Au@Pt CoreâShell Mesoporous Nanoballs and Nanoparticles as

Article pubs.acs.org/JPCC

Au@Pt Core−Shell Mesoporous Nanoballs and Nanoparticles as Efficient Electrocatalysts toward Formic Acid and Glucose Oxidation Yaovi Holade,† Anaïs Lehoux,‡,§,∥ Hynd Remita,‡,§ Kouakou B. Kokoh,† and Têko W. Napporn*,† Université de Poitiers, IC2MP, UMR CNRS 7285, Équipe SAMCat, 4, rue Michel Brunet, B27, TSA 51106, 86073 Poitiers Cedex 09, France ‡ Laboratoire de Chimie Physique, UMR 8000-CNRS Université Paris-Sud, Université Paris-Saclay, 91405 Orsay Cedex, France § CNRS, Laboratoire de Chimie Physique, UMR 8000, 91405 Orsay, France †

S Supporting Information *

ABSTRACT: By effectively bypassing the common problem of the loss of contact of platinum nanoparticles to carbon black support involved in the traditional methodology, supportless Pt, Au@Pt 3D-porous nanostructures (PtNBs and Au@PtNBs), and Aucore-Ptshell nanoparticles (Au50Pt50NPs) were synthesized. The radiolytic synthesis provides a high control on the size, morphology, and composition of the nanoparticles. Nanoballs, 85 ± 5 nm for PtNBs and 75 ± 5 nm for Au@PtNBs, are formed by 3D-interconnected nanowires leading to a giant mesoporous structure. A single Au@PtNB is formed by an ca. 30 nm Au-core, surrounded by a porous Pt-shell of 15 nm thickness made by 3D-connected nanowires of 2 nm diameter. The high catalytic activity of these nanostructures toward organic electrooxidation was highlighted in aqueous media and attributed to their particular core−shell structure. The Au@PtNBs mesoporous materials exhibit improved kinetics toward glucose electrooxidation compared to their counterpart PtNBs and are a promising anode material for direct glucose fuel cells. For formic acid electrooxidation, Au50Pt50NPs (3 nm diameter) exhibit the most specific activity. The radiolysis enables the effective synthesis of various core−shell nanostructures ranging from simple to mesoporous ones. The present work continues the research line where advanced methods are used to prepare highly active nanostructures with improved catalytic kinetics.

1. INTRODUCTION The less tolerant character of costly platinum (Pt) electrocatalyst and its sluggish kinetics toward the oxygen reduction reaction (ORR) and organics electrooxidation represent a major obstacle to a more widespread use of the polymer electrolyte membrane (PEM) fuel cells. Currently, the state-ofthe-art organics electrooxidation and ORR catalyst is in a form of fine Pt nanoparticles (NPs) dispersed on the conducting carbon supports (Pt/C).1 Unfortunately, the carbon support corrosion process, the aggregation of Pt NPs, and their dissolution lead to significant loss of the electrochemical surface area (ECSA).2,3 For a long time, the improvement of Pt/C-based nanostructures performance concentrated researchers’ attention.4 More recently, the unsupported Pt nanoscale materials were found to exhibit interesting properties with evidently alleviated ECSA loss.5,6 The exploration of novel synthesis routes to elaborate more active and cleaner materials for energy conversion and storage systems emerges as the unavoidable target for developing innovative applications. The rational control of the shape, composition, and structure of the extended alloy catalyst surfaces has resulted in significant improvements of their catalytic ability.5,7 The synthesis processes go from chemicals to physical methods. 4,8,9 Furthermore, the application of platinum group metals (PGMs) as electrode materials in catalysis is still limited by their scarcity together with their price. Their use in electro© 2015 American Chemical Society

catalysis is, however, essential to improve the electrode performances by enhancing significantly their utilization efficiency (UE) and optimizing their specific activity. To this end, various strategies based on chemical methods have been proposed: (i) core−shell nanomaterials;10 (ii) increasing the proportion of atoms exposed on the surface enables the UE of Pt atoms from 9.5 to 20% by reducing the edge length of a Pt cube from 12 to 4 nm;11 (iii) engineering nanoframes with open nanostructure with mutable ridges as thin as a few nanometers;6,12 (iv) pile up metal atoms into nanosheets to design different layers (UE up to 50%);13,14 (v) cubic or octahedral nanocages enclosed by (100) and (111) facets, with distinctive electrocatalytic activity;11 (vi) (meso)porous nanostructures;8,9,15−17 and (vii) PGMs nanotubes18,19 or hollow/ honeycomb nanosphere10,19 materials. However, radiolysis is a powerful method to synthesize nanoparticles of controlled size and shape in solutions and in heterogeneous media. Solvent radiolysis induces formation of solvated electrons and radicals that reduce the metal ions homogeneously in the medium leading to a homogeneous nucleation. Small and relatively monodispersed nanoparticles are obtained. Compared to chemical reducing processes that follow a diffusion front, Received: September 27, 2015 Revised: November 8, 2015 Published: November 16, 2015 27529

