Hydrothermal Synthesis of ZnO Nanorods

Research will be limited to:
- Material: ZnO nanorods
- Method: Hydrothermal synthesis
- Precursors: Zinc nitrate and HMTA
- Substrate: Seeded substrate

Strategy: The objective is to establish the precise chemical mechanisms and optimized growth parameters for ZnO nanorod synthesis using the zinc nitrate/HMTA hydrothermal method on seeded substrates. I will focus on the role of the seed layer morphology on nanorod alignment, the chemical kinetics of HMTA decomposition as a pH buffer, and the influence of precursor concentration and temperature on aspect ratio. This will provide the foundational evidence needed for a reproducible protocol.
Queries:
- ZnO nanorods hydrothermal synthesis zinc nitrate HMTA seeded substrate mechanism
- influence of seed layer thickness on ZnO nanorod alignment hydrothermal
- HMTA decomposition kinetics ZnO nanorod growth zinc nitrate
- optimized growth parameters ZnO nanorods aspect ratio zinc nitrate HMTA
- effect of precursor concentration ratio zinc nitrate to HMTA ZnO nanorods

Executing 5 concurrent searches:
- ZnO nanorods hydrothermal synthesis zinc nitrate HMTA seeded substrate mechanism
- influence of seed layer thickness on ZnO nanorod alignment hydrothermal
- HMTA decomposition kinetics ZnO nanorod growth zinc nitrate
- optimized growth parameters ZnO nanorods aspect ratio zinc nitrate HMTA
- effect of precursor concentration ratio zinc nitrate to HMTA ZnO nanorods

Found 7 papers. Key excerpts:

Assisted-hydrothermal Synthesis and Characterization of Flower-like ZnO Nanostructures — S. López-Romero et al., 2012

Flower-like nanostructures formed by ZnO nanorods were synthesized and
deposited on seeded silicon and glass substrates by a hexamethylenetetramine (HMTA) – assisted hydrothermal method at low temperature (90 o C) with methenamine ((CH3)6N4), as surfactant and catalyst. The substrates were seeded with ZnO nanoparticles. The structure and morphology of the nanostructures were studied by means of x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM) techniques. Influence of the seed nanoparticle on the formation of the flower-like ZnO nanostructures is demonstrated. The influence of the organic oxygenated chains on the crystalline habit during the growth process is also observed.
Keywords: Zinc oxide; Flower-like nanostructures; Seeded growth; assisted hydrothermal synthesis.
1. Introduction
Zinc oxide is one of the most studied oxide semiconductors and recently has attracted
attention due to the possibility of producing several complex nanostructures. These nanostructures might be applied for the fabrication of blue emitting lasers due to their wide band gap (3.37 eV) and high exciton binding energy (60meV), 1 transparent semiconductors, 2 piezoelectric devices, 3 short-wavelength light-emitting devices, 4 blue emitting LEDs, 5 chemical sensors, 6 solar cells, 7 etc. Several methods such as, electrodeposition, 8 evaporation, 9 vapor-liquid-solid (VLS) growth, 10 metalorganic catalyst assisted vapor-phase epitaxy, 11 aqueous thermal decomposition, 12 microwave activated chemical bath deposition (MW-CBD), 13 chemical bath deposition (CBD), 14 hydrothermal-assisted method, 15 etc. have been reported for the production of this kind of nanomaterials. However, hydrothermal-assisted (HTA) method is more convenient over others as it is less expensive with easier composition control, large area deposition, and works at lower temperatures. On the other hand, small changes in any hydrothermal parameters, such as temperature, pH, molar ratio of the precursors, or even reaction time, generate profound influence on the final products .Moreover, using HTA

