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Anal. Chem. wa4, 56,524-529
Ultrafast Voltammetry and Voltammetry in Highly Resistive Solutions with Microvoltammetric Electrodes Jonathon 0. Howell and R. Mark Wightman*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Dlsk-shaped mlcrovoltammetrlc electrodes have been fabrtcated wlth rad11 of less than 7 hm. Gold and platinum wires as well as carbon flbers have been used In rlgld Insulating materlais to construct electrodes that have sufflclent physical strength 80 that the electrodes can be polished and resurfaced as Is done wlth electrodes of Conventional size. These electrodes have been characterlzed in acetonltrlle wlth letraalkylammonlum perchlorate supportlng electrolytes. At scan rates less than 50 mV 8-' the llmltlng current at these electrodes Is found to exhibit steady-state behavior. The magnltude of the Faradaic current Is sufflclently small under these conditions that distortion of voltammograms by IR drop Is mlnlmal. Therefore, voltammograms can be obtalned wlth very low concentrations of supportlng electrolyte. At scan rates above 200 V 8-' voltammograms at these electrodes exhibit the characterlstlcs of planar dlffuslon. The reduced double layer capacltance and IR drop permit scan rates up to 20 000 V s-' to be obtained with minimal dlstortlon. Thus, the propertles of mlcrovoltammetrlc electrodes facllltate voltammetric measurements on a microsecond time scale without the need for instrumental technlques to correct the data.
Microvoltammetric disk-shaped electrodes (electrodes of radius less than 7 pm) have several properties that facilitate their use in electrochemical studies. The most striking feature that has been demonstrated to date is that they exhibit sigmoidal shaped voltammograms at low scan rates under conditions of diffusional mass transport (I). In chronoamperometric experimentsthe current is essentially time independent for times longer than 1 s (2). The steady-state features of electrochemistry at microvoltammetric electrodes have been used to advantage in applications such as anodic stripping voltammetry (3),the elimination of flow rate dependence of amperometric detectors (4), and the elimination of the effects of complex ECE or catalytic reactions (2). Two other advantageous features of these very small electrodes are the reduced double layer capacity and the small currents that are generated during voltammetry. The small double layer capacitance facilitates rapid changes in the electrode potential. The current necessary to charge the double layer at microvoltammetricelectrodes is small relative to those measured at electrodes of conventional size, because the double layer capacity is proportional to the electrode area. The absolute value of the current from the electrolysis of solution components is similarily diminished in amplitude relative to macroelectrodes. Although diminished, the magnitude of the current is sufficient that measurements are not difficult with modern instrumentation (5). However, as will be shown, a distinct advantage is realized with these small currents because voltage distortion of voltammograms, caused by iR drop, is greatly reduced. Most modern electrochemical experiments with conventional electrodes are done with three-electrode potentiostats and with iR compensation to 0003-2700/84/0356-0524$01.50/0
minimize voltage distortion (6-8).These measures may not be sufficient in very resistive solutions because reproducible results require accurate and reproducible placement of the reference electrode tip relative to the working electrode (6). Full iR compensation by electronic means is difficult because of instrumental instability (7).An alternate method to remove iR effects is by semiintegral analysis (9, 10) or derivative techniques (11). In this paper we demonstrate that voltammetry under conditions where iR effects would normally be deleterious is facilitated by the use of microvoltammetric electrodes and, in many cases does not require instrumental sophistication or elaborate data manipulation. Although charging and Faradaic currents and the double layer capacity are diminished in amplitude, the solution resistance at a disk-shaped microvoltammetric electrode is inversely proportional to the radius. This occurs because the resistance (R)of a microvoltammetricelectrode is dominated by the resistance near the electrode surface. For a disk electrode embedded in an infinitely insulating plane the resistance is given by eq 1, where p is the specific resistance of
R = p/4r
(1)
the solution and r is the radius of the electrode (12). In this paper we examine the resistance and capacitance at microvoltammetric electrodes to determine the conditions where these properties can be used to make unique electrochemical measurements. In addition to demonstrating the advantage of very small electrodes in solutions of very high resistance without the use of instrumental correction techniques, we also show that microvoltammetric electrodes facilitate cyclic voltammetry at rates that are up to 3 orders of magnitude greater than are practical at electrodes of conventional size. In these experiments, cyclic voltammetry has been employed, since it is the most useful electrochemical technique to survey the electrochemical properties of a series of compounds. Many of the features of microvoltammetric electrodes have already been used to advantage in spectroelectrochemistry experiments (13).
