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% double space list of tables and figures |
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%\AtBeginDelayedFloats{\renewcomand{\baselinestretch}{1.66}} |
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\begin{abstract} |
| 79 |
< |
We examine surface reconstructions of Pt and Au(557) under |
| 80 |
< |
various CO coverages using molecular dynamics in order to |
| 81 |
< |
explore possible mechanisms for any observed reconstructions |
| 82 |
< |
and their dynamics. The metal-CO interactions were parameterized |
| 83 |
< |
as part of this work so that an efficient large-scale treatment of |
| 84 |
< |
this system could be undertaken. The large difference in binding |
| 85 |
< |
strengths of the metal-CO interactions was found to play a significant |
| 86 |
< |
role with regards to step-edge stability and adatom diffusion. A |
| 87 |
< |
small correlation between coverage and the diffusion constant |
| 88 |
< |
was also determined. The energetics of CO adsorbed to the surface |
| 89 |
< |
is sufficient to explain the reconstructions observed on the Pt |
| 90 |
< |
systems and the lack of reconstruction of the Au systems. |
| 88 |
< |
|
| 79 |
> |
The mechanism and dynamics of surface reconstructions of Pt(557) and |
| 80 |
> |
Au(557) exposed to various coverages of carbon monoxide (CO) were |
| 81 |
> |
investigated using molecular dynamics simulations. Metal-CO |
| 82 |
> |
interactions were parameterized from experimental data and |
| 83 |
> |
plane-wave Density Functional Theory (DFT) calculations. The large |
| 84 |
> |
difference in binding strengths of the Pt-CO and Au-CO interactions |
| 85 |
> |
was found to play a significant role in step-edge stability and |
| 86 |
> |
adatom diffusion constants. Various mechanisms for CO-mediated step |
| 87 |
> |
wandering and step doubling were investigated on the Pt(557) |
| 88 |
> |
surface. We find that the energetics of CO adsorbed to the surface |
| 89 |
> |
can explain the step-doubling reconstruction observed on Pt(557) and |
| 90 |
> |
the lack of such a reconstruction on the Au(557) surface. |
| 91 |
|
\end{abstract} |
| 92 |
|
|
| 93 |
|
\newpage |
| 163 |
|
Coulomb potential. For this work, we have used classical molecular |
| 164 |
|
dynamics with potential energy surfaces that are specifically tuned |
| 165 |
|
for transition metals. In particular, we used the EAM potential for |
| 166 |
< |
Au-Au and Pt-Pt interactions.\cite{EAM} The CO was modeled using a rigid |
| 166 |
> |
Au-Au and Pt-Pt interactions.\cite{Foiles86} The CO was modeled using a rigid |
| 167 |
|
three-site model developed by Straub and Karplus for studying |
| 168 |
|
photodissociation of CO from myoglobin.\cite{Straub} The Au-CO and |
| 169 |
|
Pt-CO cross interactions were parameterized as part of this work. |
| 176 |
|
methods,\cite{Daw84,Foiles86,Johnson89,Daw89,Plimpton93,Voter95a,Lu97,Alemany98} |
| 177 |
|
but other models like the Finnis-Sinclair\cite{Finnis84,Chen90} and |
| 178 |
|
the quantum-corrected Sutton-Chen method\cite{QSC,Qi99} have simpler |
| 179 |
< |
parameter sets. The glue model of Ercolessi {\it et al}. is among the |
| 180 |
< |
fastest of these density functional approaches.\cite{Ercolessi88} In |
| 179 |
> |
parameter sets. The glue model of Ercolessi {\it et al}.\cite{Ercolessi88} is among the |
| 180 |
> |
fastest of these density functional approaches. In |
| 181 |
|
all of these models, atoms are treated as a positively charged |
| 182 |
|
core with a radially-decaying valence electron distribution. To |
| 183 |
|
calculate the energy for embedding the core at a particular location, |
| 209 |
|
The EAM, Finnis-Sinclair, and the Quantum Sutton-Chen (QSC) potentials |
| 210 |
|
have all been widely used by the materials simulation community for |
| 211 |
|
simulations of bulk and nanoparticle |
| 212 |
< |
properties,\cite{Chui:2003fk,Wang:2005qy,Medasani:2007uq} |
| 212 |
> |
properties,\cite{Chui:2003fk,Wang:2005qy,Medasani:2007uq,mishin99:_inter} |
