www.elsevier.com/locate/carbon

Carbon 45 (2007) 907–912

Quantitative assessment of carbon nanotube dispersionsby Raman spectroscopy

Christoph G. Salzmann *, Bryan T.T. Chu, Gerard Tobias, Simon A. Llewellyn,Malcolm L.H. Green

Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, OX1 3QR Oxford, United Kingdom

Received 1 August 2006; accepted 18 January 2007Available online 26 January 2007

Abstract

Aqueous dispersions of single wall carbon nanotubes (C-SWNTs), prepared using different dispersing agents, have been analysed byRaman spectroscopy. Normalising the spectra with respect to the area of the water O–H stretching transition eliminates the effects ofphoton scattering and absorption on the way through the dispersion, and the dispersions can be assessed quantitatively by comparisonof the areas of the carbon nanotube G-band. The normalised G-band areas show linear concentration dependence according to Beer’slaw. The influences of different dispersing agents and excitation wavelengths are discussed and the results are compared to the commonlyused UV–Visible spectroscopic analysis. The method presented here is semi-quantitative and it is proposed to use the most effective dis-persing agent found in this study, sodium dodecylbenzene sulfonate (SDBS), as a benchmark for future dispersion experiments.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Single wall carbon nanotubes (C-SWNTs) have emergedas a material with highly remarkable electronic, thermal,optical, mechanical, spectroscopic and chemical properties[1–8]. However, a general problem associated with theirapplication is their inherent insolubility in most solvents.The dispersion of carbon nanotubes in different solventsis therefore an important step in, for example, their chem-ical functionalisation, preparation of composites, and alsotheir characterisation [4,6–10]. Much research has focusedon achieving the highest possible degree of dispersion with-out altering the properties of the C-SWNT themselves. Forthe dispersion of C-SWNTs in aqueous media, two strate-gies have been developed. The first involves a chemicalfunctionalisation of the C-SWNT by either creating hydro-philic functional groups on the sidewalls of the C-SWNTsor by covalently binding hydrophilic molecules onto the

0008-6223/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2007.01.009

* Corresponding author. Tel.: +44 1865 272641; fax: +44 1865 272690.E-mail address: christoph.salzmann@chem.ox.ac.uk (C.G. Salzmann).

surface of the C-SWNTs [11–15]. The second approachdoes not involve chemical modification of the C-SWNTs.Instead, C-SWNTs are dispersed by using surfactants,wrapping of hydrophilic (bio)polymers, or charged nano-particles [6,10,16–25].

C-SWNT dispersions are commonly quantified or char-acterised by UV–Visible absorption spectroscopy, nearinfrared (NIR) absorption spectroscopy, fluorescence spec-troscopy, weighing after solvent removal, or just by visualinspection of the darkness of the dispersions: In UV–Visabsorption spectroscopy, the absorption value at 500 nmis often chosen to quantify different C-SWNTs dispersions[21,24,25]. However, this suffers in general from the diffi-culty of separating contributions from C-SWNTs andother species (e.g. carbonaceous impurities or dispersingagents). Also, ultrasonication time was found to increaseUV–Vis absorption at a constant C-SWNT concentration[26]. In the NIR spectral range, the electronic interbandtransitions of the C-SWNTs are observed [10,27]. Espe-cially the intensity of the S22 interband transition near1000 nm has been used for quantification of C-SWNT dis-persions [13] and also for purity evaluation of C-SWNTs in

908 C.G. Salzmann et al. / Carbon 45 (2007) 907–912

dispersion [28]. However, it was also shown that the inten-sity of the S22 peak increases with increasing surfactantconcentration. Also, C-SWNT debundling leads to blue-shifting of the peak position and changes in the fine struc-ture of the peak [29]. Band gap fluorescence measured fromC-SWNT dispersions is a powerful tool for the determina-tion of the chirality/diameter distribution present in a sam-ple [6,10]. However, due to the quenching effect of metallicC-SWNTs in a bundle, it is limited to the analysis of indi-vidually dispersed C-SWNTs [10]. Also the peak positionof the fluorescence peaks is greatly affected by the surfac-tant used [18].

Here, we present a method for quantifying carbon nano-tube dispersions by Raman spectroscopy. In this regard,the Raman spectra are normalised with respect to the areaof a solvent peak, for example the m(O–H) stretching tran-sition of water, and analysed by the resulting G-band areas.The effects of scattering and absorption of photons on theirway through the dispersion as well as changes in theexperimental parameters are thereby eliminated. Thenormalised G-band areas show linear concentration behav-iour in accordance with Beer’s law. Due to the resonanceenhanced Raman intensity of C-SWNTs [5,30], thismethod probes exclusively the C-SWNTs present in the dis-persion, whereas other carbonaceous species such as amor-phous carbon or graphite particles are neglected.

