Langmuir 1994,10,4324-4329
4324
In Situ Surface Differential Diffraction Study of Metal Monolayer Formation by Underpotential Deposition on Silver(111) Oriented Surfaces Evans D. Chabala and Trevor Rayment* Department of Chemistry, University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 lEW, U.K. Received March 31, 1994. I n Final Form: August 10, 1994@ Surface differential diffraction was used to study the process of the formation of adsorbate layers of thallium and lead, by underpotential deposition (UPD),ontoAg(ll1) electrodes. It was possible to undertake real-time in situ structural measurements during the formation of the adsorbate layers. The intensity of the differential diffraction peaks, as a function of potential in the underpotential region, was used to monitor the growth of the adsorbate layers. Thallium UPD on Ag(ll1) was found to be a gradual process over a larger underpotential region, whereas the UPD of lead occurred rather abruptly and had a much smaller underpotential shift. This difference in behavior between the two systems could be a reflection of the difference in the mechanisms of underpotential deposition for the two systems. The former could be a gradual adsorption process, while the latter a nucleation-growth process. It was also possible to measure the distance of the adsorbate plane on the substrate with respect to the interplanar spacing in the bulk of the latter. The thallium adsorbate layer gave a distance of 2.42 f 0.05 to 2.55 f 0.05 A, increasing with scan in the cathodic direction. The lead adsorbate layer gave a distance of 3.00 f 0.05 A. From these distances, we inferred the probable adsorption sites of the adsorbate species on Ag(ll1).
Introduction Atomic adsorbate layers of submonolayer to monolayer coverage are readily formed on metal substrates at the electrode-electrolyte interface by underpotential deposition (UPD).1-3 Underpotential deposition is the formation of adsorbate layers at the electrode-electrolyte interface at potentials positive to the reversible Nernst potential for bulk dep~sition.l-~ This phenomenon offers unique opportunities to study two-dimensional atomic structures under thermodynamically reversible and equilibrium conditions. The adsorbate layers formed by this process have been shown to reveal interesting properties and characteristics. In particular, metal monolayers adsorbed on metallic and semiconducting substrates have been shown to cause significant changes to the structural, optical, and electronic properties of the substrate mat e r i a l ~ . ~In- ~addition, the combination of the structural and electronic changes at the electrode-electrolyte interface due to underpotential deposition results in the modification of the reactivity of the metal electrode and influences properties such as electrocatalysis and corr ~ s i o n . ~ - lFurthermore, l these atomic adsorbate layers are the first step in electrodeposition, and they influence the further growth of bulk deposits, and this is of primary interest to electrochemistry. @
Abstract published in Advance ACS Abstracts, September 15,
1994. (1)Lorenz, W. J.; Hermann, H. D.; Wuthrich, N.; Hilbert, F. J . Electrochem. Soc. 1974, 121, 1167. (2) Kolb, D. M. In Adu. Electrochem. Electrochem. Eng.; Gerischer, H., Tobias, C., Eds.; J. Wiley and Sons Inc.: New York, 1978; Vol. 11, p 125. (3) Conway, B. E. Prog. Surf. Sci. 1984, 16, 1. (4) Kolb, D. M.; Przasnyski, M.; Gerischer, H. J . Electroanal. Chem. Interface Electrochem. 1974, 54, 25. ( 5 ) Trassati, S. In Adu. Electrochem. Electrochem. Eng.; Gerischer, H., Tobias, C., Eds.; J . Wiley and Sons Inc.: New York, 1977; Vol. 10, p 213. (6) McIntyre, J. D. E.; Kolb, D. M. Symp. Faraday Soc. 1970,4, 99. (7)Adzic, R. R.; Despic, A. R. J . Chem. Phys. 1974, 61, 3482. (8) Kokkinidis, G.; Hasiotis, K.; Sazou, D. Electrochim. Acta 1990, 35, 1957. (9) Bindra, P.; Light, D.; Molla, J. Electrochim. Acta 1991,36, 529. (10)Drazic, D. M.; Vorkapic, L. Z. Corrosion Sci. 1978, 18, 907. (11) Lafranconi, G.; Mazza, E.; Sivieri, E.; Torchio, S. Corrosion Sci. 1978, 18, 617.
