Effect of Temperature Variation on the Under-Potential Deposition of

Jun 15, 2009 - The under-potential deposition of copper on Pt(111) from aqueous 0.05 M H2SO4 + 5 mM CuSO4·5H2O is studied in the 273 ≤ T ≤ 333 K ...
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J. Phys. Chem. C 2009, 113, 12309–12316

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Effect of Temperature Variation on the Under-Potential Deposition of Copper on Pt(111) in Aqueous H2SO4 Gregory Jerkiewicz,*,† Fre´de´ric Perreault,† and Zorana Radovic-Hrapovic‡ Department of Chemistry, Queen’s UniVersity, 90 Bader Lane, Kingston, Ontario, K7L 3N6 Canada, and De´partement de Chimie, UniVersite´ de Sherbrooke, 2500 boul. de l’UniVersite´, Sherbrooke, Que´bec, J1K 2R1 Canada ReceiVed: January 16, 2009; ReVised Manuscript ReceiVed: April 21, 2009

The under-potential deposition of copper on Pt(111) from aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O is studied in the 273 e T e 333 K range using cyclic voltammetry (CV). The CV transients always reveal two cathodic peaks (CI and CII) for the entire temperature range; there is only one anodic peak (AI) in the case of 273 e T < 298 K and the second one appears in the case of T g 298 K. In the case of 273 e T < 298 K, the cathodic and anodic peaks shift toward lower potentials upon T increase, while in the case of T g 298 K, there is no observable peak displacement. The charge density associated with the CuUPD deposition and stripping does not reveal any temperature-dependence. The results demonstrate that T variation does not lead to chargedensity redistribution between the peaks, but the charge density of the first peak (CI and AI) in the depositionstripping profiles is always greater than that of the second peak (CII and AII). The experimentally determined charge density associated with the Cu deposition and stripping is lower than the value expected for one epitaxial CuUPD layer on Pt(111). This difference is due to the coadsorption of anions that occurs concurrently with UPD Cu. Introduction The formation of ultrathin metallic layers on more-noble metal substrates accomplished by electrochemical means, the so-called under-potential deposition (UPD), has been studied extensively over the past several decades.1-6 The UPD of copper on polycrystalline and single-crystal Pt surfaces, especially on the basal planes and the (111) surface in particular, has been the subject of research in several groups using various experimental techniques.7-24 Cyclic voltammetry (CV) is typically used to analyze peaks (peak potential and current density) and to determine the charge density associated with the process in relation to the electrolyte composition, while in situ scanning tunneling microscopy (STM) is used to examine the interfacial structure. Recently, microcalorimetry was employed for the first time to monitor temperature changes associated with UPD Cu on a thin polycrystalline Au electrode.25 This interesting approach revealed that the electrode absorbs heat during UPD Cu and releases it during the stripping of Cu. It is well established that metal adatoms deposited on the electrode surface significantly affect the electrode’s catalytic properties toward various electrochemical reactions. For instance, an almost complete inhibition of the oxygen reduction reaction (ORR) was observed in the presence of Cu adlayers on Pt electrodes.26,27 Even if the main driving force of UPD of metals is the metal adsorbate-metal substrate (MUPD-S) interaction, other interfacial phenomena and associated atomic/molecular interactions can also play an important role, so that the overall Gibbs energy of the entire interfacial systems is minimal at a given applied potential. The presence of strongly adsorbing anions (AN) that constitute the electrolyte is of particular importance because the * Corresponding author. E-mail: [email protected]. Tel: (613) 533-6413. Fax: (613) 533-6669. † Queen’s University. ‡ Universite´ de Sherbrooke.

AN-MUPD and AN-S interactions can modify the UPD process.28-34 In one interesting study, Horanyi first reported on the adsorption of sulfate/bisulfate on the Pt(poly) electrode that was induced by CuUPD atoms.35 It was also shown that in the case of UPD Cu on Pt single-crystal electrodes anions can markedly influence the structure of the metallic adlayer and the deposition kinetics.31 A radiochemical method showed that a CuUPD adlayer on Pt(111) develops concurrently with the bisulfate adsorption.36 The group of Kolb31 reported that an ordered layer having the (3 × 3)R30° geometry develops on Pt(111) in the case of UPD Cu from an aqueous electrolyte containing H2SO4 + CuSO4. They assigned the widely spaced (3 × 3)R30° structure to the CuUPD overlayer in the presence of coadsorbed anions (sulfate/bisulfate). The group of Abruna10,37 determined the CuUPD-CuUPD spacing using X-ray absorption spectroscopy (XAS). Their results indicate that the CuUPD overlayer assumes a hexagonal close packed structure on Pt(111) in acidic solutions. The effect of anion adsorption on UPD Cu was also investigated by many ex-situ and in situ techniques, such as low energy electron diffraction (LEED), Auger electron spectroscopy (AES), scanning tunneling microscopy (STM), and atomic force microscopy (AFM).11,17,20-24,30,31 These techniques are very useful direct or indirect methods of acquiring atomiclevel structural data. They have been instrumental in obtaining structural data about UPD Cu in acidic solutions and in providing much needed insight into the process. In particular, combination of these techniques with well-established and very precise electrochemical methods allows one to assign specific charge density values to UPD Cu and the concurrently occurring anion adsorption. Although much work has been done on UPD Cu on Pt electrodes, an aspect of research that has received little attention is the impact of temperature variation on the process. In chemical systems, the variation of temperature, pressure and concentration translates into modification of the Gibbs energy that drives them.

