Influence of Amorphous Silica Matrices on the Formation, Structure, and Chemistry of Iron and Iron Oxide Nanoparticles

摘要

Fe(CO)_5 physisorbs onto Corning's code 7930 porous Vycor glass (PVG) and dried (≤200℃), base-catalyzed (NH_3) tetramethoxysilane/methanol/water xerogels. Although chemically and structurally similar matrices, 488-nm photolysis of the physisorbed complex yields ca. equal amounts of Fe° and Fe_2O_3 in PVG, tut only Fe_2O_3 in the xerogel. Mossbauer, EXAFS, and XANES results give no indication the photoprocucts bind to either silica matrix, and consolidation of the PVG matrix leads to Fe°-Fe_2O_3 nanoparticle formation with little change in the Fe°/Fe(lll) ratio. PVG serves as a template defining the particle diameter and interparticle spacing, whereas consolidation of the xerogel does not result in nanoparticle formation. Instead, ca. 20% of the octahedrally coordinated Fe(Ⅲ) converts to tetrahedral coordination during consolidation. The photoproducts within these porous silica matrices reflect a competition between aggregation and oxidation, where the extent and most likely the rate of aggregation are functions of the correlation lengths of these amorphous matrices. With a correlation length of 22 ± 1 nm, aggregation exceeds oxidation in PVG and limits oxidation to the outer periphery, thereby creating particles whose Fe°/Fe(lll) ratio is unaffected by air or water released during consolidation of the silica matrix. The correlation length of the xerogel, <1 nm, limits aggregation of the primary photoproduct and favors smaller particles. As a result, the primary photoproducts in the xerogel do not achieve sufficient size to limit oxidation to the outer periphery of the particle, and the primary photoproduct oxidizes, forming only Fe_2O_3. Desorption of decomposition products derived from the xerogel precursors creates a dynamic surface that limits nanoparticle growth during annealing. Desorption also disrupts the growing silicate matrix, creating sites that facilitate the change from octahedrally to tetrahedrally coordinated Fe(Ⅲ) in the xerogel.