Facile aqueous-phase synthesis of uniform palladium nanoparticles of various shapes and sizes.

Nanoparticles are core base materials for implementing nanotechnology in diverse areas. Because these nanoparticles exhibit interesting sizeand shape-dependent physical and chemical properties, the synthesis of uniform nanoparticles with controlled sizes and shapes is of great importance. Furthermore, nanoparticles have a characteristic high surface-to-volume ratio, and nanoparticles of platinumgroup metals, in particular, Pd nanoparticles, have been extensively used as catalysts for many organic reactions, such as olefin hydrogenation and carbon–carbon coupling reactions. 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. 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, and the aqueous-phase synthesis of uniform nanoparticles using environmentally friendly processes is still very challenging. 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. In principle, the size of nanoparticles depends on the relative rates of nucleation and growth. 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 + ACHTUNGTRENNUNG(RCO2) as both reducing and stabilizing agents. Boutonnet et al. prepared monodisperse Pt, Pd, Rh, and Ir nanoparticles in the size range of 2–5 nm suspended in microemulsion. f] Pileni and co-workers reported sizeand shape-controlled synthesis of nanoparticles of copper and cobalt using reverse micelles as nanoscale reactors. 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. 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. Because of their low toxicity and amphiphilic nature, they have been used for drug delivery and as structure-directing agents in the synthesis of mesoporous materials. Recently, triblock Pluronic copolymers have also been used as capping agents for the synthesis of different metal nanoparticles. In the synthesis of gold nanoparticles, triblock Pluronic copolymer serves as both an efficient reductant and as a stabilizer for the growing nanoparticles. On the other hand, in the synthesis of Pt nanoparticles, H2PtCl6·6H2O was reduced with NaBH4 in the presence of the copolymer as a stabilizer. 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 0.1m 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 1A and B). The X-ray diffrac[*] Dr. Y. Piao, Y. Jang, M. Shokouhimehr, Prof. T. Hyeon National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering Seoul National University Seoul 151-744 (Korea) Fax: (+82)2-886-8457 E-mail: thyeon@snu.ac. kr