10.1021/jp900478u CCC: $40.75  2009 American Chemical Society Published on Web 06/15/2009

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Figure 1. Cyclic-voltammetry profile for the deposition and stripping of CuUPD on Pt(111) in aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O; T ) 298 K, s ) 5 mV s-1, and A ) 0.0431 cm2. The annotations CI, CII, AI, and AII stand for two cathodic and two anodic peaks. The inset presents a cyclic-voltammetry profile for Pt(111) in aqueous 0.05 M H2SO4 solution; T ) 298 K, s ) 50 mV s-1, and A ) 0.0431 cm2.

Thus, there exists a need to evaluate the influence of some of these parameters on electrochemical phenomena. In this contribution, we present the results of our study of UPD Cu on Pt(111) in aqueous H2SO4 + CuSO4 · 5H2O as a function of temperature using cyclic voltammetry. Changes in CV profiles brought about by the temperature variation are discussed in terms of the peak potential, peak current density, total adsorptiondesorption charge density, as well as the charge density of individual CV features. The strong dependence of UPD Cu on temperature suggests that surface structures adopted by the system might undergo significant changes or even disordering as temperature is raised. Experimental Section Pt(111) Electrode Preparation. The Pt(111) electrode was prepared according to the procedure developed by Clavilier38,39 and oriented using the methodology developed by Hamelin.40,41 It was subsequently polished with alumina (0.05 µm) to a mirrorlike finish. The quality of the Pt(111) surface was verified by recording a CV profile in aqueous 0.05 M H2SO4 solution in the 0.05-0.84 V potential range (versus SHE). Agreement between our results (Figure 1) and those reported in the literature37,42-44 indicate that the Pt(111) surface was of high quality, precisely oriented, and well ordered. The bead-shaped Pt single crystal was not a perfect sphere and the (111) surface obtained by its cutting and polishing was not a perfect circle. The Pt(111) electrode’s diameter (d) was determined by placing the electrode in a Vernier microscope and by taking ten sets of measurements; each new set of measurements was obtained by turning the single crystal by ∼36°. The average diameter of the crystal was d ) 0.1172 ( 0.0002 cm and the corresponding real surface area (here the roughness factor equals unity, R ) 1) was A ( ∆A ) 0.0431 ( 0.0001 cm2. Although the diameter and area of the Pt(111) surface were determined in air with great care, it is apparent that it is impossible to maintain exactly the same area exposed to the electrolyte during several flame annealing and cooling procedures. Thus, it is reasonable to assume that the experimental uncertainly of the surface area in contact with the electrolyte is significantly larger (by a factor or 5-10) and the value of ∆A ) 0.0005-0.0010 cm2. Solutions, Electrochemical Cells, and Reference Electrode. The aqueous 0.05 M H2SO4 solution was prepared from BDH Aristar grade H2SO4 and high-purity water (Nanopure, 18 MΩ

Jerkiewicz et al. cm-1). The deposition and stripping of Cu was performed in aqueous 5 mM CuSO4 · 5H2O (Aldrich, 99.999%) + 0.05 M H2SO4 electrolyte. The experiments were carried out in two identical, two-compartment electrochemical cells. The glassware was precleaned according to a well-established procedure.45-47 During the experiments, H2 gas was bubbled through the reference electrode (RE) compartment in which a Pt/Pt-black electrode was immersed; it served as a reversible hydrogen electrode (RHE); the zero potential on the RHE scale corresponds to -0.075 V on the SHE one (see below). Since the potential of CV features was measured versus RHE, it became necessary to convert it to the SHE scale. First, the potential measured versus RHE at any temperature was recalculated to the RHE at 298 K through eq 1 bearing in mind that the ∂E/∂T factor equals 8.4 × 10-4 V K-1.48

ERHE,298K ) ERHE,T*298K + (T - 298)

∂E ∂T

(1)

Second, the potential expressed versus the RHE (0.05 M aq. H2SO4) at 298 K was converted to the SHE scale at 298 K by application of eq 2.

