Underpotential Co-deposition of Au–Cu Alloys: Switching the

Article pubs.acs.org/Langmuir

Underpotential Co-deposition of Au−Cu Alloys: Switching the Underpotentially Deposited Element by Selective Complexation Defu Liang and Giovanni Zangari* Department of Materials Science and Engineering and Center for Electrochemical Science and Engineering (CESE), University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, United States S Supporting Information *

ABSTRACT: Underpotential deposition and monolayer replacement processes are widely used for the synthesis of core/shell catalysts and heterointerfaces. Conventionally, only the more noble metal can be underpotentially deposited on or replace the less noble metal, limiting the number of accessible material configurations. We show here that the reverse process is possible, using the Au−Cu pair as a model system. By tuning the redox potentials of the two components via use of strong, selective metal ion complexes, Au−Cu alloys could be synthesized at will by (i) conventional underpotential co-deposition, whereby Cu is reduced at underpotential in parallel with the overpotential deposition of Au, or (ii) the reverse process, where Au is reduced at underpotential, while Cu is deposited at overpotential. Selective complexation also draws the redox potential of Au and Cu closer, resulting in co-deposition under activation control for the noble metal and precise alloy composition control by the applied potential, enabling in principle the formation of arbitrary metal or alloy interfaces. The alloys resulting from the two processes exhibit distinct enthalpy of mixing, suggesting different degrees of short-range order and dissimilar atomic configurations. These findings open new perspectives on underpotential deposition phenomena and possibly new synthetic opportunities in electrodeposition.

1. INTRODUCTION Electrodeposition provides exquisite control over metallic film growth.1 The driving force for film formation, i.e., the potential applied to the substrate electrode, can in fact be controlled very precisely and within short times, with both features being unequaled by physical deposition methods. In addition, the energy of the metal ion precursors in solution is of the same order of magnitude as the typical solid-state effective atomic pair interactions, rendering the deposition process uniquely sensitive to these energy differences. This latter feature is exploited in underpotential deposition (UPD),2,3 whereby a monatomic layer of B is deposited on a substrate A at a potential EB/A more positive than the redox potential of the pure element B, EB/B. This process and its modifications, such as surface-limited redox replacement (SLRR),4 are being extensively used in nanomaterial synthesis, to form for example core/shell catalyst nanoparticles or conformal, atomically sharp heterojunctions.5,6 Informed in part by the work of Trasatti,7 who analyzed in detail the relationship between work function and potential of zero charge in metals, the first systematic study of UPD was undertaken by Kolb, who showed that, for a wide set of A−B pairs, the UPD shift ΔEUPD = EB/A − EB/B is proportional to the difference in work function of the metals involved.8 ΔE UPD = α(ΦB − ΦA )

with α = 0.5 V/eV

realized; this however challenges the notion that UPD is ultimately made possible by attractive interactions between B and A, which are symmetric. Since then, numerous UPD systems have been studied at the atomistic scale3 and firstprinciple calculations9,10 have been used to predict the energetics and atomic arrangements of UPD layers at surfaces. Only in a few cases, however, eq 1 has been critically assessed on thermodynamic grounds,11 confirming the essential validity of the functional form advanced by Kolb. Underpotential co-deposition (UPCD),12 i.e., the electrodeposition of an alloy A−B (EA0 > EB0) starting at a potential EB(alloy) more positive than the redox potential of the less noble component B, EB(B), consists of an ongoing UPD of B onto an alloy surface that is continuously refreshed by the overpotential deposition of A. This process is usually described by alloy thermodynamics, whereby Ei(alloy) (i = A or B) is given by a generalized Nernst equation13 z+

Ei(alloy) = Ei ,0 +

(2)

shifted positive with respect to the redox potential of elemental i, Ei(i), because of the activity of component i in the alloy ai(alloy) being EB0, it is possible by selectively varying the ion activity in solution aiz+ to invert the ranking of nobility of the two components, thus allowing in principle UPCD of A during overpotential deposition of B. Essentially, reverse UPCD could be achieved by inverting the relative nobility of the metal ions and cathodically biasing the deposition process, so that both metal ions are being reduced. In the present work, we select Au as metal A and Cu as metal B and demonstrate tuning the relative nobility of Au and Cu by selective ion complexation, to achieve at will conventional or reverse UPCD of Au and Cu. More specifically, we show the formation of Au−Cu alloys from strongly complexing electrolytes using either the conventional process involving UPCD of Cu during Au reduction at overpotential or a heretofore never reported process of Au UPCD induced by Cu overpotential deposition. Selective complexation is also found to result in alloy formation under activation control for the apparent more noble metal, leading to uniform alloy composition and dense, high-quality films.

