Electrochemical and Physicochemical Characterizations of Gold-Based Nanomaterials: Correlation between Surface Composition and Electrocatalytic Activity
Electrochemical and Physicochemical Characterizations ofGold-Based Nanomaterials: Correlation between SurfaceComposition and Electrocatalytic ActivityYaovi Holade,∗ Karine Servat, Julie Rousseau, Christine Canaff, Suzie Poulin,Teko W. Napporn,∗∗ and K. Boniface Kokoh∗∗,z
Universite de Poitiers, IC2MP, UMR 7285 - CNRS, Equipe SAMCat, 4, rue Michel Brunet, B27, TSA 51106,86073 Poitiers cedex 09, France
Manuscript submitted August 24, 2015; revised manuscript received September 16, 2015. Published October 13, 2015.
The design of cutting-edge nanomaterials to be used as advancedelectrode materials in energy conversion and storage technologies hasfocused the attention of researchers in the last decade. For a metal hav-ing a face-centered cubic (fcc) crystallographic symmetry (Au, Pt, Pd,Ag. . . ), the equilibrium shape of its nanocrystal is predicted by fun-damental thermodynamic considerations to be a cubo-octahedral.1–4
Thus, it is exclusively composed of (111) and (100) facets. Most ofthe prepared supported metal catalysts have this morphology. Vari-ous crystalline structures with high Miller indices (hkl) have beenreported only for unsupported nanoparticles.5–7 Zhang and coworkersreported the preparation of 24 high-index (720) facets of Au concavenanocubes using a combination of silver and chloride ions through aseed-mediated synthetic method.8 The group of Xia has shown the pos-sibility of obtaining nanocrystals Pt(510), Pt(720) and Pt(830) usinga kinetically-controlled method with tunable electrocatalytic proper-ties towards the well-known oxygen reduction reaction (ORR).5,9 Byusing a simple route based on seed growth approach, they obtainedPd concave nanocubes bounded by high index Pd(730) facets, wherePd nanocubes were used as seeds for the reduction of a Pd precur-sor in aqueous solution.10 The majority of these investigations hasconcerned monometallic materials even if some endeavors have beenundertaken for Au-based core-shell bimetallic materials having highindex (hkl) facets.11,12
The unsupported nanoparticles serve often as models in catalysis.For the design of realistic applications like fuel cells (FCs) technolo-gies, an electrical conducting support is necessary to first dispersethese nanomaterials and then, to enhance the current densities, whilereducing concomitantly the metal content in the electrode catalyst.Unfortunately, this support gives rise to an additional constraint. Itrestricts the controllability of nanoparticles shape and finally doesnot enable obtaining nanoparticles with desired crystallographic ori-entations. Highly dispersed catalysts provide a maximum availablesurface area for electrocatalytic reactions. On the other hand, the de-crease of particles size especially in bimetallic materials down to fewnanometers is undoubtedly accompanied by variation of numerousphysicochemical properties, thus affecting the catalytic properties.2–4
It should be noted that the entire reactions in heterogeneous cataly-sis are not size-depending. But, it is unanimously accepted that the
∗Electrochemical Society Student Member.∗∗Electrochemical Society Active Member.
presence of different elements in a nanomaterial composition has no-table effect on their electronic properties. Seminal works have shownthat bulk gold-based electrode materials exhibit good electrocatalyticactivity towards carbohydrates electrooxidation.13,14 The effective de-velopment of efficient carbohydrate-based direct alkaline fuel cells(DAFCs) offers many benefits among of which the electrochemi-cal synthesis of added-value chemical from selective electrooxida-tion of carbohydrates (abundant, renewable and non-toxic organiccompounds from biomass, an extensive and endlessly renewable re-source). Thus, various synthetic methods have emerged to preparecarbon-supported Au, Pt and Pd nanoparticles for their electrooxida-tion, especially glucose. Fundamental understanding of the correlationbetween their surface properties and electrocatalytic ones for the op-timization of performances in DAFCs is still missing.
In previous reports, the so-called “Bromide Anion Exchange,BAE”15 method has been revised to prepare various compositionsof Au-Pd, Au-Pt and Au-Pt-Pd nanomaterials supported on VulcanXC 72R carbon.16,17 Their good electrocatalytic properties have beenpointed out, particularly as anode catalysts for glucose electrooxida-tion in hybrid biofuel cell either in conditions mimicking physiologicalones18,19 or human serum.20 The most fascinating aspect of this sim-ple, fast and convenient synthetic method is an approach free fromorganic molecule as surfactant or capping agent. The present workaims at scrutinizing the direct correlation between the physicochem-ical and electrochemical properties of these nanomaterials. Herein,nanostructures were first characterized by high-resolution transmis-sion electron microscopy (HRTEM) together with energy dispersiveX-ray (EDX). Then, their surface and electronic properties were ex-amined by X-ray photoelectron spectroscopy (XPS) measurements.Carbon monoxide (CO) and glucose were used as surface probingmolecule and fuel, respectively, to evaluate the catalytic activity ofthese electrode materials.
