Effects of the SO4 Groups on the Textural Properties and Local Order Deformation of SnO2 Rutile Structure

Sulfated tin oxide was synthesized from a hydroxylated tin oxide obtained by the precipitation method, followed by ion exchange of OH groups by SO4 species with a sulfuric acid solution. The samples were characterized by X-ray diffraction, transmission electron microscopy, thermoanalysis, and nitrogen physisorption by the Brunauer−Emmett−Teller method. The rutile crystalline structure was refined by the Rietveld method. Thermal analysis suggests the following stoichiometric formulas: SnO2 - X (OH)2 X and SnO2 - X (OH) X (HSO4) X with X = 0.35 and 0.17 for non-sulfated and sulfated samples, respectively. The SO4 species remained strongly bonded at the SnO2 surface stabilizing its crystallite size against sintering, inhibiting the crystallite aggregation, and it acts as a structure porogen director mediating nanoparticle growth and assembly yielding a mesostructure form of SnO2 with wormhole morphology and high thermal stability. The interaction between SO4 2- and the SnO2 surface changes the symmetry of the representative tin−oxygen octahedron. It relaxes the four tin−oxygen bond lengths located at the basal plane of the octahedron while the two apical Sn−O bonds decrease, producing a strong deformed octahedron, which could be transformed into a higher asymmetry in the electronic distribution around the Sn4+ nuclei. The elimination of SO4 groups brings about the coalescence and crystallite growth, which collapse the mesostructure form of SnO2, decreasing the surface area and porosity.