Micro structural environments and redox states of iron in random and ordered porous silica matrices.

摘要

In our previous studies we have shown that the refractive index of porous Vycor glass can be changed by doping with iron and at the lower end of the iron loading, the refractive index shows a fairly linear increase with the loading. This allows us to create refractive index patterns in porous Vycor glass. The exact mechanisms regarding image formation in the Vycor glass and the factors that affect the image quality are still being investigated. In this study we analyzed the cross-sectional distribution of iron and the lateral diffusion of iron during the heat treatment in order to understand the contrast variations. The study also focused on microstructural changes of iron particles from the surface to the interior of the porous Vycor glass. The other objective of the study is to understand microstructural variations of iron in regular pore structured materials such as MCM-41 and random pore networks such as xerogel and PVG.;Results show that the maximum effective lateral diffusion length of iron in PVG is 10 &mgr;m at 650°C. We conclude that the particle growth which occurs at 650°C is due to a less than 10 &mgr;m diffusion length within the matrix. XANES results show that elemental iron found in the PVG immediately after photolysis is concentrated in the interior of the glass. Although some elemental iron is found on the surface of the glass they are covered with a protective layer of Fe(III) oxides. This protective layer seems to be robust enough to prevent further oxidation of elemental iron particles during the annealing process at 650°C but the elemental iron found in the interior of the glass did oxidize during the annealing process until the protective layer of Fe(III) oxide is formed. The results suggest that once the Fe(III)/Fe(0) ratio reach a critical value further oxidation is prevented. EXAFS data analysis along with EPR confirmed that the chemical nature of iron oxides formed on the surface and the interior of the PVG are identical and Fe(III) is in an octahedral environment. The Mossbauer data suggest that the Fe(0) particles in the PVG substrate are randomly oriented whereas Fe(III) has some orientation suggesting that particles are attached to the silica substrate through the oxide envelope.;Unlike Fe(CO)5 doped PVG, when Fe(CO)5 doped MCM-41 is photolyzed, it leads to formation of octahedrally and tetrahedrally coordinated iron sites within the silica matrix. Mossbauer study shows that with the increasing temperature, iron migrates from octahedral sites to tetrahedral sites.;Irons in xerogel behaves differently than iron in PVG or MCM-41. Iron migration into tetrahedral sites initiates at 650°C and the number of tetrahedral sites increase with temperature. Neither xerogel nor MCM-41 shows any evidence of elemental iron before or after heat treatments. The Fe(0) formation in PVG seems to be a unique phenomenon.