pubs.acs.org/Langmuir © 2009 American Chemical Society
Electrodeposition of Multilayered Bimetallic Nanoclusters of Ruthenium and Platinum via Surface-Limited Redox-Replacement Reactions for Electrocatalytic Applications Tumaini S. Mkwizu,*,†,‡ Mkhulu K. Mathe,‡ and Ignacy Cukrowski*,† ‡
† Department of Chemistry, University of Pretoria, NW-1 Building, Pretoria 0002, South Africa, and Energy and Processes Competence Area, Materials Science and Manufacturing Unit, Council for Scientific and Industrial Research (CSIR), P.O. Box 395, Pretoria 0001, South Africa
Received June 19, 2009. Revised Manuscript Received September 2, 2009 An electrochemical synthesis of multilayered bimetallic Ru|Pt nanoclusters, supported on glassy carbon, is reported for the first time. The novel nanoclusters were synthesized via surface-limited redox-replacement reactions involving sacrificial Cu, deposited prior to the formation of each individual noble metal layer, in a sequential fashion. It has been shown that the Cu adlayers control the morphology and electrochemical properties of the resultant nanostructures. Sequentially deposited Ru|Pt nanoclusters exhibited superior electrocatalytic activity (when compared to equivalent monometallic Pt and an alloy-type codeposited Pt-Ru nanostructures) with respect to methanol electrooxidation in an acidic medium. Moreover, it has been established that the electrochemical process taking place at the Ru|Pt nanoclusters followed the bifunctional mechanism. Electrokinetic studies of the oxygen reduction reaction (ORR) were also performed. Analysis of hydrodynamic linear sweep voltammetric experiments, performed at various flow rates on oxygen-saturated acidic medium, revealed that the Pt and Ru|Pt nanoclusters exhibited direct four- and two-electron ORR pathways, respectively. A specially designed electrochemical flow-cell was used for automated sequential electrodeposition of the multilayered nanoclusters of predefined composition and electrochemical and electrocatalytic investigations.
1. Introduction Metallic nanoparticles are known to exhibit unique advantageous properties over macro (or bulk) materials when used in electrochemical applications, such as enhancement of mass transport, catalysis, high effective surface area, and control over electrode microenvironment.1 Multimetallic nanoclusters (clusterlike nanoparticles or nanoalloys) are extensively studied due to their widespread applications in electronics, material science, and catalysis.2 In particular, bimetallic catalytic nanoparticles have been a vigorous area of research, as they have potential applications in direct alcohol fuel cells for portable electronic devices and automobiles due to the much higher energy density of liquid alcohols than that of gaseous fuels, such as hydrogen.3-6 Surface-catalyzed reactions are extremely sensitive to the atomic-level details of the catalytic surface (the arrangement of the exposed atoms, or active sites for catalysis, determines the performance of the catalyst7); hence synthesis of nanoparticles possessing predefined electrocatalytic properties by tuning their morphology and composition is of interest. Bimetallic catalysts are known to enhance alcohol as well as hydrogen *Corresponding author. Telephone: þ27-12-4202934 (T.S.M.); þ27-124203988 (I.C.). Fax: þ27-12-4204687 (T.S.M.); þ27-12-4204687(I.C.). E-mail:
[email protected] (T.S.M.);
[email protected] (I.C.). (1) Welch, C. M.; Compton, R. G. Anal. Bioanal. Chem. 2006, 384, 601. (2) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. Rev. 2008, 108, 845. (3) Watanabe, M. In Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Savinova, E. R., Vayenas, C. G., Eds.; CRC Press: Boca Raton, 2003; p 827. (4) Mathiyarasu, J.; Remona, A. M.; Mani, A.; Phani, K. L. N.; Yegnaraman, V. J. Solid State Electrochem. 2004, 8, 968. (5) Koper, M. T. M. Surf. Sci. 2004, 548, 1. (6) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 242. (7) Somorjai, A. G. Introduction to surface chemistry and catalysis; Wiley: New York, 1994; Chapter 6.
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oxidation at fuel cell anodes in fuel cells,8-10 and some binary systems have enhanced activity compared to pure platinum for oxygen reduction.3,11 It has been noted that the ratio of Pt to Ru has significant effects on the performance of the Pt-Ru alloy-type catalysts for methanol oxidation.12-14 Therefore, there is a strong interest in bimetallic nanostructured systems offering highly dispersed and maximized atomic contacts of constituent elements, and consequent catalytically large and electrochemically active surface areas that promote both bifunctional and electronic effects.15 Electrodeposition of nanostructured alloys by conventional methods is typically performed by codeposition of metals of interest from a common electrolytic bath. The deposition potential, current density, deposition time, and/or precursor concentrations have to be optimized simultaneously (and often compromised) to deposit the desired multimetallic system. If, for instance, Pt and Ru are used, then the resultant composition of the nanostructure formed, denoted here as Ptx-Ruy, to indicate that the two metals were codeposited in the form of an alloy with an average atomic ratio of Pt to Ru being x:y. Much more refined control of deposits can, in principle, be achieved by sequential electrodeposition of nanostructured and multielement catalysts, but this still remains a challenge. (8) Vigier, F.; Rousseau, S.; Coutanceau, C.; Leger, J.-M.; Lamy, C. Top. Catal. 2006, 40, 111. (9) Goddard, W., III; Merinov, B.; Van Duin, A.; Jacob, T.; Blanco, M.; Molinero, V.; Jang, S. S.; Jang, Y. H. Mol. Simul. 2006, 32, 251. (10) Antolini, E. J. Power Sources 2007, 170, 1. (11) Toda, T.; Igarashi, H.; Watanabe, M. J. Electroanal. Chem. 1999, 460, 258. (12) Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522. (13) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 2654. (14) Petrii, O. A. J. Solid State Electrochem. 2008, 12, 609. (15) Roth, C.; Benker, N.; Theissmann, R.; Nichols, R. J.; Schiffrin, D. J. Langmuir 2008, 24, 2191.
Published on Web 10/01/2009
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Figure 1. Schematic diagram of the instrumental setup developed for automated electrodeposition and electrochemical characterization using a flow-cell.
