II International Symposium on Carbon for Catalysis ABSTRACTS

PP-I-50
Computational details
First-principles calculations based on DFT have been performed by using the
CASTEP 3.2 code. The interactions between valence electrons and ion cores are represented
by the ultrasoft pseudopotentials (USP) suggested by Vanderbilt, which regards the 4s 3d
states as the valence configuration for Ni and 2s 2p states for carbon. Exchange and
correlation of the Kohn-Sham theory are treated with the generalized gradient approximation
(GGA) functional proposed by Perdew et al.. A plane wave energy cutoff of 270 eV is needed
to converge the total energy within the tolerance of 2.0e-5 eV/atom. Linear synchronous
transit/quadratic synchronous transit (LST/QST) method was performed to search the
transition state for the carbon diffusion on the metal surface. The results presented here are
based on periodic systems, i.e., a study of the Ni(111) 2 × 2 surface is essentially an infinitely
periodic surface composed of these 2 × 2 repeating units.
Results and discussion
Two alternative threefold adsorption sites are considered: one is called the fcc site, the other
the hcp site. The difference between them is that there is a Ni atom below the hcp site in the
second layer and below the fcc site in the third layer. The obtained adsorption enthalpy of a
carbon at the hcp site is 1.17 eV which is 0.05 eV lower than that at the fcc site, indicating
that the carbon adsorption at the hcp site is more energetically favorable because of the higher
coordination. Two surface diffusion pathways were investigated: the fcc-top-hcp path, a
carbon atom moves from the fcc site to the hcp site via an adjacent top site. The second
fcc-bridge-hcp path, a carbon atom reaches the hcp site via a bridge site. The activation
energy is 2.42 eV and 0.60 eV for the first and second pathway, corresponding to two
transition states with a carbon atom over the top site and bridge site, respectively.
Conclusion
During the CNF growth, if the carbon content on the gas/metal interface is low, the carbon
surface diffusion adopts the fcc-bridge-hcp pathway. And when the concentration is high,
carbon atoms may diffuse along both the two pathways. However, our previous calculation
indicates that the activation energy for the carbon bulk diffusion is 1.22 eV at the CNF
steady-state growth stage. As a consequence, carbon atoms will diffuse in the bulk Ni under
high interface concentration to produce fishbone-type or platelet CNFs while the surface
diffusion leads to the formation of tubular CNFs under dilute interface carbon content.
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