Comprehensive Study of the Application of a Pb Underpotential

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Comprehensive Study of the Application of a Pb Underpotential Deposition-Assisted Method for Surface Area Measurement of Metallic Nanoporous Materials Y. Liu,† S. Bliznakov,‡ and N. Dimitrov*,†,‡ Materials Science and Engineering Program, State UniVersity of New York at Binghamton, P.O. Box 6000, Binghamton, New York 13902-6000, and Department of Chemistry, State UniVersity of New York at Binghamton, P.O. Box 6000, Binghamton, New York 13902-6000 ReceiVed: February 19, 2009; ReVised Manuscript ReceiVed: May 15, 2009

An inexpensive, fast, selective, and sensitive technique for surface area measurement of metallic nanoporous materials (MNPM) is developed, systematically tested, and validated. The approach employed is based on underpotential deposition (UPD) of metals on foreign substrates. In this work, Pb UPD on Au is chosen to illustrate the applicability of and reveal the advantages and limitations of the proposed method. Experiments are designed for surface area measurement of nanoporous gold (NPG) electrodes with pore sizes in the range of 5-15 nm, prepared by electrochemical dealloying of single phase AupAg1-p (atomic fraction p ) 0.1, 0.2, and 0.3). Dealloying is performed galvanostatically at a current density of 1 mA cm-2 in a AgClO4 solution, acidified to pH 1. The experimental results suggest a linearly increasing charge in the Pb UPD layer with NPG thickness. This finding hints at (i) uniformity of the NPG structure and (ii) the general ability of this method to work for analysis of bulk materials. The proposed approach is tested by studying the dependence of the NPG surface area upon the original alloy composition and correlating the results with the NPG structure and morphology imaged by high-resolution scanning electron microscopy. An anomalously high surface area is registered in dealloyed Au0.1Ag0.9 samples and is attributed to the lack of a pre-existing percolation backbone. Unlike the instantaneous Pb UPD process on a flat metal surface, the slow and thickness-dependent kinetics of Pb layer formation on NPG is associated with hindered mass transport through pores. Further validation of the Pb UPD method is made by experimental monitoring of heat treatment-enforced coarsening and the basic modeling of the correlation between surface area and ligament size in NPG. Finally, a critical comparison with Brunauer-Emmett-Teller (BET) analysis reveals important advantages of the developed method for surface area measurement in MNPM specimens. 1. Introduction Metallic nanoporous materials (MNPM) can be described as metals having a three-dimensional (3D) interconnected solid-void structure in nanoscale. To date, besides nanoporous Au (NPG) that can be easily prepared by electrochemical dealloying (selective Ag dissolution) of single phase AupAg1-p alloys,1-4 successfully synthesized MNPM include nanoporous Pt,5-7 Cu,8,9 Ag,9-11 Pd,12,13 and Ni.14,15 The substantial surface area development in MNPM manifested by a several orders of magnitude increase versus the geometric area of the bulk material renders these structures attractive for a variety of upcoming technologies associated with catalysis,16-18 fuel cells,19,20 membranes,21 actuators,22 and electrochemical sensing.23,24 At present, the characterization of nanoporous metals relies mostly on field emission scanning electron microscope (FESEM) and/or transmission electron microscope (TEM), when applicable.1-23 When the determination of surface area is of interest, FESEM and TEM could provide only a qualitative picture based on measurements of average pore and ligament sizes and the assumption of a structural isotropy of the substrate of interest. However, when a quantitative result is needed, the microscopy techniques fail to accurately provide details about the actual surface area. Thus, an inexpensive, fast, accurate, selective, and sensitive technique * To whom correspondence should be addressed. E-mail: dimitrov@ binghamton.edu. Telephone: +1 607 777 4271. † Materials Science and Engineering Program. ‡ Department of Chemistry.

for surface area measurement of MNPM would not only enable efficient surface area measurement but would also help to better understand the kinetics of the structural evolution in MNPM and to assess the factors and limitations associated with mass transport through interconnected nanosized channels. The first choice for the surface area determination of a fine structure like MNPM would be today’s area characterization “work horse”, the Brunauer, Emmett and Teller (BET) method. BET is based on a physical absorption of gas molecules onto the substrate under characterization. The area can be obtained on the basis of a pressure measurement of the physically adsorbed gas isotherms in the chamber.25 It has been widely used to determine the surface area of hydrogen storage materials,26 nanoparticle assemblies,27 nanotubes,28 nanorods,29 and nanoporous materials.30 While BET is deemed accurate and reliable for many of the above listed systems, there are limitations and shortcomings associated with its efficiency for characterization of MNPM. For instance, a large amount of sample is needed (i.e., more than one gram of NPG) for the BET surface area measurement because the pressure measurement is not sensitive enough. In addition to that, an elevated temperature treatment ranging between 100 and 350 °C, recommended generally as an effective way for water/solvent and contaminant removal, could substantially enhance the diffusivity of surface atoms and in turn trigger a massive coarsening of the sample2,21,31,32 due to the Gibbs-Thomson effect.33 This would ultimately result in a significant decrease

10.1021/jp901536f CCC: $40.75  2009 American Chemical Society Published on Web 06/19/2009