Electrodeposition by using surface-limited redox-replacement (SLRR) reactions has been used by Brankovic et al.,16 who demonstrated formation of nanoclusters of single noble metals, such as Pt, Pd, and Ag, on Au(111) substrate using Cu sacrificial adlayers as a template for the deposition process. Recently, formation of mixed and alloy-type Pt-M nanoclusters (M = Au, Pd, Ir, Rh, Re, Ru, or Os) on Pd(111) single-crystal and carbon-supported Pd nanoparticles has been reported for potential use in oxygen reduction.17 Typically, this involved generation of a Cu adlayer by underpotential deposition (UPD)18 followed by codeposition of the two noble metals (as Pt-M mixture) from a common solution by the SLRR reaction. Formation of monometallic nanofilms of either Pt or Ru on Au(111) substrate was achieved via repeated SLRR cycles in a methodology called electrochemical atomic layer deposition (EC-ALD) that utilized underpotential-deposited adlayers of either Cu or Pb.19,20 Since the smallest changes in the metallic nanoparticle structure and composition might have a decisive effect on the particle’s catalytic properties, Pt-based bimetallic systems, denoted here as n(M|Pt), to indicate n adlayers of Pt and adlayers of another noble metal M, sequentially plated over each other n times (instead of alloy-type Ptx-My deposits), might be desirable electrocatalysts with unique properties. Interestingly, to the best of our knowledge, electrodeposition of multilayered n(Ru|Pt) nanoparticles (via SLRR reactions, individually implemented for deposition of each noble metal) has not been reported so far. In comparison to codeposition, wider degrees of freedom to tune the morphology and hence electrocatalytic properties should be realized. (16) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173. (17) Vukmirovic, M. B.; Zhang, J.; Sasaki, K; Nilekar, A. U.; Uribe, F.; Mavrikakis, M.; Adzic, R. R. Electrochim. Acta 2007, 52, 2257. (18) Herrero, E.; Buller, L. J.; Abruna, H. D. Chem. Rev. 2001, 101, 1897. (19) Kim, Y.-G.; Kim, J. Y.; Vairavapandian, D.; Stickney, J. L. J. Phys. Chem. B 2006, 110, 17998. (20) Thambidurai, C.; Kim, Y.-G.; Stickney, J. L. Electrochim. Acta 2008, 53, 6157.
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The aim of this study is the sequential electrodeposition of nanoclusters via extension of the SLRR reaction methodology involving sacrificial Cu adlayers in deposition of each noble metal. In particular, we have (i) examined the efficiency of the SLRR-based methodology in the multilayered mode as well as the role the sacrificial Cu plays, (ii) investigated the electrochemical and electrocatalytic properties of generated nanostructures, and (iii) compared properties of the n(Ru|Pt) nanoclusters with nanoclusters synthesized via sequential codeposition, denoted as n(Pt-Ru), and monometallic Pt nanoclusters, denoted as n(Pt), all generated under the same experimental conditions and with the use of the same electrochemical flow-cell. The various types of nanostructures obtained were probed for their electrocatalytic activity for methanol oxidation as well as oxygen reduction reactions, both in acidic media. n(Ru|Pt)
2. Experimental Section 2.1. Reagents. All chemicals were of analytical grade (>99% purity). All solutions were prepared with high-purity water obtained with a Milli-Q water purifier system (Millipore Inc.) with a resistivity of 18 MΩ 3 cm. Precursor solutions of Cu2þ, Pt4þ, and Ru3þ (each 1 mM, pH = 1 ( 0.05) were prepared from CuSO4 3 6H2O (Merck), H2PtCl6 3 6H2O, and RuCl3 3 3H2O (SA Precious Metal Ltd., South Africa), respectively, in 0.1 M HClO4 (Merck). CH3OH (Merck) 0.5 M solution was also prepared in 0.1 M HClO4. 2.2. Instrumentation. An Autolab potentiostat PGSTAT30 (equipped with GPES 4.9.007 and Frequency Response Analyzer (FRA) 4.9.007 software, EcoChemie BV, The Netherlands) was used for cyclic voltammetric (CV), hydrodynamic linear sweep voltammetric (HLSV), chronoamperometric (CA), anodic stripping voltammetric (ASV), and electrochemical impedance spectroscopic (EIS) measurements. The dedicated instrumental setup (Figure 1) for automated electrodeposition and electrochemical characterization consisted of (i) a specially designed and custombuilt three-electrode electrochemical flow-cell, all electrochemical DOI: 10.1021/la902219t
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Article experiments reported here were performed in this cell; (ii) four Dosimats (model 765, Metrohm, Switzerland) configured as piston pumps for delivery of appropriate electrolyte solutions; (iii) a five-way rotatable multivalve (model R36781 valve head and model R77810 valve drive, Hamilton, Switzerland); (iv) the Autolab potentiostat; and (v) a personal computer. Polytetrafluoroethylene (PTFE) tubing and fittings were used for delivery of reagents. All experiments in this work were performed at room temperature (ca. 25 °C). Automated experiments for the formation of well-defined nanostructures were performed utilizing custom-developed LabVIEW (National Instruments, TX) programs (virtual instruments). The pumps, the five-way valve, and the potentiostat were all computer-controlled via standard RS-232 and USB interfaces. Four solutions could be independently delivered to the flow-cell via the outlet channel of the five-way valve. Ultrahigh purity nitrogen gas was used to saturate the solutions prior to measurements. A vacuum pump was used to degas sample solutions prior to saturation with nitrogen. N2 atmosphere was maintained throughout the experiments. The flow-cell used (Figure 1) had a 120 μL flow-channel (defined by a 1 mm thick silicone rubber gasket), and it consisted of two Perspex blocks for holding of (i) the reference and counter electrodes, both in the upper block, and (ii) the working electrode. The Ag/AgCl/3 M KCl reference electrode (model 6.0727.000, Metrohm) was placed at the outlet channel of the cell, against which all potentials are reported, unless otherwise stated. The working electrode (substrate under study) was a glassy carbon (GC) rod (grade V-25, SPI Supplies). The GC rod was embedded in a brass holder that was secured via fine threading into the Perspex block exposing a disk with a geometric area of 0.2 cm2. The counter electrode was a machined stainless steel rod directly screwed into the Perspex block (exposed geometric area of 0.2 cm2). Potential-pH (E-pH) models for Pt-Cu-H2O, Ru-CuH2O, and Pt-Ru-H2O systems were generated using the thermochemical software package FactSage version 5.5.21 Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), employed to study the morphology and acquisition of chemical identities of nanoclusters, were performed with a JEOL model JSM-5800LV microscope (operated at 10 kV for SEM and 20 kV for EDX measurements). Further morphological and topographical studies using atomic force microscopy (AFM) were carried out with a DI Nanoscope IVa (Digital Instruments, Inc.) instrument in ambient tapping mode using a model RTESPW tip (Veeco Manufacturing, Inc.). 2.3. Automated Synthesis of Nanoclusters. Prior to its use, the GC disk was polished with sand paper (grit 1200), followed by alumina slurries on a polishing cloth, then washed in 0.5 M nitric acid, rinsed in ultrasonic bath in a 1:1 water-acetone mixture, and finally thoroughly rinsed in sonicated deionized water. After assembling the cell, electrochemical cleaning was done in 0.1 M HClO4 by applying CV scans between -0.1 and 1.2 V at 200 mV s-1 until stable cycles were obtained. A dedicated LabVIEW virtual instrument was used to synthesize Ru|Pt nanoclusters, supported on the GC substrate, using SLRR reactions involving sacrificial Cu adlayers (the resultant nanoclusters are denoted as n(Ru|Pt)Cu/GC). Repeatable steps (n deposition cycles) directed either a precursor solution (CuSO4, H2PtCl6, or RuCl3) or the background electrolyte (BE) solution (0.1 M HClO4) to the cell at a set flow rate and preset deposition time, tdep (by controlling the four pumps and the five-way valve, Figure 1). The potential of the working electrode and time of each operation was preprogrammed for each step. A typical single deposition cycle involved (1) rinsing the cell with the BE, followed by filling the cell with Cu2þ solution at selected applied potential Eappl, (2) Cu deposition at Edep, followed by rinsing the cell with (21) Bale, C. W.; Chartrand, P.; Decterov, S. A.; Eriksson, G.; Hack, K.; Ben Mahfoud, R.; Melznc- on, J.; Pelton, A. D.; Petersen, S. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 189.