EXPERIMENTAL SECTION Reagents. Acetonitrile (HPLC grade, Fisher, Fair Lawn, NJ) was used as received for the studies of voltammetry of ferrocene. For the voltammetric studies of organic compounds, the acetonitrile was passed through a column of neutral alumina that had been activated at 700 "C. Tetraethylammonium perchlorate (TEAP, G. F. Smith Chemical Co., Columbus, OH) was recrystallized twice from hot, doubly distilled water and was dried in a vacuum oven at 80 O C prior to use. Tetrabutylammonium perchlorate (TBAP) and lithium perchlorate (G. F. Smith) were used as received. Ferrocene, naphthoquinone (Eastman Kodak Co., Rochester, NY), and benzoquinone (MCB, Gibbstown, NJ) were purified by sublimation. Anthraquinone (Eastman Kodak Co.) was recrystallized from ethanol. Anthracene (Gold Label, Aldrich, Milwaukee, WI) was used as received. Electrodes. Microvoltammetric electrodes were constructed with fine wires of gold (6.5 pm radius, a gift from R. L. McCreery) and platinum (5 pm radius, Goodfellow Metals, Cambridge, UK) or carbon fibers (Thornell VSB-32, type P, Union Carbide Corp., New York). Individual noble metal wires were sealed in soft glass 0 1984 American Chemical Soclety
ANALYTICAL CMMISTRY. VOL. 56, NO. 3, MARCH 1984
tubes with a Bunsen burner flame. Carbon fiber electrodes were prepared by d i n g the fiber between two microsoope slides with epoxy (Epon 828 with 14% rn-phenylenediamine, Miller Stephenson, Chicago. IL). Silver epoxy (Epo-Tek HZOE, Epoxy ~ to make electrical contact Technology Inc., Billerica,MA) w a used to a copper wire that was used as the electrode connection. The surfaces of the electrodes were ground flat with 600-grit carbom d u m and 9-pm sandpaper. The electrodes were then polished to a mirror finish with 5-, 0.3-, and 0.05-pm alumina (Fisher, Cincinnati, OH). All electrodes were repolished with 0.05-pm alumina before each cyclic voltammogram. For comparison, commercial gold and platinum electrodes of conventional size ( r = 0.08 em) were employed (Bioanalytical Systems, West Lafayette, IN). A platinum working electrode of intermediate size was constructed with a platinum wire (r = 0.0406 cm) sealed in glass and ground flat. The reference electrode (Ag/AgClOJ was fabricated with a silver wire in an cracked glags tip capillary containing 0.01 M silver perchlorate in acetonitrile. The supporting electrolyte in the reference electrode chamber was the m e type and concentration as used in the test solution. A platinum wire auxiliary electrode was used. Electrochemical Methods. T w o homebuilt potentiontab of conventional, three-electrode design were employed. For sean rates helow 100 V s-' the 'slow" potentioatat employed had an RC time constant of 30 ps and LFP356 operational amplifiers (National Semiconduetor)were used throughout Voltammograms with two electrodeswere obtained with the reference and auxiliary lead shorted together. For use at higher scan rates a "fast" potentiostat was constructed with an RC time constant of 0.18 pa at the lowest gain setting, with LF357 operational amplifiern (National Semiconductor) in the current to voltage conversion stage. The magnitude of the current gain was adjusted to the desired value by replacing the feedback resistor. In this potentioatat switches were not employed and the lead from the working electrode to the current transducer was kept short ( a p proximately 4 em) to reduce stray capacitance. All experiments were conducted in a Faraday cage grounded to earth. Electronic compensation for iR wan used only with the conventionallysized electrodes. The three-electrode feature is only useful with microvoltammetric electrodes to prevent current passing through the reference electrode. Potential wave fonna were generated with a function generator (Wavetek Model 143, San Diego, CA). Voltammograms were digitized with a transient recorder that haa &bit resolution and a minimum acquisition time of 10 ns (Gould-Biomation Model 8100, Santa Clara, CA) and then sent to an IBM Personal Computer (Boca Raton, FL)for suhsquent display and analysis. Data were obtained at 1- or 2.5-mV intervals. High-speed voltammograms shown in this paper were subjected to a seven-point, moving average smooth. Resistance and capacitance measurementa of the electrochemicalcell were made hy the ac impedanoe method using a 5mV peak-bpeak sine wave and the cell response was determined with a lock-in amplifier (Princeton Applied Research C o p Model 5101, Princeton, NJ). The phase shift of the cell was corrected for instrumental distortion 88 described elsewhere (14). For reductions, all solutions were deoxygenated with argon. The electrochemid cell had a volume of 20 mL and incorporated a luggin probe. All measurements were made with the cell thermostated at 25 OC. Values are reported as the average of at least quadruplicate determinations made on two different days and are with standard deviations. RESULTS AND DISCUSSION Electrode Characterization. Scanning electron microscopy of the electrode surface reveals that there is an excellent seal between the glass and the electrode material for the gold electrode (Figure 1). The small diameter of the nobel metal wires employed 88 electrode material minimizes cracking of the glass caused by differences in the coefficients of thermal expansion between the metal and glass. The seal between the carbon fiber and epoxy does not seem complete; however, this does not distort the voltammetry at slow scan rates. The carbon electrode surface has a well-defined, disk-shaped geometty, and both surfaces appear to be smooth. The platinum
