Effect of Temperature and Growth Time on Vertically Aligned ZnO Nanorods by Simplified Hydrothermal Technique for Photoelectrochemical Cells

Some Distinct Attributes of ZnO Nanorods Arrays: Effects of Varying Hydrothermal Growth Time

This study investigates the growth time effect on the structural, morphological, optical, and photoelectrochemical characteristics of highly oriented ZnO nanorod arrays (ZNRAs). The nanorod arrays were grown on ITO substrates using the unified sol-gel spin coating and hydrothermal techniques. ZnO nanoparticles (ZNPs) were synthesized using the sol-gel spin coating method. In contrast, the hydrothermal method was used to grow the ZnO nanorods. The hydrothermal growth time investigated was between 4 and 12 h. The synthesized ZNRAs were used as the photoanode electrodes to investigate their photoelectrochemical (PEC) electrode potency. The as-prepared ZNRAs were characterized using various analytical tools to determine their structures, morphologies, optical, and photoelectrochemical traits. EDX spectra showed the presence of uncontaminated ZnO chemical composition, and FTIR spectra displayed the various functional groups in the samples. A rod-shaped ZnO nanocrystallite with mean lengths and diameters of 300-500 nm and 40-90 nm, respectively, is depicted. HRTEM images indicated the nucleation and growth of ZNRAs with a lattice fringe spacing of 0.26 nm and a growth lattice planer orientation of [002]. The optimum ZNRAs (grown at 8 h) as photoelectrode achieved a photoconversion efficiency of 0.46% and photocurrent density of 0.63 mA/cm², that was 17 times higher than the one shown by ZNPs with Ag/AgCl as the reference electrode. Both values were higher than those reported in the literature, indicating the prospect of these ZNRAs for photoelectrode applications.

Enhancement in Photovoltaic Performance of Solar Cells by Electrostatic Adsorption of Dyes on ZnO Nanorods

ZnO nanorods were formed by chemical bath deposition on fluorine-doped tin oxide (FTO) glass and the photovoltaic performance of ZnO-based dye-sensitized solar cells (DSCs) was investigated. A DSC with 8 h-grown ZnO nanorods showed a higher power conversion efficiency (PCE) than devices with 4, 6, and 10 h-grown ones. Further improvement in PCE was achieved in a cell with a silver-ion-deposited ZnO/FTO electrode. By deposition of Ag+ on the surface of the 8 h-grown ZnO nanorods, the dye-loading amount increased by approximately 210%, compared to that of pristine ZnO nanorods, resulting in a 1.8-times higher PCE. A DSC with the pristine ZnO/FTO electrode showed a PCE of 0.629%, while in a device with the silver-ion-deposited ZnO/FTO, the PCE increased to 1.138%. In addition, interfacial resistance at the ZnO/dye/electrolyte was reduced to approximately 170 Ω from 460 Ω for the control cell with the pristine ZnO/FTO. We attributed the higher dye-loading amount in the silver-ion-deposited ZnO/FTO to the electrostatic attraction between the positively charged ZnO and carboxylate anions (-COO-) of the N719 dyes.

Photocatalytic performance improvement by utilizing GO_MWCNTs hybrid solution on sand/ZnO/TiO2-based photocatalysts to degrade methylene blue dye

In this work, sand/zinc oxide (ZnO)/titanium dioxide (TiO₂)-based photocatalysts were hybridized with graphene oxide (GO) and GO_multi-walled carbon nanotubes (MWCNTs) hybrid solution. The novel hybrid was then used in photocatalysis to degrade dye contamination. The nanocomposite photocatalyst was initially fabricated by growing ZnO nanorods (NRs) via sol-gel immersion followed by synthesizing TiO₂ NRs for different times (5 and 20 h) using a hydrothermal method on sand as a substrate. Prior to the hybridization, the initial GO was synthesized using electrochemical exfoliation and further mixed with 1 wt% MWCNTs to form GO_MWCNTs hybrid solution. The synthesized GO and GO_MWCNTs hybrid solution were then incorporated onto sand/ZnO/TiO₂ nanocomposite-based photocatalysts through immersion. Various sand/ZnO/TiO₂-based photocatalysts were then tested for methylene blue (MB) dye degradation within 3 days. On the basis of UV-Vis measurement, the highest MB degradation was achieved by using sand/ZnO NRs/TiO₂ NRs (5 h)/GO_MWCNTs (92.60%). The high surface area and high electrical conductivity of GO_MWCNTs prolonged the lifetime of electron/hole separation and thus enhanced the photocatalytic performance.

