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\usepackage{natbib} |
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\usepackage{multirow} |
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\usepackage{wrapfig} |
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\usepackage{fixltx2e} |
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%\mciteErrorOnUnknownfalse |
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\usepackage[version=3]{mhchem} % this is a great package for formatting chemical reactions |
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\hline |
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\end{tabular} |
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\label{tab:co_energies} |
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\end{table} |
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|
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|
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\subsection{Validation of forcefield selections} |
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By calculating minimum energies for commensurate systems of |
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single and double layer Pt and Au systems with 0 and 50\% coverages |
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(arranged in a c(2x4) pattern), our forcefield selections were able to be |
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indirectly compared to results shown in the supporting information of Tao |
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{\it et al.} \cite{Tao:2010}. Five layer thick systems, displaying a 557 facet |
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were constructed, each composed of 480 metal atoms. Double layers systems |
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were constructed from six layer thick systems where an entire layer was |
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removed from both displayed facets to create a double step. By design, the |
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double step system also contains 480 atoms, five layers thick, so energy |
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comparisons between the arrangements can be made directly. The positions |
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of the atoms were allowed to relax, along with the box sizes, before a |
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minimum energy was calculated. Carbon monoxide, equivalent to 50\% |
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coverage on one side of the metal system was added in a c(2x4) arrangement |
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and again allowed to relax before a minimum energy was calculated. |
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|
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Energies for the various systems are displayed in Table ~\ref{tab:steps}. Examining |
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the Pt systems first, it is apparent that the double layer system is slightly less stable |
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then the original single step. However, upon addition of carbon monoxide, the |
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stability is reversed and the double layer system becomes more stable. This result |
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is in agreement with DFT calculations in Tao {\it et al.}\cite{Tao:2010}, who also show |
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that the addition of CO leads to a reversal in the most stable system. While our |
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results agree qualitatively, quantitatively, they are approximately an order of magnitude |
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different. Looking at additional stability per atom in kcal/mol, the DFT calculations suggest |
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an increased stability of 0.1 kcal/mol per Pt atom, whereas we are seeing closer to a 0.4 kcal/mol |
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increase in stability per Pt atom. |
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|
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The gold systems show a much smaller energy difference between the single and double |
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systems, likely arising from their lower energy per atom values. Additionally, the weaker |
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binding of CO to Au is evidenced by the much smaller energy change between the two systems, |
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when compared to the Pt results. This limited change helps explain our lack of any reconstruction |
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on the Au systems. |
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|
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|
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%Table of single step double step calculations |
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\begin{table}[H] |
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\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 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.} |
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\centering |
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\begin{tabular}{| c | c | c | c | c | c | c |} |
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\hline |
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\textbf{Step} & \textbf{N}\textsubscript{M} & \textbf{N\textsubscript{CO}} & \textbf{Unit-Cell Energy} & \textbf{Energy per M} & \textbf{Energy per CO} & \textbf{Difference per M} \\ |
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\hline |
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Pt(557)-S & 480 & 0 & -61142.624 & -127.381 & - & 0 \\ |
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Pt(557)-D & 480 & 0 & -61027.841 & -127.141 & - & 0.240 \\ |
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\hline |
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Pt(557)-S & 480 & 40 & -62960.289 & -131.167 & -45.442 & 0 \\ |
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Pt(557)-D & 480 & 44 & -63040.007 & -131.333 & -45.731 & -0.166\\ |
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\hline |
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\hline |
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Au(557)-S & 480 & 0 & -41879.286 & -87.249 & - &0 \\ |
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Au(557)-D & 480 & 0 & -41799.714 & -87.084 & - & 0.165 \\ |
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\hline |
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Au(557)-S & 480 & 40 & -42423.899 & -88.381 & -13.615 & 0 \\ |
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Au(557)-D & 480 & 44 & -42428.738 & -88.393 & -14.296 & -0.012 \\ |
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\hline |
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\end{tabular} |
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\label{tab:steps} |
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\end{table} |
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|
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|
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\subsection{Pt(557) and Au(557) metal interfaces} |
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Our Pt system is an orthorhombic periodic box of dimensions |
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54.482~x~50.046~x~120.88~\AA~while our Au system has |