Carbon 41 (2003) 1477–1488

P urification process for single-wall carbon nanotubesa , b b c*Karla L. Strong , David P. Anderson , Khalid Lafdi , John N. Kuhn

aAir Force Research Laboratory, Materials and Manufacturing Directorate, AFRL /MLBC, 2941 P. Street, Wright Patterson AFB,OH 45433,USA

bUniversity of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0168,USAcUniversity of Dayton, Southwestern Ohio Council for Higher Education, 3155 Research Blvd., Suite 204, Dayton, OH 45420-4015,

USA

Received 13 May 2002; accepted 24 December 2002

Abstract

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electricalconductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamentalchallenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs,along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screenand compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluatingtheir purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward,inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotubecarbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pureSWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which takeonly a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relativepurity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.Published by Elsevier Science Ltd.

Keywords: A. Carbon nanotubes; C. Raman spectroscopy; Thermal analysis; Transmission electron microscopy; X-ray photoelectronspectroscopy

1 . Introduction and background disordered carbons such as incomplete fullerenes (tubesand spheres), and onion-morphology ‘shell carbon’ which

Since the discovery of single-wall carbon nanotubes often encases the metal catalyst[3–6].(SWNTs) by Iijima [1] there have been many inves- The purpose of the present research was, first, totigations of these materials. SWNTs have exceptional develop a simple, scaleable purification process forstrength and stiffness along with high thermal and electri- SWNTs to provide materials which would be satisfactorycal conductivity[2]. These properties, in combination with for structural or multifunctional aerospace applications.their relatively low density, have brought SWNTs to the Second, an understanding of the purification mechanismattention of researchers who are striving to engineer the was sought. Finally, quick and effective analytical methodsnext generation of aerospace structural materials. However, to screen and compare the relative purity of SWNTs afterthere are many obstacles to making processable materials different treatments were required.which take advantage of these properties. One of the most Previous studies on purifying SWNTs usually includedfundamental challenges is purifying the SWNTs to remove heating in an oxidizing atmosphere to remove non-the metallic catalyst (typically Fe, Co, and/or Ni) used in nanotube carbon[3,6,7]. The presence of the metal catalystgrowth, as well as removing carbon buckyballs and at this stage of the purification process allows easier

oxidation of shell carbon surrounding the nanometer-sizedcatalyst particles. Chiang et al.[6] have stated that*Corresponding author. Tel.:11-937-255-3104; fax:11-937-exposure to wet air or wet Ar /O mixture breaches the656-4706. 2

E-mail address: karla.strong@wpafb.af.mil(K.L. Strong). carbon shell, presumably by diffusion, and converts the

0008-6223/03/$ – see front matter Published by Elsevier Science Ltd.doi:10.1016/S0008-6223(03)00014-9

1478 K.L. Strong et al. / Carbon 41 (2003) 1477–1488

metal to an oxide or hydroxide. When the metal is Eklund et al. have found that a narrowing of the G peak isoxidized, the volume increases and the metal oxide particle related to a greater graphitic nature of carbon nanosoot ascracks open the carbon shell. Researchers have also stirred it is heat-treated to progressively higher temperatures[19].or refluxed the SWNTs in HNO [3,7,8], HCl [6,9], or It is a reasonable expectation that this relationship also3

H O [9] to reduce the amount of metal catalyst. Oxidation holds true for single-wall carbon nanotubes. Although2 2

followed by acid treatment appears to be critical; the acid neither G:D ratio nor the width of the G peak can be usedeasily dissolves metallic oxides exposed by the mild as an absolute measure of SWNT purity, together they areoxidation [6]. In addition, many investigations have in- likely to provide information about the relative disorder ofcluded a final annealing step of heating at 800–10008C in the graphitic material in a sample and, therefore, thean inert gas or vacuum, which may ‘heal’ nanotubes relative effectiveness of various purification treatments.damaged by the previous purification steps[6,8,9]. Most of In addition, the RBM modes can be used to estimatethe purification processes reduced the metal content below SWNT diameter[11]. The relationship between frequency,1 wt% [3,6,7]. n, of the peaks in the radial breathing mode and the

Methods to analyze the purity of the SWNTs have nanotube diameter is given by[11]:included thermal gravimetric analysis (TGA)[3,6–9],

aRaman spectroscopy[3,6–10], X-ray diffraction (XRD) ]]]diameter (nm)5

21n (cm )[8,9], transmission electron microscopy (TEM)[3,5–9],

scanning electron microscopy (SEM)[6–9], UV–Vis– 21wherea5224 nm cm . Other values fora reported inNear-IR spectroscopy[6,7], and energy dispersive spec- 21the literature have ranged from 223.75 to 248 nm cmtroscopy (EDS)[9].[11,20–22].Recently, Alvarez et al.[21] and Rao et al.In particular, Raman spectroscopy has been shown to be[22] have modified the relationship to account for the vana powerful tool for characterizing SWNTs. The charac-der Waals interaction between the SWNTs when they areteristic peaks occur due to the radial breathing modecontained in ropes. Rao states this relationship as[22]:(RBM), disordered carbon (D band), and an out-of-phase

agraphene sheet-like vibrations (split G band)[11–13]. 21]]]]diameter (nm)5 1 14 (cm )individual tubes 21These peaks occur at approximately 170–325, 1330, and n (cm )bundle211585 cm , respectively. The radial breathing mode is

