A.G. Brolo *,1, S.D. Sharma

Transcription

1 Electrochimica Acta 48 (2003) 1375/ Using probe beam deflection (PBD) to investigate the electrochemical oxidation of silver in perchlorate media in the presence and absence of chloride ions A.G. Brolo *,1, S.D. Sharma Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6 Received 7 September 2002; received in revised form 22 November 2002 Abstract The electrochemical oxidation of silver in 0.1 M KClO 4 solutions containing KCl were investigated by cyclic voltammetry (CV) and electrochemical probe beam deflection (PBD). Ag (aq) ions were the main product of the silver oxidation in the absence of the halide. The formation of Ag (aq) provoked a beam deflection towards the electrode surface. A beam deflection away from the electrode surface was then observed during the reduction of the Ag (aq) ions. A convolution analysis yielded a diffusion coefficient of 1.2/10 9 m 2 s 1 for Ag (aq) in this medium. An anodic peak due to the formation of AgCl (s) film was observed for the oxidation of silver in solutions containing Cl (aq). As the applied potentials were made more positive in media containing chloride (after the peak due to the AgCl (s) formation), a flux of ions away from the electrode surface was clearly detected by PBD. This was assigned to the formation of Ag (aq) ions through the porous AgCl (s) film structure. Oscillations on the position of the laser beam were present during the oxidation at high chloride concentrations, due to the precipitation of AgCl (s) from the solution phase. The electrochemical and PBD data were consistent with a dissolution-precipitation mechanism for the AgCl (s) film formation. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Silver oxidation; Probe beam deflection; Silver chloride; Film formation; Mirage effect 1. Introduction The utilization of surface analytical methods to complement classical electrochemical measurements is one of the most notorious traits of modern electrochemistry [1,2]. Surface-sensitive techniques, such as electrochemical IR spectroscopy [3], surface-enhanced Raman scattering (SERS) [4] and surface second-harmonic generation [5], produce information about the chemical nature, structure and orientation of adsorbed molecules. In contrast, probe beam deflection (PBD) is an in situ spectroelectrochemical tool that provides a measure of the flux of species in the electrochemical diffusion layer [6 /8]. Therefore, the data obtained on the solution side by the PBD method is intrinsically * Corresponding author. Tel.: / ; fax: / address: agbrolo@uvic.ca (A.G. Brolo). 1 ISE member. complementary to the information obtained from techniques that probe the surface directly. A typical PBD experiment consists of directing a light beam (generally provided by a low power He/Ne laser) parallel to a surface under electrochemical control. The path of the beam is susceptible to the optical properties of the solution close to the electrochemical interface. The light beam will be deflected by an angular amount u when a refractive index gradient in the direction perpendicular to the surface (@n/@x) is created. The refractive index gradient can be established due to concentration (@c/@x) and/or temperature (@T/@x) changes. The angular deflection u is then given by Eq. (1) below [9]: u @T (1) n where L is the width of the electrode and n 0 is the refractive index of the solution. The second term in the square brackets of the right hand side of Eq. (1) is /03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)

2 1376 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/1384 generally ignored, since temperature effects are typically not significant during an electrochemical experiment in excess of supporting electrolyte [10]. The concentrative refractivity (@n/@c) can be independently determined using a refractometer. The behavior of ions in the diffusion layer, obtained by PBD, together with results from electrochemical and surface analytical methods yield a complete description of the system under investigation [11 /13]. This combined approach has been used to study both ion exchange in electroactive films [11,13] and intercalation of ions in graphite electrodes [12]. The PBD method has also been applied to the investigation of anodic processes, including metal dissolution and film formation [14/17]. For instance, in situ PBD was used to investigate the electro-oxidation of copper in alkaline media [14,15]. The concentration gradients and ion fluxes for the species involved at different stages of copper dissolution and