Parameters of bundle-structure controlled SG SWCNTs.
Hydrogen (H2) gas is an ideal clean fuel, because H2 emits only water on burning and the energy content per unit mass is much greater than that of hydrocarbon fuels (Gregory & Oerlemans, 1998). Using H2 as a fuel has been expected to prevent global warming. To achieve the effective utilization of H2 energy, the development of its efficient storage method is necessary. H2 is supercritical gas at room temperature; the critical temperature of H2 is 33 K. Thus, it is difficult to store large amount of H2 at room temperature because the supercritical gas does not liquefy even under high pressures. Efficient adsorbents for H2 storage have been actively studied to overcome this problem.
Single-wall carbon nanotube (SWCNT) is considered to be the most promising material which can contribute to construct a new sustainable chemistry (Iijima, 1991; Iijima & Ichihashi, 1993; Hirsch, 2002; Saito et al., 1998) and particularly a H2 storage system, because SWCNT bundles have both of internal and interstitial nanospaces which strongly interact even with supercritical H2 (Liu et al., 1999; Wang & Johnson, 2000; Seung & Young, 2000; Xu et al., 2007; Kim et al., 2007). One SWCNT consists of one graphene sheet rolling up. Thus, SWCNT is a special material referred to as “bi-surface nature material” because the whole carbon atoms are exposed to the both internal and external surfaces, each with different nanoscale curvatures of the SWCNT wall (Noguchi et al., 2007; Fujimori et al., 2010). A SWCNT has a huge geometrical surface area of 2630 m2 g-1, the same as graphene. The effective surface area of SWCNTs for molecules varies with its tube diameter and the target molecular size. In addition to the large surface area, the differences between surfaces with positive and negative curvature can be exploited to establish unique material science and technology. Ordinary SWCNTs associate to form an ordered bundle structure through dispersion interaction, providing interstitial pore spaces surrounded by carbon walls with positive curvature, which are the strongest molecular sites. Therefore, bundled SWCNTs have considerable potential for application to gas storage, the stabilization of unstable molecules, quantum molecular sieving (Noguchi et al., 2010), specific reaction fields, gas sensing, electrochemical energy storage and so on (Banerjee et al., 2003; Arai et al., 2007). However, when the interstitial pore width is just comparable to the size of a small molecule, the molecules preadsorbed in the interstitial nanospaces often block further adsorption, or the capacity of the interstitial pore spaces is too small compared with the internal nanospace capacity. Thus, it is necessary to establish a means for tuning the bundle structure for providing the larger capacity of internal and interstitial nanospaces with an optimum size for the target function, as the volume of the interstitial nanospaces at the strongest sites is too small.
Pillaring an SWCNT bundle is the best approach to control interstitial nanoporosity, realizing enhanced adsorptivity for supercritical gases such as H2, and strengthening the specificity of the molecular recognition function (Abrams et al., 2007; Zhao et al., 2007). Here we report the simple preparation of fullerene (Kroto et al., 1985) (C60)-pillared SWCNT bundles by sonication of SWCNTs in a C60 toluene solution and the consequent enhancement of the supercritical H2 adsorptivity of the SWCNTs (Arai et al., 2009). As C60 molecules have a conjugated π-electron structure similar to that of SWCNTs, the C60-pillared SWCNT system can be regarded as a new nanocarbon. In fact, naphthalene-pillared SWCNT have pseudo-metallic property (Gotovac-Atlagić et al., 2010).
Another approach to control the structure of SWCNT bundles is building up of the designed bundles from the isolated SWCNTs (Yamamoto et al., In press). Hata et al. succeeded to prepare mutually isolated SWCNTs of high purity using CVD method, stimulating interfacial researches on SWCNT (Hata et al., 2004). The transmission electron microscopy (TEM) image of SWCNT called as supergrowth SWCNT (SG SWCNT) is shown in Fig. 1 a. SG SWCNT has the average diameter of 2.8 nm and the length of 1 mm order. Very recently, the authors evidenced that the monolayer of N2 molecules adsorbed on the internal wall of the negative curvature of SWCNT is more ordered than that on the external wall of the positive curvature (Ohba et al., 2007). Thus, SWCNT has an explicit bi-surface nature for molecules, which should be applicable to develop intriguing and novel materials of multi-interfacial functions. If we control the bundle structure formation of the isolated SWCNTs induced by drying the SWCNTs dispersed in the solvent, many interstitial sites are formed enough to adsorb supercritical H2. The effect of surface tension of solvents, which are used to disperse SWCNTs, is focused on to control the bundle structure formation, since the SWCNTs in the bundle are bound by van der Waals force, that is, 27.910-3 and 22.110-3 N/m for toluene and methanol at 273 K, respectively.
