Oxygen Reduction Reaction and Peroxide Generation on Shape

Article pubs.acs.org/Langmuir

Oxygen Reduction Reaction and Peroxide Generation on ShapeControlled and Polycrystalline Platinum Nanoparticles in Acidic and Alkaline Electrolytes Ruttala Devivaraprasad,† Rahul Ramesh,† Nalajala Naresh,†,‡ Tathagata Kar,† Ramesh Kumar Singh,† and Manoj Neergat*,† †

Department of Energy Science and Engineering, Indian Institute of Technology Bombay (IITB), Powai, Mumbai, India 400076 IITB-Monash Research Academy, Powai, Mumbai, India 400076

‡

S Supporting Information *

ABSTRACT: Shape-controlled Pt nanoparticles (cubic, tetrahedral, and cuboctahedral) are synthesized using stabilizers and capping agents. The nanoparticles are cleaned thoroughly and electrochemically characterized in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes, and their features are compared to that of polycrystalline Pt. Even with less than 100% shape-selectivity and with the truncation at the edges and corners as shown by the ex-situ TEM analysis, the voltammetric features of the shape-controlled nanoparticles correlate very well with that of the respective single-crystal surfaces, particularly the voltammograms of shape-controlled nanoparticles of relatively larger size. Shape-controlled nanoparticles of smaller size show somewhat higher contributions from the other orientations as well because of the unavoidable contribution from the truncation at the edges and corners. The Cu stripping voltammograms qualitatively correlate with the TEM analysis and the voltammograms. The fractions of lowindex crystallographic orientations are estimated through the irreversible adsorption of Ge and Bi. Pt-nanocubes with dominant {100} facets are the most active toward oxygen reduction reaction (ORR) in strongly adsorbing H2SO4 electrolytes, while Pttetrahedral with dominant {111} facets is the most active in 0.1 M HClO4 and 0.1 M NaOH electrolytes. The difference in ORR activity is attributed to both the structure-sensitivity of the catalyst and the inhibiting effect of the anions present in the electrolytes. Moreover, the percentage of peroxide generation is 1.5−5% in weakly adsorbing (0.1 M HClO4) electrolytes and 5− 12% in strongly adsorbing (0.5 M H2SO4 and 0.1 M NaOH) electrolytes. sites.3 However, the ORR activity in 0.1 M HClO4 on the Pt {hkl} surface, based on the half-wave potentials, was found to be in the order of {110} > {111} > {100}.4,5 These differences in the ORR activity were mostly attributed to the differential adsorption of perchlorate (ClO4−) anion on the different planes of the Pt surface. Although the degree of specific adsorption of the chlorate anions on the metal surface is not as strong as that of the (bi)sulfate anions, the ORR kinetics was observed to be affected significantly on different Pt {hkl} surfaces.4 The order of ORR activity and peroxide generation of Pt {hkl} surfaces in alkaline electrolytes (0.1 M NaOH and 0.1 M KOH) was found to be {111} > {110} > {100}.7,11 This difference was attributed to the structure sensitivity of hydroxyl (OH−) ions on different surfaces and its inhibiting effect.7 The superior ORR activity of

1. INTRODUCTION Kinetics of oxygen reduction reaction (ORR) depends on the geometric factors (low-index crystallographic orientations {hkl} present on the catalyst surface) as reported on single-crystal surfaces of Pt and Pd and on electronic factors decided by the surface/subsurface/bulk composition of well-defined alloy surfaces.1−6 Structure-sensitive ORR kinetics is also significantly affected by the anion (HSO4−, ClO4−, and OH−) adsorption from the electrolyte medium (H2SO4, HClO4, and NaOH) on the single-crystal Pt electrodes. Thus, Markovic et al. and others investigated the ORR kinetics on single-crystal electrodes in different electrolytes (both acidic and alkaline).3−5,7−12 The order of ORR activity in 0.05 M H2SO4 on the Pt {hkl} surface was found to be {110} > {100} > {111}, whereas, the peroxide generation follows the order of {111} > {100} > {110}.3 The difference in activity toward ORR in 0.05 M H2SO4 electrolyte was mainly attributed to the inhibiting effect of the adsorbed (bi)sulfate (HSO4−) anion on different © 2014 American Chemical Society

Received: April 14, 2014 Revised: June 28, 2014 Published: July 1, 2014 8995

dx.doi.org/10.1021/la501109g | Langmuir 2014, 30, 8995−9006

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Such surface-cleaned nanoparticles are expected to have tremendous implications in the field of electrocatalysis. To compare the activity of Pt single-crystal electrodes with that of shape-controlled Pt nanoparticles, it is imperative to estimate the fraction of low-index planes present on the shapecontrolled nanoparticles using in-situ experimental techniques. Clavilier et al. and others investigated the electrochemical behavior of irreversibly adsorbed heteroatoms (Bi, Ge, Pb, As, Sb, and Te) on different Pt single-crystal electrodes and used it to estimate the fraction of various {hkl} orientations present on the shape-controlled and polycrystalline nanoparticles.32−36 The fraction of low-index planes present on the shapecontrolled Pt nanoparticles estimated using this in-situ probing qualitatively correlates with the ex-situ TEM results.37−39 Thus, Solla-Gullón et al. and others investigated the kinetics of methanol, formic acid, and carbon monoxide oxidation on different cleaned shape-controlled Pt nanoparticles and correlated the activity with that of the single crystal Pt electrodes.40−42 ORR, the complex four-electron reduction of oxygen to water, is one of the classical electrochemical reactions relevant to electrochemical energy conversion and storage, and it is widely investigated on well-defined surfaces and on polycrystalline nanoparticles.3,4,7,43 However, translating the facet {hkl} dependence of ORR activity to practical bulk electrocatalysts (shape-controlled nanoparticles) has met with several challenges. The major issue is the removal of the strongly-adsorbed surfactants and capping agents from the shape-controlled precious metal nanoparticles and their subsequent electrochemical characterization in the desired electrochemical medium. Thus, Sanchez-Sanchez et al. investigated ORR activity on surfactant-free shape-controlled Pt nanoparticles using the tip generation−substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM) through rapid array imaging of microsized catalysts and correlated the results with those on well-defined single crystal electrodes.44 Alexeyeva et al. investigated ORR on impurity-free Pt nanoparticles supported on single-walled and multiwalled carbon nanotubes in acidic and alkaline electrolytes.45 Otherwise, ORR and the peroxide generation on wellcleaned and electrochemically characterized shape-controlled nanoparticles using rotating ring disk electrode (RRDE) technique are yet to be investigated completely. In this article, we report the detailed surface electrochemistry and dependence of ORR activity on various low-index planes present on different well-characterized shape-controlled Pt nanoparticles such as polycrystalline (Pt-PC), cubic (Pt-NC), tetrahedral (PtTD), and cuboctahedral (Pt-CO) nanoparticles. The surface electrochemistry, ORR activity, and peroxide generation of well-cleaned and characterized shape-controlled Pt nanoparticles in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes are correlated with that of the single-crystal electrodes reported in the literature.3,4,7 The electrochemical features of the shape-controlled Pt nanoparticles are also studied through Cu stripping voltammograms in acidic electrolytes (0.5 M H2SO4 and 0.1 M HClO4). The results obtained from the in-situ probing (irreversibly adsorbed heteroatoms (Bi and Ge)) are correlated qualitatively with the Cu stripping voltammograms and ex-situ TEM analysis.