Rec

orde

d in

tens

ity

(3)

(2)

(1)

νO-H G'

G

D

2. Experimental methods

2.1. Preparation of the dispersions

Carbon nanotubes were supplied by Thomas Swan and Co. Ltd. C-SWNTs in a ‘‘wet cake’’ form (ca. 95% water by weight). According toTEM analysis, the C-SWNTs have an average diameter of 1.7 ± 0.3 nm.The water was removed at 60 �C before use. Based on literature proce-dures [10,18,20,22], different carbon nanotube dispersions were prepared:10 mg of C-SWNTs were combined with 10 mL of a 1 wt% SDBS solution(sodium dodecylbenzene sulfonate – Aldrich), a 1 wt% SDS solution(sodium dodecyl sulfate – Aldrich), a 0.1 wt% DNA solution (singlestranded Salmon DNA – Nippon Chemical Feed Co. Ltd.), a suspensionof ZrO2 nanoparticles (5 mL of 20 wt% ZrO2 nanoparticles (size 5–10 nm,NYACOL Nanotechnology) +5 mL water), and just water. The C-SWNTs were then dispersed by ultrasonic agitation in a bath sonicator(110 W) for 15 min. Additionally, a dispersion of 10 mg amorphous car-bon (carbon nanopowder 99+%, 30 nm average particle size, Aldrich) ina 10 mL solution of 1 wt% SDBS was prepared. All dispersions were cen-trifuged at 4000 r.p.m. for 30 min and the supernatant was used for furtheranalysis. A set of dispersions with concentrations (c/c0) of 1.00, 0.75, 0.50,0.25, and 0.10 was prepared by diluting the original SDBS dispersion with1 wt% SDBS solution. These dispersions were ultrasonicated for 15 minbefore analysis.

4000 3500 3000 2500 2000 1500 1000Raman shift / cm-1

(5)

(4)

Fig. 1. Raman spectra (k0 = 632 nm) recorded from a series of C-SWNTdispersions in SDBS solution obtained upon diluting and ultrasonicating.The C-SWNT concentrations (c/c0) are 1.00, 0.75, 0.5, 0.25, and 0.10 forspectra (1)–(5). Peaks arising from the C-SWNTs (D, G, and G 0 bands)and water (m(O–H)) are marked.

2.2. Characterisation of the dispersions

For recording Raman spectra, dispersions were pipetted into 2 mmdiameter quartz capillaries with 0.01 mm wall thickness (Hilgenberg com-pany). The capillaries were sealed to prevent the evaporation of water. AllRaman spectra were recorded on a Jobin Yvon spectrometer (Labram 1B)equipped with a microscope, through a 10 fold magnification objective(Olympus company), by coadding four spectra with collection times of40 s each. The distance between the wall of the capillary and the laser focuswas �200 lm for all measurements. A 20 mW He–Ne laser (632 nm) and a

40 mW argon-ion laser (514 nm) were used. The 1800 L/mm grating pro-vides a resolution starting from 1.0 at 200 cm�1 up to 0.5 cm�1 at3600 cm�1 for the He–Ne laser and from 1.5 cm�1 at 200 cm�1 up to1.0 cm�1 at 3600 cm�1 for the argon-ion laser. The abscissa was calibratedwith the 520.7 cm�1 peak of a silicon standard, and the sharp Raman shiftsare accurate within the limits of the resolution.

UV–Visible absorbance spectra of the dispersions were recorded inquartz cuvettes (light path length of 5 mm) on a GBC spectrometer (Cin-tra 10) in the range from 200 to 1000 nm with a scan rate of 500 nm min�1

and a resolution of 0.427 nm.

3. Results and discussion

For the quantitative analysis of C-SWNTs in disper-sions, a linear relationship between the concentration ofthe C-SWNTs and the measured quantity has to be estab-lished. Fig. 1 shows the Raman spectra of C-SWNT disper-sions in SDBS solutions with relative concentrations (c/c0)of 1.00, 0.75, 0.50, 0.25, and 0.10.

