Draft version
In-situ XAS measurements probing the Mn K-edge were performed at the SSRL Beam-line 6-2 ES2. The
electrochemical setup for these measurements has been reported previously.29,39 A double crystal Si(311)
monochromator equipped with Rh-coated mirror was used to operate the beamline. Furthermore, a parabolic
mirror was used to focus the beam to a spot size of 230x400 μm2 (FWHM) at the sample position. Six
spherically bent analyzer crystals of germanium ‹333› resolved the MnKα signal which could then be detected
by an Si drift detector, SDD, in photon counting mode. The spectra presented here have all been normalised to
have an edge jump of one after the linear background is subtracted. Error bars represent ±1 standard deviation
from Poisson statistics and standard error propagation.
3. Results and Discussion
To investigate the beneficial interaction between Au and Mn for oxygen evolution, thin films of MnOx mixed
with two different amounts of gold were prepared. The mixed films contain 30 % or 50 % Au and are compared
to pure MnOx and a polycrystalline Au surface. The percentage values are on a total metal basis, so that 30 %
Au means that 30 % of the manganese are now replaced with Au.
From XPS measurements the Mn 2p and Au 4d peaks were used to evaluate the Mn:Au stoichiometry. It should
be noted that a small Cu contamination (1-3 %) was found for two samples used for electrochemical tests, but
the activity towards OER remained the same as without contamination. See more details in supporting
information. The XPS spectra can be seen in Supplementary Information, Figures S1-3. The Mn2p½ satellite
distance and Mn3s multiplet splitting can also be used to evaluate the initial Mn:O stoichiometry.20,40
Comparing the results obtained for as-prepared samples to literature references, indicates an Mn:O
stoichiometry consistent with Mn3O4.
The thin films were also characterized with Glancing Angle X-ray Diffraction, to investigate whether the sputter
deposition method yields crystalline Mn oxide or Au phases. The results for pure MnOx can be seen in Figure 1a
and the results for mixed Au-MnOx films in Figure 1b. There are nine peaks from the MnOx film matching with a
Mn3O4 phase which is plausible from a low amount of oxygen present during deposition. Interestingly, the
Mn3O4 peaks vanish for the mixed films where gold peaks are instead visible, for both concentrations. For 30
and 50 % Au, four peaks can be identified which match with a gold face centered cubic (FCC) phase. These
results clearly show that Au particles are formed. The difference between the two concentrations is the
broadening of the peaks. Using Scherrers equation it is possible to compare the Au domain size for the two
films based on this broadening. For 30 % Au the domains are approximately 2 nm while for 50 % Au they are 3
4