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.