Temperature-Induced Deposition Method for Anchoring Metallic

J. Phys. Chem. B 2004, 108, 17915-17920

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Temperature-Induced Deposition Method for Anchoring Metallic Nanoparticles onto Reflective Substrates for in Situ Electrochemical Infrared Spectroscopy V. Stamenkovic´ ,* M. Arenz, P. N. Ross, and N. M. Markovic´ Materials Sciences DiVision, Lawrence Berkeley National Laboratory, UniVersity of California at Berkeley, Berkeley, California 94720 ReceiVed: July 22, 2004; In Final Form: September 9, 2004

A new temperature-induced deposition method for anchoring metallic nanoparticles onto smooth/refelective polycrystalline Au (Au-poly) and classy carbon (GC) substrates has been developed for in situ infrared reflection-absorption spectroscopy (IRAS). Characteristic vibrational spectra for CO adsorbed on anchored nanoparticles of Pt/C, Pt3Sn/C, Pt3Mo/C, and Pt2Ru3/C are presented. When Au is used as a substrate, CO adsorbed on Au can interfere with bands for CO adsorbed on nanoparticles. By contrast, if GC is used as a substrate, CO adsorbed on nanoparticles are as expected from corresponding spectra obtained from Pt and Pt-bimetallic electrodes.

1. Introduction Carbon-supported metal nanoparticles have been the subject of intensive fundamental research because highly dispersed metal particles supported on carbon and oxides make up an important class of heterogeneous catalysts.1 In electrochemical systems, Pt and Pt-bimetallic catalysts were employed in commercial prototype fuel cells even three decades ago, so the concept of nanoparticle electrocatalysts is not new. Like many industrial catalysts, however, the catalysts are put into use long before their structure and properties are clearly understood. Only recently, with the advent of new surface sensitive techniques and the development of computational methods, has it become possible to correlate the reactivity with the atomic-level surface structure of the nanoparticle catalyst. For fundamental studies of nanoparticle films in an electrochemical environment, a very important issue is how to attach metal nanoparticles onto a conducting substrate without changing their physical and chemical properties. Recently, several surface optimization methods have been proposed for anchoring metal nanoparticles onto various inert, i.e., carbon,2,3 and quasi-inert, i.e., gold4,5 or coated gold,6 substrates. For example, Weaver et al.5,7,8 initially proposed a method in which physical deposition of nanoparticles onto gold substrates from a water suspension is followed by drying and intensive rinsing. However, infrared reflection absorption spectra (IRAS) for CO adsorbed on Pt nanoparticles displayed anomalous negative absorbance. The same authors established a new procedure for attaching metal nanoparticles onto a gold substrate, namely, before attaching nanoparticles the gold substrate was previously coated with polymerized silane units.6,9 This method provided high-quality IRAS and Raman spectra10 but requires a long complicated preparation procedure. In previous reports related to in situ infrared characterization of nanoparticles in an electrochemical environment, electrooxidation of carbon monoxide and small organic molecules8,11,12 has been used as a model system to reveal the relationship between the adsorbate site occupancy and the reaction rate. To probe effects of temperature on methanol and CO oxidation, spectroelectrochemical cells capable of operation at temperatures * Address correspondence to this author. E-mail: [email protected].

