Influence of Surface Defect Structure on the Underpotential Deposition

Nov 15, 1995 - Received June 26, 1995. In Final Form: September 13, 1995X ... for the Ag(111) films decreased from 1.15 for unannealed films to 0.99 (...
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Langmuir 1996, 12, 494-499

Influence of Surface Defect Structure on the Underpotential Deposition of Pb Monolayers at Ag(111) Keith J. Stevenson, David W. Hatchett, and Henry S. White* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received June 26, 1995. In Final Form: September 13, 1995X The underpotential deposition (upd) of Pb monolayers at Ag(111) thin-film electrodes in perchlorate solutions have been investigated as a function of the substrate defect structure. Ag(111) films were grown epitaxially on muscovite mica by thermal evaporation and annealed at 300 °C at 10-6 Torr for periods between 0 and 12 h. X-ray diffraction and electrochemical scanning tunneling microscopy indicate that thermal annealing significantly increases the degree of film crystallinity and smoothness. A more ideal voltammetric response for the upd of Pb was obtained using Ag thin-film electrodes that were annealed for periods longer than 6 h. Specifically, the width of the upd voltammetric wave, as well as the splitting between the cathodic and anodic peaks, was found to decrease with an increase in annealing time. Surface roughness factors (defined as the ratio of the electrochemically-active area relative to the geometric area) for the Ag(111) films decreased from 1.15 for unannealed films to 0.99 ( 0.03 for films after 12 h of annealing. The results indicate that previously reported voltammetric features observed for the upd of Pb monolayers in perchlorate solutions are associated with Pb deposition at Ag(111) surface defect sites.

Introduction Electrochemical deposition of Pb monolayers on wellordered Ag electrodes has been the subject of numerous investigations during the past several decades.1-15 Early investigations16-18 using single-crystal Ag electrodes established that ca. one equivalent monolayer of Pb can be deposited from Pb2+ solutions onto the low-index faces of Ag at potentials 100-200 mV positive of the potential of bulk Pb deposition, E°(Pb2+/Pb). The deposition of metal atoms on a foreign metal electrode at potentials positive of the reversible bulk potential is due to work function differences between the two dissimilar metals. This process, of which the deposition of a Pb monolayer at Ag is one of many similar systems that has been extensively X Abstract published in Advance ACS Abstracts, November 15, 1995.

(1) Bort, H.; Juttner, K.; Lorenz, W. J.; Schmidt, E. J. Electroanal. Chem. Interfacial Electrochem. 1978, 90, 413. (2) Takayanagi, K.; Kolb, D. M.; Kambe, K.; Lehmpfuhl, G. Surf. Sci. 1980, 100, 407. (3) Klimmeck, M.; Juttner, K. Electrochim. Acta 1982, 27, 83. (4) Schmidt, E.; Siegenthaler, H. J. Electroanal. Chem. Interfacial Electrochem. 1983, 150, 59. (5) Jovicevic, J. N.; Jovic, V. D.; Despic, A. R. Electrochim. Acta 1984, 29, 1625. (6) Jovic, V. D.; Jovicevic, J. N.; Despic, A. R. Electrochim. Acta 1985, 30, 1455. (7) Carro, P.; Hernandez Creus, A.; Gonzalez, S.; Salvarezza, R. C.; Arvia, A. J. J. Electroanal. Chem. Interfacial Electrochem. 1991, 310, 361. (8) Jovic, V. D.; Jovic, B. M.; Despic, A. R. J. Electroanal. Chem. Interfacial Electrochem. 1990, 288, 229. (9) Melroy, O. R.; Toney, M. F.; Borges, G. L.; Samant, M. G.; Kortright, J. B.; Ross, P. N.; Blum, L. Phys. Rev. B 1988, 38, 10962. (10) Samant, M. G.; Toney, M. F.; Borges, G. L.; Blum, L.; Melroy, O. R. Surf. Sci. 1988, 193, L29. (11) Melroy, O. R.; Toney, M. F.; Borges, G. L.; Samant, M. G.; Kortright, J. B.; Ross, P. N.; Blum, L. J. Electroanal. Chem. Interfacial Electrochem. 1989, 258, 403. (12) Toney, M. F.; Gordon, J. G.; Samant, M. G.; Borges, G. L.; Melroy, O. R.; Yee, D.; Sorensen, L. B. J. Phys. Chem. 1995, 99, 4733. (13) Lorenz, W. J.; Grassa, L. M.; Schmidt, U.; Obretenov, W.; Staikov, G.; Bostanov, V.; Budevski, E. Electrochim. Acta 1992, 37, 2173. (14) Laguren-Davidson, L.; Lu, F.; Salaita, G. N.; Hubbard, A. T. Langmuir 1988, 4, 224. (15) Vitanov, T.; Popov, A.; Staikov, G.; Budevski, E.; Lorenz, W. J.; Schmidt, E. Electrochim. Acta 1986, 31, 981. (16) Bewick, A.; Thomas, B. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 127. (17) Bewick, A.; Thomas, B. J. Electroanal. Chem. Interfacial Electrochem. 1976, 70, 239. (18) Dickertmann, D.; Koppitz, F. D.; Schultze, J. W. Electrochim. Acta 1976, 21, 967.

