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Effect of oxidation on single-wall carbon nanotubes
6 October 2000
Chemical Physics Letters 328 (2000) 374±380
www.elsevier.nl/locate/cplett
Eect of oxidation on single-wall carbon nanotubes Shinichi Nagasawa a,*, Masako Yudasaka a, Kaori Hirahara c, Toshinari Ichihashi b, Sumio Iijima a,b,d a
c
Nanotubulites Project, JST-ICORP, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan b NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan Nanotubulites Project, JST-ICORP, c/o Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468-8502, Japan d Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468-8502, Japan Received 18 April 2000; received in ®nal form 9 August 2000
Abstract Two common methods of oxidation, gas-phase oxidation by heat treatment in oxygen gas and liquid-phase oxidation using nitric acid, were applied to single-wall carbon nanotubes (SWNT). The heat treatment in oxygen showed that thinner SWNTs burn more quickly. The nitric acid treatment showed that SWNTs are relatively inert to oxidation using acids. When the nitric acid treated samples were further oxidized by heat treatment in oxygen, selective oxidation of thinner SWNTs occurred. Ó 2000 Elsevier Science B.V.
1. Introduction Recent developments in synthesizing large quantities of single-walled carbon nanotubes (SWNT) have led to a signi®cant amount of research on the physical and chemical properties of this unique material [1]. Laser ablation [2,3] and arc-discharge methods [4] are commonly used to produce large quantities of SWNTs. However, the web-like deposits containing the SWNTs produced by these methods have a large amount of impurities, such as amorphous carbon, graphite pieces, and catalytic metal particles. Many methods of puri®cation have been proposed [5±9] for removing these impurities. One of the most commonly used puri®cation methods is oxidation, where amorphous carbon
*
Corresponding author. Fax: +81-298-50-1366. E-mail address: [email protected] (S. Nagasawa).
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 0 9 6 0 - X
impurities are selectively oxidized and removed. Various oxidation processes are available. One simple method is heat treatment of the web-like deposits in oxygen [10]. Another method is to re¯ux the web-like deposits in boiling nitric acid [8]. Both are eective methods of puri®cation. However, the chemical properties of the SWNTs are said to be similar to those of amorphous carbon impurities, so oxidation may cause damage or structural changes in the SWNTs. We performed experiments to clarify the eects of oxidation on the SWNTs using the two methods above. 2. Experiment We synthesized SWNT materials using the laser ablation method. Three dierent targets were made in order to produce three types of web-like deposits, which had dierent diameter
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
distributions for the SWNTs. The ®rst target was made from a sintered carbon composite (Klasta Mate by Toyo Tanso) that contained Ni (0.6 at.%) and Co (0.6 at.%) catalysts. The produced web-like deposit is labeled SCC sample in this report. The second target was made by compressing graphite powder at the pressure of 55 MPa cmÿ1 , with Ni (0.6 at.%) and Co (0.6 at.%) powder as growth catalysts into a 1 cm-diameter pellet, and the produced web-like deposit is labeled PM sample in this report. The third target was also made by compressing graphite powder, but this time with Ni nitrate (Ni:0.3 at.%) and Co nitrate (Co:0.3 at.