ESHE ) ERHE(0.1MH2SO4) - 0.075V

(2)

The above formulas were derived on the basis of the Nernst and Davis equations as explained elsewhere.48 Finally, highpurity Ar gas, presaturated with water vapor, was passed through the working electrode (WE) compartment. The counter electrode (CE) was a Pt gauze (99.998% in purity, Aesar) and its surface area was several times greater than that of WE. Setup for Temperature Measurements. The electrochemical cells were immersed in a water bath (Haake W13) and the temperature was controlled to within ( 0.5 K by means of a thermostat (Haake D1); the water level in the bath was maintained above that of the electrolyte in the cell. The temperature in the water bath and the electrochemical cells was controlled by means of mercury thermometers and a K-type thermocouple (80 TK Fluke), and were found to agree to within ( 0.5 K. The experiments were conducted at 273 K e T e 333 K with a temperature interval of ∆T ) 5 K. Copper deposition-stripping on Pt(111) using the hanging meniscus methodology was difficult to perform below 273 K or above 333 K because of the electrolyte condensation on the side of the crystal that can facilitate creeping of the electrolyte on the crystal’s side. Rigorous passing of argon gas above the electrolyte effectively prevented any electrolyte condensation. If electrolyte condensation and creeping were to be present, then the CV profile would change from one corresponding to Pt(111) to a different one characteristic of polycrystalline Pt. In addition, the Pt/electrolyte interfacial tension is such that in the case of a half-bead crystal with a droplet of water or electrolyte, there is no observable electrolyte presence on the crystal side unless one deliberately shortens the hanging meniscus. Electrochemical Measurements. The temperature-dependent studies required very meticulous manipulations that ensured excellent long-range surface order and extreme cleanliness of the entire setup. The pretreatment procedure consisted of annealing the Pt(111) electrode in a hydrogen/oxygen flame and cooling it in a 2-to-1 volume ratio mixture of Ar(g) and H2(g). The Pt(111) surface was then protected by a droplet of high purity water and transferred to the first electrochemical cell. The purity of the system and the long-range surface order were

Deposition of Copper on Pt(111) verified by recording CV profiles (s ) 50 mV s-1) in the potential range (0.06 - 0.90 V vs RHE) corresponding to HUPD and anion adsorption in aqueous 0.05 M H2SO4. Following the electrode pretreatment and cleanliness verification, the Pt(111) electrode was transferred with a droplet of H2SO4 to the second electrochemical cell that contained aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O. CV profiles were recorded in the 0.45 0.80 V vs SHE potential range at s ) 5 mV s-1. Following the deposition and stripping of CuUPD, the Pt(111) electrode was removed from the Cu-containing cell at 0.80 V vs SHE (there is no Cu deposited at this potential) and then immersed in the cell containing aqueous 0.05 M H2SO4; a CV profile was subsequently recorded (s ) 50 mV s-1) to verify the long-range order of Pt(111) and the absence of any Cu. The electrochemical instrumentation included: (a) an EG&G Model 263A potentiostat-galvanostat; (b) an EG&G model 175 analog universal programmer, (c) a Computer Boards, Inc., Scope card and Compuscope 512-1 M software package, and (d) a personal computer. All potentials were measured with respect to RHE and were subsequently converted to the SHE scale (see above).

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Results and Discussion

Figure 2. Series of cyclic-voltammetry profiles for the deposition and stripping of CuUPD on Pt(111) in aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O recorded at seven different T values in the 273-333 K range (∆T ) 10 K); s ) 5 mV s-1 and A ) 0.0431 cm2. The annotations CI, CII, AI, and AII stand for two cathodic and two anodic peaks. The inset presents a series of cyclic-voltammetry profiles for Pt(111) in aqueous 0.05 M H2SO4 solution recorded at seven different T values in the 273 - 333 K range (∆T ) 10 K) immediately following the CuUPD deposition-stripping experiments; s ) 50 mV s-1 and A ) 0.0431 cm2.

Temperature Dependence of UPD Cu in Aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O. Figure 1 shows two CV profiles for Pt(111) both recorded at 298 K. The CV profile shown in the inset was obtained at s ) 50 mV s-1 in the first cell that contained aqueous 0.05 M H2SO4. The adsorption and desorption peaks for HUPD and anion are characteristic of a well-ordered and impurity-free Pt(111), and the results are in agreement with literature standards.42-44,49 The main CV profile was obtained at s ) 5 mV s-1 in the second cell that contained aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O; it refers to UPD Cu on Pt(111) and the two sets of well-defined deposition (cathodic) and stripping (anodic) peaks are in agreement with previously published data.17,20 The peaks CI and CII are found at 0.60 and 0.56 V, and the peaks AI and AII at 0.62 and 0.60 V, respectively. We analyzed the relationship between the peak current density (jp) and s at T ) 298 K, and determined that this relationship is linear for s e 5 mV s-1. Such a linear relationship indicates that UPD Cu is not under diffusion control for s e 5 mV s-1 but becomes diffusion controlled when s > 5 mV s-1.17 In addition, we found that the peak potential (Ep) does not shift with s for s e 5 mV s-1. Thus, the charge density obtained at a given potential corresponds to an equilibrium, although the system involves not only CuUPD adatoms but also coadsorbed anions and water molecules that all interact with the Pt(111) substrate and with each other (see below). The origin of the asymmetry in the CV profiles is discussed in a later section. In a separate series of experiments, we analyzed the influence of T variation on the CV features associated with UPD Cu. The experimental work was done at thirteen different temperature values in the 273-333 K range with an interval of ∆T ) 5 K. Figure 2 presents a series of CV profiles for seven different T values in the 273-333 K range with a temperature interval ∆T ) 10 K all recorded at s ) 5 mV s-1. The results reveal two distinct tendencies, one for 273 e T e 293 K and the other for 298 e T e 333 K. The changes in the CV profiles (Figure 2) are completely “temperature-reversible” in the sense that a subsequent decrease of T from the highest value (333 K) to the lowest one (273 K) reproduces the original CV transient. It should be added that there was no pronounced effect of T variation on the geometry of the meniscus at the Pt(111)electrolyte interphase for the T range reported in this paper; the