DOI: 10.1021/acs.jpcc.5b09417 J. Phys. Chem. C 2015, 119, 27529−27539

Article

The Journal of Physical Chemistry C

is mixed with a Pt(NH3)4,Cl2 aqueous solution (0.1 mol L−1, 2 mL) in a 50 mL vial, at 50 °C, until complete dissolution of the surfactant. Then, 2.98 mL of cyclohexane is added, as a swelling agent, and vortexed until homogenization of the mixture. Final evolution of the white and opaque gel into a perfectly clear and transparent mesophase is completed through the addition of 1pentanol (∼240 μL), used as a cosurfactant. 2.2.2. Synthesis of 3D-Porous Au@PtNBs in Mesophases. Core−shell bimetallic nanostructures are prepared in two steps. First, gold nanoparticles (AuNPs) with a diameter of 30 ± 5 nm are synthesized. Then, in a second step, these nanoparticles are introduced in a platinum salt doped mesophase to synthesize Au@PtNBs. Radiolytic Synthesis of 30 nm Gold NPs. 590 mg of CTAB (cetyltrimethylammonium bromide) are dissolved in 20 mL of Millipore water at 50 °C, under mild stirring. After complete dissolution, 8.2 mg of CTAB is dissolved, followed by the addition of 1.6 mL of an aqueous solution of HAuCl4 (C = 0.025 mol L−1). The solution, initially transparent, leads to an orange solution, due to the formation of the [AuIII−CTA−] complex. After 1 h, 430 μL of acetone are added, followed, 15 min later, by 330 μL of cyclohexane. The solution is deaerated under N2 flow and quickly irradiated for 14 h at a dose rate of 2 kGy h−1. Synthesis of Mesophase for Au@PtNBs. In the second step for the preparation of bimetallic structures, 74 mg of Pt(NH3)4,Cl2·2H2O salts and 1.03 g of CTAB are dissolved in 2 mL of the gold colloidal solution ([Pt(NH3)42+] = 0.1 mol L−1 and [CTAB] = 1.41 mol L−1). To the obtained aqueous solution, 2.98 mL of cyclohexane (swelling agent) followed by 240 μL of 1-pentanol (cosurfactant) are added to prepare the hexagonal mesophase. It has to be noted that the proportion of CTAB, cyclohexane, and 1-pentanol for the mesophase preparation is the same as for PtNBs synthesis described above. Radiolytic Reduction of Metallic Salts in Mesophases. Mesophases are incorporated in glass vessels with a rubber plastic septum, centrifuged at 2000 rpm for 20 min, and deoxygenated under a N2 flow for 10 min. The absence of oxygen is crucial for the radiolytic reduction of the metallic salts contained in the mesophases to avoid oxidation reactions by O2 or by O2•− (formed by the reaction of solvated electrons with O2). Furthermore, 1-pentanol acts as a scavenger of the oxidative OH• radicals produced during radiolysis, and this reaction leads to the formation of reducing alcohol radicals. Mesophases are exposed to γ-irradiation at room temperature for 13 h (irradiation dose of 91.2 kGy, dose rate 7 kGy h−1) for a slow and homogeneous radiolytic reduction. The irradiation dose is sufficient for complete reduction of the Pt complex confined in the water phase, as shown previously by XPS analysis of Pt nanoballs synthesized under the same conditions.20 The γ-irradiation source (located at Paris-Sud University, Orsay, France), is a 60Co gamma-facility of 7000 Curies with a maximum dose rate of 7000 Gy h−1. The samples, which were initially transparent gels, turned into black opaque gels after irradiation, indicating reduction of metallic salts and nanostructures’ formation. Nanomaterials are extracted from the mesophases by several cycles of washings in 2-propanol and centrifugation in order to remove to a large extent both surfactant and residual salts. Synthesis of Au50Pt50NPs (Aucore-Ptshell) in Solution. The radiolytic synthesis of bimetallic AuPt nanoparticles has already been reported by the authors, and these nanoparticles have already been characterized.21,23,45 In brief, the procedure can be

radiolysis presents the advantage of inducing a homogeneous nucleation and growth in the whole volume of the irradiated sample.20−24 Metal nanoparticles including 1D, 2D and 3D nanostructures can be directly synthesized in surfactant mesophases.20,22,25 Subsequently, the radiolysis process can be effectively used to fabricate surface structured nanocrystals to reduce the mass of platinum per surface area, minimizing Pt amount in the electrocatalysts. 1D-mesoporous metallic materials have been synthesized without a radiolysis process, as those reported by the group of Yamauchi.26−30 Particularly, Pt−Au alloy films obtained from the electrochemical deposition28 and self-supported one-dimensional (1D) mesoporous Pt nanowires27 exhibit distinguished catalytic properties for the amperometric glucose detection and methanol electrooxidation. The energy required for portable devices such as cell phones, personal digital assistants (PDAs), and laptop computers is not fully supplied by the advanced batteries systems. Thus, the polymer electrolyte membrane (PEM)-based fuel cell becomes attractive for powering these devices. Direct formic acid fuel cells (DFAFCs) are one of these promising fuel cells, since formic acid is environmental and societal friendly.31,32 Besides, direct glucose solid alkaline membrane fuel cells (DGFC), biofuel cells (BFCs), biosensors, and bioelectronics lead to a renewed interest for the glucose electrooxidation reaction.28,33,34 The mechanisms of formic acid35−42 and glucose electrooxidation43,44 reactions were yet discussed in the literature. In the present work, the application in electrocatalysis of 3D-porous nanoballs of Pt (PtNBs) and Au@Pt (Au@PtNBs) and small bimetallic alloyed Pt−Au nanoparticles, both synthesized by radiolysis, is reported. Especially for glucose electrooxidation, the best compromise between activity and stability required platinum−gold alloys. Thus, our target is to improve the electrocatalytic performances of platinum−gold alloys during glucose electrooxidation, by engineering Au@PtNBs. Herein, carbon monoxide (CO) was used as a probe molecule, whereas formic acid and glucose were employed as fuels for the electrochemical tests. For glucose electrooxidation, we expect a high specific catalytic activity of platinum and a high surface area (improving the UE of Pt atoms) thanks to the porosity of the Au@PtNBs in order to elaborate an anode electrode catalyst for DGFC.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetrachloroauric (III) acid (HAuCl4, 99.99%), hexachloroplatinic (IV) acid (H2PtCl6, ≥99.8), and tetraamineplatinum (II) chloride (Pt(NH3)4Cl2, 99%), from Aldrich were used as metallic salts. 2-Propanol (≥99%), cetyltrimethylammonium bromide (CTAB, ≥98%), polyacrylic acid (PAA, 60%), trisodium citrate (≥99%), 1-pentanol (≥99%), cyclohexane (>99%), sodium hydroxide (NaOH, 97%), and D-(+)-glucose (99.5%) were obtained from SigmaAldrich. High purity nitrogen (N2) and carbon monoxide (CO) gases were purchased from Air Liquide. Sulfuric acid (H2SO4, 96%) and formic acid (97%) were purchased from MERCK and Alfa Aesar, respectively. All the chemicals were used as received without further purification. All named ultrapure water is Milli-Q Millipore (MQ: 18.2 MΩ cm at 20 °C). 2.2. Preparation of the Mesoporous Materials and Nanoparticles by the Radiolysis Process. 2.2.1. Synthesis of 3D-Porous PtNBs in Mesophases. Tridimensional porous Pt nanoballs (PtNBs) are prepared in hexagonal mesophases following a procedure detailed elsewhere.20,22 1.03 g of CTAB 27530


