Computer Modeling of Nanoporous Materials: An ab initio Novel Approach for Silicon and Carbon
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Computer Modeling of Nanoporous Materials: An ab initio Novel Approach for Silicon and Carbon Ariel A. Valladares1, Alexander Valladares2, and R. M. Valladares3 1 Condensed Matter, Instituto de Investigaciones en Materiales, UNAM, Apartado Postal 70-360, Mexico, D.F., 04510, Mexico 2 Physics Department, Facultad de Ciencias, UNAM, Apartado Postal 70-542, Mexico D.F., 04510, Mexico 3 Physics Department, Facultad de Ciencias, UNAM, Apartado Postal 70-542, Mexico, D.F., 04510, Mexico
ABSTRACT Carbon and silicon have been consistently proposed as elements useful in the generation of porous materials. Carbon has been insistently postulated as a promising material to store hydrogen, and crystalline silicogermanate zeolites have recently been synthesized and are being considered in catalytic processes. In the present work we report an approach to generating porous materials, in particular porous carbon and silicon, which leads to the existence of nanopores within the bulk. The method consists in constructing a crystalline diamond-like supercell with 216 atoms with a density (volume) close to the real value, then halving the density by doubling the volume (50% porosity), and subjecting the resulting supercell to an ab initio molecular dynamics process at 300 K for Si, and 1000 K for carbon, followed by geometry relaxation. The resulting samples are essentially amorphous and display pores along some of the “crystallographic” directions. We report their radial distribution functions and the pore structure where prominent. Keywords: Porous carbon; porous silicon, computational simulations INTRODUCTION The quest for new materials with a variety of porous structures has acquired an important dynamics, mainly due to their potential usefulness in several applications that go from catalytic processes to storing and filtering atoms and molecules. Recently, we started to study nanoporous structures of carbon (the size of the pores are in the nanoregime) to investigate their use as a storage (fuel) “tank” for hydrogen [1] [2]. It is well known that activated carbon has been used for a long time as a reactive cleaning agent to get rid of unwanted byproducts in catalytic processes. This reactive behavior makes the study of porous carbon as a fuel tank to store hydrogen or to trap other contaminating substances a technologically important subject. Around the time we begin our investigations an experimental work was published concerning hydrogen adsorption in different carbon structures [3] making the subject more appealing since a possible comparison of our results with experiment could be drawn [2]. Due to its chemical valence carbon appears in a variety of atomic structures and the multiplicity of bond types leads to amorphous carbon, nanotubes, nanoporous carbon and graphene nanostructures, to mention a few. One would expect that these structures would manifest themselves in amorphous
nanoporous carbon since the atomic structure is related to the density, and this varies notably depending on the porosity of the sample
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