other than ambient have been presented.11,13-15 Although improvement in our understanding of vibrational properties of CO adsorbed on nanoparticles has been obtained from previous measurements, it was recognized that the level of interpretation of these results was limited by several factors, including the following: (i) the appearance of anomalous negative absorbance originating from CO adsorbed on thick nanoparticle layers;5,7 (ii) a possible interference in absorption spectra from CO adsorbed on nanoparticles with CO adsorbed on the unmodified Au substrate;4 (iii) weak CO infrared band intensity when nanoparticles are attached to the glassy carbon support;3 and (iv) complicated preparation procedures.9 In this paper we describe a new method for anchoring metallic nanoparticles onto reflective substrates of gold and glassy carbon, which we characterize as a temperature-induced deposition (TID) method. Commonly detected negative absorbance νCO bands, observed due to the dielectric behavior induced by metal nanoparticle aggregates,5,16 are completely eliminated by utilizing our new preparation method. At the same time we introduce an upgraded spectroelectrochemical cell for experiments at elevated temperatures. 2. Experimental Section 2.1. Substrates and Nanoparticle Electrocatalysts. Commercially available high surface area carbon supported Pt-based catalysts provided by E-Tek (Pt/C: 30 wt %, d ≈ 5 nm; Pt3Sn/C: 20 wt %, d ≈ 5 nm) and by Tanaka Precious Metals Group (Pt2Ru3/C: 54 wt %, d ≈ 4 nm) were deposited onto a mirror polished polycrystalline Au electrode (hereafter denoted as Au-poly) or a glassy carbon electrode (hereafter denoted as GC) with a diameter of 10 mm. Prior to each measurement Au and/or GC electrodes were repolished by 0.3 µm alumina, rinsed with distilled water, and then flame annealed in a propane/air flame (only Au substrate). 2.2. Deposition of the Thin Nanoparticle Layers. A schematic representation of the “preparation chamber” along with the FTIR cell is depicted in Figure 1. The deposition procedure for attaching thin nanoparticle layers to the substrate consists of three steps: (i) heating Au-poly and GC substrates on a hot plate up to ca. 130 °C (the temperature was controlled

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Figure 1. Schematic representation: (a) preparation chamber for thin catalyst layers; (b) upgraded spectroelectrochemical cell for in situ FTIR measurements at elevated temperatures (initial design was presented by Iwasita18). GI is the gas inlet (bubbler), TC the thermocouple, WE the working electrode, CH the cartridge heater, CE the counter electrode, and RE the reference electrode.

by a thermocouple) in an argon-purged glass bell, (ii) depositing a dilute suspension (20 µL of ultrasonically dispersed catalyst suspension in triply pyrodistilled water; 1.0 mg of catalyst/mL) via micropipet onto the substrate, and (iii) removing the electrode from the hot plate and cooling the sample in argon stream for ca. 2 min. In the second step, the catalyst particles are uniquely spread over the polished substrate by instantaneous evaporation of aqueous matrix forming a thin and uniform film. This fast evaporation, forced by the temperature of the substrate, is the key factor in providing perfect anchoring and good dispersion of the metallic nanoparticles onto polished surfaces. The thickness of the catalyst layer, and hence its spectroelectrochemical properties, can be controlled by two parameters: the substrate temperature and the catalyst concentration in the suspension. As a guideline our values for optimal results were a temperature range from 120 to 140 °C and a concentration of 1-2 mg of catalyst/mL. As in the case for planar electrodes,17 before the transfer into an electrochemical cell, the nanoparticle film was protected from air-born contamination by a drop of triply distilled water. Depositing a thin IR reflective catalyst overlayer, as described above, requires no more than 3 min, which is an additional advantage of this experimental methodology. 2.3. Electrochemical Measurements. The substrate modified by the nanoparticle film was mounted in a rotating disk electrode and immersed into the electrolyte under potential control at ∼0.05 V vs RHE in 0.5 M H2SO4 (Baker, Ultrex). The electrolyte prepared with triple pyrodistilled water was thermostated at 293 K by a circulating constant-temperature bath connected to the water jacket of a standard three-compartment electrochemical cell. The reference electrode was a saturated calomel electrode (SCE) separated by an electrolytic bridge from the reference compartment. All potentials in this paper are, however, referenced to the reversible hydrogen electrode potential (RHE) at the same temperature (calibrated from the hydrogen oxidation reaction19) in the same electrolyte; argon, carbon monoxide, and hydrogen were bubbled through a glass frit (Air Products, 5N8 purity). All voltammograms were recorded with a sweep rate of 50 mV/s. 2.4. FTIR Spectroscopy. After the initial characterization in an electrochemical cell, the electrode was transferred to the spectroelectrochemical cell, which is designed for the IR external reflection mode in a thin electrolyte layer configuration, as shown in Figure 1b. The cell is coupled to a CaF2 prism beveled 60° from the prism base and equipped with a specially created

