R. Szargan

Reactivity of surface chemical states on fractured pyrite

Abstract Synchrotron-radiation-excited photoelectron spectroscopy was used to monitor sulfur chemical states on fractured pyrite surfaces reacted with atmospheric gases. The results demonstrate that there are at least three distinct states at the pyrite surface and each is oxidised at a very different rate in air. The two surface chemical states are more reactive than bulk sulfur, the most reactive surface sulfur component being S 2− . The second chemical state is identified as the surface atom of the first disulfide layer (S 2− 2 ), and the least reactive species are sulfur atoms of disulfide groups beneath the surface layer (i.e., all sulfur atoms having bulk coordination). A model combining the interpretation of sulfur surface species after Nesbitt et al. (Am. Mineral. in press) and the proposed oxidation mechanism of Eggleston et al. (Am. Mineral. 81 (1996) 1036) was developed to explain the initial oxidation processes on pyrite surfaces, where air oxidation of pyrite commences with the oxidation of S 2− sites at the surface.

Oxidation of pyrite surfaces: a photoelectron spectroscopic study

Surfaces of pyrite (FeS2) differently prepared in situ and ex situ have been studied before and after contact to air and air-saturated aqueous solutions of 4≤pH≤10 by means of photoelectron spectroscopy. Pyrite surfaces fractured or scraped in situ revealed FeS-like species concentrated in the surface region. Preparation (polishing, grinding, powdering) and prolonged oxidation in air mainly resulted in basic iron sulphate and iron oxide/hydroxide. A promoting effect of an increased surface roughness due to the preparation was observed for the formation of iron oxide/hydroxide compared with sulphate in contrast to the natural oxidation process. Oxidation in air also led to sulphur-rich species identified as iron-deficient regions below monolayer coverage. Similar regions were present at ground surfaces exposed to air-saturated solution of pH4 and pH5. In near-neutral to alkaline solution mainly iron hydroxy-oxide is formed the layer thickness of which was estimated in the range of 0.5 nm (pH5) to 1.7 nm (pH10).

Surface states and reactivity of pyrite and marcasite

Abstract The sulphur surface sites of pristine marcasite surfaces and their reactivity during initial air oxidation were investigated for the first time using synchrotron radiation excited photoelectron spectroscopy (SRXPS). Both S 2p and Fe 2p spectra were collected and compared to those of the polymorph pyrite. A new sulphur surface component is found additionally to the two known pyrite surface signals. Due to the non-isotropic character of the orthorhombic marcasite structure, its peak intensity depends strongly on the surface orientation. Its binding energy is close to the region of short-chained polysulphides. The signal is assigned to sulphur trimers produced by electron transfer between surface near sulphur dimers and S− formed after ruptures of SS bonds which results in S2− and S32−. In the cubic crystal arrangement of pyrite the stabilisation of S− to S2− is realised by an electron transfer from iron to sulphur only. A confirmation of this assumption is found in the Fe 2p spectra. The contribution of Fe3+ species — a result of the electron transfer process — is considerably less pronounced for marcasite. The mechanism of initial oxidation is different for marcasite and pyrite. The sulphur species responsible for the new spectroscopic component are the most reactive sites at the marcasite surface, while S2− is oxidised first in case of pyrite. The signals indicating the products of marcasite oxidation in the S 2p spectrum are less intense as expected from the intensity decreasing of the trimer peak. Therefore, an oxidation of sulphur trimers to elemental sulphur is assumed which could not be detected because of its volatility in the vacuum of the air lock system.

Hard amorphous CSixNy thin films deposited by RF nitrogen plasma assisted pulsed laser ablation of mixed graphite/Si3N4-targets

Abstract Carbon–silicon–nitride thin films were grown on (100) oriented silicon substrates by pulsed laser deposition (PLD) assisted by a RF nitrogen plasma source. Up to about 30 at% nitrogen and up to 20 at% silicon were found in the hard amorphous thin films by RBS, XPS, and SNMS in dependence on the composition of the mixed graphite/Si3N4–PLD target. Due to incorporation of 10% Si3N4 to the PLD graphite target the CSixNy films show slightly increased universal hardness value of 23 GPa (at 0.1 mN load force, reference value for silicon substrate 14 GPa), increased plastic hardness (67 instead of 61 GPa), but strongly decreased elastic modulus (from 464 to 229 GPa) compared to the corresponding carbon nitride film without silicon. The internal compressive stress of the CSixNy films showed a maximum of 5.5 GPa at a film thickness below 50 nm and decreased down to about 1.5 GPa for film thickness exceeding 100 nm. X-ray photoelectron spectroscopy (XPS) of CSixNy film surfaces shows clear correlation of binding energy and intensity of fitted features of N 1s, C 1s, and Si 2p peaks to composition of the graphite/Si3N4 target and to nitrogen flow through the plasma source, indicating soft changes of binding structure of the thin films due to variation of PLD parameters. Increasing carbon double and triple bonding of the CSixNy films in dependence on the deposition process as identified by FTIR and Raman spectroscopy correlates with decreasing nanohardness. The results demonstrate the capability of the plasma assisted PLD process to deposit hard amorphous CSixNy films with variable chemical binding structure and corresponding mechanical properties.

