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In-situ observation of structural change in MWCNTs under high-pressure H2 gas atmosphere
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Diamond & Related Materials 17 (2008) 548 – 551 www.elsevier.com/locate/diamond
In-situ observation of structural change in MWCNTs under high-pressure H2 gas atmosphere Atsuko Nakayama a,⁎, Shigenori Numao a , Satoshi Nakano b , Shunji Bandow a , Sumio Iijima a a
21st century COE program “Nanofactory”, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan b National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Available online 23 December 2007
Abstract Multi-walled carbon nanotubes having a tube-opened framework (o-MWCNTs) have been pressurized in an atmosphere of hydrogen up to 2.5 GPa and at room temperature using a diamond-anvil cell (DAC). Compression up to 0.57 GPa was accompanied by expansion of the honeycom-lattice structure of graphene sheets in spite of decreasing interlayer distance. A deviation in the in-plane C–C distance was obtained to be 3.8 × 10− 4 nm at 0.57 GPa. The expansion in the honeycom-lattice structure suggests generation of charge-transfer interaction between graphite and hydrogen. This is one of the proofs that hydrogen is intercalated in the interlayer space of o-MWCNTs. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanotubes; Radiofrequency plasma vaporization; Hydrogen storage; X-ray diffraction; High pressure
1. Introduction We have once focused on utilizing carbon nanotubes (CTs) as hydrogen (H2)-storage materials because their cage structures were expected to have spaces to adsorb a large amount of H2 gas. It was reported that the H2-gas storage-abilities of singleand multi-walled carbon nanotubes (SWCNTs and MWCNTs) were estimated to be 5 to 15 wt.% [1–3] and 0.25 wt.% [4], respectively. On the other hand we also know unfavorable results against them. Such an inconsistency may be caused by the following backgrounds: (1) no tried and tested method to measure the amount of H2 gas absorbed in SWCNTs and (2) no experimental results on electron states of SWCNTs and H2, having a relationship between the host and the guest. Experimental confirmation of the electron state of H2-doped SWCNTs is very important for us to clarify the H2 gas-storage mechanism of any CNTs, which could inform the adsorption site of H2 and the magnitude of interaction working between carbon atom and H2 molecule. ⁎ Corresponding author. Present address: Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi-Ninocho, Niigata 950-2181, Japan. Tel./fax: +81 25 262 7267. E-mail address: [email protected] (A. Nakayama). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.032
A stable bond of H2 molecule has large ionization energy [5] and the magnitude of electron affinity is close to that of carbon; we could not expect the charge-transfer interaction between graphite and H2, unless carbon and H2 are in same special condition. Pressurization is one of the methods to induce unstable electron state of H2. Recently we focused meso-carbon micro-beads (MCMBs) have a graphite structure and found that pressurization of them in an atmosphere of H2 produces a H2graphite intercalation compound [6]. We could understand that the pressurization changes in a C–C π bond composing of an inplane structure of graphene sheets and van der Waals interaction working between the layers. These facts have motivated us to investigate whether MWCNTs accept H2 under pressure. MWCNTs have many graphene sheets, which are rolled up and closely stacked each other; they should also have the ability to cause the intercalation of H2 under high pressure like graphite. Recently a radiofrequency (RF) plasma method enabled to produce metal-free and fully-packed MWCNTs having the tubeclosed framework [7]. The metal free and pure state can simply prove the influence of H2 on such carbon assemblies. Their internal-tube diameters are 0.4 nm, which are the smallest values in any CNTs. They have tips with cone angles of 19.2°. Heat treatment of MWCNTs brings about the tube-opened framework, which is expected to have a lot of H2 pockets like
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Fig. 1. HRTEM images of (a) the c-MWCNTs produced by RF plasma method and (b) the o-MWCNTs, having the tube-open framework, produced by heat treatment of the c-MWCNTs.
MCMBs. To observe and understand the intercalation of H2 into such carbon-nanostructured materials with graphite structure, may give the important hint for the material design of H2-storage materials. In this study the opened MWCNTs (o-MWCNTs) were prepared and their x-ray diffraction experiments under an atmosphere of H2 gas were carried out under pressure up to 2.5 GPa and at room temperature using a diamond-anvil cell (DAC). The lengths of a- and c-axis lengths were determined at each pressure stage. The condition of o-MWCNTs in which H2 is introduced, was discussed through the pressure changes in the a- and c-axis lengths.
