Langmuir 1993,9, 831-838
Surface Processes and Adsorption States of Methylene Blue at Graphite Electrode Surfaces in an Acidic Medium: An Electroreflectranca Study Takamasa Sagara* and Katsumi Niki Department of Physical Chemistry, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Received May 26, 1992. In Final Form: December I , 1992
The adsorption states of methylene blue (MB) at three different graphite electrode surfaces in MB solution (0.1 M HzSO4) were characterized by using electroreflectance spectroscopy (ER). Two different adsorption states were detected at a basal plane pyrolytic graphite electrode, namely, state I with a formal potential at 107 mV (vs. Ag/AgCl electrode in saturated KCl), and state I1 at 215 mV. These formal potentials were more negative than the formal potential of MB in the solution phase. In both states, positive EX spectral bands were observed in the visible region, whereas the reduced form of MB in solution phase is colorless. At an edge plane pyrolytic graphite electrode, only state I was detected. At an ab plane of highly oriented pyrolytic graphite (HOPG)electrode, a new adsorption state, which is different from states I or 11, was observed at 190 mV. State I was assigned to a specific adsorption state of MB at the edge site of the graphite. MB adsorbed on graphite electrodes exhibited various ERspedral bands depending on the properties of the graphite surfaces. An ER signal due to the adsorption-desorption process of MB could be monitored at an HOPG electrode.
Introduction The usefulness of the electroreflectance (ER) method to analyze electrode reactions of molecules adsorbed on electrode surfaces has been extensively demonstrated. In particular, the characterization of the species adsorbed on the electrode surfaces has been an attractive target.1-9 Although detailed information regarding the molecular structure of the adsorbed species cannot always be obtained using the ER method, establishment of the interpretation of the ER spectrum provides deep inslight into the state of the adsorbed s p e c i e ~ . l ~ ~ * ~ Usually, ER measurements of adsorbed species are conducted at a highly reflective electrode surface in the absence of species in the solution phase. On the other hand, ER studies on dyes reversibly adsorbed on electrode surfaces from the solution phases have been rather limited, since the electrochemical perturbation of the light absorption of the dye solution interferes with the measurement of the reflectance spectrum of the adsorbed dyes.lOJ1 Also, the use of a poorly reflective surface has not been attractive. The aim of this work is to characterize the adsorption states of methylene blue (MB) on various graphite electrode surfaces in a MB solution. In our previous paper,
* To whom correspondenceshould be addressed.
(1)Plieth, W. UV-Visible Reflectance Spectroscopy in Electrochemistry in Spectroscopic and Diffraction Techniques in Interfacial Electrochemistry; GutiBrrez, C., Melendres, C. Eds.; Kluwer Academic Publishers: The Netherlands, 1990; p 223. (2) Memming, R. Faraday Discuss. Chem. SOC.1974,58,261. (3) Hinnen, C.; Parsons, R.;Niki, K. J. Electroanal. Chem. Interfacial Electrochem. 1983,147,329. (4) Bedioui, F.;Devynck, J.; Hinnen, C.;Rouseau,A.; Bied-Charreton, C.; Gaudemer, A. J. Electrochem. SOC.1985, 132, 2120. (5) Lezna,R.O.;deTacconi,N.R.;Hahn,F.;Arvia,A. J.J.Electroanal. Chem. Interfacial Electrochem. 1991,306, 259. ( 6 ) Sagara, T.; Igarashi, S.;Sato, H.; Niki, K. Langmuir 1991,7,1005. (7) Sagara, T.;Murakami, H.; Igarashi, S.;Sato,H.;Niki, K. Langmuir 1991, 7, 3190. (8) Wang, H. X.; Sagara,T.; Sato, H.; Niki, K. J. Electroanal. Chem. Interfacial Electrochem. 1992, 331,925. (9) Sagara, T.; Iizuka, J.; Niki, K. Langmuir 1992,8, 1018. (10) Plieth,W. J.; Gruschinske,P. Ber. Bunsen-Ges.Phys. Chem. 1972, 76, 485. (11) Gorodyskii,A. V.; Kolbasov, G. Ya.;Karpov, I. I.; Taranenko, N. I. Ukr. Khim. Zh. 1985,51,488.