The early studies of underpotential deposition were dominated by the traditional electrochemical techniques.12-18 However, these techniques could not provide direct structural information on the two-dimensional adsorbate layers and on the electrode-electrolyte interface itself. Over time, a wide spectrum of techniques has been brought to bear upon the study of UPD systems. Optical and spectroscopic techniques were among the first to compliment electrochemical techniques in the study of UPD s y s t e m ~ . l ~ Techniques -~~ applied subsequently included UHV chemical and structural techniques22-26 and conventional and synchrotron X-ray technique^.^^-^^ Nonlinear optical m e t h o d ~as~well ~ , ~as~radi0tracel.3~ and (12) Schultze. J. W.: Dickertmann. D. Surf. Sci. 1976. 54. 489. (13) Conway,’B.E.; kngerstein-Kozlowska; H.; Ho, F. C. J.’Vac. Sci. Technol. 1977, 14, 351. (14) Gerischer, H.; Kolb, D. M.; Sass, J. K.Adu. Phys. 1978,27,437.
(15)SieEenthaler, H.: Juttner. K.: Schmidt, E.: Lorenz. W. J. Electrochim. Acta 1978,23, 1009. (16) Hamelin, A. J.Electroanal. Chem. 1984, 165, 167. (17) Swathirajan, S.; Bruckenstein, S. Electrochim. Acta 1983,28, 865.
(18) Swathirajan, S.; Mizota, H.; Bruckenstein, S. J . Phys. Chem. 1982,86,2480. (19) Adzic, R. R.; Yeager, E.; Cahan, B. D. J.Electrochem. Soc. 1974, 121. 474. (20) Horkans, J.; Cahan, B. D.; Yeager, E. J.Electrochem. Soc. 1975, 122. 1585.
(21)Takamura, T.; Watanabe, F.; Takamura, K. Electrochim. Acta 1981,26, 979. (22) Lawen-Davidson, L.; Lu, F.; Salaita, G. N.; Hubbard, A. T. Langmuir-l988,4, 224. (23) Beckmann, H. 0.; Gerischer, H.; Kolb, D. M.; Lehmpfuhl, G. Faraday Symp. Chem. Soc. 1977, 12, 51. (24) Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P.; Vilegas, I.; SUES. -- . D. W.: Sticknev. J. L. J. Phvs. Chem. 1991. 95. 5245. (25) Zei, M.S.; Qiab; G.; L e h m p h l , G.; Kolb, D. M.Ber. BunsenGes. Phys. Chem. 1987,91, 349. (26) Hanson, M. E.; Yeager, E. InElectrochem. Surf. Sci.: Molecular
Phenomena a t Electrode Surfaces; Soriaga, M. P., Ed.; American Chemical Society: Washington, DC, 1988; p 141. (27) Fleischmann, M.; Graves, P.; Hill, I.; Oliver, A.; Robinson, J. J . Electroanal. Chem. 1983, 150, 33. (28) Fleischmann, M.; Mao, B. W. J . Electroanal. Chem. 1988,247, 297. (29) Abruna, H. D.; White, J. H.; Albarelli, M. J.; Bommarito, G. M.; Bedzyk, K. M. J.; McMillan, M. J . Phys. Chem. 1988,92, 7045. (30) Abruna, H. D., Ed. Electrochemical Interfmes: Modern Tech-
niques for in-Situ Interfme Characterisation; VCH Publishers Inc.: New York, 1991.
