Modification of the phase transition temperatures in titania doped with various cations
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Modification of the phase transition temperatures in titania doped with various cations R. Rodr´ıguez-Talaveraa) Departmento de F´ısica, Universidad Aut´onoma Metropolitana-Iztapalapa Apdo. Postal 55–534, M´exico, D.F. 09340
S. Vargas and R. Arroyo-Murillo Departmento de Qu´ımica, Universidad Aut´onoma Metropolitana-Iztapalapa Apdo. Postal 55–534, M´exico, D.F. 09340
R. Montiel-Campos and E. Haro-Poniatowski Departmento de F´ısica, Universidad Aut´onoma Metropolitana-Iztapalapa Apdo. Postal 55–534, M´exico, D.F. 09340 (Received 21 July 1995; accepted 12 July 1996)
Titania matrices prepared by a sol-gel technique were doped with several cations (La, Zn, Al, K, Na, Ca, Ba, and Co). The effect of the dopants on the thermal and structural properties of the materials is analyzed. The dopant concentration was 2% mol with respect to titanium, and in all cases the same anion (nitrate) was used. The transition temperatures from amorphous to anatase and from anatase to rutile were measured using x-ray diffraction. The amorphous-anatase transition is independent, for almost all samples, of the type of dopant used; however, the anatase-to-rutile phase transition depends strongly on the kind of cation. This means that the temperature range where the anatase phase exists can be controlled by choosing the appropriate dopant. We have found a correlation between the anatase-rutile phase transition temperature and the radius of the cations and their electric charge.
I. INTRODUCTION
Titania is a material that has been used extensively in many applications such as microelectronics, waste water purification, inorganic membranes, and catalyst support. One of the crystalline phases of titania (rutile) has been extensively studied due to its important physical and chemical properties and industrial applications.1–3 It is usual to add dopant agents to titania in order to improve some of its properties; for example, titania is coated with aluminum oxide to improve gloss and resistance to degradation of pigments.4 It is also usual to dope titania to improve thermal stability of inorganic membranes.5 It is important to understand the role played by the dopants in the structural modification of titania. Specifically we want to identify the important characteristics of the dopant ions that are responsible for the modification of the titania phase transition temperatures. The kinetics of the crystalline phase transformation depend strongly on the characteristics of dopants: the radius and the electric charge. Some work has been reported on the modification of the anatase-to-rutile phase transformation when the following oxides are used as dopants: aluminum, copper, manganese, iron, and a)
Author to whom correspondence should be addressed. J. Mater. Res., Vol. 12, No. 2, Feb 1997
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zinc5–7 ; it was concluded that the mechanism responsible for such modification is an excess in oxygen vacancies that modifies the anatase-to-rutile transformation
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