Characterization of Large Fullerenes in Single-Wall Carbon Nanotube

36

J. Phys. Chem. C 2007, 111, 36-44

ARTICLES Characterization of Large Fullerenes in Single-Wall Carbon Nanotube Production by Ion Mobility Mass Spectrometry Carl D. Scott,*,† Michael Ugarov,‡ Robert H. Hauge,§ Edward D. Sosa,| Sivaram Arepalli,| J. Albert Schultz,‡ and Leonard Yowell† ES4 NASA Johnson Space Center, Houston, Texas 77058, Ionwerks, Incorporated, 2472 BolsoVer, Suite 255, Houston, Texas 77005, Carbon Nanotechnology Laboratory, Chemistry Deptartment, Rice UniVersity, Houston, Texas 77005, and ERC, Incorporated, NASA Johnson Space Center, Houston, Texas 77058 ReceiVed: June 12, 2006; In Final Form: October 4, 2006

Samples from nanotubes produced by various methods are analyzed by laser desorption-ionization ion mobility time-of-flight mass spectrometry (LDI-IM-TOF-MS). The measurements find mostly positive fullerene ions in the mass range from 240 to 10000 Da. These principal contaminants are produced along with single-wall carbon nanotubes, which are too large to be detected in the mass spectra. Large fullerenes in the spectra are identified by their regular pattern of two carbon atom intervals and agreement of their mobility with theory. The distribution of large fullerenes varies among the samples depending on the type of production technique. Large numbers of fullerenes are seen in samples produced by the arc process and by the laser ablation process, whereas relatively few fullerenes are seen in samples produced by CVD processes. Measured drift times are in good agreement with those calculated based on a rigid sphere model. From close examination of the drift time distributions, there appears to be isomers of lower mobility. Agreement of measured with calculated drift times suggest that these isomers may be elongated structures. To assess possible limiting shapes of isomers seen in the mobility spectra, it appears that the shapes of the large fullerenes of a given mass vary from spherical to possibly short cylinders.

Introduction During production of carbon nanotubes by various processes other forms of carbon are produced along with the nanotubes. These have been variously called amorphous carbon, turbostratic graphite, “schmutz,” fullerenes, and even polyaromatic carbon (PAC). Some of this material usually remains, even in purified product. It is well known that it is very difficult to remove these forms of carbon from nanotube samples. Solvent extraction can remove some of the smaller fullerenes and PACs. Graphitic carbon is particularly hard to remove as well as large fullerenes. Usually some of the impurities cannot be removed. The remaining amount depends on the source of the nanotubes and method of purification. The particular production method, temperature conditions, and amount and type of catalysts used all give a certain uniqueness to the impurities in the product. One of the aims of this study is to determine whether the distribution of fullerenes in nanotube samples can yield information about their origin and purification. The other major aim is to infer possible shapes of the fullerenes seen in the samples with a hope that cut nanotubes or fragments may be determined in the future from such measurements. The existence of * To whom correspondence should be addressed. Current address: 492 Enchanted Oak, New Braunfels, TX 78132. † ES4 NASA Johnson Space Center. ‡ Ionwerks, Incorporated. § Rice University. | ERC, Incorporated.

elongated fullerenes may be associated with nanotubes whose growth was terminated before becoming long nanotubes. Direct measurements to identify the specific form of impurities in nanotube samples have been difficult. Raman spectra show indirect evidence that suggests sp3 forms of carbon in samples by the D-band peak. However, this is not very specific information since many forms of carbon show this, even damaged nanotubes. A number of characterization techniques for identifying contaminants in samples from single-wall carbon nanotubes (SWCNT) production have been attempted. These include Raman spectroscopy,1,2,3 thermogravimetric analysis4,5 (TGA), inductively coupled plasma (ICP) spectroscopy,6 scanning electron microscopy (SEM),7 transmission electron microscopy (TEM),8 solvent extraction and high-pressure liquid chromatography9 (HPLC), and nuclear magnetic resonance11 (NMR). These methods can give varying amounts of information about contaminants depending on their identity. A more direct measure of certain contaminants in samples can be obtained using mass spectrometry. At least the mass (molecular weight) distribution of certain species can be obtained. Knowing the mass, then one may be able to infer the identity of the species. Different production techniques may yield different distributions. Large fullerenes have been observed in samples of carbon nanotubes obtained from various sources. Mass spectroscopy has been used to identify fullerenes in samples of soot since the discovery of C60.12,13 Many researchers have observed a variety of fullerenes in samples of nanotubes.14-17 In a

