J. Phys. Chem. C 2010, 114, 13975–13978
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Hydrogen Adsorption Property of Pore Structure Controlled Single-Walled Carbon Nanotubes with Electron Irradiation Jung-Hyun Cho,† Sung-Ryul Huh,† Sun-Taek Lim,† Cheol-Min Yang,‡ Hwan-Jung Jung,§ Katsumi Kaneko,§ and Gon-Ho Kim*,† Department of Nuclear Engineering, Seoul National UniVersity, Seoul 151-742, Korea, H-Project Team, R&D Business Labs., Hyosung Corporation, Anyang 431-080, Korea, and Department of Chemistry, Faculty of Science, Chiba UniVersity, Chiba 263-8522, Japan ReceiVed: April 2, 2010; ReVised Manuscript ReceiVed: July 4, 2010
The supercritical H2 adsorption property of single-walled carbon nanotube (SWCNT) bundles irradiated by a 300 keV electron beam was studied with aspects of structural deformations and morphological changes in defect SWCNT bundles. Electron irradiation may extract the carbon atoms from the lattice surface of the SWCNT bundles, producing a lot of dangling bonds. Random rebonding of the dangling bonds causes the shrinkage of the SWCNTs, and also, it increases the intertube linkage, reducing the micropore volume and the H2 adsorption amount per unit mass. Meanwhile, the shrinkage of the SWCNT dramatically promotes the H2 adsorptivity due to narrower micropores having a stronger interaction potential for H2. Introduction A SWCNT bundle has an advantage for H2 adsorption due to the unique pore structure consisting of the intratube and intertube micropores and the groove sites.1-3 In some cases, the entangled SWCNT bundles have even mesopores.4 The pore structures of the SWCNT bundles are potentially applicable to adsorbent structures. To obtain a better adsorptivity for the supercritical H2, the modulation of the unique pore structures of SWCNTs is required. Recently, it is reported that the acid treatment on the SWCNT bundles increases the volumetric ratio of micropores to mesopores, improving the gas adsorptivity.4 The intercalation of C60 molecules in the SWCNT bundles enhanced the H2 adsorptivity by increasing the interstitial porosity.5 Simulation studies on the interactions between the irradiated particles and the SWCNTs revealed that the number of defects was varied with the energy, dose, and ion species.6,7 In terms of the production of defects, therefore, the mass of the irradiating particle is carefully chosen due to its proportionality of the collision cross section and energy-transfer rate. The ion beam irradiation with energies of a few hundred electronvolts produced severe structural damages, such as the amorphization6 or the rupture8 of the carbon nanotubes (CNTs), whereas the electron beam irradiation with energies of a few hundred kiloelectronvolts produces only vacancies in the CNT. Because the pore size of an SWCNT sensitively affects the H2 adsorptivity,9 it is expected that the property of the SWCNT bundles can be controlled by the electron beam irradiation. The possible scenario is the following: A highly energetic electron interacts with the carbon atoms in the lattice of an SWCNT, followed by the vacancy and dangling bond formations. The vacancy is energetically unstable so that the more stable structure, such as 5-1 db (pentagon-one dangling bond), remains from the defect of a vacancy due to the electron * To whom correspondence should be addressed. E-mail:
[email protected]. † Seoul National University. ‡ Hyosung Corporation. § Chiba University.
interaction.10 With sequential electron irradiations, other types of defects, such as 5-8-5, 5-7-7-5, 5-5-5-11, or 5-7-6-7-5 defects, can be produced by removing 2, 4, 5, or 6 atoms from the SWCNTs, respectively.11 Finally a large number of the carbon atoms are extracted from the SWCNTs, resulting in the shrinkage of the tube diameter.10 In addition, the electron irradiation may produce some active defect sites for hydrogen absorption with the pore structure variation and the tube diameter reduction. For a SWCNT bundle, the irradiation causes the intertube linkage after the random rebonding12 and the structural deformations may diminish the average volume of the micropores. In this study, the micropore structural deformation and the H2 adsorptivity of the SWCNT irradiated by a 300 keV electron beam are examined. Experimental Section SWCNT bundles (ASP-100F) synthesized by using an arc discharge method, followed by the purification processes of gasphase oxidation and acid treatment, were purchased from Hanhwa Nanotech Co., Ltd. (Republic of Korea). For the electron irradiation treatment, a few sheets made of the SWCNT bundles were prepared. To fabricate a sheet target, the SWCNT bundles dissolved in ethanol were sonicated and then filtered using a membrane. After a drying process, it had a thickness of 100 µm and a diameter of 16 mm. Because the 300 keV electron beam has a penetration depth up to ∼400 µm, producing a lot of vacancies, as predicted from Figure 1, the 300 keV electron beam was chosen for the irradiation source. The 300 keV electron beam has a scattering cross section13 of 10-18 cm2, which is ∼100 times smaller than 10-16 cm2 of a few hundreds electronvolt heavy ion beam. Energy dissipation of the incident electron takes place over the layer of the SWCNT sheet, which was estimated from Monte Carlo Simulation of Electron Trajectory in Solids (CASINO)14 by assuming the carbon atom density of the SWCNT structure to be 1.35 g cm-3.15 Because the deposited energy on the SWCNT bundles from a few hundred kiloelectronvolt electron beam irradiation causes severe structural deformations, such as the amorphization of the SWCNTs16 by 1023 cm-2, the electron dose was carefully
10.1021/jp1029739 2010 American Chemical Society Published on Web 08/02/2010
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Figure 1. Energy dissipation graph of the 300 keV electron beam in a carbon target calculated by the Monte Carlo Simulation of Electron Trajectory in Solids (CASINO).
