Electroless Deposition of Platinum Nanoparticles in Room

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Electroless Deposition of Platinum Nanoparticles in Room-Temperature Ionic Liquids Da Zhang, Takeyoshi Okajima, Dalin Lu, and Takeo Ohsaka Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402429v • Publication Date (Web): 28 Aug 2013 Downloaded from http://pubs.acs.org on September 1, 2013

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Electroless Deposition of Platinum Nanoparticles in Room-Temperature Ionic Liquids

Da Zhang1, Takeyoshi Okajima1, Dalin Lu2, and Takeo Ohsaka1 *

1 Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-5, Midori-ku, Yokohama 2268502, Japan 2 Technical Department, Center for Advanced Analysis, Tokyo Institute of Technology, Nagatsuta 4259-R1-34, Midori-ku, Yokohama 226-8503, Japan

*To whom correspondence should be addressed. Phone: +81-45-9245404. Fax: +81-45-9245489 E-mail: [email protected]

Keywords: ionic liquid, platinum, electroless deposition

ACS Paragon Plus Environment

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Abstract

The electroless deposition of Pt nanoparticles (Pt-NPs) could be carried out by dissolving potassium tetrachloroplatinate(II) (K2[PtCl4]) in 1-ethyl-3-methylimidazolium (EMI+) room-temperature ionic liquids (RTILs) containing bis(trifluoromethylsulfonyl) imide (NTf2-) or tetrafluoroborate (BF4-) anion and small cations such H+, K+ and Li+. In this case no deposition of Pt-NPs occurred in RTILs without such small cations. The formation of Pt-NPs was only observed in RTILs containing trifluoromethanesulfonimide (HNTf2) and protons at high temperature ( ≥ 80 °C) when potassium hexachloroplatinate(IV) (K2[PtCl6]) was dissolved in the RTILs. The obtained Pt-NPs gave a characteristic absorption spectrum of ultra-small Pt-NPs. The ultra-small and uniform Pt-NPs of ca. 1-4 nm in diameter were produced and the Pt-NPs/EMI+NTf2- dispersion was kept stably without adding any additional stabilizers or capping molecules for several months. The identified Fourier-transform patterns along the [011] zone axis were observed for the TEM images of Pt-NPs. Based on the results obtained, a probable mechanism of the electroless formation of Pt-NPs is discussed.

Introduction

Room-temperature ionic liquids (RTILs) are molten salts composed entirely of cations and anions, which have attracted intensive interest as new possible media for electrodeposition of metals, because RTILs have many advantages such as a wide electrochemical potential window, high ionic conductivity, and good thermal stability.1-4 Also, there are a rising number of publications5-16 demonstrating that anions and cations of RTILs can influence chemical and physicochemical reactions. RTILs are not only liquids with interesting physical properties, but also they can totally alter reaction pathways and products.5-6 Previous attempts on RTILs have revealed that, as a new kind of solvents, they are favorable template for the preparation of predictable chemical nanostructures.7-11 RTILs stabilize metal nanoparticles due to their


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high ionic and dielectric properties, high polarity and the ability to form supramolecular networks. A protective shell can be fabricated because anions will coordinate with the prepared metal nanoparticles and cations form hydrogen bridges with anions.7-11 The controlled and reproducible chemical nanostructures can be prepared and keep stable for several months without any extra stabilizing molecules. Many metal NPs such as Co17, Al18, Mg19, Ta20, Ag21, Au22, Pt23, Ir24, Rh25, Te26 and Ru27 have been prepared in RTILs. Among various metal nanoparticles, the synthesis of Pt nanoparticles (Pt-NPs) has received increasing attention because of their unusual catalytic properties and the importance of their applications in extensive chemical and electrochemical reactions, e.g., in the production of hydrogen from methane,28 oxygen reduction,29 and formic acid oxidation.30 Several methods have been employed for the fabrication of Pt-NPs, such as vapor deposition and sputtering.31,32 However, these physical methods are not so satisfactory to control the morphology and crystallographic orientations. Compared with them, electroless deposition is technically convenient to operate and occurs through chemically promoted reduction of metal ions without an externally applied potential. It is effective for arbitrarily controlling the morphology and size of the nanoparticles by suitably choosing the preparation conditions such as the reducing agents, stabilizers and temperature,22,33 and has been extensively used in fields such as metal film coating, corrosion, and preparation of bimetallic catalyst particles.34-36 Recently, electroless deposition of Au was carried out by Aldous et al. by dissolving Au

complex

(H[AuCl4])

into

1-butyl-3-methylimidazolium

bis(trifluoromethyl)sulfonylimide

([C4mim][NTf2]), demonstrating that it is a more straightforward and effective method for preparing Au nanoparticles.37 Based on previous studies,5,38,39 it is expected that electroless deposition in RTILs could give a novel and promising way for the preparation of metal nanoparticles of interest. In this study, electroless deposition of Pt-NPs was carried out satisfactorily by dissolving K2[PtCl4] or K2[PtCl6] in the 1-ethyl-3-methylimidazolium RTILs containing bis(trifluoromethylsulfonyl) imide (NTf2-) or tetrafluoroborate (BF4-) anion and small cations such H+, K+ and Li+ at various temperatures. Cyclic voltammetric (CV) measurements and ultraviolet-visible (UV-vis) absorption spectroscopy were used to confirm the fabrication of Pt-NPs. Characterization of the prepared Pt-NPs was carried out by


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transmission electron microscopy (TEM) and electrochemical techniques. Oxidation of formic acid at the Pt-NPs deposited electrodes was also carried out to examine the crystalline facet of the Pt-NPs. In addition, a probable mechanism of the electroless deposition of Pt-NPs is proposed.