method, ZnO nanostructures of different morphologies could be synthesized. 16 In this work, we report on the production of flower-like nanostructures of ZnO by a two-step surfactant assisted hydrothermal method on different substrates. The nanostructures were deposited on pretreated silicon and glass substrates by seeding ZnO nanoparticles on their surfaces. It is shown that the pre-treatment of the substrates has a great influence on the growth of the flower-like nanostructures.
2. Experimental 2.1. Materials The chemical reagents used in this study were analytical reagent grade (Sigma-Aldrich) and used as received without further purification. Silicon wafers (Virginia semiconductor, inc.) with a <100> orientation and glass plates were used as substrates.
2.2 Pre-treatment of the substrates The process to obtain a colloidal solution to deposit the ZnO nanoparticle seeds on the substrates surface (ZnO seeded substrates) has been described elsewhere 17 . The procedure is based on the sol-gel method. Briefly, zinc acetate [Zn(CH3COO)2] and cetyltrimethylammonioum hydroxide (CTAOH) were taken as precursor materials. Initially 0.01M of zinc acetate was dissolved in ethyl alcohol and magnetically stirred at 60 o C for 1h. Then, cetyltrimethylammonium hydroxide was mixed into the solution with Zn/CTAOH molar ratio of 1/1.6 and then refluxed at 60 o C for 2h. By direct immersion of silicon and glass substrates into the colloidal solution, the ZnO nanoparticle seeds were deposited onto the substrate surfaces. Subsequently, the substrates were heated in dry air at 300 o C for 12h.
2.3 Preparation of flower-like nanostructures
Zinc nitrate (Zn(NO3)2.6H2O) was used as precursor and hexamethylenetetramine, also called methenamine ((CH3)6N4) as surfactant and catalysts. The precursor solution was prepared by dissolving 3.0 g of zinc nitrate and 2.8 g of methenamine in deionized

water
under vigorous stirring at 50 o C for 1h to form a 0.01 M equimolar solution. Then, the seeded silicon and glass substrates were immersed in this solution at 90 o C for 2h. It was observed that a white ZnO powder precipitated at the flask bottom. Finally, the substrates were thoroughly washed with deionized water and allowed to dry in air at room temperature.
The reaction mechanisms proposed for the hydrothermal synthesis is already reported by J.
Zhung and coworkers 14 . Based on Zhung analysis, the [Zn(OH)4] 2- role is well established and the corresponding chemical reaction for this particular hydrothermal synthesis is as follow:
(CH3)6N4, 90 ºC
Zn(NO3)2 +2H2O Zn(OH)2 + 2HNO3 (1)
Zn(OH)2 ↔ Zn 2+ + 2HO - (2)
Zn 2+ + 2HO - ↔ ZnO + H2O (3)
Zn(OH)2 + 2OH - ↔ [Zn(OH)4] 2- (4)
In reaction (1), Zn 2+ ions are combined with OH - radicals in the aqueous solution to form a Zn(OH)2 colloid through the reaction Zn 2+ + 2OH - → Zn(OH)2 . Later, in the hydrothermal process, the Zn(OH)2 is separated into Zn 2+ ions and OH - radicals according to reaction (2). Then, ZnO nuclei are formed according to the reaction (3), when the concentration of Zn 2+
ions and OH - radicals reaches a supersaturation grade. Finally, the growth units of [Zn(OH)4] 2- radicals are obtained through the reaction (4). The dissolution-nucleation cycle according to reactions (5) and (6), respectively produces:
[Zn(OH)4] 2- ↔ Zn 2- +4OH - (5)
Zn 2+
+ 2OH ↔ ZnO + H2O (6)
3. Characterization
Morphology of the sample was studied using a JEM5600-LV scanning electron
microscope. The single-crystal

Enhanced band edge luminescence of ZnO nanorods after surface passivation with ZnS — Asad Ali et al., 2018

We report on the passivation of surface defects of ZnO nanorods by surface layer deposition. ZnO nanorods and ZnS-ZnO hybrid nanostructures are grown on FTO coated glass substrate by chemical bath deposition method. XRD spectrum of ZnO nanorods shows the preferential growth along the c-axis. SEM analysis confirms the nearly aligned growth of the ZnO nanorods with a hexagonal shape. XPS measurements were performed to confirm the deposition of the surface layer and surface stoichiometry. Room temperature photoluminescence of ZnO nanorods showed two emission bands, viz. the band edge emission and the blue-green emission, with the latter being associated with the defect states arising from the surface of ZnO nanorods. The band edge emission is significantly increased as compared to blue-green emission after ZnS surface layer deposition on ZnO nanorods. The quenching of blue-green emission is explained in terms of reduced surface defects after ZnS deposition. Density functional theory (DFT) calculations are used to understand the mechanisms of defect passivation in ZnS-ZnO nanostructures and we show that the S atoms prefer the O site as compared with the Zn and interstitial sites.
1. Introduction
ZnO nanostructures have attracted considerable attention due to their excellent electrical and optical properties. These properties make ZnO one of the suitable materials for use in nextgeneration optoelectronic devices [1]. ZnO is a nontoxic material and can be synthesized using cost-effective solution process such as Chemical Bath Deposition (CBD) method. ZnO based nanostructures have wide range of applications in a device technology. They have been used in photovoltaics in different device architectures such as in planar and rod like geometries. ZnO nanostructures are also been used in gas and bio sensors, transistors, optically pumped lasers,
light emitting diodes, and piezoelectric devices etc [2-5]. The wide band gap (3.3 eV) and high exciton binding energy (60 meV) at room temperature make ZnO one of the most extensively used semiconductor materials [6]. In recent years, There has been a surge in the investigation of Photoluminescence

