Structural Recovery of Highly Oxidized Single-Walled Carbon

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Structural recovery of highly oxidized single-walled carbon nanotubes fabricated by kneading and electrochemical applications Joong Tark Han, Joon Young Cho, Jeong Hoon Kim, Jeong In Jang, Jun Suk Kim, Hye Jeong Lee, Jong Hwan Park, Ji Su Chae, Kwang Chul Roh, Wonki Lee, Jun Yeon Hwang, Ho Young Kim, Hee Jin Jeong, Seung Yol Jeong, and Geon-Woong Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00719 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Chemistry of Materials

Structural recovery of highly oxidized single-walled carbon nanotubes fabricated by kneading and electrochemical applications Joong Tark Han1,2,*, Joon Young Cho2, Jung Hoon Kim1, Jeong In Jang1, Jun Suk Kim3, Hye Jeong Lee1, Jong Hwan Park1, Ji Su Chae4, Kwang Chul Roh4, Wonki Lee5, Jun Yeon Hwang5, Ho Young Kim1, Hee Jin Jeong1, Seung Yol Jeong1,2 & Geon-Woong Lee1 1

Nano Hybrid Technology Research Center, Creative and Fundamental Research Division, Korea

Electrotechnology Research Institute (KERI), Changwon 51543, Republic of Korea 2

Department of Electro-Functionality Materials Engineering, University of Science and

Technology (UST), Changwon 51543, Republic of Korea 3

Department of Physics, Sungkyunkwan University, Suwon 16419, Republic of Korea

4

Energy Materials Center, Korea Institute of Ceramic Engineering & Technology (KICET), Jinju

52851, Republic of Korea 5

Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST),

Wanju 55324, Republic of Korea *Corresponding author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Harsh oxidation of 1 nm single-walled carbon nanotubes (SWCNTs) can lead to fatal defect structures, which can jeopardize their mechanical and electrical performances. Here, we show the structural recovery of highly oxidized SWCNTs (Ox-SWCNTs) which were rapidly (within 1h) oxidized by kneading without forming failure-causing defects inspired by flour dough. The rational oxidation of SWCNTs by kneading de-bundled the SWCNTs and led to obtain thermodynamically stable SWCNT solutions in water or even in alcohol without dispersant molecules. Importantly, the structure of the Ox-SWCNTs could be recovered by chemical, thermal, photothermal, or solvothermal reduction, enhancing the electrical conductivity of the Ox-SWCNT films from ~100 to ~1000 S cm−1. These Ox-SWCNTs and chemically reduced OxSWCNTs showed high performance in supercapacitors and Li-ion battery as electrochemical conductors, respectively.


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INTRODUCTION Surface functionalization of carbon nanotubes (CNTs) facilitates their dispersion in solution, without using a dispersant, by overcoming the van der Waals interactions between the nanoscale carbon materials1-3. Unfortunately, permanent functionalization of carbon materials can compromise some of their electrical properties4-6, while the solution processability permits their applications in electronic or electrochemical devices via solution-based fabrication. CNTs are usually oxidized by acid treatment with HNO3 or a HNO3–H2SO4 mixture at elevated temperatures to introduce oxidative moieties at the end of nanotubes for colloidal dispersion in aqueous solutions1-3. In particular, single-walled CNTs (SWCNTs) tend to be shortened and damaged under harsh oxidation conditions because their curved surfaces have higher reactivity than the graphene basal plane7–11. Strong KMnO4-based acid solutions have been used to fabricate graphene nanoribbons by shortening SWCNTs and unzipping through harsh oxidation. Therefore, the structural recovery of highly oxidized SWCNTs (Ox-SWCNTs) with fewer permanent defects and without shortening is a fascinating route for solution processing of SWCNTs for electrical or electrochemical applications, just like the method used to prepare reduced graphene oxide from graphite. Intensive studies have shown that graphene oxide can be deoxygenated to recover the electrical properties for subsequent application in electrical or electrochemical components12–18. However, there have been no reports on the reduction of highly oxidized SWCNTs because of their vulnerability to harsh oxidation conditions. In this study, we show the structural recovery of Ox-SWCNTs, particularly those fabricated by kneading; kneading is an extremely efficient method of oxidation which minimizes acid consumption and waste generation. Without using additional dispersant molecules, the OxSWCNTs could be dispersed in water, polar organic solvents, and even alcohol. The most


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important feature is that the structure of the Ox-SWCNTs could be significantly recovered by chemical, electrochemical, thermal, or photothermal reduction, which enhanced their electrical conductivity and electrochemical performance. The practical value of this strategy was demonstrated by the successful application of Ox-SWCNTs and reduced Ox-SWCNTs in electrochemical conductors.

