Reconstruction of the Chalcopyrite SurfacesâA DFT Study - The

Article pubs.acs.org/JPCC

Reconstruction of the Chalcopyrite SurfacesA DFT Study Cláudio de Oliveira, Guilherme Ferreira de Lima, Heitor Avelino de Abreu, and Hélio Anderson Duarte* Grupo de Pesquisa em Química Inorgânica Teórica-GPQIT, Departamento de Química-ICEx, Universidade Federal de Minas Gerais-UFMG. 31.270-901-Belo Horizonte-MG, Brazil S Supporting Information *

ABSTRACT: Chalcopyrite (CuFeS2) is the main source of copper in the world. The development of hydrometallurgical processes to extract copper from chalcopyrite is challenging due to the low leaching kinetics. The main difficulty is in the fact that the kinetics of the leaching process decreases very rapidly, marginally stopping the reaction. A passivation process of the surface has been proposed for explaining the low reaction kinetics. However, the leaching mechanism and the reactants which are involved in the passivation process are still a matter of debate. Therefore, understanding the chalcopyrite surface reactivity and the intricate reaction occurring in the solid/solution interface is of fundamental importance. In the present study, DFT calculations within the plane wave framework were performed to understand the reconstruction of (001), (100), (111), (112), (101), and (110) chalcopyrite surfaces. Metal and sulfur terminated surfaces have been investigated. The structural and electronic properties of the reconstructed surfaces have been discussed in detail. Three different mechanisms of the chalcopyrite surface reconstructions emerged from this study. It is clear that the chalcopyrite surface undergoes important reconstruction in which the sulfide, S2−, ions migrate to the surface which tend to oxidize, forming disulfides, S22−, and, concomitantly, reducing the superficial Fe3+ to the Fe2+. It is also observed that the metal atom moves downward to the surface, forming metallic-like bidimensional alloys underneath the surface. pointed out as passivation candidates.3,4,11−21 On the other hand, the presence of Ag(I) avoids the passivation and the leaching process is enhanced.22,23 Mikhlin et al.24 studied through X-ray photoelectron spectroscopy (XPS), X-ray emission, Mössbauer spectroscopy, and cyclic voltammetry the surface evolution on chalcopyrite leached by H2SO4 + Fe2(SO4)3 and HCl + FeCl3 at 50 °C. They identified the presence of S32− species on the chalcopyrite surface after the leaching with ferric sulfate and S42− species after leaching with ferric chloride. Parker et al.14 used XPS to analyze the surface evolution on the chalcopyrite surface leached by FeSO4 and Fe2(SO4)3 in sulfuric acid media with pH in the range 1.3−1.9. They also synthesized plenty of polysulfide compounds like Na2S4 and K2S6 to use them as standards in XPS analyses. Their analyses indicated the presence of disulfides, sulfates, and thiosulfates on the chalcopyrite surface, but they did not identify polysulfides as done by Mikhlin et al.24 On the basis of their results, Parker et al.14 proposed an oxidation mechanism in which the disulfide in the chalcopyrite surface reacts with one hexaaquairon(III) complex in aqueous solution. According to their mechanism, the oxygen atoms in thiosulfide come from water molecules and this proposal is in good agreement with the results obtained by Reddy et al.25 in their study about pyrite oxidation.

1. INTRODUCTION The main source of copper in the world is chalcopyrite (CuFeS2). Even though more than 80% of the copper is available as chalcopyrite,1 there is an increasing need to treat chalcopyrite ores by hydrometallurgical processes. Considering the limitations of the pyrometallurgical route and its environmental impact, the hydrometallurgical process emerges as a promising way to obtain copper from complex and low grade ores. Furthermore, sulfide minerals are involved in the mechanism of acid mine drainage, decreasing the pH in aquifers and releasing heavy metals to the environment.2 Nowadays, around 20% of copper in the world is obtained by a hydrometallurgical process.1 In this process, the mineral is leached in an aqueous solution followed by an electrochemical reduction of copper. The most used leaching agent is iron(III)3−5 in acid solution with sulfate4 or chloride.6−8 Leaching in the presence of microorganisms has also been used.9,10 In spite of the advantages of the hydrometallurgical route against the pyrometallurgical one, the leaching process of the chalcopyrite is not well understood. The large copper recovery of the initial steps of the chalcopyrite leaching is rapidly decreased, marginally stopping the leaching process.3,4 This behavior is not observed for other sulfide minerals. The decrease in the leaching kinetics is attributed to a passivation process on the chalcopyrite surface. However, the mechanism of surface passivation is still a matter of debate. Metal-deficient sulfides, polysulfides, elemental sulfur, and jarosites have been © 2012 American Chemical Society

