First-Principles Study Of The One-dimensional Nanomaterials

With the sustained and rapid development of the world economy, the fossil energy, such as coal, oil and natural gas, has been consumed increasingly, and leads to serious environmental pollution. To develop sustainable, efficient and environmentally friendly energy resources have become one of the most important topics of the energy strategy for a country. Due to their outstanding features, hydrogen and natural gas (methane is the major component) become the most promising new energy resources. At present, one of the main problems that impede practical uses of hydrogen and methane is their storage and transport. The common high-pressure compression is not a suitable storage means for some drawbacks, for example, energy cost, expensive equipment, safety concerns and so on. As a result, adsorption storage is promising for storing hydrogen and methane at ambient temperature and pressure, in which the research and development of high efficient, low cost nanoporous adsorption materials is a research area with priority.The pollution gases, such as oxidates of carbon, nitrogen, sulfur etc., resulting from industrial production processes is harmful for both environment and human body. Therefore, gas sensors are needed to detect and warn timely the gas content in specific environment. For better practical applications, important research areas of gas sensors are focued on the development of sensors with high sensitivity and selectivity. For their complex structure and small size, nanomaterials are regarded as ideal sensor candidates.The first-principles calculation, based on quantum mechanics, is used to study structures and properties for nanomaterials in the electronic level. Without empirical parameters, this method could reveal interaction mechanism between different materials in atomic or molecular level, and thus are widely used in the design of nanomaterials with specific adsorption or sensing properties. For the storage research of H2 and CH4, the multiscale method is usually used. The main ideals include:first, using the first-principles calculation to calculate the interaction energies between the adsorbate and adsorbent; second, fitting the results to a force field and its parameters; then, performing grand canonical ensemble Monte Carlo (GCMC) simulation to obtain the macro-properties. In the process, the bridge (coupling) between the first-principles calculation and the GCMC simulation is a pivotal step. On the other hand, for the research of a gas sensor, the effect between the gas molecule and the electronic property of the sensor material is the key factor for the sensing application. This work is contributed to storage of clean energy gas (H2 and CH4), and the sensor of toxic gases (CO and NO). By modeling the CH4 and carbon nanotubes (CNTs) and using the mature classical parameters of the Lennard-Jones (LJ) potential (CLJ), the bridge method is adopted for the multiscale simulation. By analysing of the band structure and electronic of density states, carbon nanotubes (CNTs), silicon nanotubes (SiNTs) and metal (Pd, Pt)-decorated carbon nanotubes are investigated for sensing of CO and NO. The main findings and novelty are summarized as follows.1. By using the first-principles calculation, the potential energy curves of the interaction of CH4 with (14,14) CNT and the adsorption sites are obtained. It is found that for the MP2 and PW91 methods, the binding energy increases with polarization and diffusion functions in the basis sets. The results from the MP2 method are larger than the PW91 method based on the same basis set. In addition, the basis set superposition error (BSSE) must be considered in the MP2 method, but not necessary in the PW91 method. Via the site-to-site and site-to-surface interaction calculations, the results from the first-principles calculation are fitted to LJ potential and Morse potential, respectively. It is found that the Morse-fitting potential is in good agreement with the first-principles calculation results. The fitting potentials are then put in GCMC simulation for adsorption calculations. As a result, the site-to-site method, the methane uptakes obtained by the Morse potential on the MP2/6-311G** level and the LJ potential on the MP2/6-311G* level are in agreement with the CLJ results. By using the site-to-surface method, the methane uptakes obtained by the LJ potential and PW91 method are similar with the CLJ results. It is expected that this work provides useful information for the multiscale simulation method and the selection of accurate force field models.2. By using the first-principles calculation, structural properties of a series of SiNTs, such as armchair and zigzag, are explored. It is found that these SiNTs could form a stable smooth tube with sp2 hybridization. The electronic properties of SiNTs depend on diameter and chirality. Only the zigzag SiNTs with large diameters could be semiconductor. Moreover, adsorption of CO and NO on (8,0) SiNTs are investigated. It is found that CO and NO can be physisorbed or chemisorbed on the top or bridge site outside the SiNTs, leading the puckered structures become noticeable. When CO is located on the top site, its C atom could be bound with the Si atom on the tubular surface, with the binding energy of 1.559 eV and charge transfer of 0.658 e. When NO is located on the bridge site, N atom could be bound with two Si atoms on the bridge site, with the binding energy of 2.135 eV and charge transfer of 2.064 e. Finally, the adsorption of CO and NO on (16,0) SiNTs are studied. After CO and NO adsorption, the (16,0) SiNTs become metallic from semiconducting. The change of the electrical properties of SiNTs can provide an electronical signal for detecting these gases. In particular, magnetic generation of the SiNTs can also serve as a sensitive signal for NO. In short, the SiNTs with the semiconducting structure are a promising candidate for CO and NO detection.3. By using the first-principles calculation, the adsorption of metal (Pd and Pt) on the (8,0) single-walled carbon nanotube (SWNT) is investigated. Comparing cell-1(C:Pd (or Pt)= 64:1) with cell-2 (C:Pd (or Pt)= 32:1), it is found Pd and Pt could be chemisorbed on the C-C bridge site outside the surface. Due to different densities of metal atoms in the two cells, the binding energies in cell-2 are larger. The pure (8,0) SWCNTs are semiconducting with the band gaps of 0.64 eV. The band gaps of Pd- or Pt-decorated SWNT with cell-1 are 0.59 eV and 0.51 eV, respectively. However, the band gaps of Pd- or Pt-decorated SWNT with cell-2 are 0.56 eV and 0.32 eV, respectively. Considering the single vacancy defect, the band gap of (8,0) SWNT becomes 0.35 eV. After adsorption of Pd and Pt, the values change to 0.34 eV and 0.33 eV, respectively. The doped Pd make the metallic (5,5) SWNT becomes semiconducting with the band gap of 0.25 eV. The electronic changes result from the charge transfers from Pd and Pt to the SWNT.4. By using the first-principles calculation, the sensing property for the Pd- and Pt-decorated (8,0) SWNT with cell-2 are investigated. It is found that CO and NO gas molecules can be chemisorbed to the Pd or Pt atom, with a large binding energy and significant charge transfer. For CO adsorption, the band gap of Pd-SWNT becomes 0.62 eV, and the band gap of Pt-SWNT becomes 0.55 eV. The different binding energies, charge transfer and conductance change may lead to the different responses in the CNT-based sensors, which could be used to explain experiment. Compared to CO, the adsorption of NO has lower binding energy and larger charge transfer in the same condition. As a result, NO is also could be detected by the Pd-or Pt-decorated SWNT sensor.For the single vacancy defect, the new Pd-or Pt-doped (8,0) SWNT will have stronger response to CO and NO. It is found the Pd-doped (5,5) SWNT also could be used as CO and NO sensing. In conclusion, metal-decorated SWNT exhibits strong affinity towards the gas molecules above. These results may be helpful for the design of the nanoscale sensing devices.