EXPERIMENTAL SECTION Materials. Pristine SWCNT powder produced by chemical vapor deposition (TUBALL) was purchased from OCSiAl. Fuming nitric acid, NaClO3, KOH, dimethylformamide (DMF), butanol, isopropylalcohol, and 1-methoxy-2-propanol were used without further purification for the oxidation and dispersion of the carbon materials. Oxidation of SWCNTs by kneading. Oxidation of the SWCNT was carried out by kneading, mimicking the process of kneading flour-dough to make bread or noodles. The SWCNT powder was first mixed with an oxidant powder in the dry state; an acid solution was slowly added to the powder mixture to prepare it for kneading. For example, 2 g of the SWCNT powder was first mixed with various amounts of NaClO3 powder by drying blending to prepare different batches of the mixed powder. Next, 20 mL of the fuming nitric acid solution was slowly released in droplets onto a batch of the SWCNT–NaClO3 powder mixture. After that, the mixture was kneaded with a blade for several minutes by mimicking flour-dough preparation. Kneading for a long period is generally more efficient for homogeneous oxidization of the carbon material. After resting for several hours at room temperature, water was poured over the highly acidic carbon dough and stirred for less than 1 h, followed by centrifugal or filtration purification to remove the residual acid and oxidant. The oxidized SWCNTs were dispersed in various solvents by using a


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horn sonicator or homogenizer without using additional dispersant molecules. The process was repeated for the different batches of SWCNT–NaClO3 powder mixture containing different amounts of NaClO3. Chemical reduction of Ox-SWCNTs. Filtrated Ox-SWCNT papers were chemically reduced by 12 h of immersion in an aqueous HI acid solution at room temperature for 12 h. The solvothermal solution-phase reduction of Ox-SWCNTs was performed at 120 °C for 15 h by refluxing in DMF. Thermal reduction of Ox-SWCNTs and I–V characterization of Ox-SWNT field-emission transistor. For the fabrication of the Ox-SWNT field-emission transistor (FET), the Ox-SWCNT solution with concentration of 0.1 mg mL−1, dispersed in isopropyl alcohol, was spin-coated onto a 300 nm SiO2 silicon substrate. The coated wafer was then dried at room temperature without heating because the Ox-SWCNTs could be reduced by heat. These processes were repeated several times to form a percolated random network of Ox-SWCNTs. Conventional electron-beam lithography and the lift-off technique were used for the patterning of the metal electrode. The contact metal layers (Cr/Au, 5 nm/50 nm) were deposited by electron-beam and thermal evaporation at a pressure of 10−6 Torr. The defined channel length and width of the Ox-SWCNT FET were 5 and 2 μm respectively, which were later confirmed by examination under a scanning electron microscope (SEM). All device measurements for the Ox-SWNT device were carried out in a vacuum probe station under a pressure of 10−5 Torr and equipped with an electrical parameter analyser (4200, Keithley, USA). Thermal reduction of the Ox-SWCNTs was carried out by heating a sample stage inside the vacuum chamber at a temperature ramping rate of 10 °C/min.


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Photothermal reduction of Ox-SWCNTs. Filtrated Ox-SWCNT films were exposed to intense pulsed light (IPL) with a frequency of 1 Hz and different levels of energy density in the range of 3.6 to 9.6 J cm−2. A xenon flash lamp was used as the light source with a spectrum in the range of about 400-900 nm. The flashlight energy was adjusted by modulating the applied voltage, pulse on/off time, pulse duration, number of pulses, and pulse interval. Structural characterization. Field-emission scanning electron microscopy (FE-SEM; S4800, Hitachi, japan) and atomic force microscopy (AFM; DI-3100, Digital Instruments) were used to analyse the changes in the bundle structure of the SWCNTs before and after oxidation. Highresolution images were obtained with a transmission electron microscope (TEM; Titan G2 60300, FEI, USA) at an accelerating voltage of 80 kV. The oxidation levels of the SWCNTs and graphite were determined by X-ray photoelectron spectroscopy (XPS) (Multilab2000, Thermo VG Scientific Inc., USA).

