Electroreflectance Study of the Redox Reaction of Methylene Blue

The electrode reaction of methylene blue (MB) adsorbed on a pyrolytic graphite ... absorption spectrum between reduced and oxidized forms of MB adsorb...
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Langmuir 1992,8, 1018-1025

1018

Electroreflectance Study of the Redox Reaction of Methylene Blue Adsorbed on a Pyrolytic Graphite Electrode Takamasa Sagma,* Jun Iizuka, and Katsumi Niki Department of Physical Chemistry, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Received November 7,1991. In Final Form: January 2,1992 The electrode reaction of methylene blue (MB) adsorbed on a pyrolytic graphite electrode was studied by using electroreflectance (ER)spectroscopy. It was found that ER spectra represent the difference absorption spectrum between reduced and oxidized forms of MB adsorbed on the electrode surface. The Stark effect does not contribute to the ER spectra at all. Detailed analysis of both ER spectra with the electrode potential modulation and the specular reflectance spectra at constant potentials leads to the conclusion that the reduced form of MB adsorbed on a pyrolytic graphite electrode is not colorless, though the fully-reduced form of MB in an aqueous solution, known as leuco methylene blue (leuco-MB),is colorless. A specific interaction between MB and the graphite electrode surface was suggested.

Introduction The employment of in situ spectroscopic techniques in electrochemical systems has attracted a great deal of attention. Efforts have been made to develop a variety of spectroelectrochemicaltechniques by which the electrontransfer mechanism of adsorbed redox species can become clear at a molecular 1evel.l Several new strategies have been devoted to the spectroelectrochemicalcharacterization of irreversibly adsorbed redox specie^.^-^ One of them is the measurement of the electronic spectra of the adsorbed speciesby the reflectance method with a modulation of the electrode potential, i.e., electroreflectance spectroscopy (ER).6s7 The ER method can be applied to various electrode materials except for a nonreflective one, in contrast to the measurement of UV-vis transmission spectrum in which only optically transparent electrodes can be used.* The ER method can be readily applied even to an edge plane of the pyrolytic graphite electrode, which is poorly reflective. The ER measurement with an ac-wave potential modulation can be outlined as follows? The applied electrode potential, E , is the sum of the dc electrode potential, Edc, and a sinusoidal-wave modulation, so that E = Edc + AEac sin (wt),where AEacis the ac amplitude, w is the angular frequency, and t is time. Under the steady-state illumination of the electrodesurfacewith amonochromatic light, the change of the reflectance of the electrode surface in response to the ac electrode potential modulation is detected as an ER signal. The ER response is defined as the ac ER signal divided by the total reflectance of the electrode surface (ARIR). When Edcis nearly equal to the formal potential of the electrochemically active species adsorbed on the electrode surface and the absorption spectra of reduced and oxidized forms are different, the ~

~~

~~

* To whom correspondence should be addressed. (1) Gale,R.J.,Ed.Spectroelectr0chemistry;PlenumPress:New York, 1988; see also references therein. P. J.Electroanu1.Chem.Interfacia1 (2) Jeanmaire,D.L.;VanDuyne,R. Electrochem. 1977, 77, 858. 1977,99,5215. (3) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. SOC. (4) Bewick, A.; Kunimatau, K.; Robinson, J.; Russell, J. W. J. Electroanul. Chem. Interfacial Electrochem. 1981, 119, 175. (5) T. Kuwana, Ber. Bumen-Ges. Phys. Chem. 1973, 77,858. (6) Aylmer-Kelly, A. W. B.; Bewick, A,; Cantrill, P. R.; Tunford,A. M. Faraday Discuss. Chem. SOC.1973,56,96. (7) Memming, R. Faraday Discuss. Chem. SOC.1974,58, 261. (8) Bard, A. J.; Faulkner, L. R. ElectrochemicalMethods;John Wiley & Sons: New York, 1980; Chapter 14. (9) Sagara, T.;Igarashi, S.;Sato, H.; Niki, K. Langmuir 1991,7, 1005.