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Figure 1. (a) TEM image of Au50Pt50NPs (Au-core covered by a thin layer of Pt-shell). (b) Indexed SAED pattern of the core (gold).

electrode consisted of a catalytic ink deposited onto a freshly polished glassy carbon (GC, 3 mm diameter) disk used as a conductive carrier and dried under N2 stream. Contrary to the literature, we did not use any polymer like Nafion during this process for the mechanical strength of the ink. For the optimization of the catalytic ink layer on the GC, different volumes (total metal concentration: 1 mmol L−1) of the catalytic ink were typically deposited and tested. Then, the stock solution is diluted in order to deposit 6 μL (1.1 μgmetal, meaning 16 μgmetal cm−2) for all the studies. Furthermore, to avoid oxygen interferences during electrochemical experiments, the electrolytic solution was systematically deoxygenated by bubbling N2 into the electrolyte for 30 min prior to each electrochemical experiment. The catalytic activity of the catalysts was investigated by using three different molecules: (i) CO used as probe molecule; (ii) 0.1 mol L−1 formic acid used as fuel, as well as (iii) 5 mmol L−1 D-(+)-glucose. All the electrochemical experiments were fulfilled at room temperature (21 ± 1 °C). For CO stripping experiments, CO was adsorbed at 0.10 V vs RHE for 5 min, and then, the excess (dissolved CO) was removed by bubbling N2 for at least 30 min before CV experiments. The first CV after CO oxidation attests that there is no CO remaining in the electrolytic solution. To compare the catalytic activity toward CO or formic acid oxidation, the current was normalized by the active surface of platinum. The procedure followed for its evaluation is reported in the Supporting Information (Figure S1a and b). The specific activity was accessed by correcting the obtained activity by the contribution of the supporting electrolyte.

set as follows: Metallic salts with a total concentration of 1 mmol L−1: 0.5 mmol L−1 of HAuCl4 and 0.5 mmol L−1 of H2PtCl6 are dissolved in a poly(acrylic acid) solution (PAA, 0.1 mol L−1) to stabilize the nanoparticles during their growth. 2Propanol (0.1 mol L−1) is added in the solutions to scavenge the oxidative OH• radicals, produced upon water radiolysis. The solutions are deoxygenated under a continuous flow of nitrogen for 10 min, and vials are then exposed to γ-rays for 5 h (irradiation dose of 7.5 kGy, dose rate of 1.5 kGy h−1). 2.3. Physicochemical Characterization of the Materials. 2.3.1. Transmission Electron Microscopy Observations and Characterization. A suspension of metallic nanoparticles after washings is deposited dropwise onto carbon-coated copper grids for transmission electron microscopy (TEM) observations. TEM experiments were performed on a JEOL JEM 100 CXII microscope at an accelerating voltage of 100 kV. Diffraction patterns were obtained through selected area electron diffraction (SAED) mode. Images were collected with a 4008 × 2672 pixels CCD camera (Gatan Orius SC1000). 2.3.2. X-ray Scattering Experiments. The mesophases were inserted in glass capillaries of 1.5 mm diameter and were characterized by small-angle X-ray scattering (SAXS) both before and after γ-irradiation. The experiments were performed with an in-house setup of the Laboratoire Charles Coulomb, “Réseau X et gamma”, Université Montpellier 2, France. A high brightness low power X-ray tube, coupled with aspheric multilayer optics (GeniX 3D from Xenocs), was employed, delivering an ultralow divergent beam (0.5 mrad). Scatterless slits were used to give a clean 0.8 mm diameter X-ray spot with an estimated flux around 35 Mph/s at the sample position. A transmission configuration was used. The scattered intensity was collected on a two-dimensional Schneider 2D image plate detector prototype, at a distance of 1.9 m. Experimental data were corrected for background scattering and sample transmission. 2.4. Electrochemical Measurements. All the cyclic voltammetry (CV) experiments were fulfilled with an analogical potentiostat EG&G PARC Model 362 (Princeton Applied Research) in a conventional three-electrode cell. A reversible hydrogen electrode (RHE) was used as the reference electrode in acid medium (0.5 mol L−1 H2SO4) and a mercury/mercury oxide (MOE) in alkaline medium (0.1 mol L−1 NaOH). However, for convenience, all the results reported herein were referred versus RHE. To avoid the contamination of this electrode during organics electrooxidation, it was separated from the solution by a Luggin capillary tip. A slab of glassy carbon (GC) of 6.48 cm2 geometrical surface area functioned as the auxiliary (i.e., nonreactive) electrode. The working