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C solid-state nuclear magnetic resonance (13C NMR) spectroscopy

(DSX 400 MHz, Bruker, Germany) with magic-angle spinning was also utilized to confirm the functional groups on the carbon surface. The quantities of oxidative groups attached to the carbon surface were measured by performing thermogravimetric (TG) analysis (TGA; Q500, TA Instruments, USA) under ambient condition with ramping up the temperature at 5 ℃/min. The shear viscosity of the Ox-SWCNT paste was measured using a viscometer (DV-111 Ultra, Brookfield, USA). The electrical conductivity was measured by the four-point probe method (LORESTA MCP-T610, Mitsubishi Chemical Analytech, Japan) at room temperature. The structural characteristics of the pristine and oxidized carbon materials were investigated using a confocal Raman spectrometer (NTEGRA SPECTRA NT-MDT, Spectra Instruments, Ireland) with excitation wavelengths of 532 and 633 nm. The absorbance of light in the ultraviolet and


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visible ranges by the Ox-SWCNT was recorded on an ultraviolet–visible (UV–vis) spectrometer (Varian Cary 5000, Agilent, USA). Electrochemical analysis in aqueous electrolyte. The electrochemical properties of the samples (pristine-SWCNTs and Ox-SWCNTs) in an aqueous electrolyte were measured on a potentiostat (VSP, Bio-Logic, France) using a three-electrode system in a beaker-type cell at room temperature. The three-electrode cell consisted of a platinum (Pt) plate as the counter electrode, a Ag/AgCl electrode filled with a saturated aqueous KCl solution as the reference electrode, and a 1 M H2SO4 solution as the electrolyte. A piece of SWCNT buckypaper prepared by filtration (1.5 × 1.5 cm, electrode density: 0.44 g cm−3) was attached to the Ti-plate current collector using carbon ink. Cyclic voltammetry (CV) tests were performed within a potential window of 0.9 V, and the specific capacitance was calculated at scanning rates of 1 and 500 mV s −1. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 50 kHz to 50 MHz. The specific capacitance of each SWCNT film electrode was calculated from the voltammetric charge obtained from the CV curve according to Eq. (1): t

Cp

 i(t )dt = 0

E  m ,

(1)

where Cp, i, m, and ΔE are the specific capacitance of the film electrode, the instantaneous current in the cyclic voltammogram, the mass of the film, and the potential window (ΔE = E2 − E1), respectively. Electrochemical analysis in Li-ion battery. To prepare the battery electrode, pristine and OxSWCNT powders were dispersed in deionized H2O to obtain solutions with a concentration of


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0.1 mg mL−1, and the dispersions were sonicated for 20 min. Binder-free electrodes were prepared by the vacuum-filtration method on inorganic filter membranes (Whatman Anodisc, GE Healthcare, USA). After vacuum drying, free-standing electrodes were obtained by removing the SWCNT films. A coin-type half-cell (CR2032) was then assembled by using lithium metal foil as both reference and counter electrodes. The mass values of the current collector-free SWCNT electrodes ranged from 0.5 to 1.0 mg cm-2. A polypropylene membrane (Celgard, USA) was used as the separator, and 1.0 M lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate and dimethyl carbonate (3:7, v:v) was used as the electrolyte. The mass loading of the electrodes ranged from 0.5 to 1.0 mg cm−2. A galvano/potentiostat test station (VMP3, Biologic Science Instruments, USA) was used to charge and discharge the coin-type (2032) cell using a current of 100 mA g−1 between 1.5 and 4.5 V (versus Li/Li+). CV and EIS data were also obtained using the VMP3 test station. The voltage range for the CV tests was 1.5−4.5 V at a scanning rate of 1 mV s−1. The EIS data were obtained at frequencies of 1 MHz to 10 mHz. Symmetric cells comprising a pair of the as-prepared SWCNT electrodes, a separator, and an electrolyte were examined.

