Structural, Electronic And Hydrogen Storage Properties For Clusters And Graphene Oxide

In recent years, the study of low-dimensional nano structures such as clusters, carbon nanotubes and graphene, is a hot topic in the fields of physics, chemistry and many other subjects. It as the bridge between microscopic atom/molecules and macroscopic condensed matters is of key importance to reveal and design the novel nano-materials that are technologically promising. In this dissertation, we discussed from the zero-dimensional semiconductor and metal clusters, the behavior of water clusters inside one-dimensional nanoscale enviroments, to the structure of the two-dimensional graphene oxide and its hydrogen storage properties.Semiconductor clusters and metal clusters are two hot topics in the field of atom clusters. The study of semiconductor clusters is of key importance to understand the growth mechanism of semiconductor materials with different scales and dimensions and to reveal the novel semiconductor nanostructures that are technologically promising. We have investigated the semiconductor germanium and III-V compounds (AlP and InP) clusters using density functional theory method. For the germanium clusters, we have performed an unbiased global search for the geometries in the size range of 30≤n≤39 using genetic algorithm and the Gen (n=30-39) clusters prefer the motif of supercluster structures stacked by several stable subunits such as Ge10 and Ge6 connecting via a few bridging atoms. We have obtained the lowest-energy structures for the small-sized neutral and anionic AlP and InP clusters and calculated the electronic properties including the binding energies, HOMO-LUMO gaps, electron affinities and photoelectron spectra, agree well with the experiments. Also, inspired by the concept of superatom, we investigated the H2 dissociation on the doped icosahedral Al12X (X=B, Al, C, Si, P, Mg, and Ca) clusters by means of density functional theory. The hydrogen dissociation behavior on metal clusters characterized by the activation barrier and reaction energy can be tuned by controllable doping. Among these doped clusters, Al12Ca with the highest reaction energy and lowest activation barrier is the best catalyst for dissociating H2. Thus, doped Al12X clusters might serve as highly efficient and low-cost catalysts for hydrogen dissociation.Water has been recognized as the matrix of life and plays a crucial role in many biological and chemical systems. Studying the water confined in nanoscale environments provides a better understanding of the proton transport across the channels in the cellular membrane that transport water in and out of the cell. As a result of nanoscale confinement, the chemical and physical properties of the encapsulated water are different from the bulk counterparts. We have investigated the water clusters confined inside the nonpolar and polar cavities modeled by carbon fullerene cages and carbon nanotubes, respevtively, and analyzed the equilibrium structures, electronic properties and vibration frequencies for the encapsulated water clusters. The dipole moments of water clusters in the confined phase are smaller than those in the gas phase due to the screening effect of the outer cavities. The interaction between water molecules and the outer environments is identified as physisorption, but the weak coupling effects the electronic and vibrational properties of the encapsulated water molecules. We also considered the infrared spectrum for the ordered and disorded water clusters confined inside carbon nanotubes, which can be used to distinguish the configuration of encapsulated water molecules.Graphene oxide (GO) has recently attracted resurgent interests as a parent material for producing large-scale graphene-like platelets. Experimentally synthesized graphite oxides are disordered, which makes the determination of the atomic structures difficult. We have investigated the GO structures starting from the stable zero-dimensional (0-D) structural motifs consisting of the hydroxyl and epoxy functional. The 0-D structural motifs prefer to form the chain-like one-dimensional (1-D) structural motifs, and then the stable two-dimensional GO structures are obtained. Moreover, the Raman characteristics for the local stable GO structures are simulated and the results provide useful theoretical evidences to differentiate these structures with aid of Raman spectroscopy and would be helpful to further understand the structural and vibration properties of GO. Based on this, we studied the hydrogen storage properties on the GO surface. GO contains ample hydroxyl groups, which are the active sites for anchoring Ti atoms. The Ti atoms bind strongly to the oxygen sites with binding energies as high as 450 kJ/mol, which are large enough to prevent the Ti atoms from clustering. Furthermore, each Ti can bind multiple H2 with the desired binding energies (14-41 kJ/mol per H2). The estimated theoretical gravimetric and volumetric densities can be as high as 4.9 wt% and 64 g/L, respectively.