- Email: [email protected]
Chemisorption and electrochemistry of diphenyldiselenide on the hanging mercury drop electrode
Electrochimica Acta 49 (2004) 1389–1395
Chemisorption and electrochemistry of diphenyldiselenide on the hanging mercury drop electrode Jesús L. Muñiz Álvarez, Josefa A. Garc´ıa Calzón, Juan M. López Fonseca∗ Departamento de Qu´ımica F´ısica y Anal´ıtica, Universidad de Oviedo, E-33071 Oviedo, Spain Received 7 May 2003; received in revised form 12 September 2003; accepted 4 November 2003
Abstract The interfacial and electrochemical behaviour of diphenyldiselenide (2 Se2 ) on the hanging mercury drop electrode has been studied by means of cyclic and ac voltammetry and chronocoulometry using 30% dimethylformamide–water media. Both capacitance and Faradaic current measurements reveal that a progressively thicker multilayer film is formed as a result of chemisorption of 2 Se2 at the mercury/solution interface. By controlling the thickness of the film, a gradual inhibition of the electrochemical reduction of Ru(NH3 )6 3+ is achieved. © 2003 Elsevier Ltd. All rights reserved. Keywords: Diphenyldiselenide; Chemisorption; Multilayer films; Cyclic and ac voltammetry; Chronocoulometry
1. Introduction The formation and electrochemical characterisation of films of organic molecules chemisorbed on metal surfaces have been studied extensively. Most of these films are monolayers formed by self-assembly [1,2]. Typical examples of such self-assembled monolayers (SAMs) are those of thiolic (or disulphide) compounds chemisorbed on Au surfaces [1,2]. In addition, chemisorbed monolayers of these compounds on Hg surfaces have been characterised [2–13]. On the other hand, a few studies on multilayer films resulting from chemisorption of thiocompounds on Au or Hg surfaces have been reported [14,15]. Selenoderivatives are other good candidates to form chemisorbed films on Au or Hg substrates. As a matter of fact, the early electrochemical studies performed by Nygård in protic mixtures [16–18] revealed that diphenyldiselenide, 2 Se2 , (as well as all other organic diselenides) is spontaneously chemisorbed on mercury according to: 2 Se2 + Hg → 2Se–Hgads
(1)
This reaction and the subsequent reversible charge transfer reaction (2) were considered to be responsible for the re∗
Corresponding author. Tel.: +34-985-103689; fax: +34-985-103125. E-mail address: [email protected] (J.M. L´opez Fonseca). 0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.11.003
duction of 2 Se2 at both the dropping mercury electrode (DME) and the hanging mercury drop electrode (HMDE) [16–19]. From these reactions, and assuming that a single adsorbate-saturated monolayer is formed at high 2 Se2 concentrations, the dc-, sampled dc-, normal pulse- and ac-polarographic behaviours of 2 Se2 in protic media are successfully explained [19,20]. The cyclic voltammetric behaviour of 2 Se2 at the HMDE in these media was interpreted on the basis of the same assumption [19].
(2) Besides, the self-assembly of 2 Se2 on Au surfaces has been recently reported [21,22]. It was found that the Se–Se bond is preserved upon chemisorption on Au, in contrast with the dissociative chemisorption (reaction (1)) occurring on Hg surfaces. Although the basic scheme for the reduction of organic diselenides at mercury electrodes has been established by Nygård many years ago [16–18], a systematic study about the chemisorbed film formed through reaction (1) has not been reported yet. For this reason, we have performed a detailed investigation of the interfacial and electrochemical behaviour of 2 Se2 at the HMDE in protic media. Cyclic voltammetry (CV), phase-sensitive ac voltammetry (ACV)
1390
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
and chronocoulometry (CC) were used with this purpose. In essence, our results reveal that the chemisorption of 2 Se2 leads to the formation of a multilayer film, whose thickness can be modified by changing the adsorption conditions. The application of this finding for controlling the rate of outer-sphere electrode reactions is also explored.
2. Experimental CV and CC were performed by means of a potentiostat/galvanostat -Autolab type II (from Ecochemie) attached to a personal computer with the proper software and fitted to a 663 VA stand (from Metrohm) which includes a multimode mercury electrode and the reference and auxiliary electrodes. ACV was performed using a Metrohm 693 VA processor fitted to a 694 VA stand, also from Metrohm. Measurements were made at a modulation frequency of 75 Hz and a r.m.s. modulation voltage of 10 mV. The out-of-phase (Φ = 90◦ ) component of the ac current was recorded against dc potential with a scan rate of +4 or −4 mV/s. Values of the out-of-phase component were converted into capacitance values by using the Grahame’s data for the capacitance of Hg in a 0.1 M KCl aqueous solution [23]. A HMDE, a platinum wire, and an Ag/AgCl/KCl 3 M were used as the indicator, the auxiliary and the reference electrode, respectively. The surface areas of the HMDE were 0.54 mm2 in ACV and 0.37 mm2 in CV and CC. The reference electrode was connected with the cell solution through a daily renewed KNO3 -saturated solution. pH measurements were made with a pH-meter Crison 2001. 2 Se2 was supplied by Sigma and other chemicals were Merck. Milli-Q purified water was used for preparing solutions. Stock solutions of 1 × 10−2 M 2 Se2 in dimethylformamide (DMF) were prepared everyday. Most of the electrochemical measurements were performed in a medium of 0.1 M Na2 SO4 and 0.01 M sodium acetate buffer in water containing 30% (v/v) DMF at pH 4.5 (but DMF was removed in the medium used for performing some ex situ measurements, see further). In addition, some experiments were carried out in media in which the acetate buffer was replaced with 0.01 M disodium phosphate (pH 7.1) or TRIS (pH 9.1) buffers. The solutions of 2 Se2 in the above media were purged with oxygen-free nitrogen for 20 min. A fresh mercury drop was exposed to the solutions, which firstly were stirred at 33.33 revolutions/s (unless otherwise stated) for a defined exposition time, texp , and then were maintained without stirring for a rest time, trest . During both periods, either a fixed potential, Eexp , was applied to the electrode or this was maintained in open circuit. Electrochemical measurements were performed in situ (i.e., in the above 2 Se2 solutions) or ex situ (in a 2 Se2 free solution). When a fixed potential was applied in the first type of measurements, the recordings started immediately
after the rest period from an initial potential, Ei , coincident with Eexp ; however, when the electrode was maintained in open circuit during the exposition and rest periods, Ei was maintained during 5 s. Ex situ measurements were performed after maintaining the electrode in open circuit during the exposition and rest periods. Then, the electrode was removed from the solution of 2 Se2 and introduced into a previously deareated 2 Se2 -free solution. The measurements were carried out immediately or after a fixed time was elapsed from the insertion (in the second case, the solution was purged during this interval); in both cases, Ei was maintained during 5 s. All the experiments were carried out at room temperature (23 ± 2 ◦ C).
