J. Phys. Chem. 1995, 99, 4733-4744
4733
In-Situ Atomic Structure of Underpotentially Deposited Monolayers of Pb and T1 on Au(ll1) and Ag(ll1): A Surface X-ray Scattering Study Michael F. Toney,* Joseph G. Gordon, Mahesh G. Samant, Gary L. Borges, and Owen R. Melroy IBM Almaden Research Center, IBM Research Division, San Jose, Califomia 95120 Dennis Yee and Larry B. Sorensen Department of Physics FM-15, University of Washington, Seattle, Washington 981 95 Received: November 4, 1994; In Final Form: January 12, 1995@
We describe in-situ surface X-ray scattering measurements of the electrochemical underpotential deposition (UPD) of T1 and Pb monolayers on Au( 111) and Ag( 111) with the aim of understanding the factors that influence the atomic structure and compressibility of these systems. We find that the UPD deposits form incommensurate, hexagonal monolayers that are compressed compared to the bulk metals and rotated from the substrate [Oli] direction by several degrees. These structures are similar to those found for the vapordeposited analog and for the closest-packed planes of the bulk phases. In these systems (as for other incommensurate UPD systems), the in-plane spacing between adatoms decreases with decreasing electrode potential, and from such data the monolayer compressibility (KZD) is calculated. Values of K2D are tabulated for these four systems, for Bi on Au( 111) and Ag( 11l), and for a simple model of K2D; we discuss the trends evident in this table. To understand the differences in the voltammograms of Pb/Ag( 111) in acetate and perchlorate, the monolayer structures of these systems were determined. The same compressed, incommensurate, hexagonal monolayer was found, and at any given potential, the near-neighbor spacings were nearly identical. Our results show that for these incommensurate UPD systems, as for Bi on Ag(ll1) and Au( 1 1l), (i) the adatom-adatom interaction is the primary force determining the monolayer structure, (ii) although the adatom-substrate interaction is strong and results in the formation of the monolayer, it only influences the monolayer structure slightly, and (iii) the presence of the large concentration of water molecules or adsorbing anions in contact with the monolayer does not affect its structure. For UPD Pg/Ag(lll) our results show that kinetics and not thermodynamics causes some of the observed differences in the voltammograms.
I. Introduction The electrochemical deposition of metal layers onto a foreign metal substrate frequently occurs in distinct stages with the initial formation of one (or more) layers at electrode potentials positive of the reversible potential for bulk deposition (the Nernst potential). This process is termed underpotential deposition (UPD), and these initially deposited layers have been extensively investigated in the past 25 because of their importance in processes such as adsorption, charge transfer, surface diffusion, nucleation and growth, electrocatalysis, and electrodeposition and because of their inherent fundamental interest. Knowledge of the atomic structure of UPD layers is important to understanding many of these processes but has been difficult to achieve in the past. However, substantial progress has been made in determining, in situ, the atomic structure of UPD layers with the recent application of surface X-ray scattering, extended X-ray absorption fine structure (EXAFS),3and electrochemical scanning tunneling and atomic force microscopies (STM and In this paper we briefly review our earlier X-ray scattering results and present new data for UPD T1 and Pb on the (111) faces of Ag and Au near full coverage. These systems have been previously investigated by e l e c t r ~ c h e m i c a l ~and - ~ ~in-situ o p t i ~ a l ~ Jmethods, ~ - ~ ~ and recently, aspects of the surface structure have been obtained from STM,23-28AFM,29 @
Abstract published in Advance ACS Absfracfs,March 1, 1995.
0022-365419512099-4733$09.0010
and X-ray scattering investigations. Our X-ray studies of Pb and T1 on Ag( 111) showed that the UPD monolayers of these systems form two-dimensional (2D), incommensurate, hexagonal solids that are rotated by 4-5" with respect to the ~ubstrate.~O-~~ As the applied electrode potential decreases (becomes more cathodic), the in-plane atomic spacing also decreases (e.g., the monolayer compresses), and from these data, the compressibility of the monolayer is ~ a l c u l a t e d . ~ * There ,~~-~~ are two purposes of this paper: the first is to systematically compare and contrast the monolayer structures of these four systems as well as the similar systems of UPD Bi on Ag( 111)37 and A ~ ( l l l ) ~This ~ . provides insight into the structuredetermining forces for these UPD layers deposited on these smooth substrates, where the deposited adatoms are -20% larger than the substrate atoms. The atomic forces that influence the monolayer structure include the interactions between the adatoms, between the adatoms and the solution species (the anions and the solvent, water), and between the substrate and the adatoms (which includes both the average energy for adatom deposition and the substrate corrugation energy or the change in adatom-substrate energy as an adatom moves over the substrate). Second, we wish to compare the measured compressibility of these systems (including Bi on Ag( 111) and Au(111)) to each other and to a simple model for the compressibility-that of a 2D free electron gas. This provides a better understanding of the aspects of these systems that determine the compressibility.
0 1995 American Chemical Society
4734 J. Phys. Chem., Vol. 99, No. 13, 1995
In many UPD systems, it has been observed that the electrolyte composition and the presence of adsorbing anions can have a significant influence on the UPD process. For example, the shapes of the monolayer deposition and dissolution voltammograms are greatly affected by the supporting electrolyte.12J3J5These results raise the question of whether the atomic structure of the UPD layer is influenced by different electrolytes and adsorbed anions (e.g., the thermodynamics is affected) or whether the differences in the voltammograms are due to kinetic effects. Herein, we address this question for UPD Pb/Ag( 111) in perchlorate, a weakly adsorbing electrolyte, and in acetate, a strongly adsorbing one. It also is generally observed for UPD systems that the current peaks associated with monolayer deposition (cathodic scan direction) do not occur at the same potential and have different shapes than the current peaks for dissolution or stripping (anodic scan direction).12-15 Furthermore, these peaks can have a reasonably large breadth. These observations could be caused by kinetic effects or by actual structural differences in the layers when the deposition potential is reached in either an anodic or cathodic scan. It is important to determine which of these mechanisms is active. To address this, we present data for the structure of Pb/Ag( 11l), where the deposition potential is reached in either an anodic or a cathodic scan, and describe results for the other UPD systems we have investigated. The remainder of this paper is organized as follows. In section 11, we outline the experimental details. In section 111, we describe Pb on Ag( 11l), including (a) the UPD in perchlorate and acetate, (b) the atomic structure of the monolayer, (c) the monolayer structure when the deposition potential is reached in anodic or cathodic scans, and (d) the dependence of the monolayer near-neighbor spacing on electrode potential (Le., compressibility). Sections IV and VI describe our results for UPD Tl and Pb on Au( 11l), respectively, while section V briefly reviews results for Tl/Ag( 111).35,39Discussion of our results is given in section VII, including the importance of adatomadatom, adatom-substrate, adatom-anion, and adatom-solvent interactions in determining the UPD layer structure, as well as a comparison of the compressibility for these systems.