Pt in alkaline medium than that in acidic electrolytes is because of the lower steric hindrance of hydroxyl (OH−) ions compared to that of complex oxy-anions (HSO4− and ClO4−) during O2 adsorption on the Pt-planes13,14 and due to the difference in the ORR mechanism. Kondo et al. investigated the ORR activity of palladium single crystal electrodes (Pd {hkl}) in 0.1 M HClO4 solution, and it is in the sequence of {100} > {111} > {110}.6 The order of ORR activity of Pd {hkl} single-crystal electrode surfaces in the HClO4 electrolyte is exactly opposite to that of Pt {hkl}.6 Having established the dependence of specific crystallographic orientation on the ORR activity of Pt and Pd, there is an increasing interest to develop shape-controlled nanoparticles for their application in electrocatalysis. Synthesis of shape-controlled nanoparticles was initially reported in the literature for magnetic/optical applications.15 These particles were synthesized using various surfactants, capping agents, complex stabilizing agents, and reducing agents such as polyvinylpyrrolidone (PVP), ascorbic acid, citric acid, cetyltrimethylammonium bromide (CTAB), etc.16−18 The capping agents specifically adsorb on selected facets of the initial nuclei, and the growth of the particle is effectively controlled in the desired orientation.17 Depending on the type of the capping agent and its concentration, these crystals grow in specific directions from their nuclei resulting in a variety of selected shapes.17 Various shapes of Pt nanoparticles, viz., cubic, octahedral, truncated-octahedral, triangular, hexagonal, and pyramidal were synthesized; shape-selectivity close to ∼80% is often reported.17,18 Even though the synthesis of nanoparticles with desired size, shape, composition, and structure has been reported following complex procedures through controlled-nucleation and growth, their detailed electrochemical characterization is limited because of the strong adsorption of the reagents on the colloidal nanoparticle surface and low selectivity of the desired shapes.17,18 However, when electrochemically tested, the activity of such materials was observed to be inferior to that of the standard polycrystalline Pt. Moreover, the activity comparison was not with established commercial material but with a surfactant capped polycrystalline Pt prepared under conditions similar to those of the shapecontrolled nanoparticles.19,20 The lower activity of the catalyst was mostly due to the incomplete removal of the strongly adsorbed stabilizers/capping agent, partial selectivity of the shape (20−40%), and large particle size of Pt.21−25 Better understanding is required to control the shape, achieve nearly 100% shape-selectivity, and, most importantly, to clean the surface of the shape-controlled catalysts to obtain an impressive electrochemical and catalytic response. Since nanoparticles capped with organic ligands cannot be directly used as catalysts, a variety of treatments such as thermal annealing, acid washing, and UV/ozone irradiation were applied to remove the organic moieties from the particle surface.26−30 Effective cleaning of the nanoparticles takes place with UV/ozone treatment without altering the size and shape. Vidal-Iglesias et al. investigated the effect of UV/ozone cleaning treatment on shaped-controlled Pt nanoparticles and demonstrated using electrochemical techniques in which the cleaning procedure perturbs the surface structure of the nanoparticle. 31 The resulting surface modification significantly alters the catalytic activity. They also showed that conventional TEM does not allow discriminating between shape and surface structure.31 Thus, it is pivotal to establish procedures for surfactant removal without changing the shape, size, and surface structure of the particle.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Pt-PC, Pt-NC, Pt-TD, and Pt-CO Nanoparticles. Pt-PC nanoparticles were synthesized through the sulfitocomplex route followed by conventional sodium borohydride 8996


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Figure 1. TEM image of Pt-TD nanoparticles (a), HRTEM image of single tetrahedral nanoparticle (b), and SAED pattern of Pt-TD (c); the inset to panel a shows the particle size distribution histogram.

Figure 2. TEM image of Pt-NC nanoparticles (a), HRTEM image of single cubic nanoparticle (b), and SAED pattern of Pt-NC (c); the inset to panel a shows the particle size distribution histogram.

