Systematic and detailed examination of NaYF4-Er-Yb-TiO2 photocatalytic activity under Vis–NIR irradiation: Experimental and theoretical analyses

Abstract

Enhancing the efficiency of solar light utilization and understanding the acquired mechanism of photocatalytic reactions are the critical goals in the field of heterogeneous photocatalysis. To meet growing demands, the up-conversion particles (UCPs) have become an attractive perspective to extend the absorption spectrum of UV-active photocatalysts. The systematic experimental and theoretical study of the effects of UCPs on the photocatalytic activity of TiO2, and the investigation of its excitation mechanism are vital for understanding and optimizing the photocatalytic properties. Accordingly, hydrothermal process has been employed to manufacture visible light active NaYF4-Er-Yb-TiO2 photocatalyst in the present study. The UCPs have been located at the TiO2 crystal boundaries, where Er3+ and F− surface doping has been also observed. The predicted electronic structures of UCPs-TiO2 indicate that the formation of new Er-4f states within the band gap of TiO2 is the key factor of photocatalytic performance at Vis irradiation. Luminescence spectroscopy measurements reveal up-conversion energy transfer from Yb3+ to Er3+ ions in UCPs particles. Nevertheless, UCPs-TiO2 is not active under NIR irradiation, and the photocatalytic activity under Vis irradiation is not caused by OH, but rather by other forms of reactive oxygen species, such as O2− and OOH.