Recent Advances in Tailoring Nano-Architecture of Zno Nanomaterial’s Using Different Fabrication Routes

Akbar S, Anwar A, Umm-i-Kalsoom UI and Bashir S

Published on: 2023-06-02

Abstract

Unique properties and the ease of fabrication of ZnO nanomaterial make this material an extremely interesting candidate for applications in field emission devices, piezoelectric nano power generators, biosensors, photovoltaic cells, and heterojunction diodes. Due to its unique properties such as good thermal and chemical stability, easy modification of electrical conductivity and high excitation binding energy, ZnO nanomaterials have been considered as potential applicant in opto-electronic devices. By controlling nano architecture of ZnO materials, thermal and electrical properties of these materials can be altered. Applications of this material has risen dramatically in recent years, which has promoted scientists and researchers to develop novel synthetic approaches and to modify existing fabrication routes to synthesize ZnO nanomaterials of unique morphology and nanoarchitecture. This review article provides details about the synthetic procedures adopted to synthesize ZnO nanomaterials of specific morphology. This study also elaborates the morphological features and physiochemical properties of these materials affected by manufacturing process.

Keywords

Zno; Nanomaterials; Morphology; Electronic Property; Semiconductor

Introduction

ZnO (zinc oxide) is an abundantly occurring, inexpensive, non-toxic, chemically stable, and biologically safe promising binary semiconductor. It is a naturally n-type semiconductor with radiative wide bandgap energy E_g ~ 3.37 eV, easily tuned from 2.3 to 7.9 eV with the peak emission from ultraviolet to the visible region of electromagnetic spectrum [1]. It also exhibits E_b~60 meV excitation binding energy higher than many other wide bandgap semiconductors like GaN and SiC [2]. Thus, it can withstand large electric fields and high power operations. Holding these specific electronic and optical properties, ZnO is considered as an ideal candidate for applications in optoelectronic industry such as photocatalyses, transparent conductive devices, and light emitting diodes [3-5].

The most common crystalline structure of ZnO is wurtzite structure with lattice spacing value of 0.3252 nm [6]. In the simplest form, ZnO adopts tetrahedron geometry with a non-centrosymmetric crystal structure responsible for its piezoelectric and pyroelectric properties [7]. ZnO materials with piezoelectric properties are used in acoustic wave resonators, acoustic-optic modulators and nanogenerators [8,9]. Coupled semiconducting and piezoelectric properties, ZnO nanomaterials are extremely important and hold several key advantages in energy harvesting applications [10].

Another important property of the ZnO materials is the green luminescence which appears due to point defect [11, 12]. This optical property of ZnO materials can be tuned by the incorporation of other transition metals such as Cu (copper), Mn (manganese) and Ni (nickel) [13,14]. Due to this property, ZnO is considered as an ideal candidate for biomedical imaging. It is also the most reported metal oxide used for gas sensors. It has shown excellent sensitivity towards CO, H₂, O₂, and CH₂O [15-18]. Due to its multifunctionality, ZnO sensors can also be operated in optical-resistive mode and have shown great sensitive response for O₃ detection [19]. ZnO nanoparticles show high efficiency toward light scattering in UV region and have been used in topical application such as sunscreens against free radical reactions at skin surface [20,21]. In vitro, sun protections factor was greatly improved in the presence of nanosized ZnO material [22]. This material is also known for its antifungal, antibacterial and other biological activities [23-26].

ZnO crystalizes in the wurtzite structure similar to GaN but, in contrast, its availability in bulk single crystal and reduced concentration of extended defects has stimulated many research groups to focus on its semiconducting properties. However, instable electronic conductivity of ZnO semiconductor has been a major hindrance toward its application in electronic devices [27]. ZnO naturally shows n-type conductivity and to improve its optoelectronic properties, efforts are being made to induce the p-type conductivity. Band gap tuning in ZnO materials can be achieved by alloying with MgO or CdO [28,29]. On the other hand, p-type conductivity of ZnO materials can be achieved by doping of IB, IIB, and VA column elements [30-32]. p-type when achieved using phosphorus (P), antimony (Sb), and arsenic (As), compromised the conductive stability of ZnO materials.

