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.