Understanding the Role of Oxidative Etching in the Polyol Synthesis of Pd Nanoparticles with Uniform Shape and Size Yujie Xiong, † Jingyi Chen, † Benjamin Wiley, ‡ and Younan Xia* ,† Departments of Chemistry and Chemical Engineering, UniVersity of Washington, Seattle, Washington 98195-1700 Shaul Aloni and Yadong Yin The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received March 4, 2005; E-mail: xia@chem.washington.edu Palladium nanoparticles serve as the primary catalyst for low- temperature reduction of pollutants emitted from automobiles 1 and organic reactions, such as Suzuki, Heck, and Stille coupling. 2 By tailoring the size and/or shape, one can, in principle, enhance their catalytic performance in a range of applications. 3 To date, shape- controlled synthesis has been achieved for many metals and alloys, such as Co, Ag, Au, Pt, and FePt. 4 Palladium nanoparticles of various morphologies have also been prepared in the presence of surfactants, 5 with the mediation of RNAs, 6 through the thermal decomposition of a Pd-surfactant complex, 7 and via the use of a coordinating ligand. 8 However, a more thorough understanding of the possible chemical reactions involved in the formation of Pd nanoparticles is still required in order to gain a better control over their shape, crystallinity, and yield. Here, we demonstrate that Pd cubooctahedral nanoparticles could be prepared with high yields and good uniformity using a modified polyol process, in which [PdCl 4 ] 2- was reduced by ethylene glycol (EG) at 110 °C in the presence of poly(vinyl pyrrolidone) (PVP). 9 In particular, it was found that oxidative etching of Pd nanoparticles by air might lead to the removal of twinned particles in the early stage and the dissolution of single-crystal cubooctahedra in the late stage. The key to high yields of uniform cubooctahedral nanoparticles is to utilize the former and eliminate the latter. Figure 1 shows TEM images of Pd nanoparticles sampled at different stages from a reaction performed in air. At t ) 5 min (Figure 1A and Figure S1A), the sample mainly contained cubooc- tahedra of 4-8 nm in size and 10% multiply twinned particles (MTPs). A magnified image of the 5-fold MTP is shown in the inset. As the reaction proceeded to t ) 1 h, all the twinned particles disappeared while the average size of the cubooctahedra grew to 8 nm. In the following 2 h, there was no significant change for both size and shape. Figure 1B (also see Figure S1B) shows a typical image of the sample obtained at t ) 3 h. During the next 2 h, the cubooctahedra were slowly dissolved until they reached an average diameter of 3 nm (Figure 1C). Beyond this point, the Pd particles began to grow again until they reached an average size of 10 nm by t ) 7 h 40 min (Figure 1D). By analyzing the images, the size distribution of nanoparticles was found to be broader than those sampled at t ) 3 h (Figure S2). The cubooctahedral structure was supported by high-resolution and dark-field TEM studies (Figure 2). The fringes in the HRTEM image are separated by 2.0 Å, which agrees with the {200} lattice spacing of face-centered cubic Pd. The dark-field image (upper right inset) recorded by a {200} reflection beam unambiguously il- lustrates both the single crystallinity of the nanoparticle and the formation of {100} facets on its surface. The fringe orientation in the HRTEM image implies that the nanoparticle is bound by 8 {111} facets and 6 {100} facets. As supported by the superior contrast in the dark-field TEM image, it can be concluded that the single-crystal nanoparticle has a cubooctahedral shape, similar to the model shown in the inset of Figure 2 (lower left). The powder X-ray diffraction (PXRD) and electron diffraction (ED) pattern † Department of Chemistry. ‡ Department of Chemical Engineering. Figure 1. TEM images of Pd nanoparticles prepared in air at (A) t ) 5 min; (B) t ) 3 h; (C) t ) 5 h; and (D) t ) 7 h 40 min. Twinned particles are indicated by tw. The inset of (A) shows the magnified image of a 5-fold twinned nanoparticle. Figure 2. HRTEM image of a Pd cubooctahedron prepared in air at t ) 3 h. The insets show the dark-field TEM image of a cubooctahedron using the {200} reflection beam (upper right) and the geometrical model of the cubooctahedron (lower left). Published on Web 05/03/2005 7332 9 J. AM. CHEM. SOC. 2005, 127, 7332-7333 10.1021/ja0513741 CCC: $30.25 © 2005 American Chemical Society