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

Abstract

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.