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

Abstract

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.