Morphology tuning in nontemplated solvothermal synthesis of titania nanoparticles
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Nanoparticles and nanocrystalline particles of pure anatase titania (TiO2) were synthesized by solvothermal processing of TiCl4 ethanol and isopropanol solutions at 120 to 200 °C. This one-step and nonsurfactant approach is versatile and the morphology tuning can be achieved by manipulating the growth kinetics. Dispersed nanocrystals of spherical, cubic, and acicular shapes and hollow spherical and core-shell structured micrometer-sized particles were obtained under different experimental conditions. The obtained hollow spherical- and core-shell–structured particles have an average diameter of 700 nm to 1.0 m, with an average crystallite size of 5 to 16 nm. The dependence of nucleation/crystal growth and morphology development on solvothermal medium, reaction temperature, and reactant concentration was investigated. The reaction mechanism was then suggested and tentatively discussed from coordination and solution chemistry. I. INTRODUCTION
Nanocrystalline TiO2 is one of the most studied oxides in recent decades because of its widespread applications in photocatalysis, pigment, optics, and photovoltaic cells.1–3 In recent years, TiO2 hollow spheres as a promising material for applications in medicine, pharmaceuticals, and the paint industry have attracted much attention.4–6 The function of TiO2 nanomaterials relies on the their microstructure, e.g., it was suggested that rodshaped TiO2 nanoparticles could have better photocatalytic performance than spherical nanoparticles because, for spherical crystals, the surface trapping sites will increase e−/h+ recombination,7 therefore, crystallite size control and morphology manipulation is of essential importance in the preparation of TiO2 nanopowders. There are extensive investigations on the synthesis of TiO2 nanoparticles, and the investigations can be roughly classified into two categories: study on dispersed or loosely agglomerated nanocrystals with isotropic (e.g., nearspherical) or anisotropic shapes (e.g., rods, fibers, etc.), and study on particles consisting of nanocrystallites with particular structural arrangements such as spheres, coreshell–structured particles, hollow spheres, and mesoporous materials. For solution-based synthetic routes, TiO2 nanoparticles are obtained mainly by the sol-gel precipitation method with8–13 or without calcination,7,14–19 the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0146 J. Mater. Res., Vol. 21, No. 5, May 2006
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hydrothermal20–29 or solvothermal30–35 method, and the solution combustion method.36 These solution-based synthetic methods for TiO2 nanoparticles can be categorized as hydrolytic methods (water is involved in the formation reactions) or nonhydrolytic methods (water is not involved in the formation reactions). Hydrothermal synthesis is a hydrolytic method and is known for its capability to synthesize high-quality TiO2 powders at moderately high temperatures using simple procedures and cheap reactants. H
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