Article pubs.acs.org/JPCC
Metastable AuxRh100−x Thin Films Prepared by Pulsed Laser Deposition for the Electrooxidation of Methanol Régis Imbeault, David Reyter, Sébastien Garbarino, Lionel Roué, and Daniel Guay* Institut National de la Recherche Scientifique Energie, Matériaux et Télécommunications (INRS - EMT) 1650, Boul. Lionel-Boulet, Varennes, Québec, Canada J3X 1S2 ABSTRACT: Metastable AuxRh100−x thin films were prepared by crossed beam pulsed laser deposition on two different substrates. High kinetic-energy deposition conditions were used (0.1 Torr He, 5.5 cm target-to-substrate distance). The films were characterized by X-ray diffraction and X-ray photoelectron spectroscopy, and their electrocatalytic activities for the electrooxidation of methanol were assessed by linear sweep voltammetry in 1 M NaOH. All films are made of a single fcc phase compound. For 40 < x < 85 at %, the lattice parameter deviates slightly from that expected from Vegard’s law. The XRD patterns of AuxRh100−x/C and AuxRh100−x/Si did not evolve over a period of 1 year, indicating that the solid solution is kinetically stable. However, as expected from the Au−Rh binary phase diagram, heat treatment of Au50Rh50/C at 500 °C in Ar during 24 h leads to decomposition of the metastable fcc phase and the formation of Rh and Au. X-ray photoelectron spectroscopy revealed that the Au surface composition closely follows the Au bulk content. The binding energy difference between the Rh 3d5/2 and Au 4f7/2 core level peaks shifts from 223.3 to 222.4 eV as x is increased from 5 to 95, indicating that a surface alloy is formed between Au and Rh. In the presence of methanol, AuxRh100−x/C with x = 25, 50, and 75 shows a methanol oxidation peak (a2) in a potential region (ca. −0.12 V vs Hg/HgO), where pure Au and Rh do not exhibit any electrocatalytic activity for methanol oxidation. The current of peak a2 increases with the methanol concentration. The formation of a surface alloy significantly improves the electrocatalytic activity for methanol oxidation by providing OH− species at lower potential (by 400 mV) compared with pure Au.
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INTRODUCTION Since the beginning of the sixties, the technology of fuel cells has attracted considerable attention due to its ability to transform chemical energy directly into electric power, with high fuel efficiency and little to no emission of greenhouse gases.1−4 Of particular interest is the proton-exchange membrane fuel cell (PEMFC), which has been identified as the best potential candidate for replacing the internal combustion engine inside vehicles, as well as a promising power source for portable devices.2,5−8 PEMFCs can operate on a variety of combustible (hydrogen, formic acid, methanol, ethanol, etc.). Although the electro-oxidation of methanol is a more complex reaction than hydrogen, the use of methanol offers the advantages of high storage capacity as well as the ease of transport, storage, and handling. In this context, the electrooxidation of methanol is of interest because of its relevance for direct methanol fuel cell (DMFC).9,10 In acidic solution, platinum is the most active anode catalyst for methanol oxidation. However, Pt is easily poisoned by strongly adsorbing intermediates like CO and oxygenated species resulting from the oxidation of methanol.11,12 It is generally accepted that the oxidation of methanol is easier in alkaline solution because the presence of OH− species favors the electro-oxidation of methanol and of reaction intermediates adsorbed at the surface of the catalyst. Also, the use of basic solution considerably widens the range of potentially © 2012 American Chemical Society
interesting elements due to enhanced stability in alkaline electrolyte.13 Recently, Pd-, Au-, and Ni-based materials have been used as catalysts for methanol oxidation in alkaline solutions.12,13 Among them, gold is most particularly interesting because of its resistance to poisoning.14 On smooth polycrystalline gold electrode, it is wellestablished that the fact that the oxidation of methanol in alkaline solution does not proceed before the coverage by OH− anions is significant. However, in some cases, the electrooxidation of methanol on gold could be initiated at lower potential (between 200 and 400 mV less positive) than observed on smooth polycrystalline Au. This was observed for gold nanoparticles15 and for gold electrode, whose surface is modified through potential cycling16 or by the formation and decomposition of gold amalgams.17 The formation of small crystallites that could trap OH− anions17 and a high number of surface atoms with a low coordination number15 were invoked to explain this unusual activity of gold for methanol oxidation. However, the stability of these more active gold electrodes remains a matter of concern as the methanol oxidation current observed at less positive potential decreases with time.16,17 Received: December 27, 2011 Revised: February 2, 2012 Published: February 7, 2012 5262
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films for the electro-oxidation of methanol will be investigated. A synergistic effect between Au and Rh atoms when intimately mixed at the electrode surface is observed. As a result, the onset potential for methanol oxidation in alkaline solution is ca. 400 mV less positive on AuxRh100−x alloy than on pure Au.
