DFT + U and Low-Temperature XPS Studies of Fe-Depleted Chalcopyrite (CuFeS 2 ) Surfaces: A Focus on Polysulfide Species Vladimir Nasluzov, Aleksey Shor, Alexander Romanchenko, Yevgeny Tomashevich, and Yuri Mikhlin* Federal Research Center “Krasnoyarsk Scientific Center”, Institute of Chemistry and Chemical Technology of the Siberian Branch of the Russian Academy of Sciences, Akademgorodok, 50/24, Krasnoyarsk 660036, Russia *S Supporting Information ABSTRACT: 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 chalco- pyrite 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 S 5 2− and S 3 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. 1. INTRODUCTION Chalcopyrite, CuFeS 2 , is an antiferromagnetic semiconductor with the band gap of about 0.5 eV, having a zincblende-type crystalline structure with Fe 3+ and Cu + cations in tetrahedral coordination with S 2− anions, and each S atom has two Cu and two Fe as the nearest neighbors. 1−12 Chalcopyrite shows interesting magnetic, thermoelectric, optoelectronic, and other properties (for example, refs11−16), which are influenced, especially in the case of nanomaterials and thin films, by the state of the surface and near-surface region. Furthermore, chalcopyrite is the main mineral and industrial source of copper, and its geochemical behavior, hydrometallurgical leaching, flotation, and so forth greatly depend on the character of surfaces arising in these processes. 10 Elemental sulfur is the main S-bearing product of corrosion of chalcopyrite in the atmosphere and aqueous solu- tions. 10,17−20 At the same time, numerous studies utilizing X- ray photoelectron spectroscopy (XPS), Auger electron spec- troscopy, time-of-flight secondary ion mass spectrometry, X- ray absorption, and Raman spectroscopies 21−34 have found that oxidation commonly produces surface layers strongly depleted in metal and contained di- and polysulfide anions due to the preferential release of cations from the sulfide phase; the metal-deficient regions can be as thick as a few tens of nanometers or even more. 7,23−25,31,32 These phenomena are poorly understood, and some researchers have put in doubt the existence of the metal-deficient structures and polysulfide species, attributing these to chemisorbed sulfur because of its volatility under vacuum. 33−35 The oxidation and leaching of metal sulfides are effectively retarded over a wide range of conditions because of “passivation”, the nature of which, and a role of the metal-deficient layers, are still disputable. 10,17−46 Density functional theory (DFT) methods have been widely applied to simulate the bulk structure and surfaces of chalcopyrite, 47−58 including adsorption of water, mineral acids, cations, and flotation reagents, usually at the CuFeS 2 (001) crystal face. It has been revealed, in particular, that reconstruction of the surfaces results in the formation of disulfide anions, 49−51,56−58 in accordance with photoelectron spectra of the surfaces fractured in an ultra-high vacuum, 59−61 with the energies of reconstructed surfaces laying in the range of 0.53−0.95 J/m 2 . 56 However, heavily metal-depleted structures containing polysulfide have not been explored theoretically. Recently, we found from depth-resolved high-energy photo- emission spectroscopy (HAXPES) and X-ray absorption spectroscopy that the metal-deficient regions of reacted chalcopyrite 32 and iron sulfides 62 are composed of a thin outer layer with high S excess and polysulfide species, then metal-deficient zone with mono- and disulfide anions, the Received: June 28, 2019 Revised: August 2, 2019 Article pubs.acs.org/JPCC Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.9b06127 J. Phys. Chem. C XXXX, XXX, XXX−XXX Downloaded via NOTTINGHAM TRENT UNIV on August 19, 2019 at 08:04:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.