In Situ Stress and Nanogravimetric Measurements During

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J. Phys. Chem. C 2007, 111, 17580-17586

In Situ Stress and Nanogravimetric Measurements During Underpotential Deposition of Pb on (111)-Textured Au G. R. Stafford* and U. Bertocci Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: August 20, 2007; In Final Form: September 18, 2007

The surface stress associated with the underpotential deposition (upd) of lead on (111)-textured Au is examined, using the wafer curvature method, in acidic perchlorate supporting electrolyte. The surface stress is correlated to θ, the fractional Pb coverage, by independent nanogravimetric measurements using an electrochemical quartz crystal nanobalance (EQNB). The gravimetric results are similar to the data found in the literature, showing an electrosorption valency of 2 and the formation of a hexagonal close-packed monolayer of Pb. The complete Pb monolayer causes an overall compressive surface stress change of about -1.2 N m-1. The stress-coverage curve can be divided into two linear regions separated by a plateau. The region at low to intermediate coverage is caused by the formation of Au-Pb bonds which decrease the charge density and reduce the tensile surface stress inherent to the clean Au surface. In the second linear region at high coverage, the Pb adlayer compresses and behaves as a free-standing elastic film with a biaxial modulus close to that for Pb (111) in the bulk. The plateau that separates these two linear regions corresponds to the stress relaxation “hump” that appears in the stress-potential curve. This stress relaxation is attributed to island coalescence.

Introduction The underpotential deposition (upd) of metal monolayers onto foreign metal substrates is important to both the electrodeposition and electrocatalysis communities. In metal deposition involving Stranski-Krastanov nucleation and growth, the upd layer forms prior to the formation of 3-D crystals so that an understanding of deposition processes in the upd region may help us better understand the growth and subsequent properties of bulk thin films. Monolayers and submonolayers of metals such as Bi and Pb on some noble metal surfaces have shown enhanced catalytic activity for a variety of electroreduction processes, most notably the two-electron reduction of H2O2 to H2O, often the limiting step in the reduction of O2 to H2O in aqueous fuel cells.1,2 For this reason, the upd of Pb on a variety of noble metal substrates has been extensively examined by in situ techniques such as electrochemical voltammetry/ coulometry,3-7 spectroscopy,8,9 scanning probe microscopy,10-12 X-ray scattering,13 and quartz crystal nanogravimetry,14-18 in order to better understand the formation and structure of the upd adlayer and perhaps gain some insight into the electrocatalytic activity. Pb forms an incommensurate hexagonal close-packed (hcp) monolayer on Au (111).19 This is not unexpected, since monolayers with a large substrate mismatch adopt incommensurate structures that are independent of the substrate and are nearly identical to the close packed plane of the adlayer’s bulk structure. Unlike similarly sized Bi, which forms a (2 × 2) ordered adlayer on Au (111) at low coverage, Pb does not form intermediate superlattice structures. On the nonreconstructed Au (111) surface, islands of hcp Pb nucleate initially at the Au step edges and then on terraces.10-12 Deposition continues by rapid lateral growth, followed by island coalescence. The full monolayer forms an hcp layer that compresses * To whom correspondence should be addressed.