Experimental
Preparation of the nanomaterials from a surfactant-free method:Bromide Anion Exchange (BAE).— Multimetallic nanostructures Au-Pt, Au-Pd and Au-Pt-Pd were synthesized from the revised “Bro-mide Anion Exchange, BAE” method.16 The entire optimizationis reported elsewhere.17 The singularity of this approach relies onits softness and simplicity of implementation without using an or-ganic molecule as surfactant or capping agent. Conveniently, BAEallows an effective preparation of well-dispersed nanoparticles with a
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H930 Journal of The Electrochemical Society, 162 (14) H929-H937 (2015)
good chemical yield (>90%), exhibiting high electrochemical activesurface area and excellent catalytic properties. For catalysts prepa-ration, hexachloroplatinic (IV) acid hexahydrate (H2PtCl6 · 6H2O,ACS reagent, ≥37.50% Pt basis), tetrachloroauric (III) acid trihydrate(HAuCl4 · 3H2O, ACS reagent, ≥99.9%), potassium tetrachloropalla-date (II) (K2PdCl4, 99%), sodium borohydride (NaBH4, 99%), potas-sium bromide (KBr, ≥99%) and L-ascorbic acid (AA, ≥99%) werepurchased from Sigma-Aldrich and used as-received without furtherpurification. Contrariwise, Vulcan XC 72R carbon black used as sup-port for nanoparticules dispersion was supplied from Cabot and ther-mally pre-treated in order to boost the electrocatalytic properties ofthe nanoparticles as recently highlighted.21 The reducing agent NaBH4
was used for gold-platinum nanomaterials (Au-Pt/C). To avoid hydro-gen (present in NaBH4) insertion into the palladium lattice network,22
AA was used for materials containing palladium (Au-Pd/C or Au-Pt-Pd/C). The targeted metal loading was 20 wt.%. All the solutions wereprepared with Millipore Milli-Q water (MQ, 18.2 M� cm at 20◦C).
Typically, AuxPtyPtz nanoparticles supported on Vulcan XC 72R(AuxPtyPtz/C, where x, y and z are the corresponding atomicpercentages) were synthesized using HAuCl4 · 3H2O (for Au),H2PtCl6 · 6H2O (for Pt) and K2PdCl4 (for Pd) as metal precursors.These metal salts, taken in the proportion corresponding to the de-sired atomic composition of the nanoparticles and total metal weightof 20 mg are dissolved in 100 mL MQ water (thermostated at 25◦C,using a magnetic stirrer). Then, KBr was added to the solution (φ= n(KBr)/n(metals) = 1.5) under vigorous stirring. After completesolution homogenization, 80 mg Vulcan was added under constantultrasonic homogenization for 45 min. Then, a freshly prepared re-ducing agent solution was added dropwise to reduce the Au3+, Pt4+
and Pd2+ species to their metallic state. As aforementioned, eitherNaBH4 solution (0.1 mol L−1, n(NaBH4)/n(metals) = 15) or AA (0.1mol L−1, n(AA)/n(metals) = 7) was used as reducing agent. The reac-tion continued for 2 hours at 40◦C under vigorous stirring. Finally,the obtained metal nanoparticles supported on Vulcan (AuxPtyPtz/C)were filtered on Buchner system, washed roughly three times withMQ water and dried in an oven at 40◦C for 12 hours.
Methods for physicochemical characterizations of thenanomaterials.— The metal loading was determined from dif-ferential and thermogravimetric analyses (DTA-TGA). Experimentswere conducted on TA Instruments SDT Q600 apparatus by thermallyheating a few milligrams of the sample (contained in an aluminacrucible) under air flow of 100 mL min−1 from 25 to 900◦C with a5◦C min−1 temperature rate. The elementary analysis of the sampleswas carried out by inductively coupled plasma optical emissionspectrometry (ICP-OES) using a spectrometer Optima 2000 DVprovided by Perkin Elmer after their mineralization in a reactorcontaining 10 mg of the catalyst power dissolved in aqua regia. Themorphologies, particles dispersion on the support and the size ofthe obtained materials were analyzed using transmission electronmicroscopy (TEM) on a TEM/STEM JEOL 2100 UHR (200 kV)equipped with a LaB6 filament while energy dispersive X-ray(EDX, JED Series AnalysisProgram provided by JEOL) allowed todetermine their chemical elementary composition and homogeneity.High-resolution (HRTEM) analyses were conducted on the samemicroscope. The nanoparticules’ support, Vulcan XC 72R carbonbeing electronically conducting, materials are free of nuisances likecharging.4 Thus, samples for TEM observations were easily preparedby their dispersion in ethanol and then deposited onto copper grid. Toprobe and characterize the oxidation state of the surface and electronicinteractions between the elements in the different as-synthesizednanomaterials, X-ray photoelectron spectroscopy (XPS) was em-ployed. Its suitability to study electrocatalysts has been reviewed byCorcoran.23 XPS measurements were performed in a high vacuumchamber (pressure ≤ 2 10−9 Torr) on a Kratos Axis Ultra DLDspectrometer equipped with a monochromatic radiation source AlMono (AlKα: 1486.6 eV) operating at 150 W (15 kV and 10 mA). Thesurvey spectra were performed with a step of 1 eV (transition energy:160 eV). Based on the collected basic information, high-resolution
XPS spectra were collected at a step of 0.1 eV (transition energy: 20eV). XPS peaks were fitted using Gaussian-Lorentzian (GL) profilefunction and asymmetry determined from pure metal which wasobtained after reduction under hydrogen gas atmosphere. It should benoted that a systematic subtraction of the background noise precededthe normalization of the spectra and the measurement of bindingenergies (BE) was corrected based on the energy of C 1s at 284.4 eV(Vulcan XC 72R carbon). The Fermi level was also probed and XPSspectra presented herein use C1s as reference. In addition, referencematerials (bulk Pt, Pd, Au) were also used to better analyze the XPSspectra of the prepared nanomaterials. It should be noticed that theWagner’s table for the sensitivity factors was used for quantificationsand the CASA XPS software for data acquisition and processing.
