Hydrothermal synthesis and formation mechanism of the anatase nanocrystals with co-exposed high-energy {001}, {010} and [111]-facets for enhanced photocatalytic performance

Abstract

Truncated tetragonal bipyramid anatase TiO₂ nanocrystals enclosed by {001}, {010} and {101} facets, and tetragonal cuboid anatase TiO₂ nanocrystals with co-exposed [111]- and {101} facets were hydrothermally synthesized by using the H⁺-form of the tetratitanate H₂Ti₄O₉ as the precursor and HF and H₂O₂ as the capping agent and solvent, respectively. The as-prepared anatase TiO₂ nanocrystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED) and nitrogen adsorption/desorption measurements. The transformation from the layered structure of tetratitanate HTO to anatase TiO₂ nanocrystals may experience two types of reactions including in situ topotactic transformation reaction by splitting the Ti–O–Ti bonds of the corner-shared by two TiO₆ octahedra along the [010]-direction of the HTO, and the dissolution–recrystallisation reaction along various crystal planes of the zigzag ribbon-like anatase crystal during the hydrothermal reaction process. Furthermore, the photocatalytic activities of the as-prepared anatase nanocrystals were evaluated by the photocatalytic degradation of methylene blue under UV-light irradiation at room temperature in air. Truncated tetragonal bipyramid and cuboid coexistence of anatase TiO₂ nanocrystals with a large percentage of co-exposed high-energy {001}, {010} and [111]-facets exhibit high surface photocatalytic activities for the degradation amount of MB per unit surface area of catalyst (mg (MB) per m² (TiO₂ surface area)), which can be explained by the cooperative mechanism of the surface atomic structure and surface electronic structure of the different facets.