RESULTS AND DISCUSSION In our study, the commercial SWCNT powder synthesized by plasma-enhanced chemical vapour deposition was rationally oxidized by kneading, which was inspired by the process used to knead dough for making breads or noodles. This process does not require large volumes of harsh acid solutions and washing water. In a typical kneading process in our study, the SWCNT powder was mixed with NaClO3 powder at a weight ratio of 1:7. A small amount of fuming nitric acid was then added, then the mixture was kneaded for several minutes and left to rest for 1 h to minimize acid consumption and induce a fast reaction (Figure 1a). The acidic mixture of


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SWCNTs was then purified with water by centrifugation or filtration and subsequently freezedried to obtain an oxidized carbon powder (Figure S1). To minimize irreversible defect formation in the SWCNTs during oxidation, we used a mixture of NaClO3 and fuming nitric acid, which is milder than KMnO4-based oxidizing solutions6,19. The structure of the oxidized SWCNTs with fewer defects was then recovered by chemical, electrochemical, thermal, or photothermal deoxygenation, commonly used in the processing of graphene oxide12–18,20,21. Controlled oxidation of the SWCNTs, as shown in the XPS data of the Ox-SWCNTs (Fig. 1b), was carried out by varying the amount of oxidant, NaClO3, at a constant mixing ratio of SWCNT to fuming nitric acid (1 g:10 mL). The intensity of the XPS peak at ~287 eV, corresponding to the C–O bond related to hydroxyl or epoxide, gradually increased with increasing oxidant content, indicated by the increase in oxygen atoms in the survey scan (Figure S2). The 13C solid nuclear magnetic resonance (NMR) spectrum (Figure S2(c)) also showed the presence of primarily C–O bonds (53.8 ppm). Thermogravimetric analysis (TGA) data showed a gradual increase in the decomposition of oxidative groups attached to the SWCNTs at around 200 °C (Figure S3(a)). The decrease in the decomposition temperature from 650 to 450 °C indicates de-bundling of SWCNTs stacked by π–π interaction, which agrees with the scanning electron microscopy (SEM) images of Ox-SWCNTs showing a decrease in the bundle size (Figure S3(b)). After that, stable aqueous dispersions of Ox-SWCNTs were obtained (Figure 1c). The typical blue-shifted π–π* transition peak (~235 nm) of C–C and C=C bonds in the sp2-hybridized regions was observed in the ultraviolet–visible (UV–vis) spectra of the dilute (25 mg L−1) Ox-SWCNT, which is corresponding to that observed in typical graphene oxide solutions13. Importantly, the linear increase in UV–vis absorbance with increasing concentrations and the optical images of the Ox-SWCNT solutions (Figure S4) show that the dispersion was stable up to an Ox-SWCNT concentration of 150 mg/L, even in alcoholic solvents such as butanol, isopropyl alcohol,


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methoxy-2-propanol, as well as water and dimethylformamide (DMF). Moreover, the welldispersed 0.4 wt% Ox-SWCNT paste showed high viscosity, and shear-thickening behaviour was observed in the low-shear regime, even at low SWCNT concentration (Figure 1d). This indicates the inter-tube interaction of functional groups (OH groups) attached to the nanotube surface in the concentrated solutions. The production of these SWCNT-based inks and pastes suggest that the solution processing of SWCNTs is possible without consideration of dispersion issues. According to collision theory22, the rate of a reaction depends on the frequency of successful collisions in a given unit of time. A minimum amount of energy, called the activation energy of the reaction, is required to carry out the reaction of the molecules. The fast oxidation reaction observed in this study can be attributed to the high local concentration of oxidant molecules at the liquid–solid interface on the carbon-material surface. Moreover, kneading promotes diffusion of the oxidative solution into the carbon interface. Thus, it is worth noting that oxidation by kneading dramatically reduced the reaction time to less than 1 h and lowered the amount of acid used to one-seventh of that required in a conventional process. The latter result is especially remarkable because high acid consumption is a critical drawback of the oxidation processes of carbon materials used for purification or functionalization. The most interesting observation is that the Ox-SWCNTs prepared with a SWCNT/NaClO3 weight ratio of 1:7 (hereafter denoted by Ox-SWCNTs (1:7)) were still very long (Figure 2a). The bundle diameter and length are below 10 nm and over 10 µm from AFM and TEM images, respectively. This is in stark contrast to the SWCNTs which are shortened via oxidation by KMnO48. To further investigate the structural change of the nanotubes, Raman spectroscopy with a 633 nm laser was performed on filtrated SWCNT paper. The D band intensity at 1300 cm−1 and the noise level increased in the spectra of SWCNTs oxidized with larger amounts of NaClO3 (Figure 2b), indicating that the SWCNTs were oxidized by more