beams on BL-18C at Photon Factory, High Energy Accelerator Research Organization (KEK). The x-ray beam was monochromatized to a wavelength of 0.6188 Å, and introduced to the specimen through a pinhole collimator with a 100-µm diameter. Each x-ray diffraction-pattern was obtained by exposing the detector for 1 h at room temperature. 3. Results and discussion 3.1. The structure of o-MWCNTs generated by heat treatment
2. Experiments
The HRTEM images of MWCNTs are shown in Fig. 1. We could confirm that the heat treatment removed acute tips closing the tubes and small parts of surfaces on the c-MWCNTs with
The closed MWCNTs (c-MWCNTs) were prepared by the RF plasma vaporization method [7]. The o-MWCNTs were obtained by heat treatment of the c-MWCNTs at 675 °C for 75 min under a mixed-gases atmosphere of O2 and Ar (1:4 by pressure). Generation of the tube-opened structure was confirmed by highresolution transmission-electron microscopy (HRTEM). A pair of diamond anvils, which have culet diameters of 0.6 mm and thicknesses of 2.0 mm, respectively, were used to make the high pressure. The wolfram foil with 100 µm initial thickness was indented down to 50 µm in thickness using the anvil face; a gasket was prepared by drilling a hole with 300 µm in diameter at the center of the indention on the foil. The sample was put in the gasket hole together with ruby balls for the pressuremeasurement, which was filled with high-density H2-gas compressed up to 100 MPa at room temperature [8]. Angle-dispersive powder-patterns were taken with an imaging plate as a detector using synchrotron radiation (SR)
Fig. 2. X-ray diffraction patterns of the o- and c-MWCNTs observed at 1 atm and room temperature.
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Fig. 3. X-ray diffraction patterns of the o- and c-MWCNTs observed under high-pressure H2 gas at room temperature, (a) and (b), respectively.
keeping the layered structure. The diffraction patterns of the oand c-MWCNTs in Fig. 2 were analyzed on the assumption that they have two types of structures: crystalloid and non-crystalloid components. The a- and c-axis lengths were obtained from the 110 and 002 reflections, respectively. The line shape of 100 reflection, overlapping with the 101 one, was too complex to determine the a-axis length. Each peak position and its error range were determined by fitting of the diffraction patterns with Gauss functions. The heat treatment shortens the a-axis length and extends the c-axis one. In case of crystalloid component, the magnitudes of shrink and extension of a- and c-axis lengths from the pristine ones are about 0.1% and 0.9%, respectively. The c-MWCNTs produced by RF plasma method have stressful structure of the tube tips with an angle of 19.2° [7]. Heat treatment could remove the strain of structure from the c-MWCNTs, resulting in the van der Waals and C–C π bonds of the interlayer and inplane structure, which are observed in graphite. 3.2. MWCNTs under high-pressure H2-atmosphere The x-ray diffraction patterns of o- and c-MWCNTs, pressurized in the atmosphere of H2, respectively, are shown in Fig. 3. The reflection of 002 from o-MWCNTs shows narrowing of linewidth with increasing the pressure while the cMWCNTs show little change in the line shape and intensity. In case of c-MWCNTs, H2 becomes a good pressure-transmitting medium and is effective to compress the c-MWCNTs without changing the stacking state of graphene sheets. The compression of o-MWCNTs makes a homogeneous-stacking structure in contrast to the c-MWCNTs because a degree of freedom in structure for o-MWCNTs is larger than that of c-
MWCNTs. Therefore o-MWCNTs show narrowing of the linewidth from the reflection of 002 with increasing the pressure. These results prove that heat treatment actually removes the tips from c-MWCNTs. Changes in lattice parameters of the o-MWCNTs under high-pressure H2 atmosphere are shown in Fig. 4. The a-axis length of crystalloid component increases with increasing the pressure while the c-axis one monotonously decreases. The aaxis length indicates the maximum at 0.57 GPa, which is 0.27% larger than the atmospheric value: a0 = 0.245 nm; that is a reversible pressure change. The reflection of 110, coming from non-crystalloid component, could not be extracted from the diffraction patterns under pressure below 0.57 GPa; pressurization above 0.57 GPa causes renewal of the 110 reflection. In addition the a-axis length of non-crystalloid component did not return to the original one by pressure release. Such complicated phenomena suggest that H2 causes transformation of the inplane structure