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Nitrogen adsorption characterization of aligned multiwalled carbon nanotubes and their acid modification
Journal of Colloid and Interface Science 277 (2004) 35–42 www.elsevier.com/locate/jcis
Nitrogen adsorption characterization of aligned multiwalled carbon nanotubes and their acid modification Zuojiang Li, Zhengwei Pan, Sheng Dai ∗ Chemical Sciences Division, MS-6201, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 23 January 2004; accepted 18 May 2004 Available online 5 June 2004
Abstract Aligned multiwalled carbon nanotube (CNT) arrays were synthesized by using an iron-based sol–gel catalyst and acetylene as the precursor. These CNTs show high purity, uniform diameters and pore-wall thickness. Low temperature nitrogen adsorption was employed to characterize the structural and surface properties of the as-synthesized sample and that modified with boiling concentrated nitric acid. The adsorption characteristics of the as-synthesized and modified CNTs were thoroughly investigated. High-resolution comparative αs -plot showed that the nitrogen adsorption on CNTs takes place via a multistage mechanism closely related to their structures. It was also found that the acid modification significantly increased the adsorption energy and enhanced the adsorption capacity under low pressures. High-resolution comparative method provided valuable insights about the surface and pore structures of CNTs. Published by Elsevier Inc. Keywords: Carbon nanotubes; Nitrogen adsorption; Surface modification; αs -Plot
1. Introduction The discovery of carbon nanotubes [1,2] has stimulated intensive research on the synthesis, modification and physical properties of these novel carbon materials [1–5], due to their unique one-dimensional pore structure, rolled graphitic layers as well as applications in sensors, separations, electronic devices, gas storage and quantum dots [6–8]. Carbon nanotubes can be basically classified into multiwalled and single-walled carbon nanotubes (SCNTs) or open-ended and closed CNTs according to their morphologies. The most prominent structural character of carbon nanotubes is their one-dimensional, nanoscale cavities in the micropore and mesopore range, varying from ∼1 nm for single-walled CNT and 3–10 nm for multiwalled CNTs. The availability of these inner cavities in adsorption, separation, nanosized reaction as well as templating fabrication is determined by the openness of CNT ends. As an advanced adsorbent, the adsorption phenomenon, especially the gas adsorption on carbon nanotubes, has been studied via theoretical calculations [9,10], experimental measurements [11–16] and the molecular sim* Corresponding author. Fax: +865-576-5235.
E-mail address: [email protected] (S. Dai). 0021-9797/$ – see front matter Published by Elsevier Inc. doi:10.1016/j.jcis.2004.05.024
ulations [17,18]. One of the incentives of these studies is the adsorption and storage of hydrogen in carbon nanotubes, which is believed to be superior to other adsorbent because of their distinctive tubular structure and the graphitic pore walls. However, diverse results of hydrogen storage capacity have been reported ranging from 0.1 to 10 wt%, and the reproducibility of these data is very poor [19–21]. So, it becomes critically important to characterize the structure of CNTs and predict their gas adsorption properties. The research in this area may clarify the discrepancies related to hydrogen storage in CNTs. As a reliable method in characterization of porous solids, nitrogen adsorption at low temperature has been proven to provide large quantity of information about the adsorbent, including specific surface area, external surface area, microporosity and mesoporosity and pore size distribution etc. [22–24]. The adsorption isotherm in low-pressure region can also provide insights into the surface properties such as structural and/or functional heterogeneities [25–27]. This technique has been applied to the structural analysis of other porous and nonporous carbons. There have been several reports related to nitrogen adsorption on CNTs [11–16], which provided valuable information regarding the structure of CNTs. For instance, Inoue and Kaneko et al. [13] studied
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the capillary condensation of nitrogen on multiwalled CNTs and analyzed the adsorption with comparative αs -plot. However, the reference material they employed is a nonporous carbon black instead of graphite. The analyses and classifications of nitrogen adsorption on aggregated CNTs by Yang et al. [15] is oversimplified and the calculation of pore size distribution by employing desorption branch of the isotherm is inappropriate. It is known that the as-synthesized carbon nanotubes normally contain certain amounts of catalyst particles, graphitic particles and amorphous carbon that may degrade the properties of CNTs. In order to remove these impurities and open the CNT channels, several approaches have been employed, including oxidation with acid, burning in air/oxygen, or acid oxidation followed by high temperature treatment in inert atmosphere etc. [28–32]. These treatments dramatically increase the purity of carbon nanotubes and, in many cases, open the CNT ends and increase the adsorption capacity. Pan et al. synthesized aligned multiwalled CNTs by using an iron based sol–gel catalyst and acetylene as the carbon precursor at lower temperatures (about 650 ◦ C) [33,34]. In addition to their high purities and uniform diameters, the length of these carbon nanotubes can reach as long as millimeter scale in comparison to micron scale reported previously. Pan et al. also prepared a patterned CNTs arrays using this approach [35]. The availability of these long and aligned CNTs facilitates the measurement of physical properties and their applications in electronic devices. In this paper, the as-synthesized, aligned multiwalled CNTs are treated with concentrated nitric acid and, the adsorption properties of the untreated and treated CNT are fully characterized with low temperature nitrogen adsorption. The adsorption isotherms are analyzed with high-resolution comparative αs -plot and the results are correlated to the structural characteristics of CNTs. Nitrogen adsorption is a convenient and powerful method to characterize the structural and surface properties of carbon nanotubes and, is also very useful in the prediction of gas adsorption properties of CNTs.
2. Experimental The aligned multiwalled carbon nanotubes were prepared by using an iron-based sol–gel catalyst and acetylene as the carbon precursor. Details of this method can be found in literature [33,34]. Basically, the film-like catalyst was prepared with 1.5 M Fe(NO3 )3 aqueous solution and equal volume of tetraethyl orthosilicate (TEOS) in the presence of ethanol and hydrofluoric acid, followed by drying at ambient temperature and 70 ◦ C. The catalyst was then treated in vacuum at 450 ◦ C overnight, and reduced in hydrogen at 550 ◦ C for 4 h. Aligned multiwalled CNTs were synthesized at 650 ◦ C for 6 h, with nitrogen diluted acetylene (10%) as carbon source. Well-aligned multiwalled carbon nanotubes grew perpendicularly to the substrate surface.
The as-synthesized, aligned CNTs were obtained by carefully peeling them off from the catalyst substrate using tweezers so that the original arrangement of CNTs would not be significantly damaged. The oxidation and purification of CNTs were carried out in concentrated nitric acid and refluxed at boiling temperature for 4 h. Later, the CNTs were thoroughly washed with deionized water. The obtained CNTs were filtered and dried. Felt-like CNTs were obtained after acid modification. Nitrogen adsorption was conducted at 77 K on a Quantachrome Autosorb-1 volumetric analyzer. The samples were degassed at 200 ◦ C under vacuum for 4 h in prior to the measurement. Comparative adsorption analysis was also employed to investigate the surface properties and microporosity of the nanotubes. 2.1. SEM and TEM Scanning electron microscopy (SEM) analyses were performed on a Philips XL-30 field emission scanning electron microscope operated at 10 kV. The microstructure of CNTs was examined with a Philips CM200 high-resolution transmission electron microscopy (HRTEM) operated at 200 kV.
