Non-contact atomic force microscopy study of hydroxyl groups on the spinel MgAl2O4(100) surface

Abstract

Atom-resolved non-contact atomic force microscopy (NC-AFM) studies of the magnesium aluminate (MgAl2O4) surface have revealed that, contrary to expectations, the (100) surface is terminated by an aluminum and oxygen layer. Theoretical studies have suggested that hydrogen plays a strong role in stabilizing this surface through the formation of surface hydroxyl groups, but the previous studies did not discuss in depth the possible H configurations, the diffusion behaviour of hydrogen atoms and how the signature of adsorbed H is reflected in atom-resolved NC-AFM images. In this work, we combine first principles calculations with simulated and experimental NC-AFM images to investigate the role of hydrogen on the MgAl2O4(100) surface. By means of surface energy calculations based on density functional theory, we show that the presence of hydrogen adsorbed on the surface as hydroxyl groups is strongly predicted by surface stability considerations at all relevant partial pressures of H2 and O2. We then address the question of how such adsorbed hydrogen atoms are reflected in simulated NC-AFM images for the most stable surface hydroxyl groups, and compare with experimental atom-resolved NC-AFM data. In the appendices we provide details of the methods used to simulate NC-AFM using first principles methods and a virtual AFM.