- Email: [email protected]
An approach to carbon nanotubes with high surface area and large pore volume
Microporous and Mesoporous Materials 100 (2007) 1–5 www.elsevier.com/locate/micromeso
An approach to carbon nanotubes with high surface area and large pore volume Jun Jie Niu *, Jian Nong Wang, Ying Jiang, Lian Feng Su, Jie Ma School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 17 June 2006; received in revised form 14 September 2006; accepted 2 October 2006 Available online 15 November 2006
Abstract Multi-walled carbon nanotubes (MWCNTs) with high purity were synthesized by a floating catalyst technique suitable for large-scale production. Ethanol and ferrocene were used as the carbon and catalyst precursors, respectively. Thiophene was added to assist the growth. The pristine sample was chemically activated by using KOH at high temperature and the specific surface area increased drastically from 65 m2/g to 830 m2/g while the graphite structure was maintained. Furthermore, the mean pore diameter was decreased from 21.15 nm to 7.68 nm and pore volume increased from 0.3429 cm3/g to 1.5950 cm3/g. MWCNTs with a high surface area and pore volume would have promising applications in many fields. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Carbon nanotubes; Activation; Specific surface area
1. Introduction It is now well known that carbon nanotubes (CNTs) could have applications in many fields such as electromechanical actuators [1], hydrogen storage [2], field emission display [3], and supercapacitors [4]. Particularly the applications in catalyst support, electrochemical super capacitor, gas and energy storage have attracted extensive attentions [5,6]. For these applications, CNTs are required to possess a high specific surface area (SSA). Nevertheless, most CNTs currently reported in open literatures generally have low SSAs in the range of 100–300 m2/g. This inferior property has hindered the potential applications of CNTs. In order to improve the properties of CNTs, omnifarious synthesis techniques [7–10] and post-treatment approaches have been applied [11,12]. Amongst these methods, modification via KOH activation appears to be an effective route to improve the SSA and pore size distribution [13,14]. Kim et al. investigated the development of porous CNTs by varying the activation temperature of *
Corresponding author. Tel.: +86 21 62932050. E-mail address: [email protected] (J.J. Niu).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.009
KOH. They got a SSA of 650 m2 g 1 at 900 °C [15]. Beguin et al. reported a two-step process for producing opened CNTs with a SSA of 310 m2/g [16]. Raymundo-Pinero et al. studied the KOH/NaOH activation mechanism of CNTs grown by catalytic pyrolysis of acetylene [14,17]. Although progress has been made in previous studies, an efficient approach to CNTs with a high SSA is still lacking and needs further investigation. Herein, we report a simple floating CVD process to achieve continuous production of carbon nanotubes with high purity. First, ethanol and ferrocene were used as the carbon and catalyst precursors, respectively. We then report our study of the chemical activation of the as-received CNTs by using KOH. As will be shown, the SSA of CNTs could be enhanced drastically from 65 m2/g to 830 m2/g with the graphitic structure and tubular morphology maintained. 2. Experimental CNTs were synthesized by spray pyrolysis through a floating-CVD technique. The experimental setup consisted of an electric furnace, a quartz tube with a diameter of
2
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
30 cm and length of 130 cm, a sample collector and a quartz capillary used for spraying. Ferrocene and thiophene were dissolved in ethanol at a given concentration. The concentration of ferrocene was 10 g/l and that of thiophene 50 ml/l. In a typical experiment, the quartz tube was flushed with Ar first in order to eliminate oxygen from the reaction chamber, and then heated to a temperature of 1100 °C. Ar flow was initiated at a rate of 100 l/h. Subsequently, the ethanol solution dissolved with ferrocene and thiophene was supplied by an electric squirming pump. The supplying rate was adjusted to be 50 ml/h. Pyrolysis products were collected in a glass bottle connected to the quartz tube. After reaction for 2 h, the as-grown samples were purified in 1:1 HNO3 at 120 °C for about 3 h to remove metal particles. Subsequently the sample was dried sufficiently for the next study. The activating agent of KOH was introduced with purificated CNTs (we called pristine CNTs) amongst extremely dilute de-ionized water with a mass ratio of 1:5 (CNTs:KOH). Treatment temperature was carried at 850 °C in an Ar flow of 180 ml/min and reaction time was held 1.5 h. After activation, the materials were washed with dilute acid and de-ionized water to remove the alkali compounds and impurities. Morphology of pristine and modified samples was observed by transmission electron microscopy (TEM, JEM100 and JEM2010), respectively. The crystalline
