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Hydrogen storage in carbon nanotubes modified by microwave plasma etching and Pd decoration
Carbon 44 (2006) 762–767 www.elsevier.com/locate/carbon
Hydrogen storage in carbon nanotubes modified by microwave plasma etching and Pd decoration Shi-chun Mu *, Hao-lin Tang, Sheng-hao Qian, Mu Pan, Run-zhang Yuan State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 1 October 2003; accepted 12 September 2005 Available online 25 October 2005
Abstract Microwave plasma etching and Pd decoration methods were employed for the modification of carbon nanotubes (CNTs). The defects on the nanotube wall increased after the etching process as determined from HRTEM observation and Raman measurement. The defects supplied more hydrogen accesses to the interlayers and hollow interiors of CNTs. The results of hydrogen uptake measurements showed that the etched CNTs had higher hydrogen storage capability than that of the original sample at ambient temperature and pressure of 10.728 MPa. Furthermore, the CNTs decorated with Pd showed a hydrogen storage capability of 4.5 wt.%, about three times higher than that of the non-decorated samples. The hydrogen uptake mechanism of the modified CNTs was discussed. 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Carbon composites; Gas storage; Etching; Surface treatment
1. Introduction There are various methods to increase defects in order to obtain more hydrogen accesses to CNTs and to improve the hydrogen storage performance of CNTs. Ye et al. [1] performed hydrogen adsorption on purified SWCNT samples treated with sonication. They observed that the sonicated CNTs contained more wall defects, and the defects could be acted as H2 accesses to the endohedral pores. Li et al. [2] also observed proper defects in tubes were helpful to improve the hydrogen adsorptive capacity. However, they considered that keeping the graphitization was important to bond hydrogen in tubes. In our previous work [3,4], hydrogen storage performance of CNTs decorated with Pt and Pd was studied. The function of Pd was to dissociate hydrogen molecules into protons or atoms to make hydrogen easily penetrate into the interlayers and hollow interiors of CNTs. Hydrogen atoms/protons could bond with carbon atoms by
chemisorption or charge transfer, and form hydrogen molecules in tubes, which is a multi-step hydrogen uptake process as pointed out by Lee et al. [5,6] and Cheng et al. [7]. In this paper, we employed microwave hydrogen plasma to etch the tube walls to create defects as hydrogen accesses to interlayers and interiors of tubes. Because five- or sevennumbered carbon rings may incorporate in the hexagonal network of graphite sheets [8,9], these carbon rings have lower stabilities compared with six-numbered rings and are easily subjected to destruction in an etching process. If the etching is continued, the wall defects in tubes would increase and the hydrogen accesses to tubes will be enhanced. Moreover, the addition effect of Pd to the etched CNTs on the hydrogen storage performance was also investigated. 2. Experimental 2.1. Pretreatment of CNTs
*
Corresponding author. Tel.: +86 27 87665259; fax: +86 27 87879468. E-mail address: [email protected] (S.-c. Mu).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.010
Multi-walled carbon nanotubes (MWCNTs) were synthesized by catalytic decomposition of acetylene over
S.-c. Mu et al. / Carbon 44 (2006) 762–767
cobalt nanoparticles. As-received samples was immersed and refluxed in NaOH solution for 2 h, and subjected to mild sonication for 1 h; then refluxed in a mixed acid (H2SO4:HNO3 = 3:1) for 1 h. After washing with de-ionized water, the sample was dried in vacuum at 393 K, and followed by heat treatment at 973 K under argon atmosphere. The pretreated sample was labeled as sample T6. 2.2. Hydrogen plasma etching Hydrogen plasma etching of CNTs was done in a microwave plasma generator as shown in Fig. 1. The microwave energy spread along rectangle wave-guide in the mode of TE10 and stimulated the gas into uniform plasma ball in the resonant cavity. The output of microwave plasma etching was 0.3–3 kW. To prevent graphitization of CNTs at high temperature, the etching temperature of 973–1273 K was considered. The gas pressure was 6.0 · 102– 6.0 · 103 Pa. The etching gas was hydrogen, N2, Ar or their mixture was used as protecting gas. Sample T6 was put into a silica window, and wetted by de-ionized water to prevent the sample from being flown in degassing and etching processes. The hydrogen etching of the sample was conducted at 2 kW, 1073–1193 K and 6.0 · 102 Pa under Ar gas for 2 h and 4 h, the etched samples were labeled as T6H2 and T6H4, respectively. The morphography of the CNTs before and after etching was observed by HRTEM (JEM-2010FEF). Samples were prepared by suspending the powder in ethanol in ultrasonic bath for 15 min and dropped onto microporous carbon film attached on Cu grids. Raman spectrum (U-1000 Ramanor) was employed to determine the defect growth in tubes through comparing D-band and G-band intensity. The wavelength of laser was 487.9860 nm and the power was 150 mV. The crystalline changes in CNTs were tested by XRD (NICOLET 170SX). 2.3. Synthesis of the etched CNTs decorated with Pd
