FTIR spectroscopy of alcohol and formate interactions with mesoporous TiO₂ surfaces

The effects of pH and ultraviolet (UV) light with ligated formic acid on mesoporous TiO₂ were characterized by transmission Fourier transform infrared (FTIR) spectroscopy and compared with adsorbed formate complexes. Surface-modified anatase thin films were prepared from acidic aqueous nanoparticulate anatase suspensions diluted with methanol and ethanol. Bands assigned to carboxylic acid groups displayed unique bonding character in the ligated formic acid on the anatase surface. For increased proton concentrations in the films, separation in -COO stretching bands (Δv) for formic acid increased (increase in frequency for _vC=O and decrease in frequency for vC-O). With UV exposure, surface-bound organics were rapidly removed by photocatalytic oxidation at 40 °C and 40% relative humidity (RH). In addition, the Av of the formic acid bands decreased as organics were mineralized to carbonates and CO₂ with UV light. Aqueous formic acid adsorption experiments showed a distinctly different bonding environment lacking carbonate, and the Δv for the carboxylic groups indicated a bridging bidentate coordination. The Δv of the bands increased with increasing proton concentration, with both bands shifting to higher wavenumbers. The shifts may be ascribed to the influence of protonation on surface charge and the effect of that charge on the electronegativity of carboxylate groups bound to the surface. As alcohols are used in the mesoporous TiO₂ solar cell preparation, implications of these surface modifications to dye-sensitized photovoltaics are discussed.