Anal. Chem. 1995, 67, 3115-3122
Electron Transfer Kinetics at Modified Carbon Electrode Surfaces: The Role of Specific Surface Sites Peihong Chen, Mark A. Fryling, and Richard L. McCreery*
Department of Chemistty, The Ohio State University, 120 West 78th Avenue, Columbus, Ohio 43210
The electron transfer (El') kinetics of R u ( N H ~ ) ~ ~ + / ~ +than , glassy carbon (GC).l0-l2 Third, some redox systems are I I - C ~ ~ ~ -Fc?(CN)~~-/~-, /~-, Feaq2+/3+,and Vaq2+l3+were sensitive to the presence of surface oxides, which act as catalysts for electron transfer13-15 or modify the surface charge.16 Unforexamined on several modified glassy carbon surfaces. The tunately, many common electrode surface preparation procedures kinetics of the aquated ions were very sensitive to the affect more than one of these three variables. For example, density of surface oxides, while those of the other redox electrochemical pretreatment (ECP) by anodization of the sursystems were not. In particular, chemical derhalization face7,8J7changes the carbon microstructure, removes impurities, of surface carbonyl groups decreased the rate of electron and forms surface oxides.' Thus it is difticult to determine which transfer with Fe3+l2+ by 2-3 orders of magnitude but had factor underlies the kinetic changes induced by ECP. little effect on R u ( N H ~ ) ~ ~or+ I/ ~ r c+l ~ ~ - / The ~ - . electron In several previous reports, we examined the effects of carbon trader rates for Fe31/2+correlated with surface C=O microstructure,1°-12J8clean lines^,^^^^^ and surface o x i d a t i ~ non ~,~~ density determined by resonance Raman spectroscopy. a variety of redox systems, mostly transition metal complexes. Neutral, cationic, and anionic nonspecific adsorbers Several conclusions which are relevant to the present work should decreased the rates of ET with the aquated ions a p be summarized here. First, 17 inorganic complexes and four proximately equally but had little effect on Ruorganic redox systems exhibit much slower ET rates on HOPG (NH3)62-/3f. The redox systems studied were classified compared to GC, a shortfall we attribute to the low density of into two groups: those which are catalyzed by surface electronic states in HOPG.lZ Second, disordering of HOPG by carbonyl groups and those which are not. Possible laser activation or ECP increases ET rates for most systems to catalytic mechanisms are considered. A significant effort by many laboratories has been directed toward understanding the relationship between surface structure and electron transfer reactivity for carbon Complex surface chemistry and an often unknown level of surface impurities have made it difticult to determine the important structural variables controlling carbon electrode reactivity. At least three major phenomena affect electron transfer (m? reactivity, and these vary in importance for different redox systems and solution conditions. First, many redox systems (e.g., Fe(CN),j3-14-, ascorbic acid, dopamine) are very sensitive to surface cleanliness, and observed ET rates are strongly dependent on surface h i s t ~ r y . ~ Second, .'~ the microstructure of the carbon has a large effect on most redox systems, with the basal plane of highly ordered pyrolytic graphite (HOPG) exhibiting much slower ET (1) McCreery, R. L. In Electroanalytical Chemistry; Bard, A J., Ed.; Dekker: New York. 1991; Vol. 17, pp 221-374. (2) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; Wiley: New York, 1988. (3) Sarangapani. S.. Akridge, J. R., Schumm, B., Eds. Proceedings ofthe Workshop on the Electrochemisty ofcarbon;The Electrochemical Society: Pennington, NJ, 1984. (4) Hu, I. F.; Kuwana, T. Anal. Chem. 1986,58, 3235. (5) Wightman, R M.; Deakin, M. R; Kovach, P. M.; Kuhr. W. G.; Stutts, K. J. J. Electrochem. SOC.1984,131, 1578. (6) Hu, I. F.; Karweik, D. H.; Kuwana, T. 1.Electroanal. Chem. 1985,188, 59. (7) Engstrom, R. C. Strasser, V. A Anal. Chem. 1984,56,136. (8) Cabaniss, G. E.; Diamantis. A. A; Murphy, W. R., Jr.; Linton R. W.; Meyer, T. J. J. Am. Chem. Sot. 1985,107, 1845. (9) McDermott, C. A; Kneten, K. R;McCreery, R L.]. Electrochem. Soc. 1993, 140. 2593. 0003-2700/95/0367-3115$9.00/0 0 1995 American Chemical Society
values close to those observed on clean GC. Third, Feaq2+13+, Vaq2+13+,and Eu,q2+13+are further accelerated by the presence of surface oxides to ET rates well above the outer-sphere values predicted from homogeneous self-exchange rates via Marcus Fourth, this oxide catalysis is not consistent with mechanisms based on redox mediation, hydrophobic effects, or double layer effects. Combined with the extensive literature on electrochemistry at carbon, these observations lead to a working hypothesis on the structural basis of ET reactivity at carbon.1,21Outer-sphere systems such as Ru(NH3)2+13+,IrCl&13-, etc. are sensitive to the electronic structure of the carbon (due to the electronic density of states, DOS) but are otherwise insensitive to surface functional groups or impurities. The effect of low DOS is nonspecific and applies to all 21 redox systems examined to date. In addition, (10) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J. Am. Chem. Sot. 1990,112, 4617. (11) Kneten, R IC; McCreery, R. L. Anal. Chem. 1992,64,2518. (12) Cline, K. K.; McDermott, M. T.; McCreery, R. L. J Phys. Chem. 1994,98, 5314. (13) Evans, J. F.; Kuwana, T. Anal. Chem. 1977,49, 1632. (14) Tse, D. C. S.;Kuwana, T. Anal. Chem. 1978,50, 1315. (15) Armstrong, F. A,; Bond, A. M.; Hill, H. A 0.;Olive, B. N.; Psalti, I. S. M. J. Am. Chem. Sot. 1989,111, 9185. (16) Deakin, M. R.; Stutts, K. J.; Wightman, R. M. J. Electroanal. Chem. 1985, 182,113. (17) Barbero, C.; Silber. J. J.; Sereno. L. J. Electroanal. Chem. 1988,248,321. (18) Rice, R J.; McCreery, R. L. Anal. Chem. 1989,61, 1637. (19) Poon, M.; McCreery, R. L.; Engstrom. R. Anal. Chem. 1988,60, 1725. (20) Allred. C. D.; McCreery, R L. Anal. Chem. 1992,64,444. (21) McCreery, R L.; Cline, K IC;McDermott, C. A; McDermott. M. T. Colloids S u 6 A 1994,93, 211.