3. Results and discussion 3.1. Characterization of the as-synthesized, aligned multiwalled carbon nanotubes by nitrogen adsorption at 77 K The multiwalled carbon nanotubes synthesized with the iron-based sol–gel catalyst exhibit good parallel arrays of individual tubes with uniform pore size and pore-wall thickness. In addition, this kind of CNTs can grow as long as several millimeters in perpendicular to every surface of the catalyst substrate through a tip-growing mechanism [33–35]. The catalytic chemical vapor decomposition of acetylene by the nanosized iron particles on the tips of carbon nanotubes make it possible to grow CNTs “forest” rather than a simple mixture of CNTs and amorphous carbons. Shown in Figs. 1a and 1b are SEM and TEM images of the as-synthesized multiwalled CNTs arrays. It is clearly visible that the CNTs assemble approximately parallel to each other in such a way that each nanotube is separated from another with a space of 30–100 nm. The pore walls of these CNTs are composed of well-organized graphitic layers with a concentric cylindrical cavity. Another advantage of this synthesis route is that one end of the CNTs is always open and no additional treatment is required to open the inner cavities of CNTs [34]. The purity of the CNTs prepared with this method is quite high (verified with SEM and TEM) and only very small amount of worm-like amorphous carbon coexists with CNTs on the top of the nanotubes arrays. Additionally, remarkable dimension uniformity was observed for the CNTs samples, which exhibit uniform outer and inner diameters. The TEM analyses
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(a)
Fig. 2. The nitrogen adsorption–desorption isotherm of aligned multiwalled carbon nanotubes in linear and logarithmic scales.
(b) Fig. 1. SEM (a) and TEM (b) images of as-synthesized, aligned carbon nanotubes.
showed that the out and inner diameters of these multiwalled carbon nanotubes are in the range of 15–20 and 5–8 nm, respectively. Shown in Fig. 2 is nitrogen adsorption isotherm of the as-synthesized CNTs in linear and logarithmic (inset) scale. One can notice that the as-synthesized CNT arrays exhibit a type II adsorption isotherm according to the IUPAC classification [22], with a sharp condensation step as the relative pressure approach unity. Several adsorption characteristics can be observed on this adsorption isotherm. First, very small amount of nitrogen is adsorbed at relative pressures below 0.05, suggesting the insignificant microporous and/or small mesoporous feature of the as-synthesized samples. The gradual increasing adsorption in the relative pressure region of 0.05–0.95 indicate an adsorption process on a solid with primarily mesoporous or macroporous pore structure [22,23]. This steadily increased adsorption of nitrogen is related to the multilayer adsorption phenomenon on the surface of adsorbent, i.e., the gradually increased film thickness of nitrogen and possibly capillary condensation in some mesopores. An obvious adsorption and condensation of nitrogen occurs in the relative pressure range of 0.95–1.0 that corresponds to pores larger than 40 nm. The adsorp-
tion/desorption hysteresis loop can be observed in the region of higher relative pressures, indicating the mesoporous characteristics of CNT pore structure. Apparently, the above condensation step at high relative pressures are caused by the condensation of nitrogen in spaces between individual carbon nanotubes, the dimension of which varies according to the relative positions between individual CNTs and, is in good agreement with the average interstitial space measured with SEM shown in Fig. 1a. Shown in the inset of Fig. 2 is the adsorption isotherm of CNT arrays in logarithmic scale to clearly present the adsorption characteristics of aligned CNTs under lower pressures. Very small amount of nitrogen (<5 cm3 /g STP) was adsorbed under relative pressure below 10−4 . Previous studies have shown that the adsorption in this region is closely related to the structural or functional heterogeneities of the sample [26,39]. The former is related to microporosity while the later may be caused by surface functional groups that may interact with the quadropolar nitrogen molecules. For the CNTs arrays under study, this adsorption should be caused by impurities in the sample or the structural highenergy sites such as CNT edges as well as micropores and/or intraparticular spaces. The noticeably adsorption steps in the relative pressure range of 10−4 –10−3 should be ascribed to formation of the first adsorption layer of nitrogen on the inner and/or external graphitic cavities of CNTs. Previous study of graphitized carbon blacks [27] observed a similar adsorption step in the same pressure region, and the exact position of which is dependent upon the degree of graphitic crystallinity [36]. The relatively steep adsorption step of CNTs reflects the high surface homogeneities [24,27], which in turn signifies the uniform structure and high purity of CNTs sample. However, the adsorption steps associated with the transition between a two-dimensional disordered fluid and two-dimensional ordered solid phase at relative pressure of ∼10−2 is not observed on aligned CNTs, indicating their
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surface homogeneity is lower than that of graphitized carbon blacks with low surface areas. The adsorption of nitrogen on carbon nanotubes is remarkably different from other porous carbons in several aspects. First, due to their one-dimensional tubular structure, CNTs have a surprising high aspect ratio and, the external surface available for adsorption is considerably larger than the surface area arising from inner cavities. For instance, the ratio of outer cavity surface area to inner surface area is about 1.6 for single-walled nanotubes, while this value is about 3 for the multiwalled CNTs under study. The predominance of outer cavity surface area to inner cavity surface area determines the adsorption characteristics of nitrogen on CNTs. The adsorption on the external surface of CNTs is more important than the adsorption inside the micro/mesoporous cavities. On the contrary, the nitrogen adsorption on the external surface of traditional porous carbons such as activated carbon or activated carbon fibers is negligible in comparison to that in micropores [24,37]. Another noteworthy difference should be ascribed to the interstitial space between individual carbon nanotubes. The dimension of these spaces is determined by the relative positions among CNTs. If the carbon nanotubes are closely arranged parallel to each other, the dimension of these interstitial spaces would be comparable to or even smaller than the inner tubular cavities. And, some researchers claimed that, especially in the case of hydrogen storage, the interstitial space is more crucial than the inner tubular cores [15]. The quality of interstitial space primarily relies on the relative position of individual carbon nanotubes, which is in turn determined by the synthetic conditions. The common purification process of CNTs normally involves the ultrasonic, stirring, filtration as well as other steps that may dramatically change the shape, quality and volume of the interstitial spaces. So, maintaining the as-synthesized arrays of single-walled or multiwalled CNTs is essential in applying them to adsorption and gas storage. The adsorption of nitrogen in mesopores has been interpreted in terms of the capillary condensation theory defined by the Kelvin equation [22]. The availability of highly ordered silica materials such as MCM-41, MCM-48 and SBA15 has stimulated the fundamental research regarding the molecular adsorption and capillary condensation phenomena in highly ordered structure [38]. One distinctive feature of those ordered silicas is the appearance of a sharp adsorption step on the adsorption isotherm at a relative pressure that correspond to the capillary condensation of nitrogen in the extremely uniform mesopores, a fact that can be applied to improve the Barrett–Joyner–Halenda (BJH) pore size distributions [38]. However, although the investigated CNT samples show a uniform mesoporous cavities of 5–8 nm, no steep adsorption steps were observed at relative pressures of 0.4–0.7 that correspond to the capillary condensation of nitrogen in these one dimensional, tubular pores. So far, this sharp adsorption step has not been observed in previous reports [11–16]. The lack of apparent capillary steps
