Study of the Voltammetric Spike Response of ... - ACS Publications

Figure 4 Sweep rate dependence of the spike response in the cyclic voltammogram: (A) plot of anodic peak current (ipa) and cathodic peak current (ipc)...
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Study of the Voltammetric Spike Response of Heptyl Viologen at a HOPG Electrode Horizontally Touched to a Gas/Heptyl Viologen Aqueous Solution Interface Takamasa Sagara,*,†,‡ Saori Tanaka,‡ Yumika Fukuoka,‡ and Naotoshi Nakashima‡ “Organization and Function”, PRESTO, JST and Department of Applied Chemistry, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan Received August 22, 2000. In Final Form: December 27, 2000 We investigated the electrochemistry of a basal plane highly oriented pyrolytic graphite electrode horizontally touched to a gas/heptyl viologen aqueous solution (G/S) interface using cyclic voltammetry, electroreflectance (ER) spectroscopy, potential step chronoamperometry, chronoreflectometry, and chronocoulometry. The spike response on the voltammogram, which was previously analyzed by Tokuda et al. (see: J. Electroanal. Chem. 1999, 473, 138.), was found to be observable only in restricted conditions. The experimental ER spectrum at the spike potential was in good agreement with the simulated one, indicating that the redox reaction of heptyl viologen takes place at the spike potential and the radical cation adsorbs on the electrode surface in a nonupright orientation. The voltammogram and potential step response suggested that the kinetics of the spike response can be explained in terms of a nucleation-growthcollision (NGC) mechanism. It was revealed that the transient of reflectance in response to the potential step synchronizes with the NGC current component. The plot of the charge as a function of the final potential in the potential step coulometry measurement exhibited a transition point at which the interconversion of the oxidation state amounts to more than half of the adsorbed viologen. The spike response was assigned as the first-order faradaic phase transition, presumably between a Gibbs monolayer and a Langmuir monolayer. The presence of the Gibbs monolayer of heptyl viologen at the G/S interface was confirmed by surface tension measurements.

Introduction Monolayers of some organic molecules possessing a long alkyl chain drastically change their assembling structures on an electrode surface in response to the change of the electrode potential, and a number of such processes have recently become well-known.1-21 If such dynamic processes are drastic and chemically reversible, they may be * To whom correspondence should be addressed. E-mail: sagara@ net.nagasaki-u.ac.jp. † “Organization and Function”, PRESTO, JST. ‡ Department of Applied Chemistry, Nagasaki University. (1) Bizzotto, D.; Zamlynny, V.; Burgess, I.; Jeffrey, C. A.; Li, H.-Q.; Rubinstein, J.; Merril, R. A.; Lipkowski, J.; Galus, Z.; Nelson, A.; Pettinger, B. In Interfacial Electrochemistry: Theory, Experiment, and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 405. (2) Buess-Herman, C.; Bare´, S.; Poelman, M.; Van kierken, M. In Interfacial Electrochemistry: Theory, Experiment, and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 427. (3) Nelson, A.; Auffret, N. J. Electroanal. Chem. 1988, 244, 99. (4) Nelson, A.; Leermaker, F. A. M. J. Electroanal. Chem. 1989, 273, 183. (5) Nelson, A.; Leermaker, F. A. M. J. Electroanal. Chem. 1990, 278, 73. (6) Gordillo, G. J.; Schiffrin, D. J. J. Chem. Soc., Faraday Trans. 1994, 90, 1913. (7) Gao, X.; White, S.; Chen, S.; Abruna, D. Langmuir 1995, 11, 4554. (8) Freymann, M.; Poelman, M.; Buess-Herman, C. Isr. J. Chem. 1997, 37, 241. (9) Bizzotto, D.; Noe¨l, J. J.; Lipkowski, J. Thin Solid Films 1994, 248, 69. (10) Bizzotto, D.; Noe¨l, J. J.; Lipkowski, J. J. Electroanal. Chem. 1994, 369, 259. (11) Bizzotto, D.; Lipkowski, J. J. Electroanal. Chem. 1996, 409, 33. (12) Sagara, T.; Zamlynny, V.; Bizzotto, D.; McAlles, A.; McCrindle, R.; Lipkowski, J. Isr. J. Chem. 1997, 37, 197. (13) Bizzotto, D.; Lipkowski, J. Prog. Colloid Polym. Sci. 1997, 103, 2201. (14) Bizzotto, D.; Nelson, A. Langmuir 1998, 14, 6269. (15) Burgess, I.; Jeffrey, C. A.; Cai, X.; Szymanski, G.; Galus, Z.; Lipkowski, J. Langmuir 1999, 15, 2607.

applicable to novel modified electrodes with dynamic functions. These processes may be driven by interfacial electric fields and surface charges or triggered by a redox reaction. The interplay of molecule-electrode substrate and intermolecular interactions including solvent molecules greatly contributes to the dynamic processes. Redox-triggered change of the molecular orientation of an organic monolayer is among the typical processes. It includes the spike responses of viologens observed in the voltammograms.16-21 An adsorption layer of viologen on an electrode surface produces a pair of spike peaks in the voltammogram around or at a more positive potential than the reduction potential of the viologen dication (V2+) in the solution phase to viologen radical cation (V•+). This phenomenon has been observed at Hg,17-19,22,23 Au(111),21 Pt,16 and highly oriented pyrolytic graphite (HOPG)20 electrodes in aqueous solution of viologens. The origin of the spike response has tentatively been assigned as the redox-triggered change of molecular orientation or formation of a 2D condensed film. It is known that the electronic charge accompanied by the spike peak corresponds to the monolayer amount of viologen at higher concentrations of (16) Bewick, A.; Cunningham, D. W.; Lowe, A. C. Makromol. Chem., Macromol. Symp. 1987, 8, 355. (17) Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1993, 347, 371. (18) Milla´n, J. I.; Rodrı´guez-Amaro, R.; Ruiz, J. J.; Camacho, L. Langmuir 1999, 15, 618. (19) Milla´n, J. I.; Rodrı´guez-Amaro, R.; Ruiz, J. J.; Camacho, L. J. Phys. Chem. B 1999, 103, 3669. (20) Arihara, K.; Kitamura, F.; Nukanobu, K.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1999, 473, 138. (21) Tominaga, M. Abstracts of the 36th Joint Meeting of Kyushu Divisions of Chemical Societies 1999, 4.63, 96. (22) Salas, R.; Sa´nchez-Maestre, M.; Rodrı´guez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. Langmuir 1995, 11, 1791. (23) Kobayashi, K.; Fujisaki, F.; Yoshimine, T.; Niki, K. Bull. Chem. Soc. Jpn. 1986, 59, 3715.