3. RESULTS AND DISCUSSION Radiolysis was used in order to synthesize bimetallic Au/Pt nanoparticles (Pt50Au50NPs: rich Au-core covered by a thin layer of Pt-shell) in solution, 3D-porous Pt nanostructures (PtNBs), and their bimetallic core−shell Au@Pt equivalent (Au@PtNBs) in mesophases. The hydrated electrons and the reducing radicals produced during water radiolysis induce a homogeneous reduction in water or in the confined aqueous phase (in the case of mesophases) leading to the formation of metal nanoparticles or nanostructures.21,24 3.1. Physicochemical Characterization of the Alloyed Au−Pt Nanoparticles. AuPt nanoparticles have been prepared by radiolysis. This method of reduction allows an accurate control of the reduction kinetics, since the dose rate can be adjusted finely. Previous works have highlighted that the reduction kinetics is a key factor to control both the structure (i.e., core/shell or alloyed) and the size of the nano27531


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Figure 2. (a) SAXS of the mesophase before and after irradiation, presenting peaks in a characteristic ratio for a hexagonal arrangement with 16 nm diameter tubes. HRTEM images of (b) an assembly of PtNBs and (c) a single PtNB of about D = 90 nm, highlighting pore construction, d = 15 nm.

Figure 3. (a) HRTEM image of Au@PtNBs formed by a large core made of gold and a porous Pt shell. The dashed circle shows the core. (b) Indexed SAED pattern of the core (gold).

particles.21,24,46 Fast reduction kinetics leads to a large number of small alloyed particles (2−4 nm). Slower reduction kinetics (lower dose rate) generates a smaller number of core−shell nanoparticles with a larger size (>3 nm). We prepared AucorePtshell nanoparticles (a Au-core covered by a thin layer of Ptshell) starting from a 1:1 initial atomic ratio.23,45,46 The TEM image, presented in Figure 1a, shows the particular morphology of the nanoparticles. The diameter of the particles is 5−8 nm with frequent elongated particles due to the soft stabilization offered by poly(vinyl alcohol) (PVA) and to the coalescence of close nanoparticles. The total atomic composition of Au:Pt = 53:47 (from EDX) is close to the nominal 50:50. Figure 1b displays the SAED pattern of the particles’ core, highlighting the good polycrystallinity of gold. 3.2. Physicochemical Characterization of Pt Mesoporous Nanostructures. Porous stable nanostructures were

synthesized by radiolytic reduction of the metal ions inside the water channels of a hexagonal mesophase. Several groups demonstrated the simplicity of the soft-templating method to synthesize porous metallic films and/or porous materials by electroreduction,15 metathesis,17 or gamma-reduction.20,22,25 The use of quaternary mesophases has been previously reported for the rapid preparation of various mesoporous structures, i.e., monometallic22 or bimetallic,20,25 and with a large panel of structural parameters. The hexagonal mesophases were characterized by SAXS, as presented in Figure 2a, before and after gamma-irradiation. The first three orders of the Bragg peaks are located at 0.453, 0.770, and 0.909 nm−1 and 0.430, 0.748, and 0.864 nm−1, respectively, both in the ratio 1:(3)1/2:2 expected for p6mm symmetry. The corresponding lattice parameters are, respectively, 16 and 16.8 nm. The excellent accordance of the lattice parameters before and after irradiation 27532


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Figure 4. Steady-state cyclic voltammograms recorded in 0.5 mol L−1 H2SO4 at 20 mV s−1 for (a) different volumes of Au@PtNBs ink deposited onto GC, (b) Pt50Au50NPs (3 μL deposited), and (c) 6 μL of PtNBs, Au@PtNBs, and Pt50Au50NPs inks.

around the structure still presents a 3D-porous structure similar to the pure platinum one in terms of nanowire thickness and porosity, described above. Indeed, each Au@PtNB is formed by a core of gold (ca. 30 nm) surrounded by a porous shell of platinum (15 nm of thickness) made by 3D-connected nanowires (2 nm diameter), as depicted in Figure 3a. Each Au@PtNB is about 70−80 nm, slightly smaller than the monometallic PtNB. The thickness of the Pt nanowires corresponds to the one of the water channels, subsequently attesting to the template effect of the mesophase. These bimetallic nanostructures are very similar to those already reported with AuPd.25 The Au-core was characterized by its SAED pattern (Figure 3b), showing its polycrystallinity. For the glucose electrooxidation reaction, the activity at low electrode potentials increases from Pt to Au, contrary to the long-term stability. Thus, the Au-core is expected to stabilize Pt at the shell in Au@PtNBs, leading to high electrochemical stability. 3.4. Nanostructure Surface Characterization by Electrochemical Measurements. The electrochemical behavior of the nanostructures in acidic supporting electrolyte was investigated. It began by testing different volumes of the ink in order to find out the better condition. Figure 4a displays the CVs of Au@PtNBs for 3 μL (11 μgmetal cm2), 6 μL (16 μgmetal cm2), and 9 μL (24 μgmetal cm2). From these results, an ink volume of 6 μL for a metal loading of 16 μgmetal cm−2, as currently used loadings (from 10 to 20 μgmetal cm−2),5,6,47 was used. The optimum loading depends on the dispersion of catalyst (composition, supported on carbon power or not and the initial metal content) and may differ considerably. Indeed, the comparative investigations made by Mayrhofer and co-

