DMS semiconductors and (c) semiconductors that do not exhibit magnetic behaviour, as seen in Fig. 19a–c . As seen in Fig. 19a , magnetic semiconductors are constructed solely out of the periodic alignment of magnetic components. As can be seen in Fig. 19b , DMS are composite materials consisting of magnetic components and nonmagnetic semiconductors. As can be seen in Fig. 19c , nonmagnetic semiconductors do not have any magnetic impurities present in the host lattice of the semiconductor. Doping various transition or rare earth ions into nonmagnetic semiconductors, such as Cu, Ni, Sm, Co, Cr, Eu, Fe, Mn, Gd, and so on, is one method for producing DMS ( Fig. 19b ).

Doping is the technique of inserting impurities on purpose into an intrinsic semiconductor in order to affect the material's physical properties. Doping is also known as doping an intrinsic semiconductor. A great number of research reports were distributed all at once via DMS 154–159,189 . There have also been reports of attempts to doped semiconductor nanocrystals 160,161 . There is a growing interest in studying the fundamental characteristics of DMS in various nanostructures for spintronics applications 158–160,162 . The introduction of transition or rare earth ions into semiconductors results in the formation of these materials. Because the d and f shells of transition or rare earth ions are only partially full, these doping elements have electrons that are not connected with another atom. This allows for greater doping efficiency. One of the bands in transition metals such as manganese, copper, cobalt, and nickel is only partially full or just over half-filled at most (up or down spins). The ions of the transition metal are almost always substituted for the cations that are originally present in the host semiconductor. Doping manganese into zinc oxide, for example, causes the element to offer its four s 2 electrons to the s–p 3 bonding and causes a Mn 2+ charge state to be created in the tetrahedral bonding. This is because doping manganese causes the element to give its four s 2 electrons to the s–p 3 bonding. In order to determine the tetrahedral bonding, the d bands of the transition metal hybridise with the VB bands of the host (O-p bands in zinc oxide). Because of this hybridization, the interface between the locally organised carriers in the host valence band and the three-dimensional spins is replaced, which results in the sample having a local magnetic moment. When it comes to defining the magnetic properties of materials that have been doped with transition metals, the degree of doping in the carrier density, the crystal, and the quality of the crystal all play a part.

Dulub et al. 163 provided the impetus for thinking about semiconductor oxides, specifically zinc oxide, in the context of spintronics. According to the predicted mean field theory, common diamagnetic semiconductors with five atomic percent Mn doped and a hole quantity of 3.5 × 10 20 cm 3 would have a high Curie temperature. In the case of zinc oxide and GaN, simulations show that the Curie temperature exceeds 300 K. 163 Because the Zener model suggests that magnetic properties can be modified by modifying the carrier concentration in the materials, the character of the carrier is an important part of the model that must be taken into consideration. In the beginning, the substitution integral parameter suggested that p-type materials would be ideal candidates for high Curie temperatures. Additionally, the density of states in the valence band is higher than the density of states in the conduction band. Because it is impossible to produce p-type zinc oxide, Dietl's theory does not apply to zinc oxide, which is an element.

Li et al. 164 proposed a pattern that demonstrates the dominance of defect states on DMS ferromagnetism properties.

They claim that donor defects are responsible for covering up a significant amount of doping substance as well as the establishment of a contaminated band. In the case of type zinc oxide, these donor defects can take the form of zinc interstitials or oxygen vacancies. This contaminated band is capable of interacting with the local magnetic moment if the bound magnetic polaron is made significantly larger. Within this radius, the magnetic dopants will interact with the bound carrier, and they will be able to align their spins in each bound magnetic polaron so that they are parallel to one another. In order to obtain both ferromagnetism and penetrating ferromagnetism in the DMS, the magnetic polarons that are coupled to the electrons in the material must be stacked one on top of the other to create a chain that runs the length of the material. MS nanocrystals are remarkable materials that incorporate quantum confinement effects as well as magnetic features due to the system's DMS composition. Some artificial problems are involved in the direct exchange of cations/anions of host material via dopant ions in nanocrystals. 165,166 A significant barrier that needs to be surmounted is the creation of nanocrystals that have dopant ions incorporated continuously throughout the lattice of the host substance. The utilization of nanocrystals that have a high surface area to volume ratio has begun to promote impurity separation to the surface of the nanocrystals during a process known as “self annealing” in the core. This process takes place in the core. As a consequence of this, dopants are probably just sitting on the surface, which results in a high level of entrapment. Nevertheless, the production method for generating doped nanocrystals serves a crucial function in the overall process. Several papers 162,167,168 propose successful dopant integration in host materials. Doping a very small amount of rare earth or transition atoms is the primary technique that is utilized in the process of initiating magnetism in ZnO. There are still some points of contention regarding the substitutional insertion of transition or rare earth elements in host materials and the attainment of ferromagnetism in doped host materials, both of which have been the subject of extensive research. The ferromagnetism that was explored in the host semiconductor could have been induced by the inherent magnetism of the semiconductor itself, as well as its precipitates, or the secondary magnetic phases of transition metals. If DMS is researched in a methodical manner by correlating all of its attributes, then the controversy over the presence of magnetic properties can be settled once and for all.

A great deal of curiosity has been ignited as a result of the finding that metal oxide nanocrystals exhibit ferromagnetism at normal temperature. When compared to the equivalent bulk material, nanocrystals have a high surface-to-volume ratio; hence, changes in nanocrystal size have the greatest influence on the surface effects. The influence of voluntary surface spins on saturation magnetization provides evidence that this function plays a crucial role in magnetic properties. In their bulk and nanostructure forms, metal oxides such as HfO 2 , ZnO, and Al 2 O 3 exhibit diamagnetic and ferromagnetic magnetic properties, respectively. 169 Interactions between localised electron spin moments and oxygen vacancies at nanocrystal surfaces are thought to be the cause of ferromagnetism in nanocrystalline materials. 169 Ferromagnetic behaviour was seen in chemically produced zinc oxide nanocrystals that were capped by a variety of capping agents when the samples were allowed to reach room temperature. Spin polarization can be facilitated by the alteration of the surface charge state by coupled ligand. 152 According to the findings of this study, the magnetic properties of nanocrystals are not only associated with the presence of magnetic ions, but they are also highly supported by the presence of surface defects. Additionally, the presence of magnetic ions is associated with the magnetic properties of nanocrystals. In addition to this, the magnetic properties are connected with the presence of magnetic ions, which explains why they have these characteristics. 152 Different type of ZnO doped magnetic properties graphs are displayed in the Fig. 21 (a–f) .

4.3. ZnO and their structural properties

The variation of ZnO nanostructures is determined by the growth mechanism, the growth method, the synthesis conditions, and the type of substrate. Nanowires, nanorods, nanotubes, nanocolumns, nanorings, nanobelts, nanosheet networks, nanoribbons, nanoflowers, hollow micro- and nanospheres and nanocombs are among the nanostructures.

