Fabrication and photocatalytic performance of Sn-doped titania hollow spheres using polystyrene as template

Sn-doped anatase hollow spheres were fabricated using a template method involving polystyrene spheres as core and anatase coating as shell. The synthesis route included the preparation of PS spheres, followed by their coating by Sn-doped TiO2 sol-gel precursor and subsequent removal of the PS cores by pyrolysis and re crystallization at 500 degrees C for 2 h. The observation of minor amounts of rutile suggests that Sn promotes the anatase -> rutile phase transformation. At doping levels of <= 1.0 mol% Sn, the unsaturated solubility and increasing defect densities enhanced nucleation. At 1.0-2.0 mol% Sn, the solubility remained unsaturated but increasing Sn incorporation reduced crystallinity owing to lattice deformation and partial amorphization. At 2.0-3.0 mol% Sn, solid solution saturation occurred, resulting in excess dopant precipitation, leading to grain boundary pinning and partial blockage of surface-active sites. Ionic radii, thermodynamic, phase equilibria, intervalence charge transfer, and defect chemistry considerations suggest that Sn4+ exhibits substitutional solid solubility in the TiO2 lattice. The photocatalytic performance was in the order 1.0 > 1.2 > 1.5 approximate to 0.7 > 2.0 > 0.0 > 3.0 mol% Sn. This ranking is consistent with the dominant role of crystallinity such that, at <= 1.0 mol% Sn, the performance increased owing to enhanced nucleation from low defect density and increasing crystallinity while, at 1.0-2.0 mol% Sn, the performance decreased from increased lattice strain and effective partial amorphization, and, at 2.0-3.0 mol% Sn, it decreased from maximal lattice strain and blockage of active sites.