Direct Enrichment of Metallic Single-Walled Carbon Nanotubes by

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Letter

Direct Enrichment of Metallic Single-Walled Carbon Nanotubes by Using NO as Oxidant to Selectively Etch Semiconducting Counterparts 2

Qiangmin Yu, Chuxin Wu, and Lunhui Guan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02140 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry Letters

Direct Enrichment of Metallic Single-Walled Carbon Nanotubes by Using NO2 as Oxidant to Selectively Etch Semiconducting Counterparts Qiangmin Yu,a,b,c Chuxin Wu*a,b and Lunhui Guan*a, b a

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Fuzhou 350108, China, E-mail: [email protected], [email protected]; Fax:+86591-6317 3550; Tel: +86-591-6317 3550 b

Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Fuzhou 350108, China c

University of Chinese Academy of Sciences, Beijing 100049, China

Supporting Information ABSTRACT: We report an efficient method for enriching

high-purity metallic single-walled carbon nanotubes (mSWCNTs) by using NO2 as oxidant to remove semiconducting components at 220°C. After etching, the mSWCNTs with purity higher than 90% were obtained. The surviving m-SWCNTs remain intact structure, without any extra defects on their surface.

TOC Graphic:

As-grown single-walled carbon nanotubes (SWCNTs) usually exist as the mixtures of metallic SWCNTs (mSWCNTs) and semiconducting SWCNTs (s-SWCNTs) on the basis of their electronic structures.1-3 It is extremely important to obtain SWCNTs with unique conductivity, since it will pave the technological obstacles for their widespread applications, especially in nanoelectronics devices.46 Recently, various techniques have been applied for

achieving SWCNTs with only unique electronic structures by either in situ selective growth7,8 , novel catalyst design,9 or post separation approaches by taking advantage of differences in their physical or chemical properties.10 For the post separation methods, the first step towards the separation is to disperse SWCNT bundles in solvent with surfactant to form solution of individual SWCNTs, and then the SWCNTs were separated by alternating current dielectrophoresis,11 centrifugation12 or chromatography.13 These methods are time-consuming, and can only deal with a small amount of SWCNTs at great expense. Furthermore, the dispersions by surfactant introduce unwanted impurity on the surface of SWCNTs. Selective chemical etching is an alternative approach to enrich s-SWCNTs or mSWCNTs. However, the chemical functionalization or plasma treatment inevitably introduced many defects or impurities on the surface of SWCNTs, thus destroyed their intrinsic properties.14-16 It remains a challenge to develop a nondestructive method to separate SWCNTs in large quantity, especially for obtaining m-SWCNTs, which can endure ultrahigh current densities due to ballistic electron transport, as the reactivity of m-SWCNTs is higher than that of s-SWCNTs.17,18 Recently, Liu et al presented a scalable ex-situ method to etch s-SWCNTs by SO3 at 400°C, obtaining m-SWCNTs with 80% purity.19 However, after the reaction, there still remained ~20% s-SWCNTs, and the residual m-SWCNTs contained some defects. It is desirable to find an oxidant to selectively etch s-SWNTs, obtaining mainly m-SWCNTs with high conductivity. Theoretical calculation indicated that the adsorption energy of NO2 strongly relied on the electronic structure of nanotubes.20 It is a good guideline for the separation of nanotubes by using NO2 as oxidant. In this study, we demonstrate a simple approach that makes metallic SWNTs remarkably enriched through a classical one-step chemical

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reaction of carbon with NO2 : C+NO2→CO+NO at temperature as low as 220°C. The selective etching procedure is illustrated in Scheme 1. After the etching reaction, the samples are mainly enriched with metallic tubes without any increase of the defects on side walls of the SWCNTs. The enrichment of m-SWCNTs was evidenced by Raman spectroscopy, UV-vis-NIR absorption spectroscopy, and conductivity measurements. Raw high-pressure carbon monoxide (HiPCO) SWCNTs used in this work were commercial available from Carbon Nanotechnologies Inc. They were simply purified with dilute (18 %) HCl to remove most exposed Fe nanoparticles, and used as the starting material. For selective etching, the pristine-SWCNTs (10 mg) were transferred in a U-type quartz tube, and then NO2 gas was introduced to the tube.

