Publications

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.

The present study is focused on the wet chemical synthesis and the characterization of ZnO-CdS composites. The X-ray diffraction shows that the composites contain ZnO in hexagonal wurtzite structure and CdS in cubic phase. The scanning/transmission electron microscopy images reveal flower-like structures with different sizes depending on the CdS content. The optical investigations on composites reveal that the reflectance spectra disclose two thresholds of similar to 370 nm and similar to 460 nm associated with the ZnO and CdS, respectively. The photocatalytic activity measurements evidenced that the degradation efficiency of RhB in the presence of composites is higher comparatively with pristine ZnO, depending on the catalyst morphology, which varies with CdS content and the pH value of RhB solution. The electron paramagnetic resonance revealed the presence of the paramagnetic point defects in the samples. Thus, the wet chemical approaches are suitable for a large scale production of such ZnO-CdS composites having enhanced photocatalytic activity.

Luminescent properties of Yb,Er:YAG transparent ceramics containing 5 at% Yb3+ and 0.5, 1 and 1.5 at%, respectively, Er3+ ions have been studied. It has been found that increasing of erbium ions concentration increases both efficiency of nonradiative energy transfer Yb3+ -> Er3+ (which reaches 72% for 1.5 at% Er3+) and the luminescence in the range of 650-700 nm associated with F-4(9/2), (11/2) -> I-4(15/2) transitions of Er3+ ions. It was also determined that the concentration quenching of sensitized luminescence of Er3+ ions at 1532 nm is associated with secondary excitation of metastable energy level of Er3+ followed by up conversion emission. (C) 2018 Elsevier B.V. All rights reserved.

Synthesis and characterization of iron oxide nanoparticles coated with a large molar weight dextran for environmental applications are reported. The first experiments involved the synthesis of iron oxide nanoparticles which were coated with dextran at different concentrations. The synthesis was performed by a co-precipitation technique, while the coating of iron oxide nanoparticles was carried out in solution. The obtained nanoparticles were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction spectrometry, Fourier transform infrared spectroscopy and superconducting quantum interference device magnetometry. The results demonstrated a successful coating of iron oxide nanoparticles with large molar weight dextran, of which agglomeration tendency depended on the amount of dextran in the coating solution. SEM and TEM observations have shown that the iron oxide nanoparticles are of about 7 nm in size.

By platinum deposition on a 150 nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface. (C) 2017 Elsevier B.V. All rights reserved.

Herein we report a simple and scalable route to synthesize porous cobalt/cobalt oxide - carbon sphere composites as anode material for rechargeable lithium-ion batteries. It involves the impregnation of starch-derived hydrochar spheres with a cobalt salt, followed by a heat treatment (700 degrees C) under inert atmosphere. The obtained high surface area (similar to 670 m(2) g(-1)), submicron spheres (similar to 300 nm diameter) with high-degree of microporosity (81%) consist of an amorphous carbon matrix with embedded Co/CoO nanoparticles (similar to 6 nm sized), having a total cobalt content of 6.2 wt%. The hybrid sphere anodes demonstrated superior specific capacity, rate performance and cycling stability. Discharge capacities of 520 and 310 mA h g(-1) are observed at charge-discharge rates of 0.1 and 1C respectively. No significant capacity fading is identified on prolonged cycling at various current densities. The electrode also demonstrated excellent structural stability during extended charge-discharge processes. (c) 2018 Elsevier Ltd. All rights reserved.

Silico-phosphate films doped with CdS/ZnS quantum dots (QDs) for luminescence temperature sensing devices, were deposited on silicon substrate by sol-gel method, spin coating technique. Optical absorption and photoluminescence emission of CdS/ZnS-doped silico-phosphate films were investigated. CdS/ZnS nano-crystalline particles were put into evidence by X-Ray Diffraction (XRD) spectroscopy. The specific vibration modes of the constituents were evidenced by structural analysis carried out by Raman and Fourier Transform Infrared spectroscopy (FTIR). The characteristic fluorescence emission of CdS/ZnS nanoparticles was noticed in the visible domain. The morphology of the films was investigated by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Clusters of hexagonal crystalline CdS/ZnS QDs were put into evidence.

Light induced catalytic processes have attracted significant attention during the last years for wastewater treatment due to their efficiency in decomposition of organic contaminants. In this study we report the synthesis of graphene oxide (GO)/ZnO hybrid layers with high photocatalytic efficiency using laser radiation. The results show that the hybrid layers exhibit much improved photodecomposition efficiency as compared to pure GO or ZnO both under UV and visible-light irradiation. The enhanced photocatalytic efficiency of the hybrid as compared to the reference pure ZnO and GO layers was attributed to the contribution of GO to the separation and transport of the photogenerated charge carriers. Additionally, under visible light irradiation the organic molecules can act as first sensitizers in the degradation process. The recyclability of the layers was also investigated through repetitive photodegradation cycles under UV- or visible-light irradiation. After consecutive degradation runs, the hybrid photocatalyst layers were still stable and retained high degradation efficiency, ensuring reusability. The photocatalytic activity of the layers was correlated with the gradual change of their chemical structure during consecutive degradation cycles. Owing to the high photodegradation efficiency, reusability, and ease of recovery the synthesised hybrid layers consisting of easily available materials are suitable for environmental purification applications.

It is shown that the dielectric and piezoelectric properties of Ba(Ti0.8Zr0.2)O-3-x(Ba0.7Ca0.3)TiO3 (x = 0.45) (BCTZ 45) epitaxial thin films have a nontrivial dependence on film thickness. BCTZ 45 epitaxial films with different thicknesses (up to 400 nm) have been deposited on SrTiO3 by pulsed laser deposition and investigated by different combined techniques: conventional and off-axis X-ray diffraction, high resolution transmission electron microscopy and dielectric and piezoforce microscopy. The changes occurring in epitaxial films when their thickness increases have been attributed to a partial relaxation of misfit strain, driving the induced tetragonal symmetry in very thin films to the original rhombohedral symmetry of the bulk material in the thickest film, which influences directly and indirectly the dielectric and piezoelectric properties.

We have analyzed by transmission electron microscopy silicon and GaAs crystals polished with sandpapers of different grain size. The surface damage induced a crystal permanent convex curvature with a radius of the order of a few meters. The curvature is due to a compressive strain generated in the damaged zone of the sample. Contrary to what was reported in the literature, the only defects detected by transmission electron microscopy were dislocations penetrating a few microns from the surface. Assuming the surface damage as a kind of continuous indentation, a simple model able to explain the observed compressive strain is given.