Co-existence of Bi with multiple valence states in zeolites – Controlling the optical properties by annealing atmosphere

We investigate the effect of annealing atmosphere on the optical properties of Bi doped zeolites by diffuse reflectance, steady state and time-resolved photoluminescence (PL), and PL excitation measurements. The results reveal that zeolites can be used as an excellent host material to stabilize multiple Bi centers (Bi3+, Bi2+, and Bi-related near-infrared (NIR) active centers) in the framework, which shows ultra-broadband emission from visible to NIR range. Annealing in N2 leads to the partial conversion of Bi3+ ions into Bi2+ and Bi-related NIR active centers. Our results demonstrate that the broadband NIR emission may be attributed to the electronic transition of Bi low valence state, rather than a higher valence state.

► Bi doped zeolites were annealed in air and N2, respectively. ► The optical properties are very sensitive to the annealing atmosphere. ► A tunable ultra-broadband light-source covering the visible and near-infrared range. ► Annealing in N2 leads to reduction of Bi3+ into Bi2+ and Bi-related NIR active centers. ► The NIR emission is attributed to the electronic transition of Bi low valence state.