l
l
i
i
__ Flpure 1.
SZS
.1
Scanning electron micrographs of micrOvOltammehiC
-odes: (a)g& wire sealed In glass: (b) carbon nber in epoxy. Bar is 5 pm fw each micrograph.
microvoltammetric electrode also appeared to be well sealed by electron microscopy. Ac impedance measurements were used to determine the electrode capacitance. In 0.1 M TBAP/acetonitrile solutions, the 90° phase shift of the cell u p to 10 kHz using the gold microvoltammetric electrode indicates that the capacitive reactance is the major impedance over the frequency range examined (Figure 2). From these data the double layer capacity was found to have a value of 13.8 0.5 pF (n = 10) at -0.3 V (10 fiF cm-*), and the resistive component can be estimated to he 30 kn. The low value for the capacitance confirms that solution does not creep between the electrode material and the insulation. The independence of the phase of the measured signal as a function of frequency ala0 confirms that frequency dispersion is not a problem (15). Figure 2 also contains a frequency response curve for the m n t transducer section of the potentiostat (the other elements of the circuit exhibit a higher frequency response). As can be seen. the time constant of this instrument is less than that of the electro chemical cell. Resistance values cannot accurately be obtained from the data shown in Figure 2 because of the predominance of the capacitive reactance. Therefore, the veracity of eq 1was tested with an electrode of larger radius (0.0406 cm) in acetonitrile with 0.5 M TBAP. A resistance of 246 2 Q (n = 8) was
*
*
526
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984 c
e
c
c
g
o
~
t
9 00
o 0
0
103
io4
105
106
0.4
FREQUENCY (Hz)
Flgure 2. Frequency response of the current transducer of the low time constant potentiostat with 10 kQ dummy cell and electrochemical cell with gold microvoltammetric electrode: (0) normallzed gain of current transducer; (0) phase shift of current transducer; (+) phase shift of cell containing 0.1 M TBAP in acetonitrile.
v (vs-')
Figure 3. Cyclic voltammetric response at slow scan rates for the oxidation of ferrocene in acetonitrile at a gold microvoltammetric electrode: (a) 0.1 V s-' scan rate, 0.1 M TBAP, 1.O mM ferrocene; (b) 10 V s-' scan rate, 0.1 M TBAP, 1.0 mM ferrocene; (c) maximum current as a function of scan rate ( v ) for 2.25 mM ferrocene, 0.5 M TBAP.