Pb UPD-Assisted Method for Surface Area Measurement in the surface area of MNPM consisting originally of very fine ligaments and pores. A way to address the above limitations is to employ electrochemical techniques, which can analyze nanosized layers and do not require heat treatment of the specimens. The doublelayer capacitance and charge of the underpotentially deposited (UPD) monolayer of metal on foreign substrates are two electrochemical characteristics that are directly related to the roughness of the electrode and can thereby be used for surface area measurements of MNPM. The double-layer capacitance could be derived by the analysis of electrochemical impedance spectroscopy (EIS). The ratio between the doublelayer capacitance measured on porous and flat electrodes from the same metal correlates directly with the surface area development. EIS is a powerful technique that also provides additional information on the shape of the pores,34-38 but as Cattarin et al.34 reported, the recording of impedance spectra is complicated with NPG samples. In addition, the EI spectra analysis is a complex procedure and is often dependent on critical assumptions (choice of right model, proper fitting procedures, and sensitive software). Unlike EIS, the UPD measurement is independent of the shape and uniformity of the pores, and the charge corresponding to the UPD monolayer formation or stripping could be easily determined by standard electrochemical measurements such as chronoamperometry (CA) and cyclic voltammetry (CV). Thus for MNPM, the UPD measurement appears to be a more suitable and straightforward method than EIS for surface area assessment. UPD has been widely used to characterize the surface area and/or surface roughness of generally flat single and polycrystalline metal surfaces.39,40 Most recently, UPD-based protocols have also been employed in the study of MNPM substrates.7,10,24 The UPD phenomenon is strictly limited to the formation of one monolayer and rarely two complete monolayers perfectly reproducing the morphology of the underlying substrate.39,40 This way, the charge of a UPD monolayer is directly proportional to the substrate surface area. Therefore, measurement of the roughness factor, Rf, obtained as a ratio of accurately measured UPD layer charges on developed and flat metal surfaces, would guarantee an accurate determination of the surface area development. Applications of UPD as a characterization tool emphasize the assessment of surface activity of electrochemical sensor substrates41,42 and the surface area measurement of catalytically active substrates.43-46 In the second group of applications, hydrogen UPD on carbon-supported Pt has been used for surface area assessment of fuel cell catalysts.43-45 Adding to that some random attempts for surface area characterization of electrodes including MNPM,7,10,24 one finds no systematic and comprehensive study on the kinetics and limitations of the UPD process in MNPM published so far in the literature. Another missing piece of information relates to the mechanism of UPD formation on MNPM. Slow UPD formation onto MNPM substrates lasting for a couple hundred seconds24 has been reported in contrast with only a few seconds on a metal surface.39,40 It has been speculated that the UPD might not be under pure kinetic control.5,24 If limitations are contributed to by mass transport (diffusion) control, it is not clear whether these limitations are due to a narrow pore size and/or to a low UPD metal ion concentration. Understanding the UPD process on MNPM is fundamentally and technologically important. For instance, the UPD process on MNPM could serve as a model for the faradaic process operating in typical catalytic and/or sensing scenarios. Also, surface limited redox replacement (SLRR) of UPD layers that

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12363 has been increasingly used for surface modification47,48 and thin film growth49,50 on a variety of important substrates would undoubtedly be affected by limitations considered earlier in this section. Thus, the work reported herein is aimed at (i) revealing key details and limitations associated with the mechanism and kinetics of UPD formation in architectures featuring interconnected porosity metal structures and (ii) developing an inexpensive, fast, accurate, selective, and sensitive technique for surface area measurements of MNPM. In the present paper, Pb UPD on Au as a well understood system39,40 is chosen to illustrate the applicability of and reveal the advantages and limitations of the proposed electrochemical method. Additional justification for the choice of Pb is provided by its ability to form a UPD layer on a variety of other noble metals (Ag,49 Cu,50 Pt,51 and Rh52) that could be processed as nanoporous substrates. Last but not least, Pb UPD would still be confined within the limits of a monolayer, even if surface alloying were to take place as no UPD system with Pb participation has been shown to feature mixing that goes beyond a site exchange between the UPD layer and topmost layer of the substrate.39 Experiments are designed for surface area measurement of NPG electrodes prepared by electrochemical dealloying (selective Ag dissolution) of single phase AupAg1-p, where the atomic fraction p ) 0.1, 0.2, and 0.3. These feature stable pore and ligament sizes in the range of 5-15 nm and a controlled dealloying depth to 5 µm. Instead of cyclic voltammetry (CV) conventionally used in similar cases,39,40 in this work UPD measurements were carried out using chronoamperometry (CA). First, the surface area development is determined by measuring the roughness factor, Rf, as a function of the alloy composition and dealloying depth. Second, important aspects of the UPD mechanism and kinetics are discussed in view of the duration of UPD at different dealloying depths, the difference between CA and CV measurements, and the impact of Pb2+ ion concentration on the formation of a UPD layer. Finally, sample coarsening caused by heat treatment, surface area modeling, and BET measurements are conducted to ascertain and validate the applicability of the UPD-assisted method for surface area measurement in MNPM. 2. Experimental Section 2.1. AupAg1-p Alloy Synthesis. Single phase AupAg1-p alloys (p ) 0.1, 0.2, and 0.3) were synthesized by hightemperature melting in inert atmosphere and were characterized by X-ray diffraction (Philips X’Pert MRD system with Cu KR radiation) and energy dispersive X-ray spectroscopy (EDXS) (Zeiss Supra 55 VP system with EDAX detector) to confirm the mixing and alloy composition. Precalculated amounts of Au and Ag (both 99.99%, Surepure Chemetals) were placed in a graphite crucible inside a quartz reactor, deoxygenated by ultra high purity (UHP) N2 gas (less than 1 ppb oxygen, moisture content, CO, and CO2) for at least 2 h prior to the melting. After 20 min melting at 1200 °C, the samples were slowly cooled to room temperature, rolled into 50 µm thick foil, and cut into small pieces. This procedure was repeated two more times to ensure maximum homogeneity and best mixing. Finally, the alloys were rolled into a 3 mm thick sheet, and cylindrical samples with a diameter of 6 mm were punched out by a “punch and die” press (Roper Whitney of Rockford, Inc.). These samples were then annealed in inert atmosphere at 800 °C for 20 h. Finally, linear scanning voltammetry (LSV) at a scan rate of 1 mV s-1 (NPG Preparation) was conducted on each alloy sample in an acidic solution in a three-electrode configuration to confirm their composition by measuring the dealloying critical potential.

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2.2. NPG Preparation. For processing of NPG samples the above-described alloys were first mechanically polished on alumina slurry (Buehler) to 0.05 µm. The samples were then rinsed with Barnstead Nanopure water (R > 18.2 Ω cm), subjected to annealing at 800 °C in N2 atmosphere, and cooled to room temperature. The dealloying to selectively remove Ag from the alloys was carried out in a 1 × 10-3 M AgClO4 (99.995+% metal basis, Sigma-Aldrich) and 1 × 10-1 M HClO4 (double distilled, GFS Chemicals) solution at a constant current density i ) 1 mA cm-2. The selective dissolution was conducted in a three-electrode cell controlled by a PC-based potentiostat/ galvanostat (EC Epsilon, BASi). Ag wires pretreated by etching in 8 M HNO3 were used for counter and reference electrodes. The time, t, required for the penetration of the dissolution front at a certain depth, d, was calculated by eq 1 represented by the Faraday law for electrolysis, including the atomic fraction (also volume fraction in the case of AupAg(1-p)) of Ag, (1-p) to reflect the complex nature of the dissolving sample (alloy).

