Dynamics of point defect formation, clustering and pit initiation on the pyrite surface

Efficient Photoelectrocatalytic Reduction of CO2 to Selectively Produce Ethanol Using FeS2/TiO2 p-n Heterojunction Photoelectrodes

Herein, the FeS2/TiO2 p-n heterojunction was first utilized as a photoelectrode for the PEC reduction of CO2 to selectively produce ethanol. The FeS2/TiO2 photoelectrode was fabricated through electrochemical anodization, electrodeposition, and vulcanization methods. The impact of the FeS2 loading amount and applied bias on the PEC performance was investigated. The behavior of photocurrent polarity reverse is observed depending on the FeS2 loading amount, which is related to the energy band structure of the semiconductor/electrolyte interface. The active sites for ethanol production were identified on TiO2 nanotubes rather than on the FeS2 surface. Incorporation of FeS2 not only broadened the visible light absorption range but also formed a p-n heterojunction with TiO2. FeS2/TiO2 with an electrodeposition time of 15 min exhibits the highest ethanol yield of 1170 μmol L-1 cm-2 for 3.5 h of reaction under ultraviolet-visible (UV-Vis) illumination at an applied bias of -0.7 V. Compared to TiO2, FeS2/TiO2 showed significantly higher ethanol yield due to its appropriate loading amount of FeS2 and the synergistic effect of strong UV-Vis light absorption and efficient separation and transfer of charge carriers at the p-n junction.

Electrochemical Investigations on the Corrosion Behavior of 904L Stainless Steel in LiBr Solutions

The pitting corrosion susceptibility of 904L stainless steel in aerated LiBr solutions was investigated. The influence of various experimental variables, including electrolyte concentrations, pH, scan rate, temperatures, and constant potential has been studied using electrochemical measurements, such as cyclic potentiodynamic polarization, potentiostatic techniques, and electrochemical impedance spectroscopy (EIS). The surface morphology of 904L stainless steel was examined by scanning electron microscope (SEM). The rate of uniform corrosion and susceptibility toward pitting corrosion increases with an increase in LiBr concentrations and temperature. Increasing the pH of the solution decreases the rate of both uniform and pitting corrosion. An increase in the scan rate enhances the uniform corrosion, but suppresses the pitting corrosion. EIS diagrams displayed a depressed semicircles with the center under the real axis. Bode plots support the result that the uniform corrosion resistance of the alloy in LiBr solutions decreases with increasing Br− anion concentration. The observations suggest that this one time constant may actually be the overlap of two-time constants.

Investigation of Sulfur Doping in Mn-Co Oxide Nanotubes on Surface-Enhanced Raman Scattering Properties

Doping engineering is an efficient strategy to manipulate the optoelectronic properties of metal oxides for sensing, catalysis, and energy applications. Herein, we have demonstrated the fabrication of sulfur (S)-doped Mn-Co oxides to regulate their band and surface electronic structures, which is beneficial to enhancing the charge transfer (CT) between the metal oxides and their adsorbed molecules. As expected, significantly enhanced SERS signals are achieved on S-doped Mn-Co oxide nanotubes, and the minimum detection concentration can reach as low as 10^-8 M. Furthermore, the change in the electronic structure caused by S-doping provides different microelectric fields to influence the orientation of the interaction between the probe molecules and the substrate. Additionally, the evaluation of the oxidase-like catalytic activity of the substrate proved that, with an increase in the ratio of Co²⁺/Co³⁺ content, the number of electrons on the substrate increases, which promotes the CT process and further increases the degree of CT. The nonmetallic doping route in semiconducting metal oxides can provide effective and stable SERS activity; moreover, it provides a new strategy for exploring the relationship between CT in catalysis and SERS performance of semiconductors.

