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-xZnx)(N1-xOx) 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.
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