Comparison of Oxygen Reduction Reaction at Silver Nanoparticles

Apr 23, 2012 - Adenosine 5′-triphosphate capped silver nanoparticles (ATP-Ag NPs) with diameters of 4.5 ± 1.1 nm were synthesized using a one-pot c...
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Comparison of Oxygen Reduction Reaction at Silver Nanoparticles and Polycrystalline Silver Electrodes in Alkaline Solution Poonam Singh and Daniel A. Buttry* Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States S Supporting Information *

ABSTRACT: Adenosine 5′-triphosphate capped silver nanoparticles (ATP-Ag NPs) with diameters of 4.5 ± 1.1 nm were synthesized using a one-pot chemical reduction route. After removal of the ATP capping ligands, the electrochemical activity of these NPs for the oxygen reduction reaction (ORR) in aqueous alkaline solution (0.1 M NaOH) was studied in a single layer-by-layer (1 L LbL) film of NPs and compared with bulk polycrystalline Ag using cyclic voltammetry (CV) and rotating disk electrode (RDE) experiments. For the NPs in the 1 L LbL film, the active area of catalyst available for the ORR was calculated using the charge for the underpotential deposition (UPD) of lead (Pb) on the NPs. RDE data were analyzed to determine the ORR rate constant and the number of electrons (n-value) involved in oxygen reduction. Analysis of Koutecky−Levich (K−L) plots indicates an n-value between 3 and 4 with a higher n-value under some conditions for the NPs than for bare polycrystalline Ag. Possible origins of the variation of n-values for the Ag NPs compared to bulk Ag are discussed.

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INTRODUCTION This paper describes a study of the oxygen reduction reaction at 4.5 nm diameter silver nanoparticles (NPs) incorporated into layer-by-layer films. The silver NPs are prepared by reduction of Ag+ with aqueous borohydride in the presence of adenosine 5′-triphosphate (ATP) and have previously been studied electrochemically.1 The solid NPs may be isolated and redissolved in aqueous solution, and these solutions may be used to prepare layer-by-layer films containing the NPs. Oxidation of the Ag NPs to Ag2O in aqueous base followed by reduction back to the metallic state leads to loss of the ATP ligands. This provides “bare” Ag NPs that can be studied for their electrocatalytic activity in the oxygen reduction reaction (ORR). The kinetic parameters were evaluated and compared to polycrystalline Ag using ORR experiments. The analysis of Koutecky−Levich (K−L) plots gave the rate constant and the number of electrons exchanged per oxygen molecule for the ORR (the n-value). The study is one of only a limited number providing quantitative rate data on the electrocatalytic activity of Ag NPs with well-characterized size dispersity under alkaline conditions. Development of new electrocatalysts for the ORR and understanding of the mechanistic details by which they catalyze the reaction are important for fuel cell applications. Much past effort has been focused on Pt, given its excellent activity as an ORR catalyst.2 However, its high cost and low earth abundance have made it increasingly unattractive for such applications. A resurgence of interest in alkaline fuel cells and Li air batteries has been driven by the recognition that non-Pt catalysts with high activities are available for such conditions.3,4 In particular, this resurgence has refocused attention on Ag, which is a good ORR catalyst under alkaline conditions.5−8 However, in © 2012 American Chemical Society

contrast to the case of Pt, for which a large number of studies of ORR activity of Pt NPs exist, relatively few studies have described the ORR activity of well-characterized Ag NPs. A key issue for ORR activity at Ag or other catalysts is whether O2 is reduced by two electrons to H2O2 (or HO2−) or four electrons to water (or OH−), i.e., with an n-value of 2 or 4. In general, an n-value of 4 is the preferred pathway, because of both the higher currents available and the unwanted chemical reactivity of H2O2 and its decomposition products toward various fuel cell components.9 A few earlier studies have addressed the n-value, generally finding that Ag functions as a four electron catalyst. However, the determination of the nvalue can be complicated by substrate issues because of the two electron reduction of O2 observed on the C surfaces typically used to support the metal NP catalyst.3 According to Léger et al.,10 20 wt % Ag on C (Ag/C) with a particle size close to 15 nm showed 3.6−3.7 electrons exchanged during the ORR. More recently, Léger et al.11 studied the influence of metal loading (i.e., Ag/C) on the ORR and reported 20 wt % as the optimum loading of Ag with an n-value of 3.6−3.8. Chen et al.12 reported comparable performances of 60 wt % Ag/C and 20 wt % Pt/C, again suggesting a four electron pathway for Ag under these conditions. Very recent work by Chen et al.13 described an improvement in catalytic activity of carbon supported citrate-capped Ag NPs for the ORR with increasing metal loading from 10 to 60%, with n-values near 4. In contrast, a few studies have reported n-values near 2. For example, Zhang et al.14 have shown a four electron pathway for 20 wt % Ag/C Received: February 20, 2012 Revised: April 18, 2012 Published: April 23, 2012 10656

