Positive onset potential and stability of Cu2O-based photocathodes in water splitting by atomic layer deposition of a Ga2O3 buffer layer

Abstract

The Cu₂O-based photocathode is considered as one of the most promising photocathodes for high performance water splitting under sunlight. However, the relatively negative onset potential for H₂ production of these photocathodes impedes further optimization of the solar-to-fuel conversion efficiency. Here, a thin Ga₂O₃ buffer layer is introduced between the Cu₂O absorber layer and the TiO₂ protective layer by atomic layer deposition to increase the photovoltage. For the optimized TiO₂ deposition temperature, the Pt/TiO₂/Ga₂O₃/Cu₂O electrode achieves a high cathodic photocurrent of −2.95 mA cm⁻² at 0 V vs. RHE and an extremely positive onset potential of 1.02 V vs. RHE (defined as the potential where photocathodic current reaches 20 μA cm⁻² under air-mass 1.5 global illumination), benefiting from a buried p–n junction and a favorable band alignment. The Pt/TiO₂/Ga₂O₃/Cu₂O electrodes exhibit a stable cathodic current for 2 h under continuous illumination of a 500 W Xe lamp for the TiO₂ deposition temperatures below 180 °C.