SAXS measurements of the porosity in Cu(II)-doped SnO 2 xerogels during crystallization

Abstract

The small-angle X-ray scattering (SAXS) technique was used to study the porosity which develops in Cu(II)-doped SnO 2 monolithic xerogels during crystallization. The influence of the upper temperature of heat-treatment and of Cu(II) content on the structure was determined. Previous studies of the porosity in undoped SnO 2 samples treated at temperatures ranging from 300 up to 600°C demonstrated the existence of a bimodal size distribution (one distribution was due to intra-aggregate and the other to inter-aggregate pores). However, the SAXS data from Cu(II)-doped samples heated to 500°C had a single mode distribution due only to inter-aggregate pores. Doped samples isothermically treated at 500°C were studied by the in situ SAXS technique. The time evolution of the scattering intensity function, or structure function of the porous material, exhibits a dynamical scaling property. The asymptotic behavior at high q (wave numbers) of the scaled function and consequently the nature and morphology of the porosity interface are a function of Cu(II) content. The kinetic exponents predicted by the statistical theory for the structure function suggest that the mechanism of porosity coarsening is controlled by surface diffusion.