Reduction of carbon dioxide (CO2) with photocatalysts, an artificial photosynthesis, has been widely studied from the viewpoints of contribution to the energy, environmental and carbon resource issues. The photocatalytic reduction of CO2 with water is one of the most challenging catalytic reactions, because the reduction ability of water is much lower as compared to other reduction reagents such as hydrogen (H2). Recently, we found that the photocatalytic reduction of CO2 with water proceeded over a bare Ga2O3 and the activity increased by loading Ag nanoparticles on Ga2O3 (Ag/Ga2O3). The photocatalytic activity is likely to correlate with the chemical states and/or the size of the Ag nanoparticles which depend on the preparing method of the photocatalysts.
In this study, we prepared Ag/Ga2O3 photocatalysts by using solution plasma method (SPM). The SPM is a new preparing method of metal nanoparticles without any dispersants in an aqueous solution with electrolytes. The Ag nanoparticles were synthesized by glow discharge between Ag rods in an aqueous solution. The synthesized Ag NPs were loaded on gallium oxide (Ga2O3) photocatalyst by filtering the solution with Ga2O3 powder, and the photocatalytic activities of the obtained Ag/Ga2O3 samples were evaluated.
Although the photocatalytic reduction of CO2 with water proceeded over all the Ag/Ga2O3 photocatalysts to produce CO, the CO production rates decreased with the reaction time. Measurements of DR UV-vis spectra and TEM images revealed that a part of the Ag NPs migrated and aggregated on the photocatalyst surface to become larger particles during the photocatalytic reaction, which would be related to the decrease of the photocatalytic activity. It was also found that the photoirradiation treatment on the prepared Ag/Ga2O3 sample before the use for the photocatalytic reaction improves the photocatalytic performance.