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Comparative study on modification of single wall carbon nanotubes by sodium dodecylbenzene sulfonate and melamine sulfonate superplasticiser
Applied Surface Science 255 (2009) 6359–6366
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Comparative study on modification of single wall carbon nanotubes by sodium dodecylbenzene sulfonate and melamine sulfonate superplasticiser Z. Markovic´ a, S. Jovanovic´ a, D. Kleut a, N. Romcˇevic´ b, V. Jokanovic´ a, V. Trajkovic´ c, B. Todorovic´-Markovic´ a,* a b c
Vincˇa Institute of Nuclear Sciences, P.O.B. 522, 11001 Belgrade, Serbia Institute of Physics, P.O.B. 68, 11001 Belgrade, Serbia Institute of Immunology and Microbiology, School of Medicine, University of Belgrade, Dr. Subotic´a 1, Belgrade, Serbia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 November 2008 Received in revised form 21 January 2009 Accepted 4 February 2009 Available online 14 February 2009
In this work, the results of synthesis and characterization of single wall carbon nanotubes (SWCNTs) functionalized by two surfactants (sodium dodecylbenzene sulfonate and melamine sulfonate superplasticiser) have been presented. The properties of pristine and modified SWCNTs have been compared by different techniques: Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). Raman analysis reveals the changes in vibrational spectra of SWCNTs after modification by different surfactant molecules. FTIR analysis has shown the presence of sulfonate group which is strong evidence for nanotube modification. AFM analysis has shown separation of big single wall carbon nanotube bundles into thin bundles of them. ß 2009 Elsevier B.V. All rights reserved.
PACS: 61.46.Fg 68.37.Ps 78.30.j Keywords: Single wall carbon nanotubes Atomic force microscopy Raman spectroscopy Fourier transform infrared spectroscopy
1. Introduction Single wall carbon nanotubes (SWCNTs) possess unique structure, mechanical, electrical, thermal, optical and chemical properties [1–4]. They could be used as nanocomposites, field emission displays, diodes and transistors, sensors, etc. Carbon nanotubes are considered as potential biomedical materials because of their flexible structure and propensity for chemical functionalization [5]. There are many interesting and promising pharmaceutical applications for carbon nanotubes as carriermediated delivery vehicles for biofunctional molecules, as targets for biophysical treatments and as templates for tissue regeneration [6–8]. They could be used as intracellular transporters of biomolecules as well [9]. But a general problem associated with their application is their inherent insolubility in most solvents [7,10]. A uniform distribution of SWCNTs is hard to achieve due to their mutual van der Waals interaction which leads to agglomeration as well. To successfully disperse carbon nanotubes, the dispersing medium
* Corresponding author. Tel.: +381 11 2455 451; fax: +381 11 3440100. E-mail address: [email protected] (B. Todorovic´-Markovic´). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.02.016
should be capable of both wetting the hydrophobic tube surfaces and modifying the tube surfaces to decrease tube aggregation. Surfactant-assisted dispersion is one of the possible methods used to obtain a stable dispersion [11–13]. Several commercial surfactants such as sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), sodium cholate (SC) were reported to efficiently disperse bundled nanotubes into suspensions of individual nanotubes [14–17]. Moore et al. established that a nonionic surfactant or polymer’s ability to suspend nanotubes appears to be due mostly to the size of the hydrophilic group, with higher molecular weights suspending more nanotube material [15]. Dong et al. indicated that individual SWCNTs suspended in the surfactants, SDS and SDBS, were toxic to 1321N1 human astrocytoma cells due to the toxicity of SDS and SDBS on the nanotube surfaces [18]. Bergin and co-workers observed the exfoliation of nanotubes from the bundles as the concentration of nanotubes is reduced by dilution with surfactant (SDBS) solution [19]. Sun et al. found that dispersion quality is controlled by the magnitude of electrostatic repulsive forces between coated nanotubes [20]. In this work, stable dispersions of single wall carbon nanotubes in distilled water were prepared using two surfactants: SDBS and melamine sulfonate superplasticiser (MSS). This is for the first time
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to report about successfully functionalized single wall carbon nanotubes by MSS. The structure of modified SWCNTs was investigated by atomic force microscope while the degree of modification of single wall nanotubes was investigated by Fourier transform infrared (FTIR) and Raman spectroscopy. Melamine sulfonate superplasticiser is an organic polymer. The long molecules wrap themselves around the nanotubes, giving them a highly negative charge so that they repel each other. Anion groups binding to polymer backbone are fitted in the sidewall defects of SWCNTs. This polymer disperses the nanotubes through a mechanism of electrostatic repulsion. 2. Experimental procedure A stock solutions containing 10 and 28 mg of SDBS and MSS in 100 ml distilled/deionized water was made up, respectively. A dispersion of pristine single wall carbon nanotubes in THF was ultrasonicated for comparison to functionalized ones. An amount of 50 mg of as-prepared SWCNTs (95% purity from BuckyUSA) was added to 100 mL of the stock solutions. The SWCNT solutions were sonicated (ultrasonic bath with power 750 W) for 3 h. After sonication, all samples were centrifuged at 4000 rpm for 1 h. For each sample the well-dispersed supernatant was taken for characterization leaving behind the precipitate. The concentrations of both nanotube dispersions were determined by gravimetric method: 10 ml of nanotube dispersions was dried at 60 8C in air and the mass was measured. The concentration of nanotube/SDBS suspension was 200 mg/L while the concentration of nanotube/MSS suspension was 540 mg/L. Raman spectra of pristine and modified single wall carbon nanotubes were obtained by Micro Raman Chromex 2000 and JY 1000 Raman system using 532 nm of a frequency doubled Nd:YaG laser and 647 nm from Ar ion laser with power of 2 mW, respectively. The spectral resolutions were 4 and 0.2 cm1, respectively. Raman spectra were recorded at room temperature. Samples of dried modified nanotubes were pressed in the shallow hole of indium substrate. For the FTIR spectroscopy analysis, nanotube suspensions were dried at temperature of 60 8C. Dried nanotubes were mixed with KBr powder and pellets were formed. FTIR spectra were measured at room temperature in the spectral range from 400 to 4000 cm1, on a Nicollet 380 FT-IR, Thermo Electron Corporation spectrometer. Atomic force microscopy (AFM) measurements were performed using Quesant microscope operating in tapping mode in air at room temperature. In tapping mode the cantilever oscillates close to resonance and the tip only slightly touches the surface [21]. Mica was used as a substrate. Diluted dispersions of nanotubes were deposited on mica substrate and imaged after drying. Standard silicon tips (purchased from Nano and more) with force constant 40 N/m were used. The accuracy of the AFM mean diameter determination was improved by deconvolution. The freshly cleaved mica has a very small roughness (mean roughness was 0.12 nm) favoring the formation of aggregates which appear during drying of the thin layer of colloid due to the capillary forces. To reduce the aggregation on the substrate dispersions were diluted 40 times. 3. Results and discussion
Fig. 1. RBMs of Raman spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (b) and melamine sulfonate superplasticiser (c). Laser energy was 2.41 eV.