Analytical Chemistry, Vol. 67, No. 78, September 15, 1995 31 15
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Figure 1. XPS survey spectra of (A) polished GC and (B) argonsputtered GC.
there are many redox reactions, such as that of Fe,q2+/3+,which are catalyzed by specific interactions with surface oxidesg and are therefore very sensitive to surface chemistry. The current experiments were designed to test this hypothesis in detail for several representative redox systems, including RU(NH&~+/~+ and Fe,,2+i3+. Various preparation techniques were used to vary the nature and density of surface functional groups before observing ET reactivity. By combining independent measures of oxide functional groups based on Raman spectroscopy and XPS with observations of kinetic behavior, the relationship between surface structure and ET rate was revealed. EXPERIMENTAL SECTION In all cases except in the study of argon-sputtered GC electrodes, the GC electrodes used were from Bioanalytical Systems Inc. (MF2070). In the case of argon sputtering pretreatment, Tokai GC-20 plates from Applied Industrial Material Co. were used. Polished GC electrodes were prepared by initial sanding with Sic paper when necessary and then polishing with successive slurries of 1, 0.3, and 0.05 p m alumina (Buehler) in Nanopure water on Microcloth polishing cloth (Buehler). Polished GC electrodes were sonicated in Nanopure water (Barnstead) for about 10 min before being placed into the electrochemical cell or further treatment. During transfers between each step, the surfaces of GC electrodes were kept wet to reduce contamination from air. Argon sputtering treatment was carried out in the vacuum chamber of an XPS/Auger spectrometer (VG Scientific). The vacuum chamber was pumped to Torr, then filled with argon gas to a pressure of 2 x Torr. The GC surface was sputtered with an AG21 argon source (VG Scientific) at a focus energy of 3116
Analytical Chemistry, Vol. 67, No. 18, September 15, 1995
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Figure 2. Voltammograms of (A) 5 mM Fe3+"+ in 0.2 M HCIOl at polished (dashed line), argon-sputtered (solid line), and argonsputtered/ECP (dotted line) GC electrodes (scan rate, 0.2 V/s; ECP procedure was seven cycles from 0 to 2.2 V in 1.2 M H z S 0 4 at 0.2 VIS) and (B) 1 mM R U ( N H ~ ) ~ ~ +in' ~1 +M KCI at polished (dashed line) and argon-sputtered (solid line) GC electrodes (scan rate, 20 V/S).
3.6 kV and target current of 30 pA for 10 min. XPS survey spectra were obtained before and after argon sputtering with a VG Scientific XPS spectrometer (Mg X-ray radiation source). The GC pieces were transferred into 0.2 M HClO., solution immediately after they were taken from the vacuum system, and electrochemical measurements were carried out within 5 min. 2,4DNPH derivatization of GC electrode surfaces and Raman measurement of the derivatized surfaces were performed as described previously.zz Preceding electrochemical experiments on derivatized electrodes, the electrodes were rinsed thoroughly in 20 mL of Nanopure water twice before transfer into the electrochemical cell. To derivatize the electrode surface with 3 , s dinitrobenzoyl chloride, the electrode was polished and rinsed with pyridine, immersed in 0.1 M 3,5dinitrobenzoyl chloride/pyridme solution, and heated on a hot plate for 1 min. After standing in the solution for 1 h, the electrode was thoroughly rinsed with pyridine followed by Nanopure water. 2,GAnthraquinone disulfonate (AQDS) was adsorbed to GC electrodes by placing polished GC electrodes in 10 mM 2,GAQDS aqueous solution for 10 min and rinsing in 30 mL of Nanopure water successively three times. The electrodes were then transferred into an electrochemical cell containing 0.1 M HClO4 for voltammetry. Methylene blue (MB) was adsorbed on GC electrodes by dipping polished GC in 0.1 mM MB in water for 10 min and rinsing (22) Fryling. M. A; Zhao, J.; McCreery, R. L.A n d . Chem. 1995,67, 967.
Table 1. Electron Transfer Rate Constants (in cm/s) of Some Inner-Shere and Outer-Sphere Redox Systems at Modified GC Electrode
polished GC RU(NH3)63+'2+
IrClfj-/4Fe(CN)e3-l4-
Fe,92t/3+ Eu,~*-/~+ Vaq2+/3+
(I
0.24 f 0.07" 0.50 f 0.06 0.10 i 0.02 (2.5 0.3) x (8.4 f 1.4) x (6.8 f 4.0) x
*
Standard derivatization, N
argon-sputtered GC
0.039 f 0.005 0.11 ?C 0.13 0.037 f 0.009 (2.1 f 1.3) x
