Phase Transfer of Noble Metal Nanoparticles from Ionic Liquids to

Noble metal nanoparticles (NMNPs) are very important in fields such as optics and heterogeneous catalysis.(1-4) Compared with traditional aqueous or o...
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Phase Transfer of Noble Metal Nanoparticles from Ionic Liquids to an Organic/Aqueous Medium Penglei Cui,† Hongyan He,† Dong Chen,†,‡ Hui Liu,†,‡ Suojiang Zhang,*,† and Jun Yang*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China S Supporting Information *

ABSTRACT: The synthesis of noble metal nanoparticles (NMNPs) using, or in the presence of, ionic liquids (ILs) represents a burgeoning direction in materials chemistry. Processing these nanomaterials synthesized in ILs would be drastically simplified if they could be routinely dispersed into a wide variety of polar/nonpolar solvents. We herein demonstrate the phase transfer of the as-prepared nanoparticles from ILs to an organic or aqueous medium. The protocol involves first mixing the noble metal sols in ILs and an ethanolic or a methanolic solution of transfer agent and then extracting the transfer-agent-stabilized NMNPs into toluene or aqueous phase. Electron micrographs reveal that the particles are fully dispersed after transfer, and the size/ morphology of the NMNPs could be significantly tuned by the ILs. In particular, electrochemical measurements of the Pt nanoparticles upon methanol oxidation reaction demonstrate that the particles are dominated by low-index crystal planes. aqueous phase to a nonpolar organic solvent,34−44 or ILs,45 approaches for the transfer of NMNPs from ILs to a common polar/nonpolar medium are still very lacking today; hence, the development of such approaches is undoubtedly important in order to maximize the advantages of the synthesis of NMNPs in ILs. In this work, we report the feasibility of the protocol we developed previously for the transfer of NMNPs from ILs to an organic or aqueous medium. Although not new, this protocol, which involves first mixing the noble metal sols in ILs and an ethanolic or a methanolic solution of transfer agent and then extracting the transfer-agent-stabilized NMNPs into toluene or aqueous phase, is the first extension to transfer the NMNPs from ILs to a nonpolar organic medium or an aqueous phase. Electron micrographs reveal that the particles are fully dispersed after transfer, and the size/morphology of the NMNPs could be significantly tuned by the ILs. In particular, electrochemical measurements of the Pt nanoparticles upon methanol oxidation reaction (MOR) demonstrate that the particles are dominated by low-index crystal planes.

1. INTRODUCTION Noble metal nanoparticles (NMNPs) are very important in fields such as optics and heterogeneous catalysis.1−4 Compared with traditional aqueous or organic solvents used for preparing NMNPs with controlled sizes/morphologies, ionic liquids (ILs) have garnered sustained research interest as benign solvent systems for the synthesis of nanomaterials, as they offer many distinct advantages, such as good thermal stability, high ionic conductivity, broad electrochemical potential windows, high synthetic flexibility, and environmental benefits deriving from the negligible vapor pressures.5,6 The most interesting aspect about the fabrication of NMNPs in ILs is that it can not only produce conventional NMNPs but also provide pathways toward new NMNPs with properties that cannot (or can only with great difficulty) be made via conventional processes.7−9 Over the past 15 years, a large number of NMNPs, including Au,10−19 Ag,7,8,20 Pt,10,15,21 Pd,22,23 Ru,24,25 Rh,26,27 and Ir,26,28 have been synthesized in, or in the presence of, ILs toward optical and/or catalytic applications, and the related approaches have been summarized in some nice mini-reviews and critical review articles.29−33 Although the syntheses of NMNPs in ILs have their own merits, specific applications often call for the dispersion of newly formed nanoparticles in other polar or nonpolar solvents in order to maximize the advantages of these environments based on some processing considerations. The dispersion of NMNPs in a required solvent can be achieved either by preparing the particles directly in this medium or by transferring the particles from ILs to this phase. The latter has the advantage of leveraging many existing methods for preparing the metal nanoparticles in ILs. In addition, considering the high viscosity and low volatilization of ILs, the processing of the NMNPs synthesized in ILs would be drastically simplified if they could be transferred into a wide variety of polar/nonpolar solvents. However, in comparison with the abundant work on phase transfer of NMNPs from © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. General Materials. Aqueous-soluble ILs including 1butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4, 99%) and 1-allyl-3-methylimidazolium chloride ([Amim]Cl, 99%) and organic-soluble IL (ammonium dibutyl phosphate, [AD]PO4, 98%) from Linzhou Keneng Material Technology Co.; hydrogen tetrachloroaurate(III) trihydrate (HAuCl4· 3H2O, 99.9%), chloroplatinic acid hexahydrate (H2PtCl6· 6H 2 O, ACS reagent, ≥37.5% Pt basis), sodium Received: Revised: Accepted: Published: 15909