copper oxide formation were obtained from plots of the PBD signal versus potential (cyclic deflectograms (CD)) [14]. Other anodic processes studied using PBD include the electro-dissolution of silicon in fluoride media [16] and the oxidation of hypophosphite at a nickel electrode [17]. In this work, PBD was used to examine the anodic oxidation of silver in perchlorate media in the presence and absence of chloride ions. Our interest in the electrochemical behavior of silver originates from our experience with this metal as a SERS substrate [4]. The anodic behavior of silver in chloride solutions has been widely investigated [18 /25], and the results presented in this paper provide further insights into the interpretation of these electrochemical data. 2. Experimental 2.1. Solutions The solutions were prepared using ultra-pure water (18.2 MV cm 2 ) from a Barnstead NANOpure Diamond water purification system. KCl (ACP) and KClO 4 (Aldrich) were used without further purification Cell, electrodes and electrochemical equipment The working electrode was fabricated from a 99.99% silver rod (Premion /Alfa Aesar). A silver disk of approximately 6.35 mm diameter was mounted in a Teflon holder. Electrical contact was made to the silver electrode by an externally threaded stainless-steel rod. Before each experiment, the working electrode was polished with emery paper and with progressively finer grades of alumina powder down to 0.5 mm. The polished silver electrode was rinsed with copious amounts of ultra-pure water and then transferred to the appropriate cell. A silver/silver chloride electrode (in saturated KCl) was used as the reference electrode, and all potentials in this paper are quoted against this reference. A 0.3 mm platinum wire (Alfa Aesar) was employed as the counter electrode. The experiments were performed using a single compartment cell with two windows (50 /50/ 50 mm quartz cell cuvette from Spectrocell). The cell was purged with pre-purified N 2 for 30 min prior to the measurements, and a gentle stream of nitrogen was maintained to blanket the solution during data acquisition. A specially designed Teflon cap was used to adapt the three electrodes and the nitrogen inlet/outlet to the cell. Electrochemical measurements were obtained using a potentiostat/galvanostat PAR 173 system and a Hokuto Denko HB-111 function generator Probe beam deflection setup A schematic representation of the PBD setup is given in Fig. 1. The nm output from a 2 mw He/Ne laser (Melles Griot) was focused by a plano-convex lens (15 cm focal length). The focused beam diameter was estimated to be less than 100 mm and traveled parallel to the surface of the silver electrode. It has been shown that a beam diameters smaller than 100 mm are required to obtain reliable electrochemical PBD data [26]. The cell was mounted on a mechanical stage allowing adjustments along all three axes and rotation. The detection system consists of a 6 mm Hamamatsu S3931 position sensitive detector (PSD) and a Hamamatsu C signal processing circuit. The analog readings from the PSD were fed into a Pentium computer equipped with a data acquisition board (PCI 6024E; National Instruments-NI) and a connector accessory (BNC 212; NI). The signal was then digitized using customized data acquisition software written in LABVIEW (NI). The voltage output from the PSD was further converted into beam deflection angle. The LABVIEW program also recorded the electrochemical information simultaneously with the PBD signal. 3. Results and discussion 3.1. Oxidation of silver in 0.1 M KClO 4 A cyclic voltammogram (CV) and a CD for silver in 0.1 M KClO 4 solution are presented in Fig. 2 (CD, upper curve, and CV, lower curve). Both the CV and the CD were recorded simultaneously as described in the experimental part (Section 2.3). The anodic process observed in Fig. 2 was assigned to the electrochemical formation of Ag (aq) ions. The increase of the concentration of cations at the interface provoked a deflection towards the electrode surface (see the inset (a) on Fig. 2). This angle of deflection towards the surface was

