Abstract: Without using aqueous ammonia and a surface modifier, a facile one-step method was developed to fabricate Fe nanoparticles coated with a SiO₂ shell (Fe@SiO₂) by a modified Stöber method combined with an aqueous reduction method. The Fe@SiO₂ was prepared by directly adding potassium borohydride to a mixed solution of tetraethylorthosilicate (TEOS) and anhydrous ferric chloride. The structure and morphology of the as-synthesized powders were investigated by X-ray powder diffraction (XRD), energy dispersion analysis of X-ray (EDAX), transmission electron microscopy (TEM), ultraviolet-visible (UV-Vis) absorption spectroscopy, Fourier-transform infrared (FTIR) spectrometry and X-ray photoelectron spectroscopy (XPS). The feasibility of using the prepared Fe@SiO₂ for the reductive immobilization of Cr(VI) in water was studied. The influence of TEOS addition on Cr(VI) removal by Fe@SiO₂ was investigated. The results showed that the prepared Fe@SiO₂ had a distinct core-shell structure. One or two Fe nanoparticles (20-30 nm in diameter) were homogeneously coated by a porous SiO₂ shell. With an increase in the amount of added TEOS the Fe nanoparticles had better dispersion and the thickness of the SiO₂ coating increased gradually. Compared with uncoated Fe nanoparticles, Cr(VI) removal by Fe@SiO₂ increased greatly. At a TEOS dosage of 0.1 mL the removal ability of the prepared Fe@SiO₂ was the highest. The highest removal ability of Fe@SiO₂ was 466.67 mg·g^-1 and it was only 76.35 mg·g^-1 for uncoated Fe nanoparticles.

Key words: SiO₂ shell, Fe nanoparticles, One-step synthesis, Hexavalent chromium, Reduction