Procedures for the electrochemical characterizations and tests.—Electrochemical experiments were conducted in a conventional three-electrode cell using an analogical potentiostat EG&G PARC Model362 from Princeton Applied Research. The reference electrode con-sists of a fresh home-made reversible hydrogen electrode (RHE), sep-arated from the solution by a Luggin capillary tip. The working elec-trode consisted of a catalytic ink (3 μL) deposited onto a well-polishedglassy carbon (GC) disk of 3 mm diameter (geometrical surface area:0.071 cm2) through an abrasive ADL disk with alumina powders of 1,0.3 and 0.05 μm. A slab of GC (6.48 cm2 geometrical area) was usedas counter electrode. The catalytic ink was prepared as previouslyreported16,17,21,22 and according to the procedure initiated by Wilsonand co-workers for FCs applications.24 The total metal loading in thecatalytic layer on the electrode is ca. 80 μgmetal cm−2. Cyclic voltam-mograms (CVs) were recorded in a 0.1 M alkaline solution (NaOH,97%; from Sigma-Aldrich) used as supporting electrolyte. Prior toany test, the solution was completely deoxygenated with nitrogenfor 30 min. Nanocatalyst surfaces were probed by carbon monoxide(CO, Air Liquide, from ultra-pure) stripping experiments. Basically,to make CO stripping experiment, there are 3 steps: (i) Adsorptionof CO for about 5 min on the electrode at a fixed potential (at ECO
= 0.10 V vs. RHE for Au-Pt/C electrode materials and at 0.3 V vs.RHE for those containing Pd, for avoiding hydrogen absorption atlow electrode potential); (ii) Under ECO potential control, nitrogen isbubbled for roughly 30 min in order to remove completely the non-adsorbed CO in bulk solution; (iii) Finally, the cyclic voltammetryis performed starting at ECO. In this case, only the remaining CO,which is adsorbed at the electrode surface, is electrooxidized duringthe positive scan. In the reverse scan, there is no reactive species whichwill be oxidized again. To investigate the electrocatalytic activity ofthe electrode materials, 10 mM glucose (D-(+)-glucose, 99.5%; fromSigma-Aldrich) was used as fuel. All electrochemical measurementswere conducted at controlled temperature of 25◦C.
Results and Discussion
The as-synthesized nanomaterials were successively characterizedby TGA, ICP and XRD. The targeted metal loading of 20 wt.% wasconfirmed for all materials. Furthermore, the nominal composition inmultimetallic materials is verified from ICP analyses: Au90Pd10 (forAu90Pd10), Au79Pd21 (for Au80Pd20), Au74Pt26 (for Au80Pt20), Au58Pt42
Nanomaterials characterization by high-resolution transmissionelectron microscopy (HRTEM).— Prior to HRTEM analyses, nano-materials were examined in low resolution (TEM) in order to establishhistograms of particles size distribution as well as the determination ofthe catalysts dispersion (meaning the exposed fraction of atoms).25–27
Fig. 1 shows the TEM micrographs of the monometallic and multi-metallic nanomaterials. Overall, particles are well dispersed on thesupport with mean particles size between 3 and 5 nm, except for Pd(roughly 9 nm). This size deviation is only due to the reducing agentused for Pd since ascorbic acid promoted the formation of biggerparticles than NaBH4.22 The obtained catalyst dispersion goes from
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Figure 1. TEM images of: (a) Au/C, (b) Pt/C, (c) Pd/C, (d) Au90Pd10/C, (e) Au80Pt20/C and (d) Au70Pt15Pd15/C nanomaterials.