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performed using a JEOL JEM-2100 electron microscope operating at 200 kV with the supplied software for automated electron tomography. For the TEM measurements, a drop of the nanoparticle solution was dispensed onto a 3 mm carboncoated copper grid. Excessive solution was removed by an absorbent paper, and the sample was dried under vacuum at room temperature. An energy-dispersive X-ray spectroscopy analyzer attached to the TEM was used to analyze the chemical compositions of the synthesized nanoparticles. X-ray photoelectron spectroscopy (XPS) was conducted on a VG ESCALAB MKII spectrometer. Samples for XPS analyses were concentrated from the toluene solution of NMNPs to 0.5 mL using flowing N2. Ten milliliters of methanol was then added to precipitate the nanoparticles, which were recovered by centrifugation, washed with methanol several times, and then dried at room temperature in a vacuum. 2.7. Electrochemical Measurements. Electrochemical measurements for the Pt nanoparticles transferred from IL/ water mixtures with different volume ratios were carried out in a standard three-electrode cell connected to a Bio-logic VMP3 (with EC-lab software version 9.56) potentiostat. A leak-free Ag/AgCl (saturated with KCl) electrode was used as the reference electrode. The counter electrode was a platinum mesh (1 × 1 cm2) attached to a platinum wire. For the loading of the Pt nanoparticles on Vulcan XC-72 carbon support, a calculated amount of carbon powder was added to the toluene solution of Pt nanoparticles. After the mixture was stirred for 24 h, the 20 wt % Pt on carbon support (Pt/C) was collected by centrifugation, washed thrice with methanol, and then dried at room temperature in a vacuum. The working electrode was a thin layer of Nafionimpregnated catalyst cast on a vitreous carbon disk. This electrode was prepared by ultrasonically dispersing 10 mg of the Pt/C in 10 mL of aqueous solution containing 4 mL of ethanol and 0.1 mL of Nafion. A calculated volume of the ink was dispensed onto the 5 mm glassy carbon disk electrode to produce a nominal catalyst loading of 20 μg cm−2 (Pt basis). The carbon electrode was then dried in a stream of warm air at 70 °C for 1 h. The room-temperature cyclic voltammograms of Pt/C in argon-purged HClO4 (0.1 M) were recorded between −0.25 and 1 V at 50 mV s−1 and used for the determination of electrochemically active surface areas (ECSAs) of Pt. The catalyst performance in room-temperature methanol oxidation reaction (MOR) was also measured by cyclic voltammetry (CV). For these measurements, the potential window of −0.2 to 1 V was scanned at 20 mV s−1 until a stable response was obtained. The electrolyte was methanol (1 M) in perchloric acid (0.1 M), and the current density was normalized by the ECSA to obtain the specific activities.

