Bonding of Gold Nanoclusters to Oxygen Vacancies on Rutile $\mathrm{T}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}(110)$

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile ${\mathrm{T}\mathrm{i}\mathrm{O}}_2(110)$ surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the ${\mathrm{T}\mathrm{i}\mathrm{O}}_2(110)$ system involving vacancy-cluster complex diffusion is presented.

Figure 1

(a) Clean ${\mathrm{T}\mathrm{i}\mathrm{O}}_2(110)$ surface with bridging oxygen vacancies. (b)–(d) ${\mathrm{T}\mathrm{i}\mathrm{O}}_2(110)$ surface after 4% ML Au deposition at 130, 210, and 300 K, respectively. Vacancies are indicated by squares. All images are $150\times 150{\AA }^2$.

Figure 2

(a) A closeup STM image of one of the smallest clusters observed. Vacancies are marked with squares. (b) Simulated STM image of a single Au atom trapped in an oxygen vacancy. A model of the ($2\times 2$) surface unit cell used in the calculations is included. (c) Line profiles comparing theory and experiment [see arrows in (a) and (b)].

Figure 3

Densities of vacancies and Au clusters before and after deposition of $\sim 0.04\mathrm{M}\mathrm{L}$ Au at different temperatures. $1\mathrm{M}\mathrm{L}=1\mathrm{\text{vacancy or cluster}}/{\mathrm{T}\mathrm{i}\mathrm{O}}_2(110)\mathrm{\text{unit cell}}=5.13\times {10}^{14}{\mathrm{c}\mathrm{m}}^{-2}$. The difference in initial vacancy concentrations at different temperatures can be explained by different sample cooling rates during preparation, since accumulation of vacancies at the surface occurs only in a narrow temperature interval ($\sim 500\mathrm{K}<T<700\mathrm{K}$) where vacancies are created, but bulk-surface diffusion is hindered [13].

Figure 4

Transition between adsorption on vacancies and diffusion. Solid lines show the energy for the Au disk adsorbed on a given number of oxygen vacancies (indicated in the graph). The dashed line shows the limit for diffusion, and vertical arrows indicate the actual number of Au atoms in the cluster at which the transition occurs. The inset shows the attachment energy for ${\mathrm{A}\mathrm{u}}_n$ ($n=1–3$) adsorption on ${\mathrm{T}\mathrm{i}\mathrm{O}}_2(110)$.

Figure 5

Histogram of size distributions of Au clusters on ${\mathrm{T}\mathrm{i}\mathrm{O}}_2(110)$ for different deposition temperatures and annealing. All counts compare the size distribution after 0.02 ML Au. All areas were measured at a threshold level of $1\AA$ above the medium position of the (110) plane on which the clusters were positioned.