Microwave-Assisted Polyol Synthesis of Polymer-Protected

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Microwave-assisted Polyol Synthesis of Polymer-protected Monometallic Nanoparticles Prepared in Batch and Continuous-flow Processing. Masafumi Harada, and Cong Cong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00991 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on May 4, 2016

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Industrial & Engineering Chemistry Research

Microwave-assisted Polyol Synthesis of Polymerprotected Monometallic Nanoparticles Prepared in Batch and Continuous-flow Processing Masafumi Harada*,† and Cong Congǂ †

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

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

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: Microwave-assisted polyol synthesis of monometallic Pd, Rh, Ru, and Pt nanoparticles has been demonstrated by the use of ethylene glycol and glycerol as a reducing agent in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). The size and morphology of the synthesized nanoparticles have been investigated by means of high-resolution transmission electron micrograph (HRTEM) and extended X-ray absorption fine structure (EXAFS). A comparison between batch scale-up and continuous-flow processing for the microwave-assisted polyol synthesis has been also indicated in terms of the size and morphology of various nanoparticles. In the single-mode MWassisted continuous flow processing (10 mL processing volume), the well-dispersed colloidal nanoparticles with a metal concentration equal to ~ 10 mM were successfully produced in sealed

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glass reactor at boiling temperature of the solvent at a flow rate of 1 mL/min. Furthermore, in the multi-mode MW-assisted batch processing, the colloidal nanoparticles appeared to be narrower size distribution in ethylene glycol than that in glycerol. The availability of the size regulation (i.e., particle size, size distribution) is discussed.

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

Introduction Metal nanoparticles have been used in many applications such as electronics, optics, catalysis, and sensing devices because of their unique functions as catalytic, magnetic, and electronic properties. Such properties are strongly dependent on their size, shape and impurities of metal nanopaticles. A lot of different methods of the synthesis of the metal nanoparticles have been utilized to control their size, size distribution, and morphologies effectively. One of the synthetic methods is a chemical synthesis of metal nanoparticles protected by polymer, surfactant and organic molecules in liquid phase.1-3 The polyol process4-8 is a convenient, versatile, and low-cost method for the synthesis of metal nanomaterials with various morphologies on a large scale. Current polyol synthesis is mainly based on the utilization of ethylene glycol (EG)4-6 or glycerol7,8 which plays an important role both as a solvent of the precursors and as a reducing reagent for the reaction. Microwave (MW) irradiation is one of the most promising techniques for the preparation of nanomaterials with controlled size and shape.9-13 It is well known that the microwave dielectric heating, which is caused by the interaction of the dipole moment of molecules under the highfrequency electromagnetic radiation, offers a promising method for the preparation of metallic nanoparticles because of its characteristics of rapid volumetric heating with shorter reaction time, higher reaction rate, selectivity and increase product yield, and energy savings. In the past decade,


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there has been increasing interest in the use of MW heating as compared to the conventional heating in various kinds of metallic nanoparticle syntheses.14-26 Among various solvents, polyols with high boiling points such as EG (boiling point ~ 198°C) and glycerol (boiling point ~ 290°C) are typical solvents for carrying out the microwave-assisted preparation in an open reaction system because of the high penetration depth of the microwave. For example, Komarneni et al.15 have used MW irradiation in combination with the polyol process to produce Pt and Ag nanoparticles, and the control of particle size and shape was achieved by adjusting the molecular weight of poly(N-vinyl-2pyrrolidone) (PVP) as a protective agent, and also by adjusting the pH. Varma et al.24 recently reported the synthesis of Au, Pt, and Pd nanomaterials by means of MW irradiation in the presence of various surfactants, revealing that glycerol can act as a reducing agent and their morphologies are controlled by the MW irradiation time and glycerol content. On the other hand, aside from the great advantages of MW-assisted polyol synthesis, there are also a few drawbacks. MW-assisted synthesis is not easily scalable from laboratory small-scale batch synthesis to industrial larger-scale production.27,28 The most significant limitation of the “scale-up” technique is due to the penetration depth of MW radiation into absorbing materials, which is generally in the order of a few centimeters, depending on their dielectric properties.29 Consequently, recent scale-up efforts have focused on performing microwave synthesis under continuous-flow conditions. Using either single-mode or multimode microwave instruments, successful examples of microwave-assisted continuous-flow processing have been reported in the literatures30-34 to produce nano-sized inorganic materials using a variety of different MW reactor prototypes. In this study, we have performed the comparison of the scale-up efficiencies for the polyol synthesis in the solvents (EG and glycerol) dissolving PVP as a protective agent, employing either MW-assisted batch or continuous-flow processing, to produce various kinds of monometallic nanoparticles of Pd, Rh, Ru, and Pt. Their sizes, size distributions, and morphologies have been confirmed by high-resolution transmission electron microscopy (HRTEM) and extended X-ray


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absorption fine structure (EXAFS) measurements. A special emphasis is devoted to the ability of glycerol to control the size and dispersity of monometallic nanoparticles in PVP solutions. Experimental Section Materials.