21wherea5224 nm cm .present in SWNTs only[11]. An asymmetry on the rightThe intensity of some RBM peaks is significantly higherside of the G band represents the peak characteristic of

because they are resonantly enhanced[11,19,21].With themulti-wall carbon nanotubes (MWNTs), and occurs at21 21 use of transition energies calculated from allowed optical1620 cm [14]. The peak at 1585 cm (primary G peak)

transitions for SWNT of various diameters and helicities,is due to longitudinal vibrations along the graphene sheet,21 the type of tube (metallic or semiconductor) may beand the peaks at 1567 and 1543 cm are due to transverse

derived[5,21,23,24].graphene sheet vibrations[15]. The Raman spectroscopyIn the present research, a simple purification methodolo-signature of SWNTs is unique and can be used to identify

gy, derived from one suggested by Rice University[6,7]the presence of the nanotubes.was employed. The purity of SWNTs at each stage of theThe Raman spectrum may also be used to qualitativelypurification process was studied. Several analytical meth-describe the purity of the SWNTs. An increase in the ratioods were utilized, including inductively coupled plasmaof the area of the primary G peak to that of the D peakmass spectroscopy (ICP–MS), TGA, XRD, X-ray photo-(G:D ratio) is correlated to increasing graphite crystalliteelectron spectroscopy (XPS), TEM, and Raman spectros-size[16,17] and a decrease in the amount of ‘unorganized’copy. The capabilities of each were assessed and balancedcarbon in graphitic materials[18,19]. Previously, it haswith their ease of use. It is hoped that simple, effectivebeen observed that increased purity is associated withmethods of SWNT purification and analysis will provideincreased SWNT bundle size, also known as rope diam-investigators with a stable, high-quality source of SWNTseter. It is thought that there are fewer small particlesfor use in multiple applications.present to interfere with the van der Waals attraction

between neighboring tubes[6]. We suggest that in SWNTsamples, crystallite size may be equated to rope diameter.Therefore, the G:D ratio may be a qualitative measure of 2 . Experimental procedureincreased purity.

The G peak width might also be a measure of purity of 2 .1. MaterialsSWNTs. Laspade et al. found, for several different types ofgraphitic carbon, a reduction in G peak width with The SWNTs used in this study were supplied by Riceincreased ‘graphitization’, defined as a diamagnetic sus- University. The tubes were made using the Rice Universityceptibility and average interlayer spacingd approaching the high-pressure carbon monoxide method HiPco� [25].002

that of pure single crystal graphite[17]. In addition, The tubes were received in the raw state as they were

K.L. Strong et al. / Carbon 41 (2003) 1477–1488 1479

removed from the reactor without any purification. Also, a 5–20 mg of tubes were placed in the TGA and heated atstandard for Raman spectroscopy of graphite powders was 108C/min from ambient to 10008C. The chamber wasobtained from Fisher Scientific, and was 99.9% pure. continuously swept with air at a rate of 60 ml /min.

2 .2. Nanotube purification process 2 .5. Transmission electron microscopy

The as-received tubes (AR) were placed in a ceramic Transmission electron microscopy was performed on adish and placed in a Fisher Isotemp Programmable Philips CM30 at an accelerating voltage of 100 kV.

Furnace Model 495. Wet air was introduced into the oven Samples were prepared by touching a copper grid to(the water is thought to aid the oxidation of disordered prepared SWNT/water suspensions and allowing them tocarbons) by bubbling room temperature air through water dry.

3and then into the oven at 100 cm /min. The oven wasprogrammed to heat at 58C/min from ambient to 2508C 2 .6. Micro-Raman spectroscopyand hold for 12 h. At that point, the heater was turned offand the oven was allowed to cool by natural convection. Micro-Raman spectroscopy was performed with a Re-These tubes were designated as being oxidatively heat nishaw Raman microscope. Laser light at 514.5 nm from

1treated (HT). an Ar laser was used for excitation. A power density of2The tubes were extracted with constant boiling HCl in a 1.1 mW/cm was used. Calibration was performed with

21Soxhlet extractor. The tubes (1–2 g) were held in the the Si peak at 520.7 cm . Raman data were normalized21extractor in a glass thimble with a fine frit bottom (10–25 using the primary G peak at approximately 1586 cm as

mm). Although the individual nanotubes had a smaller the reference, and peaks were fit with a Lorentziandiameter than the frit, no significant amount of the tubes lineshape. The error in the peak positions was less than

21passed through the filtering bottom due to matting and/or 62.0 cm .roping.