Nitrogen (N) is the most suitable dopant and various methodologies have been reported for the synthesis of N-doped ZnO materials [33-36]. Another approach to modify the electronic and optical properties of ZnO materials is by controlled addition of intrinsic defects [37]. Oxygen vacancy (V_o) has been the most important defect identified in the metal oxide materials. Numerous studies have reported a direct relation of V_o with optical, electronic, and charge transport properties of ZnO materials [38,39].

As a unique functional material, ZnO has capacity to adopt various morphologies depending upon its fabrication route. The main focus of the current global research is tuning of physical and chemical characteristics of a nanomaterial by altering their nanoscale dimensions and porosity. It has been observed that shape, size, and porosity of a nanomaterial have great influence on its physical and chemical characteristics. Nanoporosity offers a large surface area, which enables increased efficiency of surface catalysed chemical and physical processes. Additionally, nanoporosity creates advanced features, which allow tailored properties for different applications. ZnO nanowires, nanofibres and nanorods have shown high surface to volume ratio as compared to thin films [40]. Similarly, nanowires with smaller diameter showed higher sensitivity toward gas detection as compared to nanowires with large dimeter [41]. Different crystal facets are exposed in nanomaterials exhibiting different morphologies, thus, changing their sensitivity and selectivity. ZnO nanocombs exhibited higher sensitive response toward NO₂ molecules as compared to nanobelts [42]. Tremendous efforts are being made to design and control the nanoarchitecture of the ZnO materials using various synthetic approaches, which in turn can modify their optical, electronic, ferromagnetic, and surface properties [43-51].

Synthesis techniques used for ZnO nanomaterials can be broadly categorized into gas phase synthesis and solution phase synthesis. Gas phase synthesis includes chemical vapour deposition (CVD), physical vapour deposition (PVD), vapour phase transport, thermal evaporation, and thermal decomposition methods. On the other hand, solution phase synthesis includes hydrothermal process, sol-gel method, chemical solution route, and electrochemical method. Various novel synthetic routes adopted for the fabrication of ZnO nanomaterials are discussed in detail in this paper.

Chemical Vapor Deposition Method (CVD)

Chemical synthesis route using precursors in gas phase at elevated temperatures is called chemical vapour deposition (CVD) method. The deposition of materials from CVD method has been emerged as a major processing technique that is essential for growth and development of advanced technology [52]. Using CVD method, high quality ZnO nanomaterials have been deposited on a variety of substrates under different operating conditions (Table 1). Generally, in CVD method, a substrate is typically exposed to single or multiple volatile precursor solutions, which decompose/react at the substrate surface to produce solid deposits. A carrier gas is used to remove any volatile side-products from the reaction chamber. Depending upon the initiation of chemical reaction and operating conditions, CVD method is classified into 1)- low pressure CVD, 2)- ultra high pressure CVD, 3)- aerosol assisted CVD, and 4)- microwave plasma assisted CVD. By changing the operating conditions in CVD, structural tuning of ZnO nanomaterials has been reported in the literature. Interest in the fabrication of ZnO nanotubes and nanowires has increased dramatically. In this regard, a large number of studies have reported to fabricate nanotubes and nanowires of varying length and diameter using CVD method. A selection of CVD methods used to fabricate ZnO nanomaterials along with their morphological features are described in Table 1.

Table 1: CVD methods used to fabricate ZnO nanomaterials.

Precursors	Substrate	Carrier Gas	^Tdep(^oC)	Morphology	Diameter(nm)	Length (µm)	Ref
Zn(C₅H₇O₂)₂· xH₂O	PAA membrane	O₂	500	Nanotube	100-300	Tens of microns	[53]
O₂ and (C₂H₅)₂Z_n	Pt coated silicon	Ar	500	Nanorods	~ 100	~ 4	[54]
ZnO/Zn	Si	Ar	1300	Nanotube	30-100	> few tens	[55]
ZnO	Si	Ar	1380	Nanorods	70-300	Oct-70	[56]
Zn	SiO₂/Si wafers	Ar	750	Nanowires	100	Up to several micron	[49]
ZnO, Zn, graphite powder mixture	Saphire	O₂/N₂	945	Nanowires	50-60	∼2	[57]
ZnO: graphite powder	------	O₂/N₂	550	Microwires	30000	------	[58]
Zn	Si	O₂/Ar	650	Nanowires	100-200	Up to several micron	[16]
Zn	Alumina plate	O₂/Ar	582-617	Nanorods	~100	-------	[19]
(C₂H₅)₂Z_n	Fused quartz	N₂	450	Film	------	-------	[41]
ZnO and graphite powders	Cu	Ar	1000	Nanocombs	------	-------	[59]

