One-Step One-Phase Synthesis of Monodisperse Noble-Metallic Nanoparticles and Their Colloidal Crystals Nanfeng Zheng, Jie Fan, and Galen D. Stucky* Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106 Received January 27, 2006; E-mail: stucky@chem.ucsb.edu Noble-metallic nanoparticles have attracted increasing research attention during the past decades due to their interesting size- dependent optical, electronic, and catalytic properties. 1,2 Nanopar- ticles with a narrow size distribution can further function as building blocks for the construction of higher-ordered superlattices that exhibit collective properties of individual nanoparticles. 3-8 Although several synthetic routes of noble-metallic nanoparticles have been developed, the challenge remains of obtaining monodisperse nanoparticles with size <10 nm on a large scale. Since the first report in 1994, the syntheses of metallic nanoparticles with size less than 10 nm have been dominated by the Brust method, a two- phase protocol that can be easily scaled up to gram scale. However, the nanoparticles prepared by the Brust method and its variations typically have a continuous and broad size distribution in the range of 1-4 nm. 9,10 Similarly, the method based on the solvated metal atom dispersion technique is suitable for preparation of metal nanoparticles on the gram scale, 6,11 but post-heat treatment is generally required for good size dispersivity. Recently, efforts have been made to develop one-phase syntheses in which the reduction of metal takes place homogeneously in a selected organic solvent rather than at the two-phase interface as in the Brust method. 12-15 Even though these one-phase syntheses have been shown to significantly narrow the particle size distribu- tion, to our best knowledge, monodisperse metallic particles with size dispersivity <5% have not yet been reported by using any one-phase synthesis without a subsequent size-selection process. We report here a facile one-step one-phase synthetic route to achieve a variety of metallic nanoparticles by using amine-borane complexes as reducing agents. With the use of different metal sources, both mono- and alloyed metallic nanoparticles with a narrow size distribution can be obtained in a single step on a gram scale. The synthesized nanoparticles are ready to function as building blocks for the formation of large colloidal crystals (Figure 1) directly from the reaction mixtures. All syntheses were carried out in air by mixing metal source(s) and capping ligand (e.g., thiols) in an organic solvent, such as benzene, toluene, or chloroform. An amine-borane complex was then added to the mixture and stirred until the reduction was complete. As an example, dodecanethiol-capped gold nanoparticles were prepared as follows: 0.25 mmol AuPPh 3 Cl was mixed with 0.125 mL of dodecanethiol in 20 mL of benzene to form a clear solution to which 2.5 mmol of tert-butylamine-borane complex was then added. The color of the mixture darkened gradually and became purple-red after stirring at 55 °C for 5 min. TEM samples were prepared by dipping carbon-coated Cu TEM grids directly into the solution and drying in air for at least 2 h. As shown in Figure 1A, long-range close-packed superlattices of 6.2 nm gold nanoparticles (Figure 1A) can be obtained even without control over the evaporation rate. To prepare colloidal crystals, the mixture was sealed and cooled naturally in air upon the completion of reaction. After 2 days, tens- of-micrometer-sized colloidal crystals (Figure 1B) were obtained. Small-angle X-ray scattering and diffraction patterns of the colloidal crystals shown in Figure 1C indicate their 3D face-centered cubic structure with a ) 11.3 nm. Prior to this work, the formation of large metallic colloidal crystals required careful control over growth conditions even after a size-selection process. 3-8 In this work, we have found that the crystallization of as-made nanoparticles is readily induced by cooling the mixture or diffusing a polar solvent (e.g., ethanol) into the mixture. The use of amine-borane complexes is essential for the syntheses of monodisperse metallic nanoparticles reported here. Compared to commonly used reducing agents (e.g., NaBH 4 , LiBH 4 ), amine- borane complexes have a weaker reducing ability, which can slow the reducing rate of gold cations and allow control over the growth of nanoparticles. Upon the addition of strong reductants, such as NaBH 4 , gold cations are reduced rapidly, resulting in an immediate color change of the reaction mixture from colorless or yellow to dark red. With the use of weaker reductants applied in our syntheses, a much slower but continuous color change from colorless to yellow, pink, brown, and finally to purple-red is observed, which indicates a relatively slow reducing rate of Au(I). In addition to tert-butylamine-borane, other amine (i.e., triethylamine, morpho- line, and ammonia)-borane complexes were examined. While the triethylamine complex is a reducing agent comparable to the tert- butylamine complex, the morpholine complex exhibits much weaker reducing ability and requires longer reaction time to complete the reaction. In comparison, the reducing ability of ammonia-borane Figure 1. (A) TEM image of close-packed superlattice of 6.2 nm gold nanoparticles. (B) Optical micrograph of colloidal crystals formed directly from the reaction mixture. The inset is the dark-field micrograph. (C) Small- angle X-ray scattering (red traces) and diffraction patterns (black traces) of the colloidal crystals. The subscript “S” and “A” designate the Miller indices from the superlattice and atomic lattice of Au nanoparticles, respectively. The peak marked with an asterisk is from the window material of the scattering instrument. Published on Web 04/29/2006 6550 9 J. AM. CHEM. SOC. 2006, 128, 6550-6551 10.1021/ja0604717 CCC: $33.50 © 2006 American Chemical Society