Improved Conductivity Induced by Photodesorption in SnO2 Thin Films Grown by a Sol-Gel Dip Coating Technique

Thin films of undoped and Sb-doped SnO2 have been prepared by a sol-gel dip-coating technique. For the high doping level (2–3 mol% Sb) n-type degenerate conduction is expected, however, measurements of resistance as a function of temperature show that doped samples exhibit strong electron trapping, with capture levels at 39 and 81 meV. Heating in a vacuum and irradiation with UV monochromatic light (305 nm) improves the electrical characteristics, decreasing the carrier capture at low temperature. This suggests an oxygen related level, which can be eliminated by a photodesorption process. Absorption spectral dependence indicates an indirect bandgap transition with Eg 3.5 eV. Current-voltage characteristics indicate a thermionic emission mechanism through interfacial states.     

Investigation of New Ion Conducting Ormolytes Silica-Polypropyleneglycol

Two groups of hybrid organic-inorganic composites exhibiting ionic conduction properties, so called ORMOLYTES (organically modified electrolytes), have been prepared by the sol-gel process. The first group has been prepared from mixture of a lithium salt and 3-isocyanatopropyltriethoxysilane(IsoTrEOS),O,O-bis(2-aminopropyl)polypropyleneglycol. These materials produce chemical bonds between the organic (polymer) and the inorganic (silica) phases. The second group has been prepared by an ultrasonic method from a mixture of tetraethoxysilane (TEOS), polypropyleneglycol and a lithium salt. The organic and inorganic phases are not chemically bonded in these samples. The Li+ ionic conductivity, , of all these materials has been studied by AC impedance spectroscopy up to 100°C. Values of up to 10–6 –1 · cm–1 have been found at room temperature. A systematic study of the effects of lithium concentration, polymer chain length and the polymer to silica weight ratio on shows that there is a strong dependence of on the preparation conditions. The dynamic properties of the Li+ ion and the polymer chains as a function of temperature between –100 and 120°C were studied using 7Li solid-state NMR measurements. The ionic conductivity of both families are compared and particular attention is paid to the nature of the bonds between the organic and inorganic components.     

Formation of SnO2 Supported Porous Membranes

In this work, the effect of the substrate microstructure on the formation of SnO₂ membranes and of the sintering conditions on their porosity have been analysed. Samples have been prepared by colloidal suspensions cast on alumina or kaolin substrates. Supported membranes have been characterized by Hg porosimetry, MEV, XRD and N₂ adsorption-desorption isotherms. The results show that the narrower pore size distribution of alumina substrate allowed to prepare membranes more homogeneous and free of cracks than that supported on kaolin. The crystallite and pore sizes of the membranes could be controlled by adjusting the temperature of sintering, allowing materials with adequate microstructure with application for ultrafiltration process.     

Small angle X-ray scattering study of surface modified tin oxide nanoparticles prepared by sol-gel route

The surface properties of SnO₂ nanoparticles were modified by grafting ionic (Tiron®_, (OH)₂C₆H₂(SO₃Na)₂·H₂O) or non-ionic (Catechol®_, C₆H₄-1,2-(OH)₂) capping molecules during aqueous sol-gel processing to improve the redispersibility of powdered xerogel. The effect of the amount of grafted organic molecules on the redispersibility of powders in aqueous solution at several basic pH values was studied. The nanostructural features of the colloidal suspensions were analyzed by small angle X-ray scattering (SAXS) measurements. Irrespective of the nature and amount of grafted molecules, complete redispersion was obtained in aqueous solution at pH = 13. The redispersion at pH = 11 results in a mixture of dispersed primary particles and aggregates. The proportion of well dispersed nanoparticles and aggregates (and their average size) can be tuned by the quantity of grafted ionic molecules.     

Short Range Order Evolution in the Preparation of SnO2 Based Materials

The evolution of Eu³⁺ doped SnO₂ xerogels to the cassiterite structure observed during sintering was studied by means of Eu³⁺ spectroscopy, XRD and EXAFS at the Sn K-edge. Eu³⁺ ions adsorbed at the surface of colloidal particles present a broad distribution of sites, typical of oxide glasses. With sintering at 300°C, this distribution is still broadened. Crystallization is clearly observed by the three techniques with increasing sintering temperature. It is found that the addition of Eu³⁺ limits the crystallite growth.     