ac reflectance change, All, is mainly due to the change in the light absorption of adsorbed species with the potential modulation.1° The ER spectrum, the plot of the ER response at a Constant Edc as a function of the wavelength, A, allows the discussion on the electronic state of the adsorbed species. The ER voltammogram, the plot of the ER response at a constant wavelength as a function Of E&, enables us to obtain the information on the kinetics of the electron-transfer process, as reported in our previous paper.g It is known that an ER spectrum with the ac modulation of the electrode potential could exhibit one of two kinds of features: feature I, the first differential derivative feature of the absorption spectrum of adsorbed species arising from dipole-field interaction (the Stark effect),11J2 and feature 11,the differenceabsorption spectrum between reduced and oxidized forms.g The former reflecta the perturbation of the interaction between the transition moment of the adsorbed molecule and the surface electric field in the double-layer p~tentialregion,~~ while the lattep representsthe electrochemicalconversionbetween reduced and oxidized forms.1° Plieth and his colleagues f i s t demonstrated that the ER spectra of feature I were observed at metal electrodes on which a variety of dye molecules were adsorbed.lSl6 Several other researchers also have reported the ER spectra arising from the Stark effect.17-19 On the other hand, Hinnen and her colleagues, as well as our group, have reported the ER spectra of feature 11. So far, the ER spectra of feature 11 were obtained at gold electrodes on which cytochrome c,10**22 cytochrome c3,lo or m e t a l l o p o r p h y r i n ~are ~ ~adsorbed. ~~~ (10) Hinnen, C.; Parsons, R.;Niki, K. J. Electroanul. Chem. Interfacial Electrochem. 1983,147,329. (11) Labhart, H. Adu. Chem. Phys. 1967,13, 179. (12) Liptay, W. Angew. Chem. 1969,81, 195. (13) Plieth, W. J.; Gruschinska, P.; Hensel, H.-J. Ber. Bunsen-Gee. Phys. Chem. 1978,82,621. (14) Plieth, W. J.; Schmidt, P. H. J. Electroaml. Chem. Interfacial Electrochem. 1986,201, 163. (15)Plieth, W. J.; Schmidt, P.; Keller, P. Electrochim. Acta 1986,31, 1001. (16) Plieth, W. J.; Schmidt,P. H.Ber. Bunsen-Ges.Phys. Chem. 1987, 91, 323. (17) Nakabayaehi,S.; Kira, A. J.Electroanal. Chem.Interfacial Electrochem. 1991,300, 249. (18)Henglein, F.; Lipkoweki, J.; Kolb, D.M. J. Electroanal. Chem. Interfacial Electrochem. 1991, 303,245. (19) Lema, R. 0.;Re Tacconi, N. R.; Hahn, F.; Arvia, A. J. J. Electroaml. Chem. Interfacial Electrochem. 1991, 306,259. (20) Hinnen, C.; Niki, K. J. Electroanul. Chem. Interfacial Electrochem. 1989,264, 157.

0743-7463/92/2408-1018$03.00/00 1992 American Chemical Society

Electroreflectance Study of Methylene Blue

Langmuir, Vol. 8, No. 3,1992 1019

Usually, the ER spectra of feature I are obtained in the c w R double-layer potential region, while those of feature I1 are found in the vicinity of the formal potential of the adsorbed species. However, in our recent ER measurements in the vicinity of the formal potential for some dyes adsorbed on a pyrolytic graphite electrode, the obtained ER spectra are surprisingly different from the feature I1 spectra expected from the absorption spectra of the dyes in the solution phase. The ER spectra particularly resemble the feature I spectra in shape, though the ER response can be observed only when E d c is close to the I F formal potentials of the dyes adsorbed on the electrode. One of the examples is Nile Blue A as already r e p ~ r t e d . ~ Similar results have been obtained for methylene blue and some other phenothiazin dyes.25 In the present paper, the electrode reaction of methM ylene blue adsorbed on a pyrolytic graphite electrode is studied in detail using the electroreflectance techniques and the specular reflectance spectral measurements. What we focused on in this experiment is the interpretation of the ER spectra at the above-mentioned interface. The mirror adsorption state of MB on the pyrolytic graphite electrode Figure 1. Schematic picture of cell assembly for specular is also discussed.

A&, I\

i\

Experimental Section A pyrolytic graphite plate,purchased from Union Carbide Co., was cut and mounted in an epoxy-cement resin (Torr Seal, Barian Co.). The electrode surface was polished with a 600-grit Sic emery paper to expose a surface parallel to the ab plane. After sonication of the electrode in water, the electrode surface was peeled off, at least twice, using Scotch tape so as to expose a fresh ab plane. The typical geometrical electrode area of the graphite electrode used for ER measurements was 0.4 cm2. Immediately after the electrode surface was peeled off, the electrode was subjected to the measurements or adsorption procedure. Methchloride), ylene blue (3,7-bis(dimethylamino)phenathiazin-5-ium purchased from Wako Chemicals Co., was used without further purification. Water was distilled and purified through a Milli-Q filter (Milli Pore Co.). The resistivity of the purified water was more than 16 MR-cm. All of the other chemicals were reagent grade and were used as supplied. For pH dependence measurevents, 1 M NaC104 stock solution was used as a supporting + HzS04 was added electrolyte, and NazB40, + NaOH or to adjust pH. An Ag/AgCl electrode in saturated KC1 solution and Pt foil was used, respectively, as a reference and counter electrode. Instrumentation for electroreflectance(ER)measurementa was described in our previous paper.9 An optical blind and an optical pinhole were newly installed between the spectroelectrochemical cell and the photomultiplier in order to eliminate a scattered or reflected light from the cell wall. The modulation of the electrode potential was done with not only a sinusoidal but also a square wave. For the measurements of specular reflectance absorption spectra, a UV-vis double-beam spectrophotometer (UV2200,Shimadzu Co.) was combined witha specular reflectance attachment (Shimadzu Co.), on whicha cylindrical quartz cell was positioned as shown in Figure 1. The position of a graphite electrode (apparentarea 0.86 cm2)was fixed in such a way that the maximal reflectance was gained. The light incident angle was 5O. A graphite plate, whose ab plane was exposed, was peeled off and used as an optical reference. The specular reflectance spectra were measured at controlled electrode potentials with a sweep (21) Sagara, T.; Niwa, K.; Sone, A.; Hinnen, C.; Niki, K. Langmuir 1990, 6, 254.

