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In Situ Stress and Nanogravimetric Measurements During Underpotential Deposition of Pd 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: September 9, 2008; ReVised Manuscript ReceiVed: October 29, 2008
The surface stress associated with the electrodeposition of Pd onto (111)-textured Au cantilever electrodes in 0.1 M H2SO4 supporting electrolyte has been examined. An electrochemical quartz crystal nanobalance was also used in order to correlate the stress development with the Pd coverage. PdSO4 and H2PdCl4 were separately examined as electroactive species, in order to determine the influence of [PdCl4]2- adsorption on the stress response. The stress associated with Pd underpotential deposition (upd), as well as with the growth of subsequent adlayers, is tensile, whose primary source is the +4.9% lattice mismatch between Pd and Au. However the expected tensile stress is in part counterbalanced by the formation of Pd-Au bonds as well as anion adsorption onto the Pd surface. Pd upd causes a surface stress change of +0.4 N m-1 whereas a second monolayer produces an additional stress change of +0.6 N m-1. We attribute this larger tensile stress change for the second monolayer to the diminished electronic influence of the Au substrate. A similar stress response is observed for both H2PdCl4 and PdSO4 as sources of Pd2+. Introduction The underpotential deposition (upd) of metal monolayers onto foreign metal substrates is important to several technologies. In metal deposition involving Stranski-Krastanov nucleation and growth, the upd layer forms prior to the formation of threedimensional (3-D) crystals so that the deposition processes in the upd region can be expected to influence the growth and subsequent properties of bulk thin films. In electrocatalysis, 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 Submonolayer deposition of a second metal has been shown to significantly retard Pt poisoning by CO, a severe limitation in organic-based fuel cells.3,4 Epitaxially grown Pd overlayers on both Au(111) and Pt(111) have been used to study a wide variety of electrochemical phenomena such as hydrogen adsorption and absorption,5,6 formic acid and formaldehyde oxidation,7-9 carbon monoxide oxidation,10 and oxygen reduction.11 Modified catalytic activity has been observed for Pd overlayers consisting of a few monolayers. In order to better understand this catalytic enhancement, the electrodeposition of Pd onto Au(111) has been examined by a variety of in situ techniques such as scanning probe microscopy,5,6,12-15 X-ray scattering,16 optical methods,17 quartz crystal nanogravimetry,12,13 and standard electrochemical methods.9,18,19 In both chloride- and sulfate-containing electrolyte, Pd deposition on Au(111) proceeds two-dimensionally in a layer by layer mode and grows pseudomorphically for at least the first few monolayers. Since the lattice mismatch between Pd and Au is +4.9%, the first few Pd layers have an expanded in-plane atomic spacing. As a consequence, it has been argued that strain-induced changes in the electronic structure of the Pd overlayer are at least partially responsible for the enhanced catalytic activity and modified electrochemical behavior.9 10.1021/jp8080063
Our understanding of Pd deposition is also being enhanced by computational efforts. Monte Carlo simulations using the interatomic potentials of the embedded atom model (EAM) are in agreement with experimental data, showing that Pd films grow epitaxially and pseudomorphically with the crystallographic orientation of the Au(hkl) substrate.20 Periodic density functional theory (DFT) calculations predict variations in the surface electronic structure for pseudomorphic overlayers that can be correlated with adsorption energy shifts.21 Specifically in the case of Pd-Au(111), DFT calculations have shown a maximum in the binding energies of both atomic hydrogen and CO on two Pd overlayers on Au that might help to explain the reported higher catalytic reactivity of the overlayers with respect to that of bulk Pd.22 The authors conclude that both lattice strain and the interaction of the Pd films with the Au substrate could lead to the modified catalytic activity. An in situ probe gaining popularity in electrochemistry involves the measurement of surface and growth stress by monitoring the curvature of a wafer or cantilever while in solution and under potential control. Surface stress is the reversible work required to elastically deform a surface. The loss of bonds at a clean metal surface causes a redistribution of charge density between the remaining surface atoms, thereby increasing their attractive interaction, and causing a decrease in their equilibrium interatomic spacing. Since the surface atoms are held in place by the bulk lattice, they are stretched from their equilibrium lattice positions and a tensile surface stress arises. The adsorption of species on the surface can be expected to alter the surface stress, since the local interaction of each adsorbate will alter the bond strength between neighboring atoms on the surface.23,24 This is also true of metal monolayers formed by upd. The stress response of several upd systems has been reported in the literature, including Cu,25-29 Al,30 Ag,31,32 Pb,32-35 and Bi36 on Au(111), Pb on Ag(111),32 and Ag on Pt(111).37 Experimental data have shown that the stress change associated with a pseudomorphic (1 × 1) metal monolayer cannot be estimated from continuum elasticity theory by simply using the
This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society Published on Web 12/10/2008