Figure 3. TEM image of Pt-CO nanoparticles (a), HRTEM image of single cuboctahedral nanoparticle (b), and SAED pattern of Pt-CO (c); the inset to panel a shows the particle size distribution histogram. (NaBH4) reduction.46 Pt-TD and Pt-NC nanoparticles were synthesized from H2PtCl6 and K2PtCl4 precursors, respectively, using a sodium poly acrylate (PAA) capping agent and H2 gas reducing agent.47,48 Pt-CO nanoparticles were prepared by the NaBH4 reduction of the K2PtCl4 precursor using tetra-decyl trimethylammonium bromide (TTAB) capping agent.49 These nanoparticles were centrifuged and cleaned thoroughly for further characterization. 2.2. Physical and Electrochemical Characterizations. XRD patterns were recorded using a PANalytical X’Pert PRO machine (30 mA, 40 kV) and HRTEM images with JEOL JEM 2100 F field emission electron microscope. Electrochemical measurements were performed in a conventional three-electrode electrochemical cell using a WaveDriver 20 Bipotentiostat/Galvanostat system from Pine Research Instrumentation, USA. The thin-film catalyst layer on the rotating disk electrode was prepared following the method reported in the literature.50 Fraction of {111} terrace sites were estimated through Bi adsorption and that of {100} terrace sites through Ge adsorption. Bi and Ge adsorption on cleaned Pt nanoparticles were carried out using the procedure reported by Clavilier et al. and Gómez et al., respectively.35,36 Further details on the synthesis and cleaning procedures along with the physical and electrochemical characterizations are discussed in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Physical Characterization. Figures 1, 2, and 3 show the TEM analysis of well-cleaned shape-controlled Pt nanoparticles. The images (Figures 1a, 2a, and 3a) show that the PtTD, Pt-NC, and Pt-CO nanoparticles are well-dispersed, and the insets show the particle size distribution histograms. The average particle size of Pt-TD, Pt-NC, and Pt-CO nanoparticles is found to be ∼6.0, 12, and 11 nm, respectively, and it correlates with that determined from the XRD patterns (Figure S1, Supporting Information). Figure 1b shows the HRTEM image of a single tetrahedral particle, and from the image, it is clear that the tetrahedral structure is formed. It can be seen that the structure consists of triangular surface planes with an interplanar spacing of 0.232 nm (d-spacing) corresponding to the {111} facets. Figure 2b shows the HRTEM image of a single cubic nanoparticle with a calculated interplanar spacing of 0.203 nm corresponding to {100} facets. Figure 3b shows the HRTEM image of a single cuboctahedral particle. It is found that the particle has grown in both {111} and {100} directions with d-spacing of 0.226 and 0.190 nm, respectively. 8997


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Figure 4. CVs of Pt-PC, Pt-NC, Pt-TD, and Pt-CO catalysts in argon-saturated 0.5 M H2SO4 solution recorded at a scan rate of 50 mV s−1.

M HClO4 and 0.1 M NaOH in Supporting Information). Thorough cleaning is imperative to obtain the true voltammetric response on nanoparticles synthesized using different capping agents and stabilizers. While recording the CVs, the upper limit of the potential was restricted to 0.8 and 0.9 V in acidic and alkaline electrolytes, respectively. Potential cycling above 1 V results in the surface reconstruction and consequent loss of the characteristic faceted surface of different shapecontrolled nanoparticles. Figure 4 shows the CVs of Pt-PC, Pt-NC, Pt-TD, and Pt-CO catalysts in 0.5 M H2SO4 solution recorded at a scan rate of 50 mV s−1. The CVs show clear precious metal features, the hydrogenadsorption/desorption (Hads/des) region, double layer region, and oxide formation region at 0.0−0.4, 0.4−0.65, and >0.65 V, respectively.43 Since the surface of these shape-controlled nanoparticles is composed of facets with different Hads/des energy, which exhibits peak features at specific potentials, the hydrogenunder potential deposition (Hupd) features differ depending on the shape of the nanoparticles. The sharp voltametric features of the catalysts in the H2SO4 electrolyte are due to the combined adsorption of HSO4− anion parallel to the adsorption of hydrogen ion.51 On the basis of the investigations on singlecrystal surfaces, the peak at 0.125 V is assigned to the {110} sites and that at 0.27 V to the step contributions associated with {100}/{111} terrace sites on the catalyst surface.51 Sharp peak features pertaining to the {111} sites of the Pt-surface are not observed in the Hupd region of the CVs in 0.5 M H2SO4;

The selected area electron diffraction (SAED) patterns shown in Figures 1c, 2c, and 3c indicate that the Pt-TD, Pt-NC, and Pt-CO particles have an FCC structure with four rings, from inner to outer, corresponding to {111}, {200}, {220}, and {311} crystal planes, which is in accordance with the XRD patterns (Figure S1; see the relevant text in Supporting Information). The HRTEM analysis shows the surface faceting toward different low-index crystallographic planes according to the shape of the nanoparticle. The intense {111} ring with tetrahedral and {200} ring with cubic particles in the ED patterns indicate the shape-selectivity of the platinum nanoparticles. With Pt-TD nanoparticles, the rough shape-selectivity estimated from the TEM images is 70% tetrahedral, 20% spherical, and 10% irregular shapes. Similarly, with Pt-NC nanoparticles, 50% cubic, 30% truncated cubes, and 20% irregular shapes, and with Pt-CO nanoparticles, it is 90% cuboctahedral and 10% irregular shapes. 3.2. Voltammetric Characterization of the Nanoparticles in Acidic (0.5 M H2SO4 and 0.1 M HClO4) and Alkaline (0.1 M NaOH) Electrolytes. Figures 4, S2, and S3 (Supporting Information) show the CVs recorded in 0.5 M H2SO4, 0.1 M HClO4, and 0.1 M NaOH, respectively. The CVs were recorded to evaluate the surface faceting of different lowindex crystallographic planes on the shape-controlled platinum nanoparticles. The characteristic voltammetric features of platinum are clearly visible in both acidic and alkaline eletrolytes (see further discussion on characterization in 0.1 8998


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Figure 5. Cu stripping voltammograms of Pt-PC, Pt-NC, Pt-TD, and Pt-CO with 12 mM CuSO4 solution in argon-saturated 0.5 M H2SO4 solution at a scan rate of 10 mV s−1.

ordered (bi)dimensional terrace sites. The shoulder peak at 0.32−0.37 V due to {100} terrace sites is negligible in Pt-TD’s case as the tetrahedral structure is dominated with {111} and {110} sites. Similar kind of voltammetric features are reported by Solla-Gullón et al. with shape-controlled Pt-TD nanoparticles.41 In Pt-CO nanoparticles, the Hupd features are similar to both Pt-PC and Pt-NC. The peak at 0.125 V is well-defined and sharp, but the peak intensity is low when compared to the peak present at 0.27 V. The major difference in the voltammogram of Pt-CO when compared to the other voltammograms is the relatively higher intensity of the peak at 0.27 V in Pt-CO. This may be due to the higher density of {100}/{111} step sites on Pt-CO. The shoulder peak at 0.32− 0.37 V due to the Hads/des from {100} terrace sites and peak at 0.5 V due to anion adsorption on {111} sites are also evident in the voltammogram. Both {111} and {100} orientations are expected in the case of Pt-CO nanoparticles, and this is evident from the TEM images (see Figure 3). The CV of Pt-CO clearly shows the dominance of {100} and {111} sites, and the surface is relatively less occupied with {110} sites. The ESAs of the catalysts estimated from the Hdes region of the CVs (Figures 4 and S2, Supporting Information) are shown in Table S1 (Supporting Information). The CVs recorded in the 0.1 M HClO4 electrolyte (Figure S2, Supporting Information) do not show the {111} ordered bidimensional terrace sites in the double layer region as that observed in H2SO4. CVs in 0.5 M H2SO4 give clear features when compared to those of the 0.1 M HClO4 electrolyte. All these voltammetric features are in agreement with the Pt singlecrystal electrodes.3,4,51 Similarly, the CVs were also recorded in

