J. Phys. Chem. B 2002, 106, 3371-3374
3371
Study of Nitrogen Adsorbed on Single-Walled Carbon Nanotube Bundles Dae-Hwang Yoo,† Gi-Hong Rue,‡ Yoon-Hwae Hwang,† and Hyung-Kook Kim*,† Department of Physics and RCDAMP, Pusan National UniVersity, Pusan 609-735, Korea, and School of Electronic and Electric Engineering, Kyungpook National UniVersity, Taegu 702-701, Korea ReceiVed: August 3, 2001; In Final Form: NoVember 5, 2001
The adsorption of N2 on close-ended single-walled carbon nanotube bundles was studied at different temperatures ranging from 81 to 94 K. The isosteric heat of adsorption was obtained from the adsorption isotherm measurements, and the binding energy of N2 was estimated from the isosteric heat of adsorption data in the low-coverage region. The estimated binding energy was about 86 meV, and this value lies between those of Ne and CH4 on nanotube bundles. It was also observed that the isosteric heat of adsorption increased in regions of low coverage and decreased as the coverage increased. This was due to both interactions between N2 molecules and the heterogeneity of the nanotube bundles.
Introduction
Experimental Section
A carbon nanotube (CNT) is an ultrathin carbon fiber with a nanometer-size diameter and a micrometer-size length. CNTs were accidentally discovered by Sumio Iijima in a carbon cathode that was used in the arc-discharging process for the preparation of fullerenes, which are small carbon clusters. The structure of a CNT consists of an enrolled graphitic sheet. CNTs are classified mainly as either multiwalled or single-walled carbon nanotubes (MWNTs or SWNTs, respectively) depending on their preparation method. As novel and potential carbon materials, CNTs are far more useful and important than fullerenes from a practical point of view. They can be used in nanoscale applications such as nanoscale devices1,2 and ultrafine probes.3,4 CNTs are also useful in macroscale applications such as field emission devices,5-8 gas storage devices,9-12 and macroscale devices.13 Recently, the adsorption of gases on various adsorbents including CNTs has been intensively studied both experimentally2-8,10-18 and theoretically1,9,19-27 because of the interesting physical phenomena occurring in the adsorption process and the potential for practical applications. An academic interest in adsorption on carbon nanotubes has stemmed from the consideration that this system can provide an experimental realization of matter in one dimension. Practically, the adsorption phenomena can be used for the development of new gas storage technologies, which would have a considerable economic impact. Theoretical studies have shown that the binding energy of a gas on SWNTs is higher than that on graphites,19 indicating the possibility of using SWNTs as a gas storage material. This paper reports an experimental study of the adsorption of N2 on close-ended single-walled carbon nanotubes. From the measurements of the isothermal adsorption at different temperatures, the isosteric heat of adsorption, qst, was estimated. The dependence of these quantities on the coverage is also reported, as it is related to the interactions between N2 molecules and to the heterogeneity of the substrate.
SWNTs with half-fullerene caps at the ends, produced using the pulsed-laser vaporization method, were purchased from Rice University.28 The mean tube diameter of the SWNTs was 1.2 nm. The nanotube samples were placed in a copper cell and evacuated at 350 K for 24 h before measurements were taken. The gas handling system consisted of 1/4-in. VCR-type valves (Nupro) and a capacitance pressure gauge (MKS Baratron 127). A He closed-cycle refrigerator (CTI model 22) was used, and the temperature was controlled using a temperature controller (Lakeshore DRC-93CA) with 0.01-K precision. Measurements were performed at the five different temperatures 81.20, 83.30, 86.40, 88.45, and 93.72 K.
* Corresponding author. E-mail:
[email protected]. † Pusan National University. ‡ Kyungpook National University.
Results and Discussion Figure 1 shows the isothermal adsorption data of nitrogen on SWNTs in a linear-logarithmic plot measured at 71 K above the triple point. In the early stages, nearly all portions of the gases that flowed into the cell were adsorbed on the SWNTs. In Figure 1, a knee can be observed at low coverage. It indicates the formation of the first coverage of N2 on the substrate. The gases start to contribute to the pressure of the system after the knee. The value of the saturation pressure at 71 K was about 324 Torr, at which point the coverage diverged, as shown in Figure 1. The amount of N2 adsorbed at the knee was approximately 1 mmol/g. Yin et al.25 reported GCMC simulations for the adsorption of nitrogen on square-packed nanotube arrays with varying nanometer diameters and separations. However, the adsorbed amount in their simulation results is much larger than that in our experimental result. This might be a result of the difference in form of the nanotube arrays. Nanotubes have a natural tendency to arrange themselves trigonally and these trigonal arrays cause interstitial channel spaces smaller than those formed by square-packed arrays. This reduces the adsorption capacity. Another reason that might cause the difference in adsorbed amounts is in the sites considered to be adsorbed in theory. There are three possible sites on nanotube bundles for adsorption. These are interstitial channels, ridges, and the bundle’s outer surfaces. In the simulation results of Yin et al.,25 the interstitial channel sites were considered to be filled
10.1021/jp013004e CCC: $22.00 © 2002 American Chemical Society Published on Web 03/08/2002
3372 J. Phys. Chem. B, Vol. 106, No. 13, 2002
Yoo et al.