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investigated and reviewed in recent years,19-21 is commonly referred to as underpotential deposition (upd). Upd and Pb at single-crystal Ag electrodes has been characterized in previous voltammetric experiments as a chemically reversible wave, or set of reversible waves. The potential at which upd occurs and the observed voltammetric wave shape are sensitive to the crystallographic orientation of the electrode, reflecting different degrees of stabilization of the Pb monolayer by the substrate. The presence of adsorbing electrolyte anions has also been reported to influence the position and shape of the upd voltammetric response.5,6,8,12,14,16 For instance, a single and very narrow voltammetric wave is observed in solutions containing adsorbing anions (e.g., halides and CH3COO-), while a more complex three-wave pattern, consisting of a broader primary peak and two smaller secondary waves, is observed in solutions that contain a weakly adsorbed anion (e.g., perchlorate). Several researchers12,15,16 have speculated that this behavior may be associated with Pb deposition at surface defect sites (e.g., step edges), which are blocked to different degrees by anion adsorption (e.g., halides to a greater extent than ClO4-). In the present article we report an investigation concerning the effects of thermal annealing of Ag(111) thin-film electrodes on the voltammetric response observed during the upd of Pb. The electrodes used in this study were prepared by thermal evaporation of Ag on muscovite mica, a well-known procedure for preparing ordered Ag(111) surfaces that yields voltammetric behavior for the upd of Pb, as well as other metal adlayers, that is comparable to that observed at well-defined (111)-oriented Ag single crystals.14 The upd of Pb of Ag(111) films has been employed in our laboratory during the past year as a relatively simple diagnostic measure of the quality of Ag film electrodes employed in related studies and as a method for measuring the electrochemically-active surface area (which may be substantially different than the geometrical electrode area, as shown below). During these (19) Juttner, K.; Lorenz, W. J. Z. Phys. Chem. Neue Folge 1980, 122, 163. (20) Kolb, D. In Advances in Electrochemistry and Electrochemical Engineering; Gericher, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (21) Kolb, D. M.; Przasnyski, M.; Gericher, H. J. Electroanal. Chem. Interfacial Electrochem. 1974, 54, 25.