Palladium(II) chloride (PdCl2, Nacalai Tesque), rhodium(III) chloride hydrate

(RhCl3 ･ 3H2O, Wako Chemicals), ruthenium(III) chloride hydrate (RuCl3 ･ 3H2O, Aldrich), hexachloroplatinic(IV) acid (hydrated hydrogen hexachloroplatinate(IV), H2PtCl6 ･6H2O, Nacalai Tesque), and distilled water were used without further purification. Ethylene glycol (EG), glycerol, and hydrochloric acid were purchased from Nacalai Tesque, and poly(N-vinyl-2-pyrrolidone) (PVP, K-30, average M.W. = 40000) was purchased from Tokyo Kasei Kogyo Co., and they were used as received. The boiling point of EG and glycerol is 471 K and 563 K, respectively. Microwave apparatus and synthetic procedures. Batch synthesis in a sealed glass reactor using single-mode MW irradiation. In a typical experiment for the preparation of monometallic Pd nanoparticles (abbreviated as Pd_EG) in ethylene glycol (EG), 555.7 mg PVP was dissolved in 50 ml of EG as a solvent to make the polymer solution (PVP/EG) with a concentration of PVP equal to 100 mM. 0.2 ml of HCl was added to 88.66 mg of PdCl2 powder to dissolve it into aqueous solutions. After the addition of HCl, 50 ml of PVP/EG solution was poured to prepare the metallic solution with the concentration of [Pd] = 10 mM. In the case of the preparation of Pd nanoparticles (abbreviated as Pd_gly.) in glycerol, glycerol was used as a solvent instead of EG to make PVP/glycerol solutions. The other monometallic solutions (with the different metal concentration) were prepared as a starting solution (that is, M_EG or M_gly., [M] = 5 and 10 mM, M = Pd, Rh, Ru and Pt), according to the similar procedure to produce the corresponding nanoparticles. MWassisted nanoparticle synthesis was performed in a single-mode Discover SP (CEM Corporation) microwave instrument at 2.45 GHz controlled irradiation by continuous MW powers up to 300 W. All reactions were carried out with continuous stirring in a sealed glass tube reactor (max 10 mL volume) containing 5 – 7 mL of the respective starting solution. Reaction temperatures were monitored by an IR sensor on the outside of the reactor, and reaction times were regarded as hold


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times at the setting temperature, not to total irradiation times. The solution temperature was raised to 471 K (in case of EG) and 523 K (in case of glycerol) by heating the solutions for about 2 – 3 min, and it was maintained at this temperature for about 10 min. After naturally cooling down to room temperature by a flow of compressed air, the colloidal solutions were collected for the HRTEM and EAXFS measurements. Batch scale-up synthesis using multi-mode MW irradiation. In order to potentially achieve scale-up quantities of products, a commercially available multi-mode microwave reactor was used. For the batch scale-up experiments, a multi-mode microwave apparatus (MICROSYNTH PLUS, Milestone General K.K.) equipped with both an external IR and internal fiber-optic temperature sensor was employed in a continuous wave mode at 700 W (2.45 GHz) under the ambient condition. Since simultaneous monitoring of both temperature values showed discrepancies between the two methods to some extent, we favored to use the fiber-optic probe for establishing the reaction temperature. A 500 mL 2-neck round bottom flask was placed inside the multimode apparatus and a reflux condenser was connected to the flask through the protective mount in the ceiling of the microwave cavity. As is the same temperature profile in the batch synthesis using single-mode MW irradiation, the solution temperature was raised to 471 K (in case of EG) and 523 K (in case of glycerol) by heating the solutions for about 2 – 3 min, and it was maintained at this temperature for about 10 min. The as-prepared colloidal solutions were stored at room temperature and used for further characterizations. Continuous-flow synthesis in a glass reactor using single-mode MW irradiation. A continuous-flow reactor system has been proposed for the synthesis of metal nanoparticles taking full advantage of microwave dielectric heating. The continuous-flow experiments were carried out in the above single-mode microwave system, utilizing a continuous-flow reactor (10 mL inner volume, supplied by CEM) connected through a PTFE inlet tube (sheathed in Kevlar fiber) to a standard HPLC pump (JASCO Co., PU-1580). For the flow experiments, the HPLC pump was used with a flow rate of 1 mL/min, and the outlet was connected to a 250 psi back-pressure regulator. The reaction temperature was monitored by an internal IR sensor at the bottom of the flow reactor. Pressure


