Specular Reflectance in Thin-Layer Electrochemistry Peter T. Kissinger and Charles N. Reilley Department of Chemistry, University of North Carolina, Chapel Hill, N . C . 27514
A new approach to spectro-electrochemical studies is described. Spectral reflectance from metal film electrodes incorporated into thin-layer cells with variable barrier plates is advantageous for observation of electrochemical events via electronic and vibrational spectroscopy. Various improvements in the construction of sandwich-type thin-layer cells are discussed. Low resistance platinum film electrodes deposited on glass via thermal reduction are described and evaluated for spectro-electrochemical experiments. Rapid exhaustive electrolysis and diffusional mixing facilitate spectral analysis of primary products, n-value determination, and observation of moderately fast homogeneous post-kinetics. Spectroscopic application is demonstrated for the electrochemical oxidation of 9,lO-diphenylanthracene (DPA) and reduction of 5 3 diethylbarbituric acid (DEBA) in acetonitrile. A spectrum of the DPA radical cation is given and its halfregeneration reaction with water is confirmed. The DEBA anion is generated electrolytically through reduction of the most acidic proton.
AMONGthe multifarious links between electroanalytical and spectroscopic techniques that have recently come into fashion, the basic thin-layer concept ( I ) has unusual versatility. Optically transparent electrodes in thin-layer sandwich cells have had demonstrated utility in UV-VIS (2,3), infrared (4), and fluorescence experiments (2). Thin-film thin-layer electrodes have been reported (2). The present paper describes improvements in the previous design and presents a new technique for spectral observation of species confined in the thin-layer volume. A broad range of energies (ca. 700-50,000 cm-1) is, thus, made available for monitoring electrochemical processes in small amounts of solution. Single and twin-electrode thin-layer experiments are useful for rapid examination of electrochemical processes. The facile exhaustion of reactant permits ready determination of n-values and product spectra. Moderately fast kinetic processes can be examined with ease. Limitations on the study of very rapid or very slow transients are (with the single electrode cells used here) imposed by the inevitable iR-drop and edge effects, respectively. The available kinetic window can be extended somewhat by the use of getter-electrodes (2) and more judicious placement of the reference and auxiliary electrodes (5). For the spectral examination of very rapid kinetics and short-lived intermediates, the combination of rapid scanning spectroscopy with optically transparent electrodes recommends itself (6). The basic configuration for optical spectroscopy concurrent with thin-layer electrolysis is shown in Figure 1. This scheme has several important advantages over the previously reported
(1) C. N. Reilley, Rev. Pure Appl. Clzem., 18, 1221 (1967).
(2) A. Yildiz, P. T. Kissinger, and C. N. Reilley, ANAL.CHEM., 40, 1018 (1968). (3) W. R. Heineman, J. N. Burnett, and R. W. Murray, ibid., 40, 1970 (1968). (4) Ibid., p 1974. ( 5 ) W. R. Heineman, Ph.D. Thesis, University of North Carolina, 1968. (6) J. W. Strojek, G. A. Gruver, and T. Kuwana, ANAL.CHEM., 41, 481 (1969). 12
Figure 1. Basic configuration for specular reflectance in thinlayer electrochemistry. Reflectance losses and refraction have been neglected
transmission methods. Spectral sensitivity is enhanced by the double pass through the solution layer, without increasing the diffusional pathlength to the electrode surface. Multiple reflections may be employed when additional sensitivity is required. Because the electrode need not be transparent, a more rugged, lower resistance electrode is practicable. Luminous power loss attributable to the electrode surface is minimized because the per cent reflectance ( Z R ) of a polished opaque electrode is usually greater than the per cent transmittance of a usable thin-metal film or grid. This is less likely to be true in the UV where the Z R for most metals falls off dramatically (7). It is important that the angle of incidence be acute to minimize reflectance losses a t the barrier surfaces. Various barrier materials (quartz, glass, sodium chloride, etc.) permit easy application over a wide spectral range without altering the electrode or basic cell design. EXPERIMENTAL