4.3.1. ZnO nanorods and nanowires

One-dimensional nanostructures such as nanorods and nanowires are gaining popularity due to the multiple applications they have in photovoltaic systems, nanoelectronics, chemistry, and biosensors. These nanostructures are comprised of a single layer. In addition, the 1D nanostructures are a good candidate for the production of effective optoelectronic nanodevices due to their many important characteristics, such as a direct band-gap and a significant exciton binding energy. This makes the 1D nanostructures a good candidate for the production of efficient optoelectronic nanodevices. There have been a significant number of articles written about the synthesis of zinc oxide nanorods and nanowires, as well as their characteristics and the growth mechanisms behind them. There are two primary growth methods that have been described for the creation of zinc oxide nanowires and nanorods through a gas phase process. These approaches are as follows: vapor–liquid–solid, 170,171 and vapor-solid. 172–174 As nucleation sites for the generation of one-dimensional nanostructures, metal nanoclusters or metal nanoparticles have been utilized in the VLS mechanism, which is a process that is supported by a catalyst. Metal is the material that both of these nanoclusters and nanoparticles are composed of. The production of alloy liquid droplets takes place as a result of the gaseous reactants interacting with the catalytic particles throughout this process. The formation of 1D nanostructures is significantly aided by the participation of these droplets. The formation of precipitation begins when a droplet of liquid gets supersaturated with the medium from which it originated. If the conditions are favourable, the amount of precipitation that falls will continue to increase over the course of time, which will result in the construction of connected structures that are one-dimensional. During the process of creating 1D nanostructures, a number of different metal catalyst components, including gold, tin, copper, and cobalt, are utilised as catalysts. On the other hand, in order to successfully carry out the VS mechanism, the utilisation of a catalyst is not required in any way. When it comes to the process of generating 1D nanostructures, it is generally agreed upon that one of the most significant components is mastering the ability to regulate the level of supersaturation present. This is due to the fact that the degree of supersaturation is what is responsible for determining the primary growth morphology. For whiskers growth, a low degree of supersaturation is required; for bulk growth, a medium degree is necessary; and for powder growth, a high degree of supersaturation is necessary. The source materials are vaporised at a high temperature in the usual vapor solid process, and then they are quickly condensed onto the substrate in a zone that is characterized by a low temperature. This results in the formation of a vapor solid. This takes place in an area where the temperature is lower than the point at which vaporization can take place. Condensed molecules give rise to seed crystals after the first stage of the condensation process is complete. These seed crystals are put to use as nucleation sites for the later phases of the process of developing nanostructures. The specific vapor solid approach has been established for the purpose of manufacturing a wide variety of ZnO nanostructures in order to fulfil demand. Fig. 22a–h displaying the ZnO nanowires and rods.

4.3.2. ZnO nanotubes

ZnO nanotube arrays are almost always created by dissolving the centre of previously made nanorods. Electrochemical dissolution is a viable option., 202 Chemical dissolution is more prevalent, though. Zinc oxide is amphoteric, which means it may dissolve chemically in either an acidic (HCl) or a basic (KOH) solution. 203 The selective decomposition of the nanorods' centres while leaving the lateral faces unaffected by the process can be explained in two different ways. To begin, the metastable plans for (0001) zinc oxide have a higher surface energy than the lateral plans, which are thus more stable. 204,205 Second, the (0001) designs have more defects, which makes them more likely to break. 204 To the dissolving solution, Wang et al. added a surfactant (cetyl trimethylammonium bromide). 206 This molecule is well known for being bound to the (1000) plans and safeguarding the wall of the 1D structure throughout the process of nanorods dissolving into nanotubes. This is why it has received so much attention. 207–210 Ammonia solution is used as an etching agent.

All of the aqueous solutions were prepared with deionized water that had been acquired from Sigma-Aldrich. The resistivity of the water was 18 Ω cm. All of the chemicals that were utilised in this work were of analytical quality and did not require any additional purification prior to their usage. The manufacturing of ZnO nanotubes is a process that takes place over two stages. In the initial step of the process, the substrate, which was a piece of clean room paper measuring 50 mm by 50 mm, was washed in deionized water and then allowed to dry in the air. After that, the substrate was heated for an additional twenty minutes at a temperature of one hundred degrees Celsius in order to evaporate any trace amounts of moisture that could have been present in the paper. The paper substrate, on the other hand, has a large capacity for absorbing water, which makes it susceptible to wetting and limited in its ability to withstand low temperatures for extended periods of time. As a consequence of this, a wetting and chemical barrier layer is essential in order to shield the paper substrate from the effects of being exposed to water and chemicals. 211 This layer needs to be able to act as a barrier against chemicals and moisture, in addition to having mechanical and dielectric properties that are satisfactory. The deposition of such a barrier layer on the substrate can be accomplished using a various of methods, including sputtering, evaporation, and chemical vapour phase deposition, to name a few. 211 In addition, because these processes require a large number of intricate stages, we opted to avoid them in favour of a method that was both straightforward and highly effective. This method involved applying a protective layer to the paper substrate in order to achieve passivation or chemical resistance. For this purpose, we utilised the advanced electronics cyclotene 3022-46 resin that is manufactured by Dow Chemical Company USA. This resin is a polymer that can be used for wafer-level applications that require a thin layer, and it can do so successfully. During the synthesis process, the surface roughness and damage may be reduced thanks to this change of the surface, which may also help. In addition to this, it has the potential to assist in enhancing the alignment and homogeneity of ZnO nanotubes on the paper substrate. 212 After applying a layer of cyclotene by spin coating to the surface of the paper substrate, we baked it in a vacuum for fifty minutes at a temperature of one hundred degrees Celsius. After that, the substrate was roasted in the oven for approximately thirty minutes at a temperature of one hundred sixty degrees Celsius. After that, a spin coater was used to apply a seed layer to the substrate at a speed of 2100 rpm for approximately one minute. This step served to provide nucleation sites for the creation of ZnO nanorods. This method was carried out a total of five times in order to ensure adequate coverage.

ZnO nanoparticles were used to construct the seed layer. These nanoparticles were produced by achieving a concentration of 0.01 M in methanol by diluting zinc acetate dehydrate, which has the chemical formula (C 4 H 6 O 4 Zn·2H 2 O). After that, the solution was brought up to a temperature of sixty degrees Celsius. A second solution of KOH in methanol with a concentration of 0.03 M was dropwise added to the first solution while it was continuously stirred at a temperature of 60 °C for two hours. ZnO nanoparticles have diameters that ranged anywhere from 5 to 10 nanometers. 213 After maintaining a temperature of 180 °C on the substrate for a period of thirty minutes, it was eventually possible to consolidate the seed layer. This was made possible after the substrate was heated. Following that, the temperature of the substrate was permitted to gradually recover to that of the surrounding environment. We chose a method for developing the ZnO NRs that required a temperature that was on the lower end of the spectrum. Zinc nitrate hexahydrate [Zn(NO 3 ) 2 ·6H 2 O] and hexamethylenetetramine (C 6 H 12 N 4 ) were mixed in equal amounts in DI water and kept under continuous magnetic stirring for 30 min in order to obtain a consistent growth solution. This was done in order to obtain a consistent growth solution. This was done in order to obtain a growth solution that was consistent throughout. After that, the paper substrate, which had been preheated, was immersed in the solution and heated at a temperature of eighty degrees for a period of five hours. Following that, it was cleaned with DI water to remove any residuals that may have been on the surface, and after that, it was dried at room temperature in the air.