Received: January 20, 2012 Revised: February 17, 2012 Published: February 17, 2012 6357

dx.doi.org/10.1021/jp300713z | J. Phys. Chem. C 2012, 116, 6357−6366

The Journal of Physical Chemistry C

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Harmer et al.26 analyzed through XPS, time of fly secondary ion mass spectrometry (ToF-SIMS), and scanning electron microscopy (SEM) surface evolution on the chalcopyrite leached by HClO4 at 85 °C and initial pH of 1. They also identified the presence of polysulfides and further elemental sulfur on the chalcopyrite surface and proposed one mechanism which involves both oxidation and reduction steps. Li et al.27 also leached chalcopyrite by different acids in controlled conditions and identified using SEM and XPS the presence of polysulfides, disulfides, and sulfate on chalcopyrite surfaces. However, both Harmer et al.26 and Li et al.27 do not attribute the passivation to the presence of these species on the surface. Differences in both leaching conditions and mineral samples do not allow the convergence of results; in this way, the presence of polysulfides is still questionable and their influence on the mineral reactivity is not clear. In order to understand the first step in the chalcopyrite oxidation, the sulfur speciation was analyzed on pristine chalcopyrite samples by Harmer et al.28 They investigated fractured chalcopyrite using both synchrotron and conventional XPS identifying polysulfides and the sulfur terminated (111) as the exposed surface after the fracture. Klauber29,30 and Acres et al. 31 analyzed the chalcopyrite fractures in an inert N2 atmosphere. They used XPS and synchrotron XPS identifying the presence of disulfides; however, polysulfides were not detected in both studies. The experimental findings indicate clearly that the understanding of the reactivity of the chalcopyrite surface is a fundamental issue. The bulk chalcopyrite has been much investigated due to its semiconductor character; however, the reconstruction of the chalcopyrite surfaces has not received much attention. Density functional calculations have been extensively used to study pyrites, including the reconstruction of different surfaces,32 defects,33 adsorption of molecules,34−38 oxidation mechanism,39 and many other properties. Nevertheless, only a few studies investigated the chalcopyrite. Von Oertzen et al.40 used DFT in its plane wave implementation to obtain the Mulliken population41 on the relaxed (112) and (012) surfaces. Their results indicated sulfur atoms with different charge distribution on these surfaces in agreement with the results obtained by Harmer et al.28 In a recent work, de Oliveira and Duarte42 applied the DFT/plane waves methodology to study the (001) surface reconstruction. They analyzed both the geometry parameters and electronic structure of the sulfur and metal terminated surfaces. Their results showed the formation of disulfide bonds as experimentally reported by Acres et al.31 and Klauber29,30 in their XPS studies in an inert atmosphere. In this paper, we will extend the previous study of de Oliveira and Duarte42 to different surfaces. Chalcopyrite shows only a weak preferential cleavage along the (112) direction, although it is not clear which surface is the most relevant in terms of reactivity. Klauber30 indicated the (001) surface as the most probable to obtain reconstruction with disulfide dimers. In the present work, the structural and electronic properties of the (100), (101), (110), (111), (112), and (001) reconstructed chalcopyrite surfaces are investigated in detail.

follow valence configuration: Fe 3s2 3p6 3d6 4s2 4p0, Cu 3d10 4s1 4p0, and S 3s2 3p4 3d0. The valence states were expanded in plane waves with a kinetic energy cutoff of 30 and 300 Ry for the charge density cutoff. The integration over the Brillouin zone was performed using the Monkhorst−Pack scheme,46 and the number of k-points in the Brillouin zone was set according to the supercell model used to describe the surface, as stated in Table 1. All the k-point grids used in this work are able to Table 1. K-Point Grid, Number of CuFeS2 Units, and Supercell Parameters for Different Surfaces supercell parametersa surface (001) (100) (101) (110) (111) (112) a

K-point grid 2 2 2 3 3 1

× × × × × ×

2 2 4 2 3 3

× × × × × ×

1 1 1 1 1 1

no. of CuFeS2 units

a′

b′

16 16 16 16 12 36

10.526 10.526 14.810 7.443 6.402 18.169

10.526 10.418 5.263 10.418 7.443 7.443

In angstroms.

calculate the total energy with an error lower than 10−3 Ry in comparison with larger grids. Chalcopyrite crystallizes in the tetragonal group (space group I42̅ d) with four formulas (CuFeS2) per unit cell.47 The lattice parameters were determined to be a = b = 5.289 Å and c = 10.423 Å, and the Fe−S and Cu−S bond lengths were 2.257 and 2.302 Å, respectively.47 In the solid structure, the oxidation number of iron, copper, and sulfur is +3, +1, and −2, respectively.47 At 0 K, chalcopyrite is assumed to be an antiferromagnetic material with alternate planes of iron with spin up or down along the c direction.40 Recently, de Oliveira and Duarte42 carried out DFT calculations on the chalcopyrite's bulk structure and they were able to obtain geometric structures in good agreement with experiment. The lattice parameters, for example, were calculated to be 5.263 and 10.362 Å for a and c, respectively, while the bond lengths were calculated to be 2.241 and 2.293 Å for the Fe−S and Cu−S bonds, respectively. The bulk structure optimized by de Oliveira and Duarte was used to construct each supercell. The direction normal to the surface plane was set as c′, and the a′ and b′ direction were set keeping the orthogonality and producing supercells with a reasonable number of atoms to study the reconstruction. The chalcopyrite stoichiometry (1:1:2) was preserved in all supercells. The number of CuFeS2 units and the supercell parameters for each surface are also reported in Table 1. A vacuum of 15 Å along the c′ direction was established to avoid interactions between the top and bottom surfaces in the slab. Figures S2−S10 (Supporting Information) show the supercells for each surface studied in this work. The surfaces were reconstructed using the conjugated gradient method to optimize geometry with a force tolerance equal to 1.0 × 10−3 Ry bohr−1. In the slab approach, each supercell contains two surfaces. We optimized the interested surface while the other surface (the last two atomic layers) has been kept fixed in the bulk geometry. The effects of this approximation will be discussed in section 3.a, showing that this is an adequate model for describing the chalcopyrite surface.

