Oxygen Reduction at Nanostructured Electrodes Assembled from

DMR-9625692 and NHMFL under Cooperative Agreement DMR-0084173. ... E. R., Vayenas, C. G., Eds.; Marcel Dekker Inc: New York: 2003; Chapter 9...
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8060

J. Phys. Chem. C 2007, 111, 8060-8068

Oxygen Reduction at Nanostructured Electrodes Assembled from Polyacrylate-Capped Pt Nanoparticles in Polyelectrolyte Zaki G. Estephan, Leen Alawieh, and Lara I. Halaoui* Chemistry Department, American UniVersity of Beirut, Beirut 110236, Lebanon ReceiVed: December 19, 2006; In Final Form: March 20, 2007

Oxygen reduction and related processes are studied at nanostructured Pt electrodes assembled from polyacrylatecapped Pt nanoparticles (〈d〉 ) 2.5 ( 0.6 nm) in poly(diallyldimethylammonium)chloride (PDDA) on indium tin oxide or glassy carbon with varying nanoparticle surface coverage. The nanoparticle density was varied laterally by varying the dipping time of PDDA-modified electrodes in the nanoparticles solution, or vertically with the number of nanoparticle/polyelectrolyte (bi)layers following a layer-by-layer assembly. TEM images revealed submonolayer coverage in one bilayer at 60 min dipping with a fractal distribution, and a significant surface coverage at four bilayers with evidence of multilayer assembly. Cyclic voltammetry in oxygencontaining electrolytes showed the assemblies to be electroactive for oxygen and hydrogen peroxide reduction, with a pH-dependent oxygen reduction peak shifting by -50 mV/pH unit. OH adsorption was found to be less favored occurring at more positive potential at the nanostructured electrode compared to polycrystalline Pt, while the oxide reduction peak was negatively shifted at the former electrode, in agreement with reports of increased oxophilicity with decreased particle size. The oxygen reduction peak potential shifted positively upon increasing Pt nanoparticles coverage, consistent with the catalytic activity of Pt for oxygen reduction. The active surface area of Pt nanoparticles was measured electrochemically from the charge of hydrogen underpotential deposition at the assemblies in H2SO4, and the diffusion-limited peak current for oxygen reduction measured per real Pt surface area is reported to decrease with increasing catalyst loading, as a result of reaching a limiting effective diffusion field.

Introduction The effect of Pt crystallite size on the kinetics of the oxygen reduction reaction (ORR) is an unsolved problem in electrochemistry.1-14 Fundamental considerations and the need for the rational design of fuel cell catalysts impart to this problem its significance. In general, a comprehensive understanding of nanoscale electrocatalysis necessitates control of nanoparticle size and shape and assembly procedures that afford variation of nanoparticle surface density. Assembled structures must also support facile mass and charge transport, and surface-modified nanoparticles should retain their catalytic activity; otherwise a procedure of decapping will be necessary, which may alter the nanoparticle size/shape. To explain this crystallite size effect,3-6 Kinoshita6 proposed a structural model that assumes the fcc Pt particle as a cuboctahedron, and related the measured maximum in mass activity (A/g) for ORR at ∼3.5-5.5 nm Pt particles and the decrease in specific activity (A/cm2) with particle size to changes in the distribution of sites, implying lowest activity at edges and corners. The problem remains that this model does not explain all aspects of the dependence of catalytic activity on particle size. For instance, it is not reconciled with following studies of ORR,1,2,7,8 which revealed the (110) facet to have highest activity in acid,7 while the (111) facet has highest activity in base,8 and when the crystallite-size effect is not measured to differ between the two media.9 Another hypothesis (the territory model) was proposed by Watanabe et al., who attributed the maximum activity of ORR * Corresponding author. E-mail: [email protected].

on Pt that was reached at (calculated) interparticle distances larger than 18-20 nm10,11 to the role of diffusion or others undeterminedsparameter at short interparticle separation.10,11 Giordano et al.12 later questioned whether the reported particle size effect is one of a critical interparticle separation. Other factors such as defects in the catalyst surface,4,13 and changes in the electronic surface properties with size,14 have also been proposed to contribute to the catalytic activity on Pt. Pt particles in fuel cells are ∼2-10 nm in size. The general method for electrocatalyst preparation involves loading Pt colloids or reduction of the metal salt after impregnation in different amounts on high surface area carbon support4,6,10,12 followed by thermal annealing, thus varying the particle size but likely also interparticle separation.10,12 The shape is also rarely defined. Electrode preparation generally consists of solvent casting of carbon-supported particles and attachment with a thin Nafion film.10,12,14-16 Other preparation methods for nanoparticles-on-electrode have appeared. Pt nanoparticles of 1.4 nm size were prepared inside poly(amidoamine) dendrimers by Crooks et al., and coupled by electrooxidation to glassy carbon.17 Polyacrylate(PAC)-capped Pt nanoparticles of various shapes and sizes were synthesized by El-Sayed et al.,18 and we reported the assembly of similarly modified 2.5 nm Pt nanocrystals in a cationic polyelectrolyte.19,20 Schiffrin et al. reported deposition of colloidal Pt onto Au using C60 as linker,21 and Friedrich et al. deposited polyvinylpyrrolidone PVP-stabilized Pt colloid or citrate-stabilized Pt at opencircuit potential on Au.22 Electrophoretic deposition of colloidal Pt has also been achieved.23 Alternatively, Pt nanoparticles were grown in situ on electrodes via metal evaporation in vacuum,24 chemical deposition,25 electroless deposition on Ti,26 underpo-