Figure 2. Adsorption data for all of the temperatures in this study. The pressure (y axis) is given in the natural logarithmic scale.
Figure 1. Isotherm adsorption data for nitrogen on SWNTs at 71 K. The inset shows a comparison isotherm adsorption for nitrogen on SWNTs and on graphite. The arrow indicates the fluid-solid phase transition of nitrogen on graphite with increasing coverage. The coverage (y axis) is given in the common logarithmic scale.
prior to the other sites. However, in a report on isotherm adsorption experiments for CH4, Ne, and Xe, Talapatra et al.29 reported that all three gases were adsorbed on the same types of sites regardless of the size of the gas. In addition, the calculated effective area of the SWNT bundles for each gas was the same. From these results, they concluded that CH4, Ne, and Xe gases do not adsorb in the interstitial channels of nanotubes.29 Thus, for these gases, two types of adsorption sites on nanotube bundles are accessible. These are the ridges on the outside surface of the bundles where two tubes meet and the surfaces of the individual tubes on the outer surface of the bundles. The binding energy of the ridges is larger than that of the outer surface, so the gas has a tendency to adsorb on the ridges first. To compare the results of adsorption on different adsorbents, adsorption isotherm measurements of nitrogen on graphite were made at 71 K, and the results are shown in the inset of Figure 1. The most obvious difference is the amount of nitrogen adsorbed on the substrates. For the same pressure, the amount of adsorption on the SWNTs was about 3 times higher than that on graphite. This indicates that a higher pressure is needed to hold the same amount of adsorbate on graphite, i.e., the binding energy of nitrogen on SWNTs is higher than that on graphite. As can be seen in the inset, there is a step in the adsorption of N2 on graphite, which originates from the fluidsolid phase transition in N2. For SWNTs, however, no abrupt change in N2 adsorption was observed. Therefore, we conclude that the physical properties of SWNTs are different from those of graphite, even though a CNT has the shape of a rolled-up graphite sheet. Adsorption is an exothermic process. The energetics of the process can be described in terms of the heat of adsorption, which is the amount of heat released when an atom adsorbs on a substrate. In the limited case of T ) 0 with zero coverage, the isosteric heat of adsorption, qst, is equal to the binding energy of an atom to the substrate. However, at finite temperatures and
coverages, the isosteric heat of adsorption also reflects the interactions among the gas molecules and the characteristics of the substrate. The isosteric heat of adsorption can be measured calorimetrically, or it can be determined from adsorption isotherm data measured at different temperatures. In the latter case, the isosteric heat of adsorption is defined as30
(∂ ∂Tln P)
qst ) kT2
N
(1)
Here, k is Boltzmann’s constant, N is the amount of gas adsorbed on the nanotubes, P is the pressure of the coexisting unadsorbed gas, and T is the average value of the temperature. Data on the magnitude of the heat of adsorption and its coverage dependence can provide useful information about the nature of the surface and the adsorbed phase. According to the ideal Langmuir model, the heat of adsorption should be independent of coverage.31 This requirement, however, is seldom fulfilled in real systems because of the effect of surface heterogeneity and adsorbate-adsorbate interactions.31 We first considered the effect of interactions. The isosteric heat of adsorption of N2 was estimated for different values of N. Figure 2 shows the plot of N vs ln P for five different temperatures. In Figure 2, only a limited range of data was plotted around the knee in Figure 1. Using these data, the isosteric heat of adsorption was estimated using eq 1, and the results are shown in Figure 3. The y axis is represented in two different units: meV and kJ/mol. Four graphs in Figure 3a represent the coverage-dependent heat of adsorption curves obtained from all possible pairs of temperature intervals for the temperatures examined; that is, q1 is for 81.2 and 83.3 K, q2 is for 83.3 and 86.4 K, and so on. The curves shown in Figure 3a satisfy the condition that the values of the heat of adsorption are larger at higher temperatures. The average value of the heat of adsorption is also shown in Figure 3b. The maximum value of the data (∼95 meV) at low coverage is comparable to that at zero coverage on graphite.32 This is different from the expectation that the heat of adsorption for nanotubes is larger than that for graphite. The heat of adsorption decreased rapidly as the coverage increased when N was greater than 0.08 mmol/ g. An underlying mechanism explaining this observation must be related to the electrostatic properties of the nitrogen and the geometric shape of the nanotube bundles. Previously, Masuda et al.33 showed that the heats of adsorption of CH4, N2, and CO on various NaCaA zeolites depend on the coverage. They found that, for the quadrapolar N2 and dipolar CO molecules, the
