Highly stable copper oxide composite as an effective photocathode for water splitting via a facile electrochemical synthesis strategy

Hydrogen generation through photoelectrochemical (PEC) water splitting using solar light as an energy resource is believed to be a clean and efficient way to overcome the global energy and environmental problems. Extensive research effort has been focused on n-type metal oxide semiconductors as photoanodes, whereas studies of p-type metal oxide semiconductors as photocathodes where hydrogen is generated are scarce. In this paper, highly efficient and stable copper oxide composite photocathode materials were successfully fabricated by a facile two-step electrochemical strategy, which consists of electrodeposition of a Cu film on an ITO glass substrate followed by anodization of the Cu film under a suitable current density and then calcination to form a Cu ₂O/CuO composite. The synthesized Cu ₂O/CuO composite was composed of a thin layer of Cu ₂O with a thin film of CuO on its top as a protecting coating. The rational control of chemical composition and crystalline orientation of the composite materials was easily achieved by varying the electrochemical parameters, including electrodeposition potential and anodization current density, to achieve an enhanced PEC performance. The best photocathode material among all materials prepared was the Cu ₂O/CuO composite with Cu ₂O in (220) orientation, which showed a highly stable photocurrent of -1.54 mA cm ^-2 at a potential of 0 V vs reversible hydrogen electrode at a mild pH under illumination of AM 1.5G. This photocurrent density was more than 2 times that generated by the bare Cu ₂O electrode (-0.65 mAcm ^-2) and the stability was considerably enhanced to 74.4% from 30.1% on the bare Cu ₂O electrode. The results of this study showed that the top layer of CuO in the Cu ₂O/CuO composite not only minimized the Cu ₂O photocorrosion but also served as a recombination inhibitor for the photogenerated electrons and holes from Cu ₂O, which collectively explained much enhanced stability and PEC activity of the Cu ₂O/CuO composite. Thus, the electrochemical strategy proposed in this study for the synthesis of the Cu ₂O/CuO composite opens a new way to use copper oxides as photocathode materials in PEC cells for a highly stable and effective water splitting.