10.1021/jp0687091 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007

Oxygen Reduction at Nanostructured Electrodes tential deposition redox replacement,27 or by potentiostatic electrodeposition.28-31 Multilayered Pt films have also been prepared via layer-by-layer assembly of PtCl62- and [tetrakis(N-methylpyridyl)porphyrinato]cobalt (CoTMPyP) on GC or of citrate-Pt nanoparticles and CoTMPyP,32 PAC-capped Pt nanoparticles (〈d〉 )2.5 ( 0.6 nm) in poly(diallyldimethylammonium chloride), PDDA,19,20,33 or triarylphosphine-Pt nanoparticles in polyallylamine HCl.34 It remains that the solution synthesis of nanoparticles in the presence of surface capping agents followed by their assembly on electrodes may allow the best control of narrow size distribution, shape, and surface density. We report here an electrochemical study by cyclic voltammetry of oxygen reduction and related processes of hydrogen peroxide reduction and OH adsorption at nanostructured Pt electrodes assembled from 2.5 nm polyacrylate-stabilized Pt nanoparticles in PDDA. The electrodes are constructed while varying the nanoparticle surface density and the number of layers.19 TEM images revealed submonolayer coverage at 60 min dipping and almost full lateral coverage at four bilayers. The electrochemical behavior is related to the real catalytic Pt surface area, measured from hydrogen underpotential deposition,20 and is compared to bulk Pt. This study aims to show that this assembly, leading to active electrodes, can serve as a unique system to control the catalyst size/shape while independently varying its surface density for studies of the role of these factors on electrocatalysis, particularly considering that polyacrylate-capped Pt nanoparticles have been prepared in different sizes and shapes.18 Experimental Methods Materials. The following materials were used: Potassium hexachloroplatinate (IV), K2PtCl6 (Acros Organics, ∼ 40% Pt); poly(acrylic acid) 2100 sodium salt, PAC (Aldrich); poly(diallyldimethylammonium chloride), PDDA, 20 wt % in water, average molecular weight ∼200- 350 kDa (Aldrich); sodium citrate (Analar); sulfuric acid ∼98% (GPR); sodium phosphate dibasic anhydrous (Mallinckrodt Chemicals); sodium phosphate tribasic 98% (the British Drug Houses, Ltd); o-phosphoric acid 85% (Fischer); perchloric acid 70-72% (Baker); sodium chloride (Scharlau); ammonium hydroxide (28-30 wt %, Acros); hydrogen peroxide (30%, Fluka Chemika); ethanol (Merck). Aqueous solutions were prepared with double-distilled (dd) water. The concentration of the polymer solution is for the monomer unit. Pt Nanoparticles Synthesis. The synthesis of polyacrylate(PAC)-capped Pt nanoparticles and their characterization have been previously reported.19 A 10 mg sample of K2PtCl6 dissolved in 1.34 g sodium polyacrylate/50 mL (aq) solution (at a ratio of Pt/PAC 1:31) was added to a 0.50 g/50 mL sodium citrate (aq) solution. The pH was raised to ∼8.3 with 0.5 M NaOH, and the solution was refluxed for 3.5 h. Pt (IV) was reduced with citrate in the presence of polyacrylate to yield a light gold-brown solution. Nanostructured Electrode Assembly. PAC-Pt nanoparticles were assembled layer-by-layer in PDDA on glassy carbon disk (GC, d ) 5 mm) or indium tin oxide coated glass (In: SnO2, ITO, Delta Technologies, Rs) 8 - 12 Ω) electrodes. Prior to assembly, the GC electrode was polished with 5 µm followed by 0.5 µm alumina slurries on a polishing cloth (Buehler), and sonicated in dd water. The electrode was dipped in piranha (7:3 (v/v) 98% H2SO4:30% H2O2 aqueous solution) for 5 min, rinsed, and dried in air. ITO substrates were cleaned for 30 min in boiling ethanol and for 10 min in warm 7:3 (v/v) NH3 /H2O2 (aq) solution, and then rinsed and dried.

