J. Phys. Chem. C 2010, 114, 12095–12098
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A Generalized Method for Evaluating the Metallic-to-Semiconducting Ratio of Separated Single-Walled Carbon Nanotubes by UV-vis-NIR Characterization Liping Huang,† Hongliang Zhang,† Bin Wu,† Yunqi Liu,*,† Dacheng Wei,† Jianyi Chen,† Yunzhou Xue,† Gui Yu,† Hisashi Kajiura,*,‡ and Yongming Li‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and Material Laboratory, Sony Corporation, Atsugi Tec. No. 2 4-16-1 Okata Atsugi, Kanagawa 243-0021, Japan ReceiVed: March 14, 2010; ReVised Manuscript ReceiVed: June 9, 2010
A general and useful method has been developed to evaluate the metallic-to-semiconducting (M/S) ratio for separated single-walled carbon nanotubes (SWNTs). By virtue of measuring UV-vis-NIR spectra of a variety of solutions with different ratios of metallic-rich to semiconducting-rich SWNTs, the commercial IsoNanotubes samples as well as metallic-rich HiPCO SWNTs (HiPCO-M) separated by an Agarose gel method have been evaluated. Values of 99.5% metallic contents for IsoNanotubes-M, 98.9% semiconducting contents for IsoNanotubes-S, and 1.24 for the absorption coefficient of IsoNanotubes, whereas 80.4% metallic contents for HiPCO-M and 1.05 for the absorption coefficient of HiPCO SWNTs were obtained. This method does not need pure metallic (M-) or semiconducting (S-) SWNTs as references. Furthermore, we found that this method can also be applied to evaluate the M/S ratio for any SWNT samples. Single-walled carbon nanotubes (SWNTs) have great potential for various applications in thin-film transistors and transparent/ flexible electronics.1 Unfortunately, metallic SWNTs (MSWNTs) and semiconducting SWNTs (S-SWNTs) always coexist in as-synthesized samples due to the unselected chirality distribution, which poses one of the most critical problems in SWNT research. Along this line, many methods, including current breakdown,2 selective chemical interaction,3 gas-etching,4 density gradient ultracentrifugation,5 gel separation,6 and so on,7 have been proposed to selectively and efficiently separate SWNTs into single electrical-type SWNTs or one-type enriched samples. Meanwhile, the separating efficiency or the M-SWNTs to S-SWNTs ratio (M/S ratio) for both as-prepared and separated SWNTs is needed to be evaluated for studying the separation mechanisms as a guideline for further improvement of separation as well as for the selective synthesis of one-type enriched SWNT samples. Moreover, it is not trivial to accurately and efficiently evaluate the M/S ratio. Current methods for evaluating the M/S ratio are mainly based on electrical measurements8-10 and optical spectroscopy.11,12 Although the former electrical test is accurate in a single tube level, it is not capable of distinguishing the electrical type of SWNTs if the M- and S-SWNTs form bundles as only M-SWNTs would dominate the electrical measurement. Obviously, it is also a time-consuming process for quantitatively determining the M/S ratio with bulk samples. For the latter, an opticalspectroscopymethod,Ramanspectroscopy,andUV-vis-NIR spectroscopy are mostly used to qualify the separation efficiency according to the relationship between the peak shifts and SWNTs’ diameters and chiralities called Kataura’s plot.13 Only part of the SWNTs can be excited in a certain excitation laser energy in the case of Raman. Continuous excitation laser wavelengths in the whole spectrum range should be applied in the Raman spectroscopy method for quantitative evaluation. This * To whom correspondence should be addressed. E-mail:
[email protected] (Y.Q.L),
[email protected] (H.S.K.). † Chinese Academy of Sciences. ‡ Sony Corporation.
