Facet-Dependent Cobalt Ion Distribution on the CoO Nanocatalyst Surface.

CoO is an important catalyst widely used for CO oxidation or electrochemical water oxidation near room temperature and was also recently used as support for single-atom catalysts (SACs). CoO with a spinel structure hosts dual oxidation states of Co and Co in the lattice, leading to the complexity of its surface structure as the exposure of Co and Co has a significant impact on the performance of the catalysts. Although it is acknowledged that different facets exhibit varied catalytic activities and different abilities in hosting single atoms to provide active centers in SACs, the CoO surface structure remains under-investigated. In this study, major facets of {111}, {110}, and {100} were studied down to subangstrom scale using advanced electron microscopy. We noticed that each facet has its own most stable surface configuration. The distribution of Co and Co on each facet was quantified, revealing a facet-dependent distribution of Co and Co. Co was found to be preferentially exposed on {100} and {110} as well as surface steps. Surface reconstruction was revealed, where a subangstrom scale shift of Co was confirmed on facets of {111} and {100} due to polarity compensation and oxygen deficiency on the surface. This work not only improves our fundamental understanding of the CoO surface structure but also may promote the design of CoO-based catalysts with tunable activity and stability.