First-principles study of Au nanostructures on rutile ${\text{TiO}}_2\left(110\right)$

We report systematic density-functional theory calculations of the structure and energetics of ${\text{Au}}_n$ nanorows $\left(n=1–7\right)$ and clusters $\left(n=1–12\right)$ adsorbed on the defected (110) rutile surface. The calculations show that gold nanorows bind strongly to a missing-row defected ${\text{TiO}}_2$ surface with an adhesive binding energy of about 1.5 eV. The cohesive binding energy of Au atoms in a row amounts to about 2.5 eV/atom. An analysis of the gold row properties points to their metallic nature. The charge redistribution on adsorbed rows shows that all ${\text{Au}}_n$ rows are negatively charged compared to the free-standing structures. The adhesive bonding of gold clusters to the vacancy defected bridging oxygen row at the ${\text{TiO}}_2\left(110\right)$ is of covalent nature and is stronger than that of the Au rows. The cohesive energy per atom in a ${\text{Au}}_n$ cluster is about 2.2 eV for the $n\ge 5$ clusters and is larger $\left(\sim 2.3–2.4\text{eV}\right)$ for smaller ones. We found that all clusters studied are negatively charged with about 1.1 electron charge. This charging shows only a weak dependence on the odd-even number of gold atoms forming a cluster.

Figure 1

(Color online) Front views (along the rows) of the optimized final configurations (first and second rows) of ${\text{Au}}_n$ rows $\left(n=1–7\right)$ on the missing-row $1\times 2$ surface cell of ${\text{TiO}}_2\left(110\right)$. Gold atoms are represented by the medium-size red (gray) balls (numbered in white), the largest blue (darkest) balls represent Ti, whereas the smallest green (light gray) balls represent oxygen atoms. Only the atoms of the topmost trilayer of the (110) oriented ${\text{TiO}}_2$ slab are displayed. The noninteger numbers show the Au-Au and Au-Ti bond lengths (in angstrom). The third and bottom rows display, respectively, side view along the $\left[\overline{1}10\right]$ direction and top view of the $n=4–7$ gold rows. For the top view of $n=1–3$ rows consult Fig. 4 of Ref. 23.

Figure 2

The adhesion energy $E_{\text{adh}}$, adsorption binding energy per atom $E_{\text{coh}}$, and work function, $\Phi$, of the ${\text{Au}}_n$ rows adsorbed on the missing ${\text{O}}^{\text{br}}$ row of the ${\text{TiO}}_2\left(110\right)$ surface. The lowest panel displays the bond length between the bottom Au and the nearest surface Ti atom.

Figure 3

(Color online) Front and top views of the surfaces of constant electron-charge-density change upon adsorption of four to seven Au atom rows. Only isosurfaces $\left(0.03e/{\text{Å}}^3\right)$ of the electron-charge gain are displayed. Blue (darkest), green (light gray), and red (gray) balls represent Ti, O, and Au atoms, respectively. The isosurfaces for $n=1–3$ rows were presented in Ref. 23. The numbers $Q\left({\text{Au}}_n\right)$ give the total Bader charges (in units of the electron charge $e$) gained by the rows. The corresponding charges for the ${\text{Au}}_1$, ${\text{Au}}_2$, and ${\text{Au}}_3$ rows are 0.56, 0.58, and $0.57e$, respectively.

Figure 4

(Color online) Evolution of the local density of states on the $1\times 2$ missing-row ${\text{TiO}}_2\left(110\right)$ surface with the adsorption of the Au rows. Only the energy window close to the Fermi level (set as zero) is shown. Positive and negative values represent the spin-up and spin-down polarized electron states. The states of the top ${\text{TiO}}_2\left(110\right)$ layer atoms are plotted in yellow (gray), while red (dark gray) color marks the LDOS on the Au atoms.

Figure 5

(Color online) Side and top views of the optimized configurations of adsorbed ${\text{Au}}_n$ clusters $\left(n=1–6\right)$ on the defected ${\text{TiO}}_2\left(110\right)$ surface. Au atoms are represented by the medium-size red balls, the largest blue balls represent Ti, whereas the smallest green ones represent oxygen atoms. The numbers show the Au-Au bond lengths (in $\text{Å}$).

Figure 6

(Color online) Same as in Fig. 5 but for the $n=7–12$ gold clusters.

Figure 7

Adhesion energy $E_{\text{adh}}$, adsorption binding energy per atom $E_{\text{coh}}$, and work function, $\Phi$, of the ${\text{Au}}_n$ clusters adsorbed on the defected ${\text{TiO}}_2\left(110\right)$ surface. The lower values for $E_{\text{coh}}$ and $\Phi$ correspond to the $1b$ placement of the Au atom (cf. text).

Figure 8

(Color online) Side and top (lower row) views of the surfaces of constant electron-charge-density change upon adsorption of ${\text{Au}}_n$ clusters $\left(n=1–6\right)$. Only isosurfaces of the electron-charge gain, corresponding to $0.03e/{\text{Å}}^3$, are displayed. Blue (darkest), green (light gray), and red (gray) balls represent Ti, O, and Au atoms, respectively. The numbers $Q\left({\text{Au}}_n\right)$ give the amount of electron charge gained by the cluster.

Figure 9

(Color online) Same as in Fig. 8 but for $n=7–12$ gold clusters.

Figure 10

(Color online) The local density of states for ${\text{Au}}_n$ clusters on the defected ${\text{TiO}}_2\left(110\right)$ surface. Only the energy range close to the Fermi level (set as zero) is shown. The density of states of the top ${\text{TiO}}_2\left(110\right)$ $4\times 2$ layer is plotted in yellow (gray), while red (dark gray) color marks the LDOS of Au clusters.