A Review on Synthesis and Optoelectronic Applications of Nanostructured ZnO

Nanostructured ZnO has gained a lot of interest as a suitable material for various applications, especially sensing, energy conversion, and storage. ZnO nanostructures can be synthesized in several ways. It is one of the materials that can be prepared in a variety of morphologies including hierarchical nanostructures. This review article presents a review of current research activities on the growth of ZnO Nanorods. The article covers various water-based routes of synthesis and is further characterized by the type of substrate used for the growth. The growth factors involved in the hydrothermal and chemical bath deposition methods are discussed. These factors include the variety of precursors, time, temperature, and the seeding method employed. At the end, applications such as gas sensing and improvement in Opto-electric properties are discussed.


INTRODUCTION
Zinc Oxide (ZnO) is a wide-band-gap (3.37 eV) semiconductor of the II-VI semiconductor group. The native doping of the semiconductor due to oxygen vacancies or zinc interstitials is n-type (Ozgur et al., 2005). The application of these nanorods depends on factors like crystallinity and surface morphology, orientation, optical, and electrical properties. The synthesis of one-dimensional (1D) nanostructures of ZnO, has attracted much attention because ZnO is a potential candidate for Gas Sensing (Cittadini et al., 2015;Zhang et al., 2020), UV sensing (Fang et al., 2009;Zhou et al., 2020), and Immuno-sensors for Viruses (Sanguino et al., 2014;Han et al., 2016), in LEDs (Lupan et al., 2010;Mohammad et al., 2020), in solar cell applications such as an electron transporting layer in Perovskite Yun et al., 2020), and Dye Sensitized Solar cells (Thambidurai et al., 2012;Jung et al., 2020) etc.

Major Advantages for 1D Zinc Oxide Nanostructures Include
(i) Their growth can be carried out on a wide variety of substrates (including polymers which can be amorphous but offer the advantage of being flexible or/and biodegradable). (ii) There is a wide variety of synthesis routes for the growth of these nanostructures, each offering a certain niche advantage (Ray et al., 2020). (iii) The common defects or stress elevators in these nanorods are oxygen vacancies (Ozgur et al., 2005). The nanorods can relieve the stresses by elastic relaxation (Huang et al., 2001). (iv) Unique properties such as high bulk electron mobility or direct band equivalent to the energy of UV (ultraviolet) light. Other favorable properties include a wide band gap, good transparency, room-temperature luminescence, and high electron mobility (Roza et al., 2020).
(v) Quantum confinement can be exploited by the use of heterostructures of ultrathin layers in a single nanorod exhibiting atomically abrupt interfaces (Park et al., 2003). (vi) ZnO has a tunable optical band gap, which can be altered by changing the composition, morphology, size etc. (vii) Another distinct advantage is that ZnO is considered a biosafe and biocompatible compound, as a trace amount of Zinc is present in the human body (Jayaprakash et al., 2020).
Based on the above facts, ZnO is a very important material. Its 1D nanostructures have been proven to give improved performances in a wide variety of applications. This review article focuses on the growth methods of 1D ZnO nanorod arrays in terms of seeding layer, alignment, and the aspect ratio of the grown 1D nanorod arrays.

