Facile Aqueous‐Phase Synthesis of Uniform Palladium Nanoparticles of Various Shapes and Sizes (2007)

Uniform palladium nanoparticles of various sizes and shapes are synthesized via a simple aqueous-phase reaction between Pd2+ ions and PEOx–PPOy–PEOx triblock copolymers; the size and shape are controlled by varying reaction conditions such as the pH value (see image). The palladium nanoparticles showed high catalytic activities for Suzuki cross-coupling reactions. Nanoparticles are core base materials for implementing nanotechnology in diverse areas. Because these nanoparticles exhibit interesting size- and shape-dependent physical and chemical properties, the synthesis of uniform nanoparticles with controlled sizes and shapes is of great importance.1 Furthermore, nanoparticles have a characteristic high surface-to-volume ratio, and nanoparticles of platinum-group metals, in particular, Pd nanoparticles, have been extensively used as catalysts for many organic reactions, such as olefin hydrogenation and carbon–carbon coupling reactions.2 Compared with homogeneous palladium catalysts, palladium nanoparticles have such advantages as easy removal from reaction mixtures and catalyst recyclability. Recently, uniform palladium nanoparticles were synthesized using several different procedures, including polyol processes and thermolysis of palladium complexes.3 While most of these reported approaches involved the use of toxic organic solvents or reagents, there are only a few reports on the synthesis of uniform-sized palladium nanoparticles in an aqueous solution,4 and the aqueous-phase synthesis of uniform nanoparticles using environmentally friendly processes is still very challenging.5 Recently, Xia and co-workers and other research groups reported the synthesis of novel metal nanoparticles in aqueous solutions using poly(N-vinyl-2-pyrrolidone) (PVP) as the reducing agent.6 In principle, the size of nanoparticles depends on the relative rates of nucleation and growth.7a To date, several attempts have been made for the size-controlled synthesis of nanoparticles of palladium and related platinum-group metals. Reetz et al. reported the size-selective preparation of Pd nanoparticles by using a variety of tetra-n-octylammonium carboxylates (n-Oct)4N+(RCO2)− as both reducing and stabilizing agents.7b–7d Boutonnet et al. prepared monodisperse Pt, Pd, Rh, and Ir nanoparticles in the size range of 2–5 nm suspended in microemulsion.7e, 7f Pileni and co-workers reported size- and shape-controlled synthesis of nanoparticles of copper and cobalt using reverse micelles as nanoscale reactors.7g–7j Nagy and co-workers prepared Pt colloids in the range of 4 to 6.6 nm by varying the concentration of K2PtCl4 from 0.12 to 0.55 mM.7k Recently, Xia and co-workers reported the preparation of large (≈50 nm) palladium nanocubes by controlling the number of seeds formed at the nucleation stage.8 Triblock Pluronic copolymers, poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEOx–PPOy–PEOx) are commercially available, inexpensive, nonionic, high-molecular-weight materials that have unique and interesting properties.9 Because of their low toxicity and amphiphilic nature, they have been used for drug delivery10 and as structure-directing agents in the synthesis of mesoporous materials.11 Recently, triblock Pluronic copolymers have also been used as capping agents for the synthesis of different metal nanoparticles.12 In the synthesis of gold nanoparticles, triblock Pluronic copolymer serves as both an efficient reductant and as a stabilizer for the growing nanoparticles.12c, 12d On the other hand, in the synthesis of Pt nanoparticles, H2PtCl6⋅6 H2O was reduced with NaBH4 in the presence of the copolymer as a stabilizer.12h We report here on an efficient aqueous-phase synthesis of uniform palladium nanoparticles from the reduction of Pd salts using triblock Pluronic copolymers. We also controlled the particle size and shape of Pd nanoparticles by varying the reaction conditions. Furthermore, we successfully used these palladium nanoparticles as a catalyst for Suzuki cross-coupling reactions. The palladium nanoparticles were synthesized simply by mixing aqueous solutions of palladium salts and Pluronic copolymer. In a typical procedure, 0.1 mL of 0.1 M Na2PdCl4 was added to 10 mL aqueous solution containing 0.2 g of triblock Pluronic copolymer (P123, PEO19–PPO69–PEO19, 3.4 mM) at room temperature, and the resulting solution was vigorously stirred. Within several minutes, the color of the solution changed from light yellow to a deep brown, indicating the formation of