A.M. Botelho do Rego

Characterization of a natural and an electro-oxidized arsenopyrite: a study on electrochemical and X-ray photoelectron spectroscopy

This paper discusses the results of a detailed study on the electrochemistry of an arsenopyrite mineral and a concentrate as well as other mineral species contained in it in a chloride medium (NaCl 1.9 M+HCl 0.1 M) using cyclic voltammetry. The surface modification promoted by the anodic oxidation of arsenopyrite mineral samples (+0.8 V for 1 h) is also analysed. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy have been used to provide information on the chemical state of natural and electrooxidized surfaces. The results showed that layers of elemental sulphur are produced on the oxidized surface of arsenopyrite by the formation of an intermediate metal-deficient sulphide Fe1 � xAs1 � yS. Other surface oxidation products such as iron oxides, arsenic oxides and oxysulphur species have also been detected, confirming the interpretation of the voltammetric studies. The identified surface products show that arsenopyrite oxidation occurs by the diffusion of the metal atoms from the bulk to the interface region and by their interaction with air forming metal oxide layers (mainly iron and arsenic oxides) and leaving a predominant elemental sulphur layer.

An XPS study of the fibre-matrix interface using sized carbon fibres as a model

This work is part of a larger project whose main objective is a better understanding of the mechanism of adhesion between the surface of carbon fibres and resins in composites. The effect over the surface of commercial PAN-based carbon fibres (Courtaulds IM CG43-750) induced by several degrees of a commercial wet oxidative surface treatment (STL) as well as the nature of the fibre-size interface have been studied by X-ray photoelectron spectroscopy. As far as functional groups are concerned, a good qualitative and quantitative characterization of the fibre surface has been achieved, using a criterium based on the requirement of internal consistency between results obtained with photoelectrons from C 1s, O 1s and N 1s peaks. Oxidative treatment induces an enrichment in oxygen and nitrogen at the surface; the presence of the former on the surface is due to the treatment but strong evidence for a residual origin of nitrogen is obtained. Coating of the commercial fibres by a prepolymer similar to the resin used as the matrix, has been used to test the type of bonding between fibre and resin before curing. Evidence is given supporting the idea that the coating (size) does not entirely cover the surface of the fibre and that only a part of it is covalently bound, alcohol groups seeming to play an important role in the adhesion mechanism.

Oxidation of pinane using transition metal acetylacetonate complexes immobilised on modified activated carbon

The performance of a new solid catalyst is studied. Copper and cobalt acetylacetonate complexes are chemically anchored to functionalised activated carbon. These catalysts are active and highly selective to 2-pinane hydroperoxide in the oxidation of pinane at room temperature and atmospheric pressure.

Study of polymer film surfaces by EELS using selectively deuterated polystyrene

HREELS spectra of selectively deuterated polystyrene films show loss energies corresponding to the excitation of vibrational modes of the surface region of the organic medium. The thickness of the analyzed surface region can be varied by changing the incident electron energy and/or the scattering geometry. Selective labelling allows to better separate the contributions of main chain and of pendant-groups, and to eliminate the influence of contaminants. It is shown for instance that isotactic and atactic polymer surfaces differ, the former being mainly composed of phenyl rings. End group segregation at the surface may also be important. Multiple losses are much more intense than the corresponding combination modes in IR: the theory of interaction of low energy electrons with polymer films should be further investigated.

OXYDEHYDROGENATION OF CYCLOHEXANOL OVER CARBON CATALYSTS

Abstract The reaction of cyclohexanol over carbon catalysts with surfaces of different chemical nature was studied. The catalyst samples were prepared by treating a parent activated carbon with HNO3, H2O2, O2 and N2O. The catalytic tests were performed in a packed-bed reactor at 350 °C. Dehydration as well as dehydrogenation of the substrate occur under the reaction conditions. The main products obtained are cyclohexene, cyclohexanone, benzene and phenol. Dehydration occurs preferentially over the carbon treated with HNO3, while the best selectivity to cyclohexanone is achieved with the carbon treated with H2O2. The catalyst sample treated with N2O is highly selective to phenol. A mechanism involving nitro groups on the carbons surface is proposed.

