Chemistry Reference
In-Depth Information
in 2D aggregates as compared to 1D chains, which might be rationalized by the
higher efficiency of the linear systems to distribute the excess electrons and lower
the internal Coulomb repulsion [
48
]. The energetic preference for chains dimin-
ishes, however, for clusters containing more than 6-7 atoms, inducing a crossover
to 2D island shapes and later to 3D structure. It should be mentioned that also the
HOMO-LUMO gap shows a strong dependence on the atom number per cluster
(Fig.
15
). Whereas Au aggregates containing 10-15 atoms have experimental gap
sizes of around 1.5 eV, the HOMO-LUMO gap closes for more than 100 atoms in
the cluster, reproducing earlier results for Au clusters on TiO
2
[
61
]. The gap size
hereby follows the inverse cluster area
ʩ
according to
E
g
/ ʩ
1
, as expected for the
energy separation of eigenstates in a 2D harmonic potential [
43
,
60
].
In summary, the charge-mediated binding scheme of gold to oxide films is closely
related to the possibility to transfer electrons through the insulating spacer. The
dominant transport mechanism is electron tunneling from the Fermi level of the
metal substrate through the oxide layer. The tunneling probability depends exponen-
tially on the oxide thickness, and to a smaller extent on the gap size, as already
discussed by Cabrera and Mott [
55
,
62
]. As an interesting consequence the growth
regime of Au on thin oxide films might be altered simply by increasing the film
thickness until tunneling transport becomes impossible (Fig.
11
)[
15
]. Whereas only
2D islands are found on 3 ML MgO/Ag(001) films, a dimensionality crossover to 3D
particles occurs on 8 ML films that do not support charge transfer anymore
[
16
]. These experiments have been reproduced by DFT calculations, addressing the
shape of Au
8
clusters on 2 and 5 ML thick MgO/Ag(001) films [
55
]. With increasing
oxide thickness, the amount of charge transfer was found to decrease by 50%, causing
the interfacial adhesion to drop from 0.7 to 0.35 eV per interfacial atom. As a result,
the initially planar Au
8
cluster turned 3D on the thick film in order to increase the
number of Au-Au bonds and to counterbalance the loss in interfacial adhesion. Both
experiments and theory therefore confirm the importance of charge transfer in
determining the growth shape of metals on thin-film systems.
2.5 Metal Growth on Doped Oxide Materials
The former paragraphs have demonstrated how the equilibrium shape of metal
particles on oxide supports depends on the charge state of the deposits. It turned out
that charged species, either single atoms or small aggregates, tend to bind stronger
to ionic oxides, as they exhibit additional electrostatic and polaronic interaction
channels that are not available for neutral entities [
28
,
55
]. However, the concept of
charge-mediated control of the metal-oxide adhesion seems to be restricted to
ultrathin films, as the extra electrons are provided by a metal substrate and need
to tunnel through the insulating spacer layer. In contrast, the request to tailor the
equilibrium shape of metal particles becomes particularly large on bulk oxides as
used in heterogeneous catalysis, as the chemical properties of metal-oxide systems
were shown to depend sensitively on the particle shape [
63
-
66
]. The relation
