Formation of a Pt-Decorated Au Nanoparticle Monolayer Floating on

Apr 13, 2016 - Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603, Japan. ‡ Graduate School of Engineering, Osaka...
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Formation of a Pt-Decorated Au Nanoparticle Monolayer Floating on an Ionic Liquid by the Ionic Liquid/Metal Sputtering Method and Tunable Electrocatalytic Activities of the Resulting Monolayer Daisuke Sugioka,† Tatsuya Kameyama,† Susumu Kuwabata,‡ Takahisa Yamamoto,† and Tsukasa Torimoto*,† †

Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603, Japan Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

‡

S Supporting Information *

ABSTRACT: A novel strategy to prepare a bimetallic Au−Pt particle film was developed through sequential sputter deposition of Au and Pt on a room temperature ionic liquid (RTIL). Au sputter deposition onto an RTIL containing hydroxyl-functionalized cations produced a monolayer of Au particles 4.2 nm in size on the liquid surface. Subsequent Pt sputtering onto the original Au particle monolayer floating on the RTIL enabled decoration of individual Au particles with Pt metals, resulting in the formation of a bimetallic Au−Pt particle monolayer with a Pt-enriched particle surface. The particle size slightly increased to 4.8 nm with Pt deposition for 120 min. The shell layer of a bimetallic particle was composed of Au−Pt alloy, the composition of which was tunable by controlling the Pt sputter deposition time. The electrochemical surface area (ECSA) was determined by cyclic voltammetry of bimetallic Au−Pt particle monolayers transferred onto HOPG electrodes by a horizontal liftoff method. The Pt surface coverage, determined by ECSAs of Au and Pt, increased from 0 to 56 mol % with elapse of the Pt sputter deposition time up to 120 min. Thus-obtained Au−Pt particle films exhibited electrocatalytic activity for methanol oxidation reaction (MOR) superior to the activities of pure Au or Pt particles. Volcano-type dependence was observed between the MOR activity and Pt surface coverage on the particles. Maximum activity was obtained for Au−Pt particles with a Pt coverage of 49 mol %, being ca. 120 times higher than that of pure Pt particles. This method enables direct decoration of metal particles with different noble metal atoms, providing a novel strategy to develop highly efficient multinary particle catalysts. KEYWORDS: ionic liquid, sputter deposition, monoparticle layer, electrocatalyst, methanol oxidation, core−shell nanoparticle

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chemical deposition of a thin Pt layer on core particles.15 The composition of the surface layer of chemically synthesized AuPt alloy particles could be modified by varying the heating conditions used for post heat treatment, their electrocatalytic activities being dependent on the surface fraction of Pt.16 The formation of a Pt skin layer on a Pt3Ni nanoframe significantly improved the electrocatalytic activity for oxygen reduction reaction compared to the electrocatalytic activities of the corresponding solid particles of pure Pt or Pt3Ni alloy.17 After being subjected to oxygen reduction reaction, Pt-based alloy particles of Pt3Y18 and Pt3Co19 also produced a Pt-rich surface layer that contributed to higher catalytic activities. Besides chemical syntheses, there have been reports recently of successful preparation of metal nanoparticles and related composites via physical vapor deposition (PVD) onto liquids with very low vapor pressures, such as silicone oils,20−22

INTRODUCTION Methods for controlling the surface structure of metal nanoparticles have been extensively investigated for applications to catalysts,1,2 fuel cells,3−6 and plasmonic devices,7−9 because the morphology and composition of the particle surface can considerably influence many of their physical and chemical properties. Many efforts have been directed toward surface modification of particles with noble metals to enhance the activities of catalyst particles.10 Pt-based bimetallic nanoparticles have been considered to be promising materials as electrocatalysts for improving the performance of fuel cells and minimizing the content of Pt in catalysts.11,12 Various bimetallic nanoparticles with different surface structures have been prepared via chemical syntheses in solutions.13 For example, homogeneously alloyed spherical nanoparticles of PtCo and PtRu were prepared by chemical reduction of the corresponding metal precursors in reverse micelles, the size of which could be controlled by the molar ratio of the metal precursors to the surfactant in the preparation.14 Core−shell-structured particles composed of a Au core and a Pt shell were prepared by © XXXX American Chemical Society

