Atomically tunable photo-assisted electrochemical oxidation process design for the decoration of ultimate-thin CuO on Cu₂O photocathodes and their enhanced photoelectrochemical performances

A representative method of forming CuO thin films on Cu2O photoabsorbers is simple annealing oxidation at high temperature in a controlled oxygen atmosphere, but the typical oxidation process is very quick and irregular, resulting in a high density of defect sites. To maximize the beneficial effect of CuO/Cu2O heterojunction photocathodes, novel criteria for CuO preparation have been suggested: (1) in-plane epitaxy with Cu2O, (2) atomic layer-by-layer growth, (3) ultimate-thin and completely conformal coating, and (4) minimized internal defects. As a novel strategy to achieve these criteria, we propose a photo-assisted electrochemical oxidation (PAEO) method, where the Cu2O surface is precisely phase-changed into the ultimate-thin CuO layer with a thickness of 4 nm via fine thickness control using photoenergy and an external potential at room temperature. The produced CuO crystals are grown on Cu2O without generating structural defects by accommodating the epitaxial relationship below the critical thickness. From static and dynamic (photo)electrochemical analyses, the decoration of ultimate-thin CuO offers high electrical conductivity and fast charge transport, guarantees sufficient open-circuit potential (OCP), and substantially retains the initial OCP value by minimizing the contribution of recombination loss. Finally, the PAEO-treated photocathodes exhibit an excellent photocurrent density of 15 mA cm-2 (approaching the theoretical maximum current) and 8.3 mA cm-2 at 0 V vs. RHE in electrolytes with and without scavenger hydrogen peroxide (H2O2), respectively, as well as a OCP of 0.78 V, even with the use of suboptimal Al-doped ZnO buffer layers.