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and drying, the decorated CNTs were treated at 573 K under argon atmosphere. The results were labeled as T7, T7H2, and T7H4 respectively. TEM was performed to characterize the samples. 2.4. Hydrogen uptake of modified CNTs The hydrogen uptake and release performance of samples was measured by pressure differentia method used by relative references [3,10]. As shown in Fig. 2, a dried sample was put into the sample cell and degassed in situ under high vacuum at 573 K for 2 h; hydrogen (99.9999%) was loaded when the cell was cooled down to ambient temperature; as the hydrogen pressure in the cell was to ca 10.7 MPa, close the cell, hydrogen uptake curves were output automatically by a computer; when the hydrogen pressure reached balancing for 2–3 h, the cell was open for hours for hydrogen release, take out the sample and weigh it; finally, the sample was put into the cell again, repeat the step one, then the sample was taken out and weighed. 3. Results and discussion 3.1. Characterization of the pretreated and the modified CNTs HRTEM images of the pretreated CNTs are shown in Fig. 3(a). The pretreated sample contained more than 95% CNTs determined by TGA. The outer diameter of nanotubes ranges from 20 to 40 nm, and the inner diameter ranges from 8 to 15 nm. There are thin amorphous carbon layers adhere to the nanotube walls, while the nanotube walls possess high graphitization and have 0.34 nm of facet (0 0 2). The number of graphite layers is about 10–40. As shown in Fig. 3(b), after etching, CNTs have less numbers of graphite layers ranging from 10 to 30 nm, but the d (0 0 2) is consistent with the pretreated CNTs determined by XRD (Fig. 4). The thickness of the amorphous carbon layers is larger than that of the graphitilized layer,
Sample (0.5 g) T6, T6H2 and T6H4 was sonicated for 1 h in 200ml PdCl2 (1 g/l), respectively. Then NaBH4 (1 mol/l) as a reduction agent was added at a speed of 0.3 ml/min until the solution was colorless. After washing
Fig. 1. Microwave plasma generator: (1) micro-wave source, (2) closed loop control, (3) three-bolt adjustor, (4) coupling antenna, (5) plasma ball, (6) resonant cavity and (7) inlet of inert gas.
Fig. 2. Schematic of an instrument measuring the amount of hydrogen uptake for CNTs: (1) sample cell, (2) heating jacket (3) high accuracy sensor, (4) gas valve, (5) computer, (6) pressure gauge, (7) vacuum pump and (8) hydrogen purification train.
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which indicates the increased defects or disordered structures exist in tubes. Fig. 5 shows Raman spectra of the pretreated and the etched CNT samples. The G-band at 1580 cm 1 in CNTs is similar to the peak G of HOPG produced by the graphitic basal plane [11,12]. The appearance of G-band indicates that there are crystal graphite layers in tube structures. The Raman mode of D-band at about 1364 cm 1 is attributed to the wall defects in CNTs or cystalline size effect of graphite particles occurring in samples [12–15]. However, it is considered that D-band is related to the effects of defects or disordered structures in tubes [12,13]. Thus the ratio of D-band and G-band intensity (ID/IG) can be used to characterize the level of defect growth or disordered structures in CNTs [14,15]. As shown in Table 1, the intensity of D-band (ID) and the ratio of ID/ IG (IG is the intensity of G-band) increase significantly after being etched by hydrogen plasma. It shows wall defects in CNTs increase by the plasma etching. Fig. 6 shows the TEM images of etched CNTs decorated with Pd (sample T7H2 and T7H4). Crystalline Pd can be identified by both the ED and XRD patterns. Pd content of sample B is 20 wt.% tested by an atomic absorption spectrophotometer. The distribution and thickness of deposited Pd is not even. 3.2. Hydrogen uptake experiments of modified CNTs As shown in Fig. 7, the trends of hydrogen uptake curves of the etched CNTs are similar to the original sample (T6). At the beginning of testing, the hydrogen uptake increases rapidly, and reaches the equilibrium in
Fig. 3. HRTEM micrographs of the pretreated CNTs (a) and the etched CNTs (b).
