Theoretical Studies Of The Chemical Modification And Electronic Properties For Nanotubes Materials

Design and synthesis of new-type materials with various physical or chemical properties is very important in experimental research and industrial applications, recently. Nanotube materials (included carbon nanotubes, boron nitride nanotubes and silicon carbide nanotubes et.al), which exhibit unique structures and excellent electronic, mechanical and thermodynamic properties, have evoked a lot of research intersests and become a research topic in materials science, physics, chemistry and biology. The potential applications of nanotubes materials are numerous and will play a key role in future nanoscience and nanotechnology. Some widely unvestigated examples include the applicability to nanoscale devices, chemical sensors, nano reactor and hydrogen storage materials. However, the poor solubility and difficulties of purifying and processing of nanotubes materials have hampered the future application of nanotube materials. Chemical modification (i.e., functionalization) was not only extensively promoted as one way to overcome these problems, but also can provide an approach to design and synthesis new nanotube materials with more excellent properties. In this paper, we chose several chemical modifications of nanotubes materials with typical reagents which have been carefully studied using quantum methods, obtained some interesting results. On the basis of the molecular orbital theory, the traditional transition state theory as well as quantum chemistry theory, the systems chose have been investigated using Density Functional (DFT) Theory and energy band theory. The structures of the reagents, the reaction products and he transition states along the reaction paths have been obtained, then we got the sturctures, the reaction energies, electronic properites as well as the information of orbitals. The reaction mechanism has been discussed deeply using these data.The whole paper consists of seven chapters. Chapter 1 mainly reviews the evolution of chemical modification of nanotubes materials. The second chapter summarizes the theory of quantum chemistry and calculation methods of this paper. The contents of two chapters were the basis and background of our studies and offer us with useful and reliable quantum methods.Chapter 3 In this paper, Metallocenes (MCp2, M = Fe, Co, Ni), a well known organometallic molecule withπ-electron system, have been considered as encapsulation inside carbon nanotubes using DFT theoretical investigation, and the structural, energetic and electronic properties of MCp2@SWCNT have been obtained. We verified that such an encapsulation is actually a noncovalent functionalization, and examined binding energies and charge transfer of the MCp2@(16, 0) SWCNT system as a function of the adiabatic ionization potential of metallocene molecule. For Ip (Iv) configuration the optimal distance between the central of FeCp2 molecule and the near tube's wall is 4.70 (5.10) ?, as well as the minimum diameter of SWCNT is about 9.40 (10.20) ? to exothermically encapsulate a FeCp2 molecule. In addition, the electronic properties of MCp2@(16, 0) SWCNT systems were examined in detail, clarifying the doping effects the bandgap and revealing a sizable charge by encapsulation of CoCp2 and NiCp2, along with the advantages in not distorting the wall of the tubes.Chapter 4 A theoretical exploration of the 1,3-dipolar cycloadditions CH2N2 and N2O onto pristine and B-doped single-walled carbon nanotubes (SWCNTs) have been carried out. We can found the reaction activities have been enhanced by B-doped. For Path-CBNC, compared with the reaction of CH2N2 on pristine SWCNTs, the barrier energies and reaction energies of 1,3-dipolar cycloadditions are decreased by 0.58 eV and 0.80 eV, respectively. Likewise, for Path-NBCC, the barrier energies and reaction energies of 1,3-dipolar cycloadditions are decreased by 0.35 eV and 1.30 eV, respectively. On the other hand, for Path-OBNC, compared with the reaction of N2O on pristine SWCNTs, the barrier energies and reaction energies of 1,3-dipolar cycloadditions are decreased by 0.60 and 1.56 eV, respectively. The predicted reacitivity of our considered reaction can be understood in terms of the frontier molecular orbital (FMO) theory.Chapter 5 The reaction behavior of chemical modification of boron nitride nanotube (BNNT) with the ammonia plasmas have been investigated by density functional theory calculations. Unlike previously studied of the functionalization with NH3 and amino functional groups, we found that NH2* radical involved in the ammonia plasmas can be covalently incorporated to BNNT through a strong single B-N bond. Subsequently, the H* radical, which is also involved in the ammonia plasmas, would prefer to combine with the N atom neighboring the NH2-functionalized B atom. Our study reveal that this reaction behavior can be elucidated using the frontier orbital theory. The calculated band structures and density of states (DOS) indicate this modification is an effective method to modulate the electronic properties of BNNTs. We have discussed various defects on the surface of BNNTs generated by collision of N2+ ion. For most defects considered the reactivity of the functionalization of BNNTs with NH2* are enhanced. Our conclusions are independent of the chirality and the diameter dependence of the reaction energies is presented.Chapter 6 We systematically studied the structural, energetic and electronic properties of zigzag boron nitride nanotubes (BNNTs) functionalized by a class of substitutional carbene (CR2) where R = H, F, Cl, CH3, CN and NO2 on different absorption sites using density functional theory. For R = H, F and Cl, the open structure is preferred with a BNNT sidewall bond cleavage. While for R = CH3 and CN, the competition between the open and closed cyclopropane-like three-membered ring (3MR) structures occurs. Interestingly, for R = NO2 we find a novel double five-member-rings (5MR) structure with high reaction stability. This new structure cannot be found in BNNTs'alternative carbon nanotubes (CNTs). In addition, the electronic properties of BNNTs functionalized with carbenes are hardly changed for R = H, F, Cl, CH3 and CN, but significantly affected for R = NO2 due to the heterocyclic double 5MR structure.Chapter 7 The adsorption/dissociation of O2 molecule on the surface of silicon carbide nanotubes (SiCNTs) was investigated by density functional theory. We found several adsorption configurations, including chemisorption and cycloaddition configurations, for triplet and singlet O2. Unlike the case of carbon nanotubes, the chemisorption of triplet O2 on SiCNTs is exothermic with remarkable charge transfer from nanotubes to O2 molecule. Singlet O2 adsorption on the surface of SiCNTs can yield cycloaddition structures with large binding energies and sizeable charge transfer. The reaction mechanism studies show that for triplet O2 chemisorption configuration is favorable, but cycloaddition configuration is preferred for singlet O2. For singlet O2, we also studied the dissociation of O2 molecule, and a two-step mechanism was presented. The dissociation of molecular O2 results in formation of two three-membered rings with large binding energies. The key to the dissociation process of singlet O2 on the SiCNTs surface is the first step with the barrier energy of 0.40 eV. Finally the electronic properties of SiCNTs adsorbed with triplet and singlet O2 are shown to be dramatically influenced.