Characterization of single-wall carbon nanotubes by N 2 adsorption

Transcription

1 Carbon 42 (2004) Characterization of single-wall carbon nanotubes by N 2 adsorption Fanxing Li, Yao Wang, Dezheng Wang *, Fei Wei Department of Chemical Engineering, Tsinghua University, Tsinghua Yuan, Beijing , China Received 13 November 2003; accepted 23 February 2004 Available online 3 July 2004 Abstract N 2 adsorption isotherms at 77 K of single-wall carbon nanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs), and mixtures of these carbon nanotubes (CNTs) were analyzed for differences in their pore size distributions (PSDs). The PSDs, calculated in the microporous region by the Horvath Kawazoe method and in the mesoporous region by the BJH method, are in agreement with the structures of both types of CNTs deduced from high-resolution transmission electron microscopy. A characteristic peak in the microporous region in the PSD of SWNTs is not present in the PSDs of MWNTs and impurities such as amorphous carbon, metal residues of catalysts, etc. The evaluation of this peak is proposed as a convenient tool for the quantitative characterization of SWNT purity in carbon nanotube-containing samples. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; C. Adsorption, BET surface area; D. Microporosity 1. Introduction Since their discovery in 1993 [1,2], single-wall carbon nanotubes (SWNTs) have been extensively studied because of their unique structure and properties [3,4]. Different methods are under study for the production of SWNTs of high purity and a convenient method to characterize the presence and purity of SWNTs would be useful for these studies. Common impurities in SWNTs are amorphous carbon, multi-wall carbon nanotubes (MWNTs), carbon nanoparticles and metal particles [5,6], and due to similarities in the properties of the different forms of carbon, it can be difficult to determine accurately the purity of SWNT samples. Thermo-gravimetric analysis (TGA) has been used for the quantitative analysis of the purity of SWNTs [5], but there are often situations where the forms of carbon are so similar that the spectrum cannot be clearly resolved and this affects the accuracy of its curve fitting. Herrera and Resasco [7] reported a combined temperature programmed oxidation (TPO) and Raman spectroscopy characterization that showed that the * Corresponding author. Tel.: ; fax: address: wangdz@flotu.org (D. Wang). oxidation temperature (which in principle is similar to the weight loss temperature in TGA) of SWNTs is strongly affected by the presence of leftover catalysts. In addition, small amounts of SWNTs are not detectable in TPO when the other forms of carbon have broad peaks. Qian et al. [8] proposed Raman spectroscopy as a fast way to determine the molar ratio of SWNTs and MWNTs. Although Raman spectroscopy is fast and effective in evaluating the relative amounts of SWNTs and MWNTs, the exact purity of SWNTs is difficult to obtain and the influence of amorphous carbon is not included. Furthermore, Raman spectroscopy is based on the use of research grade instrumentation which may not be readily available, and its sensitivity to many forms of impurities is not high [7]. Transmission electron microscopy (TEM) is also useful for assessing the quality of SWNTs [9,10] but it is basically a microscopic probe. More methods that are commonly available and complementary will be helpful for the analysis of SWNT samples. In this work, N 2 adsorption isotherms at 77 K of SWNTs of different purities as well as MWNTs are reported and examined as tools for the characterization of SWNTs. Essentially, N 2 adsorption is sensitive to the pore size distribution (PSD) and when the tested materials are porous and can be differentiated on the basis of different pore dimensions, it is a useful characterization /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 2376 F. Li et al. / Carbon 42 (2004) tool. Du et al. [11] had previously reported the use of N 2 /Ar adsorption to investigate the pore structure of SWNTs. N 2 adsorption is generally used to measure pore size distributions (PSDs) in the mesoporous region (2 50 nm) but most SWNTs have pore diameters between 0.4 and 2.0 nm, which is in the microporous region in the IUPAC classification [12,13]. Thus, the measurement protocol needs to be modified for the acquisition of information in the microporous region from adsorption measurements. This requires extending the measurement of the isotherm to very low pressures (ca. 10 Pa and below), using equipment that is generally not available to standard laboratories. 