Langmuir 1991, 7, 1005-1012
1005
Voltammetric Application of Electromodulated Electroreflection Absorption Spectroscopy: Electroreflectance Voltammetry as an in Situ Spectroelectrochemical Technique Takamasa Sagma,*Satoshi Igarashi, Hisakuni Sato, and Katsumi Niki Electrochemistry Laboratory, Department of Physical Chemistry, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Received June 20, 1990.I n Final Form: September 26, 1990 It has been demonstrated that the voltammetric in situ application of electromodulated UV-visreflectance spectroscopy, electroreflectance(ER) voltammetry, is a powerful tool with which to analyze the electrode reaction of an adsorbed species on an electrode surface. The ER response, which in fact originates from the absorbance change with ac modulation of the electrode potential, was theoretically analyzed and simulated as a function of electrode potential. The results of the theory and simulation were compared to the electrode reaction of Nile Blue A adsorbed on a pyrolytic graphite electrode surface. The resulting ER voltammetry was responsive only to the faradaic processes of Nile Blue A adsorbed on a pyrolytic graphite electrode, and its turnover reaction rate was determined to be in the range of 60-120 5-1. It was revealed through ER measurements that the electronic structure as well as the redox reaction mechanism of the adsorbed Nile Blue A on a graphite electrode were significantly different from those of the native dye.
Introduction The recent development of spectroelectrochemistryhas received a great deal of attention. The combined use of in situ spectroscopictechniques and conventional methods has afforded us a detailed understanding of electrode processes on a molecular level.' A variety of structural and mechanistic information is provided, depending on the method of spectroscopy. Infrared reflection absorption spectroscopy (IRRAS) and Raman scattering (RS) spectroscopy offer information about the structural aspects in the vibrational mode, while ultraviolet-visible (UV-vis) absorption spectroscopy offers similar information in an electronic mode. A body of work on the electrode process using UV-vis absorption has been carried out to measure in situ the spectra of species in the solution phase, among which the optically transparent thin-layer electrode (OTTLE) is most widely utilized.2 Attempts have also been made to measure the spectra of the species in the vicinity of the electrode interface by using the internal reflection method with an optically transparent ele~trode.~ The reflection method in UV-vis mode to characterize solid surfaces, often referred as electroreflectance (ER), has developed in solid-state physicse4t5The method has been widely used in the studies of the electronic band structure of semiconductors. In this method, the reflectance change a t the solid surface due to the perturbation of dielectric properties of the surface is detected as a function of the surface electronic field of the solid. The ER technique has also been applied to metal electrode surfaces combined with conventional electrochemical methods where a change in surface charge causes a change in reflectance. For example, Hinnen and her
* To whom correspondence should be addressed.
(1) Gale,R.J.,Ed. Spectroelectrochemistry;PlenumPreas:New York, 1988, and referencea therein. (2) Su, C.-H.;Heineman, W. R. AMI. Chem. 1981,53,594. (3) Winograd, N.; Kuwana,T. J. Electroanul. Chem. InterfacialElectrochem. 1969,23, 333. (4) Seraphin, B. 0.; Bottka, N. Phys. Rev. 1966,145,628. (5) hpnes, D. E.; Bottka, N. In Semiconductor and Semimetals; Willardson, R. K., Speer C. A., E%.; Academic Preee: New York, Vol. 9, 1972.
