Factors determining surface oxygen vacancy formation energy in ternary spinel structure oxides with zinc

Spinel oxides are an important class of materials for heterogeneous catalysis including photocatalysis and electrocatalysis. The surface O vacancy formation energy (E-Ovac) is a critical quantity for catalyst performance because the surface of metal oxide catalysts often acts as a reaction site, for example, in the Mars-van Krevelen mechanism. However, experimental evaluation of E-Ovac is very challenging. We obtained the E-Ovac for (100), (110), and (111) surfaces of normal zinc-based spinel oxides ZnAl2O4, ZnGa2O4, ZnIn2O4, ZnV2O4, ZnCr2O4, ZnMn2O4, ZnFe2O4, and ZnCo2O4. The most stable surface is (100) for all compounds. The smallest E-Ovac for a surface is the largest in the (100) surface except for ZnCo2O4. For (100) and (110) surfaces, there is a good correlation, over all spinels, between the smallest E-Ovac for the surface and bulk formation energy, while the ionization potential correlates well in (111) surfaces. Machine learning over E-Ovac of all surface sites in all orientations and for all compounds to find the important factors, or descriptors, that decide the E-Ovac revealed that bulk and surface-dependent descriptors are the most important, namely the bulk formation energy, a Boolean descriptor of whether the surface is (111) or not, and the ionization potential, followed by geometrical descriptors that are different in each O site.

Automated summary

Spinel oxides are important for catalysis, with the formation energy of surface O vacancies critical for catalyst performance. The (100) surface is most stable on various compounds, with the smallest E-Ovac mostly on this surface. Bulk formation energy correlates well with the smallest E-Ovac on (100) and (110) surfaces, while ionization potential is important for (111) surfaces. Machine learning analysis identified bulk and surface-dependent descriptors as key factors determining E-Ovac.