highlights that the mesophase structure has not been altered, even after some hours of irradiation at a high dose rate.22 The slight modification could be rather attributed to a stabilization of the mesophase observed when it is left without disturbance. After irradiation, the template effect is evidenced by obtaining nanostructures, such as those presented in Figure 2b and c. Figure 2b shows an assembly of spherical PtNBs, with a size (diameter, D) distribution from 80 to 95 nm. As displayed in Figure 2c, each PtNB is composed of 3D-interconnected Pt nanowires to build a giant frame called nanoballs with hexagonal cells. The thickness of the nanowires is ca. 2 nm. The pores in this frame have an average porosity diameter of d = 15 nm, as expected since the hexagonal arrangement had tubes with 16 nm diameter. The growth mechanism leading to Pt nanoballs has been studied.20 These open-framework-like nanocages11 or nanoframes6 that have a high number of lowcoordinated atoms in those polycrystalline structures and high surface area (herein around 100 m2 g−1 from BET measurements) make them promising electrocatalysts. 3.3. Physicochemical Characterization of Au−Pt Mesoporous Core−Shell Nanostructures. In this paper, the synthesis of Au@PtNBs core−shell is reported for the first time, presented in Figure 3a. This structure has been prepared by inserting gold nanoparticles in the aqueous phase of the mesophase, acting as seeds for the subsequent growth of platinum nanowires. TEM observations show that these Au nanoparticles are homogeneous in diameter (about 30 nm). Even though the nanoparticles are much larger than the confined aqueous channels of the mesophase (2 nm), the mesophases are only disturbed locally. The platinum shell built 27533


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Figure 5. CO stripping experiments performed in 0.5 mol L−1 H2SO4 at 20 mV s−1 on (a) Au@PtNBs electrode material. (b) Comparison of the specific activity based on active area.

thesis of bimetallic core−shell nanoparticles (Pt50Au50NPs) and core−shell mesoporous nanostructures (Au@PtNBs). This approach can tune effectively Pt properties toward significant electrocatalytic activities. 3.5. Nanostructure Surface Probing by CO Stripping Experiments. To evaluate the electrocatalytic activity of the as-synthesized nanostructures, we performed an oxidative stripping of CO. The representative CVs recorded before, during, and after CO oxidation voltammetry are shown in Figure 5a for the Au@PtNBs sample. The whole obtained CVs for CO stripping are reported in Figure S2a−c. Figure 5b shows the adsorbed CO (COads) oxidation peak, localized at 0.88, 0.85, and 0.89 V vs RHE for PtNBs, Au@PtNBs, and Pt50Au50NPs catalysts, respectively. The high specific current density for the Pt50Au50NPs electrode material was expected, since it has the lowest active surface area, as can be seen in Figure 4c and Figure S2. Otherwise, the oxidation peak shifts toward more negative potentials for the Au@PtNBs electrode compared to its counterpart PtNBs, indicating the improvement of kinetics. The improvement of the catalytic activity of the platinum in the presence of gold is historically attributed to the synergic effect involving the d-band of the metal.45,52−54 According to Pedersen et al.,54 the reactivity of the Pt overlayer on the Au substrate increases very surprisingly. They explained that by a simple model, in which the change in the CO binding energy is directly proportional to the shift of the d-band center of the metal overlayer.54 Another pathway could be the mesoporous character of the materials (Au@PtNBs). Indeed, it was shown that the CO electrooxidation reaction on Pt nanoparticles slows down when the particle size decreases to 2 nm or less due to a marked reduction in the surface diffusivity of COads.55 Alternatively, Wang et al.56 reported that Pt−Cu porous nanomaterials show more promotional effect toward CO conversion. Consequently, the electronic interactions between the core (gold) and the shell (platinum) lead to the improvement of the electrochemical kinetics of Au@PtNBs, highlighted by the negative COads oxidation peak shift. It can be noticed that the electrooxidation of COads occurs at a potential more than 0.8 V vs RHE. This positive shift toward higher potentials was already reported for Pt-based materials which could involve the alloy structure48,57 or enough neighboring Pt atoms.58 Indeed, peak positions of 1 V vs RHE,48 0.9 V vs RHE,57 or 0.94 V vs RHE52 have been reported. This origin requires further scrutiny. Furthermore, as we will see later, the organics oxidation takes place at much lower potentials. Basically, the second cycle after CO stripping

