Explore The Photoluminescence Mechenism Of Carbon Quantum Dots Based On The Hydrathermal Synthesis And Gradiend Separation Method

Carbon quantum dots (CQDs) is becoming a rising star among the carbon nano-materials in recent years. The advantages of CQDs, such as good biocompatibility and size effect, make them an ideal functional material with good application potential in photocatalysis, elemental detection, etc., especially in bio imaging. From this point of view, in order to bring more benefits to carbon quantum dots in the medical field, researchers have been devoting efforts to optimizing the synthesis approach, and getting deep insight of the mechanism of photoluminescence. However, the luminescence mechanism of CQDs is still debated at present. This is related to the diversity of CQDs, for instance, CQDs prepared by different approaches are different in structure and optical properties. As a result, the photoluminescence mechanism is difficult to get a unified explanation. Researchers often classify the CQDs and separately investigate the origin of luminescence. In this paper, we choose the CQDs prepared from the precursors of citric acid and ethylenediamine adopting hydrothermal treatment, and explore the influence factors by adjusting the synthetic strategy as well as separation of CQDs into fractions with different quantum yield and the luminescent mechanism is summarized through the analysis. Relative researches in detail are presented as follow:First, the carbon source, different-COOH/-NH2 ratios of precursors, and the carbonation reaction time were changed on the basis of the original hydrothermal synthesis reaction. Through the analysis of the as-prepared products it was found that carboxyl, hydroxyl and existence of C=C of the precursor as well as the relative content of amino and time of carbonization had an effect on the optical properties of the CQDs. Moreover, putting CQDs into different pH environment was also tried, showing that the acid and alkali could weaken the photoluminescence. Above all, the functional groups, the degree of carbonization and the existence form of the functional groups are the influence factors of CQDs photoluminescence.Second, the CQDs were successfully separated by gradient centrifugation and different CQDs fractions with different quantum yield were gained. Through the analysis of size, structure, morphology and optical properties of different fractions, it was found that all fractions emitted blue fluorescence at the same wavelength and the fraction with highest quantum yield had highest functional group content as well as the lowest contrast in the high resolution transmission electron microscopy (HRTEM) images which means the lowest carbonation degree. On the contrary, fractions with lower quantum yield had less functional groups while their carbonized degree were higher. This suggested that the emission of blue fluorescence was most contributed by the molecular state on the surface of CQDs so that the decrease of functional groups on the surface would induce the decrease of quantum yield. The cores of CQDs tended to form graphite state which has the effect of absorption in the UV light. Both factors influence the optical property of CQDs.Gradient centrifugation was used for the separation of CQDs. In this topic, a new centrifugal method was established which was named to be "hydrophilicity gradient" centrifugation. Gradient media of the system were ethanol aqueous solutions with different water content. The as-prepared CQDswere non-sedimental in water even under powerful centrifugation. To solve this problem, CQDs were pre-aggregated first in acetone, and the aggregates settled down in the gradient medium with de-aggregation during the centrifugation. Until the settlement to the bottom of centrifuge tube, the de-aggregation finished. Through the analysis of fractions after separation andcontrol experiments, it was confirmed that the de-aggregation was realized based on different content of hydrophilic groups on the surface of CQDs, which resulted in different quantum yield. In this point, the CQDs were separated effectively according to the hydrophilicity difference.The photoluminescence of CQDs prepared by hydrothermal treatment of citric acid and ethylenediamine was finally summarized through the above experiments. The fluorescence of the CQDs was concluded to be due to the surface molecular state. The core structure turned to graphite state which could absorb the UV light. Both the surface molecular state and core structure had influence on the CQDs optical properties, while there was no connection with the size.