Processing of Nanoporous Ag Layers by Potential-Controlled

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Langmuir 2008, 24, 8332-8337

Processing of Nanoporous Ag Layers by Potential-Controlled Displacement (PCD) of Cu L. T. Viyannalage, Y. Liu, and N. Dimitrov* Department of Chemistry, State UniVersity of New York at Binghamton, P.O. Box 6000 Binghamton, New York 13902-6000 ReceiVed February 21, 2008. ReVised Manuscript ReceiVed May 5, 2008 A cementation-like process taking place under potential control and introduced in this work as a “potentialcontrolled displacement” (PCD) is developed as a new method for processing of nanoporous Ag structures with controlled roughness (porosity) length scales. Most of the development work is done in a deoxygenated electrolyte containing 1 × 10-3 M AgClO4 + 5 × 10-2 M CuSO4 + 1 × 10-1 M HClO4 using a copper rotating disk electrode at 50 rpm. At this electrolyte concentration, the Ag deposition is under diffusion limitations whereas the Cu dissolution displays a typical Butler-Volmer anodic behavior. Thus, a careful choice of the operational current density enables strict control of the ratio between the dissolving and depositing metals as ascertained independently by atomic absorption spectrometry (AAS). The roughness length scale of the resulting surfaces is controlled by a careful selection of the current density applied. The highest surface area and finest morphology is obtained when the atomic ratio of Ag deposition and Cu dissolution becomes 1:1. Preseeding of uniform Ag clusters on the Cu surface made by pulse plating of Ag along with complementary plating and stripping of Pb monolayer is found to yield finer length scale resulting in up to a 67% higher surface area. An electrochemical technique using as a reference value the charge of an underpotentially deposited Pb layer on a flat Ag surface is used for measuring the real surface area. Scanning electron microscopy (SEM) studies are conducted to examine and characterize the deposit morphology of Ag grown by PCD on Cu substrates.

Introduction The selective dissolution of homogeneous alloys, also called dealloying, has attracted considerable attention because of its suitability for processing of porous metallic structures with a variety of possible applications.1 Specific examples here could be filtration membranes, surface coatings with controlled roughness, and controlled pore-size architectures that could accommodate catalytically active metal nanoparticles.2 The process of selective dissolution of silver gold alloys has extensively been studied in terms of critical potential, anion concentration, pH, and mechanical strength of resulting nanoporous metals.3–5 The optimum Au content and dealloying threshold potential for Ag-Au alloy which gives the high degree of mechanical integrity for dealloyed nanoporous metals has been experimentally identified.2,6 A detailed description of porosity formation and surface diffusion of alloy components during dealloying has been also successfully simulated.7,8 However, dealloying as a tool for synthesis of nanoporous structures has limitations imposed by the nature and composition of the starting alloy. Namely, only dealloying of homogeneous alloys that are rich in the less-noble constituent are so far found to render the resulting structure nanoporous. Also, not all singlephase alloys would necessarily yield the porosity developed by * To whom correspondence should be addressed. E-mail: dimitrov@ binghamton.edu. (1) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (2) Senior, N. A.; Newman, R. C. Nanotechnology 2006, 17, 2311. (3) Sieradzki, K.; Movrin, D.; McCall, C.; Dimitrov, N.; Erlebacher, J. J. Electrochem. Soc. 2001, 149(8), 370. (4) Wagner, K.; Brankovic, S. R.; Dimitrov, N.; Sieradzki, K. J. Electrochem. Soc. 1997, 144(10), 3545. (5) Rong, L.; Sieragzki, K. Phys. ReV. Lett. 1992, 68(8), 1168. (6) Lu, X.; Balk, T. J.; Spolenak, R.; Arzt, E. A. Thin Solid Films 2007, 515, 7122. (7) Erlebacher, J.; Sieradzki, K. Scr. Mater. 2003, 49, 991. (8) Erlebacher, J. J. Electrochem. Soc. 2004, 151(10), C614.

