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Microwave-Assisted Polyol Synthesis of Pt/Pd and Pt/ Rh Bimetallic Nanoparticles in Polymer Solutions Prepared by Batch and Continuous-Flow Processing. Cong Cong, Sayaka Nakayama, Shinya Maenosono, and Masafumi Harada Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03154 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Industrial & Engineering Chemistry Research

Microwave-Assisted Polyol Synthesis of Pt/Pd and Pt/Rh Bimetallic Nanoparticles in Polymer Solutions Prepared by Batch and Continuous-Flow Processing Cong Cong,† Sayaka Nakayama,ǂ Shinya Maenosono,# and Masafumi Harada*,ǂ †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China ǂ

Department of Health Science and Clothing Environment, Faculty of Human Life and Environment,

Nara Women’s University, Nara 630-8506, Japan #

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai,

Nomi, Ishikawa 923-1292, Japan

CORRESPONDING AUTHOR FOOTNOTE: Prof. Dr. Masafumi Harada Tel: +81-742-20-3466; Fax: +81-742-20-3466 E-mail address: [email protected] (M. Harada)

ABSTRACT: Colloidal dispersions of Pt/Pd and Pt/Rh bimetallic nanoparticles have been synthesized by microwave-assisted polyol method using ethylene glycol and glycerol as a solvent in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). The structure of bimetallic nanoparticles has been investigated by means of high-resolution transmission electron micrograph (HRTEM), energydispersive X-ray spectroscopy (EDS) elemental mapping and extended X-ray absorption fine structure (EXAFS). The effectiveness of the batch and continuous-flow processing was demonstrated for the preparation of various bimetallic nanoparticles under multimode or singlemode microwave irradiation. In the single-mode microwave-assisted continuous-flow processing in contrast to the batch processing, the well-dispersed colloidal bimetallic nanoparticles were 1 ACS Paragon Plus Environment

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successfully produced at the boiling temperature of solvent. EXAFS analysis indicated that, in the case of Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles prepared using both batch and continuous-flow processing, the distribution of different metallic species in a particle tended to be a “cluster-in-cluster” structure.

KEYWORDS: microwave-assisted synthesis; polyol synthesis; Pt/Pd bimetallic nanoparticles; Pt/Rh bimetallic nanoparticles; PVP; EXAFS

Introduction Bimetallic nanoparticles have received considerable attention as a consequence of their possible applications in fields such as optics, heterogeneous catalysis, hydrogen storage, biological sensing and biomedicine, because their unique optical, catalytic, and biological properties can be easily tuned by controlling the size, shape and compositions.1-3 The existence of a new second metallic element causes the composition of the nanoparticles to become complex, and the desired physical and chemical properties could be improved. Homogeneity, dispersion, alloying extent, and structure play very important roles on the surface properties. In particular, either core-shell or alloy structures of bimetallic nanoparticles are beneficial to chemical reactions as catalyst, because of their high activity, selectivity, and chemical/physical stability.4-6 Detailed characterization of the Au/Pd,7 Au/Pt,8 Cu/Pd,9 Ni/Pd,10 Pt/Pd,11-13 Pt/Rh,14-16 and Pt/Ru17,18 bimetallic nanoparticles stabilized in solution phase have been reported. To the best of our knowledge, the literature on the Pt/Pd bimetallic nanoparticles reveals that the preparation conditions employed vary from polymer-protected bimetallic nanoparticles in aqueous solution to laser vaporization of bulk alloys. The colloidal dispersions of Pt/Pd and Pt/Rh bimetallic nanoparticles were previously prepared by the reduction of the corresponding salts in water/ethanol 2 ACS Paragon Plus Environment

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in the presence of PVP11,14 or in water-in-oil microemulsion systems.12 The equilibrium surface and bulk compositions were determined by X-ray absorption near-edge structure (XANES) and/or EXAFS technique to fully understand the catalytic properties and their relationship to the electronic structures of bimetallic nanoparticles.11-18 For example, Toshima and co-workers11 reported the synthesis of polymer-protected Pt/Pd bimetallic nanoparticles by simultaneously reduced the metal ions in water/ethanol solutions, and studied their structure using EXAFS technique. Touroude et al.12 reported the preparation of alloy particles of Pt/Pd in water-in-oil microemulsions, and investigated the catalytic activity for isomerization reactions. Renouprez et al.13 investigated the structures, compositions, and catalytic activity of Pt/Pd nanoparticles produced by the Nd:YAG laser vaporization of rods of bulk Pt/Pd alloys of various compositions in extensive studies. They showed that the Pt-Pd nanoparticles with a diameter of 1-5 nm are truncated octahedral which consisted of Pt-rich core and Pd-rich shell. On the other hand, microwave (MW) irradiation is one of the most sophisticated methods for the preparation of nanoparticles with controlled size and shape.19-21 In the past decade, there has been increasing attention in the use of MW heating instead of conventional heating in various kinds of monometallic and bimetallic nanoparticle syntheses,22-31 because MW heating has shown to reduce dramatically the reaction times and to enhance the purity of the nanoparticles produced. For example, Bensebaa et al.22 used MW heating to produce Pt/Ru bimetallic nanoparticles in ethylene glycol (EG) solution dissolving PVP, and demonstrated that microwave preparation parameters linked to surface composition and structure. Patel et al.23 reported the Pt/Ag and Pd/Ag bimetallic nanoparticles synthesized through MW-polyol method in the presence of PVP. They showed that the main characteristic of microwave syntheses is the uniform heating, and the particle size would result in a narrow distribution due to this property. While the MW-assisted syntheses mentioned above are quite successful in small-scale systems, particularly for the rapid synthesis of nanomaterials, there is a great desire to develop large-scale syntheses techniques under MW irradiation, which could obtain products in kilogram scale. As for the technology of MW scale-up method, there is a main physical limitation that the penetration 3 ACS Paragon Plus Environment


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depth of MW irradiation into absorbing materials (solvent or reactant) is restricted. To overcome the inherent problems associated with scale-up, two different approaches for MW synthesis have emerged. One is by the use of larger batch-type reactors,32,33 and the other is by the use of continuous flow reactor systems. This flow reactor system, which could potentially be operated for long periods of time, exemplifies one possible mode of scale-up using MW system. In recent years, several research groups have developed some continuous-flow microwave reactor systems to conduct syntheses of various nanoparticles.34,35 We have recently reported the use of EG and glycerol in the polyol process for the preparation of various monometallic nanoparticles by using continuous-flow type method (10 mL processing volume) under single-mode MW irradiation.36 The synthesis of bimetallic nanoparticles in a continuous-flow reactor system is described by the schematic drawing in Figure S1 (see Supporting Information). In this paper we aim the development of efficient large-scale preparation method for the bimetallic nanoparticles with their controllable size and size distribution. Here we present the preparation of Pt/Pd and Pt/Rh bimetallic nanoparticles protected by PVP in the solvents (EG and glycerol) using MW-assisted batch or continuous-flow processing. We investigate the internal structure of the Pt/Pd and Pt/Rh bimetallic nanoparticles using a HRTEM, a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), EDS elemental mapping, and EXAFS measurements. Experimental Section Materials.