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studies, we have observed that the voltammetric response for the upd of Pb shows more ideal behavior, concurrent with a significant decrease in surface roughness, after annealing the electrodes at elevated temperatures and reduced pressures. In contrast to previously described results for the same systems, we have also observed that the voltammetric response for Pb upd at annealed Ag films in the presence of acetate (CH3COO-) is significantly broader and less well defined than in ClO4- solutions. Thus, the purpose of this report is 2-fold. First, we wish to communicate the method of Ag film growth and annealing that we have attempted to optimize in preparing Ag(111) electrodes. Although the thermal evaporation of Ag on mica has been previously employed for preparing electrodes,22-24 it does not appear that a systematic investigation of the effect of thermal annealing on the voltammetric behavior of these electrodes has been previously described. Second, our results indicate that the anticipated effects of thermal annealing, which include an increase in the degree of crystallographic order and a decrease in surface roughness, are readily reflected in the voltammetric response of these electrodes during the upd of a Pb monolayer. These data, which are supported by X-ray diffraction analysis and in-situ electrochemical scanning tunneling microscopy of the Ag films, support previous suggestions in the literature that the dependence of the voltammetric wave shape on the electrolyte anions is associated with structural defects on the Ag(111) surface. Experimental Section Sample Preparation. Slightly modified procedures described by Jaeger et al.25 and Reichelt and Lutz26 were followed in preparing the Ag(111) electrodes. Ag films were deposited on mica in a Veeco CVC CVE-20 filament evaporator (TFS Technologies, Albuquerque, NM). Care was taken to avoid using mica substrates with visible step structures. A base pressure between 8 × 10-7 and 2 × 10-6 Torr was maintained during deposition using sorption and turbomolecular pumps. Freshly cleaved muscovite mica substrates (thickness ∼ 0.25 mm and area ∼ 3 cm2) were positioned 23 cm above the Ag source using a stainless steel substrate holder. A movable shutter separated the source and mica substrates. Prior to deposition of Ag, the mica was heated, using backside illumination from two quartz lamps, to 350 °C for 2 h to desorb surface impurities. Sample temperature was monitored using a type K thermocouple sandwiched between two pieces of mica that were attached to the sample holder. The Ag shot (99.999%, Alpha/Aesar) was resistively heated in a Mo boat for 2 min with the shutter closed to outgas impurities in the metal. The shutter was then opened, and Ag was allowed to deposit onto the mica at a rate of 2 Å/s, until a film thickness of 3000 Å was achieved. Deposition rates and film thicknesses were monitored using a Kronos QM-311 thickness monitor. After the desired film thickness was achieved, the Ag films were annealed at 300 °C for 0, 2, 6, or 12 h using the quartz lamp heaters. The lamps were turned off, and the samples were allowed to return to room temperature (∼3 h). The evaporation chamber was vented to the atmosphere, and the samples were immediately placed in a desiccator filled with N2. Electrochemical measurements and surface analyses were generally performed within 1 day after the deposition. Materials and Electrochemical Apparatus. Solutions were prepared using water obtained from a Barnstead water purification system (“E-pure”) with the feed water inlet connected to an in-house deionized water line. All chemicals (NaClO4, NaF, HClO4, CH3CO2H, CH3CO2Na, PbClO4, Pb(CH3COO)2, and PbO) were reagent grade and used as received. A conventional one-compartment, three-electrode Pyrex glass cell was used for the electrochemical measurements. Pt wire (22) Pashley, D. W. Adv. Phys. 1965, 14, 327. (23) Newman, R. C. Philos. Mag. 1957, 2, 750. (24) Pashley, D. W. Philos. Mag. 1959, 4, 316. (25) Jaeger, H.; Mercer, P. D.; Sherwood, R. G. Surf. Sci. 1967, 6, 309. (26) Reichelt, K.; Lutz, H. O. J. Cryst. Growth 1971, 10, 103.