2. COMPUTATIONAL DETAILS All calculations were performed using the density functional theory with the exchange and correlation potential proposed by Perdew and Wang (PW91)43 as implemented within the Quantum-ESPRESSO package − PWscf.44 The core electrons were described by ultrasoft pseudopotentials45 considering the 6358



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3. RESULTS Chalcopyrite exhibits just a very weak cleavage at the (112) plane, and under normal conditions, it cleaves, forming plenty of surfaces. The investigation of the chemical behavior of these surfaces is crucial for understanding the mineral reactivity. The mineral cleaves, exposing the surfaces with lower energies. Tasker48 proposed a very simple model for surface stability of the ionic covalent compound such as chalcopyrite. According to the composition of each atomic layer, Tasker48 has outlined three types of surfaces. Type 1 is that in which each layer is neutral, with a relative number of cations and anions making the overall charge equal to zero. Type 2 presents charged layers but without perpendicular charge due to stacking sequence, and type 3 is charged with perpendicular dipole moment. According to Tasker,48 type 1 and 2 surfaces can exist in nature with just small relaxation and reconstruction, while type 3 requires significant reconstruction in order to disperse the net charge. It is important to emphasize that chalcopyrite has an important covalent character and consequently some surfaces of type 1 or 2 can also reconstruct significantly. In the next sections, we discuss the reconstructions of several surfaces. The (001) surface is a type 2 surface, and according to Klauber, this surface fits with his physical model to explain the XPS results under an inert atmosphere.30 Although there is not any relation with the presence of the (100) chalcopyrite surface, it is a type 2 surface which has, at the same atomic layer, iron atoms with spin up and down, allowing several distinct reconstructions. The (110), (101), and (112) surfaces are of type 1 which are the main cleavage orientation of CuInSe2, an analogous structure to chalcopyrite.49 The (111) surface is a type 2 surface. All the cleavage planes in the chalcopyrite's unit cell are shown in Figure S1 in the Supporting Information. 3.a. (001) Surfaces. There are two possible terminations for the (001) chalcopyrite surface, the sulfur terminated surface, (001)-S, and the metal terminated surface, (001)-M. Recently, de Oliveira and Duarte42 have studied the reconstruction of both (001)-S and (001)-M surfaces using PW91/plane wave calculations; however, they have used a very small model which only allowed the study of (1 × 1) reconstructions. In the present work, the (001) surfaces have been revisited, however, using larger models which allow the study of different reconstructions, for example, the (2 × 2) reconstruction in the (001)-S surface. In our slab model, the unreconstructed (001)-S chalcopyrite surface has, in the first atomic layer, eight sulfur atoms and four iron and four copper atoms in the second atomic layer, as shown in Figure S2 in the Supporting Information. Two different reconstructions have been obtained for this surface, and they are presented in Figure 1. In both reconstructions, each sulfur atom in the first atomic layer approaches another one, forming S−S bonds. Bonds between three sulfur atoms or more, forming oligomers, are not favored. On the (1 × 1) reconstruction, Figure 1a and b, the S−S bond length is 2.15 Å, very close to the 2.16 Å calculated by de Oliveira and Duarte in their study using a smaller model.42 The bond length between the S−S and the metals on the second layer is 2.32 Å, again in good agreement with the previous work.42 For the (2 × 2) reconstruction, Figure 1c and d, the S−S bond length is quite small, about 2.12 Å, and the bond length between the S−S atoms and the metal atoms on the second atomic layer was calculated to be around 2.30 Å. Besides the similarities in the geometric point of view, both reconstructions are very similar

Figure 1. (a) Top and (b) side views of the (1 × 1) reconstructed (001)-S chalcopyrite surface. (c) Top and (d) side view of the (2 × 2) reconstructed (001)-S chalcopyrite surface. The dotted lines indicate the supercell. Distances in angstroms.

in energy. Our calculations indicated that the (2 × 2) reconstruction is just 0.1 eV/supercell more stable than the (1 × 1) reconstruction. Calculations using slabs with iron with spin up, in the second atomic layer, instead of iron with spin down were also performed, looking for differences in both geometry and energy. However, we did not observe any significant change. The unreconstructed (001)-M surface has, in its first atomic layer, four iron and four copper atoms, while its second atomic layer is composed by eight sulfur atoms, as shown in Figure S3 in the Supporting Information. This surface reconstructs, forming several metal−metal bonds, as indicated in Figure 2.

Figure 2. (a) Top and (b) side views of the reconstructed (001)-M chalcopyrite surface. The dotted lines indicate the supercell. Distances in angstroms.