The C-SWNT peaks are found at 2625 cm�1, 1590 cm�1

and 1324 cm�1. The most intense feature is the G-band at1590 cm�1 which is associated with several tangential C–Cstretching transitions of the C-SWNT carbon atoms [5,30].The broad peak at �3300 cm�1 arises from the O–Hstretching transitions (m(O–H)) of water [31]. Peak intensi-ties in Raman spectroscopy are proportional to the concen-tration of a species but are also influenced by experimentalparameters including, for example, laser output power andfrequency, laser alignment conditions, and the focus condi-tions [32]. To eliminate the influences of experimentalparameters, the dispersions were measured on the sameday and the distance between the inner wall of the glassand the focus was set to �200 lm for all measurements.

The peak areas of the m(O–H) bands and the G-bandsfor spectra (1)–(5) in Fig. 1 are shown as a function ofthe C-SWNT concentration (c/c0) in Fig. 2.

0.0 0.2 0.4 0.6 0.8 1.00

1x104

2x104

3x104

4x104

5x104

6x104

G-band

ν(O

-H)

area

/ cm

-1

G-b

and

area

/ cm

-1

c / c0

0

1x105

2x105

3x105

4x105

5x105

6x105

7x105

8x105

νO-H

Fig. 2. G-band and m(O–H) peak areas (k0 = 632 nm) obtained upondiluting the SDBS dispersion with starting concentration c0 and ultra-sonication. Dashed lines are guides to the eye. Arrows indicate theexpected concentration dependencies.

0.0 0.2 0.4 0.6 0.8 1.00.00

0.05

0.10

0.15

0.20

0.25

0.30

Nor

mal

ised

G-b

and

area

c / c0

Fig. 3. Concentration dependence of the normalised G-band area. Thelinear fit (dashed line) is forced through zero.

C.G. Salzmann et al. / Carbon 45 (2007) 907–912 909

The H–O–H bending transition (�1640 cm�1) overlapsslightly with the carbon nanotube G-band on the higher fre-quency side of the G-band. The integration of this peak wastherefore carefully avoided. With increasing C-SWNT con-centration, a significant decrease of the m(O–H) peak area isobserved. This behaviour is unexpected in the sense that theconcentration of water, as the solvent, would be expected tobe constant upon diluting the original C-SWNT dispersionwith SDBS solution. The expected behaviour is thereforeindicated by the horizontal arrow in Fig. 2. For the areaof the G-band, a linear trend is observed with increasingC-SWNT concentration at low concentrations. However,at higher C-SWNT concentrations, deviation from this lin-ear behaviour is observed and the increase of G-band areawith concentration decreases significantly. The expected lin-ear behaviour according to Beer’s law is indicated by anarrow at low concentrations. This nonlinearity representsa major problem in the quantification of C-SWNT disper-sions and was also observed by Itkis et al. in their studyon purity evaluation of C-SWNTs (cf. Fig. 10 in Ref.[28]). They concluded that this behaviour originates fromphotons undergoing ‘‘secondary scattering or absorptionon their way out of the dispersion to the detector, thusreducing the strength of the Raman signal’’. Scattering ofphotons on their way through the dispersion seems indeedlikely as the wavelength of the Raman lasers have similarorders of magnitude as the C-SWNT agglomerate sizes.Upon diluting and ultrasonicating the C-SWNT dispersion,a decrease in C-SWNT bundle size is expected [26] andtherefore secondary photon scattering should become lesslikely. This is in accordance with the larger G-band areaincreases with concentration at lower concentrations. ForC-SWNT dispersions, nonlinear transmittance behaviourwas observed at high laser powers which is thought to orig-inate from the formation of microplasmas (cf. Fig. 4 in Ref.[33]).

It can be assumed that photons arising from the G-bandand m(O–H) Raman scattering processes experience very

similar amounts of secondary scattering and absorptionon their way through the dispersion. Also, as explainedabove, the concentration of water is expected to remainunchanged upon dilution. The Raman spectra are thereforenormalised in a next step with respect to the m(O–H) areas sothat the m(O–H) areas equal unity. Fig. 3 shows a plot of theG-band areas, obtained by integration from the normalizedspectra, as a function of the C-SWNT concentration.

The normalised G-band areas show linear concentrationbehaviour in accordance with Beer’s law. It is thereforepossible to use the normalised G-band areas for quantita-tive analysis of C-SWNT dispersions. By using this kindof normalisation, it is furthermore possible to eliminatethe effects of changing experimental parameters, since thesample and reference peaks are expected to be affectedequally.