2. RESULTS 2.1. Cu UPCD with Au. In a previous work,14 we demonstrated UPCD of Cu with Au using a slightly acidic solution containing Au as a weak chloride complex; in this work, we adopt instead strong complexes for both metal ions, to controllably modify the redox potentials over a wide range. An ethylenediamine (EN)/glycine-based electrolyte was used to achieve Cu UPCD with Au. The complex Au(EN)2 is stable in neutral and alkaline conditions,15 necessitating Cu complexation by glycine16 to avoid Cu2+ precipitation. Chemical equilibria calculations (see the Experimental Section and Results of the Supporting Information) were carried out to select an electrolyte comprised of 2 mM Au(EN)2, 20 mM CuSO4, 0.2 M glycine, and 0.2 M Na2SO4 at pH 7, yielding a Cu redox potential of −0.59 V versus a saturated mercurous sulfate electrode (MSE). Cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) data were collected simultaneously at a Au electrode in the potential range from 0 to −1.1 V. Figure 1a shows the rate of mass increase dm/dt versus applied potential Eapp for the background solution with the addition of (i) 2 mM Au(EN)2, (ii) 20 mM CuSO4, and (iii) both metal ions, respectively. Au deposition (red, dashed line) starts at −0.18 V; a reduction peak is observed at −0.59 V; and diffusion-limited deposition starts at −0.75 V, as indicated by the plateau in the deposition rate. The rate of deposition under these conditions is dm/dt = 84 ng/cm2/s, corresponding to 0.16 Au monolayers/s or a current density of 0.12 mA/cm2, with this value being in agreement with calculations of the diffusion-limiting current using a 0.5 mm diffusion layer thickness and [Au3+] = 2 mM. The onset of Cu reduction from electrolyte ii (blue, dash-dotted line) occurs at −0.73 V, a potential where Au deposition has barely reached diffusionlimiting conditions, suggesting that co-deposition of Cu may occur under activation control for Au, in contrast with previous results with weakly complexing solutions.14 The gradual increase and two plateaus observed in the dm/dt data from the complete electrolyte iii (green, solid line) hinder a precise determination of the onset of Cu co-deposition, which however is clearly depolarized with respect to the deposition of elemental Cu (electrolyte ii), supporting the occurrence of UPCD. A more precise estimate of ECu(alloy) is obtained from

Figure 1. (a) Rate of mass change versus applied potential for the EN/ glycine solution: Au only (red, dashed line), Cu only (blue, dashdotted line), and Au + Cu (green, solid line), with a scan rate of 5 mV/ s at a Au quartz crystal. (b) Composition (dots) as a function of the deposition potential for alloys deposited on Ru from the EN/glycine solution, 2 mM Au + 20 mM Cu. Error bars are based on two sets of samples, with each sample measured at three different locations. The line represents the best fit with a sub-regular solution model. (c) XRD patterns for Au−Cu alloys of various compositions, deposited from the EN/glycine electrolyte, on a Ru substrate.

compositional data, collected by growing and characterizing by energy-dispersive spectroscopy a series of Au−Cu alloy films (200−300 nm thick) at various potentials and displayed versus applied potential in Figure 1b). Assuming that co-deposition is driven by the alloy enthalpy of mixing ΔHmix, a sub-regular solution model, where ΔHmix = (WG1xA + WG2xB)xAxB17 yields the following relationship between activity of B in the alloy and composition: 2567

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Langmuir aB(alloy) = x B exp

Article

[WG1 + 2(WG2 − WG1)x B](1 − x B)2 RT (3)