11. Experimental Section The X-ray experiments were performed in situ, under potential control, and at room temperature. The electrochemical cell is the same as that used in our previous investigations and has been described e l ~ e w h e r e . ~ ~To. ~eliminate ~ , ~ ~ , diffusion ~~ of atmospheric 0 2 through the polypropylene film that retains the electrolyte, the electrode was surrounded with a cylindrical cap that was constantly purged with clean n i t r ~ g e n . ~This ~.~~ effectively prevents oxidation of the UPD monolayer, and we observed no changes in the monolayer diffraction patterns over periods of up to several days. The UPD layers were deposited with the cell inflated so a relatively thick layer of electrolyte (-1 mm) covers the electrode (thick-film geometry), and all voltammetric data presented in this paper were obtained in this geometry. After monolayer deposition, the electrolyte was partially withdrawn, and the surface diffraction data were measured through the thin layer of electrolyte (S20 pm) that remained on the electrode. Data at different potentials were obtained by the following procedure: (1) inflating the electrochemical cell, (2) ramping the electrode from the previous potential to 400-800 mV positive of the reversible potential of the UPD metal (thus completely stripping off the UPD layer), (3) sweeping the electrode negative to the new potential, and (4) withdrawing the excess electrolyte and measuring the X-ray scattering. We feel that this procedure is more likely to give equilibrium than deposition in the thin film geometry.
Toney et al. The electrode substrates were epitaxially grown thin films of Ag and Au that were vapor deposited onto freshly cleaved mica.30931These films grow with the (111) direction perpendicular to the substrate surface. The epitaxy of the Ag films is excellent, and the in-plane mosaic spread is -0.2”; however, the epitaxy of the Au films is poorer, and the mosaic is 4”. One result of this large mosaic is a lower diffraction intensity for adlayers on Au( 111) substrates compared to adlayers on Ag(1 11) substrates. Despite the difference in mosaic, radial scans of the surface diffraction peaks of both substrates were sharp, and from the width of these surface peaks, the surface domain size is estimated as 400-500 A. After metal deposition, the substrates were stored in dry NZ for up to several days before use. Immediately after removal from storage, the voltammetry measured on these thin film substrates shows only weak indications of Pb and T1 UPD. However, after about 30 min of potential cycling and flushing the cell, Characteristic voltammograms were obtained, such as those shown in the following sections. We attribute the changes to a scrubbing of impurities in the system by the UPD process. The electrolyte for the T1 experiments was 0.1 M Na2S04 containing 2.5 mM Tl2SO4, and the potentials were measured relative to the Ag/AgCl (3 M KCl) reference electrode in the diffraction cell. The observed equilibrium potential for bulk T1 deposition was -700 mV for experiments of Tl/Au( 111) and -710 mV for Tl/Ag(lll).40 The electrolyte for the Pb/Au(1 11) experiments was 0.1 M HC104 containing 5 mM PbO, and potentials were measured relative to the Ag/AgCl (3 M NaCl) in the cell; the measured equilibrium potential for bulk Pb deposition was -440 mV. For the Pb/Ag(lll) experiments, two electrolytes were used: (1) 0.1 M NaC104 containing 5 mM Pb(C104)2 and 10 mM HC104 (referred to as perchlorate data), and (2) 0.5 M sodium acetate and 0.5 M acetic acid with 5 mM lead acetate (acetate data). Potentials were measured relative to the Ag/AgCl (3 M NaC1) reference electrode in the cell, and the observed equilibrium potentials for bulk Pb deposition were -445 and -535 mV for perchlorate and acetate, respectively. All chemicals were Aldrich ultrapure reagents. Deionized water was obtained from a “nanopure” (Barnstead) system. To facilitate comparison of the four systems studied, we report all potentials relative to the measured, equilibrium potential for deposition of the appropriate bulk metal. The data for T1 on Au(ll1) and Ag(ll1) and Pb on Au(ll1) were collected at the National Synchrotron Light Source beam line X2OAS4lAn incident X-ray energy of 9997 eV (1.240 A) was selected using a Si( 111) double-crystal monochromator.The X-rays were focused with a grazing incidence mirror; at the sample, this produced vertical and horizontal full widths at halfmaximum (fwhm)of 0.8 and 1.7 mm, respectively. The incident beam intensity was monitored by a NaI scintillation detector viewing a Kapton foil. The data for Pb/Ag( 111) were obtained at the Stanford Synchrotron Radiation Laboratory on an eightpole wiggler, beamline 7-2.42 A silicon (1 11) double-crystal monochromator was used to select an incident X-ray energy of 9015 eV (1.375 A), and the X-rays were focused with a grazing incidence mirror. At the sample the incident beam size was approximately 1.5 mm vertical by 2 mm horizontal, as determined by slits in front of the sample. An ionization chamber was used to monitor the incident beam intensity. In both experiments the diffracted beam was analyzed with 1 mrad Soller slits, since this resolution reasonably well matched the diffraction from the UPD layers and since these slits allowed sampling a large area on the substrates. The intensity was measured with a NaI scintillation detector. The acceptance of the diffracted beam out of the scattering plane was defined by
Monolayers of Pb and T1 on Au( 1 1 1) and Ag( 1 1 1)
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v (mv) v (mv) Figure 1. Linear sweep voltammograms for the deposition of Pb on Ag(ll1) taken in the X-ray cell (thick-film geometry). The potentials (V) were measured relative to Ag/AgCl but are reported relative to the observed, equilibrium potential for bulk Pb deposition. The scan rate was 2 mV/s. (a) The perchlorate electrolyte: 0.1 M NaC104 containing 5 mM Pb(C104)~and 10 mM HC104. (b) The acetate electrolyte: 0.5 M sodium acetate and 0.5 M acetic acid with 5 mM lead acetate. slits and was 20 mrad. The sample was mounted on a Huber four-circle diffractometer, and data were obtained in the symmetric (w = 0) mode>3 with a grazing incidence angle of about 1". These surface X-ray scattering measurements are timeconsuming; to obtain a typical data set at a given potential required at least 1 h. This contrasts to most experiments characterizing the electrochemical response of these systems, where the data collection is much faster (e.g., sweep rates in voltammograms of 1 - 1000 mV/s).
