Selection of structural type and particle size in titanium (IV) oxide
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A. Martinez Departamento de Quimica Inorganica y Bioinorganica, Facultad de Farmacia, Universidad Complutense, 28040-Madrid, Spain
J. M. Gonzalez-Calbet Instituto de Magnetismo Aplicado, RENFE-UCM, Apto. 155, Las Rozas 28230-Madrid, Spain and Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad Complutense, 28040-Madrid, Spain (Received 8 January 1993; accepted 30 April 1993)
Titanium (iv) oxide (amorphous, brookite, anatase, and rutile) fine particles have been synthesized by pyrolysis of an aerosol. As-received materials are constituted by hollow balloons of around 1-2 fim formed by very small crystalline nuclei, whose crystal size can be modified by annealing. By heating at 900 °C for 48 h individual crystals of around 1 fim are obtained. Intermediate sizes can be controlled as a function of the thermal treatment.
I. INTRODUCTION Titanium (iv) oxide is polymorphic and crystallizes in four known crystal structures,1"3 namely, rutile, anatase, brookite, and the high pressure form TiO 2 II. At room pressure and temperatures higher than 400 °C, both brookite and anatase are transformed to the rutile-type,4"6 which is the most stable structural type for TiO 2 . Applications of TiO2 obviously depend on the structural type, anatase and rutile being the most used. Among semiconductors studied so far TiO 2 is known to be the best catalyst in the photocatalytic action for wastewater treatment; the photocatalytic efficiency varies widely with its crystal forms, crystallinity, and particle size.7'8 For instance, for the degradation of C2HC13 and CH2C1COOH pure anatase exhibits the best catalytic efficiency, while pure rutile is a poor catalyst. However, rutile, which contains a certain amount of anatase, is a much better catalyst than pure rutile.9 On the other hand, rutile TiO2 has been traditionally used as a precursor for the manufacture of pigments. Later on, TiO 2 is shown to be an important catalyst support, one of the most interesting applications being that catalysts consisting of group 8-10 metals supported on TiO2 give rise to strong metal support interactions which inhibit hydrogen adsorption on the metal.10'11 More recently, and due to its semiconductor behavior, TiO2 is being used for gas sensors12'13 since its near-surface region conductivity is very sensitive to the atmosphere. It is worth mentioning that in polycrystalline semiconductors, the electric conductivity can be determined by grain boundaries. These boundaries are, 2336
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J. Mater. Res., Vol. 8, No. 9, Sep 1993
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generally, discontinuities in the crystal which facilitate gas adsorption.14 For most of these applications, it is of particular importance to synthesize a material showing very high specific surface, which is strongly dependent on the preparation method. We describe, in this paper, the synthesis of TiO2 fine particles by the pyrosol method.15 The structural transitions between the different TiO2 polymorphs, the stability domain of every phase, and the particle size a
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