Microflow reactor synthesis of palladium nanoparticles stabilized with ...

J Nanopart Res (2010) 12:951–960 DOI 10.1007/s11051-009-9645-7

RESEARCH PAPER

Microflow reactor synthesis of palladium nanoparticles stabilized with poly(benzyl ether) dendron ligands Kanjiro Torigoe Æ Yohsuke Watanabe Æ Takeshi Endo Æ Kenichi Sakai Æ Hideki Sakai Æ Masahiko Abe

Received: 6 February 2009 / Accepted: 22 April 2009 / Published online: 12 May 2009 Ó Springer Science+Business Media B.V. 2009

Abstract A simple glass capillary microflow reactor system has been applied for the synthesis of palladium nanoparticles by thermal decomposition of palladium acetate (Pd(OAc)2) in diphenyl ether in the presence of poly(benzyl ether) dendron ligands (PBED GnNH2, n = 1–3) as a stabilizer. Effect of hydrodynamic parameters (capillary diameter, linear flow rate, volume flow rate, and reaction temperature) and concentrations (precursor and stabilizer) on the particle size was investigated. The particle size can be controlled by varying linear flow rate and temperature as well as ligand/precursor concentration ratio. Volume flow rate does not affect the particle size when the linear flow rate is held constant for different capillary diameters (150–320 lm). Unlike batch systems, in this microreactor system, smaller particles are produced at low ligand concentrations when the molar ratio of the ligand to metal precursor ranged from 1 to 5.

Electronic supplementary material The online version of this article (doi:10.1007/s11051-009-9645-7) contains supplementary material, which is available to authorized users. K. Torigoe (&) T. Endo H. Sakai M. Abe Research Institute for Science and Technology, Tokyo University of Science, 2641, Yamazaki, Noda 278-8510, Japan e-mail: [email protected] Y. Watanabe K. Sakai H. Sakai M. Abe Department of Pure and Applied Chemistry, Tokyo University of Science, 2641, Yamazaki, Noda 278-8510, Japan

As another characteristic of the microreactor synthesis, the concentration of the Pd precursor can be increased (up to 27 mM) with maintaining a constant particle size (3.1 ± 0.2 nm) and a good monodispersity, while in the batch system a significant increase and broadening in the particle size are observed with increasing precursor concentration. Keywords Microflow reactor Palladium nanoparticles Thermal decomposition Dendron ligand Microfluidics Nanomanufacturing

Introduction Many properties of metal nanoparticles including catalytic activity, phase transition temperature, magnetism, optical extinction, and electron transport properties are known to be dependent on the size (Halperin 1986; Kreibig and Vollmer 1995; Schmid 2004). Therefore, the size control of nanoparticles has a crucial importance both in fundamental research and industrial applications. Strategies for the size control of metal nanoparticles involve utilization of ligands or stabilizers which strongly and densely adsorb on the metal surfaces, e.g., alkanethiol ligands for gold (Brust et al. 1994), or those forming a closely packed ligand layer, e.g., triphenylphosphine derivatives for Au55 clusters (Schmid et al. 1981).

