Review Volume 12, Issue 3, 2022, 4261 - 4292 https://doi.org/10.33263/BRIAC123.42614292 Zinc Oxide Nanoparticles Synthesis Methods and its Effect on Morphology: A Review Eric Kwabena Droepenu 1,2,* , Boon Siong Wee 1,* , Suk Fun Chin 1 , Kuan Ying Kok 3 , Muhammad Firdaus Maligan 1 1 Resource Chemistry Program, Faculty of Resource Science and Technology, Universiti Malaysia Sarawak 94300, Kota Samarahan, Sarawak, Malaysia 2 Graduate School of Nuclear and Allied Sciences, University of Ghana, AE1, Kwabenya-Accra, Ghana 3 Malaysian Nuclear Agency, Bangi, Kajang, 43000 Selangor, Malaysia * Correspondence: swboon@unimas.my (B.S.W.); kobladodzie01@yahoo.com (E.K.D.); Scopus Author ID 57194506096 Received: 28.06.2021; Revised: 1.08.2021; Accepted: 5.08.2021; Published: 14.08.2021 Abstract: Zinc oxide is an important material with numerous applications due to its unique properties. Due to their thermal and chemical stability are used in wide applications such as LEDs, sensors, catalysts, and photodetectors. Different chemical, physical, and biological methods have been adopted to achieve the intended result, as enumerated in many pieces of literature. Therefore, selecting an efficient synthesis process is essential, which is a key factor that significantly influences the efficacy of the synthesized nanocrystalline materials. The chemical synthesis of nanoparticles (NPs) via hydrothermal, solvothermal, and sol-gel routes is considered effective as high-quality crystalline structures are produced. Control of parameters of processes yields excellent morphological features of the synthesized samples. This review explored the different parameters of processes and their effect on the morphology of ZnO nanostructures via hydrothermal, solvothermal, and sol-gel techniques. Finally, some ZnO nanocomposites molecules are reviewed as per the dopant used and its effect on the sample compound synthesized. Keywords: synthesis methods; nanostructures; nanomaterials; nanoparticles; zinc oxide; morphology. © 2021 by the authors. This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). 1. Introduction Zinc oxide is one example of transition metal oxides that exhibit unique electrical, optical and mechanical properties. Due to their unique properties, they have numerous applications in fields such as pharmaceuticals, electronics, consumer goods, optical and electrical devices, and environmental remediation [1-5]. The morphology of ZnO nanostructures has a significant role in their applications, according to Jin and Jin 2019 [6]. For instance, rod-like, sheet-like, and belt-like nanostructures are suitable for solar cells, light- emitting diodes, gas sensors, and biological probes. Interestingly, researchers focus on the morphology of nano ZnO to achieve their intended purpose in its applications. Controlling the size and shape of these nanostructures during synthesis is the major focus of the researcher. Different synthesis techniques have been developed to be grouped into physical, chemical, and biogenic (green route). Physical techniques include pulsed laser deposition, magnetron sputtering, electrodeposition, and electron beam evaporation [7-11]. The chemical synthesis route comprises hydrothermal, solvothermal, sol-gel, chemical bath deposition, wet chemical process, spray pyrolysis, microemulsion, and precipitation methods [12-18]. Green https://biointerfaceresearch.com/ 4261 https://doi.org/10.33263/BRIAC123.42614292 or biogenic synthesis of metal oxide nanomaterials is plant or microbe mediated, with the extracts being biocompatible and can serve as capping agents for stabilizing the NPs [19, 20]. Although chemical synthesis poses toxicity to the environment, pure desired crystals with high stability and high yield are produced [21]. ZnO nanostructures morphology and size of ZnO nanostructures can be modified when reaction conditions (precursor and their concentrations, temperature, solvent type, and surfactants) in a particular synthesis route are controlled [22]. It was reported that, although different treatment methods and surfactants were used in the synthesis of ZnO nanostructures, similar flower-like structures were obtained using the same precursor (zinc acetate dihydrate) [23-24]. The use of surfactants (HMT and PEG 400) and annealing the synthesized ZnO nanocrystalline powder at 500 oC by Rocha et al. [23] resulted in the particle to record crystallite size of 30 nm as against free surfactant and without extra heat at a crystallite size of 7-8 nm. In another study by Vahidi et al. [25], ZnO NPs of different crystallite sizes (51, 60, and 61 nm) but similar aggregated spherical shaped particles were obtained using Pelargonum zonale leaf extract and zinc nitrate as a precursor under different heating conditions (conventional heating, autoclave, and microwave irradiation). However, samples from the conventional heating process recorded the highest antibacterial effect of the bacterial strains used for the study. When this study was compared to that of Satheshkumar et al. [26], although the same morphology of aggregated spherical shaped particles was produced from the conventional heating process, the crystallite size was far below (1.62-1.88 nm) that of Vahidi et al. These variations may be attributed to different conditions such as the concentrations of the precursor and extracts used as well as the reaction conditions under which each sample was prepared. These reports indicate that variation of synthesis parameters could affect the morphology of crystals produced. Literature suggests that different techniques are being developed for a suitable and easy means to achieve desired properties of these nanostructures. However, it was reported that wet techniques of synthesizing metal oxide nanomaterials usually produce impure particles with larger particle sizes [27]. These limitations result from the complexity of the chemical reactions during the growth and nucleation stages of the reaction mechanism, which makes it impossible for repeatability. Therefore, selecting an efficient synthesis process is essential, which is a key factor that significantly influences the efficacy of the synthesized nanocrystalline materials. This review critically examines the current development of appropriate reaction conditions that could be employed in four chemical synthesis methods (hydrothermal, solvothermal (microwave-assisted), sol-gel and hybrids) to achieve the desired features of zinc oxide nanoparticles needed for its specific application. 2. ZnO Nanoparticles Synthesis by Hydrothermal Route This route of synthesizing ZnO nanoparticles involves using high heat and pressure (100-1000 °C and 1-10,000 atm) with water as a solvent in an autoclave (Figure 1). Although good quality crystals are produced, high energy consumption in some cases coupled with expensive autoclaves is some disadvantages to this technique. The heating method may be conventional or microwave, with each having its advantages and disadvantages. Other parameters such as the type of reactant, alkaline source, doping material, surfactant, or reaction conditions (calcination temperature and reaction time) determine the characteristics of nano ZnO synthesized. Conventional heating employs the principle of conduction and convection within the reaction medium resulting in the non-uniformity of heat to the molecules. Molecules https://biointerfaceresearch.com/ 4262 https://doi.org/10.33263/BRIAC123.42614292 are subjected to heat sources via two modes; First, where there is direct contact with the reacting molecules and the heat source element. And secondly, the heating element may be in contact with the containing vessel of the reaction mixture in the case of a heating mantle. The former may result in contaminations by the heating element in direct contact with the mixture. However, the latter is mostly preferred. This effect subsequently affects the nature and quality of samples synthesized. In furtherance