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\usepackage[version=3]{mhchem} % this is a great package for formatting chemical reactions |
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% \usepackage[square, comma, sort&compress]{natbib} |
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\begin{abstract} |
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We examine surface reconstructions of Pt and Au(557) under |
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various CO coverages using molecular dynamics in order to |
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explore possible mechanisms for any observed reconstructions |
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and their dynamics. The metal-CO interactions were parameterized |
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as part of this work so that an efficient large-scale treatment of |
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this system could be undertaken. The large difference in binding |
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strengths of the metal-CO interactions was found to play a significant |
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role with regards to step-edge stability and adatom diffusion. A |
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small correlation between coverage and the diffusion constant |
87 |
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was also determined. The energetics of CO adsorbed to the surface |
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is sufficient to explain the reconstructions observed on the Pt |
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systems and the lack of reconstruction of the Au systems. |
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|
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|
91 |
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The mechanism and dynamics of surface reconstructions of Pt(557) |
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and Au(557) exposed to various coverages of carbon monoxide (CO) |
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< |
were investigated using molecular dynamics simulations. Metal-CO |
94 |
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interactions were parameterized from experimental data and plane-wave |
95 |
< |
Density Functional Theory (DFT) calculations. The large difference in |
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binding strengths of the Pt-CO and Au-CO interactions was found to play |
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a significant role in step-edge stability and adatom diffusion constants. |
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The energetics of CO adsorbed to the surface is sufficient to explain the |
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step-doubling reconstruction observed on Pt(557) and the lack of such |
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a reconstruction on the Au(557) surface. |
78 |
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The mechanism and dynamics of surface reconstructions of Pt(557) and |
79 |
> |
Au(557) exposed to various coverages of carbon monoxide (CO) were |
80 |
> |
investigated using molecular dynamics simulations. Metal-CO |
81 |
> |
interactions were parameterized from experimental data and |
82 |
> |
plane-wave Density Functional Theory (DFT) calculations. The large |
83 |
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difference in binding strengths of the Pt-CO and Au-CO interactions |
84 |
> |
was found to play a significant role in step-edge stability and |
85 |
> |
adatom diffusion constants. Various mechanisms for CO-mediated step |
86 |
> |
wandering and step doubling were investigated on the Pt(557) |
87 |
> |
surface. We find that the energetics of CO adsorbed to the surface |
88 |
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can explain the step-doubling reconstruction observed on Pt(557) and |
89 |
> |
the lack of such a reconstruction on the Au(557) surface. |
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\end{abstract} |
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|
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\newpage |
389 |
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data collection. All of the systems examined had at least 40~ns in the |