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Thermo couple

Circulation pump

Heated tube

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Thermo couple

Back pressure regulator

MW oven Cooler

reactant solution

product solution

inner volume : 10 mL

Figure 1. Schematic drawing of the continuous-flow reactor system for the production of metal nanoparticles (left). The continuous-flow processing was performed in a CEM Discover SP single-mode microwave system, utilizing a continuous-flow cell provided by CEM (right). The reaction temperature was monitored by an IR sensor on the outside of the flow cell. fluctuations of the system were prevented by the utilization of a back-pressure regulator (JASCO, SCF-BPG). The starting solution was loaded into the flow reactor via a PTFE inlet tube using HPLC pumps. The instrumental set-up of the continuous-flow reactor system is shown in Figure 1. Here it should be noted that residence time is defined as the actual time in which the material in transit through the tube in the reactor was exposed to the MW field. The sample solution prepared in the reactor was immediately cooled down by passing through a water/ice bath to prevent any further reaction, and the resultant colloidal dispersions were collected for further characterizations. Characterization. High-resolution transmission electron microscopy (HRTEM) observation was carried out by means of a JEOL JEM-2200FS microscopy, equipped with a field emission electron source and a corrector of spherical aberration for objective lens, operating at an accelerating voltage of 200 kV. Samples for HRTEM observation were prepared by placing a drop of colloidal metal solution on a carbon coated copper grid and dried at room temperature. The particle diameters and particle size distributions of the obtained nanoparticles were measured by counting at least 200 particles from the photograph. The EXAFS measurements were performed at room temperature in a transmission mode in two different beamlines. For the measurements of Pd, Rh, and Ru K-edges in monometallic Pd, Rh,


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and Ru nanoparticles and their reference samples, a Si(311) double crystal monochromator was used and the measurements were performed at beamline NW10A in PF-AR, High Energy Accelerator Research Organization (KEK) in Japan. This beamline was operated at 6.0 GeV with the maximum beam current of 55 mA. Ionization chambers served to detect the intensity I0 of incoming X-ray (using Ar gas) and the intensity I of transmitted X-ray (using Kr gas). The experiments at the Pt L3edge in monometallic Pt nanoparticles and their references were carried out at beamline BL-9C in PF using a Si(111) single crystal. This source operates at 2.5 GeV in a top-up mode set to 450 mA. The colloidal solutions used in the EXAFS measurements were loaded into glass cells (50 mm optical path length for Pd, Rh, and Ru K-edges and 10 mm for Pt L3-edge) sealed with polyimide film (KAPTON-200H, 50 m of thickness). EXAFS spectra were recorded around the respective edge in the step scanning mode. EXAFS data analyses were identical to those in our previous papers,35,36 and are briefly outlined below. The data were analyzed using REX2000 software package (supplied by Rigaku Co.) to determine the structural information. EXAFS oscillation (k) was extracted using a cubic spline method and normalized with edge height. The k3-weighted (k) was Fourier-transformed into R space in the k ranges of 30–160 nm-1 with a Hanning function window. In the curve-fitting step, the backscattering amplitude and phase shift were derived from reference samples. Especially, to acquire precise coordination numbers of metal atoms around the absorbing metal atom, EXAFS spectra of metal foils were measured as a reference. Results and Discussion Batch synthesis using single-mode MW irradiation. MW-assisted nanoparticle synthesis in polyol solutions was performed in the batch-type reactor by the single-mode MW irradiation. The morphology and the average sizes of monometallic Pd nanoparticles synthesized in EG or glycerol solvent in the presence of PVP are shown in Figure 2. The particles are nearly spherical in shape (i.e., cuboctahedra and icosahedra) and well-dispersed in the solutions. The average particle sizes were 6.7, 11.1, 6.1, and 11.0 nm for samples of Pd_EG ([Pd] = 5 mM), Pd_EG ([Pd] = 10 mM),


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(a) A.D. = 6.7 nm

(b) A.D. = 11.1 nm

(c) A.D. = 6.1 nm

(d) A.D. = 11.0 nm

Figure 2. TEM images and particle size distributions of the diameter for the colloidal Pd nanoparticles prepared in batch-type reactor by means of the single-mode MW irradiation for (a) [Pd] = 5 mM in EG solvent, (b) [Pd] = 10 mM in EG solvent, (c) [Pd] = 5 mM in glycerol solvent, and (d) [Pd] = 10 mM in glycerol solvent.


