Electrochemical Design of Atomically Conformal Ga₂O₃ Buffer Layers via Cu₂O-Catalyzed Nitrate Reduction Reaction for Efficient Cu₂O Photocathodes

Gallium oxide (Ga₂O₃), which provides ideal band alignment with Cu₂O, is one of the most effective buffer-layer materials for enhancing photovoltage. However, Ga₂O₃ deposition has thus far relied exclusively on atomic layer deposition, which requires expensive and commercially limited precursors, limiting its scalability. In this study, we propose a new electrochemical approach for the atomically conformal deposition of Ga₂O₃ buffer layers based on the use of gallium nitrate as a precursor and a Cu₂O-catalyzed nitrate reduction reaction. This approach promotes the localized generation of hydroxide ions at the Cu₂O surface, enabling the cathodic electrodeposition of uniform Ga₂O₃ films at room temperature without high-temperature annealing. Electrochemically deposited Ga₂O₃ (e-Ga₂O₃) significantly improved both the onset potential (> 1.0 V_RHE) and photocurrent density (~4 mA cm⁻² at 0.6 V_RHE) of Cu₂O by suppressing charge recombination. When integrated into an all-oxide tandem photoelectrochemical device with a BiVO₄-based photoanode, the optimized Cu₂O/e-Ga₂O₃ photocathode achieved a solar-to-hydrogen efficiency of 3.80% under bias-free conditions and demonstrated stability for over 60 h. This study presents a low-temperature and cost-effective route for developing high-performance Cu₂O photocathodes for efficient solar-driven water splitting.