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difference between the bulk lattice parameters to determine a misfit strain. When the misfit is positive (small adsorbate), elasticity theory predicts a tensile stress, yet there are several examples where the opposite is true, namely, the upd of Cu on Au(111)25-29 and Al on Au(111).30 As expected, when the misfit is negative (large adsorbate), elasticity theory predicts compressive stress, which is generally supported by experiment, although there is some discrepancy regarding the magnitude of the compressive stress.31,37 Leiva et al. have used the EAM to calculate the surface stress due to epitaxial monolayer adsorption for a variety of face-centered cubic (fcc) adsorbate-substrate combinations.38 They conclude that the surface stress change is a balance between an adsorption contribution, which is negative, and a misfit term, which can be positive or negative. 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. One is then left to question how significant the lattice misfit contribution is toward the surface stress change during upd processes. Trimble et al.28 have reported that the surface stress difference associated with epitaxial overlayer growth, ∆F, can be given by
∆F ) ∆fint + ∆h + ∆Σcohtf + ∆fpzc
(1)
where ∆fint is the intrinsic surface stress difference between the overlayer and the substrate, ∆h is the interface stress, ∆Σcohtf is the product of the coherency stress and the film thickness, and ∆fpzc is an electrocapillary term that reflects the change in capacitance and potential of zero charge (pzc) associated with changing the surface from Au to Pd. Equation 1 might be particularly useful for pseudomorphic film growth of several monolayers in which case the coherency term can be distinguished from the remaining terms that are independent of film thickness. If one assumes that ∆fpzc is small, then one can calculate the interface stress, ∆h, with knowledge of ∆fint. An examination of the stress response of Pd deposition on Au(111) might be of interest for several reasons. It would provide additional stress data for a upd system with positive lattice mismatch. Recall that Al and Cu upd on Au, both of which have positive lattice mismatch, result in compressive surface stress changes. However, the Pd-Au system is unique in that Pd grows pseudomorphically on Au for the first four monolayers.14 Whereas the surface stress response of the upd layer will have both adsorption and misfit contributions, the stress response of the subsequent overlayers should be due to misfit alone. In such a case, eq 1 might be useful in determining the various surface stress contributions. The surface stress response may also shed some light on the unique catalytic activity exhibited by the first few Pd monolayers. In this paper, we examine the surface stress associated with the deposition of Pd onto (111)-textured Au cantilever electrodes in 0.1 M H2SO4 supporting electrolyte. The deposition of Pd on Au(111) is highly irreversible unless chloride is present, and the electrochemical removal of the Pd layer is kinetically hindered in chloride-free electrolyte. In chloride-containing electrolyte, [PdCl4]2- is the electroactive species and is known to strongly adsorb onto both Au and Pd surfaces.12,14 Since surface stress is sensitive to both ionic and fully discharged adsorbates, such as metal monolayers, we examine deposition with and without chloride present in order to determine the influence of [PdCl4]2- adsorption on the stress response. We separately examine the process using an electrochemical quartz crystal nanobalance (EQNB) in an effort to correlate the stress development with the Pd coverage.
Experimental Section In situ stress measurements were made on a HeNe optical bench using the wafer curvature method.27,36 The cantilever was a borosilicate glass strip (D 263, Schott) measuring 60 mm × 3 mm × 0.108 mm. (Certain trade names are mentioned for experimental information only, in no case does it imply a recommendation or endorsement by NIST.) 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 at a temperature of 300 °C and growth rate of 0.1 nm s-1. The curvature of the substrate was monitored while under potential control by reflecting a HeNe laser off of the glass/metal interface onto a position-sensitive detector. 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 full width at half-maximum (fwhm) on the order of 2°. These films have a fiber texture; i.e., there is no in-plane orientation. We examine both the changes in surface stress of the Au cantilever in response to changes in surface chemistry in the Pd upd region as well as the in-plane stress developed in electrodeposited Pd films. The relationship between the force per cantilever beam width, Fw, exerted by processes occurring on the electrode surface and the radius of curvature of the cantilever, is given by Stoney’s equation39
Ysts2 Fw ) ) 6(1 - νs)R
∫0t σf dt f
(2)
where Ys, νs, and ts are Young’s modulus, Poisson ratio, and thickness of the glass substrate, respectively, and R is the radius of curvature of the cantilever. In the case where the force on the cantilever is due to surface processes, Fw is the surface stress. In the case where the force on the cantilever is the result of bulk metal deposition, then Fw is equal to the stress-thickness product; i.e., the film stress, σf, integrated through the thickness of the film, tf (eq 2). We report two stress quantities. The first, the average film stress (σavg), is defined as Fw divided by the film thickness (tf), while the second, the instantaneous film stress (σinst), is defined as the numerical derivative of Fw with respect to thickness. The instantaneous stress corresponds to the stress associated with the deposition of an infinitesimal amount of new material, assuming that the stress state of the previously deposited material does not change with time. These quantities are defined in eq 3.
