Experimental and theoretical interpretation of the order/disorder clusters in CeO2:La

Abstract

A spectroscopic study of nanostructured lanthanum-doped cerium oxide samples showed that a mild decrease by5.3% in the surface concentration of Ce(III) species and, a reduction by 0.2 eV in the so-called energy-gap (EF ?Ev) after lanthanum addition was sufficient to create the previously reported significant dual behavior whenexposed to carbon monoxide at 653 K. The observed X-ray diffraction patterns indicated the presence of a purefluorite-type crystalline structure. A higher presence of paramagnetic defects clusters, indicated by electronparamagnetic resonance spectroscopy measurements and confirmed by the X-ray photoelectron spectroscopy,was attributed as being responsible for its dual behavior in a CO(g) atmosphere. Theoretical studies have shownthe band structure and density of state of the pure and doped material, illustrating the orbitals that participate inthe valence band and conduction band. The addition of a small quantity of La3+ converted some Ce3+ into Ce4+,narrowed the ?effective band-gap? by 0.2 eV, and created a singly ionized oxygen vacancy species, thus changingthe total electrical resistance and creating its dual behavior. The exposure to a reducing atmosphere such ascarbon monoxide resulted in a significant increase in defects and converted some Ce4+ into Ce3+ and vice-versawith oxygen atmosphere exposure.