Suppression of photocatalysis and long-lasting luminescence in ZnGa2O4 by Cr3+ doping

Abstract

ZnGa₂O₄ powder, synthesized by the solid state method, exhibited efficient photocatalytic activity for rhodamine B (RhB) degradation under mercury lamp illumination. However, the photocatalytic activity of ZnGa₂O₄ was highly suppressed when doped with Cr³⁺ ions. We discussed the mechanism of photocatalysis based on the photoluminescence properties of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺, and the blue fluorescence lifetimes of host ZnGa₂O₄ powders with different Cr³⁺ concentrations were also measured. The results indicated that the doped Cr³⁺ ions act as recombination centers that can highly reduce the amount and lifetime of the electron–hole pairs, thus reducing the photocatalytic activity of ZnGa₂O₄. The thermoluminescence (TL) curves of ZnGa₂O₄ and ZnGa₂O₄:Cr³⁺ showed that the amount of trapped electrons/holes in ZnGa₂O₄ is almost seven times higher than that of ZnGa₂O₄:Cr³⁺. The suppressed long-lasting luminescence intensity and photocatalytic activity of ZnGa₂O₄:Cr³⁺ were supposed to have come from the decrease of trapped electrons/holes and shortened lifetimes of electron–hole pairs. Possible mechanisms for long-lasting luminescence and photocatalysis of ZnGa₂O₄ coupled with photoluminescence mechanisms of ZnGa₂O₄:Cr³⁺ were also proposed.