workers from carbon-supported catalysts (Pt/C: 1, 2, and 5 nm) and nanostructured Pt film (NSTF) showed optimum loadings of 14 and 42 μgmetal cm2 for Pt/C and the NSTF catalyst, respectively.47 Figure 4b shows the steady-state CV of Au50Pt50NPs. Figure 4c displays the steady-state CV of the three kinds of materials. As the Au amount decreases in the catalysts from bimetallics (Au@PtNBs, Pt50Au50NPs) to monometallics (PtNBs), the upper limit potential was decreased to limit oxygen evolution and Pt dissolution. Desorption of adsorbed hydrogen on the Pt surface occurs during the forward going scan (region A1). This is followed by the Pt surface oxidation, which takes place for a potential value higher than 0.8 V vs RHE (zone A2). After the reduction of this Pt oxide (region C2), the adsorption of hydrogen starts at 0.35 V vs RHE in the reverse scan (region C1). The potential of the peak in region C2 strongly depends on the upper potential limit during the positive-going sweep and the nature of the catalyst. Basically, it shifts toward lower potential when increasing the upper potential. For the same upper potential limit, it can shift toward either lower or higher potentials depending on the nature of Pt oxide for alloys.48 Typically, it is close to 0.7 V vs RHE for an upper potential limit of 1.5 V vs RHE.49 Herein, this peak position is about 0.74 V vs RHE for Au@PtNBs. Furthermore, the absence of the peak at ca. 1.2 V vs RHE during the reverse scan, characteristic of gold oxide reduction on this material, confirms clearly the formation of a core−shell structure. There was no peak over several dozens of cycles even if the upper potential rises to 1.6 V vs RHE. Thus, all Au is surrounded by Pt shell. This result indicates that the core−shell structure cannot be destroyed during electrochemical measurements (electrochemically stable). Besides, this peak is present with Pt50Au50NPs (Figure 4b). This is a confirmation of a core−shell structure with a Pt thin layer where only few gold atoms are exposed on the surface if its nominal composition exceeds 50%. Currently, bimetallic electrocatalysts can present common or separate region potentials where their surface oxides are reduced to metal. And, it is well-known that PdAu systems present a common zone50,51 conversely to PtAu systems.48,50 The Pt active surface (SA) determined from this region increases in the order Pt50Au50NPs < PtNBs < Au@ PtNBs. The normalization gives ECSA = 5 m 2 Pt g −1 (Pt50Au50NPs) < 8 m2Pt g−1 (PtNBs) < 19 m2 g−1 (Au@ PtNBs). This value is small compared to carbon-supported Pt nanoparticles where ECSA = 20−80 m2Pt g−1 but the ECSA of Au@PtNBs is 2-fold higher than PNBs. All of these electrochemical characterizations confirm the successful syn27534


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Figure 6. Formic acid (0.1 mol L−1) electrooxidation 0.5 mol L−1 H2SO4 at 20 mV s−1: (a) Steady-state cyclic voltammograms for Pt50Au50NPs catalyst. (b) Comparison of specific activity of PtNBs, Au@PtNBs, and Pt50Au50NPs catalysts.

pathway on the active sites, while those of peaks B and D may comprise the continuous oxidation of intermediate and adsorbed formate at the active sites free from intermediate adsorbed CO, respectively.42,52 Indeed, peak A is situated in the hydrogen region of Pt. Thus, the reaction which leads to this peak involves eq 1. Then, in the potential range 0.8−0.9 V vs RHE, as the formation of hydroxyl species on the Pt surface occurs, the oxidation of this adsorbed intermediate in a Langmuir−Hinshelwood mechanism (eq 2) takes place. However, the reaction schematized by eq 2 is a non-Faradaic process. Thus, the increase in the current means that there is another non-bifunctional process which can be illustrated by eq 3 as previously noticed.42 Other mechanisms involving carbon monoxide as an intermediate were proposed.52 Equation 4 consists of the overall reaction with two exchanged electrons.

does not show any peak related to the CO oxidation. However, for the Pt50Au50NPs material, we noticed a small CO oxidation peak during the second CV and no peak at the third one. In another independent experiment, we obtained the same observation, thus underpinning the conclusion that this remaining COads may be linked to the physical structure of the material. By comparing CVs before and after CO stripping, it appears that the catalyst surface is not the same. Indeed, it is well-known that the CO strongly modifies the surface after its adsorption. 3.6. Electrocatalytic Activity toward Formic Acid Electrooxidation. To gain further insights on the catalytic performance of the nanostructures, the formic acid electrooxidation was conducted in acidic medium. As aforementioned, formic acid is a potential fuel for energy conversion systems. Figure 6a depicts the electrochemical behavior of the Pt50Au50NPs electrode in 0.5 mol L−1 H2SO4 without (black curve) and with 0.1 mol L−1 HCOOH (red curve) from 0.05 to 1.6 V vs RHE. The corresponding CVs of PtNBs, Pt50Au50NPs, and Au@PtNBs are reported in Figure S3a−d. After roughly 20 complete cycles, the steady state is reached. All of the materials exhibit a low onset potential of about 0.2 V vs RHE. When the upper potential limit is set at 1.6 V vs RHE, the three electrodes show similar trends in the forward going scan with three oxidative peaks marked by A (0.5−0.6 V vs RHE), B (0.9 V vs RHE), and C (1.42 V vs RHE). Only one peak (D: 0.54 V vs RHE) is observed in the negative scanning. While the origins of peaks A, B, and D are well-defined, peak C is subject to controversy.42 The explanations (peaks A, B, and D) were provided by Capon et al.42 and confirmed by Bai et al.52 According to them, the electrooxidation of formic acid may follow an indirect dehydration conversion process.42 Besides, the adsorbed carbon monoxide identified by Beden and coworkers form spectroelectrochemical investigations as the poisoning species in the early 1980s39 remains the only adsorbate detected for over 20 years. In the early 2000s, the group of Osawa (by using surface-enhanced infrared absorption spectroscopy, SEIRAS) and others demonstrated that the bridge-bond formate (HCOO ads ) is the only reactive intermediate in the direct pathway.35−38 More recently, the different mechanisms of formic acid electrooxidation were reviewed by Cuesta and co-workers.40,41 This reveals that the electrooxidation can proceed through a dual path mechanism that involves a direct dehydrogenation pathway and an indirect dehydration one. Thus, peak A is attributed to the oxidation of adsorbed formate in an indirect