10.1021/la001219u CCC: $20.00 © 2001 American Chemical Society Published on Web 02/10/2001

Voltammetric Spike Response of Heptyl Viologen

viologen. Therefore, the spike response is believed to be a faradaic phase transition process of a viologen monolayer adsorbed on the electrode surface, though the mechanism and kinetics are not fully understood yet at a molecular level. The voltammetric spike response of methyl viologen at a Hg electrode has been investigated by Camacho and his colleagues from the viewpoint of a faradaic phase transition.22 The spike response of heptyl viologen at a Hg electrode has been extensively investigated by Tokuda and his colleagues in aqueous solution17 and by Camacho and his colleagues in a water/DMSO mixture.18,19 Camacho and his colleagues argued that one of the spike responses is due to a faradaic phase transition in which the orientation of viologen changes from a flat-lying to an edge-on orientation accompanied by a change of the orientation and conformation of alkyl chains. Tokuda and his colleagues studied the spike response at a HOPG electrode by the use of in situ IR reflection spectroscopy.20 Their results suggested that the viologen radical cation moieties assume a side-on orientation with the alkyl chain axis parallel to the HOPG surface. It is of tremendous significance to describe the molecular level mechanism and kinetics of the redox-triggered change of the monolayer structure by using the HOPG/ heptyl viologen interface as a prototypical case. To gain more insight into the mechanism and kinetics, the use of a dynamic spectroelectrochemical technique is indispensable. Because the one-electron reduction of V2+ to V•+ results in a significant UV-vis absorption spectral change, in situ UV-vis reflectance spectroscopy should appear as a strong tool for the study of the process. In the present work, we investigated in detail the spike response of heptyl viologen at a basal plane HOPG electrode by the complementary use of UV-vis reflectance spectroscopy. We first dipped a HOPG electrode in heptyl viologen solution using an electrode holder equipped with an O-ring, but we could not observe the sharp spike response at all. However, when a hanging meniscus configuration at a gas/solution (G/S) interface was used, the sharp spike response was observed reproducibly. Therefore, we focus first on the exploration of the necessary conditions that enable us to observe the spike response. Then, we describe the adsorption state of the viologen moiety producing the spike response using the results of electroreflectance measurements. We demonstrate the results of the measurements of potential step responses of both current (charge) and reflectance for the purpose of tracking the process in the time domain. We also attempt to answer the question of whether the heptyl viologen dication adsorbs on the HOPG surface. Experimental Section Heptyl viologen (N,N′-diheptyl-4,4′-bipyridinium dibromide, HV2+ 2Br-) purchased from Tokyo Kasei Kogyo Co. was recrystallized once from acetone-ethanol and dried in a vacuum. Its purity was verified by 1H NMR and elemental analysis. Water was purified through a Milli-Q Plus Ultrapure water system coupled with an Elix-5 kit (Millipore Co.). Its resistivity was over 18 MΩ cm. All other chemicals were of reagent grade and were used as received. A plate of HOPG was the product of Matsushita Electric Co. (Panasonic graphite, PGX 04 and 05; size 12 mm × 12 mm × 3 mm thickness). The difference between PGX 04 and 05 is the mosaic spread of the X-ray diffraction peak: 0.4-0.5° for PGX 04 and 0.5-0.6° for PGX05. We did not find any difference in the experimental results of the present work whichever HOPG we used. To expose a fresh basal plane, the surface of the HOPG was peeled off by the use of Scotch adhesive tape immediately before use. For the comparison to the HOPG electrode, a basal plane pyrolytic graphite (BPG, Tomoe Co., Tokyo) also was used

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Figure 1. Schematic picture depicting the lower part of the spectroelectrochemical cell for the measurement at the hanging meniscus configuration and the optical setup for the reflection measurement at a normal incidence. after peeling off. The double-layer capacity of the BPG electrode in a base solution was ca. 20 µF cm-2, whereas that of the HOPG ranged from 1.4 to 3.8 µF cm-2. To set up a hanging meniscus configuration (H-M configuration) for the HOPG or BPG electrode (Figure 1), the graphite plate was connected perpendicularly to a copper pipe (diameter, 6.5 mm) using carbon paste glue at the rear surface. The contact resistance was less than 10 Ω. When the HOPG electrode was dipped in the solution, the electrode was mounted in a homemade Kel-F holder with a fluorinated rubber O-ring.24 All the electrochemical and spectroelectrochemical measurements were made in 1.00 mM HV2+ 2Br- + 0.30 M KBr solution using a Ag/AgCl/saturated KCl reference electrode and a Au wire coil counter electrode under an Ar gas (>99.998%) atmosphere. All potentials in the present paper are referenced to this reference electrode. A quartz spectroelectrochemical cell of a cylindrical body with an optically flat bottom window was used for both voltammetric and electroreflectance (ER) measurements. The ER instrumentation used previously24 was partially modified to enable normal incidence to the surface of the HOPG electrode in the H-M configuration through the cell bottom window (Figure 1). The specular reflection light was fed to a photomultiplier (R928, Hamamatsu Photonics Co.) using a half mirror. The waveform modulating the electrode potential E is described as

E ) Edc + Eac ) Edc + ∆Eac exp(jωt)