Langmuir, Vol. 12, No. 2, 1996 495 and Ag/AgCl (3 M NaCl) electrodes were employed as the counter and reference electrodes, respectively. Ag film electrodes were prepared by cutting the larger mica/Ag substrates into ∼1 cm × 0.4 cm samples. A small spring-loaded Cu clip was used to make electrical contact to one end of the sample. The electrode was immersed to a depth of ∼0.5 cm into the solution taking precaution not to contact the Cu clip. Solutions were purged for ∼20 min with N2 before electrochemical analysis to remove O2 from the solution, and a positive pressure of N2 was maintained over the solution during the measurements. The cell temperature was 25 ( 2 °C. Voltammetric measurements were performed using an EG&G Princeton Applied Research Model 173 potentiostat/ galvanostat, a Model 175 universal programmer, and a Kipp & Zonen Model BD 90 XY recorder. Surface Characterization. X-ray diffraction analysis was performed using a Rigaku CN2005 X-ray diffractometer that employes a Cu KR1 line source. Mica/Ag samples were mounted on a rotating sample holder, and scattering intensity data were obtained in a θ/2θ geometry. The data were acquired between 35° and 85°. Data acquisition was performed every 0.05° with an integration time of 3 s. Scattering intensity data were normalized to the peak intensity of the (111) reflection (38.117°) and plotted as intensity (au) vs 2θ. The STM and control electronics were custom built and similar to those described earlier.27 Independent control of the tip and substrate potential was accomplished by modifying a Pine Instruments RDE 3 bipotentiostat and integrating it into the STM electronics. A two-piece, three-electrode electrochemical cell, machined out of Kel-F, was designed with Pt and Ag wires serving as the counter and reference electrodes, respectively. All measurements were made in 0.1 M NaF under potential control. A Pyrex bell jar modified with a nitrogen gas inlet covers the STM to provide an inert atmosphere free of oxygen. Etched tungsten or mechanically-cut Pt (90%)-Rh (10%) tips were used in all imaging experiments. The tips were insulated with Apiezon wax or hot glue to block out faradaic reactions.

Results and Discussion The voltammetric response of a Ag(111) electrode for the upd of Pb in an aqueous solution containing 0.1 M NaClO4, 0.01 M HClO4, and 0.005 M Pb(ClO4)2 is shown in Figure 1. The voltammetric response is characterized by three reversible and closely-spaced waves at potentials ∼150 mV positive of the bulk deposition of Pb (-440 mV vs Ag/AgCl). Following the notation used by Toney et al.,12 we designate the pairs of anodic and cathodic peaks as A1/C1, A2/C2, and A3/C3 (see Figure 1). In general agreement with previous reports, we observe that the wave at intermediate potentials (A2/C2) is significantly larger than the two waves at more positive (A1/C1) and negative potentials (A3/C3). This three-wave pattern is qualitatively similar to that previously reported during the past two decades by several research groups1,2,4-7,12,14-16,19 for upd of Pb at Ag(111) in solutions containing ClO4-. An examination of the literature on this subject indicates that the relative magnitudes of the A1/C1, A2/C2, and A3/C3 waves depend on the method of surface pretreatment. The three-wave pattern is also obtained at both Ag(111) singlecrystal electrodes and (111)-oriented Ag films.1-5,12,15,16 However, mechanical polishing of Ag(111) single crystals is reported to result in a noticeable increase in the side peaks (A1/C1 and A3/C3) relative to the main A2/A3 wave,6,7,16 a result that has been confirmed in our laboratory. The initial investigations of Pb upd at Ag(111) by Bewick and Thomas suggested that the above voltammetric response corresponds to the deposition of ∼1 monolayer of Pb.16,17 More recent in-situ grazing incidence X-ray diffraction (GIXS) measurements have demonstrated that the completed Pb upd layer is a closed-packed structure, (27) Zeglinski, D. M.; Ogletree, D. F.; Beebe, T. P.; Hwang, R. Q.; Somorjai, G. A.; Salmeron, M. B. Rev. Sci. Instrum. 1990, 61, 3769.

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Figure 2. Voltammetric response of Ag film electrodes as a function of annealing time. Scan rate: 20 mV/s. Geometric areas of electrodes (annealing time): 0.24 cm2 (0 h), 0.26 cm2 (2 h), 0.21 cm2 (6 h), and 0.30 cm2 (12 h).

