Determination of the Concentration of Single-Walled Carbon

Oct 24, 2006 - Department of Chemical Engineering and The Ilse Katz Center for Meso and Nanoscale Science and .... (25) Ryabenko, A. G.; Dorofeeva, T...
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Anal. Chem. 2006, 78, 8098-8104

Determination of the Concentration of Single-Walled Carbon Nanotubes in Aqueous Dispersions Using UV-Visible Absorption Spectroscopy S. Attal,† R. Thiruvengadathan,† and O. Regev*,†,‡

Department of Chemical Engineering and The Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel

Stable, homogeneous, aqueous dispersions of singlewalled carbon nanotubes (SWNTs) are prepared by nonspecific physical adsorption of surfactants enhanced by sonication. Upon centrifugation, supernatant and precipitate phases are obtained. The initial weights of the SWNTs and the surfactant are divided between these two phases, and the respective SWNT concentration in each phase is unknown. The focus of this work is on the determination of the true concentration of raw, exfoliated HiPCO SWNTs in the supernatant phase. A UV-visible absorption-based approach is suggested for a direct measurement of the SWNT and the surfactant concentration in the supernatant. UV-visible absorbance spectra of SWNTs-surfactant dispersions and surfactants alone reveal that the intensity of a certain peak, attributed to the π-plasmon resonance absorption, is unaffected by the presence of most surfactants. A calibration plot is then made by monitoring the intensity of the peak as a function of the true concentration of the exfoliated SWNTs. Thus, we are able to determine the unknown concentration of surfactant-dispersed HiPCO SWNTs in the supernatant solution, simply by measuring its optical absorbance. Moreover, we can now calculate the surfactant efficiency in dispersing SWNTs. Cryogenic-transmission electron microscopy and thermogravimetric analysis techniques are used for the characterization of these dispersions and to complement the UV-visible measurements. The discovery of carbon nanotubes1 (CNTs) in the past decade has immensely contributed to the advancement of nanoscience and technology. The potential of CNTs as transistors, chemical and biological sensors, field emission sources, and filler in polymer matrixes is attributed to their exotic physical properties, such as high aspect ratio, low density, high tensile strength, and anisotropic electrical conductivity.1-5 However, the as-synthesized * Corresponding author: (e-mail) [email protected]. † Department of Chemical Engineering. ‡ Ilse Katz Center for Meso and Nanoscale Science and Technology. (1) Iijima, S. Nature 1991, 354, 56-58. (2) Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A.; Nath, M. Chemphyschem 2001, 2, 78-105.

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single-walled carbon nanotubes (SWNTs) are bundled,6 preventing their efficient use. Several research groups have reported exfoliation of SWNT bundles into individual ones either by their direct functionalization7 or by treatment with superacids.8-10 An alternative approach is the use of dispersing agents, such as ionic surfactants (sodium dodecyl sulfate, SDS),11,12 sodium dodecylbenzenesulfonate (NaDDBS),13 nonionic surfactant (Triton X-10013), or polysaccharides (gum arabic).14 Furthermore, DNA,15 polyelectrolytes,16-19 and proteins20,21 are also shown to exfoliate and stabilize SWNTs in water. Typically, the surfactant-dispersed SWNT solution is prepared by mixing surfactant and SWNTs in water. The solution is then sonicated and centrifuged. The grayish supernatant phase contains (3) Thostenson, E. T.; Ren, Z. F.; Chou, T. W. Composites Sci. Technol. 2001, 61, 1899-1912. (4) Collins, P. G.; Avouris, P. Sci. Am. 2000, 283 (6), 62-69. (5) Snow, E. S.; Novak, J. P.; Campbell, P. M.; Park, D. Appl. Phys. Lett. 2003, 82, 2145-2147. (6) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H. J.; Petit, P.; Robert, J.; Xu, C. H.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483-487. (7) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760-761. (8) Davis, V. A.; Ericson, L. M.; Parra-Vasquez, A. N. G.; Fan, H.; Wang, Y. H.; Prieto, V.; Longoria, J. A.; Ramesh, S.; Saini, R. K.; Kittrell, C.; Billups, W. E.; Adams, W. W.; Hauge, R. H.; Smalley, R. E.; Pasquali, M. Macromolecules 2004, 37, 154-160. (9) Ramesh, S.; Ericson, L. M.; Davis, V. A.; Saini, R. K.; Kittrell, C.; Pasquali, M.; Billups, W. E.; Adams, W. W.; Hauge, R. H.; Smalley, R. E. J. Phys. Chem. B 2004, 108, 8794-8798. (10) Rai, P. K.; Pinnick, R. A.; Parra-Vasquez, A. N. G.; Davis, V. A.; Schmidt, H. K.; Hauge, R. H.; Smalley, R. E.; Pasquali, M. J. Am. Chem. Soc. 2006, 128, 591-595. (11) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331-1334. (12) Regev, O.; ElKati, P. N. B.; Loos, J.; Koning, C. E. Adv. Mater. 2004, 16, 248-251. (13) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269-273. (14) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-Rozen, R. Nano Lett. 2002, 2, 25-28. (15) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338-342. (16) Schaefer, D.; Brown, J. M.; Anderson, D. P.; Zhao, J.; Chokalingam, K.; Tomlin, D.; Ilavsky, J. J. Appl. Crystallogr. 2003, 36, 553-557. (17) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y. H.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265-271. (18) Grunlan, J. C.; Liu, L.; Kim, Y. S. Nano Lett. 2006, 6, 911-915. 10.1021/ac060990s CCC: $33.50