structure was analyzed by X-ray diffraction using CuKa radiation (XRD, D8Advance, Bruker). N2 adsorption/ desorption experiments using BELSORP measuring instruments (BELSORP-mini, JAPAN, INC.) were carried out in order to investigate the porosity. All the samples were outgassed at 250 °C under nitrogen flow for about 4 h prior to measurement. The nitrogen adsorption/desorption data were recorded at the liquid nitrogen temperature (77 K). The specific surface area (SSA) was calculated using the Brunauer–Emmett–Teller (BET) equation. The micropore radius distributions were determined by micropore analysis method (MP method). And the mesopore diameter distribution was determined by Barrett–Joyner–Halenda (BJH) method. Details about these analytical methods can be found in Ref. [18–20]. Finally, pore volumes were estimated to be the liquid volume of adsorption (N2) at a relative pressure of 0.99. 3. Results and discussion Fig. 1a displays a TEM image of pristine CNTs with well-organized tube structure. It is seen that the diameter is 20–40 nm and length of several hundreds of nanometers to one micrometer, respectively. The crystal structure is studied by XRD data which indicate a well graphite nature (Fig. 2a). By evaluation, the CNTs grew fast and the transformation ratio from ethanol to CNTs was as high as 95%
Fig. 1. TEM images of pristine (a) and activated (b, c) CNTs.
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
b
a
3
C (002)
60
160
C (002)
Intensity
Intensity
120
80
C (101)
40
C (100) C (100) C (101)
40
20
C (004)
0 20
40
60
20
80
40
60
80
2 Theta (degree)
2 Theta (degree)
Fig. 2. XRD data of the pristine (a) and activated (b) CNTs.
1000
Amount adsorbed /cm 3(STP)g-1
approximately. More importantly, CNTs were carried out of the reaction chamber and collected in a glass bottle. Thus, the whole process, including the supply of reactants and the collection of product, was continuous. This unique feature enables the present approach suitable for largescale production. Thus, fabrication cost is low and can conveniently supply plenty of samples for future applications. Considering above results, we can deduce that the pristine CNTs have a good tubular structure and smooth shell with few defects. However, after activation treatment, morphology and structure presented a visible alteration. TEM morphologies of the activated CNTs are showed in Fig. 1b and c. Compared with pristine sample in Fig. 1a, the CNTs’ length is obviously shortened to be about 200–300 nm. Furthermore, part of the hollow tubular structure is destroyed and a large quantity of defects is produced. The tips of preserved tubes are mainly opened and many flaky apertures are generated on the surface (the arrows in Fig. 1b and c). This structure on the outer surface probably is due to the deposition of pyrolytic carbon [17]. Although defects were introduced during the activation, the crystalline nature of CNTs was basically preserved. From the XRD data in Fig. 2b, peaks of C (0 0 2), C (1 0 1) and C (1 0 0) with relatively high intensity and symmetry are clearly observed. This observation suggests that the graphite structure was remained even after strong activation reaction. The N2 adsorption/desorption isotherms of pristine and activated CNTs are presented in Fig. 3. As can be seen from this figure, both micropore (diameter < 2 nm) and mesopore (diameter: 2–100 nm) structures are present in the pristine and activated CNTs. The gradual increase from p/p0 > 0.2 is attributed to the wide range of mesopores. In comparison, the N2 adsorption/desorption amount of activated CNTs is considerably higher than that of unactivated sample whatever at low or high pressure. A hysteresis is observed in line b while this is almost absent in line a. The hysteresis is attributed to the well-known capillary condensation and normally due to the intertubular structure amongst samples as well as reported in the litera-
pristine CNTs/absorption pristine CNTs/desorption
800
activated CNTs/absorption
b
activated CNTs/desorption 600
400
a
200
0 0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 3. N2 adsorption/desorption isotherms of pristine and activated CNTs.
