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Nano-mechanical cutting and opening of single wall carbon nanotubes
1 December 2000
Chemical Physics Letters 331 (2000) 125±131
www.elsevier.nl/locate/cplett
Nano-mechanical cutting and opening of single wall carbon nanotubes I. Stepanek a,b, G. Maurin c, P. Bernier a,*, J. Gavillet d, A. Loiseau d, R. Edwards b, O. Jaschinski a a
c
G.D.P.C.-UMR 5581 CNRS, University of Montpellier II Sciences and Techniques du Languedoc, cc 026, Pl. E. Bataillon, 34095 Montpellier Cedex 05, France b E.I, Joint Research Center, TP 800, Via E. Fermi, It-21020 Ispra (VA), France L.P.M.C.-UMR 5617 CNRS, University of Montpellier II Sciences and Techniques du Languedoc, cc 003, Pl. E. Bataillon, F-34095 Montpellier Cedex 05, France d L.E.M.-UMR 104, ONERA B.P. 72, 92322 Ch^ atillon, France Received 10 July 2000; in ®nal form 7 September 2000
Abstract We pioneered a simple and soft mechanical cutting process for single wall carbon nanotubes involving diamond particles as abrasive material. High resolution transmission electron microscopy observation reveals shortened nanotubes always organised in bundles. Micrograph analysis allows us to determine the breaking mechanism of the bundles. The average nanotube lengths, estimated using AFM images, are ranging from 300 to 700 nm. Adsorption measurements show a topological modi®cation of the material with the presence of new microporosities ranging in the average nanotube diameter. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The recent development of dierent synthesis methods to produce large amounts of carbon nanotubes [1,2] opens up possibilities for the use of these structures in many applications. They are potential candidates in the area of electron ®eld emission [3], electrochemical energy storage systems (e.g., lithium ion batteries) [4,5], or of gas storage systems [6,7]. Concerning gas storage, the control of the nanotube opening is a key issue. The previous approaches developed for opening multi-
*
Corresponding author. Fax: +33-4-67522504. E-mail address: [email protected] (P. Bernier).
walled [8±10] and single-walled carbon nanotube (SWNT) [11±14] caps implied a random pollution of the system by undesirable chemical species and more or less severe structural damage. To prevent the formation of such irreversible damages, we pioneered a new process involving diamond grains as abrasive material. In this Letter, we describe this nano-mechanical cutting (NMC) method, which appears to be a simple and clean process for opening nanotubes without any chemical or thermal treatment. Direct observation of the as-obtained material by high resolution transmission electron microscopy (HRTEM) reveals that nanotubes are unambiguously cut and open. The bundle length distribution after cutting is then estimated using atomic force microscopy (AFM). Finally, nitrogen adsorption
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 1 6 3 - 5
126
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
measurements at 77 K are performed to determine the topological modi®cations of the NMC treated material, thus con®rming that nanotubes remain opened after the abrasion. 2. Experimental 2.1. Synthesis of carbon nanotubes. Mechanical treatment We used SWNTs synthesised by an electric arc discharge under helium atmosphere with the graphite anode ®lled with a mixture of metallic catalysts and graphite powder (Ni:Y:C/4.2:1:94.8 at.%) [1]. We selected the collaret part of the synthesis product where the SWNT concentration is highest (60 vol % estimated from random sampling and by scanning electron microscopy (SEM) observations). Basically, SWNTs are close-packed into bundles with diameters ranging from 5 to 20 nm where nanotubes are well-aligned in a twodimensional periodic triangular lattice bonded by van der Waals interactions. The average tube diameter is around 1.3 nm and the triangular lattice parameter is 1.7 nm. While half of this material was kept to serve as a reference, the other half was ultrasonically dispersed in ethanol at ambient temperature to obtain a homogeneous suspension. Then, the nanotube suspension was poured on two diamond lapping ®lm disks, and rubbed until the alcohol evaporation was complete. The mean size of the diamond grains at the disc surface is 0.1 lm (Buehler). At the end, the thin layer of a ®ne powder that covered the discs was extracted by rinsing with ethanol followed by solvent evaporation at 100°C. 2.2. High resolution transmission electron microscopy (HRTEM) Morphologies and structures of the pristine (PNTs) and nano-mechanically cut tubes (NMCNTs) were ®rstly investigated by HRTEM using a CM20 Philips instrument. Both samples were preliminarily dispersed in ethanol assisted by ultrasound and scooped up with a holey amorphous carbon grid.
