Cerium oxide nanoparticles coated by surfactant sodium bis(2-ethylhexyl) sulphosuccinate (AOT): local atomic structures and X-ray absorption spectroscopic studies

Cerium oxide nanoparticles coated by sodium bis(2-ethylhexyl) sulphosuccinate (AOT) were prepared by using a microemulsion method. Transmission electron microscopy revealed an average particle size of 2-3 nm. X-ray diffraction showed that the cerium oxide nanoparticles retain the CeF2-type cubic structures like the bulk crystal. The intermediate valence of formally tetravalent compounds had been detected by x-ray-absorption near-edge structure (XANES) spectra of Ce L-III absorption in bulk CeO2 and the cerium oxide nanoparticles. Two well resolved white lines can be assigned to the electron configurations of 4f(0)L and 4f(1)L, respectively, where L denotes a ligand hole. At the same time, the cerium oxide nanoparticles also showed the structural features of trivalent compounds, in comparison to the trivalent Ce(NO3)(3) 6H(2)O, Four Lorentzian functions and two arctan functions were used to fit the normalized XANES spectra. The extended x-ray-absorption fine-structure (EXAFS) technique was used to probe the local atomic structures around the absorber Ce. The multielectron excitation effect on the EXAFS spectra was eliminated. A core-shell model was used to deduce the near-neighbour structural parameters around cerium. Bulk CeO2 with eight oxygen atoms located at 2.343 Angstrom was used as the reference sample to extract the backscattering amplitude and phase shift of the Ce-O bond. One half of the atoms locate at the core part with the CeF2-type cubic structures (eight oxygens at 2.343 Angstrom around Ce), the other half of the atoms are amorphous phase located in the shell part (surface of the nanoparticles) with approximately Ce2O3 structural features (averaged seven oxygens at 2.50 Angstrom ,around Ce).