σavg )
Fw tf
σinst )
d(Fw) dt
(3)
In all cases the film thickness was calculated from the charge, assuming 100% current efficiency and uniform current distribution. The electrolyte was 0.1 mol L-1 H2SO4 (Mallinckrodt) containing 1.0 mmol L-1 Pd2+. The Pd2+ was added as PdSO4 (Aldrich) or as H2PdCl4. The latter was formed by adding stoichiometric quantities of PdSO4 and HCl (Fisher).14 The distilled water was further purified to 18.3 MΩ cm 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 disk 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
Underpotential Deposition of Metal Monolayers 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 saturated K2SO4 solution. All potentials are referenced to the SSE. Prior to 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 Dell Pentium 4 computer and LabVIEW software. A more detailed description of the optical bench and stress measurement can be found in refs 27 and 36. An electrochemical quartz nanobalance (EQNB) manufactured by Maxtek Inc. was employed to measure mass changes during Pd deposition, upd, and overpotential deposition (opd). The quartz crystals employed, also manufactured by the same firm, were polished 2.54 cm AT cut disks onto which first Ti and then Au was vacuum deposited. 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 was driven by LabVIEW software on a Macintosh Power PC computer. The glass cell for the EQNB measurements had separate compartments for the working, counter, and SSE reference electrodes, the latter being connected via a Luggin-Haber 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. Before introduction into the cell, the solution was thoroughly deareated by bubbling highpurity Ar. The Maxtek, Inc., RQCM measures both the resonant frequency and the resistance R1 of the equivalent resonant circuit: R1 in all measurements changed very little, confirming that no significant roughening of the electrode surface took place.
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Figure 1. Potentiodynamic scan and charge density (a) and potentiodynamic scan and mass change (b) for (111)-textured Au in 0.1 mol L-1 H2SO4 and 1.0 mmol L-1 H2PdCl4. Sweep rate ) 2 mV s-1.
Results The Pd upd behavior, using H2PdCl4 as the source of Pd2+, can be summarized in the two plots of Figure 1. The first shows the typical voltammetry between 0.48 and 0.08 V. The quality is somewhat inferior to what the literature shows on good Au(111) single crystals, as it is well-known that the voltammetry is very sensitive to the step density of the Au.14,19 Nonetheless, the essential features are present. The pzc for Au(111) in this electrolyte is -0.19 V,14 so the Au has an adsorbed layer of [PdCl4]2- on the surface in the Pd upd region.12,14 It is believed that Pd deposition occurs by reductive discharge of the adsorbed Pd chloro complex. Pd upd begins at about 0.28 V, and the observed separation between upd and opd occurs at 0.09 V. This potential range for Pd upd is identical with that reported by Baldauf.5 It should be noted that the kinetics of the upd tends to be rather sluggish, and it is not always possible to separate upd from opd. The anodic stripping shows a shoulder at about 0.13 V, corresponding to a small amount of opd, and a peak at about 0.19 V, corresponding to the dissolution of the upd layer. Figure 1a also shows that the net charge for deposition and stripping is zero, indicating that there are no significant parasitic reactions, particularly the reduction of dissolved oxygen. The solution was purged with Ar before transferring it into the cell, but a major reason is probably that the voltage range is too positive for oxygen reduction. Figure 1b shows the mass response over the same voltammetric potential range. The mass of the upd layer is about 240 ng cm-2, which is close to the theoretical value of 245 ng cm-2
Figure 2. Plot of mass change vs charge density for the potentiodynamic scan shown in Figure 1.
for a (1 × 1) layer of Pd on Au(111). This value is often larger, reflecting the larger surface area due to the roughness of the vapor-deposited gold. The plot in Figure 2 shows that there is perfect matching between charge and mass, with the value of the m/Q ratio, taken from the slope of the line, quite close to the theoretical value of -0.55 ng µC-1, assuming a 2ereduction to Pd metal. This m/Q value indicates that the adsorbed [PdCl4]2- is not desorbed but remains on the surface of the Pd monolayer. This should be expected since the pzc for Pd is more negative than that of Au. It is also consistent with EQNB and
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Figure 4. Surface stress response to cathodic potential steps from a starting potential of +0.38 V vs SSE.
Figure 3. (a) Potentiodynamic scans and (b) change in surface stress for (111)-textured Au in 0.1 mol L-1 H2SO4 and 1.0 mmol L-1 H2PdCl4. Variation of the cathodic vertex at constant sweep rate of 5 mV s-1.