to this, there is a high loss of heat as a result of the long duration of the heating process. Also, ‘wall effect’ is a phenomenon where the maximum temperature is experienced at the reaction vessel walls, leading to the heterogeneity of the obtained products. However, microwave irradiation is characterized by a short heating time with the heating directed to the molecules (precursor and solvent) by microwaves' energy, thereby providing intense friction and collision between the molecules, which accelerates the nucleation process [28]. Microwave heating ensures the synthesis of pure samples as there is uniformity in the heating of the molecules [29, 30]. For instance, the capability of solvent molecules to be irradiated at a given temperature and frequency to heat energy known as loss factor (loss factor of deionized water is 0.123) renders microwave heating more efficient in the hydrothermal method of synthesis [31]. However, impurities in the solvent decrease the penetration depth, according to Kim et al. [32]. These factors indicate that reaction molecules have acquired an electric dipole moment in proportion to the applied electric field. Figure 1. Overview of the hydrothermal method to synthesize ZnO nanoparticles. ZnO nanocrystals were synthesized using autoclave mode of heating at temperatures between 100-150 oC for 1-5 hours by Vellakkat et al. [33]. The study reported that variation of reaction temperature and time produced diverse morphologies (flower-like, rod-like, and spherical granular). The study also reported that an increase in temperature time decreased the crystallite size (46.19, 41.66, and 34.74 nm), whereas an increase in reaction time increased the size (44.76, 53.51, and 78.21 nm). Meanwhile, high purity hexagonal wurtzite ZnO nanocrystals were produced from microwave-assisted hydrothermal technique by Ming et al. [34]. From their study, a mixture of zinc nitrate, potassium sodium citrate, and sodium hydroxide solutions was irradiated in a microwave oven (650 W, 2.45 GHz) for 20 minutes at 90 oC. High resolution of the spherical surface features of the particles revealed numerous https://biointerfaceresearch.com/ 4263 https://doi.org/10.33263/BRIAC123.42614292 interconnected nanosheets, which were displayed as microflower in the TEM analysis. In addition, the synthesized sample exhibited a high photocatalytic performance for Rhodamine B degradation due to its smaller band gap. According to Suwanboon et al. [35], the type of precipitating agent (alkaline source) used influences the morphology of the ZnO nanocrystals produced. In their study, hexagonal columnar structures changed to hexagonal platelet structures with crystallite size (55.8-57.8 nm) as the molar concentration of LiOH increased from 0 to 2 when the reaction mixture was autoclaved at 180 oC for 15. However, NaOH produced rod-like structures (Figure 2a) with crystallite size (46.7-48.0 nm) under the same conditions. The study concluded that, whereas hexagonal ZnO platelets (Figure 2b) showed better photocatalytic efficiency to methyl blue, the rod-like structures exhibited better inhibitory activity against Staphylococcus aureus. The effect of surfactants on the morphology of ZnO nanocrystals was investigated by Zhu et al. [36]. They reported that an increase in the concentration of CTAB (0.03-0.1 M) saw plate-like nanostructures aggregated to form flower-like structures (Figure 2c). A further increase in CTAB to 0.3 M collapsed the flower-like structures. The synthesis conditions were reaction time of 3 hours, autoclave temperature of 150 oC for 16 hours, and calcination temperature of 500 oC for 2 hours. These flower-like nanostructures have been investigated as suitable materials for Sulphur dioxide gas detection [37] and ethanol sensing [36]. In other studies, the effects of reactants on the morphological structures of ZnO nanostructures were investigated. Perillo et al. [38] and Alver et al. [39] employed different zinc salt precursors [Zn(CH3COO)2⋅2H2O; ZnCl2 and Zn(NO3)2⋅6H2O] and hexamethylenetetramine (HMTA) at 90 °C for 6 hours on a magnetic stirrer and 140 oC for 4 hours in an autoclave respectively. HMTA acted as a pH buffer to release the OH- from the reaction mixture. Perillo et al. recorded hexagonal nano-rods with crystallite size (40-46 nm), particle length, and diameter (400 nm-6.5 µm and 73-300 nm) for all the precursors. Although Alver et al. also produced hexagonal rod-like nanostructures from zinc acetate and zinc nitrate precursors, zinc chloride produced hexagonal rod-like and plate-like structures (impurity). This impurity confirms the assertion made by Wojnarowicz et al. [27] that zinc chloride produces a stable by-product (simonkolleite), which is considered an impurity during the synthesis of ZnO. (a) (b) (c) (d) Figure 2. ZnO morphologies:(a) rod-like; (b) hexagonal platelets; (c) flower-like; (d) wire-like nanostructures. Amin et al. [40] also investigated the effect of pH, precursor concentration, growth time, and temperature on the morphology of ZnO nanostructures. They reported that tetrapod- like, flower-like, and urchin-like structures were obtained when the pH of the reaction mixture was from 8-12.5. However, as the pH was lowered from 8 to 4.6, rod-like structures eroded gradually to wire-like nanostructures (Figure 2d). Meanwhile, when the concentration of the precursor Zn(NO3)2·6H2O, was also decreased from 400-25 mM, the micro-rods observed at pH 8 gradually thinned out to wire-like structures with a diameter (< 100 nm) and length (1.2 https://biointerfaceresearch.com/ 4264 https://doi.org/10.33263/BRIAC123.42614292 µm). The average length of the rod-like structures produced was directly proportional to growth duration. The first 6 hours increased the length from 500 nm to 1.8 µm until the final length increased to 2.2 µm at the 10th hour. There was no further increase in length after the 10 hours of duration. They also reported that the aspect ratio of the synthesized rod-like structures increased with increasing temperature from 50 oC to 95 oC beyond which there was no change. Similarly, rod-like nanostructures were reduced to thin-film macrostructures when the precursor concentration was increased from 10 mM to 500 mM [41]. Table 1.0 illustrates other literature on hydrothermal synthesis using microwave-assisted and conventional approaches in ZnO nanostructures synthesis and their applications. 2.1. ZnO nanoparticles synthesis by the solvothermal route. The solvothermal technique is a widely used technique that usually uses non-aqueous solvents under controlled temperature and pressure higher than atmospheric pressure [42]. This technique requires a simple setup (Figure 3), less expensive equipment, relatively low synthesis temperature to yield a large area of deposition [43]. The technique also allows the microstructural control of the particles produced and the dispersity of ZnO NPs [44]. Despite these advantages, some of the solvents used are very costly and are very toxic to the environment. Figure 3. Overview of the solvothermal method to synthesize ZnO nanoparticles. According to Šarić et al. [45], properties of the non-polar solvents enable the reaction mechanism and particle growth to be controlled easily as moisture to the system is eliminated. The intermediate species from the non-polar solvents used in this technique offer better characterization when analyzed with NMR spectroscopy compared to the aqueous system [46. These non-polar solvents can serve as surfactants. The type of non-polar solvent and its intermediate species formed during the synthesis process could affect the crystal growth as selectivity to the different facets of the ZnO crystal is possible [47]. In most solvothermal syntheses, ethanol and methanol (alcohol) are the most common solvents used due to their availability and cost. Other solvents, as well as surfactants used according to literature, including oleic acid, gluconic acid, tween 