Approximately 600 ml of concentrated HCl were diluted 2 .7. X-ray photoelectron spectroscopyto 1 l with deionized water. Some (|300 ml) of thisslightly over-strength solution was added to the Soxhlet X-ray photoelectron spectroscopy was performed usingextractor pot where, during the course of refluxing, the a Surface Science Instruments M-probe XPS instrument

28excess HCl was lost through the condenser to produce the operated at a base pressure of less than 1310 Torr.constant boiling acid. This refluxing acid continuously Using an Al anode, a 40031000-mm line spot, and acondensed onto the SWNTs for 68–72 h while siphoning 25-eV pass energy, the full width at half maximum of theback into the extractor pot every 5–12 min. The volume of Au 4f peak was 0.85 eV. Binding energy positions were7 / 2

the extractor was|50 ml but the actual volume of acid in calibrated against the Au 4f peak at 83.9 eV, and energy7 / 2

each siphon wash was less than that due to the volume separations were calibrated using the Cu 3s and Cu 2p3 / 2

displaced by the thimble and nanotubes. After acid ex- peaks at 122.39 and 932.47 eV, respectively. The detectiontraction, the tubes were extracted with deionized water for limit was approximately 1.0 at%. Samples were argon ion24 h and then with methanol 24 h before drying. sputtered for 90 s prior to analysis to remove surface

The HT tubes which were subsequently extracted were carbon and oxygen contamination from the atmosphere.designated as cleaned (CL), while some of the AR tubes Peak fitting was performed with a Lorentzian lineshape towere also extracted without oxidation and designated calculate chemical composition. Samples were insulating,simply as extracted (EX). so an electron flood gun was used for charge neutraliza-

tion.2 .3. Chemical analysis

2 .8. X-ray diffractionChemical analysis was performed by ICP–MS, with an

accuracy of60.2 wt%. The samples were digested in aqua X-ray diffraction scans of inorganic residues wereregia before analysis. Some of the sample remained obtained using a Rigaku D/MAX horizontal diffractometerundigested, and this remaining amount was dried and in a symmetricu–2u reflective mode. The Cu Ka radiationanalyzed with X-ray fluorescence (XRF) to determine the was produced using a rotating anode generator at 40 kVmetal content. and 150 mA and a diffracted beam crystal monochromator.

The samples were attached to a glass slide using double-2 .4. Thermogravimetric analysis sided tape such that the sample was held at the Bragg–

Brento diffracting position. Transmission XRD scans ofTGA was performed using a TA Instruments model the nanotubes were obtained on a Bruker AXS D8/Discov-

TGA 2850 Thermogravimetric Analyzer to measure the er system using a scintillation detector after 48 Soller slitsmass loss of the tubes on heating in air. Approximately and 0.58 (0.1 mm) detector slits. The transmission samples

1480 K.L. Strong et al. / Carbon 41 (2003) 1477–1488

T able 1were held between transparent tape windows—the tape inChemical composition of SWNT before and after purificationboth transmission and reflection showing in the diffractiontreatments (at%)patterns as large broad amorphous scatter. The Cu Ka

radiation was produced using a sealed tube generator at 40Sample Composition, XPS Comp., Comp.,¨kV and 40 mA with a single Gobel mirror monochromator. XPS excluding O ICP–MS

Use of a GADDS area detector was attempted but was C Fe O C Fe C Feunsuccessful due to the excess iron fluorescence from most

AR 96.3 3.9 0 96.3 3.9 94.8 5.2of the nanotube samples.HT 83.0 9.1 7.9 90.1 9.9 – –EX 92.9 3.7 3.4 96.2 3.8 – –CL 99.0 0.0 1.0 100.0 0.0 99.8 0.2

3 . Results and discussion

3 .1. Cleaning the nanotubes present. Also, it is clear that there is minimal benefit fromextraction lasting longer than 24 h.

The oxidative heat treatment was recommended by RiceUniversity [6,8] as a method of removing the disorderedcarbon, as well as converting the iron catalyst to a more 3 .2. Chemical analysissoluble oxide. Approximately 40% of the original mass ofthe nanotube mixture was lost during the oxidative heat The results of the metals analysis by ICP–MS of thetreatment. carbon nanotubes before and after purification are shown

The Soxhlet extraction of the HT with acid initially inTable 1. The AR nanotubes contained 5.2 at% iron,produced a strong orange extract, which gradually became while the CL nanotubes contained only 0.2 at% iron, aclear over the first 24 h. The color is presumably from the significant reduction, corresponding to the results of thedissolved iron catalyst as FeCl . The mass loss during acid analysis by ICP–MS above. Using this method, one3

extraction varied from 30 to 50%. would conclude that the purification process developedThe extraction efficiency was checked on a different lot was very effective in removing Fe catalyst.

of HT SWNTs from the Rice University HiPco� process However, as noted earlier, the aqua regia did notthat included both iron and nickel catalyst. The amount of completely dissolve the nanotube samples, leaving ametal in the extracting acid (1-ml aliquots periodically residue which could not be analyzed by ICP–MS. Thisremoved from outside the extraction thimble for analysis) material was shell carbon surrounding iron catalyst. Thewas measured by ICP–MS. The average of two extractions carbonaceous shell was not soluble in aqua regia, which isis shown inFig. 1. The amount of iron extracted started at acid-based, and would prevent the iron inside the shellover 1000 ppm. This amount reduced sharply with ex- from being dissolved as well. The residue was examinedtraction time, but was still decreasing slightly at less than 1 with XRF, which in this case was considered semi-quan-ppm after 24 h. The nickel extracted started at|100 ppm titative. It was discovered that the insoluble materials fromand leveled off at|1 ppm after only a few hours. Clearly, both the AR and CL samples contained iron. The insolublethe acid extraction of HT SWNTs removes a significant material from the AR sample contained approximately 4amount of the Fe catalyst, as well as the Ni catalyst, when at% iron, and the insoluble material from the CL sample

Fig. 1. Metal content in the extracting acid of the Soxhlet extractor while cleaning SWNTs.

