Marco Minicucci

The structure of lead-silicate glasses: molecular dynamics and EXAFS studies

Molecular dynamics (MD) simulations and extended x-ray absorption fine structure (EXAFS) investigations of the structure of lead-silicate glasses, xPbO(1 − x)SiO2, have been undertaken to elucidate the problem of partially contradicting experimental findings reported in the literature about basic structural units and their interconnection. The MD simulations were performed in a wide range of compositions, x = 0.1–0.9. The atoms were assumed to interact by a two-body Born–Mayer–Huggins interaction potential. The EXAFS measurements were performed for x = 0.3, 0.5 and 0.7, and also for pure crystalline (red) PbO at the L3-edge of Pb. The absorption spectra were analysed within the GNXAS approach. Our EXAFS and MD results are in good agreement, and support some previous suggestions that: (1) the PbO4 groups are the dominant structural units in lead-silicate glasses for any concentration and (2) at lower PbO concentrations the co-existence of the PbO4 and PbO3 groups is possible. The medium-range ordering in the simulated glasses has also been investigated in detail. The connectivity of the SiO4 tetrahedra network breaks at about x = 0.45, whereas the Pb structural units form a continuous (mainly edge-sharing) network even at relatively low PbO concentrations (x > 0.2). The cation–anion ring statistics is also discussed.

Single-energy x-ray absorption detection: a combined electronic and structural local probe for phase transitions in condensed matter

We describe the general principle and prototypical applications of a largely unexploited x-ray absorption technique based on single-energy detection during temperature scans. The present developments take advantage of the highly automated experimental set-ups and unique x-ray radiation characteristics available at third-generation synchrotron radiation sources. The sensitivity of the x-ray absorption cross-section to the local structural order makes it possible to investigate the occurrence of phase transitions. We show that by tuning the photon energy to a spectral feature that is highly sensitive to the sample phase it is possible to follow the occurrence and the characteristics of the phase transition. The sensitivity to the atomic fraction of the sample in a certain phase can be in the range. The energy tunability can be used to enhance the sensitivity to structural or electronic transformations. Several applications to prototypical cases are discussed in detail, including examples of solid-solid phase transitions, melting and undercooling, melting of binary alloys, impurity melting, and non-bulk phenomena.

Influence of hydrogen reduction on the structure of PbSiO3 glass: an EXAFS study

A new X-ray analysis of the structure of lead-silicate glass PbSiO3, and of its hydrogen-reduced form, is presented. The X-ray absorption spectroscopy (XAS) experiments were performed at the L3-edge of Pb, and the data have been analyzed in the framework of the multiple scattering (MS) theory. The influence of the reduction time on the glass structure is presented. In unreduced glass the Pb atom neighborhood resembles the corresponding local ordering in red PbO. On increasing the reduction time Pb neighborhood becomes more and more similar to that of fcc lead, which is compatible with the experimentally observed creation of metallic lead granules.

Visible light photoactivity of Polypropylene coated Nano-TiO2 for dyes degradation in water.

The use of Polypropylene as support material for nano-TiO2 photocatalyst in the photodegradation of Alizarin Red S in water solutions under the action of visible light was investigated. The optimization of TiO2 pastes preparation using two commercial TiO2, Aeroxide P-25 and Anatase, was performed and a green low-cost dip-coating procedure was developed. Scanning electron microscopy, Atomic Force Microscopy and X-Ray Diffraction analysis were used in order to obtain morphological and structural information of as-prepared TiO2 on support material. Equilibrium and kinetics aspects in the adsorption and successive photodegradation of Alizarin Red S, as reference dye, are described using polypropylene-TiO2 films in the Visible/TiO2/water reactor showing efficient dyes degradation.

Structure of crystalline and amorphous Ge probed by X-ray absorption and diffraction techniques

Results of experiments dedicated to the study of the structure under high pressure of amorphous Ge (a-Ge) and crystalline Ge (c-Ge) are reported. Energy-dispersive X-ray diffraction measurements of c-Ge have been collected at the DW11A beamline (DCI, LURE) using a heatable diamond anvil cell as pressure device up to 500 K. The a-Ge measurements have been performed at the ESRF, using the advanced set-up available at the BM29 beamline, which allows the simultaneous collection of X-ray absorption spectroscopy data and diffraction patterns used to monitor pressure and crystallization of a sample in a Paris–Edinburgh large-volume cell. The new structural data allowed us to obtain a reliable determination of the lattice parameters as a function of pressure and temperature in c-Ge and of the first-neighbor distance distribution in a-Ge.

