I. P. Asanov

Spectroscopic and XRD studies of the air degradation of acid-reacted pyrrhotites

Abstract Monoclinic and hexagonal pyrrhotites leached in 1 mol/L HCl and exposed to the air at 100% and ∼10% relative humidity for up to 5 months were studied using X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy, Mossbauer spectroscopy, and electron paramagnetic resonance (EPR). The amorphous, nonequilibrium, iron-depleted layer (NL) produced by the leaching amounted to half of the residue mass and was composed of predominantly low-spin ferrous iron and polysulfide anions. Elemental sulfur and goethite were the only crystalline products of the NL decomposition. FTIR spectroscopy and XPS also revealed several sulfoxy species and, at low humidity, a small amount of ferric oxide. Neither alterations of the underlying pyrrhotite nor new iron sulfide phases (pyrite, pyrrhotite, etc.) crystallized from the amorphous NL were found. The NL decomposition was faster in the wet environment than in the dry one, and the oxidation of the NL was much more rapid than that of starting pyrrhotites. The intensity and quadruple split of the Mossbauer signal from the product (an isomer shift of 0.36 mm/s) were found to increase over the aging, indicating that the NL structure becomes more rigid and the singlet Fe(II) gradually converts to Fe(III). X-ray Fe Lα,β emission spectra showed the formation of intermediate, high-spin Fe(II) within the NL oxidized in the humid environment, but not in the dry air. No unpaired electron spins were detected by EPR; lines of paramagnetic Fe3+ appeared after the samples were aged in the dry air for 49 d and even later in the humid atmosphere. These phenomena are explained in terms of the formation of defects with negative correlation energy, similar to noncrystalline semiconductor chalcogenides, and of the fast electron exchange between the iron species, respectively. Mechanisms for reactions involved with the weathering of iron sulfides, which take into consideration the NL lattice elasticity, S-S and S-O bonding, oxygen incorporation, and oxidative and spin state of iron, are discussed. It is suggested in particular that the surface layer, strongly enriched in sulfur, as well as elemental sulfur and ferric oxyhydroxides, do not inhibit sulfide oxidation and acid production under weathering conditions, but the partially oxidized, disordered, nonstoichiometric layer may be passive.

Anisotropy of chemical bonding in semifluorinated graphite C2F revealed with angle-resolved X-ray absorption spectroscopy.

Highly oriented pyrolytic graphite characterized by a low misorientation of crystallites is fluorinated using a gaseous mixture of BrF(3) with Br(2) at room temperature. The golden-colored product, easily delaminating into micrometer-size transparent flakes, is an intercalation compound where Br(2) molecules are hosted between fluorinated graphene layers of approximate C(2)F composition. To unravel the chemical bonding in semifluorinated graphite, we apply angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and quantum-chemical modeling. The strong angular dependence of the CK and FK edge NEXAFS spectra on the incident radiation indicates that room-temperature-produced graphite fluoride is a highly anisotropic material, where half of the carbon atoms are covalently bonded with fluorine, while the rest of the carbon atoms preserve π electrons. Comparison of the experimental CK edge spectrum with theoretical spectra plotted for C(2)F models reveals that fluorine atoms are more likely to form chains. This conclusion agrees with the atomic force microscopy observation of a chain-like pattern on the surface of graphite fluoride layers.

Reactivity of pyrrhotite (Fe9S10) surfaces: Spectroscopic studies

Synthetic hexagonal pyrrhotite (Fe9S10) etched in hydrochloric acid solution and then dried in air has been studied using ex situ XPS, X-ray fluorescence, Mossbauer, solid-state NMR and EPR spectroscopies. The metal-deficient non-equilibrium, up to several micrometres thick, layer (NL) formed on pyrrhotite under non-oxidative conditions has been found to be composed predominantly of low-spin Fe2+, nearly equal quantities of di- and polysulfide sulphur (probably, chains of 3–5 atoms) and no or low oxygen. When pyrrhotite with the NL is kept in air, singlet ferrous iron converts into high-spin Fe2+ and Fe3+, oxygen is incorporated into the layer and the surface enrichment in sulfur over iron decreases. A Mossbauer signal with an isomer shift of 0.36 mm s−1 and negligible quadruple splitting has been detected for the etched sample, desiccation in air gives rise to a quadruple split of up to 0.65 mm s−1 and a minor decrease in the isomer shift. The application of variable X-ray tube accelerating voltage has made it possible to obtain depth-resolved Fe-Lα,β spectra of the NL and to find several alteration zones which include different forms of iron. Slow oxidative dissolution of the material in 1 M HCl+0.01 M FeCl3 electrolyte produces only a thin NL with mostly O-bonded Fe3+ and polysulfide prevailing over mono- and disulfide species. Subsequent air-drying of this sample results in an increase in the concentrations of oxygen, S-bonded Fe, and mono- and disulfide species, along with S0 formation. No unpaired electron spins have been registered in any of these NL.

Electronic structure of the non-equilibrium iron-deficient layer of hexagonal pyrrhotite

Abstract Natural hexagonal pyrrhotite before and after 1M HCl leaching has been studied, and X-ray and UV photoelectron, X-ray emission S K α, S K β, S L 2,3 , Fe L α and S K absorption spectra have been recorded and analyzed in conjunction with earlier reported Mossbauer spectroscopy data and available SCF-X α scattered wave MO calculations of the octahedral cluster FeS 6 10− and the molecular ion S 2 2− . The formation of a thick disordered non-equilibrium iron-deficient layer at the pyrrhotite surface after chemical treatment has been confirmed. Alterations of the X-ray spectra from the mineral as a result of acid etching indicated the sulphur-sulphur bonds and agreed well with the previous conclusion about the conversion of a ferrous ion from the quintet into the singlet state. The electronic structure of the metal-deficient layer has been described using a combination of structures of the S S bond containing group like S 2 2− , of the low-spin singlet cluster FeS 6 10− , and of a system of localized states.

