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Investigation of Electrosorption of Organic Molecules onto Gold and Nickel Electrodes Using an Electrochemical Quartz Crystal Microbalance
Journal of Colloid and Interface Science 220, 281–287 (1999) Article ID jcis.1999.6544, available online at http://www.idealibrary.com on
Investigation of Electrosorption of Organic Molecules onto Gold and Nickel Electrodes Using an Electrochemical Quartz Crystal Microbalance Anhong Zhou and Naixian Xie 1 College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, People’s Republic of China Received March 18, 1999; accepted September 15, 1999
15), have been reported (16). The kinetics of deposition of nickel in the presence of BD has been studied (17–19). A chemical study (20) and analysis (21) and electro-oxidation (12) of BD have also been studied. The electrochemical quartz crystal microbalance (EQCM), a mass-sensitive detector based on the piezoelectric effect in an oscillating quartz crystal, has spurred many recent developments in interfacial process research (22–24). The present paper reports a quantitative and qualitative investigation of the adsorption of these two organic molecules (BA and BD) onto metal electrodes, electrodeposited gold and electrodeposited nickel, by a combined EQCM/CV (cyclic voltammetry) method.
The adsorption behaviors of two organic molecules, benzoic acid (BA) and 2-butyne-1,4-diol (BD), on metal electrodes have been studied using an electrochemical quartz crystal microbalance (EQCM) combined with the cyclic voltammetry technique. In the range of potentials studied, BA molecules were adsorbed onto an electrodeposited gold electrode with a saturation concentration of 5.0 3 10 14 molecules/cm 2. It was found that the Frumkin isotherm model was most suitable to depict the electrosorption behavior. The isotherm parameters by nonlinear fitting, which agreed with the literature values, implied BA was chemisorbed on the gold surface. For BD on an electrodeposited nickel electrode, the equivalent molar mass of the reaction species was calculated on the basis of the voltammetry curve and mass curve, which were obtained simultaneously during the potential scan. The analysis of EQCM data for the electrosorption of BD on gold and nickel electrodes showed an irreversible characteristic; the latter effectively inhibited the hydrogen evolution reaction. © 1999 Academic Press Key Words: electrosorption; electrochemical quartz crystal microbalance; metal electrode; benzoic acid; 2-butyne-1,4-diol.
INTRODUCTION
As has been reported before, most earlier investigations of molecular adsorption onto metal electrodes have concentrated on liquid electrodes (such as Hg, Ga, and InGa) or lowmeltingpoint solid electrodes (sp metals) (1–3). But on platinum group elements (d metals), organic molecules undergo chemisorption, in contrast to weak physisorption on sp metals. Yet the interaction of organic molecules with the surfaces of group IB metals are stronger than those with sp but weaker than those with d metal surfaces. A number of methods have been applied to study the surface behavior of benzoic acid (BA) for its anticorrosion property (4 – 8). The electrochemical behaviors of unsaturated alcohols, also of practical interest because of their inhibitory effect (9, 10), have been studied (11–13). However, few studies of 2-butyne1,4-diol (BD), used as a brightening agent in nickel plating (14, 1
To whom correspondence should be addressed. E-mail: zhouanhong@ 163.net.
EXPERIMENTAL
Instrumentation Experiments are carried out with an HDV-7 potentiostat (Fujian Sanming City Radio Factory No. 2), an XFD-8A ultralow-frequency signal generator (NingBo DongFong Radio Factory No. 2), and an LZ3 X–Y function recorder (ShangHai Great Wall Precise Electricmeter Factory). The frequency shift is counted by a SAMPO CN 3615 high-resolution frequency counter (Taiwan) controlled by a computer. The oscillator circuit and experimental apparatus diagram are similar to that in Ref. (25). A conventional three-electrode electrochemical cell is used in these experiments. The electrochemical cell is kept in a CS501 super thermostat (ChongQing Test Apparatus Factory) with its temperature fluctuation maintained within 25 6 0.1°C. The working electrode is a 10-MHz AT-cut piezoelectric quartz crystal (JA5, Beijing National 707 Factory, China). The EQCM is initially calibrated by copper deposited from a copper sulfate bath (150 g/L CuSO 4 z 5H 2O and 50 g/L H 2SO 4). The calibrated mass-sensitive coefficient is 4.33 3 10 29 g/(cm 2 Hz). All potentials are measured with respect to a saturated calomel electrode (SCE). The counter electrode is a platinum gauze.
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0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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Gold Electrode Preparation After being degreased two or three times with alcohol and acetone, quartz crystals coated with evaporated silver were dried in an air flow and then electrodeposited with gold by galvanostaic deposition (current density, 5–10 mA/cm 2 for 10 s, and then 1–3 mA/cm 2 for 2–3 min). The plating bath used for Au electrodeposition was provided by Wearnes Greatwall Circuits Co., Ltd. (ChangSha). The platinized electrodes were rinsed with doubly distilled water and dried in an air flow. After being degreased again two or three times with alcohol and acetone, the platinized gold electrodes were all kept in a dryer until the fundamental frequency in the air was stable. Nickel Electrode Preparation Before electrodeposition, the silver-coated quartz crystals were degreased as described above. The electrodes were electroplated with nickel through two procedures, first pre-electroplating (125 g/L NiSO 4 z 7H 2O, 9 g/L NaCl, 32 g/L H 3BO 3, 0.07 g/L sodium cetyl sulfate (SCS), pH 5.0, 25°C) at current density 1.0 A/dm 2 and second brightening electroplating (270
FIG. 2. The surface adsorption concentration (G) of BA on a gold electrode as a function of potential (0.01 M NaClO 4 1 1.0 mM BA, y 5 50 mV s 21).
g/L NiSO 4 z 7H 2O, 35 g/L NiCl 2 z 6H 2O, 35 g/L H 3BO 3, 0.08 g/L SCS, 0.15 g/L coumarin, pH 4.2, 48°C) at current density 1.8 A/dm 2. Other treatments on the platinized nickel electrodes were similar to those for the preparation of the gold electrode. RESULTS AND DISCUSSION
The Electrosorption of BA on a Gold Electrode
FIG. 1. EQCM/CV data on a gold electrode recorded in the potential range from 20.24 to 1.6 V/SCE in (1) 0.01 M NaClO 4 and (2) 0.01 M NaClO 4 1 1.0 mM BA (y 5 50 mV s 21): (a) mass curves; (b) current density curves.
