IR Spectroscopic Behavior of Polaronic Trapped Electrons in TiO₂ under Aqueous Photocatalytic Conditions

Attenuated total reflection infrared spectroscopy has been used to study a broad mid-IR absorption attributed to electron polarons that was exhibited during ultraviolet (UV) irradiation of anatase TiO 2 particle films under aqueous photocatalytic conditions. The influence of formate hole scavenging and proton intercalation that accompanies electron trapping was investigated in anoxic solution conditions. The UV excitation and decay kinetics of the polaronic absorption (PA) varied with chemical conditions that influenced electron and hole scavenging and proton intercalation reactions. A striking near-linear decay of the PA under certain conditions indicated saturation of surface electron and oxygen sites, giving a constant reaction rate. The PA band shape varied with absorbance and between anatase samples. Water absorption losses concomitant with the PA have been ascribed to polaron-induced variation of sample refractive index. The PA likely arises from the photoionization of large polarons.