Experimental Section

Reagents

EMINTf2, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and M+NTf2- (M+: H+, Li+ and K+) were purchased from Kanto Chemical Co., Ltd (Tokyo, Japan). K2[PtCl6], K2[PtCl4], chloroplatinic acid hexahydrate (H2PtCl6∙6H2O), sulfuric acid and formic acid (Wako Pure Chemicals Industries) were of analytical grade and were used as received. All aqueous solutions were prepared with deionized water purified by a Millipore Milli-Q system (Millipore, Japan). O CF3

O

S

N

O

S

CF3

BF4

O

N

N

EMINTf2

N

N

EMIBF4

Synthesis of Pt-NPs

All the glasswares used in the following procedures were cleaned by sonication for 15 min in purified water and dried prior to use. The formation of Pt-NPs in EMINTf2 was carried out by the following three


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procedures. First, the Pt-NPs were prepared in EMINTf2 containing K2[PtCl4] and protons (provided as trifluoromethanesulfonimide (HNTf2)). 4.1 mg K2[PtCl4] was dissolved into 1 ml EMINTf2 to prepare the solution (its color is red) containing 10 mM K2[PtCl4], and 56 mg HNTf2 was added into the solution. Nitrogen (N2) gas was bubbled directly into the solution for 10 min before the addition of HNTf 2 to obtain a N2-saturated solution. The mixture solution was sonicated for 5 min and stored in the vacuum oven at 105 °C for 12 h, which changed its color from red to black with the formation of Pt-NPs. The Pt-NPs prepared in this way were called Pt-NPs(1). Secondly, 19 mg methanesulfonic acid (CH3SO3H) was used instead of 56 mg HNTf2 and the Pt-NPs preparation was carried out in the same procedure as above. The Pt-NPs prepared in this way were called Pt-NPs(2). Thirdly, Pt-NPs(3) were prepared via the reduction of H2PtCl6 with NaBH4 in the presence of EMIBF4. A mixture of 1 ml NaBH4 solution (0.3%, w/w) and 0.1 ml EMIBF4 was added rapidly into 15 ml H2PtCl6∙6H2O aqueous solution (0.12 mmol/L) with vigorous stirring at 0 °C for 20 min.

Au-NPs were also prepared by the same procedure as used for Pt-NPs(1) except that 30 mM Na[AuCl4] was used instead of 10 mM K2[PtCl4].

Sample preparation for UV and TEM measurements

In the sample preparation for UV measurements, the Pt-NPs(1) and Pt-NPs(2) were collected from PtNPs dispersed RTILs under centrifugation (10000 rpm for 30 min) and then twice washed with 99.5% ethanol in an ultrasonic re-dispersion-centrifugation process and dispersed into 50 ml ethanol. The PtNPs(3) were collected from Pt-NPs dispersed aqueous solution under centrifugation (10000 rpm for 30 min) and then twice washed with 99.5% ethanol in an ultrasonic re-dispersion-centrifugation process and


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dispersed into 0.5 ml EMINTf2. The UV-vis absorption spectrum was measured using a JASCO V-550 UV-vis spectrometer (JASCO, Japan). In the sample preparation for TEM measurements, the Pt-NPs(1), Pt-NPs(2) and Au-NPs were collected by the same procedure as used for the preparation of the UV sample except that they were dispersed into 0.5 ml ethanol. TEM images were captured by JEM-2010 field emission transmission electron microscope (JEOL, Japan).

Electrochemical measurements

All the electrochemical measurements were performed with a three-electrode electrochemical cell containing a Pt particles-deposited glassy carbon (GC) (3 mm in diameter), Pt disk electrode (1.6 mm in diameter) or Pt micro-electrode (0.1 mm in diameter) as a working electrode, a spiral platinum wire as an auxiliary electrode, and a potassium chloride-saturated silver/silver chloride (Ag|AgCl|KCl(sat.)) as a reference electrode or a Pt wire as a quasi-reference electrode using ALS/Chi 750A electrochemical analyzer. The potential of the Pt quasi-reference electrode was monitored by dissolving a small amount of ferrocene in EMINTf2 whose potential was previously evaluated against an Ag|AgCl|KCl(sat.) electrode. The formal potential of the ferrocene/fericinium couple in EMINTf2, which is 0.22 V vs. Ag|AgCl|KCl(sat.), was 0.07 V vs. Pt wire quasi-reference electrode. In such a way, a potential of ca. −0.15 V vs. Ag|AgCl|KCl(sat.) could be estimated for the Pt quasi-reference electrode. Prior to each measurement, N2, argon (Ar) or oxygen (O2) gas was bubbled directly into the cell solution for at least 15 min to obtain a N2, Ar or O2-saturated solution, and during every measurement N2, Ar or O2 gas was flushed over the cell solution.


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The Pt particles-deposited GC working electrodes were prepared as follows. First, GC electrodes were polished on polishing microcloth (Marumoto Struers Kogyo) with alumina powder [particle diameter, 1.0 and 0.06 m (Marumoto)]/water slurries. After polishing, the GC electrodes were rinsed, sonicated for 10 min, and stored in deionized water. Pt particles-deposited GC electrodes were prepared by immersing the GC electrodes into EMINTf2 containing 10 mM K2[PtCl4] (or K2[PtCl6]) and 20 or 200 mM HNTf2, CH3SO3H, KNTf2 or LiNTf2 with stirring for 1 day. The Pt particles thus deposited on the GC electrodes were electrochemically cleaned by repeatedly scanning the potential between −0.2 and +1.5 V vs. Ag|AgCl|KCl(sat.) at a potential scan rate of 500 mV s−1 in 0.5 M H2SO4 solution under N2 gas atmosphere until CV responses, characteristic of a pure Pt, were obtained. The Pt low-index single crystalline domains [i.e., Pt(111), Pt(110), and Pt(100)] on the surface of Pt particles was examined by measuring the electro-oxidation of formic acid on the Pt particles-deposited GC electrodes in N2-saturated 0.5 M H2SO4 solution containing 0.1 M HCOOH.40-42 The real surface area of the Pt particles was estimated from the voltammograms obtained in 0.5 M H2SO4 solution by measuring the amount of charge consumed during the hydrogen desorption assuming that 210 C cm-2 corresponds to a monolayer of adsorbed hydrogens.43

Evolution gas analysis-mass spectrometeric (EGA-MS) measurements

0.5 l EMINTf2, 1.7 mg HNTf2 or 0.5 l EMINTf2 containing 200 mM HNTf2 were prepared in Arsaturated glove box. The samples were heated at a heating rate of 20 °C/min from 30 to 700 °C under helium (He) saturated atmosphere and the molecular weights of produced gases were detected by GCMSQP2010 (Shimadzu Corporation) and PY-2020iD (Frontier Laboratories Ltd.).


