Abstract
The preparation of tungsten oxide (WO3) thin film by direct current (DC) reactive sputtering magnetron method and its photoelectrocatalytic properties for water oxidation reaction are investigated using ultraviolet-visible radiation. The structural, morphological, and compositional properties of WO3 are fine-tuned by controlling thin film deposition time, and post-annealing temperature and environment. The findings suggest that the band gap of WO3 can be controlled by adjusting the post-annealing temperature; the band gap decreased from 3.2 to 2.7 eV by increasing the annealing temperature from 100 to 600 °C. The theoretical calculations of the WO3 bandgap and the density of state are performed by density functional theory (DFT). Following the band gap modification, the photoelectrocatalytic activity increased and the maximum photocurrent (0.9 mA/cm2 at 0.6 VSCE) is recorded with WO3 film heated at 500 °C. The WO3 film heated under air exhibits much better performance in photoelectrochemical water oxidation process than that of annealed under inert atmosphere, due to its structural variation. The change in sputtering time leads to the formation of WO3 with varying film thickness, and the maximum photocurrent is observed when the film thickness is approximately 150 nm. The electrical conductivity and charge transfer resistance are measured and correlated to the properties and the performance of the WO3 photoelectrodes. In addition, the WO3 photoelectrode exhibits excellent photoelectrochemical stability.
Highlights
Hydrogen energy is considered as a potential solution for addressing the world’s energy needs
This study demonstrated that the important properties of WO3 thin films can be fine-tuned by controlling deposition time, and post-annealing temperature and environment
It is discerned that the photoelectrocatalytic activity of WO3 photoelectrodes increased with decreasing bandgap, and the maximum photocurrent (0.9 mA/cm2 at 0.6 VSCE) was recorded with WO3 thin film heated at 500 ◦C with a bandgap of 2.85 eV
Summary
Hydrogen energy is considered as a potential solution for addressing the world’s energy needs. Many investigations are dedicated to application of metal oxide semiconductors, such as TiO2, ZnO, CuO, BiVO4, Fe2O4 etc., as photoanodes for photoelectrochemical water oxidation [15,16,17,18,19] Among these metal oxide semiconductors, tungsten oxide (WO3) has received much attention due to the wide range of its bandgap (~2.6 to ~2.8 eV for bulk material [20] and up to 3.2 eV for nanostructure forms [21]) that can absorb up to 480 nm, and being active in the visible light region. The optical properties, and the photocurrent density of the prepared films were dependent on the annealing temperature Another crucial factor is the film thickness, which can be controlled by adjusting the deposition time. Exceeding that optimal thickness had negative impact on the PEC performance Driven by these findings, attempted to perform a comprehensive study to investigate the influence of post-annealing temperature, annealing atmosphere, and deposition time on the photoelectrocatalytic activity of the WO3 sputtered thin films. The theoretical bandgap of WO3 film was calculated using DFT calculation and compared with the experimental value
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