scanning tunneling microscopy (STM) studies of this system that appear in the literature.12,14 Figure 3 shows the voltammetry (a) and stress response (b) of the (111)-textured Au cantilever in 0.1 mol L-1 H2SO4 containing 1.0 mmol L-1 H2PdCl4 where the cathodic vertex is adjusted to more negative values so that both Pd upd and bulk deposition occur. In addition to an increase in cathodic current when the cathodic limit is negative of +0.08 V, an additional stripping wave appears in the 0.12-0.16 V potential range, prior to the upd stripping wave at about 0.23 V. In this particular set of voltammograms, we extend the anodic limit in order to show the set of peaks at +0.49 V that is believed to be due to the replacement of [PdCl4]2- by Cl- on the Au(111) surface.14,19 The stress curves in Figure 3b show that the surface stress moves in the tensile (positive) direction from a value arbitrarily chosen as zero at 0.38 V, the starting potential. As the cathodic vertex is made more negative, two features are readily apparent. First of all, the stress transients tend to superimpose, indicating that although the kinetics is poor, the process is reversible and highly reproducible. The second important feature is the significant increase in slope that occurs near the transition between upd and opd growth. Although the deposition rate (deposit thickness) needs to be factored in as well, this latter feature suggests that the stress of the bulk Pd deposit is more tensile than that of the single monolayer deposited in the upd region. The hysteresis in the stress transients reflects the sluggishness of the deposition/stripping kinetics. However, other than some small drifting that occurs due to the relatively slow
sweep rate used in these experiments, the cantilever returns to its original position, indicating that insignificant alloying occurs between the Pd and Au for the Pd thicknesses, potentials, and times examined. Others have drawn similar conclusions.14,16 The overall shape of the stress-potential curves is quite different from that which has been reported for other pseudomorphic upd systems with positive misfit, namely, Cu25-29 and Al30 on Au(111). These curves show a stress maximum at the transition between anion desorption, which produces tensile stress, and the beginning of metal deposition, which results in compressive stress. We have attributed the compressive stress to the formation of metal adsorbate-Au bonds which partially satisfy the bonding requirements of the Au surface atoms, thereby reducing the tensile surface stress inherent to the clean Au surface. This is consistent with bond order models that appear in the literature.40 In the present case of Pd-Au(111), the upd reaction occurs at potentials well positive of the pzc for Au(111) in sulfate, so that anion desorption, in this case [PdCl4]2-, does not come into play. As a consequence, the tensile increase seen prior to upd must be due to electrocapillarity, i.e., adding negative charge to the electrode surface. It is well documented that the surface stress/charge ratio is negative at potentials near the pzc.24,41-44 At the onset of Pd deposition, at about 0.21 V, the stress continues to move in the tensile direction, unlike the compressive stress observed with the onset of Cu and Al upd on Au(111). An excellent way to distinguish the upd and opd stress response is through a series of potential pulses into the Pd deposition region. Figure 4 shows the stress response for a series of 30 s pulses from a starting potential of +0.38 V. When the cantilever electrode is pulsed to a potential in the upd region (positive of +0.09 V), the stress moves in the tensile direction and stabilizes to a value that is potential dependent. Upon reversing the potential back to a value of +0.38 V, the cantilever returns to its original position, although this may take several seconds. The surface stress change due to Pd deposition in the upd region is only a function of potential, not time, since deposition is limited to potential-dependent coverage. In contrast, when the potential is pulsed into the opd region, the surface stress initially jumps in the positive direction and continues to increase with a slope that is potential dependent. This behavior clearly reflects the kinetically controlled steady state growth that occurs in the opd region. Figure 5 shows a plot of the stress change, taken as the average of the cathodic and anodic stress response, as a function
Underpotential Deposition of Metal Monolayers
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Figure 5. Plot of average surface stress change vs cathodic charge density for potential steps shown in Figure 4.
of the total cathodic charge, determined by integrating the accompanying current transients of the potential steps of Figure 4. The plot shows data for two independent sets of experiments on the same cantilever. Two regions are clearly apparent in the plot. The first is the upd region, which is limited to a charge of about 450 µC cm-2 and a surface stress value of about +0.4 N m-1. This value is similar to that observed in Figure 3 when the voltammetry is limited to the upd region. When deposition exceeds a monolayer, the surface stress increases linearly with the amount of charge passed. On the basis of the largest charge passed in Figure 5, nearly 3 monolayers of Pd were deposited, resulting in a total surface stress change of 1.3 N m-1 in the tensile direction. As mentioned previously, Pd has a +4.9% misfit with respect to the Au substrate. As a consequence, one would expect a tensile stress change for a pseudomorphic (1 × 1) adlayer considering misfit alone. This can be quantified by the expression23
