Anal. Chem. 2001, 73, 915-920
Investigation of the Electrochemical and Electrocatalytic Behavior of Single-Wall Carbon Nanotube Film on a Glassy Carbon Electrode Hongxia Luo, Zujin Shi, Nanqiang Li,* Zhennan Gu, and Qiankun Zhuang
Department of Chemistry, Peking University, Beijing 100871, People’s Republic of China
The electrochemical behavior of a film of single-wall carbon nanotubes (SWNTs) functionalized with carboxylic acid groups was studied extensively on a glassy carbon (GC) electrode. One stable couple corresponding to the redox of the carboxylic acid group, which was supported by XPS and IR experiments, was observed. The electrode process involved four electrons, while the rate-determining step was a one-electron reduction. The SWNT filmmodified electrode showed favorable electrocatalytic behavior toward the oxidation of biomolecules such as dopamine, epinephrine, and ascorbic acid. Carbon nanotubes are new and interesting members of the carbon family offering unique mechanical and electronic properties combined with chemical stability. These tubes were found in two types of structures: the multiwall carbon nanotubes (MWNTs)1 and the single-wall carbon nanotubes (SWNTs).2,3 MWNTs are composed of concentric and closed graphene tubules, each with a rolled-up graphene sheet, formed with a range of diameters, typically from 2 to 25 nm. A SWNT is made of a single graphite sheet rolled seamlessly, with a diameter of 1-2 nm. SWNTs usually are observed arranged in a regular pattern of bundles that consist of tens to hundreds of nanotubes in parallel and in contact with each other. Since the discovery of carbon nanotubes in 1991,1 much experimental and theoretical research has been directed toward their production, purification, mechanical and electronic properties, and electrical conductivity.4 Practical applications of SWNTs as tips in scanning probe microscopy5-10 and as field-effect transistors11,12 were also reported. (1) Iijima, S. Nature (London) 1991, 354, 56-58. (2) Iijima, S.; Ichihashi, T. Nature (London) 1993, 363, 603-605. (3) Bethune, D. S.; Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature (London) 1993, 363, 605-607. (4) Ajayan, P. M. Chem. Rev. 1999, 99, 1787-1799. (5) Arie, T.; Akita, S.; Nakayama, Y. J. Phys. D: Appl. Phys. 1998, 31, L49L51. (6) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nature (London) 1996, 384, 147-150. (7) Wong, S. S.; Harper, J. D.; Lansbury, P. T., Jr.; Lieber, C. M. J. Am. Chem. Soc. 1998, 120, 603-604. (8) Nagy, G.; Levy, M.; Scarmozzio, R.; Osgood, R. M., Jr.; Dai, H.; Smalley, R. E.; Michaels, C. A.; Flynhn, G. W.; McLane, G. F. Appl. Phys. Lett. 1998, 73, 529-531. (9) Akita, S.; Nishijima, H.; Nakayama, Y.; Tokumasu, F.; Takeyasu, K. J. Phys. D: Appl. Phys. 1999, 32, 1044-1048. (10) Hafner, J. H.; Cheung, C. L.; Leiber, C. M. J. Am. Chem. Soc. 1999, 121, 9750-9751. 10.1021/ac000967l CCC: $20.00 Published on Web 01/30/2001
© 2001 American Chemical Society
Depending on their atomic structure, carbon nanotubes behave electrically as a metal or as a semiconductor.13-15 The subtle electronic properties suggest that carbon nanotubes have the ability to promote electron-transfer reactions when used as an electrode in chemical reactions. The MWNTs were first used to fabricate carbon nanotube electrodes. As they were insoluble in most solvents, the MWNTs (10 mg) were mixed with bromoform, mineral oil, or liquid paraffin and then packed into a glass capillary. Electrical attachment to a potentiostat was made through a platinum or copper wire. The resulting carbon nanotube microelectrodes were used to probe bioelectrochemcial reactions16,17 and in the electrocatalysis of oxygen.18 Their performance has been found to be superior to that of other carbon electrodes. Recently, microelectrodes constructed from individual carbon nanotubes with 80-200 nm diameters have been reported.19 Their voltammetric response is characteristic of steady-state radial diffusion. Compared with MWNTs, the SWNT is a well-defined system in terms of electronic properties. Individual SWNTs can be regarded as quantum wires.20 However, the electrochemistry of SWNTs is less studied so far. There has been only one report21 about the cast films of SWNTs on Pt and Au electrodes, but the films did not show well-resolved voltammograms. In this report, the SWNTs were treated with nitric acid during the purification process. The carboxylic acid groups were introduced on the open ends of the SWNTs.22 The nitric acid-purified SWNT solution was cast on a glassy carbon (GC) electrode to form a carbon nanotube (11) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature (London) 1998, 393, 49-52. (12) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Appl. Phys. Lett. 1998, 73, 2447-2449. (13) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992, 60, 2204-2206. (14) Wildo ¨er, J. W. G.; Venema, L. C.; Rinzler, A. G.; Smalley, R. E.; Dekker, C. Nature (London) 1998, 391, 59-62. (15) Odom, T. W.; Huang, J.