3 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/ Fig. 1. Experimental apparatus for electrochemical PBD measurements. WE, working electrode; CE, counter electrode; RE, reference electrode. Fig. 2. CV (lower curve) and CD (upper curve) for Ag in 0.1 M KClO 4. Scan rate: 5 mv s 1. (a) Schematic representation of a beam deflection towards the electrode surface during the Ag dissolution (negative deflection). In this case, the beam deflection is due to the silver oxidation and the consequently increase in the [Ag ] in the diffusion layer; (b) schematic representation of a beam deflection away from the electrode surface during the Ag reduction (positive deflection). The beam deflection is then provoked by a decrease in the [Ag ] in the diffusion layer. considered to be negative. This was consistent with our definition of an x-axis perpendicular to the surface, with the origin set at the beam position, and with positive values towards the solution. The reverse process (reduction of Ag(aq) ions to metallic silver) was observed during the cathodic scan. In this case, the ion concentration in the diffusion layer decreased, leading to a deflection away from the electrode surface (positive deflection, represented in the inset (b) in Fig. 2). The CV (lower curve in Fig. 2) clearly shows that the cathodic charge was much smaller than the anodic charge, suggesting that the oxidation product (Ag(aq)) was soluble and diffused away from the electrode surface under the conditions of this experiment (5 mv s 1 ). A temporal mismatch between the electrochemical and the PBD data is a common feature for this type of experiment [27]. This phenomenon is illustrated in Fig. 3A, which presents the same data as in Fig. 2, but Fig. 3. CV (solid line) and CD (open squares) plotted against time. System: Ag in 0.1 M KClO 4. Scan rate: 5 mv s 1. (A) Experimental measurements before the convolution analysis; (B) calculated PBD from the measured current (convolved current) and probe beam data. Convolved current calculated using D (Ag) /1.2/10 9 m 2 s 1, x/ 130 mm, and (/@/n /@/c)/1.2/10 5 m 3 mol 1. plotted against time. The electrochemical current in Fig. 3A accounts for the flux of ionic species i at the electrode surface (J i (0, t)). On the other hand, the beam deflection measured the flux of ions at a distance x (ca. 130 mm in our experiments) from the surface (J i (x, t)). Therefore, it is expected that the PBD data should lag behind the current as shown in Fig. 3A. The flux of ions at the electrode surface can be linked to the flux of ions at a distance x from the surface by the convolution of the current with the transfer function of the mass transport [27]. The flux is then given by (the asterisk (*) represents the convolution):

4 1378 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/1384 J i (x; t)f(x; t)j i (0; t) (2) Where F(x, t) is the transfer function given by: F(x; t) x x 2 pffiffiffiffiffiffiffiffiffiffiffi exp (3) 2 pd i t 3 4D i t D i is the diffusion coefficient of species i transferred to a distance x from the electrode surface. Considering an ion flux proportional to the current according to Fick s first law [28], we I(t)zFAD i where A is the electrode area, F is the Faraday constant and z is the charge of the ion. The beam deflection (u(x, t)) can then be evaluated using Eqs. (1) /(4) and neglecting any temperature effect [27]: u(x; t) F(x; t)i(t) (5) n o FAD i Fig. 3B shows the result of the described convolution analysis applied to the data in Fig. 3A. Good agreement between the PBD and the convolved current data was obtained for a diffusion coefficient of 1.2/10 9 m 2 s 1. This value is similar to the 1.3/10 9 m 2 s 1 reported in a previous electrochemical PBD work on the deposition and dissolution of silver from a gold surface [29] Oxidation of silver in 1 mm KCl/0.1 M KClO 4 The addition of 1 mm (10 3 M) KCl to the 0.1 M KClO 4 solution induced an anodic peak (labeled A1) in the CV at approximately /300 mv, as shown in Fig. 4A. No oxidation occurred at this potential in the absence of chloride ions (see Fig. 2). Fig. 4B shows the deflectogram (curve a) and the convolved current (curve b) plotted against time (obtained as described in Section 3.1 for Fig. 3B). The curves in Fig. 4B (and in the subsequent figures) are offset (stacked) for the sake of clarity. It has been established that the silver dissolution in chloride media occurs through the formation of a silver chloride film [23]. Hence, the anodic peak A1, observed in Fig. 4A and B, was assigned to the AgCl (s) film formation, according to the equation: Ag (s) Cl (aq)?agcl (s) e (6) The deflectogram presented in Fig. 4B (curve a) shows a positive deflection (away from the electrode surface) as silver started to oxidize. This behavior is opposed to the one observed for the oxidation of silver in the absence of chloride (Figs. 2 and 3). The positive deflection peaked at a point near the asterisk in Fig. 4B (curve a). This peak correlated well with the maximum observed in the anodic peak A1 from the