Owing to the discharge of sulfate anions SO 422 on a gold surface (26), NaClO 4 was chosen for the supporting electrolyte, being generally considered a nonadsorbing medium. In view of the oxidation of gold, it is necessary to observe the changes in mass (or frequency) of EQCM in electrolyte solution with and without BA. In 0.01 M NaClO 4 solution, the frequency changes of EQCM during the potential scan were synchronously recorded by a computer-controlled frequency counter. At a potential E, the frequency change relative to the frequency at the beginning of the potential scan was designated as Df(E)1. In 0.01 M NaClO 4 solution containing a certain concentration of BA, similar experiments were conducted to compare the effect of BA on the frequency response and the CV curves. At the same potential E, the frequency change relative to the individual initial frequency was designated as Df(E)2. The difference of these two frequency changes (Df(E)) was obtained as Df(E) 5 Df(E)2 2 Df(E)1. With a change in the concentration of BA in solution, the corresponding frequency response curves were obtained. The electrosorption of BA onto a gold electrode, in the potential range 20.24 –1.6 V/SCE, is shown in Figs. 1 and 2. Figure 1a shows the mass curves in the absence and presence of BA in the electrolyte solution. Figure 1b shows the corresponding voltammetry curves during the potential scan. In 0.01 M NaClO 4 solution, the CV shows no features in the “double-layer” region of potentials from 20.2 to 0.6 V. When the positive-going potential reaches the potential of the formation of gold oxide, from 0.7 to 0.8 V, the current density begins
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to increase. Correspondingly, a reduction peak of gold oxide at ca. 0.7 V is observed during the negative-going scan. The fact that the potential range of oxidation is more positive than that of reduction indicates that the redox reaction on gold is irreversible. On the other hand, the CV obtained in BA solution shows that the presence of BA hinders the formation of gold oxide to some extent, and also shows that the intensity and potential of the reduction peak do not change obviously. The difference of CVs implies that BA has substantial surface activity at the potential of the beginning of oxide formation on a gold surface. EQCM is used to quantitatively investigate the behavior of BA on a gold electrode. The frequency shifts are simultaneously recorded, accompanying the CV experiments in the two above-mentioned solutions. The EQCM is based on Sauerbrey’s equation (27), which describes a linear relationship between the mass change, Dm, and the frequency shift, Df, as follows: Dm 5 2K mDf.
[1]
So the surface concentration of the surface-active substance (G) can be written as G5
Df~E! K mN 0 , M
[2]
where Df(E) is the difference in frequency shifts between those obtained in BA solution and those in blank solution, respectively, at potential E, K m is the mass-sensitive coefficient of the quartz crystal (K m 5 4.33 3 10 29 g/(cm 2 Hz)), M is the molar mass of the adsorbed molecules (for BA, M 5 122.12 g/mol), and N 0 is the Avogadro constant. The potential dependence of BA adsorption on a gold electrode is shown in Fig. 2. At the start of the potential scan, 20.25 V, there are about 1.7 3 10 14 molecules/cm 2 of BA adsorbed on a gold electrode. This result agrees with that obtained earlier from surface-enhanced Raman spectroscopy (SERS), indicating that considerable adsorption of benzoic acid took place at potentials from 20.25 to 0.5 V (7). In the range from 0.5 to 0.8 V, the G values increase obviously and reach the peak value (i.e., the saturated concentration) 5.0 3 10 14 molecules/cm 2. Then the G values strikingly decrease at a more positive potential (ca. 0.8 V), at which the surface oxidation of the gold begins (see Fig. 1). The value of G decreases continually until the potential reverses at 1.60 V. In contrast, in the initial stage of the negative-going scan, the G values increase until the reduction potential of gold oxide is achieved at ca. 0.82 V. It is seen that the adsorption in the negative-going scan is less than that in the positive-going potential scan. These lower G values are accounted for by the fact that the surface oxide prevents the adsorption of benzoic acid on a gold electrode. After the potential climbs over 0.82 V, at which the
FIG. 3. Adsorption isotherm of BA on a gold electrode at E ads 5 1.06 V/SCE.
oxide begins to reduce, the G values decrease until the initial potential of the CV is reached. It is known that a dynamic equilibrium is established in the electrode– bulk solution interface; thus the adsorption isotherms are allowed to be used in this case. In this experiment, the adsorption potential (E ads) was held at 1.06 V. As described in Eq. [2], the surface adsorption concentrations (G) were obtained from the frequency shifts Df(E). The corresponding adsorption isotherm is shown in Fig. 3. It was found that the Frumkin isotherm is the most suitable equation for this case,
S
c exp 2
D
S
D
DG 8ads G` G exp 2 g 5 , RT G` 2 G G`
[3]
° where DG ads is the Gibbs energy of adsorption, G ` is the maximum surface concentration of the adsorbed benzoic acid at each adsorption potential, c is the concentration of BA, g is the interaction coefficient within the adsorbate, T is the temperature where adsorption occurs (in this experiment T 5 298 K), and R is a constant (58.31 J/(mol K)). The isotherm parameters were calculated by nonlinear re° gression: DG ads 5 225.4 kJ/mol, g 5 20.4, and G ` 5 6.5 3 14 10 molecules/cm 2. These values agree roughly with those ° reported in (28). The quantity of DG ads indicates benzoic acid underwent chemisorption rather than physisorption on a gold electrode.
The Effect of BD Adsorption on Hydrogen Evolution As shown in Ref. (12), BD appears to be strongly adsorbed in the potential range 0 –1.3 V (vs RHE); however, the adsorption of BD on a gold electrode in the negative potential region has not been investigated. The electrosorption of BD on a gold electrode is shown in Figs. 4 and 5. These two mass curves were obtained by a potential scan from 21.17 to 0.03 V (Fig. 4a) and in the opposite scan direction (Fig. 5), respectively. When the scan
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Second, H 3O 1 acts on a hydrogen atom adsorbed on an electrode surface and produces H 2, which is called the Heyrovsky reaction, H3O 1 1 MeH 1 e 3 Me 1 H2 1 H2 O,
[5]
where Me is the metal electrode (in this experiment, Me is referred to as a gold electrode). When the hydrogen atoms adsorbed on an electrode surface produce H 2, the Tafel reaction occurs: MeH 1 MeH 3 2Me 1 H2 .