Fig. 1. Typical HRTEM image of SWNTs: (a) raw material with high density of rope entanglements; (b) nano-mechanically treated material consisting on cut bundles. We note the presence of metallic catalyst nanoparticles.
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
2.3. Atomic force microscopy (AFM) AFM images were recorded using a Digital Instrument D3100 AFM. All the images were
127
taken in the tapping mode. The nanotubes were ®rstly dispersed in dimethylformamide (DMF) by ultrasonic agitation (ultrasound bath, 10 h). The concentration was about 0.01 mg of material per
Fig. 2. HRTEM pictures of a mechanically treated collaret using abrasive disks. Bundles are lying perpendicularly to the electron beam and are imaged by lattice fringes related to the lattice built by the nanotubes within a rope. The fringe spacing depends on the orientation of the bundle with respect to the electron beam. The eect of the abrasive treatment is to expose the bundles locally to a very high radial compression provoking their bending. The bending remains elastic and reversible up to a certain value of curvature corresponding to a certain local strain, and beyond this, kinks appear which are attributed to irreversible damage such as bond breaking. Further increasing of strain, leads to the complete fracture of the tubes. (a) and (b) Images of bundles in a typical PNTs and NMCNTs. (c) Images of bundles bent beyond the elastic limit. (d) Image of a completely fractured bundle. Note that the surface of the fracture is perpendicular to the bundle axis. The two resulting segments of the bundle are very straight and well crystallised, the segment on the right-hand side of the image displays a twisted feature as a non perturbed bundle. (e) A fractured bundle with clearly uncapped nanotubes.
128
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
Fig. 3. Tapping mode AFM images of isolated nanotube bundle after the NMC treatment. (a) bulbles with 270 and 310 nm-long. (b) 460 and 590 nm. (c) a 700 nm-long rope. These reported sizes are representative of the treated material. (d) Size distribution plots of cut bundles after the abrasive treatment. The average length is 450 nm 60 nm.
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
ml of solvent. Then, nanotubes were electrodeposited onto Si=SiO2 substrates without any further rinsing step after deposit. 2.4. Adsorption measurements NMCNTs and PNTs topologies were characterised by nitrogen adsorption at 77 K by using a volumetric (Sorptomatic 1900, Milestone) instrument. The two samples were pre-treated under vacuum at 250±300°C over one night in order to remove the water molecules and some impurities at the material surface. 3. Results and discussion The HRTEM micrographs of the PNTs and NMCNTs are shown in Fig. 1a and b, respectively. In PNTs, we typically observe entangled and elastically bent nanotubes. The NMCNT one reveals fewer entanglements and in this case, most of the ropes are cut. Fig. 2a shows a typical example of a bundle where a fraction of the nanotubes have been obviously cut into two pieces. This feature is the result of a fracture of the bundle due to the mechanical stress exerted locally by the diamond grains in radial compression. A large number of observations of dierent features permit us to describe the cutting process as follows. Bundles are ®rst elastically bent (see an example in Fig. 2b). Beyond a critical local strain, kinks are formed and correspond to the occurrence of bond breaking (Fig. 2b and c). Depending on the number of tubes in a rope, all the tubes or a fraction of them are damaged (Fig. 2c). We investigated large regions of the NMCNTs and, statistically, these kinks give rise to more or less folded bundles with curvature angle at the kinks ranging from 120° to 60°. These values are strikingly well consistent with numerical simulations of the bending of SWNT [12,13] predicting the appearance of kinks for bending angles above 120°. Finally complete fracture occurs at these kinks above a critical threshold of the local strain leading to the complete nanotube opening (Fig. 2d). The nature of the fracture is revealed by the morphology of the