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Mkwizu et al. the BE at Edep, (3) filling the cell with the Pt4þ solution at open circuit (OC), followed by the OC SLRR reaction of Cu adlayers by Pt adlayers (reaction denoted as OC SLRRPt) at quiescent conditions, (4) rinsing with BE and filling the cell with the Cu2þ solution at Eappl, (5) Cu deposition at Edep, followed by rinsing the cell with the BE at Edep, and (6) rinsing with the Ru3þ solution at OC followed by the OC SLRR reaction of Cu adlayers by Ru (OC SLRRRu) at quiescent conditions. During all these steps, the current and potential that resulted from the deposition processes were sampled as a function of time to generate online currentpotential-time (I-E-t) transients. After completion of n deposition cycles, the BE solution was injected into the cell at OC (a standby stage of the electrochemical system). For comparative studies, additional nanoclusters were synthesized. First, sequentially deposited n monometallic Pt multilayered nanoclusters (denoted as n(Pt)Cu/GC), also generated using SLRR steps involving Cu adlayers, were prepared by repeating steps (1)-(3), as described above. Second, direct spontaneous deposition of Pt and Ru adlayers at OC (without involvement of sacrificial Cu adlayers) was performed. The general procedure, as described above for generation of n(Ru|Pt)Cu/GC, was followed but with the Cu2þ solution replaced with the BE solution at relevant steps. The generated electrode is denoted n(Ru|Pt)/GC. Third, another bimetallic electrode was obtained with the use of sacrificial Cu by SLRR reactions involving codeposition of Pt and Ru (OC SLRRPt-Ru) from a mixed precursor solution (1 mM H2PtCl6 and 1 mM RuCl3 in 0.1 M HClO4). The steps employed were as for the n(Pt)Cu/GC electrode. The resultant electrode is denoted here as n(Pt-Ru)Cu/GC. Lastly, sequential spontaneous codeposition of Pt and Ru was carried out similar to the foregoing electrode in the absence of sacrificial Cu adlayers (the resultant electrode is denoted as n(Pt-Ru)/GC). 2.4. Characterization of Nanoclusters. Electrochemical behavior of synthesized nanoclusters was probed in 0.1 M HClO4 by CV between -0.2 and 1.2 V at 50 mV s-1. Electrochemical surface area (ESA) of the nanostructured electrodes was estimated using CA ran on 1 mM K4Fe(CN)6 (in 0.1 M HClO4) solution under the diffusion-controlled current regime at 0.6 V. The Cottrell equation was used to calculate ESA values using the Fe2þ/Fe3þ couple as redox probe.22 Methanol electro-oxidation studies were carried out in quiescent 0.5 M CH3OH in 0.1 M HClO4 solutions using as-prepared nanoclusters, which were conditioned initially in 0.1 M HClO4 by scanning the potential at 50 mV s-1 from -0.2 to 1.2 V for five cycles. CVs for methanol tests were ran at 50 mV s-1 for five cycles in the potential range from 0 to 1.0 V, followed by EIS measurements in freshly injected CH3OH solution, at a bias potential of 0.4 V, using a sine-wave signal of 10 mV amplitude in the frequency range of 0.5 Hz to 100 kHz. CA measurements were performed at an applied potential of 0.4 V for 300 s. Between electrochemical measurements, the flow-cell was rinsed at OC with the blank electrolyte (0.1 M HClO4). Oxygen reduction reaction (ORR) was studied in ultrahigh purity O2-saturated 0.1 M HClO4 solution (presaturated for 2 h and thereafter continuously saturated with the O2 gas in a dedicated stock bottle). The ORR on as-prepared nanoclusters was studied at hydrodynamic conditions at various flow rates (2-20 mL/min) of the 0.1 M HClO4 solution by recording cathodic linear sweep voltammograms at 50 mV s-1 in the potential range 0.8-0.1 V. A dedicated pump line of the instrumental setup and the same flow-cell were used to probe the asprepared electrocatalysts by performing various measurements. Studies involving electrochemical and electrocatalytic properties were performed with freshly prepared nanoparticles as a benchmark for further stability studies that were out of the scope of the current investigation. Ex situ microscopic investigations were carried out within 24 h of preparation of the nanoparticles. (22) Bard, A.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001; Chapter 5.
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Figure 2. (a) CV curve recorded on 1 mM CuSO4 þ 0.1 M HClO4 with bare GC electrode; (b) ASV curves recorded after electrodeposition of Cu from 1 mM CuSO4 þ 0.1 M HClO4 at -0.05 V; (c-f ) E-t curves recorded during sequential electrodeposition of (c) 8(Ru|Pt)Cu/GC, (d) 8(Ru|Pt)/GC, (e) 8(Pt)Cu/GC, and (f ) 8(Pt-Ru)Cu/GC. See text for parameters used at numbered steps.
3. Results and Discussion 3.1. Thermodynamic Modeling of Pt-Cu-H2O and Ru-Cu-H2O, and Redox-Replacement Considerations. Insight onto the effect of pH and potential on the formation of various Pt, Ru, and Cu species was gained from thermodynamic E-pH models for the Pt-Cu-H2O and Ru-Cu-H2O systems at 25 °C for 1 mM solutions; see Figure S1 in the Supporting Information. The models showed that (i) metallic Pt exists at E < 0.89 V and pH between 0 and 3.5, (ii) metallic Pt and Ru coexist below pH of 1.2 when E < 0.55 V, and (iii) metallic Cu exists in the same pH-range but only at E < 0.25 V (potentials are with respect to standard hydrogen electrode (SHE)). Based on this information, all deposition experiments were performed at pH of 1 ( 0.05. Typical stages in the reductive formation of a metal layer from precursor ions predominantly involve nucleation and subsequent growth of metal nuclei on the surface. The reduction process can either be kinetic- or diffusion-controlled.23 Further insights on the Cu electrodeposition mechanism on the GC substrate were obtained from CV performed on 0.1 M HClO4 solution, as shown in Figure 2a, where no distinct peaks for Cu UPD peaks were recorded. There was some insignificant shoulder observed at potentials between 0 and 0.1 V. This potential range overlaps with (23) Pandey, R. K.; Sahu, S. N.; Chandra, S. Handbook of Semiconductor Electrodeposition; Marcel Dekker: New York, 1996; Chapter 3.