The second step was to obtain the ZnO nanotubes, which we did using a process that involves chemical etching to convert ZnO NRs to NTs. 214 This approach has been effectively used by many research groups to produce zinc oxide NTs. 215,216 In order to do this, the ZnO NRs that were located on the paper substrate were chemically etched into ZnO NTs by placing them in an aqueous solution of KCL at a temperature of 80 °C for several hours. After that, the substrate was taken away and given a thorough cleaning with DI water in order to remove any residuals that might have been on the surface. The last step was to hang it up outside so it could dry. We used scanning electron microscopy with a 12 keV energy setting and transmission electron microscopy with a 200 keV energy setting to explore the surface morphologies and diameters of the ZnO nanotubes that were formed. An X-ray diffractometer was used with Cu K radiation, a wavelength of 1.54178 Å, 40 keV, and 100 mA for the aim of determining the crystal structure of the final products and classifying them into their respective phases. This was accomplished by using the instrument. An energy-dispersive X-ray spectroscopy that was linked to a scanning electron microscope and operated at 20 keV was used to investigate the evidence for the purity and elemental composition of the as formed ZnO nanotubes. The charge-coupled device camera, which was cooled using nitrogen, was used to carry out the measurements of CL. The luminescence was collected by a parabolic mirror, and it was then scattered by a monochromator of 0.55 metres in length and fitted with a grating measuring 600 mm −1 . Fig. 23a–h displaying the ZnO nanotubes.

4.3.3. ZnO nanobelts and nanorings

The belt-like nanostructure morphology is a structural characteristic that is maintained by functional oxides with a variety of different crystallographic structures. In reality, three different architectures can be used to generate ZnO nanobelts (NBs). 222 Planar defects can be used to grow one of these structures. ZnO NBs can be used in a variety of ways. ZnO nanorings are a remarkable type of ZnO morphology that can be produced, in addition to ZnO nanobelts, by ZnO nanobelts. 223 Planar defects are required for the development of nanorings, according to microscopic research. 224 Twins, interstitial stacking layers and conventional stacking faults created by impurity atoms are all examples of planar defects. 225 Using indium as a dopant element, Wang's team was able to create ZnO nanorings. 222 They also demonstrated that the introduction of planar defects within ZnO NBs, such as inversion domain borders, may be caused by the doping of indium ions. They came to the conclusion that the polarity of the NBs was in no way affected by the IDBs, regardless of whether they were coupled head-to-head or tail-to-tail. This was the finding that they came to. Because of the long-range electrostatic interaction between the surface polar charges on the two sides, the development of a nanoring was initiated by circularly folding a Nanobelt, and loop-by-loop winding of the nanobelt generated a full ring. This interaction was caused by the fact that the nanobelt had surface polar charges on both of its sides. This interaction was brought about as a result of the fact that the surface polar charges on both sides are charged in the opposite direction. The fact that a nanobridge can be folded into a nanotube allowed for the successful completion of this interaction. They came to the conclusion that indium would be the best material to use as the doping agent due to the significance of indium doping in the semiconductor industry. Furthermore, many research groups, including our own, have been focusing their efforts over the past few years on the development of indium-doped zinc oxide nanostructures. Tin is an important doping chemical that, in addition to indium, has the potential to open up new applications for zinc oxide. 226 It is generally knowledge that the band gap of ZnO can be adjusted to a more desirable value by alloying the material with another substance that possesses a band gap that is different. This causes a change in the wavelength of exciton emission to occur as a result. Because the inclusion of tin oxide, which has a greater band-gap than ZnO (3.6–3.97 eV), results in a widened band-gap, the ZnO/SnO 2 structure produced by alloying ZnO with SnO 2 could be a possible contender for future optoelectronic devices. This is due to the fact that the inclusion of SnO 2 , which has a greater band-gap than ZnO. In addition to this, when compared to undoped zinc oxide nanowires, the field emission properties of Sn-doped zinc oxide nanowires are significantly improved, and the resistance decreases as the amount of Sn present in the nanowires increases. ZnO's band-gap ranges between 3.6 and 3.97 ev, whereas SnO2's band-gap is between 3.6 and 3.98 ev. 227 Sn can act as a doping material in ZnO NBs, causing the production of planar defects, in addition to its impacts on the optical band gap and better electrical characteristics. 228 As a direct consequence of this, it will be possible to produce zinc oxide nanorings using tin (Sn) as the dopant material, which is something that has never been accomplished previously. On the other hand, research has not yet been conducted to determine how the concentration of tin influences the formation of zinc oxide nanorings. In addition, the vapor–liquid–solid process, which is considered to be one of the most important ways for producing one-dimensional nanostructures, has not yet been employed to create zinc oxide nanorings. Fig. 24a–h displaying the ZnO nanobelts and nanorings.

4.3.4. Hollow ZnO nano- and micro-spheres

Utilizing carbonaceous saccharide microspheres of variable diameters as templates has resulted in the development of a straightforward and generic process that is capable of producing zinc oxide hollow microspheres with a number of shells that can be adjusted. This method is also very successful. It was revealed that the triple-shelled zinc oxide hollow spheres with a large surface area displayed the highest levels of photocatalytic activity. This was discovered when the photocatalytic capabilities of the as-synthesized products were assessed by degrading methyl orange (MO) dye. In addition to this, research was conducted to determine the mechanism behind the production of multiple shelled zinc oxide hollow spheres and the rationale behind their high photocatalytic activity. 233

Every one of the elements was of reagent-grade quality, and they were all utilized in their natural state. In this experiment, the metal precursors were zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) and carbonaceous saccharide microspheres served as the sacrificial templates. The following explanation provides an outline for the typical production of single-shelled ZnO hollow microspheres. In accordance with what was previously stated, carbonaceous microspheres were produced using the emulsion polymerization of sugar in hydrothermal circumstances. 234,235 Adjusting the amount of sugar solution used and the amount of time the reaction is allowed to run can change the width of the carbon spherules that are formed. The microspheres of carbonaceous saccharide were given multiple washings in deionized water and ethanol at a concentration of one hundred percent until the filtrate became transparent.

With the assistance of ultrasonication, freshly manufactured carbonaceous microspheres (0.5 g) with diameters of 500 nm were dispersed throughout a solution of 1.5 M zinc nitrate (water/ethanol = 1 : 3, v/v, 25 mL). Following the completion of a total of 0.5 h of ultrasonic dispersion, the suspension was then aged for 8 h in a water bath maintained at a temperature of 60 °C. After that, the suspension was dehydrated in an oven at a temperature of 80 °C for 12 h, after which it was vacuum filtered, rinsed several times with deionized water, and then filtered again. The resulting black composite microspheres were heated to 350 °C for one hour, then gradually brought up to 450 °C in air at a rate of one degree per minute, and eventually held at 450 °C for two hours. This was done so that the templates could be removed. The ZnO hollow microspheres with a single shell were produced as a white powder by-product when the tube furnace was allowed to naturally cool to room temperature. A procedure that is quite similar to this one was used in the production of double- and triple-walled ZnO hollow microspheres. Burning was one of the steps in the process that resulted in the production of ZnO nanoparticles. In a nutshell, 5 mL of deionized water were used to dissolve 3 g of Zn(CH 3 COO) 2 ·2H 2 O and 1 g of CO(NH 2 ) 2 , and then NH 3 was added after that. The water was added in a very careful and methodical manner drop by drop until the solution became extremely thick gel precursors. This process took quite some time. The resultant viscous gel precursors were immediately heated to 500 °C, where they spontaneously ignited to generate white ZnO particles. This step was repeated until the desired amount of ZnO had been produced. Fig. 25a–h displaying the ZnO nano and microspheres.

