The underpotential deposition of cadmium on Pt(1 1 1): effect of the anions and CO displacement experiments

Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156 www.elsevier.com/locate/jelechem

The underpotential deposition of cadmium on Pt(1 1 1): effect of the anions and CO displacement experiments Roberto Go´mez, Juan M. Feliu * Departament de Quı´mica Fı´sica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain Received 21 October 2002; received in revised form 29 January 2003; accepted 11 February 2003 Dedicated to the memory of Mike Weaver, a friend and a respected scientist

Abstract The underpotential deposition (upd) of cadmium on Pt(1 1 1) electrodes in acidic solutions has been investigated. The formation of two upd layers prior to the onset of bulk deposition has been shown. Voltammetric and charge displacement experiments (as CO can displace Cd adatoms) allow an estimation of the Cd coverage corresponding to the upd layers. The first layer is relatively open (ucd :/0.31), probably because of important Cd
/Cd repulsion, whereas the second is more compact. Charge displacement experiments also provide an estimate for the potential of zero charge of the Cd/Pt(1 1 1) electrode, which, as expected, lies between those of Pt(1 1 1) and Cd(0 0 0 1) electrodes. Finally, the effect of chloride and bromide on the upd process has been studied. Both anions lead to an extraordinary sharpening of the voltammetric peaks, which results from the existence of strong attractive Cd
/ halide interactions at the electrode surface. In the case of bromide the main upd peak splits depending on cadmium and bromide analytical concentrations. This unusual behavior is thought to be caused by the separate adsorption of Cd2 and CdBr  species. The adsorption of CdBr  would occur at more positive potentials probably induced by the presence on the electrode surface of preadsorbed bromide. The limitations of coulometry (either from voltammetry or from charge displacement experiments) for providing values of upd adatom coverage are discussed. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Underpotential deposition; Pt(1 1 1); Cd adatoms; Anion adsorption; Potential of zero charge; Charge displacement

1. Introduction The underpotential deposition (upd) of metals on single crystal electrodes has been widely studied in the past three decades [1,2]. This research has been fueled by the ease of preparation, in this way, of monolayers of metals on metals, which could eventually be employed for solving scientific and technical problems, for instance in electrocatalysis [3]. However, most of these studies have focused on gold single crystals and, specifically, with copper and silver adatoms. In compar-

* Corresponding author. Tel.: /34-96-590-9301; fax: /34-96-5903537. E-mail address: [email protected] (J.M. Feliu).

ison, the effort devoted to platinum single crystals is modest. This is probably due to the fact that the proper preparation and use of these samples was accepted by the scientific community more recently than in the case of gold single crystals. In addition, the upd of metals on platinum is complicated by a stronger adsorption of anions and a greater tendency to irreversible adatom adsorption. Among the systems that have been studied, we find the ubiquitous case of the upd of copper on the three platinum basal planes (see for example Refs. [4
/6]) as well as on several stepped surfaces (see for example Ref. [7]). There also exist several publications on the upd of silver on the three Pt basal planes [8
/10] and a number of recent studies on the deposition of lead on Pt(1 0 0) [11,12] and, especially, on Pt(1 1 1) [13
/15]. Publications

0022-0728/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(03)00135-9

146

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

dealing with other adatoms have appeared more sporadically. To our knowledge the systems studied include: Zn on Pt(h k l) electrodes [16
/19], Tl on Pt(1 1 1)[20
/ 23] and Cd on Pt(1 1 1) [24]. One of the aspects that has attracted an increasing interest is the role that anions play in the kinetics and thermodynamics of the deposition process [4,7,11,17,18,21]. A number of years ago, we showed that the voltammetry of platinum single crystals shows currents directly linked to the adsorption of anions, which points to the exchange of charge between the metal substrate and the adsorbed anion [25]. The effect of anions has been studied in a detailed way primarily in one case, which can be considered as the model system on platinum: Cu on Pt(1 1 1) [26
/33]. Some authors [27
/31] accept that the mechanism for deposition could be schematized as follows. After the appearance of a preadsorbed state, the initial adsorption of copper would give rise to a mixed CuAn (An  /anion) adlayer which, in turn, during a second stage of the deposition process would become a copper monolayer covered by an anion adlayer. This mechanism has been proposed to explain experimental data about Cu upd on Pt(1 1 1) in the presence of bisulfate, chloride or bromide. Obviously, an extension of these studies to other upd systems is of interest. Recently, the effect of anions on Zn upd at Pt(1 1 1) electrodes has also been analyzed in some detail [17]. In addition to the intrinsic interest of this type of investigation, the study of the upd of cadmium constitutes a first stage in the electrosynthesis of semiconducting compounds such as CdS, CdSe and CdTe on well-defined metallic substrates by electrochemical atomic layer epitaxy (ECALE) [34]. This electrochemical methodology may be used to prepare semiconducting thin films of high quality given that the deposition is done atomic layer by atomic layer. While this approach has been exploited using gold [35
/37] and silver [38,39] single crystal electrodes as substrates, it is largely unexplored in the case of platinum. The upd of cadmium on monocrystalline platinum surfaces, specifically on Pt(1 1 1), has been studied by Wieckowski and co-workers [24]. In their paper, some voltammetric profiles are given but the main aim was to follow the adsorption of bisulfate on cadmium adatoms by using radiotracers. It was found that the adsorption of bisulfate on the Cd adlayer takes place at potentials significantly lower than those characteristic of the bare Pt(1 1 1) surface. This was explained on the basis of the work function, which is lower for cadmium than for platinum. In this study, the cadmium coverage was also determined by assuming that each adatom exchanged two electrons during its adsorption. The resulting value was low, not attaining half of the maximum theoretical coverage (corresponding to an hexagonal incommensurate adlayer). The final conclusion was that the deter-

mination of coverages based on coulometry is not immediate and that other faradaic and non-faradaic processes could be involved in the deposition and stripping of the adlayer. In a more general vein, the problem of coverage determination for some stripping processes has been studied in some detail previously, specifically for CO and NO adlayers [40,41]. It has been recognized that the re-establishment of the double layer upon desorption of the adlayer involves a significant amount of charge that needs to be subtracted from the raw stripping charge. One of the key concepts necessary to make the corrections is the total charge, which is available at any desired potential in the low potential adsorption region by means of charge displacement experiments [41]. In this paper, we present new results on cadmium upd on Pt(1 1 1), both in the presence and the absence of chloride and bromide. In addition to a classical voltammetric study of the system, we have performed several displacement experiments of the Cd adlayer with CO and discuss the ensemble of data so as to obtain more detailed information on the Cd adlayer.