111. Lead on Silver(ll1) The voltammetry of Pb/Ag( 1 1 1)8'10-17,20,21,23,24 consists of a large current peak about 120- 140 mV positive of the reversible Pb potential and two smaller peaks slightly more positive and negative than this (e.g., see Figure 1). The structure of the dissolution voltammogram is similar to this, but the peak positions and shapes are slightly different. It has been shown that as the supporting electrolyte is changed from perchlorate to acetate to citrate (weakly to strongly adsorbing anions), the main peaks in the monolayer deposition and dissolution currents shift toward the reversible Pb potential, and the size and shape of the smaller deposition and dissolution peaks change.12J3J5 The charge passed for UPD layer formation (area under the deposition peaks) depends slightly on the electrolyte but is approximately consistent with that expected for a monolayer of Pb, assuming full discharge of the Pb2+ ion to Pb metal in the adlayer, which was shown in refs 14 and 15. Our previous in-situ surface X-ray scattering results of Pb/ Ag( 1 1 1) in a ~ e t a t e ~ Oshowed - ~ ~ that the Pb deposit formed a monolayer that was incommensurate with the substrate, compressed compared to bulk Pb, and rotated by about 4.5" from the Ag [Oli]direction. This structure has been confirmed with in-situ STM.24 In addition, the STM observed a longwavelength pattern that consisted of "Moire" spots spaced 1518 A apart and rotated 24-29' with respect to the Pb rows. This was ascribed to a "superstructure" in the Pb layer. We discuss this Moire pattern (which is also observed in Pb/Au(111))in section VI1.D. A. Underpotential Deposition of Pb on Ag(ll1) in Acetate and Perchlorate. Figure 1 shows a typical current response of the Ag electrode to a linear potential sweep (2 mV/s) for Pb on Ag(ll1)in (a) perchlorate and (b) acetate. The predominant
J. Phys. Chem., Vol. 99, No. 13, 1995 4735 features are the large, sharp peaks associated with deposition (C2) and dissolution (A2)of the UPD layer. The deposition and dissolution peak positions occur at 125 and 155 mV, respectively, in acetate and at 140 and 170 mV, respectively, in perchlorate. Furthermore, the peaks are larger and narrower in acetate than perchlorate. Also apparent in Figure l a (perchlorate) are the small deposition peaks (Cl and C3) and their anodic counterparts (A1 and A3). Peaks A1 and C1 are absent in acetate, and A3 and C3 are barely visible. The voltammograms in Figure 1 are consistent with those previously p ~ b l i s h e d . * , ~ ~Together -'~ with our X-ray data from the Ag surface, this demonstrates the quality of our Ag( 1 11) substrates. The peaks C1 and A1 are thought to be due to Pb adsorption and desorption from surface defects (steps), and their disappearance in acetate is attributed to adsorbed acetate ions blocking these sites.11-13 Several reasons for the smaller UPD shift in acetate have been suggested including a weaker Pb-Ag bond,15 Pb complex formation in acetate,12and stronger adsorption of acetate on Ag than on Pb.13 It is unclear what causes the narrower C2 and A2 peaks in acetate relative to perchlorate and the reduced C3 and A3 peaks.12,13s15 B. Structure of the UPD Monolayer. Figure 2a-d shows radial and azimuthal diffraction scans of the (10)Bragg rod from the Pb monolayer in the perchlorate (a and b) and acetate (c and d) electrolytes. In an azimuthal scan, the intensity is measured along an arc at a constant scattering vector Q = (4d A) sin 8,while in a radial scan the intensity is measured along a radial scattering vector at constant sample orientation 4. In the radial scan, the intensity is plotted against Qll, the component of the scattering vector parallel to the surface. The X-ray data from monolayers formed in both electrolytes are essentially identical, and the data are summarized by the diffraction pattern in Figure 2e. There are two equivalent domains of the Pb monolayer that are oriented about 4~4.5"from the Ag [Oli] direction, and these cause two peaks in the azimuthal scans (at 4 = 3~4.5").The diffraction from one of the domains is marked with arrows in Figure 2e. Consistent with our previous measurement^,^^-^^ the data in Figure 2 show that the UPD layer of Pb on Ag( 1 1 1)forms an incommensurate hexagonal monolayer rotated about 4.5"from the Ag [Oli]direction. A schematic representation of this real space structure has appeared b e f ~ r e ~ Oand . ~ l is quite similar to that shown in Figure 5a for Pb/Au( 1 1 1)except that the rotation angle is S2 = 4.5" and the open circles represent the Ag( 11 1) surface atoms. The interaction between the solvent molecules and the Pb adatoms does not influence the structure of the complete monolayer, since the monolayer structure of UPD Pb/Ag( 11 1) is essentially identical to that of vapor-deposited Pb on Ag(111)near full coverage.21,30,31,44,45 The monolayer compression in the vacuum experiments (1-2%) is slightly less than for UPD Pb (1.4-3.0%,see below), although the rotation angle S2 is about the same. C. Dependence of Monolayer Structure on Anodic or Cathodic Scan Direction. As is evident in the voltammograms of Figure 1 and those reported p r e v i ~ u s l y , ' ~ -the ' ~ current peaks for deposition and dissolution do not occur at the same potential. Furthermore, these peaks are broadened with a full width at half-maximum (fwhm) of -5-10 mV. Both these features are generally observed for UPD systems, and a key question is whether they are caused by kinetic effects or by actual structural differences in layers when the deposition potential is reached in an anodic scan or cathodic scan. To address this, the monolayer structure was investigated for a given
4736 J. Phys. Chem., Vol. 99, No. 13, 1995 I
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Figure 2. X-ray diffraction from Pb on Ag(111). (a) A radial scan of the Pb (10) Bragg rod for a potential of 95 mV in the perchlorate electrolyte. In this scan the magnitude of the scattering vector Q was varied, while the azimuthal angle was fixed at Q = 4.55". (The azimuthal angle is that between the scattering vector and the Ag [Ol 11 direction.) The diffracted intensity is plotted against Qll, the component of the scattering vector parallel to the surface. (b) Azimuthal angle scan of the Pb (10) Bragg rod at 95 mV in the perchlorate electrolyte. Here the intensity is measured along an arc at fixed Qll= 2.116 A-l. (c) Radial scan of the Pb (10) Bragg rod for a potential of 95 mV in the acetate electrolyte. The azimuthal angle was Q = 4.65'. (d) Azimuthal scan of the Pb (10) Bragg rod at 95 mV with QII= 2.118 A-l in the acetate electrolyte. The diffraction peak intensities in perchlorate are larger than in acetate due to different solution layer thicknesses and the use of different substrates. From the peak width in the radial scans (a) and (c), we estimate a Pb domain size of -450 A. (e) One quadrant of the in-plane diffraction pattern for UPD Pb/Ag(111). The center of the pattern is illustrated with a plus, the Ag reflections - with open circles, and the Pb reflections with closed circles. There are two observed domains, oriented approximately h4.5" from the Ag [Ol 11 direction; one is marked by the arrows.
potential V reached via anodic and cathodic scans. For the cathodic scans, the X-ray scattering was conducted on monolayers formed by sweeping the potential from =+450 mV to the desired potential V . To investigate the monolayer structure for anodic scans, the potential was first swept from =+450 to +5 mV, the sweep direction reversed, and then the potential swept positively to the final potential V. The scan rate was always 2 mV/s. To within experimental error, we find that both in electrolytes and at potentials where the monolayer is stable, the Pb(l0) peaks, and thus the monolayer structure, are the same for both anodic and cathodic scan directions. These results show that the monolayer is in equilibrium and that the widths of the deposition and stripping peaks and the offset in potential between these peaks are likely due to kinetic effects and/or h e t e r ~ g e n e i t yof~ ~the Ag( 111) substrate.