Preparation of Monoclinic Pyrrhotite by Thermal Decomposition of Jarosite Residues and Its Heavy Metal Removal Performance

Jarosite residues produced by zinc hydrometallurgical processing are hazardous solid wastes. In this study, monoclinic pyrrhotite (M-Po) was prepared by the pyrolysis of jarosite residues in H2S atmosphere. The influence of gas speed, reaction temperature, and time was considered. The mineral phase, microstructure, and elemental valence of the solids before and after pyrolysis were analyzed using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. The performances of the prepared M-Po on the removal of Zn and Pb from aqueous solution were evaluated. The results show M-Po to be the sole product at the reaction temperatures of 550 to 575 °C. Most of the M-Po particles are at the nanometer scale and display xenomorphic morphology. The phase evolution process during pyrolysis is suggested as jarosite → hematite/magnetite → pyrite → pyrite+M-Po → M-Po+hexagonal pyrrhotite (H-Po) → H-Po. The formation rate, crystallinity, and surface microtexture of M-Po are controlled by reaction temperature and time. Incomplete sulfidation may produce coarse particles with core–shell (where the core is oxide and the shell is sulfide) and triple-layer (where the core is sulfate, the interlayer is oxide, and the shell is sulfide) structures. M-Po produced at 575 °C exhibits an excellent heavy metal removal ability, which has adsorption capacities of 25 mg/g for Zn and 100 mg/g for Pb at 25 °C and pH ranges from 5 to 6. This study indicates that high-temperature sulfidation is a novel and efficient method for the treatment and utilization of jarosite residues.

On the Mechanistic Understanding of Photovoltage Loss in Iron Pyrite Solar Cells

Considering the natural abundance, the optoelectronic properties, and the electricity production cost, iron pyrite (FeS₂ ) has a strong appeal as a solar cell material. The maximum conversion efficiency of FeS₂ solar cells demonstrated to date, however, is below 3%, which is significantly below the theoretical efficiency limit of 25%. This poor conversion efficiency is mainly the result of the poor photovoltage, which has never exceeded 0.2 V with a device having appreciable photocurrent. Several studies have explored the origin of the low photovoltage in FeS₂ solar cells, and have improved understanding of the photovoltage loss mechanisms. Fermi level pinning, surface inversion, ionization of bulk donor states, and photocarrier loss have been suggested as the underlying reasons for the photovoltage loss in FeS₂ . Given the past and more recent scientific data, together with contradictory results to some extent, it is timely to discuss these mechanisms to give an updated view of the present status and remaining challenges. Herein, the current understanding of the origin of low photovoltage in FeS₂ solar cells is critically reviewed, preceded by a succinct discussion on the electronic structure and optoelectronic properties. Finally, suggestions of a few research directions are also presented.

Surface sulfurization activating hematite nanorods for efficient photoelectrochemical water splitting

Surface treatment is an effective method to improve the photoelectrochemical (PEC) performance of photoelectrodes. Herein, we introduced a novel strategy of surface sulfurization to modify hematite (α-Fe₂O₃) nanorods grown in an aqueous solution, which triggered encouraging improvement in PEC performances. In comparison to the solution-grown pristine α-Fe₂O₃ nanorod photoanode that is PEC inefficient and always needs high temperature (>600 °C) activation, the surface sulfurized α-Fe₂O₃ nanorods show photocurrent density increased by orders of magnitude, reaching 0.46 mA cm^-2 at 1.23 V vs. RHE (reversible hydrogen electrode) under simulated solar illumination. This improvement in PEC performances should be attributed to the synergy of the increased carrier density, the reduced surface charge carrier recombination and the accelerated water oxidation kinetics at the α-Fe₂O₃/electrolyte interface, as induced by the incorporation of S ions and the formation of multi-state S species (Fe-S_x-O_y) at the surface of α-Fe₂O₃ nanorods. This study paves a new and facile approach to activate α-Fe₂O₃ and even other metal oxides as photoelectrodes for improved PEC water splitting performances, by engineering the surface structure to relieve the bottlenecks of charge transfer dynamics and redox reaction kinetics at the electrode/electrolyte interface.

Pyrite and Organic Compounds Coexisting in Intrusive Mafic Xenoliths (Hyblean Plateau, Sicily): Implications for Subsurface Abiogenesis

Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from the Hyblean Plateau, Sicily. The representative sample consists of plagioclase, Fe-oxides, clinopyroxene, pyrite and minor amounts of many other minerals. Plagioclase displays incipient albitization, clinopyroxene is deeply corroded. Pyrite grains are widely replaced by spongy-textured magnetite, which locally hosts Ca-(and Fe-)sulfate micrograins and blebs of condensed organic matter. Whole-rock trace element distribution evidences that incompatible elements, particularly the fluid-mobile Ba, U and Pb, are significantly enriched with respect to N-MORB values. The mineralogical and geochemical characteristics of the sample, and its U-Pb zircon age of 216.9 ± 6.7 MA, conform to the xenolith-based viewpoint that the unexposed Hyblean basement is a relict of the Ionian Tethys lithospheric domain, mostly consisting of abyssal-type serpentinized peridotites with small gabbroic intrusions. Circulating hydrothermal fluids there favored the formation of hydrocarbons trough Fischer-Tropsch-type organic synthesis, giving also rise to sulfidization episodes. Subsequent variations in temperature and redox conditions of the system induced partial de-sulfidization, Fe-oxides precipitation and sulfate-forming reactions, also promoting poly-condensation and aromatization of the already-formed hydrocarbons. Here we show organic matter adhering to a crystal face of a microscopic pyrite grain. Pyrite surfaces, as abiotic analogues of enzymes, can adsorb and concentrate organic molecules, also acting as catalysts for a broad range of proto-biochemical reactions. The present data therefore may support established abiogenesis models suggesting that pyrite surfaces carried out primitive metabolic cycles in suitable environments of the early Earth, such as endolithic recesses in mafic rocks permeated by hydrothermal fluids.

Transport Evidence for Sulfur Vacancies as the Origin of Unintentional n-Type Doping in Pyrite FeS2

Pyrite FeS₂ has long been considered a potential earth-abundant low-cost photovoltaic material for thin-film solar cells but has been plagued by low power conversion efficiencies and open-circuit voltages. Recent efforts have identified a lack of understanding and control of doping, as well as uncontrolled surface conduction, as key roadblocks to the development of pyrite photovoltaics. In particular, while n-type bulk behavior in unintentionally doped single crystals and thin films is speculated to arise from sulfur vacancies (V_S), proof remains elusive. Here, we provide strong evidence, from extensive electronic transport measurements on high-quality crystals, that V_S are deep donors in bulk pyrite. Otherwise identical crystals grown via chemical vapor transport under varied S vapor pressures are thoroughly characterized structurally and chemically, and shown to exhibit systematically different electronic transport. Decreased S vapor pressure during growth leads to reduced bulk resistivity, increased bulk Hall electron density, reduced transport activation energy, onset of positive temperature coefficient of resistivity, and approach to an insulator-metal transition, all as would be expected from increased V_S donor density. Impurity analyses show that these trends are uncorrelated with metal impurity concentration and that extracted donor densities significantly exceed total impurity concentrations, directly evidencing a native defect. Well-controlled, wide-range n-doping of pyrite is thus achieved via the control of V_S concentration, with substantial implications for photovoltaic and other applications. The location of the V_S state within the gap, the influence of specific impurities, unusual aspects to the insulator-metal transition, and the influence of doping on surface conduction are also discussed.

Combined DFT and XPS Investigation of Cysteine Adsorption on the Pyrite (1 0 0) Surface

The adsorption of cysteine on the pyrite (1 0 0) surface was evaluated by using first-principles-based density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) measurements. The frontier orbitals analyses indicate that the interaction of cysteine and pyrite mainly occurs between HOMO of cysteine and LUMO of pyrite. The adsorption energy calculation shows that the configuration of the -OH of -COOH adsorbed on the Fe site is the thermodynamically preferred adsorption configuration, and it is the strongest ionic bond according to the Mulliken bond populations. As for Fe site mode, the electrons are found transferred from cysteine to Fe of pyrite (1 0 0) surface, while there is little or no electron transfer for S site mode. Projected density of states (PDOS) is analyzed further in order to clarify the interaction mechanism between cysteine and the pyrite (1 0 0) surface. After that, the presence of cysteine adsorption on the pyrite (1 0 0) surface is indicated by the qualitative results of the XPS spectra. This study provides an alternative way to enhance the knowledge of microbe–mineral interactions and find a route to improve the rate of bioleaching.