we reported that the ER spectrum of MB adsorbed on an ab plane pyrolytic graphite electrode in a dye-free solution is greatly different from the spectrum predicted from the absorption spectrum of MB s o l ~ t i o n .It~ is well-known that at a metal electrode in a MB solution, pre- and postwaves due to the adsorption of MB on an electrode surface are ~ b s e r v e d . l ~In - ~the ~ potential range at which the electrode reaction of MB in the solution phase (MB,d takesplace, MB remains adsorbed at the graphite electrode surfaces. In addition, MB is a typical phenothiazin dye which exhibits a high electrocatalytic activity on various electrode surfaces. For example, some of the phenothiazin dyes adsorbed on a graphite electrode can markedly decrease the overpotential necessary for the oxidation of nicotinamide adenine dinucleotide (NADH).lSJ6 Thus, it is very important to characterize the state of the monolayer-adsorbed MB on the electrode surface for a deeper understanding of the electron-transfer reaction mechanism of MB. In the present study, the adsorption states of MB at graphite electrodes in MB solutions (0.1 M HzS04) were studied in detail by ER techniques. Attentions were directed to the detection of the adsorption state, whose voltammetric response is merged into the response of MBBoh,as well as to the evaluations of both the formal potentials and the ER spectral characteristics of the adsorbed dyes. Since it is expected that the surface properties of graphite electrodes govern the adsorption states, three different graphite electrodes were used, namely, a pyrolytic graphite exposing an ab plane (BPG), a pyrolytic graphite exposing an edge plane (EPG), and a highly oriented pyrolytic graphite (HOPG). The surface of the HOPG is a nearly prefect ab plane with little defects, that of the EPG is perpendicular to the ab plane and is (12) Wopschall, R. H.; Shain, I. Anal. Chem. 1967,39,1527. (13) SvetliEiE, V.; TomaiC, J.; ZutiE, V.; Chevalet, J. J. Electroanul. Chem. Interfacial Electrochem. 1983,146, 71. (14) SvetliEiE, V.; btiC, V.; Clavilier, J.; Chevalet, J. J. Electroanul. Chem. Interfacial Electrochem. 1985, 195, 307. (15) Persson, B. J. Electroanal. Chem. Interfacial Electrochem. 1990, 287, 61. (16) Persson, B.; Gorton, L. J . Electroanal. Chem. Interfacial Electrochem. 1990,292, 115.
0743-7463/93/2409-0831$04.00/00 1993 American Chemical Society
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832 Langmuir, Vol. 9,No. 3,1993 covered with various functional surface groups, and that of BPG consists of ab plane parta fragmented by edge plane parta.17 Experimental Section Three different graphite electrodes were prepared. A BPG electrode was prepared from a pyrolytic graphite (Union Carbide Co.) as follows. The graphite mounted in an epoxy cement resin (Torr Seal, Barian Co.) was polished in parallel to an ab plane using 600-grit Sic emery paper. Then, the electrode surface was peeled-off, at least twice, using Scotch tape so as to expose a fresh ab plane. The pyrolytic graphite (Union Carbide) was cut perpendicularly to an a b plane to prepare an EPG electrode. It was polished with 3000-grit emery paper, washed with a water jet, and then sonicated. A fresh ab plane surface of a highly oriented pyrolytic graphite (HOPG, ZYA grade, Union Carbide) was prepared by peelingoff the surface. A Kel-Felectrode holder with a silicone O-ring was used to sheathe the HOPG electrode and to expose an electrode area of 0.385 cm2. The pretreatment of these electrodes was conducted just before immersing the electrodes in the electrolyte solutions. Methylene blue (NIB, 3,7-bis(dimethylamino)phenothiazin5-ium chloride), purchased from Wako Chemcial Co., was used without further purification. Water was distilled and purified through a Mill-& filter (Millipore Co.). The resistivity of the purified water was more than 18 Mi2 cm. All other chemicals were of reagent grade and were used as supplied. In all experiments,the supporting electrolyte was 0.1 M HzS04 aqueous solution. A Ag/AgCl electrode in saturated KC1 solution and a platinum wire were used, respectively, as a reference and counter electrode. The electrode potentials given in this paper were measured with respect to this reference electrode. All of the measurements were carried out at 23 A 3 "C in an argon atmosphere. The instrumentation for electroreflectance (ER)measurements was the same as described in our previous [email protected]
In order to minimize the absorption of light by the bulk solution of MB without increasing the solution resistance, the distance between the electrode surface and inner wall of the quartz optical window of the cell was set to be about 1-2 mm. The average level of the signal from a photomultiplier was kept constant regardless of the wavelength by using feedback control of the high voltage applied to the photomultiplier.lS The ER response (ARIR),the ac component of the reflectance divided by the dc component of the reflectance, was measured by modulating the electrode potential by sinusoidal wave.
Results and Discussion Properties of Three Graphite Electrodes. It is worthwhile to first review the properties of the three types of graphite electrodes. Double layer capacitance, c d , was obtained from an ac impedance measured in the base solution (0.1 M H2S04). c d was equated to the capacitance in an equivalent circuit in which a resistance and a capacitance are connected in series. In the calculationof c d , geometricalelectrode areas were used. Values obtained for c d in the potential range between 0.6 and -0.1 V were 15-20 pF cm-2 for a BPG electrode, 110-220pF cm-2for an EPG electrode,and 2.0for an HOPG electrode. 2.5 pF It is certain that the HOPG electrode exposes a welldefined ab plane, as judged from the value of c d in comparison to the reported values.17 The surface of the HOPG electrode is highly reflective. The surface of the BPG electrode does not consist of a well-defined ab plane, as expected from an X-ray diffraction analysis. The large value of c d of the BPG electrode is due not only to the large roughness factor but also to inhomogeneity in ~~
(17)McCreery,R. L. CarbonElectrodes: Structural Effecta on Electron Transfer Kinetica inEZectroanuZyticalChemistry;Bard, A. J., Ed.;Marcel Dekker: New York, 1991; Vol. 17, p 221. (18) Sagara, T.;Sato, H.; Niki, K. Bunseki Kagaku 1991,40,641.