%) powder as growth catalysts, and the produced web-like deposits is labeled Nitrate sample. A target was placed in an electrical tube furnace and laser ablation was performed at 1200°C while argon gas passed through the quartz-tube reactor at a rate of about 300 sccm (standard cm3 minÿ1 ) with a pressure of 750 Torr. The second harmonic beam of a Nd-YAG laser (pulse width 7 ns, frequency 10 Hz) was focused to obtain an energy density of about 1 J cmÿ2 per pulse on the target's surface. The web-like deposits were then collected from the wall of the reactor. The SWNT yield of the SCC sample is approximately 10%, while the yield of the PM and Nitrate samples are approximately 20±30%. Two commonly used oxidation methods, heat treatment in oxygen and re¯uxing in nitric acid were applied to the web-like deposits. In the ®rst oxidation method, the deposits were dispersed in ethanol. A droplet of the suspension was dried on a quartz glass plate and placed in an electric furnace. Pure (99%) oxygen was introduced and the oxygen pressure was held at 250 Torr. The furnace was heated to the desired temperatures of 350, 400 and 500°C and held for 10, 20, or 30 min. These two parameters were varied to change the oxidation level. For the nitric acid treatment, approximately 10 mg of the web-like deposits were added to 10 ml of 70% nitric acid and sonicated for 20 min. The sample was then re¯uxed for 4 h at 120°C. Dense yellow vapors of nitrogen dioxide developed during this process, indicating that the oxidation proceeded in the raw material. The color of the suspension also became yellow. The suspension
375
was washed in distilled water and centrifuged in plastic tubes until the pH level of the successive washing solution became neutral. Scanning electron microscopy (SEM) and Raman spectra (excitation: Ar laser, wavelength: 487.99 nm, power: 30 mW, aperture diameter:
Fig. 1. The `radial breathing mode' of Raman spectra of the web-like deposits (SCC sample) at dierent oxidation levels by heat treatment in oxygen. Normalized: (a) original; (b) 350°C, 10 min; (c) 400°C, 10 min; (d) 500°C, 10 min.
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S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
4 lm were used to determine the eects of oxidation on the dierent web-like deposits. For the Raman spectra, the A1g `radial breathing mode' (RBM) was carefully observed to monitor the diameter distribution of the samples. 3. Results and discussion 3.1. Heat treatment in oxygen A notable change was seen in the RBM of the Raman spectra of the heat treated web-like deposits. The peaks at the RBM were separated by using the Lorentz function, and the diameter was calculated from the frequency of the peak values [11]. From the theoretical calculation, A1g `Breathing mode' frequency, xr , is given by 1:00170:0007 xr x 10;10 r 10;10 =r , where x 10;10 165 cmÿ1 and r 10;10 0:6785 nm. Fig. 1 shows an example of the change in the RBM of the SCC sample under dierent oxidation levels. The large peak at 181 cmÿ1 corresponds to that of SWNTs with a diameter of 1.22 nm. The smaller peak at 163 cmÿ1 corresponds to that of SWNTs with a diameter of 1.38 nm. Although there were no peak shifts, there was a change in the
intensity. For example, the peak intensity at 163 cmÿ1 increased in relation to the peak at 181 cmÿ1 . The intensity of the peak due to a 1.22 nm diameter was ®ve times as much to that of 1.38 nm diameter initially, but after severe oxidation, the ratio dropped to 2.4:1 (Fig. 2). This suggests that SWNTs with a diameter of 1.22 nm burn more quickly than SWNTs with a diameter of 1.38 nm. The RBM peaks indicate that the second PM sample typically contained two sizes of SWNTs, 1.22 and 1.37 nm in diameter. The original intensity ratio of these two peaks was 1.5:1. As the oxidation condition became stronger (i.e., higher temperature, longer time), the SWNTs with a 1.22 nm diameter burned much quicker than the SWNTs with a 1.37 nm diameter (Fig. 2). The third sample (Nitrate) contained much thinner SWNTs with a 1.13 nm diameter. Again, as the oxidation condition became stronger, the thinner SWNTs with 1.13 or 1.22 nm diameter burned quicker than the one with 1.37 nm diameter (Fig. 2). For the three samples, oxidation was more effective on the thinner SWNTs. The thinner SWNTs were selectively oxidized. This can be explained as follows. Thinner SWNTs are more highly strained and thus more energetically
Fig. 2. Relative peak intensity of radial breathing modes for: (j) PM sample (1.22/1.37 nm); (s) Nitrate sample (1.22/1.37 nm); (d) Nitrate sample (1.13/1.37 nm); (M) SCC sample (1.22/1.38 nm). The actual values of relative peak intensity were multiplied by the numbers on the graph for presentation.