instead, a broad and featureless hump starting from 0.06 to 0.4 V is observed without any peak center.3 The peak at 0.5 V is attributed to the HSO4− adsorption on small ordered (bi)dimensional {111} sites.3 The Hads region is exactly the mirror image of the Hdes region, and hence, we observe a highly symmetric Hupd region in the voltammograms. The Pt-PC voltammogram shows two well-defined peaks at 0.125 and 0.27 V corresponding to the {110} and {100}/{111} step sites, respectively.51 A minor hump at 0.5 V due to the anion adsorption on the {111} site is also observed. The peak at 0.125 V corresponding to {110} step sites in the Pt-NC voltammogram is suppressed and broad when compared to that of Pt-PC, where a sharp well-defined peak is observed. The Hads/des region of Pt-NC shows a sharp peak at 0.27 V, and it can be assigned to the {100}/{111} step contribution on cubic structure. In the Pt-NC nanoparticles, the surface is dominated with {100} facets, and with truncation at the corners of the cubic structure, a reasonable fraction of {111} sites is also expected. Thus, we observe a characteristic hump at 0.5 V in the voltammogram. However, the shoulder peak observed in the potential region of 0.32−0.37 V is due to the Hads/des from broad {100} terrace sites in Pt-NC nanoparticles; such features are reported for the single-crystal surface and Pt cubic nanoparticles in the literature.40 The Pt-TD nanoparticles show characteristic peaks due to dominant {111} and {110} sites. The peak observed at 0.125 V due to {110} sites is relatively more intense when compared to the peak at 0.27 V due to {100}/{111} step sites. The major difference observed in the case of Pt-TD when compared to Pt-PC is the comparatively more intense peak at 0.5 V due to the {111} 8999


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Figure 6. CVs of Ge adsorbed (dotted line) and Bi adsorbed (solid line) Pt-PC, Pt-NC, Pt-TD, and Pt-CO catalysts in 0.5 M H2SO4 solution recorded at a scan rate of 50 mV s−1.

({111}, {110}, and {100}) of varying percentage with minor occupancy of high-index facets. From the single-crystal studies, the peak at ∼0.5 V can be assigned to Cu stripping from {110} sites.52 However, the peaks present in the range of 0.72−0.75 V have contributions from both {110} and {100} sites. Similarly, the peaks present in the range of 0.625−0.675 V have contribution from both {110} and {111} sites. Figure 5 shows the background-corrected Cu stripping voltammograms of Pt-PC, Pt-NC, Pt-TD, and Pt-CO nanoparticles with 12 mM of CuSO4 in 100 mL of 0.5 M H2SO4 electrolyte recorded at a scan rate of 10 mV s−1. The stripping voltammograms of all the Pt nanoparticles show three distinct peaks at 0.45, 0.65, and 0.725 V; these peak positions are in accordance with the peaks reported by Green et al.55 In Pt-PC nanoparticles, the peak at 0.45 V can be assigned to Cu stripping from {110} sites of platinum, which is relatively more intense when compared to that on all the other Pt nanoparticles. This implies that Pt-PC has relatively more {110} sites than {111}/{100}. Among the three peaks present in Pt-NC nanoparticles, the first peak feature at 0.45 V is broad and less intense when compared with Pt-PC nanoparticles. The sharp and well-defined peak features present at 0.65 and 0.725 V can be attributed to the presence of both {100}/{111} sites along with {110} sites. However, these peaks are not distinguishable in the case of Pt-PC nanoparticles. From the Cu stripping voltammograms, we can qualitatively conclude that Pt-NC has a higher number of {100} sites than {110} sites when compared to those of Pt-PC nanoparticles. The Cu

alkaline medium to investigate the shape-selectivity of the different shape-controlled nanoparticles. The voltammetric features obtained in 0.1 M NaOH (Figure S3; also see the relevant text in Supporting Information) are in good correlation with the fraction of low-index planes obtained from the irreversible Bi and Ge adsorption in 0.5 M H2SO4 solution (discussed below) and with the ex-situ TEM analysis. 3.3. Reversible Adsorption of Copper (Cu+2) Ions on Shape-Controlled Pt Nanoparticles. Figures 5 and S1 (Supporting Information) show the Cu stripping voltammograms recorded in 0.5 M H2SO4 and 0.1 M HClO4 solution, respectively. The surface of shape-controlled Pt nanoparticles is investigated by stripping of the under-potential deposited (UPD) copper monolayer. Endo et al. investigated Cu stripping from {110} well-defined surfaces in the 0.5 M H 2SO4 electrolyte containing 1 mM Cu2+ ions after potential cycling.52 It is observed that the {110} sites exhibit three different peaks at ∼0.5, ∼0.675, and ∼0.75 V due to the structural changes occurring during the potential cycling and stripping of coadsorbed (bi)sulfate copper (1 × n) overlayer.52 Bittner et al. investigated the growth of the Cu monolayer on the Pt {100}−(1 × 1) well-defined surface with 10 mM CuSO4 in 50 mM H2SO4 supporting electrolyte.53 {100} sites exhibit a sharp peak at ∼0.72 V and a shoulder peak at ∼0.74 V due to the stripping of the Cu monolayer from the Pt surface.53 Pt {111} sites show a Cu stripping peak at ∼0.625 V with 0.05 mM CuSO4 in 50 mM H2SO4 electrolyte.54 The surface of nanoparticles generally consists of all the low-index facets 9000


