Preparation of solid nickel nanoparticles by large-scale spray pyrolysis of Ni(NO₃)₂·6H₂O precursor: Effect of temperature and nickel acetate on the particle morphology

Spherical nickel nanoparticles were prepared by a large-scale spray pyrolysis process with two continuous reactors and were investigated in terms of particle-formation mechanism, surface property, particle shape, and size with changing the temperature of two continuous reactors and the type of precursor. When nickel nitrate was used as a precursor and the second reactor temperature was over 1200 °C, solid Ni particles were obtained, but many ultra-fine particles of less than tens nanometer were simultaneously formed via a gas-to-particle conversion mechanism. The formation of such ultra-fine particles was reduced by making hollow particles in the first reactor at 500 °C and thereafter densifying them in the second reactor at 1400 °C, but could not completely prevent. The addition of about 5-10 mol% nickel acetate instead nitrate precursor was found to be very effective to avoid the formation of such ultra-fine particles as well as producing solid particles with clean and smooth surface. On the basis of the results obtained, a mechanism of particle formation in the large-scale spray pyrolysis was proposed. Finally, spherical and solid nickel nanoparticles, which had clean surface and high density (larger than 8.4 g/cm³), were prepared from the mixed precursor (nitrate/acetate) at a residence time of about 3 s without any chemical additive.