Fig. 5. Raman spectra of the CNT samples.
Table 1 Changes of the ratio of ID/IG in samples determined by Raman spectra
Fig. 4. XRD of the pretreated CNTs (T6) and the etched CNTs (T6H2 and T6H4).
Samples
T6
T6H2
T6H4
ID (cts/s) IG (cts/s) ID/IG
429 589 0.73
468 600 0.78
505 601 0.84
S.-c. Mu et al. / Carbon 44 (2006) 762–767
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Fig. 8. Hydrogen uptake for the CNTs pretreated and decorated with Pd (T7, 0.5 g) and the etched CNTs decorated with Pd (T7H2, 0.5 g; T7H4, 0.5 g) at ambient temperature and pressure of 10.735 MPa.
Fig. 6. TEM images of the etched CNTs decorated with Pd. Crystal Pd distribution on the etched CNTs for sample T7H2 (a) and sample T7H4 (b).
capacity of the etched CNTs reaches ca 1.0 wt.% and ca 1.4 wt.%, respectively, which shows that hydrogen plasma etching is useful to improve the performance of hydrogen storage of CNTs. As shown in Fig. 8, the Pd decorated CNTs show much higher hydrogen uptake capacity than the non-decorated samples. Sample T7 has 3.5 wt.% hydrogen uptake and is similar to the previous results [4]. Sample T7 was pretreated and decorated with Pd, but no etching treatment. Sample T7H2 and T7H4 have ca 4.0 wt.% and ca 4.5 wt.% hydrogen capacity, respectively, and have similar uptake characteristics with sample T7, including the first 50-min uptake plateau, rapid uptake increase region ranging from 50 to150 min, and the slow region (>150 min). The duration of uptake plateau tends to be short for sample T7H2 and T7H4 compared with sample T7, which indicates that the etched CNTs have faster uptake rates. Moreover, the desorption test shows that more than 80% absorbed H2 can be released at ambient temperature. 3.3. Hydrogen uptake mechanism of modified CNTs
Fig. 7. Hydrogen uptake for the pretreated CNTs (0.5 g) and the etched CNTs (0.5 g) vs the duration atambient temperature and pressure of 10.728 MPa.
10–20 min. After that, there are basically no changes which indicate that the hydrogen uptake is end in CNTs. The non-etched CNTs have ca 0.6 wt.% hydrogen uptake capacity under the pressure of 10.728 MPa, which is almost consistent with the previous testing results [3,4]. While after 2 h and 4 h etching processes, the hydrogen uptake
The improved performance of the etched CNTs can be attributed to the increase of wall defects or disordered structures in tubes. The increased wall defects enrich accesses for H2 to interlayers and hollow interiors of tubes (Fig. 9(a) and (b). Supposing the graphitized layers are intact (Fig. 9(a), namely there are not any wall defects embed in graphitized layers, hydrogen has to enter tubes through the open ends. This means it needs huge pressure and/or long time for hydrogen to fill in tubes due to large aspect ratio. While the defects or disordered structures in CNTs increase by etching process (Fig. 9(b), they can supply more accesses for hydrogen to enter tubes. If a mediate pressure is imposed, H2 would fill in tubes resulting in a improved performance of hydrogen storage. This is confirmed by the higher hydrogen storage performance of etched CNTs as shown in Fig. 7. The higher performance of the etched-CNTs decorated with Pd could be attributed to the change of hydrogen diffusion patterns. In previous studies [4], we had presented a model to show the hydrogen uptake mechanism in CNTs
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Fig. 9. Hydrogen uptake mechanism of the non–etched CNTs (a) and the etched CNTs (b).
