Adsorption and diffusion of a molybdenum atom on the $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ surface: A first-principles study

A study on the structure and energetics of a single molybdenum atom adsorbed on the $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ surface is reported. All possible adsorption sites have been determined. Moreover, it is found that incorporation of the Mo atom into the first surface layer leads to considerably more stable structures than on any adsorption site. Different channels for migration of the molybdenum atom have been identified. The diffusion barriers of these channels have been determined. The results on structure and energetics are discussed by analyzing the electronic properties of the $\mathrm{Mo}∕\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ systems.

Figure 1

(Color online) Ball and stick representation of a relaxed $\mathrm{Ti}{\mathrm{O}}_2$ surface. Black (red) and gray (green) balls represent oxygen and titanium atoms, respectively. Numbers 1, 2, 3, and 4 refer to the bridging oxygen, in-plane oxygen, sixfold titanium, and fivefold titanium, respectively.

Figure 2

Top view of the $(2\times 4)$ supercell. Different possible adsorption sites are labeled 1–5. Site 1 is the most stable one. Black and gray balls represent oxygen and titanium atoms, respectively. O(2) and O(3) stand for bridging oxygen and in-plane oxygen, respectively.

Figure 3

Perspective view of the two most stable structures. (a) adsorption site 1 and (b) substitutional site of Mo (bigger gray ball) on $\mathrm{Ti}{\mathrm{O}}_2$ surface. Representation is as in Fig. 2. Note that both Mo and the displaced and/or interstitial Ti share the same crystallographic (001) plane.

Figure 4

Local density of states of surface Ti atoms. (a) is for Ti(6) of the clean surface, (b) is for displaced Ti(6) upon adsorption of Mo on site 1, and (c)–(f) are for the structure with substitutional Mo. (c) Ti(6) nearest to Mo, (d) interstitial Ti, (e) Ti(6) most distant from Mo, and (f) Ti(5) nearest to Mo and interstitial Ti.

Figure 5

Partial density of states (DOS) (a) for a free Mo atom and (b) for $\mathrm{Mo}∕\mathrm{Ti}{\mathrm{O}}_2$ at absorption site 1. A positive (negative) sign is used for the majority (minority) spin DOS. The $\mathrm{Mo}4d$ DOS is shown as a solid line. In (b), the $\mathrm{O}2p$ DOS of one of the two equivalent bridging oxygen neighbors of Mo is shown as a broken line.

Figure 6

(Color online) Potential energy surface for the diffusion of a single Mo atom on the $\mathrm{Ti}{\mathrm{O}}_2$ surface. The relative adsorption energies, $\Delta E$ in eV, are calculated with respect to the adsorption site 1. The upper right panel represents the surface as in Fig. 2. The lower panels show the cuts of the potential energy surface along $\left[1\overline{1}0\right]$ and [001] including site 1.

Figure 7

Diffusion barrier of a single Mo atom from adsorption site 1 to the substitutional site. The positions of the Mo atom and sixfold Ti atom are plotted along the reaction path in (001) plane. The unit length is $1\mathrm{\AA }$. The substitutional site of Mo has been taken as the origin of the coordinate frame. The inset shows the energy barrier, with the $X$ axis being the $\left[1\overline{1}0\right]$ coordinate of the Mo atom. The relative adsorption energies (in eV) are calculated with respect to adsorption site 1.