Raman and optical absorption spectra of oriented Se8 and Se12 rings formed in zeolites: Dependence on the Se loading density

Abstract

We encapsulated selenium (Se) into AFI and LTA zeolites via Se vapour adsorption at ∼450 °C and studied Raman and optical absorption spectra (RS and OAS, respectively) of zeolite single crystals with incorporated Se. At high Se loading density ( D ), Se 8 rings are formed in the AFI channels along with Se chains. The rings are oriented by their 4-fold axis parallel to channels. Se 8 is also formed in the LTA large cavities with its 4-fold axis oriented parallel to the 4-fold axis of LTA at any D up to ∼8 Se atoms per cavity (at/c). However, at D > 0.5 at/c, nearly unexplored Se 12 rings oriented by their 3-fold axis parallel to the LTA 3-fold axis are formed along with Se 8 in neighboring cavities. We discriminate between Se 8 and Se 12 rings via 1) RS frequencies; 2) RS and OAS band intensities vs. D ; 3) RS band intensities vs. excitation wavelength and light polarization configuration. With an increase in D , the number of Se 12 rings grows faster than that of Se 8 . We extract OAS of Se 8 and Se 12 from the experimental AFI-Se and LTA-Se OAS and show that the 3.0–3.3 eV absorption of Se 12 is enhanced compared to that of Se 8 . This absorption is responsible for the observed resonant Raman effect of Se 12 at the 514.5 nm wavelength excitation. Both LTA-confined Se 8 and Se 12 display rather high thermal stability. • Se 8 rings co-exist with Se chains and dominate over Se 6 rings in high- D AFI. • Se 8 is formed in LTA at any D while Se 12 appear at D > 0.5 Se atoms per cavity. • Se 12 optical absorption spectrum is obtained for the first time. • Se 12 3.0–3.3 eV absorption causes resonant Raman effect at 514.5 nm excitation. • Se 8 and Se 12 orientations are determined and their thermal stability is shown.