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oxidant. When the SWCNT/NaClO3 weight ratio was 1:5, the D band increased more steeply. These results agree with the XPS (Figure 1d) and TGA data (Figure S3). Moreover, increasing oxidation time exponentially promoted the oxidation of SWCNTs, resulting in a steep increase of the D band of Ox-SWCNTs (1:7) after 5 h of reaction (Figure 2c and S5). However, the intensity of the G′ band relative to the G band did not decrease even for highly oxidized SWCNTs, indicating that some pristine SWCNTs remained and were surrounded by Ox-SWCNTs in the SWCNT bundle as illustrated in Figure 2(a). To confirm this, high-resolution transmission electron microscope (TEM) images were obtained. Compared with the pristine SWCNTs shown in Figure 2d, the metal catalyst particles were removed and some pristine SWCNTs were still present, without severe cutting in the core of the bundle (Figure 2e). The observation of the oxidized SWCNT surface without cutting (Figure 2f) indicates that the combination of kneading and oxidation with NaClO3–fuming-nitric-acid yielded less destruction of the carbon materials and minimized the time consumed by the process. Moreover, the G band of the Ox-SWCNTs was up-shifted from 1581 to 1600 cm–1 due to charge transfer with the oxidative functional groups (Figure 2b), which corresponded to the dramatic increase in work function of the Ox-SWCNT film from 4.73 to 5.11 eV, as shown by ultraviolet photoelectron spectroscopy (Figure 3g and h). This result has meaningful implications for the application of Ox-SWCNT films as an interfacial layer in optoelectronic devices. Ox-SWCNTs have been utilized as p-type dopants for CVD-grown graphene because the oxidative functional groups on the SWCNT surface can withdraw electrons from the carbon material surface23. p-type doping by the charge transfer from graphene to Ox-SWCNTs led to a positive shift of 55 V for the threshold voltage (Vth) in the gate-dependent I–V characteristics of Ox-SWCNT-modified graphene (Figure 2i). The electrical resistivity of a graphene film also decreased when its surface


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was treated with an Ox-SWCNT overcoating (Figure 2j). These results indicate that the SWCNT surface was highly oxidized by our kneading process with minimizing cutting. To demonstrate the structural recovery of Ox-SWCNTs, the Ox-SWCNT papers prepared by filtration were first chemically reduced by dipping in a HI acid solution at room temperature. The ratio of intensities of the D and G bands (ID/IG) in the Raman spectrum of Ox-SWCNTs prepared with a SWCNT/NaClO3 weight ratio of 1:7 (hereafter denoted by Ox-SWCNTs (1:7)) dramatically decreased from 0.7 to 0.3 after chemical reduction by the HI acid (Figure 3a). This drop corresponds with the eight-fold increase in electrical conductivity of the Ox-SWCNT (1:7) sheet from 91 to 709 S cm−1 after chemical reduction (Figure 3b), indicating the structural recovery of Ox-SWCNTs. The conductivity of the reduced Ox-SWCNT (1:5) sheet was as high as 1000 S cm−1. The recovery of radial breathing mode peaks also demonstrates the structural recovery of Ox-SWNCTs after chemical reduction (Figure S6). Furthermore, the intact structure of core SWCNTs in bundles was demonstrated by decomposing highly oxidized SWCNTs via intense irradiation by a 532 nm laser in the Raman spectrometer. As a result, the D band disappeared, and the G band shifted from 1582 to 1573 cm−1 in the Raman spectrum. The XPS spectrum of the reduced Ox-SWCNTs clearly shows deoxygenation through chemical reduction by the HI acid (Figure 3c and Table S1). These results strongly indicate that our method minimized the cutting of SWCNTs, and oxidative functional groups were detached from the surface by reduction. This is the most important aspect of the application of solution processed Ox-SWCNTs as an electrical or electrochemical conductor. Other reduction approaches, namely electrochemical, thermal, photothermal, and solvothermal processes, were also used to recover the sp2 carbon structures on the nanotube surface. The reduced Ox-SWCNTs were examined by cyclic voltammetry (CV; Figure 3d), XPS (Figure 3e), current–voltage (I–V) characterization (Figure 3f–h and Figure S7), and Raman


