Jungle-Gym Structured Films of Single-Walled Carbon Nanotubes

Article pubs.acs.org/JPCC

Jungle-Gym Structured Films of Single-Walled Carbon Nanotubes on a Gold Surface: Oxidative Treatment and Electrochemical Properties Masato Tominaga,*,†,‡ Shingo Sakamoto,† and Hiroyuki Yamaguchi† †

Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan Kumamoto Institute for Photo-Electro Organics (Phoenics), 3-11-38 Higashimachi, Kumamoto 862-0901, Japan

‡

ABSTRACT: A jungle-gym structured film of single-walled carbon nanotubes (SWCNTs) was synthesized onto a gold surface according with our protocol, which was used immediately as a working electrode in electrochemistry without pretreatment because of the high electrical conductivity between the SWCNTs and the gold surface. No removal of the SWCNTs from the gold surface in solution was necessary, which completely covered the gold surface. The electrochemical properties of as-grown SWCNTs and SWCNTs treated with UV-ozone and electrochemical oxidation were investigated by use of ferrocenecarboxylic acid (Fc) as a monitor redox species. The redox waves of Fc were analyzed by voltammetric simulation fitting method. In the case of UV-ozone treatment, the side-wall structure of the SWCNTs was well oxidized, but the deeper parts of SWCNTs films were not well-oxidized because the UVozone irradiation did not penetrate into the deeper parts of the SWCNT layer. On the other hand, electrochemical oxidation at the side-wall structure of the SWCNTs was performed uniformly. The heterogeneous electron transfer rate constant (k°′) values for Fc were as follows: (0.45−1.4) × 10−3, 7.6 × 10−3, and 0.45 × 10−3 cm·s−1 for the as-grown SWCNTs, SWCNTs treated with electrochemical oxidation, and SWCNTs treated with UV-ozone, respectively. The k°′ values for the as-grown SWCNTs were similar to those for SWCNTs treated with UV-ozone, which suggests that the UV-ozone irradiation would affect only the surface of the SWCNT layer. The electrochemically active surface areas of the electrochemical oxidized and UV-ozone-treated SWCNTs decreased to 1/15−1/30 and 1/2, respectively, in comparison with the untreated SWCNTs, because of the decomposition of the sp2-hybridized carbon structure in the SWCNTs. The simulated voltammograms for Fc at the electrochemically oxidized SWCNTs fitted perfectly, indicating that it was uniformly active at all parts of the electrochemically oxidized SWCNT surface. On the other hand, the observed voltammograms from untreated SWCNTs and UV-ozone-treated SWCNTs did not fit well because of the broad peaks in the voltammograms, which indicated that the electron transfer rate constant was broad at those SWCNT surfaces.

1. INTRODUCTION Since the discovery of carbon nanotubes (CNTs) in 1991, they have been one of the most actively studied materials because of their unique properties as atomically ordered materials and as promising materials for nanotechnological applications.1−4 CNTs are part of the sp2 carbon family such as graphene and highly oriented pyrolytic graphite (HOPG). Single-walled carbon nanotubes (SWCNTs) are described as one graphite sheet rolled up with their ends capped by fullerene-like structures. Thus, multiwalled CNTs (MWCNTs) can be viewed as additional graphene tubes around the core of the SWCNT tube with a separation distance of 0.34 nm between the inner layer and the outer layer. SWCNTs have quasi-onedimensional nature and cylindrical symmetry, and the sp2 hybridization of the carbon bonds induces interesting electrical properties that have been studied extensively by use of band structure representations of graphene as a starting point.5,6 SWCNTs show metallic or semiconducting properties, which depend on the chirality of the graphene wrap. SWCNT synthesis with selective chirality has been impossible until now, and thus a third of the synthesized SWCNTs will have metallic properties (m-SWCNTs), and the remaining two-thirds will be semiconducting (s-SWCNTs) with a band gap proportional to © 2012 American Chemical Society