Received: February 16, 2016 Accepted: April 13, 2016

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DOI: 10.1021/acsami.6b01978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces vegetable oils,23 poly(ethylene glycol),24,25 liquid crystals,26 and room temperature ionic liquids (RTILs),27−45 in which small clusters or atoms ejected from bulk metals were trapped on the liquid surface and then formed islandlike aggregates of metal particles on the liquid surface or nanoparticles uniformly dispersed in the bulk liquid phase. RTILs are particularly useful for the synthesis of nanoparticles with well-controlled sizes and shapes, because of their extremely low vapor pressure and their capabilities to dissolve many kinds of substances and to uniformly disperse a variety of solid nanoparticles. It has been reported by our groups30−39 and other researchers27,40−43 that sputter deposition of metals onto RTILs could produce the corresponding metal nanoparticles, such as Au,29 Ag,43 Cu,42 Pd,37 and Pt,38,39 uniformly dispersed in the solution without the use of any additional stabilizing agent. This RTIL/metal sputtering technique was also useful for preparing composite nanoparticles in the solution phase: alloy nanoparticles of AuAg,31 AuCu,35,44 AuPt,33 and AuPd34 were prepared by simultaneous sputter deposition of different corresponding metals,31,33−35 while successive sputter deposition of noble metals and In produced core−shell-structured particles of Au@ In2O3 and [email protected] Recently we have reported45 that a monolayer of Au nanoparticles was produced on the RTIL surface via a single-step deposition process when the RTIL contained hydroxyl-functionalized cations, with the size of the Au particles floating on the RTIL surface being tunable depending on the Au sputtering time, and that the thusobtained particle film could be easily transferred from the solution surface onto various solid substrates via a horizontal liftoff method without coalescence between particles. This technique would also be applicable to the preparation of a monolayer composed of bimetallic nanoparticles on an RTIL surface, but such an attempt has not been undertaken. Here, we report precise surface modification of Au particles sputter-deposited on an ionic liquid surface via sequential sputter deposition of Pt, producing a monolayer of bimetallic Au−Pt nanoparticles. Pt species deposited on individual Au particle surfaces made a surface layer composed of AuPt alloy, the surface coverage of Pt being controlled by the Pt sputtering time. The electrocatalytic activity of Au−Pt particles for methanol oxidation was investigated as a model reaction by using highly oriented pyrolytic graphite (HOPG) electrodes with an accumulation of bimetallic Au−Pt particle monolayers.

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such as a quartz glass plate or HOPG, by a horizontal liftoff procedure in which the surface of the solid substrate was brought down parallel to the RTIL surface and lightly touched the film formed on the solution surface, being similar to the accumulation of pure Au particle films on HyEMI-BF4 as reported in our previous paper.45 After washing with a copious amount of acetonitrile for removing the excess amount of RTIL from their surface, the particle films were used for various measurements. The shapes and sizes of thus-obtained particles were examined using a Hitachi H-7650 transmission electron microscope with an acceleration voltage at 100 kV. Samples for transmission electron microscopy (TEM) measurement were prepared by transferring the Au−Pt nanoparticle film on the RTIL surface to a copper TEM grid covered with an amorphous carbon overlayer (Oken Shoji, STEM100Cu) by the horizontal liftoff procedure, followed by rinsing with acetonitrile for removing the excess amount of RTIL and drying under vacuum. High-resolution images of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were obtained by Cs-corrected HR-STEM (ARM-200F, JEOL Co. Ltd.) with an acceleration voltage at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) analysis was simultaneously carried out during the TEM measurements. Extinction spectra of metal particle films were acquired with an Agilent Technology 8453A spectrophotometer after transferring the films onto quartz plates via horizontal liftoff. The crystal structures of metal nanoparticles were determined by measuring X-ray diffraction (XRD) patterns with a Rigaku SmartLab-3K using Cu Kα radiation. Samples for XRD measurement were prepared by accumulating the prepared metal particle films on a low-background Si sample holder. X-ray photoelectron spectroscopy (XPS) measurements of Au and Au−Pt particle films were carried out using a JEOL JPS-9000MC with Al Kα irradiation. The chemical compositions of the Au−Pt nanoparticle films were analyzed by X-ray fluorescence spectroscopy (Rigaku, EDXL-300). Electrochemical properties of a Au−Pt nanoparticle monolayer transferred onto an HOPG electrode were investigated using an electrochemical analyzer (BAS Instruments Inc., ALS model 701C). A three-electrode chemical cell with the particle-film-immobilized HOPG substrate acting as the working electrode, a Pt wire as the counter electrode, and a reversible hydrogen electrode (RHE) as the reference electrode was used for the measurements. The apparent area of the working electrode was defined by an O-ring to be 0.18 cm2. The electrochemical surface area (ECSA) of the Au−Pt nanoparticle film was determined by recording a cyclic voltammogram between 0.01 and 1.7 V vs RHE at a potential scan rate of 50 mV s−1 in a N2-saturated 0.5 mol dm−3 H2SO4 aqueous solution. The electrolyte for methanol oxidation reaction (MOR) was a N2-saturated 0.5 mol dm−3 KOH aqueous solution containing 0.5 mol dm−3 methanol. The electrocatalytic activity for MOR was assessed by cyclic voltammetry with a potential range between 0.01 and 1.3 V vs RHE and a scan rate of 50 mV s−1, in which the current density was obtained by dividing the observed current by the total ECSA value of Au and Pt. For comparison, pure Pt nanoparticles dispersed in HyEMI-BF4 were prepared by sputter deposition of Pt for 5 min onto HyEMI-BF4 in the absence of the Au nanoparticle film with a discharge current of 10 mA under an argon pressure of 20 Pa. The thus-obtained HyEMIBF4 dispersion contained Pt particles 1.0 nm in diameter with 0.88 mmol dm−3 as Pt atoms. The MOR activity of the Pt particles was determined by immobilizing Pt particles dispersed in HyEMI-BF4 onto an HOPG electrode with heat treatment as reported in our previous paper,33,36 an 80 mm3 portion of Pt particle-dispersed HyEMI-BF4 being spread on HOPG, followed by heat treatment for 30 min at 473 K in air. This was because the present conditions used for Pt sputtering could not form a nanoparticle film on the surface of pure HyEMI-BF4 but produced Pt particles uniformly dispersed in the solution phase.