10.1021/jp063649k CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006

Characterization of Large Fullerenes particularly useful and descriptive paper, Hunter and Jarrold18 studied large carbon clusters, including giant fullerenes up to about 320 carbon atoms, with ion mobility mass spectroscopy. Their drift time distributions, taken by injecting fullerenes from a laser ablation source, showed isomers that deviated significantly from spherical. They concluded that these nonspherical species having large drift times might be fullerene dimers and graphene sheets or multilayer graphitic fragments. They also found a more compact species that they think might be multishell species. More recently, fullerenes were observed in extractions from nanotube product obtained using the HiPco process. Using laserdesorption-ionization mass spectroscopy, Ramesh et al.19 found what appears to be fullerenes covering a range of C120-C400. Their evidence is based mainly on a separation of the peaks of 24 mass units (or two carbon atoms) that is consistent with closed fullerene structures. Moreover, this evidence is supported by very nice TEM pictures that show what appear to be closed, nearly spherical, or elongated objects in contact with the nanotubes.18 Not only were fullerenes observed in HiPco product, but Sadana et al.20 found them in samples from nanotubes produced by the laser-ablation oven technique at NASA Johnson Space Center and at the University of Karlsruhe and the arc vaporization technique at the University of Paris 13. Sadana et al.19 functionalized and extracted the samples, then submitted extraction droplets of each sample to laser desorption-ionization time-of-flight mass spectrometery (LDITOF-MS). Depending on the source of nanotubes, they found a variety of distributions of species attributed to large fullerenes up to about 5000 mass units. Their high-resolution TEM measurements of the arc-produced sample also revealed large fullerenes in the extraction. Additional information can be obtained if the mass spectrometer is coupled to an ion mobility measurement. Since ion mobility is a function of the ion cross section, knowing the mass we can obtain information about the particle density and, possibly, shape of the species as well. One motivation is to assess the future possibility of applying the technique to obtain structural information about short functionalized carbon nanotubes. By comparing measurements with calculations of ion mobilities for various shapes, we are able to discern possible shape distributions associated with fullerenes of a given mass. In this study, we obtain a range of possible length to diameter ratios of species in the samples. The ability to discern the structure of carbon molecules such as chains, cycles, polycycles, fullerenes, and fullerene clusters has been demonstrated in the work of the Bowers group at the University of California at Santa Barbara21,22 and by the Jarrold group at Northwestern University.23-26 In the present work, we make plausible conclusions about the shape of fullerenes in the samples produced during production of single-wall carbon nanotubes (SWCNTs) and investigate the possibility of the existence of very short nanotubes. The method involves developing expressions for the cross section of positive cluster ions of capped cylinders with helium atoms in the drift tube of the apparatus. From the prediction of cross sections, we then determine what the ion mobility and thus drift times are. Comparing the theoretical predictions with measurements, we then infer limiting possible shapes. Measurements In order to perform simultaneous ion mobility and mass measurements one must inject ions of the species from samples into a gas-filled drift tube. Prior to the present study, laser