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(a) A.D. = 5.9 nm

(b) A.D. = 5.4 nm

(c) A.D. = 2.9 nm

(d) A.D. = 2.8 nm


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(e) A.D. = 6.1 nm

Figure 3. TEM images and particle size distributions of the diameter for the colloidal metal nanoparticles prepared in batch-type reactor by means of the single-mode MW irradiation for (a) Rh nanoparticles in EG solvent, (b) Rh nanoparticles in glycerol solvent, (c) Ru nanoparticles in EG solvent, (d) Ru nanoparticles in glycerol solvent, and (e) Pt nanoparticles in EG solvent. Metal concentration of the colloidal dispersions is [Rh] = [Ru] = [Pt] = 10 mM. Pd_gly. ([Pd] = 5 mM), and Pd_gly. ([Pd] = 10 mM), respectively, demonstrating that the larger average sizes were observed at the high metal concentration ([Pd] = 10 mM) than those in the low metal concentration ([Pd] = 5 mM) irrespective of solvent species employed. Thus, the effect of the initial metal concentrations was clearly manifested in both particle size and size distribution. The MW-assisted polyol method is common and can be applied to other noble metals such as Rh, Ru, and Pt. Figure 3 shows a representative set of TEM images for monometallic Rh, Ru, and Pt nanoparticles with the metal concentration of 10 mM, prepared in EG or glycerol solvent. In the case of Rh nanoparticles, TEM images (Fig. 3(a) and 3(b)) show the average particle sizes of 5.9 nm (in EG) and 5.4 nm (in glycerol) with the majority of the particles being cubic, rectangle, or triangle in shape. On the other hand, in the case of Ru nanoparticles, all Ru nanoparticles were observed as multipod-type nanoparticles as shown in Fig. 3(c) and 3(d). This morphology was probably due to the aggregation of the small spherical Ru nanoparticles (average particle size: ca. 2.8 - 2.9 nm) to the multipod nanocrystals. In contrast to the Rh and Ru nanoparticles, a fairly uniform particle size distribution of Pt nanoparticles (average particle size: 6.1 nm) was observed from the TEM images (Fig. 3(e)), which confirmed most of the Pt nanoparticles are not perfect spherical but truncated shape. It should be noted that the completion of MW irradiation of the glycerol solution dissolving


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(a)

(b)

(c)

Figure 4. Fourier-transforms of the Pd K-edge EXAFS spectra of the colloidal Pd nanoparticles prepared in batch-type reactor by means of the single-mode MW irradiation using (a) EG and (b) glycerol as the solvents containing different amount of Pd ions ([Pd] = 5 and 10 mM). (c) Fourier-transforms of the Pt L3-edge EXAFS spectra of the colloidal Pt nanoparticles prepared in the same method using EG solvent containing different amount of Pt ions ([Pt] = 5 and 10 mM). The Fourier-transforms of reference samples are also presented such as EG solution of PdCl2 and aqueous solution of H2PtCl6･6H2O in addition to Pd foil and Pt foil. Pt ionic precursors was not succeeded in producing Pt nanoparticles, because the gas product (byproduct) was generated and hence the pressure in the sealed reactor increased rapidly. Prior to the scale-up efficiencies for the polyol synthesis, EXAFS measurements were performed. Figures 4(a) and 4(b) show the Fourier-transforms of Pd K-edge EXAFS spectra for the colloidal Pd nanoparticles produced in the solvents of EG and glycerol, respectively, at the different metal concentrations ([Pd] = 5 and 10 mM), as well as an ethanol solution of PdCl 2 (reactant) and


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Pd foil as references. The intensity of the peak, located around 2.5 Å (R = 2.0 – 3.0 Å), for the sample (Pd_EG ([Pd] = 5 mM)) is lower than that for the sample (Pd_EG ([Pd] = 10 mM)). All the colloidal Pd nanoparticles clearly show the same peak position as Pd foil, whose peak is attributed to a Pd-Pd metallic bond. A peak of the reactant solution of PdCl2 between 1.3 and 2.3 Å is assigned to the Pd-Cl bond (the average Pd-Cl bond length is 2.31 Å determined by X-ray diffraction37). This peak completely disappeared when the colloidal Pd nanoparticles were produced by the MW-irradiation. Similarly, the Fourier-transforms of Pt L3-edge EXAFS spectra for the colloidal Pt nanoparticles were demonstrated in Figure 4(c). A peak at 1.7 Å due to Pt-Cl bond disappeared and a new peak at 2.5 Å assignable to the metallic Pt-Pt bond was observed after the reduction. Furthermore, the Fourier-transforms of K-edge EAXFS spectra for the colloidal Rh and Ru nanoparticles were shown in Figure S1 of the Supporting Information. To evaluate the structural parameters of the monometallic Pd, Rh, Ru and Pt nanoparticles prepared in EG or glycerol by using single-mode MW irradiation, curve fitting analysis was carried out by means of the procedure previously reported.35,36 Table 1 shows the structural parameters, such as coordination numbers (C.N.), bond distances (r), energy shift (E), Debye-waller factor () and R factor, of these monometallic nanoparticles. In the case of the colloidal Pd nanoparticles, the average first shell Pd coordination number appears little dependent on the initial metal concentration and solvent species. In the higher metal concentration ([Pd] = 10 mM), C.N.s are likely to be higher to small extent (Table 1). However, within the uncertainty of the measurements, these C.N. values are approximately close to 10.5±0.5. This indicates that the average size in the case of [Pd] = 10 mM is nearly identical to that in the case of [Pd] = 5 mM regardless of the solvent species. Here we consider the average particle size deduced from the EXAFS analysis. If a cuboctahedral and icosahedral structure for these Pd


