5.3 Experimental validation of the concept 91
TiO2 and mixed oxides, the samples prepared with overlayers will be denoted
TiO2/MnO2. These samples were tested with a slightly dierent protocol compared
to what has been explained earlier; after initial cyclic voltammetry and a
stabilization period at 1.4 VRHE, chronoamperometry measurements were performed
for one hour at 1.7, 1.75, 1.8 and 1.85 VRHE subsequently. Furthermore,
the stability is based solely on EQCM data, since the ICP-MS equipment had
not been installed at the time. At 1.7 VRHE both current and mass losses
are negligible, hence, this data will not be included here. First, an overlayer
thickness of nominally 1.5 Å was investigated. This thickness corresponds to
approximately half a monolayer coverage assuming rutile (110) surface termination
of TiO2. Such an assumption is rather crude as these really thin layers
are not very likely to be crystalline unless the conditions are met for epitaxial
growth. In this case the MnO2 lm is not crystalline and epitaxial growth is
not expected. Furthermore, the QCM calibration of deposition rates is made for
thicker layers (15-20 Å), which means that for very thin lms the uncertainty
in rate is more critical. Depositing 1.5 Å is likely to result in too much TiO2
but on the other hand with too little TiO2 it could be dicult to measure any
dierence in stability. The results of depositing 1.5 Å TiO2 on top of MnO2 is
shown in gures 5.13a and b.
Activityafter1hour
0.05MH2SO4
1,751,801,85
5
4
3
2
1
0
jgeo/mA.cm-2
U-iR/V(vs.RHE)
MnO2
MnO2
+TiO2
+TiO2
+TiO2
MnO2
MasslossesfromEQCM
Extrapolatedto2hours
1,751,801,85
b)0
-150
-300
-450
-600
+TiO2
+TiO2
+TiO2
MnO2
MnO2
MnO2
Massloss/ngOxide.cm-2
U-iR/V(vs.RHE)
a)
Figure 5.13: a) Activity of MnO2 and TiO2/MnO2 at 1.75, 1.8 and 1.85 VRHE
measured in 0.05 M H2SO4. The activity is compared as the current density measured
after one hour. b) Mass losses of MnO2 and TiO2/MnO2 at 1.75, 1.8 and 1.85 VRHE.
The mass losses here are based on EQCM measurements of the frequency change
during one hour tests. The TiO2 layer is nominally 1.5 Å and the error bars indicate
1 standard deviation from 3 independent measurements.
The activity of the MnO2 thin lm has decreased upon addition of the TiO2
overlayer. At 1.85 VRHE the measured current is on average 17 % lower for
TiO2/MnO2. In an ideal case where the MnO2 is perfectly at, 1.5 Å of TiO2
is likely block a large fraction of the surface, leading to up to 50 % decrease
in current density. However, the samples prepared here are not perfectly at