Water-induced surface reconstruction of oxygen $\left(2\times 1\right)$ covered Ru(0001)

Low-temperature scanning tunneling microscopy and density-functional theory (DFT) were used to study the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The oxygen atoms occupy hcp sites in an ordered structure with $\left(2\times 1\right)$ periodicity. DFT predicts that water is weakly bound to the unmodified surface, 86 meV compared to the $\sim 200\text{meV}$ water-water H bond. Instead, we found that water adsorption causes a shift of half of the oxygen atoms from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms. The energy cost of reconstructing the oxygen overlayer, around 230 meV per displaced oxygen atom, is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. Water forms hydrogen bonds with the fcc O atoms in a $\left(4\times 2\right)$ superstructure due to alternating orientations of the molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen-honeycomb structure, which is metastable relative to the original $\left(2\times 1\right)$. This stable structure is not recovered until after heating to temperatures close to 260 K.

Figure 1

(Color online) (a) Atomically resolved STM image (bottom) and schematic representation (top) of the $\text{O}\left(2\times 1\right)/\text{Ru}\left(0001\right)$ surface. Red circles represent the oxygen atoms and yellow represent the Ru atoms. (b) Large scan area image of the $\text{O}\left(2\times 1\right)/\text{Ru}\left(0001\right)$ surface, showing different domains rotated $120°$ with respect to each other. White spots correspond to O vacancies. Imaging parameters: [(a) and (b)] 21.5 mV and 89.6 pA.

Figure 2

(Color online) Structures formed by oxygen on Ru(0001) at half-monolayer coverage. Two possible surface geometries are shown, along with the ($2\times 2$) cell (dark lines) used in the DFT calculations. Yellow circles correspond to Ru atoms and red circles correspond to oxygen adsorbed on Ru hollow sites. (a) $\text{O}\left(2\times 1\right)$ layer with all O atoms occupying hcp positions and (b) $2\text{O}\left(2\times 2\right)$ honeycomb structure with half of the O atoms in hcp and half in fcc sites. Both systems were calculated and the $\text{O}\left(2\times 1\right)$ structure found to be more stable by 231 meV per $\left(2\times 2\right)$ cell.

Figure 3

(Color online) (a) STM image showing individual water molecules, and groups of two and three molecules in adjacent sites on a $\text{O}\left(2\times 1\right)/\text{Ru}\left(0001\right)$ adsorbed at 140 K. During imaging at 7 K, clusters of neighboring molecules are stable while isolated molecules are vibrating. ($5.8\text{nm}\times 3.8\text{nm}$, $-380\text{mV}$, and 5.4 pA). The schematic in (b) shows that water molecules adsorb between the oxygen rows. The Ru atoms are represented in yellow, surface oxygen in red, and the oxygen of the water molecule in blue. [(c) and (d)] Images showing isolated water molecules and pairs of molecules on a well resolved image of the $\text{O}\left(2\times 1\right)/\text{Ru}\left(0001\right)$ surface ($2.9\text{nm}\times 4.7\text{nm}$, $-245\text{mV}$, and 10 pA). The schematic drawing in (e) shows the location of the water molecules relative to the ($2\times 1$) oxygen lattice.

Figure 4

(Color online) Optimized model of a hypothetical water molecule adsorbed on an unmodified $\text{O}\left(2\times 1\right)/\text{Ru}\left(0001\right)$: (a) top view and (b) side view. Two quite long and weak hydrogen bonds are formed with the oxygen atoms in the surface. The calculated binding energy of 86 meV is insufficient to ensure wetting.

Figure 5

(Color online) Models of water adsorption structures on the $2\text{O}\left(2\times 2\right)/\text{Ru}\left(0001\right)$ honeycomb surface. A $4\times 4$ cell (marked by the lines) was used for the calculations. The coverage of water is 0.25 ML. Yellow circles correspond to Ru atoms. Red circles correspond to oxygen atoms at hcp and fcc sites; the oxygen of the water molecule is represented in blue. The difference between the two structures is the orientation of the water molecule: (a) H atoms point toward the O(hcp) atoms and (b) H atoms point toward the O(fcc) atoms. Adsorption energies are given in the last row of Table . The O(fcc)-oriented molecules are $\sim 60\text{meV}$ more stable.

Figure 6

STM images of the $\text{O}\left(2\times 1\right)/\text{Ru}\left(0001\right)$ surface with different amounts of water adsorbed at 140 K: (a) $10–20\mathrm{\%}$ (b) $20–30\mathrm{\%}$, and (c) $75–85\mathrm{\%}$ water coverage. All images are $40\text{nm}\times 40\text{nm}$ in size. STM image parameters: (a) $-155\text{mV}$ and 37 pA, (b) 221 mV and 8 pA, and (c) $-385\text{mV}$ and 4 pA.

