Chemistry Reference
In-Depth Information
Fig. 17 PBE projected
state density calculated for
non-doped (
top
) and doped
(
bottom
) CaO films in
presence of an Au
adatom [
80
]
The presence of suitable dopants is, however, not the only requirement for a
stable donor characteristic, the interplay between dopants and host oxide deter-
mines the redox activity. This shall be demonstrated for two similar systems,
Cr-doped MgO and Mo-doped CaO, which still exhibit an entirely different behav-
ior [
81
-
83
]. Chromium has a similar electronic structure as Mo, i.e., the same
number of d-electrons, but is a 3d and not a 4d metal. Surprisingly, it is unable to
influence the Au growth behavior on the MgO support (Fig.
18
). Even at a high Cr
concentration, the 3D growth of gold prevails and hardly any 2D islands are found
on the surface. The reason for this poor behavior is the low energy position of the Cr
t
2g
levels in the MgO band gap, which originates from a substantial stabilization of
the Cr 3d
-
electrons in the MgO crystal field. Note that the crystal field in MgO is
substantially stronger than in CaO, due to the reduced lattice parameter [
84
]. In
addition, the ionization energies of Cr atoms are higher than for Mo, which makes
formation of Cr
4+
and Cr
5+
ions energetically more expensive [
85
]. As a result, Cr is
able to donate at maximum one single electron to gold, which compares to three for
the Mo dopants in CaO [
81
].
Moreover, this electron may not even reach the ad-metal, because it is likely to
be captured by parasitic electron traps that are present in real oxides. Typical
electron traps are cationic vacancies, e.g., Mg defects, domain boundaries, or
dislocation lines [
86
]. The cation defects (
V
-centers) preferentially develop in
oxides with a large number of high-valance dopants, as they are able to compensate
the augmental charge state of the impurities with respect to the intrinsic ions
[
84
]. According to DFT calculations, the formation energy of electron trapping
