Directly coat TiO2 on hydrophobic NaYF4:Yb,Tm nanoplates and regulate their photocatalytic activities with the core size

We demonstrate a facile method to directly coat a TiO2 layer on hydrophobic NaYF4:Yb,Tm upconversion nanocrystals. Through modifying the hydrophobic nanocrystals with a surfactant layer, a conventional sol–gel route can be applied to them for TiO2 deposition. The prepared β-NaYF4:Yb,Tm@TiO2 particles show obvious photocatalytic activity under near-infrared light as well as ultraviolet light. Based on the success in preparing upconversion cores with different sizes and subsequent TiO2 coating, we have systematically investigated the effect of core sizes on their photocatalytic performance. The results suggest that upconversion fluorescence of the core and surface of the shell both have a great effect on their photocatalytic activities. A moderate core size is thus preferred, due to the competition between these two effects.