Niobium-titanium oxide powders obtained by laser-induced synthesis: Microstructure and structure evolution from diffract
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Niobium-titanium oxide powders obtained by laser-induced synthesis: Microstructure and structure evolution from diffraction data Laura E. Depero,a) Luigi Sangaletti, Brigida Allieri, Elza Bontempi, Roberto Salari, and Marcello Zocchi Istituto Nazionale per la Fisica della Materia and Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Universit`a di Brescia, Via Branze 38-25123 Brescia, Italy
Cristina Casale and Maurizio Notaro CISE Tecnologie Innovative S.p.A., P.O. Box 12081, Milano, Italy (Received 11 November 1996; accepted 28 August 1997)
The influence of the niobium content on the anatase-to-rutile phase transition in nanopowders of Nb–Ti oxides was studied and the changes in the particle size and microstrain distribution obtained at different temperatures were analyzed. A correlation is found between the initial microstructure in the Ti1 2 x Nbx O2 (x 0.03, 0.2) powder and the niobium content. The presence of Nb was found to inhibit the growth of both the anatase and the rutile phases.
I. INTRODUCTION
One of the problems in catalytic and sensor applications of TiO2 anatase-based materials is the structural transformation to rutile, depending on temperature and time. TiO2 (anatase)-supported transition metals, such as V, Rh, Cu, and Fe, while being active catalysts in oxidation and reduction reactions,1–5 are promoters of the anatase to rutile transformation.6 –8 A simple explanation is that impurities that increase the oxygen vacancy concentration accelerate the transformation, whereas those with a valence larger than four retard it. However, in this way it is not possible to understand the opposite effect of V and Mo ions in the anatase structure which are expected to slow down the anatase to rutile transformation, with a valence state in this phase of 51 and 61, respectively.9 Therefore, the study of the effects of impurities on the structural properties of TiO2 powders is demanding. On the other hand, the structure and properties of nanocrystalline materials have a great interest, in particular in catalysis and as precursor powders in thick film preparation, since materials with high specific surface are required in catalytic and sensor applications.10,11 Unfortunately, the small size of the crystallites is one of the factors promoting the anatase to rutile transformation.12 Therefore, great interest is devoted to the kinetics of grain growth in TiO2 nanophases13 and to potential grain growth inhibitors in nanocrystalline TiO2 .14 A laser-induced process has been studied and developed for the synthesis either of pure TiO2 powders, for use as a catalyst support,3 or of mixed oxides to a)
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J. Mater. Res., Vol. 13, No. 6, Jun 1998
be used directly as catalysts in the selective catalytic reduction of nitrogen oxides with ammonia.4 Nb- or Ta-doped TiO2 thin films have been studied for diodetype gas sensors and other kinds of semiconducting electrical devices.15,16 This fact makes the Nb-doped TiO2
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