10 to 30%, which is in good agreement with the reported values forelectrocatalysts.26,27
HRTEM observations and analyses of the monometallicnanomaterials.— We next undertook a high-resolution TEM study(HRTEM) to access the morphology as well as the crystallographicorientation of the nanoparticles. Figs. 2a and 2b show HRTEM im-ages of Au/C where nanoparticles tend to adopt a truncated octahedronshape with different degrees of truncation. Nanoparticles have facetsoriented by crystallographic planes (111) and (200) in the case of Pt/C(Fig. 2c) and Pd/C (Figs. 2d and 2e). Obtaining high Miller indicessuch as (200) instead of (100) suggests that the BAE synthesis methodoffers favorable thermodynamic conditions. Contrariwise, within thesame material, different forms may coexist as it is the case of goldwith two different forms thermodynamically. In Fig. 2a, the nanopar-ticle is a truncated octahedron with facets composed of (111) and(200) planes while the other in Fig. 2b is a pseudo-cuboctahedronwhose facets are determined by the planes (100) and (110). Indeed,a real cubo-octahedron has the facets (100) and (111).2–4 The changein the crystallographic orientation could be due to the presence ofboth chloride bromide ions. In addition, the particle in Fig. 2b hasa quasi-symmetry element which is a mirror plane as indicated bythe circles in the picture. Bromide ions alone mediate the formationof nanocubes, meaning facets (100). Since the determined interpla-nar spacing of crystallographic plane is d(hkl) = 0.29, instead of d(hkl)
= 0.39 for (100), we concluded that the final shape was modified bythe presence of both ions.
Considering supported nanoparticles, the formation of facets (111)and (100) is thermodynamically more favorable since the surface en-ergy (γ) associated with different crystallographic planes follows theorder: γ(111) < γ(100) < γ(110).3 Basically, under thermodynamicequilibrium conditions, the shape of a crystal is predicted by theWulff’s theorem, which states that the minimum energy is obtained fora polyhedron whose center distances to the surfaces are proportionalto their surface energies.1,4,28 Thus, the polyhedron corresponding tothe more thermodynamic stable morphology for a nanoparticle with
a fcc crystal symmetry is a truncated octahedron.1,3,4 The latter type-structure is composed of eight hexagonal faces and six square faceswhere the crystalline orientation of the surface atoms is described byMiller indices (111) and (100), respectively.
Furthermore, the presence of the carbon support undoubtedly in-fluences the final shape of the nanoparticle. The crystal shape shown inFigs. 2b and 2e which cannot be rigorously predicted by Wulff’s theo-rem indicates that the BAE method offers better conditions than othermethods like the well-known water-in-oil one.29 It has been shown thatfor the glucose electrooxidation reaction, the activity of nanoparticlesincreases in the direction (111) < (110) < (100).30,31 Furthermore, thepresence of pseudo-cuboctahedron shape and particles with exposedfacets (200) could lead to specific catalytic performances.
HRTEM observations and EDX analyses of multimetallicnanomaterials.— HRTEM observations coupled with EDX analyseswere made on multimetallic catalysts to identifying possible preferredorientations of nanoparticles while analyzing the homogeneity of cat-alysts. For the atomic quantification, energy levels of 9.712 keV (forAu), 9.441 keV (for Pt) and 2.852 keV (for Pd) were considered. Thesignal of carbon (ca. 0.25 keV) is due to Vulcan used as nanoparti-cles’ support and that of the copper (ca. 0.95 keV, 8.15 and 8.95 keV)belongs to the grid used for microscopic observations.
HRTEM photographs in Fig. 3a show a particular shape of Au80Pt20
nanoparticle with crystallographic facets (111), (200) and finally thefacet (110). A closer examination reveals the presence of a mirrorplane, which induces the formation of a twin (top micrograph ofFig. 3a). The occurrence of twins in a crystal results from pooling twograins along a crystallographic plane. This has the effect of forming ananocrystal made of two half-crystals, their structure being the mirrorreflection of each other by the seal twin. Recently, same phenomenahave been observed in the case of PtCo and PtNi nanoparticles: sev-eral rows of atoms along the junction with simulation experiments.32
Habrioux et al.29 had already observed the twins presence in thenanocrystals Au70Pt30/C prepared by water-in-oil microemulsion. Butthe shape shown in Fig. 3a has not yet been obtained experimentally
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H932 Journal of The Electrochemical Society, 162 (14) H929-H937 (2015)
Figure 2. HRTEM micrographs of (a, b) Au/C, (c) Pt/C and (d, e) Pd/Cmonometallic nanomaterials. The insert pictures in (a), (c) and (d) are relatedto the Fourier transform of the corresponding images for the determinationof crystallographic facets. The zone axis [hkl] during the measurement isindicated.
with supported metal nanoparticles. It constitutes an advance towardsshape-controlled supported nanoparticles preparation. This nanopar-ticle has large twins with lots of atomic rows supported by the(110) plane. The formation of well-defined twins has been also ob-served with monometallic Au/C nanomaterials synthesized with L-ascorbic acid as reducing agent (not shown herein). Furthermore,the HRTEM image of Au90Pd10/C (Fig. 3b) shows a shape close tothat of an icosahedron (three-fold symmetry)2,33 or decahedron(five-fold symmetry),2,34 both having been obtained in the case of goldnanomaterials using ascorbic acid as reducing agent and some cappingagents. The nanoparticle displays the index (111) and (200) facets, asthose of the aforementioned crystal morphology. The HRTEM imageof Au70Pt15Pd15/C depicts the same facets (111) and (200). The originof the formation of the high Miller indices (200) instead of (100) onescould be due to the fact that no organic molecule was used during thesynthesis and certainly the presence of both chloride and bromide ionsas previously explained. So, the particles can grow with any constraintsfrom organic molecules. Due to the different adsorption kinetics ofhalides, nanocrystals with high Miller indices can be formed.
