SAXS measurements of the porosity in Cu(II)-doped SnO2 xerogels during crystallization

The small-angle X-ray scattering (SAXS) technique was used to study the porosity which develops in Cu(II)-doped SnO2 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 SnO2 samples treated at temperatures ranging from 300 up to 600 degrees 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 degrees C had a single mode distribution due only to inter-aggregate pores. Doped samples isothermically treated at 500 degrees 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. (C) 1997 Elsevier Science B.V.