XAFS Study on the Local Atomic Structures of CeriumOxide Nanoparticles with Surface Coatings

Four kinds of cerium oxide nanoparticles coated, respectively, with Sodium bis(2-ethylhexyl) sulfosuccinate (AOT), cetyltrimethyl ammonium bromide (CTAB), poly vinyl pyrrolidone (PVP) and those (naked) without surface coating were prepared using chemical method. The particle sizes were identified to be 2–3 nm by using transmission electron microscope (TEM) and x-ray diffraction (XRD). Mixed valences of Ce3+ and Ce4+ were detected by x-ray absorption near-edge-structure (XANES) spectra. Extended x-ray absorption fine structure (EXAFS) spectra of Ce L3-edge were fitted with a core-shell model. The core parts are almost the same as bulk CeO2 with Ce4+ characteristic, however their shell parts change with surface coatings. The local atomic structures at shell parts (surface) and the contents and thicknesses of the shell parts were evaluated. The averaged Ce-O bond lengths at surface are changed from 2.504 Å (AOT), 2.568 Å (PVP), 2.628 Å (CTAB) to 2.634 Å (naked). The contents of surface are changed from 49% (AOT), 15% (PVP), 19% (CTAB) to 15% (naked). The results demonstrate that surface coatings have different influences on the surfaces of cerium oxide nanoparticles. A detailed discussion is given in this paper.