Insights into a selective synthesis of anatase, rutile, and brookite-type titanium dioxides by a hydrothermal treatment
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Novel water-soluble titanium complexes coordinated by hydroxycarboxylic acids or amines were developed, and the hydrothermal treatment of the new complexes was carried out to elucidate the formation mechanism of the titania polymorphs including rutile, anatase, and brookite. An empirical relationship among the crystal structure of TiO2, the ligand, and the complex structure was found. Anatase, rutile, or a mixture of both was obtained by the hydrothermal treatment of the complexes coordinated by hydroxycarboxylic acids. The structure of complexes prepared using hydroxycarboxylic acids, which have one hydroxyl and one carboxylic groups, seems to be preferable for the formation of rutile. It was also found that the hydrothermal treatment of titanium complexes coordinated by amine with NAc2 structure resulted in the formation of brookite. Thus, the effect of ligand and complex structure on the crystal structure of TiO2 synthesized by the hydrothermal treatment of the complexes was proposed.
I. INTRODUCTION
Titanium dioxide (TiO 2 ) is widely recognized as an important ceramic material owing to its high chemical stability and ability as a white pigment, catalyst support, capacitor, and photocatalyst. Among titania polymorphs, anatase, rutile, and brookite exist in nature. In both industry and the laboratory, these three polymorphs have been paid attention as the main research targets, because they can be easily synthesized compared to other polymorphs such as hollandite and ramsdellite-type TiO2, which do not exist in nature. It has been well-known that each polymorph of titania shows different chemical and physical properties, e.g., band gaps of anatase and brookite are nearly equal each other to 3.2 eV and that of rutile is 3.0 eV.1 Therefore, establishment of a reliable method to enable the selective synthesis of TiO2 polymorphs is of vital importance. At solution method, it has been argued that the selective synthesis of the three polymorphs of TiO2 can be performed in terms of control of titanium species’ structures in a solution.2–12 Usually, titanium atom exists as a six-coordinated polyhedron in a solution, denoted as TiX6 nYn.10,13 When titanium atom is coordinated by many hydroxy groups in a given solution, the titanium octahedra, TiX6 n(OH)n, give rise to more frequent instances of edge sharing through oxolation between OH groups. This case may bring a result in the formation of TiO6 octahedra with a more edge-shared structure, i.e., the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.229 90
J. Mater. Res., Vol. 29, No. 1, Jan 14, 2014
http://journals.cambridge.org
Downloaded: 24 Mar 2015
preferable formation of anatase12; because a TiO6 octahedron has a four-edge shared structure by another TiO6 in the anatase structure, rutile is consisted of a two-edge shared and two-corner shared polyhedra and brookite has three-edge shared and one-corner shared polyhedra. On the other hand, TiX6 n(OH)n with less OH groups leads to the formation of rutile. Co
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