© 2006 American Chemical Society Published on Web 10/24/2006

the dispersed and mostly exfoliated SWNTs while the precipitate consists of bundled SWNTs, graphite, amorphous carbon, catalysts, and a few nondispersed SWNTs. Upon mixing the SWNTs and surfactant, the solution is unstable. Centrifugation accelerates the phase separation process and indicates the SWNTs’ state: When exfoliated, they are dispersed and do not phase separate; when bundled, they phase separate with the other impurities.19 Cryogenic-transmission electron microscopy (cryo-TEM) imaging of SWNT dispersions performed by Moore et al.19 reveals that without centrifugation both bundled and individual nanotubes are imaged while upon centrifugation only individual nanotubes remain in the supernatant. The SWNTs and the surfactant molecules are divided between the supernatant and the precipitate phases. Thus, the true concentration of the SWNTs and the surfactant in the supernatant phase is always lower than the as-prepared concentrations (the concentrations used in the preparation of the dispersion). Therefore, a quantitative method is imperative and its importance is underlined in the preparation of SWNT-based nanocomposites. In recent years, UV-visible-NIR spectroscopy has been used as a tool for analyzing a few aspects of SWNT dispersions. A few examples are as follows: the stability of SDS-SWNTs in aqueous dispersions,22 exfoliation kinetics of Carbolex and HiPCO SWNTs,23 purity of SWNTs,24 and comparison between centrifugation and oxidation processes employed to purify the SWNTs.25 Resasco and co-workers have developed a method for quantifying the dispersibility and bundle exfoliation of purified CoMoCAT SWNTs with a variety of surfactants on the basis of two parameters, namely, “resonance ratio” and “normalized width” defined from observed features in the UV-visible spectra.26 Estimation of these two parameters makes it possible to determine the relative ability of various surfactants to exfoliate the SWNTs bundles. However, this method does not help to determine the absolute concentration of exfoliated SWNTs. In a few studies, the SWNT27,28 or multi-walled nanotube (MWNT)29 concentration in solution was measured through extinction at ∼500 nm on a featureless slope, which could lead to erroneous results, especially for multicomponent systems. Rai and co-workers studied the isotropic-to-nematic phase transition of SWNTs in strong acids and calibrated the absorbance spectra to calculate the concentration of dilute dispersions of SWNTs.10 In this system, however, the SWNTs are functionalized and no dispersant is required for their solubilization. Therefore, the calculation of SWNT concentration is easier. Moreover, the strong acid purification of the SWNTs makes their (19) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379-1382. (20) Karajanagi, S. S.; Yang, H. C.; Asuri, P.; Sellitto, E.; Dordick, J. S.; Kane, R. S. Langmuir 2006, 22, 1392-1395. (21) Nepal, D.; Geckeler, K. E. Small 2006, 2, 406-412. (22) Jiang, L. Q.; Gao, L.; Sun, J. J. Colloid Interface Sci. 2003, 260, 89-94. (23) Grossiord, N.; Regev, O.; Loos, J.; Meuldijk, J.; Koning, C. E. Anal. Chem. 2005, 77, 5135-5139. (24) Landi, B. J.; Ruf, H. J.; Evans, C. M.; Cress, C. D.; Raffaelle, R. P. J. Phys. Chem. B 2005, 109, 9952-9965. (25) Ryabenko, A. G.; Dorofeeva, T. V.; Zvereva, G. I. Carbon 2004, 42, 15231535. (26) Tan, Y. Q.; Resasco, D. E. J. Phys. Chem. B 2005, 109, 14454-14460. (27) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 193-194. (28) Kang, Y. J.; Taton, T. A. J. Am. Chem. Soc. 2003, 125, 5650-5651. (29) Hill, D. E.; Lin, Y.; Rao, A. M.; Allard, L. F.; Sun, Y. P. Macromolecules 2002, 35, 9466-9471.