of ZnO nanostructures [7]. In most of the studies, two emission bands have been observed [8] such as the near band emission which corresponds to band-to-band transition of the charge carriers and the defect emission which is the result of structural defects of ZnO nanostructures, such as Zn interstitials, oxygen vacancies, dangling bonds at the surface etc. These structural inhomogeneities result in the formation of inter band states. Optoelectronic properties of nanomaterials are greatly affected by the confinement effects and surface defects [9-11]. The latter can cause the defect emission and band bending which results in the low luminescence efficiency of the nanomaterials [12-13]. Many researchers attributed the defect emission band in PL of ZnO nanostructures to the surface states [14], while others have argued that the green emission might be the result of bulk defects, such as oxygen vacancies [15]. Suppressing the surface defects to enhance the ultraviolet (UV) emission of ZnO nanorods can result in better performance of the ZnO based optoelectronic devices.
There are few reports in which the optical properties of ZnO nanostructures have been altered by surface treatments [15-17]. Richters et al. have investigated the PL properties of Al2O3/ZnO nanowire structures [15] where they have found that after coating the ZnO nanowires with Al2O3 the near band emission at low temperatures was enhanced and the deep level emission was reduced. It has also been observed that hydrogenation also results in reduced deep level emission and increased band edge emission [17]. Other surface treatments such as argon ion milling and polymer covering have also been observed to reduced deep level emission and produce increased band edge emission in ZnO nanostructures [18-19]. The reduced defect emission is expected to result in better performance of the ZnO based hybrid nanostructures. Considering the good electronic transport properties of ZnO, it has been used as an electron transport material in hybrid solar devices. However, utilization of the ZnO nanostructures in modern solar cell devices has remained limited due to its surface defects which not only contribute to reduction of Power Conversion Efficiency (PCE) but also lead to degradation of absorber materials [20-21]. As discussed above that one of the

possible ways to reduce defects in ZnO nanostructures is their surface treatments. From the device point of view one needs to select a material for the surface treatment that offers a suitable band alignment in hybrid nanostructures
to facilitate charge transport as well as serve to reduce surface defects in ZnO nanostructures. In this study, we investigate the effect of ZnS surface layer on the photoluminescence and electrical properties of ZnO nanorods, and find substantial enhancement in band-edge emission and suppression of the defect emissions after surface treatment.
2. Experimental Section

2.1. Growth of Zinc Oxide Nanorods
We first washed the FTO substrates in the baths of acetone, methanol, ethanol, and in distilled water sequentially using sonicator. After washing the substrates, we deposited ZnO seed layer using spin coating method. ZnO seed layer solution was prepared using Zinc acetate hexahydarte, ethylene glycol and diethanol amine. First we prepared 0.4 M solution of Zinc acetate in ethylene glycol. We noticed that the solution turned milky after few minute stirring at room temperature [21]. We added Diethonl ammine drop wise in the solution of Zinc acetate until it becomes transparent. We stirred the solution further for 30 minutes and the final solution was spin coated on FTO glass at 2500 rpm for 30 sec. After each spin coat the samples were dried in oven at 250 0 C for 20 minutes. We repeated this process five times and the samples were annealed at 500 0 C for two hours [22].
ZnO nanorods were grown using Chemical Bath Deposition method (CBD). The growth solution was prepared using aqueous solution of zinc nitrate and hexamethylene tetramine (HMTA). First we separately prepared the equimolar aqueous solution of HMTA and Zinc nitrate hexahydrate. After 30 minutes stirring, we pour the solution of HMTA into zinc nitrate hexahydrate solution and the final obtained solution was stirred further for one hour. Substrates were immersed in the growth solution with the support from the Teflon rod, such that the seed layer coated side of the substrates were facing down. The reaction time for the growth of ZnO nanorods was 2 hours at 90 0

Effects of growth conditions on properties of CBD synthesized ZnO nanorods grown on ultrasonic spray pyrolysis deposited ZnO seed layers — K. Mosalagae et al., 2020

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Effects of Reaction Parameters on the Geometry and Crystallinity of Hydrothermally Synthesized ZnO Nanorods for Bio-Fouling Applications — Abderrahmane Hamdi et al., 2023

