Role and function of noble-metal/Cr-layer core/shell structure cocatalysts for photocatalytic overall water splitting studied by model electrodes

The mechanism of hydrogen evolution by a core/shell noble-metal/Cr ₂O₃ particulate as a highly efficient cocatalyst for overall water splitting under visible light using the photocatalyst (Ga _1-xZn_x)(N_1-xO_x) is investigated by electrochemical and in situ spectroscopic measurements of model electrodes. The electrodes are prepared by electrochemical deposition of 1.8-3.5 nm thick Cr₂O₃ films on Rh and Pt plates and are evaluated as model systems of Rh/Cr₂O₃ and Pt/Cr₂O₃ core/shell particulates, which have previously been applied effectively as cocatalysts for hydrogen evolution in this system. Proton adsorption/desorption and H₂ evolution currents are observed for both the Cr ₂O₃-coated and the bare electrodes, and the infrared absorption band due to Pt-H stretching (2039 cm^-1) is apparent for both the coated and the bare electrodes. These observations indicate that the Cr₂O₃ layer does not interfere with proton reduction or hydrogen evolution and that proton reduction takes place at the Cr ₂O₃/Pt interface. However, the reduction of oxygen to water is suppressed only in the Cr₂O₃-coated samples. The Cr₂O₃ layer is thus permeable to protons and the evolved hydrogen molecules, but not to oxygen. Cocatalyst modification by thin films with this type of functionality thus appears to be a useful strategy for improving the efficiency of photocatalytic overall water splitting.