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However, these ligand systems might strongly attenuate or modify intrinsic surface activities of metal nanoparticles (Garcia et al. 2005). On the other hand, microflow reactor (or simply microreactor) systems are characterized by a small reaction volume which induces laminar flow structure of inner fluid and a rapid thermal exchange between their surroundings (Ehrfeld et al. 2000). The microreactor systems have become increasingly popular in organic synthesis, since the rapid thermal exchange can effectively reduce unfavorable side reactions (Ja¨hnisch et al. 2004). Other uses of the microreactor include mixing, extraction, and catalysis (Ehrfeld et al. 2000; Ja¨hnisch et al. 2004). We anticipate that these characteristics of the microreactor can also be utilized for the synthesis of monodisperse metal and other nanoparticles (Ying et al. 2008; Xu et al. 2005). Here, the particle size may be tailored by varying the hydrodynamic parameters (i.e., flow rate and temperature). Some studies have been reported on the microreactor synthesis of metal nanoparticles, e.g., on Ag (Lin et al. 2004), Au (Wagner and Kohler 2005), Ag/Au (Kohler et al. 2008), and Cu nanoparticles (Song et al. 2005). Researches on semiconductor nanoparticles are also in progress, involving CdS (Edel et al. 2002) and CdSe (Nakamura et al. 2002; Chan et al. 2003). However, the effect of hydrodynamic parameters on the size and size distribution of metal nanoparticles has yet to be completely explored. One of the reasons may be that some of these studies were performed by mixing more than two solutions in elaborate microreactors with several confluent points. In these systems, however, laminar flow in the microreactor should be perturbed at the confluence points, which may result in less efficient size control of nanoparticles. In this report, we used the simplest microreactor system composed of a syringe pump and a long straight glass capillary. Pd nanoparticles were synthesized there by thermal decomposition of palladium acetate, which does not require mixing with a reductant; hence, the effect of hydrodynamic parameters on the particle size can be clearly observed. As a ligand for stabilizing nanoparticles, we used here poly(benzyl ether) dendrons with an amino group at their focal point (PBED Gn-NH2, n = 1–3) (Nakao et al. 2002; Love et al. 2004, 2006; Mizugaki et al. 2008). These dendrons have been selected, since they can stabilize metal nanoparticles with a small number

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of molecules due to their bulkiness. Moreover, the minimum number of metal–ligand interconnection through the focal point of dendron should less perturb the intrinsic surface properties of the metal. We have investigated herein the effect of hydrodynamic parameters, i.e., capillary diameter, flow rate, and reaction temperature, as well as the effect of concentrations of metal precursor and ligand.

Experimental section Materials Palladium(II) acetate (Pd(OAc)2) was purchased from Aldrich. Other chemicals were purchased from Wako Pure Chemicals unless otherwise noted. All chemicals were of reagent grade and used without further purification. Synthesis of dendron ligands As ligands for Pd nanoparticles, first to third generations of poly(benzyl ether) ligands with an amino focal group, referred to as PBED Gn-NH2 (n = 1–3, see Fig. 1 for their chemical structure), have been synthesized by Gabriel reaction for corresponding dendron bromides Gn-Br with refer to Vo¨gtles’ method (Vo¨gtle et al. 1998) with a slight modification. G1-Br (3,5-dibenzyloxybenzyl bromide) used here was a commercial product from Tokyo Chemical Industries. G2-Br and G3-Br were synthesized according to Frećhets’ method (Hawker and Frećhet 1990). PBED Gn-NH2 was synthesized by reacting Gn-Br with 1 mol equivalent of potassium phthalimidate (TCI) in dry acetone by refluxing for 7 h in the presence of 18-crown-6 ether (0.15 mol equivalent). Then, the intermediate dendron phthalimide was hydrolyzed with 1 mol equivalent of hydrazine hydrate in mixed THF/ethanol (1/1 in v/v) by refluxing for 1 h. The average reaction yield was 67%, 78%, and 80% for G1-, G2-, and G3-NH2, respectively. Their purity was confirmed by 1H- and 13 C-NMR (JEOL JNX-ECP, 300 MHz for 1H, 75 MHz for 13C), FAB-MS (JEOL JMS-SX102A) and elemental analysis (Perkin Elmer PE2400-II). 1Hand 13C-NMR spectra of Gn-NH2 are shown in the supporting information (supplementary Figs. S1–S3).