1. IntroductionIt is well known that biological fouling (or biofouling) is a major concern, especially for the maritime industry. Indeed, biofouling organisms, ranging from bacteria to barnacles, cause numerous technical problems and incurs a heavy economic penalty [1]. Therefore, a variety of methods have been investigated. The use of antifouling coatings is the most common means for the inhibition of micro- and macro-fouling. However, most of these traditional coating solutions use organic biocides and toxic coatings, that may accumulate in the marine environment [2]. Due to the risk of leaching into the aquatic environment and toxicity to aquatic organisms, “green” alternatives to toxic-based technologies are therefore required.Several works highlight different materials to prevent this biofouling effect. Recently, zinc oxide (ZnO) has been widely reported as promising material for preventing the undesirable growth of micro- and macro-organisms [1]. This material has several advantages such as chemical stability and biocompatibility [3]. It is environmentally friendly, inexpensive, and easy to synthesize [4]. In addition, it is a well-known photocatalyst [5] that displays antibacterial [6] and antifouling properties [7]. Moreover, this material is a good candidate for oil-water separation [8].This metal oxide can exist in different forms and shapes, such as in zero dimensional 0D (nanoparticles) [4], one dimensional 1D (nanorods) [9], two dimensional 2D (thin films) [10], or three dimensional 3D (nanoflowers [11], nanosheets [12], nanowalls [13], nanoflakes [14]). Among these nanostructures, zinc oxide nanorods (ZnO NRs) have received widespread attention. These 1D nanostructures have highly active surfaces and large surface-to-volume ratios [15], fewer grain boundaries and defects, and efficient charge transport along the nanorods axis. They also exhibit multiple semiconductor, piezoelectric and pyroelectric properties [16]. Furthermore, ZnO NRs gained considerable interest for marine antifouling applications due to their antibacterial and antifouling activities [7].In the literature, Priyanka Sathe et al. successfully developed sunlight-responsive antifouling ZnO nan

orods coated fishing nets that reduce the abundance of microfouling organisms by three-fold compared to uncoated nets (control) and nets painted with commercial biocidal coatings [17]. Jiyeon Lee et al. [18] reported that ZnO NRs are potentially useful as an adhesion resistant biomaterial. They showed that the cells adhered much more to a flat substrate than to the ZnO NRs.Moreover, Al-Fori et al. [19] showed that coatings containing ZnO NRs prevented marine micro and macrofouling in static conditions. They have reported that the anti-fouling effect was attributed to the reactive oxygen species produced by photocatalysis in the presence of sunlight. In addition, Dobretsov et al. [20] performed toxicity assays on ZnO NRs and spherical ZnO nanoparticles (ZnO NPs). They showed that the lowest toxicity was for ZnO NRs whereas the highest toxicity was observed for ZnO NPs. Due to their excellent properties, a wide range of techniques have been presented to synthesize zinc oxide nanostructures. ZnO NRs have been synthesized by many techniques, such as electro-chemical deposition [21], chemical vapor deposition (CVD) [22], vapor–liquid–solid (VLS) growth [23], etc. However, these methods generally operate at high temperatures (>100 °C) and sophisticated equipment is required. The hydrothermal method was presented as an alternative to preparing ZnO nanorods.This simple method presents an inexpensive low-temperature process (<100 °C) with scalability and high yield [24]. The other advantage is that this method can be applied on different types of surfaces. Depending on the intended application, ZnO nanorods were synthesized on soft and hard substrates, such as polymer [24], fabrics, cotton [25], glass [26], and metal [27]. Several studies have reported the advantages of applying ZnO NRs to stainless steels in the field of medical, food processing, automotive, aviation, and other applications. This product displays a high resistance to corrosion and heat, high durability, high hardness, and fabrication flexibility [28].The purpose of the present research is to study the hydrothermal deposition of ZnO NRs on stainless steel. A static field immersion test was conducted in a