(1 - p)zdFAgF t) iaAg

(1)

Here, z is the number of electrons transferred for the oxidation of one Ag atom, F is the Faraday’s constant, FAg is the mass density of Ag and aAg is its atomic weight. The analysis of eq 1 suggests that after dissolving, for instance, 90% of the volume occupied by Ag atoms in a layer of Au0.1Ag0.9 with thickness d, 10% will be left as a spongy nanoporous Au. Synthesized that way, the NPG samples were rinsed with Barnstead Nanopure water immediately after the dealloying and were then immersed in water and kept for an hour to make sure all Ag ions diffused out of the pores. 2.3. UPD Measurements. UPD-assisted surface area measurements were carried out by chronoamperometry and cyclic voltammetry in a solution containing 1 × 10-2 M Pb(ClO4)2 (99.995+% metal basis, Sigma Aldrich), 1 × 10-1 M NaClO4 (99.99% Sigma Aldrich), and 1 × 10-2 M HClO4 (double distilled, GFS Chemicals). Background experiments assessing the charge contribution of side reactions and/or double-layer charging were performed in the same solution but in the absence of Pb2+ ions. The reference electrode was Pb wire that was pretreated by etching in a diluted nitric acid (1:1 ratio) solution heated to 50 °C. A Pt wire, flame annealed prior to the experiment, served as counter electrode. All UPD experiments were carried out in a solution that was deoxygenated for at least 2 h by purging with an UHP N2 gas. CA was used to register the current transients of UPD formation at a constant potential 20 mV (Pb/Pb2+) and UPD stripping at a constant potential 650 mV (Pb/Pb2+). The surface area of NPG was obtained simply by dividing either UPD formation or stripping charges by the charge density of a Pb UPD layer on a flat polycrystalline Au electrode, prepared by a procedure described elsewhere.49 CV with a scanning rate 5 mV s-1 was conducted to assess the kinetic limitations of UPD for the surface area measurement. In addition to that, regardless of the reference electrode used in this work, all potentials are reported versus a Pb/Pb2+ pseudoreference electrode (except for the dealloying where Ag/Ag+ was quoted as reference), and all solutions used were prepared with the above listed chemicals as received from the vendors and Barnstead Nanopure water (R > 18.2 Ω cm). 2.4. SEM and BET Experiments. Alternatively to UPD measurements, a high-resolution field emission scanning electron microscope (FESEM) (Zeiss Supra 55 VP) coupled with an in-

Figure 1. Current density response of AupAg1-p alloys to an anodic potential scan (sweep rate of 1 mV s-1) using LSV. Inset: Potential transient of AupAg1-p alloys with a dissolution depth of 1 µm at a current density of 1 mA cm-2 using chronopotentiometry (CP).

lens detector at an accelerating voltage of 10 kV and a working distance of 2 mm was used to characterize the NPG samples after dealloying. Also, BET measurements (Micromeritics ASAP 2020) were conducted to validate the developed UPD-assisted method and to assess the sensitivity of both approaches to heattreated samples. Specific details concerning each of these experimental procedures are described in Results and Discussion. 3. Results and Discussion 3.1. Dealloying. 3.1.1. Alloy Characterization and Electrochemical Results. Alloy specimens AupAg1-p used as testing vehicles in this work, were analyzed for homogeneity and composition. XRD work done on all three compositions (not shown) ascertained the presence of features characteristic for the Au-Ag system.53 Given the close structural similarity of Au and Ag (fcc metals with only 0.4% difference in the lattice parameters) that limits the quantitative power of XRD, we used EDXS measurements for compositional analysis. Results from six point EDXS measurements confirmed the alloy composition with an error of 0.3% for Au0.1Ag0.9, and 0.4% for Au0.2Ag0.8 and Au0.3Ag0.7. In addition, the alloy composition was verified by registering the onset of a Ag selective dissolution known as dealloying critical potential.54 It is known that critical potential governed by the delicate balance between Ag dissolution and Au surface diffusion in single-phase AupAg1-p alloys depends strictly upon alloy composition.1,54 Thus, by analyzing the critical potential data, one could derive quantitative results for the alloy composition. Figure 1 shows anodic polarization curves in perchlorate solution containing Ag ions for all three alloys. The anodic curves undoubtedly feature increasing critical potentials with the rise of the Au fraction in the alloys. A comparison of the results in Figure 1 with data reported earlier in the literature53 clearly confirms the EDXS analysis findings and thereby verifies the alloy composition. Finally, the absence of features associated with dissolution of “free” (unalloyed) Ag on the polarization curves along with the XRD and EDXS results confirms the homogeneity of the characterized alloys. 3.1.2. SEM Results. Further in this work, selective removal of Ag from the above-described AupAg1-p alloys (dealloying) is carried out to leave porous Au with ligaments and pores in nanodimension. The dealloying is performed at a constant current of 1 mA cm-2, and the potential transients registered as a result of each run are presented in the inset of Figure 1. Here, the main advantage of working at constant current is the precise control of the dealloying depth enabled by a predefined

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Figure 3. CV curves of Pb UPD (solid blue) and background (dashed red) on polycrystalline Au at a sweep rate of 20 mV s-1.

