title: Characterization of palladium nanoparticles by using X-ray reflectivity, EXAFS, and electron microscopy.
abstract: We compared the characteristics of dodecanethiolate palladium nanoparticles synthesized by two different techniques, a one-phase method and a two-phase method. From transmission electron microscopy (TEM), we determined that the particle sizes were 46 +/- 10 angstroms and 20 +/- 5 angstroms for the one- and two-phase particles, respectively. Electron diffraction confirmed that their structure was face-centered cubic (fcc). The lattice constant a0 was 3.98 +/- 0.01 angstroms and 3.90 +/- 0.01 angstroms for the one- and two-phase particles, respectively. High-resolution TEM (HRTEM) showed that the one-phase particles had an ordered core surrounded by a disordered shell structure, while the two-phase particles appeared to be crystalline throughout. The particles were also analyzed with extended X-ray absorption fine structure (EXAFS). A cuboctahedral fcc model was used to fit the data, which implied particle sizes of less than 10 angstroms for both the one- and two-phase particles. The discrepancy between the two techniques was attributed to the presence of a disordered phase, which we presumed was composed of Pd-S compounds. Compared with the bulk palladium, lattice expansion was observed in both one- and two-phase particles by electron diffraction, HRTEM, and EXAFS. At the air/water interface, a uniform film that produced surface pressure/area isotherms could only be obtained from the two-phase particles. The one-phase particles did not wet the water surface. X-ray reflectivity data indicated that the Langmuir monolayer of the two-phase particles was only 13 angstroms thick. TEM revealed the diameter of the particles in this layer to be 23 angstroms; hence the particles assumed an oblate structure after spreading. EXAFS examination of a stack of 750 Langmuir monolayers indicated far fewer Pd-S compounds, which may have dissolved in the water. The data were consistent with a model of a monolayer of truncated cuboctahedron Pd particles that were 7 angstroms thick and 19 angstroms in diameter.
authors.affiliation: Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275, USA.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Sun
authors.forename: Anatoly I
authors.initials: AI
authors.lastname: Frenkel
authors.forename: Rebecca
authors.initials: R
authors.lastname: Isseroff
authors.forename: Cheryl
authors.initials: C
authors.lastname: Shonbrun
authors.forename: Michelle
authors.initials: M
authors.lastname: Forman
authors.forename: Kwanwoo
authors.initials: K
authors.lastname: Shin
authors.forename: Tadanori
authors.initials: T
authors.lastname: Koga
authors.forename: Henry
authors.initials: H
authors.lastname: White
authors.forename: Lihua
authors.initials: L
authors.lastname: Zhang
authors.forename: Yimei
authors.initials: Y
authors.lastname: Zhu
authors.forename: Miriam H
authors.initials: MH
authors.lastname: Rafailovich
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Sokolov