The metal atoms which were in the first atomic layer, in the unreconstructed surface, go downward, while the metal atoms which were in the third atomic layer, in the unreconstructed surface, go upward. These metals occupy almost the same plane, forming an alloy-like structure, as represented in Figure 2a. This alloy-like structure has, as basic unit, rectangles formed by metal−metal bonds with the bond lengths in the range 2.61−2.64 Å, as shown in Figure 2a. The sulfur atoms that were in the second atomic layer in the unreconstructed surface go upward, forming the first atomic layer in the reconstructed surface, as shown in Figure 2b. They occupy an almost central position over the rectangles formed by metal centers, interlacing them, as shown in Figure 2a. The distance 6359



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between the sulfur and the iron atoms is about 2.24 Å, while the sulfur−copper distance is between 2.27 and 2.32 Å. The same kind of reconstruction was described by de Lima et al.50 in their study about the water adsorption on the (001) chalcopyrite surfaces. What is interesting is that de Lima et al.50 used a methodology with localized basis set and PBE as exchange and correlation functional, while here we obtained similar results using plane waves with PW91 functional. This reconstruction is not in agreement with the work of de Oliveira and Duarte;42 however, in their work, a very limited supercell was used that, in our opinion, explains the different results. As mentioned in section 2, we have worked in the reconstruction of the interested surface keeping the other surface fixed. One can argue whether the surface kept fixed would not have some influence in the reconstruction. We used the (001) surface to verify if this approach has some influence on the geometric results. For this, we duplicate the number of atomic layers in comparison to the previous slab models. We kept 15 Å of vacuum and the same k-point grid. The middle of the supercell was fixed, since the effect of reconstruction on these atoms is small. Both surfaces were relaxed simultaneously, and the relaxed structure is presented in Figure 3.

The top surface which is sulfur terminated reconstructs, forming S−S bonds, while the bottom surface which is metal terminated reconstructs, forming the alloy-like structure. In Table 2, we compared some geometric parameters between the Table 2. Comparison between the Bond Lengths Evaluated Using Different Supercell Models bond lengtha

a

atomic pairs

(001)-S

S−S S−Fe S−Cu metal−metal metal−sulfur

2.12 2.30 2.32

(001)-M

(001)-MS

2.61−2.64 2.24−2.33

2.12 2.30 2.33 2.62−2.65 2.24−2.33

In angstroms.

relaxed surfaces using the models with one surface kept fixed and the model, named (001)-MS, which allow the relaxation of both surfaces. The results in Table 2 do not indicate significant differences in both approaches. The disulfide bond length, for example, in the model where the bottom surface was kept fixed was evaluated as 2.12 Å; the same value was obtained using the larger model where both surfaces are relaxed. The bond lengths in the metal terminated surface are also in very good agreement in both models, as indicated in Table 2. The results indicate that the reconstruction is just a local phenomenon and affects few atomic layers. The results also indicate that the number of atomic layers used to analyze the reconstruction of (001)-S and (001)-M surfaces is adequate to provide insights about the geometry and electronic properties of the chalcopyrite surfaces. 3.b. (100) Surfaces. We studied the reconstruction of the sulfur terminated, (100)-S, and the metal terminated, (100)-M, chalcopyrite surfaces. Conversely to what is found for the (001) surface, the (100) surface is composed of iron atoms with either spin up or down. As in the (001)-S surface, the unreconstructed (100)-S surface has, in its first atomic layer, eight sulfur atoms and eight metal atoms in the second atomic layer, as shown in Figure S4 in the Supporting Information. Similar to the (001)-S surface, the (100)-S surface also reconstructs, forming S−S bonds. We analyzed seven different reconstructions for this surface, and six of them are in Figure S5 in the Supporting Information. Energetically, the most favorable reconstruction is indicated in Figure 4a and b. The other reconstructions are less stable by 0.01−0.03 eV/supercell. Figure S5b in the Supporting Information is the second more stable reconstruction and Figure S5a the less stable reconstruction. The most stable reconstruction has all sulfur atoms in the surface, making S−S bonds with bond lengths ranging from 2.11 to 2.13 Å, as shown in Figure 4a. The bond lengths of the S−S bonds depend on the atoms to which the sulfur are bonded. The S−S bonded to three copper atoms and one iron atom are slightly smaller than that bonded to three iron atoms and one copper atom. In the most stable reconstruction, for example, when the S−S is bonded to three copper and one iron, the bond length is 2.11 Å, while the bond length is 2.13 Å when it is bonded to three iron atoms and one copper atom. This behavior is similar in the other reconstructions for the (100)-S surface. In the reconstruction shown in Figure S5b (Supporting Information), for example, the differences are more drastic, with a S−S bond length of 2.12 Å when it is

Figure 3. Side view of the slab model with the top and bottom surfaces reconstructed simultaneously. The dotted lines indicate the supercell, and the full lines indicate the fixed atoms. 6360



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Figure 4. (a) Top and (b) side views of the most stable reconstruction of the (100)-S chalcopyrite surface. (c) Top and (d) side views of the reconstructed (100)-M chalcopyrite surface. The dotted lines indicate the supercell, and the full lines indicate the fixed atoms.

