Oxidation of Pd(553): From ultrahigh vacuum to atmospheric pressure

The oxidation of a vicinal Pd(553) surface has been studied from ultrahigh vacuum (UHV) to atmospheric oxygen pressures at elevated sample temperatures. The investigation combines traditional electron based UHV techniques such as high resolution core level spectroscopy, low-energy electron diffraction, scanning tunneling microscopy with in situ surface x-ray diffraction, and ab initio simulations. In this way, we show that the O atoms preferentially adsorb at the step edges at oxygen pressures below ${10}^{-6}\mathrm{mbar}$ and that the (553) surface is preserved. In the pressure range between ${10}^{-6}$ and $1\mathrm{mbar}$ and at a sample temperature of $300–400°\mathrm{C}$, a surface oxide forms and rearranges the (553) surface facets and forming (332) facets. Most of the surface oxide can be described as a PdO(101) plane, similar to what has been found previously on other Pd surfaces. However, in the present case, the surface oxide is reconstructed along the step edges, and the stability of this structure is discussed. In addition, the $(\sqrt{6}\times \sqrt{6})$ ${\mathrm{Pd}}_5{\mathrm{O}}_4$ surface oxide can be observed on (111) terraces larger than those of the (332) terraces. Increasing the O pressure above $1\mathrm{mbar}$ results in the disappearance of the (332) facets and the formation of PdO bulk oxide.

Figure 1

(a) Model of the Pd(553) surface. (b) STM image of the clean surface ($85\times 65{\mathrm{\AA }}^2$, $0.015\mathrm{V}$, and $0.54\mathrm{nA}$). Almost all terraces consist of one step row and three visible rows in the terrace that together with a fifth row underneath the next step make up the (553) surface structure. (c) LEED pattern from the clean Pd(553) surface. The unit cell of the (111) terraces is indicated and the splitting due to the periodicity of the steps. (d) SXRD scan from the Pd(553) surface in the $K$ direction with $H=0$ and $L=1.2$ plotted in (553) coordinates. The extra peak between $H=4$ and $H=5$ is due to {111} planes from polycrystalline parts within the sample.

Figure 2

(a) Ex situ HRCLS spectra for different oxygen exposures. Each spectrum was recorded at room temperature after exposing the sample to oxygen for $5\min$ at $350°\mathrm{C}$. (b) In situ SXRD scans in the $k$ direction with $H=0$ and $L=1.2$ for different oxygen pressures and a sample temperature of $350°\mathrm{C}$. The scans are plotted in (553) coordinates.

Figure 3

(Color online) (a) LEED image from the Pd(553) surface after exposing it to ${10}^{-6}\mathrm{mbar}$ of oxygen for $5\min$ at $350°\mathrm{C}$. Apart from the spots from the clean surface, also $(2\times 1)$-like streaks can be seen, indicating a structure with a periodicity that is well defined in the step direction. (b) In situ SXRD $K$ scans ($H=0$ and $L=1.2$) from the clean and the ${10}^{-6}\mathrm{mbar}$ oxygen exposed Pd(553) surface. (c) HRCLS from the $\mathrm{Pd}3d_{5∕2}$ level from the clean (bottom) and the ${10}^{-6}\mathrm{mbar}$ oxygen exposed surface (top) for $5\min$ at $350°\mathrm{C}$.

Figure 4

(Color online) Calculated models for the adsorption of oxygen on Pd(553): (a) the low-coverage case (1O), (b) a $p2mg$-like structure at the step (2O), (c) oxygen adsorbed on the step edge and terrace (3O), and (d) phase diagram of the relaxed phases [(a)–(c)] (lower blue lines) as well as the corresponding unrelaxed structures (light gray lines). ${\mu }_0$ is the chemical potential of oxygen and $\gamma$ the Gibbs free surface energy. High chemical potentials correspond to high oxygen pressure and/or low temperature.

Figure 5

(Color online) (a) LEED image after exposing the sample to ${10}^{-3}\mathrm{mbar}$ of oxygen at $350°\mathrm{C}$. The periodicity of the spots in the $K$ direction has changed due to faceting. New spots have emerged perpendicular to the steps, indicating a reconstruction along the steps. (b) SXRD scan along the steps reveals a periodicity of about 12. (c) SXRD scans along $K$ with $H=0$ and increasing $L$, plotted both in (553) and (332) coordinates, showing that the (553) surface has faceted into a (332) surface.

Figure 6

(Color online) (a) STM image ($V=0.95\mathrm{V}$, $I=0.64\mathrm{nA}$) after exposure to ${10}^{-3}\mathrm{mbar}$ of oxygen at $350°\mathrm{C}$. On the (221) and (775) areas (marked by arrows), a similar surface oxide as on (332) can be observed. (b) and (d) are line scans, which are marked by black lines in (a) and have been tilted to obtain flat (332) and (111) facets, respectively. (c) and (e) are the corresponding side view models. The asterisk in (a) and in model (c) marks a disordered area of the STM image.

Figure 7

(Color online) (a) STM image ($V=0.21\mathrm{V}$, $I=5.69\mathrm{nA}$) of an oxide-covered (332) area after exposure to ${10}^{-3}\mathrm{mbar}$ of oxygen at $350°\mathrm{C}$. The oval indicates the transition region between well-ordered surface-oxide stripes. The oxide unit cell is indicated by a rectangle in (a)–(d). (b) Model of the substrate below the surface oxide and (c) simulated lowest-energy structure for the surface oxide on Pd(332). The red (small) atoms are oxygen, while gray (large) atoms are Pd, and the geometric height (normal to the surface) is indicated by atom brightness. (d) Simulated STM image of the lowest-energy structure on Pd(332). States with energy between $E_F$ and $E_F+1\mathrm{eV}$ have been taken into account.

Figure 8

(Color online) HRCLS from the surface oxide on Pd(553): left the $\mathrm{O}1s$ region, and right the Pd $3d_{5∕2}$ region.

Figure 9

(Color online) Phase diagram for the ${\mathrm{Pd}}_5{\mathrm{O}}_4$ surface oxide (“$\sqrt{6}$”) on Pd(111) as well as the PdO(101)-like surface oxide layer on Pd(332), as shown in Fig. 7c. The surface energy for the $\sqrt{6}$ oxide is multiplied by 1.1078 to account for the higher surface area that fictional (111) and $\left(11\overline{1}\right)$ faceted surfaces would result in. The vertical gray line corresponds to the stability limit of the PdO bulk oxide.

Figure 10

(Color online) (a) Intensity distribution in the $H=0$ plane together with a schematic model of the PdO unit cell. (b) Illustration of the alignment of the PdO on the Pd(553) as deduced from (a).