convolved current (Fig. 4B, curve b). The convolved current in Fig. 4B was calculated using a diffusion coefficient of Cl(aq) ions equal to 2.2/10 9 m 2 s 1. The positive beam deflection (away from the electrode surface) was due to the depletion of Cl(aq) in the diffusion layer during the oxidation (according to Eq. (6)). The anodic current increased as the applied potential became more positive than the peak A1, suggesting the onset of another anodic process, labeled A2. The process A2 provoked a dip on the deflectogram (marked with an arrow in Fig. 4B, curve a). The beam deflection to the negative direction indicates that a flux of ions away from the electrode surface became important at these potentials. In contrast with Fig. 2 (in the absence of chloride), the cathodic peak C1, obtained during the reverse scan in Fig. 4A, presented a charge of similar magnitude as observed for the oxidation process. This confirms that the main oxidation product in the presence of chloride was an insoluble salt that did not diffuse away from the electrode surface. The nature of the negative deflection during A2 should be related to either the spontaneous dissolution of the AgCl (s) surface precipitate or to the formation of soluble silver oxidation products through the film. Considering that K sp for AgCl (s) is of the order of 2.0/10 10 [30], it is unlikely that the film dissolution would dominate under these conditions. In fact, Altukhov and Shatalov [22] and Kolodziej [25] have demonstrated that significant dissolution of AgCl (s) films only occurs in concentrated chloride solutions due to the formation of soluble complexes of type [AgCl x ] 1x (aq). Moreover, the negative deflection was accompanied by an increase in the total current, indicating that the ions in the flux were originated electrochemically. The negative deflection can then only be assigned to the formation of soluble Ag(aq) ions. The amount of AgCl (s) adsorbed to the surface can be readily estimated from the charge of the peak C1. This cathodic charge was around 8.0 C m 2 (within 5%) for the CV presented in Fig. 4A, yielding an absolute number of Cl - at the surface of the order of 5/10 19 Cl ions m 2.A monolayer of silver surface atoms contains approximately 1.5/10 19 atoms of silver m 2 [31]. This approximated calculation shows that enough AgCl (s) to completely cover the surface was produced during the anodic scan. It is then suggested that the production of Ag(aq) ions occurred through the film, since it has been established that the surface was fully covered by AgCl (s). This is in agreement with previous electrochemical work which demonstrated that the AgCl (s) layer, formed during the anodic dissolution of silver in chloride medium, is always very porous [18/25]. The deflectogram (Fig. 4B, curve a) shows a negative deflection during the cathodic scan, correlated to the cathodic peak C1. The negative deflection during the reduction of the AgCl (s) film (inverse of the process represented by

5 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/ Fig. 4. System: Ag in 1 mm KCl/0.1 M KClO 4, scan rate: 5 mv s 1. (A) CV, anodic limit: /400 mv; (B) (curve a) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /400 mv; (C) CVs, anodic limits: (curve a)/500 mv, (curve b) /600 mv; (D) (curve a) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /600 mv. Convolved current calculated using D (Cl )/2.2/10 9 m 2 s 1, x/130 mm, and (/@/n /@/c)/9.4/10 6 m 3 mol 1. Eq. (6)) was a consequence of the increase in the Cl (aq) concentration close to the electrode surface. Fig. 4C shows CVs for the same system (Ag/KCl 1 mm/0.1 M KClO 4 ), but the positive limits of the CVs were extended to /500 mv (curve a) and /600 mv (curve b). The redox peaks at approximately /300 mv (anodic) and approximately /200 mv (cathodic), labeled A1 and C1, respectively, were present in both CVs. When the anodic limit was set to /500 and /600 mv, the cathodic charges for C1 (Fig. 4C) were 8.3 and 7.8 C m 2, respectively. These values can be considered identical, within an estimated experimental uncertainty of 9/5%, to the one observed for Fig. 4A. Hence, the increase in the anodic limit did not lead to more film deposition for this low chloride concentration. The cathodic peak due to the reduction of the soluble Ag (aq) species is labeled C2 in Fig. 4C. The charge under peak C2 was significantly higher than for C1 for both CVs presented in Fig. 4C. The PBD data and the convolved current for this medium with the anodic limit set to / 600 mv are presented in Fig. 4D (curves a and