An aqueous synthesis of photocatalyst by selective dissolution of titanium oxide/hydroxyapatite composite

Abstract

A novel synthesis method of a highly active photocatalyst was proposed. Titanium dioxide (TiO 2) nano-particles were prepared by three-step procedure, precipitation of hydroxyapatite (HAp) on TiO 2 particles, heat treatment of the TiO 2/HAp composites, and acid treatment in hydrochloric acid. The unique point of this procedure is the selective dissolution of HAp to obtain exposed TiO 2 surfaces. The HAp precipitation was achieved by stirring TiO 2 powders in the mixtures of Ca(NO 3) 2 and NH 4H 2PO 4 aqueous solutions at pH 8.5. Then, the heat-treated TiO 2/HAp composites were treated with hydrochloric acid. The precipitated HAp avoided the direct contact of TiO 2 particles and suppressed the phase transformation from anatase-to-rutile >200 °C. The HAp also suppressed a decrease of specific surface area of TiO 2 during the heat treatment. The photocatalytic activities were evaluated from an absorbance decrease of methylene blue (MB) under ultraviolet (UV) irradiation. The MB photodecomposition was approximated to the first-order reaction and the reaction rate constants of the obtained TiO 2 powders heated at various temperatures were higher than those of conventional TiO 2 powders heated at same temperatures. The enhanced photocatalytic activity is attributed to the suppression effects for the phase transformation to rutile phase and the decreasing of specific surface area in the heat treatment.