Table 1. Composition of Surfactant-Dispersed 0.1 wt % SWNT in Aqueous Samples surfactant system

surfactant concn (wt %)

SWNTs to surfactant ratio (w/w)

SWNT recoverya (%)

NaDDBS NaDDBS NaC SDS CTAB CTAT CTAT

0.1 1 10 1 0.5 0.2 0.2

1:1 1:10 1:100 1:10 1:5 1:1 1:2

35 65 25 45 40 40 30

a The ratio between the weights of the true SWNT dispersed in the supernatant and the initial SWNT powder taken for the preparation (Based on results presented in Table 2).

electronic structure and optical properties different from the raw SWNTs. Spectroscopic techniques are therefore expected to yield a quantitative value for the exfoliated SWNT concentration in the suspensions, while electron or atomic force microscopy imaging sheds light mostly on morphological issues such as dispersion quality and exfoliation state, but fails to give quantitative information on exfoliated SWNT concentration. For example, in the preparation of surfactant-dispersed SWNTs-polymer composites, the exfoliated SWNT concentration in the matrix is only approximated based on the purity of the SWNT powder.12,30-35 In this paper, we present two UV-visible-based approaches for calculating the true concentration of surfactant-dispersed exfoliated SWNTs in solution. In these methods, the concentration of the surfactant in the supernatant is first estimated directly from the UV-visible spectrum and then the concentration of SWNTs is calculated based on the total weight of the dried supernatants. This eventually facilitated the construction of calibration curves indicating the true concentration of SWNTs. Thus, one can easily determine the concentration of SWNTs in a given dispersion simply by isolating their UV absorbance. EXPERIMENTAL SECTION Materials. Raw HiPCO SWNTs (70% purity) were purchased from Carbon Nanotechnologies Inc. NaDDBS, SDS, cetyltrimethylammonium p-toluenesulfonate (CTAT), cetyltrimethylammonium bromide (CTAB), and sodium cholate (NaC) were purchased from Sigma Aldrich. All chemicals were used as received. Deionized (DI) water (18.2 MΩ cm) was used in the preparation of all the samples. SWNTs Dispersions. Solutions of as-prepared 0.1 wt % SWNTs were dispersed by a few surfactants indicated in Table 1 (As-prepared concentration refers to the initial SWNT concentration used in the preparation of the dispersions.). We chose the surfactant to SWNT weight ratio according to optimum values (30) Ham, H. T.; Koo, C. M.; Kim, S. O.; Choi, Y. S.; Chung, I. J. Macromol. Res. 2004, 12, 384-390. (31) Ramasubramaniam, R.; Chen, J.; Liu, H. Y. Appl. Phys. Lett. 2003, 83, 29282930. (32) Grunlan, J. C.; Mehrabi, A. R.; Bannon, M. V.; Bahr, J. L. Adv. Mater. 2004, 16, 150-153. (33) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513-2517. (34) Bryning, M. B.; Islam, M. F.; Kikkawa, J. M.; Yodh, A. G. Adv. Mater. 2005, 17, 1186-1191. (35) Subramanian, G.; Andrews, M. J. Nanotechnology 2005, 16, 836-840.

