Effect of hydrolysis catalyst on the Ti deficiency and crystallite size of sol-gel-TiO 2 crystalline phases
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T. Lopez, E. Sanchez, and R. Gomez Department of Chemistry, Universidad Autonoma Metropolitana-Iztapalapa, A. P. 54-534, 09340 Mexico D. F., and Universidad de Guanajuato, Lascurdn de Retana 5, Guanajuato 36000, Guanajuato, Mexico (Received 29 March 1995; accepted 13 July 1995)
We prepared sol-gel titania by using different hydrolysis catalysts, and characterized it by x-ray powder diffraction. The structure of the crystalline phases—brookite, anatase, and rutile—in the samples annealed between 70 and 900 °C was refined by using the Rietveld technique. From the refinement we obtained the structure parameters, the concentration of each phase, and their average crystallite size. These quantities and their evolution with temperature depended on the hydrolysis catalyst. Anatase and rutile were deficient in Ti, suggesting that their crystalline structure contained hydrogen atoms, forming OH" ions inside. In anatase this deficiency depended on its crystallite size, but it was constant in rutile. When anatase was annealed, it dehydroxylized, producing either crystallite growing up or its conversion into rutile. From the analysis we also found the conditions for obtaining single-phase samples that could be used as precursors for making up titania single-phase thin films. I. INTRODUCTION Titania (TiO2) has chemical and electrical properties that make it an attractive material for different applications. It works well as photocatalyst,' as support in catalysis,2 and as electrode in electrochemistry.3 In electronics it is not only used in capacitors,4 in humidity and gas sensors,5'6 but also in photovoltaic solar cells.7 To help understand the catalytic and electronic properties of titania, its electronic structure has been obtained from x-ray photoelectron spectroscopy,8 Auger,9 and energy-loss10 electron spectroscopies. Since titania is photoactive and is used as a thin film in many applications, its optical properties have also been studied in detail.11"13 At normal pressures titania can have three different crystalline phases—brookite, anatase, and rutile. In all these phases Ti atoms are inside deformed oxygen octahedra. The number of sharing edges of these octahedra distinguishes the different crystalline phases. Three octahedra edges are shared in brookite, four in anatase, and two in rutile,14 which causes a different mass density for each phase. Pure titania with large crystallite sizes is stoichiometric, dielectric, and not useful in catalysis. Its electronic and catalytic properties, however, change when it is doped with other atoms, when oxygen vacancies are created, or when the valence of some Ti atoms15 is reduced from 4+ to 3+. Here, its chemical and electronic properties depend on the local defect density and on the type of impurities introduced into its crystalline structure.16 2788
J. Mater. Res., Vol. 10, No. 11, Nov 1995
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Another way of changing the chemical and electronic properties of titania is to prepare it having a very small crystallite size,
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