is probably related to several factors such as small adsorption capacity of inner cavity in relative to external surface, the openness of tubular mesopores, the uniformity of pore sizes as well as possible amorphous carbon inside the tubular pores. Previous studies have showed that the cavities of some CNTs are closed in both ends and, complicated treatments are required to make them available in adsorption. Nevertheless, this reason should be excluded for the multiwalled CNTs under investigation because the SEM and TEM studies have proven that one end of the carbon nanotubes is often always [34]. The porous and surface properties of the aligned carbon nanotubes is examined by high-resolution αs -plot method [23,39,40], one of the most reliable ways to analyze the microporosity/mesoporosity and surface heterogeneities because it does not involve any geometrical assumptions about the pore structure. Conventionally, αs -plot method is used to analyze the microporosity and external surface area of porous solids. The αs -plot is constructed by plotting the volume adsorbed on the studied sample against the standard reduced adsorption αs , which is a function of relative pressure and defined on the reference solid as the ratio of the amount adsorbed at a given relative pressure to the adsorption capacity at relative pressure 0.4. The slope of the linear part on αs -plot is proportional to the ratio of external surface for the investigated sample with respect to the reference material, whereas the intercept of this linear region to the “adsorption quantity” axis is related to the microporosity. In this study, it is shown that αs -plot analysis can provide valuable information regarding the multistage adsorption of nitrogen on CNTs. A graphitized carbon black Carbopack F [27] was employed as the reference adsorbent. Presented in Fig. 3 is the high-resolution αs -plot for the as-synthesized, aligned carbon nanotube arrays. The dotted line is the projected αs curve for CNTs if adsorption on their
Fig. 3. High-resolution αs -plot for the aligned CNTs vs a graphitized carbon black reference material—Carbopack F. The dashed line denotes a predicted αs -plot if the adsorption on CNTs takes place via a multilayer adsorption as that on graphitized carbon black.
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surface is similar to that on the graphitized reference material. A deviation from this linearity stands for an adsorption process different from multilayer adsorption, which might be caused by the pore fillings [37] or the energetic differences between two solids compared [39]. Unlike conventional microporous carbons that exhibit distinct upward step at lower αs -value due to the micropore filling [37,41], CNTs show a downward deviation at low αs values (αs < 0.7). According to previous theoretical studies [39], this variation is caused by the lower average energy of carbon nanotubes in comparison to that of the graphitized carbon black. Similar results were also observed by Inoue and Kaneko when they compared CNTs with a carbon black reference adsorbent [13]. The αs -plot analysis certifies the negligible amount of micropores and/or small mesopores in CNTs, as evidenced by the extremely small intercept of the αs curve on the adsorption axis in Fig. 3. In the αs range of 0.55–0.65, the adsorption on CNTs gradually approaches the projected multilayer adsorption line, suggesting the formation of nitrogen monolayer on the carbon nanotubes is completed and, the effect of carbon surface on the continued adsorption is decreasing as the adsorption film thickness is increased. Later, in the αs range of 0.65–1.0 that corresponds to a relative pressure range of 0.18–0.4, the nitrogen adsorption on CNTs take place via the same multilayer adsorption mechanism as that on the compared reference graphitized carbon black. A second deviation can be observed in higher αs region (1.0 < αs < 1.3), showing an enhanced adsorption caused by the capillary condensation inside the tubular cavities and/or interstitial spaces between CNTs. The corresponding relative pressure range for this capillary condensation process is 0.4–0.65. The Kelvin equation establishes a relationship between the cylindrical pore size and the condensation relative pressure as follows [22]: ln
2σ Vm 1 p , =− p0 RT (r − t)
where σ is the surface tension of liquid adsorbate, Vm is the molar volume of nitrogen at the measurement temperature and r is pore radius and t is the film thickness on adsorbent. R and T are gas constant and absolute temperature, respectively. Hence, the calculated pore width for CNTs is between 5.3 and 8.5 nm respectively, if the nitrogen film thickness on the carbon surface defined by Halsey [42] is taken into account. This result signifies narrow pore-size distributions for inner cavities and the high-quality CNTs sample under study, which is also in good agreement with the TEM analyses. A secondary linearity can be observed in the αs range of 1.3–1.55 that correspond to a relative pressure range of 0.65– 0.77, showing that in this adsorption range, the capillary condensation inside the inner cavities is completed. Therefore, the adsorption takes places exclusively on the external surface of the carbon nanotubes. The downward deviation in the larger αs region (αs > 1.55) should be caused by the
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huge interparticular capillary condensation on the reference solid Carbopack F [27]. Besides the two condensation steps discussed above, Inoue and Kaneko also observed a third deviation for the adsorption in the interparticle spaces [13], which may be related to either the aggregated CNT samples or the carbon black reference material in their studies. It should be emphasized here that the interstitial space in the aligned CNT sample in this study is much bigger than that in aggregated CNTs, which is supported by the adsorption/desorption hysteresis in high-pressure region and the SEM image in Fig. 1a. Based on the analyses above, it is reasonable to claim that the formation of a monolayer of nitrogen is completed on the internal and external surface of carbon nanotubes as the αs value reaches 0.65. Therefore, the amount adsorbed at this point can be used to evaluate the surface area of multiwalled CNTs investigated, leading to a surface area of 305 m2 /g. This value is very close to the BET surface area of 293 m2 /g for the aligned CNTs, suggesting that the surface area arising from other sources such as micropores is negligible. 3.2. Nitric acid modification and its effects on the adsorption properties of multiwalled carbon nanotubes Considerable efforts have been put into the purification of as-synthesized CNTs based on the different oxidation rate [31,32] between carbon nanotubes and the amorphous carbons. The employed oxidizing agents include hydrogen peroxide [28], nitric acid [29,30], air or oxygen [31]. The end-caps and the surface of tubular CNTs are oxidized to carboxylic acid, phenolic groups as well as other weakly acidic functional groups. Although these purification endeavors increase the purity of single-walled and multiwalled CNTs, it was found that these treatments may damage the original structure of CNTs and create some defects on the surface. This phenomenon is more significant when the CNTs was purify with strong oxidizing reagent [30]. Previous studies also [43] confirmed that the acid oxidation of active carbons and carbon blacks dramatically change their surface and structural properties. Encouragingly, nitrogen adsorption especially low-pressure adsorption isotherms can be used to investigate the surface and structural properties. In order to investigate the effects of oxidation treatments on the surface and structural properties of carbon nanotubes, the CNT sample was treated in concentrated, boiling nitric acid for 4 h and the nitrogen adsorption isotherm was measured. However, in this case the original alignment of the as-synthesized CNTs was destroyed and the sample become an aggregated CNT felt after oxidation followed by washing with deionized water and drying in oven. Fig. 4 is the nitrogen adsorption isotherm at 77 K for the strongly oxidized sample in linear and logarithmic (inset) scale. The corresponding αs -plot is shown in Fig. 5. The Brunauer–Emmett–Teller (BET) surface area, mesopore surface area and the total pore volume for the as-synthesized and the oxidized carbon nanotubes are summarized in Ta-
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Table 1 The pore structure parameters for as-synthesized, aligned CNTs and the acid modified CNTs Sample
As-synthesized, aligned CNTs
Acid oxidized CNTs
BET surface area (m2 /g) Mesopore surface area (m2 /g) Total pore volume (cm3 /g)* Micropore volume (cm3 /g)**
293 283 0.51 Not detected
516 248 1.03 0.11
* Total pore volume was calculated by converting the adsorption capacity at relative pressure of 0.99 into volume of liquid nitrogen using a factor of 0.001546. ** Micropore volume was computed by using α range of 0.7–1.0. s
Fig. 4. The nitrogen adsorption–desorption isotherm for the nitric acid treated multiwalled carbon nanotubes in linear and logarithmic scales.
