Phosphate-Ion-Induced Formation of a Cu2O/Cu3(PO4)2 p–n Junction with Type-II Band Alignment for Improved Photoelectrochemical Hydrogen Production

Abstract Cu2O is an attractive photocathode for solar-driven hydrogen production via photoelectrochemical (PEC) water splitting; however, its performance is fundamentally limited by severe bulk and interfacial recombination arising from inefficient charge separation and transfer. Engineering a p–n junction at the Cu2O interface offers a viable route to overcome these limitations, yet controllable approaches remain scarce. Here, we present a facile phosphate-ion-induced surface conversion strategy to construct a type-II Cu2O/Cu3(PO4)2 p–n heterojunction. Upon integration with a NiO hole transport layer (HTL) and a RuOx hydrogen evolution catalyst, the resulting NiO/Cu2O/Cu3(PO4)2/RuOx photocathode delivers a photocurrent density of 3.7 mA cm–2 at 0 VRHE under standard illumination, representing an approximately 3-fold enhancement over bare NiO/Cu2O (1.3 mA cm–2). The IMPS and PEIS studies revealed an enhanced charge-transfer efficiency through suppressed recombination and reduced interfacial charge-transfer resistance. The favorable band alignment among the NiO, Cu2O, and Cu3(PO4)2 layers promotes directional charge transport, leading to enhanced PEC hydrogen evolution. However, despite the significant role of Cu3(PO4)2 in charge separation, it does not provide adequate protection against photocorrosion. Using a TiO2 overlayer was essential to stabilize the electrode and achieve a Faradaic efficiency close to unity.