Mixing properties of Al2O3(0001)-supported M 2O3 and MM′O3 monolayers (M, M′ = Ti, V, Cr, Fe)

Considering the importance of sub-monolayer transition metal oxides supported on another oxide in many industrial processes, with the help of a DFT + U approach, we provide information on the structural and electronic properties of pure M O-2(3) and mixed MM ' O-3 oxide monolayers (M, M ' = Ti, V, Cr, Fe) supported on an alpha-Al2O3(0001) support. With their structure in the prolongation of the alumina corundum lattice, the monolayers have non-equivalent surface and interface cations, which leads to two different cation configurations in the mixed oxides. In all cases, the interfacial charge transfer is weak, but strong cation-cation electron redistributions may take place as in TiVO3, TiFeO3, VFeO3, and TiCrO3 in which actual redox processes lead to cation oxidation states different from the expected +3 value. We show that the tendency to mixing relies on the interplay between two very different driving forces. Cation-cation redox reactions, in most cases, strongly stabilise mixed configurations, but preference for a given cation position in the monolayer, because of surface energy reasons, may strengthen, weaken or even block the mixing tendency. By comparison with results obtained in bulk ilmenite, in free-standing monolayers and in MLs deposited on transition metal substrates, we evidence the flexibility of their electronic structure as a function of size, dimensionality and nature of support, as a lever to tune their properties for specific applications.

Automated summary

With the help of a DFT + U approach, the structural and electronic properties of sub-monolayer transition metal oxides supported on α-Al2O3 are investigated. The study reveals non-equivalent surface and interface cations in mixed oxides, as well as cation oxidation states different from the expected values. The formation of mixed oxides is influenced by the interplay between cation-cation redox reactions and surface energy considerations. The flexibility of the electronic structure, depending on size, dimensionality, and nature of support, allows for tuning the properties of these oxides for specific applications.