cartridge heater, thermocouple, and power supply, allowing experiments under temperature control (Figure 1b). A cartridge heater and thermocouple were placed in a thin Teflon sleeve to protect the heater/thermopar body from the acid environment. The whole system for temperature control was previously calibrated in the range between 20 and 80 °C. Intensive bubbling through the gas inlet allows constant mixing of electrolyte and prevents local overheating. During the experiment at elevated temperatures, a time interval of 15 min has been used for thermal equilibrium between the electrolyte and the CaF2 optical window. This step minimizes heat transfer through the cell window and prevents the existence of a possible thermal gradient in the thin layer configuration due to the temperature difference between the reflective working electrode and the optical window.13 Prior to each measurement a cyclic voltammogram was recorded to confirm the cleanliness of the electrode surface and the electrolyte. Subsequently the solution was saturated with CO for at least 5 min, holding the electrode potential at 0.01 V. The spectra were recorded with a resolution of 8 cm-1. All measurements were performed with p-polarized light. To obtain a single beam spectrum 50 scans were collected at each potential resulting in a recording time of 25 s. Reflectance spectra were calculated as the ratio ∆R/R0, where R and R0 are the reflectance values corresponding to the sample and reference spectra, respectively. Reference spectra were recorded at either 0.9 or 0.01 V, where adsorbed CO (COad) is completely oxidized or well before the onset of COad oxidation, respectively. A microcalomel electrode protected from leaking controlled the reference potential in the spectroelectrochemical cell. All in situ FTIR measurements were done with a Nicolet Nexus 670 spectrometer purged with nitrogen and equipped with a MCT detector cooled with liquid nitrogen. 3. Results and Discussion As mentioned above, depending on the sample preparation method for in situ IRAS measurements, different shapes of the adsorbed C-O bands can be observed on Pt nanoparticles attached to either Au or GC substrates.3,5 For example, if a Pt catalyst layer is attached to the substrate by using a conventional method, resulting in an optically “thick” layer of catalysts, then in agreement with previous results5 the C-O spectra (Figure 2 a) show a characteristic bipolar (or “negative”) band in the range between 2065 and 2110 cm-1. The interpretation and discussion of the observed bipolarity/abnormality, which grows signifi-

Anchoring Nanoparticles onto Reflective Substrates

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Figure 2. Spectra of adsorbed CO at 0.05 V in CO saturated 0.5 M H2SO4: (a) a thick catalyst layer of Pt/C produces a bipolar and/or negative absorbance band on different substrates; (b) a thin catalyst layer enables adsorption of CO on Au sites. The ideal thickness of the catalyst layer obtained by TID method for (c) Pt/C, Pt2Ru3/C on gold, and bare gold substrate and (d) Pt-poly, Pt/C on glassy carbon, and bare glassy carbon substrate. All reference spectra were taken at 0.9 V.

cantly with an increase of the nanoparticle layer thickness or nanoscale thin metal films, are given in refs 5, 16, and 20. Briefly, it is associated with a complex dielectric behavior induced by aggregated nanoparticles. The analysis of these anomalous bipolar bands can be fraught with difficulty, therefore it is desirable to the extent possible to acquire unipolar spectra comparable to those in IRAS measurements on planar (bulk) electrodes.21 As we demonstrate below, to reach this goal, it is necessary to attach a uniform and thin nanoparticle layer on the reflective substrate. At this point it is important to note that while GC is completely inert toward CO adsorption, the interaction of CO with Au is not insignificant22 and characteristic frequencies for COad coordinated to Au atoms can be observed, i.e., 19401990 cm-1 for the 3-fold bridging, 2005-2070 cm-1 for the 2-fold bridging, and 2115-2140 cm-1 for the terminal position. Consequently, if deposited nanoparticle layers are not uniformly dispersed onto the Au surface, leaving small patches of the substrate uncovered by the catalysts (which usually occurs if too dilute suspensions are used), then the spectral features characteristic for the Au-CO interaction may be superimposed on the CO spectra from the nanoparticles. To demonstrate this, a representative set of COad spectra obtained from nonuniform nanoparticle layers of Pt/C and Pt3Sn/C attached to Au is given in Figure 2b. Shown as a baseline in the same figure is a typical CO spectrum from polycrystalline Au, which is used as a substrate. As reported previously22 for the Au(110)-CO system, rather complex CO spectra are found for the mirror-polished and flame-annealed Au-poly surface: a band near 1965 cm-1, with the second, weaker band shifted positively by about 45 cm-1 and, finally, a band near 2115 cm-1. Corresponding CO spectra on Pt/C and Pt3Sn/C in Figure 2b show a strong νCO band at 2069 cm-1 and four weaker bands centered at 1875, 1965, 2008, and 2117 cm-1. Following the assignments made for surfaces of Pt, Pt3Sn(110),21,23,24 and Au, the bands at 2069 and 1875 cm-1 can be ascribed to a-top and bridging sites of CO on Pt, respectively; the remaining three frequencies are assigned to the 3-fold bridging (1965 cm-1), the 2-fold bridging (2008 cm-1), and the atop (2117 cm-1) positions of CO on Aupoly. Consequently, the weak but clearly visible CO band at 2117, 2008, and 1965 cm-1 for the Pt/C and Pt3Sn/C samples might erroneously be ascribed to “weakly” (2117 cm-1) or