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propanol. This process was repeated five times to eliminate free ATP from the NP sample. Characterization. To prepare transmission electron microscopy (TEM) samples, a dilute solution of ATP-Ag NPs (0.2 mg/mL) was drop-pipetted onto a carbon film supported on a copper TEM grid, and the sample was allowed to dry in air for 30 min. Samples were imaged using a Philips CM12S TEM operating in bright field mode at 80 kV. Electrochemical Characterization for Oxygen (O2) Electroreduction. The cyclic voltammetry (CV) experiments on NPs were performed with a CHI760C potentiostat in a 50 mL conical flask which was used as an electrochemical cell. Rotating disk electrode (RDE) experiments on NPs were performed in a 400 mL RDE electrochemical cell with a rotating disk electrode (RDE) attached to a Pine AFMSRX rotator coupled with a CHI760C bipotentiostat. RDE experiments on polycrystalline Ag electrodes were performed on a Pine AFMSRX rotator attached to an AFRDE5 bipotentiostat. The working electrodes used for CV and RDE voltammetric experiments were a gold disk electrode (diameter = 2 mm), an RDE gold electrode (diameter = 5 mm), and an RDE Ag electrode (diameter = 6 mm). A spiral Pt wire counter electrode and a Ag/AgCl reference electrode (saturated with NaCl) were used in all experiments. All potentials are referenced to this Ag/AgCl reference electrode. Working electrodes were polished sequentially with aqueous slurries of 0.3, 0.1, and 0.05 μm alumina powders on a polishing microcloth. The electrodes were then sonicated in DI water for 10 min between polishing steps. The RDE Ag electrode was polished sequentially with 0.1 and 0.05 μm alumina powders followed by sonication in DI water for 10 min and then dried under N2 for 10 min. This polishing procedure is known to produce a surface roughness of 1.1−1.2; a value of 1.1 was used for all calculations.20 Polished gold electrodes were cycled at 100 mV/s in 0.5 M H2SO4 between 0 and 1.43 V for approximately 10 min until a CV characteristic of a clean gold surface was obtained. The gold electrodes were then rinsed with DI water, dried under N2, and used as substrates for immobilizing NPs using the LbL film growth procedure described below. All the solutions used for electrochemical measurements were made up with DI water and were extensively purged with N2 or O2 gas before use. Current densities for all plots were calculated using the geometric surface area of the electrodes except for the ORR kinetic response of the NPs, where the electrochemical surface area was obtained using underpotential deposition (UPD) of Pb on the Ag NP surface (see below). Layer-by-Layer (LbL) Assembly and Characterization. ATP-capped Ag NPs were immobilized on a previously cleaned gold electrode for electrochemical characterization using the LbL assembly method.17,21 The clean gold electrode was dipped in 10 mM 3-mercaptopropionic acid (MCP, Alfa-Aesar, 99%) in ethanol for 60 min to create an MCP self-assembled monolayer (SAM) followed by rinsing in ethanol and drying under N2. The carboxylic acid groups at the exterior of the MCP SAM were deprotonated by placing the electrode into a 0.1 M NaOH (EMD chemicals, 97%) solution for 5 min. The modified Au electrode was then placed in a solution of 2 mg/ mL poly(diallyldimethylammonium) hydrochloride (PDDA, Aldrich) in 0.1 M NaOH (pH ∼12) for 30 min, thus producing the attachment of the first cationic PDDA layer. The electrode was then washed in DI water for 1 min, after which it was placed in an ATP-Ag NP solution (1.5 mg/mL) for 30 min,