Figure 5. (a) Top and (b) side views of the reconstructed (111)-S chalcopyrite surface. (c) Top and (d) side views of the reconstructed (111)-M chalcopyrite surface. The dotted lines indicate the supercell. Distances in angstroms.

bonded to three copper atoms and one iron atom and 2.20 Å when it is bonded to three iron atoms and one copper atom. The unreconstructed (100)-M surface has, in its first atomic layer, eight metal atoms, with four copper and four iron atoms, while the second atomic layer is composed only by sulfur atoms, as shown in Figure S6 in the Supporting Information. This surface, similar to (001)-M, also reconstructs, forming an alloy-like structure, as indicated in Figure 4c and d. One important difference is that in the (001)-M surface there are rows of copper and rows of iron, as shown in Figure 2a, while in the (100)-M surface metals are mixed in the rows, as indicated in Figure 4c. This explains the differences in the bond lengths between the (001)-M and (100)-M surfaces and also the distortion observed in Figure 4c and d. The behavior of the sulfur atoms in the (100)-M surface is similar to that in the (001)-M surface. They occupy alternate positions almost in the center of squares formed by the metallic atoms. 3.c. (111) Surface. Similarly to the (001) and (100) chalcopyrite surface, the (111) surface has also two possible terminations, the sulfur-terminated, (111)-S, and the metalterminated surfaces, (111)-M, as shown in Figures S7 and S8 in the Supporting Information. The unreconstructed (111)-S surface has, in its first atomic layer, four sulfur atoms, each one bonded to one metal atom, as shown in Figure S7 in the Supporting Information. Similarly to the (001)-S and (100)-S, this surface reconstructs, forming S−S bonds. Nevertheless, different from the previous sulfur terminated surfaces, the (111)-S surface reconstructs, forming a chain of four sulfur atoms, as indicated in Figure 5. Two S−S bonds, which form these sulfur chains, are within the supercell, and their bond lengths were calculated to be 2.04 Å, around 0.1 Å shorter than that obtained for the (001)-S and (100)-S chalcopyrite surfaces. The S−S bond between two supercells was calculated to be 2.26 Å. The unreconstructed (111)-M surface has two iron and two copper atoms in the first atomic layer, as shown in Figure S8 in

the Supporting Information. This surface follows the same tendency of the (001)-M and (100)-S and reconstructs, forming metal−metal bonds. However, on the (111)-M surface, there is not a virtually infinite plane of metals, like in (001)-M and (100)-M, but small units, similarly to a lozenge, formed by four Fe−Cu bonds and one bond between the two iron atoms on the diagonal, as shown in Figure 5c. The Fe−Cu bond lengths are in the range 2.37−2.45 Å, and the difference is due to the fact that the four metal atoms are not exactly in the same plane. The Fe−Fe bond was calculated to be 2.47 Å long. The small angles Fe−Cu−Fe are 61 and 58°, and the larger angles Cu−Fe−Cu are 116 and 120°, once more, is not exactly equal in a regular lozenge because of this small deviation in the metal plane. 3.d. (112) Surface. The (112) surface is a stepped surface with superficial atoms in three different altitudes, as shown in Figure S9 in the Supporting Information. In this surface, each atomic layer is composed by sulfur and metal atoms forming a type 1 surface, according to Tasker's definition.48 The reconstructed surface is showed in Figure 6. Following the same tendency observed on (001)-S and (100)-S, the (112) surface reconstructs, forming S−S bonds. One S−S bond is formed between sulfur atoms in the same supercell and another between sulfur atoms in different supercells. The S−S bond lengths were calculated to be 2.26 Å. A remarkable difference of this surface in comparison with the previously discussed ones is a movement of the surface metal atoms, as shown in Figure 6b. With the reconstruction, these atoms move downward, in comparison with the unreconstructed surface. 3.e. (110) Surface. The unrelaxed (110) surface is a stepped surface with atomic layers composed by sulfur−metal− sulfur chains, as indicated in Figure S10 in the Supporting Information. This surface does not reconstruct, forming new 6361



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the relaxed surface change in comparison to the unrelaxed one. On the unrelaxed surface, the Fe−S and Cu−S bonds are around 2.24 and 2.29 Å,42 respectively, while in the relaxed surface all bond lengths decrease to 2.20 Å for the Fe−S bond and 2.25 Å for the Cu−S bond. All the bond lengths for the relaxed surface are indicated in Figure 7a. 3.f. (101) Surface. The (101) chalcopyrite surface is also a stepped surface, and it is also formed by a sulfur−metal− sulfur chain. In Figure S11 in the Supporting Information, we presented the unrelaxed surface, where the chain in the first atomic layer has, in the same plane, both sulfur and metal atoms. Similarly to the (110) surface, no reconstruction with new bonds has been observed after the optimization process, but the metal atoms in the first atomic layer go downward to an intermediate position between the first and second atomic layer, as shown in Figure 8. The bonds along the chains, as

Figure 8. (a) Top and (b) side views of the reconstructed (101) chalcopyrite surface. The dotted lines indicate the supercell. Distances in angstroms.

occurred on the (110) surface, are a little shorter in comparison with the unrelaxed surface. The Fe−S bond decreases from 2.24 to around 2.19 Å, and the Cu−S bond decreases from 2.29 to around 2.22 Å. Figure 6. (a) Top and (b) side views of the reconstructed (112) chalcopyrite surface. The dotted lines indicate the supercell. Distances in angstroms.