3. Results and discussion 3.1. Characterisation of the film 3.1.1. In situ measurements The cyclic voltammograms shown in Fig. 1 were recorded in a medium of 0.1 M Na2 SO4 and 0.01 M acetate buffer in 30% DMF–water at pH 4.5, containing 1 × 10−5 M 2 Se2 . These voltammograms were obtained with Eexp = −0.25 V after various exposition times. As seen in this figure, a single cathodic peak (a) at −0.73 V appears during the forward scan for texp = 5 s (curve 1). With increasing texp , a second (b) and a third (c) cathodic peaks are successively developed at less negative potentials (curves 2 and 3). Further increasing in texp leads first to a splitting of peak c in peaks c and c (curve 4) and then to a sharp increase of peak c together with a change in its peak potential towards more negative values (curve 5). Fig. 1 also shows that the peak current increases
Fig. 1. Cyclic voltammograms obtained in a solution of 1 × 10−5 M 2 Se2 , 0.1 M Na2 SO4 and 0.01 M acetate buffer in 30% DMF–water at pH 4.5, after an exposition period, texp , of 5 s (1), 15 s (2), 30 s (3), 90 s (4) and 150 s (5) with stirring of the solution. Rest period, trest = 5 s. Each voltammogram was recorded using a fresh mercury drop to which a potential, Eexp = −0.25 V was applied during the exposition and rest periods. Initial potential, Ei = −0.25 V; scan rate, v = 100 mV/s.
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
Fig. 2. Cyclic voltammograms obtained in solutions of 5 × 10−5 M (1) and 1 × 10−4 M 2 Se2 (2) in the same medium as in Fig. 1. texp = 30 s; other conditions are the same as in Fig. 1.
and the peak potential changes negatively for peak b with increasing texp . In contrast, neither the peak current nor the peak potential for peak a depend on texp . As a consequence of the different effects of texp on peak currents for peaks a, b and c , the peak a is not detected at long values of texp (curve 5). The cyclic voltammograms shown in Fig. 2 were obtained in the above medium containing 5 × 10−5 or 1 × 10−4 M 2 Se2 , with texp fixed in 30 s (curves 1 and 2, respectively). From voltammogram 3 in Fig. 1 and those in Fig. 2, it is seen that the effect of increasing 2 Se2 concentration at constant texp on the electrochemical behaviour of 2 Se2 is similar than that of increasing texp at constant 2 Se2 concentration. Furthermore, peaks b and c also increase continuously with increasing texp at the 2 Se2 concentrations of Fig. 2 and similar, but slower increases, were observed when the solution was not stirred during the exposition period.
1391
On the other hand, strong decreases in peaks b and c (and conversion of c in c) but no changes in peak a were found in the successive potential scans when both texp and the 2 Se2 concentration were fixed. This is illustrated in Fig. 3, where some voltammograms recorded by means of repetitive CV are shown. In addition, the effect of the scan rate, v, on the peak currents, Ip , for peaks a, b and c (in the first potential scan) was studied in a solution of 2 × 10−5 M 2 Se2 in the above medium and with texp = 30 s. In the range assayed (10–100 mV/s), the ratio Ip /v was found to be constant for peaks a and b. For peak c, the relationship Ip /v1/2 was constant at v ≤ 50 mV/s but it increased gradually with increasing v between 50 and 100 mV/s. It was also found that the peak potentials for a, b and c in the voltammograms of Fig. 2 (obtained at pH 4.5) coincide with the respective values in the voltammograms recorded in solutions buffered at pH 7.1 and 9.1, other conditions being the same as in this figure. This reveals that no subsequent protonation of the Se− species—see reaction (2)—occurs at pH ≥ 4.5. Ludv´ık and Nygård have reported on the cyclic voltammetric behaviour of 2 Se2 at the HMDE under experimental conditions rather different than those used in this work (specifically, solutions in 40% ethanol–water at pH 7.4 were used with texp = trest = 0) [19]. Two cathodic peaks similar to those labelled a and b in this work were described in [19] and considered to be an adsorption post-peak and a main (diffusion) peak, respectively; thus, it was assumed that a saturation coverage by the adsorbate has been reached [1]. In addition, a peak like our peak c was observed by Ludv´ık and Nygård at high 2 Se2 concentrations and it was attributed to the effect of a product adsorption. The previously described behaviour of peak a is, in fact, that expected for an adsorption peak in conditions of full
Fig. 3. Voltammograms obtained by means of repetitive CV during the first (1), second (2) and fifth (3) potential scans. These voltammograms were recorded using a single mercury drop in a solution of 7.5 × 10−5 M 2 Se2 in the same medium as in Fig. 1. texp = 30 s; other conditions are the same as in Fig. 1.
1392
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
coverage by a reactant monolayer, according to the theoretical treatments [1]. From reaction (2) and the charge obtained by integration of peak a in voltammogram 1 of Fig. 1 (in which peak a—with its maximum height—is the only developed), a value of 9.4 × 10−11 mol/cm2 was obtained for the surface concentration of Se–Hg in the adsorbate-saturated monolayer. It should be noted that the measured charge includes a contribution of the double-layer charging and, thus, the real surface concentration of the adsorbate should be less than the above value. Nevertheless, a main peak cannot appear at low reactant concentration; it should be independent on texp and its peak current should be proportional to v1/2 , according to [1]. Consequently, both the behaviour of b shown in Fig. 1 and its dependence with v exclude the possibility that peak b could be a main peak. On the other hand, the effect of v on peak c is not that expected for an adsorption peak; in addition, the very low anodic peaks (with respect to the cathodic peaks) observed in the reverse scans of the cyclic voltammograms at high texp and/or high 2 Se2 concentration (see Figs. 1 and 2) reveal that the product of the electrode reaction is not adsorbed at the interface; thus, peaks c or c cannot be caused by a product adsorption. In an attempt to elucidate the origin of peaks b and c or c , the capacitance or pseudocapacitance of the interface were measured by means of ACV. Fig. 4A shows the capacitance–dc potential curve obtained with Eexp = 0.00 V in a solution of 1 × 10−4 M 2 Se2 in the medium at pH 4.5. The overlapped peaks appearing at negative potentials in the presence of 2 Se2 correspond to the cathodic peaks observed in the cyclic voltammograms under similar exposition conditions; so, they are pseudocapacitance peaks including Faradaic contributions. At less negative potentials, a
Fig. 5. Effect of the exposition period, texp , on the capacitance of the pit shown in Fig. 4A. Eexp = 0.00 V.
wide capacitance pit is developed. According to both theoretical and experimental results, the appearance of capacitance pits is due to the formation of either an ordered monolayer [24] or a multilayer film [25]. The capacitance, C, in the pit region of Fig. 4A is as low as 0.7 F/cm2 , under the conditions of this figure, and this capacitance even decreases with increasing texp , as shown in Fig. 5. These C values allow one to estimate the film thickness, d, by means of the Helmholtz model, i.e., by using the equation C = ε0 εFr /d, where ε0 is the permittivity of the vacuum, 8.85 × 10−12 C2 /(N m2 ), and εFr is the dielectric constant inside the film. By assuming εFr ∼ = 7 (the calculated value for the SAM of 2 Se2 on Au [21,22]), values of d ranging between 88 and 176 Å are obtained from the data in Fig. 5. In fact, the film studied in this work and the SAM of 2 Se2 on Au have a different structure (vide infra) and, so, the respective dielectric constants
Fig. 4. Capacitance–dc potential curves obtained in solutions of 1 × 10−4 M 2 Se2 and 0.1 M Na2 SO4 in 30% DMF–water, buffered with 0.01 M acetate at pH 4.5 (A); 0.01 M phosphate at 7.1 (B), and 0.01 M Tris at pH 9.1 (C). Each curve results from the combination of two ac voltammograms, each of which was obtained using a fresh mercury drop. The drop was exposed to the stirred solution during texp = 30 s under a dc potential Eexp = 0.00 V. After trest = 5 s, the ac voltammogram was recorded from an initial potential Ei = Eexp toward either more negative (with a scan rate v = +4 mV/s) or more positive (with v = −4 mV/s) potentials. Modulation amplitude (r.m.s.), 10 mV; modulation frequency, 75 Hz. Curve 0 in Fig. 4A was obtained in the absence of 2 Se2 and without exposition period; other conditions are the same as in this figure.
