The modification of the surface reception properties of nanocrystalline structures is of great interest in environmental, catalysis and energy related applications. For instance, an oxide surface covered with a layer of another oxide opens the possibility of creating the nanosized counterparts of bulk catalytic systems. A relevant example is the TiO2-WO3, which is an active catalysts in a broad range of reactions. The chemical synthesis of the colloidal, nanocrystalline version of such system will first be exposed, by coupling suitable sol-gel chemistry with solvothermal processing. Then, the range of obtained structures will be discussed, ranging from WOx-surface modified TiO2 to TiO2-WO3 heterojunctions. The complex structural evolution of the materials will be discussed, depending on the W concentration. A summary of the acetone sensing properties of these systems will be shown. In particular, the surface activation of the otherwise almost inactive pure TiO2 by surface deposition of WO3-like layers will be highlighted. Addition of the smallest W concentration boosted the sensor response to values comparable to those of pure WO3, ranging over 2-3 orders of magnitude of conductance variation in presence of ethanol or acetone gases. Simple analysis of the sensing data will evidence that the combination of such nanocrystalline oxides results in catalytic activation effects, with exactly opposite trend, with respect to pure TiO2, of the activation energies and best responses.
We present a structural, magnetic and magneto-optical (MO) study of Co nanoparticles sputter-deposited at different temperatures and embedded in three different matrices (two insulators such as MgO and AlN and a metal such as Pt). MgO capping layer does not affect the magnetism of the nanoparticles as demonstrated by in situ transversal and ex situ polar Kerr loops. The structure of the nanoparticles was investigated by TEM and a Co crystalline core surrounded by an amorphous crust was observed. From the analysis of the MO spectral response of the nanoparticles we demonstrate that the evolution of the MO constants as a function of Co concentration can be explained with the Maxwell-Garnett model. It is also observed that the reduction of nanoparticles size gives rise to a decrease of the relaxation time of the electrons into them. The deposition of Pt capping gives rise to the magnetic connection of the islands mediated by the polarised Pt, with the formation of different Co-Pt compounds as was observed with TEM. We observe that in the case of AlN capping destroys the magnetism of the samples due to a strong nitridation of Co.
Due to their simple implementation, low cost and good reliability for real-time control systems, semiconductor gas sensors offer good advantages with respect to other gas sensor devices. As gas adsorption is a surface effect, one of the most important parameter to tailor the sensitivity of the sensor material is to increase the surface area. For these propose, mesoporous oxides have been synthesized. Nanostructured mesoporous materials present a large and controllable pore size and high surface are. For the preparation of ordered nanostructure arrays, a hard template method has been used. This method presents some advantages when compared with a soft template method, especially in its specific topological stability, veracity, predictability and controllability. Moreover, with this hard template method we can obtain crystalline mesoporous oxides, with small particle size and high surface area. We have used SBA-15 (two-dimensional hexagonal structure) and KIT-6 (three-dimensional cubic structure) as a template for the synthesis of different crystalline mesoporous WO3 with a particle size about 8-10 nm and high surface area. Low angle XRD spectra show a high order mesoporous structure, without rests of silica template. TEM confirms that the silica host has been completely removed; therefore, the nanowires constitute a self-supported superlattice. HRTEM studies have been focused on the detailed structural characterization of these materials. Electrical characterization of the sensor response in front of NO2 has been performed. Some catalytic additives have been also introduced, in order to increase the sensitivity of the material.