Photoassisted hydrogen evolution over spinel CuM2O4 (M=Al, Cr, Mn, Fe and Co)

The photocatalytic ability of CuM2O4 (M=Al, Cr, Mn, Fe and Co) crystallizing with spinel-type structure has been evaluated according to the H2-evolution. The oxides are black and displayed a semiconducting behavior where the electronic balance comes from a small over stochiometry (CuCo2O4.016). Electron hopping occurs between similar sites in normal spinel with a low activation energy whereas larger energies were found in inverse spinel. The electrodes were characterized photoelectrochemically and acquired the characteristic of p-type semiconductors. The conduction band edge, determined from intensity–potential curves, is located below the H2O/H2 potential leading to a thermodynamically favorable H2-liberation under visible light. The photoactivity was dependent on preparative conditions and the best results were obtained over CuCo2O4 prepared through nitrate route in presence of SO32− (3.6 ml h−1 g−1). Improved photoactivity may be interpreted in terms of flat band potential and electronegativity. The presence of cobalt contributes to increase the electron affinity, a desirable property for p-type specimen. It has been observed that the amount of evolved H2 rose using stronger reductors by increasing the band bending. The initial performance of catalyst was almost restored using a fresh reactant solution. With time, the water reduction slowed down because the end products Sn2- and S2O62− compete with the adsorbed water for the photoelectrons.