DFT + U and Low-Temperature XPS Studies of Fe-Depleted

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DFT + U and Low-Temperature XPS Studies of Fe-Depleted Chalcopyrite (CuFeS2) 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

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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 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 Sn2− 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 S52− and S32− 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, CuFeS2, is an antiferromagnetic semiconductor with the band gap of about 0.5 eV, having a zincblende-type crystalline structure with Fe3+ and Cu+ cations in tetrahedral coordination with S2− 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 solutions.10,17−20 At the same time, numerous studies utilizing Xray photoelectron spectroscopy (XPS), Auger electron spectroscopy, time-of-flight secondary ion mass spectrometry, Xray absorption, and Raman spectroscopies21−34 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 © XXXX American Chemical Society

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 CuFeS2(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/m2.56 However, heavily metal-depleted structures containing polysulfide have not been explored theoretically. Recently, we found from depth-resolved high-energy photoemission spectroscopy (HAXPES) and X-ray absorption spectroscopy that the metal-deficient regions of reacted chalcopyrite32 and iron sulfides62 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

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DOI: 10.1021/acs.jpcc.9b06127 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the surface (012), and a = 754.5 pm, b = 1054.4 pm for the surface (110). In the c-direction, the slabs were separated by vacuum gaps of 1500 and 900 pm for (012) and (110) surfaces, respectively. Ten-layer slabs built using (a,b) or (2a,1b) in-plain translation vectors and Cu16Fe14S32 or Cu32Fe30S64 unit cells, respectively, represented the surface structures formed upon deletion of two neighboring Fe atoms. The formation energy Ef of defects with x Fe vacancies corresponds to the energy of an arbitrary reaction

thickness of which varied from few nm to tens nm, and nearly stoichiometric defective underlayers down to hundred nm in depth. DFT + U simulation32 specified the most stable defects in the inner layers; particularly, the centers involving disulfide anions near double Fe vacancies in the “disulfide” structure, while polysulfide anions were essentially unstable in the “bulk”. The aim of the current research was to examine surfaces of chalcopyrite depleted in Fe both theoretically with DFT + U simulation taking into account a Hubbard-type correlation energy U for Coulomb repulsion and exchange interaction on Fe sites,11,32,63 and experimentally, using the method of lowtemperature XPS of the fast-frozen particulate material developed by Shchukarev.64,65 This technique allows preserving polysulfide and elemental sulfur, which are volatile in an ultra-high vacuum, along with a thin frozen film of the adjacent aqueous solution,44,66,67 making the spectra more sensitive to the surface of the solid. The evolution of sulfur species formed under different etching conditions and after warming up to room temperature was monitored and compared with the results of DFT + U calculations. It was found, particularly, that the surface centers involving three and five bound sulfur atoms are very stable, while isolated disulfide anions are not, in contrast to the underlying layer32 and reconstructed stoichiometric surfaces.57−60 The findings are discussed in terms of mechanisms behind the formation of the thick nonstoichiometric underlayers, essentially retarded release of elemental sulfur and “passivation” of chalcopyrite oxidation, and physical properties of the surfaces.

Cu nFenS2n + 3x /2H 2Osol + 3x /4O2 → Cu nFen − xS2n + x Fe(OH)3sol

(1)

where n = 16 or 32, x = 1 or 2, and index “sol” indicates that the total molecular energies of H2O and Fe(OH)3 included contributions of solvent effects.75 To compare different structures, we also considered the formation energies Ef(1) per one removed iron atom. 2.2. XPS of Fast-Frozen Samples. Natural chalcopyrite (Primorskoye, Russia) with no visible inclusions of foreign phases was ground in an agate mortar to a size of −75 μm (about 0.5 g)67 and added to 20 mL of water or aqueous solutions of 0.05 M HCl, 0.05 M FeCl3 + 0.05 M HCl, 0.025 M Fe2(SO4)3 + 0.05 M H2SO4 or similar media, and agitated in a thermostated glass, typically at 25 °C or 50 ± 0.5 °C, for a predetermined time. The relatively low concentrations of ferric salts and reaction times were chosen in such a way that the quantities of reagents and products allow observation of the modified sulfide surfaces. Then the slurry was transferred into a polypropylene beaker, centrifuged at 5000 rpm for 10 min, and the supernatant was discarded. The wet precipitate after centrifugation was placed on a molybdenum sample holder and frozen in the spectrometer air-lock under dry N2(g) for 60 s prior to pumping to a vacuum of 10−6 mbar.44,65−67 The sample was transferred to the precooled manipulator in the analytical chamber; the temperature was maintained at −160 ± 5 °C, and the vacuum was 10−9 mbar over the experiment. After the measurement, the samples were warmed up in a vacuum overnight and re-examined at room temperature. For comparison, the photoelectron spectra were also collected from several samples after centrifugation, which were dried in the lock chamber vacuum or air and measured at room temperature. The photoelectron spectra were acquired with a SPECS photoelectron spectrometer (SPECS Surface Nano Analysis GmbH) equipped with a PHOIBOS 150 MCD9 energy analyzer at electron take-off angle 90° using a monochromated Al Kα source (1486.7 eV) operated at 180 W. The binding energy (BE) scale was referenced to the C 1s peak of aliphatic carbon at 285.0 eV of an adventitious contamination layer. The high-resolution spectra were fitted with Gaussian−Lorentzian peak profiles after Shirley background subtraction with CasaXPS software. Spin−orbit energy splitting and an intensity ratio for the S 2p3/2,1/2 doublet were assumed to be 1.19 eV and 2:1, respectively, with the components of the doublet having equal linewidth.

