surface, since TiO2 is stable in acid up to more than 2.1 VRHE.28 At the same time the terrace sites,
critical for the activity towards the oxygen evolution reaction, are less likely to be occupied by TiO2.
For the remainder of this paper, we shall experimentally verify the theoretical prediction that TiO2
decorated MnO2 should exhibit improved stability, relative to MnO2, without compromising the OER
activity.
2.2 Experimental results
Several options exist for the experimental realization of the theoretically predicted catalyst; we chose
to do so by sputter-depositing thin films of mixed Mn and Ti oxides. We adapted the deposition
methods from our previous study on pure oxides.32 The films had a nominal total thickness of 40 nm
(based on the average deposition rates-see Experimental Methods section). The upper 5 nm of the Ti
modified samples contain 20 % TiO2. The mixed oxide layers were obtained by simultaneous cosputtering
of Mn and Ti targets. We anticipate that this should form a mixed film with high dispersion
of Ti in MnO2. Although we cannot rule out phase separation, we find this possibility unlikely, given
that the mobility of the material would be low at the deposition temperature of 200 ᵒC. Figure 5a
shows a schematic of the prepared thin films. The procedure we employ is based on our earlier study
where we tested the activity and stability of pure oxides.32
Grazing Angle X-Ray Diffraction, GA-XRD, and X-ray Photoelectron Spectroscopy, XPS, were used to
establish the structure and composition. From GA-XRD, no peaks were found, indicating that the films
are highly disordered and possibly amorphous (Figure S1). However, from XPS analysis the oxidation
state of the Mn was consistent with MnO2 spectra in literature. The XPS spectra of the Mn3s and
Mn2p½ can be seen in Figure S2a and b, and the analysis of the oxidation state is based on the
multiplet splitting of the Mn3s and the distance for the satellite feature of the Mn 2p½.73,74
Furthermore, the ratio Mn:Ti in the surface was found to be 80:20, based on the Mn2p and Ti2p
(Figure S2c) peaks, which is in good agreement with the measured deposition rates.
The samples were also characterized with Scanning Electron Microscopy, SEM, in order to elucidate
surface morphology and possible surface changes from electrochemical testing. In Figure 4a