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

Hierarchical porous gamma-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 Stober method and subsequent TiO2 layer by vapor-thermal route. Notably, the interior black Fe3O4 templates are converted into dark-brown gamma-Fe2O3 during the vapor-thermal assisted TiO2 deposition. By introducing a SiO2 layer between the gamma-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 gamma-Fe2O3 core and TiO2 shell. Firstly, a direct contact and electron injection from TiO2 to gamma-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. (C) 2011 Elsevier B.V. All rights reserved.