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
unstable than thicker ones. Therefore, we believe that they are highly reactive towards oxidation. The electron diraction patterns of the original and the sample heat treated at 500°C were taken. (Not shown.) The diraction pattern of the original showed clear diraction spots which suggest that the high crystllinity of SWNT lattice within the bundle. On the other hand, the diraction pattern of the sample heat trated at 500°C did not
377
show the clear diraction spots, which suggests that the SWNTs were not well aligned. We suspect that the selective oxidation of the thinner SWNTs led to gaps within the bundle, and thus the distortion of the SWNT lattice had occurred. The low crystallinity helps to explain the selective oxidation of the thinner SWNTs. SEM observations were used to clarify the condition of the web-like deposits at the intermediate
Fig. 3. Scanning electron microscopy images of web-like deposits (Nitrate sample) after dierent oxidation treatments: (a) original; (b) heat treatment in oxygen (HT) 350°C, 10 min; (c) HT 400°C, 10 min; (d) HT 500°C, 10 min; (e) nitric acid treatment.
378
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
stages of oxidation. Fig. 3a shows the condition of the original sample. The SEM image of the weblike deposits heat-treated at low temperature 350°C showed no indication of any changes (Fig. 3b), while sharp tips of the SWNTs were frequently observed for 400°C (Fig. 3c). The SEM
images showed no sign of bundle thinning (Fig. 3). This coincides with the results of Morishita et al. [12], who reported that SWNTs are gasi®ed from the tips. At 500°C, most of the SWNTs were gasi®ed, and only catalytic metal particles and large graphite pieces (2±20 lm) remained (Fig. 3d).
Fig. 4. Raman spectra of web-like deposits (Nitrate sample) after dierent oxidation treatments. Not normalized: (a) original; (b) nitric acid treatment; (c) nitric acid treatment followed by heat treatment in oxygen (400°C, 10 min); (d) nitric acid treatment followed by heat treatment in oxygen (500°C, 10 min).
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
3.2. Nitric acid treatment The samples were also treated with nitric acid to see if nitric acid treated samples had results similar to the selective oxidation. Selective oxidation of the thinner SWNTs was again observed for nitric acid treated samples of SCC, PM, and Nitrate that were further oxidized by heat treatment in oxygen. For example, Fig. 4c,d show the selective oxidation that was seen in the Nitrate sample. The SEM picture (Fig. 3e) of the web-like deposits (Nitrate sample) after nitric acid treatment showed that the SWNTs were uniformly coated with a `sticky' material. This sticky material was especially noticeable where the bundles crossed or split. Also, the catalytic metal particles were effectively removed (Fig. 3e). The transmission electron microscopy (TEM) observation (Fig. 5) showed that the bundles were covered with impurities, and this would correspond to the sticky material observed by SEM (Fig. 3e). Due to the impurities on the side of the bundles, we could not analyze the damage or defects caused to the SWNTs by the nitric acid treatment. The Raman spectra of the nitric acid treated SWNTs (Fig. 4b) showed a decrease in peak intensity and a peak shift in the E2g G-line (at around 1592 cmÿ1 ) to the higher energy side
379
(approximately) 6±8 cmÿ1 ) when compared to the typical Raman spectra of web-like deposit samples before nitric acid treatment (Fig. 4a). Also, enhanced peak intensity in the D-line (at around 1350 cmÿ1 ) due to amorphous carbon was seen. Further oxidation by heat treatment in oxygen gas removed this material on the surface of the bundles. The Raman spectra were obtained, showing the typical characteristics of the SWNTs after the nitric acid treatment (Fig. 4c,d). It has been already clari®ed in the recent publications that the sticky materials are the decomposed products from the amorphous carbon impurities. Having carboxyl, aldehyde, and other oxygen containing functional groups [13], these decomposed products are highly reactive to oxidation. Surface modi®cation of the SWNTs, due to the attachment of hydrophilic groups, also took place, which makes it possible to chemically attach dierent types of ions or functional groups to the surface of SWNTs [14].
