4138
Langmuir 2002, 18, 4138-4141
Oxidation and Porosity Evaluation of Budlike Single-Wall Carbon Nanohorn Aggregates E. Bekyarova,*,†,‡,§ K. Kaneko,*,‡,| D. Kasuya,⊥ K. Murata,†,‡ M. Yudasaka,† and S. Iijima†,⊥,# JCORP-JST, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, Center for Frontier Electronics and Photonics, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya 468-8502, Japan, and NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan Received November 29, 2001. In Final Form: February 21, 2002 Budlike single-wall carbon nanohorns (SWNHs), being capped hollow materials, were oxidized by heating in oxygen to produce nano-order windows in the walls. The opening of budlike SWNHs was studied with transmission electron microscopy and nitrogen adsorption at 77 K. The adsorption isotherms show development of the porosity with increasing the temperature of the SWNHs oxidation. Thus, the micropore volume of SWNH after oxidation in O2 at 693 K increases three times. The adsorption analysis of both substructures, formed by the nanohorn interstices and their internal cavities, is discussed. The oxidation does not change the size and shape of the budlike SWNH bundles. It is an effective way to produce SWNHs with a big micropore volume.
Introduction The interest in carbon nanotubes and related materials has been growing since their discovery in 1991.1 There is a great advance in understanding the physical properties and potential applications of these unique materials as a result of the increased research activity all over the world. A single-wall carbon nanohorn (SWNH) is a new fullerenerelated structure, which is synthesized recently by laser ablation of pure graphite.2 It was found that the morphology of SWNHs depends on the buffer gas atmosphere during the laser ablation.3 The primary SWNH particles are tubules with cone caps resembling a horn. When nanohorns are produced in an Ar atmosphere, the tubules aggregate together to form spherical particles, which resemble dahlia flower.2,3 Dahlia-like SWNHs are produced with high yield (>90%) and purity. SWNHs, prepared in He atmosphere, have a spherical assembly with budlike shape and are named budlike nanohorns. The most distinctive feature of SWNHs is that the individual nanohorns are completely closed and therefore they cannot accept substances inside. For adsorption utilization, opening of the potential adsorption space represented by interior of the nanohorns, is of crucial importance. In the case of single-wall carbon nanotubes (SWNTs), an effective way to cut and shorten the nanotubes is the polymer-assisted ultrasonication.4 How* To whom correspondence should be addressed. E-mail: kaneko@ pchem2.s.chiba-u.ac.jp and
[email protected]. † JCORP-JST, c/o NEC Corporation. ‡ Department of Chemistry, Faculty of Science, Chiba University. § Permanent address: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. bl.11, Sofia-1113, Bulgaria. | Center for Frontier Electronics and Photonics, Chiba University. ⊥ Meijo University. # NEC Corporation. (1) Iijima, S. Nature (London) 1991, 56, 354. (2) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; Kokai, F.; Takahashi, K. Chem. Phys. Lett. 1999, 165, 309. (3) Kasuya, D.; Yudasaka, M.; Takahashi, K.; Kokai, F.; Iijima, S. Proceedings of The 21th Fullerene Symposium, Tsukuba 2001, p 27.
ever, the most common method for pore opening is heat treatment in oxygen.5,6 We report a study on the nanohorn opening of budlike SWNHs by thermal treatment in oxygen atmosphere and its effect on the pore structure development. The adsorption of N2 at 77 K on the internal tubular pores and the interstitial pores among SWNH particles are discussed in this paper. Experimental Section The budlike SWNH aggregates were produced by CO2 laser irradiation (wavelength 10.6 µm, power density ∼20 kW/cm2, pulse width 500 ms, frequency 1 Hz) of graphite with He buffer gas atmosphere (760 Torr) at room temperature. The sample is denoted by b-SWNH. The oxidation of the bud-SWNH aggregates was conducted in a quartz tube in flowing pure oxygen (760 Torr) at 623 and 693 K for 10 min. The samples oxidized at 623 and 693 K are named b-SWNH-ox-623 and b-SWNH-ox-693, respectively. Oxidation at temperatures above 693 K was not carried out since at higher temperature the bud-SWNHs are not stable and burn very fast. Transmission electron microscopy (TEM) micrographs were obtained with a conventional transmission electron microscope (Topcon EM-002B) at 200 kV accelerating voltage. The adsorption isotherms of N2 were measured volumetrically at 77 K with an Autosorb-1, Quantachrom. The samples were evacuated at 423 K and 10-4 Pa for 2 h prior to the adsorption measurements. The particle density was estimated from the buoyancy in He. Displacement of equivalent helium volume is often used for particle density evaluation since helium has low adsorption at room temperature, which is normally assumed to be negligible. The high-pressure He buoyancy was measured gravimetrically at 303 K in the pressure range of 0.01-10 MPa. The system is equipped with an electronic microbalance (Cahn 1100) and highpressure transducers (MKS Baratron). The pretreatment conditions were the same as those for N2 adsorption. (4) Yudasaka, M.; Zhang, M.; Jabs, C.; Iijima, S. Appl. Phys. A 2000, 71, 449. (5) Morishita, K.; Takarada, T. J. Mater. Sci. 1994, 34 1169. (6) Nagasawa, S.; Yudasaka, M.; Hirahara, K.; Ichihashi, T.; Iijima, S. Chem. Phys. Lett. 2000, 328, 374.
