The Luminescence And Photocatalytic Properties Of Ion-doped Tungstate Or Fluorophosphate Matrix Phosphors

With the development of human society,people are facing increasingly serious problems in energy and the environment.Photocatalysis with the characteristics of no pollution and low cost can effectively degrade pollutants,so it has a good application prospect.Zinc tungstate is a kind of self-activating materials,and has attracted extensive research as a luminescence material.In addition,zinc tungstate also possesses good photocatalytic properties.Therefore,zinc tungstate has both good luminescence and photocatalytic properties.As we all know,the introduction of defects is an effective way to improve physical properties of materials and can explore new functions in semiconductor materials.The most common method for introducing traps is ions doping in the material.Therefore,to achieve the purpose of introducing defects,we designed ZnWO₄ matrix doped Dy³⁺,Bi³⁺,Ca²⁺and Li⁺ions,respectively.We also studied the effects of ions doping on the luminescent and photocatalytic properties of ZnWO4.ZnWO₄:Li⁺phosphors were prepared by the conventional high-temperature solid-state method.Their crystal structures were studied by X-ray diffraction（XRD）.The XRD patterns of ZnWO₄:Li⁺shifted to the lower diffraction angle,indicating that Li⁺has entered the ZnWO₄ lattice.The emission spectra show that the luminescence intensity of the ZnWO₄:Li⁺phosphor is reduced due to the doping of Li⁺.The reason is that the doping of Li⁺introduces more electron traps and reduces the recombination rate of electrons and holes,resulting in lower emission intensity.The analysis of diffuse reflectance spectra shows that ZnWO₄:Li⁺has smaller band gap energy,resulting in more electron-hole pairs.The photocatalytic degradation of RhB shows that the doping of Li⁺can enhance the photocatalytic activity.The result shows that Li⁺doping in ZnWO₄ can enhance light harvest capability to generate more electron-hole pairs,and can act as a trap center by decreasing the recombination of photogenerated electrons and holes.ZnWO₄:Dy³⁺,Ca²⁺phosphors were prepared by the traditional high-temperature solid-state method.The crystal structure was studied by X-ray diffraction（XRD）.XRD shows Ca²⁺has entered the lattice and replaced the position of some Zn²⁺ions.The ZnWO₄:Dy³⁺,Ca²⁺phosphor has the feature of Dy³⁺emission and lower emission intensity due to more electron traps produced by the doping of Ca²⁺indicates.The analysis of diffuse reflectance spectra shows that Dy³⁺and Ca²⁺doping can extend the photo-response of ZnWO₄ toward long-wavelength light.The photocatalytic results show that Ca²⁺and Dy³⁺co-doped ZnWO₄ phosphors have higher photocatalytic activity than pure ZnWO₄:Dy³⁺,which could be ascribed to more electron-hole pairs generated and the combination of electrons and holes decreased in the co-doped phosphors,resulting in higher photocatalytic activity.ZnWO₄:Dy³⁺,Bi³⁺were synthesized via high temperature solid-state reaction method.The as-prepared samples were characterized by X-ray diffraction（XRD）.The elemental composition and valence were analyzed using X-ray photoelectron spectroscopy（XPS）.The ZnWO₄:Dy³⁺,Bi³⁺phosphor shows photoluminescence with a broad band and the Dy³⁺emission feature,and a high photocatalytic activity in degradation of RhB under simulated UV irradiation.The result shows that Bi³⁺doping in ZnWO₄ can enhance light harvest capability to generate more electron-hole pairs,and act as a trap center by decreasing the recombination of photogenerated electrons and holes.The work suggests that ZnWO₄:Dy³⁺,Bi³⁺phosphors are promising materials for the photocatalytic decomposition of pollutants.A series of Sr₃Gd_1-xLi（PO₄）₃F:xSm³⁺（x=0.02,0.04,0.06,0.08）phosphors were synthesized by a high-temperature solid state method.The Sm³⁺-activated Sr₃Gd Li（PO₄）₃F phosphors can be efficiently excited by the wavelengths in the range from 350 nm to 450 nm,which matches perfectly with that of the commercial n-UV LED chips.The optimal doping concentration of Sr₃Gd_1-xLi（PO₄）₃F:xSm³⁺phosphors was determined to be x=0.04,corresponding to the quantum efficiency of 2.23%,and the CIE chromaticity coordinates（x=0.5172,y=0.4641）.The concentration quenching mechanism of Sm³⁺in Sr₃Gd Li（PO₄）₃F host is mainly attributed to the dipole-dipole interaction,which was confirmed by the fluorescent lifetimes.The effect of temperature on the photoluminescence property of Sr₃GdLi（PO₄）₃F:Sm³⁺was investigated.90%of the intensity is preserved at 150℃.In addition,a WLED lamp was fabricated by a 405nm n-UV LED chip coated with Sr₃Gd_0.96Sm_0.04Li（PO₄）₃F phosphor and commercial yellow phosphor(YAG:Ce³⁺)of a certain mass ratio.The present work indicates that the Sr₃GdLi（PO₄）₃F:Sm³⁺orange–red-emitting phosphors tend to be potential application in n-UV white LED.However,it does not show good photocatalytic activity for RhB under UV light irradiation.ZnWO₄:Li⁺,ZnWO₄:Dy³⁺,Bi³⁺,and CaZnWO₄:Dy³⁺phosphors were prepared by high temperature solid-state reaction method,respectively.The diffraction peak intensity of these three materials was high,and their crystallinity was excellent.The results showZnWO₄:Dy³⁺,Bi³⁺and CaZnWO₄:Dy³⁺phosphors havecharacteristic emission peaks of Dy³⁺under UV light excitation.Due to the doping of ions,cation traps are increased,and the recombination center is suppressed,and the luminescence intensity is reduced.The photocatalytic degradation of Rhodamine B shows that these three kinds of phosphors have excellent photocatalytic properties.By Li⁺,Ca²⁺and Bi³⁺doping,a new trap center was introduced,which can increase the separation rate of photogenerated electrons and holes and improve catalytic performance.For ZnWO₄:Dy³⁺,Bi³⁺,which exhibited the best photocatalytic effect and the degradation rate of Rhodamine B under ultraviolet irradiation,could reach 99.9%within 40minutes.