Synthesis Of Nayf₄ Matrix Of Up-conversion Phosphors By A Special Reverse Micelle Method

The properties of nanoparticles, such as small size effect, quantum size effect, etc., have equipped the nanomaterials with many unique features that make them quite different from the routine materials in optics, calorifics, electricity and magnetics, etc. Based on their special properties, nanoparticles have been applied in many high-tech fields recently.With the development of biochips, up-conversion fluorescent materials have gradually become a new research focus because of their potential applications as biochips. Until now, ytterbium and erbium co-doped hexagonal sodium yttrium fluoride (NaYF₄: Yb, Er) is among the most efficient up-conversion phosphors. Its potential applications in biological and medical fields have attracted many scientists.Up to now, the main methods to synthesize cubical type NaYF₄ are coprecipitation method, solid-liquid biphase method, etc. Well-dispersed cubical NaYF₄ nanocrystals have been achieved successfully. But the reaction conditions needed to gain hexagonal NaYF₄ nanocrystals are still tough, the lowest temperature needed is about 160℃.A special reverse micelle synthesis method is adopted in this paper to synthesize NaYF₄ nanocrystals. Our research begin with a well-chosen benchmark sample, and then makes extensive studies about the influences of reaction time, reaction temperature, surfactant's concentration, etc. on the NaYF₄'s crystalline types. ICP-AES elementary analysis is also used to test the Y element's concentration variance. Together with XRD patterns' analysis, a lot of novel and meaningful achievements are gained:(1) Well-dispersed, homogeneous NaYF₄ fluorescent matrix nanocrystals have been synthesized by the special reverse micelle method with high yield. The smallest diameter achieved is only around 7 nm.(2) Many influence factors's effects on NaYF₄'s crystalline type are investigated, and the outcome goes as follows:(a) The variance of reaction time will not affect NaYF₄'s crystalline type obviously.(b) When the reaction temperature is between 20℃and certain organic solvent's boiling point, it will not affect NaYF₄'s crystalline essentially; when the reaction temperature is a little bit higher then 16.3℃, NaYF₄'s crystalline can be controlled at cubical form; but when the temperature is below 16.3℃, the product is in an amorphous state.(c) When the reaction temperature is above 16.3℃, and when, at this temperature, sodium oleate/oleic acid's solubility in oil is not affected, the ratio of hexagonal type to cubical type of NaYF₄ increases with the rise of sodium oleate's concentration. When sodium oleate's concentration is above 0.4 mol/L, under normal temperature, pure hexagonal nanocrystals are achieved.(d) Different organic solvents have different polarities, the differences cause their different solubilities to sodium oleate/oleic acid, and the differences cause different crystalline types.(e) When the products dispersed in oil phase are collected for high-temperature, high-pressure treatment, NaYF₄ nanoparticles are still well-dispersed and homogeneous, with crystalline type unchanged.(f) Y(ClO₄)₃'s pH value variations won't affect NaYF₄'s crystalline type.(g) Another anionic surfactant AOT can't take sodium oleate's place to achieve similar experimental outcomes.(h) Ionic strength in water phase will affect the surfactant's function. (i) Fluorescent materials synthesized by this special reverse micelle methed still have good up-conversion luminescence properties. (3) The reaction principles to synthesize NaYF₄ is discussed:(a) The microenvironments formed by oil, water and surfactant have made the formation of NaYF₄ nanoparticles much slower, compared with the ones formed in aqueous solution.(b) The reaction mechanisms are also demonstrated according to the ICP testing results and corresponding XRD patterns.By this special reverse micelle method, not only well-dispersed, homogeneous and size-variable nanocrystals can be processed like the normal reverse micelle method. Controllable crystalline type and high-yield can also be achieved under normal temperature.