Pt + HCOOH → Pt(COOH)ads + H+ + e−

(1)

Pt(OH)ads + Pt(COOH)ads → 2Pt + CO2 + H 2O

(2)

Pt(OH)ads + HCOOH → Pt + CO2 + H 2O + H+ + e− (3) +

HCOOH → CO2 + 2H + 2e

−

(4)

As can be seen in Figure 6a, the decrease in the current density at ca. 1.2 V vs RHE was reported to be likely due to PtO layer formation, which inhibits the subsequent formic acid oxidation.42 Surprisingly, another oxidation peak (C) takes place above 1.4 V vs RHE, as the interaction of HCOOH with an active form of oxygen electrochemically adsorbed on the PtO surface. Indeed, this feature, postulated in 1971 by the group of Tyurikova and emphasized by Capon et al.,42 is in line with the formation of PtO(O)ads species according to eqs 5 and 6.42 Then, this PtO(O)ads species is capable of reacting with HCOOH which, according to the authors, is adsorbed on the inactive PtO surface, eq 8 via eq 7.42 PtO + H 2O → PtO(O)ads + 2H+ + 2e−

(5)

PtO(O)ads → PtO2

(6)

PtO + HCOOH → PtO(HCOOH)ads

(7)

PtO(O)ads + PtO(HCOOH)ads → 2PtO + CO2 + H 2O (8)

The comparative activity of the three kinds of catalysts is displayed in Figure 6b. In terms of activity, all three kinds of 27535


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Figure 7. Cyclic voltammograms recorded during the electrooxidation of 5 mmol L−1 of D-(+)-glucose in 0.1 mol L−1 NaOH electrolyte solution; at a scan rate of 20 mV s−1. (a) Au@PtNBs and (b) comparison of PtNBs and Au@PtNBs catalyst activity.

peak positions A, B, C, and D move with the scan rate. This indicates that the electrochemical reaction is irreversible. 3.7. Electrocatalytic Activity toward Glucose Electrooxidation. We then expanded the mesoporous nanoball catalytic activity exploration by looking at glucose detection. For that, a low concentration of glucose (5 mmol L−1) was used for electrochemical studies in 0.1 mol L−1 NaOH. For glucose electrooxidation, we did not normalize the current by the active sites of platinum. Indeed, both Pt and Au are active toward glucose electrooxidation. As gold in the bimetallic material did not show a useful reduction peak for its active surface area evaluation, it is not possible to use this method. As evidenced in Figure 7a and b, the glucose electrooxidation occurs without overpotential on PtNBs and Au@PtNBs electrodes. In Figure 7a, the black curve is the catalyst response in 0.1 mol L−1 NaOH solution free of glucose, and the red one is recorded in the presence of 5 mmol L−1 glucose. Basically, the glucose electrooxidation reaction on platinum is first induced by the dehydrogenation process at ca. 0.3 V vs RHE followed by gluconate desorption at higher potentials during the forward scanning.43,44 It is well-known that gold does not exhibit a hydrogen region. However, glucose dehydrogenation can occur on its surface at higher potentials.61 A synergistic effect in AuPt NPs toward glucose electrooxidation was already advanced.48,62 This is evidenced at 0.3 V vs RHE where Au@ PtNBs is at least 2-fold more active than PtNBs. The oxidation peak current is 9 and 16 A g−1 for PtNBs (0.64 V vs RHE) and Au@PtNBs (0.69 V vs RHE), respectively. At a concentration of 5 mmol L−1 glucose, the present nanostructures exhibit good activity compared to the literature where higher concentrations are used,50,62,63 for example, at 0.7 V vs RHE, a current density of 3 A g−1 on Pt/C or Au/C and 12 A g−1 on AuPt/C alloy with 10 mmol L−1 glucose.62 More importantly, the peak position shifts more negatively toward lower potential than those reported (ΔE > 100 mV).63,64 These remarkable performances can be assigned to the mesoporous structure of the as-synthesized nanoscale materials instead of surface orientation where glucose reactivity over Au(hkl) increases in the order (111) < (110) < (100).65 This means that the core gold via electronic interactions, as aforementioned, boosts the electrocatalytic activity of the Pt shell. Furthermore, the reverse oxidation peak is associated with the freshly chemisorbed species or nonoxidized intermediates heaped on the surface during the forward scanning. Indeed, after the positive going scan, the catalyst surface is saturated of oxide species and potential strongly adsorbed intermediates. Then, it is

