Formation of Magnetic FexOy/Silica Core-Shell Particles in a One-Step Flame Aerosol Process

In this study, iron silicon oxide particles were generated in a one-step flame assisted spray pyrolysis (FASP) process using H-2/air or H-2/O-2 diffusion flames. A colloidal precursor solution was used, which contained dissolved iron nitrate and stably suspended silica nanoparticles. H-2/air flames resulted in magnetic FexOy/silica core-shell particles. There was a correlation between particle size and particle structure; particles larger than 500 nm had the coreshell structure, but smaller particles had non-core-shell structures. H-2/O-2 flames only resulted in nanoparticles that had non-coreshell structures. The core-shell particles had a iron oxide core that was hermetically enclosed in a uniform silica shell with a typical thickness of approximately 100 nm; they were superparamagnetic with a room-temperature saturation magnetization greater than 24 emu/g. Temperature history of the particles may be used to explain the correlation between flame type and particle structure. The correlation between particle size and structure may be due to size-dependent thermodynamic stability of the structures, or kinetics of heat and mass transfer. The results from this study suggest that micrometer sized iron oxide silica core-shell magnetic particles could be generated from a one-step flame aerosol process, but FexOy/silica nanoparticles (<100 nm) with the coreshell structure cannot be generated in a one-step flame aerosol process.