T_dep: deposition temperature, PAA: porous anodic alumina, Zn(C₅H₇O₂)₂·xH₂O: zinc acetylacet-onate hydrate, (C₂H₅)₂Zn: diethylzinc, Ref: reference.

CVD method has also been used for the production of 3D ZnO nanomaterials. For example, flower shaped and comb like nanostructures have been produced using Si as a substrate and Cu sheet in later using the same Ar as a carrier gas [59,60]. CVD is an effective chemical technique also used for the production of p-type ZnO films, where doping of ZnO nanomaterials has been carried out to engineer their optical and structural properties. Thin films are deposited on the substrate surface when heated at different temperatures. Various studies have reported the fabrication of p-type ZnO films using CVD method are given in Table 2.

Table 2: p-type ZnO materials produced using CVD method.

Source material	Dopant	Substrate	Carrier Gas	^Tdep(^oC)	Ref
Diethyzinc and NO	NO	Glass	N₂	200-500	[61]
Zinc diacetylacetonate	HAuCl₄/Silver acetylacetonate/ copper (II) acetylacetonate/ (Al(NO₃)₃	Silica coated glass	N₂	250-450	[62]
Diethylzinc	AsH₃	Si wafers	O₂	450–600	[63]
Zinc triflate	Aluminium acetylacetonate	Glass	N₂	600	[64]
ZnO and metallic zinc	NH₃	Saphire	NH₃and H₂	650-800	[65]

In CVD deposited ZnO materials, structural control was achieved by altering operating conditions, such as deposition temperature, time, and substrate material. Deposition temperature and the substrate material were found to directly affect the film orientation as degree of orientation increased with increasing deposition temperature [66]. Another study reported, a change in the alignment of nanorods with respect to deposition time. Nanorods grown for longer time are less aligned as compared to those grown for only 20 min [67]. Substrate material also has a direct impact on the alignment behaviour of growing nanostructures. Saphire and platinum coated Si substrate have shown to promote the growth of highly aligned nanoneedles and nanorods [54]. Another study showed that morphology and orientation of CVD grown nanostructures was also affected by the distance of substrate from Zn source material regardless of the nature of substrate. Nanowires grown close to the source material are more aligned as compared to those grown far from the source [68]. Alignment in CVD grown ZnO nanotubes has also been achieved through template assisted technology. Highly aligned nanotubes were grown using porous alumina as a template [53].

Well oriented ZnO nanostructures like; nanowires, nanobelts, and nanorods have great potential towards application in energy harvesting devices like piezoelectric nano-generators as well [9]. All these examples show that CVD is highly effective method for controlling the orientation and alignment of nanostructures. Higher degree of orientation in CVD grown nanowires was reported as compared to those grown by hydrothermal method [68]. Two different substrates viz. silicon and FTO coated glass were used to grow nanowires. In both cases, the substrates were first coated with ZnO seed layer to promote the growth of nanowires.

Thermal Evaporation Method

Thermal evaporation method is regarded as one of the top conventional physical vapour deposition (PVD) techniques. The method is simple and inexpensive which involves thermal evaporation of a material at high temperature, which is then condensed on to a cold substrate [69,70]. This method has been used to synthesize ZnO nanotubes, nanowires, nanobelts, and nanorods. A list of studies using different starting materials and deposition temperatures are given in Table 3. In most of the studies, silicon wafers were used as a substrate and argon is used as a carrier gas for the outflow of volatile side-products.