Hexachloroplatinic (IV) acid (H2PtCl6 ･6H2O, Nacalai Tesque), palladium (II)

chloride (PdCl2, Nacalai Tesque), rhodium (III) chloride hydrate (RhCl3･3H2O, Wako Chemicals), EG (Nacalai Tesque), glycerol (gly.) (Nacalai Tesque), hydrochloric acid (Nacalai Tesque) and PVP (K-30, average M.W. = 40000, Tokyo Kasei Kogyo Co.) were used as received. In addition, distilled water was not further purified, and boiling points for EG and glycerol are 471 K and 563 K, respectively.

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Microwave Apparatus and Preparation Method. Batch Type Method Using Multimode MW Irradiation. Colloidal dispersions of Pt/Pd and Pt/Rh bimetallic nanoparticles were prepared from the corresponding ionic precursor (H2PtCl6･6H2O, PdCl2, RhCl3･3H2O) by MW irradiation in EG or glycerol combined with PVP. The typical synthesis of equal molar ratio of Pt/Pd bimetallic nanoparticles in EG (Pt/Pd (1/1)_EG) can be described as the following: Dissolve 555.7 mg PVP in 50 ml EG to make a 100 mM polymer solution (PVP/EG), then, add 129.47 mg of H2PtCl6･6H2O to the PVP/EG solution. Next, add 0.1 mL HCl to 44.33 mg PdCl2 powder, then, pour 50 ml PVP/EG containing H2PtCl6 ･6H2O to PdCl2/HCl to finally make [Pt] = [Pd] =5 mM metallic solution. Similar method was utilized to synthesize Pt/Pd bimetallic nanoparticles in glycerol (Pt/Pd_gly.) by the replacement of EG with glycerol. To investigate the effect of Pt/Pd molar ratios on the size and internal structure of nanoparticles, the molar ratios of Pt to Pd were varied as 4/1, 1/1, and 1/4 with the total amount of both metals equal to 10 mM. In addition, in the case of the preparation of Pt/Rh bimetallic nanoparticles in EG (Pt/Rh_EG) and glycerol (Pt/Rh_gly.), the similar procedure was applied to produce the corresponding nanoparticles with various molar ratios of Pt/Rh. The reaction system used here was a commercially available multimode microwave apparatus (MICROSYNTH PLUS, Milestone General K.K.). MW irradiation was performed in a non-stop wave mode at 700 W (maximum power of 1000 W, 2.45 GHz) employed by internal fiber-optic probe under the ambient condition. A two-necked round bottom flask (500 mL) was put into the apparatus and a reflux condenser outside the microwave cavity was connected by a glass joint of the flask. The fiber-optic temperature probe was directly inserted into the reaction solution, and the reaction temperature was controlled by automatic adjusting of microwave power. The solution was heated to the temperature of 471 K (in EG) and 523 K (in glycerol) under MW irradiation for about 2-3 min, then kept the temperature for about 10 min with continuous stirring using a magnetic stir bar. After being cooled to room temperature, the colloidal dispersions were obtained for the TEM and EXAFS measurements.



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Batch Type Method Using Single-Mode MW Irradiation. The preparation of bimetallic nanoparticles using the single-mode MW irradiation, 5-7 mL of the reaction solution containing metallic ion and PVP was loaded in a 10 mL sealed glass tube, and placed in a single-mode microwave apparatus (Discover SP, CEM Corporation) at 300 W under 2.45 GHz. The internal IR sensor was placed into the bottom of the instrument, and monitored and controlled the reaction temperatures. The experimental solution was then heated to 471 K (in EG) and to 523 K (in glycerol) for about 2-3 min. After reaching the desired temperature, it was maintained for about 10 min. After cooling the solution to room temperature naturally, the obtained colloidal dispersions were used for the HRTEM and EXAFS measurements. For further characterizations by means of the inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), the nanoparticles were precipitated by adding acetone into the colloidal dispersions followed by centrifugation with a rotational speed of 5000 rpm for 5 min. Then, methanol was poured into the precipitate to re-disperse the nanoparticles. Finally, acetone was added into the dispersion followed by centrifugation with a rotational speed of 5000 rpm for 3 min. Continuous-Flow Type Method Using Single-Mode MW Irradiation. The colloidal dispersions of bimetallic nanoparticles were prepared in continuous-flow type reactor system36 (see Figure S1) employing single-mode microwave apparatus. The mixture of metallic ion and PVP solution was pumped by a high pressure liquid chromatography pump (HPLC pump, JASCO Co., PU-1580) into a continuous-flow type reactor with 10 mL inner volume (supplied by CEM). The temperature of solution was monitored and controlled by an internal IR sensor. It should be noted that the flow type reactor system was demonstrated to be an efficient method to maximize the reaction time. The system pressure was monitored by a back-pressure regulator (JASCO, SCF-BPG). As for this experiment, the flow rate of the HPLC pump was set to 1 mL/min. The experimental solution passed through the flow type reactor was cooled down by a water/ice bath immediately, and then the colloidal dispersions was obtained without filtering for characterizations. Characterization. HRTEM was conducted by a JEOL JEM-2200FS microscopy with a working voltage of 200 kV. HAADF-STEM imaging and EDS elemental mapping were conducted 6 ACS Paragon Plus Environment

