Photocatalytic reduction of CO2 with water promoted by Ag clusters in Ag/Ga2O3 photocatalystsElectronic supplementary information (ESI) available. See DOI: 10.1039/c5ta04815j

Ag loaded Ga 2 O 3 (Ag/Ga 2 O 3 ) photocatalysts were prepared by an impregnation method and examined for the photocatalytic reduction of CO 2 with water where CO, H 2 and O 2 were formed as products. TEM and X-ray absorption near edge structure (XANES) measurements revealed that around 1 nm sized Ag clusters were formed predominantly in an active Ag/Ga 2 O 3 sample while partially oxidized large Ag particles with the size of several-several tens of nm were observed in a less active Ag/Ga 2 O 3 sample. Both Ag L 3 -edge and O K-edge XANES analyses suggested that the small Ag clusters accepted more electrons in the d-orbitals as a result of the strong interaction with the Ga 2 O 3 surface. In situ FT-IR measurements of the Ag/Ga 2 O 3 samples showed CO 3 stretching vibration bands assignable to monodentate bicarbonate and bidentate carbonate species chemisorbed on the Ga 2 O 3 surface, and to monodentate carbonate species on the large Ag particles. Among these chemisorbed species, the monodentate bicarbonate and/or the bidentate carbonate species changed to bidentate formate species, as the reaction intermediate, under UV light irradiation. The bidentate formate species was formed not by the plasmonic excitation of the Ag nanoparticles but by the photoexcitation of the Ga 2 O 3 semiconductor, and the formation process would be promoted at the perimeter of the Ag clusters on the Ga 2 O 3 surface by the effective separation of electron-hole pairs. The structure and function of the Ag cocatalyst in Ag loaded Ga 2 O 3 photocatalysts for photocatalytic CO 2 reduction with water were investigated. It was found that around 1 nm sized Ag clusters had strong interactions with the Ga 2 O 3 surface and promoted the formation of bidentate formate species as the reaction intermediate for the CO production.