obtained which compares favorably with the value of 240 s2 calculated when a value for the specific resistivity of 39 s2 cm (16)is employed. Thus, eq 1 can be used with the measured capacitance to calculate the cell time constant. For 0.6 M TEAP (used later in this paper) the value is 0.2 ps. Voltammetry at Low Scan Rates. The shape of voltammograms obtained at microvoltammetric electrodes depends on the scan rate that is employed. At low scan rates the voltammograms are sigmoidal in shape because the predominant mode of diffusion to the electrode is steady state. This occurs because a significant fraction of the current is passed at the edges of the electrode (17-19). At higher scan rates more conventional peak-shaped voltammograms are obtained (Figure 3). Because the relation between current and the other electrochemical variables has not been experimentally determined at microvoltammetricelectrodes during cyclic voltammetry,it was of interest to examine this behavior. The oxidation of ferrocene a t a carbon fiber microvoltammetric electrode in acetonitrile with 0.2 M LiC104was selected, because the diffusion coefficient for this system has been reported (2.4 X 10" cm s-l, ref 20). In addition, the radius of the carbon fiber, which is a well-defined disk as seen in the electron micrograph, is also known (r = 5.1 pm, ref 2). The relationship for the current (i) given by eq 2 has pre-
i = 4nFDrC
(2)
viously been derived for the current under mass-transport limited, steady-state conditions at a disk-shapedelectrode (17))
0.2
0.0 -0.2 0.4 0.2 E (V) vsAg/AgCI04
0.0
-0.2
Figure 4. Cyclic voltammograms obtained with a gold microvoitammetric electrode of ferrocene in acetonttrlle as a function of supporting electrolyte concentration: (a) 1.1 mM ferrocene, 100 mM TBAP; (b) 1.1 mM ferrocene, 0.1 mM TBAP; (c) 1.1 mM ferrocene, 0.01 mM TBAP, arrows indicate direction of scan; (d) as (c) but with 0.11 mM ferrocene.
where n is the number of electrons per mole of compound electrolyzed, F is the Faraday, D is the diffusion coefficient, r is the radius of the electrode, and C is the bulk concentration of depolarizer. With the use of the known values for D, C, F, n, and r, and the measured value of the current, a value of the coefficient of 4.0 + 0.1 was determined, in agreement with the theoretical value. This value differs from that we reported earlier (2)because of a difference in the insulation geometry (21). This relationship provides a simple method for the determination of diffusion coefficients and, alternatively, with compounds of known values of D,can be used to calculate the effective radius of an electrode. This was done for the gold electrode shown in Figure 1which does not have a perfect disk-shaped geometry. An effective radius of 6.5 f 0.1 pm was determined which is in agreement with the manufacturers specified radius. Voltammetry in Resistive Solutions. As stated in the introduction, the very small currents which occur at microvoltammetric electrodes should facilitate the use of these electrodes in solution of high resistance. Very small electrodes have previously been employed by Parker for the voltammetry of aromatic compounds in benzene and chlorobenzene (22); however, the advantages of steady-state voltammetry under these conditions were not explored. We have employed a range of supporting electrolyte concentrations during the oxidation of ferrocene in acetonitrile (Figure 4) to test these properties. Virtually undistorted voltammograms are obtained even with a two-electrode configuration. This is the expected result since most of the resistance is within a few micrometers of the electrode tip (12))and with microvoltammetric electrodes the luggin probe cannot be placed sufficiently close to compensate for the resistance. We have recently heard that A. Bond, M. Fleischmann,and J. Robinson have shown that voltammograms for the oxidation of ferrocene in acetonitrile can be obtained in the absence of supporting electrolyte with electrodes of 0.5 p m radius. For a reversible system at low sweep rates the currentvoltage curve at a microvoltammetric electrode should follow the relation E = ElIz R T / n F In ((il - i ) / i ) (3)
+
where il is given by eq 2. Thus, a plot of the natural logarithm of the normalized current function vs. potential should give a slope of 38.92 V-l. Significant iR drop would have the effect of decreasing the theoretical value of the slope. The measured slopes are compared to those calculated, after consideration of iR drop, in Table I. For supporting electrolyte concentrations down to the millimolar level, the solution resistance
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
527
Table I. Effect of Solution Resistance on Voltammetry concn.mM (in acetonitrile) ferrocene TBAP 1.1 1.1 1.1 1.1 1.1 0.11
slope, V-I
38.0 39.2 37.0 29.5 21.4 30.5
100 10 1 0.1 0.01 0.01
calcd slope, a V-I
38.9 38.3 34.2 16.1
2.35 16.1
a Calculated from ref 23 by using a specific resistance of 59.6 a cm M divided by the concentration of TBAP.
