Electronic states and magnetic structure at the Co ₃O ₄(110) surface: A first-principles study

Tricobalt tetraoxide (Co ₃O ₄) is an important catalyst and Co ₃O ₄(110) is a frequently exposed surface in Co ₃O ₄ nanomaterials. We employed density-functional theory with the on-site Coulomb repulsion U term to study the atomic structures, energetics, and magnetic and electronic properties of the two possible terminations, A and B, of this surface. These calculations predict A as the stable termination in a wide range of oxygen chemical potentials, consistent with recent experimental observations. The Co3 ⁺ ions do not have a magnetic moment in the bulk, but become magnetic at the surface, which leads to surface magnetic orderings different from that in the bulk. Surface electronic states are present in the lower half of the bulk band gap and cause partial metallization of both surface terminations. These states are responsible for the charge compensation mechanism stabilizing both polar terminations. The computed critical thickness for polarity compensation is four layers.