Apparatus. The sandwich-type thin-layer cells were constructed as shown in Figure 2. All work reported here utilized electrode surfaces deposited on microscope slides by thermal reduction of platinum paint (Hanovia Liquid Bright Platinum 05-X) (8). The slides were first drilled with a ‘Il6 in. diamond bit to permit attachment of the glass inlet tube. After cleaning the glass with alcoholic potassium hydroxide and isopropanol, the desired electrode area was masked with Scotch tape, and the paint was lightly applied with a camel hair brush. After the paint dried, the tape was carefully removed and the slide placed in a previously warmed muffle furnace at ca. 600 “C for ca. hour. The inlet tube was then attached, and the cells were assembled as previously described (2). All of the cells used for this work were constructed with a = 0.5-1.0 cm, b = 1.0 cm, c = 1.5 cm, and 8 (cell thickness) = 100 I.(. Metals deposited by thermal evaporation under vacuo (2) were found to have more uniform surfaces and higher reflectivity. The difficulty of preparing opaque electrodes by this method (which is an excellent source of transparent electrodes), and the relative instability of these very thin surfaces at extreme anodic potentials (particularly in chloride media) made reduction of platinum from solution the method (7) G. Hass in “Applied Optics and Optical Engineering,” Vol. 111, R. Kingslake, Ed., Academic Press, New York, N. Y., 1965, p 313. (8) B. S. Pons, J. S. Mattson, L. 0. Winstrom, and H. B. Mark, Jr., ANAL.CHEM., 39,685 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
G
W
BARRIER
B
a
b
C
Figure 2. Thin-film thin-layer electrochemical cell
Figure 4. Infrared cell design-front and side views
Typical electrode element with Teflon spacer in position b. Transparent barrier plate c . Side view of assembled cell
a.
B. G. R. S. T. W.
of choice. Analysis of the platinum films by spark source mass spectrometry and differential pulse polarography indicates that, whereas the vapor deposited films were better than 99.9% platinum, the painted films were only about 95% platinum. The major contaminant in the latter is bismuth (ca. 4%) with tin, silver, and lead present in trace quantities. Figure 3 illustrates a general scheme found useful for various experiments with sandwich-type thin-layer cells. In a typical experiment, the cup is filled with supporting medium, and a deaerated solution is pumped from the reservoir into the thinlayer cell. The use of a syringe pump (Sage Instruments Model 234) permits experiments with flowing solution. Electrodes in the reservoir facilitate ordinary electroanalytical experiments on the same sample. These electrodes, the pump, and the overflow are luxuries not necessary for the simplest thin-layer experiments. Pumping solution into the cell from above is found to be far superior to drawing solution from the cup by suction (2, 3). This mode of operation facilitates deaeration of solution by eliminating the need for a glove bag. A single 10-ml syringe filled with deaerated solution is sufficient supply for many experiments considering the thin-layer volume is typically 10 pl or less. In addition, we have been able to perform experiments with as little as 1 ml of solution by this means. Teflon (Du Pont) tubing and various fittings were obtained from the Hamilton Company and Chromatronix, Inc. For systems particu-
Brass plate Glass slide Rubber gasket NaCl barrier plate Teflon spacer Working electrode contact
larly sensitive to oxygen, stainless-steel or glass tubing is advisable. Cells for the infrared region were constructed using a sodium chloride window for the barrier plate, Figure 4. Plates 2 in. X 1 in. X 6 mm may be obtained from International Crystal Laboratories. The glass-Teflon-NaC1 sandwich is clamped between rubber gaskets affixed to 1/16-in. brass plates. Although usually unnecessary, doping the cell edges with Tygon paint (Carboline K-63 white, Carboline Co., St. Louis, Mo.) assures a tight seal. Figure 5 illustrates the cell holder used for reflectance experiments in all spectral regions. The 3/s-in. aluminum base plate is compatible with the sample compartment of the Cary 14 spectrophotometer and includes provision for kinematic mounting in the Beckman IR 12 infrared instrument. The front surface mirrors, M1 and M2, are manufactured locally by vacuum deposition of aluminum on glass microscope slides and are conveniently affixed to machined mounts with double-stick Scotch tape. Provision is made for the inclusion of entrance and exit slits, and the barrier between M1 and M2 minimizes stray light reaching the photodetector. In a typical experiment, the light beam is focused on the electrode with M1, M2 is adjusted to maximize detectcr response, and the two mirrors are then locked in position. The angle of incidence for these studies was maintained at about 20 degrees.
