Electro-oxidation of COchem on Pt Nanosurfaces: Solution of the Peak

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Electro-oxidation of COchem on Pt Nanosurfaces: Solution of the Peak Multiplicity Puzzle Patrick Urchaga,† Steve Baranton,*,† Christophe Coutanceau,† and Gregory Jerkiewicz*,‡ †

Laboratoire de Catalyse en Chimie Organique, Equipe Electrocatalyse, Universite de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France ‡ Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada

bS Supporting Information ABSTRACT: An understanding of the oxidation of chemisorbed CO (COchem) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and COchem oxidative stripping behavior. Oxidative stripping voltammograms are obtained for COchem preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in COchem oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for COchem oxidative stripping unambiguously.

1. INTRODUCTION The emerging hydrogen economy addresses environmental and energy challenges faced by society by envisaging the production of electricity using polymer electrolyte membrane fuel cells (PEMFC). The success of this approach relies heavily on the broad availability of hydrogen that can be produced either by water electrolysis or reforming hydrocarbons and alcohols. Water electrolysis is a renewable yet expensive method of producing high-purity hydrogen. The reforming of carboncontaining compounds (e.g., hydrocarbons and alcohols) is less expensive but produces hydrogen containing significant amounts of carbon monoxide (CO). PEMFC anode materials contain Pt because of its ability to dissociate H2 to chemisorbed H (Hchem) and to oxidize Hchem to H+. The formation of H+ with the release of an electron is the key anodic reaction taking place in PEMFC. However, CO accompanying H2 readily chemisorbs and poisons Pt-containing electrocatalysts, thus hindering their ability to dissociate H2 and oxidize Hchem. Consequently, a comprehension of Pt
COchem interactions is of paramount importance to the design and fabrication of novel CO-tolerant electrode materials. The desorption of COchem from Pt is endergonic (ΔrG° > 0, where ΔrG° is the reaction’s standard Gibbs energy) and requires the application of a high anodic potential; it is recognized to give rise to a prepeak and two peaks in a voltammetric COchemstripping transient (Figure 1).1,2 The relative peak ratios are vaguely interpreted in terms of the 3D morphology and 2D surface structure of the electrode material. Elsewhere,3,4 it was r 2011 American Chemical Society

proposed that the process follows a Langmuir
Hinshelwood mechanism (eqs 1 and 2).
Pt þ H2 O f Pt
OHads þ Hþ aq þ e

ð1Þ


Pt
COchem þ Pt
OHads f Pt þ CO2 þ Hþ aq þ e

ð2Þ In the case of a two-electron process such as COchem oxidative stripping, potential differences of 0.05 (peaks 1 and 2) and 0.25 V (prepeak and peak 1) between two voltammetric features translate into differences of 9.6 and 48 kJ mol
1 in ΔrG°, respectively. The phenomenon that results in peak multiplicity and corresponds to a significant change in ΔrG° is poorly understood. The oxidative stripping of COchem was investigated using low Miller index single-crystal Pt electrodes,5 stepped Pt surfaces,6,7 supported Pt nanoparticles,8,9 and unsupported Pt nanoparticles10 in order to analyze its structure sensitivity, but the assignment of the three voltammetric features (Figure 1) to specific steps in the reaction or particle size or structural effects remains unclear. Data obtained using single-crystal electrodes can hardly be applied to Pt nanoparticles because of the proximity of edges and corners to flat domains that give rise to synergetic effects, not to mention Received: July 27, 2011 Revised: October 14, 2011 Published: October 20, 2011 3658

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Figure 1. Three voltammetric features typically observed in COchem oxidative stripping measurements.

the inherent size difference that should also affect the potential of COchem oxidative stripping peaks. Because membrane
electrode assemblies (MEA) used in PEMFC contain Pt nanoparticles, research on the electrocatalytic properties of unsupported and preferentially oriented Pt nanoparticles is expected to shed light on the missing link among their reactivity, size, and structure.10,11 Various interpretations, which include particle size, nanoparticle agglomeration, and surface structure effects, were proposed to explain the peak multiplicity but did not succeed in relating the data obtained for nanoparticles to those acquired using bulk materials.12,13 This contribution reports new results on the oxidative stripping of COchem on preferentially oriented Pt nanoparticles and on nanostructured polyoriented Pt surfaces that are mesoscopic in size. It reports on the existence of a new voltammetric feature that has been overlooked but is instrumental in explaining the origin of COchem oxidation peak multiplicity.

