National Institute Of Materials Physics - Romania

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.

Small samples of 12.5 mm in diameter made from pure tungsten were exposed to a dense plasma jet produced by a coaxial plasma gun operated at 2 kJ. The surface of the samples was analyzed using a scanning electron microscope (SEM) before and after applying consecutive plasma shots. Cracks and craters were produced in the surface due to surface tensions during plasma heating. Nanodroplets and micron size droplets could be observed on the samples surface. An energy-dispersive spectroscopy (EDS) analysis revealed that the composition of these droplets coincided with that of the gun electrode material. Four types of samples were prepared by spark plasma sintering from powders with the average particle size ranging from 70 nanometers up to 80 mu m. The plasma power load to the sample surface was estimated to be 4.7 MJ m(-2) s(-1/2) per shot. The electron temperature and density in the plasma jet had peak values 17 eV and 1.6 x 10(22) m(-3), respectively. (C) 2017 National Institute for Laser, Plasma, and Radiation Physics (INFLPR). Published by Elsevier B.V. All rights reserved.

Ceria materials doped by Gd and/or Pr were prepared by a precipitation route and calcined in air at 500 and 900 degrees C. The effects of doping and thermal treatment on the materials' surfaces were studied by X-ray photoelectron and Raman spectroscopies. The redox properties were investigated using temperature-programmed reduction experiments in H-2 and CH4 followed by temperature-programmed oxidation in O-2. The catalytic properties of these materials in CH4/H2O reaction in the absence/presence of H2S were evaluated at 750 degrees C in a large excess of CH4 with respect to H2O. The materials were classified by order of reactivity as follows: CPO > CGPO > CeO2 > CGO, which can be primarily related to the surface area variation. Comparing the activity per m(2) of various samples indicates that the catalytic activity is correlated with the Ce3+ concentration at the surface as measured by XPS. Gd-doped and undoped samples exhibit the highest surface Ce3+ concentration as well as the highest activity in SMR per m(2), while Pr-doped samples are the least active (activity per m(2)) with the lowest surface Ce3+ concentrations. H2S has a sharp promoting effect on the catalytic behavior of ceria-based materials, improving the production of H-2 by almost one order of magnitude.

Analyte sensitivity for gas sensors based on semiconducting metal oxides should be highly dependent on the film thickness, particularly when that thickness is on the order of the Debye length. This thickness dependence has previously been demonstrated for SnO2 and inferred for TiO2. In this paper, TiO2 thin films have been prepared by Atomic Layer Deposition (ALD) using titanium isopropoxide and water as precursors. The deposition process was performed on standard alumina gas sensor platforms and microscope slides (for analysis purposes), at a temperature of 200 degrees C. The TiO2 films were exposed to different concentrations of CO, CH4, NO2, NH3 and SO2 to evaluate their gas sensitivities. These experiments showed that the TiO2 film thickness played a dominant role within the conduction mechanism and the pattern of response for the electrical resistance towards CH4 and NH3 exposure indicated typical n-type semiconducting behavior. The effect of relative humidity on the gas sensitivity has also been demonstrated.

Annealing temperature of green bodies has been shown to influence greatly the optical properties and laser characteristics of Y3Al5O12:Nd3+ (1 at%) ceramics. Increase the annealing temperature above some critical one (800 degrees C) results in appearance of submicron pores due to Y4Al2O9 phase formation accompanied by specific volume expansion. The energy changes associated with the chemical reaction can lead to the development of microstructures that possess lower sinterability. As a result, Y3Al5O12:Nd3+ ceramics prepared from unannealed green bodies and those annealed at 600 and 800 degrees C possess higher optical transmittance and enhanced slope efficiency (63-67%) compared with that obtained using green bodies annealed at 1000 degrees C (41%). Further investigations are necessary in order to explain this behavior.