The Study On Fluorescence Analysis Of Detection And Photocatalytic Performance Of Nitrogen Doped Carbon Quantum Dots

In this paper, three kinds of Nitrogen doped carbon quantum dots(N-CQDs) with different QYs of 26.9%,44.7%,66.8% were synthesized via a one-pot simple and eco-friendly hydrothermal process. After that, the QY 66.8% N-CQDs were used for the selective and sensitive detection of Hg2+ and I". In addition, TiO2 composited with N-CQDs(CQDs/P25) of different QYs was used in the field of visible-light-driven photocatalytic performance for H2 evolution. After that, the results and mechanism with the influence of QYs for the composites were discussed.With the use of nitrogen-rich materials and the adjustment of the amounts and ratio of raw materials, three kinds of N-CQDs with different QYs at 26.9%,44.7%,66.8% were synthesized via a one-pot eco-friendly hydrothermal process. The three kinds of N-CQDs exhibit great solubility and stability in water and have similar sizes and morphology. However, They are different from each other in the composition and structure. With the increase of the addition of ethylenediamine, the ratio of N/C increases, contents of C-N and the contents of surface groups such as CO-NH increase, leading to the improvement of QY for N-CQDs obtained.In this study, with the addition of various kinds of metal ions, the QY 66.8% N-CQDs have shown high selectivity for mercury ions through a fluorescence quenching process. Furthermore, the fluorescence quenching intensity was proportional to the Hg2+ concentrations from 10 nM to 20 μM with a detection limit of 8.6 nM (1.72 ppb), which is lower than the standard of World Health Organization(WHO) and the U.S. Environmental Protection Agency(EPA). After that, the quenching state of N-CQDs-Hg2+ system was proved to have great selectivity for iodine ions with the addition of various anions through a recovery process. And then with the addition of various concentrations of I-, the recovered fluorescence intensity has shown linear relationships with the concentrations of I- in the range of 0.5 μM to 40 μM with a detection limit of 0.354 μM, which has shown the potential for the detection of I- in human urine. At last, with the results of UV-Vis absorption spectra and fluorescence decays, it could be confirmed that the fluorescence quenching of N-CQDs by Hg2+ was due to the charge transfer between N-CQDs and Hg2+, and with the addition of I-, the formation of HgI2 contributed to the decrease of charge transfer process, and led to the recovery of N-CQDs.N-CQDs with different QYs composite(N-CQDs/P25) with TiO2 via a physical method. Three kinds of N-CQDs/P25 exhibit improved photocatalytic H2 evolution under UV-Vis and visible light irradiation, compared to pure P25.And with the increase of QY, the performance of H2 evolution improved. The mechanism was explored by the structure and composition of different N-CQDs. The improvement of QY, was caused by the doping of nitrogen and passivation, which could make the surface groups become more uniform and richer, and decrease the surface defects. The N-CQDs with less surface defects contribute to the electron reservoir more effectively. Therefore, under UV-Vis light irradiation N-CQDs act as an electron reservoir to improve the efficient separation of the photoinduced electron-hole pairs of P25. However, under visible light irradiation N-CQDs act as a photosensitizer to sensitize P25 into a visible light response "dyade" structure for H2 evolution more effectively.