(22) Sagara, T.; Murakami, H.; Igarashi, S.; Sato, H.; Niki, K. Langmuir 1991, 7, 3190. (23) Bedioui, F.; Devynck, J.; Hinnen, C.; Rouseau, A.; Bied-Charreton, C.; Gaudemer, A. J. Electrochem. SOC.1986,132, 2120. (24) Ngameni, E.;Laouhan, A.; L'Her, M.; Hinnen, C.; Hendricks, N. H.; Collman, J. P. J. Electroanal. Chem. Interfacial Electrochem. 1991, 301, 207. (25) Sagara, T.; Iizuka, J.;Sato, H.; Niki, K. Unpublishedresulta, 1991.

reflectance spectra measurement: A, Ag/AgCl electrode in saturated KCl solution;B, vycor tip; C, counter electrodeterminal; D, Pt counter electrode; E, pyrolytic graphite electrode; F, quartz cell; R, reference electrode terminal; W, working electrode terminal.

rate of a wavelength of 57 nm min-l, a slit width of 2.0 nm, and a sampling pitch of 0.5 nm. Smoothing of the curve of the reflectance spectrum was carried out by a convolution method using 25 sampling points with an interval of 4 points. All of the measurements were carried out at 25 2 "C in Ar atmosphere and in the absence of dye in the solution phase, unless otherwise stated. All of the electrode potentials given in the present paper are measured with respect to the above-mentioned Ag/AgCl reference electrode. The film-transfer procedure to adsorb methylene blue on the graphite electrode surface was as follows: immediately after the graphite electrode was peeled off, a drop of ca. 40-100 p L of a saturated solution of methylene blue in water was placed on the surface of the graphite electrode for 10 min. Then, the electrode was rinsed with the same buffer solution as that used in the subsequent measurement.

*

Results and Discussion Dc Voltammetric Study. Figure 2 shows a cyclic dc voltammogram of a pyrolytic graphite electrode on which methylene blue (MB) was adsorbed by the film-transfer procedure. A phosphate buffer solution (50 mM, pH 7.9) was used as a supporting electrolyte. The peak current isproportionaltosweeprates, u,up to 200mVs-',indicative of the reaction of MB adsorbed on the electrode surface. The anodic and cathodic peak currents are identical, and the wave shapes are symmetrical with respect to the peak potentials. The adsorbed layer of MB is so stable that the desorption of the dye within the period of measurement was negligible at this pH. The formal potential, which was taken as the average of the anodic and cathodic peak potentials, is -242 mV. Assuming that the number of electrons involved in the electron-transfer reaction is 2, the amount of MB adsorbed on t h e electrode surface is calculated as being 4.0 X 10-lo mol cm-2 from the charge of the peaks. If a surface roughness factor of 3 is assumed, the surface area occupied by one MB molecule is ca. 130 A2,which is identical to the value estimated for a flat monolayer adsorption according to the molecular size.26 The half-width of the peak is variant with u (for example, (26) Kipling, J. J.; Wilson, R. B. J. Appl. Chem. 1960, 10, 109.

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1020 Langmuir, Vol. 8,No. 3,1992

( pH 2/ e 5.6 )

(%??"&y I H

\

-(w2N (MB)

,6)

- H4

t--+ H' ( PIC,

=

5.6)

H

(hm-MB )

Figure 4. Redox reaction mechanism of MB adsorbed on a pyrolytic graphite electrode surface.

111111111111 ao

-05

0.5

/ V

E vs. E,,,,Mcl

Figure 2. Cyclicvoltammogramat a pyrolyticgraphiteelectrode

on which methylene blue is adsorbed. The electrolyte solution is 50 mM phosphate buffer, pH 7.9. Sweep r a w (mV 8 - 9 are a, 200; b, 150; c, 100; d, 50; e, 25. A plot of anodic (closed circle) and cathodic peak currents (open circle) as a function of sweep ratk (u) is also shown.

e

,

o . o ~ ~ , ~, ~ ~ ' l

4

, 0

5

,,,

10

PH

1

14

Figure 3. Formal potential obtained from cyclic dc voltammogram, E O ' , for adsorbed methylene blue as a function of pH in 1 M perchlorate electrolyte. The straight lines are drawn by the least-squares fitting calculation.

130 mV at 200 mV s-1 and 105 mV at 25 mV s-l), though the voltammetric response is nearly reversible, Therefore, the apparent number of electrons involved in the chargetransfer process, n,, cannot be determined accurately from the half-width of the peak, but it is estimated to be less than unity. To discuss the reaction mechanism in more detail, the pH dependence of the cyclic dc voltammogram was examined. In acidic solutions (pH < 3.51, the dc voltammetric response decayed and completely disappeared during a few cycles of the electrode potential. Before the disappearance of the dc voltammetric response, the height of the anodic peak was always less than that of the cathodic peak. These facta indicate that the reduction product of MB easily desorbs from the electrode surface, probably because of the high solubility of the reduction product in acidic solutions. When pH values are higher than 3.7, desorption of MB during the voltammetric measurement is not observed. Both the amount of adsorbed MB calculated from the peak area and the W w i d t h of the peak are independent of pH in the pH range between 4 and 12. Figure 3 shows the formal potential of MB adsorbed on a pyrolytic graphite electrode as a function of pH. The

