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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. [ABSTRACT FROM AUTHOR]

We report here a simple hydrothermal synthesis of 100–200 nm flakes of tochilinite (Fe1−xS)·n(Mg,Fe)(OH)2 constructed by interchanging atomic sulfide and hydroxide sheets as a representative of a new platform of multifunctional two-dimensional materials. The reliable formation of tochilinites was ensured by an excess of sodium sulfide, with the assembly of the metal sulfide and hydroxide sheets driven by their opposite electric charges. X-ray photoelectron spectroscopy found that the hydroxide layers involved Fe3+ cations from 10 to 40% of total iron tuned by addition of Al and Li entering the layers; the Fe1−xS sheets comprised comparable amounts of high-spin Fe3+ and Fe2+ centers, and minor S–S bonding. The room-temperature Mössbauer spectra fitted with several doublets (chemical shift of 0.35–0.4 mm s−1 and varying quadrupole splitting) transformed to three six-line patterns (hyperfine fields of ∼290, 350 and 480 kOe) due to magnetic ordering at 4.2 K, albeit the paramagnetic behavior observed in SQUID experiments. A series of UV-vis absorption maxima were explained in terms of both the high-index all-dielectric Mie resonance, in line with the permittivity measurement data, and the ligand-metal charge transfer resembling that in Fe–S clusters in proteins. Prospective properties and applications of the materials are discussed. [ABSTRACT FROM AUTHOR]

The dynamics of the system of two elastically coupled pendulums located in a magnetic field is considered when the masses of the pendulums differ significantly. In this case, both coupled and localized nonlinear normal modes (NNMs) can be observed. Such modes are constructed by the multiple scales method. The stability of NNMs is studied using a numerical-analytical procedure, which is related to the known Lyapunov stability criterion. The stability of NNMs is assessed by estimating the orthogonal deviations from the corresponding trajectories in the system configuration space. Regions of such modes of instability in the system parameter space are obtained. Numerical modelling includes the construction of phase diagrams, trajectories in the configuration space and frequency spectrum, which allows us to detect regular or complex behavior of the system, both for small and significant initial deviations of the pendulums. • Stability of nonlinear normal modes (NNMs) in the system of two coupled pendulums in a magnetic field is considered.. • The multiple scales method is used to construct coupled and localized nonlinear normal modes (NNMs). • The stability of NNMs is studied using a numerical-analytical procedure, which is related to the Lyapunov stability criterion.. • Influence of the system parameters and initial deviations to the NNM stability is analyzed. [ABSTRACT FROM AUTHOR]

Gold and silver sulfoselenides are of interest as materials with high ionic conductivity and promising magnetoresistive, thermoelectric, optical, and other physico-chemical properties, which are strongly dependent on composition and structure. Here, we applied X-ray photoelectron spectroscopy and Ag L 3 X-ray absorption near-edge structure (XANES) to study the electronic structures of low-temperature compounds and solid solutions Ag 2 S x Se 1–x (0 < x < 1), AgAuS, and Ag 3 AuS x Se 2–x (x = 0, 1, 2). Upon substitution of Se with S, a steady increase in the positive charge at Ag(I) sites and only minor changes in the local charge at chalcogen atoms were found from the photoelectron Ag 3d, S 2p, Se 3d, and Ag M 4,5 VV Auger spectra. The intensity of the Ag L 3 -edge peak, which is known to correlate with hole counts in the Ag 4d shell having a formal d 10 configuration, was enhanced by 20–25% from Ag 2 Se to Ag 2 S and from Ag 3 AuSe 2 to Ag 3 AuS 2 . The effect of gold is more pronounced, and the number of Ag d holes and the negative charge of S and Se notably decreased for Au-containing compounds; in particular, the Ag L 3 -edge peak is about 35% lower for AgAuS relative to Ag 2 S. At the same time, the Au 4f binding energy and, therefore, charge at Au(I) sites increase with increasing S content due to the transfer of electron density from Au to Ag atoms. It was concluded that the effects mainly originate from shortening of the metal–chalcogen and especially the Au Ag interatomic distances in substances having similar coordination geometry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nano-, submicro- and micrometer mineral particles may have an important role in beneficiation of metal ores and environmental impact, but their origin and characteristics are poorly understood. Here, we report data for the yield and the composition of fine fractions, and surfaces of several ground Cu-Ni sulfide ores studied using laser diffraction, scanning electron microscopy and energy dispersive X-ray analysis, X-ray photoelectron spectroscopy. Colloidal particles were characterized using dynamic light scattering, zeta-potential measurement, transmission electron microscopy and electron diffraction. The production of fines by dry milling was found to increase from about 0.01 vol.% to 0.2 vol.% for submicrometer particles and from ~ 0.5 vol.% to about 1.5 vol.% for particulate material less than 5 μm in the following order: Noril'sk disseminated low sulfide ore ≤ Noril'sk Cu-rich sulfide ore < Noril'sk valleriite ore < Kingash ore. For wet milling, the yield may be several times higher. Both surfaces of the milled ores and colloids were enriched in O, Mg, Si (largely as serpentine slimes) and depleted in sulfur, basic metals and iron, but colloidal valleriite, chalcopyrite, and oxidized pyrrhotite were found in the respective supernatants too. Typically, the colloidal particles form aggregates with an average hydrodynamic diameter of about 1 μm and a smaller number of ~ 5 μm species, except for valleriite ore, which exhibits a single size distribution peak at 2.7 μm. Zeta-potential, which characterizes the electric charge of the particles and dispersion stability of colloids, changed from − 25 mV for the low sulfide ore to about 0 mV for valleriite ore, and to + 15 mV for Kingash ore. Poor flotation recovery of metal from Kingash ore and Noril'sk valleriite ore is suggested to be due to both the large quantities and positive charge of hydrophilic ultrafine serpentine and/or magnesium hydroxide minerals. Resistance to oxidation and hence stability against aggregation of copper-bearing sulfide colloids in waste waters is expected to result in a negative impact on the environment. [ABSTRACT FROM AUTHOR]