J. Phys. Chem. C, Vol. 111, No. 1, 2007 37

Figure 1. Schematic of LDI-IM-TOF mass spectrometer

irradiation of samples with and without a chemical matrix has been used to desorb and ionize molecules in samples. In the present experiments, no additional matrix was used. The pulsed laser, operated at relatively low power (typically below 100 mJ/ cm2), desorbs the species and ionizes them. They are accelerated into the segmented helium-filled drift cell by an electrostatic field. The electrostatic field applied across the cell causes the ions to drift through the gas. The velocity of the ions, hence their traverse time, depends on their cross section, the voltage, and the gas density. After exiting the drift cell, the ions are then directed into a mass spectrometer for mass determination. The LDI-IM-TOF-MS system was developed at Ionwerks, Inc.27 It has been extensively used for the structural analysis of a variety of biological samples including peptides, proteins, lipids, as well as real tissues and microorganisms. Compared to conventional mass spectrometers, the instrument allows one to obtain simultaneously additional information about the shape of the ionized species by measuring variations in the ion-gas collision cross sections. Recently, this technique was applied to new classes of inorganic and organic materials,27 including samples of fullerenes and nanotubes. It was apparent that distributions of large molecules or clusters could be seen in the spectra. Special software is used to display the drift time versus mass/charge ratio to assess both the mass distribution as well as the collision cross section distribution. The data can be displayed in a three-dimensional space, number of ion counts versus mass/charge and drift time. Laser Desorption-Ionization Ion Mobility Time-of-Flight Mass Spectrometry. Samples taken from the product of nanotube growth were suspended (not extracted) in a solvent (either methanol or chloroform) using low-intensity sonication. Three of the samples had been previously purified by standard means prior to suspension in the solvents. Five-microliter droplets of the suspension were placed on a stainless steel plate and allowed to dry in air for up to 1 week. The measurements were performed using the LDI-IM-TOF-MS system at Ionwerks, Inc., Houston, TX. A schematic of the system is shown in Figure 1. A 349-nm Nd:YLF (fourth harmonic) pulsed laser beam impinged on the sample spots at a rate of 200 Hz. The spot size of the laser is up to 200 µm. To prevent depletion of a unique spot on the sample, the plate is continuously moved to intercept a fresh surface. Since this is done by hand, the motion is not necessarily consistent, making it difficult to obtain absolute ion count comparisons between samples. However, typically a few thousand to tens of thousand pulses were used

38 J. Phys. Chem. C, Vol. 111, No. 1, 2007

Scott et al.

TABLE 1. Comparison of Measured and Calculated Mobilities and Pressure Determined from Corrected Drift Times Cn, n

mass

60 70 120 140 240

720 840 1440 1680 2880

60 240

720 2880

1/Ko, m2/Vs Shvartsburg et al.(1998) 2343 2606 3610 4030 6000 Mesleh et al. (1996) 2321 5900

1/K, m2/V s 9.824 10.93 15.14 16.90 25.16 9.73 24.74

1/K, m2/V s

relative error in 1/K, %

measured ∆t, µs

P, Torr

-2.73 -1.06 -1.81 -0.87 1.81

110.7 123.4 180.3 200.6 287.8

3.42 3.43 3.62 3.60 3.47

-3.71 0.15

110.7 287.8

3.45 3.53

spherical shell theory 10.09 11.04 15.41 17.04 24.70 spherical shell theory 10.09 24.70