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Table 1. Structural parameters from EXAFS analysis for the colloidal metal nanoparticles prepared in batch-type reactor by means of the single-mode MW irradiation. Sample Pd_EG ([Pd] = 5 mM) Pd_EG ([Pd] = 10 mM) Pd_gly. ([Pd] = 5 mM) Pd_gly. ([Pd] = 10 mM)

Edge Pd-K Pd-K Pd-K Pd-K

Bond Pd-Pd Pd-Pd Pd-Pd Pd-Pd

C.N.a 10.5 10.9 10.5 11.0

r/Å 2.74 2.74 2.81 2.81

E / eV -0.205 -0.932 -0.710 -0.935

/Å 0.070 0.065 0.072 0.079

R / %b 0.560 0.208 0.977 0.315

Rh_EG ([Rh] = 5 mM) Rh_EG ([Rh] = 10 mM) Rh_gly. ([Rh] = 5 mM) Rh_gly. ([Rh] = 10 mM)

Rh-K Rh-K Rh-K Rh-K

Rh-Rh Rh-Rh Rh-Rh Rh-Rh

10.2 10.7 8.9 9.4

2.68 2.68 2.68 2.68

-0.659 -0.410 -2.159 -1.373

0.065 0.065 0.071 0.067

0.046 0.018 0.155 0.048

Ru_EG ([Ru] = 5 mM) Ru_EG ([Ru] = 10 mM) Ru_gly. ([Ru] = 5 mM) Ru_gly. ([Ru] = 10 mM)

Ru-K Ru-K Ru-K Ru-K

Ru-Ru Ru-Ru Ru-Ru Ru-Ru

10.6 10.9 8.2 8.8

2.66 2.65 2.66 2.66

0.378 0.690 -2.733 -2.658

0.073 0.075 0.075 0.076

0.366 0.199 0.859 0.678

Pt_EG ([Pt] = 5 mM) Pt-L3 Pt-Pt 9.8 2.76 Pt_EG ([Pt] = 10 mM) Pt-L3 Pt-Pt 10.3 2.76 a Coordination numbers of the corresponding metallic bond.

-0.907 -1.265

0.065 0.064

0.356 0.045

The R factor is defined as as [k3(k)obs - k3(k)calc]2/[k3(k)obs]2  100. The error bar of r and C.N. were estimated by varying the E value (±10 eV) and the  values (±0.01 Å), respectively. The error bars in the C.N. and r values are estimated to be ±10% and ±0.03 Å, respectively. b

nanoparticles is assumed, the average particle size deduced from the C.N. ranges between 3 and 4 nm.38,39 This average particle size estimated from the C.N. is much smaller than that expected from the TEM image (Figure 2), which has been often found in our previous literatures.35,36 In order to explain the discrepancy between the size from the EXAFS spectrum and from the TEM image, we have proposed that the larger size particles observed by TEM is constituted of a cluster of small elementary particle.40 The trend in C.N.s for the Pd nanoparticles is similar to that observed for the Pt nanoparticles. The C.N.s of Pt-Pt bond are 9.8 (for Pt_EG ([Pt] = 5 mM)) and 10.3 (for Pt_EG ([Pt] = 10 mM)). These values are close to those obtained for the Pd nanoparticles within the experimental error, revealing that the particle size of Pd and Pt nanoparticles is neither strongly influenced by the metal concentration or by the reaction temperature (that is, solvent species). On


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the other hand, in the case of Rh and Ru nanoparticles, the different trends are observed that the C.N. becomes larger as the metal concentration is higher. However, C.N. of Rh-Rh bond is larger in Rh samples prepared in EG solvent than in those prepared in glycerol solvent, which is consistent with the particle size obtained from TEM. For example, the C.N.s of Rh-Rh are 10.7 (for Rh_EG ([Rh] = 10 mM)) and 9.4 (for Rh_gly. ([Rh] = 10 mM)), suggesting that the size of Rh nanoparticles prepared in glycerol is relatively smaller than those prepared in EG. The same trend is also seen in the C.N. of Ru-Ru bond, which is observed as 10.6 (for Ru_EG ([Ru] = 5 mM)), 10.9 (for Ru_EG ([Ru] = 10 mM)), 8.2 (for Ru_gly. ([Ru] = 5 mM)), and 8.8 (for Ru_gly. ([Ru] = 10 mM)). Thus, in accordance with the TEM images of Figures 2 and 3, the EXAFS data shows that the particle sizes of Rh and Ru nanoparticles are strongly affected by the solvent species employed, as shown in Figure 4 and Table 1. The particle size and size distribution in polyol synthesis is greatly correlated to the viscosity of the reaction medium.41 As expected, the higher viscosity of glycerol (1412 cP at 293 K) than EG (23.5 cP at 293 K) leads to the formation of smaller nanoparticles with narrow size distribution. Batch scale-up synthesis using multi-mode MW irradiation. In order to attempt to scale up the production quantities of metal nanoparticles, MW-assisted polyol synthesis was performed in the batch-type reactor (~ 500 mL) under the multi-mode MW irradiation conditions. It was found that product yields (>99%, as-synthesized) were similar to the ones observed under the single-mode MW irradiation conditions. TEM images of the monometallic Pd nanoparticles, as shown in Figure 5, confirm a majority of cuboctahedral and truncated cubic nanoparticles with a narrow size distribution. The average particle sizes were 7.9, 10.0, 5.1, and 9.8 nm for the samples of Pd_EG ([Pd] = 5 mM), Pd_EG ([Pd] = 10 mM), Pd_gly. ([Pd] = 5 mM), and Pd_gly. ([Pd] = 10 mM), respectively. The higher Pd concentration leads to larger particle size in the formation of Pd nanoparticles. A representative set of TEM images for other monometallic Rh, Ru, and Pt