tures [15,21]. After activation, not only the tube tip was opened, but also many pore structures with small size were generated (see the arrows in Fig. 1b and c). Especially, the uptrend of hysteresis appears at lower pressures of p/p0 > 0.4 than that in Ref. [15] which began at p/p0 > 0.8. This implies that the CNTs after the present activation possessed more small pores and could give rise to a high surface area. The detailed features of micropores and mesopores analyzed by the BET equation and T-plot method are presented in Table 1. As can be seen from the table, the total SSA and pore volume of activated CNTs reach 830 m2/g and 1.5950 cm3/g, respectively. These values are drastically increased from those for pristine CNTs by about 13 and 5 times, respectively. Such increases correspond to a decrease in mean pore diameter from 21.15 to 7.68 nm. Moreover, the micropore area and volume are improved almost by 70 and 90 times, respectively. Despites the mesopore surface area and volume increase, the considerable amount of micropores obviously takes a dominant role. This also can be shown from the ratios of pore area
4
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
Table 1 Pore structures of the pristine and activated CNTs Sample
Total surface area (m2/g)
Total pore volume (cm3/g)
Mean pore diameter (nm)
Micropore area (m2/g)
External area (mesopore mainly) (m2/g)
Micropore volume (cm3/g)
Mesopore (mainly) volume (cm3/g)
Pristine CNTs Activated CNTs Ratio of activated and pristine CNTs
64.8 830.0 12.8
0.3429 1.5950 4.65
21.15 7.68 0.36
6.1 421.0 69
58.7 409.0 7
0.0025 0.2233 89
0.3404 1.3717 4
150
pristine CNTs activated CNTs
dVp /drp
100
10.5 nm
50
0 1
2
5
10
20
30
40
50
Pore radius (rp )/nm
Fig. 4. Pore size (radius) distribution of pristine and activated CNTs obtained by BJH method.
1500
pristine CNTs activated CNTs
1000
dVp /ddp
and volume to total area and volume. The ratio of micropore area to total surface area is enhanced from 9% to 51% and the ratio of volume to total pore volume from 0.7% to 14% while the corresponding ratios for mesopore are decreased (Table 1). The mesopore and micropore size distributions by applying the BJH and MP methods are shown in Figs. 4 and 5, respectively. As can be seen from Fig. 4, the mesopore radius (rp) distribution of the activated CNTs is similar with that of the pristine sample in the region of tens of nanometers. The stronger peak is located at rp = 10.5 nm approximately. This observation demonstrates that CNTs maintained at least partly tubular structure even after activation. The big difference lies in that the volume distribution (dVp/drp) of activated sample abruptly increases, especially near to the micropore region, whereas the pristine sample remains within a very low level. This indicates the presence of plenty of micropores and mesopores after KOH modification. The actual pore diameter distribution of micropore can be obtained from the MP-plot as shown in Fig. 5. For the active action is not uniform, the micropore diameter shows a distribution range while the majority concentrates in 0.9 nm (see Fig. 5). Whatever the line intensity and smallest diameter, activated CNTs both have a large improvement compared with pristine sample. This result also suggests that a great deal of micropores was generated by using the KOH activation.
0.6nm
500
0.8nm
0 0.5
1.0
1.5
2.0
Pore diameter (dp)/nm Fig. 5. Pore size (diameter) distribution of pristine and activated CNTs obtained by MP method.
It is well-known that CNTs activation via KOH is a high-efficient technique for producing a large scale of porous structure. The controlling factors include KOH/CNTs ratio, activation temperature, reaction time and gas flow rate [13]. The activation mechanism is normally suggested to include independent hydroxide and redox processes during the reaction [17,22]. The overall reaction can be summarized as: C + 6KOH M 2K + 3H2 + 2K2CO3. The generated potassium will be intercalated between the graphitic layers [17] or form graphite–potassium intercalation compounds [23]. Thus, a few of graphic structure will be destroyed and generates quantity of micropores and mesopores even with open apertures (Figs. 1 and 3). This shows that the structure of activated CNTs is porous like the activated carbon. A quantity of inner surface in the porous structure leads to a relatively high total SSA. On the other hand, the tips of original CNTs are almost closed and thus the inner tube surface could not contribute to the SSA. That the very long CNTs entangle each other also will reduce the useful surface area. After activation, the entangled CNTs will be shortened and the dispersion is strongly increased. The most important is that the closed tips are opened and the wall thickness may be decreased. All of the above mentioned cases induce an obvious improvement in total SSA. Furthermore, a plenty of surface defects will be produced anywhere among the CNTs. Consequently, the variations in CNTs caused by the activation intensively enlarged the SSA and pore volume as shown in Table 1.