4.3.5. Star- and flower-shaped ZnO nanostructures

Using a method that is based on a solution and performed at a low temperature, sulphur doping of hexagonal ZnO nanowires using thiourea (SC(NH 2 ) 2 ) produces hexagram-shaped ZnO nanostructures (“nanostars”). According to scanning electron microscopy, the amounts of sulphur doping have a significant impact on the morphology of the nanostructure when viewed in cross-section (SEM). According to the calculations of the density functional theory, the transformation from hexagonal nanowires to nanostars takes place as a result of sulphur atoms preferentially bonding to the vertices of hexagonal structures while the structures are developing. The observed hexagram structure is most likely the result of the resultant shift in the local chemical environment. X-ray photoelectron spectroscopy and photoluminescence spectroscopy were utilized to demonstrate the presence of sulphur in the nanostars. The involvement of sulphur in the formation of nanostars was validated in control tests using the sulfur-free counterpart urea (OC(NH 2 ) 2 ).

The processes developed by Greene and Pacholski for making ZnO nanowires in solution were followed to make the nanowires. In the first step of this procedure, solutions of zinc acetate dihydrate [(CH 3 CO 2 ) 2 Zn·2H 2 O (Fluka, assay 99.5%)] and sodium hydroxide [NaOH (Fisher Chemical, assay = 98.6%)] were prepared in methanol at concentrations of 0.01 mol L −1 and 0.03 mol L −1 , respectively. The mixture of 13.68 mL of a solution containing 0.03 mol L −1 of NaOH and 26.32 mL of a solution containing 0.01 mol L −1 of (CH 3 CO 2 ) 2 Zn·2H 2 O was then stirred for two hours at a temperature of 60 °C. ZnO seed crystals were produced by applying a drop-coating of the solution that had been produced to a silicon substrate, then rinsing the substrate with methanol and blow-drying it with air. This method of drop-coating was carried out a number of times. After that, the substrate was annealed for twenty minutes at a temperature of three hundred and fifty degrees Celsius in order to form ZnO seed crystals. After placing the substrate in an aqueous solution that contains 0.025 mol L −1 of zinc nitrate [Zn(NO 3 ) 2 · x H 2 O (Alfa Aesar, assay = 99%)] and 0.025 mol L −1 of hexamine (hexamethylenetetramine) [(CH 2 ) 6 N 4 (Alfa Aesar, assay = 98%)], heat the mixture at 90–95 °C for The ZnO nanostars that were used for this study were produced by hydrothermally growing them with varying concentrations of a thiourea [SC(NH 2 ) 2 (Alfa Aesar, test = 99%)] doping solution. This method was used to explore the nanostars' properties. The amounts of thiourea were changed (0.025 mol L −1 , 0.05 mol L −1 , 0.1 mol L −1 , 0.2 mol L −1 , and 0.5 mol L −1 ), and a control experiment was performed using urea at a concentration of 0.1 mol L −1 [OC(NH 2 ) 2 (Acros Organics, assay = 99%)]. In every single experiment, 10 mL of each reactant solution were utilized.

It is explained how each of the thiourea-infused growth treatments works. The entire sample occupied a space of 30 mL. In order to characterise each of the products, we utilized SEM (FEI XL 30), PL (Kratos Analytical Axis Ultra), and XPS (Kratos Analytical Axis Ultra). Measurements of photoluminescence (PL) were carried out at room temperature using a HORIBA Jobin Yvon LabRAM ARAMIS grating spectrometer in conjunction with the 325 nm line of a HeCd laser. 239 Fig. 26a–h displaying the ZnO nanostars and nanoflowers.

4.4. Fabrication technique

Many synthetic approaches were utilized to fabricate zinc oxide nanoparticles. They are primarily classified into three categories: chemical fabrication, physical fabrication, and biological fabrication.

4.4.1. Chemical fabrication

The process of transforming the basic materials or reactant into a product through the utilization of one or more chemical processes is referred to as chemical fabrication. The process of chemical fabrication can be divided into two distinct phases: the gas phase and the liquid phase. The liquid phase can be further subdivided into precipitation/co-precipitation technique, colloidal technique, sol–gel technique, oil microemulsion technique, hydrothermal technique, and solvothermal technique, while the gas phase can be further separated into pyrolysis and gas condensation techniques. 243

4.4.1.1. Co-precipitation technique/precipitation technique

In order to convert a solution into a solid using this method, either an insoluble form or a higher saturation level must be utilized. The treatment of zinc compounds begins with dilute hydrochloric acid, followed by dilute hydrochloric acid. The reaction is carried out at room temperature with gentle stirring, and a solution containing NaOH, KOH, and NH 4 OH is added drop by drop to act as a precursor. When the pH reaches a range between 8 and 10, the base solution addition process is stopped. The mixture described above is heated to 85 °C for six hours, then centrifuged, brought down to room temperature, and filtered. The white powder is formed as a result of precipitating the substance with distilled water in order to remove any impurities. 244–246 Fig. 27 displaying the flow chart of co-precipitation method.

4.4.1.2. Sol–gel technique

The sol–gel method is a wet chemical procedure that can be used to produce a three-dimensional network. This approach is also known as the way of producing sol–gel materials. This process begins with the formation of a colloidal suspension, which is referred to as a sol, and is then followed by the gelation of the sol in a constant liquid phase, which is referred to as a gel. In this phase, the zinc compound is heated to 50 °C while being dissolved in double-distilled water. A magnetic stirrer is used throughout the process of gradually adding alcohol at a concentration of 100%, which is then followed by the dropwise addition of hydrogen peroxide until the solution becomes transparent. The solution was left to ferment for twenty-four hours before being dried at eighty degrees Celsius for a number of hours in order to generate white zinc oxide nanoparticles. In order to get rid of any traces of by products, wash many times in water that has been through two distillation processes, and then dry in an oven heated to 80 °C. During the drying process, zinc oxide is completely converted. 247 Fig. 28 displaying the flow chart of sol gel method.

4.4.1.3. Microemulsion technique

Microemulsion is a liquid solution that is optically isotropic, thermodynamically stable, and is made up of water, oil, and amphiphile. In this particular investigation, zinc oxide nanoparticles were produced by a process known as reverse microemulsion. The substances n -heptane, glycerol and dioctyl sulfosuccinate sodium in that order, are utilised for the roles of surfactant, polar phase, and non-polar phase, respectively. The synthesis results in two different microemulsions, each of which has a different ratio of surfactants. Dissolving dioctyl sulfosuccinate sodium in n -heptane at room temperature while stirring continuously will result in the production of a microemulsion. After the ingredients have been combined, the solution should be cut into two equal parts and labelled solution A and solution B. The zinc compound is stirred into solution A while constantly being stirred while the other half of the glycerol is dissolved in the zinc compound. In the same manner, add some sodium hydroxide (NaOH) that has been dissolved in glycerol to solution B. The aforementioned two solutions were combined in a continuous mixing process at room temperature until they produced a solution that was clear. After that, gradually blend solution B with solution A while stirring constantly for twenty-four hours at a temperature between sixty and seventy degrees Celsius. Centrifuge the mixture for twenty minutes at a speed of 10 000 rpm to obtain a white solid powder. After being washed in a mixture of methanol and chloroform and centrifuged for ten minutes at ten thousand revolutions per minute, the product was dried for one hour at one hundred degrees Celsius in an open-air drying oven and then placed overnight in a vacuum drier at room temperature. This process took a total of twenty-four hours. Calcinated in an air atmosphere for three hours at temperatures ranging from 300 to 500 °C. 248,249 Fig. 29 displaying the flow chart of microemulsion method.

