Continuous Hydrothermal Synthesis of Nickel Ferrite Nanoparticles Using a Central Collision-Type Micromixer: Effects of Temperature, Residence Time, Metal Salt Molality, and NaOH Addition on Conversion, Particle Size, and Crystal Phase

• Published in: Industrial & engineering chemistry research Vol. 50; no. 16; pp. 9625 - 9631
• Main Authors: Sue, Kiwamu, Aoki, Mitsuko, Sato, Takafumi, Nishio-Hamane, Daisuke, Kawasaki, Shin-ichiro, Hakuta, Yukiya, Takebayashi, Yoshihiro, Yoda, Satoshi, Furuya, Takeshi, Sato, Toshiyuki, Hiaki, Toshihiko
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 17.08.2011
• Subjects: Applied sciences; Chemical engineering; Concentration (composition); Conversion; Exact sciences and technology; Iron; Materials and Interfaces; Nanoparticles; Nickel; Nickel ferrites; Particle size; Synthesis; Temperature effect; Particle size; Micromixing; Nanoparticle; Collision
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/ie200036m

Continuous hydrothermal synthesis of nickel ferrite nanoparticles from Fe(NO3)3 and Ni(NO3)2 was performed using a central collision-type micromixer developed for rapid heating of a starting solution to the reaction temperature and homogeneous nucleation. Temperature, residence time, and nitrate molality were varied in the ranges 573–673 K, 0.02–2.00 s, and 0.05–0.50 mol/kg, respectively. The effects of temperature, residence time, nitrate molality, and NaOH addition on conversion, Ni/Fe molar ratio, particle size, and crystal phase were examined using ICP spectroscopy, EDX spectroscopy, TEM, and XRD. In the cases without NaOH, the Ni conversion was less than 2% at temperatures up to 623 K and increased dramatically to around 50% at 673 K, whereas the Fe conversion was more than 94% at all temperatures. In terms of conversion, the Ni/Fe molar ratio was less than 0.01 at temperatures up to 623 K, and stable nickel ferrite was not produced. By contrast, at 673 K, the Ni/Fe molar ratio increased sharply to more than 0.2, and stable nickel ferrite could be obtained. With increasing residence time at 673 K, the Ni conversion and Ni/Fe ratio increased, and the lattice parameter decreased from 8.35 to 8.34 Å. These results indicate that the products at an early stage of the reaction are similar in structure to γ-Fe2O3 and can be considered as a Ni-deficient NiFe2O4 whereas the products at a later stage have a structure close to that of NiFe2O4. In addition, the average particle size increased slightly from 5.2 to 7.4 nm at 0.05 mol/kg and increased markedly from 5.8 to 12.3 nm at 0.50 mol/kg with increasing temperature despite the high Fe conversion of >97% at the shortest residence time of 0.02 s. In the cases with NaOH, smaller nanoparticles of less than 5.0 nm with a stoichiometric Ni/Fe molar ratio of 0.5 were produced at 673 K. On the basis of these results, the mechanisms of nucleation and growth in the nickel ferrite synthesis are discussed.