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

Sue, Kiwamu, Kawasaki, Shin-ichiro, Sato, Takafumi, Nishio-Hamane, Daisuke, Hakuta, Yukiya, Furuya, Takeshi
Industrial & Engineering Chemistry Research 2016 v.55 no.28 pp. 7628-7634
aqueous solutions, calcium, calcium nitrate, crystal structure, crystallites, engineering, heat, hydrolysis, nanoparticles, particle size, potassium hydroxide, titanium, titanium dioxide
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 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 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.