Germanium-substituted Zn2TiO4 solid solution photocatalyst for conversion of CO2 into fuels

Photocatalytic CO2 reduction conjugated with H2O oxidation is regarded as a promising artificial photosynthesis system because it can simultaneously solve energy and environment problems. Here, a novel solid solution, Zn2Ti1-xGexO4 (0 <= x <= 0.15), with high photocatalytic activity for the reaction was successfully synthesized via a facile molten salts route. The Zn-based solid solutions with size about 200 nm have a homogeneous inverted cubic spinel structure (Fd3m) and a continuously modulated band gap with the Ge content. For the CO2 reduction reaction with H2O under simulated solar irradiation, the Zn2Ti1-xGexO4 solid solutions display not only high activity for the conversion of CO2 into CH4 and CO fuels, but also long-term stability (>60 h of catalytic reaction). Experimental results and theoretical calculations indicated that the conduction and the valence bands of the cubic spinet Zn2TiO4 are positively shifted by introducing Zn2GeO4 with a pseudocubic inverse spinel structure, but the band gaps of solid solutions are simultaneously modulated with the introduction of germanium. Nevertheless, the Zn2TiO4 affords a light-carrier effective mass and strong electron delocalization by forming a solid solution with Zn2GeO4, which is beneficial to improving migration of photogenerated electrons and holes. As a synergistic result of band gap narrowing and high carrier diffusion, good conversion efficiencies for production of solar fuels through the reactions of CO2 reduction with H2O are achieved. (C) 2019 Elsevier Inc. All rights reserved.