It has been observed that during thermal evaporation, temperature difference inside the tube furnace has dramatic effect on the morphology of As-synthesized ZnO nanomaterials. Lee et al [48]. reported the formation of mixture of nanobelts and nanorods at high temperature zone (900-1030 ^oC) within tube furnace. At low temperature zone (350-450 ^oC), ZnO nanowires were obtained. The width/diameter of the nanostructures was gradually reduced with respect to the temperature in furnace as shown in figure 1. Nanorods and nanobelts were found to be more optically active as compared to nanowires [56].

Figure 1: SEM images of the ZnO nanomaterials grown in (a) high-temperature zone (900-1030 ^oC) and (b) low-temperature zone (350-450 ^oC). Reprinted (adapted) with permission from Lee et al. (2003), Copyright (2003) Elsevier [56].

Table 3: Thermal evaporation methods used for the synthesis of ZnO nanostructures.

Precursors	Substrate	^Tdep(^oC)	Morphology	Ref
Ball milled ZnO powder	Si	1380	NRs, NBs, NWs	[56]
Zn and ZnO powder	Silicon wafers	1300	NTs	[71]
Zn metal	Si wafers	750	NWs	[49]
Zn powder	Si	600-650	NCs	[45]
ZnO films and metallic Zn	ZnO film coated saphire	900	TPs	[43]

NRs: nanorods, NBs: nanobelts, NWs: nanowires, NTs: nanotubes, NCs: nanocombs, TPs: tetrapods, Ref: reference

Table 3 gives a systematic comparison of different studies reported for the synthesis of ZnO nanomaterials of various morphologies. From comparison, it is clear that thermal evaporation technique involves high temperatures for sublimation of precursor materials. Low synthetic temperatures for the fabrication of ZnO nanomaterials are registered in the presence of catalytic materials. However, it has been demonstrated that purity of the nanomaterial is compromised in the presence of a catalyst, which ultimately affects the material properties. Feng et al. reported synthesis of ZnO material with tetrapod like morphology using catalyst free hydrothermal process (Figure 2) [43]. Huang et al [36]. documented a novel evaporation method that involves low temperature synthesis of ZnO nanomaterials without involvement of a catalyst.

Figure 2: SEM images of ZnO nanostructures with tetrapod-like morphology at different magnifications. Reprinted (adapted) with permission from Feng et al. [43] Copyright (2010) Elsevier.

Vapour Transport Method

To date, chemical vapour transport (CVT) method has been found as a simple and versatile technique used for the preparation of ZnO nanostructures. In this method, typically a solid material is volatilized in the presence of a transport agent and then directed to deposit on specific target substrates. For this purpose, precursors are loaded in a quartz boat and substrate is normally placed downstream of the precursor materials. The whole assembly is then placed inside a tube furnace, which is then heated to a specific temperature under constant flow of inert gas [72]. A number of studies reported synthesis of ZnO nanomaterials using vapour transport method are compared in Table 4.

Table 4: list of studies reported synthesis of ZnO nanostructures using vapour transport method.

R	Substrate	Carrier Gas	T(^oC)	Morphology	Dia (nm)	Ref
ZnO/graphite powder	Si	Ar	1200	NWs	45	[73]
ZnO/graphite powder	Au coated Si	Ar	900-925	NWs	80-120	[74]
ZnO/graphite powder	Si	Ar and O₂	450	NRs	300	[50]
ZnO/graphite powder	Si	Ar and O₂	600	NWs	40	[50]
ZnO/Zn powder mixture	Si	Ar	1300	NTs	~60	[55]

NRs: nanorods, NWs: nanowires, NTs: nanotubes, Ref: reference

Table 4 gives a comparison of deposition conditions reported in different studies. In most of the studies, a mixture of ZnO and graphite powder is used as starting materials and Si substrates are being reported as the most used substrates. In CVT, temperature of the furnace plays an important role during the synthesis of nanomaterials. Nanowires and nanorods of varying lengths and diameters have been produced at different temperatures. For example, single crystal ZnO nanowires of diameter 45 nm were prepared when the precursor material was heated to 1200 ^oC for 1 h under constant flow of argon gas [73]. Huang [65]. reported the catalytic growth of highly crystalline ZnO nanowire of diameter 80-120 nm and length of 10-20 µm when the starting materials were heated to 900-925 ^oC for 5-30 min under a constant flow of argon gas. High temperature and long reaction time promoted the growth of nanowires with large diameter. The same group reported synthesis of ZnO nanowires of large diameters varying from 20-150 nm and length of about 2-10 µm. In this study, the starting material was heated to 880-905 ^oC and growth time was 2-10 min [75]. In another study, ZnO nanorods of diameter 300 nm and 1.0 µm length were synthesized by heating the precursor materials at 450 ^oC. Furthermore, heating the precursor material at 600 ^oC promoted the growth of nanowires of diameter 40 nm and length of 2-6 µm [50]. Another study reported synthesis of ZnO nanotubes with a regular polyhedral shape (diameter 60 nm, length few tens µm) using mixture

