In Situ Electrochemical Raman Spectroscopy of Air-Oxidized

Article pubs.acs.org/JPCC

In Situ Electrochemical Raman Spectroscopy of Air-Oxidized Semiconducting Single-Walled Carbon Nanotube Bundles in Aqueous Sulfuric Acid Solution Shin-ichi Ogino,†,‡ Takashi Itoh,§ Daiki Mabuchi,† Koji Yokoyama,† Kenichi Motomiya,† Kazuyuki Tohji,† and Yoshinori Sato*,†,∥ †

Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki, Aoba-ku, Sendai 980-8579, Japan Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8579, Japan ∥ Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research, Shinshu University, Asahi 3-1-1, Matsumoto 390-8621, Japan §

S Supporting Information *

ABSTRACT: In this study, we oxidized approximately 90% semiconducting, highly crystalline single-walled carbon nanotube (hc-SWCNT) bundles in the atmosphere at 450 °C for 30 min to obtain SWCNTs modified with oxygen-containing functional groups and investigated not only the influence of air oxidation on the electrochemical doping of the air-oxidized SWCNT (AOSWCNT) bundles in aqueous sulfuric acid solution using in situ Raman spectroscopy, but also the relationship between the in situ electrochemical Raman data and the properties of electric double-layered supercapacitors (EDLSCs). By oxidizing the hcSWCNTs in air, AO-SWCNTs with a small diameter distribution could be prepared. When a negative charge was applied to the AO-SWCNTs used as a working electrode in a three-electrode electrochemical cell for in situ Raman spectroscopy, a large downshift of the G+ line of the AO-SWCNTs was observed compared to that before air oxidation. On increasing the ratio of small-diameter nanotubes/total nanotubes, the Raman data obtained in situ revealed that the effect of the weakening of the C−C bond was stronger than that of the renormalization of the phonon energy. In contrast, in the case of applying a positive charge to the AO-SWCNTs, the magnitude of the upshift of the G+ line for the AO-SWCNTs was slightly larger than that for the hcSWCNTs. The influent electric charges per unit mass and the specific capacitances of the AO-SWCNT electrodes for the maximum magnitude of the shift of the G+ line (10.7 cm−1) were 60.1 C/g and 50.1 F/g, respectively, which are larger than those of hc-SWCNT electrodes. In situ Raman spectroscopy is a useful method to simultaneously assess the increase or decrease in the diameter distribution of small nanotubes and the specific capacitances of electric double-layered supercapacitors of chemically functionalized SWCNTs by the magnitude of the shift of the G+ line compared to unfunctionalized SWCNTs. capacitance of the carbon materials1 (i.e., the electronic density of state (DOS) of the carbon materials). Changes in these factors also affect the strength of the C−C bond.2 Furthermore, the renormalization of the phonon energy depends on the strength of the electron−phonon interaction, as the carrier

1. INTRODUCTION The charging of an electric double-layered supercapacitor (EDLSC) corresponds to the simultaneous polarization of both the positive and the negative carbon material electrodes. During this polarization, injection and ejection of electrons occur at the negative (anode) and the positive (cathode) electrodes, respectively. The injection and ejection of electrons lead to a change in the number of mobile charge carriers. This charging electrode affects the electronic conductivity and space charge © XXXX American Chemical Society

Received: December 9, 2015 Revised: March 16, 2016

A

DOI: 10.1021/acs.jpcc.5b12057 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

always corresponds to an upshift in the G+ line. In fact, Kalbac et al. reported that a SWCNT with a diameter as small as 1.37 nm31 or bundles of SWCNTs with diameters of 1.32−1.40 nm36 exhibited a downshift in the G+ line for negative charging and an upshift in the G+ line for positive charging. Recently, it was reported that metallic SWCNTs are easier to oxidize than semiconducting ones and that small-diameter SWCNTs can be oxidized more readily than large-diameter ones.37 In addition, it is known that the DOS of SWCNTs changes after chemical modification.11−15 Since in situ Raman measurements can be used to investigate the physical properties of chemically functionalized SWCNTs as well as their DOS, the obtained results should help to elucidate whether or not chemically modified SWCNTs improve the characteristics of the polarized electrodes used in EDLSCs. Here, we oxidized bundles of approximately 90% semiconducting, highly crystalline SWCNTs in the atmosphere at 450 °C for 30 min to obtain SWCNTs modified with oxygen-containing functional groups. We investigated not only the influence of air oxidation on the electrochemical doping of the air-oxidized SWCNTs using in situ Raman spectroscopy, but also the relationship between the in situ electrochemical Raman data and the properties of EDLSCs.

