Effects of cationic dopants on the phase transition temperature of titania prepared by the sol-gel method
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Effects of cationic dopants on the phase transition temperature of titania prepared by the sol-gel method S. Vargas Departamento de Fı´sica, UAM-Iztapalapa, Apdo. Postal 55-534, Me´xico, D.F. 09340
R. Arroyo Departamento de Quı´mica, UAM-Iztapalapa, Apdo. Postal 55-534, Me´xico, D.F. 09340
E. Haro and R. Rodrı´gueza) Departamento de Fı´sica, UAM-Iztapalapa, Apdo. Postal 55-534, Me´xico, D.F. 09340 (Received 21 July 1998; accepted 21 June 1999)
The effect of different cations in the anatase–rutile phase transition temperatures for titania prepared by the sol-gel method was studied. The metal dopants were chosen from different periods and groups of the periodic table to see the role played by the electronic configuration, the oxidizing state, the atomic size, etc. on these temperature modifications. Linear relationships between the anatase–rutile phase transition temperatures and the ionic radii for alkali metal, alkaline earth metal, and group 3 and 13 elements were obtained. For elements of the period 4, there was not such a defined tendency; for most of them the modification of the phase transition temperature was too small. The cations used were Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Al3+, Y3+, La3+, Er3+, Ti4+, Co2+, Ni2+, Cu2+, and Zn2+. In all cases the dopant’s concentration was 2 mol% with respect to titanium, and the same anion (nitrate) was used for all salts. A variation of more than 330 °C in the anatase–rutile phase transition temperatures was obtained by using these dopants. The transition temperatures from amorphous to anatase and from anatase to rutile phases were obtained from the x-ray diffractograms.
I. INTRODUCTION A. Periodic table trends
It is important to consider the properties of individual metals in various oxidation states to understand the role played by them in the properties of, in this particular case, titania when it is doped with these cations. The electron configuration controls the atomic properties such as oxidizing states, atomic sizes, electronegativities, etc.1–4 In the case of alkali metals, because the probability to lose two electrons is very unfavorable, the chemistry of this group is defined in terms of a +1 oxidation state. This is an important fact when these kinds of atoms are introduced in an environment where there is a strong interaction with many oxygens from the TiO2 matrix: they produce a strong local oxygen deficiency, which modi-
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Address all correspondence to this author. e-mail: [email protected] Also affiliated with Instituto de Fı´sica, UNAM, Apdo. Postal 1-1010, Queretaro, Qro. 76001.
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J. Mater. Res., Vol. 14, No. 10, Oct 1999
fies some bulk properties of the material, especially the transitions to different crystalline phases; this is because these phase transformations require atomic mobility to happen, and this is provided by the empty spaces left by the oxygen deficiency sites.5–9 On the other side, elements of group 2 (alkaline earth metals) are less strongly reducing agents than the alkali me