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Results and Discussion

Effect of proton, anion and temperature

Figure 1 shows that no Pt deposition was observed in H2SO4 solution when GC electrodes were immersed in EMINTf2 containing 10 mM K2[PtCl4] for 1 day, but the Pt particles were prepared by adding organic acids (HNTf2 or CH3SO3H) into EMINTf2 containing 10 mM K2[PtCl4] under N2 gas atmosphere, as seen from the fact that a characteristic current-potential curve of Pt was obtained in H2SO4 solution, and the amount of Pt deposition varies with temperature. A typical CV obtained at the bare GC electrode in H2SO4 is shown in the inset of Figure 1A and the redox response around 0.4 V is ascribed to the redox reaction of the quinone moiety on the GC electrode surface.44 Similar CVs were also obtained at the GC electrodes immersed in EMINTf2 containing 10 mM K2[PtCl4] at different temperatures (25 and 105 °C) for 1 day (Figure 1A), indicating that in this case no formation of Pt particles takes place on the GC electrode surface. Same results were also obtained when K2[PtCl6] was used instead of K2[PtCl4]. On the other hand, a typical characteristic response of Pt was observed when the GC electrode was immersed in EMINTf2 containing 10 mM K2[PtCl4] and 200 mM HNTf2 for 1 day (see Fig. 1B). Similar CV response with a different current intensity was obtained, when CH3SO3H was used as organic acid instead of HNTf2. The difference in current intensity between the CVs obtained when HNTf2 and CH3SO3H were used may reflect the differences in their acid dissociation constants in EMINTf2 and in the size and morphology of the produced Pt particles (i.e., Pt-NPs(1) and Pt-NPs(2)). As shown in Figure 1C, the current intensity at the Pt-NPs deposited GC electrodes gradually increased with increasing temperature when the GC electrodes were immersed in EMINTf2 containing 10 mM K2[PtCl4] and 200 mM HNTf2. When K2[PtCl6] was used instead of K2[PtCl4], the characteristic response of Pt was observed actually when the GC electrode was immersed in EMINTf2 containing 10 mM K2[PtCl6] and 200 mM HNTf2


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above 80 °C and the current intensity increased gradually with temperature (Figure 1D). In Figure 2, 10 mM K2[PtCl4] was dissolved in EMIBF4 instead of EMINTf2. It is obvious based on the hydrogen evolution current at −0.2 V that no and a little bit of deposited Pt particles could be observed when the CH3SO3H was absent and present, respectively (Figures 2A and 2B). In contrast to the case of EMINTf2 + HNTf2 (Fig. 1C), the current intensity increased only slightly with increasing temperature (Figure 2B) and no Pt deposition could be observed when K2[PtCl6] was used instead of K2[PtCl4] (Figure 2C).

Thus, it may be concluded that the formation of a large quantity of Pt particles took place when EMINTf2 contained K2[PtCl4] and protons, while a very small quantity of Pt particles was formed when EMIBF4 was used instead of EMINTf2 keeping the other conditions constant. When K2[PtCl6] was used instead of K2[PtCl4], a large amount of Pt particles could be formed only in EMINTf2 containing HNTf2 at high temperature (≥80 °C).

Characterization of Pt-NPs by UV-Vis spectroscopy and TEM

The deposited Pt-NPs were of the Pt black type, being different from the gray-white color of usual Pt plate or particle, because they are small enough to confine their electrons and to produce quantum effect which could give unique optical properties of nanoparticles.45 This is confirmed by the UV-vis absorption spectra of the as-prepared Pt-NPs(1) and Pt-NPs(2) shown in Figure 3A and a rather broad absorption continuum which extends throughout the visible region was observed, being coincident with the reported spectra of Pt-NPs.46-48 Figure 3B shows the absorption spectra of EMINTf2 containing 10 mM K2[PtCl4] after adding 200 mM HNTf2 with heating at 105 °C for different time. The absorption spectra increased gradually in the visible region with increasing the heating time after adding 200 mM HNTf2. The


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spectrum observed after heating for 24 h is almost the same as that of Pt-NPs(3) dispersed in EMINTf2, indicating that the Pt-NPs formation proceeds with heating in EMINTf2 containing HNTf2.

TEM measurements were also carried out to characterize the morphology and crystalline domains of the prepared Pt-NPs(1) and Pt-NPs(2). Ultra-small nanoparticles were found for both Pt-NPs(1) and PtNPs(2) as shown in Figures 4a and 4b, respectively. The ultra-small Pt-NPs of about 1-3 nm and 2-4 nm in diameter, as shown in the histograms (Figure 4a inset and 4b inset), were produced and the PtNPs/EMINTf2 dispersion was kept stably without adding any additional stabilizers or capping molecules for several months, i.e., no aggregation and precipitation were found, because the prepared Pt-NPs may be considered to be surrounded by the unique ionic liquid network as reported previously.7-11 The same identified Fourier-transform patterns recorded along the [011] zone axis49 (Figures 4c and 4d) indicate that the ultra-small particles of Pt-NPs(1) and Pt-NPs(2) possess the same shape which is probably of truncated octahedron covered by a mix of {100} and {111} facets for the sake of minimizing the total interfacial free energy.50,51

Formic acid oxidation

Polycrystalline Pt surfaces have mainly three low-index crystallographic orientations, i.e., the Pt(111), Pt(100), and Pt(110) planes and the particle shapes are closely related to the crystallographic surfaces.52 However, we could not clarify the crystalline facets of every prepared Pt-NP because the surface was strongly coated with the network of RTIL which would make the TEM measurements obscure. With an aim to explore the crystalline facets of the deposited Pt particles, the oxidation of formic acid at the PtNPs electrodes was carried out. Figure 5 illustrates the characteristic CVs for the oxidation of formic acid obtained at the Pt-NPs(1) and Pt-NPs(2)-deposited GC electrodes. These both CVs are on the whole