manipulated within the range of 8.0 × 1014 to 8.0 × 1015 cm-2. The dose was controlled by the irradiation time with a fixed electron beam current of the linear accelerator in the Korea Atomic Energy Research Institute. N2 and H2 adsorption isotherms were used for the characterization of the pore structures and the evaluation of H2 adsorptivity. The pristine and the electron-beam-irradiated SWCNT bundles were measured by a volumetric adsorption apparatus (Autosorb-1-MP, Quantachrome) at 77 K after preevacuation at 423 K and 10-4 Pa for 2 h. The pore structure parameters were determined by the subtracting pore effect (SPE) method2 that was performed by using high-resolution Rs plots constructed based on the standard adsorption data for highly crystalline nonporous carbon black (Mitsubishi 4040B). The images of the bundles were observed with a transmission electron microscope (TEM, JEM2100F, JEOL). For TEM observation, some bundles were dispersed on a microgrid after sonication with ethanol. The SWCNT sheet and the bundles on the grid were irradiated by the electron beam, simultaneously. The defective structure was excited by a 514 nm Ar ion laser and evaluated qualitatively with the Raman spectroscopy using a spectroscope (T64000, Jobin-Yvon).
Figure 2. Transmission electron microscope images of (a) pristine SWCNT bundles and SWCNT bundles irradiated by the 300 keV electron beam with doses of (b) 8.0 × 1014 and (c) 8.0 × 1015 cm-2.
Results and Discussion Figure 2 shows the images of the pristine and irradiated SWCNT bundles at different doses. The SWCNT bundles irradiated by the 300 keV electron beam indicate no serious damages due to the small collision cross section, as predicted previously. The morphology of the SWCNT bundles irradiated with a dose of 8.0 × 1014 cm-2 shown in Figure 2b sustains a good crystalline structure as the pristine SWCNT bundles in Figure 2a. By increasing the dose up to 8.0 × 1015 cm-2 in Figure 2c, the crystalline structure becomes slightly blunt on the rough surfaces of the bundles. The formation of a large number of defects in the SWCNT bundles could be induced by the intensive electron beam irradiation. The formation of defects can be evaluated from the D-band intensity17 of Raman spectra, as shown in Figure 3. The intensities of the D and G bands reflect the amounts of the defects of the SWCNTs and the well-crystalline graphitic structures in the SWCNTs, respectively. Thus, the intensity ratio of D to G bands, ID/IG, of Raman spectra represents the damageto-crystal ratio on the surface of the SWCNT bundles. As shown
Figure 3. Raman spectra for pristine and 300 keV electron-irradiated sheets made of the SWCNT bundles: the intensity of the D band represents the amount of defects. The inset image is the enlarged spectrum of the D band.
in the inset plot of Figure 3, ID/IG ratios are calculated as 0.007 for no exposure, 0.038 for the dose of 8.0 × 1014 cm-2, and 0.092 for the dose of 8.0 × 1015 cm-2. The results could be expected from TEM images of Figure 2 as well. The N2 adsorption isotherms are utilized to confirm the structural changes induced by the pores in the SWCNT bundles
H2 Adsorption of SWCNTs with Electron Irradiation
J. Phys. Chem. C, Vol. 114, No. 33, 2010 13977 TABLE 1: Pore Structure Parameters of SWCNT Samples N2 adsorption (77 K): subtracting pore effect (SPE) method
Figure 4. (a) N2 adsorption isotherms at 77 K. The micropore volume of the sheet of the SWCNT bundles is estimated by these isotherms. (b) Comparison plots of the amounts of adsorbed N2 on pristine and electron-irradiated SWCNT bundles. The dashed line indicates adsorption on mutually equivalent surfaces. (c) Supercritical H2 adsorption isotherms at 77 K.