1/d, where d is the diameter of the SWCNT, and on the order of ∼0.5 eV.7 In the case of MWCNTs, only one of the concentric tubes needs to have metallic properties for the overall electronic character to be essentially metallic. The density states in the band structures for m- and s-SWCNTs are low, at around the Fermi level, in comparison with a typical metal.8,9 From the viewpoint of electrochemical fields, CNTs have the following advanced points: the combination of a high aspect ratio, nanometer-sized dimensions, high electrical conductivity, and low capacitance in the pristine state, which indicate that CNTs have great potential as promising electrode materials for electrochemisty. To clarify the special characteristics of SWCNTs and MWCNTs in electrochemistry, as compared with other carbon electrode materials such as glassy carbon and HOPG, voltammetric analyses were carried out. The MWCNTs could be described as HOPG-like: the electrochemical properties for the side wall of the MWCNTs could be analogous to the basal plane of HOPG. The open-ended Received: November 22, 2011 Revised: April 10, 2012 Published: April 10, 2012 9498

dx.doi.org/10.1021/jp2112473 | J. Phys. Chem. C 2012, 116, 9498−9506

The Journal of Physical Chemistry C

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2. EXPERIMENTAL SECTION 2.1. Instrumentation. The synthesized CNTs were characterized by use of the following instruments. Transmission electron microscopic (TEM) characterizations were performed on a JEOL, 2000FX electron microscope with an acceleration voltage of 200 kV. Field-emission scanning electron microscopic (FE-SEM) characterizations were performed on a Hitachi, SU-8000. Raman spectroscopic measurements were carried out on a Horiba (Jobin Yvon) LabRAM HR-800 instrument with 514 nm (2.41 eV) laser excitation. All images were captured with a digital charge-coupled device (CCD) camera. The wavenumber calibration was performed with the 520 cm−1 emission of silica slides used for analysis. The laser was focused at 2 mm with a laser power of 0.2 mW using a 50× long lens. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo VG Scientific, Sigma Probe HA6000II. This instrument uses a focused monochromatic Al Kα X-ray (1486.68 eV) source for excitation, a spherical section analyzer, and a six-element multichannel detection system. The percentage of individual elements detected was determined from the relative composition analysis of the peak band areas. A UV-ozone treatment system (model OC-2503, Eye Graphics Co., Japan) was used in this study. The UV light had main emissions at 185 and 254 nm. The density at 230− 280 nm (sensitivity peak ca. 255 nm) was evaluated as 11 mW·cm−2 by an ultraviolet ray integration illumination meter (UVPF-A1, Eye Graphics Co., Japan). The concentration of ozone was ∼100 ppm. Cyclic voltammetric measurements and controlled-potential electrolyses were performed with an electrochemical analyzer (ALS/CHI, model 600A) using a conventional three-electrode cell with Ag/AgCl (saturated KCl) as the reference electrode and a Pt plate as the counterelectrode. For electrolytic studies, the working electrode and counterelectrode were separated by a glass filter. All potentials were reported with respect to the Ag/AgCl (saturated KCl) electrode. The electrolyte solution was purged with high-purity argon before any measurements were taken. Simulations of the cyclic voltammograms were performed with a cyclic voltammetric simulator, DigiSim 2.0, Bioanalytical Systems.36 2.2. Chemicals. Ferrocenecarboxylic acid (Fc, 99.0%) was purchased from Wako Pure Chemical (Japan). Cobalt(II) acetate tetrahydrate [(CH 3 COO) 2 Co, 99.0%] and molybdenum(II) acetate dimer [[(C2H3O2)2Mo]2, 98.0%] were purchased from Nacalai Tesque (Japan) and Aldrich and were used as received. A plate of basal-plane highly oriented pyrolytic graphite (HOPG basal plane, Panasonic graphite, PGX 05, electrode area 0.44 cm2) was from Panasonic Co., Japan. Prior to use, the surface of the HOPG (basal plane) was peeled off by adhesive tape to expose a fresh basal plane. Glassy carbon (GC, BSA, electrode area 0.07 cm2) was used as the other working electrode. Prior to use, the GC electrode was polished with a 0.05 mm alumina slurry, followed by sonication in pure water for 10 min. Water was purified with a Millipore Milli-Q water system. All other chemical reagents were of analytical grade and were used without further purification. 2.3. Synthesis of SWCNTs on a Gold Electrode. The catalytic ink was prepared by dissolving cobalt(II) acetate tetrahydrate, ca. 10 wt %, and molybdenum(II) acetate dimer, ca. 1.4 wt %, in 10 mL of ethanol and then mixing them with zeolite in order to obtain the semisolid form. This semisolid