2. Experimental The electrodes employed in this study were prepared essentially following the technique developed by Clavilier et al. [42]. Before each experiment, all the electrodes were subjected to a high-temperature flame treatment and then cooled either in air or in a hydrogen/argon atmosphere. Next, the electrodes were quenched in ultrapure water saturated with the corresponding gases. This procedure gives rise to reproducible and welldefined voltammetric responses, which witnesses the quality of the crystals and the cleanliness of working solutions. Potentials were measured against and are quoted versus the reversible hydrogen reference electrode (RHE). All the experiments were performed at room temperature. Test solutions were prepared from concentrated H2SO4 or HClO4 (Merck Suprapur) using ultrapure water (Millipore MilliQ water) and degassed by bubbling Ar through the solution for at least 20 min. Cadmium was deposited from solutions prepared with 3CdSO4 ×/8H2O (Merck pro analysi) or cadmium carbonate (Aldrich) dissolved in the test electrolyte. Chloride and bromide solutions were prepared from NaCl and KBr (both Merck Suprapur). The procedure employed for the CO charge displacement experiments has been described in detail elsewhere [25,41]. Carbon monoxide (N47) was obtained from Air Liquide.

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

3. Results 3.1. Voltammetric experiments Fig. 1 shows the voltammetric profiles obtained for a Pt(1 1 1) electrode in 0.5 M H2SO4 in the presence of two different concentrations of Cd2: 103 M (solid line) and 102 M (dashed line) at a sweep rate of 5 mV s 1. The response is characterized by an important blockage of the adsorption processes (of hydrogen and anions) that usually take place in the test solution (see inset in Fig. 1). On the other hand, it is remarkable that both the adsorption and the stripping profiles show two peaks. For 1 mM concentration the peaks are located at 0.535 and 0.550 V in the positive-going sweep and at 0.515 and 0.545 V in the negative-going one. Under these experimental conditions, bulk cadmium deposition would be expected to take place around the corresponding equilibrium potential, /0.45 V RHE. In this way the underpotential shift is as high as 1 V. It is worth pointing out that the deposition/stripping process occurs at potentials more positive than those of (bi)sulfate adsorption on the bare Pt(1 1 1) electrode. It is noteworthy that the upd does not involve disruption of the long range order of the substrate. Once the cadmium adatoms have been desorbed the voltammetric profile shows the features characteristic of a well-ordered Pt(1 1 1) electrode, particularly the pair

Fig. 1. Voltammograms at 5 mV s 1 for Pt(1 1 1) electrodes in 0.5 M H2SO4 electrolyte. (Solid line) 1 mM Cd2 ; thin solid line corresponds to the baseline for integration (dashed line) 10 mM Cd2 . (Inset) Voltammogram for a Pt(1 1 1) electrode in 0.5 M H2SO4 at 50 mV s 1. Arrows indicate one of the voltammetric signatures for a well-defined and clean Pt(1 1 1) electrode in 0.5 M H2SO4.

147

of peaks located at around 0.7 V. This is apparent in the voltammogram corresponding to 1 mM Cd2 concentration (solid line in Fig. 1). In addition, the voltammetric profile remains stable upon successive cycling. As observed, an increase in the concentration of the depositing cations leads to a shift of the peaks toward more positive values, as expected. The charge integrated under the upd peaks is not very high. If we take the apparent baseline (see Fig. 1), the charge attained is just 151 mC cm 2. This indicates that whatever the corrections to be made in order to obtain the Cd coverage, its maximum value is relatively low. The experiments shown in Fig. 1 could cast some doubts on the capability of cadmium to block the platinum adsorption sites. First, the (pseudo)capacitive currents are quite important in the potential range between 0.06 and 0.50 V and second, the charge densities implied in the stripping of the adlayer are lower than for other adatoms. In order to shed light on this particular point, the window was opened toward more negative values. In this way we were able to check to what extent the hydrogen evolution reaction (her) was impeded by the presence of the electrodeposited layer (Fig. 2). The dashed line corresponds to a blank voltammogram in the absence of cadmium, whereas the solid line was obtained in a 103 M Cd2 solution.

Fig. 2. Voltammograms at 50 mV s 1 for Pt(1 1 1) electrodes. (Solid line) 0.5 M H2SO4/1 mM Cd2 ; thin solid line corresponds to the baseline for integration. (Dashed line) 0.5 M H2SO4. (Inset) Detail of the voltammogram recorded in 0.5 M H2SO4/1 mM Cd2 at 20 mV s 1. Peaks a and b correspond to the stripping of the second and first upd layer, respectively.

