Effect of thermal treatment on the 4f-hopping conductivity of CeO2 exposed to CO(g) atmosphere

•A well-defined morphology could be seen for the sample calcined at 900 °C.•The area-volume relation affects the number of charge carriers, reflecting in the pre-exponential factor A of conductivity.•Reducing gases are responsible for the increase in the number of available carriers for conduction.•The annihilation of oxygen vacancies, as a consequence of oxygen exposure, otherwise reduces the amount of Ce(III) species along with a lower number of carriers. The thermal treatment of CeO2 at 600 and 900 °C on the film response against CO is analyzed. A fluorite-type crystalline structure was observed, with the later presenting a well-defined morphology formed by the set of (100), (110) and (111) surfaces. The band-gap was 3.35 and 3.37 eV, for the 600 and 900 °C samples. We have proposed an equation based on the mass action law that shows the dependence of the carriers density as a function of the grain size, demonstrating that the area-volume relation can affect the number of charge carriers, which reflects in the pre-exponential A factor of conductivity. Reducing gases generate a greater number of carriers available for conduction, reflected on the A factor. When vacancies are annihilated by exposing to an oxygen atmosphere, the Ce (III) amount reduces. The activation energy and pre-factor A in air, vacuum and CO were calculated for correlating with the sample morphologies.