Equation 3 was plugged into eq 2 to fit the composition xB versus potential data; a least-squares fit yields values for the redox potential of Cu, ECu(Cu) = −0.723 V, very close to the value of −0.73 V determined by EQCM (Figure 1a), and for the two Margules parameters WG1 and WG2 that will be used later to plot the alloy enthalpy of mixing of the alloy films. A discrepancy between the experimental data points and the fitting curve (Figure 1b) is observed at the most negative potentials, where Cu is co-deposited at or close to overpotential (OPD) conditions. Compositional data demonstrate that Cu co-deposition occurs already at potentials more positive than −0.53 V, where 14 atomic % Cu is incorporated; this implies that the UPD shift of Cu deposition would be larger than 200 mV, comparable to that observed in weakly complexing solutions.14 This confirms that UPCD can be observed even in complexing solutions, despite the reduced reversibility of the Cu UPD process. A more precise assessment of the onset of codeposition by compositional data is hindered by the slow deposition rate of Au-rich films. X-ray diffraction (XRD) patterns for Au−Cu alloys in Figure 1c show (111) and (200) reflections from a face-centered cubic (FCC) solid solution, indicated by arrows, shifting to progressively higher angles (smaller lattice constant) with an increasing Cu fraction, evidencing the formation of a continuous series of solid solutions. In Cu-rich films, however, additional reflections from a pure Cu phase are observed, suggesting the precipitation of a second phase, which occurs only when Cu is co-deposited close to or at OPD conditions. On the basis of the width of the XRD peaks, a typical grain size of the pure Cu phase is ∼50 nm, larger than that of the alloy (∼20 nm). 2.2. Au UPCD with Cu. A sulfite-based electrolyte18 was used to reverse the nobility ranking of Cu and Au and thus demonstrate Au UPCD with Cu. The solution contained 20 mM HAuCl4, 2 mM CuSO4, and 0.168 M Na2SO3, and pH was brought to 8 using H2SO4; the large excess of sulfite insures full complexation of both metal ions. CV and EQCM data were collected simultaneously at the Au electrode in the potential range from −0.4 to −1.35 V. Figure 2a shows dm/dt data versus Eapp for the background solution with the addition of (i) 2 mM CuSO4, (ii) 20 mM HAuCl4, and (iii) both Au and Cu, respectively. The data clearly show that Cu (blue, dash-dotted line) now behaves as the more noble metal; the onset of Cu deposition occurs at −0.75 V, and a constant deposition rate is observed starting at about −1.0 V. Au reduction (red, dashed line) starts only at about −1.15 V, behaving as the less noble element; the deposition rate increases monotonously in the whole range investigated. The alloy electrolyte iii (green, solid line) exhibits an onset of Cu deposition at about −0.68 V, ∼70 mV more positive than that of the pure Cu solution, suggesting a higher free [Cu+] in solution after the addition of the Au salt; this is expected due to competing complexation equilibria occurring upon Au addition. The deposition rate increases above the diffusion-limiting value for Cu at about −0.9 V, indicative of the onset of alloy deposition at an UPD shift of about 250 mV, comparable to that observed with the previous solution. The composition of Au−Cu films (200−300 nm thick) is displayed versus applied potential in Figure 2b; 10 atomic % Au is already co-deposited at about −0.95 V, while 25 atomic % Au alloys are grown at −1.1 V, around the onset of

Figure 2. (a) Rate of mass change versus potential for the sulfite solution: Au only (red, dashed line), Cu only (blue, dash-dotted line), and Au + Cu (green, solid line), with a scan rate of 5 mV/s at a Au quartz crystal. (b) Composition (dots) as a function of the deposition potential for alloys deposited on Ru from the sulfite solution, 2 mM Cu + 20 mM Au. Error bars are based on two sets of samples, with each sample measured at three different locations. The line represents the best fit with a sub-regular solution model. (c) XRD patterns for Au−Cu alloys of various compositions, deposited from the sulfite electrolyte, on a Ru substrate.

pure Au deposition. Again, a more precise determination of the UPCD onset is hindered by the slow deposition rate of Cu-rich films. The data above confirm the occurrence of reverse UPCD, whereby Au co-deposition is depolarized in the presence of Cu on the electrode surface. Sulfur incorporation is observed in this set of films (Figure 2b); the S fraction is below 5% down to −1.2 V and saturates at 9 atomic %. The incorporation of sulfur from sulfite solutions is often observed, e.g., in Au18 and Au− Ni19 deposition, and is attributed to the disproportionation of SO32− to SO42− and elemental sulfur, which is then physically adsorbed on the surface and trapped in the growing film. We assume therefore that sulfur incorporation has a negligible effect on the thermodynamics of the alloy system. Fitting the composition versus potential data using the sub-regular solution 2568

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model17 gives a redox potential of −1.25 V for Au deposition, slightly more negative than the value of −1.15 V determined by EQCM. As before, a fit to the experimental data fails to predict compositions measured close to or at overpotential conditions. XRD patterns for Au−Cu alloys deposited from sulfite solutions are displayed in Figure 2c; the alloy reflections indicated by arrows shift progressively to lower angles, suggesting the formation of increasingly Au-rich FCC solid solutions at more negative potentials. Films grown close to or at Au OPD show evidence of a second phase, corresponding to pure Au. The typical grain size of the alloy phase estimated by the XRD peak widths is about 15 nm. Fitting of alloy composition versus potential yields two Margules parameters, WG1 and WG2, which provide an estimate of the integral enthalpy of mixing of the two sets of electrodeposited alloys.17 These functions are plotted in Figure 3 together with high-temperature data available for bulk alloys

