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.

Two-dimensional (2D) materials and phenomena attract huge attention in modern science. Herein, we introduce a family of layered materials inspired by the minerals valleriite and tochilinite, which are composed of alternating “incompatible”, and often incommensurate, quasi-atomic sheets of transition metal chalcogenide (sulfides and selenides of Fe, Fe-Cu and other metals) and hydroxide of Mg, Al, Fe, Li, etc., stacked via electrostatic interaction rather than van der Waals forces. We survey the data available on the composition and structure of the layered minerals, laboratory syntheses of such materials and the effect of reaction conditions on the phase purity, morphology and composition of the products. The spectroscopic results (Mössbauer, X-ray photoelectron, X-ray absorption, Raman, UV-vis, etc.), physical (electron, magnetic, optical and some others) characteristics, a specificity of thermal behavior of the materials are discussed. The family of superconductors (FeSe)·(Li,Fe)(OH) having a similar layered structure is briefly considered too. Finally, promising research directions and applications of the valleriite-type substances as a new class of prospective multifunctional 2D materials are outlined.

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 4f_7/2 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 AuSe_1−xS_x/Au nanocomposites.

We report the reliable synthesis of 2D iron sulfide-magnesium hydroxide nanoflakes. The sulfide and hydroxide sheets assemble via opposite electric charges. Comparable amounts of high-spin Fe³⁺ and Fe²⁺ centers occur in the sulfide layers.

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.

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.