also will be a very complicated process. UV-vis-NIR spectroscopy, based on the different absorption wavelengths and intensities depending on the SWNTs’ diameters and chiralities, in principle, should be simple, efficient, and more accurate for quantitatively evaluating the M/S ratio for SWNT samples. However, the derivation is involved in accurate determination of the quantity defined as the absorption coefficient associated with the corresponding M- and S-SWNT absorption spectroscopy, as introduced by Kataura’s group.8 The value (1.2) of the absorption coefficient is obtained by assuming the sorted SWNTs as pure M- and S-SWNTs, which can be used as standard reference samples and only applies to the SWNT samples with a diameter distribution of 1.1-1.3 nm. Here, we describe a general model that allows the accurate determination of the absorption coefficient by a programmed experimental approach. Our method works well for evaluating any separated samples through any separation method by only using UV-vis-NIR spectroscopy without using pure M- and S-SWNTs as standard reference samples. We started our model by first considering the defined absorption coefficient (f) for pristine SWNT samples, where 1 1 1 1 f ) M11 /S22 and M11 and S22 are the absorption intensities of the M11 or S22 peak for one M-SWNT or one S-SWNT, respectively.
f)
M111 S122
)
M11 /nM S22 /nS
(1)
where M11 and S22 are the absorption intensity of the M11 or S22 peak in the solution and nM and nS are the number of M- and S-SWNTs in the solution, respectively. The ratios of M-SWNTs or S-SWNTs in the solution are then given by
10.1021/jp102316c 2010 American Chemical Society Published on Web 06/28/2010
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RM )
J. Phys. Chem. C, Vol. 114, No. 28, 2010
nM ) nM + nS
Huang et al.
1 1+f or
V ) 1/2,
S22 M11 RS )
f)
′′ /[1/2c(1 - RSa) + (1 - RSb)] M11 ′′ /(1/2cRSa + RSb) S22
(6) nS ) nM + nS
1 M11 1+ S22 f
(2)
V ) 1/3,
f)
M 11 /[1/3c(1 - RSa) + (1 - RSb)] S 22 /(1/3cRSa + RSb)
(7) In eq 2, if we can know the value of f, then we can easily get the percentage of M- or S-SWNTs through measuring the peak areas of M11 and S22 by UV-vis-NIR. For a given SWNT sample, there is an absorption coefficient f corresponding to it with a given diameter distribution, such as the value 1.2 for SWNTs with the diameter distribution of 1.1-1.3 nm reported by Kataura’s group in ref 8. Once we get the value of f, then eq 2 can be applied for any SWNT samples within the diameter distribution. However, the value of 1.2 is just available for SWNTs with diameters of 1.1-1.3 nm, and the method they used to get the value was with the help of assumed pure Mand S-SWNTs within this diameter distribution as references, which is very hard, even impossible, for us to move on for any further evaluation. We found that, if we have two separated SWNT solutions, such as metallic-rich (M-rich) and semiconducting-rich (S-rich) SWNT solutions mixed with different ratios, we should be able to calculate f and M/S ratios based on the UV-vis-NIR spectroscopy measurement of the mixed solutions. This method does not need any pure single electrical-type SWNTs as reference. For example, the concentrations for these two solutions are ca and cb, whereas the ratios of semiconducting SWNTs for both solutions are RSa and RSb, respectively. The subscript a and b indicate metallic-rich and semiconducting-rich SWNT solutions. When mixing these two solutions with known volumes, such as Va and Vb, we can get
f)
M111 S122
)
M11 /nM M11 /(nMa + nMb) ) ) S22 /nS S22 /(nSa + nSb) M11 /[caVa(1 - RSa) + cbVb(1 - RSb)] S22 /(caVaRSa + cbVbRSb)
(3)
V ) 1/4,
f)
M 11 /[1/4c(1 - RSa) + (1 - RSb)] S 22 /(1/4cRSa + RSb)
(8) It is very complicated to solve the values for these four unknown factors from the above four equations. Given that the concentration ratio of two solutions c can be manually controlled in some extent, we just suppose the case of c ) 1 to simplify the calculation process. By using the three eqs 5-7, we can then simply get that
f)
