DFT + U and Low-Temperature XPS Studies of Fe-Depleted Chalcopyrite (CuFeS₂) Surfaces: A Focus on Polysulfide Species

The initial release of cations upon oxidation of metal sulfides commonly produces a metal-deficient surface and undersurface layers, which should greatly affect the properties of materials but are still poorly understood. We employed density functional theory + U simulation of chalcopyrite (012) and (110) surfaces with up to a half of surface iron removed together with X-ray photoelectron spectroscopy (XPS) of fast-frozen chalcopyrite oxidized in aqueous solutions. It was calculated that the centers comprising tri- or pentasulfide anions or tri- and disulfide complexes have the negative formation energy of 1.2–1.5 eV per one extracted Fe atom, while defects with disulfide anions are disadvantageous. The surfaces are typically “metallic” with comparable densities of S sp and Cu 3d states at the Fermi level. Upon performing cryo-XPS studies, it was found that sulfide surfaces depleted in iron but not in copper, and polysulfide anions S_n^2– with n ≥ 5 arose. As oxidation progresses, a deficit of Cu occurs, and S–S chains grow. Upon warming up to room temperature, polysulfide species partially volatilize, so S₅^2– and S₃^2– anions appear to prevail, while the minor contribution of disulfide remains unchanged. The high stability of “polysulfide” centers is considered responsible for retarded oxidation and leaching (“passivation”) of chalcopyrite; metallic DOS is important for the physical properties of the surfaces.