Environment Effects on the Charge States of Metallic and

Publication Date (Web): February 6, 2019. Copyright © 2019 American Chemical Society. *(T.S.) E-mail: [email protected]. Telephone: +81-29-861...
0 downloads 0 Views 3MB Size
Download PDF
Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Environment Effects on the Charge States of Metallic and Semiconducting SWCNTs during Their Separation by the ElectricField Induced Layer Formation Method Yuki Kuwahara,†,‡ Fusako Sasaki,‡ and Takeshi Saito*,†,‡ †

National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan Technology Research Association for Single Wall Carbon Nanotubes, Tsukuba 305-8565, Japan

‡

J. Phys. Chem. C Downloaded from pubs.acs.org by LMU MUENCHEN on 02/06/19. For personal use only.

S Supporting Information *

ABSTRACT: The mechanism of separating metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWCNTs) by electric-field induced layer formation (ELF) was investigated, by examining effects from the environmental conditions, that is, the pH and surfactant concentration on the separation process. Time course analysis showed that the pH and surfactant concentration in the cell become inhomogeneous during the ELF separation process. The ζ potential measurements revealed that the s-SWCNTs are much more negatively charged than m-SWCNTs in the specific pH range. The mechanism of ELF separation of m- and s-SWCNTs by forming layers is attributed to the dynamic changing/balancing of the electrophoretic and electroosmotic forces acting on the SWCNTs. The applicability of various surfactants for the ELF method was also discussed in terms of the surfactant types and the molecular structures.

■

method.23 The setup of the ELF method is quite simple. A DC electric field is applied in the longitudinal direction to a SWCNT dispersion containing the nonionic surfactant of polyoxyethylene (100) stearyl ether (Brij S100, SigmaAldrich). Interaction with the electric field led to the accumulation of m- and s-SWCNTs into the upper and lower parts of the dispersion, respectively, forming divided layers (see the movie in Supporting Information). It has been proved that the ELF separation can accommodate a relatively wide range of SWCNT diameters, from ca. 1.0 to 1.7 nm,23,24 while it is less selective in terms of the diameter and chirality. Therefore, the diameter distribution of SWCNTs is approximately retained after the ELF separation.23 In addition, the sSWCNT dispersion obtained from ELF separation is promising as an ink in printed electronics, because it is free from ionic contamination such as Na+, which is a concern in the semiconductor process. We have fabricated s-SWCNT thin film transistors by using the ELF-separated s-SWCNT dispersion as a semiconductor ink, and demonstrated that these devices possess high performance, small hysteresis, and low variability in their characteristics.25,26 Because of the similarity between ELF separation and electrophoresis system, the charge states of SWCNTs in the

INTRODUCTION Single wall carbon nanotubes (SWCNTs) have a quasi-onedimensional structure, and each of them can be formed by rolling a graphene sheet into a seamless cylinder. Difference in this rolling-up process leads to their varied structural features such as the diameter and chirality. This could further cause differences in their electronic states, i.e., the different chiralities of SWCNTs lead to their metallic- or semiconducting natures, and the diameter is closely related with the value of the band gap.1 Therefore, SWCNTs have been considered not only for metallic applications such as transparent conductive films and electrodes2−4 but also for semiconducting applications such as field effect transistors,5,6 near-infrared fluorescence imaging,7,8 thermoelectric devices,9 THz imaging,10 and solar cells.11 Since the as-produced SWCNTs are generally a mixture of those with metallic (m-) and semiconducting (s-) electrical properties,12 their separation is desirable, especially for the semiconducting applications because these are extremely sensitive to impurities. Various SWCNT separation methods have been reported, i.e., density gradient centrifugation (DGU),13,14 electrophoresis,15 gel column separation,16,17 aqueous two-phase (ATP),18 charge sign reversal (CSR) method,19 column separation using ssDNA,20,21 selective specific SWCNT dispersion using polyfluorene-based derivatives,22 and so on. To separate the m- and s-SWCNTs, we have recently developed the electric-field-induced layer formation (ELF) © XXXX American Chemical Society

Received: October 19, 2018 Revised: January 18, 2019

A

DOI: 10.1021/acs.jpcc.8b10192 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) Schematic setup of the SWCNT separation method by ELF. (b) Photographs of the separation cells after 0, 1, 24, and 96 h. Each area divided by dash lines (f1−f10) indicates a collected fraction. UV−vis−NIR absorption spectra of f2 (c) and f9 (d) at each elapsed time. Time course measurements of pH distribution (e) and Brij-S100 concentration distribution (f) along the height of the separation cell.

