Quantitative structure determination of adsorbed formate and surface hydroxyls on Fe₃O₄(001)

Using the chemically specific techniques of normal incidence X-ray standing waves and photoelectron diffraction, we have investigated the dissociative adsorption of formic acid on the Fe₃O₄(001) surface, specifically probing the local structures of both the adsorbed formate and resulting surface hydroxyl. Using model independent direct methods, we reinforce the observations of a previous surface X-ray diffraction study that the formate molecule adsorbs with both oxygens atop octahedrally coordinated surface Fe cations and that ∼60% of the formate is adsorbed in the so called tet site. We additionally determine, for the first time, that the surface hydroxyl species are found at the so called int site. This confirms previous DFT predictions and reinforces the pivotal role the surface hydroxyl plays in lifting the subsurface cation vacancy termination of the Fe₃O₄(001) surface.