0743-7463/94/2410-4324$04.50/00 1994 American Chemical Society
Monolayer Formation by UPD on Silver(ll1) oscillating quartz crystal3smethods have also been used to study UPD systems. Most recently, scanning probe m i c r o s c ~ p i e s ~have ~ - ~ ~been added to the plethora of techniques that have been used to study UPD systems. The end result has been that a wealth of information has been gathered for UPD systems mostly for complete monolayer coverages. However, it has not been easy to obtain in situ structural information of submonolayer coverages under conditions of continuously varying potential. Although the formation of monolayers by underpotential deposition is a precisely controlled and readily reproducible process, this aspect of UF'D systems has not been fully exploited in most of the in situ studies that have been done hitherto. Since the electrode-electrolyte interface is buried between two condensed phases, in situ structural measurements could only be done by minimizing the amount of water at the interface to avoid the attenuation of the measuring probe. This meant that the formation of the adsorbate layer by UPD and in situ structural measurements could not be done simultaneously; structural measurements could only be done effectivelyunder static conditionsupon completed adsorbate layers.30Even then, data collection periods were of very long hours for conventional sources of x-ray^.^',^^ The latter constraint has been addressed by the advent of synchrotron radiation sources29whereas the former has only recently started receiving attention with the advent of scanned probe microscopies. With scanned probe microscopies, it is possible to make measurements in situ progressively as the adsorbate layer is being f ~ r m e d . ~However, ~-~~ interpretation of images obtained from these techniques is not yet straightforward. In this paper, we present preliminary results from a method which is being developed to make dynamic in situ X-ray diffraction measurements of the formation of adsorbate layers, by underpotential deposition, at the electrode-electrolyte interface. The in situ electrochemical cell was designed such that ample quantities of electrolyte were maintained at the interface so that no restrictions or constraints were imposed upon the electrochemical control of the formation of the adsorbate layers while structural measurements were made at all coverages." This was achieved by making the electrode upon which underpotential deposition was performed a very thin metal film on mica, and the X-rays were incident upon the interface through the back of the film. Despite the use of a laboratory source of X-rays, the intensity diffracted from the thin metal films was so high that (31)Samant, M. G.; Toney, M. F.;Borges, G. L.;Blum, L.; Melroy, 0. R. J. Phys. Chem. 1988,92,220. (32)Melroy, 0.R.; Toney, M. F.;Borges, G. L.; Samant, M. G.; Kortright, J. B.; Ross, P. N.; Blum, L. Phys. Rev. B 1988,38,10962. (33)Toney, M. F.;Gordon, J. G.; Samant, M. G.; Borges, G. L.; Melroy, 0. R.; Yee, D.; Sorensen, L. B. Phys. Rev. B 1992,45,9362. (34)Materlik, G.; Schmah, M.; Zegenhagen, J.; Uelhoff, W. Phys. Rev. B 1985,32,5502. (35)Furtak, T. E.;Miragliotta, J.; Korenowski, G. M. Phys. Rev.B 1987,35,2569. (36)Koos, D.A,; Richmond, G. L. J . Phys. Chem. 1992,96, 3770. (37)Horanyi, G.; Vertes, G. Electrochim. Acta 1986,31,1663. (38)Hepel, M.; Kanige, K.; Bruckenstein, S. J . Electroanal. Chem. 1989,2661409. (39)Itaya, K.; Tomita, E. Surf. Sci. 1988,201,L507. (40)Szklarczvk, M.; Bockris, J. Om.J . Electrochem. SOC.1990,137, 452. (41)Magnussen, 0.M.; Hotlos, J.;Beitel, G.; Kolb, D. M.; Behm, R. J. J . Vac. Sci. Technol. E 1991,9,969. (42)Chen, C.-H.; Vesecky, S. M.; Gewirth, A. A. J.Am. Chem. SOC. 1992,114,451. (43)Manne, S.; Hansma, P. K.; Mamie, J.; Elings, V. B.; Gerwith, A.A.Science 1991,251,183. (44)Chabala, E. D.;Bashir, H. H.; Rayment, T.; Archer, M. D. Langmuir 1992,8,2028.
Langmuir, Vol. 10,No. 11, 1994 4325 saturation of the position-sensitive detector occurred at full generator power. This strong signal from the thin metal films was used to monitor the formation of the adsorbate layer during underpotential deposition by monitoring the coherent interference between scattering from the crystal planes of the substrate and that from the adsorbate layer. This phenomenon is known as surface differential diffraction (SDD)or Bragg peak interference. The relative amount of material in the adsorbate layer at different potentials (coverages) and the distance of the adsorbate layer on the substrate, with respect to the interplanar spacing in the latter, are obtained from the surface differential diffractionmeasurements. Details of this phenomenon are given elsewhere."-48