after the electron beam irradiation (Figure 4a). The N2 adsorption isotherm of the pristine SWCNT bundles is close to the type II by the definition of the IUPAC classification; the initial uptake is very marked compared with the typical IUPAC type II. The N2 adsorption amount of the pristine SWCNT bundles at low pressures (P/P0) could be classified mainly from the adsorption in intertube pores and partially opened intratube pores. Electron beam irradiations seem to have little effects on the shape of the N2 adsorption isotherm, as all of the N2 adsorption isotherms show similar trends. However, the initial N2 adsorption amounts
sample
micropore surface area (m2 g-1)
micropore volume (mL g-1)
pristine De ) 8.0 × 1014 cm-2 De ) 8.0 × 1015 cm-2
365 162 34
0.16 0.07 0.02
of the irradiated SWCNT bundles at low P/P0 remarkably decrease compared with that of the pristine SWCNT bundles. The pores at a low pressure are blocked by the intertube linkages, and the spaces of the intratubes in the SWCNT bundles shrank after the electron beam irradiation. The variation of micropore volume can be estimated by the SPE method2 based on the N2 adsorption isotherms in Figure 4a. The micropore volume was drastically reduced from 0.16 mL/g for pristine bundles to 0.07 mL/g for the bundles irradiated by the 300 keV electron beam with a dose of 8.0 × 1014 cm-2; the micropore volume decreased to 0.02 mL/g by irradiation with a larger electron dose of 8.0 × 1015 cm-2 (Table 1). Figure 4b is a plot comparing N2 adsorption amounts on the pristine SWCNT bundles (x axis) to the electron-beam-irradiated SWCNT bundles (y axis). Because the dotted line indicates the N2 adsorption amounts on the pristine SWCNT bundles, the plot provides visible information on the volumetric changes by the pores on the SWCNT bundles after the electron beam irradiation. In both of the electron-beam-irradiated cases, the clear downward deviations are observed at the low-pressure region, indicating the decrease in the surface area of narrow micropores. On the other hand, a downward deviation at the high-pressure region should be attributed to the decreased surface area of the wide micropores and mesopores. The downward deviation becomes more severe by increasing the electron beam dose. It implies that the defect formation induced by the intertube linkages and shrunken intratube pore is enhanced with increased electron beam doses. Accordingly, the micropore entrance structure can be regulated by the electron beam irradiation. The supercritical H2 adsorptivity should be sensitive to the structural changes of the micropores. Figure 4c shows H2 adsorption isotherms at 77 K, where the H2 adsorption amount decreases by increasing the electron beam doses. The reduced H2 adsorption of the electron-irradiated SWCNTs is induced by the decreased micropore volume.9 The changes of the micropore volume must be mainly caused by the strong linkages of intertube pores and the shrunken intratube pores of the SWCNT bundles. Carbon atoms of the SWCNTs are extracted or recoiled from the collisions with the energetic incident electrons, remaining the dangling bonds on the carbon atoms which induce the intertube linkages12 inside. A large number of linkages produce the intertube pores, whereas the recoiled carbon atoms from the lattice can induce the shrinkage of the SWCNTs inside the bundles.10 At the first stage of shrinkage of the bundle, the intratube pore is effectively reduced, as indicated by curves 2 and 1 in Figure 4c. More electron irradiation reduces the intertube pore of the SWCNTs. It induces the increase of the mass density of electron-irradiated SWCNT bundles, and the weight density of hydrogen adsorption to the irradiated SWCNT bundles is decreased with increasing the dose of electron irradiation. In Figure 5, the H2 adsorptivity per unit micropore volume is estimated for the investigation of the pore structure after the electron beam irradiation. The absorptivity per unit micropore volume is increased with increasing the dose of electron
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Cho et al. and shrink the tube diameter, which are indispensable for the SWCNT applications. The comparative examination of N2 and H2 adsorption is a quite efficient method for explaining the fine structural changes of the micropores. Acknowledgment. This work was supported by the Korea Electrical Engineering and Science Research Institute (Grant No. R-2005-7-085), by the Korea Science and Engineering Foundation (Grant No. 20090078525), by BK21 Research Division of Seoul National University for Energy Resources, and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (Grant No. NRF-2010-0001829). References and Notes
Figure 5. Amount of supercritical H2 adsorption per unit micropore volume. After electron irradiation, the property of the adsorption per unit volume is improved in comparison with that before irradiation.
irradiation; however, it is not significantly changed for that of the lower dose irradiation with the comparison of pristine SWCNTs. As expected in Figure 4, the interaction of energetic electron irradiation with the SWCNT increases the number of pores and also it reduces the surface of pores due to shrinkage of the SWCNT and the bundle. Thus, for the lower dose of electron irradiation, the hydrogen adsorptivity is not improved, as indicated by curve 2 in Figure 5. With increasing the dose, the dangling bonds in the pore area are enhanced, then the adsorption of hydrogen per unit pore volume is increased. From the adsorptivity measurements, the H2 adsorptivity seems to be an appropriate standard for assessing the volumes of the micropores, whereas the N2 adsorptivity is insensitive to the volumetric changes of the pores. From the results of the adsorprivity measurements, the electron beam irradiation can control the sizes of the tube opening and the tube diameter as well. The control of the SWCNT dimensions has been inaccessible from other chemical and physical treatments. Conclusions A new technique for the structural changes of the SWCNTs using high-energy electron irradiation is introduced in the study. The electron beam irradiation could close the open tube spaces
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