density of single-walled carbon nanotubes (SWCNTs) also changes.3 Given the high specific surface area, carrier mobility, and conductivity of carbon nanotubes (CNTs), SWCNTs have been investigated for use as polarized electrodes in EDLSCs.4−9 In chemically functionalized SWCNTs, it is reported that the specific capacitances are larger than those of unfunctionalized SWCNTs.8−10 The reason for this is believed to be the greater electrolyte ion adsorption on the surface of functionalized SWCNTs as a result of the improved wettability of the surface of SWCNTs. The DOS of SWCNTs changes with chemical functionalization,11−15 and the injection and ejection of electrons into and from the functionalized SWCNTs lead to further change in the DOS, which is inferred to have an effect on the properties of the EDLSCs. In addition, the specific surface area of SWCNTs changes on varying the diameter distribution of nanotubes by chemical functionalization, and the specific capacitances of the EDLSCs will change as a consequence. In situ Raman spectroelectrochemistry is an established method for investigating changes in the physical properties of SWCNTs during electrochemical charging in an aqueous or nonaqueous electrolyte solution.16−36 Changes in the C−C bond length and the renormalization of the phonon energy of SWCNTs during charging reflect the frequency shift of the Raman lines in the tangential displacement (TG) mode. On the other hand, when the DOS of the SWCNTs changes, which exhibits a resonant Raman effect involving electronic transitions between the π valence and π* conduction bands, it is revealed that the intensity of the Raman lines in the TG mode changes. Thus, the resonant Raman intensity depends strongly on the position of the Fermi level within the DOS. In fact, most research groups studying this topic report that the positive or negative doping of SWCNTs causes an upshift or downshift, respectively, of the G+ line of the TG mode; the exception are Wang et al.23−25 who reported no change in the D and G+ lines with the potential over the entire range of applied voltages. Changes in both the C−C bond length and the phonon energy renormalization process determine the behavior of the G+ line of the TG mode of semiconducting SWCNTs during electrochemical charging. The magnitude and direction (upshift and downshift) of the vibrational frequency of the TG mode has been found to be dependent on the diameter of the semiconducting SWCNTs.31,36 In the case of the negative charging of SWCNTs, the small-diameter SWCNTs exhibit a downshift, while the large-diameter SWCNTs show an upshift.31,36 This behavior, explained by the upshift effect of the renormalization of the phonon energy and the downshift effect of the weakening of the C−C bond, causes the G+ line to exhibit opposite signs, which may cancel each other. The weakening of the C−C bond leads to a general downshifting of the G+ line due to the filling of the antibonding orbitals. In contrast, the renormalization of the phonon energy leads to a general upshifting of the G+ line, and this upshift is larger for large-diameter tubes (≈2.4 nm).3 Thus, for smalldiameter tubes, the G+ line downshifts because the effect of the weakening of the C−C bond is stronger than that of the renormalization of the phonon energy. Further, for largediameter tubes, the G+ line upshifts because the effect of the weakening of the C−C bond becomes weaker than that of the renormalization of the phonon energy. On the other hand, positive charging leads to a strengthening of the C−C bond and the consequent upshifting of the G+ line. The shift in the G+ line due to the renormalization of the phonon energy is independent of the type of charging (negative or positive) and

2. EXPERIMENTAL SECTION 3.1. Sample Preparation. We synthesized SWCNTs by the arc discharge method using a mixture of Fe/Ni particles as the metal catalyst.9,38 In order to remove any amorphous carbon, 1.0 g of the as-formed soot was burned in air at 450 °C for 30 min. The resulting burnt soot was then burned again in air at 500 °C for 30 min by continuous air oxidization. Next, the oxidized soot was introduced into a flask containing a 6.0 mol/ L aqueous hydrochloric acid solution in order to dissolve the metal particles. The resultant suspension was filtered using a polytetrafluoroethylene (PTFE) membrane filter with an average pore diameter of 0.1 μm, and the filtered cake was washed with purified water. The resulting sample was dried in air at 60 °C for 12 h. Following this procedure, the dried material was again burned in air at 500 °C for 30 min and treated with a 6.0 mol/L aqueous hydrochloric acid solution, in order to remove the remaining amorphous carbon and catalytic metal particles, respectively. Finally, the resulting suspension was filtered using a PTFE membrane filter with an average pore diameter of 0.1 μm, and the filtered cake was washed with purified water. The resulting sample was dried in air at 60 °C for 12 h and then dried in vacuum at 200 °C for 24 h. The purified SWCNTs were annealed in high vacuum, in order to remove the attached functional groups and restore the hexagonal graphene network. The annealing procedure was as follows: 150 mg of the resultant sample was put into a graphite crucible, which was placed on a graphite plate around a C/C composite heater. After the chamber had been evacuated, the sample was heated under high vacuum (2.0 × 10−5 Pa) at 1200 °C for 3 h. The resulting SWCNTs are referred to as “hcSWCNTs”. Next, chemically functionalized SWCNTs were prepared by air oxidation. The hc-SWCNTs (50 mg) were placed in an aluminum oxide boat. The aluminum oxide boat was then placed at the center of a quartz tube (inner diameter of 24 mm), which was kept at the center of a tubular electric furnace. The resulting materials were annealed in a flow of dry air (400 mL/min) at 450 °C for 30 min. The weight loss of the materials was 5.5 mg. The functionalized SWCNTs are hereafter referred to as “AO-SWCNTs.″ B


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Figure 1. (a) HRTEM image and (b) bundle diameter distribution of the hc-SWCNTs. (c) HRTEM image and (d) bundle diameter distribution of the AO-SWCNTs.

