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 ∼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.