structure derived from the oxidation of the end cap (the cap structure consists of both six- and five-membered rings), and the locations on the tube axis where the graphitic sheets terminate, could be analogous to the edge-plane steps on the HOPG surface.10−12 The side walls of the MWCNTs and the basal plane of HOPG are essentially inert, and edge-plane-like defects, including the open ends in MWCNTs or the step edge, dominate their electrochemical properties. In the case of SWCNTs, edge-plane-like defects such as the open ends derived from the oxidization of the cap in SWCNTs are also characterized as active sites electrochemically.13−15 On the other hand, some research results indicated that the side walls of SWCNTs are active electrochemically.16,17 The differences between these results would be due to defect effects and purity. The main methods for synthesis of CNTs are electric arc discharge, laser ablation, high-pressure carbon monoxide, and catalyzed chemical vapor deposition (CVD).18−23 The CNTs synthesized by these methods contain metallic nanoparticles, which exist on the inner and outer surface of the CNT tubes. Thus, purification procedures are necessary to use the CNTs, even if commercially available CNTs are used. The most commonly used purification technique is oxidative treatment, usually employing strong oxidizing acids and refluxing the CNTs in HNO3, H2SO4, HCl, or mixtures of these acids.24−26 Furthermore, mixtures of concentrated HNO3 and H2SO4 are particularly effective in removing amorphous carbon. HCl treatment with sonication is effective to remove metallic nanoparticles. On the other hand, it has been reported that CNTs contain metallic nanoparticles, even after being washed with concentrated acidic solutions at elevated temperatures, because these metallic nanoparticles are sheathed by the graphene sheets.27 Furthermore, an important consideration is the secondary effect of the acid purification process. CNTs can have damage such as opening of the CNT end caps and oxidizing damage at the sidewall, which will form a sp3 structure in CNTs; in the other words, the CNTs are functionalized with oxygen-containing groups such as carboxylic acids, ketones, and alcohols.28−31 Such acidic functional groups are likely to affect the behavior of the CNT-based electrodes, because the presence of side-wall defects causes significant changes in the CNT electronic states near the Fermi level, typically resulting in increased electrical resistance of the CNT, and also oxygencontaining surface groups can aid electrochemical reactions.32−35 The careful characterization of CNTs is essential in order to understand these electrochemical processes. In this study, the protocol for preparing a jungle-gym structured film of SWCNTs onto a gold surface is presented, and detailed characterization of the resulting material and voltammetric data by use of a ferrocene derivative is performed. The prepared electrode can be used immediately as a working electrode in electrochemistry without pretreatment, which has advances as follows. First, the electrical conductivity between SWCNTs and gold surface was sufficient to allow electrochemical measurements. Second, SWCNTs were not easily removed from the gold surface in solution, because the SWCNTs bonded strongly to the gold surface. Third, the gold surface was covered completely by SWCNTs, because the characteristic electrochemical behavior of a gold surface was not observed at all. Fourth, the metallic nanoparticles, which act as a catalyst for SWCNT synthesis, were deposited into the interior of the cylindrical structure of the SWCNTs, which suggested that those metallic nanoparticles were inactive in electrochemistry. 9499



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mixture was used as the catalytic ink for CNTs. A gold wire (99.999%, 0.8 mm diameter) was modified with the catalytic ink via a dip-coat method. The CNTs were synthesized in a quartz tube (inner diameter 35 mm, length 840 mm) equipped with temperature and gas-flow control systems. The catalytic ink on the gold wire was first reduced in H2 for 10 min at ca. 850 °C, and then a mixture of ethanol (99.8%) and H2 gas was introduced into the chamber at a flow rate of 100 cm3·min−1 H2 at 850 °C for 10 min.37−39 The synthesized CNTs were characterized by Raman spectroscopy, FE-SEM, and TEM.

3. RESULTS AND DISCUSSION 3.1. Characterization of Jungle-Gym Structured Film of SWCNTs. Figure 1 shows photographs of a gold wire before

Figure 1. Photographs of a gold wire surface (a) before and (b) after the SWCNTs synthesis process.

and after the CNT synthesis process. The CNTs were recognized on the gold wire surface, because the surface color changed to black. Figure 2 shows FE-SEM images of the CNT surface on a gold wire. Bundled CNTs of 5−50 nm diameter were observed. The thickness of the film was estimated to be ca. 18 μm from the cross-section of the FE-SEM image. The CNTs on the gold wire seemed to be a jungle-gym structured film. From the TEM image in Figure 2c, several pieces of SWCNTs bundles were recognized, and the diameter of the SWCNTs was evaluated at ca. 1 nm. Raman spectra of SWCNTs synthesized on a gold electrode are shown in Figure 3. Raman spectroscopy plays an important role for the characterization of sp2-hybridized structures in carbon materials and yields information about defects and the crystalline structure.40,41 The prominent features of the Raman spectra of the CNTs are the G-band appearing at ca. 1590 cm−1 and the D-band at ca. 1350 cm−1.40,41 The G-band is a doubly degenerate phonon Raman-active mode for sp2-structured carbon networks, whereas the D-band is localized where the lattice structure is not perfect, mostly at the edges and the defects of the sp2-hybridized carbon structure. The G/D ratio of the SWCNTs synthesized was evaluated to be ca. 30 when a laser at 514.5 nm was used for excitation. These results indicated that the synthesized SWCNTs were highly crystalline in their sp2-hybridized carbon structure. From the radial breathing mode (RBM) of the SWCNTs, the diameter distribution of the SWCNTs can be estimated from d = 248/ v, where d is the diameter of a SWCNT (nanometers) and v is the Raman shift (cm−1).42,43 From the peaks in the RBM

