Hydrogen Atom Adsorption on Aluminum Clusters: An Electronic Structures Density Functional Study
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Hydrogen Atom Adsorption on Aluminum Clusters: An Electronic Structures Density Functional Study Phung Thi Viet Bac, Hiroshi Ogawa Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba 305-8568, JAPAN ABSTRACT Properties of hydrogenated icosahedral aluminum clusters were investigated using density functional theory in comparison with those of aluminum bulk systems. Two surface models simulating f.c.c. and icosahedral (111) surfaces were introduced. Results show that the H atom interacts weakly with surface of clusters when the cluster size is increased. The migration energy of H atom between neighboring T and O sites becomes smaller for icosahedral subsurface than for either bulk material or the f.c.c. subsurface. The results indicate that the icosahedral surface is more favored for H atom to adsorp than f.c.c. surface, the icosahedral surface increases the migration barriers of H atom from the surface to the subsurface. INTRODUCTION Nanomaterials have attracted great interest in recent years because of their unusual mechanical, electrical, electronic, optical, magnetic and surface properties. The high surface/volume ratio of these materials presents important implications with respect to energy storage. Nanostructured materials are promising for hydrogen storage because of their unique features such as adsorption on the surface, intergrain and intragrain boundaries, and bulk absorption [1]. Nanostructured and nanoscale materials strongly influence the thermodynamics and kinetics of hydrogen absorption and dissociation by increasing the diffusion rate and by decreasing the required diffusion length. Investigation of suitable materials for hydrogen storage has driven attention to different lightweight nanostructures. Among various hydrogen storage materials, hydrogen adsorption by aluminum clusters of different sizes, aluminum nanowires, and aluminum f.c.c. clusters can be considered as promising candidates for fuel cell hydrogen storage devices [2, 3]. These findings are important because light metals such as aluminum are inexpensive and can store large amounts of hydrogen, which is crucial for fuel cell design [4]. Crane and Nuzzo [5] obtained evidence related to H adsorption on Al(111). Through careful examination of temperatureprogrammed desorption data, they suggested that chemisorbed H on Al(111) occupies both surface and subsurface sites. Using low-energy ion spectroscopy Bastasz [6] showed that the occupied site of atomic hydrogen exists on the top layer of the surface. Based on both experimentation and simulation[7, 8, 9, 10, 11], hydrogen is known to adsorb strongly on the surface of aluminum clusters, thereby greatly changing the structure of the clusters. It is important to understand the properties of aluminum clusters and their interaction with hydrogen at a fundamental level. In previous reports [12, 13], we demonstrated the electronic properties of aluminum clusters from small clusters up to several hundre
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