K.L. Strong et al. / Carbon 41 (2003) 1477–1488 1481

T able 2contained approximately 1 at% iron. Therefore, the valuesFinal TGA residue mass results for SWNTs tested in airshown inTable 1are somewhat inaccurate. Since most of

athe material did dissolve in the aqua regia, it is expected Nanotube Residue Ironthat the values reported inTable 1 are within 0.5 at%. clean-up mass (%) (at%)These results show that when using ICP–MS, nanotube levelsample preparation is critical to obtain accurate results. A AR 27.7 4.92different method of atomizing the sample, such as an arc AR 28.0 4.98flash, would be highly preferable. EX 30.5 5.51

HT 42.7 8.403 .3. Thermogravimetric analysis HT 56.4 12.29

CL 0.73 0.11aThe mass losses for the samples at various stages of Assuming all the iron in the SWNTs is converted to Fe O in2 3

clean-up are shown inFig. 2.Variations in the final residue the final residue.mass are shown inTable 2. The variability in the HTsamples is a result of the variability of oxygen exposure in the onset of oxidation of the carbonaceous material bysamples sitting in the oven. The top of the sample is nearly 1508C. The stability of the cleaned nanotubes byexposed to more air than the bottom of the sample. The TGA is indicative of the oxidative loss of the disorderedtumbling of the nanotubes during the initial oxidative heat carbon and fullerenes during the oxidative heat treatment.treatment suggested by Park and co-workers[26,27] would Additional carbonaceous materials is lost during the ex-therefore be more appropriate in cleaning larger lots of traction in acid after being partially oxidized and morenanotubes. easily flushed from the nanotube rope mats.

Below 3008C, a mass gain is observed in the AR and The values of 4.9–5.0 at% Fe in the AR tubes and 0.11EX samples. This is presumably the uptake of oxygen by at% in the CL nanotubes are smaller than the values fromthe iron catalyst. Note that without the oxidation cycle, ICP–MS (5.2 and 0.2). This could be due to the inac-extraction of the catalyst is ineffective; Fe remains in the curacies of the ICP–MS mentioned earlier, or due to theEX sample, which is oxidized during TGA. Simultaneous- assumption that all the residual mass is in the form ofly, there is a mass loss from the conversion of disordered Fe O . While this is the phase identified in the XRD (q.v.)2 3

carbon to CO (between approximately 325 and 4008C). other minor phases, such as FeO and Fe O , may also be2 3 4

In the EX and AR samples, the shoulder between 350 and present which could change the calculated at%.4008C is the oxidation of iron, which occurs during the As expected from the small amount of material lostoxidation stage. In the HT sample, all the exposed iron has during extraction of the AR tubes, the EX tubes lost veryalready been oxidized, so no additional O uptake is seen little iron relative to AR. Since heat-treating the nanotubes2

below 3008C. However, as the non-nanotube carbon is in air oxidized much of the non-nanotube carbon, theconverted to CO , fresh Fe is exposed and is oxidized. The relative amount of iron left in the samples was higher than2

CL sample also shows no mass increase below 3008C, in the AR tubes. After extraction of the HT nanotubes toindicating an absence of Fe. produce the cleaned nanotubes, there is very little iron

Also interesting to note in the CL tubes is the delay of remaining.

Fig. 2. Typical TGA scans of the four levels of nanotube purification.

1482 K.L. Strong et al. / Carbon 41 (2003) 1477–1488

Fig. 4. Bright field image of grown concentric spheres and carbonFig. 3. TEM micrograph of SWNTs, as received. Arrows indicate lamellas.the presence of particles of metal catalyst.

demonstrate the peaks characteristic of SWNTs discussed213 .4. Transmission electron microscopy above. None had the peak at 1620 cm characteristic of

MWNTs, and therefore little or no MWNTs were present.The as-received SWNTs consist of a large amount of Table 3 shows the calculated G to D peak area ratios

metal catalyst (single arrows inFig. 3) of various sizes and (G:D) compared to that of reagent grade graphite anda number of carbon types which have different shapes and highly ordered pyrolytic graphite (HOPG). The G:D ratiothe molecular organizations: (1) onion-like carbon particles increased dramatically as the SWNTs were purified, in-(carbon shells or carbon blacks), (2) carbon lamellas, (3) dicating removal of non-nanotube carbon material. Thecatalytic carbons, (4) CVD carbon grown on carbon, (5) G:D ratio increased by a smaller amount in the EX and HTsingle-walled carbon nanotubes trapped in amorphous samples. Extraction could wash away small particles ofcarbon, and (6) free single-walled carbon nanotubes. carbon and iron oxide, while oxidative heat treatment

Most of these carbon forms have been described previ-ously in nanotube samples except onion-like carbon par-

ticles and carbon lamellae[28]. The onion-like and lamel-lar carbons can form at any carbon surface where carbonfree radials exist which nucleate and subsequently addconcentric or parallel carbon layers. The onion-like par-ticles, which grow in the gas phase, are similar to carbonblack in their crystallographic structure[29]. Significantamounts of onion-like (single arrows inFig. 4) andlamellar carbon (double arrows inFig. 4) are present inthis sample.