Hard X-ray photoelectron spectroscopy (HAXPES) using an excitation energy range of 2 keV to 6 keV in combination with Fe K- and S K-edge XANES, measured simultaneously in total electron (TEY) and partial fluorescence yield (PFY) modes, have been applied to study near-surface regions of natural polycrystalline pyrite FeS 2 and pyrrhotite Fe 1−x S before and after etching treatments in an acidic ferric chloride solution. It was found that the following near-surface regions are formed owing to the preferential release of iron from oxidized metal sulfide lattices: (i) a thin, no more than 1–4 nm in depth, outer layer containing polysulfide species, (ii) a layer exhibiting less pronounced stoichiometry deviations and low, if any, concentrations of polysulfide, the composition and dimensions of which vary for pyrite and pyrrhotite and depend on the chemical treatment, and (iii) an extended almost stoichiometric underlayer yielding modified TEY XANES spectra, probably, due to a higher content of defects. We suggest that the extended layered structure should heavily affect the near-surface electronic properties, and processes involving the surface and interfacial charge transfer. [ABSTRACT FROM AUTHOR]

Despite the important role of metal xanthates in a number of industrial processes and emerging applications, no attempts have been made to prepare the metal xanthate nanoparticles and to study colloidal solutions of insoluble heavy metal xanthates. Here, we examined the formation of colloidal copper xanthate particles during the reactions of aqueous solutions of cupric sulfate and various potassium xanthates, which occur in flotation and water treatment slurries and can be used to manufacture nanoparticles for materials science (e.g., as precursors for copper sulfide nanoparticles and biomedicine). The products were characterized using UV-vis absorption, dynamic light scattering, zeta potential measurements, transmission electron microscopy (TEM), electron diffraction, Fourier transform infrared spectroscopy, thermogravimetry, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XANES). Colloidal copper xanthates with compositions of ROCSSCu (R = ethyl, isopropyl, butyl, isobutyl, and amyl groups), disordered structures and average diameters of 20–80 nm easily formed and aggregated and were stable for at least several hours, especially if excessive xanthate was used. The hydrodynamic diameters of the nanoparticles were smaller at lower temperatures. Dixanthogens, which were produced in the reactions along with ROCSSCu, seemed to promote nanoparticle aggregation and precipitated with the copper xanthate, affecting their thermal decomposition. The TEM micrographs and S K- and Cu K-edge XANES spectra revealed core/shell particle morphologies, likely with Cu(i) bonded to four S atoms in the core and reduced copper coordination in the shell. [ABSTRACT FROM AUTHOR]

Although mining and mineral processing industry is a vast source of heavy metal pollutants, the formation and behavior of micrometer- and nanometer-sized particles and their aqueous colloids entered the environment from the technological media has received insufficient attention to date. Here, the yield and characteristics of ultrafine mineral entities produced by routine grinding of the Pb–Zn sulfide ore (Gorevskoe ore deposit, Russia) were studied using laser diffraction analysis (LDA), dynamic light scattering (DLS) and zeta potential measurement, microscopy, X-ray photoelectron spectroscopy, with most attention given to toxic lead species. It was revealed, in particular, that the fraction of particles less that 1 μm in the ground ore typical reaches 0.4 vol. %. The aquatic particles in supernatants were micrometer size aggregates with increased content of zinc, sulfur, calcium as compared with the bulk ore concentrations. The hydrodynamic diameter of the colloidal species decreased with time, with their zeta potentials remaining about −12 mV. The colloids produced from galena were composed of 20–50 nm PbS nanoparticles associated with lead sulfate and thiosulfate, while the surface oxidation products at precipitated galena were largely lead oxyhydroxides. The size and zeta potential of the lead-bearing colloids decreased with time down to about 100 nm and from −15 mV to −30 mV, respectively. And, conversely, lead sulfide nanoparticles were mobilized before the aggregates during redispersion of the precipitates in fresh portions of water. The potential environmental impact of the metal-bearing colloids, which is due to the large-scale production and relative stability, is discussed. [ABSTRACT FROM AUTHOR]

We acquired low-temperature X-ray photoelectron spectra of fast-frozen wet pastes prepared by centrifugation of the natural sphalerite slurries containing potassium n-butyl xanthate (KBX), without washing or any additional treatment, before and after the mineral activation in 0.1 mM CuSO 4 solution. Zeta potentials of the slurry particles were also measured. The method allowed characterizing the mineral surfaces and adsorbates, volatile substances and hydrated species within the interfacial layers under more realistic flotation-related conditions. The uptake of xanthate by unactivated mineral was low even in 10 mM KBX, while the surface became metal-deficient (enriched in sulfur), more hydrophobic and contaminated with carbonaceous matter. Dibutyl dixanthogen was the major interfacial product of the interaction of Cu-activated sphalerite having the surface atomic Zn/Cu ratio of 3 and excess of sulfur with the KBX solutions. The spectra revealed only minor quantities of Zn and Cu xanthates and oxyhydroxides at all the samples; some hydrated K + counter-ions were detected at the negatively charged surfaces after the xanthate treatment. The dixanthogen was concluded to form via oxidation of xanthate at the interface, probably catalyzed by surface cuprous species but not direct interaction with cupric ions. [ABSTRACT FROM AUTHOR]

