Shape-dependent surface energetics of nanocrystalline TiO₂

We report the direct determination of surface enthalpies for nanophase TiO₂ anatase with different morphologies derived from drop solution calorimetry in a molten sodium molybdate (3Na₂O4MoO₃) solvent at 702 °C. The energetics of surface hydration has been measured using a Calvet microcalorimeter coupled with a gas dosing system. The surface enthalpies of hydrated surfaces for anatase TiO₂ nanoparticles, nanowires and sea-urchin-like assemblies are 0.51 ± 0.05, 1.07 ± 0.28, and 1.29 ± 0.16 J m^-2, respectively, whereas those of anhydrous surfaces are 0.74 ± 0.04, 1.24 ± 0.28, and 1.41 ± 0.16 J m^-2, respectively. The trend in TiO₂, which shows higher surface enthalpies for more complex nanostructures, is consistent with that reported in ZnO. The shape-dependent surface enthalpy at the nanoscale level is discussed in terms of exposed surface structures. The enthalpies of hydration appear to be similar for all morphologies.