-L.; Kim, P.; Lieber, C. M. Nature (London) 1998, 391, 62-64. (16) Britto, P. J.; Santhanam, K. S. V.; Ajayan, P. M. Bioelectrochem. Bioenerg. 1996, 41, 121-125. (17) Davis, J. J.; Coles, R. J.; Hill, H. A. O. J. Electroanal. Chem. 1997, 440, 279-282. (18) Britto, P. J.; Santhanam, K. S. V.; Rubio, A.; Alonso, J. A.; Ajayan, P. M. Adv. Mater. 1999, 11, 154-157. (19) Campbell, J. K.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 37793780. (20) Tans, S. J.; Devoret, M. H.; Dai, H.; Thess, A.; Smalley, R. E.; Geerligs, L. J.; Dekker, C. Nature (London) 1997, 386, 474-477. (21) Liu, C. Y.; Bard, A. J.; Wudl, F.; Weitz, I.; Heath, J. R. Electrochem. Solid State Lett. 1999, 2, 577-578.
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film. The film showed very stable electrochemical behavior and can be used to catalyze the electrochemical reaction of some biomolecules such as dopamine, epinephrine, and ascorbic acid. EXPERIMENTAL SECTION Preparation of SWNTs Functionalized with Carboxylic Acid Groups. The SWNTs were prepared by a direct current arc-discharge method23 and purified to >90%.24 The preparation of SWNTs functionalized with carboxylic acid groups was described elsewhere.25 Instruments and Chemicals. Cyclic voltammetric measurements were performed with a Princeton Applied Research (PAR) model 273 potentiostat/galvanostat and a PAR model 270 electrochemical system with a conventional three-electrode cell. The working electrode was a glassy carbon electrode with a diameter of 4 mm, the auxiliary electrode consisted of a platinum wire, and a saturated calomel electrode (SCE) was used as the reference electrode. Water was triply distilled with a quartz apparatus. Highpurity nitrogen was used for deaeration. Dopamine was from Aldrich, and epinephrine was from Sigma. All other reagents were of analytical grade. All cyclic voltammetric experiments were carried out at room temperature (about 15 °C). The scanning electron microscope (SEM) image was obtained using a Hitachi S-4200 microscope. X-ray photoelectron spectroscopy (XPS) measurement was performed using a PHI 5300 ESCA electron spectrometer. Fourier transform (FT) IR spectra were recorded on a Nicolet Magna-IR 750 spectrometer. Preparation of SWNT Film. One milligram of purified SWNTs (functionalized with carboxylic acid groups) was dispersed with the aid of ultrasonic agitation in 10 mL of N,N-dimethylformamide (DMF) (AR) to give a 0.1 mg/mL black solution. The working electrode was carefully polished with emery paper (No. 1500) and chamois leather containing Al2O3 slurry and then ultrasonically cleaned in distilled water and ethanol. The SWNT film was prepared by dropping a solution of SWNT (10 µL, 0.1 mg/mL) in DMF on the GC electrode surface and then evaporating the solvent under an infrared heat lamp. RESULTS AND DISCUSSION Physical Characterization. The SEM image of the SWNT film on a pretreated GC disk (4 mm in diameter) is shown in Figure 1. Many SWNT bundles with general diameters of 20-30 nm can be observed. The length of the SWNT bundle could not be measured since both ends of the bundle were not visible at the same time. Some bundles twisted together and became indiscernible. From the image, it can be seen that the SWNT sample also contained a small portion of amorphous carbon impurities. Electrochemical Behavior of the SWNT Film. When a SWNT film-modified GC electrode was placed into pH 6.9 BrittonRobinson (B-R) buffer solution, a pair of reduction/reoxidation waves was observed. The peak potentials and peak currents (22) Niu, C. M.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl. Phys. Lett. 1997, 70, 1480-1482. (23) Shi, Z.; Lian, Y.; Zhou, X.; Gu, Z.; Zhang, Y.; Iijima, S.; Zhou, L.; Yue, K. T.; Zhang, S. Carbon 1999, 37, 1449-1453. (24) Shi, Z.; Lian, Y.; Liao, F.; Zhou, X.; Gu, Z.; Zhang, Y.; Iijima, S. Solid State Commun. 1999, 112, 35-37. (25) Li, B.; Lian, Y.; Shi, Z.; Gu, Z. Chem. J. Chinese Univ. 2000, 21, 16331635.