EXHAUST
N
SYRINGE PUMP
GLASS
OVERFLOW
II
4
Figure 3. Schematic of auxiliary apparatus for thin-layer electrochemistry
Pi Figure 5. Cell holder for specular reflectance studies
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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(nm)
Figure 6. Spectral study of 9,lQ-diphenylanthracene oxidation. Deaerated solution of 2mM DPA, 0.1M TBAP in acetonitrile. Product spectrum (DPA+) obtained after application of +1.3 V us. SCE. Background obtained for supporting electrolyte only
40
30
Ultraviolet-Visible Region. Utilization of the cells described above for electronic spectroscopy is illustrated in Figure 6. These spectra were recorded us. a 20% T neutral density filter in the reference beam. The increasing base line toward the blue is principally a result of a decreasing reflective efficiency for the electrode surface. Useful spectra can be obtained down to about 200 nm, below which absorption by the platinum film (low Z R ) becomes pronounced. A flat base line is easily obtained when a reference cell is used; however, the decrease in system efficiency must be kept in mind. The electrochemical generation of 9,lO-diphenylanthracene (DPA) radical cation (DPA+), and its chemical and spectroscopic properties have been discussed by Sioda (9). The instability of DPA+ in acetonitrile solutions has been attributed to reaction with residual water forming trans-9,lOdihydro-DPA.
2DPA+
+ 2H20
--c
2H+
+ DPA + DPA(OH)2
The rapid exhaustive electrolysis intrinsic to thin-layer electrochemistry affords a well resolved spectrum of DPA+ without interference from DPA absorbance even in relatively high (> 100 mM) concentrations of water. This would, of course, be impossible with bulk electrolysis and subsequent transmission spectroscopy. Consider an experiment in which the (9) R. E.Sioda, J. Phys. Chem., 72,2322 (1968). 14
I
I
I800
1700
WAVENUMBER
A simple potentiostat constructed from two Philbrick P65AU operational amplifiers and boosted (P66A) for 100 mA capability was used for all spectro-electrochemical experiments. Chemicals. Eastman Spectroquality acetonitrile, polarographic grade tetrabutylammonium perchlorate (TBAP) obtained from Southwestern Analytical Chemicals, and 9,lOdiphenylanthracene and K & K Laboratories, Inc., were used without further purification. Barbital (5,5-diethylbarbituric acid) obtained from Fisher Scientific Co. was recrystallized thrice from acetonitrile. RESULTS AND DISCUSSION
I 1900
1600
CM"
Figure 7. Infrared study of barbital reduction. Reflectance unit in reference beam contains supporting electrolyte only A . lOmM barbital, 0.2M TBAP, acetonitrile; at open
circuit B. After application of -1.40 V us. SCE AN. Region of strong solvent absorption
thin-layer volume is rapidly (10 seconds) exhausted of millimolar DPA in wet (ca. lOOmM water) acetonitrile, and the circuit to the working electrode is then opened. In this condition, the concentration of DPA+ decays to zero, and 50% of the original DPA is regenerated, in agreement with Sioda's results. Infrared Region. The cells described here are particularly useful for infrared work where the advantages of a single, easily modified, barrier plate, and high overall transmittance are manifest. Any attempt to combine infrared spectroscopy with electrochemistry is beset with a rather formidable solvent problem. Three requirements for the ideal solvent are: the availability of a transparent window in the spectral region(s) of interest; sufficient dielectric constant to support a high concentration of electrolyte; and sufficient sample solubility. Solvents meeting all three criteria are seemingly not available. Examination of a large number of possibilities leads us to suggest three compromises: acetonitrile, dichloromethane, and dichloroethane. The latter two are advisable when relatively high solution resistance can be tolerated. The reflectance efficiency of the platinum electrode and auxiliary mirrors is very high throughout the infrared region, and little loss can be attributed to this cause. The barrier plate and lack of focusing optics In the system are primary culprits. Nevertheless, the unit transposed ca, 50z of the incident radiation in regions with no solution absorbance. Refinement of the configuration used here is possible by utilizing concave front surfaced mirrors (M1 and M2) and a hemicylindricai barrier.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
As an illustration of the capabilities of this technique, we have considered the reduction of acetonitrile solutions of barbital (5,5-diethylbarbituric acid) 0.2M in TBAP. Cyclic voltammetry of this system reveals an irreversible one-electron cathodic wave on platinum, gold, and mercury electrodes with peak potentials of -1.25, -1.70, and -2.40 V os. SCE, respectively. Spectroscopic studies indicate that in acetonitrile the molecule exists almost entirely in the un-ionized keto form. Figure 7 shows the infrared spectrum in the carbonyl region before and after electrolysis in a reflectance thin-layer cell. The disappearance of the principal carbonyl band after application of the potential step is illustrated in Figure 8. The resulting product has a spectrum characterized by lower energy carbonyl bands and a strong band attributable to C=N stretch. In fact, the entire product spectrum is exactly that of the barbital anion. This immediately suggests the overall reaction
0.4
ON
E
0.3
A a2
0.1
0.0
0
50
100
150
200
TIME, SEC.
