Ultrasound Driven Deposition and Reactivity of Nanophasic Amorphous Iron Clusters with Surface Silanols of Submicrospherical Silica

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• 作者：Sivarajan Ramesh ; Ruslan Prozorov ; and Aharon Gedanken
• 刊名：Chemistry of Materials
• 出版年：1997
• 出版时间：December 1997
• 年：1997
• 卷：9
• 期：12
• 页码：2996 - 3004
• 全文大小：301K
• 年卷期：v.9,no.12(December 1997)
• ISSN：1520-5002

文摘

Aggregates of nanophasic amorphous iron particles in the sizerange of 5-10 nm havebeen deposited on Stober's silica submicrospheres exhibiting varyingdegrees of surfacereactivity, by an ultrasound-driven decomposition of iron pentacarbonylin Decalin medium.The reaction of the surface-active species such as adsorbed water,silanols, and the bridgingoxygens of the strained siloxane links with elemental iron and theirinfluence on the natureof the target iron oxide nanoparticles have been examined by X-raydiffraction (XRD),transmission electron microscopy (TEM), FT-IR, BET nitrogen adsorption,and magneticsusceptibility measurements. X-ray amorphous, nanophasic ironclusters instantaneouslyreact with untreated silica surface to form an amorphous oxyhydroxideprecursor, which oncrystallization under argon yields nanocrystallineFe₃O₄. Silica microspheres heattreatedat 450 mages/entities/deg.gif">C under argon or in vacuum (10^-5Torr) still contained enough surface-reactive speciesto form Fe₃O₄ andmages/gifchars/alpha.gif" BORDER=0>-Fe₂O₃, respectively. Nanophasicclusters of amorphous, elemental ironcould be deposited only on silica heated to 750 mages/entities/deg.gif">C. The extremereactivity of the amorphousiron nanoparticles toward ammonia was exploited to synthesizeFe₃N nanocrystals weaklyadhered to silica microspheres. Pure nanocrystalline mages/gifchars/alpha.gif" BORDER=0>-Festrongly adhered to silicamicrospheres was obtained by the reduction of the amorphous ironprecursor under flowinghydrogen. Magnetic susceptibility measurements showed all theamorphous and nanocrystalline samples to be superparamgnetic, except polycrystallinemages/gifchars/alpha.gif" BORDER=0>-Fe/SiO₂, which reachedsaturation and exhibited a hysteresis loop characteristic offerromagnetically orderedmaterials. Values of interfacial magnetization coefficientsextracted from the M-H dataindicated a significant diamagnetic contribution from the substratesilica core, particularlyin the case of crystallized samples indicative of anonmagnetic/antiferromagnetic impurityphase formed in the interface. The formation of iron oxides on thesilica surface has beendiscussed primarily in terms of a redox reaction between the surfacemages/gifchars/sigma.gif" BORDER=0 >-O-H groups and thezerovalent metal to form an oxidized iron species.