Robust, double-shelled ZnGa2O4 hollow spheres for photocatalytic reduction of CO2 to methane

Robust, double-shelled ZnGa2O4 hollow spheres were successfully fabricated by hydrothermally treating an aqueous solution containing Zn(II), Ga(III), and citric acid, followed by annealing at 600 °C, 700 °C, or 800 °C in air to remove the carbon species. The hollow structure is expected to trap incident photons to enhance the light absorbance. The sample annealed at 700 °C exhibited the optimized photocatalytic performance in the reduction of CO2 in the presence of water vapor to methane. This property is ascribed to the improved crystallinity of the sample, which has fewer defect centers for the recombination of electron–hole pairs compared with that annealed at 600 °C. The reduced performance of the sample done at 800 °C relative to the one annealed at 700 °C is attributed to the formation of additional impurities besides ZnGa2O4, possibly due to partial Zn(II) evaporation at higher temperature leading to segregation of potential Ga-based oxides. RuO2 and Pt were loaded onto the sample surface to greatly enhance the photocatalytic performance. The best photocatalytic performance was observed in the sample co-loaded with Pt and RuO2.