Figure 2. SEM images of (a) top view and (b) cross-section of SWCNTs synthesized on a gold surface. (c) TEM micrographs of SWCNTs synthesized on a gold surface .

Figure 3. Raman spectrum of SWCNTs synthesized on a gold surface. Excitation laser wavelength: 514.5 nm (2.41 eV).

region, a relatively narrow diameter distribution ranging between 0.9 and 1.6 nm was recognized, which was in good agreement with the estimated value of the diameters from the TEM images. From a detailed comparison with the plot of Kataura et al.,44 the Raman peaks around 150−220 cm−1 were assigned to the semiconductor SWCNTs, whereas the peaks around 230−300 cm−1 were assigned to the metallic SWCNTs, thus indicating that the jungle-gym structured films were composed of a mixture of semiconductor and metallic 9500



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SWCNTs. The Raman peak around 2700 cm−1 corresponded to the second-order Raman spectrum of a crystalline sp2hydribized structure carbon, which was termed the G′ (2D) band.45,46 It is known that the shape of the G′-band is strongly dependent on the number of graphene layers in the sample. The G′-band for a single graphene sheet is composed of a single Lorentzian peak with a full width at half-maximum (fwhm) = 30 cm−1.45,46 Furthermore, it is known that SWCNTs show a sharp G′ peak similar to that of graphene.47 The fwhm of the CNTs in the jungle-gym structured film was evaluated at ca. 65 cm−1, which is approximately 2-fold larger than the fwhm of a single graphene layer. In comparing the various features that are observed in the Raman scattering process for SWCNTs, the effect of the tube diameter on the mode frequencies must be considered.48 The larger fwhm would be due to the diameter distribution (0.9−1.6 nm) of the synthesized SWCNTs and structural defects of the SWCNTs. The overall results led us to conclude that the synthesized CNTs could be considered as SWCNTs. It is not easy to estimate surface area of jungle-gym structured films of SWCNTs. The double-layer capacitance (C) enables one to calculate the accessible surface area in jungle-gym structured film as follows. The capacitive charge− discharge current (I) is I = C dE/dt, where dE/dt is the linear potential sweep rate in the cyclic voltammograms, and due to the double layer at the electrode/electrolyte interface, it is proportional to the effective surface area. Here, one must pay attention that this equation is confirmed under the conditions of a planar electrode in the Gouy−Chapman model. Compton and co-workers49 investigated the effect of varying the nanotube radius on the differential capacitance density. The results of comparisons between the experimental and simulated values for capacitance of varying the nanotube radius indicated that the measurable effect on the capacitance would be small for a large cylinder radius (ca. 10 nm or more) or a high electrolyte concentration (ca. 100 mmol·dm−3 or more). Under our experimental conditions of 100 mmol·dm−3 phosphate buffer solution, use of the equation I = C dE/dt would be reasonable. The surface specific capacitance per geometric area was evaluated to be ca. 5 × 103 μF·cm−2 for jungle-gym structured films of SWCNTs by calculating from the potential region of 0.2−0.4 V in a phosphate buffer solution (100 mmol·dm−3, pH 7). The SWCNT can be described as one graphite sheet rolled up with both ends normally capped by fullerene-like structures. The capacitance of the HOPG basal plane was measured at ca. 5 μF·cm−2 by calculating from the potential region of 0.2−0.4 V, which was relatively higher than the reported value of 2−3 μF·cm−2.50 Thus, the effective surface area in a jungle-gym structured film of SWCNTs would be almost 103-fold higher than the apparent surface area (0.25 cm2) of the modified electrode. The gold plate surface was completely covered with SWCNTs, because the characteristic electrochemical behavior of a gold surface was not observed at all: No oxidation reaction of the gold surface was observed by electrochemistry. No glucose oxidation reaction was observed in alkaline solution in the presence of glucose.51,52 The electrical conductivity between the prepared jungle-gym structured film of SWCNTs and the gold surface was sufficient to allow electrochemical measurements, and the SWCNT film was not easily removed from the gold surface in solution, because the SWCNTs bonded strongly to the gold surface. The metallic nanoparticles, which act as a catalyst for SWCNT synthesis, were deposited