responsible for the reduction wave. The large dependence of the voltammetric peak potential on the nature of the electrode is consistent with a prekinetic dissociation followed by hydrogen evolution. Confirmation is obtained from reflectance experiments in the UV where the product band at 242 nm matches that of the anion produced by chemical means. After many 10-pl samples were reduced in a series of experiments with the same cell, a noticeable amount of the barbital anion became incorporated into the surface of the sodium chloride barrier. This was easily confirmed by disassembling the cell. A transmission spectrum of the barrier plate proved to be identical with that of sodium barbital in a sodium chloride pellet. Naturally, this suggests that some caution is appropriate when considering the chemical inertness of the barrier material selected for a given experiment. At least as of this writing, it appears that the present approach provides, in the infrared region, sensitivity, and time response superior to previously reported ATR methods (10-12).
Surface Studies. Changes in electrode reflectivity have potential for surface studies. Film formation or alternation of surface structure can greatly change the ZoR as a function of wavelength. Reflectance spectroscopy has been shown to be useful for passivation and adsorption studies (13, 14). Multiple specular reflectance of oxide layers on gold electrodes has been reported (15). The utility of specular reflectance for examination of a variety of electrochemical surface phenomena has recently been discussed by McIntyre (16). Although such effects would have been detrimental to
(10) H. B. Mark, Jr. and B. S. Pons, ANAL.CHEM., 38,119 (1966). (11) T. Takamura and H. Yoshika, Abstracts, 17th Meeting CITCE, Tokyo, Japan, September 1966, No. C301, p 260. (12) D. R. Tallant and D. H. Evans, ANAL.CHEM.,41, 835 (1969). (13) K. J. Cathro and D. F. Koch, J. Electrochem. Soc., 111, 302 (1966). (14) D. F. A. Koch and D. E. Scaife, ibid., 113,302 (1966). (15) T. Takamura, K. Takamura, W. Nippe, and E. Yeager, Abstracts, Spring Meeting of the Electrochemical Society, New York, N. Y., May 1969, No. 257A. (16) J. D. E. McIntyre, ibid., No. 232.
Figure 8. Transient absorbance of 1703 cm-1 barbital peak after application of -1.40 V us. SCE. Solution as per Figure 7 the purposes of the present investigation, they appear to be negligible for most systems. Preliminary results in these laboratories indicate that reflectance provides a potentially useful means of monitoring the electrodeposition of metals. Semi-Infinite Diffusion. Specular reflectance from metal films or polished bulk metals is a viable alternative to transparent electrodes (6, 17) for spectro-electrochemical studies in semi-infinite solution. Particular advantage is seen for fast high-current experiments where the surface resistance attendant with transparent electrodes may be encumbering. In a rather imaginative experiment multiple specular reflectance within a silvered cylinder has been used to search for solvated electrons intermediate to hydrogen evolution (18). Reflectance from a mercury pool electrode has been reported (19). Studies in progress indicate that thin mercury films (ca. 1 pm thick) electrolytically deposited on platinum will prove useful when a mercury surface is required. These highly reflecting very smooth films are less subject to disturbing mechanical motions derived either from the surroundings or from rapid potential excursions. Vibrations of this sort can be prohibitive when short-time high-sensitivity optical measurements are attempted. ACKNOWLEDGMENT
The authors thank J. R. Hass and R. J. Lewis for the spark source mass spectral and pulse polarographic analyses, respectively. R. P. Van Duyne is thanked for stimulating discussion. RECEIVED for review August 14, 1969. Accepted October 21, 1969. This investigation was supported by the National Science Foundation, Grant GP-8669 and Air Force Office of Scientific Research, Grant AFOSR 69-1 625. (17) T. Kuwana and J. W. Strojek, Discussions Faraday SOC.,45, 134 (1968). (18) D. C. Walker, ANAL.CHEM., 39, 896 (1967). (19) V. I. Shapoval and V. N. Turchak, Ukr. Khim. Zh., 34, 525 (1968).
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