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 CeL_III absorption in bulk CeO₂ and the cerium oxide nanoparticles. Two well resolved white lines can be assigned to the electron configurations of 4f⁰L and 4f¹L, 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 · 6H₂O. 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 Å 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 Å 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 Å around Ce).