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Figure 1. SEM image of the SWNT film on a GC disk, magnification 50000×.
Figure 2. Cyclic voltammograms of the SWNT film on a GC electrode in pH 6.9 B-R buffer, 0.1 V s-1 scan rate between 0.4 and -0.4 V. (a) The first, (b) the second, and (c) the third cycle, and (d) after 24 h.
remained very stable after the second cycle (Figure 2). At a scan rate of 0.1 V s-1, the cathodic and anodic peak potentials were -0.126 and -0.024 V vs SCE, respectively. The reduction and reoxidation waves of the SWNT film were extremely broad. Similar to the case with the MWNT microelectrode,16,17 the background current of the SWNT film was apparently large. This might be attributed to the increased surface charge. Similar results were obtained on different electrode materials (GC, Au, and pyrolytic graphite). After the cyclic voltammetric experiment, the SWNT film-modified GC electrode was removed from the solution, rinsed in water and ethanol, and exposed to the air for 24 h, and then the same cyclic voltammetric experiment was performed again. The cyclic voltammogram is shown in Figure 2d. Comparing the results of Figure 2c, little change in the peak potentials and peak currents was found. These results indicate that the SWNT film was fairly stable. In addition, the amorphous carbon impurities were proved to have little effect on the electrochemical behavior of the SWNT film. Characteristics of the Electroactive Group. To study which group was involved in the electrode reaction, XPS and IR experiments were performed. The XPS spectrum of SWNT film on a glassy carbon disk (4 mm in diameter) showed a significant
Figure 3. XPS spectrum of the SWNT film on a GC disk.
Figure 4. Microscopic FTIR spectra of solid SWNT (top) and the reduced product of the SWNT film (bottom), with baseline correction.
amount of oxygen (Figure 3). The appearance of the peaks corresponding to the carboxylic acid group at 1729 cm-1, and the carboxylate group at 1588 cm-1, on the microscopic FTIR spectrum of solid SWNTs (Figure 4, top), which is in accordance with the literature,26,27 indicates that carboxylic acid groups and (26) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95-98. (27) Hamon, M. A.; Hui, J. C.; Chen, Y.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Adv. Mater. 1999, 11, 834-840.
the carboxylate groups were present on the surface of the SWNT. Since no alkali was used during the purification process, the existence of carboxylate groups might be attributed to the electroionization of the carboxylic acid group. As mentioned above, the SWNT film on an Au electrode had the same electrochemical behavior as that on a GC electrode. A 100-µL portion of 0.1 mg/mL SWNT solution was added dropwise on a 1.5 cm × 3 cm piece of Au foil to form a film with an approximate area of 15 mm2. The film was scanned from 0.4 to -0.5 V in pH 6.9 B-R buffer and left at -0.5 V for 10 min to allow the reduction to become more complete, and then the Au foil was removed from the solution, rinsed with water and ethanol, and dried under an infrared lamp. The reduced product was scraped carefully under a magnifier, and its microscopic FTIR spectrum was recorded (Figure 4, bottom). Compared with the FTIR spectrum of solid SWNT, the FTIR spectrum of the reduced product showed new peaks at 3338 and 1056 cm-1, which were due to the -OH and C-OH stretch modes, respectively. The peak at 2914 cm-1 was from the C-H stretch mode in methane. The peaks of COOH and COO- at 1736 and 1584 cm-1 were also present, but they were much less intense than other peaks, due to the incomplete reduction of the SWNT film. Therefore, during the redox process of the SWNT film, the carboxylic acid groups were the electroactive species and were reduced to CH2OH coupled with four electrons. The Effect of pH on the Electrochemical Behavior of the SWNT Film. The cyclic voltammetric results of the SWNT film in different media, including 0.1 M HAc-NaAc (pH 5.2), 0.1 M KH2PO4-Na2HPO4 (pH 6.6), 0.1 M KCl, 0.1 M trihydroxymethylaminomethane-HCl (pH 7.5), and 0.1 M NH3-NH4Cl (pH 8.3), showed that, in all the above media except KCl, electrochemical behavior similar to that in the BR buffer was obtained. In 0.1 M KCl solution, the peaks became flatter. Figure 5 shows the electrochemical behavior of the SWNT film in B-R buffer at different pH values. It was clear that both the cathodic and anodic peak potentials negatively shifted with increasing pH. The cathodic and anodic peak potentials depended linearly on the pH according Analytical Chemistry, Vol. 73, No. 5, March 1, 2001
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irreversible at higher scan rates. For the irreversible electrode reaction, the relationship between peak potential and the scan rate follows Laviron’s equation:29,30
EPc ) E ° ′ + EPa ) E ° ′ +
Figure 5. Cyclic voltammograms for the SWNT film at 0.1 V s-1 in (a) pH 2.3, (b) pH 3.2, (c) pH 5.1, (d) pH 6.9, and (e) pH 10.2 B-R buffer.