Fig. 5. High-resolution αs -plot for the acid-treated, multiwalled CNTs vs a graphitized carbon black reference material—Carbopack F. The intercept of dashed line on the adsorption axis indicates the micropore volume.
ble 1. The oxidized carbon nanotubes still show a Type II adsorption isotherm but its adsorption capacity is significantly increased. The obvious adsorption step in the relative pressure of 0.8–1.0 and the appearance of the huge adsorption/desorption hysteresis are related to the notably amount of interstitial space between individual carbon nanotubes due to the aggregated felt-like morphology. Moreover, the acid treated sample exhibits enhanced adsorption at low relative pressures. The adsorption capacity of the oxidized sample was dramatically increased to about 105 cm3 /g at lower (p/p0 = 0.05) relative pressure, in comparison to that of about 30 cm3 /g on the as-synthesized sample. The enhanced adsorption capacity at low pressure is normally caused by either the microporosity or the surface functionalities. The boiling nitric acid can change the original porous structure of carbons, in addition to increase the surface hy-
drophilicity [43]. Therefore, the strong acid treatment may not only remove the iron catalyst and the amorphous carbons but also create some structural defects or micropores on outer and inner surface of CNTs [30]. The micropore volume in Table 1 was calculated by extrapolating αs curve in the range of 0.7–1.0 to the adsorption axis and converting the intercept into micropore volume. The oxidized sample has a micropore volume of 0.11 cm3 /g while it is negligible for the as-synthesized sample. The oxidized sample also show a smoother adsorption step in the relative pressure of 10−4 –10−3 compared to the as-synthesized CNTs, which is indicative of a more heterogeneous carbon surface for the modified sample. After acid-oxidation treatment, both the BET surface area and the pore volume were increased approximately by a factor of two in comparison to those of the as-synthesized sample. Yang et al. [16] also found that the acid-purification treatment can remarkably increase the BET surface area and micropore volume for single-walled carbon nanotubes. Simultaneously, they observed the decrease of mesopore volume by purification, which was believed caused by the oriented assembly structure of purified single-walled carbon nanotubes (SCNTs). However, in this study, the mesopore volume increases after the acid treatment. Consequently, the dramatic increase of the surface area and micropore volume for this multiwalled CNTs should be related to the removal of iron catalyst and amorphous carbon, the formation of aggregated felt structure of treated sample, and oxygen functional groups on the edge or tube surfaces. The shape of the high-resolution αs curves for the multiwalled CNTs is distinctive from those of the single-walled carbon nanotubes reported by Yang et al. [16]. The former exhibits a downward deviation from the projected adsorption on graphitic surface whereas the latter displays an apparent upward deviation in the lower αs range (αs < 0.5). This should be attributed to the much smaller cavities (∼1 nm) in single-walled carbon nanotubes, which lead to an enhanced adsorption potential [24] inside these cylindrical pores. According to previous theoretical studies, an upward deviation is caused by the higher average energy of SCNTs relative to the referenced graphitized carbon black. This can probably explain the advantages of SCNTs over multiwalled CNTs
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in the gas storage and other adsorptive applications. On the other hand, for the adsorptive applications, the multiwalled carbon nanotubes do not show significant advantages over the conventional microporous carbons from the analyses of nitrogen adsorption isotherms. These observations may partially explain the controversial results on hydrogen storage and poor reproducibility of the data on single-walled or multiwalled CNTs. Another phenomenon worth discussion is the nitrogen adsorption on the single-walled carbon nanotubes. The nitrogen adsorption properties of the as-synthesized as well as the acid-modified multiwalled CNTs are close to the theoretical calculations. Nevertheless, even though the theoretical calculation and molecular simulations [18,44] predicted a very large surface area of ∼3000 m2 /g for SCNTs, so far, the BET surface area and pore volumes in the experimental studies [11–16] are several to ten times smaller than those predictions. This large discrepancy has not been paid much attention. We believe the possible causes include: (a) the inapplicability of Brunauer–Emmett–Teller model for the evaluation of the surface area of single-walled CNTs in which an enhanced potential is always observed; (b) the lower purity of the samples which may contain double-walled or multiwalled CNTs as well as other impurities such as residual amorphous carbons and catalyst and; (c) the accessibility of nitrogen molecules to small or closed cylindrical cavities or interstitial spaces. The purity of the sample would be a crucial factor in future adsorptive characterization of singlewalled carbon nanotubes.
4. Conclusions Nitrogen adsorption was successfully used to monitor the structural and surface properties of multiwalled carbon nanotubes before and after acid modification. Analyses of nitrogen adsorption isotherms and high-resolution comparative αs -plots of the as-synthesized and acid-modified CNTs showed that the nitrogen adsorption on CNTs takes place via a four-stage process: submonolayer adsorption; multilayer adsorption on the external and internal surface of cylindrical cavities; the capillary adsorption inside the cylindrical cavities; and the capillary condensation inside the interstitial spaces. The external surface of CNTs plays an essential role in the adsorption process. Nitric acid modification at boiling temperature significantly enhanced the adsorption capacity of carbon nanotubes at low pressures. This study also shows that multiwalled carbon nanotubes do not display apparent advantages over the conventional porous carbon for gas storage from the viewpoint of nitrogen adsorption.
Acknowledgments This research was supported in part by an appointment to the Oak Ridge National Laboratory Postdoctoral Research
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Associates Program administered jointly by the Oak Ridge Institute for Science and Education and the Oak Ridge National Laboratory (ORNL). The authors also acknowledge the ORNL’s SHaRE Collaborative Research Center for the use of their electron microscopy facilities.