“strongly” (2008 cm-1 and 1965 cm-1) 23,24 adsorbed CO on the Pt nanoparticles but in fact originate from the Au substrate. These two artifacts, namely the occurrence of bipolar bands on thick catalyst layers and/or a possible appearance of νCO on Au, can be completely controlled by a correct implementation of our TID method for depositing metallic particles on either Au or GC substrates. For example, if Pt/C and Pt2Ru3/C catalysts are attached to Au by using this method (Figure 2c), then only frequencies for a CO adlayer adsorbed on the respective metallic nanoparticles are obtained in IRAS measurements. Absence of a negative component in all spectra is obvious as well as a superior signal-to-noise ratio, with (∆R/R)CO of almost 4%. A further inspection of spectra in Figure 2c reveals that with the Au substrate the CO band is markedly asymmetrical, an observation that is in accord with previous reports.5,7,8 By contrast, if GC is used as a substrate (Figure 2d) a symmetrical line shape is observed for CO adsorbed on Pt/C, indicating that when using Au as a substrate one should keep in mind that the tailing of the νCO band toward lower wavenumbers is the influence of the substrate on the vibrational properties of COL and not an inherent property of a CO adlayer adsorbed on nanoparticle catalysts. Notice that the position of the COL band is substrate insensitive, i.e., the same wavenumber for COL on the Pt/C sample was measured on Au and glassy carbon substrates. It is also important to note that if glassy carbon is used as a substrate for metallic nanoparticles deposits, the signalto-noise ratio is lower (due to the inferior reflective properties of glassy carbon) than that for Au. The former (∆R/R)CO are, however, very close (ca. 1%) to corresponding (∆R/R)CO obtained from a planar Pt electrode, so that the analysis of those spectra is as convenient as that for the Pt-CO system. For comparison, spectra obtained from the mirror-polished bulk surface of polycrystalline Pt and Pt/C nanoparticles attached on glassy carbon are given in Figure 2d. The significant shift in position of linearly adsorbed COL, which is 2074 cm-1 on Pt-poly and 2060 cm-1 on Pt/C nanoparticles, is well understood, so no further discussion regarding this issue will be provided in this paper; for details the readers are referred to work by Lamy and co-workers3 and Weaver and co-workers.7,25 Having established νCO properties for CO adsorbed on different types of metallic nanoparticle films attached to Au and GC, in the following we present characteristic potential-

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Figure 3. In situ CO oxidation on Pt/C catalyst attached on the glassy carbon in 0.5 M H2SO4 at 293 K. Series of infrared spectra obtained during progressive oxidation of CO: (a) CO2 evolution (the reference spectrum was recorded at 0.01 V), (b) adsorbed CO (the reference spectrum was recorded at 0.9 V), and (c) cyclic voltammetry of the bare glassy carbon substrate (gray curve) and Pt/C catalyst attached on the glassy carbon (black curve) at a sweep rate of 50 mV/s.