We next use this quantification method to assess C-SWNT dispersions using different dispersing agents in water[10,18,20,22]. Fig. 4a shows the normalised Raman spectra(He–Ne laser) of dispersions using (1) SDBS, (2) SDS, (3)DNA, (4) ZrO2 nanoparticles, (5) just water, and (6) a dis-persion of amorphous carbon in SDBS solution.

The areas of the carbon nanotube G-bands at 1590 cm�1

were determined next by integration from the normalisedspectra: (1) 0.288, (2) 0.156, (3) 0.126, (4) 0.059, (5) 0,and (6) 0. These quantities can now be used for directquantitative comparison of the different C-SWNT disper-sion. It is possible for example to conclude that SDBS isalmost twice as effective as SDS in dispersing C-SWNTs.SDS and DNA show similar dispersing effects whereasthe dispersion prepared using ZrO2 nanoparticles is com-paratively poor. The method presented here is semi-quanti-tative which means that the results are scored on anarbitrary scale. For tests with potential dispersing agents,we propose to use SDBS, the most effective dispersingagent found here, as a benchmark.

The normalised G-band area of the C-SWNT dispersionin pure water (5) is, as expected, zero as the C-SWNTs floc-culate immediately after ultrasonic agitation. The disper-sion of amorphous carbon does not give a detectable

*

G' D

0.0041

0.0110

0.0139

A(G-band) = 0.0220

A(νO-H

) = 1

(6)

(5)

(4)

(3)

(2)

b λ0 = 514 nm

a λ0 = 632 nm

(1)

G

4000 3500 3000 2500 2000 1500 1000

˚

˚

˚

*

*

*

Nor

mal

ised

inte

nsity

Raman shift / cm-1

(6)

(5)

(4)

(3)

(2)

(1)

0.059

0.126

0.156

A(G-band) = 0.288

A(νO-H

) = 1

˚

Fig. 4. Raman spectra of C-SWNT dispersions using (a) a He–Ne laser(632 nm) and (b) an argon-ion laser (514 nm). The dispersions wereprepared using (1) SDBS, (2) SDS, (3) DNA, (4) ZrO2 nanoparticles, (5)and just water as dispersing agents. Spectrum (6) was recorded from adispersion of amorphous carbon in SDBS. Spectra were normalised bydividing by the areas of the O–H stretching transitions. The areas of G-bands are given in (a) and (b). The peaks arising from C-SWNTs (D, G,and G 0 bands) are indicated in (a). Asterisks in (a) and (b) mark the peakpositions of acetic acid used for stabilisation of the ZrO2 nanoparticles.Peaks marked by circles could arise from a small fraction of fluorescingnanoparticles.

910 C.G. Salzmann et al. / Carbon 45 (2007) 907–912

G-band intensity in spectrum (6) which demonstrates thepoor Raman scattering properties of amorphous carboncompared to C-SWNTs. In general, the Raman spectraof C-SWNT samples are dominated by the spectral featuresof the C-SWNTs and not by other carbonaceous impuritiessuch as amorphous carbon or graphitic particles [5]. Forthe quantification method presented here, this is highlyadvantageous as it allows for the quantification of C-SWNTs alongside other carbonaceous impurities presentin the C-SWNT samples.

The high Raman intensity of the C-SWNTs arises froma resonance Raman scattering process [30,34]. Dependingon the wavelength of the laser, C-SWNTs of different diam-eters and chiralities fulfil the resonance conditions and con-tribute hence to the spectrum. Therefore, by using a specificwavelength, only subsets of the carbon nanotube species

present in dispersion are probed. In order to assess theselectivity of a dispersing agent towards dispersing C-SWNTs of certain diameters or chiralities, the Raman spec-tra of the dispersions were recorded using a different laserwavelength. Fig. 4b shows the spectra of the different dis-persions using an argon-ion laser (514 nm). The spectrawere again normalised with respect to the area of the O–H stretching transition, as described above. The areas ofG-bands in the normalised spectra are: (1) 0.0220, (2)0.0139, (3) 0.0110, (4) 0.0041, (5) 0, and (6) 0. All G-bandareas in Fig. 4b are smaller when compared with the spec-tra in Fig. 4a. The reasons for this could be i.a. that the res-onance conditions are met less accurately using the argon-ion laser (514 nm), that the concentration of C-SWNTs inresonance is lower, or that the Raman scattering intensitiesof the C-SWNTs in resonance are weaker. However, thesequence of the normalised G-band areas is the same asfound for the He–Ne laser in Fig. 1a. A plot of the norma-lised G-band areas, obtained using the He–Ne laser,against the values found for the argon-ion laser shows alinear trend with a slope of 12.6 ± 0.8 (R = 0.993) (notshown). This implies that the C-SWNTs in resonance at632 nm or 514 nm are dispersed very similarly by the differ-ent dispersing agents. It should be emphasised that theslope of 12.6 ± 0.8 is expected to change if different carbonnanotube samples with different diameter distributionswere used.

