Optically Transparent Vitreous Carbon Electrode V. E. Norvell and Gleb Mamantov“ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 379 16
Development of optically transparent electrodes (OTEs) for simultaneous spectroscopic and electrochemical studies (I),particularly of electrochemically generated reactants (21, has received considerable attention. A special type of OTE, the optically transparent thin-layer electrode (OTTLE) has been especially useful in such studies because the small cell volume of a typical OTTLE allows complete electrolysis of the thin-layer solution within a short period of time (2, 3). OTTLEs have proved to be useful for determining values of Eo’and n for several complex biological redox systems (2). Thus far, UV-visible spectroelectrochemistry using OTEs has involved the use of an electrode consisting either of a film of metal or metal oxide on a transparent substrate, or a metal wire mesh minigrid held between two optical windows (2). OTEs based on a germanium plate ( 4 , 5 ) or carbon film (6, 7) have been reported for IR studies. The recent development of reticulated vitreous carbon (RVC) (8)has made possible the construction of an OTE with an RVC electrode. The structure of RVC allows the RVCOTE to exhibit many of the characteristics of an O’M’LE, even though the volume of solution electrolyzed is 10-20 times greater than for a typical minigrid OTTLE. The fabrication and evaluation of the RVC-OTE are discussed in this paper. EXPERIMENTAL Construction of the RVC-OTE. RVC (trademark of Chemotronics) with porosity of 100 pores per inch (ppi) was obtained from Chemotronics International, Inc., Ann Arbor, Mich. This material was cut into slices of approximate dimensions 2 cm x 4.5 cm with thicknesses ranging from 0.8 to 1.2 mm for use as electrodes. Electrical contact to the RVC was made in the following manner: a fine Cu wire was threaded through a corner of the electrode and folded over to secure it; Liquid Organic Silver No.9167 (Englehard Industries, East Newark, N.J.) was then applied to the surrounding area and the organic portion burned away leaving a silver coating; this was repeated several times until a sturdy contact was achieved. The OTE was assembled by sealing the RVC electrode between two clean microscope slides with epoxy in a configuration similar to that suggested by DeAngelis and Heineman (3). The RVC was positioned a few mm from the bottom of the slides and extended about 1 cm beyond the slides on each side. To prevent accidental breakage of the RVC at the edge of the slides, the whole exposed portion of the electrode, including the silvered contact region, was coated with epoxy. The Teflon spacers used by DeAngelis and Heineman (3)were not necessary here since the thickness of the cell was determined by the thickness of the RVC slice. A small diameter Teflon tube was connected to the top of the cell to allow solution to be drawn into the cell with a syringe. To dislodge air bubbles from the RVC electrode, it was necessary to draw solution above the top of the electrode and then “pump” the solution up and down rapidly. Purging the OTE with nitrogen before filling helped minimize this problem. Small residual bubbles that remained in the electrode after this procedure did not affect the operation of the OTE. The solution level was maintained above the top of the electrode by clamping the filling tube. Cells constructed according to the above procedure had volumes of 400 t o 550 p L depending on the thickness of the electrode. These volumes were determined by coulometry with a standard solution of K3Fe(CN)Gusing the calculation procedure given by DeAngelis and Heineman (3). Cell thicknesses were calibrated by comparing the absorbance of a KMn04 solution in the OTE with the absorbance of the same solution in a 1.0-cm cuvette with a 0.9-cm spacer. Apparatus. Cyclic voltammetry was performed with a PAR 173/179 Potentiostat/Coulometer and a PAR 175 Universal 1470
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