In the literature related to acidic polymer electrolyte fuel cell, there is a vast amount of works describing how a more nucleophilic element like Ru and Rh can be added to Pt to improve the CO-tolerance of the anode catalyst. 18−20 According to the bifunctional mechanism, which was devised to explain the beneficial effect of Ru on the CO-tolerant properties of Pt, the OH− moiety on a nearby Ru atom participates in the electro-oxidation of CO bonded to Pt. Because the formation of an OH− moiety on the nucleophilic element is occurring at a less positive potential than on pure Pt, CO bonded to a Pt atom can be oxidized at a less positive potential than it would be on pure Pt.21,22 As mentioned above, the oxidation of methanol on gold can proceed at less positive potential inasmuch as OH− moieties are present. The idea at the basis of this study was to try to mimic the approach that was followed in acidic polymer electrolyte fuel cell by mixing Au with a more nucleophilic element to stabilize the OH− moiety and achieve oxidation of methanol at a less positive potential than it normally happens on smooth gold electrode. It was shown in a recent ab initio computation study of the crystal structure of materials that Au does not form stable compound with Rh and Ru.23 In the case of Rh, which is of particular interest for the present study, this has been confirmed experimentally. At 1060 °C, the Au−Rh temperature− composition phase diagram indicates that the fcc terminal solid solution, Au(Rh), has ∼1.6 at % solid solubility of Rh in Au, whereas the fcc terminal solid solution, Rh(Au), has ∼0.5 at % solid solubility of Au in Rh.24 At that temperature, the bulk miscibility gap is close to 98 at %. However, synthetic strategies exist to form alloy nanoparticles within the miscibility gap, relying on conucleation through fast reduction kinetics or cosequestration within a confined space. More recently, simple room-temperature borohydride coreduction of appropriate aqueous metal salt solutions in the presence of molecular and polymeric surface stabilizers yields alloy nanoparticles with composition falling in the miscibility gap.25 In particular, this was demonstrated for the Au−Pt and Au−Rh systems. However, the organic ligand stabilizers can be difficult to remove. Different methods have been devised for cleaning such nanoparticles such as heating in different atmospheres or submitting the nanoparticles to electrochemical decontamination by surface oxidation. These methods could produce a change of the surface structure. It was shown that modification to the surface structure could be limited if electrochemical decontamination is performed under the right conditions. However, those residual surfactant molecules were still present, causing an incomplete deposition of the nanoparticles (floating) at the surface of the substrate.26 From a more practical point of view, the realization of an electrode from an assembly of such nanoparticles is also a challenge. In recent years, pulsed laser deposition (PLD) has emerged as an effective method for preparing thin films and nanoparticles with unusual properties, and it has established itself as a versatile method for the synthesis of nanocrystalline and cluster-assembled films when deposition is performed in a moderate pressure.27 It was shown recently that PLD can be used to prepare thin films that are made of AuxPt100−x solid solution over the entire range of composition.28 In the present study, AuxRh100−x thin films are prepared with x varying from 0 to 100. It will be shown that solid solution of Au and Rh can be prepared over the whole composition. The surface composition and the electrocatalytic properties of these
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EXPERIMENTAL SECTION Thin Film Preparation. Thin films of AuxRh100−x were deposited by cross-beam pulsed-laser deposition (CBPLD) in a moderate He background gas pressure. The setup used to perform the CBPLD experiments was designed according to Tselev et al.29 and a detailed description of our experimental equipment can be found elsewhere.30 Two laser beams (KrF @ 248 nm, 17 ns pulse width, repetition rate 50 Hz, fluences 3.0− 18.0 J cm−2) are synchronously focused onto two pure solid targets: Au (99.99%, Kurt J. Lesker) and Rh (99.8%, Kurt J. Lesker). The plasma plumes are directed toward a substrate that is held at equal distance (5.5 cm) from the two targets. Both targets were kept in continuous rotational and translational motion to obtain a uniform ablation over their entire surface. The deposition was performed in a stainless-steel ultra high vacuum chamber turbo pumped to a base pressure of 10−5 Torr and then filled with high-purity He (Helium N5.0, Praxair) at a fixed background pressure of 100 mTorr. To modify the Au/Rh concentration ratio of the films, we varied the laser pulse energy on the two targets independently with the aid of attenuating plates placed in