AgxAu(1-x) dealloying.9 Such limitations narrow significantly the number of nanoporous metals that could be processed by dealloying. In general, suitable for this purpose over certain compositional range would be single-phase alloys, featuring a considerable difference in the redox potentials of their constituents. These limitations, for instance, substantially narrow the choice of alloys suitable for development of nanoporous silver by dealloying. Another disadvantage of dealloying as a tool for synthesis of nanoporous materials is associated with cost effectiveness. It is clear that an alloy to be dealloyed needs first to be prepared. However, all methods for alloy preparation are rather expensive. That is why today’s interest in nanoporous metal architectures with various functionality1,2 warrants additional efforts for development of alternative pathways for synthesizing the structures and morphologies of interest. In this paper we report on an exploratory development work associated with the applicability of a cementation-like process for generation of metal nanoporous structures with tunable pore size and morphology length scale. The cementation reaction has largely been used for years for recovering metals such as Cu from leach baths and Ag from Cu electrorefining solutions as well as heavy metal remediation.10–13 Despite the variety of its application not much is known about the detailed mechanism and factors governing the cementation phenomenon. Recent developments in this field suggest an increasing interest toward the factors controlling the deposit morphology resulting from the cementation process.14,15 Cementation of silver on copper surface in the presence of sulfuric (9) Dimitrov, N.; Mann, J. A.; Vukmirovic, M.; Sieradzki, K. J. Electrochem. Soc. 2000, 147, 3283. (10) Stefanowicz, T.; Osifiska, M.; Napieralska-Zagozda, S. Hydrometallurgy 1997, 47, 69. (11) Puvvada, G.; Tran, T. Hydrometallurgy 1995, 37, 193. (12) Brent, H. J.; Jaeheon, L. Hydrometallurgy 2003, 69(1-3), 45. (13) Gamboaa, G. V.; Noyolaa, M. M.; Valdiviesob, A. L. Hydrometallurgy 2005, 76, 193. (14) Sulka, G. D.; Jaskula, M. Hydrometallurgy 2004, 72, 93.

10.1021/la800569t CCC: $40.75  2008 American Chemical Society Published on Web 07/10/2008

Processing of Nanoporous Ag Layers

acid in terms of kinetics and other factors has been extensively studied. Silver cementation on copper follows a first-order rate constant, while the concentration of copper sulfate in the solution strongly influenced the morphology of silver deposit. Formation of Ag dendrites has been observed in a wide range of Ag ion concentrations during the cementation process.16 However, when the copper concentration is higher dendrites are found to disappear. The disappearance of dendrites on the surface is explained as a competitive process occurring in the bulk of the solution in which Cu(I) ions reduce additional silver ions.11,17 Generally, the kinetics and mechanism of various metal ions cementation from electrolyte has been extensively investigated for the past decade, but morphological and structural evolution as a result of the cementation process have not been among the priorities in this field.12,18–21 Most recently cementation-based protocols have been employed for surface modification,22 deposition of ultrathin epitaxial metal and compound semiconductor layers,23,24 and growth of controlled nanostructures such as Au-Ag nanoparticles.25,26 In this work, we introduce a new approach for processing of nanoporous metal structures taking advantage of the cementation layout. In particular, our interest is associated with a cementationlike scenario where an external potential control serves to partially decouple the reduction reaction from the oxidation one, thus providing better and more independent control of the entire process. Presently, we name this approach potential-controlled displacement (PCD). As a galvanic displacement the cementation is a corrosion process in which redox exchange takes place between a less noble metal substrate (S) that is being attacked by ions of a more noble metal (Mz+) present in solution. Viewing the cementation as a self-sustaining process with a long-term duration in which the dissolving metal has to find its way out it is clear that the newly grown phase must feature a certain level of porosity. This property could serve as a foundation for the development of materials with tunable roughness and porosity length scale. In the exploratory research that we present herein, PCD is investigated in a system involving silver deposition on a dissolving copper substrate in sulfate solution. Our experiments are carried out under potential control enforced by galvanostatic polarization. The interest to PCD in this work is warranted by the suitability of this process for controlled formation of nanoporous structures prepared so far only by dealloying of singlephase alloys. The main outcome is associated with an understanding of factors governing the evolution of structural and morphological features of architectures generated by PCD.