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
The CNTs with a high SSA will intensively improve the dispersion of catalyst such as platinum and then optimize catalytic characteristics. More importantly, CNTs with well-maintained graphic structure ensure the excellent conductivity in electrode application. Therefore, the present CNTs with a high SSA can be used as catalyst carrier and supercapacitor, which have an important application in fields of novel fuel cell and energy storage devices. 4. Conclusion In summary, multi-walled CNTs were continuously fabricated by a floating CVD technique. The pristine CNTs were chemically modified by using KOH at high temperature. The CNTs with a high SSA of 830 m2/g and pore volume of 1.5950 cm3/g were achieved while the graphite structure was preserved. Such activated CNTs with a high SSA could have extensive applications in many fields. Acknowledgments This work was supported by the Shanghai-Applied Materials Research and Development Fund (Item No. 06SA06), National Natural Science Foundation of China, and Youth Teacher Fund of Shanghai Jiaotong University (A2306B). We would like to thank Instrumental Analysis Center of Shanghai Jiaotong University, for their great helps in measurements. References [1] R.H. Baughman, C. Cui, A.A. Zakhidov, Z. Iqbal, J.N. Barisci, G.M. Spinks, Science 284 (1999) 1340.
5
[2] X. Li, H. Zhu, L. Ci, C. Xu, Z. Mao, B. Wei, Carbon 39 (2001) 2077. [3] A.G. Rinzler, J.H. Hafner, P. Nikolaev, L. Lou, S.G. Kim, D. Tomanek, Science 269 (1995) 1550. [4] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennent, Appl. Phys. Lett. 70 (1997) 1480. [5] A.M. Zhang, J.L. Dong, Q.H. Xu, H.K. Rhee, X.L. Li, Catal. Today 93–95 (2004) 347. [6] C.J. Liu, T.W. Wu, L.S. Hsu, C.J. Su, C.C. Wang, F.S. Shieu, Carbon 42 (2004) 2635. [7] Y. Soneda, L. Duclaux, F. Beguin, Carbon 40 (2002) 965. [8] H. Okuno, J.P. Issi, J.C. Charlier, Carbon 43 (2005) 864. [9] G. Yu, J. Gong, D. Zhu, S. He, Z. Zhu, Carbon 43 (2005) 3015. [10] L.F. Su, J.N. Wang, F. Yu, Z.M. Sheng, H. Chang, C. Pak, Chem. Phys. Lett. 420 (2006) 421. [11] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature 372 (1994) 159. [12] F. Rodriguez-Reinoso, M. Molina-Sabio, M.T. Gonzalez, Carbon 33 (1995) 15. [13] Q. Jiang, Y. Zhao, Micropor. Mesopor. Mater. 76 (2004) 215. [14] E. Raymundo-Pinero, D. Cazorla-Amoros, A. Linares-Solano, S. Delpeux, E. Frackowiak, K. Szostak, Carbon 40 (2002) 1614. [15] S.M. Lee, S.C. Lee, J.H. Jung, H.J. Kim, Chem. Phys. Lett. 416 (2005) 251. [16] S. Delpeux, K. Szostak, E. Frackowiak, F. Beguin, Chem. Phys. Lett. 404 (2005) 374. [17] E. Raymundo-Pinero, P. Azais, T. Cacciaguerra, D. Cazorla-Amoros, A. Linares-Solano, F. Beguin, Carbon 43 (2005) 786. [18] R.S. Mikhail, S. Brunauer, E.E. Bodor, J. Colloid Interface Sci. 26 (1968) 45. [19] B.C. Lippens, J.H. Boer, J. Catal. 4 (1965) 319. [20] E.P. Barrett, P.B. Joyner, P. Halenda, J. Amer. Chem. Soc. 73 (1951) 373. [21] M. Eswaramoorthy, R. Sen, C.N.R. Rao, Chem. Phys. Lett. 304 (1999) 207. [22] M.A. Lillo-Rodenas, J. Juan-Juan, D. Cazorla-Amoros, A. LinaresSolano, Carbon 42 (2004) 1371. [23] R. Xue, Z. Shen, Carbon 41 (2003) 1862.