FIG. 4. EQCM/CV data on a gold electrode recorded in the potential range 21.17 to 0.03 V/SCE in (1) 0.01 M NaClO 4 and (2) 0.01 M NaClO 4 1 1.0 mM BD (y 5 50 mV s 21): (a) mass curves; (b) current density curves.
goes from 21.17 to 0.03 V (Fig. 4), the mass change in the absence of BD (curve 1) is more remarkable than that in the presence of BD (curve 2). A similar effect on mass is also observed in the case of the opposite scan direction (Fig. 5). On the other hand, Fig. 4b shows that the current density in the presence of BD (curve 2) is slightly compressed, compared with that in blank solution (curve 1). It is worth noting that the mass changes in the electrolyte solution (the solid lines in Figs. 4a and 5) are accounted for by the effect of hydrogen evolution. The adsorption of hydrogen would lead to increasing hydrophilic properties of the electrode surface, and the presence of hydrogen bubbles can cause additional mass loading. This effect resulted in mass increase during the negative-going potential scan, whereas the presence of BD hindered this cathodic mass increase, especially in the case of Fig. 4a. Because no obvious current peak is observed in the range 21.00 – 0.03 V, the electrosorption of BD molecules on a gold electrode inhibited the mass change during the potential scan. Furthermore, once formed, the adsorbate BD was not easily removed from the electrode. The discharge of hydrogen on the metal electrode consisted of two processes, as follows (29). First, the discharge of H 3O 1 leads to the formation of a hydrogen atom adsorbed on an electrode surface, which is called the Volmer reaction: H3O 1 1 Me 1 e 3 Me–H 1 H2 O.
[4]
[6]
Hydrogen gas would diffuse from the electrode surface into the bulk solution or even would escape out of the solution. The reactions [4]–[6] were all prevented by the presence of BD in solutions. When the scan began at negative potential (as illustrated in Fig. 4a), the electrode surface had been concentrated by so many electroactive BD molecules that the subsequent potential change does not result in a similar change in mass as that in the electrolyte solution only. Meanwhile, the electrosorption reduced the number of naked atoms of metal Me in reaction [4]. Determination of Equivalent Molar Mass (EMM) of Reaction Species By definition, the value of n i M i /n is the equivalent molar mass (EMM) of the component i. Thus, for all the compounds formed in the vicinity of either of the electrodes, the Faraday’s law can be rewritten by
O Dm F
i
IDt
i
5
O ~n M /n!, i
i
[7]
i
where I is the total current density passed through the electrode
FIG. 5. The mass curves of EQCM recorded on a gold electrode in the potential range 0.03 to 21.17 V/SCE in (1) 0.01 M NaClO 4 and (2) 0.01 M NaClO 4 1 1.0 mM BD (y 5 50 mV s 21).
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TABLE 1 The Partial EQCM Data on the Nickel Electrode in Different Solutions Investigated solution
I
Scan direction a
Potential, E (V)
Time interval, Dt (s)
Current density, i (mA/cm 2)
Mass change, Dm (310 29 g/cm 2)
Equivalent molar mass, EMM (g/mol)
0 to 20.74 V
20.095 20.245 20.665 20.385 20.25 20.06 20.165 20.495 20.70 20.45
2.3 7.1 0.71 1.2 6.4 1.2 5.0 12.3 2.0 7.1
0.0247 20.00707 20.0918 20.0141 0.0141 0.00636 0.0053 0.00177 20.0106 0.102
4.33 24.33 17.32 24.33 4.33 4.33 4.33 4.33 28.66 24.33
6.4 6.7 224.3 19.7 ` 54.8 14.3 9.9 19.0 27.3
20.74 to 0 V 0 to 20.74 V
II 20.74 to 0 V
Note. (I) 0.01 M NaClO 4 1 0.1 mM BD. (II) 0.01 M NaClO 4 1 0.5 mM BD. a Scan rate: 20 mV/s.
(for the anodic reaction, I . 0, and for the cathodic reaction, I , 0), M i is the molecular mass of the component i, n . 0 is the total number of electrons exchanged in the reaction, Dt is the time interval in which the mass changes by Dm i for either of the electrodes, n i are the stoichiometric coefficients of the reaction species (for the oxidized forms, n i . 0, and for the reduced forms, n i , 0), and F is the Faraday constant. In a practical EQCM experiment, the current density and the mass change are measured simultaneously during the potential scan. The value in the left part of Eq. [7] can be easily computed in every experimental point using a conventional discrete data acquisition. It is evident that the sign and magnitude of the obtained effective equivalent molar mass give a clue to the identification of the reaction species. The EQCM Analysis of Electrosorption Behaviors of BD on the Nickel Electrode The partial EQCM data on brightened nickel in different solutions are listed in Table 1. The curves obtained in the solution 0.01 M NaClO 4 1 0.1 mM BD are shown in Fig. 6. A typical mass curve (Fig. 6a) shows that the electrode mass remains almost constant at the beginning of the cathodic-going scan (from 0 to 20.3 V) and slightly increases at more negative potentials (from 20.3 to 20.66 V). Consequently, it is seen that the mass significantly increases with the potential from 20.66 to 20.74 V. However, the current curve (Fig. 6b) tends to decrease during the potential scan. None of the current peaks observed even achieves 20.74 V at the end of the cathodicgoing scan. The anodic-going part of the voltammetry curve increases gradually as the potential reverses at 20.74 V, accompanyied by anodic mass increase in Fig. 6a. Although the mass curve (Fig. 6a) and the current curve (Fig. 6b) are not characterized by any oxidation or reduction peak in a cathodic- and anodic-going scan, the EMM curves (Figs. 6c and 6d) show that new species have been produced on the electrode during the potential scan. As for solution I, when
FIG. 6. The EQCM data analysis of BD on a brightened Ni electrode in 0.01 M NaClO 4 1 0.1 mM BD (y 5 20 mV s 21): (a) mass curve; (b) current density curve; (c) equivalent molar mass (from 0 to 20.74 V/SCE); (d) equivalent molar mass (from 20.74 to 0 V/SCE).
