Anal. Chem. 2003, 75, 6949-6957
Electrochemical Characterization of Binderless, Recompressed Exfoliated Graphite Electrodes: Electron-Transfer Kinetics and Diffusion Characteristics P. Ramesh and S. Sampath*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India
Exfoliated graphite (EG) is prepared by the thermal exfoliation of graphite intercalation compounds at different temperatures. Surface and bulk physicochemical properties of EG are followed by spectroscopic and analytical methods and are observed to be a function of exfoliation temperature. EG particles can be recompressed without any binder and used as surface-renewable electrodes. Surface preparation is accomplished by either polishing or roughening the electrode surface using emery sheets. Effects of exfoliation temperature and the surface preparation on the electron-transfer kinetics and on the diffusion characteristics have been followed by electrochemical methods using several benchmark redox systems. It is found that the electron-transfer kinetics and the diffusion of K4[Fe(CN)6] are affected by the nature of the EG surface while that of iron(II)(1,10-phenanthroline)3 and cobalt(II)(1,10-phenanthroline)3 are not affected by the surface preparation. The redox systems are classified into different groups according to their kinetic sensitivity. Diffusion of electroactive species toward the EG electrodes is found to nonlinear. Current-time plots suggest that the recompressed EG electrodes can be modeled as fractals. Several investigations have been carried out to understand the heterogeneous electron transfer at carbon surfaces due to their diverse applications in the area of electrochemistry.1-10 The surface properties of carbon have a profound effect on the background current, adsorption, and observed electron-transfer rates.1,2 However, attempts to correlate surface properties to * Corresponding author. E-mail:
[email protected]. (1) McCreery, R. L. Carbon Electrodes: Structural Effects on Electron-Transfer Kinetics. In Electroanalytical Chemistry; A Series of Advances; Bard, A. J., Ed.; Dekker: New York, 1991; Vol. 17, pp 221-374. (2) McCreery, R. L. Electrochemical Properties of Carbon Surfaces. In Interfacial Chemistry: Theory, Experiment and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; pp 631-647. (3) Kneten, K. R.; McCreery, R. L. Anal. Chem. 1992, 64, 2518. (4) Allred, C. D.; McCreery, R. L. Anal. Chem. 1992, 64, 444. (5) Fagan, D. T.; Hu, I. F.; Kuwana, T. Anal. Chem. 1985, 57, 2759. (6) Poon, M.; McCreery, R. L. Anal. Chem. 1986, 58, 2745. (7) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J. Am. Chem. Soc. 1990, 112, 4617. (8) Rice, R. J.; McCreery, R. L. Anal. Chem. 1989, 61, 1637. (9) Sternitzke, K. D.; McCreery, R. L. Anal. Chem. 1990, 62, 1339. (10) Ranganathan, S.; Kuo, T.-C.; McCreey, R. L. Anal. Chem. 1999, 71, 3574. 10.1021/ac034833u CCC: $25.00 Published on Web 11/14/2003
© 2003 American Chemical Society
electroanalytical performance of carbon-based electrodes have not been very effective due to the dependence of the observed rate on the surface pretreatment, surface history, and redox system structure.3-10 Unambiguous conclusions can be made only when well-established surface pretreatment and benchmark systems are used to characterize these electrodes.1,2 Several pretreatment procedures such as polishing, electrochemical pretreatment, laser activation, solvent extraction, and vacuum heat treatment have been developed to achieve the goal of “rapid electron transfer” on carbon surfaces.5-20 The redox systems that have been used to understand the electron-transfer kinetics on carbon-based electrodes consist of three classes based on the electrochemical reversibility. They are quasi-reversible, inorganic systems, quasi-reversible, organic systems, and irreversible systems.1 The outer-sphere electron transfer associated with Fe(CN)63-/4- has been extensively used to benchmark various electrodes.21 To understand the effect of surface pretreatment and the mechanism of activation, the ideal choice of an electrode material is highly oriented pyrolytic graphite (HOPG). It exhibits high anisotropy in the observed rate.3,4,8 Additionally, it serves as a model surface for other graphitic carbons due to its highly oriented, defect-free surface.3 Electron-transfer kinetics on another popular electrode, glassy carbon (GC), is also extensively studied.11 Highly polished GC is observed to give a fast electron transfer for many different analytes.11-13 Electrochemical pretreatment (ECP) of GC is reported to result in the fast electron transfer (11) Kamau, G. N. Anal. Chim. Acta 1988, 207, 1. (12) Hu, I. F.; Karweik, D. H.; Kuwana, T. J. Electroanal. Chem. 1985, 188, 59, (13) Kamau, G. N.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1985, 57, 545. (14) Armstrong, F. A.; Cox, P. A.; Hill, H. A. O.; Lowe, V. J.; Oliver, B. N. J. Electroanal. Chem. 1987, 217, 331. (b) Szucs, A.; Novack, M. J. Electroanal. Chem. 1995, 383, 75. (15) Chen, P.; McCreery R. L. Anal. Chem. 1996, 68, 3958. (16) Rice, R.; Allred, C. R.; McCreery, R. L. J. Electroanal. Chem. 1989, 263, 163. (17) Engstom, R. C.; Diamantis, A. A.; Murphy, W. R., Jr.; Linton, R. W.; Meyer, T. J. J. Am. Chem. Soc. 1985, 107, 1845. (18) Bowling, R.; Packard, R. T.; McCreery, R. L. Langmuir 1989, 5, 683. (19) Cai, X.; Kalcher, K.; Neuhold, C.; Ogorevc, B. Talanta 1994, 41, 407. (20) Chi, Q.; Gopel, W.; Ruzgas, T.; Gorton, L.; Heiduschka, P. Electroanalysis 1997, 9, 357. (21) Goldstein, E. L.; Van De Mark, M. R. Electrochim. Acta 1982, 27, 1079. (b) Sohr, R.; Muller, L. Electrochim. Acta 1975, 20, 451, (c) Kuta, J.; Yeager, E. J. Electroanal. Chem. Interfacial Electrochem. 1975, 59, 110. (d) Peter, L. M.; Durr, W.; Bindra, P. Gerisher, H. J. Electroanal. Chem. 1976, 71, 31.