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the bulk (3D) Cu deposition that theoretically starts at E < 0.05 V (The Nernst formal potential for the Cu2þ/Cu(s) couple is 0.046 V (vs Ag/AgCl/3 M KCl) at 1 mM Cu2þ). The shoulder observed in Figure 2a could be indicative of a nucleation step as was demonstrated elsewhere by in situ Cu nucleation mechanistic studies by electrochemical AFM.24 Several attempts to plate Cu at Edep = 0.05 V resulted in no significant deposits at reasonable and acceptable time intervals. Because of that we have opted for the use of relatively small overpotentials in relation to ECu2þ/Cu where deposition of Cu(s) occurs most likely through kinetically controlled 3D nucleation and progressive growth of nanoclusters. Effect of deposition time was probed by ASV at Edep = -0.05 V for tdep ranging from 5 to 150 s. Analysis of variation in total charge density (derived from the integrated ASV curves, Figure 2b) led to the deduction that about 1 monolayer equivalent of Cu nanoclusters was obtained for tdep of about 90 s (the term monolayer equivalent is used here to indicate formation of Cu nanoclusters on the GC substrate’s geometric area equivalent to deposition of a hypothetical monolayer of Cu calculated from a theoretical charge density required for such monolayer deposition25). Studies performed on Cu electrodeposition on GC substrate26 indicate that (24) Grujicic, D.; Batric, P. Electrochim. Acta 2002, 47, 2901. (25) Furuya, N.; Motoo, S. J. Electroanal. Chem. 1979, 98, 189. (26) Zapryanova, T.; Hrussanova, A.; Milchev, A. J. Electroanal. Chem. 2007, 600, 311.
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indeed fast electrocrystallization of well-dispersed Cu nanoclusters occurs at kinetically controlled deposition overpotential as employed in Figure 2b. Assuming the preferential redox-reaction pathways, leading to zero-valent metallic states, as described by eqs 1-4, GC þ Cu2þ þ 2e - f Cu=GC
ð1Þ
2Cu=GC þ PtCl6 2 - f Pt=GC þ 2Cu2þ þ 6Cl -
ð2Þ
Pt=GC þ Cu2þ þ 2e - f Cu=fPt=GCg
ð3Þ
3Cu=fPt=GCg þ 2Ru3þ f 2Ru=fPt=GCg þ 3Cu2þ
ð4Þ
the following electrochemical parameters were selected for each stage employed to generate n(Ru|Pt)Cu/GC (see Figure 2c): (1) rinsing the cell with BE at Eappl = 0.2 V for 20 s, followed by rinsing with the Cu2þ solution at Eappl for 20 s; (2) Cu deposition at Edep = -0.05 V for 90 s, followed by rinsing with BE at Edep for 20 s; (3) rinsing with the Pt4þ solution at OC for 20 s, followed by the OC SLRRPt for 180 s; (4) rinsing, initially with BE, followed by the Cu solution at Eappl for 20 s (for both solutions); (5) Cu deposition at Edep for 90 s, followed by rinsing with BE at Eappl for 20 s; and (6) rinsing with the Ru3þ precursor solution at OC for 20 s, followed by the OC SLRRRu 180s. This protocol was repeated for 1 e n e 8 cycles in order to systematically generate larger bimetallic nanoclusters. In the comparative studies to obtain (i) 8(Ru|Pt)/GC, the same sequence of stages was followed, but the Cu solution was replaced by BE, as depicted by E-t transient shown in Figure 2d, and (ii) E-t transients for 8(Pt)Cu/ GC and 8(Pt-Ru)Cu/GC are shown in Figure 2e,f, where stages 1 and 2 are exactly the same as those employed for the generation of 8(Ru|Pt)Cu/GC. For clarity, only four initial cycles are shown in Figure 2c,d and eight cycles are shown in Figure 2e,f. Variations in OC potential (EOC) during Pt and Ru redoxexchange (stages 3 and 6 in Figure 2c), to form n(Ru|Pt)Cu/GC, clearly indicate that there are two distinctive processes taking place that might be used as signatures for the OC SLRRPt and OC SLRRRu reactions. It was observed that EOC during OC SLRRPt reaches a maximum and constant value of about 0.66 V; variation in EOC (as a maximum value attained) for all eight cycles during the deposition of Pt (as well as Ru) is shown in Figure S2 of the Supporting Information. When the E-t transient of stage 3 in Figure 2c is compared with equilibrium potentials shown in Figure S1 in the Supporting Information, metallic Pt(s) is indeed deposited; for most of the deposition period, EOC is within the potential range where Pt(s) and Cu2þ are thermodynamically predicted to coexist and only when EOC reaches its maximum value in stage 3 a possibility of coexistence of Pt(s), Pt2þ, and Cu2þ might be considered. In the case of Ru deposition, EOC approaches a value of about 0.45 V in the first cycle and gradually decreases, reaching a constant maximum value of about 0.32 V (see Figure S2 in the Supporting Information). The variation in the maximum value of EOC suggests that, initially, besides Ru(s), RuO2(s) might also be formed, but after the fifth cycle the only thermodynamically predicted equilibrium (Figure S1b in the Supporting Information) involves Ru(s) and Cu2þ. The experimentally obtained OC potentials strongly suggest deposition of metallic Pt(s) and Ru(s). A very different reaction mechanism appears to take place when 8(Ru|Pt)/GC was generated; see Figure 2d. From the second 574 DOI: 10.1021/la902219t
cycle onward, there is not a large difference in EOC at stages 3 and 6 where the maximum EOC approached 0.65 and 0.60 V during OC SLRRPt and OC SLRRRu, respectively, but an obvious pattern emerges. This strongly suggests that during stage 3 metallic Pt is formed, but at stage 6 most likely RuO2 is deposited as predicted from thermodynamic E-pH modeling, where the calculated equilibrium potential for the coexistence of RuO2(s) and Cu2þ matches well the experimentally observed EOC. These observations indicate that, during spontaneous deposition of Pt and Ru, without involvement of sacrificial Cu, preferentially RuO2(s) rather than Ru(s) adlayers are generated on metallic Pt. Figure 2e shows that the sequential deposition of Pt(s) took place, to form 8(Pt)Cu/GC, via the same mechanism (OC SLRRPt) as discussed above for plating of Pt in case of 8(Ru|Pt)Cu/GC. Maximum EOC of about 0.48 V, observed at stage 3 in Figure 2f, is well within the potential windows where Pt(s) and RuO2(s) are predicted to coexist with Cu2þ (Figure S1 in the Supporting Information). This observation suggests that during the OC SLRRPt-Ru process RuO2-Pt rather than Pt-Ru deposits were generated at stage 3. An E-pH diagram, computed for the mixture of Pt and Ru (Figure S3 in the Supporting Information), predicts that at pH = 1 metallic forms of these two noble metals exist only at E < 0.55 V vs SHE (0.33 V vs Ag/AgCl; the observed EOC of 0.48 V in Figure 2f is significantly more positive). Interpretation of Ru oxide deposits from analysis of Figure 2 correlates very well with data reported by Wieckowski and Chrzanowski27 for spontaneous Ru deposition on Pt (singlecrystal to polycrystalline) electrodes. In our studies, it appears that, only in the case of the sequential OC SLRR reactions involving sacrificial Cu, and separate solutions of Pt and Ru, these two elements are preferentially deposited in their metallic forms when Ru3þ ions are in contact with metallic Cu. In the case of the common mixture of Pt with the less noble Ru ions, Pt deposits preferentially and the spontaneous deposition of Ru must result in formation of RuO2(s) on Pt(s). However, separating their deposition leads to some kind of leverage in favor of both noble metals (in this regard, the nobility of Pt and Ru being referenced to Cu, the sacrificial element). It is possible that Ru oxides formed during OC deposition stages, as noted above, have been reduced at stage 2 (upon application of Edep to deposit Cu at second and subsequent cycles), but this does not apply to the last deposition cycle where the final Ru layer is formed. Results discussed further strongly indicate that the final layer deposited during the OC SLRR reaction has a decisive impact on electrochemical and electrokinetic activities of generated nanostructures. 