In this chapter, we report the preparation of C60-pillared SWCNT bundles and SWCNT bundles induced with capillary force-aided drying method, whose the supercritical H2 adsorptivities are enhanced by C60-pillaring and by the bundle formation.
2. Experimental section
2.1. Preparation and characterization of C60-pillared SWCNT bundle
We used SWCNT samples prepared by the laser ablation of a graphite rod in the presence of Ni and Co (@ Institute of Research and Innovation: IRI) (Yudasaka et al., 1999; Kokai et al., 2000). The produced SWCNT was purified by the following method: SWCNT (200 mg) was added to a 15% hydrogen peroxide solution, and this solution was refluxed with a water bath at 373 K for 5 h to remove amorphous carbons. The residual catalysts of Ni and Co were removed by a 1 M hydrogen chloride solution. Then, SWCNT was filtrated, washed with doubly distilled water, and left at room temperature overnight. The TEM image of the purified SWCNT is shown in Fig. 1b. Characterization data for the purified SWCNT are shown in Fig. 2. Figure 2a shows thermogravimetry (TG) and differential thermogravimetry (DTG) curves measured in the N2/O2 flow. The estimated content of Co-Ni catalyst is about 8 wt%. X-ray diffraction (XRD) pattern shown in Fig. 2b measured using CuKα exhibits a clear peak due to their well-ordered hexagonal bundle structure. The peak at 2θ=6.12º corresponds to 1.44 nm of interlayer distance d of SWCNT. Raman spectra in the radial breathing mode (RBM) band and G- and D-bands regions are shown in Fig. 2c. Very small peak at D-band indicates high-quality of the purified SWCNT. The tube diameter (dSWCNT) is 1.37 nm, determined by the relation of dSWCNT = 248/w, where w is the wavenumber of the RBM (Kataura et al., 1999). Closed SWCNT samples were used to clearly show the effect of C60-pillaring. Figure 2d shows the N2 adsorption isotherms of the purified SWCNT at 77K. The BET specific surface area is 337 m2 g-1, indicating that the purified SWCNTs were closed.
For C60-pillaring, we applied the methods used for the adsorption of organic substances on SWCNTs (Gotovac et al., 2007) and the preparation of peapod SWCNTs (Yudasaka et al., 2003). C60-pillared SWCNTs were prepared by a simple sonication of SWCNT in C60 toluene solution with different concentrations. Purified SWCNTs (10 mg) were ultrasonically treated at 28 Hz for 6 h in C60-dissolved toluene solutions of different concentrations up to 2.8 g L-1Toluene in an ice storage. Then, the samples were stood for 24 h, filtrated, and dried in a vacuum at 333 K for 24 h. The amount of C60 on the SWCNT bundles was determined by the weight change of the samples before and after C60-pillaring treatment. The C60-pillared SWCNTs are designated as SWCNT-C60(x), where x is the amount in gram of C60 doped to 1 g of SWCNTs. Here, SWCNT-C60(0), which was ultrasonically treated in toluene without C60, was also prepared for comparison.
The SWCNT-C60(x) samples were characterized with N2 adsorption at 77 K, XRD, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TGA), and high resolution transmission electron microscopy (HR-TEM). The H2 adsorptivity of the SWCNT-C60(x) samples was examined at 77 K by volumetric method. Samples were pre-evacuated at 423 K and 1 mPa for 2 h for the adsorption measurements of N2 and H2 at 77 K. TGA experiments were carried out in N2 flow (100 ml/min) from ambient temperature to 1273 K at a rate of 5 K/min.
