Cutting of multi walled carbon nanotubes

Transcription

1 Applied Surface Science 252 (2006) Cutting of multi walled carbon nanotubes Jinyong Li *, Yafei Zhang National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Institute of Micro and Nano Science and Technology, Shanghai Jiaotong University, Shanghai , PR China Received 28 March 2005; received in revised form 30 April 2005; accepted 30 April 2005 Available online 3 June 2005 Abstract Multi walled carbon nanotubes (MCNT) synthesized by CVD method have been successfully cut into different lengths by controlling H 2 SO 4 /HNO 3 (5:3) oxidation time. During the cutting process H 2 SO 4 and HNO 3 were added independently and the oxidation processes were carried out at a lower temperature to void excess weight loss and damage to MCNT. The resulting shorted MCNT (s-mcnt) formed stable dispersion state in the polar solvents without the help of surfactants that provided possibility for further functionalization and application. Moreover, NaOH solution was used to determine the total percentage of acidic sites and the total percentage of acidic sites are about 0.2 1%. # 2005 Elsevier B.V. All rights reserved. PACS: De; Jb; Tw Keywords: Multi walled carbon nanotubes (MCNT); Cutting; Titration; Scanning electron microscopy 1. Introduction Since the discovery of carbon nanotubes by Iijima [1], fundamental research on carbon nanotubes and various potential applications [2 7] have been proposed. Recently, carbon nanotubes (CNT) with relatively short lengths have been attracting strong attention because such CNT are expected to provide connectors and components in molecular electronic devices [8] and have rich chemistry [9,10]. Therefore, * Corresponding author. Tel.: ; fax: address: ljy@sjtu.edu.cn (J. Li). different methods have been developed to cut CNT either physically or chemically. It is reported that the fluorination of purified CNT followed by pyrolysis of the fluorinated nanotubes at temperatures up to C was found to have cut the nanotubes to a range of short lengths but the process is very complicated and energy consuming [11]. It is reported that STM and AFM were also successfully used to produce short CNT however these method were only used for individual CNT [12,13]. Recently, ball mills [14 18] has become an efficient method to cut CNT however it generally takes over 100 h to cut CNT to an average length of nm. Moreover, sometimes carbon nanotubes were partially or completely /$ see front matter # 2005 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 J. Li, Y. Zhang / Applied Surface Science 252 (2006) damaged or carbon nanoparticles formed at the broken end of nanotubes. Therefore, here we developed a facile, effective and controllable method to obtain shorted multi walled carbon nanotubes (s-mcnt) in a shorter time. The concentration of acidic sites introduced by mixture acid was decided by acid base titration method. The characteristics of MCNT were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and the resulted functional groups were characterized using infrared spectroscopy (FTIR). preferentially. As a result, the mass was reduced to relatively short and straight carbon nanotubes. The representative SEM images of s-mcnt are shown in Fig. 1. After mixture acid treatment for 4 h, purified 2. Experimental As prepared MCNT synthesized by chemical vapor deposition (CVD) method contain many catalysts [19,20] therefore the as prepared MCNT were put into concentrated HCl with magnetic agitation for 5 h then filtered to eliminate these catalysts. The cutting experiments were carried out as follows: 250 mg pretreated MCNT was suspended in 300 ml of concentrated H 2 SO 4 with magnetic agitation for 30 min at room temperature then added 180 ml of concentrated HNO 3. The mixture was heated at 60 8C for 4, 8 and 12 h, respectively. The s-mcnt were collected on a polytetrafluoroethylene (PTFE) filter with the pore size of 200 nm and washed with deionized water until the PH value reach neutral. s-mcnt was ready for the titration experiment by heating at 100 8C for 1 h under vacuum to degas absorbed carbon dioxide and water. Then 100 mg of the s-mcnt was added in 0.03 M NaOH solution under vacuum for 50 h and stir was required to allow the s-mcnt equilibrate with NaOH solution. The mixture was then filtered through a filter with pore size of 200 nm and MCNT collected on the filter were washed with de-ionized water to remove all of the NaOH residues. The excess NaOH in the combined filtrated and washings were titrated with 0.03 M aqueous HCl to reach the neutral PH. 3. Results and discussion Because carbon atom hexagons are more stable than pentagons and defect sites, these pentagons or defect sites of CNT are destroyed by mixture acid Fig. 1. SEM images of s-mcnt after mixture acid oxidation for (a) 4, (b) 8 and (c) 12 h.

