Coadsorption phases of $\mathrm{C}\mathrm{O}$ and oxygen on Pd(111) studied by scanning tunneling microscopy

The adsorption of $\mathrm{C}\mathrm{O}$ on an oxygen precovered Pd(111) surface was investigated between 60 and 300 K. Applied methods were variable temperature scanning tunneling microscopy (STM) and video STM to analyze the coadsorption structures. The STM data are compared with simulated STM images for the various surface phases in order to identify the appropiate structural model for each case. Low-energy electron diffraction and reaction isotherms by means of mass spectrometry were used to correlate the phases with the reaction yielding $\mathrm{C}{\mathrm{O}}_2$. The video-STM data recorded during $\mathrm{C}\mathrm{O}$ adsorption at 300 K on the ${(2\times 2)}_{\mathrm{O}}$ phase show a fast phase transition into the $(\sqrt{3}\times \sqrt{3})R{30}_{\mathrm{O}}^{\mathrm{\circ }}$ structure, followed by reaction to $\mathrm{C}{\mathrm{O}}_2$. The reaction only starts after completion of the phase transition, indicating that the $(\sqrt{3}\times \sqrt{3})R{30}_{\mathrm{O}}^{\circ }$ structure plays a crucial role for the reaction. At temperatures between 170 and 190 K the phase transition is slow enough to be monitored with STM. The experimental images of both the ${(2\times 2)}_{\mathrm{O}}$ and the $(\sqrt{3}\times \sqrt{3})R{30}_{\mathrm{O}}^{\circ }$ structures are well reproduced by the simulations. Further $\mathrm{C}\mathrm{O}$ adsorption caused a second phase transition into a $p{(2\times 1)}_{\mathrm{O}}$ structure. The STM simulations strongly support a pure oxygen $p(2\times 1)$ structure, rather than a mixed $\mathrm{O}+\mathrm{C}\mathrm{O}$ structure, in contrast to previous experimental work. The $\mathrm{C}\mathrm{O}$ molecules form the same structures between the $\mathrm{O}$ islands that are known from the pure Pd(111)/$\mathrm{C}\mathrm{O}$ system. At lower temperatures, between 110 and 60 K, a so far unknown $(2\times 2)$ phase was observed. The formation of this structure, and its imaging by the STM, show that it constitutes a mixed $p{(2\times 2)}_{\mathrm{O}+\mathrm{C}\mathrm{O}}$ structure, where the oxygen atoms remain unchanged, and the $\mathrm{C}\mathrm{O}$ molecules occupy hcp sites between the $\mathrm{O}$ atoms.

Figure 1

Density of states (DOS) projected on the first surface atoms for the SIESTA calculation (dashed line) and the extended Hückel theory (EHT) fit (solid lines) for (a) the $p{(2\times 2)}_{\mathrm{O}\mathrm{fcc}+\mathrm{C}\mathrm{O}\text{top}}$ model of Fig. 12c, (b) the clean Ir tip, and (c) the oxygen terminated Ir-$\mathrm{O}$ tip. The insets at the top of the figures give side and normal views of each system. The solid thick lines in the normal views indicate the unit cell. For the tips, although the SIESTA calculations are performed in a $p(3\times 3)$ lattice, the STM simulations assume an isolated apex, as indicated in the figures. The examples show the level of agreement achieved for the structures considered in this work.

Figure 2

Four STM images $(9\mathrm{nm}\times 9.5\mathrm{nm})$ from a series recorded with the video-STM at 300 K, the $\mathrm{C}\mathrm{O}$ pressure was $1\times {10}^{-7}\mathrm{Torr}$. (a) Initial surface covered with the $p{(2\times 2)}_{\mathrm{O}}$ structure $(t=0\mathrm{s})$, (b) the $(\sqrt{3}\times \sqrt{3})R{30}_{\mathrm{O}}^{\circ }$ structure starts to form at the lower-left of the image $(t=20\mathrm{s})$, (c) complete $(\sqrt{3}\times \sqrt{3})R{30}_{\mathrm{O}}^{\circ }$ structure; the local $\mathrm{O}$ coverage has changed from $\theta =1∕4$ to $\theta =1∕3(t=22\mathrm{s})$, (d) final stage, with the surface completely covered by $\mathrm{C}\mathrm{O}$. The $p(1\times 1)$ periodicity of the palladium substrate is weakly resolved $(t=48\mathrm{s})$. The frames are from the same area, see arrow pointing to the same defect. Tunnel parameters are $V=+0.32\mathrm{V}$, $I=1.4\mathrm{nA}$.

