Extraction of High-Purity Single-Chirality Single-Walled Carbon

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Extraction of High-Purity Single-Chirality Single-Wall Carbon Nanotubes Through Precise pH Control Using Carbon Dioxide Bubbling Yota Ichinose, Junko Eda, Yohei Yomogida, Zheng Liu, and Kazuhiro Yanagi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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The Journal of Physical Chemistry

Extraction of High-purity Single-chirality Singlewall Carbon Nanotubes through Precise pH Control Using Carbon Dioxide Bubbling Yota Ichinose,† Junko Eda, † Yohei Yomogida,† Zheng Liu,‡ and *Kazuhiro Yanagi† †Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0372, Japan ‡National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, Aichi 4638560, Japan

ABSTRACT

The preparation of single-chirality single-wall carbon nanotubes (SWCNTs) is of great importance to comprehensively understand their physical properties and applications. The highpurity separation of single-chirality SWCNTs has been reported using gel chromatography with a mixed surfactant system, but further improvements are necessary to enhance the purity of the resulting single-chirality SWCNTs. In this study, we focused on using pH as a parameter to improve the separation purity and investigated its influence on mixed surfactant-based purification. The pH of the mixed surfactant solution was precisely controlled with carbon

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dioxide bubbling with an accuracy of approximately 0.1. Using this eluent with a series of different pH values, we achieved diameter-selective separation of the SWCNTs. Moreover, we found that the residual metallic SWCNTs could be completely removed under the proper pH condition. Finally, on the basis of these findings, we produced single-chirality SWCNTs with a purity of more than 99% without residual metallic SWCNTs.

TEXT Introduction

Developing techniques for preparing single-chirality single-wall carbon nanotubes (SWCNTs) is very important for comprehensively understanding the physical properties of SWCNTs in thin films and their transistor and biomedical applications

1-3

. To date, various types of purification

techniques for single-chirality SWCNTs have been reported, such as DNA-based separation density gradient ultracentrifugation (DGU) gel chromatography

3, 13-16

1, 7-9

, aqueous two-phase extraction (ATP)

10-12

4-6

,

, and

. Most of the methods have employed several different surfactants,

such as sodium dodecyl sulfate (SDS), sodium cholate (SC) and sodium deoxycholate (DOC), to produce monodispersions of SWCNTs and extract SWCNTs with a specific electronic structure. In the case of the gel chromatography method, it was recently revealed that using a mixed surfactant system, containing SC, DOC and SDS, is crucial for high-purity separations of singlechirality SWCNTs 3. In this method, precisely controlling the parameters (e.g., the composition of the mixed surfactants) is crucial for achieving high-purity separation. However, in the extracted solutions of the single-chirality solution, other types of chirality have still been present


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(3-7%) 3 and thus finding other controllable parameters is important for improving the purity of separated single-chirality SWCNTs as well as their performance in applications.

In 2011, Hirano et al. reported that pH was the important parameter in gel chromatography using a single surfactant system of SDS, which influenced the metallic/semiconducting separation of SWCNTs

17

. They reported that a decrease in the pH led to oxidation of the

SWCNTs and an increase in their SDS coverage, which changed the interactions between the SDS-wrapped SWCNTs and the gel. According to their results, semiconducting SWCNTs became nonadsorbed to the gel at pH values less than 7. It is known that pH of an SDS solution can reach ~5.7 due to atmospheric carbon dioxide (CO2) 18; thus, such separation processes can be sensitive to the atmosphere, and their proposed pH control is important for stable separations. In contrast, in mixed surfactant systems, it is necessary to use not only SDS but also SC and DOC, and it has been reported that SC- and DOC-wrapped SWCNTs become unsusceptible to oxidation, i.e., pH

19

. Therefore, the effect of pH may be different from that in usual SDS

systems. However, studies regarding the influence of pH on mixed surfactant-based purification processes for single-chirality SWCNTs have not been conducted. In this study, we systematically investigated the effects of pH on gel chromatography using a mixed surfactant system and demonstrated further purification of single-chirality SWCNTs using precise pH control. The pH was controlled using CO2, which is known to decrease the pH of SDS systems. The pH was easily changed using CO2 bubbling, and hence, we prepared a series of solutions with different pH values. Chirality separation was conducted using these eluents with a series of different pH values. We observed diameter selective desorption of the SWCNTs from the gel, in which the larger diameter species were more easily eluted as the pH became


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lower. This diameter selectivity was similar to that in found in usual SDS systems, but the pH required for elution of the SWCNTs was different. Moreover, we found that the residual metallic SWCNTs could be completely removed under the proper pH condition. Finally, we demonstrated that further purification of high-purity single-chirality SWCNTs is possible on the basis of these findings.