W
I . .. . ' ' ~ I " ' ' . " ' ' ' . " " " I -2.0
-2.5
E (VI
VS
Ag/AgC104 1
L
I
I
-2 0
-2 5
-2.0 E (V)
-2 5
-20
-25
Ag/AgC104
Figure 6. Cyclic voltammograms for the reduction of anthracene (2.22 mM) In acetonitrile with 0.6 M TEAP at a gold microvoltammetric electrode: (a) 1000; (b) 2000; (c) 5000; (d) 10000; (e) 20000; (f) 50 000;and (9) 100 000. Scan rates are in V s-'. 10
100
1000
lopoo
IO0,C
I
v (Vs-')
Figure 5. Variation of normallzed peak current for an electrode of 6.5 pm radius with scan rate: (dashed line) calculated result considerlng only planar diffusion; (solid line) calculated result for both planar and steady-state contributions (seetext);(points)experimentally determined values for the reduction of anthracene in acetonitrile with 0.6 M TEAP at a gold microvoltammetric electrode. is in agreement with the calculated value. At lower concentrations of supporting electrolyte, the measured slope always exceeds that of the calculated value. This may arise because impurities in the solvent contribute to the solution conductivity. It is also likely that the resistance decreases during the course of electrolysis-the oxidation product of ferrocene is positively charged and will contribute to the conductivity of the solution in the region of the electrode surface. This is evidenced by the hysteresis which is apparent with 0.01 mM TBAP (Figure 4c). Thus, when data are obtained under conditionswhere the concentration of depolarizer exceeds that of the supporting electrolyte, the effects of migration should be considered. Further, the data with 0.11 mM ferrocene in Table I show that the iR drop is proportional to the depolarizer concentration. These data demonstrate that undistorted voltammetric measurements can be made in resistive media. Correction of data obtained under steady-state conditionsis straightforward since the effects of iR can be simply removed. Combination of eq 1 and 2 suggests that further reduction of the electrode radius will not improve the results in resistive media since iR drop is independent of the electrode radius under steady-state conditions. However, recent data suggest that this is not the case (24). Voltammetry at Fast Scan Rates. For voltammograms obtained a t fast scan rates (>200 V s-l) the diffusion layer extends only a few micrometers into solution. Therefore, edge effects are less important a t these very small electrodes and the currents should exhibit the time dependence expected of a planar electrode (25). The peak current (i,) in cyclic voltammograms under planar conditions is proportional to the square root of the scan rate (u). This behavior is observed within 9% at a scan rate of 200 V s-l at the gold microvoltammetric electrode. This is demonstrated for the reduction
of anthracene in Figure 5. The solid line was calculated by using data for the spherical, steady-state correction tabulated by Nicholson and Shain (25). Although their results are only valid for spherical electrodes, we have replaced A / r o with 4r in eq 2 of their Table I to adjust the magnitude of the steady-state term, and a remarkably good fit is obtained. These results are also in agreement with those that have been digitally simulated for cyclic voltammetry (26). Some of the voltammograms from which the data shown in Figure 5 were obtained are shown in Figure 6. Very high scan rates with microvoltammetric electrodes are possible because of the low time constant of the electrochemical cell and minimal distortion from iR drop with respect to results obtained at large electrodes. Instrumental distortion is negligible up to 20 000 V s-' and distortion caused by iR drop on the forward going peak at this scan rate is less than 15 mV for the example in Figure 6. At higher scan rates the data are useful only to demonstrate that Faradaic information can be obtained. However, it should be noted that these data could be instrumentally corrected for iR distortion as is normally done with electrodes of conventional size. Practical cyclic voltammetry with electrodes of conventional size is restricted to an upper limit of 100 V s-l because of instrumental and iR limitations. This upper limit is essentially the lower limit in which these microvoltammetric electrodes can be used because of the ill-defined nature of the current when it is a composite of steady-state and time-dependent contributions. Several reporb of extremely fast voltammetry do exist in the literature (27-29), but all of these required some form of iR correction. Both the Faradaic current at high scan rates and the charging current, which tends to dominate the electrode response at very high sweep rates, are proportional to the electrode area. Thus, as the electrode radius is reduced, these contributions to iR drop are reduced proportionately, and voltammogramscan be obtained on a microsecond time scale without the need for care in electrode placement or iR compensation. However, in cyclic voltammetry with a disk electrode of any size, the ratio of the charging current to the Faradaic current increases with the square root of the scan rate so there is a practical limit to the scan rate that can be obtained. For example, the Faradaic information in the
528
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Table 111. Measured Heterogeneous Rate Constants for Reductions at Gold and Platinum Microvoltammetric Electrodesa k" k" (at D(x105), (at gold), platinum), cmz s'' cm s-l cm 6 - l compound benzoquinone naphthoquinone anthraquinone anthracene
2.13 i 0.07 0.39 i 0.10 0.24 i 0.04 1.86 i 0.05 0.73 * 0.12 0.61 k 0.09 1.98 k 0.10 1.78 i 0.35 2.00 i 0.05 3.46 t 0.55 a Measured in acetonitrile with 0.6 M TEAP at scan rates between 500 and 1 0 000 V s-l.