with a mean particle size of 174 nm, but reported a two electron pathway for 0.5 wt % Ag/C with a mean particle size of 4.1 nm. Similarly, during a study of Pt−Ag bimetallic catalysts, Ticianelli et al.15 reported an n-value of 2.3 for 20 wt % Ag/C with 47.7 nm as the particle size. We suspect that the low nvalues in these last two studies were likely influenced by the C substrate reactivity, since C is known to catalyze the ORR with an n-value of 2.3,13 To summarize, to our knowledge there are only a few previous studies of the ORR electrocatalytic activity, and especially the n-value, of Ag NPs.10,13−15 Further, all of the previous kinetic studies have used Ag NPs on C substrates. As mentioned above, this can substantially complicate the interpretation of the n-value in basic solutions, since the C substrate reduces O2 by two electrons at a potential near where the metal NPs catalyze the four electron reduction. An advantage of the present study is that the Ag NP catalytic activity is examined in a LbL film on a Au substrate electrode, modified in such a way as to completely poison the substrate electrode toward the ORR. Thus, it is possible here to directly examine the Ag NP catalytic activity in the absence of any possible contribution from the substrate. Previous groups16,17 have investigated the immobilization of Ag NPs in layer-bylayer (LbL) assemblies. However, we are aware of no previous studies of the ORR at Ag NPs immobilized using the LbL method. A final point to be made is that several of the previous studies have employed relatively polydisperse Ag NPs, typically prepared directly on the C substrate. Because of the difficulties of maintaining NP size control under such synthetic conditions, this approach might obscure any influence of size on the electrocatalytic activity, as has been observed, for example, in the case of Pt.18 In the present study, the synthetic approach used allows for independent, substrate-free production of Ag NPs whose size distribution can be carefully characterized prior to their immobilization in the LbL film and use in the ORR. This provides a better characterized catalyst sample that opens the possibility for future studies of the influence of size on the catalytic reactivity.

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EXPERIMENTAL SECTION NP Synthesis and Purification. All chemicals including silver nitrate (AgNO3, 99.999%), adenosine 5′-triphosphate disodium salt hydrate (Na2ATP, 99%), and sodium borohydride (NaBH4, 99.9%) were purchased from Sigma-Aldrich. Deionized (DI) water of resistivity 18.3 MΩ from a Millipore purification system was used for all solutions. ATP-capped silver NPs were prepared by a method similar to one used in a previous report of ATP-capped Au NPs as described by Brook et al.19 Briefly, AgNO3 (10 mM, 3 mL) and Na2ATP (10 mM, 15 mL) were added to 138 mL of DI water in a sealed glass vial under bubbling N2, and the solution was stirred at room temperature for 20 min. Freshly prepared NaBH4 (100 mM, 5 mL) was then quickly added with continued stirring. A light yellow color appeared immediately upon addition of NaBH4, indicating the formation of ATP-capped silver nanoparticles (ATP-Ag NPs). The formation and growth of the NPs was allowed to proceed for 4−5 h. Following this, the NP solution was concentrated from 161 mL to 0.5−1 mL on a rotary evaporator. Solid NPs were obtained by addition of 0.1−0.5 mL of 2-propanol to 0.5−1 mL of concentrated NP solution, which caused precipitation of the NPs. They were isolated by centrifugation at 1000 rpm for 5 min. The solid NPs can be readily redispersed in water and precipitated again using 210657

