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

• Flame spray pyrolysis (FSP) was employed for synthesis of Fe/Al2O3 nanoparticles.
• In the absence of iron precursor, FSP produced a mixture of two Al2O3 polymorphs.
• Formation of a solid solution (FexAl2O3 + x) was observed.

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 (FexAl2O3 + 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.

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