An approach to carbon nanotubes with high surface area and large pore volume Jun Jie Niu *, Jian Nong Wang, Ying Jiang, Lian Feng Su, Jie Ma School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 17 June 2006; received in revised form 14 September 2006; accepted 2 October 2006 Available online 15 November 2006
Abstract Multi-walled carbon nanotubes (MWCNTs) with high purity were synthesized by a floating catalyst technique suitable for large-scale production. Ethanol and ferrocene were used as the carbon and catalyst precursors, respectively. Thiophene was added to assist the growth. The pristine sample was chemically activated by using KOH at high temperature and the specific surface area increased drastically from 65 m2/g to 830 m2/g while the graphite structure was maintained. Furthermore, the mean pore diameter was decreased from 21.15 nm to 7.68 nm and pore volume increased from 0.3429 cm3/g to 1.5950 cm3/g. MWCNTs with a high surface area and pore volume would have promising applications in many fields. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Carbon nanotubes; Activation; Specific surface area
1. Introduction It is now well known that carbon nanotubes (CNTs) could have applications in many fields such as electromechanical actuators [1], hydrogen storage [2], field emission display [3], and supercapacitors [4]. Particularly the applications in catalyst support, electrochemical super capacitor, gas and energy storage have attracted extensive attentions [5,6]. For these applications, CNTs are required to possess a high specific surface area (SSA). Nevertheless, most CNTs currently reported in open literatures generally have low SSAs in the range of 100–300 m2/g. This inferior property has hindered the potential applications of CNTs. In order to improve the properties of CNTs, omnifarious synthesis techniques [7–10] and post-treatment approaches have been applied [11,12]. Amongst these methods, modification via KOH activation appears to be an effective route to improve the SSA and pore size distribution [13,14]. Kim et al. investigated the development of porous CNTs by varying the activation temperature of *
Corresponding author. Tel.: +86 21 62932050. E-mail address: [email protected] (J.J. Niu).
1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.009
KOH. They got a SSA of 650 m2 g 1 at 900 °C [15]. Beguin et al. reported a two-step process for producing opened CNTs with a SSA of 310 m2/g [16]. Raymundo-Pinero et al. studied the KOH/NaOH activation mechanism of CNTs grown by catalytic pyrolysis of acetylene [14,17]. Although progress has been made in previous studies, an efficient approach to CNTs with a high SSA is still lacking and needs further investigation. Herein, we report a simple floating CVD process to achieve continuous production of carbon nanotubes with high purity. First, ethanol and ferrocene were used as the carbon and catalyst precursors, respectively. We then report our study of the chemical activation of the as-received CNTs by using KOH. As will be shown, the SSA of CNTs could be enhanced drastically from 65 m2/g to 830 m2/g with the graphitic structure and tubular morphology maintained. 2. Experimental CNTs were synthesized by spray pyrolysis through a floating-CVD technique. The experimental setup consisted of an electric furnace, a quartz tube with a diameter of
2
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
30 cm and length of 130 cm, a sample collector and a quartz capillary used for spraying. Ferrocene and thiophene were dissolved in ethanol at a given concentration. The concentration of ferrocene was 10 g/l and that of thiophene 50 ml/l. In a typical experiment, the quartz tube was flushed with Ar first in order to eliminate oxygen from the reaction chamber, and then heated to a temperature of 1100 °C. Ar flow was initiated at a rate of 100 l/h. Subsequently, the ethanol solution dissolved with ferrocene and thiophene was supplied by an electric squirming pump. The supplying rate was adjusted to be 50 ml/h. Pyrolysis products were collected in a glass bottle connected to the quartz tube. After reaction for 2 h, the as-grown samples were purified in 1:1 HNO3 at 120 °C for about 3 h to remove metal particles. Subsequently the sample was dried sufficiently for the next study. The activating agent of KOH was introduced with purificated CNTs (we called pristine CNTs) amongst extremely dilute de-ionized water with a mass ratio of 1:5 (CNTs:KOH). Treatment temperature was carried at 850 °C in an Ar flow of 180 ml/min and reaction time was held 1.5 h. After activation, the materials were washed with dilute acid and de-ionized water to remove the alkali compounds and impurities. Morphology of pristine and modified samples was observed by transmission electron microscopy (TEM, JEM100 and JEM2010), respectively. The crystalline