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thus attaching a quantity of ATP-Ag NPs to the PDDA surface layer. In the present case, no final layer of PDDA was used in order to avoid complications from PDDA adsorption and surface contamination. The surface morphology of the LbL films was investigated using noncontact atomic force microscopy (NcAFM). NcAFM samples of single layer-bylayer (1 L LbL) films were prepared on a flat gold film of 150 nm thickness with exclusively (111) orientation that was obtained by electron beam deposition on freshly cleaved mica at 340 °C.22 NcAFM measurements were carried out using phosphorus doped silicon cantilevers (Veeco, Model No. RTESPA-M) with a force constant of 20−80 N/m. Prior to surface area characterization by Pb underpotential deposition (see below) and use in the ORR, the ATP-Ag NPs were characterized using cyclic voltammetry under aqueous alkaline conditions. As described briefly below, under these conditions the Ag NPs can be oxidized completely to the Ag2O redox state. This process is chemically reversible (i.e., both redox states, Ag and Ag2O, are chemically stable under the conditions). This oxidation causes the NPs to lose their ATP capping ligands. However, NcAFM images show that they retain their discrete identities as NPs (i.e., they do not agglomerate) and remain within the LbL film after such cycling. Thus, this procedure was used to remove the ATP capping ligands so that the surface area and ORR activity of the “bare” Ag NPs could be assayed. Determination of Electrochemical Surface Area Using Pb Underpotential Deposition (UPD). The electrochemically accessible surface area of the Ag NPs available for ORR was determined using Pb UPD.23−25 This was done on 1 L LbL films of ATP-Ag NPs on gold electrodes in a solution of 5 mM lead oxide (PbO, Sigma-Aldrich), 28 mM perchloric acid (HClO4, Sigma-Aldrich, 70%), and 0.1 M sodium perchlorate (NaClO4, Sigma-Aldrich). The area under the Pb UPD stripping peak was used to calculate the available, true Ag surface area using the theoretical Pb UPD surface charge on Ag of 260 μC cm−2.24 This available area was used to calculate the current density per unit area of the active electrocatalyst in RDE measurements for the Ag NPs.

Figure 1. TEM of ATP-Ag NPs. Inset shows the histogram. Size = 4.5 ± 1.1 nm; N = 2176.

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RESULTS AND DISCUSSION Figure 1 shows a typical TEM image of the ATP-Ag NPs. The inset shows the particle size distribution histogram confirming the average diameter of 4.5 ± 1.1 nm for N = 2066. X-ray diffraction (XRD) of a powder sample of the NPs (Figure S1 in the Supporting Information) confirms that they have the facecentered-cubic structure of Ag. An NcAFM image of a 1 L LbL film of ATP-Ag NPs on an atomically flat region of a (111) textured gold film on mica is shown in Figure 2. The image clearly shows homogeneously distributed NPs in the film with a relatively high particle density. The vertical height measurement in the AFM image (which is more accurate than the width of the NP due to tip-convolution effects26,27) confirms the presence of NPs with a height of ∼4.5 nm. These data show that the one-pot synthetic approach allows synthesis of high quality ATP-Ag NPs with a relatively narrow size distribution and their use in the production of high quality LbL films containing the ATP-Ag NPs. Figure 3 shows cyclic voltammetric data for a 1 L LbL film of ATP-Ag NPs in N2 saturated 0.5 M NaCl solution in the potential range of −0.4 to +0.25 V. Scans were done before and after ORR experiments to assess the stability of the LbL film under ORR conditions (see below for a discussion of the results

Figure 2. NcAFM of 1 L LbL film of ATP-Ag NPs. Scan area: 1 μm × 1 μm.

Figure 3. Cyclic voltammogram of 1 L LbL film of Ag NPs in N2 saturated 0.5 M NaCl before (black) and after (red) O2 reduction experiments. Scan rate: 10 mV/s.

after ORR). Considering the black (“before ORR”) curve, the positive scan shows an anodic wave with a peak potential of +0.08 V, and the negative scan shows the corresponding 10658