EXPERIMENTAL SECTION

A monolayer film of Au nanoparticles was sputter-deposited on an ionic liquid of 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate (HyEMI-BF4) by a previously reported method.45 A portion of HyEMI-BF4, purchased from Ionic Liquids Technologies Inc., was dried for 3 h at 373 K under vacuum before use. An 80 mm3 portion of HyEMI-BF4 was spread on a glass plate (4.0 cm2) that was horizontally set in a sputter coater (JEOL, JFC-1300). The surface of the RTIL was located at a distance of 35 mm from the gold target (99.99% in purity, diameter 5 cm). Sputter deposition of Au on HyEMI-BF4 was carried out for 2.5 min with a discharge current of 10 mA under an argon pressure of 20 Pa at room temperature, resulting in the formation of a monolayer composed of Au nanoparticles 4.2 nm in diameter. Subsequent sputter deposition of Pt was carried out on the thusobtained original Au particle monolayer by replacing the Au target with a Pt plate (99.99% in purity, diameter 5 cm). The sputtering conditions used for Pt deposition were the same as those used for Au deposition except for a sputtering time of 5, 20, 40, 80, or 120 min. Thus-obtained bimetallic Au−Pt particle films floating on the surfaces of RTILs could be transferred onto the surfaces of solid substrates,

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RESULTS AND DISCUSSION Bimetallic Au−Pt Particle Monolayer Prepared by Sequential Sputter Deposition of Au and Pt on the B

DOI: 10.1021/acsami.6b01978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces HyEMI-BF4 Surface. Various kinds of monometallic particle monolayer films were prepared in our previous study.45 They were formed when metal sputter deposition was carried out onto RTILs containing hydroxyl-functionalized cations, though the sputter deposition conditions were dependent on the kind of metal used. The sputter deposition of Au, Ag, or Pd produced the corresponding uniform nanoparticle monolayers on the solution surface. In contrast, we have reported that Pt particles were uniformly dispersed in the HyEMI-BF4 solution phase but could not form a film on the solution surface by Pt sputter deposition with a discharge current of 10 mA probably due to the significantly slow rate of trapping of Pt species on the RTIL surface, though a monolayer film composed of Pt particles was predominantly produced on the surface when the discharge current was increased to 40 mA. Thus, for the purpose of selectively modifying Au particles with Pt atoms in the present study, we selected Pt sputtering conditions in which Pt particle films would not be formed on the surface of pure HyEMI-BF4, that is, a discharge current of 10 mA under an argon pressure of 20 Pa. Pt sputter deposition varied the optical properties of the original Au particle films. Figure 1 shows photographs of

of Pt sputter deposition, and then cracks easily occurred in the film during the horizontal liftoff procedure as shown in Figure 1, though the reason for this is not clear at the present stage. The surface composition of Au particles was modified with elapse of Pt sputter deposition as described later. This is expected to vary the adsorption strength of RTILs on the particle surface, resulting in a change in the stability of particle films on the RTIL surface. Furthermore, the RTILs, exposed by the removal of surface particle films, were browner after prolonged Pt deposition. This is in contrast to the solution having no color under an original Au particle film, with Au sputter deposition on HyEMI-BF4 producing only a Au particle monolayer on the liquid surface without formation of any Au nanoparticles in the solution phase film, as reported in our previous paper.45 Figure 2 shows extinction spectra of films transferred onto quartz glass plates. The monolayer film of original Au particles

Figure 2. Extinction spectra of bimetallic Au−Pt particle films transferred onto quartz glass plates. The numbers in the figure represent the time in minutes used for Pt sputter deposition.

