pubs.acs.org/NanoLett
High-Yield Synthesis and Structure of Double-Walled Bismuth-Nanotubes Regine Boldt,† Martin Kaiser,† Daniel Ko¨hler,† Frank Krumeich,‡ and Michael Ruck*,† †
Department of Chemistry and Food Chemistry, Dresden University of Technology, D-01062 Dresden, Germany, and ‡ Laboratory of Inorganic Chemistry, ETH Zu¨rich, CH-8093 Zu¨rich, Switzerland ABSTRACT A new convenient room-temperature template-free route for high-yield synthesis of double-walled bismuth nanotubes through the treatment of solid bismuth monoiodide with n-butyllithium is presented. Scanning electron microscopy and transmission electron microscopy observations of the product show uniform one-dimensional nanoparticles with high aspect ratios and lengths up to several hundred nanometers. Investigations of the cross sections of the bismuth nanotubes reveal an inner diameter of about 4.5 nm and an outer diameter of 6 nm. The tube walls consist of two coaxial cylinders, and the estimated thickness of the double-wall of about 0.75 nm matches quite properly two layers in the rhombohedral bismuth bulk structure. KEYWORDS Bismuth, template-free synthesis, double-walled nanotubes
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ince the first reports on carbon nanotubes by Iijima1 in 1991, and on inorganic nanotubes by Tenne2 in 1992, tremendous experimental and theoretical investigations have been focused on nanoscale tubular particles because of their interesting physical properties and the wide range of potential applications.3-7 Nanotubes generally have a strong phonon blocking effect due to their hollow tube channels and the inner and outer surface involved, resulting in reduced lattice thermal conductivity and different electronic transport properties compared to nanowires or nanorods. Up to now, numerous families of inorganic nanotubes (NTs) have been synthesized, for example, transition metal chalcogenide NTs,8-12 boron- or silicon-based NTs,13-15 transition metal halogenide NTs,16 mixed-phase and metal-doped NTs,17,18 as well as metal NTs.19-22 In the past few years especially, the interest in tubular structures of bismuth increased.
smaller than that of SWBiNTs, and in the case of DWBiNTs the band gap decreases with increasing diameter of the tube.34 Since the first report of the synthesis of bismuth nanotubes (BiNTs) by Li et al.35 in 2001, different strategies for synthesis followed either low-temperature hydrothermal reduction,36 room-temperature aqueous chemical approaches,37 solvothermal methods,38 embedding in anodic aluminia membranes via electro-deposition,39 or microwave heating of bulk bismuth.40 Up to now, all published syntheses of bismuth nanotubes imply a typical chemical bottomup approach by reduction and self-assembling of atomic or molecular precursors of bismuth. We report a convenient room-temperature template-free route for high-yield synthesis of double-walled bismuth nanotubes (DWBiNTs) through the treatment of solid bismuth monoiodide (BiI)41 with n-butyllithium.42 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations of the product show uniform one-dimensional nanoparticles with high aspect ratios and lengths up to several hundred nanometers (Figure 1). In conventional high-resolution (HR)TEM as well as in high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) investigations, tubular structures with contrast along their axes are visible (Figure 2a). The tubular character is evident in particles or fragments that are orientated in such a way that their cross sections can be observed (Figure 2b,c). This first observation of the cross sections of bismuth nanotubes allows to distinguish between the two main types of nanotube walls, namely scrolled layers and concentric cylinders.43,44 The cross sections of the bismuth nanotubes reveal an inner diameter of about 4.5 nm and an outer diameter of 6 nm (Figure 2b,c). The tube walls consist of two coaxial cylinders. The bismuth atoms in the double-walled tubes can be seen with dark contrast in bright-field TEM images (Figure 2b) and bright in HAADF-
Bulk bismuth has a quasi-layered rhombohedral structure that allows the formation of nanowires and especially nanotubes. Furthermore bulk bismuth is a semimetal with a very small indirect band overlap and unusual electronic properties resulting from its small electron effective-mass, low charge carrier density, and large mean free path.23,24 Therefore bismuth in nanostructured form is regarded as an excellent object in order to study finite-size effect, quantum confinement, magneto resistance, and thermoelectric effects.25-31 Quantum chemical simulations suggest that single-walled bismuth nanotubes (SWBiNTs)32,33 as well as double-walled bismuth nanotubes (DWBiNTs)34 are stable and semiconducting. The band gap of DWBiNTs should be
* To whom correspondence should be addressed. E-mail: michael.ruck@ chemie.tu-dresden.de. Fax: (+49)-351-463-37287. Received for review: 10/2/2009 Published on Web: 12/17/2009 © 2010 American Chemical Society
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FIGURE 2. (a) High-resolution TEM image of the prepared bismuth nanotubes. (b) High-resolution TEM image of the cross sections of two typical nanotubes. (c) HAADF-STEM image of the nanotubes and their typical double-layered cross sections. Because of the Z-contrast method the bismuth atoms occur with bright contrast in the HAADF. FIGURE 1. Representative (a) scanning electron micrograph and (b) TEM image of bismuth nanotube samples. Both images demonstrate the sample homogeneity.