dependent infrared spectra for COad at two different temperatures as well as corresponding concomitant development of spectra for CO2 dissolved in solution (the latter being established by monitoring the development of O-C-O stretch of CO2 at 2343 cm-1) from electrochemical oxidation of COad. The following examples are used to demonstrate how the electrolyte temperature affects the attachment of the catalyst to the substrate and the vibrational properties of the CO adsorbed on Pt/C at the different temperatures, which correspond to the real operating condition of fuel cells. Cyclic voltammograms of Pt/C on GC electrodes are well established,2 i.e., as in Figure 3a the Hupd potential region is first followed by the double layer potential region and then by the adsorption of oxygenated species. Cyclic voltammetry of Pt/C on GC, Figure 3a, recorded prior to and/or after infrared measurements was the same, confirming that the nanoclusters are strongly and permanently attached to the substrate. Displayed in Figure 3a is also a series of representative potential dependent IRAS spectra in CO saturated solution at 293 K, referenced to the single-beam spectrum at 1.2 V. In agreement with Pt-poly (see Figure 2d), for the Pt/C-GC system two CO stretching bands are obtained in CO-saturated solution: a major band in the range of 2060-2074 cm-1 and a relatively weak, yet readily discernible, C-O stretch in the range of 1858-1852 cm-1. Further analysis of the COad spectra revealed that COL band positions shift linearly with increasing potential in the positive direction at a rate of dE/dνCO ) 27 cm-1 V-1. The fact that a very similar behavior is obtained for Pt-poly demonstrates the binding site occupancy of COad on metallic nanoparticles can be monitored with the same accuracy as in the case for planar surfaces.21,23 Another important characteristic of this method of attachment is that the Pt/C-GC electrode can be pressed and lifted against the optical window several times without damaging the catalyst layer. Cyclic voltammetry of polycrystalline Au along with Pt/C and PtRu/C nanoparticle layers attached to Au are shown in

Stamenkovic´ et al.

Figure 4. In situ CO oxidation on Pt/C catalyst attached to gold in 0.5 M H2SO4 at 333 K. Series of infrared spectra obtained during progressive oxidation of CO: (a) CO2 evolution (the reference spectrum was recorded at 0.01 V), (b) in situ electrooxidation of adsorbed CO (the reference spectrum was recorded at 0.9 V), and (c) cyclic voltammetry in 0.5 M H2SO4 at 293 K with a sweep rate of 50 mV/s; (solid curve) thin catalyst layer of Pt/C on Au substrate, (dashed curve) thin catalyst layer of PtRu/C on Au substrate, and (gray curve) pure Au substrate.

Figure 4c. These voltammograms were recorded in the hanging meniscus configuration in argon purged 0.5 M H2SO4 at 20 °C. The voltammogram for Au shows a typical double layer potential region (0.05 V < E < 1.0 V) that is followed first by OH adsorption and then by oxide formation. Not surprisingly, the voltammogram for Pt/C attached to Au is almost identical with the one shown in Figure 3 for the glassy carbon substrate. As for the Pt/C-GC system, the data at 333 K are used primarily to confirm that the catalyst layer remains unchanged during experiments, confirming that nanoparticles have excellent physical and electrical contact with the Au substrate. Furthermore, the fact that no new features are obtained in the CV for Pt/C is an important test that Au substrate and Pt clusters did not form an alloy in the process of attachment at elevated temperatures. Corresponding potential-dependent CO adsorption spectra are shown in Figure 4b. In the potential range of 0.08 V < E < 0.65 V two C-O stretching bands are obtained in CO-saturated solution; a major band in the range of 2058-2068 cm-1 corresponds to a-top COL and the C-O stretch around 1842 cm-1 corresponds to COB adsorbed at bridging sites. The analysis of these spectra (not shown) revealed that the COL band position shifts linearly with increasing the potential in the positive direction at a rate of dE/dνCO ) 22 cm-1 V-1, which is slightly different from the rate of shift at 293 K (either GC or Au substrates). The production of CO2 in Figure 4c and thus the oxidation kinetic of CO is clearly enhanced at the elevated temperature. As a result, a positive Stark tuning slope is evidenced only until 0.55 V (notice in Figure 3b the positive dE/dνCO is obtained up to 0.65 V) and at more positive potentials the electrochemical Stark tuning slope is compensated by loss of dipole coupling due to the onset of COad oxidation and thus the decrease of the COad coverage. Details related to the temperature effects on CO oxidation on Pt/C and other Pt bimetallic nanoparticles will be the subject of our future reports.

