Self Reduction And Spectral Tunability Of Eu³⁺ In Aluminate Phosphors

Eu²⁺,as an activator material,has been usually produced by reducing Eu³⁺from Eu₂O₃ in a reduced atmosphere and commonly used in LED phosphors.This production process has such problems as high energy consumption and high risk.In this work,based on the fact the Eu³⁺can be reduced in air in some particular host materials,we proposed the sites selectivity of Eu²⁺and Eu³⁺,chose strontium aluminates and barium aluminates as the host materials,and synthesized Eu^2+/3+co-activated direct-white-emitting phosphors.The luminescence properties and chemical stability of the as-synthesized phosphors have been investigated.SrAl₂O₄ was employed as the host material;Eu^2+/3+were employed as the activator ions.The phosphors SrAl₂O₄:Eu^2+/3+was synthesized in air by solid-state method.The co-existence of Eu²⁺and Eu³⁺in the host lattice was verified by photoluminescence spectrum,diffuse reflection spectrum and X-ray photoelectron spectroscopy.The effects of Eu³⁺concentration and synthesis conditions（i.e.calcination temperature and holding time）on the self-reduction of Eu³⁺were studied by photoluminescence spectrum.The increased content of Eu³⁺promoted the formation of V’’_Srr vacancies and free electrons,and thermal excitation accelerated the release and transfer of electrons from V’’_Srr vacancies.BaAl₁₂O₁₉9 was employed as the host material;Eu^2+/3+were employed as the activator ions;Li⁺was employed as the charge compensator.The phosphors BaAl₁₂O₁₉:Eu^2+/3+was synthesized in air by solid-state method.The co-existence of Eu²⁺and Eu³⁺in the host lattice was verified by photoluminescence spectrum,diffuse reflection spectrum and X-ray photoelectron spectroscopy.The effects of Li⁺concentration on the self-reduction of Eu³⁺was studied by photoluminescence spectrum.The increased concentration of Li⁺caused a decrease in the number of electron donors in the host lattice,leading to a lower probability of combination between Eu³⁺and electrons.Sr₄Al₁₄O₂₅,with hexa-coordinate Al sites,was employed as the host material;Eu^2+/3+and Mn⁴⁺were employed as the activator ions.The phosphor Sr₄Al₁₄O₂₅:Eu^2+/3+,Mn⁴⁺was synthesized in air by solid-state method.By means of the efficient energy transfer between Eu²⁺and Mn⁴⁺,a series of phosphors with tunable emitting color were achieved by changing the doping concentration of Mn⁴⁺.When the concentrations of Eu³⁺and Mn⁴⁺were 1 mol%and 0.1 mol%respectively,the emitting color of the phosphor was located in warm white area with a CIE coordinates of（0.3465,0.2906）and correlated color temperature of 4554 K.Sr₄Al₁₄O₂₅,Eu^2+/3+and Tb³⁺were employed as the host material and the activator ions respectively.The phosphor Sr₄Al₁₄O₂₅:Eu^2+/3+,Tb³⁺was synthesized in air by solid-state method.A series of phosphors with tunable emitting color were achieved by changing the doping concentration of Tb ions.When the concentrations of Eu³⁺and Tb ions were 1 mol%and 6 mol%respectively,the emitting color of the phosphor was located in white area with a CIE coordinates of（0.2799,0.3059）and correlated color temperature of 9209 K.In order to improve the moisture stability of aluminate phosphors,SiO₂ coatings were deposited homogeneously on the surface of the commercial SrAl₂O₄:Eu²⁺,Dy³⁺particles.The thickness of SiO₂ coating deposited per cycle was estimated to be around 0.14 nm by transmission electron microscope.The luminescence intensity and moisture stability of the studied phosphor SrAl₂O₄:Eu²⁺,Dy³⁺were improved significantly with different coating cycles,among which 35 cycles provided the optimum improvement.