Publications

This paper reports the sensing mechanism of SnO2-CuWO4 mixed oxides towards H2S detection revealed through Pulsed Temperature Mode (PTM). After thick film deposition onto commercial substrates, the final annealing temperature enhances the material reliability through the changes occurred during the experimental conditions. PTM approach represents a tradeoff in terms of sensing mechanism versus sensing performance, in order to get insights about the fundamental aspects. The measurements have been done under dynamic gas flow conditions, in the presence of 50% relative humidity (RH). The gas sensors based on SnO2-CuWO4 exhibit sensor signals higher than 100 to 10 ppm H2S, low cross-sensitivity to common gaseous interfering agents (CO, CH4, NH3, SO2), average selective sensitivity of 8.64 towards the main interfering NO2 gas, moderate response (4 min)/recovery (5 min) transients and power consumption. In order to emphasize the role of moisture towards H2S detection mechanism, the photoacoustic spectroscopic insights were correlated with Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy results. Such approach has shown promising aspects towards further operational sensitive devices. (c) 2017 Elsevier B.V. All rights reserved.

In this work, we report on the variability of the Schottky effect in solution processed Ba1-xSrxTiO3 films (BST, x = 0, 0.5) grown on 0.5% Nb doped SrTiO3 substrates with top Pt electrodes (NSTO/BST/Pt). The films display leakage currents accompanied by varying degrees of hystereses in the current-voltage measurements. The magnitude of the leakage and hystereses depend on the Sr content. We focus on the current-voltage (I-V) behavior of our samples in the light of thermodynamic theory of ferroelectrics coupled with equations of semiconductors. Our calculations allowed us to unambigously determine the electronic character of the defects and related band bending effects in our samples. The extent of asymmetry and the hystereses in the I-V curves for x = 0 and 0.5 are shown to be controlled by the polarization in qualitative agreement with our calculations. Amplitude of the ferroelectric polarization, which is a function of composition here, has a strong impact on leakage currents in forward bias while this effect is much weaker under negative bias. The latter occurs as polarization pointing away from the NSTO semiconducting substrate causes depletion of carriers at the NSTO side of the NSTO/BST interface, increasing resistance to current flow through the stack. Such an occurence also increases the energy gap between the Fermi level and the conduction bands of the films, thereby reducing the bulk conduction through the film as well. The dependence of leakage currents on polarization direction points out to the possibility of a non-destructive read-out route in ferroelectric films much thicker than tunnel junctions. (c) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

New magneto-optical vitreous materials were obtained by melting-quenching technique comprising wet route raw materials preparation. The glass has the following composition in oxide mol. % 10 Li2O, 9 Al2O3, 5 ZnO, (35; 20; 50) B2O3,,(35; 50; 20) P2O5, 3 Bi2O3, 3 PbO, phosphorus and boron oxide being the vitreous network formers. It was also prepared a similar reference glass composition but without Bi2O3 and PbO. Optical and structural characterization by ultraviolet-visible (UV-Vis), Fourier Transform Infrared (FTIR) and Raman Spectroscopy of the bulk glasses showed a transmission over 90%, metaphosphate structure of glass together with Q(2) boron oxide units and P-O-B bonds. The mechanical parameters, hardness (H), Youngs modulus (E) and fracture toughness (KO of boron phosphate glasses, evaluated by micro- and nanoindentation techniques, demonstrated mostly higher values in comparison with those for alumino-phosphate glasses due to mixed borophosphate network. Thermal behavior was investigated by Differential Scanning Calorimetry (DSC) putting in evidence the vitreous transition temperature which decreases with about 45 degrees C when Bi and Pb oxides were added and two crystallization effects. The diamagnetic character of a highly transparent Bi and Pb oxide co doped boron phosphate glass was confirmed by ellipsometry, and the glass presented high magneto-optical properties at the top of the commercial bulk products.

