Preparation And Properties Of Carbon Quantum Dots And Their Composite Structures

Carbon quantum dots(CQDs) are fluorescent carbon nanomaterials, that have good dispersion. In a wide range of visible and near infrared light, CQDs can convert strong fluorescence and upconversion emission by single photon and two-photon absorption. And they can produce light-induced charge transport as electron donor or acceptor. Therefore, CQDs have important application value in biological detection, photovoltaic devices and catalytic fields. The size and morphology control of CQDs are very important for their performance and application. In addition, the catalytic efficiency of the single CQDs is relatively low as a photocatalyst, which makes the composite catalyst of CQDs become a trend. Due to low price, high chemical stability and the possibility of using visible light, semiconductor TiO2 catalyst attract people’s attention. Noble nanometal has a strong absorption in the visible region because of the surface plasmon resonance effect, which attracts increasing interest.In this thesis, we discussed the synthesis and surface decoration of carbon quantum dots, and preparation and properties of CQDs/TiO2 or CQDs/Ag composite material. Furthermore, we explored photocatalytic mechanism of such composite photocatalysts. The specific work included following sections:(1) Ascorbic acid as the carbon source, a method using reverse microemulsion was used to synthesize fluorescent CQDs. By UV-visible(UV-Vis), IR and photoluminescence(PL), we investigated their optical properties. Carbon quantum dots had the excitation wavelength dependence and many oxygen containing groups of their surface, such as OH,C=O,C-O-C, and so on. In addition, CQDs have graphite carbon structure. Size tunability was gained by adjusting water-surfactant molar ratio(w) without any size selection procedure, including 1.92 nm,2.86 nm,3.72 nm and 4.94 nm. With increasing the value of w, the polydispersity of the HDA-CQDs broadened. Hexadecylamine was used to functionize CQDs. The synthesized CQDs had high fluorescence quantum yield of maximum 47% at the 360 nm excitation wavelength. Finally, the effects of different surface active agents and different carbon sources were discussed.(2)Using ethylene glycol as the carbon source, CQDs were synthesize in a autoclave, and the average particle size was about 3.5nm. After modification of carbon quantum dots by amino and chlorine, oxygen containing groups decreased, C-N or C-Cl stretching vibration was showed in infrared spectrum. Otherwise, fluorescence intensity of carbon quantum dot increased, the best optimal excitation and emission peaks were red shift and the absorption peak moved to visible region. But the catalytic degradation of carbon quantum dots was relatively low. Based on this, design of composite photocatalyst was used to degradate organic dyes using visible light. Finally, we explored the mechanism of photoluminescence and surface modification of CQDs.(3)TiO2/CQDs composite photocatalyst was prepared by hydrothermal method. Making use of visible light, methylene blue was degraded. Titanium dioxide can quench the fluorescence of CQDs, and the absorption spectrum of composite catalyst moved to the visible region. Carbon quantum dots modified with ethylenediamine can photodegrade methylene blue efficiently. Finally, an explanation of such photocatalyst’s mechanism was discussed. Particle size of TiO2 particles prepared by hydrothermal method was smaller, its crystal stucture containied a lot of anatase and a small amount of rutile. TiO2/CQDs had better degradation ability. In terms of TiO2/CQDs in-situ synthesis by hydrothermal method, TiO2 crystal structure was based on rutile. When CQDs and TiCl3 were mixed according to volume ratio of 3:5, composit had the most catalytic degradation ability.(4)CQDs/Ag composite catalysts was synthized by using ethylene glycol and CQDs prepared in hydrothermal reaction kettle. When Ag and CQDs were mixed accoding to the volume ratio of 1:2, the catalytic efficiency of composites was the highest. Making use of reduction of hydroxyl groups on the surface of CQDs, silver ion was reduced. Then, CQDs/Ag composites were in-situ synthized, which was more stable. CQDs and ethylene glycol with the volume ratio of 4:1 were dissloved into the solution, which reduced silver ion to gain CQDs/Ag. Its catalytic efficiency was higer than single CQDs or Ag/CQDs prepared by mechanical mixing method. Due to the surface plasmon resonance effect, silver was excited electrons under the visible light irradiation. These electrons formed schottky defects in the interface of composites interface, which accelerated the catalytic degradation. Catalytic activity enhancement of Ag/Cl-CQDs can promote the degradation of methyl orange. 50 m L, 50 mg/L methyl orange was degraded completely in 12 minutes. Ag/Cl-CQDs can be used as efficient catalysts for methyl orange under visible light irradiation.