Theoretical Study on the Enhancement of Oxygen Adsorption by Superlattice Structures of Transition Metal Oxides

Abstract

Transition metal oxides (TMO) materials have been an indispensable material for catalyst and cathode applications. However, its applicability on oxygen evolution reaction (ORR) was quite challenging because of the low efficiency. The crucial problem of TMO materials was found to be the weak adsorption of oxygen molecule on the surface, which is the first step of ORR. This issue could be resolved by the superlattice structure of TMO, which possibly generates the dipole on the surface, thus changes the adsorption energy of oxygen molecule. Here, we investigated the effect of superlattice structure on the improvement of oxygen adsorption via density functional theory (DFT) calculations. Preferentially, we selected rutile-type materials (i.e., TiO2, VO2, CrO2, RuO2, and SnO2), which are simple yet fundamental, to understand the basic properties of superlattice structures. OTFG-Ultrasoft pseudopotential showed a high accuracy on the lattice parameter (less than 1.85% errors for all materials of our interest). TiO2 and SnO2 showed band gaps while RuO2 showed metallic band structure. Especially, the band structures of CrO2 and VO2 indicated half metallic characteristics, where only alpha spin exhibited conductivity. Based on these electronic properties, the superlattice structures were constructed by combining two different rutile-type materials with less than 5% of lattice mismatch, where the electronic distributions near the interface and the surface were investigated. Note that the surfaces of superlattice structures were modeled with (110) oxygen-bridge termination, which was the most stable termination of rutile-type materials. The dipole effect at the interface led to the electron re-distribution of the surface. The work function of TMO changed, inducing polarization along [110] direction. We found that strong polarization enhance the oxygen binding energy at the electron dominant surface. This theoretical study reveals that that the construction of the dipole-induced superlattice structure is a way of improving ORR.