NANO LETTERS
Effect of Purification on Pore Structure of HiPco Single-Walled Carbon Nanotube Aggregates
2002 Vol. 2, No. 4 385-388
Cheol-Min Yang,† Katsumi Kaneko,*,§,| Masako Yudasaka,† and Sumio Iijima†,‡ Japan Science and Technology Corporation, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan; Department of Physics, Meijo UniVersity, 1-501 Shiogamaguchi, Tempaku, Nagoya 468-8502, Japan; Department of Chemistry, Faculty of Science, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan; Center for Frontier Electronics and Photonics, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan Received October 26, 2001; Revised Manuscript Received February 12, 2002
ABSTRACT HiPco single-walled carbon nanotube (HPNT) containing Fe was purified by one-step purification with HCl-washing (D-method) and two-step purification with HCl-washing after air oxidation (GD-method). The N2 adsorption isotherm of HPNT at 77 K is of type II and it changes to type IV after purification. Purification by the D and GD methods increases the total surface area and micropore volume, and it decreases the average pore width from 3.5 nm to 1.7 and 1.0 nm, respectively.
Introduction. Carbon nanotubes,1 which are one of the recently discovered novel carbon nanomaterials, have received much attention both from scientific and technological aspects and they are expected to hold a great potential for application in various fields. In particular, the reversible storage of hydrogen on carbon nanotubes has been studied extensively.2-4 Because the pore structure of carbon nanotube aggregates plays an essential role in gas adsorption, it is necessary to determine accurately the pore structure of carbon nanotube. The single-walled carbon nanotubes (SWCNTs) with catalytic method contain metallic impurities such as Fe, Ni, and Co in addition to various forms of carbon such as graphite, amorphous carbon, and nanoparticles.5-7 These authors have studied gas adsorption using single-walled carbon nanohorns (SWNHs) which can be obtained in a pure form.8,9 As SWNH has a slightly different morphology from SWCNT, we need highly pure SWCNTs to understand the gas adsorption properties exactly. A better purification method must be developed in order to study intrinsic properties of carbon nanotubes. Hence, various purification techniques for carbon nanotubes have been reported.10-13 However, many purification methods such as chemical and mechanical treatments give rise to structure change of carbon nanotubes. Yudasaka et al. reported that metal content of HiPco SWCNT (HPNT), which was about 30 wt. %, †
Japan Science and Technology Corporation, NEC Corporation. Department of Physics, Meijo University. Department of Chemistry, Faculty of Science, Chiba University. | Center for Frontier Electronics and Photonics, Chiba University. ‡ §
10.1021/nl015652f CCC: $22.00 Published on Web 03/07/2002
© 2002 American Chemical Society
decreases to 2 wt. % by heat treatment at 2073 K accompanying the diameter enlargement of HPNTs.14 Chiang et al.15 reported the purification and characterization of HPNT aggregates, but they did not characterize the pore structure of HPNT aggregates. In this study, the purification of HPNT containing Fe was performed by using oxidation in air and washing with hydrochloric acid (HCl) in order to remove metal catalysts from the carbon nanotube. Then, pore structure parameters of HPNT aggregates before and after purification were investigated by high-resolution N2 adsorption measurement at 77 K. Experimental. HPNT was purchased from Carbon Nanotechnologies Incorporated. The purification of HPNT was performed by two methods. In the dissolution (D) method, HPNT was immersed in 35% HCl and stirred at room temperature for 48 h to dissolve nanoparticles of Fe. The precipitates were filtrated and then washed with distilled water three times. After the sample was dried in air for 24 h at room temperature, the sample was heated in Ar at 523 K for 4 h. These processes were repeated two times. In the gasification-dissolution (GD) method, HPNT was heated in air flow of 100 mL‚min-1 at 623 K for 30 min. This process was performed in order to remove the carbon coating around the Fe nanoparticle before dissolution. The oxidized HPNT was further purified by the D method. The micropore structures were determined by adsorption of N2 at 77 K using volumetric equipment (Quantachrome
Figure 1. TEM images of pristine and purified HPNT aggregates: (a) pristine HPNT; (b) GD-HPNT.
