First-principles Studies Of Doping Effect On Carbon Nanomaterials

Carbon nanomaterials are carbon materials with one or more dimensions in the scale of nanometer. There are several types of carbon nanomaterials, such as fullerene, carbon nanoparticles (carbon nanospheres, carbon nanocapsules), carbon nanotubes (carbon nanofibers), graphene and other nanoporous carbons. Due to their characteristic properties, carbon nanomaterials are of great potentials in a wealth of applications such as mechanics, electrics, magnetism, optics and catalysis etc. Thereby, carbon nanomaterials have aroused widespread interests since their discovery and became the most important research interest of nano-science. Until now, research on the properties and technologies of carbon nanomaterials is still the cutting edge of modern Science.Along with the development of technology, the properties of pure carbon materials cannot meet our needs for new materials and new properties. Various methods and technologies are used to modify the properties of carbon materials. For instance, doping is widely used to modify the properties of carbon nanomaterials, but the detailed mechanism of doping is still unknown to us due to the lack of numerous experimental data for analysis. Therefore, setup theoretical models that have potentials to unveil the doping mechanism are of critical importance. In the field of nano materials, quantum mechanics is the fundamental law for these micro-systems. But fully solving the schrdinger equation which is the key of quantum mechanics is too hard to complete. In light of this dilemma, approximate solutions of the schrodinger equation has emerged as an alternative method. By applying several approximations, schrdinger equation can be solved in a much easier and quicker way. With the quick development of computer science in recent twenty years, the cost of computation reduces greatly and computational chemistry plays a more and more important role in theoretic simulations. Being quick and reliable in calculations, first principle calculations based on density functional theory (DFT) have turned into the most important method in computational chemistry and becames an important simulation method in material science and condensed matter physics for its high efficiency and accuracy. In this thesis, we investigated the doping mechanism and their influences on the properties of carbon nanomaterials by first principle calculations.The thesis includes five chapters. In the first chapter, we briefly introduce the research methods in computational chemistry with the focus on the framework of density functional theory, their developments and common exchange correlation functionals. The critical point in density functional theory is to establish single electron model:A non-interaction system having the same charge density with the real system. The kinetic energies and Hartree potentials of non-interaction system are therefore used to approximate of the kinetic and interaction energies of the real system. The resultant differences between non-interaction system and real system are included in the exchange correlation item. Hence, the density functional theory is accurate from a theoretical point of view, provided that a suitable exchange correlation functional is found to reduce the difference between simulations and experiments. In the latter chapter, we introduce the methods for band calculation in solid state materials that have been widely applied in solid state physics. In the meantime, we introduce the GW method with quasi-particle model and time dependent density functional theory. Since we involved many calculations on energy barriers of chemical reactions, we briefly introduce the computational methods of calculating energy barriers in chemical reaction. Finally, we introduce some information about software packages based on density functional theory.In the second chapter, we mainly studied the effects of fullerenes doping on the transportation properties of carbon nanobubes. In the beginning, we briefly introduced the development of carbon nanotubes, especially a contradict experiment result that might be explained by theoretical calcuations. With the encapsulating of azafullerenes, the same carbon nanotubes (called’ nanopeapods’) show different unilateral conductivities as has been confirmed by different research groups. By using first principles studied on geometric structures and band structures, we found the possible reasons for such contradictions:carbon nanotubes with different structures can lead to different transportation behavior. Only the carbon nanotubes with a5-8-5defect can change from p-type semiconductor to n-type semiconductor, with the encapsulating of azafullerenes.In the third chapter, we research on the catalytic characteristics of nitrogen doped carbon nanomaterials as electrode materials in oxygen reduction reaction processes. These nitrogen doped carbon nanomaterials as metal-free catalysts show high electrocatalytic activity and have great potential to replace the traditional Pt based electrode materials, since Pt is expensive and is easy to be poisoned by CO and have poor tolerance and stability to methanol. Although these nitrogen doped carbon nanomaterials still have a lower activity than Pt based materials, their potential as novel electrode materials to replace Pt based counterparts have been confirmed by various experimental and theoretical results. Due to the complex of catalysis, the mechanism of doping nitrogen into carbon nanomaterials, especially the catalyzing center is still under debate and the true mechanism of catalysis is still under study. In this chapter, we calculate the energy barriers of oxygen molecule dissociation on carbon nanostructures with nitrogen doping by applying a simple model to simulate oxygen dissociation on nitrogen doped carbon nanomaterials. Our results show nitrogen doping improves the electrocatalytic activities of carbon nanotubes. The graphite-like nitrogen doping increases the electrocatalytic activity most.The fourth and fifth chapters are the extended work of chapter three. Since nitrogen doped carbon nanomaterials showed good catalytic activity towards oxygen reduction, other elements such as boron and sulfur have been doped into the carbon nanomaterials. Although doping with these elements do not show a comparable activity as nitrogen doped one, codoping boron/nitrogen, sulfur/nitrogen showed an improved activity. In Chapter four, we mainly studied the boron doping and boron/nitrogen codoping carbon nanotubes as electrode materials and their catalytic activity in terms of oxygen reduction. We calculated the energy barriers of oxygen dissociation on these materials. Our calculation indicates that simple boron doping does not contribute a high catalytic activity while boron/nitrogen codoping has a higher activity than nitrogen doped carbon nanotube. These results are consistent with experimental results. In the fifth chapter, we investigated the sulfur/nitrogen codoping on the oxygen reduction capability of carbon nanotubes, our results suggested that sulfur/nitrogen codoping can reduce the energy barrier of oxygen reduction.