Role and Function of Noble-Metal/Cr-Layer Core/Shell Structure Cocatalysts for Photocatalytic Overall Water Splitting Studied by Model Electrodes

Abstract

The mechanism of hydrogen evolution by a core/shell noble-metal/Cr 2 O 3 particulate as a highly efficient cocatalyst for overall water splitting under visible light using the photocatalyst (Ga 1− x Zn x )(N 1− x O 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 Cr 2 O 3 films on Rh and Pt plates and are evaluated as model systems of Rh/Cr 2 O 3 and Pt/Cr 2 O 3 core/shell particulates, which have previously been applied effectively as cocatalysts for hydrogen evolution in this system. Proton adsorption/desorption and H 2 evolution currents are observed for both the Cr 2 O 3 -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 Cr 2 O 3 layer does not interfere with proton reduction or hydrogen evolution and that proton reduction takes place at the Cr 2 O 3 /Pt interface. However, the reduction of oxygen to water is suppressed only in the Cr 2 O 3 -coated samples. The Cr 2 O 3 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.