Experimental Section A Cu polycrystalline cylinder (99.999%, Alfa AESAR) 5 mm in diameter and 2 mm thick was used as a working electrode in rotating disk electrode (RDE) configuration in experiments carried out in an oxygen-evacuated three-electrode cell. The crystal was first mechanically polished with a water-based alumina slurry (suspension of Buehler Micropolish II deagglomerated alumina polishing powder) down to 0.05 µm. Following the mechanical polishing, the crystal (15) Sulka, G. D.; Jaskula, M. HelV. Chim. Acta 2006, 89, 427. (16) Sulka, G. D.; Jaskula, M. Electrochim. Acta 2006, 51, 6111. (17) Sulka, G. D.; Jaskula, M. Hydrometallurgy 2003, 70, 185. (18) Nguyen, H. H.; Tran, T.; Wong, P. L. M. Hydrometallurgy 1997, 46, 55. (19) Ornelas, J.; Marquez, M.; Genesca, J. Hydrometallurgy 1998, 47, 217. (20) Sulka, G. D.; Jaskula, M. Hydrometallurgy 2002, 64, 13. (21) Karavasteva, M. Hydrometallurgy 2005, 76, 149. (22) Brankovic, S. R.; Wang, J. X.; Adziæ, R. R. Surf. Sci. 2001, 474, L173. (23) Vasilic, R.; Viyannalage, L. T.; Dimitrov, N. J. Electrochem. Soc. 2006, 153, C648. (24) Stickney, J. L. Electroanal. Chem. 1999, 75, 25. (25) Sun, Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 3892. (26) Yin, Y. D.; Erdonmez, C.; Aloni, S.; Alivisatos, A. P. J. Am. Chem. Soc. 2006, 128, 12671.

Langmuir, Vol. 24, No. 15, 2008 8333 was rinsed with 2 L of Barnstead Nanopure water (>18.3 MΩ). It was then electrochemically polished in a 5:3:2 ratio mixture of concentrated H3PO4:ethylene glycol:water at anodic dc current density (0.1-0.15 A cm-2) for 10-15 s. After electrochemical polishing, the crystal was thoroughly rinsed with 0.2 L of ethyl alcohol and 2 L of Barnstead Nanopure (>18.3 MΩ) water. Finally, the Cu surface was terminated by a droplet of, pH 1, perchloric acid solution in order to deter possible oxidation. Then the electrode was mounted on a rotating disk holder (Pine Instruments) following steps described in detail elsewhere.27 All PCD experiments were carried out in a solution containing 1 × 10-3 M AgClO4 (99.999% metal basis, GFS Chemicals) + 5 × 10-2 M CuSO4 (99.995% metal basis, Sigma-Aldrich) + 1 × 10-1 M HClO4 (double distilled, GFS Chemicals). The rotating rate was controlled by a Pine Instrument MSRX speed controller. All PCD experiments were conducted at a fixed rotation speed of 50 rpm. The electrode potential was controlled by a BAS Epsilon potentiostat operated remotely by a PC. BAS-EC software version 1.61 served for both control of the experiments and data acquisition. The surface area measurements were carried out by stripping voltammetry and/or chronoamperometry in a solution containing 3 × 10-3 M Pb(ClO4)2 (99.995+% metal basis, Sigma-Aldrich) + 1 × 10-1 M NaClO4 (99.99% Sigma-Aldrich) + 1 × 10-2 M HClO4, typically utilizing stripping of underpotentially deposited Pb monolayer.28,29 Silver seeding experiments were conducted by potentialpulse experiments in a solution containing 1 × 10-1 M Pb(ClO4)2 + 1 × 10-3 M AgClO4 + 1 × 10-2 M HClO4. Here, the experimental protocol was generally adopted from the so-called defect-mediated thin film growth protocol,30 where higher nucleation density was artificially created by metal seeding to facilitate layer by layer growth mode in electrodeposition. The three-electrode glass cells used in the above detailed experiments were cleaned successively in concentrated HNO3 and concentrated H2SO4 heated to 70 °C. Then, the cell was rinsed with 2 L of deionized water and finally terminated by Barnstead Nanopure water. A mercury sulfate electrode (MSE) was used as a reference electrode. Unless stated otherwise, all potentials in the text are presented versus MSE. A platinum wire served as a counter electrode. Prior to each experiment the electrolyte was deoxygenated for at least 2 h using ultra-high-purity nitrogen gas with less than 1 ppb oxygen, CO, CO2, and moisture content. The exact amount of deposited silver and dissolved copper in the PCD runs was determined by a flame atomic absorption spectrophotometer (AAS) (Perkin-Elmer, model AAnalyst 300) equipped with silver and copper. Lumina Hollow Cathode Lamps External standard calibration method was used at wavelengths of 328.1 and 222.6 nm for silver and copper, respectively. The surface and cross-section morphology of the nanoporous Ag layer processed by PCD was analyzed by field emission scanning electron microscopy (model LEO 1550) equipped with a secondary electron detector. An Agilent 4500 with STM 300S scanner, Agilent 5100 controller, and Agilent Pico Scan software were used for imaging on seeded Au(111) surfaces. Tips for the STM experiments were made by etching of a Pt80%-Ir20% wire in a 1:2 mixture of saturated CaCl2 solution and water at 25 V (ac).

