J. Phys. Chem. C 2007, 111, 2901-2905
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Structural Changes in Double-Walled Carbon Nanotube Strands Induced by Ultraviolet Laser Irradiation Yong Zhang,* Tao Gong, Jinquan Wei, Wenjin Liu, Kunlin Wang, and Dehai Wu Key Laboratory for AdVanced Manufacturing by Materials Processing Technology, Department of Mechanical Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: October 11, 2006; In Final Form: December 8, 2006
Centimeters-long double-walled carbon nanotube strands were irradiated by a 355 nm pulsed laser. Multiwalled carbon nanofibers, hollow and solid onion-like nanoparticles, were found to be changed from DWNTs, induced by UV laser irradiation. As-grown Fe catalysts were also found to aggregate and were conveniently able to be removed by a dilute HCl solution. The structural changes in the DWNTs strand depend importantly on the laser energy input. On the basis of high-resolution transmission electronic microscope (HRTEM) observations, we propose a process of the structural change.
Introduction Carbon has many kinds of molecule structures, such as graphite, diamond, fullerene, and carbon nanotubes (CNTs), etc. It is known that allotropes can transform into one another in certain conditions. Because of their unique structures, CNTs have attracted much attention. It is interesting to find out how CNTs perform and what they can change into, under various conditions. One basic principle is introducing a certain energy to break the C-C bond, leading to reconstruction, which finally resulted in a structural change of CNTs. For the convenient use of transmission electronic microscopy (TEM), electron beam irradiation is likely the most popular method used in the research1-4 on morphology changes of CNTs. Through the examination of TEM, various structural changes have been observed. As another source of high energy beams, lasers were first introduced to perform the synthesis of CNTs, which made graphite transform into CNTs.5 Then, lasers were used to treat CNTs. During the processing, CNTs could change into other kinds of carbon materials (e.g., diamonds,6 carbon nanoparticles,7 and sub-micrometer sized structures).8 It was also found that no damage could be observed when laser parameters were appropriate, such as preferential destruction of metallic and semiconducting single-walled carbon nanotubes (SWNTs)9 and the removal of amorphous carbon10 and catalyst particles11 from multiwalled carbon nanotubes (MWNTs). In these papers, the structural change of CNTs was influenced by laser power density. At a low power density, CNTs may have no damage nor sinter together, while at a high power density, CNTs may be damaged, and they could be transformed into nanoparticles or even burned out. From that research, SWNTs and MWNTs provide different properties of laser irradiation, due to their different structure stabilities. Since double-walled carbon nanotubes (DWNTs) are a simple type of MWNT with minimum layers, and their diameter is close to that of SWNT, the property of DWNT is between that of SWNT and MWNT. When DWNT is irradiated by a laser beam, it can also be transformed into other kinds of * Corresponding author. Tel.:
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Figure 1. (a) HRTEM image of a DWNTs bundle tip. The outer diameter is ∼2.1 nm, and the inner diameter is ∼1.3 nm. (b) SEM image of a DWNTs strand without purification.
structures. In this paper, DWNTs strands were treated by a 355 nm ultraviolet (UV) laser. The structural changes in the DWNTs strands were observed to be regular with respect to the laser energy input.
10.1021/jp0666838 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/26/2007
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TABLE 1: Laser Parameters in the Experiment sample
power (mW)
frequency (Hz)
power density (mW cm-2)
energy input (mW cm-2/mm)
1 2 3 4 5
1 10 40 150 2000
1 10 75 300 2500
7.96 × 100 7.96 × 101 3.18 × 102 1.19 × 103 1.59 × 104
7.96 × 100a 2.65 × 102 7.96 × 103 1.19 × 105 1.33 × 107
a
The scanning speed was 1 mm/s on this condition.