Experimental Section The underpotential deposition ofthallium and lead onto silver(111)single crystalline oriented electrodes was studied using the procedure detailed in a previous publication." Silver films were made by vacuum evaporation of silver wire (99.99%;Advent Research Materials) onto freshly cleaved mica at 250 "C in a vacuum pressure of less than 10+ Torr. The films were subsequently annealed for at least 1.5 h at 300 "C, thus making films with (111)single crystalline orientation, with the (111) planes parallel to the mica substrate s u r f a ~ e . The ~ ~ ,absence ~~ of the 200 reflection in the X-ray diffraction measurements and the fact that transmission experiments showed hexagonal symmetry of the (220) atomic planes that are orthogonal t o the (111)planes confirmed that the films had single crystalline orientation with single domains. The films were mosaic crystals with mosaic spreads of between 1.5 and 2.0" FWHM. The thickness of the films was measured using a film thickness monitor or by monitoring resistance. Films between 150 and 300 A were used. The underpotential deposition ofthallium on silver was studied from two solutions: 0.002 M thallous nitrate (BDH Laboratory Reagents) in 0.01 M perchloric acid (Aristar, BDH Chemicals) and 1.0 M sodium perchlorate (AR, BDH Chemicals); and 0.002 M thallium(1) acetate (Johnson Matthey) in 0.5 M acetic acid (A R, Fisons Ltd.) and 1.0 M sodium acetate (AnalaR, BDH Chemicals). All electrolyte solutions were made in triply distilled water and were purged with nitrogen gas for at least 15 min before being used in the electrochemical cell. The potential of the silver thin film electrode was cycled between 100 and -700 mVwith respect to the saturated calomel electrode (SCE) to investigate the underpotential deposition of thallium. Two dominant peaks were observed on the cyclic voltammogram before bulk deposition: the first stripping peak of thallium UPD was observed at ca. -450 mV and the second peak at ca. -660 mV vs SCE. The charge density measured in both peaks was 386 f 50 pC cm-2.2J5 During X-ray measurements, the potential of the silver thin film electrode was cycled continuously and the underpotential adsorbate layer was repeatedly electrosorbed and desorbed for the duration required to collect sufficient diffraction data. Scan rates of between 0.5 and 50 mV s-1 were employed, and the typical time for accumulation of sufficient counts in a diffraction pattern was between 30 and 120 min, dependingon the thickness and quality of the silver thin film. The cyclic voltammograms for the underpotential deposition ofthallium on silver(ll1) oriented thin films were reproducible over this period oftime. Figure l a shows a typical cyclic voltammogram of the underpotential deposition of thallium on silver(111)single crystalline oriented thin film electrodes in the cell used for in situ diffractionmeasurement~.~J~ (45)Taub, H.;Carneiro, K.; Kjems, J. K.; Passell, L.Phys. Rev. E 1977,16,1551. (46)Rayment, T.;Thomas, R. K.; Bonchil, G.; White, J. H. Mol. Phys. 1981,43,601. (47)Beaufds, J. P.; Barbaux, Y. J . Appl. C y s t . 1982,15,301. (48)Thorel. P.: Croset. B.: Marti. C.: Coulomb, J. P. In Proc. Conf. Neutron Scattkriig; Moon, R.' M., Ed.; Oak Ridge National Laboratory: Gatlinburg, TN, 1976;p 85. (49)Buchholz, S.;Fuch, S. H., Rabe, J. P. J . VUC.Sci. Technol. B 1991,9,857. (50)Golan, Y.; Margulis, L.; Rubinstein, I. Su$. Sci. 1992,264,312.
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4326 Langmuir, Vol. 10, No. 11, 1994
potential were stored in different segments of memory. The photon counts in each diffraction pattern were accumulated over the duration of the experiment as the potential of the electrode was continuously cycled between the anodic and cathodiclimits. The interference effect of the adsorbate layer on the diffraction from the thin film electrode was observed by substacting a diffractionpattern collected before any UPD adsorptionoccurred from all the other diffraction patterns collected. With this procedure, it was possible to observe the presence of even submonolayer quantities of the adsorbate layer on the thin film electrode.
. 4
3.
u
Results The differential diffraction patterns obtained from the surface differential diffraction study of the thallium and lead UPD adsorbate layers were analyzed as detailed in a previous publication.44 -600
Thallium Underpotential Deposition on Silver-
0
E l m V vs. SCE I 1
8ooil i
400 -
. 4 v
O I -400 -
-800
1
1
,
; -400
!
-200
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0
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Figure 1. (a, Top) Current vs potential voltammogram for thallium underpotential deposition on Ag(ll1) from 0.002 M thallium(1)acetate in 0.1M acetic acid and0.5M sodium acetate; potential scan rate 2.5 mV/s. (b, Bottom)Current vs potential curve for lead underpotential deposition onAg(ll1)from 0.002 M lead(I1)acetate inO.1 M acetic acid and 0.5 M sodium acetate; potential scan rate 20 mV/s.