The EDX analyses have given a consistent composition forAu78Pt22 (Au80Pt20/C). In contrast, the composition Au90Pd10/C andthose of trimetallic catalysts are heterogeneous (trimetallics havingsome gold islands). In the case of Au90Pd10/C, different compositionsas Au94Pd6 and Au86Pd14 were observed. For trimetallic catalysts,the compositions with the three metals are Au48Pt17Pd35 (+ gold is-lands) for Au70Pt15Pd15/C and Au30Pt16Pd54 for Au60Pt20Pd20/C (+
Figure 3. EDX spectra and HRTEM micrographs of (a) Au80Pt20/C, (b)Au90Pd10/C and (c) Au70Pt15Pd15/C multimetallic nanomaterials.
gold islands). The more heterogeneous materials are those where L-ascorbic acid was used as reducing agent. This heterogeneity could beexplained by a thermodynamic approach than that involving the metalsalts reduction kinetics, when L-ascorbic acid is used. Indeed, the re-duction of Au3+ into Au+ is very fast; but kinetics of the second stepfrom Au+ to Au0 as well as the reduction kinetics of Pt4+ and Pd2+
species into Pt0 and Pd0 depend on the experimental conditions. Thepresence of other metal species as Pt4+ and Pd2+ could substantiallycatalyze the last reduction of Au+ to Au0. Subsequently, homoge-neous core@shell nanoparticles having Au as core (thus, less atoms atthe surface) is expected. As, it will be shown in sections Electrochem-ical evidence of synergistic effect in Au-Pd bimetallic nanostructuresand Surface state and electronic properties in the nanomaterials: X-ray photoelectron spectroscopy (XPS) measurements, multimetallicnanoparticles have less Au atoms at their surface. Such reduction ki-netics pathway leading to the formation of core-shell structures cannotexplain the presence of gold islands. So, we believe that two processesoccur during the synthesis. A part of Au4+ is swiftly reduced into Au+
followed by the co-reduction of Au+ by Pt4+ and Pd2+ species. Then,the repulsion effect between these first multimetallic nanoparticlesand the remaining Au4+ species leads definitely to the formation ofisolated gold nanoparticles (Au islands) because of the metal mis-cibility limit at nanoscale level. With NaBH4, the reduction occursso quasi-instantaneously that no segregation effect can take place.Consequently, the miscibility of the three metals would limit ther-modynamics, which should lead to homogeneous core@shell (a corecomposed of Au atoms and a shell made of Pd and/or Pd atoms)structure for multimetallic nanomaterials.
Electrochemical activity of the nanomaterials.— The electro-catalytic activity of some as-prepared nanomaterials has beenrecently compared by their polarization curves towards glucose
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Journal of The Electrochemical Society, 162 (14) H929-H937 (2015) H933
Figure 4. Specific activities, evaluated at different electrode potentials from polarization curves on (a) Au-Pd/C, (b) Au-Pt/C and (c) Au-Pt-Pd/C electrodematerials: Polarization curves were recorded at 20 mV s−1 in a 0.1 mol L−1 NaOH + 10 mmol L−1 glucose at 25◦C. (d) Typical curves on Au80Pt20/C electrodematerial in the absence (i), presence of 10 mmol L−1 glucose (ii) and after correction with the electrolyte contribution (iii) at 20 mV s−1 and 25◦C.