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by a JEOL ARM-200F microscopy, which equipped with a spherical aberration corrector operated at 200 kV with a resolution of 0.8 Å. For the preparation of samples to be characterized by HRTEM and HAADF-STEM, one can put a drop of colloidal dispersions onto the copper grid with carbon coated and then dry at ambient temperature. At least 200 nanoparticles of the photograph were measured, and then the average diameter and size distributions of the samples were obtained. ICPOES analysis was carried out on a Shimadzu ICPS-7000. XRD was performed by X-ray diffractometer (Rigaku MiniFlex600) at room temperature equipped with Cu K radiation (1.5418 Å). XPS analysis was operated by a high-performance XPS system (Shimadzu Kratos Axis-Ultra DLD) using monochromated Al K radiation. The EXAFS measurements were carried out at the beamline 9C of Photon Factory (PF) and the beamline NW10A of Photon Factory Advanced Ring (PF-AR), in High Energy Accelerator Research Organization (KEK, Japan). At beamline 9C, a Si(111) single crystal monochromator was equipped that were detuned to 70% of the intensity to remove higher harmonics. Pt L3-edge EXAFS data of the Pt/Pd and Pt/Rh bimetallic nanoparticles and their reference samples were recorded in transmission mode using ionization chambers as the detectors at room temperature. At beamline NW10A, a Si(311) double crystal was used as monochromator. Pd and Rh K-edge EXAFS data of these bimetallic nanoparticles were recorded in transmission mode with the detector of ionization chambers. The data of the corresponding metal foils was acquired for energy calibration. The colloidal dispersions were placed into different optical path length glass cells (10 mm for Pt L3-edge and 50 mm for Pd and Rh K-edges) sealed with 50 m thickness polyimide film (KAPTON-200H). EXAFS spectra were provided by the step scanning mode around the respective edge. The detailed method for EXAFS measurements can be found elsewhere.11,14 The EXAFS data analysis was obtained by commercially available REX2000 software package (Rigaku Co.). A cubic spline method was applied for background subtraction and the height of the edge was employed for spectra normalization. Furthermore, from k space to R space, the Fourier transformation (FT) of the k3-weighted EXAFS data was collected with a Hanning function window spanning from 30 to 160 nm-1 for k range. In the curve-fitting process, we employed empirically 7 ACS Paragon Plus Environment


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derived parameters for the phase shift and the amplitude functions compared to foils of alloy either between Pt and Pd or Pt and Rh as references.37,38 Results and Discussion In order to systematically investigate the effect of MW irradiation protocol (multimode or single-mode) on the size and internal structure of various bimetallic nanoparticles in polyol synthesis (EG or glycerol), HRTEM, HAADF-STEM, EDS, and EXAFS measurements were used to investigate the bimetallic nanoparticles with the total metal concentration of 10 mM produced in the presence of PVP. Batch Type Method Using Multimode MW Irradiation. Pt/Pd Bimetallic Nanoparticles. The bimetallic nanoparticles of Pt/Pd were prepared by using batch-type reactor system under multimode MW irradiation. A representative set of TEM images for the Pt/Pd (1/1) bimetallic nanoparticles and corresponding size distributions in diameter was shown in Figure 1(a) and 1(b) prepared in EG and glycerol solvent, respectively. Although both of their shapes are similar to spherical and their size distributions are roughly ranging from 3 to 12 nm, the nanoparticle dispersity in EG solvent is relatively higher than that in glycerol solvent. The average diameters of Pt/Pd (1/1) prepared in EG and glycerol are 6.2 nm and 9.3 nm, respectively.


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(a) A.D. = 6.2 nm

(b) A.D. = 9.3 nm

Figure 1. TEM images for the Pt/Pd (1/1) bimetallic nanoparticles and corresponding size distributions in diameter prepared by multimode MW irradiation through batch-type method: (a) EG solvent, (b) glycerol solvent.

Figure 2 shows the FT-EXAFS spectra of Pt/Pd bimetallic nanoparticles at edges of both Pt-L3 and Pd-K with different Pt/Pd molar ratio of 1/0, 4/1, 1/1, 1/4, and 0/1. In the Pt-L3 edge spectra (Figure 2(a)), the height of peak located around 2.6 Å is reduced with a decrease of the molar ratio of Pt atoms. Particularly, because of the contribution of Pt-Pd and Pt-Pt bond in a particle,11,14,37,38 9 ACS Paragon Plus Environment


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the split peaks are clearly observed between 2 and 3 Å at the Pt/Pd ratio of 1/1. On the other hand, in the Pd-K edge spectra (Figure 2(b)), the similar trend is observed as in the case of Pt-L3 edge, that is, the height of peak located around 2.5 Å decreases and the shoulder peak around 2.2 Å appears with a decrease of the molar ratio of Pd atoms. This indicates that Pt/Pd bimetallic nanoparticles were produced in EG solvent combined with PVP. Furthermore, Fourier transforms of EXAFS spectra for the Pt/Pd (1/1) colloidal dispersions in glycerol solvent are shown in Figures S2(a) and S2(b) (see Supporting Information). It is found that split peaks appear between 2 and 3 Å both in the Pt-L3 and Pd-K edge, indicating the formation of Pt-Pd bond in the bimetallic nanoparticles.

(a)


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(b)

Figure 2. FT-EXAFS spectra of Pt/Pd bimetallic nanoparticles at Pt/Pd ratio = 1/0, 4/1, 1/1, 1/4, and 0/1 prepared in batch-type reactor under multimode MW irradiation in EG: (a) at the Pt-L3 edge; (b) at the Pd-K edge.

The data achieved for the Pt-L3 edge and Pd-K edge were Fourier-filtered over 1.9－3.1 Å and 1.7－3.0 Å, respectively. Then, a two-shell curve-fitting including Pt-Pt and Pt-Pd as well as Pd-Pd and Pd-Pt were utilized to analyze Pt/Pd bimetallic nanoparticles. The structural parameters including coordination number (C.N.), bond distances (r), energy shift (E), and Debye-waller factor () achieved by above method for a series of the Pt/Pd bimetallic nanoparticles prepared in EG solvent under the multimode MW irradiation are listed in Table 1. As for the Pt/Pd (1/1) bimetallic nanoparticles, the C.N. of the Pt-Pt and Pt-Pd bond of bimetallic nanoparticles are 7.7 and 2.7 in the Pt-L3 edge, respectively, and the average C.N. at this edge is 5.2. One the other hand, the C.N. of the Pd-Pd and Pd-Pt bond for the same bimetallic nanoparticles are 7.3 and 4.2 in the Pd-K edge, respectively. The average C.N. is 5.75 at this Pd-K edge. Hence, we can derive the average C.N. of Pt/Pd (1/1) bimetallic nanoparticles equal to 10.95. 11 ACS Paragon Plus Environment


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According to the relation between the average C.N. and particle diameter, we can roughly estimate the diameter of the bimetallic nanoparticles prepared in EG is expected to be around 4.0 nm from the average C.N. of 10.95. The diameters for these bimetallic nanoparticles are not consistent with those estimated from TEM observation, implying that there is an aggregation between the bimetallic nanoparticles. In the Pd-K edge spectra, the C.N. values of Pd-Pt bond become lower when the Pt molar ratio decreased from Pt/Pd (4/1) to Pt/Pd (1/4) through Pt/Pd (1/1). This is a reason of fewer reduced Pt atoms around the central Pd. Furthermore, as for the Pt/Pd (4/1) bimetallic nanoparticles, there is no contribution of the PtPd bond at the Pt-L3 edge in Table 1. It is expected that the larger nanoparticles of Pt were formed because of the higher molar ratio of Pt in the systems. A few smaller and well-dispersed Pd nanoparticles were embedded in these larger Pt nanoparticles. This means that the Pt-Pd bond was minority in the system. Hence, it was difficult to observe the contribution of Pt-Pd bond even if bimetallic nanoparticles formed. On the contrary, as for the Pt/Pd (1/4) bimetallic nanoparticles, structural parameters of them could be detected because larger Pd nanoparticles might not be formed despite the higher molar ratio of Pd. Similarly, as for the bimetallic nanoparticles of Pt/Pd (1/1) prepared in glycerol, the average C.N. is regarded as 12.05 by the same consideration above described in Table S1 (see Supporting Information).