2. METHODS 2.1. Computational Details. The DFT calculations were performed using the GGA + U scheme63 with the PW91 parameterization of the exchange−correlation functional68,69 and the Hubbard-type on-site U parameter as implemented in the Vienna ab initio simulation package.70,71 The U correction was set equal to 4 eV for Fe 3d states and 0 eV for the Cu site, as we justified previously.32 Spin unrestricted eigenfunctions of the projector augmented wave method72,73 with explicit treatment of the 3s23p4 electrons for S atoms, 4s13d10 electrons for Cu atoms, and 3d74s1 electrons for Fe atoms were generated using plane waves restricted by an energy cut off of 400 eV. Numerical integration in the reciprocal space was carried out using a 5 × 4 × 1 Monkhorst−Pack k-points grid.74 A 10 atomic layer slab with Cu16Fe15S32 unit cells was employed to simulate CuFeS2(012) and (110) surfaces (Figure 1) with isolated Fe vacancies, where every second Fe atom of the upper cationic layer is missing. In-plane lattice translations were fixed to match equilibrium geometry of the bulk lattice with antiferromagnetic order of magnetic moments of Fe ions. The translation vectors were a = 533.5 pm, b = 1499.5 pm for

3. RESULTS 3.1. DFT + U Calculations. 3.1.1. Intrinsic CuFeS2. Figure 1 shows optimized structures of unreconstructed stoichiometric CuFeS2(012) and (110) surfaces. Both surfaces are composed of the valleys with four-coordinated S centers at the bottom and the S-terminated ridges with three-coordinated top

Figure 1. Structures of intrinsic, stoichiometric (012) and (110) surfaces of CuFeS2; blue, brown, and yellow spheres represent Cu, Fe, and S atoms, respectively. Cations of the first (U) and second (D) atomic metal layers, and anions of the top S-layer are marked for following the discussion of the formation of defects in Figures 2−4. B


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U(Fe) vacancy adjacent to S1 and S2 atoms at the (012) surface (Figure 1) is occupied by the S1 atom. On the (110) surface, similar transfer of a top S atom to the U(Fe) vacancy results in structure 2 with the formation energy Ef of −1.20 eV. If the S atoms occupy vacant U(Cu) positions (structures 3 and 4), the energies Ef are calculated to be −0.65 and −0.91 eV, respectively. The relocated Cu atoms are in interstitial positions (CuI) and coordinate S atoms forming nearly linear S−Cu−S chains. The S−S bond lengths in the structures 1−4 are of 208−214 pm, and Cu−S and Fe−S bonds vary in the range of 213−241 and 234−243 pm, respectively (for more details see Table S1). In addition to S3 groups, each of the defects 1−4 contains one three-coordinated metal atom in the D-layer, i.e. Cu3c in the first two structures, and Fe3c in the other two structures (Table S1). The variations in the coordination environment of the S3 groups in U(S3) and U(CuIS3) defects result in energy differences of about 0.3 and 0.8 eV on the two surfaces. On both surfaces, the configurations incorporating disulfide S2 groups (structures 5 and 10 in Figure 2) are much less stable than those with S3 groups. 3.1.3. Surface Defects Formed upon Deletion of Two Adjacent Fe Atoms. Figures 3 and 4 show the most stable

S atoms. One of the walls of the valley has three-coordinated cations (“upper-” or U-atoms) and the other wall has fourcoordinated cations (“down-”, or D-atoms). The CuFeS2(012) surface exposes sites with two adjacent U-Fe atoms, whereas there are no adjacent U-Fe atoms on the (110) surface. Both slabs exhibit an antiferromagnetic order of magnetic moments of Fe3+ ions. Magnetic moments of the two adjacent ferric ions of the U-layer on the (012) surface have opposite directions, while the U-ion moments on the (110) surface feature one direction, opposite to that of the D-layer moments. The magnitudes of local magnetic moments varying in the range of 3.5−3.7 μB are similar to those calculated and measured (3.7− 3.85 μB) for Fe atoms in CuFeS2.1,2,10 The average Fe−S and Cu−S bond lengths between the low coordinated atoms are 226 and 223 pm, respectively, compared with the value of ∼230 pm for the bulk CuFeS2.3 3.1.2. Surface Defects Formed upon Extraction of One Fe Atom. The deletion of a single Fe atom from the top cation layer on the (110) and (012) surfaces of the Cu16Fe16S32 unit cell was calculated to yield structures 8 and 9; under the oxidation conditions (reaction 1), these conversions are 0.01 and 0.14 eV endothermic (Figure 2). A relocation of

Figure 3. Equilibrium structures and formation energies Ef for the defects at the CuFeS2(012) surface produced by the removal of two Fe atoms from the upper cationic layers (UU-vacancies) of the Cu16Fe16S32(1a) or Cu32Fe32S64(2a) unit cells. Red balls represent atoms of S2 and S3 groups, and black and blue balls marked with crosses indicate positions of the U(Fe) vacancies and relocated Cu atoms. See the text for further explanation.

