We introduce here a multifunctional material composed of alternating atomic sulfide sheets close to CuFeS2 and Mg-based hydroxide ones (valleriite), which are assembled due to their electric charges of opposite sign. Valleriite particles of 50-200 nm in the lateral size and 10-20 nm thick were synthesized via a simple hydrothermal pathway using various concentrations of precursors and dopants, and examined with XRD, TEM, EDS, X-ray photoelectron spectroscopy, reflection electron energy loss spectroscopy (REELS), Mössbauer, Raman and UV-vis-NIR spectroscopies, magnetic, dynamic light scattering, zeta potential measurements. The electronic, magnetic and optical characteristics are found to be critically dependent of the charge (electron density) at the narrow-gap sulfide layers containing Cu+ and Fe3+ cations, and can be tuned via the composition of hydroxide part. Particularly, substitution of Mg2+ with Al3+ increases the negative charge of the hydroxide layers and reduces the content of Fe3+-OH centers (10-45% of total iron); the effects of Cr and Co dopants entering both layers are more complicated. Mössbauer doublets of paramagnetic Fe3+ detected at room temperature transform to several Zeeman sextets at 4.2 K; the hyperfine fields up to 500 kOe and complex magnetic behavior, but not pure paramagnetism or antiferromagnetism, were observed for valleriites with the higher positive charge of the sulfide sheets, probably due to the depopulation of the minority-spin 3d states of S-bonded Fe3+ ions. Aqueous colloids of valleriite show optical absorption at 500 - 750 nm, which, along with the peaks at the same energies in REELS, may arise due to quasi-static dielectric resonance involving the vacant Fe 3d band and being dependent on the composition of both layers too. These and other findings call attention to the of valleriites as a new rich family of 2D materials for a variety of potential applications.

Valleriite, (Cu,Fe)S2n(Mg,Al,Fe)(OH)2, and related layered minerals are of interest due to their unusual two-dimensional structure, formation mechanisms, physical and chemical properties, and potential involvement into mineral processing and materials science applications. Here, we have studied Kingash Cu-Ni ore samples containing 10-25% of valleriite in association with serpentines (lizardite and chrysotile) and magnetite using scanning electron microscopy and electron microprobe analysis, Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), thermal analysis and zeta potential measurement. The data are compared with those for Al-doped valleriite synthesized via a hydrothermal route. It was found that the Kingash valleriite contains excessive iron relative to CuFeS2 stoichiometry, which mainly occurs, leaving aside magnetite, as Fe3+-OH species in hydroxide layers of valleriite and minor Fe centers in serpentines. Thermal dihydroxylation of hydroxide layers of valleriites occurs near 500 oC in inert atmosphere; in air, sulfide sheets oxidize with an exothermal peak at 447 oC, and sulfur oxides don’t volatilize but react with hydroxide groups of valleriite rather than serpentines. Zeta potential measurements of coarse ore particles using the flow potential technique suggested that the surface of valleriite is negatively charged in a wide pH range while the positive values at low pHs for fine particles are inflicted by serpentine. The findings demonstrate close resemblance of the natural and synthetic Al-doped valleriites, and the key role of valleriite, despite its moderate content, for the interfacial characteristics of the valleriite-bearing ores.

We report here the application of the low-temperature X-ray photoelectron spectroscopy (cryo-XPS) of fast-frozen dispersions as a quasi in situ technique for a case study of metal sulfides reacted in acidic aqueous solutions under non-oxidizing and moderate oxidizing conditions. The sulfide surfaces are known to tend to be depleted in metals, producing essentially sulfur-enriched surfaces and extended underlayers on Fe- and Cu-bearing sulfides, which have previously been examined using depth-sensitive HAXPES and cryo-XPS. The current study is focused on zinc and lead sulfides (natural sphalerite and galena), for whom both the experiment and theoretical DFT simulations suggest a low stability of sulfur-excessive structures. Cryo-XPS revealed the complicated behavior of the minerals under non-oxidative etching conditions, in particular, a notable concentration of polysulfide for PbS in dilute perchloric acid and a very minor one in hydrochloric acid. Oxidative etching with Fe3+ cations produced polysulfide anions and then elemental sulfur, which both volatized in the ultra-high vacuum at room temperature; the species can, nonetheless, be distinguished by considering the binding energies, electrostatic charging and evaporation rates. The cryo-XPS also detected interfacial products, e.g., ferrous chloride. DFT found that S-excessive centers are unstable in the undersurface regions of both materials, but are less unfavorable for ZnS surfaces, in agreement with the experimental data. It was concluded that cryo-XPS allows us to greatly reduce distortions of the interface composition in comparison with conventional techniques.