of ZnO powder and metallic Zn as a starting material, which was heated to 1300 ^oC under constant flow of Ar [55].

The method has also been used to synthesize other morphologies like nanobridges and nanonails as shown in Figure 3. In a study, a mixture of ZnO, In₂O₃, and graphite powders was used as a precursor material. Low In₂O₃ratio, high temperature (1000 ^oC) and air pressure (2.0 Torr) favoured the formation of ZnO nanobridges, whereas slightly low temperature and air pressure favoured the formation of ZnO nanonails [46].

Figure 3: Scanning electron microscopic images of the ZnO nanobridge variations (a, b), ZnO nanonails flower top view (c) and nanonails side view (d). Reprinted (adapted) with permission from Lao et al. [46]. Copyright (2003) American Chemical Society.

Hydrothermal Process

In hydrothermal process, precursor solutions are subjected to specific temperature for specific period of time, which results in the production of highly crystalline material [76]. The hydrothermal method is more economical and includes simple experimental steps as compared to CVD and CVT methods. By adjusting the experimental conditions during hydrothermal process, morphology and crystallinity of as synthesized nanomaterial can be controlled which in turn can greatly affect the optical properties of the material [77]. The method has been used to generate ZnO nanomaterials of various morphologies ranging from 1D nanoparticles to 2D nanorods, and nanowires, and even more sophisticated 3D flower shape structures. Details are given in table 5.

In hydrothermal process, a variety of precursor materials are heated at specific temperature using specific solvents to produce ZnO nanomaterials of various morphologies. Precursor materials used in various studies and resulting nanomaterials are compared in Table 5.

Table 5: List of precursor materials used in hydrothermal methods reported for the synthesis of ZnO nanostructures.

Precursors	T(^oC)	Morphology	Ref
[Zn(CH₃COO)₂·2H₂O]		NRs	[78]
zinc acetate dihydrate	160	NRs	[79]
Zn(OH)₄^2-	140	NWs	[80]
Zinc acetate hydrate, urea and ethylene glycol	120	NFs	[44]
Ethylenediamine with zinc acetate and NaOH	80-100	NPs	[77]
Zinc acetate and triethanolamine	Low T	NPs	[81]
Zinc chloride and ammonium hydroxide	150	NPs	[82]
Zinc nitrate and methenamine	90	NTWs	[83]

NRs: nanorods, NWs: nanowires, NFs: Nanoflowers, NPs: Nanoparticles, NTWs: nanotowers

Crystalline nature of ZnO nanomaterials greatly affects the optical and electronic properties of these materials [77]. Hydrothermal process produced highly crystalline ZnO nanomaterials. For example, Foo et al. [69] synthesized single crystal ZnO nanorods using hydrothermal process. The study reported the use of four different solvent; methanol, ethanol, isopropanol and 2-methoxyethanol, and their influence on structural and optical properties of ZnO nanorods. The highest bandgap (3.21 eV) was reported for ZnO nanorods that grew from the 2-methoxyethanol seeded layer. In another study, hydrothermal process was used to synthesize single crystalline ZnO nanorods with wurtzite structure while utilizing very simple experimental conditions. The diameter was observed to be 100-800 nm and length of several micrometer [84]. In another study, zinc acetate dihydrate was hydrolysed in the presence of water to synthesize ZnO nanorods with wurtzite crystallographic structure [79].