148

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

As observed, the inhibition of the her is very large. The hydrogen evolution appears at overpotentials of around /300 mV. In fact, the hindrance is much more important than that found for some irreversibly adsorbed adatoms previously studied such as As [43] and Bi [44] or even for the adsorption of a CO monolayer [45]. The her voltammetric response consists of broad negative peaks centered at /0.40 and /0.25 V in the negative and positive-going sweeps, respectively. The anodic feature at 0.3 V, appearing after the excursion to negative potentials, corresponds to the oxidation of the evolved H2. Stirring of the working solution prevents its appearance. This would not be the case if Cd deposition were responsible for the broad cathodic features. In addition, it appears that a second layer develops before the onset of the massive deposition process. The inset in Fig. 2 shows a voltammogram obtained under the same conditions at 20 mV s 1. Deposition/stripping peaks for the second upd layer appear at /0.47 and /0.40 V, respectively, i.e. at potentials slightly more positive than those corresponding to bulk deposition/ stripping. Bulk deposition of cadmium leads to the large currents at /0.49 V in the inset of Fig. 2. The underpotential shift in this case amounts to around 40 mV only. It is interesting to compare the charges under the stripping peaks corresponding to the first and second cadmium layers (peaks b and a in Fig. 2, respectively). Whereas peak b integrates to 150 mC cm 2, peak a integrates to more than 400 mC cm 2, which implies that the second layer is much more compact than the first. The evaluation of the charge under peak a is based on an apparent horizontal baseline (see figure). A more precise value is not available since the stripping process signal overlaps hydrogen evolution currents to some extent. In any case an approximate value can be given confidently because of the sharpness of the cadmiumstripping peak, which contrasts with the broad feature associated with hydrogen reduction on the Pt(1 1 1) surface covered by the first Cd upd layer. Let us see the effect of more strongly adsorbed anions, namely chloride and bromide, on the upd process. We employ perchloric acid as a supporting electrolyte to which we add different amounts of NaCl and KBr. Fig. 3 shows the voltammetric profiles for a 102 M Cd2 solution and two different concentrations of NaCl: 10 2 M (solid line) and 10 3 M (broken line). As observed, an extraordinarily sharp peak and a broad voltammetric contribution at potentials slightly more positive, characterize the profiles. By comparison with a voltammogram recorded under the same conditions in the absence of cadmium (see inset in Fig. 3), it can be stated that the broad contribution is linked to the desorption/adsorption of chloride on the platinum basal plane. The sharp peaks for cadmium upd appear at potentials slightly more negative than for sulfuric acid electrolytes and there is a shift toward more negative values as the

Fig. 3. Voltammograms at 5 mV s 1 on Pt(1 1 1) in 0.1 M HClO4/1 mM Cd2 (solid line) 10 mM NaCl; thin solid line corresponds to the baseline for integration. (Dashed line) 1 mM NaCl. (Inset) Voltammogram recorded for a Pt(1 1 1) electrode in 0.1 M HClO4/10 mM NaCl at 50 mV s 1. Arrows indicate the same pseudocapacitive process.

chloride concentration is increased. The actual potentials for the stripping process are 0.535 V for an anion concentration of 1 mM and 0.525 V if the anion concentration is increased up to 10 mM. The charge density integrated in this case amounts to 162 mC cm 2, a value slightly larger than that found in sulfuric acid. The addition of bromide triggers more important changes in the voltammetric profile. Fig. 4 depicts the voltammograms obtained at a sweep rate of 5 mV s 1 corresponding to 10 mM Br  solution for two different concentrations of cadmium (1 and 10 mM). If we compare the peak potential in the presence of bromide with the cases of chloride or (bi)sulfate, we observe a more important shift toward negative values as expected from the strong interaction of bromide with Pt(1 1 1). More strikingly, the upd process is associated now with two extremely sharp voltammetric peaks. This is in contrast with the behavior observed in the case of chloride, where a single pair of peaks is observed in all cases. An increase in Cd2 concentration leads to the commonly observed shift of the upd peaks toward more positive values (26 and 31 mV dec 1 for the couples at less and more positive potentials, respectively). It is remarkable that for a relatively high sweep rate (5 mV s 1), the degree of reversibility is high as is deduced from the symmetry of the voltammograms: in the case of the 10 mM Cd2/10 mM Br  solution, peak potentials are Ep,a /0.410 and Ep,c /0.405 V for the couple at less positive potentials and Ep,a /Ep,c /0.425 V for that

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

Fig. 4. Voltammograms at 5 mV s 1 on Pt(1 1 1) in 0.1 M HClO4/10 mM KBr. (Solid line) 10 mM Cd2 ; (dashed line) 1 mM Cd2 .

at more positive potentials. Fig. 5 was obtained at a sweep rate of 50 mV s 1 for a Pt(1 1 1) electrode in contact with a solution containing 10 mM KBr and 10 mM Cd2. The shape of the voltammogram is evidence that the first step in the cadmium deposition process is a fast process (the difference of potentials for the positive

149

and negative-going peaks is only 10 mV at 50 mV s 1). In Fig. 5 the voltammogram obtained under the same conditions but in the absence of cadmium is also shown (dashed line). From the comparison of both voltammetric profiles, the total blockage of the platinum adsorption processes in the presence of Cd is evident. More importantly, the upd process occurs at potentials where bromine adatoms are stable at the Pt(1 1 1) surface. Thus, it is not controlled by the desorption of bromide from the Pt(1 1 1) surface. This behavior is in contrast with what has been observed in the presence of chloride. One of the most interesting aspects of this system is revealed in Fig. 6. The positive-going sweeps for Cd upd are presented for two different bromide concentrations, 1 mM (curve a) and 10 mM (curve b), in the presence of 10 mM Cd2 concentration. As observed the morphology of the voltammetric profile corresponding to the Cd stripping depends very much on the ratio [Cd2]/[Br ] (analytical concentrations). Obviously, an increase in the initial bromide concentration (not merely the introduction of the anion) leads to the splitting of the stripping peak. The same behavior is observed for 1 mM Cd2 when the bromide concentration is changed from 1 to 10 mM. In any case, the charge density under the peak(s) does not depend in a significant way on this particular detail, scaling to 180 mC cm 2.

3.2. Charge displacement experiments Results corresponding to the displacement experiments by potentiostatic adsorption of CO are shown in

Fig. 5. Voltammograms at 50 mV s 1 on Pt(1 1 1) in 0.1 M HClO4/ 10 mM KBr in the presence (solid line) and the absence (dashed line) of 10 mM Cd2 .

Fig. 6. Positive-going potential scans corresponding to a Pt(1 1 1) electrode in contact with 0.1 M HClO4/10 mM Cd2 solution. (Curve a ): with 10 mM KBr; (curve b ) with 1 mM KBr. Sweep rate: 5 mV s 1.

