NMR, ESR, and Luminescence Characterization of Bismuth Embedded Zeolites Y

Abstract

Thermal treatment of bismuth-embedded zeolite Y yields luminescent Bi⁺ substructures without the formation of metallic nanoparticles. The structural and photophysical features of the resulting zeolite Y have been thoroughly characterized by using extensive experimental techniques including nuclear magnetic resonance (NMR), electron spin resonance (ESR), 2-dimentional excitation–emission and absorption spectra. NMR and ESR results indicate that some Al and oxygen are expelled from the zeolite Y framework after undergoing thermal treatment. The detailed analyses of luminescence and absorption spectra, coupled with TDDFT calculations, suggest that all Bi⁺ substructures (i.e., Bi₄⁴⁺, Bi₃³⁺, Bi₂²⁺, and Bi⁺) are optically active in the near-infrared (NIR) spectral range. It is found that Bi⁺, Bi₂²⁺, Bi₃³⁺, and Bi₄⁴⁺ units result in NIR emissions peaking at ca. 1050, 1135, 1145, and 1240/1285 nm, respectively. The emission lineshapes under diverse excitation wavelengths greatly depend on the Bi concentration and annealing temperature, as a result of the change in the relative concentration and the spatial distribution, as well as local structural features of Bi active species. Specifically, the above analyses imply that the reducing agents for Bi³⁺ are water molecules as well as framework oxygen. These findings represent an important contribution to the understanding of the processes involved in the formation of Bi⁺ and of the luminescence mechanisms of Bi⁺ substructures in zeolite Y frameworks, which are not only helpful for the in-depth understanding of experimentally observed photophysical properties in other Bi-doped materials but also important for the development of novel photonic material systems activated by other p-block elements