Cryogenic XPS of wet particulate samples separated via centrifugation and fast-frozen allows quasi in situ examination of solid surfaces, adsorbates, and reaction products, largely preventing the loss both of volatiles and hydrated species at mineral/water interfaces. Here, the cryo-XPS has been applied to characterize the surfaces and interfacial layers of natural pyrite (FeS 2 ), chalcopyrite (CuFeS 2 ), and galena (PbS) in solutions of a common flotation collector, potassium n -butyl xanthate (KBX), in conjunction with zeta-potential measurement. It was found, in particular, that dibutyl dixanthogen was the major adsorbate at pyrite in 0.1 mM KBX and 10 mM KBX solutions; dixanthogen and cuprous xanthate in the next stage were formed on chalcopyrite, and predominant chemisorbed butyl xanthate was present at galena, including in 10 mM KBX solution. The results may suggest that the production of dixanthogens at the interface has been underestimated while the quantities of surface metal xanthates could be over evaluated in previous studies. Pronounced differential charging effects were observed in the XPS experiment for the samples moderately hydrophobized by the xanthate treatment; we proposed that the effect was due to electrically isolated mineral particles with hydrophobic and ice-repellent surfaces, which retained, however, some frozen water islets. [ABSTRACT FROM AUTHOR]

Experiments were performed to determine crystallization of Fe,S-melts (pyriti≿ and troilitic with molar ratio S/Fe ratios of 2 and 1, respectively) containing traces of gold and silver at (Ag/Au) wt ratios varying from 10 to 0.1. The solid products were studied by optical microscopy, scanning electron microscopy, X-ray powder diffraction (XRD), microprobe analysis, and X-ray photoelectron spectroscopy (XPS) in order to reveal the concentration limits of “invisible” gold and silver in magmatic iron sulfides, and to determine the influence of sulfur on forms of precious metals in the Fe–S system with different Ag/Au ratios. Au–Ag phases do not form inclusions but instead concentrate on the grain boundaries in the synthetic pyrrhotite and troilite, while pyrite comprises micro- (1–5 μm) and macroinclusions of Au–Ag alloys and Au–Ag sulfides. In “pyriti≿” systems, the fineness of alloys increases from 650 to 970‰ and the composition of sulfides changes from acanthite (Ag 2 S) to uytenbogaardtite (Ag 3 AuS 2 ) and petrovskaite (AgAuS) as the Ag/Au ratio decreases. The concentrations of “invisible” precious metals revealed in troilite were 0.040 ± 0.013 wt.% Au and 0.079 ± 0.016 wt.% Ag. Measured concentrations in pyrite and pyrrhotite were <0.024 wt.% Au and <0.030 wt.% Ag. The surface layers of iron sulfides probed with XPS were enriched in the precious metals, and in silver relative to gold, especially in the systems with Fe/S = 1, probably, due to depletion of the metallic alloy surfaces with gold. Au- and Ag-bearing iron sulfides crystallized primarily from melts may be the source of redeposited phases in hydrothermal and hypergene processes. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM) and force spectroscopy techniques have emerged as powerful tools for in situ studies of interactions between solid surfaces, reactants and air bubbles which are of fundamental importance for understanding the mechanisms and increasing the efficiency of froth flotation. Such studies, however, have very rarely involved real mineral sulfides and flotation collectors to date. Here, the tapping mode AFM, force spectroscopy with conventional silicon cantilevers, and ex situ XPS were used to characterize surfaces of single crystals of natural PbS and, for comparison, highly oriented pyrolytic graphite (HOPG) in pure water before and after their pre-treatment in 0.1 mM and 10 mM potassium n -butyl xanthate (KBX) solutions. Submicrometer domains with the height about 3–8 nm were observed on the PbS surfaces with chemisorbed xanthate. The force–distance curves measured upon retraction of the tip from PbS and HOPG surfaces pre-treated in KBX solutions revealed the attraction forces, often with two sudden pull-off steps, acting at the separation distances longer than 100 nm. We attributed the soft domains to gas nanobubbles; the long-range attraction was explained in terms of the capillary forces induced by a vapor cavity arising between the tip and the hydrophobic substrates upon their disconnection, and coalescence of the cavity with the pre-existing nanobubbles. The role of these phenomena in flotation is briefly discussed. [ABSTRACT FROM AUTHOR]

X-ray photoelectron spectra of core levels and the valence bands were acquired, and the density-of-states (DOS) were calculated using periodic density functional theory (DFT) for synthetic ternary compounds AgAuS and Ag 3 AuS 2 , known as minerals petrovskaite uytenbogaardtite, respectively, for the first time. The chemical shift of the Au 4f binding energy as large as 0.65 eV relative to bulk elemental gold was found for Ag 3 AuS 2 , in contrast to only about 0.3 eV for AgAuS. The DFT analysis showed, in particular, that the S 3p states are largely pushed out from the Ag 4d and Au 5d bands affording two sub-bands in the valence region of both substances, but the overlapping of the S 3p and Au 5d states is larger for Ag 3 AuS 2 . It was demonstrated that the energy position of Au 4f levels is mainly influenced by Au–Ag interaction, and increasing the number of Ag atoms in the first coordination shell of S–Au–S units on going from AgAuS to Ag 3 AuS 2 results in the positive shift of the Au 4f bands consistent with the experimental data. [ABSTRACT FROM AUTHOR]

Highlights: [•] Uncoated and capped Ag NPs of about 3–5nm and 10–12nm in diameter were studied. [•] Resistive switching was found in the tunneling spectra at smaller uncapped NPs. [•] Anodic oxidation of uncapped NPs to Ag2O and AgO nontrivially depends on their size. [•] Capping of Ag NPs impedes the anodic oxidation. [ABSTRACT FROM AUTHOR]