Figure 1. Voltammetric response of a 0.22 cm2 mica/Ag(111) film in a N2-purged solution containing 5 mM Pb(ClO4)2, 0.1 M NaClO4, and 10 mM HClO4. Scan rate: 10 mV/s. The solid curve corresponds to a well-annealed (12 h) Ag(111) film.

rotated from the Ag(111) surface by ∼4.5°.9-12 This structure has been recently confirmed by in-situ STM28,29 for the same solution conditions and will be assumed in the discussion below. We routinely observe that the voltammetric response of the evaporated Ag films on mica is similar to that reported in the literature but with significantly reduced A1/C1 and A3/C3 peaks. Furthermore, extended lengths of thermal annealing frequently, but not always, yield a voltammetric response in ClO4- solutions in which both the A1/C1 and A3/C3 peaks are completely absent. For instance, the voltammogram indicated by the solid line in Figure 1 corresponds to the response of a mica/Ag(111) electrode that was thermally annealed at 300 °C for 12 h at a pressure of ∼10-6 Torr. Clearly, both the A1/C1 and A3/C3 waves are substantially reduced in the voltammetric response. This voltammetric response is typical of Ag films annealed for periods g6 h and has been reproduced in our laboratory using electrodes prepared in different thermal depositions. The voltammetric curve in Figure 1 showing the three-wave pattern (dashed line) is, in fact, atypical of the behavior of mica/Ag(111) films prepared in our laboratory. Representative voltammograms for the deposition of Pb at mica/Ag(111) electrodes in perchlorate solutions after 0, 2, 4, and 12 h annealing periods are presented in Figure 2. The effect of annealing over extended periods of time has four noticeable effects. First, the baseline capacitive current at potentials positive and negative of the upd wave decreases by ∼50% (see also Figure 1). Since the capacitance of the electrode is expected to be roughly proportional to the electrode area, this decrease in baseline current is indicative of a decrease in the true electrode area. Second, the splitting between the anodic and (28) Muller, U.; Carnal, D.; Siegenthaler, H.; Schmidt, E.; Lorenz, W. J.; Obretenov, W.; Schmidt, U.; Staikov, G.; Budevski, E. Phys. Rev. B 1992, 46, 12899. (29) Obretenov, W.; Schmidt, U.; Lorenz, W. J.; Staikov, G.; Budevski, E.; Carnal, D.; Miller, U.; Siegenthaler, H.; Schmidt, E. Faraday Discuss. 1992, 94, 107.

Figure 3. Voltammetric peak splitting (∆Ep (b)) and anodic peak fwhm (9) for the Pb upd A2/C2 wave as a function of annealing time. All data were recorded at a scan rate of 20 mV/s. The average value and estimated errors (one standard deviation) at each time are based on at least five different electrodes.

cathodic peak potentials of the A2/C2 wave decreases from ∼35 mV for an unannealed Ag film to ∼10 mV for films annealed for 6 and 12 h. Third, the full width at halfmaximum (fwhm) of both the A2 and C2 peaks decreases by approximately a factor of 2. Finally, as noted above, the side peaks (A1/C1 and A3/C3) are significantly reduced relative to the A2/C2 wave. Figure 3 shows values of ∆Ep and fwhm as a function of annealing time. The data suggest that most of the effect of the thermal annealing is realized during the first 6 h. The error bars in Figure 3 (representing one standard deviation and computed on the basis of voltammetric measurements using at least five mica/Ag(111) electrodes at each annealing time) provide a semiquantitative indication of the reproducibility of the effect of thermal annealing. We observed that there is some variability in the voltammetric response, even between electrodes cut from the same mica/Ag substrate. In particular, the relative magnitudes of the A1/C1 and A3/C3 waves vary slightly from one electrode to another but are always less pronounced (relative to the A2/C2 wave) than previously reported in the literature. The apparent roughness, R, of the mica/Ag(111) electrodes was computed as the ratio of the electrochemicallyactive area relative to the geometric area. The electro-

