Effect of temperature on chemical activation of carbon nanotubes

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Solid State Sciences 10 (2008) 1189e1193 www.elsevier.com/locate/ssscie

Effect of temperature on chemical activation of carbon nanotubes Jun Jie Niu*, Jian Nong Wang School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 31 October 2007; received in revised form 26 November 2007; accepted 10 December 2007 Available online 23 December 2007

Abstract Multi-walled carbon nanotubes (CNTs) with a high specific surface area (SSA) present extensive applications in many fields. In this work, we investigate the nanoporous structures of CNTs activated by KOH under various temperatures of 700, 800, 850, and 900  C. A mild activation will generate a plenty of micro-structures and a strong activation will produce mesopores. A highest specific surface area and a highest pore volume were obtained at a suitable temperature of 850  C. The temperature played an important effect on the activation process. It is suggested that the formation of nanoporous structure on the surface of CNTs may be related to appropriate potassium intercalation at a reasonable temperature. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Carbon nanotubes; Activation; Temperature dependence

1. Introduction Single-walled and multi-walled carbon nanotubes (CNTs), for their special structures and excellent physical and chemical properties, are being studied widely for many applications including hydrogen storage [1e4], field emission display [5], and others [6e8]. The activated carbon nanotubes (ACNTs) with a high specific surface area (SSA) have a potential prospect in supercapacitors [9], catalyst support, adsorbent [10,11], selective molecular filtering [12], and so on. Until now, it has been demonstrated that activation by KOH appears to be a very effective method to increase the SSA and pore size distribution [13e20]. Kim et al. investigated the development of porous CNTs activated by KOH and obtained a SSA of 650 m2/g [16]. Raymundo-Pinero et al. studied the KOH/ NaOH activation principles of CNTs [14,21]. A few factors affect the activation of CNTs by KOH. Among them, temperature appears to be one of the most effective ones. Although previous studies showed that the reaction time, gas flow, mass ration, and temperature would have influence on the activated efficiency, as we know, the exact analysis

* Corresponding author. Tel.: þ86 21 62932050. E-mail address: [email protected] (J.J. Niu). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.12.016

on temperature has not been studied. In this paper, CNT activation with KOH (mass ration 1:5) at the temperature from 700 to 900  C for 1.5 h was studied in detail. The mild activation would generate a plenty of micro-structures and strong activation generates mesopores. The total SSA and pore volume were increased with the temperature enhancement and climbed to the highest point at 850  C. Inversely, an excessive activation at a higher temperature of 900  C induced a decreased SSA. The activation mechanism with different temperatures is discussed in detail. 2. Experimental CNTs were synthesized by a spray pyrolysis through a floating-CVD technique as previous reported [22]. The as-grown samples were purified in HNO3 (1:1 ratio) at 120  C for about 3 h to remove metal particles. Thus, the desired pristine CNTs were obtained for the next study. The KOH agent was introduced with pristine CNTs in dilute de-ionized water with a mass ratio of 5:1 (KOH/CNTs). Here the water made the compounds more sufficiently mixed and could be removed in the next dealing at high temperature. Activation temperatures were varied among 700, 800, 850, and 900  C in an Ar flow at a rate of 180 ml/min and reaction time was held at 1.5 h. After modification, the as-received samples were mixed

J.J. Niu, J.N. Wang / Solid State Sciences 10 (2008) 1189e1193

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Fig. 1. TEM images of pristine (a) and activated (850  C) (b) CNTs. The inset in (b) is a TEM image of the tip of ACNT.

3. Results and discussion The TEM morphologies of pristine and activated CNTs are presented in Fig. 1. The long CNTs with a uniform size are observed in the pristine sample (Fig. 1a) while the shortened ones with rough surface are achieved after activation (Fig. 1b). Especially from the inset of Fig. 1b, the uniform outside of the tube has been destroyed with a plenty of slices everywhere. Although the tubular structure was partially destroyed, the major graphitic nature was preserved. As illustrated in previous results, the main peak of C (002) with high intensity is clearly observed, together with those for C (101) and C (100). The obtained N2 adsorption/desorption isotherms of ACNTs are displayed in Fig. 2. As can be seen from the figure, all of the isotherms should be of nos. 1e4 type, which are often

associated with micropores (diameter: <2 nm) and mesopores (diameter: 2e100 nm), based on the classification recommended by IUPAC [26]. The absorbed amount for ACNTs is obviously heightened compared with that of pristine sample whatever in the zone of low pressure (P/P0) or in the zone of high pressure. The enlarged hysteresis loop space between the adsorption/desorption isotherms for ACNTs indicates the presence of a large quantity of mesopores, and the line climbs steeply in the area of low pressure indicating a plenty of micropores existing after activation. When analyzed in detail, in the zones of both low and high pressures, the lines exhibit a gradual improvement with the activation temperature until the highest value at 850  C is reached. However, as the temperature arose to 900  C, the absorbed amount was decreased (Fig. 2).