b, respectively). The A1 peak in the convolved current was correlated with a small positive beam deflection (not clear in this scale, but marked with an asterisk in Fig. 4D). As the anodic current for the A2 process increased (due to the silver oxidation through the AgCl (s) film), the deflectogram presented a very significant negative deflection with the maximum of the convolved current (Fig. 4D, curve b) coinciding with the minimum of the deflectogram (Fig. 4D, curve a, marked with an arrow). The anodic current reached a maximum value at the anodic potential limit (/600 mv), when the potential sweep was reversed the convolved current decreased. Simultaneously, the absolute beam deflection became less negative and presented a distinct series of shoulders. The discussion regarding the nature of these shoulders will be delayed until Section 3.6, after the data for the oxidation of silver at higher concentrations of KCl have been presented. A positive beam deflection due to the reduction of Ag (aq) (C2) and a negative beam deflection at C1 due to the reduction of the AgCl film were observed. The convolution analysis using only one constant diffusion coefficient may not yield a perfect fit between the convolved current and the measured deflectogram. For instance, the differences between the convolved current and the deflectogram observed in Fig. 4D in the A2 region are a consequence of the complexity of the system. It is very likely that different species are participating in distinct processes at a given potential in that region. A more complete analysis would require the determination of contributions from individual ionic

6 1380 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/1384 fluxes to the total current at a given potential, considering the effects of the counter ions [27] and the supporting electrolyte [32]. The use of a constant diffusion coefficient, however, can be regarded as a useful first approximation. This approximation will be used throughout this paper since it reduces the time mismatch between the deflectogram and the current allowing a semi-quantitative interpretation of the experimental data Oxidation of silver in 5 mm KCl/0.1 M KClO 4 Fig. 5 presents a set of CVs, deflectograms and convolved current plots for 5 mm KCl in 0.1 M KClO 4 at several reversal (anodic) potentials. The onset of the oxidation of silver is expected to shift in the direction of more negative potentials as the concentration of chloride increases [18 /25,30]. In fact, the cyclic votammogram for a reversal potential limit of /300 mv, shown in Fig. 5A, already presents considerable formation of AgCl (s). The charge under the cathodic peak C1 is approximately 12 C m 2. This charge is significantly higher than that obtained for the solution containing 1 mm KCl (Fig. 4A) when the anodic limit was /400 mv (ca. 8 C m 2 ). The deflectogram and the convolved current that correspond to the CV in Fig. 5A are presented in Fig. 5B. The measured deflectogram (Fig. 5B, curve a) presents a positive deflection during the formation of AgCl (s) and a negative deflection during its reduction. The convolved current (Fig. 5B, curve b) agrees well with the PBD trace in this case. The anodic current at potentials more positive than the A1 peak decreased its intensity and assumed a virtually constant value until the onset of A2 process. This behavior is illustrated for reversal anodic potentials of /400 and /500 mv, presented in Fig. 5C (curves a and b). The cathodic charges under the C1 peak from Fig. 5C were approximately 29 and 28 C m 2 for curves a and b, respectively. These charges were the same, within experimental error, and correspond to the reduction of more than 15 monolayers of AgCl (s). The PBD trace and the convolved current when the anodic limit was set to /400 mv are presented in Fig. 5D. The main features observed in Fig. 5D are similar to those observed in Fig. 4B. The deflectogram (Fig. 5D, curvea) shows a positive deflection in the A1 region (marked with an asterisk) and a dip in the A2 region (marked with an arrow). The deflection in the negative direction, provoked by the formation of soluble Ag(aq) ions, became much more pronounced when the anodic limit was extended to values more positive than /400 mv (not shown). Fig. 5. System: Ag in 5 mm KCl/0.1 M KClO 4, scan rate: 5 mv s 1. (A) CV, anodic limit: /300 mv; (B) (curve a) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /300 mv; (C) CVs, anodic limits: (curve a)/400 mv, (curve b) /500 mv; (D) (curve a) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /400 mv.