10.1021/jp0766914

as the potential is made more negative.13 During compression, there is a rotational change of a few degrees in the position of the adlayer with respect to the Au surface. An additional in situ probe gaining popularity in electrochemistry involves the measurement of surface and growth stress. The sensitivity of surface stress to both ionic and fully discharged adsorbates makes this measurement particularly relevant for upd studies, where both processes tend to occur simultaneously.20,21 Several investigators have examined the surface stress changes associated with the upd of Pb on (111)textured Au,22-24 and all have reported similar stress signatures, namely the generation of compressive stress in the early stages of upd followed by a relaxation in the tensile direction which has been linked to the rotational change of the adlayer reported by Toney et al.13 The stress relaxation is followed by additional compressive stress as the result of electrocompression, the magnitude of which is close to that expected from the reported strain.13,22,23 There is however a discrepancy in the literature between the fractional coverage θ at which the adlayer rotation occurs and that observed for the stress relaxation. In the surface X-ray scattering measurements of Toney et al., adlayer rotation occurs at a potential of 130 to 160 mV positive of the reversible Pb/ Pb2+ potential, which is within the voltage range of full monolayer coverage.13 This appears to coincide with a slope change (albeit still compressive) in the surface stress measurements of Brunt et al.22 However, more recent surface stress measurements show stress relaxation at more positive potentials where θ is on the order of 0.5-0.8 of a full monolayer.23,24 This apparent discrepancy in surface coverage and the fact that the data of Toney et al. clearly shows that the adlayer rotation causes no corresponding change in the nearest-neighbor spacing of the adlayer suggest that the stress relaxation may actually be associated with island coalescence rather than adlayer rotation.

This article not subject to U.S. Copyright. Published 2007 by the American Chemical Society Published on Web 11/03/2007

Deposition of Pb on (111)-Textured Au

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In this paper, we examine the surface stress associated with the upd of Pb onto (111)-textured Au cantilever electrodes in perchloric acid supporting electrolyte. In order to correlate the stress development with the Pb coverage, we separately examine the process using an electrochemical quartz crystal nanobalance (EQNB). Experimental Section In situ stress measurements were made on a HeNe optical bench using the wafer curvature method.25,26 The cantilever was a borosilicate glass strip (D 263, Schott) measuring 60 mm × 3 mm × 0.108 mm. The glass had a Young’s modulus of 72.9 × 109 N m-2 and a Poisson ratio of 0.208, as specified by the vendor. Onto one side of the cantilever, a 4 nm thick adhesion layer of titanium and a subsequent 250 nm film of gold were vapor-deposited by electron-beam evaporation. The glass-metal interface provided the reflective surface for the laser beam. Prior to use, the electrodes were cleaned in piranha solution (3:1 volume mixture of concentrated H2SO4:30% H2O2). The Au electrodes had a strong (111) crystallographic orientation. The 200 reflection was not apparent in θ - 2θ X-ray scans and rocking curves of the 111 reflection generally yielded a fullwidth half-maximum (FWHM) on the order of 2°. These films have a fiber texture; i.e., there is no in-plane orientation. Fieldemission scanning electron microscopy shows the Au grain size to be on the order of 25 nm. The electrolyte was 0.1 mol L-1 HClO4 (Sigma-Aldrich, 99.999%) containing 10 mmol L-1 Pb(ClO4)2 (Sigma-Aldrich, 99.999%). The distilled water was further purified using an “EASY pure UV” ultrapure water system (Barnstead). The electrochemical cell was a single compartment Pyrex cell covered by a perfluorethylene cap. A glass disc was joined to the back of the cell to allow it to be held and positioned by a standard mirror mount on the optical bench. The counter electrode was a platinum foil placed parallel to and in the same solution as the working electrode. The reference electrode was a saturated Hg sulfate electrode (SSE) that was separated from the working compartment by a Vycor-tipped bridge filled with 1.0 mol L-1 HClO4. Prior to making a measurement, the electrolyte was purged with nitrogen. Potential control was maintained using an EG&G Princeton Applied Research Corp. (PARC) model 273 potentiostat-galvanostat that was controlled by a Macintosh Power PC computer and LabView software. A more detailed description of the optical bench and stress measurement can be found in refs 25 and 26. Mass changes during Pb upd were monitored by means of the electrochemical quartz nanobalance (EQNB). The measuring apparatus was a model RQCM from Maxtek, Inc., also the manufacturer of the quartz crystals used. The crystals were polished 2.54 cm diameter AT cut blanks, onto which first Ti and then Au were evaporated. The resonance frequency was 5 MHz. The Au deposit had a strong (111) crystallographic orientation, and rocking curves of the 111 reflection generally yielded a FWHM on the order of 3°. The EQNB as well as the EG&G 273 potentiostat-galvanostat were driven by Labview software on a Macintosh Power PC computer. The cell in which the EQNB measurements were made had separate compartments for the working, counter, and SSE reference electrodes, the latter being connected via a LugginHaber capillary. The cell had a magnetic stirrer, and high purity Ar was either bubbled in the main compartment or swept above the solution to maintain a small overpressure inside the cell. The Maxtek, Inc. RQCM measures both the resonant frequency and the resistance R1 of the equivalent resonant circuit: R1 in