Spectra (1), (2), (4), and (5) in Fig. 4b show broad peakscentred at �2200 cm�1. These features possibly arise froma small fraction of fluorescing nanoparticles present in thedispersions. Such particles could be isolated from a C-SWNT sample by electrophoresis [35] and from the super-natant of a SDS dispersion [36]. Also, very recently, wehave isolated fluorescing carbonaceous fragments in highconcentrations from a C-SWNT sample after nitric acidtreatment [37] and we have shown that fluorescence ofthese fragments is quenched by adsorption onto the C-SWNTs. The fluorescence detected here could thereforeoriginate from similar fragments in the dispersion afterdesorption from the C-SWNTs. The He–Ne laser(632 nm) is most probably too low in energy to excite fluo-rescence as these broad features are not observed in Fig. 4a.

For quantitative analysis presented here it is necessarythat the Raman spectroscopic properties of the G-bandare not significantly altered by using different dispersingagents. This was previously stated in Refs. [18,38] and wealso find that the peak positions and relative intensities ofthe G, D, and G 0-bands remain unchanged in spectra (1–4) in Fig. 4a or b. We have shown furthermore that, despiteprogressive debundling of the C-SWNTs (cf. above), a lin-ear behaviour between normalised G-band area and C-SWNT concentration is found. To investigate further theinfluence of a surfactant on the Raman spectroscopic prop-erties of the G-band, a SDS dispersion with c/c0 = 1 wasdiluted with the same volume of a 1 wt% SDBS solutionand ultrasonicated for 5 min. This is thought to producean at least statistic mixture of SDS and SDBS surfactant

400 600 800 1000

0

1

2

3

(5)

(6)

(4)

(3)

(2)

Abs

orba

nce

Wavelength / nm

(1)

Fig. 5. UV–Visible absorption spectra (upper) and photographs (lower) ofC-SWNT dispersion using (1) SDBS, (2) SDS, (3) DNA, (4) ZrO2

nanoparticles, (5) just water, and (6) a dispersion of amorphous carbon inSDBS solution.

C.G. Salzmann et al. / Carbon 45 (2007) 907–912 911

molecules in the C-SWNT micelle. Due to the p-stackingcapability of SDBS, it can be argued that SDBS is probablyenriched at the surfaces of the C-SWNTs. The C-SWNTconcentration after this procedure is c/c0 = 0.5. The nor-malised G-band area was found to halve as a consequenceof this experiment. This shows that the Raman scatteringproperties of the C-SWNT G-band are not significantlyaffected by the substitution of SDS with SDBS.

Finally, we compare the results obtained here by Ramanspectroscopy with that obtained by UV–Visible absorptionspectroscopy which is the method most commonly used toanalyse carbon nanotube dispersions [21,24,25]. Fig. 5shows the UV–Visible absorption spectra of the differentdispersions.

The absorptions at 500 nm are 1.152, 0.974, 0.659,0.307, 0, and 3.19 for the dispersions using (1) SDBS, (2)SDS, (3) DNA, (4) ZrO2 nanoparticles, (5) just water,and (6) a dispersion of amorphous carbon in SDBS. Theabsorption values for the carbon nanotube samples (1–5)follow the same trend as the normalised G-band areasfound in Fig. 4a and b. However, the values found forthe dispersion of amorphous carbon in a solution of SDBS(6) are very different. In UV–Visible absorption, this dis-persion gives an absorbance of �3.19 at 500 nm which isthe highest absorbance value of all samples. Also, visually,this sample is one of the darkest dispersions (cf. Fig. 5(bot-tom)). With Raman spectroscopy on the other hand, amor-phous carbon did not contribute to the spectrum. This

illustrates the advantage of the quantitative measurementspresented here using Raman spectroscopy as varyingamounts of dispersed amorphous carbon could obscurethe results obtained by UV–Visible spectroscopy.

Acknowledgments

The authors thank Thomas Swan Co. Ltd. for supplyingthe C-SWNT samples used in this study. We acknowledgethe financial support provided through the AustrianScience Funds (FWF; project J2446) (C.G.S.) and the 6thEuropean Community Framework Programme (MarieCurie Intra-European Fellowship; MEIF-CT-2006-024542) (G.T.).

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