Synthesis, Characterization and Optical Behavior of Nanocrystalline CoWO₄

Nanocrystalline CoWO₄ sampled were synthesized using a simple mechanochemical approach and a solid-state reaction, respectively. The formation of nanocrystalline CoWO₄ was characterized by X-ray diffraction (XRD) and infrared spectroscopy (IR). The optical properties of the obtained samples were explored by diffuse reflectance UV-visible (DRS) and photoluminescence (PL) techniques. A milling speed of 850 rpm led to the direct synthesis of monoclinic CoWO₄ with a short reaction time (1 h). The complete reaction did not occur in the solid-state synthesis. The obtained samples had monoclinic crystal systems with different lattice parameters. The average crystallite sizes of CoWO₄ were in the range of 20 to 180 nm. The TEM investigation showed that the morphology of the CoWO₄ particles differed depending on the preparation conditions. The values of the determined optical bandgap of CoWO₄ were the range of 1.89 to 2.18 eV, according to diffusion reflectance spectroscopy in the ultraviolet-to-visible range. Broader blue-green emission spectra with peaks at 430 nm were observed for samples prepared via both routes. The CIE color coordinates of the CoWO₄ samples lay in the blue and purple regions. The quantum yields of the CoWO₄ samples synthesized after 1 h and 5 h milling times at 850 rom were 0.34 and 0.67%, respectively. This study proposes an affordable mechanochemical approach for blue-green phosphors that could possibly be used in various light-emitting diodes (LEDs).