Anchoring Nanoparticles onto Reflective Substrates

Figure 5. In situ CO oxidation on various bulk materials and corresponding nanoparticle catalysts attached on gold in 0.5 M H2SO4 at 293 K: (a) CO2 evolution obtained after potential hold at 0.1 V after 7 min (the reference spectra were recorded at 0.01 V); (b) COad on Pt-poly, Pt3Ru, and Pt3Mo; and (c) COad on Pt/C, Pt2Ru3/C, and Pt3Mo/C nanoparticles (the reference spectra were recorded at 0.9 V).

Finally, to demonstrate that TID is indeed a convenient method to study vibrational and kinetic properties of CO on different Pt-bimetallic nanoparticle catalysts, we summarize in Figure 5 representative sets of CO stretching frequencies along with corresponding CO2 production for Pt, Pt3Ru2, and Pt3Mo. For comparison, the relevant IRAS results for planar electrodes with the same/similar composition of the alloying component are also included in Figure 5. The right-hand side of the triptych in Figure 5 displays typical CO spectra on Pt and Pt-bimetallic nanoparticles attached to Au. The central part of the figure corresponds to νCO frequencies on planar (bulk) electrode surfaces. The fact that very similar spectra for a CO adlayer on nanoparticles and planar electrodes are obtained in separate measurements is the best confirmation that the attached nanoparticle films are indeed of good quality. Another important point is the ability to follow the appearance of the CO2 band at low potentials where CO oxidation takes place at very low rate and cannot be detected in electrochemical measurements.23 Comparison between the three different catalysts in Figure 5c reveals that CO2 production is the most extensive on the Pt3Mo catalysts followed by Pt-Ru and finally on Pt no detectable CO2 band is observed at 0.1 V. This suggests that Pt-Mo is the best CO oxidation catalyst, a fact that is consistent with our previous electrochemical results.26,27 Complete studies regarding the relationship between the onset of CO oxidation and CO-tolerance for different bimetallic nanoparticles as well as a study of particle size effect and dependence of COad stretching frequencies vs ΘCO coverage on nanoparticles will be presented in our follow-up reports. 4. Summary A new method for anchoring metallic nanoparticles onto smooth substrates has been developed. It has been demonstrated that an ultrathin layer of carbon-supported Pt-based catalyst can be deposited onto mirror-polished Au and/or GC surfaces by implementing a temperature-enforced procedure. This method provides excellent reflective properties of the sample probe and