τ)
Y(111) ε d θ 1 - ν(111) mf (111)
(4)
where Y(111) is Young’s modulus for the (111) surface of Pd, ν(111) is the Poisson ratio, εmf is the misfit strain, d(111) is the height of the Pd monolayer (0.224 nm), and θ is the number of monolayers on the surface. It should be noted that τ is equivalent to ∆Σcohtf in eq 1. The elastic constants, calculated from the elastic compliances for Pd,45 are Y(111) ) 136 GPa and ν(111) ) 0.526. Inserting these values into eq 4 for a monolayer of Pd results in a misfit stress of +3.2 N m-1. It is clear from Figure 5 that the upd of Pd results in a tensile stress that is about an order of magnitude less than that predicted from lattice misfit. If we examine the slope of the stress-charge curve in the opd region of Figure 5, we obtain a value of -0.14 V (units of N m C-1). The second and third monolayers each have a surface stress of 0.62 N m-1, based on a monolayer charge of 446 µC cm-2. This is equivalent to a biaxial stress of 2.7 GPa. The stress in the second and third monolayers is about 50% higher than the stress of the upd layer and may reflect the fact that any compressive contribution to the surface stress due to the formation of Pd-Au bonds vanishes for additional growth on the Pd monolayer. There are several reasons why a simple elasticity model would fail to predict the stress change associated with the growth of a pseudomorphic monolayer. The most obvious is that a
geometric argument ignores the fact that chemical bonds are formed between the substrate and the adlayer. As mentioned previously, when adsorption energy is considered together with lattice misfit, there is qualitative agreement between EAM calculations and experimental data. For a monolayer of Pd on Au(111), Leiva’s EAM calculations38 indicate that the negative adsorption contribution to the surface stress essentially balances the positive misfit term, resulting in a total stress change of -0.10 N m-1. Our experimental value of +0.4 N m-1 is clearly closer to the EAM value than that based on misfit alone. An additional complication, not considered in these calculations, is the layer of adsorbed anions on the metal monolayer. Electronegative adsorbates will remove charge from the surface and induce an additional compressive stress.23,24 Adsorbates can also modify the lattice spacing of the metal monolayer, thereby alleviating some of the coherency strain imposed by the substrate. Moire´ patterns in STM images of the ordered 3 × 7 adlayer of sulfate on Cu(111) single crystal have been interpreted to be caused by a 4% expansion in the top layer of the copper.28,46 As a consequence it has been argued that the coherency strain in a Cu upd layer on Au(111) is reduced by the sulfate adsorbed on the Cu surface. In the case of Pd upd on Au(111), the adsorbed layer of [PdCl4]2- on the Pd adlayer would be expected to add a compressive component to the surface stress; however, there is no evidence that the Pd lattice spacing deviates from that dictated by the Au substrate, at least for the first four monolayers.14 The standard potential of the Pd2+ + 2e- ) Pd reaction is 0.951 V vs NHE,47 which places the equilibrium potential in 0.1 M H2SO4 containing 1 mM of Pd2+ close to 0.21 V vs SSE. The potential for Pd deposition is then about 0.4 V positive of the pzc for Au(111). The significance of this, in terms of a surface stress measurement, is that anion desorption and subsequent readsorption onto the upd layer will not occur and will therefore not contribute to the stress transient. One is then left to question whether or not this will alter the basic shape of the stress-potential curve during upd. As mentioned previously, the shape for Pd upd, shown in Figure 3b is clearly different from the shape reported for Cu and Al on Au(111)25-30 where a stress maximum is observed. In chloride-containing electrolyte, [PdCl4]2- is adsorbed on the Au(111) surface at potentials positive of Pd deposition and remains on the surface at more negative potentials where Pd is deposited. In order to examine any possible role of [PdCl4]2- to the surface stress response, a series of experiments was performed where PdSO4 was added as the electroactive species. Although changing the anion does not negate the fact that Pd deposition occurs well positive of the pzc for Au(111), it does occur negative of the phase transition of the sulfate adlayer to the ordered (3 × 7)R19.1° structure which may alter the energetics for Pd deposition.15 Although it has been demonstrated that pseudomorphic Pd layers can still be deposited from sulfate-based solutions, the reaction is highly irreversible unless chloride is present, and the electrochemical removal of the Pd layer is difficult since Pd forms a rather stable oxide in the absence of chloride.15 Figure 6 shows the voltammetric and stress response for the Au cantilever electrode in 0.1 mol L-1 H2SO4 with and without the addition of 1 mmol L-1 PdSO4. When Pd2+ is not present, the voltammetric current is essentially zero in the potential range of interest. The stress response is typical for (111)-textured Au in H2SO4 where the positive change in stress is due to electrocapillarity and sulfate desorption, which is known to begin at about 0.2 V.27 In contrast, when PdSO4 is present, Pd upd begins at about 0.32 V and the upd/opd transition is difficult to
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Figure 6. Potentiodynamic scan and change in surface stress for (111)textured Au in 0.1 mol L-1 H2SO4 (b) and 0.1 mol L-1 H2SO4 + 1.0 mmol L-1 PdSO4 (s). Sweep rate ) 5 mV s-1. Inset shows the first three voltammetric scans in the presence of Pd2+.