the spectra of pristine and functionalized nanotubes was normalized to yield the same intensity for the G-band. 3.1.1. RBM band analysis Raman active radial breathing mode (RBM) of pristine single wall carbon nanotubes, nanotube/MSS and nanotube/SDBS observed in range from 100 to 300 cm1 is presented in Fig. 1. In this mode, all carbon atoms are moving in-phase in the radial direction. The important point about RBM band is the fact that the energy (wavenumber) of these vibration modes only depends on the diameter of single wall carbon nanotubes. For 532 nm excitation, RBM signals in the regions of 130–150, 150–215 and 230–300 cm1 originate from the van Hove electronic transitions of semiconducting single wall carbon nanotubes (s-SWCNT-E44s, s-SWCNT-E33s) and metallic single wall carbon nanotubes (m-SWCNT-E11m), respectively. As a result, relative content of m- and s-SWCNTs before and after functionalization in this region can be evaluated from the resonant Raman spectra. Ratio of semiconducting to metallic nanotubes after treatment was determined in accordance with procedure developed earlier [24]. The ratio of the number of m-SWCNTs to number of s-SWCNTs in the observed samples, hereafter referred to the M:S ratio can be found by approximating the mean diameter distributions for mand s-SWCNTs. Results of fitting procedure assuming that the ratio of metallic to semiconducting nanotubes is 1:2 are presented in Table 1. Results in Table 1 indicate us that MSS interacts better with metallic nanotubes and mean diameter of functionalized metallic nanotubes is almost identical to pristine single wall carbon nanotubes. It means that only few MSS molecules are necessary for functionalization of metallic nanotubes. For functionalization of sSWCNTs, more MSS molecules are necessary which results in significantly larger mean diameter than that of pristine nanotubes.
Table 1 The enrichment factors of m- and s-SWCNTs and mean diameters of nanotubes in these fractions at Elaser = 2.41 eV.
3.1. Raman spectroscopy Raman spectroscopy is very useful tool for the characterization of single wall carbon nanotubes in diameter range of 2 nm [22]. Raman scattering in SWCNTs is a diameter-selective resonant scattering process [23]. Prior to band analysis, the intensity of all
Pristine SWCNTs Nanotube/MSS Nanotube/SDBS
s-SWCNTs (%)
m-SWCNTs (%)
ds-SWCNTs ðnmÞ
dm-SWCNTs ðnmÞ
67 55.6 61
33 44.4 39
1.65 1.70 1.66
0.98 0.98 0.99
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Fig. 2. Raman spectra of melamine sulfonate superplasticiser (a), pristine (b) and modified single wall carbon nanotubes by melamine sulfonate superplasticiser (c) and SDBS (d). Elaser = 2.41 eV.
SDBS interacts better with semiconducting single wall carbon nanotubes. Mean diameter of s-SWCNTs is only slightly larger than that of pristine s-SWCNTs. Also, it can be noticed that more SDBS molecules are required for functionalizing metallic nanotubes which results in diameter increase. 3.1.2. G- and D-band analysis In Fig. 2, Raman spectra of G- and D-band of melamine sulfonate superplasticiser, pristine SWCNTs, and modified nanotubes by melamine sulfonate superplasticiser (nanotube/MSS) and SDBS (nanotube/SDBS) are presented. The laser energy was 2.41 eV. As can be seen from Fig. 2, in the 1230–1750 cm1 region, two Raman bands are observed: a relatively broad band near 1300 cm1 and a stronger band with structure in the 1580–1600 cm1 [25,26]. The band with maximum near 1300 cm1 is common in disordered sp2 carbon matrix and has been called D-band. It is activated by disorder in sp2 carbon network. The band at 1590 cm1 is close to that observed in well ordered graphite and it is called G-band. The G-band in well ordered nanotubes has several components that stem from perfect cylindrical symmetry of nanotubes. G-band at 2625 cm1 is a second-order related harmonic [26]. Metallic and semiconducting SWCNTs have been found to exhibit different Raman lineshapes for the G-band. Early studies of ensemble of SWCNT indicate that laser radiation with energy 2.41 eV resonates dominantly with s-SWCNTs. Red and infrared laser excitations resonate better with m-SWCNTs [27]. Profile of Gband of s-SWCNTs is composed of basically four Lorenzian components [28]. Raman spectra of m-SWCNTs exhibit only two strong peaks with Lorentzian and Breit Wigner Fano (BWF) line shapes [29]. To track changes in the G-band positions and width we have carried out lineshape analysis designed to minimize the number of independent fitting parameters. The intensity of all peaks was normalized to yield the same intensity for the G-band at 1590 cm1. In the case of pristine nanotubes, we have fitted the region from 1500 to 1750 cm1 with the sum of four Lorentzian components. As for modified nanotubes, the region from 1500 to 1750 cm1 with the sum of two Lorentzian functions has been fitted. In Fig. 3, fitted Raman spectra of G-band of pristine and modified single wall carbon nanotubes are presented. As could be observed in Fig. 3a, four components at 1533, 1565, 1587 and 1602 cm1 could be identified. The intensity of components at 1533 and 1602 cm1 is very small. After nanotube functionalization, only two peaks (G and
Fig. 3. Deconvoluted Raman spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (b) and melamine sulfonate superplasticiser (c). Elaser = 2.41 eV.
