The Synthesis And Catalytic Application Of Fluorescent Nano Carbon Dots

Carbon nanostructures have attracted a lot of attentions of scientists, because they have a lot of unique and innovative properties. By introducing the background and development of fluorescent carbon dots, this thesis explores the synthesis of different kind of C-Dots using ultrasonic and alkali-assisted method, which use glucose, activated carbon, graphite as the carbon sources, respectively. The as-prepared C-Dots have a lot of fluorescent properties. Based on such properties, especially the upconversion fluorescent property, we introduce that the C-Dots/TiO2and C-Dots/Cu2O composites which could be used as wide spectrum utilizing or near infrared (NIR) photocatalyst with high efficiency. Furthermore, we explore photocatalytic mechanism of such composite photocatalysts. We also investigate the fluorescent C-Dots as a catalyst for efficient selective catalytic oxidation reaction and acid catalyst under NIR or visible light. In this thesis, the specific work includes following sections:1. Glucose or activated carbon as the carbon source, a method using ultrasonic method was used to synthesize two different fluorescent C-Dots. By UV-visible (UV-Vis), IR, photoluminescence (PL) and a fluorescence microscope test, we investigate their optical properties, focusing on the functional group of surface and their PL properties under different excitation wavelengths. The surface of the C-Dots contain a lot of hydroxyl groups (-OH), carbonyl (=O)-containing group. The study found that these C-Dots not only have downconversion PL properties, but also have excellent upconversion PL properties. Finally, based on the different ultrasonic reaction conditions and carbon sources, we explore the synthesis mechanism of C-Dots.2. Using graphite as the carbon source, the carbon quantum dots (CQDs) synthesized by alkali-assisted electrochemical method have a diameter in1.2-3.8nm range, which have the PL properties of size-dependent (quantum size effect) and excellent upconversion PL properties. More importantly, based on such excellent conversion PL properties of CQDs, the titanium dioxide/carbon quantum dots (TiO2/CQDs) composite photocatalyst were designed and synthesized, it can use a wide spectrum of sunlight (extend to visible light) to photodegrade MB efficiently. Finally, an explanation of such photocatalyst’s mechanism was given. Such electrochemical method achieves the controlled synthesis of CQDs, while the CQDs can be used as a powerful energy conversion component which can be used in the design of the photocatalyst.3. Using glucose, sodium hydroxide and copper sulfate as raw materials, a one-step ultrasonic method was used to prepare the CQDs/Cu2O composite nanomaterial. By SEM and TEM results, we found that the surface of such composite material had a protruding structure. Solid UV absorption spectrum and the diffuse reflectance spectra test results show that the composite can absorb the light within a range of600-2500nm (near infrared). By using such material to photodegrade MB experiment under NIR light, we firstly demonstrate that this photocatalytic system can take advantage of (near) infrared light to improve the photocatalytic activity, and given the catalytic mechanism of such catalyst under NIR light illumination. By comparison, CQDs/Cu2O nanomaterials’ unique protruding structure allows near-infrared light multiple reflections between vacancies and protruding particles, which can be a greater extent of using of the light source, which enhance its photocatalytic activity. Finally, the protruding structure’s excellent light reflection capabilities and CQDs’ upcon version PL properties make that such composite material can be used as an efficient and stable NIR light sensitive photocatalyst.4. Through comparing the selective oxidation reactions (benzyl alcohol to benzaldehyde) under irradiated by different light resources or without irradiation, it was found that1-4nm CQDs as a catalyst can efficiently catalytic oxidation benzyl alcohol to benzaldehyde (conversion rate:92%; selectivity:100%) under the NIR light. The test of using terephthalic acid photoluminescence probing assay (TA-PL) to detect the HO· and the catalytic reaction of using toluene as substrate indicate that HO· is the main active oxygen species in such selective oxidative reaction. Electrocatalytic activity experiments show that CQDs can catalytic the decomposition of H2O2. In addition, NIR light irradiation can inhibit the catalytic activity of CQDs for decomposing H2O2, additionally, it was also found that CQDs have the ability of photo-induced electron transfer under NIR irradiation. Finally, the proposed mechanism for the efficient controllable selective oxidation of benzyl alcohol to benzaldehyde catalyzed by NIR light driven photocatalyst CQDs was given out, the catalytic activity of CQDs is dependent on their photocatalytic activity for H2O2decomposition and NIR light induced electron transfer property.5.5-10nm CQDs prepared by electrochemical cutting graphite method (using water as the electrolyte) can give longer wavelength emission of500-850nm range, such emission could be quenched efficiently by electron donor molecules in solution, confirming that CQDs are excellent electron acceptors to generate protons under visible light. Based on the photo-induced proton property, after a series of catalytic experiments (Esterification, Beckmann rearrangement, Aldol condensation) which need acid as catalyst were catalyzed by CQDs or contrast samples under irradiation or not, we found that CQDs under visible light irradiation can promote the reactions achieving a high conversion (around40%). The approach curves by SECM (under visible light or not) and the change of pH value under different light illumination intensity or temperature demonstrated that CQDs can generate protons under visible light irradiation. Finally, the catalytic mechanism of CQDs as a visible acid catalyst was given out.Because of the excellent PL properties of C-Dots prepared by electrochemical and ultrasonic methods mentioned above, these C-Dots can be composited with different semiconductor metal oxide to broaden the light range of the catalyst using. Furthermore, based on the characteristics of the photoinduced electron transfer,1-4nm carbon dots can be used as catalyst for selective catalytic oxidation reaction under NIR light. While the5-10nm C-Dots can be used as a visible control efficient acid catalyst based on their photoinduced proton ability.