10.1021/la0117348 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/18/2002
Oxidation of Carbon Nanohorm Aggregates
Langmuir, Vol. 18, No. 10, 2002 4139
Figure 1. Transmission electron microscope images of as-grown b-SWNH.
Figure 2. (a) Nitrogen adsorption isotherms at 77 K on b-SWNH (2), b-SWNH-ox-623 (b), and b-SWNH-ox-693 (9). Open and closed symbols denote adsorption and desorption, respectively. (b) Nitrogen adsorption isotherms with a logarithmic relative pressure axis.
Figure 3. Rs-plots of nitrogen adsorption isotherms at 77 K on b-SWNH (2), b-SWNH-ox-623 (b), and b-SWNH-ox-693 (9).
Morphology and N2 Adsorption Isotherms of Budlike SWNH Aggregates. Typically, the bud-SWNHs are produced with a yield of 70-80%. A representative TEM micrograph of as-grown b-SWNH, used in this study, is illustrated in Figure 1. SWNHs aggregate in spherical bundles of about 70 nm in diameter. The bundles are not as ragged as those of dahlia-SWNH2 and only few nanohorns are observed to stick out of the bundles. The bud nanohorn bundles retain their shape and size after heat treatment in oxygen at 623 and 693 K as revealed by the TEM observations (not shown here). The oxidation produces many holes in the nanohorns as a result of the reaction between the oxygen molecules and carbon atoms at defect sites and/or caps. However, it is quite difficult to evaluate quantitatively the open nanohorns by TEM. Since any nanohorn opening will affect strongly the adsorption of gas molecules, the effect of the heat treatment in oxygen on the structure of b-SWNH was examined with nitrogen adsorption at 77 K. The adsorption isotherms of N2 at 77 K for the budlike SWNH, as-grown and oxidized in O2 at 623 and 693 K, are given in Figure 2. b-SWNH exhibits an isotherm of type II according to IUPAC classification, without hysteresis and with a relatively high plateau. The high uptake in the adsorption isotherm at low relative pressure indicates presence of micropores. The type of the nitrogen adsorption isotherm for b-SWNH is similar to that for dahliaSWNHs.7 The oxidation of b-SWNH in oxygen results in higher uptake at low pressure, but the shape of the isotherm does not change. This is interpreted as a result
of the opening of closed nanohorns without additional changes of the aggregate structure. A significant adsorption uptake is observed with rising of the oxidation temperature. The logarithmic scale (Figure 2b) gives a more detailed picture of the adsorption process at low relative pressures, where mainly the micropore filling occurs. In the experimental range of relative pressure the adsorption curves do not exhibit any clear steps, typically associated with energetically homogeneous adsorption space. This fact suggests a broad distribution in the micropore size of the samples studied. The adsorption isotherms were analyzed with the subtracting pore effect method (SPE) for high-resolution Rs-plot.8 Rs-plot on highly crystalline nonporous carbon black (Mitsubishi 4040B) was used as a standard isotherm. Figure 3 shows the Rs-plots of the SWNH samples. All plots have an explicit filling swing (FS) at low Rs-region (Rs < 0.5). FS originates from mono-layer-like filling enhanced by strong molecule-micropore wall interactions.8 The cooperative swing (CS), associated with filling of the residual space between the monolayers on both micropore walls, is also observed. Typically, the total surface area, at, is obtained from the slope of the line passing through the origin of the plot; the external surface area, aext, and the total micropore volume, Vmit, are calculated from the slop and the intercept, respectively, of the line fitting the data at the high Rs-region. The pore structure parameters estimated by the SPE method are summarized in Table 1. The oxidation process is accompanied with a substantial increase of the total surface area. Also, the amount of the micropores increases significantly after oxidation. Thus, the micropore volume of SWNH after oxidation in O2 at 693 K increases three
(7) Murata K.; Kaneko, K.; Steele, W. A.; Kokai, F.; Takahashi K.; Kasuya, D.; Yudasaka M.; Iijima S. Nanoletters 2001, 1, 197.
