Comparative study of core-shell nanostructures based on amino-functionalized Fe-4@SiO2 and CoFe2O4@SiO2 nanocomposites

Fe3O4@SiO2 and CoFe2O4@SiO2 and their corresponding amino-functionalized nanocomposites were successfully synthesized by a process of two steps including the preparation by coprecipitation or hydrothermal synthesis of the corresponding magnetic cores, the coating of its surface with a silica coating followed by its subsequent functionalization with 3-aminopropyltriethoxysilane (APTES). All magnetic samples were characterized by XRD using FULPROFF program, FTIR analysis, TEM and M-H hysteresis loops. The results showed diffraction maxima indexed in a cubic symmetry of S. G. Fd-3m with Z = 8 compatible with an inverse spinel-type structure. FTIR spectra of all samples show the characteristic bands of the magnetic cores and others bands corresponding to the asymmetric vibration of O-Si-O and Si-O-Si bonds of silica. The TEM images confirm that all the nanoparticles are coated, finding the largest thickness of the coating in the Fe3O4 sample prepared hydrothermally, which are the smaller ones. An expected reduction of the saturation magnetization of the magnetic cores is achieved with the coating and functionalization, although the behaviour of the Fe3O4 -samples remains practically superparamagnetic while the corresponding ones of cobalt are still ferrimagnetic. Fe3O4 nanocomposites respond to more quickly in the presence of an external magnetic field, something important against the removal of contaminating species in aqueous media. UV-Vis spectroscopy studies confirm the adsorption capacity of Cu2+ in aqueous solutions of the prepared nanocomposites, having found that a small thickness of the coating leads a greater adsorption, so that the best adsorption is found for CPFe3O4@SiO2-APTES nanocomposite. (C) 2018 Elsevier B.V. All rights reserved

Spark plasma versus conventional sintering in the electrical properties of Nasicon-type materials

Li_(1+x)M_(x)Ti_(2−x)(PO_(4))_(3) powders with x = 0 and 0.3 and M = Al, Cr and Fe have been sintered by conventional sintering (CS) and Spark Plasma Sintering (SPS), and the electrical properties have been compared. The use of SPS allows preparing samples with higher density at lower temperature and shorter time than the CS, avoiding segregation of secondary phases and with reduced crystallite size. The introduction of aluminum, chromium and iron in the LiTi_(2)(PO_(4))_(3) (LTP) clearly enhances ionic conductivity even if the samples have similar densities. Despite the different level of density reached with CS and SPS, the activation energies of dc and grain boundary contributions are very similar and the differences in ionic conductivity are determined by pre-exponential factors. The samples produced by SPS showed a well-defined grain boundary meaning a more homogenous electrical contact

Synthesis of Li_(1+x)M^(III)_(x)Ti_(2-x)(PO_(4))_(3) with nasicon structure, using sol-gel methods. Study of the relationship microstructure-electrical properties

© Sociedad Española de Cerámica y Vidrio. National Meeting on Electroceramics (9. 2009. Madrid). Este trabajo ha sido realizado gracias al proyecto PR34/07-15895 BSCH-UCM.Haciendo uso de la química sol-gel, se han preparado ortofosfatos de composición LiTi_(2)(PO_(4))_(3) y Li_(1.05)(Cr/Fe)_(0.05)Ti_(1.95)(PO_(4))_(3) a temperaturas moderadas mediante el método Pechini. Estas fases han sido caracterizadas estructural y microestructuralmente por difracción de rayos X de polvo y microscopía electrónica de barrido (SEM), encontrándose que todas cristalizan en una estructura tipo NASICON, con parámetros de red muy similares. El dopaje con Fe y Cr permite aumentar la densidad de las muestras en la sinterización, mejorando de forma apreciable su conductividad iónica. Se ha observado un incremento de hasta cuatro órdenes de magnitud en la conductividad a temperatura ambiente obteniéndose una energía de activación de 0.29 eV para el material dopado con Cr.Compounds of formula Li_(1+x)M^(III)_(x)Ti_(2-x)(PO_(4))_(3) with M^(III)=Cr,Fe and x=0 and 0.05 have been prepared at soft temperatures using the Pechini synthesis method, based on sol-gel chemistry. The structural and microstructural characterization by X-ray diffraction and Scanning Electron Microscopy (SEM), shows that all of them crystallize in a NASICON-type structure with similar lattice parameters. Doping with Fe and Cr, causes an increase of the density of the samples after sinterization what clearly improves the ionic conductivity of the original material, LiTi_(2)(PO_(4))_(3) until values of 9x10^(-4) S cm^(-1) at room temperature in the chromium-doped material.Depto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEBSCH-UCMpu

Superparamagnetic and light-emitting bifunctional nanocomposites of iron oxide and erbium or thulium doped yttrium orthovanadate

Advances in biomedical research have increased interest in obtaining and studying new bifunctional materials for use in theragnostic. Here we describe in detail the preparation of new magnetic-fluorescent bifunctional (Y0.9Ln0.1VO4/Fe3O4)@SiO2 and [(Y0.9Ln0.1VO4 @SiO2)/Fe3O4)@SiO2] nanocomposites with Ln = Er or Tm. In addition, their magnetic and optical properties were carefully analyzed. The influence of Fe3O4 content and the silica shell thickness on the fluorescent emission in the VIS-NIR region of Y0.9Ln0.1VO4 cores was evaluated as well as their use as display systems with the possibility of directing them by means of external magnetic fields. Samples were prepared using wet chemistry methods involving low temperatures and short reaction times. Y0.9Ln0.1VO4 samples that are not easily oxidizable were prepared by a hydrothermal method, while Fe3O4 sample was synthesized by a coprecipitation process in which the mixture of precursors was treated at very low temperature to avoid oxidation. The powder amalgamation of both Y0.9Ln0.1VO4 and Fe3O4 samples was possible due to the silica polymeric network synthetized by a modified Stöber method. The purity of all samples was ensured by XRD and FTIR techniques. Diffraction profiles of Y0.9Ln0.1VO4 samples show diffraction maxima that can be indexed to a tetragonal symmetry of space group I41/amd, compatible with the zircon structure-type of YVO4 host. All reflections present in the diffraction profile of Fe3O4 sample can be indexed to a cubic symmetry of space group Fd3̅m, characteristic of an inverse spinel structure-type. The amorphous silica incorporation on the samples was also evaluated by TEM images. Studies of the magnetic behavior and luminescent emission intensity of the investigated samples showed their dependence on both, the silica coating thickness, and the contact or not between the luminescent samples and the magnetic powder