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cathodic wave with a peak potential of −0.05 V. These are very close to the reversible potential of 0.0 V for the Ag/AgCl redox couple under these conditions. As discussed previously, the redox process responsible for these two waves is the chemically reversible oxidation of the Ag NPs to AgCl NPs.1,28 More specifically, the redox process causes complete conversion of the Ag in the ATP-Ag NPs to the AgCl phase and back, which corresponds to a reversible redox driven phase transformation of the NPs. The anodic and cathodic charges are equal, indicating chemical reversibility. The surface coverage of NPs in a typical film prepared as described in the Experimental Section is 30−50% of a full, close-packed monolayer, so the NPs are both entangled in the PDDA polymer used to form the LbL film and effectively isolated from one another. This result is consistent with the NcAFM image in Figure 2. It should be noted that lateral tip-convolution effects influence imaging of the NPs in the layer,26,27 so the NcAFM image should be taken only as a qualitative representation of the lateral distribution of NPs in the film. As will be seen below, electrochemical ORR studies of these NP films result in a loss of approximately 8% of the NPs. This shows that a major fraction of the NPs are sufficiently tightly bound in the LbL film to survive the ORR conditions. Cycling through the oxidation and reduction processes shown in Figure 3 completely removes the ATP capping ligands, leaving “bare” Ag NPs entrapped within the PDDA layer in the LbL film. This is shown by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM−EDS) on NP samples before and after the electrochemical cycle. The EDS spectrum of a bulk sample of the ATP-Ag powder (Figure S2 in the Supporting Information) shows a P/Ag atomic ratio of 0.26. This gives a mass fraction of ligand in the NP sample of approximately 1/3. This is consistent with a relatively close-packed coverage of ATP capping ligands on the NP surface. The most likely mode of binding is via interaction of the N7 nitrogen and exocyclic amine from the adenine headgroup of the ATP, based on earlier reports of adenine adsorption at Ag surfaces.29 To explore the fate of the capping ligands during redox cycling, the ATP-Ag NPs in a 7 L LbL NP film were oxidized to AgCl and subsequently reduced back to Ag NPs on indium−tin oxide. Ligand loss was verified from the disappearance of the phosphorus X-ray fluorescence peak in the EDS spectrum, but a strong Ag signal remains (Figure S3 in the Supporting Information). Thus, the capping ligands are lost as a result of redox cycling between Ag and AgCl in a chloride medium, but the resulting bare Ag NPs appear to remain entangled in the PDDA and therefore trapped in the film. It is not clear how the presence of the PDDA affects the electrochemical response of the NPs since no data are available on the behavior of the NPs in the absence of the polymer. Following removal of the ATP capping ligands, the electrochemically active surface area of the “bare” Ag NPs was characterized by Pb UPD. Figure 4 shows a cyclic voltammogram for Pb UPD for a 1 L LbL film of ATP-Ag NPs. Figure 4a shows the scan of a freshly made LbL film. The cathodic peak at −0.27 V corresponds to the deposition of the Pb UPD layer, and the anodic peak at −0.37 V corresponds to its removal. Figure 4b shows the result after removal of the ATP capping ligands by cycling in a chloride medium as described above. In this case, the current peaks are less separated, showing cathodic and anodic peak potentials of −0.33 and −0.29 V, respectively. Also, the UPD charge is obviously larger after ATP removal. This larger charge

Figure 4. Underpotential deposition of Pb on Ag NPs and polycrystalline Ag in 5 mM PbO in 28 mM HClO4 in 0.1 M sodium perchlorate. Scan rate: 10 mV/s.

shows the increased UPD area available after removal of the ATP capping ligands. This is consistent with the expectation that UPD would be partially or completely blocked by the presence of a strongly adsorbed capping ligand at the NP surface.23 For comparison, Figure 4c shows the UPD of Pb on a clean polycrystalline Ag surface. This curve has essentially the same features as those in Figure 4b, consistent with the notion that the Ag NP surfaces are free of the strongly adsorbing ATP capping ligands after oxidation in basic solution. The electrochemical area available for UPD was calculated from the UPD charge using the theoretical Pb UPD surface charge on Ag of 260 μC cm−2,24 and then used in kinetic calculations for the ORR reactions described below. This assumes that the area available for UPD deposition is identical to the area available for the ORR, which seems reasonable. Judging from the similarity between the UPD response at bare Ag and at Ag NPs that have lost their ATP ligands, entanglement within the PDDA polymer does not appear to significantly affect the UPD response of the NPs. The UPD results can be compared to the few previous studies of UPD on metal nanoparticles. Schevchenko and coworkers studied Pb UPD on 6−8 nm diameter Ag colloids adsorbed onto Ti electrodes.30 They observed peak morphologies very similar to those reported here. However, quantitative comparisons are not possible since they did not report the surface coverage of metal NPs adsorbed at the electrode surface. Both the present results and those reported by Shevchenko and co-workers are in contrast to previous studies of UPD on metal NPs by Compton and co-workers. They studied Pb UPD on Ag NPs in two size ranges, 20−40 nm and 80−120 nm,31 and Tl UPD on Au NPs in two size ranges, either small (10 nm diameter) or large (diameters in the range 30−60 nm).32 They reported that UPD of Pb on Ag NPs is observed for larger Ag NPs in the size range size 80−120 nm, but not for small diameter Ag NPs with sizes in the 20−40 nm range. Similarly, they reported that Tl UPD on Au NPs is not observed for 10 nm diameter Au NPs, but can be observed for larger NPs with diameters of 30−60 nm. The origin of this discrepancy is not clear, though the sensitivity of the UPD process to adsorbates (as shown in Figure 4) may have some bearing on these observations.23 Nevertheless, the results in Figure 4 and those presented by Shevchenko and co-workers show unequivocally that Pb UPD can occur on Ag NPs in the size range 4−8 nm. Interestingly, the peak morphology for the UPD process at these small diameter NPs is similar to that at polycrystalline Ag,24 but substantially different from those on 10659