Hydrothermal process has also been used for the preparation of 3D nanostructures. The porous flower like ZnO nanostructures shown in figure 4 were obtained by following two step procedure. In the first step, zinc carbonate precursor was obtained after stirring (pH 5.0) and autoclaving (120 ^oC for 12 h) a solution zincacetate hydrate, urea and ethylene glycol. In the second step, Zn carbonate precursor was annealed at 500 ^oC in a muffle furnace for 2 h to obtain flower like ZnO nanostructures. The obtained ZnO nanostructures were used in gas sensors application [44].

Figure 4: Scanning electron microscopic images of (a) flower-like zinc carbonate precursor and (b) resulting porous flower-like ZnO nanostructures obtained after annealing. Reprinted (adapted) with permission from Gu et al. (2011), Copyright (2011) Elsevier [44].

Pal. et al. [68] reported hydrothermal process at low temperature induced synthesis of ZnO nanomaterials of different morphologies with high reproducibility. For their preparation, precursors like ethylenediamine with zinc acetate and NaOH were heated at 80-100 ^oC and nanoarchitecture of ZnO materials were simply achieved by adjusting the initial and final pH values. Another study reported the growth of ZnO nanotower bundles on electrodeposited ZnO films using simple hydrothermal process. For this, ITO plates coated with ZnO films were immersed in aqueous solutions of zinc nitrate and methenamine in an autoclave unit and heated at about 90 ^oC for 3 h. Size and shape of ZnO nanomaterial were controlled by growth temperature and nature of the substrate used [83].

Hydrothermal process has also been used to produce ZnO nanoparticle coating on metals. Shen et al [72]. reported a simple low temperature hydrothermal process to coat Ag (silver) Nanowires with ZnO nanoparticles. For this, zinc acetate was used as a main precursor while triethanolamine was used as a nucleation agent and a binder of ZnO nanoparticles to Ag nanowires.

Pulsed Laser Deposition/Ablation

Pulsed laser deposition (PLD) has emerged as a result of the most recent technological development in physical deposition techniques, where a high power pulsed laser is used to decompose a targeted material to produce plasma plume to be deposited on a substrate material [85]. It is an advanced technique which enables investigators to synthesize nanomaterials with controlled morphology and orientation. Extensive studies have been documented on the physics of PLD. For instant, it has been demonstrated that during PLD process, pressure of background gas, distance between substrate and target, laser energy, and nature of target substrate have a pronounced effect on oxide film formation, its morphology, and epitaxial growth which can directly affect the device properties and efficiency [86].

PLD grown nanoparticles are free of surfactants and their colloidal solutions are very stable. Colloidal suspensions of ZnO nanoparticles were produced using pure zinc metal as a target material in the presence of deionized water/isopropanol/ acetone at room temperature. The ablation in the presence of deionized water and isopropanol yielded spherical nanoparticles of 14-20 nm in diameter, whereas ablation in acetone produced spherical and platelet shaped particles with 100 nm in diameter and 40 nm thickness, respectively [87]. PLD has also been used to produce tetrapod like ZnO nanostructures with root diameter in the range of 200-500 nm and tip with an average diameter of about 30 nm by heating ZnO and pure metallic zinc at 900 ^oC [43]. This technique was also used for doping in ZnO nanomaterials. High-pressure pulsed-laser deposition method was used to prepare phosphorus doped ZnO nanowires. In this method, ZnO and P₂O₅ were heated at 830 ^oC in the presence of gold as a catalyst [88].

Sol-Gel Method

Sol-gel method is a wet chemical route to synthesize high surface area nanomaterials. Being very affective in surface modification, sol-gel provides new approach to coat a substrate surface with stable polymeric network of nanomaterial. The technique involves; hydrolysis of precursors and - polycondenstation of the hydrolysed species. The method offers great degree of control of reaction conditions under which precursors are polymerized, which in turn can drastically affect the structure and morphology of synthesized nanomaterials [89].

Sol-gel is also considered to be the most suitable method for thin film depositions. Generally, precursor material is dissolved in solvent to prepare sol which is then coated onto a substrate using different coating techniques; dip coating, spin coating etc. Different studies reported sol-gel method for the synthesis ZnO thin films and nanostructures are given in Table 6.