3.2. Structural Characterization. The morphologies of the two samples were determined using scanning electron microscopy (SEM) (S-4100, Hitachi, Japan) and highresolution transmission electron microscopy (HRTEM; HF2000, Hitachi, Japan); both imaging systems were equipped with a field emission gun. The SEM and HRTEM systems were operated at 5 and 200 kV, respectively. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS-ULTRA system (Shimadzu Ltd., Japan) with a monochromatized Al Kα radiation source. The Brunauer−Emmett−Teller specific surface areas were measured using a NOVA 1200 porosimeter (Quantachrome Instruments, U.S.A.) on the basis of N2 adsorption at −196 °C. The contact angles of the samples were measured using a contact angle meter (DM-500, Kyowa Interface Science Co., Ltd., Japan). The visible-near-infrared (Vis-NIR) spectra were measured using a UV−vis spectrometer (U-3900, Hitachi, Japan) and a FT-IR/NIR spectrometer (Frontier, PerkinElmer, U.S.A.). Transmissive SWCNT thin films were prepared by the spraying method.39,40 3.3. In Situ Electrochemical Raman Spectroscopy. A specially designed electrochemical cell (Figure S1) was used for in situ Raman spectroscopy in this study.41 The cell was manufactured from polychlorotrifluoroethylene and was equipped with a sapphire plate for optical measurements and three electrode ports for electrochemical measurements. Thin films of the SWCNTs formed on glassy carbon (GC, Tokai Carbon Co., Ltd., Japan) were used as the working electrodes. The potential of the working electrode was controlled using a Ag/AgCl (saturated KCl) reference electrode. First, the surface of the GC electrode was cleaned by polishing the sample successively with 0.05- and 0.1-μm-diameter alumina slurries. This procedure was followed by acid etching in a 10% aqueous nitric acid solution and ultrasonification in Milli-Q water. Next, thin films of the SWCNTs were fabricated on the GC

electrodes by the drop-casting method. A suspension of the SWCNTs in ethanol was formed and 70 μL of this suspension was dropped on the GC electrode. The suspension was dried at 60 °C in an electric oven for 15 min. Its weight was determined using an electromicrobalance (UMX2, Mettler Toledo, U.S.A.). The effective area of the electrode in contact with the electrolyte was 0.20 cm2. An aqueous sulfuric acid solution was used as the electrolyte; it was prepared from high-purity sulfuric acid (special grade, Wako Pure Chemical Industries, Ltd., Japan) and Milli-Q water. The electrolyte was purged with pure argon gas (99.999%, Taiyo Nippon Sanso Co., Japan) for 30 min to eliminate air before the cyclic voltammograms (CV) and in situ Raman spectra were measured. The electrolyte used for the experiments was a 3.5 mol/L aqueous sulfuric acid solution, since this concentration corresponds to a high electric conductivity (approximately 0.73−0.75 S/cm). According to theoretical calculations of the acid dissociation constant, the [H+], [HSO4−], and [SO42−] values were 4.6, 2.3, and 1.1 mol/ L, respectively.42 Raman measurements of a 3.0 mol/L aqueous sulfuric acid solution yielded the values of [HSO4−] and [SO42−], which were 2.0 and 1.0 mol/L, respectively.43 In a 3.5 mol/L aqueous sulfuric acid solution, the [HSO4−] value is twice that of [SO42−]. CV measurements were performed using a specially designed electrochemical cell at a scan rate of 10 mV/s between −0.2 and +1.0 V. The CV curve of the SWCNT electrodes shows higher currents at all the potentials (Figure S2a) compared to that of the bare GC electrode as a supporting substrate for the SWCNT electrodes, indicating that the electrochemical data corresponding to the SWCNTs were accurate. The CV curves were almost similar to those obtained using a conventional electrochemical cell (Figure S2b).33 Therefore, the specially designed electrochemical cell used in this study was found to be suitable for spectroelectrochemistry. C


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The Journal of Physical Chemistry C For the in situ Raman spectroelectrochemistry analyses,41 an argon ion laser (Innova 300, Coherent, Inc., U.S.A., 488.0 nm, 100 mW) was focused on the surface of the working electrode at an angle of approximately 60°; the spot size was approximately 0.1 mm × 1.0 mm. The optical window of the electrochemical cell allowed light scattered from the electrode surface to be transmitted, was collected by an achromatic lens and focused on the entrance slit of a single-stage spectrometer (iHR320, Horiba Jobin-Ybon, Japan); Rayleigh scattering was prevented by using notch filters (Super Notch Plus, Kaiser Optical Systems, U.S.A.). The Raman spectra were captured using a high-sensitivity charge-coupled device array detector (Symphony, Horiba Jobin-Ybon, Japan). This Raman spectroscopy system allowed us to capture Raman spectra for 20 s under potentiostatic conditions. We focused on the 100−300 cm−1 range for the radial breathing mode (RBM) and the 1200−1800 cm−1 range for the TG mode of the SWCNTs for every 0.1 V between −0.2 and +1.0 V in one go. The effective spectral width of the entrance slit was approximately 32 cm−1. 3.4. Calculation of Specific Capacitance from CV Curve. The capacitance of the EDLSCs was calculated from the CV curve. The specific capacitances were estimated according to the following equation: Cp = C /m = (Q /ΔV )/m =

{∫ (I·dt)/ΔV }/m

where Cp is the specific capacitance; C is the total capacitance; m is the weight of the dropped SWCNT electrode; Q is the total charge (integral area of CV curve when the current is greater than zero); I is the current; dt is the measurement interval; and ΔV is the applied voltage.