Figure 7

(Color online) (a) High-resolution STM image from a water domain revealing the hexagonal structure of the adsorbed water. (b) Model structure of showing water adsorbed on the $2\text{O}\left(2\times 2\right)/\text{Ru}\left(0001\right)$. Adapted from Ref. 1. STM image parameters: (a) 221 mV and 8 pA, and (b) 221 mV and 7 pA.

Figure 8

(Color online) Overview of relaxed geometries from the DFT calculations for different relative orientations of the water molecules within the layer. (a)–(d) correspond to different configurations in which all the molecules are H bonded to O(fcc) atoms. In panel (e) one molecule out of four is H bonded to two O(hcp) atoms and in panel (f) 50% of the molecules are H bonded to O(hcp) atoms. In (a) all water dipoles are aligned while in (b) their directions alternate. These two configurations are the most stable and are energetically degenerated, with an adsorption energy $E_{\text{ads}}=896\text{meV}/{\text{H}}_2\text{O}$. The other configurations are slightly less stable with adsorption energies lower by $21\text{meV}/{\text{H}}_2\text{O}$ for (c), $17\text{meV}/{\text{H}}_2\text{O}$ for (d), $28\text{meV}/{\text{H}}_2\text{O}$ for (e), and $35\text{meV}/{\text{H}}_2\text{O}$ for (f).

Figure 9

(Color online) Experimental STM image showing the presence of slight deviations from the perfect ($2\times 2$) alignment in the position of the maxima corresponding to water molecules, represented by the blue lattice. Every second row is slightly displaced.

Figure 10

(Color online) [(a)–(c)] Optimal configurations for the adsorption of water molecules on the oxygen-honeycomb reconstruction showing (a) one, (b) two, and (c) three different molecular orientations. Structures (a) and (b) are energetically degenerate while (c) is slightly less optimal by $\sim 17\text{meV}/{\text{H}}_2\text{O}$. In all the configurations, the water molecules are slightly displaced $\left(\sim 0.2\text{Å}\right)$ along the $xy$ plane, with respect to the Ru top sites, in order to form hydrogen bonds with the O(fcc) atoms (bond length $2.25\text{Å}$). [(d)–(f)] Corresponding simulated constant current images at $+400\text{mV}$ bias. In panels (e) and (f) the slightly displaced centers of the molecular protrusion are marked in blue.

Figure 11

Water adsorbed at 140 K on the O/Ru(0001) surface followed by annealing to 180 K. The residual water molecules form lines several nanometers long. The lines do not grow over monatomic steps nor decorate them as seen in (a). Image parameters: (a) $80{\text{nm}}^2$, $-340\text{mV}$, and 9 pA, (b) $40{\text{nm}}^2$, $-310\text{mV}$, and 5.5 pA, and (c) $6.6{\text{nm}}^2$, $-309\text{mV}$, and 5.4 pA.

Figure 12

STM images of the oxygen-honeycomb structure after desorbing the water by annealing to (a) 185 K $\left(15\text{nm}\times 15\text{nm}\right)$ and (b) to 220 K $\left(25\text{nm}\times 25\text{nm}\right)$. Image parameters: (a) $-148\text{mV}$ and 11 pA, and (b) $-312\text{mV}$ and 26 pA.

Figure 13

(Color online) (a) High-resolution image $\left(9.2\text{nm}\times 6.8\text{nm}\right)$ of the O honeycomb structure with a small patch of the original $\text{O}\left(2\times 1\right)$ structure. The resolution of the images in the $\text{O}\left(2\times 1\right)$ patch is sufficient for identification of the adsorption sites in the surrounding honeycomb structure. The nodes of the superimposed lattice located over the white protrusions in the honeycomb structure represent Ru(0001) top sites. (b) The dark stripes are domain boundaries between different honeycomb patches as can be seen more clearly with the help of the yellow lines. The nodes of the lattice in (b) represent Ru(0001) fcc sites $\left(9\text{nm}\times 5.2\text{nm}\right)$. (c) Section through the $\text{O}\left(2\times 1\right)$ patch and honeycomb structure in the right image, showing that the corrugation in the honeycomb structure is about five times higher than in the $\text{O}\left(2\times 1\right)$ structure. According to the simulated STM images, the double peak structure might be attributed to the O(fcc) and Ru top atoms. STM parameters: $-21\text{mV}$ and 11 pA.

Figure 14

(Color online) Calculated constant current STM image at $+400\text{mV}$ bias and topographic profile for the oxygen-honeycomb reconstruction. Inset: red circles correspond to O atoms and yellow circles correspond Ru atoms.