0743-7463/91/2407-1005$02.50/0
colleagues applied the ER method to the observation of change in surface charge as a function of electrode potential a t a chloride-adsorbed gold electrode surfaceesJ McIntyre and Kolb used ER to track the formation of a metal oxide film on an electrode surface.8 Bewick et al. also used ER for the detection of an intermediate of COS reduction.9 In the electrochemical in situ measurement of ER, ac modulation of the electrode potential (electromodulation),sJ the double beam method: continuouswave modulation of the illumination intensity: the multiplication of rapid sweep cyclic voltammetry lo have been used. Electromodulated ER spectroscopy has been applied also to the investigation of the electrode reactions of adsorbed organic species. Electrochromism of dye molecules adsorbed on electrode surfaces has been studied by Plieth et al.11-13 and Memming.14 Hinnen, Parsons, and Niki first used ER techniques with electromodulation to analyze the electrode reaction of an adsorbed protein possessing large extinction coefficienta.15 In this study, the mode of measurement is in fact the optical absorption of the adsorbed species at the electrode surface. Since this method enables us to measure the difference in spectra between the reduced and oxidized forms involved in the electron transfer process, the electronic structure of the adsorbed species can be explored.15J6 This method has also been (6) Dalbera, J. P.; Hinnen, C.; Rouseeau, A. J. Phys. C Solid State Phys. 1977,38, 185. (7) Hinnen. C.: Huonn. C. N. V.: Roueseau. A.: Dalbera. J. P. J . Electroanul. Cheh. Ihterfadal Electrochem. 1979, $5, 131. . (8) MaIntyre, J. D. E.; Kolb, D. M. Symp. Faraday SOC.1970,4,99. (9) Aylmer-Kelly,A. W. B.; Bewick, A.; Cautrill,P. R . ; W o r d , A. M. Faraday Discuss. Chem. SOC.1975,56,96. (10) Collae,C.; Beden,B.;Lager,J. M.; Lamy, C. J. Electroanid. Chem. Interfacial Electrochem. 1986, 186, 287. (11) Plieth, W. J.; Gruschinake, P.; Hensel, H. J. Ber. Bumen-Gee. Phys. Chem. 1978,82,615. (12) Schmidt, P.; Pleith, W. J. J. Phys. (Paris) 1983, 4 , C10-175. (13) Schmidt, P. H.: Plieth, W. J. J. Electroanal. Chem. Interfacial Electrochem. 1986,201, 163. (14) Memming, R. Faraday Discuss. Chem. SOC.1974,68,261. (15) Hinnen,C.:Parsom,R.:Niki. K. J.Electroanal. Chem.Interfacial Electrochem. 1983,147,329. (16) Bedioui, F.; Devynck, J.; Hinnen, C.; Roueeau, A.; Bied-Chamton, C.; Gaudemer, A. J . Electrochem. SOC.1986,132, 2120.
0 1991 American Chemical Society
1006 Langmuir, Vol. 7, No. 5, 1991
Sagara et al.
applied to the study of the role played by surface modifiers in the electrode reaction of cytochrome c.I7J8 The remarkable features of electromodulated electroreflectance absorption spectroscopy as an in situ spectroelectrochemical technique can be summarized as follows: (i) Information about the electronic structure of the adI I sorbed species at the electrode can be obtained, as mentioned above. (ii) The electrode reaction mechanism can be analyzed in great detail. (iii) The faradaic process I A I/ I can be selectively observed. This is because the response I L I A involves only a faradaic process, unless the absorbance of LpF I the adsorbed species is very small. AIthough this third feature is expected to make the analysis of the mechanism and kinetics of the electrode process simpler and more sensitive, the utility of the voltammetric application of this method has not yet been lac 1 kjc = A R I R ( A Eo, W , A E ) firmly demonstrated. Figure 1. Block diagram of the instrumentation for ER volThe present paper describes theoretical aspects of tammetric measurement: L, Xenophoto lamp; FL, optical filter; measuring the ER voltammetric signal as a function of M, monochromator; S, slit; C, spectroelectrochemical cell; PS, electrode potential, that is, ER voltammetry. This paper potentiostat; FG, function generator; OSC, oscillator; HV, highalso outlines the practical application of ER voltammetry voltage supply; PM, photomultiplier; FB, feedback circuit; PA, preamplifier; NF, noise filter; LPF, low-pass filter; LIA, lockto the redox reaction of an adsorbed dye, Nile Blue A. in-amplifier; PC, personal computer (PC9801, NEC). Nile Blue A plays a role as a mediator of the electrode reaction of dihydronicotinamide adenine din~c1eotide.l~ of reflected light, Rat, divided by the dc component of the intensity Its catalytic activity seems to be strongly dependent on of reflected light, R, was given as a function of the dc electrode the nature of the adsorbed dye, but to our knowledge, potential (ER voltammetry) or as a function of wavelength (ER there are few spectroelectrochemical studies of the dye.Ig spectra). Here we present a simple case in which the electrode Materials. Nile Blue A (3-amino-7-(diethylamino)-1,2-benreaction of the dye is conducted on a pyrolytic graphite zophenoxazine), a product of Eastman Kodak Co., was supplied electrode. The results of a detailed study of the dye as an as perchlorate salt and was used without further purification. electrochemical catalyst will be described in a subsequent Water was purified to 16 Mil cm through a Milli-Q filter (Millipore Co.). All other chemicals were reagent grade. Pyrolytic paper.20 graphite was purchased from Union Carbide Co. and contained In the present paper, the ER voltammetric response as about 1% impure planes in the cleaved ab-plane, as verified by a function of wavelength of the incident light is denoted X-ray diffraction. as “ER spectra” and as a function of dc electrode potential Procedure. The graphite electrode surface was polished as “ER voltammogram”. parallel to the ab-plane with 600 to 3000 grit emery paper and then with 1-pm alumina powder to produce a mirrorlike hydroExperimental Section phobic surface. After sonication of the graphite electrode in pure ~