structure was analyzed by X-ray diffraction using CuKa radiation (XRD, D8Advance, Bruker). N2 adsorption/ desorption experiments using BELSORP measuring instruments (BELSORP-mini, JAPAN, INC.) were carried out in order to investigate the porosity. All the samples were outgassed at 250 °C under nitrogen flow for about 4 h prior to measurement. The nitrogen adsorption/desorption data were recorded at the liquid nitrogen temperature (77 K). The specific surface area (SSA) was calculated using the Brunauer–Emmett–Teller (BET) equation. The micropore radius distributions were determined by micropore analysis method (MP method). And the mesopore diameter distribution was determined by Barrett–Joyner–Halenda (BJH) method. Details about these analytical methods can be found in Ref. [18–20]. Finally, pore volumes were estimated to be the liquid volume of adsorption (N2) at a relative pressure of 0.99. 3. Results and discussion Fig. 1a displays a TEM image of pristine CNTs with well-organized tube structure. It is seen that the diameter is 20–40 nm and length of several hundreds of nanometers to one micrometer, respectively. The crystal structure is studied by XRD data which indicate a well graphite nature (Fig. 2a). By evaluation, the CNTs grew fast and the transformation ratio from ethanol to CNTs was as high as 95%
Fig. 1. TEM images of pristine (a) and activated (b, c) CNTs.
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
b
a
3
C (002)
60
160
C (002)
Intensity
Intensity
120
80
C (101)
40
C (100) C (100) C (101)
40
20
C (004)
0 20
40
60
20
80
40
60
80
2 Theta (degree)
2 Theta (degree)
Fig. 2. XRD data of the pristine (a) and activated (b) CNTs.
1000
Amount adsorbed /cm 3(STP)g-1
approximately. More importantly, CNTs were carried out of the reaction chamber and collected in a glass bottle. Thus, the whole process, including the supply of reactants and the collection of product, was continuous. This unique feature enables the present approach suitable for largescale production. Thus, fabrication cost is low and can conveniently supply plenty of samples for future applications. Considering above results, we can deduce that the pristine CNTs have a good tubular structure and smooth shell with few defects. However, after activation treatment, morphology and structure presented a visible alteration. TEM morphologies of the activated CNTs are showed in Fig. 1b and c. Compared with pristine sample in Fig. 1a, the CNTs’ length is obviously shortened to be about 200–300 nm. Furthermore, part of the hollow tubular structure is destroyed and a large quantity of defects is produced. The tips of preserved tubes are mainly opened and many flaky apertures are generated on the surface (the arrows in Fig. 1b and c). This structure on the outer surface probably is due to the deposition of pyrolytic carbon [17]. Although defects were introduced during the activation, the crystalline nature of CNTs was basically preserved. From the XRD data in Fig. 2b, peaks of C (0 0 2), C (1 0 1) and C (1 0 0) with relatively high intensity and symmetry are clearly observed. This observation suggests that the graphite structure was remained even after strong activation reaction. The N2 adsorption/desorption isotherms of pristine and activated CNTs are presented in Fig. 3. As can be seen from this figure, both micropore (diameter < 2 nm) and mesopore (diameter: 2–100 nm) structures are present in the pristine and activated CNTs. The gradual increase from p/p0 > 0.2 is attributed to the wide range of mesopores. In comparison, the N2 adsorption/desorption amount of activated CNTs is considerably higher than that of unactivated sample whatever at low or high pressure. A hysteresis is observed in line b while this is almost absent in line a. The hysteresis is attributed to the well-known capillary condensation and normally due to the intertubular structure amongst samples as well as reported in the litera-
pristine CNTs/absorption pristine CNTs/desorption
800
activated CNTs/absorption
b
activated CNTs/desorption 600
400
a
200
0 0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 3. N2 adsorption/desorption isotherms of pristine and activated CNTs.
