Hydrothermal synthesis of highly pure brookite-type titanium oxide powder from aqueous sols of titanate nanosheets

Abstract

Hydrothermal synthesis of brookite-type titanium dioxide was examined using aqueous sols of titanate nanosheets, which were prepared with Ti[OCH(CH3)2]4 and N(CH3)4OH by a bottom-up process in aqueous solutions. Highly pure brookite powders were yielded at hydrothermal temperatures as low as 120 °C and N(CH3)4OH/Ti[OCH(CH3)2]4 molar ratios greater than 2. Although titanate nanosheets are formed at N(CH3)4OH/Ti[OCH(CH3)2]4 ≥ 0.4, the addition of excess N(CH3)4OH was critical in forming highly pure brookite powders. Excess N(CH3)4+ caused the precipitation of layered titanates, which were formed by stacking titanate nanosheets. Additionally, the crystal nucleation of brookite occurred in the precipitates. Then, the adsorption of titanate nanosheets on brookite particles caused the particles to transform into rod-like shapes elongated along the c-axis. The adsorbed titanate nanosheets were structurally converted into brookite under the influence of the underlying brookite crystal. Despite having a rough surface, the resulting brookite particles were single crystal-like. Moreover, the use of any tetraalkylammonium hydroxides other than N(CH3)4OH did not yield brookite. The addition of N(CH3)4OH to the reaction sols yielded precipitates of small titanate nanosheets. The morphology of the titanate nanosheets could have a significant influence on brookite crystallization.