Unique Anatase TiO2 Twinning Crystals Formed by High-Energy {001} Facets and the Improved Photocatalytic Activity

For the first time, anatase TiO2 twin crystals have been synthesized by a simple hydrothermal method. The samples are characterized using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, ultraviolet–visible diffused reflectance spectra, and nitrogen sorption isotherms. It is found that a prolonged reaction time significantly contributes to the structure evolution and that twin crystals form via the oriented attachment (OA)- and Ostwald ripening (OR)-controlled secondary growth mechanism. Remarkably, the photoactivity of TiO2 twin crystals is 2.57 and 8.74 times higher than that of single-crystalline anatase TiO2 nanosheets and rutile TiO2, respectively. This has been mainly ascribed to the unique twin crystal structure. This work is a big step toward further improvement of the photocatalytic properties of of TiO2.