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Pt4{100}‐Ge + H 2O → Pt4{100}‐GeO + 2H+ + 2e−

stripping volatmmogram of Pt-TD nanoparticles is similar to that of Pt-PC; the only difference is the relatively sharper and well-defined peak at 0.65 V in the former case which can be attributed to the presence of more {111} terrace sites. The Cu stripping voltammogram of Pt-CO is entirely different when compared to the voltammograms of the other shape-controlled nanoparticles. Here too we can observe three peaks: the first peak at 0.45 V due to {110} sites, the second peak at 0.65 V, and a more intense third peak at 0.725 V, which can be attributed to the presence of both {100}/{111} sites along with {110} sites. Similar results are observed with Cu stripping in 0.1 M HClO4 shown in Figure S4 (see the relevant discussion in Supporting Information). These results are in accordance with the ex-situ TEM analysis and the observed voltammetric features in both 0.5 M H2SO4 and 0.1 M HClO4. Cu stripping is a relatively slow process when compared to the Hupd. Therefore, it is difficult to distinguish the surface faceting with different coordinations like terrace, steps, and kinks. Hence, the peaks corresponding to different facets merge together, and it is difficult to assign peaks to a particular site. The information gathered from the Cu stripping voltammetry of a well-defined surface can be used to understand the surface occupancy of different sites on Pt shape-controlled nanoparticles in a qualitative way. However, for a meaningful correlation of surface faceting on nanoparticles with ORR and peroxide formation, a quantitative estimation of the different surface sites is required. The TEM analysis shows the approximate particle size and selectivity toward different shapes of the nanoparticles depending on the synthesis method. Even though the percentage of formation of the desired shape can be evaluated from TEM images, the fraction of various low-index crystallographic planes present on the catalyst surface can be estimated through the irreversible adsorption of Ge and Bi on {100} and {111} facets, respectively, of the shape-controlled Pt nanoparticles. 3.4. Irreversible Adsorption of Bismuth (Bi) and Germanium (Ge) on Shape-Controlled Pt Nanoparticles. Figure 6 shows the voltammograms of the irreversible adsorption of Bi (solid line) and Ge (dotted line) on different shape-controlled Pt nanoparticles in 0.5 M H2SO4 electrolyte recorded at a scan rate of 50 mV s−1. The fraction of {111} and {100} facets is quantitatively estimated using irreversible adsorption of Bi and Ge, respectively. The upper limit of the potential was restricted to 0.75 and 0.6 V while recording voltammograms on Bi and Ge adsorbed Pt surfaces, respectively, to avoid the surface reconstruction and consequent loss of characteristic features. The characteristic surface redox peak for the Bi adatom adsorption on Pt {111} sites is observed at ∼0.62 V.35 Solla-Gullόn et al. proposed that since the Bi atom is bigger than Pt, it will be blocking three sites by the exchange of 2 electrons.37 The surface reaction is written as follows:37

(2)

In Pt-PC, Bi adsorption is observed at ∼0.63 V; this is slightly at a higher potential when compared to that of the Bi redox peak for Pt-NC, Pt-TD, and Pt-CO, where the peak is observed at ∼0.62 V. The slight shift in Bi peak position in the case of Pt-PC is attributed to the {111} terrace size; a very small shift toward higher potential is observed when the terrace size becomes narrower.37 The adsorption of the Bi adatom is slightly strong at the steps when compared to that at the terrace sites, and it will first cover all the steps sites before the adsorption on {111} terrace sites. Thus, the broad and less intense Bi adsorption peak in the case of Pt-PC can be attributed to the increase in step density. The Bi adsorption peak with Pt-NC is sharper when compared to that of Pt-PC. Pt-NC and Pt-CO have Bi redox peaks of comparable intensity, and both have reasonable percentages of {111} terrace contribution. The Bi redox peak is more intense in the case of Pt-TD, and it is expected since the tetrahedral shape mostly consists of {111} sites. The Ge redox peak in the case of Pt-PC is broad when compared to that of the other catalysts. Pt-NC and Pt-CO show very sharp Ge redox peaks, and this feature is in good agreement with the TEM results. The cube and cuboctahedral have a {100} dominant surface. The {100} facet observed in the case of Pt-TD may be due to the truncation of the shape that creates {100} facets at the surface and also because of the presence of the other shapes of nanoparticles. From the CVs shown in the Figure 4 (referring to normal CVs recorded in 0.5 M H2SO4), the charge associated with each low-index plane contributing to the Hupd region can be estimated provided each peak is purely arising from a selected plane. This Hupd charge does not involve any charge associated with the adsorption of foreign adatoms. The charge obtained from the oxidation of adsorbed adatoms, Bi and Ge (Figure 6), and the Hupd charge obtained from the blank voltammograms (Figure 4) can be used to estimate the fraction of {100} and {111} sites. The percentage of {100} and {111} sites thus calculated is shown in Table 1. It can be observed that in the Table 1. Fraction of {111} and {100} Sites of ShapeControlled Platinum Nanoparticles Pt nanoparticles

{111} sites Bi %

{100} sites Ge %

Pt-PC Pt-NC Pt-TD Pt-CO

2 13 60 30

21 52 14 45

case of Pt-PC nanoparticles, the fraction of {100} is ∼21% and that of {111} is ∼2%, which is relatively less as there is no specific orientation dominance in Pt-PC nanoparticles. In PtNCs, as expected for a cubic-shaped Pt nanoparticle, the fraction of {100} sites is ∼52%, and it is higher as compared to the fraction of {111} sites (∼13%). In Pt-TD nanoparticles, the fraction of {111} (∼60%) is higher than the {100} (∼14%) sites. However, in the case of Pt-CO nanoparticles the fraction of {100} is ∼45% and that of {111} is ∼30% due to the presence of both these orientations in the single cuboctahedral nanoparticles. These results are in accordance with the ex-situ TEM analysis and the qualitative results obtained from CVs and Cu stripping voltammograms.