Exfoliation of graphite into graphene in aqueous solution: an application as graphene/TiO2 nanocomposite to improve visible light photocatalytic activity

The aim of this study was the preparation of a graphene/TiO2 heterogeneous catalyst supported on polypropylene for visible light photocatalysis. Aqueous graphene dispersions were prepared by liquid-phase exfoliation of graphite in the presence of a non-ionic surfactant, Triton X-100. The obtained graphene dispersion was characterized by X-ray diffraction, dynamic light scattering and UV-Visible spectroscopy and was subsequently used for the preparation of graphene/TiO2 photocatalyst. As-prepared photocatalysts were tested for the photocatalytic degradation of a refractory dye, Alizarin Red S, in water solutions as target pollutant. Graphene/TiO2 nanocomposites showed higher adsorption of Alizarin Red S on the catalyst surface and higher photocatalytic activity for its degradation under visible light irradiation, in respect to those obtained with pure TiO2.

Anomalies in the structure of solid Cd under pressure: an x-ray diffraction study

The results of an x-ray diffraction energy-dispersive study on solid Cd under high-pressure conditions at room temperature are presented. The trends of the lattice parameters and of the axial ratio c/a as a function of pressure show that an anomalous slope change, previously discussed in connection with an electronic topological transition (ETT), is still observed using a different pressure transmitting medium for relative volume compression V/V0~0.86. A detailed study of the peak positions shows that some Bragg reflections, in particular those related to the length of the c axis, are shifted with respect to the calculated positions for a typical hcp structure. This anomalous behaviour, observed for pressures above 4 GPa, is briefly discussed in terms of possible oriented lattice strains and non-hydrostatic effects.

EXAFS study on liquid gallium under high pressure and high temperature

EXAFS (Extended X-ray Absorption Fine Structure) spectra of liquid gallium near the Ga K-edge have been collected in wide pressure and temperature ranges. Reliable short-range pair distribution functions have been determined using advanced ab-initio calculations (GNXAS) taking into account the medium and long-range structure obtained by previous neutron and x-ray-scattering studies.

Testing interaction models by using x-ray absorption spectroscopy: solid Pb

Structural models obtained using classical molecular dynamics (MD) simulations and realistic interatomic potentials for solid metals are tested using experimental results obtained by x-ray absorption spectroscopy (XAS). Accurate L-edge extended x-ray absorption fine-structure (EXAFS) measurements of Pb grains dispersed in BN and graphite matrices have been collected for temperatures up to the melting point. The thermal expansion of the grains was measured by energy-dispersive x-ray diffraction techniques and found to be coincident with that of pure Pb up to the limit of the present measurements. L3-edge EXAFS measurements of solid Pb at various temperatures have been analysed using advanced data-analysis techniques (GNXAS) based on exact spherical-wave multiple-scattering simulation of the absorption cross-section. Realistic structural models for solid Pb were obtained from MD simulations using an empirical pair potential (Dzugutov, Larsson and Ebbsjo (DLE)), a tight-binding (TB) square-root functional, and an embedded-atom (EA) model potential parametrized by us. The short-range pair distribution function g(r) reconstructed by means of EXAFS is compared with those obtained by MD simulations. The empirical DLE potential, originally designed for the liquid state, is too soft, showing too-large values for the average distance R, variance σ2, and skewness β. The TB and EA potentials are both compatible with XAS data as regards the average distance and skewness of the first neighbours. The distance variance, associated with the thermal vibration amplitudes, is underestimated for the TB potential, while the EA model is found to be in agreement with XAS data. The present results are also compared with those from a previous EXAFS study on solid lead, where the cumulant expansion and a simple one-dimensional anharmonic oscillator model were used. The need for realistic interaction models and appropriate simulation schemes for reliable XAS data analysis is emphasized, while differences from and improvements with respect to previous approaches are only briefly discussed.

SOLID AND LIQUID AgI AT HIGH PRESSURE AND HIGH TEMPERATURE: A X-RAY ABSORPTION SPECTROSCOPY STUDY

We report about recent X-ray absorption spectroscopy (XAS) measurements on solid and liquid AgI under high pressure. The structural behaviour of AgI has been investigated to pressures P∼4.3 GPa at room temperature and to P∼1.8 GPa at 1100 K. The high temperature/high pressure conditions have been obtained by means of a large-volume press of the Paris-Edinburgh type, coupled with a 10 mm boron/epoxy biconical gasket. The absorption spectra have been collected in transmission mode, both at the K-edge of Ag and I, and the samples have been characterized in situ by energy scanning X-ray diffraction at fixed angles. Our XAS results for solid AgI are compatible with previous X-ray diffraction measurements. For liquid AgI, we observe a slight change in the intensity and a shift in the frequency of the XAS oscillations with respect to what obtained in the case of the ambient pressure liquid.