ZnO thin films with wurtzite structure fabricated onto a glass substrate showed excellent sensitivity toward alcohol vapours [90]. In another study, ZnO nanostructured thin films showed enhanced optical and conductivity properties [91]. Thin films deposited on quartz substrates were observed to exhibit excellent optoelectronic properties [97]. ZnO nanostructures were fabricated onto a SiO₂/Si substrate under acidic and neutral environment. Nanostructures produced in neutral solutions exhibited rod-like morphology. However, mixed shape structures (rod-like and platelets) were obtained in acidic solution. Rod like nanostructures showed stronger luminescence as compared to platelets [92]. Another study reported synthesis of ZnO nanofibers by sol gel method using simple and inexpensive electrospinning techniques [93].

Table 6: Studies showing sol-gel method for the synthesis of ZnO thin films and nanostructures.

Precursors	Solvent/stabilizer	Coating method	Morphology	Ref
Zinc acetate	Diethanolamine	Dip coating	Thin films	[90]
Zinc acetate dihydrate	2-Methoxethanol and monoethanolamine	Spin coating	Thin films	[91]
Zinc nitrate hexahydrate	Water/ methenamine	Immersion in aqueous solution	Rod like and platelets	[92]
Zinc acetate and poly vinyl alcohol (PVA)	Water	Electrospinning	Nanofibers	[93]
Zinc acetate dihydrate	2-methoxyethano/ monoethanol-amine	Spin coating	Thin films	[94]
zinc nitrate hexahydrate	distilled water	Gel-coating	Nanoparticles	[95]
zinc acetate	ultrapure water	-------	Nanoparticles	[96]

Electrochemical Methods

Electrochemical methods are very versatile, simple, and inexpensive. These are the least studied methods used for the fabrication of ZnO nanomaterials. Only a few studies have reported the use of anodization, galvanostatic cathodic deposition, and potentiostatic cathodic deposition of ZnO nanomaterials.

Zheng et al [86]. used one-step electrochemical deposition method on porous ZnO/Al membrane system by anodization process to further use in optical, electric and gas sensitive nanodevices. Likewise, He et al. [87] also used anodization method for the deposition of ZnO thin film. In this method, electro-polished zinc foil was used as an anode and graphite plate/Zn foil was used as a cathode. The electrodes were maintained at a distance of 50 nm. Anodization was carried out at an applied voltage of 9.7 to 16.7 V in an electrolyte solution of water, hydrofluoric acid and methanol for 0.5 to 30 min. Bingqiang et al [88]. produced ZnO nanoneedles by electrochemical deposition method. Galvanostatic cathodic deposition of nanoneedles was carried out at 0.9 mA current on Au coated silicon substrate. Zinc sheet was used as an anode. Zinc nitrate aqueous solution was used as an electrolyte. In another study, ZnO thin films were simply electrodeposited on ITO plates. Zinc chloride and potassium chloride solutions were used as electrolytes. Electrochemical cell consisted of platinum wire counter electrode, Ag/AgCl reference electrode, whereas ITO plates were used as working electrodes. Deposition was carried out at -0.6 V at 60 °C for 0.5 h [83].

Other Methods

There are a number of other synthetic procedures adopted for the synthesis of ZnO nanomaterials. Chemical solution route has been used to fabricate ZnO nanomaterials with flower like structures. ZnO flower like nanostructures were grown on glass substrate using aqueous chemical growth method. For this synthesis, Zn(NO₃)₂.6H₂O and C₆H₁₂N₄ were heated at 95 ^oC for 1-20 h. The nanomaterial was tested for ozone sensing [98]. ZnO microflowers were obtained using zinc nitrate hexahydrate, hydroxyl amine hydrochloride, and NaOH solution refluxed at 90 ^oC for 20 min. These nanostructures were used for biomedical applications [99]. The same study also reported the formation of ZnO nanoparticles when zinc acetate dihydrate solution in methanol and aniline were used as a precursor solution. The solution was refluxed at about 65 ^oC for 6 hrs to produce nanoparticles.