E ( V ) v s .Ag/AgC104
Flgure 7. Cyclic voltammograms for the reduction of anthraqulnone (1-90 mM) in acetonitrile wRh 0.6 M TEAP at a gold microvoltammetric electrode: (a) 200; (b) 1000; (c) 5000; (d) 10000. Scan rates are in
v s-1.
Table 11. Measured Heterogeneous Rate Constant for the One-Electron Reduction of Anthraquinonea u, kV s-l
k.", cm s-l
10
1.81 i 0.31
u,
kV s-l
k " , cm s-l
1.78 f 0.23 5 1.86 i 0.38 1.52 i 0.28 2 1.71 i 0.46 0.2 1.02 f 0.13 a Measured in acetonitrile with 0.6 M TEAF' at a gold electrode ( r = 6.5 p m ) . 1 0.5
voltammogram obtained at 1OOOOO V s-l is almost hidden by the charging current. The Measurement of Electrochemical Kinetics. Voltammograms for the reduction of several organic compounds in acetonitrile with 0.6 M TEAP as supporting electrolyte have been examined at sweep rates from 100 to 20000 V s-l. All of the compounds examined to date have exhibited separation of the peak currents at the highest scan rates that is greater than the 59 mV expected for a reversible compound. Each of the compounds tested has exhibited a different degree of peak separation, indicative of a kinetic origin. This is clearly seen in the voltammetry of anthraquinone (Figure 7). At 200 V s-l the peak separation for the first wave (75 mV), corresponding to the formation of the radical anion, is not much less than that for the second wave (110 mV), the reduction of the radical anion to the dianion. At 10 000 V s-l the peak separation of the second wave (250 mV) has increased considerably more than that of the first (120 mV). These data demonstrate a greater kinetic limitation for the reduction of the radical anion than the neutral compound, however, it should be noted that these rates have not been corrected for double layer effects. The variation of the heterogeneous rate constants (k') for the reduction of anthraquinone to its radical anion as a function of sweep rate is given in Table 11. The rate constants were obtained by using the method of Nicholson (30),which is based on the degree of peak separation between the forward and reverse scans, and requires that the diffusion coefficient be known. In this work, the diffusion coefficients were determined from the value of the limiting current for voltammograms obtained at 20 mV s-l. The values obtained at scan rates less than 200 V s-l appear to be low, and this arises because these voltammograms also contain some contributions from edge effects. A t higher scan rates and when the peak separation is sufficiently large for Nicholson's method to be accurate, reasonable repeatability of the heterogeneous rate constant is obtained a t different scan rates. The measured heterogeneous rate constants for several compounds obtained by this technique are summarized in
Table 111. The values for the quinones increase with molecular size, which can be attributed to the greater ability of the larger molecule to delocalize charge (31). The values for benzoquinone and naphthoquinone are similar to those reported previously at platinum and gold electrodes (31) under similar experimental conditions but with electrodes of conventional size. (A similar value of the rate constant (0.33 f 0.05 cm s-l) is obtained in our laboratory for the reduction of benzoquinone at a conventionally sized gold electrode.) However, the value for anthraquinone that is reported here is greater than that reported previously and is likely to be more correct because of the limitations of conventional cyclic voltammetry techniques at conventionallysized electrodes to measure very rapid rate constants. The uncorrected charge transfer rate constant for the reduction of anthracene reported here is slightly lower than that at a mercury electrode in dimethylformamide (32),but still establishes anthracene as a compound with one of the fastest heterogeneous rates that has been reported.
