X-ray Absorption Spectroscopy Investigation of Different Crystallographic Structures of Cobalt Nanoparticles
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1074-I10-28
X-ray Absorption Spectroscopy Investigation of Different Crystallographic Structures of Cobalt Nanoparticles Vadim Palshin, Rohini DeSilva, and Challa SSR Kumar J. Bennett Johnston Sr., Center for Advanced Microstructures & Devices, Louisiana State University, 6980 Jefferson Hwy., Baton Rouge, LA, 70806 ABSTRACT The ultimate goal of nanotechnology is to design successful syntheses to produce nanomaterials with required properties. The properties of nanoparticles mainly depend on their size, shape and crystal structure, hence control over these factors is a major issue in this field. In this study, Co nanoparticles with four different crystallographic structures (hcp, fcc, bcc and epsilon) have been wet-chemically synthesized by varying precursors and surfactants in the reaction. Co K-edge absorption spectra were collected in transmission mode at the XMP beamline at the Center for Advanced Microstructures and Devices (CAMD). The X-ray Absorption Near Edge Structure (XANES) and Fourier Transformed Extended X-ray Absorption Fine Structure (FT EXAFS) spectra were analyzed using the “fingerprint method”, i.e. by comparing them with either standard compounds or with the simulated spectra that were calculated using the FEFF8 code. EXAFS spectra were also analyzed by fitting them with relevant crystallographic models in r-space including higher coordination shells using the IFEFFIT analysis program. All four crystal structure types were conclusively determined by using this approach.
INTRODUCTION Based on X-ray and electron diffraction patterns, nanomaterials are often characterized as “amorphous” when they give broad and diffuse ring patterns. The X-ray Absorption Fine Structure (XAFS) spectroscopy does not require the existence of longrange order in the sample due to its nature as a local probe technique; and can be successfully used to determine the structure of relatively disordered systems, such as nanoparticles. Control of the crystal structure is one of the key issues in nanoparticle synthesis since it has a direct effect on the physical and chemical properties. This is particularly important when several crystal structures with small energy differences exist, e.g. in the case of cobalt nanoparticles where at least four crystal structures are known. It should also be noted that some samples may have a mixed phase structure. In this case the crystal structure can be successfully identified by XAFS only if one phase is present in significantly greater amounts than the others, and all structure assignments will be referring to this dominant phase. In this study, Co nanoparticles have been synthesized by wet-chemical methods starting from two precursors: acetylene-bridged dicobalt hexacarbonyl (ADH) and dicobalt octacarbonyl (DCO), in the presence of a number of surfactants such as oleic acid (OA), trioctylphosphine oxide (TOPO), octadecylamine (ODA), and trioctylphosphine (TOP). The four samples presented in this study were produced using the following precursor/surfactant combinations: ADH + OA/TOPO (samp
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