Figure 2. Equilibrium structures 1−10 and formation energies Ef for the defects at (A) CuFeS2(012) and (B) CuFeS2(110) surfaces produced by removal of one Fe atom from the upper cationic layers (U-vacancies) of the Cu16Fe16S32 unit cells. Red spheres represent atoms of S2 and S3 groups, and black and blue ones marked with crosses indicate the positions of the U(Fe) vacancies and relocated Cu atoms. See the text for further explanation.

equilibrium structures containing Sn groups for slab models with Cu16Fe14S32 and Cu32Fe30S64 unit cells. For the (012) surface (Figure 3), defects 13, 19, 25, and 26 are different configurations of the extended unit cell slab Cu32Fe30S64 (2a and 1b in-plane translations), and the structures 15, 17, 21, and 23 were optimized in the calculations of the smaller Cu16Fe14S32 unit cell (1a and 1b in-plane translations). Formation energies Ef of the double U(Fe)-vacancy defects vary from −1 to −2.75 eV (i.e., Ef(1) changes from −0.53 to −1.38 eV for one Fe atom removed). The S−S bond lengths range from 208 to 220 pm for S2, 201 to 220 pm for S3 and S4 groups, and S5 species feature a broader range from 198 to 224

neighboring U(Cu) atoms to the position of the U(Fe) vacancy results in structures 6 and 7, which are about 0.2 eV more favorable. The relocated Cu atoms are only slightly displaced from the U(Fe) positions and retain their threefold coordination as in the initial U(Cu) sites. The energy gains are higher for the structures 1−4 formed when S atoms of the top layer move into the empty cationic sites of the defects 6−9. This yields defects in which each of the two terminal atoms of the S3 group contacts two or three cations (Figure 2 and Table S1, Supporting Information). In particular, the structure 1 with Ef = −1.48 eV forms when the C


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The Journal of Physical Chemistry C pm. The first coordination spheres of the Sn groups contain from three to seven cations; particularly, each atom of S2 anions and terminal atoms of S3 groups coordinate two or three cations with the Cu−S and Fe−S bonds of 221−253 and 232−261 pm, respectively. The nearest environment of the bridging S atoms within S3 groups does not contain cations, except for the S atom coordinating one Cu atom in the defect structure 21. Each of the two terminal atoms of the S4 group in structure 24 bond to one Fe atom, and one terminal S atom and one bridging S atom contact with one Cu atom. Terminal atoms of the S5 chain are engaged in three (structures 14, 19, 25) or two sulfur−metal bonds (structure 26). In the last structure with an open S5 chain, the middle S atom is connected to one Cu and one Fe cation, and one of the bonds of the terminal atoms to two Fe atoms, while another terminal atom bond to two Cu atoms. The calculated length of the terminal S−S({−Cu}2) bond of 198 pm turns out to be the shortest among all S−S contacts. The bridging S atoms of the S5 groups do not participate in metal−sulfur bonding. In addition to disulfide and polysulfide groups, most of the defects also expose three-coordinated metal cations located in the second cation layer, replacing four-coordinated cations of the D-layer of regular surfaces; the lowest-energy structures 11−13, 15 and structures 20, 23, 24 contain Cu3c cations, defects 16 and 22 include Fe3c cations, and defects 14, 19, 21, 25, and 26 contain both Cu3c and Fe3c centers (Tables S2 and S3). The lowest-energy structure 13 calculated for the extended unit cell slab contains one S2 group and one S3 group. To form this defect with two adjacent Fe atoms deleted in the top cationic layer, atom S1 of the top S-layer in the first coordination sphere of one of the U(Fe) vacancies moves to fill this U-vacancy while atom S2 contacting both U(Fe) sites occupies the second U-vacancy (see Figures 1 and 2). Structure 15 of the smaller unit cell is analogous to the structure 13, and its formation energy is only 0.1 eV less advantageous. Configuration 17 also incorporates adjacent S2 and S3 species, and one of the Cu atoms of the D-layer relocates to an interstitial position between these groups forming two additional Cu−S bonds. Nevertheless, this center is less stable by 0.22 eV than the configuration 15. Defect 21 with Ef of −2.2 eV contains two S3 groups. To form this structure, one of the S atoms of the top surface layer fills a U(Fe)-vacancy and one of the Cu atoms of the U-layer relocates to an interstitial position in the vicinity of another U(Fe)-vacancy and three S atoms, and another top layer S atom occupies U(Cu) vacancy. Structure 19 exposing the S5 group as part of a boat-like CuS5 six-member ring turns out to be one more double Fe-vacancy defects with the formation energy exceeding −2.3 eV (Figure 3). The scheme of emergence of this defect is similar to that of defects 13 and 15, as both U(Fe)-vacancy sites are occupied with S atoms of the top layer but now with atoms S1 and S3 instead of atoms S1 and S2 (Figures 2 and 3). We have also considered, among other double Fe-vacancy defects, the structure 23 with an isolated S3 group, the structure 25 with a six-ring FeS5, and the structure 26 with an open chain S5 group, the formation schemes for which include displacement of one Cu atom into an interstitial position near U(Fe)-vacancies. In the configuration with the FeS5 ring, two relocated top layer S atoms occupy U(Fe)-vacancy and U(Cu)-vacancy. For the defects 23 and 25, the energies Ef are close to −1.8 eV. The emergence of the open S5 chain structure occurs via the displacement of the