After purification most of the metal catalyst was re-moved. However, there are still some remaining catalystparticles surrounded by large masses of carbon (seeFig.5). The CL sample consists of two types of single-walledcarbon nanotubes: trapped SWNTs (roped) in amorphousmatrix and free SWNTs. This can be seen inFig. 6.

3 .5. Micro-Raman spectroscopy

Fig. 7 compares the Raman spectra for each of the fourpurification levels.Fig. 8 shows a typical example of aSWNT Raman spectra in the RBM region along with a Fig. 5. Image showing catalysts trapped in a mass of amorphousLorentzian peak fit. All of the experimental samples carbon in CL sample.

K.L. Strong et al. / Carbon 41 (2003) 1477–1488 1483

delayed onset of oxidation in the CL sample, indicatinghigher purity. Raman spectroscopy together with TGA,while not quantitative, are very easy and sufficient meth-ods for monitoring the relative purity of the SWNTs.

Also shown inTable 3is information about the diameterof the SWNTs derived from the Raman RBM peaks. UsingRao’s equation for nanotubes in a bundle[22], andassuming an error in the position of the Raman peaks of

2162.0 cm , the range of nanotube diameters apparent inthe sample and the diameter of the SWNT which generatedthe largest RBM peak are calculated. One must keep inmind, however, that only certain SWNT diameters andchiralities are sensitive to a specific laser energy, so thatthe diameter distribution may appear discontinuous al-though it may be, in fact, a smooth distribution[6,11].Therefore, the range of the diameters calculated withseveral laser wavelengths is a better measure of thediameters of nanotubes present.

The largest RBM peaks are likely those resonantlyenhanced by the 514.5-nm wavelength laser. The resonant-ly enhanced SWNT peaks present in these samples occur

21at the wave numbers of 263–267 cm . This correspondsto nanotubes of diameters 0.89 to 0.91 nm. When using a514.5-nm wavelength laser (photon energy52.414 eV), ithas been shown that resonantly enhanced peaks from tubesof this diameter are due to metallic nanotubes[5,21,23,24].

3 .6. X-ray photoelectron spectroscopy

Typical XPS spectra of the C and Fe peaks are shown inFig. 9. Table 4shows the experimental binding energy ofthe C 1s electron, along with the corrected bindingenergies of the Fe 2p and O 1s electrons for each of the3 / 2

four samples, both before and after sputtering. Peakshifting due to the charge neutralization gun complicatedthe analysis of the peak positions. Therefore, the peakpositions for Fe and O were corrected, assuming that theassociated carbon peak remained constant at 284.5 eV. Thisis a reasonable assumption, as the variation in the ex-

Fig. 6. (a) High-resolution images showing lattice fringes of free perimental peak positions for carbon appear to apply to alland isolated single-walled carbon nanotubes in CL sample. (b) the experimental peaks for a given sample, so the variationHigh-resolution images showing lattice fringes of SWNT trapped observed probably does not have any significance. Dif-in amorphous matrix (single arrows) in CL sample. ferentiating between the carbon contained in carbon

nanotubes, disordered carbon, and carbonaceous materialalone would oxidize the small carbon particles and iron adsorbed on the surface of the nanotubes from the atmos-

2particles. However, the iron oxide would remain in the phere, which are all likely to be sp bonded to at least onesample until removed with the acid reflux. Also shown in other carbon atom, may require the use of synchrotronTable 3is the full width at half maximum (FWHM) of the photoemission such as that employed by Suzuki et al.[30].primary G peak for each material. In general, the FWHM It was not attempted in this study. In some cases, twodecreases with purification treatment, indicating less dis- peaks were required to fit the data. Both peaks are reportedordered carbon. The oxidative heat treatment alone seems inTable 4.to have little effect, signifying that the low temperatures The binding energy of the Fe peaks is quite revealing. Itwere not sufficient to increase the order of the carbon is important to know that the binding energy of metallicmaterial in the sample. Together, the G:D and FWHM of iron occurs near 707 eV, and that of iron in Fe O occurs2 3

the primary G peak indicate that the SWNTs were purified. over a narrow range at 710.8[31]. For the AR sample, allThis is consistent with the TGA results, which showed a the iron is metallic, both on the surface and in the interior,

1484 K.L. Strong et al. / Carbon 41 (2003) 1477–1488

Fig. 7. Raman spectroscopy of SWNTs at the four stages of purification. An expanded scan of the radial breathing mode is included foreach. The data for each step have been vertically off-set for clarity.

Fig. 8. Example of Raman spectroscopy and Lorentzian curve fitting for SWNTs.

with a binding energies near 707 eV. The situation is moreT able 3 complicated for the HT sample. Here, the Fe on the surfaceCharacteristics of Raman spectra for SWNT samples and stan- occurs at 710.8 eV, exactly that expected for Fe O .2 3dards

However, after sputtering, two peaks occur at 707.2 andSample G:D G Diameter Diameter 708.9 eV. This may indicate partial oxidation of the iron in

FWHM range largest RBM the interior of the sample. The situation is similar for the21(cm ) (nm) peak (nm) EX sample, except even at the surface the iron is only

AR 7.7 31 0.74,D,1.13 0.89 partially oxidized, as indicated by the peak at 708.7 eV. ItEX 8.1 28 0.76,D,1.19 0.91 is likely that extraction alone begins to oxidize a smallHT 10.7 30 0.76,D,1.17 0.90 amount of the exposed iron.CL 15.1 26 0.75,D,1.17 0.90 The oxygen binding energy data confirm the conclusionsGraphite 2.5 23 drawn from the Fe data. Here it is important to note thatHOPG ` 16 the binding energy for oxygen in a metal oxide would

K.L. Strong et al. / Carbon 41 (2003) 1477–1488 1485

Fig. 9. Examples of X-ray photoelectron spectroscopy data for SWNTs at different elemental ranges: (a) carbon, (b) iron.