mean

to acquire spectra that have a significant number of counts. Positive ions produced by laser ablation enter the ion mobility cell (drift tube) through a channel 3 mm in diameter. The segmented mobility cell is filled with helium at a pressure of a few Torr. A chain of resistors connects the segments of the mobility cell to provide a relatively uniform voltage gradient along the length of the tube. A potential of about 1700 V is applied across the 14-cm length of the cell. The segments are shaped to provide a certain amount of nonuniformity in the electric field that helps confine or focus the ions on the axis of the cell. A set of differentially pumped chambers with gas skimmers is placed at the exit of the cell through which the ions flow into a high-vacuum region, where they are accelerated and orthogonally extracted by high-voltage pulses into the reflector of the TOF-MS. Reflected ions are subsequently detected using a microchannel plate detector. The laser, extraction voltages, and detector output are synchronized under electronic control and computer data acquisition. The two-dimensional mobility mass spectrum is acquired by performing a series of one-dimensional mass spectrum acquisitions during the relatively long period of the ion drift through the mobility cell (typically up to a few milliseconds). The timeof-flight of ions in the TOF-MS are significantly smaller (up to 200 µs). There is an additional time that ions spend between the exit of the cell and the extraction region of the TOF-MS that has to be taken into account in order to estimate the actual duration of the ion interaction with the buffer gas in the mobility cell. The typical mass resolution of the instrument is 2500, and the mobility resolution is around 30. The mass was calibrated using samples of C60 and C70. Higher masses were corrected by observing the peaks that differ by 24 mass units. The pressure, held constant within 1% or less during each set of experiments, was determined from drift times for each calibrating species C60, C70, C120, and C140 using their published mobilities (Shvartzburg et al.25) and C240 (Mesleh et al.26); see Table 1, which shows the pressure calculated for each species. The average pressure is 3.50 Torr. Correction of Drift Times by Extrapolation to Infinite Accelerating Voltage. To compare the measured and calculated drift times, it is necessary to account for the time that the molecules spend outside the drift tube in the measurements. If the voltage across the mobility cell were infinitely large, then the ions would spend virtually no time crossing the cell. In that case, the measured time would only be that associated with motion in the entrance and exit regions of the apparatus plus any delay associated with timing of the laser pulse. We can estimate the actual time spent in the mobility cell by measuring the total time at various cell voltages and extrapolating the time to infinite voltage. To obtain the infinite-voltage drift time, a set of measurements is made at six voltages across the mobility cell from 1100 to 1800 V. All other parameters are held constant.

3.50

The total measured drift times for a set of mass/charge ratios is determined. For each mass/charge the time is plotted versus 1/V. By extrapolation to 1/V ) 0 (infinite V) the drift time correction is determined. This set of times is correlated to the mass of the fullerenes by a curve fit, and the drift time correction for other masses is found by interpolation. This amount of time is subtracted from the total drift time to obtain the mobility-cell drift time. Theory Ion Mobility (Cross Section). The applied voltage or electric field in the mobility cell accelerates ions to a terminal speed determined by the balance of forces between the accelerating field and the momentum transferred to the atoms of gas through which they move. Ion mobility is defined as the drift velocity divided by the accelerating electric field

K ) Vd/E

(1.1)

McDaniel and Mason28 derived the expression for the low electric field limit giving the mobility in terms of the properties of the gas and the ions

K)

[

]

(18π)1/2 1 1 + 16 m mb

ze 1 (kTEF)1/2 Ω(1,1) avg N

1/2

(1.2)

where m and mb are the mass of the ions and atoms in the background gas, respectively, k is the Boltzmann constant, TEF ) T + mbVd2/3k is the effective gas temperature, N is the gas number density, ze is the charge on the ion, and Ωavg(1,1) is the collision integral averaged over all orientations of the ions. The drift velocity correction to the effective gas temperature is negligible due to the relatively low drift velocity in this study. We will use this form of the mobility along with eq 1.1 to help identify or confirm the presence of fullerenes in our samples and assess their shapes. Calculation of collision integrals will be described in the section to follow. Calculations of Drift Times for Spheres and Capped Cylinders Based on Hard-Sphere and Hard-Rod Estimates of Collision Cross Sections. Hard-Sphere Collision Integral. The cross section and average collision integrals of large fullerenes with helium atoms is determined in the present analysis assuming that the fullerenes are hollow spherical shells having a surface density of carbon atoms approximately equal to that of graphene sheets. The surface density R ) 38.2 atoms/ nm2 is based on the interatomic distance of 0.142 nm of the hexagonal pattern of carbon atoms in graphene sheets.29 A more precise estimate of surface density can be determined taking into account the pentagons. However, for large fullerenes this leads to a negligible correction since all closed spherical shells

Characterization of Large Fullerenes


Figure 2. Typical drift time spectrum of species seen in carbon nanotube samples.

made of hexagons and pentagons have only 12 pentagons. The diameter of a sphere defined by the locus of centers of the carbon atoms in the fullerene is determined by the number of carbon atoms n

ds )

xn/Rπ

(1.3)

Figure 3. High-resolution LDI-TOF-MS spectrum from unpurified laser-produced nanotube sample showing 24-Da interval that is characteristic of large fullerenes (red) and 12-Da interval (red and blue) characteristic of doubly charged large fullerenes.