4. DISCUSSION Considering the results presented in the previous section, we can outline three different mechanisms of reconstruction for the nine surfaces investigated. In Table 3, we summarized the main geometric characteristics and also the mechanism of reconstruction for each surface. The (001)-S, (100)-S, (111)-S, and (112) surfaces have reconstructed, forming S−S bonds, and the S−S bond length are in the range 2.04−2.26 Å, as indicated in Table 3. To understand this reconstruction mechanism, it is important to remember that, in the bulk, each sulfur atom is coordinated to four metallic atoms in a tetrahedral geometry. With the cleavage, the sulfur atoms on the surface have lost two bonds and the new S−S bonds are formed by the overlap of the dangling bond of these sulfur atoms. In order to better understand the nature of this new S−S bond, electron localization functions (ELF)51−53 were evaluated in the plane of the S−S bond aiming to provide insights about the bond nature. The ELF is close to 1 in the region where the electrons are paired to form a covalent bond. On the other hand, ELF is around 0.5 in regions where the bonds have metallic character. In Figure 9, we show the ELF of the (001)-S chalcopyrite surface in its (1 × 1) reconstruction.

bonds, as occurred in the (001), (100), (111), and (112) surfaces. However, after the optimization process, the metal atoms in the first atomic layer go out of the plane, occupying an intermediate position between the sulfur atoms which remain in the first atomic layer and the sulfur−metal chain in the second layer, as shown in Figure 7a and b. The bond lengths in

Figure 7. (a) Top and (b) side views of the reconstructed (110) chalcopyrite surface. The dotted lines indicate the supercell. Distances in angstroms. 6362



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Table 3. Structural Properties and Reconstruction Mechanism for the Chalcopyrite Surfaces structural properties on the reconstructed surface (in angstroms) surface

S−S

(001)-S

2.12

(001)-M (100)-S

Cu−Cu: 2.61 Fe−Cu: 2.63 Fe−Fe: 2.61 2.12

(100)-M (111)-S

Cu−Cu: 2.67 Fe−Cu: 2.66 Fe−Fe: 2.67 2.04a 2.26b

(111)-M (112)

M−M

Cu−Cu: Fe−Cu: 2.45 Fe−Fe: 2.47 2.26

(101) (110) a

M−S

reconstruction mechanism

Cu−S: 2.30 Fe−S: 2.30 Cu−S: 2.27−2.32 Fe−S: 2.24 Cu−S: 2.33 Fe−S: 2.30 Cu−S: 2.32 Fe−S: 2.23

reconstruction with formation of S−S bonds reconstruction with formation of an alloy-like structure reconstruction with formation of S−S bonds reconstruction with formation of an alloy-like structure

Cu−S: 2.34 Fe−S: 2.31 Cu−S: 2.20−2.21 Fe−S: 2.15−2.19 Cu−S: 2.20−2.33 Fe−S: 2.19−2.25 Cu−S: 2.22 Fe−S: 2.19 Cu−S: 2.20−2.28 Fe−S: 2.18−2.23

reconstruction with formation of S−S bonds reconstruction with formation of metallic aggregates reconstruction with formation of S−S bonds no reconstruction; relaxation exposing the sulfur atoms no reconstruction; relaxation exposing the sulfur atoms

S−S bond within the supercell. bS−S bond between two supercells.

surface in its (1 × 1) reconstruction. The scale of the curves was kept the same to allow a better comparison. The DOS projected on the sulfur atoms, presented in Figure 10a, corresponds to the sum of the electronic states of all sulfur atoms in the first atomic layer in both unreconstructed and reconstructed (001)-S surface. Analyzing it, it is possible to see a decrease in the number of states in the region between −1 and 0 eV due to the reconstruction process. This suggests that the new S−S bond is formed in an oxidative process, forming disulfide groups, S22−, as described by eq 1. Figure 9. Calculated ELF of the (001)-S chalcopyrite surface.

Reconstruction

(CuFeS2)−2S2 − ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (CuFeS2)−S2 2 − + 2e−

The ELF in the region between the S−S atoms is around 0.5 which indicates bonding between the sulfur atoms. The ELF of the (001)-S surface in its (2 × 2) reconstruction, (100)-S, (111)-S, and (112) surface shows similar behaviors, and they are presented in Figures S12−S15 in the Supporting Information. In the chalcopyrite's bulk, sulfur, iron, and copper atoms have oxidation numbers −2, +3, and +1, respectively.47 To investigate the reconstruction's effects on the electronic structure, density of states (DOS) projected over the surface atoms were evaluated. In Figure 10, we show the DOS projected on the sulfur, iron, and copper atoms in the (001)-S

(1)

The formation of disulfide groups is consistent with the molecular orbital diagram for the S−S bond, shown in Figure S16, in the Supporting Information. The S22− species has a bond order equal to one, and the σu* orbital is unoccupied and the π* is fully occupied. The S−S bond lengths observed in the reconstructed surfaces are also in good agreement with that obtained for the disulfide. The S−S bond lengths were calculated to be between 2.04 and 2.26 Å in the reconstructed chalcopyrite surfaces, as indicated in Table 3. These values are closer to the PBE/6-311G(d,p) estimate of 2.276 Å for the

Figure 10. Projected density of states on the (a) sulfur atoms, (b) iron atoms, and (c) copper atoms in the (1 × 1) reconstructed (001)-S chalcopyrite surface. The curves are in the same scale to allow a direct comparison. The black dashed and red straight lines correspond to the unreconstructed and reconstructed surfaces, respectively. 6363