slopes of the two linear parts in Figure 3 are obtained by the least-squares fitting calculation, and they are -60 f 3 mV/pH when pH I5.6 and -27 mV/pH when pH 2 5.6. These values reveal that the redox reaction at pH I5.6 is a two-electron two-proton-transfer process while that process. at pH 15.6 is a two-electron-one-proton-transfer The reaction mechanism is written as shown in Figure 4. The pH value of the cross point in Figure 3, 5.6, is considered to be the PK, value of the dimethylamino nitrogen of leuco methylene blue (leuco-MB). It is known that the protonation of two dimethylaminonitrogens are available in an aqueous solution, and the PKa values are pKa,1 = 5.02 and pKa,2 = t1.87.~' It is interesting to note that a slope of -90 mV/pH, corresponding to a twoelectron-three-proton-transfer process, was observed in the aqueous solution at pH values lower than 5.% The result of the present experiment, the slope at 4 < pH < 5.6 is not -90 mV/pH but -60 mV/pH, indicates that the PK, value for the protonation of the second dimethylamino nitrogen of leuco-MB adsorbed on the pyrolytic graphite electrode is less than 4. (If the PK, value is higher than 4, as in the solution phase, then either another cross point should appear in Figure 3 or the slope at the acidic region should be -9OmV/pH.) That is, the chemical property of leuco-MB as the reduction product of MB adsorbed on the pyrolytic graphite electrode is different from that of leuco-MB in the solution phase. A similar pH dependence of the formal potential to that obtained in this work was reported by Ye and Baldwin for MB adsorbed on a polished graphite electrode surface, though they reported a PK, value of 4.4.29 In our separate experiment, the redox response of adsorbed MB was observed at 100 mV at a pyrolytic graphite electrode/MB solution interface in 0.1 M HzS04 solution at pH 0.4.30This formal potential is the same as the value which is obtained by extrapolation of the plot of the acidic region in Figure 3 to pH 0.4. This fact suggests that one of the two dimethylaminonitrogens of leuco-MB adsorbed on the pyrolytic graphite electrode cannot be protonated even if the pH is as low as 0.4. The formal potential of the adsorbed MB at pH 7 obtained in the present work is about 100 mV more negative than that of MB in the solution and is ca. 90 mV more negative than that of MB adsorbed on a mercury electrode?' The extrapolation of the straight line in the acidic region in Figure 3 to pH 0 gives rise to a formal potential as being 105 mV, which is 140 mV more negative than the formal potential of MB in the solution (27) Nikol'skii,.B. P.; Zak+r'evekii, M. S.; Pal'chevekii. Uch. Zap. Lenmgr. Goa. Unrv., Ser. Khrm. Nauk 1957,15,26; Chem. Abstr. 1968, 52, 9812. (28) Vettar, K. J. Electrochemical Kinetics, Theoretical and Experimental Aspects; Academic Preea: New York, 1967; p 487. (29) Ye, J.-N.; Baldwin, R. P. Annl. Chem. 1988,60,2263. (30) Sagara, T.; Niki, K. Manuscript in preparation. (31) Chen, X.-M.; Zhuang, J.-H.; He, P.-X. J. Electroanul. Chem. Interfacial Electrochem. 1989, 271, 257.

Electroreflectance Study of Methylene Blue

500

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A I nm

Figure 5. ER spectra for MB adsorbed on a pyrolytic graphite electrode surface at pH 4.90 (solid line) and pH 9.53 (broken line). For the comparison of spectral curves, ARIR was presented in arbitrary scale. Ede was set at E O ’ , the frequency was 14.24 Hz, AEacwas 28 mV, the sweep rate was 0.42nms-l, and the time constant was 1 s. The dotted line represents the difference absorption spectrum measured for MB solution (absorption spectrum of the oxidized form is subtracted from that of the reduced form).

phase.29 The negative shift of the formal potential of MB upon adsorption on the graphite electrode would be a common feature of phenothiazin dyes as pointed out by Perss0ns.3~ The number of electrons to complete the reduction of MB, n, is 2 as determined from Figure 3. In contrast, the value of n, obtained from the dc voltammogram is less than 1 a t any pH. It is known that the reduction of MB follows an ECE mechanism (two one-electron-transfer processes with an intervening proton-transfer p r o ~ e s s ) . ~Since ~ - ~ ~the intervening proton-transfer reaction is a very rapid process, one may regard the reduction process as two consecutive one-electron-transfer rea~tions.3393~ In an aqueous solution, n, is 1.63, which was interpreted as a result of the relation between the two formal potentials being Ez” = Elo’ + 51 mV, where Elo’ and E 2 O ’ are the reduction potentials of the first and the second electron-transfer processes, re~pectively.~~ Two sources of the discrepancy between n and n, for MB adsorbed on the graphite electrode can be cited as follows: (i) the potential separation between the first and the second reduction potentials, A E O ’ = E 2 O ‘ - El0’,for the adsorbed MB is more negative than that of MB in the solution phase, and (ii) the formal potential of the adsorbed MB depends on the heterogeneous nature of the adsorption sites on the graphite electrode surface, and the formal potential is distributed in some potential range. Only from the dc voltammetric measurements, one cannot discuss the reaction mechanism in more detail. Then, keeping the above mentioned results in mind, the electrode reaction of MB was further analyzed by using electroreflectance techniques. Electroreflectance Study. ER spectra and ER voltammograms were measured at 16 different pH values. Figure 5 shows ER spectra at the formal potentials at pH (32)Persson, B. Ph.D. Thesis, Lund University, LUNKDL-NKAK1017,1990. (33)Wopschall, R. H.;Shain, I. Anal. Chem. 1967, 39, 1527. (34)ZutiE, V.;SvetliCiC, V.; Clavilier, J.; Chevalet, J. J.ElectroanaL Chem. Interfacial Electrochem. 1987 219, 183. (35)Loviri6,M.; Komorsky-LovriC;4.J.ElectroanaL Chem.Interfacial Electrochem. 1988, 248, 239. (36)Sagara, T.; Niki, K. Unpublished results, 1990.