We then add the diameter of carbon atoms dC to obtain the overall diameter of the spherical fullerene

dF ) ds + dC

(1.4)

If we assume rigid elastic sphere collisions, then the collision diameter of a spherical shell of diameter dF with a helium atom of diameter dHe is

1 d ) (dF + dHe) 2

(1.5)

Then the collision cross section for elastic spheres is

[x

1 1 σ ) d2 ) 4 4

n + dC + dHe πR

]

2

(1.6)

By integrating over angle and collision energy, the collision integral 2 Ω(1,1) avg ) πd ) π

[x

n + dC + dHe πR

]

2

(1.7)

Τhis simple form results because of the assumption of hardsphere collisions that are independent of the collision energy. The diameters dHe ) 0.2575 nm and dC ) 0.3354 nm were assumed. Rigid Elastic Tube (Capped Cylinder) Collision Integral. Closed fullerene-like shells can come in various isomer configurations having shapes that may vary from oblate spheroids to spheres to prolate spheroids to capped cylinders. Since we do not know the exact shape of all but a few particular fullerenes, e.g., C70 and C84, we will take spheres and capped cylinders as limiting cases. Capped cylinders may also represent nanotubes, which in the present study would have to be very short. By analogy with the hard elastic sphere, collision integrals of elongated fullerenes or cut nanotubes may be estimated. We assume that they are simply cylinders capped on the ends by spheres. The surface area is then the sum of the area of the

Figure 4. Calculated ion mobility cell drift times for singly and doubly charged spherical shells (fullerenes) compared with fits to measured drift times.

cylinder plus a sphere of the same diameter

As ) πd2s + πdsX

(1.8)

where X is the length of the cylindrical portion of the molecule and L is the overall length, including the spherical end caps. The approximate number of carbon atoms in this structure is n ) AsR. As for spheres, we add the diameter of carbon atoms to obtain the overall diameter of the cylindrical fullerene

dT ) ds + dC

(1.9)

For a given diameter ds and a given number of carbon atoms in the graphene cylinder with fullerene (spherical) end caps, we can solve eq 1.8 for the length of the cylinder

X)

n - ds πdsR

(1.10)

For X to be positive there must be a minimum number of atoms


Scott et al.

Figure 5. Comparison of mobility spectra showing effects of solvent and time after target preparation for JSC laser sample number 171.5.

for a given diameter tube. It also should be noted that the number of atoms in the tube must be increased by the number of atoms in a ring of carbon atoms. Therefore, not all masses of cylindrical tubes are possible. For example, C70 has a single ring of 10 carbon atoms between its end caps made of 30 atoms per end cap. It is a very short, capped cylinder. Another commonly seen fullerene, C84, would be in another class. It could not be a cylinder capped by two halves of a C60 because the difference in number of atoms is not a multiple of 10. Three possible isomers are nearly spherical as shown in Figure 1 of ref 30, Figures 13a, 14a, and 15a of ref 31, and ref 32. To determine the cross section of capped cylinders we start with their geometry. The effective cross section will be the sum of the spherical part and the cylindrical part averaged over all angles. The contribution of the cylindrical part is zero when the axis of the cylinder is aligned along the collision velocity. It is greatest when the axis is perpendicular to the collision velocity. The spherical part of the collision integral is the same as for a sphere. We now consider the cylindrical contribution. At 90° the diameter is

d′ ) ds + dC d′ )

n + dC πRX

(1.11) (1.12)

Then the cross section is

1 σT ) (d′ + dHe)X cos θ 2

(1.13)

Due to their random molecular motion, the molecules will be tumbling. Therefore, their average drag through the mobility cell depends on the orientation-averaged cross section. We

average over the angles to get the collision integral

Ωcyl )