(1)

where Edc is the dc potential, Eac is the ac potential, ∆Eac is the ac amplitude, j ) x-1, ω ) 2πf, which is the angular frequency (f is the frequency of the potential modulation), and t is the time. The ER signal is defined as the ac component of the reflectance divided by the time-averaged reflectance and designated as (∆R/R)ER. Both the real part (in-phase component with respect to Eac) and the imaginary part (90° out-of-phase component) of (∆R/R)ER were simultaneously monitored as a function of the incident light wavelength, λ. The transient of the reflected light intensity in response to the potential step was measured using the same instrument as the ER measurement. The reflected light intensity signal from the photomultiplier was offset to be zero at the initial potential. The change of the signal in response to the potential step was recorded using an analyzing recorder (model AR1200, Yokogawa). The transient signal was normalized by the reflected light intensity at the initial potential and designated as (∆R/R)step. The transient curves of (∆R/R)step for 32 steps sampled at 100 kHz were accumulated simultaneously with the current transient and then (24) Sagara, T. Res. Dev. Phys. Chem. 1998, 2, 159.

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Figure 2. Surface excess (Γs)-concentration (cHV) curve obtained from the surface tension measurement for HV2+ 2Br(cHV) + 0.3 M KBr aqueous solution at 24 °C. linearly averaged to reduce the random noise. In all electrochemical measurements, a potentiostat (model HECS-315B, Fuso) was used. Surface tension measurement was made by the use of a Wilhelmy balance with a Pt plate (Surface Tensiometer CBVPA3, Kyowa Interface Co.). The simulation of the ER spectrum was conducted using a homemade program. Regression analysis in the curve fitting calculation was made by the help of commercial software.

Results and Discussion I. Heptyl Viologen Adsorption to Air/Aqueous Solution Interface from the Solution Phase. The spike response of interest appears at a HOPG electrode in the H-M configuration.20 To set up this configuration, the electrode surface was lowered from the gas phase to the G/S interface until the electrode surface came into contact with the solution surface. Then, the electrode was elevated vertically to attain a hanging meniscus, keeping the contact (Figure 1). Because a viologen bearing long alkyl chains may behave as an amphiphile, it may adsorb to the G/S interface to form a Gibbs monolayer. Figure 2 shows the plot of the surface excess of HV2+ at the G/S interface, Γs, as a function of the bulk concentration of HV2+ 2Br-, cHV. This plot was obtained from the surface tension (γ)-cHV curve for an air/HV2+ 2Br- (cHV) + 0.3 M KBr solution interface at 24 °C using the Gibbs equation:

Γs ) -

1 dγ 2RT d ln cHV

(2)

It was revealed that heptyl viologen molecules adsorb to the G/S interface from the solution phase to form a Gibbs monolayer. Therefore, the HOPG surface initially touches the Gibbs monolayer when the H-M configuration is prepared. II. Voltammetric Study. Figure 3 shows a typical cyclic voltammogram at a HOPG electrode in the H-M configuration on 1 mM HV2+ 2Br- + 0.3 M KBr solution. Our experiments reproduced the results reported by Tokuda and his colleagues.20 Reduction of heptyl viologen dication (HV2+) in the solution phase to heptyl viologen cation radical (HV•+) as an insoluble deposit on the surface was diffusion-controlled and gave rise to a cathodic peak on the voltammogram at -523 mV. Reoxidation of the insoluble deposit of HV•+ salt to soluble HV2+ gave an anodic peak around -435 mV. The voltammetric curve shown in Figure 3 was recorded in a more positive potential region than the above-mentioned potentials of the redox reaction of the bulk species. A cathodic spike peak at -0.375 V and an anodic spike peak at -0.300 V were

Figure 3. Cyclic voltammogram of a HOPG electrode (A ) 1.44 cm2) in the H-M configuration on 1 mM HV2+ 2Br- + 0.3 M KBr solution at a sweep rate of 80 mV s-1. The potential scanning range was restricted to the region of spike response.

observed at v ) 80 mV s-1, where v is the potential sweep rate. The spike response identical with Figure 3 was reproducibly observable whether or not the cathodic end potential was set to include the bulk reduction to twoelectron reduced form (HV0) down to -0.90 V. When continuous potential cycling was repeated in the potential range as in Figure 3, the change of the peak current was not observed within a period well over 5 h. Figure 4 shows the features of the cyclic voltammograms at various v. The peak current is not proportional to v, indicating that the redox response is not of an ideal thin layer electrochemistry of a Langmuir monolayer of noninteracting species. A simple model for a nucleationgrowth-collision (NGC) process proposed by Camacho and his colleagues25 predicted that the peak current, ip, is proportional to v0.6 and ∆Ep (peak separation) and ∆W1/2 (full-width at the half-height) are proportional to v0.4. The experimental v dependence of ipa (anodic peak current), ipc (cathodic peak current), and ∆W1/2 for the anodic peak is almost in line with the model. Even at v ) 0.1 mV s-1, ∆Ep of 12 mV is observed (Figure 4B). The asymmetry of the voltammetric response between anodic and cathodic processes is highlighted by the faster shift of the cathodic peak potential with v than its anodic counterpart. This indicates that the electron-transfer elemental step is associated with a significant change of the state of the viologen layer, the kinetics of which is different between anodic and cathodic processes. The peak charge of both cathodic and anodic peaks was 1.17 × 10-5 C cm-2, independent of v. The differential capacity-potential curve obtained from an ac voltammogram (not shown here) in the potential region of Edc > -270 mV was virtually continuous to the curve in the potential region of Edc < -360 mV. Therefore, the peak charge is predominantly due to the faradaic process but not to the interfacial capacity jump at the spike potential. Assuming that the charge is fully due to the one-electron redox reaction of surface-confined viologen moieties, the surface excess, Γ, is 1.2 × 10-10 mol cm-2, corresponding to the area occupied by one heptyl viologen molecule of 1.4 nm2. This value of Γ is ca. 15% smaller than that reported by Tokuda and his colleagues.20 If heptyl viologen (25) Sa´nchez-Maestre, M.; Rodrı´guez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1994, 373, 31.

