Incorporation of Upconverting LiYF4:Yb3+, Tm3+ Nanoparticles with High Quantum Yield in TiO2 Metallogels for Near Infrared-Driven Photocatalytic Dye Degradation

By using anhydrous conditions, we report here an optimized synthesis of small but highly emitting upconverting nanoparticles (UCNPs) of LiYF4:Yb3+, Tm3+ without any surface passivation layers of silica or undoped matrix. For this, anhydrous precursors [Li(TFA)(monoglyme)] and [Ln(TFA)3(monoglyme)] (Ln = Y, Gd, Tm, Yb; TFA = trifluoroacetate) were synthesized via a facile one-pot synthetic protocol, which showed excellent thermal decomposition compatibility to afford Ln3+-doped LiLnF4 NPs by simultaneous decomposition in the solution phase. A detailed study of the influence of many key parameters such as the ratio of dopant ions and solvents, concentration of precursors, reaction time, and temperature over crystalline phase, size, shape, morphology, and upconverting properties of the NPs produced followed by a mechanism for the formation of LiYF4-based NPs is presented. The high quality of these NPs is manifested by the fact that their upconversion efficiency is quite comparable to that of a bulk single crystal of the same composition (bulk materials are believed to have far better UC performance due to low surface area and less defects). Successful incorporation of these UCNPs in transparent TiO2 metallogels provided an alternative strategy of highly homogeneous UCNP-TiO2 composites of variable composition (1–10% of NPs) having very high specific surface area (435–580 m2/g).