Observation and Destruction of an Elusive Adsorbate with STM: ${\mathbf{O}}_2/{\mathrm{TiO}}_2(110)$

When a slightly defective rutile ${\mathrm{TiO}}_2(110)$ surface is exposed to ${\mathrm{O}}_2$ at elevated temperatures, the molecule dissociates at defects, filling O vacancies ($V_{\mathrm{O}}$) and creating O adatoms (${\mathrm{O}}_{\mathrm{ad}}$) on ${\mathrm{Ti}}_{5c}$ rows. The adsorption of molecular ${\mathrm{O}}_2$ at low temperatures has remained controversial. Low-temperature scanning tunneling microscopy of ${\mathrm{O}}_2$, dosed on ${\mathrm{TiO}}_2(110)$ at a sample temperature of $\approx 100\mathrm{K}$ and imaged at 17 K, shows a molecular precursor at $V_{\mathrm{O}}$ as a faint change in contrast. The adsorbed ${\mathrm{O}}_2$ easily dissociates during the STM measurements, and the formation of ${\mathrm{O}}_{\mathrm{ad}}$’s at both sides of the original $V_{\mathrm{O}}$ is observed.

Figure 1

Successive STM images ($V_{\mathrm{sample}}=+1.8\mathrm{V}$, $I=0.05\mathrm{nA}$, $T_{\mathrm{sample}}=17\mathrm{K}$, 120 seconds per image) of a reduced rutile ${\mathrm{TiO}}_2(110)$ surface after exposure to 0.045 L ${\mathrm{O}}_2$ at $\approx 100\mathrm{K}$. White box: Oxygen vacancy ($V_{\mathrm{O}}$). Circles: O adatoms (${\mathrm{O}}_{\mathrm{ad}}$). Dotted circles are ${\mathrm{O}}_{\mathrm{ad}}$’s formed from ${\mathrm{O}}_2$ in the previous image. Yellow (bright) arrows: Adsorbed ${\mathrm{O}}_2$. Ovals: Two bean-shaped ${\mathrm{O}}_{\mathrm{ad}}$’s from dissociation of ${\mathrm{O}}_2$. (a1) Streakiness due to ${\mathrm{O}}_{\mathrm{ad}}$ creation (arrows); (a2) averaged line profile across adsorbed ${\mathrm{O}}_2$ recorded along ${\mathrm{O}}_{\mathrm{br}}$ rows (one marked by black arrows). (b1),(b2) Comparison of ${\mathrm{O}}_{\mathrm{ad}}$ pairs with and without $V_{\mathrm{O}}$ in between.

Figure 2

Successive STM images ($V_{\mathrm{sample}}=+1.8\mathrm{V}$, $I=0.03\mathrm{nA}$, $T_{\mathrm{sample}}=17\mathrm{K}$) of a reduced rutile ${\mathrm{TiO}}_2$ surface after exposure to 0.045 L ${\mathrm{O}}_2$ at 100 K. Black arrows point at scan lines where an ${\mathrm{O}}_2$ molecule is converted into two ${\mathrm{O}}_{\mathrm{ad}}$’s and one of the ${\mathrm{O}}_{\mathrm{ad}}$’s disappears by filling a vacancy. (f) Schematic showing the species involved.