150

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

Fig. 7. Current vs time curves corresponding to charge displacement experiments by potentiostatic CO adsorption. Arrows indicate either the instant when CO is admitted into the cell or an increase in the CO flow.

Fig. 7 for a 1 mM Cd2 in 0.5 M H2SO4 working solution. Displacements have been performed at two different potentials: 0.10 and 0.40 V. In both cases, the displaced currents are positive, corresponding to a net oxidation process since the displacement is dominated by the desorption of cadmium adatoms. This result is in stark contrast with those obtained in the absence of cadmium, where a negative current systematically developed at 0.40 V [41], given that only adsorbed anions are displaced in that case. This confirms once again that the CO molecule acts as a neutral probe, and that the experimental results respond faithfully to the composition of the interfacial region. However, the displaced charge density depends on potential: the value obtained for 0.10 V amounts to 117 mC cm 2, whereas that corresponding to 0.40 V scales to 72 mC cm 2. This change can be explained on the basis of the voltammogram shown in Fig. 1, where the existence of a significant charge exchange between 0.10 and 0.40 V is observed. The displacement kinetics are relatively fast, although there is an effect of the potential at which the displacement is performed. The rate of displacement is significantly higher in the experiment done at 0.40 V. Fig. 8 shows the voltammetric profile corresponding to the oxidation of the saturated CO adlayer formed during a previous displacement experiment. The subsequent recovery of the typical profile for the adsorption/desorption of cadmium adatoms is also shown. It is clear from these profiles that the displacement of the metallic deposit by CO is total and that the Pt(1 1 1) structure does not show signals of disordering as a consequence of the Cd displacement and subsequent oxidation of CO. Moreover, the CO stripping charge coincides (within the experimental error) with the value obtained in similar experiments done in the absence of cadmium [41].

Fig. 8. Voltammetric profile corresponding to the stripping of the CO adlayer resulting from the displacement experiment. (Dashed line): second positive-going sweep. Sweep rate: 50 mV s 1.

4. Discussion 4.1. Apparent charge transfer Let us describe from a thermodynamic point of view the process of upd including the presence of adsorbed anions both on the Pt surface and on the upd adlayer. We will follow a treatment similar to that given by Taguchi and Aramata [17] but we will assume that Faraday’s law is preserved during the adsorption process, i.e., we will assume that when partial charge transfer occurs, no charge flows through the external circuit [46]. In the case of Cd upd on Pt(h k l) in sulfuric acid solutions we assume that sulfate/bisulfate adsorbs both on platinum and on the cadmium adlayer as is shown by radiotracers [24]. In addition we will assume that the main cadmium species present in solution is Cd2, given that no stable complexes are seemingly formed with (bi)sulfate. The deposition reaction can thus be represented as: Pt(1 1 1)(HSO4 )m qCd2 (2qmn)e XPt(1 1 1)(Cd)q (HSO4 )n (mn)HSO 4

(1)

The stoichiometric coefficients for the different adsorbates are defined per Pt surface atom. Assuming that the above process is at equilibrium we may write down a pseudo-Nernstian equation: o Eupd Eupd 

RT (2q  m  n)F

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

ln

a(Cd2 )q × a(HSO4 =Pt)m mn a(Cd=Pt)q × a(HSO4 =Cd=Pt)n × a(HSO 4 )

(2)

Making the derivative with respect to ln a (Cd2), we obtain:   @Eupd q RT (3)  @ln a(Cd2 ) T;a(i"Cd2 ) (2q  m  n) F Taking into account that there is an excess of supporting electrolyte we may substitute the latter equation by:   @Eupd q 2:303RT (4)  2 F @log[Cd ] T;a[i"Cd2 ] (2q  m  n) where the concentrations of all the species different from Cd2(aq) are maintained constant both in solution and at the surface. We may estimate the latter derivative based on the shift of the upd peak potentials (where the adlayer composition is assumed to be constant). This yields a value of /0.030 V dec1 at room temperature, which implies that m :/n [R1], i.e., the coverage by adsorbed anions does not seem to change significantly upon cadmium adsorption. This fact is not surprising given that several metal electrode surfaces with hexagonal symmetry exhibit similar structures for adsorbed bisulfate as evidenced by scanning tunneling microscopy [47]. This result has important implications regarding coulometric evaluation of coverages: in the present case the straightforward application of Faraday’s law (without taking anions into account) will be approximately correct (see below). 4.2. Influence of anion adsorption Eq. (1) indicates that the presence of adsorbable anions (or adsorbable neutral species) may change the upd process from both a kinetic and thermodynamic point of view. This influence derives from the tendency of most anions to adsorb both on the metal substrate and on the upd layer. In addition, the possibility exists of complexation of the metallic species to be deposited in solution (and even at the surface). We have added both chloride and bromide to a 0.1 M HClO4 test electrolyte in order to assess the effect of typical specifically adsorbed anions on the Cd upd process. The presence of anions triggered remarkable changes in the voltammetric profiles. The addition of chloride causes a noticeable sharpening and reversibilization of the voltammetric profile associated with the cadmium upd process. One of the aspects that should be discussed first is possible cadmium complexation occurring upon the introduction of chloride. Taking into account that Cd
/Cl complexes are not particularly stable [48,49] and that the chloride concentrations used are not in

151

large excess with respect to cadmium concentration, the effect of solution complexation can be neglected in a first approximation. Therefore the upd process can be represented by the equation: Pt(1 1 1)(Cl)m qCd2 (2qmn)e XPt(1 1 1)(Cd)q (Cl)n (mn)Cl

(5)