Methods We investigated the effect of pH on chirality separation using a pH-controlled aqueous solution containing mixed surfactants. The pH of the solutions was precisely controlled by carefully mixing a CO2-treated solution (pH: ~6.0) with the original solution (pH: ~8.8) (Figure S1, Supporting Information). The CO2 treatment was conducted by bubbling CO2 into a solution containing SDS (Sigma-Aldrich) and SC (Sigma-Aldrich). After measuring the pH using a pH meter (±0.02 pH accuracy, S2K922, IFSETCOM Co., Ltd.), the bottle was refilled with nitrogen to avoid subsequent pH changes during the experiment. For separation, CoMoCAT (0.8 nm in diameter, Sigma-Aldrich) and HiPco SWCNTs (1.0±0.3 nm in diameter, Unidym) were dispersed in an aqueous solution containing 0.5% SDS and 0.5% SC, using sonication followed by ultracentrifugation. The dispersion was loaded into a column filled with gel beads (Sephacryl S-200 HR, GE Healthcare). After eluting the unbound metallic SWCNTs, the adsorbed semiconducting SWCNTs were eluted and collected after a stepwise decrease in pH from 8.4 to 7.6 in increments of 0.1.

Results and Discussion


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Diameter-selective separation of SWCNTs using precise pH control Figure 1a and 1b shows the optical absorption spectra of the eluted SWCNTs at different pH values using the HiPco and CoMoCAT SWCNTs, respectively. Both results show that the large diameter SWCNTs were eluted earlier than the small diameter ones. This trend was clearly observed in the HiPco SWCNT sample, which contained many kinds of (n,m) species with a wide range of diameters. This diameter-selective elution was similar to that of a previous study using a single surfactant system of SDS 20. In contrast, the pH required for eluting the SWCNTs was higher in the SDS/SC system than in the SDS single system, in which nearly all the SWCNTs were eluted at a pH of approximately 3-4 17, 20. This observation indicates that the SC influenced the SWCNTs and/or micelle structure around the SWCNTs, resulting in a change in the sensitivity to pH. The detailed pH dependence of the eluted (n,m) species is discussed later.

Purification of chirality-enriched SWCNTs by precise pH control The pH-controlled diameter-selective elution of SWCNTs enables further purification of chirality-enriched SWCNTs. In this study, we applied this method to purify 4 (n,m) species separated using a previously reported method 3, in which the SWCNTs were eluted with DOC (Kanto-Kagaku Co.). The nearly single-chirality (6,5) and (7,5) samples were obtained using CoMoCAT SWCNTs, and the (9,4) and (10,3) samples were obtained using HiPco SWCNTs (Figure S2, Supporting Information). To make these original samples adsorb to the gel again, the DOC concentration of the dispersion was diluted to half of the original using a solution containing SDS and SC 3. After loading the dispersion, the adsorbed SWCNTs were eluted and collected by decreasing the pH in the same manner as the previous separation. Figure 2a-2d


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shows the optical absorption spectra of the purified SWCNTs compared to the original SWCNTs. Each spectrum of the purified SWCNTs shows sharp S11, S22 and S33 peaks (Sii indicates ith optical transition of the semiconducting type chirality of SWCNTs) due to the disappearance of other absorption peaks around the target (n,m) species after purification. This observation indicates that the purities of these (n,m) species were highly improved after the pHcontrolled purification. The detailed elution profiles (Figure S3, Supporting Information) revealed that the diameter selection had an important role during purification in this experiment, which was especially noted in the (7,5) and (9,4) samples that originally contained several (n,m) species. For instance, the purification of the (7,5) and (9,4) samples exhibited gradual changes in the eluted species from large-diameter species to small-diameter species at pH 8.1-8.0 and pH 8.2-8.1, respectively. In contrast, the (6,5) sample, which was originally high purity, did not exhibit changes in the S11 absorption but did exhibit changes in the residual metallic SWCNT adsorption at approximately 400-500 nm (see Figure 2d). The detailed analysis regarding the residual metallic SWCNTs is discussed later. The diameter dependence affected the target species elution under suitable pH conditions. According to the S11 peak intensity of the target (n,m) species plotted as a function of the eluent pH (Figure S4, Supporting Information), the different samples were eluted at slightly different pH values: approximately 8.1 for (9,4) and (10,3), 8.0 for (7,5) and 7.9 for (6,5). Figure 2e shows the relation between the diameters of the respective chiralities and the pH required for their elution. Here, we chose only the chiralities that could be identified clearly in their absorption spectra. These results clearly exhibited the linear relation between the diameters of the eluted


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SWCNTs and pH, which are the same as the results using pristine HiPco and CoMoCAT SWCNTs (Figure 1).