| 213 |
|
melting,\cite{Belonoshko00,sankaranarayanan:155441,Sankaranarayanan:2005lr} |
| 214 |
< |
fracture,\cite{Shastry:1996qg,Shastry:1998dx} crack |
| 215 |
< |
propagation,\cite{BECQUART:1993rg} and alloying |
| 216 |
< |
dynamics.\cite{Shibata:2002hh} One of EAM's strengths |
| 214 |
> |
fracture,\cite{Shastry:1996qg,Shastry:1998dx,mishin01:cu} crack |
| 215 |
> |
propagation,\cite{BECQUART:1993rg,Rifkin1992} and alloying |
| 216 |
> |
dynamics.\cite{Shibata:2002hh,mishin02:b2nial,zope03:tial_ap,mishin05:phase_fe_ni} One of EAM's strengths |
| 217 |
|
is its sensitivity to small changes in structure. This arises |
| 218 |
|
because interactions |
| 219 |
|
up to the third nearest neighbor were taken into account in the parameterization.\cite{Voter95a} |
| 224 |
|
surface structures. Additionally, the similarity of EAM's functional |
| 225 |
|
treatment of the embedding energy to standard density functional |
| 226 |
|
theory (DFT) makes fitting DFT-derived cross potentials with adsorbates somewhat easier. |
| 225 |
– |
\cite{Foiles86,PhysRevB.37.3924,Rifkin1992,mishin99:_inter,mishin01:cu,mishin02:b2nial,zope03:tial_ap,mishin05:phase_fe_ni} |
| 227 |
|
|
| 228 |
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|
| 229 |
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|
| 230 |
|
|
| 231 |
+ |
|
| 232 |
|
\subsection{Carbon Monoxide model} |
| 233 |
|
Previous explanations for the surface rearrangements center on |
| 234 |
|
the large linear quadrupole moment of carbon monoxide.\cite{Tao:2010} |
| 390 |
|
data collection. All of the systems examined had at least 40~ns in the |
| 391 |
|
data collection stage, although simulation times for some Pt of the |
| 392 |
|
systems exceeded 200~ns. Simulations were carried out using the open |
| 393 |
< |
source molecular dynamics package, OpenMD.\cite{Ewald,OOPSE} |
| 393 |
> |
source molecular dynamics package, OpenMD.\cite{Ewald,OOPSE,openmd} |
| 394 |
|
|
| 395 |
|
|
| 396 |
|
|
| 462 |
|
|
| 463 |
|
%Evolution of surface |
| 464 |
|
\begin{figure}[H] |
| 465 |
< |
\includegraphics[width=\linewidth]{ProgressionOfDoubleLayerFormation_yellowCircle.png} |
| 465 |
> |
\includegraphics[width=\linewidth]{EPS_ProgressionOfDoubleLayerFormation} |
| 466 |
|
\caption{The Pt(557) / 50\% CO system at a sequence of times after |
| 467 |
|
initial exposure to the CO: (a) 258~ps, (b) 19~ns, (c) 31.2~ns, and |
| 468 |
|
(d) 86.1~ns. Disruption of the (557) step-edges occurs quickly. The |
| 521 |
|
|
| 522 |
|
%Diffusion graph |
| 523 |
|
\begin{figure}[H] |
| 524 |
< |
\includegraphics[width=\linewidth]{DiffusionComparison_errorXY_remade_20ns.pdf} |
| 524 |
> |
\includegraphics[width=\linewidth]{Portrait_DiffusionComparison_1} |
| 525 |
|
\caption{Diffusion constants for mobile surface atoms along directions |
| 526 |
|
parallel ($\mathbf{D}_{\parallel}$) and perpendicular |
| 527 |
|
($\mathbf{D}_{\perp}$) to the (557) step-edges as a function of CO |
| 577 |
|
|
| 578 |
|
%Discussion |
| 579 |
|
\section{Discussion} |
| 580 |
< |
We have shown that a classical potential model is able to model the |
| 581 |
< |
initial reconstruction of the Pt(557) surface upon CO adsorption as |
| 582 |
< |
shown by Tao {\it et al}.\cite{Tao:2010}. More importantly, we were |
| 583 |
< |
able to observe features of the dynamic processes necessary for |
| 584 |
< |
this reconstruction. Here we discuss the features of the model that |
| 585 |
< |
give rise to the observed dynamical properties of the (557) reconstruction. |
| 580 |
> |
We have shown that a classical potential is able to model the initial |
| 581 |
> |
reconstruction of the Pt(557) surface upon CO adsorption, and have |
| 582 |
> |
reproduced the double layer structure observed by Tao {\it et |
| 583 |
> |
al}.\cite{Tao:2010}. Additionally, this reconstruction appears to be |
| 584 |
> |
rapid -- occurring within 100 ns of the initial exposure to CO. Here |
| 585 |
> |
we discuss the features of the classical potential that are |