T able 4 occur at approximately 530 eV, while that of oxygen boundX-ray photoelectron spectroscopy results for the carbon nanotubes to carbon in an organic compound (e.g., –C=O or –C—

O—H) is about 2 eV higher[31]. In the HT sample, beforeSample XPS peak positions (eV)sputtering, all the oxygen exists as the iron oxide. After

Exp. C Corr. Fe Corr. O sputtering, about 50% of the oxygen occurs in the ironAR Surface 281.8 706.9 – oxide, and the remaining amount is in an organically

Sputtered 280.9 707.4 bound oxygen such as that found in surface contamination.Such contamination would not be expected to be complete-HT Surface 282.0 710.8 530.5ly removed by sputtering, as nanotube surfaces are presentSputtered 281.8 707.2 531.2throughout a sample of the material. The oxygen might708.9 529.4also be bonded to the surface of nanotubes; it would be

EX Surface 277.3 708.7 529.9difficult to differentiate these two types of oxygen. OxygenSputtered 279.3 707.3 532.0on the surface in the EX sample appears to be in the iron708.8oxide, whereas after sputtering, it exists as an organically

CL surface 282.1 – 532.5 bound oxygen. This would confirm that extraction alonesputtered 282.2 533.2 begins to oxidize exposed iron. Both before and after

1486 K.L. Strong et al. / Carbon 41 (2003) 1477–1488

sputtering of the CL sample, all the oxygen is organically screening tools for the presence of metallic catalysts withbound. There is no evidence of the presence of Fe O in experience. This is an advantage, as both TGA and XPS2 3

the CL sample. These results are consistent with the TGA are quicker and require less sample preparation than ICP–results where the iron is observed to be in the metallic state MS.for the AR and EX samples, and mostly oxidized in the HTsample. 3 .7. X-ray diffraction

Chemical composition as determined by XPS for thefour samples is compared to the ICP–MS data inTable 1. Transmission XRD (Fig. 10) was able to identify theThese composition data were derived from sputtered intertube spacings in the SWNT ropes, near 1.1 nm. Whilesamples to reduce the possibility of including oxygen and this is most visible in the AR and CL samples, it can becarbon from surface contamination. The data clearly show seen in the others as well. This value is within the range ofthat the as-received SWNTs have a significant amount of values obtained by Raman spectroscopy. There is noFe at 3.9 at%. Oxidative heat treatment oxidizes some of graphitic (002) diffraction indicating there is very little orthe C, causing it to be lost, thus increasing the relative no MWNTs in these samples, also confirmed with Ramanamount of Fe. The amount of O present in the sample also spectroscopy and TEM. Some (10) diffraction is visibleincreases, indicating that oxygen has reacted with the between 40 and 508 2u but is not very distinct; this issample to produce iron oxide and possibly also pendant consistent with the small persistence lengths expected ingroups on the nanotubes themselves, consistent with the the curved and flexible graphene tubes of SWNTs. There isXPS peak position data. Extraction alone does not appear some amorphous scatter near 208 2u which may be fromto change the amount of Fe present in the sample, and amorphous carbon in the sample or from the samplecauses some oxidation of the sample. However, complete holder.cleaning lowers the amount of Fe below the XPS detection The XRD of SWNTs after air oxidation, which burns offlimit of approximately 1 at%, and apparently retains some most of the carbon, reveals the rust red residue to beoxygen groups on the surface of the nanotubes, as de- Fe O , most likely hematite. An example of this diffrac-2 3

termined from the XPS peak position data. tion is shown inFig. 11. This is critical information inWhen comparing the compositions of the samples as order to use TGA as a tool for estimating the amount of

determined by XPS and ICP–MS, it is clear that XPS metal present in a sample. The iron phases in theunderestimates the amount of Fe present in these samples, nanotubes lack coherent lengths large enough to produceespecially if sputtering is not performed to expose Fe in recognizable diffraction.shell carbon. The XPS data should also be compared to theTGA composition data shown inTable 2. Again, XPSappears to underestimate the amount of iron, but consider- 4 . Conclusionsing the inherent inaccuracies of both estimation methods,the results are remarkably similar. However, both TGA We have demonstrated an easy method of cleaningand XPS appear to be sensitive enough to iron to be used SWNTs and evaluating their purity, and have elucidatedto determine the relative effectiveness of a purification the chemical mechanism of this purification method. First,treatment. TGA and XPS may therefore be used as as the sample is heated in moist air, much of the fullerenes

Fig. 10. Transmission XRD of the carbon nanotubes. The 1.1-nm inter-tube spacing in the nanotube ropes is shown with an arrow and ismost visible in the as-received and clean CNT. Note that for clarity the AR tube scan has been vertically displaced from overlaying the HTscan.