390 |
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data collection stage, although simulation times for some Pt of the |
391 |
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systems exceeded 200~ns. Simulations were carried out using the open |
392 |
< |
source molecular dynamics package, OpenMD.\cite{Ewald,OOPSE} |
392 |
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source molecular dynamics package, OpenMD.\cite{Ewald,OOPSE,openmd} |
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|
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|
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|
461 |
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|
462 |
|
%Evolution of surface |
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|
\begin{figure}[H] |
464 |
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\includegraphics[width=\linewidth]{EPS_ProgressionOfDoubleLayerFormation.pdf} |
464 |
> |
\includegraphics[width=\linewidth]{EPS_ProgressionOfDoubleLayerFormation} |
465 |
|
\caption{The Pt(557) / 50\% CO system at a sequence of times after |
466 |
|
initial exposure to the CO: (a) 258~ps, (b) 19~ns, (c) 31.2~ns, and |
467 |
|
(d) 86.1~ns. Disruption of the (557) step-edges occurs quickly. The |
520 |
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|
521 |
|
%Diffusion graph |
522 |
|
\begin{figure}[H] |
523 |
< |
\includegraphics[width=\linewidth]{Portrait_DiffusionComparison_1.pdf} |
523 |
> |
\includegraphics[width=\linewidth]{Portrait_DiffusionComparison_1} |
524 |
|
\caption{Diffusion constants for mobile surface atoms along directions |
525 |
|
parallel ($\mathbf{D}_{\parallel}$) and perpendicular |
526 |
|
($\mathbf{D}_{\perp}$) to the (557) step-edges as a function of CO |
576 |
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|
577 |
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%Discussion |
578 |
|
\section{Discussion} |
579 |
< |
We have shown that a classical potential model is able to model the |
580 |
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initial reconstruction of the Pt(557) surface upon CO adsorption as |
581 |
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shown by Tao {\it et al}.\cite{Tao:2010}. More importantly, we were |
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< |
able to observe features of the dynamic processes necessary for |
583 |
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this reconstruction. Here we discuss the features of the model that |
584 |
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give rise to the observed dynamical properties of the (557) reconstruction. |
579 |
> |
We have shown that a classical potential is able to model the initial |
580 |
> |
reconstruction of the Pt(557) surface upon CO adsorption, and have |
581 |
> |
reproduced the double layer structure observed by Tao {\it et |
582 |
> |
al}.\cite{Tao:2010}. Additionally, this reconstruction appears to be |
583 |
> |
rapid -- occurring within 100 ns of the initial exposure to CO. Here |
584 |
> |
we discuss the features of the classical potential that are |
585 |
> |
contributing to the stability and speed of the Pt(557) reconstruction. |
586 |
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|
587 |
|
\subsection{Diffusion} |
588 |
< |
The perpendicular diffusion constant |
589 |
< |
appears to be the most important indicator of double layer |
590 |
< |
formation. As highlighted in Figure \ref{fig:reconstruct}, the |
591 |
< |
formation of the double layer did not begin until a nucleation |
592 |
< |
site appeared. And as mentioned by Williams {\it et al}.\cite{Williams:1991, Williams:1994}, |
593 |
< |
the inability for edges to cross leads to an effective edge-edge repulsion that |
594 |
< |
must be overcome to allow step coalescence. |
595 |
< |
A greater $\textbf{D}_\perp$ implies more step-wandering |
596 |
< |
and a larger chance for the stochastic meeting of two edges |
597 |
< |
to create a nucleation point. Parallel diffusion along the step-edge can help ``zipper'' up a nascent double |
598 |
< |
layer. This helps explain why the time scale for formation after |
599 |
< |
the appearance of a nucleation site was rapid, while the initial |
600 |
< |
appearance of the nucleation site was unpredictable. |
588 |
> |
The perpendicular diffusion constant appears to be the most important |