4.4.1.4. Hydrothermal technique

It is a method for the creation of single crystals that is predicated on the solubility of minerals in hot water that is subjected to intense pressure. To prepare stock solutions, zinc component is first stirred into methanol, then dissolved in the solvent. To modify the pH to a range between 8 and 11, NaOH that has been dissolved in methanol is added to the stock solution while it is being stirred continuously. After that, the solution was autoclaved in stainless steel autoclaves lined with Teflon for 6 and 12 h at temperatures ranging from 100 to 200 °C under autogenous pressure before being allowed to naturally cool down to ambient temperature. Following the completion of the reaction, the white solid product was extracted by washing it with methanol, filtering it, and then drying it in a laboratory oven at 60 °C. 250,251 Fig. 30 displaying the flow chart of hydrothermal method.

4.4.1.5. Solvothermal technique

It is a method in which the solvent is added at a pressure and temperature ranging from moderate to high, which makes it easier for the precursors to interact with one another throughout the synthesis. In this particular experiment, ethylene glycol and ethanol were mixed together and used in the capacity of a solvent. For a period of twenty minutes, the zinc component should be mixed into the solvent solution. In order to reach the required temperature, the sealed chamber is kept inside a box furnace that has been preheated for a period of twelve hours. The experiment was carried out at a variety of temperatures, including 200 °C, 150 °C, and 135 °C, in order to calibrate the size of the nanoparticles. After that, the precipitate was collected, after that it was washed three times with ethanol and water, and finally it was dried in the air at room temperature. 252 Fig. 31 displaying the flow chart of solvothermal method.

4.4.1.6. Pyrolysis technique

The process known as pyrolysis begins with the atomization of a precursor solution, continues with the solution's evaporation, and concludes with the solution's decomposition into films and particles. In order to produce the precursor solution, the zinc component is first dissolved in the distillate water. Nebulization occurs in response to the pressure exerted by the surrounding air. In a reactor maintained at a temperature of 1200 °C, the droplets disintegrate. A cold precipitator is used to create nanoparticles, which are subsequently collected and dried in an oven at a temperature of 100 °C. Washing the product in water helped get rid of any unreacted zinc compound that was present in it. 253,254 Fig. 32 displaying the flow chart of pyrolysis method.

4.4.1.7. Gas condensation technique

Zinc compound is introduced into a chamber that is under vacuum. By utilising induced current and keeping the vacuum pressure and vaporisation temperature constant, the substance is melted and then evaporated into gas before being vaporised. An inert gas and material vapour have a collision inside of a vacuum chamber. After that, it travels to a collecting surface that is cooled to a low temperature, where it produces nanoparticles as it settles. We are able to simply manage the pressure and temperature by maintaining optimal conditions within the vacuum chamber. This is possible due to the fact that the temperature of the collection surface rises when liquid nitrogen flows continuously through the collector while it is located inside the vacuum chamber. Nanoscale production of metal nanoparticles begins with nucleation of the particles. The nanoparticles are amassed on the surface of the collector by the processes of vaporisation and condensation. 255 Fig. 33 displaying the flow chart of gas condensation method.

4.4.2. Physical fabrication

Evaporating the material is the first stage in this bottom-up technique to the synthesis of nanostructural materials. The second step, rapid controlled condensation, is used to acquire particle size and is the second step. The three processes that fall under the category of physical synthesis are high-energy ball milling; solid, chemical and physical vapour deposition and laser ablation.

4.4.2.1. High energy ball milling technique

The milling of ZnO powder takes anywhere from two hours to fifty hours, depending on the temperature and humidity of the surrounding air. Hardened steel balls are used in the milling process. In a horizontal oscillating mill, the milling process was carried out mechanically at a rate of 25 Hz. The ratio of zinc oxide powder to steel balls in the combination is 1 : 15, based on the weight of the individual components. The processing of the material was place without the use of any additional milling agents. 256 Fig. 34 displaying the flow chart of high energy ball milling method.

4.4.2.2. Laser ablation

First, prepare the solution by dissolving sodium dodecyl sulphate in double-distilled water. Next, irradiate a piece of zinc metal with Nd:YAG lasers at a frequency of 10 Hz for attentive output of secondary harmonics, with a focal length of 250 nm for 60 min and a total energy of 100 mJ. Nanoparticles of zinc were synthesized. 257 Fig. 35 displaying the flow chart of laser ablation method.

4.4.3. Biological fabrication

This method refers to bioremediation, in which biological processes are used to degrade and metabolise chemical substances, restoring environmental quality. Plant-mediated and microbe-mediated biological synthesis are the two types of biological synthesis.

4.4.3.1. Plant mediated technique

In this process, nanoparticles are made by bioreducing metal ions to their most basic form utilising plants or plant components. 258 Fig. 36 displaying the flow chart of plant mediated method ( Table 2 ).

4.4.3.2. Microbes mediated technique

The autoclave is used to make and sterilise the nutrition broth. Then, under aseptic conditions, bacterial strains are introduced, and the temperature is maintained overnight. The appearance of turbidity confirms bacterial growth, after which the supernatant and pellet were separated and studied under FTIR, XRD, UV spectrophotometer, and SEM. 260 Fig. 37 displaying the flow chart of microbes mediated method.

4.5. Application of ZnO

Zinc oxide's vast range of useful chemical and physical properties have led to its application in a diverse range of industries. It has many different applications, ranging from ceramics to tyres, agriculture to pharmaceuticals, and chemicals to paints. It is also employed in a wide number of different industries ( Fig. 38 ).

(1) The global production of zinc oxide is approximately 10 5 tonnes per year, with the rubber sector consuming the majority of it for the creation of various cross-linked rubber products. 261 It is possible to increase the heat conductivity of traditional pure silicone rubber by adding thermal conductivity fillers such as metal oxides, metal powders, and inorganic particles. Although traditional pure silicone rubber has a poor heat conductivity, this property can be improved. It is possible for certain types of thermal conductivity powders, such as AlN 3 , MgO, Al 2 O 3 , ZnO, and SiO 2 , to increase the thermal conductivity of silicone rubber while maintaining its high electrical resistance. Because of this, these powders are attractive options for high-performance engineering materials. It is feasible to achieve high thermal conductivity even at a low filler content by using nanoscale fillers, which are used in the manufacturing process. However, as a result of the weak connection that exists between the surface of the nanoparticles and the polymer matrix, the ZnO nanoparticles have a tendency to group together in the polymer matrix to form larger particles. Techniques that modify the surface are utilised in order to improve the interaction of nanoparticles with the polymer in order to solve this problem. Das et al. 262 demonstrate how, during the curing process, a hydrosilylation procedure is used to incorporate unmodified and surface-modified ZnO nanoparticles into the silicone rubber. Both of these types of nanoparticles include the vinyl silane group. Both the silicone rubber/zinc oxide (SR/ZnO) and the silicone rubber/silicon dioxide at zinc oxide (SR/SiVi@ZnO) nanocomposites were investigated in terms of their related structure, morphology, and properties. Yuan et al. employed a sol–gel approach to generate zinc oxide nanoparticles (with an average size of less than 10 nm). After that step was completed, the silicone coupling agent VTES was successfully incorporated onto the surface of the nanoparticles. Both the thermal conductivity and the mechanical properties of the SR/SiVi@ZnO nanocomposites were significantly enhanced as a result of the formation of a cross-linking structure within the silicone rubber matrix as well as an improvement in the dispersion of the nanocomposites within that matrix.