Microwave plasmas applied for the synthesis of free standing graphene sheets

Self-standing graphene sheets were synthesized using microwave plasmas driven by surface waves at 2.45 GHz stimulating frequency and atmospheric pressure. The method is based on injecting ethanol molecules through a microwave argon plasma environment, where decomposition of ethanol molecules takes place. The evolution of the ethanol decomposition was studied in situ by plasma emission spectroscopy. Free gas-phase carbon atoms created in the plasma diffuse into colder zones, both in radial and axial directions, and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the hot region of the plasma in the outlet plasma stream where nanostructures assemble and grow. Externally forced heating in the assembly zone of the plasma reactor has been applied to engineer the structural qualities of the assembled nanostructures. The synthesized graphene sheets have been analysed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. The presence of sp3 carbons is reduced by increasing the gas temperature in the assembly zone of the plasma reactor. As a general trend, the number of mono-layers decreases when the wall temperature increases from 60 to 100 °C. The synthesized graphene sheets are stable and highly ordered.

Controlled growth of Cu2O nanoparticles bound to cotton fibres

A green, safe and fast procedure is presented for in situ generation of nanoparticles (NPs) of cuprous oxide (Cu2O) onto cotton fibres at room temperature using water as a solvent. The method is based on a mild surface oxidation of cellulose fibres to generate in a controlled way carboxylic groups acting as a binding site for the adsorption of Cu(2+) via electrostatic coordination. Then, the adsorbed Cu(2+) ions were readly converted into Cu2O by dipping the treated cotton fibres into a aqueous solution of a reducing agent. Field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), as well as UV-vis absorption and emission spectroscopic methods were used to analyse the size, morphology, chemical composition and the crystalline structure of the generated nanoparticles on the fabrics. The morphology of the ensuing Cu2O nanoparticles was shown to be dependent on the reduycing agent used. Antibacterial properties of the modified fibres were also investigated.

XPS investigation of BxNyCz coatings deposited by laser assisted chemical vapour desposition

Abstract Some of the most interesting new materials are located in the boron-carbon-nitrogen composition triangle. Laser assisted chemical vapour deposition (LCVD) using different gas phase precursors and processing conditions yields BxNyCz coatings with varying crystallographic structure and stoichiometry. In this work we investigate the capabilities of the X-ray photoelectron spectroscopy (XPS) technique for the characterisation of BxNyCz films produced by LCVD from diborane, ethane and ammonia or diborane and dimethylamine reactive atmospheres, using a continuous wave CO2 laser. The XPS results are interpreted together with those obtained by glancing incidence X-ray diffraction and electron microprobe analysis in order to study the chemical bonds present in the coatings.

Oligothiophene films under electron irradiation: electron mobility and contact potentials

Abstract This work compares mobility in various films of α-oligothiophenes with chainlengths ranging from four to six thiophene rings (4T, 5T and 6T), which were studied by high-resolution electron energy-loss spectroscopy (HREELS) in the same conditions of beam intensity and geometrical configuration. The increase of electron mobility going from 4T to 6T was estimated from an analysis of the energy extent in the complete spectra of the electrons backscattered from the sample surface. The same method was still applied to two films of 6T deposited on different substrates, gold and highly oriented pyrolitic graphite (HOPG), where a change in the molecular orientation is expected. The same value was found for the electron mobility in both films. However, different contact potentials were observed and no charge accumulation was detected. On the other hand, work functions were estimated for both films using the work functions values found in the literature for gold and HOPG, which are 4.6 and 4.5 eV, respectively. In this case, it was not possible to conclude if a different molecular orientation produced a change in the work function or not, as the difference between the work function values of the substrates given in the literature is larger than 0.1 eV.

Electronic excitation and secondary electron emission studies by low-energy electrons backscattered from thin polystyrene film surfaces

Thin atactic polystyrene films were studied by backscattering of low-energy electrons. Energy losses corresponding to the excitation in the film of triplet and singlet electronic excitation of the pendant groups are observed. The variation of the corresponding cross sections with incident electron energy are in good agreement with theoretical calculations for benzene. The variation of the backscattered intensity integrated over electron energy in a given direction, yields a ionization threshold of about 7 eV and shows evidence for structure of the valence band.