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.