Sol-gel processes for protection and synthesis of luminescent materials

Sol-gel chemistry has a very broad application area such as thin films and coatings, monoliths, powders, grains and spheres, fibers, composites, porous gels and membranes. From thin film to powder, different kind of materials with excellent control of stoichiometry, density and microstructure can be synthesized using simple equipment without the need for vacuum at relatively low temperatures. Using sol-gel, material properties can be modified by changing only one parameter during the preparation. Rare earth doped alkaline earth binary and ternary sulfides have a very special place in luminescent materials because of their relatively low synthesis temperature and broad emission spectra upon doping with europium and cerium. Particularly to obtain red or orange emission CaS:Eu2+ and SrS:Eu2+ are considered suitable candidates. Ca1−xSrxS:Eu2+ phosphors with a strong absorption in the blue region are currently also used in white-light emitting diodes. In addition to wavelength converters in LEDs, alkaline earth sulfide phosphors are employed in different areas such as display applications, electroluminescent devices and optical information storage. Nevertheless, the lack of stability with respect to water and other atmospheric components hinders their usage as phosphor hosts. A number of encapsulation techniques have been utilized to improve the stability of sulfide phosphors and sol-gel is the one of the most attractive techniques. This work focuses on thin films, protection coatings, powders and luminescent materials prepared using sol-gel chemistry. TiO2 and Al2O3 powders, thin films and protection layers were synthesized via water free sol-gel. In addition to the optical and structural properties of the products, the effect of the coating on the stability of sulfide particles was investigated. Al2O3 synthesized via sol-gel was also compared with Al2O3 synthesized via ALD (atomic layer deposition) as protection layer. Lastly, undoped and persistent luminescent Eu and Nd-doped CaAl2O4 powders were synthesized via water free sol-gel technique. Effect of the calcinations temperature and the doping concentration on the structure and photoluminescence properties was investigated

Optical and structural properties of aluminium oxide thin films prepared by a non-aqueous sol-gel technique

Clear aluminium oxide sols without precipitation were synthesized via a non-aqueous sol-gel technique using three different alcohols (ethanol, isopropanol and n-butyl alcohol) as solvent, aluminium sec-butoxide as a precursor and acetyl acetone as a chelating agent. Although all sols could be successfully used to prepare thin films, the most stable one was prepared with n-butyl alcohol. Highly transparent, homogenous and amorphous aluminium oxide thin films were obtained on Si substrates after a heat treatment at 500 A degrees C. X-Ray photoelectron spectroscopy (XPS) and Fourier transform infrared absorption (FT-IR) spectroscopy revealed all films were hydroxide free. The optical and structural properties of the films were particularly investigated. Any significant difference except from thickness on the film properties was not observed by changing the alcohol. Refractive index was used as an indication of the porosity of the films and ranged from 1.54 to 1.60

Sol-gel preparation of pure and doped TiO₂ films for the photocatalytic oxidation of ethanol in air

Stable sols of TiO2 were synthesized by a non-aqueous sol-gel process using titanium (IV) isopropoxide as precursor. The microstructure, optical and morphological properties of the films obtained by spin-coating from the sol, and annealed at different temperatures, were investigated using scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy and ellipsometry. The crystalline structure of the films was characterized by X-ray diffraction and their photocatalytic activity was evaluated for the oxidation of ethanol in air. The influence of the calcination temperature, pre-heat treatment and the number of layers was studied. Simultaneous thermo-gravimetric and differential thermal analysis measurements were carried out to ascertain the thermal decomposition behavior of the precursors. In order to obtain a higher photoresponse in the visible region, a series of vanadium-, niobium- and tantalum-doped TiO2 catalysts was synthesized by the same sol-gel method. For V doping two different precursors, a vanadium alkoxide and V2O5, were used. The effect on the crystallization and photocatalytic activity of the doped TiO2 films was investigated. Furthermore, to identify the effective composition of the samples, they were characterized by X-ray photoelectron spectroscopy and the surface area of the powders was measured by N-2 adsorption. The 10 wt.% doped catalysts exhibit high photocatalytic activity under visible light and among them the best performance was obtained for the sample containing Ta as dopant. The crystallite sizes are closely related to the photocatalytic activity