Experimental Procedures The DWNTs used in the laser treating experiment were macroscopic strands with lengths of 5-6 cm, which were directly pulled out from the initial DWNTs filaments prepared
by chemical vapor deposition, as described in ref 12. The details were modified in this study. Ferrocene and sulfur were dissolved in xylene (Fe/S ) 100:12, atomic ratio), and the concentration of the solution was 0.1 g/mL. The reaction temperature was 1180 °C, and the flow rates of argon and hydrogen were 2000 and 500 sccm, respectively. Figure 1a shows a high-resolution transmission electronic microscope (HRTEM) of the tip of an as-grown DWNTs bundle. Two concentric graphite cylinders can be clearly seen. In the bulk sample, DWNTs bundles form a strand, as seen in Figure 1b, a scanning electronic microscope (SEM) image of a DWNTs strand. From Figure 1b, we can see that DWNTs are continuous and well-oriented.
Figure 2. HRTEM images of carbon nanostructures transformed from DWNTs after UV laser irradiation. (a) Platelet hollow nanofiber. (b) Cap of panel a. The outer diameter is ∼20 nm. (c) Onion-like nanoparticle with several concentric circles and a hollow center. Its outer diameter is ∼56 nm. (d) Onion-like nanoparticles. The nanoparticles indicated by A and B are 60-70 and 50-60 nm, respectively. The inset is the magnification of the sidewall of A. (e) Edge of a large onion-like nanoparticle with diameter of ∼100 nm. (f) Carbon nanofiber. The laser energy input was 7.96 × 100 mW cm-2/mm (sample 1) for panels a and b, 7.96 × 103 mW cm-2/mm (sample 3) for panels c and d, and 1.33 × 107 mW cm-2/mm (sample 5) for panels e and f, respectively.
Figure 3. TEM images of the morphology changes in the DWNTs strand after UV laser irradiation. From panels a-e, the laser energy input vs parameters from samples 1-5 are shown, which are listed in Table 1, respectively. (f) Sample 3 after 10% HCl (wt %) treatment for 1 h. In each image, A and B indicate the carbon nanostructure and Fe particle, respectively.
Double-Walled Carbon Nanotubes
Figure 4. Raman spectra of the DWNTs before and after UV laser irradiation. (a) RBM and (b) D- and G-band. In both panels a and b, lines in magenta, black, red, green, blue, and cyan are vs the as-grown DWNTs strands 1-5, respectively. Sample numbers are consistent with those listed in Table 1.
In the laser processing experiment, the laser source was a pulsed Nd+/YAG laser made by Lambda Physik. It had a wavelength of λ ) 355 nm (third harmonics of 1064 nm). The spot diameter was 4 mm, and the scanning speed was 3 mm/s. The samples were put into a chamber under an argon atmosphere protection and irradiated by the UV laser. The laser spot moved along the DWNTs strands and was repeated 10 times. The magnitude order of the laser power density was varied from 100 to 104 W/cm2. It needs to be pointed out that the laser power is related to the frequency for this laser system; thus, the energy input was found to be more effective in depicting the structural change during the laser processing. Parameters are listed in Table 1. Results and Discussion After UV laser irradiation, many other nanostructures formed, as shown in Figure 2. From Figure 2a-f, the structural transformation process of DWNTs can be clearly seen. The
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2903 morphology, size, and degree of graphitization of these structures strongly depends on the laser energy input. Figure 2a,b shows that DWNTs are sintered together and form a platelet hollow nanofiber when the energy input was low (7.96 × 100 mW cm-2/mm). However, the formation of this multiwalled structure was not successful, as its sidewall was not smooth. As the energy input increased to 7.96 × 103 mW cm-2/mm, hollow and solid onion-like nanoparticles formed, as shown in Figure 2c,d, respectively. In Figure 2d, two solid nanoparticles can be seen. The nanoparticle indicated by B is more spherical but smaller than A. It shows that newly formed small particles are more stable than large particles in this condition. When the energy input was up to 1.33 × 107 mW cm-2/mm, a large solid onionlike nanoparticle was formed, which was spherical and uniform, as shown in Figure 2e. Carbon nanofibers, which were solid in the tube, can also be found, as seen in Figure 2f. We can see that the higher the energy input is, the more regular and larger the shape of the newly formed nanostructure is, which leads to higher stability. Meanwhile, from Figure 2a-f, the structure turns from hollow to solid. Therefore, the newly formed nanostructure tends to have a higher stability and larger diameter as the laser energy input increases. Furthermore, from Figure 2a-f, the interlayer distances of the nanostructures are 0.370.38, 0.37-0.39, 0.35-0.37, 0.34, 0.34-0.35, and 0.36-0.37 nm, respectively. Except for the solid carbon nanofiber, the interlayer distance presents a tendency of decreasing toward 0.34 nm, which is the most stable distance of the graphite layer. Combined with these results, it could be seen that with respect to the increase of the