(8) Kaneko, K.; Ishii, K.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075.
Results and Discussion
4140
Langmuir, Vol. 18, No. 10, 2002
Bekyarova et al.
Table 1. Pore Structure Parameters, Particle Density, and Closed Pore Volume of SWNHs internal microporosity
total microporosity sample
at (m2‚g-1)
aexta (m2‚g-1)
Vmit (cm3‚g-1)
Vme (cm3‚g-1)
Vmiinternal (cm3‚g-1)
d (nm)
particle density (g‚cm-3)
closed pore volume, (cm3‚g-1)
b-SWNH b-SWNH-ox-623 b-SWNH-ox-693
320 600 830
100 135 170
0.11 0.23 0.34
0.21 0.29 0.32
0.12 0.24
1.8 1.9
1.31 1.82 1.91
0.32 0.11 0.08
a
In the estimated value for the external surface area there is a contribution of the mesopore area.
Figure 4. A schematic representation of a bundle of b-SWNHs (a), two adjacent SWNHs creating interstitial pore in as-grown b-SWNH (b), and internal pores in oxidized b-SWNHs with open nanohorns (c).
times. The mesopore volume, Vme, estimated as a difference between the total pore volume (the volume filled at P/Po ) 0.98) and the micropore volume, is also given in Table 1. The mesopore volume increases after oxidation as some of the nanohorns in the bundle are with a diameter bigger than 2 nm and opening of such pores contributes to the mesopore volume. Internal and Interstitial Micropores. The microporosity structure of the SWNHs aggregates is subdivided into two substructures. The first substructure observed in SWNHs is formed between the individual single-wall nanohorns, which aggregate in bundles. The distances between adjacent nanohorns are in the micropore range as suggested by TEM observations. The second substructure originates from the inner hollow cavity of the nanohorns and it becomes available for adsorption only after oxidation as the nanohorns grow with closed caps. A schematic diagram of the pores present in SWNHs is shown in Figure 4. There is no established terminology for both pore types in the literature. We adopt the terms internal pores for the adsorption space inside the nanohorn (Figure 4c) and interstitial pores for the interstitial channels between the nanohorns in the bundle (Figure 4b). The interaction potential profile calculated using the Lennard-Jones potential shows that the attractive potential energies of the interstitial and internal pores are very different.9,10 Therefore, to understand better the role of both types of porosity in the adsorption process, (9) Murata K.; Kaneko, K.; Steele, W. A.; Kokai, F.; Takahashi K.; Kasuya, D.; Hirahara K.; Yudasaka M.; Iijima S. J. Phys. Chem. 2001, 105, 10210. (10) Cheng, H.-M.; Yanh, Q.-H.; Liu, C. Carbon 2001, 39, 1447.