Programmer. The PAR 173/179 instrument was also used for coulometry. Spectra were recorded with a Bausch and Lomb Spectronic 505 spectrophotometer. All potentials were measured vs. the saturated calomel electrode. Reagents. Standard ACS grade reagents and deionized water were used for all solutions without further purification. R E S U L T S A N D DISCUSSION The RVC-OTE is unique among OTEs because of the composition and special structure of the electrode material. RVC is a glasslike carbon with a porous structure having a free void volume of about 97% and a surface area of about 65 cm2/cm3 (8). An electrode of substantial surface area can thus be made from a relatively small piece of RVC. RVCOTEs of the dimensions given Bbove have surface areas ranging from 30 to 40 cm2 (based on the calibrated cell thickness). This compares to a surface area for a typical minigrid OTTLE of 1-2 cm2 (9,10).When RVC is cut into thin slices, its light transmittance is comparable to that of wire mesh minigrids (13%-45% for 100 ppi RVC 1.2 mm to 0.5 mm thick) and is essentially constant over the entire UVvisible spectrum. The three-dimensional structure of RVC allows the RVC-OTE to behave in a fashion similar to a minigrid OTTLE, i.e., electrochemical equilibrium is rapidly achieved and electrolysis of the solution in the immediate vicinity of the electrode can be carried out to completion within a short period of time with diffusion as the sole means of mass transport. This is because most of the solution in the vicinity of the electrode is actually inside the electrode, so that each electroactive entity has a relatively short diffusional path to the electrode surface. This behavior is illustrated by the chronocoulogram shown in Figure 1. A standard solution of K3Fe(CN)6 was reduced by stepping the potential of the RVC-OTE from 0.6 V to 0.0 V at time t = 0. The charge is seen to increase rapidly, fiially leveling off to a constant value, QT, when complete electrolysis has been achieved. By subtracting the background, QB,the cell volume and approximate thickness can be calculated (3). The background correction was determined with a solution of the supporting electrolyte, and was typically 2 to 8% of the total charge (Table I). The results of coulometry were very reproducible both in the shape of the chronocoulogram and the total charge required for complete electrolysis (Table I). The cyclic voltammetric behavior of the RVC-OTE (Figure 2) is very similar to that of a minigrid OTTLE (3). The important feature to note here is the rapid drop in current to near zero following the peak in both scan directions. This is an indication of complete electrolysis of the electroactive species (3). The measured values of iPaand i,, are identical, indicating complete retention of the electroactive material within the small cell volume. Diffusion into the bulk solution is negligible. The RVC-OTE has been applied to the spectroelectrochemistry of o-tolidine (11)and Mn04- (12,13),and the results are illustrated in Figures 3 and 4. These spectra were obtained by applying a potential at which only the reduced form was present (Mn04-was aonverted to Mn02- by the reaction 4Mn04- 40H4Mn042- O2 2H20), then applying potential steps to generate the oxidized form. The current was allowed to decay for two or three minutes to background level before each spectrum was recorded. For both o-tolidine and permanganate, the same sets of spectra were obtained
+
-
+
+
Table I. Coulometric Data Reproducibility for the RVC-OTEa RVC-OTE NO. '2
RVC-OTE NO. '1 Q,
QB
0.1630 0.1617 0.1638 0.1655 0.1635 * 0.0016
QT
0.0125 0.0122 0.0126 0.0147 0.0130 ?r 0.0011
QB
0.2164 0.2167 0.2160 0.2162 0.2163 * 0.0003
a Solution: 4.0 X loF3M K3Fe(CN), in 1.0 M KCl. Potential stepped from 0.6 V to 0.0 V. cell thickness: -0.92 mm. Cell volume: -550 p L , cell thickness: -1.20 mm.
0.0043 0.0043 0.0043 0.0031 0.0040 ?r 0.0006 Cell volume: -400 pL,
f 07
04
I
/
a 03
1
c[
,
1
-
L
-
-
I
L
1
I
1
1
4w
450
500
550
i.nm
800
650
I
700
Figure 4. RVC-OTE spectra of 3.0 X M Mn04- In 4.0 M KOH, for dlfferent values of applied potenthl: ( a ) 0.550,( b )0.400,(c)0.380,
(d)0.360,(e)0.340,(f)0.320,(g)0.300,(h)0.280,(i)0.260,Ci)0.150 V vs. SCE. Cell thickness: -0.92 mm, transmittance: 22%
36
05
,
I
C I
c4 E
v
VI
02
,
0 ,
,
0
I -0,
5:E
cyclic voltammogram of 4.05X M K3Fe(CN), in 1.0 M KCI. Scan rate: 1 mVls. Cell volume: -550 pL. Cell 1.2mm thickness: Flgure 2. RVC-OTE
-
when starting with the oxidized form and reducing stepwise. The limited transmittance of the OTE was corrected for by placing in the reference beam of the spectrophotometer a piece of RVC of a thickness similar to that used in the OTE. The RVC-OTE has several characteristics which overcome some of the disadvantages of wire minigrid or metal oxide OTEs, while combining the advantages of both of these types of electrodes. Because RVC is a rather chemically inert matrix of vitreous carbon, strongly acidic or basic solutions, or strongly oxidizing or reducing solutions have little or no effect on the operation of the electrode. The electrode is also electrochemically unreactive, exhibiting a usable potential