80 [48], ethylene glycol [49- 51], triethanolamine, TEA [43]. Surfactants are amphiphilic molecules classified as anionic, nonionic, cationic, or amphoteric in terms of charge present in the hydrophilic portion of the molecule after dissolution in an aqueous solution. They usually form water-in-oil micro- emulsions which may be of two or three categories; the first metal complex dissolved in the https://biointerfaceresearch.com/ 4265 https://doi.org/10.33263/BRIAC123.42614292 water pools, another metal salt complex, and the reducing agent. An example is the mixture of both platinum and palladium salts in the presence of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) as surfactant and hydrazine as a reducing agent [52]. The surfactants tend to restrict the growth of the particles as the size of the emulsion, therefore, tend to determine the particle size. Šarić et al. [45] established that increase in the mole ratio of the surfactant and type of alcohol use increases the particle size. According to a review by Wojnarowicz et al. [27], surfactants such as Triton X100, polyethylene glycol 400 (PEG-400), polyvinylpyrrolidone (PVP) produces rod-like nanostructures [53], polyvinyl alcohol (PVA) results in the production of flake-like nanostructures [54], whiles polyvinylpyrrolidone (PVP) and cetyltrimethylammonium-bromide (CTAB) produces flower-like nanostructures [55]. The reaction kinetics of the molecules are mainly governed by the transport of species through the heat transfer within the system. As indicated previously, thermal decomposition of reacting molecules is efficient when microwave irradiation is used compared to conventional heating. One microstructural property of ZnO NPs that influences their application is the agglomeration of the synthesized crystals. Literature has reported that Diethanolamine (DEA) and Triethanolamine (TEA) are suitable solvents used as polymerization and stabilization agents in controlling the morphology of ZnO structures by the aggregation of primary nanoparticles [43,56]. In a study by Saric et al. [45], Triethanolamine (TEA) was investigated to have an impact on the growth of particles. The result established that TEA acted as both a suppressor and modifier of the particles formed at a reaction temperature of 170 oC. Furthermore, the growth and aggregation of the particles depended on the mole concentration ratio of the zinc precursor and TEA. In other to develop the morphology of ZnO NPs to suit a particular application, Idiawati et al. [57] demonstrated the effect of growth time using a two-stage approach to grow ZnO nano-rods on Indium Tin Oxide (ITO) substrate. The first reaction stage (seed layer formation) employed zinc acetate dihydrate and Monoethanolamine (MEA) reaction at 70 °C for 2 hours. Then follows the growth stage where zinc nitrate tetrahydrate [Zn(NO3).4H2O], Hexamethylenetetramine [HMT], and ITO were reacted at 90 °C with the growth-time varied at 4, 6, and 9 hours for each concentration. The as-prepared ZnO nanorods gave the crystallite size, nanorod length, and diameter to be 44-51 nm, 570-1280 nm, and 138-236 nm, respectively. It has been proven that these rod-like nanostructures are showed resistance to wood decay fungus [58]. Nevertheless, Ming et al. [34] investigated the effect of different zinc salts (zinc sulfate, zinc nitrate, and zinc acetate) on Rhodamine B dye's morphology and photocatalytic property. The microwave operating conditions (650 W, 2.45 GHz) produced microsphere structures covered with nanosheets and nanoparticles (nitrate and sulfate zinc) and microflower structures covered with nanosheets (zinc acetate). Nitrate-ZnO sample exhibited the best photocatalytic degradation of 93.63% after 120 minutes compared to acetate-ZnO and sulphate-ZnO samples with percentage degradation of 82.35% and 64.66%, respectively. Other studies carried out using this route but with different conditions are illustrated in Table 2.0. 2.2. ZnO nanoparticles synthesis by sol-gel route. Sol-gel as a chemical technique in metal oxide nanoparticle synthesis was a technology used to fabricate glass and ceramic materials some four decades ago, as reported by Dimitriev et al. [59]. The method is made up of aqueous and non-aqueous processes involving water and organic solvents. Aqueous sol-gel chemistry converts the precursor (mostly inorganic metal https://biointerfaceresearch.com/ 4266 https://doi.org/10.33263/BRIAC123.42614292 salt or metal alkoxides) into inorganic solid by virtue of the water molecules. On the other hand, non-aqueous sol-gel chemistry transforms the precursor (usually metal acetylacetonates and acetates, organometallic compounds, metal alkoxides, inorganic metal salts) via thermal decomposition. Sol-gel was adopted because of its considerable use of reduced temperature, which was economical compared to physical methods. This technique also offers the desired rate of thermal stability, good flexibility of crystal formation that is reproducible, better control of the particle size and shape to suit a wide range of applications, and relatively inexpensive apparatus set-up [60, 61]. Devoid these advantages, there are a few fundamental problems regarding the aqueous process, according to Livage et al. [62]. First, many reaction parameters (pH, precursor concentration, temperature, method of mixing, oxidation rate, hydrolysis, and condensation) need careful control to achieve the desired result. Also, the synthesized sample is generally amorphous, which needs the right annealing temperature to get the required crystalline structure. However, the non-aqueous process tends to overcome some of these major limitations of the aqueous process. Sol-gel employs either a chemical solution (sol) or colloidal particles to produce an integrated network (gel) involving aqueous or organic solvent. The process involves various steps, including hydrolysis and polycondensation, gelation, aging, drying, and crystallization, as illustrated in Figure 4. Figure 4. Overview of the sol-gel processes to synthesize ZnO nanoparticles. Oxygen produced by the reaction solvent (water or alcohol) aids in forming the metal oxide in the hydrolysis stage. Alternatively, the use of an acid or alkaline source could also aid the hydrolysis of the precursor used. After the hydrolysis stage, the solvent condenses to form a colloidal mixture (gel), which yields hydroxyl (-OH) or oxo (-O) species to connect the metal molecules. The gelling process continues through the third stage until it is subjected to the type of drying, as illustrated in Figure 4. The type of drying process adopted affects the structure of the gel formed. Also, the nature of nanoparticles to be formed also depends on the drying process and relative humidity. For instance, nano-films may require low humidity for the stability of the sample formed. As already stated, calcination of the sample also influences the nature of the sample formed in terms of morphology [64, 65]. Shaikh and Ravangave [66] used the sol-gel method to synthesize nanorod structures with crystallite size in the range of 33-43 nm when zinc acetate and sodium hydroxide were https://biointerfaceresearch.com/ 4267 https://doi.org/10.33263/BRIAC123.42614292 used as starting materials. The reaction time was varied between 2-8 hours, after which the as- prepared samples were calcined at 100 oC for 2 hours. Hasnidawani et al. [67] synthesized rod- like nanostructures with an average particle size of 81-85 nm with similar starting materials. Differences could be attributed to calcination and the longer reaction time employed by Shaikh and Ravangave. According to Iwamura et al. [68], reaction time and purity of the nanomaterial can be enhanced with the use of the microwave-assisted sol-gel technique. Irradiation of zinc acetate dihydrate and N, N-dimethylacetoamide (DMAc) with a temperature of 64-118 °C for 1.5-4.0 min duration yielded spherical nanocrystalline structures with a particle