D. Potential Dependence of the Monolayer Structure. In acetate for 0 .e V 5 135 mV and in perchlorate for 0 < V 5 145 mV, the Pb layer structure is the same: the incommensurate lattice rotated approximately 3.8-4.5" from the Ag [ O l i ] direction and compressed compared to the case of bulk Pb. There are, however, small differences in the lattice constants (nearneighbor spacing) and in the rotation angle Q, which will be quantified below. For 0 < V 5 130 mV (acetate) and 0 < V 5 120 mV (perchlorate), the monolayer structure is stable for at least several hours (e.g., our measurement time), and the diffraction intensities do not systematically depend on potential, electrolyte, or scan direction. From this, we conclude that in these potential ranges the monolayer is stable and in thermodynamic equilibrium. This conclusion is consistent with voltammetric and potential pulse experiments (for
Monolayers of Pb and T1 on Au( 111) and Ag( 111)
J. Phys. Chem., Vol. 99, No. 13, 1995 4737
3.44
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Figure 3. Dependence of the Pb monolayer near-neighbor distance (ann)on the electrode potential for UPD Pb/Ag(111). A typical error bar is shown for the datum at 40 mV. The near-neighbor spacing for bulk Pb is 3.501 A, well above the range of the plot. The solid and dashed lines
are linear least-squares fits to the data for perchlorate and acetate, respectively. These lines have slopes of 0.424 f 0.01 &V (perchlorate) and 0.420 f 0.01 &V (acetate);their offsets are 3.390 8, (perchlorate) and 3.395 8, (acetate). The random errors in the offsets are f0.001 A, but when we include the possible systematic error due to the uncertainty in measurement of the reversible Pb potentials (S5 mV), the error in the offsets become f0.002 A. that show the stripping characteristics of the monolayer are unaffected by holding the potential in this region for 235 min. For V = 145 mV in perchlorate and V = 135 mV in acetate, diffraction from the incommensurate Pb monolayer was observed, but there were indications that this structure may be metastable (e.g., slow intensity decrease with time). Furthermore, although the peak positions were the same for both anodic and cathodic scan directions, the intensities were smaller for the cathodic scans and, for both scan directions, were less than expected for a full monolayer. These observations could result from a metastable UPD layer (consistent, in perchlorate, with results from electrochemical measurements8,l0)or from a slight nonuniformity in the electrode potential; further measurements are needed to clarify the stability of the UPD monolayer near these potentials. In perchlorate at V = 165 mV (just positive of midway between Az and Cz), we observed no diffraction near the expected Pb (10) peak position for the incommensurate layer. Thus, such a layer is not stable at this potential. As with the previous systems we i n ~ e s t i g a t e d , ~ ~ the ,~~-~’ positions of the Pb diffraction peaks shift to larger Qll as the electrode potential decreases. This corresponds to a decrease in the near-neighbor spacing and a compression of the monolayer. Such behavior is readily understood in terms of an increase in the monolayer chemical potential with decreasing electrode potential, as described e l s e ~ h e r e . ~ ~ ,Here ~~-~ we’ focus on how this behavior is influenced by anions: Figure 3 shows the dependence of the near-neighbor distance u,,,, on electrode potential in perchlorate (squares) and acetate (circles). The open and filled symbols represent data obtained from cathodic and anodic scan directions, respectively, and the lines are linear least-squares fits to these data. Figure 3 shows that at a given electrode potential there is little difference between the near-neighboring spacing of the hexagonal incommensurate monolayer in the perchlorate and acetate electrolytes (e.g., 50.005 A). The linear least-squares
fits have slopes of 0.424 f 0.01 &V (perchlorate) and 0.420 f0.01 &V (acetate), which are identical, to within experimental error. The offset in the linear fit is slightly larger in acetate (3.395 A) than perchlorate (3.390 A). However, this difference is close to that due to the systematic error ( 5 5 mV) that results from our experimental uncertainty in measuring the reference potential (the reversible Pb potential). We conclude that the presence of a strongly adsorbing anion does not effect the monolayer structure and only weakly influences the nearneighbor spacings, if at all. The data in Figure 3 can be used to determine the 2D isothermal compressibility of the monolayer K ~ D . ~ ~We, ~ have
where CP is the 2D spreading pressure, a is the atomic area, Z is the number of electrons transferred per atom deposited, and Vis the electrode potential. Using this and the slope from Figure 3, we calculate the compressibility as K2D = 1.25 f 0.05 A2/ eV, which is about 20% larger than we previously obtained for Pb/Ag( 111) in acetate.32 We attribute this to a slight oxidation of the Pb monolayer in our initial experiments, which was caused by diffusion of atmospheric 0 2 through the polypropylene film in the X-ray cell. This was eliminated in the present experiments by surrounding the electrode with the cylindrical cap that was constantly purged with n i t r ~ g e n . ~ ~ . ~ ~ The dependence of rotation angle S2 on the electrode potential is not completely reproducible, and with time and potential cycling, there is a slow, irreversible decrease in S2 (from about 4.5’ to about 3.8’). We observe similar behavior for TVAg(111)35 and suggest that this due to the adsorption of trace impurities (probably organics) when the electrochemical cell is inflated and the potential cycled. The reader is referred to ref
~ - ~ ~
4738 J. Phys. Chem., Vol. 99, No. 13, 1995
Toney et al.
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Figure 4. Linear sweep voltammogram for the deposition of Pb on Au( 11 1) in 0.1 M HC104. Data were taken in the X-ray cell (thickfilm geometry). The potentials were measured relative to Ag/AgCl but are reported relative to the observed, equilibrium potential for bulk Pb deposition. The scan rate was 10 mV/s.
35 for details. We emphasize that the near-neighbor spacing is reproducible and does not show any time dependence.
IV. Lead on Gold(ll1)
Figure 5. Schematic real-space representation of one domain of monolayer Pb on Au( 1 1 I). The open circles represent atoms of the Au( 11 1) surface and have a diameter proportional to their nearestneighbor spacing (2.884 A). The shaded circles represent the Pb adatoms and have a diameter proportional to their average nearestneighbor spacing. (a) The rotational angle between the Ag and Pb lattices is S2 = 2S0, and the average near-neighbor spacing of the Pb monolayer is 3.43 A. This structure is for V 5 130 mV. (b) The rotation angle between the Ag and Pb lattices is S2 = 0".and the average near-neighbor spacing of the Pb monolayer is 3.49 A. This structure is for V 2 160 mV. These structures ignore the local modulation in near-neighbor positions that result because the adatoms tend to move toward the lowest energy sites on the substrate.
