Enhanced photoluminescence of hollow CaWO4 microspheres: the fast fabrication, structural manipulation, and exploration of the growth mechanism

Abstract

Calcium tungstate (CaWO₄) has been extensively studied for optical applications due to the high luminous efficiency and photoluminescence (PL) emissions generated from its scheelite-type tetragonal architecture. Nevertheless, how to fabricate CaWO₄ materials with a controllable morphology using a rapid dynamic strategy is still an intractable challenge. In this work, hollow CaWO₄ microspheres of different sizes with homogeneous diameters and high porosity were prepared via a high-efficiency microwave irradiation method. Through X-ray diffraction analyses, it was observed that the as-prepared products displayed a highly pure phase of CaWO₄. In addition, the overall topology and component nanoparticle dimensions of the microstructures were identified using field-emission scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy images. These as-obtained hollow CaWO₄ microspheres, assembled by uniform nanospheres with a high crystallinity, demonstrated excellent luminescence properties, as determined through emission spectra with a maximum wide band of 390–420 nm, which is validated by PL measurements. This is assigned to the transfer of ¹T₂ → ¹A₁ that resulted from the charge-transfer of the tetrahedral [WO₄] groups of CaWO₄. Furthermore, the formation mechanism of hollow CaWO₄ microspheres is summarized into “self-assembly–dissolution-recrystallization–Ostwald-ripening” by monitoring the changes in the shapes and microstructures under continuous dwell time.