Enhanced hydrothermal conversion of surfactant-modified diatom microshells into barium titanate replicas
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Enhanced hydrothermal conversion of surfactant-modified diatom microshells into barium titanate replicas Eric M. Ernst, Ben C. Church, Christopher S. Gaddis, Robert L. Snyder,a) and Kenneth H. Sandhageb) Center for Biologically Enabled Advanced Manufacturing, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 23 August 2006; accepted 27 December 2006)
The three-dimensional nanostructured SiO2-based microshells of diatoms have been converted into nanocrystalline BaTiO3 via a series of shape-preserving reactions. The microshells, obtained as diatomaceous earth, were first exposed to a surfactant-induced dissolution/reprecipitation process [C.E. Fowler, et al., Chem. Phys. Lett. 398, 414 (2004)] to enhance the microshell surface area, without altering the microshell shape. The SiO2 microshells were then converted into anatase TiO2 replicas via reaction with TiF4 gas and then humid oxygen. Hydrothermal reaction with a barium hydroxide-bearing solution then yielded three-dimensional nanocrystalline microshell replicas composed of BaTiO3. The enhanced surface area of the surfactant-treated microshells resulted in faster conversion into phase-pure BaTiO3 at 100 °C.
The attractive electronic, optical, and chemical properties exhibited by barium titanate-based compositions have led to the use of these ceramics as capacitors, thermistors, actuators, sensors, phosphors, and other devices.1–8 A variety of approaches (e.g., mixed oxide, mixed salt, sol-gel, polymeric precursor, hydrothermal, microemulsion, mechanochemical, and combustion syntheses) have been used to synthesize BaTiO3 powders with fine particle and crystal sizes.9–19 Nanocrystalline barium titanate-based ceramics have exhibited relatively high room-temperature dielectric constants that are temperature- and voltage-stable for integrated capacitors,20,21 high sensitivity to water vapor and carbon dioxide for gas sensors,22–24 and enhanced response of fluorescence to temperature changes for real-time temperature monitoring.25 The worldwide interest in nanoscale ferroelectric devices has also led to the recent syntheses of BaTiO3 nanowires,26 nanorods,27 nanoshell tubes,28 and nanoshell spheres.29,30 However, the scal-
a)
This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/jmr_policy. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0169 J. Mater. Res., Vol. 22, No. 5, May 2007
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able fabrication of complex three-dimensional (3D) BaTiO3-based nanostructures in a variety of wellcontrolled morphologies via synthetic methods has been a significant challenge. Nature, on the other hand, provides impressive examples of 3D microscale to nanoscale mineral assembly.31–39 For example, coccolithophorids (Haptophyta) and diatoms (Bacillariophyta) are single-celled
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