Continuous Hydrothermal Synthesis of Pr-Doped CaTiO3 Nanoparticles from a TiO2 Sol

Continuous hydrothermal synthesis of Pr doped CaTiO3 nanoparticles from Pr(NO3)(3), Ca(NO3)(2), a TiO2 sol (crystallite diameter of 5 nm), and various aqueous solutions of KOH was carried out at 673 K and 30 MPa. The synthesis at a very short residence time of 0.02 s could be examined by using a T-type micromixer for rapid heating of the aqueous solutions to 673 K. Pr-doped CaTiO3 nano particles having an average particle diameter of 22 nm and a strong red emission peak of about 613 nm were continuously produced at a residence time of 5.0 s. Further, the effects of residence time and KOH molality on the Ca/Ti ratio, particle diameter, crystallite diameter, and crystal structure of the products were carefully studied in order to discuss the formation mechanism of mainly the CaTiO3 structure in Pr doped CaTiO3 nanoparticles from a system containing a solid oxide (TiO2). With increasing residence time and also KOH molality, the Ca/Ti ratio of the product increased up to a stoichiometric ratio of CaTiO3 (1.0), major crystal phase was changed from TiO2 (anatase, tetragonal) to CaTiO3 (orthorhombic), the diameter of TiO2 decreased, and that of CaTiO3 increased. On the basis of the results, the following formation mechanism is proposed: dissolution of the TiO2 sol, formation of a hydroxide precursor including Ca2+ and Ti4+ though hydrolysis, and nucleation growth of CaTiO3 through dehydration condensation.