Draft version
higher concentration of Au, indicating that a significant amount of the gold in the 30% sample is oxidized, not
just the surface.
Figure 5. a) Au LIII-spectra for Au(30%)-MnOx (blue) and Au(50%)-MnOx (red) at open circuit conditions. b) Au LIII-spectra for
Au(30%)-MnOx (blue) and Au(50%)-MnOx (red) at 1.65 VRHE.
Since the XANES results showing a higher oxidation for MnOx film mixed with gold, it raises the question; why
does a higher oxidation state lead to higher OER activity. The binding energies for Mn sites in a higher oxidation
state is closer to the optimal strengths. This notion can be justified with the DFT calculations reported in 8,36
where MnO2 is predicted to have a lower overpotential than Mn2O3, Mn3O4 and MnO.
It is therefore interesting that the two Au concentrations results in very similar Mn oxidation state from the insitu
XANES, while the activity enhancement is much more significant at 50 % Au. An important difference
between the two concentrations is the Au domain size, which was markedly larger at 50 %. Furthermore, the
Au LIII-edge spectra revealed that the sample with 30% gold was oxidized at high potentials, whereas the
sample with 50 % was not. Assuming the domain size estimated with XRD to be a suitable measure of the
particle size it should be noted that size effects for oxygen electrochemistry is very significant in the range of 1-
5 nm.50 A possible explanation for the observations is that a certain size of Au particles is necessary for the full
enhancement. This enhancement could be related to the type of Au sites available at the surface of such
particles during oxygen turnover.30,51 Greeley et al. have reported that the binding to OER intermediates for Au
surfaces is stronger for undercoordinated sites.52 Specifically, *OH binding to Au(211) was 0.17 eV stronger
than to Au(100). Furthermore, the number of undercoordinated sites for Au particles increase dramatically
when the size of particles is decreased from 5 to 1 nm.51 For the smaller Au domains this is consistent with
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