dispersion are expected to play an important role in the ELF separation. Hitherto, lots of research groups had reported the charge states of SWCNTs.19,27−29 For example, the charge state of SWCNTs was described in some pioneering works on coating SWCNTs onto a substrate, in which the aminomodification of substrate30−32 and Coulomb interaction through the electrodeposition phenomenon33,34 were utilized. Most of these studies reported that the SWCNTs in the dispersion are negatively charged.35 It is well-known that SWCNTs have some defects36,37 that are generally modified by functional groups such as carboxyl groups.35 The ionization of such functional groups on SWCNTs is possible cause of the negative charge. We had first explained the ELF separation mechanism by difference in the charge states between micelles containing sand m-SWCNTs, because this separation is induced by the application of a DC electric field.23 However, in our previous study,23 the m- and s-SWCNTs separated by ELF showed no difference in their electrophoretic behavior, which obviously contradicted the proposed mechanism. Even until now, the detailed mechanism of ELF separation remains an open question, despite the numerous researches that apply the ELFseparated s-SWCNTs.10,25,26,38 As much as the authors know, there have been only limited studies on the charge states or the ζ potential of SWCNTs dispersed in the aqueous solution of nonionic surfactant.19,31,34

However, such information may be the key to clarify the ELF separation mechanism. One of these studies examined the chromatographic separation of m- and s-SWCNTs, which were found to have different ζ potentials when dispersed in pHadjusted aqueous solutions. The proposed reason for this difference was that positive charge is only generated on mSWCNTs in the acidic environment due to the effect of the selective hole doping in the metallic electronic structure. Such environment effects could also cause significantly different charged states of the SWCNTs in ELF separation. In this study, the environment effects, namely the pH and Brij-S100 concentration in the SWCNT dispersion during ELF separation were investigated. The ζ potentials, as an important property for evaluating the charge state and the influence of the electric field, were measured for the m- and s-SWCNTs after the separation. It has been revealed that both types of SWCNTs are negatively charged in the dispersion, yet the magnitudes of their ζ potentials are significantly different. On the basis of these results and the discussion on the dynamically changing charge states of m- and s-SWCNTs during the separation process, we have proposed the plausible mechanism of the ELF separation.

■

EXPERIMENTAL SECTION SWCNTs were produced by an enhanced direct-injection pyrolytic synthesis (eDIPS) method.39 The average diameter B

DOI: 10.1021/acs.jpcc.8b10192 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

measured for the main fractions, f2 and f9, at the respective time points. These results clearly suggest that the separated layers are of m- and s-SWCNTs: M11 and M22 peaks that are representative of m-SWCNTs became intense at f2, while S22 and S33 representative of s-SWCNTs diminished as the separation progressed over time. The opposite trends were observed for f9. Although the shapes of the absorption spectra in f2 and f9 were approximately conserved after 24 h of separation, the m- and s-SWCNTs were gradually enriched in these layers during this time. The purity of s-SWCNTs in f2 after 96 h was estimated by Raman analysis to be 94.4%. We have measured the pH value of each fraction to characterize the dynamically changing pH in the cell under the electric field. Figure 1e shows the time course of pH distribution within the cell during ELF separation. Before applying the electric field, the dispersion had pH = 4.5. Under the electric field, pH on the cathode side was momentarily increased while that on the anode side slightly decreased. As a result, a nonuniform gradient of pH was formed in the separation cell as time progressed. In particular, relatively large changes in the pH were observed in the vicinity of the electrodes. A small amount of bubble was also generated on the surfaces of both electrodes during the separation process, indicating mild water electrolysis reaction as given by eq 1 (at cathode) and eq 2 (at anode):