Figure 1. Potentiodynamic scan and mass changes on (111)-textured Au in 10 mmol L-1 Pb(ClO4)2 + 0.1 mol L-1 HClO4 (solid lines) and mass changes in Pb2+-free 0.1 mol L-1 HClO4 (dashed line). Sweep rate ) 10 mV s-1. Solid points are steady-state mass changes obtained potentiostatically from a starting potential of 0.0 V/SSE.

all upd measurements changed very little, confirming that no significant roughening of the electrode surface took place. EQNB measurements in the pure acid were also carried out in order to determine the voltage range where anion adsorption and desorption occurred. All voltages mentioned in this work are given with respect to the SSE electrode. Results Figure 1 shows both the voltammetric and gravimetric response of the Au-covered quartz electrode for the Pb upd region in 0.1 mol L-1 HClO4 containing 10 mmol L-1 Pb2+. The voltammetry consists of a broad set of peaks centered at -0.40 V and a set of sharp, reversible peaks at about -0.65 V, very similar to that reported in the literature for (111)-textured and single-crystal Au.3,8,10,13,24 The broad set of peaks is reported to be irreversible, and the magnitude and position are easily influenced by sweep rate as well as the potential history of the Au electrode.3,8,10 It is generally believed that in this potential region (-0.35 to -0.60 V) islands of hcp Pb nucleate and grow at step edges and eventually on the Au terraces. The sharp peak centered at -0.65 V is associated with island coalescence and the formation of a homogeneous hcp adlayer. We, and others, have observed that the latter set of sharp peaks centered at -0.65 V become split at faster sweep rates.8,13,22 The compression region reported by Toney et al. occurs in the potential region between the sharp peak at -0.65 V and the bulk deposition of Pb, which in this electrolyte begins at -0.865 V. Several prior investigations have concluded that Pb2+ is completely discharged,6,14,17 and if deposition is restricted to a monolayer, then no alloying with the Au substrate occurs.6 The charge required to form a fully compressed Pb monolayer can then be calculated to be 320 µC cm-2, assuming the radius of the Pb atom is 0.17 nm, somewhat smaller than the bulk value of 0.175 nm. We have integrated the current density from several voltammograms with sweep rates ranging from 5 to 20 mV s-1 and typically obtain charge density values of 350 ( 10 µC cm-2. This is consistent with an electrode roughness factor of 1.1 which we independently obtain from the charge necessary to completely reduce a monolayer of Au oxide, assuming that one oxygen is adsorbed for each surface atom of Au.27 Figure 1 also shows the gravimetric response of the Aucovered quartz electrode for the Pb upd region. In addition to the potentiodynamic curves, gravimetric results from stepwise

