In Situ ATR-FTIR Investigation of the Effects of H2o and D2o Adsorption on the TiO2 Surface

Abstract

Surface hydroxyl groups on TiO2 play an important role for the photoinduced hydrophilicity as well as for photocatalytic reactions. Adsorbed species such as water and organic compounds are expected to affect the hydroxyl group behaviour on the TiO2 surface, however, their influences have to date not been clearly determined. Herein, a detailed investigation of H2O and D2O adsorption on the TiO2 surface (anatase) in the dark and under UV(A) irradiation is reported using ATR-FTIR spectroscopy. The adsorption and photocatalytic degradation of acetate on the TiO2 surface was also investigated in H2O and D2O. Adsorption of H2O with different ratios of D2O revealed an isotopic exchange reaction occurring on the surface of TiO2 in the dark. This isotopic exchange leads to a change in the behaviour of acetate with the intensity of the OH bending band between the carboxylate and the OH group at 900 cm-1 decreasing and ultimately disappearing in the dark. Upon illumination with UV(A) light both, OH and OD groups are formed, which in turn increase the hydrophilicity of the TiO2 surface. The amount of OH and OD groups was found to increase in the presence of molecular oxygen. This increase is most likely caused by the photoadsorption of H2O and D2O due to photoinduced charge transfer processes [1]. On the other hand, the investigations show that during UV(A) irradiation the acetate molecules can adsorb again on the TiO2 surface which is explained by an increased surface area due to a de-aggregation of the particles agglomerates [2]. Based upon the observed correlation between the carboxylate and the OH and/or OD groups at neutral pH (pH ≈ pHzpc) it is obvious that acetate preferentially adsorbs on the positively charged anatase in a monodentate structure. As an in-situ technique, ATR-FTIR studies provide evidence of the adsorption behaviour of water and D2O on TiO2 surfaces thus yielding new insights into the mechanism of photoinduced hydrophilicity and of photocatalytic reactions.