2. Experimental This work assesses the possibility of using N 2 adsorption isotherms to assay the quality of SWNTs in a sample. The methodology is to use well-characterized samples with different SWNT contents to verify the information deduced from the N 2 adsorption isotherms. It is now known that SWNTs possess micropores but the impurities in SWNTs do not have such small pores. The basis of this work is the use of the presence and volume of micropores in a sample to determine the existence and proportion of SWNTs in samples that may also contain MWNTs, amorphous carbon, and residual metal catalyst as impurities Apparatus and experimental modifications for micropore determination The details of an adsorption isotherm vary with different adsorbents and adsorbates but a general conclusion is that the presence of enhanced gas uptake at very low pressure implies the presence of micropores [12 15]. Therefore, the adsorption isotherm in the very low pressure region is of high significance in this work. Conner [14] has discussed the necessary experimental conditions for accurate measurements of N 2 uptake at very low pressures. These include: (1) adequate pumping speed in the sample cell, (2) pressure sensors accurate in the low pressure region, and (3) the capability for slow dosing. Further considerations and more details are given by Conner [14]. An apparatus for measuring N 2 adsorption isotherms was modified to satisfy the above experimental requirements. It used a 150 l s 1 turbomolecular pump with a mechanical backing pump to provide a base vacuum of Torr. Adequate pumping speed at the sample was provided by using tubings that have a minimum inner diameter of 8 mm to provide high conductance. Two pressure sensors with full scales of 1000 and 10 Torr (MKS Baratron Type 223B) were used to cover a large pressure range. The specification accuracy of the sensors is that their experimental uncertainties are equal to or less than 0.3% of the reading and they were factory-modified to give a 10-fold increase in the voltage outputs for higher precision. The outputs of the sensors were recorded into a computer by a 16 bit A/ D card (Daheng AC6112; China). Gas dosing was performed by means of a leak valve (Shenyang Scientific Instruments Co., China) that is a type of throttling valve commonly used for dosing gases in ultra-high vacuum chambers and which can dose the sample with adequately small amounts of adsorptive. About 0.1 g sample was used for each measurement. The sample was first evacuated at 500 K for >5 h. The procedure to determine a N 2 adsorption isotherm followed that given by Conner [14], which was designed to ensure adsorption equilibrium and sufficiently detailed data in the microporous region. At the beginning of the isotherm measurement, the amount of N 2 per dose was usually as low as ml (STP) so that there was sufficient resolution in the low pressure region and adequate time was given (about half an hour) for equilibrium to be reached. As the pressure increased, the dose amount was increased at the rate of about three times between consequent doses and the equilibrium time could be reduced. When the cell pressure was high enough (>1 Torr), a continuous flow technique was used to acquire high-resolution data [12]. In this region, the isotherm was measured under quasi-equilibrium after experimental verification that quasi-equilibrium was achieved. The leak rate was adjusted to give a pressure rise in the sample cell of about Torr/h. The calculation of the PSD used a standard procedure [12,13] with the difference that this was extended to the microporous region. In this region, the Horvath Kawazoe (HK) model [15,16,25] was used in place of the Kelvin equation. A linear interpolation was used between experimentally measured values when the calculation required adsorbed amounts at specific P=P 0 values that were between experimental points. In using an adsorption isotherm and the HK or Kelvin equation to derive a PSD, the interval between PSD abscissa values, DL (spacing of pore diameter), used was chosen to be small enough (0.01 nm) so that there was no visually discernible difference in the PSD with when a smaller DL was used Sample preparation The SWNT samples used in the experiments were prepared by methane decomposition on a Fe/MgO catalyst as described by Yan et al. [17]. The raw sample was washed with 35% HCl to remove most of the metal and metal oxide catalysts. The SWNT content of the purified sample was over 80 wt.% as estimated from DTA TGA characterization (TA Instruments, TGA2050) and TPO experiments (using a homemade apparatus made from a