Pt3{111}‐Bi + 2H 2O → Pt3{111}‐Bi(OH)2 + 2H+ + 2e− (1)

The characteristic surface redox peak for the Ge adatom adsorption on Pt {100} sites is observed at ∼0.5 V.36 The doubler-layer charge increases after the adsorption of Ge since it is dependent on the size of the terrace sites in the case of a surface with a narrow terrace.37 It was proposed that Ge will be blocking two platinum sites by the exchange of 4 electrons.37 The surface reaction is written as follows:37 9001


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It should be noted that CVs and ORR polarization curves are recorded up to 0.8 and 1 V, respectively, thus hindering the surface restructuring process. 3.5. ORR Activity and Peroxide Formation in Acidic (0.5 M H2SO4 and 0.1 M HClO4) and Alklaine (0.1 M NaOH) Electrolytes. Figure 7 shows the ORR voltammo-

An important factor that needs to be considered while correlating the catalytic activity with surface orientations or shape of the nanoparticle is the role of surface defects (step type and kink type). In general, defects may enhance or inhibit the reaction through both geometric and electronic effects. The reactions that are limited by weak adsorption of a reactant or intermediate in CO and methanol oxidation can be promoted by defects. Spendelow et al. and Lebedeva et al. showed that CO oxidation on Pt {111} in alkaline medium is a defectfavored reaction.56,57 Since the oxidation of CO requires adsorption of OH, and OH adsorption on terraces is negligible below 0.65 V, the reaction proceeds mainly at the defects.56,57 Deliberate introduction of defects, in the form of small Pt islands, increases the rate of CO oxidation in this potential region.57 In contrast, reactions that proceed by strong adsorption of a reactant, intermediate, or spectator species are inhibited by defects and occur primarily on terraces. The ORR on Pt {111} in alkaline media is a model of a terracefavored reaction. It is known to be limited by the strong adsorption of OH.11,58 Here, the addition of defects does not increase ORR activity since the defects are completely blocked by adsorbed OH/O.57 Similar kind of results were reported with the Pt {111} vicinal surfaces in the HClO4 electrolyte.10 Feliu et al. investigated ORR activity on Pt well-defined surfaces. The Pt {111} terrace sites are modified with {100}/ {111} steps, in order to determine the effect of the steps and the symmetry of the terrace on ORR in acidic electrolytes.10,12 It is observed that the ORR activity increases with increase in the step site density. Importantly, structure-sensitivity of ORR depends explicitly on two factors: anion adsorption and oxide formation on the electrode surface; whereas, the oxygen adsorption energy on different sites plays a secondary role in determining the activity of the catalyst.10 Lee et al. observed that unlike the case of methanol oxidation and CO oxidation, ORR is not affected by the surface steps of particles having size ∼2 nm.59 However, there is no other report that translates the influence of ORR activity of surface steps on Pt single crystal electrodes to Pt nanoparticles. On the basis of the earlier reports by Feliu et al., with nanoparticles, contributions from different facets (with terrace and step sites) are assigned to different potentials in the cyclic voltammogram as follows.51 The peak at 0.125 V in the Hupd region is due to the contribution from {110} sites. The peak at 0.27 V corresponds to the {100} step sites on {111} terrace sites, along with contribution from {100} steps close to {100} terraces at the same potential. The peak feature between 0.35− 0.37 and at 0.5 V corresponds to the {100} and {111} bidimensionally ordered terrace sites, respectively. Thus, the effect of step sites at 0.27 and 0.125 V on ORR activity is minimal as ORR is a terrace-favored reaction in the case of the NaOH electrolyte and to some extent in the case of the HClO4 electrolyte also (perchlorate ions are weakly adsorbed). However, in the case of the sulfuric acid electrolyte one cannot rule out the influence of step sites on ORR. Considering the above facts and the complexity involved in estimating the steps and kinks of nanoparticles, we restricted our discussion to the correlation of the order of ORR activity observed on different nanoparticles with the fractional availability of low-index planes estimated through Ge and Bi adsorption. Pt-TD nanoparticles with predominant {111} sites shows higher ORR activity in NaOH and HClO4 electrolytes, whereas Pt-NC/Pt-CO nanoparticles with dominant {100} terrace sites show higher ORR activity in the H2SO4 electrolyte.

Figure 7. (a) ORR voltammograms (|jlim| normalized) of Pt-PC, PtNC, Pt-TD, and Pt-CO in 0.5 M H2SO4; the inset to panel a shows ORR voltammograms without normalization. (b) H2O2 oxidation current (ring current) recorded in parallel to the ORR voltammograms shown in panel a. The solid line indicates a positive sweep, and the dotted line indicates a negative sweep; the inset to panel b shows the fraction of H2O2 formation as a function of potential during O2 reduction.

grams and the corresponding peroxide oxidation features recorded in 0.5 M H2SO4. Figure 7a shows the limiting-current normalized |jlim|, background CV-corrected ORR voltammograms of Pt-PC, Pt-NC, Pt-TD, and Pt-CO catalysts recorded in oxygen-saturated electrolyte at a scan rate of 20 mV s−1; inset shows the ORR voltammograms, those are not normalized with their respective limiting-current values. These voltammograms are recorded to investigate the surface-sensitivity of ORR on different shape-controlled Pt nanoparticles. Markovic et al. and other groups investigated ORR on Pt{hkl} extended surfaces.7−9,11,12 Single crystal studies reveal that the ORR activity and peroxide evolution are dependent on the type of sites (terraces, kinks, and steps) present on the metal or catalyst 9002


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surface.7−9,11,12 It is reported that {110} and {111} sites show higher ORR activity than that of {100} sites in both HClO4 and NaOH electrolytes, whereas the latter shows higher ORR activity in the H2SO4 electrolyte.3,4,7,11,12 The surface of Pt nanoparticles, either polycrystalline or of specific shape (cubic, tetrahedral, and cuboctahedral), can be described by a mixture of {111} and {100} ordered domains connected by sites in which the atoms have low-coordination number. Thus, the presence of specific atomic sites and geometric disposition on the surfaces (terraces, kinks, and steps) can be attributed to the shape of the Pt nanoparticles. However, through in-situ irreversible Bi and Ge adsorption, we estimated the fraction of low-index sites ({111} and {100}) present on the surface of shape-controlled Pt nanoparticles as discussed earlier (Table 1). The high potential limit of ORR polarization was restricted to 1 and 0.9 V in acidic and alkaline electrolytes, respectively. The corresponding fraction of peroxide formation is calculated using the following formula: X H2O2 = 2