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Figure 4. Roughness factor (R ) electrochemically active area/ geometrical area) of Ag films as a function of annealing time. The average value and estimated errors (one standard deviation) at each annealing time are based on 7-21 different electrodes.

chemically-active area was determined by dividing the integrated charge associated with either the anodic or cathodic waves by the theoretical charge expected for a close-packed monolayer of Pb on a Ag(111) surface. To compute the theoretical charge, we used the nearestneighbor distance for the Pb layer in ClO4- solutions recently measured by Toney et al. using GIXS.12 The Pb upd layer undergoes a slight potential-dependent compression on Ag(111), giving rise to a nearest-neighbor distance that decreases from 3.44 to 3.39 Å in the potential range between the upd and bulk deposition of Pb. An average value of 3.415 was assumed in our calculations, yielding a theoretical charge of 318 µC/cm2. Figure 4 shows that R decreases from 1.15 ( 0.03 for unannealed Ag films to 0.99 ( 0.03 for films annealed for 12 h (these statistics are based on 7-21 independent measurements performed at each annealing time). Similar to the fwhm and ∆Ep data, there is no significant decrease in R for films annealed longer than 6 h. The decrease in ∆Ep, fwhm, and R with increasing annealing time suggests that thermal annealing produces a more well-ordered and smoother Ag(111) surface. On the basis of the results of previous investigations of Ag deposition on mica,24,26 this conclusion is anticipated since it is generally observed that deposition or annealing at temperatures above 300 °C results in a significant decrease in the density of twin boundaries, dislocations, and other defect structures. In situ electrochemical STM and glancing angle X-ray diffraction analyses of the mica/ Ag(111) samples were performed to confirm that the dependence of the voltammetric response on annealing time resulted from an increase in the overall quality of the Ag layer. X-ray diffraction patterns for mica/Ag(111) electrodes annealed at 0, 2, 6, and 12 h are presented in Figure 5. These data were reproduced several times with qualitatively similar results. The effect of annealing is apparent in the decrease of the (200), (220), and (311) diffraction peaks, indicating preferential growth of (111)-oriented grains. The line widths for the (111) also decrease with annealing. We also frequently observe that the (111) diffraction peak is split for the unannealed Ag films and those annealed for 2 h, a finding that we speculate is due to twinning of the Ag(111) films. Twinning of thermallydeposited Ag(111) films on mica has been previously

Figure 5. X-ray diffraction patterns of 3500 Å-thick Ag films annealed at 300 °C for 0, 2, 6, and 12 h.

reported to occur in the 〈112〉 direction.30 The splitting of the (111) peak is absent in the diffraction patterns of all samples annealed for 6 and 12 h, indicating that thermal annealing is effective in eliminating this feature. It is also reasonable to assume that the number and size of twins present in an annealed sample is substantially reduced. As in the case of the voltammetric results, the X-ray diffraction data suggest that very little improvement in the film quality is obtained by annealing for periods longer than 6 h. Figure 6 shows in-situ STM images recorded in 0.1 M NaF of unannealed and annealed Ag films. We find that imaging the Ag surfaces in air, even at relatively low resolution, is significantly more difficult than corresponding measurements in air on flame-annealed Au and Pt samples. This difficulty most likely results from the spontaneous formation of a surface AgxO layer in air. Although Ag(111) surfaces can be readily imaged with atomic resolution when protected by an adlayer of halogen31-33 or sulfur34 atoms, all attempts to image the bare surface in air at similar high resolution have been unsuccessful. However, imaging the Ag films in situ under potential control greatly enhances the resolution. Near atomic resolution was obtained for these Ag films in 0.1 NaF solutions over a potential range between -1.5 and -0.3 V (vs Ag wire). STM images of unannealed films, Figure 6a, indicate that deposition, without thermal annealing, produces films that contain a larger number of channels and holes. Images of unannealed films, however, also show regions (30) Hosford, W. F. The Mechanics of Crystals and Textured Polycrystals; Oxford University Press: New York, 1993; p 163. (31) Schott, J. H.; White, H. S. J. Chem. Phys. 1994, 98, 291. (32) Schott, J. H.; White, H. S. Langmuir 1994, 10, 46. (33) Kawasaki, M.; Ishii, H. Langmuir 1995, 11, 832. (34) Stevenson, K. J.; White, H. S. Unpublished results, University of Utah, 1995.

