Anal. Chem. 1899, 65, 1552-1558
1552
Effects of High-Intensity Ultrasound on Glassy Carbon Electrodes Honghua Zhangt and Louis A. Coury, Jr.' Department of Chemistry, Duke University, Durham, North Carolina 27708-0346
Glassy carbon electrodes which have been irradiated with 20-kHz ultrasound from a 475-W generator in dioxane are shown to exhibit enhanced heterogeneous electron-transfer rates for a variety of aqueous redox probes. When sonications are performed in water, however, no significant enhancement effects are observed. Several electroanalytical techniques with different time scales are employed along with scanning electron microscopy to characterize surfaces before and after ultrasonicmodificationin different solvents. Results indicate that surface roughness does not change appreciably after brief sonication in dioxane, although a small amount of surface pitting occurs. These electrodes are demonstrated to remain active for up to 5 days and to be more prone to adsorb aromatic redox probes in aqueous media than polished electrodes. After sonication in water, carbon surfaces are highly pitted and show evidence of an increase in the density of electroactive surface oxides. Thus, the improvement in kinetics observed after sonication in dioxane is probably not associated with either increased microscopic electrode area or mediated electron transfer between surface oxides and solution analytes, but instead is likely to involve surface cleaning.
INTRODUCTION We have recently undertaken a comprehensiveinvestigation of the effects of ultrasonically induced cavitation on electrochemical processes and on electrode surface morphology. Sonochemistry commands continuing interest, due to the unique chemical and physical processes which occur as a consequence of the extreme temperatures, pressures, and turbulence associated with cavitation.14 When electrochemical experiments are conducted during sonication, tremendous increases in currents are observed,7-10which in principle may be attributed to any combination of (i) increased mass transport rate, (ii) changes in electrode area, or (iii) increased rate of electron transfer (at low overpotentials). In the latter
* To whom correspondence should be addressed.
+ Present address: Enzyme Technology Research Group, Inc., 710 W. Main, Durham, NC, 27701. (1) Suslick, K. S. Science 1990,247, 1439-1445. (2) Suslick, K. S. In Ultrasound Its Chemical,Physical,andBiological Effects; Suslick, K. S., Ed.; VCH New York, 1988. (3) Flint, E. B.; Suslick, K. S. Science 1991,253, 1397-1399. (4) Doktycz, S. J.; Suslick, K. S. Science 1990,247, 1067-1069. (5) Suslick, K. S.; Doktycz, S. J. Adu. Sonochem. 1990, I , 197-230. (6) Grinstaff, M. W.; Suslick, K. S. R o c . Natl. Acad. Sci. U.S.A. 1991,
aa,7708-7710. (7) Bard, A. J. Anal. Chem. 1963,35, 1125-1128. (8) Huck, H. Ber. Bunsenges. Phys. Chem. 1987,91,648-654. (9) Dewald, H. D.; Peterson, B. A. Anal. Chem. 1990,62, 779-782. (10) Hagan, C. R.; Coury, L. A., Jr., manuscript in preparation. 0003-2700/93/0365-1552$04.00/0
case, the effect may be due to temperature gradients established by interfacial cavitation (viz., Arrhenius behavior and/or decreased local viscosity) or due to alteration of the electrode itself. Changes in the physical and chemical properties of electrode surfaces caused by ultrasound must be understood before quantitative electrochemical measurements during sonication may be attempted. We report here changes in glassy carbon surfaces which occur due to brief periods of high-intensity sonication by observingthe interfacial kinetic properties of electrodes before and after sonication. The utility of this procedure for activation of glassy carbon will be considered. Numerous methods have been previously reported for activation of carbon surfaces including in situ laser irradiation,l1-l8 vacuum heat treatment,19-21 electrochemical oxidation,17?21-26 and various mechanical procedures (polishing18p21,26and fracturing16-17727). Our results differ from previous studies in that significant activation is achieved only when the activation medium is an organic solvent with a low vapor pressure. The advantages of our approach include the simplicity and low cost of the equipment used and the longevity (several days) of the active surface.
