Center for Functional Nanomaterials Seminar

Oxide nanoparticle catalysis is a field currently undergoing tremendous growth, and much work has been done to engineer catalysts with high reactivity and selectivity in order to lower energy demands. Although relatively well characterized in the bulk, surface effects are magnified for particles <5nm where the percentage of atoms considered to be in a bulk-like state becomes vanishingly small. For many systems, the assumption of surface structures related to a simple bulk truncation is inadequate as it fails to account for the high energy of dangling bonds. This problem is magnified for polar surfaces which classically exhibit infinite surface energy and surface dipoles if the valence imbalances are not properly accounted for. Understanding the driving forces for surface structure modification is critical to uncovering atomistic relations between nanoparticle structure and performance.

In this talk I will present experimental and theoretical results for a series of structures on the polar MgO (111) surface, which is a model system for highly ionic rocksalt oxides. The experimental data clearly shows that there are substantial coverages of hydroxyl groups on these surfaces even after ultra-high vacuum (UHV) annealing. Utilizing a combination of transmission high-energy electron diffraction (THEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations, the transitions between these structures can be described by a kinetic model involving mobile protons and hydroxyl groups over a relatively stationary cation framework. Notably, many of the observed structures are not those predicted by DFT thermodynamics, but are kinetically metastable. The inaccessibility of thermodynamically predicted structures is a problem which is likely to be magnified by the short processing times required in real catalysts to minimize coarsening and agglomeration effects during nanoparticle synthesis.