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O

O

O

O O

O

O

O

O

O

O O

O

O O

O

O

O O

NH2

(a) G1-NH2

O

NH2

(b) G2-NH2

O

O

NH2

(c) G3-NH2

Fig. 1 Chemical structures of PBED Gn-NH2

G1-NH2 (C21H21NO2): FAB(?)MS; 320.1646 (found), 320.1645 (calculated). Elemental analysis; C 78.65%, H 6.23%, N 4.06% (found), C 78.97%, H 6.63%, N 4.39% (calculated). G2-NH2 (C49H45NO6): FAB(?)MS; 744.3318 (found), 744.3319 (calculated). Elemental analysis; C 79.03%, H 6.11%, N 1.86% (found), C 79.11%, H 6.10%, N 1.88% (calculated). G3-NH2 (C105H91NO14): FAB(?)MS; 1592.666 (found), 1592.666 (calculated). Elemental analysis; C 78.88%, H 5.93%, N 0.98% (found), C 79.17%, H 5.88%, N 0.88% (calculated). Synthesis of Pd nanoparticles First as a precursor solution, palladium(II) acetate was mixed with PBED Gn-NH2 in diphenyl ether. This solvent was selected due to its high boiling point (259 °C) and no reducibility for Pd(II) ions. Concentration of Pd(OAc)2 was fixed at 1 mmol dm-3 and the molar ratio of Gn-NH2 to Pd(OAc)2 was 5 unless otherwise noted. In order to completely dissolve Pd(OAc)2, it was vigorously stirred for 1–24 h, depending on the concentration, at ambient temperature. Then, the precursor solution was charged in a glass syringe (Hamilton gastight syringe, 5 mL) and set at a syringe pump (Bioanalytical Systems BASi Bee MD-1000 & 1001). The syringe pump was further connected to a silica capillary tube with different inner diameters (Polymicro Technologies TSP series, id 150, 200, 300 lm). Total length of the capillary tubes was set to 2 m, in which 1.5 m of the intermediate part was immersed in a silicone oil bath. Temperature of the heat bath was varied from 140 to

200 °C and controlled within ±1 °C. The precursor solution was pushed out from the syringe at a volume flow rate of 2.5–500 lL min-1. After passing through the heat bath, these samples were quickly and spontaneously cooled down to ambient temperature and collected at the outlet. The dendron-stabilized Pd nanoparticles were characterized by means of TEM (Hitachi H-7650, 120 kV) and UV–vis spectrophotometry (Agilent 8453A). For TEM, 2 lL of each sample was mounted on carbon-coated Cu grids using a micropipette and left stand overnight in a hood chamber to evaporate the solvent. Size analysis was performed by measuring more than 200 particles on fivefold enlarged TEM images at direct magnification of 100,000. UV–vis extinction spectra of Pd NPs dispersions were recorded at 1 cm of light pass length.

Results and discussion Thermal decomposition of palladium(II) acetate Pd(OAc)2 as solid state is reported to occur at 205 °C (Wikipedia). In solution, however, it takes place at lower temperature (Esumi et al. 1989). We have investigated the effect of reaction temperature. Here, the reaction time (residence time in the heat bath) was constant at 29 s by fixing volume flow rate (250 lL min-1) and capillary diameter (320 lm). Figure 2b compares UV–vis extinction spectra of the precursor solution and samples after the reaction at 140–200 °C. The precursor solution exhibits pale yellow color (supplementary Fig. S4) and does not absorb the light at longer wavelengths than 500 nm. Absorption bands of Pd(OAc)2 can be observed at

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180°C

160°C

10 5 0

= 3.9 nm σ = 0.5 nm

20 15 10 5

1

1.6 2.2 2.8 3.4

4

4.6 5.2 5.8 6.4

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1.8

2.4

3

3.6

4.2

4.8

5.4

6

6.6

= 4.2 nm σ = 0.7 nm

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0

20 nm 20 15

3

3.6 4.2 4.8 5.4

6

6.6

= 4.8 nm σ = 0.8 nm

10 5 0

1.8 2.4

Particle diameter / nm



(b)

Frequency / %

Frequency / %

Frequency / %

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= 2.7 nm σ = 0.9 nm

20 nm

20 nm

20 nm 20 15

200°C

Frequency / %

(a) 140°C

1.8

2.4

3

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2.0 200°C 180°C 160°C 140°C precursor