tropical coastal marine environment in Singapore to evaluate the material for its ability to prevent biofouling settlement. To the best of our knowledge, there are few papers reporting the impact of the ZnO seed layer deposited on stainless steel on the formation of vertically aligned nanorods in antifouling applications. 2. Experimental 2.1. MaterialsEthanol, acetone, and methanol Sodium hydroxide (98%), Zinc acetate dihydrate (98%), zinc nitrate hexahydrate (98%), and hexamethylenetetramine (99%), were supplied by Alfa Aesar. Octadecyltrimethoxysilane (ODS) was purchased from Sigma Aldrich. All chemicals used were of analytical grade. In our experiments, we used distilled water to clean surfaces and to prepare solutions. 2.2. Experimental SetupIn this work, we used stainless steel 430 Martensitic SS (X12Cr16) with 0.12% Carbon and 16% Chromium as a substrate in our experiments. These substrates were cleaned in three different baths, namely acetone, ethanol, and water (5 min each in an ultrasonic bath), then dried with N2 gas. In order to prepare ZnO seed layer and ZnO NRs, we relied on the work of Ali et al. [27] with some changes in the synthesis protocol. Figure 1 at the top of the dashed line shows the ZnO seed preparation protocol. Zinc acetate (91 mmol/L) and sodium hydroxide (67 mmol/L) were added separately in methanol (stirring for 15 min). Zinc acetate was used as a source of zinc while sodium hydroxide was used as a source of hydroxyl ions. The solutions were stirred until all of the solids dissolved to give transparent solutions. Then, sodium hydroxide (NaOH) was added drop by drop onto the zinc acetate (Zn(CH3CO2)2·2H2O) at 60 °C, with a frequency of 60 drops per minute. The obtained solution was stirred for 3 h at 20 °C to form a stable and clear solution, preventing the agglomeration of ZnO nanoparticles. For the deposition of the above solution prepared solution on substrates, the chemical bath deposition (CBD) method was applied as it is a simple and low

Deposition Time and Annealing Effects of ZnO Seed Layer on Enhancing Vertical Alignment of Piezoelectric ZnO Nanowires — Taoufik Slimani Tlemcani et al., 2019

1. IntroductionZnO is both a piezoelectric and semiconducting material [1], with an energy band gap of about 3.37 eV. A broad range of applications have been demonstrated for one-dimensional (1D) ZnO nanostructures, for example as a photoconductor [2], field emitter [3], logic gate [4], and waveguide [5]. Recently, the application of ZnO nanowires in energy conversion, for example in dye sensitized solar cells [6] and gas sensors [7], has attracted increasing research interests. These developments have as a goal to conserve and extract energy from the environment by exploring the physical properties of ZnO nanostructures. In recent years, ZnO nanowire (NW) array based nanogenerators (NGs) have shown great potential to convert mechanical to electrical energy by using the coupling effects of the semiconducting and piezoelectric properties of ZnO [1,8,9]. However, to acquire sufficient piezoelectric NG performance for practical applications, further developments of ZnO NWs for mass production require full control of the synthesis process and nanowire morphology, as well as their crystal quality. Currently, there are numerous methods to grow ZnO NWs, such as vapor-liquid-solid processes (VLS) [10], metal-organic chemical vapor deposition (MOCVD) [11], or thermal evaporation methods [12].However, these processes require high pressures and temperatures, as well as acid-resistant environments, which makes them difficult to integrate with standard fabrication methods and future flexible electronics. Thus, new green approaches are crucial for the development of novel nanostructures. Recent solution methods for the synthesis of ZnO NWs have been engaged because of their low growth temperatures, low cost, and the potential for scaling up. Additionally, low temperature solution based growth techniques have been investigated for the formation of ZnO NWs on the surface of materials such as carbon fibers, tows, or fabrics in order to enhance the interfacial strength, excluding weakening of the composite materials during high temperature processing [13]. Hydrothermal synthesis has many merits that can allow the production of uniform and well distributed ZnO NW arrays. Moreover, the diversity of materials into which ZnO NWs can be incorporated and on which they can be grown addresses challenges in multiple

technological domains, such as the military, with the improvement of Kevlar and composite fabrics [14], as well as health care and environmental applications with the development of multifunctional wearable sensors [15]. In this regard, our research group has reported a hydrothermal process for producing arrays of ZnO NWs on conductive substrates below 100 °C [16,17,18,19,20,21,22].In addition, the seed layer is very important for the growth of high quality ZnO NWs, and plays a remarkable role in their properties [18,23,24]. As is largely known, to synthesize continuous and well aligned ZnO NWs, it is necessary to use a ZnO seed layer. Many authors have reported the effect of ZnO seed layer with different thicknesses on the growth of ZnO NWs [23,24,25,26]. These reports showed thicknesses ranging from 20 to 1000 nm, all of which influence the alignment of ZnO NWs; parameters which affect the alignment are not entirely known yet, and the study of such parameters still remains a challenging issue for the scientific community. In particular, it is suggested that the enhancement of the surface condition and the interfacial properties of ZnO NWs are key factors in determining nanodevice performance through the creation of a functional gradient between the NWs and matrix, which has been shown to improve load transfer [18,27,28]. Besides the seed layer thickness effect on the growth of NWs, there is an effective method to modify the surface condition of ZnO NWs, which consists of thermal annealing of the ZnO seed layer. In particular, the annealing treatment improves the crystallinity of the ZnO seed layer and the adhesion on the substrate, which contributes to well-aligned ZnO NWs and could consequently have a significant effect on piezoelectric nanogenerators.The present work reports the growth of ZnO NWs on a ZnO seed layer by hydrothermal synthesis method. The influences of deposition time and annealing treatment of the ZnO seed layer on the ZnO NWs were studied. 2. Experimental DetailsThe substrate cleaning was carried out with hydrofluoric acid HF (50%), hydrogen peroxide H2O2 (30%), and sulfuric acid H2SO4 (96%), which were supplied by K

