Influence of amorphous silica matrices on the formation, structure, and chemistry of iron and iron oxide nanoparticles.
Fe(CO)(5) physisorbs onto Corning's code 7930 porous Vycor glass (PVG) and dried (<or=200 degrees C), 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(0) and Fe(2)O(3) in PVG, but only Fe(2)O(3) 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 Fe(0)-Fe(2)O(3) nanoparticle formation with little change in the Fe(0)/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 Fe(0)/Fe(III) ratio is unaffected by air or water released during consolidation of the silica matrix. The correlation length of the xerogel, <or=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(2)O(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.
Sunil
Dehipalawage
D
Department of Physics, Queensborough Community College, City University of New York, Bayside, New York 11364, USA.
Dong
Jinquan
J
Gafney
Harry D
HD