of the prepared SWCNTs was 1.3 nm, as characterized by Raman and optical absorption spectroscopies. Into 30 mL of 1 wt % Brij-S100 aqueous solution was dispersed 3 mg of SWCNTs by a bath-type sonicator (Bransonic 1510J-DTA, Branson) for 10 min and then by a tip-type sonicator (VCX500, Sonics) with a power of 150 W for 10 min in an ice bath. The obtained solution was centrifuged for 30 min at 250 000 × g (himac CS100GXII, Hitachi) to eliminate large bundled SWCNTs. The supernatant solution was collected for use in ELF separation. As shown in Figure 1a, cylindrical separation cells (cell volume: 20 mL) with platinum electrodes on their upper (cathode) and lower (anode) sides were prepared for the separation of SWCNTs by ELF. The upper and lower electrodes were placed on the 19 mL and the 1 mL lines, respectively. ELF separation was carried out by applying a DC potential of 30 V (Model 2400 Source Meter, Keithley). For the time course analysis of ELF separation, we prepared seven identical separation cells and analyzed these at different elapsed times. In a given cell, the SWCNT dispersions were carefully collected in 2 mL fractions from top to bottom, and named f1−f10. This makes a total of 10 fractions from each cell with different elapsed times. For each fraction, the UV− vis−NIR absorption spectrum (UV-3600, Shimadzu Corporation), pH (LAQUA twin-pH-11B, Horiba, Ltd.), and refractive index (RA620, Kyoto Electronics Manufacturing Co., Ltd.) were measured. The purity of the final s-SWCNTs was characterized in terms of Raman spectra (Ex. 633 nm, SENTERRA, Bruker Corporation) as follows. The peaks of the radial breathing mode (RBM) originated from s- and m-SWCNTs at 145 and 196 cm−1, respectively, were fitted with Lorentzian functions on an appropriate background to evaluate the peak area. Here, we assumed that 2/3 of the pristine SWCNTs are s-SWCNTs and 1/3 are m-SWCNTs as theoretically predicted under the assumption of equal synthetic probability.1,40,41 On the basis of this assumption and the analysis result of RBM peak areas for the pristine SWCNTs, we calculated the compositional ratio of s-SWCNTs to the total SWCNTs contained in the fraction as the purity of s-SWCNTs in this work. From the refractive index of each fraction, we can easily calculate the concentration of Brij S100 in the solution (see the calculation method in SI). For the ζ potential measurement, the Brij-S100 concentrations of m- and s-SWCNTs dispersions were adjusted by monitoring the refractive index as follows. Dialysis (Biotech Cellulose Ester Membrane, 1000 kDa MWCO) was carried out for the obtained s-SWCNT dispersion to remove most of the surfactant. After that, both m- and s-SWCNTs were either diluted with a solvent, or added with a conc. Brij-S100 solution to prepare 1 wt % Brij-S100 aqueous dispersion. The pH of the dispersion was adjusted by adding aqueous solutions of hydrochloric acid or sodium hydroxide.

H 2O + 2e− → H 2 + 2OH−

(1)

2H 2O → O2 + 4H+ + 4e−

(2)

Thus, the pH gradient observed in the separation cell is due to the diffusion of H+ and OH− generated by these electrochemical reactions. As we will discuss later, this generated pH gradient is important in the mechanism of ELF separation. As another environment condition, we have measured the time course of the Brij-S100 concentration distribution in the separation cell. The result is shown in Figure 1f. Brij S100 is a nonionic surfactant used to disperse SWCNTs in water, and its concentration in each fraction was calculated from the measured refractive index. In our previous study,23 we observed a slight gradient in the solution density along the vertical direction of the cell after ELF separation, due to the electrophoresis of negatively charged Brij-S100 micelles to the anode. Nevertheless, the detailed behavior and distribution of Brij S100 was not characterized. In this study, the time course analysis clearly shows that Brij S100 migrates to the anode side under the electric field at a nearly constant velocity. This migration causes an approximately linear concentration distribution of Brij S100 as shown in Figure 1f, which contrasts to the discrete distribution of SWCNTs accumulating in layers. As described above, the s-SWCNTs and Brij S100 migrate to the anode side, whereas only m-SWCNTs migrate to the cathode side in the ELF separation. The time course of the pH distribution and the Brij-S100 concentration, shown in parts e and f of Figure 1, confirms that the SWCNT separation mainly progresses in the range of pH 4 to 6 and around 1 wt % of BrijS100 concentration, while the layers in which the separated mand s-SWCNT accumulate have more extreme local environments. Table 1 lists the values of pH and Brij-S100 concentration in f2 and f9 after 96 h of separation. Thus, it was revealed that the as-separated dispersions of m- and s-

