Morphology control of the NaGdF4:Yb,Tm@NaGdF4 core–shell nanostructure by tailoring the ratio of core to shell

Abstract

Morphology control of upconversion nanoparticles (UCNPs) is of fundamental and technological importance to manipulate their shape/size-dependent properties for special applications. Here, we developed a method to control the morphology of the NaGdF₄:Yb³⁺,Tm³⁺@NaGdF₄ core–shell nanostructure by tailoring the ratio of precursors in the core and shell forming process. Through a core–shell approach, “flower-like” and “sea chestnut-like” hierarchical nanostructured UCNPs have been synthesized via a thermal decomposition method. The molar ratio of core to shell is a key factor in determining the morphology of the NaGdF₄:Yb³⁺,Tm³⁺@NaGdF₄ core–shell nanostructure. TEM images show that the UCNPs range from 15 nm to 100 nm with sphere, hexagonal plane, and tetrahedron morphologies and flower-like aggregates were prepared by varying the precursor amount of the core while keeping the precursor amount of the shell constant. To further understand the factors affecting the morphology of the core–shell structure, the effect of the size of the core and the heating rate on the morphology of NaGdF₄:Yb³⁺,Tm³⁺@NaGdF₄ were also evaluated. The results indicate that when the size of the core was about 6 nm, the atomic ratio of core/shell was 0.25, and the heating rate was 10 °C min⁻¹, “sea chestnut-like” NaGdF₄:Yb³⁺,Tm³⁺@NaGdF₄ UCNP aggregates were formed. The selected area electron diffraction (SAED) pattern showed a ring-like pattern, indicating a polycrystal and multi-layer structure. This morphology control strategy may provide fundamental guidance for preparing hierarchical core–shell structured nanocrystals.