Valleriite is of interest as a mineral source of basic and precious metals and as an unusual material composed of two-dimensional (2D) Fe–Cu sulfide and magnesium hydroxide layers, whose characteristics are still very poorly understood. Here, the mineral samples of two types with about 50% of valleriites from Noril'sk ore provenance, Russia, were examined using Cu K- and Fe K-edge X-ray absorption fine structure (XAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), 57Fe Mössbauer spectroscopy, and magnetic measurements. The Cu K X-ray absorption near-edge structures (XANES) spectra resemble those of chalcopyrite, however, with a higher electron density at Cu+ centers and essentially differ from those of bornite Cu5FeS4; the Fe K-edge was less informative because of accompanying oxidized Fe-containing phases. The post-edge XANES and extended XAFS (EXAFS) analysis reveal differences in the bond lengths, e.g., additional metal–metal distances in valleriites as compared with chalcopyrite. The XPS spectra confirmed the Cu+ and Fe3+ state in the sulfide sheets and suggest that they are in electron equilibrium with (Mg, Al) hydroxide layers. Mössbauer spectra measured at room temperature comprise central doublets of paramagnetic Fe3+, which decreased at 78 K and almost disappeared at 4.2 K, producing a series of hyperfine Zeeman sextets due to internal magnetic fields arising in valleriites. Magnetic measurements do not reveal antiferromagnetic transitions known for bornite. The specific structure and properties of valleriite are discussed in particular as a platform for composites of the 2D transition metal sulfide and hydroxide (mono)layers stacked by the electrical charges, promising for a variety of applications.

Iron sulfides are important minerals and materials, whose surface properties are largely determined by the interface with aqueous phases. Here, natural pyrrhotite (Fe9S10) and pyrite (FeS2) were ground, reacted in water and acidic non-oxidative and oxidative ferric chloride and sulfate solutions, then centrifuged, fast-frozen and examined with X-ray photoelectron spectroscopy at -160 degrees C (cryo-XPS) in comparison with the samples thawed in vacuum, dried in vacuum or air and measured at room temperature (RT). The interfacial layers were composed mainly of ferric (hydr)oxides and aqueous iron salts as the reaction products rather than reactants; RT-XPS found the products modified due to dehydration, further reactions and redistribution over the surface. The cryo-XPS revealed plentiful surface Fe-S centers with various spin and oxidation states at pyrrhotite and pyrite, particularly in water, missed in conventional XPS. The surfaces of pyrrhotite and, much less, pyrite are enriched with polysulfide anions S--S-n-S-, which partially volatilize at RT both under non-oxidative and oxidative conditions; the number n can be larger than 6 at pyrrhotite. It was concluded, nevertheless, that elemental sulfur insignificantly contributes to the spectra under the conditions employed. Pros and cons of the technique in the case study of iron sulfides are discussed.