range extending from about 1.2 V to -1.0 V at pH 7 . These limits represent a current density of approximately 5 pA/cm2 of electrode surface area. This range is comparable to that of a tin oxide or indium oxide OTE, whereas the range of a metal film or wire minigrid OTE is more restricted (2). The spectral characteristics of the RVC-OTE, on the other hand, are very similar to those of a wire minigrid OTE, Le., practically constant transmission over the UV-visible spectrum (2). This is in contrast to the widely varying transmission of metal film or metal oxide OTEs (2). The RVC-OTE thus combines the electrochemical advantages of metal oxide electrodes with the spectroscopic advantages of the minigrid OTE. Preliminary experiments have indicated that it may be possible to extend the negative potential limit of the RVCOTE by deposition of a mercury film onto the RVC surface. This procedure has been successfully applied to Pt fiim OTEs (14) and nickel (10) and gold (15) minigrid OTEs. Our success with this technique has been limited because of poor Hg film formation; however, a smooth mercury coating has been ANALYTICAL CHEMISTRY, VOL. 49,NO. 9,AUGUST 1977
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achieved by depositing mercury on nickel-plated RVC (26). The use of the RVC-OTE for IR spectroelectrochemistry in aqueous solutions has been considered, but the thickness of the electrode (see below) would appear to make direct IR transmission spectroscopy difficult. Internal reflectance spectroscopy appears to be more promising. A cell of the type used by Laser and Ariel (17),in which the internal reflection element is surrounded by an OTE, could be easily constructed with an RVC electrode. Bulk RVC is available in various porosities, and can easily be cut into any desired shape. The choice of several porosities allows control over the optical characteristics as well as the surface area-to-volume ratio of any particular electrode. The minimum thickness to which RVC can be cut (thus the maximum optical transmittance) is dependent on the porosity. We have found the minimum thickness for 100 ppi RVC to be about 0.5 mm. RVC is also very inexpensive. A complete OTE can be made with less than $0.15 worth of material. In conclusion, we have demonstrated that the RVC-OTE is a versatile new optically transparent electrode which has properties unlike any previously reported OTE. We feel that the RVC-OTE, or modifications thereof, will be applicable to a number of spectroelectrochemical studies, particularly in nonaqueous or corrosive solutions at elevated temperatures, e.g., molten salt systems.
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LITERATURE CITED T. Kuwana, Ber. B u m e m , phys. Chem.,77,856 (1973), and references therein. T. Kuwana and W. R. Heineman, Acc. Chem. Res., 9,241 (1976), and references therein. T. P. DeAngells and W. R. Heineman, J. Chem. Educ., 53,594 (1976). H. B. Mark, Jr., and B. S. Pons, Anal. Chem., 38, 119 (1966). D. R. Tallant and D. H. Evans, Anal. Chem., 41, 835 (1969). J. S. Mattson and C. A. Smith, Anal. Chem., 47, 1122 (1975). J. S. Mattson and T. T. Jones, Anal. Chem., 48, 2164 (1976). Bulletin No. 176, Chemotronics International, Inc., Ann Arbor, Mlch. W. R. Helneman, J. N. Burnett, and R. W. Murray, Anal. Chem., 40, 1970 (1966). W. R. Heineman, T. P. DeAngells and J. F. Qoelz, Anal. Chem., 47, 1364 (1975). R. W. Murray, W. R. Heineman, and 0 . W. O'Dom, Anal. Chem., 39, 1666 (1967). D. B. Freeman, Ph.D. Dissertation, University of Tennessee, Knoxville, Tenn., 1971. D. B. Freeman and G. Mamantov, Electrochlm. Acta, 21, 257 (1978). W. R. Helneman arid T. Kuwana, Anal. Chem., 43, 1075 (1971); 44, 1972 (1972). M. L. Meyer, T. P. DeAngells, and W. R. Helneman, Anal. Chem., 49, 602 (1977). W. A. Rice arid C. H. Franklin, ChefmWonics International, Inc., Ann Arbor, Mlch., private communication. D. Laser and M. Ariel, J. Elecfroanal. Chem., 41, 381 (1973).
RECEIVED for review April 22, 1977. Accepted May 27, 1977. We would like to acknowledge the support by the Energy Research and Development Administration under Contract E(40-1)-5053.