J. Phys. Chem. C, Vol. 111, No. 22, 2007 8061 The (GC or ITO) electrode was dipped in 10 mM PDDA/0.4 M NaCl(aq) solution for 30 min, rinsed 2 × 1 min with dd water, and air-dried for 10 min. The PDDA-modified electrode was then dipped in the as-prepared Pt nanoparticles solution for 1 h, rinsed in water for 1 min, and dried for 15 min. This resulted in the assembly of a PDDA /nanoPt bilayer. n bilayers are deposited by repeating this procedure n times using a software-controlled multilayer assembly robot (StratoSequence VI, NanoStrata, Inc.); the films are referred to as (PDDA/ nanoPt)n. To vary the surface density of Pt nanoparticles within the same layer, the PDDA-modified electrode was dipped in the Pt nanoparticles solution for a time t (min) ) 2, 5, 10, 20, 30, 60, and 120; the resulting electrode is termed PDDA/nanoPtt. Electrochemical Measurements. Electrochemical measurements were acquired in a three-electrode electrochemical cell with the PDDA/nanoPt assembly or a polycrystalline Pt disk (d ) 2 mm) as the working electrode. The Pt disk electrode was polished with 5 and 0.05 µm alumina slurries, sonicated in dd water, and electrochemically activated in deaerated 1 M H2SO4 by cycling the potential between 2 and -1 V at 500 mV/s until the characteristic hydrogen adsorption peaks were resolved. A Pt wire served as the counter electrode, and a Ag/AgCl as the reference electrode (Analytical Sensors, Inc., or homemade, in 3.5 M KCl). The potential of the reference electrode was assessed in 5 mM Fe(CN)63-/5 mM Fe(CN)64- in 0.1 M KNO3 with a Pt disk working electrode prior to measurements. Cyclic voltammograms (CV) were recorded using a home-interfaced bipotentiostat (model AFCBP1, Pine Instruments Company) and Pine 2.7 software. CVs were acquired in deoxygenated 1 M H2SO4 at assemblies of Pt nanoparticles to measure the underpotential deposition of hydrogen (Hupd) prior to measurements of oxygen reduction. Solutions were deoxygenated by bubbling nitrogen for 30 min and maintaining a nitrogen blanket during measurement. Oxygen reduction was studied in air-saturated or oxygensaturated electrolyte solutions of 1 M H2SO4, 0.1 M KOH, 0.1 M HClO4, and 0.25 M phosphate buffers at pH 4, 7, and 11. To saturate solutions with oxygen, the gas was bubbled in solution for 30 min and an O2 blanket was maintained. The area of ITO dipped in solution was all covered with PDDA/ nanoPt film, and was in the range of ∼2-2.5 cm2. The cross sectional (geometric) area (Ageom) of PDDA/nanoPt on the electrode is used in reporting current densities, unless it is indicated that the current is relative to the real (microscopic) Pt area (Areal) measured from Hupd. The diffusion field area from eq 1 is designated by A. TEM Imaging. PDDA/nanoPt films assembled on SiOxcoated 300 mesh Cu grids (SPI) in one, two, and four bilayers were imaged by TEM. The same assembly procedure described above was followed except that grids were floated on the respective solution/air interface with the SiO film facing down (to cover one face). Images were acquired on a JEOL-2010 highresolution TEM operated at 200 kV with a point and lattice resolution of 0.23 and 0.14 nm, respectively (NHMFL, Florida State University, images by Dr. Yan Xin). Results and Discussion General Electrochemical Characteristics of PAC-Capped Pt Nanoparticles/PDDA Assemblies. Polyacrylate-capped Pt nanoparticles have been characterized earlier by TEM and HRTEM, which revealed the growth of face-centered cubic (fcc) nanocrystallites of 2.5 ( 0.6 nm average diameter.19 Nanostructured Pt electrodes were assembled from polyacrylatecapped Pt nanoparticles in PDDA on ITO (or GC) electrodes;

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Figure 1. TEM images of Pt nanoparticles assembled on PDDAmodified SiO/Cu grid in 1 layer (a), 2 bilayers (b), and 4 bilayers (c). Scale bar is 20 nm.

on 1 layer of PDDA or in multilayers by virtue of electrostatic and hydrophobic interactions. To obtain a view of the surface distribution and density, TEM images were acquired of Pt nanoparticles assembled directly on PDDA-modified SiOx film in one, two, and four bilayers. The TEM micrographs in Figure 1 show the Pt nanoparticles to assemble at submonolayer coverage on 1 PDDA layer at t ) 60 min dipping (1 bilayer), and accordingly a significant fraction of the substrate surface area (PDDA on SiOx) remains uncovered with Pt. The nanoparticle density is shown in the images to be higher for the (PDDA/nanoPt)2 assembly, but coverage remains less than a full monolayer. When four bilayers were assembled, TEM images showed a significant surface coverage (of PDDA on SiOx) with Pt nanoparticles, reaching almost full lateral coverage across most of the surface. This is shown later to be consistent with Pt surface area measurements from Hupd. Although the TEM micrographs present only a view of the horizontal topography, the contrast in some areas is indication of nanoparticles assembled in underlying layers. A fractal surface distribution of Pt nanoparticles is observed in the TEM images starting at one bilayer, where nanoparticle assembly takes place with a framework along circular lines with internal voids. This distribution is notably maintained even as more PDDA layers are adsorbed atop the Pt layer in multilayered

