Morphology and microstructural properties of TiO 2 nanopowders doped with trivalent Al and Ga cations
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L. Sangaletti Istituto Nazionale per la Fisica della Materia and Dipartimento di Matematica e Fisica, Universita` Cattolica del Sacro Cuore, Via Trieste 17, 25121 Brescia, Italy
C. Casale and M. Notaro Ente Nazionale per l’Energia Elettrica, Struttura Ricerca, Via Reggio Emilia 39, 20090 Segrate, Italy (Received 18 February 1999; accepted 30 June 2000)
The effects of doping on the morphological and microstructural properties of TiO2 nanopowders produced by laser pyrolysis were investigated mainly by x-ray diffraction (XRD) and electron microscopy. Samples of TiO2 powders were prepared by doping with different trivalent cations (Al and Ga). The powders were calcined at different temperatures in the range 400–1000 °C for 18 h, as well as at constant T ⳱ 700 °C up to 160 h. After each thermal treatment, XRD patterns were collected. The analysis of XRD patterns allowed us to estimate the microstrains and average crystallite size and to observe the evolution of the microstructural parameters with temperature. Both Al and Ga inhibited the crystallite growth of TiO2 anatase and the rutile phases, this effect being larger in the Al-doped powders.
I. INTRODUCTION
Nanocrystalline materials are widely studied for catalysis and as precursor powders in thick film preparation, because materials with high specific surface are required in catalytic and gas sensor applications.1,2 Therefore great interest is devoted to the kinetics of grain growth in TiO2 nanophases3 and to potential grain growth inhibitors in nanocrystalline TiO2.4 A large number of studies have been carried out on the anatase-torutile transformation in the nanoscale regime involving both thermodynamics aspects5–11 and relationships between morphological and structural properties and the synthesis routes.12–17 Indeed, even if the rutile is the stable phase for TiO2, the anatase can be obtained in nanocrystalline powders and thin films. An important factor controlling the transformation temperature is the grain size.5,18,8 It was shown that anatase becomes more stable than rutile for particle size below approximately 14 nm,10 and thus surface free energy and surface stress play important roles in the stability of this phase. Indeed, during thermal treatments of anatase nanopowders the phase transformation and the grain growth occur simultaneously and are correlated.11,15 The anatase-to-rutile transformation can also be enhanced or inhibited by the presence of a dopant in the anatase structure. For example, the presence of Cu, 2080
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J. Mater. Res., Vol. 15, No. 10, Oct 2000 Downloaded: 28 Mar 2015
Mn, Fe, and Zn19–23 promotes the transformation, while sulfur, chloride, fluoride, Ni, and W24–26 are inhibitors. The simplest explanation of the change in the transition temperature is that the introduction of substitutional cations with oxidation state less than 4 induces oxygen deficiency in the anatase structure and then it promotes the transformation. However this is not sufficient to explain these changes, as for example the case o
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