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large atomically flat terraces at single crystal Ag(111) surfaces,25 which are very sharp while those in Figure 4 are quite broad. Turning now to the ORR reaction, Figure 5 shows the cyclic voltammogram for a 1 L LbL film of “bare” Ag NPs in O2

results of steady state RDE ORR experiments in O2 saturated 0.1 M NaOH at rotation rates from 100 to 2500 rpm for Ag NPs and bulk polycrystalline Ag, respectively. The onset of the ORR is observed for both cases near −0.1 V, with diffusion limited behavior being achieved between −0.4 and −0.6 V depending on rotation rate. The expected increase in diffusion limited current density is observed as a function of rotation speed, with somewhat higher diffusion limited currents at the bulk Ag electrode than at the Ag NP film. This discrepancy is likely due to small differences in the actual electrochemically active surface area between the two samples. Nevertheless, the behavior at 4.5 nm diameter Ag NPs and bulk polycrystalline Ag is quite similar. Two significant features can be observed in the RDE data. First, both the Ag NP film and the bulk Ag electrode show a decrease in the diffusion limited current as the potential is made more negative in the diffusion limited region. This may suggest a decrease in the n-value in this region, a phenomenon that has been previously observed at Ag single crystal surfaces.5 Second, the RDE curves show an obvious hysteresis between the negative-going (forward) and positive-going (reverse) potential scans. Specifically, at higher rotation rates the currents are higher on the reverse scans than on the forward scans. This effect is somewhat more pronounced for the Ag NP film than for the bulk Ag electrode, and hysteresis is observed even though the scan rate (10 mV/s) is slow enough for steady state conditions to prevail. We return to these points below. To determine the overall stability of the Ag NPs in the LbL film toward the Pb UPD and ORR experimental conditions, the electrochemical charge for the Ag/AgCl redox process was used to monitor any possible loss of NPs after the ORR studies. The results are shown in Figure 3. Comparison of the red (“after”) curve with the black (“before”) curve reveals a loss of 8% of the NP oxidation charge after the ORR experiments. This suggests some loss of a small fraction of the NPs from the film or erosion of the NPs during the reduction. However, the stability of the Ag NPs in the LbL film is sufficiently robust to allow their ORR activity to be further characterized. In order to quantitatively evaluate the ORR kinetics (including n-values) at the bulk Ag electrode and Ag NP film, analysis of the RDE data was done using the Koutecky− Levich (K−L) formalism as given by33

Figure 5. Cyclic voltammograms for O2 reduction on LbL film of ATP-Ag NPs in O2 saturated 0.1 M NaOH at different scan rates.

saturated 0.1 M NaOH between potentials 0 and −0.8 V at three different scan rates. The cathodic wave with a peak potential between −0.3 and −0.4 V is attributed to O2 reduction. The linear increase of the peak current with the square root of the scan rate confirms diffusion controlled O2 reduction at the NP film surface. Figures 6 and 7 show the

Figure 6. Steady state polarization curves of O2 reduction on 1 L film of ATP-Ag NPs in O2 saturated 0.1 M NaOH under different rotation rates. Potential scan rate: 10 mV/s.

1 1 1 1 1 = + = + 2/3 −1/6 0 1/2 j jk jd nFkC 0 0.62nFDO2 ν Cω (1)

where j is the measured current density at a given potential, jk and jd are the kinetic and diffusion limited current densities, k is the (potential dependent) rate constant for O2 reduction, n is the number of electrons involved in the oxygen reduction reaction, F is the Faraday constant (96 485 C mol−1), ω is the rotation rate (rad s−1), C0 is the saturation concentration of O2 in 0.1 M NaOH at 1 atm O2 pressure (1.26 × 10−6 mol cm−3),6 DO2 is the diffusion coefficient of O2 (1.93 × 10−5 cm2 s−1),6 and ν is the kinematic viscosity of the solution (1.09 × 10−2 cm2 s−1).6 The disk currents at several rotation rates for Ag NPs and bulk Ag (shown in Figures 6 and 7, respectively) were used to construct K−L plots for each of several potentials as shown in Figures 8 and 9. The K−L plots are linear at a given potential, but the slopes and intercepts vary as a function of potential for the bulk Ag electrode and the Ag NP film. Table 1 shows the n-values (from the slopes) and rate constants (from

Figure 7. Steady state polarization curves of O2 reduction at bare Ag in O2 saturated 0.1 M NaOH under different rotation rates. Potential scan rate: 10 mV/s.