129
fractured surface. As shown in Fig. 2a and d, this surface is completely ¯at and perpendicular to the bundle axis and the lips of the open tubes (Fig. 2e) are very sharp and clean, with no damaged carbonaceous material at the surface. All these features are typical of a very brittle behaviour which has been theoretically predicted to occur at high strain whatever the value of the helicity [14] and indicating that eective high strain can be achieved during the abrasive treatment. However, these strains are very localised and only aect a short fragment of the bundle with respect to its length. Once the bundle is cut, it recovers a very straight shape meaning that all the strain has been released in a reversible manner and that, except at the kinks, the bundle was elastically distorted. The present behaviour is very dierent from that observed during tensile deformation tests performed recently in situ in a microscope, where the bundles are plastically elongated up to the ®nal rupture [15]. Furthermore, X-ray diraction on our cut samples shows a decrease of the intensity peak at ÿ1 indicating a slight perturbation of the 0.42 A organisation of the nanotubes in the bundles. The further advantages of this cutting process with respect to other cutting and opening methods are that it can occur at any place of the tubes. It is too delicate to estimate the length of the cut bundles and eciency of the NMC process with only HRTEM images, consequently we investigated the NMCNT lengths using tapping mode AFM images. Fig. 3a±c show typical cut nanotube ropes images. The various scans obtained allow us to establish statistics on nanotube length. This result is particularly reliable because the sample preparation involves surfactant-free dispersion and thus no rinsing is necessary. Consequently, there is no risk of unintentional removing of nanotubes. Secondly, this estimation is reinforced by considering about hundred images. The length distribution plot (Fig. 3d) indicates nanotubes sizes ranging from 350 to 700 nm. These values are ®ve times lower than those usually observed for the nanotubes in the raw material. This result con®rms the eciency of our technique. We performed nitrogen adsorption measurements to determine if a new kind of pores can be reached by the gas in NMCNTs, and if there is an
130
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
Fig. 4. Pore size distribution of the sample before and after the abrasion. The adsorption measurements were done at 77 K and 1 atmosphere of nitrogen, using a Sorptomatic 1900 apparatus (Milestone). Analyses of the measurements were performed using the Horvath±Kawazoe theory based on cylindrical pores. ÿ1 is plotted The dierential pore volume dV =dR in cm3 gÿ1 A versus the pore diameter. Solid line: PNTs; dot line: NMCNTs.
increase of the speci®c surface area of the material. The experimental surface BET of the as-obtained material after the abrasion is unfortunately less important than the pristine one. However, this result could be foreseeable: the presence of diamond grains increases the density and thus the weight of the given sample and secondly, if we cut the tube, the mesopores disappear in large quantity. As this characterisation is not conclusive, we analyse the experimental data using the Horvath±Kawazoe investigation method [16]. The limits of validity of this calculation are well known, but it is still a convenient way for comparing the material before and after the abrasion. In Fig. 4, the dierential ÿ1 is pore volume dV =dR expressed in cm3 gÿ1 A plotted versus the diameter of the pores in the studied material. We note a clear modi®cation of the pore size pro®le, which is due to the appearance of new microporosities ranging from 0.9 to 1.8 nm, thus including the average size of our nanotube diameters. This observation con®rms that new pores are accessible to nitrogen molecules and therefore that some nanotubes are opened. 4. Conclusion In this Letter, we have underlined the eciency and the softness of the nano-mechanical process to
Fig. 5. Typical HRTEM picture of an uncapped multi-walled carbon nanotube resulting from the abrasion using diamond grains.