laser energy input, DWNTs could be transformed into more and more stable structures. Figure 3 shows the distribution of newly formed nanostructures. Generally, these nanoparticles congregate to be clustered. The existence of DWNT bundles reveals that structural changes in the DWNTs strands are localized. In Figure 3a, hollow nanofibers can be seen, which are magnified in Figure 2a,b. The black particles are Fe particles. From Figure 3b-e, onion-like carbon nanoparticles can be seen, which have been shown in Figure 2b-e. As the energy input increases, the size of the nanoparticles keeps growing, and the quantity of the nanoparticles has no distinct increase. This means that more carbon atoms participate in the structural change as the energy input increases. Figure 3 also depicts the morphology transformation of Fe particles in DWNTs with respect to the increasing laser energy input. In Figure 3a, Fe particles are spherical and larger than the Fe catalyst remaining in the as-grown DWNTs. It is due to the fusion and aggregation of the initial Fe catalysts under the high energy input of a UV laser. When the energy input increases to 2.65 × 102 mW cm-2/mm, Fe particles grow even larger, as is displayed in Figure 3b. It is interesting that among all the figures shown in Figure 3, Fe particles shown in Figure 3c have the largest size, some of which have a diameter of ∼200 nm. As compared with Figure 3c, Figure 3d reveals the reduction in diameter of Fe particles. However, the energy input is higher than the previous values. Finally, in Figure 3e, large Fe particles are seldom seen, although the energy input is the highest. Interestingly, most of the Fe particles are spherical and smooth on the edge, and many of them have nothing wrapped on their outside surface, while Fe catalysts are wrapped by carbon layers or amorphous carbon in the as-grown DWNTs. It indicates that the UV laser treatment removed the outside carbon coatings. Therefore, Fe impurities could be easily eliminated by a simple acid treatment. Sample 3 was put into a dilute HCl solution. After acid treatment for 1 h, these large Fe particles were nearly
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Figure 5. Schematic illustration of UV laser induced structural change of DWNT. (a-e) Top view and (f-j) front view of the evolution: (a and f) DWNTs; (b) broken and (g) short DWNTs; coalescence of fragments both (c) in tip and (h) at sidewall; broken fragments (h) into smaller fragments (i); further coalescence and growth of panels c and i to form panels d and j, respectively, leaving a hollow center; and contract of empty space and growth on the surface to form panel e.
dissolved, as is shown in Figure 3f. Empirically, the remaining Fe particles in the image were mainly as-grown Fe catalysts, which could be estimated by their morphology. As compared with the as-grown DWNTs, the Fe catalysts distinctly reduced. Raman spectra were also obtained with a Renishaw 2000 Raman spectroscope, which are plotted in Figure 4. The excited laser wavelength was 632.8 nm. Since the structural change in the DWNTs strand mainly occurred in the area that was exposed to the laser beam, a large proportion of the DWNTs remained in the strands. It is consistent with the results that the radial breathing modes (RBM) can be identified for all the samples (Figure 4a). As compared with the as-grown DWNTs strand, both the D- and the G-band shifted for all the samples after UV laser irradiation, which indicates that the structural changes occurred in the DWNTs mentioned previously (Figure 4b). Since a large proportion of the DWNTs remained, the shifting was not remarkable. The D-band peak of the as-grown DWNTs strand can hardly be identified, which shows the high quality of our samples. Similarly, the D-band peaks of samples after UV laser irradiation are also very low, as compared with the G-band. Since the ratio between the intensity of the G-band and the intensity of the D-band (IG/ID) can be used to characterize the crystallization of the carbon materials, IG/ID of the as-grown DWNTs strand and samples 1-5 are calculated to be 44.2, 35.5, 22.1, 11.8, 20.8, and 48.7, respectively. Introducing the irregular shaped carbon nanostructures influences the crystallization. Evidently, the crystallization becomes the worst in sample 3. Combined with TEM observations, it can be concluded that 7.96 × 103 mW cm-2/mm is the turning point of the energy input in our experiment. The mechanism of structural transformation of SWNTs2 and onion-like carbon particles13 under electron irradiation has been discussed. According to the previous discussion, the explanation of the structural changes in the DWNTs strand under UV laser irradiation might be the combination of these two mechanisms. Here, we just give a simple suggestion about the growth process of the nanostructures and their shapes with respect to the laser energy input. At extremely low energy inputs, the photonic energy of the UV laser mainly dominates the laser processing. In addition, the impulsion of the photons cannot be ignored.
