Reoxidation of $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ via Ti interstitials and line defects

The interaction of oxygen with line defects of Ti interstitials on a $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ surface is investigated using scanning tunneling microscopy (STM) measurements and first-principles calculations. Ab initio molecular dynamics calculations show that an oxygen molecule dissociatively adsorbs on a row of Ti interstitials. Oxygen atoms subsequently surround a Ti interstitial to form the equatorial plane of a partially complete octahedron, a basic building block for single- and double-strand formation. Upon the exposure of single strands to oxygen at room temperature, bright spots preferentially agglomerate on and along strands. The STM images of the bright spots, interpreted with density functional theory, indicate that they are $\mathrm{Ti}{\mathrm{O}}_x$ $(x<2)$ aggregates and that strands may serve as nucleation sites for the surface growth.

Figure 1

(Color online) (a) The most stable structure of Ti interstitials after being oxidized to TiO (Ti, blue; O, red in ball-and-stick models) calculated with DFT and its simulated STM contrast compared to the experimental STM image ($8\times 8{\mathrm{nm}}^2$, $1.2\mathrm{V}$, and $0.5\mathrm{nA}$). The DFT-relaxed structures of (b) a single strand of ${\mathrm{Ti}}_2{\mathrm{O}}_2$ after one Ti and one O rows and (c) a double strand of ${\mathrm{Ti}}_2\mathrm{O}$ on $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ after only one Ti row added to the TiO row in (a). Only the top few layers of the actual system are shown here.

Figure 2

(Color online) The effect of various annealing conditions on regenerating $(1\times 1)$ $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$: (a) $100\mathrm{nm}\times 100\mathrm{nm}$ of STM topographic image after $15\min$ of Ar sputtering ($1\mathrm{keV}$, sample current: $0.4\mu \mathrm{A}$) followed by $20\min$ of annealing at $585°\mathrm{C}$. For larger scan areas $(400\mathrm{nm}\times 400\mathrm{nm})$ after $15\min$ of Ar sputtering ($1\mathrm{keV}$, $⩽0.3\mu \mathrm{A}$) followed by annealing for (b) $15\min$ at $630°\mathrm{C}$, (c) $20\min$ at $645°\mathrm{C}$, and (d) $36\min$ at $655°\mathrm{C}$.

Figure 3

(Color online) STM topographic images $(50\mathrm{nm}\times 50\mathrm{nm})$ of $(1\times 1)$ $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ from (a) a “fresh” sample, after $18\text{cycles}$ of sputtering and annealing, and (b) an aged sample, after 65 such cycles.

Figure 4

(Color online) STM topographic images of single strands after the exposure to $8\mathrm{L}$ of ${\mathrm{O}}_2$ at room temperature taken with $1.48\mathrm{V}$ and $0.5\mathrm{nA}$: (a) $(50\mathrm{nm}\times 50\mathrm{nm})$, (b) a close-up view of a strand $(7.8\mathrm{nm}\times 10.9\mathrm{nm})$, and (c) after subsequent heating to $400\mathrm{K}$ for $10\min$ ($40\mathrm{nm}\times 40\mathrm{nm}$, $1.2\mathrm{V}$, and $0.5\mathrm{nA}$).

Figure 5

(Color online) Molecular dynamics of ${\mathrm{O}}_2$ interacting with a row of Ti interstitials on $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$: (a) $0\mathrm{fs}$, (b) $12\mathrm{fs}$, (c) $13\mathrm{fs}$, (d) $18\mathrm{fs}$, (e) $148\mathrm{fs}$, and (f) $201\mathrm{fs}$. The time is referenced to the initial frame (a).

Figure 6

(Color online) DFT-relaxed structural model of a $\mathrm{Ti}{\mathrm{O}}_2$ molecule adsorbed on (a) the single-strand defect and (b) the pristine $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ surface. The adsorbed molecule is highlighted by an oval of dashed line on each surface. Three major structural distortions of each surface are indicated with green arrows. The $(2\times 2)$ supercells built on top of four layers of $\mathrm{Ti}{\mathrm{O}}_2$ were used to calculate the energetics of the adsorption of $\mathrm{Ti}{\mathrm{O}}_2$ molecule.