Removal of residual metallic SWCNTs by precise pH control To investigate the detailed effects of pH-controlled purification in terms of the removal of residual metallic SWCNTs, we analyzed the absorption spectra at approximately 400-500 nm, which were assigned to the M11 band of the CoMoCAT metallic SWCNTs. Figure 3a shows the results of the (6,5) sample. The spectra of the original (6,5) sample exhibited an S22 peak at 570 nm as well as the M11 peaks of residual metallic SWCNTs. The peaks at 400 and 470 nm were assigned to (5,5) and (7,4), respectively. We found that these residual metallic SWCNTs were completely missing at pH values less than 7.9. Figure 3b shows the S11 peak intensity of (6,5) and the content rate of the metallic SWCNTs. The content rate was estimated as the ratio of the M11 peak intensity (470 nm) to the S11 peak intensity of (6,5). The metallic SWCNTs were eluted before elution of (6,5) SWCNTs and then the ratio of (6,5) increased. As a result, we could obtain high-purity single-chirality (6,5) SWCNTs by collecting the eluted sample at pH values less than 7.8. This pH control was also effective during the adsorption process. We confirmed that the (6,5) SWCNTs could only be adsorbed without adsorption of the residual metallic SWCNTs by applying the pH-controlled sample onto the gel. It is known that decrease of pH induces oxidation of nanotubes, making them p-type and the positively charged state of the nanotube affects the adsorption of surfactants surrounding nanotubes17,21,22. For example, SDS, which is an anionic surfactant, is more attracted to the positively charged nanotubes. The presence of large amount of SDS on the surface of nanotubes


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reduces the adsorption of the nanotubes to the gel17, 21, 22. The SWCNTs with smaller band gap tend to be more easily oxidized than the nanotubes with larger band gap 23, 24. Therefore, metallic SWCNTs tend to be more influenced by pH than semiconducting SWCNTs, and the semiconducting SWCNTs with larger diameters are more influenced than the nanotubes with smaller diameters. In this study, similar behavior is observed. The metallic SWCNTs and the semiconducting SWCNTs with large diameter are easily eluted by decrease in pH compared to the semiconducting SWCNTs with small diameter (Figure 2).

Evaluation of purity of purified (6,5) Figure 4a shows the optical absorption spectra of the high-purity (6,5) SWCNTs obtained after pH-controlled purification. Since the M11 peaks at 400 and 470 nm completely disappeared after purification, the purity of the semiconducting SWCNTs was thought to be very high. Next, the purity of the single-chirality SWCNTs was evaluated by fitting the absorption spectrum to simulated peaks of the individual (n,m) species with linear baseline absorption, and the ratio of the area of the dominant absorption peak to the sum of all peak areas was estimated. Figure 4b shows the results of the peak fitting for the high-purity (6,5) SWCNTs. There were two recognizable peaks, except for the (6,5) SWCNTs: a peak corresponding to the sub-band of (6,5) at approximately 850 nm and a small peak corresponding to (9,1) at approximately 910 nm. The absorbance data did not allow for a quantitative evaluation of the purity beyond 99%; however, judging from the content of the other species, such as (9,1), being less than 1%, the purity of the single-chirality (6,5) SWCNTs was estimated be more than 99%. Such high purity was confirmed also by transmission electron microscope (TEM) measurements (Figure S5 & S6,


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Supporting Information). The purity was almost the same as that of (6,5) SWCNTs obtained by the state-of art of DNA purification techniques6. Moreover, this purity was estimated for the situation in which the metallic SWCNTs were absent. We believe that this sample has better or comparable quality compared to those from other studies 4, 6, 7, 13, 16, 25.