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of catechols.17 Polishing and ECP increase the surface oxygencontaining functional groups as confirmed by X-ray photoelectron spectroscopic (XPS) studies.11,13 Hydrogen modification of a GC surface has been found to have no marked effect on the peak potential difference (∆Ep) of reaction involving K4[Fe(CN)6] and dopamine, but the ∆Ep of Fe2+(aq)/Fe3+(aq) is found to be affected.22 Monolayers of nitrophenyl, methylene blue, or anthraquinonesulfonic acid on GC have been used to characterize the electron-transfer kinetics of different redox systems.15 Electrontransfer kinetics on carbon fiber is extensively studied since it is well suited for in vivo measurements.23-26 Electron-transfer kinetics on carbon paste electrodes has also been reported for various systems.1,20 The surface activation procedures result in exposure of active edge planes on the HOPG electrodes.1 A faster electron-transfer rate is observed on active sites than that observed on the less active (smooth) sites. The surface is considered to be kinetically heterogeneous when both active and less active sites are simultaneously present. The surface heterogeneity can have a profound effect on the observed voltammetry. The assumption of planar diffusion is no more valid when the surface heterogeneity is of the order of the diffusion layer thickness, xDt, where D is the diffusion coefficient and t time of the experiment. The size and shape of the active edge sites may vary depending on the surface pretreatment. A laser activation method can be used to precisely control the active sites on the HOPG basal plane to produce microarray electrodes.9 Composite electrodes are known to show microarray electrode behavior. This depends on the particle size and the loading of carbon particles in the inert matrix. Deviation of the current versus (time)-1/2 plots from Cottrell behavior is an indication of a nonlinear type of diffusion behavior.27-29 Several researchers have considered the rough or partially blocked electrodes as fractals.30-32 Fractals are self-similar structures. Pajkossy reported on the diffusion of electroactive species toward rough and partially blocked electrodes.31 He has used the log (I)-log (t) response of an electrode to determine the fractal nature of an electrode surface. Exfoliated or expanded graphite is a low-density material with high temperature resistance.33,34 When the graphite intercalation compounds are subjected to a thermal shock, the intercalates undergo a rapid phase transition resulting in the formation of an expanded, puffed-up material known as exfoliated graphite (EG).35 (22) Kuo, T.-C.; McCreery, R. L. Anal. Chem. 1999, 71, 1553. (23) Garris, P. A.; Wightman, R. M. In Neuromethods: Voltammetric Methods in Brain Systems; Boulton, A. A., Baker, G. B., Adams, R. N., Eds.; Humana Press, Totowa, NJ, 1995; Vol. 27, Chapter 6 (24) Pihel, K.; Walker, Q. D.; Wightman, R. M. Anal. Chem. 1996, 68, 2084. (25) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G. E.; Travis, E. R.; Wightman, R. M. Anal. Chem. 1996, 68, 3180. (26) Bath, B. D.; Michael, D. J.; Trafton, B. J.; Joseph, J. D.; Runnels, P. L.; Wightman, R. M. Anal. Chem. 2000, 72, 5994. (27) Lipka, S. M.; Cachen, Jr, G. L.; Stoner, G. E.; Scribner, L. L., Jr.; Gileadi, E. J. Electrochem. Soc. 1988, 135, 368. (28) Anderson, J. E.; Tallman, D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051. (29) Weisshaar, D. E.; Tallman, D. E. Anal. Chem. 1983, 55, 1146. (30) Pajkossy, T. J. Electroanal. Chem. 1991, 300, 1. (31) Pajkossy, T.; Nyikos, L. Electrochim. Acta 1989, 34, 171. (b) Pajkossy, T.; Nyikos, L. Electrochim. Acta 1989, 34, 181. (32) Kant, R. J. Phys. Chem. B 1997, 101, 3781. (b) Dassas, Y.; Duby, P. J. Electrochem. Soc. 1995, 142, 4175. (33) Chung, D. D. L. J. Mater. Sci. 1987, 22, 4190. (34) Chung, D. D. L. J. Mater. Eng., Perform. 2000, 9,161.