Absorbance

1.5

1.0

0.5

0.0 300

400

500

600

700

800

Wavelength / nm

Fig. 2 a TEM images of Pd NPs prepared at different temperatures (volume flow rate 250 lL min-1; [G1-NH2]/[Pd(OAc)2] = 3; capillary id = 320 lm). b UV–vis extinction spectra of the precursor and Pd NPs dispersions in diphenyl ether

400 nm (d–d transition band), 272 nm (CT band), and 206 nm (p–p* band) when dissolved in methanol, while in diphenyl ether, the latter two bands are concealed by an intense absorption below 350 nm by the solvent. Reaction at 140 °C does not cause a significant change in the UV–vis spectrum, implying that the thermal decomposition of Pd(OAc)2 hardly takes place. Meanwhile, the reaction at 160 °C or higher temperature brings about a remarkable spectral change leading to a continuously increasing absorbance from the near-IR region toward shorter wavelengths. This indicates the formation of Pd nanoparticles (Esumi et al. 1989; Henglein 2000) by thermal decomposition of Pd(OAc)2. With increasing temperature, the absorbance at longer wavelengths increases and the slope toward shorter wavelengths becomes less steep. This can be attributed to three factors: increase in the number of Pd nanoparticles, increase in the particle size, and/or increase in the

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number of aggregates (Esumi et al. 1989; Henglein 2000). On the other hand, TEM images in Fig. 2a indicate that the mean particle size increases with increasing reaction temperature, from 2.7 nm at 140 °C to 4.8 nm at 200 °C. Also, we find that not only the particle size but also the number of particles and aggregation state are all dependent on the reaction temperature. In the sample prepared at 140 °C, the number of particles is obviously smaller than the other samples, which is another proof for the incomplete reaction. Meanwhile, at 200 °C, a significant amount of aggregates is observed apart from dispersed nanoparticles. Therefore, we fixed hereafter the reaction temperature at 180 °C which appears to be the optimal condition. Subsequently, we investigated the effect of volume flow rate on the size of Pd nanoparticles. These experiments were performed at a fixed capillary

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diameter (320 lm). Two main observations can be deduced from TEM images shown in Fig. 3a. First, the mean particle size decreases from 5.2 to 3.4 nm with increasing volume flow rate from 2.5 to 250 lL min-1. This implies that the crystal growth by incorporation of nuclei becomes less easy with

25 µ l/min

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= 5.2nm σ= 0.7nm

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=4.6nm σ=0.5nm

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50 nm

500 µ l/min

=3.4nm σ=0.3nm 1.8 2.4 3

3.6 4.2 4.8 5.4 6 6.6


(b)

10

50 nm

250 µ l/min

30 25 20 15 10 5 0

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Frequency / %

Frequency / %

20

50 µ l/min

20

30

Frequency / %

(a)

increasing volume flow rate. Second, a remarkable aggregation occurs at the lowest flow rate investigated. This is because adsorption of bulky dendron ligands to the metal surface is easier at higher flow rate. On the other hand, UV–vis spectra in Fig. 3b indicates that the absorbance at longer wavelengths

50 nm

50 nm

2.0 500 µl / min 250 µl / min 50 µl / min 25 µl / min 2.5 µl / min precursor

Absorbance

1.5

1.0

0.5

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500 600 Wavelength / nm

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Fig. 3 a TEM images of Pd NPs prepared at different volume flow rates (180 °C, [G2-NH2]/[Pd(OAc)2] = 5, capillary id.: 320 lm). b UV–vis extinction spectra of Pd NPs dispersions

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200 µ m

Frequency / %

Frequency / %

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30

= 3.3 nm σ = 0.4 nm

20 10

0 1.8 2.8 3.8 4.8 5.8 6.8


(b)

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20 nm

Frequency / %

(a)