tures [15,21]. After activation, not only the tube tip was opened, but also many pore structures with small size were generated (see the arrows in Fig. 1b and c). Especially, the uptrend of hysteresis appears at lower pressures of p/p0 > 0.4 than that in Ref. [15] which began at p/p0 > 0.8. This implies that the CNTs after the present activation possessed more small pores and could give rise to a high surface area. The detailed features of micropores and mesopores analyzed by the BET equation and T-plot method are presented in Table 1. As can be seen from the table, the total SSA and pore volume of activated CNTs reach 830 m2/g and 1.5950 cm3/g, respectively. These values are drastically increased from those for pristine CNTs by about 13 and 5 times, respectively. Such increases correspond to a decrease in mean pore diameter from 21.15 to 7.68 nm. Moreover, the micropore area and volume are improved almost by 70 and 90 times, respectively. Despites the mesopore surface area and volume increase, the considerable amount of micropores obviously takes a dominant role. This also can be shown from the ratios of pore area
4
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
Table 1 Pore structures of the pristine and activated CNTs Sample
Total surface area (m2/g)
Total pore volume (cm3/g)
Mean pore diameter (nm)
Micropore area (m2/g)
External area (mesopore mainly) (m2/g)
Micropore volume (cm3/g)
Mesopore (mainly) volume (cm3/g)
Pristine CNTs Activated CNTs Ratio of activated and pristine CNTs
64.8 830.0 12.8
0.3429 1.5950 4.65
21.15 7.68 0.36
6.1 421.0 69
58.7 409.0 7
0.0025 0.2233 89
0.3404 1.3717 4
150
pristine CNTs activated CNTs
dVp /drp
100
10.5 nm
50
0 1
2
5
10
20
30
40
50
Pore radius (rp )/nm
Fig. 4. Pore size (radius) distribution of pristine and activated CNTs obtained by BJH method.
1500
pristine CNTs activated CNTs
1000
dVp /ddp
and volume to total area and volume. The ratio of micropore area to total surface area is enhanced from 9% to 51% and the ratio of volume to total pore volume from 0.7% to 14% while the corresponding ratios for mesopore are decreased (Table 1). The mesopore and micropore size distributions by applying the BJH and MP methods are shown in Figs. 4 and 5, respectively. As can be seen from Fig. 4, the mesopore radius (rp) distribution of the activated CNTs is similar with that of the pristine sample in the region of tens of nanometers. The stronger peak is located at rp = 10.5 nm approximately. This observation demonstrates that CNTs maintained at least partly tubular structure even after activation. The big difference lies in that the volume distribution (dVp/drp) of activated sample abruptly increases, especially near to the micropore region, whereas the pristine sample remains within a very low level. This indicates the presence of plenty of micropores and mesopores after KOH modification. The actual pore diameter distribution of micropore can be obtained from the MP-plot as shown in Fig. 5. For the active action is not uniform, the micropore diameter shows a distribution range while the majority concentrates in 0.9 nm (see Fig. 5). Whatever the line intensity and smallest diameter, activated CNTs both have a large improvement compared with pristine sample. This result also suggests that a great deal of micropores was generated by using the KOH activation.
0.6nm
500
0.8nm
0 0.5
1.0
1.5
2.0
Pore diameter (dp)/nm Fig. 5. Pore size (diameter) distribution of pristine and activated CNTs obtained by MP method.
It is well-known that CNTs activation via KOH is a high-efficient technique for producing a large scale of porous structure. The controlling factors include KOH/CNTs ratio, activation temperature, reaction time and gas flow rate [13]. The activation mechanism is normally suggested to include independent hydroxide and redox processes during the reaction [17,22]. The overall reaction can be summarized as: C + 6KOH M 2K + 3H2 + 2K2CO3. The generated potassium will be intercalated between the graphitic layers [17] or form graphite–potassium intercalation compounds [23]. Thus, a few of graphic structure will be destroyed and generates quantity of micropores and mesopores even with open apertures (Figs. 1 and 3). This shows that the structure of activated CNTs is porous like the activated carbon. A quantity of inner surface in the porous structure leads to a relatively high total SSA. On the other hand, the tips of original CNTs are almost closed and thus the inner tube surface could not contribute to the SSA. That the very long CNTs entangle each other also will reduce the useful surface area. After activation, the entangled CNTs will be shortened and the dispersion is strongly increased. The most important is that the closed tips are opened and the wall thickness may be decreased. All of the above mentioned cases induce an obvious improvement in total SSA. Furthermore, a plenty of surface defects will be produced anywhere among the CNTs. Consequently, the variations in CNTs caused by the activation intensively enlarged the SSA and pore volume as shown in Table 1.