Under the dual effect, DWNTs are broken into many fragments and are also shortened (Figure 5b,g). Then, they coalesce both in the tip and at the sidewall (Figure 5c,h). Since the energy is not high enough, the coalescence is not good. Thus, the tip is not so spherical and has a hollow space (Figure 5c), and the sidewall is not smooth (Figure 5i). As the energy input increases, a heat effect is induced. The strengthened impulsion and heat make the multiwalled structure broken, and more carbon atoms aggregate to form nanoparticles. Generally, a multiwalled structure can transform into some nanoparticles, as it is long in length. It is proven by images shown in Figure 3 that nanoparticles are clustered. As heat still provides insufficient energy, the growth of the nanoparticle stops, while there is a hollow part in it (Figure 5d,j). When the energy input is extremely high, the heat effect and impulsion of the photon become notable. Heat provides sufficient energy to make the carbon atoms nearby keep aggregating and growing on the surface, forcing the hollow space to contract and finally leading to the formation of a large solid particle (Figure 5e). Conclusion In summary, we have investigated various structural changes in the DWNTs strand under UV laser irradiation. The formation and morphology of carbon nanostructures depend on the energy input. When the energy input is higher, larger and more stable particles are found. It was also found that Fe catalysts are aggregated and isolated from the DWNTs, leading to easy removal. Acknowledgment. This work is supported by the National Natural Science Foundation of the People’s Republic of China (Grant 50475013) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant 20060003071) and partly by the National Center for Nano Science and Technology, P. R. China. References and Notes (1) Ajayan, P. M.; Ravikumar, V.; Charlier, J. C. Phys. ReV. Lett. 1998, 81, 1437. (2) An, K. H.; Park, K. A.; Heo, J. G.; Lee, J. Y.; Jeon, K. K.; Lim, S. C.; Yang, C. W.; Lee, Y. S.; Lee, Y. H. J. Am. Chem. Soc. 2003, 125, 3057. (3) Smith, B. W.; Luzzi, D. E. J. Appl. Phys. 2001, 90, 3509.
Double-Walled Carbon Nanotubes (4) Banhart, F. J. Mater. Sci. 2006, 41, 4505. (5) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H. J.; Petit, P.; Robert, J.; Xu, C. H.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (6) Wei, B.; Zhang, J.; Liang, J.; Liu, W.; Gao, Z.; Wu, D. J. Mater. Sci. Lett. 1997, 16, 402. (7) Ma, R. Z.; Wei, B. Q.; Xu, C. L.; Liang, J.; Wu, D. H. Carbon 2000, 38, 636. (8) Kichambare, P. D.; Chen, L. C.; Wang, C. T.; Ma, K. J.; Wu, C. T.; Chen, K. H. Mater. Chem. Phys. 2001, 72, 218.
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2905 (9) Huang, H. J.; Maruyama, R.; Noda, K.; Kajiura, H.; Kadono, K. J. Phys. Chem. B 2006, 110, 7316. (10) Bai, X. D.; Li, D.; Du, D.; Zhang, H. J.; Chen, L. F.; Liang, J. Carbon 2004, 42, 2125. (11) Kim, J. S.; Ahn, K. S.; Kim, C. O.; Hong, J. P. Appl. Phys. Lett. 2003, 82, 1607. (12) Wei, J. Q.; Jiang, B.; Wu, D. H.; Wei, B. Q. J. Phys. Chem. B 2004, 108, 8844. (13) Ugarte, D. Carbon 1995, 33, 989.