IR ⎛ I ⎞ /⎜ID + R ⎟ N ⎝ N⎠

higher when compared to that on Pt-PC and Pt-TD nanoparticles. Therefore, the shape-controlled Pt nanoparticles with more {100} orientation is expected to show higher ORR activity in 0.5 M the H2SO4 electrolyte. From Figure 7a, it can be seen that the half-wave potential of Pt-PC is 844 mV and that on Pt-NC/Pt-CO it is shifted to a higher potential by ∼10 mV, i.e, 854 mV, whereas the half-wave potential on Pt-TD is shifted to a lower potential by ∼30 mV, i.e, 814 mV. These results are attributed to the relative changes in the fraction of low-index sites available on the shape-controlled Pt nanoparticles. Figure S5 (Supporting Information) shows the MA and SA of Pt-PC, Pt-NC, Pt-TD, and Pt-CO catalysts at 0.9 V in the H2SO4 electrolyte. The activity parameters (MAs and SAs) at potentials of 0.8, 0.85, and 0.9 V are shown in Table S2 (Supporting Information). The specific and mass activities of Pt-PC are comparable with the activity of polycrystalline Pt reported in the literature.60 At 0.9 V, the SA of Pt-TD is 170 μA cm−2, which is lower when compared to that of Pt-PC (219 μA cm−2). Pt-CO shows a SA of 285 μA cm−2, but the SA of PtNC (310 μA cm−2) is higher than that of the other catalysts; a similar trend is observed with MA as well. The increment in SA of Pt-NC when compared to that of Pt-PC is because of the higher number of {100} sites on the surface of the nanoparticles. The higher specific and mass activities of PtNC and Pt-CO nanoparticles when compared to those of PtTD and Pt-PC nanoparticles are due to the faceting occurring on the surface toward {100} sites, and the ORR activity is in the order of Pt-NC > Pt-CO > Pt-PC > Pt-TD. Figure 7b shows the H2O2 oxidation current (ring current) on Pt-PC, Pt-NC, Pt-TD, and Pt-CO corresponding to the ORR voltammograms shown in Figure 7a; the forward and reverse scans are shown with solid and dotted lines, respectively. The fraction of peroxide formation is shown in the inset to Figure 7b. The peroxide generation features depend on the active catalyst site density, catalyst layer thickness, reaction pathways, and adsorbed impurities.61−63 Schneider et al. investigated the effect of site density on peroxide oxidation current.62 With decreasing Pt coverage/loading, the probability for the successful readsorption/reduction of H2O2 on Pt sites decreases, which in turn increases the amount of H2O2 escaping out from the active catalyst layer and thus leads to a significant increase in the H2O2 yield at the ring. A similar observation of decrease in peroxide percentage with increase in thickness of the electrode and active sites (loading) is also reported in the literature.62 As expected, Pt-CO/Pt-NC shows more peroxide oxidation current due to the slightly higher particle size; the slight increase in particle size decreases the active site density and therefore the readsorption/reduction of H2O2 on Pt sites resulting in a higher yield of H2O2. Thus, the peroxide formation on Pt-TD and Pt-PC nanoparticles (6 and 8 nm) is lower when compared with that on the other shape-controlled Pt nanoparticles. Otherwise, the contribution from the differences in active site density is negligible in our case since the dispersion of shape-controlled catalysts and catalyst loading are comparable; moreover, catalyst layer thickness is also similar since all of the catalysts are unsupported and form similar thin-films on the glassy carbon disk electrode. However, Pt-CO/Pt-NC has a {100}-dominated surface which favors the adsorption of oxygenated species (OHads) resulting in a higher peroxide generation through the dual path mechanism. The percentage of {100} facets in these nanoparticles is above 45%; here, the peroxide generation features are affected by surface

(3)

The ring current (IR) observed due to peroxide oxidation is lower in magnitude when compared with the disk current (ID). The collection efficiency (N) of the RRDE used was 22%. The characteristic ORR voltammetric features of Pt are clearly visible in both acidic and alkaline eletrolytes as the surface is cleaned from capping/stabilizing agents. Only forward scan is presented in all ORR voltammograms since the difference between the positive and negative sweeps is minimal. ORR current of all the shape-controlled Pt nanoparticles is diffusioncontrolled below 0.65 V, mixed-diffusion kinetic-controlled in the region of 0.65−0.85 V, and purely kinetic-controlled above 0.85 V, i.e, up to the on-set potential. There is a small variation in the current densities in the mass-transport region (diffusioncontrolled) of ORR polarization curves due to structuresensitivity and inhibition effect of the electrolytes. However, ORR voltammograms are normalized with the respective limiting-current densities for better comparison among the different shape-controlled Pt nanoparticles.12 Specific activity (SA) is calculated by normalizing the mass-transfer corrected kinetic current (ik) with ESA of the catalyst on the disk, and mass activity (MA) is calculated by normalizing kinetic current with platinum loading on the disk. The kinetic current (ik) is calculated from the ORR voltammograms in Figures 7a and S6a (Supporting Information) using the following equation:

1 1 1 = + im ik il

(4)

where il is the mass-transfer limiting current, and im is the measured current. The platinum loading on the disk is 6 μg. Single-crystal studies in the H2SO4 electrolyte show strong dependence of hydrogen adsorption capabilities on Pt{hkl} orientation, thus making a lower number of sites available for oxygen or any oxygenated species to adsorb on the surface.3 However, hydrogen strongly adsorbs in the region of 0.06−0.4 V. Sulphito oxy-anions adsorb specifically on ordered (bi)dimensional {111} terrace sites at 0.5 V and inhibit the adsorption of oxygen or oxygenated species even at higher potentials, which eventually makes {111}-site-dominated Pt nanoparticles show lower ORR activity when compared with that of {100} sites. From Table 1, the fraction of {100} sites present in the case of Pt-NC and Pt-CO is observed to be 9003