Results and Discussion Formation of a two-phase interpenetrating solid-void structure grown by displacement of elemental Cu by Ag ions under potential control (maintained in a constant current mode) was investigated. In this section, after presentation of a simple example of the morphology of interest (Figure 1), a detailed coverage of the electrochemical background of the PCD process along with an independent quantitative AAS study ascertaining the electro(27) Markovic, N. M.; Gasteiger, H. A. J. Phys. Chem. 1995, 99, 3411. (28) Brisard, G. M.; Zenati, E.; Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. Langmuir 1995, 11, 2221. (29) Sackmann, J.; Bunk, A.; Potzschke, R. T.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 1998, 43(19-20), 2863. (30) Sieradzki, K.; Brankovic, S. R.; Dimitrov, N. Science 1999, 284, 138.

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Figure 1. SEM image of two-phase interpenetrating solid-void morphology of Ag grown by PCD for 30 min on dissolving Cu electrode in a solution containing 5 × 10-2 CuSO4 + 1 × 10-3 M AgClO4 +1 × 10-1 M HClO4 at a current density of 80 µA/cm2.

Figure 2. Cathodic curve for Ag in 1 × 10-3 M AgClO4 (pH 1) solution and anodic curve for Cu in 1 × 10-4 M CuSO4 (pH 1) at sweep rate of 1 mV · s-1. Ec (I ) 0) is the cementation potential (one copper atom dissolves and two silver atoms plate); E1 is a potential where the Cu to Ag ratio is 1:4; E2 is a potential where the Cu to Ag ratio is 1:1.

chemical findings is presented. Next, following a general introduction of an accurate technique for measurement of developing surface area in metal/alloy structures, ways to control the morphology length scale (pore and ligament size) along with experiment where PCD is carried out on the electrode surface with uniformly preseeded Ag clusters are shown and critically compared. Finally, arguments about the structural evolution of PCD grown layers with the depth of penetration are considered, and a SEM cross-section image illustrating a vertical profile of a porous layer generated by PCD is demonstrated. Electrochemistry of the PCD Scenario. The concomitant reaction for Ag deposition and Cu dissolution in a natural cementation process can be expressed by the following equation in which stoichiometrically the Ag to Cu atomic ratio is 2:1. 2e-

2Ag+ + Cu y\z 2Ag + Cu2+ It is obvious that the galvanic displacement process could be regarded as two separate reactions that are driven by the difference between Cu and Ag redox potentials. In the first of these processes 2Ag+ ions accept two electrons and reduce themselves to Ag atoms, and in the other one a Cu atom yields two electrons and oxidizes itself to Cu2+ ion. Figure 2 shows the results of two independent experiments that were carried out separately to study the silver deposition on Ag and copper dissolution behavior. The Ag bulk deposition on Ag was studied in deoxygenating electrolyte