3.2. Electrochemical Characterization of the Nanoclusters. The influence of increasing deposition cycles on electrochemical activity is demonstrated in a set of CVs (Figure 3a), involving the n(Ru|Pt)Cu/GC electrode and recorded in N2-saturated 0.1 M HClO4 solution after n = 2, 4, 6, and 8 cycles. The constancy in the reduction peak potential at about 0.45 V (reduction of surface oxides formed during anodic scans between 0.2 and 1.2 V), an abrupt increase in oxidation current at about 1.2 V, and adsorption/desorption peaks of hydrogen (between 0 and -0.2 V) suggest that the morphology of deposits does not change significantly form cycle to cycle but the surface area of the nanostructures increases because recorded currents were progressively larger. Hence, for further comparison, results obtained on electrodes generated after eight cycles were used. CVs obtained on the 8(Pt)Cu/GC and 8(Ru|Pt)Cu/GC electrodes (Figure 3b) confirm that (i) Pt nanostructures were generated in the case of 8(Pt)Cu/GC (well-known electrochemical signature of (27) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 5974.
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Figure 3. Comparison of CV curves recorded in 0.1 M HClO4 at 50 mV s-1 for the as-prepared specified nanoclusters after indicated sequential deposition cycles. In all cases, the fifth CV scan is shown. Initial scan direction is indicated by the broken arrow.
Pt electrode22,28 in acidic medium is observed), (ii) Ru was deposited in the case of 8(Ru|Pt)Cu/GC; a rapid rise in oxidation current between 1 and 1.2 V is observed that is consistent with electrochemical formation of surface RuO2, considered a reversible oxide at E < 1.3 V (vs Ag/AgCl) as noted elsewhere,29 and (iii) electrochemical and catalytic properties of Pt adlayers have not been influenced by deposited Ru adlayers as indicated by almost identical hydrogen adsorption/desorption peaks observed for the two CVs. The influence of Cu sacrificial adlayers is clearly shown in Figure 3c, obtained with electrodes generated from OC SLRR and direct spontaneous sequential deposition of Pt and Ru without sacrificial Cu. Even though both CVs show similarities (confirming the presence of Pt and Ru), the activity of Pt when Cu was not involved appears to be significantly decreased. This might be attributed to (and correlates very well with) different morphologies of deposits obtained, as predicted in the previous section, namely, Pt and Ru in metallic forms for the 8(Ru|Pt)Cu/ GC and metallic Pt with RuO2(s) for the electrode generated without Cu involvement (see also Figure 2d). A similar decrease in the activity of Pt is also observed in the case of codeposited noble metals (Figure 3d) where the formation of metallic Pt and RuO2(s) was also predicted from E-t transients (see Figure 2f ). At the same time, a large anodic current starting at about 1.2 V confirms that ruthenium was indeed codeposited with metallic Pt, supporting observations related to the relevant E-t transients. 3.3. Microscopic Characterization. Figure 4 depicts SEM images and corresponding EDX spectra for 8(Pt)Cu/GC, 8(Ru| Pt)Cu/GC, and 8(Pt-Ru)Cu/GC. Reasonably uniform coverage was obtained after eight deposition cycles with the GC surface providing electrochemically active sites for preferential growth of the nanoclusters. Somewhat different in size nanostructures were attained for the 8(Pt)Cu/GC and 8(Ru|Pt)Cu/GC electrodes (Figure 4, parts a and b), with the latter one showing smaller number and larger in size particles.
Since the only difference in generating the two electrodes was the use of one or two precursor solutions of the same analytical concentrations and they all were used in eight deposition operations of 180 s, the deposited stack of (Ru|Pt) layers (the overall deposition time was 8 2 180 s) resulted in larger particles when compared with the stack of Pt layers generated in half of that time, 8 180 s. It is also possible that with the prolonged deposition time the adjacent small particles underwent conglomeration and hence the smaller number of nanoparticles is observed in case of the 8(Ru|Pt)Cu/GC electrode. The most striking difference in morphology, however, is observed between the deposits seen in Figure 4a,b and those in Figure 4c. The observed difference correlates well with the fact that metallic Pt(s) and (Ru|Pt)(s) deposits were formed when single metal ion solutions were used, but a mixture of RuO2(s) and Pt(s) was generated in case of the codeposition process. It appears that the formation of RuO2(s) during the SLRR codeposition process has a profound influence on the deposit morphology: large nanostructures are not present, but rather highly dispersed and uniformly distributed layers of RuO2(s) and Pt(s) were obtained as seen in Figure 4c. Interestingly, similar morphology was obtained when a spontaneous codeposition without the use of sacrificial Cu (to obtain the n(Pt-Ru)/GC electrode) took place (Figure S4 in the Supporting Information); the analysis of the corresponding EOC (not shown) also indicated the formation of RuO2. The relevant EDX spectra (Figure 4) of the 8(Pt)Cu/GC, 8(Ru|Pt)Cu/GC, and 8(Pt-Ru)Cu/GC electrode surfaces qualitatively confirmed (i) the presence of Pt and Ru (where expected) and (ii) quantitative (or close to) replacement of Cu by Pt as well as Ru during the OC SLRR reactions, as EDX showed no detectable traces of Cu. Relevant literature suggests difficulty of quantitative deductions of nanoscale Pt and Ru deposits on carbonic substrates from EDX examination.30,31 Efforts are being pursued for further ex situ characterization studies by
(28) Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327, 353. (29) Hadzi-Jordanov, S.; Angerstein-Kozlowska, H.; Vukovic, M.; Conway, B. E. J. Electrochem. Soc. 1978, 125, 1471.
