Coexistence of an anatase/TiO2(B) heterojunction and an exposed (001) facet in TiO2 nanoribbon photocatalysts synthesized via a fluorine-free route and topotactic transformation

Abstract

In this work, we report a novel approach to fabricate hierarchical TiO₂ microspheres (HTMS) assembled by ultrathin nanoribbons where an anatase/TiO₂(B) heterojunction and high energy facet coexist. The as-adopted approach involves (1) nonaqueous solvothermal treatment of a mixture of tetrabutyl titanate and acetic acid and (2) topotactical transformation into HTMS via thermal annealing. By this approach, the TiO₂(B) phase usually synthesized from an alkaline treatment route could be initially formed. Subsequently, phase transition from TiO₂(B) to anatase TiO₂ occurs upon thermal treatment. It is demonstrated that such phase transition is accompanied by crystallographic orientation along the c-axis of anatase and TiO₂(B) crystals, resulting in not only a coherent interface between two phases but also oriented attachment of anatase mesocrystals along the [001] direction, and finally high-energy (001) facet exposure. Interestingly, this work provides an alternative fluorine-free route for the synthesis of TiO₂ crystals with high-energy (001) facet exposure. The structural analysis reveals that lattice-match induced topotactic transformation from TiO₂(B) to anatase is the sole reason for the (001) facet exposure of anatase TiO₂. The photocatalytic test for acetaldehyde decomposition shows that HTMS with anatase/TiO₂(B) heterojunction and high-energy (001) facet exhibits superior photocatalytic efficiency compared with the relevant commercial product P25, which can be ascribed to the synergistic effect of large surface area, anatase/TiO₂(B) heterojunction as well as high-energy facet exposure.