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similar in shape, but the current intensity (expressed as current densities calculated using each real surface area) at the Pt-NPs(1)-deposited GC electrode is bigger than that at the Pt-NPs(2)-deposited GC electrode. Two oxidation peaks were observed in the anodic and cathodic potential sweeps. During the anodic sweep, the peak at ca. 0.35 V vs. Ag|AgCl|KCl(sat.) is ascribed to the direct oxidation of formic acid to CO2 on Pt(111) plane.40,53 The peak at ca. 0.73 V corresponds to the oxidation of formic acid to CO on Pt(110) and Pt(100) crystalline planes and the surface is poisoned by the intermediate CO.40,53 During the cathodic sweep, a much larger and broad oxidation peak was observed, in which the oxidation of the poisonous intermediate CO on Pt(110) plane and the direct oxidation of formic acid on Pt(111) plane are considered to be observed at ca. 0.54 and 0.35 V, respectively38,51 and both oxidation peaks are overlapped actually. The oxidation of the poisonous intermediate CO on Pt(100) plane at ca. 0.23 V is not so conspicuous, compared with that on Pt(110) plane. This result that Pt-NPs(1) and Pt-NPs(2) possess similar low-index crystallographic orientations ratios means that the particle shapes of them are probably same. The difference in crystallographic low-index facet ratios of Pt-NPs, estimated from the TEM and formic acid oxidation measurements, may be due to the facet change of primary ultra-small Pt-NPs during the electrochemical pretreatment in sulfuric acid aqueous solution (see Experimental section).

Mechanism of Electroless Deposition of Pt

The disproportionation reaction of [PtCl4]2- to [PtCl6]2- and Pt is expected to take place also in the RTILs used here (EMINTf2 and EMIBF4) as previously found for [PtCl4]2- in N, N-diethyl-N-methyl-N(2-methoxyethyl) ammonium tetrafluoroborate (DEMEBF4) where [PtCl4]2- was electrogenerated by 2electron reduction of [PtCl6]2-.6 The formation of Au metal via the disproportionation of [AuCl2]- to [AuCl4]- and Au in [C4mim][NTf2] containing protons was proposed by Aldous et al.,37 although the mechanism of the “proton-induced” disproportionation remains to be clarified. Thus, the fact that the


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electroless deposition of Pt metal could be observed at 25 °C in EMINTf2 and EMIBF4 containing 10 mM [PtCl4]2- and protons (Fig. 1C and Fig. 2B), but not in the absence of protons (Fig. 1A) may suggest that the proton-induced disproportionation reaction of [PtCl4]2- might have occurred. Figure 6 supports this presumption, that is, Pt deposition was observed when GC electrodes were immersed in EMINTf2 containing 10 mM K2[PtCl4] and 20 or 200 mM HNTf2 at 25 °C for 1 day. The characteristic CVs of Pt were observed and the current intensity increased with increasing HNTf2 concentration. We found that the Pt formation takes place also in the presence of Li+ and K+, although their effect on the Pt formation is significantly small compared with that of H+, as can be readily seen from the comparison of the current intensity of the CVs shown in Fig. 6. The disproportionation reaction of [PtCl4]2- is a divalent anion-divalent anion reaction and thus this bimolecular reaction could be largely effected electrostatically by cations which may effectively approach the “reaction domain” of the disproportionation of [PtCl4]2- through the so-called ionic liquid organized network.7-11 As mentioned above, this disproportionation reaction does not occur actually in the EMINTf 2 containing K2PtCl4 in the absence of H+, suggesting that [PtCl4]2- anions can not be brought so close together that their disproportionation reaction does not take place, probably because of their electrostatic repulsion. Therefore, the addition of small, freely mobile cations in the EMINTf2 containing K2PtCl4 can be expected to enable this disproportionation, because they can effectively approach the “reaction domain” of the disproportionation of [PtCl4]2- and consequently reduce the electrostatic repulsion between [PtCl4]2anions. The diffusion coefficients of cations can be considered as a reasonable parameter reflecting their mobile degree. The diffusion coefficient of H+ is larger than that of Li+, which were estimated from the steady-state voltammograms obtained at a Pt micro-electrode (100 m in diameter) for the reduction of H+ or Li+ in Ar-saturated EMINTf2 containing 200 mM HNTf2 or LiNTf2 (Table 1).54 The diffusion coefficient of K+ is larger than that of Li+ in [C4mpyrr][NTf2] as reported by Wibowo et al..55 Thus, it can be thought that the diffusion coefficients of H+, K+ and Li+ in EMINTf2 increase in the order of H+ > K+ > Li+. The data in Fig. 6 suggests that H+ is much more effective than K+ and Li+ on the disproportionation