Abstract: The new method of the forced resonance vibrations construction in mechanical systems with internal resonance is represented. According to this approach, the generalized theory of non-linear normal vibration modes by Shaw and Pierre, the modified Rauscher method and the harmonic balance method are combined with a new iterative computation procedure. The proposed approach is used in analysis of the single-disk rotor system with the isotropic-elastic shaft and the non-linear supports of Duffing type. Gyroscopic effects, asymmetrical disposition of the disk on the shaft and internal resonance are also taken into account. The NNM approach allows reducing the 8-DOF problem of the rotor dynamics to the 2-DOF non-linear system for each non-linear normal mode. Both the model of massless supports and the model of supports with inertial effects are considered. It is shown that in last case all resonance regimes are separated into two different kinds. First kind corresponds to cyclic symmetric trajectories in a system's configuration space; the second kind corresponds to centrally symmetric ones. Regimes of the first kind can be evaluated by the use of the simplified mathematical model proposed in this work. Simplified model consists only of four generalized coordinates instead of the eight initial ones. [Copyright &y& Elsevier]

Abstract: The nanoscale products deposited on pyrite at room temperature from 0.1mM tetrachloroaurate or silver nitrate aqueous solutions and colloidal solutions (sols) of Au0, Ag0 and gold sulfide nanoparticles, which were prepared beforehand by the reduction of the Au(III) and Ag(I) complexes with sodium citrate, borohydride or sulfide, have been studied using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning tunneling microscopy (STM) and tunneling spectroscopy (STS). The quantity of metals spontaneously immobilized from the solutions is basically higher than from the sols and depends on the reaction conditions. In particular, the surface concentration of gold passes a maximum at the initial molar ratio Na2S to HAuCl4 of about 2 in the sulfidic sols. Ag and Au sulfide phases deposit as plain islands of 10–30nm in the lateral size while elemental metals form nanoparticles. Although XPS showed predominant lines of metallic gold in all the cases, XAS in the transmission mode revealed that disordered gold sulfide is the main product after the interaction of pyrite with the media with Na2S to HAuCl4 ratios more than 1.5–2. The discrepancy was rationalized in terms of decomposition of surface layer of Au2S yielding Au0 in air and the spectrometer vacuum. Silver spontaneously deposits preferentially as Ag2S, and metallic nanoparticles arise with increasing reaction time; the precipitation of Ag nanoparticles (NPs) from their sols results also in some silver sulfide. A sharp current surge was found in tunneling spectra at silver sulfide and Ag NPs under positive or both positive and negative threshold biases of several tenths of a volt, respectively. The effect is most likely due to an increase in the local conductance (known as resistive switching) as a result of enrichment of acanthite (α-Ag2S) by Ag producing conducting argentite (β-Ag2S) and then Ag0. The switching allows identifying Ag0 and Ag2S phases on nanoscale; it can play an important role in surface reactions of silver in the ore deposition and processing. [Copyright &y& Elsevier]

Abstract: The reduction of tetrachloroaurate by citrate ions in aqueous solutions yielding gold nanoparticles (GNPs) has been studied using in situ tapping mode atomic force microscopy (AFM), UV–vis absorption and dynamic light scattering (DLS) spectroscopies, small-angle X-ray scattering (SAXS) along with ex situ TEM, EDX and XPS. Special attention is given to mesoscale intermediates responsible for the intense coloring of the transient solutions and their role in nucleation and crystal growth. AFM detects liquid droplet-like domains, globules 30–50nm in diameter arranged in submicrometer aggregates in the gray and blue solutions, and well separated individual particles in the final red sols. DLS shows abrupt appearance of species about 30nm and larger but not growing Au nanoparticles, while SAXS reveals gradually increasing nanoparticles and no aggregates. The mesoscale structures observed in TEM become looser as the reaction proceeds; they contain signatures of oxidized Au and other solutes. The results are interpreted in terms of decomposition of supersaturated solutions to afford domains (“dense droplets”) enriched by gold, and then, after nucleation and coalescence of Au nuclei inside them, rather slow growth of gold nanoparticles within the associated globules; the color changes of the transient solutions are due to increasing interparticle distances. [Copyright &y& Elsevier]

Abstract: Gold species spontaneously deposited on pyrite and chalcopyrite, pyrrhotite, galena, sphalerite from HAuCl4 solutions at room temperature, as well as the state of the reacted mineral surfaces have been characterized using synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS), scanning tunneling microscopy and tunneling spectroscopy (STM/STS). The deposition of silver from 10−4 M AgNO3 has been examined for comparison. Gold precipitates as metallic nanoparticles (NPs) from about 3nm to 30nm in diameter, which tends to aggregate forming larger particles, especially on pyrite. The Au 4f binding energies increase up to 1eV with decreasing size of individual Au0 NPs, probably due to the temporal charging in the final state. Concurrently, a positive correlation between the tunneling current and the particle size was found in STS. Both these size effects were observed for unusually large, up to 20nm Au particles. In contrast, silver deposited on the minerals as nanoparticles of semiconducting sulfide showed no shifts of photoelectron lines and different tunneling spectra. The quantity of gold deposited on pyrite and other minerals increased with time; it was lower for fracture surfaces and it grew if minerals were moderately pre-oxidized, while the preliminary leaching in Fe(III)-bearing media inhibited the following Au deposition. After the contact of polished minerals with 10−4 M solution (pH 1.5) for 10min, the gold uptake changed in the order CuFeS2 >ZnS>PbS>FeAsS>FeS2 >Fe7S8. It was noticed that the open circuit (mixed) potentials of the minerals varied in approximately the same order, excepting chalcopyrite. We concluded that the potentials of minerals were largely determined by Fe(II)/Fe(III) couple, whereas the reduction of gold complexes had a minor effect. As a result, the deposition of gold, although it proceeded via the electrochemical mechanism, increased with decreasing potential. This suggests, in particular, that the accumulation of “invisible” gold in arsenian pyrites and arsenopyrite under hydrothermal conditions may be explained by the low electrochemical potentials but not structural relationships between As and Au in solids. [Copyright &y& Elsevier]