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(a) A.D. = 7.9 nm

(b)

A.D. = 10.0 nm

20 nm


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(c) A.D. = 5.1 nm

(d) A.D. = 9.8 nm

Figure 5. TEM images and particle size distributions of the diameter for the colloidal Pd nanoparticles prepared in batch-type reactor by means of the multi-mode MW irradiation for (a) [Pd] = 5 mM in EG solvent, (b) [Pd] = 10 mM in EG solvent, (c) [Pd] = 5 mM in glycerol solvent, and (d) [Pd] = 10 mM in glycerol solvent.


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nanoparticles with metal concentration of 10 mM is shown in Figure S2 of the Supporting Information. These monometallic Rh, Ru, and Pt nanoparticles were formed in the size range of 1.9 – 5.0 nm, which are comparable with the results obtained from EXAFS analysis (shown later). The morphology of respective nanoparticles prepared by means of the multi-mode MW irradiation is almost identical to that (Figure 3) prepared by means of the single-mode irradiation. Interestingly, the average particle size is strongly related to the solvent species employed. The particle sizes of individual nanoparticles prepared in glycerol are smaller than those prepared in EG, which is observed in all the monometallic nanoparticles. Furthermore, when the same reactions were performed in the single-mode irradiation conditions, we were able to obtain larger average particle sizes (see Figures 2 and 3), provided satisfactory proof of the different results between the singlemode and multi-mode MW synthesis techniques. This difference might be due to the initial nucleation rate of small nanoparticles, which would be a focus of future research. The relations of particle size to the EXAFS spectra are reflected in the coordination numbers obtained in the fits to the EXAFS data. Figures 6(a) and 6(b) show the Fourier-transforms of Pd Kedge EXAFS spectra for the colloidal Pd nanoparticles prepared in the solvents of EG and glycerol, respectively, using the multi-mode MW irradiation. The similar trends were observed in the case of the synthesis using the single-mode MW irradiation (Figures 4(a) and 4(b)). The Fourier-transforms of Pt L3-edge EXAFS spectra for the colloidal Pt nanoparticles prepared in the EG and glycerol were also demonstrated in Figure 6(c) and 6(d). Additionally, the Fourier-transforms of K-edge EXAFS spectra for the colloidal Rh and Ru nanoparticles were shown in Figure S3 of the Supporting Information. These EXAFS results are displayed in Table S1 of the Supporting Information.


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(a)

(b)

(c)

(d)

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Figure 6. Fourier-transforms of the Pd K-edge EXAFS spectra of the colloidal Pd nanoparticles prepared in batch-type reactor by means of the multi-mode MW irradiation using (a) EG and (b) glycerol as the solvents containing different amount of Pd ions ([Pd] = 5 and 10 mM). (c) Fourier-transforms of the Pt L3-edge EXAFS spectra of the colloidal Pt nanoparticles prepared in the same method using (c) EG and (d) glycerol containing different amount of Pt ions ([Pt] = 5 and 10 mM). In the case of the colloidal Pd nanoparticles prepared in EG or glycerol, the average first shell Pd coordination numbers are obtained as approximately 11.0 ± 0.3 (See Table S1). Similar phenomenon is found in the case of the colloidal Pt nanoparticles. The Pt nanoparticles have C.N.s around 10.5±0.2, which is neither noticeably dependent on the metal concentration or the solvent species (See Table S1). In a similar manner, Rh and Ru nanoparticles have larger C.N.s prepared in EG than those prepared in glycerol, indicating the particle sizes of Rh and Ru nanoparticles are strongly controlled by the solvent species (See Table S1). Comparing TEM images in Figure 5 with


