Optimization of hydrothermal synthesis of Ce3+ co-doped NaYF4, Yb3+, Er3+upconversion nanoparticles and investigation of their size, morphology, and phase transitions.

Abstract

In this study, the synthesis of water-soluble polyethyleneimine (PEI) modified UCNPs which were prepared by a hydrothermal method have been reported. The resultant UCNPs were small size and hexagonal β-phase structure thanks to the Ce3+ co-doping effect. Without Ce3+ ion co-doping, NaYF4,Yb3+,Er3+ (80% Y,18% Yb, 2% Er) UCNPs were micro-rods shape with ≈5 μm length and pure hexagonal phase. After introducing 3% of Ce3+ to the NaYF4,Yb3+,Er3+ (72% Y,20% Yb, 5% Er) UCNPs, the sizes of the UCNPs have been reduced to 90 nm diameter with hexagonal phase and 30 nm diameter with cubic phase. The cubic-tohexagonal phase transition process was approved by X-ray diffraction (XRD) and their sizes were also calculated from the Debye-Scherrer equation from the XRD data which are coherent with the particle size measured from transmission electron microscope (TEM). Additionally, the UCNPs display three emission bands at 520, 540 and 654 nm under 980 nm laser due to the 4f-4f transitions of Er3+ ions.