The underpotential deposition of lead was performed from solutions of 0.005M lead(I1)acetate (GradeI, Johnson Matthey) in 0.5 M sodium acetate (A R)and 0.1 M acetic acid (as above), andlead(I1)fluoride (GradeI, Johnson Matthey)in 1.0 M sodium perchlorate (99+%, Aldrich Chemical)and 0.01M perchloricacid (asabove). Lead(I1)perchlorate(0.005M, 99%, JohnsonMatthey) was alsoused in the latter solution. The underpotentialstripping peak ofthe lead onAg(ll1)single crystallineoriented electrodes was observed at ca. -370 mVvs SCE, and the charge discharged at the electrode due to the stripping of the lead adsorbate layer was 296 f 30 pC cm-2.2 During X-ray measurements, the potential of the silver thin film electrode was continuously cycled between 100 and -500 mV vs SCE by cyclic voltammetry, repeatedly electrosorbingand strippingthe lead adsorbate layer. The cyclic voltammograms were reproducible for the duration of X-ray diffraction data collection, which was the same as for the thallium system. Figure l b gives a typical cyclic voltammogram ofthe underpotential deposition oflead onto the silver(l11)single crystallineoriented thin film electrode in the cell used for in X-ray diffraction measurements. The formation of the UPD adsorbate layers of thallium and lead on silver(ll1)oriented electrodeswas monitored by collecting X-ray diffraction patterns from the thin film electrode in predetermined intervals (steps) of potential. The diffraction patterns were stored in a multichannel analyzer (MCA) such that the diffraction patterns collected in different intervals of
(111) Oriented Electrodes. The differential diffraction patterns obtained from the silver(ll1) single crystalline oriented thin film electrode during the underpotential deposition of thallium are given in Figure 2a for the desorption scan. The diffraction pattern collected in the potential interval from 100to 0 mVvs SCE was subtracted from all the other diffraction patterns collected in intervals of 100mV to obtain the patterns given in Figure 2a. Figure 2b shows the fitting of the calculated interference patterns to the experimental differential diffraction peaks. The information obtained by fitting the calculated interference patterns to the differential diffraction peaks is given in Table 1. The scale factors of the calculated interference patterns can be used as a measure of the relative intensity ofthe differential peaks. The differential diffraction peak of the potential interval from -600 to -700 mV vs SCE for the adsorption scan was taken as representative of full coverage (i.e. 100%). All the other differential intensities are quoted relative to this. The intensity of the differential diffraction peaks was observed t o increase gradually from practically no signal in the potential interval from -300 to -400 mV vs SCE to a maximum in the potential interval from -600 to -700 mV. The relative change in the charge density discharged during the UPD of thallium at the electrode-electrolyte interface in the potential interval from -300 to -700 mV vs SCE was compared with the relative change in the intensity of the differential diffraction peaks as shown in Figure 3a. It can be seen that there is a correlation between the intensity of the differential diffraction peaks and the charge density between -300 and -600 mV, which corresponds to the region containing the first underpotential deposition peak. However, between -600 and -700 mV, while the charge transferred to the electrode increases by ca. 90%, the intensity of the surface differential diffraction increases by only ca. 8%. Figure 3b gives a comparison of the intensities of the differential diffraction as a function of potential for the anodic and cathodic potential sweeps, which shows that they are essentially the same. The surface differential diffraction phenomenon provides a means of measuring the perpendicular distance of the adsorbate plane with respect to the interplanar spacing in the bulk of the electrode materia1.45,46The distances measured for the thallium underpotential adsorbate layer on the silver(ll1) oriented thin film electrode are given in Table 1 for the potential interval in which the differential diffraction peaks were observed. These distances ranged from 2.42 & 0.05 to 2.55 f 0.05 A. The anodic (stripping) scan showed a decrease in the distance measured from 2.55 f 0.05 to 2.42 f 0.05 A as the adsorbate layer was stripped off the electrode. This same trend could be true of the cathodic scan, taking into
Langmuir, Vol. 10, No. 11, 1994 4327
Monolayer Formation by UPD on Silver(l11)
e Charge Dcluiiy ........ P e a Intcntily
140-
90
-
40
I
..
