Anal. Chem. 1995, 67, 3089-3091
Fabrication of Poly(chlorotrifluoroethylene)/ Precious Metal Composite Electrodes Jeffrey B. Montgomery and Jeffrey E. Anderson*
Department of Chemistty, Murray State Universify, Murtay, Kentucky 42071
W oprocedureswere examined for the fabrication of poly(chlorotrifluoroethylene)/precious metal composite electrodes in an attempt to reduce the size of the active sites on the surface. A grinding procedure used with graphite was unsuccessful with platinum in that individual platinum particles became isolated from one another. As a result, the electrode material was nonconducting. A procedure which involved sputter coating the poly(chlorotn'fluoroethylene)with gold prior to compression molding was successful. Electrodes fabricated with this procedure had active areas less than about 1%of their geometric area and behaved as an array of isolated ultramicroelectrodesat times less than 0.1 s. Composite electrodes fabricated from a mixture of a conducting material and an inert binder have been studied and used for voltammetry for many years. The inert binders used have varied from organic oils to thermoplastics. The conducting material used has been predominately graphite, as in the case of carbon paste electrodes and po~(chlorotrinuoroethylene)/gtaphitecomp~sites.~-~ More recently, various precious metal composites have been examined. Gold and platinumgas well as silverlohave been used in the fabrication of composite electrodes with poly(ch1orotrifluoroethylene) (Kel-I?) as the inert binder. These precious metal composites have been shown to have active sites on the order of 20-30pm in diameter.7s8J1Since the precious metal particle size used in making these electrodes is on the order of 1pm, the active sites actually consist of aggregates of many particles intertwined through the Kel-F matrix, resulting in the conductivity of these electrodes. As with other composite electrodes, these electrodes behave as an array or ensemble of microelectrodes, which leads to their advantage: increased current densities per unit active area as a result of the contribution from nonlinear diffusion. (1) Tallman, D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978,50, 1051-1056. (2) Chesney, D. E.; Anderson, J. L.; Weisshaar, D. E.; Tallman, D. E. Anal. Chim. Acta 1981,124, 321-331. (3) Weisshaar, D. E.; Tallman, D. E.; Anderson, J. L. Anal. Chem. 1981,53, 1809-1813. (4) Tallman, D. E.; Weisshaar, D. E.]. Liq. Chromafogr. 1983,6, 2157-2172. (5) Anderson, J. L.; Whiten, K K; Brewster, J. D.; Ou, T. Y.; Nonidez, W. IC Anal. Chem. 1985,57,1366-1373. (6) Cope, D. IC;Tallman, D. E. /. Electroanal. Chem. Intelfncial Electrochem. 1986,205, 101-123. (7) Weisshaar, D. E.; Tallman, D. E. Anal. Chem. 1983.55,1146-1151. (8) Anderson, J. E.; Hopkins, D.; Shadrick, J. W.; Ren, Y. Anal. Chem. 1989, 61, 2330-2332. (9) Petersen, S. L.; Tallman, D. E. Anal. Chem. 1990,62, 459-465. (10) Petersen, S. L;Tallman, D. E. Anal. Chem. 1988,60, 82-86. (11) Petersen, S. L.; Weisshaar, D. E.; Tallman, D. E.; Schulze, R IC; Evans, J. IC;DesJariais, S. E.; Engstrom, R. C. Anal. Chem. 1988,60, 2385-2392.
0003-2700/95/0367-3089$9.00/0 0 1995 American Chemical Society
Some time ago, we reported on a new procedure for reducing the active site size of Kel-F/graphite composite electrodes.12 By grinding the poly(chlorotrifluoroethy1ene)/graphite mixture in a micronizing mill prior to the compression moldings of the mixture, the active site size of the graphite was reduced from -25 pm to 1-2 pm. This reduction in active site size yielded a significant increase in the current density, as predicted from t h e 0 ~ 7 . lIn ~ addition, electrodes with '30% (w/w) graphite behaved as though the surface was 100%active on the time scale of the chronoamperometric experiments performed (10 ms to 1 s). This is a result of complete overlap of adjacent diffusion layers of the active sites. In this work, we examine two procedures for the reduction of the active site sizes of Kel-F/precious metal composites. The first consisted of applying the grinding procedure used successfully with the graphite composite electrodes. The second procedure consisted of coating the Kel-F particles with a precious metal prior to compression molding. EXPERIMENTAL SECTION
Chemical Reagents. All chemicals used were of reagent grade. The acetonitrile was obtained from American Scientific Products, the tetrabutylammonium perchlorate (TBAP) from Eastman Kodak Co., and the ferrocene from Aldrich Chemical Co., Inc. The Kel-F rod was obtained from Plastic Profiles Inc. (East Hanover, NJ), and the Kel-F 81 powder was supplied by 3M Commercial Chemicals Division (St. Paul, MN). The Kel-F powder was sieved to ~ 1 5pm 0 prior to use. The platinum powder (0.5-2.5 pm) was obtained in 99.9%purity from AESAR Johnson Matthey Inc. Instrumentation. The experiments were performed using a system consisting of an ACSO9 single board microcomputer (Datricon Corp., Portland, OR) interfaced to an IBM PS/2 Model 30 286 and a potentiostat (IBM EC/225 voltammetric analyzer). Communication between the 6809 processor of the ACSO9 and the PC was accomplished via an RS232 interface. The 6809 system was equipped with an analog-to-digital converter (ADC) board (12 bit) and a digital-to-analogconverter (DAC) board (16 bit) connected to the 6809 board via an STD bus. The ADC and DAC boards were made in-house. The potential of the electrode was controlled with the DAC, and the resulting current was measured with the ADC. The software for the 6809 system was written in machine language, which was downloaded from the PC via the RS232 interface with the assistance of the on-board monitor of the 6809. The PC software for setting up the experiments, communicating with the 6809, and data analysis was written in QuickBasic (Microsoft Inc.). The micrographs of the (12) Anderson, J. E.; Montgomery, J. B.; Yee, R. Anal. Chem. 1991,63, 653656. (13) Shoup, D.; Szabo, A/. Electroanal. Chem. 1984,160, 19-26.
