Phase-selective magnetron sputtering deposition of copper oxide polymorphs: Unraveling the superiority of Cu2O over metastable Cu4O3 in photoelectrochemical water splitting

Copper oxides are promising photocathodes for solar hydrogen production, but the role of the metastable intermediate phase, paramelaconite (Cu 4 O 3 ), remains obscure due to its narrow thermodynamic stability window. Herein, we report the controllable synthesis of pure Cu 2 O, Cu 4 O 3 and CuO thin films via radio-frequency reactive magnetron sputtering by precisely manipulating the oxygen partial pressure. A systematic phase diagram was established to isolate the challenging Cu 4 O 3 phase, facilitating comprehensive comparative studies of the inherent correlations between crystal structure, electronic properties, and photoelectrochemical (PEC) behaviors. Mott-Schottky plots and band alignment analysis reveal that Cu 2 O exhibits the most favorable electronic properties, featuring the highest hole carrier density (∼ 3.1 × 10 19 cm −3 ) and the most negative conduction band potential, providing the strongest thermodynamic driving force for hydrogen evolution. In contrast, although the Cu 4 O 3 also demonstrates a high carrier density (∼ 2.7 × 10 19 cm −3 ), comprehensive PEC evaluations identify its rapid surface recombination and significant photocorrosion. This stems from a high density of deep-level traps acting as recombination centers, which drives severe surface charge recombination and consequently triggers rapid photocorrosion. Conversely, the Cu 2 O phase shows superior kinetic stability with rectangular photoresponses and distinctive self-activation behavior. This work unravels the trade-off between conductivity and stability, affirming that the defect tolerance and phase purity of Cu 2 O render it as the optimal candidate material for durable solar water splitting.