2.2. Preparation and characterization of predominant bundle formation of isolated SWCNTs
The high purity isolated SWCNTs (SG SWCNTs) were produced by the CVD method (Hata et al., 2004). The SG SWCNTs were sonicated in toluene and methanol around 273 K for 12 h. Then, the SG SWCNTs in each solvent were filtrated and dried at 333 K for formation of the bundle structure by using the capillary force. The obtained SWCNT samples using toluene and methanol are denoted SWCNT/Tol and SWCNT/Met, respectively. The removal condition of residual toluene or methanol on SWCNT samples was determined by TGA before adsorption measurements. The nanopore structures of SWCNT/Tol and SWCNT/Met were evaluated by N2 adsorption measurements at 77 K after pretreatment at 423 K and 10-4 Pa for 2 h. The high pressure adsorption isotherms of supercritical H2 were measured at 77 K by gravimetrical method after the pretreatment same as the N2 adsorption measurement.
3. Results and discussion
3.1. C60-pillared SWCNT bundle formation and its H2 adsorptivity
The C60-doped amount against the C60 concentration of the toluene solution (g L-1Toluene) is shown in Figure 3. The C60 uptake versus the C60 concentration curve has a step near 0.7 g g-1SWCNT of uptake and 0.5 g L-1Toluene of the C60 concentration; the step indicates the formation of a stable structure between C60 and the SWCNTs. The uptake at the step closely corresponds to amount required for perfect filling of the interstitial spaces by C60 molecules, as estimated from the interstitial spaces in the model structure of an SWCNT bundle and the uptake of C60 for a trigonal arrangement as shown in Fig. 4 (Williams & Eklund, 2000).
Figure 5 shows the N2 adsorption isotherms of C60-pillared SWCNT bundles. The N2 adsorption amount was dramatically changed by C60-pillaring treatment. The amount adsorbed on SWCNT-C60(0.646) is the greatest. The adsorption isotherms of SWCNT-C60(0.646) and SWCNT-C60(1.68) have an extremely great uptake below P/P0=0.1; the marked low pressure adsorption is more evidently observed in the isotherms of which abscissa is expressed by the log P/P0. Accordingly the C60-pillaring treatment increases the nanoporosity having a very strong interaction potential.
The stable structure, that is, SWCNT-C60(0.646), provides the maximum nanopore volume in N2 adsorption measurements at 77 K. Figure 6 shows the relation between the nanospace volume and the C60-doped amounts x (See Table 1 for detail). The nanospace volume of SWCNT-C60(0.646) is 0.15 cm3 g-1, which is 1.36 times higher than that of SWCNT-C60(0). Hence, SWCNT-C60(0.646) should have the optimum C60-pillared structure for the acceptance of molecules in the interstitial nanospaces expanded by C60-pillaring. A C60 concentration higher than 1.0 g L-1Toluene should induce further C60-pillaring and coating of the external surface of the SWCNT bundle, thus reducing the nanopore volume.
Figure 7 shows HR-TEM images of the bundle structure of SWCNT-C60(0.646), which should have the optimum structure to adsorb molecules. The wide-range observation (Fig. 7a) shows a well-aligned bundle sheet even after ultrasonic C60-pillaring treatment in the toluene solution. The side-view observation (Fig. 7a) indicates that C60 molecules are present on the SWCNT surfaces and there are no peapod SWCNTs (i.e., C60 in the SWCNTs). Figure 7b,c shows cross sections of the SWCNT-C60(0.646) bundle having expanded hexagonal and tetragonal arrays, respectively. The intertube distance of the expanded bundle is estimated to be ~2.2 nm, leading to an interlayer distance d′=1.9 nm for hexagonal symmetry and d″=1.8 nm for tetragonal symmetry, under the assumption of uniform bundle structure for each symmetry. The interlayer distances d′ and d″ of the bundles of hexagonal and tetragonal superlattices of the C60-pillared SWCNT bundle are 2.03 and 1.92 nm, respectively, according to geometrical evaluation, which were close to the values determined from TEM, as shown in Fig. 8. Thus, SWCNT-C60(0.646) should have a mixed C60-pillared structure with both hexagonal and tetragonal symmetries.