two top S atoms to U(Fe)-vacancy and D(Cu)-vacancy positions; its energy Ef(1) of about −0.5 eV is relatively low. Of particular interest is that the creation of the defects with two S3 groups (21) and with S5 groups (19, 25, 26) exposes new three-coordinated Fe cations (Table S2), extraction of which is expected to be more favorable upon further oxidative dissolution than that of the regular four-coordinated cations in the second metal layer. The double vacancy defects of the CuFeS2(110) surface (Figure 4 and Table S3) were optimized using the Cu16Fe14S32

Figure 4. Equilibrium structures and formation energies Ef for the defects at the CuFeS2(110) surface produced by the removal of two Fe atoms from the upper cationic layers (UU-vacancies) of the Cu16Fe16S32(1a) and Cu32Fe32S64 unit cells (2a). Red balls represent atoms of S2 and S3 groups, and black and blue balls marked with crosses indicate positions of the Fe vacancies and relocated Cu atoms. See the text for further explanation.

slab model (1a and 1b in-plane translations, structures 18 and 24) and extended slab Cu32Fe30S64 (2a and 1b in-plane translations, structures 11, 12, 14, 16, 20, and 22). The three lowest-energy structures on the extended slab surface having Ef(1) close to −1.38 eV per Fe atom removed are found to be the structure 12 containing one S2 group and one S3 group, and the structures 11 and 14 with isolated S3 and S5 species, respectively. These defects arise through the displacement of one Cu atom of the U-layer to the U(Fe)-vacancy position; the Cu atom remains three-coordinated in the configurations 11 and 12, while it bounds two S atoms in the structure 14. The structure 16 with two S3 groups located in two different surface valleys is by 0.3 eV less stable than the lowest-energy defect with the relocated Cu atom and the isolated S3 group. The defect configuration 18 of the minimum slab model with one Cu atom shifted to the U(Fe) position and two S2 groups in the same valley is by 0.1 eV less stable than the structure 16. The configuration 20 (Ef = −2.28 eV) contains S2 and S3 groups similar to the structure 12; however, the central S atom of the S3 group in the structure 12 is situated in the vicinity of the U(Fe)-vacant center while in the structure 20 that fills the U(Cu)-vacancy formed after the Cu atom relocation to an interstitial position near the U(Fe)-vacancy. Two less favorable configuration shows the energies Ef of −1.7 and −2 eV (Figure 4). Structure 24 is the only one with the S4 group and a Fe vacancy in the D-layer. The rearrangement producing this structure involves relocation of one U-layer Cu atom to the D


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The Journal of Physical Chemistry C D(Fe)-vacancy position, one S atom of the second S-layer fills the U(Fe)-vacancy to form the S3 group, and one more S atom of the second layer adjacent to the U(Cu)-vacancy moves to an interstitial position bonding the terminal atom of the S3 group. The CuFeS2(110) surface has no sites with close U-layer Fe cations, so the creation of defects with the adjacent S2 and S3 groups in the same surface valley, as in the structures 13 and 15, is only possible if U(Cu)-vacancy arises due to the relocation of the Cu atom into one of the Fe vacancies, like the structure 12. The structure 22 formed without relocation of Cu atoms contains S2 and S3 groups in different surface valleys; it is ∼0.8 eV less favorable than the lowest-energy defects on the (012) and (110) surfaces. The defect with two S3 species (16) and that with the S5 group (14), and also the ones with the adjacent S2 and S3 groups (22) expose three-coordinated iron centers Fe3c of the second metal layer, similar to the (012) surface, and Cu3c centers substituting Cu4c centers are created in the defect structures 11, 12, 14, 20, and 24 (Table S3). The density-of-states (DOS) near the Fermi level is shown in Figure 5; these and additional DOS plots in a wider energy