K.L. Strong et al. / Carbon 41 (2003) 1477–1488 1487

Fig. 11. Example of XRD of nanotube residue with hematite, Fe O , diffraction shown.2 3

and disordered carbon are oxidized, leaving primarily iron technique such as ICP–MS, which does not rely on acidoxide, largely in cracked-open carbon shells, SWNTs, and digestion of the sample, is needed. XPS can assess thea small amount of iron protected from oxidation by bonding state of the carbon and catalysts if more detail issurrounding shell carbon. Upon continuous extraction in needed about the state of purification. TEM is necessary iffreshly distilled HCl for at least 24 h, the catalyst is almost images of the carbons and catalysts are desired. These lastentirely removed, leaving approximately 0.2 at% Fe. The several techniques are significantly more difficult to con-remaining cleaned material (the CL sample of this study) duct and interpret than the first few, and would probablyis largely SWNTs with some remaining shell carbon. not serve well as routine screening tools. However, they

The cleaning method employed is simple, inexpensive, were very helpful in determining the mechanism of theand fairly effective. It was demonstrated on HiPco� purification procedure used in this study. It is hoped thatSWNTs, but is thought to be applicable to carbon this demonstration of an easy method of cleaning SWNTsnanotubes made by any manufacturing process. The reader and evaluating their purity will aid future researchers asis cautioned that to obtain optimum results, the processing well as those seeking to employ SWNTs in commercialconditions may have to be modified for different carbon applications.nanomaterials. The efficiency of any purification methoddescribed in this paper or in published articles is dependentstrongly on the raw materials. If the material precursor is A cknowledgementsrich in catalysts covered in thick carbon shells, 100% cleanmaterial is unlikely to occur without compromising the The authors would like to thank Dr. Robert H. Hauge ofSWNT yield. Rice University for supplying the single-wall nanotubes for

It was also confirmed that TGA and Raman spec- this study. We would also like to thank Mr. Michael Jubaratroscopy, both of which take only a few minutes and of Chemsys for the ICP–MS analysis, and Drs. Fred E.require little sample preparation, are sufficient as quali- Arnold and Thuy Dang of AFRL/MLBP for first sug-tative screening tools to determine the relative purity of gesting the use of the Soxhlet extractor. This research wasSWNTs. As reported by previous researchers[11–13], funded by the United States Air Force Research Labora-Raman spectroscopy can detect if SWNTs are present andtory at Wright-Patterson Air Force Base, OH, partiallyto estimate their diameters. This study has shown that the through contracts F33615-00-D-5006 and F33615-97-2-G:D ratio and G peak width are indicators which can be 5840.used to rank the purity of SWNT samples. TGA gives agood measure of the amount of metal catalyst in thematerials (providing the type of metal oxide formed is R eferencesverified by some other means). It is thought that theseanalytical methods are the leading candidates for quality

[1] I ijima S. Helical microtubules of graphitic carbon. Natureassurance tools in industrial production of SWNTs. XRD 1991;354(6348):56–8.can be used to determine the inter-tube spacing in ropes [2] S alvetat JP, Bond JM, Thomson NH, Kulik AJ, Forro L,(an estimate of tube diameter) and to evaluate oxidized Benoit W, Zuppiroli L. Mechanical properties of carbonresidue of tubes to identify the metal catalysts if needed. nanotubes. Appl Phys A, Mater Sci Proc 1999;69(3):255–60.

For a quantitative measure of purity, a mass analysis [3] D illon AC, Gennet T, Jones KM, Alleman JL, Parilla PA,

1488 K.L. Strong et al. / Carbon 41 (2003) 1477–1488

Heben MJ. A simple and complete purification of single- [18] D illon RO, Woollam JA, Katkanant V. Use of Ramanwalled carbon nanotube materials. Adv Mater scattering to investigate disorder and crystallite formation in1999;11(16):1354–7. as-deposited and annealed carbon films. Phys Rev B

[4] D uesberg GS, Muster J, Byrne HJ, Roth S, Burghard M. 1984;29(9):3482–9.Towards processing of carbon nanotubes for technical appli- [19] E lkund PC, Holden JM, Jishi RA. Vibrational modes ofcations. Appl Phys A, Mater Sci Proc 1999;69(3):269–74. carbon nanotubes; spectroscopy and theory. Carbon

[5] K ataura H, Kumazawa Y, Maniwa Y, Umezu I, Suzuki S, 1995;33(7):959–72.Ohtsuka Y, Achiba Y. Optical properties of single-wall [20] K urti J, Kuzmany H, Burger B, Hulman M, Winter J, Kressecarbon nanotubes. Synthetic Metals 1999;103(1-3):2555–8. G. Resonance Raman investigation of single wall carbon

[6] C hiang IW, Brinson BE, Huang AY, Willis PA, Bronikowski nanotubes. Synthetic Metals 1999;103(1-3):2508–9.MJ, Margrave JL, Smalley RE, Hauge RH. Purification and [21] A lvarez L, Righi A, Rols S, Anglaret E, Sauvajol JL, Munozcharacterization of single-wall carbon nanotubes (SWNTs) E, Maser WK, Benito AM, Martinez MT, de la Fuento GF.obtained from the gas-phase decomposition of CO (HiPco Diameter dependence of Raman intensities for single-wallprocess). J Phys Chem B 2001;105(35):8297–301. carbon nanotubes. Phys Rev B 2001;63(15):1534011–4.