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
must be different. Nevertheless, even though the actual εFr were appreciably higher than 7, the calculated values of d largely exceed the thickness of a monolayer of Se–Hg, which should depend on the molecular orientation but cannot be greater than around 7 Å according to the space-filling model. Thus, it is concluded that increasing texp leads to the formation of a progressively thicker multilayer of Se–Hg on the HMDE, provided that the Eexp value is inside the potential range where no electroreduction occurs. Fig. 4B and C reveal that the formation of multilayer films of Se–Hg also occurs in solutions buffered at pH 7.1 and 9.1. In addition, Fig. 4A and B show that the presence of thicker films inhibits totally the electrode reaction of mercury oxidation (which yields the high pseudocapacitance peak in curve 0 of Fig. 4A) in neutral or acidic solutions up to, at least, a potential of +0.90 V. This conclusion was corroborated by means of CV. Nevertheless, Fig. 4C shows that the mercury oxidation reaction is not totally inhibited by the film of Se–Hg in alkaline solutions (under other conditions identical than those in Fig. 4A and B) as reveals the small pseudocapacitance peak appearing at +0.3 V. It should be remarked that all the above results were obtained by performing the exposition step under an applied potential which assures that no electroreduction of 2 Se2 occurs during the exposition (and the rest) period. However, essentially the same results were also found when the HMDE was maintained in open circuit during the exposition and rest periods and the recordings started from an initial potential where 2 Se2 is not reduced. Subsequently, the charge resulting from the electrode reaction was determined by means of CC. These experiments were performed in a solution of 5 × 10−5 M 2 Se2 in the same medium as in Figs. 1–3, using values of texp between 30 and 120 s. The resulting charge versus t1/2 plots exhibit two linear segments as shown in Fig. 6, instead of a single segment resulting from the contribution of dissolved reactant as expected for the reduction of a monolayer at the long
1393
sampling times of this figure [1]. The slopes of the second segment in all the curves of Fig. 6 are virtually coincident and, then, this segment corresponds to the referred diffusion contribution. The charge at the end of the first segments increases with texp and, thus, results from the slow reduction of a progressively thicker film (note that the contribution of double-layer charging should be negligible for the high charges measured at the end of these segments). According to reaction (2), these charges correspond to the reduction of amounts of Se–Hg ranging between 5.3 × 10−9 mol/cm2 (texp = 30 s) and 1.5 × 10−8 mol/cm2 (texp = 120 s). The first value is some greater than that obtained from the integration of the cathodic peaks in voltammogram 1 of Fig. 2, ca. 3 × 10−9 mol/cm2 . At all events, the calculated amounts of adsorbate largely exceed that obtained for the reduction of an adsorbate-saturated monolayer (vide supra). From all the above results, it is concluded that the reduction of a single, Se–Hg-saturated monolayer can only be observed under well defined adsorption conditions (as the exposition conditions and 2 Se2 concentration of voltammogram 1 in Fig. 1); then, peak a, with its maximum peak current, is the only produced. Nevertheless, the interface is not in equilibrium when such a monolayer is formed and, consequently, more and more adsorbate is accumulated onto the mercury surface with increasing texp and/or 2 Se2 concentration. This results in the formation of a progressively thicker multilayer film of Se–Hg, the slow reduction of which is the responsible for the appearance of peaks a, b and c or c in the voltammograms. In this situation, peak a should correspond to the reduction of the first (inner) monolayer in this film. Note that reaction (2) is like those responsible for the electrochemical dissolution of films formed in some anodic electrodeposition processes, where reduction of bulk (outer) layers goes before that reduction of the electrode adjacent-monolayer. Thus, the formation of a Se–Hg-saturated monolayer on the HMDE is followed by the continuous growth of a
Fig. 6. Plots of charge against square root of time obtained from chronocoulometric experiments after an exposition period, texp , of 30 s (1), 90 s (2), and 120 s (3). Measurements were performed in a solution of 5 × 10−5 M 2 Se2 in the same medium as in Fig. 1. Each curve was obtained using a fresh mercury drop with Eexp = −0.25 V; the initial potential (=Eexp ) was applied during the rest period (5 s) and the step potential was −0.90 V. Charges were measured between 0.1 and 5 s at intervals of 0.1 s.
1394
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
multilayer film, meaning that such a monolayer does not prevent the subsequent occurrence of reaction (1). This is attributed to the fact that intermolecular forces were not strong enough as required for the formation of a highly packed monolayer. So, a surface concentration of adsorbate less than 9.4 × 10−11 mol/cm2 implies that a mean electrode area greater than 177 Å2 is occupied by one Se–Hg molecule in the adsorbate-saturated monolayer, and this area largely exceeds those calculated for closed packing Se–Hg molecules according with the space-filling model (even if a parallel orientation of rings relative to the electrode surface is assumed). Consequently, a fraction of the mercury surface is not actually coated by the Se–Hg species when an adsorbate-saturated monolayer is formed and reaction (1) can proceed on the uncovered surface: the resultant molecules can be aggregated due to the hydrophobic interactions between their benzene rings yielding a multilayer film. An aggregation resulting from such interactions has been suggested in the case of films of thiophenol on Au(1 1 1) on the basis of their (featureless) STM images [26]. 3.1.2. Ex situ measurements Each voltammogram shown in Fig. 7 was obtained after a mercury drop, at open circuit potential, was exposed to a solution of 2 Se2 as indicated in the caption of this figure. Then, the coated electrode was introduced into an aqueous solution of 0.1 M Na2 SO4 and 0.01 M acetate buffer at pH 4.5 and, immediately or after a defined time elapsed from the insertion, a cyclic voltammogram was recorded. The sole difference between the voltammogram 1 in Fig. 7 (obtained immediately after the insertion into the 2 Se2 -free solution) and the one obtained in the same solution used to form the chemisorbed film (with texp = 40 s and trest = 15 s) is in peak c , which is some smaller in the first voltammogram. Nevertheless, this peak, as well as peaks a and b, strongly decrease with the time elapsed between their recordings and
Fig. 7. Cyclic voltammograms obtained in ex situ experiments after the coated electrode was in contact with a 2 Se2 -free solution during 0 min (1), 1 min (2), 5 min (3), 10 min (4), and 15 min (5). The voltammograms were recorded in an aqueous solution of 0.1 M Na2 SO4 and 0.01 M acetate buffer at pH 4.5. The initial potential Ei = −0.25 V was applied during 5 s and the scan rate was 100 mV/s. Each voltammogram was obtained using a mercury drop which was previously introduced, in open circuit, into a solution of 5 × 10−5 M 2 Se2 , 0.1 M Na2 SO4 and 0.01 M acetate buffer in 30% DFM-water during 40 s (with stirring of the solution) and other 15 s (without stirring).
