C. Sánchez

About the band gap nature of FeS2 as determined from optical and photoelectrochemical measurements

Abstract We discuss in this paper the validity of some approaches currently used to determine the energy gap value (Eg) of semiconducting pyrite (FeS2) from optical and photoelectrochemical measurements. It is concluded that the wide dispersion of the reported experimental values of Eg might be due to the inadequacy of those approaches.

Structural and microstructural features of pyrite FeS 2− x thin films obtained by thermal sulfuration of iron

Structural and microstructural properties of synthetic thin films of pyrite (FeS 2− x ), prepared by thermal sulfuration of iron layers, were investigated from Rietveld refinements of x-ray diffraction data, collected by step/scan mode. From this refinement lattice constant, a , and sulfur position parameter, u , nearest neighbor Fe–S and S–S bond distances and tetrahedral and octahedral bond angles have been determined. Moreover, sulfur deficit in the samples, surface and volume-weighted crystallite size and microstrains were also obtained. From these data, the influence of temperature and time of sulfuration and sulfur pressure on their structural and microstructural properties has been established. Stoichiometric pyrite thin films are obtained by sulfurating the iron films at low temperatures ( T s ∼ 600–700 K) during short times ( t s ∼ 0.5–2 h). These experimental conditions yield films with the highest a , u , Fe–S bond distance, and microstrains, as well as S/Fe ratios about 2.00, i.e., null sulfur vacancies, the smallest S–S bond distances, and crystallite size. Finally, the possible influence of these structural and microstructural characteristics on some physical properties (optical absorption, electrical resistivity …) of the films is discussed.

Simultaneous differential scanning calorimetry and thermal desorption spectroscopy measurements for the study of the decomposition of metal hydrides

An innovative experimental method to investigate the thermal decomposition of metal hydrides is presented. The method is based on an experimental setup composed of a differential scanning calorimeter connected through a capillary tube to a mass spectrometer. The experimental system allows the simultaneous determination of the heat absorbed and the hydrogen evolved from a metal hydride during thermal decomposition. This arrangement constitutes a coupled differential scanning calorimetry (DSC) and thermal desorption spectroscopy (TDS) technique. It has been applied to metal hydride materials to demonstrate the capability of the experimental system. A method to obtain the heat of decomposition of metal hydrides is described. It involves the measurement of an apparent decomposition heat as a function of the carrier gas flow.

Characterization of iron pyrite thin films obtained by flash evaporation

Abstract Thin films of pyrite (FeS2) have been prepared by flash evaporation and the necessary experimental conditions (substrate temperature and bell pressure) have been established. Under these conditions films are composed of pyrite and pyrrhotite phases, as shown by X-ray diffraction patterns. This sulphur deficiency can be compensated by a further treatment in a sulphur atmosphere. Electrical resistivity and optical absorption measurements have been performed to characterize the FeS2 films.

Influence of the martensitic transformation on the hydrogenation properties of Ti50−xZrxNi50 alloys

Abstract Ti50−xZrxNi50 alloys with 0≤x≤24 develop either austenitic or martensitic crystal structures when prepared by melt-spinning or induction melting, respectively. This outcome is a consequence of the particular alloy microstructure resulting from each preparation method, which induces a difference of 100°C on the martensitic transformation temperatures for alloys with the same composition. Austenitic alloys absorb hydrogen up to 1.5 hydrogen atoms per AB unit (H/AB) at 130°C and 20 bar, without displaying any plateau pressure for hydrogen pressures between 0.1 and 10 bar. In contrast, martensitic alloys exhibit a plateau pressure with hydrogen concentrations between 1 and 2.1 H/AB, and reach a maximum hydrogen concentration of 2.6 H/AB under the same thermodynamic conditions. Consequently, martensitic alloys form a dihydride compound that, for the representative case of Ti32Zr18Ni50 alloy, has a formation enthalpy of −12.3±0.2 kcal mol H2−1.

Pyrite thin films: Improvements in their optical and electrical properties by annealing at different temperatures in a sulfur atmosphere

Iron pyrite thin films prepared by flash evaporation of pyrite powder have been annealed at different temperatures in a sulfur atmosphere. We present some results on the influence of the annealing temperature (from 250 to 450u2009°C) on the optical and electrical properties of three groups of samples with different thicknesses (≂0.3, 0.6, and 1 μm, respectively). Sulfuration temperature has a clear influence on the optical absorption and electrical resistivity of the films, with some differences in their behavior depending on the film thickness. In light of the available present knowledge of pyrite thin films, interpretation of the obtained results is difficult, it suggests that the shape of the optical absorption curves (and their absorption edge) at low photon energies is determined by the density of point defects, which decreases on increasing the annealing temperature. On the other hand, the electrical resistivity seems to be influenced by both the film grain size and point defect density.

Titanium trisulfide (TiS3) : A 2D semiconductor with quasi-1D optical and electronic properties

We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance.

Temperature dependence of the optical absorption edge of pyrite FeS2 thin films

Optical absorption edge measurements of pyrite thin films prepared by flash evaporation have been made at different temperatures in the range 10-300 K. Absorption edge values vary from 1.05 to 0.99 eV in that temperature interval. Experimental data have been fitted to the Varshni formula and to an expression proportional to the statistical Bose-Einstein factor used previously by other workers. The main phonon energy calculated from the fitting of the experimental points to the Bose-Einstein expression is in good agreement with the average energy of the active optical phonons as determined by Raman spectroscopy carried out on the same pyrite thin film.

Titanium trisulphide (TiS3) nanoribbons for easy hydrogen photogeneration under visible light

First evidence of hydrogen evolution by using titanium trisulphide (TiS3) as a photoanode in a photoelectrochemical cell (PEC) is reported. Synthesized TiS3, composed of numerous nanoribbons, has been structurally, morphologically and photoelectrochemically characterized. Moreover, the value of its flat band potential has been estimated (Vfb = −0.68 ± 0.05 V vs. Ag/AgCl) by Electrochemical Impedance Spectroscopy (EIS) measurements. This value has been used to depict the band energy levels of the TiS3/electrolyte interface. Finally, flows of photogenerated hydrogen up to 1.80 ± 0.05 μmol H2 min−1 have been quantified by Mass Spectrometry (MS) at a (Ag/AgCl) bias potential of 0.3 V, yielding a photoconversion efficiency of about 7%.

Physical properties of Cu-doped FeS2 pyrite thin films

Abstract Cu-doped FeS2 thin films have been grown and their physical properties investigated. For this purpose a series of Fe-Cu (Cu nominal concentration of 1at%) evaporated films has been sulfurated (sulfur pressure of 600 Torr) at different temperatures (200 to 500°C). Structural, electrical and optical characterization have shown that the films are n-type and their resistivity changes with the sulfuration temperature from 0.002 to 3.28 Ω·cm. Values of the film optical absorption edge vary from 0.8 to 1.1 eV with the sulfuration temperature. Discussion is made by comparing the present results with those from undoped FeS2 films prepared by the same method.