exhibited a localized surface plasmon resonance (LSPR) peak at 540 nm. With an increase in the Pt sputtering time, the LSPR peak intensity decreased and then finally vanished with sputter deposition of Pt for more than 80 min. It has been reported that the intensity and position of the LSPR peak for Au-based binary particles were sensitive to the composition of the particles, especially the surface composition.31,35 For example, alloying Au with Ag or Cu caused a shift in the LSPR peak of the resulting particles depending on the chemical composition. Furthermore, surface coating of Au particles with a Pt or Pd thin layer decreased the LSPR peak intensity of the Au particle cores.46,47 Therefore, the results shown in Figure 2 suggested that sputter deposition of Pt onto the Au particle monolayer floating on HyEMI-BF4 caused a change in the surface chemical composition of individual Au particles, such as homogeneous alloying between Au and Pt or surface coating of Au particles with a Pt-rich layer, without the formation of particle films composed of a mixture of pure Au and pure Pt particles. TEM images of thus-obtained Au−Pt particle films are shown in Figure 3. It can be seen that the film that was formed on the HyEMI-BF4 surface by sequential sputter deposition of Au and Pt was a monolayer of spherical particles. The original Au particle film was composed of spherical Au particles with an average size of 4.2 nm, which was larger than the crystallite size of 3.4 nm estimated from the diffraction peak in the corresponding XRD pattern (shown below) with Scherrer’s equation, indicating that the individual particles were composed

Figure 1. Top-view photographs of an original Au particle film (a) and Au−Pt bimetallic particle films prepared with Pt sputtering times of 5 (b), 20 (c), 40 (d), and 80 (e) min on the HyEMI-BF4 surface. A photograph of transfer of half of each particle film on the RTIL surface onto a glass plate is also shown beside the corresponding top-view image. (f) Schematic illustration of horizontal liftoff of a Au−Pt particle monolayer film onto a solid substrate from the RTIL surface.

bimetallic Au−Pt particle films formed on the HyEMI-BF4 surface. Regardless of the Pt sputtering time, each film floating on HyEMI-BF4 could be transferred onto a quartz glass plate. Although an original Au particle film had a red-blue color, the film on the solution surface turned brown after Pt sputter deposition, the degree of change in color being enhanced by an increase in the Pt sputtering time. It should be noted that the particle films became more fragile with an increase in the time C

DOI: 10.1021/acsami.6b01978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Average diameter of Au−Pt bimetallic particles (a), their particle density in monolayer films (b), and Pt content in the films (c) as a function of the Pt sputtering time. The error bars in panel a represent the size distribution of the particles.

Figure 3. TEM images of an original Au particle monolayer film (a) and Au−Pt bimetallic particle films with Pt sputtering times of 5 (b), 20 (c), 40 (d), 80 (e), and 120 (f) min. The films were transferred from the HyEMI-BF4 surface to TEM grids. The scale bars in the figure represent a length of 20 nm.

The nanostructure of thus-obtained Au−Pt particles was investigated by HAADF-STEM and EDS measurements. Figure 5a shows a high-resolution HAADF-STEM image of bimetallic Au−Pt particles prepared with Pt sputter deposition for 80 min. Clear lattice fringes were observed in individual particles, and grain boundaries could be recognized inside most of the particles, indicating that each particle was polycrystalline. The spatial distribution of Pt atoms in a Au−Pt particle was evaluated by STEM−EDS analysis. The change in chemical composition along line AB in Figure 5a is shown in Figure 5b. The elements of both Au and Pt were detected in the whole particles, but their contents varied depending on the position in the particle. The particle core included a significantly large content of Au, >80%, while a composition with a Au/Pt ratio of ca. 0.6/0.4 was observed at the particle surface or the region just below the particle surface, indicating that thus-obtained bimetallic particles were not AuPt alloy having a homogeneous composition but were composed of a Pt-enriched surface layer and Au-dominant core. Pt sputter deposition onto Au-particle-deposited HyEMIBF4 also resulted in the production of Pt particles homogeneously dispersed in the solution phase as well as an increase in the size of the Au particles floating on the HyEMIBF4 surface. As mentioned above, HyEMI-BF4 solutions below bimetallic particle films exhibited a brown color (Figure 1). Their extinction spectra were very broad as shown in Figure S2, the profile being very similar to that of pure Pt particles uniformly dispersed in HyEMI-BF4. TEM measurements of the HyEMI-BF4 solution phase (Figure S1b) revealed that Pt particles with a size of 1.2 nm were homogeneously dispersed