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containing 1 × 10-3 M AgClO4 and 1 × 10-1 M HClO4 at a constant rotation speed of 50 rpm (solid line). A similar experiment was performed to study copper dissolution in deoxygenated 5 × 10-2 M Cu2SO4 and 1 × 10-1 M HClO4 solution (dotted line). At this concentration of Ag+ ions in solution Ag deposition is a mass-transport-limited process that is manifested by (i) the potentially independent current at 50 rpm (solid line) and (ii) the dependence of the current upon the rotation speed applied (short and long dashed lines registered at 10 and 100 rpm, respectively). At the same time the Cu dissolution process displays a typical Butler-Volmer anodic behavior that results in a current-voltage curve featuring (i) an exponential increase with anodic sweep of the potential and (ii) independence upon the rotation speed. Given the independently registered current-voltage behavior (Figure 2) our approach in a system where both processes are enabled is to control the Cu dissolution rate by changing the anodic current densities while keeping Ag deposition constant as guaranteed by the wide range of potential- independent Ag deposition current. Thus, decoupled deposition and dissolution fluxes would enable direct control of the atomic ratios of depositing and dissolving metals. Thus, different anodic current densities, measured at potentials E1, Ec, and E2, would give 1:4, 1:2, and 1:1 Cu:Ag atomic ratio, respectively, as the Ag deposition current density is constant at a given concentration of Ag+ ions and fixed rotation speed. This result indicates that a change of the anodic current density could generate a range of different Ag deposition to Cu dissolution atomic ratios. In our work these findings were confirmed experimentally by a carefully designed AAS experiment. In order to determine the amount of deposited Ag and dissolved Cu during the PCD, two different AAS assays were carried out. In the first one (experiment A) the exact amount of dissolved copper was determined by subjecting to AAS assay for Cu the entire solution (30 mL) X M Cu2+ + 1 × 10-3 M AgClO4 + 1 × 10-1 M HClO4 that was used for PCD of Cu by Ag at constant current density 80 (1:1) and 240 (1:2) µA/cm2 for 60 min. In the second experiment (experiment B) copper electroplated on Au disks as a film with approximate thickness of 5 µm was first used as the working electrode in the abovedescribed PCD experiment. Then, the exact amount of the dissolved Ag was obtained by AAS assay of a solution with composition X M Ag+ + Y M Cu2+ + 0.1 M HClO4 that was prepared by quantitative electrodissolution in 40 mL of 0.1 M HClO4 of the entire copper layer carrying the porous Ag generated by PCD. The results of the AAS experiment are presented in Table 1 as the experimentally measured amount (mol) for both experiments, respectively. For comparison, in Table 1 the expected amounts (mol) calculated coulometrically under the assumption of a 1:1 and 1:2 deposition to dissolution atomic ratio (based on the background results in Figure 2) are also presented. It is clearly seen from Table 1 that in agreement with the experimental layout the amount of Cu increases about two times in experiment A and no difference in the Ag content is registered in experiment B. Also, only statistically negligible differences are observed between the calculated and experimentally measured amounts of Cu and Ag participating in the PCD process. Finally, the mass balance derived from the AAS experiment is in perfect agreement with previous electrochemical quartz crystal microbalance work by G. Ertl et al.,31 suggesting that Cu dissolution in halide-free electrolyte goes directly to the Cu2+ state. This agreement indicates that Cu+ generation and disproportionation is not an issue in the present case. (31) Doblhofer, K.; Wasle, S.; Soares, D. M.; Weil, K. G.; Ertl, G. J. Electrochem. Soc. 2003, 150, C657.

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Table 1. Concentration of Ag and Cu Obtained by AAS during Deposition and Dissolution

a

metal electrode area 0.2 cm2

current density applied (µA · cm-2)

Cu exp. A Ag exp. B

80 240 80 240

final concentrationa (ppm)

expected amount (mol)

experimentally measured amount (mol)

1.33 ( 0.02 2.70 ( 0.04 1.58 ( 0.04 1.57 ( 0.04

5.97 × 10-7 1.19 × 10-6 5.97 × 10-7 5.97 × 10-7

(6.28 ( 0.09) × 10-7 (1.28 ( 0.18) × 10-6 (5.86 ( 0.15) × 10-7 (5.82 ( 0.15) × 10-7

The initial concentration is 0 in all runs.

Figure 3. Anodic stripping curves (sweep rate of 1 mV · s-1) of a Pb monolayer deposited on Ag nanoporous metal surfaces processed by different current densities. (Inset) Stripping of a Pb monolayer from the Ag (111) surface at a sweep rate of 1 mV · s-1.

