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

Abstract

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). ETY-CTC possesses higher photocatalytic activities compared to Er³⁺/Tm³⁺/Yb³⁺ tridoped (CaWO₄@TiO₂) under NIR and UV-vis-NIR light irradiations, since more ˙OH and O₂˙⁻ 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.