4. Conclusion We have applied two dierent types of oxidation to SWNTs. Gas-phase oxidation by heart treatment in oxygen revealed that SWNTs with smaller diameter are more reactive towards oxidation than larger diameter SWNTs. Nitric acid treatment revealed that SWNTs are relatively inert to oxidation by acids. Selective oxidation of thinner SWNTs were again observed for the nitric acid treated samples that were further oxidized by heat treatment in oxygen.
References
Fig. 5. Transmission electron microscopy image of web-like deposits (Nitrate sample) after nitric acid treatment.
[1] R. Saito, G. Dresselhaus, M. Dresselhaus, Physical Properites of Carbon Nanotubes, Imperial College Press, London, 1998. [2] A. Thess, et al., Science 273 (1996) 483. [3] T. Guo, et al., Chem. Phys. Lett. 243 (1995) 49. [4] C. Journet, et al., Nature 388 (1997) 756. [5] T.W. Ebbesen, P.M. Ajayan, H. Hiura, K. Tanigaki, Nature 367 (1994) 519. [6] K. Tohji, et al., Nature 383 (1996) 679. [7] S. Bandow, et al., J. Phys. Chem. B 101 (1997) 8839.
380
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
[8] E. Dujardin, T.W. Ebbesen, A. Krishnan, M.M.J. Treacy, Adv. Mater. 10 (1998) 611. [9] A.G. Rinzler, et al., Appl. Phys. A 67 (1998) 29. [10] K. Morishita, T. Takarada, J. Mater. Sci. 34 (1994) 1169. [11] R. Saito, T. Takeya, T. Kimura, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B 57 (1998) 4145.
[12] K. Morishita, T. Takarada, Carbon 35 (1997) 977. [13] K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, Wiley, New york, 1998 (Chapter 3). [14] T.W. Ebbesen, H. Hiura, M.A. Bisher, M.M.J. Treacy, J.L. Shreeve-Keyer, R.C. Haushalter, Adv. Mater. 8 (2) (1996) 155.
Chemical Physics Letters 328 (2000) 374±380
www.elsevier.nl/locate/cplett
Eect of oxidation on single-wall carbon nanotubes Shinichi Nagasawa a,*, Masako Yudasaka a, Kaori Hirahara c, Toshinari Ichihashi b, Sumio Iijima a,b,d a
c
Nanotubulites Project, JST-ICORP, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan b NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan Nanotubulites Project, JST-ICORP, c/o Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468-8502, Japan d Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468-8502, Japan Received 18 April 2000; received in ®nal form 9 August 2000
Abstract Two common methods of oxidation, gas-phase oxidation by heat treatment in oxygen gas and liquid-phase oxidation using nitric acid, were applied to single-wall carbon nanotubes (SWNT). The heat treatment in oxygen showed that thinner SWNTs burn more quickly. The nitric acid treatment showed that SWNTs are relatively inert to oxidation using acids. When the nitric acid treated samples were further oxidized by heat treatment in oxygen, selective oxidation of thinner SWNTs occurred. Ó 2000 Elsevier Science B.V.
1. Introduction Recent developments in synthesizing large quantities of single-walled carbon nanotubes (SWNT) have led to a signi®cant amount of research on the physical and chemical properties of this unique material [1]. Laser ablation [2,3] and arc-discharge methods [4] are commonly used to produce large quantities of SWNTs. However, the web-like deposits containing the SWNTs produced by these methods have a large amount of impurities, such as amorphous carbon, graphite pieces, and catalytic metal particles. Many methods of puri®cation have been proposed [5±9] for removing these impurities. One of the most commonly used puri®cation methods is oxidation, where amorphous carbon
*
Corresponding author. Fax: +81-298-50-1366. E-mail address: [email protected] (S. Nagasawa).
