Structure and Electronic Properties of Molecularly-capped Metal Nanoparticles: The effect of Nano-size, Metal Core and C
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Structure and Electronic Properties of Molecularly-capped Metal Nanoparticles: The effect of Nano-size, Metal Core and Capping Molecule Probed by X-ray Absorption Spectroscopy Peng Zhang and Tsun-Kong Sham* Department of Chemistry, University of Western Ontario, London, ON, N6A5B7, Canada ABSTRACT We have conducted an investigation using TEM, XRD and synchrotron based Xray absorption fine structure (XAFS) to probe the structural and electronic characteristics of several molecularly capped metal nanoparticles (NPs). Three series of NPs were employed in the XAFS study to address the effect of nano-size, metal-core and capping molecules on the structure and properties of the NPs. They are (1) thiol-capped Au NPs from 1.6nm to 4.0nm, (2) thiol-capped Au and Pd NPs of ~2nm and (3) Au NPs of ~4nm capped by thiol and tetraoctylammonium bromide (TOAB). The results on their local structure, bonding and d-charge distribution are presented. INTRODUCITON The study of local structure and electronic properties of nanoparticle (NP) materials such as bonding and charge distribution have attracted intense interests in that at nanometer scale significant changes of these properties can be realized by simply modifying their local environment [1]. A better understanding of the local phenomena of NP materials may finally enable scientists to tailor their properties and functions using the “scaling law”. Among the techniques employed in the investigation of the local properties of NPs, X-ray absorption fine structure (XAFS), or X-ray absorption spectroscopy (XAS), is a unique and powerful tool because it can provide reasonably accurate structural and electronic information such as bond distance and charge redistribution upon capping and size changes [2,3]. It is particularly well suited to the study of various nanophase materials in that it does not require long-range order and it can study the local structure and bonding of samples in both liquid and solid states. Here we show the representative results of a systematic XAFS study on the structure and electronic properties of a series of molecularly capped metal NPs. The emphasis is placed on using XAFS to probe the change of bonding and electronic behavior of the NPs induced by varying their local environments: the size of the NP, the metal-core and the capping molecules. EXPERIMENTAL Thiol and tetraoctylammonium bromide (TOAB)-capped Au and Pd NPs were synthesized using the two-phase method according to the literature[4]. TEM measurements were performed by casting a few drops of the NPs (in tolune) onto “holey” carbon grids and then examined with a Philips CM20 TEM operated at 200 KeV. Wide
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angle X-ray diffraction patterns of the NPs were collected with a Rigaku RTP 300RC Xray diffractometer using Co Kα1 radiation. The XAFS studies were conducted at room temperature at the PNC-CAT BM line (Au L3- and Pd K-edge) at the Advanced Photon Source (APS), Argonne National Laboratory and at the CSRF-DCM (S K-edge) at the Synchrotron Radiation Center (SRC), University of Wisc
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