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orbitals from metals with the sp3 orbitals from sulfur atoms. When the surfaces (001)-M, (100)-M, and (111)-M are formed, metal atoms on the surface lose two bonds and the new metal−metal are δ bonds formed by the overlap of the dxy, dxz, and dyz orbitals. The metal−metal bonds are in the range 2.45−2.67 Å, as indicated in Table 3. ELFs were calculated in the plane of the metal−metal bonds for the (001)-M, (100)-M, and (111)-M surfaces and the results are presented in Figures 11 and S20 and S21 in the

S22− molecule in the gas phase, compared to 1.935 Å for the neutral S2 molecule. In Figure 10b, we show the sum of the DOS projected on the iron atoms in the second atomic layer. Comparing the DOS of the unreconstructed and reconstructed surfaces, it is possible to see a shift of the unoccupied electronic states toward the Fermi level through the reconstruction process. Similar behavior is not observed for the DOS projected on the copper atoms, Figure 10c. These results suggest that the electrons released with the formation of the disulfide groups (eq 1) are transferred to iron atoms. In this case, the iron atoms on the surfaces which reconstruct forming disulfide groups are reduced from +3 to +2. The DOS projected over the sulfur, iron, and copper atoms in the (001)-S surface in its (2 × 2) reconstruction, in the (100)-S surface, and in the (111)-S surface are also presented in Figures S17−S19 in the Supporting Information. The analysis of these DOS projected on the surface atoms allows similar conclusions. The reconstruction of the (111)-S surface is slightly different, because a chain of four sulfur atoms is observed. ELF data, in Figure S14 in the Supporting Information, show the S−S bonds between the four sulfur atoms, and DOS projected on the surface atoms, shown in Figure S19 in the Supporting Information, also indicated an oxidation of sulfur atoms and a reduction of the iron atoms. Experimentally, disulfide groups have been pointed out in the chalcopyrite surface when the mineral is cleaved under an inert atmosphere. Klauber, in 2003,29 analyzing XPS S2p spectra and, more recently, Acres et al.31 using synchrotron radiation attributed the band 161.9 eV to a disulfide group. The presence of sulfur oligomers, Sn2−, as observed in the reconstruction of the (111)-S surface, was also pointed out experimentally. In the Harmer et al.28 analysis of S2p synchrotron XPS spectra, they attributed a band centered at 161.88 eV to sulfur polymers. In their analysis, such reconstruction was attributed to the sulfur terminated (111) surface, as we observed in this work. Mikhlin,24 in 2004, and Harmer et al.,26 in 2006, combined XPS with other techniques and also identified the formation of Sn2− groups in the chalcopyrite surface. From our results, it is clear that the chalcopyrite surface reconstructs, exposing the sulfur atoms to the surface for disulfides, which is the precursor for forming polysulfides. The oxidation of the S2− to S22− is followed by the reduction of the Fe3+, which has the antibonding states occupied, explaining the experimental observation that the iron atoms are first released from the surface and then the copper atoms.20,54 The second reconstruction mechanism was observed for the (001)-M, (100)-M, and (111)-M surfaces. These surfaces reconstruct, forming several metal−metal bonds, as indicated in Table 3. In these surfaces, before the reconstruction, the first atomic layer is formed by metal atoms, as shown in Figures S3, S6, and S8 in the Supporting Information. In the (001)-M and (100)-M, the surface reconstructs, forming a plane with several metal−metal bonds, resulting in an alloy-like structure. Another interesting characteristic of these surfaces is the movement of the sulfur atoms, which were in the second layer in the unreconstructed surface, toward the first atomic layer. In the (111)-M, metal−metal bonds are formed, resulting in metalliclike aggregates. To understand the reconstruction mechanism for this surface, it is important to remember that, at the bulk, each metal atom is coordinated to four sulfur atoms in a tetrahedral geometry. In this case, the bond between metal and sulfur atoms is formed by the overlap of the dxy, dxz, and dyz

Figure 11. Calculated ELF of the (001)-M chalcopyrite surface.

Supporting Information, respectively. These results show bonding between the metal atoms with a value of ELF between 0.30 and 0.45. The value of ELF is relatively lower in these metal terminated surfaces, but one has to note that the metal atoms are not exactly in the same plane. DOS projected on the surface atoms were also investigated. In Figure 12, we show the DOS projected on the iron atoms

Figure 12. Projected density of states on (a) iron atoms and (b) copper atoms in the (001)-M chalcopyrite surface. The curves are in the same scale to allow a direct comparison. In part a, the black dashed, red straight, and blue dotted lines correspond to unreconstructed and reconstructed (Fe up) and reconstructed (Fe up) surfaces, respectively. In part b, the black dashed and red straight lines correspond to the unreconstructed and reconstructed surfaces, respectively.

and also on the copper atoms in the (001)-M surface. We do not observe significant changes in the DOS projected on the metal atoms undergoing reconstruction. The more significant change is the fact that the electronic states are spread out in a larger range of energy as expected for metallic systems. In the 6364