| 586 |
> |
contributing to the stability and speed of the Pt(557) reconstruction. |
| 587 |
|
|
| 588 |
|
\subsection{Diffusion} |
| 589 |
< |
The perpendicular diffusion constant |
| 590 |
< |
appears to be the most important indicator of double layer |
| 591 |
< |
formation. As highlighted in Figure \ref{fig:reconstruct}, the |
| 592 |
< |
formation of the double layer did not begin until a nucleation |
| 593 |
< |
site appeared. And as mentioned by Williams {\it et al}.\cite{Williams:1991, Williams:1994}, |
| 594 |
< |
the inability for edges to cross leads to an effective edge-edge repulsion that |
| 595 |
< |
must be overcome to allow step coalescence. |
| 596 |
< |
A greater $\textbf{D}_\perp$ implies more step-wandering |
| 597 |
< |
and a larger chance for the stochastic meeting of two edges |
| 598 |
< |
to create a nucleation point. Parallel diffusion along the step-edge can help ``zipper'' up a nascent double |
| 599 |
< |
layer. This helps explain why the time scale for formation after |
| 600 |
< |
the appearance of a nucleation site was rapid, while the initial |
| 601 |
< |
appearance of the nucleation site was unpredictable. |
| 589 |
> |
The perpendicular diffusion constant appears to be the most important |
| 590 |
> |
indicator of double layer formation. As highlighted in Figure |
| 591 |
> |
\ref{fig:reconstruct}, the formation of the double layer did not begin |
| 592 |
> |
until a nucleation site appeared. Williams {\it et |
| 593 |
> |
al}.\cite{Williams:1991,Williams:1994} cite an effective edge-edge |
| 594 |
> |
repulsion arising from the inability of edge crossing. This repulsion |
| 595 |
> |
must be overcome to allow step coalescence. A larger |
| 596 |
> |
$\textbf{D}_\perp$ value implies more step-wandering and a larger |
| 597 |
> |
chance for the stochastic meeting of two edges to create a nucleation |
| 598 |
> |
point. Diffusion parallel to the step-edge can help ``zipper'' up a |
| 599 |
> |
nascent double layer. This helps explain the rapid time scale for |
| 600 |
> |
double layer completion after the appearance of a nucleation site, while |
| 601 |
> |
the initial appearance of the nucleation site was unpredictable. |
| 602 |
|
|
| 603 |
|
\subsection{Mechanism for restructuring} |
| 604 |
< |
Since the Au surface showed no large scale restructuring in any of |
| 605 |
< |
our simulations, our discussion will focus on the 50\% Pt-CO system |
| 606 |
< |
which did exhibit doubling featured in Figure \ref{fig:reconstruct}. A |
| 607 |
< |
number of possible mechanisms exist to explain the role of adsorbed |
| 608 |
< |
CO in restructuring the Pt surface. Quadrupolar repulsion between |
| 609 |
< |
adjacent CO molecules adsorbed on the surface is one possibility. |
| 610 |
< |
However, the quadrupole-quadrupole interaction is short-ranged and |
| 611 |
< |
is attractive for some orientations. If the CO molecules are ``locked'' in |
| 612 |
< |
a specific orientation relative to each other, through atop adsorption for |
| 613 |
< |
example, this explanation would gain credence. The energetic repulsion |
| 614 |
< |
between two CO molecules located a distance of 2.77~\AA~apart |
| 615 |
< |
(nearest-neighbor distance of Pt) and both in a vertical orientation, |
| 616 |
< |
is 8.62 kcal/mol. Moving the CO to the second nearest-neighbor distance |
| 617 |
< |
of 4.8~\AA~drops the repulsion to nearly 0. Allowing the CO to rotate away |
| 618 |
< |
from a purely vertical orientation also lowers the repulsion. When the |
| 619 |
< |
carbons are locked at a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is |
| 620 |
< |
reached when the angle between the 2 CO is $\sim$24\textsuperscript{o}. |
| 621 |
< |
The barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so |
| 622 |
< |
repulsion between adjacent CO molecules could increase the surface |
| 623 |
< |
diffusion. However, the residence time of CO on Pt suggests that these |
| 624 |
< |
molecules are extremely mobile, with diffusion constants 40 to 2500 times |