Nitrogen Adsorbed on Carbon Nanotube Bundles
J. Phys. Chem. B, Vol. 106, No. 13, 2002 3373 that both the quadrapolar property of nitrogen and the heterogeneity of the substrate contribute to the decrease of the adsorption heat. Figure 3 also shows an abnormal increase of the heat of adsorption at coverages below 0.08 mmol/g. This region corresponds to the adsorption of N2 on the ridges, and N2 can be considered as a one-dimensional gas. As the coverage increases, N2 molecules start to experience interactions between molecules. Therefore, we expect qst to increase with N in this region. We need to study the coverage-dependent heat of adsorption using different molecules for a more complete understanding. A detailed study is currently in progress in our laboratory, and the results will be published in the future. The condition of thermodynamic equilibrium between nitrogen molecules adsorbed on nanotubes and those in the vapor allows the binding energy to be determined. When the system reaches equilibrium, the value of the chemical potential of the adsorbed nitrogen must be equal to that of the vapor that coexists with N2 adsorbed inside the cell. We also expect that, at very low coverage, the adsorbate can be considered as a onedimensional ideal gas and the vapor can be treated as a threedimensional ideal gas. With these conditions, the binding energy can be approximated in terms of the heat of adsorption as30
qst ) Eb + 2kT
Figure 3. (a) Isosteric heat of adsorption curve for all pairs of temperature intervals examined in this study. q1 is for 81.2 and 83.3 K, q2 for 83.3 and 86.4 K, q3 for 86.4 and 88.45 K, and q4 for 88.45 and 93.72 K. (b) Average value of the isosteric heat of adsorption over all different pairs of temperatures.
decrease in heat of adsorption with increasing coverage was more pronounced than that for the nonpolar CH4 molecules. This leads to the conclusion that the decrease of qst in Figure 3 originates from the quadrapolar property of N2. We now consider the effect of surface heterogeneity. The heats of adsorption on the ridges and the outer surface will be different for nanotube bundles. During adsorption, as the coverage increases, the preferred adsorption sites change from the ridges, having a higher binding energy, to the outer surface, having a lower binding energy. This change also causes a decrease in the heat of adsorption. Masuda et al.33 reported that the heat of adsorption decreases to about 80% of the maximum value of the heat of adsorption with increasing coverage. In our study, the rapid decrease of the heat of adsorption to 35 meV on nanotube bundles, which is less than one-half of the maximum value of the heat of adsorption, supports the idea
(2)
Here, 2kT is related to the difference in the number of degrees of freedom between the vapor and the adsorbed phase. This relation was applied to the coverage below the peak in Figure 3. The estimated average value of the binding energy, Eb, was 85.6 meV. In Figure 3, the value of the heat of adsorption is 35 meV near the knee. This indicates that the binding energy of nitrogen on nanotube bundles might be lower than that on graphite near the knee. This seems to be a natural result in that the number of carbon atoms on the outer surface of CNTs is smaller than the number on graphite, which follows from the geometric shape of the nanotube bundles. The value of the binding energy lies between the Eb values of Ne (52 meV) and CH4 (222 meV).34 This difference might originate from the ratio of the size of the adsorbates to the corrugation of the adsorbent. The adsorbates whose size is smaller than the corrugation of the adsorbent would experience the microscopic potential of the adsorbent. In contrast, the surface of the substrate would appear almost energetically uniform to a larger molecule, which sees only the potential averaged over a larger region. We also speculate that the difference in the values of the binding energies might originate from the molecular size of adsorbed gas under the assumption that the larger the size of the molecule, the greater the interaction between the gas and the substrate. Conclusions We measured the isothermic adsorption of nitrogen at the five different temperatures 81.20, 83.30, 86.40, 88.45, and 93.72 K and estimated the isosteric heat of adsorption, qst, from these data. We observed that the isosteric heat of adsorption increased at low coverage, because of the interactions between N2 molecules, and decreased as the coverage increased because of the interactions between N2 molecules and the heterogeneity of the nanotube bundles. The binding energy of nitrogen adsorbed on SWNTs at low coverage was estimated to be about 86 meV. The obtained binding energy lies between those of Ne and CH4 on nanotube bundles. Acknowledgment. This work was supported by Grant 20001-11400-004-2 from the Basic Research Program of the Korea Science and Engineering Foundation.
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