ATR-IR Spectroscopic Studies of Trapped Electrons in Titanium Dioxide

Abstract

The research presented in this thesis predominantly explored two topics using attenuated total-internal reflection infrared (ATR-IR) spectroscopy. Firstly, the adsorption of small molecules to nanoparticulate TiO2 films was investigated to better characterise the spectroscopic properties of interfacial species on hydrated TiO2. Rehydration of dried samples revealed an intense absorption at ~890 cm-1 that was unobserved in aqueous conditions, which was attributed to a rocking vibrational mode of highly restricted water in the 2nd hydration layer. Studies of the influence of pH variation on TiO2 surface speciation revealed several bands between 1100 and 800 cm-1 that were attributed to deformation vibrations of Ti–OHx groups, which have received little characterisation under aqueous conditions. Adsorption of fluoride ions displaced adsorbed hydroxyls and confirmed that these modes originated from Ti–OHx species. Additionally, a prominent absorption loss exhibited 835 cm-1, which was concomitant with fluoride adsorption, was attributed to a surface phonon mode. Lastly, adsorption studies of small carboxylic acids such as formate, oxalate, and α-hydroxy acids, are presented and the nature of the adsorbed species discussed.

Secondly, the fundamental behaviour of a broad, asymmetric IR absorption exhibited during UV irradiation of TiO2, and attributed to trapped electrons, was investigated. This shallow trap infrared absorption or STIRA was characterised through studies of the influence of pH, dissolved oxygen, hole scavenging and fluoride adsorption. Oxygen suppressed the STIRA under all conditions as a result of electron scavenging and oxygen concentration directly influenced the rate of STIRA decay after termination of UV irradiation. In the absence of oxygen, the STIRA was enhanced by increased [H+] and this was ascribed to a facilitating role of proton intercalation, as supported by fluoride adsorption studies that pointed to little influence from adsorbed H+. Formate and oxalate also enhanced the STIRA via hole scavenging (electron injection) and the intensity of the STIRA correlated with the IR absorptions from adsorbed species. Thus, the rate of hole scavenging was directly related to adsorbed species surface coverage. Hole scavenging by adsorbed species also generated a secondary STIRA phenomenon and this was attributed to electrons trapped near the TiO2 particle surfaces. The final chapter of this thesis reconciles many experimental and theoretical observations relating to the STIRA. The chemical sensitivity of the trapped electrons, the physical nature of the trap state, and the origin of the IR absorption are discussed.