Voltammetric Spike Response of Heptyl Viologen

Figure 4. Sweep rate dependence of the spike response in the cyclic voltammogram: (A) plot of anodic peak current (ipa) and cathodic peak current (ipc), (B) plot of peak potentials, and (C) plot of the full width at the half-height of the spike peak (∆W1/2).

molecules assume upright or flat-lying orientations, a bipyridinium moiety should occupy 0.23 or 0.59 nm2, respectively, ignoring the extra projection area of counteranions.26 The experimental value of Γ indicates that both the viologen moiety and alkyl chains are in contact with the electrode surface, provided that a closely packed monolayer is formed. Even when the solution was intentionally contaminated with O2 gas, we could still observe the spike response. Catalytic reduction of O2 as a cathodic EC process at the spike potential was not observed. This fact indicates that the reduction product of the spike response does not react with O2. The reduced form of viologen, HV•+, generally exhibits high susceptibility to O2 in the solution phase so that the radical cation is immediately oxidized. Therefore, (26) Cotton, T. M.; Kim, J.-H.; Uphaus, R. A. Macrochem. J. 1990, 42, 44.

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this observation indicates that O2 molecules in the solution cannot gain access to the HV•+ produced by the spike response. Presumably, the viologen radical cation moiety produced at the spike potential is not exposed to the solution, and the alkyl chains effectively block the access of O2 molecules to the moiety. Note that minor peaks were seen in Figure 3 at -0.1 and -0.15 V. These peaks were not always seen, and the presence of them never affected the characteristics of the spike response. It is important to note that the spike response was insensitive to the electrode potential at which the H-M configuration is set. That is, the touching procedures at an open circuit potential, at 0.0 V (positive to the spike response) and at -0.4 V (negative to the spike response), all gave rise to a voltammogram identical with that in Figure 3. We also measured the voltammograms at a BPG electrode in the H-M configuration. However, no spike response was observed, though the bulk reduction of the soluble HV2+ was observed to be the same as at a HOPG electrode at negative potentials. We tested the use of an edge plane of a HOPG electrode at the H-M configuration. Then, no spike response was observed at all. These facts reveal that the use of a basal plane HOPG electrode is essential to produce the spike response and that the size of the graphite (0001) terrace is an important factor. The necessary conditions for the appearance of the spike response at a HOPG electrode are further detailed in the next section. III. Exploration of Necessary Conditions of Spike Response Appearance. We tested the electrochemistry of heptyl viologen using a variety of conditions including an electrode configuration other than H-M. As a conclusion, it was revealed that the sharp spike response is observable only in restricted conditions. When we used an O-ring to hold a HOPG electrode, we never observed the sharp spike response. A HOPG electrode with a freshly exposed basal plane was held in a Kel-F electrode holder with an O-ring24 to expose a circular area of basal plane surface to the solution. This electrode was immersed into 1 mM HV2+ 2Br- + 0.3 M KBr solution in such a way that the HOPG surface crosses the G/S interface vertically. Then, we observed no trace of the spike peak or quite broad response at the spike response with the maximal peak height as small as ca. 1/25 of that at the H-M configuration. Even when we first dipped the electrode with an O-ring in 0.3 M KBr solution and then dissolved HV2+ 2Br-, we did not observe the sharp spike. As an additional experiment, we made two different surface pretreatments of the HOPG electrode held by an O-ring: (1) electrochemical oxidation27 and (2) physical scratching using a hard stainless steel edge. As a result of both pretreatments, we again observed a broad and small response at the spike potential (the peak height was as small as ca. 1/20 of that at an untreated HOPG electrode at the H-M configuration). The defects of the HOPG basal surface do not provide the conditions to produce the spike response when the HOPG electrode is held by the holder and dipped in the solution. The horizontal touching of the HOPG to the gas/ solution interface may be essential to produce the spike response. (27) In 0.1 M KNO3 solution, the electrode was polarized at 1.95 V for 2 min and subsequently at 0.0 V for 30 s. This pretreatment induces surface defects (graphite edge sites) due to the electrochemical oxidation of the HOPG surface (refs 28 and 29).

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Table 1. Results of the Examinations of the Conditions for the Appearance of the Spike Response at a HOPG Electrode in the H-M Configuration entry

initial setting of H-M configurationa or another initial procedure

0 1

H-M on HV solnc surface H-M on base solnd surface

2

H-M on base soln surface

3

H-M on HV soln

4

H-M on HV soln

5

H-M on HV soln

6

H-M on HV soln

7

H-M on HV soln

8

C18OH monolayer spreading at G/S interfacef

additional procedure none addition of HV soln on the solution surface injection of HV soln into the solution phase (i) continuous potential cyclinge for period of tm (ii) detached from HV soln and remaining droplet on the surface was removed by a capillary (iii) set H-M on HV soln (i) detached 10 min after H-M setting from HV soln and remaining droplet on the surface was removed by a capillary (ii) set H-M on HV soln (i) continuous potential cyclinge for period of 15 min (ii) detached from HV soln and remaining droplet on the surface was removed by a capillary (iii) set H-M on base soln push the HOPG electrode down into the HV soln to immerse the edge planes C18OH monolayer spreading at G/S interfacef H-M on C18OH monolayer-covered HV soln

resulting voltammetric response at the spike potentialb ++ (see Figures 3 and 4) ++ (sharp spike grew and then decayed because of low final HV concentration) ++ (sharp spike grew and then decayed because of low final HV concentration) tm ) 10 min: + (peak current ca. 60% of entry 0) tm ) 1 h: + (peak current ca. 25% of entry 0) tm ) 5 h: - - (no response) ++ (peak current ca. 90% of entry 0)

- - (no response)

++ (same as entry 0 superimposed on enhanced double-layer charging current) ++ (same as entry 0) - - (no response)

a Initial H-M setting was made at an open circuit potential. b (++) Sharp spike with similar magnitude as entry 0, (+) sharp spike but with smaller magnitude, (- -) no response. c HV soln: 1 mM HV2+ 2Br- + 0.3 M KBr aqueous solution. d Base soln.: 0.3 M KBr aqueous solution. e At 80 mV s-1 between 0.0 and -0.4 V. f 1-Octadecanol (C18OH) in chloroform was spread to form a Langmuir monolayer of C18OH at the G/S interface.