Abstract: We have used ex situ atomic force microscopy (AFM), scanning tunneling microscopy and spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) to study the surfaces of natural arsenopyrite samples that were electrochemically polarized in 1M HCl, or leached in acidic solutions containing ferric iron salts, and then reacted with aqueous gold (III) chloride at ambient temperatures. For arsenopyrite oxidized on a positive-going potential sweep, progressively increasing amounts of surface Fe(III)–O and As–O species, and of S/Fe and S/As ratios in a non-stoichiometric sulfidic layer were found. The products formed in the sweep to a potential of 0.6V (Ag/AgCl) of the passivity region are shaped in about 100nm protrusions of two sorts, which are arranged in micrometer-size separate areas, while they are largely mixed at higher, “transpassive” potentials. The quantities of surface alteration substances notably decrease after leaching in ferric chloride and ferric sulfate acidic solutions. Passivation of arsenopyrite was suggested to associate with the disordered, metal-deficient surface layer having moderate excess of sulfur rather than with the products of arsenopyrite oxidation. Exposure of arsenopyrite to 10−5–10−3 M (pH 2) solutions results in the deposition of 8–50nm gold particles; only a small fraction of the gold is present as Au(I)–S species. The electrochemical oxidation at 0.6V or ageing of arsenopyrite in air promotes the subsequent gold deposition; in contrast, the amount of Au deposited on arsenopyrite that was treated by leaching in ferric chloride and sulfate solutions was about 10 times smaller than with polished arsenopyrite samples. It has been concluded that reducing agents formed as intermediates of arsenopyrite decomposition facilitate the Au0 cementation although other factors related to the surface state of the arsenopyrite play a role as well. A decrease in the tunneling current magnitudes with decreasing the Au0 particle size has been revealed using STS. This effect along with the increase by 0.2–0.5eV in the XPS Au 4f binding energies were tentatively ascribed to retarding the electron transitions by emerging electrostatic charge on gold nanoparticles (Coulomb blockade). Possible mechanisms for the effects, and their potential role in the deposition and hydrometallurgy of “invisible” gold are discussed. [Copyright &y& Elsevier]

Abstract: Natural n-type PbS single crystals have been studied using AFM, STM and STS after long-term oxidation in air at ambient temperatures and extensive etching in aqueous acid solutions, in contrast to previous work devoted to initial corrosion of fresh surfaces. The exposure of PbS to atmosphere at high relative humidity for several days yields widespread loose oxidation products; the process is much slower at low humidity. Surface morphologies diverge after the treatment in 1M perchloric and hydrochloric acid solutions at room temperature and become widely different at elevated temperatures, displaying commonly etch pits up to several micrometers in size and depth along with rather uniformly distributed 20–100nm protrusions of PbS phase. The changes both in topography and semiconducting properties of PbS found by tunneling spectroscopy have been explained in terms of the non-uniform distribution of donor- and acceptor-type defects D+/D− in the metal depleted surface layer, which are generated by chemical reactions and, in turn, determine the rates of the PbS corrosion. In particular, the D− centers exhibit a self-catalyzing effect on the non-oxidative local dissolution of PbS in HCl media, resulting in the deep etch pits. [Copyright &y& Elsevier]

Abstract: The nonlinear two-degrees-of-freedom system under consideration consists of a linear oscillator with a relatively big mass which is an approximation of some continuous elastic system, and an essentially nonlinear oscillator with a relatively small mass which is an absorber of the main linear system vibrations. Free and forced vibrations of the system are investigated. Analysis of nonlinear normal vibration modes shows that a stable localized vibration mode, which provides the vibration regime appropriate for an absorption, exists in a large region of the system parameters. In this regime amplitudes of vibrations of the main elastic system are small; simultaneously vibrations of the absorber are significant. Frequency response of the system under external periodic force is obtained. The dynamical interaction of elastic string under impact impulse and the essentially nonlinear absorber is considered too. Absorption of a longitudinal traveling wave in the system is analyzed. [Copyright &y& Elsevier]

High resolution Fe L-, Cu L-, S L-edge and O K-edge X-ray absorption near-edge structure (XANES) spectra of synthetic bornite and the samples oxidized in air and leached in a ferric chloride solution have been recorded and compared with the spectra of chalcopyrite. The pre-edge shoulder in the Cu L-edge spectrum of bornite and the strong leading peak in the spectrum of chalcopyrite were attributed to the electron transitions to the minor density of unoccupied Cu 3d states, and the post-edge features were related to the states of s- and p-type. The Fe L-edge XANES showed the presence of predominant Fe2+ for chalcopyrite, and some quantity of Fe2+ for bornite cannot be excluded despite XPS and Mössbauer spectroscopy had previously detected only Cu+ and Fe3+ species in both minerals. The Cu L- and S L-edge spectra of bornite considerably modify and become similar to those of chalcopyrite after the oxidative leaching that produces the heavily metal-depleted surface layers, and the Fe L-edge XANES shows only a slightly increased contribution of Fe2+. The multiparticle configurations involving Fe2+ and Cu2+ or holes at S atoms may play a role in the L-spectra. [Copyright &y& Elsevier]