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 0 9 6 0 - X
impurities are selectively oxidized and removed. Various oxidation processes are available. One simple method is heat treatment of the web-like deposits in oxygen [10]. Another method is to re¯ux the web-like deposits in boiling nitric acid [8]. Both are eective methods of puri®cation. However, the chemical properties of the SWNTs are said to be similar to those of amorphous carbon impurities, so oxidation may cause damage or structural changes in the SWNTs. We performed experiments to clarify the eects of oxidation on the SWNTs using the two methods above. 2. Experiment We synthesized SWNT materials using the laser ablation method. Three dierent targets were made in order to produce three types of web-like deposits, which had dierent diameter
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
distributions for the SWNTs. The ®rst target was made from a sintered carbon composite (Klasta Mate by Toyo Tanso) that contained Ni (0.6 at.%) and Co (0.6 at.%) catalysts. The produced web-like deposit is labeled SCC sample in this report. The second target was made by compressing graphite powder at the pressure of 55 MPa cmÿ1 , with Ni (0.6 at.%) and Co (0.6 at.%) powder as growth catalysts into a 1 cm-diameter pellet, and the produced web-like deposit is labeled PM sample in this report. The third target was also made by compressing graphite powder, but this time with Ni nitrate (Ni:0.3 at.%) and Co nitrate (Co:0.3 at.%) powder as growth catalysts, and the produced web-like deposits is labeled Nitrate sample. A target was placed in an electrical tube furnace and laser ablation was performed at 1200°C while argon gas passed through the quartz-tube reactor at a rate of about 300 sccm (standard cm3 minÿ1 ) with a pressure of 750 Torr. The second harmonic beam of a Nd-YAG laser (pulse width 7 ns, frequency 10 Hz) was focused to obtain an energy density of about 1 J cmÿ2 per pulse on the target's surface. The web-like deposits were then collected from the wall of the reactor. The SWNT yield of the SCC sample is approximately 10%, while the yield of the PM and Nitrate samples are approximately 20±30%. Two commonly used oxidation methods, heat treatment in oxygen and re¯uxing in nitric acid were applied to the web-like deposits. In the ®rst oxidation method, the deposits were dispersed in ethanol. A droplet of the suspension was dried on a quartz glass plate and placed in an electric furnace. Pure (99%) oxygen was introduced and the oxygen pressure was held at 250 Torr. The furnace was heated to the desired temperatures of 350, 400 and 500°C and held for 10, 20, or 30 min. These two parameters were varied to change the oxidation level. For the nitric acid treatment, approximately 10 mg of the web-like deposits were added to 10 ml of 70% nitric acid and sonicated for 20 min. The sample was then re¯uxed for 4 h at 120°C. Dense yellow vapors of nitrogen dioxide developed during this process, indicating that the oxidation proceeded in the raw material. The color of the suspension also became yellow. The suspension
375
was washed in distilled water and centrifuged in plastic tubes until the pH level of the successive washing solution became neutral. Scanning electron microscopy (SEM) and Raman spectra (excitation: Ar laser, wavelength: 487.99 nm, power: 30 mW, aperture diameter:
Fig. 1. The `radial breathing mode' of Raman spectra of the web-like deposits (SCC sample) at dierent oxidation levels by heat treatment in oxygen. Normalized: (a) original; (b) 350°C, 10 min; (c) 400°C, 10 min; (d) 500°C, 10 min.
