Enhancement of hydrogen physisorption on single-walled carbon nanotubes resulting from defects created by carbon bombardment

The defect effect on hydrogen adsorption on single-walled carbon nanotubes (SWNTs) has been studied by using extensive molecular dynamics simulations and density functional theory (DFT) calculations. It indicates that the defects created on the exterior wall of the SWNTs by bombarding the tube wall with carbon atoms and ${\mathrm{C}}_2$ dimers at a collision energy of $20\mathrm{eV}$ can enhance the hydrogen adsorption potential of the SWNTs substantially. The average adsorption energy for a ${\mathrm{H}}_2$ molecule adsorbed on the exterior wall of a defected (10,10) SWNT is $\sim 150\mathrm{meV}$, while that for a ${\mathrm{H}}_2$ molecule adsorbed on the exterior wall of a perfect (10,10) SWNT is $\sim 104\mathrm{meV}$. The ${\mathrm{H}}_2$ sticking coefficient is very sensitive to temperature, and has a maximum value around $70\text{to}90\mathrm{K}$. The electron density contours, the local density of states, and the electron transfers obtained from the DFT calculations clearly indicate that the ${\mathrm{H}}_2$ molecules are all physisorbed on the SWNTs. At temperatures above $200\mathrm{K}$, most of the ${\mathrm{H}}_2$ molecules adsorbed on the perfect SWNT are soon desorbed, but the ${\mathrm{H}}_2$ molecules can still remain on the defected SWNTs at $300\mathrm{K}$. The detailed processes of ${\mathrm{H}}_2$ molecules adsorbing on and desorbing from the (10,10) SWNTs are demonstrated.

Figure 1

The scattering and adsorption process of ${\mathrm{H}}_2$ molecules colliding at $0.1\mathrm{eV}$ onto a defected (10,10) SWNT: (a) the initial positions of the ${\mathrm{H}}_2$ molecules, from where they are colliding onto the tube wall; (b) the configuration of the system at $t=600\mathrm{fs}$ after the collisions, showing some of the ${\mathrm{H}}_2$ molecules scattered from the tube wall and others adsorbed tentatively on the wall; (c) at time $t=1500\mathrm{fs}$, most of the ${\mathrm{H}}_2$ molecules are scattered away; and (d) the stably adsorbed ${\mathrm{H}}_2$ molecules near the defect sites at $300\mathrm{K}$, the inset of this figure shows some typical defects formed on the tube wall.

Figure 2

${\mathrm{H}}_2$ molecules adsorbed on a defected (10,10) SWNT upon the collisions at $0.01\mathrm{eV}$: (a) at time $t=1500\mathrm{fs}$ after the collisions, only about 20% of the ${\mathrm{H}}_2$ molecules are scattered; (b) the configuration of the system at $t=10\mathrm{ps}$ after the collisions; (c) the configuration of the system after annealing at $300\mathrm{K}$ for $300\mathrm{ps}$; and (d) the configuration of the system after annealing at $300\mathrm{K}$ for $1\mathrm{ns}$, showing very stable adsorption of the ${\mathrm{H}}_2$ molecules on the defected tube wall.

Figure 3

${\mathrm{H}}_2$ molecules adsorbing on and desorbing from a perfect (10,10) SWNT: (a) the initial positions of the ${\mathrm{H}}_2$ molecules, from where they are colliding at $0.01\mathrm{eV}$ onto the perfect SWNT; (b) at time $t=1500\mathrm{fs}$ following the collisions, more than half of the ${\mathrm{H}}_2$ molecules are scattered off; (c) the configuration of the system after it is annealed at $300\mathrm{K}$ for $300\mathrm{ps}$; and (d) annealing at $300\mathrm{K}$ for $1\mathrm{ns}$; the last adsorbed ${\mathrm{H}}_2$ molecule is desorbed.

Figure 4

The sticking coefficient as a function of incident energy of ${\mathrm{H}}_2$ molecules for the cases of ${\mathrm{H}}_2$ on the defected SWNT and of ${\mathrm{H}}_2$ on the perfect SWNT, showing the peaks around an equivalent temperature ranging from $70\mathrm{K}\text{to}90\mathrm{K}$.

Figure 5

The interaction energy between a ${\mathrm{H}}_2$ molecule and a perfect (10,10) SWNT versus radial distance from the adsorption site, comparing the curve obtained from SIESTA with that from the empirical van der Waals potential.

Figure 6

The electron density contours during a ${\mathrm{H}}_2$ molecule desorbing from a perfect (10,10) SWNT and the local DOS of the perfect (10,10) SWNT before and after the adsorption of a ${\mathrm{H}}_2$ molecule: (a) the radial distance between the ${\mathrm{H}}_2$ molecule and the adsorbing site as a function of time; (b) the electron density contour of a ${\mathrm{H}}_2$ molecule adsorbed on the perfect (10,10) SWNT, this ${\mathrm{H}}_2$-perfect SWNT complex is formed by the ${\mathrm{H}}_2$ colliding onto the tube wall and followed by annealing at $100\mathrm{K}$ for $1\mathrm{ps}$, a physisorption is confirmed; (c) the electron density contour at annealing time, $t=36\mathrm{ps}$ ($100\mathrm{K}$ for $30\mathrm{ps}$ and $200\mathrm{K}$ for $6\mathrm{ps}$), showing that the ${\mathrm{H}}_2$ molecule is desorbing from the adsorbing site; and (d) the electron density contour showing that the ${\mathrm{H}}_2$ molecule is completely desorbed from the tube wall at annealing time, $t=50\mathrm{ps}$ ($100\mathrm{K}$ for $30\mathrm{ps}$ and $200\mathrm{K}$ for $20\mathrm{ps}$); (e) the local DOS of the perfect SWNT before the ${\mathrm{H}}_2$ adsorption; and (f) the local DOS of the SWNT after the ${\mathrm{H}}_2$ adsorption.

Figure 7

Stable adsorption at $300\mathrm{K}$ for a ${\mathrm{H}}_2$ molecule adsorbed near a defect that is a chemically adsorbed ${\mathrm{C}}_2$ dimer on the defected (10,10) SWNT: (a) the radial distance between the ${\mathrm{H}}_2$ molecule and the adsorbing site on the tube as a function of annealing time at $300\mathrm{K}$; (b) the electron density contour showing the ${\mathrm{H}}_2$ is physisorbed on the tube wall; (c) the local DOS of the defected (10,10) SWNT before the ${\mathrm{H}}_2$ adsorption; and (d) the local DOS of the defected (10,10) SWNT after the ${\mathrm{H}}_2$ adsorption.