Tuning of electron transfer by Ni–P decoration on CeO2–TiO2 heterojunction for enhancement in photocatalytic hydrogen generation

Abstract

Photocatalytic hydrogen generation using hybrid transition metal oxides has received significant attention recently. However, low response of the catalysts under visible light driven water splitting due to the wide band gap and the high rate of electron–hole recombination suppresses their efficiency. This can be encountered by suitable surface modifications of the catalysts. In the present study, CeO2–TiO2 hybrid oxide is explored as a good photo catalyst and its activity is further enhanced by a simple but effective surface decoration using Ni–P. The enhanced activity is due to narrowing of the band gap of CeO2–TiO2 to 2.4 eV and decreasing of electron–hole recombination rate due to the decoration process. The relay of electrons from valence band of the hybrid oxide to the hybrid P band of the phosphorus and then to the conduction band (CB) of the hybrid oxide decreased the required energy for electron excitation and recombination of electrons and holes. The Ni metal as a co–catalyst can trap the electrons and generates additional reaction sites on the catalyst surface. This is due to the transfer of electrons from CB of CeO2–TiO2 to Ni through Schottky barrier right up until the Fermi level of the two becomes equal. The Ni atoms on the surface act as additional reaction sites during hydrogen evolution. Moreover the O–H group on the surface of the catalyst scavenges the holes in the valence band during photo–excitation and boosts its activity by further reducing electron-hole recombination. Metal (Ni) and non–metal (P) decoration on semiconductor oxides with active heterojunctions is a good method to enhance the photocatalytic activity of the oxides. The thermal decomposition followed by a deposition/reduction process adopted in this study is proved to be an efficient method to develop Ni–P decorated CeO2–TiO2 composite photocatalysts. The Ni–P decorated CeO2–TiO2 produces 1300 μmol of 99% pure hydrogen by catalyzing the water spitting reaction without any sacrificial agent.