VARIOUS ROUTES FOR GROWTH OF ZINC OXIDE NANOROD STRUCTURES
There are many different routes for the manufacturing of 1D nanostructures of ZnO. The present review deals with the solution-based methods only. The solution-based synthesis routes for ZnO nanorod arrays under discussion are: (i) Hydrothermal Growth (ii) Chemical Bath Deposition (iii) Electrochemical Deposition (iv) Pulsed LASER Deposition (v) Spray Pyrolysis As the present study deals with the orientation and the aspect ratio of the ZnO nanorods, the nature of the substrate plays an important role in the growth process. Most substrates have a large difference in lattice parameters with ZnO, so a seed layer is deposited on the substrate before the growth of nanorods is carried out. The seed layer required for the growth can be deposited in the form of ZnO thin film of various thicknesses by sputtering, spin, or dip coating and/or drop casting, Atomic layer deposition etc. The following section provides a detailed description of various solution-based methods to grow aligned nanorod arrays of ZnO via solution-based methods.  Wang et al. (2006) Silicon (100) The substrate is placed in reaction media having zinc chloride and ammonia at 90°C for 4 h Gas sensing (NH 3 , CO and H 2 ) application, and promising results for nanorods grown at less than 100°C Solís-pomar et al.
Silicon (100) Nanorods grown on ALD-ZnO films, the thickness of this ZnO seed layer was varied: (a) 40 nm, At low thickness or short term ALD, the nanostructures formed are seed-like and aligned, while if the thickness of the seed layer increases, there is also a significant increase in surface defects which lead to the formation of nanorods (b) 80 nm (c) 120 nm (d) 180 nm Hussain et al. (2015) Al doped zinc oxide glass substrate Nanorods with various growth times of 1, 2, 3, and 4 h The growth time effects the density of the nanorods, the increasing time results in denser growth Sarkar and Basak. (2011) Glass Nanorods were grown using precursor zinc acetate, CTAB and HMTA in following conditions: (a) Without excess zinc; The presence of zinc powder has not significantly affected the morphology of the nanorods. The excess zinc although shows improved UV emission for 30 mM of excess zinc used (b) 5 mM excess zinc (c) 15 mM excess zinc (d) 30 mM excess zinc Excess zinc was introduced by using zinc metal powder Gaddam et al. (2015) Phynoxy alloy (co 40%, Cr 20%, Ni 16%, 7% Mo) ZnO nanorods are grown in zinc nitrate and HMTA at different growth temperatures. (a) 60°C, (b) 70°C, (c) 80°C, and (d) 90°C The length and diameter of the ZnO nanorods increase with increasing temperature Jiaqiang et al. (2006) No substrate CTAB and zinc powder were treated hydrothermally at 182°C for 24 h Nanorods ranging from 40 to 80 nm in diameter are obtained Rai et al. (2009) No substrate CTAB and zinc nitrate were treated hydrothermally at 200°C for 10 h Nanorods ranging from 100 ± 10 nm in diameter and 900 ± 100 nm in length were obtained Frontiers in Materials | www.frontiersin.org April 2021 | Volume 8 | Article 613825 2

Hydrothermal Method
Hydrothermal synthesis is a high-pressure aqueous solutionbased growth process performed in a sealed steel vessel (autoclave). The process involves crystallizing substances at a high temperature and pressure.
The hydrothermal process offers the advantages of providing uniform growth (Qiaoping et al., 2020;Sutradhar et al., 2020). The growth temperature plays an important role in the overall crystallinity and aspect ratio of the grown nanorods. The commonly used growth temperatures range between 80 and 100°C, while the synthesis duration usually ranges from 1 to 4 h (Abdelouhab et al., 2020;Ghosh et al., 2020). The growth substrates commonly include silicon wafer or coated glass slides. Table 1 enlists the reported work about hydrothermal synthesis of ZnO nanorods. Hassanpour et al. (Hassanpour et al., 2017) compare the growth on both glass and Si (100) wafer substrate; the seed layer was drop casted on both the substrates, then masked by PMMA which was patterned by Electron Beam Lithography followed by annealing for 10 min at 90°C. The growth occurs in 50 mM solutions of zinc nitrate hexahydrate and 50 mM hexamethylenetetramine (HMTA) in deionized water at 85°C for 2 h. They evaluate the effect of the seed layer on the growth and report that annealing at 300°C provides the best seed layer for growth, as shown in Figures 1A-C.

Growth of Zinc Oxide Rods on Si Wafer Substrate
The growth of ZnO nanorods on a silicon wafer requires a seamless seed layer to overcome the lattice difference between the ZnO and the silicon wafer. An unbroken seed layer is required to grow vertically aligned nanorods with fewer defects (Tang et al., 2020). The deposition techniques like sputtering and atomic layer deposition offer an unbroken layer. Sputtering also has the advantage of in situ monitoring and allows for control of the thickness of the thin film of ZnO being deposited as the seed layer.