nanoparticles. Although the reaction was complete within several hours (see Supporting Information, Figure S1), the reaction mixture was kept stirring for 24 h at room temperature. The addition of propanol and hexane (2:1 in volume) followed by centrifugation isolated the palladium nanoparticles in the form of a dark-brown powder. The resulting powder was readily redispersible in water, which afforded a transparent brown suspension of well dispersed nanoparticles. The aqueous dispersion was stable and no precipitate occurred for more than five months at room temperature. Transmission electron microscopy (TEM) confirmed the generation of highly crystalline nanoparticles with a spherical shape and an average diameter of (4.8±0.5) nm (Figure 1 A 1and B). The X-ray diffraction (XRD) pattern of the nanoparticles revealed a face-centered cubic (fcc) Pd crystal structure (Figure 2). 2The particle size calculated using the Scherrer formula was found to be 4.73 nm, which matched well with that obtained from the TEM image. The lattice-spacing value of 0.224 nm in the high-resolution (HR) TEM image (Figure 1 C) 1matched very well with the (111) lattice plane of palladium metal. The electron-diffraction pattern, shown in Figure 1 D, 1exhibited four diffused rings, which were assigned to the (111), (200), (220), and (311) reflections of fcc Pd. A, B) TEM images of the palladium nanoparticles prepared using P123. The inset in image (B) is a photograph of the palladium nanoparticle solution. C) HRTEM image of a single nanoparticle. The inset shows the corresponding fast Fourier transform (FFT) pattern. D) Typical electron-diffraction pattern of palladium nanoparticles. Powder X-ray diffraction pattern of the palladium nanoparticles. The particle size and shape was controlled by varying the pH value of the reaction mixtures. When the pH was varied by adding the appropriate amount of HCl to the reaction mixture, uniform Pd nanoparticles with particle sizes of 5.6 nm (pH 6.4), 6.9 nm (pH 3.8), 8.2 nm (pH 3.2), and 10.9 nm (pH 2.8) were produced (Figure 3 A–D). 3When significantly more HCl was added to the reaction mixture, clustered dendritic nanoparticles with much larger overall sizes were produced. For example, when 5 mM (pH 2.5) and 10 mM (pH 2.3) of HCl were added to the reaction mixture, dendritic nanoparticles13 with overall sizes of 21 nm and 27 nm, respectively, were produced (Figure 3 E 3and F). Additional TEM images of the dendritic nanoparticles are shown in Figure S2 and S3 of the Supporting Information. Similar results were obtained when HCl was replaced with either H2SO4 or HNO3 (see Figure S4–S6 and Table S1 of the Supporting Information), whereas there was no obvious change in the particle size when KCl, K2SO4, or KNO3 was used instead of HCl. These results clearly suggest that it is the acid itself that has a strong influence on the nanoparticle size. TEM images of the palladium nanoparticles synthesized in P123 aqueous solution with the HCl concentration of A) 0.1 mM (pH 6.4), B) 0.5 mM (pH 3.8), C) 1 mM (pH 3.2), D) 2 mM (pH 2.8), E) 5 mM (pH 2.5), and F) 10 mM (pH 2.3). In a previous report, it was shown that the reduction of metal ions was performed by the PEO part of Pluronic copolymer.12c, 12d In low-pH conditions, it is known that H+ ions disturb the binding of the metal ions to the oxyethylene group.14 It was suggested that H+ ions could prevent the PEO part from the reduction of metal ions, and that the number of nuclei was consequently decreased. The overall increase in the particle size after the addition of acid can be explained by the decrease in the number of nuclei.14 Another possibility might be the decreased coordination ability of Pluronic copolymer at low pH.15 Furthermore, the formation of the dendritic nanoparticles at very low pH can also be explained by the significantly decreased coordination ability of the Pluronic copolymer to the growing nanoparticles.15 We used different triblock Pluronic copolymers in the synthesis. When the reaction was carried out using P L64 (PEO13–PPO30–PEO13), P103 (PEO17–PPO60–PEO17), and F108 (PEO132–PPO50–PEO132) copolymers instead of P123 but using the same reaction conditions described above, palladium nanoparticles with average diameters of (5.4±0.5) nm, (4.3±0.5) nm, and (3.8±0.4) nm, respectively, were obtained (Figure 4). 