Visible and Near-Infrared Emission in Ba₃Sc₄O₉:Bi Phosphor: An Investigation on Bismuth Valence Modification

Bismuth (Bi)-activated luminescence materials have attracted much attention for their tunable broad emissions ranging from a visible to near-infrared (NIR) region. However, it remains a challenge to regulate the Bi valence state and achieve NIR emission via a facile way. Here, we report the design and preparation of Ba₃Sc₄O₉:Bi phosphors, which emit visible and NIR emissions simultaneously even prepared in the air condition. The self-reduction mechanism of Bi³⁺ species in Ba₃Sc₄O₉ with a rigid crystal structure is illustrated based on the charge compensation model, and the coexistence of different Bi-active centers, Bi³⁺ for visible emission, while Bi⁺ and Bi⁰ for NIR emission, is confirmed by the spectroscopic data and X-ray photoelectron spectroscopy (XPS) analysis. The enhanced NIR emission was further achieved through controlled reducing treatment and the related mechanism has also been clarified. This work paves a new way to control bismuth valence and tune the emission of Bi-based luminescence materials for emerging photonics applications.