"I 0 400
h I nm Figure 1. Absorption spectrum of a solution of methylene blue in oxidized form (12.8 pM) in 0.1 M HzS04.
character as discussed e l s e ~ h e r e . The ~ value of C d is reproducible in several BPG electrodes. The surface of the BPG electrode is less reflective than the HOPG electrode. The surface of the EPG electrode is the least reflective. When a newly prepared EPG electrode is subjected to the potential scan in the negative direction from the zero-current potential, an oxygen reduction wave is observed. To remove the oxygen contamination of the surface, several cycles of sweeping the electrode potential to -0.6 V are needed. A weak redox response of surface groups is also detected at the EPG electrode at 100 mV in the base solution,but it is not so significant as to disturb the measurements of the MB redox response even in a dilute MB solution. Before the EPG electrodeis immersed in a dye solution, an oxidation-reduction cycle treatment was conducted in the base solution to obtain a steadystate background. For all of the three types of graphite electrodes, the background ER spectra in the base solution were of zerolevel and exhibited no characteristic spectral structure. Thus, in the preeent ER measurements in the dye solutions, background subtraction was not used. Absorption Spectrum of M B in 0.1 M H2S04. In the present experiments, a solution of oxidized form of MB in 0.1 M H&04 was used. The concentration of MB, CME, ranged from 2 to 40 pM. Figure 1 shows a typical absorption spectrum of MB solution. The solution of the oxidized form of MB (MB,) shows three absorption bands: 665-nm peak of the monomer, 613-nm peak of the dimer, and a band at around 741 nm, which is probably due to aggregated MB,, molecules. The 741-nm band is not observed in the neutral pH range. The reduced form of MB (MBrd) is known as leuco-methylene blue, and its aqueous solution is colorless. Voltammetric Study at the BPG Electrode. Figure 2 shows a dc cyclic voltammogram at a BPG electrode in 32 pM MB, solution in 0.1 M HzS04. Two redox responses are observed. Keeping in mind that the voltammetric peak height is proportional to sweep rate, u, for the redox reaction of the adsorbed species but is proportional to the square root of u for the diffusion-controlledredox reaction of the solution species,19Figure 2 was analyzed. The height of the peak at 105 mV, the peak I, is proportional to u up to 200 mV 8-1, and the peak separation is almost zero. The electrode reaction at the peak I is assigned to a quasi-reversible electron-transfer reaction of MB adsorbed on the electrode surface. The charge of the peak I corresponds to approximately one or two monolayers of MB provided that MB molecules adsorb on the electrode surface in a flat orientation. (19) Bard, A.J.;Faulkner,L. R. ElectrochemicalMethods;John Wiley & Sons: New York, 1980; Chapters 6 and 12.
Adsorption States of Methylene Blue
Langmuir, Vol. 9, No. 3, 1993 833
Figure 2. dc cyclic voltammogram at a BPG electrode (geo-
metrical electrode area, 0.40 cm2)in 32 pM MB,. solution in 0.1 M HzS04: initial potential, 0.6 V;vertexpotential,4.3 V. Sweep rates in mV s-1 were as follows: a, 2; b, 5; c, 10;d, 20; e, 50; f, 100; g, 200.
Figure 3. ac voltammogram at a BPG electrode in 32 pM MB,,
The height of the peak at the positive potentials, peak
11,is proportional to the square root of u when u I10 mV 8-l but is proportional to u when u 1 50 mV s-I. The peak separation is 30-40 mV. At a slow sweep rate, the average of the anodic and cathodic peak potentials is 245 mV, which is equal to the formal potential of MB in 0.1 M HzS04 solution.a22 These facta suggest that the electrode reaction at the peak I1 involves a reaction of MB in the solution phase (MB,,h) but it is not a pure diffusioncontrolled process. Note that the cathodic peak height of the peak I1 is greater than the anodic peak height. Later in the concluding remarks, we will discuss the reason for this phenomenon. Figure 3 shows ac voltammogram at a BPG electrode obtained in the same conditions as in Figure 2. Peaks I and I1 in Figure 3 correspond to those in Figure 2. The peak pseudocapacitance, C,, of peak I is nearly constant when frequency, f , is smaller than 15 Hz. On the other hand, the value of C, of peak I1 increases steeply with decreasing f. When f is increased to 640 Hz, the value of C, of peak I1 is 1 order of magnitude greater than the Warburg (diffusion-controlled)~ a p a c i t a n c ecalculated ~~ for the reversible electrode reaction of MB,,h. Therefore, the electrode reaction at peak I1 involves not only the redox reaction of MB,,h but also that of adsorbed MB. The concentration dependence of both dc and ac voltammograms was also examined at a BPG electrode. When CMB = 2.2 pM, dc voltammetric peak currents of both peaks I and I1 were proportional to u in the range of 1-200 mV s-l. When CMB = 10pM, the linear relationship between the peak current at peak I and u was still held, but that at peak I1 deviated at u < 20 mV s-l. When CMB = 20 pM, the proportionality between peak current at peak I1 and u was seen only at u > 50 mV s-I. These results of the voltammetric measurements lead us to conclude that the electrode reaction at peak I (20)Vetter, K.J. Electrochemical Kinetics, Theoretical and Erperimental Aspects; Academic Press: New York, 1967;p 487. (21)Ye, J.-N.; Baldwin, R. P. Anal. Chem. 1988,60, 2263. (22)Lu, Z.-L.;Dong, S.-J.J. Chem. SOC.,Faraday Tram 1 1988,84, 2979. (23)The capacitance due to the diffusion process is given by l/uw1/2 where u is aconstant proportional to the reciprocal of diffusion coefficient. For details, see Chapter 9 of ref 19.
