Preparation, Structure, and Orientation of Pyrite FeS2{100} Surfaces: Anisotropy, Sulfur Monomers, Dimer Vacancies, and a Possible FeS Surface Phase (original) (raw)
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The Journal of Physical Chemistry, 2014
Sulfur dimer (S 2 2− ) terminated pyrite FeS 2 {100} surfaces with a low energy electron diffraction (LEED) pattern of 2 × 1 symmetry are reported. The 2 × 1 symmetry correlates with the orientation of the anisotropic surface structure and external symmetry of macroscopic striations on the pyrite cube face. The basic condition to form these surfaces is a mild 200 V Ne + sputtercleaning procedure followed by a 570 K anneal of the sample in a 10 −7 Torr S 2 (g) atmosphere. Controlled amounts of surface sulfur monomers (S 2− ) can be introduced by mild sputtering of the sulfur dimer terminated surfaces. At low monomer concentrations the surface displays the same characteristic 1 × 1 LEED pattern as that for fracture-generated surfaces. With increasing sulfur depletion, a (1/√2 × 1/√2)R45°LEED pattern emerges, and soft Xray photoelectron spectroscopy (XPS) results show a sulfur dimer deficient near-surface region and a new high binding energy sulfur spectral component suggesting the presence of local coordination environments where sulfur monomers are coordinated by four Fe ions compared to three as in the pyrite structure. The plausible formation of a defective FeS-like surface phase where monomeric sulfurs are coordinated by four Fe ions, and bond counting energetics favoring surface sulfur monomer recombination around Fe vacancy sites on pyrite FeS 2 {100}, both imply surface sulfur dimer vacancy sites with unique adsorption and reactivity properties. Taken together, our results suggest a very rich and dynamic defect structural landscape at pyrite FeS 2 {100} surfaces with direct implications for its surface chemical activity.
Understanding the Surfaces and Crystal Growth of Pyrite FeS2
Materials Research, 2018
Pyrite is a common sulfide mineral, which has arisen early interest by its euhedral shape and by its metallic glow similar to gold. However, it is only in our century that we began to understand pyrite crystal growth, considering the thermodynamic and kinetic aspects of crystal formation as a function of temperature and concentration of the elements present in the medium. This article reports an analysis by molecular mechanics of 11 surfaces associated to observed morphologies in order to explain the growth of natural and synthesized minerals. The lowest surface and attachment energies (respectively 1.04 J/m 2 and-20.3 kJ/mol) were obtained for the (001) plane, indicating that it is the most stable surface and that kinetic growth also preferentially occurs on this plane. Less known properties, such as crystal striations along the <100> directions, are also discussed.
Density-functional theory studies of pyrite FeS2() and () surfaces
Surface Science, 2002
We have performed density-functional theory calculations using both plane wave-pseudopotential and Gaussian basis set approaches on the (1 0 0), planar (1 1 0) and microfacetted (1 1 0) surfaces of pyrite (FeS 2 ). Our calculations indicate that the (1 0 0) surface is more stable than the planar (1 1 0) surface, which is predicted to have a higher surface energy. Creation of microfacets on the (1 1 0) surface resulted in a lower surface energy. Relatively small differences in calculated surface energy between the ideal and relaxed (1 0 0) and (1 1 0) surfaces were found. The (1 0 0) and surfaces are predicted to be essentially bulk-terminated, with a relatively small amount of relaxation. Electrostatic effects induced by loss of coordination at surface Fe atoms are likely to be responsible for surface ionic displacements. Our calculations indicate that surface Fe atoms of fourfold coordination, present on the (1 1 0) surfaces, are spin polarized, while those of fivefold coordination are fully spin-paired. These results suggest that magnetic species, such as O 2 , are more prone to react at low Fe coordination defect sites on real FeS 2 (1 0 0) surfaces.