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Table 1. Structural Parameters of Pt/Pd Bimetallic Nanoparticles Obtained from EXAFS Analysis Prepared in EG Employing Batch-Type Reactor under Multimode MW Irradiation. Sample

Edge

Bond

C.N.a

r/Å

E / eV

/Å

R / %b

Pt/Pd (1/0)

Pt-L₃

Pt-Pt

10.4

2.76

-1.020

0.066

0.108

Pt-Pt

11.4

2.77

3.429

0.080

Pt/Pd (4/1)

Pt-L₃ Pt-Pd

-

-

-

-

Pd-Pd

5.0

2.77

0.696

0.065

Pd-Pt

6.8

2.75

-0.380

0.065

Pt-Pt

7.7

2.74

-2.495

0.071

Pt-Pd

2.7

2.75

-1.600

0.059

Pd-Pd

7.3

2.77

-0.815

0.073

Pd-Pt

4.2

2.74

-1.720

0.070

Pt-Pt

4.0

2.75

-2.459

0.060

Pt-Pd

6.9

2.77

0.500

0.060

Pd-Pd

8.8

2.76

-1.048

0.067

Pd-Pt

2.6

2.75

3.376

0.067

Pd-Pd

10.7

2.74

-0.757

0.065

2.430

Pd-K

Pt/Pd (1/1)

4.315

0.333

Pt-L₃

Pd-K

Pt/Pd (1/4)

0.255

0.717

Pt-L₃

Pd-K

Pt/Pd (0/1)

Pd-K

a

Coordination numbers.

b

The R factor, see ref 36.

0.185

0.273

Pt/Rh Bimetallic Nanoparticles. As for the Pt/Rh (1/1) bimetallic nanoparticles, the bimetallic nanoparticles are well dispersed both in EG and glycerol, but the distributions of the diameter are narrow to some extent (Figure 3(a) and 3(b)). The average diameter of bimetallic nanoparticle is nearly the same, i.e., 4.2 nm and 4.0 nm, respectively. 13 ACS Paragon Plus Environment


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(a) A.D. = 4.2 nm

(b) A.D. = 4.0 nm

Figure 3. TEM images for the Pt/Rh (1/1) bimetallic nanoparticles and corresponding size distributions in diameter prepared by multimode MW irradiation through batch-type method: (a) EG solvent, (b) glycerol solvent.

Fourier transforms of EXAFS spectra containing Pt-L3 and Rh-K edge for Pt/Rh bimetallic nanoparticles at different molar ratios are shown in Figure 4. As for the Pt/Rh (1/1) bimetallic


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nanoparticles, there is a split peak between 2 and 3 Å at the Pt-L3 edge and Rh-K edge, which confirms the formation of bimetallic nanoparticles in the EG solvent. After MW irradiation, the peaks of Pt/Rh (4/1) and Pt/Rh (1/4) between 2 and 3 Å are both split at the edge of Pt-L3. This implies that, with the increasing of Rh molar ratio, the phase remarkably changes because the strength of contribution from Pt-Pt bond decreases, meanwhile the strength of contribution from PtRh bond increases. Similarly, at the edge of Rh-K, the main peak around 2.4 Å becomes split into two while the Pt molar ratio was increased, especially in the case of Pt/Rh (1/1) and Pt/Rh (4/1). Moreover, in the case of glycerol solvent, Fourier transforms of EXAFS spectra for Pt/Rh (1/1) bimetallic nanoparticles are shown in Figures S2(c) and S2(d) (see Supporting Information). These spectra strongly indicate the formation of Pt-Rh bond of the bimetallic nanoparticles.

(a)



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(b)

Figure 4. FT-EXAFS spectra of Pt/Rh bimetallic nanoparticles at Pt/Rh ratio = 1/0, 4/1, 1/1, 1/4, and 0/1 prepared in batch-type reactor under multimode MW irradiation in EG: (a) at the Pt-L3 edge; (b) at the Rh-K edge.

Table 2 shows the structural parameters (C.N., r, E and ) of a series of the Pt/Rh bimetallic nanoparticles prepared in EG solvent by means of the multimode MW irradiation. These parameters are determined the same as previous method in Pt/Pd case mentioned above. At the Pt-L3 edge, the C.N. of the Pt-Rh bond of Pt/Rh (4/1), Pt/Rh (1/1), and Pt/Rh (1/4) nanoparticles are 1.8, 5.1, and 7.9, respectively. The values of C.N. increase when the concentration of Rh becomes higher. Similarly, in the Rh-K edge spectra, the C.N. values of Rh-Pt bond of Pt/Rh (4/1), Pt/Rh (1/1), and Pt/Rh (1/4) nanoparticles decrease with the higher concentration of Rh. In the similar trend as is observed in Table1, the hetero atoms around the absorbing atom (Pt-Rh bond at Pt-L3 edge, Rh-Pt bond at Rh-K edge) changed with its concentration. The C.N. of the Pt/Rh (1/1) nanoparticles 16 ACS Paragon Plus Environment

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prepared in EG are 6.3 (Pt-Pt contribution) and 5.1 (Pt-Rh contribution) at the Pt-L3 edge, and 4.1 (Rh-Rh contribution) and 6.9 (Rh-Pt contribution) at the Rh-K edge. As a result, the average C.N. is calculated as 11.2. The same consideration described above for Pt/Pd can derive the average C.N. of Pt/Rh (1/1) bimetallic nanoparticles in glycerol as 11.4 showing in Table S1 (see Supporting Information).

