Carbon nanotubes may be decorated with different metals by different techniques. These surfaces obtained are of technological importance due to the fact that they may be suitable material for gas storage. This property is especially worth being analyzed in the case of the hydrogen storage, since this gas is the optimal candidate as the energy carrier in an economy based on renewable resources.
In a recently theoretical study, Yildirim et al [1] found that titanium decoration of carbon nanotubes remarkably increases the capacity for hydrogen storage. For this system, we demonstrated that a problem may arise during the manufacture of a titanium decorated graphene sheet, because titanium is irreversibly oxidized to titanium dioxide in presence of small oxygen concentrations remaining in the experimental vacuum [2] and the titanium dioxide has a lower hydrogen storage capacity.
As graphene is a new carbonate material of high surface are, we studied the hydrogen storage capacity of the graphene layer decorated with nickel in order to contribute to the development of the storage systems, sincea single nickelatominvacuumcan coordinateuntilfive hydrogenmolecules and does not oxidize as easily as titanium.
In order to anticipate the potential problems that could arise during the manufacturing process, handling and operation of the storage system, we studies by means of density functional theory (DFT): 1- The adsorption of on graphene, 2- Reduction reaction fromto , 3- The interaction of the decorated with the air components, 4- The possibility of hydride formation during the storage operation and 5- The hydrogen storage capacity. In all cases we investigate if the decorated remains adsorbed and how the modification affects the storage capacity.
The study of the minimum energy path for the different reactions was undertaken using nudged elastic band method (NEB) [5], the local minima where found through the conjugate gradient (CG) technique, employing Density Functional Theory (DFT) calculations with spin polarization (sp) as implemented in SIESTA [3] in all cases. Exchange and correlation effects were described using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional [4]. The basis set used was a double- one plus polarization quality.
References
[1] T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94 175501 (2005).
[2] M.I. Rojas, E.P.M. Leiva, Phys. Rev. B 76 155415 (2007).
[3]J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón, D. Sánchez-Portal, J. Phys.: Condens. Matter 14 2745 (2002).
[4] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 3865 (1996).
[5] G. Henkelman, B.P. Uberuaga, H. Jonsson, J. Chem. Phys. 113 (2000) 9901-04.
