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Microscopic Processes and Phenomena at Oxide Surfaces and Interfaces

Final Report Summary - OXIDESURFACES (Microscopic Processes and Phenomena at Oxide Surfaces and Interfaces)

Metal oxides are the most common inorganic materials in the Earth’s crust, and exhibit an enormous variety in their physical and chemical properties. Their surfaces are central in many applications, most notably in energy-related technologies such as photo-, electro-, and heterogeneous catalysis or fuel/electrolysis cells. At the same time the complexity of metal oxides, their many compositions, structures, and defects, makes it notoriously difficult to obtain reliable atomic-scale information about the microscopic surface processes. This project has provided molecular-scale insights are needed to better understand, and ultimately control, these promising materials.

In a surface science approach we have studied idealized model systems (single crystals with a controlled crystal structure, composition, and defects) under tightly controlled conditions (usually ultrahigh vacuum, UHV). We have combined with state-of-the-art microscopy and spectroscopic techniques with the development of novel methodologies to study surfaces atom-by-atom and molecule-by-molecule, in three main areas:

(1) Defects at the surface and in the bulk: The surface chemistry of metal oxides is largely defect-controlled. At the outset of this project little was known about the interaction between the defects located at the surface and inside the material. With the tip of our Scanning Tunneling Microscope (STM) we found ways to move single oxygen vacancies to and from the surface, and identify adsorbed molecules. We also identified the unique role of missing metal ions: ordered subsurface cation vacancies result in special surface structures, which are useful for studying single-atom processes related to catalysis. We have probed surface reactivity with small organic molecules, CO, H2O, and O2, and we have shown how to charge and discharge single molecules with the tip of our atomic force microscope (AFM).
(2) Multi-elemental oxides: While binary metal oxides are already quite complicated, the addition of a third element adds another dimension of complexity. We have taken up this challenge to prepare well-defined samples necessary for meaningful surface science investigations, focusing on the versatile materials class of perovskite oxides. We refined ways to change surface structure via stoichiometry, have interfaced advanced surface characterization with pulsed laser deposition (PLD) growth of complex oxides, and have used cleaved single crystals of layered, perovskite-type structure for insights into the surface chemistry of these materials. As an important outcome we have transferred physics approaches (for sample treatment, film growth, and concepts relying on the rotation/tilt of the octahedral units that constitute the perovskite structure) to surface reactivity.
Many of our experiments were performed in close collaboration with theoretical groups. This allowed deeper insights into electronic structure of metal oxides, specifically the degree of localization of excess charge, and the formation of so-called polarons.
(3) Oxide surfaces in aqueous solution: It important to make connect UHV-based surface science experiments to more relevant environments, in particular to studies in liquid water. We have worked on developing appropriate methodologies and procedures to study semiconducting metal oxides in water with a high degree of cleanliness and control.