The upd process shifts toward less positive potentials in the presence of chloride, which should be interpreted in terms of differences of chemical potential of the anion adlayer on Pt(1 1 1) and Cd/Pt(1 1 1). As expected the upd peak potential depends on the concentration of chloride for a constant Cd concentration. From the pseudo-Nernstian equation it is possible to deduce:   @Eupd (m  n) 2:303RT (6)   F @log[Cl ] T;a[i"Cd2 ] (2q  m  n) An estimation of the values of m/n in different cases can be obtained assuming that:     @Eupd DEupd : (7) @log[Cl ] T;a[i"Cd2 ] Dlog[Cl ] T;a[i"Cd2 ] The following estimate results: m/n :/0.4q . Seemingly, coverage on the bare substrate is larger than on Cd/Pt(1 1 1). This probably derives from the fact that the first upd layer is an open structure with coadsorbed anions, while a more compact anion adlayer can be formed on bare Pt(1 1 1) as there is no coadsorbate competing for the sites. The same conclusion is drawn if concentrations for chloride and cadmium are corrected for complexation or even if we consider CdCl  as the depositing species. The case of bromide deserves a more detailed discussion. In addition to the sharpening of the profiles already observed in the case of chloride, the deposition/stripping process for the first layer occurs, depending on bromide and cadmium concentration, either in one or in two steps. Splitting of the upd peaks triggered by coadsorption of anions is a known phenomenon observed for instance in the case of Cu upd on Pt(1 1 1) [27]. The anion concentration required is typically low (around 50 mM). However in this case the splitting develops when [Br ] is increased from 1 to 10 mM for a constant [Cd(II)] (either 1 or 10 mM). This suggests that speciation in solution could be at the origin of this striking behavior as we will discuss in the following. The stability constant for CdBr  is taken to be 36 (on the molar scale) according to the values given in Ref. [50]. The concentration of Cd2 and CdBr  calculated for the solutions investigated are reported in Table 1. Complexes with more bromide ligands are not considered as their concentration is always negligible for the bromide and cadmium concentrations used in this work. It may be assumed that, at high bromide concentrations the upd process begins with a first step consisting

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

152

Table 1 Equilibrium solution concentrations for Cd2 and CdBr  for different analytical concentrations of cadmium ions and bromide Solution

[Cd2 ]/ mM

[CdBr  ]/ mM

[Cd2 ]/ [CdBr  ]

1 mM Cd(II)/1 mM bromide 1 mM Cd(II)/10 mM bromide 10 mM Cd(II)/1 mM bromide 10 mM Cd(II)/10 mM bromide

0.97

0.034

29

0.74

0.26

2.8

9.74

0.26

37

7.8

2.2

3.5

in the deposition of CdBr  followed by the deposition, in a second step, of uncomplexed cadmium cations until the complete first upd layer is attained. The standard potentials for both deposition processes would be separated by a few mV, being more positive for that of the complex. Separate peaks appear only if the ratio [Cd2]/[CdBr ] approaches unity under the experimental conditions employed in this work. For larger ratios both peaks merge into a single one. As seen in Fig. 6, an increase in bromide concentration triggers the appearance of the split-off peak at more positive potentials. Therefore, and from a formal viewpoint, bromide would have a positive (mixed) electrosorption valency. This anomalous behavior for bromide would actually reflect the fact that the adsorbing species is CdBr  whose concentration increases almost proportionally with bromide concentration for a constant cadmium concentration (see Table 1). There is however an apparent inconsistency as we are assuming that the stripping/deposition of the complex occurs at more positive potentials than that of the uncomplexed cation. The deposition of CdBr  would be assisted by the formation of a surface complex with preadsorbed bromide ions, i.e., it would be an example of anion induced adsorption of CdBr . This is a wellknown phenomenon reported in earlier works with mercury electrodes [51]. The different experimental observations can be rationalized if we consider the following reversible processes as responsible for the split-off and main voltammetric pairs of peaks, respectively: Pt(1 1 1)(Br)a qCdBr (qab)e XPt(1 1 1)(Br)b (CdBr)q (ab)Br

(8)

Pt(1 1 1)(Br)b (CdBr)q rCd2 (2rqbc)e XPt(Cd)qr (Br)c (qbc)Br

(9)

It is interesting to point out that Eq. (8) would formally correspond to a Cd monoelectronic adsorption process. In this respect, the formation of surface Cd(I)

species during cadmium electrodeposition has been proposed in the past [52]. The behavior reported here for cadmium upd is unprecedented as several conditions must be fulfilled in order for it to develop: (i) the anion that is preferentially adsorbed at the substrate electrode must be a good ligand for the cation to be deposited; (ii) the equilibrium potential for the adsorption/desorption of the complex must be more positive than that for the uncomplexed cation and (iii) there must be a particularly stable intermixed adlayer at intermediate coverage in such a way that the stripping/deposition process occurs in two separate steps. These are stringent conditions, which would cause this behavior to be unusual. In fact, similar experiments in the presence of chloride did not show such behavior, nor did experiments done with Pt(1 0 0) electrodes under similar conditions [53]. Significantly, the introduction of halide anions causes a dramatic sharpening of the voltammetric profiles, which attests to the existence of large attractive lateral interactions. This fact is not surprising given that the electronegativity of Cd is lower than that of Pt, thus giving rise to an intense surface dipole. In the case of chloride and bromide a surface dipole also appears, though inverted. Therefore the Cdd adatoms can interact attractively with Xd yielding the sharp profiles observed in the voltammograms. In addition, and even more importantly, the interaction between cadmium and halogens could include covalent (non-ionic) contributions linked to the complexing abilities of chloride and bromide. 4.3. Interpretation of the charge displacement experiments The technique of charge displacement by CO adsorption has been applied to a number of adsorbates [25,41]. In most cases the species displaced have been anions (including anion-like species), either reversibly or irreversibly adsorbed. In addition, and most frequently, the displaced species has been adsorbed hydrogen, which can be considered as an adsorbed cation. In this work we extend the applicability of the displacement technique to underpotentially deposited Cd adlayers for the first time. Obviously, these studies are feasible since the adsorption of the displacing agent, CO in our case, is stronger than that of the Cd adatom. Recently, the displacement of metallic layers on Pt(h k l ) electrodes by CO has been demonstrated by Lucas et al. [54,55] although no quantitative displacement charge measurements were reported. Let us discuss the relationship existing between the displaced charge at any potential and the cadmium coverage. Recently, we have shown the lack of a standard procedure in order to estimate the coverage from stripping charges and have provided a general