Figure 2. Top view FESEM micrographs of dealloyed (a) NPG0.1, (b) NPG0.2, and (c) NPG0.3 samples.

dissolution rate. The dissolution time calculation for alloys with different composition is described in the Experimental Section. It is shown in the inset of Figure 1, that a time of 30-50 s is needed for the potential to reach steady state. This steady state then operates for dissolution fronts penetrating to 5 µm, which is the maximum dealloying depth considered in this work. Also, the reaching of a steady-state potential for each alloy composition implies uniformity and isotropy of the substrate structure developed by dealloying. Following the dealloying, the NPG layers are first characterized using FESEM. An interconnected solid-void 3D structure typical for dealloyed AupAg1-p alloys is observed with dimensions ranging from 5 to 20 nm for different compositions of interest (Figure 2). It is also clearly seen that the ligament and pore size depend upon the original alloy composition. Thus, NPG prepared by the dealloying of the Au0.3Ag0.7 alloy (NPG0.3) features the finest pore and ligament size (Figure 2c). At the same time, while the dimensions generally increase with a decrease in Au content in the alloy, no clear trend could be elucidated solely by simple morphology comparison of alloy samples used in this work. For instance, opposing the abovementioned length scale trend, NPG0.1 appears to feature a smaller pore and ligament size than those of NPG0.2. Also, its structure is considerably less uniform than that of NPG0.2 and NPG0.3. Finally, as is shown later in this paper, the surface area of NPG0.1 is almost twice as high compared to that of NPG0.2. Possible

reasons for the discrepancy observed at low Au fractions will be proposed later in this paper. Another interesting observation resulting from the analysis of the SEM images presented in Figure 2 is associated with the similarity between ligament and pore size. This observation suggests a massive material rearrangement during the dealloying process in which pre-existing percolation ligaments of the more noble metal serve as nucleation centers for the structuring and growth of the new interconnected 3D architecture. 3.2. UPD Measurement. Pb UPD on flat polycrystalline Au is first carried out by CV to investigate the general system behavior. Then, after determining the formation and stripping potentials, Pb UPD on NPG is performed using CV and CA. Further in this section, only results of NPG0.1 are shown as an illustration of the applicability of the proposed method. Results featuring identical qualitative and quantitative trends (not shown) are also obtained for NPG0.2 and NPG0.3. 3.2.1. Pb UPD on Polycrystalline Au. Figure 3 shows the reversible Pb UPD on a flat polycrystalline Au electrode. The registered CV allows for determining the formation and stripping potential and the UPD charge density that is needed as a normalization factor for the surface area determination of NPG. The voltammetry presented in Figure 3 shows a solid curve that is similar to those registered by others on polycrystalline Au.55-57 It is noteworthy that the sharp peak registered at 0.200 V is typical for Au (111), and the broader wave appearing at 0.450 V is characteristic for the (100) and (110) faces of Au.57 The CV suggests also that at the potential of 0.020 mV the Au surface is covered with a monolayer of Pb. Bulk deposition of Pb becomes apparent at potentials more negative than 0.000 V. Also, no Pb atoms are expected on the Au surface at potentials higher than 0.650 mV. Given the fact that the potential of 0.650 mV is negative to the critical potential of Ag dealloying from AupAg1-p alloys,54 the UPD measurement would not affect the alloy intactness. The best choice for “formation” and “stripping” potentials in further experiments would be 0.020 and 0.650 V, respectively. The UPD charge density can easily be obtained by simply integrating the current density over time using either the CV formation or stripping curves and knowing the potential scan rate. Similar to other literature sources57 and after taking into account the correction (∼3.5%) for the background current (Figure 3, dotted curve), our results suggest a charge density of 300 ( 10 µC cm-2 for UPD of Pb on a Au polycrystalline flat surface. 3.2.2. Pb UPD on NPG Using CV. Potential cycling between 0.650 and 0.020 V is applied to determine the charge of Pb UPD on NPG. As shown in Figure 4a, the formation peaks are

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Figure 4. (a) Pb UPD CV curves of NPG0.1 samples featuring dealloying depths from 0.5 to 5 µm at the scanning rate of 5 mV s-1. (b) Surface area development, calculated from the corresponded charges of the formation and stripping CV peaks in Figure 4a, as a function of dealloyed depth.

less distinctive for Pb UPD on NPG0.1 (Figure 4a) as compared to those for polycrystalline Au (Figure 3). At the same time, two relatively separate stripping peaks at lower dealloying depths (less than 1 µm), merge to form a dominant peak at the higher depths (Figure 4a). Indeed, a careful analysis of the results presented in Figure 4a reveals many aspects of a typical diffusion-limited deposition scenario operating for the Pb UPD process on NPG, when a CV experiment is carried out. For instance, there is a relatively well-shaped peak registered for the UPD formation when the dealloying depth is 0.5 µm. For other depths, the cathodic current density (about -1 mA cm-2) remains relatively constant upon potential scans from 0.250 to 0.020 V, and no peak could be registered. Then, in the reversed potential scan (anodic direction), the deposited Pb monolayer begins stripping off immediately only for the sample dealloyed to the depth of 0.5 µm. Unlike the CV run for a 0.5 µm thick layer, the UPD formation process for the rest of the dealloying depths is still proceeding even when the reverse potential scan is initiated. A further manifestation of this behavior is represented by negative currents registered to higher potentials (as high as 0.220 V) at which a sharp (anodic) stripping peak would occur if a Pb UPD layer were stripped from a flat Au electrode (compare Figures 3 and 4). These observations undoubtedly suggest a slow formation process of the Pb UPD layer that becomes even slower with an increase in dealloying depth. Mass transport (diffusion) limitations are also hinted at by the results in Figure 4b, where the surface area obtained by CV formation and stripping charges is plotted as a function of the depth of dealloying (Figure 4b). Apparently, the insufficient amount of Pb becomes more and more obvious as the dealloying depth increases, thereby, leading to a leveling of the roughness factor Rf, defined as the ratio between the actual and geometric surface area. It is to be noted that no difference in the surface area measurements done either way is generally observed, suggesting that the time of formation of a complete Pb monolayer exceeds substantially the time of registration using CV at the scan rate used in our experiments. To ascertain the correlation between the leveling of the Rf and structural aspects of NPG warranting slow diffusion of Pb2+ ions, we registered SEM micrographs on cross-sectioned NPGp, where p ) 0.1, 0.2, and 0.3 atomic fraction. For the sake of clarity, only a portion of the cross section of interest is presented in Figure 5. While a careful morphology comparison reveals some length scale differences (reported also by Cattarin, et al.34 for NPG0.25) between the topmost surface of NPG (thickness less than 50 nm) and the cross-sectioned one, the results presented in Figure

Figure 5. FESEM images of mechanically cross-sectioned (a) NPG0.1, (b) NPG0.2, and (c) NPG0.3 samples after dealloying.