solution in 0.1 M H2S04: initial potential, 0.6 V; end potential, 4 . 3 V, sweep rate, 2 mV s-l; ac amplitude, 5 mV. Pseudocapacitance in a series equivalent circuit is shown.
h I nm
Figure 4. ERspectra for a BPG electrode in 32 rM MB, solution
in 0.1 M HzSOr: modulation, 10 mV and 14.3 Hz with a sine wave. The real parts at 105 mV (A), 280 mV (B), and 250 mV (C)are shown in an arbitrary scale to make the comparison of spectral shape easier. The actual signal intensities at the positive peaks are as follows: A, 2.2 X l(r; B, 3.9 X 1od;C, 1.4 X lo-‘.
corresponds to the redox process of adsorbed MB and that the electrode reaction at peak I1 involves the redox processes of both adsorbed MB and MB,,h. When the height of peak I1 is proportional to u1I2 in Figure 2 (IJ I 5 mV s-9, the average of the anodic and cathodic peak potentials of peak I1 is 245 mV, which is the same as the formal potential of MB,,h in 0.1 M H2S04.21922 When u is increased from 10 to 200 mV s-l, the average of the anodic and cathodic peak potentials of the peak I1 shiftstoward the negative direction. Therefore,the formal potential of adsorbed MB at the peak I1 was estimated to be slightly more negative than that of MB,h,. ER Study at the BPG Electrode. Figure 4 shows ER spectra at a BPG electrode in 32 pM MB, solution. Curve A is the ER spectrum measured at &e (dc potential for ER spectral measurement) = 105 mV with
834 Langmuir, Vol. 9, No. 3, 1993
Figure 5. ER voltammograms at a BPG electrode in 32 pM MB,, solution in 0.1 M HzS04: sweep rate, 2 mV s-l; initial potential, 0.6 V; end potential, -0.3 V; frequency, 14 Hz; modulation amplitude, 10 mV; wavelength, 700 nm (left, a) and 609 nm (right, b). an effective amplitude of potential-modulation of 10 mV.
Under these measurement conditions, the redox response rat the peak I1 does not effect the ER response. The light absorption of MBm1n does not influence the ER response, either. Therefore, curve A represents the ER spectrum of adsorbed MB with formal potential around 105 mV. Curve B is measured at 280 mV, which is 35 mV more positive than the formal potential of MBmh. A pronounced negative peak at 663 nm is observed on curve B. This wavelength corresponds to the absorption maximum of MB, monomer in the solution phase (see Figure 1). The ER spectrum due to the redox reaction of the solution species represents the difference absorption spectrum between the reduced and oxidized forms. Since MB,,d in the solution phase is colorless, the ER spectrum due to the redox reaction of MB,,h is the same as the absorption spectrum of MB,, in the solution phase multiplied by -1. Therefore, curve B reveals the contribution of the redox reaction of MB,h to the ER response a t 280mV. However, the positive peak at 570 nm is not the ER response of MB,,h but of adsorbed MB on the electrode surface, because positive ER response cannot arise from MB,,h. Curve B is the sum of the redox responses of MB adsorbed on the electrode surface and of the MBs0h. The difference in the wavelength of the positive peak between curves A and B suggests that the adsorption state of MB observed at 280 mV (curve B) is different from that observed a t 105 mV (curve A). Curve C represents the ER spectrum at E d c = 250 mV. The heights of the positive and negative peaks are nearly the same, indicative of the domination of the ER response of the adsorbed MB over the ER response of MB,,h. The positive peak wavelength is the same as that of curve B. Curve C is identical in shape to the spectrum obtained by parallel shifting of curve A toward the blue direction by about 80 nm. Figure 5 shows ER voltammograms measured under the same conditions as used to obtain Figure 4. The data shown in Figure 5a were obtained a t 700 nm. This wavelength is very near to the wavelength of zero ER response, pze, of curve A in Figure 4 (703 nm). The ER response a t peak 11,therefore, can be selectively measured at 700 nm. The small ER voltammetric response, whose real part is positive, is observed between 0 and 150 mV. This ER response is due to the electrode reaction at the peak I, since the wavelength for the measurement (700 nm) is a little bit shorter than the pze of curve A in Figure 4 (703 nm). In the potential region around peak 11,a peak at 245 mV and a shoulder at 210 mVare observed in Figure 5a.