3 F. Li et al. / Carbon 42 (2004) Fig. 1. (a) HRTEM photo of the SWNTs and (b) histogram of the pore diameters of the SWNTs from HRTEM photos. The pore diameter is measured as the distance between the centers of diametrically opposite atoms of the tube walls. mass flow controller, temperature controller and thermal conductivity detector). The Fe catalyst content was around 10 wt.% and the rest of the sample were carbon impurities. The carbon phase in the purified sample other than SWNT was mostly amorphous carbon, often observed scattered around the tubes or gathered around the metal catalyst core to form carbon shells (mostly around 10 nm). Fig. 1a shows a high-resolution transmission electron microscope (HRTEM; JOEL 2010) photo of the purified SWNTs used. The statistical distribution of the pore diameter of more than 100 SWNTs measured from eight different HRTEM photos is shown in Fig. 1b. MWNT samples were chosen for comparison with SWNTs due to their similarities and furthermore, MWNTs are often present as an impurity in SWNTs. The MWNT sample was prepared from methane decomposition on a Fe/Al 2 O 3 catalyst in a nanoagglomerated fluidized-bed reactor (NAFBR) [18]. The sample was annealed under a vacuum of 10 3 Pa and a temperature of 1700 C. This treatment results in a purified sample with a MWNT purity of over 99% [19]. HRTEM photos of the MWNT samples have been previously reported [20]. Different amounts of SWNTs and MWNTs were accurately weighted and were well mixed by shearing. First, an agate mortar was used to shear the SWNT and MWNT samples to blend them initially. Then, a rotation mixer (CMC M800; China) at 2500 rpm was used to further mix the samples for 20 min. TEM photos indicate that the shearing did not change the pore structures, and this is corroborated by the fact that the adsorption isotherms of sheared and original SWNTs did not differ visibly. The use of the mortar was necessary because the purified MWNT samples were small granules that were much larger than those of the SWNT sample. A mixture of purified SWNTs and MWNTs with a ratio of 1:1 (denoted as OxyA) was oxidized in the TPO apparatus under different temperatures to investigate the influence of carbon impurities other than MWNTs. The SWNT contents of the mixed samples and the oxidation temperatures are listed in Table Results and discussion 3.1. Isotherms and PSDs of SWNTs and MWNTs Nitrogen adsorption isotherms at 77 K of the purified SWNT and MWNT samples are shown in Fig. 2. Both SWNTs and MWNTs showed Type II adsorption isotherms (IUPAC classification). This is in agreement with the work of Yang et al. [21]. Fig. 2b is an expanded view of the very low pressure region that shows considerable differences between the adsorption isotherms of SWNTs and MWNTs: the uptake amount by SWNTs is much higher than that by vacuum-annealed MWNTs. The large amount of adsorption on the SWNTs at very low pressures indicates the presence of micropores [12,13]. The uptake of nitrogen at very low pressures is due to micropore filling from the enhanced adsorbent adsorbate interactions in the micropores and is distinct from Table 1 The various CNT samples used in the experiments Sample ID PurS MixA MixB MixC MixD PurM OxyA OxyB OxyC Purified SWNT content (%) or treatment condition 100% 96.9% 88.7% 81.1% 47.9% 0% (MWNT) 49.2% OxyA oxidized at 420 C for 40 min OxyA oxidized at 550 C for 40 min

4 2378 F. Li et al. / Carbon 42 (2004) Amount Adsorbed (ml, STP/g) Amount Adsorbed (ml, STP/g) 500 PurS PurW MixA 400 MixB MixC MixD PurS PurW MixA 40 MixB MixC 30 MixD Relative Pressure Purified SWNT Purified MWNT (a) (b) 4.0x x x10-4 Relative Pressure [25], which is the distance between the edges of the atoms. The former value is larger than the latter by the diameter of a carbon atom, which is 0.34 nm. In the mesoporous region (2 50 nm), the BJH method based on the Kelvin equation [33] is conventionally used to calculate the PSDs. Thus, this work used a combination of the HK and BJH methods, namely, the HK method for the <2 nm region and the BJH method for the 2 50 nm region. The two methods did not give the same pore diameter for the pores that are filled with fluid at a same pressure in the 2 nm region where they conjoin, but this was not a problem in this study because N 2 uptakes were very small in this small region. Fig. 3a shows the calculated PSD of the SWNT sample. The sample has both micropores and