Synthesis and characterization of Al2O3 nanoparticles by flame spray pyrolysis (FSP): Role of Fe ions in the precursor

Iron doped aluminium oxide nanoparticles are of interest for number of applications (e.g. water treatment, catalytic conversion of exhaust gases) due to their high surface area, hardness, catalytic and magnetic properties. In the present study, flame spray pyrolysis (FSP) was employed for the synthesis of Fe/Al₂O₃ nanoparticles. Precursor solutions of aluminium acetylacetonate (0.2 mol·L^-1) and ferrocene (0 to 0.2 mol·L^-1) in toluene were used to synthesise pure and iron (Fe) doped Al₂O₃. The particle composition and morphology were studied and effect of iron concentration was analysed. It was found that in the absence of the iron precursor, FSP produced a mixture of two Al₂O₃ polymorphs: θ-Al₂O₃ and η-Al₂O₃. The addition of ferrocene as an iron precursor was found to suppress formation of θ-Al₂O₃. At an iron molar concentration of 0.2 mol·L^-1 mainly hercynite, FeAl₂O₄, was observed. Furthermore, increasing the iron concentration caused a linear shift of the X-ray diffraction peaks from positions corresponding to η-Al₂O₃ to those of FeAl₂O₄. This indicates the formation of a solid solution (Fe_xAl₂O_3+x) at intermediated concentrations. It was also found that the primary particle size, which was below 10 nm, did not significantly change with the increased iron concentration and was comparable to the mean crystallite size indicating that size of these single crystalline primaries is determined by the synthesis process rather than the chemistry of the product. However, the hydrodynamic size was around 180 nm indicating that the particles are agglomerates in the water suspension. Additionally, zeta potential of the nanoparticles was found to decrease slightly with increasing iron content, though in all cases it was above 50 mV. Finally, the potential of synthesized nanoparticles was examined for the removal of fluoride because fluoride causes harmful health effects to human health at elevated concentrations. The results of fluoride removal using synthesized nanoparticles produced in this study showed that the highest fluoride removal efficiency was observed for the sample having no iron content.