-700
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-100
-400
Potential I mV vs. SCE
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-
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-700 to -600
---700 I
-600to -500 -500 to -400
-3
Potential I mV vs. SCE
-IT
.35.0
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Figure 2. (a, Top) Differential diffraction patterns for the underpotential deposition of thallium on Ag(ll1) measured in 100 mV steps. The emergence of the surface differential diffractionpeaks and their growth coincidedwiththe adsorption peaks in the cyclic voltammogram for thallium underpotential deposition on Ag(ll1) (compare Figure la). (b, Bottom) Differential diffraction patterns (crosses)fitted with calculated interference patterns (solidlines) for thallium adsorbate layers on Ag(ll1). Only differential diffraction patterns collected in the potential region hom -400 to -700 mVvs SCE had sufficient intensity to be fitted accurately. The top three patterns are for the cathodic scan and the bottom three are for the anodic scan (compare Figure la). Table 1. Thallium Underpotential Deposition on Silver(111)' potentialrange distance measured (A) scale factors (%) (mV vs SCE) anodic cathodic anodic cathodic 110 100 -700 to -600 2.55 f 0.05 2.45 f 0.05 77 94 -600 to -500 2.45 f 0.05 2.44 f 0.05 2.42 f 0.05 2.50 f 0.1 33 33 -500 to -400 The distancesmeasured fromthesurfacedifferential diffraction for thallium underpotentialdeposition onAg(ll1)at Merent stages of coverage are given for the anodic and the cathodic scans. The relative intensityis the fraction of the scalefactor of the differential diffraction patterns measured in different intervals of potential; 100%refers to the differential diffraction intensity after the first full monolayer peak in the cyclic voltammogram (compareFigure la). account the larger error bar of the distance measured in the potential interval from -400 to -500 mV vs SCE. Therefore, there seems to be an increase, slight as it may be, in the distance of the adsorbate plane on the electrode surface with an increase in coverage. This could be due
Figure 3. (a, Top) Relative charge density due to the underpotentialdepositionofthalliumonAg(ll1)comparedwith the differential diffraction intensity, both as a function of potential. The charge density of the first underpotential adsorption peak is assigned loo%, corresponding to the first full monolayer coverage; the same is done for the differential diffraction intensity measured after the first full monolayer, i.e. in the potential interval from -500 to -600 mV vs SCE. Error bars are &12%for the charge density and f5% for the differential diffraction intensity. (b, Bottom) Differential diffraction intensities for thallium underpotential deposition on Ag(ll1) compared for the cathodic and anodic scans of potential, as a function of potential. The points on the plot are coincident, except the last two. to the fact that the adspecies in the adsorbate layer are not fully discharged upon initial adsorption and that further discharging occurs with further adsorption at more cathodic potentials. This could be the source of the very small peaks between the two large underpotential peaks (see Figure la). Another reason could be that as more adspecies are adsorbed onto the electrode surface, the adsorbate layer expands laterally, slightly pushing the adspecies out of the initial adsorption sites. Although the distances measured are, strictly speaking, those of the adsorbate plane with respect to the interplanar spacing in the bulk of the electrode, it is very tempting to relate these distances to the different adsorption sites available to an atomic adspecies on the electrode surface. Such adsorption sites on a predominantly (111)surface will be the a-top, the bridge, and the threefold hollow sites. Through the use of a hard-sphere model and the metallic radii of silver and thallium atoms as 1.44 and 1.71 A, re~pectively,~~ the adsorbate plane would be at the distances of 3.15, 2.80, and 2.67 respectively, with respect to the surface layer of the electrode surface, for the different adsorption sites. In a comparison of this
A,
~~~
~
(51)Wells, A. F. StructurullnorgunicChemistry,Clarendon: Oxford, 1984.
Chabala and Rayment
4328 Langmuir, Vol. 10, No. 11, 1994 Potential Interval I mV vs. SCE
-400 to -500
-400 to -300
-*$%A.
..
*
The distances measured for the lead underpotential adsorbate layer on the Ag(ll1) oriented electrode with respect to the interplanar spacings in the latter were between 3.00 f0.05 and 3.2 f0.1 A. Seekingto interpret these distances in terms of adsorption sites using the same hard-sphere model indicates that the lead adspecies would have been adsorbed between the a-top and the bridge sites. This seems to be consistent with the observation that the lead adsorbate layer was formed on the silver electrode to full monolayer coverage rather abruptly, and therefore, the Pb-Pb interactions would have been more significant than the Pb-Ag interactions. The large radii misfit between Pb and Ag (rpdrAg = 1.21) should also be taken into account in this case.52This is very unlike the thallium ' P . adsorbate layer on the Ag(ll1)electrodewhich underwent I gradual adsorption, and hence, it seems very likely that the T1-Ag interactions played a significant role in the early stages of the formation of the adsorbate layer as reflected in the measured distances given above.
