Preparation And Fluorescence Properties Of Hexagonal Boron Nitride Quantum Dots

Hexagonal boron nitride quantum dots(BNQDs)have potential applications in disease diagnosis,fluorescence imaging,metal ion detection and other fields due to their good biocompatibility,low cytotoxicity and excellent photoluminescence properties.However,the as-prepared BNQDs at present have some problems,such as large size,uneven particle size distribution and single luminous color.Therefore,in this study,hexagonal boron nitride nanosheets(h-BNNSs)were prepared by liquid nitrogen ultrasonic freeze-thaw exfoliation method.The as-prepared h-BNNSs were used as precursors to prepare BNQDs and sulfur-doped BNQDs(S-BNQDs).The optical properties of QDs are regulated by changing the solvent types and adjusting the experimental parameters in order to obtain QDs with good fluorescence properties.The main research contents include the following aspects:(1)h-BNNSs was prepared with hexagonal boron nitride(h-BN)as raw material by liquid itrogen ultrasonic freeze-thaw exfoliation method.The effects of different freeze-thaw exfoliation times on the morphology and particle size of h-BNNSs were investigated.The results showed that after 5 freeze-thaw exfoliation,the h-BNNSs size became smaller(100-300 nm),the thickness became thinner(about 3 nm),and showed good crystallinity,indicating that most of the h-BN powders were effectively exfoliated into monolayer or few-layer nanosheets.The potential value of the h-BNNSs measured by the Zeta potentiometer is-23.8 m V,the high surface charge ensures that the h-BNNSs repels each other in the dispersion process and is not easy to agglomerate,which makes the h-BNNSs dispersion show high stability.(2)Ethanol-BNQDs(W1-BNQDs)and DMF-BNQDs(W2-BNQDs)with average lateral size of 4.76 nm,1.65 nm,quantum yield of 0.21%,2.49%and good crystallinity were prepared by solvothermal method,using the above prepared h-BNNSs as precursors,and ethanol and N-N dimethylformamide(DMF)as solvents.The optical properties of W1-BNQDs and W2-BNQDs were investigated.The results showed that both the as-prepared W1-BNQDs and W2-BNQDs emitted bright blue fluorescence and exhibited nanosecond decay under UV-Vis(365 nm).With the increase of excitation wavelength,the W1-BNQDs exhibits excitation independent emission behavior,while the W2-BNQDs exhibits excitation-dependent emission behavior,which can be attributed to the different polarity of different solvents.Through adjusting the reaction parameters in the solvothermal process,it is found that the reaction temperature is 180? and the reaction time is 12 h,the as-prepared BNQDs fluorescence intensity is the strongest.(3)Using thiourea as dopant and h-BNNSs as precursor,the S-BNQDs was prepared by solvothermal method.The results show that the as-prepared S-BNQDs has uniform particle size distribution,size of about 3 nm and blue light under UV-Vis(365 nm).It was found that the emission peak of S-BNQDs showed regular red shift from 410 nm to 525 nm with the increase of photoluminescence excitation wavelength by fluorescence spectroscopy.In conclusion,thiourea has a good regulation effect on the optical properties of BNQDs.