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spectroscopy (Figure S7). The cyclic voltammetry (CV) curves of the Ox-SWCNT working electrode show the electrochemical reduction of Ox-SWCNTs (Figure 3d). The redox peak at 1.9 V related to C–O bonds disappeared after the second cycle, and the redox peak at 2.6 V related to C=O bonds remained. This clearly shows that the C–O bonds dissociated during the electrochemical process, and the C=O or COOH bonds with high binding energy were stable, which was confirmed by the XPS results (Figure 3e). Another way to deoxygenate oxidized carbon materials is by thermal treatment in vacuum. The electrical current of Ox-SWCNT bundles were monitored by thermal treatment with holding durations of 30 min at elevated temperatures under a vacuum of 10−5 Torr. As shown in the plot of source-to-drain current vs. source-to-drain voltage (Ids–Vds) (Figure 3f), the electrical resistance of the Ox-SWCNT bundle dramatically decreased at 425 K, which means that deoxygenation of Ox-SWCNTs took place. In addition, Ox-SWCNT field-effect transistors (FETs) were fabricated and their current–voltage (I– V) characteristics revealed the carrier-transport phenomenon on the Ox-SWCNTs. The Ids vs. gate-to-source voltage (Ids–Vgs) curve in Figure 3g shows that the Ox-SWCNTs exhibited typical p-type characteristics at 300 K, with Vth located in the region of positive gate bias owing to the presence of oxygen moieties. As the temperature increased, Vth dramatically shifted by over 50 V to the negative gate bias region at 475 K, indicating that the Fermi level of the Ox-SWCNTs shifted from the valence band to the conduction band as a result of deoxygenation24-26. If oxidative functional groups on the nanotube surface remained, the p-type characteristics of nanotubes would have remained even at elevated temperature owing to charge transfer. After exposure of the thermally reduced SWCNTs to air, the value of Vth was up-shifted via p-doping by O2 in air at room temperature (Figure 3h)27. In the third alternative approach, the Ox-SWCNT film was reduced by intense pulsed light (IPL) via a photothermal process (Figure S7). These


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results also support that our oxidation process is highly efficient to rationally oxidized vulnerable SWCNTs by kneading. To investigate whether the characteristics of Ox-SWCNTs and reduced Ox-SWCNTs impart useful properties, we used the Ox-SWCNTs and reduced Ox-SWCNTs as supercapacitors and cathodes in Li-ion batteries, respectively. First, the electrochemical performance of OxSWCNTs in an aqueous electrolyte (1 M H2SO4) was characterized. Analysis of the CV curves showed that the Ox-SWCNTs had a big impact on the overall specific capacitance of the SWCNT film through the combination of the electric double-layer capacitance and pseudo-capacitance (Figure 4a and b). This was possible owing to the oxidative functional groups attached to the SWCNT surface and the high electrical conductivity of the Ox-SWCNT film even after oxidation. Moreover, compared to the high impedance of pristine SWCNTs, the stable and low impedance of densely packed Ox-SWCNT papers clearly illustrates the beneficial effect of the highly conducting Ox-SWCNTs with small bundle size on the supercapacitor performance (Figure S8). Next, we examined the electrochemical performances of Li-ion batteries employing OxSWCNTs (1:7) and reduced Ox-SWCNTs as cathode materials. The performance of SWCNT buckypaper was examined by preparing coin-type half-cells, in which Li metal served as the counter and reference electrodes. Upon oxidation, the initial gravimetric capacity of the OxSWCNT cathode was markedly enhanced from that of the pristine SWCNT cathode (Figure 4c) owing to Faradaic reactions (Figure 4d), while the double-layer capacitance was dominant in the pristine SWCNTs (Figure 4e). The pristine SWCNT electrode exhibited an initial discharge capacity of ~50 mAh g−1, whereas the Ox-SWCNT electrode showed a remarkably enhanced initial discharge capacity of ~450 mAh g−1 at a current density of 0.1 A g−1, probably owing to the increase in oxidative functional groups and the high surface area of the Ox-SWCNT electrode resulting from the de-bundling of SWCNTs during oxidation. However, capacity fading of the