G+) could be identified. Recently, Husanu et al. showed that only two G peaks could be observed in Raman spectra after nanotube functionalization because nanotubes were debundled [30]. The data concerning the positions and bandwidths of G and G+ of pristine and modified nanotubes are presented in Table 2. The G+ feature is associated with carbon atom vibrations along nanotube axis (LO phonon mode) and its frequency vGþ is sensitive to charge transfer from dopant additions to SWCNTs [22]. The G feature is associated with vibrations of carbon atoms along circumferential direction of the SWCNTs (TO phonon) and its lineshape is highly sensitive to whether the SWCNTs is metallic or semiconducting.
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Table 2 The values of position and bandwidth of corresponding peaks of pristine and modified nanotubes. Elaser = 2.41 eV. G peak position and bandwidth (cm1)
G+ peak position and bandwidth (cm1)
Pristine SWCNTs
1565 59.5
1588 24.41
SWCNTs/SDBS
1572 43.43
1591.7 22.88
SWCNTs/MSS
1567.5 48.1
1589.6 24.91
After SDBS and MSS processing, we have found that position of G- and G+-band components are upshifted several cm1. Upshift is larger for nanotube/SDBS suspension. Previous Raman studies of modified single wall carbon nanotubes have shown that removing electrons from single wall carbon nanotubes results in an upshift on the G+-band peak [23] as observed here. In fact, G+-band upshift is probably associated with electron transfers from nanotube to benzene ring of SDBS. Furthermore, the G+-band upshift in the case of nanotube/SDBS dispersion is probably associated with SO3 group. In the case of modified nanotubes by melamine sulfonate, G+-band upshift is very small. The number of anionic groups of melamine sulfonate superplasticiser bonded to nanotube skeleton is larger than that of SDBS. Therefore, the interaction mechanism of surfactants with s-SWCNT is different. In order to investigate the vibrational properties of metallic SWCNTs, Raman spectra of pristine and modified nanotubes were measured by red laser with energy of 1.91 eV as well. In Fig. 4, deconvoluted Raman spectra of G-band of pristine and modified nanotubes are presented. The spectra were fitted by the sum of BWF and Lorentzian functions. The lower frequency G modes from metallic tubes were fitted by applying BWF function, while the G+ mode for semiconducting tubes was fitted by Lorentzian function, in accordance with resonance Raman studies on isolated SWCNTs [27]. The data concerning the positions and bandwidths of pristine and functionalized SWCNTs are listed in Table 3. Charge transfer to SWCNTs can lead to an intensity increase or decrease of the BWF feature [22]. As can be seen from Fig. 4, the intensity of BWF feature of functionalized nanotubes by MSS decreases while the intensity of functionalized nanotubes by SDBS is not varied. The decrease of BWF contribution was occurred as a consequence of the decrease in electronic density and therefore of the electron–phonon coupling effect in metallic nanotubes. In fact, it has been suggested that the plasmon coupling between tubes controls the intensity of the BWF contribution to the G-band for metallic tubes [31]. Based on data presented in the diagram that relates positions of BWF and Lorentzian with nanotube diameter [22], the mean diameters of metallic and semiconducting SWCNTs were calculated and listed in Table 4.
Table 3 The values of position and bandwidth of corresponding peaks of pristine and modified nanotubes. Elaser = 1.91 eV. G position and bandwidth (cm1)
G+ position and bandwidth (cm1)
Pristine SWCNTs
1550 12
1589 26.88
SWCNTs/SDBS
1565 15.22
1590 32.66
1550 21.67
1587 24.04
SWCNTs/MSS
Fig. 4. Deconvoluted Raman spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (b) and melamine sulfonate superplasticiser (c). Elaser = 1.91 eV.
Concerning the values of mean diameters of pristine and modified SWCNTs, we concluded that melamine sulfonate supeplasticiser functionalized metallic single wall carbon nanotubes better than semiconducting ones while SDBS functionalized Table 4 Mean diameters of s-SWCNTs and m-SWCNTs.
Pristine SWCNTs SWCNTs/MSS SWCNTs/SDBS
ds-SWCNTs ðnmÞ
dm-SWCNTs ðnmÞ
1.82 1.85 1.47
1.39 1.39 1.82
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superplasticiser produces little or no change in the carbon skeleton of single wall carbon nanotubes. However, it is not the case with nanotubes modified by SDBS. In the case of interactions among SDBS and nanotubes very noticeable change in the Raman spectrum could be observed (D-band), even by examining the Raman spectra by eye. 3.2. FTIR spectroscopy
Fig. 5. FTIR spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (d) and MSS (e). FTIR spectra of SDBS (b) and MSS (c) are inserted as well.
In Fig. 5, FTIR spectra of SDBS, MSS, pristine and modified single wall carbon nanotubes are presented. As could be seen from the diagram, the peaks at 2933 and 832 cm1 are assigned to SDBS. The peaks at 1035 and 1169 cm1 are assigned to ionic sulfonate SO3 group [32]. The presence of these peaks indicates the functionalization of SWCNTs by SDBS and MSS. Besides, the peaks at 1622 and 3250 cm1 could be attributed to the stretching mode of trace water in SWCNTs [32]. Further, FTIR spectra show peaks at 1992, 2082, 2119 and 2342 cm1 which stem from pristine SWCNTs [33,34]. 3.3. Atomic force microscopy
semiconducting SWCNTs better. These results are in a good agreement with results concerning mean diameters of pristine and modified nanotubes calculated from RBM spectra. The intensity of D-band of nanotube/SDBS is almost two times lower compared to the D-band of pristine nanotubes and nanotube/MSS. The intensity of D-band could indicate the degree of chemical modification which has disordered the structure of carbon nanotubes. Disorder originates from two factors: added functional groups and inhomogeneous decoration of the tube walls by sodium cations. It is well known that inducing disorder to a sp2 carbon network induces broad disorder D-band at 1350 cm1. The decreasing of D-band in SDBS nanotubes could indicate that defects in the skeleton of a nanotube are removed by the insertion of benzene rings from SDBS into the skeleton. There is no change in intensity of D-band of MSS nanotubes. It indicates that polymer backbone is wrapped around nanotubes and is not inserted into defects in the nanotube wall. The detailed explanation of stabilization mechanism of SWCNTs is given later in the manuscript. Based on the obtained results of Raman analysis, we have concluded that chemical modification by melamine sulfonate
The distribution of lengths and diameters of pristine and functionalized single wall carbon nanotubes were characterized by AFM operating in tapping mode. The overall analysis of the characteristics of pristine and functionalized nanotubes has been performed based on AFM observation of twenty SWCNTs. Top view AFM image of pristine single wall carbon nanotube bundles is presented in Fig. 6. Pristine SWCNTs were dispersed in organic solvent-tetrahydrofuran (THF). The lengths of pristine SWCNT bundles are about 2–3 mm. The mean diameter of pristine SWCNT bundles is 150.5 nm while the height of these tubes was 23 nm. In Fig. 7, top view AFM images of modified single wall carbon nanotubes by SDBS are presented. The lengths of nanotubes varied from several hundreds nanometers to 2–3 mm. The mean diameter of tubes was 41.49 nm. The surfactant molecules have been adsorbed along the graphite surface of nanotubes (Fig. 7b). When zooming in Fig. 7a, the tube height was measured to be 0.75 nm (Fig. 7c). In Fig. 8, modified SWCNT bundles by melamine sulfonate superplasticiser are presented. One could observe that the tubes are bent likely as a result of ultrasonic treatment. The lengths of
Fig. 6. Top view AFM image and height profile of pristine single wall carbon nanotube bundle dispersed in THF.