Effects of growth conditions on properties of CBD synthesized ZnO nanorods grown on ultrasonic spray pyrolysis deposited ZnO seed layers

ZnO nanorods were synthesized on a seed layer coated glass substrate using chemical bath deposition (CBD). Prior to growth, a seed layer had been prepared via ultrasonic spray pyrolysis method. The aim was to explore the influence of varying the chemical bath deposition conditions namely: growth time, bath temperature and concentration levels of the precursor on the orientation, structural, optical and vibrational properties of the subsequently grown nanorods. The presence of ZnO nanorods resembling the hexagonal-wurtzite structure having preference of orientation along the c-axis and varying crystallinity under different growth parameters was confirmed by X-ray diffraction (XRD). Scanning Electron Microscopy (SEM) acquired images of uniformly arranged and vertically oriented ZnO nanorods grown at a relatively higher bath temperature of 90 °C and shorter growth period of 2 h. UV/Vis/NIR spectrophotometer measurements revealed an optical transmittance of between 50 – 70 % for the nanorods. Raman spectroscopy results confirmed the presence of Raman active E_2(low) and E_2(high) modes corresponding to 98 cm⁻¹ and 478 cm⁻¹ belonging to the hexagonal ZnO phase. This work shows that the orientation, structural, optical and vibrational properties of the grown nanorod structures are controlled via alteration of the growth parameters.

The Design of ZnO Nanorod Arrays Coated with MnOx for High Electrochemical Stability of a Pseudocapacitor Electrode

Tremendous efforts have been made on the development of unique electrochemical capacitors or pseudocapacitors due to the overgrowing electrical energy demand. Here, the authors report a new and simple strategy for fabricating hybrid MnOx-coated ZnO nanorod arrays. First, the vertically aligned ZnO nanorods were prepared by chemical bath deposition (CBD) as a template providing a large surface area for active material deposition. The manganese oxide was subsequently coated onto the surface of the ZnO nanorods to form a hybrid MnOx-coated ZnO nanostructure by anodic deposition in a manganese acetate (MnA)-containing aqueous solution. The hybrid structure of MnOx-coated ZnO nanorod arrays exhibits a large surface area and high conductivity, essential for enhancing the faradaic processes across the interface and improving redox reactions at active MnOx sites. A certain concentration of the deposition solution was selected for the MnOx coating, which was studied as a function of deposition time. Cyclic voltammetry (CV) curves showed that the specific capacitance (SC) of the MnOx-coated ZnO nanostructure was 222 F/g for the deposition times at 10 s when the concentration of MnA solution was 0.25 M. The unique hybrid nanostructures also exhibit excellent cycling stability with >97.5% capacitance retention after 1200 CV cycles. The proposed simple and cost-effective method of fabricating hybrid nanostructures may pave the way for mass production of future intelligent and efficient electrochemical energy storage devices.

Microscale electrohydrodynamic printing of in situ reactive features for patterned ZnO nanorods

Patterning of zinc oxide (ZnO) nanorods has attracted considerable interests to enhance the performance of ZnO-based functional devices. Most of the existing techniques for patterned ZnO nanorods are based on conventional microfabrication methods that commonly require cleanroom environment, high-cost equipment and complicated processes. In this study, electrohydrodynamic (EHD) printing strategy was accommodated to fabricate microscale ZnO nanorods patterns based on in situ reactive inks. Smaller working voltage and larger nozzle-to-collector distance facilitated the formation of thinner PEO-Zn(NO₃)₂ filaments, which were decomposed into ZnO nanoparticles to serve as the seeding template for the hydrothermal growth of ZnO nanorods. The resultant ZnO nanorods can be flexibly tuned by the EHD printed patterns. The effect of growth time on the size and morphology of ZnO nanorods was investigated. Compared with the spin-coating method, the photoelectrochemical property of patterned ZnO nanorods was well controlled and showed enhanced photoelectrochemical stability. The presented method provides a flexible and rapid way to customize patterned ZnO nanorods that can be potentially used in the fields of optical detectors, biosensors or solar-driven devices.