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of [PtCl4]2- ions and the effectiveness is in the order of H+ > K+ > Li+ in agreement with that of their diffusion coefficients. Here it should be noted that the disproportionation of [PtCl4]2- ions was observed in DEMEBF4 without adding these small cations,6 probably due to the weak hydrogen-bond strength between DEME+, containing only aliphatic C-H groups, and BF4-, namely, the ionic liquid network structure in DEMEBF4 is not so rigid compared with that in EMINTf2 and EMIBF4, where the imidazolium ring protons (e.g., aromatic C-H groups) in EMI+ possess a preferential affinity as hydrogenbond donors to F atoms of NTf2- or BF4-,56-58 and thus DEME+ ions may easily approach the “reaction domain” of the disproportionation of [PtCl4]2-. In addition, the fact that, in the presence of proton, Pt-NPs formation in EMIBF4 is much less than in EMINTf2 may be understood based on the much smaller diffusion coefficient of proton in RTIL containing BF4- anion than that in RTIL containing NTf2- anion.59 Next, we will consider the possibility of the reduction of [PtCl4]2- and [PtCl6]2- by carbon monoxide (CO) generated via the decomposition of HNTf2. Figure 7 shows the formation of CO when 0.5 l EMINTf2 containing 200 mM HNTf2 was heated from 30 to 700 °C. The decomposition of EMINTf2 and HNTf2 begins at ca. 400 and 60 °C, respectively (Fig. S1 and Fig. S2). However, HNTf2 starts to decompose at ca. 80 °C, when dissolved in EMINTf2 (Fig. 7) and the generation of CO between 60 to 200 °C is shown in the inset of Figure 7. The inset of Figure 8 shows typical CVs obtained at poly-Pt electrode in Ar- and CO-saturated EMINTf2. From the comparison of the CVs in this figure, we can see that the oxidation peak at ca. 0.9 V corresponds to the oxidation of CO in EMINTf 2. Figure 8 depicts the CVs at poly-Pt electrode in EMINTf2 containing 200 mM HNTf2 at 200 °C. The new anodic peak was observed at ca. 0.9 V after the elapse of 10 min, being in agreement with the oxidation of CO shown in the inset of Fig. 8. This demonstrates the generation of CO in EMINTf2 containing 200 mM HNTf2, probably due to the decomposition of HNTf2. In this connection, the fact that the Pt deposition from [PtCl4]2- and [PtCl6]2- in EMINTf2 containing HNTf2 was increased with increasing temperature above 80 °C (Fig.1C and D) can be understood reasonably based on the results regarding the Pt deposition on the GC electrode immersed in EMINTf2 containing 10 mM K2[PtCl4] or K2[PtCl6]under CO gas bubbling


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(0.4 bar) for 1h at 90 and 130 °C (Fig. 9). That is, CO can reduce [PtCl4]2- to produce Pt metal and a larger characteristic CV response of Pt was obtained when the GC electrode was immersed in EMINTf2 at higher temperature (e.g., 130 °C), reflecting a more significant decomposition of HNTf2 to generate CO. These results indicate that [PtCl4]2- and [PtCl6]2- are reduced by CO gas generated from the decomposition of HNTf2 at high temperatures (typically ≥80 °C). In summary, the electroless Pt deposition from [PtCl4]2- in EMINTf2 containing H+, K+ or Li+ takes place via a small cation-assisted disproportionation of [PtCl4]2- to [PtCl6]2- and Pt, that is, this disproportionation reaction is enhanced in the order of H+ > K+ > Li+ as well as via the reduction of [PtCl4]2- by CO gas generated by the decomposition of HNTf2 at high temperature (≥80 °C), while the Pt deposition from [PtCl6]2- occurs via its reduction by the CO gas.

Conclusions

The electroless deposition of Pt-NPs was examined using K2[PtCl4] or K2[PtCl6] in the RTILs (EMINTf2 and EMIBF4) containing M+NTf2- (M+: H+, K+ and Li+) at various temperatures for the first time, and the CVs and UV-vis spectra confirmed the formation of Pt-NPs. From the TEM images, ultrasmall (1-4 nm in diameter) Pt-NPs were found to be formed and the Pt-NPs/EMINTf2 dispersion could be kept stably, i.e., no aggregation of nanoparticles occurs actually for several months. Furthermore, it was found that the ultra-small single-crystalline Pt-NPs have a [011] zone axis. A probable mechanism of the electroless deposition of Pt-NPs is proposed: the formation of Pt-NPs occurs vis a small cation-assisted disproportionation reaction of [PtCl4]2- to [PtCl6]2- and Pt at low temperature (typically 25 and 50 °C) when the ionic liquids contain small cations such as H+, K+ and Li+, i.e., the disproportionation reaction is


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enhanced in the order of H+ > K+ > Li+, while [PtCl4]2- and [PtCl6]2- could be reduced to Pt by CO gas produced by the decomposition of HNTf2 at high temperature (≥80 °C).

Acknowledgments

The present work was financially supported by Grant-in-Aid for Scientific Research (A) (No. 19206079) to T.O. from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and to D. Z. from Tokyo Institute of Technology Global COE Program for Energy Science. D. Z. gratefully acknowledges the Government of Japan for a MEXT Scholarship.

References

(1) Sheldon, R. Catalytic Reactions in Ionic Liquids. Chem. Commun. 2001, 2399-2407.

(2) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Ionic Liquids for the Convenient Synthesis of Functional Nanoparticles and Other Inorganic Nanostructures. Angew. Chem., Int. Ed. 2004, 43, 49884992.

(3) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2002. (4) Rogors, R. D.; Seddon, K. R. Ionic Liquids Industrial Applications to Green Chemistry; American Chemical Society: Washington, DC, 2002.

(5) Oyama, T.; Okajima, T.; Ohsaka, T. Electrodeposition of Gold at Glassy Carbon Electrodes in Room-Temperature Ionic Liquids. J. Electrochem. Soc. 2007, 154, D322-D327.


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Page 16 of 42

(6) Zhang, D.; Chang, W.; Okajima, T.; Ohsaka, T. Electrodeposition of Platinum Nanoparticles in a Room-Temperature Ionic Liquid. Langmuir 2011, 27, 14662-14668.

