Spectroscopic evidence of defective species in nanostructured lanthanum (La)-doped cerium oxide (CeO₂) gas sensors

Full text: A spectroscopic study of nanostructured lanthanum-doped cerium oxide samples showed that a mild decrease by 5.3% in the surface concentration of Ce(III) species and, a reduction by 0.2eV in the so-called energy-gap (E_F – E_v) after lanthanum addition was sufficient to create the previously reported significant dual behavior when exposed to carbon monoxide at 653K. The observed X-ray diffraction patterns indicated the presence of a pure fluorite-type crystalline structure. A higher presence of paramagnetic defects clusters, indicated by electron paramagnetic 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 shown the band structure and density of state of the pure and doped material, illustrating the orbitals that participate in the valence band and conduction band. The addition of a small quantity of La³⁺ converted some Ce³⁺ into Ce⁴⁺, narrowed the "effective band-gap" by 0.2eV, and created a singly ionized oxygen vacancy species, thus changing the total electrical resistance and creating its dual behavior. The exposure to a reducing atmosphere such as carbon monoxide resulted in a significant increase in defects and converted some Ce⁴⁺ into Ce³⁺ and vice-versa with oxygen atmosphere exposure. (author)