ABSTRACT
The optical response of europium ions in the parent (non-silylated) and silylated microporous-mesoporous Zeogrid materials was investigated in detail in relation to Zeogrid structure. All materials were characterized using nitrogen adsorption isotherms, powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry, and time-resolved photoluminescence spectroscopy. A two europium species distribution with distinct luminescence spectra and lifetimes was found for both parent and silylated Zeogrid. In the parent Zeogrid, the short-lived europium species is characterized by the intensity ratio R=I(5D0-(7)F2)/I(5D0-(7)F1) or asymmetry values of approximately 0.4-0.7 and photoluminescence (PL) lifetimes of 110-125 micros and therefore is assigned to an almost fully hydrated europium species. In the silylated Zeogrid, the short-lived europium species is characterized by asymmetry values of 1.0-2.4 and lifetimes of 160-180 micros suggesting a relatively distorted europium environment. The long-lived europium species exhibits similar asymmetry ratios in the parent and silylated Zeogrid, which vary between 5.0 and 6.2 with increasing Si to Al ratio from 25 to 150 and slightly different PL lifetimes. The mechanism responsible for the intensity of the electric and magnetic forbidden 5D0-(7)F0 transition was determined to be J-mixing of the 7F2 into the 7F0 state through the axial second-order crystal-field potential. The comparison between the photoluminescence properties of europium in the parent and silylated Zeogrid demonstrates that the effects of rehydration were strongly suppressed following silylation.