a-Pristine CNTs d-Pristine CNTs a-ACNTs (700°C) d-ACNTs (700°C) a-ACNTs (800°C) d-ACNTs (800°C) a-ACNTs (850°C) d-ACNTs (850°C) a-ACNTs (900°C)

800

Amount adsorbed/cm3(STP)g-1

with dilute acid and de-ionized water to remove the alkali compounds and impurities. N2 adsorption/desorption using BELSORP measuring instruments (BELSORP-mini, JAPAN, INC.) was carried out in order to investigate the pore properties of the samples. All the samples were degassed 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 SSA was calculated using the BrunauereEmmette Teller (BET) equation. The mesopore radius distribution was determined by BarretteJoynereHalenda (BJH) method and the micropore diameter distributions were determined by micropore analysis method (MP method) and t-plot method [23e25]. Finally, pore volumes were estimated to be the liquid volume of adsorption (N2) at a relative pressure of 0.99. Morphology and micro-structure of pristine and ACNTs were observed by transmission electron microscopy (TEM, JEM100 and JEM2010). The graphitic nature was checked by X-ray diffraction using Cu Ka radiation (XRD, D8Advance, Bruker).

400

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 2. N2 adsorption/desorption isotherms of pristine and activated CNTs at 700, 800, 850, and 900  C.

J.J. Niu, J.N. Wang / Solid State Sciences 10 (2008) 1189e1193

SSA Total pore volume

800

1.6

SSA (m2/g)

1.4 700 1.2 600 1.0 500

Total pore volume (m3/g)

The complete pore structure data for pristine and activated CNTs are presented in Table 1. It clearly shows that the SSA and total pore volume are strongly increased when the activation temperature of 850  C was applied. Nevertheless, the SSA and pore volume drop back to 582.5 m2/g and 0.7889 m2/g, respectively, again at a higher temperature of 900  C. This variation trend is be more visually displayed in Fig. 3. In statistics, the total SSA and pore volume are mainly contributed by the separated mesopores and micropores produced on the samples. As shown in Fig. 4a, the mesopore radius distribution has a stronger peak located at w10.5 nm and shows a larger upwarping when the size is close to the region of micropores. The detailed SSA and pore volume of mesopores are shown in Table 1 and Fig. 4b. Based on these data, the variation of SSA and mesopore volume with temperature is similar to the total SSA and pore volume shown in Fig. 2. When the activation temperature reached 850  C, the SSA and mesopore volume are 409.0 m2/g and 1.3717 cm3/g, which contribute to the total pores of 49.3% and 86%, respectively. The variation of micropores shows a slight difference as displayed in Fig. 5 and Table 1. At the temperature of 850  C, the SSA and micropore volume are close to the values at 700 and 800  C, respectively. But, when the activation is processed at a higher temperature of 900  C, the values are continually increased to 428.7 m2/g and 0.2356 cm3/g, respectively. This indicates that the population of micropores is enhanced even when the temperature is increased to 900  C. This is in contrast with that observed for the mesopores. After KOH activation, the long CNTs will be shortened and the closed tips are opened. As a result, the total SSA and pore volume of CNTs are both intensively increased. As shown above, the quantity of mesopores is increased as the temperature is increased up to 850  C, but is decreased as the temperature is further increased to 900  C. But the micropores were contiguously increased up to 900  C. The activation redox process can be summarized as: C þ 6KOH 4 2 K þ 3H2 þ 2 K2CO3 [21,27]. The produced potassium will be intercalated between the graphitic layers or form graphiteepotassium intercalation compounds. As a result, some graphitic structures will be destroyed and a large quantity of micropores and mesopores are generated. Obviously, the strong interaction between the potassium and CNTs will produce quantity of mesopores while a mild interaction induces micropores. Here one case should be mentioned, that is, the quality of pristine CNTs will take an important role on the activation. If the nature of pristine sample is different,

1191

0.8 700

750

800

850

900

Activation Temperature (degree) Fig. 3. Variation of total SSA and pore volume with activation temperature.