Langmuir, Vol. 8, No. 3,1992 1021

4.9 (solid line) and at pH 9.5 (broken line). These spectra were measured with a 20-mV sinusoidal-wave potential modulation (peak-to-peak amplitude of 57 mV). It is known that the spectral shape represents the difference spectrum between reduced and oxidized forms of adsorbed dye, unless the Stark effect contributes to the ER s p e ~ t r u m . ~The J ~ spectral shape in Figure 5, however, appears markedly different from the difference absorption spectrum (spectrum of leuco-MB from which that of the oxidized form of MB is subtracted; see dotted line in Figure 5) of MB in the solution phase. The spectra shape rather resembles the red-shifted first differential derivative curve of the absorption spectrum of the oxidized form of MB in the solution phase. In comparison with the first differential derivative of the absorption spectrum of the homogeneous solution of the oxidized form of MB, the ER spectra in Figure 5 are red-shifted by about 35 nm and show significant tailing toward both redder and bluer wavelength regions. The characteristic wavelengths at the positive peak (631 nm on the solid line in Figure 51, the point of zero ER response (695 nm, abbreviated as pze), and the negative peak (733nm) are almost pH independent in the pH range between 4 and 12. The standard deviations are f15 nm at the positive peak, *6 nm at pze, and f 1 2 nm at the negative peak. Since the slit width in these ER measurements is as much as 8 nm, the derivations of these characteristic wavelengths are likely to be originated from a low resolution. The ratio of the peak heights, (AI?/ R)650 nm to (AI?/R)730 nm, is also pH independent. The protonation of the dimethylamino nitrogen at pH < PKa occurs after the two-electron reduction of MB.33*34 Thus, the fact that the ER spectra at pH < pK, is the same as those at pH > pK, suggests that the protonation of the dimethylamino nitrogen at pH < PKa does not give rise to the change in color. When E d c >> Eo’ + 200 mV, the ER spectrum in the presence of the adsorbed MB was the same as that in the absence of the adsorbed MB regardless of pH. Even if the modulation amplitude was increased up to 200 mV in the double-layer potential region, the ER response was always at the background level, and no spectral feature of MB or leuco-MB was observed. These facts mean that the absorption spectrum of the adsorbed MB (oxidized form) is independent of the modulation amplitude of the electrode potential in the double-layer region. Lema and his colleagues recently reported that the ER responses of MB adsorbed at a sulfur-modified gold electrode in the double-layer region display the change in the extinction coefficient of adsorbed MB with the electrode potential due to the dipole-field intera~ti0n.l~ Such a feature was not observed at the graphite electrode used in this work. The contribution of the Stark effect to the light-absorbing property of MB, therefore, is negligible in the doublelayer region. Figure 6 shows ER voltammograms at pH 4.7 and 8.5. The ER voltammetric curve is symmetrical with respect to the peak potential. According to our simulation, the ER voltammetric curve is not symmetrical when the amount of the reduction intermediate cannot be ignored even if the intermediate is colorless.36 That is, the ER voltammograms in Figure 6 indicate that the presence of the reaction intermediate is negligible. It is, therefore, likely that the E 2 O ’ is much more positive than El0’. The peak potentials of real and imaginary parts in ER voltammograms are equal to each other and are identical to the formal potentials obtained from dc cyclic voltammograms. Except for the shift of the formal potential

Sagara et al.

1022 Langmuir, Vol. 8, No. 3, 1992

-1-

600

700 h / nm

-0.4 -0.2 0.0 E VS. E N M C I I V

Figure 6. ER voltammograms for methylene blue adsorbed on a pyrolytic graphite electrode: a, real part of ER response at pH 4.69; b, imaginary part at pH 4.69; c, real part at pH 8.47; d, imaginary part at pH 8.47. The wavelength was 660 nm, the frequency was 14.24 Hz, AEae= 14.1 mV, and the sweep rate was 2 mV s-l.

with pH, the shape of the ER voltammetric curve is almost pH independent. The half-width of the ER voltammetric peak as well as a ratio of peak height of the real part of the ER signal to that of the imaginary part is indicative of redox reaction parameters as demonstrated in our previous paper? if the ER response shows the difference feature (feature 11). To add more details to the reaction mechanism, we should first establish the interpretation of the ER spectra. Interpretation of ER Spectra. As mentioned above, it is unnecessary to take into account the Stark effect in the double-layer potential region. There are, however, still two possible interpretations of the ER spectra in Figure 5.