Figure 2. XPS O1s spectra of the (a) hc-SWCNTs and (b) AOSWCNTs. The dotted lines are the deconvoluted peaks.

3. RESULTS AND DISCUSSION 3.1. Characterization. The obtained HRTEM images of the hc-SWCNTs showed the lattices of the hc-SWCNTs clearly. Further, the SWCNTs possess an almost perfect nanotube-framework (Figure 1a). The diameters of the bundles were 4.0−60 nm, and the average bundle diameter was 14.2 nm, which were determined by fitting a Gaussian function (Figure 1b). On the other hand, the lattice images of the AOSWCNTs were wavy and suggested the presence of defects (Figure 1c). Further, the average bundle diameter was 12.9 nm, while the bundle diameters were 4.0−40 nm (Figure 1d). The average specific surface areas of the hc-SWCNTs and AOSWCNTs were 320.9 ± 39.0 and 488.3 ± 34.3 m2/g, respectively. These results suggested that the hc-SWCNTs reacted with oxygen atoms and were burned partly and that defects were induced in the nanotubes. In addition to the oxidation of the nanotubes, the surfaces of the nanotubes were modified with oxygen-containing functional groups. Figure 2a,b shows the O1s XPS spectra of the two samples. The O1s peak seen in the high-resolution scan could be deconvoluted into five peaks. The peaks labeled 1, 2, 3, 4, and 5 were attributable to the carbonyl oxygen of quinones (531.0−531.9 eV), the carbonyl oxygen atoms in esters and anhydrides and the oxygen atoms in hydroxyl groups (532.3−532.8 eV), the ether-type oxygen atoms in esters and anhydrides (533.1−533.8 eV), the oxygen atoms in carboxyl groups (534.3−535.4 eV), and the H2O and O2 molecules adsorbed on the surfaces of the nanotubes (536.0−536.5 eV), respectively.44,45 Table 1 shows the relative percentages of the oxygencontaining functional groups with respect to the total oxygen species, as calculated from the integrals of the corresponding

Table 1. Ratio of the Number of Oxygen Atoms to the Number of Carbon Atoms on the hc-SWCNTs and AOSWCNTs and the Relative Percentages of the Various Oxygen-Containing Functional Groups, As Estimated from the Results of the XPS Analyses O1s (%) materials

O/C

peak 1

peak 2

peak 3

peak 4

peak 5

hc-SWCNTs AO-SWCNTs

0.015 0.029

6.0 9.5

25.3 24.8

35.3 31.4

26.3 28.5

7.1 5.8

deconvoluted peaks. As a result, the number of oxygencontaining functional groups increased after air oxidation (see the ratio of the oxygen atoms to the carbon atoms in Table 1). In particular, the numbers of carbonyl oxygen of the quinones and the ether-type oxygen atoms in the ester and anhydrides increased significantly. The contact angles of the two samples, measured using droplets of the 3.5 mol/L aqueous sulfuric acid solution, were 96.1° ± 2.3 and 88.8° ± 0.8, respectively, indicating that the oxygen-containing functional groups made the nanotubes hydrophilic. Figure 3 shows the RBM and TG bands in the non-in-situ Raman spectra. The Raman frequencies in the two strong RBM bands were approximately 161 cm−1 (peak (i) and 181 cm−1 (peak (ii); Figure 3a, Table 2). The diameter of SWCNTs can be approximated using the following equation:46 ω(cm−1) = 217.8/d(nm) + 15.7

Hence, the positions of the highest-intensity RBM bands corresponded to tube diameters of approximately 1.50 and 1.32 D


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Figure 3. Non-in-situ Raman spectra for (a) RBM and (b) TG modes of the hc-SWCNTs and AO-SWCNTs when excited using 2.54 eV laser radiation. Peaks i and ii are at 161 and 181 cm−1, respectively. The inset in (b) shows a magnified view of the D band.

Table 2. Raman Shifts in Peak i, Peak ii, D Band, G− Line, and G+ Line for the hc-SWCNTs and AO-SWCNTs; Ratio of the Raman Intensity of the D Band to That of the G+ Line (ID/IG; R Value); and the Ratio of the Raman Intensity of Peak i to That of Peak ii (Ii/Iii) for the hc-SWCNTs and AO-SWCNTs materials

peak i (cm−1)

peak ii (cm−1)

D band (cm−1)

G− line (cm−1)

G+ line (cm−1)