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the C.N.s in Table S1 confirmed that the EXAFS results are almost in accordance with the TEM results, except for the colloidal Pd nanoparticles. Continuous-flow synthesis using single-mode MW irradiation. There is increasing interest in the scaling-up potential of microwave dielectric heating. Because the manipulation with a stirred reactor is limited by the penetration depth of microwaves, the best opportunities for the fine chemical industry are considered to lie in a continuous-flow operation.42 Thus, a continuously operated reactor is likely to be useful for the application of microwave technology. In the present study for the synthesis of metal nanoparticles, the same concentration as in the batch processing could be available for flow processing without any pumping problems, hence allowing a direct comparison between the two methods. The optimized batch conditions (471 K in the case of EG solvent, hold time of 10 min) were directly applied into a flow processing in this study. The continuous-flow process (temperature: 471K) was run using the flow reactor (inner volume of 10 mL) with a flow rate (1 mL/min) of the reductant (reaction mixture) through the reactor. This flow rate corresponds to a residence time of 10 min, which is required for full conversion on the basis of the batch microwave experiments. The polyol synthesis of monometallic nanoparticles was carried out under continuous-flow microwave irradiation, and product yields (>90%, as-synthesized) were lower to some extent than the ones in the batch scale-up experiment. This is due to the product losses caused by the plugging of the reaction tubes during longer reaction time. Figure 7 shows the representative TEM images of monometallic Pd, Rh, Ru, and Pt nanoparticles synthesized in EG by means of the continuous-flow processing using the single-mode MW-irradiation. The Pd nanoparticles have irregular shape (mixture of sphere, cubic, and triangle) and show aggregation of the particles (average size of the individual particles: 5.5 - 6.7 nm), while the other Rh, Ru, and Pt nanoparticles possess nearly spherical shape and fairly narrower size


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(a)

(b)

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A.D. = 5.5 nm

A.D. = 6.7 nm

(c) A.D. = 3.4 nm

(d)

A.D. = 2.0 nm


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(e) A.D. = 3.6 nm

Figure 7. TEM images and particle size distributions of the diameter for the colloidal metal nanoparticles prepared in flow-type reactor by means of the single-mode MW irradiation for (a) Pd nanoparticles ([Pd] = 5 mM), (b) Pd nanoparticles ([Pd] = 10 mM), (c) Rh nanoparticles ([Rh] = 10 mM), (d) Ru nanoparticles ([Ru] = 10 mM), and (e) Pt nanoparticles ([Pt] = 10 mM) in EG solvent. distribution. When the metal concentration of these nanoparticles is equal to 10 mM, the average diameters are 6.7, 3.4, 2.0, and 3.6 nm for samples of Pd_EG ([Pd] = 10 mM), Rh_EG ([Rh] = 10 mM), Ru_EG ([Ru] = 10 mM), and Pt_EG ([Pt] = 10 mM), respectively. The Fourier-transforms for the colloidal monometallic Pd, Rh, Ru, and Pt nanoparticles by using continuous-flow processing were shown in Figure S4 of the Supporting Information, and the structural parameters calculated from EXAFS analysis were presented in Table 2. While the obtained C.N. values for the respective metal-metal bond deviated to some extent from those in the scale-up batch synthesis (shown in Table S1 of the Supporting Information), it can be seen that the average first shell C.N. of metal-metal bond appears to be ca. 10.3 (for Pd_EG ([Pd] = 10 mM)), 10.4 (for Rh_EG ([Rh] = 10 mM)), 10.3 (for Ru_EG ([Ru] = 10 mM)), and 10.6 (for Pt_EG ([Pt] = 10 mM)). These C.N. values are not distinguishable, and they are nearly identical to those for the corresponding metal-metal bond of the metal nanoparticles prepared in the batch synthesis (the respective values are 10.7, 10.3, 11.0, and 10.4 in Table S1 of the Supporting Information). Consequently, the facile scalability of continuous-flow processing can make this flow process an attractive alternative to the batch approach.


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Table 2. Structural parameters from EXAFS analysis for the colloidal metal nanoparticles prepared in continuous-flow-type reactor by means of the single-mode MW irradiation. Edge Pd-K Pd-K

Bond Pd-Pd Pd-Pd

C.N.a 9.8 10.3

r/Å 2.74 2.74

E / eV -0.400 -0.303

/Å 0.066 0.064

R / %b 0.637 0.527

Rh_EG ([Rh] = 5 mM) Rh-K Rh_EG ([Rh] = 10 mM) Rh-K

Rh-Rh Rh-Rh

10.4 10.4

2.68 2.67

0.073 -0.363

0.067 0.065

0.134 0.049

Ru_EG ([Ru] = 5 mM) Ru-K Ru_EG ([Ru] = 10 mM) Ru-K

Ru-Ru Ru-Ru

11.4 10.3

2.65 2.65

0.562 0.293

0.076 0.074

0.179 0.222

Pt_EG ([Pt] = 5 mM) Pt-L3 Pt-Pt 10.6 2.76 Pt_EG ([Pt] = 10 mM) Pt-L3 Pt-Pt 10.6 2.76 a Coordination numbers of the corresponding metallic bond.