mesopores. The PSD shows a maximum at the pore size of 1.17 nm. This is in good agreement with the histogram shown in Fig. 1b derived from visual inspection of the SWNT pore diameters in HRTEM photos. There is a secondary maximum at a pore size of around 2.8 nm and there are pores in the larger microporous and mesoporous regions, but all these pore distributions are much smaller than the primary maximum. It is believed the larger pores are the slits between bundles of SWNTs. The PSD of the SWNTs is in good qualitative agreement Fig. 2. Adsorption isotherms of purified SWNTs, MWNTs and their mixtures: (a) full isotherm; (b) expanded view of low pressure region. (PurS: purified SWNT sample; PurW: purified MWNT sample; MixA: 96.9% SWNT 3.1% MWNT; MixB: 88.7% SWNT 11.3% MWNT; MixC: 81.1% SWNT 18.9% MWNT; MixD: 47.9% SWNT 52.1% MWNT). adsorption in mesopores that is due to multilayer adsorption and capillary condensation. Different methods using different models have been proposed for the calculation of the PSDs of microporous materials, e.g., the Dubinin Stoeckli method [22], the Jaroniec Choma method [23], the Horvath Kawazoe (HK) method [15,16,24,25], density functional theory [26,27] and molecular simulation methods [28 30], in order of increasing complexity. Reviews of the methods have been given by Rouquerol et al. [12], Gregg and Sing [13], Yang and coworkers [31], and Jaroniec et al. [32]. This work used the HK method for a cylindrical pore model [16,25] for calculating the PSDs in the microporous region because the necessary physical parameters for microporous carbon and nitrogen are available in a paper by Rege and Yang [25], who showed that the calculated PSDs using the modified HK method are in good agreement with the known characteristics of the relevant samples. In order to allow a direct comparison between the calculated pore size and the HRTEM results, the pore diameter used in this work is the distance between the centers of diametrically opposite atoms of the pore wall. This differs slightly from the effective pore diameter Fig. 3. PSDs from N 2 adsorption of (a) SWNTs and (b) enlarged plot of the microporous regions of the PSDs from N 2 adsorption of SWNTs and MWNTs, and the PSD from HRTEM photos of SWNTs. V pore denotes the volume of liquid condensate in the pore; L denotes pore diameter.

5 F. Li et al. / Carbon 42 (2004) with that shown in [11] but their calculated micropore diameters are smaller and are about half of those in Fig. 3. The choice of a slit pore model in their work is likely to be the cause of the discrepancy. In order to show more clearly the differences in the micropore distribution of the SWNT sample and that of the MWNTs in it, Fig. 3b focuses with an expanded view on the microporous region of the SWNTs, the MWNTs, and the corresponding distribution derived from HRTEM photos of the SWNTs. To compare the HRTEM results directly with the PSDs calculated from N 2 adsorption, an equal total pore volume was used for the pores in the distribution calculated from the HRTEM photos, that is, equal to the total pore volume of the pores in the distribution calculated from N 2 adsorption for the pore diameter range of interest ( nm). Using the distribution of percentages of different pore sizes shown in Fig. 1b, the PSD for the HRTEM results is calculated and is shown in Fig. 3b. It can be seen that the SWNT PSD calculated from HRTEM agrees qualitatively with the PSD calculated from the SWNT adsorption isotherm. The difference between them is probably due to insufficient statistics and the fact that the length of the different pores cannot be measured in the HRTEM photos. It can be seen in Fig. 3b that the MWNTs also have micropores, but the micropore volume of the MWNTs is distinctively smaller and the peak pore size is about 1.39 nm, more than 0.2 nm larger than that of the SWNTs. The presence of micropores in the MWNTs is most likely not due to the presence of small pore diameters in the MWNTs since such small tubes have not been observed by electron microscopy. It is believed that these micropores are small apertures between the walls of the MWNTs that are formed when they twist together tightly. Due to the large outer diameter of the MWNTs (around 10 nm from HRTEM), the apertures are not likely to be very small because of geometric constraints, and this results in the almost complete absence of micropores smaller than 1.2 nm. Therefore, the HK pore size distribution peak at a diameter of 1.17 nm is proposed as a characteristic peak of SWNTs. This corresponds to