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

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 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.

CO2 Adsorption and Activation on the (110) Chalcopyrite Surfaces: A Dispersion-Corrected DFT + U Study

“…Recently, Mikhlin et al 24 have studied possible configurations arising at CuFeS 2 (110) and (012) surfaces because of the removal of adjacent Fe atoms in the top layers by applying DFT calculations and also performed X-ray photoelectron spectroscopy studies of the chalcopyrite samples slightly oxidized in water. 25…”

Towards Understanding the Role of Surface Gas Nanostructures: Effect of Temperature Difference Pretreatment on Wetting and Flotation of Sulfide Minerals and Pb-Zn Ore

“…However, it should be taken in consideration that modification of the sulfide surfaces occurs ex situ upon handling the samples and XPS experiment in the ultra-high vacuum, including volatization of water, elemental sulfur, polysulfide anions, dixanthates, and decomposition of solid/water interface and some compounds. The species adsorbed due to the interaction with xanthate collector and other flotation reagents at particulate sulfide minerals [ 18 , 19 ] and initial oxidation of chalcopyrite [ 42 ] were studied previously using low-temperature XPS of the fast-frozen specimens. We applied this method for examination of the oxidation of galena, sphalerite, and pyrite; the results to be published elsewhere.…”

X-ray Photoelectron Spectroscopy in Mineral Processing Studies

“…The studies [62][63][64][65][66][67][68] performed with the aim to understand the surface S-S species have elucidated that the end atoms in polysulfide anions bear negative charges similar to that in disulfide groups with the S 2p 3/2 BE about 162.5 eV. A small charge may be localized at S atoms nearest to the terminal ones [66], while intermediate atoms are close to elemental sulfur with almost no charge and BE between 163 and 164 eV.…”

“…The occurrence of polysulfides on reacted sulfide mineral surfaces was confirmed with Raman spectroscopy (not quite surely, though) [57,69,70] and TOF-SIMS [71] and other techniques. Polysulfides are thought to be stable in the spectrometer vacuum, in contrast to elemental sulfur [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][63][64][65][66][67][68].…”

“…The inconsistency seems to be resolved by using hard XPS depth profiling [89,90] which elucidated that the metal-depleted surfaces are composed of several layers with only the outer one containing polysulfide anions. Nasluzov et al [68] found with DFT+U simulation and cryo-XPS of fast-frozen chalcopyrite (Figure 1) that the polysulfides as intermediates to elemental sulfur are surprisingly stable and hinder the release of elemental sulfur (see Section 3.3 for details).…”