Estephan et al. assembly as shown in the images of (PDDA/nanoPt)2 and (PDDA/nanoPt)4. Such assembly appearing to favor particleparticle interaction upon solvent drying could possibly be caused by the morphology of the underlying PDDA polyelectrolyte layer adsorbed from 0.4 M NaCl concentration. A typical CV in deoxygenated 1 M H2SO4 at (PDDA/nanoPt)4 on ITO is presented in Figure 2a,i. Three potential regions are characteristics of CVs at Pt surfaces:13,14 the oxide region, the double-layer region and the region of hydrogen underpotential deposition (Hupd). Hupd at PDDA/nanoPt assemblies has been reported earlier and is therefore discussed only briefly here.20 The CV at (PDDA/nanoPt)4 shows the onset of atomic hydrogen adsorption at ∼ 170 mV following double-layer adsorption, and the onset of hydrogen evolution at -115 mV ( 10 mV. Two main H-adsorption states were identified, a strongly adsorbed hydrogen state H(S) at (100) sites, and a weakly adsorbed state H(W) at (110) sites at more negative potentials.20,35-38 Similar results were obtained at PDDA/nanoPt on GC (results not shown). Negative shifts of -25 and -63 mV in the H-adsorption potentials were previously reported at 4 and 14 bilayers, respectively, relative to polycryst-Pt, but a similar shift in the desorption potential was not observed (at 20 mV/s). We had attributed this shift to a kinetic limitation in the H adsorption step as a result of a “different local Pt surface environment” caused by surface modification and assembly, although it was not possible to separate the effect of nanocrystalline size in the absence of studies of similarly modified nanoparticles of different sizes.20 Following that paper, Mayrhofer et al.14 reported a negative shift of -25 mV in the Hupd potential region (both adsorption and desorption) for 1 nm compared to 30 nm Pt-on-C particles, attributed to a -35 mV shift in the pztc, thus showing that electronic factors due to size variations can also affect Pt surface adsorption processes. Reports of Hupd at Pt nanoparticles differed in resolving the adsorption states at small nanoparticles, which could be a result of different preparation methods yielding different surface oxidation states and modification, or of variations in deposition procedures. Penner et al. observed reversible Hupd peaks for large Pt nanocrystals electrodeposited on basal plane oriented graphite, but not for sizes less than 3 nm.29 Hupd states were not resolved at Pt colloids, deposited on Au, modified with PVP (∼4 nm size), with tetraoctylammonium bromide (10 nm size), or unmodified.21 On the other hand, clearly defined hydrogen Hupd states were recorded by Mayrhofer et al.14 and Arenz et al.13 at 1-30 nm Pt-on-carbon, by Cherstiouk et al.25 at 1.3-7.5 nm Pt on GC, and at 6 nm citrate-stabilized Pt nanoparticles in CoTMPyP.33 In these studies13,14,25,33 the potential was scanned to the region of oxide formation, but it was not indicated whether this was necessary for observing the adsorption states. Hupd states were resolved here at the 2.5 nm modified Pt nanocrystals without scanning into the oxide region (positive of 800 mV).39 The Pt nanoparticle assemblies are shown in the CV at (PDDA/nanoPt)4 in air-saturated sulfuric acid to exhibit catalytic activity for ORR, with an onset of oxygen reduction at 730 mV and a well-defined irreversible reduction peak at a peak potential (Ep) of 480-495 mV (Figure 2a,ii). After purging with oxygen, the onset of reduction was detected at 780 mV and the peak potential shifted to 428 mV with a significantly larger peak current density (inset). A comparison with the CV in nitrogensaturated solution confirms this reduction to be that of molecular oxygen. O2 reduction at Pt is a multielectron charge-transfer reaction in which the rate-determining step is believed to be the addition of the first electron to an O2 adsorbed species.1,2 The peak current ip in cyclic voltammetry of such a reaction is

Oxygen Reduction at Nanostructured Electrodes

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Figure 2. (a) CVs at (PDDA/nanoPt)4 on ITO in deoxygenated (i), air-saturated (ii), and oxygen-purged (inset) 1 M H2SO4. Scan rate is 20 mV/s. (b) Plots of Jp (peak current density) vs ν1/2 and Ep vs ln ν, where ν is scan rate between 5 and 100 mV/s.

expected to depend linearly on the square root of the scan rate according to the following relation (at 25 °C):40 1/2 ip ) (2.99 × 105)nR1/2AC/o D1/2 o ν

(1)

and the peak potential Ep to depend logarithmically on the scan rate, according to40

Ep ) Eo′ -

[

( )

D1/2 o RFν RT 0.780 + ln o + ln RF RT k

1/2

( )

]

(2)

where n is the total number of electrons transferred, R is the transfer coefficient, A is the electrode area, C/o is the oxidized species bulk solution concentration, Do is its diffusion coefficient, ν is the scan rate, and Eo′, F, R, T, and ko have their usual definition.40 The linear dependence of jp (current density) on ν1/2 and of Ep on ln ν (Figure 2b) measured between 5 and 100 mV/s at the nanoPt assembly in sulfuric acid showed the expected relationships. Figure 3 shows CVs at a polycryst-Pt in deoxygenated (i) and in air-saturated (ii) and oxygenated 1 M H2SO4 (inset) for

comparison. The adsorption peaks of atomic hydrogen at this surface are distinctly resolved (scan i) and correspond from most positive to long-range adsorption at (100) terraces, at (100) sites, (110) sites, and the small resolved shoulder (at -102 mV) possibly to adsorption at (111) sites.35 In air-containing 1 M H2SO4, reduction of dissolved oxygen took place with an onset at ∼790 mV and an Ep of 605 mV, in a more facile process than that at (PDDA/nanoPt)4 on ITO. In oxygen-purged solution, the onset of oxygen reduction occurred at ∼710 mV and Ep at 440 mV with a significantly higher peak current density. A significant difference between the nanostructured electrode and polycryst-Pt is the onset of OH adsorption. At polycrystPt, OH adsorption takes place in what appears to be two states, the first with an onset at 480 mV and another beginning at 700 mV (Figure 4iii). Upon scanning the potential from 1.25 V a well-defined oxide reduction peak is measured with Ep at 613 mV. The potential region of oxide formation and its reduction therefore coincides with that of oxygen reduction at this surface. On the other hand, OH adsorption was less favored at the Pt nanocrystals assemblies, taking place with an onset positive of 800 mV (Figure 4i,ii), meaning that there is a smaller OH

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Figure 3. CVs at polycryst-Pt disk electrode in deoxygenated (i), air-saturated (ii), and oxygen-purged (inset) 1 M H2SO4. Scan rate is 20 mV/s.