STEM images (Figure 2c) due to Z-contrast.45 The estimated thickness of the double-wall of about 0.75 nm matches quite properly two layers in the rhombohedral bismuth bulk structure (2/3c ) 2/3 × 1.18 nm ) 0.79 nm).46 Because of the low melting point of bismuth the nanotubes are very sensitive to the electron beam, and hence it is challenging to obtain undisturbed HRTEM images with atomic resolution. Nonetheless, the lattice fringe of the nanotubes along their axes could be estimated to about 0.8 nm. This matches with calculated lattice fringes of double-walled bismuth nanotubes of zigzag type Bi(n,0).34 Combining these observations with the results from firstprinciple calculations on DWBiNTs with smaller diameters34 converges in a consistent structure model. Figure 3 (right) shows a constructed regular (34,0) at (40,0) DWBiNT, that is, two coaxial tubes of zigzag type with circumferences of 34 respectively 40 bismuth combs. The structure is quasicommensurate, since translational symmetry holds parallel to the tube only. The misfit of rotational symmetry between inner and outer wall results in varying interwall distances. In the XRD pattern, a substantial tailing of the reflection 012 of rhombohedral bismuth represents the range 330 pm g d g 290 pm, and thus the distances between neighboring bismuth atoms in the DWBiNTs. © 2010 American Chemical Society
FIGURE 3. Scheme of the reaction of bismuth monoiodide with n-butyllithium. (Left) One possible way of reconstruction of [Bi0BiIII4/2]-chains by self-assembling. BiI consists of parallel infinite [Bi0BiIII4/2]-chains with strong covalent bonding inside and only weak interactions in between. In the crystal structure of bismuth monoiodide Bi1 is bonded to bismuth atoms only, whereas Bi2 has bonds to one bismuth and four iodine atoms.41 (Right) Constructed regular (34,0) at (40,0) DWBiNT; Din ) 4.75 nm, Dout ) 5.64 nm, taxis ) 0.7874 nm, distances Bi-Bi 302-307 pm in the walls, >315 pm between the walls. The structure is quasi-commensurate since translational symmetry holds parallel to the tube only.
The mechanism of the formation of the double-walled bismuth nanotubes is not clear yet and necessitates further experimental and theoretical investigations. Nevertheless there are suggestions, which are based on the particular structure of the solid precursor. BiI consists of parallel infinite [Bi0BiIII4/2]-chains with strong covalent bonding inside and only weak interactions in between (Figure 3, left).41 We assume that the preformed bismuth backbone is preserved during the reduction with n-butyllithium. The reduction step 209
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should be immediately followed by self-assembling of the unsaturated bismuth zigzag chains. At least two ways of reconstruction that result in the formation of a tube are imaginable. Either formation of a double layer followed by curving and rolling up, as proposed by Li et al.,35 or a helical nanobelt-twist-join-growth, which has been suggested for the formation of tellurium nanotubes.47 A supporting fact for the essential role of preformed bismuth fragments is the observation that BiI3, which does not include any Bi-Bi bonds, is not a suitable precursor in this room-temperature synthesis. In view of the outlined reconstruction mechanism and in apparent contrast to the syntheses of bismuth nanotubes via classical bottom-up strategies from molecular precursors, we classify our concept as a chemical top-down-bottom-up approach. The availability of samples with a high portion of doublewalled bismuth nanotubes with uniform diameter, which is achieved by the solid-state precursor based synthesis, allows further characterization as well as evaluation of possible applications. Besides, the synthetic concept can also be applied to intermetallic systems, for example, reduction of Bi12Ni4I3 to Bi3Ni.48
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Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft. Electron microscopy investigations were performed at EMEZ (ETH Zu¨rich). This paper is dedicated to Professor Arndt Simon on the occasion of his 70th birthday. Supporting Information Available. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
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