Pyrite surface environment drives molecular adsorption: cystine on pyrite(100) investigated by X-ray photoemission spectroscopy and low energy electron diffraction

We have demonstrated that the annealing process for cleaning pyrite surfaces is a critical parameter in promoting ordering on the surface and driving surface reactivity. Furthermore, we describe a spectroscopic surface characterization of the presence or absence of the surface ordering, as indicated by the Low Energy Electron Diffraction (LEED) pattern, as a function of the surface annealing process. Complementary X-ray photoemission spectroscopy (XPS) results provide evidence that longer annealing processes of over 3 hours repair the sulfur vacancies in the pyrite, making FeS species partially disappear in favor of FeS₂ species. These features play an important role in molecular adsorption. We show that in the case of the cystine amino acid on the (100) pyrite surface, the substrate structure is responsible for the chemical adsorption form. The presence of an ordered structure on the surface, as indicated by the LEED pattern, favors the cystine NH₃⁺ chemical form, whereas the absence of the surface ordering promotes cystine NH₂ adsorption due to the sulfur-deficient surface. The cystine molecule could then act by changing its chemical functionalities to compensate for the iron surface coordination. The chemical molecular adsorption form can be selected by the surface annealing conditions, implying that environmental conditions could drive molecular adsorption on mineral surfaces. These findings are relevant in several surface processes, and they could play a possible role in prebiotic chemistry surface reactions and iron-sulfur scenarios.

Origin of Photocarrier Losses in Iron Pyrite (FeS2) Nanocubes

Iron pyrite has received significant attention due to its high optical absorption. However, the loss of open circuit voltage (Voc) prevents its further application in photovoltaics. Herein, we have studied the photophysics of pyrite by ultrafast laser spectroscopy to understand fundamental limitation of low Voc by quantifying photocarrier losses in high quality, stoichiometric, and phase pure {100} faceted pyrite nanocubes. We found that fast carrier localization of photoexcited carriers to indirect band edge and shallow trap states is responsible for major carrier loss. Slow relaxation component reflects high density of defects within the band gap which is consistent with the observed Mott-variable range hopping (VRH) conduction from transport measurements. Magnetic measurements strikingly show the magnetic ordering associated with phase inhomogeneity, such as FeS2-δ (0 ≤ δ ≤ 1). This implies that improvement of iron pyrite solar cell performance lies in mitigating the intrinsic defects (such as sulfur vacancies) by blocking the fast carrier localization process. Photocarrier generation and relaxation model is presented by comprehensive analysis. Our results provide insight into possible defects that induce midgap states and facilitate rapid carrier relaxation before collection.

Synthesis, characterization and fabrication of ultrathin iron pyrite (FeS2) thin films and field-effect transistors

We report the synthesis of an ultrathin FeS₂thin filmviathermal sulfurization of an iron thin film. The FeS₂based transistor not only broadens the applications of pyrite, but also provides a platform for investigating FeS₂materials.

Phase Stability and Stoichiometry in Thin Film Iron Pyrite: Impact on Electronic Transport Properties

The use of pyrite FeS2 as an earth-abundant, low-cost, nontoxic thin film photovoltaic hinges on improved understanding and control of certain physical and chemical properties. Phase stability, phase purity, stoichiometry, and defects, are central in this respect, as they are frequently implicated in poor solar cell performance. Here, phase-pure polycrystalline pyrite FeS2 films, synthesized by ex situ sulfidation, are subject to systematic reduction by vacuum annealing (to 550 °C) to assess phase stability, stoichiometry evolution, and their impact on transport. Bulk probes reveal the onset of pyrrhotite (Fe(1-δ)S) around 400 °C, rapidly evolving into the majority phase by 425 °C. This is supported by X-ray photoelectron spectroscopy on (001) crystals, revealing surface Fe(1-δ)S formation as low as 160 °C, with rapid growth near 400 °C. The impact on transport is dramatic, with Fe(1-δ)S minority phases leading to a crossover from diffusive transport to hopping (due to conductive Fe(1-δ)S nanoregions in an FeS2 matrix), followed by metallicity when Fe(1-δ)S dominates. Notably, the crossover to hopping leads to an inversion of the sign, and a large decrease in magnitude of the Hall coefficient. By tracking resistivity, magnetotransport, magnetization, and structural/chemical parameters vs annealing, we provide a detailed picture of the evolution in properties with stoichiometry. A strong propensity for S-deficient minority phase formation is found, with no wide window where S vacancies control the FeS2 carrier density. These findings have important implications for FeS2 solar cell development, emphasizing the need for (a) nanoscale chemical homogeneity, and (b) caution in interpreting carrier types and densities.