Hydrothermal synthesis of perovskite-type MTiO₃ (M = Zn, Co, Ni)/TiO₂ nanotube arrays from an amorphous TiO₂ template

Ordered perovskite-type MTiO₃/TiO₂ nanotube arrays (NTAs) (M = Zn, Co, Ni) are prepared by a general hydrothermal route based on amorphous TiO₂ NTAs via electrochemical anodization of Ti foil. The as-anodized amorphous TiO₂ is not stable and can react with H₂O in solution producing soluble Ti(OH)₆ ^2- to form anatase nanoparticles (NPs) via water-induced dissolution and recrystallization. The pH and salt content in the solution play important roles in the morphology and composition of the hydrothermal products. In the presence of a metal acetate, the reaction between Ti(OH)₆ ^2- and H⁺ is dramatically restricted and the reaction proceeds preferentially between Ti(OH)₆ ^2- and M²⁺ (M = Zn, Co, Ni) to produce insoluble MTiO₃ NPs which adhere onto the original architecture in situ to form perovskite-type MTiO₃/TiO₂ NTAs. This study elucidates the role of the amorphous structure in the formation of MTiO₃ and provides a general means of synthesizing nanostructured MTiO₃.