inside the cylindrical structure of the SWCNTs, which indicated that the metallic nanoparticles were inactive in electrochemistry.53,54 Thus, the prepared SWCNT film would be useful in clarifying the natural electrochemical properties of the SWCNTs as follows. 3.2. SWCNTs Treated with UV-Ozone and Electrochemical Oxidation. UV-ozone and electrochemical oxidation treatments have been performed to activate carbon material electrodes.55−57 The UV-ozone and electrochemical oxidation processes would generate a variety of oxygenated functional groups such as aldehydic, alcoholic, ketonic, esteric, and carboxylic moieties at the ends of the CNTs, and in particular at the structural defect sites along the side wall of the CNTs. Figure 4 shows Raman spectra from SWCNTs of the junglegym structured film during the UV-ozone treatment process.

Figure 4. (a) Raman spectra of SWCNTs during the UV-ozone treatment process and (b) a plot of the G-band/D-band (G/D) ratio over those spectra as a function of treatment time. Excitation laser wavelength: 514.5 nm (2.41 eV).

The decrease in the G-band and the increase in the D-band were observed with increasing UV-ozone treatment time, thus demonstrating that oxygenated functional groups were generated on the SWCNT surface. The XPS results also supported the generation of oxygenated functional groups on the SWCNT surface as described below. XPS is a useful tool for the analysis of chemical state changes in carbon materials. Figure 5 shows a high-resolution C(1s) peak in the XPS spectra of the jungle-gym structured film of SWCNTs during the UVozone oxidation treatment process. The data can be fitted to values close to those for graphite, and more specifically, to those reported in the literature for CNTs.58 The main peak at 284.4 eV was assigned to the C(1s) binding energy for sp2 hybridization of the graphene sheet in a SWCNT. The shoulder of the main peak was composed of four different carbon stages: sp3-hybridized carbon atoms (−CH2− at 285.2 eV), the alcohol/ether group (C−O at 286.6 eV), the carbonyl group (CO at 288.0 eV), and the carboxy acid/ester group (O− CO at 289.2 eV). The higher binding energies correlated 9501



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Figure 5. XPS results in the C(1s) region for SWCNTs after UV-ozone treatment, and its deconvolution analysis. The UV-ozone treatment times were (a) 0, (b) 1, (c) 5, and (d) 15 min.

with the increased numbers of oxygen atoms bonded to the carbon, because the electronegative oxygen atoms induced a positive charge on the carbon atom.57,58 A small plateau at the higher binding energy side at ∼290.5 eV was assigned to the π−π* transitions accompanying the excitation of the typical sp2-hybridized carbon. An increase in the size of the shoulder of the main peak was observed with increasing treatment time as shown in Figure 5, indicating that the relative percentages of oxygenated functional groups generated on the SWCNT surface were increased. Table 1 summarizes the relevant results. Table 1. Relative Percentages of Surface Functional Oxygenated Groups Obtained from Curve Fitting of C(1s) Peaks of SWCNTs Treated with UV-Ozone UV-ozone treatment time, min

C−C, % (284.4, 285.2 eV)

C−O, % (286.6 e V)

0 1 5 15

74 72 54 53

13 20 32 36

O−CO, CO, % % (288.0 eV) (289.2 eV) 3 5 3 2

2 3 5 6

π−π* shakeup, % (290.5 eV)

Figure 6. TEM image of CNTs after UV-ozone treatment for 15 min.

8 7 6 3

SWCNTs was observed with increasing numbers of potential cycles. Table 2 shows the summarized results from the highresolution C(1s) peak in the XPS spectra of the SWCNTs during electrochemical oxidation. The relative percentages of alcohol/ester groups (C−O) of the SWCNTs treated with electrochemical oxidation for 20 cycles were approximately 230% higher in comparison with the as-grown SWCNTs. On the other hand, only a slightly increased proportion of carbonyl groups (CO) and no increase in carboxy acid/ester groups (O−CO) were observed. These results led us to conclude that the sp2-hybridized carbon was cleaved by the electrochemical oxidation treatment. The sp2-hybridized carbon of the SWCNTs was oxidized by both UV-ozone and electrochemical methods. The differences in oxidation effect between these two treatments will be discussed below. 3.3. Characteristic Properties of Electrode Reactions of Fc at SWCNTs. To clarify the special characteristics of SWCNTs and MWCNTs in electrochemistry as compared with other carbon electrode materials such as glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) carbon, voltammetric analyses were carried out. Fc was used as a redox species to characterize the properties of the SWCNT surface in an electrochemical reaction. Figure 7 shows the typical cyclic voltammogram for Fc at the jungle-gym structured film of as-grown SWCNTs in a buffer solution (pH 7). The oxidation and reduction peaks of Fc were observed at 0.45 and 0.31 V, respectively. The redox potential was evaluated at 0.38 V, which was in good agreement with a previous report.59 Figure 8 shows a plot of the oxidation peak