electrode-electrolyte contact area remained constant within the experimental uncertainty over the entire temperature range studied. The qualitative changes can be summarized as follows: (i) at low T values (273 e T e 298 K), there are two cathodic peaks (CI and CII) and an anodic one (AI); (ii) CI is quite sharp peak, while CII is broad; (iii) AI is asymmetric and seems to represent two superimposed, irresolvable features (at 273 e T < 298 K); (iv) both CI and CII become narrower and sharper upon an increase of T; (v) AI evolves into two well-defined anodic peaks (AI and AII) at 298 e T e 333 K; (vi) all peaks become narrower and their peak current density (jp) increases when T is raised; and (vii) the pronounced changes in the peaks’ morphology (blunt f sharp) occur at T = 298 K. We wish to emphasize that the CV features associated with HUPD and anion adsorption/desorption were re-examined in aqueous 0.05 M H2SO4 (without Cu2+) after the layer of CuUPD was stripped and the Pt(111) electrode was removed from the first cell at 0.80 V (CuUPD-free surface) and transferred to the first cell (inset in Figure 2); these CV profiles are in agreement with previously published data.50 They lead to the observation that the Pt(111) electrode preserves its long-range surface order upon CuUPD deposition and stripping, and that there is no noticeable Pt-Cu surface alloy formation. Presence of such an alloy in the nearsurface region would dramatically alter the CV profile for Pt(111) after its transfer to the second cell containing only aqueous 0.05 M H2SO4. In addition, if a surface alloy were to form, then the cathodic (deposition) and anodic (stripping) charge density values would not balance (see later section). Analysis of the Temperature Dependence of UPD Cu in Aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O. In order to provide a detailed analysis of the impact of T variation on the CV characteristics for UPD Cu on Pt(111), we evaluated the behavior of the following parameters as a function of T: (a) the potential of the cathodic and anodic peaks (Ep,C and Ep,A, respectively); (b) the current density of the cathodic and anodic peaks (jp,C and jp,A, respectively); (c) the total cathodic and anodic charge density (qC,T and qA,T, respectively); and (d) the charge density of the individual cathodic and anodic peaks. In Figures 3 and 4, we show the variation of Ep,C and Ep,A as a function of T. For both Cu deposition and stripping, we observed two qualitatively distinct tendencies associated with

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Figure 3. Relationship between Ep,C and T for the cathodic peaks shown in Figure 2.

Figure 4. Relationship between Ep,A and T for the anodic peaks shown in Figure 2.

Figure 5. Relationships between jp and T for the cathodic and anodic peaks shown in Figure 2.

the temperature increase in the two specific ranges: (i) 273 e T e 293 K and (ii) 298 e T e 333 K. In the case of the first temperature range (273 e T e 293 K), Ep of CI and CII shifts linearly toward higher values; the slope of the Ep,C versus T relationships (∂Ep,C/∂T) equals 4.41 × 10-4 V K-1 for CI and 1.25 × 10-3 V K-1 for CII. Ep of AI shifts toward lower values and the slope of the Ep,A versus T relationship (∂Ep,A/∂T) equals -7.65 × 10-4 V K-1 (AII does not appear in this temperature range). In the case of the second temperature range (298 e T e 333 K), there is hardly any variation of Ep,C or Ep,A with T. In Figure 5, we show the jp versus T relationships for the two cathodic and two anodic peaks. The results demonstrate that the peak current density varies significantly with T variation. The values of jp for CII continuously increase as T is raised,

Jerkiewicz et al.

Figure 6. Relationships between the total cathodic (qC,T) and total anodic (qA,T) charge density as a function of T determined on the basis of the experimental results presented in Figure 2. The inset shows the value of δqT ) qC,T - qA,T as a function of T.