Growth of Seed Layer via Spin Coating
In spin coating, the substrates were coated, by spinning the seed solution on the sample (Al-Asedy and Al-Khafaji, 2020). The viscosity and overall lamination of the layer are dependent upon the solution used, and the temperature at which the seed layer was annealed. Srivastava et al. (Srivastava et al., 2015) reported spin coating for making a seed layer by spinning solution (0.5 M of zinc acetate in 20 ml of isopropanol with dropwise addition of 0.5 M of ethanolamine followed by room temperature aging and then stirring it at 50°C for 1 h) on a 1 × 1 cm Si wafer at 3,000 rpm for 30 s. The spinning process was repeated three times followed by an annealing step (80°C for 30 min) after every deposition and was finally annealed at 1100°C for 1 h. The growth of nanorods occurs when the substrate was placed vertically in 0.05 M Zinc Nitrate and 0.05 M Hexamethylene Tetramine (HMTA) at 80°C for 4 h, as shown in Figures 1D,E.

Growth of Seed Layer via Sputtering
In sputtering, the target purity and the chamber pressure, i.e. the pressure of the argon gas, plays an important role in the deposition of the seed layer (Sannakashappanavar et al., 2020). The role of chamber pressure is that it effects the sputtering efficiency, while for deposition of ZnO, if the target used is of metallic zinc, the oxygen environment will ensure the deposition of ZnO, moreover, it counteracts the most common defect of oxygen vacancies in the growth of ZnO. Mbuyisa et al. (Mbuyisa et al., 2015) reported the use of metallic zinc target (Purity of 99.995% and Diameter 50.8 mm) and an oxygen environment in the direct current magnetron sputtering system for the preparation of ZnO film on Si (100) substrate at room temperature. The substrate was cleaned by ultra-sonication and pre-sputtered for 2 min to clean it further in the argon atmosphere. The deposition occurs at a sputtering power of 60 W for 15 min with three different chamber pressures (0.4, 1.2, and 8 Pa) followed by an in-situ annealing process (400°C for 2 h at 0.4 Pa oxygen pressure). For nanorod growth, they followed J. X. Wang et al. (Wang et al., 2006) (Figures 2A-C) and growth took place in 0.05 M zinc chloride and ammonia at pH 11 at 90°C for 60 min followed by air drying of the samples. The characterization of the samples reveals that with an increasing chamber pressure (0.4-12 Pa) the nanorods become highly oriented and their aspect ratio decreases.

Growth of Seed Layer via Atomic Layer Deposition
Atomic layer deposition is a type of Chemical Vapor Deposition (CVD), that deposits atoms layer by layer. The seed layer formed by this process has fewer defects as one atomic layer is being deposited allowing for the atoms to arrange precisely. The

Growth of Zinc Oxide Rods on Glass Substrates
The glass substrates include ordinary glass or some coated glass slides (ITO, FTO). The coated glass slides offer the distinct advantage of being conductive and as such the growth of the nanorods can be done on these substrates and provide an electrical application as they can work as electrodes in various devices (Umar et al., 2009) (solar cells or sensors).

Growth of Seed Layer via Drop Casting
Drop casting, requires the use of less sophisticated equipment, as the solution is deposited on the substrates and then dried either by heating or leaving it to air dry. Using this method the thickness of the layer cannot be accurately controlled and can affect the overall crystallinity and growth of the Nanorods (Aé et al., 2010;Farooqi and Srivastava, 2019). Q. Hussain et al. (Hussain et al., 2015) deposited the seed layer using a solution of 5 mM Zinc acetate dihydrate and 5 mM anhydrous KOH via drop casting. The growth of ZnO Nanorods occurs in 25 mM Zinc Nitrate 0.025 M Hexamethylene Tetramine (HMTA) and Frontiers in Materials | www.frontiersin.org April 2021 | Volume 8 | Article 613825 4 0.1 M poly-ethylenimine (PEI) at 90°C for 1-4 h. When the growth time increases from 1 to 2 h the haze ratio increases (78.21%) but after a successive increase in the growth time the roughness also increases, causing a decrease in haze ratio, as shown in Figure 3.

