TiO₂ stability landscape: Polymorphism, surface energy, and bound water energetics

The energetics of pure-phase rutile nanorods and spherical anatase nanoparticles have been studied by high-temperature drop solution calorimetry in 3Na₂O-4MoO₃ solvent at 975 K and water adsorption calorimetry in a wide range of particle sixes (surface area) from 6 to 40 nm (5-270 m²/g). The calorimetric surface enthalpies for rutile and anatase. calculated as 2.22 ± 0.07 and 0.74 ± 0.04 J/m ², respectively, are in general agreement with Ranade et al.'s results (Proc. Natl. Acad, Sci. U.S.A., 2002, 99, 6476), although their numerical values are somewhat different because of impurities and unaccounted bound water in the previous work. This study supports the energy crossovers previously proposed for the TiO₂ polymorphs. The energetics of water adsorption were measured using a commercial Calvet microcalorimeter coupled with a gas dosing system. This permitted the calculation of differential and integral enthalpies of water adsorption that characterize how tightly water binds to rutile and anatase surfaces and the calculation of adsorption entropies, which reflect the surface mobility of adsorbed water. The integral enthalpy of tightly bound water (relative to liquid H₂O standard state) is -18 kJ/mol for anatase and -40 kJ/mol for rutile. As seen previously for Al₂O₂, the TiO₂ polymorphs with higher surface energy bind water more tightly. The calculated entropy changes for the adsorption of water on TiO₂ are more negative than the entropy changes for the condensation of gaseous water to hexagonal ice. This finding suggests highly restricted mobility of molecules adsorbed at initial stages of adsorption (low coverage) and, possibly, dissociative adsorption on both rutile and anatase surfaces. However, nanoparticles contain both tightly bound water and loosely bound water. The latter is characterized by energetics of bulk water. The stabilizing water contribution to the overall energy of the system makes the hydrated nanophase samples more stable. The recommended transformation enthalpy for bulk anatase to bulk rutile is -1.7 ± 0.9 kJ/mol.