size of 312-509 nm. However, an increase in time to 6 min altered the morphology to nanowires of length and diameter, 1334 and 127 nm, respectively. Also, the good and quality yield of cubic octamethylsilsesquioxane was synthesized using microwave-assisted technique [69]. The type of reagent used in sol-gel synthesis also influences the morphology as well as the optical properties of the synthesized samples. According to Vanaja and Rao [70], potassium and sodium hydroxide were used as precipitating agents on the precursor, zinc nitrate. Potassium hydroxide produced a smaller crystallite size (21.59 nm) of ZnO as compared to 36.89 nm when sodium hydroxide was used. Both agents produced irregular spherical-shaped structures with sizes 17-25 nm and 30-50 nm for KOH and NaOH, respectively. Apart from the solvent controlling the reaction mechanism in non-aqueous sol-gel processes, surfactants can also be used in the presence of the precursor in the transformation process, thereby acting as stabilizing ligands at temperature ranges of 250-350 oC. The surfactant’s ability to select the specific crystal face during the growth stage enables the control of the morphology and agglomeration as well as the surface properties of the nanoparticles. However, surface-adsorbed surfactant compromises the gas sensing effect or catalysis of some nanomaterials as well as their toxicity [71]. Thorn-like ZnO nanostructures were synthesized through the sol-gel method by Khan et al. [72], when zinc acetate, sodium hydroxide, and cetyltrimethylammonium bromide (CTAB) was reacted under different stirring conditions (500, 1000, 1500, and 2000 rpm) for half an hour. Although other nanostructures such as nanoflowers, nanorods, nanowhiskers, nanobelts, nanotubes, nanorings, and nanocolumns were also identified, the thorn-like structures (Figure 5) were dominant with size determined at < 50 nm by TEM analysis. Figure 5. Thorn-like ZnO nanostructures. Brintha and Ajitha [73] compared the morphological differences when ZnO nanostructures were prepared using sol-gel, hydrothermal and aqueous solution techniques. https://biointerfaceresearch.com/ 4268 https://doi.org/10.33263/BRIAC123.42614292 Although sol-gel employed high annealing temperature of 450 oC for 6 hours, the other techniques had their reaction temperatures between 80-100 oC for 6 hours. Hydrothermal and sol-gel techniques produced spherical and flower-like structures with 14 nm and 18 nm crystallite, respectively. On the other hand, the aqueous solution technique recorded a mixture of the two structures with a crystallite size of 13 nm. Results from this study were similar to that of [74] when ZnO was doped with Ni and Al in a sol-gel process. Other studies carried out using the sol-gel method of synthesis with different conditions and useful application of the as- prepared samples are illustrated in Table 3.0. 2.3. ZnO hybrid/composite synthesis route. The intent of producing suitable changes in electrical, optical, and magnetic properties of metal oxide nanomaterials for their intended practical purposes is through the use of some selected elements in the periodic table (Group I, II, III, transition metals, metalloids, non- metals, and lanthanides) as doping materials [75]. Since these transition elements are semiconductors, creating defects in the lattice of these semiconductors or forming hybrid materials with their required properties helps make the fabricated composite material an effective photocatalyst. Elements that are suitable as dopants in ZnO nanostructures should have a small ionic radius as that of Zn (0.74 Å). This enables the ions to easily migrate over the crystal to occupy the interstitial positions, thereby affecting the nanocomposite compound's structural, morphological, and optical properties. According to Yousefi et al. [76], Li-doped ZnO showed better crystal quality than Na and K-doped ZnO as well as the undoped ZnO. It was also evident that, when alkaline earth metals (Mg2+, Ca2+, Sr2+, and Ba2+) was used as a dopant in a co-precipitation technique by Hameed et al. [77], XRD patterns of Mg and Ca- doped-ZnO showed no addition phase due to their ionic radii (0.66 Å & 0.99 Å) been close to that of Zn where the others were far above (1.13 Å for Sr2+ and 1.35 Å for Ba2+). The doped samples showed better enhancement in the crystalline nature as compared to the undoped sample. Transition metal (TM) doped ZnO nanostructures are explored due to the magnetic and electronic properties they exhibit as a result of the electron spins in the 3d-orbital. Composite nanostructures synthesized from these species have better applications in electrical, electronics, and magnetic devices as compared to the bulk. The ionic radius of TMs was established to be one of the determining parameters for producing a very pure doped nanocrystalline structure. Zak et al. [78] identified Ni, Mn, and Co-doped ZnO to show a decreased particle size compared to the undoped. This result was attributed to the similar ionic radius of the dopants, Ni (0.69 Å), Co (0.745 Å), and Mn (0.46 Å) to Zn, where their lattice strain was replaced by Zn in the lattice structure. From a study by Kaur et al. [79], Gd-doped-ZnO exhibited a paramagnetic behavior with a weak ferromagnetic component at low Gd-doping compared to the undoped. The doped sample was synthesized using zinc acetate dihydrate and gadolinium nitrate in 2-methoxy ethanol as solvent at 60 oC for 2 h. After adding monoethanolamine, the sol was aged 24 h at room temperature, dried at 500 oC for 1 h, and finally annealed at 500 oC for another 1 h. Results from this study showed the formation of rod-like nanostructures with an average crystallite size of 25-37 nm. These hybrid nanostructures can be synthesized using different techniques. Table 4.0 illustrates doped-ZnO nanostructures synthesized by different techniques. https://biointerfaceresearch.com/ 4269 https://doi.org/10.33263/BRIAC123.42614292 Table 1. Characteristics of ZnO nanostructures synthesized via different hydrothermal routes. Method of synthesis Precursor/Reactant Synthesis condition Properties Applications Reference Microwave-assisted Zn(Ac)2 ·2H2O, Reaction time: 10 min; Autoclave Hexagonal prism-like structures; Particle [80] without extra heat NH4OH/NaOH, AOT temp: 80, 100, 120 or 140 °C for 5, 10, size: 200-300 nm treatment 20 min; Microwave power: 300, 600 and 1200 W; Drying time: 65 °C for 3 h. Microwave-assisted + Zn(Ac)2∙2H2O, N2H4, Irradiation: 15/10 min 510/680 W; Needle-like, Flower-like, Spherical: [81] heat treatment Zn(NO o3)2∙6H2O, NaOH, Drying: 100 C 2 h; Irradiation: 15 Particle diameter: 50-150 nm NH3 min 150 W; Drying: 100 oC 2 h; Calcination temp: 600 oC 3 h Microwave-assisted Zn(Ac)2∙2H2O, NaOH Reaction temp: 100 oC & 140 oC for Plate-like, rounded plate-like, brush-like, Photocatalyst against [82] without extra heat (solvent = water/ethyl 45 and 60 min; Microwave power: 800 and flower-like; Band gap: 3.17-3.24 eV; Rhodamine-B treatment alcohol; 50/50 v/v & 100/0 W; Drying temp: 80 oC overnight; pH Surface area: 8.46-10.70 m2/g v/v) = 10 Microwave-assisted Zn(NO3)2, NaOH pH = 8.3; Microwave irradiation: 1-5 Rod-like, flower-like; Particle size Bio-imaging and drug [83] without extra heat min; Drying temp: 70 oC (diameter): 100-200 nm; Hydrodynamic (Quercetin) delivery treatment size (DLS): 135-361 nm Microwave-assisted ZnCl2, NaOH, CTAB, reaction temp: 50 oC for 90 min; Cone-like (surfactant free & Pluronic Photocatalyst in the [84] without extra heat Pluronic F127 Microwave power: 2.45 GHz; 130 W F127), Plate-like (CTAB); Surface area: degradation of methyl treatment for 5 min; Drying temp: 60 oC for 24 h 15.5-24.8 m2/g; Crystallite size: 19.6-21.0 blue (MB) nm; Particle size: 92.8 nm; 58.1 nm (CTAB); 80.2 nm (Pluronic F127); Band gap: 3.36 eV (free surfactant and CTAB); 3.34 eV (Pluronic F127) Microwave-assisted ACF Fabric Preparation: Reaction time: 1 h; Drying temp: 50 Rod-like structures