Growth of Seed Layer via Sputtering
S. Sarkar and D. Basak (Sarkar and Basak, 2011) deposited a ZnO seed layer of 40 nm on the ordinary glass substrate using the DC reactive magnetron sputtering technique. The growth of the nanorods occurred in 10 mM Zinc acetate di-hydrate, 10 mM HMTA, and 1 mM CTAB solution at 80°C for 12 h. The addition of Zinc metal powder during growth causes an excess of zinc metal (5, 10, 15, 20, and 30 mM) for enhancing the ultraviolet emission properties, as shown in Figure 4 (Dwivedi et al., 2020).

Other Substrates
Common substrates include silicon wafer or glass slides and polymers. The polymer substrates have additional properties of being lightweight, flexible, or biodegradable depending upon the application.

Growth of Seed Layer via Sputtering
Gaddam et al. (Gaddam et al., 2015) used Phynox alloy substrate for the growth of ZnO nanorods. The substrate was electrically conductive and biocompatible. The seed layer of 100 nm was deposited by DC reactive-magnetron sputtering and the growth was carried out in 25 mM zinc nitrate and 25 mM hexamethylenetetramine solution at growth temperatures 60°C, 70°C, 80°C, and 90°C for 4 h. An increase in the length and diameter of the ZnO nanorods was observed with an increasing growth temperature, as shown in Figure 5A-D.

Growth of Powder
The growth of ZnO Nanorods can also be performed without a substrate and the rods grown are collected as precipitates at the end of the procedure (Bhushan et al., 2019). The growth of these nanorods without a seeded substrate requires the use of surfactant, which allows  the c-axis growth which is paramount in the formation of ZnO nanorods. Jiaqiang et al. (Jiaqiang et al., 2006;Akshaykranth et al., 2020) and Prabhakar Rai (Rai et al., 2009)

Chemical Bath Deposition
Chemical Bath Deposition (CBD) requires only solution containers and substrate mounting devices. It involves two steps, nucleation and particle growth, and is based on the formation of a solid phase from a solution (Abdulrahman et al., 2020;Alshamarti and Alkhayatt, 2020). The substrate is immersed in a solution containing the precursors. This method depends upon parameters like Bath temperature, pH of the solution, molarity, and time. Boukous et al. (Boukos et al., 2007) reported the growth of ZnO nanorods on Si (111) substrate (with and without deposition of ZnO seed layer) and glass slides. The seed layer of ZnO (100 nm) was deposited using an electron beam in the presence of Zinc Metal foil and formamide for 1-24 h, and temperature was varied (40-80°C). Their results indicate that the most defects occur in the bare Si wafer (without seed layer) while all other samples show lower defects at higher temperatures (Ungula and Swart, 2019). Table 2 gives a summary of the chemical bath deposition of ZnO nanorods.

Growth of Seed Layer via Pulsed LASER Deposition
Yu et al. (Yu et al., 2017) used a N-type Si (100) wafer and deposited thin ZnO films (for 10, 20 and 30 min) via PLD, as shown in Figures 6D-F. For nanorod's growth, they used Zinc metal foils as a source of formamide solution and the bath temperature was kept constant at 65°C for 24 h and annealed at 600°C for 1 h.

Growth of Seed Layer via Dip Coating
The dip coating method, as the name suggests, requires the substrates to be dipped in the seed solution for a short period. The dipping action is performed for a short time and the substrate is taken out and is either air dried, heated, or a blow of N 2 inert gas is used after every successive dipping step followed by annealing. Senthil et al. (Senthil et al., 2013)

Growth of Seed Layer via Spin Coating
Zhaolin Yuan (Yuan, 2015) produced a Gold/Zinc Oxide nanorods array based schottky barrier diode by evaporating Gold (Au -150 nm) under pressure (2.2 × 10-3 Pa) on top of ZnO nanorods, as shown in Figures  7A,B. The seed layer is deposited by spin coating the seed layer solution (5 mM Zinc Acetate and ethanol) on ITO substrate and annealed at 350°C for 20 min. The substrates were then immersed in 0.05 M Zinc nitrate and HMTA in a flask at 93°C for 1 h, air drying at 50°C.

