Ag-modified TiO₂/SiO₂/Fe₃O₄ sphere with core-shell structure for photo-assisted reduction of 4-nitrophenol

Nitrogen-containing contaminants, such as 4-nitrophenol (4-NP), cause detrimental effects when discharged into the environment and thus should be reduced or removed from ecosystems. In this study, an Ag-loaded TiO₂-SiO₂-Fe₃O₄ (TSF) with a core-shell structure was employed for the photo-assisted reduction of 4-NP. Fe₃O₄, SiO₂, and TiO₂ in the core-shell structure served as a magnetic center, protective layer, and light absorber, respectively. To improve the reduction activity of 4-NP, Ag was loaded onto TSF under stirring, with a variation of the temperature (2-130 °C) and reaction time (1, 2, and 4 h). Under the optimized conditions, 5Ag-TSF (with 5 wt% of Ag) could promote the reduction of aqueous 4-NP solution (2 × 10^-4 M, 75 mL) in the presence of NaBH₄ (0.1 M, 5 mL) under irradiation by a metal halide lamp, affording over 98% reduction within 5 min and a rate constant of 0.185 min^-1, demonstrating its promising activity. Moreover, due to the advantages of the core-shell structure, the magnetic properties of Fe₃O₄ were sufficient to enable facile recycling of the sample for further reaction; SiO₂ could protect the Fe₃O₄ center from oxidation or reduction; TiO₂ enabled Ag accommodation and absorbed light to generate electron-hole pairs. In summary, an Ag-loaded TiO₂-SiO₂-Fe₃O₄ sphere with high activity and recyclability for 4-NP reduction was prepared via a facile and simple stirring method, where the sample can be used as a promising material in environmental remediation.