Removal of tetracycline [85] without extra heating ACF + HNO oC (oven), 100 o3 C for 3 h (furnace) Growth of ZnO: Zn(Ac) + NH4OH, on ACF Reaction time: 3 h fabric Microwave power: 1120W, 2450 MHz for 30 min x 3 steps; Drying temp: 50 °C for 24 h; pH = 10- 11 Microwave-assisted Zn(Ac)2∙2H2O, TEOA, Microwave power: 2.45 GHz, 800 W, Dumb-bell (pH 8), Photoelectrode [86] without extra heating BTCA, KOH 150°C, 30 min; Drying temp: 100 °C; Spherical (pH 9) pH = 12/13 Hexagonal bi-pyramidal (pH 10); Surface area: 35, 15, 25 m2/g (Dumb-bell, spherical, hexagonal bi-pyramidal); Particle size: 195, 430, 60 nm (Dumb- bell, spherical, hexagonal bi-pyramidal) https://biointerfaceresearch.com/ 4270 https://doi.org/10.33263/BRIAC123.42614292 Method of synthesis Precursor/Reactant Synthesis condition Properties Applications Reference Microwave-assisted ZnO nanorods synthesis Microwave power: 450 W, 5 min; Plate-like, flower-like structures; Energy Photoanode, [87] with extra heating Zn(Ac)2∙2H2O, NaOH Drying temp: 80 °C for 24 h band gap: 3.201-3.217 eV Photocatalyst in the (Mole ratio: Zn2+: OH- (1: Reaction time: 1.5 h; Drying temp: 80 degradation of methyl 0,3,6,9,12,15) °C for 30 min; Calcination temp: 450 blue (MB) ZnO photoanode synthesis °C for 1.5 h ZnO, PEG, C2H5OH Microwave-assisted Zn(NO3)2∙6H2O, NaOH Microwave power: 70-100 W, 100- Particle size: 20-250 nm; Crystallite size: [88] without extra heating 240 °C, 3-7 min, 9-49 bars; Reactor 22-30 nm; parameters: 120-200 oC, 30 min, 1-14 Particle size: 30-400 nm; Crystallite size: bars; Drying temp: 50 °C for 2 h 30-32 nm Microwave-assisted Zn(NO3)2∙6H2O, NaOH, Microwave power I: 350 W for 2 min; Spherical; Crystallite size: 25 nm (2 min); [89] without extra heating Gum Arabic Microwave power II: 350 W for 2, 4, 6 Particle size (DLS): 160-180 nm (2-6 & 10 min; Drying temp: 80 oC; pH = min); 225 nm (10 min); Particle size 10 (FESEM): 20-40 nm (non-aggregate); 150-200 nm (aggregates) Microwave-assisted Seed layer preparation: Annealing temp: 300 °C for 30 min Rod-like and rod-like thin film structures; Electronic switch [90] without extra heating Zn(Ac)2∙2H2O, C3H8O (3x); Microwave power I: 900 W for 2 Nanorod diameter: 40-50 nm (one step), Growth of ZnO nanorod: min; < 50 PSI; Microwave power II: 600 nm (two-stem); Crystallite size: 1-10 Substrate, Zn(NO3)2∙6H2O, 100 & 1600 W, 80 oC, for 5 & 15 min µm (one step), 200 nm (two step) HMTA Microwave-assisted Zn(NO3)2∙6H2O, NaOH (4, Microwave power: 110 oC for 1 h; Star-like and chrysanthemum flower-like Photocatalyst for [91] without extra heating 5 & 6 mmol of NaOH) Drying temp: 80 oC for 24 h structures; Energy band gap: 3.21, 3.22 & reduction of 3.24 eV; Crystallite size: 26, 24, 21 nm chromium(VI) respectively. Microwave-assisted Zn(Ac)2∙2H2O, NaOH Reaction temp: 75 oC for 2 h; Spherical shape (8-24 h), Spherical & [92] without extra heating Autoclave temp: 100, 125, 150, 175 & rod-like (100-200 oC); Crystallite size: 200 oC for 8, 12, 16, 20, & 24 h; 31-38 nm (8-24 h); Drying temp: 75 oC for 6 h 27-35 nm (100-200 oC); Particle size (HR-TEM): 30-40 nm; Energy band gap: 2.90-3.78 eV Microwave-assisted Zn(NO3)2∙6H2O, NaOH Microwave power: 2.45 GHz, 1000 Spindle-like; Crystallite size: Antidiabetic & [93] without extra heating W, 10 min; Drying temp: 60 oC 21 nm; Surface area: 11.06 m2/g (ZnO) antibacterial inhibitor overnight; pH = 10 Microwave-assisted Zn(NO3)2, NaOH Irradiation time: 15 min; Drying temp: Spherical shape; Crystallite size: Antidiabetic activity [94] without extra heating 60 oC for 24 h; pH = 10 15.5 nm (ZnO) Microwave-assisted Seed layer preparation: Annealing temp: 310 °C for 60 min Rod-like; Particle diameter & length: 26- [95] without extra heating Zn(Ac)2∙2H2O, C3H8O Microwave Power: 2.45 GHz, 105 oC 32 nm & 440 nm (75 mM), 35–39 nm & for 5, 10 & 15 min; Drying: N2 gas https://biointerfaceresearch.com/ 4271 https://doi.org/10.33263/BRIAC123.42614292 Method of synthesis Precursor/Reactant Synthesis condition Properties Applications Reference Precursor growth: 600 nm (1.0 M), 78–84 nm & 1.5 µm (2.0 Zn(NO3)2∙6H2O (75 mM, M) 0.1 M, 0.2 M), C6H12N4 22-26 nm & 430 nm (0.1 M @ 5 min), 35-40 nm & 60 nm (0.1 M @ 10 min), 35-39 nm & 605 nm (0.1 M @ 15 min) Microwave-assisted Zn(Ac)2∙2H2O, NaOH Microwave Power: 2450 MHz, 700 Irregular sheet-like (pH 6), Agglomerated Antibacterial activity [96] without extra heating W, 6 min; Drying temp: 60 °C for 4 h; uniform microstructures (pH 8 & 10); pH = 6, 8, & 10 Particle size: ~100 nm (pH 8 & 10) Microwave-assisted Seed layer preparation: Si, Sputtering parameters: 100 W for 100 Rod-like; Particle size (TEM): 90 nm [97] without extra heating ZnO deposit by RF s, < 7×10-4 Pa and 1.0 Pa magnetron sputtering Precursor growth: Microwave Power: 60-110 oC, 5-40 Zn(Ac)2∙2H2O (0.01~0.04 min mol/L), C6H12N4 Microwave-assisted Synthesis of 1-carboxy4- Reaction time: 2 h; Refluxing temp: Hexagonal wurtzite structure; Crystallite Bactericidal agent [98] without extra heating methylpyridin-1-ium 70 oC for 6 h; Microwave power: 20 size: 30.6 nm (uncapped); 22.9 nm (3% C6H7N, C2H3ClO2 min; Drying temp: 60 oC for 8 h dye-capped); Energy band gap: 3.6 eV Synthesis of Dye; {4-[(E)- 2-(furan-2-yl) ethenyl]pyridin-1-ium-1- yl}acetate C6H7NO2, C5H5O2, C5H4O2, C2H5OH, C5H5N ZnO synthesis Zn(Ac)2∙2H2O, Dye (1% & 3%), NaOH Microwave-assisted Zn(Ac)2∙2H2O, TRIS Microwave power: 300 W for 3 min; Spherical; Energy band gap:  3.49 eV Antifungal agent [99] without extra heating (20%) Drying temp: 80 °C overnight Microwave-assisted Zn(Ac)2·2H2O, Phthalic Microwave power: 2.45 GHz, 800 W, Platelet shape (SDA), star-like (TPA), [100] without extra heating acid (PA)/ Isophthalic acid 150 °C for 30 min; Drying temp: 100 rod-like (IPA), plate-like (PA); Particle (IPA)/ Terephthalic acid °C for 12 h; pH = 7, 10 & 12 size: 50-500 nm (free SDA), 100-150 nm (TPA) (presence of SDA) Microwave assisted Zn(NO3)2∙6H2O, NaOH Microwave power: 2.45 GHz, 320 & Needle-like (320 W), Rod-like (480 W); Photocatalyst in the [101] with extra heating (1:15 molar ratio), PEG 480 W for 5 s & 15 s; Drying temp: 80 Particle size (PEG free/PEG): < 500 nm/ degradation of methyl °C for 24 h; Calcination temp: 450 °C 300 nm length (320 W), 2 μm/3 μm blue (MB) for 1 h length (480 W); Energy band gap: 3.24 Ev (PEG free), 3.10-3.23 eV (PEG) Microwave-assisted NaC12H25SO4, C3H8O, Reaction temp: 80 ℃ for 2 h; Fake-like (0.12 W/g), spherical (0.12 & Photocatalyst in the [102] with extra heating NH3, Zn(Ac)2·2H2O Microwave power: 2.45 GHz, 100-800 0.56 W/g); Crystallite size: 38.84 nm degradation of phenol https://biointerfaceresearch.com/ 4272 https://doi.org/10.33263/BRIAC123.42614292 Method of synthesis Precursor/Reactant Synthesis condition Properties Applications Reference W for 2 h, 0.12, 0.37 and 0.56 W/g; (0.12 W/g), 34.52 nm (0.37 W/g), 31.08 Calcination temp: 550 °C for 3 h nm (0.56 W/g); Energy band gap: 2.89 eV (0.12 W/g), 2.39 eV (0.37 W/g), 2.26 eV (0.56 W/g); Surface area: 13.10 m3/g (0.12 W/g), 13.57 m3/g (0.37 W/g), 14.35 m3/g (0.56 W/g) Microwave-assisted Synthesis of Polystyrene Reaction temp: 70 oC for 4 h; Drying Hollow with cavities; Particle size Adsorbent [103] with extra heating (PS) template: temp: 27 oC; Microwave irradiation: (TEM): 35 nm; 1483.50 nm C8H8, PVP (2:1 w/w), 120 oC for 10 min; Calcination temp: (hydrodynamic diameter by DLS); Cavity C o2H5OH, Na2S2O8 527 C; Sonicate and stir mixture for 1 diameter: 34 nm; Surface area: 17.1 m2/g Hollow ZnO NPs h (h-ZnO), 12.3 m2/g (Glu-h-ZnO) synthesis: PS, Zn(Ac)2·2H2O, NH4OH Glutathione-hollow ZnO NPs synthesis: Glu, h-ZnO (1:1), CH3OH Microwave-assisted Seed layer preparation: Deposit spraying temp: 350 oC; Rod-like; Particle size: Photocatalyst in the [104] with extra heating Glass substrate, Microwave power: 90 oC, 180 W for ~ 4.3 µm (ave. length) degradation of phenol Zn(Ac)2·2H2O deposit by 45 min (4x); Drying temp: 90 oC 100 nm (ave. diameter); Energy band gap: spraying Calcination temp: 