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995 3089
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Figure 1. Scanning electron micrograph of goldIKel-F composite electrode 2. SEM conditions are given in the micrograph.
Figure 2. Plot of potential step data for gold/Kel-F composite electrode 2 from experiments performed on 1 mM ferrocene in 0.10 M TBAPIacetonitrile with current (I) vs time (O-".
electrode surfaces were obtained using a JEOL JSM scanning electron microscope. Sputter coating of the Kel-F powder with gold was accomplished using a Hummer N sputter coater unless otherwise noted. The micronizing mill was obtained from McCrone Accessories and Components (Chicago, IL). Eleehode Fabrication. ' h e platinum/Kel-F composite electrodes were fabricated by first grinding a mixture of platinum and Kel-F powder (15.12% Pt w/w) for 1 h in the micronizing mill. The mixture was then compression molded at 2M) "C and loo0 psi for 4 min in an apparatus described previously? The resulting pellet was then machined down to 3 mm in diameter and -6 mm long. The pellet was presefit into a Kel-F tube. Electrical contact was made with silver conducting paint and a brass rod inserted into the opposite end of the Kel-F tube. The gold/Ke!-F composite electrodes were fabricated by first sputter coating the surface of the Kel-F powder with a gold film. One electrode, designated 2, was fabricated from Kel-F which was sputter coated in the Hummer N sputter coater eight times for 5 min each. The Kel-F powder was stirred and shaken between each 5 min coating. The resulting goldcoated Kel-F was then compression molded and fashioned into an electrode, as was the platinum composite. Another electrode, designated 1, was fabricated from Kel-F powder which was sputter coated compliments of Denton Vacuum Inc. The sputter coating was accomplished in a similar fashion; however, precise times are not available.
and Kel-F. To prevent charging of the nonconducting regions of the surface, an acceleration voltage of 1kV was used instead of the usual 10-20 kV. With this lower acceleration voltage, the conducting regions on the surface appear white, and the nonconducting regions are dark At high acceleration voltages, the image is reversed (nonconducting regions appear white because of charging effects), and it becomes very difficult to focus. With this assignment of gold and Kel-F regions in mind, it is apparent that there is connectivityof the gold film on the individual particles through the matrix. Initial conductance measurements with an ohmmeter were not as promising. Assuming that light colored regions are the gold, it is obvious that the active regions are far apart, and the probability of touching the surface at the exact points to complete a circuit with the ohmmeter probes would be rather low. Closer examination with the SEM indicates that the gold lines on the surface are -1 pm across or less. Electrochemical experiments immediately indicated that there are connective pathways through the Kel-F matrix. Given the size of the gold sites on the surface and the relatively large distance between the sites, we might expect a different electrochemical response from these electrodes as compared with previous poly(ch1orotritluoroethylene) composite electrodes. In past chronoamperomebic experiments performed on graphite electrodes fabricated using the grinding process, overlap of adjacent diffusion layers began or was complete at short times, depending on the composition. As mentioned above, electrodes with >30%graphite behaved as though the surface was 100%active at times longer than 10 ms, indicating that overlap of diffusion layers is already complete. With 10%graphite electrodes, the overlap begins at -30 ms. Although the active sites are small with these electrodes, they are also relatively close together. Given the relatively large distances between active regions for the gold electrodes fabricated here, we expect that the diffusion layers will not overlap at such short times. Therefore, these electrodes should behave as isolated microelectrodes at much longer times. This limiting behavior is advantageous since the theory of the behavior of these electrodes is much simpler than when overlap ping diffusion layers must be taken into account Figure 2 depicts the chronoamperometric response of gold electrode 2 for the oxidation of ferrocene in acetonitrile (0.10 M telmbutylammonium perchlorate). The 1/P12plot is linear prior
RESULTS AND DISCUSSION
The platinum composite electrodes fabricated using the grinding procedure were a failure in that they were nonconducting. Examination of the surfaces of these electrodes using the SEM indicated why. Although the grinding procedure obviously decreased the size of the platinum sites on the surface to 1-5 pm, the sites were isolated from one another. Therefore, no connective pathway existed through the nonconductive matrix It is possible that increasing the percentage of platinum would eliminate this problem, but it would undoubtedly increase the size of the active sites. Figure 1 is a micrograph of the surface of one of the poly(chlorotriiluoroethylene)/gold composite electrodes obtained using the SEM. Unlike biological specimens, the surface of the electrode was not coated with a conductive coating prior to examination, as this made it difficult to distinguish between gold 3090 Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