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Fig. 7. Top view AFM images and height profile of modified SWCNT bundles by sodium dodecylbenzene sulfonate.
nanotube bundles varied from several hundreds nanometer to 1– 2 mm. When zooming in Fig. 8a, the tube height was measured to be 1.6 nm (Fig. 8b). The wrapping of SWCNTs by melamine sulfonate could be seen in Fig. 8a as well. This micrograph clearly reveals the MSS wrapping and significant decrease in diameter of SWCNTs after the functionalization. The mean diameter of nanotube bundles is about 34.12 nm. Surfactant aggregates known as micelles can be seen around the observed nanotube bundles (Fig. 8a). Based on AFM study, we could conclude that even after functionalization the formation of nanotube bundles took place.
However, it appears that the average thickness of the bundles is reduced after functionalization as shown in the presented height profile. 3.4. The stabilization mechanism of prepared nanotube dispersions Single wall carbon nanotubes have been dispersed in sodium dodecylbenzene sulfonate, an anionic surfactant that adsorbs at the surface of the nanotubes bundles. The obtained nanotube/SDBS dispersion has black colour and apparently homogenous one is
Fig. 8. Top view AFM image and height profile of modified SWCNT bundles by melamine sulfonate superplasticiser.
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Fig. 9. Reaction scheme for the modification of SWNTs using sodium dodecylbenzene sulfonate.
obtained. The amount of surfactant in the suspension is very important for the stability of investigated dispersion because surfactant molecules induce electrostatic repulsions that could counterbalance van der Waals attractions. As for the stabilization mechanism of nanotube/SDBS dispersion we supposed that surfactant molecules could adsorb onto the surface of nanotubes like hemimicelles that sheath the graphite surface-Fig. 9 [35,36]. Furthermore, the alkyl chain of the used surfactant molecules probably lies flat on the graphitic tube surface [14]. p–like stacking of the benzene rings onto the surface of graphite is believed to increase the binding and surface coverage of surfactant molecules to graphite significantly [37]. Melamine sulfonate superplasticiser is an organic polymer and could be dispersed in aqueous media. This polymer has sodium sulfonate group like hydrophilic head of SDBS. These facts prompt us to disperse single wall carbon nanotubes in aqueous phase by using melamine sulfonate superplasticiser. The dispersion is achieved by the interaction between inner hydrophobic site of the polymer and hydrophobic surface of SWCNTs together with a suitable orientation of outer sodium sulfonate group to water. Possible mechanism of stabilization of nanotube/melamine sulfonate dispersion is presented in Fig. 10. As could be observed
from Fig. 10, SO3 anions binding to polymer backbone are fitted in the sidewall defects of SWCNTs while melamine polymer backbone itself is wrapping around nanotubes and p–p interactions are established between aromatic rings and graphitic tube surface [38,39]. The p–p interaction between the polymers and the SWCNTs surfaces contributes to the solubilization of the SWCNTs [40]. Furthermore, previous studies had shown that the wrapping of SWCNTs by polymers was believed to be a general phenomenon, driven largely by thermodynamics to eliminate hydrophobic interface between the tubes and the aqueous medium [38,41]. Based on the obtained results we could conclude that single wall carbon nanotubes are better dispersed by melamine sulfonate superplasticiser compared to SDBS. The presence of several SO3 anioins bonded to polymer backbone enables better nanotube stabilization by MSS than by SDBS. In the case of nanotube/SDBS dispersion one amphiphilic molecule contains only one SO3 anion per surfactant molecule. In order to verify the stabilization of these two dispersions by electrostatic repulsive forces we caused the precipitation of nanotube dispersion by adding 1 wt.% NaCl solution. The precipitation was occurred during several hours.
Fig. 10. Reaction scheme for the modification of SWNTs using melamine sulfonate.