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the cathodic-going scan reached the potential 20.245 V, the time interval Dt equals 7.1 s (Table 1). This time is longer than the others, commonly within 1–2 s. It is reasonably considered that the longer time interval is a characteristic of frequency shift, owing to the electrosorption of organic molars on a nickel electrode. Moreover, when the anodic-going scan reaches 20.655 V, the EMM attains 224.3 g/mol, corresponding to a small peak in Fig. 6d. The negative sign of EMM in the cathodic region corresponds to the mass increase due to the produced reduced forms as in the discussion of Eq. [7]. This cathodic reaction is allowed for the electroreduction of BD on a nickel electrode despite the absence of a reduction peak in the current curve (Fig. 6b). In the case of 1,4-butanediol, the molar mass is equal to 90.6 g/mol, 224.3 5 290.6/n, so n 5 3.7. The following four-electron-exchanged reaction has taken place: HOCH2 C'CCH2OH 1 4H 1 1 4e 3 ~2-butyne-1,4-diol! HOCH2CH2CH2CH2OH. ~1,4-butanediol!
The oscillation of signs of EMM in the course of the potential scan (Figs. 6c and 6d) imply that it may be possible to produce many reaction species (intermediates) at the electrode surface. The nickel atom possesses the electron configuration [Ar] 3d 8 4s 2 . The empty 3d orbitals can act with the p electrons of the ethynyl (OC'CO) (i.e., the polarization effect of electrons), which result in physical adsorption of BD on the nickel surface. On the other hand, these empty orbitals can also result from interaction between the electron donor (the isolated electron in the oxygen atom of OOH) and the electron acceptor (the empty 3d orbitals), which result in chemical adsorption by a chemical bond force. Additionally, this kind of specific adsorption would lead to two molecular orientations at the electrode surface, one is the flat orientation and the other is the vertical orientation. It is necessary to combine the present analysis with other analysis techniques (such as molecular spectroscopy) to study further the molecular orientation of organic molecules on solid electrodes. CONCLUSION
[8]
The sign of the mass change (positive) indicates that the product of reaction [8], 1,4-butanediol, is still adsorbed on the nickel electrode, not quickly diffusing into the bulk solution. At 20.385 V, the EMM attains 19.7 g/mol. Because of the negative sign of the current and the mass change, it is also considered that the cathodic reaction has occurred. Thus, the EMM value at 20.385 V corresponds to the oxidized forms, as described in Eq. [7]. In the case of BD, the molar mass is equal to 80.6 g/mol, 19.7 5 80.6/n, so n 5 4. The existence of reaction [8] is also demonstrated. Similarly, no reduction peak has been observed in the current curve (Fig. 6b). At a more negative potential of 20.25 V, the time interval Dt equals 6.4 s and the mass change is positive, which are explained by the fact that BD in blank solution would diffuse to the electrode surface and adsorption would occur in case the BD adsorbed on the electrode surface had been reduced as in reaction [8]. In the event of higher concentrations of BD (solution II in Table 1), BD has been adsorbed in the region from 0 to 20.495 V due to the longer time intervals of 5.0 s (at 20.165 V) and 12.3 s (at 20.495 V). In the opposite scan (from 20.74 to 0 V), the EMM value achieves 19.0 g/mol as the potential scans to 20.70 V, where the signs of i and Dm are both negative, 19.0 5 80.06/n, so n 5 4.2. The EMM value (19.0 g/mol) is also accounted for by the presence of the electroreduction reaction [8] in a higher BD concentration solution. The reduction potential in solution II (20.7 V) is more negative than that in solution I (20.655 V). At 20.45 V, the time interval Dt equals 7.1 s, and Dm is negative, which also results from the desorption of 1,4-butanediol and/or 2-butyne-1,4-diol on a nickel surface.
Under all conditions studied, the electrosorption of organic molecules on metal electrodes show a certain irreversibility. The results from the qualitative and quantitative interpretations of EQCM data for the electrosorption behavior of these molecules are reliable and satisfying. The Frumkin isotherm equation is found to describe the adsorption of BA on a gold electrode. The fitted adsorption parameters agree well overall with the values given in the literature. The adsorption of BD molecules on an electrode surface obviously inhibits the hydrogen evolution, which is connected with the cathodic mass increase. Three curves, i.e., the mass curve, the current curve, and the EMM curve, are obtained simultaneously during the potential scan. The presence of electrosorption of BD on a brightened nickel electrode is demonstrated. Since the oscillation of EMM suggests the formation of more complicated species, it is necessary to further investigate the mechanism of BD electroreduction on metal electrodes by means of other effective approaches. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China and the Natural Science Foundation of Hunan Province (China).
REFERENCES 1. Damaskin, B. B., Pettrii, O. A., and Batrakov, V. V., Eds. “Adsorption of Organic Compounds on Electrodes.” Plenum, New York, 1971. 2. Parson, R., Chem. Rev. 90, 813 (1990). 3. Lipkowski, J., and Stolberg, L., in “Adsorption of Molecules at Metal Electrodes” (J. Lipkowski and P. N. Ross, Eds.), VCH, New York, 1992. 4. Venkataraman, H. S., and Dandapani, B., J. Proc. Inst. Chem. (India) 37, 71 (1967). 5. Katoh, K., and Schmid, G. M., Bull. Chem. Soc. Jpn. 44, 2007 (1971).
ELECTROSORPTION AT GOLD AND NICKEL ELECTRODES 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Niki, K., and Shirato, T., J. Electroanal. Chem. 42, App.7 (1973). Gao, P., and Weaver, M. J., J. Phys. Chem. 89, 5040 (1985). Corrigan, D. S., and Weaver, M. J., Langmuir 4, 599 (1988). Bartos, M., and Hackerman, N., J. Electrochem. Soc. 139, 3428 (1992). Bartos, M., Kapusta, S. D., and Hackerman, N., J. Electrochem. Soc. 140, 2604 (1993). Pastor, E., Wasmus, S., and Iwasita, T., J. Electroanal. Chem. 353, 81 (1993). Holze, R., Luczak, T., and Beltowska-brzezinska, M., Electrochim. Acta 39, 991 (1994). Roquet, L., Belgsir, E. M., Leger, J. M., and Lamy, C., Electrochim. Acta 41, 1533 (1996). Raub, E., Boba, N., Knobler, M., and Stalzer, M., Trans. Inst. Met. Finish 42, 108 (1963). Epelboin, I., Froment, M., and Wiart, R., Compt. Rendus 254, 4021 (1962). Volk, O., and Fisher, H., Electrochim. Acta 5, 112 (1961); 4, 251 (1961).