Figure 3

Mass spectrometer data showing the $\mathrm{C}{\mathrm{O}}_2$ production rate for different $\mathrm{C}\mathrm{O}$ pressures between $5\times {10}^{-9}$ and $5\times {10}^{-7}\mathrm{Torr}$. (a) Plotted as a function of time. The integral of the curves are constant, as is expected for a fixed oxygen coverage $(\sim 0.25\mathrm{ML})$. (b) Plotted as a function of $\mathrm{C}\mathrm{O}$ dose in $\mathrm{L}$ $(1\mathrm{L}={10}^{-6}\mathrm{Torr}\mathrm{s})$. The inset shows LEED intensities for the $(1∕3,1∕3)$ and $(1∕2,1∕2)$ spots at $9.9\times {10}^{-9}\mathrm{Torr}$ $\mathrm{C}\mathrm{O}$. Note the induction phase for $\mathrm{C}{\mathrm{O}}_2$ production, where the $\mathrm{C}{\mathrm{O}}_2$ signal stays low. After a $\mathrm{C}\mathrm{O}$ exposure of $\sim 0.55\mathrm{L}$ the rate increases rapidly. This point also corresponds to a maximum of the $(1∕3,1∕3)$ and a minimum of the $(1∕2,1∕2)$ LEED spot intensities.

Figure 4

STM images $(12\mathrm{nm}\times 12\mathrm{nm})$ recorded during adsorption of $\mathrm{C}\mathrm{O}$ at 170–190 K. (a) Oxygen $p{(2\times 2)}_{\mathrm{O}}$ phase; $V=-0.4\mathrm{V}$, $I=1\mathrm{nA}$, $T=170\mathrm{K}$. (b) $p{(2\times 2)}_{\mathrm{O}}$ structure with bright features; $V=+0.4\mathrm{V}$, $I=1\mathrm{nA}$, $T=170\mathrm{K}$. (c) Dark areas appear; $V=+0.4\mathrm{V}$, $I=0.9\mathrm{nA}$, $T=173\mathrm{K}$. (d) $(\sqrt{3}\times \sqrt{3})R{30}_{\mathrm{O}}^{\circ }$ structure; $V=-1.1\mathrm{V}$, $I=0.7\mathrm{nA}$, $T=181\mathrm{K}$. (e) $p{(2\times 1)}_{\mathrm{O}}$ and $\mathrm{c}{(4\times 2)}_{\mathrm{C}\mathrm{O}}$ phases; $V=1.75\mathrm{V}$, $I=0.66\mathrm{nA}$, $T=184\mathrm{K}$. (f) $p{(2\times 1)}_{\mathrm{O}}$ islands and $\mathrm{C}\mathrm{O}$ covered surface; $V=-1.7\mathrm{V}$, $I=0.75\mathrm{nA}$, $T=176\mathrm{K}$. Images (a)–(e) correspond to “reversed-contrast” tip conditions, with oxygen imaged as bright bumps. (f) corresponds to “normal-contrast” with oxygen imaged dark.

Figure 5

Formation of the $p{(2\times 2)}_{\mathrm{C}\mathrm{O}+\mathrm{O}}$ mixed structure. (a) STM image $(15.5\mathrm{nm}\times 15.5\mathrm{nm})$ recorded at 60 K showing $\mathrm{C}\mathrm{O}$ molecules starting to form a $(2\times 2)$ structure of bright features. The $\mathrm{C}\mathrm{O}$ dose is $2\mathrm{L}$. $V=-0.85\mathrm{V}$, $I=0.9\mathrm{nA}$. (b) Image $(13.4\mathrm{nm}\times 13.4\mathrm{nm})$ after $8\mathrm{L}$ $\mathrm{C}\mathrm{O}$; $V=-1.4\mathrm{V}$, $I=0.9\mathrm{nA}$, $T=90\mathrm{K}$. (c) STM image $(18.4\mathrm{nm}\times 18.4\mathrm{nm})$ at 102 K, with the $p{(2\times 2)}_{\mathrm{C}\mathrm{O}+\mathrm{O}}$ almost completed. $V=-1.7\mathrm{V}$, $I=0.9\mathrm{nA}$.

Figure 6

STM image $(8.1\mathrm{nm}\times 8.5\mathrm{nm})$ with the completed $p{(2\times 2)}_{\mathrm{C}\mathrm{O}+\mathrm{O}}$. $\mathrm{C}\mathrm{O}$ molecules around defects appear brighter. $V=-1.7\mathrm{V}$, $I=0.9\mathrm{nA}$, $T=103\mathrm{K}$.

Figure 7

STM image $(15.4\mathrm{nm}\times 21.8\mathrm{nm})$ recorded at 176 K of the oxygen covered Pd(111) surface showing the $p{(2\times 2)}_{\mathrm{O}}$ structure. At this temperature the oxygen atoms (dark dots, i.e., “normal contrast”) are almost immobile, so that the structure and the vacancies (bright) are well resolved. At the domain boundaries, indicated by the line, some oxygen atoms occupy $\sqrt{3}$ positions (marked with arrows). $V=-1.4\mathrm{V}$, $I=1.05\mathrm{nA}$.