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

approach for such estimates [40]. It is worthwhile trying to extend such a procedure to underpotentially deposited adatoms. Let dlCd and dlCO stand for the double layer structure on Cd/Pt(1 1 1) and CO/Pt(1 1 1) surfaces, respectively. The process occurring upon CO adsorption (taking into account that CO is capable of displacing cadmium and adsorbed anions from the surface): Pt(1 1 1)=(Cd)u (dlCd )CO 0 Pt(1 1 1)=CO(dlCO )uCd2 2ue

(10)

Therefore the charge exchanged during the displacement process, qdis, at potential E will be: qdis (E)2F GCd qTCO (E)qTCd (E) 2qPt(1

T T 1 1) uCd qCO (E)qCd (E)

(11)

where Gcd is the surface concentration of Cd in the adlayer, qTCO and qTCd correspond to the total charges supported by the CO/Pt(1 1 1) and the Cd/Pt(1 1 1) electrodes, respectively, at potential E . On the other hand qPt(1 1 1) stands for the Pt(1 1 1) atomic surface density expressed in electrical units and uCd for the cadmium coverage expressed as the number of Cd adatoms per Pt surface atom. Moreover, taking into account that the double layer capacity for the CO/ Pt(1 1 1) is very small, we can neglect qTCO without significant error: qdis (E):2F GCd qTCd (E)2qPt(1

T 1 1) uCd qCd (E)

(12)

Obviously from charge displacement experiments, it is not possible to assess simultaneously the value of the cadmium coverage and the total charge supported by the Cd/Pt(h k l ) electrode. As the potential is made more positive qTCd(E ) increases and obviously qdis(E) decreaces, as is observed experimentally. It is informative though to perform the charge displacement at least at two different potentials in the range of stability of the upd layer. Let E1 and E2 be the potentials at which the displacements are done. From Eq. (12) it is apparent that: qdis (E1 )qdis (E2 ):qTCd (E2 )qTCd (E1 )

g

E2

j dE E1 v

(13)

Implicitly we have assumed that the Cd adlayer coverage does not change in the potential range (E1,E2). Our results confirm that the latter equation is approximately fulfilled, illustrating once more that the CO molecule acts as a neutral probe. The limitations that are in evidence in the coverage determination by means of charge displacement experiments are reproduced in the determination by voltammetric coulometry. It is interesting to discuss the corrections needed in order to obtain reliable values for the coverage. Let Ea and Eb be the potentials just

153

before and after Cd voltammetric stripping occurs. At potential Ea (the electrode is covered by cadmium) the electrode bears a total charge equal to qTCd(Ea) whereas at potential Eb (the electrode is free of cadmium) the total charge is qTPt(Eb). Therefore:

g

Eb

j

Ea

v

dE qTPt (Eb )2F GCd qTCd (Ea )

(14)

In this equation j needs to be corrected for the double layer contribution (apparent baseline). Again, the coverage cannot be determined through this equation since the total charge on the cadmium adlayer is not known. However, if we accept that the total charge of both Cd/Pt(1 1 1) and Pt(1 1 1) electrodes are dominated by anion specific adsorption it is likely that qTPt(E ):/qTCd(E ), especially if we take into account that the (bi)sulfate coverage does not seem to change upon the formation of a cadmium adlayer (see above). This fact supports the coulometric evaluation of the cadmium coverage through direct integration of the voltammetric peak. This situation is in contrast with the coulometric evaluation of coverages of molecular adsorbates such as NO and CO [40,41]. In such a way and in the case of sulfuric acid working solutions, a cadmium coverage of 0.31 is attained in the first upd layer. The fact that cadmium attains such a low coverage is linked to the large underpotential shift observed in this case (around 1 V), a consequence of the large difference in work functions between the adatom and the substrate. Cadmium adatoms are expected to have a substantial positive charge. This does not mean that Cd(II) exchanges less than 2e  during its deposition, but that there is a charge redistribution between the substrate and the adatom once the adatom has been adsorbed. The net repulsion existing between cadmium adatoms, deriving from their large partial positive charge together with the low attractive Cd
/Cd interaction (as deduced from the relatively low Cd enthalpy of vaporization) favor an open structure for the first Cd layer. Anions can be coadsorbed on such a layer, accounting for the attractive total lateral interaction deduced from the half-width of the voltammetric peaks. Finally, the large mismatch between Cd and Pt also favors the formation of open Cd adlayers on Pt(1 1 1). The displacement experiment can be used to obtain an estimation for the total charge of the Cd/Pt(1 1 1) electrode. If we take into account the charge displaced at 0.1 V (117 mC cm 2) we can estimate, based on Eq. (12), the potential of zero total charge (pztc) for the modified electrode by taking an average value for the (pseudo)capacity of 150 mF cm 2, observed between 0.10 and 0.40 V. The value obtained, /0.13 V, is significant lower than that of Pt(1 1 1) but is not as negative as in the case of the Cd(0 0 0 1) electrode

154

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

(/1.00 V SCE in NaF solutions [56], i.e. about /0.8 V vs RHE in the working electrolyte used in this work). The strong partial positive charge probably retained by the Cd adatoms together with the open adlayer structure (low coverage of Cd) would explain the difference with respect to the bulk Cd(0 0 0 1) electrode. In any case, the pztc for the Cd/Pt(1 1 1) electrode is expected to lie between those of Pt(1 1 1) and Cd(0 0 0 1). In the preceding discussion we have assumed that the cadmium adlayer is included in the metallic side of the interphase. Of course, we can consider that cadmium belongs to the solution side of the interphase, i.e. that Cd2 is a specifically adsorbed cation, especially if one takes into account that the adlayer is at equilibrium with cadmium ions in solution. In such a case Eq. (12) would read: qdis (E):qTPt(1

1 1) (E)