Electrochemical Method for Determining Surface Area. According to results described in the previous paragraphs it is clear that the PCD protocol employed in the Ag/Cu system yields a porous metal structure. In the dealloying practice different methods have been used for determining surface area. We developed for this system a method that is based on Pb underpotential deposition (UPD). In this method we take advantage of the ability of Pb to form an UPD monolayer on both Ag and Cu.28,29,32–36 Given that, a comparison of Pb UPD charge registered prior to and after the PCD process would give us the factor associated with area development in the course of the displacement. While the foundation and details of the development of this method will be presented elsewhere, we show in Figure 3 the stripping of Pb UPD layer from nanoporous Ag films made by PCD with two different anodic current densities resulting in a different deposition and dissolution atomic ratio for Ag and Cu. As a reference curve we also present Pb stripping behavior registered in the UPD range on plain Ag electrode (dashed curve). The latter serves as a background experiment yielding charge density that could be used as a reference for subsequent area measurements. It should be noted that similar approaches for monitoring the real surface area were applied to other systems.37–39 Thus, in Figure 3 the dotted curve represents the surface area of the nanoporous layer processed by PCD in the Ag/Cu system at open-circuit potential for 30 min at 50 rpm (32) Vasilic, R.; Vasiljevic, N.; Dimitrov, N. Electroanal. Chem. 2005, 580(2), 203. (33) Vasiljevic, N.; Dimitrov, N.; Sieradzki, K. J. Electroanal. Chem. 2006, 595(1), 60. (34) Brisard, G. M.; Zenati, E.; Gasteiger, H. A.; Markovic, N. M. Langmuir. 1997, 13(8), 2390. (35) Obretenov, W.; Dimitrov, N.; Popov, A. J. Cryst. Growth 1996, 167(1/2), 253. (36) Stevenson, K. J.; Hatchett, D. W.; White, S. Langmuir 1996, 12(2), 494.

(simple cementation scenario). The charge measured here is equivalent to what one would expect from four Pb monolayers of on Ag surface. This result suggests a 4-fold increase of the total surface area. The solid line illustrates the stripping curve for a Pb UPD monolayer from the surface which was processed by maintaining a 1:1 deposition to dissolution atomic ratio. The comparison between the solid and dotted curves indicates that a 62.5% larger area has developed by increasing the copper dissolution rate. This in turn suggests that by varying the current density applied one could exercise a certain level of control over the developing surface area. The latter argument is considered quantitatively and in more detail in the next paragraph, where area measurements are correlated with high-resolution scanning electron microscopy (SEM) imaging. It should be noted that the Pb stripping curve in Figure 3 is almost identical in shape to the stripping curve registered on single-crystalline Ag surface with orientation (111), Figure 3 (inset). Given the remarkable sensitivity of Pb UPD to the crystallographic orientation of Ag29 resulting in the unique shape of the current-voltage curve for each crystal face this result suggests that the deposition and regrouping of Ag atoms during the PCD process takes place in a way that preferentially favors growth of (111) facets. SEM Studies of the Morphology Developed by PCD. The morphology of Ag nanoporous metals processed by PCD of Cu by Ag with different anodic current densities resulting in a different dissolution to deposition ratio was investigated by SEM. Figure 4 demonstrates top-view SEM micrographs of a porous silver phase grown on a concurrently dissolving copper substrate by PCD at different current density that warrants a different Cu-Ag atomic ratio. As mentioned before, the morphology presented here is strikingly similar shapewise to the morphology that has been reported by several research groups for selective dissolution of Ag from Ag-Au alloy in different electrolytes.1,2 Dealloyed samples exhibit interconnected nanoporosity with finer length scales depending to some extent on the initial alloy composition.6,7 Most often, pore/ligament sizes of 5-20 nm have been developed by dealloying of Ag-Au alloy. In the case of PCD regarded in this work, coarser structures with ligament size in the range of 50-150 nm were synthesized. Correlating the surface morphology of these structures with the dissolution to deposition atomic ratio in Figure 4 implies the presence of an optimal PCD regime for synthesizing interconnected pore/ligament structures with a length scale smaller than 100 nm (Figure 4B). Here the area evolution during both Ag deposition only (ratio 1:0) and copper dissolution only (ratio 0:1) is also presented for comparison as limiting values on the bar graph. Figure 4 also demonstrates that when the atomic ratio for deposition and dissolution metals Ag:Cu becomes 2:1, the interconnected ligaments length scale is relatively coarse (Figure 4A). This inevitably results in registration of less total surface area. The reason for the coarser length scale in that case (37) Sanchez, P. L.; Elliott, J. M. Analyst. 2005, 130, 715. (38) Thorp, J. C.; Sieradzki, K.; Tang, L.; Crozier, P. A.; Misra, A.; Nastasi, M.; David Mitlin, D.; Picraux, S. T. Appl. Phys. Lett. 2006, 88, 033110. (39) Elliott, J. M.; Birkin, P. R.; Bartlett, P. N.; Attard, G. S. Langmuir 1999, 15(22), 7411. (40) Yeh, F. H.; Tai, C. C.; Huang, J. F.; Sun, I. W. J. Phys. Chem. B 2006, 110, 5215.