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Nitrogen adsorption characterization of aligned multiwalled carbon nanotubes and their acid modification Zuojiang Li, Zhengwei Pan, Sheng Dai ∗ Chemical Sciences Division, MS-6201, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 23 January 2004; accepted 18 May 2004 Available online 5 June 2004
Abstract Aligned multiwalled carbon nanotube (CNT) arrays were synthesized by using an iron-based sol–gel catalyst and acetylene as the precursor. These CNTs show high purity, uniform diameters and pore-wall thickness. Low temperature nitrogen adsorption was employed to characterize the structural and surface properties of the as-synthesized sample and that modified with boiling concentrated nitric acid. The adsorption characteristics of the as-synthesized and modified CNTs were thoroughly investigated. High-resolution comparative αs -plot showed that the nitrogen adsorption on CNTs takes place via a multistage mechanism closely related to their structures. It was also found that the acid modification significantly increased the adsorption energy and enhanced the adsorption capacity under low pressures. High-resolution comparative method provided valuable insights about the surface and pore structures of CNTs. Published by Elsevier Inc. Keywords: Carbon nanotubes; Nitrogen adsorption; Surface modification; αs -Plot
1. Introduction The discovery of carbon nanotubes [1,2] has stimulated intensive research on the synthesis, modification and physical properties of these novel carbon materials [1–5], due to their unique one-dimensional pore structure, rolled graphitic layers as well as applications in sensors, separations, electronic devices, gas storage and quantum dots [6–8]. Carbon nanotubes can be basically classified into multiwalled and single-walled carbon nanotubes (SCNTs) or open-ended and closed CNTs according to their morphologies. The most prominent structural character of carbon nanotubes is their one-dimensional, nanoscale cavities in the micropore and mesopore range, varying from ∼1 nm for single-walled CNT and 3–10 nm for multiwalled CNTs. The availability of these inner cavities in adsorption, separation, nanosized reaction as well as templating fabrication is determined by the openness of CNT ends. As an advanced adsorbent, the adsorption phenomenon, especially the gas adsorption on carbon nanotubes, has been studied via theoretical calculations [9,10], experimental measurements [11–16] and the molecular sim* Corresponding author. Fax: +865-576-5235.
E-mail address: [email protected] (S. Dai). 0021-9797/$ – see front matter Published by Elsevier Inc. doi:10.1016/j.jcis.2004.05.024
ulations [17,18]. One of the incentives of these studies is the adsorption and storage of hydrogen in carbon nanotubes, which is believed to be superior to other adsorbent because of their distinctive tubular structure and the graphitic pore walls. However, diverse results of hydrogen storage capacity have been reported ranging from 0.1 to 10 wt%, and the reproducibility of these data is very poor [19–21]. So, it becomes critically important to characterize the structure of CNTs and predict their gas adsorption properties. The research in this area may clarify the discrepancies related to hydrogen storage in CNTs. As a reliable method in characterization of porous solids, nitrogen adsorption at low temperature has been proven to provide large quantity of information about the adsorbent, including specific surface area, external surface area, microporosity and mesoporosity and pore size distribution etc. [22–24]. The adsorption isotherm in low-pressure region can also provide insights into the surface properties such as structural and/or functional heterogeneities [25–27]. This technique has been applied to the structural analysis of other porous and nonporous carbons. There have been several reports related to nitrogen adsorption on CNTs [11–16], which provided valuable information regarding the structure of CNTs. For instance, Inoue and Kaneko et al. [13] studied
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the capillary condensation of nitrogen on multiwalled CNTs and analyzed the adsorption with comparative αs -plot. However, the reference material they employed is a nonporous carbon black instead of graphite. The analyses and classifications of nitrogen adsorption on aggregated CNTs by Yang et al. [15] is oversimplified and the calculation of pore size distribution by employing desorption branch of the isotherm is inappropriate. It is known that the as-synthesized carbon nanotubes normally contain certain amounts of catalyst particles, graphitic particles and amorphous carbon that may degrade the properties of CNTs. In order to remove these impurities and open the CNT channels, several approaches have been employed, including oxidation with acid, burning in air/oxygen, or acid oxidation followed by high temperature treatment in inert atmosphere etc. [28–32]. These treatments dramatically increase the purity of carbon nanotubes and, in many cases, open the CNT ends and increase the adsorption capacity. Pan et al. synthesized aligned multiwalled CNTs by using an iron based sol–gel catalyst and acetylene as the carbon precursor at lower temperatures (about 650 ◦ C) [33,34]. In addition to their high purities and uniform diameters, the length of these carbon nanotubes can reach as long as millimeter scale in comparison to micron scale reported previously. Pan et al. also prepared a patterned CNTs arrays using this approach [35]. The availability of these long and aligned CNTs facilitates the measurement of physical properties and their applications in electronic devices. In this paper, the as-synthesized, aligned multiwalled CNTs are treated with concentrated nitric acid and, the adsorption properties of the untreated and treated CNT are fully characterized with low temperature nitrogen adsorption. The adsorption isotherms are analyzed with high-resolution comparative αs -plot and the results are correlated to the structural characteristics of CNTs. Nitrogen adsorption is a convenient and powerful method to characterize the structural and surface properties of carbon nanotubes and, is also very useful in the prediction of gas adsorption properties of CNTs.
2. Experimental The aligned multiwalled carbon nanotubes were prepared by using an iron-based sol–gel catalyst and acetylene as the carbon precursor. Details of this method can be found in literature [33,34]. Basically, the film-like catalyst was prepared with 1.5 M Fe(NO3 )3 aqueous solution and equal volume of tetraethyl orthosilicate (TEOS) in the presence of ethanol and hydrofluoric acid, followed by drying at ambient temperature and 70 ◦ C. The catalyst was then treated in vacuum at 450 ◦ C overnight, and reduced in hydrogen at 550 ◦ C for 4 h. Aligned multiwalled CNTs were synthesized at 650 ◦ C for 6 h, with nitrogen diluted acetylene (10%) as carbon source. Well-aligned multiwalled carbon nanotubes grew perpendicularly to the substrate surface.
The as-synthesized, aligned CNTs were obtained by carefully peeling them off from the catalyst substrate using tweezers so that the original arrangement of CNTs would not be significantly damaged. The oxidation and purification of CNTs were carried out in concentrated nitric acid and refluxed at boiling temperature for 4 h. Later, the CNTs were thoroughly washed with deionized water. The obtained CNTs were filtered and dried. Felt-like CNTs were obtained after acid modification. Nitrogen adsorption was conducted at 77 K on a Quantachrome Autosorb-1 volumetric analyzer. The samples were degassed at 200 ◦ C under vacuum for 4 h in prior to the measurement. Comparative adsorption analysis was also employed to investigate the surface properties and microporosity of the nanotubes. 2.1. SEM and TEM Scanning electron microscopy (SEM) analyses were performed on a Philips XL-30 field emission scanning electron microscope operated at 10 kV. The microstructure of CNTs was examined with a Philips CM200 high-resolution transmission electron microscopy (HRTEM) operated at 200 kV.