Discussion
100 3.00 f 0.05 100 -25 -25 3.1 & 0.1 3.2 & 0.1 The distances measured fromthe surfacedifferentialdiffraction for lead underpotential deposition on Ag(ll1) at different stages of coverage are given for the anodic and the cathodic scans. The relative intensity is the fraction of the scale factor of the differential diffraction pattems measured in different intervals of potential; 100% refers to the differential diffraction intensity after the monolayer peak in the cyclic voltammogram (compare Figure lb). -500 to -400 -400 to -300
3.00 f 0.05
with the distances measured for the underpotential deposition ofthallium on the silver(ll1) oriented thin film electrode, it seems to follow that the thallium adspecies were adsorbed in the threefold hollow sites, as previously proposed by Siegenthaler et al.15 and Rawlings et a1.62 There is a discrepancy of4.5-9.4% between the measured distances and that calculated for the adsorption ofmetallic thallium adatoms in threefold hollow sites of a Ag(ll1) surface based on the hard-sphere model. Consideringthe crude model used, this discrepancy seems to be acceptable, especially taking into account also the nature of the interface and the adspecies, as discussed below.
Lead Underpotential Deposition on the Silver(111) Oriented Electrode. The underpotential deposition and stripping peaks of lead onto the silver(ll1) oriented electrode were observed at -400 and -370 mV vs SCE, respectively. The underpotential deposition shift was small, and therefore, the overall potential interval in which the interference effect of the UPD adsorbate layer was studied was only 200 mV. The differential diffraction peaks were observed by subtracting the Ag(ll1)diffraction pattern obtained in the potential interval 100-0 mV vs SCE from all the other diffraction patterns. The experimental differential diffraction patterns were fitted with the calculated interference patterns as shown in Figure 4. The information obtained from these fits is given in Table 2. The relative intensity of the differential diffraction peaks increased abruptly to maximum intensity in the potential interval from -300 to -500 mV vs SCE, reflecting the abrupt and sharp peak of the lead underpotential deposition on the silver(ll1) oriented electrode surface and the smaller underpotential shift.
The results presented above show different trends in the underpotential deposition systems of thallium and lead onto the Ag(ll1) oriented electrode. This was observed both in the nature of the electrochemistry of the systems as obtained in the cyclic voltammograms and in the variation of the intensity of the differential diffraction peaks as a function of potential in the underpotential region. The thallium system showed a gradual increase in the intensity of the differential diffraction peaks, while the lead system showed a rather abrupt increase. Furthermore, different adsorption sites were inferred from the distances measured of the adsorbate planes on the Ag(ll1) thin film electrodes for the two systems, despite the similarity in the sizes of the species adsorbed. These observations could be pointing to the fact that the two systems undergo different adsorption mechanisms. The mechanism of thallium underpotential deposition seems to be that of the gradual adsorption to a full and complete adsorbate layer1~53~54 while that of lead seems to be consistent with a first-order phase transition into a 2D solid phase (nucleation and growth m e ~ h a n i s m ) .This ~~ trend has been observed before for thallium and lead underpotential deposition on Au(ll1) oriented thin film electrodes where it was shown that the thallium adsorbate layer was formed gradually on the gold electrode while the lead adsorbate layer was formed rather abruptly.44 Furthermore, underpotential shifts (i.e. the difference in potential between the most anodic underpotential stripping peak and the equilibrium Nernst potential for stripping the bulk) are used as a measure of the binding of the electrosorbate layer to the ~ u b s t r a t e .Thallium ~ UPD on Ag(ll1) has a larger underpotential shift than lead UPD, and taking this into account, thallium would be expected to bind more strongly to the silver substrate than lead. This seems to be reflected in the difference in the measured distances. The distances deduced from the surface differential diffractionpatterns do not agree perfectly with calculations made using hard spheres. This is not at all surprising. The atomic radii are derived from metals in which the coordination number is 12. At the surface of an electrode (52) Engelsmann,K.; Lorenz, W. J.;Schmidt, E. J .Electroanal. Chem. 1980,114,1. (53)Bosco, E.; Rangarajan, S. K. J.Electroanal. Chem. 1981,129, 25. (54) Staikov, G.; Juttner, K.; Lorenz, W. J.;Schmidt,E. Electrochim. Acta 1978,23,305. (55) Schultze, J. W.; Dickertmann, D. Ber. Bunsen-Ges Phys. Chem. 1978,82,528,
