Crystallization of nanosized titania particles prepared by the sol-gel process
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R. Arroyo-Murillo Departamento de Quimica, Universidad Autonoma Metropolitana-Iztapalapa, Apdo. Postal 55-534, Mexico, D.F. 09340 (Received 29 November 1993; accepted 11 April 1994)
Sols of titania were obtained by the sol-gel method and their size profile was followed by dynamical light scattering. In the early stages of the reaction an unstable behavior was detected. After this unstable regime the particle size reaches a steady state where the sols have a constant size while increasing in number. Once the sol concentration reaches its overlap value, the gelation regime takes place. For samples prepared in this way Raman spectra and x-ray diffractometry were used to characterize the kinetics of crystallization of the material.
I. INTRODUCTION The main applications of titanium dioxide are as pigments, adsorbents, and as catalytic supports. In almost all of these, the size of the titania particles is an important factor for the performance of the material. Nanoparticles of titania are widely used for sintering processes1'2 where the morphology, the size, and the size distribution control to a large extent the sintering performance because it affects directly the compact density of the material. Nanophase titania has been characterized by various techniques such as high resolution scanning electron microscopy,1"3 x-ray powder diffraction,1"4 Raman spectroscopy,5"8 and Mossbauer spectroscopy9 among others. Raman spectroscopy of TiO2 single crystals has been widely studied under a variety of conditions, in particular, as a function of pressure and temperature.10 Titanium dioxide is known to crystallize in rutile (tetragonal), brookite (orthorhombic), and anatase (tetragonal) phases. Several types of samples have been analyzed by Raman spectroscopy: initially natural and synthetic single crystals and more recently powders. The material in powder can be obtained by various techniques having important technological applications. For instance, ultrafine-grained ceramics are fabricated by precipitation of fine particles from chemical precursors.2 One drawback of this method is that the resultant powder must be fired at high temperatures to drive off contaminants before sintering can be carried out. Another technique combines inert gas condensation of metal or ceramic vapors and in situ collection and compaction of the ceramic powder2 leading to a lower thermal processing.
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J. Mater. Res., Vol. 9, No. 8, Aug 1994
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Recently, detailed studies of nanophase TiO2 have been realized by macro-Raman5 and micro-Raman6 spectroscopies. The samples were obtained by the condensation method described before. The macroscopic measurements showed that both anatase and rutile phases of TiO2 were present in the samples5; however, the majority of the samples were dominated by rutile. Furthermore, the anatase phase could be completely removed by annealing the samples above 875 °C. It was found that the grain size had no effe
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