| 625 |
< |
larger than surface Pt atoms. This mobility suggests that the CO are more |
| 626 |
< |
likely to shift their positions without dragging the Pt along with them. |
| 604 |
> |
Since the Au surface showed no large scale restructuring in any of our |
| 605 |
> |
simulations, our discussion will focus on the 50\% Pt-CO system which |
| 606 |
> |
did exhibit doubling. A number of possible mechanisms exist to explain |
| 607 |
> |
the role of adsorbed CO in restructuring the Pt surface. Quadrupolar |
| 608 |
> |
repulsion between adjacent CO molecules adsorbed on the surface is one |
| 609 |
> |
possibility. However, the quadrupole-quadrupole interaction is |
| 610 |
> |
short-ranged and is attractive for some orientations. If the CO |
| 611 |
> |
molecules are ``locked'' in a vertical orientation, through atop |
| 612 |
> |
adsorption for example, this explanation would gain credence. The |
| 613 |
> |
calculated energetic repulsion between two CO molecules located a |
| 614 |
> |
distance of 2.77~\AA~apart (nearest-neighbor distance of Pt) and both |
| 615 |
> |
in a vertical orientation, is 8.62 kcal/mol. Moving the CO to the |
| 616 |
> |
second nearest-neighbor distance of 4.8~\AA~drops the repulsion to |
| 617 |
> |
nearly 0. Allowing the CO to rotate away from a purely vertical |
| 618 |
> |
orientation also lowers the repulsion. When the carbons are locked at |
| 619 |
> |
a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is reached when the |
| 620 |
> |
angle between the 2 CO is $\sim$24\textsuperscript{o}. The calculated |
| 621 |
> |
barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so |
| 622 |
> |
repulsion between adjacent CO molecules bound to Pt could increase the |
| 623 |
> |
surface diffusion. However, the residence time of CO on Pt suggests |
| 624 |
> |
that the CO molecules are extremely mobile, with diffusion constants 40 |
| 625 |
> |
to 2500 times larger than surface Pt atoms. This mobility suggests |
| 626 |
> |
that the CO molecules jump between different Pt atoms throughout the |
| 627 |
> |
simulation, but can stay bound for significant periods of time. |
| 628 |
|
|
| 629 |
< |
Another possible mechanism for the restructuring is in the destabilization of strong Pt-Pt interactions by CO adsorbed on surface Pt atoms. To test this hypothesis, a number of configurations of CO in varying quantities were arranged on the upper plateaus around a step on an otherwise clean Pt(557) surface. A few sample configurations are displayed in Figure \ref{fig:SketchGraphic}, with energy curves corresponding to each configuration in Figure \ref{fig:SketchEnergies}. Certain configurations of CO, cases (e), (g) and (h) for example, can provide significant energetic pushes for Pt atoms to break away from the step-edge. |
| 629 |
> |
A different interpretation of the above mechanism which takes the |
| 630 |
> |
large mobility of the CO into account, would be in the destabilization |
| 631 |
> |
of Pt-Pt interactions due to bound CO. Destabilizing Pt-Pt bonds at |
| 632 |
> |
the edges could lead to increased step-edge breakup and diffusion. On |
| 633 |
> |
the bare Pt(557) surface the barrier to completely detach an edge atom |
| 634 |
> |
is $\sim$43~kcal/mol, as is shown in configuration (a) in Figures |
| 635 |
> |
\ref{fig:SketchGraphic} \& \ref{fig:SketchEnergies}. For certain |
| 636 |
> |
configurations, cases (e), (g), and (h), the barrier can be lowered to |
| 637 |
> |
$\sim$23~kcal/mol by the presence of bound CO molecules. In these |
| 638 |
> |
instances, it becomes energetically favorable to roughen the edge by |
| 639 |
> |
introducing a small separation of 0.5 to 1.0~\AA. This roughening |
| 640 |
> |
becomes immediately obvious in simulations with significant CO |
| 641 |
> |
populations. The roughening is present to a lesser extent on surfaces |
| 642 |
> |
with lower CO coverage (and even on the bare surfaces), although in |
| 643 |
> |
