Chemistry Reference
In-Depth Information
between chemical reactivity and geometry of the active metal species has been
explored in detail for gold [
8
]. Raft-shaped Au islands on iron oxide, for example,
have been identified as the active entities in the low-temperature oxidation of CO
[
67
,
68
]. Also in the Au/TiO
2
system, bilayer deposits turned out to be the most
active [
61
,
69
]. Both results suggest a special role of the perimeter sites of metal
deposits, which enable molecules to interact simultaneously with the metal and the
oxide support. As those sites are most abundant on flat metal islands, a close
interrelation between structure and reactivity is not surprising. We note in passing
that the shape affects also other fundamental properties of metal deposits, e.g., their
electronic structure and optical response [
70
,
71
], which renders a careful shape
control
relevant
for
applications
in microelectronics,
nano-optics,
and
photocatalysis as well.
One possibility to extend the concept of charge-mediated particle growth to bulk
oxides is the insertion of suitable charge sources directly into the oxide material,
preferentially into a near-surface region to allow for charge exchange with adsor-
bates. By this means, all advantageous effects of charge control could be
maintained for oxide slabs of arbitrary thickness. The fundamental approach to
insert charge centers into a material is doping, and the underlying concepts have
been introduced and brought to perfection already in the mature field of semicon-
ductor technology. Surprisingly, the art of doping is less advanced to what oxide
concerns, which relates to a number of peculiarities in these materials. Oxides are
subject to self-doping either by native defects or unwanted impurities, the concen-
tration of which is difficult to control experimentally [
72
]. Both lattice defects and
impurity ions may adopt different charge states in the oxide lattice [
73
,
74
], a
variability that leads to pronounced compensation effects and is less common in
semiconductors. And finally, the dopants may be electrically inactive in a wide-gap
insulator, as thermal excitation is insufficient to rise the electrons from defect states
into the bulk bands. As a result, the excess charges remain trapped at the host ions
and are unavailable for charge transfer. The following examples demonstrate,
however, that doping is a versatile approach to control the growth of metals even
on bulk-like oxide materials [
17
,
18
,
75
-
79
]. The underlying concepts are thereby
similar to the charge-transfer picture developed for thin films before.
In general, doping is carried out with impurity ions that adopt either a higher or
lower valence state than the substituted ions in the oxide lattice. In rare case, also
charge-preserving doping is realized, and geometric and strain effects and not
charge transfer become relevant in these cases. Whereas high-valence dopants
may serve as charge donors and provide extra electrons, undervalent dopants
have acceptor character and may accommodate electrons from suitable adsorbates.
Based on the above considerations we now expect that charge donors in an oxide
lattice have a similar influence on the particle shape as the metal support below a
thin oxide films.
The impact of doping on the growth morphology of gold has first been realized
for crystalline CaO(001) doped with Mo in the sub-percent range [
80
]. On the
doped oxide, the gold was found to spread out into extended monolayer islands,
while the conventional 3D growth regime prevailed on the pristine, non-doped