3. Results and discussion 3.1. Characterization of the as-synthesized, aligned multiwalled carbon nanotubes by nitrogen adsorption at 77 K The multiwalled carbon nanotubes synthesized with the iron-based sol–gel catalyst exhibit good parallel arrays of individual tubes with uniform pore size and pore-wall thickness. In addition, this kind of CNTs can grow as long as several millimeters in perpendicular to every surface of the catalyst substrate through a tip-growing mechanism [33–35]. The catalytic chemical vapor decomposition of acetylene by the nanosized iron particles on the tips of carbon nanotubes make it possible to grow CNTs “forest” rather than a simple mixture of CNTs and amorphous carbons. Shown in Figs. 1a and 1b are SEM and TEM images of the as-synthesized multiwalled CNTs arrays. It is clearly visible that the CNTs assemble approximately parallel to each other in such a way that each nanotube is separated from another with a space of 30–100 nm. The pore walls of these CNTs are composed of well-organized graphitic layers with a concentric cylindrical cavity. Another advantage of this synthesis route is that one end of the CNTs is always open and no additional treatment is required to open the inner cavities of CNTs [34]. The purity of the CNTs prepared with this method is quite high (verified with SEM and TEM) and only very small amount of worm-like amorphous carbon coexists with CNTs on the top of the nanotubes arrays. Additionally, remarkable dimension uniformity was observed for the CNTs samples, which exhibit uniform outer and inner diameters. The TEM analyses
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(a)
Fig. 2. The nitrogen adsorption–desorption isotherm of aligned multiwalled carbon nanotubes in linear and logarithmic scales.
(b) Fig. 1. SEM (a) and TEM (b) images of as-synthesized, aligned carbon nanotubes.
showed that the out and inner diameters of these multiwalled carbon nanotubes are in the range of 15–20 and 5–8 nm, respectively. Shown in Fig. 2 is nitrogen adsorption isotherm of the as-synthesized CNTs in linear and logarithmic (inset) scale. One can notice that the as-synthesized CNT arrays exhibit a type II adsorption isotherm according to the IUPAC classification [22], with a sharp condensation step as the relative pressure approach unity. Several adsorption characteristics can be observed on this adsorption isotherm. First, very small amount of nitrogen is adsorbed at relative pressures below 0.05, suggesting the insignificant microporous and/or small mesoporous feature of the as-synthesized samples. The gradual increasing adsorption in the relative pressure region of 0.05–0.95 indicate an adsorption process on a solid with primarily mesoporous or macroporous pore structure [22,23]. This steadily increased adsorption of nitrogen is related to the multilayer adsorption phenomenon on the surface of adsorbent, i.e., the gradually increased film thickness of nitrogen and possibly capillary condensation in some mesopores. An obvious adsorption and condensation of nitrogen occurs in the relative pressure range of 0.95–1.0 that corresponds to pores larger than 40 nm. The adsorp-
tion/desorption hysteresis loop can be observed in the region of higher relative pressures, indicating the mesoporous characteristics of CNT pore structure. Apparently, the above condensation step at high relative pressures are caused by the condensation of nitrogen in spaces between individual carbon nanotubes, the dimension of which varies according to the relative positions between individual CNTs and, is in good agreement with the average interstitial space measured with SEM shown in Fig. 1a. Shown in the inset of Fig. 2 is the adsorption isotherm of CNT arrays in logarithmic scale to clearly present the adsorption characteristics of aligned CNTs under lower pressures. Very small amount of nitrogen (<5 cm3 /g STP) was adsorbed under relative pressure below 10−4 . Previous studies have shown that the adsorption in this region is closely related to the structural or functional heterogeneities of the sample [26,39]. The former is related to microporosity while the later may be caused by surface functional groups that may interact with the quadropolar nitrogen molecules. For the CNTs arrays under study, this adsorption should be caused by impurities in the sample or the structural highenergy sites such as CNT edges as well as micropores and/or intraparticular spaces. The noticeably adsorption steps in the relative pressure range of 10−4 –10−3 should be ascribed to formation of the first adsorption layer of nitrogen on the inner and/or external graphitic cavities of CNTs. Previous study of graphitized carbon blacks [27] observed a similar adsorption step in the same pressure region, and the exact position of which is dependent upon the degree of graphitic crystallinity [36]. The relatively steep adsorption step of CNTs reflects the high surface homogeneities [24,27], which in turn signifies the uniform structure and high purity of CNTs sample. However, the adsorption steps associated with the transition between a two-dimensional disordered fluid and two-dimensional ordered solid phase at relative pressure of ∼10−2 is not observed on aligned CNTs, indicating their
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surface homogeneity is lower than that of graphitized carbon blacks with low surface areas. The adsorption of nitrogen on carbon nanotubes is remarkably different from other porous carbons in several aspects. First, due to their one-dimensional tubular structure, CNTs have a surprising high aspect ratio and, the external surface available for adsorption is considerably larger than the surface area arising from inner cavities. For instance, the ratio of outer cavity surface area to inner surface area is about 1.6 for single-walled nanotubes, while this value is about 3 for the multiwalled CNTs under study. The predominance of outer cavity surface area to inner cavity surface area determines the adsorption characteristics of nitrogen on CNTs. The adsorption on the external surface of CNTs is more important than the adsorption inside the micro/mesoporous cavities. On the contrary, the nitrogen adsorption on the external surface of traditional porous carbons such as activated carbon or activated carbon fibers is negligible in comparison to that in micropores [24,37]. Another noteworthy difference should be ascribed to the interstitial space between individual carbon nanotubes. The dimension of these spaces is determined by the relative positions among CNTs. If the carbon nanotubes are closely arranged parallel to each other, the dimension of these interstitial spaces would be comparable to or even smaller than the inner tubular cavities. And, some researchers claimed that, especially in the case of hydrogen storage, the interstitial space is more crucial than the inner tubular cores [15]. The quality of interstitial space primarily relies on the relative position of individual carbon nanotubes, which is in turn determined by the synthetic conditions. The common purification process of CNTs normally involves the ultrasonic, stirring, filtration as well as other steps that may dramatically change the shape, quality and volume of the interstitial spaces. So, maintaining the as-synthesized arrays of single-walled or multiwalled CNTs is essential in applying them to adsorption and gas storage. The adsorption of nitrogen in mesopores has been interpreted in terms of the capillary condensation theory defined by the Kelvin equation [22]. The availability of highly ordered silica materials such as MCM-41, MCM-48 and SBA15 has stimulated the fundamental research regarding the molecular adsorption and capillary condensation phenomena in highly ordered structure [38]. One distinctive feature of those ordered silicas is the appearance of a sharp adsorption step on the adsorption isotherm at a relative pressure that correspond to the capillary condensation of nitrogen in the extremely uniform mesopores, a fact that can be applied to improve the Barrett–Joyner–Halenda (BJH) pore size distributions [38]. However, although the investigated CNT samples show a uniform mesoporous cavities of 5–8 nm, no steep adsorption steps were observed at relative pressures of 0.4–0.7 that correspond to the capillary condensation of nitrogen in these one dimensional, tubular pores. So far, this sharp adsorption step has not been observed in previous reports [11–16]. The lack of apparent capillary steps
