Metal Sulfide Systems Research Articles

Achieving accurate energetics beyond (semi-)local density functional theory: Illustrated with transition metal disulfides, Cu2ZnSnS4 , and Na3PS4 related semiconductors

Understanding, designing, and processing functional metal sulfides are significant challenges in part because of the high temperatures and pressures and the number of secondary phases encountered in these complex systems. In particular, the underlying thermochemical properties are not well understood that would allow prediction of equilibrium conditions and driving forces. In addition, obtaining accurate values for the energetics of metal sulfide systems is far from complete, suggesting application of density functional theory (DFT) calculations. Here, the results of an examination of 69 phases by DFT using 12 exchange-correlation (X-C) functionals indicate that (i) the key source of error in predicting the Gibbs energy of a phase is the enthalpy calculated at 0 K rather than entropy at finite temperatures from phonon calculations and (ii) an improved prediction of the thermodynamic properties at 0 K relies on the selected nonlocal X-C functional such as the hybrid potential. Regarding metal sulfides, we conclude from the present DFT results that (1) the secondary phase ${\mathrm{Cu}}_{2}{\mathrm{ZnSn}}_{3}{\mathrm{S}}_{8}$, associated with the desired photovoltaic material ${\mathrm{Cu}}_{2}{\mathrm{ZnSnS}}_{4}$, is not stable at 0 K, but it becomes slightly stable with increasing temperature (i.e., g800 K), primarily due to the vibrational entropy, which makes it difficult to be detected in a typical thin-film growth process; (2) the hybrid X-C functional improves the predicted energetics for most of the layered transition-metal disulfides such as ${\mathrm{TiS}}_{2}$, ${\mathrm{MoS}}_{2}$, and ${\mathrm{WS}}_{2}$, but not for the nonlayered ${\mathrm{RuS}}_{2}$, ${\mathrm{OsS}}_{2}$, and ${\mathrm{IrS}}_{2}$ as well as the layered ${\mathrm{PdS}}_{2}$; and (3) the formation of the solid-state electrolyte ${\mathrm{Na}}_{3}{\mathrm{PS}}_{4}$ is thermodynamically favored. We further conclude that accurate energetics as a function of temperature for the materials of interest is feasible to be achieved beyond the semilocal DFT calculations with the key being enthalpy predicted at 0 K.

Construction of novel S/CdS type II heterojunction for photocatalytic H2 production under visible light: The intrinsic positive role of elementary α-S

Traditional view points that the photocorrosion product sulfur clusters have negative effect on the activity of CdS system. Differently, in this paper, little amount of crystalline elementary α-sulfur (α-S) nanoparticles were loaded on the surface of CdS nanorods and resulted in enhanced photocatalytic H2 production activity under visible light (λ>420nm). The photocatalytic H2 production over 10% S/CdS composite was 8.14mmol and further rose to 30.02mmol in the presence of K2PtCl6 after 3h irradiation. In the novel S/CdS system, largely improved separation efficiency of photoexcited charges were achieved through the heterojunction interface role between α-S nanoparticles and CdS nanorods. The matching type II energy band structure ensures that α-S could quickly trapped electrons from the host CdS nanorods for reducing H+ to produce H2. After 10 cycles experiment, S/CdS still kept excellent photocatalytic H2 production activity. This study clearly demonstrates the intrinsic promoting effect of α-S in CdS photocatalyst even in other metal sulfide systems for enhancing photocatalytic H2 production activity under visible light.

Heterostructured metal sulfide (ZnS–CuS–CdS) photocatalyst for high electron utilization in hydrogen production from solar water splitting

H2-production from solar water splitting attracts great attention due to its cleanness and renewable characteristics in converting solar energy into chemical energy. A sequential fabrication approach for metal sulfide photocatalysts consisting of three components (ZnS, CuS, and CdS) is designed in order to achieve efficient charge flows by introducing CuS and CdS on ZnS surfaces. ZnS particles are firstly synthesized as a base material through colloidal precipitation, and then the ZnS surfaces are sequentially modified by adding CuS (cation exchange reaction) and CdS (ionic reaction) precursors. For the final products (ZnS–CuS–CdS), atomic compositions are carefully analyzed. The ZnS–CuS–CdS composite photocatalyst reaches a maximum H2-production rate (837.6μmol/gh) at Cu 0.81wt% and Cd 14.7wt% under the standard solar irradiation (1kW/m2, AM 1.5G) compared to the ZnS only case (13.5μmol/gh). This highly enhanced photocatalytic activity is due to synergistic effects of heterostructured ZnS–CuS–CdS photocatalyst which can improve charge flow and light absorption. This study shows a great possibility of manufacturing multi-component metal sulfide systems for improving H2-production activity from solar water splitting.