17582 J. Phys. Chem. C, Vol. 111, No. 47, 2007 potentiostatic measurements are shown. The agreement is excellent. Potentiodynamic scans at different sweep rates (1 to 100 mV s-1) have also been performed, with practically identical results. Anion desorption is the dominant feature as the potential approaches -0.15 V from more positive values. The gravimetric response in the absence of Pb2+ (dashed line) shows that ClO4desorption is essentially complete prior to Pb upd. Since the potential of zero charge (pzc) for Au (111) in 0.1 mol L-1 HClO4 is -0.21 V/SSE28,29 perchlorate desorption is expected in this potential range. Pb upd occurs in three steps that can be identified by changes in slope of the gravimetric curve. The first two steps roughly correspond to the current peaks in the voltammetry. In the potential range of -0.35 to -0.60 V, corresponding to the broad voltammetric waves, the gravimetric response shows hysteresis, reflecting the irreversibility of these processes. At -0.60 V, the slope of the gravimetric curve increases, in agreement with the increased deposition rate associated with island coalescence. In addition, the hysteresis is reduced which is consistent with the reversibility of the corresponding voltammetric waves. The slope of the gravimetric curve then decreases in the compression region. The total mass gained for the complete monolayer of Pb was typically 370 ( 13 ng cm-2 which is consistent with the expected value of 378 ng cm-2 for a compressed hcp monolayer of Pb with roughness factor of 1.1. The fact that the measured mass increase can be accounted for by the mass of a Pb monolayer suggests that neither H2O nor ClO4- play a significant role in the upd process, in agreement with reports in the literature.6,9,18 The surface stress response of the Au cantilever in 0.1 mol L-1 HClO4 containing no Pb2+, in the potential region of Pb upd, is shown in Figure 2a. The figure shows the stress response for a potentiodynamic scan (10 mV s-1) as well as for a series of steady-state cathodic potential steps from two different initial potentials. The surface stress moves in the tensile (positive) direction from a value arbitrarily chosen as zero at 0.2 V. The stress response reflects the desorption of ClO4- from the gold surface. This is consistent with Ibach’s surface-induced charge redistribution model, where electron acceptors such as adsorbed anions cause compressive stress, because they reduce the electron density in the surface.30 As the potential approaches the pzc from the positive direction, ClO4- is desorbed and its compressive contribution to the surface stress is diminished. Contrary to reports in the literature,23 we do not see a maximum in the surface stress at the pzc as one would expect from a traditional electrocapillary curve. Rather, in the absence of Pb2+, the surface stress continues to move in the tensile direction, even into the onset of hydrogen evolution. Our stress response is similar to that reported by Seo and Yamazaki in Pb2+-free 0.5 mol L-1 NaClO4 electrolyte at pH 3.0.24 Figure 2(b) shows the voltammetric and surface stress response of the Au cantilever in 0.1 mol L-1 HClO4 containing 10 mmol L-1 Pb2+. At potentials positive of Pb upd, -0.30 V, the stress response is similar to that when Pb2+ is not present; that is, the surface stress moves in the tensile (positive) direction due to the desorption of ClO4- from the gold surface. At about -0.43 V, where discrete islands of hcp Pb are formed, the surface stress moves in the compressive direction. It is interesting to note that the stress maximum occurs well into the upd region, near the completion of the first voltammetric peak. At more negative potentials, the stress continues in the compressive direction except for a small relaxation, which coincides with the reversible voltammetric wave at -0.64 V. This relaxation has been reported by several authors, which they attribute to

Stafford and Bertocci

Figure 2. (a) Surface stress response to potentiodynamic scan (10 mV s-1) on (111)-textured Au in 0.1 mol L-1 HClO4. Solid points are stress changes obtained potentiostatically from initial potentials of +0.2 V (b) and +0.4 V (9); (b) potentiodynamic scan (10 mV s-1) and surface stress on (111)-textured Au in 10 mmol L-1 Pb(ClO4)2 + 0.1 mol L-1 HClO4.

rotation of the Pb monolayer.22-24 The surface stress continues to move in the negative direction in the compression region as one approaches the bulk deposition of Pb. When the potential scan is reversed, the stress response of the return sweep is approximately the mirror image of that of the forward scan, although some hysteresis is present. The overall upd process is quite reversible, and our results are consistent with reports in the literature that no alloy formation between Au and Pb occurs if deposition is limited to a monolayer.6 We have made several measurements like that in Figure 2b, varying the sweep rate from 2 to 500 mV s-1 and find that, whereas the voltammetry changes as expected for a surface-confined reaction (peak current changes linearly with sweep rate), the surface stress curves essentially superimpose, except for some loss of detail in the relaxation region at the higher sweep rates. We can correlate the gravimetric and surface stress results in two different ways. Figure 3 shows coverage-stress-time plots where the gravimetric data are normalized to the total mass recorded in the upd region. The normalized gravimetric data can be thought of as the fractional coverage θ with respect to the fully formed and electrocompressed hcp Pb monolayer. The surface stress was arbitrarily set to zero at the potential where the experiment was initiated, 0.0 V. The plots are shown for sweep rates of 5, 10, 50, and 100 mV s-1. The interesting feature of this series of graphs is the consistency in the value of the Pb fractional coverage θ which corresponds to the stress relaxation “hump” on the cathodic sweep. The value of θ is 0.56 ( 0.02