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20 10 0

= 4.5nm σ= 0.5nm 1.8 2.8 3.8 4.8 5.8 6.8


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1.5

Absorbance

i.d. 320 µm i.d. 200 µm i.d. 150 µm

1.0

0.5

0.0 400

450

500

550

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700

Wavelength / nm Fig. 4 a TEM images of Pd NPs synthesized in glass capillaries with different inner diameters at a constant volume flow rate (25 lL min-1, [G2-NH2]/[Pd(OAc)2] = 5, 180 °C). b UV–vis extinction spectra of Pd NPs dispersions

(k C 500 nm) first increases with increasing volume flow rate from 2.5 to 25 lL min-1. The increment of the absorbance becomes very small when increasing the flow rate from 25 to 50 lL min-1, and it decreases at higher flow rate. It should be noted that here the reaction time decreases with flow rate. The low absorbance for the sample at 25 lL min-1 can likely be attributed to the precipitation of large aggregates. For the samples of 25 and 50 lL min-1, their spectra are almost identical because of very similar particle size and high dispersion state. The decrease of absorbance at a higher flow rate is likely due to the decrease in the reaction yield. This is supported from the small number of Pd nanoparticles at 500 lL min-1. Next, we have investigated the effect of capillary diameter on the size of Pd nanoparticles by using three capillaries of different inner diameters (150, 200, and 320 lm). At a constant volume flow rate (25 lL min-1), the particle size increases with increasing diameter of the glass capillaries, as shown

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the TEM images in Fig. 4a. This result is consistent with the increase in the UV–vis absorbance with increasing capillary diameter (Fig. 4b). Note that here the linear flow rates are different for these three capillaries. Meanwhile, at a constant linear flow rate, the size of Pd nanoparticles hardly changes with capillary diameter, as shown the TEM images in Fig. 5a. Besides, the change in UV–vis absorbance with capillary diameter is very small (Fig. 5b). These two comparative experiments clearly indicate that the particle size is dominated not by the volume flow rate but by the linear flow rate, and that the capillary diameter is not a factor to determine the particle size. The most plausible reason is that the reaction time is a function of linear flow rate. At a constant volume flow rate, the linear flow rate decreases proportionally with the square of the capillary diameter. Hence, the reaction time decreases with increasing capillary diameter, which leads to the decrease in the particle size. On the other hand, at a constant linear flow rate, the reaction time does not change with the capillary

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200 µ m

Frequency / %

Frequency / %

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=3.2nm σ=0.4 nm 1.8 2.4

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Fig. 5 a TEM images of Pd NPs synthesized in glass capillaries with different inner diameters at a constant linear flow rate (50 mm s-1, [G2-NH2]/[Pd(OAc)2] = 5, 180 °C). b UV–vis extinction spectra of Pd NPs dispersions

r=3

Frequency / %

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r =1

= 3.1 nm σ = 0.5 nm 1.8 2.4 3 3.6 4.2 4.8 Particle diameter / nm

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20 10 0 1.8

20 nm

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Fig. 6 TEM images of Pd NPs synthesized at different ligand/precursor concentration ratios r = [G2-NH2]/[Pd(OAc)2] (volume flow rate 250 lL min-1, capillary id. 320 lm, 180 °C)

diameter, leading to a constant particle size. Capillary diameter is not a controlling factor for the particle size because the particle size is very small (*10-5) compared with the capillary diameter.