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Ox-SWCNT electrode originated primarily from the degradation of C–O groups of Ox-SWCNTs during cycling, as previously shown in Figure 3d. Therefore, solvothermal reduction of OxSWCNTs was also performed by refluxing in dimethylformamide (DMF)28. The resulting dramatic drop in intensity of the C–O peak in the C 1s XPS spectrum demonstrates that the hydroxyl groups on the nanotube surface could be easily removed without sacrificing the C=O groups (Figure 4f). The CV curves of the reduced Ox-SWCNT electrode with different upper and lower potential limits indicate that the Li ions could be reversibly stored on the surface of the reduced Ox-SWCNTs by both Faradaic reactions and double-layer charging (Figure 4g); the reduced Ox-SWCNTs exhibited a Faradaic capacitance of 35 F g−1 and double-layer capacitance of 97 F g−1. The reduction peak at 2.6 V indicates that, as previously reported29, the Faradaic reactions occurred between the Li ions and carbonyl (C=O) groups of the reduced Ox-SWCNT electrode. However, the pristine SWCNT electrode showed a total capacitance of 58 F g−1, which is attributed almost entirely to the double-layer capacitance (Figure 4e). The charge–discharge behaviour of the reduced Ox-SWCNT electrode was more stable than that of the Ox-SWCNT electrode: the shapes of the charge–discharge curves of the reduced Ox-SWCNT electrode hardly changed with increasing cycle number. Moreover, the electrochemical impedance spectroscopy data confirmed that the charge-transfer resistance of the reduced Ox-SWCNT electrode was much lower than that of the Ox-SWCNT electrode (Figure 4h). The reduced Ox-SWCNT electrode delivered a reversible initial capacity of 180 mAh g−1, which is 3.6 times that of the pristine SWCNT electrode, and it exhibited a capacity loss of only 2.5% after 100 cycles (Figure 4c). The rate performance also improved: a specific capacity of ~25 mAh g−1 was observed at 25 A g−1 (~130C) owing to the enhanced kinetics in Li-ion storage on the SWCNT surface (Figure S9).


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CONCLUSIONS Solution processability and structural controllability of carbon materials are prerequisite for these applications. Our results demonstrate that remarkable electrical or electrochemical performances were obtained after deposition of SWCNT inks or pastes that were dispersed in water and various organic solvents (without additional dispersant). It is equally important that SWCNTs were oxidized by kneading with NaClO3 and fuming nitric acid to minimize defect formation and acid consumption. But most important of all, Ox-SWCNTs were successfully deoxygenated by chemical, electrochemical, thermal, solvothermal, or photothermal reduction to recover their sp2 carbon structures. The electrical conductivity, Raman analysis, and XPS data all demonstrate the structural recovery of highly oxidized SWCNTs without compromising their electrical or electrochemical performances. Supercapacitors and LIB cathode electrodes using Ox-SWCNTs and reduced Ox-SWCNTs showed high electrochemical performances compared to those employing pristine SWCNTs. The high solution dispersibility and structural recovery of Ox-SWCNTs are expected to facilitate their application as electrical or electrochemical components in next-generation devices. We also anticipate that kneading can be applied to largescale fabrication of oxidized carbon materials.

ASSOCIATED CONTENT Supporting Information These materials are available free of charge via the Internet at http://pubs.acs.org. This includes photographs of oxidation of SWCNTs by kneading, additional characterization data of OxSWCNTs with thermogravimetric, spectroscopic and microscopic analysis, and electrochemical performances.


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AUTHOR INFORMATION Corresponding Authors [email protected] Author Contributions J.T.H. organized the experiments, supervised the synthesis of materials and wrote the manuscript. J.Y.C. and J.H.K. synthesized and characterized the materials. J.S.K performed the FET measurements. H.J.L and H.H.P. performed the Li-ion battery experiments. J.S.C and K.C.R. characterized the electrochemical properties. H.Y.K. and M.J.K characterized the materials. H.J.J., S.Y.J., and S.H.S supervised the XPS and Raman measurements and analysed the XPS spectra for the chemical structure. G.-W.L. supervised the synthesis and characterization of the materials. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This research was supported by the Center for Advanced Soft-Electronics as Global Frontier Project (2014M3A6A5060953) funded by the Ministry of Science, ICT, and by the Primary Research Program (19-12-N0101-23) of the Korea Electrotechnology Research Institute. ABBREVIATIONS Ox-SWCNTs, highly oxidized single-walled carbon nanotubes;

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Figure captions: Figure 1. (a) Scheme of oxidation of SWCNTs via a direct mixing with oxidant powders and strong acids, followed by kneading of the SWCNTs dough. Their structures can be recovered by chemical, electrochemical, solvothermal, thermal, photothermal deoxygenation methods. (b) XPS spectra of rationally oxidized SWCNTs with different amounts of oxidant (NaClO3) relative to the SWCNTs. (c) UV–vis spectra of the aqueous Ox-SWCNT solution with a concentration of 25 mg L−1, dispersed without dispersant molecules. (d) Shear viscosity of Ox-SWCNT paste (0.4 wt%).