EXPERIMENTAL SECTION Reagents. Potassium ferricyanide (Fisher),sodium iron(II1) ethylenediaminetetraacetate dihydrate (Fe(EDTA)-)Aldrich), FeClz-4Hz0(Fisher), 4-methylcatechol (4MC) (9996,Aldrich), 3,4-dihydroxyphenethylaminehydrochloride (dopamine,Sigma), and 1,4-dioxane(99+% ,Aldrich) were used as received. All other
chemicals were of at least reagent grade. Solutionswere prepared with 1.0 or 0.5 M KCl as the supporting electrolyte, and some were buffered with phosphate or phthalate. Solvents used for ultrasonic activation of glassy carbon electrodes were not degassed, but electrochemicalsolutions were saturated with argon. (11) Poon, M.; McCreery, R. L.; Engstrom, R. Anal. Chem. 1988,60, 1725-1730. (12) Poon, M.; McCreery, R. L. Anal. Chem. 1987,59, 1615-1620. (13) Poon, M.; McCreery, R. L. Anal. Chem. 1986,58, 2745-2750. (14) Huang, W.; McCreery, R. L. J. Electroanal. Chem. 1992, 326, 1-12. (15) Rice, R. J.; McCreery, R. L. J.Electroanal. Chem. 1991,310,127138. (16) Pontikos, N. M.; McCreery, R. L. J. Electroanal. Chem. 1992, 324, 229-242. (17) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J.Am. Chem. SOC. 1990, 112, 4617-4622. (18) Bodalbhai, L.; Brajter-Toth, A. Anal. Chem. 1988,60,2557-2561. (19) Fagan, D. T.; Hu, I.-F.; Kuwana, T. Anal. Chem. 1985,57,27592763. (20) Stutta, K. J.; Kovach, P. M.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1983,55, 1632-1634. (21) Wightman, R. M.; Deakin, M. R.; Kovach, P. M.; Kuhr, W. G.; Stutts, K. J. J.Electrochem. SOC.1984,131, 1578-1583. (22) Cabaniss, G. E.; Diamantis, A. A.; Murphy, W. R., Jr.; Linton, R. W.; Meyer, T. J. J.Am. Chem. SOC.1985,107, 1845-1853. (23) Engstrom, R. C. Anal. Chem. 1982,54, 2310-2314. (24) Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984,56, 136-141. (25) Rice, M. E.; Galus, Z.; Adams, R. N. J. Electroanal. Chem. 1983, 143,89-102. (26) Hu, 1.-F.;Karweik,D. H.; Kuwana,T. J.Electroanal. Chem. 1985, 188,59-72. (27) Rice, R.; Allred, C.; McCreery, R. J. Electroanal. Chem. 1989, 263, 163-169.
0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1993 0.25
I
20 r
...
r
1553
n
10 -0.1 5 -0.25
750
3
v
0
.e
375
0 -375 -750 E I mV
Flgure 1. Cyclic voltammograms of 1.46 mM Fe(EDTA)-/*- at polished
-10 -20
L L
-20
L
GC (dashed curve) and at GC electrodes after sonication in water (w) and dioxane (d). Scan rate, 0.1 VIS.
Carbon Electrodes. In most electrochemical experiments, glassy carbon disk electrodes from Bioanalytical Systems (BAS) were employed. Glassy carbon (GC) electrodes for scanning electron microscopy measurements were constructed by cutting a rod of Tokai GC-30 (Electrosynthesis Co., 3 mm in diameter) into cylinders each about 2 mm in length which were sealed in epoxy resin (Epo-Tek 301-2, Epoxy Technology, Inc). The manufacturers' data sheets report indistinguishable densities, thermal limits, gas permeabilities, and thermal expansion coefficients for the two types of GC, and we observed no differences in the electrochemical properties of each material. Before each activation experiment, electrodes were polished first with fine emery paper (600A),followed by polishing successively with 5-, 1-,0.3-,and0.05-pmaluminasuspensions(Buehler) ona polishing cloth (Microcloth, Buehler) until a mirror finish was obtained. In all cases, electrodes were thoroughly rinsed with water and sonicated briefly in a Branson 1200 low-intensity ultrasonic cleaningbath. Water used in these studies wm purifiedby reverse osmosis (Barnstead ROpure-LP) followed by deionization (Barnstead NanoPure) to yield a minimum resistivity of 18.1M k m . Sonication Procedure. Ultrasonic irradiation of GC electrodes was performed in a sonication cell which was thermally jacketed by ethylene glycol coolant delivered though a Brinkman RC-20 Lauda refrigerated recirculating bath. Coolant temperature in the bath was held at 12 OC, and the temperature in the sonochemicalcell was monitored during sonication with a digital thermometer (Fluke Instruments). In most cases, the cell temperature increased from 12 to 50 f 3 "C during a 5-min sonication. Electrodes were positioned parallel to the l/z-in.diameter Ti horn tip and were thus directly in the center of the cavitational plume generated by a 475-W Heat Systems XL2010 ultrasonic processor. A power output level of 10 (120-pm peakpeak tip amplitude) at a frequency of 20 kHz was employed. The separation distance between the Ti horn and the surface of the glassy carbon electrodes was 2 mm in all experiments. Unless otherwise noted, electrodes were sonicated for 5 min. After sonication, electrodes were rinsed with the sonication solvent, which rapidly evaporated before the electrodes were immersed in aqueous solutions of various redox probes. Instrumental Measurements. Electrochemical experiments were performed with a BAS-100Belectrochemical analyzer (BAS) or a VersaStat (EG&G Princeton Applied Research) each interfaced to a laboratory PC (Zeos). Electrochemical cells were of conventional design with Pt auxiliary and BAS RE-5 AgIAgC1 (3M NaCl) reference electrodes. Areas of polished and sonicated GC electrodeswere determined as described previously.2s Scanning electron microscopy (SEM) measurements were carried out with a Philips 501 electron microscope operated at 15-kV accelerating voltage. Glassy carbon disks were sputter-coated with Au-Pd for 2 min before SEM measurements.