Figure 7. Change in surface stress for the first five consecutive voltammetric scans for (111)-textured Au in 0.1 mol L-1 H2SO4 + 1.0 mmol L-1 PdSO4. Sweep rate ) 5 mV s-1.
detect. On the basis of charge, about 4 monolayers are deposited by the time the potential reaches 0.0 V at a sweep rate of 5 mV s-1. On the return sweep, the dissolution current is very small, reflecting the irreversible nature of the process in the absence of chloride ion in solution. The stress response looks quite similar to that when [PdCl4]2- is present. Initially the stress response follows the Pd2+-free curve; however, when Pd deposition begins at 0.32 V, the stress deviates in the tensile direction. At a potential of about 0.1 V, the slope of the stress curve increases further, likely marking the transition between upd and opd growth. Interestingly, the stress returns to its predeposition value by the time the potential returns to 0.6 V, suggesting that the Pd is removed and that sulfate is readsorbed onto the Au surface. However the inset in Figure 6 is a plot of the first three voltammetric waves, recorded sequentially, with the return sweeps removed for clarity. Only the first transient shows upd behavior, whereas the subsequent transients reflect overpotential deposition alone. This is a fairly clear indication that the upd layer is not removed during the anodic portion of the sweep. The answer to this seemingly reversible stress behavior may be found by examining the stress transients for the first five voltammetric waves, shown in Figure 7. The curve marked 1 is the initial scan, also shown in Figure 6. As mentioned above, the stress returns to its initial value at the end of the anodic
Stafford and Bertocci
Figure 8. Change in mass for three consecutive voltammetric scans for (111)-textured Au EQNB electrode in 0.1 mol L-1 H2SO4 + 1.0 mmol L-1 PdSO4. Sweep rate ) 2 mV s-1.
sweep. The stress response of the second cathodic sweep is clearly different from the first. Initially the stress is rather flat, reflecting both the more negative potentials required for overpotential deposition and the fact that sulfate remains adsorbed on the Pd surface in this potential region due to the electrode’s more negative pzc. The very slight decrease in stress at the beginning of the second cathodic sweep is likely due to Pd dissolution. On the return sweep, the irreversibility of the deposition/stripping process is clearly evident in the second and subsequent cycles where a remnant tensile stress is observed after each cycle. Only the first scan shows no net change in stress after a complete voltammetric cycle. Several changes to the electrode surface are unique to the first cycle, namely, a shift in the pzc of about -0.33 V as the surface changes from Au to Pd15 and, as a consequence, readsorption of sulfate onto the Pd surface, most of which takes place on the return sweep. Sulfate forms an ordered (3 × 7)R19.1° adlayer on both Au(111) and Pd(111), although only small domains have been found on Pd adlayers by STM.15 First principles calculations show that the surface stress for clean, unreconstructed Pd(111)48 is 3.68 N m-1 whereas that for Au(111)49 is 2.77 N m-1. One could then conclude that the adsorbate-induced compressive surface stress due to sulfate would be greater on Pd than on Au which would give rise to a more negative stress at the same electrode potential. In addition, the negative shift in pzc would increase the electrocapillary contribution of the surface stress for Pd over that of Au, again moving the stress in the negative direction. For example, an electrocapillary stress change of -0.1 N m-1 has been reported for Cu upd on Au.28 These changes to the electrode surface during the first cycle add a negative surface stress which could balance the tensile stress in the Pd layers that remain on the surface, thus returning the stress to its original point despite the Pd remaining on the surface. The subsequent cycles show the gradual increase of tensile stress with Pd deposition. In order to confirm that Pd remains on the surface during a complete voltammetric cycle, we performed a similar set of voltammetric experiments using the EQNB. These results are shown in Figure 8. In each sweep about 120 ng cm-2 were deposited, less than a full monolayer. On the return sweep, about 50-60 ng cm-2 were redissolved. It is conceivable that although Pd deposition from a solution not containing chlorides is on the whole irreversible, some Pd atoms, perhaps in low coordina-
Underpotential Deposition of Metal Monolayers
Figure 9. (a) Stress × thickness product and (b) average and instantaneous biaxial stress for Pd deposition onto (111)-textured Au in 0.1 mol L-1 H2SO4 and 1.0 mmol L-1 H2PdCl4. Nominal deposit thicknesses of 4.7 ML (0.05 V), 4.2 ML (0.06 V), and 3.4 ML (0.07 V) are based on a monolayer charge density of 446 µC cm-2.
