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/cm(2) 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.