Ii/Iii

ID/IG

hc-SWCNTs AO-SWCNTs

162 163

181 182

1349 1352

1568 1569

1595 1595

1.68 1.47

0.009 0.012

If one assumes that the diameter of an oxygen molecule (0.346 nm) is the almost same as that of a nitrogen molecule (0.364 nm),53 then it can be concluded that SWCNTs might be oxidized from their tips or from the outsides of the bundles, because the oxygen molecules would be unable to penetrate into the interstitial channels. Based on the bundle diameter distribution of the AO-SWCNTs, determined from the HRTEM images (Figure 1d), it could be concluded that the hc-SWCNTs were oxidized from the outsides of the bundles. We checked the diameter distribution of the outermost nanotubes of 18 SWCNT bundles before and after air oxidation (Figure 4). The average diameters of the hc- and AO-SWCNTs were 1.45 and 1.37 nm, respectively, which were determined by fitting a Gaussian function, and the hc-SWCNTs was found to possess large diameters for the outermost nanotubes of the bundles. From here onward, the large-diameter nanotubes are suggested to be present on the outsides of the bundles and could react easily with O2. However, we cannot assert that these nanotubes are semiconducting SWCNTs at this stage. We calculated the percentages of the semiconducting nanotubes with respect to the total number of metallic nanotubes contained in the samples on the basis of the integral intensities (after baseline subtraction) of the first optical transition band (M11) and the second optical transition band (S22) in the visNIR spectra of the two samples (Figure S3).54−57 The percentages of the semiconducting SWCNTs in the hcSWCNT and the AO-SWCNT bundles were estimated to be 88.9 and 87.2%, respectively. Figure 5 shows the CV curves of the two samples measured using an in situ Raman cell. The open-circuit potentials of the hc-SWCNTs and AO-SWCNTs were +0.31 and +0.27 V (vs Ag/AgCl), respectively. Electric double layers are formed on SWCNT electrodes when the H+ ions adsorb on the SWCNT electrode at negative potentials between −0.2 and 0.0 V (vs Ag/AgCl), and HSO4− and SO42− ions adsorb at positive

nm. SWCNTs whose RBM intensity is enhanced by the resonance Raman effect when a 488 nm (2.54 eV) laser is used are semiconducting SWCNTs.46 The disorder-induced mode (D band) was seen at approximately 1350 cm−1 (inset in Figure 3b). In addition, the main components of the TG mode (G band), which are referred to as the G− line (1566 cm−1) and the G+ line (1595 cm−1), were also detected (Figure 3b). After air oxidation, the intensities of the RBM and TG peaks decreased, and the ratio of the intensity of the D band to that of the G+ line (ID/IG; known as the “R value”) increased (Table 2). It has been reported that an increase in the intensity of the D band occurs not only owing to an increase in the number of topological defects in CNTs,47 but also owing to an increase in the number of sp3-hybridized covalent bonds formed by the CNTs.48−51 The obtained non-in-situ Raman data indicated that topological defects were induced in the nanotube frameworks of the AO-SWCNTs and that the surfaces of the nanotubes were modified with oxygen-containing functional groups; this was consistent with the results of the structural characterization performed using HRTEM. In the case of the RBM bands, the ratio of the intensity of peak i to that of peak ii (Ii/Iii) decreased after air oxidation (Table 2), suggesting that semiconducting SWCNTs with a diameter of 1.50 nm are easier to oxidize than those with a diameter of 1.32 nm. Using in situ Raman spectroscopy, Li-Pook-Than et al. found that the thermal oxidation of SWCNT bundles at 300−600 °C results in air etching.37 Their data suggested that the etch rate for smalldiameter semiconducting SWCNTs is higher than that for large-diameter semiconducting SWCNTs. In addition, they found that metallic SWCNTs could be etched more rapidly than could semiconducting ones. In our previous studies, using cryogenic thermal-desorption spectroscopy, we have shown that, in the case of hc-SWCNTs, hydrogen38 and nitrogen molecules52 do not penetrate the interstitial channels, which are nanosized interspaces formed between the bundled nanotubes. E


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oxygen-containing functional groups. The increase was also attributed to an increase in the specific surface area of the nanotubes due to the decrease in the diameter distribution of the nanotube bundles and due to the formation of topological defects in the nanotubes. Here, we would like to state that we confirmed whether the SWCNTs were electrochemically oxidized or reduced under the experimental conditions used in this study. For this issue, we performed the electrochemical experiments while carefully monitoring the potential of the oxygen evolution reaction (OER) and confirmed that the evolution of oxygen gas was not observed during the CV cyclings. In the CV curve of the hc-SWCNTs, no peaks related to the quinone-hydroquinone reaction were detected. In the CV curve of the AO-SWCNTs, the current responses corresponding to the peaks related to the quinone-hydroquinone reaction did not increase, and CO2 gas, which is produced when the electrochemical oxidation of the oxygencontaining functional groups occurs,59 was not observed. It is known that oxygen-containing functional groups modify the surfaces of carbon materials when a positive potential of +1.2 V (vs RHE) is applied to carbon black60 and to multiwalled carbon nanotubes (MWCNTs)60,61 in a dilute aqueous sulfuric acid solution (0.5−1.0 mol/L H2SO4). Since a positive potential of +1.2 V (vs RHE) was not applied to the samples used in this study, it appears that the surface oxidation to the samples did not occur. In contrast, MWCNTs oxidized by nitric acid undergo a hydrogen evolution reaction (HER) at −0.1 V (vs RHE) in a 0.5 mol/L aqueous sulfuric acid solution,62 while SWCNTs undergo a hydrogen evolution reaction (HER) at 0.0 V (vs RHE) in a 1.5 mol/L aqueous sulfuric acid solution.63 In this study, we performed the electrochemical experiments while carefully monitoring the hydrogen evolution potentials and confirmed that hydrogen gas was not produced. With the reduction of the oxygen-containing functional groups on carbon materials, the surface oxygen groups of graphene oxide are reduced at −0.75 V (vs SCE) in a 1.0 mol/L aqueous sulfuric acid solution.64 For the potential range used in this study (−0.2 to +1.0 V (vs Ag/AgCl)), the surface oxygen groups of the AO-SWCNTs are not reduced. Therefore, it was confirmed that the HER and OER did not occur and that the hc- and AO-SWCNTs were not electrochemically oxidized or reduced, respectively. 3.2. In Situ Raman Spectroscopy. Figure 6 shows the development of the RBM bands in the in situ Raman spectra of the two samples during electrochemical charging. The changes in the Raman spectra were reversible with respect to the potentials applied. Before in situ Raman experiments for SWCNTs, we have confirmed in situ Raman spectra for GC electrode under potential control, the RBM bands were not observed in the in situ Raman spectrum of the bare GC electrode (see Figure S4a). In the case of the hc-SWCNTs, the highest intensity of peak i (163 cm−1) was observed around −0.2 V (vs Ag/AgCl). Additionally, its intensity decreased on positive polarization; however, the position of the peak remained unchanged. From the RBM bands of the non-insitu Raman spectrum of the hc-SWCNTs (Figure 3a), the intensity ratio of peak i to that of peak ii (181 cm−1) (Ii/Iii) was calculated and found to be 1.68. On the other hand, in the case of the RBM bands of the in situ Raman spectrum of the hcSWCNTs in the open-circuit mode (Figure S5), the Ii/Iii value was 0.85. As mentioned before, it is likely that large-diameter semiconducting nanotubes existed at the outsides of the bundles. Further, since the open-circuit potential was +0.3 V