■

RESULTS AND DISCUSSION Figure 1 shows the time course of ELF separation. In Figure 1b, after 1 h of voltage application, the upper interface of the SWCNT dispersion moved downward, and a wispy bluish layer was formed around there. After 24 h, clear layers of bluish and rusty colors were formed at around the locations of f2 and f9, which are close to the cathode and anode, respectively, and an almost colorless region was formed under the upper bluish layer. Parts c and d of Figure 1 show the absorption spectra C

DOI: 10.1021/acs.jpcc.8b10192 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Table 1. Enriched SWCNT Electronic Types, pH, and Concentration of Brij S100 for f2 and f9 Containing the Separated SWCNT Dispersions

f2 f9

enriched electronic types of SWCNTs

pH

concentration of Brij S100 (wt %)

metallic semiconductive

6.9 3.7

0.44 1.69

SWCNTs had totally different pH environment and Brij-S100 concentration. As described in the introduction, it is already known that the pH environment possibly influences the charge state of SWCNTs. Therefore, we have investigated the ζ potentials of m- and s-SWCNTs in their dispersion under different pH and Brij-S100 concentration. Figure 2 plots the ζ potentials of

Figure 3. ζ potentials of m- and s-SWCNTs dispersed in the BrijS100 aqueous solutions of (a) 0.3 and (b) 2.0 wt %. The error bars represent the standard error.

might be related to the selective doping on m-SWCNTs. According to the recent study by Wang et al.,19 SWCNTs are doped through the water formation reaction given by eq 3.

Figure 2. ζ potentials of m- and s-SWCNTs (blue square and red diamond, respectively) dispersed in 1.0 wt % Brij-S100 aqueous solution. The shadow region indicates the pH range corresponding to that in the separation cell. The error bars represent the standard error.

4(SWCNT) + 4H+ + O2 V 4(SWCNT)+ + 2H 2O

(3)

It is known that m-SWCNTs are predominantly oxidized by this doping reaction due to the nonzero density of states at the Fermi level.42,43 Under acidic conditions, the forward reaction in eq 3 becomes faster and the hole doping progresses, especially in m-SWCNTs. This is also a convincing reason why the negative ζ potential decreases with decreasing pH, as shown in Figure 2 and 3. Furthermore, the above-mentioned selective doping in m-SWCNTs simply reflects the difference in their electronic structure from the s-SWCNTs, so this also explains the insensitivity of the ELF method toward the SWCNT diameter. From the above results, the micelles of m- and s-SWCNTs (and also that of Brij S100 itself) are negatively charged, which is also supported by their electrophoretic behaviors reported in our previous study.23 This clearly rules out the possibility of separation by simple electrophoresis, which requires charges of different signs. When a voltage is applied to negatively charged particles, the electrophoretic force pulls them toward the anode side. On the other hand, if there are the substances with zero or minor charge nearby, these substances conversely migrate toward the cathode side as they are pushed by the electroosmotic flow of the liquid media in general. In the system of ELF separation, the effect from electroosmotic flow on the m-SWCNTs should be stronger than that from electrophoresis, because the m-SWCNTs carry considerably less charge than the s-SWCNTs and Brij S100 within pH 4−6.

the separated m- and s-SWCNTs dispersions with 1.0 wt % Brij S100 under various pH, in which the shadow region indicates pH range in the ELF separation cell. Since SWCNTs disperse in aqueous solutions by forming micelles with Brij S100, the ζ potential measured here is actually that of the slipping plane of the micelles involving m- or s-SWCNTs. As shown in Figure 2, both types of dispersions are negatively charged, and the charge magnitude increases with increasing pH in the range from 3.5 to 7.5, that is, under the same pH environment as in the separation cell. The amount of charge is obviously larger for the s-SWCNTs than the m-SWCNTs, and this difference becomes maximum at pH = 4−6. In addition, the ζ potential of the 1 wt % Brij-S100 aqueous solution was almost the same as that of the s-SWCNTs dispersion, although this is not shown in the graph. The ζ potentials of m- and s-SWCNT dispersions in 0.3 and 2.0 wt % Brij S100 were similarly measured. The corresponding pH dependency, shown in Figure 3, is qualitatively the same as in Figure 2. However, the difference in ζ potential between m- and s-SWCNTs becomes smaller as the Brij-S100 concentration increases. This result suggests that excess Brij S100 reduces the negative charge in the micelles. Such difference in the ζ potential between m- and s-SWCNTs D

DOI: 10.1021/acs.jpcc.8b10192 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