(7) Redel, E.; Kramer, J.; Thomann, R.; Janiak, C. Synthesis of Co, Rh and Ir Nanoparticles from Metal Carbonyls in Ionic Liquids and Their Use as Biphasic Liquid-liquid Hydrogenation Nanocatalysts for Cyclohexene. J. Organomet. Chem. 2009, 694, 1069-1075. (8) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis, 2nd ed.; Wiley-VCH: Weinheim, 2007; Vol. 1. (9) Redel, E.; Thomann, R.; Janiak, C. Use of Ionic Liquids (ILs) for the IL-anion Size-dependent Formation of Cr, Mo and W Nanoparticles from Metal Carbonyl M(CO)6 Precursors. Chem. Commun. 2008, 15, 1789-1791. (10) Kramer, J.; Redel, E.; Thomann, R.; Janiak, C. Use of Ionic Liquids for the Synthesis of Iron, Ruthenium, and Osmium Nanoparticles from Their Metal Carbonyl Precursors. Organometallics 2008, 27, 1976-1978. (11) Redel, E.; Thomann, R.; Janiak, C. First Correlation of Nanoparticle Size-Dependent Formation with the Ionic Liquid Anion Molecular Volume. Inorg. Chem. 2008, 47, 14-16. (12) Park, J. W.; Yamauchi, K.; Takashima, E.; Tachikawa, N.; Ueno, K.; Dokko, K.; Watanabe, M. Solvent Effect of Room Temperature Ionic Liquids on Electrochemical Reactions in Lithium-Sulfur Batteries. J. Phys. Chem. C 2013, 117, 4431-4440. (13) Ueno, K.; Inaizumi, S.; Hata, K.; Watanabe, M. Colloidal Interaction in Ionic Liquids: Effects of Ionic Structures and Surface Chemistry on Rheology of Silica Colloidal Dispersions. Langmuir 2009, 25, 825-831.


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Page 17 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(14) Ueno, K.; Inaba, A.; Kondoh, M.; Watanabe, M. Colloidal Stability of Bare and Polymer-Grafted Silica Nanoparticles in Ionic Liquids. Langmuir 2008, 24, 5253-5259.

(15) Yu, P.; Yan, J.; Zhang, J.; Mao, L. Cost-effective Electrodeposition of Platinum Nanoparticles with Ionic Liquid Droplet Confined onto Electrode Surface as Micro-media. Electrochem. Commun. 2007, 9, 1139-1144. (16) Yu, P.; Qian, Q. Wang, X.; Cheng, H.; Ohsaka, T.; Mao, L. Potential-controllable Green Synthesis and Deposition of Metal Nanoparticles with Electrochemical Method. J. Mater. Chem. 2010, 20, 58205822. (17) Fukui, R.; Katayama, Y.; Miura, T. Electrodeposition of Cobalt from a Hydrophobic RoomTemperature Molten Salt System. Electrochemistry 2005, 73, 567-569.

(18) Stafford, G. R.; Kongstein, O. E.; Haarberg, G. M. First-principles Calculations of the Electrochemical Reactions of Water at an Immersed Ni(111)/H2O Interface. J. Electrochem. Soc. 2006, 153, C207-C212.

(19) NuLi, Y.; Wang, J.; Wang, J.; Xu, J.; Wang, P. Electrochemical Magnesium Deposition and Dissolution with High Efficiency in Ionic Liquid. Electrochem. Solid-State Lett. 2005, 8, C166-C169.

(20) Fl Abedin, S. Z.; Farag, H. K.; Moustafa, E. M.; Welz-Biermann, U.; Endres, F. Synthesis of Propylene/1-butene copolymers with Ziegler-Natta Catalyst in Gas-phase Copolymerizations, 1: Kinetics and Macromolecular Properties. Phys. Chem. Chem. Phys. 2005, 7, 2333-2341.

(21) Borissov, D.; Aravinda, C. L.; Freyland, W. Comparative Investigation of Underpotential Deposition of Ag from Aqueous and Ionic Electrolytes: An Electrochemical and In Situ STM Study. J. Phys. Chem. B 2005, 109, 11606-11615.


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(22) Zhang, H.; Cui, H. Synthesis and Characterization of Functionalized Ionic Liquid-Stabilized Metal (Gold and Platinum) Nanoparticles and Metal Nanoparticle/Carbon Nanotube Hybrids. Langmuir 2009, 25, 2604-2612. (23) Scheeren, C. W.; Machado, G.; Dupont, J.; Fichtner, P. F. P.; Texeira, S. R. Nanoscale Pt(0) Particles Prepared in Imidazolium Room Temperature Ionic Liquids: Synthesis from an Organometallic Precursor, Characterization, and Catalytic Properties in Hydrogenation Reactions. Inorg. Chem. 2003, 42, 47384742. (23) Dupont, J.; Fonseca, G. S.; Umpierra, A. P.; Fichtner, P. F. P. Transition-metal Nanoparticles in Imidazolium Ionic Liquids: Recyclable Catalysts for Biphasic Hydrogenation Reactions. J. Am. Chem. Soc. 2002, 124, 4228-4229. (25) Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P. The Use of Imidazolium Ionic Liquids for the Formation and Stabilization of Ir0 and Rh0 Nanoparticles: Efficient Catalysts for the Hydrogenation of Arenes. Chem. Eur. J. 2003, 9, 3263-3269. (26) Zhu, Y. J.; Wang, W. W.; Qi, R. J.; Hu, X. L. Microwave-assisted Synthesis of Single-crystalline Tellurium Nanorods and Nanowires in Ionic Liquids. Angew. Chem., Int. Ed. 2004, 43, 1410-1414. (27) Silveira, E. T.; Umpierre, A. P.; Rossi, L. M.; Machado, G.; Morais, J.; Soares, G. V.; Baumvol, I. L. R.; Teixeira, S. R.; Fichtner, P. F. P.; Dupont, J. The Partial Hydrogenation of Benzene to Cyclohexene by Nanoscale Ruthenium Catalysts in Imidazolium Ionic Liquids. Chem. Eur. J. 2004, 10, 3734-3740. (28) Yongquan, Q.; Alexander, M. S.; Jennifer, L.; George, D. S.; Ting, G. Probing Site Activity of Monodisperse Pt Nanoparticle Catalysts Using Steam Reforming of Methane. J. Phys. Chem. Lett. 2010, 1 (1), 254-259. (29) Tritsaris, G. A.; Greeley, J.; Rossmeisl, J.; Noskov, J. K. Atomic-scale Modeling of Particle Size Effects for the Oxygen Reduction Reaction on Pt. Catal. Lett. 2011, 141, 909-913.