[7] C hiang IW, Brinson BE, Smalley RE, Margrave JL, Hauge [22] R ao AM, Chen J, Richter E, Schlecht U, Eklund PC, HaddonRH. Purification and characterization of single-wall carbon RC, Venkateswaren UD, Kwon YK, Tomanek D. Effect ofnanotubes. J Phys Chem B 2001;105(6):1157–61. van der Walls interactions on the Raman modes in single

[8] R inzler AG, Liu J, Dai H, Nikolaev P, Huffman CB, walled carbon nanotubes. Phys Rev Lett 2001;86(17):3895–Rodriguez-Macias FJ, Boul PJ, Lu AH, Heymann D, Colbert 8.DT, Lee RS, Fischer JE, Rao AM, Eklund PC, Smalley RE. [23] J orio A, Saito R, Hafner JH, Lieber CM, Hunter M, McClureLarge scale purification of single wall carbon nanotubes: T, Dresselhaus G, Dresselhaus MS. Structural (n, m) de-process, product and characterization. Appl Phys A termination of isolated single-wall carbon nanotubes by1998;67(1):29–37. resonant Raman scattering. Phys Rev Lett 2001;86(6):1118–

[9] G orelik OP, Nikolaev P, Arepalli S. Purification procedures 21.for single-wall carbon nanotubes. NASA Technical Report [24] R ao AM, Bandow S, Richter E, Eklund PC. Raman spec-NASA/CR-2000-208926, May 2001. troscopy of pristine and doped single wall carbon nanotubes.

[10] B andow S, Asaka S, Saito Y, Rao AM, Grigorian L, Richter Thin Solids Films 1998;331(1-2):141–7.E, Eklund PC. Effect of the growth temperature on the [25] N ikolaev P, Bronikowski MJ, Bradley RK, Rohmund F,diameter distribution chirality of single-wall carbon Colbert DT, Smith KA, Smalley RE. Gas-phase catalyticnanotubes. Phys Rev Lett 1998;80(17):3779–82. growth of single-walled carbon nanotubes from carbon

[11] R ao AM, Richter E, Bandow S, Chase B, Eklund PC, dioxide. Chem Phys Lett 1999;313(1-2):91–7.Williams KA, Fang S, Subbaswamy KR, Menon M, Thess [26] P ark YS, Choi YC, Kim KS, Chung DC, Bae DJ, An KH,A, Smalley RE, Dresselhaus G, Dresselhaus MS. Diameter- Lim SC, Zhu XY, Lee YH. High yield purification ofselective Raman scattering from vibrational modes in carbon multiwalled carbon nanotubes by selective oxidation duringnanotubes. Science 1997;275(5297):187–90. thermal annealing. Carbon 2001;39(5):655–61.

[12] K urti J, Kresse G, Kuzmany H. First-principles calculations [27] M oon JM, An KH, Lee YH, Park YS, Bae DJ, Park GS.of the radial breathing mode of single-wall carbon nanotubes. High-yield purification process of singlewalled carbonPhys Rev B 1998;58(14):R8869–72. nanotubes. J Phys Chem B 2001;105(24):5677–81.

[13] S aito R, Takeya T, Kimura T, Dresselhaus G, Dresselhaus [28] O berlin A. In: Thrower PA, editor, Chemistry and physics ofMS. Raman intensity of single-wall carbon nanotubes. Phys carbon, vol. 22, New York: Marcel Dekker, 1989, pp. 1–141.Rev B 1998;57(7):4145–53. [29] H eidenreich RD, Hess WM, Ban LL. A test object and

[14] R ao AM, Jorio A, Pimenta MA, Dantas MSS, Saito R, criteria for high resolution electron microscopy. J ApplDresselhaus G, Dresselhaus MS. Polarized Raman study of Crystallogr 1968;1(1):1–19.aligned multiwalled carbon nanotubes. Phys Rev Lett [30] S uzuki S, Bower C, Kiyokura T, Nath KG, Watanabe Y,2000;84(8):1820–3. Zhou O. Photoemission spectroscopy of single-walled carbon

[15] K asuya A, Sugano M, Maeda T, Saito Y, Tohji K, Takahashi nanotube bundles. J Electron Spectrosc Relat PhenomenaH, Sasaki Y, Fukushima M, Nishina Y, Horie C. Resonant 2001;114–116:225–8.Raman scattering and the zone-folded electronic structure in [31] M oulder JF, Stickle WF, Sobol PE, Bomben KD. In:single-wall nanotubes. Phys Rev B 1998;57(9):4999–5001. Chastian J, King Jr. RC, editors, Handbook of X-ray

[16] T uinstra F, Koenig JL. Raman spectrum of graphite. J Chem photoelectron spectroscopy, Eden Prairie, MN: PhysicalPhys 1970;53(3):1126–30. Electronics, Inc, 1998, pp. 44–5, See also pp. 80–81.

[17] L aspade P, Marchand A, Couzi M, Cruege F. Characteriza-tion of carbon matrixes with Raman microspectroscopy.Carbon 1984;22(4/5):375–85.