the Fermi level within the valence band, suggesting a metallic character of the surfaces. As a general trend, the Fermi level shifts deeper into the valence band, and the DOS at the Fermi level increases with the increasing oxidation of the surfaces, and growing deficit of Fe and S−S bonding; these states have about equal S sp and Cu 3d contributions. 3.2. XPS Analysis of Chalcopyrite Surfaces. Typical Xray photoelectron spectra of particulate chalcopyrite conditioned in water or acidic solutions, centrifuged, and fastfrozen in the lock chamber prior to vacuum pumping are shown in Figures 6 and 7. The atomic concentrations of elements determined from the survey spectra and the shares of S, Cu, Fe, and O in different chemical states found by fitting the high-resolution spectra are presented in Figure 6 too. The spectra of copper with the major Cu 2p3/2 peak at 932.2−932.5 eV, negligible intensity of shake-up satellites at 940−944 eV, and the position of the Auger Cu L3MM peak at about 918 eV are characteristic of Cu+ bonded to S in chalcopyrite,8−10,22−34 and these parameters insignificantly change upon the chemical treatment and warming. Some asymmetry of the Cu 2p3/2 peak at higher BEs may be fitted with two minor maxima at 933.5 and ∼935 eV attributable to oxidized Cu species (e.g., CuO and Cu(OH)2);76 their relative intensities somewhat alter in favor of the latter after warming up all of the samples, possibly due to the interaction of CuO with water at ambient temperature. Alternatively, the high BE tail can be due to shake-off satellites originating from transitions to the continuous states above the Fermi level, in agreement with the DOS plots (Figures 5 and S1), which show that such states arise at the polysulfide surfaces, and should decrease after the loss of sulfur upon warming up (see below). The Fe 2p spectra can be fitted with two sets of multiplet lines (four narrow peaks and a wider satellite) from Fe3+ bonded to S (the binding energy of Fe 2p3/2 peaks of 708 eV and higher), and from Fe3+−O species at the BEs above 710 eV.77 Since several oxidized iron substances and iron chlorides may contribute to the spectra, the fittings are not precise but allow estimating the composition of the sulfide surfaces and near-surface aqueous phases interacting with the solid;65 the latter is, however, beyond the scope of this paper. The amount of oxidized Fe is about 40% of the total for the sample contacted with water, and it decreases after the reaction with hydrochloric acid solution and considerably increases upon oxidation in the ferric ion bearing media, in particular, 0.05 M FeCl3 solutions. The enhanced concentrations of oxygen, particularly, O2− and OH− species, and chloride ions suggest that Fe3+ is bonded both with O and Cl atoms. The Fe spectra undergo small changes after warming the samples in a vacuum due to evaporation of water, and likely crystallization of Fe−O species. This concurs with O 1s spectra, which show decreasing concentrations of physically (535 eV) and chemically adsorbed water (533.2 eV) and, to a smaller extent, OHgroups (∼531.8 eV). The reacted surface layers are depleted in Fe and exhibit the total S/Fe atomic ratio of 2.4 and the ratio related to Fe in the sulfide phase of ∼4 for chalcopyrite shortly treated with water (in effect, after aerial oxidation), and more strongly for chalcopyrite reacted in the acidic solutions. It is important that the S/Cu ratios remain close to the initial value of 2 after conditioning in water and hydrochloric acid, and it decreases after warming and evaporating of excessive sulfur (see below). After the ferric ion etching (samples c, c′), the S/Cu ratios diminish well below 2 implying that not only Fe but also Cu

Figure 5. Total and partial density-of-states for the stoichiometric unreconstructed CuFeS2(012) and (110) surfaces and selected Fedeficit surfaces (defect structure types are denoted in the plots) as calculated using DFT + U. The Fermi level positions are marked (−5.91, −5.66, etc.).

range are presented in Figure S1 (Supporting Information). Both the stoichiometric and Fe-deficient (012) surfaces have a significant density-of-states immediately above the Fermi level, and thus “metallic” properties. The stoichiometric slab (110) surface exhibits a forbidden gap of 0.46 eV, compared with the gap of ∼0.5−0.7 eV for the bulk of chalcopyrite.11 The structure 2 with S3 groups centered at isolated Fe vacancies also shows the Fermi level located in the narrow band gap. The lowest-energy double Fe-vacancy structures 14-UU(CuIS5), 11-UU(CuFeS3), and 12-UU(CuFeS2S3) (see Figure S1) have E


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Figure 6. X-ray photoelectron spectra of particulate chalcopyrite conditioned in (a, a′) water for 1 min (25 °C), (b, b′) 0.1 M HCl for 30 min (50 °C), and (c, c′) 0.05 M FeCl3 + 0.01 M HCl for 10 min (50 °C), centrifuged and fast-frozen, and measured at −160 °C (a−c), and warmed up to +25 °C (a′−c′). The histogram shows atomic concentrations of elements in various chemical states determined using Cu 2p, Fe 2p, O 1s, C 1s, S 2p, Cl 2p lines.

Figure 7. X-ray photoelectron S 2p spectra of particulate chalcopyrite conditioned in water (1 min), 0.1 M HCl (30 min), and 0.05 M FeCl3 + 0.01 M HCl (10 min), centrifuged and fast-frozen, and measured at −160 °C (a, a′), −40 °C (a″), and warmed up to +25 °C (b, b′). The spectra a′ and b′ measured employing an electron flood gun (0.5 eV, 12 mkA).

polysulfide species, respectively, and a wider satellite at about 164.5 eV, that is due to charge transfer to vacant Fe 3d states.59−61 When the sample was warmed up to room temperature in a vacuum chamber (spectra b), the intensity of the polysulfide component somewhat decreased, and that of the monosulfide increased, so the mono-/di-/polysulfide ratio changes to 1:0.1:0.2. At the same time, the peak S 2p3/2 of polysulfide shifts by 0.3 eV to a lower BE (163.2 eV), while the position of monosulfide line remains the same. It is also interesting that irradiation of the samples with slow electrons applying a flood gun results in some increase in disulfide signals at the expense of polysulfide ones, probably, owing to electrons caught by “polysulfide” atoms. The origin of the lines

atoms are extracted from the sulfide lattice under these conditions. 3.3.2. S 2p Spectra. The spectra of Cu, Fe, and O provide limited information on the structure of modified chalcopyrite surfaces, and we further focus on the spectra of sulfur. Figure 7 represents S 2p spectra acquired from the chalcopyrite samples described above (Figure 6); additional examples can be found in Figures S2−S4 (Supporting Information). The S 2p spectra from the chalcopyrite sample slightly oxidized upon contact with air and then water can be fitted utilizing three doublets with the S 2p3/2 peaks at 161.6, 162.6 ± 0.1, and 163.5 eV with the relative intensities of 1:0.15:0.26, which are commonly attributed to monosulfide, disulfide, and F