while those for CI increase until T ) 318 K is reached and past this value there is only a small decrease of jp. The values of jp for AII increase only slightly, while those for AI increase until T ) 323 is reached and past this value these is only a small decrease of jp. It is interesting to observe that the jp versus T relationships for the anodic peaks are mirror images of those for the cathodic peaks. We also notice that the T variation “intensifies” more the first deposition-stripping peak set than the second one. In Figure 6, we show the relationship between the total cathodic and anodic charge densities (qC,T and qA,T, respectively) as a function of T determined on the basis of the experimental results presented in Figure 2. The results indicate that qC,T and qA,T are unaffected by the T variation. The average value of qC,T is 405 ( 0.6 µC cm-2 and that of qA,T is 402 ( 0.8 µC cm-2; thus, they agree to within 3 µC cm-2. These charge density values are in good agreement with those reported in the literature.16,31 In addition, they indirectly support our notion that we were able to have a reproducible Pt(111)-electrolyte contact that resulted in the same electrode area exposed to the electrolyte. We observe a small increase of qC,T from 400 to 406 µC cm-2 for 273 e T e 293 K and no change for 298 e T e 333 K. The value of qA,T decreases from 410 to 399 µC cm-2 for 273 e T e 293 K and remains practically constant for 298 e T e 333 K. Although we observe a small difference between qC,T and qA,T, (δqT ) qC,T - qA,T), its value never exceeds ( 10 µC cm-2 and falls within 2.5% of the total charge density value; thus, within the experimental uncertainty of our measurements (inset in Figure 6). It is important to emphasize that the total charge density measured experimentally corresponds to both the under-potential deposition of Cu and the anion adsorption.31 The theoretical charge density for the deposition of a complete monolayer of CuUPD from Cu2+ (twoelectron process) on Pt(111) is 481.6 µC cm-2. Thus, the experimentally determined average charge density value for CuUPD deposition (405 µC cm-2) and stripping (402 µC cm-2) is some 77 - 80 µC cm-2 lower than the expected value for 1 ML of CuUPD. On the other hand, it is well established that the adsorption of bisulfate anions (HSO4-) occurs concurrently with UPD Cu on Pt(111) and that they adopt the (3 × 3)R30° structure; the latter corresponds to an anion surface coverage of 0.33.17,19,31 Bearing in mind that the anion oxidative adsorption is a one-electron process, the charge density associated with the process is 80.2 µC cm-2. The latter value could account for the “charge density deficit” (versus the value that one expects for 1 ML of CuUPD) that we observe by integrating the CV

Deposition of Copper on Pt(111)

Figure 7. Dependence of the charge density (qC) of the cathodic peaks (CI and CII) on T determined on the basis of the experimental results presented in Figure 2.

Figure 8. Dependence of the charge density (qA) of the anodic peaks (AI and AII) on T determined on the basis of the experimental results presented in Figure 2.

profiles. If the charge density associated with the bisulfate adsorption (80.2 µC cm-2) is added to our experimentally determined value of qC,T ) 405 or qA,T ) 402 µC cm-2, then the overall charge density (485.2 and 482.2 µC cm-2, respectively) is remarkably close to the value of 481.4 µC cm-2 that one expects for 1 ML of CuUPD. However, Ito et al.30 proposed on the basis of IR, LEED and STM measurements that the bisulfate undergoes transition to sulfate concurrently with the deposition of CuUPD adatoms. Clearly, UPD Cu is a complex process that needs to be carefully examined. The new results that we report in this contribution, and which reveal a qualitatively new behavior, call for a further analysis of the process that should also take into account the temperature effect (see below). In order to advance our analysis of the influence of T variation on UPD Cu, we examine the charge density of individual cathodic and anodic peaks (Figures 7 and 8). The charge density values for CI are scattered between 205 and 235 µC cm-2, with the average value being 225 ( 2.5 µC cm-2, while those for CII are scattered between 171 and 195 µC cm-2, with the average value being 180 ( 2.2 µC cm-2. The analysis of AI and AII is limited to a narrower temperature range (T g 298 K) because the second anodic peak (AII) appears in CV profiles only when T reaches or exceeds 298 K. We observe that there is no significant variation of the charge density of AI or AII with temperature; the average value of the charge density for AI is 219 ( 3.9 µC cm-2 and that for AII is 180 ( 3.7 µC cm-2. Thus, our results show that the charge density for AI is always greater than that for AII (for 298 e T e 333 K) for reasons that

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Figure 9. Dependence of δqI and δqII on T determined on the basis of the experimental results presented in Figures 7 and 8.

still remain unclear. We also analyzed the difference between the respective cathodic and anodic charge densities (δqI and δqII, respectively) for the peaks I and II in the 298 e T e 333 K range, thus in the range where two peaks are observed (Figure 9). The results indicate that δqI decreases from positive to negative values and δqII increases from negative to positive values. Importantly, these two relationships are almost mirror images and the magnitude of δqI and δqII never exceeds some 30 µC cm-2. Importance and Significance of the New Results. The above-presented results and their qualitative analysis offer an original and new contribution, which demonstrates that the variation of temperature has a pronounced impact on UPD Cu and anion adsorption on Pt(111). These results as well as those for UPD H,48,50 UPD Ag,51 Asads,52 and Biads53 on Pt(111) prove that there exists a general trend that demonstrates a pronounced impact of temperature variation on various surface electrochemical processes. However, it is important to perform a more detailed analysis of our results and to elaborate on the importance of our findings. It is essential to analyze the impact of T variation on the magnitude of CuUPD deposition and anion adsorption in the light of the explanations proposed by Ito et al.30 and Kolb et al.31 The interpretation put forward by Kolb et al. (Scheme 1, eqs 3 and 4) involves a reductive deposition of Cu2+ (eq 3) and a concurrently occurring oxidative adsorption of HSO4- (eq 4). The overall process implies transfer of one electron from each HSO4- that undergoes adsorption to only a fraction of Cu2+ ions that become adsorbed, because the surface coverage of HSO4 ads (θHSO4) is smaller than that of CuUPD (θCu). The process can be interpreted as charge redistribution between adsorbing species that is mediated by the Pt(111) substrate and driven by the externally applied electric field. Importantly, the charge density associated the charge redistribution when anions are oxidatively adsorbed while cations are reductively deposited cannot be determined from CV profiles by their integration because there is no external charge supplied to drive the process.