the introduction of the electrode into the 2 Se2 -free solution (see Fig. 7). An aqueous solution was used for performing these measurements because of aqueous media are commonly employed for the ex situ characterisation of self-assembled films [2]. When the solution used in the ex situ measurements also contains 30% DMF, essentially the same behaviour shown in Fig. 7 was observed. All these experiments evidence the progressive destruction of the multilayer film when it is put in contact with 2 Se2 -free solutions. 3.2. Effects of the film on the electrochemistry of the Ru(NH3 )6 3+/2+ couple The rate of an outer-sphere electrode reaction decreases with increasing distance between its reaction plane and the electrode surface [27]. Accordingly, a method for controlling the rate of such reactions is based on coating the electrode with various SAMs, the thickness of which are fixed by the molecular length of the respective adsorbate [28]. Although the film studied in this work is never in equilibrium with a solution of 2 Se2 , the thickness of the film can be controlled by changing the adsorption conditions (i.e., the exposition period, the 2 Se2 concentration and/or the stirring conditions). Thus, it should be possible to obtain films with different thickness which can be used for controlling the rate of outer-sphere reactions. From this point of view, the electrode reactions involving the Ru(NH3 )6 3+/2+ couple was studied in a solution of 1 × 10−5 M 2 Se2 and 1 × 10−3 M Ru(NH3 )6 3+ with various exposition periods. The voltammograms 1–4 shown in Fig. 8 were obtained with Ei = Eexp = 0.00 V and using the same texp values as in voltammograms 1–3 and 5 of Fig. 1, respectively. The voltammogram 1 in Fig. 8 was obtained at an electrode coated by a Se–Hg-saturated monolayer film (see voltammogram 1 in Fig. 1). A pair of peaks yielded by the Ru(NH3 )6 3+/2+ couple appears in this voltammogram, both the cathodic and anodic peaks being identical to those obtained at a bare HMDE (i.e., in a 2 Se2 -free solution). This reveals that the respective electrode reactions are not
Fig. 8. Cyclic voltammograms obtained in a solution containing 1×10−3 M Ru(NH3 )6 3+ and 1 × 10−5 M 2 Se2 after an exposition time, texp , of 5 s (1), 15 s (2), 30 s (3), and 150 s (4). The composition of the medium and other conditions are the same as in Fig. 1.
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
inhibited by a (poorly packed) monolayer film. Nevertheless, an inhibition of the electrochemical reduction of Ru(NH3 )6 3+ occurs as soon as a multilayer film is formed, being more strong in proportion as the film growth (compare the voltammograms in Figs. 1 and 8). Surprisingly enough, the electrochemical oxidation of Ru(NH3 )6 2+ is quasi-totally inhibited (within the potential range of Fig. 8) by the multilayer film as soon as it begins to be formed. Obviously, the kinetics of the Ru(NH3 )6 3+ electroreduction, but not that of the Ru(NH3 )6 2+ electrooxidation, can be gradually controlled by means of a simple change in the exposition time. The origin of these “asymmetric” effects remains unclear at the present. It can be speculated that the multilayer could be permeable to both Ru(NH3 )6 2+ and Ru(NH3 )6 3+ species, but the permeability is more restricted for the first ion due to its greater size; then, the charge transfer should occur at different planes during the cathodic and anodic reactions. In this respect, the inhibitory effects on other electrode reactions are being currently studied.
4. Conclusions In protic media, the formation of a Se–Hg-saturated monolayer on the HMDE is followed by the continuous growth of a multilayer film and, thus, such a monolayer does not prevent the subsequent occurrence of reaction (1). This is attributed to the fact that intermolecular forces are not strong enough as required for the formation of a highly packed monolayer; since a fraction of the mercury surface is not actually coated by the Se–Hg species when an adsorbate-saturated monolayer is formed, reaction (1) can proceed on the uncovered surface. The Se–Hg molecules formed through this reaction can be aggregated due to the hydrophobic interactions between benzene rings yielding a multilayer film. The instability of this film in 2 Se2 -free solutions is attributed to the weakness of such interactions. The growth of the Se–Hg multilayer at the HMDE is related to the amount of 2 Se2 transported to the electrode surface during the exposition period. The fact that such a growth was not detected at the DME [16–20] should be attributed to the very small amounts of 2 Se2 that may be transported to the surface of this electrode during its drop time. For this reason, when the electrochemical reduction of 2 Se2 is studied by means of polarographic techniques, the observed behaviour is explained by assuming that only a monolayer of Se–Hg could be formed on the mercury electrode [16–19]. The thickness of the film characterised in this work can be controlled by changing the adsorption conditions.
1395
This allows one a gradual control on the kinetics of the Ru(NH3 )6 3+ electroreduction. Acknowledgements This work was supported by Project PB98-1537 from the Ministerio Educación y Cultura. References [1] A.J. Bard, L.R. Faulkner, Electrochemical Methods. Fundamental and Applications, 2nd ed., Wiley, New York, 2001 (Chapter 14). [2] H.D. Finklea, in: A.J. Bard, I. Rubinstein (Eds.), Electroanalytical Chemistry, vol. 19, Dekker, New York, 1996, p. 109, and references therein. [3] A. Demoz, D.J. Harrison, Langmuir 9 (1993) 1046. [4] I. Turyan, D. Mandler, Anal. Chem. 66 (1994) 58. [5] N. Muskal, I. Turyan, D. Mandler, J. Am. Chem. Soc. 117 (1995) 1147. [6] N. Muskal, I. Turyan, D. Mandler, J. Electroanal. Chem. 409 (1996) 131. [7] K. Slowinski, R.V. Chamberlain, R. Bilewicz, M. Majda, J. Am. Chem. Soc. 118 (1996) 4709. [8] K. Slowinski, R.V. Chamberlain, C.J. Miller, M. Majda, J. Am. Chem. Soc. 119 (1997) 11910. [9] J.M. Sevilla, T. Pineda, R. Madueño, A.J. Román, M. Blázquez, J. Electroanal. Chem. 442 (1998) 107. [10] K.J. Stevenson, M. Mitchell, H.S. White, J. Phys. Chem. B 102 (1998) 1235. [11] K. Slowinski, K.V. Slowinska, M. Majda, J. Phys. Chem. B 103 (1999) 8544. [12] F.T. Buoninsegni, L. Becucci, M.R. Moncelli, R. Guidelli, J. Electroanal. Chem. 500 (2001) 395. [13] Z. González Arias, J.L. Muñiz Álvarez, J.M. López Fonseca, J. Colloid Interface Sci. 250 (2002) 295. [14] Y. K´ım, R.L. McCarley, A.J. Bard, Langmuir 9 (1993) 1941. [15] N. Muskal, D. Mandler, Electrochim. Acta 45 (1999) 537. [16] B. Nygård, Acta Chem. Scand. 15 (1961) 1039. [17] B. Nygård, Acta Chem. Scand. 20 (1966) 1710. [18] B. Nygård, Polarographic investigations of organic diselenides and disulphides, Abstract of Uppsala Dissertation in Science, No. 104, 1967. [19] J. Ludv´ık, B. Nygård, J. Electroanal. Chem. 423 (1997) 1. [20] R. Danielsson, B.L. Johansson, B. Nygård, B. Persson, Chem. Scr. 20 (1982) 19. [21] K. Bandyopadhyay, K. Vijayamohanan, Langmuir 14 (1998) 625. [22] K. Bandyopadhyay, K. Vijayamohanan, M. Venkataramanan, T. Pradeep, Langmuir 15 (1999) 5314. [23] D.C. Grahame, J. Am. Chem. Soc. 71 (1949) 2975. [24] R. de Levie, Chem. Rev. 88 (1988) 599. [25] P. Nikitas, J. Electroanal. Chem. 451 (1998) 249. [26] T. Sawaguchi, F. Mizutani, S. Yoshimoto, I. Taniguchi, Electrochim. Acta 45 (2000) 2861. [27] A.J. Bard, L.R. Faulkner, Electrochemical Methods. Fundamental and Applications, 2nd ed., Wiley, New York, 2001 (Chapter 3). [28] S. Terrettaz, J. Cheng, C.J. Miller, R.D. Guiles, J. Am. Chem. Soc. 118 (1996) 7857.