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some SWCNTs, while leaving the electronic structures of the surviving SWCNTs unchanged. The high concentration of m-SWCNTs in the NO2 treated samples were evidenced by UV-vis-NIR absorption spectra, a powerful tool to characterize the electronic states, diameter distribution and chirality of SWCNTs. Figure 2a shows the absorption spectra of the pristine-SWCNTs (black curve) and NO2-treated SWCNTs (red curve). It displays the energy transitions between the van Hove singularities of the density of states: first interband transitions (M11, 1.9-2.3 eV) for m-SWCNTs, and first (S11, 0.6-1.2 eV) and second (S22, 1.2-1.9 eV) interband _____________________________________________

Scheme 1. Schematic illustration of SWCNTs separation by selectively etching s-SWCNTs using NO2 gas at 220°C. When the air in the tube was totally replaced by NO2, the quartz tube was heated at 220°C in oil bath. The reaction was kept for 16 h with continuous NO2 flowing. The obtain material was dried at 120°C under vacuum for 6 h. The final product weighed 1.2 mg. (See supporting information for more experimental details). Figure 1a shows optical images of the pristine-SWCNTs and the NO2-treated SWCNTs, both in solid forms and their dispersions in N, N-Dimethyl formamide (DMF). The pristine-SWCNTs appear typical black in color. After etching with NO2, the SWCNTs became prominent golden yellow in color. The dispersion in DMF appears brown, quite similar with that of the m-SWCNTs in DMF reported by previous results.21 Detailed scanning electron microscopy (SEM) images indicate that the NO2-treated SWCNTs became much fluffy compared with their pristine counterparts. The microstructure characterization also reveals that after etching, the NO2-treated SWCNTs maintain their tube morphology. X-ray photoelectron spectroscopy data (Fig. S2) further confirm the absence of defects or adsorbed NO2 molecules on side walls of SWCNTs after gas etching. The results imply that treating with NO2 selective destroyed

Figure 1. Optical images of the Pristine-SWCNTs and the NO2-treated SWCNTs both in solid forms and their dispersions in DMF. Enlarged are SEM images of the samples. transitions for s-SWCNTs.22-24 In comparison with the pristine-SWCNTs, most of the peaks ascribed to S11 and S22 of s-SWCNTs disappear after etching with NO2. Some peaks attributed to m-SWCNTs with small diameters also decrease dramatically, whereas the peaks of m-SWCNTs with larger diameters are well-retained. To further determine the diameter distribution, we performed assignments of the diameter range of the absorption spectra. Figure 2b shows the diameter distribution in different region (on the top scale, the diameter are calculated from the transition energies25,26). As shown in the S22 region for the NO2-treated SWCNTs, the peaks of s-SWCNTs with diameter from 0.9 to 1.1 nm disappear significantly. Only a small peak as-

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cribed to s-SWCNTs with diameter larger than 1.2 nm remains. SWCNTs with smaller diameters are preferentially etched over larger ones because of the higher radius of curvature and higher strain in the C-C bonding configuration. Meanwhile, the M11 peaks of the NO2-treated SWCNTs, especially those near 607 and 555 nm, ascribed to (9,9) and (8,8) tubes increase, indicating highly content of metallic tubes in the NO2-treated SWCNTs samples. A general model27 indicate a higher than 90% concentration of the mSWCNTs in the NO2-treated SWCNTs samples. To our best knowledge, it is the highest value ever reported for mSWCNTs obtained by selective etching method.

M-SWNTs ((9,9) (8,8)) for NO2-treated SWCNTs are stronger than that of the Pristine-SWCNTs. From Figure 3c with 532 nm laser excitation, small diameter m-SWNTs (8,5) located in higher RBMs were totally removed from the Pristine-SWCNTs, while the lower peaks ((15,3) (13,4)) are better retained. This implies that smaller m-SWNTs could be destroyed using the proposed method because of the higher radius of the ________________________________________________