cut the nanotubes. HRTEM micrographs show that SWNTs are unambiguously opened. The opening mechanism through a brittle fracture process leads to chemically and structurally undamaged nanotubes. Thus, the method can be also applied to multi-walled nanotubes as well attested by the HRTEM image of Fig. 5. The simplicity of this mechanical treatment suggests that it can be easily scaled up and optimised. This mechanical process could solve the severe limitations in the possibility of ®lling carbon nanotubes with various substances, as for example hydrogen. References [1] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer, Nature (London) 388 (1997) 756. [2] A. Thess, R. Lee, P. Nikolaev, H. Hai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.C. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [3] S.J. Tans, ARM. Verschueren, C. Dekker, Nature (London) 393 (1998) 49. [4] G. Maurin, Ch. Bousquet, F. Henn, P. Bernier, R. Almairac, B. Simon, Chem. Phys. Lett. 312 (1999) 14. [5] P. Chen, X. Wu, J. Lin, K.L. Tan, Science 285 (1999) 91. [6] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature (London) 386 (1997) 377.
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131 [7] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [8] S.C. Tsang, P.J.F. Harris, M.L.H. Green, Nature 362 (1993) 520. [9] P.M. Ajayan, et al., Nature 362 (1993) 522. [10] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature 372 (1994) 159. [11] Liu, et al., Science 280 (1998) 1253.
131
[12] S. Iijima, C. Brabec, A. Maiti, J. Bernholc, J. Chem. Phys. 104 (5) (1996) 2089. [13] D. Srivastava, M. Menon, K. Cho, Phys. Rev. Lett. 83 (15) (1999) 2973. [14] M.B. Nardelli, B.I. Yakobson, J. Bernholc, Phys. Rev. Lett. 81 (21) (1998) 4656. [15] O. Lourie, H.D. Wagner, Appl. Phys. Lett. 73 (1998) 3527. [16] G. Horvath, K.J. Kawazoe, Chem. Eng. Jpn. 16 (1983) 470.
Chemical Physics Letters 331 (2000) 125±131
www.elsevier.nl/locate/cplett
Nano-mechanical cutting and opening of single wall carbon nanotubes I. Stepanek a,b, G. Maurin c, P. Bernier a,*, J. Gavillet d, A. Loiseau d, R. Edwards b, O. Jaschinski a a
c
G.D.P.C.-UMR 5581 CNRS, University of Montpellier II Sciences and Techniques du Languedoc, cc 026, Pl. E. Bataillon, 34095 Montpellier Cedex 05, France b E.I, Joint Research Center, TP 800, Via E. Fermi, It-21020 Ispra (VA), France L.P.M.C.-UMR 5617 CNRS, University of Montpellier II Sciences and Techniques du Languedoc, cc 003, Pl. E. Bataillon, F-34095 Montpellier Cedex 05, France d L.E.M.-UMR 104, ONERA B.P. 72, 92322 Ch^ atillon, France Received 10 July 2000; in ®nal form 7 September 2000
Abstract We pioneered a simple and soft mechanical cutting process for single wall carbon nanotubes involving diamond particles as abrasive material. High resolution transmission electron microscopy observation reveals shortened nanotubes always organised in bundles. Micrograph analysis allows us to determine the breaking mechanism of the bundles. The average nanotube lengths, estimated using AFM images, are ranging from 300 to 700 nm. Adsorption measurements show a topological modi®cation of the material with the presence of new microporosities ranging in the average nanotube diameter. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The recent development of dierent synthesis methods to produce large amounts of carbon nanotubes [1,2] opens up possibilities for the use of these structures in many applications. They are potential candidates in the area of electron ®eld emission [3], electrochemical energy storage systems (e.g., lithium ion batteries) [4,5], or of gas storage systems [6,7]. Concerning gas storage, the control of the nanotube opening is a key issue. The previous approaches developed for opening multi-
*
Corresponding author. Fax: +33-4-67522504. E-mail address: [email protected] (P. Bernier).
