Search

Abstract: The complete understanding of the properties of a material goes in close relationship with a correct description of their structure. This is particularly true for mesoporous silica that exhibits different shapes and sizes of the porous framework as well as different degrees of crosslinking of the silica network itself. These variations can affect drastically the specific properties of these materials as well as their hydrothermal resistance. Hydrothermal resistance and adsorption properties are governed by the surface structure, but the later parameter is not frequently studied because it can be difficult to characterize. Therefore a simple parameter such as surface roughness could be helpful in relating the properties of materials with their surface structure. The direct determination of surface roughness is impossible, but it has been demonstrated that a single mathematical parameter, the surface fractal dimension Ds, can be related within a certain limit with surface roughness. Here we demonstrate using nitrogen adsorption data that a correlation can be drawn between the values of the surface fractal dimension Ds used as a gauge of the surface roughness and the structural evolution of a series of mesoporous silicas. Among the different adsorption models for obtaining Ds that are available nowadays, we selected the one proposed by Avnir and Jaroniec. We demonstrate in this report that there exists a real correlation between the evolution of Ds, when applied to a series of samples prepared under different synthesis conditions, with the evolution of their structure and properties. By example, this study allowed us to link the range of hydrothermal resistance of different mesoporous silicas (e.g. the HMS, MSU, MCM, SBA families of silicas) with their surface roughness. Moreover, we found that HMS silica, a very resistant material toward hydrothermal treatment, exhibits actually the lowest surface roughness. We could demonstrate too how different synthesis parameters such as temperature, can modify this surface roughness, hence the final properties of the mesoporous silica. Finally, we show that this method can even be applied to water ellipsoporometry and help to follow the crystallization of thin layers of metal oxide. Bearing in mind that Ds is a parameter that can readily be determined using the existing software – based on different models – associated with most commercial adsorption equipment, we urge the broader use of the Ds parameter for the structural characterization of surface roughness in mesoporous materials and metal oxide thin layers. [Copyright &y& Elsevier]

Abstract: This paper reports the preparation of meso-structured rare earth (Eu3+, Sm3+ and Er3+)-doped Y2O3 films using the evaporation induced self-assembly (EISA). Thin films with ∼30% of porous volume and pore diameter of 9nm, stability up to 680°C were prepared using a specific polyethylene oxide based copolymer structuring agent. X-ray diffraction analysis was performed to study the crystallization of the inorganic network and the porosity. Time-resolved small angle X-ray scattering (SAXS), atomic force microscopy (AFM) and environmental ellipsometry porosimetry (EEP) techniques were used to complete the characterization of the films. The luminescent properties of these layers, i.e. fluorescence spectra and decay profiles, were investigated, showing the optical features characteristic of these lanthanide ions incorporated in the Y2O3 lattice. [Copyright &y& Elsevier]

Abstract: The thermal stability of anatase TiO2 mesoporous thin films has been considerably increased up to 800°C by substituting a fraction of Ti cations by Nb. Mesoporous mixed oxide films Ti1−x Nb x O2, with x =Nb/(Nb+Ti) ranging from 0 to 1, and homogeneous crystalline walls have been successfully prepared from chloride precursors and non-ionic block copolymer templates. They all exhibit a uniaxially distorted Im3m bcc mesostructure with the a lattice parameter ranging from 14 to 19nm. Films have been characterised by simultaneous in situ SAXS and WAXS investigations during thermal treatment. Transmission electron microscopy and UV–visible ellipsometry were used to confirm the structure, the porosity and the good optical quality. A high porosity of 40% (pore size estimated at 7nm) was maintained for the 10% Nb-doped films above the temperature of crystallisation and a bandgap of 3.40eV has been measured after annealing at 500°C. [Copyright &y& Elsevier]