Sol-gel processing of nanocrystalline haematite thin films
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Sol-gel processing of nanocrystalline haematite thin films L. Armelaoa) Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Universit`a di Padova, via Loredan 4, 35131 Padova, and Centro di Studio sulla Stabilit`a e Reattivit`a dei Composti di Coordinazione, Consiglio Nazionale delle Ricerche, via Marzolo 1, 35131 Padova, Italy
A. Armigliato Instituto LAMEL, Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy
R. Bozio Dipartimento di Chimica Fisica, Universit`a di Padova, via Loredan 2, 35131 Padova, Italy
P. Colombo Dipartimento di Ingegneria Meccanica, Settore Materiali, Universit`a di Padova, via Marzolo 9, 35131 Padova, Italy (Received 11 March 1996; accepted 14 February 1997)
The microstructure of Fe2 O3 sol-gel thin films, obtained from Fe(OCH2 CH3 )3 , was investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. Samples were nanocrystalline from 400 ±C to 1000 ±C, and the crystallized phase was haematite. In the coatings, the a –Fe2 O3 clusters were dispersed as single particles in a network of amorphous ferric oxide.
In recent years, nanometer-scale materials have been the object of continuously growing interest.1,2 As a matter of fact, nanoclusters represent an intermediate state between discrete molecules and extended-network solids. Consequently, nanocrystalline materials can widely diversify their properties as a function of particle size and geometry. This is of great scientific interest in relation to scale-dependent quantum properties like quantum dots or quantum walls and for thermodynamic properties connected to phase transitions.3 Nanomaterials are also appealing for various applications in many technological fields. For instance, the linear and nonlinear optical properties of such materials can be widely modulated by varying the nanocluster dimensions. Moreover, as far as the chemical reactivity is concerned, the fact that nanoclusters have a very high number of surface atoms compared to the inside located atoms emphasizes the importance of geometrical effects in determining the application of nanomaterials as sensors and catalysts. In the domain of metal-oxide materials, a –Fe2 O3 nanocrystalline coatings have been proposed in the literature as promising catalysts in dehydrogenation reactions,4 as materials for solar energy conversion,5 as sensors,6 for magnetic,7 and nonlinear optical devices.8 In principle, these films can be prepared by means of many deposition techniques such as rf-sputtering or chemical vapor deposition. Compared to other synthesis methods, however, the sol-gel route may offer unique
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Author to whom correspondence should be addressed. J. Mater. Res., Vol. 12, No. 6, Jun 1997
http://journals.cambridge.org
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possibilities for an accurate control over thin film microstructure and crystallinity.9 A ceramic material, in fact, is obtained by heating coatings deposited at low temperatures from a liquid precursor. The aim
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