Memory effect of ZrO2 matrix on surface Co3O4–CoO transition

Abstract

Cobalt oxide system Co3O4-CoO has been studied on the ZrO2 matrix surfaces with FTIR and XPS. The tetragonal and monoclinic ZrO2 matrix materials have been synthesized from the Zr-n-propoxide-acetylacetone-water-isopropanol system. The study shows that the ZrO2 matrix is able to retain the relative Co3O4:CoO population at elevated temperatures. The thermodynamically stable oxide population (Co3O4:CoO) at room temperature for ZrO2-supported Co3O4-CoO is about 50:50 (Co2+ :Co3+ = 2:1), which is markedly different from the 100:0 case (Co2+ :Co3+ = 1:2) for an unsupported Co3O4-CoO surface oxide system. The relative Co3O4 :CoO ratio in the surface region of the ZrO2 is temperature dependent but matrix-polymorph independent. The composition of an oxide solid solution formed by the Co3O4-CoO and matrices of ZrO2 is determined to have a cobalt molar percentage of 4.5%. Diffusion thermodynamic quantities are investigated, and the measured diffusion activation energy for a cobalt ion in the ZrO2 matrices is 0.21 eV. The mechanism of the ZrO2 memory effect on surface Co3O4-CoO transition will also be addressed.