Functionalization of carbon-based nanostructures with light transition-metal atoms for hydrogen storage

In a recent letter [T. Yildirim and S. Ciraci, Phys. Rev. Lett. 94, 175501 (2005)], the unusual hydrogen storage capacity of Ti decorated carbon nanotubes has been revealed. The present paper extends this study further to investigate the hydrogen uptake by light transition-metal atoms decorating various carbon-based nanostructures in different types of geometry and dimensionality, such as carbon linear chain, graphene, and nanotubes. Using first-principles plane-wave method we show that not only outer but also inner surface of a large carbon nanotube can be utilized to bind more transition-metal atoms and hence to increase the storage capacity. We also found that scandium and vanadium atoms adsorbed on a carbon nanotube can bind up to five hydrogen molecules. Similarly, light transition-metal atoms can be adsorbed on both sides of graphene and each adsorbate can hold up to four hydrogen molecules yielding again a high-storage capacity. Interestingly, our results suggest that graphene can be considered as a potential high-capacity ${\mathrm{H}}_2$ storage medium. We also performed transition state analysis on the possible dimerization of Ti atoms adsorbed on the graphene and single-wall carbon nanotube.

Figure 1

(Color online) (a) Final optimized atomic structure of a linear $\mathrm{Ti}\text{−}{\mathrm{C}}_5$ complex holding five ${\mathrm{H}}_2$ with average binding energy $E_b=0.42\mathrm{eV}$ per ${\mathrm{H}}_2$ and gravimetric density $g_d=0.085$. (b) Same for ${\mathrm{C}}_3$ chain capped by two Ti atoms from both ends holding ten ${\mathrm{H}}_2$ with $g_d=0.133$. (c) Same for ${\mathrm{C}}_2$ molecule with $g_d=0.144$ (or $14.4\mathrm{wt}\%$). Respectively, small, medium, and large balls indicate H, C, and Ti atoms.

Figure 2

(Color online) (a) A $(2\times 2)$ pattern of Ti atoms occupying the hexagonal sites. (b) Two TM (Sc, Ti, or V) atoms shown by large dark ball adsorbed below and above the center of a hexagon of carbon atoms in graphene. Each TM atom binds up to four ${\mathrm{H}}_2$ molecules. (c) A $(2\times 2)$ cell of graphene binding two $\mathrm{TM}+4{\mathrm{H}}_2$ ligand; one above and the other below the atomic plane.

Figure 3

(Color online) Atomic configuration of Ti atom adsorbed on the surface of the (12,0) SWNT binding multiple ${\mathrm{H}}_2$ molecules: (a) Ti adsorbed externally (i.e., ${\mathrm{Ti}}_{\mathit{ex}}$) and holding four ${\mathrm{H}}_2$ one of which is dissociated. (b) Ti atom adsorbed internally (i.e., ${\mathrm{Ti}}_{\mathit{in}}$) and holding four ${\mathrm{H}}_2$. (c) Two Ti atoms adsorbed at the same time (i.e., ${\mathrm{Ti}}_{\mathit{ex}}$ and ${\mathrm{Ti}}_{\mathit{in}}$). One Ti is adsorbed externally and holding four ${\mathrm{H}}_2$, and another Ti adsorbed internally and holding four ${\mathrm{H}}_2$. (d) Externally adsorbed V binding five ${\mathrm{H}}_2$. (e) Internally adsorbed V binding four ${\mathrm{H}}_2$. (f) ${\mathrm{V}}_{\mathit{ex}}$ and ${\mathrm{V}}_{\mathit{in}}$ each holding four ${\mathrm{H}}_2$ molecules.

Figure 4

(Color online) (a) Variation of total energy in transition of a single Ti atom from any hexagonal site of a $(4\times 4)$ graphene to the bridge site and the involved energy barrier $Q$. Variations of total energy by fixing and relaxing the graphene atoms are indicated by square and crosses, respectively. (b) Variation of the total energy and involved energy barrier $Q$ in transition from two Ti atoms adsorbed at two hexagonal sites (leaving one empty hexagonal site between them) to a ${\mathrm{Ti}}_2$ dimer as a final state II. Here, while the left adsorbed Ti atoms is moved to the right in increments, its $y$ and $z$ coordinates and all the coordinates of the left Ti atom are relaxed. $d_i$ and $d_f$ are initial and final (after relaxation) distances between two Ti atoms. Carbon and Ti atoms are indicated by gray and dark balls, respectively.

Figure 5

(Color online) A zigzag chain of Ti atoms is formed, when two adjacent rows of Ti atoms are displaced from hexagonal site toward the bridge sites as shown by arrows. The system reconstructs from a $(2\times 2)$ to a $(4\times 4)$.

Figure 6

(Color online) (a) Variation of total energy in the translation of a single Ti atom from the hexagonal site of the (12,0) SWNT to the bridge site and the involved energy barrier $Q$. (b) Variation of total energy and involved potential barrier from the initial state I to the final state II corresponding to ${\mathrm{Ti}}_2$ dimer adsorbed on SWNT. The barrier occurs at the center of the right bridge. Overcoming the barrier the left Ti jumps to the adjacent hexagonal site to form ${\mathrm{Ti}}_2$ with the right Ti atom. $d_i$ and $d_f$ are initial and final distances between two Ti atoms. $c_0$ is the lattice parameter of the (12,0) SWNT.