The nanomaterials could be developed by all three means i.e., chemical, physical and biological methods which is shown in . Among them, the physical approaches include sputtering [
64], laser ablation [
65], pyrolysis [
66], lithography [
67], and hot and cold plasma [
67]. Meanwhile, the chemical methods that are used most frequently are lyotropic liquid crystal templates [
68], electrochemical deposition [
69], electroless deposition [
70], hydrothermal [
71] and solvothermal techniques [
72], sol gel technique [
73,
74], laser chemical vapor deposition technique [
75], laser pyrolysis [
76], and chemical vapor deposition [
77]. The nanomaterials could also be synthesized by biological approaches like microbial [
78] and plant derived materials [
79]. The microbial synthesis of nanomaterials [
80] employs the utilization of microorganisms like algae [
81], fungi [
53], and bacteria [
82]. The main drawback is that when there is a utilization of commercial precursor for the synthesis of nanomaterials by any of the above-mentioned approaches, the process, as well as the product, become expensive. So, in order to obtain a cost-effective material, the precursor should be lower in terms of cost. One such material is industrial waste [
83], biological waste, or agricultural waste [
19].
3.1. Physical Methods for Synthesis of 2D NSMs
The 2D NSMs could be synthesized by various physical methods [
84] such as evaporation [
85], lithography [
86], sputtering, phase condensation, hot and cold plasma spray pyrolysis [
87], inert gas phase condensation [
88], pulsed laser ablation method [
89], and sonochemical reduction [
90]. These methods (physical) are generally used for the synthesis of nanowalls [
53], nanoprisms [
91], nanosheets [
92], nanoplates [
93], and nanodisks [
34]. The nanomaterials synthesized by the physical method are homogenous in nature and ordered. Dai et al., 2002 developed the SnO nanodisks [
64] alumina plates using the thermal evaporation method under optimized environmental conditions [
94]. Here, firstly, SnO or SnO
2 powders were kept in an alumina boat, which was in turn placed in a quartz tube reactor (evaporation source), where alumina acted as a substrate which was placed one by one downstream. The physical techniques provide an environment-friendly approach for the development of 0D, 1D, 2D, and 3D nanomaterials, which are shown below in .
Figure 4. Physical methods for the synthesis of 2D nanomaterial.
3.2. Chemical Methods for Synthesis of Nanomaterials
Chemical methods have contributed to the fabrication of materials at the nanoscale [
95]. The Chemical methods have several advantageous properties over physical methods as the previous one involves mixing of chemical at the molecular level which ensures good chemical homogeneity [
74,
96]. Chemical reduction methods are reported to have numerous drawbacks for instance utilization toxic reagents and solvents, generation of unwanted by-products due to which there are several extra steps is needed for removal of impurities, time-consuming [
97]. The most common chemical methods are electroless deposition [
98], lyotropic liquid crystal templates [
34], hydrothermal and solvothermal method, sol-gel technique, electrochemical deposition, chemical vapor deposition (CVD), laser pyrolysis, and laser chemical vapor deposition techniques (LCVD), which are utilized frequently for the production of different NSMs. The above-mentioned techniques are shown in .
Figure 5. Chemical methods of synthesis of nanomaterials.
Chen et al., in 2018, reported the synthesis of two-dimensional metallic nanomaterials from various routes [
99]. Yang et al., in 2019, reported the synthesis, engineering, and applications of fly ash from various routes like physical and chemical but the emphasis was given mainly on the precursor mediated synthesis, not on the waste-based materials [
100]. In 2015, Paul et al. reported the thin film deposition of Feo on the Pt(111) by the ferrocene adsorption and oxidation method [
101]. Zhang et al. reported the synthesis of multifunctional flexible 2-dimensional carbon nanostructured N-nets reported their importance in electronics, energy, and the environment [
102].