376
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
4 lm were used to determine the eects of oxidation on the dierent web-like deposits. For the Raman spectra, the A1g `radial breathing mode' (RBM) was carefully observed to monitor the diameter distribution of the samples. 3. Results and discussion 3.1. Heat treatment in oxygen A notable change was seen in the RBM of the Raman spectra of the heat treated web-like deposits. The peaks at the RBM were separated by using the Lorentz function, and the diameter was calculated from the frequency of the peak values [11]. From the theoretical calculation, A1g `Breathing mode' frequency, xr , is given by 1:00170:0007 xr x 10;10 r 10;10 =r , where x 10;10 165 cmÿ1 and r 10;10 0:6785 nm. Fig. 1 shows an example of the change in the RBM of the SCC sample under dierent oxidation levels. The large peak at 181 cmÿ1 corresponds to that of SWNTs with a diameter of 1.22 nm. The smaller peak at 163 cmÿ1 corresponds to that of SWNTs with a diameter of 1.38 nm. Although there were no peak shifts, there was a change in the
intensity. For example, the peak intensity at 163 cmÿ1 increased in relation to the peak at 181 cmÿ1 . The intensity of the peak due to a 1.22 nm diameter was ®ve times as much to that of 1.38 nm diameter initially, but after severe oxidation, the ratio dropped to 2.4:1 (Fig. 2). This suggests that SWNTs with a diameter of 1.22 nm burn more quickly than SWNTs with a diameter of 1.38 nm. The RBM peaks indicate that the second PM sample typically contained two sizes of SWNTs, 1.22 and 1.37 nm in diameter. The original intensity ratio of these two peaks was 1.5:1. As the oxidation condition became stronger (i.e., higher temperature, longer time), the SWNTs with a 1.22 nm diameter burned much quicker than the SWNTs with a 1.37 nm diameter (Fig. 2). The third sample (Nitrate) contained much thinner SWNTs with a 1.13 nm diameter. Again, as the oxidation condition became stronger, the thinner SWNTs with 1.13 or 1.22 nm diameter burned quicker than the one with 1.37 nm diameter (Fig. 2). For the three samples, oxidation was more effective on the thinner SWNTs. The thinner SWNTs were selectively oxidized. This can be explained as follows. Thinner SWNTs are more highly strained and thus more energetically
Fig. 2. Relative peak intensity of radial breathing modes for: (j) PM sample (1.22/1.37 nm); (s) Nitrate sample (1.22/1.37 nm); (d) Nitrate sample (1.13/1.37 nm); (M) SCC sample (1.22/1.38 nm). The actual values of relative peak intensity were multiplied by the numbers on the graph for presentation.
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
unstable than thicker ones. Therefore, we believe that they are highly reactive towards oxidation. The electron diraction patterns of the original and the sample heat treated at 500°C were taken. (Not shown.) The diraction pattern of the original showed clear diraction spots which suggest that the high crystllinity of SWNT lattice within the bundle. On the other hand, the diraction pattern of the sample heat trated at 500°C did not
377
show the clear diraction spots, which suggests that the SWNTs were not well aligned. We suspect that the selective oxidation of the thinner SWNTs led to gaps within the bundle, and thus the distortion of the SWNT lattice had occurred. The low crystallinity helps to explain the selective oxidation of the thinner SWNTs. SEM observations were used to clarify the condition of the web-like deposits at the intermediate
Fig. 3. Scanning electron microscopy images of web-like deposits (Nitrate sample) after dierent oxidation treatments: (a) original; (b) heat treatment in oxygen (HT) 350°C, 10 min; (c) HT 400°C, 10 min; (d) HT 500°C, 10 min; (e) nitric acid treatment.
378
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
stages of oxidation. Fig. 3a shows the condition of the original sample. The SEM image of the weblike deposits heat-treated at low temperature 350°C showed no indication of any changes (Fig. 3b), while sharp tips of the SWNTs were frequently observed for 400°C (Fig. 3c). The SEM
images showed no sign of bundle thinning (Fig. 3). This coincides with the results of Morishita et al. [12], who reported that SWNTs are gasi®ed from the tips. At 500°C, most of the SWNTs were gasi®ed, and only catalytic metal particles and large graphite pieces (2±20 lm) remained (Fig. 3d).
Fig. 4. Raman spectra of web-like deposits (Nitrate sample) after dierent oxidation treatments. Not normalized: (a) original; (b) nitric acid treatment; (c) nitric acid treatment followed by heat treatment in oxygen (400°C, 10 min); (d) nitric acid treatment followed by heat treatment in oxygen (500°C, 10 min).