Conclusions We revealed the effect of pH on gel chromatography using mixed surfactant systems and applied the obtained knowledge to further purify single-chirality SWCNTs. The pH of the solution containing SDS and SC was precisely controlled in a range from 8.4 to 7.6 with an accuracy of approximately 0.1 by mixing the solution (pH: ~6) treated with CO2 bubbling with an untreated solution (pH: ~9). By applying this eluent with a series of different pH values to the gel with adsorbed SWCNTs, we observed diameter-selective desorption of the SWCNTs from the gel, in which the larger-diameter species were easily eluted as the pH decreased. This diameter selectivity was useful for the subsequent purification of 4 nearly single-chirality (n,m) species, including (6,5), (7,5), (9,4), and (10,3). Moreover, we found the metallic SWCNTs were eluted before elution of the semiconducting SWCNTs, and hence, we could obtain high-purity semiconducting single-chirality (6,5) SWCNTs by collecting the eluted sample at the proper pH. Finally, on the basis of these findings, the purity of the purified single-chirality (6,5) samples reached more than 99% without residual metallic SWCNTs. We believe that these high-purity single-chirality SWCNTs will enable high-performance semiconductor applications, such as in field-effect transistors 26 and thermoelectric devices 27.


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Figure 1. Diameter-selective separation of SWCNTs by precise pH control. (a, b) Optical absorption spectra of the eluted SWCNTs at different pH values. CoMoCAT (a) and HiPco SWCNTs (b) were used as the original starting materials, as shown in the respective top columns. These spectra corresponded to a pH range of 8.4 to 7.6 in increments of 0.1 in order from the top to the bottom.


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Figure 2. Purification of the chirality-enriched SWCNTs by precise pH control. (a-d) Optical absorption spectra of the single-chirality (n,m) species, including (10,3) (a), (9,4) (b), (7,5) (c), and (6,5) (d), before and after the pH-controlled elution. These spectra were normalized by their S11 intensities. (e) Relationship between the diameters of the respective chiralities and the pH required for their elution.


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Figure 3. Removal of the residual metallic SWCNTs by precise pH control.(a) Optical absorption spectra of the eluted SWCNTs of (6,5) at different pH values in the SDS/SC system. These spectra were normalized at 280 nm, vertically shifted and focused at 300-700 nm for comparison. (b) Relationship between the S11 peak intensity of (6,5) and pH, and the content rate of the metallic SWCNTs.


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Figure 4. Evaluation of the purity of (6,5).(a) Optical absorption spectrum of the (6,5) SWCNTs. (b) Evaluation of the purity of the single-chirality (6,5) SWCNTs by fitting the absorption spectrum.

ASSOCIATED CONTENT Supporting Information. Change in pH of mixed surfactant solution, eluted SWCNTs at different DOC concentrations, the eluted (n,m) species, Relationship between the eluted (n,m) species and pH of eluent, a TEM image of purified (6,5) and analyses results of the diameters of 16 samples of purified (6,5), the electron diffraction pattern of a bundle of purified (6,5) samples taken by TEM

AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Numbers JP16H00919, JP17K14088, JP25107003. We thank Professor Sumio Iijima for his kind supports, advices and discussions about the TEM measurements. ABBREVIATIONS SWCNTs, single-wall carbon nanotubes; SDS, sodium dodecyl sulfate; SC, sodium cholate; DOC, sodium deoxycholate.


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Figure 1. Diameter-selective separation of SWCNTs by precise pH control. (a, b) Optical absorption spectra of the eluted SWCNTs at different pH values. CoMoCAT (a) and HiPco SWCNTs (b) were used as the original starting materials, as shown in the respective top columns. These spectra corresponded to a pH range of 8.4 to 7.6 in increments of 0.1 in order from the top to the bottom.


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Figure 2. Purification of the chirality-enriched SWCNTs by precise pH control. (a-d) Optical absorption spectra of the single-chirality (n,m) species, including (10,3) (a), (9,4) (b), (7,5) (c), and (6,5) (d), before and after the pH-controlled elution. These spectra were normalized by their S11 intensities. (e) Relationship between the diameters of the respective chiralities and the pH required for their elution.



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Figure 3. Removal of the residual metallic SWCNTs by precise pH control.(a) Optical absorption spectra of the eluted SWCNTs of (6,5) at different pH values in the SDS/SC system. These spectra were normalized at 280 nm, vertically shifted and focused at 300-700 nm for comparison. (b) Relationship between the S11 peak intensity of (6,5) and pH, and the content rate of the metallic SWCNTs.


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Figure 4. Evaluation of the purity of (6,5).(a) Optical absorption spectrum of the (6,5) SWCNTs. (b) Evaluation of the purity of the single-chirality (6,5) SWCNTs by fitting the absorption spectrum.


Extraction of High-Purity Single-Chirality Single-Walled Carbon

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