XPS, Fe Lα,β and Cu Lα,β X-ray emission and Fe L-, Cu L-, S L-edge and O K-edge absorption spectroscopies, Mo¨ssbauer spectroscopy and cyclic voltammetry were applied to study reacted surface layers of natural chalcopyrite, CuFeS2. The surfaces became metal-depleted after the anodic oxidation in 1 M HCl and the leaching in 1 M H2SO4+0.2 M Fe2(SO4)3 or 1 M HCl+0.4 M FeCl3 solutions, with the sulfur excess and iron/copper ratio been higher in the last instance, and were enriched in copper after the electrochemical reduction. The electronic structures of the metal-deficient layers up to several tenths of micrometer thick were similar to that of chalcopyrite, except that the density of the highest occupied states depended on sulfur anions formed (predominant S3-anions after the ferric sulfate treatment, S4-anions after the ferric chloride leaching or the potential sweep to 0.9 V, etc.). The layers created by the preliminary oxidation had only a small effect on the chalcopyrite voltammetry. We suggest a new reaction mechanism considering a role of the surface changes, including disordering and Anderson localization of the electronic states. [Copyright &y& Elsevier]

[Display omitted] • Pyrrhotite and pyrite conditioned in water and acidic solutions were studied. • The results of cryogenic and room-temperature XPS techniques are compared. • Amounts of aqueous products at the interface are determined by the reaction rates. • Plentiful Fe-S centers with different state of Fe were detected. • Polysulfide are considered as intermediates of oxidation of iron sulfides. Iron sulfides are important minerals and materials, whose surface properties are largely determined by the interface with aqueous phases. Here, natural pyrrhotite (Fe 9 S 10) and pyrite (FeS 2) 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 °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. [ABSTRACT FROM AUTHOR]

Colloidal copper sulfides produced in reactions of aqueous copper and sulfide ions are important for many materials applications, environment and mineral processing. Here, Cu x S 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 Cu 0.7 S nanoparticles comprised polysulfide species at this and higher sulfide concentrations; upon aging, the particles release sulfur and transform to chalcocite-like structure (Cu 2-x S, x < 1). Conversely, at the "excess" of copper ions, Cu 2-x S-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 CuS 2 clusters form initially, and following conversion of disulfide to polysulfide and then monosulfide ions rather than release of Cu determines the character of Cu x S nanoparticles. Image 1 • Cu 0.7 S particles of 4–6 nm formed for S2− to Cu2+ ions ratio of 2 and higher. • Covellite-type Cu x S NPs released S at the expense of polysulfide upon aging. • Cu 2-x S-type NPs formed and evolved to 14 nm covellitic ones for "excess" of Cu2+ ions. • NIR absorption maxima at 1100–1200 nm arose and increased upon aging. • Polysulfide species stabilized Cu-depleted covellite and reduced the density of holes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The pyrosynthesis of gold chalcogenides AuX (AuTe 0.7 Se 0.2 S 0.1), AuX 2 (AuTe 1.8 Se 0.2 –AuTe 1.8 Se 0.1 S 0.1) and Au 3 X 10 (Au 3 Te 6 Se 3 S–Au 3 Te 6 Se 2.5 S 1.5) was performed from a mixture of elemental substances taken in the atomic ratios X/Au (X = Σ(Te + Se + S)) of AuTe 0.666 Se 0.167 S 0.167 (X/Au = 1), AuTeSe 0.5 S 0.5 (X/Au = 2), AuTe 2 Se 1.125 S 0.375 , AuTe 2 Se 0.75 S 0.75 (X/Au = 3.5) and AuTe 2.5 SeS 0.5 (X/Au = 4). Optical and scanning electron microscopy, electron microprobe analysis, X-ray powder diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were applied to characterize the samples. It was established that Au chalcogenides of composition AuX 2 were analogous to mineral calaverite (AuTe 2), Au 3 X 10 phases were close to S-bearing mineral maletoyvayamite and the AuX substance seemed to be a synthetic analog of a phase Au 0.99–1.00 Te 0.70–0.71 Se 0.25–0.27 S 0.03–0.06 found in the Gaching area of the Maletoivayam ore field (Central Kamchatka volcanic belt). The results on complex Au chalcogenides in the quaternary Au–Te–Se–S system are of interest for chemistry, mineralogy and geochemistry of Au, and potential applications in materials science. Image 1 • New gold chalcogenides AuX and Au 3 X 10 were synthesized in the Au–Te–Se–S system. • AuX and Au 3 X 10 had compositions AuTe 0.7 S 0.1 Se 0.2 and Au 3 Te 6 Se 3 S–Au 3 Te 6 Se 2.5 S 1.5 • AuX and Au 3 X 10 were analogs of unnamed minerals (Maletoivayam deposit, Kamchatka). • Synthesized AuX 2 (AuTe 1.8 Se 0.2 , AuTe 1.8 Se 0.1 S 0.1) was an analog of calaverite. • XPS revealed the oxidation state of the elements; gold was +1 in all chalcogenides. [ABSTRACT FROM AUTHOR]