Figure 6. Semilogarithmic dependence of the cathodic peak potential (b), the anodic peak potential (9), and the scan rate for a SWNT film in pH 6.9 B-R buffer.
to the following equations: EPc ) 0.312 - 0.0608 pH and EPa ) 0.377 - 0.0545 pH, respectively. The slopes of the plots of EPc and EPa vs pH (-60.8 and -54.5 mV pH-1) were very close to the theoretical value of - 57.0 mV pH-1 at 15 °C. This shows that, during the electrode reaction, four protons are involved in the four-electron redox reaction of the SWNT film. Kinetic Measurement of the SWNT Film. As shown in Figure 6, the potential scan rate also had an influence on the cyclic voltammetric behavior of the SWNT film. A higher scan rate resulted in a higher current flow, as expected for a surface wave.28 The reduction peak current iPc was proportional to the scan rate over the range 0.02-5 V s-1. When the scan rate was lower than 0.1 V s-1, the cathodic peak potential EPc remained unchanged with an increase in the scan rate, but it would shift negatively at higher scan rates. The anodic peak potential did not change when the scan rate was lower than 0.5 V s-1, while it shifted positively as the scan rate increased. So the separation of peak potentials increased with increasing scan rate. When the scan rate was higher than 1 V s-1, the wave shape distorted severely. This indicates that the electrode reaction becomes electrochemically (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 522.
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RT RTks RT ln ln ν RnF RnF RnF
(1)
RTks RT RT ln + ln ν (1 - R)nF (1 - R)nF (1 - R)nF (2)
where R is the electron-transfer coefficient, ks is the standard rate constant of the surface reaction, v is the scan rate, and E ° ′ is the formal potential. Figure 6 showed the plot of EP vs ln v of the SWNT film in pH 6.9 B-R buffer solution. The E ° ′ () -0.073 V) could be estimated as the midpoint between the cathodic and the anodic peak potentials at low scan rate. At high scan rate, the plots of EP vs ln v were linear. The equations of the straight line were
EPc ) -0.141 - 0.0799 ln v
(3)
EPa ) -0.0347 + 0.0609 ln v
(4)
According to eqs 1 and 3, Rn was evaluated to be 0.31 and ks to be 5.3 s-1 at 15 °C. Thus, it was possible to estimate that the ratedetermining step of the reduction process was a single electrontransfer reaction (n ) 1, R ) 0.31). From eqs 2 and 4, the values of n (1 - R), and ks were obtained as 0.41 and 2.2 s-1, respectively. These results suggest that the rate-determining step of the reduction and reoxidation process might not be the same step. According to refs 31 and 32 and the results mentioned above, the mechanism of the electrode process of the SWNT film could be described as follows. The reduction process:
The reoxidation process:
Electrocatalysis of the SWNT-Modified Electrode toward the Oxidation of Biomolecules. As mentioned above, the SWNT film had very stable electrochemical behavior. It might be used as a chemically modified electrode to explore biological and analytical applications. In this work, biomolecules such as dopam(29) Laviron, E. J. Electroanal. Chem. 1974, 52, 355-393. (30) Laviron, E. J. Electroanal. Chem. 1979, 101, 19-28. (31) Baizer, M. M.; Lund, H. Organic Electrochemistry; Marcel Dekker: New York, 1983; p 379. (32) Baizer, M. M.; Lund, H. Organic Electrochemistry; Marcel Dekker: New York, 1983; p 507.
