(302d) Hollow Spherical Rare-Earth-Doped Yttrium Oxysulfate: A Novel Structure for Upconversion

Rare-earth (RE) ions exhibit unique luminescent properties, including the ability of converting the near infrared long-wavelength excitation radiation into shorter visible wavelengths by photon upconversion (UC) due to the incompletely filled 4f electron shell. Compared to organic fluorophores and semiconducting nanocrystals, rare-earth-based nanomaterials display higher photochemical stability, sharp emission bandwidths, large anti-Stokes shifts and Stokes (up to 500 nm) that separate discrete emission peaks from the ultraviolet/infrared excitation. In recent years, rare-earth-doped UC materials have been increasingly applied as a novel type of luminescent optical labels in biological assays and medical imaging and have become promising alternatives to organic fluorophores and quantum dots, etc. Nonetheless, these rare-earth-doped systems were mainly based on the NaREF₄, LiYF₄, REF₃, etc. crystal structure. There are few reports on rare-earth-based RE₂O₂SO₄ structure as an alternative for the UC application.

In this work, a facile hydrothermal route followed by annealing was applied for the preparation of monoclinic yttrium oxysulfate (Y₂O₂SO₄) hollow spheres withother rare earth ions (Yb³⁺ and Er³⁺) doped. The formation of hollow spheres may involve the Ostwald ripening. The hybrid materials were firstly demonstrated for upconversion application. The compatibility of host crystal structure enables the easy doping of a second rare-earth metal element without significantly changing the host lattice. The photoluminescent properties were affected by the ratio and concentration of doped rare earth metal ions due to the featured energy transfer and symmetry of crystal field. The type of luminescent center and the crystallinity of samples were also proved to have remarkable influences on the optical properties of as-prepared products.