Other Substrates
Chemical bath deposition of ZnO nanorods on polymer or graphite substrates has also been reported. The shift from conventional substrates is due to the requirement of some specific property that these substrates provide because of the focused application for the ZnO nanorods. Zainelabdin et al. (Zainelabdin et al., 2010) reported using PEDOT:PSS substrate and coated two polymers by spin coating. TFB (poly (9,9dioctylfluorene-co-N-(4-butylpheneylamine) diphenylamine) is spin coated first and then PFO (poly (9,9-dioctyl-fluorene)) polymer is spin coated on TFB, as shown in Figure 7C. ZnO nanoparticles are then spin coated on top and placed in a chemical bath of 0.15 M Zinc Nitrate hexahydrate and 0.01 M HMTA as the growth medium at 50°C. Shabannia et al. used Polyethylene Naphthalate (PEN) (Shabannia and Hassan, 2014) and Porous Silicon (PS) (Shabannia and Hassan, 2013) as substrates for the growth of ZnO nanorods. RF magnetron sputtering is employed to deposit the ZnO seed layer of thickness 70 nm and the growth occurs by placing the substrates vertically in 0.05 M zinc nitrate hexahydrate and 0.05 M HMTA at 80°C, as shown in Figure 7D.
Shabania (Shabannia, 2016) reported the effect of growth time by varying the time (2, 3.5, 5, and 8 h) which affected diameter, varying from 22 to 55 nm from 2 to 8 h, while the nanorods formed at 5 h had better surface morphology), as shown in Figure 8.
B. Yuliarto et al. (Yuliarto et al., 2017) developed a SO 2 sensor on alumina substrate with a Silver inter-digitated array electrode. The substrate was dip coated in seed solution (0.2 M Zinc Acetate tetrahydrate in ethylene glycol and Diethanol Amine) and dried at 120°C for 15 min and annealed at 450°C for 30 min. The bath precursor for growth included 0.015 M Zinc Nitrate hexahydrate in 3:1 Deionized Water:ethanol and 0.02 M Hexamethylene Tetramine (HMTA). The Seeded substrate was immersed in the bath for 1 h at 90°C and Annealed at 450°C for 1 h. Two-Times CBD sample is obtained by immersing the substrate obtained after annealing in the bath for 16 h and then annealed again. For a three time CBD sample the process of immersing the substrate was carried out again for 16 h at 90°C, as shown in Figure 9A.
O. F. Farhat et al. (Farhat et al., 2015) reported a flexible Low power UV sensor and used Teflon (PTFE) as a substrate and RF Magnetron Sputtering (Power 150 W, Ar Pressure 5.5 mTorr, Temperature 200°C). After deposition of the seed layer, the substrate was annealed at 200°C for 1 h. The growth occurred by placing this annealed substrate vertically in 0.05 M Zinc Nitrate and 0.05 M Hexamethylene Tetramine (which were heated separately at 80°C before mixing them). The bath temperature was maintained at 95°C for 3 h for the growth of the Nanorods.
Z. Zhang et al. (Zhang et al., 2013) reported growth of ZnO Nanorods, on graphite substrate using a ceramic ZnO target to deposit the seed layer on the substrate using RF magnetron sputtering (Power 120 W and Ar Pressure 3.0 Pa) and growth was performed in equimolar (25 mM) of Zinc Acetate Nanorods and Hexamethylene Tetramine at 95°C for 3 and 5 h, as represented in Figures 9B,C.