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reported in the literature.12,13,26 The issue of SWNT recovery mentioned in Table 1 is discussed in the last part of the Results and Discussion section. All the solutions were bath-sonicated for 24 h (model Elmasonic S, 30 W, 37 kHz). The resulting suspensions were centrifuged for 30 min at 6240g using a Megafuge 1.0 (Heraues) to remove the precipitates. Electron Microscopy. A low-temperature cryo-TEM technique was employed to image the SWNT aqueous dispersions. Sample preparation was carried out using a Vitrobot at room temperature.36 A drop of the solution was deposited on a TEM grid coated by a holey carbon film (lacey carbon, 300 mesh, Ted Pella, Inc.), automatically blotted with a filter paper, and plunged into liquid ethane at its freezing point. The vitrified samples were stored under liquid nitrogen before being transferred to a TEM (Technai 12, FEI) using a Gatan workstation and cryoholder for imaging at 98 K. The microscope was operated at 120 kV in lowdose mode and with a few micrometers underfocus to increase phase contrast. Images were recorded on a Gatan 794 CCD camera and analyzed by a Digital Micrograph 3.6 software. Thermogravimetric Analysis (TGA). The supernatants were first oven-dried at 45 °C. The dried powders were measured by Mettler Toledo Star System (Mettler TC 15) under N2 or ambient air at a flow rate of 200 mL/min and at a heating rate of 10 °C/ min from room temperature to 1000 °C. UV-Visible Spectroscopy. Measurements were made on the supernatant phase in quartz cuvettes using a double-beam UVvisible spectrophotometer (Jasco V-530) in the 200-800-nm range. Since the Beer-Lambert law is not obeyed at strong absorbance (high analyte concentrations),37 the supernatants were diluted appropriately to keep the measured values of absorbance in the range of 0.1-2 in the entire wavelength range.27 Two approaches are used in this work to estimate the concentration of SWNTs in the supernatants, namely, the “overlapping method” and the “subtraction method” (vide infra). In both approaches, the surfactant weight is directly estimated from the absorbance spectra and subsequently subtracted from the dried supernatant weight to give the SWNT weight. For both methods, the same dilutions and the same original SWNT dispersions were used. Typically, the following dilutions were used: 10, 20, 30, 40, and 60 µL of dispersion in a 4 mL of DI water. In the overlapping method, DI water is used in the reference channel. Some surfactants such as NaDDBS absorb light in the same wavelength region as that of SWNTs while others do not (e.g., CTAB). Hence, a comparison of the absorbance spectra of SWNTs dispersed by these two kinds of surfactants makes it possible to estimate the concentration of the surfactant (e.g., NaDDBS) in the supernatant. In a double-beam spectrophotometer, the source light is split into two identical beams, which pass through both reference and sample channels. The absorbance of the sample is computed by subtracting the intensity of the light measured at the sample channel from that of the reference channel. The subtraction method relies on appropriate baseline correction. Hence in this (36) Frederik, P.; Bomans, P.; Electron Microscopy Unit, University of Maastricht, Faculty of Medicine. http://www.maastrichtinstruments.nl/projects/VR/ DesignPhilosophy.html, 2001. (37) Offersgaard, J. F.; Ojelund, H. Appl. Spectrosc. 2002, 56, 469-476.

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Figure 1. Cryo-TEM micrograph of 0.1 wt % SWNT aqueous dispersions by NaDDBS (1:10 w/w). The relatively small black dots (single arrow) are catalyst particles, and the big spots (double arrow) are cubic ice.

method, the absorbance of the sample was measured with respect to the corresponding aqueous surfactant solution in the reference channel. A typical procedure employed for the subtraction method is as follows: A baseline spectrum is first recorded keeping identical concentrations of NaDDBS aqueous solutions in both reference and sample cuvettes. The cuvette in the sample channel is then replaced by a SWNTs-NaDDBS dispersion sample of an unknown concentration. The absorbance of the sample is measured, and the baseline recorded earlier accounts for the subtraction of the absorbance due to surfactant. The above procedure is repeated for several baselines (prepared for different NaDDBS concentrations). The absorbance of the same sample is measured each time after recording a baseline. The intensity of the surfactant peak is monitored to ensure correct subtraction. In this work, we demonstrate that both approaches estimate the SWNT concentration of surfactant-dispersed solutions. RESULTS AND DISCUSSION Surfactant-dispersed SWNTs solutions were investigated by cryo-TEM, TGA, and UV-visible spectroscopy techniques. In this section, we first present our cryo-TEM observations followed by TGA results. Finally, the data obtained from the UV-visible measurements are detailed, and the UV-visible-based approach for estimating the SWNT concentration is depicted. A calibration curve developed from this analysis summarizes this work. Electron Microscopy. Cryo-TEM micrographs of 0.1 wt % SWNT solution dispersed by NaDDBS is shown in Figure 1. NaDDBS exfoliates the HiPCO SWNT bundles in a better manner in comparison to other surfactants.13 Indeed, we find mostly exfoliated SWNTs, as shown in Figure 1. The information extracted from cryo-TEM observation is mostly morphological and could indicate the degree of exfoliation, size of catalyst particles, or nanotube dimensions.19 The enhanced ability of NaDDBS molecules to disperse SWNTs in aqueous solutions is attributed to the presence of the benzene ring in the molecules, and the consequent π-stacking of the benzene rings with the SWNT walls.13,26 Thermogravimetric Analysis. A TGA plot of as-purchased dried HiPCO SWNT powder was performed under air ambient (Figure 2, inset). The initial weight gain around 240-280 °C is attributed to the oxidation of metal catalysts while the SWNT