(15)

where qTPt(1 1 1)(E ) corresponds to the total charge of the Pt(1 1 1) electrode in contact with a Cd2 solution (cadmium is included in the solution side of the interphase). The positive value of the charge displaced at 0.40 V indicates that the pztc is located at more positive potentials. By voltammogram integration, this particular pztc can be located with accuracy at 0.53 V, which is the potential for the first stripping peak. This value is higher than that found in the same electrolyte in the absence of cadmium (0.32 V) [41] as expected for the specific adsorption of a cation. The definition of two different pztc depending on the conceptual location of the metallic adlayer has been discussed recently by Langkau and Baltruschat for the Ag/Pt(1 1 1) system [57]. The kinetics of the displacement process also deserve some attention. As mentioned before, the displacement is faster at positive potentials (near the stripping potential). Obviously, as we apply a more negative potential, we increase the activation energy for the displacement process since we are displacing a cationic species. In the case of iodine [58] the displacement became more sluggish as the potential was made more positive, in contrast with the behavior observed for Cd. It is not surprising though, for iodine is desorbed as an anion. Finally we will comment on the formation of a second upd layer. The charge integrated in this process is well above 400 mC cm 2 (a more precise evaluation is risky since the her occurs simultaneously). In any case, such a large value indicates that the second layer is probably a close-packed layer of cadmium adatoms (whose coverage would be 0.79 in terms of Cd adatoms per platinum surface atom). However the charge density exchanged exceeds that corresponding to the closed-packed layer, which points to the fact that the deposition of the second layer also includes the compaction of the first. It could

be argued that in such a case the charge exchanged should be about 600 mC cm 2. Double layer effects could account for the apparent discrepancy. In fact, if we assume that, prior to the second layer formation, the first one becomes compacted, the whole process may be sketched as: Pt(1 1 1)=(Cd)a (dlCd )(2ba)Cd2 2(2ba)e 0 Pt(1 1 1)=Cdb =Cdb dlCd?

(16)

where a and b stand for the Cd coverages for the first and second upd layers, respectively, and dlCd and dlCd? for the corresponding double layer structures. The charge exchanged in the stripping of the second layer (qsl) would be: qsl  2(2ba)qPt(1 qTCd

T T 1 1) qCd qCd?

(17)

qTCd?

and correspond, respectively, to the total where charge of the Pt(1 1 1) electrode with one and two Cd upd layers. Based on the locations of the pztc (/0.13 V for the electrode modified with one Cd monolayer and presumably about /0.8 V for the electrode modified with two monolayers) and the peak potential for the stripping of the second layer (/0.39 V), we infer that qTCd B/0 and qTCd? /0, both contributions diminishing the charge value measured in the second layer stripping and explaining the apparent contradiction.

5. Conclusions We have reported results on the upd of cadmium on Pt(1 1 1) electrodes. The deposition of two upd layers prior to the onset of bulk deposition has been shown. As expected, the upd process is strongly influenced by the presence of anions. We have dealt in detail with the influence of chloride and bromide. In both cases an extraordinary sharpening of the voltammetric profiles is observed as a consequence of the appearance of strong attractive interactions in the cadmium/halogen adlayer. The addition of bromide in certain concentrations triggers the splitting of the upd peak, which is attributed to the separate adsorption of CdBr  and Cd2 species together with the formation of a stable intermixed layer of cations and anions. It is remarkable that the adsorption of the CdBr  cation occurs at more positive potentials than for Cd2, reflecting the fact that bromine adatoms may induce preferential adsorption of the complex.In addition to the classical voltammetric experiments we have performed charge displacement experiments. The charge displaced depends basically on the adatom coverage and on the total charge corresponding to the modified electrode. Knowing the value of the adatom coverage, the value for the total charge

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

for the cadmium-covered electrode can be estimated and eventually a value for the pztc may be obtained. In the present case a value for the cadmium coverage of 0.31 is obtained from coulometry and a value for the pztc of / 0.13 V is deduced from the charge displacement experiment. The latter value corresponds to a l ocation of cadmium in the metallic side of the interphase. If we take cadmium as a specifically adsorbed cation, belonging to the solution side of the interphase, a value of 0.53 V is deduced for the pztc, which lies in the middle of the upd process. In general, the different results are explained assuming that Faraday’s law is preserved during electrodeposition of cadmium adatoms, which does not preclude a local redistribution of charge in the Cd /Pt bond, a consequence of the difference in work-function existing between the two metals. This local redistribution would not require a flow of charge through the external circuit.

Acknowledgements This research was carried out with the support of the Generalitat Valenciana, OCYT, under contract GV01400 and the MCYT (Spain) through project BQU20000240.

References [1] A. Aramata, in: R.E. White, J.O’M. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 31, Kluwer Academic/ Plenum Publishers, New York, 1997, pp. 181
/250. [2] E. Herrero, L.J. Buller, H.D. Abrun˜a, Chem. Rev. 101 (2001) 1897. [3] P.N. Ross, in: J. Lipkowski, P.N. Ross (Eds.), Electrocatalysis, Wiley, New York, 1998, pp. 43
/74. [4] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, Electrochim. Acta 44 (1998) 1009. [5] Z.-L. Wu, S.-L. Yau, Langmuir 17 (2001) 4627. [6] O. Endo, N. Ikemiya, M. Ito, Surf. Sci. 514 (2002) 234. [7] L.J. Buller, E. Herrero, R. Go´mez, J.M. Feliu, H.D. Abrun˜a, J. Phys. Chem. B 104 (2000) 5932. [8] F. El Omar, R. Durand, R. Faure, J. Electroanal. Chem. 160 (1984) 385. [9] A.M. Bittner, J. Electroanal. Chem. 431 (1997) 51. [10] J.X. Wang, N.S. Marinkovic, R.R. Adzic, B.M. Ocko, Surf. Sci. 398 (1998) L291. [11] C.A. Lucas, N.M. Markovic, P.N. Ross, Langmuir 13 (1997) 5517. [12] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, J. Chem. Soc., Faraday Trans. 94 (1998) 3373. [13] R.L. Borup, D. Sauer, E.M. Stuve, Surf. Sci. 293 (1993) 10. [14] B.N. Grgur, N.M. Markovic, P.N. Ross, Langmuir 13 (1997) 6370. [15] N. Hoshi, I.T. Bae, D.A. Scherson, J. Phys. Chem. B 104 (2000) 6049. [16] S. Taguchi, A. Aramata, Md.D. Quaiyyum, M. Enyo, J. Electroanal. Chem. 274 (1994) 275.