electrooxidation reaction in alkaline medium.17 But, the presentstudies aim at examining the origin of the noticed difference. Fig. 4depicts the Volcano plots of the specific activity at different electrodepotentials. The specific activity is obtained by correcting the collectedcurrent during glucose electrooxidation by the supporting electrolytecontribution. Typical full curves at 20 mV s−1 on Au80Pt20/C electrodematerial in the absence, presence of 10 mmol L−1 glucose and aftercorrection with the electrolyte contribution at 25◦C are provided inFig. 4d. We especially focused on electrode potentials less than 0.7 Vvs. RHE to mimic their performances as anode materials in glucose-based FCs. Indeed, in alcohol FCs, the electrode potential is expectedto not exceed 0.7 V vs. RHE during operation. This enables to havelarge cell voltage and subsequently a high output power. Amongall gold-palladium bimetallic nanomaterials, the most synergisticeffect is observed when 10 at.% of gold is replaced by palladium. At0.4 V vs. RHE, Au90Pd10/C is almost 2-fold more efficient than themonometallics. Table I shows Tafel plot tests; the exchanged currentdensity is very high on Au90Pd10/C compared to monometallics Au/Cand Pd/C. This enhancement can be primary explained by a possiblesynergistic effect. We will see in section Surface state and electronicproperties in the nanomaterials: X-ray photoelectron spectroscopy
(XPS) measurements that XPS spectra give sound evidences on theelectronic effects synonymous of interactions between each element,responsible for increasing glucose oxidation reaction kinetics at lowelectrode potentials. Additionally, the presence of either facet (200)or (110) may also positively contribute to this kinetic improvementsince same nanomaterials prepared from the water-in-oil method withonly (100) and (111) have lower performances.29
Concerning gold-platinum binaries (Fig. 4b), the entire compo-sitions show better reaction kinetics than monometallics. Even ifPt/C shows high specific activity for E > 0.5 V vs. RHE, chronoam-perometry tests have shown that this catalyst is quickly deactivatedduring the reaction. It is a common phenomenon in electrocatalysiswhen considering organic molecule electrooxidation. We report inFig. 4c the Volcano plots for the optimized trimetallic nanomaterialsused as electrode materials. It clearly indicates the synergisticeffect between the three metals. This phenomenon is the desiredbehavior for anode materials catalyzing organic molecules oxidationin FCs. Compared with the literature (appropriate normalization), thenanocatalysts developed here for the glucose electrooxidation exhibitbetter performances in terms of the onset potential, current density(more than 2-fold increase) and stability over time. To gain clear
Table I. Exchanged current density, j0, of nanocatalysts obtained from the polarization curves performed at a scan rate of 2 mV s−1 in presenceof 10 mM glucose in 0.1 M NaOH at 25◦C.
Tafel plots for E < 0.6 V vs. RHE.52 64 261 646 238 109
Tafel plots for E > 0.6 V vs. RHE.j0(μA · cm−2) 62 1 801 1024 961 230
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Figure 5. CO stripping experiments performed at 20 mV s−1 in 0.1 mol L−1 NaOH electrolyte at 25◦C. On Au-Pd/C electrode materials: (a) Recorded CVs and(b) Surface atomic composition determined from the Rand and Woods method. (c) Au-Pt electrode materials steady-state CVs at 50 mV s−1. Background correctedCO stripping curves at (d, f) Au-Pt/C, (e) Au-Pt-Pd/C electrode materials. In (f), the current was normalized using Pt active surface.
understanding about these improved catalytic properties, we haveperformed series of deep investigations. CO stripping experimentswere first carried out on Au-Pd bimetallic nanomaterials. Then, XPSanalysis might provide complete evidences.
Electrochemical evidence of synergistic effect in Au-Pd bimetal-lic nanostructures.— The nanomaterials surface has been electro-chemically characterized by CO stripping experiments, as shown inFig. 5a for Au-Pd/C bimetallic catalysts. During the positive scan,the adsorbed CO (COads) is oxidized at ca. 0.8 V vs. RHE. Then, the
catalyst surface being free from COads, is oxidized at high electrodepotential. Finally, these oxides are reduced during the reverse scan.Gold oxides are reduced at 1.09 V vs. RHE and those of palladiumare reduced at around 0.62 V vs. RHE. This figure contains two ma-jor findings. First of all, the oxidation peaks of COads shift towardslow electrode potentials for the bimetallic catalysts. The Au/C cat-alyst has been also tested towards CO stripping. It does not showany significant catalytic activity, despite the theoretical predictions ofNørskov35 and experimental results from bulk Au(111).36 But in bulksolution (meaning a CO-saturated alkaline solution), it exhibits good
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catalytic activity.36–39 The additional constraint imposed by the “COstripping” experiments is the maintenance of the CO molecule on thesurface. It seems that this binding energy is so low that the adsorptionand retention on the electrode are difficult. Consequently, the weaklyadsorbed CO is lost during the dissolved CO removal under nitro-gen atmosphere (which lasts roughly 20–30 min). Subsequently, thereaction kinetics improvement can be explained by electronic interac-tions between gold and palladium in bimetallics. Such experimentalobservations confirm the noticed excellent catalytic activity towardsglucose electrooxidation.
For intermediate atomic compositions in Au-Pd systems, thereis only one peak where the oxide of bimetallic materials is reduced.More importantly, it shifts towards higher potentials when gold contentincreases in the bimetallic material. On other hand, only one phase,that of alloy, is formed. To gain further insights about its composition,we used the electrochemical method proposed by Rand and Woods40
to estimate the surface atomic composition of the Au-Pd electrodesaccording to Equation 1.
EAuPdpeak = χPdEPd
peak+χAuEAupeak ⇒ χPd = 100
(EAuPd
peak − EAupeak
EPdpeak − EAu
peak
)[1]
χPd and χAu(χPd = 100-χAu) are the Pd and Au surface atomic per-centages (surf.at.%) and EPd
peak , EAupeakand EAuPd
peak are the oxide reductionpeak potentials for Pd, Au and AuPd, respectively.
