| 324 |
|
\hline |
| 325 |
|
& Calculated & Experimental \\ |
| 326 |
|
\hline |
| 327 |
< |
\multirow{2}{*}{\textbf{Pt-CO}} & \multirow{2}{*}{-1.84} & -1.4 \bibpunct{}{}{,}{n}{}{,} |
| 327 |
> |
\multirow{2}{*}{\textbf{Pt-CO}} & \multirow{2}{*}{-1.81} & -1.4 \bibpunct{}{}{,}{n}{}{,} |
| 328 |
|
(Ref. \protect\cite{Kelemen:1979}) \\ |
| 329 |
|
& & -1.9 \bibpunct{}{}{,}{n}{}{,} (Ref. \protect\cite{Yeo}) \\ \hline |
| 330 |
|
\textbf{Au-CO} & -0.39 & -0.40 \bibpunct{}{}{,}{n}{}{,} (Ref. \protect\cite{TPDGold}) \\ |
| 334 |
|
\end{table} |
| 335 |
|
|
| 336 |
|
|
| 337 |
< |
\subsection{Validation of forcefield selections} |
| 338 |
< |
By calculating minimum energies for commensurate systems of |
| 339 |
< |
single and double layer Pt and Au systems with 0 and 50\% coverages |
| 340 |
< |
(arranged in a c(2x4) pattern), our forcefield selections were able to be |
| 341 |
< |
indirectly compared to results shown in the supporting information of Tao |
| 342 |
< |
{\it et al.} \cite{Tao:2010}. Five layer thick systems, displaying a 557 facet |
| 343 |
< |
were constructed, each composed of 480 metal atoms. Double layers systems |
| 344 |
< |
were constructed from six layer thick systems where an entire layer was |
| 345 |
< |
removed from both displayed facets to create a double step. By design, the |
| 346 |
< |
double step system also contains 480 atoms, five layers thick, so energy |
| 347 |
< |
comparisons between the arrangements can be made directly. The positions |
| 348 |
< |
of the atoms were allowed to relax, along with the box sizes, before a |
| 349 |
< |
minimum energy was calculated. Carbon monoxide, equivalent to 50\% |
| 350 |
< |
coverage on one side of the metal system was added in a c(2x4) arrangement |
| 351 |
< |
and again allowed to relax before a minimum energy was calculated. |
| 337 |
> |
\subsection{Forcefield validation} |
| 338 |
> |
The CO-Pt cross interactions were compared directly to DFT results |
| 339 |
> |
found in the supporting information of Tao {\it et al.} |
| 340 |
> |
\cite{Tao:2010}, while the CO-Au results are interpreted on their own. |
| 341 |
> |
These calculations are estimates of the stabilization |
| 342 |
> |
energy provided to double-layer reconstructions of the perfect (557) |
| 343 |
> |
surface by an overlayer of CO molecules in a $c (2 \times 4)$ pattern. |
| 344 |
> |
To make the comparison, metal slabs of both Pt and Au that were five atoms thick and |
| 345 |
> |
which displayed a (557) facet were constructed. Double-layer |
| 346 |
> |
(reconstructed) systems were created using six atomic layers where |
| 347 |
> |
enough of a layer was removed from both exposed (557) facets to create |
| 348 |
> |
the double step. In all cases, the metal slabs contained 480 atoms |
| 349 |
> |
and were minimized using steepest descent under the EAM force |
| 350 |
> |
field. Both the bare metal slabs and slabs with 50\% carbon monoxide |
| 351 |
> |
coverage (arranged in the $c (2 \times 4)$ pattern) were used. The |
| 352 |
> |
systems are periodic along and perpendicular to the step-edge axes |
| 353 |
> |
with a large vacuum above the displayed (557) facet. |
| 354 |
|
|
| 355 |
< |
Energies for the various systems are displayed in Table ~\ref{tab:steps}. Examining |
| 356 |
< |
the Pt systems first, it is apparent that the double layer system is slightly less stable |
| 357 |
< |
then the original single step. However, upon addition of carbon monoxide, the |
| 358 |
< |
stability is reversed and the double layer system becomes more stable. This result |
| 359 |
< |
is in agreement with DFT calculations in Tao {\it et al.}\cite{Tao:2010}, who also show |
| 360 |
< |
that the addition of CO leads to a reversal in the most stable system. While our |
| 361 |
< |
results agree qualitatively, quantitatively, they are approximately an order of magnitude |
| 362 |
< |
different. Looking at additional stability per atom in kcal/mol, the DFT calculations suggest |
| 363 |
< |
an increased stability of 0.1 kcal/mol per Pt atom, whereas we are seeing closer to a 0.4 kcal/mol |
| 364 |
< |
increase in stability per Pt atom. |
| 355 |
> |
Energies calculated using our force field for the various systems are |
| 356 |
> |
displayed in Table ~\ref{tab:steps}. The relative energies are calculated |
| 357 |
> |
as $E_{relative} = E_{system} - E_{M-557-S} - N_{CO}*E_{M-CO}$, |
| 358 |
> |
where $E_{M-CO}$ is -1.8 eV for CO-Pt and -0.39 eV for CO-Au. Our |
| 359 |
> |
calculated CO-Pt minimum is actually at -1.83 eV at a distance of 1.53~\AA, |
