Fluorochromic Properties of Undoped and Ln 3+ -Doped CaWO 4 Phosphor Particles

Abstract

Undoped and Ln3+-doped (Ln3+ = Eu3+, Tb3+) CaWO4 phosphor particles were synthesized by a hydrothermal method, aiming at generating fluorochromic properties in response to redox reactions. The phosphor particles obtained were first treated with an aqueous L(+)-ascorbic acid solution as a reductant at room temperature, which resulted in a considerable decrease in photoluminescence (PL) intensity of both CaWO4 (the host-lattice blue emission) and CaWO4:Ln3+ (the 4f-4f red or green emission). The quenched PL could then be recovered by treating the particles with a sodium hypochlorite solution as an oxidant also at room temperature. This PL switching behavior was predominantly caused by redox reactions between the solid particles and the redox species in the solutions based on a change of W6+/W5+ valance states of the CaWO4 host crystal. More interestingly, CaWO4:Eu3+ exhibited a change of emission colors, depending on the excitation wavelength, due to different sensitivity of the host emission and the Eu3+ emission to the redox treatment. Such a changeable fluorochromism is regarded as a newly developed function of the phosphor materials for use in facile optical detectors and sensors.