J.J. Niu et al. / Microporous and Mesoporous Materials 100 (2007) 1–5
The CNTs with a high SSA will intensively improve the dispersion of catalyst such as platinum and then optimize catalytic characteristics. More importantly, CNTs with well-maintained graphic structure ensure the excellent conductivity in electrode application. Therefore, the present CNTs with a high SSA can be used as catalyst carrier and supercapacitor, which have an important application in fields of novel fuel cell and energy storage devices. 4. Conclusion In summary, multi-walled CNTs were continuously fabricated by a floating CVD technique. The pristine CNTs were chemically modified by using KOH at high temperature. The CNTs with a high SSA of 830 m2/g and pore volume of 1.5950 cm3/g were achieved while the graphite structure was preserved. Such activated CNTs with a high SSA could have extensive applications in many fields. Acknowledgments This work was supported by the Shanghai-Applied Materials Research and Development Fund (Item No. 06SA06), National Natural Science Foundation of China, and Youth Teacher Fund of Shanghai Jiaotong University (A2306B). We would like to thank Instrumental Analysis Center of Shanghai Jiaotong University, for their great helps in measurements. References [1] R.H. Baughman, C. Cui, A.A. Zakhidov, Z. Iqbal, J.N. Barisci, G.M. Spinks, Science 284 (1999) 1340.
5
[2] X. Li, H. Zhu, L. Ci, C. Xu, Z. Mao, B. Wei, Carbon 39 (2001) 2077. [3] A.G. Rinzler, J.H. Hafner, P. Nikolaev, L. Lou, S.G. Kim, D. Tomanek, Science 269 (1995) 1550. [4] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennent, Appl. Phys. Lett. 70 (1997) 1480. [5] A.M. Zhang, J.L. Dong, Q.H. Xu, H.K. Rhee, X.L. Li, Catal. Today 93–95 (2004) 347. [6] C.J. Liu, T.W. Wu, L.S. Hsu, C.J. Su, C.C. Wang, F.S. Shieu, Carbon 42 (2004) 2635. [7] Y. Soneda, L. Duclaux, F. Beguin, Carbon 40 (2002) 965. [8] H. Okuno, J.P. Issi, J.C. Charlier, Carbon 43 (2005) 864. [9] G. Yu, J. Gong, D. Zhu, S. He, Z. Zhu, Carbon 43 (2005) 3015. [10] L.F. Su, J.N. Wang, F. Yu, Z.M. Sheng, H. Chang, C. Pak, Chem. Phys. Lett. 420 (2006) 421. [11] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature 372 (1994) 159. [12] F. Rodriguez-Reinoso, M. Molina-Sabio, M.T. Gonzalez, Carbon 33 (1995) 15. [13] Q. Jiang, Y. Zhao, Micropor. Mesopor. Mater. 76 (2004) 215. [14] E. Raymundo-Pinero, D. Cazorla-Amoros, A. Linares-Solano, S. Delpeux, E. Frackowiak, K. Szostak, Carbon 40 (2002) 1614. [15] S.M. Lee, S.C. Lee, J.H. Jung, H.J. Kim, Chem. Phys. Lett. 416 (2005) 251. [16] S. Delpeux, K. Szostak, E. Frackowiak, F. Beguin, Chem. Phys. Lett. 404 (2005) 374. [17] E. Raymundo-Pinero, P. Azais, T. Cacciaguerra, D. Cazorla-Amoros, A. Linares-Solano, F. Beguin, Carbon 43 (2005) 786. [18] R.S. Mikhail, S. Brunauer, E.E. Bodor, J. Colloid Interface Sci. 26 (1968) 45. [19] B.C. Lippens, J.H. Boer, J. Catal. 4 (1965) 319. [20] E.P. Barrett, P.B. Joyner, P. Halenda, J. Amer. Chem. Soc. 73 (1951) 373. [21] M. Eswaramoorthy, R. Sen, C.N.R. Rao, Chem. Phys. Lett. 304 (1999) 207. [22] M.A. Lillo-Rodenas, J. Juan-Juan, D. Cazorla-Amoros, A. LinaresSolano, Carbon 42 (2004) 1371. [23] R. Xue, Z. Shen, Carbon 41 (2003) 1862.