(30) Richarz, F.; Wolmann, B.; Vogel, U.; Hoffschulz, H.; Wandely, K. Surf. Sci. 1995, 335, 361. (31) Zheng, M.-S.; Sun, S.-G.; Chen, S.-P. J. Appl. Electrochem. 2001, 31, 749.
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Figure 4. SEM micrographs of nanoparticles obtained after eight sequential deposition cycles and corresponding EDX spectra (insets) of (a) Pt, (b) Ru|Pt, and (c) codeposited Pt-Ru, all obtained with SLRR cycles involving Cu.
surface-responsive techniques, such as X-ray photoelectron spectroscopy and/or Auger electron spectroscopy. Also a quantitative analysis of total Pt and Ru loadings by a sensitive analytical technique, such as inductively coupled plasma spectroscopy, might be required in this regard to provide insights on stoichiometric composition of the nanoclusters studied. Additional and important information on the morphology and topography was obtained from the AFM images obtained for the 8(Ru|Pt)Cu/GC electrode, that was of main interest in this work (see Figure 5). The AFM topographic image shows individual as well as densely packed nanoparticles (small needlelike islands), and this most likely can be attributed to the random distribution of electroactive centers on the GC substrate on which the growth of the nanoparticles preferentially occurs. Cross-sectional analysis of the AFM images showed that many of the particles attained the height of about 10-50 nm. Interestingly, the higher the nanostructure, the larger its diameter, most likely indicating that even though the vertical growth might have been preferential (many free-standing nanoclusters were observed), 3D growth could have taken place too with consecutive deposition cycles, resulting in a some coalesced neighboring nanoclusters. 3.4. Methanol Oxidation Studies. Representative CVs recorded on 0.5 M CH3OH þ 0.1 M HClO4 solution for three 576 DOI: 10.1021/la902219t
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electrocatalysts, 8(Pt)Cu/GC, 8(Ru|Pt)Cu/GC, and 8(Pt-Ru)Cu/ GC, are shown in Figure 6a. The onset potentials on the CVs for the monometallic and bimetallic systems, synthesized with the same deposition parameters, clearly show a shift from about 0.45 V for the 8(Pt)Cu/GC electrode to about 0.40 V for the 8(Ru| Pt)Cu/GC electrode. Also, a significant increase in current density of a methanol oxidation reaction (MOR) is observed for the bimetallic system (Ru|Pt) (trace (i), Figure 6a). However, the forward and reverse peak potentials appear to be very much the same for electrocatalysts (i) and (iii), suggesting that the overall reaction mechanism of MOR is the same with largely increased catalytic activity in case of Ru|Pt nanoclusters. Such observations are in line with the bifunctional mechanism12,32 of methanol oxidation involving promotional effects induced on Pt(s) by Ru(s); the applicable and generally accepted reaction mechanism is provided in the Supporting Information. Very different morphology and composition of the codeposited Pt-Ru electrode (trace (ii), Figure 6a), where RuO2(s) was formed, resulted in somewhat decreased activity toward the MOR when compared with monometallic Pt nanoclusters (trace (iii), Figure 6a) and largely decreased activity when compared with bimetallic Ru|Pt deposits (trace (i), Figure 6a); this correlates well with literature reports.14 Both forward and reverse peaks are still present, most likely suggesting that also in this case the overall mechanism of MOR remains the same. Figure 6b shows representative EIS results, in Nyquist format, from studies involving the same set of electrodes and medium as used in CV (Figure 6a). A semicircular spectrum was obtained for the 8(Ru|Pt)Cu/GC electrode, whereas near-semicircular spectra were recorded for the 8(Pt)Cu/GC and 8(Pt-Ru)Cu/GC electrodes, indicative of significantly different electrokinetic activity among them, with bimetallic n(Ru|Pt) nanostructures being most active. An equivalent circuit model (inset in Figure 6b) is similar to literature models used for probing electrocatalysis in MOR by EIS.33 Modeling of the impedance response was achieved by fitting the circuit consisting of Rs representing the solution resistance, with CPE being a constant-phase element associated with the double layer capacitance and Rct standing for the overall charge-transfer resistance associated with MOR. Table 1 summarizes the parameters extracted from the analysis of the EIS results. The consistency in the Rs values (8.0 Ω ( 0.6) reflects the fact that the electrochemical cell configuration as well as electrolyte used were the same for all systems investigated. Also, an acceptably small value of χ2 = 0.1 (a statistical parameter related to the overall fit) gave some sort of assurance where the reliability of fitted parameters is concerned. The observed variation in the CPE might be attributed to the morphology of the deposits and size of the nanoparticles. It appears that for thinner layers the CPE parameter is somewhat smaller, but at the same time the empirical constant R (extracted from EIS data and used to calculate CPE) does not very much (on average it was 0.82 ( 0.04). The RCt parameter correlates very well with results obtained from the CV experiments; it is smallest for the 8(Ru| Pt)Cu/GC electrode for which largest methanol oxidation currents were recorded, and largest for the electrode generated by the codeposition process. It is interesting to note from Table 1 that with an increase in the number of cycles also an increase in the electrocatalytic activity for MOR is observed for the n(Ru|Pt)Cu/ GC electrode that correlates well with EIS results presented in Figure 6c. (32) Maillard, F.; Lu, G.-Q.; Wieckowski, A.; Stimming, U. J. Phys. Chem. B 2005, 109, 16230. (33) Ocampo, A. L.; Miranda-Hernandez, M.; Morgado, J.; Montoya, J. A.; Sebastian, P. J. J. Power Sources 2006, 160, 915.
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Figure 5. AFM images for the Ru|Pt nanoclusters obtained after eight deposition cycles with OC SLRR involving Cu: (a) Ru|Pt nanoclusters’ morphology on the GC substrate and (b) 3D topographic image of the same area seen in (a).
