Superparamagnetic γ-Fe2O3@SiO2@TiO2 composite microspheres with superior photocatalytic properties

Hierarchical porous γ-Fe2O3@SiO2@TiO2 (FST) composite microspheres with sandwich-like structure were fabricated by an effective three-step approach. Specifically, the preformed monodisperse Fe3O4 spheres were used as templates for directing the sequential deposition of SiO2 layer by modified Stöber method and subsequent TiO2 layer by vapor–thermal route. Notably, the interior black Fe3O4 templates are converted into dark-brown γ-Fe2O3 during the vapor–thermal assisted TiO2 deposition. By introducing a SiO2 layer between the γ-Fe2O3 core and TiO2 shell, while the superparamagnetic properties was remained to great extent, the photocatalytic activity towards degradation of rhodamine B (RhB) dye molecules was also significantly improved, as compared to that of the Fe3O4@TiO2 (FT) counterparts. This enhancement in photoactivity is mainly attributed to two positive effects of the interbedded SiO2 layer between the γ-Fe2O3 core and TiO2 shell. Firstly, a direct contact and electron injection from TiO2 to γ-Fe2O3 is blocked by this wide bandgap SiO2 electronic barrier, which avoids charge recombination at the interface. Secondly, a SiO2 middle layer is a good adsorbent towards dye molecules, which enhances the enrichment of pollutant dye molecules around the porous titania photoactive layer and thus the photocatalytic efficiency. Such a novel multifunctional photocatalytic integrated microspheres can effectively degrade organic pollutants and can be easily recovered by a magnet, which was reused at least five times without any appreciable reduction in photocatalytic efficiency.

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► Hierarchical porous γ-Fe2O3@SiO2@TiO2 composite microspheres with sandwich-like structures are fabricated.
► SiO2 middle layer is crucial for retention of photocatalytic active of TiO2.
► SiO2 middle layer is a good adsorbent towards dye molecules in water.
► Composite photocatalyst can effectively degrade organic pollutants and easily recovered by a magnet.