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Figure 4. Surface area development during PCD for 30 min with different ratios of deposition/dissolution current densities. SEM images (A-D) showing the surface morphology at different Ag to Cu ratios.

is associated with the domination of the Ag deposition over the Cu dissolution that in turn leads to higher probability for the incoming Ag atoms to land on pre-existing Ag cluster rather than form a new nucleation site on the dissolving copper surface. It should be noted that a further increase of the dominant Ag deposition role results in large surface spots where plain silver deposit forms instead of interconnected porosity structure (not shown here). Maximum area and finest surface morphology was observed when the Ag:Cu atomic ratio become 1:1 (Figure 4B). Apparently, as seen from the bar graph (Figure 4E), this change in the dissolution to deposition ratio results in an area increase of 67% in comparison with the open-circuit potential (cementation) regime (ratio 2:1). A selective increase of Cu dissolution impact by applying more positive current density, bringing the deposition to dissolution ratio to 1:2, results in a slightly coarser length scale (Figure 4C). It is interesting to note that with increasing further the Cu dissolution rate the structure length scale does not change significantly (Figure 4D) while the surface area steadily decreases. This phenomenology could be most likely associated with the increasing disproportion between dissolving and depositing metals that either prevents uniform nucleation of Ag or destroys already formed Ag nuclei. As a result of this trend lower resolution images registered at a deposition to dissolution ratio of 1:5 (not shown here) reveal large spots of bare Cu between areas featuring interconnected porosity. It should be noted that the morphology obtained in the present work is qualitatively similar but finer to the morphology of samples processed by alloying/dealloying of Ag-Zn mixtures.31 Although few literature sources have reported the effect of the electrolyte anion and temperature on the morphology evolution of Ag layers obtained on the Cu surface by the simple cementation process,15,16 our work at this stage is limited to the proof of concept results suggesting applicability of PCD for synthesis of two-phase interpenetrated solid-void Ag/Cu layers with limited tunable porosity length scale. Parameters of that kind will be considered in a future work where a detailed experimental optimization is intended. Preseeding of Silver on Copper Surface with Complementary Pb UPD. Silver was preseeded on the copper surface prior to the PCD experiment in an attempt to extend the length scale tunability. The idea here is to uniformly prenucleate on the surface growing metal clusters of a given size that later would set the length scale in a subsequent PCD experiment. Technically,

Figure 5. STM image of the Ag surface after two short potential pulses from +0.35 to +0.05 V and back to 0.35 V vs Pb/Pb2+ pseudo-reference electrode. Image size ) 1 × 1 µm. Z range ) 3 nm.

the preseeding of Ag clusters was done using a protocol implemented for enhancing the nucleation density in electrochemical defect-mediated growth.30 According to this protocol the seeded metal (Ag in this case) is deposited by a potential pulse along with formation of a Pb UPD layer. Then the Pb UPD layer is stripped and the surface remains coated uniformly by Ag clusters of 30-50 nm lateral size. Here, a Au (111) surface with large atomically flat terraces is chosen instead of Cu for better illustration of the seeding concept. It should be noted that in this approach the cluster size could be controlled by the (i) pulse duration, (ii) number of pulses, and (iii) silver ion concentration in the solution. In this work, we seeded silver clusters on the surface of copper specimens from solution containing 1 × 10-3 M AgClO4 + 1 × 10-1 M Pb(ClO4)2 + 1 × 10-2 M HClO4 by two 3 s potential pulses described in the caption of Figure 5. In our experiments the preseed with Ag clusters Cu samples were subjected to PCD. We found, as a result of these experiments, that the Ag seed creates cathodic sites that upon initiation of the displacement reaction preferentially accommodate the depositing Ag atoms, thus facilitating propagation of the predetermined length scale. The bar graph in Figure 6 illustrates the difference of the area development of porous silver surfaces processed by

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Figure 7. SEM image showing a cross-sectioned Cu sample processed by PCD at a current density corresponding to a deposition dissolution metal ratio of 1:1 for 60 min.