these cases it is likely due to random fluctuations that squeeze out |
| 644 |
> |
step-edge atoms. Step-edge breakup by continuous single-atom |
| 645 |
> |
translations (as suggested by these energy curves) is probably a |
| 646 |
> |
worst-case scenario. Multistep mechanisms in which an adatom moves |
| 647 |
> |
laterally on the surface after being ejected would be more |
| 648 |
> |
energetically favorable. This would leave the adatom alongside the |
| 649 |
> |
ledge, providing it with 5 nearest neighbors. While fewer than the 7 |
| 650 |
> |
neighbors it had as part of the step-edge, it keeps more Pt neighbors |
| 651 |
> |
than the 3 an isolated adatom would have on the terrace. In this |
| 652 |
> |
proposed mechanism, the CO quadrupolar repulsion still plays a role in |
| 653 |
> |
the initial roughening of the step-edge, but not in any long-term |
| 654 |
> |
bonds with individual Pt atoms. Higher CO coverages create more |
| 655 |
> |
opportunities for the crowded CO configurations shown in Figure |
| 656 |
> |
\ref{fig:SketchGraphic}, and this is likely to cause an increased |
| 657 |
> |
propensity for step-edge breakup. |
| 658 |
|
|
| 627 |
– |
|
| 659 |
|
%Sketch graphic of different configurations |
| 660 |
|
\begin{figure}[H] |
| 661 |
< |
\includegraphics[width=0.8\linewidth, height=0.8\textheight]{COpathsSketch.pdf} |
| 662 |
< |
\caption{The dark grey atoms refer to the upper ledge, while the white atoms are |
| 663 |
< |
the lower terrace. The blue highlighted atoms had a CO in a vertical atop position |
| 664 |
< |
upon them. These are a sampling of the configurations examined to gain a more |
| 665 |
< |
complete understanding of the effects CO has on surface diffusion and edge breakup. |
| 666 |
< |
Energies associated with each configuration are displayed in Figure \ref{fig:SketchEnergies}.} |
| 661 |
> |
\includegraphics[width=\linewidth]{COpaths} |
| 662 |
> |
\caption{Configurations used to investigate the mechanism of step-edge |
| 663 |
> |
breakup on Pt(557). In each case, the central (starred) atom is |
| 664 |
> |
pulled directly across the surface away from the step edge. The Pt |
| 665 |
> |
atoms on the upper terrace are colored dark grey, while those on the |
| 666 |
> |
lower terrace are in white. In each of these configurations, some |
| 667 |
> |
number of the atoms (highlighted in blue) had a CO molecule bound in |
| 668 |
> |
a vertical atop position. The energies of these configurations as a |
| 669 |
> |
function of central atom displacement are displayed in Figure |
| 670 |
> |
\ref{fig:SketchEnergies}.} |
| 671 |
|
\label{fig:SketchGraphic} |
| 672 |
|
\end{figure} |
| 673 |
|
|
| 674 |
|
%energy graph corresponding to sketch graphic |
| 675 |
|
\begin{figure}[H] |
| 676 |
< |
\includegraphics[width=\linewidth]{stepSeparationComparison.pdf} |
| 677 |
< |
\caption{The energy curves directly correspond to the labeled model |
| 678 |
< |
surface in Figure \ref{fig:SketchGraphic}. All energy curves are relative |
| 679 |
< |
to their initial configuration so the energy of a and h do not have the |
| 680 |
< |
same zero value. As is seen, certain arrangements of CO can lower |
| 681 |
< |
the energetic barrier that must be overcome to create an adatom. |
| 682 |
< |
However, it is the highest coverages where these higher-energy |
| 683 |
< |
configurations of CO will be more likely. } |
| 676 |
> |
\includegraphics[width=\linewidth]{Portrait_SeparationComparison} |
| 677 |
> |
\caption{Energies for displacing a single edge atom perpendicular to |
| 678 |
> |
the step edge as a function of atomic displacement. Each of the |
| 679 |
> |
energy curves corresponds to one of the labeled configurations in |
| 680 |
> |
Figure \ref{fig:SketchGraphic}, and are referenced to the |
| 681 |
> |
unperturbed step-edge. Certain arrangements of bound CO (notably |
| 682 |
> |
configurations g and h) can lower the energetic barrier for creating |
| 683 |
> |
an adatom relative to the bare surface (configuration a).} |
| 684 |
|