The voltammetry of Pb/Au( 111)'*.25-29.~1,34,47-51 shows a large, sharp current peak about 210 mV positive of the reversible potentials. This voltammogram is similar to those obtained Pb potential and several much smaller peaks at potentials previous1y .29.47-5 1 positive to this (see Figure 4). On stripping, the anodic peak Our in-situ X-ray data for Pb/Au( 111) are quantitatively corresponding to the sharp deposition peak often splits into two consistent with our previous data31-34 and the STM and A F M peaks (especially at fast potential scan rates), and there is a shift ~ t u d i e s . ~ *However, *~~ in the present experiments we were able to about 600 mV of the anodic peak that corresponds to the to determine the rotation angle S2 and find that, for V S 130 deposition peak at about 270 mV.29,49Studies have shown that mV, S2 * 2S0, while for more positive potentials ( V 2 160 the Pb adlayer consists of neutral Pb ad atom^.^^.^^ We previmV), S2 collapses to zero. Because of the large in-plane mosaic ously used in-situ X-ray scattering to determine the atomic spread for the Au(ll1) thin film substrate, we did not follow structure of the Pb adlayer at V = 70 mV and found it formed the potential dependence of S2 nor measure it accurately. The an incommensurate, hexagonal monolayer that was compressed Pb adlayer forms an incommensurate hexagonal structure, and relative to bulk Pb. This structure was subsequently confirmed Figure 5 shows a schematic representation of one domain for by STM and AFM.25.26*2**29 These studies also showed that on both S2 = 2.5" (a) and S2 = 0" (b). The open circles represent unreconstructed Au( 111) surfaces Pb deposition initially occurs atoms of the Au( 111) surface, while the shaded circles represent at step edges (V * 300 mV). Subsequently, small Pb (or Pb the Pb adatoms. This structure is an average; it ignores the oxyhydroxide) islands are formed on the surface (V * 250 mV), subtle local modulation in near-neighbor positions that results and these islands coalesce to form the full monolayer at V because the adatoms tend to move toward the lowest energy 200 mV. Hysteresis in the formation and dissolution of these sites on the substrate. We have measured this substrate-induced islands may cause the irreversibility in the voltammetry spatial modulation for Tl/Ag( 111) and find that it is mentioned above. Both STM and AFM observed a Moire we expect a comparable result for Pb/Au( 111) and for the other pattern in the full m ~ n o l a y e r , ~although * - ~ ~ different periodicities systems described in this paper. As with previous systems (Pb, were reported. The formation of these patterns is discussed in TI, and Bi on Ag(l1 l)), the monolayer structure of UPD Pb/ section VI1.D. In one of the STM studies,26it was reported Au( 111) is essentially identical to that of vapor-deposited Pb that the Au( 111) electrode is substantially roughened after on Au( 111) near full coverage?2 although the compression in deposition and stripping of the Pb monolayer (e.g., a large the vacuum experiments is slightly less than for UPD Pb/Auincrease in the number of small Au islands and pits). This was (111). interpreted as showing that the UPD layer consists of a PbIn the potential range we have investigated (full coverage, 0 Au alloy on the surface. However, roughening of the Au( 111) < V 5 190 mV), the Pb monolayer adopts the hexagonal electrode was not observed in other STM or AFM s t ~ d i e s . ~ * . ~ ~incommensurate structure. There is a small dependence of the In this section we describe our results for the full monolayer near-neighbor spacing on potential, although S2 appears to structure (0 < V 5 190 mV). Figure 4 shows a typical change discontinuously at V = 160 mV. Again, the hexagonal voltammogram for the UPD of Pb/Au( 111) in 0.1 M HClO4 at structure is stable for at least several hours, and the structure 10 mV/s. This shows the large, sharp deposition peak at about and near-neighbor spacing do not systematically depend on scan 210 mV, the corresponding, split stripping peaks, and several direction, showing that in this potential range the monolayer is much smaller stripping and deposition peaks at more positive stable and in thermodynamic equilibrium.
J. Phys. Chem., Vol. 99, No. 13, 1995 4139
Monolayers of Pb and T1 on Au( 111) and Ag( 111)
P b / A u ( l l l ) ti A g ( l l 1 ) 3.52
.
0
3.50
0
3.48
cq: Y
Pb/Au(lll) Pb/Ag(11 l), acetate Pb/Ag(l 1 l), perchlorate ,
3.46
c
3.44 3.42 3.40 3.38
0
50
1 00 potential [mv]
150
200
Figure 6. Dependence of the Pb monolayer near-neighbor distance (ann)on the electrode potential for Pb/Au(111) (circles), Pb/Ag(111) in acetate
(filled squares), and Pb/Ag(111) in perchlorate (open squares). The dashed line is a quadratic least-squares fit to the data for Pb/Au(l 11) (e.g., ann = CO C1V CzV, where a,,,, is in 8, and Vis in mV). The solid lines are linear least-squares fits to the Pb/Ag(lll) data. For Pb/Au(lll), the and C2 = 6.7 x lo-'. For Pb/Ag(lll) CZ 0; the best fit result in perchlorate is CO= 3.390 best fit result is CO= 3.3977, C1 = 0.4149 x while in acetate, CO= 3.395 and C1 = 0.420 x and C1 = 0.424 x
+
+
Figure 6 shows the potential dependence of the near-neighbor spacing for Pb/Au( 111) (circles), Pb/Ag( 111) in acetate (filled squares), and Pb/Ag( 111) in perchlorate (open squares). The lines in Figure 6 are linear (Pb/Ag( 111)) and quadratic (Pb/ Au( 111)) least-squares fits to the data. As is evident, the nearneighboring spacing for Pb/Ag(lll) is fit by a straight line, while the data for Pb/Au( 111) cannot adequately be fit to a straight line but require a quadratic function to give the curvature in the data. Such curvature is reasonable. It implies a decrease in the compressibility with smaller near-neighbor spacing where the adatom-adatom repulsive force becomes increasingly stronger and it is increasingly difficult to pack the adatoms close together. The reason the data for Pb/Ag( 111) do not show any curvature likely results because the UPD range (and hence the range in a,) is significantly smaller than for Pb/Au( 111); thus, any curvature in the Pb/Ag( 111) data is simply not apparent. Using eq 1, the average compressibility for Pb/Au( 111) (at the midpoint, V = 100 mV) is calculated as K2D = 1.69 & 0.09 &/eV. Note that the data are not sufficiently accurate to determine the dependence of K ~ Don a n n . As mentioned above, one of the STM studiesz6reported that the Au( 111) electrode is substantially roughening after deposition and stripping of the Pb monolayer, and this was interpreted as showing that the UPD layer consists of a Pb-Au surface alloy. We see no evidence for this. Roughening of the electrode would cause a decrease in the surface X-ray scattering from the Au(ll1) electrode (e.g., the crystal truncation rod).53 We checked for this and it was not observed. In addition, the near-neighbor spacing for the UPD layer of Pb/ Au( 111) (see Figure 6) is close to that for UPD Pb/Ag( 11l), and if a Pb-Au alloy were formed, then a, would be much smaller (e.g., closer to that for Au, ann= 2.884 A). Recall also that no roughening of the electrode was observed in other STM or AFM s t u d i e ~ . ~ ~ ~ ~ ~
V. Thallium on Silver(ll1) Our results for TI on Ag( 111) have been described in previous publication^.^^,^^ Here we describe only the essentials. At potentials between 235 and 30 mV (vs the measured, reversible potential for bulk T1 deposition), the T1 deposit forms an incommensurate, hexagonal monolayer compressed relative to bulk T1 by 1.4-3.0% and rotated from the Ag[Oli] direction by 51 = 4-5" (see the inset to Figure 7). As with other systems, this structure is the same as that of vapor-deposited TUAg(1 11).& For the UPD system, the in-plane spacing between T1 adatoms decreases with decreasing electrode potential with an average 2D compressibility of K2D = 1.54 f 0.10 A*/eV. The rotation angle 51 depends on electrode potential and adatom spacing, but again irreversibly decreases with potential cycling. Despite this irreversibility, the dependence of S2 on adatom spacing qualitatively agrees with t h e 0 1 - y . ~Investigations ~~~~ of the structure of monolayers where the deposition potential is reached in either an anodic or a cathodic scan were also conducted, and as with Pb/Ag( 11l), we found these structures to be identical. This, together with the observation that the monolayer structure does not change over at least 24 h (the longest we waited), shows that the monolayer is in thermodynamic equilibrium and that the finite width and offset of the peaks in the deposition curves are due to kinetics, the influence of adsorbed anions, and/or substrate heterogeneity. At potentials between the monolayer region and bulk deposition, Tl forms a bilayer that also has an incommensurate hexagonal struct~re.~~
VI. Thallium on Gold(ll1) The UPD of Tl/Au( 111) in neutral or weak acid has been investigated with ~ o l t a m m e t r y , 4second ~ ~ ~ ~harmonic generation (SHG),18,57 and in-situ X-ray d i f f r a c t i ~ n . ~The ~ , ~voltammetry ~
Toney et al.