′ M11 ′′ M ′ M11 ′′ S ′ S22 ′′ M S22 11 + 2M11 22 - 3M11 11 ′ S22 ′′ S ′ S22 ′′ M ′ M11 ′′ S M11 22 + 2S22 11 - 3S22 22
(9)
By substituting the value of f into eqs 5 and 6, we can then get the value of RSa and RSb as follows
RSa )
′ f ′′ f 4S22 3S22 ′ + S22 ′ f ′′ + S22 ′′ f M11 M11
(10)
RSb )
′′ f ′ f 3S22 2S22 ′′ + S22 ′′ f ′ + S22 ′ f M11 M11
(11)
The detailed calculation process is shown in the Supporting Information (Part 7). Figure 1 shows the corresponding UV-vis-NIR results with mixed solutions (see experimental details in the Supporting Information). From the absorption spectra, we can clearly see that, with the addition of S-rich SWNT samples, the peaks between 600 and 800 nm remain unchanged, while the other peaks are obviously increased. The peaks between 600 and 800
If we define the concentration ratio as c ) ca/cb and the volume ratio as V ) Va/Vb, then the equation can be simplified as
f)
M11 /[cV(1 - RSa) + (1 - RSb)] S22 /(cVRSa + RSb)
(4)
In eq 4, M11 and S22 are measured as peak areas and V is known. By simply varying the value of V, we will be able to establish four equations including four unknown factors: f, c, RSa, and RSb. For example, if we just choose V ) 1, 1/2, 1/3, and 1/4 (the cases of V ) 1, 2, 3, 4 are shown in the Supporting Information), the four equations are shown as follows
V ) 1,
′ /[c(1 - RSa) + (1 - RSb)] M11 f) ′ /(cRSa + RSb) S22
(5)
Figure 1. UV-vis-NIR spectra for solutions mixed with different volume ratios of separated SWNT samples. Va:Vb refers to the volume ratio of M-rich (IsoNanotubes-M, denoted as a) to S-rich (IsoNanotubesS, denoted as b) SWNT solutions.
Evaluating the M/S Ratio of SWNTs by UV-vis-NIR
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TABLE 1: Intensity (Peak Areas) of M11 and S22 for Different Volume Ratios of Separated SWNT Solutions Va:Vba
M11 × 103
S22 × 103
1:1 1:2 1:3 1:4
8.18 8.21 8.22 8.23
6.52 12.91 19.16 24.70
a Va:Vb refers to the volume ratio of M-rich (IsoNanotubes-M, denoted as a) to S-rich (IsoNanotubes-S, denoted as b) SWNT solutions.
nm are attributed to M11 and the peaks between 900 and 1200 nm are attributed to S22, according to Kataura’s plot. The whole measured peak area results are listed in Table 1 (see calculation details in the Supporting Information). When these results are put into eq 9, the absorption coefficient f ) 1.24 is obtained. Similarly, by using eqs 10 and 11, RSa ) 0.5% and RSb ) 98.9% are obtained as well, which means 99.5% metallic contents for IsoNanotubes-M and 98.9% semiconducting contents for IsoNanotubes-S. To verify the results, we used the calculated f value 1.24 to evaluate the M/S ratio for the fourth mixed solutions with eq 2. Substituting the peak areas of M11 and S22 of the fourth mixed solutions (shown in Table 1.) into eq 2, we can get that the semiconducting content Rs is about 78.8%, compared with 79.2% calculated by mixing one part of a with RSa ) 0.5% with four parts of b with RSb ) 98.9%. We take this result as credible, and this absorption coefficient can be used for this kind of SWNT sample for further evaluation. This method has also shown that we do not need pure metallic and pure semiconducting SWNT samples as references. However, there are some prerequisite conditions that should be considered. Above all, to get the accurate absorption results, monodispersed SWNT solutions should be prepared. For many separation processes, such as density gradient ultracentrifugation, gel separation, etc., the separated SWNT solutions are monodispersed. For other separation processes, such as gas etching, the separated samples are not in the form of monodispersed solution, so a dispersion process should be conducted first. Another one is the value of the concentration ratio c. In our experiment, the concentrations of M-rich and S-rich SWNT samples are nearly the same, so we just discuss the case of c ) 1. Sometimes, the separated SWNT samples have different concentrations. In this case, it would be better to prepare the same concentration of these two solutions by diluting or redispersing to