3 2946 J. Li, Y. Zhang / Applied Surface Science 252 (2006) Fig. 2. Histograms of the lengths of s-mcnt for mixture acid oxidation for (a) 8 h and (b) 12 h. MCNT were obtained and the lengths of MCNT are almost several micrometers (Fig. 1a). The images of s- MCNT after mixture acid oxidation for 8 and 12 h are shown in Fig. 1b and c, respectively, and it is observed that the long MCNT were cut into different lengths. A statistical evaluation of the size distribution was performed using the MCNT lengths determined from a number of SEM images. There is a significant difference in the lengths distribution of s-mcnt in histograms (Fig. 2a and b) that the average CNT lengths were calculated to be 605 and 287 nm, respectively, for 8 and 12 h. Moreover, we found that the s-mcnt could be dispersed in the polar solvents such as DMF and ethanol more easily than as prepared. The degree of the dispersion of s-mcnt in DMF was >0.3mg/ml (Fig. 3a and b), but the as prepared MCNT agglomerate and sediment at the bottom (Fig. 3c). These dispersion liquids of the s-mcnt were stable without aggregation for more than several months. This would be due to the reduction of their length and the effect of the introduced hydrophilic group, i.e., the carboxyl group. Acids are known to introduce acidic groups onto the surface of carbonaceous materials in general, and carbon nanotubes in particular [21,22]. Additional peaks introduced by mixture acid (Fig. 4b). One at 1719 cm 1 is associated with C O stretching of carboxylic acids whilst increased strength of the signal at 1222cm 1 may be associated with C O stretching in the same functionalities. The peak at 1579 cm 1 is assigned to C C stretching, which originates from Fig. 3. Dispersibility of s-mcnt in DMF (a) 0.25 mg/ml, (b) 0.30 mg/ml for 2 weeks and as prepared MCNT in DMF (c) for 1 day. Fig. 4. FTIR spectra of (a) as prepared MCNT, (b) MCNT treated by mixture acids for 8 h and (c) MCNT neutralized by NaOH.