Table 2. Structural Parameters of Pt/Rh Bimetallic Nanoparticles Obtained from EXAFS Analysis Prepared in EG Employing Batch-Type Reactor under Multimode MW Irradiation. Sample

Edge

Bond

C.N.a

r/Å

E / eV

/Å

R / %b

Pt/Rh (1/0)

Pt-L₃

Pt-Pt

10.4

2.76

-1.020

0.066

0.108

Pt-Pt

9.2

2.76

-1.027

0.063

Pt/Rh (4/1)

Pt-L₃ Pt-Rh

1.8

2.75

3.886

0.074

Rh-Rh

0.7

2.71

-9.282

0.070

Rh-Pt

7.1

2.71

-1.162

0.070

Pt-Pt

6.3

2.75

-0.632

0.056

Pt-Rh

5.1

2.73

0.044

0.079

Rh-Rh

4.1

2.70

0.455

0.076

Rh-Pt

6.9

2.70

-1.648

0.068

Pt-Pt

3.3

2.71

-8.741

0.050

Pt-Rh

7.9

2.70

-2.588

0.066

Rh-Rh

7.9

2.69

0.159

0.071

Rh-Pt

3.9

2.69

-0.638

0.062

Rh-Rh

10.3

2.68

-0.659

0.064

0.125

Rh-K

Pt/Rh (1/1)

5.386

0.118

Pt-L₃

Rh-K

Pt/Rh (1/4)

0.800

0.260

Pt-L₃

Rh-K

Pt/Rh (0/1)

Rh-K

a


b


0.290


0.111


Page 18 of 38

Batch Type Method Using Single-Mode MW Irradiation. Pt/Pd Bimetallic Nanoparticles. MW-assisted bimetallic nanoparticle synthesis was carried out in batch-type reactor under singlemode MW irradiation. As shown in Figure 5, HRTEM image combined with electron diffraction pattern of the Pt/Pd (1/1) bimetallic nanoparticles conﬁrms a majority of spherical-like cubic nanoparticles with well-dispersion. The average particle diameter is 7.1 nm, and the pattern of Pt/Pd (1/1) bimetallic nanoparticles consists of sharp diffraction lines which show [111] and [200] reflections, on the contrast, [220] and [311] of the fcc (face-centered cubic) lattice are reflected by weak diffraction lines. This is the same as the monometallic Pt and Pd nanoparticles shown in Figures S3(a) and S3(b) (see Supporting Information). XRD pattern of the Pt/Pd (1/1) bimetallic nanoparticles is shown in Figure S4(a), indicating that this sample has several peaks which can be related to diffraction lines of the fcc structure for [111], [200], [220], [311], and [222]. The [111] peak is located between Pt (fcc) and Pd (fcc) suggesting target Pt-Pd alloy. The particle size is

A.D. = 7.1 nm

11/nm

Figure 5. HRTEM image and electron diffraction pattern for Pt/Pd (1/1) bimetallic nanoparticles and corresponding size distribution in diameter prepared by single-mode MW irradiation through batch-type method in EG solvent. obtained as 6.9 nm from the XRD data with the Scherrer equation for the [111] peak. It is obvious 18 ACS Paragon Plus Environment

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that this size is almost identical to that from HRTEM observation, indicating that the nanoparticles are monocrystalline. Figures S5(a) and S5(b) present Fourier transforms of EXAFS spectra of Pt/Pd (1/1) colloidal dispersions above Pt-L3 and Pd-K edge (see Supporting Information), respectively. There are clearly split peaks between 2 and 3 Å, this strongly confirms the formation of Pt-Pd bond in the bimetallic nanoparticles. The structural parameters of a series of the Pt/Pd bimetallic nanoparticles prepared in EG employed by single-mode MW irradiation are listed in Table S2 (see Supporting Information). In the Pt-L3 edge spectra, no contribution of the Pt-Pd bond of Pt/Pd (4/1) was observed due to the fact that well-dispersed Pd nanoparticles were embedded in these larger nanoparticles of Pt. As for the Pt/Pd (1/1) and Pt/Pd (1/4), the C.N. value of Pt-Pd bond is 5.3 and 7.6, respectively. These results demonstrate a higher Pd concentration leads to higher C.N. value of Pt-Pd bond. One the other hand, the C.N. values of Pd-Pt bond in Pt/Pd (4/1), Pt/Pd (1/1) and Pt/Pd (1/4) bimetallic nanoparticles are 9.0, 6.2 and 2.4, respectively. The result indicates that the lower Pd concentration leads to higher C.N. value of Pd-Pt bond above Pd-K edge. In addition, the average C.N. of Pt/Pd (1/1) bimetallic nanoparticles can be calculated as 12.8.

Pt/Rh Bimetallic Nanoparticles. HRTEM image and electron diffraction of the Pt/Rh (1/1) bimetallic nanoparticles is shown in Figure 6, and it conﬁrms a majority of spherical-like cubic nanoparticles with well-dispersion. The diameter range of these nanoparticles is from 3 nm to 10 nm, and the average diameter of them is 5.4 nm. The electron diffraction pattern has sharp in [111] and [220] reflections and it has weak in [220] and [311] of the fcc lattice, as can be seen from Figures S3(a) and S3(c) of monometallic Pt and Rh nanoparticles (see Supporting Information). Figure S4(b) present XRD pattern of the Pt/Rh (1/1) bimetallic nanoparticle. The XRD pattern is nearly the same as the electron diffraction pattern, showing the fcc structure of the Pt/Rh alloy nanoparticles. The particle size is estimated as 5.0 nm from the XRD data by means of the Scherrer equation for the [111] peak. Moreover, the peak between 2 and 3 Å splits into two indicates the formation of PtRh bond (Figures S5(c) and S5(d)). 19 ACS Paragon Plus Environment


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A.D. = 5.4 nm

Figure 6. HRTEM image and electron diffraction pattern for Pt/Rh (1/1) bimetallic nanoparticles and corresponding size distribution in diameter prepared by single-mode MW irradiation through batch-type method in EG solvent. As shown in Table S3 (see Supporting Information), the C.N. values of Pt-Rh bond at Pt-L3 edge and Rh-Pt bond at Rh-K edge have a similar trend with the case of Pt/Pd bimetallic nanoparticles. In the case of EG solvent, the C.N. values of Pt/Rh (1/1) bimetallic nanoparticles are 7.1 and 3.8 at the Pt-L3 edge, and 4.0 and 7.6 at the Rh-K edge. So the average C.N. of bimetallic Pt/Rh (1/1) nanoparticles in EG can be calculated as 11.3. It is also found that the C.N. values of PtRh are 2.5 and 7.5 and that of Rh-Pt are 11.0 and 4.7 in the cases of Pt/Rh (4/1) and Pt/Rh (1/4). On the basis of our findings, it is recognized that the C.N. values of Pt-Rh become higher as Pt molar ratio decreases and/or Rh molar ratio increases. Also, the C.N. values of Rh-Pt become lower with the decrease of Pt concentration. These are consistent with the results already shown in Tables 1 and 2.

