Synthesis, characterization, and photocatalytic properties of pyrochlore Bi 2 Ti 2 O 7 nanotubes
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Stanislaus S. Wonga) Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400; and Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973 (Received 8 August 2006; accepted 23 August 2006)
Bismuth titanate (Bi2Ti2O7) nanotubes were successfully synthesized with an alumina template-based sol-gel technique. As-synthesized nanotubes are smooth and uniform with diameters ranging from 180 to 330 nm and lengths varying from 7 to 12 m. Extensive characterization of as-prepared samples has been performed using x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive x-ray spectroscopy (EDS), and selected-area electron diffraction (SAED). Photocatalytic studies indicate that as-prepared nanotubes possess higher photocatalytic activity than the corresponding bulk sample prepared without the use of an alumina template.
I. INTRODUCTION
Inorganic pyrochlore oxides can be described by the general formula of A2B2O7 and display diverse, unusual properties ranging from colossal magnetoresistance, ferroelectricity, to superconductivity.1,2 Closely related layered manganite materials have been shown to confine electronic motion to two dimensions and possess interesting optical and electrical properties.3 The structure of pyrochlore oxides can best be summarized as A2B2O6O⬘, comprising two interpenetrating sublattices with the formulae of A2O⬘ and BO6, in other words, a juxtaposition of two different types of cationic coordination. A number of materials belonging to this family of oxides have been successfully synthesized, including Cd2V2O7, Cd2Re2O7, Tl2Ru2O7, and Zn2Nb2O7.4–7 A particularly interesting pyrochlore oxide, namely Bi2Ti2O7, is characterized by the presence of Ti-O octahedral frameworks coupled with Bi-O tetrahedral units. Structural analysis of bismuth titanate (Bi2Ti2O7) indicates that it possesses a face-centered cubic structure with Fd3m symmetry (a ⳱ 20.68 Å). Bismuth titanate is interesting for a number of reasons. This material has been considered as a highly promising high-k dielectric material because of its high permittivity and low leakage current density. Specifically, because of a)
Address all correspondence to this author. e-mail: [email protected] or [email protected] DOI: 10.1557/JMR.2006.0362 J. Mater. Res., Vol. 21, No. 11, Nov 2006
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its relatively high-k dielectric properties (dielectric constant, k ⳱ 110∼160), Bi2Ti2O7 has attracted considerable attention as a useful component of (i) gate insulators in advanced metal oxide semiconductor (MOS) transistors,8–10 (ii) storage capacitors in dynamic randomaccess memory (DRAM) applications, and (iii) buffer layers to improve the electrical properties of ferroelectrics such as Pb(Zr0.5Ti0.5)O3 (PZT) and Bi4Ti3O12.11–13 Moreover, it has recently been found that bismuth titanate can be used for oxidizing
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