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
3.2. Nitric acid treatment The samples were also treated with nitric acid to see if nitric acid treated samples had results similar to the selective oxidation. Selective oxidation of the thinner SWNTs was again observed for nitric acid treated samples of SCC, PM, and Nitrate that were further oxidized by heat treatment in oxygen. For example, Fig. 4c,d show the selective oxidation that was seen in the Nitrate sample. The SEM picture (Fig. 3e) of the web-like deposits (Nitrate sample) after nitric acid treatment showed that the SWNTs were uniformly coated with a `sticky' material. This sticky material was especially noticeable where the bundles crossed or split. Also, the catalytic metal particles were effectively removed (Fig. 3e). The transmission electron microscopy (TEM) observation (Fig. 5) showed that the bundles were covered with impurities, and this would correspond to the sticky material observed by SEM (Fig. 3e). Due to the impurities on the side of the bundles, we could not analyze the damage or defects caused to the SWNTs by the nitric acid treatment. The Raman spectra of the nitric acid treated SWNTs (Fig. 4b) showed a decrease in peak intensity and a peak shift in the E2g G-line (at around 1592 cmÿ1 ) to the higher energy side
379
(approximately) 6±8 cmÿ1 ) when compared to the typical Raman spectra of web-like deposit samples before nitric acid treatment (Fig. 4a). Also, enhanced peak intensity in the D-line (at around 1350 cmÿ1 ) due to amorphous carbon was seen. Further oxidation by heat treatment in oxygen gas removed this material on the surface of the bundles. The Raman spectra were obtained, showing the typical characteristics of the SWNTs after the nitric acid treatment (Fig. 4c,d). It has been already clari®ed in the recent publications that the sticky materials are the decomposed products from the amorphous carbon impurities. Having carboxyl, aldehyde, and other oxygen containing functional groups [13], these decomposed products are highly reactive to oxidation. Surface modi®cation of the SWNTs, due to the attachment of hydrophilic groups, also took place, which makes it possible to chemically attach dierent types of ions or functional groups to the surface of SWNTs [14].
4. Conclusion We have applied two dierent types of oxidation to SWNTs. Gas-phase oxidation by heart treatment in oxygen revealed that SWNTs with smaller diameter are more reactive towards oxidation than larger diameter SWNTs. Nitric acid treatment revealed that SWNTs are relatively inert to oxidation by acids. Selective oxidation of thinner SWNTs were again observed for the nitric acid treated samples that were further oxidized by heat treatment in oxygen.
References
Fig. 5. Transmission electron microscopy image of web-like deposits (Nitrate sample) after nitric acid treatment.
[1] R. Saito, G. Dresselhaus, M. Dresselhaus, Physical Properites of Carbon Nanotubes, Imperial College Press, London, 1998. [2] A. Thess, et al., Science 273 (1996) 483. [3] T. Guo, et al., Chem. Phys. Lett. 243 (1995) 49. [4] C. Journet, et al., Nature 388 (1997) 756. [5] T.W. Ebbesen, P.M. Ajayan, H. Hiura, K. Tanigaki, Nature 367 (1994) 519. [6] K. Tohji, et al., Nature 383 (1996) 679. [7] S. Bandow, et al., J. Phys. Chem. B 101 (1997) 8839.
380
S. Nagasawa et al. / Chemical Physics Letters 328 (2000) 374±380
[8] E. Dujardin, T.W. Ebbesen, A. Krishnan, M.M.J. Treacy, Adv. Mater. 10 (1998) 611. [9] A.G. Rinzler, et al., Appl. Phys. A 67 (1998) 29. [10] K. Morishita, T. Takarada, J. Mater. Sci. 34 (1994) 1169. [11] R. Saito, T. Takeya, T. Kimura, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B 57 (1998) 4145.
[12] K. Morishita, T. Takarada, Carbon 35 (1997) 977. [13] K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, Wiley, New york, 1998 (Chapter 3). [14] T.W. Ebbesen, H. Hiura, M.A. Bisher, M.M.J. Treacy, J.L. Shreeve-Keyer, R.C. Haushalter, Adv. Mater. 8 (2) (1996) 155.