Characterization of surface defect sites on bulk and nanophase anatase and rutile TiO₂ by low-temperature specific heat

The specific heats of highly hydrated 7 nm nanoparticles and lightly hydrated 200 nm particles of rutile and anatase show a small, broad maximum below 5 K. The excess heat capacity can be modeled with a two-level system having very small molar weighting factors (∼10^-3 for nanoparticles, ∼10^-5 for bulk) and energy separations of approximately (4.88 ± 0.06) K (7 nm particles) and (2.5 ± 0.3) K (200 nm particles). The weighting factors correspond to the number of very high-energy surface sites identified by adsorption calorimetry and to the number of isolated 3-coordinate Ti defects observed on models built to the nanoparticle morphologies. We attribute the features to water adsorbed at defect sites on the particle surfaces. The very low energies associated with the Schottky-type heat capacity maxima are consistent with tunnel splittings of the ground state of a multiple-well potential. A double-well potential model for the hydrogen bonding of an OH group associated with the defects is consistent with the maxima observed for both the 7 and 200 nm particles.