Field emission luminescence of nanodiamonds deposited on the aligned carbon nanotube array

Detonation nanodiamonds (NDs) were deposited on the surface of aligned carbon nanotubes (CNTs) by immersing a CNT array in an aqueous suspension of NDs in dimethylsulfoxide (DMSO). The structure and electronic state of the obtained CNT–ND hybrid material were studied using optical and electron microscopy and Infrared, Raman, X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. A non-covalent interaction between NDs and CNT and preservation of vertical orientation of CNTs in the hybrid were revealed. We showed that current-voltage characteristics of the CNT–ND cathode are changed depending on the applied field; below ~3 V/µm they are similar to those of the initial CNT array and at the higher field they are close to the ND behavior. Involvement of the NDs in field emission process resulted in blue luminescence of the hybrid surface at an electric field higher than 3.5 V/µm. Photoluminescence measurements showed that the NDs emit blue-green light, while blue luminescence prevails in the CNT–ND hybrid. The quenching of green luminescence was attributed to a partial removal of oxygen-containing groups from the ND surface as the result of the hybrid synthesis.

Edge state magnetism in zigzag-interfaced graphene via spin susceptibility measurements

Development of graphene spintronic devices relies on transforming it into a material with a spin order. Attempts to make graphene magnetic by introducing zigzag edge states have failed due to energetically unstable structure of torn zigzag edges. Here, we report on the formation of nanoridges, i.e., stable crystallographically oriented fluorine monoatomic chains, and provide experimental evidence for strongly coupled magnetic states at the graphene-fluorographene interfaces. From the first principle calculations, the spins at the localized edge states are ferromagnetically ordered within each of the zigzag interface whereas the spin interaction across a nanoridge is antiferromagnetic. Magnetic susceptibility data agree with this physical picture and exhibit behaviour typical of quantum spin-ladder system with ferromagnetic legs and antiferromagnetic rungs. The exchange coupling constant along the rungs is measured to be 450 K. The coupling is strong enough to consider graphene with fluorine nanoridges as a candidate for a room temperature spintronics material.

Wrinkled reduced graphene oxide nanosheets for highly sensitive and easy recoverable NH3 gas detector

Highly wrinkled reduced graphene oxide nanosheets were prepared by chemical exfoliation from ball-milled graphite powder. This wrinkled graphene nanosheets showed higher sensitivity and simpler recovery ability than the flat reduced graphene oxide nanosheets when used as the NH3 gas detector. According to both experimental analysis and theoretical calculation, the favourable sensing properties were attributed to a specific curved structure which allowed a stronger energy change in the response process and a free diffusion space for sensor recovery.

Study on thermal decomposition of double complex salt [Pd(NH3)4][PtCl6]

Thermal decomposition of double complex salt [Pd(NH3)4][PtCl6] in a helium atmosphere has been studied by ex situ X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy. Statistical multivariate curve resolution approach to XPS and Auger spectra analysis was applied. This approach was compared with a convenient XPS analysis, demonstrating comparable results and its effectiveness in analysis of sample containing components with various oxidation states, but a little difference in composition. The result obtained allows the complex salt to use as a perspective single-source precursor in one-pot synthesis of PdPt alloy nanoparticles. To examine the precursor, thermal decomposition and shock of [Pd(NH3)4][PtCl6] were performed under various conditions in inert and reduction atmospheres and the final products were studied.

Pyrrhotite Electrooxidation in Acid Solutions

The electrooxidation of pyrrhotite, including that preliminarily etched in acid solutions, is studied by voltammetry, scanning electron microscopy, and X-ray electron spectroscopy. The oxidation mechanism includes the formation; “reversible” oxidation (which predominantly involves an increase in the amount of polysulfide ions) of a near-surface nonequilibrium metal-deficient layer (NL); and the layer destruction. The pyrrhotite passivation is caused not by a thick NL, but rather by a thin layer that contains oxidized iron, probably by a top, oxygen-containing zone of NL.

Conditions for the Formation of a Nonequilibrium Nonstoichiometric Layer on Pyrrhotite in Acid Solutions

Conditions for the formation of a nonequilibrium nonstoichiometric metal-deficient layer (NL) on natural pyrrhotites are studied, together with the NL electroreduction and the role NL plays in a nonoxidizing dissolution of pyrrhotite in solutions of sulfuric and hydrochloric acids. To estimate the NL weight, the charge connected with a cathodic peak at about –0.2 V (Ag/AgCl) is used. The peak reflects the irreversible reduction of NL with the formation of hydrogen sulfide, which is confirmed by SEM and XES data. Bulky NL forms in 0.5 M H2SO4at –0.1 to 0.0 V, where the nonoxidizing dissolution rate sharply alters, and at 0.5–1.1 V, where the pyrrhotite oxidation rate is high. The NL growth is controlled by solid-state diffusion, whereas nonoxidizing dissolution of iron is limited by diffusion at 20–30°C and a kinetic stage of dissolution of sulfur at higher temperatures.