116 The benecial interaction between Au and MnOx
From the Pourbaix diagram of gold in aqueous solutions, the surface is expected
to oxidise to a +3 state at 1.46 VRHE 138. However, since the reactivity for
small Au nanoparticles can be drastically increased it is perhaps not surprising
that the Au(30%)-MnOx show some oxidation already at 1.2 VRHE. In conclusion,
a distinct dierence between the two Au concentrations is observed: the
Au atoms in the lower concentration lm are on average more oxidised than the
Au atoms in the higher concentration lm.
6.2.3.2 Discussion
Throughout the experimental part of this chapter, several characterization methods
have been employed with the aim of studying Mn-Au interaction. From XPS
the concentrations of Au were found to be 30 % and 50 % while the Mn oxide
lm matched a Mn3O4 stoichiometry. Glancing Angle XRD conrmed the
Mn3O4 phase of the pure Mn oxide lm, however, the mixed lms only showed
peaks for small gold domains. The size of these domains could be estimated
from the XRD peak broadening, which indicated 2 nm for 30 % and 3 nm for
50 %. Surface morphology was assessed with SEM, indicating a slightly more
open and less densely packed structure. This porosity was also in line with
electrochemical evaluation of the surface area. Judging from capacitance measurements
the mixed gold lms had capacitances up to 80 % higher than the
pure Mn oxide. However, the increased capacitance was not sucient to explain
the increase in activity, which was up to 5.5 times higher than Mn3O4 for the
lms with 50 % gold. From literature reports, it was suggested that Mn3+ states
were promoted by gold interactions, which was thought to improve the activity.
However, the in-situ XANES spectra presented here are in strong contrast to
that conclusion. Instead, the mixed lms were signicantly more oxidised compared
to the Mn3O4. Finally, the Au L3-edge spectra indicated that the Au
atoms for 30 % concentration were on average more oxidised than the gold in a
50 % concentration, a dierence that was observed already at 1.2 VRHE.
From the combined set of experiments, it can be concluded that a benecial
interaction between Mn and Au for OER catalysis is observed for co-sputtered
thin lms. The results further indicate that the interaction depends on the size
of the Au domains. For small domains, roughly 2 nm, the activity enhancement
is low and can almost be explained by increase in surface area. These
small domains also oxidise under reaction conditions which can be interpreted
as detrimental for obtaining a high OER activity. On the other hand for larger
domains, 3 nm, the particles show only little oxidation. It would be interesting
to combine these ndings with the theoretical model explained in section 6.1.
In this model it was suggested that Au sites act as proton acceptors, but it
was assumed that these sites would exhibit the properties of bulk gold. While
the experiments done so far can not directly conrm the theoretical concept at
this stage it is interesting to note that binding energies for the larger particles
are expected to be more similar to bulk gold compared to the smaller particles.