Figure 4. CVs at (PDDA/nanoPt)1 (i), (PDDA/nanoPt)2 (ii) on ITO, and at polycryst-Pt disk (iii) in deoxygenated 1 M H2SO4. Currents are normalized to Areal of Pt calculated from the charge of Hupd for each electrode. The inset shows current densities per geometric surface area (Ageom).

coverage at lower potentials at this electrode compared to polycryst-Pt. Once formed, the oxide reduction peak however took place with an Ep at 540 mV, negatively shifted from polycryst-Pt, indicating a more difficult desorption (strong adsorption state) at the nanocrystalline electrode. The CVs in Figure 4 normalized to the respective real surface areas of each electrode, measured from Hupd (details below), clearly reveal these differences and show the quasi-reversible process of oxide formation/reduction to be more irreversible at the Pt nanoparticle assemblies. The dependence of the oxophilicity of Pt nanoparticles on particle size has been subject of study because of the action of an OHadlayer in affecting the kinetics of catalyzed reactions at Pt, such as the ORR and CO oxidation.1,2,8,13,14 Penner et al. observed that the peak separation of oxide formation/reduction in 0.05 M sulfuric acid at Pt nanocrystals electrodeposited on HOPG increased with decreasing particle size from 15.5 to 1.5 nm, indicating a more irreversible process at small particles.29 An oxidation peak corresponding to oxide formation was positively shifted by about 200 mV at 4.3 nm compared to 15.5 nm particles (estimated from Figure 11 of ref 29).29 Cherstiouk

Estephan et al. et al. reported a negative shift in the Pt oxide reduction peak in 0.1 M sulfuric acid at 1.3 nm Pt particles (by 200 mV) compared to 7.5 nm particles or polycrystalline Pt.25 Mayrhofer et al. studied OH adsorption at 1-30 nm Pt-on-C particles and attributed differences denoting increased oxophilicity with decreased dimension to a negative shift in the pztc.14 CVs normalized to the real Pt surface area showed the desorption peak of oxygenated species to shift negatively by more than 130 mV from 30 and 1 nm particles, but the authors reported that on the anodic sweep a negative shift in OH adsorption was not clearly apparent in the voltammograms.14 Mukerjee et al. using in situ X-ray absorption fine structure and X-ray absorption near edge spectroscopy studies at 2.5-9 nm Pt particleson-C in 0.1 M HClO4 showed that OH adsorption becomes stronger at particles smaller than 5 nm, which the authors attributed to an increase in the fraction of low-coordination sites.41 The electrochemical characteristics of the Pt nanoparticle films here indicated a hindered oxygenated species adsorption on the anodic sweep, and a more irreversible surface oxidation (less favored desorption on the cathodic sweep) compared to polycryst-Pt. The negatively shifted Pt-oxide reduction potential agrees with the above cited reports of a stronger oxygenated species adsorption at small Pt particles.14,25,29,41 However, the measured positive shift in the onset of OH adsorption (of about 300 mV relative to polycryst-Pt) has only been reported to our knowledge in this work, in the positive shift in the oxide formation peak reported by Penner et al. at unmodified Pt nanocrystals on HOPG though the onset of OH adsorption was not indicated29 and by You et al., who reported a positive shift in the oxidation potential of 2.5 nm Pt particles dispersed in a graphite film relative to bulk Pt.42 You et al. measured in XPS spectra a higher binding energy (by ∼0.9 eV) for 4f7/2 and 4f5/2 electrons compared to bulk Pt, and attributed the more difficult surface oxidation of Pt nanoparticles to the f electrons higher binding energy.42 Schull et al. also measured a higher binding energy of 4f electrons in XPS spectra of 1.7 nm tris(4-phosphonatophenyl phosphine)-stabilized Pt nanoparticles.34 This could explain the positive shift in OH adsorption potential compared to bulk Pt. It is speculative to state if this shift could also have contribution here from the presence of a negatively charged surface carboxylate capping agent hindering OH adsorption, and answering this conclusively requires a study of similarly modified nanoparticles of different sizes and/or of 2.5 nm Pt nanoparticles with different surface modification. The effect of the ligand stabilizer on the catalytic activity and electronic properties of metal nanoparticles has been reported in the literature.43 In any case, a smaller OH coverage at potentials where O2 reduction is feasible is of significance, because it potentially minimizes the effect of this site-blocking species in competing with ORR.1,2,8 The Pt nanoparticles were also electroactive and the assemblies were stable and supported oxygen reduction in different electrolytes and pH range as shown in the CVs at (PDDA/ nanoPt)4 in air-saturated phosphate buffers of pH 4, 7, and 11, 0.1 M HClO4, and 0.1 M KOH in Figure 5. The oxygen reduction peak potential shifted by -50 mV per pH unit (inset), though notably Ep in the higher pH perchloric acid electrolyte occurred more positive than in sulfuric acid. This is consistent with reported pH and electrolyte dependence of oxygen reduction at Pt.44,7,8 The more positive Ep in HClO4 relative to H2SO4 can be attributed to the site blocking effect of bisulfate,7 as opposed to the nonadsorbing perchlorate ions.

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Figure 5. CVs at (PDDA/nanoPt)4 in air-saturated 0.1 M HClO4 (i), 0.25 M phosphate buffer pH 4 (ii), 7(iii), 11(iv), and 0.1 M KOH (v) at 20 mV/s scan rate. The inset shows a plot of Ep as a function of pH.