This paper is an extension of the previous review, done by the same authors (Mikhlin, Y., and Avramov, K. V., 2010, “Nonlinear Normal Modes for Vibrating Mechanical Systems. Review of Theoretical Developments,” ASME Appl. Mech. Rev., 63(6), p. 060802), and it is devoted to applications of nonlinear normal modes (NNMs) theory. NNMs are typical regimes of motions in wide classes of nonlinear mechanical systems. The significance of NNMs for mechanical engineering is determined by several important properties of these motions. Forced resonances motions of nonlinear systems occur close to NNMs. Nonlinear phenomena, such as nonlinear localization and transfer of energy, can be analyzed using NNMs. The NNMs analysis is an important step to study more complicated behavior of nonlinear mechanical systems.This review focuses on applications of Kauderer–Rosenberg and Shaw–Pierre concepts of nonlinear normal modes. The Kauderer–Rosenberg NNMs are applied for analysis of large amplitude dynamics of finite-degree-of-freedom nonlinear mechanical systems. Systems with cyclic symmetry, impact systems, mechanical systems with essentially nonlinear absorbers, and systems with nonlinear vibration isolation are studied using this concept. Applications of the Kauderer–Rosenberg NNMs for discretized structures are also discussed. The Shaw–Pierre NNMs are applied to analyze dynamics of finite-degree-of-freedom mechanical systems, such as floating offshore platforms, rotors, piece-wise linear systems. Studies of the Shaw–Pierre NNMs of beams, plates, and shallow shells are reviewed, too. Applications of Shaw–Pierre and King–Vakakis continuous nonlinear modes for beam structures are considered. Target energy transfer and localization of structures motions in light of NNMs theory are treated. Application of different asymptotic methods for NNMs analysis and NNMs based model reduction are reviewed. [ABSTRACT FROM AUTHOR]

The study of ruthenium catalysts for ammonia decomposition on carbonized and non-carbonized Al 2 O 3 nanofibers (ANF) showed that the activity of catalysts with carbonized supports (ANFC) was 2–3 times higher compared to non-carbonized ones. Thus, on Ru/ANFC and Ru/ANF the release of hydrogen reached 133.5 and 34.7 mmol H 2 /(min·g cat), respectively, whereas on Ru-BaAc/ANFC and Ru-BaAc/ANF, only 118.8 and 58.6 mmol H 2 /(min·g cat), respectively. On the average, the activation energy of ammonia decomposition on ANFC-supported catalysts is 15 kJ/mol lower than that value for ANF-supported catalysts. According to TEM data, Ru particles on ANFC are larger than on ANF, but are more evenly distributed. An increase in the activity of the catalyst correlates with a change in the electronic state of the active component. XPS data for Ru indicate a shift in the binding energy towards lower values when going from ANF to ANFC. [Display omitted] • 1D materials as supports for ammonia decomposition catalysts. • Ru supported on carbonized (ANFC) and non-carbonized (ANF) Al 2 O 3 fibers was studied. • Ruthenium particles on ANFC exceed in size those on ANF. • H 2 production over ruthenium ANFC-supported catalysts was 133.5 mmol H 2 /(min·g cat). • Activity of ANFC-supported catalysts 2-3 times exceeded that of ANF-supported ones. [ABSTRACT FROM AUTHOR]

Two principal concepts of nonlinear normal vibrations modes (NNMs), namely the Kauderer–Rosenberg and Shaw–Pierre concepts, are analyzed. Properties of the NNMs and methods of their analysis are presented. NNMs stability and bifurcations are discussed. Combined application of the NNMs and the Rauscher method to analyze forced and parametric vibrations is discussed. Generalization of the NNMs to continuous systems dynamics is also described. [ABSTRACT FROM AUTHOR]

Hydrogenation of flax shives in ethanol over bifunctional Ni/C catalysts at 225 °C has been studied. It has been shown that the 10% Ni/C catalyst enhances the yield of monomeric products from 1.1 to 9.7 wt %, decreases the solid product content from 45 to 35 wt %, and increases the degree of delignification to 91%. The main monomeric compounds obtained during hydrogenation are propyl guaiacol and propenyl guaiacol. It has been established that an increase in the mass transfer intensity via increasing the stirring rate or decreasing the catalyst grain size leads to an increase in the total yield of monomeric compounds and the propanol guaiacol yield. Alkaline and acid pretreatment increases the cellulose content in the solid product from 42 to 73 wt %. The proposed sequential scheme of the transformation and formation of monomeric products over the bifunctional nickel catalyst is lignin—coniferyl alcohol—4-propanol guaiacol—4-propenyl guaiacol—4-propyl guaiacol. [ABSTRACT FROM AUTHOR]

The development of inexpensive, easy-to-produce, and easy-to-use analytical tools for detection of harmful and toxic substances is a relevant research problem with direct applications in environmental monitoring and protection. In this work, we propose a novel composite material based on alumina nanofibers and detonation nanodiamonds for detection of phenol in aqueous medium. The composite material was obtained by mixing an aqueous suspension of alumina nanofibers with a diameter of 10–15 nm and a length of several microns and a hydrosol of nanodiamonds with an average cluster size of 70 nm. The mechanisms underlying the interaction of these nanomaterials are clarified and the physicochemical properties of the composite are investigated. The SEM and TEM studies show that the obtained composite has a network structure, in which clusters of nanodiamonds (10–20 nm in diameter) are distributed over the surface of nanofibers. Coupling of nanomaterials occurs due to opposite signs of their zeta potentials, which results in electrostatic attraction and subsequent chemical bonding as indicated by the X-ray photoelectron spectroscopy and simultaneous thermal analysis. The bonding apparently occurs between functional groups (mainly carboxyl) on the surface of nanodiamonds and amphoteric hydroxyl groups on the surface of alumina nanofibers. The proposed composite allows an easy-to-perform colorimetric analysis for qualitative and quantitative determination of phenol in aqueous samples with linear response over a wide range of concentrations (0.5–106 μM). Multiple tests have shown that the composite is reusable and retains its catalytic function for at least 1 year during storage at room temperature. [ABSTRACT FROM AUTHOR]