MG ULTRA PURE CHEMICALS SAS (Saint Fromond, France), and all of which were used without further purification. The chemicals used for the ZnO NW growth included zinc nitrate hexahydrate Zn(NO3)2∙6H2O (98%), hexamethylenetetramine (HMTA) (CH2)6N4 (>99,5%) purchased from Sigma-Aldrich S.a.r.l (Saint-Quentin Fallavier, France), and ammonium hydroxide NH4OH (29%) solution from KMG ULTRA PURE CHEMICALS SAS (Saint Fromond, France), also used as received. The deposition of metallic layer was done with physical vapor deposition (PVD) equipment (Plassys MP 650 S, Marolles-en-Hurepoix, France), and the ZnO seed layer deposition with a PVD equipment (Plassys MP 550 S, Marolles-en-Hurepoix, France). By offering the opportunity to work on small as well as large substrate areas up to 8 inches, this PVD equipment underlines the scale–up potential and industrial interest of the fabrication processes described in the following section. A tubular furnace (Thermolyne 79300, Dubuque, IA, USA) was used for the ZnO seed layer annealing treatment. A stainless steel temperature controllable autoclave (Parr Instrument Company, Moline, IL, USA) was used to operate the synthesis of ZnO NWs.The entire study was performed using n-type, 500 µm thick silicon wafer (100) cut into 2 × 2 cm² samples. The standard cleaning procedure [29] consists of first immersing the substrates in a bath of a mixture of sulfuric acid and hydrogen peroxide (H2SO4:H2O2, 1:1) for 10 min at 110 °C, in order to remove any metallic and organic contaminants, followed by a rinse session in deionised (DI) water. Secondly, they are cleaned for 2 min in diluted hydrofluoric acid (25 %) to remove the native oxide layer of SiO2 usually formed at the Si surface, before a last rinsing in DI water. Last but not least, the substrates are dried under nitrogen flow.On

The effect of seed layer on morphology of ZnO nanorod arrays grown by hydrothermal method — Yinglei Tao et al., 2010

With a large direct band gap (3.37 eV), large exciton binding energy (60 meV), excellent chemical and thermal stability, ZnO is one of the most important multifunctional semiconductors due to its wide range of potential photo-electrochemical applications such as light-emitting diodes, optical waveguides, dye-sensitized photovoltaic cells, conductive gas sensors and transparent electrodes, etc. [1] , [2] , [3] , [4] , [5] . Therefore, fabrication of ZnO nanostructures in highly oriented, aligned and ordered arrays is of critical importance for the development of novel devices. During the past several years, various methods have been developed for the synthesis of oriented arrays of ZnO nanorods and nanowires, including vapor–liquid–solid (VLS) [6] , metal–organic chemical vapor deposition (MOCVD) [7] , template-assisted [8] , and solution method [9] . Among the various growth techniques developed, the low cost, low temperature hydrothermal method holds great promise for devices application. Recently, several studies have grown highly oriented ZnO nanorods, via two-step process, on substrates with the use of preexisting textured ZnO seeds such as a ZnO nanoparticle layer or ZnO films [10] , [11] , [12] . Alignment of the ZnO nanocrystals is substrate-independent and occurs on flat surfaces regardless of their crystallinity or surface chemistry, including Al 2 O 3 single crystals, transparent conducting oxides such as indium tin oxide (ITO) and fluorine doped tin oxide (FTO), amorphous oxides including glass and silicon with its native oxide. For the integrated applications of nanodevices, it is necessary to control the exact growing positions of the zinc oxide nanorods on the target substrate. Several methods have demonstrated the selective-area growth of ZnO nanowires using selective-area chemical vapor deposition (CVD) or electrochemical process [13] , [14] . Unfortunately, these approaches need high temperature environment, expensive single crystalline substrates, and expensive low-write speed electron beam lithography. A method combined with conventional lithography methods need to be developed to exhibit advantages of the solution-based ZnO growth method,