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17. Chassaing, E., Joussellin, M., and Wiart, R., J. Electranal. Chem. 157, 75 (1983). 18. Bressan, J., and Wiart, R., J. Appl. Electrochem. 9, 615 (1979). 19. Epelboin, I., and Wiart , R., J. Electrochem. Soc. 118, 1577 (1971). 20. Damboise, M., Mathieu, D., and Piron, D. L., Talanta 35, 763 (1988). 21. Von Drs., and Schmitz, J. H. A., Galvanotechnik 77, 1579 (1986). 22. Deakin, M. R., and Buttry, D. A., Anal. Chem. 61, 1147A (1989). 23. Buttry, D. A., and Ward, M. D., Chem. Rev. 92, 1355 (1992). 24. Schumacher, R., Angew. Chem. 29, 329 (1990). 25. Bruckenstein, S., and Shay, M., Electrochim. Acta 30, 1295 (1985). 26. Kouznetsov, D., Asugier, Ropital, F., and Fiaud, C., Electrochim. Acta 40, 1513 (1985). 27. Sauerbrey, G., Z. Phys. 155, 206 (1959). 28. Zelenay, P., Waszczuk, P., Dobrowolska, K., and Sobrowski, J., Electrochim. Acta 39, 655 (1994). 29. Bockris, J. O. M., and Khan, S. U. M., Eds., “Surface Electrochemistry.” Plenum Press, New York, 1993.
Investigation of Electrosorption of Organic Molecules onto Gold and Nickel Electrodes Using an Electrochemical Quartz Crystal Microbalance Anhong Zhou and Naixian Xie 1 College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, People’s Republic of China Received March 18, 1999; accepted September 15, 1999
15), have been reported (16). The kinetics of deposition of nickel in the presence of BD has been studied (17–19). A chemical study (20) and analysis (21) and electro-oxidation (12) of BD have also been studied. The electrochemical quartz crystal microbalance (EQCM), a mass-sensitive detector based on the piezoelectric effect in an oscillating quartz crystal, has spurred many recent developments in interfacial process research (22–24). The present paper reports a quantitative and qualitative investigation of the adsorption of these two organic molecules (BA and BD) onto metal electrodes, electrodeposited gold and electrodeposited nickel, by a combined EQCM/CV (cyclic voltammetry) method.
The adsorption behaviors of two organic molecules, benzoic acid (BA) and 2-butyne-1,4-diol (BD), on metal electrodes have been studied using an electrochemical quartz crystal microbalance (EQCM) combined with the cyclic voltammetry technique. In the range of potentials studied, BA molecules were adsorbed onto an electrodeposited gold electrode with a saturation concentration of 5.0 3 10 14 molecules/cm 2. It was found that the Frumkin isotherm model was most suitable to depict the electrosorption behavior. The isotherm parameters by nonlinear fitting, which agreed with the literature values, implied BA was chemisorbed on the gold surface. For BD on an electrodeposited nickel electrode, the equivalent molar mass of the reaction species was calculated on the basis of the voltammetry curve and mass curve, which were obtained simultaneously during the potential scan. The analysis of EQCM data for the electrosorption of BD on gold and nickel electrodes showed an irreversible characteristic; the latter effectively inhibited the hydrogen evolution reaction. © 1999 Academic Press Key Words: electrosorption; electrochemical quartz crystal microbalance; metal electrode; benzoic acid; 2-butyne-1,4-diol.
INTRODUCTION
As has been reported before, most earlier investigations of molecular adsorption onto metal electrodes have concentrated on liquid electrodes (such as Hg, Ga, and InGa) or lowmeltingpoint solid electrodes (sp metals) (1–3). But on platinum group elements (d metals), organic molecules undergo chemisorption, in contrast to weak physisorption on sp metals. Yet the interaction of organic molecules with the surfaces of group IB metals are stronger than those with sp but weaker than those with d metal surfaces. A number of methods have been applied to study the surface behavior of benzoic acid (BA) for its anticorrosion property (4 – 8). The electrochemical behaviors of unsaturated alcohols, also of practical interest because of their inhibitory effect (9, 10), have been studied (11–13). However, few studies of 2-butyne1,4-diol (BD), used as a brightening agent in nickel plating (14, 1
To whom correspondence should be addressed. E-mail: zhouanhong@ 163.net.
EXPERIMENTAL
Instrumentation Experiments are carried out with an HDV-7 potentiostat (Fujian Sanming City Radio Factory No. 2), an XFD-8A ultralow-frequency signal generator (NingBo DongFong Radio Factory No. 2), and an LZ3 X–Y function recorder (ShangHai Great Wall Precise Electricmeter Factory). The frequency shift is counted by a SAMPO CN 3615 high-resolution frequency counter (Taiwan) controlled by a computer. The oscillator circuit and experimental apparatus diagram are similar to that in Ref. (25). A conventional three-electrode electrochemical cell is used in these experiments. The electrochemical cell is kept in a CS501 super thermostat (ChongQing Test Apparatus Factory) with its temperature fluctuation maintained within 25 6 0.1°C. The working electrode is a 10-MHz AT-cut piezoelectric quartz crystal (JA5, Beijing National 707 Factory, China). The EQCM is initially calibrated by copper deposited from a copper sulfate bath (150 g/L CuSO 4 z 5H 2O and 50 g/L H 2SO 4). The calibrated mass-sensitive coefficient is 4.33 3 10 29 g/(cm 2 Hz). All potentials are measured with respect to a saturated calomel electrode (SCE). The counter electrode is a platinum gauze.
