Ce, Eu incorporation through doping of ALD-ZnO thin films for enhancing their photoluminescent properties.

Nanostructured ZnO nanoarrays deposited on silicon oriented substrates is a very promising area in the study of the control of physicochemical properties, in which photoluminescence plays a crucial role. This optical property inherent to ZnO, can be favorably modified through the inclusion of doping elements, with the purpose of appropriately modifying their optical absorption and luminescence. Following this objective, in the present work we present the development of Zn(1-x-y)Ce(x)Eu(y)O nanostructured thin films. The samples were produced in two steps process by atomic layer deposition technique followed by a solvothermal synthesis. The purpose of Cerium and Europium incorporation into the ZnO compound is to enhance the photoluminescence in ZnO thin films. In a first stage textured thin films were obtained from diethylzinc at a temperature of 190 °C and a pressure of 3.29 × 10-4 atm, on silicon substrates (111). Subsequently, the perpendicular growth of nanostructures was induced under a solvothermal process, where Zn(NO3)2 was used as Zn precursor and hexamethylene-tetramine operating as a dual-ligand to promote the linking of Zn2+ ions. The growth of cerium-europium ZnO nanostructures was promoted with Ce(C2H3O2)3·H2O and Eu(NO3)3·5H2O. The obtained Zn(1-x-y)Ce(x)Eu(y)O nanostructured thin films, were examined through SEM-microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and photoluminescence studies. The attained results show that it is feasible to produce Ce-Eu-doped ZnO nanostructures with tailored photoluminescence and crystal size. Interestingly the Ce-Eu doping induces a strong shift in comparison to the typical UV emission of ZnO; an effect that can be related with the increase of lattice defects in ZnO.

Reversible Self-Assembly (fcc-bct) Crystallization of Confined Granular Spheres via a Shear Dimensionality Mechanism.

By combining vibrational annealing and shear dimensionality, we experimentally show (1) a fast reversible crystallization fcc-bct (face-centered cubic-body-centered tetragonal) in a granular system that is composed of dissipative millimeter-sized dry spheres, (2) a two-dimensional (planar) shear promotes self-assembly into an fcc crystal, while one-dimensional shear produces a bct crystal, and (3) in analogy with heterogeneous nucleation, a granular temperature gradient leads to the formation of crystal domains showing competition of polymorphic phases in the cold regions. Our findings suggest that controlling the directionality of the interactions steers to reversible crystallization of hard spheres, adds clues for theoretical studies, and provides a novel mechanism for the technological development of the applications of self-assembling phononic crystals.

"Tailoring the TiO2 phases through microwave-assisted hydrothermal synthesis: Comparative assessment of bactericidal activity".

Nanocrystalline titania (TiO2) is one of the most investigated crystalline nanostructured systems in the field of materials science. The technological applications of this material are related to its optoelectronic and photocatalytic properties, which in turn are strongly dependent on the crystal phase (i.e., anatase, brookite, and rutile), particle size, and surface structure. However, systematic comparative studies of all its crystal phases are scarce in literature due to difficulties in providing a controlled synthesis, which is primarily important in obtaining the brookite phase. In this report, the synthesis of TiO2 nanoparticles in the anatase, brookite, and rutile structures was explored, using amorphous TiO2 as a common precursor under microwave-assisted hydrothermal conditions. The influence of parameters such as temperature, acidity, and precursor concentration on phase crystallization were investigated. The TiO2 materials (amorphous and crystalline phases as well as commercial Degussa P25) were systematically characterized using Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-visible reflectance spectroscopy, and dynamic and electrophoretic light scattering. The bactericidal activity and photocatalytic antibacterial effectiveness of each material were evaluated through the determination of the minimum inhibitory and bactericidal concentrations, and via the mortality kinetic method under ultraviolet (UV) illumination under similar conditions with two bacterial groups of unique cellular structures: Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus). The results are discussed with particular emphasis on the relationship between the synthesis parameters (acidity, precursor concentration, temperature and reaction time) and the bactericidal properties.

Photocatalytic performance of nitrogen doped ZnO structures supported on graphene oxide for MB degradation.