XANES investigation of chemical states of nitrogen in polyaniline

Abstract X-ray absorption and photoelectron spectra of polyaniline were investigated in order to characterize the chemical bonding of its nitrogen atoms. The identification of neutral and charged nitrogen groups is of special interest. X-ray absorption near-edge structure (XANES) at the nitrogen K edge shows characteristic resonance structures reflecting transitions from the N1s core-level into unoccupied states. For comparison N1s X-ray photoelectron spectra were recorded representing the initial state of the N1s−12p( π ∗ σ ∗ ) transitions. Due to the increasing energy differences between the N1s initial state and the unoccupied π and σ states, discrete final states, originating from the different chemical bonding at the nitrogen atom (N, NH+  and NH), were resolved in the N K XANES.

Nitrogen K-shell excitations in complex molecules and polypyrrole

Abstract Experimental and theoretical studies of nitrogen K-shell near-edge absorption in complex compounds such as thiourea, benzalaniline, pyrrole, polypyrrole, tetraethylammonium, tetrabutylammonium and substituted 2-hydroxy-3-methoxy-benzyl-idene-aniline compounds were carried out. The quasi-atomic model was used in order to calculate the spectral distribution of oscillator strengths for transitions from the N 1s level to excited discrete and continuum states, to explain general trends in the changes of K-shell excitation spectra and to reveal their relationships with chemical bonding of the N atom in the compounds. The performed analysis of theoretical and experimental spectra evidences the central role of splitting of an intense N 1s −1 2p resonance in N K XANES formation in all compounds under study. The influence of the electronic charge transfer, bond conjugation and localization on the energies and intensities of π ∗ and σ ∗ resonances are discussed in detail. The strong connection between interference and chemical bonding effects on the main resonance features of X-ray absorption in the compounds under study is revealed.

XPS investigation of chemical states in monolayers: Recent progress in adsorbate redox chemistry on sulphides

Abstract The paper presents new results on the adsorption of 2-mercaptobenzothiazole (MBT) and 2-mercaptobenzoxazole (MBO) on galena (PbS) and pyrite (FeS 2 ). Adsorption of MBT and MBO on galena and pyrite surfaces from 10 −5 M aqueous solution results in monolayers of chemisorbed deprotonated molecules bound to the surface via the thiolate group. A secondary interaction between the lone pair of the nitrogen and the sulphide surface may be possible. From the unequal intensity of the two S2p emissions in MBT an upright orientation of the molecule can be assumed. Adsorption of MBT from 10 −4 M aqueous solution leads to the formation of 2,2′-dithiobis(benzothiazole) as an oxidation product of MBT. Oxidation of MBO is not observed which may be the result of its 100 mV higher redox potential. Optical activation of the MBT oxidation on CdS and FeS 2 in contrast to PbS is suggested from the increase of the S2p signals attributed to the bridging –S–S– group of BBTD during illumination. We conclude that a surface photovoltage reducing the band bending may be responsible for the higher oxidation rate at the pyrite/electrolyte interface during illumination with respect to the dark. Oxidation of sulphide surfaces by UV light in air produces lead(II) sulphate soluble in aqueous solution. Dissolved Pb 2+ ions lead to the formation and precipitation of a Pb(mbt) 2 complex in alkaline MBT solution.

Influence of substituents on the N K X-ray absorption near-edge structure of pyrrole derivatives

Abstract The X-ray absorption and electron yield spectra of monomeric pyrrole including derivatives with different substituents, thiazole and oxazole have been investigated. Inductive and mesomeric effects on the spectral features are discussed. Inductive effects create energy shifts of the π ∗ resonances: mesomeric effects cause a splitting and energy shifts of the π ∗ resonances.

Competing interaction of different thiol species on gold surfaces

Abstract Self-assembled monolayers (SAMs) of thiol molecules show very interesting properties. We have investigated the possibilities of creating mixed thiol films, consisting of two structurally and chemically very different thiol species. We used the aliphatic hexadecanethiol and the aromatic 2-mercapto-benzothiazol (MBT). The interaction of both species after successive adsorption on gold was investigated using XPS. We found, that hexadecanethiol displaces the MBT from gold surfaces, similar to the removal of carbon contamination by the alkanethiol. In contrast, MBT cannot replace the adsorbed alkanethiol layer. Therefore, the creation of mixed films has to start with a partial coverage of hexadecanethiol, followed by the MBT, which now can assemble to the uncovered surface without disturbing the alkanethiol.

New methods in flotation research—application of synchrotron radiation to investigation of adsorbates on modified galena surfaces

Abstract Synchrotron radiation-excited photoelectron spectra (SRXPS) were used in order to elucidate flotation-related problems arising from adsorption of sulphur-containing heterocyclic collectors like 2-mercaptobenzothiazole (MBT) on galena. MBT is considered to be representative for a new group of selective collectors for flotation of sulphide minerals. SRXPS provides a strongly enhanced surface sensitivity compared to conventional XPS. A further advantage is the increased excitation cross-section of S compared to Pb which allows the extraction of quantitative results from the S2p spectra of the adsorbates. Additional components besides the signal of the bulk sulphide were observed in the S2p spectra of cleaved as well as preoxidized galena samples proving the chemisorption of MBT and the multilayer formation of oxidation products. Using the intensity ratios of adsorbates vs. bulk signals in dependence on the adsorption conditions, a model is developed for the composition and the structure of the overlayer.