5.3 Experimental validation of the concept 93
overlayer samples were also tested for a prolonged time after the initial short
term chronoamperometry. Specically they were tested at 1.8 for 10 hours and
then at 1.85 V for 8 hours, while the mass losses were evaluated with the EQCM
frequency measurement. The results are shown in gure 5.15a and b, for current
density and mass losses respectively.
MnO2
024681012141618
5
4
3
2
1
0
+5ÅTiO2
024681012141618
0
-400
-800
-1200
-1600
-2000
MnO2
jgeo/mA.cm-2
Time/hours
1.85VRHE
1.8VRHE
+5ÅTiO2
+0.5ÅTiO2
+1.5ÅTiO2
b)
+0.5ÅTiO2
MnO2
Massloss/ngOxide.cm-2
Time/hours
+1.5ÅTiO2
1.8VRHE1.85VRHE
a)
Figure 5.15: a) Current density of MnO2 and TiO2/MnO2 samples at 1.8 and
1.85 VRHE, measured in 0.05 M H2SO4. b) Mass losses of MnO2 and TiO2/MnO2 at
1.8 and 1.85 VRHE. The mass losses here are based on EQCM measurements of the
frequency change. MnO2 is in red, 0.5 Å TiO2/MnO2 in green, 1.5 Å TiO2/MnO2 in
red and 5 Å TiO2/MnO2 in purple.
The activities of the TiO2/MnO2 are in this case signicantly lower than the
pure MnO2. It should be noted that compared to the average for pure MnO2,
the sample used for long term testing had more than 30 % higher current density.
Regarding the stabilizing eect of the overlayers it is again clear that the 1.5
Å thickness yields the most signicant eect. After 10 hours at 1.8 VRHE the
1.5 Å TiO2/MnO2 lost 93 % less mass compared to the pure MnO2. After 18
hours the combined mass loss for 1.5 Å TiO2/MnO2 is 70 % less than what
was observed for the pure MnO2. However, the two other overlayer thicknesses,
0.5 and 5 Å, exhibited stability improvements less impressive than the 1.5 Å
overlayer.
With overlayer deposition it is important that the mobility of the Ti atoms is
suciently high so that the undercoordinated sites are blocked by incoming Ti
atoms. The mobility can be tuned by changing substrate temperature. For
all the samples presented here the substrate temperature was kept at 200 oC
during deposition. It is possible that this is not high enough to give the titanium
mobility at the surface and it is therefore an important parameter to investigate
in future studies. At the same time there is a chance that the TiO2 will simply
form particles and minimize the interaction with the MnO2 surface. In principle,
the Ti atoms are sputtered o the target as single atoms or clusters consisting
of very few atoms, however, if larger clusters were to form it could explain