4 J. Li, Y. Zhang / Applied Surface Science 252 (2006) Table 1 Acidity of shorted MCNT Sample Mixture acid oxidation time (h) NaOH (ml) HCl (ml) Acidic site (%) MCNT weight loss (%) inherent structure of carbon nanotubes. Neatralization with NaOH leads to the formation of carboxylate ion (Fig. 4c), which manifests itself as a large peak at 1590 cm 1 [23]. The remaining, reduced peak at 1719 cm 1 may either be the result of incomplete reaction or due to the presence of carbonyl groups, but its unchanged appearance after neutraliztion carried out at 100 8C instead of room temperature suggests the latter. Further information was provided by the titration results, which indicated the presence of acidic groups of varying strength. By calculation, the amount of acidic sites is given in Table 1 together with the weight loss of MCNT. The mole percentages of acidic sites of s-mcnt samples are 0.288, and 0.970%, which are attained for oxidation for 4, 8 and 12 h. The weight loss percentages of MCNT are 8.74, and 24.02%, respectively, as Table 1 shown. X-ray diffraction was also applied in order to follow the composition changes of MCNT. Fig. 5 compares the XRD data for the as prepared MCNTand the sample after HCl treatment. It shows that HCl treatment evidently increases the intensity of graphite and nearly eliminates the catalyst impurities. Fig. 5. XRD spectra of (a) as prepared MCNT and (b) MCNT treated by concentrated HCl. 4. Conclusion In summary, we have successfully cut MCNT in different lengths under consideration of minimal weight loss and damage to MCNT by a facile, effective and controllable method. The total percentage of acidic sites which can serve as reactive sites for further functionalization and application were calculated by acid base titration about 0.2 1%. Moreover, the s-mcnt can form stable dispersion state in the polar solvents without the help of surfactants. Further investigations such as the electrical properties are carrying on in order to develop novel CNT-based functional materials. Acknowledgements This work was supported by the National Natural Science Foundation of China under award No , and the developing foundation of Shanghai Science and Technology under award No. 0452nm056. References [1] S. Iijima, Nature 354 (1991) 56. [2] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [3] S. Roth, M. Burghard, V. Krstic, K. Liu, J. Muster, G. Philipp, G.T. Kim, J.G. Park, Y.W. Park, Curr. Appl. Phys. 1 (2001) 56. [4] S. Wong, A. Wolley, E. Joeslevich, C. Lieber, Chem. Phys. Lett. 306 (1999) 219. [5] P. Ajayan, L. Schalder, C. Giannaris, A. Rubion, Adv. Mater. 12 (2000) 750. [6] Z. Jin, K.P. Pramoda, G. Xu, S.H. Goh, Chem. Phys. Lett. 337 (2001) 43. [7] Q. Wang, S.R. Challo, D.S. Sholl, J.K. Johnson, Phys. Rev. Lett. 82 (1999) 956. [8] T. Saito, K. Matsushige, K. Tanaka, Physica B 323 (2002) 280. [9] J. Chen, M.A. Hamon, H. Hu, Y.S. Chen, A.M. Rao, P.C. Eklund, R.C. Haddon, Science 282 (1998) 95. [10] S.S. Wong, A.T. Woolley, E. Joselevich, C.L. Cheung, C.M.J. Lieber, Am. Chem. Soc. 120 (1998) [11] Z. Gu, H. Peng, R.H. Hauge, R.E. Smalley, J.L. Margrave, Nano Lett. 2 (2002) [12] A. Rubio, S.P. Apell, L.C. Venema, C. Dekker, Eur. Phys. J. B 17 (2000) 301. [13] I. Stepanek, G. Maurin, P. Bernier, J. Gavillet, A. Loiseau, R. Edwards, O. Jaschinski, Chem. Phys. Lett. 331 (2000) 125.

5 2948 J. Li, Y. Zhang / Applied Surface Science 252 (2006) [14] K. Niesz, A. Siska, I. Vesselényi, K. Hernadi, D. Méhn, G. Galbács, Z. Kónya, I. Kiricsi, Catal. Today 76 (2002) 3. [15] À. Kukovecz, T. Kanyó, Z. Kónya, I. Kiricsi, Carbon 43 (2005) 994. [16] Z. Kónya, J. Zhu, K. Niesz, D. Mehn, I. Kiricsi, Carbon 42 (2004) [17] E. Farkas, M.E. Anderson, Z.H. Chen, A.G. Rinzler, Chem. Phys. Lett. 363 (2002) 111. [18] Y.B. Li, B.Q. Wei, J. Liang, Q. Yu, D.H. Wu, Carbon 37 (1999) 493. [19] P.X. Hou, S. Bai, Q.H. Yang, C. Liu, H.M. Cheng, Carbon 40 (2002) 81. [20] W. Huang, Y. Wang, G.H. Luo, F. Wei, Carbon 41 (2003) [21] D.B. Mawhinney, V. naumenko, A. Kuzenetsova, J.T.J. Yates, J. Liu, R.E. Smalley, J. Am. Chem. Soc. 122 (2000) [22] M.A. Hamon, H. Hu, P. Bhowmik, S. Niyogi, B. Zhao, M.E. Itkis, R.C. Haddon, Chem. Phys. Lett. 347 (2001) 8. [23] M.S.P. Shaffer, X. Fan, A.H. Windle, Carbon 36 (1998) 1603.