We further examined the necessary conditions for the appearance of the spike response at the H-M configuration to make clear the nature of the spike response. The procedures of the examinations and the results are summarized in Table 1. The results of entries 1 and 2 indicate that when the H-M configuration is used, the spike response emerges regardless of the initial existence or subsequent addition of heptyl viologen in the solution phase. The decay of the spike response is due to the decrease in the concentration of HV2+ near the electrode with time. Adsorption of HV2+ on the HOPG surface, if it occurred, is a reversible process. The results of entries 3 and 4 indicate that once the electrode is detached and the H-M configuration is then set again on HV2+ 2Br- solution the magnitude of the spike response is not fully recovered. The dependence on tm can be due to a change of the surface structure of HOPG,30,31 such as surface process-induced delamination. In fact, as the period passed during which the first H-M configuration was prolonged the vertical width of the cyclic voltammogram in the double-layer potential region became greater, though the characteristics of the spike response itself were unchanged at least in a period of 8 h. Work is currently under way to clarify this time evolution. In entry 5, film transfer of viologen molecules was not attained, at least when they are in oxidized form. This (28) Bowling, R. J.; Packard, R. T.; McCreery, R. L. J. Am. Chem. Soc. 1989, 111, 1217. (29) Sagara, T.; Fukuda, M.; Nakashima, N. J. Phys. Chem. B 1998, 102, 521. (30) Andersen, J. E. T.; Jensen, M. H.; Møller, P.; Ulstrup, J. Electrochim. Acta 1996, 41, 2005. (31) He, Y.; Wang, Y.; Zhu, G.; Wang, E. J. Electrochem. Soc. 1999, 146, 250.

result again supports the reversible adsorption of HV2+. The result of entry 6 reveals that the process originating the spike response does not take place on the edge plane and that the presence of a three-phase junction perimeter (HOPG basal plane/gas/solution) is not necessarily required to produce the spike response. The tests in entries 7 and 8 were conducted to see the effect of the coexistence of an insoluble amphiphile, which has a tendency to form a Langmuir monolayer at a G/S interface. In entry 7, the C18OH monolayer at the free G/S interface did not interfere with the spike response. In entry 8, the presence of C18OH monolayer on the HOPG surface completely diminished the spike response.32 However, the bulk redox reaction of heptyl viologen in the solution phase at more negative potentials was observed to be the same as in the case of the absence of the C18OH G/S interfacial film. These results reveal that the appearance of the spike response requires the direct contact of heptyl viologen solution with the HOPG surface. To sum up, the HOPG basal plane should experience the horizontal touching to the G/S interface and should not be subsequently detached from the solution in order for the sharp spike response with a constant Γ to be observed. From a comparison of the H-M configuration and the setup with an O-ring, the contact of the boundary (32) It is known that the presence of a C18OH monolayer on a Au(111) electrode lowers the interfacial differential capacity and that phase change of the layer is observed with potential change (refs 1, 9, 10, and 13). These phenomena do not take place on HOPG (or even if they do, it cannot be observed as capacity or current change), because HOPG is not a metal. The interfacial capacity of a HOPG electrode is not governed by the double layer but is totally dominated by the spacecharge layer in the HOPG (refs 33-37). Note that for the same reason, attempts to see the adsorption of heptyl viologen on the HOPG surface by using only capacity measurements should be abandoned.

Voltammetric Spike Response of Heptyl Viologen

Figure 5. Electroreflectance (ER) spectrum of a HOPG electrode in the H-M configuration on 1 mM HV2+ 2Br- + 0.3 M KBr solution surface: (solid line) real part of the experimental ER spectrum at normal incidence (Edc ) -0.313 V, f ) 14 Hz, ∆Eac ) 20 mVrms) and (broken line) simulated ER spectrum by the use of the anisotropic optical constant for HOPG.

between basal and edge planes to the solution phase may be one of the keys to produce the spike response. On a speculative basis, this boundary may act as a nucleation site. IV. Electroreflectance Spectrum. The solid line in Figure 5 represents the real part of the ER spectrum measured at the midpoint potential between the anodic and cathodic peaks at a normal incidence. The real part is shown as a representative, because the spectral profiles of both real and imaginary parts were identical if not of the same intensity. The ER signal of a HOPG electrode in the absence of heptyl viologen was zero in the potential region of interest. The ER signal in the presence of heptyl viologen in the solution phase also was zero at more positive potentials than the spike response. The experimental ER spectral profile in Figure 5 is far different from the difference absorption spectrum between HV•+ and HV2+ (because HV2+ is colorless at λ > 315 nm, the ER spectrum as the difference absorption spectrum should be the same profile as the absorption spectrum of HV•+). This feature is in sharp contrast to viologen-thiol monolayers on a Au electrode.38,39 Because of the differences in the substrate optical properties between HOPG and Au, any qualitative interpretation of the ER spectrum at HOPG in the light of that at a Au electrode is not justified. Therefore, we simulated the ER spectral profile in the same way as our previous work40 to confirm whether viologen moieties are confined on the HOPG electrode surface and undergoing a redox reaction of HV2+/•+ at the spike potential. We assumed that the one-electron redox reaction of the heptyl viologen monolayer adsorbed on the HOPG surface with a flat-lying or edge-on orientation is the origin of the ER response. Briefly, the simulation procedure was as follows. (33) Kneten Cline, K.; McDermott, M. T.; McCreery, R. L. J. Phys. Chem. 1994, 98, 5314. (34) Gerischer, H.; McIntyre, R.; Scherson, D.; Storck, W. J. Phys. Chem. 1987, 91, 1930. (35) Gerischer, H. J. Phys. Chem. 1985, 89, 4249. (36) Gerischer, H.; McIntyre, R. J. Chem. Phys. 1985, 83, 1363. (37) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1972, 36, 257. (38) Sagara, T.; Kaba, N.; Komatsu, M.; Uchida, M.; Nakashima, N. Electrochim. Acta 1998, 43, 2183. (39) Sagara, T.; Maeda, H.; Yuan, Y.; Nakashima, N. Langmuir 1999, 15, 3823. (40) Sagara, T.; Murase, H.; Komatsu, M.; Nakashima, N. Appl. Spectrosc. 2000, 54, 316.