is probably related to several factors such as small adsorption capacity of inner cavity in relative to external surface, the openness of tubular mesopores, the uniformity of pore sizes as well as possible amorphous carbon inside the tubular pores. Previous studies have showed that the cavities of some CNTs are closed in both ends and, complicated treatments are required to make them available in adsorption. Nevertheless, this reason should be excluded for the multiwalled CNTs under investigation because the SEM and TEM studies have proven that one end of the carbon nanotubes is often always [34]. The porous and surface properties of the aligned carbon nanotubes is examined by high-resolution αs -plot method [23,39,40], one of the most reliable ways to analyze the microporosity/mesoporosity and surface heterogeneities because it does not involve any geometrical assumptions about the pore structure. Conventionally, αs -plot method is used to analyze the microporosity and external surface area of porous solids. The αs -plot is constructed by plotting the volume adsorbed on the studied sample against the standard reduced adsorption αs , which is a function of relative pressure and defined on the reference solid as the ratio of the amount adsorbed at a given relative pressure to the adsorption capacity at relative pressure 0.4. The slope of the linear part on αs -plot is proportional to the ratio of external surface for the investigated sample with respect to the reference material, whereas the intercept of this linear region to the “adsorption quantity” axis is related to the microporosity. In this study, it is shown that αs -plot analysis can provide valuable information regarding the multistage adsorption of nitrogen on CNTs. A graphitized carbon black Carbopack F [27] was employed as the reference adsorbent. Presented in Fig. 3 is the high-resolution αs -plot for the as-synthesized, aligned carbon nanotube arrays. The dotted line is the projected αs curve for CNTs if adsorption on their
Fig. 3. High-resolution αs -plot for the aligned CNTs vs a graphitized carbon black reference material—Carbopack F. The dashed line denotes a predicted αs -plot if the adsorption on CNTs takes place via a multilayer adsorption as that on graphitized carbon black.
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surface is similar to that on the graphitized reference material. A deviation from this linearity stands for an adsorption process different from multilayer adsorption, which might be caused by the pore fillings [37] or the energetic differences between two solids compared [39]. Unlike conventional microporous carbons that exhibit distinct upward step at lower αs -value due to the micropore filling [37,41], CNTs show a downward deviation at low αs values (αs < 0.7). According to previous theoretical studies [39], this variation is caused by the lower average energy of carbon nanotubes in comparison to that of the graphitized carbon black. Similar results were also observed by Inoue and Kaneko when they compared CNTs with a carbon black reference adsorbent [13]. The αs -plot analysis certifies the negligible amount of micropores and/or small mesopores in CNTs, as evidenced by the extremely small intercept of the αs curve on the adsorption axis in Fig. 3. In the αs range of 0.55–0.65, the adsorption on CNTs gradually approaches the projected multilayer adsorption line, suggesting the formation of nitrogen monolayer on the carbon nanotubes is completed and, the effect of carbon surface on the continued adsorption is decreasing as the adsorption film thickness is increased. Later, in the αs range of 0.65–1.0 that corresponds to a relative pressure range of 0.18–0.4, the nitrogen adsorption on CNTs take place via the same multilayer adsorption mechanism as that on the compared reference graphitized carbon black. A second deviation can be observed in higher αs region (1.0 < αs < 1.3), showing an enhanced adsorption caused by the capillary condensation inside the tubular cavities and/or interstitial spaces between CNTs. The corresponding relative pressure range for this capillary condensation process is 0.4–0.65. The Kelvin equation establishes a relationship between the cylindrical pore size and the condensation relative pressure as follows [22]: ln
2σ Vm 1 p , =− p0 RT (r − t)
where σ is the surface tension of liquid adsorbate, Vm is the molar volume of nitrogen at the measurement temperature and r is pore radius and t is the film thickness on adsorbent. R and T are gas constant and absolute temperature, respectively. Hence, the calculated pore width for CNTs is between 5.3 and 8.5 nm respectively, if the nitrogen film thickness on the carbon surface defined by Halsey [42] is taken into account. This result signifies narrow pore-size distributions for inner cavities and the high-quality CNTs sample under study, which is also in good agreement with the TEM analyses. A secondary linearity can be observed in the αs range of 1.3–1.55 that correspond to a relative pressure range of 0.65– 0.77, showing that in this adsorption range, the capillary condensation inside the inner cavities is completed. Therefore, the adsorption takes places exclusively on the external surface of the carbon nanotubes. The downward deviation in the larger αs region (αs > 1.55) should be caused by the
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huge interparticular capillary condensation on the reference solid Carbopack F [27]. Besides the two condensation steps discussed above, Inoue and Kaneko also observed a third deviation for the adsorption in the interparticle spaces [13], which may be related to either the aggregated CNT samples or the carbon black reference material in their studies. It should be emphasized here that the interstitial space in the aligned CNT sample in this study is much bigger than that in aggregated CNTs, which is supported by the adsorption/desorption hysteresis in high-pressure region and the SEM image in Fig. 1a. Based on the analyses above, it is reasonable to claim that the formation of a monolayer of nitrogen is completed on the internal and external surface of carbon nanotubes as the αs value reaches 0.65. Therefore, the amount adsorbed at this point can be used to evaluate the surface area of multiwalled CNTs investigated, leading to a surface area of 305 m2 /g. This value is very close to the BET surface area of 293 m2 /g for the aligned CNTs, suggesting that the surface area arising from other sources such as micropores is negligible. 3.2. Nitric acid modification and its effects on the adsorption properties of multiwalled carbon nanotubes Considerable efforts have been put into the purification of as-synthesized CNTs based on the different oxidation rate [31,32] between carbon nanotubes and the amorphous carbons. The employed oxidizing agents include hydrogen peroxide [28], nitric acid [29,30], air or oxygen [31]. The end-caps and the surface of tubular CNTs are oxidized to carboxylic acid, phenolic groups as well as other weakly acidic functional groups. Although these purification endeavors increase the purity of single-walled and multiwalled CNTs, it was found that these treatments may damage the original structure of CNTs and create some defects on the surface. This phenomenon is more significant when the CNTs was purify with strong oxidizing reagent [30]. Previous studies also [43] confirmed that the acid oxidation of active carbons and carbon blacks dramatically change their surface and structural properties. Encouragingly, nitrogen adsorption especially low-pressure adsorption isotherms can be used to investigate the surface and structural properties. In order to investigate the effects of oxidation treatments on the surface and structural properties of carbon nanotubes, the CNT sample was treated in concentrated, boiling nitric acid for 4 h and the nitrogen adsorption isotherm was measured. However, in this case the original alignment of the as-synthesized CNTs was destroyed and the sample become an aggregated CNT felt after oxidation followed by washing with deionized water and drying in oven. Fig. 4 is the nitrogen adsorption isotherm at 77 K for the strongly oxidized sample in linear and logarithmic (inset) scale. The corresponding αs -plot is shown in Fig. 5. The Brunauer–Emmett–Teller (BET) surface area, mesopore surface area and the total pore volume for the as-synthesized and the oxidized carbon nanotubes are summarized in Ta-
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Table 1 The pore structure parameters for as-synthesized, aligned CNTs and the acid modified CNTs Sample
As-synthesized, aligned CNTs
Acid oxidized CNTs
BET surface area (m2 /g) Mesopore surface area (m2 /g) Total pore volume (cm3 /g)* Micropore volume (cm3 /g)**
293 283 0.51 Not detected
516 248 1.03 0.11
* Total pore volume was calculated by converting the adsorption capacity at relative pressure of 0.99 into volume of liquid nitrogen using a factor of 0.001546. ** Micropore volume was computed by using α range of 0.7–1.0. s
Fig. 4. The nitrogen adsorption–desorption isotherm for the nitric acid treated multiwalled carbon nanotubes in linear and logarithmic scales.