Sagara and Niki
Figure 5b was recorded at 609 nm, the wavelength of which is the pze of the ER spectrum at 280 mV (curve B in Figure 4). Two peaks are clearly observed at 107 and 215 mV. The latter peak potential corresponds to that of the shoulder in Figure 5a. The ER response at 245 mV is not detected in Figure 5b, in contrast to Figure 5a. This fact implies that the peak at 245 mV in Figure 5a corresponds to the ER response of MBsoln, since the ER response of MB,,l, is expected to be very small a t 609 lam from Figure 1. In turn, the peaks at 107 and 215 mV in Figure 5b correspond to the formal potentials of the adsorbed MB. These ER voltammetric results reveal that the electrode reaction at the peak I1 involves two redox couples: the adsorbed MB with the formal potential at 215 mV and MB,,h with the formal potential at 245 mV. These ER voltammetric observations agree with the dc and ac voltammetric data in terms of the potentials of maximal responses. It is important to note that the ER spectrum of the adsorbed MB with the formal potential at 215 mV is clearly different from that of the adsorbed MB with the formal potential at 107 mV. As mentioned earlier, when CMB is 2.2 pM, the contribution of the reaction of MB,,h to the redox response at peak I1 is negligibly small. The shape of ER spectrum measured at 250 mV in the 2.2 pM MB,. solution is identical with that of curve C in Figure 4. This fact provides evidence that curve C in Figure 4 represents the ERspectrum of the adsorbed MB with the formal potential at 215 mV. We conclude that there are two types of the adsorption states of MB on the BPG electrode in MB solution (0.1 M HzS04). Namely, curve A in Figure 4 represents the ERspectrum of the adsorbed MB with the formal potential a t 107 mV (ER state I) and curve C represents the ER spectrum of the adsorbed MB with the formal potential at 215 mV (ER state 11). In our previous work: we adsorbed MB on the BPG electrode through a film-transfer procedure and measured the ER spectrum in a dye-free solution at pH > 4. The adsorption state of MB found in the previous work is identical with ER state I because (i) the ER spectral shapes are identical and (ii) the formal potential of ER state I falls on the extrapolated line of the formal potential-pH plot obtained in the previous work. Voltammetric Study at the EPG Electrode. Figure 6 shows dc and ac voltammograms of an EPG electrode in 17 pM MB, solution. Two redox responses are observed as in the case of the BPG electrode, though the relative peak height of peak I1 (230 mV) to peak I (105 mV) is much smaller than that a t the BPG electrode. The value of C, at peak I1 decreases steeply with increase of the frequency and completely disappears a t f = 640 Hz, in contrast to the results at the BPG electrode. This fact implies that the electrode reaction at peak I1 on the EPG electrode does not involve reaction of the adsorbed MB. Note that the cathodic peak height of peak I1 is greater than the anodic peak height. We will discuss the reason for this difference in the concluding remarks. ER Study at the EPQ Electrode. The EPG electrode is poorly reflective, but ER measurement was etill possible with a sensitivity of ARIR = 3 X 10-5. Figure 7 shows ER spectra at E&= 105 mV (solid line) and 190 mV (broken line). Since the imaginary part of the ER response at the EPG electrode is more intense than the real part, the imaginary part is shown. When Figure 7 is compared to the ER spectra in Figure 4, the sign of the ER response should be changed. A t 105 mV,
Adsorption States of Methylene Blue
Langmuir, Vol. 9, No. 3, 1993 835 30
EwAS, / V
Figure 6. dc and ac voltammograms at an EPG electrode (geometricalelectrode area, 0.36cm2)in 18pM MB,, solution in 0.1 M HzS04. As for dc voltammogram, sweep rates (in mV s-l) are as follows: a, 5; b, 10; c, 20;d, 50; e, 100, f, 200. As for ac voltammogram, modulation amplitude is 5 mV. ac voltammograms are presented in an arbitrary scaleand are shifted vertically
for the sake of comparison of the curve shape.
E vs. EN-
0.5 I V
Figure 8. dc and ac voltammograms at a HOPG electrode in 17 WMMB,, solution in 0.1 M HzS04solution. As for dc voltammogram, sweep rates (in mV s-l) are as follows: a, 20; b, 50; c, 100. As for the ac voltammogram modulation amplitude is 5 mV, and frequencies (in Hz) are as follows: d, 4.0;e, 14.3;f, 64; g, 640.