tetrachloropalladate(II) (Na2PdCl4, 98%), sodium citrate dehydrate (≥99%), dodecylamine (DDA, 98%), sodium borohydride (NaBH4, 98%), glutathione (GSH, 99+%), and oleylamine (OLA, technical grade, 70%) from Sigma-Aldrich; methanol (99%), ethanol (99.5%), tetramethylammonium hydroxide pentahydrate (TMAH, 95.0+%), and toluene (99.5%) from Beijing Chemical Works; aqueous HClO4 solution (70%, ACS reagent) and Nafion 117 solution (5% in a mixture of lower aliphatic alcohols and water) from Aladdin Reagents; and Vulcan XC-72 carbon powders (BET surface area of 250 m2 g−1 and average particle size of 40−50 nm) from Cabot were used as received. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with deionized water before drying in an oven. 2.2. Synthesis of NMNPs in the Presence of WaterSoluble ILs. For the synthesis of Pt nanoparticles in the presence of water-soluble ILs, 2 mL of 10 mM aqueous H2PtCl6 solution was diluted with 20 mL of [Bmim]BF4/water mixture with the volume ratio of 0/100, 20/80, 40/60, 60/40, 80/20, or 100/0, respectively, and then 1 mL of 100 mM aqueous solution of sodium citrate was added. Under vigorous stirring, 0.8 mL of 100 mM aqueous NaBH4 solution was introduced dropwise, and the mixture was continuously stirred for 30 min for the sufficient reduction of H2PtCl6 in the mixture of [Bmim]BF4 and water. With substitution of [Bmim]BF4 by [Amim]Cl, the syntheses of Au and Pd nanoparticles in the presence of water-soluble ILs followed the same protocol for the preparation of Pt nanoparticles in IL/water mixtures. 2.3. Phase Transfer of NMNPs from Water-Soluble ILs to a Nonpolar Organic Medium. The transfer of NMNPs from water-soluble ILs to toluene followed an ethanol-mediated protocol, which we reported previously.41,42,44 Typically, each of the NMNP sols in IL/water mixtures was mixed with 50 mL of ethanol containing 1 mL of dodecylamine, and the mixtures were stirred for 3 min. A 20 mL volume of toluene was added to each of the mixtures and stirring continued for another 3 min. Dodecylamine-stabilized NMNPs were extracted into the toluene layer rapidly, leaving behind colorless IL/water solutions. 2.4. Synthesis of NMNPs in the Presence of OrganicSoluble ILs. For the synthesis of Pt nanoparticles in the presence of organic-soluble ILs, 20.5 mg of HAuCl4, 26 mg of H2PtCl6, or 15 mg of Na2PdCl4 was added to 10 mL of [AD]PO4, followed by the addition of 1 mL of oleylamine serving as reducing agent. The mixture was heated to 150 °C under flowing N2 and kept at these conditions for 1 h for the reduction of metal precursors (Au3+, Pt4+, or Pd2+) by oleylamine. After reaction, the mixture was cooled to room temperature for the next transfer to the aqueous phase. 2.5. Phase Transfer of NMNPs from Organic-Soluble ILs to Aqueous Phase. With slight modification, the transfer of NMNPs from organic-soluble ILs to aqueous phase followed a protocol established for the transfer of NMNPs from toluene to water.43 In brief, GSH (60 mg, 0.2 mmol) and TMAH (108 mg, 0.6 mmol) were dissolved in 10 mL of methanol to form the glutathione tetramethylammonium salt (GTMA, 20 mM). Next, 10 mL of methanolic solution of GTMA was mixed with the as-prepared Au, Pt, or Pd sol in organic-soluble IL. With rapid stirring for 5 min, 10 mL of water was introduced. The mixture was left to stand, and the upper (aqueous) phase was collected after the complete separation of two phases. 2.6. Particle Characterizations. Transmission electron microscopy (TEM) and high-resolution (HR) TEM were

3. RESULTS AND DISCUSSION As syntheses of NMNPs in various solvents have their own merits and deficiencies, specific applications may call for the transfer of newly formed nanoparticles between different environments after synthesis. This makes phase transfer an evidently important technique in the development of nanoscience and nanotechnology. We prefer to focus this work on the application of the phase-transfer protocol we established previously for the migration of NMNPs from ILs to a nonpolar organic solvent or an aqueous phase for a given application, for example, highly efficient loading of Pt nanoparticles on carbon support for electrochemical measurements. 15910

