Species-related luminescence-structure relationships in europium-exchanged mesoporous material

Abstract

Europium exchanged into a mesoporous material (Zeotile-1) was extensively characterized with respect to the Si/Al ratio and surface silylation by using time-resolved emission spectroscopy. Qualitative as well as quantitative details of the europium species-related luminescence-structure relationships were obtained and discussed such as the decay associated spectra, local distortion and structure of the bonding environment, crystal-field strength, radiative relaxation rates, and the quantum efficiency. Thus, two europium species were found in the parent as well as in the silylated materials: one species located on the internal surface and the second inside the 2–2.5 nm pores. The species located on the internal surface is characterized by photoluminescence decay times of 105 μs<τ<125 μs, an asymmetry value R of 0.6<R<0.8, and a quantum efficiency of 2%–2.5%. Upon silylation, the photoluminescence decay times, the asymmetry values, and the quantum efficiency were increased to 160 μs<τ<180 μs, 1.7<R<2.1, and ∼4%, respectively. Following silylation, the number of water molecules is reduced in the first coordination shell of the europium species located on the internal surface from eight to nine to about five. On the other hand, the europium species located inside the pores showed a much longer photoluminescence decay time (460 μs<τ<560 μs) and a much higher asymmetry ratio (5<R<6.5). The related photoluminescence efficiency was 26%–30%. An average of one up to two water molecules in the first coordination shell of the europium species located inside the pores was estimated for both parent and silylated materials.