Photocatalytic optimization of ZnO–Ga₂O₃ composite thin films for PEC water splitting: effects of thickness, environment, and annealing temperature

This study reports a systematic investigation into the photoelectrochemical (PEC) performance of Ga₂O₃/ZnO (GZO) composite thin films fabricated <i>via</i> RF magnetron sputtering. GZO films were deposited on FTO/Glass and titanium (Ti) foil substrates, with key fabrication parameters - namely deposition time, annealing gas atmosphere, and annealing temperature - systematically varied to optimize photocatalytic activity. Surface morphology and crystallinity were evaluated using SEM and XRD, respectively, revealing that both deposition time and annealing conditions significantly influence grain structure and crystallinity, which in turn affect PEC performance. Among the tested conditions, films deposited for 25 minutes and annealed in air exhibited optimal performance, with annealing at 600 °C on Ti foil substrates yielding the highest photocurrent density of 1.7 × 10^-4 A cm^-2 at 1.23 V <i>vs.</i> RHE. Electrochemical impedance spectroscopy (EIS) confirmed improved charge transfer properties at this temperature, although stability testing indicated potential trade-offs between performance and long-term durability. These findings highlight the critical role of thermal and atmospheric control during post-deposition treatment in tailoring the structural and electronic properties of GZO thin films. The optimized GZO photoanodes demonstrate strong potential for low-cost, efficient, and scalable solar hydrogen production, contributing to the advancement of sustainable energy technologies.