Chemisorption and electrochemistry of diphenyldiselenide on the hanging mercury drop electrode Jesús L. Muñiz Álvarez, Josefa A. Garc´ıa Calzón, Juan M. López Fonseca∗ Departamento de Qu´ımica F´ısica y Anal´ıtica, Universidad de Oviedo, E-33071 Oviedo, Spain Received 7 May 2003; received in revised form 12 September 2003; accepted 4 November 2003
Abstract The interfacial and electrochemical behaviour of diphenyldiselenide (2 Se2 ) on the hanging mercury drop electrode has been studied by means of cyclic and ac voltammetry and chronocoulometry using 30% dimethylformamide–water media. Both capacitance and Faradaic current measurements reveal that a progressively thicker multilayer film is formed as a result of chemisorption of 2 Se2 at the mercury/solution interface. By controlling the thickness of the film, a gradual inhibition of the electrochemical reduction of Ru(NH3 )6 3+ is achieved. © 2003 Elsevier Ltd. All rights reserved. Keywords: Diphenyldiselenide; Chemisorption; Multilayer films; Cyclic and ac voltammetry; Chronocoulometry
1. Introduction The formation and electrochemical characterisation of films of organic molecules chemisorbed on metal surfaces have been studied extensively. Most of these films are monolayers formed by self-assembly [1,2]. Typical examples of such self-assembled monolayers (SAMs) are those of thiolic (or disulphide) compounds chemisorbed on Au surfaces [1,2]. In addition, chemisorbed monolayers of these compounds on Hg surfaces have been characterised [2–13]. On the other hand, a few studies on multilayer films resulting from chemisorption of thiocompounds on Au or Hg surfaces have been reported [14,15]. Selenoderivatives are other good candidates to form chemisorbed films on Au or Hg substrates. As a matter of fact, the early electrochemical studies performed by Nygård in protic mixtures [16–18] revealed that diphenyldiselenide, 2 Se2 , (as well as all other organic diselenides) is spontaneously chemisorbed on mercury according to: 2 Se2 + Hg → 2Se–Hgads
(1)
This reaction and the subsequent reversible charge transfer reaction (2) were considered to be responsible for the re∗
Corresponding author. Tel.: +34-985-103689; fax: +34-985-103125. E-mail address: [email protected] (J.M. L´opez Fonseca). 0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.11.003
duction of 2 Se2 at both the dropping mercury electrode (DME) and the hanging mercury drop electrode (HMDE) [16–19]. From these reactions, and assuming that a single adsorbate-saturated monolayer is formed at high 2 Se2 concentrations, the dc-, sampled dc-, normal pulse- and ac-polarographic behaviours of 2 Se2 in protic media are successfully explained [19,20]. The cyclic voltammetric behaviour of 2 Se2 at the HMDE in these media was interpreted on the basis of the same assumption [19].
(2) Besides, the self-assembly of 2 Se2 on Au surfaces has been recently reported [21,22]. It was found that the Se–Se bond is preserved upon chemisorption on Au, in contrast with the dissociative chemisorption (reaction (1)) occurring on Hg surfaces. Although the basic scheme for the reduction of organic diselenides at mercury electrodes has been established by Nygård many years ago [16–18], a systematic study about the chemisorbed film formed through reaction (1) has not been reported yet. For this reason, we have performed a detailed investigation of the interfacial and electrochemical behaviour of 2 Se2 at the HMDE in protic media. Cyclic voltammetry (CV), phase-sensitive ac voltammetry (ACV)
1390
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
and chronocoulometry (CC) were used with this purpose. In essence, our results reveal that the chemisorption of 2 Se2 leads to the formation of a multilayer film, whose thickness can be modified by changing the adsorption conditions. The application of this finding for controlling the rate of outer-sphere electrode reactions is also explored.
2. Experimental CV and CC were performed by means of a potentiostat/galvanostat -Autolab type II (from Ecochemie) attached to a personal computer with the proper software and fitted to a 663 VA stand (from Metrohm) which includes a multimode mercury electrode and the reference and auxiliary electrodes. ACV was performed using a Metrohm 693 VA processor fitted to a 694 VA stand, also from Metrohm. Measurements were made at a modulation frequency of 75 Hz and a r.m.s. modulation voltage of 10 mV. The out-of-phase (Φ = 90◦ ) component of the ac current was recorded against dc potential with a scan rate of +4 or −4 mV/s. Values of the out-of-phase component were converted into capacitance values by using the Grahame’s data for the capacitance of Hg in a 0.1 M KCl aqueous solution [23]. A HMDE, a platinum wire, and an Ag/AgCl/KCl 3 M were used as the indicator, the auxiliary and the reference electrode, respectively. The surface areas of the HMDE were 0.54 mm2 in ACV and 0.37 mm2 in CV and CC. The reference electrode was connected with the cell solution through a daily renewed KNO3 -saturated solution. pH measurements were made with a pH-meter Crison 2001. 2 Se2 was supplied by Sigma and other chemicals were Merck. Milli-Q purified water was used for preparing solutions. Stock solutions of 1 × 10−2 M 2 Se2 in dimethylformamide (DMF) were prepared everyday. Most of the electrochemical measurements were performed in a medium of 0.1 M Na2 SO4 and 0.01 M sodium acetate buffer in water containing 30% (v/v) DMF at pH 4.5 (but DMF was removed in the medium used for performing some ex situ measurements, see further). In addition, some experiments were carried out in media in which the acetate buffer was replaced with 0.01 M disodium phosphate (pH 7.1) or TRIS (pH 9.1) buffers. The solutions of 2 Se2 in the above media were purged with oxygen-free nitrogen for 20 min. A fresh mercury drop was exposed to the solutions, which firstly were stirred at 33.33 revolutions/s (unless otherwise stated) for a defined exposition time, texp , and then were maintained without stirring for a rest time, trest . During both periods, either a fixed potential, Eexp , was applied to the electrode or this was maintained in open circuit. Electrochemical measurements were performed in situ (i.e., in the above 2 Se2 solutions) or ex situ (in a 2 Se2 free solution). When a fixed potential was applied in the first type of measurements, the recordings started immediately
after the rest period from an initial potential, Ei , coincident with Eexp ; however, when the electrode was maintained in open circuit during the exposition and rest periods, Ei was maintained during 5 s. Ex situ measurements were performed after maintaining the electrode in open circuit during the exposition and rest periods. Then, the electrode was removed from the solution of 2 Se2 and introduced into a previously deareated 2 Se2 -free solution. The measurements were carried out immediately or after a fixed time was elapsed from the insertion (in the second case, the solution was purged during this interval); in both cases, Ei was maintained during 5 s. All the experiments were carried out at room temperature (23 ± 2 ◦ C).
3. Results and discussion 3.1. Characterisation of the film 3.1.1. In situ measurements The cyclic voltammograms shown in Fig. 1 were recorded in a medium of 0.1 M Na2 SO4 and 0.01 M acetate buffer in 30% DMF–water at pH 4.5, containing 1 × 10−5 M 2 Se2 . These voltammograms were obtained with Eexp = −0.25 V after various exposition times. As seen in this figure, a single cathodic peak (a) at −0.73 V appears during the forward scan for texp = 5 s (curve 1). With increasing texp , a second (b) and a third (c) cathodic peaks are successively developed at less negative potentials (curves 2 and 3). Further increasing in texp leads first to a splitting of peak c in peaks c and c (curve 4) and then to a sharp increase of peak c together with a change in its peak potential towards more negative values (curve 5). Fig. 1 also shows that the peak current increases
Fig. 1. Cyclic voltammograms obtained in a solution of 1 × 10−5 M 2 Se2 , 0.1 M Na2 SO4 and 0.01 M acetate buffer in 30% DMF–water at pH 4.5, after an exposition period, texp , of 5 s (1), 15 s (2), 30 s (3), 90 s (4) and 150 s (5) with stirring of the solution. Rest period, trest = 5 s. Each voltammogram was recorded using a fresh mercury drop to which a potential, Eexp = −0.25 V was applied during the exposition and rest periods. Initial potential, Ei = −0.25 V; scan rate, v = 100 mV/s.
