ZnO/ZnS Nanoparticles on NaYF₄:Yb,Tm for Near-Infrared-Activated Photocatalytic Cr(VI) Reduction

The essential requirement for photocatalysis─utilization of sunlight energy─was addressed by combining ZnO/ZnS nanoparticles (NPs) with NaYF4:Yb3+,Tm3+ upconversion nanoparticles (UCNPs) in a core–shell composite structure that can convert near-infrared (NIR) light to UV light through energy transfer (ET). The material was examined with advanced characterization techniques and computational methods, which allowed a better understanding of the interface and provided insights into possible conditions for the ET between UCNPs and ZnO, which has not been studied before. In addition, this work suggests a simple method for the in situ formation of a heterojunction on ZnO while it is attached to UCNP, which has not been applied to UCNP-coupled photocatalysts to date. The in situ-formed heterostructure of ZnO and ZnS was proven to enhance NIR-driven photocatalytic activity via efficient charge separation through the Z-scheme mechanism: 70% of Cr(VI) reduction was obtained within 3 h of NIR laser irradiation with UCNP@ZnO/ZnS, while 48% reduction of Cr(VI) was achieved by UCNP@ZnO. Reactive oxygen species (ROS) were detected during NIR-triggered photocatalysis, proving the energy conversion from UCNPsto photocatalysts.