-1.202 -1.064

0.064 0.063

0.171 0.122

Sample Pd_EG ([Pd] = 5 mM) Pd_EG ([Pd] = 10 mM)

The R factor is defined as as [k3(k)obs - k3(k)calc]2/[k3(k)obs]2  100. The error bar of r and C.N. were estimated by varying the E value (±10 eV) and the  values (±0.01 Å), respectively. The error bars in the C.N. and r values are estimated to be ±10% and ±0.03 Å, respectively. b

In this context we have previously investigated the synthesis of monometallic Rh, Ru, and Pt nanoparticles in high-temperature and high-pressure water, ethanol, and water-ethanol mixtures in the presence of PVP by using the continuous-flow reactor system.35,36,43 The colloidal Rh, Ru, and Pt nanoparticles of small size (2 - 3 nm in average particle size) were obtained at 473 K and 25 MPs in this system. By using the high-temperature conditions, we anticipated that it might be possible to change the rate limiting process of the nucleation of nanoparticles and thus to control the particle size. In contrast to previous findings43-45 in the synthesis under high-temperature and high-pressure conditions, it is proposed from the present study that a variety of monometallic nanoparticles were efficiently synthesized in relatively lower-pressure conditions by means of continuous-flow processing under MW-irradiation. According to the present results, continuous-flow microwave reactors distribute many of the advantages offered by microwave batch systems. An important safety consideration is that heating is direct, and when the power is turned off, thermal input stops immediately. Another advantage is that


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only relatively small volumes of material are subjected to processing at any time, which lowers the risk of serious mishap. Reactions can be sampled and analyzed while the run is in progress and the conditions altered without shutting down. However, considering the scale-up potential and the fact that the current microwave batch approach has probably reached its limits with reactor volume of 500 mL in a batch, the continuous-flow approach will probably be a key method for further scale-up, applying numbering-up or similar strategies. Conclusion We have systematically examined the size and morphology of various monometallic nanoparticles synthesized in polyol solvent (EG or glycerol) by means of the MW irradiation. A continuous-flow-type reactor and a batch-type reactor system have been demonstrated to facilitate the scaling-up production for the MW-assisted polyol synthesis of metal nanoparticles. In the singlemode MW-assisted continuous-flow processing, well-dispersed Rh, Ru, and Pt nanoparticles were obtained at a flow rate of 1 mL/min (the residence time in tube reactor is about 10 min), while an aggregation of small nanoparticles occurs in the case of Pd nanoparticles. In the multi-mode MWassisted batch processing, the particle size of Rh or Ru nanoparticles (except for Pd nanoparticles) prepared in glycerol is relatively smaller than those prepared in EG, revealing that the particle size is strongly influenced by both the solvent and the metal concentration. Furthermore, a very attractive capability of the multi-mode MW-assisted heating was demonstrated to produce highly uniform metallic nanoparticles using glycerol solvent dissolving PVP as a protective regent. Acknowledgments. We acknowledge the Photon Factory Advisory Committee (PAC) (Proposal Nos. 2011G005 and 2013G005) at High Energy Accelerator Research Organization (KEK) for EXAFS measurements. A part of this work was supported by Kyoto University microstructural


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characterization platform (KUMCP) as a program of “Nanotechnology Platform” (No. A-15-KT0015) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Supporting Information Available: TEM images and particle size distributions of the diameter for various colloidal metal nanoparticles prepared in batch-type reactor by means of the multi-mode MW irradiation; Fouriertransforms of EXAFS spectra and the structural parameters of the colloidal metal nanoparticles prepared in batch- or flow- type reactor by means of the MW irradiation in EG or glycerol. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents Graphics and Synopsis Manuscript title: Microwave-assisted Polyol Synthesis of Polymer-protected Monometallic Nanoparticles Prepared in Batch and Continuous-flow Processing Author list: Masafumi Harada*,† and Cong Congǂ Thermo couple

Circulation pump

Heated tube

Thermo couple

Back pressure regulator

MW oven Cooler product solution

reactant solution Flow reactor

Pd nanoparticles ([Pd] = 5 mM) synthesized in glycerol

Microwave-assisted polyol syntheses of monometallic Pd, Rh, Ru, and Pt nanoparticles have been demonstrated by the use of ethylene glycol and glycerol as the reducing agents in the presence of PVP. The size and morphology of the synthesized nanoparticles have been investigated by means of HRTEM and EXAFS. A comparison between batch scale-up and continuous-flow processing has also been indicated.


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Microwave-Assisted Polyol Synthesis of Polymer-Protected

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