Continuous-Flow Type Method Using Single-Mode MW Irradiation. Images of TEM and histograms of Pt/Pd (1/1) and Pt/Rh (1/1) are visualized in Figure 7. Through flow-type method, the bimetallic nanoparticles were prepared in EG under the single-mode MW irradiation. The average 20 ACS Paragon Plus Environment

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diameters are 5.9 nm for Pt/Pd (1/1) (Figure 7(a)) and 2.7 nm for Pt/Rh (1/1) (Figure 7(b)) with a better dispersion, respectively. These suggest that a smaller nanoparticle size can be obtained in continuous-flow processing than in batch-type processing, compared with Figures 5 and 6. These results indicate that the aggregations hardly occur in continuous-flow processing.

(a) A.D. = 5.9 nm

(b)

A.D. = 2.7 nm

Figure 7. TEM images for the bimetallic nanoparticles and corresponding size distributions in diameter prepared by single-mode MW irradiation through flow-type method in EG: (a) Pt/Pd (1/1) and (b) Pt/Rh (1/1). The EXAFS spectra of Pt/Pd (1/1) prepared in flow-type method under single-mode MW irradiation are shown in Figures S6(a) and S6(b) (see Supporting Information). In the Pt-L3 edge 21 ACS Paragon Plus Environment


Page 22 of 38

spectra, the peak is clearly split into two ranging between 2 and 3 Å, this strongly demonstrating the formation of Pt-Pd bond in a Pt/Pd (1/1) bimetallic nanoparticle. Even after the MW irradiation there is almost one peak in the FT spectra of the Pd monometallic nanoparticles above Pd-K edge. Although the height of peak between 2 and 3 Å is reduced, the peak has a trend of the appearance of shoulder peak around 2.1 Å, suggesting the generation of bimetallic Pt/Pd (1/1) nanoparticles. On the other hand, the EXAFS spectra of Pt/Rh (1/1) at Pt-L3 edge and Rh-K edge are shown in Figures S6(c) and S6(d), respectively (see Supporting Information). After MW irradiation, there is a split peak between 2 and 3 Å both above Pt-L3 edge and Rh-K edge, which indicates the formation of bimetallic Pt/Rh (1/1) nanoparticles. From Table 3, in the Pt-L3 edge spectra, the C.N. of Pt-Pt and Pt-Pd are 8.6 and 3.3, respectively. As for the Pd-K edge, the C.N. of Pd-Pd is 7.4, the C.N. of Pd-Pt is 6.8. Similarly, we can calculate the average C.N. of Pt/Pd (1/1) bimetallic nanoparticles is 13.05, which might indicate some error bar in C.N. In addition, in the Pt-L3 edge and Rh-K edge spectra, the C.N. values from contributions of Pt-Pt, Pt-Rh, Rh-Rh and Rh-Pt are 6.0, 3.4, 3.8 and 8.0, respectively, resulting in the average C.N. of 10.6 for Pt/Rh(1/1). These average CNs closely reflects the particle size obtained by TEM.


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Table 3. Structural Parameters of Bimetallic Nanoparticles Obtained from EXAFS Analysis Prepared in EG Employing Continuous-Flow-Type Reactor under Single-mode MW Irradiation Sample Pt/Pd (1/1)_EG

Edge Pt-L3 Pd-K

Pt/Rh (1/1)_EG

Pt-L3 Rh-K

a


b


Bond Pt-Pt Pt-Pd Pd-Pd Pd-Pt

C.N.a 8.6 3.3 7.4 6.8

r/Å 2.76 2.75 2.78 2.75

E / eV -1.429 -1.245 0.395 -0.173

/Å

R / %b

0.056 0.043 0.077 0.088

1.315

Pt-Pt Pt-Rh Rh-Rh Rh-Pt

6.0 3.4 3.8 8.0

2.76 2.76 2.70 2.69

1.429 7.336 2.869 -0.818

0.056 0.068 0.072 0.076

1.189

0.553

1.373

Structural Model of the Bimetallic Nanoparticles. Bimetallic nanoparticles contain two kinds of metal elements, and the crystal structure of them is similar to the bulk alloy. There has been increasing use of XAFS to characterize bimetallic nanoparticles, revealing several possibility for the two metals to be distributed within a nanoparticle, for example, random alloy,6,39 core-shell,40,41 cluster-in-cluster,14,38,42 and separated model. In the random alloy structure, two kinds of metal elements form nanoparticle irregularly. In the core-shell structure, two different metal elements form an inner core and a shell, respectively. In the cluster-in-cluster structure, the nanoparticle is made up of nanoclusters which are formed by two different metal elements, respectively. In the separate structure, one metal element forms a half part of the nanoparticle and other metal element forms another half. Figure 8 presents the schematic models of the bimetallic nanoparticles. To estimate the atomic compositions and the internal location of atoms in a particle,43,44 we further examined the HAADF-STEM images and the EDS elemental mapping images of equal molar ratio of Pt/Pd and Pt/Rh (Figure 9 and Figure 10). The atomic compositions of the Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles detected by ICP-OES, XPS, and EDS are summarized in Table S4 (see Supporting Information). As for the Pt/Pd (1/1) bimetallic nanoparticles, the atomic 23 ACS Paragon Plus Environment