2. EXPERIMENTAL METHODS 2.1. Preparation of Platinum Materials. Platinum materials were prepared using previously described methods. Briefly, spherical Pt nanoparticles were synthesis by the water-in-oil microemulsion method (W/O method). Preferentially shaped Pt nanoparticles were prepared using either the tetradecyltrimethylammonium bromide method (TTAB method) or the polyacrylate method (PA method).10,14,15 The TTAB method involved the reduction of 1.0  10
3 M K2PtCl4 in an aqueous medium at 50 °C by adding an ice-cooled NaBH4 solution with TTAB employed as a capping agent.16
18 The PA method involved the reduction of K2PtCl6 in an aqueous solution of sodium polyacrylate (PA, with a mean molecular mass of 2100 g mol
1). The Pt/PA molar ratio and pH value of the reaction medium were varied in order to obtain either octahedral or cuboctahedral nanoparticles. To obtain octahedral nanoparticles, the pH was adjusted to 7.0; this procedure is referred to as the PA-1 method. To obtain cuboctahedral nanoparticles, the pH was adjusted to 9.0; this procedure is referred to as the PA-2 method. After the PA solution was added to that containing Pt4+, ultra-high-purity Ar gas was vigorously bubbled through the solution for 30 min to remove any dissolved O2. Subsequently, H2 that acted as a reducing agent was bubbled for 10 min, and then the reactor was sealed for ca. 12 h. All Pt nanoparticles were then cleaned by repetitive dispersion
precipitation steps in a large volume of ultra-high-purity water and finally stored in water as a diluted suspension. The bulk Pt(poly) electrode consisted of a 99.99% rod (3.5 mm in diameter) that was polished in stages with an

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alumina suspension (1, 0.3, and 0.05 μm) to obtain a mirrorlike finish. The thermal treatment that produced atomically flat crystallites was achieved by heating with a hydrogen flame (∼1000 °C) for 1 min followed by quenching in ultra-high-purity water. 2.2. Physical Characterization. Transmission electron microscopy (TEM) analysis of Pt nanoparticles was carried out using a JEOL 2100 ultra-high-resolution microscope (200 kV) equipped with a LaB6 filament; it offered a resolution of 0.35 nm. Metallographic characterization of the Pt(poly) electrode was carried out with a JEOL 840A scanning electron microscope (SEM). Prior to the SEM analysis, each Pt(poly) electrode was etched in aqua regia for 5 min followed by thorough rinsing with ultra-high-purity water. 2.3. Electrochemical Measurements. Cyclic voltammetry (CV) and oxidative stripping experiments were conducted using an EG&G PAR model 362 analogue potentiostat that was interfaced for data acquisition and analysis. All potentials were measured with respect to a reversible hydrogen electrode (RHE). Unsupported Pt nanoparticles were deposited on a clean gold substrate until a greyish deposit was observed. All experiments were carried out at room temperature (20 ( 1 °C) in a classical three-compartment glass electrochemical cell. The electrolyte was a 0.5 M aqueous (18.2 MΩ cm) H2SO4 solution. Ultrahigh-purity N2 gas was bubbled through the electrolyte to expel any traces of reactive gases (O2, H2, and CO). The counter electrode was a glassy carbon plate. The reference electrode was connected to the working-electrode compartment with a Luggin capillary to minimize any ohmic resistance. To perform oxidative stripping experiments of COchem, a CO saturating layer on Pt materials was prepared by bubbling high-purity CO through the working-electrode compartment for 5 min at a constant chemisorption potential of Echem = 0.1 V. Afterward, N2 was bubbled for 30 min at the same potential to expel any remaining CO.