589 |
> |
indicator of double layer formation. As highlighted in Figure |
590 |
> |
\ref{fig:reconstruct}, the formation of the double layer did not begin |
591 |
> |
until a nucleation site appeared. Williams {\it et |
592 |
> |
al}.\cite{Williams:1991,Williams:1994} cite an effective edge-edge |
593 |
> |
repulsion arising from the inability of edge crossing. This repulsion |
594 |
> |
must be overcome to allow step coalescence. A larger |
595 |
> |
$\textbf{D}_\perp$ value implies more step-wandering and a larger |
596 |
> |
chance for the stochastic meeting of two edges to create a nucleation |
597 |
> |
point. Diffusion parallel to the step-edge can help ``zipper'' up a |
598 |
> |
nascent double layer. This helps explain the rapid time scale for |
599 |
> |
double layer completion after the appearance of a nucleation site, while |
600 |
> |
the initial appearance of the nucleation site was unpredictable. |
601 |
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|
602 |
|
\subsection{Mechanism for restructuring} |
603 |
< |
Since the Au surface showed no large scale restructuring in any of |
604 |
< |
our simulations, our discussion will focus on the 50\% Pt-CO system |
605 |
< |
which did exhibit doubling. A |
606 |
< |
number of possible mechanisms exist to explain the role of adsorbed |
607 |
< |
CO in restructuring the Pt surface. Quadrupolar repulsion between |
608 |
< |
adjacent CO molecules adsorbed on the surface is one possibility. |
609 |
< |
However, the quadrupole-quadrupole interaction is short-ranged and |
610 |
< |
is attractive for some orientations. If the CO molecules are ``locked'' in |
611 |
< |
a specific orientation relative to each other, through atop adsorption for |
612 |
< |
example, this explanation would gain credence. The calculated energetic repulsion |
613 |
< |
between two CO molecules located a distance of 2.77~\AA~apart |
614 |
< |
(nearest-neighbor distance of Pt) and both in a vertical orientation, |
615 |
< |
is 8.62 kcal/mol. Moving the CO to the second nearest-neighbor distance |
616 |
< |
of 4.8~\AA~drops the repulsion to nearly 0. Allowing the CO to rotate away |
617 |
< |
from a purely vertical orientation also lowers the repulsion. When the |
618 |
< |
carbons are locked at a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is |
619 |
< |
reached when the angle between the 2 CO is $\sim$24\textsuperscript{o}. |
620 |
< |
The calculated barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so |
621 |
< |
repulsion between adjacent CO molecules bound to Pt could increase the surface |
622 |
< |
diffusion. However, the residence time of CO on Pt suggests that these |
623 |
< |
molecules are extremely mobile, with diffusion constants 40 to 2500 times |
624 |
< |
larger than surface Pt atoms. This mobility suggests that the CO molecules jump |
625 |
< |
between different Pt atoms throughout the simulation, but will stay bound for |
626 |
< |
significant periods of time. |
603 |
> |
Since the Au surface showed no large scale restructuring in any of our |
604 |
> |
simulations, our discussion will focus on the 50\% Pt-CO system which |
605 |
> |
did exhibit doubling. A number of possible mechanisms exist to explain |
606 |
> |
the role of adsorbed CO in restructuring the Pt surface. Quadrupolar |
607 |
> |
repulsion between adjacent CO molecules adsorbed on the surface is one |
608 |
> |
possibility. However, the quadrupole-quadrupole interaction is |
609 |
> |
short-ranged and is attractive for some orientations. If the CO |
610 |
> |
molecules are ``locked'' in a vertical orientation, through atop |
611 |
> |
adsorption for example, this explanation would gain credence. The |
612 |
> |
calculated energetic repulsion between two CO molecules located a |
613 |
> |
distance of 2.77~\AA~apart (nearest-neighbor distance of Pt) and both |
614 |
> |
in a vertical orientation, is 8.62 kcal/mol. Moving the CO to the |
615 |
> |
second nearest-neighbor distance of 4.8~\AA~drops the repulsion to |
616 |
> |
nearly 0. Allowing the CO to rotate away from a purely vertical |
617 |
> |
orientation also lowers the repulsion. When the carbons are locked at |
618 |
> |
a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is reached when the |
619 |
> |
angle between the 2 CO is $\sim$24\textsuperscript{o}. The calculated |
620 |
> |
barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so |
621 |
> |
repulsion between adjacent CO molecules bound to Pt could increase the |
622 |
> |
surface diffusion. However, the residence time of CO on Pt suggests |
623 |
> |
that the CO molecules are extremely mobile, with diffusion constants 40 |
624 |
> |
to 2500 times larger than surface Pt atoms. This mobility suggests |
625 |
> |
that the CO molecules jump between different Pt atoms throughout the |
626 |
> |
simulation, but can stay bound for significant periods of time. |
627 |
|
|
628 |
< |
A different interpretation of the above mechanism, taking into account the large |
629 |
< |
mobility of the CO, looks at how instantaneous and short-lived configurations of |
630 |
< |
CO on the surface can destabilize Pt-Pt interactions leading to increased step-edge |
631 |
< |
breakup and diffusion. On the bare Pt(557) surface the barrier to completely detach |
632 |
< |
an edge atom is $\sim$43~kcal/mol, as is shown in configuration (a) in Figures |
633 |
< |
\ref{fig:SketchGraphic} \& \ref{fig:SketchEnergies}. For certain configurations, cases |
634 |
< |
(e), (g), and (h), the barrier can be lowered to $\sim$23~kcal/mole. In these instances, |
635 |
< |
it becomes quite energetically favorable to roughen the edge by introducing a small |
636 |
< |
separation of 0.5 to 1.0~\AA. This roughening becomes immediately obvious in |
637 |
< |
simulations with significant CO populations. The roughening is present to a lesser extent |
638 |
< |
on lower coverage surfaces and even on the bare surfaces, although in these cases it is likely |
639 |
< |
due to stochastic vibrational processes that squeeze out step-edge atoms. The mechanism |
640 |
< |
of step-edge breakup suggested by these energy curves is one of the most difficult |
641 |
< |
processes, a complete break-away from the step-edge in one unbroken movement. |
642 |
< |
Easier multistep mechanisms likely exist where an adatom moves laterally on the surface |
643 |
< |
after being ejected so it ends up alongside the ledge. This provides the atom with 5 nearest |
644 |
< |
neighbors, which while lower than the 7 if it had stayed a part of the step-edge, is higher |
645 |
< |
than the 3 it could maintain located on the terrace. In this proposed mechanism, the CO |
646 |
< |
quadrupolar repulsion is still playing a primary role, but for its importance in roughening |
647 |
< |
the step-edge, rather than maintaining long-term bonds with a single Pt atom which is not |
648 |
< |
born out by their mobility data. The requirement for a large density of CO on the surface |
649 |
< |
for some of the more favorable suggested configurations in Figure \ref{fig:SketchGraphic} |
650 |
< |
correspond well with the increased mobility seen on higher coverage surfaces. |
628 |
> |
A different interpretation of the above mechanism which takes the |
629 |
> |
large mobility of the CO into account, would be in the destabilization |
630 |
> |
of Pt-Pt interactions due to bound CO. Destabilizing Pt-Pt bonds at |
631 |
> |
the edges could lead to increased step-edge breakup and diffusion. On |
632 |
> |
the bare Pt(557) surface the barrier to completely detach an edge atom |
633 |
> |
is $\sim$43~kcal/mol, as is shown in configuration (a) in Figures |
634 |
> |
\ref{fig:SketchGraphic} \& \ref{fig:SketchEnergies}. For certain |
635 |
> |
configurations, cases (e), (g), and (h), the barrier can be lowered to |
636 |
> |
$\sim$23~kcal/mol by the presence of bound CO molecules. In these |
637 |
> |
instances, it becomes energetically favorable to roughen the edge by |
638 |
> |
introducing a small separation of 0.5 to 1.0~\AA. This roughening |
639 |
> |
becomes immediately obvious in simulations with significant CO |
640 |
> |
populations. The roughening is present to a lesser extent on surfaces |
641 |
> |
with lower CO coverage (and even on the bare surfaces), although in |
642 |
> |
these cases it is likely due to random fluctuations that squeeze out |
643 |
> |
step-edge atoms. Step-edge breakup by continuous single-atom |
644 |
> |
translations (as suggested by these energy curves) is probably a |
645 |
> |
worst-case scenario. Multistep mechanisms in which an adatom moves |
646 |
> |
laterally on the surface after being ejected would be more |
647 |
> |
energetically favorable. This would leave the adatom alongside the |
648 |
> |