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Gold Electrode Preparation After being degreased two or three times with alcohol and acetone, quartz crystals coated with evaporated silver were dried in an air flow and then electrodeposited with gold by galvanostaic deposition (current density, 5–10 mA/cm 2 for 10 s, and then 1–3 mA/cm 2 for 2–3 min). The plating bath used for Au electrodeposition was provided by Wearnes Greatwall Circuits Co., Ltd. (ChangSha). The platinized electrodes were rinsed with doubly distilled water and dried in an air flow. After being degreased again two or three times with alcohol and acetone, the platinized gold electrodes were all kept in a dryer until the fundamental frequency in the air was stable. Nickel Electrode Preparation Before electrodeposition, the silver-coated quartz crystals were degreased as described above. The electrodes were electroplated with nickel through two procedures, first pre-electroplating (125 g/L NiSO 4 z 7H 2O, 9 g/L NaCl, 32 g/L H 3BO 3, 0.07 g/L sodium cetyl sulfate (SCS), pH 5.0, 25°C) at current density 1.0 A/dm 2 and second brightening electroplating (270
FIG. 2. The surface adsorption concentration (G) of BA on a gold electrode as a function of potential (0.01 M NaClO 4 1 1.0 mM BA, y 5 50 mV s 21).
g/L NiSO 4 z 7H 2O, 35 g/L NiCl 2 z 6H 2O, 35 g/L H 3BO 3, 0.08 g/L SCS, 0.15 g/L coumarin, pH 4.2, 48°C) at current density 1.8 A/dm 2. Other treatments on the platinized nickel electrodes were similar to those for the preparation of the gold electrode. RESULTS AND DISCUSSION
The Electrosorption of BA on a Gold Electrode
FIG. 1. EQCM/CV data on a gold electrode recorded in the potential range from 20.24 to 1.6 V/SCE in (1) 0.01 M NaClO 4 and (2) 0.01 M NaClO 4 1 1.0 mM BA (y 5 50 mV s 21): (a) mass curves; (b) current density curves.
Owing to the discharge of sulfate anions SO 422 on a gold surface (26), NaClO 4 was chosen for the supporting electrolyte, being generally considered a nonadsorbing medium. In view of the oxidation of gold, it is necessary to observe the changes in mass (or frequency) of EQCM in electrolyte solution with and without BA. In 0.01 M NaClO 4 solution, the frequency changes of EQCM during the potential scan were synchronously recorded by a computer-controlled frequency counter. At a potential E, the frequency change relative to the frequency at the beginning of the potential scan was designated as Df(E)1. In 0.01 M NaClO 4 solution containing a certain concentration of BA, similar experiments were conducted to compare the effect of BA on the frequency response and the CV curves. At the same potential E, the frequency change relative to the individual initial frequency was designated as Df(E)2. The difference of these two frequency changes (Df(E)) was obtained as Df(E) 5 Df(E)2 2 Df(E)1. With a change in the concentration of BA in solution, the corresponding frequency response curves were obtained. The electrosorption of BA onto a gold electrode, in the potential range 20.24 –1.6 V/SCE, is shown in Figs. 1 and 2. Figure 1a shows the mass curves in the absence and presence of BA in the electrolyte solution. Figure 1b shows the corresponding voltammetry curves during the potential scan. In 0.01 M NaClO 4 solution, the CV shows no features in the “double-layer” region of potentials from 20.2 to 0.6 V. When the positive-going potential reaches the potential of the formation of gold oxide, from 0.7 to 0.8 V, the current density begins
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to increase. Correspondingly, a reduction peak of gold oxide at ca. 0.7 V is observed during the negative-going scan. The fact that the potential range of oxidation is more positive than that of reduction indicates that the redox reaction on gold is irreversible. On the other hand, the CV obtained in BA solution shows that the presence of BA hinders the formation of gold oxide to some extent, and also shows that the intensity and potential of the reduction peak do not change obviously. The difference of CVs implies that BA has substantial surface activity at the potential of the beginning of oxide formation on a gold surface. EQCM is used to quantitatively investigate the behavior of BA on a gold electrode. The frequency shifts are simultaneously recorded, accompanying the CV experiments in the two above-mentioned solutions. The EQCM is based on Sauerbrey’s equation (27), which describes a linear relationship between the mass change, Dm, and the frequency shift, Df, as follows: Dm 5 2K mDf.
[1]
So the surface concentration of the surface-active substance (G) can be written as G5
Df~E! K mN 0 , M
[2]
where Df(E) is the difference in frequency shifts between those obtained in BA solution and those in blank solution, respectively, at potential E, K m is the mass-sensitive coefficient of the quartz crystal (K m 5 4.33 3 10 29 g/(cm 2 Hz)), M is the molar mass of the adsorbed molecules (for BA, M 5 122.12 g/mol), and N 0 is the Avogadro constant. The potential dependence of BA adsorption on a gold electrode is shown in Fig. 2. At the start of the potential scan, 20.25 V, there are about 1.7 3 10 14 molecules/cm 2 of BA adsorbed on a gold electrode. This result agrees with that obtained earlier from surface-enhanced Raman spectroscopy (SERS), indicating that considerable adsorption of benzoic acid took place at potentials from 20.25 to 0.5 V (7). In the range from 0.5 to 0.8 V, the G values increase obviously and reach the peak value (i.e., the saturated concentration) 5.0 3 10 14 molecules/cm 2. Then the G values strikingly decrease at a more positive potential (ca. 0.8 V), at which the surface oxidation of the gold begins (see Fig. 1). The value of G decreases continually until the potential reverses at 1.60 V. In contrast, in the initial stage of the negative-going scan, the G values increase until the reduction potential of gold oxide is achieved at ca. 0.82 V. It is seen that the adsorption in the negative-going scan is less than that in the positive-going potential scan. These lower G values are accounted for by the fact that the surface oxide prevents the adsorption of benzoic acid on a gold electrode. After the potential climbs over 0.82 V, at which the
FIG. 3. Adsorption isotherm of BA on a gold electrode at E ads 5 1.06 V/SCE.
oxide begins to reduce, the G values decrease until the initial potential of the CV is reached. It is known that a dynamic equilibrium is established in the electrode– bulk solution interface; thus the adsorption isotherms are allowed to be used in this case. In this experiment, the adsorption potential (E ads) was held at 1.06 V. As described in Eq. [2], the surface adsorption concentrations (G) were obtained from the frequency shifts Df(E). The corresponding adsorption isotherm is shown in Fig. 3. It was found that the Frumkin isotherm is the most suitable equation for this case,
S
c exp 2
D
S
D
DG 8ads G` G exp 2 g 5 , RT G` 2 G G`
[3]
° where DG ads is the Gibbs energy of adsorption, G ` is the maximum surface concentration of the adsorbed benzoic acid at each adsorption potential, c is the concentration of BA, g is the interaction coefficient within the adsorbate, T is the temperature where adsorption occurs (in this experiment T 5 298 K), and R is a constant (58.31 J/(mol K)). The isotherm parameters were calculated by nonlinear re° gression: DG ads 5 225.4 kJ/mol, g 5 20.4, and G ` 5 6.5 3 14 10 molecules/cm 2. These values agree roughly with those ° reported in (28). The quantity of DG ads indicates benzoic acid underwent chemisorption rather than physisorption on a gold electrode.