Figure 6. Area development of a silver nanoporous layer generated on silver-seeded and nonseeded copper substrate. (Inset) SEM micrograph showing the morphology evolution resulting from PCD (1:1 ratio) on preseeded with Ag clusters Cu surface.

PCD of both silver preseeded and bare copper substrates. It is seen that a 65-70% increase of the surface area is registered on Ag preseeded copper substrates at a current density corresponding to a deposition to dissolution atomic ratio of 1:1. Careful analysis of the SEM micrograph presented in Figure 6 suggests that not only the ligament size of the porous Ag layer is finer but also the appearance of the morphology of the accordingly generated structure is different. It is also to be noted that the majority of clusters seen in the SEM image in Figure 6 are of a size similar to the prenucleated clusters presented in Figure 5. This result gives us reason to believe that the preseeding with Ag clusters quantitatively and even qualitatively influences the subsequent PCD process. While the quantitative aspect is generally expected owing to the above presented arguments, no plausible explanation could be given at this time for the qualitative differences manifested by the morphology in both cases. It should also be noted that owing to postdeposition coarsening of the resulting 3D porous structure the effect of seeding would be preserved only in relatively thin layers generated by PCD. Vertical Profile and Cross-Section Analysis of Layers Processed by PCD. A key question in the discussion on growth of layers by PCD is what happens deeper into the substrate after several layers featuring interconnected porosity are organized on top of the substrate. Work of others indicates that in a standard cementation scenario dendrites nucleate and grow with the process evolution.14 At the same time, it is clear that the displacement sustains and yet the porous layer stays intact on the dissolving substrate. This indicates that dissolution of less noble metal from the substrate bulk features irregularity that would result in some type of porosity. This way, the growing metal most likely only coats by a thin film the channels drilled by the dissolution deeper into the substrate. Figure 7 shows a cross-sectioned Cu sample that was subjected to PCD for 60 min in acidified sulfate electrolyte at a current density that provides a 1:1 deposition to dissolution atomic ratio. The overall charge collected under these experimental conditions corresponds to formation of a 1.2 µm thick, uniform porous layer with the length scale seen in pictures A-D in Figure 4. However, while the length scale of the topmost

layer in Figure 7 looks familiar, it is clearly seen that the depth of penetration of the dissolution front (confined for the sake of clarity between the dashed lines) ranges from 1.5 to 2.2 µm. It is also obvious that the structure deeper into the substrate is coarser than the one seen on the topmost layers. This suggests that after completion of several layers with interconnected porosity diffusion limitations associated with transport of both Ag+ ions into the pores and Cu2+ out in the solution start playing role in the PCD process. Thus, the amount of Ag atoms participating directly in the structuring of the uniform porous layer steadily decreases. As a result of this trend, the dissolution front penetrates deeper than expected owing to the dominant role of Cu dissolution. The observed phenomenology ascertains the hypothesis for stringent transport limitations for the Ag+ and Cu2+ ions at deeper penetration of the dissolution front that lead eventually to coarsening and deviation from the porous layer uniformity.

Summary This paper demonstrates for the first time generation of nanoporous Ag structures with tunable length scale by potentialcontrolled displacement of Cu by Ag ions. The proposed method uses a constant current regime to establish a range of controlled Cu dissolution to Ag deposition ratios and thus manipulate the porosity (surface roughness) length scale. In addition, preseeding of Ag clusters is found to influence qualitatively and quantitatively the subsequent PCD process to yield eventually higher surface area. The proposed strategy for synthesis of porous substrates features limitations in the vertical propagation of the processed structure attributed to hindered mass transport. The results of the present work suggest tunability of the porosity length scale in the range of 50-150 nm, resulting in a 3-10-fold increase of the geometric surface area for Ag layers grown by PCD of Cu up to a thickness of 1 µm. It is also shown that transport restrictions in the vertical direction limit the growth of a uniformly porous Ag layer to a thickness of 2 µm. Acknowledgment. The authors acknowledge the support of this work by the National Science Foundation, Division of Materials Research (DMR-0603019). Mick Thomas (Cornell Center for Nanoscale Systems) and Dave King (Endicott Interconnects) are acknowledged for their help with the SEM work. LA800569T