the laser beam trajectory. The maximum incident energies onto the Au and Rh targets were ∼180 and ∼135 mJ per pulse, respectively. The number of laser pulses was fixed at 60 000. The AuxRh100−x thin films were deposited on silicon plates (Prime grade 100 mm p-type (100) doped with boron, University Wafer) and disks of amorphous graphite (99.997%, Ø = 5 mm, Goodfellow Corporation). Before deposition, the silicon plate was cut into small ∼1 cm2 pieces, which were then ultrasonically cleaned in a solution of hexane (ACS reagent, HPLC grade, Fisher Scientific) and ethylacetate (ACS reagent, Fisher Scientific) and rinsed in methanol (ACS reagent, HPLC grade, Fisher Scientific). They were then heated to 100 °C during 1 h. The graphite substrates were first polished and then cleaned according to the procedure described above. In all cases, the thicknesses of the films were ca. 200 nm. Physico-Chemical Characterizations. The surface morphology of the films was investigated by scanning electron microscopy (SEM, JSM-6300F), whereas their composition was measured by energy-dispersive X-ray fluorescence (EDX) (Oxford Link ISIS, ATW2, cold-cathode field emission with a resolution of 133 eV at 5.9 keV). The composition of the films was determined at low magnification (×250), by taking the average of four different measurements on each sample. In each case, the deviation from the mean value was 18 MΩ). Before each experiment, the electrolyte solution was purged with high-purity argon (Argon N5.0, Praxair) for at least 30 min, and a light flow of argon was continuously maintained over the solution during electrochemical analyses. The apparatus used was a VMP3 multipotentiostat from BioLogic, controlled by EC-Lab software. The electrochemically active surface area of each electrode was determined by measuring the double-layer capacitance, Cdl, obtained from cyclic voltammograms recorded in a purely capacitive region between −600 to −400 mV versus Hg/HgO at different scan rates (between 20 to 200 mV/s) in 1 M NaOH and by using the equation Cdl = i/(dV/dt) where i is the current measured at −500 mV versus Hg/HgO and dV/dt is the scan rate. The real surface area of the electrodes was estimated by assuming Cdl0 = 20 μF/cm2.
Figure 1. X-ray diffraction patterns of AuxRh100−x thin films deposited on Si substrates: (A) x = 100, (B) x = 81, (C) x = 66, (D) x = 51, (E) x = 35, (F) x = 23, (G) x = 8, and (H) x = 0. For clarity, the region where the diffraction peak of the substrate appears (at 2θ ≈ 54°) has been omitted.
larger 2θ angle values as the Rh content of the film is increased, indicating that the lattice parameter decreases as the Rh content is increased. This shift toward higher 2θ angle values is accompanied by a slight increase in the full width at half-maximum (fwhm) of the diffraction peaks. Similar XRD patterns have been obtained for AuxRh100−x prepared by room-temperature borohydride coreduction of appropriate mixtures of HAuCl4·3H2O and RhCl3·xH2O in the presence of polyethylene glycol.25 To estimate the position and the fwhm of the diffraction peaks, we fitted the experimentally measured diffraction data with pseudo-Voigt functions having a mixed Gaussian− Lorentzian line shape. Then, the lattice parameter was calculated from the peak position using Bragg’s formula (eq 1). ahkl =
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λ h2 + k2 + l 2 2 sin θhkl
(1)
The various lattice parameters (a111, a200, a220, and a311) obtained from the (111), (200), (220), and (311) diffraction peak of the fcc phase do not deviate by more than 0.03 Å from the average value. Figure 2 depicts the variation of the lattice parameter, a, of the AuxRh100−x thin films with respect to the bulk Au content. The lattice parameter of pure Au (4.08 Å) and pure Rh (∼3.81 Å) correspond well to the values found in the literature (4.08 Å for Au (JCPDS 4-0784) and 3.80 Å for Rh (JCPDS 5-0685)). As seen in Figure 2, the lattice parameter deviates slightly from that expected based on Vegard’s law, which holds that a linear relation exists, at constant temperature, between the crystal lattice parameter of an alloy and the concentrations of the constituent elements. This is in qualitative agreement with results shown elsewhere.25 This positive deviation may be attributed to the energy of interaction between Au and Rh atoms, which is greater than the mean value of Au−Au and Rh− Rh interactions.32 The decrease in the lattice parameter as gold atoms are replaced by rhodium atoms is consistent with the variation of the atomic radius between these two elements. (The atomic radii of Au and Rh are 1.44 and 1.35 Å, respectively.) The temperature−composition phase diagram of the AuxRh100−x system was previously reported.24 The miscibility gap is quite large. At ∼1060 °C, the maximum equilibrium solid