3. RESULTS The top part of Figure 2A shows a scanning electron microscopy (SEM) image of a polished, etched Pt(poly) electrode (the method is referred to as etching), which reveals crystallites of different orientations and ca. 50
200 nm in size separated by grain boundaries. The top part of Figure 2B shows an SEM image of the same region after annealing in a hydrogen flame, followed by quenching in ultra-high-purity water (the method is referred to as thermal treatment). Because the two images are the same, one concludes that the thermal treatment does not change the crystallite shape or size. The center parts of Figure 2A,B (black curves) show cyclic voltammetry (CV) profiles in the region of adsorption/desorption of underpotential-deposited H (HUPD) in 0.5 M aqueous H2SO4 for the etched Pt(poly) prior to (Figure 2A) and after (Figure 2B) thermal treatment. The CV profile for etched Pt(poly) reveals two peaks at 0.12 and 0.26 V that closely resemble those obtained for a typical bulk Pt electrode.19 The CV profile for thermally treated Pt(poly) reveals two peaks at the same potentials, with the first being slightly more pronounced and the second being very sharp, and a broad voltammetric wave in the 0.30
0.45 V range. In general, sharp CV features are characteristic of atomically ordered monocrystalline Pt electrodes.20 The CV profile in Figure 2B closely resembles that obtained for a bead-shaped Pt single crystal with all faces being in contact with the electrolyte and thus with all faces contributing to the CV response.21 Figure 2A,B (red curves) show voltammetric profiles for the COchem oxidative stripping. The etched electrode gives rise to a very low intensity prepeak (0.40
0.65 V) and a well-defined peak at 0.726 V with a small shoulder at 0.785 V that might correspond to an overlapping peak. (The first derivative ∂I/∂E, with I being the current 3659

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Figure 2. Voltammograms obtained using (A) polished and (B) polished and thermally treated Pt(poly) electrodes. (Top) Scanning electron microscopy images of the Pt(poly) surfaces. (Center) COchem oxidative stripping voltammograms (red lines) and subsequent cyclic voltammograms (black lines) for CO-free Pt(poly) electrodes; scan rate s = 20 mV s
1. (Bottom) The first derivative, ∂I/∂E, versus E curve for the COchem oxidative stripping transients for (A) polished and (B) polished and thermally treated bulk Pt(poly) electrodes.

and E being the potential, in the bottom part of Figure 2A provides evidence for the existence of second peak.) The thermally treated electrode gives rise to a low-intensity prepeak (0.35
0.65 V) and two well-defined, similar intensity peaks at 0.738 and 0.796 V, respectively; the second peak may be related to the shoulder in Figure 2A. An analysis of the voltammetric transients for the adsorption/desorption of HUPD and the oxidative stripping of COchem as well as the existing literature21 allows one to conclude that the thermal treatment creates a Pt electrode with randomly oriented but atomically flat surface domains. Nanoparticles synthesized by the W/O method (inset in Figure 3A; also see the Methods section) display a mean diameter of ca. 3 nm with a narrow size distribution. Their 2D projections generated by transmission electron microscopy (TEM) do not provide evidence of any preferential shape development, thus they are assumed to be spherical (Supporting Information, Figure SI1a). The size distributions and 3D structures of the preferentially oriented nanoparticles prepared by TTAB, PA-1, and PA-2 (Methods section) were also determined by TEM (Supporting Information, Figure SI1b
d). The TTAB method produces mainly well-defined cubic nanoparticles with an average size of ca. 12 nm (inset in Figure 3B). The PA-1 method generates mainly octahedral nanoparticles that produce rhombohedral projections with an average size of ca. 10 nm (inset in Figure 3C), whereas the PA-2 method produces cuboctahedral nanoparticles that yield hexagonal projections with an average size of ca. 9 nm (inset in Figure 3D). The concepts of size and the preferential surface orientation of nanoparticles are nontrivial. In the case of nanoparticles other than cubic, all side lengths and angles between sides are required