7 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/ Oxidation of silver in 50 mm KCl/0.1 M KClO 4 Further increase in the bulk concentration of Cl - ions is accompanied by an increase in the amount of AgCl (s) film formed during the silver oxidation. The cathodic charges due to the reduction of the AgCl (s) film (C1) were 116 C m 2 for a reversal potential of /250 mv (Fig. 6A, curve a); 340 C m 2 for a reversal potential of /350 mv (Fig. 6A, curve b) and 445 C m 2 for a reversal potential of /600 mv (Fig. 6C). Fig. 6B shows the behavior of the deflectogram and the convolved current for a reversal potential of /350 mv (CV shown in Fig. 6A, curve b). In this case, formation of Ag (aq) was still not observed (no beam deflection in the negative direction). The PBD graph and convolved current present the expected behavior for the flux of only one ionic (Cl ions) species. A plateau in the anodic current was observed after the process A1 in Fig. 6C. The second oxidation wave started after the plateau at potentials more positive than /500 mv (Fig. 6C). The PBD curve (Fig. 6D, curve a) followed the convolved current (Fig. 6D, curve b) up to the peak A1 for this potential limit (/600 mv). However, the negative beam deflection observed during the process A2 presented complex oscillatory behavior (indicated by an arrow in Fig. 6D). These non periodical oscillations are reminiscent of the shoulders presented in Fig. 4D (Section 3.2). The correlation between the deflectogram (Fig. 6D, curve a) and the convolved current (Fig. 6D, curve b) in the C1 region is illustrated in the inset in Fig. 6D. It is clear that a good fit was obtained up to the minimum current in C1. After that, while the convolved current increased sharply, the beam deflection shows a slower evolution. Another interesting aspect was observed in the shaded region of Fig. 6C. It can be seen that the cathodic current after the C1 peak did not drop to zero, as observed for instance with the CVs shown in Fig. 6A. In fact, a considerable cathodic current, of the order of at least /100 ma, persisted as the applied potential was swept to the negative direction Oxidation of silver in 0.1 M KCl/0.1 M KClO 4 The electrochemical and PBD behavior during the oxidation of silver in 0.1 M KCl/0.1 M KClO 4 for reversal anodic potentials up to /400 mv are similar to the ones presented in Section 3.4 (Fig. 6). For instance, the overall shape of the deflectogram when the reversal potential was set to /300 mv, presented in Fig. 7B, is well correlated with the results observed in Fig. 6B. No evidence of ion flux away from the electrode surface was Fig. 6. System: Ag in 50 mm KCl/0.1 M KClO 4, scan rate: 5 mv s 1. (A) CVs, anodic limits: (curvea)/250 mv, (curveb)/350 mv; (B) (curvea) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /350 mv; (C) CV, anodic limit: /600 mv; (D) (curve a) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /600 mv; inset is the superposition of the beam deflection curve (solid line) and the convolved current (open squares) between 350 and 500 s.

8 1382 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/1384 Fig. 7. System: Ag in 0.1 M KCl/0.1 M KClO 4, scan rate: 5 mv s 1. (A) CVs, anodic limits: (curve a)/300 mv, (curve b)/400 mv; (B) (curve a) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /400 mv; (C) CV, anodic limit: /600 mv; (D) (curve a) measured beam deflection and (curve b) calculated beam deflection from the measured current (convolved current). Anodic limit: /600 mv; inset is the superposition of the beam deflection curve (solid line) and the convolved current (open squares) between 320 and 480 s. observed (beam deflection in the negative direction) during the oxidation in either case. A slight negative deflection began to be observed in the anodic limit when the reversal potential was extended to /500 mv, yielding a deflectogram similar to Fig. 5D. The CV when the anodic limit was increased to /600 mv is presented in Fig. 7C. A region of constant anodic current was again observed after the A1 peak. The anodic current started to increase around /500 mv due to the A2 process (formation of Ag(aq) through the porous AgCl (s) layer). A residual cathodic current was also observed after the C1 peak (shaded area in Fig. 7C). The deflectogram and the convolved current calculated using the data from Fig. 7C are shown in Fig. 7D. A random fluctuation (marked with an arrow) was observed during the negative deflection after the A1 peak (marked with an asterisk). A significant mismatch was observed between the convolved current and the deflectogram after the cathodic peak C1 (see inset in Fig. 7D). In this case, however, a shoulder is evident in the deflectogram during the cathodic scan, with no counterpart of the convolved current. The