ledge, providing it with 5 nearest neighbors. While fewer than the 7 |
649 |
> |
neighbors it had as part of the step-edge, it keeps more Pt neighbors |
650 |
> |
than the 3 an isolated adatom would have on the terrace. In this |
651 |
> |
proposed mechanism, the CO quadrupolar repulsion still plays a role in |
652 |
> |
the initial roughening of the step-edge, but not in any long-term |
653 |
> |
bonds with individual Pt atoms. Higher CO coverages create more |
654 |
> |
opportunities for the crowded CO configurations shown in Figure |
655 |
> |
\ref{fig:SketchGraphic}, and this is likely to cause an increased |
656 |
> |
propensity for step-edge breakup. |
657 |
|
|
658 |
|
%Sketch graphic of different configurations |
659 |
|
\begin{figure}[H] |
660 |
< |
\includegraphics[width=0.8\linewidth, height=0.8\textheight]{COpathsSketch.pdf} |
661 |
< |
\caption{The dark grey atoms refer to the upper ledge, while the white atoms are |
662 |
< |
the lower terrace. The blue highlighted atoms had a CO in a vertical atop position |
663 |
< |
upon them. These are a sampling of the configurations examined to gain a more |
664 |
< |
complete understanding of the effects CO has on surface diffusion and edge breakup. |
665 |
< |
Energies associated with each configuration are displayed in Figure \ref{fig:SketchEnergies}.} |
660 |
> |
\includegraphics[width=\linewidth]{COpaths} |
661 |
> |
\caption{Configurations used to investigate the mechanism of step-edge |
662 |
> |
breakup on Pt(557). In each case, the central (starred) atom is |
663 |
> |
pulled directly across the surface away from the step edge. The Pt |
664 |
> |
atoms on the upper terrace are colored dark grey, while those on the |
665 |
> |
lower terrace are in white. In each of these configurations, some |
666 |
> |
number of the atoms (highlighted in blue) had a CO molecule bound in |
667 |
> |
a vertical atop position. The energies of these configurations as a |
668 |
> |
function of central atom displacement are displayed in Figure |
669 |
> |
\ref{fig:SketchEnergies}.} |
670 |
|
\label{fig:SketchGraphic} |
671 |
|
\end{figure} |
672 |
|
|
673 |
|
%energy graph corresponding to sketch graphic |
674 |
|
\begin{figure}[H] |
675 |
< |
\includegraphics[width=\linewidth]{Portrait_SeparationComparison.pdf} |
676 |
< |
\caption{The energy curves directly correspond to the labeled model |
677 |
< |
surface in Figure \ref{fig:SketchGraphic}. All energy curves are relative |
678 |
< |
to their initial configuration so the energy of a and h do not have the |
679 |
< |
same zero value. As is seen, certain arrangements of CO can lower |
680 |
< |
the energetic barrier that must be overcome to create an adatom. |
681 |
< |
However, it is the highest coverages where these higher-energy |
682 |
< |
configurations of CO will be more likely. } |
675 |
> |
\includegraphics[width=\linewidth]{Portrait_SeparationComparison} |
676 |
> |
\caption{Energies for displacing a single edge atom perpendicular to |
677 |
> |
the step edge as a function of atomic displacement. Each of the |
678 |
> |
energy curves corresponds to one of the labeled configurations in |
679 |
> |
Figure \ref{fig:SketchGraphic}, and are referenced to the |
680 |
> |
unperturbed step-edge. Certain arrangements of bound CO (notably |
681 |
> |
configurations g and h) can lower the energetic barrier for creating |
682 |
> |
an adatom relative to the bare surface (configuration a).} |
683 |
|
\label{fig:SketchEnergies} |
684 |
|
\end{figure} |
685 |
|
|
686 |
< |
While configurations of CO on the surface are able to increase diffusion, |
687 |
< |
this does not immediately provide an explanation for the formation of double |
688 |
< |
layers. If adatoms were constrained to their terrace then doubling would be |
689 |
< |
much less likely to occur. Nucleation sites could still potentially form, but there |
690 |
< |
would not be enough atoms to finish the doubling. For a non-simulated metal surface, where the |
691 |
< |
step lengths can be assumed to be infinite relative to atomic sizes, local doubling would be possible, but in |
692 |
< |
our simulations with our periodic treatment of the system, the system is not large enough to experience this effect. |
693 |
< |
Thus, there must be a mechanism that explains how adatoms are able to move |
694 |
< |
amongst terraces. Figure \ref{fig:lambda} shows points along a reaction coordinate |
