Effects of Titania Surface Orientation on the Heterogeneous Formation of Barium Titanate by Hydrothermal Synthesis

Abstract

We investigated the mechanism of hydrothermal synthesis of BaTiO3 in the system of Ba(OH)2 and TiO2 in the early growth stage, based on model experiments using various single-crystal rutile TiO2 substrates with (001), (110), (100), and (101) orientations as starting reactants and a practical case study of widely utilized anatase TiO2 powder. The BaTiO3 products represented heterogeneous island growth directly on rutile TiO2 substrates and morphological features depending on the surface orientations of the substrates. Well-defined orientation relationships between BaTiO3 and the TiO2 substrate depended on the surface crystal planes of rutile TiO2 and were classified into four independent crystallographic types, including the unique topotaxial relationship already reported in barium diffusion-induced solid-state reactions. The hydrothermally treated anatase powder exhibited a dumbbell-like precipitation of BaTiO3 on the anatase TiO2 particles, preserving a crystallographic relationship of (001)[100] BaTiO3 ∥ (001)[100] anatase. The experimental findings revealed the heterogeneous dissolution–precipitation mechanism via surface orientation-dependent epitaxial growth of BaTiO3 directly on rutile and anatase TiO2 in the initial stage of hydrothermal reactions, suggesting the importance of controlling surface morphology and crystallinity of TiO2 as hydrothermal synthesis sources.