Four batches of cerium oxide powders (with nanocrystallite size of 6.9 nm-572 nm) were prepared from four precursor nanopowders by thermal decomposition of Ce-propionate and annealing in air between 250 degrees C-1200 degrees C for 10 min-240 min. Ceria formation reactions, structure, vibrational, luminescence and magnetic properties were investigated by differential scanning calorimetry, x-ray diffraction, electron microscopy, infrared spectroscopy, photoluminescence and SQUID. All the samples exhibit room temperature ferromagnetism, RTFM, (with coercivity, H-c, of 8 Oe - 121 Oe and saturation magnetization, M-s, of up to 6.7*10(-3) emu/g) and a broad defect-related photoluminescence, PL, emission in the visible range. The samples derived from the same precursor show M-s proportional to the peak area of defect-related PL emission whereas this is not valid for the samples derived from the different precursors. An improvement of ferromagnetism and intensity of defect-related PL emission was observed when annealing the products in which nanocrystalline cerium oxide coexists with Ce - oxicarbonate traces, Ce2O2CO3. The experimental results were explained based on the following considerations: room temperature ferromagnetism was induced by the defective ceria with high concentration of oxygen vacancies generated by decomposition of Ce-propionate; oxygen vacancies of the starting precursor nanopowders could be redistributed (at the surfaces/grain boundaries, GBs) upon heating under conditions that promote an inert local environment; the decomposition of Ce2O2CO3 residues can provide an excess of oxygen vacancies at the nanoparticles surfaces or GBs, which can induce or enhance ferromagnetism; surfaces/GBs rather than bulk defects appear responsible for RTFM - this can explain the (often reported in literature) inconsistency between oxygen vacancies concentration and M-s. (C) 2018 Elsevier B.V. All rights reserved.

This work is focused on a novel class of hybrid materials exhibiting enhanced optical properties and high surface areas that combine the morphology offered by the vinyl substituted silica host, and the excellent absorption and emission properties of 5,10,15,20-tetrakis(N-methyl-4-pyridyl) porphyrin-Zn(II) tetrachloride as a water soluble guest molecule. In order to optimize the synthesis procedure and the performance of the immobilized porphyrin, silica precursor mixtures of different compositions were used. To achieve the requirements regarding the hydrophobicity and the porous structure of the gels for the successful incorporation of porphyrin, the content of vinyltriacetoxysilane was systematically changed and thoroughly investigated. Substitution of the silica gels with organic groups is a viable way to provide new properties to the support. An exhaustive characterization of the synthesized silica samples was realised by complementary physicochemical methods, such as infrared spectroscopy (FT-IR), absorption spectroscopy (UV-Vis) and photoluminescence, nuclear magnetic resonance spectroscopy (Si-29-MAS-NMR) transmission and scanning electron microscopy (TEM and SEM), nitrogen absorption (BET), contact angle (CA), small angle X ray and neutron scattering (SAXS and SANS). All hybrids showed an increase in emission intensity in the wide region from 575 to 725 nm (Q bands) in comparison with bare porphyrin. By simply tuning the vinyltriacetoxysilane content, the hydrophilic/hydrophobic profile of the hybrid materials was changed, while maintaining a high surface area. Good control of hydrophobicity is important to enhance properties such as dispersion, stability behaviour, and resistance to water, in order to achieve highly dispersible systems in water for biomedical applications.

The synthesis of metal-free graphene-based photocatalysts has received great attention recently due to their expected contributions to the development of solar-based hydrogen generation via water-splitting in a low cost and ecological manner. In this work, a new method for the generation of nitrogen-doped graphene-based powder employing an alternative solution to commonly used toxic and hazardous organic solvents is presented. The procedure involves ultraviolet pulsed laser irradiation of graphene oxide (GO) flakes dispersed in 1-butyl-3-methylimidazolium [bmim]-based ionic liquids using both chloride and acetate anions. The structural and compositional analysis using transmission electron microscopy, X-ray photoelectron and infrared spectroscopy indicate that the irradiated GO becomes partially reduced and doped with graphitic, pyrrolic and pyridinic nitrogen species. Interestingly, the relative content of the nitrogen functionalities is controlled by the anion in the ionic liquid and its concentration, with the obtained graphene-based powders showing higher photocatalytic activity than GO. Furthermore, a remarkable synergistic effect is observed for GO-[bmim]-acetate powder (acting as co-catalyst) in combination with anatase TiO2 nanoparticles. The presented method opens new research avenues for the cost-effective mass production of graphene-based photocatalysts for water splitting applications. (C) 2018 Elsevier Ltd. All rights reserved.