Zinc sulfide surface formation on Hg electrode during cyclic voltammetric scan: an implication for previous and future research studies on metal sulfide systems

Cyclic voltammetry on the Hg electrode was used to investigate the electrochemical behavior of NaCl/NaHCO3 electrolyte solutions supersaturated with respect to Zn sulfide phases. The voltammetric results clearly show how an Hg electrode, due to exchange between Hg2+ from an HgSadlayer and Zn2+ from solution, becomes the site for surface ZnSadlayer formation in the potential range −0.45 to −0.6 V. The exchange reaction is reversible, and the surface-formed ZnSadlayer persists at the Hg electrode surface until −1.3 V during cathodic scans. Near −1.3 V, it is reduced. In the same solution, evidence for reduction of bulk Zn sulfide species including nanoparticles was not obtained. The approach emphasized here can be readily extended to any other system consisting of metal electrode and chalcogenide anions, pointing to the importance of choosing experimental conditions (i.e., deposition potential, stirring, and accumulation times) to avoid artifacts and wrong interpretation of data due to surface formation of metal sulfide species.

Determination of S−H Bond Strengths in Dimolybdenum Tetrasulfide Complexes

Homolytic solution bond dissociation free energies (SBDFE) for S−H bonds have been determined for soluble dimolybdenum tetrasulfide complexes through thermochemical cycles using electrochemical potentials and pKa values in acetonitrile. In spite of the importance and extensive use of metal sulfide catalysts, these S−H bond strengths are among the first experimentally determined values for metal sulfide systems. For [Cp*Mo(μ-S)(μ-SMe)2(μ-SH)MoCp*]+ (S4Me2H+), [Cp*Mo(μ-S)(μ-SMe)(μ-SH)2MoCp*]+ (S4MeH2+), and [Cp*Mo(μ-S)(μ-SH)3MoCp*]+ (S4H3+), the pKa values were determined to be 5.6 ± 0.4, 5.3 ± 0.3, and 4.9 ± 0.3, respectively. The E1/2 values for S4Me2•+/o, S4MeH•+/o, and S4H2•+/o were measured to be −0.02 ± 0.02, +0.04 ± 0.05, and +0.07 ± 0.07 V vs FeCp2+/o, respectively. Using these experimental values, the homolytic S−H SBDFE for S4Me2H+ to S4Me2•+, S4MeH2+ to S4MeH•+, and S4H3+ to S4H2•+ were determined to be 60.8 ± 1.0, 61.8 ± 1.6, and 61.9 ± 2.0 kcal/mol, respectively. These SBDFE values can be used ...

Simulation experiments for shocked primitive materials in the Solar System

Shock recovery experiments were conducted on porous mixtures (initial porosity 35 ± 5%) in silicate–metal and silicate–metal–sulfide systems in an attempt to simulate impact phenomena of unconsolidated porous materials similar to ordinary chondrites in a wide range of shock pressures from 20 to 70 GPa. The textures and chemical compositions of shocked samples were investigated in detail. Features such as grain deformation, fracture density decrease, and heterogeneous meltings are found. At < 30 GPa, mechanical effects of the shock process, such as silicate fracturing and metal elongation, are dominant. Morphological analysis of metal grains in shocked samples reveals good correlation between the degree of deformation and the shock pressure. The metal grain aspect ratio may be a good indicator of shock pressure. At higher pressures, thermal effects, which are enhanced in the shock compression of porous media, become prominent. Above 30 GPa, silicate grain fractures disappear, and silicate darkening dominates. Shock-induced melting in the shock veins and melt networks is observed and interpreted as localized in situ melting features, the heat source of which is frictional heating between grains. Thin veins and melt pockets of sulfide–metal melt are the principal characteristics in this type of sample. Melting features of metal–sulfide systems alter the spectral property of meteoritic bodies. The initial porosity of a target may have been an important factor in impact processes in the early Solar System.

Surface reactions in aqueous metal sulfide systems. 1. Fundamental surface reactions of hydrous PbS and ZnS

Proton adsorption at hydrous sulfide mineral surfaces has been investigated. Potentiometric titrations and solubility studies of freshly precipitated PbS, commercially available PbS and ZnS as well as the natural minerals galena and sphalerite were performed. An experimental method for accurate determination of the concentration of reacted protons at the metal sulfide surface is presented. In slightly acidic solutions an ion exchange reaction between protons and surface metal ions was discovered leading to quite high metal ion concentrations in the bulk solution.

