Modifying the Adsorption Characteristic of Inert Silica Films by Inserting Anchoring Sites

The adsorption properties of thin silica films on Mo(112) have been tailored by embedding single Pd atoms into the nanopores of the oxide material. The embedded Pd is able to anchor metal adatoms that would not bind to the inert silica surface otherwise. Several adsorption structures, e.g., $\mathrm{Pd}\mathrm{\text{−}}\mathrm{Pd}$, $\mathrm{Ag}\mathrm{\text{−}}\mathrm{Pd}$, and $\mathrm{Au}\mathrm{\text{−}}\mathrm{Pd}$ complexes, have been prepared in this way and analyzed with the STM and density functional theory. The binding strength of the different adatoms to the surface is determined by the number of electrons in their frontier orbitals, which introduce a repulsive interaction with the oxide electronic states and weaken the covalent bond to the Pd anchor.

Figure 1

(a) STM topographic image of ${\mathrm{SiO}}_2/\mathrm{Mo}(112)$ after insertion of ${\mathrm{Pd}}_{\mathrm{sub}}$ species ($U_s=1.2\mathrm{V}$, $14\times 14{\mathrm{nm}}^2$). (b) Same surface after deposition of single Pd (0.5 V), (c) Au (0.5 V), and (d) Ag atoms (1.9 V, $14\times 14{\mathrm{nm}}^2$). While only ${\mathrm{Pd}}_{\mathrm{sub}}$ species are observed in (a), new ad-structures become visible in (b–d). The inset in (a) shows a structure model of a ${\mathrm{Pd}}_{\mathrm{sub}}$ in a silica pore (Si, small yellow; O, small red; and Mo, large blue circles).

Figure 2

(a) STM topographic image of ${\mathrm{Pd}}_{\mathrm{sub}}$ and $\mathrm{Pd}\mathrm{\text{−}}{\mathrm{Pd}}_{\mathrm{sub}}$ species on ${\mathrm{SiO}}_2/\mathrm{Mo}(112)$ ($U_s=1.0\mathrm{V}$, $10\times 10{\mathrm{nm}}^2$). (b) As in (a) but after controlled removal of the adatom marked by the square box. (c) As in (b) but with enhanced contrast to show the silica atomic structure. Below the removed atom, the signature of a ${\mathrm{Pd}}_{\mathrm{sub}}$ becomes visible that has initially anchored the ad-species. The circles indicate other $\mathrm{Pd}\mathrm{\text{−}}{\mathrm{Pd}}_{\mathrm{sub}}$ complexes as guides for the eye; the dashed arrows mark a silica domain boundary.

Figure 3

(a) $dI/dV$ spectra of a ${\mathrm{Pd}}_{\mathrm{sub}}$, a $\mathrm{Pd}\mathrm{\text{−}}{\mathrm{Pd}}_{\mathrm{sub}}$ complex, and the bare silica surface (set point 2.5 V). (b) Calculated state density of the supported silica film (gray region). The line spectra depict the Pd $5s$ (solid line), Ag $5s$ (dash-dotted line), and Au $6s$ (dashed line) contribution to the hybrid state formed with the O $2p$ orbitals, when the respective adatom binds to a ${\mathrm{Pd}}_{\mathrm{sub}}$ anchor.

Figure 4

Topographic image and $dI/dV$ maps of a ${\mathrm{Pd}}_{\mathrm{sub}}$ and a ${\mathrm{Au}\mathrm{\text{−}}\mathrm{Pd}}_{\mathrm{sub}}$ species on ${\mathrm{SiO}}_2\mathrm{\text{/}}\mathrm{Mo}(112)$ taken at different sample biases ($6\times 6{\mathrm{nm}}^2$). The increase of the $dI/dV$ signal at the ${\mathrm{Au}\mathrm{\text{−}}\mathrm{Pd}}_{\mathrm{sub}}$ at $-0.2$ and $+1.55\mathrm{V}$ marks two intrinsic states of the complex, whereas the ringlike enhancement at 1.0–1.5 V is related to a deformation of the silica lattice upon Au adsorption.