(2) Zinc oxide is an extremely efficient and often used crosslinker for carboxylated elastomers. 263,264 It is possible to produce vulcanizates that have a high tensile strength, resistance to tearing, and level of hardness as well as hysteresis. The increased mechanical capabilities of ionic elastomers are primarily the result of their high stress relaxation capacity. This is because the enhanced mechanical capabilities of ionic elastomers are caused by the slippage of elastomer chain molecules on the surface of ionic clusters and the reformation of ionic bonds when the sample is deformed externally. In addition, ionic elastomers have thermoplastic properties, and when they are in a molten state, they can be handled just like any other thermoplastic polymer. 265 However, zinc oxide-crosslinked carboxylic elastomers have a few drawbacks that need to be considered. Their scorchiness, complete absence of flex, and high compression set are the features that stand out the most. In order to prevent searchability, carboxylated nitrile elastomers are cross-linked utilising systems consisting of zinc peroxide or zinc peroxide/zinc oxide. Although the development of ionic crosslinks accounts for the vast majority of the vulcanization of XNBR with zinc peroxide, the action of the peroxide also results in the formation of covalent connections between the individual chains of the elastomer. However, longer vulcanization durations are required in order to achieve vulcanizates with equivalent tensile strength and crosslink density to zinc oxide-crosslinked vulcanizates. This is because zinc oxide is a crosslinking agent. In the case of XNBR vulcanization utilising zinc peroxide/zinc oxide systems, the curing process is comprised of at least three stages: the rapid synthesis of ionic crosslinks as a result of the initial zinc oxide present; peroxide crosslinking resulting in the development of covalent bonds (peroxide action); and ionic crosslinking as a result of the production of zinc oxide as a result of the peroxide decomposing. The third stage, which takes place gradually over long periods of time and results in degradation, very certainly involves the production of ionic species in some way. The vulcanization periods that can be achieved with XNBR that has been crosslinked with zinc oxide are substantially longer than those that can be achieved with XNBR that has been vulcanised using any other method. Despite the fact that it can cause burns in some situations, zinc oxide is nevertheless extensively utilised as a cross-linking agent in carboxylated nitrile rubbers. This is the case despite the fact that zinc oxide can induce burns. When it comes to the process of cross-linking, the parameters that are most important in determining its activity are the particle size, surface area, and shape of the zinc oxide. This is due to the fact that zinc oxide reacts with elastomer carboxylic groups to form carboxylic salts, which are also known as ionic crosslinks. The nature of the interphase that develops between the cross-linking agent and the elastomer chains is determined by these factors. 266

(3) Hamed et al. 267 used a solid-state pyrolytic process to synthesize nano zinc oxide. Studies on surface area and microscopic pictures showed that the generated zinc oxide had a surface area that ranged from 12 to 30 m 2 g −1 and had particle sizes that ranged from 15 to 30 nm. The particle sizes ranged from 15 to 30 nm. The neoprene rubber that the researchers employed had a trace amount of zinc oxide, which served as a curing agent and was incorporated into the substance. It was demonstrated that a modest dosage of ZnO was optimal, particularly in comparison to the ZnO that is found in commercial items. Evaluations were conducted on the rubber's curing qualities as well as its mechanical properties, and the results were compared to those of conventionally cured rubbers made with zinc oxide. It was discovered that a low concentration of zinc oxide was sufficient to provide comparable curing and mechanical properties to neoprene rubber that utilised a higher concentration of commercial zinc oxide. This was discovered by comparing the properties of neoprene rubber that used a higher concentration of commercial zinc oxide. This was a finding that came about as a result of the observation that a low concentration of zinc oxide was sufficient. It all started with the observation that a small amount of zinc oxide was sufficient.

(4) Zinc oxide is commonly employed in the creation of several types of pharmaceuticals due to its drying, antibacterial, and disinfecting qualities. 268,269 Historically, it was utilized in the treatment of epilepsy, and later on, it was utilized in the treatment of diarrhoea. At the moment, it is employed in the region, most frequently in the form of creams and ointments, but less frequently in the form of liquid powders and dusting powders. Dusting powders and liquid powders are utilized less frequently. Zinc oxide is frequently used in dermatological goods intended to treat inflammation and irritation because of its capacity to stimulate wound healing. These products can be found on the market. Peeling of the skin can occur when it is exposed to higher amounts of the substance. In addition to that, it is also available in suppositories. Additionally, it has use in the field of dentistry, primarily as a component of dental pastes and additionally for the production of temporary fillings. Zinc oxide is also utilized to give dietary zinc in a variety of dietary supplements and nutritional items. This is necessary because zinc is essential for human health. 270

(5) Before the introduction of nanoparticles of TiO 2 and ZnO, sun lotions had heavy formulations that did not rub well into the skin and were cosmetically unpleasant. Because of their ability to absorb UVA and UVB rays, these chemicals began to be used in creams. A new cream formula including a combination of TiO 2 and ZnO solved the problem of an insufficiently white layer, resulting in a more clear, less sticky, and much simpler to rub into the skin medium. 271 Titanium and zinc oxides have been demonstrated in a number of studies to be excellent sun cream media because they absorb UV rays, do not irritate the skin, and are quickly absorbed into the skin. 272–274

(6) Zinc oxide is biocompatible for textile applications, and nanostructured zinc oxide coatings are more air-permeable and UV-blocking efficient than their bulk equivalents. 275 As a result, ZnO nanostructures have attracted a lot of interest as UV-protective textile coatings. Using ZnO nanostructures, various ways for producing UV-protecting textiles have been reported. For example, UV-blocking capabilities of hydrothermally produced ZnO nanoparticles in SiO 2 -coated cotton fabric were outstanding. 276 After synthesis in a homogeneous phase reaction at high temperatures, deposition of ZnO nanoparticles on wool and cotton fibres resulted in a significant increase in UV-absorbing activity. 277 UV protection was equally impressive with zinc oxide nanorod arrays grown on a fibrous substrate utilising a low-temperature growing method. 278

(7) Zinc oxide is a unique and crucial semiconductor with numerous electronic and electrotechnology applications. 279–281 At ambient temperature, zinc oxide has a wide energy band of 3.37 eV and a high bond energy of 60 meV, making it useful in photoelectronics 282 and electronic equipment, 283 devices emitting a surface acoustic wave, 284 field emitters, 285 sensors, 286–289 ultraviolet lasers, 290 and solar cells. 291 ZnO also shows luminescence (most notably photoluminescence—the production of light in the presence of electromagnetic radiation). It is employed in field emission display equipment, such as televisions, because to this property. It outperforms typical materials such as sulphur and phosphorus (phosphorescent compounds) in terms of UV resistance and electrical conductivity. The photoluminescent properties of zinc oxide are affected by the crystals size, the presence of defects in the crystalline structure, and the temperature. 292–295 ZnO is a semiconductor, and thin films made of it have strong conductivity and visible light permeability. It can be used to make light-permeable electrodes for solar batteries because of these qualities. It's also a promising material for ultraviolet-emitting devices and could be used as a transparent electrode in photovoltaic and electroluminescent equipment. 296,297