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
Fig. 2. Cyclic voltammograms obtained in solutions of 5 × 10−5 M (1) and 1 × 10−4 M 2 Se2 (2) in the same medium as in Fig. 1. texp = 30 s; other conditions are the same as in Fig. 1.
and the peak potential changes negatively for peak b with increasing texp . In contrast, neither the peak current nor the peak potential for peak a depend on texp . As a consequence of the different effects of texp on peak currents for peaks a, b and c , the peak a is not detected at long values of texp (curve 5). The cyclic voltammograms shown in Fig. 2 were obtained in the above medium containing 5 × 10−5 or 1 × 10−4 M 2 Se2 , with texp fixed in 30 s (curves 1 and 2, respectively). From voltammogram 3 in Fig. 1 and those in Fig. 2, it is seen that the effect of increasing 2 Se2 concentration at constant texp on the electrochemical behaviour of 2 Se2 is similar than that of increasing texp at constant 2 Se2 concentration. Furthermore, peaks b and c also increase continuously with increasing texp at the 2 Se2 concentrations of Fig. 2 and similar, but slower increases, were observed when the solution was not stirred during the exposition period.
1391
On the other hand, strong decreases in peaks b and c (and conversion of c in c) but no changes in peak a were found in the successive potential scans when both texp and the 2 Se2 concentration were fixed. This is illustrated in Fig. 3, where some voltammograms recorded by means of repetitive CV are shown. In addition, the effect of the scan rate, v, on the peak currents, Ip , for peaks a, b and c (in the first potential scan) was studied in a solution of 2 × 10−5 M 2 Se2 in the above medium and with texp = 30 s. In the range assayed (10–100 mV/s), the ratio Ip /v was found to be constant for peaks a and b. For peak c, the relationship Ip /v1/2 was constant at v ≤ 50 mV/s but it increased gradually with increasing v between 50 and 100 mV/s. It was also found that the peak potentials for a, b and c in the voltammograms of Fig. 2 (obtained at pH 4.5) coincide with the respective values in the voltammograms recorded in solutions buffered at pH 7.1 and 9.1, other conditions being the same as in this figure. This reveals that no subsequent protonation of the Se− species—see reaction (2)—occurs at pH ≥ 4.5. Ludv´ık and Nygård have reported on the cyclic voltammetric behaviour of 2 Se2 at the HMDE under experimental conditions rather different than those used in this work (specifically, solutions in 40% ethanol–water at pH 7.4 were used with texp = trest = 0) [19]. Two cathodic peaks similar to those labelled a and b in this work were described in [19] and considered to be an adsorption post-peak and a main (diffusion) peak, respectively; thus, it was assumed that a saturation coverage by the adsorbate has been reached [1]. In addition, a peak like our peak c was observed by Ludv´ık and Nygård at high 2 Se2 concentrations and it was attributed to the effect of a product adsorption. The previously described behaviour of peak a is, in fact, that expected for an adsorption peak in conditions of full
Fig. 3. Voltammograms obtained by means of repetitive CV during the first (1), second (2) and fifth (3) potential scans. These voltammograms were recorded using a single mercury drop in a solution of 7.5 × 10−5 M 2 Se2 in the same medium as in Fig. 1. texp = 30 s; other conditions are the same as in Fig. 1.
1392
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
coverage by a reactant monolayer, according to the theoretical treatments [1]. From reaction (2) and the charge obtained by integration of peak a in voltammogram 1 of Fig. 1 (in which peak a—with its maximum height—is the only developed), a value of 9.4 × 10−11 mol/cm2 was obtained for the surface concentration of Se–Hg in the adsorbate-saturated monolayer. It should be noted that the measured charge includes a contribution of the double-layer charging and, thus, the real surface concentration of the adsorbate should be less than the above value. Nevertheless, a main peak cannot appear at low reactant concentration; it should be independent on texp and its peak current should be proportional to v1/2 , according to [1]. Consequently, both the behaviour of b shown in Fig. 1 and its dependence with v exclude the possibility that peak b could be a main peak. On the other hand, the effect of v on peak c is not that expected for an adsorption peak; in addition, the very low anodic peaks (with respect to the cathodic peaks) observed in the reverse scans of the cyclic voltammograms at high texp and/or high 2 Se2 concentration (see Figs. 1 and 2) reveal that the product of the electrode reaction is not adsorbed at the interface; thus, peaks c or c cannot be caused by a product adsorption. In an attempt to elucidate the origin of peaks b and c or c , the capacitance or pseudocapacitance of the interface were measured by means of ACV. Fig. 4A shows the capacitance–dc potential curve obtained with Eexp = 0.00 V in a solution of 1 × 10−4 M 2 Se2 in the medium at pH 4.5. The overlapped peaks appearing at negative potentials in the presence of 2 Se2 correspond to the cathodic peaks observed in the cyclic voltammograms under similar exposition conditions; so, they are pseudocapacitance peaks including Faradaic contributions. At less negative potentials, a
Fig. 5. Effect of the exposition period, texp , on the capacitance of the pit shown in Fig. 4A. Eexp = 0.00 V.
wide capacitance pit is developed. According to both theoretical and experimental results, the appearance of capacitance pits is due to the formation of either an ordered monolayer [24] or a multilayer film [25]. The capacitance, C, in the pit region of Fig. 4A is as low as 0.7 F/cm2 , under the conditions of this figure, and this capacitance even decreases with increasing texp , as shown in Fig. 5. These C values allow one to estimate the film thickness, d, by means of the Helmholtz model, i.e., by using the equation C = ε0 εFr /d, where ε0 is the permittivity of the vacuum, 8.85 × 10−12 C2 /(N m2 ), and εFr is the dielectric constant inside the film. By assuming εFr ∼ = 7 (the calculated value for the SAM of 2 Se2 on Au [21,22]), values of d ranging between 88 and 176 Å are obtained from the data in Fig. 5. In fact, the film studied in this work and the SAM of 2 Se2 on Au have a different structure (vide infra) and, so, the respective dielectric constants
Fig. 4. Capacitance–dc potential curves obtained in solutions of 1 × 10−4 M 2 Se2 and 0.1 M Na2 SO4 in 30% DMF–water, buffered with 0.01 M acetate at pH 4.5 (A); 0.01 M phosphate at 7.1 (B), and 0.01 M Tris at pH 9.1 (C). Each curve results from the combination of two ac voltammograms, each of which was obtained using a fresh mercury drop. The drop was exposed to the stirred solution during texp = 30 s under a dc potential Eexp = 0.00 V. After trest = 5 s, the ac voltammogram was recorded from an initial potential Ei = Eexp toward either more negative (with a scan rate v = +4 mV/s) or more positive (with v = −4 mV/s) potentials. Modulation amplitude (r.m.s.), 10 mV; modulation frequency, 75 Hz. Curve 0 in Fig. 4A was obtained in the absence of 2 Se2 and without exposition period; other conditions are the same as in this figure.