to describe their shape and volume. Furthermore, noncubic nanoparticles have different sides with different arrangements of surface atoms. In the case of cubic nanoparticles, an edge length is adopted to represent their size, whereas in the case of octahedral and cuboctahedral nanoparticles, the length of the largest dimension is accepted to represent their size. With regard to the nanoparticles that are preferentially oriented (the nanoparticle face that contributes the most to the electrochemical signal), the TTAB method produces mainly (100) domains, the PA-1 method mainly produces (111) domains, and the PA-2 method mainly produces a combination of (100) and (111) domains (Table 1). The black curves in Figure 3A
D show CV profiles for the adsorption/desorption of HUPD in 0.5 M aqueous H2SO4. All samples reveal reversible peaks at 0.12 and 0.26 V, which are broader in the case of spherical nanoparticles (Figure 3A) than in the case of preferentially oriented nanoparticles (Figure B
D). In all cases, the two peaks are related to the adsorption/ desorption of HUPD on edges mimicking (110) domains and atomically ordered sides having a (100) orientation.22 In the case of cubic nanoparticles (Figure 3B), there are several smallintensity overlapping peaks that correspond to atomically ordered, short- and long-range (100) domains. There is also a small, broad peak at ca. 0.5 V that is assigned to anion adsorption/desorption on (111) domains. In the case of octahedral nanoparticles (Figure 3C), again there are low-intensity overlapping peaks that correspond to atomically ordered, short- and long-range (100) domains, but the fraction of surface domains having the (100) orientation is smaller than in the case of cubic nanoparticles. However, a well-defined, broad peak at ca. 0.5 V points to the presence of a significant fraction of domains having 3660

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Figure 3. Voltammograms obtained for Pt nanoparticles of various shapes: (A) spherical, (B) cubic, (C) octahedral, and (D) cuboctahedral. (Top) COchem oxidative stripping voltammograms (red lines) and subsequent cyclic voltammograms (black lines) for CO-free Pt nanoparticles; scan rate s = 20 mV s
1. (Bottom) First derivative, ∂I/∂E, vs E curve for the COchem oxidative stripping transients. The transmission electron microscopy images are given in the insets.

the (111) orientation. In the case of cuboctahedral nanoparticles (Figure 3D), there are several overlapping peaks that correspond to atomically ordered, short- and long-range (100) domains that are comparable in size to those obtained in the case of cubic nanoparticles. In addition, there is a broad, low-intensity peak at ca. 0.5 V that represents (111) domains. The interpretation of the CV profiles in terms of contributions from (100) and (111) domains is in good agreement with the TEM images that show (i) the absence of preferentially oriented domains in the case of spherical nanoparticles (Figure 3A); (ii) the presence of (100) domains in the case of cubic nanoparticles (Figure 3B); (iii) (111) domains in the case of octahedral nanoparticles (Figure 3C); and (iv) both (100) and (111) domains in the case of cuboctahedral nanoparticles (Figure 3D). The COchem oxidative stripping experiments were carried out after CO chemisorption (preadsorption) at a constant potential of Echem = 0.10 V. The red curves in Figure 3A
D show COchem oxidative stripping profiles for the respective Pt nanoparticles. In all cases, the current is zero until an onset potential of ca. 0.35
0.40 V is reached, at which the prepeak develops. As the potential is increased further, the stripping profiles reveal a high-intensity signal that is due to the superposition of peaks 1

(0.72 V) and 2 (0.76 V). The current profiles and peak potentials are in agreement with previously published data for nanoparticles treated under the same experimental conditions.23 It is worthwhile to note that the relative intensity of peak 2 at 0.76 V increases with an increasing fraction of (100) surface domains within the nanoparticles. The black curves in Figure 3A
D are the first CV profiles for HUPD adsorption/desorption right after COchem oxidative stripping. Their close resemblance to those obtained before CO chem oxidative stripping (Supporting Information, Figure SI2) indicates that the chemisorption and oxidative stripping of CO neither alter the morphology of nanoparticles nor modify the structure of surface domains.

4. DISCUSSION The above-presented results (Figures 2A and 3A) demonstrate that both etched Pt(poly) and spherical Pt nanoparticles (which mimic polyoriented surfaces) generate very similar voltammetric profiles for the HUPD adsorption/desorption and oxidative stripping of COchem. In addition, the potentials of the voltammetric features are the same to within (5 mV, thus 3661

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Table 1. Estimation of the Surface-Ordered Domain Ratios by the Bismuth and Germanium Adatom Methods20 sample

% (100)

Table 2. Potential Ranges and Assignation of the COchem Oxidative Stripping Peaks to Specific Surface Domains and Sites

% (111)

unsupported Pt-w/o

10

10

Pt-TTAB

42

18

Pt-PA-1

14

Pt-PA-2

26

prepeak

peak

peak

peak

1a

1b

2 0.76

0.35
0.60

0.68
0.70 0.70
0.72

44

potential range (V versus RHE)