an effective SWNT inner wall-to-wall average pore diameter of 0.83 nm for the SWNTs investigated in this study. The PSD of the MWNTs also showed a maximum at 3.3 nm in the mesoporous region (not shown). This can be assigned to the inner pore diameter of the MWNTs since they showed a maximum at about 3.5 nm in HRTEM photos. Our interest is in the possible use of the pore size distribution in the microporous region as a characterization of SWNTs. This section examines the contributions to N 2 adsorption from the other substances present in the SWNT sample. The main impurities are amorphous carbon, MWNTs and metal particles covered by carbon shells. N 2 uptake by the metal particles is very small because their surface constitutes only a small fraction of the total surface. This was verified by measuring the adsorption isotherm of an untreated SWNT sample with a carbon content of 3.4%, in which most of the remaining contents are the catalyst residues of MgO and Fe. The amount of N 2 adsorbed by this sample at 77 K was the equivalent of a BET surface area of 37 m 2 /g. Since the amount of catalyst residues in the purified sample is decreased by about 10-fold, it can be concluded that N 2 adsorption at 77 K is almost completely due to the carbon phases for this typical catalyst. To verify that the amorphous carbon impurities do not contribute to N 2 adsorption at pore size 1.17 nm, experiments were performed on sample OxyA, which was a mixture of purified MWNTs and SWNTs with a weight ratio about 1:1. First, TPO and DTA TGA data for sample OxyA, the mixture, were obtained (Fig. 4). Fig. 4 shows three weight loss peaks. Many studies [5 7,34,35] have reported that the oxidation sequence of the carbon phases is amorphous carbon, SWNTs, and MWNTs in order of increasing temperature. The TPO curve indicates that amorphous carbon is oxidized below 450 C, SWNTs are oxidized between 3.2. N 2 adsorption characteristics of other impurities Fig. 4. (a) DTA TGA curve and (b) TPO curves of mixture A. The experiments used a heating rate of 10 C/min.

6 2380 F. Li et al. / Carbon 42 (2004) and 600 C and MWNTs between 600 and 700 C. Based on this, three comparison measurements were made with three samples: OxyA, the original mixture of SWNTs and MWNTs with a composition close to 1:1; OxyB, a mixture treated under 5% oxygen in argon at 420 C for 40 min; and OxyC, a mixture treated under 5% oxygen at 550 C for 40 min. The 5% oxygen used was to keep to the same operating condition as the TPO experiment. Subsequent TEM and energy dispersive spectrometry (EDS; RINK 300) examinations (data not shown) showed that there was very little amorphous carbon in sample OxyB, the carbon shells surrounding the metal particles had been oxidized, and some of the metal particles had clustered together. In sample Oxy- C,much fewer SWNTs were found under HRTEM examination. The N 2 isotherms of the three samples and their corresponding PSDs are shown in Figs. 5 and 6. Fig. 6 shows that the characteristic peak height of sample OxyA is about half that of the purified SWNTs, which conforms to the fact that half of the mixture is MWNTs. Sample OxyB, which had amorphous carbon removed, shows a larger SWNT peak. The increase in peak height is somewhat higher than that expected by solely subtracting the amount of oxidized-off carbon phases in Fig. 5. N 2 adsorption isotherms of samples OxyA, OxyB and OxyC. Fig. 6. An expanded view of PSDs of samples OxyA, OxyB and OxyC in the microporous region ( nm). sample A (normalized to a per weight basis). This suggests that amorphous carbon not only does not contribute to N 2 adsorption in the micropore region, but actually reduces it. A possible explanation is that some amorphous carbon had obstructed the entrance of some SWNTs and rendered the interior of some tubes inaccessible to nitrogen molecules. Another possible reason is that some of the SWNTs were closed, that is, their ends were originally sealed caps, and these were opened by the oxidation at high temperature. In any event, either of these interpretations imply that low pressure N 2 adsorption is not particularly sensitive to closed CNTs. A small shift in the average pore size towards smaller size was also observed. This may be due to the fact that the smaller graphite caps that seal thinner tubes are more easily oxidized and easier to remove. Sample OxyC shows a much lower peak height in agreement with oxidation of the SWNTs. These results confirm that the peak at 1.17 nm is only due to SWNTs Illustration of the determination of SWNT