RESULT AND DISCUSSION Structural Characterization. XRD characterization was performed on AuxRh100−x thin films deposited on oriented Si substrates. This substrate shows only a sharp (and intense) diffraction peak at 2θ ≈ 54° (not shown) in a region where the deposited films have no diffraction peaks. Figure 1 shows the XRD patterns of a few selected AuxRh100−x/Si thin films, with x varying from 0 to 100. For the sake of clarity, the region where the sharp diffraction peak of Si is occurring was omitted. These diffractograms exhibit only one series of diffraction peaks that can be indexed to a face-centered cubic (fcc) phase. The fact that only one set of diffraction peaks is observed is a clear indication that Au and Rh have formed a solid solution. For x = 100, the diffraction peaks are located at 2θ = 38.2° (111), 44.4° (200), 64.5° (220), 77.4° (311), and 81.9° (222), which is in good agreement with the peak position expected for pure Au (JCPDS, card no. 4-0784). For x = 0, the diffraction peaks are at 2θ = 41.0° (111), 47.2° (200), 69.6° (220), and 784.2° (311), in good agreement with what is expected for pure Rh (JCPDS, card no. 5-0685). For 0 < x < 100, the diffraction peaks are located between those of pure Au and pure Rh and move toward the 5264
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Figure 2. Variation of the fcc lattice parameter a of AuxRh100−x/Si (empty circles) and AuxRh100−x/C (filled triangles) thin films with respect to the bulk Au content. The a lattice parameters of metallic Au and Rh (JCPDS card Au (JCPDS card nos. 4-0784 and 5-0685, respectively) are shown as open stars.
Figure 3. X-ray diffraction patterns of Au50Rh50/C before (A) and after (B) annealing at 500 °C for 24 h.
peak increases from ∼0.4 for x = 0 (pure Rh) to 1.7 for x = 30 before decreasing again to ∼0.4 as x increases to 100 (pure Au). In comparison, the (220)/(111) integrated intensity ratios of polycrystalline Au (JCPDS, card no. 4-0784) and Rh (JCPDS, card no. 5-0685) are 0.32 and 0.26, respectively. This clearly points to the fact that thin films are growing along a preferred crystallographic orientation and that this phenomenon occurs in a restricted range of composition centered at x = 40, as highlighted in Figure 4A.
solubility of Rh in Au and Au in Rh is about 1.6 and 0.5 at %, respectively. On the basis of these data, one would have expected the XRD patterns of AuxRh100−x thin films to exhibit two distinct sets of diffraction peaks. This is not the case, and the data of Figures 1 and 2 clearly indicate that a metastable single fcc phase is formed at all compositions when AuxRh100−x thin films are prepared by CBPLD. The XRD patterns of AuxRh100−x/C and AuxRh100−x/Si were recorded over a period of 1 year without any noticeable change, indicating that the solid solution is kinetically stable. Similar results were obtained for AuxRh100−x thin films deposited on graphite substrates. (See Figure 2.) In an attempt to emphasize the fact that AuxRh100−x thin films prepared by CBPLD are in a metastable state, a AuxRh100−x/C sample with x = 50 was heated to 500 °C under an inert atmosphere during 24 h. The chemical composition of the film, as determined from EDX measurements, remains the same and was not affected by the thermal treatment. The XRD patterns of as-prepared Au50Rh50/C and after heat treatment are shown in Figure 3. After heat treatment, the single set of diffraction peaks characteristic of an fcc phase is replaced by two distinct sets of diffraction peaks that are assigned to two different fcc phases and, as far as we can tell, no peak of the initial solid solution remains. The lattice parameters of these fcc phases are a = 3.806 and 4.076 Å, significantly different from that of the as-prepared Au50Rh50 thin film, which is 3.974 Å. According to the data of Figure 2, this corresponds to almost pure Rh and Au, respectively, indicating that phase separation between Au and Rh has occurred. On the basis of the Au−Rh phase diagram, this is not unexpected, as the mutual solubility of Au and Rh is very low (98 at % at 500 °C, and these films are metastable. However, at room temperature, no structural evolution was noticed over a period of 1 year. As evidenced by XPS, Au and Rh atoms are alloyed in the utmost outer layer of the films. A significant improvement of the electrocatalytic activity for the MOR was observed on Au−Rh alloy thin films, as illustrated by a negative shift of 400 mV of the MOR onset potential on Au25Rh75 and Au50Rh50, as compared with pure gold. This is attributed to a bifunctional mechanism, whereby hydroxyl species present at Rh surface sites participate in the oxidation of methanol taking place on nearby gold atoms.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 5268
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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chair program. R.I. would like to thank the “Fonds de recherche du Québec − Nature et Technologies” (FQRNT) for an Energy Research scholarship.
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