Chemistry Reference
In-Depth Information
{111} slabs were modelled using six atomic layers, the bottom one being fixed at
mean bulk positions, while the atoms of the other five layers are free to experience
thermal motions. Each crystallographic plane contains 128 and 90 atoms for the
{100} and {111} orientations, respectively. The thermal expansion of Au predicted
by the Gupta potential was used to set the lattice constant for a given temperature.
A deposition rate of 1 atom/ns was applied. Figure
8
shows snapshots from the MD
simulations of the deposition of 1 monolayer (ML) of metal M (M
¼
Rh, Pd and Pt)
on both the {100} and {111} Au surfaces, for temperatures of 300 and 500 K.
6.2.1 Rh/Au
From the first column in Fig.
8
, it is evident that Rh forms surface clusters on the
Au substrate. Their average size depends on the surface mobility of the adatoms
(and on the deposition rate). For instance, two sub-clusters can be distinguished for
{100}/300 K, where the surface mobility is relatively low because of the low
temperature and the higher coordination of an adatom on the {100} surface compared
with the {111} surface. The surface cluster is more compact at
T
500 K. A single
Rh cluster is observed on the {111} substrate for both temperatures. The formation of
3D Rh clusters on Au surfaces is consistent with the strong immiscibility of the two
elements in the bulk. If low-coordinated Au atoms exist on the surface, they display a
tendency to attach themselves to the Rh clusters and to climb onto their surface. The
illustration at the bottom of column 1 of Fig.
8
shows a snapshot of a Rh cluster on the
Au{111} substrate annealed at 500 K for 20 ns together with 8 Au adatoms.
This effect, which is consistent with the Au-Rh exchange previously observed
[
75
,
88
,
89
], results from the greater Au-Rh bond strength compared with that of
the Au-Au bond and the much lower surface energy of Au than Rh (Table
1
). If
low-coordinated Au atoms are present on the surface, they would aggregate together
at step or kink sites of the growing Rh island and could be buried by newly deposited
Rh atoms. This is a possible scenario to explain the observed mixing at the interface
of Rh-coated Au nanorods [
51
].
¼
6.2.2 Pd/Au
The second column in Fig.
8
shows MD snapshots for Pd deposition on Au. In
contrast to Rh, Pd effectively wets the Au surfaces at this deposition rate, even at
low temperatures. This makes it possible for an ideal layer-by-layer growth to take
place until the accumulated strain resulting from the lattice mismatch causes some
surface reconstructions. If, however, at this point the Pd layer is sufficiently thick
the system may be kinetically trapped in a state which is far from equilibrium,
where a mixed bulk phase and a surface enriched in Au are expected. This could be
a kinetically driven mechanism for the formation of the very sharp Au-Pd interface
observed in Fig.
7
. It is evident from Fig.
8
that the energy barrier for the
interchange of Pd adatoms and surface Au atoms on Au{100} is low enough to
