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 () 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 is very challenging. We obtained the for (100), (110), and (111) surfaces of normal zinc-based spinel oxides ZnAlO, ZnGaO, ZnInO, ZnVO, ZnCrO, ZnMnO, ZnFeO, and ZnCoO. The most stable surface is (100) for all compounds. The smallest for a surface is the largest in the (100) surface except for ZnCoO. For (100) and (110) surfaces, there is a good correlation, over all spinels, between the smallest for the surface and bulk formation energy, while the ionization potential correlates well in (111) surfaces. Machine learning over of all surface sites in all orientations and for all compounds to find the important factors, or descriptors, that decide the 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.