content using N 2 adsorption The results above indicate that the height (or area) of the characteristic peak around 1.17 nm can be used to characterize the presence of SWNTs. Further experiments were carried out to test the use of this peak to determine the purity of SWNTs. Purified SWNTs and MWNTs were blended in different proportions and the isotherm of each mixture was measured. Different proportions of purified SWNTs and MWNTs were weighed and mixed following the procedure described in Section 2. The proportions of purified SWNTs used in the samples are listed in Table 1. The isotherms of the mixtures are shown in Fig. 2, which shows a relationship between the adsorption amount in the microporous region and the proportion of SWNTs. The isotherms of the mixtures showed a distinct trend. The calculated PSDs showed that the intensity of the peak corresponding to micropores around 1.17 nm got smaller as the proportion of MWNTs increased. The peak height and the peak area between 1.1 and 1.2 nm are shown in Table 2. Fig. 7, which to avoid cluttering only shows the samples with 81.1% and 47.9% purified SWNT, shows some representative PSDs. This example depends on the SWNTs having open pores with diameters in the microporous region while the other constituents present do not. The reports in the literature indicate that this is a reasonable assumption [11,36,37]. Although SWNTs produced using different methods may have different physical micropore structures, provided that a process produces SWNTs that have similar pore size distributions and proportion of open pores, a fairly pure sample can be used as the standard sample for the calibration of the microporous

7 F. Li et al. / Carbon 42 (2004) Table 2 SWNT contents in SWNT/MWNT mixtures estimated from HK pore size distribution (PSD) and BET surface area calculated using nitrogen adsorption isotherms Sample ID wt.% purified SWNT PSD peak height, h (ml/g/nm) ðh p =h ps Þ100% PSD peak area, A (ml/g) ða p =A ps Þ100% BET surface area (m 2 /g) BET area ratio 100% Corrected a BET area ratio 100% PurS MixA MixB MixC MixD PurM OxyA OxyB b N/A b OxyC c N/A c a Corrected BET area ratio ¼ (BET area ) 240.1)/(619.1 ) 240.1). b Sample OxyA after oxidation in 5% oxygen at 420 C for 40 min. c Sample OxyA after a further oxidation in 5% oxygen at 550 C for 40 min. Fig. 7. Pore size distributions of purified CNTs and their mixtures. region. If there is no purified SWNT sample composed of pure SWNTs, but one has available a standard sample with a known proportion of SWNTs, the peak size of pure SWNTs can be simply estimated by dividing the peak size of the standard sample by the percentage of pure SWNTs in it. This example suggests that the peak height or area in the micropore region can be used to determine the purity of a SWNT sample. That is, the ratio of peak size h p (or area A p ) of a sample to that of the purified sample h ps (or A ps ) is the percent of purified SWNTs in the sample (denoted as W x ): Fig. 8 compares the calculated results with the mixture compositions known from the preparation. There is some deviation when A p, the peak area, is used in the calculation when the proportion of SWNTs is very low. This is because A p is calculated by integration over the range of pore size chosen (here, nm) and Eq. (1b) assumes that there is no N 2 uptake in this range by the other components in the mixture, but there is some adsorption by MWNTs, albeit small. Thus, when the proportion of SWNTs is low (e.g. <30%), and the contribution of the SWNTs to the N 2 uptake in this range is not much more than that due to the other components, Eq. (1b) is not very accurate as a measure of SWNTs. In this situation, the use of h p in Eq. (1a) is more sensitive because it is calculated from N 2 uptake data by a differentiating process. However, because h p values are derived by (numerical) differentiating, its value at a particular abscissa position can be sensitive to the abscissa point spacing and the spacing between successive P=P 0 data points in the raw data in the P=P 0 region of interest. Accurate h p values require very detailed measurements in the very low pressure region. On the other W x ¼ h p 100% h ps or ð1aþ W x ¼ A p 100% ð1bþ A ps The h p values are read directly from the PSD curves. The A p values are obtained by numerical integration of the PSD curves from pore diameters of nm (the range was chosen by inspection of the characteristic peak of the SWNT sample in Fig. 3b). Fig. 8. Comparison between the actual and calculated percentage of SWNTs.

