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

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.