(I) The surface charge is perturbed as a result of the redox reaction of MB adsorbed on the electrode surface. This perturbation induces the modulation of the surface electric field in response to the electrode potential modulation. Then, the obtained ER spectrum exhibits the Stark effect in the vicinity of the formal potential. If the oxidized form of MB adsorbed on the electrode surface possesses a strong absorption band in the visible region and the reduced form is colorless, as when they are in the solution then the ER spectrum shows a line shape identical to the first differential derivative of the spectrum of the oxidized form. In practice, the ER spectrum in Figure 5 can be seen as the red-shifted first differential derivative curve of the absorption spectrum of the oxidized form of MB. (11)The characteristics of the absorption bands of MB and leuco-MB adsorbed on the electrode surface are completely different from those of the dye in the solution phase because of the strong interaction between the dye and the graphite electrode surface and among the adsorbed dye molecules. The ER spectrum shows a difference absorption spectrum between reduced and oxidized forms. The ER spectrum, therefore, displays the spectrum of the reduced form adsorbed on the electrode surface from which that of the oxidized form is subtracted.9 The change in the strength of the electric field induced by the redox reaction of MB should be a function of the amplitude of the potential modulation, because the change in the surface charge originates from the perturbation of (37) Obata,

H.Bull. Chem. SOC.Jpn. 1961, 34, 1057.

800

-4-

600

700 h / nm

800

Figure 7. ER spectra measured with square-wave potential modulation for MB adsorbed on a pyrolytic graphite electrode in 30 mM phosphate buffer solution, pH 7.0. The positive vertex potential is -40 mV, and the negative vertex potentials are a, -0.40 V; b, -0.30 V; c, -0.27 V; d, -0.25 V; e, -0.23 V; f, -0.21 V. The frequency was 14.26 Hz, the sweep rate was 0.42 nm s-l, and the time constant was 3 a. the field strength. It is, therefore, expected in case I that the characteristic wavelengths of the ER spectra shift successively with the change in Edc. In case 11, the ER spectrum shows the difference absorption spectrum of two discrete oxidation states, and thus the spectral shape as well as the characteristicwavelengths of ER spectra should always be the same regardless of the amplitude of the potential modulation, unless the reduction intermediate contributes to the ER response. To examine the dependence of ER spectra upon the amplitude of the potential modulation, the measurements of ER spectra were further conducted by using a squarewave potential modulation instead of the sinusoidal wave. In the use of sinusoidal-wavemodulation, the ER spectrum does not exactly represent the difference absorption spectrum at the positive and negative maxima of the sinusoidal wave when the absorption band shiftssuccessively with the electrode potential as in case I. The ER spectrum, measured by using the square-wave modulation, is expected to represent the difference absorption spectrum at the two vertex potentials more explicitly. Figure 7 shows the ER spectra with square-wave modulation for MB adsorbed on a pyrolytic graphite electrode in 30 mM phosphate buffer solution, where the formal potential is -225 mV and the amount of adsorbed MB is nearly a monolayer. The positive vertex potential of the square-wave modulation was set to be constant at -40 mV where the MB absorbed on the electrode surface remains in the fully oxidized form, as verified from the dc cyclic voltammogram. The negative vertex potential was set at nine different potentials. All the other conditions for the measurements were the same throughout the measurements. Although it took about 150 min to complete the measurements, the decrease in the voltammetric response during this period was less than 5%. It is very important to point out that the spectral shapes presented in Figure 7 are all the same. In other words, if the scale of the vertical axis is linearly expanded or reduced with an appropriate factor, all of the spectral curves are superimposable. The characteristic wavelengths are 616 f 3 nm at the positive peak, 690 f 1nm at pze, and 727 f 6 nm a t the negative peak. The value of pze is independent of the potential range of modulation, indicative of no wavelength shift of the light-absorption bands of adsorbed dye with the electrode potential. These facts clarify that the ER spectra do not represent the difference absorption spectrum of MB in an oxidation

Langmuir, Vol. 8, No. 3, 1992 1023

Electroreflectance Study of Methylene Blue

( E - E”) /

400

v

600

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A / nm

state between two electrode potentials, but the difference absorption spectrum of two oxidation states. The absorption band of MB does not shift successivelywith the electrode potential. That is, no evidence of the contribution from the dipole-field interaction is seen in Figure 7. It is important to note that the amount of the reduction intermediate and the absorption by the intermediate are negligible, because otherwise pze shifts with the change in the negative vertex potential. The ER voltammograms in cases I and I1 also are expected to be different from each other. The magnitude of the modulation of the surface electric field induced by the redox reaction of the adsorbed dye molecules in case I is approximately proportional to the change in the surface charge and also to the change in the amount of the reduced form of MB, if the thickness of the compact layer is constant. Therefore, in case I, the amplitude of the ER response should be proportional to the product off and dfldE. Here, f is a fraction of the reduced MB in the adsorbed dye layer. The ER voltammetric curve in case I1 has been already calculated as reported in our previous paper.g Figure 8 shows the simulated plots of ARIR calculated for cases I and I1 as a function of ( E - EO’). A simulated curve of the ER voltammetric curve for a reversiblereaction of the adsorbed species (dotted line) is drawn on the basis of the difference feature? which corresponds to case 11. Here, n, = 1 is assumed; that is, f is written as f = [l + exp{F(E- EO’)/RT)l-’