5 validate the overall uniformity of the dealloyed layers. Thus, considering the SEM micrographs in Figures 2 and 5, a rough estimate suggests ligament and pore sizes of 7-10, 12-15, and 5-6 nm for NPG0.1, NPG0.2, and NPG0.3, respectively. Overall, the absence of structural anisotropy in dealloyed layers considered in this work confirms mass transport limitations as a compelling reason for saturation of the roughness factor, Rf, with the depth of dealloying in the CV experiment. That is why an accurate charge-based surface area measurement in any MNPM structure would require sufficient time. Thus, a viable way to address this issue would be an infinitely slow CV or even a current measurement at constant potential (CA). The second option will be described in detail in the next section. 3.2.3. Pb UPD on NPG Using CA. The main benefit to using CA instead of CV for studying UPD on NPG is that the process can be monitored until a complete Pb monolayer is formed or stripped. In these experiments, the potential is pulsed from the open circuit potential (OCP, normally higher than 0.650 V) to



Figure 6. (a) Current density transients of Pb UPD formation on NPG0.1 samples after dealloying to different depths (to 5 µm) at a potential of 0.020 V. (b) Surface area development versus dealloying depth.

Figure 7. (a) Current density transients of Pb UPD stripping on NPG0.1 with dealloying depth (to 5 µm) at the potential of 0.650 V. (b) Surface area development versus dealloying depth.

0.020 V, and the current transient is registered until the background current level is reached. In this way, the entire process of Pb UPD formation on NPG can be precisely monitored regardless of the time needed for the completion of the monolayer at different dealloying depths (Figure 6a). By integrating the current density over time and normalizing by division to the monolayer charge of UPD Pb on a plain Au polycrystalline electrode, we obtained a linear relationship of the Rf versus dealloying depth as shown in Figure 6b. In another run, a stripping experiment presented in Figure 7 is carried out in the following way. After a full monolayer of Pb is formed (Figure 6a), the potential is pulsed from 0.020 to 0.650 V for the UPD stripping process to take place. While taking longer than the stripping from a flat surface, the Pb layer stripping (Figure 7a) is faster than its formation counterpart. The overall stripping time also depends upon the depth of dealloying. In general, trends for UPD stripping are quite similar to those observed in the UPD formation process. It should be noted that prior to the normalization procedure, the charge obtained from the results in Figures 6a and 7a is reduced by a factor of 0.120. This correction takes into account double-layer charging effects (∼0.085) manifested by the comparison of charges under blue and green curves in Figure 8 and background charges (∼0.035) seen best through the comparison of blue and red curves in Figure 3. Also, the close match between the blue and red curves in Figure 8 clearly rules out any possibility for additional charge accumulation caused by the possible surface alloying known to take place in this system as a site exchange process strictly confined within the topmost substrate layer.58,59 Continuing the analysis of Figures 6b and 7b one finds an indisputably linear relationship between the roughness factor and dealloying depth. The linear dependence shown in Figures

Figure 8. Current transients of Pb UPD stripping on a flat polycrystalline Au electrode at a potential of 0.650 V. Solid blue and red dashed curves represnet Pb UPD stripping carried out at 30 and 630 s after the layer formation, respectively. Double-layer charging in a background solution is represented by the green dotted curve.

6b and 7b is an anticipated result given the overall uniformity and isotropy of the dealloyed samples shown in Figures 2 and 5 and reported numerous times in the literature (see ref 1). That is why the linearity here is regarded as proof of the applicability of the proposed approach for surface area measurements. Also, the detailed analysis of the results presented in Figures 6-8 suggests that the CA procedure successfully addresses issues with the incomplete Pb monolayer seen earlier with the CV runs. It is now interesting to analyze the effect of the Au content on the Rf and accuracy of the chronoamperometric data obtained separately using formation and stripping of the UPD layer. The relationship of the Rf versus dealloying depth is linear for all alloy compositions considered in this analysis as shown in Figure

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Figure 9. Surface area development at different dealloying thicknesses obtained from (a) UPD formation and (b) UPD stripping using CA.

TABLE 1: Results of Linear Regression Analysis of Data Presented in Figure 9 UPD formation -1

UPD stripping

area development

p% Au

slope (µm )

correlation coefficient

slope (µm )

correlation coefficient

surface area density (m2 g-1)

10 20 30

233 157 277

0.993 0.992 0.998

173 116 224

0.999 0.999 0.999

65-70 20-25 30-35

9. In general, qualitatively identical trends of the surface area development are registered regardless of whether formation or stripping is considered (Figure 9a,b). It can also be seen that the developed surface area at a given dealloying thickness decreases in the order of NPG0.3 > NPG0.1 > NPG0.2. Generally, without considering structural aspects of the alloys prior to dealloying, the reason for the above trend can be attributed to the argument that smaller pore size is associated with larger surface area. This way, the order determined by these measurements is in agreement with the morphology length scale of the SEM micrographs in Figure 5. In the case of the deep dealloying considered here, cross-sectioned samples are preferred as more relevant than top view samples (Figure 2). Indeed, SEM images (not shown) suggest that the topmost surface morphology is confined within a layer as thin as 50 nm. While no qualitative difference in the charge trends is found (Figures 9a,b), there certainly is a quantitative difference between the area measurements using formation and stripping transients. The fitting slopes associated with the measurements in Figure 9 are summarized in Table 1. Theoretically, the charges obtained using the UPD formation and stripping should be equal as an identical amount of atoms participate in both processes. Practically, however, about 22% more surface area is registered on the basis of formation in comparison with stripping of the UPD layer. In addition to that, the straight lines obtained from the UPD stripping data feature better fitting intercepts that are closer to the theoretical value of 1. One way of realizing the charge discrepancy in the “formation-stripping” comparison in Table 1 is to attribute the additional charge accumulation to side processes taking place concomitantly with the UPD. A side process taking place in that potential range could be the oxygen reduction reaction (ORR) on a Au surface.60 Apparently, ORR would affect the charge accumulated more during the slower UPD formation process. Because of that argument, UPD stripping appears to provide more accurate information about the real surface area development. The surface area development could also be analyzed through the surface area density defined as area developed per gram of Au in the alloy. While this parameter is not in the focus of our present work, its importance is justified by the immediate association with general criteria for cost effectiveness. Because