695 |
< |
where an adatom along the step-edge with an adsorbed CO ``burrows'' into the |
696 |
< |
edge displacing an atom onto the higher terrace. This mechanism was chosen |
697 |
< |
because of similar events that were observed during the simulations. The barrier |
698 |
< |
heights we obtained are only approximations because we constrained the movement |
699 |
< |
of the highlighted atoms along a specific concerted path. The calculated $\Delta E$'s |
700 |
< |
are provide a strong energetic support for this modeled lifting mechanism. When CO is not present and |
701 |
< |
this reaction coordinate is followed, the $\Delta E > 3$~kcal/mol. The example shown |
702 |
< |
in the figure, where CO is present in the atop position, has a $\Delta E < -15$~kcal/mol. |
703 |
< |
While the barrier height is comparable for both cases, there is nearly a 20~kcal/mol |
704 |
< |
difference in energies and makes the process energetically favorable. |
686 |
> |
While configurations of CO on the surface are able to increase |
687 |
> |
diffusion and the likelihood of edge wandering, this does not provide |
688 |
> |
a complete explanation for the formation of double layers. If adatoms |
689 |
> |
were constrained to their original terraces then doubling could not |
690 |
> |
occur. A mechanism for vertical displacement of adatoms at the |
691 |
> |
step-edge is required to explain the doubling. |
692 |
|
|
693 |
+ |
We have discovered one possible mechanism for a CO-mediated vertical |
694 |
+ |
displacement of Pt atoms at the step edge. Figure \ref{fig:lambda} |
695 |
+ |
shows four points along a reaction coordinate in which a CO-bound |
696 |
+ |
adatom along the step-edge ``burrows'' into the edge and displaces the |
697 |
+ |
original edge atom onto the higher terrace. A number of events similar |
698 |
+ |
to this mechanism were observed during the simulations. We predict an |
699 |
+ |
energetic barrier of 20~kcal/mol for this process (in which the |
700 |
+ |
displaced edge atom follows a curvilinear path into an adjacent 3-fold |
701 |
+ |
hollow site). The barrier heights we obtain for this reaction |
702 |
+ |
coordinate are approximate because the exact path is unknown, but the |
703 |
+ |
calculated energy barriers would be easily accessible at operating |
704 |
+ |
conditions. Additionally, this mechanism is exothermic, with a final |
705 |
+ |
energy 15~kcal/mol below the original $\lambda = 0$ configuration. |
706 |
+ |
When CO is not present and this reaction coordinate is followed, the |
707 |
+ |
process is endothermic by 3~kcal/mol. The difference in the relative |
708 |
+ |
energies for the $\lambda=0$ and $\lambda=1$ case when CO is present |
709 |
+ |
provides strong support for CO-mediated Pt-Pt interactions giving rise |
710 |
+ |
to the doubling reconstruction. |
711 |
+ |
|
712 |
|
%lambda progression of Pt -> shoving its way into the step |
713 |
|
\begin{figure}[H] |
714 |
< |
\includegraphics[width=\linewidth]{EPS_rxnCoord.pdf} |
715 |
< |
\caption{ Various points along a reaction coordinate are displayed in the figure. |
716 |
< |
The mechanism of edge traversal is examined in the presence of CO. The approximate |
717 |
< |
barrier for the displayed process is 20~kcal/mol. However, the $\Delta E$ of this process |
718 |
< |
is -15~kcal/mol making it an energetically favorable process.} |
714 |
> |
\includegraphics[width=\linewidth]{EPS_rxnCoord} |
715 |
> |
\caption{Points along a possible reaction coordinate for CO-mediated |
716 |
> |
edge doubling. Here, a CO-bound adatom burrows into an established |
717 |
> |
step edge and displaces an edge atom onto the upper terrace along a |
718 |
> |
curvilinear path. The approximate barrier for the process is |
719 |
> |
20~kcal/mol, and the complete process is exothermic by 15~kcal/mol |
720 |
> |
in the presence of CO, but is endothermic by 3~kcal/mol without.} |
721 |
|
\label{fig:lambda} |
722 |
|
\end{figure} |
723 |
|
|
724 |
< |
The mechanism for doubling on this surface appears to require the cooperation of at least |
725 |
< |
these two described processes. For complete doubling of a layer to occur there must |
726 |
< |
be the equivalent removal of a separate terrace. For those atoms to ``disappear'' from |
727 |
< |
that terrace they must either rise up on the ledge above them or drop to the ledge below |
728 |
