One-step synthesis and characterization of core–shell Fe@SiO2 nanocomposite for Cr (VI) reduction

A facile one-step method was developed to fabricate mono-dispersed Fe nanoparticles (Fe NPs) coated with SiO2 shell by aqueous reduction method combined with modified Stöber method. Borohydride was acted not only as a reductant for iron salt but also as a catalyst for hydrolysis and polycondensation reaction of tetraethylorthosilicate (TEOS), and more importantly, there was no need to use surface primer for the generation of Fe NPs and catalyst NH4OH for SiO2. Both the Fe NPs agglomeration and SiO2 shell thickness can be controlled through the synthetic conditions. Lower potassium borohydride (KBH4) injection speed was preferable to assemble Fe NPs. The SiO2 shell thickness increased gradually with the increase of TEOS amount. Under the condition of TEOS amount of 0.1 mL and KBH4 injection speed of 5 mL/min, 25 nm single Fe NP was coated with SiO2 shell with thickness of about 9 nm. The resulting nanoporous SiO2 shell was proved to allow reactant to reach the Fe NPs while at the same time protect them from aggregation. The reactivity characterization of the SiO2-coated Fe nanoparticles (Fe@SiO2) showed that both TEOS concentration and KBH4 injection speed had effect on Cr (VI) degradation ability. The highest removal capacity of Fe@SiO2 can reach 467 mg Cr/g Fe at an initial Cr (VI) concentration of 70 mg/L under pH 6.0 ± 0.1. XPS and TEM results showed that Cr (VI) was converted to nontoxic Cr (III) and the reaction product was completely adsorbed to SiO2 shell.

► We prepare, characterize monodispersed core–shell Fe@SiO2 nanocomposites, then use them for Cr (VI) reduction. ► The facile one-step synthesis method excludes using surface primer for Fe NPs and catalyst NH4OH for SiO2 generation. ► Nanoporous SiO2 shell permit reactant to reach the Fe NPs while at the same time protected them from aggregation. ► Under the optical condition, 25 nm single Fe NP was homogeneously coated with a 9 nm SiO2 shell. ► Removal capacity of Fe@SiO2 can reach 467 mg Cr/g Fe which was much higher than that of reported zerovalent iron particles.