The relative percentages of alcohol/ester (C−O) and carboxy acid/ester (O−CO) groups of SWCNTs treated with UVozone for 15 min were measured at ca. 300% higher in comparison with those of the as-grown SWCNTs. On the other hand, only a slightly increased proportion of carbonyl groups (CO) was observed. Such generation of oxygenated functional groups on the SWCNT surface has also been reported in previous studies that used air oxidation, plasma oxidation, acid oxidation, and electrochemical oxidation.57 These results led us to conclude that, after simply exposing them to UV-ozone, the sp2-hybridized carbons were cleaved without the presence of a cleaving agent. Figure 6 shows a typical TEM image of SWCNTs after UV-ozone treatment for 15 min. After this treatment, it was observed that the end cap of the CNTs was opened (red dotted circles in Figure 6), and there was much damage to the side wall of CNTs. These results suggest that carbon−carbon bond cleavage occurred due to the UV-ozone treatment. Electrochemical oxidation treatment for the SWCNTs was performed in a neutral aqueous solution by potential cycling over a range of 0−1.5 V at a potential sweep rate of 20 mV·s−1. From the Raman spectra excited by a 514.5 nm (2.41 eV) laser, a decrease in the G/D ratio of the G- and D-bands of the 9502



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Table 2. Relative Percentages of Surface Functional Oxygenated Groups Obtained from Curve Fitting of C(1s) Peaks of SWCNTs Treated with Electrochemical Oxidation electrochemical oxidation treatment, cycles

C−C, % (284.4, 285.2 eV)

C−O, % (286.6 eV)

CO, % (288.0 eV)

O−CO, % (289.2 eV)

π−π* shakeup, % (290.5 eV)

0 5 10 20

74 71 63 63

13 20 27 30

3 4 5 6

2 3 4 1

8 2 1 1

subtracted voltammograms were also simulated as shown in Figure 10. The overall shapes of the simulated voltammograms were less fitting to the experimentally observed voltammograms except in Figure 10B. This reason would be due to the broad electron transfer rate constant as described below. The L value was calculated at ca. 17 μm for the UV-ozone treated SWCNTs, which was in good agreement with the actual thickness of the jungle-gym structured film of SWCNTs. On the other hand, L was calculated to be an unlimited value (L = ∞ mm) for SWCNTs treated with electrochemical oxidation, which indicated that the redox reaction was infinite electrochemical diffusion. This difference in electrochemical behavior would be due to the treatment characteristics between UVozone and electrochemical oxidation treatments. In the case of UV-ozone treatment, the side-wall structure of the SWCNTs was well oxidized, and the deep parts of the SWCNT film would not be oxidized because the UV-ozone irradiation would not penetrate deeply into the SWCNT layer. On the other hand, the electrochemical oxidation was performed uniformly at the side-wall structure of the SWCNTs and throughout the layers. The oxidation treatment resulted in a lack of conductivity in the CNT, because the sp2-hybridized carbon structure in the SWCNTs was decomposed. In fact, the electrochemically active surface areas of the electrochemically oxidized and UV-ozone-treated SWCNTs decreased to 1 /15−1/30 and 1/2, respectively, in comparison with the untreated SWCNTs. Typically, the surface area of the electrochemically oxidized SWCNTs was roughly twice the apparent surface area, indicating that the SWCNT surface was inactive for electrochemistry. Thus, thin-layer electrochemical behavior would not be observed. Furthermore, the simulated voltammograms for Fc at the electrochemically oxidized SWCNTs fitted well in comparison with the SWCNTs and UV-ozone-treated SWCNTs, which was due to the uniform activity at all parts of the electrochemically oxidized SWCNT surface. On the other hand, the experimentally observed voltammograms from the SWCNTs and UV-ozone treated

Figure 7. Typical cyclic voltammograms from jungle-gym structured films of SWCNTs in a phosphate buffer solution (pH 7) in the presence (solid line) and absence (broken line) of 1 mmol·dm−3 ferrocenecarboxylic acid (Fc). Potential sweep rate: 160 mV·s−1.