Applications of silver nanoparticles (Ag NPs) in modern technologies require environmentally friendly methods of large-scale production of the nanoparticles with controlled morphology and surface state in the form of high-concentration metal sols with minimal quantities of organic stabilizers. Herein, we report a procedure based on reduction of aqueous silver nitrate with ferrous sulfate in the presence of citrate ions. We studied the effect of various factors on the chemical reaction by applying transmission electron microscopy, ultraviolet-visible absorption spectroscopy, and dynamic light scattering (DLS) and proposed protocols with reduced quantities of the reagents allowing preparation of uniform spherical Ag NPs of 5 to 15 nm in diameter. A DLS study of sols after dilution was employed to estimate the tendency of colloidal particles to interact in order to optimize post-synthetic purification and concentration procedures. Particularly, filtration instead of centrifugation and electrolytic coagulation with trisodium citrate in place of sodium nitrate were utilized to produce extremely concentrated, more than 1000 g/L Ag, and stable silver hydrosols with no additional stabilizers. The chemical, X-ray photoelectron spectroscopy, and thermogravimetric analyses demonstrated that the Ag NPs contained citrate-derived capping ligands, and low amounts of Fe are appropriated for chemical and low-temperature sintering, surface functionalization, nanofluidics, and other applications.

Surface nanobubbles at hydrophobic interfaces now attract much attention in various fields but their role in wetting-related phenomena is still unclear. Herein, we report the effect of a preliminary contact of “hot” solids with cold water previously proposed for generation of surface nanobubbles, on wettability of compact materials and flotation of particulate galena (PbS), sphalerite (ZnS), and Pb-Zn sulfide ore. Atomic force microscopy was applied to visualize the nanobubbles at galena crystals heated in air and contacted with cold water; X-ray photoelectron spectroscopy was used to characterize the surface composition of minerals. Contact angles measured with the sessile drop of cold water were found to increase when enhancing the support temperature from 0 to 80 °C for sphalerite and silica, and to pass a maximum at 40–60 °C for galena and pyrite (FeS2) probably due to oxidation of sulfides. The temperature pretreatment depressed the recovery of sulfides in collectorless schemes and improved the potassium butyl xanthate-assisted flotation both for single minerals and Gorevskoye Pb-Zn ore. The results suggest that the surface nanobubbles prepared using the temperature difference promote flotation if minerals are rather hydrophobic and insignificantly oxidized, so the addition of collector and activator (for sphalerite) is necessary.

The reaction of reduction solution of Ag (I) by Fe (II) citrate complex was studied herein. This allows you to receive silver nanoparticles with high stability with a concentration above 60 g/l. It was determined that the nanoparticles size depends on the injection rate, mixing rate, reagent concentration, pH and some post-synthetic operations on the average size of nanoparticles. It was shown that decreasing the concentration of Ag (I) and increasing the concentration of stabilizer also bringing pH to 7 lead to small and uniform particles. Optimal conditions were found that made it possible to reduce particle size and reduce the concentration of reagents by 33 % in the results. According to XPS, TEM, DLS and FTIR datas, nanoparticles of metallic silver with a size of 6.5±1.8 nm were obtained, which stabilized by the product of partial decay of the citrate ion

Colloidal copper sulfides produced in reactions of aqueous copper and sulfide ions are important for many materials applications, environment and mineral processing. Here, CuxS nanoparticles formed and aged at varying copper sulfate to sodium sulfide ratios were studied using in situ UV-vis-NIR spectroscopy, dynamic light scattering, X-ray absorption spectroscopy, ex situ TEM, X-ray photoelectron spectroscopy and Raman scattering, and DFT + U calculations. It was established that the ratio of aqueous Cu2+ to S2- ions of 1:2 is critical for the reaction, which yields disordered covellite-like 4-6 nm Cu0.7S nanoparticles comprised polysulfide species at this and higher sulfide concentrations; upon aging, the particles release sulfur and transform to chalcocite-like structure (Cu2-xS, x < 1). Conversely, at the "excess" of copper ions, Cu2-xS-type particles grew into 12-14 nm "covellitic" nanoparticles. The optical absorbance at 1100-1200 nm commonly attributed to localized surface plasmon resonance increased with time and was lowest for Cu2+/S2- = 1:2. DFT + U calculations found that polysulfide stabilizes copper-deficit covellite, while Cu vacancies in chalcocite are more favorable and destabilized by S-S bonding; the Fermi level energy increases and the hole density decreases with polysulfide formation. We believe that CuS2 clusters form initially, and following conversion of disulfide to polysulfide and then monosulfide ions rather than release of Cu determines the character of CuxS nanoparticles.