< |
them. The presence of CO helps with the energetics of both of these situations. There must be sufficient |
729 |
< |
breakage of the step-edge to increase the concentration of adatoms on the surface and |
730 |
< |
these adatoms must then undergo the burrowing highlighted above or some comparable |
731 |
< |
mechanism to traverse the step-edge. Over time, these mechanisms working in concert |
732 |
< |
lead to the formation of a double layer. |
724 |
> |
The mechanism for doubling on the Pt(557) surface appears to require |
725 |
> |
the cooperation of at least two distinct processes. For complete |
726 |
> |
doubling of a layer to occur there must be a breakup of one |
727 |
> |
terrace. These atoms must then ``disappear'' from that terrace, either |
728 |
> |
by travelling to the terraces above of below their original levels. |
729 |
> |
The presence of CO helps explain mechanisms for both of these |
730 |
> |
situations. There must be sufficient breakage of the step-edge to |
731 |
> |
increase the concentration of adatoms on the surface and these adatoms |
732 |
> |
must then undergo the burrowing highlighted above (or a comparable |
733 |
> |
mechanism) to create the double layer. With sufficient time, these |
734 |
> |
mechanisms working in concert lead to the formation of a double layer. |
735 |
|
|
736 |
|
\subsection{CO Removal and double layer stability} |
737 |
< |
Once a double layer had formed on the 50\%~Pt system it |
738 |
< |
remained for the rest of the simulation time with minimal |
739 |
< |
movement. There were configurations that showed small |
740 |
< |
wells or peaks forming, but typically within a few nanoseconds |
741 |
< |
the feature would smooth away. Within our simulation time, |
742 |
< |
the formation of the double layer was irreversible and a double |
743 |
< |
layer was never observed to split back into two single layer |
734 |
< |
step-edges while CO was present. To further gauge the effect |
735 |
< |
CO had on this system, additional simulations were run starting |
736 |
< |
from a late configuration of the 50\%~Pt system that had formed |
737 |
< |
double layers. These simulations then had their CO removed. |
738 |
< |
The double layer breaks rapidly in these simulations, already |
739 |
< |
showing a well-defined splitting after 100~ps. Configurations of |
740 |
< |
this system are shown in Figure \ref{fig:breaking}. The coloring |
741 |
< |
of the top and bottom layers helps to exhibit how much mixing |
742 |
< |
the edges experience as they split. These systems were only |
743 |
< |
examined briefly, 10~ns, and within that time despite the initial |
744 |
< |
rapid splitting, the edges only moved another few \AA~apart. |
745 |
< |
It is possible with longer simulation times that the |
746 |
< |
(557) lattice could be recovered as seen by Tao {\it et al}.\cite{Tao:2010} |
737 |
> |
Once a double layer had formed on the 50\%~Pt system, it remained for |
738 |
> |
the rest of the simulation time with minimal movement. Random |
739 |
> |
fluctuations that involved small clusters or divots were observed, but |
740 |
> |
these features typically healed within a few nanoseconds. Within our |
741 |
> |
simulations, the formation of the double layer appeared to be |
742 |
> |
irreversible and a double layer was never observed to split back into |
743 |
> |
two single layer step-edges while CO was present. |
744 |
|
|
745 |
+ |
To further gauge the effect CO has on this surface, additional |
746 |
+ |
simulations were run starting from a late configuration of the 50\%~Pt |
747 |
+ |
system that had already formed double layers. These simulations then |
748 |
+ |
had their CO forcibly removed. The double layer broke apart rapidly |
749 |
+ |
in these simulations, showing a well-defined edge-splitting after |
750 |
+ |
100~ps. Configurations of this system are shown in Figure |
751 |
+ |
\ref{fig:breaking}. The coloring of the top and bottom layers helps to |
752 |
+ |
exhibit how much mixing the edges experience as they split. These |
753 |
+ |
systems were only examined for 10~ns, and within that time despite the |
754 |
+ |
initial rapid splitting, the edges only moved another few |
755 |
+ |
\AA~apart. It is possible that with longer simulation times, the (557) |
756 |
+ |
surface recovery observed by Tao {\it et al}.\cite{Tao:2010} could |
757 |
+ |
also be recovered. |
