Influence of Amorphous Silica Matrices on the Formation, Structure, and Chemistry of Iron and Iron Oxide Nanoparticles
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- 作者：Dehipalawage Sunil ; Jinquan Dong ; Harry D. Gafney
- 刊名：Journal of the American Chemical Society
- 出版年：2009
- 出版时间：October 21, 2009
- 年：2009
- 卷：131
- 期：41
- 页码：14768-14777
- 全文大小：284K
- 年卷期：v.131,no.41(October 21, 2009)
- ISSN：1520-5126
文摘
Fe(CO)5 physisorbs onto Corning’s code 7930 porous Vycor glass (PVG) and dried (≤200 °C), base-catalyzed (NH3) tetramethoxysilane/methanol/water xerogels. Although chemically and structurally similar matrices, 488-nm photolysis of the physisorbed complex yields ca. equal amounts of Fe0 and Fe2O3 in PVG, but only Fe2O3 in the xerogel. Mossbauer, EXAFS, and XANES results give no indication the photoproducts bind to either silica matrix, and consolidation of the PVG matrix leads to Fe0−Fe2O3 nanoparticle formation with little change in the Fe0/Fe(III) 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 Fe0/Fe(III) 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 Fe2O3. 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.