Figure 6. (a) CV curves recorded on 0.5 M CH3OH þ 0.1 M HClO4 (forward scans are solid lines, and reverse scans are dotted lines) for (i) 8(Ru|Pt)Cu/GC, (ii) 8(Pt-Ru)Cu/GC, and (iii) 8(Pt)Cu/GC at scan rate of 50 mV s-1 (starting potential is indicated with broken arrow, fifth scan is shown). (b) EIS plots (solution as in (a)) of (i) 8(Ru|Pt)Cu/GC, (ii) 8(Pt-Ru)Cu/GC, and (iii) 8(Pt)Cu/GC; inset shows the equivalent circuit used in curve fitting (solid lines) of the impedance responses. (c) Comparison of EIS responses of n(Ru|Pt)Cu/GC after n = 4 and 8 deposition cycles. (d) CA responses of (i) 8(Ru|Pt)Cu/GC, (ii) 8(Pt-Ru)Cu/GC, and (iii) 8(Pt)Cu/GC, recorded at 0.4 V for 300 s. Current density ( j) values in (a) and (d) are with respect to ESA.
CA was also employed to investigate the performance of the electrocatalysts; the potential was stepped from open circuit to 0.4 V and held constant for 300 s (Figure 6d). Results obtained correlate very well with CV and EIS data; the same order of activity, 8(Ru| Pt)Cu/GC > 8(Pt)Cu/GC > 8(Pt-Ru)Cu/GC, is observed. 3.5. Oxygen Reduction Studies. The oxygen reduction reaction (ORR) is the topic of widespread investigations, both at the fundamental and applied levels in fuel cells and electrochemical generation of hydrogen peroxide, among other applications.34,35 The (34) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (35) Alcaide, F.; Cabot, P.-L.; Brillas, E. J. Power Sources 2006, 153, 47.
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ORR is a complex process that involves two parallel reaction pathways in acidic medium. A direct four-electron reaction in which O2 is reduced to H2O is represented by eq 5 ð5Þ O2 þ 4Hþ þ 4e - f 2H2 O and the two-electron pathway forming H2O2 shown in eq 6: O2 þ 2Hþ þ 2e - f H2 O2
ð6Þ
The Pt, Ru|Pt, and Pt-Ru nanoclusters were tested for their activity toward the ORR in the O2-saturated 0.1 M HClO4 solution in separate experiments that used as-prepared nanoclusters (the synthetic procedures reported in this work proved to be reproducible). Figure 7a shows typical HLSV curves recorded for DOI: 10.1021/la902219t
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Table 1. Representative Electrochemical Parameters Obtained from EIS and HLSV Measurements for Methanol Oxidation Reaction (MOR) in 0.5 M CH3OH þ 0.1 M HClO4 and Oxygen Reduction Reaction (ORR) in 0.1 M HClO4 of Platinum-Based Electrodes Investigated in This Work MOR (EIS)
ORR (HLSV)
deposition cycles
Rs/Ω
Rct/Ω
CPE/F
R
8(Pt)Cu/GC
8
7.91 (1.40%)c
11.9 103 (3.07%)c
5.47 10-5 (2.04%)c
0.84 (0.42%)c
3.1a 3.9b
1.3 10-2
4(Ru|Pt)Cu/GC
4
8.51 (5.01%)c
7.03 103 (5.63%)c
2.1 10-6 (9.48%)c
0.82 (1.38%)c
-
-
8(Ru|Pt)Cu/GC
8
7.39 (2.66%)c
3.62 103 (3.37%)c
4.56 10-5 (4.61%)c
0.86 (0.86%)c
1.6a 2.1b
2.3 10-2
electrode material
no. of electrons
kc (cm/s)
8.62 15 103 1.9 10-6 0.78 1.2a (2.17%)c (4.04%)c (3.45%)c (0.86%)c 2.0b 5.1 10-2 a b c Analysis performed at E = 0.502 V, using electrochemical surface area. Analysis performed at E = 0.502 V, using geometric area. Relative errors of computed values. 8(Pt-Ru)Cu/GC
8
Figure 7. (a) HLSV curves recorded on 0.1 M HClO4 with the 8(Pt)Cu/GC electrode, scan rate 50 mV s-1. (b) Comparison of HLSV curves for (i) 8(Ru|Pt)Cu/GC, (ii) 8(Pt)Cu/GC, and (iii) 8(Pt-Ru)Cu/GC. (c) Plots of 1/i versus 1/ν -1/3 generated from the HLSV curves on (i) 8(Ru|Pt)Cu/ GC, (ii) 8(Pt)Cu/GC, and (iii) 8(Pt-Ru)Cu/GC electrocatalysts.
the 8(Pt)Cu/GC electrode (equivalent HLSV data obtained for the 8(Ru|Pt)Cu/GC and 8(Pt-Ru)Cu/GC electrodes are shown in Figures S5 and S6 in the Supporting Information). Examples of superimposed HLSV curves recorded on all the electrodes at the flow rate of 10 mL/min are shown in Figure 7b. Analysis of HLSV curves36 to extract kinetic parameters related to ORR involved the following expressions 1 1 1 ¼ þ i ik nAFZO2 CO2 ν1=3
ð7Þ
ZO2 ¼ 0:925DO2 2=3 ðhx1 Þ -1=3
ð8Þ
ik ¼ nAFkc CO2
ð9Þ
-
(36) Heller-Ling, N.; Poillerat, G.; Koenig, J. F.; Gautier, J. L.; Chartier, P. Electrochim. Acta 1994, 39, 1669.
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that are applied to the mixed kinetic and diffusion controlled potential region. Kinetic currents were computed using eqs 7 and 8, where i is the measured current at the applied potential E, ν is the linear velocity (m s-1) related to the volume flow rate by ν = νf/hd, νf is the volume flow rate (m3 s-1), h is the half-height of the flow-channel (0.5 10-3 m), d is the width of the flow channel (6 10-3 m), x1 is the diameter of the disk electrode in the direction along the flow, DO2 is the diffusion coefficient of O2 (1.93 10-5 cm2 s-1),37 CO2 is the saturation concentration of O2 (1.22 10-6 mol cm-3),33 n is the number of electrons transferred, kc is the rate constant of the overall reaction, and A is the surface area of the electrode. The number of electrons was evaluated from eq 7 by plotting 1/i versus 1/ν1/3 (Figure 7c), and the data obtained using electrochemical as well as geometrical surface areas are presented in Table 1. From the slopes seen in Figure 7c, it is evident that similar electrochemical processes took place when Pt and Ru were deposited as opposed to monometallic Pt deposits. (37) Adzic, R. R.; Wang, J.; Ocko, B. M. Electrochim. Acta 1995, 40, 83.
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Figure 8. Schematic of the electrochemical synthetic protocols used to obtain bimetallic Ru|Pt nanoclusters via SLRR reactions and the proposed resultant active sites that lead to their enhanced bifunctional electrocatalytic activity.