Successively, the effect of the concentration and generation of the dendron stabilizer has been studied. Figure 6 presents TEM images of Pd nanoparticles prepared at different concentrations of the dendron

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Frequency / %

= 4.7 nm σ = 0.7 nm

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(c) G3-NH2 / Pd

(b) G2-NH2 / Pd

(a) G1-NH2 / Pd

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20 nm

Fig. 7 TEM images of Pd NPs stabilized with PBED of different generations (volume flow rate 250 lL min-1, [Gn-NH2]/ [Pd(OAc)2] = 5, [Pd(OAc)2] = 1 mM, capillary id. 320 lm, 180 °C)

Batch reactor (180 °C)

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20 nm

Microreactor (180 °C , 250 µ l /min)

= 3.2 nm σ = 0.3 nm

10 0

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(h)

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Frequency / %

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20

(g)

(f)

Frequency / %

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20 nm

Fig. 8 Comparison of Pd NPs synthesized with a batch reactor (a–d) and a microreactor (e–h). [Pd(OAc)2] = 1 mM (a, e); 3 mM (b, f); 9 mM (c, g); 27 mM (d, h). [G2-NH2]/[Pd(OAc)2] = 5

ligand (G2-NH2). Interestingly, the particle size increases from 2.2 to 3.1 nm with increasing ligand/ precursor molar ratio (r) from 1 to 3 at a constant precursor concentration (1 mmol dm-3). In the range of 3 B r B 5, the increase levels off. At higher ligand concentration, i.e., r = 10, reduction of Pd(II) complex is strongly attenuated due to the formation of a stable complex between the amino group at the focal

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point of dendron and Pd(II) ion. Production of smaller nanoparticles at lower ligand concentration (down to r = 1) appears to be unique in the microreactor system, although at much lower concentration (i.e., r = 0.5, data not shown), a significant aggregation of nanoparticles took place. High concentration of bulky dendron molecules in a small space of the microreactor would retard the adsorption

Particle size / nm

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10 batch

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10 Pd precursor conc. / mM

100

Fig. 9 Comparison of the size of Pd NPs synthesized by batch system and by microreactor. [G2-NH2]/[Pd(OAc)2] = 5, 180 °C

of the ligand to nucleic metal particles, which results in a less effective suppression of the crystal growth. Subsequently, the effect of dendron generation has been investigated. Figure 7 shows that the particle size significantly decreases with increasing generation of dendron: 4.7, 3.2, and 2.8 nm for G1-, G2-, and G3-NH2, respectively, at a fixed concentration. This result suggests that the crystal growth of Pd nanoparticles is efficiently suppressed by steric hindrance of dendron molecules. The same trend have been reported for Au nanoparticles ligated with lysine-based dendron molecules (Love et al. 2004). Finally, we have compared the microflow reactor system with a batch system at various precursor concentrations. The results are presented in Figs. 8 and 9. In the batch system, the mean particle size increases from 4.3 to 8.9 nm with increasing precursor concentration from 1 to 27 mmol dm-3, associated with a significant increase in the polydispersity. Meanwhile, in the microreactor system, the mean particle size is always smaller than that of the corresponding samples in the batch system. In addition, the particle size keeps a constant value (3.1 ± 0.2 nm) in this precursor concentration range, as well as an excellent monodispersity (B10%). This is a noteworthy advantage of the microreactor system.

Conclusions We have studied here the microflow reactor system for the synthesis of Pd nanoparticles via thermal decomposition of palladium acetate Pd(OAc)2 in diphenyl ether by using amino-focalized poly(benzyl ether) dendron (PBED Gn-NH2, n = 1, 2, 3) as a

ligand for the stabilization. Effects of hydrodynamic parameters (capillary diameter, temperature, volume flow rate, and linear flow rate), concentrations (precursor and stabilizer), and dendron generation have been investigated. As significant results, it was found that the particle size depends not on the volume flow rate but on the linear flow rate, because reaction time is defined by the latter. In this microreactor system, smaller particles were produced at low concentrations of the ligand, which is significantly different from the general trend in batch systems. Moreover, in the microreactor system, the precursor concentration can be increased up to 27 mmol dm-3 with keeping the mean particle size and size dispersion constant at 3.1 ± 0.2 nm, which is a remarkable advantage compared with batch systems. Experiments on the chemical properties of these dendronstabilized Pd nanoparticles are now in progress. Acknowledgment This study has been financed by Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) Grant-in-Aid for Scientific Research (C) (Project No. 18510088).

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