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EG powder can be recompressed or restacked without any binder, and the recompressed material is amenable to polishing or surface renewal by mechanical means.36 EG-based materials have been used as seals, high-temperature gaskets, catalyst supports, and adsorption substrates owing to the near-perfect crystallographic face that it provides.33,37 Very few studies deal with the use of EG-based material for electrochemical applications.38-40 Fukuda and co-workers38 have reported the use of foliated natural graphite as anodes for rechargeable lithium ion cells. Preliminary cyclic voltammetric data on the EG electrodes have been reported by Frysz and Chung.39 However, the use of bare EG in electroanalytical chemistry has not been explored so far. We have recently reported on the use of covalently functionalized EG for determination of analytes such as NADH and alcohol.41 In this paper, we report on the detailed electrochemical characterization of bare EG-based electrodes including electron-transfer kinetics and diffusion characteristics of various redox systems. EXPERIMENTAL DETAILS Exfoliation of natural graphite particles (particle size 300-400 µm, Stratmin Graphite) were carried out as follows: Natural graphite particles were intercalated by immersing the particles in a mixture of concentrated H2SO4/HNO3 (3:1 by volume) for 24 h at ambient conditions. The material was washed with distilled water and air-dried. Exfoliation was then carried out by introducing the material in a preheated furnace. Exfoliation was carried out at 800, 600, and 400 °C in air for 1 min, and the resulting materials are denoted as SE800, SE600, and SE400, respectively. The electrodes are prepared by compressing ∼150 mg of EG as a pellet at a pressure of 6 tons/cm2 for ∼5 h for SE800 andSE600 and at a pressure of 4 tons/cm2 for 8 h for SE400. The pellet was cut in to small pieces using a clean, sharp knife and mounted on Pyrex glass tubes using conducting silver epoxy and copper wire and made as electrodes. The electrodes were subsequently polished well with SiC paper of 600- and 1500-grit emery sheets. A smooth surface was obtained by further polishing using 4/0, 5/0, and 6/0 emery polishing papers. Rough surfaces were obtained by scratching the surface against different grades of emery sheets in the same direction. Roughened electrodes are represented with the grit number of emery sheet used. Electrodes were washed with double-distilled water after the surface preparation and used for electrochemical studies. Surface area measurements were carried out using a Quantachrome surface area analyzer (Quantasorb). Porosity measurements were carried out using a mercury porosimeter (Quantachrome Porosimeter, Autoscan 60) on pellet samples. The elemental analysis was carried out using a Carlo Erba CHN analyzer. The samples were degassed thoroughly before the measurements. Density measurements were carried out by finding out the mass of 20 mL of EG without applying any external pressure. The (35) Anderson, S. H.; Chung, D. D. L. Carbon 1984, 22, 253 (36) Dowell, M. B.; Howard, R. A. Carbon 1986, 24, 311 (37) Gilbert, E. P.; Reynolds, P. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1998, 94, 1861. (38) Fukuda, K.; Kikuya, K.; Isono, K.; Yoshio, M. J. Power Sources 1997, 69, 165. (39) Frysz, C. A.; Chung, D. D. L. Carbon 1997, 35, 858. (40) Kao Corp., Jpn. Kokai Tokkyo Koho JP 59 78 204, 1984; Chem. Abstr. 1984, 101, 131358z. (41) Ramesh, P.; Sampath. S. Anal. Chem. 2000, 72, 3369. (b) Ramesh, P.; Sivakumar, P.; Sampath, S. J. Electroanal. Chem. 2002, 528, 82.
apparent density of the EG was determined by the ratio of mass to volume. The conductivity of recompressed EG pellets has been measured using a four-probe method. The four contact points were formed by attaching thin copper wires to the EG pellet through conducting silver epoxy. A programmable current source (model 220) and a multimeter (model 2000) from Keithley Instruments, Inc. were used for the measurements. Functional group analysis was carried out as follows: 100 mg of EG was stirred with 20 mL of an appropriate base of 0.01 M under ambient conditions for 24 h. Excess base was then titrated against standard HCl solution to find out the concentration of the remaining base. NaHCO3 and Na2CO3 were used to determine the amount of carboxyl and lactone functional groups, respectively. pHPZC measurements were carried out using a batch equilibrium method.42 Briefly, 20 mg of EG was shaken with 10 mL of 0.1 M KNO3 and equilibrated. The initial pH of the KNO3 solution was adjusted by the addition of 0.1 M HNO3 or KOH, and the ionic strength was maintained constant. After equilibration for 24 h, the final pH of the solution was measured. The consumption of H+ or OH- ions was calculated from the difference in the initial and the final concentration of these ions. The plateau region of the intial pH versus final pH plots corresponded to the pHPZC. The electrochemical cell contained the EG pellet as the working electrode, platinum foil as the counter electrode, and calomel as the reference electrode. All the measurements were carried out under nitrogen atmosphere. The measurements were carried out using either a CHI 660A electrochemical analyzer from the CH Instruments or Versastat II from EG&G PARC. Impedance spectra were obtained using a lock-in amplifier (model 5210) coupled with a potentiostat/galvanostat (263A, EG&G, PARC). The spectra were collected in the frequency range of 100 kHz-100 mHz with a 5-mV rms ac perturbation at the formal potential. Scanning electron microscope (SEM), Raman, and XPS spectra were carried out using pellets of EG with polished and different rough surfaces. Scanning electron micrographs were obtained using a JEOL microscope (JSM 840A) operating at 20 kV. FT-IR experiments were performed on a Bruker Equinox 55 FT-IR spectrometer from 400 to 4000 cm-1 with a resolution of 4 cm-1. FT-Raman experiments were carried out using a Bruker IFS 100/S spectrometer with an excitation wavelength of 1064 nm in the backscattering geometry. Measurements were carried out between 50 and 4000 cm-1 with a resolution of 4 cm-1. All the spectra were averaged over 200 scans. XPS spectra were recorded using a VG Scientific ESCA LAB (Mark IV, England) spectrometer. Redox systems used in the present study are composed of K4[Fe(CN)6], iron(II)(1,10-phenanthroline)3 [Fe(phen), 2 mM ferrous ammonium sulfate + 6 mM 1,10-phenanthroline] and cobalt(II)(1,10-phenanthroline)3 [Co(phen), 2 mM CoCl2 + 6 mM 1, 10 phenanthroline] in 1 M KCl; Fe2+ at pH 4.0 and dopamine, ascorbic acid, and NADH in phosphate buffer, pH 7.0. Analytical reagent grade chemicals and double-distilled water were used all for the measurements. RESULTS AND DISCUSSION Physicochemical Properties of Exfoliated Graphite. Surface morphology of EG powder resembles a wormlike structure, (42) Babic, B. M.; Milonjic, S. K.; Polovina, M. J.; Kaludierovic. B. V. Carbon 1999, 37, 477.