758 |
|
|
749 |
– |
|
759 |
|
%breaking of the double layer upon removal of CO |
760 |
|
\begin{figure}[H] |
761 |
< |
\includegraphics[width=\linewidth]{EPS_doubleLayerBreaking.pdf} |
762 |
< |
\caption{(A) 0~ps, (B) 100~ps, (C) 1~ns, after the removal of CO. The presence of the CO |
763 |
< |
helped maintain the stability of the double layer and its microfaceting of the double layer |
764 |
< |
into a (111) configuration. This microfacet immediately reverts to the original (100) step |
765 |
< |
edge which is a hallmark of the (557) surface. The separation is not a simple sliding apart, rather |
766 |
< |
there is a mixing of the lower and upper atoms at the edge.} |
761 |
> |
\includegraphics[width=\linewidth]{EPS_doubleLayerBreaking} |
762 |
> |
\caption{Dynamics of an established (111) double step after removal of |
763 |
> |
the adsorbed CO: (A) 0~ps, (B) 100~ps, and (C) 1~ns after the removal |
764 |
> |
of CO. The presence of the CO helped maintain the stability of the |
765 |
> |
double step. Nearly immediately after the CO is removed, the step |
766 |
> |
edge reforms in a (100) configuration, which is also the step type |
767 |
> |
seen on clean (557) surfaces. The step separation involves |
768 |
> |
significant mixing of the lower and upper atoms at the edge.} |
769 |
|
\label{fig:breaking} |
770 |
|
\end{figure} |
771 |
|
|
772 |
|
|
762 |
– |
|
763 |
– |
|
773 |
|
%Peaks! |
774 |
|
%\begin{figure}[H] |
775 |
|
%\includegraphics[width=\linewidth]{doublePeaks_noCO.png} |
783 |
|
%Don't think I need this |
784 |
|
%clean surface... |
785 |
|
%\begin{figure}[H] |
786 |
< |
%\includegraphics[width=\linewidth]{557_300K_cleanPDF.pdf} |
786 |
> |
%\includegraphics[width=\linewidth]{557_300K_cleanPDF} |
787 |
|
%\caption{} |
788 |
|
|
789 |
|
%\end{figure} |
791 |
|
|
792 |
|
|
793 |
|
\section{Conclusion} |
794 |
< |
The strength of the Pt-CO binding interaction as well as the large |
795 |
< |
quadrupolar repulsion between CO molecules are sufficient to |
796 |
< |
explain the observed increase in surface mobility and the resultant |
797 |
< |
reconstructions at the highest simulated coverage. The weaker |
798 |
< |
Au-CO interaction results in lower diffusion constants, less step-wandering, |
799 |
< |
and a lack of the double layer reconstruction. An in-depth examination |
800 |
< |
of the energetics shows the important role CO plays in increasing |
801 |
< |
step-breakup and in facilitating edge traversal which are both |
793 |
< |
necessary for double layer formation. |
794 |
> |
The strength and directionality of the Pt-CO binding interaction, as |
795 |
> |
well as the large quadrupolar repulsion between atop-bound CO |
796 |
> |
molecules, help to explain the observed increase in surface mobility |
797 |
> |
of Pt(557) and the resultant reconstruction into a double-layer |
798 |
> |
configuration at the highest simulated CO-coverages. The weaker Au-CO |
799 |
> |
interaction results in significantly lower adataom diffusion |
800 |
> |
constants, less step-wandering, and a lack of the double layer |
801 |
> |
reconstruction on the Au(557) surface. |
802 |
|
|
803 |
+ |
An in-depth examination of the energetics shows the important role CO |
804 |
+ |
plays in increasing step-breakup and in facilitating edge traversal |
805 |
+ |
which are both necessary for double layer formation. |
806 |
|
|
796 |
– |
|
807 |
|
%Things I am not ready to remove yet |
808 |
|
|
809 |
|
%Table of Diffusion Constants |
827 |
|
% \end{table} |
828 |
|
|
829 |
|
\begin{acknowledgement} |
830 |
< |
Support for this project was provided by the National Science |
831 |
< |
Foundation under grant CHE-0848243 and by the Center for Sustainable |
832 |
< |
Energy at Notre Dame (cSEND). Computational time was provided by the |
833 |
< |
Center for Research Computing (CRC) at the University of Notre Dame. |
830 |
> |
We gratefully acknowledge conversations with Dr. William |
831 |
> |
F. Schneider and Dr. Feng Tao. Support for this project was |
832 |
> |
provided by the National Science Foundation under grant CHE-0848243 |
833 |
> |
and by the Center for Sustainable Energy at Notre Dame |
834 |
> |
(cSEND). Computational time was provided by the Center for Research |
835 |
> |
Computing (CRC) at the University of Notre Dame. |
836 |
|
\end{acknowledgement} |
837 |
|
\newpage |
838 |
|
\bibliography{firstTryBibliography} |