Shallow Electron Trap, Interfacial Water, and Outer-Sphere Adsorbed Oxalate IR Absorptions Correlate during UV Irradiation of Photocatalytic TiO2 Films in Aqueous Solution

IR spectroscopic analysis of thin particle film anatase-containing TiO2 systems during UV irradiation and when immersed in acidic anoxic aqueous solution reveals a striking broad asymmetric IR peak similar to 880 cm(-1) extending to over 2000 cm(-1), which has been previously attributed to electron excitations from a shallow trap level. The observations of this broad IR absorption in quasi-vacuum conditions or in aqueous solutions appear contradictory but are explained by the absence of electron scavengers or hole scavengers, respectively. The broad IR absorption associated with the shallow electron trap becomes more pronounced with a decrease in pH from 5 to 2 and is greatly enhanced in the presence of adsorbed oxalate. Oxalate ion adsorption/desorption kinetics has been followed with IR spectroscopy during UV irradiation to show that the shallow trap IR absorption correlates with that of outer-sphere adsorbed oxalate acting as a hole scavenger. The IR spectra indicate the involvement of interfacial water in the electron trapping, probably via bridged hydroxyl groups which are more prevalent under acidic solution conditions.