112 The benecial interaction between Au and MnOx
In gures 6.14 and 6.15 the shoulder feature at 6552 eV completely disappears
and a splitting of the pre-edge is observed already from an electrochemical potential
of 1.0 VRHE. To highlight the dierences between the pure Mn3O4 and
the Au modied lms the spectra taken at 1.65 VRHE are compared in gure
6.16.
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
+30%Au
+50%Au
1.65VRHE
Mn3O4
6535654065456550655565606565
NormalizedIntensity/arb.units
PhotonEnergy/eV
Figure 6.16: Mn K-edge XAS of the three Mn oxide lms at 1.65 VRHE in 1M
KOH. Mn3O4 in black, Au(30%)-MnOx in blue and Au(50%)-MnOx in red.
The splitting of the pre-edge has been reported to follow oxidation of Mn and is
only visible for average oxidation state of 3 or higher 235. From this qualitative
analysis of the XAS features it seems that the Au modication leads to higher
oxidation state of Mn under anodic polarisation of the electrode. This nding
is in contrast to the report from Kuo et al. 226. However, the spectra can
also be quantied. This is often done by selecting a rather arbitrary point on
the XAS spectrum to measure the edge shift. By using reference measurements
the shift can then be related to an oxidation state 156, 236, 237. A problem
associated with using a single point of reference for the edge shift is that the edge
features depend on structure as well as oxidation state. Unless the structures
of samples used for comparison are well dened, two compounds with the same
nominal oxidation state could be evaluated dierently using a single reference
point 237. Instead it is possible to evaluate the overall shift of a spectrum by
using a rst moment method as described in section 3.2.3 238, 239. The edge
shift was calculated for all the spectra of Mn3O4 and Au-MnOx. The results