Page 24 of 38

ratio of Pt to Pd is relatively higher in case of XPS than in case of ICP-OES, suggesting that having more Pt atoms being located at the surface than Pd. As can be seen from Figure 9(a), HAADFSTEM image shows that the contrast of the particles is almost uniform in the entire area of the particles, implying the formation of uniform alloyed nanoparticles. Figure 9(b) to 9(d) shows the EDS elemental mapping images for Pt, Pd, and the overlaid image, respectively. Pt and Pd elements are almost uniformly distributed, but partially Pt rich at the surface, within a particle. This is consistent with the XPS result. Figure 9(e) shows the EDS line profile, indicating that the Pt signal is slightly larger at the surface than the Pd signal. According to EDS data, the atomic ratio of Pt to Pd is determined as Pt/Pd = 50/50. Moreover, for the XPS spectra (Figures S7(a) and S7(c)), both Pt 4f and Pd 3d binding energies increased by 2 eV, in contrast to those of bulk Pt and Pd metals. The initial and final state effects contribute to these changes.45 Concerning to the elucidation of the internal structure of Pt/Pd (1/1) bimetallic nanoparticles, the structural parameters shown in Table 1 should be carefully considered. As is already described above, the average C.N. of the Pt/Pd (1/1) bimetallic nanoparticles prepared in EG is 10.95, indicating an approximate particle diameter is about 4.0 nm. In general, it is well known that, if a particle possessing ca. 4.0 nm in diameter is consisted of centered six atoms and six layered structures containing total 923 atoms (i.e., the diameter of a particle is composed of 13 atoms as shown in Figure 8, consisting of 461 Pt atoms and 462 Pd atoms.), the random structure has all C.N.s of the bonds (Pd-Pd, Pd-Pt, Pt-Pt, and Pt-Pd) equal to 5.2. In the core-shell structure model, the C.N. of Pd-Pd and Pd-Pt above Pd-K edge is estimated 6.7 and 2.0, respectively, and that of PtPt and Pt-Pd in the Pt-L3 edge is estimated 10.0 and 2.0, respectively. In the cluster-in-cluster structure model, the C.N. of Pd-Pd and Pd-Pt above Pd-K edge is 5.8 and 4.0, respectively, and the C.N. of Pt-Pt and Pt-Pd in the Pt-L3 edge is 7.1 and 4.0, respectively. Furthermore, in the separate model structure, the C.N.s estimated from Pd-K edge and Pt-L3 edge are 9.6 (Pd-Pd and Pt-Pt) and 0.7 (Pd-Pt and Pt-Pd), respectively. Based on these models, the C.N. of the Pt/Pd (1/1) bimetallic nanoparticles (experimental values) is found to be very similar to that (calculated values) of the


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cluster-in-cluster model structure within an error bar. Therefore, on the basis of the EDS elemental mapping and EXAFS results, we can deduce that the Pt/Pd (1/1) bimetallic nanoparticles prepared in EG have the cluster-in-cluster structure (as shown in Tables 1 and S2).

(a)

(b)

(c)

(d)

Bond

Random (a)

Core-Shell (b)

Cluster-in-Cluster (c)

Separate (d)

MA-MA

5.2

6.7

5.8

9.6

MA-MB

5.2

2.0

4.0

0.7

MB-MB

5.2

10.0

7.1

9.6

MB-MA

5.2

2.0

4.0

0.7

Figure 8. The schematic model: (a) random, (b) core-shell, (c) cluster-in-cluster and (d) separated model for 6 layered nanoparticle consisting of 923 atoms.



(a)

(d)

(b)

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(c)

(e) (f)

Figure 9. (a) HAADF-STEM image, (b) Pt M-edge, (c) Pd L-edge, (d) overlay of EDS elemental mapping image, (e) EDS line profile along with the red line in (a), and (f) EDS spectra for the Pt/Pd (1/1) prepared in batch-type method in EG undersolvent by means of the single-mode MW irradiation.

The C.N. values of the Pt/Pd (1/1) bimetallic nanoparticles prepared in glycerol (Table S1) and prepared by continuous-flow type in EG (Table 3) are very similar to those prepared in EG. Hence the structure of Pt/Pd (1/1) is also cluster-in-cluster. Therefore, we can conclude that the internal construction of Pt/Pd (1/1) bimetallic nanoparticles prepared in batch type and continuous-flow type method under MW irradiation is cluster-in-cluster structure. On the other hand, as for the Pt/Rh (1/1) bimetallic nanoparticles, the molar ratio of Pt and Rh is very similar in all the cases of ICP-OES, XPS, and EDS. This suggests the formation of uniform alloyed Pt/Rh nanoparticles. The EDS elemental mapping images (Figure 10(b) to Figure 10(d)), EDS line profile (Figure 10(e)) as well as the HAADF-STEM image (Figure 10(a)) deduced the fact 26 ACS Paragon Plus Environment

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(a)

(d)

(b)

(c)

(e) (f)

Figure 10. (a) HAADF-STEM image, (b) Pt M-edge, (c) Rh L-edge, (d) overlay of EDS elemental mapping image, (e) EDS line profile along with the red line in (a), and (f) EDS spectra for the Pt/Rh (1/1) prepared in batch-type method in EG under single-mode MW irradiation. that Pt and Rh elements were uniformly distributed in a particle and the averaged atomic ratio is determined as Pt/Rh = 59±3 / 41±3. As shown in Figures S7(b) and S7(d), XPS spectra show that both Pt 4f and Rh 3d binding energies decreased by 1 eV, compared with those of bulk Pt and Rh metals. This is due to the fact that the electron transfer from the C=O group in PVP to Pt (or Rh) takes place in PVP-coated Pt/Rh nanoparticles when the diameter of nanoparticles is less than 7 nm.46 Here, the internal structure in a particle can be considered based on the structural parameters from EXAFS, as shown in Tables 2 and S3. The C.N. of the cluster-in-cluster model structure corresponds well with that of Pt/Rh (1/1) prepared in our study within an error. Therefore, we conclude that the model structure of Pt/Rh (1/1) bimetallic nanoparticles prepared in glycerol is cluster-in-cluster (Table S1) and prepared by continuous-flow type in EG (Table 3). The structure 27 ACS Paragon Plus Environment


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for Pt/Pd (1/1) bimetallic nanoparticles was proposed to be the same model of Pt/Rh (1/1) bimetallic nanoparticles. The particle size and the internal structure are summarized in Table 4 for the comparison between Pt/Pd and Pt/Rh bimetallic nanoparticles employing different MW synthesis systems.

Table 4. Pt/Pd and Pt/Rh Prepared in EG under MW Irradiation. Batch-Type Sample Molar Ratio

Multimode

Single-mode

Size / nm Structure

Size / nm

Structure

Size / nm

Structure

7.1

cluster-incluster cluster-incluster

5.9


a

Pt/Pd

1/1

6.2(9.3 )

Pt/Rh

1/1

4.2 (4.0a)

a

Continuous-Flow


5.4

2.7

prepared in glycerol solvent

Moreover, the structures of Pt/Pd (4/1) and Pt/Rh (4/1) bimetallic nanoparticles were proposed by the coordination numbers of them. The Pt atom has a higher molar ratio in these cases, leading to the formation of larger Pt nanoparticles. A few and well-dispersed smaller Pd (Rh) nanoparticles were embedded in these larger Pt nanoparticles. We assume that they have a cluster-in-cluster like structure, in other words, they consist of a fewer and smaller Pd (Rh) microclusters located on the surface of larger Pt clusters (core). On the contrary, in the Pt/Pd (1/4) and Pt/Rh (1/4) bimetallic nanoparticles cases, the structure is a particle with fewer and smaller Pt microclusters deposited on the surface of larger Pd (Rh) clusters (core). The internal structure is summarized in Table 5 for the comparison between Pt/Pd and Pt/Rh bimetallic nanoparticles with different molar ratios.