it is of significant importance to distinguish them. The evaluation of the parameters of both substructures is problematic because of the difficulties to distinguish the adsorption processes taking place in each substructure. However, if we assume that the oxidation does not change the aggregate structure of the bud form, subtracting the adsorption isotherms of the oxidized and as-grown SWNHs provides the adsorption isotherm of the internal pores only. The structure parameters of the internal porosity can be determined by Rs-analysis of the subtracted isotherm. This procedure is used here to estimate the structure parameters of the internal micropores. The data from the analysis are presented in Table 1. The main parameters of the interstitial substructure are the interstitial micropore volume (Vmicintrst) and the pore width (w). The adsorption methods necessitate recourse to a model of pore shape. In most of the adsorption methods, two pore models (cylindrical and slit-shape) are used in the computational procedures. As shown in the TEM images (Figure 1) and the morphological sketch (Figure 4), it is difficult to simplify the shape of the interstitial pores. Approximately, the space between two adjacent nanohorns could be assumed as a quasi-slit-like pore. This approximation is justified by the fact that in the interstitial space, as in the slit-shaped pores, there is a filling limitation for the gas molecules only in one direction, i.e., between the two carbon walls. Then, the pore width is calculated for a slitlike model as follows
ω ) 2Vmicintrst/amicintrst
(1)
where amicintrst is the surface area of the interstitial micropores (amicintrst ) at - aext). The estimated interstitial pore width of b-SWNH is 1.0 nm. The internal pore is approximately treated as a cylinder (Figure 4). The main parameters of the internal pore structure are the pore volume of the nanohorns Vmicinternal and their diameter, obtained by geometrical calculation
d ) 4Vmicinternal/amicinternal
(2)
The volume of the internal micropores increases significantly with the oxidation temperature. The increase of the microporosity is obviously a result of nanohorn opening. The oxidation at 693 K is accompanied with a slight enlargement of the average pore diameter of SWNHs. Since neither the oxidation nor the heat treatment at the temperatures used for the oxidation is likely to alter the nanotubular diameter,11 the increase should be due to enlargement of the voids formed on the nanohorn walls as the temperature of oxidation increases. The particle density (Fp), calculated from the He buoyancy, is presented in Table 1. The method for particle density evaluation from He buoyancy measurements is (11) Yudasaka M.; Kataura, H.; Ichihashi, T.; Qin, L.-C.; Kar, S.; Iijima S. Nanoletters 2001 1, 487.
Oxidation of Carbon Nanohorm Aggregates
Langmuir, Vol. 18, No. 10, 2002 4141
given in detail elswhere.12 As expected the particle densities of the oxidized SWNHs are much higher than that of b-SWNH since many of the nanohorns were opened after the oxidation. b-SWNH does not have open nanohorns. Each unit of SWNHs consists of a graphitic layer rolled up into a cylinder. Consequently, if all nanohorns are open, the particle density should be the same as the density of graphite (2.27 g‚cm-3). A difference between the densities of graphite and b-SWNH indicates the existence of closed nanohorn space, not accessible for the He molecules. The specific volume is inversely proportional to density, and from the relation
Vmic )
1 Fp
SWNHs
-
1 Fp
graphite
(3)
the closed pore volume Vmic can be estimated. The data are given in Table 1. Consequently, the opened pore volumes of b-SWNH-ox-623 and b-SWNH-ox-693 are 0.21 and 0.24 cm3‚g-1, respectively. The heat treatment in oxygen at 693 K opens 75% of the closed pore space of b-SWNH. Since the internal micropore volume represents the volume of pores opened by oxidation, it should correspond to the opened pore volume, estimated by the He buoyancy. Both volumes, obtained from nitrogen adsorption and He buoyancy, are equal for b-SWNH-ox-693. However, there is a big discrepancy in the values estimated for b-SWNH (12) Murata K.; Kaneko, K.; Kokai, F.; Takahashi K.; Yudasaka M.; Iijima S. Chem. Phys. Lett. 2000, 331, 14.
-ox-623 with a higher value obtained by He buoyancy. One possible explanation is that heat treatment in oxygen at 623 K opens on the nanohorn walls many windows, which are too narrow to admit nitrogen molecules. N2 molecules (0.363 nm), being bigger than He (0.26 nm) cannot pass through these narrow windows and thus nitrogen does not measure the whole pore volume accessible for He. Thus, a possible molecular sieving effect is expected for bud-SWNHs heat treated in oxygen at 623 K. Conclusions The adsorption study showed that the budlike SWNH aggregates possess micropores despite the closed individual nanohorns. A distinctive feature of these micropores is the small average pore width of 1.0 nm. Heat treatment in oxygen opens the closed nanohorns and, thus, increases the micropore space available for adsorption. Adsorption analysis can provide a reliable means for evaluation of the pore structure parameters of the interstitial and internal microporosity. The oxidation affects mostly the closed pores by creating windows on the walls and does not change the bundle structure as well as the interstitial microporosity. Acknowledgment. The authors thank Dr. F. Kokai and Dr. K. Takahashi for useful discussions about the synthesis of the budlike single-wall carbon nanohorns and Dr. T. Ichihashi for the help in TEM observation. E.B. is grateful to the Japan Science and Technology Corporation (JST) for the STA fellowship. LA0117348