\label{fig:SketchEnergies} |
| 685 |
|
\end{figure} |
| 686 |
|
|
| 687 |
+ |
While configurations of CO on the surface are able to increase |
| 688 |
+ |
diffusion and the likelihood of edge wandering, this does not provide |
| 689 |
+ |
a complete explanation for the formation of double layers. If adatoms |
| 690 |
+ |
were constrained to their original terraces then doubling could not |
| 691 |
+ |
occur. A mechanism for vertical displacement of adatoms at the |
| 692 |
+ |
step-edge is required to explain the doubling. |
| 693 |
|
|
| 694 |
+ |
We have discovered one possible mechanism for a CO-mediated vertical |
| 695 |
+ |
displacement of Pt atoms at the step edge. Figure \ref{fig:lambda} |
| 696 |
+ |
shows four points along a reaction coordinate in which a CO-bound |
| 697 |
+ |
adatom along the step-edge ``burrows'' into the edge and displaces the |
| 698 |
+ |
original edge atom onto the higher terrace. A number of events similar |
| 699 |
+ |
to this mechanism were observed during the simulations. We predict an |
| 700 |
+ |
energetic barrier of 20~kcal/mol for this process (in which the |
| 701 |
+ |
displaced edge atom follows a curvilinear path into an adjacent 3-fold |
| 702 |
+ |
hollow site). The barrier heights we obtain for this reaction |
| 703 |
+ |
coordinate are approximate because the exact path is unknown, but the |
| 704 |
+ |
calculated energy barriers would be easily accessible at operating |
| 705 |
+ |
conditions. Additionally, this mechanism is exothermic, with a final |
| 706 |
+ |
energy 15~kcal/mol below the original $\lambda = 0$ configuration. |
| 707 |
+ |
When CO is not present and this reaction coordinate is followed, the |
| 708 |
+ |
process is endothermic by 3~kcal/mol. The difference in the relative |
| 709 |
+ |
energies for the $\lambda=0$ and $\lambda=1$ case when CO is present |
| 710 |
+ |
provides strong support for CO-mediated Pt-Pt interactions giving rise |
| 711 |
+ |
to the doubling reconstruction. |
| 712 |
|
|
| 713 |
|
%lambda progression of Pt -> shoving its way into the step |
| 714 |
|
\begin{figure}[H] |
| 715 |
< |
\includegraphics[width=\linewidth]{lambdaProgression_atopCO_withLambda.png} |
| 716 |
< |
\caption{A model system of the Pt(557) surface was used as the framework |
| 717 |
< |
for exploring energy barriers along a reaction coordinate. Various numbers, |
| 718 |
< |
placements, and rotations of CO were examined as they affect Pt movement. |
| 719 |
< |
The coordinate displayed in this Figure was a representative run. relative to the energy of the system at 0\%, there |
| 720 |
< |
is a slight decrease upon insertion of the Pt atom into the step-edge along |
| 721 |
< |
with the resultant lifting of the other Pt atom when CO is present at certain positions.} |
| 715 |
> |
\includegraphics[width=\linewidth]{EPS_rxnCoord} |
| 716 |
> |
\caption{Points along a possible reaction coordinate for CO-mediated |
| 717 |
> |
edge doubling. Here, a CO-bound adatom burrows into an established |
| 718 |
> |
step edge and displaces an edge atom onto the upper terrace along a |
| 719 |
> |
curvilinear path. The approximate barrier for the process is |
| 720 |
> |
20~kcal/mol, and the complete process is exothermic by 15~kcal/mol |
| 721 |
> |
in the presence of CO, but is endothermic by 3~kcal/mol without.} |
| 722 |
|
\label{fig:lambda} |
| 723 |
|
\end{figure} |
| 724 |
|
|
| 725 |
+ |
The mechanism for doubling on the Pt(557) surface appears to require |
| 726 |
+ |
the cooperation of at least two distinct processes. For complete |
| 727 |
+ |
doubling of a layer to occur there must be a breakup of one |
| 728 |
+ |
terrace. These atoms must then ``disappear'' from that terrace, either |
| 729 |
+ |
by travelling to the terraces above of below their original levels. |
| 730 |
+ |
The presence of CO helps explain mechanisms for both of these |
| 731 |
+ |
situations. There must be sufficient breakage of the step-edge to |
| 732 |
+ |
increase the concentration of adatoms on the surface and these adatoms |