Table 2. Concentration of SWNTs in Supernatants Determined Using TGA and UV-Visible Absorption Spectroscopya SWNTs obtained in the supernatants (wt %)

Figure 2. Thermograms of dried supernatants of (a) SWNT-SDS (1:10 w/w) and (b) SWNT-NaDDBS (1:10 w/w) performed under N2 ambient at a flow rate 200 mL/min and a heating rate of 10 °C/min from room temperature to 1000 °C. The inset shows TGA of as-purchased raw HiPCO SWNT powder performed under air ambient.

weight loss occurs in the 300-470 °C range.38 Oxidation is a measure of the thermal stability of SWNTs or the overall quality of the SWNTs. The oxidation at relatively low-temperature range confirms that the sample is raw and not pure.38 The residue above 800 °C is ∼28%. The surfactants used in the dispersion of SWNTs have melting points in the range of 240-500 °C. By performing TGA of dried supernatants in nitrogen ambient, simultaneous weight loss due to the burning away of surfactants and the oxidation of SWNTs in the same temperature range can be avoided.39 TGA plots of dried supernatants of SWNTs dispersed by SDS and NaDDBS performed under nitrogen ambient are shown in Figure 2. The loss of weight around 220 and 475 °C shown in TGA plots of SDS-SWNTs and NaDDBS-SWNTs (Figure 2) are attributed to the loss of SDS and NaDDBS, respectively. The second weight loss occurs at 625-750 °C and attributed to the burning of SWNTs. Furthermore, the derivative TGA data show only one peak in this temperature range (625-750 °C) (see Supporting Information, Figure S1) indicating the effective removal of the amorphous carbon during centrifugation. Otherwise, two peaks (very close to each other; Figure S2 of Supporting Information) would have been observed in the derivative data for the amorphous carbon impurity and SWNTs.38,40-43 The results obtained from the analysis of TGA plots are summarized in Table 2. The results in Table 2 clearly indicate that the amount of SWNTs dispersed by NaDDBS is higher than that obtained in SDS or CTAB dispersions, confirming a few recent studies.13,26 Quantitative TGA is not trivial although it appears straightforward.44 Sample inhomogenity, sample compaction during drying, and oxide formation of the catalysts are critical factors that could (38) Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H. J. Phys. Chem. B 2001, 105, 8297-8301. (39) Liu, J. Guide to Practice-Purity. TGA, 2005. NIST: Measurement Issues in Single Wall Carbon Nanotubes. http://www.msel.nist.gov/Nanotube2/ Practice%20Guide_Section%202_TGA.pdf. (40) Dillon, A. C.; Gennett, T.; Jones, K. M.; Alleman, J. L.; Parilla, P. A.; Heben, M. J. Adv. Mater. 1999, 11, 1354-1358. (41) Arepalli, S.; Nikolaev, P.; Gorelik, O.; Hadjiev, V. G.; Bradlev, H. A.; Holmes, W.; Files, B.; Yowell, L. Carbon 2004, 42, 1783-1791. (42) Zhang, M. F.; Yudasaka, M.; Koshio, A.; Iijima, S. Chem. Phys. Lett. 2002, 364, 420-426. (43) Li, J. Y.; Zhang, J. F. Physica E (Amsterdam) 2005, 28, 309-312.

system

SWNTs surfactants ratio

SWNTs- NaDDBS SWNTs- NaDDBS SWNTs-NaC SWNTs- SDS SWNTs- CTAB SWNTs-CTAT SWNTs-CTAT

1:1 1:10 1:100 1:10 1:5 1:1 1:2

a

TGA ((20%) 0.06 0.02 0.03 0.04 0.02

UV-vis ((5%) 0.035 0.065 0.025 0.045 0.04 0.04 0.03

As-prepared SWNTs concentration in all samples is 0.1 wt %.

