SnO2/SiO2 and Sn/Sb-oxide/SiO2 gel-derived composites. Part 1: Structural evolution from a Mössbauer study

Abstract: SiO₂/SnO₂=80/20 gel samples are prepared from Si(OEt)₄ and SnCl₄ or Sn(OEt)₄. Samples doped with Sb oxide contents up to 4% are also studied. The evolution of these samples after treatment from room temperature to 1050°C is studied by ¹¹⁹Sn Mössbauer spectroscopy and X-ray analysis. Results indicate that SnO₂ separation depends on preparation method and Sn- or Sb-oxide precursors. In the case of Sn(OEt)₄, hydrolysis is less complete than for SnCl₄ and thermal curing affords charcoal separation which leads to extensive reduction of Sn(IV) to Sn(II). Products obtained above 800°C are constituted by a well-defined SnO₂ phase in the SiO₂ matrix. Segregation of Sn(II) and subsequent oxidation to Sn(IV) are invoked to account for this final morphology. The presence of Sb oxide lowers Sn(IV) reduction and favours both SnO₂ and SiO₂ crystallization, via the occurrence of a Sn/Sb-oxide metastable phase observed in the 700–800°C interval.