This work investigates the catalytic properties toward sulfide oxidation in wastewater for three composites which are functional materials obtained from red mud waste following its neutralization, chemical activation and functionalization of the iron by treatment with disodium salt of ethylenediaminetetraacetic acid, trisodium citrate or a combination of these two organic ligands. X-ray diffraction and diffuse reflectance Fourier transformed infrared spectroscopy characterizations indicated the coexistence of the corresponding iron chelates phases along with hematite the main crystallographic phase from red mud. The most active catalyst was the red mud-derived material obtained by functionalization with the mixture of ethylenediaminetetraacetate and citrate ligands. The results obtained after its testing in multiple reaction cycles showed that the decrease in conversion after 10 reaction cycles was less than 5%. Considering the results of diffuse reflectance ultraviolet visible narrow infrared spectroscopical analysis which revealed that this solid contains species with lower bond strength, it has been inferred that both the higher catalytic activity, as well as the enhanced stability, is directly related to the versatility of the active species.

Gold nanoparticles were decorated on/into the SBA-15 channels covalently functionalized by new class of symmetrical tridentate NNN-pincer ligand. This provides a unique support for stabilization of Au NPs preventing agglomeration due to the strong ligation of pincer ligand. The catalyst proved to be highly active for the reduction of nitroarenes. The kinetics study showed that the rate constant for the current protocol are comparable or larger than the reported ones in literature. The catalyst also showed high recoverability and reusability up to ten times without significant loss of activity and leaching or aggregation of Au NPs (overall TOF = 8798).

Spectroscopic investigation of Dy3+ doped Ca-3(Nb,Ga)(5)O-12 (CNGG) and Ca-3(Li,Nb,Ga)(5)O-12 (CLNGG) single crystals were performed in order to assess their potential as laser materials for yellow emission. Dy: CNGG and Dy: CLNGG single crystals were grown by the Czochralski method and investigated by high-resolution spectroscopic measurements for the first time. The Judd-Ofelt intensity parameters for the f-f transitions of Dy3+ in CNGG and CLNGG single crystals, were used to determine radiative transition rates A(r), branching ratios beta, and radiative lifetime tau(r) of the fluorescent Dy3+ levels. Based on low temperature absorption and emission spectra, partial energy levels diagram of Dy3+ doped CNGG and CLNGG single crystals were obtained. The emission cross-sections for the F-4(9/2) -> H-6(13/2) transition of special interest for laser application were determined. The room temperature decay curves were analyzed through the framework of Inokuti-Hirayama (I-H) model and the results shows that electric dipole-dipole interaction are responsible for the energy transfer processes between Dy3+ ions in CNGG and CLNGG single crystals. (c) 2017 Elsevier B.V. All rights reserved.

Si and Ge nanocrystals in oxides are of a large interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in nanocrystals. In this work, dense Ge nanocrystals suitable for enhanced photoconduction were fabricated from 60% Ge in TiO2 amorphous layers by low temperature rapid thermal annealing at 550 degrees C. An exponential increase of the photocurrent with the applied voltage was observed in coplanar structure of Ge nanocrystals composite films deposited on oxidized Si wafers. The behaviour was explained by field effect control of the Fermi level at the Ge nanocrystals-TiO2 layer/substrate interfaces. The blue-shift of the absorption gap from bulk Ge value to 1.14 eV was evidenced in both photocurrent spectra and optical reflection-transmission experiments, in good agreement with quantum confinement induced bandgap broadening in Ge nanocrystal with sizes of about 5 nm as found from HRTEM and XRD investigations. A nonmonotonic spectral dependence of the refractive index is associated to the Ge nanocrystals formation. The nanocrystal morphology is also in good agreement with the Coulomb gap hopping mechanism of T-1/2 -type explaining the temperature dependence of the dark conduction.