Another synthetic route adopted in various studies to fabricate ZnO nanomaterials is thermal decomposition method. In thermal decomposition or thermolysis, main precursor material is chemically decomposed using heat to produce desired nanomaterial. The method has been used to produce ZnO nanomaterials of different nanoarchitecture. A study reported using Zinc acetate dihydrate as precursor material which was heated to 300-450 ^oC in a quartz crucible to synthesize ZnO nanowires and nanorods [100]. Flower like ZnO nanostructures were prepared by thermolysis of the zinc-ethylenediamine [101]. ZnO nanobundles were synthesized by thermal decomposition of crystals of zinc oxalate dihydrate by heating at 400 ^oC in a furnace [102]. In another study, ZnO nanoparticles with wurtzite structure of high crystallinity were synthesized by thermal decompositions of ZnC₂O₄·2H₂O at 450 ^oC for 30 min [103]. ZnO quantum rods were prepared by a thermal decomposition of zinc acetate in organic solvents in the presence of oleic acid [104].

Another PVD technique is sputter deposition process. For industrial applications, sputtering process is often the most favoured vapour deposition technique, commonly used for the synthesis of single crystal nanowires. As compared to thermal evaporation, sputter deposition offers high deposition rate for fabrication of dense films with enhanced adhesion quality. A large variety of metals and composite materials can be sputtered than evaporated. In sputtering process, ions from an inert gas are generated, which are then accelerated and strike the target material. This results in knocking off the target atoms and deposit onto the substrate [105]. A study reported the deposition of single crystal ZnO nanowires on the Cu/Ti/Si substrate under an atmospheric environment. The average diameter of ZnO nanowires determined to be 55 nm and 45 nm depending upon their growth time ranging from 5 to 30 min [106].

Atomic layer deposition (ALD) is a type of vapour phase technique where substrate surface is exposed to different precursors introduced sequentially. ZnO nanorods and nanotubes were fabricated on a variety of substrates using ALD. ZnO nanostructures were grown on ZnO-coated substrate using diethyl zinc and trimethyl aluminium as precursor materials. Pure ZnO and 1% Al doped ZnO nanorods were obtained at 10 h growth time. For the synthesis of nanotubes, growth time was extended to 20 h. The diameter was found to vary according to the reactant concentration, temperature, and pH [107].

Precipitation methods are also available in the literature to synthesize ZnO nanorods and nanoparticles which have shown great potential toward humidity sensor applications. For the synthesis of ZnO nanorods, sodium oxalate and zinc sulphate solutions were mixed at room temperature. The precipitates obtained were calcinated at temperatures above 450 ^oC [108]. For the synthesis of ZnO nanoparticles, NaOH was mixed in C₂H₅OH and Zn(CH₃COO)₂ was mixed in ethanol separately followed by stirring at 70 ^oC. The two solutions were mixed and stirred in an ice bath for 30 min. ZnO particles were precipitated out and centrifuged followed by washing to remove ions [109].

Conclusion

The fabrication routes and synthetic examples discussed in this article give a brief overview of scientific approaches most commonly used for the preparation of ZnO nanomaterials. Chemical methods with simple steps at minimal cost and time are drawing researchers’ interest, which will lead to develop future ideas in designing of ZnO nanomaterials of specific properties according to demand, requirement, and applications. Chemical vapour deposition technique and chemical vapour transport are the most widely used techniques for the synthesis of ZnO 2D nanomaterials like nanorods, nanowires, and nanotubes. Physical features like nanowire diameter and length are easily controlled by controlling the furnace temperature. Hydrothermal process involves low temperature synthesis of nanomaterials as compared to CVD and CVT. It has been widely used for the synthesis of highly crystalline ZnO nanomaterials. For deposition of ZnO thin films, sol gel method is widely applicable. Thin films deposited via sol gel method showed excellent optoelectronic properties. Electrochemical way of synthesizing nanomaterial is the least studied method in literature. This synthesizing method will more likely to develop new ideas about formation of nanomaterials of different architecture with versatile properties that will lead to the development and incorporation of new Nano devices. By comparing different synthetic procedures, we have shown that choice of synthetic route is often driven by material property required for specific application purpose.

Declaration

Funding and/or Conflicts of interests/Competing interests

The authors have no competing interests to declare that are relevant to the content of this article.

No funding was received for conducting this study.

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