The as-synthesized product was studied by transmission electron microscopy [TEM] (Figure ). The image shows mostly the impurity-free CNTs. Only a small amount of catalyst particles introduced to the inner space of nanotubes is visible. The diameters of CNTs are mostly in the range of 2 to 10 nm (Figure , inset). However, larger CNTs (approximately 20 nm) can also be observed. The phase analysis of a non-purified CNT was performed by an X-ray diffraction technique. The XRD spectrum measured in a wide range of angles is plotted in Figure . The broad diffraction peak at Bragg angle 2θ of approximately 26° corresponds to the (002) peak of the hexagonal graphite structure. The other peaks are attributed only to the MgO (Fm/3 m) and Mo2C (P63/3 mm) phases; no other phases were noticed. The absence of any other peaks assigned to iron or iron-containing phases could be explained by the small amount of these phases which cannot be detected owing to the resolution limit of the used XRD setup.
TEM image of as-prepared CNTs. The inset shows the histogram of the diameter distribution of CNTs.
However, the Mössbauer spectroscopy allows the study of the local iron ion structures. The hyperfine interaction parameter, isomer shift [IS], deduced through the Mössbauer (Figure ) spectrum is δ = 0.32 mm/s. From the value of the IS parameter, one might conclude the presence of Fe-C chemical bonds, where iron ions are in the state of Fe3+. We would like to emphasize that the local surroundings of iron ions have high symmetry as what appears from the fact of a single peak in the Mössbauer spectrum.
The Mössbauer spectrum of fabricated CNTs.
The magnetization measurements show the presence of magnetic phases. The M(T) curves measured in the zero field in the heating and cooling modes are presented in Figure . The magnetic phase has the Curie temperature [TC] of approximately 440 K. Taking into account both the Mössbauer measurements and M(T) dependence, the magnetic phase was assigned to cementite, Fe3C. The decreased TC, as compared to the bulk value (approximately 480 K), could be explained by the small size of Fe3C particles.
The magnetization measurements of CNTs. The inset shows M2 vs. T plot for Curie temperature evaluation.
The Raman spectrum of as-grown CNTs is shown in Figure . The G line, which is attributed to the twice-degenerated deformation oscillations of the hexagonal ring in the E2 g electronic configuration of D46 h crystal symmetry, and the D line, corresponding to the ruinous hexagonal lattice and not fully ordered forms of carbon structure, are located at 1,591 cm-1 and 1,348 cm-1, respectively. The integrated area ratio AD/AG between the D and G bands indicates good crystalline quality of the as-grown nanotubes.
There are few peaks in the range below 300 cm-1
in the Raman spectrum. Commonly, peaks in this range are assigned to the radial breathing mode [RBM] of SWCNTs. Using the simple inverse relation v
= 9 +235/d
is the frequency in units of inverse centimeter, d
is diameter of nanotubes in nanometers), we estimated the diameters of the nanotubes in the range of 0.9 to 1.7 nm; the larger diameters cannot be estimated from the spectrum because of the spectrometer frequency cutoff. Besides, these peaks in the range below 300 cm-1
, apart from the RBM mode of SWNTs, can be attributed to the RBM mode of the inner tubes of FWNTs [21
Figure represents the high-resolution TEM [HRTEM] images of CNTs. As it is seen from Figure , both the few-wall and multi-wall carbon nanotubes of different diameters can be found. They usually have 3 to 10 walls and are closed-tipped (Figure ). A bamboo-like CNT structure is presented in Figure . As shown in Figure , double-wall nanotubes of relatively large diameters are also found in the array. As shown in Figure , besides the single CNTs, CNT bundles can also be found. Figure shows the presence of metal inclusions in CNTs. As shown in Figure , a large aspect ratio of metal inclusions can be found in the nanotube channels. As shown in Figure , inclusions have a single crystal structure. The accurate measurement of the crystal interplane distance of the inclusion performed using Fourier transform gives a value of approximately 0.2349 nm (see the inset), which is very close to the Fe2MoC (241) interplane distance (0.2345 nm). Finally, beside the central channels, metallic inclusions can also be found inside the CNT walls as well (Figure ).
Figure 7 HRTEM images of selected CNTs. (a) Few-wall and multi-wall CNTs of various diameters, (b) a closed-tipped CNT, (c) CNT having a bamboo-like structure, (d) a double-wall CNT (indicated by an arrow), (e) CNT bundles, (f) a single CNT demonstrating a good (more ...)
The selected area electron diffraction patterns both from the impurity-free and the area containing inclusion particles are presented in Figure , respectively. The diffusion rings in Figure are assigned to carbon nanotubes. Figure shows an overlap of reflections from the carbon nanotubes and crystal structure related to the particles encapsulated into the nanotubes. The detailed analysis of the diffraction pattern allows the assumption of the presence of Mo2C and γ-Fe phases.
Selected area electron diffraction images of CNTs. (a) Impurity-free CNTs and (b) CNTs with encapsulated nanoparticles.
The purity of the product was investigated by thermogravimetric analysis [TGA]. TGA thermogram curves of the as-prepared CNTs (Figure , left axis, black) demonstrates that carbon content is approximately 64% of the total mass of the product. The thermogram curve (Figure , left axis, red) for CNTs synthesized at the same conditions, except that after 45 min, the gas pressure was increased up to 2 bars for another 20 min, showed the increasing carbon content of up to 83%. Differential scanning calorimetry curves (Figure , right axis) for both pressures showed only one exothermic transition peak at approximately 650°C. It is known that amorphous carbon burns at temperatures lower than 580°C to 600°C; defect-free SWNTs, at 600°C to 620°C; and pure MWNTs with 10 layers and more, at 750°C to 790°C [22
]. No peaks related to amorphous phase burning were observed. The thermal analysis data are in good agreement with our Raman and TEM findings.
TGA and DSC data for CNTs synthesized at different gas pressures. The black curves corresponded to 1 bar gas pressure synthesis; the red curves correspond to 2 bar gas pressure synthesis.