Figure 1. Scanning electron micrographs of SE800 pellet (A) polished and (B) rough surfaces. Surface roughness is created using a 1500-grit emery sheet.
and it is found to change during recompression. SEM pictures of polished and rough pellets of SE800 are given in Figure 1. The SE800 pellet with a polished surface (Figure 1A) has a preferential orientation of the basal planes along the polishing direction. However, there are defect sites of the order of 3-4 µm, with an average distance of 8-15 µm between them. The polished SE600 pellets show similar morphology while that of the polished SE400 surface shows a lot of defects. The surface profiles of polished SE800, SE600, and SE400 pellets show an average roughness of 0.3, 1, and 2 µm, respectively. The variation is likely to be due to the fact that the EG microparticles are relatively well oriented due to compression in the case of SE800, while in the case of SE600 and SE400, they are not. The rough electrodes show lineshaped defects that expose a large fraction of reactive edge planes (Figure 1B). The average roughness of different rough EG pellets is found to be of the order of 10 µm. The roughness on an electrode surface may affect the diffusion characteristics of a freely diffusing species present in the bulk of the solution toward the electrode surface.1 In the case of EG electrodes, the defect size or the distance between them will give important input on the diffusion characteristics of various redox systems. In the case of recompressed EG electrodes, nonlinear diffusion is expected depending on the roughness. Table 1 shows the surface area, apparent density, and crystallite parameters of EG as a function of exfoliation temperatures. The surface area of EG is found to increase while the density decreases Analytical Chemistry, Vol. 75, No. 24, December 15, 2003
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Table 1. Surface Area, Apparent Density, and Crystallite Parameter of EG Samples sample natural graphite SE400 SE600 SE800
surface apparent area/m2‚g-1 density/g‚L-1 La/nm Lc/nm d/nm 0.37 2.23 8.34 40.5
793 24 7.2 3.3
7.74 12.6 16.1 61.8
81.86 19.01 48.47 51.13
0.340 0.344 0.337 0.337
as the exfoliation temperature is increased. The EG prepared at higher exfoliation temperatures are more crystalline than the ones prepared at lower temperatures (XRD not shown). This may be due to the presence of residual intercalates and also the oxygencontaining functional groups that may be present on EG at lower temperatures. This will, in turn, reduce the stacking in the c direction giving rise to different crystallinity. The XRD patterns of the SE800 pellet are very similar to that of the powder sample though the intensities are high with the pellets. The graphitic nature of EG is not affected during recompression as confirmed by the unaltered peak position and peak width after recompression. Lc and La are calculated from the line width using the Debye-Sherrer equation.38 It is observed that the Lc value decreases as the exfoliation temperature is decreased. Mercury porosimetry results show that the recompressed pellets of SE800 and SE600 show pores of radius 20-100 and 50-200 nm, respectively. Raman spectra of the EG samples show two distinct bands (Figure 2A). The origin of the 1581-cm-1 band (referred to as the G band) is the first-order scattering of E2g symmetry while the origin of the 1290-cm-1 band (referred to as the D band) is due to the presence of the distributed edge planes.43 A second-order scattering band at 2600 cm-1 is also observed, and it is referred to as the G′ band (not shown). The shoulder that appears at 1610 cm-1 called the D′ band, is due to a disorder in the c direction.43 The intensities of the D and D′ bands increase as the exfoliation temperature is decreased. The intensity ratios of D/G (ID/IG) for SE400, SE600, and SE800 are observed to be 0.65, 0.59, and 0.25, respectively. These observations clearly indicate that the disorder increases as the exfoliation temperature is decreased. This correlates well with the XRD observations. This disorder is likely to be due to the presence of oxygen functional groups and also due to the leftover intercalates at low temperatures. Recompressed EG pellets with a polished surface show a band at 1600 cm-1 (G band) and another band at 1290 cm-1 (D band). The E2g peak that is generally observed at 1581 cm-1 broadens and shifts to a higher frequency with decreasing La and Lc. This shift can be correlated with the orientation, microstructure, and heat treatment history to some extent, but definite conclusions are not possible at present. The compression of EG particles to form a pellet probably changes the microstructure that results in the shift of the E2g peak to 1600 cm-1. The Raman spectrum of powder EG resembles that of a rough SE800 surface, where the relative intensity of the D band is less than that of the G band and the ratio ID/IG is 0.32. Hence, the rough surface can be thought of as representing the bulk structure of the EG. The Raman spectra (43) Katagiri, G.; Ishida, H.; Ishitani, A. Carbon 1988, 26, 565. (b) Leung, S. Y.; Dresselhaus, M. S.; Dresselhaus, G. Physica 1981, 105B, 375.
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Figure 2. (A) Raman spectra of EG powder samples: (a) SE800, (b) SE600, and (c) SE400. (B) C 1s region of the XPS spectra of recompressed EG pellet with 400-grit surface roughness: (a) SE800 and (b) SE400.
are recorded with an incident laser spot of size of 0.1 mm. The SEM pictures show the defect sizes to be of the order of 3-4 µm, separated by 8-15 µm. Hence, all the exposed defects would be picked up to show a band at 1290 cm-1. Rough SE600 and SE400 pellets show spectra similar to that of the powder sample as observed in the case of SE800 graphite. The XPS spectrum of carbon exhibits asymmetry like that of metals. The asymmetry is explained by the many-body scattering effect based on the joint density of states of electron and hole pair excitation.44 Line width and symmetry are two important parameters in evaluating the graphitic nature of the EG. It is observed that the line width of EG increases from 2 to 2.8 eV when the exfoliation temperature is decreased from 800 to 400 °C (Figure 2B). Complete graphitic structure such as HOPG shows a line width of 1.25 eV.44 Though the differences in the line width are not clear, it may still be inferred from the trend that SE800 is relatively more graphitic than SE600 and SE400. The line width of evaporated carbon film is comparable to that of SE800 (2 eV).44 The IR spectrum of SE800 shows peaks at 1059, 1225, and 1650 cm-1 that correspond to phenolic, alcoholic, and carboxyl functional groups, respectively. The broad band that appears at 34003450 cm-1 is due to the OH functional group. SE600 shows a similar spectrum, but SE400 shows the presence of a large amount of water molecules associated with it. Table 2 shows the relative variation of functional groups determined by the titration method, (44) Takahagi, T.; Ishitani, A. Carbon 1988, 26, 389.
Table 2. Oxygen Concentration and Point of Zero Charge of EG Samples sample
O/%a
carboxyl/b mequiv/g
lactone/b mequiv/g
natural graphite SE400 SE600 SE800
13.6 4.5 3. 6 1.5
2.0 6.9 1.5 1.1
4.0 1.6 0.2
a
O/Cc
pHPZC
0.12 0.05 0.04
3.75 6.0 6.7
Elemental analysis. b Titrations. c XPS.
