Synthesis of rutile titania powders: Agglomeration, dissolution, and reprecipitation phenomena
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David J. Pine Chemical Engineering Department, University of California at Santa Barbara, Santa Barbara, California 93106
Fred F. Lange Materials Department, University of California at Santa Barbara, Santa Barbara, California 93106 (Received 10 September 2002; accepted 25 March 2003)
Rutile titania powders were synthesized via a sol-gel/hydrothermal process using nitric acid as the catalyst. A molar acid to alkoxide ratio of 10 and a water to alkoxide molar ratio of 250 produced 100% rutile powders when precipitated below 45 °C. Higher temperatures yielded initially either anatase or mixtures of anatase and rutile. Spherulitic growth produced cauliflower-shaped agglomerates with a mean size of 760 nm. The agglomerates could be broken apart into approximately 100-nm large broomlike agglomerates via a dissolution and reprecipitation process when reacted with approximately 2.4 molar nitric acid. Transmission electron microscopy observations showed that the broomlike agglomerate consisted of linear clusters of rodlike agglomerates composed of crystallographically aligned, primary particles approximately 4 nm in size.
I. INTRODUCTION
Titania (TiO2) has three polymorphs: anatase, brookite, and rutile. All are constructed with Ti–O6 octahedra and differ only in their octahedral linkage. In anatase, four of twelve octahedral edges are shared with neighboring octahedral; in brookite three and in rutile two octahedral share edges.1 Although rutile is the thermodynamically stable form of titania, generally anatase, and occasionally brookite, crystallizes first during synthesis. Anatase starts to transform to rutile at >600 °C.2 The different crystalline structures of titania have different material properties including density, index of refraction, and catalytic properties. Titania, a wide band gap semiconductor, can be used in a variety of applications including gas sensors,3 catalysts,4 photovoltaic solar cells,5 and pigments.6 Recently, titania also gained interest as a material used in photonic band gap crystals for the visible spectrum of light due to its high index of refraction (nrutile ≈ 2.9)7 and low absorption.8–10 Industrially, titania is produced by the sulfate and chloride processes.11 Other synthesis methods include inert gas condensation,12 oxidation-hydrothermal synthesis of metallic Ti,13 and hydrothermal methods.14–16 Most applications for titania require specific material properties and therefore, a specific crystalline structure. Depending on the synthesis recipe, the crystalline J. Mater. Res., Vol. 18, No. 6, Jun 2003
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structure of anatase,17 rutile,18 or brookite,19 as well as mixtures of the three can be synthesized by the hydrothermal method. The rutile structure can be obtained by heating the other polymorphs to high temperatures >600 °C to cause a phase change.2 Generally, the initial synthesis method of titania determines the approximate phasetransformation temperature.20 Other variables that also influence the transformation temperature
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