Electrochemical Deposition
For electrochemical deposition, the basic requirement for the electrolytic path to be completed is that the substrate should be conductive (Wang et al., 2019;Mohamed et al., 2020;Zhou et al., 2020). Gao et al. (Gao et al., 2007) reported the growth of ZnO Nanorods by Electrodeposition, on a Glass substrate exclusively for a seed layer by 20 deposition cycles. T. H. C. Son et al. (Son et al., 2014) studied the effect of current densities for different periods on the growth of ZnO Nanorods on ITO substrate. The precursors for the growth were 5 mM Zinc Nitrate hexahydrate and 5 mM Hexamethylene Tetramine (HMTA). They reported a strong influence of the current density on the morphology and orientation. At a low current, the rods that formed were tilted and hexagonal in shape while the increase in current density resulted in rods that were aligned with an obelisk shape. When a sequence of low and high current density was introduced, the features became controllable. A. Rokade et al. (Rokade et al., 2016) reported the controlled growth of ZnO Nanorods in Chlorine medium rather than the acetate or nitrate medium conventionally used. They deposited the nanorods at two different bath temperatures [1) 70°C, 2) 80°C] in 5 mM Zinc Chloride and 50 mM Potassium Chloride. The effect of temperature was observed, as the increase in temperature improved crystallinity, and led to an increase in diameters of the rods formed, as shown in Figures 9D-H. Table 3 presents a summary of reported work on electrochemical deposition of ZnO nanorods.

Microwave Irradiation
Denthaje Krishna Bhat (Bhat, 2008) reported a microwave assisted growth method for the synthesis of ZnO Nanorods, using a molar ratio of 1:4 of Zinc Acetate to hydrazine Hydrate. The mixture is irradiated (Power 150 W, Time 10 min) leading to clear white precipitates, which are filtered, washed, and dried (60°C for 4 h) in a vacuum, as shown in Figure 10A.

Pulsed LASER Deposition
In this method, a pulsing LASER hits the target and then forms a plume, which is then condensed on a substrate. Qi et al.

Sol-Gel Reflux Method
The sol-gel reflux method allows for better temperature control than the commonly used Chemical bath deposition. The reflux allows for the temperature to be maintained in the case of the sol-gel reflux method. The growth results in precipitates of ZnO Nanorods being formed. The difficulty can be observed in growth on the substrate as the substrate has to be placed in the bottom of solution container for growth to occur (Amakali et al., 2020;Babar et al., 2020). Li et al. (Li et al., 2011) reported a synthesis method that yields ZnO nanorod growth in the solution of 2 mM zinc acetate dodecyl benzene sulfuric acid sodium salt (DBS), dimethyl benzene, and hydrazine monohydrate (diluted by ethanol) stirred for 1 h, then boiled, and then refluxed for 5 h. The precipitates are filtered, washed, and dried (at 70°C in air), as shown in Figure 10D.

Other Techniques
Other techniques for the synthesis of ZnO nanorods include spray pyrolysis, thermal evaporation, and chemical vapor deposition. One Step pyrolytic synthesis of ZnO nanorods involves the precursor zinc acetate, whose pyrolysis occurs at 550°C (Huang et al., 2015). Pneumatic spray pyrolysis technique with precursors: ZnCl 2 and thiocarbamide has been established and leads to the formation of thinner ZnO nanorods (Dedova et al., 2007). A thermal evaporation process on a patterned Si wafer (110) which has silver (Ag) is a template for growth (Panda and Jacob, 2009), at 700°C for 2 h on Nickel based buffer layer (Sputtered layer and Spin coated) on Silicon wafer and sapphire substrate (Kuo et al., 2012). Chemical Vapor Deposition, for growth of ZnO nanorods has also been established using ZnSe particles as the seed layer (Guo et al., 2006). Catalyst free growth on Si substrates has also been reported (Wang et al., 2008). Vertically aligned epitaxial ZnO nanorods have been grown using a Diethyl zinc and oxygen environment on seeded Alumina substrate (Park et al., 2004), as shown in Figure 11.

MODIFICATION OF ZINC OXIDE NANORODS
The elementary ZnO properties can be improved by modifying the surface or doping the ZnO Nanorods with different entities. The improved properties result in various improved properties of the device based on Nanorods. The photochemical properties of ZnO nanorods are affected by sensitizing the rods with lead sulfide (PbS) nanoparticles. The ZnO nanorods are synthesized on Soda lime and FTO (Fluorine doped Tin oxide) coated glass substrates, and synthesis occurred via the reflux method and PbS thin films were deposited by the SILAR (successive ionic layer adsorption and reaction) method (Khot et al., 2015). Liquid Junction solar cells based on ZnO/Al:ZnO/ZnS or ZnSe/CZTS (Copper-zinc-tin-sulfide), lead to efficient photon absorption and shorter pathways for charge transport (Akram et al., 2016). A one-dimensional nanoarray of ZnO/CuIn x Ga 1−x Se 2 allows for better carrier separation and transport without using a toxic CdS buffer layer (Akram et al., 2015).

