Photocatalytic reduction of CO2 with water promoted by Ag clusters in Ag/Ga2O3 photocatalysts

Abstract

Ag loaded Ga₂O₃ (Ag/Ga₂O₃) photocatalysts were prepared by an impregnation method and examined for the photocatalytic reduction of CO₂ with water where CO, H₂ and O₂ 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/Ga₂O₃ sample while partially oxidized large Ag particles with the size of several–several tens of nm were observed in a less active Ag/Ga₂O₃ sample. Both Ag L₃-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 Ga₂O₃ surface. In situ FT-IR measurements of the Ag/Ga₂O₃ samples showed CO₃ stretching vibration bands assignable to monodentate bicarbonate and bidentate carbonate species chemisorbed on the Ga₂O₃ 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 Ga₂O₃ semiconductor, and the formation process would be promoted at the perimeter of the Ag clusters on the Ga₂O₃ surface by the effective separation of electron–hole pairs.