of polycrystalline Au. Although extremely small Pt particles ca. 1 nm in size were produced in the solution phase by Pt sputter deposition onto pure HyEMI-BF4 as shown in Figure S1a, sequential sputter deposition of Au and Pt induced a slight increase in the average size of particles in the monolayer without formation of additional particles as small as Pt particles in pure HyEMI-BF4 solution. The average diameter and size distribution of the particles were determined by measuring the sizes of more than 100 particles in TEM images and are plotted as a function of the Pt sputtering time in Figure 4a. The average diameter was slightly increased from 4.2 to 4.8 nm with elapse of the Pt sputtering time from 0 to 120 min, with the size distribution being almost constant at 16% of the average diameter. On the other hand, the two-dimensional particle density in a monolayer was constant at ca. 3.8 × 1012 particles cm−2 regardless of the Pt sputtering time. These results indicated that the Pt species trapped on the Au-film-deposited HyEMI-BF4 did not produce any nuclei to develop additional particles in the film but were simply deposited on individual Au particles to increase their size. Figure 4c shows the change in the chemical composition of bimetallic Au−Pt films. The Pt content in the films monotonously increased with the Pt sputtering time and finally reached ca. 44 mol % at 120 min of Pt sputter deposition, which roughly corresponded to the content of Pt, ca. 39 mol %, estimated from the enlargement in the particle size from 4.2 to 4.8 nm if Pt atoms were assumed to be alloyed with Au particles or to be deposited on their surface without formation of separate Pt particles in the films. D

DOI: 10.1021/acsami.6b01978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Schematic illustration of Pt deposition on individual Au particles floating on the HyEMI-BF4 surface.

Figure 5. (a) High-resolution HAADF-STEM image for Au−Pt bimetallic particles prepared by Pt sputter deposition for 80 min. (b) EDS analysis along line AB in panel a. Fractions of Au (open circles) and Pt (solid circles) are plotted.

in the solution phase after Pt sputter deposition for 120 min onto HyEMI-BF4 having a Au particle monolayer, though these small Pt particles were not observed in TEM images of Au−Pt particle monolayers (Figure 3) formed on the solution surface. The size of the Pt particles was comparable with that of Pt particles formed in pure HyEMI-BF4 (Figure S1a) in the absence of Au particle films. These results suggested that sputtered Pt species were trapped in bulk HyEMI-BF4 solution to form pure Pt particles unless they were deposited on Au particles floating on the solution surface. Consequently, we propose the mechanism by which a bimetallic Au−Pt particle monolayer is formed on the HyEMIBF4 surface as illustrated in Figure 6. First, sputtered Au species are trapped on the HyEMI-BF4 surface via strong adsorption with hydroxyl groups protruding toward a vacuum from the liquid surface as reported in our previous paper,45 resulting in selective nucleation on the liquid surface followed by particle growth to form a Au particle monolayer on the liquid surface. Subsequent sputtering of Pt results in the deposition of a thin Pt layer on Au particles acting as nuclei, resulting in the formation of a Pt-rich shell layer on the surface of individual Au core particles and a size increment of particles floating on the RTIL surface. However, when the sputtered Pt species are directly injected into the RTIL solution through a space between Au particles that can be seen in Figure 3a, small pure Pt particles 1−1.2 nm in diameter are formed in the solution phase, because Pt sputter deposition onto a pure HyEMI-BF4 surface could predominantly form Pt particles dispersed in the solution phase under the present conditions. The nanostructure of bimetallic Au−Pt particles was supported by XRD and XPS measurements. The crystal structures of the particles were analyzed by XRD measurements. The original Au particle film exhibited diffraction peaks at ca. 38° and 44° as shown in Figure 7, which were assigned to a face-centered cubic (fcc) crystal structure of Au. These peaks

Figure 7. XRD patterns of an original Au particle film (a) and Au−Pt bimetallic particle films with Pt sputtering times of 5 (b), 20 (c), 40 (d), 80 (e), and 120 (f) min. The standard diffraction patterns of Au and Pt (PDF card nos. 00-004-0784 and 00-004-0802) are also shown.

were also observed for bimetallic Au−Pt particle films regardless of the Pt sputtering time, and the peak positions did not vary even when the Pt sputtering time was prolonged. On the other hand, peaks assignable to an fcc Pt crystal could not be seen, though a peak shoulder seemed to appear at ca. 40° in patterns e and f for Au−Pt particle films, assigned to diffraction from (111) lattice planes of Pt or AuPt alloy. It was reported that the diffraction peaks of homogeneous AuPt alloy nanoparticles appeared between the corresponding peaks of Au and Pt, the peak position being dependent on the alloy composition.33,48 Therefore, these results suggested that the deposited Pt species in films were not homogeneously alloyed with Au particles but formed small Pt-rich domains on the Au particles, which showed very broad diffraction peaks, being similar to the results reported for Au particles (ca. 5 nm in size) chemically modified with Pt particles.48 E

DOI: 10.1021/acsami.6b01978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces As shown in Figure 8, XPS spectra of the original Au particle films exhibited signals at 87.6 and 83.9 eV, which were

Figure 8. XPS spectra for Au 4f and Pt 4f of an original Au particle film (a) and Au−Pt bimetallic particle films with Pt sputtering times of 5 (b), 20 (c), 40 (d), and 80 (e) min. Dotted lines represent the positions of the Au 4f and Pt 4f signals of the corresponding bulk pure metals.

