${\mathrm{NH}}_3$ on anatase ${\mathrm{TiO}}_2\left(101\right)$: Diffusion mechanisms and the effect of intermolecular repulsion

We utilized scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations to study the diffusion of ammonia $\left({\mathrm{NH}}_3\right)$ on anatase ${\mathrm{TiO}}_2\left(101\right)$. From time-lapsed STM imaging, we observed monomeric and dimeric diffusion channels, and a general tendency to higher diffusion rates with increasing ${\mathrm{NH}}_3$ coverage. In surface regions where several ${\mathrm{NH}}_3$ molecules are adsorbed within a few sites, we further observed the diffusion of ${\mathrm{NH}}_3$ molecules occurring in cascades, where the diffusion of one adsorbate triggers that of others. This eventually leads to apparent diffusion barriers that are lower than expected within a single-jump model. From the DFT calculations, we obtained mechanistic insights into the two observed ${\mathrm{NH}}_3$ diffusion channels. Within the dimeric ${\mathrm{NH}}_3$ diffusion channel, one ${\mathrm{NH}}_3$ swings around another adsorbed ${\mathrm{NH}}_3$ and experiences a reduced diffusion barrier, owing to the intermolecular bonding during the event.

Figure 1

(a) STM image (300 Å×300 Å; $V_S=+1.0V$; $I_T=0.1nA$; RT) of the $a\text{−}{\mathrm{TiO}}_2\left(101\right)$ surface recorded following ${\mathrm{NH}}_3$ exposure at RT. A repeated grayscale is used for the presentation of this image. The bright features on the terraces arise from ${\mathrm{NH}}_3$ molecules. (b) Superposition of two sequential STM images from an STM movie (200 Å×200 Å) recorded within the same experiment. White protrusions are stationary ${\mathrm{NH}}_3$ molecules. Five ${\mathrm{NH}}_3$ diffusion events occurred: Diffused ${\mathrm{NH}}_3$ molecules in the first (second) STM image are shown in green (pink). (c) Arrhenius plot of the ${\mathrm{NH}}_3$ hopping rate Γ. For each point, we recorded STM movies and analyzed sequences of 50–100 STM images. The ${\mathrm{NH}}_3$ coverage was 0.04–0.06 ML.

Figure 2

(a)–(d) Sequence of STM images recorded at 130 K (50 Å×100 Å; 20 s/image), showing the formation, (a)→(b), diffusion, (b)→(c), and breakup, (c)→(d), of an ${\mathrm{NH}}_3$ dimer. (e), (f) STM height profiles along the lines drawn in (c). The profiles of an (e) ${\mathrm{NH}}_3$ dimer and (f) ${\mathrm{NH}}_3$ monomer are shown in red and blue, respectively. Within the dimer, two ${\mathrm{NH}}_3$ molecules occupy nearest-neighbor 5f-Ti sites.

Figure 3

DFT modeling of monomeric ${\mathrm{NH}}_3$ diffusion. (a)–(e) Pathway across the ${\mathrm{O}}_{\mathrm{br}}$ rows: Side and top views are shown. Surface O atoms (small red balls), bulk O atoms (small light red balls), and surface Ti atoms (large gray balls) are indicated. The initial and final 5f-Ti adsorption sites are highlighted by black circles. (f)–(j) Pathway parallel to the ${\mathrm{O}}_{\mathrm{br}}$ rows. Presentation of the structures as in (a)–(e). (k) Corresponding energy landscapes. Diffusion pathways across (dashed line) and parallel to the ${\mathrm{O}}_{\mathrm{br}}$ rows (solid line) are directly compared. Arrows point to the corresponding computed energies.

Figure 4

DFT modeling of dimeric ${\mathrm{NH}}_3$ diffusion parallel to the ${\mathrm{O}}_{\mathrm{br}}$ rows. (a)–(d) The moving ${\mathrm{NH}}_3$ molecule takes the pathway on the right around the stationary ${\mathrm{NH}}_3$ molecule. Side and top views are shown. Presentation of the structures as in Fig. 3. (e)–(h) The moving ${\mathrm{NH}}_3$ molecule takes the pathway on the left around the stationary ${\mathrm{NH}}_3$ molecule. (i) Direct comparison of the two pathways (top view). (j) Diffusion energy landscapes. Dashed and solid lines correspond to the pathways on the right and left around the stationary ${\mathrm{NH}}_3$ molecule, respectively. Arrows point to the corresponding computed energies.

Figure 5

(a)–(f) Sequence of STM images ($40\AA \times 50\AA$; 20 s/image) showing “cascade diffusion” of five ${\mathrm{NH}}_3$ molecules at RT. The imaged five ${\mathrm{NH}}_3$ molecules did not hop within 120 s (six images) prior to the image shown in (a). From image (a) to image (b), several hopping events occurred, and this cascade of hopping events of neighboring ${\mathrm{NH}}_3$ molecules continued for ∼100 s until in image (f), the five molecules reached an arrangement that persisted for four images (∼80 s). (g) Hopping rate at RT as a function of the ${\mathrm{NH}}_3$ coverage.