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Page 19 of 42

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(30) Chan, K. Y.; Ren, J. W.; Cheng, S. A.; Tsang, K. Y. Supported Mixed Metal Nanoparticles as Electrocatalysts in Low Temperature Fuel Cells. J. Mater. Chem. 2004, 14, 505-516. (31) Fiordaliso, E. M.; Dahl, S.; Chorkendorff, I. Strong Metal Support Interaction of Pt and Ru Nanoparticles Deposited on HOPG Probed by the H-D Exchange Reaction. J. Phys. Chem. C 2012, 116, 5773-5780. (32) Yoo, S. J.; Cho, Y.-H.; Park , H.-S.; Lee, J. K.; Sunga, Y.-E. High Utilization of Pt Nanocatalysts Fabricated Using a High-pressure Sputtering Technique. J. Power Sources 2008, 178, 547-553. (33) Redel, E.; Walter, M.; Thomann, R.; Vollmer, C.; Hussein, L.; Scherer, H.; Kruger, M.; Janiak, C. Synthesis, Stabilization, Functionalization and, DFT Calculations of Gold Nanoparticles in Fluorous Phases (PTFE and Ionic Liquids). Chem. Eur. J. 2009, 15, 10047-10059. (34) Lu, P.; Shi, Z.; Walker, A. V. Selective Electroless Deposition of Copper on Organic Thin Films with Improved Morphology. Langmuir 2011, 27, 13022-13028. (35) Vitry, V.; Sens, A.; A.-F. Kanta, A.-F.; Delaunois, F. Wear and Corrosion Resistance of Heat Treated and as-plated Duplex NiP/NiB Coatings on 2024 Aluminum Alloys. Surface and Coating Technology 2012, 206, 3421-3427. (36) Chio, I.; Ahn, S. H.; Kim, J. J.; Kwon, O. J. Preparation of Ptshell-Pdcore Nanoparticle with Electroless Deposition of Copper for Polymer Electrolyte Membrane Fuel Cell. Applied Catalysis B: Environmental 2011, 102, 608-613. (37) Aldous, L.; Silvester, D. S.; Pitner, W. R.; Compton, R. G.; Lagunas, M. C.; Hardacre, C. Voltammetric Studies of Gold, Protons, and [HCl2]- in Ionic Liquids. J. Phys. Chem. C. 2007, 111, 84968503.


19

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Page 20 of 42

(38) Jang, G. G.; Blake, P.; Roper, D. K. Rate-Limited Electroless Gold Thin Film Growth: A Real-Time Study. Langmuir 2013, 29, 5476-5486. (39) Miyoshi, K.; Aoki, Y.; Kunitake, T.; Fujikawa, S. Facile Fabrication of Silver Nanofin Array via Electroless Plating. Langmuir 2008, 24, 4205-4208. (40) Claviler, J.; Parsons, R.; Durand, R.; Lamy, C.; Leger, J. M. Formic Acid Oxidation on Single Crystal Platinum Electrodes. Comparison with Polycrystalline Platinum. J. Electroanal. Chem. 1981, 124, 321-326. (41) Tripkovic, A.; Popovic, K. Dj.; Lovic, J. D. Comparison of Formic Acid Oxidation at Supported Pt Catalyst and at Low-index Pt Single Crystal Electrodes in Sulfuric Acid Solution. J. Serb. Chem. Soc. 2003, 68, 849-857. (42) Hwang, S.; Bonevich, J. E.; Kim, J. J.; Moffat, T. P. Formic Acid Oxidation on Pt100-xPbx Thin Films Electrodeposited on Au. J. Electrochem. Soc. 2011, 158, B1019-B1028. (43) Trasatti, S.; Petrii, O. A. Real Surface Area Measurements in Electrochemistry. Pure Appl. Chem. 1991, 63, 711-734. (44) McCreery, R. L.; Carbon electrodes: structural effects on electron transfer kinetics. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1991; Vol. 17, pp 221-364. (45) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; New York: Springer, 1995. (46) Luo, Y.; Sun, X. One-step Preparation of Poly(vinyl alcohol)-protected Pt Nanoparticles through a Heat-treatment Method. Materials Letters 2007, 61, 2015-2017. (47) Creighton, J. A.; Eadon, D. G. Ultraviolet-visible Absorption Spectra of the Colloidal Metallic Elements. J. Chem. Soc. Faraday Trans. 1991, 87(24), 3881-3891.


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Page 21 of 42

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(48) Henglein, A.; Ershov, B. G.; Malow, M. Absorption Spectrum and Some Chemical Reactions of Colloidal Platinum in Aqueous Solution. J. Phys. Chem. 1995, 99, 14129-14136. (49) Edington, J. W. Monographs in Practical Electron Microscopy in Materials Science; Philips: East Kilbride, 1974. (50) Lim, B.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science 2009, 324, 1302-1305. (51) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. A General Approach to the Size- and ShapeControlled Synthesis of Platinum Nanoparticles and Their Catalytic Reduction of Oxygen. Angew. Chem. Int. Ed. 2008, 7, 3588-3591. (52) Wang, Z. L. Transmission Electron Microscopy of Shape-Controlled Nanocrystals and Their Assemblies. J. Phys. Chem. B 2000, 104, 1153-1175. (53) Gullon, J. S.; Rodriguez, P.; Herrero, E.; Aldaz, A.; Feliu, J. M. Surface Characterization of Platinum Electrodes. Phys. Chem. Chem. Phys. 2008, 10, 1359-1373. (54) Meng, Y.; Aldous, L.; Belding, S. R.; Compton, R. G. The Hydrogen Evolution Reaction in a Room Temperature Ionic Liquid: Mechanism and Electrocatalyst Trends. Phys. Chem. Chem. Phys. 2012, 14, 5222-5228. (55) Wibowo, R.; Aldous, L.; Jones, S. E. W.; Compton, R. G. The Group I Alkali Metals in Ionic Liquids: Electrodeposition and Determination of Their Kinetic and Thermodynamic Properties. ECS Transactions 2010, 33(7), 523-535. (56) Mele, A.; Tran, C. D.; Lacerda, S. H. D. The Structure of a Room-Temperature Ionic Liquid with and without Trace Amounts of Water: The Role of C━H⋅⋅⋅O and C━H⋅⋅⋅F Interactions in 1-n-Butyl-3Methylimidazolium Tetrafluoroborate. Angew. Chem., Int. Ed. 2003, 42, 4364-4366.


