Carbogenic Nanodots Derived From Organo-templated Zeolites And Their Photoluminescent Properties

Molecular sieves are a class of inorganic microporous materials with regular porous structure and a broad range of applications in fields of adsorption/separation,catalysis and ion exchange.Organic amines or ammoniums as the structure-directing/templating agent are occluded in the channels or cages of the molecular sieves.Carbogenic nanodots?CNDs?are a kind of carbon-based nanomaterials with carbogenic core coupled with surface containing oxygen/nitrogen groups and a size of less than 10 nm.Since their first discover in 2004,CNDs have attracted increasing attention due to their intriguing advantages compared with the conventional semiconductor quantum dots,such as the tunable emission color,low toxicity,photobleaching resistance and so on.In this thesis,luminescent CNDs have been prepared by in situ carbonization of organic templates confined in the pore of molecular sieves.The diverse nanoporous architectures of the host zeolites,different organic templates and carbonization conditions may provide more opportunities to prepare CNDs with variable compositions,sizes and distinct surface chemistry.Thus,CNDs with interesting luminescence properties may be achieved by using this method.The main results are as follows:1.The magnesium aluminophosphate zeolite Mg APO-44 with CHA zeotype topology was prepared under hydrothermal conditions in a reaction system of 1.0 Al₂O₃: 1.0 MgO: 2.2 P₂O₅: 5.0 N-methylpiperazine?NMPZ?: 350 H₂ O.Organic template NMP is confined in cha cages of Mg APO-44.The as-synthesized zeolite product was heated at 400 oC for 4 hours in air to produce CNDs@Mg APO-44 composite material.Isolated CNDs-z can be obtained by acid/alkali etching inorganic framework and purification.Study indicates that the synthesized CNDs-z with size of 2.5 nm possess graphite-like core and highly functionalized surface by oxygen/nitrogen based groups.Different levels of aggregation will occur in the solution of CNDs-z with increasing concentration?0.04 g L-1 1.36 g L-1?.3D fluorescence excitation–emission matrix indicates that the emission wavelength of CNDs-z red-shifts with increasing concentration.Thus,fine-tuning of the fluorescence emission wavelength across the entire visible spectrum can be easily achieved by varying the concentration of the CNDs-z dispersions.2.The magnesium aluminophosphate zeolite Mg APO-44 with CHA zeotype topology was prepared under hydrothermal conditions in a reaction system of 1.0 Al₂O₃: 1.0 MgO: 2.2 P₂O₅: 5.0 N-methylpiperidine?NMPD?: 350 H₂ O.The as-synthesized zeolite product was heated at 450 oC for 4 hours in air to produce CNDs@Mg APO-44 composite material.Isolated CNDs-d can be obtained by acid/alkali etching inorganic framework and purification.Compared to CNDs-z,the as-synthesized CNDs-d possess similar graphite-like core but different functionalized surface by oxygen/nitrogen based groups.The diameter of CNDs-d is about 2.5 nm and the height is about 1.8 nm.3D fluorescence excitation–emission matrix indicates that CNDs-d possesses multicenter photoluminescence.The blue emission center shows excitation-dependent emission behavior,while green emission center shows excitation-independent emission behavior.Two emissions at about 460 nm and 515 nm are presented in CNDs-d when excited at 390 nm,which exhibit different responses to temperature,p H and hydroxyl radical.This indicates CNDs-d with multicenter luminescence can be used as multi-sensors.3.By using the thin layer chromatography,two main batches CNDs-A and CNDs-B with different polarity can be separated from CNDs-d.Fluorescence analysis indicates that CNDs-A and CNDs-B are corresponding to green?emission center G?emission and blue?emission centers B?and cyan?emission center C?emission in CNDs-d,respectively.FTIR,UV-vis and XPS characterization combined with steady state and time-resolved spectra have been used to elucidate the origins of such CNDs fluorescence.CNDs-A with green emission possesses excitation-independent?in steady state?and time-independent?in time-resolved?PL properties.Structure and composition analysis shows CNDs-A have less C=C,C=O,C-OH but rich NH₂ groups.The green emission of CNDs-A at 515 nm?emission center G in CNDs-d?can be attributed to NH₂ related quasi-molecule state.CNDs-B with blue-cyan emission mainly contain C=C,C=O,C-OH groups.Their excitation-dependent?in steady state?and time-dependent?in time-resolved?emission at 350 nm⁵20 nm?emission center B in CNDs-d?can be attributed to polar surface groups,while the photoluminescence with excitation wavelength at 240 nm²80 nm?emission center C in CNDs-d?can be assigned to ?-? conjugated domains.The influence of hydrogen peroxide on the multicenter photoluminescence of CNDs-d and the contrast experiment of carbonization of Mg APO-44 in argon also confirm the above inferences.