∫0π (1 - cos θ)(21(d′ + dHe)X cos θ)sin θ dθ

(1.14)

The total average collision integral for the capped cylinder is

ΩCapCyl ) ΩSph + ΩCyl

(1.15)

π π ΩCapCyl ) (ds + dC + dHe)2 + (ds + dC + dHe)X (1.16) 4 4 Results and Discussion Typical LDI-IM-TOF-MS spectra are shown in Figure 2, where we can see several “traces”. This figure shows the relative number of particles (shown as color scale) as function of measured drift time and m/z. At lower masses and longer drift times there are traces of low-density (or large cross section) species that correspond to linear, cyclic, and polycyclic hydrocarbons. The evidence that these are organic molecules can be seen in high resolution, where all masses are seen. Since we are dealing with organic material, only molecules containing hydrogen can exhibit a difference in mass of unity. Over the mass range of about 600-9000, we can see a more dense concentration of molecules along a definite trace that we will attribute to fullerenes. At masses corresponding to the calibration species, we calculated the mobilities using spherical shell theory described in a preceding section. These values correspond within about 2% of the values in the literature (see Table 1). Just below, over a smaller m/z range, we see a similar parallel curve that we attribute to doubly charged fullerene ions. Identification of Fullerenes. At high resolution, Figure 3, we can see that the difference in mass along the main fullerene



curve is 24 mass units. This difference corresponds to two carbon atoms, characteristic of a progression of fullerenes. Assuming closed spherical shell structures, we calculate the ion mobilities, then the drift time of fullerene-like molecules using eqs 1.1, 1.2, and 1.7. The drift time is given by

∆t ) L/Vd ) L2/VK

(1.17)

where V ) LE is the voltage applied across length L of the mobility cell. Figure 4 compares the corrected measured drift times with calculated spherical shell drift times. These measurements and theory agree very well, confirming that the assumption of closed-shell fullerene structures is the correct interpretation for these species. The spread of mobilities seen in the measurements gives an indication of the measurement accuracy, although it is possible that these species deviate from strictly spherical shapes and may be elongated or have attached groups. Identification of Doubly Charged Fullerene Ions. If we assume that the cross section of doubly charged spherical shells is the same as singly charged spherical shells but the mass to charge ratio is halved, then we can calculate the drift time for doubly charged species. The drift time should decrease by a factor of one-half due to the double charge, and m/z decreases by a similar factor. Examination of the lower (doubly ionized) curve in high resolution shown in Figure 3 reveals an m/z separation of 12, lending support that these correspond to doubly ionized fullerenes (i.e., z ) 2). As seen in Figure 4, there is good agreement with the measured curve that parallels the main fullerene curve. We interpret these results as doubly charged positive fullerene ions. Both curves in Figure 4 have been corrected by the same infinite-voltage drift time and for differences in time associated with the laser pulse delay between the pulse initiation and actual laser energy on the target. Effects of Solvent and Age. To place the nanotube samples on the support plate it is necessary to disburse them in a solvent and drop the dispersed samples onto the plate. Fullerenes and other species, e.g., nanotubes and lower mass hydrocarbons, are not extracted by this process, only suspended as a slurry. However, it was observed that the 2D spectra of the lower mass and higher drift time species varied from sample to sample due to solvent and time after application to the support plate. There was little difference in the spectra of the large fullerenes due to solvent or time. These observations are shown in Figure 5. The type of solvent (methanol or chloroform) made a difference in the composition and abundance of the low-mass species. Generally, a relatively higher signal was recorded for these species when the sample had been prepared using methanol and/ or kept in air for a few days before analysis. Since the appearance of the spectra differs in these cases, it appears that these species are products of the laser desorption process and are not inherent in the samples. They are probably products of oxidation of clusters or the medium, which may contribute to easier ionization of these low-mass species by absorbing moisture and other contaminants. In the discussion below, the fullerene trend lines are analyzed since they are not sensitive to the sample preparation procedures. Effect of Desorption Laser Power on Mass Distribution. To determine if there is an effect of the desorption laser power on the measured mass distribution, the incident power of the desorption laser was varied using neutral density filters. Two samples were examined, one from the laser ablation process of the Oak Ridge National Laboratory (ORNL) and the other from JSC. For some samples, like the one from the ORNL, one can see (Figure 6a) that there is very little difference in the spectra at various laser powers. However, in other measurements a