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(i) We assumed that the solution absorption spectra of HV2+ and HV•+ are identical with those of methyl viologen and that the content of the viologen dimer in the reduced state is ca. 10%. Because of quite low solubility of HV•+Brin water, accurate absorption spectral measurement is difficult. However, we confirmed experimentally that the absorption spectral profile of HV•+ is indistinguishable from that of the methyl viologen radical cation. Therefore, we used the spectrum of methyl viologen instead of that of heptyl viologen. (ii) We assumed that the viologen moieties form a continuous, optically homogeneous, and two-dimensionally isotropic film of a thickness of 0.36 nm (corresponding to the effective van der Waals thickness of a viologen moiety)26 with the superficial concentration equal to the experimental surface excess (1.2 × 10-10 mol cm-2). Then, the complex refractive index of the film was calculated with the help of the Kramers-Kronig relationship by taking the flat-lying orientation of the viologen longitudinal axis into consideration. (iii) The interface of interest was modeled as a threephase stratified structure: water (refractive index n1 ) 1.333), a film of the viologen moiety, and a basal plane of HOPG. The isotropic or anisotropic dielectric constant of HOPG was taken from previous ellipsometric data29 or the literature,41 respectively. (iv) The relative reflectance difference between the filmcovered electrode and the film-free electrode, (∆R/R)f, was calculated by adopting an approximation that the film is much thinner than the wavelength of the incident light.40,42 The equation used is

[ ]

(∆R/R)f ) (8πdn1/λ) Im

ˆ 2 - ˆ 3 1 - ˆ 3

(3)

where d is the thickness of the monolayer, 1 is the dielectric constant of the solution phase and is equal to n12, and ˆ 2 and ˆ 3 are the complex dielectric constants of the monolayer and HOPG, respectively. The calculation of (∆R/R)f was made for both fully reduced film (the film of the viologen radical cation) and fully oxidized film (the film of the viologen dication). (v) Using the result of (iv), a simulated ER spectrum was deduced using the equation40 ox

(∆R/R)ER )

red

(∆R/R) f - (∆R/R) f ox

1+

red

(∆R/R) f + (∆R/R) f

(4)

2

We ignored the presence of the layer composed of the alkyl chains. This approximation is rationalized even though the viologen monolayer must be optically divided into two phases: the layer composed of viologen moieties and that composed of alkyl chains. The presence of the latter layer can be taken into account by adding an additional term in the same form of eq 3.42,43 This additional term is independent of the oxidation state of viologen and thus never affects the final profile of the ER spectrum. It should be noted that, within the linear approximation, we cannot know the sequence of these two layers.42 (41) Johnson, L. G.; Gresselhaus, G. Phys. Rev. B 1973, 7, 2275. (42) Kolb, D. M. In Spectroelectrochemistry: Theory and Practice; Gale, R. J., Ed.; Prenum Press: New York, 1988; p 87. (43) Takamura, T.; Takamura, K.; Watanabe, F. Surf. Sci. 1974, 44, 93.

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The broken line in Figure 5 is the simulated ER spectral profile. The resulting spectral profile was almost the same whichever dielectric constant of ˆ 3, isotropic or anisotropic, was used. The up-and-down features of the simulated spectrum are well in accord with those of the experimental spectrum. However, characteristic wavelengths (peak wavelengths and the wavelengths of the zero-ER signal) and fine structures are different between experimental and simulated spectra. These differences may arise from the large bandwidth in the ER spectral measurement (ca. 20 nm) as well as from the difference of the content of viologen radical cation dimer between the real monolayer and our model. The resemblance of the spectra allows us to interpret the experimental ER spectrum as showing that viologen moieties (or, at least, radical cation moieties) are in contact with the HOPG surface and assume a flatlying or edge-on orientation on the surface. If the viologen is not confined to the electrode surface, the ER spectrum should be nearly the same as the solution absorption spectrum of HV•+ (vide supra). We cannot exclude an oblique orientation of the viologen moiety, but we can exclude an upright orientation. Viologen radical cation moieties in an upright orientation do not give an ER signal at all when normal incidence is used, because the electric field of the light is perpendicular to the electric dipole responsible for the light absorption. Keeping in mind that all heptyl viologen molecules are in a fully oxidized state at more positive potentials than the spike response, we can rule out the possibility that the ER signal is solely due to the change in the orientation of the HV•+ moiety or adsorption/desorption of HV•+. Our result is in line with the edge-on orientation of the viologen radical cation moiety on a HOPG surface proposed by Tokuda and his colleagues.20 In the model proposed by Camacho for HV•+ on a Hg electrode surface in a water/ DMSO mixture at a high DMSO content, the HV•+ moiety assumes two types of orientations, in both of which the longitudinal axis of the viologen moiety is parallel to the surface.18,19 We cannot speculate as to whether HV2+ forms a monolayer on the electrode surface at positive potentials, because no spectral information of colorless HV2+ is involved in the ER data. The experimental spectral profile around 550-650 nm is blue-shifted compared to the simulation. One possible explanation is that the content of HV•+ dimer in the reduced state is greater than 10%. We are currently working on the simulation using the dimer absorption spectrum for the reduced state and on the ER spectral measurements at a higher wavelength resolution. Note that the scale of the spectral signal is different between the experimental and simulated spectra. The simulation is based on the interconversion between fully reduced and fully oxidized monolayers. On the other hand, the experimental spectrum corresponds to partial interconversion and its signal intensity is also affected by kinetics. In the simulation, even though the surface excess was reduced to be 1/10 of the experimental value virtual change of the simulated spectral profile was not seen. This provides a rationale behind the comparison of the spectral profiles of the two spectra in Figure 5 though the scale is different. The use of a three-phase model is justified even if the viologen moieties are diluted by the alkyl chains in the monolayer. V. Potential Step Chronoamperometry and Chronoreflectometry. We conducted the simultaneous measurements of the transients of both current and reflected light intensity in response to the potential step. The results of the large amplitude potential step over the spike response are shown in Figure 6 by curves b, c, e, and f.