Density-functional theory studies of pyrite FeS 2( 1 0 0 ) and ( 1 1 0 ) surfaces
Surface Sci, 2002
We have performed density-functional theory calculations using both plane wave-pseudopotential and Gaussian basis set approaches on the (1 0 0), planar (1 1 0) and microfacetted (1 1 0) surfaces of pyrite (FeS 2). Our calculations indicate that the (1 0 0) surface is more stable than the planar (1 1 0) surface, which is predicted to have a higher surface energy. Creation of microfacets on the (1 1 0) surface resulted in a lower surface energy. Relatively small differences in calculated surface energy between the ideal and relaxed (1 0 0) and (1 1 0) surfaces were found. The (1 0 0) and (1 1 0) surfaces are predicted to be essentially bulk-terminated, with a relatively small amount of relaxation. Electrostatic effects induced by loss of coordination at surface Fe atoms are likely to be responsible for surface ionic displacements. Our calculations indicate that surface Fe atoms of fourfold coordination, present on the (1 1 0) surfaces, are spin polarized, while those of fivefold coordination are fully spin-paired. These results suggest that magnetic species, such as O 2, are more prone to react at low Fe coordination defect sites on real FeS 2 (1 0 0) surfaces.
XPS study of the oxidized pyrite surface
Surface Science, 2004
The core level spectra of pristine and oxidized mineral pyrite FeS 2 surfaces were studied using synchrotron excited photoelectron spectroscopy. In addition to the bulk feature, three surface core level shifted components were identified in the sulphur 2p spectra. After measuring the spectra of the clean pristine surface, the sample was oxidized for 12 h in O 2 atmosphere. The effect of this treatment of the specimen, besides causing weakening in the signal intensities, was observed as changes in the surface core level shifts (SCLS). The surface components of the oxidized pyrite spectra were shifted towards higher binding energies compared to those of the pristine sample. The reason for this phenomenon is briefly discussed.
Acta Materialia, 2021
This investigation provides novel data on the structure and chemical composition of pyrite thin films and new hints concerning their formation mechanism. From TEM-HAADF data, it has been found that the films are composed of two different layers: one is very compact and the other one is quite porous with many voids separating a few groups of grains. This porous layer is always in direct contact with the substrate, and its thickness is quite similar to that of the original Fe film. The average size of pyrite grains is equal in both layers, what suggests that the same process is responsible for their formation. Concentration profiles of sulfur, iron and some impurities (mainly sodium and oxygen from the glass substrate) through both layers are given in this work, and thus chemical inhomogeneities of the films are proved by the obtained stoichiometric ratios (⁄). Moreover, Na from sodalime glass substrates mainly accumulates at the pyrite grain boundaries and barely dopes them. The obtained results support the hypothesis that the iron sulfuration process essentially induces the diffusion of iron atoms, what leads to the porous layer formation as a manifestation of the Kirkendall Effect. Therefore, it seems that the same mechanisms that operate in the synthesis of surface hollow structures at the nanoscale are also active in the formation of pyrite thin films ranging from several tens to hundreds of nanometers.
Korean Journal of Chemical Engineering, 2018
Iron pyrite (FeS 2) thin films were fabricated by spin coating the solution of FeS 2 nanocrystals of ~40 nm in size on glass substrates, followed by annealing in a sulfur environment at different temperatures. The effect of sulfurization temperature on the morphology, structural, optical and electrical properties was investigated. With increase of the sulfurization temperature, the grain size and crystallinity of the films was improved, although some cracks and voids were observed on the surface of thin films. The band gap of the FeS 2 films was decreased at higher sulfurization temperature. The electrical properties were also changed, including the increasing in resistivity and the decrease in Hall mobility, with increase of sulfurization temperature. The change in the optical and electrical properties of the FeS 2 thin films was explained based on the changes of phase, morphology, surface, and grain boundary property.
XPS study of the sulphur 2p spectra of pyrite
Surface Science, 2003
The surface core level shifts have been measured and interpreted for cleaved FeS 2{1 0 0}. A new feature has been observed at about 2.0 eV from the S 2p bulk line on its low binding energy side. This small peak is the most surface sensitive component. The origin of this feature is thought to be the breakage of sulphur-sulphur bonds. The strongest surface peak is ascribed to the fracture of the Fe-S bond. An estimate for the effective attenuation lengths of photoelectrons in pyrite at kinetic energies 40, 180 and 610 eV supports the interpretation of the features.