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(57) Wu, B.; Liu, Y.; Zhang, Y. M.; Wang, H. P. Probing Intermolecular Interactions in Ionic LiquidWater Mixtures by Near-Infrared Spectroscopy. Chem. Eur. J. 2009, 15, 6889-6893. (58) Noda, A.; Hayamizu, K.; Watanabe, M. Pulsed-Gradient Spin-Echo 1H and 19F NMR Ionic Diffusion Coefficient, Viscosity, and Ionic Conductivity of Non-Chloroaluminate Room-Temperature Ionic Liquids. J. Phys. Chem. B 2001, 105, 4603-4610. (59) Meng, Y.; Aldous, L.; Belding, S. R.; Compton, R. G. The Formal Potentials and Electrode Kinetics of the Proton/hydrogen Couple in Various Room Temperature Ionic Liquids. Chem. Commun. 2012, 48, 5572-5574.


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Figure 1. CVs obtained at (A) GC electrodes, immersed in EMINTf2 containing 10 mM K2PtCl4 at 25 °C (solid line) or 105 °C (dashed line) for 1 day, (B) Pt particles-deposited GC electrodes, which were prepared by immersing GC electrodes in EMINTf2 containing 10 mM K2[PtCl4] and 200 mM HNTf2 (solid line) or 200 mM CH3SO3H (dashed line) at 105 °C for 1 day, and (C, D) Pt particles-deposited GC electrodes, which were prepared by immersing GC electrodes in EMINTf2 containing 10 mM K2[PtCl4] (C) or K2[PtCl6] (D) and 200 mM HNTf2 at (a) 25, (b) 50, (c) 80, (d) 90, (e) 105 and (f) 130 °C for 1 day, in N2-saturated 0.5 M H2SO4 solution. Inset in (A) is CV obtained at bare GC electrode in N2-saturated 0.5 M H2SO4 solution. Potential scan range: −0.2 V~1.5 V. Potential scan rate: 500 mV s-1. Figure 1. Zhang et al.


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Figure 2. CVs obtained at (A) GC electrodes, immersed in EMIBF4 containing 10 mM K2PtCl4 at 25 °C (solid line) or 140 °C (dashed line) for 1 day, and (B, C) Pt particles-deposited GC electrodes, which were prepared by immersing GC electrodes in EMIBF4 containing 10 mM (B) K2[PtCl4] or (C) K2[PtCl6] and 200 mM CH3SO3H at 25 °C (solid line), 105 °C (dashed line) and 140 °C (dotted line) for 1 day, in N2saturated 0.5 M H2SO4 solution. Potential scan range: −0.2 V~1.5 V. Potential scan rate: 500 mV s-1.



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0 -10 -20

B

-30 -40 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6


Figure 6. CVs obtained at Pt particles-deposited GC electrodes, which were prepared by immersing GC electrodes in EMINTf2 containing 10 mM K2[PtCl4] and (A) 20 mM or (B) 200mM HNTf2 (solid line), LiNTf2 (dashed line) and KNTf2 (dotted line) at 25 °C for 1 day, in N2-saturated 0.5 M H2SO4 solution. Potential scan range: −0.2 V~1.5 V. Potential scan rate: 500 mV s-1.



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250000

Relative amount of CO generation / a.u.

3500 3000

Intensity / a.u.

2500 2000 1500 1000 500 0

200000

150000

100000

50000

0

-500 0

50

100

150

200

250

60

80

100

Temperature /C

120

140

160

180

200

Temperature /C

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Temperature / °C Figure 7. EGA-MS data obtained by heating 0.5 l EMINTf2 containing 200 mM HNTf2 at a heating rate of 20 °C / min from 30 to 700 °C under He atmosphere. Insets show the intensity of CO (left) and relative total amount of CO generation (right) at various heating temperatures. The numerical values in the ordinate represent the molecular weights of the species produced by the decomposition. Figure 7. Zhang et al.


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100 50 0 -50

I / A

-100 -150

100 80

-200 60

I / A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-250

40 20

-300

0 -20

-350

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

E / V vs. Pt wire

-400 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

E / V vs. Pt wire

Figure 8. CVs obtained at polycrystalline Pt electrode in Ar-saturated EMINTf2 containing 200 mM HNTf2 kept at 200 °C for 0 min (solid line) or 10 min (dashed line). Inset shows CVs obtained at polycrystalline Pt electrode in Ar-saturated (solid line) or CO-saturated (dashed line) EMINTf2. Potential scan range: −1.7~1.0 V vs. Pt wire. Potential scan rate: 100 mV s-1.



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60

40

20

I / A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

-20

-40

-60 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

E / V vs. Ag/AgCl(KCl sat.) Figure 9. CVs obtained at Pt particles-deposited GC electrodes, which were prepared by immersing GC electrodes in EMINTf2 containing 10 mM K2[PtCl4] (solid line) or K2[PtCl6] (dashed line) under the CO gas bubbling (0.4 bar) at 130 °C for 1 h, in N2-saturated 0.5 M H2SO4 solution. Potential scan range: −0.2 V~1.5 V. Potential scan rate: 500 mV s-1.



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Scheme 1. Electroless deposition of Pt-NPs from [PtCl4]2- and [PtCl6]2- in EMINTf2 at low and high temperatures.

(e.g., 25 and 50 °C)

(e.g., ≥ 80 °C)

Scheme 1. Zhang et al.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 1. Diffusion coefficients of H+ and Li+ in EMINTf2 at 25 °C Ionic liquids EMINTf2 EMINTf2 EMINTf2

Cations H (HNTf2) H+ (HNTf2) Li+ (LiNTf2) +

1012 D / m2 s-1 34 25 18

References This work 54 This work

Table 1. Zhang et al.


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Supporting Information. EGA-MS data of EMINTf2 and HNTf2. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of contents only


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Electroless Deposition of Platinum Nanoparticles in Room

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