decorated with Pd: hydrogen molecules are dissociated into atoms or protons on the surface of metallic Pd with lower dissociation energy, these atoms or protons have smaller diameter and lower resistance effects and can easily penetrate Pd layers and tube walls. While hydrogen atoms or protons can be combined to H2 in the interlayer and hollow interiors. However, the inert property of the surface of CNTs determines that it is impossible to let the tubes entirely enwrapped by metallic Pd. For the uncovered surface, it consists of high graphitized layers, hydrogen molecules are very difficult to penetrate them unless there exist some defects in the graphitized layers. Fig. 10 shows the hydrogen uptake model of the decorated etched-CNTs. In this model, hydrogen diffuses into the tubes in the forms of molecules and dissociated atoms/protons: H2 enters tubes via the wall defects and open ends, at the same time, H2 is dissociated into atoms/protons catalyzed by Pd which enwraps the tubes, the atoms/protons penetrate Pd layers and tube walls. These processes improve hydrogen capacity and reduce hydrogen uptake time (see Figs. 7 and 8). However, the increase of defects in graphitized layers should be limited, or the structure of CNTs would be destroyed which leads to lowering of interaction or potential energy between hydrogen molecules and carbon atoms. DillonÕ group [16] found that the heavily destroyed CNTs by long ultrasonic treatment did not absorb hydrogen. Hitherto it is not clear which level of defect growth is optimum to absorb hydrogen. This will be studied in the future.
Fig. 10. Hydrogen uptake mechanism of etched CNTs decorated with Pd (other signs have the same meaning as in Fig. 7).
S.-c. Mu et al. / Carbon 44 (2006) 762–767
4. Conclusions It is feasible to improve the hydrogen storage performance of CNTs through increasing their wall defects or disordered structures by hydrogen plasma etching. The increased defects can supply more accesses for hydrogen entering into interlayers and hollow interiors of tubes. The etched-CNTs decorated with Pd have more hydrogen diffusion patterns in addition to the increased accesses to tubes: H2 is dissociated into atoms/protons by metallic Pd with lower dissociation energy, these atoms/protons easily penetrate tube walls and are combined into H2 in interlayers and hollow interiors. Therefore, the etchedCNTs decorated with Pd can further improve hydrogen storage capacity and accelerate hydrogen uptake. Acknowledgements Financial support by the National Natural Science Foundation of China (NSFC) (40302011) and the Postdoctor Foundation of China (2002031265) are gratefully acknowledged. References [1] Ye Y, Ahn CC, Witham C, et al. Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes. Appl Phys Lett 1999;74(16):2307–9. [2] Li XS, Zhu HW, Ci LH, et al. Effects of structure and surface properties on carbon nanotubesÕ hydrogen storage characteristics. Chin Sci bull 2001;16(46):1358–9.