-1

of that and for the sake of accurate data presentation, only ranges of surface area density are shown in Table 1. Presenting ranges instead of values is warranted by the fact that the roughness factor, Rf, is measured for a calculated depth of penetration, assuming dissolution of the entire Ag during dealloying (eq 1). At the same time, work of others61 and our own EDS results suggest that up to a third of that Ag could remain trapped in the NPG, depending upon the temperature and/or completion of the dealloying process. Thus, an additional depth that is unaccounted for in our estimate develops, leading to a reduction of the surface area density that is not always trivial to quantify. Taking into account the semiquantitative area density ranges provided in Table 1, we see that higher surface area does not necessarily mean a higher area density. Thus, NPG prepared from a 10% Au alloy has a lower Rf per 1 µm dealloying depth than NPG0.3, but it has a higher surface area density. The area density ranges of dealloyed 20% and 30% Au alloys are in reasonably good agreement with the results of others on NPM prepared by dealloying. The most recently reported data measured by BET, impedance, and/or UPD-based approaches range from 5 to 41 m2 g-1.7,34,62 Values that are well within the above range could also be indirectly derived by structural analysis of recently reported NPG structures with stable ligament and pore sizes on the order of 3-6 nm.61,63 Given the above comparison, the surface area density obtained for NPG0.1 (Table 1) appears to be unexpectedly high. While no data is available in the literature for dealloyed 10% Au alloys, we believe that the lack of a pre-existing percolation Au structure at this low Au fraction is critical for explanation of this anomaly. Indeed, the lowest Au fraction at which such structure exists in this alloy is believed to be 0.198.64,65 The absence of a percolation Au structure leaves no pre-existing nucleation skeleton for development of the interconnected porous structure resulting from the dealloying process in NPG0.1. This, in turn, would lead to nonuniformity of the structure and to more volume shrinkage due to the plastic deformation and/or possible collapse of the finest ligament ensembles.66 As a result, the accordingly processed material would have poorer mechanical properties compared to those of a standard bicontinuous solid-void structure (like NPG0.3) and would rather represent a more open structure with weakly connected nanoligaments (nanoparticles)


Figure 10. Surface area development at different dealloying depths, obtained from UPD stripping using CA and CV.

Figure 11. UPD formation time at different dealloying depths, obtained from UPD stripping using CA. Inset: Effect of Pb concentration on UPD formation duration. ∆C is the factor of Pb2+ concentration change with respect to 0.05 M.

and large macro cracks. Structures of that kind that have been synthesized by agglomeration of supported or unsupported (mainly) Pt nanoparticles (atomic weight 195 versus 197 for Au), featuring surface area densities nearing 100 m2 g-1.67,68 Additional analysis of the surface area development and discussion of the anomalously high surface area density obtained by dealloying 10% Au alloy will be presented in Validations of the Proposed Method for Surface Area Measurement as well as in a more detailed study aiming at further correlating the alloy composition with the surface area density in the near future. 3.3. Kinetic Aspects of Pb UPD on NPG Substrates. It has been demonstrated in the previous section that the UPD formation is a slow process. Indeed, a comparison between Pb UPD-assisted surface area measurements at different dealloying depth of NPG0.1, conducted by CA and CV and shown in Figure 10, visualizes best this conclusion. Identical results (not shown) are also observed for NPG0.2 and NPG0.3. It is clearly seen that there is generally no difference in the surface area measurement using either method at dealloying depths less than 1 µm. At depths of dealloying exceeding 1 µm, the surface area keeps increasing linearly in the CA-based protocol, and it eventually levels when a CV measurement is performed. In addition, the time of UPD formation increases linearly with dealloying depth for all three alloy compositions (Figure 11), with the process being much slower than the one carried out on a flat surface. Taking into account that the reversibility of Pb UPD on a flat Au electrode implies fast charge transfer kinetics (Figure 3), it is apparent that mass transport (diffusion) limitations are a critical impeding factor that governs the slow Pb UPD formation


Figure 12. Minimum concentration of Pb2+ required for having instantaneous UPD formation at different pore sizes.

on NPG. The diffusion limitations are a direct consequence of the insufficient Pb ion concentration inside the pores for instantaneous UPD formation. This deficiency can be best understood by looking into an estimate of the minimum Pb2+ concentration, C, required to form instantaneously a monolayer in pores with a very high aspect ratio. For this purpose, a simple model treating the pore of NPG as a cylinder with a radius R associated with the pore size allows for a derivation of the final formula for calculating C in the following way. The charge density, σ, of a Pb UPD monolayer on Au is multiplied by the area of the cylinder wall to be covered (no bases as the pore is hollow) and is then divided by the product of a mol of electrons (z is the state of oxidation of the Pb ions, and F is the Faraday constant) and the volume of the cylinder (eq 2).

C)

σ × Acyl 2σ σ × 2πRh ) ) 2 zF × Vcyl zFR zF × πR h

(2)

Thus, for R < 8 nm, a Pb concentration higher than 4 M is needed to provide a sufficient amount of atoms to instantaneously cover NPG with one layer of Pb. Apparently, when R is less than 3 nm, even a solution of Pb(ClO4)2 with the highest concentration close to its solubility at ambient temperature (∼11 mol/L)69 will be unable to provide ions for instantaneous UPD formation. To further support this diffusion limitations argument, the inset in Figure 11 shows a 5-fold faster UPD layer formation with a 5-fold increase in Pb2+ ion concentration. For a better idea about the big frame encompassing the mass transport limitations as a function of pore size, Figure 12 summarizes data obtained by eq 2. It could be seen that if R is on the order of 3-4 nm, the concentration needed for instantaneous UPD formation exceeds 8 - 12 mol/L. The above discussion depicts quantitatively the reason for slowing the Pb UPD process on NPG given the fact that solutions no higher than 0.01 M are most frequently used for studying the UPD systems.39 Apparently, higher concentration solutions could be used to quicken the UPD-based surface area measurements. It ought to be noted, however, that other effects of such an approach should not be discounted. For instance, counterion shielding effects observed at higher electrolyte concentration could also invoke additional limitations in the UPD layer formation process. In addition, if the solution has a concentration close to the maximum electrolyte solubility prior to the UPD stripping process, the peak amount of dissolved atoms would exceed the solubility limit and a sudden precipitation before the outward transport manages

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Figure 14. Pb UPD stripping transients of 3 µm thick NPG0.2 samples, annealed at different temperatures for 10 min. Inset: Surface area change for the annealed samples.