current in the voltammograms as a function of the potential sweep rates. The peak current was proportional to the potential sweep rate, v, over the range 0−40 mV·s−1, and was proportional to the square root of v at 160−640 mV·s−1, which is typical of thin-layer electrochemical behavior.60 This behavior can be understood by examining the jungle-gym structured film of SWCNTs on the gold electrode. A cyclic voltammetric simulation was performed under the conditions of finite electrochemical diffusion to demonstrate the thin-layer electrochemical behavior. The simulated voltammograms for Fc are shown in Figure 9 and were then compared with the experimentally obtained, charging current-subtracted voltammograms. When the value of the finite diffusion layer, L, was 13 μm for the simulation conditions, the overall shape of the simulated voltammograms fitted well to the experimentally observed voltammograms at various potential sweep rates. The calculated value for L was in good agreement with the thickness (ca. 18 μm) of the jungle-gym structured film of SWCNTs. For SWCNTs treated with UV-ozone and electrochemical oxidation, the experimentally obtained charging current-

Figure 8. Plots of the oxidation peak current in cyclic voltammograms from jungle-gym structured films of SWCNTs in a phosphate buffer solution (pH 7) of 1 mmol·dm−3 ferrocenecarboxylic acid (Fc) as a function of the potential sweep rates. 9503



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Figure 9. Typical charging current-subtracted voltammograms (solid line) from jungle-gym structured films of as-grown SWCNTs in a phosphate buffer solution (pH 7) of 1 mmol·dm−3 ferrocenecarboxylic acid (Fc) at potential sweep rates of (a) 80, (b) 160, and (c) 320 mV·s−1, and simulated voltammograms with a finite diffusion layer of thickness L = 13 μm. The electrochemical parameters used for the simulated voltammograms were as follows: electrode area 13.5 cm2; heterogeneous electron transfer rate constant 0.5 × 10−3 cm·s−1; transfer coefficient 0.5; and diffusion coefficient 5.8 × 10−6 cm2·s−1.61,62

Figure 10. Typical charging current-subtracted voltammograms (solid line) from jungle-gym structured films of SWCNTs in a phosphate buffer solution (pH 7) of 1 mmol·dm−3 ferrocenecarboxylic acid (Fc) at potential sweep rates of (a) 80, (b) 160, and (c) 320 mV·s−1, and simulated voltammograms. The SWCNTs were treated with (A) UV-ozone for 15 min and then treated with (B) electrochemical oxidation for 20 cycles. Electrochemical parameters used for the simulated voltammograms are as follows: (A) finite diffusion layer thickness L = 17 μm; electrode area 8.4 cm2; heterogeneous electron transfer rate constant 0.45 × 10−3 cm·s−1; transfer coefficient 0.5; and diffusion coefficient 5.8 × 10−6 cm2·s−1. Electrochemical parameters used for the simulated voltammograms of (B): finite diffusion layer thickness L = ∞ mm; electrode area 0.51 cm2; heterogeneous electron transfer rate constant 7.6 × 10−3 cm·s−1; transfer coefficient 0.5; and diffusion coefficient 5.8 × 10−6 cm2·s−1.61,62

SWCNTs were broad, especially for the as-is SWCNTs, which was due to the broad electron transfer rate constant at their surfaces, since the electron transfer rate constant is usually strongly dependent on the electrode surface. These results also suggested that the electrochemical oxidation at the SWCNTs was performed uniformly. The heterogeneous electron transfer rate constant (k°′) for Fc at the SWCNT, HOPG (basal plane), and GC electrodes, calculated by fitting methods between empirical and simulated voltammograms, are summarized in Table 3. The experimentally obtained cyclic voltammograms for Fc at the HOPG and GC electrodes were perfectly fitted to the simulated voltammograms under the conditions of infinite electrochemical diffusion (not shown). It is well-known that surface defects in carbon materials composed of the sp2-hybridized carbon structure are essential for rapid electron transfer reactions.62 GC is less crystallographically defined and consists of interwoven ribbons of graphite with sp2-hybridized carbon structure. Edge-planelike defects in the GC could be recognized along the edges of the graphitic ribbons. A rapid k°′ value of 16 × 10−3 cm·s−1 was calculated at the GC electrode, which was similar to the value published elsewhere.63 On the other hand, the k°′ value at the HOPG (basal plane) electrode was 2.8 (±0.3) × 10−3 cm·s−1,

Table 3. Heterogeneous Electron Transfer Rate Constants Measured from Carbon Electrodesa heterogeneous electron transfer rate constant, 10−3cm·s−1 SWCNTs treated with UV-ozone SWCNTs treated with electrochemical oxidation as-grown SWCNTs HOPG GC

