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Figure 2. Phase separation in a C-BN-C composite nanotube. Reprinted with permission from [23], K. Suenaga et al., Science 278, 633 (1997). © 1997, American Association for the Advancement of Science.

micrograph of such a B-C-N MWNT and its elemental profile measured by scanning electron beam across the tube [23]. The high-resolution image cannot give any information about phase separation because all possible phases in B-C-N system, BN, BC2N, BC3, have very similar crystal structure with graphite, and thus give the same image in the transmission electron microscope. The elemental profile, however, can give detailed phase information of all shells assuming each shell is composed of a single phase. In a cross-section elemental profile of single-phase cylindrical structure, maximum only occurs at the position that corresponds to the inner edge parallel to the probing electron beam. This is because this position has the largest equivalent thickness through which electron beam transmits and loses its energy. The elemental profile of a multiwalled nanotube can be simulated by adding up the profiles of all the shells with each shell as a single-phase cylinder. For a single-phase MWNT, there should be only two maxima in the profile because the probe size of the STEM, 0.5-1 nm, cannot resolve adjacent individual shells in a MWNT. Figure 2B shows four maxima in the C profile and two maxima in the B and N profiles. The positions of the two maxima in the B and N profiles coincide with two minima of the C profile. The result can be interpreted as a C-BN-C three-layered tubular structure as modeled in Figure 2D. Simulated elemental profiles (Fig. 2C) from the model agree with the experimental results very well. The analysis of the C-BN-C nano-tubes has provided the first clear evidence of intershell phase separation (with atomic-scale sharp interfaces) in composite nanotubes.

More recently, some other composite nanotubes have been reported. One example is the TiO2-SiO2 composite nanotubes synthesized by sol-gel template method presented in [29]. The coaxial structure is made by sequential formation of amorphous SiO2 nanotubes in the pores of anodic alumina and TiO2 nanotubes within the SiO2 nano-tubes. The composite nanotubes have a relatively large diameter of 200-250 nm. Another type of novel composite nanotubes are produced by growth of NbS2 nanotubes on carbon nanotube templates. High-resolution transmission electron microscopy and energy dispersive X-ray analysis revealed that the uniform well-crystallized NbS2 is nucleated and grown from an intermediate phase of NbO2 which is unevenly wrapped on the carbon nanotube templates. A multipoint nuclei site growth mechanism has been proposed to account for nanotube formation [30].

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