| Lanthanide doped upconversion luminescent materials are capable of emitting in the UV or visible region upon excitation with a NIR light source. At present, these optical materials have been widely used in infrared upconvertion imaging, biological imaging, laser technique, optical storage, solar cell and optical fiber communication. But the low upconvertion efficiency of the luminescent material is still hindering its applications. Fluoride material and oxide material are the mainly two types of upconversion materials, wherein the rare-earth doped Na YF4 is of the highest luminous efficiency matrix material, which has been subjected to researchers’ attention. Most previous studies have been taken on the powder of Na YF4 nanocrystal, but powder is not compatible with the application in solar cells, photocatalytic films and so on. Therefore, the preparation of upconversion luminescent films become the basis for its applications. In this paper, we introduced a preparation method of Yb3+,Tm3+ codoped Na YF4 upconversion luminescent films, modified with nano silver, and discussed the application feasibility of the films in near-infrared photocatalysis. The main works are described in the following:(1) Pure hexagonal-phase Na YF4:Yb3+,Tm3+ UC films were prepared with different doping concentrations of Yb3+, Tm3+ ions by a sol-gel reaction and heat treatment using trifluoroacetate solution. The microstructures, morphologies and optical properties were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD) and luminescent spectra. The results showed that the as-prepared samples ware pure hexagonal Na YF4. The surface of Na YF4:Yb3+,Tm3+ films are constituted by stick nanoparticles, which have a long strip shape with a diameter of 150 nm. By discussing the effect of Yb3+ and Tm3+ doping concentration on the emission efficiency of the films, we find that the optimum concentration is Yb3+ 40%, Tm3+0.5 %, and the ultraviolet emission and the blue emission at 452 nm of Tm3+ were depended on the cross relaxation process of 3F2,3 + 3F2,3 â†' 3H6 + 1D2.(2) The spherical silver nanoparticles have been prepared by reducing silver nitrate with Na BH4 in ice bath, which has a uniform size of 25 to 40 nm. We introduced a dip coating method to modify the surfaces of the as-prepared Na YF4:Yb3+,Tm3+ UC films. Ag nanoparticles were successfully attached on the surfaces of Na YF4 to form nanocomposite structures. The emission spectrum of the Na YF4:Yb3+,Tm3+ thin films during the Ag attachment stage shows firstly increased and then decreased in emission intensity as the number of attached Ag nanoparticles rising. The optimum luminescence enhancement was 2.3 for emission band peaking at 363 nm which occurs at the dosage of 120 μl/cm2 for Ag nanoparticles. The related interaction and coupling mechanisms of UC emissions of Na YF4:Yb3+,Tm3+ UC films with plasmon modes in Ag nano-structures are analyzed and discussed.(3) Ti O2 photocatalytic film were prepared by a sol-gel reaction and spin coating method. The microstructures and morphologies were characterized by AFM and XRD. The photocatalytic activity of the samples were characterized upon excitation with ultraviolet light using methylene blue solution as simulate pollutant. The results showed that the Ti O2 thin films can effectively degrade organics under UV irradiation. Na YF4:Yb3+,Tm3+/Ag/Ti O2 composite films were prepared by a spin coating method on the Na YF4:Yb3+,Tm3+/Ag substrate. The emission spectrum of the samples were collected by luminescent spectra. The difference of the emission spectrum at UV area before and after Ti O2 coated, shown that the UV emission of the UC film were mostly absorbed by Ti O2 film, which could be used in photocatalysis. The mechanism and feasibility of near-infrared photocatalysis based on Na YF4:Yb3+,Tm3+/Ag/Ti O2 composite films was discussed. |
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