Synthesis of hollow porous nanospheres of hydroxyl titanium oxalate and their topotactic conversion to anatase titania
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(Received 27 January 2011; accepted 21 April 2011)
A one-step wet chemistry route has been explored to synthesize hollow hydroxyl titanium oxalate nanoscale spheres under mild experimental conditions. The hollow spheres were ;200 nm in diameter, with a shell thickness of ;30 nm. The nanospheres were formed by smaller aggregated colloidal subunits. The influence of temperature and solvent on the structure of the nanospheres was investigated. The formation of hollow interiors in the nanospheres may be rationalized by Ostwald ripening mechanism. Simple thermal treatment topotactically transformed the chemical composition into anatase TiO2. The high-order hollow porous spherical structure was preserved, with smaller crystalline anatase TiO2 nanoparticles as building units. Dense hydroxyl titanium oxalate nanospheres and their corresponding non-hollow porous anatase TiO2 nanospheres were also successfully achieved in suitable reaction conditions. The method and procedure reported herein may be extended in principle for the fabrication of other functional materials.
I. INTRODUCTION
There is increasing interest in the fabrication of hollow nanostructures, which find important functional applications in catalysis, drug delivery, energy storage, solar cells, absorption, photonics, chemical sensors, and reactors in confined space at nanoscale.1–5 Various fabrication methods have been developed for the preparation of hollow nanostructures, and new routes of fabrication are of continued interest. Noticeably, templating techniques are widely used to generate the hollow interiors. Both soft templates (e.g., emulsion droplets,6,7 surfactants8) and hard templates (e.g., silica particles, carbon spheres) as supports have been used for the fabrication of hollow nanomaterials. The materials of interest are first deposited or encapsulated on template supports, and subsequently the templates are removed by dissolving/etching/burning to achieve the desired interiors. To avoid complications arising from the templating or final removing process, it is also desirable to fabricate hollow nanostructures by onestep template-free routes.9–12 For example, hollow magnetite, titania, and tin dioxide nanostructures have been synthesized by hydrothermal synthesis.10,11,13 The hydrothermal synthesis usually involves high temperature and a)
Present address: Parsons Laboratory for Environmental Science and Engineering, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Address all correspondence to these authors. b) e-mail: [email protected] c) e-mail: [email protected] DOI: 10.1557/jmr.2011.149 J. Mater. Res., Vol. 26, No. 12, Jun 28, 2011
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high pressure with special sealed reactors. On the other hand, wet chemistry methods under mild reaction conditions (below 100 °C) using nontoxic solvents in conventional flasks for the fabrication of hollow structures is highly desirable, but less explored.14 In addition, it is preferre
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