An efficient near infrared photocatalyst of Er3+/Tm3+/Yb3+ tridoped (CaWO4@(TiO2/CaF2)) with multi-stage CaF2 nanocrystal formation

The formation process of CaF2 is critical for the improvement of upconversion properties of the CaF2 based upconversion photocatalysts, and for this purpose a near-infrared (NIR) photocatalyst of Er3+/Tm3+/Yb3+ tridoped (CaWO4@(TiO2/CaF2)) (ETY-CTC) was synthesized. CaF2 nanocrystals are converted from CaWO4 precursors in a multi-stage process, and the remaining CaWO4 microspheres are wrapped in CaF2 and TiO2 nanocrystals to form the heterostructure of the photocatalyst. CaF2 is found to connect with TiO2 nanocrystals, instead of being coated by TiO2, resulting in a higher upconversion luminescence efficiency of ETY-CTC than that of pure Er3+/Tm3+/Yb3+ tridoped (CaWO4@CaF2). ETYCTC possesses higher photocatalytic activities compared to Er3+/Tm3+/Yb3+ tridoped (CaWO4@TiO2) under NIR and UV-vis-NIR light irradiations, since more (OH)-O-. and O-2(.-) radicals, and higher electron-hole separation efficiency are obtained in the ETY-CTC system. The multi-stage formation of luminescence agents can be an attractive method for the synthesis of NIR photocatalysts with enhanced upconversion properties and photocatalytic activities.