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surfaces, three different reconstruction mechanisms emerged for the chalcopyrite surfaces. Surfaces which have, in its first atomic layer, relatively close sulfur atoms with lower coordination number than in the chalcopyrite's bulk reconstruct, forming S−S bonds. The (001)-S, (100)-S, and (112) surfaces are examples of this reconstruction which occurs due to the dangling bonds on sulfur atoms which overlap, forming new bonds. Analyses of the projected density of states on the surface atoms indicate an oxidative process in the formation of the S−S bond. Our results suggest an increase of the sulfur oxidation number from −2 to −1, forming disulfide groups on the chalcopyrite surfaces followed by the reduction of iron with oxidation number decreasing from +3 to +2. The (111)-S reconstructs similarly to the other sulfur terminated surfaces, however, forming a chain of four sulfur atoms, the S42− group. Surfaces terminated in metal atoms such as (001)-M, (100)-M, and (111)-M reconstruct, forming several metal−metal bonds. The reconstructions on the (001)-M and (100)-M surfaces are clearly forming alloy-like structures. An interesting point in this surface is although the unreconstructed surfaces have, in the first atomic layer, metal atoms, after the reconstruction the sulfur atoms are exposed on the surface. Finally, the third mechanism observed is just a relaxation of metal atoms and it occurs on the (101), (110), and part of (112) surfaces. In these surfaces, the metal atoms move downward in order to increase the sulfur−metal−sulfur angle to around 120°. Our results indicate that the pristine cleavage of the chalcopyrite leads undoubtedly to strong reconstruction of the surfaces, exposing the sulfur atoms to the surface and the metal atoms migrating to the underneath, forming metallic-like bidimensional alloys. Although the disulfides have been experimentally detected in the pristine chalcopyrite surfaces, the metal−metal bond formation is also an important issue that deserves to be investigated in detail.

unreconstructed (001)-M surface, for example, the iron atoms have several electronic states localized close to −3 eV below the Fermi level, as shown in Figure 12a; however, with the reconstruction, these electronic states are distributed in a wider range of energy. This happens because of the new δ bonds between the metal atoms which results in several molecular orbitals close in energy, forming a band. Similar behavior is observed for the electronic states localized on the copper atoms, which are concentrated near −2 eV, in the unreconstructed surface, and, with the reconstruction, are spread out in energy, as shown in Figure 12b. For the sulfur atoms, no important changes were observed in the density of states. The DOS projected on the (100)-M and (111)-M surfaces show similar behavior as already discussed previously, and they are presented in Figures S22−23 in the Supporting Information. The alloy-like structures formed with the reconstruction of the (001)-M and (100)-M surfaces were investigated. We evaluated the spin polarization, which corresponds to the difference between the spin densities up and down. The reconstructed (001)-M and the reconstructed (100)-M surfaces have both iron atoms with spin up and down in the alloy-like structure. With this reconstruction, there is a partial coupling of spins between the iron atoms. In chalcopyrite bulk, the Löwdin population on the iron atoms was calculated to be ±3.22, while, in the reconstructed (001)-M and (100)-M, the Löwdin population in the iron atoms of the alloy-like structure was calculated to be ±2.62 and ±2.77, respectively. To the best of our knowledge, experimental evidence for this kind of reconstruction has never been detected. This is not a surprise, since such reconstructions are limited to a very few atomic layers and studies about these first atomic layers are very difficult. However, such reconstructions could have a relevant influence in the leaching mechanism. If a reconstruction forming several metal−metal bonds occurs during the chalcopyrite's leaching, this could affect the leaching kinetics. Experimental investigations have to be carried out in order to understand the chalcopyrite surface reconstruction. For the (101), (110), and part of the (112) surfaces, a third reconstruction mechanism was observed. It is not exactly a reconstruction, because new covalent bonds are not formed, but it is a relaxation of the metal atoms. Comparing Figures 6b, 7b, and 8b with Figures S9b, S10b, and S11b in the Supporting Information, it is clear that with the optimization procedure the metal atoms, in the first layer, move downward. All of these surfaces are stepped surfaces whose metal atoms, in the first atomic layer, are coordinated to three sulfur atoms. At the bulk, each metal atom is coordinated to sulfur atoms in a tetrahedral framework, and the angle ∠S−M−S is around 109° in the unrelaxed surfaces. In the relaxed surfaces, the metal atoms move downward to increase this S−M−S angle and decrease the angular strength. In the relaxed (101) surface, for example, the angle ∠S−M−S increases from 109 to 118°, while, in the (110) surface, the same angle increases from 109 to 117°.

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ASSOCIATED CONTENT

S Supporting Information *

Cleavage planes of the chalcopyrite's bulk, unreconstructed supercells, other reconstructions for the (100)-S surface, electron localization functions (ELF), and density of states (DOS) for chalcopyrite surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Brazilian Initiative National Institute of Science and Technology for Mineral Resources, Water and Biodiversity − INCT-ACQUA (http://www.acqua-inct.org). The financial support from the Brazilian Agencies Conselho Nacional ́ para o Desenvolvimento Cientifico e Tecnológico − CNPq and Fundaçaõ de Amparo a Pesquisa do Estado de Minas Gerais − FAPEMIG is gratefully acknowledged. C.d.O. would like to thank the CNPq for the postdoctoral fellowship (PNPD program).

5. CONCLUSIONS Plane wave density functional calculations have been applied to study the reconstruction of the (001), (100), (111), (112), (101), and (110) chalcopyrite surfaces. The (001), (100), and (111) cleavages have two possible terminations, the sulfur and the metal ones. The reconstructions of all of them have been investigated in detail. On the basis of analyses of these nine 6365



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