Surface reactions in aqueous metal sulfide systems: 2. Ion exchange and acid/base reactions at the ZnS [sbnd]H 2O interface

The amphoteric properties of hydrous ZnS surfaces as well as an ion exchange reaction between surface coordinated Zn 2+ ions and H + ions of the aqueous phase have been established. The measurements were performed as potentiometric titrations in 0.1 M NaClO 4 medium at 25°C. Besides measuring −log[H +] by using a glass electrode, −log[S 2−] was recorded (Ag/Ag 2S selective electrode). In addition, the total concentrations of H +, Zn 2+, and S were analyzed as a function of −log[H +] of the bulk solution. Synthetically prepared ZnS and the mineral sphalerite were studied. Bro¨nsted acidity constants pK 1 s = 6.91 and pK 2 s = 10.28 for synthetic ZnS, and pK 1 s = 7.14 and pK 2 s = 10.29 for sphalerite, were established, which implies −log[H +] of zero proton condition equal to 8.60 and 8.72, respectively. Furthermore, the ion exchange reaction SZn + 2H + ⇆ SH 2 + Zn 2+ was characterized within the range 4 ⩽ −log[H +] ⩽ 7, with log K = 9.59 and 9.65. Experimental data were evaluated on the basis of the constant capacitance model by using the computer program FITEQL. The “best” fit to experimental data was obtained by assuming no charge dependence of K 1 s and K 2 s.

Aqueous Solubilities, Solubility Products and Standard Oxidation‐Reduction Potentials of the Metal Sulfides

Available thermodynamic data, incorporating a new free energy of formation for aqueous S2− of 111 (±2) kJ/mol at 25°C, indicate that solubility products of the insoluble metal sulfide salts are several orders of magnitude smaller than previously considered. Thermodynamic characterization for a variety of soluble and insoluble aqueous metal sulfide systems is reported. Trends in the aqueous solubilities of the alkali salts solubilities are compared and discussed. New aqueous solubility products, , and standard redox potentials, , for over forty metal sulfides are determined. The new values of and at 25°C include: , and . Significant free S2− occurs only in extremely alkaline media. The suitability of , rather than , based solubility products and redox potentials is discussed and these alternate values determined.

Electrical properties of La 2S 3-doped calcium sulfide solid electrolyte

In order to consider the application of La 2S 3-doped calcium sulfide to a solid electrolyte for metal sulfide systems, its electrical properties have been investigated, based on a galvanic cell technique and conductivity measurements at various temperatures and sulfur pressures. The following cell was employed; Cu,Cu 2S |CaS (La 2S 3)| FeS, Fe. The values of the electromotive force obtained in this cell were roughly consistent with the theoretical ones. Based on conductivity measurements under various sulfur pressures at 700–900°C, the electrical conduction of La 2S 3-doped calcium sulfide was found to be ionic at sulfur pressures higher than 10 −6 atm and n-type semiconducting at lower sulfur pressures. The apparent activation energy obtained for the ionic conduction was 19.02 kcal/mol, which was reasonable for the ionic conduction through migration of a Ca 2+ ion.

低硫黄分圧下におけるCaS-1 wt%Y&lt;SUB&gt;2&lt;/SUB&gt;S&lt;SUB&gt;3&lt;/SUB&gt;固体電解質の導電率

The CaS-1 wt%Y2S3 solid electrolyte has been expected to be effective for thermodynamic studies of metal-sulfide systems. An acceptable solid-sulfide electrolyte must exhibit essentially complete ionic conductivity. The conductivity of the CaS-1 wt%Y2S3 solid solutions was determined by an a.c. bridge measurement-complex impedance plot combination method. Sulphur partial pressures were established by using metal-metal sulfide coexistence electrodes.The isothermal conductivities were constant at a sulphur partial pressures lower than the equilibrium sulphur pressure of Cu-Cu2S coexistence and a gentle fall in conductivity was observed at the Ag-Ag2S equilibrium sulphur pressure. The result suggested that the conductivity of the CaS-1 wt%Y2S3 was ionic with the current carried by the Ca2+ ions via cationic vacancies. However, the conductivity value found in this study, 7.6×10−4 S/m at 1073 K for the CaS-1 wt%Y2S3 solid solution, is much smaller than the order of 10−1 S/m which has been obtained for the ZrO2-CaO, ThO2-Y2S3 and CaF2 solid electrolytes at the same temperature.