4740 J. Phys. Chem., Vol. 99, No. 13, 1995 15
10
5 h
N
E
2
Y
0
v
.-'e
-5
-10
-15 Potential (mv)
Figure 7. Linear sweep voltammogram for the deposition of Tl on Ag( 1 1 1) in 2.5 x M T12S04 and 0.1 M Na2S04. The potentials were measured relative to Ag/AgC1(3 M KCl) but are reported relative to the observed, equilibrium potential for bulk T1 deposition. The scan rate was 2 mV/s, and arrows indicate the scan directions. The inset is a schematic real-space representation of one domain of the average structure of monolayer TVAg( 1 1 1). The rotation angle between the Ag and T1 lattices is S2 = 4.5", and the average near-neighbor spacing of the TI monolayer is 3.36 A. The open circles represent atoms in the Ag( 1 1 1) surface, and the shaded circles represent the Tl atoms. I
I
I
I
I
I
I
20
10
n
NE 2Y v
-lC
-2(
-3(
100 200 300 400 500 Potential (mv)
600 70(
Figure 8. Linear sweep voltammogram for the deposition of Tl on Au( 1 1 1) in 0.1 M Na2S04 containing 2.5 mM T12S04. The potentials were measured relative to Ag/AgCl but are reported relative to the observed, equilibrium potential for bulk T1 deposition. The scan rate was 10 mV/s.
for this system shows two deposition and two stripping peaks (see Figure 8). although there is a rather large discrepancy in the charge reported for complete monolayer formation (potentials negative of the second peak), ranging from 48 pC/cm2 (ref 58) to 160 ,uC/cm2 (ref 56). Second harmonic experiments observed a large drop in the SHG intensity coincident with the deposition of T1.18 This is similar to that observed for Pb/Au(1 11) and suggests a structural similarity between these two systems. Recently, Wang et al. reported an in-situ X-ray
diffraction study of Tl/Au( 111) in alkaline solutions.59 For the complete UPD layer (V < 290 mV vs TVTl+), Tl/Au( 111) forms an incommensurate, rotated hexagonal monolayer containing neutral metal adatoms. The near-neighbor spacing varies between 3.34 and 3.43 A, although a slight hysteresis is reported. The rotation angle shows considerable hysteresis and is in the range 5.2-6.2". For more positive potentials (V > 290 mV), Wang et al. observe an open, incommensurate hexagonal structure that is aligned along the Au substrate;this is apparently stabilized by OH- adsorption. The goal of our Tl/Au( 111) experiments is to determine the monolayer structure and compressibility in neutral solutions and to compare this to TI/Ag( 111). Figure 8 shows the voltammogram for the UPD of TI on Au( 111) in 0.1 M Na2S04 and shows two prominent stripping and deposition peaks, the one at more positive potentials being much larger than that at more negative potentials.18748*56-58 The diffraction pattern deduced from our in-situ X-ray data for TYAu( 111) is essentially the same as for Tl/Ag( 111),35.39 and Tl/Au( 111) forms the incommensurate hexagonal monolayer, just like Tl/Ag(111) and Tl/Au( 111) in alkaline solutions. The two equivalent domains of the T1 monolayer are oriented f5-5.5" from the Au [Oli] direction, consistent with that reported for Tl/Au( 111) in alkaline solutions. However, due to the large in-plane mosaic spread for the Au( 111) thin film substrate, we did not follow the potential dependence of the rotation angle SZ nor did we measure it accurately. A schematic representation of the average real-space structure of Tl/Au( 111) is essentially identical to the inset to Figure 7 (with the open circles representing the Au( 111) surface atoms, of course). Again, this is an average and ignores the substrate-induced modulation in T1 positions. We could find no vacuum experiments on vapor-deposited T1 on Au( 111) to compare with the UPD system. For 0 V 5 300 mV the TI monolayer adopts the rotated hexagonal, incommensurate structure with a small dependence of the near-neighbor spacing on potential. This hexagonal structure is stable for at least several hours (our measurement time), and the structure and near-neighbor spacing are independent of scan direction. This shows that in these potential ranges the monolayer is stable and in thermodynamic equilibrium. These observations are different from the reported behavior for Tl/Au( 111) in alkaline solutions, where a slight hysteresis is reported in the near-neighbor spacing.59 This suggests that long times are needed for TI/Au( 111) to come to equilibrium in alkaline solutions (at least when the monolayer is deposited in a thin film geometry). In our experiments at V = 350 mV (midway between the two cathodic peaks in the voltammogram, Figure 8), we did not observe any diffraction near the expected Tl(10) peak position for the incommensurate layer, and so such a layer is not stable at this potential. We did not search for open structures of the type reported by Wang et Figure 9 shows the potential dependence of the near-neighbor spacing for both Tl/Au(111) (triangles) and Tl/Ag( 111) (squares). The Tl/Au( 111) near-neighbor spacings are 3.33-3.42 A, which correspond to surface coverages of (0.99- 1.04) x loi5 atoms/ cm2 or 159-167 pC/cm2, consistent with Wang et al. for TI/ Au(ll1) in alkaline solution59and with the voltammetry of Schultze et al.56 (assuming full discharge of the T1+ ions, as found in alkaline solution^).^^ The lines in Figure 9 are quadratic least-squares fits to the data. As is evident, the nearneighbor spacings require a quadratic fit to give the curvature in the data, as explained above. Some curvature is also apparent
Monolayers of Pb and T1 on Au( 111) and Ag( 111)
J. Phys. Chem., Vol. 99, No. 13, 1995 4741
Tl/A~(lll) & A g ( l l 1 ) 3.43
3.41
-
3.39
ca u C
0
3.37
3.35
3.33
0
50
100
150
200
250
300
potential [mv] Figure 9. Dependence of the T1 monolayer near-neighbor distance (ann)on the electrode potential for TVAu(ll1) (triangles) and WAg(ll1) (squares). The solid and dashed lines are quadratic least-squares fits to the data for WAu(ll1) and WAg(1ll), respectively (e.g., u,,,, = Co CIV + CzV, where a n n is in 8, and Vis in mV). For TVAu(lll), the best fit result is CO= 3.3285, CI = 2.177 x and C2 = 3.317 x lo-’, while and Cz = 3 x lo-’. for TVAg(lll), CO= 3.3232, CI = 1.743 x
+
in the ann vs V data for Tl/Au(lll) in alkaline solution,59 although these data were fit to a straight line. From the least-squares fit in Figure 9, one can calculate the compressibility for Tl/Au(lll) using eq 1. The average compressibility (that at the midpoint in the potential, V = 150 mV) is KZD = 1.88 f 0.05 A2/eV, and again the data are not sufficiently accurate to determine the dependence of K2D on ann. This value of KZD is the same as can be calculated from the data in Wang et al.59for Tl/Au( 111) in alkaline solution, KZD = 1.9 f 0.1 A2/eV, which illustrates that for these close-packed UPD layers the electrolyte does not strongly influence the monolayer structural properties.