• Metal-free CVD on γ-alumina nanofibers can be used to obtain carbon structures. • Selected CVD mode allows to form thin carbon layer with thickness of several nm. • With the increase of deposition time, the amount and ordering of carbon increases. Catalyst-free chemical vapor deposition is used to form thin (1–2 nm) carbon layers on the surface of alumina nanofibers resulting in carbon-alumina nanocomposites. Thermal analysis, X-ray fluorescent microanalysis, Raman spectroscopy, and electrical resistance measurements of these composites show that increasing of synthesis time not only increases the amount of carbon on alumina surface, but also the ordering and density of the carbon layers. Nitrogen adsorption data reveal the decrease of total pore volume with increasing the synthesis time. The obtained composite material could be employed for the preparation of ion-selective membranes with switchable ion transport, electroconductive ceramics, and electrochemical sensors. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Background: In crustaceans, several mechanisms provide for the mechanical strength of the cuticular "tools" (dactyli, claws, jaws), which serve to catch and crush food objects. Studies on the mandibles of the endemic Baikal amphipod Acanthogammarus grewingkii by means of electron microscopy and elemental analysis have revealed specific structural features of these mouthparts. Methodology: The fine structure of the mandible has been studied by means of SEM, TEM, and AFM; methods used to analyze its elemental and phase composition include XEPMA, XPS, SEM-EDS analysis, and XRD. Conclusion: Functional adaptations of the mandible in A. grewingkii provide for the optimum combination of mechanical hardness and fracture resistance, which is achieved due to a complex structure and composition of its cutting parts. Teeth of the mandible are covered by a thin layer of silica (10-20 mm). Their epicuticle is characterized by a high density, consists of three layers, and increases in thickness toward the tooth apex. The epicuticle is enriched with Br, while the concentrations of Ca and P reach the peak values in the softer internal tissues of the teeth. These data broaden the view of the diversity of adaptation mechanisms providing for the strengthening of cuticular "tools" in crustaceans [ABSTRACT FROM AUTHOR]

Abstract: Stable sols of copper nanoparticles from 10nm to 50nm in diameter synthesized in aqueous solutions using several reducing agents and immobilized on pyrographite were characterized by XPS, Cu L3MM Auger and X-ray absorption spectroscopy (Cu L-edge TEY XANES and Cu K-edge XANES and EXAFS in transmission mode) along with UV–vis spectroscopy, AFM, TEM. It was found that the nanoparticles produced in the hydrazine hydrate and sodium borohydride assisted synthesis are composed of surface Cu(II) oxyhydroxides, and the shells formed by cuprite Cu2O and distorted Cu(I) oxide, which showed the Cu L-edge peak shifted to higher energies, above the metallic core. The relative amount of cuprite was usually larger in the hydrazine systems, but it depended on the sample preparation protocol. The nanoparticles prepared using ascorbic acid as a reactant consisted of rather thick Cu(II) overlayer and largely distorted Cu(I) oxide underneath. The post-synthetic sample handling notably affected the results; in particular, sedimentation (instead of drying of the colloidal solution) favored reduction of the oxide layers to metallic copper and agglomeration of the Cu particles in the precipitate, even in the case of ascorbic acid, while the colloidal particles in the supernatant were oxidized. [Copyright &y& Elsevier]

The dissolution of the rotating disk electrode (RDE) of natural lead sulfide (galena) and the ground mineral in nitric acid solutions has been studied as a function of electrode potential, HNO3 concentration and temperature. The rate of dissolution producing hydrogen sulfide slowly increases as the potential varies from +0.1 to −0.4 V (Ag/AgCl). The reaction order on nitric acid concentration has been found to be 1.2±0.15 at 0.2 V and 0.9 at −0.4 V (40 °C), and the apparent activation energy is 35 kJ mol−1 in 1 M HNO3 at 0 V, suggesting that the process is controlled by a chemical or electrochemical reaction. At higher biases the RDE of PbS dissolves for the most part anodically, showing the highest rate at ∼0.7 V, whereas the rate as a function of acid concentration is maximal in 1 M HNO3. The yield of sulfate increases with potential and is small for the leaching of both compact and ground galena, while it reaches 50% in the case of a flotation lead concentrate. Ferric ions catalyze the dissolution of compact and, especially, ground galena, with the peak rate at the potential of immersed platinum electrode of 0.4–0.5 V. The Fe3+/Fe2+ couple is concluded to act as an intermediator for the electron transfer between nitrate ions and the solid, indicating that the dissolution is electrochemical in nature. [Copyright &y& Elsevier]

[Display omitted] • One-dimensional materials as supports for ammonia decomposition (AD) catalysts. • The coating of alumina nanofibers (ANF) by carbon increases TOF by 2 times. • The introduction of cesium to the carbon coated ANF increases TOF by 6 times. • The coating of ANF by carbon is very favorable for high activity of ruthenium in AD. The study of ruthenium catalysts for ammonia decomposition on carbonized and noncarbonized Al 2 O 3 nanofibers (ANF) showed that activity of the catalysts with carbonized supports (ANFC) was twofold higher as compared to noncarbonized ones. Thus, on Ru/ANFC and Ru-Cs/ANFC the release of hydrogen reached 8.7 and 18.3 mmol H 2 /(min·g cat), respectively, whereas on Ru/ANF and Ru-Cs/ANF, only 4.4 and 9.6 mmol H 2 /(min·g cat), respectively. According to TEM, Ru particles on ANFC have a greater size than on ANF but are distributed more uniformly. As shown by TEM, XRD and XPS data, ANFC and ANF fibers are strongly different. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]