AS-1-MP) after preevacuation at 423 K and 10-4 Pa for 2 h. The micropore structural parameters were obtained from high-resolution Rs-plots using the standard adsorption data of highly crystalline nonporous carbon black (Mitsubishi 4040B).8 Results and Discussion. The transmission electron microscope (TEM) image of the HPNT aggregates without any purification is shown in Figure 1a. Disordered carbon and spherical nanoparticles of Fe or Fe-C coated with graphene layers with diameter of several nm are observed in the pristine HPNT aggregates. Figure 1b shows the TEM image of the HPNT purified by the GD method (GD-HPNT). The TEM image of the GD-HPNT aggregates shows that the Fe or Fe-C spherical nanoparticles decrease as a result of purification. The nanotube diameter is in the range of 0.79 to 1.20 nm according to a Raman study on pristine HPNT reported by Yudasaka et al.14 Fe contents of pristine HPNT, D-HPNT, and GD-HPNT, which are determined by thermogravimetric analysis, were 27, 18, and 6 wt %, respectively. Figure 2a shows N2 adsorption isotherm (77 K) of pristine HPNT. The isotherm of pristine HPNT is of Type II in IUPAC classification without adsorption hysteresis. HPNT shows a great saturated adsorption amount of 1200 mg‚g-1 at P/P0 ) 1. A steep increase of N2 adsorption is observed 386
Figure 2. N2 adsorption isotherms on pristine and purified HPNTs at 77 K: (a) pristine HPNT; (b) D-HPNT; (c) GD-HPNT. The solid and open symbols indicate adsorption and desorption branches, respectively.
below P/P0 ) 0.1, suggesting the presence of micropores. On the other hand, a gradual uptake of N2 is observed at the medium P/P0. HPNT also shows predominant adsorption of N2 on the external surface. Figure 2b shows N2 adsorption isotherms of HPNT purified by the dissolution method (D-HPNT). The N2 isotherm of D-HPNT is close to Type IV, having a hysteresis loop. In the low pressure it is close to Type II. The N2 isotherm of GD-HPNT shows a similar shape to the D-HPNT, as shown in Figure 2c. In generally, the hysteresis loop is associated with the different filling and emptying processes of the mesopores by capillary condensaNano Lett., Vol. 2, No. 4, 2002
Table 1: Pore Structure of Pristine and Purified HPNTs Determined by SPE Method
sample HPNT D-HPNT GD-HPNT
Figure 3. Rs-plots of the N2 adsorption isotherms on pristine and purified HPNTs at 77 K: (a) pristine HPNT; (b) D-HPNT; (c) GDHPNT.
tion mechanism. The appearance of the clear hysteresis of N2 isotherms indicates that both purification procedures produce mesopores. The removal of Fe or Fe-C nanoparticles should alter the pore network due to vacancies produced during the purification. N2 adsorption amount of GD-HPNT increases remarkably at below P/P0 ) 0.1 compared with that of pristine sample, indicating further development of micropores, although purification with the D method does not markedly increase the micropore volume. The subtracting pore effect (SPE) method with Rs-plots16 was used to determine the pore structure of nanotube samples. Figure 3 shows the high-resolution Rs-plots for N2 Nano Lett., Vol. 2, No. 4, 2002
total external average surface surface micropore mesopore pore area area volume volume width (m2‚g-1) (m2‚g-1) (mL‚g-1) (mL‚g-1) (nm) 524 587 861
436 364 334
0.15 0.19 0.27
1.09 0.86 0.79
3.5 1.7 1.0
adsorption isotherms at 77 K. The slopes of solid and dotted lines give information about the total and external surface areas, respectively. The micropore volume was calculated from the ordinate-intercept of the dotted line. The average pore width was obtained by the simple slit pore approximation. The pore structure parameters of the samples are summarized in Table 1. The mesopore volume was determined by the difference of the micropore volume and total pore volume which is obtained from adsorbed N2 amount at P/P0 ) 0.98. The total specific surface area and micropore volume of pristine HPNT are 524 m2 ‚g-1 and 0.15 mL‚g-1, respectively. HPNT shows very high external surface area of 436 m2‚g-1, which should be mainly associated with large amount of amorphous carbon. The HPNT has the average pore width of about 3.5 nm, which is close to mesopore. The purification by the D method slightly increases the total surface area and micropore volume. On the other hand, the purification with the GD method remarkably enlarges the total surface area and micropore volume due to development of micropore structure. Furthermore, the purification treatment by the D and GD methods decreases remarkably the average micropore width to 1.7 and 1.0 nm, respectively. This should stem from removal of Fe impurities and amorphous carbon, forming a well oriented assembly structure. The decrease in the mesopore volume by purification also indicates the formation of the oriented assembly structure. In conclusion, the purification methods applied in this study have advantage for further development of the micropores of HPNT aggregates in addition to the effective removal of metal. The microporosity development of HPNT aggregates should come from an oriented assembly structure formation of nanotubes, which is induced by removal of metal nanoparticles, and from partial opening of nanotubes. Thus, purification considerably affects the pore structures of SWNT aggregates. Acknowledgment. We would like to thank the Japan Science and Technology Corporation for the International Cooperative Research Project. References (1) Iijima, S. Nature 1991, 354, 56. (2) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. S. Nature 1997, 386, 377. (3) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (4) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 2307. 387
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NL015652F
Nano Lett., Vol. 2, No. 4, 2002