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4. Conclusion In this paper, single wall carbon nanotubes were functionalized by using two different surfactants: sodium dodecylbenzene sulfonate and melamine sulfonate superplasticer. Atomic force microscopy, Raman and FTIR spectroscopy were used for the characterization of surfactant-stabilized nanotube dispersions. Raman analysis was shown that the upshift of G+-band of deconvoluted Raman spectra was associated with the electron transfer from nanotubes to surfactant molecules. The upshift of Gband is greater for nanotube/SDBS dispersion compared to nanotube/MSS one. SDBS has only one sulfonate group/surfactant molecule and much more electrons are needed for stabilization of this dispersion. Deconvoluted Raman spectra of modified tubes by SDBS and MSS indicate that the interaction of SDBS with semiconducting nanotubes is stronger while the MSS interacts better with metallic nanotubes. FTIR analysis has verified the presence of sulfonate group in modified nanotube dispersions. AFM study has shown the exfoliation of nanotube bundles into thin nanotube bundles after modification by SDBS and MSS. Acknowledgement This research was supported by the Ministry of Science of Republic of Serbia (project no. 145073). References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, CA, 1996. [2] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Nanotubes, Imperial College Press, London, 1998. [3] F. Li, B.S. Cheng, G. Su, M.S. Dresselhaus, Appl. Phys. Lett. 77 (2000) 3161–3163. [4] B.W. Smith, Z. Benes, D.E. Luzzi, J.E. Fischer, D.A. Watters, M.J. Casavant, J. Schmidt, R.E. Smalley, Appl. Phys. Lett. 77 (2000) 663–665. [5] C. Klumpp, K. Kostarelos, M. Prato, A. Bianco, Biochim. Biophys. Acta 1758 (2006) 404–417. [6] L. Lacereta, A. Bianco, M. Prato, K. Kostarelos, Adv. Drug Delivery Rev 58 (2006) 1460–1470. [7] M. Foldvari, M. Bagonluri, Nanomed. Nanotechnol. Biol. Med. 4 (2008) 173–182. [8] N. Sinha, J. Yeow, IEEE Trans. Nanobiosci. 4 (2005) 180–195. [9] V. Raffa, G. Ciofani, S. Nitadas, T. Karachalios, D. D’Alessandro, M. Masini, A. Cuschiori, Carbon 46 (2008) 1600–1610. [10] K.D. Ausman, R. Piner, O. Lourie, R.S. Ruoff, M. Korobov, J. Phys. Chem. B 104 (2000) 8911–8915.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Comparative study on modification of single wall carbon nanotubes by sodium dodecylbenzene sulfonate and melamine sulfonate superplasticiser Z. Markovic´ a, S. Jovanovic´ a, D. Kleut a, N. Romcˇevic´ b, V. Jokanovic´ a, V. Trajkovic´ c, B. Todorovic´-Markovic´ a,* a b c
Vincˇa Institute of Nuclear Sciences, P.O.B. 522, 11001 Belgrade, Serbia Institute of Physics, P.O.B. 68, 11001 Belgrade, Serbia Institute of Immunology and Microbiology, School of Medicine, University of Belgrade, Dr. Subotic´a 1, Belgrade, Serbia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 November 2008 Received in revised form 21 January 2009 Accepted 4 February 2009 Available online 14 February 2009
In this work, the results of synthesis and characterization of single wall carbon nanotubes (SWCNTs) functionalized by two surfactants (sodium dodecylbenzene sulfonate and melamine sulfonate superplasticiser) have been presented. The properties of pristine and modified SWCNTs have been compared by different techniques: Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). Raman analysis reveals the changes in vibrational spectra of SWCNTs after modification by different surfactant molecules. FTIR analysis has shown the presence of sulfonate group which is strong evidence for nanotube modification. AFM analysis has shown separation of big single wall carbon nanotube bundles into thin bundles of them. ß 2009 Elsevier B.V. All rights reserved.
PACS: 61.46.Fg 68.37.Ps 78.30.j Keywords: Single wall carbon nanotubes Atomic force microscopy Raman spectroscopy Fourier transform infrared spectroscopy
1. Introduction Single wall carbon nanotubes (SWCNTs) possess unique structure, mechanical, electrical, thermal, optical and chemical properties [1–4]. They could be used as nanocomposites, field emission displays, diodes and transistors, sensors, etc. Carbon nanotubes are considered as potential biomedical materials because of their flexible structure and propensity for chemical functionalization [5]. There are many interesting and promising pharmaceutical applications for carbon nanotubes as carriermediated delivery vehicles for biofunctional molecules, as targets for biophysical treatments and as templates for tissue regeneration [6–8]. They could be used as intracellular transporters of biomolecules as well [9]. But a general problem associated with their application is their inherent insolubility in most solvents [7,10]. A uniform distribution of SWCNTs is hard to achieve due to their mutual van der Waals interaction which leads to agglomeration as well. To successfully disperse carbon nanotubes, the dispersing medium
* Corresponding author. Tel.: +381 11 2455 451; fax: +381 11 3440100. E-mail address: [email protected] (B. Todorovic´-Markovic´). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.02.016
should be capable of both wetting the hydrophobic tube surfaces and modifying the tube surfaces to decrease tube aggregation. Surfactant-assisted dispersion is one of the possible methods used to obtain a stable dispersion [11–13]. Several commercial surfactants such as sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), sodium cholate (SC) were reported to efficiently disperse bundled nanotubes into suspensions of individual nanotubes [14–17]. Moore et al. established that a nonionic surfactant or polymer’s ability to suspend nanotubes appears to be due mostly to the size of the hydrophilic group, with higher molecular weights suspending more nanotube material [15]. Dong et al. indicated that individual SWCNTs suspended in the surfactants, SDS and SDBS, were toxic to 1321N1 human astrocytoma cells due to the toxicity of SDS and SDBS on the nanotube surfaces [18]. Bergin and co-workers observed the exfoliation of nanotubes from the bundles as the concentration of nanotubes is reduced by dilution with surfactant (SDBS) solution [19]. Sun et al. found that dispersion quality is controlled by the magnitude of electrostatic repulsive forces between coated nanotubes [20]. In this work, stable dispersions of single wall carbon nanotubes in distilled water were prepared using two surfactants: SDBS and melamine sulfonate superplasticiser (MSS). This is for the first time