of o-MWCNTs. In case of MWCNTs, their structural disorder is supposed to come from (1) turbostraticstacking of graphene sheets, (2) amorphous carbons generated by the heat treatments to remove the tips and (3) scrolled forms of graphene sheets [9]. It is difficult to have further discuss about the origin of non-crystalloid component of MWCNTs. Therefore we hereafter refer to the pressure-induced structural change in the crystalloid component. What happened to the a-axis in o-MWCNTs? We suppose that the transient enlargement in the a-axis length is caused by chargetransfer interaction between the o-MWCNTs and hydrogen (H2 molecule or H atom) accompanied by the intercalation. We will confirm the electron state of hydrogen intercalated in o-MWCNTs in the next study. The value of deviation from the initial state in the in-plane C–C distance, δc–c, correlates with the magnitude of
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sation between the compression and the charge-transfer interaction. The change in c-axis length caused by the intercalation of H2 is considerably sensitive comparing with a-axis length. The enhancement of c-axis length may be occurred at the pressures lower than 0.2 GPa, which is the initial pressure of this observation. Acknowledgements This work has been carried out on the 21st century COE program for Nanofactory of ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [2] M.R. Pederson, J.Q. Broughton, Phys. Rev. Lett. 69 (1992) 2689. [3] Q. Wang, J.K. Johnson, J. Chem. Phys. 110 (1999) 557. [4] X.B. Wu, P. Chen, J. Lin, K.L. Tan, Int. J. Hydrogen Energy 25 (2000) 261. [5] J.W. Robinson, Handbook of Spectroscopy, vol. 1, CRC Press, Boca Raton, 1974, p. 257. [6] A. Nakayama, S. Nakano, T. Taniguchi, Y. Koga, private communication. [7] A. Koshio, M. Yudasaka, S. Iijima, Chem. Phys. Lett. 356 (2002) 595. [8] K. Takemura, P.C. Sahu, Y. Kunii, Y. Toma, Rev. Sci. Instrum. 72 (2001) 3873. [9] Y. Maniwa, R. Fujiwara, H. Kira, H. Tou, E. Nishibori, M. Takata, M. Sakata, A. Fujiwara, X. Zhao, S. Iijima, Y. Ando, Phys. Rev., B 64 (2001) 073105.
Fig. 4. Pressure-induced structural-changes in the a- and c-axis lengths of oMWCNTs under high-pressure H2 gas.
Coulomb repulsion working between electrons on the adjacentcarbon atoms on the layers. The value of deviation gives a maximum of δc–c = +3.8 × 10− 4 nm at 0.57 GPa. In this observation the value of the deviation may be determined by compen-
Diamond & Related Materials 17 (2008) 548 – 551 www.elsevier.com/locate/diamond
In-situ observation of structural change in MWCNTs under high-pressure H2 gas atmosphere Atsuko Nakayama a,⁎, Shigenori Numao a , Satoshi Nakano b , Shunji Bandow a , Sumio Iijima a a
21st century COE program “Nanofactory”, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan b National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Available online 23 December 2007
Abstract Multi-walled carbon nanotubes having a tube-opened framework (o-MWCNTs) have been pressurized in an atmosphere of hydrogen up to 2.5 GPa and at room temperature using a diamond-anvil cell (DAC). Compression up to 0.57 GPa was accompanied by expansion of the honeycom-lattice structure of graphene sheets in spite of decreasing interlayer distance. A deviation in the in-plane C–C distance was obtained to be 3.8 × 10− 4 nm at 0.57 GPa. The expansion in the honeycom-lattice structure suggests generation of charge-transfer interaction between graphite and hydrogen. This is one of the proofs that hydrogen is intercalated in the interlayer space of o-MWCNTs. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanotubes; Radiofrequency plasma vaporization; Hydrogen storage; X-ray diffraction; High pressure
1. Introduction We have once focused on utilizing carbon nanotubes (CTs) as hydrogen (H2)-storage materials because their cage structures were expected to have spaces to adsorb a large amount of H2 gas. It was reported that the H2-gas storage-abilities of singleand multi-walled carbon nanotubes (SWCNTs and MWCNTs) were estimated to be 5 to 15 wt.% [1–3] and 0.25 wt.% [4], respectively. On the other hand we also know unfavorable results against them. Such an inconsistency may be caused by the following backgrounds: (1) no tried and tested method to measure the amount of H2 gas absorbed in SWCNTs and (2) no experimental results on electron states of SWCNTs and H2, having a relationship between the host and the guest. Experimental confirmation of the electron state of H2-doped SWCNTs is very important for us to clarify the H2 gas-storage mechanism of any CNTs, which could inform the adsorption site of H2 and the magnitude of interaction working between carbon atom and H2 molecule. ⁎ Corresponding author. Present address: Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi-Ninocho, Niigata 950-2181, Japan. Tel./fax: +81 25 262 7267. E-mail address: [email protected] (A. Nakayama). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.032