Monolayer Formation by UPD on Silver(ll1) the environment is very different,56fj9160 the adspecies may not be fully discharged,4-6956J7and the binding may involve specificbonds with several neighboring substrate atoms.56 Finally, since the distances measured are with respect to the interplanar distances in the bulk of the electrode material, and not with respect to a rigid top layer, any relaxation or reconstruction in the surface layer of the Ag(ll1) substrate might lead to a smaller measured distance than expected from a hard-sphere model. In previous work done on the UPD of thallium on Ag(111)employing X-rays, both conventionalz8and synchrot r ~ nit,was ~ ~concluded in the first case that the thallium adsorbate bilayer enhanced the Ag(l11) diffraction peak due to the superposition ofthe Tl(11)peak of a compressed commensurate adsorbate layer. In the work with a synchrotron radiation source, it was concluded that the thallium adsorbate layer was hexagonal close packed and rotated f4.6"with respect to the substrate surface. The nearest-neighbor distance was found to be between 3.32 and 3.38A with lateral compression of -3% being observed at potentials just before bulk deposition. In both cases, the diffraction measurements were made on fully formed monolayers and/or bilayers of the thallium adsorbate. From results presented here, it should be noted that what was previously thought to be the enhancement of the Ag(ll1) peak due to the superposition of the Tl(l1) peak in the bilayer of the adsorbate is actually the interference effect between the adsorbate layer and the substrate, and that this occurred even at submonolayer coverage.
Conclusion Surface differential diffraction has been used to study the underpotential deposition of thallium and lead onto a Ag(ll1) electrode surface during continuous potential cycling. The intensity of the differential diffraction peaks was used to monitor the growth of the adsorbate layer, and it was possible to determine the perpendicular (56)Juttner, K;Lorenz, W. J. 2.Phys. Chem. N.F.1980,122,163. (57)Conway, B. E.;Angerstein-Kozlowsak, H.; Sharp, W. B. A. 2. Phys. Chem. N P . 1975,98,61. (58) Schultze, J. W.; Koppitz, F. D. Electrochim.Acta 1976,21,327. (59)Bardeen, J. Phys. Rev. 1936,44,653. (60)Schultze, J. W.; Vetter, K. J. J.Electroanal. Chem. 1973,44,63. (61)Vetter, K.J.; Schultze, J. W. J . Electroanal. Chem. 1974,53,67. (62)Rawlings, K J.; Gibson, M. J.; Dobson, P. J. J.Phys. D 1978, 11, 2059.
Langmuir, Vol. 10, No. 11, 1994 4329 distance between the latter and the average interplanar spacingof the Ag(ll1) electrode from which the adsorption sites of the adsorbate species were inferred. We concluded that different mechanisms were involved in the underpotential deposition of the thallium and lead: a gradual adsorption process for the former and a nucleation-growth process for the latter. Finally, it has been properly suggested by a critical reviewer of this study that the results presented in this paper are inferior to those produced at synchrotron facilities and therefore that it is not worth attempting to develop a technique using laboratory X-ray sources. In response we would like to comment that it is not our intention to defend SDD, but rather to investigate its utility for electrochemistry. It may be of help to readers with less direct experience of laboratory- and synchrotronbased X-ray experiments if we give an explanation of our motivation. First and foremost we wish to stress that synchrotron radiation has been used to produce really fine results in surface crystallography. Although these results cannot be produced using laboratoryx-ray sources, it is fact that synchrotron facilities are very few in number compared with laboratory-based diffractometers and therefore any techniques based on the latter which offer complementary information should be investigated. Indeed, as experienced users of synchrotron radiation, we find that the best use of limited beam time is made where background data are available from laboratory-based experiments. Surface differential diffraction as used here gives information limited to the vertical distribution of adsorbates above the electrode surface; but the technique is rapid and, with care, very reproducible. Hence it is possible to investigate the structural consequences of sweep rate, potential steps, and variation of counterions. Thus far we have presented only preliminary results. Improvements to the equipment, being made at present, will permit experiments with higher real-time and potential resolution. Within 3 years it should be possible to judge whether surface differential diffraction has any lasting value. If it transpires that the opinion of the reviewer noted above is correct,then we will inform readers accordingly. Until such time we think that it is a worthwhile topic for study.
Acknowledgment. E.D.C. acknowledgesfundingfrom the Cambridge Commonwealth Trust and Trinity College, Cambridge, U.K.
