Draft version
environment it seems unlikely that a high concentration of Mn3+ should be present at the surface while turning
over oxygen.36,37
In the present study, we will characterize the behavior of MnOx in situ under oxygen evolution conditions, using
XANES to measure the Mn K-edge. To achieve a high concentration of Mn/Au neighbors the samples
investigated are co-sputtered thin films.
2. Experimental details
Thin films of MnOx and Au-MnOx, with a nominal thickness of 40 nm, were prepared with reactive sputter
deposition using a method previously reported.38 The amount of material deposited was controlled by an inchamber
Quartz Crystal Microbalance, with which the deposition rates of both Mn and Au were calibrated. Mn
was deposited with 140 W and Au with power dependent on the desired concentration (30 or 50 %). The
deposition pressure was kept at 5 mTorr consisting of Ar and O2 in a 25/3 ratio and the substrate temperature
was 200 ᵒC. Glassy carbon disks or wafers were used as substrates (Sigradur G, HTW GmbH), both polished to a
mirror finish. To facilitate film adhesion, the glassy carbon substrates were cleaned with radio frequency (RF)
sputtering in an argon atmosphere for 10 minutes prior to deposition. The glassy carbon wafer preparation has
been reported previously.29 They were prepared to be 100-200 μm thick from GC rods and polished to an RMS
roughness of less than 50 nm.
Tests of activity towards oxygen evolution were performed in a standard three electrode glass cell using 1 M
KOH. A carbon rod was used as counter electrode and a Hg/HgO electrode as reference. The reference
electrode potential was calibrated with a reversible hydrogen electrode (RHE) in the same electrolyte by
bubbling hydrogen over a platinum mesh. All potentials are reported with respect to the RHE scale and have
been corrected for Ohmic losses, evaluated with Electrochemical Impedance Spectroscopy; range 1-200000 Hz
and DC potential 10 mV. The Ohmic resistance was between 5-9 Ω for all samples.
Ex-situ characterisation of the thin films was performed using X-ray Photoelectron Spectroscopy (Thermo-
Fisher, base pressure of 5 x 10-10 mbar and X-ray source monochromatized AlKα 1486.7 eV), Scanning Electron
Microscopy (SEM, FEI, Magellan, secondary electron detector, beam voltage of 5 kV, beam current of 50 pA)
and X-ray Diffraction (PANanalytical X’pert PRO equipment having an X-ray wavelength of 1.54 Å for the CuKα
line).
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