Surface structure and reactivity of anatase TiO₂ crystals with dominant {001} facets

Hydrofluoric acid (HF)-assisted hydrothermal/solvothermal methods are widely used to synthesize anatase TiO₂ single crystals with a high percentage of {001} facets, which are generally considered to be highly reactive. We have used Density Functional Theory calculations and first principles molecular dynamics simulations to investigate the structure of these facets, which is not yet well understood. Our results suggest that (001) surfaces exhibit the bulk-terminated structure when in contact with concentrated HF solutions. However, (1 × 4)-reconstructed surfaces, as observed in UHV, become always more stable at the typical temperatures, 400-600 C, used to clean the as-prepared crystals in experiments. Since the (1 × 4)-reconstructed surfaces are only weakly reactive, our results predict that synthetic anatase crystals with dominant {001} facets should not exhibit enhanced photocatalytic activity, consistent with recent experimental observations.