p-Type CuYO2 as hydrogen photocathode

A new photochemical system based on a CuYO2 dispersion in aqueous S2− or SO32− solution as hole scavengers is proposed. The delafossite CuYO2, doped with calcium, is a low p-type semiconductor with a hole mobility of 7.3×10−7 m2 V−1 s−1. The band gap is 3.50 eV and the transition is directly allowed. The valence band edge, located at 5.23 eV below vacuum, is made up from Cu-3d type typical of delafossite oxides. The flat band potential VfbVfb (+0.17 Vsce) lies below the H2O/H2 potential permitting a spontaneous H2-formation under band gap illumination in aqueous S2− or SO32− electrolytes. p-CuYO2 loaded with n-Cu2O exhibited a higher performance with a H2 evolution rate of 0.4 cm3 h−1 in 0.1 M S2− (pH 13.4). The decrease of photoactivity over time is due to the formation of yellow polysulfides Sn2- which compete with H2O in the reduction process.