Fig. 5. High-resolution αs -plot for the acid-treated, multiwalled CNTs vs a graphitized carbon black reference material—Carbopack F. The intercept of dashed line on the adsorption axis indicates the micropore volume.
ble 1. The oxidized carbon nanotubes still show a Type II adsorption isotherm but its adsorption capacity is significantly increased. The obvious adsorption step in the relative pressure of 0.8–1.0 and the appearance of the huge adsorption/desorption hysteresis are related to the notably amount of interstitial space between individual carbon nanotubes due to the aggregated felt-like morphology. Moreover, the acid treated sample exhibits enhanced adsorption at low relative pressures. The adsorption capacity of the oxidized sample was dramatically increased to about 105 cm3 /g at lower (p/p0 = 0.05) relative pressure, in comparison to that of about 30 cm3 /g on the as-synthesized sample. The enhanced adsorption capacity at low pressure is normally caused by either the microporosity or the surface functionalities. The boiling nitric acid can change the original porous structure of carbons, in addition to increase the surface hy-
drophilicity [43]. Therefore, the strong acid treatment may not only remove the iron catalyst and the amorphous carbons but also create some structural defects or micropores on outer and inner surface of CNTs [30]. The micropore volume in Table 1 was calculated by extrapolating αs curve in the range of 0.7–1.0 to the adsorption axis and converting the intercept into micropore volume. The oxidized sample has a micropore volume of 0.11 cm3 /g while it is negligible for the as-synthesized sample. The oxidized sample also show a smoother adsorption step in the relative pressure of 10−4 –10−3 compared to the as-synthesized CNTs, which is indicative of a more heterogeneous carbon surface for the modified sample. After acid-oxidation treatment, both the BET surface area and the pore volume were increased approximately by a factor of two in comparison to those of the as-synthesized sample. Yang et al. [16] also found that the acid-purification treatment can remarkably increase the BET surface area and micropore volume for single-walled carbon nanotubes. Simultaneously, they observed the decrease of mesopore volume by purification, which was believed caused by the oriented assembly structure of purified single-walled carbon nanotubes (SCNTs). However, in this study, the mesopore volume increases after the acid treatment. Consequently, the dramatic increase of the surface area and micropore volume for this multiwalled CNTs should be related to the removal of iron catalyst and amorphous carbon, the formation of aggregated felt structure of treated sample, and oxygen functional groups on the edge or tube surfaces. The shape of the high-resolution αs curves for the multiwalled CNTs is distinctive from those of the single-walled carbon nanotubes reported by Yang et al. [16]. The former exhibits a downward deviation from the projected adsorption on graphitic surface whereas the latter displays an apparent upward deviation in the lower αs range (αs < 0.5). This should be attributed to the much smaller cavities (∼1 nm) in single-walled carbon nanotubes, which lead to an enhanced adsorption potential [24] inside these cylindrical pores. According to previous theoretical studies, an upward deviation is caused by the higher average energy of SCNTs relative to the referenced graphitized carbon black. This can probably explain the advantages of SCNTs over multiwalled CNTs
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in the gas storage and other adsorptive applications. On the other hand, for the adsorptive applications, the multiwalled carbon nanotubes do not show significant advantages over the conventional microporous carbons from the analyses of nitrogen adsorption isotherms. These observations may partially explain the controversial results on hydrogen storage and poor reproducibility of the data on single-walled or multiwalled CNTs. Another phenomenon worth discussion is the nitrogen adsorption on the single-walled carbon nanotubes. The nitrogen adsorption properties of the as-synthesized as well as the acid-modified multiwalled CNTs are close to the theoretical calculations. Nevertheless, even though the theoretical calculation and molecular simulations [18,44] predicted a very large surface area of ∼3000 m2 /g for SCNTs, so far, the BET surface area and pore volumes in the experimental studies [11–16] are several to ten times smaller than those predictions. This large discrepancy has not been paid much attention. We believe the possible causes include: (a) the inapplicability of Brunauer–Emmett–Teller model for the evaluation of the surface area of single-walled CNTs in which an enhanced potential is always observed; (b) the lower purity of the samples which may contain double-walled or multiwalled CNTs as well as other impurities such as residual amorphous carbons and catalyst and; (c) the accessibility of nitrogen molecules to small or closed cylindrical cavities or interstitial spaces. The purity of the sample would be a crucial factor in future adsorptive characterization of singlewalled carbon nanotubes.
4. Conclusions Nitrogen adsorption was successfully used to monitor the structural and surface properties of multiwalled carbon nanotubes before and after acid modification. Analyses of nitrogen adsorption isotherms and high-resolution comparative αs -plots of the as-synthesized and acid-modified CNTs showed that the nitrogen adsorption on CNTs takes place via a four-stage process: submonolayer adsorption; multilayer adsorption on the external and internal surface of cylindrical cavities; the capillary adsorption inside the cylindrical cavities; and the capillary condensation inside the interstitial spaces. The external surface of CNTs plays an essential role in the adsorption process. Nitric acid modification at boiling temperature significantly enhanced the adsorption capacity of carbon nanotubes at low pressures. This study also shows that multiwalled carbon nanotubes do not display apparent advantages over the conventional porous carbon for gas storage from the viewpoint of nitrogen adsorption.
Acknowledgments This research was supported in part by an appointment to the Oak Ridge National Laboratory Postdoctoral Research
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Associates Program administered jointly by the Oak Ridge Institute for Science and Education and the Oak Ridge National Laboratory (ORNL). The authors also acknowledge the ORNL’s SHaRE Collaborative Research Center for the use of their electron microscopy facilities.
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