Figure 4. Diameter distribution of the outermost nanotubes of the bundles of (a) hc-SWCNTs and (b) AO-SWCNTs. Each inset shows a typical HRTEM image of the bundles.

Figure 5. CV curves of the hc-SWCNTs (black) and AO-SWCNTs (blue) at 10 mV/s in a 3.5 mol/L aqueous sulfuric acid solution.

potentials from 0.0 to +1.0 V (vs Ag/AgCl). In contrast to the currents per unit electrode mass of the hc-SWCNT electrodes, those of the AO-SWCNT electrodes increased significantly at approximately −0.1 and +0.4 V. The CV curves of the two samples did not change even after hundreds of cycles. The current densities increased slightly at approximately +0.4 V in the case of the AO-SWCNTs, owing to the faradaic pseudocapacitance associated with the oxygen-containing functional groups on nanotubes such as quinones. This phenomenon can be expressed by the following equilibrium reaction (quinone-hydroquinone reaction):17,58

The specific capacitances of the hc- and AO-SWCNT electrodes were 38.1 and 50.0 F/g, respectively. The reason for the increase in the currents was the increase in the number of ions adsorbed on the nanotubes because of the improvement in the wettability of the nanotubes owing to the presence of F


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curvature-dependent electron−phonon coupling.65 Thus, although the RBM bands of metallic SWCNTs exhibit downshifts, the phenomenon that the intensity of the RBM band (peak (ii) of semiconducting SWCNTs is enhanced by the resonance Raman effect resulting from the use of a laser beam with an excitation energy of 2.54 eV has not been reported previously. The reason for these downshifts still remains unclear. However, it is likely that they are related to the structure-dependent electron−phonon coupling in the semiconducting SWCNTs. In the case of AO-SWCNTs, the in situ Raman spectrum exhibited behavior similar to that of the spectrum of the hc-SWCNTs; only the intensity of peak i decreased, owing to the oxidation of the large-diameter nanotubes. Figure 7 shows the development of the TG bands in the in situ Raman spectra of the two samples during electrochemical

Figure 6. In situ Raman spectroelectrochemical RBM vibration for the (a) hc-SWCNTs and (b) AO-SWCNTs for electrode potentials of −0.2 to +1.0 V (vs Ag/AgCl; from top to bottom).

(vs Ag/AgCl), we believe that anions such as HSO4− and SO42−, which are adsorbed onto large-diameter nanotubes, existed at the outsides of the bundles. Thus, the Fermi energy of these large-diameter nanotubes shifted and the resonance Raman effect was weakened. On the other hand, the position of peak ii was downshifted and its intensity decreased upon positive polarization. Rafilov et al. reported the results of in situ Raman measurements performed on laser-ablation-synthesized SWCNTs with a diameter distribution of 1.25−1.45 nm in a 1.0 mol/L KCl or HCl aqueous solution.33 They measured the in situ Raman spectra of metallic SWCNTs on the basis of the resonance Raman effect using a laser beam with an excitation energy of 1.92 eV and observed the downshifting of the RBM bands. They suggested that the reason the observed downshift occurred was that some of the Cl ions had penetrated into the interstitial channels of the SWCNT bundles, and some of the bundles had debundled into individual nanotubes. Farhat et al. analyzed the frequency and line width of the RBM bands of individual metallic SWCNTs using in situ Raman measurements performed in an organic electrolyte (poly(ethylene oxide)/LiClO4).35 They experimentally confirmedthat small downshift occurs in the case of the RBM bands of metallic SWCNTs. The measured downshifts were in agreement with

Figure 7. In situ Raman spectroelectrochemical TG mode for the (a) hc-SWCNTs and (b) AO-SWCNTs for electrode potentials of −0.2 to +1.0 V (vs Ag/AgCl; from top to bottom).

charging. The changes in the Raman spectra were reversible with respect to the applied potentials used in the study. The TG bands were clearly different from that of the bare GC electrode (see Figure S4b). Sumanasekera et al. has reported the results of in situ electrochemical Raman spectroscopy of SWCNT bundles in concentrated sulfuric acid.16 When positive G