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
must be different. Nevertheless, even though the actual εFr were appreciably higher than 7, the calculated values of d largely exceed the thickness of a monolayer of Se–Hg, which should depend on the molecular orientation but cannot be greater than around 7 Å according to the space-filling model. Thus, it is concluded that increasing texp leads to the formation of a progressively thicker multilayer of Se–Hg on the HMDE, provided that the Eexp value is inside the potential range where no electroreduction occurs. Fig. 4B and C reveal that the formation of multilayer films of Se–Hg also occurs in solutions buffered at pH 7.1 and 9.1. In addition, Fig. 4A and B show that the presence of thicker films inhibits totally the electrode reaction of mercury oxidation (which yields the high pseudocapacitance peak in curve 0 of Fig. 4A) in neutral or acidic solutions up to, at least, a potential of +0.90 V. This conclusion was corroborated by means of CV. Nevertheless, Fig. 4C shows that the mercury oxidation reaction is not totally inhibited by the film of Se–Hg in alkaline solutions (under other conditions identical than those in Fig. 4A and B) as reveals the small pseudocapacitance peak appearing at +0.3 V. It should be remarked that all the above results were obtained by performing the exposition step under an applied potential which assures that no electroreduction of 2 Se2 occurs during the exposition (and the rest) period. However, essentially the same results were also found when the HMDE was maintained in open circuit during the exposition and rest periods and the recordings started from an initial potential where 2 Se2 is not reduced. Subsequently, the charge resulting from the electrode reaction was determined by means of CC. These experiments were performed in a solution of 5 × 10−5 M 2 Se2 in the same medium as in Figs. 1–3, using values of texp between 30 and 120 s. The resulting charge versus t1/2 plots exhibit two linear segments as shown in Fig. 6, instead of a single segment resulting from the contribution of dissolved reactant as expected for the reduction of a monolayer at the long
1393
sampling times of this figure [1]. The slopes of the second segment in all the curves of Fig. 6 are virtually coincident and, then, this segment corresponds to the referred diffusion contribution. The charge at the end of the first segments increases with texp and, thus, results from the slow reduction of a progressively thicker film (note that the contribution of double-layer charging should be negligible for the high charges measured at the end of these segments). According to reaction (2), these charges correspond to the reduction of amounts of Se–Hg ranging between 5.3 × 10−9 mol/cm2 (texp = 30 s) and 1.5 × 10−8 mol/cm2 (texp = 120 s). The first value is some greater than that obtained from the integration of the cathodic peaks in voltammogram 1 of Fig. 2, ca. 3 × 10−9 mol/cm2 . At all events, the calculated amounts of adsorbate largely exceed that obtained for the reduction of an adsorbate-saturated monolayer (vide supra). From all the above results, it is concluded that the reduction of a single, Se–Hg-saturated monolayer can only be observed under well defined adsorption conditions (as the exposition conditions and 2 Se2 concentration of voltammogram 1 in Fig. 1); then, peak a, with its maximum peak current, is the only produced. Nevertheless, the interface is not in equilibrium when such a monolayer is formed and, consequently, more and more adsorbate is accumulated onto the mercury surface with increasing texp and/or 2 Se2 concentration. This results in the formation of a progressively thicker multilayer film of Se–Hg, the slow reduction of which is the responsible for the appearance of peaks a, b and c or c in the voltammograms. In this situation, peak a should correspond to the reduction of the first (inner) monolayer in this film. Note that reaction (2) is like those responsible for the electrochemical dissolution of films formed in some anodic electrodeposition processes, where reduction of bulk (outer) layers goes before that reduction of the electrode adjacent-monolayer. Thus, the formation of a Se–Hg-saturated monolayer on the HMDE is followed by the continuous growth of a
Fig. 6. Plots of charge against square root of time obtained from chronocoulometric experiments after an exposition period, texp , of 30 s (1), 90 s (2), and 120 s (3). Measurements were performed in a solution of 5 × 10−5 M 2 Se2 in the same medium as in Fig. 1. Each curve was obtained using a fresh mercury drop with Eexp = −0.25 V; the initial potential (=Eexp ) was applied during the rest period (5 s) and the step potential was −0.90 V. Charges were measured between 0.1 and 5 s at intervals of 0.1 s.
1394
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
multilayer film, meaning that such a monolayer does not prevent the subsequent occurrence of reaction (1). This is attributed to the fact that intermolecular forces were not strong enough as required for the formation of a highly packed monolayer. So, a surface concentration of adsorbate less than 9.4 × 10−11 mol/cm2 implies that a mean electrode area greater than 177 Å2 is occupied by one Se–Hg molecule in the adsorbate-saturated monolayer, and this area largely exceeds those calculated for closed packing Se–Hg molecules according with the space-filling model (even if a parallel orientation of rings relative to the electrode surface is assumed). Consequently, a fraction of the mercury surface is not actually coated by the Se–Hg species when an adsorbate-saturated monolayer is formed and reaction (1) can proceed on the uncovered surface: the resultant molecules can be aggregated due to the hydrophobic interactions between their benzene rings yielding a multilayer film. An aggregation resulting from such interactions has been suggested in the case of films of thiophenol on Au(1 1 1) on the basis of their (featureless) STM images [26]. 3.1.2. Ex situ measurements Each voltammogram shown in Fig. 7 was obtained after a mercury drop, at open circuit potential, was exposed to a solution of 2 Se2 as indicated in the caption of this figure. Then, the coated electrode was introduced into an aqueous solution of 0.1 M Na2 SO4 and 0.01 M acetate buffer at pH 4.5 and, immediately or after a defined time elapsed from the insertion, a cyclic voltammogram was recorded. The sole difference between the voltammogram 1 in Fig. 7 (obtained immediately after the insertion into the 2 Se2 -free solution) and the one obtained in the same solution used to form the chemisorbed film (with texp = 40 s and trest = 15 s) is in peak c , which is some smaller in the first voltammogram. Nevertheless, this peak, as well as peaks a and b, strongly decrease with the time elapsed between their recordings and
Fig. 7. Cyclic voltammograms obtained in ex situ experiments after the coated electrode was in contact with a 2 Se2 -free solution during 0 min (1), 1 min (2), 5 min (3), 10 min (4), and 15 min (5). The voltammograms were recorded in an aqueous solution of 0.1 M Na2 SO4 and 0.01 M acetate buffer at pH 4.5. The initial potential Ei = −0.25 V was applied during 5 s and the scan rate was 100 mV/s. Each voltammogram was obtained using a mercury drop which was previously introduced, in open circuit, into a solution of 5 × 10−5 M 2 Se2 , 0.1 M Na2 SO4 and 0.01 M acetate buffer in 30% DFM-water during 40 s (with stirring of the solution) and other 15 s (without stirring).