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Figure 7. Voltammetric response of a 6 h annealed, 0.35 cm2 mica/Ag(111) film in a N2-purged solution containing (a) 5 mM Pb(ClO4)2, 10 mM HClO4, and 0.1 M NaClO4 (solid line, ν ) 5 mV/s) and (b) 5 mM Pb(CH3COO)2, 0.5 M NaCH3COO, and 0.5 M CH3COOH (dashed lines, ν ) 5, 10, and 20 mV/s).

Figure 6. In situ STM images of mica/Ag(111) films under potential control in 0.1 M NaF. All images are slope subtracted to remove drift but are otherwise unfiltered. (A) 350 × 350 nm2 constant current image of unannealed film, Vb ) -106 mV, it ) 2.2 nA, Vs ) -0.7 V vs Ag wire. (B) 100 × 100 nm2 constant current image of unannealed film, Vb ) -106 mV, it ) 2.2 nA, Vs ) -1.3 V vs Ag wire. (C) 100 × 100 nm2 constant current image of 6 h annealed film, Vb ) -66.3 mV, it ) 2.2 nA, Vs ) -0.7 V vs Ag wire.

that appear atomically flat; straight facet steps exhibiting 3-fold symmetry and rounded step edges associated with island growth are evident. Figure 6b shows a single hole (∼13 Å deep) surrounded by a highly stepped surface (step sizes ∼ 3 Å). Holes similar to the one shown in Figure 6b were commonly observed on both unannealed films and those annealed for 2 h. Ag(111) films annealed for 6 h (Figure 6c) visually appear to be smoother over larger distances relative to unannealed films. These surfaces are characterized by ∼3 Å high, atomically flat, islands and are essentially free of the hole and channel structures observed on the unannealed and 2 h annealed films. A few surface disruptions can still be observed in some of these images, but these are far less prevalent. In a related AFM study,35 the epitaxial growth of Ag on mica as a function of film thickness and substrate deposition temperature was found to proceed through several stages: (1) nucleation and coalescing island growth, (2) channel and network development, (3) hole formation, and (4) continuous film formation. Our results appear to indicate that thermal annealing of the Ag films after deposition yields similar structures. The X-ray diffraction and STM results support the conclusion that the observed decreases in ∆Ep, fwhh, and (35) Baski, A. A.; Fuchs, H. Surf. Sci. 1994, 313, 275.

R upon thermal annealing are associated with preferential growth of (111)-oriented grains as well as an increase in film smoothness. It is also likely that the substantial reduction of the A1/C1 and A3/C3 voltammetric waves (Figure 1) after extended thermal annealment is associated with the increase in film quality and, in particular, a reduction of step edges associated with channels and holes in the film. In a related study15 of the dependence of the upd voltammetric response of electrolytically-grown Ag(111) crystals, the magnitudes of the A1/C1 and A3/C3 waves were shown to increase as the step density increased (although the peak heights for the electrolytically-grown Ag(111) crystals are always significantly smaller than those observed at chemically- and mechanically-polished Ag(111) single crystals). On the basis of this previous interpretation, the decrease in the A1/C1 and A3/C3 waves observed at thermally-annealed films is most likely due to a similar decrease in the density of surface step structures. The voltammetric response for the upd of Pb at annealed Ag(111) films in the presence of CH3COO- is significantly different from that obtained in solutions containing ClO4-. Figure 7 shows representative voltammetric curves obtained using the same Ag(111) electrode (6-h annealing period) in solutions containing either CH3COO- or ClO4as the supporting electrolyte anion. The upd response in the CH3COO- solution is clearly broader and less well defined, with significantly reduced peak heights for the A2/C2 waves. However, the integrated charge associated with the Pb deposition is the same in both solutions (within 6%). The response observed in the CH3COO- solution is also different than that previously reported in the literature for single Ag(111) crystals in which a single and narrow (A2/C2) wave is observed.5,16 Using thermallyannealed films, we reproducibly observe essentially opposite behavior, with the curves obtained in ClO4solutions appearing much more ideal. Current speculation in the literature concerning the effect of the anion is based on the assumption that the