In the present work, the photocatalytic efficiency of a novel system based on ZnO doped with nitrogen (ZT) and supported on graphene oxide (GO) is investigated. ZnO synthesis and their N doping were carried out in a microwave reactor using thiourea as nitrogen source, while the GO was prepared through a variation of the Hummers' method. Structural, morphological and photochemical characterization of the developed material was performed by X-ray diffraction (XRD), UV-Vis spectroscopy, energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), analysis by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The compounds were used to photodegrade the methylene blue molecule, which confirms the efficiency of nitrogen doped supported system compared to pristine ZnO. The degradation percentage of MB under UV energy using nitrogen-doped ZnO/GO, in a time of 35 min, reached 98% degradation; while using visible light 93% of degradation was reached.

Direct interaction of colloidal nanostructured ZnO and SnO2 with NO and SO2.

A novel and easy synthesis pathway of small SnO2 nanoparticles is reported. The method consists of the spontaneous hydrolysis of SnCl4 x 5H2O in dimethyl sulfoxide (DMSO), containing 3% water, at room temperature. The structure of the SnO2 nanocrystals corresponds to that of the cassiterite phase, as shown by powder X-ray diffraction and HR-TEM. The UV-visible electronic absorption and emission spectra of the SnO2 colloids are discussed. The reactions of NO(g) and SO2(g) with ZnO (wurtzite phase) and SnO2 nanocolloids are studied. The interaction of NO with ZnO nanoparticles generates the dissolution of the particles and it is quite probable that NO3(-1), NOs(-2), N2O and N2 are formed; while its contact with SO2 probably yields SO4(-2), SO3(-2) and also the dissolution of the particles is observed. When these gases are reacted with SnO2, then NO3(-1), NO2(-1), SO3(-2) and SO4(-2), were respectively obtained.

Optical absorbance of colloidal suspensions of silver polyhedral nanoparticles.

The optical absorption of colloidal suspensions made of silver nanoparticles with polyhedral shapes is studied experimentally and theoretically. The influence of the shape on the optical response is investigated by comparing the measured absorbance with theoretical results for icosahedral, decahedral, and cuboctahedral silver nanoparticles. The theoretical spectra are obtained within the discrete dipole approximation. We find that colloidal suspensions of silver nanoparticles with a small dispersion of size distribution show very few structural shapes.

The nucleation kinetics of ZnO nanoparticles from ZnCl2 in ethanol solutions.

The first stages of the synthesis of ZnO nanoparticles by forced hydrolysis of ZnCl2 with NaOH and water in ethanol have been investigated using UV-Vis spectrophotometry. At sufficiently low water concentrations, focusing of the nanoparticle size distribution was observed during the nucleation and growth phase, followed by a defocusing phase when coarsening becomes significant. During nucleation and growth, only the smaller particles grow while the larger particles have an essentially zero growth rate, indicating that the growth rate decreases rapidly with particle size. As the average particle size remains nearly constant in this regime, the absorbance increase with time can be used to determine the nucleation rate. The nucleation rate was found to depend on both the water concentration and the reactant concentrations. The results are discussed in terms of a mechanism where water determines the precursor formation kinetics thus controlling the nucleation rate.

Phase-pure TiO(2) nanoparticles: anatase, brookite and rutile.

We report on the synthesis of phase-pure TiO(2) nanoparticles in anatase, rutile and brookite structures, using amorphous titania as a common starting material. Phase formation was achieved by hydrothermal treatment at elevated temperatures with the appropriate reactants. Anatase nanoparticles were obtained using acetic acid, while phase-pure rutile and brookite nanoparticles were obtained with hydrochloric acid at a different concentration. The nanomaterials were characterized using x-ray diffraction, UV-visible reflectance spectroscopy, dynamic light scattering, and transmission electron microscopy. We propose that anatase formation is dominated by surface energy effects, and that rutile and brookite formation follows a dissolution-precipitation mechanism, where chains of sixfold-coordinated titanium complexes arrange into different crystal structures depending on the reactant chemistry. The particle growth kinetics under hydrothermal conditions are determined by coarsening and aggregation-recrystallization processes, allowing control over the average nanoparticle size.