Insight into the crystal lattice formation of brookite in aqueous ammonia media: the electrolyte effect

Abstract

A feasible approach to synthesize pure brookite was achieved in an aqueous ammonia system via a hydrothermal process. By hydrothermal treatment of the titanate that directly hydrolyzed from tetrabutyl titanate (TBOT), a phase transition from titanate to TiO₂ was observed with XRD. Anatase TiO₂ was produced with an unobstructed closure of titanate layers. The phase transition process can be controlled by adding an electrolyte such as NaCl into the hydrothermal media. Na⁺ ions locally stop the direct closure of titanate layers at their adjacent position, which induce the formation of a brookite-like structure. The optimal hydrothermal reaction parameters for the crystal lattice formation of brookite in aqueous ammonia media were found to be 180 °C, 0.5 M NaCl and ≥72 h. The phase transition process of titanate precursor to TiO₂ depends heavily on the electrolyte. Metal cations play a crucial role in conducting the transition procedure from layered titanate into brookite or anatase. Na⁺ is preferred to conduct the phase transition from titanate to brookite rather than K⁺ and Li⁺. Although anions do not determine the phase transition rule during the hydrothermal process, they are influential in the phase transition process, as they can promote the phase transition by interacting with the TiO₆ octahedra.