Sagara et al.

Figure 6. Results of the simultaneous measurements of the transients of current and transients of (∆R/R)step at 541 nm in response to potential steps for a HOPG electrode (A ) 1.44 cm2) in the H-M configuration on 1 mM HV2+ 2Br- + 0.3 M KBr solution surface: (A) curve a, initial potential Ei ) -0.200 V and final potential Ef ) -0.300 V; curves b and c, Ei ) -0.200 V and Ef ) -0.375 V; (B) curve d, Ei ) -0.250 V and Ef ) -0.150 V; curves e and f, Ei ) -0.375 V and Ef ) -0.200 V.

When the spike potential was not included in the step, an exponential decay due to the double-layer charging (more specifically, space-charge layer charging of HOPG) was observed (curves a and d). Note that the response time of the potentiostat used was 5 µs. The uncompensated solution resistance evaluated by impedance measurements was smaller than 30 Ω. Because the interfacial differential capacity in the double-layer potential region was in the order of 10-6 F, the cell time constant of the experiment is shorter than 0.3 ms. Therefore, the potentiostat contribution to the transient responses need not be taken into account. Current transients over the spike response for both anodic and cathodic steps showed a typical feature of the NGC process.2,22,44 One or two humps were overlapping with a fast exponential decay of double-layer charging current. The current decay for the anodic step was much

Voltammetric Spike Response of Heptyl Viologen

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Figure 8. Typical cathodic current transient curves as the result of chronoamperometry with potential steps to two different final potentials for a HOPG electrode (A ) 1.44 cm2) in the H-M configuration on 1 mM HV2+ 2Br- + 0.3 M KBr solution. The initial potential was -0.250 V, and the final potentials were (a) -0.355 V and (b) -0.340 V.

and the second represents the NGC process. For limiting cases, m ) 2 for instantaneous nucleation and m ) 3 for progressive nucleation.44 However, for both m ) 2 and m ) 3 the attempts to fit to the equations failed. Two possible assignments of the spike response are written as

HV2+solution + e- a HV•+monolayer (2D condensed phase) (6) HV2+monolayer + e- a HV•+monolayer (2D condensed phase) (7)

Figure 7. Correlation between (∆R/R)step and integrated current: (A) corresponding to curves b and c in Figure 6 and (B) corresponding to curves e and f.

faster than the cathodic step. This fact is in line with the asymmetric shift of peak potentials in Figure 4. Importantly, the transients of (∆R/R)step at 541 nm synchronized with the current response as demonstrated in Figure 7. A linear relationship between (∆R/R)step and the charge (the integral of the current) is seen in the range of 1.3-4.0 ms for the anodic step (Figure 7A) and in the range of 3.8-10.0 ms for the cathodic step (Figure 7B). The horizontal axis width of the linear part corresponds well to the value of Γ obtained from the cyclic voltammogram. The deviation in the shorter time domain is due to the overlap of the double-layer charging current. The transients of (∆R/R)step do not synchronize with the doublelayer charging component but with the following current hump(s). Because the change in (∆R/R)step at 541 nm represents the redox reaction of heptyl viologen, the current hump(s) synchronized with the faradaic process. In an attempt to gain more quantitative information from the current transient, we tried to fit it to the simple equations representing NGC kinetics. We assumed that the transient can be expressed by the sum of the two independent contributions:

i(t) ) a exp(-bt) + ptm-1 exp(-qtm)

(5)

where a, b, m, p, and q are time-independent constants. The first term in eq 5 represents the double-layer charging, (44) Demir, U.; Shannon, C. Langmuir 1996, 12, 6091 and references therein.

The question here is whether HV2+ is adsorbed on the HOPG surface at positive potentials. The decay time for the cathodic step (curve c in Figure 6) is ca. 10 ms. If the adsorption process in the cathodic step of eq 7 is totally diffusion-controlled, the time required to achieve a monolayer adsorption with a surface excess of Γ, τ, is described as

τ)

πΓ2 4DcHV2

(8)

where D is the diffusion coefficient of HV2+. Assuming that D ) 5 × 10-6 cm2 s-1 and the concentration in the vicinity of the electrode surface is homogeneous at cHV ) 1 mM, we obtain τ ) 2.3 ms. This value is much shorter than the above-mentioned decay time (ca. 10 ms). Therefore, the transient data does not allow us to distinguish eq 6 as in the case of underpotential deposition45 and eq 7 as a pure NGC process of monolayer phase transition,46 even though the voltammetric response can be adopted to the simple NGC model proposed by Camacho et al.25 On the other hand, the tendency of HV2+ to form a Gibbs monolayer at a G/S interface (section I) may also emerge at a highly hydrophobic HOPG basal plane. Although further evidence is required to distinguish eqs 6 and 7, we tentatively assign that a Gibbs monolayer of HV2+ is present on the HOPG surface at positive potentials. We also made detailed measurements of chronoamperometry and chronocoulometry. We found two important features of the spike response. First, the current transient strongly depends on the final potential of the potential step, Ef. Figure 8 shows two typical cathodic transient curves as examples. For (45) Ho¨lzle, M. H.; Zwing, V.; Kolb, D. M. Electrochim. Acta 1995, 40, 1237. (46) Bosco, E.; Rangarajan, S. K. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1673.

