Reaction pathways at the iron-microspherical silica interface: Mechanistic aspects of the formation of target iron oxide
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Israel Felner Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel
Yuri Koltypin and Aharon Gedankenb) Department of Chemistry, Bar-Ilan University, Ramat Gan-52900, Israel (Received 24 September 1999, accepted 29 December 1999)
Oxidative hydrolysis of elemental iron nanoclusters on hydroxylated surfaces such as silica or alumina is known to be influenced by the degree of hydration of the surface. The understanding and control of this process is crucial in the synthesis of iron oxide coated silica microspheres with a desired magnetic property. The hydrolysis of iron nanoparticles followed by heat treatment in the case of a hydrated microspherical silica surface results in the formation of maghemite (␥–Fe2O3), whereas a dehydrated surface yielded hematite (␣–Fe2O3) nanoparticles. The influence of adsorbed water on the formation of intermediate iron oxides/oxidehydroxides and the mechanistic aspects of their subsequent thermal dehydration iron oxide phases were investigated by thermogravimetric analysis, Fourier transform infrared, and Mössbauer spectroscopies. The reactions on both the hydrated and the dehydrated surfaces were found to proceed through the formation of an x-ray amorphous lepidocrocite [␥–FeO(OH)] intermediate and its subsequent dehydration to maghemite (␥–Fe2O3). Maghemite to hematite transformation was readily facilitated only on a dry silica surface. The retardation of the lepidocrocite → maghemite → hematite transformation in the case of a hydrated silica surface is suggested to arise from strong hydrogen-bonded interactions between the substrate silica and the adsorbed nanoparticles.
I. INTRODUCTION
The chemistry of the silica surface is probably the most studied interfacial phenomenon over the past few decades and still continues to exhibit rich variations in terms of the surface chemical properties.1–6 The degree of hydroxylation and hence the reactivity of the silica surface are highly influenced by the origin of the silica. Sol-gel-derived silica is normally obtained by the hydrolysis of an alkoxide, catalyzed by an acid or a base. Acid-catalyzed hydrolysis proceeds through an electrophilic reaction to an alcogel, whereas base-catalyzed reactions follow a nucleophilic pathway, leading to the formation of hydrated silica powders.7 Stober et al.’s method8 of hydrolysis of tetraethyl orthosilicate (TEOS)
a)
Present address: Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, New York 14853. b) Address all correspondence to this author. e-mail: [email protected] 944
J. Mater. Res., Vol. 15, No. 4, Apr 2000
in the presence of ammonia or an organic base as a morphological catalyst is known to yield microspherical silica with a narrow size distribution over a wider range. The surface of the microspheres consists of single and geminal silanols, silanols connected by an extensive hydrogen-bonded network, and adsorbed water. Hydroxylated oxide surfaces tend to react with transition metal clusters in their elemental state, with an “ox
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