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Figure 3. Pb fractional coverage θ based on gravimetric data and surface stress as a function of time for the complete Pb monolayer deposition and stripping cycle. The sweep rates are (a) 5, (b) 10, (c) 50, and (d) 100 mV s-1.

and is independent of sweep rate. On the anodic sweep, θ at the onset of stress relaxation is 0.76 ( 0.04. Figure 4a shows the Figure 3 data with the surface stress plotted vs θ obtained gravimetrically. These data pertain to relatively slow sweep rates, from 5 to 100 mV s-1. In Figure 4b on the contrary, θ is based on the voltammetric charge measured from the cantilever electrode and normalized to the total charge recorded in the upd region. Of necessity, this is restricted to fairly high sweep rates (100 to 500 mV s-1) where the charge associated with parasitic reactions such as O2 reduction can be neglected. The stress-coverage curves in Figure 4a are nearly identical to those in Figure 4b and show only a minor dependence on the sweep rate. At very low θ, the surface stress moves in the tensile direction before turning compressive. This is likely due to desorption of the remnant ClO4- from the Au surface while Pb is nucleating at the step edges. Similar results have been observed for Bi upd on (111)textured Au.26 The remainder of the stress transient is comprised of two linear regions, separated by an intermediate region where the stress changes very little, although there is an appreciable change in coverage. The first linear region spans θ from about 0.2 to 0.56. The stress response then remains essentially flat until the Pb coverage approaches a value of about 0.85, where the adlayer begins to compress and the stress becomes linear again but with a steeper slope. The complete Pb adlayer causes an overall stress decrease of about 1.2 N m-1 with respect to its maximum value at a Pb coverage of 0.05. The slope of the first linear region at lower θ is very close to the value reported by Seo and Yamazaki.24 Haiss et al. observed a linear correlation between surface stress and surface charge for a variety of anions adsorbed onto Au.31 Ibach also observed a linear change of surface stress with surface coverage for a variety of electronegative adsorbates, such as S and O, on Pt and Ni surfaces32 and attributed this to adsorbate-induced changes in the charge density of the substrate surface atoms. Leiva et al. have used the embedded-atom method to calculate

the surface stress due to epitaxial monolayer adsorption for a variety of face-centered cubic (fcc) adsorbate-substrate combinations.33 When adsorption energy is considered together with lattice misfit, they find qualitative agreement with experimental data. The adsorption contribution is particularly apparent for systems with positive misfit (small adsorbate), such as Cu on Au. The embedded-atom calculation predicts compressive stress, consistent with experimental data,25,34,35 whereas considering the +11.4% misfit alone leads to tensile stress. Since upd is at least partially driven by the free energy of dissimilar bonding, one would always expect the adsorption contribution to favor compressive stress. We therefore attribute the compressive stress response at low coverage to the formation of Pb-Au bonds that partially satisfy the bonding requirements of the Au surface atoms, decreasing the charge density and reducing the tensile surface stress inherent to the clean Au surface. As a consequence, one would also expect a linear stress change with coverage/charge at low and intermediate θ. We now examine the second linear region observed at high coverage. This is the electrocompression region where additional Pb atoms are incorporated into the adlayer as the potential is made more negative. Following a treatment used by Friesen et al.,23 we convert our stress-potential data into stress-strain by using Toney et al. near-neighbor distance data in the electrocompression region,13 assuming that the zero-strain condition is reached at the near-neighbor distance for bulk Pb (0.35 nm), which occurs at a potential of 0.19 V positive of the reversible potential for bulk Pb deposition (-0.675 V/SSE). Figure 5 shows the stress-strain data associated with the return (anodic) sweep for sweep rates ranging from 2 to 500 mV s-1. Although the actual value of the stress change shows a sweep rate dependence, all of the data can be fit to a straight line over the entire compression region, with an average slope of 16.8 ( 0.3 N m-1. The Pb monolayer is then treated as a free-standing