walled [8±10] and single-walled carbon nanotube (SWNT) [11±14] caps implied a random pollution of the system by undesirable chemical species and more or less severe structural damage. To prevent the formation of such irreversible damages, we pioneered a new process involving diamond grains as abrasive material. In this Letter, we describe this nano-mechanical cutting (NMC) method, which appears to be a simple and clean process for opening nanotubes without any chemical or thermal treatment. Direct observation of the as-obtained material by high resolution transmission electron microscopy (HRTEM) reveals that nanotubes are unambiguously cut and open. The bundle length distribution after cutting is then estimated using atomic force microscopy (AFM). Finally, nitrogen adsorption
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 1 6 3 - 5
126
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
measurements at 77 K are performed to determine the topological modi®cations of the NMC treated material, thus con®rming that nanotubes remain opened after the abrasion. 2. Experimental 2.1. Synthesis of carbon nanotubes. Mechanical treatment We used SWNTs synthesised by an electric arc discharge under helium atmosphere with the graphite anode ®lled with a mixture of metallic catalysts and graphite powder (Ni:Y:C/4.2:1:94.8 at.%) [1]. We selected the collaret part of the synthesis product where the SWNT concentration is highest (60 vol % estimated from random sampling and by scanning electron microscopy (SEM) observations). Basically, SWNTs are close-packed into bundles with diameters ranging from 5 to 20 nm where nanotubes are well-aligned in a twodimensional periodic triangular lattice bonded by van der Waals interactions. The average tube diameter is around 1.3 nm and the triangular lattice parameter is 1.7 nm. While half of this material was kept to serve as a reference, the other half was ultrasonically dispersed in ethanol at ambient temperature to obtain a homogeneous suspension. Then, the nanotube suspension was poured on two diamond lapping ®lm disks, and rubbed until the alcohol evaporation was complete. The mean size of the diamond grains at the disc surface is 0.1 lm (Buehler). At the end, the thin layer of a ®ne powder that covered the discs was extracted by rinsing with ethanol followed by solvent evaporation at 100°C. 2.2. High resolution transmission electron microscopy (HRTEM) Morphologies and structures of the pristine (PNTs) and nano-mechanically cut tubes (NMCNTs) were ®rstly investigated by HRTEM using a CM20 Philips instrument. Both samples were preliminarily dispersed in ethanol assisted by ultrasound and scooped up with a holey amorphous carbon grid.
Fig. 1. Typical HRTEM image of SWNTs: (a) raw material with high density of rope entanglements; (b) nano-mechanically treated material consisting on cut bundles. We note the presence of metallic catalyst nanoparticles.
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
2.3. Atomic force microscopy (AFM) AFM images were recorded using a Digital Instrument D3100 AFM. All the images were
127
taken in the tapping mode. The nanotubes were ®rstly dispersed in dimethylformamide (DMF) by ultrasonic agitation (ultrasound bath, 10 h). The concentration was about 0.01 mg of material per
Fig. 2. HRTEM pictures of a mechanically treated collaret using abrasive disks. Bundles are lying perpendicularly to the electron beam and are imaged by lattice fringes related to the lattice built by the nanotubes within a rope. The fringe spacing depends on the orientation of the bundle with respect to the electron beam. The eect of the abrasive treatment is to expose the bundles locally to a very high radial compression provoking their bending. The bending remains elastic and reversible up to a certain value of curvature corresponding to a certain local strain, and beyond this, kinks appear which are attributed to irreversible damage such as bond breaking. Further increasing of strain, leads to the complete fracture of the tubes. (a) and (b) Images of bundles in a typical PNTs and NMCNTs. (c) Images of bundles bent beyond the elastic limit. (d) Image of a completely fractured bundle. Note that the surface of the fracture is perpendicular to the bundle axis. The two resulting segments of the bundle are very straight and well crystallised, the segment on the right-hand side of the image displays a twisted feature as a non perturbed bundle. (e) A fractured bundle with clearly uncapped nanotubes.
