Structure and chemical transformation in cerium oxide nanoparticles coated by surfactant cetyltrimethylammonium bromide (CTAB): An x-ray absorption spectroscopic study

Cerium oxide nanoparticles coated by cetyltrimethylammonium bromide (CTAB) were prepared using reverse micelles and microemulsion method. The as-prepared nanoparticles (25 degreesC) were annealed at 200, 350, 500, and 700 T for 2 h. Their particle sizes are, respectively, 230, 50, 2, 6, and 10 nm as identified by transmission electron microscopy and X-ray diffraction patterns. A chemical transformation from amorphous Ce3+ component to crystalline Ce4+ component was found in the temperature range of 200 degreesC to 400 degreesC. X-ray absorption spectroscopy was used to reveal the electronic and atomic structures. The intermediate spectroscopic valence of formally tetravalent compounds and four features A, B, C, and D were detected from the X-ray absorption near edge structure (XANES) spectra of Ce-L-III absorption for samples annealing at 350, 500, and 700 degreesC. As in trivalent bulk Ce(NO3)(3).6H(2)O, the Ce3+ characteristic was also found for samples annealing at 25, 200, and 350 degreesC. Multielectron excitation effect on the EXAFS spectra was eliminated. Local atomic structures of these nanoparticles were probed by using extended X-ray absorption fine structure (EXAFS) technique. The obtained structural parameters for bulk Ce(NO3)(3)-6H(2)O were in excellent agreement with its crystallographic data. For nanoparticles annealed at 500 degreesC and 700 degreesC, similar atomic structures with bulk CeO2 were found. For the as-prepared nanoparticles without annealing (25 degreesC), a single shell (10 x 2.59 Angstrom) is sufficient to describe its local structures, while two shells had to be used for the nanoparticles annealed at 200 T and 350 'C. One shell is corresponding to the Ce3+ Component. Another is corresponding to the Ce4+ component. All Ce4+ components have the same structural parameters except different Debye-Waller factors or atom-pair distribution functions. A core-shell model was used to explain the structural parameters around cerium for the two samples annealed at 200 degreesC and 350 degreesC. The core part is the Ce4+ component with crystalline CaF2-type cubic structures (eight oxygens at 2.343 Angstrom around Ce); the shell part is an amorphous Ce3+ component with different structures. The mechanism of chemical transformation was discussed in this paper.