Enhanced Photocatalysis from Truncated Octahedral Bipyramids of Anatase TiO₂ with Exposed {001}/{101} Facets

In this study, we develop a new synthetic method to grow anatase TiO₂ crystals composed of truncated octahedral bipyramids (TOBs) with exposed {001} and {101} facets by a vapor-solid reaction growth (VSRG) method. The VSRG method employs TiCl_4(g) to react with CaO_(s)/Ca(OH)_2(s) at 823-1043 K under atmospheric pressure. The O-deficient pale-blue TOB TiO₂ crystals display high amount of both {001} and {101} facets. Together, they decompose methylene blue photocatalytically under UV-visible (UV-vis) light irradiation. The most-efficient TOB catalyst VT923 (grown at 923 K, average edge length 400 nm, average thickness 200 nm, and surface area 4.20 m²/g) shows a degradation rate constant k, 0.0527 min^-1. This is close to that of the P25 standard 0.0577 min^-1. However, the surface area of P25 (46.8 m²/g) is about 12 times that of VT923. The extraordinary performance of VT923 is attributed to the presence of high amount of coexisting {001} and {101} facets to form effective surface heterojunctions. They would separate photogenerated electrons and holes effectively on {101} and {001} surfaces, respectively. For VT923, the {001}/{101} ratio is 0.764, which is close to 1, the highest value observed for all TOB samples grown in this study. The surface heterojunctions prolong the electron-hole separation so that VT923 demonstrates the excellent photocatalytic capability. In addition, residual Cl atoms on the exposed faces are easily removed to show clean TiO surface layers with sufficient amount of O-deficient sites in the current samples.