The photocatalytic activity of TiO₂ is the result of an interplay between a considerable number of parameters, e.g., phase composition, electronic structure, particle size, exposed surface area, degree of aggregation, mobility of charge carriers, presence of impurities, amount and kind of defects, adsorption molecules from gas or aqueous phase, lateral interactions between adsorbed species, nature of solvent, etc. The specific function and influence of a given feature for the photocatalytic performance of a TiO₂ sample is difficult to characterize since many of the parameters are strongly coupled. Consequently, due to this plurality of variables driving the nature of the photocatalytic activity, it is of primary importance to develop a method to understand and control these properties (or at least some of them).
As a first approach to this complex problem we have carried out a combined experimental and theoretical study in order to explore these phenomena. The adsorption of oxalic acid from the aqueous phase at the surface of rutile nanoparticles has been investigated by ATR-FTIR measurements. The aqueous system was also studied quantum-chemically using the semiempirical method MSINDO. Geometry optimizations have been performed, and the vibration spectra of the most stable surface complexes have been calculated. A sequence of model types has been applied in the quantum-chemical calculation in order to take into account the effect of interaction of water and oxalic acid on the adsorption mechanism and the vibration spectra. It was found that the presence of the aqueous phase significantly changes the stability of the oxalic acid surface complexes compared with the bare TiO₂ surface. The combination of experimental and theoretical information allowed identification of three species as the main contributors to the surface speciation. Two bidentate species were found with the C-C bond parallel to the TiO₂ surface, one monoprotonated and one deprotonated, and a third species being monodentate and monoprotonated. These results have been used to interpret the experimental data obtained upon photocatalytic studies performed with this system.