110 The benecial interaction between Au and MnOx
for Mn atoms in a formal 2+ state, which is consistent with the Mn3O4 phase,
comprising both 2+ and 3+ 156,235. From a qualitative point of view, the edge
can be analysed by looking at the overall position of the edge. When a feature
or the whole edge moves towards higher photon energies it is indicative of an
oxidation of the Mn atoms, consistent with more tightly bound core electrons. In
the following, only the XANES section of the results will be shown, to highlight
the changes in the edge prole as function of applied potential.
When introducing the electrolyte and applying a potential, changes in these
features are observed. For the pure Mn3O4 lm, XAS spectra following the
gradual increase in electrochemical potential can be seen in gure 6.13. The
edge is shifting towards higher photon energies when the potential is increased,
as indicated with the arrow. At the same time the shoulder at 6552 eV seems to
decrease slightly and with a closer look an extra pre-edge around 6543 eV can
be observed for the highest potential. These small changes and shift of the edge
indicates an average oxidation of the Mn atoms present in the sample, which is
certainly expected from the anodic potential applied.
1,6MnOx
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
OCV
0.80V
1.00V
1.20V
1.40V
1.65V
HigherU
In1MKOH
6535654065456550655565606565
NormalizedIntensity/arb.units
PhotonEnergy/eV
Figure 6.13: Mn K-edge XAS of the Mn3O4 lm under potential control in 1M
KOH. Error bars are based on standard deviations from Poisson statistics on several
scans.
The spectra obtained from the Au(30%)-MnOx sample can be seen in gure
6.14 and from the Au(50%)-MnOx in gure 6.15. From both sets of spectra a
very clear shift of the entire edge can be seen.