simplify the calculation process. If the concentration ratio of these two solutions is hard to determine, we can also first use our method to evaluate an SWNT sample with the above-mentioned process and then use this sample as a standard reference to evaluate any other SWNT samples as the following process. The last one we will emphasize is that there are fewer measurement errors for the areas of M11 and S22 in the UV-vis-NIR spectra if there is more difference between the M/S ratios of the two original enriched SWNT samples. The results will then be more accurate. The above discussions are all based on the separated M-SWNTs and S-SWNTs from the same pristine SWNT sample. However, in some cases, we may just get only one kind of SWNT through the separation method, such as gas etching, and current breakdown. In this case, we can use another known SWNT sample to mix with the separated samples to follow our proposed evaluation method. The unknown SWNT sample just needs to be dispersed into solution with the same dispersant and concentration with the known SWNT solution. An average
Figure 2. UV-vis-NIR spectra for solutions mixed with different volume ratios of separated SWNT samples. Va:Vb refers to the volume ratio of M-rich (HiPCO-M separated by an Agarose gel method, denoted as a) to S-rich (IsoNanotubes-S, denoted as b) SWNT solutions.
absorption coefficient jf is then introduced instead of f in the above equations. Though this average absorption coefficient does not apply to any kind of SWNT, this method can easily calculate the M/S ratio for unknown SWNT samples. Besides, we can also get the absorption coefficient of the unknown samples fu by further calculation as eq S14 (Supporting Information). This can also be used to calculate the M/S ratio for any kind of SWNT sample and the absorption coefficient for SWNT samples with any diameter distributions. HiPCO SWNT samples are the most popular kind of SWNT used for separation. We also separated HiPCO SWNT samples with an Agarose gel method as reported by Kataura’s group6 and try to evaluate the M/S ratio and f value with our proposed method. The experimental section for Agarose gel separation is described in the Supporting Information. After the separation, metallic SWNTs (denoted as HiPCO-M) were obtained in the solution, while semiconducting SWNTs (denoted as HiPCO-S) were in the gel, which is hard to accurately characterize by UV-vis-NIR because it undergoes gelation very quickly at room temperature. It is not suitable to evaluate the separated SWNT samples by mixing the HiPCO-M solution and HiPCO-S in gel with different volume ratios. However, we can choose another S-rich SWNT sample, such as the IsoNanotubes-S, which we have evaluated above to mix with the HiPCO-M solution. Figure 2 shows the UV-vis-NIR results with different volume ratios of the HiPCO-M solution to the IsoNanotubes-S solution. Because the diameter distributions for the HiPCO samples and IsoNanotubes have big differences, the peak areas of S22 and M11 for the mixed solutions cannot be directly measured. Nevertheless, we can still determine the peak areas by comparing the mixed solutions with the pristine HiPCO-M and IsoNanotubes-S (see the detailed process in the Supporting Information). The results are shown in Table 2. Substituting the peak areas of S22 and M11 and the semiconducting percentage (98.9%) of the IsoNanotubes-S into eqs S1-S3 (Supporting Information), we can get the average jf value (1.69), the semiconducting percentage for the HiPCO-M (19.6%), as well as the concentration ratio c (0.15). The metallic percentage for the HiPCO-M is 80.4%. Using this value and the peak areas of S22 and M11 for the HiPCO-M solution (Va:Vb ) 1:0) and eq 2, we can then get the f value (1.05) for the HiPCO SWNT samples, which can be used for further evaluation for any separated HiPCO SWNT samples. We attribute the little difference between this f value and the result (1.2) reported by Kataura’s group to their assumption that the sorted samples have a sufficiently high purity (>99%).