J. Phys. Chem. B, Vol. 108, No. 46, 2004 17919 the possibility to examine CO-nanoparticle interaction by IRAS on the molecular level. It also enables complete control of the commonly observed anomalous bipolar COad band obtained from nanoparticle aggregates. The same ultrathin layers can be used for characterization of catalytic/CO tolerance properties of nanoparticle electrocatalysts, either by monitoring the production of CO2 in IRAS measurements or by classical electrochemical methods. Reflective infrared spectra obtained for molecular species adsorbed on metallic nanoparticles were compared to spectra originating from corresponding planar electrodes. If Au-poly is used as a substrate, then a markedly asymmetrical CO band, which exhibits peak intensities of almost 4%, is found in IRAS measurement. By contrast, if GC is used as a substrate a symmetrical CO band is observed for the same catalysts, indicating that the tailing of the νCO band toward lower wavenumbers is an influence of the Au-poly substrate on the vibrational properties of COad and not an inherent property of the CO adlayer adsorbed on nanoparticle catalysts. Finally, if GC is used as a substrate the signal-to-noise ratio is higher (due to the inferior reflective properties of glassy carbon) than that for the same nanoparticle films attached to Au. The COad band intensities are, however, very close (ca. 1%) to corresponding intensities obtained from a planar Pt electrode, so that the analysis of those spectra is as convenient as that for the PtCO system. Combined with an upgraded spectroelectrochemical cell designed for measurements at elevated temperatures, the developed TID method enables a fast and accurate characterization of a large number of different nanomaterials under the simulated fuel cell’s operating conditions. Acknowledgment. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences, U.S. Department of Energy under Contract No. DE-AC03-76SF00098. M.A. acknowledges the German Humboldt Foundation for a Feodor-Lynen Scholarship. References and Notes (1) Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Savinova, E., Vayenas, C., Eds.; Marcel Dekker: New York, 2003. (2) Schmidt, T. J.; Gasteiger, H. A.; Sta¨b, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 2354-2358. (3) Rice, C.; Tong, Y. Y.; Oldfield, E.; Wieckowski, A.; Hahn, F.; Gloaguen, F.; Leger, J. M.; Lamy, C. J. Phys. Chem. B 2000, 104, 58035807. (4) Friedrich, K. A.; Henglein, A.; Stimming, U.; Unkauf, W. Colloids Surf. A 1998, 134, 193-206. (5) Park, S.; Tong, Y.; Wieckowski, A.; Weaver, M. J. Electrochem. Commun. 2001, 3, 509-513. (6) Park, S.; Wieckowski, A.; Weaver, M. J. J. Am. Chem. Soc. 2003, 125, 2282-2290. (7) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 9719-9725. (8) Park, S.; Tong, Y. T.; Wieckowski, A.; Weaver, M. J. Langmuir 2002, 18, 3233-3240. (9) Park, S.; Weaver, M. J. J. Phys. Chem. B 2002, 106, 8667-8670. (10) Park, S.; Yang, P. X.; Corredor, P.; Weaver, M. J. J. Am. Chem. Soc. 2002, 124, 2428-2429. (11) Vijayaraghavan, G.; Gao, L.; Korzeniewski, C. Langmuir 2003, 19, 2333-2337. (12) Gao, L.; Huang, H. L.; Korzeniewski, C. Electrochim. Acta 2004, 49, 1281-1287. (13) Kardash, D.; Huang, J. M.; Korzeniewski, C. J. Electroanal. Chem. 1999, 476, 95-100. (14) Lin, W. F.; Christensen, P. A.; Hamnett, A. J. Phys. Chem. B 2000, 104, 12002-12011. (15) Batista, E. A.; Hoster, H.; Iwasita, T. J. Electroanal. Chem. 2003, 554, 265-271. (16) Bjerke, A. E.; Griffits, P. R.; Theiss, W. Anal. Chem. 1999, 71, 1967-1974. (17) Markovic, N. M.; Adzic, R. R.; Cahan, B. D.; Yeager, E. J. Electroanal. Chem. 1994, 377, 249-259.

17920 J. Phys. Chem. B, Vol. 108, No. 46, 2004 (18) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271-340. (19) Markovic, N. M.; Grgur, B. N.; Ross, P. N., Jr. J. Phys. Chem. B 1997, 101, 5405-5413. (20) Sun, S.-G. Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Savinova, E., Vayenas, C., Eds.; Marcel Dekker: New York, 2003; pp 785-826. (21) Markovic, N. M.; Lucas, C. A.; Rodes, A.; Stamenkovic, V.; Ross, P. N. Surf. Sci. Lett. 2002, 499, L149-L158. (22) Blizanac, B. B.; Arenz, M.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2004, 126, 10130-10141.

Stamenkovic´ et al. (23) Stamenkovic, V.; Arenz, M.; Lucas, C.; Gallagher, M.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2003, 125, 2736-2745. (24) Stamenkovic, V.; Arenz, M.; Blizanac, B. B.; Ross, P. N.; Markovic, N. M. J. New Mater. Electrochem. Syst. 2004, 7, 125-132. (25) Park, S.; Wasileski, S. A.; Weaver, M. J. Electrochim. Acta 2002, 47, 3611-3620. (26) Grgur, B. N.; Zhuang, G.; Markovic, N. M.; Ross, P. N., Jr. J. Phys. Chem. B 1997, 101, 3910-3913. (27) Grgur, B. N.; Markovic, N. M.; Ross, P. N., Jr. J. Electrochem. Soc. 1999, 146, 1613-1619.