Carey Lea silver hydrosol is a rare example of very concentrated colloidal solutions produced with citrate as only protective ligands, and prospective for a wide range of applications, whose properties have been insufficiently studied up to now. Herein, the reactivity of the immobilized silver nanoparticles toward oxidation, sulfidation, and sintering upon their interaction with hydrogen peroxide, sulfide ions, and chlorocomplexes of Au(III), Pd(II), and Pt(IV) was investigated using SEM and X-ray photoelectron spectroscopy (XPS). The reactions decreased the number of carboxylic groups of the citrate-derived capping and promoted coalescence of 7 nm Ag NPs into about 40 nm ones, excluding the interaction with hydrogen peroxide. The increased nanoparticles form loose submicrometer aggregates in the case of sulfide treatment, raspberry-like micrometer porous particles in the media containing Pd(II) chloride, and densely sintered particles in the reaction with inert H2PtCl6 complexes, probably via the formation of surface Ag-Pt alloys. The exposure of Ag NPs to HAuCl4 solution produced compact Ag films along with nanocrystals of Au metal and minor Ag and AgCl. The results are promising for chemical ambient temperature sintering and rendering silver-based nanomaterials, for example, for flexible electronics, catalysis, and other applications.

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 S-n(2-) 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 S-s(2-) and S(3)(2- )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.

The electrochemical behavior of metallic palladium nanoparticles obtained by reducing H2PdCl4 with formaldehyde and the products of the interaction of H2PdCl4 with sodium sulfide with a different ratio of reagents in a solution of hydrochloric acid was studied by the method of cyclic voltammetry. Voltammograms of metallic and sulfide nanoparticles were obtained, and interpretation was given to the observed electrochemical processes. The initial and oxidized systems with different potential values were characterized by X-ray photoelectron spectroscopy (XPS). It is shown that palladium sulfide nanoparticles, obtained at high molar ratios of reagents (S/Pd = 3:1), are more resistant to oxidative and reducing effects than those obtained at low molar ratios (S/Pd =1:1).

Citrate is an important stabilizing, reducing, and complexing reagent in the wet chemical synthesis of nanoparticles of silver and other metals, however, the exact nature of adsorbates, and its mechanism of action are still uncertain. Here, we applied X-ray photoelectron spectroscopy, soft X-ray absorption near-edge spectroscopy, and other techniques in order to determine the surface composition and to specify the citrate-related species at Ag nanoparticles immobilized from the dense hydrosol prepared using room-temperature reduction of aqueous Ag+ ions with ferrous ions and citrate as stabilizer (Carey Lea method). It was found that, contrary to the common view, the species adsorbed on the Ag nanoparticles are, in large part, products of citrate decomposition comprising an alcohol group and one or two carboxylate bound to the surface Ag, and minor unbound carboxylate group; these may also be mixtures of citrate with lower molecular weight anions. No ketone groups were specified, and very minor surface Ag(I) and Fe (mainly, ferric oxyhydroxides) species were detected. Moreover, the adsorbates were different at AgNPs having various size and shape. The relation between the capping and the particle growth, colloidal stability of the high-concentration sol and properties of AgNPs is briefly considered. (C) 2017 Elsevier B.V. All rights reserved.

The structure and electrochemical properties of the products of the thermochemical transformation of aspen modifi ed and unmodifi ed with zinc chloride have been studied.It is shown that the addition of ZnCl 2 to the lignin-cellulose mass leads to a signifi cant increase of the specifi c surface area of the carbonized product up to 1800 m 2. g -1 and the formation of ZnO crystalline phase. The infl uence of the conditions of material synthesis and electrolyte concentration (1M and 3M KOH)on the shape of voltammetric curves is revealed by cyclic voltammetry.It is established that aspen wood after prolonged exposure in water can be successfully used to produce porous carbon electrode materials.