Figure 2. (a) Atomic force microscope image of the Ox-SWCNT film showing long SWCNT bundles (indicated by arrows). (b) Raman spectra of Ox-SWCNTs prepared by 1 h of oxidation; the Ox-SWCNTs were prepared different NaClO3/SWCNT weight ratios, indicated by the numbers above the spectra, in the kneading dough. The Raman peak intensity was normalized against the G band intensity. (c) Ratio of the intensity of D and G bands (ID/IG) in the Raman spectra of Ox-SWCNTs (1:7) prepared after different durations of oxidation (Supplementary Fig. 6). (d-f) TEM images of pristine SWCNTs (d) and Ox-SWCNTs (e, f). The insets in (d) and (e) show electron energy loss spectroscopy images of pristine SWCNTS and Ox-SWCNTs, indicating the attachment of oxygen functional groups. The high-resolution TEM image in (f) shows the surfaces of oxidized nanotubes (indicated by the arrow). (g) Ultraviolet photoelectron spectroscopy data of pristine and Ox-SWCNTs; the number shown above each plot indicates the weight ratio between NaClO3 and SWCNTs. (h) Changes in work function of SWCNTs with increasing amount of NaClO3 used as an oxidant. (i) Gate-dependent I–V characteristics (drain voltage: 0.01 V) and (j) I–V plots (gate voltage: 0 V) of graphene synthesized by chemical vapor


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deposition (‘Graphene’) and graphene overcoated with Ox-SWCNTs (‘Graphene + OxSWCNTs’).

Figure 3. (a) Raman spectra of pristine SWCNTs, Ox-SWCNTs (1:7), HI-reduced Ox-SWCNTs (1:7), and Ox-SWCNTs after exposure to 532 nm laser; the plots on the right show the positions of the G-band in the spectra of the samples. (b) Electrical conductivity of Ox-SWCNT buckypaper before and after reduction by HI acid at room temperature. (c) XPS spectrum of OxSWCNTs reduced by HI acid. (d) Cyclic voltammograms of Ox-SWCNT electrode from the first to the third cycle; metallic lithium was used as the reference/counter electrode, and 1.0 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate–dimethyl carbonate was used as the electrolyte solution. (e) Deconvoluted C 1s XPS spectrum of Ox-SWCNTs after the 3rd CV cycle; the strong C=O bond peak originated from the used carbonate compound. (f) I–V plots (gate voltage: 0 V) and (g) gate-dependent I–V characteristics (drain voltage: 0.1 V) of Ox-SWCNT (1:7) bundles measured after thermal treatment at various temperatures for 30 min under a vacuum of 10−5 Torr; the measurements were performed at elevated temperatures. The SEM image in the inset of (f) shows the SWCNT bundle network (scale bar: 2 µm). (h), Gatedependent I–V characteristics of Ox-SWCNTs (1:7) which were thermally treated at 425 K after exposure to air.

Figure 4. (a) Cyclic voltammograms (CVs) of Ox-SWCNT (1:7) paper obtained at scanning rates of 1–500 mV s−1. (b) Specific capacitance of pristine SWCNT and Ox-SWCNT (1:7) papers as a function of the scanning rate. (c) Specific capacity retention of pristine SWCNT, Ox-SWCNT


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and solvothermally reduced Ox-SWCNT electrodes. (d, e) Cyclic voltammograms of OxSWCNTs (d) and pristine SWCNTs (e); the yellow-coloured region represents the redox reaction between Li+ and oxygen functional groups. (f) XPS spectrum of Ox-SWCNTs reduced by refluxing in DMF at 120 °C. (g) Cyclic voltammograms with different potential windows of reduced-SWCNTs which were solvothermally deoxygenated by DMF. In the CV curves, the examined potential windows were 3.0–4.2 V vs. Li/Li+ (red), 1.5–3.0 V vs. Li/Li+ (blue), and 1.5–4.2 V vs. Li/Li+ (black) at a scanning rate of 1 mV s−1. (h) Nyquist plots of Ox-SWCNT and reduced Ox-SWCNT electrodes.


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Figure 1. Han et al.


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Structural Recovery of Highly Oxidized Single-Walled Carbon

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