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low-index sites are qualitatively correlated with the peak intensities in the CVs recorded in HClO4 and H 2 SO4 electrolytes and with the Cu stripping peak intensities. The TEM images show that the shape-selectivity of Pt nanoparticles is less than 100% and that any shape-controlled synthesis may lead to a minor fraction of the other shapes as well. Moreover, the edges and corners of the shape-controlled nanoparticles are truncated; even then, the voltammograms of these nanoparticles resemble those of the respective well-defined surfaces with minor contribution from other orientations. On the basis of the relative availability of {100} and {111} sites on the shape-controlled nanoparticles, the ORR activities and peroxide formation of Pt shape-controlled nanoparticles are compared in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes. In the 0.5 M H2SO4 electrolyte, the {100} site-dominated Pt-NC and Pt-CO show higher ORR activity than that of Pt-PC and Pt-TD nanoparticles. The order of ORR activity and peroxide formation is Pt-NC > Pt-CO ≈ Pt-PC > Pt-TD and Pt-CO > Pt-NC > PtTD > Pt-PC, respectively. The difference in ORR activity order is due to the structure-sensitive specific adsorption of HSO4− ions on {111} terrace sites, which allows the oxygen to adsorb strongly on {100} sites. Apart from this, the trend in peroxide formation and the peroxide features depend on the active site density, thickness of the catalyst layer, reaction mechanism, and surface blocking by the adsorbed impurities. In the 0.1 M HClO4 electrolyte, there is a strong adsorption of O2 on {111} sites and a lack of specific adsorption by perchlorate anions. Therefore, the {111} site-dominated Pt-TD nanoparticles show higher ORR activity than all other Pt shape-controlled nanoparticles. The order of ORR activity and peroxide formation is Pt-TD > Pt-PC > Pt-NC > Pt-CO and Pt-CO > Pt-NC > Pt-TD ≈ Pt-PC, respectively. The discrepancy in the order of peroxide oxidation is attributed to the adsorbed hydrogen, which inhibits the adsorption of oxygen or any oxygenated species. Moreover, the decrease in readsorption/ reduction of H2O2 on Pt sites with increase in particle size results in an increased H2O2 yield. In 0.1 M NaOH, less sterically hindered (OH−) ions adsorb on the Pt surface. Similar to that in the HClO4 electrolyte, the {111} sitedominated Pt-TD nanoparticles show higher ORR activity in NaOH than that of all the other Pt nanoparticles. The order of ORR activity and peroxide formation is Pt-TD > Pt-PC ≈ PtNC > Pt-CO and Pt-CO > Pt-PC > Pt-TD > Pt-NC, respectively. Here, the ORR activity order is attributed to the structure-sensitivity of the (OH) adsorption and its inhibiting effect, whereas the trend in peroxide formation is attributed to the adsorbed hydrogen, which inhibits the adsorption of oxygen. It is observed that the peroxide formation is in the range of 5−12% in strongly adsorbing H2SO4 and NaOH electrolytes and it is 1.5−5% in the HClO4 electrolyte. Thus, electrocatalytic properties are strongly dependent on the shape of nanoparticles and the electrolyte used.

facets. However, Ruvinskiy et al. explained the trends in peroxide formation using the dual path mechanism and established that the peroxide yield also depends on the reaction pathway.61 After the one electron reduction step of O2ads to form HO2ads, the reaction may proceed either by a series path involving adsorbed H2O2ads as the reaction intermediate (where the H2O2ads further dissociates into two OHads entities or desorbs from the Pt surface), or the HO2ads dissociates into Oads and OHads, followed by an electrochemical reduction to water. Thus, in the potential range of 1.0−0.8 V, the peroxide oxidation current is negligible due to 4e− reduction of oxygen to water. In the 0.8−0.4 V range, a gradual increase in H2O2 formation is observed with a decrease in potential. This is because the series mechanism involving the electrochemical reduction of HO2ads to form H2O2ads is operating parallel to the direct dissociation mechanism. The transformation of HO2ads to H2O2ads is a potential-activated step, and it accelerates toward negative potentials. At a potential below 0.8 V, the direct path may be discarded, and the ORR predominantly occurs via a H2O2-mediated pathway. H2O2 further reacts via a chemical decomposition step rather than a direct electrochemical reduction.61 Finally, at a potential below 0.4 V, the fraction of H2O2 detected is independent of potential, and it explicitly depends on the adsorbed-hydrogen which inhibits the adsorption of oxygen or any oxygenated species. Peroxide generation changes are based on the availability of sites for the adsorption of oxygen. The peroxide features observed with our catalysts are comparable with the results reported in the literature.61,62 Thus, all of the shape-controlled Pt nanoparticles follow a similar kind of mechanism. The order of formation of H2O2 on different catalysts is Pt-CO > Pt-NC > Pt-TD > PtPC, which is in accordance with the order observed in the case of the extended surface reported in the literature.3 The ORR voltammograms and the corresponding peroxide oxidation features recorded in 0.1 M HClO4 and 0.1 M NaOH electrolytes are shown in Figures S6 and S8 (Supporting Information), respectively. Unlike in the H2SO4 electrolyte, from single-crystal studies, it is observed that in the HClO4 electrolyte there is a lack of specificity of adsorbing oxy-anions on {111} terrace sites. Thus, {111} terrace sites which can facilitate strong O2 adsorption tend to show higher ORR activity than {100} sites in the HClO4 electrolyte. Hence, PtTD with dominant {111} sites shows higher activity in 0.1 M HClO4 (Figures S6 and S7 and Table S3; see the relevant discussion in Supporting Information). Complex oxy-anions are adsorbed on the Pt{hkl} in acidic electrolytes; on the contrary, less sterically hindered (OH−) ions are adsorbed in alkaline electrolytes. In the 0.1 M NaOH electrolyte, {111} sites show higher ORR activity similar to that in the 0.1 M HClO4 electrolyte where there is no specific adsorption of oxyanion on the catalyst surface. Thus, Pt-TD nanoparticles show higher ORR activity in 0.1 M NaOH as well (Figure S8, Supporting Information). The ORR activity order with our shapecontrolled Pt nanoparticles in 0.1 M HClO4, 0.5 M H2SO4, and 0.1 M NaOH electrolytes is in accordance with that of the single-crystal electrodes reported in the literature.3,4,7

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedures; physical and electrochemical characterizations; shape-sensitivity of the different shapecontrolled nanoparticles investigated through CVs; ORR and peroxide formation in 0.1 M HClO4 and 0.1 M NaOH electrolytes; and qualitative analysis of surface sites through Cu stripping volatmmograms in 0.1 M HClO4. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS Pt-PC, Pt-NC, Pt-TD, and Pt-CO nanoparticles are synthesized and electrochemically characterized in acidic and alkaline electrolytes. The fractions of low-index sites present on the surface of these nanoparticles are quantitatively estimated through irreversible adsorption of Bi and Ge. The estimates of 9004


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AUTHOR INFORMATION

Corresponding Author

*Tel: +91 22 2576 7893. Fax: +91 22 2576 4890. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Department of Science and Technology (DST), India, is acknowledged for financial support of the project through grant SR/S1/PC-68/2012. Department of Metallurgical Engineering and Material Science and Sophisticated Analytical Instrumentation Facility (SAIF), both at IITB, are acknowledged for physical characterization of the samples.

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Oxygen Reduction Reaction and Peroxide Generation on Shape

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