0.45 7.6 0.45−1.4 2.8 (±0.3) 16

a

(a) SWCNTs were treated with UV-ozone for 15 min; (b) SWCNTs were treated with electrochemical oxidation for 20 cycles from 0 to 1.5 V in a neutral solution.

which was decreased by a factor of ca. 5-fold in comparison with the GC electrode and is likely due to the defect-free structure of the HOPG electrode. For the as-grown SWCNTs, SWCNTs treated with electrochemical oxidation, and SWCNTs treated with UV-ozone, the k°′ values were (0.45− 1.4) × 10−3, 7.6 × 10−3, and 0.45 × 10−3 cm·s−1, respectively. The k°′ value of the as-grown SWCNTs was similar to that of SWCNTs treated with UV-ozone. This result also supported the hypothesis that the UV-ozone irradiation would be 9504



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performed only at the surface of the SWCNT layer. Note that it should be expected that the k°′ values of the as-grown SWCNTs were similar to the HOPG value, because SWCNTs are described as one graphite sheet rolled up with both ends normally capped by fullerene-like structures. However, the k°′ values of the as-grown SWCNTs and SWCNTs treated with UV-ozone were decreased by a factor of ca. 2−5-fold in comparison with the HOPG electrode. The reason for this is still unclear. It was expected that the electrochemical charge transfer at the SWCNTs would depend on the SWCNT’s electronic band structure from the results of theoretical experiments.16,64 The kinetics of the electron transfer reaction of Fc would be affected by the occupation and alignment of the electronic states of the SWCNTs in solution. The k°′ value of SWCNTs treated with electrochemical oxidation was higher than those of the other SWCNTs and was almost half the rapid electron transfer rate of GC. This result also suggests that the electrochemical oxidation was performed uniformly at the side-wall structure of the SWCNTs, even in the deep parts of the film.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; tel +81-96-342-3655; fax +81-96-342-3655. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research (M.T.) from the Ministry of Education, Culture, Science, Sports and Technology, Japan. M.T. also acknowledges the Sumitomo Foundation.

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REFERENCES

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4. CONCLUSIONS A jungle-gym structured film of SWCNTs was synthesized onto a gold surface, which was used immediately as a working electrode in electrochemistry without pretreatment. The effective electrochemical surface area in the jungle-gym structrured film of SWCNTs was ca. 103-fold higher than the apparent surface area, when the film thickness was ca. 18 μm. The SWCNT film was investigated by use of Fc as a redox species. The redox behavior of Fc at the SWCNT film showed typical thin-layer electrochemical behavior. The calculated finite diffusion layer fitted well to the actual film thickness evaluated from FE-SEM. The effect of the oxidation treatment on the SWCNTs in terms of the electrochemical properties was investigated by fitting voltammetric simulation analysis for Fc. The evaluated k°′ value for Fc at the HOPG (basal plane) electrode was 2.8 (±0.3) × 10−3 cm·s−1, which was decreased by a factor of ca. 5 in comparison with the GC electrode (16 × 10−3 cm·s−1). For the as-grown SWCNTs, SWCNTs treated with electrochemical oxidation, and SWCNTs treated with UVozone, the k°′ values were (0.45−1.4) × 10−3, 7.6 × 10−3, and 0.45 × 10−3 cm·s−1, respectively. The k°′ value of the as-grown SWCNTs was similar to that of SWCNTs treated with UVozone. This result suggested that the UV-ozone irradiation would be performed only at the surface of the SWCNT layer. Oxidation treatment resulted in lack of conductivity in the CNT, because of the decomposition of the sp2-hybridized carbon structure in the SWCNTs. The electrochemically active surface areas of the electrochemically oxidized and UV-ozone treated SWCNTs decreased to 1/15−1/30 and 1/2, respectively, in comparison with the untreated SWCNTs. The simulated voltammograms for Fc at the electrochemically oxidized SWCNTs fitted well in comparison with the SWCNTs and UV-ozone-treated SWCNTs, which was due to the uniform activity at all parts of the electrochemically oxidized SWCNT surface. On the other hand, the experimentally observed voltammograms from the SWCNTs and UV-ozone-treated SWCNTs were broad, especially for the SWCNTs, which was due to the broad electron transfer rate constant value at their surfaces. 9505



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(64) Heller, I.; Kong, J.; Williams, K. A.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2006, 128, 7353−7359.

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Jungle-Gym Structured Films of Single-Walled Carbon Nanotubes

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