Surface oxides on Pd(111): STM and density functional calculations

The formation of one-layer surface oxides on Pd(111) has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Besides the ${\mathrm{Pd}}_5{\mathrm{O}}_4$ structure determined previously, structural details of six different surface oxides on Pd(111) will be presented. These oxides are observed for preparation in oxygen-rich conditions, approaching the thermodynamic stability limit of the PdO bulk oxide at an oxygen chemical potential of $-0.95\text{to}-1.02\mathrm{eV}$ ($570–605\mathrm{K}$, $5\times {10}^{-4}\mathrm{mbar}$ ${\mathrm{O}}_2$). Sorted by increasing oxygen fraction in the primitive unit cell, the stoichiometry of the surface oxides is ${\mathrm{Pd}}_5{\mathrm{O}}_4$, ${\mathrm{Pd}}_9{\mathrm{O}}_8$, ${\mathrm{Pd}}_{20}{\mathrm{O}}_{18}$, ${\mathrm{Pd}}_{23}{\mathrm{O}}_{21}$, ${\mathrm{Pd}}_{19}{\mathrm{O}}_{18}$, ${\mathrm{Pd}}_8{\mathrm{O}}_8$, and ${\mathrm{Pd}}_{32}{\mathrm{O}}_{32}$. All structures are one-layer oxides, in which oxygen atoms form a rectangular lattice, and all structures follow the same rules of favorable alignment of the oxide layer on the Pd(111) substrate. DFT calculations were used to simulate STM images as well as to determine the stability of the surface oxide structures. Simulated and measured STM images are in excellent agreement, indicating that the structural models are correct. Since the newly found surface oxides are clearly less stable than ${\mathrm{Pd}}_5{\mathrm{O}}_4$, we conclude that ${\mathrm{Pd}}_5{\mathrm{O}}_4$ is the only thermodynamically stable phase, whereas all newly found structures are only kinetically stabilized. We also discuss possible mechanisms for the formation of these oxide structures.

Figure 1

STM images of a surface covered by various one-layer surface oxides (marked by letters $a–f$) grown on a 3-ML thin Pd film (a) overview ($15\mathrm{nm}\times 15\mathrm{nm}$; $-18\mathrm{mV}$, $0.54\mathrm{nA}$) with the unit cells of structures $a$ and $c$ marked, (b) close-up of an area covered by structure $d$ and (c) atomically resolved image of an ${\mathrm{Pd}}_5{\mathrm{O}}_4$ area grown on Pd(111) ($-1.8\mathrm{mV}$, $4.38\mathrm{nA}$, Pd is visible).

Figure 2

Atomically resolved STM image of an oxidized Pd(111) surface covered by the surface oxides $a$, $b$, $c$, $e$, and ${\mathrm{Pd}}_5{\mathrm{O}}_4$ ($-1.8\mathrm{mV}$, $4.38\mathrm{nA}$; Pd atoms in the “segment” are indicated by circles). The rhomb in the bottom left side is the ${\mathrm{Pd}}_{32}{\mathrm{O}}_{32}$ unit cell.

Figure 3

STM image of a fully oxide covered Pd(111) surface ($-0.5\mathrm{V}$, $0.54\mathrm{nA}$). Excess Pd forms monolayer-high islands during oxidation (bright areas). Islands are covered mainly by ${\mathrm{Pd}}_5{\mathrm{O}}_4$, whereas island-free parts are covered by various surface oxides.

Figure 4

(Color online) One-layer surface oxide structures as determined from STM. In the oxide layer, Pd atoms are indicated by large (blue) circles, O by small (red) circles, and the topmost substrate Pd(111) layer is indicated by lines with Pd at crossing points.

Figure 5

(Color online) Calculated structure models (left) of (a) ${\mathrm{Pd}}_{20}{\mathrm{O}}_{18}$, (b) ${\mathrm{Pd}}_{19}{\mathrm{O}}_{18}$, (c) ${\mathrm{Pd}}_{10}{\mathrm{O}}_9$, (d) ${\mathrm{Pd}}_9{\mathrm{O}}_8$, (e) ${\mathrm{Pd}}_{32}{\mathrm{O}}_{32}$, and (f) ${\mathrm{Pd}}_8{\mathrm{O}}_8$. Oxygen atoms are shown as smaller and darker (red) balls. The buckling of the surface layer in the structure models is indicated by the atom brightness. Simulated and measured STM images are shown at the top and bottom of the right sides, respectively. The experimental STM images in (a) and (b) show the transition between ${\mathrm{Pd}}_{20}{\mathrm{O}}_{18}$ and ${\mathrm{Pd}}_{19}{\mathrm{O}}_{18}$; thus, only the upper side of the unit cell corresponds to the calculation.

Figure 6

(Color online) Calculated phase diagram. For each value of the chemical potential, the most stable structure is that represented by the bottommost line. The dashed area indicates preparation conditions. A gray vertical line indicates the border of the PdO stability regime above $-1\mathrm{eV}$. $\Delta \gamma$ is the difference of surface energy between a given structure and the Pd(111) surface [${\gamma }_{\mathrm{Pd}\left(111\right)}=0.57\mathrm{eV}∕(1\times 1)$ cell].