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[3] Mu SC, Tang HL, Pan M, et al. Synthesis and evaluation on performance of hydrogen storage of multi-walled carbon nanotubes decorated with platinum. J Wuhan Univ Technol—Mater Sci Ed 2003;18(3):33–5. [4] Mu SC, Tang HL, Qian SH, et al. Performance of hydrogen storage of carbon nanotubes decorated with Pd. Trans Nonferous Met Soc China 2004;14(5):996–9. [5] Lee SM, An KH, Lee YH, et al. A hydrogen storage mechanism in single-walled nanotubes. J Am Chem Soc 2001;123:5059–63. [6] Lee SM, Park KS, Choi YC, et al. Hydrogen adsorption and storage in carbon nanotubes. Synth Met 2000;113:209–16. [7] Cheng H, Pez GP, Cooper AC. Mechanism of hydrogen sorption in single-walled carbon nanotubes. J Am Chem Soc 2001;123:5845– 6. [8] Endo M, Kroto HW. Formation of carbon nanofibers. J Phys Chem 1992;96:6941–4. [9] Saito R, Dresselhaus G, Dresselhaus MS. Topological defects in large fullerenes. Chem Phys Lett 1992;195:537–42. [10] Liu C, Fan YY, Liu M, et al. Hydrogen storage in singlewalled carbon nanotubes at room temperature. Science 1999;286: 1127–9. [11] Cardona M, Guntherodt G. Light scatterering in solids. Berlin: Springer-Verlag; 1982, p. 8. [12] Hiura H, Ebbesen TW, Tanigaki K, et al. Raman studies of carbon nanotubes. Chem Phys Lett 1993;202:509–12. [13] Bacsa WS, Ugarte D, Chatelain A, et al. High resolution electron microscopy and inelastic light scattering of purified multishell carbon nanotubes. Phys Rev B 1994;50:15473–6. [14] Saito R, Dresselhas G, Dresselhas MS. Physical properties of carbon nanotubes. London: Imperial College Press; 1998. [15] Kastner J, Pichler T, Kuzmany H, et al. Resonance Raman and infrared spectroscopy of carbon nanotubes. Chem Phys Lett 1994;221:53–8. [16] Dillon AC, Gilbert KEH, Parilla PA, et al. Hydrogen storage in carbon single- wall nanotubes. In: Proceeding of the 2002 US DOE hydrogen program review, NREL/CP-610-32405, 2002.
Hydrogen storage in carbon nanotubes modified by microwave plasma etching and Pd decoration Shi-chun Mu *, Hao-lin Tang, Sheng-hao Qian, Mu Pan, Run-zhang Yuan State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 1 October 2003; accepted 12 September 2005 Available online 25 October 2005
Abstract Microwave plasma etching and Pd decoration methods were employed for the modification of carbon nanotubes (CNTs). The defects on the nanotube wall increased after the etching process as determined from HRTEM observation and Raman measurement. The defects supplied more hydrogen accesses to the interlayers and hollow interiors of CNTs. The results of hydrogen uptake measurements showed that the etched CNTs had higher hydrogen storage capability than that of the original sample at ambient temperature and pressure of 10.728 MPa. Furthermore, the CNTs decorated with Pd showed a hydrogen storage capability of 4.5 wt.%, about three times higher than that of the non-decorated samples. The hydrogen uptake mechanism of the modified CNTs was discussed. 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Carbon composites; Gas storage; Etching; Surface treatment
1. Introduction There are various methods to increase defects in order to obtain more hydrogen accesses to CNTs and to improve the hydrogen storage performance of CNTs. Ye et al. [1] performed hydrogen adsorption on purified SWCNT samples treated with sonication. They observed that the sonicated CNTs contained more wall defects, and the defects could be acted as H2 accesses to the endohedral pores. Li et al. [2] also observed proper defects in tubes were helpful to improve the hydrogen adsorptive capacity. However, they considered that keeping the graphitization was important to bond hydrogen in tubes. In our previous work [3,4], hydrogen storage performance of CNTs decorated with Pt and Pd was studied. The function of Pd was to dissociate hydrogen molecules into protons or atoms to make hydrogen easily penetrate into the interlayers and hollow interiors of CNTs. Hydrogen atoms/protons could bond with carbon atoms by
chemisorption or charge transfer, and form hydrogen molecules in tubes, which is a multi-step hydrogen uptake process as pointed out by Lee et al. [5,6] and Cheng et al. [7]. In this paper, we employed microwave hydrogen plasma to etch the tube walls to create defects as hydrogen accesses to interlayers and interiors of tubes. Because five- or sevennumbered carbon rings may incorporate in the hexagonal network of graphite sheets [8,9], these carbon rings have lower stabilities compared with six-numbered rings and are easily subjected to destruction in an etching process. If the etching is continued, the wall defects in tubes would increase and the hydrogen accesses to tubes will be enhanced. Moreover, the addition effect of Pd to the etched CNTs on the hydrogen storage performance was also investigated. 2. Experimental 2.1. Pretreatment of CNTs