8 2382 F. Li et al. / Carbon 42 (2004) hand, A p values are derived by the (numerical) integration of h p values, which is a smoothing process, and it is more reliable to use Eq. (1b) when the SWNT content is not very low (e.g. >30%). This method of characterization, that is, the use of the adsorption isotherm at low pressures to determine the quantity of SWNTs, is quite convenient and existing N 2 adsorption apparatuses can be easily adapted to make the measurements Characterization using the BET surface area and its problems Some previous studies of N 2 adsorption on CNTs have used the BET surface area rather than the microporous PSDs for characterization [38,39]. The presumption is that a high BET surface implies a high proportion of SWNTs. A debundled SWNT sample was reported to have a BET surface area as high as 1600 m 2 /g [39]. However, characterization by the BET surface area can be ambiguous when applied to microporous materials, e.g., activated carbon [12,13]. BET theory is based on estimating the monolayer capacity from multilayer adsorption data at relative pressures generally ranging from 0.05 to around 0.3. However, micropores (<2 nm) are filled below the relative pressure of 0.05, and with microporous samples, there is more N 2 uptake than would be the case with multilayer physical adsorption. This often leads to the calculated BET area being an over-estimate of the actual surface area [12,14]. The BET surface areas of the samples used in this work are shown in Table 2. The BET areas of the mixtures of SWNTs and MWNTs do show a trend of higher surface areas with higher contents of SWNTs. Also listed in Table 2 are BET area ratios and corrected BET area ratios, which include a correction for the contribution by MWNTs, to show the accuracy and possible error bars in the use of these for estimating the proportion of SWNTs in these samples. A comparison of samples OxyA, OxyB and OxyC indicates that other factors also affect the BET surface area. The use of the BET area to characterize the presence of SWNTs would be erroneous in these cases. Sample OxyB has a much larger BET surface area (24% larger) than sample OxyA, although it cannot contain many more SWNTs than sample OxyA, judging from the small weight loss during the first oxidation step (the content of SWNTs of OxyB is 3% larger than OxyA from the PSD estimation). In this case, it is likely that the oxidation treatment had roughened the surface and caused the BET surface area to increase. Another probable case of the ambiguity of BET characterization is provided by the work of Cinke et al. [39]. The BET surface area of debundled SWNTs is distinctively larger than that of the untreated sample while the content of SWNTs remained unchanged. Thus, the pore size distribution is a better characterization tool than the BET area to quantify the presence of SWNTs in SWNTcontaining samples. 4. Conclusion The N 2 adsorption isotherm at low pressures (below 10 Pa) is sensitive to the presence of open SWNTs, while impurities common in SWNTs such as amorphous carbon, residue catalysts and MWNTs do not show much N 2 uptake in this pressure region. Therefore, information derived from N 2 adsorption data within this low pressure range, in the form of pore size distribution peaks, is proposed as a means to distinguish SWNTs from other impurities. The experimental requirements only need minor modifications to most standard N 2 adsorption isotherm apparatuses. Acknowledgements We are very grateful to Dr. Weizhong Qian and Dr. Hansheng Li for helpful discussions and experimental guidance. We thank Mr. Wei Huang and Mr. Yi Liu for their CNT samples. 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