On the cathode side where the pH is high and the Brij-S100 concentration is low, the ζ potential of the m-SWCNTs gradually becomes more negative, and thus the electrophoretic force and electroosmotic force are eventually balanced on the m-SWCNTs. In this way, the layer of m-SWCNTs is formed on the cathode side. On the other hand, due to the predominant electrophoretic force, s-SWCNTs with more negative charge quickly migrate toward the anode. Since the pH is low and the Brij-S100 concentration is high near the anode, the ζ potentials of both m- and s-SWCNTs become small, although the magnitude is still relatively larger for the s-SWCNTs. In this situation, the electrophoretic migration of s-SWCNTs becomes extremely slow, while the electroosmotic flow would gradually push the m-SWCNTs toward the cathode side. As a result, the layer of s-SWCNTs is formed on the anode side. So far, the separation of SWCNTs by ELF method has been confirmed only for nonionic surfactants such as the Brij series (i.e., Brij S100 and Brij L23).23 To check the applicability of different surfactants, we measured the ζ potentials of SWCNT dispersions prepared with typical nonionic and anionic surfactants (Table 2, together with the molecular structures).

As a result, the difference between the effects of electroosmotic force and electrophoretic force can contribute to the ELF separation of m- and s-SWCNTs, as illustrated in Figure 4a.

Table 2. ζ Potential (ZP) of SWCNT Dispersions in 1 wt % Surfactants and Their pH

Figure 4. (a) Schematic representation of the proposed mechanism for ELF separation. Under the electric field, m-SWCNTs, s-SWCNTs, and Brij S100 experience different magnitudes of electrophoretic force toward the anode according to their negative ζ potentials. At the same time, the electrophoresis generates the counterforce, as the result of electroosmotic flow that propels the substances with zero or minor charge such as m-SWCNTs to migrate toward the cathode side. (b) Schematic of forces on the SWCNTs in different pH environments. In the separation cell, the anode side (bottom) shows low pH and the cathode side (top) shows high pH.

As discussed above, the ζ potential of SWCNTs in the ELF separation system was found to be quite sensitive to the environment conditions of pH and Brij-S100 concentration. Furthermore, the applied electric field induces inhomogeneous distributions of the pH and Brij-S100 concentration in the separation cell. Therefore, the magnitude of the ζ potential of the SWCNTs changes according to the position in the cell. Thus, the electrophoretic and electroosmotic forces acting on m- and s-SWCNTs change dynamically during the separation process. Figure 4b schematically shows the relationship between these two forces on the two types of SWCNTs at each cell position. At the center of the separation cell under the electric field and pH = 4 to 6, the s-SWCNTs migrate toward the anode because of the electrophoretic force, while the mSWCNTs migrate toward the cathode because the electroosmotic flow has a stronger effect on them than the electrophoretic force.

For the anionic surfactants, namely sodium cholate (SC), sodium deoxycholate (SDOC), sodium dodecyl sulfate (SDS), and sodium dodecyl benzene sulfate (SDBS), the ζ potentials of SWCNT dispersion were much larger than those of nonionic surfactants due to the ionization of anionic surfactant molecules. Considering such a large amount of negative charge on the micelles of anionic surfactants containing SWCNTs, it seems that the effect of the hole doping in m-SWCNTs on the micelle charge is negligible. Actually, we chromatographically E

DOI: 10.1021/acs.jpcc.8b10192 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C separated and prepared m- and s-SWCNTs in the aqueous solution of anionic surfactants (SC and SDS) as previously reported.16 The obtained m- and s-SWCNT dispersions in these solutions had ζ potentials of −48.0 and −50.5 mV, respectively. Thus, it supports that the difference in negative charge carried by m- and s-SWCNTs is minor when we use anionic surfactants. Our ELF separation experiments using anionic surfactants showed no evidence of the separation of mand s-SWCNTs. These results indicate that anionic surfactants could not be used for ELF separation, because of the lack of the adequate difference between the m- and s-SWCNTs in their charge states. On the other hand, the ζ potentials of SWCNTs dispersed with some other nonionic surfactants, such as Triton X405, Tween 80, and Brij S20, are extremely small compared with anionic surfactants, as shown in Table 2. When we carried out the ELF separation experiments using Triton X405 and Tween 80, signs of separation appeared in both cases. However, the SWCNTs gradually agglomerated and, as a result, the degree of separation was lower than that of Brij series surfactants. Therefore, features in the molecular structure of nonionic surfactants, such as the structure of the lipophilic group and the length of the polyoxyethylene chain, are also important for the applicability in ELF separation from the viewpoint of the dispersion stability. On the basis of these results, a more advanced ELF separation technique, with higher purity and selectivity in terms of the diameter and chirality, might be realized by optimizing the molecular structure of nonionic surfactants as well as the strict control of charge states in SWCNTs.

