Near-infrared light active core−shell TiO2 nanoparticles were synthesized, where upconverting luminescent material (YF3 codoped with Yb3+/Tm3+ codoped) is used as the core and TiO2 as the shell. During the photoexcitation process, Yb3+
absorbs NIR radiation and transfer the energy to Tm3+, which emits UV radiation and in turn excites TiO2 to generate
photoexcited electrons and holes. To further enhance the photocatalytic efficiency of these systems, nanocrystals of TiO2 and a narrow-band semiconductor such as CdS were linked
with NaYF4 codoped with Yb3+ and Tm3+.
Upon NIR excitation, Yb3+ acts as a sensitizer and transfers the energy to
Tm3+, which in turn activates CdS or TiO2 by Förster resonance energy transfer or photon reabsorption mechanism and generates photoinduced electron and hole required for
mineralization of organic chemicals. The presence of CdS/TiO2 heterojunction further enhances the photocatalytic efficiency by facilitating the separation and migration of charge carriers at the interface.