NIR-Responsive Photocatalytic Activity and Mechanism of NaYF4:Yb,Tm@TiO2 Core–Shell Nanoparticles

Core–shell structured nanoparticles for near-infrared (NIR) photocatalysis were synthesized by a two-step wet-chemical route. The core is composed of upconversion luminescence NaYF4:Yb,Tm prepared by a solvothermal process, and the shell is anatase TiO2 nanocrystals around NaYF4 particles formed via a method similar to a Stöber process. Methylene blue compound as a model pollutant was used to investigate the photocatalytic activity of NaYF4:Yb,Tm@TiO2 composites under NIR irradiation. To understand the nature of NIR-responsive photocatalysis of NaYF4:Yb,Tm@TiO2, we investigated the energy transfer process between NaYF4:Yb,Tm and TiO2 and the origin of the degradation of organic pollutants under NIR radiation. Results indicate that the energy transfer route between NaYF4:Yb,Tm and TiO2 is an important factor that influences the photocatalytic activity significantly and that the degradation of organic pollutants under NIR irradiation is caused mostly by the oxidation of reactive oxygen species generated in the photocatalytic reaction, rather than by the thermal energy generated by NIR irradiation. The understanding of NIR-responsive photocatalytic mechanism helps to improve the structural design and functionality of this new type of catalytic material.