O/C ratio from the XPS, and pHPZC for the exfoliated graphite prepared at different temperatures. The results are in conformity with the fact that the higher exfoliation temperature results in lower oxygen content and functional groups. Consequently, the pHPZC of SE800 shifts toward a value higher than that of SE600 and SE400 EG. Electrochemical Characterization of Recompressed EG Electrodes. EG powder can be restacked or recompressed without any binder material. The restacking is reported to involve interlocking of graphene sheets yielding a compact, strong pellet.36 The conductivities of SE800, SE600, and SE400 pellets are observed to be 1521 ( 24, 578 ( 60, and 2.1 ( 0.15 S cm-1, respectively. Stable voltammetric background response is observed from -1 to 1.2 V in aqueous media of various pH values. The background charging current and the associated double layer capacitance are of the order of 120 µA/cm2 and 2.4 mF/cm2, respectively, for the polished surface of SE800 electrodes. Geometric area is used for the determination of double layer parameters. The polished surface contains basal planes as well as defects arising out of interparticle contact. Hence, the surface is poorly defined for the determination of actual electrochemically active surface area. The roughness created on EG shows an increase of only about 10-20% in the double layer capacitance. A comparison with HOPG is appropriate at this point. The basal plane has been reported to show a capacitance of 2 µF cm-2 that is attributed to a space charge layer caused by the semimetal character of HOPG. The basal plane of HOPG behaves as a nearly ideal capacitor, with no frequency dispersion.1 The edge plane, on the other hand, shows a capacitance of 60-70 µF cm-2. The capacitance is reported to be a weighted average of edge and basal contribution.1 The polished EG electrodes show fairly large frequency dispersion in the impedance spectra, and a pure capacitive behavior is not observed (not shown). The observed capacitance is a contribution of double layer capacitance and pseudocapacitance arising out of the functional groups present on the EG surface. The background currents and the associated capacitance increase with a decrease in the exfoliation temperature as, SE800 < SE600 < SE400. This is in line with the O/C ratio based on the XPS analysis as explained earlier. Electrochemical characterization of carbon-based electrodes has been reported using various benchmark redox systems. These redox systems are classified into several groups according to their kinetic sensitivity.15 The EG electrodes are characterized using various redox systems. The experiments have been carried out using recompressed SE800, SE600, and SE400 electrodes. Detailed discussion of electron-transfer kinetics and diffusion characteristics is given for SE800 electrodes. The electrochemical characteristics
Figure 3. (A) Cyclic voltammograms of K4[Fe(CN)6] on SE800 electrode (a) polished surface and (b) 400-grit rough surface. Electrolyte used: 1 M KCl containing 6 mM K4[Fe(CN)6]. Scan rate used: 10 mV/s. Inset: cyclic voltammogram of polished SE800 electrode in 1 M KCl containing 8 mM [K4Fe(CN)6]. Scan rate used: 10 V/s. (B) Impedance spectra of polished SE800 electrode. Inset: High-frequency region; dc bias used, 0.085 V. Electrolyte: 1 mM K4Fe(CN)6] in 1 M KCl. (C) Impedance spectra of 400-grit rough SE800 electrode; dc bias used, 0.1 V. Electrolyte: 1 mM K4Fe(CN)6] in 1 M KCl.
of EG prepared at other temperatures (SE600 and SE400) are briefly explained at the end of each section. a. Surface-Sensitive, Oxygen-Insensitive System: K4[Fe(CN)6]. The redox system K4[Fe(CN)6]/K3[Fe(CN)6] is widely used to characterize any new electrode material. The EG electrodes have been characterized using K4[Fe(CN)6] in KCl or KNO3 supporting electrolyte. Figure 3A shows the cyclic voltammograms of K4[Fe(CN)6] on SE800 electrodes with varying roughness, in 1 M KCl supporting electrolyte. It is observed that the peak potential differences (∆Ep) for polished and rough EG electrodes are 0.25 and 0.1 V, respectively, for a concentration of 6 mM K4[Fe(CN)6] at a scan rate of 10 mV/s. The predominantly basal plane oriented polished surface leads to a slow electron transfer while the edge planes associated with the rough electrode lead to a fast electron transfer. It is reported that the electron-transfer kinetics of K4[Fe(CN)6] is faster on the edge plane than that on the basal plane of HOPG.18 Various other activation procedures for carbon-based electrodes report a similar difference in the electron-transfer kinetics.3-20 The perception of involvement of oxygen functional groups in the electron-transfer step is inconsistent with different activation procedures reported.3-20 The polishing and electrochemical pretreatment may introduce oxygen to the exposed edge planes while laser activation and vacuum heat treatment do not intentionally introduce oxygen to the carbon Analytical Chemistry, Vol. 75, No. 24, December 15, 2003
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surface. However, the laser-activated and vacuum heat-treated carbon electrodes show a fast electron-transfer rate as well. Hence, fast electron transfer on an activated electrode is attributed to the interaction of K4[Fe(CN)6] with the exposed edge planes and not to an interaction with oxygen-containing functional groups. In the case of SE800 electrodes, the change in the electron-transfer kinetics of K4[Fe(CN)6] may as well be attributed to the exposure of active edge planes. It is observed that the ∆Ep is found to increase from 0.25 to 0.3 V when the scan rate is changed from 10 to 20 mV/s for polished EG electrodes. However, the voltammogram on a polished surface at a high scan rate of 10 V/s shows the presence of two sets of peaks corresponding to the redox reaction at active and less active sites (Figure 3A inset). The polished surface has an irregular distribution of edge sites on the surface (Figure 1A). The separation of the two peaks is, however, not observed at slow scan rates. This may be due to the time scale involved