Preparation And Properties Of Rare Earth Ions Doped NaBiF₄ Upconversion Materials

Upconversion materials have many advantages such as tunabale emission and near-infrared light as excitation,which have attracted wide attention of scientists in recent decades.The matrix material as a main part has a large influence on the luminescence efficiency of the upconversion material.Compared with rare earth compounds,Bi?NO₃?₃ is inexpensive,replacing the common rare earth fluorides such as NaREF₄?RE=Y,La,Eu,etc.?with NaBiF₄ as a matrix material can effectively reduce the cost of raw materials.The NaBiF₄:Yb³⁺/Er³⁺samples prepared by the precipitation method at room temperature have low upconversion luminescence intensity and it is difficult to control particle size via this method.The solvothermal preparation of NaBiF₄:Yb³⁺/Er³⁺samples has higher upconversion luminescence intensity,but the morphology is irregular.Here,in order to optimize the two preparation methods,the upconversion luminescence enhancement,and control of particle size and morphology of NaBiF₄:Yb³⁺/Er³⁺samples were achieved by impurity ions doping and solvent change.For the deficiency of NaBiF₄:Yb³⁺/Er³⁺prepared via precipitation method at room temperature,Li⁺ions with smaller radius were co-doped with rare earth ions.When doped by the positive precipitation method,most of the Li⁺ions can enter the NaBiF₄ lattice by entering the interstitial sites or substituting for the the Na⁺ions.The fomer will lead to charge imbalance,at this moment,a transient electric dipole with positive pole pointing outward was formed on the particle surface.The transient electric dipole on the crystal surface accelerate the diffusion of F^-ions from the solution to the crystal driven by charge attraction,promoting NaBiF₄:Yb³⁺/Er³⁺growth.Under the premise of regular morphology,increasing the Li⁺ion doping concentration can control the particle size from 360 nm to 871 nm.The increase in size and lattice distortion caused by Li⁺ion doping can promote the upconversion luminescence enhancement.When the Li⁺ion doping concentration was 15 mol%,the green and red emissions increased to 27.1 and 19.3 times,respectively.Most of the Li⁺ions can enter the NaBiF₄ lattice.On the contrary,when the Li⁺ion was doped by the reverse precipitation method,it is difficult for the Li⁺ions to enter the NaBiF₄ crystal lattice and formed impurities such as BiF₃,at the same time,the upconversion luminescence intensity was greatly reduced and the morphology was greatly changed.The effects of reaction time and reaction temperature on the phase composition,morphology and upconversion luminescence of NaBiF₄:Yb³⁺/Er³⁺samples were investigated by solvothermal method.The results showed that in the solvothermal stage,increasing the reaction temperature and prolonging the reaction time could increase the crystallinity of NaBiF₄:Yb³⁺/Er³⁺and increase the upconversion luminescence efficiency.However,when the temperature was higher than 220°C Bismuth was easily formed.It was easier to obtain samples with a relatively regular morphology at a low temperature.Changing the ratio of ethylene glycol to ethanol could promote the transition of the samples from spherical to rod.In addition,the upconversion luminescence intensity of NaBiF₄:Yb³⁺/Er³⁺could be further enhanced by Rb⁺ion doping.When the doping concentration of Rb⁺ion was 5 mol%,the green and red emissions were increased to 1.73 and 1.5 times,respectively.Finally,based on the fluorescence intensity ratio?FIR?technique,the FIR curve of the samples in the temperature range of 300 K⁵00 K was studied.The results showed that the NaBiF₄:Yb³⁺/Er³⁺samples has potential application value in the temperature sensing field.