Cerium oxide nanoparticles coated by surfactant sodiumbis(2-ethylhexyl) sulphosuccinate (AOT): local atomic structures and x-rayabsorption spectroscopic studies

Abstract

Cerium oxide nanoparticles coated by sodium bis(2-ethylhexyl) sulphosuccinate(AOT) were prepared by using a microemulsion method. Transmission electronmicroscopy revealed an average particle size of 2-3 nm. X-ray diffractionshowed that the cerium oxide nanoparticles retain the CeF2-type cubicstructures like the bulk crystal. The intermediate valence of formallytetravalent compounds had been detected by x-ray-absorption near-edgestructure (XANES) spectra of Ce LIII absorption in bulk CeO2 and the cerium oxide nanoparticles. Two well resolved white lines canbe 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 oftrivalent compounds, in comparison to the trivalent Ce(NO3)3·6H2O. Four Lorentzian functions and two arctanfunctions were used to fit the normalized XANES spectra. The extendedx-ray-absorption fine-structure (EXAFS) technique was used to probe the localatomic structures around the absorber Ce. The multielectron excitation effecton the EXAFS spectra was eliminated. A core-shell model was used to deducethe near-neighbour structural parameters around cerium. Bulk CeO2with eight oxygen atoms located at 2.343 Å was used as the reference sampleto extract the backscattering amplitude and phase shift of the Ce-O bond. Onehalf of the atoms locate at the core part with the CeF2-type cubicstructures (eight oxygens at 2.343 Å around Ce), the other half of theatoms are amorphous phase located in the shell part (surface of thenanoparticles) with approximately Ce2O3 structuralfeatures (averaged seven oxygens at 2.50 Å around Ce).