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3.1. Syntheses of NMNPs in Aqueous-Soluble ILs and Phase Transfer to Toluene. Upon the dropwise addition of NaBH4 solution to the noble metal precursors in IL/water mixtures with different volume ratios, the solutions underwent a series of color changes before finally arriving at a wine red (for Au), dark brown (for Pd), or light brown (for Pt) color, indicating the formation of Au, Pd, or Pt nanoparticles, respectively, as shown in Figure 1a. The transfer of NMNPs

Figure 2. TEM images and corresponding HRTEM images (inset in each TEM image) of Pt nanoparticles transferred from IL/water with volume ratios of 0/100 (a), 20/80 (b), 40/60 (c), 60/40 (d), 80/20 (e), and 100/0 (f).

nanospheres, and the average size decreased from 4.6 nm at an IL/water volume ratio of 0/100 to 4.4 nm at an IL/water volume ratio of 20/80, 3.2 nm at an IL/water volume ratio of 40/60, 2.6 nm at an IL/water volume ratio of 60/40, 1.7 nm at an IL/water volume ratio of 80/20, and 1.2 nm at an IL/water volume ratio of 100/0. The same trends were also observed for Pd and Au systems, as exhibited by the TEM and HRTEM images in Figure S1 and Figure S2 in the Supporting Information (SI). The solvents for syntheses and the average size and size distributions of the Pt, Pd, and Au nanoparticles after phase transfer from IL/water mixture to toluene are summarized in SI Table S1. Adopting the strategy reported previously, the efficiency of the phase transfer for the NMNPs could be estimated by calculating the mass of the particles recovered from toluene.41 The efficiencies of the transfer for all NMNPs were evaluated to be more than 85%. The losses were likely caused by centrifugation and nanoparticle attachment to the container walls. On the other hand, the actual efficiencies could also be lower due to surface oxidation of the metal particles during drying and the residual presence of dodecylamine; both of these would add to the measured product weights. Although the effect of the phase-transfer process on the size/ morphology of the NMNPs cannot be ruled out, the tuning of the NMNPs by ILs is apparent. In colloidal chemistry methods, the nanoparticle could be formed via two processes, i.e., nucleation and crystal growth. 46,47 Nucleation, mainly determined by the supersaturation and temperature, is the starting point of crystallization, while the particle growth is the process of assembling atoms on the surface of nuclei. The growth is controlled by the diffusion of atoms to the growing

Figure 1. Phase transfer of NMNPs from IL/water mixture to an organic medium (toluene): metal sols in IL/water mixture (a), mixture of metal sols and ethanolic DDA solution (b), and NMNPs transferred into toluene located at the upper layer (c).

from IL/water mixture to toluene followed an ethanolmediated protocol we developed previously.41,42,44 The NMNP sols in IL/water mixtures were first mixed with an ethanolic solution of DDA, followed by addition of nonpolar organic solvent. After the ethanol−noble metal sol mixtures were stirred for about 3 min, the initially transparent noble metal sols turned turbid, and a purple (for Au) or deep brown liquid (for Pd and Pt) began to appear as droplets suspended near the top of the mixture or adhered to the container walls (Figure 1b). This indicated that DDA had displaced citrate from the surface of the NMNPs. The purple or brown liquid could be easily extracted into the nonpolar organic phase by adding toluene and stirring the mixture, as demonstrated finally in Figure 1c. Analogous to the transfer mechanism we discussed before,41 ethanol, which is IL/water miscible and a good solvent for DDA, was used in lieu of toluene to increase the interfacial contact between citrate-stabilized NMNPs and alkylamine, therefore inducing the phase transfer of NMNPs from IL/water mixture to toluene, a nonpolar organic medium. Figure 2 shows the TEM images of Pt nanoparticles transferred from IL/water with volume ratios of 0/100, 20/ 80, 40/60, 60/40, 80/20, and 100/0, respectively. As indicated, with the increasing volume ratio of ILs in the IL/water mixture, changes in both the morphology and average size of the Pt nanoparticles were observed. The Pt nanoparticles with irregular shape prepared in pure water evolved into Pt 15911

