Bismuth nanotubes: a rational low-temperature synthetic route.

The discovery of carbon nanotubes has initiated an exciting, intellectually challenging, and rapidly expanding research field for one-dimensional (1D) nanostructures.1 Over the past several years, considerable efforts have been placed on the synthesis of nanotubes or nanowires. A particularly significant breakthrough in MX2 (M: Mo or W, X: S or Se) nanotube synthesis was made by Tenne and co-workers.2 Various approaches to other nanotubes, such as BN,3 BxCyNz, NiCl2, vanadium oxide,6 and InS7 have also been reported, which implies that substances possessing layered structures might be able to form nanotubes under favorable conditions. Here we report the synthesis of bismuth metal nanotubes with diameters ∼5 nm and lengths ranging between 0.5-5 μm. A low-temperature hydrothermal reduction method with bismuth nitrate [Bi(NO3)3] and aqueous hydrazine solution (N2H4‚H2O) at 120 °C has been successfully used to synthesize large quantities of nested bismuth nanotubes. The resulting Bi nanotubes were characterized by X-ray powder diffraction, transmission electron microscopy and composition analysis. Metallic bismuth (R-Bi) (Figure 1a) has a pseudolayered structure very similar to that of rhombohedral graphite and black phosphorus.8 In each layer, one Bi atom is connected with three other Bi atoms according to the 8-N rule and thus forms a trigonal pyramid. These pyramids further form a folded bismuth layer by vertex-sharing. The distances between one Bi atom and its three neighbors in the same layer and the neighboring layer are 3.072 and 3.529 Å, respectively, and the Bi-Bi-Bi bonding angle is 95.5°. The analogy between the layered structures of R-Bi (Figure 1a) and graphite/phosphorus suggests that the Bi nanotubes (Figure 1b) may also exist. In fact, theoretical simulation indeed indicates that phosphorus nanotubes are stable.9 More recently, both theoretical and experimental studies have established the existence of multishell gold nanotubes.10 Here, we report the synthesis of the Bi-nanotubes (NT) with uniform diameters of 5 nm and lengths ranging between 0.5-5 μm. Bismuth’s small effective mass (∼0.001m0) and large mean free path (∼0.4 mm at 4 K) make Bi nanotube an interesting system for studying quantum confinement effects.11 In addition, nanoscaled bismuth materials have recently been suggested to have enhanced thermoelectric properties at room temperature.12 However, owing to the relatively low melting point of Bi (271.3 °C), the synthesis of Bi nanotubes is much more difficult than that for carbon or MX2 analogies. Most of the existing hightemperature approaches, such as arc-discharge evaporation, laser ablation, or chemical vapor deposition are inappropriate for synthesis of Bi nanotubes. Hence, the investigations of rational synthesis, physical properties, and applications of the Bi nanotubes or nanowires remain as challenges to materials scientists. We have developed a low-temperature controlled hydrothermal reduction method13 to prepare Bi nanotubes. The chemical reaction we employed for synthesis of the Bi nanotubes can be formulated as