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 $\ensuremath{-}0.95\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{-}1.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ ($570--605\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{mbar}$ ${\mathrm{O}}_{2}$). 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.