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particles more positively charged and therefore enhances their binding affinity to the electrons. 3.2. Syntheses of NMNPs in Organic-Soluble ILs and Phase Transfer to Water. The synthesis of NMNPs in organic-soluble ILs was analogous to that in common organic solvents (e.g., oleylamine or ethylene glycol).51−53 Upon heating and vigorous stirring, the noble metal precursors (HAuCl4, Na2PdCl4, or H2PtCl6) could be completely dissolved in IL ([AD]PO4) and reduced by oleylamine to form NMNPs at elevated temperature. However, unlike those prepared in oleylamine, we found experimentally that the NMNPs synthesized in [AD]PO4 could not be precipitated by the common methanol precipitation method. In addition, it is difficult to directly collect the NMNPs from the IL by centrifugation due to the high viscosity of [AD]PO4. Therefore, transfer the NMNPs to an aqueous medium might be an effective way for further processing of the NMNPs. In addition, the stable dispersion of nanoparticles in water is important to many applications.54 The phase transfer from IL to water could also overcome the inherent deficiencies of water-based synthesis of NMNPs, e.g., low reactant concentration.55 Analogous to the transfer of NMNPs from aqueous-soluble ILs to an organic phase, the direct transfer of NMNPs from organic-soluble IL ([AD]PO4) to water by mixing the NMNP sol in [AD]PO4 with an aqueous solution of GTMA was not successful. The NMNPs were aggregated at the interface between IL and water instead of transferring into the aqueous phase. As the exchange between the original stabilizers (oleylamine or [AD]PO4) of NMNPs and GTMA could only occur at the interface of the IL and water, the failure to transfer the particles could be reasonably attributed to the poor contact between the two phases due to their lack of mutual solubility. With this in mind, methanol, which is miscible with IL and a good solvent for GTMA, was selected in place of water to increase the interfacial contact between oleylamine/[AD]PO4stabilized NMNPs and GTMA. Upon mixing and stirring the mixture of NMNP sols in IL and methanolic solution of GTMA for several minutes, the blur of the mixture indicated the exchange of GTMA and oleylamine/[AD]PO4. After introduction of water, the transfer of GTMA-coated nanoparticles from IL/methanol mixture to aqueous phase took place. As a comparison, a previous study for the transfer of gold and Fe2MnO4 nanoparticles from organic phase to water through an interfacial process, which is driven by the hydrophobic van der Waals interactions between the primary alkane of the stabilizing ligand and the secondary alkane of the surfactant, does not involve the ILs and ligand exchange.56 Since water has lower density than that of [AD]PO4, the transfer process of NMNPs from IL to aqueous phase could also be demonstrated using the photos shown in Figure 1, in which the DDA and toluene would be replaced with GTMA and water, respectively. Figure 4 showed the TEM images of the Au, Pd, and Pt nanoparticles in water, which were transferred from [AD]PO4. As displayed, the particles were spherical in shape and had average sizes of 12.2 nm for Au, 7.6 nm for Pd, and 13.4 nm for Pt. The HRTEM images (Figure 4b,d,f, respectively) illustrated the lattice planes in these nanoparticles, confirming that these NMNPs were of high crystallinity. The noble metal hydrosols thus obtained were highly stable, and no agglomeration was observed after several months of storage in air. 3.3. Electrochemical Properties of Pt Nanoparticles. The transfer of NMNPs from aqueous-soluble ILs into toluene

surface, followed by their incorporation into the lattice. The incorporation process might be associated with the formation of chemical bonds, which could be regarded as the reaction step. Therefore, the particle growth might be regulated by the diffusion step and the reaction step. Both the diffusion step and the reaction step can be rate-determining for the particle formation. The compromise between diffusion step and reaction step might be an important prerequisite to facilitate the formation of nanoparticles with uniform size and narrow size distribution.48 The IL in the IL/water mixture could affect the diffusion of noble metal atoms generated from the reduction of metal precursors by effectively regulating the viscosity and dielectric constant of the mixture, resulting in the formation of Pt nanoparticles with different physical features. As we will discuss in the next section, the ILs could not only affect the particle size/morphology but also lead to particles having different crystal planes from those prepared in the absence of ILs. As a typical example, the Pt nanoparticles transferred from IL/water mixtures with different volume ratios were examined by XPS analyses to determine their chemical compositions. As shown by Figure 3, the Pt 4f region for all samples could be