Figure 7. ER spectrum (imaginary part) for an EPG electrode in 18 pM MB solution in 0.1M HzSO4: modulation, 26 mV and 14.5Hz; A (solid line), E d c = 105 mV; B (broken line), E d c = 190 mV. the positive peak wavelength and the pze are 673 and 711 nm, respectively. The shape of the ER spectrum at 190 mV is nearly the same as that at 105 mV. The spectral shape exhibits the same feature as ER state I at the BPG electrode, although the pze is at ca. 10 nm longer wavelength than curve A in Figure 4. At Edc = 190 mV, no features of the ER state I1 or MB,,b are observed, but the same ER spectral shape as the solid line is seen. ERvoltammetriccurves were measured at 673nm,which is the peak wavelength of the solid line in Figure 7, and at 711 nm, which corresponds to the pze. The ER voltammogram at 673 nm showed only one peak at 100 mV, and the peak shape was symmetrical with respect to the peak potential. At 711 nm, the ER voltammetric response was not more than the background level in the potential range from 400 to -200 mV. These results reveal that the same adsorption state of MB as ER state I at the BPG electrode exists also at an EPG electrode, but the ER state I1 does not. The dc and ac voltammetric responses of peak I1at the EPG electrode, therefore, correspand to the electron-transfer reaction of MBsoln. Voltammetric Study at the HOPG Electrode. Figure 8 shows dc and ac voltammograms of an HOPG electrode in 17 pM MB,. solution. In both voltammograms, only one redox response appears. Both anodic and cathodic voltammetric peak heights are proportional to uO.65 in the range of 2-200 mV s-l.
Regardless of u, the cathodic peak height is 1.9 times as much as the anodic peak height. We will discussthe reason for this difference in the concluding remarks. The average of the anodic and cathodic peak potentials shifts toward the positive direction with decrease in u, i.e. 231 mV at 200 mV s-1and 244 mV at 2 mV s-l. The average of the anodic and cathodic peak potentials at very low sweep rate is equal to the formal potential of MBsoln. These results suggest that the electrode reaction involves two different redox processes: one is of the adsorbed MB and the other is of MB,b. In the ac voltammogram, the peak potential of the pseudocapacitanceshiftatoward the positive direction with decrease in the frequency, i.e. 177 mV at 640 Hz and 222 mV at 4 Hz. This fact implies that the formal potential of the adsorbed MB is more negative than that of MB,h. ER Study at the HOPG Electrode. Three different ER spectroscopic features are observed at the HOPG electrode depending on Edc and the frequency of the potential modulation. Figure 9 shows three typical ER spectra. Curve A was recorded with 173-Hzpotential modulation & E d c = 160mV which is near to the peak potential of the ac voltammogram at higher frequencies. At 173 Hz,the contribution of the redox reaction of MB,b to the ER response is negligibly small, because the redox reaction of MB,,h cannot follow the potential modulation at 173 Hz. The spectral shape is similar to the ER spectrum of ER state I at the BPG electrode (Figure 4A). Since the characteristic wavelengths,615-nm positive peak, 696-nm pze, and 725-nm negative peak, are independent Of Edcin the range between 120 and 195 mV, it is unnecessary to take into consideration the absorption-band-shift due to the field-dipole intera~tion.~ Curve B in Figure 9 was recorded with 8.0-Hz potential modulation at Edc= 265 mV, which is 20 mV more positive than the formal potential of MB,h. The spectral feature is similar to that of curve B in Figure 4. A negative peak is at 665 nm, which corresponds to the absorption maximum of MB,, monomer in the solution phase (Figure
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836 Langmuir, Vol. 9, No. 3, 1993
A / nm A / nm A / nm Figure 9. ER spectra for a HOPG electrode in 17 pM MB,, solution in 0.1 M HzS04. A Edc= 160 mV; frequency, 173Hz; modulation amplitude, 20 mV. B: Edc= 265 mV; frequency, 8.0 Hz; modulation amplitude, 20 mV. c: Edc= 550 mV, frequency, 8.0 Hz; modulation amplitude, 50 mV.