catalyst exhibit the best performance compared to the reported results.10,52,59 At 0.6 V vs RHE, the activity of Pt50Au50NPs is at least 10-fold higher than that of Bai and co-workers at the same potential.52 If we consider that they used 0.5 mol L−1 HCOOH and 50 mV s−1 scan rate, we could find a factor of 20. It was reported that the electrocatalytic activity of Pt toward formic oxidation reaction is exalted by the presence of the Pt(111) facet.59 Since gold does not show any significant activity toward formic acid electrooxidation,52 the improvement of the catalytic activity is due to the synergic effect involving the d-band of the metal as aforementioned for the CO52−54 and/or the surface orientation. Indeed, the electronic exchanges between the 5d orbitals of Au (full: 5d10) and Pt (unfilled: 5d9) induce the modification of their valence band energy. It is precisely the electrons of these bands which determine the chemical reactivity of the elements. As evidenced in this graph, Pt50Au50NPs exhibit a highly unexpected activity compared to the others. These results support those of the CO stripping where the same material showed the highest activity. As the Pt(111) surface is more active toward formic acid electrooxidation, the difference of reactivity between bimetallic (Pt50Au50NPs) and mesoporous (PtNBs, Au@PtNBs) catalysts could be assigned to the structure of the materials. The facets (111) are easily reachable with Pt50Au50NPs more than mesoporous materials. For the specific activity (using the active surface area), the activity toward formic acid follows the order Pt50Au50NPs > Au@PtNBs > PtNBs. This is a fundamental approach. However, in real applications like fuel cells, the catalysts are compared in terms of their “mass activity: normalization with the deposited metal weight”. Thus, by normalizing the current with the metal weight and geometrical surface area, Figure S3d shows that Au@PtNBs is the best active catalyst among the three electrode materials. It is currently assumed that the accumulation of oxidation residues during the catalytic processes can be evaluated by the ratio of the forward anodic peak to the reverse peak current (If/Ib). The higher the ratio, the more tolerant is the catalyst toward the strongly adsorbed species oxidation. This ratio is about 0.39, 0.40, and 0.42 for PtNBs, Au@PtNBs, and Pt50Au50NPs, respectively. Consequently, PtNBs is the most poisoned and Pt50Au50NPs the least poisoned catalyst. The latest is more tolerant than those reported.10,52,60 This makes the nominal Pt50Au50NPs composition a potential candidate as an anode material in DFAFC. Finally, the scan rate effect study reveals that the current increases with it (Figure S4a−c). The current 27536



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progressively reduced during the negative potential scanning. This reaction releases freshly active sites for further adsorption. The scan rate depending on the oxidation current reveals that this peak is under an adsorption process (results not shown herein). This confirms well the fact that it corresponds to the dehydrogenation process. Due to its high sensitivity to glucose, these nanomaterials could be used as anode electrode materials in direct glucose fuel cells and biofuel cells for producing electrochemical energy, as well as in biosensors for glucose detection devices. The potential use of mesoporous Pt−Au nanostructures for the design of highly sensitive amperometric glucose sensors has been proven by Li et al.28 Indeed, the design of the amperometric glucose sensor based on the optimized mesoporous Pt51Au49 alloy films enables measuring low levels of glucose with good reproducibility, in the concentration range from 6.0 μM to 11 mM with a sensitivity of 352 μA cm−2 mM−1.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 5 49 45 3967. Fax: +33 5 49 45 3580. Present Address ∥

A.L.: Hokkaido University, Catalysis Research Center, Kita 21, Nishi 10, Sapporo, Japan. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Patricia Beaunier (LRS, UPMC) and Laurence Ramos (L2C, Montpellier II) are acknowledged, respectively, for TEM observations and SAXS experiments. The authors acknowledge the GDR Nanoalliages for its support.

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4. CONCLUSION To summarize, we have synthesized bimetallic nanostructures (mesoporous nanoballs (NBs) and nonmesoporous) with at least 2 times activity gains over a monometallic Pt electrocatalyst. To achieve this improved activity, we have used radiolysis that enables effective control of particle size and morphology. Core−shell nanoparticles made from the Au-core covered by a thin layer of the Pt-shell with a size of 5−8 nm were first prepared. High-resolution transition electron microscopy (HRTEM) characterization of 3D-mesoporous materials reveals that platinum nanoballs (PtNBs) are composed of small Pt particles interconnected as nanowires to build a giant frame called nanoballs with a thickness of ca. 2 nm and a frame porosity of 15 nm. The controlled growth of PtNBs around a gold nanoparticle leads to mesoporous core− shell nanoballs Au@PtNBs with a diameter of 70−80 nm. HRTEM analyses indicate that the Au-core has a diameter of 35 nm, surrounded by a porous shell of platinum (15 nm of thickness) made by 3D-connected nanowires (2 nm diameter). The porosity and favorable surface composition of the electrocatalysts are the potential sources of this high activity. The very low onset potential and good kinetics at lower potential were highlighted by cyclic voltammetry studies during formic acid and glucose electrooxidation reactions. The high sensitivity of Au@PtNBs bimetallic nanostructures at a low concentration of glucose makes them potential candidates in glucose electrochemical energy conversion devices. Subsequently, the synthesis, structure, and activity relationships uncovered herein pave ample room for new rational pathways about more active bimetallic nanomaterial tailoring. Undoubtedly, radiolysis is a promising route for nanoscale materials synthesis for electrochemical applications. The results highlighted herein could serve as guidelines for future works concerning energy conversion and storage devices.

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Article

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09417. Description of the method used for the electrochemical surface area (ECSA) evaluation and additional cyclic voltammograms (PDF) 27537


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