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Stafford and Bertocci

Figure 4. Surface stress as a function of θ on (111)-textured Au in 10 mmol L-1 Pb(ClO4)2 + 0.1 mol L-1 HClO4. The coverage in (a) is based on mass from the EQNB, whereas that in (b) is based on charge from the cantilever electrode.

Figure 5. Stress-strain curve in the electrocompression region (anodic sweep) for Pb upd on (111)-textured Au in 10 mmol L-1 Pb(ClO4)2 + 0.1 mol L-1 HClO4. Sweep rates ranging from 2 mV s-1 to 500 mV s-1 were examined.

elastic film, for which the stress-strain relationship is given by

σ ) c Y′111 d111

(1)

where Y′111 is the biaxial modulus, c is the elastic strain due to compression, and d111 is the film thickness.23 The slope of the

stress-strain data in Figure 5 is then the quantity Y′111d111. Using the elastic compliance values found in,36 we calculate a value of 6.21 × 1010 Pa for Y′111, which, with 0.2858 nm for the interplanar spacing for Pb, leads to a value of 17.75 N m-1 for Y′111d111. This is nearly identical to the average slope obtained from Figure 5. Although it is not obvious why the modulus of a monolayer on a free surface should have the same value as the same layer in the bulk solid, the good agreement between experimental and calculated misfit stress reported in the literature for Stranski-Krastanov film growth suggests that the elastic constants of even ultrathin films are close to the bulk values.37 We now discuss the plateau that appears between the two linear regions just discussed. It is clear that this stress plateau is associated with the stress relaxation “hump” that coincides with the reversible voltammetric wave at -0.64 V, as seen in the voltammetry in Figure 2b. Due to the relatively high current associated with Pb upd in this potential region, the Pb coverage changes significantly over a short period of time, see Figure 3. At fast sweep rates, the plateau in Figure 4b is rather flat, whereas at lower sweep rates in Figure 4a, the tensile relaxation can be detected. The fact that the stress relaxation occurs at fairly low coverage (0.56) and covers a fairly wide range of fractional coverage (0.56-0.86), makes us question its association with the adlayer rotation, the current interpretation in the literature.22-24 The surface X-ray scattering measurements of Toney et al. indicate that adlayer rotation occurs at a potential of 130 mV to 160 mV positive of the reversible Pb potential (-0.735 to -0.705 V/SSE), which is within the potential range of full monolayer coverage.13 Their data also show that the adlayer nearest-neighbor distance changes continuously in this potential region due to compression and that there appears to be no dimensional change associated with the rotation of the monolayer. We find it unlikely that one would measure a biaxial stress relaxation with no apparent change in the in-plane elastic strain. These points taken together suggest that the most probable cause of this relaxation is the moment when the various Pb islands coalesce. The phenomenon can be seen as a percolation threshold: the value for the site percolation threshold for 2-dimensional lattices varies from a high of 0.696 for the honeycomb (hexagonal) to 0.593 for a cubic and 0.500 for a triangular lattice.38 In the real case, one should keep in mind that shapes and sizes of the separate Pb islands are not uniform. The agreement with the experimental value of 0.56 ( 0.02 appears reasonably good. The fact that Pb forms an upd layer on Au indicates that there is some interaction between the Pb and Au. However, the fact that the Pb adlayer is incommensurate with respect to the Au surface and that upd proceeds nonuniformly by island formation are clear indications that the substrate plays a small role in determining the structure of the adlayer. In the intermediate stages of upd where discrete islands exist, Chen et al. have shown by AFM that the interaction between these islands and the Au(111) surface is relatively weak, so that the islands can be scraped away when a stronger scanning force is applied.10 They also conclude that the hysteresis in the deposition and stripping of the islands may be due to the relatively large attractive lateral forces between the individual Pb atoms composing the islands. It seems reasonable to conclude that the near-neighbor distance between the adlayer atoms, i.e., the inplane strain, is determined by interactions within the adlayer rather than between the Pb and the Au. Obviously the Pb-Au interface requires some traction for stress to develop in the adlayer and be detected by a wafer curvature measurement.