VII. Discussion This section provides a discussion of the data presented in the previous sections, including (a) the importance of the adatom-adatom and adatom-substrate interactions in determining the UPD layer structure, (b) the influence of adatom-anion and adatom-solvent forces on the adlayer structure, (c) the effect that different anions have on the voltammetry for Pb/ Ag(lll), (d) a comparison of K2D for the TI, Pb, and Bi on Au( 111) and Ag( 111) and with that calculated for a free electron gas model of the monolayer, and (e) the Moire patterns observed with STM and AFM. A. UPD Layer Structure: The Importance of AdatomAdatom Interactions and the Effect of the Substrate. For all four systems described herein, the UPD monolayers adopt incommensurate structures that are independent of the substrate and are almost identical to the close-packed plane of the bulk phases that are stable at room temperature. From this we conclude that for these systems the adatom-adatom interactions are most important in determining the adlayer structure. We also believe the same conclusion holds for Bi/Ag( 111) and the full monolayer structure of Bi/Au( 111). These systems also adopt a monolayer structure that is similar to close-packed planes
of bulk hexagonal Bi (e.g., the (102) planes including atoms slightly above and below these planes),37 and although these systems form uniaxially commensurate, rectangular struct u r e ~ ,the ~ ~commensurability ,~~ in one direction is due to a fortuitous match between the atomic spacings of the substrate and these close-packed bulk Bi planes. Thus, for T1, Pb, and Bi on Ag( 111) and Au( 111)-systems where the adatoms are significantly larger than the substrate atoms-the adatomadatom interactions determine the adlayer structure. We speculate that for other such systems (larger adatom than substrate) the adatom-adatom interactions will also determine the adlayer structure. However, this conclusion will likely not apply to UPD systems where the adatom is smaller or about the same size as the substrate atoms. For T1, Pb, and Bi on Ag(ll1) and Au( 11l), the interaction between the adatoms and substrate is strong and results in the formation of a monolayer rather than bulk clusters (e.g., Stranski-Krastanov rather than Volmer-Weber growth). However, the corrugation or spatial variation in the adatom-substrate interaction is weak; it only influences the adlayer structure by creating a small spatial modulation in the adatoms positions as mentioned above and described in ref 39. The substrate has an additioinal small effect on the adlayer structure apparent from a comparison of the ann data for T1 and Pb on Au( 111) and Ag( 111) (Figures 6 and 9, respectively). This shows that when the substrate is Ag( 111) the near-neighbor spacing is less than when the substrate is Au(l1 l), although the difference is rather modest (50.01 A). This difference between substrates is found at all potentials but is largest for more positive potentials. A comparison of the data for Bi/Au(111)38 with Bi/Ag(111)37 shows that a similar difference is also observed here. This difference can be understood within the framework of effective medium t h e ~ r y , ~ Owhich - ~ ~ we briefly review here. In this theory the environment of an atom is modeled as a homogeneous electron gas, and the binding energy of the atom
4742 J. Phys. Chem., Vol. 99, No. 13, 1995 in this environment (e.g., in a solid or at a surface) is related to the embedding energy of the atom in this homogeneous electron gas. The density of the electron gas is called the embedding density and is an average of the electron density from the neighboring atoms in the system. Consequently, the embedding density is a monotonically decreasing function of atomic spacing. First consider a simple bulk solid. The binding energy of an atom in this environment is the embedding energy plus an electrostatic term that accounts for the attraction between the "embedded" atom and the electron density tails from neighboring atoms. Both the embedding and electrostatic energies are functions of the embedding density. Since this density is related to the atomic spacing, the equilibrium atomic spacing is determined by the minimum in the binding energy as a function of embedding density.60q61 Now consider a free-standing monolayer, and notice that the coordination number of the atoms in this layer is less than in the bulk solid. Thus, if the monolayer atomic spacing is the same as in the solid, the monolayer embedding density is less than the optimum density that gives the minimum binding energy. To reduce the binding energy, the embedding density must increase closer to the optimum density. This is achieved by a contraction or compression of the monolayer atomic spacing (compared to the bulk solid). For the more realistic case of an incommensurate monolayer on a substrate, the substrate surface atoms will also contribute to the embedding density. Thus, the atomic spacing will be larger in a freestanding monolayer but will still be smaller than bulk, since the coordination number is still smaller than in the solid. This explains why the UPD monolayer near-neighbor spacings are smaller than the bulk spacing. In addition, since gold is more electron rich than silver, an Au( 111) substrate will contribute more electron density to the monolayer embedding density than an Ag( 111) substrate. The monolayer near-neighbor spacing will, consequently, be larger (more bulklike) for an Au( 111) substrate than for Ag( 11l), which is just what we observe. B. Anion and Solvent Effect on UPD Layer Structure. One can obtain insight into the effect the solvent (water) has on the UPD layer structure by comparing this structure to the vapor-deposited analog. As noted above, we have done this for Pb, Bi, and T1 on Ag( 111) and Pb/Au( 111) (where reliable structures for the vapor-deposited systems have been reported), and in all cases, the monolayer structures are essentially identical. This remarkable similarity in two very different environments shows that in these systems the interaction between the solvent molecules and the adatoms does not influence the monolayer structure. In addition, our result that Pb/Ag(lll) forms the same hexagonal structure in both perchlorate and acetate shows that the presence or absence of a strongly adsorbing anion does not influence the UPD adlayer structure. From a comparison of Bi on Ag( 111) in chloridefree and chloride-containing electrolytes, we came to the same c o n c l ~ s i o n .Thus, ~ ~ we find that for these UPD systems, which form close-packed structures, neither the anion nor the solvent influences the structure. Note, though, that in UPD systems forming open structures the anion can have an effect on the monolayer ~tructure.~.65 Although the Pb/Ag( 111) monolayer structure is the same in perchlorate and acetate, there may be a small dependence of the near-neighbor spacing on electrolyte. The data in Figure 3 suggest that between perchlorate and acetate there is a change in annof 0.005 f 0.004 8, at a given potential. The cause of this difference is not entirely clear, and although this change is close to the error bar, it could reflect a dependence of the AgPb bond strength on anion adsorption. As argued in the previous
Toney et al.
