The Magnetic Properties Of The Pristine And Nitrogen-doped Graphite-based Carbon Materials

In recent years, the magnetism of carbon-based materials has aroused great interest from researchers. In general, all-carbon materials are diamagnetic and their susceptibilities are in the range of-10-5～-10-7emu·g-1·Oe-1. However, many groups have theoretically and experimentally demonstrated that the paramagnetism and/or ferromagnetism can be induced by localized spins in all-carbon materials. It implies that it is hopeful to obtain light, non-metallic carbon-based magnet with a Curie point well above room temperature. The carbon-based magnets have attracted much attention owing to their outstanding physical and chemical properties, and greatly potential applications in spintronics.Up to now, most of the reported paramagnetic or ferromagnetic carbon materials have a graphite-based microstructure. Namely, these carbon materials base on the sp2carbon six-member ring network, such as C60polymers, highly oriented pyrolytic graphite (HOPG), carbon nanotubes (CNTs), and graphene, etc. The ferromagnetism or paramagnetism in graphite-based materials are often considered to be induced by the defects of the six-member ring network, such as vacancies (point or line vacancies), frustrations, topological defects, edges, heptagons, or doping with light non-metallic atoms (N, H, B, O, and F), etc.Despite that the theoretical study demonstrates that the incorporation of heptagons can introduce negative curvature which can induce localized unpaired spins, no experimental work has been reported up to now. Interestingly, the HCNTs are formed by the periodic incorporation of pentagons and heptagons in the hexagonal carbon network. Therefore, the magnetic properties of HCNTs are expected. Moreover, doping nitrogen into carbon materials, as a promising way to change their electronic structure and improve the electronic, mechanical, chemical, and optical properties, its influence on the magnetic properties aroused much attention too.In this thesis, we have synthesized graphite-based carbon materials (including HCNTs, carbon micro-spheres, and graphite flakes) by means of chemical vapor deposition (CVD), and have successfully doped nitrogen into the carbon materials by annealing the pristine samples in NH3atmosphere. We concentrated on the studies of the magnetic properties of the pristine and nitrogen-doped graphite-based carbon materials. The photoluminescence (PL) properties of the pristine and nitrogen doped HCNTs have also been investigated. Our main work includes the following four parts:Ⅰ. Investigations on the synthesis and magnetic properties of HCNTs.1. The high-purity HCNTs were synthesized by means of acetylene pyrolysis at425℃using Ni nanoparticles as catalyst. It is demonstrated that the HCNTs have a graphite-based microstructure with lots of defects. The Ni nanoparticles transformed into Ni3C because of the carbonization and there are only a few metallic Ni residues.2. Both the metallic Ni residues and the HCNTs contribute to the magnetization of the samples. By subtracting the contribution of metallic Ni, the magnetism of the HCNTs was roughly investigated. Moreover, by annealing at different temperature in Ar atmosphere, the magnetism of the annealed HCNTs was systematically investigated. The origin of the magnetism of HCNTs has also been discussed.3. The result demonstrates that the HCNTs possess strong Curie-like paramagnetism and high-density of localized unpaired spins. The unpaired spins may induced by vacancies, H adatoms, and negative Gaussian curvatures introduced by heptagons, while the negative Gaussian curvatures introduced by heptagons dominate the magnetism of HCNTs.Ⅱ. Investigations on the PL and magnetic properties of the pristine and N-doped HCNTs. 1. A series of N-doped HCNTs were successfully synthesized by annealing the pristine HCNTs at different temperatures in NH3atmosphere.2. The main nitrogen doping types are pyridinic and pyrrolic. At relatively low annealing temperatures of450and500℃, the N atoms bond to C just on the edge or defect of the HCNTs in NH3atmosphere. However, N atoms could enter the graphitic layers of the HCNTs and substitute the inner C atoms in the six-member ring networks at high temperatures of700℃.3. Magnetic studies demonstrate that the magnetic moment of HCNTs can be enhanced by annealing in NH3atmosphere. The N-doped HCNTs also possess strong Curie-like paramagnetism and high-density of localized unpaired spins.4. The HCNTs show ultraviolet PL at368nm. By doping of pyridinic and graphitic N into the pristine HCNTs, the PL can be enhanced.III. Investigations on the magnetic properties of interconnected carbon micro-spheres1. The interconnected graphitic-based carbon micro-spheres with diameters in the range of3～5μm were successfully synthesized on a SiO2substrate by means of CVD without using metallic catalyst. The carbon micro-spheres have an onion-like structure with lots of native defects.2. Magnetic investigations demonstrate that the paramagnetism of the carbon micro-spheres originates from the native defects. The defects contribute to unpaired spins with magnetic moment of1μB and approximate per1,000carbon atoms contribute1.1μB unpaired spins. Because these unpaired spins are non-interaction, the sample shows typical Curie-like paramagnetism.IV. Investigations on the magnetic properties of the pristine and N-doped graphite flakes.1. Large area of graphite flakes without any magnetic metallic impurities were synthesized on a SiO2substrate by means of CVD. The graphite flakes have a incomplete layered structure with lots of defects such as bending, distortion, and crack etc.2. Magnetic investigations demonstrate that the paramagnetism of the graphite flakes originates from the native defects. The defects contribute to unpaired spins with magnetic moment of1μB and approximate per1,000carbon atoms contribute0.8μB unpaired spins. Because these unpaired spins are non-interaction, the sample shows typical Curie-like paramagnetism.3. A series of heavy nitrogen doped graphite flakes were successfully synthesized by annealing the pristine sample at different temperature in NH3atmosphere. All of the N-doped graphite flakes show Curie-like paramagnetism. The spin density of the N-doped sample annealed at500℃shows a greatly enhancement, while that of the other N-doped samples show a slight decreasing.