The Effect of BD Adsorption on Hydrogen Evolution As shown in Ref. (12), BD appears to be strongly adsorbed in the potential range 0 –1.3 V (vs RHE); however, the adsorption of BD on a gold electrode in the negative potential region has not been investigated. The electrosorption of BD on a gold electrode is shown in Figs. 4 and 5. These two mass curves were obtained by a potential scan from 21.17 to 0.03 V (Fig. 4a) and in the opposite scan direction (Fig. 5), respectively. When the scan
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Second, H 3O 1 acts on a hydrogen atom adsorbed on an electrode surface and produces H 2, which is called the Heyrovsky reaction, H3O 1 1 MeH 1 e 3 Me 1 H2 1 H2 O,
[5]
where Me is the metal electrode (in this experiment, Me is referred to as a gold electrode). When the hydrogen atoms adsorbed on an electrode surface produce H 2, the Tafel reaction occurs: MeH 1 MeH 3 2Me 1 H2 .
FIG. 4. EQCM/CV data on a gold electrode recorded in the potential range 21.17 to 0.03 V/SCE in (1) 0.01 M NaClO 4 and (2) 0.01 M NaClO 4 1 1.0 mM BD (y 5 50 mV s 21): (a) mass curves; (b) current density curves.
goes from 21.17 to 0.03 V (Fig. 4), the mass change in the absence of BD (curve 1) is more remarkable than that in the presence of BD (curve 2). A similar effect on mass is also observed in the case of the opposite scan direction (Fig. 5). On the other hand, Fig. 4b shows that the current density in the presence of BD (curve 2) is slightly compressed, compared with that in blank solution (curve 1). It is worth noting that the mass changes in the electrolyte solution (the solid lines in Figs. 4a and 5) are accounted for by the effect of hydrogen evolution. The adsorption of hydrogen would lead to increasing hydrophilic properties of the electrode surface, and the presence of hydrogen bubbles can cause additional mass loading. This effect resulted in mass increase during the negative-going potential scan, whereas the presence of BD hindered this cathodic mass increase, especially in the case of Fig. 4a. Because no obvious current peak is observed in the range 21.00 – 0.03 V, the electrosorption of BD molecules on a gold electrode inhibited the mass change during the potential scan. Furthermore, once formed, the adsorbate BD was not easily removed from the electrode. The discharge of hydrogen on the metal electrode consisted of two processes, as follows (29). First, the discharge of H 3O 1 leads to the formation of a hydrogen atom adsorbed on an electrode surface, which is called the Volmer reaction: H3O 1 1 Me 1 e 3 Me–H 1 H2 O.
[4]
[6]
Hydrogen gas would diffuse from the electrode surface into the bulk solution or even would escape out of the solution. The reactions [4]–[6] were all prevented by the presence of BD in solutions. When the scan began at negative potential (as illustrated in Fig. 4a), the electrode surface had been concentrated by so many electroactive BD molecules that the subsequent potential change does not result in a similar change in mass as that in the electrolyte solution only. Meanwhile, the electrosorption reduced the number of naked atoms of metal Me in reaction [4]. Determination of Equivalent Molar Mass (EMM) of Reaction Species By definition, the value of n i M i /n is the equivalent molar mass (EMM) of the component i. Thus, for all the compounds formed in the vicinity of either of the electrodes, the Faraday’s law can be rewritten by
O Dm F
i
IDt
i
5
O ~n M /n!, i
i
[7]
i
where I is the total current density passed through the electrode
FIG. 5. The mass curves of EQCM recorded on a gold electrode in the potential range 0.03 to 21.17 V/SCE in (1) 0.01 M NaClO 4 and (2) 0.01 M NaClO 4 1 1.0 mM BD (y 5 50 mV s 21).
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TABLE 1 The Partial EQCM Data on the Nickel Electrode in Different Solutions Investigated solution
I
Scan direction a
Potential, E (V)
Time interval, Dt (s)
Current density, i (mA/cm 2)
Mass change, Dm (310 29 g/cm 2)
Equivalent molar mass, EMM (g/mol)
0 to 20.74 V
20.095 20.245 20.665 20.385 20.25 20.06 20.165 20.495 20.70 20.45
2.3 7.1 0.71 1.2 6.4 1.2 5.0 12.3 2.0 7.1
0.0247 20.00707 20.0918 20.0141 0.0141 0.00636 0.0053 0.00177 20.0106 0.102
4.33 24.33 17.32 24.33 4.33 4.33 4.33 4.33 28.66 24.33
6.4 6.7 224.3 19.7 ` 54.8 14.3 9.9 19.0 27.3
20.74 to 0 V 0 to 20.74 V
II 20.74 to 0 V
Note. (I) 0.01 M NaClO 4 1 0.1 mM BD. (II) 0.01 M NaClO 4 1 0.5 mM BD. a Scan rate: 20 mV/s.