Figure 3. UV-visible absorption spectra of aqueous NaDDBS and CTAB solutions with and without SWNTs. The initial concentration of SWNTs in NaDDBS and CTAB are 0.000 25 and 0.001 wt % in 1:10 and 1:5 w/w, respectively. The initial concentrations of NaDDBS and CTAB in water are 0.02 and 0.005 wt %, respectively. The inset shows the plot of absorbance value recorded at the 220-nm peak as a function of NaDDBS concentration alone.

cause substantial errors in the obtained TGA data analysis. Thus, the results presented here can serve only as a relative measure for surfactant efficiency in dispersing SWNTs.44 UV-Visible Absorption Spectroscopy. A double-beam spectrophotometer was employed to measure the absorbance of all samples in 800-200 nm. CTAB-SWNT System. The absorbance spectrum of a pure CTAB aqueous solution (Figure 3) does not reveal any significant absorption in the entire wavelength region of 800-200 nm. However, the spectrum of CTAB-dispersed SWNT solution shows two absorption peaks at 225 (photon energy of 5.5 eV) and 273 nm (4.5 eV) (Figure 3). These peaks are therefore attributed to the SWNTs alone and originate from surface excitations (4.5 eV peak)45 and bulk π-plasmon46 (5.2 eV). The background in the SWNTs-surfactants spectra is attributed to absorption by metal catalysts.25 (44) Itkis, M. E.; Perea, D. E.; Jung, R.; Niyogi, S.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 3439-3448.

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Figure 4. Absorbance of NaDDBS-SWNT dispersions with varying concentration of NaDDBS keeping the SWNTs constant at 0.000 15 wt %.

NaDDBS-SWNTs System. Since NaDDBS also contains the benzene group, we expect an overlap in this spectral region of π-plasmon excitation. Indeed, its absorbance spectrum in aqueous solution (Figure 3) is characterized by two absorption peaks at 254 (weak) and 220 nm (strong). The absorbance spectrum of NaDDBS-SWNTs (Figure 3) reveals an intense peak at 225 nm and two weak peaks at 254 and 273 nm. UV-visible absorbance measurements were performed on the NaDDBS-SWNTs system at different surfactant concentrations keeping the SWNTs concentration constant. Figure 4 shows a plot of absorbance at 225 and 273 nm, measured as a function of NaDDBS concentration. While the intensity of the 225-nm peak increases linearly with increasing NaDDBS concentration, the intensity of the 273-nm peak remains constant. Thus, the intensity of the 273-nm peak could be used in the construction of the calibration plots for the true SWNT concentration. Evaluation of the SWNT Concentration in the Supernatant. The supernatant contains mostly surfactants and wellexfoliated SWNTs.12,14 Therefore, we calculate the SWNT concentration by subtracting the surfactant weight from the total dry weight of the supernatant: To estimate the surfactant concentration, we employ two approaches in the analysis of UV-visible data discussed below. Overlapping Method. Since CTAB does not absorb in the entire measured wavelength range, it is evident that the observed absorbance in the CTAB-SWNT system is attributed to the SWNTs alone (Figure 3). To estimate the surfactant concentration in the supernatant of the NaDDBS-SWNT system, we first prepared a set of CTAB-SWNT solutions with different SWNTs concentrations having 1:5 w/w SWNT/CTAB. Among these CTAB-SWNT spectra, we chose a spectrum that overlaps the NaDDBS-SWNT spectrum. Then, we subtracted the absorption values of the CTAB-SWNT (y2) from the one obtained in the NaDDBS-SWNT (y1) system. The surfactant concentration in the supernatant is then calculated using a NaDDBS calibration curve (Figure 3 inset). Consequently, the dried supernatant (at 45 °C for 24 h) is weighed. (45) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555-2558. (46) Reed, B. W.; Sarikaya, M. Phys. Rev. B 2001, 64, art. no. 195404.