Sensing Applications
Ultraviolet (UV) light sensing can be performed using ZnO Nanorods, when the light shines on ZnO, whose direct band gap is 3.3eV, equivalent to UV energy, it allows the system (ZnO) to  At low density (a) titled hexagonal column shaped nanorods were formed. While at high densities (b, c) vertically aligned obelisk shaped ZnO nanorods were formed (b) 0.6 mA/cm 2 (c) 1.2 mA/cm 2 For different periods (10-40 min) Rokade et al. (2016) • ITO (indium doped tin oxide) The growth medium was chloride based (zinc chloride, potassium chloride). The pH of the bath was kept at 6.0 while temperature of bath was varied At the cathodic potential of -0.75 V due to oxygen reduction good quality deposition is observed. As observed by SEM image, the average diameter of the ZnO nanorods increases with increased temperature (a) 70 (b) 80 FIGURE 11 | FESEM images of Aligned ZnO nanorods (Park et al., 2004).  (Bhat, 2008); SEM image of Seed layer (B) and ZnO nanorods (C) ; (D) SEM image of ZnO nanorods formed (Li et al., 2011).
Frontiers in Materials | www.frontiersin.org April 2021 | Volume 8 | Article 613825 12 experience an increase in conductivity. The increase in the conductivity correlates to the intensity of the UV light. Gas sensing can also be performed using ZnO Nanorods. The gasses that oxidize can be sensed, as, when they chemically interact they induce a change in conductivity, just as in the case of UV, and are therefore sensed. A volatile organic compound sensor, that works at room temperature by doping ZnO Nanorods with Palladium in an electrochemical deposition technique, using the precursors Zinc chloride, potassium chloride, palladium acetate, dimethyl sulfoxide (DMSO) has been reported (Öztürk et al., 2016). Similarly, Hydrogen gas can be sensed by loading Platinum-Gold (Pt-Au) nanoparticles on ZnO Nanorods. The loading was done by ultra-sonicating the nanorods in presence of Pt-Au nanoparticle suspension. This loading improves the gas sensing properties of the Nanorods by several orders compared to sensing response observed in pure ZnO or ZnO supported Au and/or ZnO supported Pt (Fan et al., 2017). Core shell nanorods of Copper oxide and Zinc Oxide (CuO: ZnO) synthesized using chloride salts of Zinc and Copper, have been reported as a selective sensor (nanorods are deposited on flat silver electrode) with a fast response to selective bicarbonate in the buffer system (Rahman and Asiri, 2015) as shown in Figures 12A,B.

Improvement in Opto-Electronic Properties
The Opto-electronic properties allow for the application of ZnO Nanorods in the field of LEDs etc. Doping of Sodium (Na) on ZnO nanorods grown on Silicon substrates has been reported to improve the optoelectronic properties. The improvement occurs because of the formation of an acceptor, which introduces p-type conductivity (Ye et al., 2017), as shown in Figures 12C-E. The growth of Mn-doped ZnO nanorods on a Porous silicon pillar array (PSPA) is shown in Figure 12F,G, and it can potentially add a red section of the visible emission, improving the color rendering index of ZnO .

CONCLUSION
This paper provides an overview of a variety of solution-based techniques and touches on a few physical techniques that have been developed for fabricating ZnO Nanorods. Every technique offers a certain advantage and has inescapable weaknesses or disadvantages. This paper compares the effect of time, temperature, and various salts on the morphology and structure of the nanorods and summarizes those in tabulated form. If the various factors in these solutions-based techniques are manipulated to the fullest, low-cost growth of ZnO Nanorods is possible, which is highly desirable in future research and applications.