Table 1. Gold Electrode Data for Experiments in 0.10 M TBAP In Acetonitrile (ImM Ferrocene)’
geometric area (cm2) active area (cm? P / A (cm-I) capacitance (uF/cm2) Id or Zp (uAlcm2) SD (Id or Zp)
solid gold
electrode 1
electrode 2
0.0314 0.0314 (100%) 20 30 777 UP) 0.6 (1.3%)
0.0707 0.0014 (2%) 1111 347 2136 (I$) 0.08 (1.3%)
0.0707 5E-5 (0.1%) 4454 166 6400 (Zd) 0.02 (3.7%)
P / A is the perimeter to area ratio determined from potential step experiments. Zp is the peak current from data in Figure 3. Id is the diffusion wave height from the data in Figure 3. (I
-64 700
to -0.1 s (t1/2= 3), after which overlap of diffusion layer appears to begin. Given eq 1for the current (i) at an inlaid electrode, the
i = nFCD[A/(JtDt)”2+ P / 2 + ...I
(1)
active area (A) of the electrode can be determined from the slope of the line at short times, and the perimeter of the active sites (p) can be determined from the intercept. The determined area and perimeter to area ratio for the two gold composite electrodes fabricated, as well as those for a solid gold electrode, are given in Table 1. The capacitance data listed in Table 1 were estimated from cyclic voltammetric experiments on blank solutions using the difference in the background at 0 V and the fact that C = Ai/(2dE/dt). Table 1 also contains data pertaining to cyclic staircase voltammetry data obtained on ferrocene, to be discussed shortly. Table 1 clearly indicates that the percent active area for the two composite electrodes is very small (2 and 0.1%). This and the small size of the gold bands or lines lead to very large perimeter to area ratios, a measure of expected current enhancement due to nonlinear diffusion. The capacitance of these electrodes is higher than that found for the pure conductor on an active area basis. This has been observed with other precious metal composites at low frequencies and has been attributed to leakage of solution into the interface, where the conductor makes contact with itself.” Figure 3 shows the results from cyclic staircase voltammetry on 1 mM ferrocene for the three electrodes. For comparative purposes, the data have been normalized by plotting current density vs potential. As expected from the perimeter to area data, there is a signiticant enhancement in the current for the two composite electrodes compared with that of the solid gold electrode. This current enhancement is also summarized in Table 1as the limiting current density (Zd)for electrode 2 and the peak current density (Zj) for the solid gold electrode and electrode 1. Comparison of these data with the capacitance data indicates that
600
500
400
300
200
’
Potential (mV)
Figure 3. Cyclic voltammetric data obtained using a solid gold electrode (A) and gold/Kel-F composite electrodes 1 (B) and 2 (C) (see Table 1). Experiments were performed in a 0.10 M TBAP/ acetonitrile solution with 1 mM ferrocene (scan rate, 0.5 V/s) using a Ag/AgCI reference electrode. The currents are normalized to the active surface areas of the electrodes. An intentional offset was added to the curves for plotting purposes.
the current enhancement more than compensates for the unusually high capacitances observed with the composites, particularly with respect to electrode 2. The standard deviations in the Id and Z’ values are also given in Table 1. It should be noted that the electrodeswere polished between each of the five experiments performed and averaged to obtain these values. CONCLUSIONS
The sputter coating of gold onto the Kel-F particles prior to electrode fabrication has proven to be a useful approach for fabricatinggold composite electrodes with low active areas. When the active area is small enough, these electrodes have the advantage of behaving as an array of isolated ultramicroelectrodes. Future refinement of the sputter coating procedure (duration of coating and Kel-F particle size) should provide control over the percent active area as well as thickness of the active sites. Finally, this process should be applicable to the fabrication of other precious metal composite electrodes. ACKNOWLEDGMENT
We acknowledge the Committee on Internal Studies and Research at Murray State University WSU) for financial support of this work. We also thank the staff of the Hancock Biological Station at MSU for use of the scanning electron microscope. Received for review January 19, 1995. Accepted June 1995.@
5,
AC950063D @
Abstract published in Advance ACS Abstracts, July 15, 1995.
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
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