100 and 350 oC for 3.2 eV (100 oC), 2.9 eV (350 oC) Precursor growth: 1 h Seeded substrate, Zn(NO3)2∙6H2O, HMTA Microwave-assisted Seed layer preparation: Reaction temp: 60 oC for 1 h; Drying Rod-like; Crystallite size: 9-25 nm (seed UV Sensor [105] with extra heating Glass substrate, temp: 300 oC for 10 min; Calcination layer), 38-56 nm (nanorods); Particle Zn(Ac)2·2H2O, C2H7NO, temp: 100-500 oC; Microwave power: size: 180-350 nm (nanorod length); C o3H8O2 deposit by spin 90 C, for 60 min Energy band gap: 3.22-3.30 eV (seed coating layer), 3.17-3.98 eV (nanorods) Precursor growth: Seeded substrate, Zn(NO3)2∙6H2O, HMTA Microwave-assisted Seed layer preparation: Reaction temp: 100 oC for 15 min; Rod-like; Particle size: 80 nm (diameter CO gas detector [106] with extra heating FTO substrate, Annealing temp: 350 oC for 1 h; of the microwave-assisted sample), 30 nm Zn(Ac)2·2H2O, C2H5OH Microwave power: 1100 W for 20 s (diameter of the hydrothermal sample) deposit by spin coating (microwave-assisted hydrolysis); Precursor growth: Oven temp: 90 oC for 45 min FTO substrate, (hydrothermal process); Drying Zn(NO3)2∙6H2O, HMT samples with N2 gas. https://biointerfaceresearch.com/ 4273 https://doi.org/10.33263/BRIAC123.42614292 Method of synthesis Precursor/Reactant Synthesis condition Properties Applications Reference Microwave-assisted Zn(NO3)2∙6H2O, TEA Microwave power: 640 W for 10 min; Crystallite size: ~27 nm Photocatalyst in the [107] without extra heating (1:30 v/v) Vacuum drying degradation of CTAB Microwave-assisted Zn(Ac)2·2H2O, NH4OH, Microwave power: 2450 MHz, 640 W Flake-like (zinc acetate), Rod-like (zinc Gas sensor for Volatile [108] with extra heating ZnCl2, Zn(NO3)2∙6H2O, for 10 min; Vacuum drying: 90 oC for nitrate), Spherical (zinc sulfate), Organic Compounds ZnSO4.7H2O 12 h; Calcination temp: 600 oC for 2 h; Hexagonal tubular (zinc chloride) (VOC) pH = 9 Microwave-assisted Zn(NO3)2∙6H2O, HMT, Microwave power: 110, 310, & 710 Plate-like; Crystallite size: 22.40-24.83 [109] with extra heating NaOH W, 90, 150 & 220 oC for 15 min; nm; Energy band gap: 3.28-3.38; Surface Calcination temp: 200 oC for 2 h; pH area: 10.44-18.09 m2/g; Particle size: = 13 400-600 nm (diameter) Microwave-assisted Zn(NO ) ∙6H O, PEG 400, Microwave power: 800 W, 100 o3 2 2 C for Quasi-spherical shape (NaOH with & Catalyst for biodiesel [110] without extra heating NaOH/NH4OH 5 min; Drying temp: 110 oC for 12 h without PEG), flower-like (NH4OH with synthesis & without PEG); Crystallite size: 29.5 nm (NaOH without PEG), 25.8 nm (NH4OH without PEG), 34.9 nm (NaOH with PEG), 29.4 nm (NH4OH with PEG); Particle size: 5 µm (NaOH with & without PEG), > 5 µm (NH4OH with & without PEG); Surface area: 14.88 m2/g (NaOH without PEG), 2.75 m2/g (NH4OH without PEG), 7.64 m2/g (NaOH with PEG), 2.03 m2/g (NH4OH with PEG) Microwave-assisted Precursor growth process: Microwave power: 50 oC for 2 min; Rod-like; Particle size (height & width): [111] without extra heating GaN substrate, Drying temp: < 100 oC 2.2 µm/ 2.5 µm (pH = 6.75), 1.5 µm/0.7 Zn(Ac)2·2H2O, NaOH µm (pH = 7.25), 1.5 µm/0.5 µm (pH = 7.75) Microwave-assisted Zn(Ac)2·2H2O, KOH Microwave power: 1000 W for 0, 0.5, Rod-like (0 min), Tetrapod (0.5 min), Photocatalyst for the [112] with extra heating 1, 1.5 & 2 min; Drying temp: 80 °C; Flower-like (1-2 min); Crystallite size: degradation of Calcination temp: 400 °C for 1 h 53, 59.3, 69.2, 74.5 and 82.4 nm (0-2 rhodamine B (RhB) min); Particle size (length): 255 nm (0.5 min), 397 nm (1-2 min); Energy band gap: 3.24 eV, 3.23 eV, 3.21 eV, 3.19 eV and 3.17 eV (0-2 min) Microwave-assisted Seed layer preparation: p- Annealing temp: 400 oC for 1 h; Rod-like; Crystallite size: 52.08 nm; UV detector [113] without extra heating Si (100) substrate, Microwave power: 90 oC for 2 h; Particle size (length): 1 µm Zn(NO3)2∙6H2O, C6H12N4 Sampled dried with N2 gas. deposit by RF sputtering Precursor growth: https://biointerfaceresearch.com/ 4274 https://doi.org/10.33263/BRIAC123.42614292 Method of synthesis Precursor/Reactant Synthesis condition Properties Applications Reference FTO substrate, Zn(NO3)2∙6H2O, HMT Microwave-assisted Zn(NO3)2∙6H2O, C6H12N4 Microwave power: 750 W, ~120 °C Spherical to hexagonal rod-like Antimicrobial agent [114] with extra heating (mole ratio of 3:20, 5:20, for10 min; Drying temp: 80 °C for 24 structures; Crystallite size: 16.7-57.9 nm; 12:20, 20:20 and 30:20) h; Calcination temp: 400 °C for 1 h Particle size: 25 nm to micro/sub micro sizes Microwave-assisted Zn(Ac)2·2H2O, NH4OH Microwave power: 150 ℃ for 30 min; Hexagonal prism (pH = 7), flower-like Photoelectrochemical [115] without extra heating (0.25, 0.5, 1.0, 1.5, 2.0 or Drying temp: 150 °C for 2 h (pH = 11); Particle size (width & length): agent 3.0 ml) pH = 7-11 1 & 5 µm (pH = 7), 100 nm (pH = 8-10) Microwave-assisted Zn(Ac)2·2H2O, NaOH Reaction temp: 60 oC for 2 h; Spherical; Particle size : 29 nm [116] without extra heating Microwave power: 90 oC for 20 min Table 2. Characteristics of ZnO nanostructures synthesized via different solvothermal routes. Method of Precursor/Reactant Synthesis condition Properties Applications Reference synthesis Solvothermal with Zn(Ac)2·H2O, ethanol/1- Autoclave temp: 170 °C for 4 h Spherical (1-butanol), rod-like (ethanol, 1- [117] heat treatment propanol/1-butanol/1- propanol, 1-pentanol, 1-octanol); Particle pentanol/1-octanol size: ∼12 nm (1-butanol); Crystallite size: 28 nm (ethanol), 14 nm (1-propanol), 12 nm (1-butanol), 13 nm (1-pentanol), 18 nm (1- octanol) Microwave-assisted Zn(NO3)2∙6H2O, PVA, Microwave power: 2.45 GHz, Spherical; Band gap: 3.35 eV; Crystallite Photocatalyst [118] with extra heat Ascorbic acid 400 W for 3 min; Drying temp: size: 20-22 nm; Particle size: against treatment 105 °C for 3 h; Calcination temp: 70-90 nm (A), 230-280 nm (B), 580-630 nm Rhodamine-B 500 °C for 4 h (C); Surface area: 8.46-10.70 m2/g Antibacterial agent Solvothermal Zinc salt, dimethyl sulfone, Reaction temp: 60 ℃ for 3 & 12 Spherical; Crystallite size: 4 nm and 10 nm; Antibacterial [119] synthesis with KOH h; Drying temp: 65 ℃ for 12 h Particle size (TEM): 4 nm and 10 nm agent conventional heating Solvothermal Zn(Ac)2∙2H2O, ethylene Refluxing temp: 195 °C for 3 h; Spherical to wire-like; Crystallite size: 14.6, Photoanodes [120] synthesis with glycol, PVP (0.001-0.004 Drying temp: 80 °C for 2 h; 12.8, 47.0 and 81.0 nm; Particle size: 14.8, conventional mol) Calcination temp: 350 °C for 2 h 12.5, 47.0 & 81.0 nm; Particle length & heating diameter: 20 µm & 22 nm; Surface area: 91, 121, 71 & 53 m2/g Solvothermal Zn(Ac)2∙2H2O, DMAc:H2O Reaction temp: 95 oC for 3 h; Quasi-spherical, dumbbell shape, rod-like; [121] synthesis with (1:0, 4:1, 3:2, 2:3, 1:4) Drying temp: 80 oC for overnight; Ave. diameter: 90 nm-0.7 µm; Energy band pH = 6.12-6.86 gap: 3.27-3.42 eV https://biointerfaceresearch.com/ 4275 https://doi.org/10.33263/BRIAC123.42614292 Method of Precursor/Reactant Synthesis condition Properties Applications Reference synthesis conventional heating Solvothermal Zn(Ac)2∙2H2O, ethanol, Reaction temp: 110 °C for 10 h Spherical (surfactant free), Hexagonal disc Photocatalyst in [122] synthesis with NaOH, SDS/PVP/PEG- (SDS presence), Hexagonal bilayer disk the degradation of conventional 10000 (PVP), Flower-like (PEG-10000); Particle Rhodamine-B heating size: 80-150 nm (surfactant free), (Rh-B). 