observed UPCD effect is originated by solid-state atomic interactions and is mostly independent of the degree of complexation of the metallic species in solution. An equivalent formulation of this concept is presented in Figure 3, where it is shown that the estimated enthalpy of mixing ΔHmix function for alloys obtained from the various electrolytes are quite close but not identical. The estimated ΔHmix functions depend upon not only the phase structure, which indeed appears to be the same, but also the precise atomic configuration of such a phase, determined by assigning the identity of each atom at every lattice site;21 this dictates the strength of the interatomic interactions and ultimately the bonding energy. In direct UPCD, the more noble element A is deposited at overpotential and nucleates the film, determining its phase structure; B is deposited at underpotential and forms an adsorbed layer on the existing surface, occupying available nucleation sites without forming a new phase. No overvoltage in fact is available to form a distinct phase. In reverse UPCD, the opposite occurs, with B atoms forming nuclei and element A adsorbing on and being incorporated by the growing film. The atomic configuration may therefore be quite different, leading to distinct enthalpy functions.20 This hypothesis is supported by the fact that the two sets of alloys obtained by direct UPCD (ref 14 and this paper) present similar mixing enthalpies, while the alloys obtained by reverse UPCD exhibit a distinct mixing enthalpy; it is less clear why the latter alloy would exhibit a ΔHmix similar to that of high-temperature bulk alloys. To obtain a wide range of alloy compositions using one of these two techniques, alloys rich in the (apparent) less noble element should be grown at OPD for this element, which as observed may lead to a deviation from the thermodynamic predictions and also to phase separation. This result can be rationalized by the fact that OPD conditions for the less noble metal thermodynamically enable its own independent nucleation;22 in addition, with a partial current of the less noble metal being significantly larger than that of the more noble metal (Figures 1a and 2a), the statistical likelihood to form nuclei of the pure less noble element is high. The possibility to use either of these growth techniques will therefore enable the formation of both Au- and Cu-rich films at low overpotentials, allowing for close compositional control because the incorporation of the less noble element approaches the thermodynamic limit, and avoiding precipitation of extraneous phases. Additionally, the ability to tune independently the redox potentials of the two components allows for co-deposition to occur under activation conditions for the more noble metal, resulting in dense, shiny films with an overall better quality. Finally, we comment on an obvious extension of the present work: the possibility to exploit selective complexation to demonstrate Au UPD on Cu. Reverse UPCD occurs via Au UPD on Cu, while Cu is being reduced at OPD; the applied potential bias helps to avoid any galvanic displacement process that may occur. In a rigorous UPD experiment, the initial concentration of Cu ions in solution is negligible, the driving force for Cu dissolution is very large, and avoiding Cu dissolution in the presence of Au ions, even if complexed, becomes difficult. Preliminary CV data from a 0.02 M HAuCl4 and 0.168 M Na2SO3 at pH 8 solution at a Cu(111) polycrystalline surface are consistent with a depolarization of Au deposition, but the observed anodic behavior cannot be interpreted by Au dissolution alone and may also involve Cu dissolution. We believe that, to rigorously demonstrate the

Figure 3. Estimated enthalpy of mixing for various alloy films grown by underpotential co-deposition in comparison to bulk alloys: Cu UPCD with Au from weakly complexing solution,14 Cu UPCD with Au from EN/glycine solution, and Au UPCD with Cu from sulfite solution and bulk alloys together with known intermetallic compounds.20

and ordered intermetallic phases20 as well as with similar data obtained for electrodeposited alloys grown from non-complexing solutions.14 The trace corresponding to the EN/glycine solution exhibits a maximum exothermic alloying enthalpy of ∼ −8 kJ/mol, similar to alloys grown from weakly complexing solutions. The position of this maximum is on the Au-rich side, precisely at 36 atomic % Cu. In contrast, the curve corresponding to the sulfite solution shows a maximum of about −6 kJ/mol, on the Cu-rich side, ∼57 atomic % Cu, comparable to those of bulk alloys. The enthalpy of mixing curves does not suggest any tendency toward the formation of ordered phases.