cathodic charges due to the reduction of AgCl (s) (peak C1) were measured from the CVs presented in Fig. 7A and C. Their values were of the order of 850, 935 and 1145 C m 2 for anodic reversal limits of /400, /500 and /600 mv, respectively. The increase in the cathodic charge of the C1 peak with the anodic limit was observed only when the KCl concentration was higher than 50 mm Discussion The initial oxidation of silver in chloride media provoked a beam deflection in the positive direction due to the formation of AgCl (s). This process led to the peak (A1) in the CVs. A current plateau was observed as the potential was swept to more positive values than the A1 peak. The PBD data still showed a positive angle of deflection during the anodic current plateau, indicating that the main process at these potentials was the diffusion-controlled thickening of the AgCl (s) film. The potential range of the plateau region increased with the concentration of chloride ions. Further excursions into positive potentials led to an increase in the anodic current indicating that another oxidation process, labeled as A2 in our voltammograms, became significant. This increase in current was accompanied by a beam deflection in the negative direction, related to an ion flux away from the electrode surface. This process was assigned to the formation of soluble Ag(aq) species through the porous AgCl (s) film structure. The ratio between the maximum of the current for both anodic processes (I A1 /I A2 ) increased with the bulk chloride concentration.

9 A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/ The negative beam deflection that occurred during the anodic oxidation (A2 process) contained fluctuations. These oscillations were apparent at higher positive potential limits and at higher chloride concentrations. The origin of these fluctuations in the beam deflection can be understood in terms of a simple diffusion layer model. The steady-state current achieved after the A1 peak indicates that the AgCl (s) film formation is limited by diffusion, i.e. the concentration of chloride at the surface is virtually zero. The size of the Cl - diffusion layer during the initial Ag(aq) formation can be estimated using the time elapsed between the start of processes A1 and A2. For instance, this time difference is approximately 30 s for a bulk chloride concentration of 5 mm (Fig. 5D). The diffusion layer thickness (d) is related to the time (t) by the equation d :/(D i t) 1/2 [28]. Thus, a diffusion layer of at least 250 mm for chloride ions (D Cl /2.2/10 9 m 2 s) can be estimated. Considering that the concentration varies linearly with distance inside the diffusion layer, it is found that the local concentration at the laser beam position was at most equal to half of the bulk concentration of Cl (laser beam was located at ca. 130 mm). The AgCl film deposits at the surface when the product [Ag ] [Cl ] (interfacial concentrations in this case) is greater than the solubility product, K sp (K sp for AgCl is approximately 2/10 10 [30]). The film formation decreases the interfacial [Cl ] to a limit where the precipitation no longer occurs and the Ag(aq) is free to diffuse away from the electrode. This process (labeled A2) also creates a concentration gradient for aqueous Ag ions. This mechanism fully supports the fact that, for bulk chloride concentrations below 50 mm, the charge under the peak C1 is independent of the positive reversal potential after the start of the anodic process A2. When the laser beam was positioned in a region of the diffusion layer where the product [Ag ][Cl ] was smaller than the K sp, a negative beam deflection was observed during the process A2 and no hysteresis or fluctuations were observed in the deflectogram. In the situations where either the bulk concentration of Cl was high and/or the system was maintained in a high anodic current regime, an appreciable concentration of silver and/or chloride ions at the position of the laser beam was induced. The intermittent precipitation of AgCl (s) at the solution side should then take place, leading to erratic oscillations in the beam deflection. For bulk chloride concentrations lower than 50 mm, this solution precipitate did not deposit onto the surface, since the charge under C1 did not increase after the potential range was extended into the A2 region. However, at high concentrations of Cl (higher than 50 mm), the charge under C1 increased with the anodic potential limit (even when the anodic limit was extended to a region where the negative beam deflection fluctuated). These observations are in accordance with a dissolutionprecipitation mechanism for film formation. In fact, a dissolution-precipitation mechanism for the electrochemical formation of AgCl has been proposed