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The deposition of Pb onto Au(111) follows a StranskiKrastanov growth mode. The initial two-dimensional growth is driven by surface free energy, while the large lattice mismatch leads to an adlayer structure that is incommensurate with respect to the substrate. Since growth is not epitaxial, there is an absence of misfit stress. One can also expect that the stress generated at island coalescence will differ from that observed for epitaxially grown films. For example, during Volmer-Weber epitaxial growth (Ag, Cu, and Au on mica(001)) huge compressive stress develops at domain walls during coalescence due to the mismatch between the islands’ nucleation centers.37,39 Since coalescence occurs at an average film thickness of 20-60 nm, atoms that arrive at the domain walls close the gap by incorporating into the deepest layer, thereby generating compressive stress. This does not apply to our situation where coalescence occurs in the monolayer and growth is not epitaxial. A treatment developed by Hoffman for the coalescence of three-dimensional islands, based on a local strain mechanism generated at the boundary,40,41 might be more appropriate for examining the elastic distortions that accompany island coalescence associated with Pb upd. The biaxial tensile stress in the film is given by

σ ) Y′

∆f ∆ ) w d111

(2)

where Y′ is the biaxial modulus of the film, w is the island diameter, and ∆ is the distortion at the boundary. The stress generated can also be equated to the change in surface stress ∆f divided by the film thickness, in our case the thickness of the Pb adlayer (0.2858 nm). Estimates of ∆ have been calculated from the interaction potential between two atoms using a hard sphere model, and values on the order of 0.8 Å have been reported.41,42 Chen et al. have shown by AFM that the Pb islands are typically 15 nm in diameter. For Y′ we use the biaxial modulus for the (111) plane of Pb, 6.21 × 1010 Pa, which we calculated and used in our analysis of the compression region. From eq 2, we calculate a coalescence-induced tensile stress of 330 MPa which corresponds to a surface stress of 0.09 N m-1. This is very close to the stress relaxation observed in Figure 2b. One can also rationalize the stress response on the return sweep. In the compression region, the stress moves in the tensile direction as Pb atoms are removed from the layer and relieve the compressive elastic strain. The strain can then become slightly positive as the near-neighbor distance exceeds the 0.35 nm value for bulk Pb. The X-ray data of Toney et al. shows a single point at about 190 mV (vs Pb/Pb2+) where this does in fact occur.13 It is reasonable to assume that the adlayer is under slight tension prior to the breakup of the continuous layer. Green et al. have shown by STM that Pb removal occurs first at the step edges and is followed by pit formation in the adlayer terrace plane, leaving isolated islands on the surface.11 Once the monolayer is broken up, the tensile stress in the individual islands relaxes and produces the compressive “hump” in the stress curve. It is interesting to note that the stress response for Pb on (111)textured Au in HClO4 electrolyte is quite similar in shape to that reported for Bi.26 This might be expected since the two have a similar lattice mismatch with Au and both the Au-Pb bond (130 kJ mol-1) and the Au-Bi bond (290 kJ mol-1) are energetically favorable. The primary difference in the two stresscoverage curves is that Bi upd has no plateau separating the two linear regions. The linear response at intermediate coverage extends directly into the compression region. The explanation