The obtained surface atomic compositions are represented inFig. 5b. It is worth mentioning that values determined from the steady-state CVs at 20 mV s−1 are quite the same. It can be concluded thatthe electrochemical surface of the electrode is mostly composed ofpalladium. A such phenomenon has been observed for Au-M (M= Pt, Pd).41 When gold is associated either with palladium or plat-inum, the latter elements go on the surface under potential cycling.Since HRTEM observations coupled with EDX analyses have shownthat a real core-shell structure was not formed, the present result sug-gests that gold and palladium form an alloy phase with an enrichmentin Pd atoms at its surface. The surface composition of Au90Pd10/C (38surf.at.% Au and 62 surf.at.% Pd) electrode material is particularlyinteresting. For organic molecules (and particularly carbohydrates)electrooxidation, the activity at low electrode potentials increases inthe order Pt > Pd > Au. But for the stability, the order is Pt < Pd< Au. Finally, the surface composition of Au90Pd10/C electrode ma-terial explains well the improved performances previously noticed.As displayed in Fig. 5c, the Au-Pt electrodes show two distinguishedpotentials of oxide reduction as already reported.29 No surface quan-tification was thus done; the Rand and Woods method is no longervalid because of the presence of two peaks.40 Furthermore, Pt and Auoxides reduction peaks in the CVs (Fig. 5c) were not used to quantifytheir relative surface composition because these peaks are not welldefined when the metal loading is less than 20 wt.% on Vulcan forthe present nanocatalysts compared to 40 wt.% elsewhere.15,29 Onlya small AuOx reduction peak can be seen in the CVs. Thus, as wehave only one common reduction peak (where both AuOx and PtOx
are reduced), it is quite difficult to find the suitable monolayer chargevalue as reference. Figs. 5d and 5e show the background corrected COstripping results for Au-Pt/C and Au-Pt-Pd/C nanocatalysts, respec-tively. The current is normalized using the weight of metals whichare active for CO stripping. These figures highlight the enhancementof the electrocatalytic activity for the multimetallic catalysts at lowelectrode potential. It should be emphasized that the same trend isobserved when CO stripping curves are normalized using the electro-chemical active surface (ECSA) of material of Pt (Fig. 5f). In this case,the ECSA of Pt was determined from hydrogen region for Au-Pt/Celectrode materials. The ECSA cannot be precisely determined forAu-Pd/C and Au-Pt-Pd/C electrodes, because they present commonpotential window where the oxides are reduced and there is no refer-ence value for the monolayer charge. As no activity of gold is hereinobserved for CO stripping, we attribute this highly enhancement to theexistence of synergistic effect, with certainly a substantial contributionof electronic interactions between elements on these nanoparticles.
Surface state and electronic properties in the nanomaterials: X-ray photoelectron spectroscopy (XPS) measurements.— To gain fur-ther insights into how the different elements interact in multimetallicmaterials, we performed XPS measurements. This surface techniqueprovides deep information on the chemical composition of the ana-lyzed sample together with the possible charge transfer between thedifferent chemical elements. A survey spectrum is first recorded forall materials to have an overview and a qualitative analysis. Then,a high-resolution spectrum is recorded for the different core-levelsof interest. Monometallics are first characterized in order to serve asbenchmarks. The high-resolution XPS spectra of Au 4f, Pt 4f andPd 3d regions are displayed in Figs. 6a, 6b and 6c, respectively. Theobserved doublets for the same band are related to the spin-orbit split-ting (±1/2). We underscore important current trends that the obtainedmaterials are not (Pt/C) or less oxidized (Au/C and Pd/C). Furtherexamination of decomposed results in Fig. 6c indicates that there aretwo kinds of oxides for Pd 3d. The doublet of gold metal are situatedat 83.9 eV for Au 4f7/2 and 87.6 eV for Au 4f5/2. Those of Pt 4f arelocated at 71.1 eV (Pt 4f7/2) and 74.5 eV (Pt 4f5/2) in agreement withthe literature.12,23,42,42 The doublets Pd 3d5/2 and Pd 3d3/2 are situatedat 335.3 eV and 340.6 eV.12 The presence of the oxide is simply dueto the natural oxidation of noble metal when exposed to ambient air.Currently, a thin protective layer covers the metal surface from deepoxidation (immunity layer). From high-resolution XRD patterns (notshown herein), no additional oxide peak was observed, which endorsescompletely the conclusion that the oxide amount is very insignificant.