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stoichiometric value of 2 again, but usually stays somewhat higher. In general, therefore, these surfaces fall beyond the range of the above DFT + U simulations. The S 2p spectra are best fitted with an increased component at the BE approaching 164 eV, much smaller disulfide doublet (S 2p3/2 peak at 162.7 eV), and the line of monosulfide at 161.7 eV, along with a weaker satellite. The contribution of S−O species is negligible, except for adsorbed sulfate (169 eV) for the samples reacted with ferric sulfate solutions (Figure S2). The ratios of poly- and disulfide signals are usually higher than 3 for the frozen samples, so the number n of S atoms in polysulfide Sn2− should be 8 and more, or there exists a contribution of S0 species. Similar to other samples, the polysulfide component decreases and shifts to lower BE, and the monosulfide doublet stays at the same BE and notably grows during warming the specimens. The proportion of polysulfide to disulfide signals decreases but the net intensity of the disulfide signal remains roughly the same, as one can see from the difference of the spectra (Figure 8). Again, low-energy electrons of a flood gun reduce the BE (up to 0.2 eV) and magnitude of the polysulfide maxima and increase the intensity of disulfide lines. It should be underlined that the S 2p spectra measured from the samples with a higher excess of S, which was warmed in the vacuum, and those with comparable composition after oxidation under milder conditions exhibit similar BEs and relative intensities of polysulfide maxima. The sample that reacted under the same conditions but studied using conventional XPS routine, that is, vacuumed at room temperature without freezing, shows the spectra analogous to the fast-frozen and then warmed specimens described above. It is important to discriminate between polysulfide species (intermediate S atoms in polysulfide anions) and elemental sulfur, which both tend to volatilize in the ultra-high vacuum. The evaporation of elemental sulfur is expected to expose the underlying surface with the simultaneous growth of di- and monosulfide signals but this is not the case (Figures 7 and 8). Further arguments that the species under consideration are mainly polysulfide, that is S−S chains with some disulfide S atoms bound to metals, are as follows: (i) BEs of polysulfide maxima are lower than those of elemental sulfur and depend on the quantities of total S and polysulfide; (ii) no or a very low electrostatic charging of tentative dielectric sulfur is observed but the signal of disulfide enhances under slow electron irradiation, probably, due to a charge transfer within S−S chains; (iii) the species are partially kept after warming and very prolonged aging in a vacuum; (iv) the spectra of the surfaces obtained under various conditions, including after the warming, reproduce the same features (the ratios of di- and polysulfide species, decreasing BEs, etc.). Therefore, the XPS examination supports the DFT + U results in terms of the composition of the Fe-deficient surfaces and the nature of S species, particularly, the low contribution of disulfide anions, in the early stages of chalcopyrite oxidation when the S/Cu ratio stays close to 2. Further oxidation produces the surfaces with depletion in Cu, and a higher deficit of Fe in comparison with those employed in the DFT + U calculations, and finally it yields elemental sulfur. Elemental sulfur was not surely detected in the photoelectron spectra of chalcopyrite even when reacted under severer oxidation conditions (Figures S2−S4). This may be due to insufficient resolution of polysulfide and elemental sulfur lines in the spectra or/and the formation of bulky sulfur entities having a small surface area. Nevertheless, the S 2p spectra largely

at 163−164 eV is still a matter of some controversy. The binding energies lower than that of elemental sulfur (usually accepted to be 164.0 eV (we measured the BE of bulk crystalline α-S8 to be 164.2 eV)) imply a small negative charge localized at S atoms. This is believed to be valid for intermediate S atoms in polysulfide anions Sn2−, whereas the terminal atoms are negatively charged similar to disulfide anions S22−.78,79 However, some researchers ascribe the species with the BEs slightly less than 164 eV to adsorbed elemental sulfur because their signal decreases upon aging and evaporation in a vacuum.35 The total excess of sulfur, S−S bonding, and the BE of polysulfide line (remaining insignificantly below 164 eV) increase after the oxidation of chalcopyrite. Under mild reaction conditions, the surfaces become more enriched in sulfur and depleted in iron while keeping the proportion S/Cu of 2, in agreement with the DFT + U simulation. For example, in the case of 0.1 M HCl (Figures 6 and 7) the ratios S2−/ S22−/Sn alter to 1:0.29:0.56 and 1:0.22:0.28 for the fast-frozen sample and for the one warmed up overnight, respectively. Keeping in mind different local charges of intermediate and terminal S atoms and the DFT findings, one could expect their proportion of 1.5 for S52− species, 0.5 for S32− anions, and 0.2 for the S32− S22− complex, which were calculated (Figure 3) to dominate the surfaces with two Fe vacancies in the model slab. The experimental proportion of polysulfide to disulfide is between 1 and 2, and this implies the occurrence of S52− and/ or anions having even larger n together with a smaller number of S32− and/or S32− S22− species. As the samples were warmed up, a part of excessive sulfur has been lost in the vacuum. The difference spectra demonstrate (Figure 8 represents a couple of