TABLE 1: Two-Dimensional Compressibilities of Several UPD Systems KD(A*/ev) system structure observed calculated WAu(111) hexagonal, SZ = 5.5" 1.88 f 0.05 0.44 WAu(lll), hexagonal, SZ = 4.5' 1.9 f 0.13 0.44 alkaline [59] hexagonal, SZ = 4.5" 1.52 f 0.05 TllAg(111) 0.44 hexagonal, SZ 2.5" 1.69 f 0.1 Pb/Au(111) 0.3 hexagonal, SZ = 4.5" 1.25 f 0.05 0.3 Pb/Ag(111) Bi/Au(lll) [38] rectangular 0.2 1.15 f 0.05 0.79 f 0.04 0.2 BUAg(ll1) [37] rectangular section, the electron density in the UPD layer determines its near-neighbor spacing, and a lower electron density leads to a shorter spacing. Since we expect a stronger Pb-Ag bond to lead to a more electron-rich environment in the Pb monolayer (Le., larger uM), the slightly larger uM in acetate suggests that the Pb- Ag bond is stronger in acetate than perchlorate. C. Pb/Ag(lll): Effect of Anions on the Voltammetry. As shown in Figure 1 and mentioned above, the voltammogram for the UPD of Pb on Ag(ll1) is significantly different in perchlorate and a ~ e t a t e . ~ ~Our J ~observations J~ shed some light on the differences. Recall that the Pb monolayer structures are the same in both electrolytes and do not depend on scan direction. Consequently, kinetic effects, and not thermodynamics, must cause the features in the voltammograms that are affected by the electrolyte. Specifically, different kinetics cause the different offsets in the anodic and cathodic peaks, the narrower main peaks C2 and A2 in acetate compared to perchlorate, and the much smaller peaks C3 and A3. These conclusions confirm previous suggestions based on electrochemical measurement^.^^^^^ D. Compressibilities of the UPD Layers. The decrease in uM with decreasing electrode potential, which is observed in all the systems studied, is understood in terms of an increase in the monolayer chemical potential with decreasing electrode potential, as described e l s e ~ h e r e . ~ These ~ , ~ ~data - ~ ~for u M vs potential allow calculation of KZD, and Table 1 shows the measured average values of K ~ Dfor the UPD systems investigated herein, for Bi/Ag( 111),37for Bi/Au( 111),38and for Tl/ Au( 111) in alkaline solution^.^^ Note that, for the same UPD deposit, K ~ Dis smaller on Ag( 111) than on Au( 11l), and so the substrate has an influence on the adlayer compressibility. Although the reasons for this difference are not completely clear, it could be due to the different electronic interactions between the Ag and Au substrates and the UPD layer or, perhaps, to different substrate energy corrugations (e.g., variation in the substrate-adatom interaction energy). For most bulk metals, the compressibility is dominated by the electron and one expects a similar domination for metal monolayers. Thus, a 2D free electron gas model of the monolayer should provide a reasonable estimate of K2D. Table 1 shows the compressibilities estimated with such am~del:~~,~
where m is the electron mass, h is Plank's constant over 2n, and n is the free electron density (n = z / a , where Z, is the number of free, or valence, electrons in the metal monolayer, and u is the monolayer unit cell area). As can be seen in Table 1, the model is in qualitative agreement with the measured KID, which provides further evidence that these UPD layers are metallic and that the metal ions fully discharge upon deposition. The free electron model also correctly predicts the trend of
J. Phys. Chem., Vol. 99, No. 13, I995 4743
Monolayers of Pb and T1 on Au( 111) and Ag( 111) decreasing K2D from T1 to Pb to Bi (increasing free electron density), but it is not quantitatively correct and predicts that K2D is independent of substrate, in contrast to the observations. A more quantitative explanation of these data awaits a better calculation of K2D for these heavy metals, such as could be obtained from a local density a p p r ~ x i m a t i o n . ~ ~ E. Moire Pattern. For Pb on Ag( 111) and Au( 11l), S T M and AFM observations found long wavelength superstructures or Moire patterns in the mono la ye^-.^^^*^.^^ This pattern is a manifestation of the periodic modulation in the adatom positions caused by the substrate, and the STM/AFM likely senses the perpendicular displacements of the Pb adatoms (e.g., Pb adatoms atop Au or Ag atoms are higher than those in hollow or bridge sites). The wave vector of the modulation or superstructure is the difference between the first-order reciprocal-lattice vectors of the monolayer and the substrate surface and thus are functions of am and the rotation angle S2.68 Since these both depend on electrode potential, the wavelength of the superstructure or Moire pattern is also potential dependent:* which has been experimentally verified for Pb/Ag(l 1l).69 Note that different periodicities for the Moire pattern were reported for PbIAu(1 1l).28,29 Since the periodicity depends on S2 and since this is very sensitive to small quantities of i m p ~ r i t i e swe , ~ ~believe that this difference is due to slightly different S2 in the two experiments.
VIII. Summary and Conclusions In this paper we have described the results of surface X-ray scattering measurements of the atomic structure of UPD Pb and T1 on Ag(ll1) and Au(ll1). For these systems, the UPD deposits near full coverage form incommensurate, hexagonal monolayers that are compressed compared to the bulk metals by 0.1-3.2% and rotated from the substrate [Oli] direction. The rotation angle varies from -5.5" (TUAu(ll1)) to -4-5" (T1 and Pb on Ag( 111)) to -2.5" or zero (Pb/Au( 11l), where the latter value is for V 2 160 mV). The monolayer structures for these UPD systems, as well as for Bi/Ag(111)37 and Bi/ Au( 111),38are the same those of the close-packed plane of the bulk phase, and for systems where a comparison can be made, they are essentially identical to those of the vapor-deposited analogs. For UPD Pb/Ag( 111) in both perchlorate and acetate, the same compressed, incommensurate, hexagonal monolayer was found, and at any given potential the near-neighbor spacings are almost identical (Le., am is larger in acetate than perchlorate by 0.005 f 0.004 A). These results show that for T1, Pb, and Bi on Ag( 111) and Au( 111) the adatom-adatom interaction plays the major role determining the structure within the monolayer. Although the adatom-substrate interaction is strong and results in the formation of a monolayer rather than bulk clusters, its effect on the monolayer structure is small. For any given potential, the different substrates cause slightly different near-neighbor spacings, and the variation (or corrugation) in the adatomsubstrate interaction creates the small spatial modulation in the adlayer. Our results also show that the presence of the large concentration of water molecules or adsorbing anions in contact with the monolayer surface does not affect its structure, which can be attributed to the fact that these metal monolayers form close-packed, metallic 2D layers where the dominant structure determining interaction is between the metal adatoms. For the systems we have studied in this paper, the UPD monolayers are compressed compared to the bulk metals by as much as 3.2%, and this compressive strain can significantly influence the subsequent growth of bulk metal deposits.70 The compression was explained above and in ref 35 using effective
medium t h e ~ r y , and ~ - ~these ~ ideas can also be used to explain the larger near-neighbor distances for adlayers on Au( 111) compared to Ag( 111). In all incommensurate UPD systems studied, the in-plane spacing between adatoms decreases with decreasing electrode potential (due to the corresponding increase in monolayer chemical p ~ t e n t i a l ~ ~ . ~From ~ - ~such ~ ) . data, one can determine the monolayer compressibility (K2D). which are shown in Table 1. These data show that, for the same UPD deposit, K2D is smaller on Ag( 111) than on Au( 11l), although the reasons for this are unclear. We also find that K2D estimated for a freeelectron gas model of the m ~ n o l a y e qualitatively r ~ ~ ~ ~ ~agrees ~~ with the measurements but is not quantitatively correct. A more quantitative explanation of these data awaits a better calculation of K2D for these heavy metals. The Pb/Ag( 111) monolayer structures are the same in both perchlorate and acetate and do not depend on scan direction, which shows that kinetics and not thermodynamics cause some of the differences in the voltammograms-specifically, the offsets in the anodic and cathodic peaks, the narrower main peaks in acetate, and the much smaller C3 and A3 peaks in acetate. These results confirm previous suggestions based on electrochemical measurements. l 2 3 l 3
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