1). When the sign-inverted absorption spectrum of MB,, in the solution phase is subtracted from curve B so that the value at 696 nm becomes zero, the resulting spectrum mimics curve A in Figure 9.24This fact reveals that curve B is the sum of the ER response due to the redox reaction of the adsorbed MB, which has the same characteristics as curve A, and the ER response due to the redox reaction of MB,,h. Curve C in Figure 9 was recorded with 8.0-Hz potential modulation at Edc= 550 mV. Under the measurement of curve C, MB is in its fully oxidized form and no redox reaction takes place. Thus, curve C represents a nonfaradaic ER response. A negative peak at 665 nm and a shoulder at 613 nm correspond, respectively, to the absorption maxima of MB,, monomer and MB, dimer in the solution phase as shown in Figure 1. Two ER spectra at 500 mV, one measured by using a perpendicularly polarized light with respect to the plane of incidence and the other measured by using a parallel polarized light, are superimposable in the range of 500-690 nm. Thus, the negative ER response is due to the absorption of MB,, in the solution phase. The decrease of the amount of MB,, in the solution in the vicinity of the electrode surface along the light path upon the negative change of the electrode potential induces the nonfaradaic ER response. This ER response represents the reversible adsorption-desorption dynamics of MB,, at the electrode surface. The amount of the adsorbed MB, a t the electrode surface increases with the change of the electrode potential to the negative direction. A positive ER response is seen a t around 720 nm on curve C. This positive response exhibits a polarization depenence so that the ER response with a parallel polarized light is greater than that with a perpendicularly polarized light. This fact indicates that the positive response at 720 nm originates from the adsorbed MB, on the electrode surface. MB,, in 0.1 M H2S04 solution possesses an extra absorption band at around 741 nm as shown in Figure 1. This 741-nmband produces the negative ER response upon the adsorption of MB,, from the solution phase. Thus, the adsorbed MB,, possesses an absorption band in the range of 720-740 nm. It is very important to note that this exhibits absorption band of the adsorbed MB,, observed on curve C coincides in position with the negative ER spectral band of MB,, observed on curve A in Figure 9. (24) It is interesting to note that, precisely speaking, the resulting curve after the subtraction is slightly different from curve A in Figure 9 in the range of 615 f 15 nm. The difference corresponds to the lack of dimer band in curve B in Figure 9. This fact may imply that (i) the dimer of MB,,, is reduced after it is dissociated into monomers or (ii) the reduction of dimer is an extremely slow process. Detailed kinetic study is now underway.
Figure 10. ER spectra for a HOPG electrode in 13 pM MB,, solution in 0.1 M HzSOd. Edc= 500 m y modulation amplitude, 70 mV. Frequencies were as follows: a and b, 4.0 Hz; c, 33 Hz; d, 173 Hz; e, 1300 Hz. Curve a is the imaginary part spectrum, while curves b through e are real part spectra. The vertical scale is normalized to the ER response at 665 nm. The actual signal intensities at the 665-nm peaks are as follows: a, 1.2 X 10-4;b, -3.0 X 10-4; c, -1.8 X 10-4;d, -1.1 X 10-4, e, -2.9 X 106.
Both monomer and dimer of MB,, are involved in the adsorption-desorption dynamics because their absorption bands are observed on curve C in Figure 9. However, the kinetics of adsorption for monomer and dimer are different from each other. Figure 10 shows ER spectra measured at various frequencies a t E& = 500 mV. The imaginary part a t 4.0 Hz exhibits the most pronounced dimer band compared with the monomer band. With an increase of the frequency, the dimer absorption band decays more rapidly than the monomer band. At frequencies higher than 1.3 kHz, the dimer band is no longer observable. These results reveal that the adsorption of MB,, monomer from the solution phase is much faster than the adsorption of MB,, dimer. Figure 11 shows typical ER voltammograms at the HOPG electrode. Parts a and b of Figure 11 were obtained from the measurement at 560 nm. At 560 nm, the real part of the ER response of the adsorbed MB has a positive sign while that of MB,h has a negative sign (see curve A in Figure 9). At 173 Hz (Figurella), the ER voltammetric peak appears only at 182 mV, and the nonfaradaic ER response is seen at potentials more positive than 0.3 V. The redox reaction of MB,,h cannot follow the potentialmodulation kinetically at 173 Hz. Therefore, the peak potential represents the formal potential of the adsorbed
Adsorption States of Methylene Blue a
Langmuir, Vol. 9,No. 3, 1993 a37 MB,, in solution
l E,,]” adsorptiondesorption
M soln,ox B
30 0 d . r
Figure 12. Reaction mechanism of methylene blue at a HOPG
0.0 0.5 E 13s. .EAdAgC, / V
Figure 11. ER voltammograms at a HOPG electrode in 28 pM
MB,, solution in 0.1 M HzS04: sweep rate, 2 mV s-l;modulation, 20 mV. a: wavelength, 560 nm; frequency, 173 Hz. b wavelength, 560 nm; frequency, 4.08 Hz. c: wavelength, 665 nm; frequency, 4.08 Hz.
MB. On the other hand, at 4.08 Hz (Figure llb), another ER voltammetric peak with a positive imaginary part appears at potentials more positive than 200 mV and is assigned to the ER response due to the redox reaction of MBsohThe redox ER response of MB,,h can be more explicitly seen in the ER voltammogram measured at 665 nm as shown in Figure llc. As we have demonstrated in our previous paper: the magnitude of the ER response due to a quasi-reversibleelectron-transfer reaction of adsorbed where o is the species is proportional to (w2 + 4k0~)-l/~, angular frequency of the potential modulation and ko is the turn-over electron-transfer reaction rate constant of the adsorbed species. In contrast, in the case of the diffusion-controlled redox reaction of the species in the solution phase, the magnitude of the ER signal is proportional to o-3/2.25Note that this power of the frequency dependence is steeper than that of Warburg impedance.23 Therefore, the ER signal which increases steeply with the decrease in frequency, i.e. 240-mV peak in Figure llc, is due to the redox response of MBsoh. From the results of ER voltammetric measurements at various frequencies and wavelengths, the formal potential of the adsorbed MB is determined to be 190 mV and that of MBsohis 243 mV. Concluding Remarks The nonfaradaic ER response due to the adsorptiondesorption dynamics of MB,, is clearly observed at the HOPG electrode. At the BPG electrtode, a small nonfaradaic response, the sign of which is negative at 609 and (25) Sagara, T.; Niki, K.