tion sites, can be removed. Tang et al. have shown similar results in sulfate electrolyte,15 where upon cycling the electrode up to 0.5 V vs SSE at 5 mV/s, about a third of the Pd monolayer deposit is dissolved per cycle. Not surprisingly, our EQNB results indicate that the mass to charge ratio is less than the theoretical value in most of the voltage range, because the mass increase due to Pd deposition is counterbalanced by the sulfate desorption from the Au that is not covered by Pd. As discussed previously, sulfate adsorption, at least in part, may occur again on the Pd deposit because of the negative shift of the pzc from Au to Pd. This allows us to conclude that although the kinetics of Pd deposition is significantly different whether H2PdCl4 or PdSO4 is the electroactive species, the biaxial tensile stress that develops in the Pd adlayer(s) is essentially the same. Finally we examine the stresses associated with the deposition of thicker Pd films onto (111)-textured Au from H2PdCl4 electrolyte. Figure 9a shows the stress-thickness product (Fw in eq 2) while Figure 9b shows both the average and instantaneous biaxial film stress. The instantaneous stress is simply the numerical derivative of the curve in Figure 9a with thickness plotted in nanometers rather than monolayers. The curves represent data for three separate films having nominal thicknesses between 3.4 and 4.7 monolayers (ML) that were electrodeposited at three different potentials in the opd region (0.05, 0.06, and 0.07 V). The first four monolayers are reported
J. Phys. Chem. C, Vol. 113, No. 1, 2009 267 to be pseudomorphic and in registry with the Au(111) surface.14 The reproducibility is excellent and the general shape of the stress-thickness product curve in Figure 9a is identical to that in Figure 5 that was obtained from the potential pulse experiments. The upd layer has a surface stress of about +0.35 N m-1, which then increases with continued growth. If we linearize the portion of the curve immediately following the upd layer, we obtain a slope of 0.48 N m-1 ML-1 which corresponds to a coherency stress of about 2.0 GPa. This is slightly smaller than the 2.7 GPa stress obtained for the second and third monolayers from the potential step data shown in Figure 5. Both experimental values are considerably less than the theoretical value of 14.2 GPa based on the +4.9% lattice misfit (eq 4). Linearizing the stress-thickness data also yields a y intercept of -0.2 N m-1. Referring back to eq 1, if we assume that ∆fpzc is -0.1 N m-1 (similar to the experimental value reported for Cu on Au(111)28) and use the first principles surface stress values of 3.7 N m-1 for Pd(111)48 and 2.8 N m-1 for Au(111)49 to obtain a ∆fint of +0.9 N m-1, then one can calculate an interface stress, ∆h, of -1.0 J m-2 from the stress-thickness data in Figure 9a. This negative value for ∆h indicates that the Pd-Au interface favors expansion. It is also clear from the Figure 9a data that the stress-thickness curve deviates from linearity as the Pd film thickens, suggesting that the Pd begins to lose coherency with the Au substrate. We now examine the stress curves in Figure 9b. The instantaneous stress, being the stress state of the newly deposited material, determines in which direction the average stress moves. If the instantaneous stress is larger than the average stress, as seen in the 1-2 ML region, then the average stress will increase. Likewise if the instantaneous stress is less than the average stress, as seen after 2.5 ML, then the average stress will decrease. Excluding the high stresses calculated in the submonolayer region, both stresses reach a maximum value in the early stages of opd growth. For the average stress, this occurs when the Pd film is 2-3 ML thick, largely as the result of material being deposited with relatively high tensile stress during the latter stages of the second monolayer. Although at present we do not yet understand the electronic and elastic strain contributions that help to make up the overall stress signature, they are also likely responsible for the unique catalytic activity that has been reported for the first few Pd monolayers. Conclusions We have shown that the stress associated with Pd upd on (111)-textured Au, as well as the growth of subsequent adlayers, is tensile. The primary source of the tensile stress is the +4.9% lattice mismatch between Pd and Au. However the tensile stress calculated from lattice mismatch is not realized. This is partially due to additional factors such as the formation of Pd-Au bonds and anion adsorption onto the Pd surface. Pd upd causes a surface stress change of +0.4 N m-1 whereas additional growth, reported to be pseudomorphic with the first, produces an additional tensile contribution that causes a maximum in the average film stress when the thickness reaches 2-3 monolayers. Interestingly this is the thickness regime that shows unique catalytic activity. We attribute this larger tensile stress to the diminished electronic influence of the Au substrate and the subsequent decrease in tensile stress to the loss of coherency as the Pd film thickens. Acknowledgment. The authors gratefully acknowledge the technical contributions of Carlos Beauchamp and discussions with Daniel Josell. Certain trade names are mentioned for
268 J. Phys. Chem. C, Vol. 113, No. 1, 2009 experimental information only, in no case does it imply a recommendation or endorsement by NIST. References and Notes (1) Sayed, S.; Juttner, K. Electrochm. Acta 1983, 28, 1635. (2) Chen, C.-H.; Gewirth, A. J. Am. Chem. Soc. 1992, 114, 5439. (3) Kolb, D. M. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1978; p 125. (4) Adzic, R. R. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1984; p 159, (5) Baldauf, M.; Kolb, D. M. Electrochim. Acta 1993, 38, 2145. (6) Hernandez, F.; Baltruschat, H. Langmuir 2006, 22, 4877. (7) Baldauf, M.; Kolb, D. M. J. Phys. Chem. 1996, 100, 11375. (8) Naohara, H.; Ye, S.; Uosaki, K. J. Electroanal. Chem. 2001, 500, 435. (9) Kibler, L. A.; El-Aziz, A. M.; Kolb, D. M. J. Mol. Catal. A 2003, 199, 57. (10) El-Aziz, A. M.; Kibler, L. A. J. Electroanal. Chem. 2002, 534, 107. (11) Naohara, H.; Ye, S.; Uosaki, K. Electrochim. Acta 2000, 45, 3305. (12) Naohara, H.; Ye, S.; Uosaki, K. J. Phys. Chem. B 1998, 102, 4366. (13) Naohara, H.; Ye, S.; Uosaki, K. Colloids Surf., A 1999, 154, 201. (14) Kibler, L. A.; Kleinert, M.; Randler, R.; Kolb, D. M. Surf. Sci. 1999, 443, 19. (15) Tang, J.; Petri, M.; Kibler, L. A.; Kolb, D. M. Electrochim. Acta 2005, 51, 125. (16) Takahasi, M.; Hayashi, Y.; Mizuki, J.; Tamura, K.; Kondo, T.; Naohara, H.; Uosaki, K. Surf. Sci. 2000, 461, 213–218. (17) Awatani, T.; Yagi, I.; Noguchi, H.; Uosaki, K. J. Electroanal. Chem. 2002, 524-525, 184. (18) El-Aziz, A. M.; Kibler, L. A.; Kolb, D. M. Electrochem. Commun. 2002, 4, 535. (19) Quayum, M. E.; Ye, S.; Uosaki, K. J. Electroanal. Chem. 2002, 520, 126. (20) Rojas, M. I.; Popolo, M. G. D.; Leiva, E. P. M. Langmuir 2000, 16, 9539. (21) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K. J. Mol. Catal. A: Chem. 1997, 115, 421. (22) Roudgar, A.; Gross, A. J. Electroanal. Chem. 2003, 548, 121. (23) Ibach, H. Surf. Sci. Rep. 1997, 29, 193. (24) Haiss, W. Rep. Prog. Phys. 2001, 64, 591.