■

AUTHOR INFORMATION

Corresponding Author

*(T.S.) E-mail: [email protected]. Telephone: +81-29861-4863. ORCID

Yuki Kuwahara: 0000-0002-3441-0476 Takeshi Saito: 0000-0001-5664-6407 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by the Nippon Sheet Glass Foundation for Materials Science and Engineering (NSG Foundation), JSPS KAKENHI Grant Number 16K17492, and the New Energy and Industrial Technology Development Organization (NEDO).

■

REFERENCES

(1) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Electronic-Structure of Chiral Graphene Tubules. Appl. Phys. Lett. 1992, 60, 2204−2206. (2) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; et al. Transparent, Conductive Carbon Nanotube Films. Science 2004, 305, 1273−1276. (3) Jeon, I.; Chiba, T.; Delacou, C.; Guo, Y.; Kaskela, A.; Reynaud, O.; Kauppinen, E. I.; Maruyama, S.; Matsuo, Y. Single-Walled Carbon Nanotube Film as Electrode in Indium-Free Planar Heterojunction Perovskite Solar Cells: Investigation of Electron-Blocking Layers and Dopants. Nano Lett. 2015, 15, 6665−6671. (4) Jeon, I.; Cui, K.; Chiba, T.; Anisimov, A.; Nasibulin, A. G.; Kauppinen, E. I.; Maruyama, S.; Matsuo, Y. Direct and Dry Deposited Single-Walled Carbon Nanotube Films Doped with MoOx as Electron-Blocking Transparent Electrodes for Flexible Organic Solar Cells. J. Am. Chem. Soc. 2015, 137, 7982−7985. (5) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Logic Circuits with Carbon Nanotube Transistors. Science 2001, 294, 1317− 1320. (6) Wang, C.; Zhang, J.; Ryu, K.; Badmaev, A.; De Arco, L. G.; Zhou, C. Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display Applications. Nano Lett. 2009, 9, 4285−4291. (7) Welsher, K.; Sherlock, S. P.; Dai, H. Deep-Tissue Anatomical Imaging of Mice Using Carbon Nanotube Fluorophores in the Second Near-Infrared Window. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 8943−8948. (8) Yomogida, Y.; Tanaka, T.; Zhang, M.; Yudasaka, M.; Wei, X.; Kataura, H. Industrial-Scale Separation of High-Purity Single-Chirality Single-Wall Carbon Nanotubes for Biological Imaging. Nat. Commun. 2016, 7, 12056. (9) Blackburn, J. L.; Ferguson, A. J.; Cho, C.; Grunlan, J. C. CarbonNanotube-Based Thermoelectric Materials and Devices. Adv. Mater. 2018, 30, 1704386. (10) Suzuki, D.; Ochiai, Y.; Nakagawa, Y.; Kuwahara, Y.; Saito, T.; Kawano, Y. Fermi-Level-Controlled Semiconducting-Separated Carbon Nanotube Films for Flexible Terahertz Imagers. ACS Appl. Nano Mater. 2018, 1, 2469−2475. (11) Bindl, D. J.; Wu, M.-Y.; Prehn, F. C.; Arnold, M. S. Efficiently Harvesting Excitons from Electronic Type-Controlled Semiconducting Carbon Nanotube Films. Nano Lett. 2011, 11, 455−460.

■

CONCLUSIONS We have investigated the effects of pH and Brij-S100 concentration on the charge state of SWCNTs in the ELF separation. The time course analysis showed that these environmental conditions gradually become inhomogeneous in the separation cell during the ELF process. The environmental dependence of the ζ potential revealed that the m- and s-SWCNTs carry different amounts of negative charge in the pH range and Brij-S100 concentration used in the separation cell, with s-SWCNTs being much more negatively charged. The relatively reduced negative charge on the m-SWCNTs is attributed to the selective hole doping on them. Besides the electrophoretic force, the electroosmotic force on the mSWCNTs also plays an important role in the ELF separation mechanism. From the above viewpoint, we propose that the ELF method utilizes the difference in the amount of negative charges between the m- and s-SWCNTs, which is conceptually unique compared with other conventional methods utilizing the difference in the interaction between SWCNTs and surfactant molecules for the separation of m- and s-SWCNTs. Furthermore, the ELF method may be extended from SWCNTs to the separation of other nanomaterials as a simple and low-cost method.