The reactions of aqueous gold complexes with H2Se and H2S are important for transportation and deposition of gold in nature and for synthesis of AuSe-based nanomaterials but are scantily understood. Here, we explored species formed at different proportions of HAuCl4, H2Se and H2S at room temperature using in situ UV-vis spectroscopy, dynamic light scattering (DLS), zeta-potential measurement and ex situ Transmission electron microscopy (TEM), electron diffraction, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Metal gold colloids arose at the molar ratios H2Se(H2S)/HAuCl4 less than 2. At higher ratios, pre-nucleation “dense liquid” species having the hydrodynamic diameter of 20–40 nm, zeta potential −40 mV to −50 mV, and the indirect band gap less than 1 eV derived from the UV-vis spectra grow into submicrometer droplets over several hours, followed by fractional nucleation in the interior and coagulation of disordered gold chalcogenide. XPS found only one Au+ site (Au 4f7/2 at 85.4 eV) in deposited AuSe, surface layers of which partially decomposed yielding Au0 nanoparticles capped with elemental selenium. The liquid species became less dense, the gap approached 2 eV, and gold chalcogenide destabilized towards the decomposition with increasing H2S content. Therefore, the reactions proceed via the non-classical mechanism involving “dense droplets” of supersaturated solution and produce AuSe1−xSx/Au nanocomposites.

The interaction of aqueous solutions bearing platinum-group elements (PGEs) with sulfides is important for understanding the formation and weathering of PGE ore deposits, mineral processing, and synthesis of nanomaterials. Here, the surface species formed upon the contact of the main sulfide minerals (pyrite, pyrrhotite, galena, chalcopyrite and valleriite) with the solutions of H2PtCl6 (pH 1.5, 20 °C) have been studied using X-ray photoelectron spectroscopy (XPS). Uptake of Pt increased gradually with increasing interaction time, and depended, as well as the composition of immobilized products, on the mineral nature and the state of its surface, e.g., the chemical pre-treatment. The highest rate of Pt deposition was observed on galena and valleriite and the lowest on pyrite and pyrrhotite. The preliminary moderate oxidation of pyrrhotite promoted Pt deposition, which, however, was hindered under harsh reaction conditions. The pre-oxidation of pyrite in all cases resulted in a decrease of the Pt deposition. Initially, Pt(IV) chloride complexes adsorb onto the mineral surface, and then the reduction of Pt(IV) to Pt(II) and substitution of chloride ions with sulfide groups occur forming sulfides of Pt(II) and then, Pt(IV). The reduction of Pt species to the metallic state was observed at valleriite after 24 h, probably due the negative charge of the sulfide nanolayers of this sulfide-hydroxide composite mineral.

The depletion of oxidized metal sulfide surfaces in metals due to the preferential release of cations is a common, but as yet poorly understood phenomenon. Herein, X-ray photoelectron spectroscopy using excitation energies from 1.25 keV to 6 keV, and Fe K- and S K-edge X-ray absorption near-edge spectra in total electron and partial fluorescence yield modes was employed to study natural chalcopyrite oxidized in air and etched in an acidic ferric sulfate solution. The metal-depleted undersurface formed was found to consist of a thin, 1-4 nm, outer layer containing polysulfide species, a layer with a pronounced deficiency of metals, mainly iron, and an abundant disulfide content but negligible polysulfide content (about 20 nm thick after the chemical etching), and a defective underlayer which extended down to about a hundred nm. DFT+U was used to simulate chalcopyrite with increasing numbers of removed Fe atoms. It was found that the structure with disulfide anion near double Fe vacancies, and the 'defective' structure comprising Cu in the position of Fe and Cu vacancy are most energetically favorable, especially when using a higher Hubbard-type parameter U, and have a large density of states at the Fermi level, whereas polysulfide anions are stable only near the surface. We propose a mechanism explaining the formation of the layered undersurface and 'passivation' of metal sulfides by (i) arrested decomposition of a nearly stoichiometric sulfide surface, and (ii) faster interfacial transfer and solid diffusion of cations towards the surface; (iii) stability limits for specific defect structures, promoting their expansion in depth rather than through compositional changes, excluding surface layers; (iv) decay of surface polysulfide layer yielding elemental sulfur.