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

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Continuous hydrothermal synthesis of Pr-doped CaTiO 3 nanoparticles from Pr(NO 3 ) 3, Ca(NO 3 ) 2, a TiO 2 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 CaTiO 3 nanoparticles 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 CaTiO 3 structure in Pr-doped CaTiO 3 nanoparticles from a system containing a solid oxide (TiO 2 ). With increasing residence time and also KOH molality, the Ca/Ti ratio of the product increased up to a stoichiometric ratio of CaTiO 3 (1.0), major crystal phase was changed from TiO 2 (anatase, tetragonal) to CaTiO 3 (orthorhombic), the diameter of TiO 2 decreased, and that of CaTiO 3 increased. On the basis of the results, the following formation mechanism is proposed: dissolution of the TiO 2 sol, formation of a hydroxide precursor including Ca 2+ and Ti 4+ though hydrolysis, and nucleation–growth of CaTiO 3 through dehydration condensation.