3. DISCUSSION The Au−Cu system lends itself to fundamental electrochemical deposition studies because it is one of the few binary alloys that combine a relatively large exothermic enthalpy of mixing with elemental components having a sufficiently high redox potential that their reduction may occur without concurrent hydrogen evolution, even in a strongly complexing electrolyte. Using selective complexation, we have demonstrated the formation of Au−Cu alloy films by both conventional UPCD and a reverse UPCD process. The observed UPD shift for the apparent less noble metal is similar in the two electrolytes discussed here as well as in weakly complexing solutions,14 suggesting that the 2569

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Platinum monolayer fuel cell electrocatalysts. Top. Catal. 2007, 46, 249−262. (7) Trasatti, S. Work function, electronegativity, and electrochemical behaviour of metals. J. Electroanal. Chem. Interfacial Electrochem. 1971, 33, 351. (8) Kolb, D. M.; Przasnyski, M.; Gerischer, H. Underpotential deposition of metals and work function differences. J. Electroanal. Chem. Interfacial Electrochem. 1974, 54, 25−38. (9) Leiva, E. Recent developments in the theory of metal UPD. Electrochim. Acta 1996, 41, 2185−2206. (10) Pašti, I.; Mentus, S. First principles study of adsorption of metals on Pt(111) surface. J. Alloys Compd. 2010, 497, 38−45. (11) Sudha, V.; Sangaranarayanan, M. V. Underpotential deposition of metalsProgress and prospects in modelling. J. Chem. Sci. 2005, 117, 207−218. (12) Mallett, J. J.; Bertocci, U.; Bonevich, J. E.; Moffat, T. P. Compositional control in electrodeposited Pt100 − xCux alloys. J. Electrochem. Soc. 2009, 156, D531−D542. (13) Moffat, T. P. Electrodeposition of Ni1 − xAlx in a chloroaluminate melt. J. Electrochem. Soc. 1994, 141, 3059−3070. (14) Mallett, J. J.; Shao, W.; Liang, D.; Zangari, G. Underpotential codeposition of Cu−Au alloys. Electrochem. Solid-State Lett. 2009, 12, D57−D60. (15) Zhu, S.; Gorski, W.; Powell, D. R.; Walmsley, J. A. Synthesis, structures, and electrochemistry of gold(III) ethylenediamine complexes and interactions with guanosine 5′-monophosphate. Inorg. Chem. 2006, 45, 2688−2694. (16) Aksu, S.; Doyle, F. M. Electrochemistry of copper in aqueous glycine solutions. J. Electrochem. Soc. 2001, 148, B51−B57. (17) Hardy, H. K. A “sub-regular” solution model and its application to some binary alloy systems. Acta Met. 1953, 1, 202−209. (18) Okinaka, Y. Some recent topics in gold plating for electronics applications. Gold Bull. 1998, 31, 3−13. (19) Rouya, E.; Stafford, G. R.; Bertocci, U.; Mallett, J. J.; Schad, R.; Begley, M. R.; Kelly, R. G.; Reed, M. L.; Zangari, G. Electrodeposition of metastable Au−Ni alloys. J. Electrochem. Soc. 2010, 157, D396− D405. (20) Orr, R. L. Heats of formation of solid Au−Cu alloys. Acta Met. 1960, 8, 489−493. (21) Ruban, A.; Abrikosov, I. A. Configurational thermodynamics of alloys from first principles: Effective cluster interactions. Rep. Prog. Phys. 2008, 71, 046501. (22) Budevski, E.; Staikov, G.; Lorenz, W. J. Electrochemical Phase Formation and Growth; VCH: Weinhiem, Germany, 1996.

occurrence of this process, detailed scanning tunneling microscopy (STM) characterization may be needed.

4. CONCLUSION By exploiting strong and selective metal complexation in solution, we have tuned the relative nobility of Au and Cu in solution to successfully demonstrate the electrochemical formation of Au−Cu alloy films by both conventional and reverse underpotential co-deposition processes. UPD shifts of about 200 mV are observed in both cases, suggesting a codeposition process induced by solid-state atomic interactions. Estimated mixing enthalpies depend upon the growth procedure, which could be explained by the formation of distinct atomic configurations and short-range order in otherwise apparently identical FCC phases. This has been ascribed to the asymmetric process of alloy formation, whereby the more noble metal nucleates on the surface, forming a templating phase, while the less noble metal is being adsorbed at underpotential and eventually incorporated in the growing film. The redox potentials of the two metals draw closer because of complexation, leading to the formation of highquality, dense films with high compositional uniformity. Finally, the possibility to use both direct and reverse UPCD improves compositional control and enables the synthesis of single-phase films in a wider range of composition than allowed by any of the two processes alone.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, a discussion of electrochemical equilibria in the EN/glycine-based electrolyte, and additional CV data for the two electrolytes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-434-243-5474. Fax: +1-434-982-5799. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of the National Science Foundation through the Award NSF CMMI 1131571 is gratefully acknowledged. REFERENCES

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