■

amount of SWCNTs in each fraction after the separation (PDF) Behavior of SWCNTs during separation by ELF method (AVI)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10192. Method for calculating the concentration of Brij S100 in the separation cell, Raman spectra of SWCNTs, Thermogravimetric analysis of pristine SWCNTs, and F

DOI: 10.1021/acs.jpcc.8b10192 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (12) Asada, Y.; Nihey, F.; Ohmori, S.; Shinohara, H.; Saito, T. Diameter-Dependent Performance of Single-Walled Carbon Nanotube Thin-Film Transistors. Adv. Mater. 2011, 23, 4631−4635. (13) Arnold, M. S.; Stupp, S. I.; Hersam, M. C. Enrichment of Single-Walled Carbon Nanotubes by Diameter in Density Gradients. Nano Lett. 2005, 5, 713−718. (14) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nanotechnol. 2006, 1, 60−65. (15) Tanaka, T.; Jin, H.; Miyata, Y.; Kataura, H. High-Yield Separation of Metallic and Semiconducting Single-Wall Carbon Nanotubes by Agarose Gel Electrophoresis. Appl. Phys. Express 2008, 1, 114001. (16) Moshammer, K.; Hennrich, F.; Kappes, M. M. Selective Suspension in Aqueous Sodium Dodecyl Sulfate According to Electronic Structure Type Allows Simple Separation of Metallic from Semiconducting Single-Walled Carbon Nanotubes. Nano Res. 2009, 2, 599−606. (17) Liu, H.; Nishide, D.; Tanaka, T.; Kataura, H. Large-Scale Single-Chirality Separation of Single-Wall Carbon Nanotubes by Simple Gel Chromatography. Nat. Commun. 2011, 2, 309. (18) Khripin, C. Y.; Fagan, J. A.; Zheng, M. Spontaneous Partition of Carbon Nanotubes in Polymer-Modified Aqueous Phases. J. Am. Chem. Soc. 2013, 135, 6822−6825. (19) Wang, J.; Nguyen, T. D.; Cao, Q.; Wang, Y.; Tan, M. Y. C.; Chan-Park, M. B. Selective Surface Charge Sign Reversal on Metallic Carbon Nanotubes for Facile Ultrahigh Purity Nanotube Sorting. ACS Nano 2016, 10, 3222−3232. (20) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. DNA-Assisted Dispersion and Separation of Carbon Nanotubes. Nat. Mater. 2003, 2, 338−342. (21) Tu, X.; Manohar, S.; Jagota, A.; Zheng, M. DNA Sequence Motifs for Structure-Specific Recognition and Separation of Carbon Nanotubes. Nature 2009, 460, 250−253. (22) Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J. Highly Selective Dispersion of Single-Walled Carbon Nanotubes Using Aromatic Polymers. Nat. Nanotechnol. 2007, 2, 640−646. (23) Ihara, K.; Endoh, H.; Saito, T.; Nihey, F. Separation of Metallic and Semiconducting Single-Wall Carbon Nanotube Solution by Vertical Electric Field. J. Phys. Chem. C 2011, 115, 22827−22832. (24) Sasaki, F.; Ihara, K.; Saito, T.; Nihey, F. Highly Pure Semiconducting Single-Wall Carbon Nanotubes Obtained by Stable Electric-Field-Induced Layer Formation. The 43rd Fullerenes-Nanotubes-Graphene General Symposium Sendai, Japan; 2012; p 85. (25) Numata, H.; Ihara, K.; Saito, T.; Endoh, H.; Nihey, F. Highly Uniform Thin-Film Transistors Printed on Flexible Plastic Films with Morphology-Controlled Carbon Nanotube Network Channels. Appl. Phys. Express 2012, 5, 055102. (26) Ohmori, S.; Ihara, K.; Nihey, F.; Kuwahara, Y.; Saito, T. Low Variability with High Performance in Thin-Film Transistors of Semiconducting Carbon Nanotubes Achieved by Shortening Tube Lengths. RSC Adv. 2012, 2, 12408−12412. (27) Hirano, A.; Tanaka, T.; Urabe, Y.; Kataura, H. pH- and SoluteDependent Adsorption of Single-Wall Carbon Nanotubes onto Hydrogels: Mechanistic Insights into the Metal/Semiconductor Separation. ACS Nano 2013, 7, 10285−10295. (28) Gui, H.; Streit, J. K.; Fagan, J. A.; Hight Walker, A. R.; Zhou, C.; Zheng, M. Redox Sorting of Carbon Nanotubes. Nano Lett. 2015, 15, 1642−1646. (29) Ding, J.; Li, Z.; Lefebvre, J.; Du, X.; Malenfant, P. R. L. Mechanistic Consideration of pH Effect on the Enrichment of Semiconducting SWCNTs by Conjugated Polymer Extraction. J. Phys. Chem. C 2016, 120, 21946−21954. (30) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Controlled Deposition of Individual Single-Walled Carbon Nanotubes on Chemically Functionalized Templates. Chem. Phys. Lett. 1999, 303, 125−129.