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to report about successfully functionalized single wall carbon nanotubes by MSS. The structure of modified SWCNTs was investigated by atomic force microscope while the degree of modification of single wall nanotubes was investigated by Fourier transform infrared (FTIR) and Raman spectroscopy. Melamine sulfonate superplasticiser is an organic polymer. The long molecules wrap themselves around the nanotubes, giving them a highly negative charge so that they repel each other. Anion groups binding to polymer backbone are fitted in the sidewall defects of SWCNTs. This polymer disperses the nanotubes through a mechanism of electrostatic repulsion. 2. Experimental procedure A stock solutions containing 10 and 28 mg of SDBS and MSS in 100 ml distilled/deionized water was made up, respectively. A dispersion of pristine single wall carbon nanotubes in THF was ultrasonicated for comparison to functionalized ones. An amount of 50 mg of as-prepared SWCNTs (95% purity from BuckyUSA) was added to 100 mL of the stock solutions. The SWCNT solutions were sonicated (ultrasonic bath with power 750 W) for 3 h. After sonication, all samples were centrifuged at 4000 rpm for 1 h. For each sample the well-dispersed supernatant was taken for characterization leaving behind the precipitate. The concentrations of both nanotube dispersions were determined by gravimetric method: 10 ml of nanotube dispersions was dried at 60 8C in air and the mass was measured. The concentration of nanotube/SDBS suspension was 200 mg/L while the concentration of nanotube/MSS suspension was 540 mg/L. Raman spectra of pristine and modified single wall carbon nanotubes were obtained by Micro Raman Chromex 2000 and JY 1000 Raman system using 532 nm of a frequency doubled Nd:YaG laser and 647 nm from Ar ion laser with power of 2 mW, respectively. The spectral resolutions were 4 and 0.2 cm1, respectively. Raman spectra were recorded at room temperature. Samples of dried modified nanotubes were pressed in the shallow hole of indium substrate. For the FTIR spectroscopy analysis, nanotube suspensions were dried at temperature of 60 8C. Dried nanotubes were mixed with KBr powder and pellets were formed. FTIR spectra were measured at room temperature in the spectral range from 400 to 4000 cm1, on a Nicollet 380 FT-IR, Thermo Electron Corporation spectrometer. Atomic force microscopy (AFM) measurements were performed using Quesant microscope operating in tapping mode in air at room temperature. In tapping mode the cantilever oscillates close to resonance and the tip only slightly touches the surface [21]. Mica was used as a substrate. Diluted dispersions of nanotubes were deposited on mica substrate and imaged after drying. Standard silicon tips (purchased from Nano and more) with force constant 40 N/m were used. The accuracy of the AFM mean diameter determination was improved by deconvolution. The freshly cleaved mica has a very small roughness (mean roughness was 0.12 nm) favoring the formation of aggregates which appear during drying of the thin layer of colloid due to the capillary forces. To reduce the aggregation on the substrate dispersions were diluted 40 times. 3. Results and discussion
Fig. 1. RBMs of Raman spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (b) and melamine sulfonate superplasticiser (c). Laser energy was 2.41 eV.
the spectra of pristine and functionalized nanotubes was normalized to yield the same intensity for the G-band. 3.1.1. RBM band analysis Raman active radial breathing mode (RBM) of pristine single wall carbon nanotubes, nanotube/MSS and nanotube/SDBS observed in range from 100 to 300 cm1 is presented in Fig. 1. In this mode, all carbon atoms are moving in-phase in the radial direction. The important point about RBM band is the fact that the energy (wavenumber) of these vibration modes only depends on the diameter of single wall carbon nanotubes. For 532 nm excitation, RBM signals in the regions of 130–150, 150–215 and 230–300 cm1 originate from the van Hove electronic transitions of semiconducting single wall carbon nanotubes (s-SWCNT-E44s, s-SWCNT-E33s) and metallic single wall carbon nanotubes (m-SWCNT-E11m), respectively. As a result, relative content of m- and s-SWCNTs before and after functionalization in this region can be evaluated from the resonant Raman spectra. Ratio of semiconducting to metallic nanotubes after treatment was determined in accordance with procedure developed earlier [24]. The ratio of the number of m-SWCNTs to number of s-SWCNTs in the observed samples, hereafter referred to the M:S ratio can be found by approximating the mean diameter distributions for mand s-SWCNTs. Results of fitting procedure assuming that the ratio of metallic to semiconducting nanotubes is 1:2 are presented in Table 1. Results in Table 1 indicate us that MSS interacts better with metallic nanotubes and mean diameter of functionalized metallic nanotubes is almost identical to pristine single wall carbon nanotubes. It means that only few MSS molecules are necessary for functionalization of metallic nanotubes. For functionalization of sSWCNTs, more MSS molecules are necessary which results in significantly larger mean diameter than that of pristine nanotubes.
Table 1 The enrichment factors of m- and s-SWCNTs and mean diameters of nanotubes in these fractions at Elaser = 2.41 eV.
3.1. Raman spectroscopy Raman spectroscopy is very useful tool for the characterization of single wall carbon nanotubes in diameter range of 2 nm [22]. Raman scattering in SWCNTs is a diameter-selective resonant scattering process [23]. Prior to band analysis, the intensity of all
Pristine SWCNTs Nanotube/MSS Nanotube/SDBS
s-SWCNTs (%)
m-SWCNTs (%)
ds-SWCNTs ðnmÞ
dm-SWCNTs ðnmÞ
67 55.6 61
33 44.4 39
1.65 1.70 1.66
0.98 0.98 0.99
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Fig. 2. Raman spectra of melamine sulfonate superplasticiser (a), pristine (b) and modified single wall carbon nanotubes by melamine sulfonate superplasticiser (c) and SDBS (d). Elaser = 2.41 eV.
SDBS interacts better with semiconducting single wall carbon nanotubes. Mean diameter of s-SWCNTs is only slightly larger than that of pristine s-SWCNTs. Also, it can be noticed that more SDBS molecules are required for functionalizing metallic nanotubes which results in diameter increase. 3.1.2. G- and D-band analysis In Fig. 2, Raman spectra of G- and D-band of melamine sulfonate superplasticiser, pristine SWCNTs, and modified nanotubes by melamine sulfonate superplasticiser (nanotube/MSS) and SDBS (nanotube/SDBS) are presented. The laser energy was 2.41 eV. As can be seen from Fig. 2, in the 1230–1750 cm1 region, two Raman bands are observed: a relatively broad band near 1300 cm1 and a stronger band with structure in the 1580–1600 cm1 [25,26]. The band with maximum near 1300 cm1 is common in disordered sp2 carbon matrix and has been called D-band. It is activated by disorder in sp2 carbon network. The band at 1590 cm1 is close to that observed in well ordered graphite and it is called G-band. The G-band in well ordered nanotubes has several components that stem from perfect cylindrical symmetry of nanotubes. G-band at 2625 cm1 is a second-order related harmonic [26]. Metallic and semiconducting SWCNTs have been found to exhibit different Raman lineshapes for the G-band. Early studies of ensemble of SWCNT indicate that laser radiation with energy 2.41 eV resonates dominantly with s-SWCNTs. Red and infrared laser excitations resonate better with m-SWCNTs [27]. Profile of Gband of s-SWCNTs is composed of basically four Lorenzian components [28]. Raman spectra of m-SWCNTs exhibit only two strong peaks with Lorentzian and Breit Wigner Fano (BWF) line shapes [29]. To track changes in the G-band positions and width we have carried out lineshape analysis designed to minimize the number of independent fitting parameters. The intensity of all peaks was normalized to yield the same intensity for the G-band at 1590 cm1. In the case of pristine nanotubes, we have fitted the region from 1500 to 1750 cm1 with the sum of four Lorentzian components. As for modified nanotubes, the region from 1500 to 1750 cm1 with the sum of two Lorentzian functions has been fitted. In Fig. 3, fitted Raman spectra of G-band of pristine and modified single wall carbon nanotubes are presented. As could be observed in Fig. 3a, four components at 1533, 1565, 1587 and 1602 cm1 could be identified. The intensity of components at 1533 and 1602 cm1 is very small. After nanotube functionalization, only two peaks (G and
Fig. 3. Deconvoluted Raman spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (b) and melamine sulfonate superplasticiser (c). Elaser = 2.41 eV.