can be found in the respective adlayer structures. The Bi adlayer does not form discrete islands that later coalesce, rather it forms a uniform (2 × 2) structure that, with additional coverage, is replaced by a uniaxially commensurate (p × x3)-2Bi adlayer which at more negative potential is compressed in the incommensurate direction.1,43-46 The Bi adlayer forms homogeneously, without island coalescence, so there is no structural basis for stress relaxation. Conclusions We present a gravimetric and surface stress examination of Pb upd on (111)-textured Au in perchloric acid supporting electrolyte. The complete Pb monolayer causes an overall surface stress change of about -1.2 N m-1 (compressive). The stress-coverage curve is comprised of two linear regions separated by a plateau. The linear region at low to intermediate coverage we attribute to the formation of Au-Pb bonds which partially satisfy the bonding requirements of the Au surface atoms, decreasing the charge density and reducing the tensile surface stress inherent to the clean Au surface. The second linear region is seen at high Pb coverage where the Pb adlayer compresses. In this region, the adlayer behaves as a free-standing elastic film where the stress-strain proportionality has a value equal to the biaxial modulus for Pb(111) in the bulk. The plateau that separates these two linear regions corresponds to the stress relaxation “hump” that appears in the stress-potential curve. There are several factors, including a large discrepancy in the fractional Pb coverage that cause us to associate this stress relaxation with island coalescence rather than adlayer rotation, which is the current interpretation in the literature. The extent of the relaxation, about 0.1 N m-1, is consistent with that calculated from a simple elastic model developed for threedimensional island coalescence. Acknowledgment. The authors gratefully acknowledge the technical contributions of Jonathan Guyer and Carlos Beauchamp. Certain trade names are mentioned for experimental information only; in no case does it imply a recommendation or endorsement by NIST. References and Notes (1) Chen, C.-H.; Gewirth, A. J. Am. Chem. Soc. 1992, 114, 5439. (2) Sayed, S.; Ju¨ttner, K. Electrochm. Acta 1983, 28, 1635. (3) Engelsmann, K.; Lorenz, W. J.; Schmidt, E. J. Electroanal. Chem. 1980, 114, 1-10. (4) Engelsmann, K.; Lorenz, W. J.; Schmidt, E. J. Electroanal. Chem. 1980, 114, 11-24. (5) Hamelin, A. J. Electroanal. Chem. 1984, 165, 167-180. (6) Hamelin, A.; Lipkowski, J. J. Electroanal. Chem. 1984, 171, 317330. (7) Kirowa-Eisner, E.; Bonfil, Y.; Tzur, D.; Gileadi, E. J. Electroanal. Chem. 2003, 552, 171-183. (8) Adzic, R.; Yeager, E.; Cahan, B. D. J. Electrochem. Soc. 1974, 121, 474-484. (9) Futamata, M. Chem. Phys. Lett. 2001, 333, 337-343. (10) Chen, C.-h.; Washburn, N.; Gewirth, A. A. J. Phys. Chem. 1993, 97, 9754-9760. (11) Green, M. P.; Hanson, K. J.; Carr, R.; Lindau, I. J. Electrochem. Soc. 1990, 137, 3493-3498. (12) Tao, N. J.; Pan, J.; Li, Y.; Oden, P. I.; DeRose, J. A.; Lindsay, S. M. Surf. Sci. Lett. 1992, 271, L338-L344. (13) Toney, M. F.; Gordon, J. G.; Samant, M. G.; Borges, G. L.; Melroy, O. R.; Yee, D.; Sorenson, L. B. J. Phys. Chem. 1995, 99, 4733. (14) Deakin, M. R.; Melroy, O. J. Electroanal. Chem. 1988, 239, 321331. (15) Henderson, M. J.; Bitziou, E.; Hillman, A. R.; Vieil, E. J. Electrochem. Soc. 2001, 148, E105-E111. (16) Hepel, M.; Kanige, K.; Bruckenstein, S. Langmuir 1990, 6, 10631067.

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