Insitu FTIR studies of the photo-electrochemical behaviour of thermal TiO2 films as a function of temperature

Abstract

The photo-electrochemical behaviour of thermal TiO₂ films in aqueous methanol electrolytes has been investigated as a function of temperature with insitu Fourier transform infrared (FTIR) spectroscopy. Although the electrochemical behaviour of the films at 10 and 50°C was apparently identical on the basis of cyclic voltammetry, both in the light and in the dark, investigation with FTIR showed very different product distributions at these two temperatures, which could be correlated with other data, suggesting a significant reduction in methanol adsorption at the higher temperature. The O₂ bubble formation observed under irradiation in the absence of methanol at both temperatures was completely suppressed at 10°C in the presence of methanol, and only observed at higher potentials at 50°C. Adsorbed methanol was photo-oxidised completely to CO₂, but solution-phase methanol was oxidised only to formaldehyde at lower temperatures and to a mixture of the latter and formic acid at 50°C. An explanation of this second difference, and of the fact that photo-oxidation at the higher temperature could also lead to O₂ formation, can be given in terms of the enhanced mobility of adsorbed ^•OH species at 50°C. Finally, a significant number of the spectra obtained were dominated by unusual water-like features; these features can be assigned to changes in the spectral response of water as a function of ionic strength, and considerable caution is needed in the interpretation of such spectra.