Influence of Hydrogen on Growth of Carbon Nanotubes
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Influence of Hydrogen on Growth of Carbon Nanostructures Maxim P. Belov1, Andrey A. Knizhnik1, Irina V. Lebedeva1, Alexey V. Gavrikov1, Boris V. Potapkin1, Timothy J. Sommerer2 and Chris M. Eastman2 1 2
Kintech Lab Ltd, 1 Kurchatov Sq., Moscow, 123182, Russia GE Global Research Center, Niskayuna NY, 12309, USA
ABSTRACT The influence of hydrogen on the growth of carbon nanostructures in thermal chemical vapor deposition is studied using density functional theory calculations. It is shown that hydrogen adatoms effectively bind to edges of graphitic structures on the Ni (111) surface. This is found to result in a significant decrease of the rate of carbon attachment to the growing graphitic structures. However, it is also demonstrated that the edges of graphitic structures which are attached to steps on the Ni surface should not be hydrogenated. INTRODUCTION Chemical vapor deposition (CVD) is considered a technique that enables control of structure of CNTs and graphene. In particular, experimental data suggest that the amount of hydrogen in the gas flow affects the number of walls, diameter, length and defects of CNTs [1]. The molar ratio of hydrogen to hydrocarbons was shown to influence the growth rate of CNTs [2]. The use of hydrogen as a carrier gas was found to decrease the density of CNTs [3].Though numerous attempts to control the CNT growth were already made, the effect of the synthesis parameters on the structure and growth rate of CNTs is still not clear and observed dependences need to be explained. We believe that hydrogen has a three-fold effect on the nucleation and growth of сarbon nanostructures. First, hydrogen can lead to a modification of the gas composition in the CVD reactor [4], which would influence the carbon supply rate onto the catalyst surface. Second, hydrogenation of carbon species on the catalyst surface should enhance desorption of hydrocarbons. However, here we consider the possibility for hydrogen to influence the growth of carbon nanostructures through hydrogenation of edges of the growing structures. As it was shown [5], there is a critical size that a graphitic structure must exceed to nucleate graphene. Hydrogen should stabilize the edges of the graphitic structures and, as a result, decrease the critical size for graphene nucleation. On the other hand, hydrogenation of graphitic structures and carbon adatoms should slow down the growth of these structures. Using density functional theory (DFT) calculations, we demonstrate that hydrogen effectively binds to edges of graphitic structures on the Ni (111) surface and does not bind to edges attached to steps on the Ni surface. We also calculate kinetic parameters of the attachment of a carbon adatom to a graphitic structure for different configurations with and without hydrogen in the system.
METHODS The kinetic parameters for the growth of graphitic structures on the Ni (111) surface were obtained using density functional theory (DFT) calculations. The VASP code [6] with PerdewWang exchange-correlation functional [7] w
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