Figure 8. Differences between the photoelectron S 2p spectra measured at −160 °C, and warmed up to +25 °C of particulate chalcopyrite conditioned in 0.1 M HCl (30 min), and 0.05 M FeCl3 + 0.01 M HCl (10 min), measured without (black) and with (red) an electron flood gun (0.5 eV, 12 mkA).

examples) that the intensities in the spectral region of monosulfide doublet increase very slightly after the treatment in 0.1 M HCl when S/Cu ∼2 and that of disulfide stays constant, whereas the polysulfide signal decreases. This can be interpreted in terms of volatilization of intermediate S atoms in S−S chains, leaving disulfide atoms bound to metals on the surface. Figure 7 also shows typical S 2p spectra of chalcopyrite reacted with an acidic solution of ferric ions. When Fe3+ ions are used as oxidants, the excess of S increases and the ratio of S/Cu enhances well above 2, probably because of the extraction of Cu, along with Fe, from the surface layer of chalcopyrite. This may also be a signature of the formation of the elemental sulfur phase. As some sulfur evaporated upon warming the sample, the S/Cu ratio approaches the G


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copper, and sulfur release and the transfer of cations to the surface become roughly equal. In this case, the total rate of oxidation and dissolution is comparable with that of solid-state transport of cations, which is generally slow, and depends on the electrode potential and semiconducting properties of metal sulfide, composition of aqueous solutions (acidity, anions, oxidants), and so on. A particular factor can be ratedetermining for specific reaction conditions and time. For example, the heterogeneous sub-micrometer lateral distribution of polysulfide species revealed by Gerson and Li using electron spectroscopy for chemical analysis microscopy29,30 may be due to the slightly different energies of the centers for various crystalline faces, and/or fluctuations of semiconducting properties in the mineral. The hindered dissolution of chalcopyrite and other metal sulfides is considered in the literature as passivation related to species screening the surface, such as elemental sulfur, metaldeficient layers, jarosite,10 and so on. At the same time, plentiful polysulfide species have been usually observed, for instance, under anodic polarization in “transpassive” regions where the oxidation becomes comparably fast at the high potentials.24,31,36−40 It becomes clear that this occurs due to accelerated release and interfacial transfer of cations but still retarded formation of elemental sulfur. This research demonstrates that the reason for the arrested yield of elemental sulfur is the formation of energy-favorable Fe-deficient surfaces with polysulfide anions in the initial stages of oxidation instead of a direct, congruent decomposition of chalcopyrite. It can be added here that the negligible quantities of thiosulfate, sulfate, etc., the yield of which is minor for chalcopyrite even in alkaline solutions,17 means insignificant role of reactions of the sulfur species with OH− ions and similar reagents. Under certain conditions, copper-rich sulfides (covellite CuS, bornite Cu 5 FeS 4 , and some others) have been reported as intermediates of chalcopyrite reactions10 but no signatures of such phases were observed in this research. The findings suggest a metallic conductivity of the surface layers due to the Fermi level position below the valence band edge (Figure 5) at almost all polysulfide surfaces. The underlying disulfide and “defect” iron-deficient layers of chalcopyrite are also metallic,32 in contrast to semiconducting chalcopyrite bulk. Therefore, the mechanism considered the nonstoichiometric layers like amorphous chalcogenide semiconductors24,36,37 and suggested a low mobility and electron conductivity as the main reason for retarding the reaction that appears incorrect. The density-of-states near the Fermi level and the number of conduction electrons depend on the nature of dominating polysulfide centers (Figure 5). This should affect many properties of chalcopyrite and related materials, for example, the intensity of localized surface plasmon resonance that is observed for CuFeS2 quantum dots at 2.4 eV.82,83 The redox processes involving S−S bonding are of interest for a number of applications of transition metal chalcogenides,84,85 in particular in Li−S batteries, and are often considered to be controlled by the occupation of the sulfur p-type states at the valence band top. The current research shows that the states of sulfur are mixed with those of metals, and the reactions are complex and different for the surface and underlying solid. These phenomena require further study.

originate from poly, di-, and monosulfide species, whereas the part of elemental sulfur is insignificant.

4. DISCUSSION The previous HAXPES and DFT + U examination of metal sulfides32,62 revealed that the Fe-deficient near-surface regions are composed of several layers, and polysulfide anions can exist only in the outermost layer. The current DFT + U simulation combined with the cryo-XPS study of the reacted surfaces of chalcopyrite showed that the lowest-energy defect centers composed of single Fe vacancies and S32− anions with the negative formation energies of 1.2−1.48 eV (Figure 2) are notably more profitable than the Fe-depleted surfaces with disulfide anions32 (and the stoichiometric reconstructed surfaces containing disulfide species56). The extraction of one more ferric ion from the surfaces produces defect centers with enhanced S−S bonding and roughly the same energy effect per Fe atom removed, so the surface further stabilizes as half of the surface Fe atoms have transferred into ferric hydroxide or aqueous phases. The scheme in Figure 9 illustrates the emergence of the surface nonstoichiometry and evolution of the surface regions