leading to a planar overlapping diffusion profile that does not differentiate the two sites at low scan rates. In the case of rough surfaces, the ∆Ep does not vary much (0.100-0.115 V) for a wide range of scan rates. The insensitivity of ∆Ep for scan rate indicates that the electron transfer on rough surfaces is faster than that observed on polished ones. The electron-transfer kinetics of K4[Fe(CN)6] is known to be affected by the identity and concentration of the supporting electrolyte.21 The K+ ion in the supporting electrolyte is known to form a bridge between the edge plane of graphite and the [Fe(CN)6]4- species.1 It is observed that the concentration of K+ in the solution affects the electron-transfer kinetics in the case of SE800 electrodes as well. The change in ∆Ep when the KCl concentration is varied from 1 to 0.1 M is only 0.012 V on a rough surface while it is found to vary to a large extent on a polished SE800 electrode (0.585 V), at a scan rate of 10 mV/s. This variation of ∆Ep with the concentration of K+ confirms that K+ forms a bridge between [Fe(CN)6]4- and the graphite. This effect is observed to be predominant on a polished surface where the redox system is found to be quasi-reversible and kinetically less facile. The bridging effect does not affect the electron-transfer kinetics considerably on the rough electrodes. The change of anion in the supporting electrolyte from KCl to KNO3 does not have any effect, confirming that the anion does not participate in the bridging effect. Cyclic voltammetric results on SE600 and SE400 show that the ∆Ep values observed on a polished surface are 0.090 and 0.095 V, respectively, while it is 0.25 V on a SE800 electrode at a scan rate of 10 mV/s. The fraction of basal planes is higher in the case of polished SE800 electrodes than on polished SE600 and SE400 electrodes. This may be explained based on the preferential orientation of SE800 when compared to SE600 and SE400 electrodes. Additionally, an increase in the oxygen content on the EG surface by an intentional oxidation/reduction pretreatment41 does not have any effect on the electrochemical characteristics. Hence, it can be inferred that [Fe(CN)6]4- is insensitive to oxygencontaining functional groups. Impedance characteristics of K4[Fe(CN)6] on EG electrodes are shown in Figure 3B,C. The polished electrode shows a higher charge-transfer resistance than that observed with rough electrodes. It is evident from impedance spectroscopy that the electron 6954 Analytical Chemistry, Vol. 75, No. 24, December 15, 2003
Table 3. Charge-Transfer Resistance (Rct) and Heterogeneous Rate Constant (k°) Values for the SE800 Electrodes with Polished and 400-Grit Roughness at the Formal Potentiala redox systems K4[Fe(CN)6] Fe(phen) Co(phen)
polished Rct/Ω‚cm2 k°/cm‚s-1 65.5 0.35 1.08
3.03 × 10-3 3.78 × 10-1 1.07 × 10-1
400-grit rough Rct/Ω‚cm2 k°/cm‚s-1 0.35 0.30 0.57
3.80 × 10-1 4.43 × 10-1 2.16 × 10-1
a Electrolyte used: 2 mM K [Fe(CN) ], Co(phΩen), and Fe(phen) 4 6 in 1 M KCl.
transfer occurs at two different rates on edge and basal planes. The polished surface, however, shows two semicircles corresponding to two different rates at the distributed edge and basal planes, respectively. This is amplified in the expanded highfrequency region shown as an inset in Figure 3B. The chargetransfer resistance and the corresponding rate constants are given in Table 3. The rate constant increases by 2 orders of magnitude when the surface is changed from polished to rough. However, different roughnesses do not change the rate constant very much. b. Surface-Insensitive Systems: Fe(phen) and Co(phen). Redox systems whose electron-transfer kinetics do not show considerable anisotropy depending on the surface roughness and associated oxygen-containing functional groups are classified as “surface insensitive”. The voltammograms of Co(phen) on polished and rough surfaces of the SE800 electrode in 1 M KCl, at a scan rate of 50 mV/s, are shown in Figure 4A. Both polished and rough surfaces show peak-shaped voltammograms. The ∆Ep values for polished and rough SE800 electrodes are 0.12 and 0.074 V, respectively, at a scan rate of 50 mV/s. Based on the observed ∆Ep values, it is clear that the redox systems are fairly reversible on both surfaces. The small increase in peak currents on roughening the surface may be attributed to the changes in the area of the electrode. The electron-transfer rates of Co(phen) and Fe(phen) are found to be unaffected by surface preparation. These systems do not have anisotropy in their rate as observed in the rate constant values. The rate constants are determined using impedance data on polished and rough SE800 electrodes (Table 3). The charge-transfer resistance and the rate constants do not differ considerably showing that the electron-transfer rate is not affected by roughening the electrode surface. The electrochemical responses on SE600 and SE400 EG electrodes are found to be similar to that observed on SE800. This leads to the conclusion that the amount of oxygen-containing functional groups does not affect the redox kinetics of the Fe(phen) and Co(phen) systems. This is similar to the results reported for HOPG and GC electrodes for these redox systems.3,15 c. Specific Functional Group-Sensitive Redox System: Fe2+/3+(aq) and NADH. Oxidation of NADH is known to be catalyzed by o-quinone functional groups. High overpotential, of the order of 1.2 V, is observed for the oxidation of NADH (reduced form of nicotinamide adenine dinucleotide) at pH 7.0 on both polished and rough SE800 electrodes. This indicates that the exposure of edge planes by roughening the surface does not catalyze the NADH oxidation. This is likely to be due to the lack of carbonyl functional groups on the EG surface. We have
Scheme 1. Classification of Redox Systems on EG Electrodes
Figure 4. (A) Cyclic voltammograms on SE800 electrodes in 2 mM Co(phen) in 1 M KCl. (B) Cyclic voltammograms on SE800 electrodes in 0.1 M acetate buffer, pH 4.0 containing 6 mM Fe2+: (a) polished and (b) 400-grit rough surfaces. Scan rate used, 50 mV/s.