| 733 |
+ |
must then undergo the burrowing highlighted above (or a comparable |
| 734 |
+ |
mechanism) to create the double layer. With sufficient time, these |
| 735 |
+ |
mechanisms working in concert lead to the formation of a double layer. |
| 736 |
+ |
|
| 737 |
|
\subsection{CO Removal and double layer stability} |
| 738 |
< |
Once a double layer had formed on the 50\%~Pt system it |
| 739 |
< |
remained for the rest of the simulation time with minimal |
| 740 |
< |
movement. There were configurations that showed small |
| 741 |
< |
wells or peaks forming, but typically within a few nanoseconds |
| 742 |
< |
the feature would smooth away. Within our simulation time, |
| 743 |
< |
the formation of the double layer was irreversible and a double |
| 744 |
< |
layer was never observed to split back into two single layer |
| 674 |
< |
step-edges while CO was present. To further gauge the effect |
| 675 |
< |
CO had on this system, additional simulations were run starting |
| 676 |
< |
from a late configuration of the 50\%~Pt system that had formed |
| 677 |
< |
double layers. These simulations then had their CO removed. |
| 678 |
< |
The double layer breaks rapidly in these simulations, already |
| 679 |
< |
showing a well-defined splitting after 100~ps. Configurations of |
| 680 |
< |
this system are shown in Figure \ref{fig:breaking}. The coloring |
| 681 |
< |
of the top and bottom layers helps to exhibit how much mixing |
| 682 |
< |
the edges experience as they split. These systems were only |
| 683 |
< |
examined briefly, 10~ns, and within that time despite the initial |
| 684 |
< |
rapid splitting, the edges only moved another few \AA~apart. |
| 685 |
< |
It is possible with longer simulation times that the |
| 686 |
< |
(557) lattice could be recovered as seen by Tao {\it et al}.\cite{Tao:2010} |
| 738 |
> |
Once a double layer had formed on the 50\%~Pt system, it remained for |
| 739 |
> |
the rest of the simulation time with minimal movement. Random |
| 740 |
> |
fluctuations that involved small clusters or divots were observed, but |
| 741 |
> |
these features typically healed within a few nanoseconds. Within our |
| 742 |
> |
simulations, the formation of the double layer appeared to be |
| 743 |
> |
irreversible and a double layer was never observed to split back into |
| 744 |
> |
two single layer step-edges while CO was present. |
| 745 |
|
|
| 746 |
+ |
To further gauge the effect CO has on this surface, additional |
| 747 |
+ |
simulations were run starting from a late configuration of the 50\%~Pt |
| 748 |
+ |
system that had already formed double layers. These simulations then |
| 749 |
+ |
had their CO forcibly removed. The double layer broke apart rapidly |
| 750 |
+ |
in these simulations, showing a well-defined edge-splitting after |
| 751 |
+ |
100~ps. Configurations of this system are shown in Figure |
| 752 |
+ |
\ref{fig:breaking}. The coloring of the top and bottom layers helps to |
| 753 |
+ |
exhibit how much mixing the edges experience as they split. These |
| 754 |
+ |
systems were only examined for 10~ns, and within that time despite the |
| 755 |
+ |
initial rapid splitting, the edges only moved another few |
| 756 |
+ |
\AA~apart. It is possible that with longer simulation times, the (557) |
| 757 |
+ |
surface recovery observed by Tao {\it et al}.\cite{Tao:2010} could |
| 758 |
+ |
also be recovered. |
| 759 |
|
|
| 689 |
– |
|
| 690 |
– |
|
| 691 |
– |
|
| 692 |
– |
|
| 760 |
|
%breaking of the double layer upon removal of CO |
| 761 |
|
\begin{figure}[H] |
| 762 |
< |
\includegraphics[width=\linewidth]{doubleLayerBreaking_greenBlue_whiteLetters.png} |
| 763 |
< |
\caption{(A) 0~ps, (B) 100~ps, (C) 1~ns, after the removal of CO. The presence of the CO |
| 764 |
< |
helped maintain the stability of the double layer and upon removal the two layers break |
| 765 |
< |
and begin separating. The separation is not a simple pulling apart however, rather |
| 766 |
< |