Figure 9 shows XRD patterns which support the above-mentioned C60-pillared SWCNT structure. SWCNT-C60(0), which was ultrasonically treated in toluene without C60, gives an explicit peak at 2.81° (X-ray source: MoKα), corresponding to the interlayer distance (d=1.44 nm (experimental)) of a hexagonal lattice of SWCNT arrays (see Fig. 8a), whereas individual SWCNTs have no diffraction peak in the concerned diffraction angle-range. This peak is weakened by the C60-pillaring treatment and a broad peak appears around 2θ=2.0°. The new peak corresponds to an interlayer distance of ~2.0 nm, which is the average of 2.03 and 1.92 nm, derived from the TEM-derived two-structure models. Thus, XRD clearly indicates the formation of C60-pillared SWCNT bundles. However, the pillared structure is not necessarily regular; hexagonal and tetragonal structures coexist, and therefore a broad superlattice peak is observed.
TGA data as shown in Fig. 10 revealed pillaring of C60 in the SWCNT bundles. Even SWCNT-C60(0), which has no C60 exhibits a remarkable weight decrease (≈ 10%) at 1200 K, which is caused by the elimination of amorphous carbon and oxygen functional groups. The weight decrease at high temperatures increases with increase in the C60-doped amount x. The sample weight decrease is much less than the C60-doped amounts, except in the region of low C60-doped amount. The considerable weight decrease in the low C60 region is caused by the elimination of amorphous carbon and oxygen functional groups. On the other hand, only a part of the C60-doped amounts adsorbed on the external surface of the SWCNT bundles can be eliminated in the high C60 region. This indicates that major C60 molecules inserted in the strong potential sites of interstitial pores cannot be eliminated.
Surface composition analysis with XPS also supports pillaring of C60 molecules in SWCNT bundles. Figure 11 shows the XPS spectra of C60-pillared SWCNT bundles. XPS analysis of the C1s spectrum of SWCNT-C60(0.646) by fitting with the C1s spectra of SWCNT and C60 shows the presence of 10% of C60 on the bundle surface, much less than the bulk content (39%) (Utsumi et al., 2007). XPS detects electrons only from surface layers in the order of 1 nm; predominant C60 molecules are not on the external surface of the SWCNT bundle, but inside of the SWCNT bundle as pillars. The absence of C60 on the bundle surface of SWCNT-C60(0.646) is revealed by Raman spectroscopy shown in Fig. 12, which is also surface sensitive. The Raman peak for C60 at 1467 cm-1 appears only for the SWCNT-C60(x) samples whose x is larger than 1.68 g g-1SWCNT (Dresselhaus et al., 1996; Rao et al., 1997). Thus, all characterization results confirm that SWCNT-C60(0.646) has a promising C60-pillared structure with adequate nanoporosity.
The change in the interaction strength of the adsorption sites can be sensitively detected by supercritical H2 adsorption. H2 adsorption isotherms of SWCNT-C60(x) at 77 K are shown in Fig. 13. For comparison, the isotherm of SWCNT-C60(0) at 40 K is also shown. As the critical temperature of H2 is 33 K, H2 at 77 K and 40 K is supercritical, and thereby the adsorption of H2 needs intensive assistance with interaction potential from solid nanospaces (Kaneko & Murata, 1997;
Xu et al., 2007). As shown in Fig. 13, a large amount of H2 can be adsorbed at enough low temperature such as 40 K (≈ 1 wt% at 0.1 MPa), even though H2 is supercritical. However, the H2 adsorption amount at 77 K was less than the half of the amount at 40 K. H2 adsorptivity of C60-pillared SWCNT bundle was enhanced when the pillaring structure wasoptimum, while excessive C60-doping reduced the H2 adsorption amounts. Upward concave H2 adsorption isotherms at relatively low pressure stems from the presence of strong adsorption sites, even for supercritical H2. The adsorbed amounts per the weight and per the nanopore volume of SWCNT-C60(x) of supercritical H2 at 77 K are plotted against the C60-doped amount x, as shown in Fig. 14. H2 adsorptivity of SWCNT-C60(x) per the sample weight markedly enhances the adsorption of H2, providing almost twice the adsorption amount of SWCNTs in the low-pressure region, and 1.3 times higher in the ambient pressure region. On the other hand, H2 adsorptivity of SWCNT-C60(x) per the nanopore volume remarkably decreased by C60-pillaring. These results indicate that H2 adsorptivity of SWCNT-C60(x) enhanced due to the increase in the nanopore volume, even though the interaction between the nanopore and supercritical H2 were weakened.