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Table 5. Pt/Pd and Pt/Rh Prepared by Batch-Type Method in EG under MW Irradiation. Sample Pt/Pd

Pt/Rh

Molar Ratio

Structure Multimode

Single-mode

4/1

CIC-Aa

CIC-Aa

1/4

CIC-Bb

CIC-Bb

4/1

CIC-Aa

CIC-Aa

1/4

CIC-Bb

CIC-Bb

a

fewer and smaller Pd (Rh) microclusters deposited on the surface of larger Pt clusters (core). CIC is the abbreviation of cluster-in-cluster. b

fewer and smaller Pt microclusters deposited on the surface of larger Pd (Rh) clusters (core).

Finally, the role of polymer protection is discussed on the basis of the XPS spectra. Figure S8 shows the XPS spectra of C 1s region in Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles prepared in single-mode MW method using batch-type reactor (see Supporting Information). XPS spectrum of C1s has been fitted by multiple Gaussians. In the case of Pt/Pd (Figure S8(a)), five peaks at 284.6, 285.6, 287.0, 288.3 and 289.8 eV are seen. Similar peaks are also detected in the case of Pt/Rh (Figure S8(b)). Those peaks are assigned to C–C, C–N, C–O, C=O and O–C=O, respectively.47 Contrast to the reference binding energies of 286.6 eV for C–O and 287.6 eV for C=O, the C1s peaks for C–O and C=O moved to higher binding energy for both bimetallic nanoparticles, which is presumably because of the coordinate bond between PVP and nanoparticles. However, the intensity of the C1s peaks for C–O, C=O, and O–C=O is much stronger in the case of Pt/Pd than Pt/Rh (Figure S8(a)), revealing that Pt/Pd (1/1) bimetallic nanoparticles seem to contain more amount of hydroxyacetic acid or oxalic acid at the particle surface, which might be generated from the oxidation of EG during the reaction. In other words, it means that Pt/Rh (1/1) bimetallic nanoparticles tend to be more surrounded by PVP than Pt/Pd (1/1) bimetallic nanoparticles. This might be related to the fact that Rh atom is more easily coordinated to PVP than Pd atom, leading to larger aggregation of Rh particles in PVP solutions rather than those of Pd particles due to the entanglement of polymer chains.11,14,48 29 ACS Paragon Plus Environment


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The reaction temperature and the heating rate of MW as well as a sort of the solvent (EG and glycerol) have significant effects on the structure and composition of bimetallic nanoparticles. In the case of bimetallic nanoparticles synthesized in other solvents without MW heating,49-52 the coreshell model was mostly applicable and reliable. However, in our present study, the cluster-in-cluster model was applied to Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles especially prepared in a high experimental temperatures. The combination of in situ EXAFS technique and the MW-assisted continuous-flow system would be useful for future studying bimetallic nanoparticles in the investigation of nucleation as well as growth kinetics.

Conclusion Bimetallic nanoparticles of Pt/Pd (1/1) and Pt/Rh (1/1) have been synthesized by MW heating in the presence of PVP, and their size, internal structure and morphology of various bimetallic nanoparticles prepared by different MW heating has been examined by means of HRTEM, HAADF-STEM, EDS elemental mapping and EXAFS. Preparation of bimetallic nanoparticles involves of both batch-type method and continuous-flow-type method, which can occur under the condition of microwave. In the multimode MW-assisted batch-type method, the average size of Pt/Pd (1/1) bimetallic nanoparticles (6.2 nm) prepared in EG was 3.1 nm smaller than that (9.3 nm) prepared in glycerol. This reveals that the solvent plays critical part in controlling particles size contrast to the preparation of Pt/Rh (1/1). In the single-mode MW-assisted batch-type method, the average size of Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles prepared in EG became larger than that in the multimode MW-assisted batch-type method. Furthermore, in the continuous-flow processing at 1 mL/min, smaller bimetallic nanoparticles were obtained than those in the batch processing. Analysis of the EXAFS, HAADF-STEM, and EDS elemental mapping gives us the fact that the internal structure of Pt/Pd (1/1) and Pt/Rh (1/1) nanoparticles is similar to cluster-in-cluster


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model. The MW-assisted heating proved highly effective preparation of different kinds of bimetallic nanoparticles protected by PVP.

Acknowledgments. We gratefully thank the Photon Factory Advisory Committee (PAC) (Proposal Nos. 2013G005 and 2015G027) at High Energy Accelerator Research Organization (KEK) for approval of EXAFS measurements. We are indebted to Koichi Higashimine of Japan Advanced Institute of Science and Technology (JAIST) for his support in performing HAADF-STEM measurements. We also thank Mari Takahashi, Ryoichi Kitaura, Chiko Shijimaya and Priyank Mohan of JAIST for their help with XPS, XRD and ICP-OES measurements. HRTEM observations were supported by Kyoto University microstructural characterization platform (KUMCP) as a program of “Nanotechnology Platform” (No. A-15-KT-0015) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX. Schematic drawing of the continuous-flow reactor system; Electron diffraction patterns of monometallic Pt, Pd and Rh nanoparticles; XRD and XPS spectra of Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles; Atomic composition of Pt/Pd (1/1) and Pt/Rh (1/1) bimetallic nanoparticles; Fourier-transforms of EXAFS spectra and the structural parameters of the Pt/Pd and



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Pt/Rh colloidal dispersions which were prepared in batch- or flow- type reactor in EG or glycerol under MW irradiation.

Author Information ORCID Cong Cong: 0000-0001-6464-0347 Shinya Maenosono: 0000-0003-2669-8219 Masafumi Harada: 0000-0003-1864-8641


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For Table of Contents Graphic

A.D. = 4.2 nm

A.D. = 6.2 nm

Pt/Pd(1/1) bimetallic nanoparticles synthesized in ethylene glycol

cluster-in-cluster structure

Pt/Rh(1/1) bimetallic nanoparticles synthesized in ethylene glycol

Microwave-assisted polyol syntheses of Pt/Pd and Pt/Rh bimetallic nanoparticles have been demonstrated by the use of ethylene glycol and glycerol as the reducing agents in the presence of PVP. From HRTEM, XPS, EDS, and EXAFS analysis, the “cluster-in-cluster” structure has been applicable for the model of bimetallic nanoparticles. A comparison between batch and continuous-flow processing has also been indicated.


Rh Bimetallic

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