CONCLUSIONS The use of microvoltammetric electrodes allows the exploration of new chemical and physical domains with cyclic voltammetry. The primary reason for this increased scope is the reduction of iR drop at these very small electrodes. This facilitates their use in solutions of high resistance, an area that has been virtually unexplored by electrochemistry. In addition, voltammetricresults can be obtained on a microsecond time scale which should enable the analysis of chemical processes which accompany electron transfer as well as the measurement of the rates of charge transfer processes. The extremely rapid measurements that can be made with these electrodes should facilitate the measwement of charge-transfer rate constants with cyclic voltammeltryup to 20 cm s-l, if they exist. Other techniques such as ac methods should be useful at these electrodes for even faster kinetic measurements. Voltammetry at scan rates less than 50 mV s-l is goverened by steady-state behavior, while scan rates greater than 200 V s-l give voltammogramsgoverened by planar diffusion. A t intermediate scan rates, the voltammograms give results which are a composite of steady-state and planar behavior. However, results obtained at the two extremes can provide electrochemical information which complements and expands the information obtained with conventional techniques. ACKNOWLEDGMENT The aid in computerization of this project by Jon Meek is gratefully acknowledged. Registry No. Benzoquinone, 106-51-4; naphthoquinone, 130-15-4; anthraquinone, 84-65-1; anthracene, 120-12-7; gold, 7440-57-5;platinum, 7440-06-4; ferrocene, 102-54-5. LITERATURE CITED (1) Wlghtman, R. M. Anal. Chem. 1981, 5 3 , 1125A-1130A. (2) Dayton, M. A,; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1080, 52, 946-950. (3) Cushman, M. R.; Bennett, 8 . G.; Anderson, C. W. Anal. Chim. Acta 1081, 130, 323-327.
Anal. Chem. 1984, 56, 529-534 (4) Caudili, W. L.; Howell, J. 0.;Wlghtman, R. M. Anal. Chem. 1982, 5 4 , 2532-2535. ( 5 ) Ewlng, A. 0.;Dayton, M. A.; Wightman, R. M. Anal. Chem. 1981, 53, 1842-1 847. (6) Nemec, L. J . Electroanal. Chem. 1984, 8 , 166-170. (7) Brltz, D. J . Elechoanal. Chem. 1978, 88, 309-352. (8) Mumby, J. E.; Perone, S. P. Chem. Instrum. 1971. 3 , 191-227. (9) Vandenborn, M. W.; Evans, D. M. Anal. Chem. 1974, 46, 643-646. (10) Imbeaux, J. C.; Saveant, J. M. J . Elechoanal. Chem. 1973, 4 4 , 169- 187. (11) Ahiberg, E.; Parker, V. D. J . Elecfroanal. Chem. 1981, 727, 57-71. (12) Newman, J. J . Nectrochem. Soc. 1966, 713, 501-502. (13) Robinson, R. S.; McCurdy, C. W.; McCreery, R. L. Anal. Chem. 1982, 5 4 , 2356-2361. (14) Kojlma, H.; Bard, A. J . Electroanal. Chem. 1975, 6 3 , 117-129. (15) Newman, J. J . Electrochem. Soc. 1970, 777, 196-203. (16) House, H. 0.;Feng, E.; Peet, N. P. J . Org. Chem. 1971, 36. 237 1-2375. (17) Saito, Y. Rev. Polarogr. 1968, 15, 178-187. (18) Aoki, K.; Osteryoung, J. J . Elechoanal. Chem. 1981, 722, 19-35. (19) Shoup, A.; Szabo, A. J . Electroanal. Chem. 1982, 140, 237-245. (20) Kuwana, T.; Bublitz, D. E.; Hoh, G. J . Am. Chem. Soc. 1960, 82, 5811. (21) Shoup, A.; Szabo A. J . Elechoanal. Chem., In press. (22) Lines, R.; Parker, V. D. Acta Chem. Scan., Ser. B 1977, 37, 369-374.
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(23) Kratochvii, B.; Yeager, M. L. Forfsch. Chem. Forsch. 1972, 27, 1-58. (24) Bond, A. M.: Fieischmann, M.; Robinson, J., submitted for publication in J . Am. Chem. SOC. (25) Nicholson, R. S.; Shain, I. Anal. Chem. 1984, 36, 706-723. (26) Heinze, J. Ber. Bunsenges. f h y s . Chem. 1981, 85, 1096-1103. (27) Cummings, T. E.; Jensen, M. A.: Eking, P. J. Electrochim. Acta 1978, 23, 1173-1184. (28) Saveant, J. M.; Tessier, D. J . Elechoanal. Chem. 1977. 77, 225-235. (29) Perone, S. P. Anal. Chem. 1986, 38, 1158-1163. (30) Nicholson, R. S. Anal. Chem. 1965, 37, 1351-1355. (31) Samueisson, R.; Sharp, M. Electrochim. Act8 1973, 18, 315-317. (32) KoJima, H.; Bard, A. J. J . Am. Chem. Soc. 1975, 97, 6317-8324.