4The sizes of the resulting nanoparticles roughly followed the trend of a decreasing chain length of the PEO blocks that act as the reductant. Interestingly, when the synthesis was performed using PEO homopolymer,16 highly aggregated and interconnected nanostructures were precipitated (see Figure S7 of the Supporting Information), which is most likely due to the absence of PPO chains that bind and subsequently stabilize the nanoparticles. TEM images of the Pd nanoparticles prepared using A) P123, B) P L64, C) P103, and D) F108. The insets show the corresponding particle size distributions. By varying the concentration of Na2PdCl4 from 0.1 to 1 mM, we were able to vary the size of Pd nanoparticles from 4.1 to 4.8 nm (see Figure S8 of the Supporting Information). These results demonstrate that the initial concentration of Na2PdCl4 does not significantly affect the particle size of the produced Pd nanoparticles. Size control of the palladium nanoparticles was also achieved via a well-established seed-mediated-growth approach.17 For example, palladium nanoparticles in the size range of (5.0±0.5) nm to (6.1±1.7) nm were obtained by slowly adding Na2PdCl4 (1 drop min−1) to a dispersion containing the previously synthesized nanoparticles with a particle size of 4.8 nm (see Figure S9 of the Supporting Information). When 0.02 M Na2PdCl4 solution was added extremely slowly to a dispersion containing 4.1-nm-sized Pd nanoparticle seeds using a syringe pump at the injection rate of 0.1 mL h−1, highly faceted Pd nanoparticles were produced (Figure 5).5 A) Low and B) relatively high magnification TEM images of the angled palladium nanoparticles grown on a 4.1-nm seed with the addition of 1 mL of a 0.02 M Na2PdCl4 solution using a syringe pump at a rate of 0.1 mL h−1. Typical HRTEM images of single C) triangular and D) rectangular nanoparticles. The insets in (C) and (D) show the corresponding FFT. The synthetic method developed here is potentially useful for the large-scale production of uniform-sized palladium nanoparticles. For example, when 4 mmol of aqueous Na2PdCl4 solution was reacted in an aqueous solution containing 80 g of P123 in 4 L of water, 0.42 g of uniform Pd nanoparticles with a particle size of 4.8 nm was produced (yield 98 %; photograph and TEM images are shown in Figure S10 of the Supporting Information). We tested the catalytic activity of the 4.8-nm-sized Pd nanoparticles for Suzuki cross-coupling reactions (Table 1). The palladium nanoparticles showed high catalytic activity for iodo- and bromoaryl substrates using a relatively small amount of the nanoparticle catalyst (0.5 mol %). In general, Suzuki cross-coupling reactions have been performed using expensive and large amounts of organopalladium complexes.18 In an attempt to expand the application of the 4.8-nm-sized nanoparticle catalyst, we performed the coupling of 1-chloro-4-nitrobenzene with phenylboronic acid under the same conditions for 24 h. However, no coupled product was obtained. Entry R X t [h] Yield [%][b] 1 H I 8 97 2 4-COCH3 I 8 99 3 2-CH3 I 8 93 4 4-COCH3 Br 12 91 5 4-CN Br 12 95 In order to evaluate the dependency of particle size on the catalytic performance, we performed the coupling of 4-bromoacetophenone with phenylboronic acid using Pd nanoparticles of two different sizes (10.9 and 4.8 nm). When 0.5 mol % of 10.9-nm-sized nanoparticle catalyst was used, the conversion was much lower (51 %) than that of 4.8-nm-sized Pd nanoparticles (95 %) under the same conditions. The results showed that the smaller nanoparticles with larger surface area exhibit better catalytic performance compared to the larger nanoparticles.19 In summary, uniform palladium nanoparticles were synthesized via a simple aqueous-phase reaction between Pd2+ ions and PEOx–PPOy–PEOx -type triblock copolymers. The size and shape of the nanoparticles could be easily controlled by varying the experimental conditions, such as the pH value of the initial reaction mixture. The palladium nanoparticles showed high catalytic activity in the Suzuki cross-coupling reactions. The current synthetic procedure is very simple and can be readily applied to large-scale production; for example, as much as 0.42 g of uniform 4.8-nm-sized Pd nanoparticles was synthesized from a single reaction. Materials: Water was deionized by using a Barnsted Nanopure system. Triblock Pluronic copolymers (P123, P L64, P103, and F108) were purchased from BASF Corp. Na2PdCl4, KCl, K2SO4, KNO3, HCl, H2SO4, and HNO3 were purchased from Aldrich Chemical Corp. Characterization: All TEM and HRTEM images were obtained using a JEOL EM-2010 microscope at an acceleration voltage of 200 kV. The X-ray diffraction pattern was taken by a Rigaku Dmax 2500 diffractometer system. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2296/2007/z600402_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.