Phase diagram of oxygen adsorbed on platinum (111) by first-principles investigation

A complete phase diagram of oxygen atoms adsorbed on a $\mathrm{Pt}\left(111\right)$ surface with oxygen coverages below half a monolayer has been computed and compared with the surface phase diagrams of related systems. Effective interaction parameters of a lattice model for the triangular lattice of the fcc sites of the $\mathrm{Pt}\left(111\right)$ surface were determined from first-principles computations. Oxygen forms on the platinum (111) surface two stable ordered phases, which persist up to high temperatures. They are the $p\left(2\times 2\right)$ and $p\left(2\times 1\right)$ phases, having coverages of $1∕4$ and $1∕2$ monolayer, respectively. At the coverage of $2∕5$ monolayer, another stable phase consisting of $p\left(2\times 1\right)$ rows but with every two rows offset by an empty site is predicted by our model, but this phase is stable only below $250\mathrm{K}$. All three phases undergo continuous phase transitions to the disordered state upon heating. At coverages lower than $1∕4$ monolayer and at low temperatures, oxygen atoms cluster into $p\left(2\times 2\right)$ islands, in agreement with observations from a scanning tunneling microscope study. The formation of $p\left(2\times 2\right)$ oxygen islands is a consequence of attractive third-nearest-neighbor interactions, despite the strong repulsion between the first and second nearest neighbors. Two regions separated by first-order phase boundaries are found at coverages between 0.26 monolayer and 0.37 monolayer and coverages between 0.43 monolayer and 0.5 monolayer.

Figure 1

Adsorbed oxygen configurations calculated using DFT-GGA computations. Large open circles represent Pt atoms and small solid circles represent O atoms. Unit cells are marked in each configuration.

Figure 2

Lateral interactions between and among oxygen atoms adsorbed on Pt (111). Small circles represent oxygen atoms and large circles represent platinum atoms.

Figure 3

Formation energies of adsorbed oxygen atoms on Pt (111) (the square point was not included in the fitting of the lateral interaction parameters, but was used as a test for our cluster expansion). Labels (a)–(o) correspond to those in Table .

Figure 4

Comparison among the changes of projected density of states (pDOS) of Pt atoms at different distances from an adsorbed oxygen atom. (The differences are with respect to Pt atoms of the same symmetry at clean surfaces.)

Figure 5

Side view of configuration $p\left(2\times 2\right)-\mathrm{O}$.

Figure 6

Phase diagram of $\mathrm{O}∕\mathrm{Pt}\left(111\right)$: the solid lines denote continuous phase transitions and the dotted lines denote first-order phase transitions.

Figure 7

A sample configuration of the ordered oxygen phase having the symmetry $p\left(2\times 1\right)$ on Pt (111) with $p\left(2\times 1\right)$ rows rotated by $120°$ with respect to each other.

Figure 8

Regions characterized as $p\left(2\times 2\right)$ oxygen islands in a snapshot of a GCMC calculation ($T=100\mathrm{K}$, $\mu =-979$, and $\theta =0.09\mathrm{ML}$).

Figure 9

Oxygen on Pt (111) dosed at $82\mathrm{K}$ and warmed to $152\mathrm{K}$. Bright atoms are Pt and dark atoms are O. $70\mathrm{\AA }\times 70\mathrm{\AA }$. Reproduced with permission from Ref. 4.