Figure 8

Comparison between experimental and simulated STM images for the oxygen $p{(2\times 2)}_{\mathrm{O}}$ structure on Pd(111). (a) Model with oxygen on the fcc position. (b) STM image $(2\mathrm{nm}\times 2\mathrm{nm})$ obtained with normal contrast showing oxygen atoms as dark and vacancies as bright features; $V=-1.4\mathrm{V}$, $I=1.05\mathrm{nA}$. (c) Simulated image calculated for the same tunnel conditions as in (b) but with the iridium tip depicted in Fig. 1b. (d) STM image $(2\mathrm{nm}\times 2\mathrm{nm})$ obtained with inverted contrast, where oxygen atoms appear bright; $V=-0.4\mathrm{V}$, $I=1\mathrm{nA}$. (e) Simulated image at the same tunnel conditions as in (c) for an oxygen terminated iridium tip [depicted in Fig. 1c].

Figure 9

The $(\sqrt{3}\times \sqrt{3})R30°$ oxygen structure. (a) Model with oxygen atoms on threefold fcc positions. (b) STM image $(2\mathrm{nm}\times 2\mathrm{nm})$; $T=181\mathrm{K}$, $V=-1.1\mathrm{V}$, $I=0.7\mathrm{nA}$; corrugation is 0.3 Å. (c) Simulation with an Ir(111) tip, in which the oxygen atoms appear dark. (d) STM image $(2\mathrm{nm}\times 2\mathrm{nm})$; $V=+0.32\mathrm{V}$, $I=1.4\mathrm{nA}$. (e) Simulation with an oxygen terminated tip where the oxygen atoms appear bright.

Figure 10

The $p(2\times 1)$ structure. (a) Model with oxygen on fcc sites. (b) Model with alternating lines of oxygen atoms and $\mathrm{C}\mathrm{O}$ molecules. (c) STM image $(5.6\mathrm{nm}\times 5.6\mathrm{nm})$ obtained in the “normal-contrast” state; $T=63\mathrm{K}$, $V=-0.3\mathrm{V}$, $I=1\mathrm{nA}$; corrugation is 0.1 Å. (d),(e) Simulated STM images performed with an iridium tip for the models in (a) and (b). (f) STM image $(4.3\mathrm{nm}\times 4.3\mathrm{nm})$ obtained with “reversed contrast”; $V=+0.7\mathrm{V}$; $I=1\mathrm{nA}$; corrugation is 0.11–0.14 Å. (g),(h) Simulated STM images with an oxygen-terminated tip for the models in (a) and (b).

Figure 11

(a) STM image $(3.3\mathrm{nm}\times 3.3\mathrm{nm})$ recorded during formation of the mixed $p{(2\times 2)}_{\mathrm{C}\mathrm{O}+\mathrm{O}}$ structure, $T=59\mathrm{K}$ and $2.5\mathrm{L}$ $\mathrm{C}\mathrm{O}$. With the applied tunneling parameters $(V=-1.4\mathrm{V};I=1.1\mathrm{nA})$ the $\mathrm{C}\mathrm{O}$ molecules and the underlying $p{(2\times 2)}_{\mathrm{O}}$ become simultaneously visible. (b) Same image with overlaid $p(2\times 2)$ lattice. The $\mathrm{C}\mathrm{O}$ molecules are located on the same sites as the dark positions of the $p{(2\times 2)}_{\mathrm{O}}$ oxygen structure. Atom-resolved images with two terraces reveal that the $\mathrm{C}\mathrm{O}$ molecules occupy hcp sites.

Figure 12

Models calculated for the $p{(2\times 2)}_{\mathrm{C}\mathrm{O}+\mathrm{O}}$ phase. (a) Oxygen on fcc and $\mathrm{C}\mathrm{O}$ on fcc sites. (b) Oxygen on fcc and $\mathrm{C}\mathrm{O}$ on hcp sites. (c) Oxygen on fcc sites and $\mathrm{C}\mathrm{O}$ on top. (d) Oxygen on fcc sites and two $\mathrm{C}\mathrm{O}$ molecules on hcp sites and on top. The simulated STM images are similar for all of these structures. In all of them, only one $\mathrm{C}\mathrm{O}$ molecule per unit cell is seen [in (d) only the on top].

Figure 13

Comparison between experiment and theory. (a) STM image of the mixed $p{(2\times 2)}_{\mathrm{C}\mathrm{O}+\mathrm{O}}$ structure $(2\mathrm{nm}\times 2\mathrm{nm})$; $T=103\mathrm{K}$, $V=-1.7\mathrm{V}$, $I=0.9\mathrm{nA}$; corrugation is 0.23 Å. (b) Simulated image for the $\mathrm{C}\mathrm{O}$-hcp model in Fig. 12b with the same tunnel conditions. The comparison indicates a good agreement for this model.

Figure 14

Tentative temperature vs $\mathrm{C}\mathrm{O}$ coverage phase diagram with the different structures observed in this study.