Figure 6. Effects of relative laser power on mass distribution of fullerenes of samples from (a) Oak Ridge National Laboratories and (b) the Johnson Space Center. The ratios between the low, medium, and high power are 1:1.24:1.56 and 1:1.11:1.26 for the experiments in the parts a and b, respectively.

general trend toward observing larger fullerenes at higher laser powers was detected; see, for example, a clear difference in spectra for the sample from in Figure 6b, even though there was a smaller variation in incident laser power compared to the ORNL sample. The observed trend is likely due to the local conditions of laser desorption of different sizes of clusters at different laser powers. Obviously, these conditions strongly depend on the sample morphology; therefore, the results vary significantly from one sample to another. The mass spectra in Figure 6a and 6b have been averaged over a range of masses to be able to see overall trends in the spectra more clearly. Effect of Nanotube Source and Purification on Fullerene Mass Distributions. One of the primary reasons for measuring the mass distribution in nanotube samples was to determine whether the type of production facility had an influence on the distributions. Nanotubes from a number of different sources were obtained and processed in the same way for the measurements. In addition, we wanted to see if purification removed all or just some of the fullerenes. Figure 7 compares fullerene distributions for CVD-produced samples obtained from Southwest Nano-


Scott et al.

Figure 7. Comparison of fullerene distributions for several samples of growth products.

technologies, Inc. (SWeNT), arc-produced samples from the Johnson Space Center (JSC), raw HiPco samples from Rice University, and two raw and one purified laser ablation samples from JSC. The two similar raw laser samples from JSC are compared to assess sample-to-sample consistency. In the case of JSC samples, purification involves extractions with toluene, methanol, and reflux in mixtures of acetic acid, HCl, and HNO3.9 The purification process of Southwest Nanotechnologies, Inc. involves reflux in hydrofluoric acid. We can see that the CVDproduced sample differs considerably from high-temperature ablation samples. In addition, purification alters the distribution considerably, although it appears that C60 and C70 are not removed as much as larger fullerenes. This is peculiar since heavier fullerenes are known to be less soluble than C60 and C70. During purification, acids and oxidation may preferentially degrade higher mass fullerenes. It should be noted that the purification process for samples from various sources was not the same. Each process had its own purification protocol. In an attempt to find a numerical indicator of the source of carbon nanotubes, it was observed that the mass distribution was notably different at low masses and at high masses for various sources of nanotubes. One possible index is the ratio of number of low-mass fullerenes in the range of 850-2000 Da to the number of high-mass fullerenes in the range of 20008900 Da. These ratios are shown in Figure 8. There is not much variation in the ratio for raw JSC laser-produced nanotubes, for ORNL raw and purified nanotubes, and for Langley Research Center (LARC) raw laser-produced nanotubes. Raw and purified SWeNT tubes have very different distributions of low-mass to high-mass fullerenes. Likewise, the arc-produced materials from the Universite´ Paris 13, Laboratoire d’Inge´nierie des Mate´riaux et des Hautes Pressions (LIMHP arc), and the laser-produced material from the University of Karlsruhe (UKG-L02-350)

Figure 8. Ratio of low-mass fullerenes to high-mass fullerenes in samples before and after purification.

differ considerably. Therefore, it may be possible to use this ratio as an indicator of source or production technique, but it cannot uniquely indicate the source. Spread of Drift Times Associated with Fullerene Shape. For closed-shell fullerenes of a given mass, the average mobility or drift time depends on their shape. On the basis of TEM observations of fullerenes in HiPco samples, many short elongated species were seen attached to carbon nanotubes. We postulate that a limiting form for these elongated structures is a cylinder or capped cylinder. We can parametrize this limiting shape by assuming varying overall length to diameter ratios L/ds. The difference in drift times for a given mass for various L/ds



Figure 11. Relative drift time distributions of the average of C334-C416 fullerenes measured from the raw JSC laser-produced sample. Figure 9. Calculated drift times for positive-ion spherical shells and capped cylinders compared with a fit to measured fullerene drift times. Also shown are drift times based on mobility measurements of Shvartsburg et al. (1998) and Mesleh et al. (1996). Mobility cell voltage is 1700 V, and helium pressure is 3.5 Torr.

