Surface And Adsorption Behaviors Of Several Low-Dimensional Materials

Zero-dimensional nanoparticles, one-dimensional nanowire, and two-dimensional slab are so-called low-dimensional materials. These materials show some special physical behaviors due to their different structures. One of the prominent characteristics of the low-dimensional material is its high density of grain boundaries, which renders a very high surface/volume ratio. In this case, their atomic arrangements cannot remain long range order in solids, and their thermodynamic properties are different from their corresponding bulk crystals, due to their high density grain boundary and their special configuration of atoms around the interface. Another prominent characteristic of low-dimensional material is its size effect. For example, the geometry and symmetry of a cluster can be completely different just by adding or deleting one atom. In fact, the distribution of electronic level changes with the dimensional and size of the nanomaterials, which renders the dimensional- and size-dependent properties. Therefore, systematically study the surface (or interface) properties, adsorption behaviors, geometric parameters, and electric structures are beneficial for us to further understand the physical mechanism of the low-dimensional materials.Hydrogen (H2) has been recognized as an attractive alternative energy carrier, which is lightweight, non-polluting, highly efficient, and easily derived. The so-called H2 fuel economy, however, faces various hurdles, such as safe and reliable storage concepts that can be used to deliver and store H2 in a cost effective way. Traditional storage approaches, such as compressed gas and liquefaction, are not suitable for H2, because of safety issues and relatively high energy costs associated with these approaches. A review of the current literature shows that carbon-based nanostructures, including nanotubes, fullerenes, and graphenes have emerged as attractive candidate hydrogen storage materials. Implementation of hydrogen storage systems requires moderate bonding strength. However, this goal has remained a challenge: on the one hand, the weak binding strength between H2 and the solid surface should be strengthened; on the other hand, extremely strong binding is not ideal either since finally both adsorption and desorption have to be considered. Previous works have demonstrated that the binding strength of H2 is related to the extra dipolar moment of the entire system. Therefore, one may hypothesize that the uptake capacity will increase if more dopants are added. In addition, the binding would be strengthened if more charges are transferred between the dopants and nanostructures.With the development of computational ability, computer simulation technologies are being widely applied in all kinds of scientific fields, which have become an effective complementary tool to the traditional experiments. For example, experimental study of size-dependent catalytic behavior is challenging due to the difficulties associated with the preparation of uniform samples with varying dimensions. In this case, computer simulation, in particular the density functional theory (DFT) has evolved as an essential tool in the study of catalytic behavior. Apparently, it is much easier to″prepare″a sequential growth of metal cluster″samples″for size effect analysis in the computer than it is in the laboratory. In addition, DFT offers distinct advantages in electronic structure determination, such as charge transfer and orbital hybridization, which is beneficial for us to understand the reaction process and mechanism in the electronic level.The detailed contents are listed as follows:1. Ab initio calculation performed by DFT and the broken bond model are utilized to systemically determine the surface energies of ceramics with B1 or NaCl structureγ100, where the subscript shows the index of surfaces. The ceramics includes the transition metal carbides (TMC), the transition metal nitrides (TMN), and the alkaline metal oxides (AMO). The results show that calculatedγ100 values of these compounds correspond to other available theoretical and simulation results well. Moreover,γvalues of AMOs on different surfaces determined have a size order ofγ100 <γ110 <γ111.γand work functionΦof twelve III-V semiconductors on (110) surfaces are calculated. The obtained values are proportional to the corresponding cohesive energy Ec, and are in good agreement with available experimental data and theoretical models. The linear relationship among Ec,γ, andΦare interpreted by analyzing their electronic properties.2. The atomic structure, thermodynamic properties, and electronic structures of NiAl(110)/Cr(110) interface are studied using first-principle density functional plane-wave ultrasoft pseudopotential method. Theγvalues of different NiAl surfaces are compared with those obtained based on the classical broken-bond rule. Simulation results indicate that the structure of Ni and Al placed in the hollow-sites of Cr atoms at the interface is more thermodynamically stable, and the NiCr bonding is dominated by 3d electrons of Ni and Cr. It is found that NiAl(110)/Cr(110) alloying could lower brittleness of NiAl compounds. With simulated values of adhesion work Wad and interface energyγi for NiAl(110)/Cr(110) system, its mechanical and thermodynamic properties are also discussed.3. DFT calculations with the All Electron Relativistic (AER) core treatment method are used to determine adsorption of CO on close-packed surfaces of Ru, Rh, Pd, Os, Ir, and Pt. The adsorption energy Ead andΦorders are obtained, which are Pd > Pt > Rh > Ru > Os > Ir and Os > Ir > Ru > Rh > Pt > Pd. In terms of the plot of electron density difference and the values of Mulliken analysis, it is found that charges transfer from metallic surfaces to CO molecules. In addition, the interaction of CO, H, and C with a sequential growth of Cu clusters with a special structure is also studied. The Ead(l) functions are found to be parabola-like for all adsorption systems, with the maximum values at layer number l = 5?6. In this case dave≈2.56 ?, which is approximately equal to the atomic distance of Cu in bulk crystals. The binding strength between the adsorbate and substrate, or ?Ead(l), is inversely proportional to their corresponding bond length d.4. We demonstrate that optimizing the number and position of dopants, a configuration of 8 Li dispersed at the hollow sites above the hexagonal carbon rings can lead to an extremely high H2 storage capacity of 13.45 wt %. Moreover, our local density approximation (LDA) calculations predict that the average Ead = ?0.17 eV/H2, which is close to the lowest requirement (?0.20 eV/H2) as proposed by the U.S. Department of Energy. The electronic analysis demonstrates two salient points, namely that the best dopants are those whose bands overlap strongly with those of H2 and the nanotube simultaneously; second, all carbon atoms in the nanotube are fully ionized and thus the high capacity is attainable.5. Using an 8-Li-doped carbon nanotube, it is found that H2 binding can be externally enhanced (or weakened) via superimposition of a positive (or negative) electric field. The calculated Ead = ?0.58 eV/H2 under F = +0.010 au is 93.33 % lower than that in the absence of a field (F indicates the field intensity). This is because the positive field produces an extra dipole moment. In contrast, Ead increases from ?0.30 to ?0.20 eV/H2 when F = ?0.010 au. In view of the fact that storage systems are insensitive to small unexpected field fluctuations, the application of the electric field as a reversible switch makes practical sense. Similar results can also be found in the Li-doped single-layer and bilayer graphenes. Our results show that the binding strength increases by 88 % when a field with a magnitude of +0.020 au is imposed. Hirshfeld charge analysis results suggest that an increase in the binding strength will occur as long as the Li (or C) carries more positive (or negative) charges.