Thermoanalysis and emission properties of Eu3+/Eu2+ in Eu3+-doped xerogels

A series of Eu3+-doped silica sol-gels are prepared under various condition s: (1) pH values of the sols are controlled at 3.0, 5.5, and 7.5; (2) the c ounterions of dopants are selected from acetate, chloride, and nitrate of e uropium (III) compounds; (3) two chelating agents, namely ethylenediaminete traacetic acid (EDTA) and ethylenediaminetriacetic (HEDTA) are used for com plexing the europium (III) dopants; (4) a sol-gel matrix containing low O-H functionality is synthesized by using deuterated solvents (D2O and C2D5OD) and under an extremely dry N-2 environment; and (5) a mixture of 1% of alu minum or antimony alkoxide and 99% of silicon alkoxide is adopted as precur sors. The conditions that these differences have on the network structures of a gel matrix are examined in order to determine the optimal conditions f or the creation of structural defects in an SiO2 network, generation of ele ctron-hole centers and utilization of them to reduce EU3+ to EU2+ during th e sol-gel processing. Both differential scanning calorimetric (DSC) measure ments and thermogravimetric and differential thermal analysis (TG/DTA) are employed to illustrate which gel samples are the most liquidlike and have t he greatest cross-linking density. The results from thermoanalysis are then correlated to the emission intensity and lifetimes of each Eu3+-doped samp le. The relative emission intensity of Eu2+/EU3+ gives the degree of conver sion of EU3+ to EU2+ which is produced from the defect electron-hole pair g eneration. The absolute emission intensity of EU3+ and EU2+ is strongly que nched by the presence of OH groups in xerogels and is shown to be enhanced by laser irradiation due to water evaporation. The results show that a basi c gel prepared by the mixed metal alkoxides most efficiently converts EU3to Eu2+ because or its liquidlike nature, reduced cross-linking density, an d low OH quenching.