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Figure 2. (a) UV-vis-NIR absorption spectra of the Pristine-SWCNTs (black line) and NO2-treated SWCNTs (red line) both dispersed in DMF. (b) Optical absorption spectra of the Pristine-SWCNTs (black line) and NO2-treated SWCNTs (red line) after abscissa transformation. The abscissa has been transformed from wavelength, in (a), to photon energy, in (b). These results demonstrate that our method etch s-SWCNTs from the pristine-SWCNTs with high efficiency, especially for the tube with diameters less than 1.2 nm. The high concentration of metallic, especially (8,8) and (9,9) tubes, were double-checked by resonant Raman scattering measurement, one of the most powerful characterization tools to demonstrate the separation efficiency. To identify the electric types and chirality of SWNTs etched by NO2 gas, three excitation energies, 785 nm (1.58 eV), 633 nm (1.96 eV) and 532 nm (2.33 eV) were chosen. The 785 nm excitation line is mainly resonance for s-SWCNTs, the 633 nm excitation line is resonance for both m- and sSWCNTs (in 1:1 ratio), and the 532 nm excitation line is mainly resonance with m-SWNTs within the range of RBM peaks from 150 to 280 cm-1.28,29 According to the radial breathing mode (RBM) spectrum analysis, the s-SWCNTs were almost destroyed by NO2 gas. The RBM peaks originating from m- and s-SWCNTs are highlighted according to the modified Kataura plot.30 As shown in Figure 3a, most of the s-SWCNTs peaks ((10,5) and (7,6)) of NO2-SWCNTs disappeared, and the peak of (12,5) was reduced drastically. Figure 3b presents RBM peaks of the Raman spectra for the pristine-SWCNTs and the NO2-treated SWCNTs samples with 633 nm laser excitation. The RBM peaks of the sSWCNTs are completely removed, while the RBM peaks of

Figure 3. Raman spectra at three excitation wavelengths (left: RBM regions; right G and D regions): (a) 785 nm (1.58 eV); (b) 633 nm (1.96 eV); (c) 532 nm (2.33 eV). In each spectrum, the black line and red line represent Pristine-SWCNTs and NO2-treated SWCNTs, respectively. The assignments of peaks can be found in Supplementary Table S5-S7. curvature and the strain in the C¬C bonding configuration.31 The enrichment of m-SWCNTs was also proven by analyzing the G-band of the samples (Figure 3 right panel); in the region, s-SWNTs are characterized by a Lo-renz curve, while m-SWNTs are characterized by a broad, low-energy Breit–Wigner–Fano tails. The Fano component in mSWNTs downshifts and broadens as the tube diameter decreases. The G-band of the pristine-SWCNTs and the NO2treated SWCNTs suggest that NO2 gas mainly destroyed sSWCNTs and m-SWNTs with diameters less than 1.1 nm,

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leaving the m-SWCNTs with larger diameters intact. Additionally, to demonstrate the well-structure of etched SWCNTs; the intensity ratio of G/D bands was introduced.25,32 The G/D value of NO2- treated SWCNTs samples decreased slightly than that of the pristineSWCNTs for all excitation energies, indicating that the defects in SWCNTs were not enhanced enormously during the etching process.Additionally, the resistivity of the pristineSWCNTs and the NO2-treated SWCNTs films were measured by using four-point probe techniques. The SWCNTs films were prepared by a vacuum filtration method (See Supporting Information for details).33 The sheet resistance

Figure 4. Transmittance versus sheet resistance for a series of transparent conductive films generated from NO2-treated SWCNTs and Pristine-SWCNTs at 550 nm wavelength. The inset images are transparent conductive films at different transmittance.

of the NO2-treated SWCNTs film is about 1500 Ω/□ at ~90% transmittance, which is much lower than that of the pristine-SWCNTs (~9000 Ω/□) at 90% transmittance. The results are similar with that of the monodisperse metallic SWCNTs prepared by the density gradient ultracentrifugation method24, 34, and in qualitative agreement with the optical absorption and Raman data. In summary, we demonstrate that semiconducting components in bulk SWCNTs can be selective etched by using NO2 as etchant. The direct gas-etching method at relative low temperature keeps the structures of the surviving metallic components unchanged. Based on the results of adsorption and Raman spectroscopy characterization, the purity of the m-SWCNTs reached over 90%. It paves a new way for the low-cost and scalable enrichment of m-SWCNTs, which are promising materials for nanodevice.

ACKNOWLEDGMENT This research was supported by National Natural Science Foundation of China (Grant no. 21171163, 21501174), the Science and Technology Planning Project of Fujian Prov-

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ince (Grant No. 2014H2008) and the strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA09010402).

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