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A1/C1 and A3/C3 waves are associated with Pb deposition at step edges that separate Ag(111) terraces. Since CH3COO- is expected to be more strongly adsorbed than ClO4-, the absence of the A1/C1 and A3/C3 waves in CH3COO- solutions has been associated with blocking of the step edges by CH3COO- anions. In-situ STM imaging of the Ag(111) surface during upd of Pb supports this hypothesis.29,31 The fact that we observe a more ideal voltammetric response and greatly diminished A1/C1 and A3/C3 waves in ClO4- solutions, while also observing a very broad wave in CH3COO- solutions, suggests that the nucleation and growth of a Pb upd layer may be accelerated by surface defect structures. This conclusion is supported by the observation that while the voltammetric wave shape in ClO4- solutions is rather insensitive to the scan rate, the voltammetric response in CH3COObecomes more irreversible as the scan rate is increased. Figure 7, for example, shows that the peak splitting (∆Ep) increases from 22 to 38 mV in the CH3COO- solution when the scan rate, ν, is increased from 5 to 20 mV/s. In ClO4solutions, ∆Ep is not only typically smaller (∼13 mV at ν ) 5 mV/s) but increases by only 1-3 mV over the same range of scan rates. This difference in behavior suggests that the formation of the Pb upd layer is kinetically less facile at the thermally-annealed Ag(111) surfaces in CH3COO- solutions than in ClO4- solutions. A more detailed investigation of the dependency of upd kinetics on the combined effects of substrate structure and the chemical nature of the anion is necessary to unravel this complex voltammetric behavior. Finally, Toney et al. recently reported that a break-in period of ∼30 min is required for mica/Ag(111) electrodes (prepared by thermal evaporation) before the characteristic Pb upd waves were observed in ClO4- or CH3COOsolutions.12 These authors attributed the required breakin period to the desorption of surface contaminants. Our experience is that the voltammetric response for upd of Pb on thermally-annealed films is always observed on the initial voltammetric scan, with negligible changes in peak

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height or width occurring upon subsequent scanning. We have also stored the mica/Ag(111) samples in air-tight vials for several months without any significant effect on the voltammetric response. We have no explanation for the remarkably different behaviors observed by Toney et al.12 and ourselves. Conclusion Thermal annealing of mica/Ag(111) electrodes results in a significant improvement in the voltammetric response observed for the upd of Pb monolayers in ClO4- solutions. The dependence of the voltammetric response, and surface roughness, on annealing time has been shown to result from an increase in film quality as measured by STM and glancing angle X-ray diffraction. We have shown that the A3/C3 and A1/C1 voltammetric side peaks observed for upd of Pb at Ag are greatly diminished after thermal annealing. These results support previous suggestions that these waves are associated with Pb deposition at defect surface structures. Because Ag(111) electrodes are routinely employed in electrochemical investigations of surface adsorbates, the method of electrode preparation described here may find general application in preparing high-quality Ag(111) surfaces that have electrochemical responses that are comparable to electrolytically-grown Ag(111) single crystals. We anticipate that the voltammetric responses corresponding to surface adsorbates other than Pb will also be dependent upon the length of the thermal annealing. For instance, recent studies in our laboratory indicate that the voltammetric response observed during the reversible absorption of HS- and alkanethiolates on mica/Ag(111) has a more ideal wave shape when the Ag(111) is thermally annealed under reduced pressure. These studies will be the subject of a future report. Acknowledgment. This work was supported by the Office of Naval Research. LA950509L