Influence of Formate Adsorption and Protons on Shallow Trap Infrared Absorption (STIRA) of Anatase TiO2 During Photocatalysis

ATR-IR spectroscopy has been used to probe the spectral changes resulting from UV irradiation of thin particulate films of anatase TiO2 under aqueous anoxic conditions. Irradiation under circumneutral solutions removed adsorbed impurities and produced a transient broad IR absorption peaking at similar to 880 cm(-1) that has been attributed to excitations associated with shallow electron traps. This shallow trap IR absorption (STIRA) was long-lived under acidic conditions with peak absorbance rising with decreasing pH. Formate was used as a hole scavenger to probe the impact of hole removal on the electron trapping processes. The adsorption of formate to anatase TiO2 was investigated in the absence of UV irradiation with the spectra showing a mixture of inner- and outer-sphere adsorbed species. UV irradiation of anatase films in the presence of formate at low concentrations enhanced STIRA intensity substantially. The STIRA scaled with TiO2 mass was more pronounced from larger crystallite anatase and was sensitive to [H+], suggesting that formation of the shallow trap state is accompanied by proton intercalation.