300 & 200 nm for length & thickness (SDS), 6 µm and 4 µm for length & thickness (PVP), ~200 nm as average diameter (PEG-10000); Band gap: 3.24 eV (surfactant free), 3.20 eV (SDS), 3.15 eV (PVP), and 3.24 eV (PEG- 10000) Microwave-assisted Zn(Ac)2∙2H2O, ethylene Reaction temp: 70 oC; Microwave Spherical shape, hexagonal shape; Particle [123] without heat glycol, H2O (1.5 wt. %) radiation: 1, 2, & 3 kW, 2.45 size: 25-50 nm; Crystallite size: 23-48 nm; treatment GHz, 12 min, 4 bar; Drying: Surface area: 40.1-40.6 m2/g freeze drying Solvothermal Zn(Ac)2∙2H2O, ethanol, Reaction temp: 60 °C for 3 h; Spherical; Crystallite size: 10.08 nm; Particle [124] synthesis with KOH Drying temp: room temp. size (TEM/BET): 7.4/9.7 nm; Surface area: conventional 101.32 m2/g heating Solvothermal Zn(Ac)2∙2H2O, ethanol, Reaction temp: 2 h at room Rod-like; Particle size: 85 nm & 1.5 μm UV resistor [125] synthesis with NaOH, CTAB, wood temperature; Autoclave temp: 90 (diameter & length) conventional samples, FAS-17 °C for 4 h; Drying temp: 45 °C heating for 24 h. Microwave-assisted Zn(NO3)2∙6H2O, urea (1:5 Microwave irradiation: 150 °C for Flower-like; Particle size (breadth): 10 nm to [126] without heat molar ratio), 15 min; Drying temp: 50 oC micron treatment overnight Microwave-assisted Zn(Ac)2.2H2O (1, 2, 4 & 8 Microwave power: 250 °C for 15 Spherical; Particle size: 4-14 nm [127] without heat mM), diethylene glycol, min; Drying temp: 80 °C treatment oleic acid Microwave-assisted Zn(Ac)2.2H2O, Isopropanol, Microwave power: 150, 175 & Energy band gap: 3.38-3.94 eV Photodetector [128] with heat treatment Diethanolamine 200 °C; Calcination temp: 100 °C for 2 h Microwave-assisted Zn(Ac)2.2H2O, NaOH, Microwave power: 300 W, 1 h; Spherical; Crystallite size: 24 nm and 26 nm Dye (methyl [129] with heat treatment Dimethylformamide Drying temp: 60 °C for 4 h; (calcined sample); Energy band gap: 3.26 eV orange) removal Calcination temp: 500 °C and 3.20 eV (calcined sample) agent https://biointerfaceresearch.com/ 4276 https://doi.org/10.33263/BRIAC123.42614292 Method of Precursor/Reactant Synthesis condition Properties Applications Reference synthesis Microwave-assisted Zn(Ac)2.2H2O, NaOH, Microwave power: 300 W for 3 Rod-like (H2O & ET), Flower-like (EG); Photocatalyst in [130] without heat Triton X-100, H2O / 2- min; Drying temp: room temp for Crystallite size: 52.64 nm (H2O), 63.70 nm the degradation of treatment ethoxyethanol (ET) / 72 h (ET), 24.02 nm (EG); Energy band gap: 3.35 methylene blue ethylene glycol (EG). eV (ET), 3.38 eV (H2O), 3.42 eV (EG) Microwave-assisted ZnCl2, C18H33NaO2, Microwave irradiation: 125, 150, Spherical; Particle size (radius/width): 2.6/0.2 [131] without heat C16H37NO, CH3OH, 175 & 200 oC for 5 min nm (125 oC), 2.7/0.3 nm (150 oC), 3.1/0.3 nm treatment C36H66O4Zn, C4H8O (175 oC), 3.8/0.5 nm (200 oC); Energy band gap: ): 3.44 eV (125 oC), 3.40 eV (150 oC), 3.38 eV (175 oC), 3.36 eV (200 oC); Crystallite size: ): 4.55 nm (125 oC), 5.88 nm (150 oC), 6.84 nm (175 oC), 8.00 nm (200 oC) Microwave-assisted Zn(Ac)2.2H2O, diethylene Microwave power: 220 & 250 oC Particle size : 5-12 nm Optoelectronic [132] with and without glycol, oleic acid, Toulene, for 10 & 15 min; Drying temp: 80 devices heat treatment Si substrate °C for 30min; Drying temp of substrate: 100 oC for 3 h; Calcination temp of substrate: 400 oC for 3 h Table 3. Characteristics of ZnO nanostructures synthesized via different sol-gel routes. Method of synthesis Precursor/Reactant Synthesis condition Properties Applications Reference Sol-gel without heat Zn(Ac)2.2H2O, CH3OH, Sonication power: 750 W for 30 Particle size: 1.3 nm (pH, 7) and 73.8 nm [133] treatment NaOH min; Drying temp: 80 oC; pH = 5- (pH, 12) 12 Crystallite size: 10.94 (pH, 7), 17.44 (pH, 8), 38.27 (pH, 10), 74.04 (pH, 12) Sol-gel without heat ZnCl2, NaOH Reaction temp: 50-90 °C; Dripping Crystallite size: 21-37 nm Photocatalyst in [134] treatment time: 20-60 min; Drying temp: 70 the degradation of °C dyes Sol-gel with heat ZnSO o4·7H2O, NaOH (1:1, Reaction time: 8, 10 & 12 h; Drying Nanoplatelets (1:1, 10 h, 300 C), Reinforcement [135] treatment 1:2, & 1:3 mole ratio) temp: 100 oC; Calcination temp: nanospheres (1:2, 8 h, 700 oC), agent for 300, 500, & 700 oC for 2 h nanoplatelets and nanorod (1:3, 12 h, 500 polymers oC); Particle size: 164-197 nm (300 oC), 1.73-167 nm (500 oC), 85-157 nm (700 oC); Crystallite size: 40.57 nm Sol-gel with heat Zn(NO3)2∙6H2O, PVA Reaction temp: 80 oC for 60 h; Spherical; Crystallite size: 15-51 nm Photocatalyst in [136] treatment Drying temp: 100 oC for 24 h; (400-550 oC); Particle size (TEM): 15 nm the degradation of Calcination temp: 400-700 oC (400 oC), 25 nm (500 oC) phenol https://biointerfaceresearch.com/ 4277 https://doi.org/10.33263/BRIAC123.42614292 Table 4. Characteristics of doped-ZnO nanostructures synthesized via different techniques. Method of Composite Precursor/Reactant Synthesis condition Properties Applications Reference synthesis Solvothermal Ag-ZnO & Pt- Zn(NO3)2·6H2O, AgNO3, Reaction temp: room temp for 1 Nanowires; Crystallite size: 23-15.9 Photocatalyst in the [137] ZnO Pt(C5H7O2)2, CTAB h; Autoclave temp: 120 oC 3 & 6 nm (ZnO), 193.6-63.6 nm (Pt-ZnO), degradation of dyes h; Drying temp: 80 oC 12.9-11.5 nm (Ag-ZnO); Particle size (TEM): 67-138 nm (ZnO), 133- 143 nm (Ag-ZnO), 101-194 nm (Pt- ZnO) Surface area: 5-33 m2/g (ZnO), 23- 43 m2/g (Ag-ZnO), 4-15 m2/g (Pt- ZnO); Energy band gap: 3.21-3.24 eV (ZnO), 3.23-3.29 eV (Ag-ZnO), 3.23-3.24 eV (Pt-ZnO) Hydrothermal Ce-ZnO ZnO, CeO2, NaOH/KOH, n- Autoclave temp: 100 oC for 12 h Hexagonal Photocatalyst in the [138] butyl amine degradation of synthetic wastewater Hydrothermal ZnO-CdS-Ag Zn(NO3)2·6H2O, C2H4O3, Reaction temp (ZnO): 60 oC for 2 Rod-like; Crystallite size: 23 & 15 Photocatalyst against [139] KOH, h; Autoclave temp (ZnO): 120 oC nm (ZnO & CdS); Surface area: E. coli Cd(NO3)2·4H2O, C2H5NS, for 48 h; Drying temp (ZnO): 120 18.9 m2/g (ZnO-CdS-Ag) AgNO3, N2H o4 C for 8 h; Reaction time (ZnO- CdS): 1 h; Drying temp (ZnO- CdS): 70 oC; Reaction time (ZnO- CdS-Ag): 1 h; Drying temp: 70 oC Hydrothermal ZnO-CdS (25:75, Zn(NO3)2·6H2O, ZnCl2, Reaction time (ZnO): 1 h; pH = Flake-like (ZnO-CdS), Hexagonal [140] 50:50 and 75:25) Thiourea 12; Autoclave temp (ZnO): 150 (ZnO), cubic (CdS); Crystallite size: °C for 3 h; Autoclave temp 13-33 nm (25:75), 17-35 nm (CdS): 160 °C for 12 h; Drying (50:50), 21-38 nm (75:25); Energy temp (CdS): 100 °C; Calcination band gap: 3.71 eV (25:75), 3.49 eV temp (ZnO-CdS): 500 °C for 3 h (50:50), 3.35 eV (75:25) Microwave- ZnO-Graphene Zn(NO3)2·6H2O, PVP, Microwave power (ZnO/ZnO- Star-like (ZnO), flake-like sheets Electrocatalyst [141] assisted oxide (GO) HMTA GO): 300 W, 100 oC for 1 h/45 (GO); Particle size: 550 nm (ZnO); hydrothermal min; Drying temp (ZnO/ZnO- Surface area: 34.3 m2/g (ZnO-GO) GO): 85 oC https://biointerfaceresearch.com/ 4278 https://doi.org/10.33263/BRIAC123.42614292 Method of Composite Precursor/Reactant Synthesis condition Properties Applications Reference synthesis Microwave- Ag–ZnO Carbinol, Zn(Ac)2.2H2O, Microwave power (ZnO): 900 W, Spherical (ZnO-5 min), hexagonal Sensor to [142] assisted AgNO3, CTAB, NaBH4 5, 10 & 15 min; Microwave (ZnO-10 min), spike-like (ZnO-15 biomolecules hydrothermal power (Ag-ZnO): 900 W, 20 min min), spike-like (Ag-ZnO-20 min); Particle size: 400-450 nm (ZnO); Length & thickness of Ag-ZnO was 1 µ & 30-40 nm; Energy band gap: 3.72 eV (Ag-ZnO) Microwave- CTAB, Zn(Ac)2.2H2O, Reaction time: 30 min, Autoclave Rod-like stacked into a Photocatalyst in the [143] assisted AgNO3, o Na2O2 mole ratio; temp: 200 C, Microwave chrysanthemum-like (flower) degradation of RhB hydrothermal CTBA:Zn:Ag = 1:1.5:0.05) irradiation: 300 W, 200 oC for 3, structure; Crystallite size: 4.1, 3.3, 4.5 & 6 h, Drying temp: 80 oC, 3.2 & 3.4 nm (ZnO, Ag-ZnO-3, 4.5 Calcination temp: 300 oC for 2 h & 6 h); Energy band gap: 3.21, 3.17, 3.14, 3.12 eV (ZnO, Ag-ZnO-3, 4.5 & 6 h); Surface area: 1.05, 6.71, 6.40, 6.74 m2/g (ZnO, Ag-ZnO-3, 4.5 & 6 h) Sol-gel Al-ZnO Zn(Ac)2.2H2O, Al (NO3)3 (0, Reaction temp: 80 oC for 1 h; Rod-like (ZnO), spherical (Al-ZnO); Sensor to NH3 gas [144] 3, 6, 9 & 12% %wt) Drying temp: 100 oC; Calcination Crystallite size: 78.8, 49.6, 29.9, temp: 600 oC for 4 h 35.0 & 52.4 nm (Al-ZnO-0, 3, 6, 9 & 12%) Microwave- ZnO-N Zn(Ac)2.2H2O, NaOH, urea Reaction time (ZnO/ZnO-N): 10 Flower-like with array of nanorods Photocatalyst in the [145] assisted (5%) min; Microwave power Particle size (Length): 1.2 - 1.7 µm degradation of Rh-B hydrothermal (ZnO/ZnO-N): 