500

(1)

where F is the Faraday constant, R is the gas constant, T is the temperature, and Eo’is the formal potential. The curve off (df/aE) is asymmetrical with respect to the peak potential, which is 18mV more positive than the peak potential of the simulated ER voltammetric curve for case 11. As shown in Figure 6, the peak potentials of the ER voltammograms obtained in our experiments are equal to the formal potentials obtained from dc voltammograms, and their line shapes are symmetrical with respect to the peak potentials. These facta again support the assumption that interpretation I is not the case. If interpretation I1can be adopted, the ER spectral curve should be in accordance with the difference spectra of the specular reflectance recorded at controlled electrode potentials. Specular reflectance spectrum at a controlled potential displays the absorption spectrum of the adsorbed dye under equilibrium plus background reflection. That is, potentiometry in terms of reflectance spectra is permitted. The results may correspond to the ER spectral curve. The specular reflectance spectra were measured at seven different electrode potentials. Since it is quite

difficult to record the background curve (Le., the spetrum in the absence of adsorbed species) with a fixed light path, another way is to present the difference spectrum between a given potential and a potential where MB is in the fully oxidized state. Figure 9 shows the difference specular reflectance spectra at six different potentials. The curves in Figure 9 were obtained by subtracting the specular reflection spectra at six different controlled potentials from the specular reflectance spectrum at -40 mV. Because the absorbance of the adsorbed species is much smaller than unity, the specular reflectance, R,, at a given electrode potential, E,, is written as

Rs(EJ = 1 - cox(1- f(Ec)J- €red@‘,) (2) Here, f is the same notation as before and cox and tred are the apparent absorption coefficients of the adsorbed species, respectively, when adsorbed MB is fully oxidized cf = 0) and when the adsorbed MB is fully reduced cf = 1). Using f(-40 mV) = 0, the difference spectra at a controlled potential of E, is represented as The line shape of the difference specular reflection spectrum (plot of AR, versus A) is identical with that of the ER spectrum of feature II.9 As shown in Figure 9, the wavelength at which the reflectance difference is zero is 699 nm, regardless of E,. Although there are slight differences in the characteristic wavelengths in comparison with those of the ER spectra (Figure 71, the spectral shape shown in Figure 9 is the same as that shown in Figure 7 when E , I-250 mV. There is another absorption band at 665 nm. This band would appear due to a trace amount of MB in the solution phase as a result of the desorption of MB from the electrode surface, since this band became more intense with time. In the case of the ER spectrum, the light absorption due to the species in the solution phase is negligible unless it reacts in response to the electrode potential modulation, On the other hand, in the case of the specular reflectance measurement at a given potential, all the light-absorbing speciespresent in the light path contribute to the spectrum. Figure 10 shows the plot of the value of AR,(E,) at 737 nm given in Figure 9 as a function of E,. The solid line is the nonlinear least-squares fitted line to an equation derived from eq 3

AR, = Ac

1 +c 1 + exp[n,F(E, - Eo’)/RT]

(4)

where Ac = €red - cox and c is a constant introduced in order to compensate for the change in reflectance from the

Sagara et al.