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Figure 9. Plot of Qt as a function of Ef for a HOPG electrode (A ) 1.44 cm2) in the H-M configuration on 1 mM HV2+ 2Br+ 0.3 M KBr solution: (A) cathodic potential step with an initial potential of -0.250 V and (B) anodic potential step with an initial potential of -0.400 V.

the sake of clarity of the presentation, the top part of the initial component was omitted. It is clear that the transient involves at least two components showing two maxima corresponding to the NGC process except for the doublelayer charging component. The positions of the current maxima depended on Ef (see also Figure 6). The time at which the current component takes a NGC maximum became longer with shifting Ef to more positive values. When Ef was more positive than -316 mV, the NGC components were hardly seen in the current transient. Importantly, in contrast to the cathodic current transient the anodic current transient always exhibited a single maximum, the position of which was dependent on Ef. We will touch on this difference between cathodic and anodic transients later. Second, we found that the total charge, Qt, as the response of potential step jumps at a certain final potential. Figure 9 shows the plots of anodic and cathodic Qt values against Ef. The value of Qt corresponds to the integral of the current transient with respect to the time from the application of the step to the infinite time. Experimentally, Qt values were obtained by the same procedure as that used in the potential step train experiments explored by Lipkowski and his colleagues.47 The typical time period needed to track the charge along the time course to obtain the value of Qt was shorter than 1.2 s. In all step experiments, the current value has already been constant after this period so that current integration over a period of 1.2 s gave rise to a unique value of Qt. Note that Qt is the sum of the charges of faradaic and nonfaradaic processes. (47) Lipkowski, J.; Stolberg, L. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; p 171.

Sagara et al.

In the cathodic step, a 1 mV change of the Ef value from -316 to -317 mV gave a drastic jump of the value of Qt. The difference of Qt between -316 and -317 mV corresponded to ca. 50% of Γ obtained from the cyclic voltammogram peak charge. This potential of the transition point was 3 mV more negative than the peak potential of the cyclic voltammogram at v ) 0.1 mV s-1. The jump of the Qt value was also recorded in the anodic step between -304 and -305 mV. The difference in Qt corresponded to ca. 75% of Γ obtained from the cyclic voltammogram peak charge. This potential of the transition point was 3 mV more negative than the peak potential at v ) 0.1 mV s-1. The cyclic voltammogram peak potentials at the infinitesimal sweep rate (Figure 4B) tend to converge to the transition point. Note that there is a small side peak at a few millivolts positive potential to the transition point for both anodic and cathodic Qt-Ef plots. This is not an experimental artifact, though we have no interpretation of this phenomenon at present. The presence of the transition point for both directions is the indication of the first-order phase transition. The difference in the potential of the transition point between anodic and cathodic potential steps, 11 mV, may reflect the hysteresis of the initiation conditions of nucleation between HV2+ and HV•+ states and may correspond to the supercooling effect of the phase transition. Because HV2+ is water soluble whereas HV•+ is not, we can tentatively propose a hypothesis that the spike response is due to the first-order faradaic phase transition between a Gibbs monolayer and a Langmuir monolayer (the latter can also be referred to as a “2D condensed film”). In the aspect of the dynamics, the cathodic and anodic processes are highly asymmetric. In the cathodic process, a gaseous Gibbs monolayer is transformed into a 2D condensed film leading to a Langmuir monolayer formation. The surface excess is different between the two phases. These features explain the difference in kinetics demonstrated by the difference in the rate of peak potential shift with v in the voltammogram and in the decay time of the transient. The presence of two NGC maxima only in the cathodic transient may reflect the two steps of growth or the presence of two isolated types of nucleation-growth of the condensed film. To verify this hypothesis, direct visualization of the surface process is required. Conclusions The process giving rise to the voltammetric spike response of heptyl viologen at a HOPG electrode horizontally touched to the gas/heptyl viologen aqueous solution interface was described using the results of cyclic voltammetry, potential step, and in situ UV-vis reflectance (electroreflectance) measurements. One of our new findings is that the conditions of the spike peak appearance are restricted. Spike response appears when a HOPG electrode was set in the hanging meniscus configuration on the surface of the heptyl viologen solution, where HV2+ forms a Gibbs monolayer as confirmed by the surface excess obtained from the surface tension measurement. When a HOPG electrode is held by a holder with an O-ring and dipped in the solution, no spike response was observed even though defects were introduced with the intention to create more nucleation centers. Presumably, in order for the sharp spike response to be observed a pristine basal plane of HOPG possessing an exposed boundary between basal and edge planes should contact the solution. The requirements for a HOPG electrode to produce the spike response are more severe than at Hg and Pt electrodes.16-19,22

Voltammetric Spike Response of Heptyl Viologen

The results of the measurements of cyclic voltammograms are consistent with the interpretation that the kinetics of the spike response is the nucleation-growthcollision (NGC) type. We tracked the process producing the spike response, for the first time in the present work, by the spectroscopic signal and the reflectance transient as a response to the potential step. The ER spectrum indicated that the viologen radical cation responsible for the spike response is in contact with the HOPG surface and the viologen moiety is reduced and oxidized at the spike peaks. The kinetics was explained in terms of a NGC mechanism, and the reflectance signal synchronizes with the simultaneously measured NGC components in the current (charge) transient. The highlight of the present work is the finding of the presence of definite transition points in the potential step coulometry data. The spike response is not due to a sluggish transition but a first-order transition.

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As a conclusion, we assigned the spike response to the first-order faradaic phase transition between a Gibbs monolayer and a Langmuir monolayer. The phase transition follows the kinetics of NGC. We are currently performing reflectance measurements at an oblique incidence, which may be useful to see the orientation of the HV•+ moiety more precisely, and microscopic observation of the NGC process. Acknowledgment. This work was the research project of “Organization and Function”, PRESTO, JST. Additionally, this work was supported financially in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Culture and Sports of Japan (to T.S.) and The Asahi Glass Foundation (to T.S.). LA001219U