G+) could be identified. Recently, Husanu et al. showed that only two G peaks could be observed in Raman spectra after nanotube functionalization because nanotubes were debundled [30]. The data concerning the positions and bandwidths of G and G+ of pristine and modified nanotubes are presented in Table 2. The G+ feature is associated with carbon atom vibrations along nanotube axis (LO phonon mode) and its frequency vGþ is sensitive to charge transfer from dopant additions to SWCNTs [22]. The G feature is associated with vibrations of carbon atoms along circumferential direction of the SWCNTs (TO phonon) and its lineshape is highly sensitive to whether the SWCNTs is metallic or semiconducting.
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Table 2 The values of position and bandwidth of corresponding peaks of pristine and modified nanotubes. Elaser = 2.41 eV. G peak position and bandwidth (cm1)
G+ peak position and bandwidth (cm1)
Pristine SWCNTs
1565 59.5
1588 24.41
SWCNTs/SDBS
1572 43.43
1591.7 22.88
SWCNTs/MSS
1567.5 48.1
1589.6 24.91
After SDBS and MSS processing, we have found that position of G- and G+-band components are upshifted several cm1. Upshift is larger for nanotube/SDBS suspension. Previous Raman studies of modified single wall carbon nanotubes have shown that removing electrons from single wall carbon nanotubes results in an upshift on the G+-band peak [23] as observed here. In fact, G+-band upshift is probably associated with electron transfers from nanotube to benzene ring of SDBS. Furthermore, the G+-band upshift in the case of nanotube/SDBS dispersion is probably associated with SO3 group. In the case of modified nanotubes by melamine sulfonate, G+-band upshift is very small. The number of anionic groups of melamine sulfonate superplasticiser bonded to nanotube skeleton is larger than that of SDBS. Therefore, the interaction mechanism of surfactants with s-SWCNT is different. In order to investigate the vibrational properties of metallic SWCNTs, Raman spectra of pristine and modified nanotubes were measured by red laser with energy of 1.91 eV as well. In Fig. 4, deconvoluted Raman spectra of G-band of pristine and modified nanotubes are presented. The spectra were fitted by the sum of BWF and Lorentzian functions. The lower frequency G modes from metallic tubes were fitted by applying BWF function, while the G+ mode for semiconducting tubes was fitted by Lorentzian function, in accordance with resonance Raman studies on isolated SWCNTs [27]. The data concerning the positions and bandwidths of pristine and functionalized SWCNTs are listed in Table 3. Charge transfer to SWCNTs can lead to an intensity increase or decrease of the BWF feature [22]. As can be seen from Fig. 4, the intensity of BWF feature of functionalized nanotubes by MSS decreases while the intensity of functionalized nanotubes by SDBS is not varied. The decrease of BWF contribution was occurred as a consequence of the decrease in electronic density and therefore of the electron–phonon coupling effect in metallic nanotubes. In fact, it has been suggested that the plasmon coupling between tubes controls the intensity of the BWF contribution to the G-band for metallic tubes [31]. Based on data presented in the diagram that relates positions of BWF and Lorentzian with nanotube diameter [22], the mean diameters of metallic and semiconducting SWCNTs were calculated and listed in Table 4.
Table 3 The values of position and bandwidth of corresponding peaks of pristine and modified nanotubes. Elaser = 1.91 eV. G position and bandwidth (cm1)
G+ position and bandwidth (cm1)
Pristine SWCNTs
1550 12
1589 26.88
SWCNTs/SDBS
1565 15.22
1590 32.66
1550 21.67
1587 24.04
SWCNTs/MSS
Fig. 4. Deconvoluted Raman spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (b) and melamine sulfonate superplasticiser (c). Elaser = 1.91 eV.
Concerning the values of mean diameters of pristine and modified SWCNTs, we concluded that melamine sulfonate supeplasticiser functionalized metallic single wall carbon nanotubes better than semiconducting ones while SDBS functionalized Table 4 Mean diameters of s-SWCNTs and m-SWCNTs.
Pristine SWCNTs SWCNTs/MSS SWCNTs/SDBS
ds-SWCNTs ðnmÞ
dm-SWCNTs ðnmÞ
1.82 1.85 1.47
1.39 1.39 1.82
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superplasticiser produces little or no change in the carbon skeleton of single wall carbon nanotubes. However, it is not the case with nanotubes modified by SDBS. In the case of interactions among SDBS and nanotubes very noticeable change in the Raman spectrum could be observed (D-band), even by examining the Raman spectra by eye. 3.2. FTIR spectroscopy
Fig. 5. FTIR spectra of pristine (a) and modified single wall carbon nanotubes by SDBS (d) and MSS (e). FTIR spectra of SDBS (b) and MSS (c) are inserted as well.
