Nonhydrolytic sol-gel synthesis: Microstructural and morphological study on nickel ferrite nanocrystals coated with olei
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Nickel ferrite nanostructured particles coated with chemisorbed oleic acid were successfully synthesized by nonhydrolytic sol-gel method. By varying the composition of metal precursors, two microstructures were obtained, i.e., dispersed nanocrystals (9.7 ± 1.8 nm) and submicron aggregates (152 ± 21 nm) consisted of many nanocrystals (8.1 ± 1.3 nm). Because oleic acid could form complex with iron (III) ions, but not with nickel (II), increasing the concentration of iron precursor consumed more oleic acid and led to insufficient oleic acid coating on particle surface. Strong intercrystallite interaction was induced from less protected nanocrystals, and aggregation thus occurred between different crystallites.
I. INTRODUCTION
Nickel ferrite nanoparticles find various applications, including magnetic recording,1 ferrofluids,2,3 biomedical applications,4–6 etc. Different methods, such as chemical coprecipitation,7 hydrothermal synthesis,8 hydrolysis of metal carboxylate in organic solvent,9 and mechanical milling10 have been used to synthesize nickel ferrite nanoparticles. However, these methods are unable to control particle size efficiently and effectively. Synthesis of monodisperse and size-controlled magnetic ferrite nanoparticles by applying surfactants have been intensively pursued in the past several years.11–13 The dominating factor of size control was the molar ratio of precursors to surfactants. However, most of these works focused on iron oxides.11,14 In addition to control of particle size, surfactant coating may also prevent agglomeration and enhance stability of nanoparticles. If surfactants have functional terminal groups (such as –COOH groups in oleic acid), they may also provide useful binding sites that are essential for biomedical applications.15–17 It is therefore desirable to use a simple synthetic method that allows control of particle size and introduction of desirable surface coating simultaneously. More recently, nonhydrolytic sol-gel method was used to synthesize TiO218 and ZnO19 nanocrystals with controllable shape and size. Oleic acid was used as surfactant in that work and was successfully coated onto the surface of
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0239 1922
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 7, Jul 2008 Downloaded: 27 Mar 2015
nanocrystals. In nonhydrolytic sol-gel method, an oxobridge forms through a condensation reaction between two different functional groups bonded to two metal centers, by eliminating a small organic molecule. Because condensation reactions may reduce the difference in reactivity of two precursors, nonhydrolytic sol-gel method is suitable for preparation of homogeneous bicomponent oxide. A number of mixed transition metal oxides with a high level of homogeneity have been recently prepared, such as SiO2–TiO2,20 ZrTiO4,21 and HfxZr1−xO2.22 Nonhydrolytic sol-gel method has not been reported for the synthesis of magnetic ferrite nanoparticles, which are an important group of transiti
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