SAXS STUDIES OF POROUS GELS AND GLASS-NANOCRYSTAL MATERIALS. A. F. Craievich, LNLS/CNPq, Campinas and IFUSP, Sao Paulo, Brasil

Introducing some degree of order in disordered systems may help to obtain interesting and useful materials. We will review recent synchrotron radiation small-angle scattering (SAXS) studies of materials having in common an initial amorphous structure and a final structure composed of nanocrystalline regions embedded in a disordered matrix.

The first investigation to be reported concerns a xerogel of SnO₂ produced using the sol-gel route. The starting xerogel is a disordered and porous material. An X-ray diffraction study indicates the formation and slow growth of small crystallites ( 1). SAXS measurements during isothermal treatment demonstrated that the first stage of porosity evolution is governed by a coarsening mechanism, similar to that observed in biphasic alloys and glasses (2). Other investigated materials are silicate and borosilicate glasses in which semiconducting nanocrystals (CdS, CdSe, PbTe) nucleate and grow. The determination and control of the average size and the dispersion in size of the nanocrystals are necessary in order to tailor materials having prespecified properties. SAXS investigations helped to establish the heat-treatment procedure and composition choise which lead to materials composed of a glassy matrix containing spherical nanocrystals with radii in the range of 2-4 nm having a narrow size distribution (3). A third correlated research aimed at the obtention of a size-monodisperse system of semiconducting nanocrystals (CdS, CdSe) embedded in an amorphous and porous matrix. It involves the preparation of fine porous silicate sonogels using the sol-gel method assisted by ultrasound. The basic idea is that this matrix may induce heterogenous nucleation of semiconducting nanocrystals with a narrow size distribution. The characterization of this complex three-phase material, composed of an amorphous matrix, nanopores and nanocrystals, was performed by classical SAXS (4) and, more recently, using the anomalous SAXS technique (5).

References:

1-G.E.S. Brito et al., J. Of Sol-Gel Sci. and Tech. To be published (1996).

2-C.V. Santilli et al., Phys. Rev. B, 5, 8801 (1995).

3-A.F. Craievich et al., Jouranl de Physique IV C8, 3,376 (1993).

4-A.F. Craievich et al., Nanostructured Materials, 5,363 (1995).

5-D. Hummel et al., J. Of Sol-Gel Sci. And Tech. To be published (1996).