In Fig. 5, FTIR spectra of SDBS, MSS, pristine and modified single wall carbon nanotubes are presented. As could be seen from the diagram, the peaks at 2933 and 832 cm1 are assigned to SDBS. The peaks at 1035 and 1169 cm1 are assigned to ionic sulfonate SO3 group [32]. The presence of these peaks indicates the functionalization of SWCNTs by SDBS and MSS. Besides, the peaks at 1622 and 3250 cm1 could be attributed to the stretching mode of trace water in SWCNTs [32]. Further, FTIR spectra show peaks at 1992, 2082, 2119 and 2342 cm1 which stem from pristine SWCNTs [33,34]. 3.3. Atomic force microscopy
semiconducting SWCNTs better. These results are in a good agreement with results concerning mean diameters of pristine and modified nanotubes calculated from RBM spectra. The intensity of D-band of nanotube/SDBS is almost two times lower compared to the D-band of pristine nanotubes and nanotube/MSS. The intensity of D-band could indicate the degree of chemical modification which has disordered the structure of carbon nanotubes. Disorder originates from two factors: added functional groups and inhomogeneous decoration of the tube walls by sodium cations. It is well known that inducing disorder to a sp2 carbon network induces broad disorder D-band at 1350 cm1. The decreasing of D-band in SDBS nanotubes could indicate that defects in the skeleton of a nanotube are removed by the insertion of benzene rings from SDBS into the skeleton. There is no change in intensity of D-band of MSS nanotubes. It indicates that polymer backbone is wrapped around nanotubes and is not inserted into defects in the nanotube wall. The detailed explanation of stabilization mechanism of SWCNTs is given later in the manuscript. Based on the obtained results of Raman analysis, we have concluded that chemical modification by melamine sulfonate
The distribution of lengths and diameters of pristine and functionalized single wall carbon nanotubes were characterized by AFM operating in tapping mode. The overall analysis of the characteristics of pristine and functionalized nanotubes has been performed based on AFM observation of twenty SWCNTs. Top view AFM image of pristine single wall carbon nanotube bundles is presented in Fig. 6. Pristine SWCNTs were dispersed in organic solvent-tetrahydrofuran (THF). The lengths of pristine SWCNT bundles are about 2–3 mm. The mean diameter of pristine SWCNT bundles is 150.5 nm while the height of these tubes was 23 nm. In Fig. 7, top view AFM images of modified single wall carbon nanotubes by SDBS are presented. The lengths of nanotubes varied from several hundreds nanometers to 2–3 mm. The mean diameter of tubes was 41.49 nm. The surfactant molecules have been adsorbed along the graphite surface of nanotubes (Fig. 7b). When zooming in Fig. 7a, the tube height was measured to be 0.75 nm (Fig. 7c). In Fig. 8, modified SWCNT bundles by melamine sulfonate superplasticiser are presented. One could observe that the tubes are bent likely as a result of ultrasonic treatment. The lengths of
Fig. 6. Top view AFM image and height profile of pristine single wall carbon nanotube bundle dispersed in THF.
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Fig. 7. Top view AFM images and height profile of modified SWCNT bundles by sodium dodecylbenzene sulfonate.
nanotube bundles varied from several hundreds nanometer to 1– 2 mm. When zooming in Fig. 8a, the tube height was measured to be 1.6 nm (Fig. 8b). The wrapping of SWCNTs by melamine sulfonate could be seen in Fig. 8a as well. This micrograph clearly reveals the MSS wrapping and significant decrease in diameter of SWCNTs after the functionalization. The mean diameter of nanotube bundles is about 34.12 nm. Surfactant aggregates known as micelles can be seen around the observed nanotube bundles (Fig. 8a). Based on AFM study, we could conclude that even after functionalization the formation of nanotube bundles took place.
However, it appears that the average thickness of the bundles is reduced after functionalization as shown in the presented height profile. 3.4. The stabilization mechanism of prepared nanotube dispersions Single wall carbon nanotubes have been dispersed in sodium dodecylbenzene sulfonate, an anionic surfactant that adsorbs at the surface of the nanotubes bundles. The obtained nanotube/SDBS dispersion has black colour and apparently homogenous one is
Fig. 8. Top view AFM image and height profile of modified SWCNT bundles by melamine sulfonate superplasticiser.
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Fig. 9. Reaction scheme for the modification of SWNTs using sodium dodecylbenzene sulfonate.
obtained. The amount of surfactant in the suspension is very important for the stability of investigated dispersion because surfactant molecules induce electrostatic repulsions that could counterbalance van der Waals attractions. As for the stabilization mechanism of nanotube/SDBS dispersion we supposed that surfactant molecules could adsorb onto the surface of nanotubes like hemimicelles that sheath the graphite surface-Fig. 9 [35,36]. Furthermore, the alkyl chain of the used surfactant molecules probably lies flat on the graphitic tube surface [14]. p–like stacking of the benzene rings onto the surface of graphite is believed to increase the binding and surface coverage of surfactant molecules to graphite significantly [37]. Melamine sulfonate superplasticiser is an organic polymer and could be dispersed in aqueous media. This polymer has sodium sulfonate group like hydrophilic head of SDBS. These facts prompt us to disperse single wall carbon nanotubes in aqueous phase by using melamine sulfonate superplasticiser. The dispersion is achieved by the interaction between inner hydrophobic site of the polymer and hydrophobic surface of SWCNTs together with a suitable orientation of outer sodium sulfonate group to water. Possible mechanism of stabilization of nanotube/melamine sulfonate dispersion is presented in Fig. 10. As could be observed
from Fig. 10, SO3 anions binding to polymer backbone are fitted in the sidewall defects of SWCNTs while melamine polymer backbone itself is wrapping around nanotubes and p–p interactions are established between aromatic rings and graphitic tube surface [38,39]. The p–p interaction between the polymers and the SWCNTs surfaces contributes to the solubilization of the SWCNTs [40]. Furthermore, previous studies had shown that the wrapping of SWCNTs by polymers was believed to be a general phenomenon, driven largely by thermodynamics to eliminate hydrophobic interface between the tubes and the aqueous medium [38,41]. Based on the obtained results we could conclude that single wall carbon nanotubes are better dispersed by melamine sulfonate superplasticiser compared to SDBS. The presence of several SO3 anioins bonded to polymer backbone enables better nanotube stabilization by MSS than by SDBS. In the case of nanotube/SDBS dispersion one amphiphilic molecule contains only one SO3 anion per surfactant molecule. In order to verify the stabilization of these two dispersions by electrostatic repulsive forces we caused the precipitation of nanotube dispersion by adding 1 wt.% NaCl solution. The precipitation was occurred during several hours.
Fig. 10. Reaction scheme for the modification of SWNTs using melamine sulfonate.
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