Manuscript in preparation.
electrode in 0.1 M HzS04 solution. The ER spectral peak wavelengths for the species of methylene blue are given as p (positivepeak) and n (negative peak). The adsorption-desorption dynamics of the reduced form of methylene blue cannot be followed in the present experiment. Table I. Characteristics of Adsorption States of MB on Three Different Graphite Electrode Surfaces BPG EPG HOPG Cd/pF cm-2 15-20 110-220 2.0-2.5 Formal Potentials of MB Adsorbed on the Graphite [email protected]
ERstateI 107mV 105mV none ERstateII 215mV none new state, at 190 mV ER Spectrum of Adsorption State of MB: Characteristic Wavelengths in nm for pp, pze, and npb ER state I pp, 673 pp, 673 pze, 703 pze, 711 np, 735 np, 737 ER state I1 pp, 580 (new state at HOPG) pze, 630 PP, 615 np, 680 pze, 696 np, 725
Formal potentials are given with respect to a Ag/AgCl electrode in saturated KCl solution. pp, positive peak; pze, wavelength of zero ER response; np, negative peak.
700 nm, is seen at potentials more positive than 0.3 V (Figure 5). Although it was difficult to obtain an ER spectrum at this potential region because of the lower sensitivity of ER measurement at the BPG electrode than at the HOPG eletrode, it is likely that the adsorptiondesorption process of MB,, occurs not only at the HOPG but also at the BPG electrode. It is not certain whether the adsorption-desorption process occurs also at the EPG electrode or not. Further improvement of the sensitivity of the instrument for the ER measurements is now underway in order to achieve more sensitive ER measurements in the double-layer potential region to analyze the adsorption-desorption dynamics at the BPG and EPG electrodes. In Figure 12, the reaction mechanism of MB on a welldefined ab plane of the graphite electrode surface in MB,, solution is illustrated. Since all of the experiments in the present paper were conducted in MB,, solution, there is no evidence as to whether the adsorption-desorption of MB,,d occurs or not. The characteristics of the adsorption states of MB on three types of graphite electrode surfaces are tabulated in Table I. ER state I, the formal potential of which is 105 mV, is dominant at the EPG electrode, while it is not detected at the HOPG electrode. The ER spectrum of
838 Langmuir, Vol. 9, No. 3, 1993
ER state I is the same for the BPG and EPG electrodes. In view of the properties of the graphite electrode surfaces, ER state I is an adsorption state of MB at the edge plane part of the graphite surface. ER state I1 is not present at the EPG electrode. Therefore, ER state I1 is not the adsorption state of MB at the edge plane of the graphite. Although the formal potential of MB of ER state I1 on the BPG electrode is the same as that of the adsorption state of MB on the HOPG electrodes, the ER spectra are different. The area exposing the ab plane on the HOPG electrode is quite large compared to the molecular size of MB. On the other hand, the ab plane parts on the BPG electrode are highly dispersed as expected from the value of c d . The difference between the ER spectra of BPG and HOPG electrodes might suggest that the electronic structure of an adsorbed dye molecule is not only determined by the property of an adsorption site for one molecule but also considerably affected by its neighbors. The interaction between the adsorbed dye molecules is an important factor governing the structure of the molecule adsorbed on an electrode surface. The formalpotentials of the adsorbed MB on the HOPG electrode and of ER state I1 are more negative than but very near to the formal potential of MB,h. This fact gives an explanation of the difference between anodic and cathodic peak heights in the dc voltammograms in Figures 2 and 8. The adsorbed M B r d can mediate the reduction of MB,, in the solution phase. On the other hand, the reoxidation of M B r e d in the solution phase cannot be
Sagara and Niki
mediated by the adsorbed MB, because the mediation process is thermodynamically uphill. Therefore, the reosidation of M B d in the solution phase is always partially inhibited by the adsorbed MB. However, the peak height difference at the EPG electrode (Figure 6) cannot be explained. One possibility is the participation of surface functional groups on the EPG electrode. More detailed study on the effect of adsorbed MB upon the redox reaction of MB,h is necessary. Last, it is noteworthy that the ER voltammetric technique is very useful to observe separably the redox reaction of the adsorbed species, whose dc voltammetric responses are merged into the electrode reaction of the species in the solution. Wide variability of the parameters, for example, wavelength and frequency, is the remarkable feature of ER voltammetry. In addition, ER technique is a powerful tool to analyze the adsorption-desorption dynamics at the electrode interface. The ER spectral data might further involve information on the opticalproperties of the adsorption layer of MB. The quantitative consideration of the electronicstructures and surface orientations of various adsorption states of MB is of great interest and is 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 Encouragement of Young Scientists (A) (No.03855168forT.S.) and to the Shimazdu Science Foundation for the financial support to T.S.