*
Corresponding author. Tel.: +86 27 87665259; fax: +86 27 87879468. E-mail address: [email protected] (S.-c. Mu).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.010
Multi-walled carbon nanotubes (MWCNTs) were synthesized by catalytic decomposition of acetylene over
S.-c. Mu et al. / Carbon 44 (2006) 762–767
cobalt nanoparticles. As-received samples was immersed and refluxed in NaOH solution for 2 h, and subjected to mild sonication for 1 h; then refluxed in a mixed acid (H2SO4:HNO3 = 3:1) for 1 h. After washing with de-ionized water, the sample was dried in vacuum at 393 K, and followed by heat treatment at 973 K under argon atmosphere. The pretreated sample was labeled as sample T6. 2.2. Hydrogen plasma etching Hydrogen plasma etching of CNTs was done in a microwave plasma generator as shown in Fig. 1. The microwave energy spread along rectangle wave-guide in the mode of TE10 and stimulated the gas into uniform plasma ball in the resonant cavity. The output of microwave plasma etching was 0.3–3 kW. To prevent graphitization of CNTs at high temperature, the etching temperature of 973–1273 K was considered. The gas pressure was 6.0 · 102– 6.0 · 103 Pa. The etching gas was hydrogen, N2, Ar or their mixture was used as protecting gas. Sample T6 was put into a silica window, and wetted by de-ionized water to prevent the sample from being flown in degassing and etching processes. The hydrogen etching of the sample was conducted at 2 kW, 1073–1193 K and 6.0 · 102 Pa under Ar gas for 2 h and 4 h, the etched samples were labeled as T6H2 and T6H4, respectively. The morphography of the CNTs before and after etching was observed by HRTEM (JEM-2010FEF). Samples were prepared by suspending the powder in ethanol in ultrasonic bath for 15 min and dropped onto microporous carbon film attached on Cu grids. Raman spectrum (U-1000 Ramanor) was employed to determine the defect growth in tubes through comparing D-band and G-band intensity. The wavelength of laser was 487.9860 nm and the power was 150 mV. The crystalline changes in CNTs were tested by XRD (NICOLET 170SX). 2.3. Synthesis of the etched CNTs decorated with Pd
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and drying, the decorated CNTs were treated at 573 K under argon atmosphere. The results were labeled as T7, T7H2, and T7H4 respectively. TEM was performed to characterize the samples. 2.4. Hydrogen uptake of modified CNTs The hydrogen uptake and release performance of samples was measured by pressure differentia method used by relative references [3,10]. As shown in Fig. 2, a dried sample was put into the sample cell and degassed in situ under high vacuum at 573 K for 2 h; hydrogen (99.9999%) was loaded when the cell was cooled down to ambient temperature; as the hydrogen pressure in the cell was to ca 10.7 MPa, close the cell, hydrogen uptake curves were output automatically by a computer; when the hydrogen pressure reached balancing for 2–3 h, the cell was open for hours for hydrogen release, take out the sample and weigh it; finally, the sample was put into the cell again, repeat the step one, then the sample was taken out and weighed. 3. Results and discussion 3.1. Characterization of the pretreated and the modified CNTs HRTEM images of the pretreated CNTs are shown in Fig. 3(a). The pretreated sample contained more than 95% CNTs determined by TGA. The outer diameter of nanotubes ranges from 20 to 40 nm, and the inner diameter ranges from 8 to 15 nm. There are thin amorphous carbon layers adhere to the nanotube walls, while the nanotube walls possess high graphitization and have 0.34 nm of facet (0 0 2). The number of graphite layers is about 10–40. As shown in Fig. 3(b), after etching, CNTs have less numbers of graphite layers ranging from 10 to 30 nm, but the d (0 0 2) is consistent with the pretreated CNTs determined by XRD (Fig. 4). The thickness of the amorphous carbon layers is larger than that of the graphitilized layer,
Sample (0.5 g) T6, T6H2 and T6H4 was sonicated for 1 h in 200ml PdCl2 (1 g/l), respectively. Then NaBH4 (1 mol/l) as a reduction agent was added at a speed of 0.3 ml/min until the solution was colorless. After washing
Fig. 1. Microwave plasma generator: (1) micro-wave source, (2) closed loop control, (3) three-bolt adjustor, (4) coupling antenna, (5) plasma ball, (6) resonant cavity and (7) inlet of inert gas.