RESULTS AND DISCUSSION Intense sonication has several noticeable effects on glassy carbon (GC) electrodes. Background currents observed by cyclic voltammetry (CV) generally increase upon sonication, improved rates of heterogeneouselectron transfer are observed after sonication in certain solvents, and pitting of electrode surfaces may be observed by microscopy. Figure 1 shows a (28) Coury, L. A,, Jr.; Heineman, W. R. J.ElectroanaL Chem. 1988, 256,327-341.
0.8 0.6 0.4 0.2 0.0 -0.2 E(V ) vs. Ag/AgCl
0.8 ,0.6 0.4 0.2 0.0 -0.2 E(V) vs. Ag/AgCI
Flgure 2. Cyclic voltammograms of 1.00 mM Fe(CN)e3-I4-(pH 7, 0.5 M KCI, 0.1 V/s) at electrodes sonicated in indicated solvents.
series of cyclic voltammograms for the kinetically sluggish Fe(EDTA)P- redox couple at a polished electrode (dashed curve) and at electrodes after sonication in water (w) and dioxane (d). Several effects are evident in the CVs shown. First, there is a significant improvement in the rate of electron transfer to Fe(EDTA)- in aqueous media after sonication in dioxane, as is evident from the decrease in peak separation in the v o l t a m m ~ g r a m Second, .~~ no analogous improvement is observed upon sonication in water, and in fact, heterogeneous kinetics are degraded in the case of Fe(EDTA)-/Z-. Third, background currents increase after sonication, as may be seen in the figure between 400 and 500 mV. This is most apparent in the case of aqueous sonication media, but also occurs to some extent for organic solvents. Each of these effects will be discussed in more detail below. Solvent Dependence of Sonication Effects. It has been demonstrated in homogeneous sonochemical experimentsthat phenomena such as cavitation intensity and cavitational threshold are solvent dependent.2 Most important of the solvent properties affecting cavitation is the vapor pressure.30831 Water, dioxane, and decane were primarily selected as solvents in our work since they have been commonly used in other studies in the field of sonochemistry.1-45 Acetonitrile and benzonitrile were also examined since they are widely used in electrochemical studies. When electrodes are ultrasonically irradiated by following the procedure described above, a considerable dependence of sonication effects on the solvent employed is observed. Figure 2 compares typical cyclic voltammograms for Fe(CN)e3- at polished (dashed curve) and sonicated (solid curve) glassy carbon electrodes after sonication for 5 min in the solvents indicated. As in the case of Fe(EDTA)V- (cf. Figure l), sonication of polished electrodes in dioxane results in a significant decrease in peak separation for Fe(CN)63-IP, indicative of enhanced heterogeneous electron-transfer rates. (No attempt has been made to calculate apparent hetero(29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (30) Suslick, K . S.;Gawienowski, J. J.; Schubert, P. F.; Wang, H. H. Ultrasonics 1984, 22, 33-36. (31) Niemczewski, B. Ultrasonics 1980, 18, 107-110.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1993
Table 11. Reproducibility of Sonochemical Effects on Glassy Carbon Electrodesa
Table I. Effect of Sonication in Dioxane on Peak Separations for Several Redox Systems u p ,
system
sonication time (min)
before
Fe(CN)&, 0.5 M KCl (pH 7) Fe(EDTA)-, 0.5 M KCl (pH 7) Fe(EDTA)-, 0.5 M KCl (pH 4) 4MC, 0.5 M KCl (pH 7) 4MC, 0.5 M KC1 (pH 4) dopamine, 0.5 M KCl (pH 7)
5 5 2 5 2 5
196 192 240 249 312 133
a
mt after 95.6 99 128 123 161" 106
~
dioxane
Peak distorted due to adsorption; see Figure 7. water
geneous rate constants (k")from peak separations due to the uncertainties associated with assuming linear diffusion to these pitted electrode surfaces; see below.) However, if sonications are performed in water, increases in peak separation are routinely observed. The extent of deactivation upon aqueous sonication ranges from moderate to severe for the redox systems Fe(CN)63-14-,Fe(EDTA)-12-, Fe(Hz0)3+/2+,and substituted catechols. This result is both surprising and interesting given that cavitation is generally more facile in water than some organic liquids, and thus activation effects a t least as great as those in organic