36

related surface

(100)

(111)

site

Figure 4. Schematic representation that summarizes the four voltammetric features observed in the COchem oxidative stripping voltammograms and assigns them to specific surface domains and sites.

indicating that the size of the electrode material does not play any significant role in either electrochemical reaction. The COchem oxidative stripping voltammograms reveal two overlapping peaks, with the charges under peaks 1 and 2 being ca. 90 and 10% of the total charge, respectively. Peak 2 is assigned to COchem oxidative stripping on (100) preferentially oriented domains; therefore, the results indicate that the polyoriented surface preserves a small fraction of (100) domains. The thermal treatment of Pt(poly) (Figure 2B) brings about changes in the HUPD adsorption/desorption and COchem oxidative stripping voltammetric transients that point to a significant increase in the fraction of (100)-oriented domains. As these domains become more pronounced, the COchem oxidative stripping transient resembles the analogous transient for Pt cubic nanoparticles that are (100) preferentially oriented (Figure 2B, ca. 40% of the total surface). Because there is a smaller fraction of (100) domains within the thermally treated Pt(poly) than in the case of Pt cubic nanoparticles, the intensity of peak 2 is lower, as expected. We observe that the thermal treatment of the ordered mesoscopic domains also increases the current of the prepeak (Figure 2B) that becomes a well-defined voltammetric feature. Consequently, it is suggested that the prepeak is a characteristic of atomically ordered domains containing surface defects. The COchem oxidative stripping transients for octahedral and cuboctahedral Pt nanoparticles (Figure 3C,D) show peaks 1 and 2, with peak 1 being either asymmetrical (Figure 3C) or resembling two overlapping peaks (Figure 3D). To determine the peak potential precisely, the first derivate ∂I/∂E was calculated. The value of E at which the first derivative equals zero and changes sign from positive to negative as E increases corresponds to a maximum that is a peak potential. However, the ∂I/∂E versus

and (111)

low-coordination (100) sites

E profiles also reveal inflection points that could be interpreted in terms of two overlapping peaks. Therefore, it is proposed that peak 1 is composed of two overlapping peaks whose potentials are close to each other. The spherical and cubic nanoparticles have a small fraction of (111) preferentially oriented domains, and in this case, the ∂I/∂E versus E profile does not show an inflection point, thus indicating that peak 1 is a single voltammetric feature or that it contains a second feature that is too small to be easily distinguished. The octahedral and cuboctahedral nanoparticles have a significant fraction of (111) preferentially oriented domains and produce COchem stripping transients that reveal peaks 1 and 2, with peak 1 most likely being a superposition of two closely located peaks. The first contribution is attributed to the oxidative stripping of COchem adsorbed on (111) surface domains (peak 1a in Figure 4), whereas the second one, at higher potentials, is assigned to the oxidative stripping of COchem on low-coordination sites (peak 1b in Figure 4). This detailed analysis demonstrates that the COchem oxidative stripping transients consist of up to four features, as illustrated in Figure 4: the prepeak and three peaks, the existence of which strongly depends on the nature and surface orientation of Pt materials. Table 2 summarizes the potential ranges and the assignation of COchem peaks in oxidative stripping voltammograms to specific surface domains and sites.

5. CONCLUSIONS It is shown that the voltammetric features for COchem oxidative stripping are due to the presence of specific surface domains within mesoscopic and nanoscopic Pt materials. It is demonstrated for the first time that the COchem oxidative stripping voltammograms consist of up to four features that can be assigned to surface domains that are either preferentially oriented or disordered (Figure 4). The chemisorption and stripping of CO do not modify Pt nanoparticles that preserve their shape and surface orientation. This article offers a new interpretation of the COchem stripping voltammograms in terms of structure-related effects taking place on Pt nanoparticles. ’ ASSOCIATED CONTENT

bS

Supporting Information. Transmission electron micrographs of platinum nanoparticles. Cyclic voltammograms recorded in 0.5 M H2SO4 for Pt nanoparticle electrodes before the COchem oxidative stripping. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] and gregory.jerkiewicz@ chem.queensu.ca. 3662

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