the introduction of the electrode into the 2 Se2 -free solution (see Fig. 7). An aqueous solution was used for performing these measurements because of aqueous media are commonly employed for the ex situ characterisation of self-assembled films [2]. When the solution used in the ex situ measurements also contains 30% DMF, essentially the same behaviour shown in Fig. 7 was observed. All these experiments evidence the progressive destruction of the multilayer film when it is put in contact with 2 Se2 -free solutions. 3.2. Effects of the film on the electrochemistry of the Ru(NH3 )6 3+/2+ couple The rate of an outer-sphere electrode reaction decreases with increasing distance between its reaction plane and the electrode surface [27]. Accordingly, a method for controlling the rate of such reactions is based on coating the electrode with various SAMs, the thickness of which are fixed by the molecular length of the respective adsorbate [28]. Although the film studied in this work is never in equilibrium with a solution of 2 Se2 , the thickness of the film can be controlled by changing the adsorption conditions (i.e., the exposition period, the 2 Se2 concentration and/or the stirring conditions). Thus, it should be possible to obtain films with different thickness which can be used for controlling the rate of outer-sphere reactions. From this point of view, the electrode reactions involving the Ru(NH3 )6 3+/2+ couple was studied in a solution of 1 × 10−5 M 2 Se2 and 1 × 10−3 M Ru(NH3 )6 3+ with various exposition periods. The voltammograms 1–4 shown in Fig. 8 were obtained with Ei = Eexp = 0.00 V and using the same texp values as in voltammograms 1–3 and 5 of Fig. 1, respectively. The voltammogram 1 in Fig. 8 was obtained at an electrode coated by a Se–Hg-saturated monolayer film (see voltammogram 1 in Fig. 1). A pair of peaks yielded by the Ru(NH3 )6 3+/2+ couple appears in this voltammogram, both the cathodic and anodic peaks being identical to those obtained at a bare HMDE (i.e., in a 2 Se2 -free solution). This reveals that the respective electrode reactions are not
Fig. 8. Cyclic voltammograms obtained in a solution containing 1×10−3 M Ru(NH3 )6 3+ and 1 × 10−5 M 2 Se2 after an exposition time, texp , of 5 s (1), 15 s (2), 30 s (3), and 150 s (4). The composition of the medium and other conditions are the same as in Fig. 1.
´ J.L. Muñiz Alvarez et al. / Electrochimica Acta 49 (2004) 1389–1395
inhibited by a (poorly packed) monolayer film. Nevertheless, an inhibition of the electrochemical reduction of Ru(NH3 )6 3+ occurs as soon as a multilayer film is formed, being more strong in proportion as the film growth (compare the voltammograms in Figs. 1 and 8). Surprisingly enough, the electrochemical oxidation of Ru(NH3 )6 2+ is quasi-totally inhibited (within the potential range of Fig. 8) by the multilayer film as soon as it begins to be formed. Obviously, the kinetics of the Ru(NH3 )6 3+ electroreduction, but not that of the Ru(NH3 )6 2+ electrooxidation, can be gradually controlled by means of a simple change in the exposition time. The origin of these “asymmetric” effects remains unclear at the present. It can be speculated that the multilayer could be permeable to both Ru(NH3 )6 2+ and Ru(NH3 )6 3+ species, but the permeability is more restricted for the first ion due to its greater size; then, the charge transfer should occur at different planes during the cathodic and anodic reactions. In this respect, the inhibitory effects on other electrode reactions are being currently studied.
4. Conclusions In protic media, the formation of a Se–Hg-saturated monolayer on the HMDE is followed by the continuous growth of a multilayer film and, thus, such a monolayer does not prevent the subsequent occurrence of reaction (1). This is attributed to the fact that intermolecular forces are not strong enough as required for the formation of a highly packed monolayer; since a fraction of the mercury surface is not actually coated by the Se–Hg species when an adsorbate-saturated monolayer is formed, reaction (1) can proceed on the uncovered surface. The Se–Hg molecules formed through this reaction can be aggregated due to the hydrophobic interactions between benzene rings yielding a multilayer film. The instability of this film in 2 Se2 -free solutions is attributed to the weakness of such interactions. The growth of the Se–Hg multilayer at the HMDE is related to the amount of 2 Se2 transported to the electrode surface during the exposition period. The fact that such a growth was not detected at the DME [16–20] should be attributed to the very small amounts of 2 Se2 that may be transported to the surface of this electrode during its drop time. For this reason, when the electrochemical reduction of 2 Se2 is studied by means of polarographic techniques, the observed behaviour is explained by assuming that only a monolayer of Se–Hg could be formed on the mercury electrode [16–19]. The thickness of the film characterised in this work can be controlled by changing the adsorption conditions.
1395
This allows one a gradual control on the kinetics of the Ru(NH3 )6 3+ electroreduction. Acknowledgements This work was supported by Project PB98-1537 from the Ministerio Educación y Cultura. References [1] A.J. Bard, L.R. Faulkner, Electrochemical Methods. Fundamental and Applications, 2nd ed., Wiley, New York, 2001 (Chapter 14). [2] H.D. Finklea, in: A.J. Bard, I. Rubinstein (Eds.), Electroanalytical Chemistry, vol. 19, Dekker, New York, 1996, p. 109, and references therein. [3] A. Demoz, D.J. Harrison, Langmuir 9 (1993) 1046. [4] I. Turyan, D. Mandler, Anal. Chem. 66 (1994) 58. [5] N. Muskal, I. Turyan, D. Mandler, J. Am. Chem. Soc. 117 (1995) 1147. [6] N. Muskal, I. Turyan, D. Mandler, J. Electroanal. Chem. 409 (1996) 131. [7] K. Slowinski, R.V. Chamberlain, R. Bilewicz, M. Majda, J. Am. Chem. Soc. 118 (1996) 4709. [8] K. Slowinski, R.V. Chamberlain, C.J. Miller, M. Majda, J. Am. Chem. Soc. 119 (1997) 11910. [9] J.M. Sevilla, T. Pineda, R. Madueño, A.J. Román, M. Blázquez, J. Electroanal. Chem. 442 (1998) 107. [10] K.J. Stevenson, M. Mitchell, H.S. White, J. Phys. Chem. B 102 (1998) 1235. [11] K. Slowinski, K.V. Slowinska, M. Majda, J. Phys. Chem. B 103 (1999) 8544. [12] F.T. Buoninsegni, L. Becucci, M.R. Moncelli, R. Guidelli, J. Electroanal. Chem. 500 (2001) 395. [13] Z. González Arias, J.L. Muñiz Álvarez, J.M. López Fonseca, J. Colloid Interface Sci. 250 (2002) 295. [14] Y. K´ım, R.L. McCarley, A.J. Bard, Langmuir 9 (1993) 1941. [15] N. Muskal, D. Mandler, Electrochim. Acta 45 (1999) 537. [16] B. Nygård, Acta Chem. Scand. 15 (1961) 1039. [17] B. Nygård, Acta Chem. Scand. 20 (1966) 1710. [18] B. Nygård, Polarographic investigations of organic diselenides and disulphides, Abstract of Uppsala Dissertation in Science, No. 104, 1967. [19] J. Ludv´ık, B. Nygård, J. Electroanal. Chem. 423 (1997) 1. [20] R. Danielsson, B.L. Johansson, B. Nygård, B. Persson, Chem. Scr. 20 (1982) 19. [21] K. Bandyopadhyay, K. Vijayamohanan, Langmuir 14 (1998) 625. [22] K. Bandyopadhyay, K. Vijayamohanan, M. Venkataramanan, T. Pradeep, Langmuir 15 (1999) 5314. [23] D.C. Grahame, J. Am. Chem. Soc. 71 (1949) 2975. [24] R. de Levie, Chem. Rev. 88 (1988) 599. [25] P. Nikitas, J. Electroanal. Chem. 451 (1998) 249. [26] T. Sawaguchi, F. Mizutani, S. Yoshimoto, I. Taniguchi, Electrochim. Acta 45 (2000) 2861. [27] A.J. Bard, L.R. Faulkner, Electrochemical Methods. Fundamental and Applications, 2nd ed., Wiley, New York, 2001 (Chapter 3). [28] S. Terrettaz, J. Cheng, C.J. Miller, R.D. Guiles, J. Am. Chem. Soc. 118 (1996) 7857.