Comparison of Decanethiolate Gold Nanoparticles Synthesized by One-Phase and Two-Phase Methods Yuan Sun, ² Anatoly I. Frenkel, ‡ Henry White, ² Lihua Zhang, § Yimei Zhu, § Huiping Xu, ⊥ Judith C. Yang, ⊥ Tadanori Koga, ² Vladimir Zaitsev, ² Miriam H. Rafailovich,* ,² and Jonathan C. Sokolov ² Department of Materials Science and Engineering, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-2275, Department of Physics, YeshiVa UniVersity, New York, New York 10016, Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973-5000, and Department of Materials Science and Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261 ReceiVed: January 20, 2006; In Final Form: July 26, 2006 We investigated the differences between the decanethiolate gold nanoparticles synthesized by two different routes: one-phase and two-phase methods. Their properties were compared in bulk and at the air-water interface by transmission electron microscopy (TEM), X-ray reflectivity (XR), extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray powder diffraction (XRD), thermal gravimetric analysis (TGA), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), electron paramagnetic resonance (EPR), and Langmuir-Blodgett technique. The mean nanoparticles sizes obtained by EXAFS and XRD were found to be smaller than those by the TEM measurements. We explained these differences by the structural disorder and multiple twinning in the nanoparticles. The one-phase particles were found by EXAFS to be smaller and had a higher grafting density of thiol chains than the two-phase particles. We attributed these differences to the enhanced disorder of the one-phase particles. At the air-water interface, the one-phase particles did not spread, while the two-phase particles spread and formed Langmuir films. TEM and XR results revealed that the close-packed monolayer of the two-phase particles collapsed and folded into multilayer films upon further compression. 1. Introduction Nanoscale particles have attracted remarkable research interest due to their unique properties and potential applications in catalysis, optical, magnetic, and electronic devices. 1-6 In the past few years, a lot of semiconductor nanoparticles, such as silicon, germanium, and cadmium sulfide, and diverse metallic nanoparticles including copper, zinc, nickel, cobalt, iron, rhodium, palladium, platinum, gold, and silver were synthesized and studied. In particular, much attention has been paid to the synthesis of stable colloidal gold. Combining a two-phase approach introduced by Faraday 7 with ion extraction and monolayer self-assembly of alkanethiol, Brust et al. 8 developed a mild method for the synthesis of alkanethiol-functionalized gold nanoparticles, which is now widely used. However, this method is not suitable for ω-substituted thiols because of the difficulty in product purification. Yee et al. 9 overcame this problem by exploring a novel one-phase method with no phase- transfer reagent involved in the system. To understand the physical and chemical properties of alkanethiolate gold nanoparticles synthesized by the one-phase (Yee et al. 9 ) and two-phase (Brust et al. 8 ) routes, a systematic comparison between the results obtained by different synthetic techniques is needed. In this paper, we report our synthesis of the decanethiolate gold nanoparticles by both one-phase and two-phase methods under the condition of the same initial Au/ thiol mole ratio (1:2). As suggested before, 8,9 these gold nano- particles comprise gold cores with face-centered cubic (fcc) structure and monolayer self-assembly of thiol covering the cores. In principle, the surface of the nanoparticles should be highly hydrophobic because of the thiol coating. However, we were surprised to find that the particles made by the two-phase method were easily spread at the air-water interface producing uniform Langmuir films. To understand the differences between the two synthesis methods, we compared particles made by the two techniques using a battery of complementary methods: transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), X-ray powder diffraction (XRD), and thermal gravimetric analysis (TGA) were used to compare the particle sizes, core structures, Au-Au and Au-S bonding properties, and the thiol coverage. X-ray reflectivity (XR) was used to characterize the Langmuir-Blodgett films produced from the two-phase particles at different surface pressures. Time-of-flight secondary-ion mass spectrometry (TOF-SIMS), the most sensitive surface analysis technique, was used to determine the quantitative elemental or isotopic composition of the nanoparticle surfaces. Although the decanethiolate gold nanoparticles made by the one-phase and two-phase methods were protected by the same ligand and had similar sizes, they varied in grafting density and behavior at the air-water interface. The comparison helps us in selecting an appropriate synthesis method according to various purposes and specific properties of the nanoparticles. * Corresponding author e-mail: miriam.rafailovich@sunysb.edu. ² State University of New York at Stony Brook. ‡ Yeshiva University. § Brookhaven National Laboratory. ⊥ University of Pittsburgh. 23022 J. Phys. Chem. B 2006, 110, 23022-23030 10.1021/jp060432h CCC: $33.50 © 2006 American Chemical Society Published on Web 11/02/2006