Figure 7. (a) CVs at (PDDA/nanoPt)n bilayers on ITO with n ) 2, 4, 6, and 12; (inset of a) at PDDA/nanoPtt (on 1 layer of PDDA) at t ) 10, 20, 30, and 120 min, acquired in deoxygenated 1 M H2SO4. Scan rate is 20 mV/s. (b) Total charge Q for Hupd and the corresponding real Pt surface area (per cm2 of geometric area) of the Pt nanostructured electrodes vs the total dipping time (in Pt nanoparticle solution): 9 for multilayers (60 min ≡ 1 bilayer), and 2 for 1 layer assembly. Open symbols correspond to Areal on secondary axis. The inset shows the data for the assembly on 1 layer of PDDA at different t. Figure 6. CVs at (PDDA/nanoPt)4 on ITO (i) and at polycryst-Pt (ii) in deoxygenated 5 mM H2O2/PBS pH 7.0. Scan rate is 20 mV/s.

Oxygen reduction to water at Pt has been proposed to take place either via a “direct” 4e- transfer or a “series” mechanism involving a 2e- reduction to H2O2, with formation of H2O2ads, followed by its reduction to water.2,45 Markovic et al.1,2 reported rotating ring disk electrode studies that indicated a series pathway at Pt and Pt bimetallic surfaces, while in a theoretical study using molecular dynamics simulations Wang et al. showed the two mechanisms operating in parallel at Pt(111), but with evidence that the direct 4e- pathway is the dominant one.45 Because H2O2 reduction to water may be involved as a step in the ORR, the feasibility of this reaction at the Pt nanoparticle assemblies was investigated. Figure 6 shows CVs at (PDDA/ nanoPt)4 on ITO and at polycryst-Pt disk in 5 mM H2O2/ deaerated phosphate buffer pH 7. The reduction of hydrogen peroxide took place at an onset of 330 mV and Ep at 40 mV at (PDDA/nanoPt)4, compared to an onset at 365 mV and Ep at 176 mV at polycryst-Pt, indicating an overpotential at the nanostructured electrode. This is a similar potential region as for oxygen reduction at (Pt/PDDA)4 in air-saturated PBS pH 7 (that took place with an onset at 370 mV and Ep at 171 mV; cf. Figure 5). Characterization and Oxygen Reduction at Assemblies with Varying Pt Nanoparticle Surface Density and Multilayer Number. PAC-capped Pt nanoparticles were assembled on ITO (or GC) electrodes modified with one layer of PDDA by varying the dipping time in the nanoparticle solution between

2 and 120 min, and the electrochemical behavior of oxygen reduction was investigated by cyclic voltammetry in relation to surface coverage. Furthermore, since full surface coverage is only obtained at multilayered assemblies, oxygen reduction was also investigated at (PDDA/nanoPt)n from n ) 2 to 12.19,20 Pt coverage was estimated from electrochemical measurement of the catalytically available Pt surface area (taken as Areal) calculated from the charge of Hupd, Q(Hupd), and which agreed qualitatively with TEM images. CVs in deoxygenated 1 M H2SO4 at (PDDA/nanoPt)n multilayers with n ) 2, 4, 6, and 12 are presented in Figure 7a, while CVs for a one layer assembly PDDA/nanoPtt on ITO with t ) 10, 20, 30, and 120 min are shown in the inset. The peaks corresponding to different Pt H-adsorption sites only became well resolved at increased nanoparticle loading, but the corresponding charge was determined between the onset of Hupd and hydrogen evolution after subtraction of double layer charging, and is shown in Figure 7b as a function of the total dipping time in Pt nanoparticle solution (9 for multilayers 60 min ≡ 1 layer, 2 for 1 layer assembly). Areal was computed by assuming a charge of 210 µC per cm2 of Pt, which corresponds to a hydrogen atom adsorbed for each Pt surface atom and a surface atomic density of 1.30 × 1015 atoms/cm2.46 The charge of Hupd corresponding to full monolayer coverage of PAC-capped 2.5 nm Pt nanoparticles was estimated as 660 µC/cm2geom by assuming closely packed 2.8 nm particles (comprising 2.5 nm spheres and 0.3 nm thickness assumed for the organic capping material).20 The fractional overage θ of Pt nanoparticles calculated with respect to such a monolayer (θ ) Areal/Amonolayer, shown in Figure S1)

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Estephan et al.

Figure 9. Peak current density (9) per real Pt surface area (Areal) and Ep (2, secondary axis) for oxygen reduction plotted vs the real surface area for Pt nanoparticle assemblies on ITO in air-saturated 1 M H2SO4. Data obtained from CVs acquired at 20 mV/s.