The number of electrons, very close to the theoretically predicted value of 2, was obtained when geometrical rather than electrochemical surface area was used, and the reason for that was explained in detail by Van Brussel et al.38 The two-electron oxygen reduction (eq 6) observed for the two Ru-containing electrodes correlates well with the literature reports that suggest that surface oxides of ruthenium lower the activity of H2O2 decomposition to H2O.39 It is seen in Table 1 that the reaction rate constant obtained for the 8(Pt-Ru)Cu/GC electrode is 4 times larger when compared with the 8(Ru|Pt)Cu/GC electrode. From that one might conclude that the 8(Pt-Ru)Cu/GC electrodes could be more suitable for applications involving the ORR when predominant formation of peroxide is of main interest. The reaction path, in the case of monometallic Pt nanoclusters, follows the four-electron direct reduction of O2 to H2O as described by eq 5; the slope obtained for the 8(Pt)Cu/GC electrode seen in Figure 7c is half of that obtained for the other two electrodes.
4. Conclusions It has been demonstrated that the use of OC SLRR reactions involving sacrificial copper, when sequentially implemented to grow Ru|Pt nanoclusters from separate Pt and Ru precursor solutions, leads to preferential formation of metallic states of both Pt and Ru. The presence of a sacrificial element, in this case copper, has been shown to be a necessary condition for the sequential deposition of metallic adlayers of ruthenium on metallic platinum. However, the direct spontaneous sequential (38) Van Brussel, M.; Kokkinidis, G.; Vandendael, I.; Buess-Herman, C. Electrochem. Commun. 2002, 4, 808. (39) Lee, J.-W.; Popov, B. N. J. Solid State Electrochem. 2007, 11, 1355.
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deposition of Pt followed by Ru without SLRR steps, or codeposition of Pt and Ru involving SLRR generates metallic Pt nanoclusters on which the spontaneous deposition of ruthenium oxides takes place. The analysis of open-circuit potentials recorded during the various deposition experiments in combination with theoretical models (here thermodynamic data were used to model the Pt-Cu-H2O, Ru-Cu-H2O, and Pt-Ru-H2O systems) proved to be an excellent predictive tool of deposits generated. The generated nanostructures were characterized using CV, HLSV, CA, EIS, SEM, AFM, and EDX. The resultant nanostructured electrode, n(Ru|Pt)Cu/GC (Figure 8 schematically summarizes the deposition steps that led to these unique Ru|Pt nanoclusters), is characterized by more positive onset potentials, lower charge-transfer resistances, and higher methanol oxidation currents when compared with all the nanostructures investigated in this work. Codeposition of Pt-Ru via simultaneous SLRR steps led to least active nanostructures, suggesting that formation of Ru oxides decreases the activity toward methanol oxidation. In this work, our focus was on active surfaces enhanced with a number of atomic contacts between Pt and Ru. To this effect, a large number of 3D bimetallic nanoclusters (rather than flat and large surface monolayers deposited on top of each other) were generated that significantly enhanced the number of active sites where bifunctional catalytic mechanisms can take place (see Figure 8). The mechanism of ORR was also investigated on the nanocluster electrodes described in this work. The four-electron pathway was favored by the monometallic Pt nanoclusters, but the Ru-containing Pt nanoclusters favored the two-electron ORR pathway. DOI: 10.1021/la902219t
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The methodology and instrumentation described here allows reproducible generation of different structures, such as mono-, bior multimetallic deposits coated as individual elements or mixtures of two, or more, metals. One can also generate nanostructures containing metals and oxides that might have unique composition allowing fine-tuning of their electrocatalytic properties for a specific reaction of interest. Surface composition of the resulting multimetallic clusters may be adjusted by (i) the inherent rates of mass transport to the sites of cluster growth, (ii) the concentration of precursors used, (iii) the nature of the electrolyte used, (iv) the rates of reduction of the templating metal on the more noble metals, (v) the thermodynamic properties of the metals of interest, and (vi) the nature of interactions between the metals of interest with their support, for instance, single-crystalline substrates, and nucleation and growth mechanisms involved. We continue to systematically investigate the above aspects. Acknowledgment. The authors wish to thank CSIR and University of Pretoria (UP) for research funding through parliamentary grants and provision of various research facilities to carry out the work. Special thanks to Nico Van Vuuren of the mechanical workshop at UP, who helped with fabrication of the flow-cell and to Thomas Malwela and Dr. Thembela Hillie of NCNM, CSIR, for help with AFM studies. T.S.M. thanks CSIR for a doctoral scholarship.
n(Ru|Pt)/GC
Glassy carbon supported n-layered nanostructure obtained through n cycles, each involving OC spontaneous deposition of a single noble metal (without the use of a sacrificial metal layer)
n(Pt)
n-Layered nanostructure obtained through n cycles of sequential deposition of Pt from its precursor salt
n(Pt)Cu/GC
Glassy carbon supported n-layered nanostructure obtained through n cycles of sequential OC SLRR of a sacrificial Cu adlayer by metallic Pt from its precursor solution
Pt-Ru
Alloy-type bimetallic nanoparticle obtained through codeposition from a mixed precursor solution of Pt and Ru salts
n(Pt-Ru)
n-Layered nanostructure obtained through n cycles of codeposition as defined for Pt-Ru
n(Pt-Ru)Cu/GC
Glassy carbon supported n-layered nanostructure obtained through n cycles, each involving OC SLRR of a sacrificial Cu adlayer by Pt and Ru from a mixed precursor solution to form alloy-type bimetallic layers
OC SLRRM
Open circuit surface-limited redox replacement of a sacrificial metal by more noble metal M from its precursor solution
OC SLRRPt-Ru
Open circuit surface-limited redox replacement of a sacrificial metal by indicated noble metals from a mixed precursor solution.
Appendix: List of Uncommon Symbols Used
Ru|Pt
n(Ru|Pt)
n(Ru|Pt)Cu/GC
Glossary Bimetallic nanoparticle consisting of Pt and Ru adlayers sequentially deposited from separate precursor solutions n-Layered nanostructure of bimetallic nanoparticle as described for Ru|Pt with Pt being deposited first and the final layer made of Ru (the same sequence of metal deposition applies to all n-layered nanostructures indicated below) Glassy carbon supported n-layered nanostructure, as described for n(Ru|Pt), obtained through n cycles, each involving an open circuit (OC) surface-limited redox replacement (SLRR) of sacrificial Cu adlayers by a single noble metal
580 DOI: 10.1021/la902219t
Supporting Information Available: (1) Potential-pH (E-pH) thermodynamic models of Pt-Cu-H2O, Ru-CuH2O, and Pt-Ru-H2O, (2) variation in maximum open-circuit potential as function of deposition cycles during Pt and Ru surface-limited redox-replacement reaction stages involving Cu, (3) SEM micrograph of sequentially codeposited Pt-Ru nanoparticles, (4) additional hydrodynamic linear sweep voltammograms in oxygen-reduction reaction studies, and (5) bifunctional mechanism for methanol oxidation reaction. This material is available free of charge via the Internet at http://pubs.acs.org.
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