Scheme 1 Cu2++2e- f CuUPD(θCu)

(3)

HSO4--e- f HSO4 ads(θHSO4)

(4)

The interpretation proposed by Ito et al. (Scheme 2, eqs 3-5) is more complex and involves a reductive desorption

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of HSO4 ads (eq 5; the species is present on the Pt(111) surface prior to UPD Cu), a reductive deposition of Cu2+ (eq 3), an oxidative adsorption of HSO4- (eq 4), and an oxidation of HSO4 ads to SO4 ads (eq 6).

Scheme 2 HSO4 ads(θ1,HSO4) + e- f HSO4-

(5)

Cu2++2 e- f CuUPD(θCu)

(3a)

HSO4--e- f HSO4 ads(θ2,HSO4)

(4a)

HSO4 ads(θ2,HSO4) - e- f SO4 ads(θ2,SO4) + H+

(6)

The overall charge density associated with all these processes depends on the coverage values of the individual adsorbates and the number of electrons transferred in each process. The theoretical charge density associated with the transfer of one electron (n ) 1) per each atom of the Pt(111) surface whose real surface area (Ar) equals 1.00 cm2 is qPt(111) ) 240.8 µC cm-2. In the case of the interpretation proposed by Kolb et al. (Scheme 1),28 the CuUPD coverage is known to be 1 ML (θCu ) 1) and the anion (bisulfate or sulfate) coverage is one-third of ML (θHSO4 ) θSO4 ) 0.33). The charge density (in µC cm-2) supplied externally to the Pt(111) electrode to drive the UPD Cu and anion adsorption processes is given as follows:

Charge Density for Scheme 1 qtheor ) qPt(111) × θCu × nCu + qPt(111) × θHSO4 × nHSO4 ) 481.6 - 79.5 ) 402.1 (7) In the case of the interpretation propose by Ito et al.,27 the initial surface coverage of HSO4 ads (θ1,HSO4) is 0.33 (anions present on the Pt(111) surface before UPD Cu commences), the CuUPD coverage is 1 ML (θCu ) 1), and the anion (bisulfate or sulfate) coverage is one-third of ML (θ2,HSO4 ) θ2,SO4 ) 0.33). The charge density supplied externally to the Pt(111) electrode to drive the UPD Cu and anion adsorption processes is given as follows:

Charge Density for Scheme 2 qtheor ) qPt(111) × θ1,HSO4 × n1,HSO4 + qPt(111) × θCu × nCu + qPt(111) × θ2,HSO4 × n2,HSO4 + qPt(111) × θ2,SO4 × n2,SO4 ) 79.5 + 481.6 - 79.5 - 79.5 ) 402.1

(8) where nCu ) 2, n1,HSO4 ) 1, n2,HSO4 ) -1 and n2,SO4 ) -1. Our experimental values of qC,T ) 405 or qA,T ) 402 µC cm-2 agree remarkably well with the mechanism proposed by Ito et al.27 as well as the mechanism proposed by Kolb et al.28 Therefore, at the present time both Schemes describe quantitatively the process not only for room temperature but also for the range of temperatures (273 e T e 333 K) reported in this paper. However, the mechanistic analysis proposed by Ito et al.27 seems more thorough in the sense that it takes into account the initial

