92 Towards a stable and inexpensive catalyst for OER in acid
as was observed with Scanning Electron Microscopy, gures 5.7a-d. It is also
possible that excess TiO2 forms particles with low adhesion to the surface. These
factors complicate the evaluation of an appropriate amount of TiO2 used to
block the undercoordinated sites. However, from the stability results in gure
5.13b it is evident that the TiO2 overlayer leads to lower mass losses. At 1.85
VRHE the mass losses are on average 55 % lower. It is therefore likely that less
undercoordinated sites are available for dissolution. However, since the mass
loss is not zero some of these sites are still not covered, which could indicate
that the mobility of TiO2 on MnO2 is too low at 200 oC. To test the eect of
overlayer thickness two other samples were prepared; TiO2/MnO2 with 0.5 Å
and 5 Å TiO2. The mass losses measured for the short term tests are shown in
gure 5.14.
MnO2
55%lower
massloss
0,00,51,01,55
0
-150
-300
-450
-600
-750
Massloss/ngOxide.cm-2
Tioverlayerthickness/Å
Figure 5.14: Mass losses at 1.85 VRHE for MnO2 and TiO2/MnO2 at 1.85 VRHE,
measured in 0.05 M H2SO4, as a function of overlayer thickness. The mass losses here
are based on EQCM measurements of the frequency change during one hour tests.
A TiO2 overlayer thickness of 0.5 Å results in lower mass losses compared to
the pure MnO2, 35 %, but the stabilization is less drastic than for 1.5 Å. Furthermore,
a thicker overlayer of 5 Å only leads to a stabilization of around 37
% compared to pure MnO2. These results indicate that there is an optimum
in overlayer thickness, which is around 1.5 Å. It is especially surprising that
the stabilizing eect of 5 Å is lower than for 1.5 Å but it could be explained
with excess TiO2 forming particles on the surface. Such particles would have
a very dierent interaction with the MnO2 surface and are not likely to selectively
block undercoordinated sites. In fact, with the very low reactivity of TiO2
particles it is possible that they will simply detach under reaction conditions.
At the same time a low mobility of TiO2 on MnO2 could be the reason why a
0.5 Å layer is not stabilizing the surface as signicantly as 1.5 Å. These three