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Figure 5. Representative absorption spectra of as-prepared 2.5 × 10-4 wt % SWNTs in 2.5 × 10-3 wt % NaDDBS aqueous solution recorded with different NaDDBS concentration in the reference. The intensities of the absorbance have been shifted in the main figure for clarity. The as-measured spectra (not shifted) are shown in the inset.

In a typical preparation of a 0.1 wt % SWNT dispersion with SWNTs:NaDDBS ) 1:10, (total amount of 10 g), the initial used weight of NaDDBS and SWNTs are 100 and 10 mg, respectively. After sonication and centrifugation, the total dried supernatant (NaDDBS and SWNTs) weighs 102 mg. The amount of NaDDBS calculated from optical absorption measurements (Figure 3, inset) is 96 mg. Hence, the amount of raw SWNT powder present in the supernatant is 6 mg. This means that 4 mg of NaDDBS and 4 mg of raw SWNTs were precipitated during the centrifugation process. Thus, the true concentration of exfoliated SWNTs in the supernatant phase is 0.06 wt %. Subtraction Method. As mentioned in the Experimental Section, the amount of surfactant present in the supernatant could also be calculated through an alternative method based on the operating principle of a double-beam spectrophotometer. Briefly, choosing a proper reference will eliminate the contribution to the absorption due to any component(s) other than the sample itself. To eliminate the contribution of NaDDBS from the absorbance, an aqueous solution of NaDDBS is used as the reference (instead of pure water in the overlapping method). When the NaDDBS concentration in the reference and the sample channels are the same, the 225-nm peak is due to SWNTs only. In other words, the unknown SWNTs-NaDDBS dispersion is measured with different NaDDBS concentrations of the reference samples. The intensity of the 225-nm peak is monitored (Figure 5). The intensity of the peak exhibits a crossover from positive to negative when the concentration of the NaDDBS in the reference cuvette becomes higher than that in the sample cuvette. Representative absorption spectra of as-prepared 2.5 × 10-4 wt % SWNTs in 2.5 × 10-3 wt % NaDDBS aqueous solution recorded with different baselines is shown in Figure 5. In the main figure, the scales along the y-axis have been shifted to show the effect of subtraction on the intensity of the 225-nm peak. The as-measured spectra shown in the inset indicate that the intensity of the peak at 273 nm remains unaffected and independent of the NaDDBS concentration used in the reference cuvette. The intensity of the 225-nm peak gradually decreases with increasing concentration of NaDDBS in the reference from 0 to 2.375 × 10-3 wt %. Above reference NaDDBS concentration of 2.375 × 10-3 wt % the 225-

Figure 6. Calibration curves before and after determining the true concentration of SWNTs in aqueous dispersions with two different surfactants (NaDDBS and CTAT) using the overlapping method. Samples were prepared by mixing 0.1 wt % HiPCO with the ratio of NaDDBS to SWNTs and CTAT to SWNTs being 10:1 (w/w) and 2:1 (w/w), respectively.

nm peak is not seen. A further increase in NaDDBS concentration to 2.75 × 10-3 wt % in the reference channel causes inversion of the peak from positive to negative. This shows that the NaDDBS concentration in the reference has certainly exceeded the NaDDBS concentration present in the sample channel. It is therefore concluded that the true concentration of NaDDBS present in the SWNT dispersion is 2.375 × 10-3 wt %. While free molecules of NaDDBS exhibit an absorption peak at 220 nm, this peak slightly shifts to 225 nm in the NaDDBS-SWNTs system, as mentioned earlier (thus, the shift in the absorption wavelength is not large). Since NaDDBS and SWNTs absorption peaks lie very close to each other, it is difficult to deconvolute the superimposed peak to separate their individual contributions to the absorption. The present calculation of the surfactant weight in the supernatant is based on the assumption that both free molecules of NaDDBS present in the aqueous phase and adsorbed NaDDBS species to the side walls of SWNTs absorb light in the same wavelength. From the analysis of the above example it seems that 95% (2.375 × 10-3/2.5 × 10-3) of the initial concentration of NaDDBS used in the preparation of a SWNTs-NaDDBS dispersion is present in the supernatant phase. Since the weight of the dried SWNTsNaDDBS supernatant is 102 mg, the weight of the SWNTs alone must be 7 mg. This is in good agreement with the value (6 mg) estimated from the overlapping method. Although the SWNT concentration in the supernatants determined by UV-visible and by TGA data (Table 2) agree with each other, the UV-visible results are more reliable (note the possible errors in TGA measurements mentioned earlier). Construction of SWNT Calibration Curves. We have performed similar measurements for different concentrations of NaDDBS-dispersed SWNT aqueous solution and repeated the same procedure for CTAT-SWNTs (weight ratio of 2:1) at different SWNT concentrations. We calculated the concentration of surfactant and SWNTs in the supernatant using the overlapping procedure and constructed calibration curves, respectively (Figure 6). We earlier stated that the absorption peak at 273 nm is a signature of the surface π-plasmon excitation of the SWNTs. More importantly, surfactants do not contribute to the absorbance at