assignable to Au 4f5/2 and Au 4f7/2, being in good agreement with those of bulk Au, 87.7 and 84.0 eV, respectively.49 On the other hand, the signals of Au 4f5/2 and Au 4f7/2 slightly shifted to lower energies of 87.2 and 83.4 eV, respectively, in bimetallic Au−Pt particle films. Furthermore, signals originating from Pt 4f5/2 and Pt 4f7/2 were observed at 74.8 and 71.6 eV in bimetallic films, being slightly higher than those of bulk Pt, 74.2 and 70.9 eV, respectively.50 These behaviors agreed with the shifts of XPS signals observed in AuPt alloy particles reported in the literature,51,52 indicating that sputter-deposited Pt species formed a AuPt alloy layer on the Au particle surface, in which the electron transfer occurred from Pt to Au atoms. Electrochemical Activities of Bimetallic Au−Pt Particle Films. The surface properties of bimetallic Au−Pt particles were characterized by cyclic voltammetry. Electrochemical measurements were carried out with a particle monolayer immobilized on an HOPG electrode by the horizontal liftoff method, an AFM image of which (Figure 9a) revealed that the particles were densely immobilized on the HOPG surface without large aggregation. Figure 9b shows cyclic voltammograms of Au−Pt particle monolayers with various Pt contents in a N2-saturated 0.5 mol dm−3 H2SO4 aqueous solution. A pure Au particle monolayer exhibited a pair of redox peaks, that is, an anodic peak at 1.6 V vs RHE and a cathodic peak at 1.16 V vs RHE, which were assigned to oxidation of Au atoms on the surface and reduction of the resulting AuOx monolayer on the Au particle, respectively. The reduction peak at 1.16 V vs RHE was also observed for bimetallic Au−Pt particle electrodes, the intensity being smaller with elapse of the Pt sputtering time. These results indicated that the surface of the Au−Pt particles had Au atoms exposed to the solution even after Pt deposition but that the surface coverage of Au atoms decreased with an increase in the Pt sputtering time. Furthermore, in the case of Au−Pt particle films, a pair of redox peaks, assignable to the adsorption and desorption of hydrogen on Pt atoms exposed

Figure 9. (a) AFM image of a Au−Pt bimetallic particle film prepared with a Pt sputtering time of 80 min. (b) Cyclic voltammograms of an original Au particle monolayer and Au−Pt particle monolayers immobilized on HOPG electrodes in a 0.50 mol dm−3 H2SO4 solution at a potential scan rate of 50 mV s−1. The numbers represent the Pt sputter deposition time in minutes.

on the particle surface, appeared at ca. 0.02−0.04 V vs RHE, accompanied by a reduction peak of the platinum oxide layer on the particle surface at 0.6 V vs RHE. The intensities of peaks characteristic of Pt atoms on the surface were enlarged with an increase in the Pt sputtering time from 5 to 120 min. The electrochemical surface areas (ECSAs) could be determined by measuring the charges of the reduction peak of the AuOx surface layer at 1.16 V vs RHE for Au atoms on the surface and of the hydrogen desorption peak at 0.04 V vs RHE for Pt atoms on the surface, with reported values of 400 and 210 μC/cm2 being used as the charges required for reduction of the AuOx monolayer on the Au surface and for hydrogen desorption on the Pt surface, respectively.53,54 Figure 10a shows the ratios of Au and Pt ECSAs of immobilized particles to the geometrical surface area of the HOPG electrode (0.18 cm2), SNP(Au)/SHOPG and SNP(Pt)/SHOPG, respectively, as a function of the Pt sputter deposition time. With elapse of the Pt sputtering time, SNP(Au)/SHOPG and SNP(Pt)/SHOPG decreased and increased, respectively, though the sum of them seemed to be roughly constant at ca. 2. We also estimated the total SNP/ SHOPG by using the average diameters of the particles and twodimensional particle densities in the monolayers determined by TEM measurements (Figure 4a,b). The obtained values, plotted in Figure 10a, were constant at ca. 2.5, being slightly larger than the total SNP/SHOPG ratios determined from ECSAs, except for the original Au particles and those obtained with Pt F