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potential of −0.6 V. In contrast, for the Ag NP film the n-value decreases only to 3.7 over this same potential range, suggesting that the four electron pathway is better preserved at the Ag NP surface than the bulk Ag surface as the potential is made more negative. Note that, in contrast to previous studies of n-values for Ag NP ORR electrocatalysis,10,13−15 the Ag NPs in the present study are not supported on a C substrate. Thus, there is no interference from the substrate on the n-values reported here. Comparison of the rate constants for the bulk Ag electrode and the Ag NP film reveals that they are similar in the kinetically limited region, but deviate as the reaction becomes diffusion controlled at more negative potentials, with the bulk Ag electrode showing a roughly 2-fold larger rate constant in this region. It is interesting that this higher rate at the bulk Ag electrode compared to the Ag NP film is coincident with the emergence of the two electron reaction pathway at more negative potentials for the bulk Ag electrode. The potential dependent change in n-value for the two catalyst materials can be interpreted in the context of past work on the ORR at (111), (100), and (110) single crystal Ag surfaces by Ross and co-workers.5,6 To briefly summarize their findings, they showed that all three principal surfaces are effective catalysts for the ORR in alkaline media, with similar rate behaviors for all three. However, they did observe small differences in the onset potential for the ORR suggesting the following ordering of reactivity: (110) ≥ (111) ≥ (100). They also observed a slight dependence of n-value on potential for all three surfaces, with the two electron pathway representing approximately 1−4% of the reaction pathway at potentials between 100 and 400 mV past the onset of the diffusion limited region. They described the ORR kinetics as influenced by two factors: (1) the competition between O2 and adsorbed OH species (OHads) for surface adsorption sites and (2) the balance between the availability of free sites and the energy of adsorption of O2.5 They also argued that the rate differences between the various crystal faces can be understood in terms of a more favorable adsorption energy for O2 on the more active faces. They further speculated that the balance between the two electron and four electron pathways is controlled by the availability of neighboring adsorption sites at the metal surface.6 In their model, O2 chemisorption with both O atoms interacting strongly with the surface is required for the four electron pathway, thus requiring neighboring sites. In a related discussion, they proposed that hysteresis in RDE experiments of the type shown in Figures 6 and 7 arises because of the potential dependence of the OH ads surface coverage.5 Specifically, while Pt shows a strong hysteresis because of the need to reductively remove OHads to completely activate the surface, the Ag single crystal faces showed a weaker hysteresis, suggesting weak potential dependence of OHads surface coverage as well as a lower overall surface coverage of OHads on Ag compared to Pt in alkaline solutions. Following their analysis, we propose the following for the ORR at bulk Ag and Ag NP films. First, the significant hysteresis in the RDE scans, which we also attribute to potential dependent OHads, and the presence of a substantial two electron pathway suggest that OHads is more significant for both bulk Ag and Ag NPs than for the single crystal surfaces studied by Ross and co-workers. This is likely due to the presence of a larger fraction of defective surface sites (e.g., steps and kinks) at the bulk Ag and Ag NP surfaces than at the single crystal surfaces. Second, the observation that the two electron pathway

Figure 8. Koutecky−Levich plots from Figure 6 for the O2 reduction at ATP-Ag NPs in O2 saturated 0.1 M NaOH for electrode potentials −0.35, 0.4, −0.45, −0.5, −0.55, and −0.6 V vs Ag/AgCl/KCl (saturated). As a guide to the eye, the red solid lines show the linear fits for potentials of −0.35 and −0.6 V.

Figure 9. Koutecky−Levich plots from Figure 7 for the O2 reduction at bulk polycrystalline silver in O2 saturated 0.1 M NaOH for electrode potentials −0.35, 0.4, −0.45, −0.5, −0.55, and −0.6 V vs Ag/AgCl/ KCl (saturated). As a guide to the eye, the red solid lines show the linear fits for potentials of −0.35 and −0.6 V.

