Mesocrystalline Nanocomposites of TiO₂ Polymorphs: Topochemical Mesocrystal Conversion, Characterization, and Photocatalytic Response

Abstract

Four kinds of platelike mesocrystalline nanocomposites of TiO2 polymorphs were successfully synthesized for the first time based on a topochemical mesocrystal conversion mechanism. In this conversion process, a [010]-oriented titanate H1.07Ti1.73□0.27O4 (□: vacancy of Ti) single crystal with lepidocrocite-like structure and platelike morphology was successively transformed into [001]- and [102]-oriented TiO2(B) phases including a {010}-faceted TiO2(B) twinning, [010]-oriented anatase phase, and [110]-oriented rutile phase. The platelike particle morphology is retained in the topochemical conversion process. The platelike particles are constructed from nanocrystals which well-aligned in the same orientation for the same phase, resulting in the formations of HTO/TiO2(B), HTO/TiO2(B)/anatase, TiO2(B)/anatase, and anatase/rutile mesocrystalline nanocomposites. The reaction mechanism and the crystallographic topological correspondences between the precursor, intermediates, and the final product were given on the basis of the nanostructural analysis results. The mesocrystalline nanocomposite of anatase/rutile polymorphs exhibits unexpectedly high surface photocatalytic activity, which can be explained by the superior electron–hole separation effect and the high activity of {010}-faceted anatase surface in the mesocrystalline nanocomposite. Such mesocrystalline anatase/rutile nanocomposite is an ideal photocatalytic system.