state of the Pt(111) electrode (presence of adsorbed anions, HSO4 ads) before UPD Cu commences. It ought to be emphasized that in order to maintain electroneutrality of the interphase, the hydronium cations must be present in the double layer region. Since they are not discharged or adsorbed, they do not contribute to CV features. We wish to recall that the first dissociation constant of H2SO4 is infinity (pKa1 ) ∞), while the second one has a finite value (pKa2 ) 1.98). The latter implies that the concentration of HSO4- in electrolyte is much higher than that of SO42-; thus the direct adsorption of SO42- is very unlikely. It is important to elaborate on the meaning of the morphological changes observed in the CV profiles brought about by T variation. In order to propose a reasonable hypothesis, it is necessary to recall what we already know about the temperature behavior of CV for the Pt(111) in aqueous H2SO4 and to evaluate what happen within an electrochemical system when T is increased. Elsewhere48 and in the inset of Figure 2, we showed that in the case of Pt(111) in 0.05 aqueous H2SO4 an increase of T does not change the part of CV profile corresponding to UPD H but gradually decreases the sharpness of the sharp peak (spike-like) overlapping a broad CV wave that is assigned to the anion adsorption and desorption; the broad CV wave (semicircle-like) is unaffected by the T variation. In general, sharp and narrow CV peaks are associated with either a longrange phase formation or a phase transition. Thus, decrease of the intensity of the sharp peak (spike) associated with the adsorbed anions suggest that the development of a long-range phase comprising anions is strongly affected by the increase of T, and becomes less pronounced as T is raised (anions are present at the interface but do not develop any ordered phase). This proposal is reasonable because as T is raised, the thermal energy of anions, cations and water molecules constituting the electrolyte increases, while the degree of order decreases. Because in the case of UPD Cu on Pt(111) in aqueous 0.05 H2SO4 we observe that the CV peaks become narrower and sharper as T is raised, and because this behavior is just the opposite of the one observed for H2SO4 alone, we conclude that this is a unique characteristic of UPD Cu on Pt(111) in 0.05 M aqueous H2SO4 + 5 mM CuSO4. Coupled CV and in situ electrochemical STM measurements23 for the same system indicate that the two cathodic peaks correspond to the formation of a (3 × 3)R30° structure and a disordered phase, respectively. A long-range ordered (3 × 7) oblique structure was imaged after a full monolayer of Cu was deposited and it was tentatively assigned to the (bi)sulfate layer residing on-top of the Cu overlayer. The results of our T-dependent measurements demonstrate that the order-disorder transition within the Cu overlayer and the anion adsorption are strongly affected by T variation. In addition, increase of the sharpness of the CV peaks points to some synergetic effects between the adsorbed species. Clearly, one would expect the CV features associated with the anion adsorption to gradually lose their sharpness upon T increase. However, a completely opposite behavior is observed and we propose two possible explanations. Explanation 1: the sharp CV features correspond only to the deposition of CuUPD in two stages because the thermal energy of adsorbed anions becomes so large that it overcomes their adsorption energy; only CuUPD adatoms interact with the Pt(111) substrate and the process gives rise to two sharp CV peaks that might be associated with the development of two ordered and/or disordered structures. Explanation 2: the sharp CV features correspond to the deposition of CuUPD in the presence of adsorbed anions; the thermal energy of anions is sufficient to overcome

Deposition of Copper on Pt(111) anion-anion lateral interactions that are responsible for the formation of an ordered overlayer but is not high enough to make them desorb; the two CV peaks corresponds to the deposition of CuUPD in the presence of coadsorbed anions and the development of two ordered and/or disordered structures. It is also interesting to elaborate on the meaning of the morphological changes of the CV peaks as T is varied within the framework of the modeling of CV transients performed by Conway et al.54,55 Their modeling did not take into account any T variation and is based on the assumption that a CV wave corresponds to the adsorption of one species that is accompanied by the transfer of one electron; they did not take into account any phase formation or phase transition phenomena. They concluded that an increase of the sharpness of a CV peak corresponds to an increase of energy of lateral interaction. However, the system analyzed in this contribution is very complex and includes coadsorbed species that interacts with one another as well as phase formation and disorder phenomena. Thus, the results presented in this contribution cannot be analyzed within the framework of the modeling of Conway et al. We are not aware of any temperature-dependent electrochemical STM or FTIR studies. Consequently, our interpretation presented here is limited to a detailed electrochemical analysis, although we aspire to gradually perform structural studies using other experimental techniques. Finally, it is essential to discuss the issue of asymmetry in our CV profiles. As we have discussed it above, the asymmetry is not scan rate dependent for s e 5 mV s-1 and, consequently, it is not due to kinetic irreversibility of the process. In our theoretical treatment of UPD,56 we explained that when asymmetry in CV profiles arises for nonkinetic reasons, then its origin lies in thermodynamic aspects of the process. Specifically, a complete CV transient (where the initial and final potentials are the same, Ei ) Ef) refers to a close thermodynamic cycle and I∆G° ) 0. In the case of UPD H on Pt(111) in aqueous H2SO4 or HClO4 solutions, the cathodic and anodic components of the CV profiles are mirror images and the corresponding ∆G°ads and ∆G°des have the same absolute values for a give T and θH. An asymmetry of the cathodic and anodic parts of CV profiles indicates that the absolute values of ∆G°ads and ∆G°des are not the same. Their difference (δ∆G° ) ∆G°ads + ∆G°des, with ∆G°ads being negative and ∆G°des being positive) can be assigned to other interfacial processes, such as surface reconstruction (∆G°reconst), surface compression (∆G°comp), reorientation of adsorbed anions (∆G°reorient), or changes in interfacial hydration (∆G°hydr). Clearly, an unmodified Pt(111) and a CuUPDcovered Pt(111) interact differently with H2O molecules in the double layer region. Any changes in the H2O coverage and orientation cannot be monitored directly by electrochemical techniques because there is no charge transfer. However, they indirectly contribute to electrochemical characteristics (such as CV profiles) that subsequently can be attributed to these phenomena. In summary, we present new and original results on the CV behavior of UPD Cu on Pt(111) brought about by T variation. Our results reveal a strong dependence of the process on temperature that point to structural phenomena and changes of thermodynamic state functions of the process. Clearly, other experimental techniques will have to be employed in order to fully appreciate and explain their meaning. However, the absence of structural data does not diminish the significance of our data because there are practically no other results that demonstrate how T variation affects metallic UPD. We intend to advance our study of the influence of temperature variation

