Adsorption of oxalate on rutile particles in aqueous solutions: a spectroscopic, electron-microscopic and theoretical study

The adsorption of oxalic acid from the aqueous phase at the surface of rutile nanoparticles has been investigated by attenuated total-reflection Fourier-transformed infrared (ATR-FTIR) measurements. A combination of high resolution transmission electron microscopy (HRTEM) and Wulff-type construction was used to elucidate the typical morphology of the nanocrystals. It is estimated that (110)-type facets present more than 85% of the exposed surface in the powder. 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 calculations 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.