128
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
Fig. 3. Tapping mode AFM images of isolated nanotube bundle after the NMC treatment. (a) bulbles with 270 and 310 nm-long. (b) 460 and 590 nm. (c) a 700 nm-long rope. These reported sizes are representative of the treated material. (d) Size distribution plots of cut bundles after the abrasive treatment. The average length is 450 nm 60 nm.
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
ml of solvent. Then, nanotubes were electrodeposited onto Si=SiO2 substrates without any further rinsing step after deposit. 2.4. Adsorption measurements NMCNTs and PNTs topologies were characterised by nitrogen adsorption at 77 K by using a volumetric (Sorptomatic 1900, Milestone) instrument. The two samples were pre-treated under vacuum at 250±300°C over one night in order to remove the water molecules and some impurities at the material surface. 3. Results and discussion The HRTEM micrographs of the PNTs and NMCNTs are shown in Fig. 1a and b, respectively. In PNTs, we typically observe entangled and elastically bent nanotubes. The NMCNT one reveals fewer entanglements and in this case, most of the ropes are cut. Fig. 2a shows a typical example of a bundle where a fraction of the nanotubes have been obviously cut into two pieces. This feature is the result of a fracture of the bundle due to the mechanical stress exerted locally by the diamond grains in radial compression. A large number of observations of dierent features permit us to describe the cutting process as follows. Bundles are ®rst elastically bent (see an example in Fig. 2b). Beyond a critical local strain, kinks are formed and correspond to the occurrence of bond breaking (Fig. 2b and c). Depending on the number of tubes in a rope, all the tubes or a fraction of them are damaged (Fig. 2c). We investigated large regions of the NMCNTs and, statistically, these kinks give rise to more or less folded bundles with curvature angle at the kinks ranging from 120° to 60°. These values are strikingly well consistent with numerical simulations of the bending of SWNT [12,13] predicting the appearance of kinks for bending angles above 120°. Finally complete fracture occurs at these kinks above a critical threshold of the local strain leading to the complete nanotube opening (Fig. 2d). The nature of the fracture is revealed by the morphology of the
129
fractured surface. As shown in Fig. 2a and d, this surface is completely ¯at and perpendicular to the bundle axis and the lips of the open tubes (Fig. 2e) are very sharp and clean, with no damaged carbonaceous material at the surface. All these features are typical of a very brittle behaviour which has been theoretically predicted to occur at high strain whatever the value of the helicity [14] and indicating that eective high strain can be achieved during the abrasive treatment. However, these strains are very localised and only aect a short fragment of the bundle with respect to its length. Once the bundle is cut, it recovers a very straight shape meaning that all the strain has been released in a reversible manner and that, except at the kinks, the bundle was elastically distorted. The present behaviour is very dierent from that observed during tensile deformation tests performed recently in situ in a microscope, where the bundles are plastically elongated up to the ®nal rupture [15]. Furthermore, X-ray diraction on our cut samples shows a decrease of the intensity peak at ÿ1 indicating a slight perturbation of the 0.42 A organisation of the nanotubes in the bundles. The further advantages of this cutting process with respect to other cutting and opening methods are that it can occur at any place of the tubes. It is too delicate to estimate the length of the cut bundles and eciency of the NMC process with only HRTEM images, consequently we investigated the NMCNT lengths using tapping mode AFM images. Fig. 3a±c show typical cut nanotube ropes images. The various scans obtained allow us to establish statistics on nanotube length. This result is particularly reliable because the sample preparation involves surfactant-free dispersion and thus no rinsing is necessary. Consequently, there is no risk of unintentional removing of nanotubes. Secondly, this estimation is reinforced by considering about hundred images. The length distribution plot (Fig. 3d) indicates nanotubes sizes ranging from 350 to 700 nm. These values are ®ve times lower than those usually observed for the nanotubes in the raw material. This result con®rms the eciency of our technique. We performed nitrogen adsorption measurements to determine if a new kind of pores can be reached by the gas in NMCNTs, and if there is an
130
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131
Fig. 4. Pore size distribution of the sample before and after the abrasion. The adsorption measurements were done at 77 K and 1 atmosphere of nitrogen, using a Sorptomatic 1900 apparatus (Milestone). Analyses of the measurements were performed using the Horvath±Kawazoe theory based on cylindrical pores. ÿ1 is plotted The dierential pore volume dV =dR in cm3 gÿ1 A versus the pore diameter. Solid line: PNTs; dot line: NMCNTs.