A stable bond of H2 molecule has large ionization energy [5] and the magnitude of electron affinity is close to that of carbon; we could not expect the charge-transfer interaction between graphite and H2, unless carbon and H2 are in same special condition. Pressurization is one of the methods to induce unstable electron state of H2. Recently we focused meso-carbon micro-beads (MCMBs) have a graphite structure and found that pressurization of them in an atmosphere of H2 produces a H2graphite intercalation compound [6]. We could understand that the pressurization changes in a C–C π bond composing of an inplane structure of graphene sheets and van der Waals interaction working between the layers. These facts have motivated us to investigate whether MWCNTs accept H2 under pressure. MWCNTs have many graphene sheets, which are rolled up and closely stacked each other; they should also have the ability to cause the intercalation of H2 under high pressure like graphite. Recently a radiofrequency (RF) plasma method enabled to produce metal-free and fully-packed MWCNTs having the tubeclosed framework [7]. The metal free and pure state can simply prove the influence of H2 on such carbon assemblies. Their internal-tube diameters are 0.4 nm, which are the smallest values in any CNTs. They have tips with cone angles of 19.2°. Heat treatment of MWCNTs brings about the tube-opened framework, which is expected to have a lot of H2 pockets like
A. Nakayama et al. / Diamond & Related Materials 17 (2008) 548–551
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Fig. 1. HRTEM images of (a) the c-MWCNTs produced by RF plasma method and (b) the o-MWCNTs, having the tube-open framework, produced by heat treatment of the c-MWCNTs.
MCMBs. To observe and understand the intercalation of H2 into such carbon-nanostructured materials with graphite structure, may give the important hint for the material design of H2-storage materials. In this study the opened MWCNTs (o-MWCNTs) were prepared and their x-ray diffraction experiments under an atmosphere of H2 gas were carried out under pressure up to 2.5 GPa and at room temperature using a diamond-anvil cell (DAC). The lengths of a- and c-axis lengths were determined at each pressure stage. The condition of o-MWCNTs in which H2 is introduced, was discussed through the pressure changes in the a- and c-axis lengths.
beams on BL-18C at Photon Factory, High Energy Accelerator Research Organization (KEK). The x-ray beam was monochromatized to a wavelength of 0.6188 Å, and introduced to the specimen through a pinhole collimator with a 100-µm diameter. Each x-ray diffraction-pattern was obtained by exposing the detector for 1 h at room temperature. 3. Results and discussion 3.1. The structure of o-MWCNTs generated by heat treatment
2. Experiments
The HRTEM images of MWCNTs are shown in Fig. 1. We could confirm that the heat treatment removed acute tips closing the tubes and small parts of surfaces on the c-MWCNTs with
The closed MWCNTs (c-MWCNTs) were prepared by the RF plasma vaporization method [7]. The o-MWCNTs were obtained by heat treatment of the c-MWCNTs at 675 °C for 75 min under a mixed-gases atmosphere of O2 and Ar (1:4 by pressure). Generation of the tube-opened structure was confirmed by highresolution transmission-electron microscopy (HRTEM). A pair of diamond anvils, which have culet diameters of 0.6 mm and thicknesses of 2.0 mm, respectively, were used to make the high pressure. The wolfram foil with 100 µm initial thickness was indented down to 50 µm in thickness using the anvil face; a gasket was prepared by drilling a hole with 300 µm in diameter at the center of the indention on the foil. The sample was put in the gasket hole together with ruby balls for the pressuremeasurement, which was filled with high-density H2-gas compressed up to 100 MPa at room temperature [8]. Angle-dispersive powder-patterns were taken with an imaging plate as a detector using synchrotron radiation (SR)
Fig. 2. X-ray diffraction patterns of the o- and c-MWCNTs observed at 1 atm and room temperature.
550
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Fig. 3. X-ray diffraction patterns of the o- and c-MWCNTs observed under high-pressure H2 gas at room temperature, (a) and (b), respectively.