(8) The addition of zinc oxide cuts down on the amount of time needed for manufacture and boosts the resistance of concrete to the action of water. Additionally, the incorporation of zinc oxide into Portland cement causes a delay in the processes of hardening and quenching ( i.e. , it causes a delay in the progressive generation of heat), and it also results in an improvement in the whiteness and final strength of the cement. Zinc silicates are compounds that are resistant to water and fire that is formed when zinc oxide is combined with silicates (for example, sodium silicate). These compounds are utilized as paint binders. These compounds, which are resistant to fire and act as adhesives, are put to use in the building industry to bind cements. A Cu/ZnO/Al 2 O 3 catalyst is utilized in the production of methanol, which is the third most important product produced by the chemical industry. The active component of this catalyst is minute Cu particles, which are driven by their interaction with the zinc oxide substrate. 298

(9) In addition, zinc oxide is utilized in the production of typographic as well as offset inks. It offers superior qualities as a printing medium (high fluidity). The utilization of zinc oxide helps to improve the covering power, pure shade, and longevity of the inks and it also prevents darkening from occurring. Zinc oxide is another ingredient that gives colours their shine. 299

(10) It can be discovered in a wide variety of meals, such as breakfast cereals, among others. Zinc is utilized from zinc oxide as a source of zinc, which is an important nutrient. Due to the unique chemical and antifungal properties of ZnO and its derivatives, they are also utilized in the manufacturing and packing of animal products (such as fish and meat) and vegetable items. These applications include: ( e.g. , peas and sweetcorn). 300

(11) Fungi and moulds are hampered in their attempts to form and grow when exposed to ZnO or one of its derivatives. Zinc oxide is often used as a means of boosting the effectiveness of fungicides. As zinc oxide encourages healthy growth in animals, it is increasingly being used as an addition in animal feed. This trend is expected to continue. Additionally, it is utilized in the production of synthetic fertilizer. 301

(12) In addition, zinc oxide can be utilized in the field of criminology, particularly in the process of mechanical fingerprint analysis. It is also included in cigarette filters because of its ability to selectively remove certain components of the smoke produced by tobacco products. To eliminate substantial quantities of H 2 S and HCN from tobacco smoke in a manner that does not result in the production of an offensive odour, filters are made of charcoal that has been loaded with zinc oxide and iron oxide. In addition to this, it is capable of extracting sulphur and compounds of sulphur from a wide variety of gases and liquids, most notably waste gases from industrial processes. In addition to this, zinc is capable of removing H 2 S from hydrocarbon gases and desulfurizing H 2 S together with other forms of sulphur. 302

(13) In addition, zinc oxide and its derivatives find use in the automotive industry as a lubricant additive, where they help reduce fuel consumption and protect against oxygen corrosion. In addition, zinc oxide has been utilized in a range of lubricants, including solid lubricants, vibration-resistant lubricants, and EP additives, amongst other applications. In the not-too-distant future, the adhesive properties of ZnO could perhaps be utilized. 303

(14) Arnold et al. 303 used individual nanobelts to create field effect transistors (FETs). Using ultrasonication, large packets of zinc oxide nanobelts were diffused in ethanol until the majority of the nanobelts were separated. This process was repeated several times. In order to conduct an atomic force microscopy investigation, these distributed nanobelts were heated on a substrate made of SiO 2 /Si. Depositing zinc oxide nanobelt diffusions on SiO 2 /Si(p+) substrates and then heating them in an oxygen atmosphere at 800 °C for two hours was the process that was used to produce ZnO field effect transistors (FETs). These substrates were first spun coated with a polymer called poly methyl meth acrylate (PMMA), then baked, and finally exposed to electron-beam lithography so that electrode arrays could be developed and characterised. The leftover PMMA was removed using hot acetone, and then a 30 nm titanium layer was formed using electron-beam evaporation to act as the source and drain for these electrodes. By altering the gate voltage, this FET device would be able to control the amount of current that was flowing from the source to the drain.

(15) ZnO nanoparticles have excellent luminous characteristics. Costenaro et al. 304 used co-precipitation method to create ZnO nanoparticles with varying amounts of aminopropyltriethoxy silane (APTS). They created LED devices using these ZnO nanoparticles and reported improved luminous characteristics. Liu et al. 305 used a carbon cloth template hydrothermal technique to create a photodetector using flexible nanoparticle-assembled ZnO cloth. Under UV irradiation, more than 600 separate measurements of the device's conductance were taken, and the response and decay periods came out to approximately 3.2 and 2.8 s, respectively.

(16) Bagabas et al. 306 advised ZnO nanoparticles for environmental applications. They spotted the photodegradation of cyanide ions while creating ZnO nanoparticles using cyclohexylamine in aqueous and ethanolic media. They found that the structure was essential to boosting the photocatalytic breakdown proficiency of cyanide ion, and they discovered this. Hong et al. 307 used a precipitation process to create ZnO nanoparticles, which had a high photocatalytic activity.

(17) Because of their exceptional biocompatibility, ZnO nanoparticles have the potential to be utilized in various applications, including medication delivery and bioimaging 308 and low cost. 309 Nanoparticles with magnetic and luminous properties have potential as drug carriers and detecting probes, among other things.

(18) Matsuyama et al. 310 suggested biomedical uses of silica-layered zinc oxide quantum dots with biotin as fluorophore for cell-labeling applications, as well as cautious destruction in malignant cell applications. 311

(19) Zinc oxide nanomaterials have shown that they can be used to detect DSSC substances 312 and are used as UV blockers in sunscreen lotion. 313 Based on their drug transport, anticancer, bioimaging activity, antibacterial, and anti-inflammatory properties, ZnO nanoparticles have demonstrated potential for usage in a variety of biomedical applications. 314

(20) Sun et al. 315 proposed that ZnO nanostructures have the potential to be utilized in applications involving the collection of energy as a stretchable nanogenerator.

The chemistry of nanomaterials (NMs) and nanoparticles (NPs) are a burgeoning field of research and a rapidly expanding technological sector in a wide variety of application domains. Nanoparticles are separated into their respective categories based on their morphology, which refers to their structure, as well as their size and shape. One-dimensional nanomaterials, two-dimensional nanomaterials, and three-dimensional nanomaterials are the three types of nanomaterials on the basis of their dimension. The most important innovations of the 21st century are the design and fabrication of nanoscale materials made of metal oxides, metals, carbon allotropes and chalcogenides. These materials are used in a vast range of fields, such as energy storage, catalysis and biosensors, conversion devices and biomedical applications. In particular, the unique physiochemical properties of semiconducting metal oxides, such as SnO 2 , ZnO, and TiO 2 , which vary depending on size and shape, have been extensively researched and exploited. One of the most stable n-type semiconducting materials for chemical and thermal applications is ZnO, which is available in a variety of forms including pellets, bulk crystal and thin film for use in everything from luminescent materials to batteries, supercapacitors and solar cells to biomedical and photocatalysis sensors. The variation of ZnO nanostructures is determined by the growth mechanism, the growth method, the synthesis conditions, and the type of substrate. Nanowires, Nanorods, nanotubes, nanocolumns, nanorings, nanobelts, nanosheet networks, nanoribbons, nanoflowers, hollow micro- and nanospheres and nanocombs are among the nanostructures. These nanostructures can be fabricated quite easily at very low temperature, and a variety of different growth techniques for ZnO nanostructures have been documented, including chemical, physical and biological techniques. The process of chemical fabrication can be divided into two distinct phases: the gas phase and the liquid phase. The liquid phase can be further subdivided into precipitation/co-precipitation technique, colloidal technique, sol–gel technique, oil microemulsion technique, hydrothermal technique, and solvothermal technique, while the gas phase can be further separated into pyrolysis and gas condensation techniques (CVD) and thermal evaporation. The three processes that fall under the category of physical synthesis are high-energy ball milling, solid, chemical and physical vapour deposition and laser ablation. Plant-mediated and microbe-mediated biological synthesis are the two types of biological synthesis. Because of the growth procedures, disciplines and applications that were discussed above, ZnO has the potential to become one of the most significant candidates for use in future research and applications. Zinc oxide's vast range of useful chemical and physical properties have led to its application in a diverse range of industries. It has many different applications, ranging from ceramics to tyres, agriculture to pharmaceuticals, and chemicals to paints. It is also employed in a wide number of different industries.