Table 1. Comparison of the Koutecky−Levich Data for Polycrystalline Silver and LbL Film of Ag NPs bare Ag

LbL film of ATP-Ag NPs

potential (V)

n

k (cm/s) × 10−2

n

k (cm/s) × 10−2

−0.35 −0.4 −0.45 −0.5 −0.55 −0.6

4.1 3.7 3.5 3.4 3.4 3.2

0.860 1.74 2.90 4.03 4.61 5.54

4.1 4.0 3.9 3.8 3.7 3.7

0.678 1.15 1.55 1.96 2.15 2.16

the intercepts) extracted from the K−L analysis. Rate constants for the bulk Ag electrode and the Ag NP film were calculated based on the electrochemically active surface area, determined from the surface roughness (1.1) and the UPD area, respectively. In the kinetically limited region (near −0.35 V), an n-value of 4 is observed at both catalysts, verifying the four electron reduction of O2. However, both materials show a decrease in the four electron pathway and an increase in the two electron pathway (resulting in a lower apparent n-value) as the potential is made more negative, as previously observed for cases where anion adsorption causes a mixed n-value between 4 and 2.6 Interestingly, this effect is most pronounced for the bulk Ag electrode, for which the n-value decreases to 3.2 at a 10661

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supported by the National Science Foundation under Grant CHE-0957122.

becomes more significant at more negative potentials for the bulk Ag and Ag NP surfaces suggests that the OHads surface coverage may become more significant at potentials in the diffusion limited region. Both results argue that OHads may exist at relatively high surface coverages over a wide potential range on the bulk Ag and Ag NP surfaces, in contrast to the behavior postulated for the single crystal surfaces. Further, the smaller contribution of the two electron pathway on the Ag NP surface compared to the bulk Ag surface may suggest, somewhat counterintuitively, that the Ag NP surface has a smaller fraction of high energy adsorption sites for OHads, thereby permitting a larger proportion of the reaction to proceed along the four electron pathway. Figure S4 in the Supporting Information shows the high resolution transmission electron microscopy (HRTEM) image of a NP illustrating a relatively smooth spherical surface. This is in contrast to the very rough surface produced by simple polishing of bulk Ag.34 Finally, we note that the finding of a two electron pathway for the Ag NPs in the present case cannot be attributed to an interfering two electron pathway from the support, since the support used in the present case is a thiol-passivated Au surface. This is in contrast to past studies of Ag NPs, all of which used C supports capable of independently catalyzing the two electron pathway in alkaline conditions. Thus, this represents the first unambiguous determination of the n-value for Ag NPs under basic conditions.

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CONCLUSIONS LbL films of ATP-Ag NPs were prepared and electrochemically treated so that the ATP capping ligands were removed. Removal of the capping ligands was shown using UPD of Pb on the Ag NPs. These same measurements also provided the electrochemically active surface area for the ORR electrocatalysis at the Ag NP films. These “bare” Ag NPs were then compared with bulk Ag as electrocatalysts for the ORR under alkaline conditions. The results demonstrate a predominantly four electron pathway for both the Ag NPs and bulk Ag. The higher fractional contribution of the two electron pathway on bulk Ag was attributed to a more defective surface on bulk Ag, at which stronger adsorption of hydroxyl species prevents the two-site chemisorption of O2 that is required for the four electron pathway. The LbL film approach was also shown to prevent interference in interpretation of the mechanistic pathway from the C substrate, as has been observed in past studies.

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ASSOCIATED CONTENT

S Supporting Information *

XRD diffraction data, EDS spectra, and HRTEM image. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the electron microscopy laboratory in the ASU School of Life Sciences Bioimaging Facility for providing access to the TEM facility. This material is based upon work 10662

dx.doi.org/10.1021/jp301676n | J. Phys. Chem. C 2012, 116, 10656−10663

The Journal of Physical Chemistry C

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(34) Dharmadhikari, C. V.; Kshirsagar, R. B.; Ghaisas, S. V. Europhys. Lett. 1999, 45, 215−221.

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dx.doi.org/10.1021/jp301676n | J. Phys. Chem. C 2012, 116, 10656−10663