Red upconversion emission of WO3 nanosheets under near-infrared laser excitation

The upconversion (UC) emission behavior occurs usually in rare-earth doped compounds but rarely in pure semiconductors. Herein, the UC emission behavior of WO 3 was investigated. The oxygen-vacancy-rich WO 3 nanosheets were rapidly synthesized by high-power laser irradiation. The red UC emission peaked at ∼1.75 eV is observed in the synthesized WO 3 nanosheets under continuous 980 nm or 1940 nm laser excitation. The UC emission stems from the abundant oxygen-deficient levels and belongs to a multiphoton absorption behavior. Meanwhile, the laser-power-dependent UC behavior could be attributed to the transformation of different oxygen defect states. On this basis, a variable output color phosphor based on Er-Yb co-doped WO 3 [WO 3 :(Er, Yb)] was developed. The WO 3 :(Er, Yb) phosphor can change the UC color from green to red with increasing the 980-nm laser power. Moreover, the upconversion behavior of WO 3 can be utilized to assist WS 2 photocatalysis. This work develops a pure semiconductor with an interesting UC emission and clarifies the UC mechanism.