RECEIVED for review October 10, 1983. Accepted December 5,1983. This research was supported by the National Science Foundation and the U.S. Army Research Office (CHE 8121422). R.M.W. is a Sloan Fellow and the recipient of a Research Career Development Award from the National Institutes of Health (KO4 NS 356). This r: aper was presented in part at the 186th Meeting of the American Chemical Society, August 30, 1978.
Glassy Carbon and Graphite Electrodes with a Hole fcr Long Path Length Thin-Layer Spectroelectrochemistry Marc D. Porter and Theodore Kuwana*
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
A new type of optlcally transparent electrode (OTE) has been developed by drllllng a small dlameter hole, ca. 500 bm, through solld conductor materlals. This deslgn has the following advantages over conventlonal OTEs: (1) long optlcal path length, (2) use of opaque electrode materials, (3) short electrolysls tlme due to enclosed cyllndrlcal dlffuslon, (4) small electrolysls volume, (5) thln-layer electrochemical behavlor, and (6) ease of fabrlcatlon. The optlcal and electrochemical responses for holey electrodes made from glassy carbon and graphlte were evaluated by use of cyclic voltammetry (CV), and potentlal step methods, Le., dlffuslon llmlted potentlal step (DLPS) and steady-state potentlal step (SSPS). Comparlsons between DLPS slmulated data for a h e a r and an enclosed cyllndrlcal dlffuslon mass transport and between simulated and experimental DLPS spectral data were made. The dlffusional model for the dlgltal slmulatlon was considered to conslst of a serles of concentrlc cyllnders. FerrWferrocyanlde was the electroactlve redox chromophore used for testlng the performance of thls electrode and cell conflguratlon.
The coupling of optical and electrochemical methods, spectroelectrochemistry(SEC), has been employed for several years to investigate a wide variety of organic, inorganic, and biological redox systems (1-15). The advantage of SEC is, of course, the cross-correlation of information from the simultaneous optical and electrochemical measurements. In recent years, several SEC cell configurationsfor solution measurements have been described including, for example, optically transparent thin-layer electrode (OTTLE) cells where the optical path length is on the order of 100-200 pm. Such cells employ electrodes rendered transparent by using either 0003-2700/84/0358-052980 1.50/0
a thin film of conducting material on a transparent support or a minigrid electrode (16-18). Longer optical path lengths have been achieved by reflecting the light beam at glancing incidence or by passing a beam adjacent and parallel to the electrode surface (19-25). In these latter cases, the electrode may be opaque with respect to the optical bez-n. In this paper the evaluation of a new type of electrode for SEC iwestigatinns is described. The transparency of the electrode is brozght about by drilling a small diameter hole through a conductor such as glassy carbon (GC) or graphite. The diameter of the hole is sufficiently small that the diffusional characteristics of a thin-layer cell can be achieved. The advantagesof this electrode, called the “holey”electrode (HE), are that: (1) the optical path length is much longer than for conventional OTTLE cells; (2) HEs can be readily [abricated with opaque conducting materials; and (3) small volumes and short electrolysis times can be achieved. The spectroelectrochemical characteristics of the HE cell were evaluatad with the electrochemicalmethods of cyclic voltammetry (CV) and potential step perturbations, i.e., diffusion limited poiential step and steady-state potential step (DLPS and SSPS). The diffusional mass transport of the test species within the cylindrically shaped hole was modeled by assuming that the finite volume elements consisted of a series of concentric cylinders. The modeling was digitally simulated by using the method of finite differences (26,27). The experimentaloptical and electrochemical responses will be compared to those obtained from the digital simulation for DLPS. The redox couple of ferri-/ferrocyanide served as the test system. EXPERIMENTAL SECTION Electrode Preparation. The HE was fabricated by drilling a small diameter cylinder through solid graphite or GC. For the graphitic electrodes (FPurity Ultra Carbon, Union Carbide Co., Cleveland, OH) a carbide drill bit of 0.051 cm diameter was used to drill the hole. For GC, a 2.5 X 2.5 cm sheet of Tokai GC (grade 0 1984 American Chemical Society