systems would be expected. (In fact, recent studies by Perusich and Alkire have demonstrated the utility of high-intensity ultrasonication in aqueous systems for depassivating Fe electr0des.3~) However, in addition to increases in peak separation, sonication of GC electrodes in water also produces higher background currents compared to the organic solvents studied. The occurrence of concomitant deactivation reactions of the carbon surface involving aqueous sonolysis products is a possible explanation, consistent with both of the above observations. Deactivation reactions would be expected to increase the amount of surface functionalization and hence the pseudocapacitance of carbon electrodes, resulting in higher charging currents.33~~~ A significant degree of activation was also observed when electrodeswere sonicated in benzonitrile, although this solvent proved to be unstable during sonication, as indicated by its rapid discoloration. Sonication in the chemically similar but more volatile solvent acetonitrile caused negligible activation, as is apparent in Figure 2. Other studies conducted in decane gave inconsistent results, sometimes resulting in deactivation and sometimes having little effect. Since results in dioxane were the most reproducible and consistent (i.e., significant activation with minimal increases in background current), it was used for the majority of subsequent studies. Table I presents typical voltammetric results for a variety of redox species observed before and after ultrasonic irradiation of GC electrodes in dioxane. As can be seen, improvement in kinetics is observed for each of the redox couples examined. The reproducibility of this effect is shown in Table I1 for replicate measurements using Fe(EDTA)-. Several trends are evident in these data. First, the peak separations seen for Fe(EDTA)- a t polished (unsonicated) electrodes vary significantly, similar to published reports for catechoP3 (although somewhat more variable than observed for ferricyanide17Jg). Second, significant enhancement in heterogeneous kinetics is observed only after sonication in dioxane. In one case for the dioxane data, however, a drastic degradation in kinetics occurred after sonication. Although the reason for this is not clear, contamination occurringduring transfer of the electrode from the nonaqueous sonochemical (32) Perusich, S. A.; Alkire, R. C. J.Electrochem. SOC.1991,138,708713. (33) Jurgen, D.; Steckhan, E. J. Electroanal. Chem. 1992,333, 177193. (34) Kepley, L. J.; Bard, A. J. Anal. Chem. 1988, 60, 1459-1467.
afterb
initial
144 122 219 119 129 146 196 71 207 161 196 121 116 121
1.03 1.03 1.14 1.03 1.04 1.05 1.10 1.00 1.22 1.07 1.09 1.03 1.05 1.05
75 282 68 86 153 74 115 96 261 192 184 219 97 155
1.05 1.36 1.12 1.15 1.12 1.12 1.06 1.04 1.29 1.12 1.09 1.31 1.04 1.14
-69 +160 -151 -33 +24 -12 -81 +25 +54 +31 -12 +158 -19 +34
1.20mMFe(EDTA)-inl.OMKCl;scanrate,O.l VIS. Sonication for 5 min in 37 mL of solvent indicated. Ratio of cathodic-to-anodic peak currents from cyclic voltammetry.
cell to the aqueous Fe(EDTA)- cell is perhaps a possibility. However, 70% of the electrodes showed activation, with a mean value for AAEwak of -81.8 mV. Finally, significant decreases in AEpeak are never observed after sonication in water, despite the observation of significantly greater surface pitting relative to dioxane-sonicated electrodes, and hence a concomitant increase in the area available for electron transfer (vide infra). Similarly, 70% of the electrodes showed deactivation after sonication in water, with a mean value for AAE,,,k of +60.4 mV. In addition to influencing electrochemical kinetics, ultrasonic irradiation also increases background currents observed by CV to various extents, depending on sonication time, solvent used, and distance between the sonicator horn and the surface of the GC electrode. For example, aqueous sonication media reproducibly gave higher background currents than did "dryn dioxane. Using dioxane which had been allowed to absorb moisture from the atmosphere yielded larger background currents than were observed using dry solvent (manufacturer's specification of