the real activation condition will present a slight difference [18]. When the temperature varies from 700 to 850  C, the activation degree is gradually enhanced. Consequently, a plenty of CNTs is shortened and a high quantity of defects including micropores and mesopores is generated. In fact, the weak interaction for generating micropores is easy to be processed, thus inducing a slight change when the temperature is increased from 700 to 850  C. On the contrary, the stronger interaction for generating mesopores is improved with the activated temperature increased from 700 to 850  C. Under 850  C, the majority of the sample is completely reacted and there are no residual pristine CNTs. Subsequently, when the temperature is increased further to 900  C, there are no new mesopores to be generated. Reversely, the mesopores are interacted with the potassium and consequently some micropores are generated. During this process, the graphitic structure of CNTs is seriously destroyed, giving rise to an amorphous nature. Thus, the SSA and volume of micropores are slightly increased while those of mesopores are decreased (Table 1, Figs. 4 and 5). In theory, more micropores and mesopores amongst the CNTs would conspicuously enhance the total SSA and pore volume. In our experiments, the micropores are continually improved with a slow ratio while the mesopores are first improved and then decreased with the activated temperatures varying from 700 to 900  C. Correspondingly, the ratio of micropore SSA to the total one is enhanced from w9% for pristine CNTs to w74% for ACNTs and the ratio of volume

Table 1 Pore structures of the pristine CNTs and ACNTs Samples

Total surface area (m2/g)

Total pore volume (cm3/g)

Mean pore diameter (nm)

Micropore area (m2/g)

Mesopore area (m2/g)

Micropore volume (cm3/g)

Mesopore volume (cm3/g)

Pristine ACNTs ACNTs ACNTs ACNTs

64.8 476.0 532.3 830.0 582.5

0.3429 0.7991 1.0628 1.5950 0.7889

21.15 6.72 7.99 7.68 5.42

6.1 348.9 269.3 421.0 428.7

58.7 127.1 263.0 409.0 153.8

0.0025 0.1965 0.2234 0.2233 0.2356

0.3404 0.6026 0.8394 1.3717 0.5533

CNTs (700  C) (800  C) (850  C) (900  C)

J.J. Niu, J.N. Wang / Solid State Sciences 10 (2008) 1189e1193

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a

b 200

Mesopore SSA (m2/g)

dVp/drp

150

100 ~10.5 nm 50

0

1.4

Mesopore SSA Mesopore volume

400

1.2 300

1.0 0.8

200 0.6 0.4

100 1

5

2

10

20

30

40

50

60

Pore radius (rp) /nm

Mesopore volume (m3/g)

Pristine CNTs ACNTs (700°C) ACNTs (800°C) ACNTs (850°C) ACNTs (900°C)

700

750

800

850

900

Activation Temperature (degree)

Fig. 4. Mesopore radius distribution of pristine and ACNTs obtained by BJH method (a) and variation of mesopore SSA and pore volume with activation temperature (b).

from w0.7% to w30% while the corresponding ratios for mesopores are decreased. Therefore, the highest SSA and pore volume of CNTs with better graphitic structure are determined by an appropriate proportion of micropores and mesopores at an activated temperature of 850  C (Table 1 and Fig. 3). The ratio of micropore SSA to the total one is w51% and the ratio of pore volume is w14% and thus the highest SSA of 830.0 m2/g and total pore volume of 1.5950 cm3/g are achieved.

Acknowledgements

4. Conclusions

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The KOH activation on CNTs at varying temperatures of 700, 800, 850, and 900  C was investigated. A mild activation will generate a plenty of micro-structures and a strong activation will produce mesopores. A highest specific surface area and a highest pore volume were obtained at a suitable temperature of 850  C.

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1500

~0.9 nm

Pristine CNTs ACNTs (700°C) ACNTs (800°C) ACNTs (850°C) ACNTs (900°C)

dVp/ddp

1000

~0.6 nm 500

0 0.4

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1.2

1.6

2.0

Pore diameter (dp) /nm Fig. 5. Micropore diameter distribution of pristine and ACNTs obtained by MP method.

This work was sponsored by the Shanghai-Applied Materials Research and Development Fund (No. 06SA06), Shanghai Educational Development Foundation (No. 2007CG14), and Youth Teacher Fund of Shanghai Jiao Tong University (No. A2306B). We would like to thank Instrumental Analysis Center of Shanghai Jiao Tong University, for their great helps in measurements.

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