1024 Langmuir, Vol. 8, No. 3, 1992

of MB in the solution phase (dotted line in Figure 5 ) very closely. This fact implies that the light absorption properties of both oxidized and reduced forms of MB adsorbed on the MFE surface are the same as those in the solution, in contrast to those on the graphite electrode. It is quite likely that the strong interaction between MB and the ab plane of the graphite electrode would be due to the surface electronic structure of the graphite, at whose 0.0 -04 -0.3 -0.2 -0.1 00 surface dangling ?r orbitals are available.40 Electrode Reaction Parameters of MB Adsorbed E v s E“/V on a Pyrolytic Graphite Electrode. Because it is Figure 10. AR,at 737 nm (see Figure 9) as a function of the confirmed that the ER response shows feature 11,we can electrode potential. The solid line is the least-squares fitted line now discuss the reaction parameters obtained from the to eq 4; for details, see text. The vertical axis is in the unit of ER voltammograms on the basis of the equations derived percent R. in our previous paperqg Although the pH dependence of the formal potential electrode with electrode potential. The least-squares (Figure 3) elucidated that adsorbed MB on the pyrolytic fitting calculation gives rise to the parameters Eo‘ = -222 graphite electrode takes place via a two-electron reaction, f 2 mV, n, = 0.96 f 0.2, Ac X lo2 = -0.42 f 0.09, and c the apparent number of electrons, na, determined from X lo2 = 0.07 f 0.01. The value of Eo’ is the same as the the half-width of the ER voltammogram ranged from 0.7 formal potential obtained from both dc and ER voltamto 1.2. It is important to recall that the cyclic dc voltammograms. mogram leads to a value of na less than 1. The value of The good agreement between the plot of Figure 10 and n,obtained from the plot in Figure 10also is approximately eq 4 is evidence that the difference specular reflectance unity. spectra can be expressed by eq 3. Furthermore, the results The surface of the pyrolytic graphite electrode used in in Figures 9 and 10 in comparison to Figure 7 allow us to this work is not highly oriented as was suggested from the conclude that the ER spectra at the graphite/MBad, double-layer capacitance, which is ca. 70 pF*cm-2. This interface show a differencefeature. In turn, this fact leads value is much larger than the reported value at the ab to an important conclusion that leuco-MB adsorbed on plane of the highly oriented pyrolytic graphite electrode the pyrolytic graphite electrode is not colorless. The (3 pFq~m-~).~O The surface of the pyrolytic graphite positive and negative bands in both ER and difference electrode used in this work is not a pure ab plane but specular reflectance spectra are responsible, respectively, inhomogeneous. That is, it is likely that the adsorption to the absorption bands of the reduced and oxidized forms state of MB on the graphite electrode surface is inhomoof MB adsorbed on the pyrolyticgraphite electrode surface. geneous. Thus, leuco-MB adsorbed on the graphite electrode We hereby introduce two approximations: (i) MB possesses an absorption band at around 620-630 nm. The molecules possess site-specificformal potentials, E’, and oxidized form possesses a band at around 730 nm. The the number of the MB molecules possessing E’ can be isosbestic point is at 695-700 nm. represented by a Gaussian distribution, and (ii) the The absorption spectrum of the oxidized form of MB electron-transfer process at any site can be regarded as a in a dilute aqueous solution is the sum of two bands, a one-step two-electron-transfer process (this is true when 665-nm peak for the monomer and a 613-nm peak for the E2” is much more positive than El0’). dimer. In contrast, the reduced form, leuco-MB, is Then, the value of G ( E ) ,the fraction of the reduced it shows ayellowthough colorlessin an aqueous form in the adsorbed dye layer on the inhomogeneous orange color in aprotic solvents such as dimethyl sulfoxsurface, at a given electrode potential, E, is written as ide.33 The red shift by 70 nm of the absorption band of the G ( E )= oxidized form of MB from that of the MB in the solution N exp(-1/2z2)[1 + e x p ( ~-(E~~ / R T ) I -d’ ~ (5) ’ phase upon the adsorption on the graphite electrode surface could be due to the formation of a J-aggregatedwith like layer of MB where the transition moment of the MB z = (E’ - Eotmem)/u molecule aligns in parallel to the surface so that a headwhere N is a normalization factor to make G(E=-=) = 1, to-tail configurationamong the MB molecules is created.38 The transition moment of MB lies in the ring ~ l a n e . 3 ~ u is the standard deviation in the distribution of the sitespecific formal potential, and EO’,,, is the mean formal Thus, it is likely that MB molecules lie flat on the ab plane potential among the site-specificformal potentials. When of a graphite surface with a T-T interaction. One could u = 37.5 mV, the plot of G(E)versus E and the plot of f ( E ) speculate that such a strong interaction would bring about with n. = 1versus E are superimposable. It is, therefore, the absorption band of leuco-MB in the visible region. It very likely that the value of na being unity or less than is reasonable that MB molecules lie flat, since otherwise unity has arisen from the inhomogeneous property of the the molecular orientation is preferable to result in the graphite electrode surface used. These results are also Stark effect.I4 consistent with the fact that the presence of the interIn order to compare the ER spectrum at the graphite mediate is negligible. electrode to that at a metal electrode, a preliminary test The electrode-transfer rate constants, k,, were deterof the ER measurement was undertaken at a mercury film mined from the ratio of the peak heights in the ER volelectrode (MFE)/MB solution interface. In the vicinity t a ” ~ g r a m . ~The values were scattered in the range of the formal potential of the adsorbed MB on the MFE, between 60 and 400 s-l, but pH dependence was not the ER spectral curve resembles the difference spectrum observed. It is known that the value of k, at a mercury/

J-1

(38) Kemnitz, K.; Tamai,N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986,90, 5094. (39) Bergmann, K.; O’Konski, C. T. J. Phys. Chem. 1963, 67, 2169.

(40) MaCreery, R. L. In Electroanalytical Chemistry;Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17.

Electroreflectance Study of Methylene Blue MB,d, interfere is very rapid. For example, Chen and his colleagues reported a value of 5 X lo5 s-l measured by using a cathodic-stripping rapid-scanning dc voltammogram.31 On the pyrolytic graphite electrode, the strong interaction between the graphite surface and adsorbed MB may decelerate the electron-transfer rate. Last, it is noteworthy that both ER spectra and ER voltammograms show no considerable influence by the counteranion in the solution and the ionic strength. This fact shows sharp contrast to the mercury/MB solution interface, where properties of the layer of leuco-MB accumulated on the mercury surface are governed by the anions to a great extent.41 In the adsorption state of MB on the pyrolytic graphite electrode used, the interaction between MB and the anion would be limited.

Concluding Remarks Although the ER spectrum in Figure 5 is not completely identical with the first differential derivative of the absorption spectrum of the homogeneous solution of the oxidized form of MB, the close resemblance between the (41)SvetliCiC, V.; TomaiC, J.; ZutiC, V.; Chevalet, J. J. Electroanal. Chem. Interfacial Electrochem. 1983, 146, 71.

Langmuir, Vol. 8, No. 3, 1992 1025 two is striking. The experimental results in the present paper can, at least, deny the contribution from the Stark effect. However, the reason why leuco-MB, which is colorless in a homogeneous aqueous solution, has a strong absorption band at around 620 nm is still open for further study. To confirm our interpretation, theoretical consideration in the light of reflection theory as well as ER studies at various electrodes by using polarized light are now underway.

Acknowledgment. We are grateful to the Ministry of Education, Science and Culture of Japan for the financial support of a Grant-in-Aid for Promoted ScientificResearch (A) (No. 03855168 to T.S.) and to the Shimadzu Science Foundation for the financial support to T.S. We thank Dr. T. Takada (Shimadzu Co.) for his kind discussion with us and making it possible to use the specular reflectance attachment. We also express appreciation to Mr. H. Murakami for his technical assistance. Registry No. MB, 61-73-4;leuco-MB, 613-11-6;leuco-MBH, 138857-11-1; NaC104, 7601-89-0; Na2B407, 1330-43-4; NaOH, 1310-73-2;H3P04,7664-38-2; HzS04, 7664-93-9; graphite, 778242-5.