(31) Lewenstein, J. C.; Burgin, T. P.; Ribayrol, A.; Nagahara, L. A.; Tsui, R. K. High-Yield Selective Placement of Carbon Nanotubes on Pre-Patterned Electrodes. Nano Lett. 2002, 2, 443−446. (32) Opatkiewicz, J. P.; LeMieux, M. C.; Bao, Z. Influence of Electrostatic Interactions on Spin-Assembled Single-Walled Carbon Nanotube Networks on Amine-Functionalized Surfaces. ACS Nano 2010, 4, 1167−1177. (33) Abe, Y.; Tomuro, R.; Sano, M. Highly Efficient Direct Current Electrodeposition of Single-Walled Carbon Nanotubes in Anhydrous Solvents. Adv. Mater. 2005, 17, 2192−2194. (34) Seemann, L.; Stemmer, A.; Naujoks, N. Local Surface Charges Direct the Deposition of Carbon Nanotubes and Fullerenes into Nanoscale Patterns. Nano Lett. 2007, 7, 3007−3012. (35) Hu, H.; Bhowmik, P.; Zhao, B.; Hamon, M. A.; Itkis, M. E.; Haddon, R. C. Determination of the Acidic Sites of Purified SingleWalled Carbon Nanotubes by Acid−Base Titration. Chem. Phys. Lett. 2001, 345, 25−28. (36) Charlier, J. C. Defects in Carbon Nanotubes. Acc. Chem. Res. 2002, 35, 1063−1069. (37) Vichchulada, P.; Cauble, M. A.; Abdi, E. A.; Obi, E. I.; Zhang, Q.; Lay, M. D. Sonication Power for Length Control of Single-Walled Carbon Nanotubes in Aqueous Suspensions Used for 2-Dimensional Network Formation. J. Phys. Chem. C 2010, 114, 12490−12495. (38) Sekiguchi, A.; Tanaka, F.; Saito, T.; Kuwahara, Y.; Sakurai, S.; Futaba, D. N.; Yamada, T.; Hata, K. Robust and Soft Elastomeric Electronics Tolerant to Our Daily Lives. Nano Lett. 2015, 15, 5716− 5723. (39) Saito, T.; Ohshima, S.; Okazaki, T.; Ohmori, S.; Yumura, M.; Iijima, S. Selective Diameter Control of Single-Walled Carbon Nanotubes in the Gas-Phase Synthesis. J. Nanosci. Nanotechnol. 2008, 8, 6153−6157. (40) Hamada, N.; Sawada, S.; Oshiyama, A. New One-Dimensional Conductors: Graphitic Microtubules. Phys. Rev. Lett. 1992, 68, 1579− 1581. (41) Tanaka, K.; Okahara, K.; Okada, M.; Yamabe, Y. Electronic Properties of Bucky-Tube Model. Chem. Phys. Lett. 1992, 191, 469− 472. (42) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connell, M. J.; Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh, S.; et al. Reversible, Band-Gap-Selective Protonation of Single-Walled Carbon Nanotubes in Solution. J. Phys. Chem. B 2003, 107, 6979− 6985. (43) Zheng, M.; Diner, B. A. Solution Redox Chemistry of Carbon Nanotubes. J. Am. Chem. Soc. 2004, 126, 15490−15494.

G

DOI: 10.1021/acs.jpcc.8b10192 J. Phys. Chem. C XXXX, XXX, XXX−XXX