Supercritical gaseous molecules are concentrated in nanopores by the strong molecule-pore interaction and a supercritical gas adsorbed in nanopores is transformed into a quasi-vapor. The Dubinin-Radushkevich (DR) equation extended for a quasi-vapor supercritical gas is expressed in the terms of the quasi-saturated vapor pressure P0q and the inherent nanopore volume VL by
where V is the nanopore volume at pressure P, E0 the characteristic adsorption energy, and β the affinity coefficient. The isosteric heat of adsorption at the fractional filling θ of 1/e can be calculated using β E0 and the enthalpy of vaporization ΔHv at the boiling point from the relation of
The DR plot, that is, the plot of [ln(VL/V)]1/2 versus lnP gives both values of P0q under the nanopore field and (Kaneko et al., 1992; Kaneko & Murata, 1997). The typical DR plot for SWCNT-C60(1.68) is shown in Fig. 15a, which exhibits a linear relationship. The isosteric heat of H2 adsorption evaluated from the supercritical DR plot is shown in Fig. 15 b. The isosteric heat is in the range of 9.5 to 9.7 kJ mol-1, being almost constant regardless of C60-pillaring. Thus, enhancement of the H2 adsorptivity of C60-pillared SWCNTs results from the increase in the nanospace volume. This value is much greater than the condensation enthalpy of H2 molecules at the boiling point (~20 K; 0.22 kJ mol-1) and the isosteric heat on the interstitial sites of (12, 12) SWCNT bundles (~9 kJ mol-1) or on the slit pores of carbon (~8 kJ mol-1) evaluated from Grand Canonical Monte Carlo simulation, indicating strong H2 molecule-interstitial pore interaction in the C60-pillared SWCNT bundles.
3.2. Predominant bundle formation from isolated SG SWCNTs
Figure 16 shows the N2 adsorption isotherms of SG SWCNT, SWCNT/Tol, and SWCNT/Met at 77 K. The N2 adsorption isotherm of SG SWCNT is of IUPAC type II, showing that SG SWCNT is mutually isolated and the caps are closed. On the other hand, the isotherms of SWCNT/Tol and SWCNT/Met are close to IUPAC Type I, indicating the presence of predominant micropores. The specific surface area (SSA) determined by the αs method (Kaneko et al., 1998) is shown in Table 2. The SSAs of SWCNT/Tol and SWCNT/Met (750 m2 g-1 and 760 m2 g-1) are considerably decreased by the sonication treatment, indicating the formation of the bundle structure with interstitial sites which are not accessible by N2 molecules. SWCNT/Tol and SWCNT/Met should have enough bundle structure. Figure 17a shows the high pressure H2 adsorption isotherms at 77 K, which are Langmuirian suggesting the presence of considerably strong interaction between H2 and each SWCNT sample. Figure 17b shows adsorption isotherms expressed by the absolute H2 amounts per the sample weight, which clearly show the formation of efficient surface for H2 adsorption. Consequently, interstitial sites were efficiently introduced in the SWCNT bundle by the compression on drying with the aid of capillary force.
Two methods for tuning the bundle structure of SWCNT to enhance its H2 adsorptivity were proposed; a one-step method for C60-pillaring in SWCNT bundles by the cosonication of C60 and SWCNT in toluene and predominant bundle formation of the isolated SWCNTs by drying SWCNTs dispersed in toluene or methanol. C60-pillared SWCNT with expanded hexagonal and distorted tetragonal arrays has enhanced H2 adsorptivity, providing almost twice the adsorption amount of SWCNTs in the low-pressure region, and 1.3 times higher in the ambient pressure region. Isolated SWCNTs treated with toluene and methanol should make enough bundle structure by the compression on drying with the aid of capillary force, forming efficient surface for H2 adsorption. These results indicate simple and promising tuning routes for SWCNT bundle structures, allowing the utilization of interstitial nanopore spaces for various fields, such as electrochemical, adsorption, sensor, and separation technologies.
We are grateful to Dr. Kunimitsu Takahashi at Institute of Research and Innovation (IRI) for providing SWCNT samples. We also thank Mr. Daisuke Noguchi for the H2 adsorption measurements at 40 K. This research was founded with Grant-in-Aids for Scientific Research (S) from Japanese Government. K.K. was supported by Exotic Nanocarbons, Japan Regional Innovation Strategy Program by the Excellence, Japan Science and Technology Agency.