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hydrogen atoms, if any, was too small to significantly influence the determination of the Pt ECSA of immobilized Au−Pt particles. This was also supported by the fact that the values of the total SNP/SHOPG determined by TEM measurements, ca. 2.5, were not considerably different from those obtained from ECSAs, ca. 2, as aforementioned. The relationship between the surface Pt coverage and Pt content in particles is of importance to evaluate the enrichment of Pt on the particle surface. The surface Pt coverage can be calculated from ECSA values of Au and Pt. As shown in Figure 10b, the Pt coverage on the particle surface was increased with an increase in Pt content in bimetallic particles. However, the obtained values were higher than those expected from homogeneous AuPt alloy particles (straight line in Figure 10b), except for Pt sputtering for 5 min. These results indicated that bimetallic Au−Pt particles had a Pt-enriched surface, being in good agreement with the results of STEM−EDS analysis shown in Figure 5b. Consequently, it was concluded that the degree of surface modification of Au particles floating on HyEMI-BF4 with Pt atoms could be controlled by just changing the Pt sputter deposition time. It is well-known that the difference in the surface conditions of metal particles remarkably influences their catalytic activity. In fact, bimetallic Au−Pt nanoparticles have been reported to exhibit greater electrocatalytic activity than the monometallic counterparts for methanol oxidation, the activity being dependent on the chemical composition and the surface structure of the particles.15,56 Thus, we evaluated the electrocatalytic activities of the present Au−Pt particles for methanol oxidation as a function of the Pt surface coverage. Figure 11 shows cyclic voltammograms for MOR recorded at HOPG electrodes modified with Au−Pt particles, in which the current density was obtained by dividing the detected current by the total ECSA of bimetallic Au−Pt particles. Although negligible activity was evident for pure Au particles, an anodic peak attributed to methanol oxidation was observed in the positive potential scan for each Au−Pt particle electrode, with the onset potential of oxidation current appearing at around 0.4−0.5 V vs RHE. The peak current intensity was considerably enlarged with a slight positive shift of the peak potential to 0.82−0.86 V vs RHE with an increase in the Pt sputter deposition time up to 80 min. For comparison, the HOPG electrode immobilized with pure Pt particles was also investigated, with Pt particles homogeneously dispersed in HyEMI-BF4 (average diameter 1.0 nm) being immobilized on the HOPG surface by thermal treatment. The size of the immobilized Pt particles was determined to be ca. 1.5 nm from the height of the AFM image shown in Figure S3. As shown in Figure 11a, the thus-obtained pure Pt particle electrode exhibited MOR activity, but the current peak observed at 0.75 V vs RHE in the positive potential scan was much smaller than those of Au−Pt particle electrodes. These results indicated that deposition of a small amount of Pt on Au particles significantly changed the catalytic properties of the resulting particles and that sputter-deposited Pt on individual particles worked as an efficient electrocatalytic reaction site for MOR, as previously reported.57 To quantitatively compare MOR activities of Pt species on the particle surface, we obtained the relationship between the peak current density for MOR in the positive potential scan and the Pt surface coverage of catalyst particles. The current density was normalized by the ECSA of Pt because of negligible activity of the bare Au surface for MOR. As shown in Figure 12, a

Figure 10. (a) Ratios of Au and Pt ECSAs of immobilized particles to the surface area of the HOPG electrode as a function of the Pt sputter deposition time. Ratios of the total surface areas of immobilized particles, obtained by ECSA analyses and by TEM measurements, to the surface area of the HOPG electrode are also plotted in the figure. (b) Relationship between the surface coverage of Pt on bimetallic Au− Pt particles determined by ECSA analyses and the Pt content in the particles. The straight line in the figure represents the case of homogeneous AuPt alloy particles. The numbers in panel b indicate the Pt sputter deposition time in minutes.

sputtering for 5 min. These results indicated that most of the Au−Pt particles that had accumulated were electrically connected to the HOPG electrode used as a substrate. It was reported55 that small Pt particles less than 1.5 nm in diameter exhibited the spillover of hydrogen atoms adsorbed on Pt to the support surfaces of F-doped SnO2 electrodes modified with and without graphene particles, in which an anodic peak resulting from the desorption of spilled-over hydrogen appeared at 0.4 V vs RHE. This may cause the overestimation of the Pt ECSA determined by using the hydrogen desorption peak. However, in the present case, any prominent peaks could not be observed at around 0.4 V vs RHE in cyclic voltammograms of Au−Pt particle monolayers on HOPG electrodes (Figure 9b), probably due to the larger particle size (ca. 4.2−4.8 nm) and/or the presence of Au atoms on the Au− Pt particle surface, suggesting that the amount of spilled-over G

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des.59 Zhong et al. investigated the mass activity of chemically synthesized AuPt alloy particles for MOR in a basic solution and found that the activity was optimal at a Au content of around 65−85%.56 Shao-Horn et al. reported that AuPt alloy particles exhibited the highest catalytic activity for MOR at the surface Pt concentration of ca. 70% in an acidic solution.16 In those previous studies, Pt species on the catalyst surface were considered to act as MOR sites, the activity being modulated depending on the surface composition of AuPt. The Pt species on the bimetallic AuPt surface exhibited larger activity for MOR than those on the pure Pt surface because the electronic structure of surface Pt atoms was modulated by nearby and subsurface Au atoms in AuPt alloy16,59 and because poisonous species for catalytic reactions, such as CO, formed on Pt sites could be effectively removed by reactions with oxygenated species adsorbed on surface Au atoms (bifunctional catalytic mechanism).56 These effects were considered to be enhanced with a decrease in Pt coverage. On the other hand, it was proposed that methanol molecules were dissociatively adsorbed on three adjacent Pt sites on the surface.52 With an increase in Pt coverage on the AuPt alloy surface, the amount of methanol adsorption sites on the surface became large, resulting in enhancement of the MOR activity. Consequently, the MOR activity of bimetallic AuPt catalysts can be characterized by the balance between these factors, that is, the enhancement of the catalytic activity of surface Pt atoms via interactions with adjacent Au atoms and the amount of surface Pt sites available for methanol adsorption.