Figure 9. Scheme illustrating the formation and structure of the metal-deficient reacted surface and undersurface layers as a function of Ef, nonstoichiometry and a distance from the surface. The initial release of iron from intrinsic sulfide lattice produces a Fe-deficit surface (left) after the layered structure arises via coupled extraction of Fe (Feaq3+ stays for iron solutes and oxyhydroxides) and then Cu and S, and solid-state transfer of cations toward the interface.

as a function of the composition violations, energy, and expansion in depth. As the deficit of iron becomes high enough, copper release occurs (Figures 6, 7, and S2−S4), creating another type of surfaces with a tentative composition Cu1−yFe1−zS2, and longer polysulfide chains, which is beyond the DFT + U calculations performed here. These surfaces are expected to easily lose excessive sulfur of polysulfides upon heating and aging at room temperature in the vacuum as Sn molecules. The sulfur vapor pressure is known to increase with increasing the number n (for n ≤ 8),80,81 and it is very likely that the yield of elemental sulfur from such surfaces speeds up both in the atmosphere and oxidizing aqueous solutions. The mechanisms behind the transformation of polysulfides into elemental sulfur require exploration. The rate of sulfur release also depends on the kinetics of interfacial transfer of iron and copper and the delivery of cations to the surface from the bulk. The solid-state diffusion creates an Fe-deficient undersurface containing defects with disulfide anions (disulfide layer),32 the stability of which is restricted in a narrow composition range, and this causes the expansion of the structure in depth upon the extraction of surface metal atoms. The transportation of cations to the surface slows down with thickening the underlayers, and a stationary regime would be reached when the rates of iron, H


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5. CONCLUSIONS We have studied possible configurations arising at CuFeS2(110) and (012) surfaces due to the removal of one and two adjacent Fe atoms from 10 atomic layer Cu16Fe16S32 (or Cu32Fe32O64) slabs, that is up to 50% of iron of the top layers, by applying DFT calculation with the GGA(PW91) + U scheme. It was revealed that the most stable Fe -deficient structures on both surfaces are much more favorable under oxidative conditions than those involving Fe vacancies and disulfide anions in the underlying layers,32 and reconstructed stoichiometric surfaces with disulfide species.56,57 The (012) surfaces with double U(Fe)-vacancies and S3 + S2 groups and S5 group have the formation energies per one Fe atom removed (e.g., Ef(1) = −1.38 and −1.17 eV for the structures 13 and 19, respectively) only slightly less profitable than those with isolated U(Fe)-vacancy and trisulfide anions on the surface (Ef of −1.48 eV for the structure 1). The most stable double vacancy defects do not include the relocation of Cu atoms to Fe-vacancy positions. Similar formation energies were obtained for the (110) surface, but the most stable centers with S3, S3 + S2, and S5 anionic groups are formed via moving Ulayer Cu atoms to the vicinity of vacant U(Fe) positions. Configurations with isolated disulfide anions are not advantageous at all of the surfaces. The Fermi level is usually located in the valence band, determining the metallic properties of the polysulfide surfaces. The density of states at the Fermi level having almost equal Cu 3d and S sp characters in general increases with enhancing deficit of Fe and S−S bonding. XPS studies of the chalcopyrite samples slightly oxidized in water, 0.1 M HCl, and fast-frozen to liquid nitrogen temperature to preserve volatile sulfur species found that the deficit of Fe and an excess of sulfur correlate with the intensity of S 2p spectra at the binding energies 163.95−163.2 eV, which is attributable to polysulfide species (middle atoms in Sn2− anions) with n ≥ 5 rather than elemental sulfur. Upon warming up the specimens to room temperature, the total content of S decreases and the polysulfide lines reduce and shift to lower BEs, while the signal of monosulfide increases and minor lines of disulfide (including terminal atoms in Sn2− chains) remain the same; the spectra suggest that S52− and S32− anions appear to prevail, in agreement with the DFT + U results. The S/Cu ratios stay close to 2 at the lower S excess, but the ratios and the polysulfide lines increase during further oxidation, going beyond the range of current DFT + U simulation, due to extraction of copper in addition to iron. The high stability of surface polysulfide centers arising in the initial stages of oxidation is believed to cause the formation of undersurface metal-deficient layers instead of the release of elemental sulfur, and passivation of chalcopyrite. Generally, the Fermi level shifts deeper into the valence band, and the DOS at the Fermi level increases with increasing oxidation of the surfaces. The characteristics of the polysulfide surfaces should determine the number of surface properties of chalcopyrite and related materials, for example, localized surface plasmon resonances in the nanoparticles.

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of Fe atoms (Tables S1−S3); total and partial densityof-states for the stoichiometric CuFeS2(012) and (110) surfaces and a series of the Fe-deficit surfaces in a wide energy range (Figure S1); XPS spectra of additional samples etched in ferric sulfate and ferric chloride solutions (Figures S2−S4) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuri Mikhlin: 0000-0003-1801-0947 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Russian Foundation for Basic Research, project 18-03-00526 (DFT + U studies) and the Russian Science Foundation, project 18-17-00135 (leaching and XPS experiments).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b06127. Formation scheme and selected characteristics of the defects formed via deletion of single Fe atoms and pairs I


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