Cuprous/Cupric Heterojunction Photocathodes with Optimal Phase Transition Interface via Preferred Orientation and Precise Oxidation

To effectively transport photogenerated charges within a cuprous oxide (Cu₂O) photocathode and to transfer them to the electrolyte, cupric oxide (CuO) as a heterojunction material enable to provide the additional built-in electric field is perfectly suitable band position and can be produced from the original Cu₂O films by simple oxidation process. However, their different crystal structures and lattice constants inevitably induce crystal distortions and the formation of a defective interface, resulting in the reduction of the photocurrent and photovoltage. To alleviate these intrinsic problems and improve its photoelectrochemical (PEC) performance, we fabricated a Cu₂O/CuO interface with high crystallinity and a well-aligned atomic arrangement by preparing a Cu₂O absorber underlying layer preferentially oriented with [111] direction and forming a thin CuO overlayer (20-30 nm) via oxidation process at precisely controlled reduced oxygen partial pressure. At 5 Torr, the resultant Cu₂O double-layer structure exhibited smooth crystallographic phase transition boundary from cubic Cu₂O to monoclinic CuO and well-aligned lattice fringes. This exhibited considerable improvement in photocurrent density (2.8 mA/cm² at 0 V vs RHE) and onset potential (0.83 V), compared with those of pristine Cu₂O. Importantly, these enhancements were achieved without coating of photocatalytic materials on the photoelectrodes.