Facile fabrication and photoluminescence properties of rare-earth-doped Gd₂O₃ hollow spheres via a sacrificial template method.

Rare-earth-doped gadolinium oxide (Gd₂O₃) hollow spheres were successfully fabricated on a large scale by using PS spheres as sacrificed templates and urea as a precipitating agent, which involved the deposition of an inorganic coating Gd(OH)CO3 on the surface of PS spheres and subsequent calcination in the air. Various approaches including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), as well as photoluminescence spectroscopies were used to characterize the samples. The results indicate that the sample is composed of uniform hollow Gd₂O₃ spheres with a mean particle size of about 2.3 μm and these hollow spheres have the mesoporous shell that are composed of a large amount of nanoparticles. The possible mechanism of evolution from PS spheres to the amorphous precursor and to the final hollow Gd₂O₃ spheres have been proposed. The as-obtained samples show strong light emission with different colors corresponding to different Ln³⁺ ions under ultraviolet-visible light and electron-beam excitation. Under 980 nm NIR irradiation, Gd₂O₃:Ln³⁺ (Ln³⁺ = Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺ and Yb³⁺/Ho³⁺) exhibit characteristic up-conversion (UC) emissions of red (Er³⁺, ²H11/2, ⁴S3/2, ⁴F9/2 → ⁴I15/2), blue (Tm³⁺, ¹G₄ → ³H₆) and green (Ho³⁺, ⁵F₄, ⁵S₂ → ⁵I₈), respectively. These merits of multicolor emissions in the visible region endow these kinds of materials with potential applications in the field of light display systems, lasers, optoelectronic devices, and MRI contrast agents.