Stafford and Bertocci (25) Haiss, W.; Nichols, R. J.; Sass, J. K. Surf. Sci. 1997, 388, 141– 149. (26) Haiss, W.; Sass, J. K. J. Electroanal. Chem. 1995, 386, 267. (27) Kongstein, O. E.; Bertocci, U.; Stafford, G. R. J. Electrochem. Soc. 2005, 152, C116. (28) Trimble, T.; Tang, L.; Vasiljevic, N.; Dimitrov, N.; van Schilfgaarde, M.; Friesen, C.; Thompson, C. V.; Seel, S. C.; Floro, J. A.; Sieradzki, K. Phys. ReV. Let. 2005, 95, 166106. (29) Seo, M.; Yamazaki, M. J. Solid State Electrochem. 2007, 11, 1365. (30) Stafford, G. R.; Beauchamp, C. R. J. Electrochem. Soc. 2008, 155, D408. (31) Brunt, T. A.; Chabala, E. D.; Rayment, T.; O’shea, S. J.; Welland, M. E. J. Chem. Soc., Faraday Trans. 1996, 92, 3807. (32) Friesen, C.; Dimitrov, N.; Cammarata, R. C.; Sieradzki, K. Langmuir 2001, 17, 807. (33) Brunt, T. A.; Rayment, T.; O’Shea, S. J.; Welland, M. E. Langmuir 1996, 12, 5942–5946. (34) Seo, M.; Yamazaki, M. J. Electrochem. Soc. 2004, 151, E276– E281. (35) Stafford, G. R.; Bertocci, U. J. Phys. Chem. C 2007, 111, 17580. (36) Stafford, G. R.; Bertocci, U. J. Phys. Chem. B 2006, 110, 15493. (37) Grossmann, A.; Erley, W.; Hannon, J. B.; Ibach, H. Phys. ReV. Lett. 1996, 77, 127. (38) Leiva, E. P. M.; Del Popolo, M. G.; Schmickler, W. Chem. Phys. Lett. 2000, 320, 393. (39) Stoney, G. G. Proc. R. Soc. London, Ser. A 1909, 82, 172. (40) Feibelman, P. J. Phys. ReV. B 1997, 56, 2175. (41) Haiss, W.; Nichols, R. J.; Sass, J. K.; Charle, K. P. J. Electroanal. Chem. 1998, 452, 199. (42) Weissmu¨ller, J.; Viswanath, R. N.; Kramer, D.; Zimmer, P.; Wu¨rschum, R.; Gleiter, H. Science 2003, 300, 312. (43) Heaton, T.; Friesen, C. J. Phys. Chem. C 2007, 111, 14433. (44) Umeno, Y.; Elsa¨sser, C.; Meyer, B.; Gumbsch, P.; Nothacker, M.; Weissmu¨ller, J.; Evers, F. EPL 2007, 78, 13001. (45) Simmons, G.; Wang, H. Single Crystal Elastic Constants and Calculated Aggregate Properties: A Handbook; The M.I.T. Press: Cambridge, MA; 1971. (46) Wilms, M.; Broekmann, P.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1998, 416, 121. (47) CRC Handbook of Chemistry and Physics, 75th ed.; Weast, R. C., Lide, D. R., Eds.; CRC Press, Inc.: Boca Raton, FL, 1995. (48) Ackland, G. J.; Finnis, M. W. Philos. Mag. A 1986, 54, 301. (49) Fiorentini, V.; Methfessel, M.; Scheffler, M. Phys. ReV. Lett. 1993, 71, 1051–4.
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