Figure 8. CVs in deoxygenated (1) and air-saturated (2) 1 M H2SO4 at (PDDA/nanoPt)t at t ) 2 [i], and 10 [ii] min (a), 30 [i], and 120 [ii] min (b), and at 2 bilayers [i] and 12 bilayers [ii] (c) on ITO. Scan rate is 20 mV/s.

indicates a low coverage at 1 bilayer (θ ) 0.26) and a coverage of 1.02 at the (PDDA/nanoPt)4 film (in Figure 7), which agrees with TEM results showing submonolayer coverage at one bilayer and almost full surface coverage and particles assembled in underlying layers at the (PDDA/nanoPt)4 assembly. The electrochemically determined Pt Areal increased with increasing the dipping time in a way consistent with the dynamics of PAC-nanoPt assembly on PDDA previously reported to follow 2 kinetics regimes at 2 separated time scales:19 the first regime is characterized with the higher adsorption rate at times shorter than ∼1 h, followed by a slower rate of adsorption and pseudo-saturation reached at longer times.19 Q(Hupd) also increased linearly with increasing the number of bilayers, affirming a reproducible surface charge reversal per PDDA layer. Adsorption of a PDDA layer onto PDDA/nanoPt results in increased particle adsorption compared to a surface otherwise assembled at the same total dipping time. This is evidenced from the higher Pt surface area of two bilayers [(PDDA/ nanoPt)2] compared to PDDA/nanoPtt)120, and could be due to the onset of adsorption onto a new layer of PDDA which serves to reverse the surface charge and increases the adsorption rate. TEM images and surface area measurements therefore provide some evidence of vertical growth in a multilayer architecture. CVs acquired in deoxygenated (1) and air-saturated (2) 1 M sulfuric acid (Figure 8) at PDDA/nanoPtt assemblies on ITO: at t ) 2, 10 (a), 30, and 120 min (b); and at (PDDA/nanoPt)2

and (PDDA/nanoPt)12 (c) show the effect of Pt surface density on the electrochemical behavior. At the smallest nanoparticle loading (t ) 2 min) on PDDA - a surface with a coverage of θ ) 0.02 - reduction of dioxygen took place at 600-620 mV onset potential and in a well-shaped reduction peak with Ep at 104 mV. Ep shifted positively with increasing Pt coverage in 1 layer from 104 mV at PDDA/nanoPtt)2 min to 360 mV at PDDA/ nanoPtt)120 min, and continued shifting with increasing the number of bilayers until four bilayers were assembled, reaching the ∼limiting value equal to 475-492 mV at 4-12 bilayers on ITO (cf. Figure 9). The peak potential shift with increased Pt loading is consistent with the catalytic activity of Pt for ORR leading to a faster reaction rate with increasing the number of catalytic sites when this number is limiting. A similar trend was observed at PDDA/nanoPt assemblies on GC (shown in Figure S2), and in general in 0.1 M KOH. The only notable difference between assemblies on ITO or GC was the positively shifted Ep at the latter support, where Ep in sulfuric acid occurred at 590 mV at (PDDA/nanoPt)12 on GC and at 500 mV at (PDDA/ nanoPt)4 on GC. On the other hand, oxygen reduction diffusion-limited peak currents measured per Areal of Pt were found to be highest at the lowest Pt nanoparticle loading, and decreased significantly with increasing Pt mass at submonolayer coverage and in multilayered electrodes, indicating an indirect relationship to the miscroscopic Pt area as shown in Figure 9. This behavior was also observed on GC support in 1 M H2SO4 and in 0.1 M KOH electrolyte. Oxygen reduction at surface-modified Pt nanoparticle assemblies must involve several rates: (1) adsorption of oxygen and charge transfer to this O2-adsorbed species at the modifiednanocrystal surface, which could in some cases and depending on the binding be accompanied by desorption of the surface modifier-ligand;47 (2) charge transport in the films possibly via a mechanism of charge hopping between nanoparticles in different layers and between nanoparticles and the PDDAmodified substrate; and (3) mass transport of oxygen and of ions in the film to maintain electroneutrality, when the electrochemical process is taking place beyond the outermost layer. The negative shift of 100 mV in Ep at (PDDA/nanoPt)12 on ITO compared to GC (Ep ) 590 mV), the latter comparable to Ep at unmodified polycryst-Pt, could be attributed either to electronic properties of the ITO support causing a resistance to charge transport from the first layer, or by not achieving full lateral coverage of the large ITO surface (2-2.5 cm2) with Pt. The similarity between Ep at (PDDA/nanoPt)12 on GC and at polycryst-Pt may indicate that O2 reduction at the modified nanoparticles or charge transfer/transport in the films are not