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12315 on this interfacial electrochemical phenomenon (metallic UPD) by employing vibrational spectroscopy (FTIR) and surface imaging (STM) techniques. Because nobody has ever performed T-dependent EC-STM or FTIR experiments, it will most likely take a considerable effort, time and technical advances before a molecular-level insight into the process can be gained. Conclusions We used cyclic-voltammetry to investigate the underpotential deposition of Cu on Pt(111) as a function of temperature. We present new data that reveal hitherto unknown temperature-dependent behavior of UPD Cu on Pt(111) in aqueous 0.05 M H2SO4 + 5 mM CuSO4 · 5H2O. Our results point to considerable changes in the potential and current density of CV peaks brought about by T variation. Although the total charge density of cathodic and anodic peaks is temperature-independent, we observe charge density redistribution among the CV peaks as T is varied. The peak potential displacement is observed only at low temperature values (273 e T e 293 K). There are two cathodic and two anodic peaks in the CV profiles, but the second anodic peak appears only at high temperature values (T g 298 K). There is no change of the total charge density that could be assigned to T variation. The existence of two asymmetric cathodic and two anodic peaks is due to some surface structural effects, lateral interaction phenomenon or surface compression that involve both the CuUPD overlayer and the adsorbed anions. Acknowledgment. We acknowledge financial support from the NSERC of Canada (Discovery Grant, Reseach Tools and Instruments Grants), FCAR du Que´bec (Research Team Grant), and Queen’s University (Research Initiation Grant). References and Notes (1) Haissinsky, M. J. Chim. Phys. 1946, 43, 21. (2) Kolb, D. M.; Przasnyski, M.; Gerischer, H. J. Electroanal. Chem. 1974, 54, 25. (3) Kolb, D. M. In: AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley Interscience: New York, 1978; Vol. 11. (4) van der Eedern, J. P.; Staikov, G.; Kashchiev, D.; Lorenz, W. J. Surf. Sci. 1979, 82, 364. (5) Budevski, E. B.; Staikov, G. T.; Lorenz, W. J. Electrochemical Phase Formation and Growth; VCH: New York, 1996. (6) Paunovic, M.; Schlesinger, M. Fundamentals of Electrochemical Deposition; John Wiley & Sons: New York, 1998. (7) Andricacos, P. C.; Ross, P. N. J. Electroanal. Chem. 1984, 167, 301. (8) Hubbard, A. T.; Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P.; Song, D. J. Electroanal. Chem. 1983, 150, 165. (9) Scortichini, C. L.; Reilley, C. N. J. Electroanal. Chem. 1983, 152, 2555. (10) White, J. H.; Abruna, H. D. J. Electroanal. Chem. 1989, 274, 185. (11) White, J. H.; Abruna, H. D. J. Electroanal. Chem. 1991, 300, 521. (12) Bhatt, D. P.; Twomay, T.; Plieth, W. J. Electroanal. Chem. 1992, 322, 279. (13) Markovic, N.; Ross, P. N. Langmuir 1993, 9, 580. (14) Yee, H. DS.; Abruna, H. D. J. Phys. Chem. 1993, 97, 894. (15) Yee, H. S.; Abruna, H. D. Langmuir 1993, 9, 2460. (16) Nashikara, O.; Nozoye, H. J. Electroanal. Chem. 1995, 386, 75. (17) Sashikata, K.; Furuya, N.; Itaya, K. J. Electroanal. Chem. 1991, 316, 361. (18) Buller, L. J.; Herrero, E.; Gomez, R.; Feliu, J. M.; Abruna, H. D. J. Chem. Soc. Faraday Trans. 1996, 92, 3757. (19) Ogasawara, H.; Inukai, J.; Ito, M. Surf. Sci. 1994, 311, L665. (20) Abe, T.; Swain, G. M.; Sashikata, K.; Itaya, K. J. Electroanal. Chem. 1995, 382, 73. (21) Tsay, J. S.; Mangen, T.; Linden, R.-J.; Wandelt, K. Surf. Sci. 2001, 482, 866. (22) Tsay, J. S.; Mangen, T.; Linden, R.-J.; Wandelt, K. J. Vac. Sci. Technol. A 2001, 19, 2217. (23) Wu, Z.-L.; Zang, Z.-H.; Yau, S.-L. Langmuir 2000, 16, 3522.

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