Growth and electronic properties of Co-doped Mn₃O₄ thin films: a combined experimental and theoretical investigation

In this study, we have prepared and investigated the electronic properties of a new and promising cobalt doped Mn₃O₄ oxide surface by site-selective and element-sensitive X-ray-absorption (XAS) and photoemission spectroscopy (XPS and resonant PES) combined with density functional theory (DFT) calculations. The crystallinity of both pristine and Co doped thin films was ensured by low energy electron diffraction measurements, in which similar diffraction patterns were obtained for both films suggesting the inclusion of the dopant species within the crystalline Mn₃O₄ thin film. According to our combined experimental data and theoretical calculation results, XAS measurements and replace energy calculations could identify Co impurities adopting preferentially a 2+ oxidation state substituting a Mn²⁺ cation on tetrahedral sites (80%) as well as the Mn³⁺ on octahedral sites (20%). Direct evidence of these findings could be found by comparing the pristine Mn₃O₄ electron absorption and photoemission spectral features with those of the doped ones. For instance, the formation of oxygen vacancies related to the formation of Co²⁺ in an octahedral site could be directly observed. Remarkably, the valence band spectrum of Co-Mn₃O₄ thin films presents additional spectral features close to the Fermi edge that can be directly attributed to Co states when compared to the PDOS obtained by the DFT calculations. It is noteworthy that the formation and stabilization of these Co dopant species in the host Mn₃O₄ surface could potentially affect and obviously modulate its capability for adsorption of molecular species and transfer of electrons, which makes the cobalt doped Mn₃O₄ surface potentially promising for catalytic applications.