800 W, 100 oC for (ZnO), 2.0 - 2.4 µm (ZnO-N) & Cr (VI) 1 h Microwave Co-Mn-ZnO Zn(Ac)2.2H2O, Reaction temp: 70 oC; Microwave Spherical; Particle size: 20–40 nm; [146] solvothermal Mn(Ac)2.4H2O, power: 100%, 190 oC, 25 min; Surface area: 45.8-56.4 m2/g (Co- Con(Ac)2.4H2O, Drying temp: freeze drying Mn-ZnO), 39.8 m2/g (ZnO); Crystallite size: 19-22 nm (Co-Mn- ZnO), 22 nm (ZnO) Sol-gel Zr-Cr/ZnO Zn(NO3)2·6H2O, Aging conditions: 2 h at room Hexagonal nanoplates and nano- Catalyst [147] Cr(NO3)3·9H2O, oxalic acid temperature, 75 °C for 2 h and polyhedron shapes 120 °C for another 10 h in an oven; Calcination temp: 400 °C for 6 h https://biointerfaceresearch.com/ 4279 https://doi.org/10.33263/BRIAC123.42614292 Method of Composite Precursor/Reactant Synthesis condition Properties Applications Reference synthesis Microwave- Mn-ZnO Mn(Ac)2·4H2O (1, 5, & 10 Reaction temp: 30 oC; Microwave Rod-like; Particle size (diameter): Sensors [148] assisted wt%), Zn(Ac)2.2H2O power: 300W, 95 oC for 10, 20 & 600-50 nm hydrothermal 30 min; Drying temp: 60 oC for 24 h; Calcination temp: 500 oC for 2 h. Microwave Zn(Ac)2.2H2O, Reaction temp: 70 °C; Microwave Spherical shape, flower-like (5-25 [149] solvothermal Mn(Ac)2·4H2O (1, 5, 10, 15, power: 600 W, 2.45 GHz, 200 °C mol %); Particle size: 30–35 nm to synthesis 20, 25 mol % Mn) for 25 min; Drying mode: freeze 15–25 nm (0-20 mol %.), 20–25 nm drying (25 mol %); Crystallite size: 22, 21, 18, 17, 18, 16, & 19 nm (0, 1, 5, 10, 15, 20 & 25 mol %); Specific area: 28, 30, 24, 20, 21, 17 & 19 m2/g (0, 1, 5, 10, 15, 20 & 25 mol %) Microwave Co-ZnO Zn(Ac)2.2H2O, C4H6CoO4, Reaction time: 1 h; Microwave Crystallite size 18 – 28 nm [150] assisted method Urea, Ethylene glycol temp: 80 oC; Calcination temp: Energy band gap: 2.24-3.26 eV 300 °C for 1 h Spherical Microwave Zn(Ac)2.2H2O, diethylene Microwave power: 250 oC for 25 Hexagonal; Crystallite size: 8.9-9.9 Electroluminescent [132] assisted method glycol (DEG), Co(II)(Acac)2, min; Calcination temp: 400 oC; nm, 16–19 nm (calcined sample); material for polymer oleic acid Drying temp: 80 °C Particle size: 15–35 nm; Energy light emitting band gap: 2.79-3.23 eV diodes Microwave- AL-ZnO, Ga- Zn(Ac)2.2H2O, AlCl3·6H2O, Reaction time: 1 h; Microwave Spherical; Particle size: 100 nm Conducting material [151] Assisted ZnO, Al-Ga-ZnO Ga(NO3)3·xH2O, diethylene power: 2.45 GHz, 1500 W, 200 (Ga-ZnO), 250 nm (Al-ZnO), ∼85 for Transparent Synthesis glycol (DEG) °C for 30 min; Drying temp: 60 nm (Al-Ga-ZnO) conducting oxides °C for 1 h; Calcination temp: 450 (TCO) coatings °C for 1 h Reflux method Cu-ZnO Zn(NO3)2·6H2O, CuCl2.2H2O Reflux time (ZnO): 3 h; Reflux Cube-like, maize corn seed-like, Photocatalyst in the [152] time (Cu-ZnO): 3-9 h; Drying rod-like (ZnO), maize corn seed-like degradation of temp (Cu-ZnO): 80 °C. (Cu-ZnO); Crystallite size: 28 nm methyl orange (MO) (ZnO), 20 nm (Cu-ZnO); Particle https://biointerfaceresearch.com/ 4280 https://doi.org/10.33263/BRIAC123.42614292 Method of Composite Precursor/Reactant Synthesis condition Properties Applications Reference synthesis size (diameter & length): 30–35 nm & 90 nm (Cu-ZnO); Energy band gap: 3.38-3.46 eV (Cu-ZnO) Solvent-thermal Fe3O4/ZnO C6H5Na3O7·2H2O, CH3- Reaction temp (ZnO): 40 oC for 3 Spherical; Surface area: 22.30 m2/g Photocatalyst in the [153] technique COONa, FeCl3·6H2O, h; Drying temp (ZnO & (composite with 10% Fe3O4), 27.52 degradation of 4- diethylene glycol (DEG), composite): 70 oC for 3 h; m2/g (composite with 20% Fe3O4), Nitrophenol (4-NP) ethylene glycol (EG), Microwave temp (composite): 20.10 m2/g (composite with 30% tetraethyl orthosilicate 160 oC for 15, 30, 60 min, Fe3O4); Energy band gap: 3.21-3.24 (TEOS) eV Solvothermal CeO2-ZnO Zn(Ac)2.2H2O, CeCl3·7H2O, Autoclave temp: 200 °C for 10 h; Chain-like structure; Crystallite size: Gas sensor and in [154] Na3C6H5O7, Calcination temp: 500 °C for 2 h 9.5 nm; Surface area: 40.9 m2/g, diagnosis of diabetes and chemical detection Microwave Fe-ZnO Zn(OAc)2.2H2O, NaOH, Microwave power: 140 W for 2 Energy band gap: 3.20-3.24 eV; Anti-reflector [155] method Fe(NO3)3·9H2O min (ZnO); Drying temp: 200 °C Particle size (diameter): 17.4-11.29 coating for 1 h (ZnO); Drying temp: 120 nm (Fe-ZnO) °C for 4–5 min (Fe-ZnO); Calcination temp: 300 °C Microwave- Zn(NO3)2·6H2O, Microwave power: 700 W, 5.8– Rod-like, hexagonal prism; Useful in biomedical [156] assisted Fe(NO3)3·9H2O (0, 1, 3, 5, 7 6.2 MPa, 20 min; Drying temp: Crystallite size: 130 nm (5% Fe), 60 applications hydrothermal & 10% mole), 60◦C by 18 h; pH = 9.5 nm (7% Fe), 30 nm (10% Fe), 60 nm (< 5% Fe); Particle size: 392- 296 nm (0-10% Fe) Hydrothermal Zn-Al-Gd Zn(Ac)2.2H2O, Reaction time: 3 h; Autoclave Mixture of urchin-like and rod-like [157] method Al(NO3)3·2H2O, temp: 160 °C for 24 h; calcination structures; Crystallite size: 67-47 Gd(NO)3·6H2O, NH4OH temp: 600 °C; pH = ~9 nm Hydrothermal Eu-ZnO Zn(Ac)2.2H2O, Reaction time: 20 min; Autoclave Spherical (undoped & doped); Optoelectronic [158] method Eu(CH3CO2)3·xH2O (1, 3 & temp: 150 °C for 8 h; Drying Crystallite size: 26 nm (undoped), devices 5 wt%) temp: 80 °C for 24 h, pH = 10 24, 15, & 18 nm (1%, 3% & 5% Eu); Energy band gap: 3.18 eV (undoped), 3.05, 3.00, & 2.94 eV (1%, 3% & 5% Eu) Single-step ZnO-Carbon dots Zn(NO3)2·6H2O, C6H8O7, Reaction time: 1 h; Heating temp: Spherical Photocatalyst for [159] aerosol process 500, 550, 600 & 650 oC; CO2 reduction Annealing temp: 600 oC for 3 h https://biointerfaceresearch.com/ 4281 https://doi.org/10.33263/BRIAC123.42614292 Method of Composite Precursor/Reactant Synthesis condition Properties Applications Reference synthesis Microwave- Al2O3-ZnO Commercial ZnO NPs, Grinding time: 10 min; Particle size: 20-30 nm (ZnO), Photocatalyst in the [160] assisted Aluminium triisopropoxide Microwave temp: 500 W, 2.45 degradation of (0.5, 1.0 & 1.5 g) GHz, 70 oC for 5 min; Aging methylene blue time: 3 h; Drying temp: 110 oC (MB) for 2 h https://biointerfaceresearch.com/ 4282 https://doi.org/10.33263/BRIAC123.42614292 3. Conclusions The multi-functionality of ZnO nanomaterials continues to grow as the developments of new methods to their synthesis are explored. These new methods allow for the control of the morphology, which subsequently affects their properties in producing innovative devices for its various applications. The improvement of microwave technology has made it easy to synthesize different desirable morphologies of nanomaterials with speed and purity. This technology is applied to different synthesis methods (solvothermal, hydrothermal, and sol-gel), it was possible to obtain desirable morphologies as the reaction mechanism could be controlled. Besides the heating mechanism, other factors such as the type of precursor, surfactants, alkaline source, annealing temperature, and dopants affect the morphological structure. Funding This research was funded by Universiti Malaysia Sarawak, Tun Openg Chair, with research grant code: F07/TOC/1738/2018. Acknowledgments The authors would like to thank the funding provided by Universiti Malaysia Sarawak, Tun Openg Chair, with research grant code: F07/TOC/1738/2018. We also acknowledge the contribution of colleagues from the Faculty of Resource Science and Technology (FRST), Geochemistry Laboratory, and Analytical Laboratory, Universiti Malaysia Sarawak. Conflict of Interest The authors declare no conflict of interest. References 1. Jiang, J.; Pi, J.; Cai, J. The advancing of zinc oxide nanoparticles for biomedical applications. Bioinor. Chem. Appls 2018, 1-18, https://doi.org/10.1155/2018/1062562. 2. Chaudhary, S.; Umar, A.; Bhasin, K.K.; Baskoutas, S. Chemical sensing applications of ZnO nanomaterials. Mater. 2018, 11, 1-38, https://doi.org/10.3390/ma11020287. 3. Sarmah, K.; Pratihar, S. Synthesis, characterization and photocatalytic application of iron oxalate capped Fe, Fe-Cu, FeCo, and Fe-Mn oxide nanomaterial. ACS Sustain. Chem. 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