Electrochemical Design of Atomically Conformal Ga ₂ O ₃ Buffer Layers via Cu ₂ O‐Catalyzed Nitrate Reduction Reaction for Efficient Cu ₂ O Photocathodes

ABSTRACT Gallium oxide (Ga ₂ O ₃ ), which provides ideal band alignment with Cu ₂ O, is one of the most effective buffer‐layer materials for enhancing photovoltage. However, Ga ₂ O ₃ 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 Ga ₂ O ₃ buffer layers based on the use of gallium nitrate as a precursor and a Cu ₂ O‐catalyzed nitrate reduction reaction. This approach promotes the localized generation of hydroxide ions at the Cu ₂ O surface, enabling the cathodic electrodeposition of uniform Ga ₂ O ₃ films at room temperature without high‐temperature annealing. Electrochemically deposited Ga ₂ O ₃ ( e ‐Ga ₂ O ₃ ) significantly improved both the onset potential (> 1.0 V _RHE ) and photocurrent density (~4 mA cm ⁻² at 0.6 V _RHE ) of Cu ₂ O by suppressing charge recombination. When integrated into an all‐oxide tandem photoelectrochemical device with a BiVO ₄ ‐based photoanode, the optimized Cu ₂ O/ e ‐Ga ₂ O ₃ 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 Cu ₂ O photocathodes for efficient solar‐driven water splitting.