Figure 3. 4f XPS spectra of Pt nanoparticles transferred from IL/water with volume ratios of 0/100 (a), 20/80 (b), 40/60 (c), 60/40 (d), 80/ 20 (e), and 100/0 (f).

deconvoluted into two pairs of doublets. The more intense doublet in each XPS spectrum is a signature of Pt in the zerovalent state, while the second and weaker doublet, with binding energy 1.4 eV higher than that of Pt(0), could be assigned to the Pt(II) oxidation state as in PtO and Pt(OH)2.37,40,49,50 In comparison with the same peaks in the Pt nanoparticles prepared in pure water (Figure 3a), the binding energies of the Pt 4f7/2 and 4f5/2 peaks of Pt nanoparticles prepared in the presence of IL were shifted appreciably to higher values. In addition, the shifts were increased with the decrease of the particle size (Figure 3b−f). Analogous phenomena were also observed in Pd nanoparticles transferred from IL/water mixtures (SI Figure S3), revealing that it is increasingly more difficult to detach the electrons from metal particles with smaller sizes. The high binding energy of NMNPs with small sizes might be attributed to stronger interactions between the small particles and surrounding stabilizers (citrate or ILs), which renders the surface of the 15912

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adsorbed on the surface and different sizes of the Pt nanoparticles. The ECSA-based voltammograms of methanol oxidation shown in Figure 5 were obtained in the potential window of 0− 1 V at a sweeping rate of 20 mV s−1. From the comparison of current densities, the Pt nanoparticles transferred from IL/ water mixtures with different volume ratios showed comparable catalytic activities. However, it was worthy to note that the features of the voltammograms over the Pt nanoparticles transferred from IL-containing mixtures (Figure 5b−d) were apparently different from those of the voltammogram over Pt nanoparticles transferred from pure water (Figure 5a). Clearly, for the Pt nanoparticles transferred from IL-containing mixtures, a high voltammetric current was detected in the forward sweep, while there is no big current measured on the negative-going sweep, indicating that Pt(110) is the dominant plane in these Pt nanoparticles.58,59 Although the formation mechanism is, as of now, unclear, we believe that it is the presence of ILs that gives rise to the (110)-dominated Pt nanoparticles. Similar to the Ag+ cations, which were usually used to control the facets of NMNPs,60−62 the ILs may selectively bind to some specific facets to facilitate the final structure of NMNPs. Further, as exhibited by the comparison in Figure 5, the onset potentials (crosspoint between the x axis and the linear segment of the CV curves) of methanol oxidation on Pt nanoparticles transferred from IL-containing mixtures with volume ratios of 20/80, 60/40, and 100/0 are 0.237 V (Figure 5b), 0.228 V (Figure 5c), and 0.242 V (Figure 5d), respectively, much lower than that of Pt nanoparticles transferred from pure water (ca. 0.336 V, Figure 5a), indicating that the oxidation of methanol was easier on the Pt nanoparticles from IL-containing mixtures than that on the Pt from pure water. In contrast to the low onset potentials for the methanol oxidation on bimetallic PtRu catalysts63,64 or Pt nanoparticles on single-walled carbon nanotubes (SWCNTs),65,66 which were induced by the reduced work function of Ru or SWCNT as compared to Pt, the low onset potentials on the Pt nanoparticles transferred from ILcontaining mixtures might be attributed to their dominant (110) plane, which is the most significant crystal plane favoring the MOR at an earlier potential.67 The high ratio of current densities in forward scan and backward scan usually means that the Pt catalysts are more COtolerant.60 This could be confirmed by the CO stripping voltammograms of Pt nanoparticles transferred from IL/water mixtures with different volume ratios. As shown in SI Figure S5, the CO stripping peaks of the Pt nanoparticles from ILcontaining mixtures shifted to a more negative potential as compared to those of the Pt particles from pure water, indicating a more facile CO removal and, hence, an improved CO tolerance in practice.