keeping the layered structure. The diffraction patterns of the oand c-MWCNTs in Fig. 2 were analyzed on the assumption that they have two types of structures: crystalloid and non-crystalloid components. The a- and c-axis lengths were obtained from the 110 and 002 reflections, respectively. The line shape of 100 reflection, overlapping with the 101 one, was too complex to determine the a-axis length. Each peak position and its error range were determined by fitting of the diffraction patterns with Gauss functions. The heat treatment shortens the a-axis length and extends the c-axis one. In case of crystalloid component, the magnitudes of shrink and extension of a- and c-axis lengths from the pristine ones are about 0.1% and 0.9%, respectively. The c-MWCNTs produced by RF plasma method have stressful structure of the tube tips with an angle of 19.2° [7]. Heat treatment could remove the strain of structure from the c-MWCNTs, resulting in the van der Waals and C–C π bonds of the interlayer and inplane structure, which are observed in graphite. 3.2. MWCNTs under high-pressure H2-atmosphere The x-ray diffraction patterns of o- and c-MWCNTs, pressurized in the atmosphere of H2, respectively, are shown in Fig. 3. The reflection of 002 from o-MWCNTs shows narrowing of linewidth with increasing the pressure while the cMWCNTs show little change in the line shape and intensity. In case of c-MWCNTs, H2 becomes a good pressure-transmitting medium and is effective to compress the c-MWCNTs without changing the stacking state of graphene sheets. The compression of o-MWCNTs makes a homogeneous-stacking structure in contrast to the c-MWCNTs because a degree of freedom in structure for o-MWCNTs is larger than that of c-
MWCNTs. Therefore o-MWCNTs show narrowing of the linewidth from the reflection of 002 with increasing the pressure. These results prove that heat treatment actually removes the tips from c-MWCNTs. Changes in lattice parameters of the o-MWCNTs under high-pressure H2 atmosphere are shown in Fig. 4. The a-axis length of crystalloid component increases with increasing the pressure while the c-axis one monotonously decreases. The aaxis length indicates the maximum at 0.57 GPa, which is 0.27% larger than the atmospheric value: a0 = 0.245 nm; that is a reversible pressure change. The reflection of 110, coming from non-crystalloid component, could not be extracted from the diffraction patterns under pressure below 0.57 GPa; pressurization above 0.57 GPa causes renewal of the 110 reflection. In addition the a-axis length of non-crystalloid component did not return to the original one by pressure release. Such complicated phenomena suggest that H2 causes transformation of the inplane structure of o-MWCNTs. In case of MWCNTs, their structural disorder is supposed to come from (1) turbostraticstacking of graphene sheets, (2) amorphous carbons generated by the heat treatments to remove the tips and (3) scrolled forms of graphene sheets [9]. It is difficult to have further discuss about the origin of non-crystalloid component of MWCNTs. Therefore we hereafter refer to the pressure-induced structural change in the crystalloid component. What happened to the a-axis in o-MWCNTs? We suppose that the transient enlargement in the a-axis length is caused by chargetransfer interaction between the o-MWCNTs and hydrogen (H2 molecule or H atom) accompanied by the intercalation. We will confirm the electron state of hydrogen intercalated in o-MWCNTs in the next study. The value of deviation from the initial state in the in-plane C–C distance, δc–c, correlates with the magnitude of
A. Nakayama et al. / Diamond & Related Materials 17 (2008) 548–551
551
sation between the compression and the charge-transfer interaction. The change in c-axis length caused by the intercalation of H2 is considerably sensitive comparing with a-axis length. The enhancement of c-axis length may be occurred at the pressures lower than 0.2 GPa, which is the initial pressure of this observation. Acknowledgements This work has been carried out on the 21st century COE program for Nanofactory of ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [2] M.R. Pederson, J.Q. Broughton, Phys. Rev. Lett. 69 (1992) 2689. [3] Q. Wang, J.K. Johnson, J. Chem. Phys. 110 (1999) 557. [4] X.B. Wu, P. Chen, J. Lin, K.L. Tan, Int. J. Hydrogen Energy 25 (2000) 261. [5] J.W. Robinson, Handbook of Spectroscopy, vol. 1, CRC Press, Boca Raton, 1974, p. 257. [6] A. Nakayama, S. Nakano, T. Taniguchi, Y. Koga, private communication. [7] A. Koshio, M. Yudasaka, S. Iijima, Chem. Phys. Lett. 356 (2002) 595. [8] K. Takemura, P.C. Sahu, Y. Kunii, Y. Toma, Rev. Sci. Instrum. 72 (2001) 3873. [9] Y. Maniwa, R. Fujiwara, H. Kira, H. Tou, E. Nishibori, M. Takata, M. Sakata, A. Fujiwara, X. Zhao, S. Iijima, Y. Ando, Phys. Rev., B 64 (2001) 073105.
Fig. 4. Pressure-induced structural-changes in the a- and c-axis lengths of oMWCNTs under high-pressure H2 gas.
Coulomb repulsion working between electrons on the adjacentcarbon atoms on the layers. The value of deviation gives a maximum of δc–c = +3.8 × 10− 4 nm at 0.57 GPa. In this observation the value of the deviation may be determined by compen-