Reaction pathways at the iron-microspherical silica interface: Mechanisticaspects of the formation of target iron oxide phases

Oxidative hydrolysis of elemental iron nanoclusters on hydroxylated surface s such as silica or alumina is known to be influenced by the degree of hydr ation of the surface. The understanding and control of this process is cruc ial in the synthesis of iron oxide coated silica microspheres with a desire d magnetic property. The hydrolysis of iron nanoparticles followed by heat treatment in the case of a hydrated microspherical silica surface results i n the formation of maghemite (gamma-Fe2O3). whereas a dehydrated surface yi elded hematite (alpha-Fe2O3) nanoparticles. The influence of adsorbed water on the formation of intermediate iron oxides/oxidehydroxides and the mecha nistic aspects of their subsequent thermal dehydration iron oxide phases we re investigated by thermogravimetric analysis, Fourier transform infrared, and Mossbauer spectroscopies. The reactions on both the hydrated and the de hydrated surfaces were found to proceed through the formation of an x-ray a morphous lepidocrocite [gamma-FeO(OH)] intermediate and its subsequent dehy dration to maghemite (gamma-Fe2O3). Maghemite to hematite transformation wa s readily facilitated only on a dry silica surface. The retardation of the lepidocrocite --> maghemite --> hematite transformation in the case of a hy drated silica surface is suggested to arise from strong hydrogen-bonded int eractions between the substrate silica and the adsorbed nanoparticles.