i.e., low temperature, low cost, high growth rate, and scale-up possibilities.
Although the influences of growth conditions in hydrothermal method such as growth temperature, deposition time and the concentration of the precursors on the morphology and the alignment of ZnO nanorod arrays have been demonstrated [15] , and the characteristics of ZnO thin films prepared by magnetron sputtering with different parameters have been widely studied [16] , to our best knowledge, the effect of seed layers prepared under different sputtering parameters on the morphology of ZnO nanorod arrays prepared by two-step hydrothermal method has never been investigated yet. In this study, highly oriented ZnO nanorods were successfully synthesized on various sputtered ZnO seed layers through aqueous solution method at low temperature. The effect of sputtering parameters such as oxygen partial pressure and annealing treatment of seed layers on the morphology of ZnO nanorod arrays were discussed. Besides, a simple aqueous solution route to selective-area grow ZnO nanorods on the substrates was demonstrated. Positions of the ZnO nanorods grown by hydrothermal process were controlled via conventional lithography.
To grow well-aligned ZnO nanorods, different ZnO thin films were deposited on silicon wafers using RF magnetron sputtering depositions. These ZnO thin films were then used as seed layers for growing nanorods. For sputtering, argon and oxygen were used as the working gases with at least 99% purity. Prior to deposition, the Zn target was sputter-cleaned. Films were sputtered onto [1 0 0] silicon wafers which were heated to 200 °C. The working pressure was 3 × 10 −5 Pa with a sputtering power of 100 W. The volume ratio between argon and oxygen of seed layer A and seed layer B were 20:3 and 20:7, respectively. Films with same sputtering parameters as seed layer B were annealed at 400 °C in the air for 1 h after sputtering, marked as seed layer C. Detailed preparing parameters to prepare different ZnO seed layers are summarized in Table 1 . The film thickness was around 100 nm.
ZnO nanorods were grown on ZnO seed layer coated substrates by an aqueous chemical method in 60 ml of

aqueous solution containing 0.1 M zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) and 0.1 M hexamethylenetetramine ((CH 2 ) 6 N 4 ) in a sealed teflon lined autoclave. The ZnO seed layer coated substrates were immersed parallel with the sidewall of the autoclave into aqueous solution. The autoclave was kept in a conventional laboratory oven at a constant temperature of 95 °C for 8 h. After the reaction, the substrates were rinsed with de-ionized water and dried in air at room temperature to remove residual salts and organic materials. To control the growth position of ZnO nanorod arrays, photoresist AZ5214 was spin coated on the ZnO–C seed layer and formed a photolithographic mask covered substrates. Using conventional lithography, circular hole array patterns were made on the organic mask after exposure and development. Then, the mask covered substrates were immersed in equimolar mixed nutrient solution of 0.1 M Zn(NO 3 ) 2 ·6H 2 O and hexamethylenetetramine and that of 0.025 M Zn(NO 3 ) 2 ·6H 2 O and hexamethylenetetramine at 95 °C for 6 h, respectively. Selective growth was achieved by the absence of ZnO nucleation sites on photoresist “mask”. Morphological microstructures of ZnO nanorods were characterized by field emission scanning electron microscopy (FE-SEM).
Fig. 1 shows the plane view of the nanorod arrays grown on various ZnO seeded substrates, when the reaction time was fixed at 8 h. As observed, the morphologies of the nanorods are different in terms of the shape, diameter, and orientation. The nanorod arrays grown on ZnO-A seed layers deposited via RF magnetron sputtering at Ar/O 2 ratio of 20:3 had an average diameter of about 200 nm and grew on the substrate in a high density. These nanorods exhibited a relatively poor alignment and disordered polygonal shapes on the end. ZnO nanorods on ZnO-B seed layers which deposited at Ar/O 2 ratio of 20:7 grew in a direction almost perpendicular to the

Investigation on the material properties of ZnO nanorods deposited on Ga-doped ZnO seeded glass substrate: Effects of CBD precursor concentration — Jatani Ungula et al., 2022