155

[17] S. Taguchi, A. Aramata, J. Electroanal. Chem. 396 (1995) 131. [18] A. Aramata, S. Taguchi, T. Fukuda, M. Nakamura, G. Hora´nyi, Electrochim. Acta 44 (1998) 999. [19] K. Igarashi, A. Aramata, S. Taguchi, Electrochim. Acta 46 (2001) 1773. [20] J. Clavilier, J.P. Ganon, M. Petit, J. Electroanal. Chem. 265 (1989) 231. [21] D.R. Wheeler, J.X. Wang, R.R. Adzic, J. Electroanal. Chem. 387 (1995) 115. [22] R.R. Adzic, J.X. Wang, O.M. Magnussen, B.M. Ocko, J. Phys. Chem. 100 (1996) 14721. [23] I. Oda, Y. Shingaya, H. Matsumoto, M. Ito, J. Electroanal. Chem. 409 (1996) 95. [24] K. Varga, P. Zelenay, G. Horanyi, A. Wieckowski, J. Electroanal. Chem. 327 (1992) 291. [25] J.M. Orts, R. Go´mez, J.M. Feliu, A. Aldaz, J. Clavilier, Electrochim. Acta 39 (1994) 1519. [26] R. Michaelis, M.S. Zei, R.S. Zhai, D.M. Kolb, J. Electroanal. Chem. 339 (1992) 299. [27] N. Markovic, P.N. Ross, Langmuir 9 (1993) 580. [28] N.M. Markovic, H.A. Gasteiger, C.A. Lucas, I.M. Tidswell, P.N. Ross, Surf. Sci. 335 (1995) 91. [29] R. Go´mez, H.S. Yee, G.M. Bommarito, J.M. Feliu, H.D. Abrun˜a, Surf. Sci. 335 (1995) 101. [30] N.M. Markovic, H.A. Gasteiger, P.N. Ross, Langmuir 11 (1995) 4098. [31] N.M. Markovic, C.A. Lucas, H.A. Gasteiger, P.N. Ross, Surf. Sci. 372 (1997) 239. [32] H. Bludau, K. Wu, M.S. Zei, M. Eiswirth, H. Over, G. Ertl, Surf. Sci. 402
/404 (1998) 786. [33] Z.-L. Wu, Z.-H. Zang, S.-L. Yau, Langmuir 16 (2000) 3522. [34] J.L. Stickney, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 21, Marcel Dekker, New York, 1999, p. 75. [35] B.W. Gregory, D.W. Suggs, J.L. Stickney, J. Electrochem. Soc. 138 (1991) 1279. [36] D.W. Suggs, J.L. Stickney, Surf. Sci. 290 (1993) 375. [37] L.P. Colletti, D. Teklay, J.L. Stickney, J. Electroanal. Chem. 369 (1994) 145. [38] F. Forni, M. Innocenti, G. Pezzatini, M.L. Foresti, Electrochim. Acta 45 (2000) 3225. [39] M. Innocenti, G. Pezzatini, F. Forni, M.L. Foresti, J. Electrochem. Soc. 148 (2001) C375. [40] R. Go´mez, J.M. Feliu, A. Aldaz, M.J. Weaver, Surf. Sci. 410 (1998) 48. [41] V. Climent, R. Go´mez, J.M. Orts, A. Rodes, A. Aldaz, J.M. Feliu, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, Inc, New York, 1999 (Ch. 26). [42] J. Clavilier, D. Armand, S.G. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267. [43] J. Clavilier, J.M. Feliu, A. Ferna´ndez-Vega, A. Aldaz, J. Electroanal. Chem. 294 (1990) 193. [44] R. Go´mez, J.M. Feliu, A. Aldaz, Electrochim. Acta 42 (1997) 1675. [45] R. Go´mez, J.M. Feliu, Electrochem. Soc. Proc. 97-16 (1997) 118. [46] B.E. Conway, H. Angerstein-Kozlowska, W.B.A. Sharp, Z. Phys. Chem. (Neue Folge) 98 (1975) 61. [47] M. Nakamura, Y. Shingaya, M. Ito, Surf. Sci. 502
/503 (2002) 474. [48] C.E. Vanderzee, H.J. Dawson, J. Am. Chem. Soc. 75 (1953) 5659. [49] R. Tomas, M. Visic, I. Tominic, V. Sokol, Croat. Chem. Acta 74 (2001) 91. [50] E. Casassas, C. Arin˜o, J. Electroanal. Chem. 213 (1986) 235. [51] F.C. Anson, D.C. Barclay, Anal. Chem. 40 (1968) 1791.

156

R. Go´mez, J.M. Feliu / Journal of Electroanalytical Chemistry 554
/555 (2003) 145
/156

[52] V. Daujotis, R. Raudonis, J. Electroanal. Chem. 326 (1992) 253. [53] R. Go´mez, J.M. Feliu, unpublished results. [54] C.A. Lucas, N.M. Markovic, B.N. Grgur, P.N. Ross, Surf. Sci. 448 (2000) 65.

[55] [56] [57] [58]

C.A. Lucas, N.M. Markovic, P.N. Ross, Surf. Sci. 448 (2000) 77. E. Lust, K. Lust, A. Ja¨nes, J. Electroanal. Chem. 413 (1996) 111. T. Langkau, H. Baltruschat, Electrochim. Acta 47 (2002) 1595. J. Clavilier, R. Albalat, R. Go´mez, J.M. Orts, J.M. Feliu, J. Electroanal. Chem. 360 (1993) 325.