Luminescence switching behavior through redox reaction in Ce3+ co-doped LaPO4:Tb3+nanorods: Re-dispersible and polymer film

Abstract

Re-dispersible Tb³⁺ doped LaPO₄ nanorods have been prepared using ethylene glycol (EG) as a capping agent as well as reaction medium at a relatively low temperature of 150 °C. The X-ray diffraction study reveals that all the doped samples are well crystalline with a monoclinic structure of the LaPO₄ phase. The luminescence intensity of ⁵D₄→⁷F₅ transition at 543 nm (green) is more prominent than that of ⁵D₄→⁷F₆ transition at 487 nm (blue) for all the samples. This is related to the polarizing effect from [PO₄]³⁻ to the Tb³⁺ site. Concentration dependent luminescence study shows that the luminescence intensity of Tb³⁺ increases up to 10 at.% and decreases above this. This is due to the concentration quenching effect arising from cross relaxation among Tb³⁺–Tb³⁺ ions. The results show that nanoparticles prepared in EG medium gives an enhanced luminescence compared to that prepared in water. This is attributed to the multiphonon relaxation effect from O–H groups surrounding over nanoparticles as well as the extent of increase of agglomeration among particles for samples prepared in water. Significant enhancement in the emission of Tb³⁺ is also observed when Ce³⁺ is used as the sensitizer in LaPO₄:Tb³⁺nanorods. The optimum concentration of Ce³⁺ for maximum luminescence is found to be 7 at.% in Ce³⁺ sensitized LaPO₄:Tb³⁺ (5 at.%). Based on the energy transfer process from Ce³⁺ to Tb³⁺, the luminescence of Tb³⁺ can be switched OFF and ON by performing oxidation and reduction of Ce³⁺↔Ce⁴⁺ using KMnO₄ and ascorbic acid, respectively. The samples are re-dispersible in water, methanol and can be incorporated into polyvinyl alcohol (PVA) films. They show a dark green emission under ultraviolet radiation.