Sub-5nm Nitrogen-doped Oxide Graphene Quantum Dots:Large-scale Preparation And Acid- Induced Catalysis And Full Color-tunable Fluorescence

The graphene quantum dots ?GQDs? is defined as a product obtained by cutting a graphene monolayer into 2-20nm pieces ?plates?. Due to high surface area,good biocompatibility and low toxicity, GQDs are emerging in the fields of biological imaging,biosensing, photovoltaic devices, light-emitting diodes, photodetectors, photocatalysis and lithium-ion batteries.In this paper, nitrogen-doping graphene quantum dots ?N-GQDs? were prepared by ball milling. In addition to the corresponding characterization of the structure of the GQDs,the unique catalytic and optical properties of the N-GQDs were studied in detail.In the second chapter,we propose a new preparation route by ball milling to prepare N-GQDs. Graphite is carbon material, and the melamine as nitrogen source. The potassium hydroxide played a key role to provide chemical shear force. N-GQDs are rich in oxygen-containing functional groups, which is successfully exfoliated via the mechanical shear force of the grinding balls and the chemical shear force of KOH. N-GQDs products shown an average diameter of less than 5nm, thickness between 1-2nm, about 2 to 3 layers.The method is costly, simple and efficient, and realizes the pure mechanical method,large-scale production of water-soluble GQDs.In the third chapter, we found interesting experimental phenomena: N-GQDs were involved in an ultrafast chemical reaction. We added hydrochloric acid to the solid powder of N-GQDs-300 to observe the release of large amounts of CO₂. When the acid was added to the aqueous solution of N-GQDs-300, the suspensions were found.Through a series of characterization, the results show that the N-GQDs-300 structure undergoes a strong change in the environment of acid, which shown the decreasing of oxygen functional groups ?including C-O, C=O group?, SP2 carbon content ?including C-C,C=C and C-H groups? increasing. In addition, Breaking the covalent bond in the N-GQDs-300 structure make the number of N-GQDs-300 defects significantly increased.Experiments on the overall reaction mechanism showed that the aqueous solution of N-GQDs-300 produced reactive oxygen species ?ROS? including singlet oxygen ?1O2?,hydroxyl radical ?·OH? and superoxide anion ?·O2-? and hydrogen peroxide ?H2O2? during chemical conversion. Based on the exploration of the chemical transformation process of N-GQDs-300 with acid, we speculate that N-GQDs structure plays a very important role.During the whole reaction, oxygen-containing functional groups on the surface of N-GQDs capture O2. Accepting electrons and H+, O2 transformed to a variety of reactive oxygen free radicals. In the other hand, H2O accepting holes transformed to hydroxyl radical ?·OH?.The resulting ROS synergistically oxidizes the oxygen-containing functional groups on the N-GQDs and eventually releases CO₂.In the fourth chapter,we found that N-GQDs aqueous solution showed multicolor fluorescence emission when the excitation wavelength changed from 325nm to 665nm. It was the first time to achieve white, green, yellow, red four fluorescence excitation dependent behavior in a single aqueous solution. Especially white fluorescence emission,through a simple ball milling process, no longer undergo any modification or material composite, is very rare. For the optical properties of the N-GQDs series, we found that the large-scale preparation of N-GQDs-500 showed a very good fluorescence quantum yield,which reached 26.61%. And the QY of the reduced graphene quantum dots ?N-rGQDs?after the reaction with the strong acid was still high, reaching 23.79%.