Figure 11. Cyclic voltammograms for methanol oxidation in a N2saturated 0.50 mol dm−3 KOH aqueous solution containing 0.50 mol dm−3 methanol. The electrodes used were (a) HOPG substrates immobilized with an original Au particle monolayer or pure Pt particles and (b) those with a monolayer of Au−Pt particles. The numbers in panel b indicate the Pt sputter deposition time in minutes.

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CONCLUSIONS Au nanoparticles floating on an ionic liquid were successfully surface-modified with Pt atoms through RTIL/metal sputtering without aggregation of component particles. The resulting bimetallic Au−Pt particles also formed a particle monolayer containing no isolated Pt particles on the surface of the ionic liquid, since the conditions used for Pt sputter deposition produced no Pt particles floating on the surface of the pure ionic liquid. The bimetallic Au−Pt particles had a Pt-enriched shell layer. The advantages of this technique over conventional chemical synthesis are easy control of the surface coverage of Pt atoms on individual Au−Pt particles by varying the Pt sputter deposition time and immobilization of thus-obtained particle monolayer films onto various solid substrates by a simple horizontal liftoff method without aggregation or coalescence of particles. These advantages enable rapid optimization of the surface composition of bimetallic particles for application to catalysts. In fact, we showed the usefulness of the obtained bimetallic particles for an electrocatalyst: A Au−Pt particle monolayer transferred onto an HOPG substrate exhibited electrocatalytic activity for methanol oxidation superior to that of the monometallic counterparts, with the activity being controlled by changing the Pt surface coverage on the catalyst particles. Optimal MOR activity was obtained with bimetallic Au−Pt particles of ca. 50 mol % Pt coverage, being ca. 120 times higher than that of pure Pt particles. Furthermore, efficient activities were observed for the bimetallic nanoparticles even without any intricate pretreatments of catalyst particles such as heat treatment at relatively high temperatures, which may cause changes in the particle size or surface composition. This is because bimetallic particles prepared by this RTIL/ metal sputtering technique require no stabilizing agents strongly adsorbed on the particle surface such as thiols or polymers. Although only preparation of Au−Pt bimetallic

Figure 12. Relationship between the peak current density of methanol oxidation and the Pt surface coverage of Au−Pt particles immobilized on an HOPG electrode. Peak currents observed in positive potential scans in Figure 11 were normalized to Pt ECSAs of the electrodes used.

volcano-type dependence was observed: the catalytic activity was enhanced with an increase in the Pt surface coverage, reaching a maximum at a Pt coverage of ca. 50 mol %, which was achieved with Au−Pt particles prepared by Pt sputter deposition for 80 min, whereas the activity began to decrease with an excess increase in surface Pt coverage. It should be noted that the optimal activity of Au−Pt particles was ca. 120 times higher than that of pure Pt particles. Similar behavior has been observed in previous studies: the MOR activity was reported by Lee et al. to be greatly dependent on the surface coverage of a Pt atomic layer deposited on Au single-crystal electrodes, with 60% Pt coverage giving optimal activity,58 while Moffat et al. reported that a Pt submonolayer coverage of 75% on Au thin film electrodes gave higher MOR activity than thick Pt-film-deposited Au electroH

DOI: 10.1021/acsami.6b01978 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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nanoparticles on the surface of HyEMI-BF4 is described in this paper, the technique enables direct decoration of various kinds of metal particles with different noble metal atoms, providing a novel strategy to develop highly efficient multinary particle catalysts. Studies along this line are currently in progress.

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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/acsami.6b01978. TEM images of Pt particles, size distributions of Pt particles, extinction spectra of Pt particles, AFM image of an HOPG electrode immobilized with Pt particles, and height profile of the AFM image (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Photosynergetics” (No. 15H01082) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by a Grant-in-Aid for Scientific Research (B) (No. 15H03876), a Grant-in-Aid for Challenging Exploratory Research (No. 15K13805), and a Grant-in-Aid for Young Scientists (B) (No. 26790006) from the Japan Society for the Promotion of Science. D.S. expresses his appreciation to the Program for Leading Graduate Schools “Integrative Graduate Education and Research in Green Natural Sciences” from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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REFERENCES

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