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I
Instrumentation. Figure 1shows the schematic diagram of the instrument for ER measurement. The incident light of constant intensity emittedfrom a400-W Xenophotolamp, HLX64663, Osram Co., was transmitted to the electrode surface through an IR cut filter, monochromator (NClON,Ritsu Applied Optics Co.), and a lens to focus the light at the electrode surface. The incident angle was nearly perpendicular to the electrode surface. The reflected light intensity from the electrode surface contained an ac component as a result of sinusoidal modulation of the electrode potential. In order to separate the reflected light at the cell wall from that at the electrode surface, the electrode was positioned so that the electrode surface was displaced by about 5O angle from the parallel position against the cell wall, and two mirrors were inserted prior to light detection. The light intensity of the reflected light was measured by a photomultiplier (PM), R928, Hamamatsu Photonics, Co. The intensity of both the dc and ac components of the reflected light was measured. The dc component was obtained from the preamplified output signal from a PM through a low-pass filter (cut-off frequency 5 Hz).The signal from a PM was also fed to a lockin-amplifier, Model 5204, PAR Corp. Co., to obtain the in-phase and out-of-phaseComponents of the ac component. The dc output was made constant by regulating the source voltage of the PM tube. The resulting signals were processed by using a personal computer, NEC PC-9801VF. The wavelength of the incident light or applied electrode potentia1 was also controlled by the personal computer. The final data, ac components of intensity (17) Hinnen, C.; Niki, K. J. Electroanal. Chem. Interfacial Electrochem. 1989,264, 157. (18) Sagara, T.;Niwa, K.; Sone, A.; Hinnen, C.; Niki, K. Langmuir 1990,6, 254. (19) Ni, F.; Feng, H.; Corton, L:;Cotton, T. M. Langmuir 1990,6,66. (20) Sagara, T.; Igarashi, S.; Niki, K. Manuscript in preparation.
water, a drop of ca. 1mL of the saturated Nile Blue A aqueous solution was placed on the surface for 3 min. Then the electrode was rinsed 3 times with the same buffer solution as the solution into which the electrode was immersed. The buffer solutions were 250 mM phosphate. To achieve pH 4, a small amount of HC1 was added. The electrode, on which the Nile Blue A was adsorbed using the procedure mentioned above, was measured in dye-free buffer solution using three methods: dc cyclic volta”ograms, ER spectra, and ER vo1ta”ograms. An Ag/AgCl electrode in saturated KCl solution and Pt foil were used, respectively, as a reference and counter electrode. All of the measurements were carried out at 25 1 OC under anaerobic atmosphere. In ER measurements, a modulation frequency of 14.24 Hz (angular frequency 89.47 rad s-l) was used to prevent beating with the commercial electric frequency (50 Hz). The effective value of the amplitude of the electromodulation was 10 mV (28.3 mV,,). All of the electrode potentials given in the present paper are with respect to the above mentioned Ag/AgCl reference electrode.
*
Theory and Simulation We are concerned with the electrode interface where the adsorbed, electrochemicallyactive speciesabsorbs light and the electrode surface can be regarded as a mirror in the wavelength region of interest. We will deal with only the reflection phenomenon due to the absorption of the adsorbed speciesand neglect photoemission and scattering. When the incident light with an intensityof Io is irradiated at the interface, the intensity of the reflected light, I,, is written as21
ER Voltammetry
Langmuir, Vol. 7, No. 5, 1991 1007
where Fox and Fred are the amounts of adsorbed oxidized generally much smaller than 1, the ER signal, Rac/Rdc, is and reduced forms, respectively, in units of mol cm-2, and proportional to the ac component of f, fat. The calculation of in-phase (real) and out-of-phase (imaginary) compowhere K ~ and , K A are the apparent absorption coefficients of the adsorbed species, respectively, when Fox= 1 mol nents with angular frequency u of fac as the solution of eq 4 gives the response to be measured. In the case of a cm-2 and Fred = 0 mol cmm2and when Fox = 0 mol cm-2 and reversible reaction (ko>> u),eq 4 can be solved analytically I'd = 1mol cm-2. Note that the following conditions are assumed in eq 1: (i) the values of K~~ and red are for anyvaluesof a and AE. In the case of a quasi-reversible reaction, a solution to eq 4 at E = Eo' is easily obtained independent of IO; (ii) the total amount of the adsorbed species of interest, Ft, is independent of the electrode by assuming a = 0.5 and AE