demonstrated the electrocatalysis of NADH on o-quinone functionalized EG electrodes. Oxidation of NADH is observed at 0.15 V on modified electrodes.41 The voltammograms obtained for the Fe2+/3+ redox reaction in acetate buffer of pH 4.0 on polished and rough SE800 electrodes are given in Figure 4B. The ∆Ep is very large, of the order of 0.4 V, at a scan rate of 50 mV/s. This is likely to be due to the lack of carbonyl functional groups on EG as observed from XPS and IR data. It is known that the carbonyl functional groups present on an electrode surface catalyze the electron transfer of Fe2+(aq).22 Hydrogen-modified GC with low oxygen content has been reported to show a high ∆Ep while anthraquinone-adsorbed GC shows a small ∆Ep for Fe2+/3+ reaction in H2SO4 medium. The ∆Ep for Fe2+/3+ reaction is not changed by roughening the EG surface though the peaks are better defined on edge sites than on smooth sites. This leads to the conclusion that both basal and edge planes have the same activity toward this system. The absence of carbonyl functional groups is responsible for this effect. SE600 and SE400 electrodes show similar voltammograms as
observed on the SE800 electrodes. Though the amount of oxygencontaining functional groups is higher on SE600 and SE400 than SE800, the reaction is not catalyzed since the functional groups are not carbonyl-based. Hence, NADH and Fe2+ are classified as the systems that are catalyzed by specific functional groups. d. Organic Systems: Dopamine and Ascorbic Acid. Dopamine and ascorbic acid are often used to characterize carbon electrode surfaces.1,2 A high overpotential of 0.45 V for the oxidation of dopamine is observed on the polished SE800 electrode at pH 7.0, and on the rough electrode, the oxidation is observed at 0.125 V. The trend is similar to the observations reported for HOPG electrodes.18 The oxidation potential of dopamine remains unchanged on SE800, SE600, and SE400 at pH 7.0 indicating that dopamine oxidation is oxygen functional group insensitive. Oxidation of ascorbic acid, however, is found to be affected by the surface preparation as well as the oxygen functional groups. Oxidation of ascorbic acid is observed at 0.45 V on a polished SE800 electrode at pH 7.0 whereas the oxidation is observed at a very low potential of -0.1 V on a rough electrode. Ascorbic acid is already known to be a surface-sensitive system.16 The oxidation potential of ascorbic acid is found to increase as the exfoliation temperature is decreased. The pHPZC of SE800, SE600, and SE400 are determined to be 6.7, 6.0, and 3.75, and hence, negatively charged ascorbate anion is repelled by the negatively charged surface functional groups at pH 7.0. Hence, the overpotential also increases as the exfoliation temperature is decreased. A detailed discussion on these two systems will be reported elsewhere.45 e. Classification of Redox Systems on EG Electrodes. Based on the above discussion, the redox systems can be classified into different groups according to their electrochemical response of the EG electrodes (Scheme 1). The classification is mainly based on the variation of electrochemical kinetics with the surface preparation and the redox system structure. Diffusion Characteristics. The rate of heterogeneous electron transfer mainly depends on the redox system structure, pretreatment, and surface chemistry.1-20 Surface pretreatment resulting in the exposure of edge planes can alter the physical properties of the electrode surface. Surface oxygen functional groups may also be introduced depending on the pretreatment conditions. If the surface pretreatment results in heterogeneity, (45) Ramesh, P.; Suresh, G. S.; Sampath. S. J. Electroanal. Chem. In press.
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Table 4. Peak Shapes as a Function of Roughness and Scan Rates Observed on SE800 Electrodesa electrode surface polished 1500-grit rough 600-grit rough 400-grit rough a
scan rate range
peak shape
2-20 mV/s and 5-10 V/s 20 mV/s-2 V/s 2-20 mV/s and 5-10 V/s 50 mV/s-2 V/s 2-50 mV/s and 2-10 V/s 75 mV/s -1 V/s 2-125 mV/s and 1-10 V/s 150-500 mV/s
peak sigmoidal peak sigmoidal peak sigmoidal peak sigmoidal
Electrolyte used: 8 mM K4Fe(CN)6] in 1 M KCl.
Figure 5. Normalized cyclic voltammograms on 400-grit rough SE800 electrodes at different scan rates: (A) 20 mV/s; (B) 200 mV/ s; (C) 2 V/s. Electrolyte used: 8 mM K4Fe(CN)6] in 1 M KCl.
then it may also affect the diffusion of electroactive species toward the electrode surface. It is already reported that the diffusion of electroactive species is affected by surface preparation when surface heterogeneity, in the form of roughness, is of the order of few micrometers.9,27 SEM studies on the EG electrode surface show that the polished electrode has defect sites distributed on the predominantly basal plane oriented surface. Roughening the surface leads to the exposure of a high fraction of edge planes. In the case of EG, it is observed that the shape of the voltammograms change depending on the surface roughness and scan rate. We have used the K4[Fe(CN)6] redox system to probe the diffusion characteristics toward various EG surfaces since it is known to be a system with an outer-sphere electron-transfer mechanism.1 Figure 3A shows the effect of surface roughness on the shape of the voltammograms. Sigmoidal- and peak-shaped responses are observed for the polished as well as rough surfaces. The shape also depends on the scan rate employed. Figure 5 shows the variation of peak shape with respect to the scan rate. Table 4 gives an account of the dependence of the shape of the voltammograms on the scan rate with different roughnesses. At sufficiently slow scan rates, a peak-shaped voltammogram is observed, while at moderately high scan rates, a sigmoidal shape is observed. At very fast scan rates of 1 V/s and above, a peakshaped response is again observed. The transition from peak shape to sigmoidal shape as a function of scan rate is very clear in the case of 400- and 600-grit roughness while it is very diffused and unclear in the case of polished and 1500-grit rough electrodes. Laser microfabricated carbon electrodes are reported to show similar observations.9 This electrode has been shown to behave as an ensemble of microelectrodes where the active edge sites are at predetermined shapes, sizes, and distances. In the case of polished EG electrodes, the active site area, shape, and distance 6956 Analytical Chemistry, Vol. 75, No. 24, December 15, 2003
Figure 6. Chronoamperometric responses of SE800 electrodes in 4 mM K4[Fe(CN)6] in 1 M KCl. (A) and (C) polished (step potential 0.6 V) and (B) and (D) 400-grit rough surfaces (step potential 0.35 V).
