Characterization of Core/Shell Nanoparticles by X-Ray Absorption Spectroscopy
S. Calvin, E. A. Carpenter, C. J. Riedel, and V. G. Harris
Naval Research Laboratory, US
Keywords: magnetic nanoparticles, x-ray spectroscopy,
X-ray absorption spectra of nanoparticles with iron cores and oxide shells, synthesized using the reverse micellar method, were collected at the National Synchrotron Light Source. Comparison of the x-ray absorption near edge structure (XANES) to standards indicated approximately 60% of the iron atoms reside in the metallic core. Extended x-ray absorption fine structure (EXAFS) suggested that the particles comprised amorphous iron oxide, amorphous close-packed iron, and, in some cases, nanocrystalline bcc iron. The presence of the bcc iron was controlled by the surfactant used to form the micelles: a mixture of NP-5 and NP-9 yielded an entirely amorphous metallic core; while CTAB yielded a metallic core that was partially nanocrystalline. Analogues of the nanoparticles created out of mixtures of bulk materials yielded similar spectra, confirming the phase assignments and ratios. In addition, by analyzing a series of crystalline palladium nanoparticles, we found that a homogenous spherical model is as accurate as an atom-by-atom approach for particles greater than one nanometer in diameter. This model was then applied to the nanocrystalline iron present in the core/shell structure to determine the approximate size of the nanocrystalline inclusions.
NSTI Nanotech 2003 Conference Technical Program Abstract