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The Journal of Physical Chemistry C potentials of more than +0.5 V (vs SCE) were applied to the SWCNT bundles, the G+ line in the Raman spectra was irreversible in the backward scan direction from +0.5 to 0.0 V, indicating that the SWCNTs were irreversibly oxidized via the formation of C−O bonds. Based on the differences in their data and ours, it can be concluded that the degree of perfection of carbon network of the SWCNTs used in the two studies might be related to whether electrochemical oxidation occurred or not. With a change in the degree of electrochemical doping, the intensities of the G+ bands decreased and their Raman lines were shifted to higher frequencies. The attenuation in the Raman intensities can be explained by the filling/depleting of the Van Hove singularities and the corresponding bleaching of the E33 optical transition energy. We deconvoluted the TG bands at each potential using a Gaussian function (Figure S6) and plotted the vibrational frequency of the G+ line as a function of the applied potential (Figure 8). We considered the vibrational frequency corresponding to the G+ Raman line in the non-in-situ Raman spectrum of the hc-SWCNTs (on which electrolyte ions were not adsorbed), namely, 1595 cm−1 as the reference and found that the G+ line for both samples downshifted for a change from −0.2 to +0.4 V (vs Ag/AgCl) and upshifted for a change from +0.4 to +1.0 V (vs Ag/AgCl). For potentials ranging from −0.2 to 0.0 V (vs Ag/AgCl), electrons are injected into the nanotubes, and H+ ions are primarily adsorbed onto the nanotubes, while for potentials ranging from 0 to +1.0 V (vs Ag/AgCl), electrons are ejected from the nanotubes and mainly HSO4− and SO42− ions adsorbed onto them. Near 0.0 V (vs Ag/AgCl), the value of Δω/ΔV at positive potential is larger than that at negative potential, where Δω and ΔV are the change in the G+ line and potential, respectively. The number of adsorbed ions estimated from the CV curves is proportional to the value of Δω. It has been reported that the direction and magnitude of the frequency of the TG band depends on the diameter of the semiconducting nanotubes.31,36 During negative charging to the open circuit potential of a electrochemical cell, small-diameter nanotubes exhibit a downshift of the G+ Raman line, while large-diameter ones exhibit an upshift. This behavior is determined by competing processes of phonon renormalization and the weakening of the C−C bond. These two effects result in the movement of the vibrational frequency of the TG band in opposite directions. During negative charging, the G+ line will be downshifted for small-diameter nanotubes because the effect of the weakening of the C−C bond is stronger than that of the renormalization of the phonon energy. On the other hand, for large-diameter nanotubes, the G+ line will be upshifted because the effect of the weakening of the C−C bond is weaker than that of the renormalization of the phonon energy. Positive charging leads to the strengthened vibration of the C−C bond and the upshifting of the G+ line. The shifting of the G+ line due to the renormalization of the phonon energy is independent of the type of charging and is always an upshift. In this study, since the diameters of the hc-SWCNTs were small (1.32−1.50 nm), the downshifting of the G+ line was observed for negative charging. On the other hand, the magnitude of the downshift in the case of the AO-SWCNTs was larger than that for the hc-SWCNTs. In the AO-SWCNTs, which were formed after air oxidation, the proportion of the small-diameter semiconducting nanotubes was higher (these were produced by the oxidation of the large-diameter nanotubes) in contrast to the hc-SWCNTs. Therefore, since the effect of the weakening of the C−C bond became stronger than the effect of the

Figure 8. (a) Changes in the Raman shift of the G+ line as a function of the electrode potential. (b) Influent electric charges per unit mass and (c) specific capacitances as a function of the magnitude of the shift of the G+ line on forward scanning from −0.2 V (vs Ag/AgCl) to arbitrary electric potentials. The black circles and blue squares correspond to the hc-SWCNTs and AO-SWCNTs, respectively.

renormalization of the phonon energy, the magnitude of the downshift for the AO-SWCNTs was large given the decrease in nanotube diameter distribution. In addition, in the case of positive charging, the magnitude of the upshift for the AOSWCNTs was slightly larger than that for the hc-SWCNTs. Although the effect of the renormalization of the phonon energy is manifested strongly when diameter of the nanotubes is large,3,31 it seemed to be suppressed in the case of the AOSWCNTs for positive charging, since the AO-SWCNTs were mainly small-diameter semiconducting nanotubes. Coupled with the effect of the change in diameter distribution, it might be possible that, since excess electrons are injected or ejected not to the covalently bonded carbon atoms with functional groups but to the carbon atoms (Ctube) of the nanotube frame, H