(for the anodic reaction, I . 0, and for the cathodic reaction, I , 0), M i is the molecular mass of the component i, n . 0 is the total number of electrons exchanged in the reaction, Dt is the time interval in which the mass changes by Dm i for either of the electrodes, n i are the stoichiometric coefficients of the reaction species (for the oxidized forms, n i . 0, and for the reduced forms, n i , 0), and F is the Faraday constant. In a practical EQCM experiment, the current density and the mass change are measured simultaneously during the potential scan. The value in the left part of Eq. [7] can be easily computed in every experimental point using a conventional discrete data acquisition. It is evident that the sign and magnitude of the obtained effective equivalent molar mass give a clue to the identification of the reaction species. The EQCM Analysis of Electrosorption Behaviors of BD on the Nickel Electrode The partial EQCM data on brightened nickel in different solutions are listed in Table 1. The curves obtained in the solution 0.01 M NaClO 4 1 0.1 mM BD are shown in Fig. 6. A typical mass curve (Fig. 6a) shows that the electrode mass remains almost constant at the beginning of the cathodic-going scan (from 0 to 20.3 V) and slightly increases at more negative potentials (from 20.3 to 20.66 V). Consequently, it is seen that the mass significantly increases with the potential from 20.66 to 20.74 V. However, the current curve (Fig. 6b) tends to decrease during the potential scan. None of the current peaks observed even achieves 20.74 V at the end of the cathodicgoing scan. The anodic-going part of the voltammetry curve increases gradually as the potential reverses at 20.74 V, accompanyied by anodic mass increase in Fig. 6a. Although the mass curve (Fig. 6a) and the current curve (Fig. 6b) are not characterized by any oxidation or reduction peak in a cathodic- and anodic-going scan, the EMM curves (Figs. 6c and 6d) show that new species have been produced on the electrode during the potential scan. As for solution I, when
FIG. 6. The EQCM data analysis of BD on a brightened Ni electrode in 0.01 M NaClO 4 1 0.1 mM BD (y 5 20 mV s 21): (a) mass curve; (b) current density curve; (c) equivalent molar mass (from 0 to 20.74 V/SCE); (d) equivalent molar mass (from 20.74 to 0 V/SCE).
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the cathodic-going scan reached the potential 20.245 V, the time interval Dt equals 7.1 s (Table 1). This time is longer than the others, commonly within 1–2 s. It is reasonably considered that the longer time interval is a characteristic of frequency shift, owing to the electrosorption of organic molars on a nickel electrode. Moreover, when the anodic-going scan reaches 20.655 V, the EMM attains 224.3 g/mol, corresponding to a small peak in Fig. 6d. The negative sign of EMM in the cathodic region corresponds to the mass increase due to the produced reduced forms as in the discussion of Eq. [7]. This cathodic reaction is allowed for the electroreduction of BD on a nickel electrode despite the absence of a reduction peak in the current curve (Fig. 6b). In the case of 1,4-butanediol, the molar mass is equal to 90.6 g/mol, 224.3 5 290.6/n, so n 5 3.7. The following four-electron-exchanged reaction has taken place: HOCH2 C'CCH2OH 1 4H 1 1 4e 3 ~2-butyne-1,4-diol! HOCH2CH2CH2CH2OH. ~1,4-butanediol!
The oscillation of signs of EMM in the course of the potential scan (Figs. 6c and 6d) imply that it may be possible to produce many reaction species (intermediates) at the electrode surface. The nickel atom possesses the electron configuration [Ar] 3d 8 4s 2 . The empty 3d orbitals can act with the p electrons of the ethynyl (OC'CO) (i.e., the polarization effect of electrons), which result in physical adsorption of BD on the nickel surface. On the other hand, these empty orbitals can also result from interaction between the electron donor (the isolated electron in the oxygen atom of OOH) and the electron acceptor (the empty 3d orbitals), which result in chemical adsorption by a chemical bond force. Additionally, this kind of specific adsorption would lead to two molecular orientations at the electrode surface, one is the flat orientation and the other is the vertical orientation. It is necessary to combine the present analysis with other analysis techniques (such as molecular spectroscopy) to study further the molecular orientation of organic molecules on solid electrodes. CONCLUSION
[8]
The sign of the mass change (positive) indicates that the product of reaction [8], 1,4-butanediol, is still adsorbed on the nickel electrode, not quickly diffusing into the bulk solution. At 20.385 V, the EMM attains 19.7 g/mol. Because of the negative sign of the current and the mass change, it is also considered that the cathodic reaction has occurred. Thus, the EMM value at 20.385 V corresponds to the oxidized forms, as described in Eq. [7]. In the case of BD, the molar mass is equal to 80.6 g/mol, 19.7 5 80.6/n, so n 5 4. The existence of reaction [8] is also demonstrated. Similarly, no reduction peak has been observed in the current curve (Fig. 6b). At a more negative potential of 20.25 V, the time interval Dt equals 6.4 s and the mass change is positive, which are explained by the fact that BD in blank solution would diffuse to the electrode surface and adsorption would occur in case the BD adsorbed on the electrode surface had been reduced as in reaction [8]. In the event of higher concentrations of BD (solution II in Table 1), BD has been adsorbed in the region from 0 to 20.495 V due to the longer time intervals of 5.0 s (at 20.165 V) and 12.3 s (at 20.495 V). In the opposite scan (from 20.74 to 0 V), the EMM value achieves 19.0 g/mol as the potential scans to 20.70 V, where the signs of i and Dm are both negative, 19.0 5 80.06/n, so n 5 4.2. The EMM value (19.0 g/mol) is also accounted for by the presence of the electroreduction reaction [8] in a higher BD concentration solution. The reduction potential in solution II (20.7 V) is more negative than that in solution I (20.655 V). At 20.45 V, the time interval Dt equals 7.1 s, and Dm is negative, which also results from the desorption of 1,4-butanediol and/or 2-butyne-1,4-diol on a nickel surface.
Under all conditions studied, the electrosorption of organic molecules on metal electrodes show a certain irreversibility. The results from the qualitative and quantitative interpretations of EQCM data for the electrosorption behavior of these molecules are reliable and satisfying. The Frumkin isotherm equation is found to describe the adsorption of BA on a gold electrode. The fitted adsorption parameters agree well overall with the values given in the literature. The adsorption of BD molecules on an electrode surface obviously inhibits the hydrogen evolution, which is connected with the cathodic mass increase. Three curves, i.e., the mass curve, the current curve, and the EMM curve, are obtained simultaneously during the potential scan. The presence of electrosorption of BD on a brightened nickel electrode is demonstrated. Since the oscillation of EMM suggests the formation of more complicated species, it is necessary to further investigate the mechanism of BD electroreduction on metal electrodes by means of other effective approaches. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China and the Natural Science Foundation of Hunan Province (China).
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