Fig. 2. Schematic of an instrument measuring the amount of hydrogen uptake for CNTs: (1) sample cell, (2) heating jacket (3) high accuracy sensor, (4) gas valve, (5) computer, (6) pressure gauge, (7) vacuum pump and (8) hydrogen purification train.
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which indicates the increased defects or disordered structures exist in tubes. Fig. 5 shows Raman spectra of the pretreated and the etched CNT samples. The G-band at 1580 cm 1 in CNTs is similar to the peak G of HOPG produced by the graphitic basal plane [11,12]. The appearance of G-band indicates that there are crystal graphite layers in tube structures. The Raman mode of D-band at about 1364 cm 1 is attributed to the wall defects in CNTs or cystalline size effect of graphite particles occurring in samples [12–15]. However, it is considered that D-band is related to the effects of defects or disordered structures in tubes [12,13]. Thus the ratio of D-band and G-band intensity (ID/IG) can be used to characterize the level of defect growth or disordered structures in CNTs [14,15]. As shown in Table 1, the intensity of D-band (ID) and the ratio of ID/ IG (IG is the intensity of G-band) increase significantly after being etched by hydrogen plasma. It shows wall defects in CNTs increase by the plasma etching. Fig. 6 shows the TEM images of etched CNTs decorated with Pd (sample T7H2 and T7H4). Crystalline Pd can be identified by both the ED and XRD patterns. Pd content of sample B is 20 wt.% tested by an atomic absorption spectrophotometer. The distribution and thickness of deposited Pd is not even. 3.2. Hydrogen uptake experiments of modified CNTs As shown in Fig. 7, the trends of hydrogen uptake curves of the etched CNTs are similar to the original sample (T6). At the beginning of testing, the hydrogen uptake increases rapidly, and reaches the equilibrium in
Fig. 3. HRTEM micrographs of the pretreated CNTs (a) and the etched CNTs (b).
Fig. 5. Raman spectra of the CNT samples.
Table 1 Changes of the ratio of ID/IG in samples determined by Raman spectra
Fig. 4. XRD of the pretreated CNTs (T6) and the etched CNTs (T6H2 and T6H4).
Samples
T6
T6H2
T6H4
ID (cts/s) IG (cts/s) ID/IG
429 589 0.73
468 600 0.78
505 601 0.84
S.-c. Mu et al. / Carbon 44 (2006) 762–767
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Fig. 8. Hydrogen uptake for the CNTs pretreated and decorated with Pd (T7, 0.5 g) and the etched CNTs decorated with Pd (T7H2, 0.5 g; T7H4, 0.5 g) at ambient temperature and pressure of 10.735 MPa.
Fig. 6. TEM images of the etched CNTs decorated with Pd. Crystal Pd distribution on the etched CNTs for sample T7H2 (a) and sample T7H4 (b).
capacity of the etched CNTs reaches ca 1.0 wt.% and ca 1.4 wt.%, respectively, which shows that hydrogen plasma etching is useful to improve the performance of hydrogen storage of CNTs. As shown in Fig. 8, the Pd decorated CNTs show much higher hydrogen uptake capacity than the non-decorated samples. Sample T7 has 3.5 wt.% hydrogen uptake and is similar to the previous results [4]. Sample T7 was pretreated and decorated with Pd, but no etching treatment. Sample T7H2 and T7H4 have ca 4.0 wt.% and ca 4.5 wt.% hydrogen capacity, respectively, and have similar uptake characteristics with sample T7, including the first 50-min uptake plateau, rapid uptake increase region ranging from 50 to150 min, and the slow region (>150 min). The duration of uptake plateau tends to be short for sample T7H2 and T7H4 compared with sample T7, which indicates that the etched CNTs have faster uptake rates. Moreover, the desorption test shows that more than 80% absorbed H2 can be released at ambient temperature. 3.3. Hydrogen uptake mechanism of modified CNTs
Fig. 7. Hydrogen uptake for the pretreated CNTs (0.5 g) and the etched CNTs (0.5 g) vs the duration atambient temperature and pressure of 10.728 MPa.