To obtain a measure of the range of possible shapes we examined the drift time distribution of a number of species near C400. Measured relative drift time distributions for 17 fullerenes near C400 (C384-C416) were averaged by first adjusting the drift time by the drift time centroid of each distribution from one species to another. The relative drift time distributions were then averaged for all 17 species (including their isotopic variants) to obtain the drift time distribution shown in Figure 11. The most abundant fullerene is likely nearly spherical or spherical because the peak has little or no left shoulder, and its drift time fits very well with spherical theory. If this is the case, we attribute the asymmetry with the existence of elongated fullerenes. There appears to be a distribution of elongated fullerenes (or conceivably extremely short nanotubes) extending up to L/d of about 5, but most of them have L/d less than 3. Conclusions

Figure 10. Drift time distribution for capped cylinders of various length-to-diameter ratios.

can give us an estimate of the range of shapes possible in the samples. Figure 9 shows calculated drift times over the range of masses from C60 to C500 for diameters from 0.7 to 2.0 nm. Also shown are drift times based on published fullerene mobilities of Shvartzburg et al.25 and Mesleh et al.26 By investigating the spectra in more detail than that shown in Figures 9 and 4, it can be seen (Figures 3 and 5) that there is a spread in the drift time distribution for a given mass. The measured data shown in Figures 9 and 4 represent a curve fit along the locus of the maximum number of molecules in curves shown in figures such as those seen in Figures 3 and 5. However, for each mass there are a number of possible isomers that allow various shapes. They may vary from spheres, to prolate spheroids, to capped cylinders. For example, ref 30 describes a number of possible isomers of C84. For each mass the mobility time distribution follows a peaked shape as implied in Figures 3 and 5. The distribution is unsymmetrical, indicating a presence of differently shaped fullerenes. Figure 10 shows a calculated range of drift times associated with each mass by lines of constant L/d, lending plausibility that various shaped fullerenes give rise to the spread of drift times seen in the 3D spectra of Figures 3 and 5.

Using laser desorption-ionization ion mobility time-of-flight mass spectrometry, we analyzed single-wall carbon nanotube growth products from a number of different sources. Fullerenes were the confirmed species identity seen in the samples by comparing ion mobility measurements with theoretical simulations and data found in the literature. Their masses range from around 720 to about 9000 Da and differ by two carbon atoms. It appears that these species are mostly closed nearly spherical shells. The measured amount or distribution of these fullerenes varied according to the source of the sample, e.g., laser ablation products, arc vaporization products, and chemical vaporization products. We showed that the ratio of specific features of the fullerene distribution varies with type or source of product. The CVD-produced nanotube samples had the least amount of fullerenes. Likewise, purification altered the distribution of fullerenes seen in the samples. It also alters the relative amounts of lower and higher mass fullerenes. A range of shapes (drift times) is seen for each mass, consistent with various elongated isomers of a given molecular weight. The preponderance of them are close to spherical, and most would have a length to diameter ratio less than three. A variety of lighter molecular species was detected in ion mobility mass spectra. They were ascribed to different linear or planar carbon and hydrocarbon structures and impurities. Their detection efficiency strongly depended on the sample preparation. Acknowledgment. The authors acknowledge the following who furnished samples of carbon nanotubes for this study: Samir Farhat of the Universite´ Paris 13, Laboratoire d’Inge´nierie

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Characterization of Large Fullerenes in Single-Wall Carbon Nanotube

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