Preparation And Up-Conversion Emission Property Of Rare Earth Doped NaYF₄

Semiconductor catalyst is a green material which can use sunlight to realize hydrogen production and degradation of organic pollutants,and gradually arouse the interest of scholars in their applications in the field of environment and energy.In the past researches,the main aim is to study the catalysts with UV-light response.But UV-light only accounts for about 4%in the solar spectrum.Because of the low utilization of sunlight,novel semiconductor photocatalysts attract more and more attention.Graphitic carbon nitride has good thermal stability,chemical stability and suitable oxidation potential so that it can be applied in many fields,such as fuel cells,gas adsorption and photocatalysis.On the other hand,up-conversion luminescence materials can convert long-wave radiation with low energy to short-wave linear light with high energy,so that they can convert absorbed infrared light from sunlight to visible light,which can be utilized in photocatalysis of semiconductor photocatalysts.Based on this mechanism,we can prepare a composite photocatalyst by combining rare earth-doped up-conversion luminescence material with graphitic-carbon nitride to improve the utilization efficiency of sunlight and the degradation efficiency of organic compounds.In this paper,we carried out researches according to the two aspects as follows:First,crystals of Yb and Er doped NaYF₄:Yb,Er rare earth fluorides were synthesized by hydrothermal method with rare earth nitrate as the raw material,ethylenediamine tetraacetic acid as a chelating agent,and the up-conversion luminescence was realized.We have changed the amount of chelating agent,reaction time,hydrothermal temperature and F^-concentration.Crystals with different crystal phase structures,different luminescence intensities and different morphologies were prepared.Finally,we obtained the crystals with hexagonal phase showing the best up-conversion ability when the synthesis temperature was 185℃,hydrothermal time was 6 h,the moles ratio of complex agent and rare earth ion（EDTA:RE）was 0.5,the moles ratio of fluorine ion and rare earth（F^-:RE）was 8.We concluded that the luminescence properties of up-conversion materials are extremely related to the crystal phase,rare earth doping ratio and synthetic method.Second,pure graphitic carbon nitride was synthesized by thermal polymerization at 550℃.The degradation experiment of rhodamine B was carried out under the illuminations with different wavelength ranges provided by simulated sunlight using composite photocatalysts by grinding the mixture of NaYF₄:Yb,Er crystals and graphitic carbon nitride.Under both of UV-light filtered visible and UV-light filtered infrared irradiations,the degradation efficiency increased with the increase of carbon nitride doping ratio,which shows that the synthesized carbon nitride has good photocatalytic activity and the stability test for composite photocatalyst with rare earth/graphitic carbon nitride ratio of 1:4 shows that the composite photocatalyst has good photodegradation stability.Under the illumination of infrared light with wavelength range above 780 nm,the photodegradation performance of rhodamine B with composite photocatalyst shows that the visible light produced by the up-conversion material can be used in the semiconductor graphitic carbon nitride for degradations of organic dyes.Finally,under the illumination of simulated sun light without filtration composite photocatalyst with rare earth/graphitic carbon nitride ratio of 1:4 shows the RhB degradation efficiency up to 91.16%.This study provides new ideas for investigating the degradations of organic pollutants,and shows good development prospects.