Article


magnitude of the upshift in the case of the AO-SWCNTs was slightly larger than that for the hc-SWCNTs in the case of positive charging. Since excess electrons are ejected not to the covalently bonded carbon atoms with functional groups but to the carbon atoms of the nanotube frame, the effect of the spring force constant of the C−C bond being stronger was manifested in terms of the G+ line during positive charging. With respect to the RBM bands of both samples, although a Raman shift was not seen in the case of the 161 cm−1 (peak i), the position of the 181 cm−1 (peak ii) was downshifted and its intensity decreased upon positive polarization. This is the first report of this phenomenon, namely, that peak ii undergoes a downshift. The reason for this downshift remains unclear. In comparison to hc-SWCNTs, AO-SWCNTs showed high influent electric charges per unit mass and specific capacitances for the maximum magnitude of the shift of the G+ line frequency. The specific capacitances of AO-SWCNTs were found to increase with more ions adsorbing on the nanotubes due to not only the high wettability as a consequence of chemical functionalization but also the high specific surface area, a result of increasing the diameter distribution of small nanotubes, and an increase in the number of defects that provide ions access into the nanotubes. In situ Raman spectroscopy can be used to simultaneously estimate the rate of the increase or decrease in the diameter distribution of small nanotube diameter distribution and the specific capacitances of EDLSCs by the magnitude of the shift of the G + line compared to unfunctionalized SWCNTs. This is a useful method to investigate the performance of chemically functionalized SWCNT electrodes for EDLSCs.

the effect of the decrease or increase in the spring force constant of the C−C bond was manifested in terms of the shifting of the G+ line during negative or positive charging. We intend to perform experiments to test this presumption. The relationships between the influent electric charges per unit mass (Figure 8b) and specific capacitances (Figure 8c) were plotted as a function of the magnitude of the shift of the G+ line during a forward scan from −0.2 V (vs Ag/AgCl) to arbitrary electric potentials. The magnitude of the shift of the G+ line was calculated by subtracting the frequency at −0.2 V (vs Ag/AgCl) from the frequency at arbitrary electric potentials. The influent electric charges per unit mass were calculated by dividing the integral of CV curves in the range from −0.2 V (vs Ag/AgCl) to arbitrary electric potentials by the mass of SWCNT electrodes. The specific capacitances were calculated by dividing the influent electric charges per unit mass by the applied voltage. The influent electric charges per unit mass and the specific capacitances of the hc-SWCNT electrodes for the maximum magnitude of the shift of the G+ line (6.1 cm−1) were 45.8 C/g and 38.1 F/g, respectively. On the other hand, the influent electric charges per unit mass and the specific capacitances of the AO-SWCNT electrodes for the maximum magnitude of the shift of the G+ line (10.7 cm−1) were 60.1 C/g and 50.1 F/g, respectively, which are larger than the corresponding value for hc-SWCNT electrodes. In the hcSWCNTs, the electrolyte ions adsorb only the outermost nanotubes of the SWCNT bundles. In the AO-SWCNTs, on the other hand, more electrolyte ions appear to adsorb onto and into the bundles because of the high wettability resulting from chemical functionalization and the high specific surface area resulting from increasing the diameter distribution of small nanotubes and an increase in the number of defects that provide ions access into the nanotubes. From the data of in situ electrochemical Raman spectroscopy, when the magnitude of the downshift of the G+ line of functionalized semiconducting SWCNTs with diameters of 1.3−1.5 nm is larger than that of unfunctionalized SWCNTs during negative charging to the open-circuit potentials, we can conclude that the diameter distribution of small nanotubes increases. Additionally, the maximum magnitude of the shift of the G+ line over the scanned range of electric potentials is effective in assessing the increase or decrease in the specific capacitance compared to unfunctionalized SWCNTs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12057. (I) Illustrations of the electrochemical cell used for in situ Raman spectroscopy; (II) CV curves of the hcSWCNTs obtained in a 3.5 mol/L aqueous sulfuric acid solution using the in situ Raman electrochemical cell and the conventional cell; (III) Vis-NIR spectra of the thin hc-SWCNT and AO-SWCNT films; (IV) In situ Raman spectra of the bare glassy carbon for electrode potentials of −0.2 to +1.0 V (vs Ag/AgCl); (V) In situ Raman RBM bands of the hc-SWCNTs under an open-circuit potential; (VI) Deconvoluted peaks of the in situ Raman TG mode of the hc-SWCNTs (PDF).

4. CONCLUSIONS In summary, we investigated the influence of air oxidation on the electrochemical doping of semiconducting hc-SWCNTs bundles in a 3.5 mol/L aqueous sulfuric acid solution using in situ Raman spectroscopy. Non-in-situ Raman measurements showed that when the bundles consisting of primarily semiconducting hc-SWCNTs were oxidized in air at 450 °C for 30 min, AO-SWCNTs with a smaller diameter distribution were formed by the burning and chemical functionalization of the large-diameter SWCNTs. The AO-SWCNTs were modified mainly with oxygen-containing functional groups, including the carbonyl oxygen of quinones and the ether-type oxygen atoms in ester and anhydrides. In the case of negative charging, the downshift in the G+ line of the AO-SWCNTs was found to be larger compared to that before air oxidation. The in situ Raman data showed that, when the ratio of the small-diameter nanotubes to the total nanotubes was high, the effect of the weakening of the C−C bond was stronger than that of the renormalization of the phonon energy. On the other hand, the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. and Fax: +81-22795-3215. Present Address ‡

JX Nippon Mining and Metals Corporation, Kitaibaraki 319− 1535, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.S. was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers 23686092, 25630290, and 15H04131. T.I. was supported by the JSPS I


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KAKENHI Grant Number 15H03847. K.T. was supported by the JSPS KAKENHI Grant Number 26220104.

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In Situ Electrochemical Raman Spectroscopy of Air-Oxidized

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