increase of the speci®c surface area of the material. The experimental surface BET of the as-obtained material after the abrasion is unfortunately less important than the pristine one. However, this result could be foreseeable: the presence of diamond grains increases the density and thus the weight of the given sample and secondly, if we cut the tube, the mesopores disappear in large quantity. As this characterisation is not conclusive, we analyse the experimental data using the Horvath±Kawazoe investigation method [16]. The limits of validity of this calculation are well known, but it is still a convenient way for comparing the material before and after the abrasion. In Fig. 4, the dierential ÿ1 is pore volume dV =dR expressed in cm3 gÿ1 A plotted versus the diameter of the pores in the studied material. We note a clear modi®cation of the pore size pro®le, which is due to the appearance of new microporosities ranging from 0.9 to 1.8 nm, thus including the average size of our nanotube diameters. This observation con®rms that new pores are accessible to nitrogen molecules and therefore that some nanotubes are opened. 4. Conclusion In this Letter, we have underlined the eciency and the softness of the nano-mechanical process to
Fig. 5. Typical HRTEM picture of an uncapped multi-walled carbon nanotube resulting from the abrasion using diamond grains.
cut the nanotubes. HRTEM micrographs show that SWNTs are unambiguously opened. The opening mechanism through a brittle fracture process leads to chemically and structurally undamaged nanotubes. Thus, the method can be also applied to multi-walled nanotubes as well attested by the HRTEM image of Fig. 5. The simplicity of this mechanical treatment suggests that it can be easily scaled up and optimised. This mechanical process could solve the severe limitations in the possibility of ®lling carbon nanotubes with various substances, as for example hydrogen. References [1] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer, Nature (London) 388 (1997) 756. [2] A. Thess, R. Lee, P. Nikolaev, H. Hai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.C. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [3] S.J. Tans, ARM. Verschueren, C. Dekker, Nature (London) 393 (1998) 49. [4] G. Maurin, Ch. Bousquet, F. Henn, P. Bernier, R. Almairac, B. Simon, Chem. Phys. Lett. 312 (1999) 14. [5] P. Chen, X. Wu, J. Lin, K.L. Tan, Science 285 (1999) 91. [6] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature (London) 386 (1997) 377.
I. Stepanek et al. / Chemical Physics Letters 331 (2000) 125±131 [7] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [8] S.C. Tsang, P.J.F. Harris, M.L.H. Green, Nature 362 (1993) 520. [9] P.M. Ajayan, et al., Nature 362 (1993) 522. [10] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature 372 (1994) 159. [11] Liu, et al., Science 280 (1998) 1253.
131
[12] S. Iijima, C. Brabec, A. Maiti, J. Bernholc, J. Chem. Phys. 104 (5) (1996) 2089. [13] D. Srivastava, M. Menon, K. Cho, Phys. Rev. Lett. 83 (15) (1999) 2973. [14] M.B. Nardelli, B.I. Yakobson, J. Bernholc, Phys. Rev. Lett. 81 (21) (1998) 4656. [15] O. Lourie, H.D. Wagner, Appl. Phys. Lett. 73 (1998) 3527. [16] G. Horvath, K.J. Kawazoe, Chem. Eng. Jpn. 16 (1983) 470.