Surface Termination of Fe₃O₄(111) Films Studied by CO Adsorption Revisited

Although the (111) surface of Fe₃O₄ (magnetite) has been investigated for more than 20 years, substantial controversy remains in the literature regarding the surface termination proposed based on structural and adsorption studies. The present article provides density functional theory results that allow to rationalize experimental results of infrared reflection-absorption spectroscopy and temperature-programmed desorption studies on CO adsorption, thus leading to a unified picture in which the Fe₃O₄(111) surface is terminated by a ¹/₄ monolayer of tetrahedrally coordinated Fe³⁺ ions on top of a close-packed oxygen layer as previously determined by low energy electron diffraction. However, surface defects play a crucial role in adsorption properties and may dominate chemical reactions on Fe₃O₄(111) when exposed to the ambient.