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TABLE 2: Intensity (Peak Areas) of M11 and S22 for Different Volume Ratios of HiPCO-M to IsoNanotubes-S Solutions Va:Vba
M11 × 103
S22 × 103
0:1 1:0 1:1 2:1 3:1
0.26 2.81 3.02 5.87 8.82
13.00 0.65 14.13 14.75 15.41
a Va:Vb refers to the volume ratio of M-rich (HiPCO-M separated by an Agarose gel method, denoted as a) to S-rich (IsoNanotubes-S, denoted as b) SWNT solutions.
In conclusion, we have proposed a simple, but efficient, method for generalized M/S ratio evaluation of separated SWNTs. By only measuring the UV-vis-NIR spectra of mixed solutions with different ratios of separated metallic-rich and semiconducting-rich SWNT samples, we can easily calculate the M/S ratio for each sample, as well as the absorption coefficient f for the pristine SWNT samples. Through simple experiments, we have evaluated the M/S ratio and f value for commercial IsoNanotubes, and using the evaluated S-rich IsoNanotubes-S solution as a reference, we can also get the M/S ratio for M-rich HiPCO-M samples obtained by an Agarose gel separation method as well as the f value for HiPCO SWNTs. Furthermore, we also find that this method can be used to calculate the M/S ratio for any one kind of SWNT and the absorption coefficient f for SWNTs with any diameter distributions. Acknowledgment. This work is supported by the National Natural Science Foundation of China (60736004, 20721061, 60911130231, 20825208, and 20973184), the Major State Basic Research Development Program (2006CB806203, 2006CB932103, and 2009CB623603), and the Chinese Academy of Sciences.
Huang et al. Supporting Information Available: Details for experimental sections and calculation procedures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hierold, C. Carbon Nanotube DeVices: Properties, Modeling, Integration and Applications; Advanced Micro & Nanosystems; WileyVCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008; Vol. 8, pp 1-37. (2) Collins, P. C.; Arnold, M. S.; Avouris, P. Science 2001, 292, 706– 709. (3) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853–1859. (4) Zhang, H. L.; Liu, Y. Q.; Cao, L. C.; Wei, D. C.; Wang, Y.; Kajiura, H.; Li, Y. M.; Noda, K.; Luo, G. F.; Wang, L.; Zhou, J.; Lu, J.; Gao, Z. X. AdV. Mater. 2009, 21, 813–816. (5) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60–65. (6) Tanaka, T.; Jin, H. H.; Miyata, Y.; Fujii, S.; Suga, H.; Naitoh, Y.; Minari, T.; Miyadera, T.; Tsukagoshi, K.; Kataura, H. Nano Lett. 2009, 9, 1497–1500. (7) Hersam, M. C. Nat. Nanotechnol. 2008, 3, 387–394. (8) Kim, W.-J.; Lee, C. Y.; O’brien, K. P.; Plombon, J. J.; Blackwell, J. M.; Strano, M. S. J. Am. Chem. Soc. 2009, 131, 3128–3129. (9) Li, Y. M.; Mann, D.; Rolandi, M.; Kim, W.; Ural, A.; Hung, S.; Javey, A.; Cao, J.; Wang, D. W.; Yenilmez, E.; Wang, Q.; Gibbons, J. F.; Nishi, Y.; Dai, H. J. Nano Lett. 2004, 4, 317–321. (10) Kim, W.; Choi, H. C.; Shim, M.; Li, Y. M.; Wang, D. W.; Dai, H. J. Nano Lett. 2002, 2, 703–708. (11) Miyata, Y.; Yanagi, K.; Maniwa, Y.; Kataura, H. J. Phys. Chem. C 2008, 112, 13187–13191. (12) Jorio, A.; Santos, A. P.; Ribeiro, H. B.; Fantini, C.; Souza, M.; Vieira, J. P. M.; Furtado, C. A.; Jiang, J.; Saito, R.; Balzano, L.; Resasco, D. E.; Pimenta, M. A. Phys. ReV. B 2005, 72, 075207. (13) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555–2558.
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