Classical MD Simulation of Hydrogen Absorption in F.C.C. and B.C.C. Nanoparticles
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Classical MD Simulation of Hydrogen Absorption in F.C.C. and B.C.C. Nanoparticles Hiroshi Ogawa and Phung Thi Viet Bac, NRI, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
ABSTRACT Hydrogen absorption in metallic nanoparticles was investigated by classical molecular dynamics (MD) simulation. We used a simple model composed of an isolated f.c.c. or b.c.c. nanoparticle of 1, 1.4, 2, 4, 6, 8 and 10 nm in diameter and surrounding hydrogen atoms. The simulated particle sizes are which correspond to about 50 to 44000 atoms. In the case of f.c.c. nanoparticles, atomic configuration with five-fold symmetries was observed in both hydrogenfree and hydrogenated particles smaller than 2 nm. The f.c.c. structure was maintained in larger particles than 4 nm with lattice deformation which varies with M-H interaction. The b.c.t. structure was observed in hydrogenated b.c.c. nanoparticles. Number of H atoms absorbed in a nanoparticle varies depending on particle size and M-H interaction: it increases with increasing particle size and M-H bond strength.
INTRODUCTION Metallic nanoparticles are regarded as a potential candidate for hydrogen storage, especially because of their high hydrogen absorption and desorption rates attributable to their large surface-to-bulk ratio. Hydrogen absorption in metallic nanoparticles has been studied mostly on Pd [1–9]. Pundt and coworkers [6,7] reported that the crystal structure of hydrogenated Pd nanoparticles varies depending on the particle size. It exhibits not only f.c.c. structure, but also the icosahedral phase. Icosahedral phases were also observed in hydrogenated Co and Ni nanoparticles [10]. Yamauchi et al. [8,9] described size-dependent P–C isotherms of Pd nanoparticles lacking a plateau region observed in the bulk phase. A large difference between the P–C isotherms of nanoparticles and bulk was attributed to surface effects. It is also expected that hydrogenated nanoparticles include lattice defects which might affect hydrogen dynamics [7]. Recent progress in computer science makes possible to analyze the structural and electronic properties of hydrogen storage materials [11–13]. Classical molecular dynamics (MD) simulation is applicable to complex systems including surface or defects at desired temperatures although it requires suitable potential functions for target composition. The present authors recently carried out classical MD simulation on hydrogen storage in metallic nanoparticles with assuming the potential parameters as phenomenological variables [14]. In this study, the authors extend the target to various nanoparticle size and crystal structures.
MD SIMULATION Classical MD simulation was carried out in the same manner of our previous studies [14– 17]. We used a simple model of one-particle in a periodic MD cell. Spherical b.c.c and f.c.c. nanoparticles with 1, 1.4, 2, 4, 6, 8 and 10 nm in diameter were assumed. The number of metallic atoms are 55 to ca. 44 000. Surrounding hydrogen gas is in monoatomic phase forme
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