Figure 7. Cyclic voltammograms at a bare GC electrode (a) and SWNT film-modified electrode (b and c) in the absence of dopamine (b) and in 0.1 mM dopamine (a and c) in pH 6.9 B-R buffer. The potential scan rate is 0.1 V s-1.
Figure 9. Cyclic voltammograms at a bare GC electrode (a) and at a SWNT film-modified electrode (b and c) in the absence of epinephrine (b) and in 0.1 mM epinephrine (a and c) in pH 6.9 B-R buffer. The potential scan rate is 0.1 V s-1.
Figure 8. Cyclic voltammograms of 0.1 mM dopamine at a SWNTmodified GC electrode in pH 6.9 B-R buffer at scan rates of (a) 0.01, (b) 0.02, (c) 0.05, and (d) 0.1 V s-1 between -0.1 and +0.4 V.
Figure 10. Cyclic voltammograms at a bare GC electrode (a) and at a SWNT film-modified electrode (b and c) in the absence of ascorbic acid (b) and in 0.2 mM ascorbic acid (a and c) in pH 6.9 B-R buffer. The potential scan rate is 0.1 V s-1.
ine, epinephrine, and ascorbic acid have been measured on the SWNT modified electrode. The results showed that the SWNTmodified electrode had favorable catalytic activity with the biomolecules. In B-R buffer (pH 6.9), dopamine showed quasi-reversible behavior with an oxidation at 0.26 V and a reduction at 0.094 V on a bare GC electrode at a scan rate of 0.1 V s-1, as shown in Figure 7a. At the SWNT film-modified GC electrode, the anodic peak shifted negatively to 0.182 V, the cathodic peak shifted positively to 0.129 V, and the peak current increased significantly (see Figure 7c). Figure 8 shows the cyclic voltammetric response of dopamine at a SWNT-modified electrode at various scan rates. The separation of the peak potentials of dopamine increased with the increasing scan rate. When the scan rate was higher than 1 V s-1, the wave shape distorted severely. The anodic and cathodic peak currents depended linearly on the square root of the scan rate over the range of 0.01-0.5 V s-1, which suggested that dopamine was undergoing a diffusion-controlled process. In the range of 1.0 × 10-6-2.0 × 10-4 M, the anodic peak current increased linearly with the concentration of dopamine. The redox waves of the SWNT film itself did not change with the presence of dopamine. The fact that the cyclic voltammetric behaviors of
one GC electrode modified with the SWNT five times in 0.1 mM dopamine solution were similar and the voltammetric responses of one SWNT-modified electrode in eight dopamine solutions were the same showed that the SWNT-modified electrode had good reproducibility and stability. Epinephrine had a similar catalytic behavior on the SWNT filmmodified electrode. As shown in Figure 9, epinephrine demonstrated an irreversible oxidation at +0.31 V on a bare GC electrode. On the SWNT-modified electrode, the oxidation peak shifted negatively to +0.24 V, the peak current increased significantly, and a small reduction peak at +0.19 V was observed in the reversal scan. The peaks of the SWNT film itself did not change after the addition of epinephrine. For the case of ascorbic acid, its irreversible oxidation peak was located at +0.24 V on a bare GC electrode, while peak potential shifted to +0.16 V on the SWNT-modified electrode (Figure 10). It was clear that the SWNT-modified electrode also catalyzed the oxidation of ascorbic acid. CONCLUSION The films of SWNTs functionalized with carboxylic acid groups showed very stable cyclic voltammetric behavior in buffer soluAnalytical Chemistry, Vol. 73, No. 5, March 1, 2001
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tions, involving the reduction of the carboxylic acid groups on the SWNTs. With a high aspect ratio, the SWNT film had a much larger surface area than the bare GC electrode, which led to a large charge current. Compared with that of other carbon nanotube electrodes,16-19 the preparation of the SWNT-modified electrode was an economical, simple, and convenient way to utilize carbon nanotubes in electrochemistry. The SWNT-modified electrode showed promising electrocatalytic behavior toward several biomolecules. This modified electrode might be used in biosensors to study the electrochemistry of biosystems. Further
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application of the SWNT-modified electrode will be explored in the future. ACKNOWLEDGMENT This project was supported by the National Natural Science Foundation of China (Grants Nos. 29835110 and 29981001) and by the China Postdoctoral Science Foundation. Received for review August 15, 2000. Accepted November 29, 2000. AC000967L
