Synthesis of spindle and square bipyramid-shaped anatase-type titanium dioxide crystals by a solvothermal method using ethylenediamine

Morphological control as well as formation of nanoparticles of anatase-type titanium dioxide has been achieved by a solvothermal treatment of a water-soluble citratoperoxotitanium (CPT) complex using ethylenediamine (EDA) as a key solvent in the presence of a given amount of water. When the EDA concentration in the whole solvent (EDA + H₂O) remained low less than 12 M, the powder obtained after the solvothermal treatment consisted of spindle-like anatase particles. With an increase of the EDA concentration in the solvent, the shape of the obtained particles changed from spindle to square bipyramid. When another water-soluble oxoglycolatoperoxotitanate complex or TiCl₄ was used as a starting source of Ti, similar shaped anatase particles could be obtained by a careful choice of synthetic conditions. Using a mixed solvent of EDA and water, sheet-like titanic acid was produced as an intermediate phase, regardless of the EDA concentration. Therefore, the formation of anatase in the current system may take place through dissolution-recrystallization of titanic acid. The formation of square bipyramidal anatase particle achieved in the present study is not agreement with a reported theoretical calculation of equilibrium phase, because the calculation reveals that truncated square bipyramid-like anatase is obtained under an acidic condition and spindle-like anatase was formed under a basic condition. An adsorption of EDA on particle surface and the pH of the solvent may contribute to the achievement of morphological control.