Zirconia foams prepared by integration of the sol–gel method and dual soft template techniques

This paper presents a new method to produce basic zirconium sulfate foams showing hierarchic porosity, based on a dual pores templating process by oil drops and gas bubbles dispersed into a hydrosol, followed by its fast gelation to form the porous patterned inorganic network. As revealed by mercury porosimetry, the hierarchical structures of final inorganic foams are composed by large gas bubble templated macro-pores (modal size 7–33 μm) and emulsion-templated supermeso-pores of modal size tunable around 3 and 0.4 μm. The relative population and modal pore size of each family and the overall porosity of the final inorganic foam can be varied by adjusting the emulsification and air–liquid foaming stirring speed, and the oil/sol ratio.     

Chitosan/(ureasil–PEO hybrid) blend for drug delivery

The effect of chitosan addition on the structure and drug release properties of ureasil–polyethylene oxide (U–PEO) hybrid materials was examined. The hybrids and the blends were prepared by the sol–gel route and their structural features were characterized by infrared spectroscopy, differential scanning calorimetry, X-ray powder diffraction, and small angle X-ray scattering. The different characterization techniques revealed the formation of transparent, rubbery, flexible, water-insoluble, and low crystalline chitosan/(U–PEO hybrid) blends, which could easily dissolve pramoxine (used as a model drug). The results showed that the combined addition of chitosan and pramoxine improved the compatibility of the organic and inorganic components of the U–PEO matrix, and provided a means of tailoring the amount of drug released at steady state equilibrium.     

Study of Hybrid Silica-Polyethyleneglycol Xerogels by Eu3+ Luminescence Spectroscopy

Good optical quality Eu3+-doped silica-polyethyleneglycol hybrids were prepared by the sol-gel process. Thermomechanical analysis showed an increase of the glass transition temperature, due to the stiffness of the polymeric network, as the amount of Eu3+ increased. Europium luminescent properties were used to study structural evolution during the sol-gel transition. For lower doping concentrations dried gels present statistical distributions of Eu3+, typical of an amorphous environment, while for higher concentrations a crystalline-like environment of Eu3+ was observed. A broad emission band was observed in the visible part of the electromagnetic spectrum and assigned to the intrinsic emission from the hybrid polymeric network.     

Stimuli-responsive controlled growth of mono- and bidimensional particles from basic zirconium sulfate hydrosols

A thermostimulated sol-gel transition in a system prepared by mixing a ZrOCl(2) acidified solution to a hot H(2)SO(4) aqueous solution was studied by dynamic rheological measurements and quasi-elastic light scattering. The effect of temperature and of molar ratio R(S) = [Zr]/[SO(4)] on the gelation kinetics was analyzed using the mass fractal aggregate growth model. This study shows that the linear growth of aggregates occurs at the early period of transformation, while bidimensional growth occurs at the advanced stage. The bidimensional growth can be shifted toward monodimensional growth by decreasing the aggregation rate by controlling the temperature and/or molar ratio R(S). EXAFS and Raman results gave evidence that the linear chain growth is supported by covalent sulfate bonding between primary building blocks. At the advanced stage of aggregation, the assembly of linear chains through hydrogen bonding gave rise to the growth of bidimensional particles.     

Controlling the growth of zirconia needles precursor from a liquid crystal template

This work reports the directional growth of macroscopic ceramic needles by combining a thermo-responsive sulfated hydroxy zirconyl (SHZ) hydrosol with a swollen hexagonal liquid crystal (SHLC). The effect of the molar ratio R_s = [Zr⁴⁺]/[SO₄^2?] on the thermo-stability of both the SHZ hydrosol and the SHLC, and also on the nucleation and 1D-growth of ceramic particles was investigated. The dynamics of the thermo-induced aggregation growth process was monitored in situ by rheological property measurements. The results show that by increasing the R_s, the kinetic stability of the system in the sol state increases, leading to the formation of a less branched framework during the thermo-induced aggregation process. Furthermore, the small-angle X-ray diffraction (XRD) results show that the thermo-stability of the SHLC employed as soft template is improved by increasing the R_s. Both effects allow us to induce the particle's nucleation by heating the SHLC template to a moderate temperature (≈50 °C), while the kinetics of the needle's growth is enhanced by increasing the [Zr⁴⁺]/[SO₄^2?] molar ratio.