between them are not uniform (Figure 1) and are difficult to characterize. Depending on the size and spacing between the active edge sites, spherical diffusion occurs to the edges and planar diffusion occurs toward the basal plane.9 It is found that the electron transfer of K4[Fe(CN)6] has two different rates associated with basal and edge surfaces. In case of SE800 electrodes, the ip/ν1/2 versus ν1/2 is nonlinear (not shown). Chronoamperometric responses of SE800 electrodes in the presence of 4 mM K4[Fe(CN)6] in 1 M KCl are given in Figure 6A,B. It is observed that the I-t-1/2 plots (corrected for background currents) show deviation from Cottrell behavior at short time scales while it merges with the expected behavior at long time scales. It is also observed that there are varying slopes before the linear diffusion takes over at long time scales. The diffusion layer thickness, xDt calculated for K4[Fe(CN)6] at 20 ms is 3.56 µm. This is of the order of the average defective edge size determined based on SEM (Figure 1). On rough electrodes, the overlap occurs faster than on the polished electrodes. In the case of EG, the edge planes act as active sites and the basal plane region acts as a low-active or inactive region. Nonlinear diffusion is also reported to occur on polymer-carbon composite elec-
trodes.27-29 The polymer acts as the blocking surface and the carbon particles act as the active sites. The time scale at which the linear diffusion starts to be predominant changes with the roughness associated with the surface. It may be speculated that the diffusion is likely to be cylindrical at short time scale at the distributed active sites. The diffusion characteristics overlap to result in spherical and finally linear diffusion at long time scales. The electrode surface, however, is not very well defined in terms of the edge site size and the distance between them. Despite this, a trend is observed between polished and rough surfaces (Figure 3A). The rough EG surface shows active and edge sites distributed over varying degrees of sizes and depths (Figure 1). The chronoamperometry results, as already explained, show nonlinear diffusion. The surface is mostly rough and partially active. These types of surfaces are frequently modeled as fractals. The fractal nature can be formed by the mechanism of formation of a solid, for example, aggregation of colloid or dendrite growth. The fractal geometry to characterize electrode surfaces has been used by several researchers.30-32 The EG surface is also characterized to check whether a fractal type of geometry is present based on the analysis of current-time response. As reported earlier,32 the fractal dimension, Df, can be derived from a [log (I)-log (t)] chronoamperometric response of an electrode. The slope of the transition between linear and nonlinear regimes can give the fractal dimension Df , according to the following equations.32
log(I) ) const - (Df - 1)/2 log(t)
(1)
log [A()] ) const - (2 - Df) log()
(2)
where A() ) I/(nFcbulkxD/πt) and ) xDt, I is current, D is diffusion coefficient, t is time of experiment, n is number of electrons, F is Faraday’s constant, cbulk is concentration of the redox species, and Df is fractal dimension. Plots of log I versus log t for polished and rough SE800 electrodes are given in Figure 6C and D. The fractal dimensions Df, calculated based on the slopes of the transition region, are 2.02 and 2.3 for polished and rough surfaces, respectively. In the case of SE800 polished electrodes, the transition is observed between 100 ms and 1 s whereas it is between 10 and 100 ms for rough electrodes. The fractal dimensions calculated from the plots of log [A()] versus log() (not shown) are 1.9 and 2.42 for polished and rough surfaces, respectively. The diffusion characteristics of Fe(CN)64- species toward the EG electrodes have been analyzed to be nonlinear based on voltammetry and chronoamperometry. Preliminary analysis of the rough EG electrode surface leads to a fractal dimension close to 2.3. The fractal dimensions presented here are only representative figures, particularly in the
light of the defects associated with the polished surface. Moreover, the difference between the Df of polished and rough electrodes is very small. However, this is a useful parameter to assess the actual surface when the surface irregularities could be characterized by a single parameter. The diffusion characteristics in the case of Co(phen) and Fe(phen) systems on SE800 electrodes with varying roughnesses follow behavior similar to that observed in the case of [Fe(CN)6].4Plots of log I versus log t for these systems are found to give a nonlinear response as observed in the case of [Fe(CN)6]4- (not shown). CONCLUSIONS Exfoliated graphite can be prepared from natural graphite, and the exfoliation temperature is found to alter the physicochemical properties. Roughening the electrode surface can easily expose active edge planes. The smooth as well as edge sites are shown to be electrochemically active. Electron-transfer kinetics is found to depend on the redox system structure and surface chemistry. Roughness associated with the surface affects the diffusion of the analyte toward the EG electrode surface. Amperometric measurements provide information about the fractal dimension of the electrodes. The use of EG electrodes in electroanalytical applications depends on the surface requirement for a particular application in terms of oxygen content, exposure of edge planes, etc. As shown in Table 2, the O/C ratio and pzc of the EG vary depending on the temperature of exfoliation. Hence, it is possible to tune the surface conditions according to the requirement. For example, the determination of dopamine in the presence of ascorbic acid at neutral pH values has been observed to depend on the preparation and surface treatment history of the electrodes.45 The rough SE800 electrodes whose pzc is 6.7 with an O/C ratio of 0.037 is found to be more suitable for dopamine determination than the SE600 and SE400 EG electrodes. A word of caution is appropriate at this point. The so-called smooth surface still contains a few defects, and hence, the electrochemistry will have a contribution from the edge planes as well. ACKNOWLEDGMENT The authors acknowledge the DST and the CSIR, New Delhi, India, for financial support. Mr. P. Bera and Mr. Shyamsundar are thanked for their help in recording XPS and surface profile studies.
Received for review July 22, 2003. Accepted October 8, 2003. AC034833U
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