We further expand our knowledge about possible electronicinteractions in multimetallic nanomaterials by recording their XPSspectra. The high-resolution spectra are shown in Figs. 6d (Au 4f) and6e (Pd 3d) for Au90Pd10/C, 6f (Au 4f) and 6g (Pt 4f) for Au80Pt20/C,and 6h (Au 4f), 6i (Pt 4f) and 6j (Pd 3d) for Au70Pt15Pd15/C samples.In the material Au80Pt20/C, Pt 4f7/2 is situated at 70.8 eV, while that of4f7/2 level of PtO and PtOx (unknown real composition) are situatedat 71.6 eV and 74.7 eV (Fig. 6g), respectively. When comparing theBE of Pt 4f7/2 between the monometallic Pt/C material (71.1 eV) andthe bimetallic Au80Pt20/C one (70.8 eV), it appears that there is ashift of 0.3 eV towards low BE. Such downshift has been reportedand attributed to a strong electronic interaction between gold andplatinum.41,43 According to Pederson et al,43 an electronic transferfrom the sub-layer 5d10 (filled) of Au to that of Pt (5d9, unfilled)occurs in Au-Pt alloy structures. Furthermore, Au90Pd10/C systemcompared to Pd/C shows ca. 0.3 eV downshift of the BE of the Pd 3d(3d5/2 at 335.0 eV for Au90Pd10/C) and less than 0.2 eV (the data beingcollected each 0.1 eV) for Pt 4f7/2 and Pd 3d5/2 in the Au70Pt15Pd15/Cmaterial. It should be particularly pointed out that Pd 3d spectrumin Au90Pd10/C and Au70Pt15Pd15/C materials includes a contributionfrom Au 4d and Pt 4d. As these peaks are quite wide, the differenceof 0.2 eV could include the error resulting from the fitting process.Furthermore, quantifications show a surface enrichment in Pt and Pdatoms as pointed out above in Electrochemical evidence of synergisticeffect in Au-Pd bimetallic nanostructures section and summarized inTable II. Indeed, the surface atomic composition of Au90Pd10/C is 65surf.at.% Au and 35 surf.at.% Pd. This composition is different tothat evaluated from the electrochemical method: 38 surf.at.% Au and62 surf.at.% Pd. But the value of 35 surf.at.% Pd is 3.5 times higherthan the nominal composition (10 at.%) that has been confirmedby ICP analyses. This highlights a surface enrichment when wecompare the ICP and XPS values, as observed from electrochemicalcharacterizations. For Au80Pt20/C, proportions are 50 surf.at.% Auand 50 surf.at.% Pt. Considering the trimetallic Au70Pt15Pd15/Ccatalyst, 25 surf.at.% Au, 35 surf.at.% Pt and 40 surf.at.% Pd wereobtained. In the both cases, Pd and Pt contents are at least 2-foldhigher than the nominal ones. Obviously, the electrochemical andphysical methods lead to the same conclusions about the structureof the as-prepared nanomaterials from BAE method. It is thereforeevident that the multimetallic surface enrichment in platinum and/orpalladium at the expense of gold combined with the strong electronicinteractions between the different chemical elements is undoubtedlyresponsible for the enhancement of their electrocatalytic properties.
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H936 Journal of The Electrochemical Society, 162 (14) H929-H937 (2015)
Figure 6. High-resolution XPS spectra of monometallic (a-d), bimetallic (d-g) and trimetallic (h-j) nanostructures.
Table II. Atomic composition of multimetallic nanomaterials: bulk(from ICP-OES analyses) and surface (from XPS measurements).
Nominal atomiccomposition Au90Pd10 Au80Pt20 Au70Pt15Pd15ICP: Real atomiccomposition Au90Pd10 Au74Pt26 Au74Pt10Pd16
XPS: Surfaceatomiccomposition(surf. at.%)
Au 65 50 25
Pd 35 - 40Pt - 50 35
Conclusions
This study was aiming to establish the correlation between thephysico(electro)chemical properties and catalytic performances ofgold-platinum, gold-palladium and gold-platinum-palladium nanos-tructures synthesized from a recently initiated soft and convenientchemical method. We underscore important physicochemical trendsthat were then substantiated by electrochemical ones. The transmis-sion electron microscopy (TEM) analyses showed that these nanopar-ticles are finely and well dispersed on the support (2-10 nm). Thesenanoparticles have good proportion of metal atoms at their surface (ex-posed ratio of atoms or dispersion), reaching 35%. The high-resolutionimages HRTEM allowed identifying the presence of twins leading tothe formation of crystalline facets (110) in Au80Pt20/C nanomaterials.
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Journal of The Electrochemical Society, 162 (14) H929-H937 (2015) H937
EDX and electrochemical analyses revealed that the structural com-position of the bimetallic catalysts was composed of alloying struc-tures which have less Au atoms at their surface. Furthermore, X-rayphotoelectron spectroscopy (XPS) measurements stressed out a down-shift of ca. 0.3 eV of the BE of Pt 4f (in Au80Pt20/C) and Pd 3d (inAu90Pd10/C) electrons.This shift demonstrates the strong electronicinteraction between the different elements, which enables reinforcingthe electrocatalytic properties of the obtained nanocatalysts. TheseXPS analyses have definitely confirmed platinum or palladium sur-face enrichment of the gold-based bi and tri-metallic catalysts as pre-viously mentioned. In other words, the alloy phase contains less Auatoms at their surface. We effectively linked the structural propertiesof these nanomaterials prepared without surfactant to their electrocat-alytic performances towards glucose and CO oxidation.
Acknowledgments
This work was supported by funding from the French National Re-search Agency (ANR) through the financial grants “ChemBio-Energyprogram 2012–2015.”
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