Construction of oxygen-rich vacancy Bi 3 O 4 Br:Yb 3+ ,Er 3+ nanosheets for enhanced photoreversible color switching and upconversion luminescence

Bi 3 O 4 Br:Yb 3+ ,Er 3+ (BYE-OV) nanosheets generate more oxygen vacancies under UV irradiation, causing the color to change from light yellow to brown-black. At the same time, the generated intermediate energy levels enhance upconversion luminescence.