Reactive co-sputter deposition of Ta-doped tungsten oxide thin films for water splitting application.

This study aimed to investigate the structural, optical, and electronic properties of WO thin films modified by Ta-doping, considering their potential application in photoelectrochemical (PEC) water splitting. Due to its unique physical and chemical properties, WO films have been commonly suggested as a promising photoanode for hydrogen production. However, the wide bandgap and unsuitable band edge positions of WO limit its PEC efficiency. Doping have been extensively applied as an effective strategy for bandgap engineering. Here, post-annealed WO films with different concentrations of Ta dopant were synthesized via reactive magnetron co-sputtering, while DC and RF sputtering powers were varied with the aim of achieving the desired properties. EDX analysis showed that Ta atoms were doped into WO in the range of 0-3.93 at%. As evident from SEM and AFM images, the surface morphology was significantly affected by increasing Ta doping, the formation of a granular structure with well-defined boundaries and increasing surface roughness (1.79-47.94 nm). XRD patterns confirmed that the incorporation of Ta atoms into a monoclinic WO improved the crystallinity, especially in the (002) direction. Most importantly, a decrease in the average transparency (92.82-74.27%), an increase in visible absorption, a red shift of the fundamental absorption edge corresponding to a favorable drop in the optical bandgap energy (3.07-2.61 eV) were found with increasing Ta concentration. Notably, the substitution of W ions with Ta dopant (0-3.93 at%) led to an upward shift in the valence band maximum (3.62-3.31 eV) and a downward shift in the conduction band minimum (0.55-0.70 eV). The WO photoanode doped with 3.93 at% Ta exhibited the maximum photocurrent density of 0.65 mA/cm (at 1 V vs. Ag/AgCl) under simulated sunlight. Furthermore, WO photoanode doped with 3.93 at% Ta showed excellent photoresponsivity and slow electron-hole recombination. The obtained results predict the potential of Ta-doping coupled with post-annealing to optimize the structural and optoelectronic properties of sputtered WO thin films as photoanode for use in efficient PEC water splitting.