Figure 7. Comparison of the calibration curves obtained for SWNTNaDDBS dispersions using subtraction and overlapping methods.

this wavelength. Thus, we build a calibration curve (Figure 6) by monitoring the absorbance value for this peak at 273 nm as a function of SWNT concentration. The linear calibration curves (Figure 6) show the true concentration of exfoliated SWNTs in the given dispersion (in agreement with Beer-Lambert law). With these curves in hand, we are now able to estimate the real concentration of raw HiPCO SWNTs in an aqueous dispersion. In this procedure, the supernatant absorbance of the given dispersion is measured and the SWNT concentration is determined from the calibration curve. It is noteworthy to observe that the calibration plots determined for NaDDBS- and CTAT-dispersed SWNTs overlap within the experimental errors (Figure 6). The obtained calibration plots are independent of the surfactants used in the preparation of the dispersions. While the ratio of CTAT to SWNTs used in the preparation of the dispersions is 2:1, the ratio of NaDDBS to SWNTs used is 10:1. The SWNT-dispersing efficiency varies, however, from one surfactant to another. From Figure 6, we are now able to calculate the SWNT recovery in the supernatant phase: The NaDDBS recovers 65% of the initial weight to SWNTs in the supernatant while CTAT recovers average value of 35% (see also Tables 1 and 2). This is indicative of the superior dispersing ability of NaDDBS as also reported by several researchers.13,26 Figure 7 shows the SWNT calibration curves using both the overlapping and subtraction methods. This again shows that the two methods yield the same calibration curves within the experimental errors. Furthermore, we found that, upon variation of the surfactant to SWNT ratios, the calibration curves obtained are unaffected (see Figure S-3 of Supporting Information). These calibration curves are useful in the estimation of HiPCO SWNT concentration in a given dispersion. Our method is applicable to aqueous dispersions prepared with surfactants that do not absorb light at 273 nm. First, the given SWNT dispersion is diluted to appropriate levels. Then the absorbance of the dispersion is measured at 273 nm. The corresponding concentration of SWNTs is then simply determined from the calibration curve. CONCLUSION The present work presents results and analysis of SWNTsurfactant aqueous dispersions obtained from three independent characterization techniques. Cryo-TEM observation of NaDDBSAnalytical Chemistry, Vol. 78, No. 23, December 1, 2006

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SWNT aqueous solution clearly indicates that mostly individual nanotubes are dispersed in the supernatant phase. Though TGA is a useful tool in the determination of the purity of SWNT soot, it is difficult to quantitatively analyze the data obtained for SWNTsurfactant aqueous dispersions due to large errors in the thermograms. Our work demonstrates that UV-visible spectroscopy (measured at peak positions) is a simple, accurate, convenient, and rapid characterization tool for monitoring the SWNT concentration in surfactant-dispersed aqueous systems. The errors associated with the UV-visible measurements are minimal in comparison with TGA. We used two approaches, namely, subtraction and overlapping methods, to estimate the surfactant concentration in the supernatant, and this eventually facilitated the determination of SWNTs concentration.

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ACKNOWLEDGMENT The authors are very grateful to the reviewers for their excellent comments, which helped in improving the presentation style and the quality of the manuscript. SUPPORTING INFORMATION AVAILABLE Derivative TGA data and calibration plots for different ratios of surfactants to SWNTs. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review May 30, 2006. Accepted September 9, 2006. AC060990S