there is a mixing of the lower and upper atoms at the edge.} |
| 762 |
> |
\includegraphics[width=\linewidth]{EPS_doubleLayerBreaking} |
| 763 |
> |
\caption{Dynamics of an established (111) double step after removal of |
| 764 |
> |
the adsorbed CO: (A) 0~ps, (B) 100~ps, and (C) 1~ns after the removal |
| 765 |
> |
of CO. The presence of the CO helped maintain the stability of the |
| 766 |
> |
double step. Nearly immediately after the CO is removed, the step |
| 767 |
> |
edge reforms in a (100) configuration, which is also the step type |
| 768 |
> |
seen on clean (557) surfaces. The step separation involves |
| 769 |
> |
significant mixing of the lower and upper atoms at the edge.} |
| 770 |
|
\label{fig:breaking} |
| 771 |
|
\end{figure} |
| 772 |
|
|
| 773 |
|
|
| 704 |
– |
|
| 705 |
– |
|
| 774 |
|
%Peaks! |
| 775 |
|
%\begin{figure}[H] |
| 776 |
|
%\includegraphics[width=\linewidth]{doublePeaks_noCO.png} |
| 784 |
|
%Don't think I need this |
| 785 |
|
%clean surface... |
| 786 |
|
%\begin{figure}[H] |
| 787 |
< |
%\includegraphics[width=\linewidth]{557_300K_cleanPDF.pdf} |
| 787 |
> |
%\includegraphics[width=\linewidth]{557_300K_cleanPDF} |
| 788 |
|
%\caption{} |
| 789 |
|
|
| 790 |
|
%\end{figure} |
| 792 |
|
|
| 793 |
|
|
| 794 |
|
\section{Conclusion} |
| 795 |
< |
In this work we have shown the reconstruction of the Pt(557) crystalline surface upon adsorption of CO in less than a $\mu s$. Only the highest coverage Pt system showed this initial reconstruction similar to that seen previously. The strong interaction between Pt and CO and the limited interaction between Au and CO helps explain the differences between the two systems. |
| 795 |
> |
The strength and directionality of the Pt-CO binding interaction, as |
| 796 |
> |
well as the large quadrupolar repulsion between atop-bound CO |
| 797 |
> |
molecules, help to explain the observed increase in surface mobility |
| 798 |
> |
of Pt(557) and the resultant reconstruction into a double-layer |
| 799 |
> |
configuration at the highest simulated CO-coverages. The weaker Au-CO |
| 800 |
> |
interaction results in significantly lower adataom diffusion |
| 801 |
> |
constants, less step-wandering, and a lack of the double layer |
| 802 |
> |
reconstruction on the Au(557) surface. |
| 803 |
|
|
| 804 |
+ |
An in-depth examination of the energetics shows the important role CO |
| 805 |
+ |
plays in increasing step-breakup and in facilitating edge traversal |
| 806 |
+ |
which are both necessary for double layer formation. |
| 807 |
+ |
|
| 808 |
|
%Things I am not ready to remove yet |
| 809 |
|
|
| 810 |
|
%Table of Diffusion Constants |
| 828 |
|
% \end{table} |
| 829 |
|
|
| 830 |
|
\begin{acknowledgement} |
| 831 |
< |
Support for this project was provided by the National Science |
| 832 |
< |
Foundation under grant CHE-0848243 and by the Center for Sustainable |
| 833 |
< |
Energy at Notre Dame (cSEND). Computational time was provided by the |
| 834 |
< |
Center for Research Computing (CRC) at the University of Notre Dame. |
| 831 |
> |
We gratefully acknowledge conversations with Dr. William |
| 832 |
> |
F. Schneider and Dr. Feng Tao. Support for this project was |
| 833 |
> |
provided by the National Science Foundation under grant CHE-0848243 |
| 834 |
> |
and by the Center for Sustainable Energy at Notre Dame |
| 835 |
> |
(cSEND). Computational time was provided by the Center for Research |
| 836 |
> |
Computing (CRC) at the University of Notre Dame. |
| 837 |
|
\end{acknowledgement} |
| 838 |
|
\newpage |
| 839 |
|
\bibliography{firstTryBibliography} |
| 840 |
|
%\end{doublespace} |
| 841 |
|
|
| 842 |
|
\begin{tocentry} |
| 843 |
< |
%\includegraphics[height=3.5cm]{timelapse} |
| 843 |
> |
|
| 844 |
> |
\includegraphics[height=2.8cm]{TOC_doubleLayer} |
| 845 |
> |
|
| 846 |
> |
A reconstructed Pt(557) surface after having been exposed to a dosage of CO equivalent to half a monolayer of coverage. |
| 847 |
> |
|
| 848 |
|
\end{tocentry} |
| 849 |
|
|
| 850 |
|
\end{document} |