Figure 13. FESEM micrographs of NPG0.2, dealloyed to a depth of 3 µm and annealed for 10 min at (a) 200 °C, (b) 300 °C, and (c) 400 °C.

to lower the local concentration could affect the measurement quality by introducing noise in the signal.70 3.4. Validations of the Proposed Method for Surface Area Measurement. 3.4.1. Sample Coarsening by Thermal Annealing. The first and most straightforward way to assess the applicability of UPD measurements for surface area determination in MNPM is to apply the proposed approach to a system with well-known and documented behavior. It has been observed by SEM that the structure of NPG becomes coarser by thermal annealing.2,21,31,32 Figure 13 shows the FESEM images of dealloyed NPG0.2 samples that are subjected to annealing for 10 min in N2 atmosphere. In agreement with previous reports in the literature,31,32 the ligaments and pores become larger as the annealing temperature increases from ambient (Figure 2b) to 400 °C (Figure 13 a-c). To register quantitatively the anticipated reduction in the surface area development, we carried out an UPD measurement on the samples shown morphologically in Figure 13 (top view). The respective UPD stripping curves are presented in Figure 14. A lower stripping current density and shorter stripping time are observed with the temperature increase. The annealing temperature impact on the surface area of NPG is even better presented in the inset of Figure 14. The reason for the (nearly) linear relationship of the surface area reduction with respect to the annealing temperature is not immediately clear at this point, and no discussion is dedicated to this result. It is only concluded that the UPD-assisted method could easily register quantitative trends that have been only qualitatively studied and documented so far in the literature.31,32 3.4.2. Surface Area Modeling of NPG. A step forward in testing and qualitatively confirming the results obtained by

Figure 15. Dependences of the modeled NPG surface area development as a function of the pore size. Inset: “Building block” unit cell of NPG that is left after dealloying.

the UPD-assisted method proposed herein could be made by a simple surface area modeling of NPG. The model presented below is developed by assuming a “lattice” type structure of the interconnected Au fraction remaining after dealloying as shown by the “building block” unit cell, depicted in the inset of Figure 15. As shown in the drawing, a is the “lattice parameter” of the unit cell, x and 2(a-x) represent the ligament (Au) and pore (void left from the Ag dissolution) size of the NPG structure, respectively. Similar modeling, albeit for cylindrical and not intersecting pores, was reported previously in the literature.71 While representing a very rough approximation of the remarkable complexity typical for a real solid-void interpenetrating structure resulting from dealloying, the proposed model should still be useful in providing basic qualitative trends of the surface area evolution in porous structures. A comparison of the NPG structure as shown in Figures 3 and 5 with the one assumed by the simplistic model in Figure 15 undoubtedly reveals limitations that would most likely lead to underestimation of the predicted surface area. Noting that the perfection of the proposed surface area model is not among the top priorities of this work, one registers qualitatively similar trends comparing the surface area calculated herein and the one obtained by a previously reported model.71 While no details of the presented model have yet been published, the Supporting Information provided with this work reveals sequential steps in the derivation of eq 3 that links the roughness factor, Rf, and the ligament size, x of NPG developed by dealloying of AupAg(1-p) alloy


Rf )

1500 × x

[

(

-1.5

p

) ] )]

cos-1(-p0.5) -1 3 cos-1(-p0.5) 3 cos 3

2p-0.5cos

[ (


(3)

Figure 15 represents the roughness factor, Rf, as calculated according to eq 3 in 1 µm thick NPGp, where p ) 0.1, 0.2, and 0.3. Generally, key points of interest here are the power law increase in the surface area clearly seen at pore sizes less than 20 nm and the increase of the surface area developed at a given dealloying depth with the increase in the original Au fraction in the alloy. On the basis of data in Figure 15 and the average pore size determined (to the extent it is possible) by SEM for each composition (using the cross-sectioned layers in Figure 5), a comparison between experimentally determined and predicted-by-modeling roughness factors for the alloys studied is presented in Figure 16. The comparison reveals a clear trend in the ratio between experimental and modeling results going from nearly 3.3 to about 1.3 with the increase of the original Au fraction in the alloy. Interestingly, the analysis of the ratio between pore and ligament size as determined by the model suggests also a similar trend, from 4.1 to about 1.6, respectively (Supplemental Information). Given literature reports on the similarity between pore and ligament size in dealloyed structures72 and considering our own experience suggesting ratios between pore and ligament size not exceeding 1.4 regardless of the original alloy composition (Figures 2 and 5 and Supporting Information), one could anticipate the better match between the model and experiment at higher Au fractions. At the same time, the calculated value would also be less than the experimentally measured one because even for NPG0.3 the model portrays a more opened structure in comparison with the real one. In summary, while the proposed model has a certain value in revealing qualitative trends that correlate roughness factor, ligament size, and the original Au fraction, other approximations that take into account the structural evolution during dealloying are needed for quantitatively matching the experimental results. 3.4.3. BET Measurement. BET is the most widely used technique for the surface area measurements of materials with a fine structure. While rather expensive, BET measurements in UHV could be done with minimum pretreatment steps. The most standard setup for BET would include two stages: first degassing and then analysis. The main obstacle encountered for the usage of BET specifically for surface area measurement of NPG samples occurs in the degassing step, when high-temperature heating for a long period of time is required to remove water and absorbed gas that affect the accuracy of the measurement. For instance, holding the temperature at 90 °C for 1 h is recommended for water removal, and treatment at 350 °C for 3 h is dedicated to the removal of absorbed gaseous contaminants. The holding temperature and time might be changed depending upon the sample itself and the accuracy of the measurement. As shown in Figure 17, without a temperature treatment, it is clearly seen that the BET measurement yields a much lower surface area density for NPG0.2 than either the UPDassisted method measurement or the surface area modeling. No significant change (or a slight increase) is registered by BET after the sample is heated at 90 °C for 1 h. This result contradicts findings of others2 and the results in the present work (Sample Coarsening by Thermal Annealing), suggesting a significant reduction of the surface area upon heat treatment. This behavior could be attributed to a dynamic balance between an area increase caused by the removal of water and thermal treatment driven coarsening. In the presented validation experiments,

Figure 16. Determination of surface area development (roughness factor) at 1 µm dealloying depth of alloys with different Au fractions using the UPD-assisted experiments and modeling method.