Conflicts of interest

The authors declared no potential conflicts of interest.

Supplementary Material

Acknowledgments.

Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R152), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Biographies

Mr. Muhammad Adil Mahmood is a PhD student in the Department of Physics, University of Lakki Marwat, KPK, Pakistan. He is working on Spintronics based nanomaterials.

Professor Aurangzeb Khan is a full Professor of physics at Abdul Wali Khan University in Mardan, KPK, Pakistan. Dr Khan earned his Ph.D. in physics (nanosciences) from Ohio University' in Athens, Ohio, USA in 2006, and he completed a postdoctoral fellowship in 2007. Additionally, he worked at Ohio University as a BNNT postdoc for two years. In 2020–2023, Dr Khan was the Vice Chancellor at the University of Lakki Marwat, KP Pakistan. Dr Khan is interested in nanomaterials and their applications. Additionally, Dr Khan has worked on both theoretical and experimental research projects in applied and fundamental physics.

Dr Rajwali Khan is an Assistant Professor and Associate Dean for research at the Department of Physics, University of Lakki Marwat, Khyber Pakhtunkhwa, Pakistan. He completed his BSc degree in Physics from Islamia College Peshawar Pakistan (2006–2008), where he also obtained a Master degree in Physics from 2008–2011. He holds a PhD degree in Physics (Strongly Correlated Electron Systems) from Zhejiang University, Hangzhou, China, in 2012–2016. He was also an Assistant Professor at Abdul Wali Khan University Mardan from 2017–2019. He did his first postdoc with a TWAS-Fellowship in collaboration with Brazil and the United States of America, in 2020. He also did his second postdoc with a Shenzhen Government Fellowship from Shenzhen University China from 2020–2021.

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Nanoscience and Nanotechnology

p-ISSN: 2163-257X    e-ISSN: 2163-2588

2013;  3(3): 62-74

doi:10.5923/j.nn.20130303.06

Semiconductor Nanomaterials, Methods and Applications: A Review

Sagadevan Suresh

Department of Physics, Sree Sastha Institute of Engineering and Technology, Chembarambakkam, Chennai, 600123

Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved.

When the size of semiconductor materials is reduced to nanoscale, their physical and chemical properties change drastically, resulting in unique properties due to their large surface area or quantum size effect. Currently, semiconductor nanomaterials and devices are still in the research stage, but they are promising for applications in many fields, such as solar cells, nanoscale electronic devices, light-emitting nano devices, laser technology, waveguide, chemicals and biosensors. Further development of nanotechnology will certainly lead to significant breakthroughs in the semiconductor industry. This paper deals with the some of the current initiatives and critical issues in the improvement of semiconductors based on nanostructures and nanodevices.

Keywords: Semiconductors, Nanomaterials, Solar Cells, Light Emitting Nano Devices

Cite this paper: Sagadevan Suresh, Semiconductor Nanomaterials, Methods and Applications: A Review, Nanoscience and Nanotechnology , Vol. 3 No. 3, 2013, pp. 62-74. doi: 10.5923/j.nn.20130303.06.

Article Outline

1. introduction, 2. introductions to nanoscience and nanotechnology, 3. semiconductor nanoparticles, 4. classifications of semiconductor nanostructures, 4.1. zero dimensional (0d) nanostructures.

4.2. Quasi One Dimensional (1D) Nanostructures

4.3. two dimensional (2d) nanostructures, 4.4. three dimensional (3d) nanosystems, 5. core-shell nanostructures, 5.1. types of core-shell nanocrystals, 6. quantum confinement effects, 6.1. weak confinement regime.

6.2. Moderate Confinement Regime

6.3. strong confinement regime.

7. Nanoparticles Synthesis Methods

7.1. wet chemical methods, 7.2. sol-gel.

7.3. Solvothermal/Hydrothermal Method

7.3.1. main parameters governing solvothermal reactions.

7.4. Surface Modification of Nanocrystals and Interparticle Forces in Solution

7.5. Van der Waals Forces

7.6. Magnetic Dipolar Forces

7.7. Electrostatic Forces

7.8. Steric Forces

7.9. Solvation Forces

8. application of semiconductor nanomaterials, 9. semiconductor nanomaterials for hydrogen production, 10. silicon semiconductor nanomaterials and devices, 11. research on nano optoelectronic sensors and photovoltaic devices, 12. organic optoelectronic materials and devices, 13. conclusions, acknowledgements.

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Critical Writing Program: Climate Science and Action: Earth in Crisis - Fall 2024: Researching the White Paper

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Research the White Paper

Researching the white paper:.

The process of researching and composing a white paper shares some similarities with the kind of research and writing one does for a high school or college research paper. What’s important for writers of white papers to grasp, however, is how much this genre differs from a research paper.  First, the author of a white paper already recognizes that there is a problem to be solved, a decision to be made, and the job of the author is to provide readers with substantive information to help them make some kind of decision--which may include a decision to do more research because major gaps remain. 

Thus, a white paper author would not “brainstorm” a topic. Instead, the white paper author would get busy figuring out how the problem is defined by those who are experiencing it as a problem. Typically that research begins in popular culture--social media, surveys, interviews, newspapers. Once the author has a handle on how the problem is being defined and experienced, its history and its impact, what people in the trenches believe might be the best or worst ways of addressing it, the author then will turn to academic scholarship as well as “grey” literature (more about that later).  Unlike a school research paper, the author does not set out to argue for or against a particular position, and then devote the majority of effort to finding sources to support the selected position.  Instead, the author sets out in good faith to do as much fact-finding as possible, and thus research is likely to present multiple, conflicting, and overlapping perspectives. When people research out of a genuine desire to understand and solve a problem, they listen to every source that may offer helpful information. They will thus have to do much more analysis, synthesis, and sorting of that information, which will often not fall neatly into a “pro” or “con” camp:  Solution A may, for example, solve one part of the problem but exacerbate another part of the problem. Solution C may sound like what everyone wants, but what if it’s built on a set of data that have been criticized by another reliable source?  And so it goes. 

For example, if you are trying to write a white paper on the opioid crisis, you may focus on the value of  providing free, sterilized needles--which do indeed reduce disease, and also provide an opportunity for the health care provider distributing them to offer addiction treatment to the user. However, the free needles are sometimes discarded on the ground, posing a danger to others; or they may be shared; or they may encourage more drug usage. All of those things can be true at once; a reader will want to know about all of these considerations in order to make an informed decision. That is the challenging job of the white paper author.     
 The research you do for your white paper will require that you identify a specific problem, seek popular culture sources to help define the problem, its history, its significance and impact for people affected by it.  You will then delve into academic and grey literature to learn about the way scholars and others with professional expertise answer these same questions. In this way, you will create creating a layered, complex portrait that provides readers with a substantive exploration useful for deliberating and decision-making. You will also likely need to find or create images, including tables, figures, illustrations or photographs, and you will document all of your sources. 

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