Spectroscopic Studies of the Microstructure and Distribution of Photodeposited Metal Oxides in Porous Vycor Glass.

The microscopic structure, distribution, bond length, co-ordination of photodeposited Fe in Porous Vycor Glass (PVG) has been studied by several spectroscopic techniques: Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Mossbauer spectroscopy, Rutherford Backscattering spectroscopy (RBS), X-ray microprobe and Transmission Electron Microscopy (TEM). Although the photolysis and subsequent heatings are carried out in air, spectroscopic data reveal two surprisingly different forms of iron. One is similar to alpha -Fe_2O_3 and consists of an Fe^{3+} ion octahedrally surrounded by six oxygen atoms at a distance of 1.8 A. The second compound which comprises more than 50% of the reaction product, is mainly elemental Fe, in which a central Fe atom is surrounded by another set of iron atoms at a distance of 2 A. Heating to 650 ^circC or 1200^circ C has only little effect on product type, ratio, or distribution. But the size of the Fe particles increases with temperature. After heating to 1200^ circC, particles are large enough to show magnetic hyperfine fields at room temperature. The observed hyperfine fields are 370 and 425 kG. The deviations of these values from the bulk values can be explained from the effect of particle size and interaction of surface atoms with the surrounding matrix.