| 360 |
> |
which was obtained from single-atom liftoffs from a Pt(111) surface. The |
| 361 |
> |
arrangement of CO on the single and double steps however, leads to a |
| 362 |
> |
slight displacement from the minimum. For a 1 ps run at 3 K, the single |
| 363 |
> |
step Pt-CO average bond length was 1.60~\AA, and for the double step, |
| 364 |
> |
the bond length was 1.58~\AA. This slight increase is likely due to small |
| 365 |
> |
electrostatic interactions among the CO and the non-ideality of the surface. |
| 366 |
|
|
| 367 |
< |
The gold systems show a much smaller energy difference between the single and double |
| 368 |
< |
systems, likely arising from their lower energy per atom values. Additionally, the weaker |
| 369 |
< |
binding of CO to Au is evidenced by the much smaller energy change between the two systems, |
| 370 |
< |
when compared to the Pt results. This limited change helps explain our lack of any reconstruction |
| 371 |
< |
on the Au systems. |
| 367 |
> |
For platinum, the bare double layer is less stable then the original single |
| 368 |
> |
(557) step by about 0.25 kcal/mole per Pt atom. However, addition of carbon |
| 369 |
> |
monoxide to the double step system provides a greater amount of stabilization |
| 370 |
> |
when compared to single step system with CO on the order of 230 kcal/mole |
| 371 |
> |
for this system size. The absolute difference is minimal, but this result is in |
| 372 |
> |
qualitative agreement with DFT calculations in Tao {\it et al.}\cite{Tao:2010}, |
| 373 |
> |
who also showed that the addition of CO leads to a reversal in stability. |
| 374 |
|
|
| 375 |
+ |
The gold systems show a smaller energy difference between the clean |
| 376 |
+ |
single and double layers when compared to platinum. Upon addition of |
| 377 |
+ |
CO however, the single step surface becomes much more stable. These |
| 378 |
+ |
results, while helpful, need to be tempered by the weaker binding energy |
| 379 |
+ |
of CO to Au. From our simulations we see that at the elevated temperatures |
| 380 |
+ |
we are running at, it is difficult for the gold systems to maintain > than 25\% |
| 381 |
+ |
coverage, despite their being enough CO in the system. |
| 382 |
|
|
| 383 |
|
%Table of single step double step calculations |
| 384 |
|
\begin{table}[H] |
| 385 |
< |
\caption{Minimized single point energies of unit cell crystals displaying (S)ingle or (D)double steps. Systems are periodic along and perpendicular to the step-edge axes with a large vacuum above the displayed 557 facet. The relative energies are calculated as $E_{relative} = E_{system} - E_{M-557-S} - N_{CO}\Delta E_{CO-M}$ , where $E_{CO-M}$ is -1.84 eV for Pt-CO and -0.39 eV for Pt-CO. The addition of CO in a 50\% c(2x4) coverage acts as a stabilizing presence and suggests a driving force for the observed reconstruction on the highest coverage Pt system. All energies are in kcal/mol.} |
| 385 |
> |
\caption{Minimized single point energies of (S)ingle and (D)ouble |
| 386 |
> |
steps. The addition of CO in a 50\% $c(2 \times 4)$ coverage acts as a |
| 387 |
> |
stabilizing presence and suggests a driving force for the observed |
| 388 |
> |
reconstruction on the highest coverage Pt system. All energies are |
| 389 |
> |
in kcal/mol.} |
| 390 |
|
\centering |
| 391 |
|
\begin{tabular}{| c | c | c | c | c | c |} |
| 392 |
|
\hline |
| 393 |
|
\textbf{Step} & \textbf{N}\textsubscript{M} & \textbf{N\textsubscript{CO}} & \textbf{Relative Energy} & \textbf{$\Delta$E/M} & \textbf{$\Delta$E/CO} \\ |
| 394 |
|
\hline |
| 395 |
|
Pt(557)-S & 480 & 0 & 0 & 0 & - \\ |
| 396 |
< |
Pt(557)-D & 480 & 0 & 114.783 & 0.239 & -\\ |
| 397 |
< |
Pt(557)-S & 480 & 40 & -124.546 & -0.259 & -3.114\\ |
| 398 |
< |
Pt(557)-D & 480 & 44 & -34.953 & -0.073 & -0.794\\ |
| 396 |
> |
Pt(557)-D & 480 & 0 & 119.788 & 0.2495 & -\\ |
| 397 |
> |
Pt(557)-S & 480 & 40 & -109.734 & -0.2286 & -2.743\\ |
| 398 |
> |
Pt(557)-D & 480 & 48 & -110.039 & -0.2292 & -2.292\\ |
| 399 |
|
\hline |
| 400 |
|
\hline |
| 401 |
|
Au(557)-S & 480 & 0 & 0 & 0 & - \\ |
| 402 |
< |
Au(557)-D & 480 & 0 & 79.572 & 0.166 & - \\ |
| 403 |
< |
Au(557)-S & 480 & 40 & -157.199 & -0.327 & -3.930\\ |
| 404 |
< |
Au(557)-D & 480 & 44 & -123.297 & -0.257 & -2.802 \\ |
| 402 |
> |
Au(557)-D & 480 & 0 & 83.853 & 0.1747 & - \\ |
| 403 |
> |
Au(557)-S & 480 & 40 & -253.604 & -0.5283 & -6.340\\ |
| 404 |
> |
Au(557)-D & 480 & 48 & -156.150 & -0.3253 & -3.253 \\ |
| 405 |
|
\hline |
| 406 |
|
\end{tabular} |
| 407 |
|
\label{tab:steps} |
