Density functional theory study of a graphene sheet modified with titanium in contact with different adsorbates

The present work is based on the theoretical study of the behavior of a graphene sheet decorated with titanium, in contact with different molecules. When the substrate is exposed only to hydrogen molecules, it is found to store up to four molecules per adatom, as already seen in the literature for single wall carbon nanotubes. Thus, titanium decoration is seen to considerably improve the hydrogen storage capacity of these carbon systems. However, it is found that low quantities of oxygen present in the gas phase should yield the oxidation of the titanium atoms, even when hydrogen is stored in the system. It is concluded that if the experimental system is exposed to air, titanium atoms on these surfaces are expected to oxidize to titanium dioxide, showing oxygen molecules to be very reactive species. Other chemicals present in air such as nitrogen or water molecules could also be chemisorbed onto the titanium adatom, but are less competitive with hydrogen.

Figure 1

(Color online) Hydrogen adsorption onto Ti/graphene. (a) CG energy changes in (eV) as a function of the minimization steps. The energy zero is taken to be the hydrogen molecule infinitely far from the rest of the system. (b) Energy along reaction coordinate obtained with the NEB method for the hydrogen molecule dissociation process. The energy zero corresponds to the initial state as depicted in the figure. CG equilibrium configurations for multiple hydrogen molecules adsorbed onto Ti/graphene system, in the case of (c) two, (d) three, and (e) four ${\mathrm{H}}_2$ molecules: Blue, C; magenta, Ti; and cyan, H.

Figure 2

(Color online) [(a) and (c)] Energy as a function of CG steps for ${\mathrm{O}}_2$ molecular adsorption on the titanium adatom/graphene system through different approach directions. (a) corresponds to a direction perpendicular to the surface, (c) to a direction parallel to the graphene sheet. [(b) and (d)] Energy along the reaction path for dissociation of the ${\mathrm{O}}_2$ molecule on the titanium adatom/graphene system to yield $\mathrm{Ti}{\mathrm{O}}_2$, as obtained using the nudged elastic band method. (b) corresponds to a direction perpendicular to the surface, (d) to a direction parallel to the graphene sheet. Different geometries of the $\mathrm{Ti}{\mathrm{O}}_2$ species considered for the calculations of the ${\mathrm{O}}_2$ and $\mathrm{Ti}{\mathrm{O}}_2$ binding energies reported in Table : (e) hollow, (f) bridge, (g) top, (h) rotated bridge, and (i) rotated top. Blue, C; magenta, Ti; and yellow, O.

Figure 3

(Color online) Oxidation reaction in the presence of stored hydrogen. (a) CG energy changes (eV) vs minimization steps for the molecular oxygen adsorption onto Ti in the presence of a dissociated hydrogen molecule. (b) Energy along reaction coordinate obtained with the NEB method for the ${\mathrm{O}}_2$ dissociation reaction. The two hydrogen atoms rebuilt the ${\mathrm{H}}_2$ molecule, and titanium yields $\mathrm{Ti}{\mathrm{O}}_2$. The ${\mathrm{H}}_2$ molecule remains adsorbed close to the $\mathrm{Ti}{\mathrm{O}}_2$. (c) CG energy changes (eV) vs minimization steps for the molecular oxygen adsorption onto Ti in the presence of an adsorbed hydrogen molecule. (d) Energy along reaction coordinate obtained with the NEB method for the ${\mathrm{O}}_2$ dissociation reaction. The hydrogen molecule remains adsorbed during all the process. [(e)–(h)] Equilibrium configurations as obtained from CG minimization for different numbers of hydrogen molecules adsorbed on the $\mathrm{Ti}{\mathrm{O}}_2$/graphene system: (e) one, (f) two, (g) three, and (h) four ${\mathrm{H}}_2$ molecules. Blue, C; magenta, Ti; yellow, O; and cyan, H.

Figure 4

(Color online) CG equilibrium configurations for the adsorption of (a) one and (b) two water molecules adsorbed on the Ti/graphene system. (c) Energy along the reaction coordinate as obtained with the NEB method for the oxidation of Ti by two water molecules. The energy zero is taken at the initial state. (d) CG equilibrium configuration for the adsorption of nitrogen on Ti/graphene: Blue, C; magenta, Ti; yellow, O; cyan, H; and green, N.