10–20 min. After that, there are basically no changes which indicate that the hydrogen uptake is end in CNTs. The non-etched CNTs have ca 0.6 wt.% hydrogen uptake capacity under the pressure of 10.728 MPa, which is almost consistent with the previous testing results [3,4]. While after 2 h and 4 h etching processes, the hydrogen uptake
The improved performance of the etched CNTs can be attributed to the increase of wall defects or disordered structures in tubes. The increased wall defects enrich accesses for H2 to interlayers and hollow interiors of tubes (Fig. 9(a) and (b). Supposing the graphitized layers are intact (Fig. 9(a), namely there are not any wall defects embed in graphitized layers, hydrogen has to enter tubes through the open ends. This means it needs huge pressure and/or long time for hydrogen to fill in tubes due to large aspect ratio. While the defects or disordered structures in CNTs increase by etching process (Fig. 9(b), they can supply more accesses for hydrogen to enter tubes. If a mediate pressure is imposed, H2 would fill in tubes resulting in a improved performance of hydrogen storage. This is confirmed by the higher hydrogen storage performance of etched CNTs as shown in Fig. 7. The higher performance of the etched-CNTs decorated with Pd could be attributed to the change of hydrogen diffusion patterns. In previous studies [4], we had presented a model to show the hydrogen uptake mechanism in CNTs
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Fig. 9. Hydrogen uptake mechanism of the non–etched CNTs (a) and the etched CNTs (b).
decorated with Pd: hydrogen molecules are dissociated into atoms or protons on the surface of metallic Pd with lower dissociation energy, these atoms or protons have smaller diameter and lower resistance effects and can easily penetrate Pd layers and tube walls. While hydrogen atoms or protons can be combined to H2 in the interlayer and hollow interiors. However, the inert property of the surface of CNTs determines that it is impossible to let the tubes entirely enwrapped by metallic Pd. For the uncovered surface, it consists of high graphitized layers, hydrogen molecules are very difficult to penetrate them unless there exist some defects in the graphitized layers. Fig. 10 shows the hydrogen uptake model of the decorated etched-CNTs. In this model, hydrogen diffuses into the tubes in the forms of molecules and dissociated atoms/protons: H2 enters tubes via the wall defects and open ends, at the same time, H2 is dissociated into atoms/protons catalyzed by Pd which enwraps the tubes, the atoms/protons penetrate Pd layers and tube walls. These processes improve hydrogen capacity and reduce hydrogen uptake time (see Figs. 7 and 8). However, the increase of defects in graphitized layers should be limited, or the structure of CNTs would be destroyed which leads to lowering of interaction or potential energy between hydrogen molecules and carbon atoms. DillonÕ group [16] found that the heavily destroyed CNTs by long ultrasonic treatment did not absorb hydrogen. Hitherto it is not clear which level of defect growth is optimum to absorb hydrogen. This will be studied in the future.
Fig. 10. Hydrogen uptake mechanism of etched CNTs decorated with Pd (other signs have the same meaning as in Fig. 7).
S.-c. Mu et al. / Carbon 44 (2006) 762–767
4. Conclusions It is feasible to improve the hydrogen storage performance of CNTs through increasing their wall defects or disordered structures by hydrogen plasma etching. The increased defects can supply more accesses for hydrogen entering into interlayers and hollow interiors of tubes. The etched-CNTs decorated with Pd have more hydrogen diffusion patterns in addition to the increased accesses to tubes: H2 is dissociated into atoms/protons by metallic Pd with lower dissociation energy, these atoms/protons easily penetrate tube walls and are combined into H2 in interlayers and hollow interiors. Therefore, the etchedCNTs decorated with Pd can further improve hydrogen storage capacity and accelerate hydrogen uptake. Acknowledgements Financial support by the National Natural Science Foundation of China (NSFC) (40302011) and the Postdoctor Foundation of China (2002031265) are gratefully acknowledged. References [1] Ye Y, Ahn CC, Witham C, et al. Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes. Appl Phys Lett 1999;74(16):2307–9. [2] Li XS, Zhu HW, Ci LH, et al. Effects of structure and surface properties on carbon nanotubesÕ hydrogen storage characteristics. Chin Sci bull 2001;16(46):1358–9.
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