Figure 4. TEM images (a,c,e) and HRTEM images (b,d,f) of Au (a,b), Pd (c,d), and Pt nanoparticles (e,f) transferred from organic-soluble ionic liquid.

is very important for further electrochemical characterization. On one hand, after the phase-transfer treatment, the NMNPs synthesized in IL/water with different volume ratios would have the same stabilizer molecules (dodecylamine) adsorbed on their surfaces. On the other hand, in comparison with the highly stable ILs, the common nonpolar organic solvent (e.g., toluene) is easy to evaporate, and the particles in it could be conveniently collected by precipitation using some polar solvents, such as methanol and acetone. Hence, the phase transfer of NMNPs from IL/water mixtures to a nonpolar organic medium could dramatically simplify the loading of NMNPs on carbon support for the electrochemical measurements. This compensates the deficiency of synthesis of metal nanoparticles in or in the presence of ILs. After phase transfer, we loaded the Pt nanoparticles synthesized in a number of IL/water mixtures on Vulcan XC72 carbon support and characterized their electrocatalytic activities toward MOR, the key reaction in direct methanol fuel cells.57 The electrochemically active surface areas (ECSAs) of the Pt nanoparticles transferred from [Bmim]BF4/water volume ratios of 0/100, 20/80, 60/40, and 100/0, respectively, were determined using CV, as shown in SI Figure S4. The specific ECSAs, based on the unit weight of Pt and calculated by integrating the charge associated with the hydrogen adsorption/desorption potential region after double-layer correction, were 35 cm2 mgPt−1 for Pt from IL/water ratio of 0/100, 20 cm2 mgPt−1 for Pt from IL/water volume ratio of 20/ 80, 18 cm2 mgPt−1 for Pt from IL/water volume ratio of 60/40, and 6 cm2 mgPt−1 for Pt from IL/water volume ratio of 100/0. The differences in ECSAs among the Pt-containing NMNDs were most likely due to the presence of residual impurities

4. CONCLUSION In summary, we extended in this work the protocol we developed previously to the transfer of noble metal nanoparticles synthesized in, or in the presence of, ionic liquids to an organic or aqueous medium. The protocol involves first mixing the noble metal sols in ILs and an ethanolic or a methanolic solution of transfer agent and then extracting the transfer agentstabilized NMNPs into toluene or aqueous phase. The ease in operation might drastically simplify the processing of metal nanomaterials synthesized in ILs for a given application. The electron micrographs revealed that the particles after transfer 15913

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Figure 5. Cyclic voltammograms of Pt nanoparticles transferred from IL/water with volume ratios of 0/100 (a), 20/80 (b), 60/40 (c), and 100/0 (d) in argon-purged HClO4 (0.1 M) with 1 M methanol and sweeping rate of 20 mV s−1.

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were highly stable, and the size/morphology of the NMNPs could be significantly tuned by the ILs. In particular, The Pt particles transferred from IL-containing mixtures were dominated by the (110) crystal plane, which resulted in better CO tolerance in catalyzing the methanol oxidation reaction.

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ASSOCIATED CONTENT

* Supporting Information S

Summary of particle character, TEM and HRTEM images, XPS spectra, CV, and CO stripping tests for additional characterization of the nanomaterials synthesized in this study. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

* Fax 86-10-8254 4915; Tel 86-10-8254 4915; E-mail sjzhang@ ipe.ac.cn (S.Z.). *E-mail [email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the 100 Talents Program of the Chinese Academy of Sciences, National Natural Science Foundation of China (Nos. 21173226, 21376247), and State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences (MPCS-2012-A-11), is gratefully acknowledged. 15914

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