Figure 17. Surface area measured by the Pb UPD-assisted method (crossed line-filled bars) and by BET (plain black bars).

almost identical results are obtained by comparing a BET measurement done after water removal and degassing on one hand and a subsequent Pb UPD-assisted area determination on the other hand (Figure 17, 200 °C, 1 h). The latter result demonstrates clearly the key limitations of a standard BET setup for surface area measurement of NPG. Indeed, when the result obtained by BET is deemed reliable, the original NPG architecture would have coarsened to yield at least a five times lesser surface area density. This finding, along with the requirement for at least 1 g of sample for running an accurate BET analysis, gives a distinct edge to the Pb UPD-assisted method for surface area measurement of MNPM with a fast coarsening rate at low to moderate temperatures. 4. Conclusions In summary, the present work illustrates systematically the applicability of and reveals in detail the limitations of a Pb UPDassisted method for surface area determination of MNPM exemplified by NPG. A comprehensive comparison between CV and CA in the formation and stripping of a Pb UPD layer on NPG (i) identifies the registration of stripping current transients as the best approach for measurement of surface area and (ii) reveals distinct diffusion limitations slowing the UPD process on NPG with a thickness in the single digit micrometer range. The diffusion limitations governing the Pb UPD on NPG substrates are attributed to the insufficient bulk concentration of Pb2+ ions for the formation of a full monolayer at a high surface to volume aspect ratio. An estimate confirming the latter statement suggested that less than 10% of the Pb monolayer could be instantaneously formed when a Pb concentration of 1-50 mM is used. The method developed in this work shows

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an overall trend in the surface area developed by dealloying in the order of NPG0.3 > NPG0.1 > NPG0.2. While no clear-cut understanding of the surface area trends with the original Au fraction has yet been developed, the anomalously high surface area density registered in NPG0.1 could have important practical implications in the design and synthesis of low metal loading catalysts. A qualitative validation of the Pb UPD-assisted method for surface area determination by a comparison between the dealloyed and heat-treated NPG specimens shows reasonable agreement with literature data. A simple surface modeling approach provides qualitative trends in surface area development in NPG and suggests that the closest match with experimental data could be obtained by model structures featuring similar pore and ligament size. A coarsening effect taking place during the removal of water and gaseous contaminations limits the use of standard BET protocols for surface area determination of NPG. However, a quantitative agreement is found between BET and Pb UPD-assisted surface measurements for samples treated for water and gas removal. Overall, the Pb UPD-assisted method is proven as an inexpensive, fast, selective, and sensitive technique for the surface area measurement of MNPM. Acknowledgment. The authors acknowledge the support of this work by the National Science Foundation, Division of Materials Research (DMR-0603019). Supporting Information Available: Sequential steps in the derivation of eq 3 and analysis of the ratio between pore and ligament size. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (2) Ji, C.; Searson, P. C. J. Phys. Chem. B 2003, 107, 4494. (3) Snyder, J.; Livi, K.; Erlebacher, J. J. Electrochem. Soc. 2008, 155, C464. (4) Dong, H.; Cao, X. J. Phys. Chem. C 2009, 113, 603. (5) Elliott, J. M.; Birkin, P. R.; Bartlett, P. N.; Attard, G. S. Langmuir 1999, 15, 7411. (6) Liu, H.; He, P.; Li, Z.; Li, J. Nanotechnology 2006, 17, 2167. (7) Thorp, J. C.; Sieradzki, K.; Tang, L.; Crozier, P. A.; Misra, A.; Nastasi, M.; Mitlin, D.; Picraux, S. T. Appl. Phys. Lett. 2006, 88, 033110. (8) Lu, H.; Li, Y.; Wang, F. Scr. Mater. 2007, 56, 165. (9) Jia, F.; Yu, C.; Deng, K.; Zhang, L. J. Phys. Chem. C 2007, 111, 8424. (10) Viyannalage, L. T.; Liu, Y.; Dimitrov, N. Langmuir 2008, 24, 8332. (11) Yeh, F.; Tai, C.; Huang, J.; Sun, I. J. Phys. Chem. B 2006, 110, 5215. (12) Kim, J.; Dohnalek, Z.; Kay, B. D. Surf. Sci. 2005, 586, 137. (13) Yi, Q.; Huang, W.; Liu, X.; Xu, G.; Zhou, Z.; Chen, A. J. Electroanal. Chem. 2008, 619-620, 197. (14) Chang, J.; Hsu, S.; Sun, I.; Tsai, W. J. Phys. Chem. C 2008, 112, 1371. (15) Fukumizu, T.; Kotani, F.; Yoshida, A.; Katagiri, A. J. Electrochem. Soc. 2006, 153, C629. (16) Zeis, R.; Lei, T.; Sieradzki, K.; Snyder, J.; Erlebacher, J. J. Catal. 2008, 253, 132. (17) Xu, C.; Xu, X.; Su, J.; Ding, Y. J. Catal. 2007, 252, 243. (18) Ding, Y.; Chen, M.; Erlebacher, J. J. Am. Chem. Soc. 2004, 126, 6876. (19) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (20) Zeis, R.; Mathur, A.; Fritz, G.; Lee, J.; Erlebacher, J. J. Power Sources 2007, 165, 65. (21) Ding, Y.; Erlebacher, J. J. Am. Chem. Soc. 2003, 125, 7772. (22) Biener, J.; Wittstock, A.; Zepeda-Ruiz, L. A.; Biener, M. M.; Zielasek, V.; Kramer, D.; Viswanath, R. N.; Weissmuller, J.; Baumer, M.; Hamza, A. V. Nat. Mater. 2009, 8, 47. (23) Jia, F.; Yu, C.; Ai, Z.; Zhang, L. Chem. Mater. 2007, 19, 3648. (24) Sanchez, P. L.; Elliott, J. M. Analyst 2005, 130, 715. (25) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.

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