Oxygen Reduction at Nanostructured Electrodes causing significant additional resistances at this time scale slowing the ORR at assemblies with high nanoPt surface coverage. For an array of nanoparticles assembled in an insulating matrix, a limiting behavior where the current is a direct function of the particles surface area is expected only at very low nanoparticle density and/or short experimental time, when (hemi-) spherical diffusion to each nanoparticle is in effect and the diffusion field is the sum of the individual diffusion fields (to each nanoparticle). At longer times when the diffusion layer thickens and as the nanoparticle density increases, the individual diffusion fields eventually overlap resulting in effectively linear diffusion. This bears similarity to an array of microelectrodes embedded in an insulating matrix.48 Material is drawn to the electrode from this diffusion area, which determines the masslimited current, and increasing coverage does not necessarily increase the diffusion area in proportion with the increase in catalyst surface area. The mass-transport limited current normalized to Areal, determined from Hupd (a surface adsorption process), can therefore be understood to decrease with increasing nanoparticle density according to the following picture. It is evident from TEM images that nanoparticles adsorb in some areas with significant particle-particle proximity therefore resulting in the surface area of each nanoparticle not remaining fully accessible to adsorption of oxygen, and causing the effective diffusion field in these areas not to increase in proportion with more particle adsorption. At the same time some nanoparticles also adsorb on a polymer surface area with previously very low coverage, and diffusion to these particles will contribute to the mass-limited current; but it appears that the first picture of adsorption could be of more importance (otherwise the diffusion field will significantly increase with nanoparticles coverage, which was not the case). Supporting this picture, the area of the effective diffusion field (A) for submonolayer assembly or (PDDA/nanoPt)n was estimated from the slope of ip versus ν1/2 using eq 1 and calculating R from the slope of Ep versus ln ν using equation [2] at scan rates between 5 and 100 mV/s, and making the assumptions that n ) 4, and Do ) 1.7 × 105 cm2 s and C/o ) 2.9 × 10-4 M, which are reported values in 0.5 M H2SO4.32b The transfer coefficient R was found to be equal to 0.29-0.44 at the nanoPt assemblies and 0.36 at polycryst-Pt. According to these calculations, the diffusion area A (of eq 1) varied in submonolayer assembly at PDDA/nanoPtt on ITO between 0.31 cm2 at t ) 2 min, 0.35 cm2 at t ) 10 min, 0. 47 cm2 at t ) 30 min, to 0.48 cm2 at t ) 120 min (for Ageom)1 cm2), thus indicating a small increase in the diffusion field with increasing the density of nanoparticles but not in proportion with the increase in the nanoparticle density. Increasing the number of bilayers did not result in increasing the diffusion area which was found to be between 0.6 and 0.8 cm2 (for 1 cm2 Ageom),49 compared to a calculated A at polycryst-Pt equal to 1.0 cm2. The diffusion area to the real area A/Areal therefore varied considerably (between 5.3 at PDDA/nanoPtt)2 min to 0.48 at PDDA/nanoPtt)120 and down to 0.06 at (PDDA/nanoPt)12 on ITO, and 0.18 at polycryst-Pt), which shows (similar to the presentation in Figure 9) the dependence of the electrochemical behavior at these electrodes on surface coverage. Conclusions and Final Remarks Nanostructured Pt electrodes with varying nanoparticle surface distribution were assembled from solution-prepared polyacrylate-capped Pt nanoparticles by virtue of electrostatic and hydrophobic interactions with a cationic polyelectrolyte,

J. Phys. Chem. C, Vol. 111, No. 22, 2007 8067 PDDA. This assembly method is shown to allow control of the nanoparticle density, separate from other factors that affect the nanoparticle size and shape, since processes of nanoparticle preparation and surface deposition are not coupled. Electrochemical characterization of PDDA/nanoPt assemblies on ITO (or GC) showed to date that these surfaces are catalytically active for many reactions that are catalyzed at unmodified Pt, such as Hupd, oxygen reduction, hydrogen peroxide reduction and oxidation, in addition to hydrogen evolution and oxidation.20,33 The electrochemical behavior studied by cyclic voltammetry showed variation at the differently structured electrodes as a function of nanoparticle surface coverage, in terms of the peak potential for oxygen reduction and the diffusionlimited peak current measured with respect to the available catalytic area. A more irreversible process of oxide formation/ reduction took place at the nanocrystalline assemblies relative to polycryst-Pt, and notably a positive shift in the onset of OH adsorption was measured at the former electrode, indicating a lower OH coverage at potentials where O2 reduction takes place. This may have implications on the rate of oxygen reduction since OH is a site blocking species competing with O2 adsorption at Pt. Current ongoing studies on this system in our laboratory involve measurements using rotating disk electrodes of the catalytic activity for ORR at polyacrylate-capped Pt nanoparticles of different sizes and shapes assembled in PDDA on GC at varying nanoparticle surface coverage, to understand the effect of size, shape, and surface coverage on the observed crystallite size effect of this reaction. A study of OH adsorption as a function of the ligand stabilizing the nanoparticle surface, particularly of its charge, and as a function of the nanoparticle size/shape, is believed essential to understand the effect of these factors on OH adsorption, and hence on the catalytic activity of surface-modified Pt nanoparticles toward the ORR. Acknowledgment. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (PRF #40548-B10), and to the American University of Beirut University Research Board (URB, Grant 2005-2006) for financial support of this research. We thank Dr. Yan Xin at the National High Magnetic Field Laboratory, Florida State University (Tallahassee, Florida) for acquiring the TEM images, with acknowledgment to NSF Grant No. DMR-9625692 and NHMFL under Cooperative Agreement DMR-0084173. Supporting Information Available: Estimated fractional surface coverage of Pt nanoparticles assembled in 1 layer and multilayers on ITO; and plots of peak current density per real Pt area, and of peak potential vs. real Pt area for PDDA/ nano-Pt assemblies on GC support. This material is available free of charge via the Internet at http://pubs.acs.org. Note Added after ASAP Publication. This article was published ASAP on May 16, 2007. In the Introduction Section, the second sentence of the fifth paragraph has been modified. The correct version was published on May 17, 2007. References and Notes (1) Markovic´, N. M.; Radmilovic, V.; Ross, P. N., Jr. In Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Savinova, E. R., Vayenas, C. G., Eds.; Marcel Dekker Inc: New York: 2003; Chapter 9. (2) Markovic´, N. M.; Schmidt, T. J.; Stamenkovic´, V.; Ross. P. N. Fuel Cells 2001, 1, 105. (3) Bregoli, L. J. Electrochim. Acta 1978, 23, 489. (4) Sattler, M. L.; Ross, P. N. Ultramicroscopy 1986, 20, 21.

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