Among all the metallic nanoparticles silver nanoparticles has gained used consideration due to their exceptional properties and applications. Silver nanoparticles of different shapes and sizes have an important role in medicine, the biomedical field and drug delivery [
103]. Till now silver NPs of various shapes and sizes has been reported by numerous investigators. Nanoprisms are one of the examples of 2D nanomaterial, which had gained huge attention in the biomedical field [
103]. Silver nanoprisms were synthesized silver salts by chemical reduction and photochemical method where the earlier method is more preferred than the later one due to their more controlled growth of nanoprisms which finds application in the industries [
104]. Monodispersed hematite (a-Fe
2O
3) nanodiscs of size (50 ± 10 nm in diameter and thickness of 6.5 nm) synthesized under mild conditions through a facile hydrothermal method, i.e., hydrolysis of ferric chloride [
105]. The reported method was quite unique as there was no use of surfactants, no toxic or hazardous chemical precursors, and no high temperature decomposition of iron precursors in non-polar solvents. The synthesized hematite nanodiscs were further characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), and superconducting quantum interface device (SQUID). The synthesis of Ta
3N
5 nanoplates was reported by Jie Fu and Sara E. Skrabalak, 2016, for the photocatalytic application [
106]. A simple technique developed for the production of hexagonal-shaped Ag nanoplates whose diameter was in the range of 15–20 nm with a smooth nanobulk of 120 nm [
107]. The silver nanoplates were prepared by a kinetically controlled solution growth method under the following conditions: polyvinyl pyrrolidone (PVP) as a capping agent, dextrose as a reducing agent, and urea as a habit modifier at 50 °C and the crystalline structure of silver nanoplates analyzed by the XRD and TEM.
Xin He et al., 2009 synthesized triangular/hexagonal silver nanoplates, nanobelts and chain-like nanoplate assemblies by utilizing N,N-dimethylformamide (DMF) along with PVP [
108]. The results revealed that due to the strong interaction between Ag
+ and PVP, there was the formation of individual nanoplates and external features of nanoplates were controlled by the ratio of AgNO
3 and PVP. Sial et al., in 2018, synthesized multimetallic nanosheets which were utilized for the manufacturing of fuel cells [
109]. Zheng et al., 2011 synthesized Palladium NSs by using CO as a reducing agent [
110]. Yan-song Zhou et al., in 2016, reported an ultra-facile and generalized approach for the synthesis of metal oxide nanosheets (TiO
2, Co
3O
4, Fe
2O
3, ZnO) with a larger surface and applied them for energy applications [
111]. Jianxing Liu et al. reported the synthesis of hematite nanosheets by using a large-sized particles of iron red and found that the shape of hematite have important effect on the magnetic and optical properties [
112]. All the above-mentioned chemical processes revealed a simple, reliable, and useful approach towards the synthesis of 2D NSMs. The shape, size, and composition of the 2D NSMs can be varied by precursor solutions, conditions of deposition and substrate materials [
84].
Besides all the above-mentioned techniques for the synthesis of nanomaterials, there are a few less applied chemical mediated approaches. One such technique is electrochemical synthesis mainly by anodization and cathodization. Though both the techniques are commonly used in an electrochemical based industry but rarely known for the synthesis of nanomaterials. Several investigators have reported the synthesis of 1D, 2D, and 3D nanomaterials by using electrochemical methods. Dai et al., in 2019, reported the synthesis of a 1D nanomaterial by anodization method, and also highlighted their importance for manufacturing energy storage devices. Anodization is an electrochemical oxidation technique for depositing metal, metal oxides or semiconductors on a surface in order to increase the thickness of the metal. Nowadays, porous materials are also synthesized for enhanced applications in the field of energy and wastewater treatment. By using this technique, mainly nanotubes are synthesized. The anodization mechanism depends on the various physical parameters like pH, time, potential, electrolyte temperature and water content. All these factors govern the morphology, porosity, wall thickness, and length of the synthesized nanomaterials. Till now, by applying such a technique, the following metal and non-metal oxides have been synthesized: ZnO, ZrO
2, α-Fe
2O
3, WO
3, Ta
2O
5, Nb
2O
5, HfO
2, CuO/Cu
2O, and NiO [
113]. Kawde et al. reported the synthesis of AuNPs modified-graphite pencil electrode by cathodization. Further, it was used for the non-enzymatic sensitive voltametric detection of glucose [
114]. Numerous investigators also reported the electrochemical based synthesis of either 1D, 2D, or 3D nanomaterials [
115,
116,
117].