1 INTRODUCTION

     Device application of one-dimensional (1D) zinc oxide (ZnO) nanostructures has drawn the attention of many researchers in the recent times.1-3 Scientists worldwide in both industrial and academic sectors have been fascinated by these ZnO nanostructures due to their usability as building blocks of other nanostructures beside the easy synthesis method.4, 5 These nanostructures can be used in various applications such as chemical sensors, dye sensitized solar cells (DSSCs), and light emitting diodes.6, 7 For instance, ZnO nanorods (ZNRs) can be used in DSSCs due to their larger surface areas as compared to the bulk ZnO and ZnO thin films as proposed by Liu et al.8 The large surface area is needed to offer attaching spots for the dyes, while the ZNRs act as a direct conducting path for charge transmission to the electrode from the exciton production point.9

     Since various characteristics such as diameter, the length, and alignment of ZNRs are known to significantly affect the performance of DSSCs, many studies aim to control these parameters to engineer desired device properties.10 To date, it is possible to prepare ZNRs independently or grow them on pre-seeded substrates. Vayssieres et al.11 successfully prepared ZNRs on un-seeded substrates by using Zn (NO3)2 solution and methenamine precursors, while Zhao et al.12 observed the absence of ZNRs when grown on a bare indium tin oxide (ITO) slide or glass using Zn (NO3)2 solution with NaOH. Evaluations to compare the ZNRs grown on the plain and the pre-treated substrates were conducted by Yang et al.,13 who summarized that better alignment of ZNRs can be obtained by using a pre-treated substrate. Furthermore, when ZNRs are doped during synthesis, the optoelectrical features are greatly enhanced hence a desirable method of engineering the ZNRs properties for a specific desired application.14-16 It has been realized that doping of ZnO nanostructures with Group III elements enhances their conduct

ivity greatly because of their ionization energy which is reported to be low.17 Owing to this property, ZnO doped with gallium (GZO) is ideal to be used as a seed layer due to favorable properties such as its good stability compared to other materials like aluminum in addition to the comparable values of the covalent radius and ionic radius of Ga and Zn.18 Growing of ZNRs on a Ga-doped ZnO layer and controlling their growth has not been studied extensively. The as-prepared well-aligned and crystalline ZNRs were grown on a pre-treated Ga-doped ZnO layer using a facile two-step process involving pre-seeding of the substrates and controlled synthesis of the ZNRs using the chemical bath deposition (CBD) method. It has been reported that the morphology and orientation of the ZnO crystallites grown by CBD technique are greatly affected by various growth parameters such as the solution pH, temperature of growth solution, levels of saturation, type of substrate, and precursor concentrations.19 As such, the control of these parameters remains a necessity for the formation of large surface area nanomaterials for use in photovoltaic and optoelectronic devices.
2 EXPERIMENTAL PROCEDURE

     The chemicals used are methenamine (C6H12N4, 99.9% purity), zinc nitrate hexahydrate (Zn (NO3)2. 6H2O, 99.9% purity), sodium hydroxide (NaOH), and gallium nitrate hexahydrate (Ga (NO3)3.6H2O, 99.9% purity). The aqueous solutions were prepared using double distilled water. The details of synthesis process of nanorods using the two-step CBD growth method are described as follows.

     A diagram showing chemical bath deposition process which involved two steps, the first step involves depositing the seeded layer of GZO on glass substrate by spin coating, is shown in Figure 1. This was followed by the growth of ZNRs on the GZO seeded substrate by the CBD at different growth concentrations. The spin coating technique was used to deposit GZO

seed particles on a glass substrate. A mixture of methanol and acetone was ultrasonically used to clean the glass substrate for 30 min followed by rinsing with deionized water and ethanol for about 5 min before deposition. Finally, the substrate was dried by blowing high-pressure N2 gas over it. The coating solution was prepared from as-synthesized GZO nanopowders. The details of preparation and optimization of GZO nanopowders were described in Ungula et al.18 The GZO powder was ground, dissolved in suitable solvent, and mixed into a homogenous paste to form the spin coating solution. A 10 ml of distilled water was added to make a 70 ml solution in 100 ml beaker reaction vessel. The reaction vessel containing the reaction bath and the suspended GZO seeded substrate was then placed in a water bath. For optimization of growth concentration in CBD setup, the precursor concentration was varied from 0.005–0.1 M.

              FIGURE 1Open in figure viewerPowerPoint

              A diagram displaying a CBD growth of ZNRs on a pre-treated substrate by two-step process




     The structural properties of ZNRs were investigated using a Bruker D8 Advanced X-ray diffractometer (XRD), while the compositional and morphological analysis was carried out using a JEOL JSM-7800F model of SEM with an inbuilt Energy dispersive X-ray spectrophotometer (EDS). A Shimadzu model SPM-9600 of atomic force microscope (AFM) was used to study the