by Katan et al. [18,19]. In order to illustrate that the observed oscillations were not due to direct contact between the laser beam and the growing solid phase, we estimated the thickness of the AgCl (s) film for the highest cathodic charge (C1) measured in this work (1145 C m 2 in Fig. 7C). In this case, using 5.56/10 3 kg m 3 as the bulk density for the AgCl (s) [33], the thickness would be around 0.5 mm. It can be assumed that the position of the laser relative to the surface was unchanged during all measurements, since the thickest film produced was at least two orders of magnitude smaller than the distance between the surface and the laser beam. The cathodic region presented two peaks, labeled C1 and C2, related to the anodic processes A1 and A2; respectively. The C2 peak was only significant at low chloride concentrations and was assigned to the reduction of the soluble Ag(aq) ions. The C1 peak was due to the reduction of the AgCl (s) film. Two interesting features were observed in the C1 region during the reduction in media containing Cl concentrations higher than 50 mm. Firstly, a mismatch between the deflectogram and the convolved current was observed after the C1 peak (see inset in Figs. 6D and 7D). Secondly, a significant cathodic current was still present after the cathodic peak (shaded area in Figs. 6C and 7C). The origin of the mismatch could be related to differences in the kinetics of film reduction from distinct sites. Burstein and Misra investigated the AgCl film formation from scratched silver surfaces [20,21] and observed two types of oxidized monolayers at distinct potential ranges [21]. These works appear to agree with the XPS observation that indicate that chloride ions exist in two types of sites on silver surfaces [31]. However, no evident electrochemical signatures for these different films were observed in the CVs (Figs. 6D and 7D). It is also possible to ascribe the differences between the convolved current and the deflectogram in this region (insets of Figs. 6D and 7D) to the complex structure of the reduction product. The reduction of AgCl (s) leads to a rough structure that has been characterized by imaging [34 /36]. Chloride ions can then be trapped inside this rough, sponge-like structure. This would slow down the diffusion away from the electrode, provoking the mismatch observed in the insets of Figs. 6D and 7D. These kinetic factors can also be related to the constant cathodic current after the C1 peak (shaded areas of the voltammograms in Figs. 6C and 7C). Moreover, we have demonstrated that some precipitation of AgCl (s) took place in the solution phase, leading to the oscillations in the beam deflection. The reduction of these AgCl (s) particles from solution can also account for the residual cathodic current after the C1 peak.

10 Conclusions A.G. Brolo, S.D. Sharma / Electrochimica Acta 48 (2003) 1375/1384 References The electrochemical behavior of a silver electrode in perchlorate solutions in the presence and absence of chloride was investigated by electrochemical PBD. The PBD data, obtained in situ, provided insights into the movements of ions in the diffusion layer during these electrochemical processes. The deflectogram in the absence of chloride displays a beam deflection in the negative direction during the silver oxidation and a positive beam deflection during the reduction of the soluble Ag(aq). The introduction of chloride ions into the solution induces the formation of AgCl (s) film in the early stage of the oxidation. The film formation is accompanied by a beam deflection in the positive direction. The electrochemical and PBD data were compared after a convolution analysis reduced the temporal discrepancy between the measured current and the flux of ions in the diffusion layer. The convolution analysis yielded good fits between the convolved current and the deflectograms in all situations that could be described by the flux of one ionic species. The deflectograms indicate that the formation of Ag(aq) also occurred even in the presence of chloride at very positive potentials. In this case, the dissolution of the silver electrode occurs through the porous AgCl (s) structure. In the limit of low chloride concentrations, the diffusion-controlled film growth is followed by a significant decrease in the chloride concentration close to the electrode surface. Consequently, the product [Ag ] [Cl ] at the surface becomes smaller than the K sp, allowing the soluble Ag(aq) species to diffuse away towards the bulk solution. At high chloride concentrations, this secondary anodic process (formation of soluble Ag ions) provokes non periodic oscillations in the deflectogram (superimposed on the negative beam deflection). 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