6.2 Experimental investigation of Au-MnOx catalysts 105
Au. With the Scherrer equation it is possible to evaluate domain size based on
the peak broadening, as explained in chapter 3. This analysis indicates that 30
% Au in the lm results in Au domains of about 2 nm in size. At 50 % the domains
reach 3 nm. So despite the co-sputtering method there are clearly particle
formation taking place under deposition conditions. However, these nanosized
Au domains are still expected to be dispersed into the Mn3O4 matrix, yielding
a high density of Mn-Au sites. Due to the lack of Mn oxide peaks for the mixed
lms, they will be denoted Au-MnOx from this point.
Scanning Electron Microscopy was employed to get an overview of the surface
morphology. The images shown in this section are taken with secondary electron
detection so that the contrast stems from surface morphology. In gure 6.7a and
b the surface of a pure Mn3O4 lm can be seen with two magnications. The
micrographs indicate that the surface consists of pyramid shaped features in the
size range of 10 to 20 nm. At the same time these features are quite densely
packed and the surface does not appear porous. It should also be noted that the
surface of this lm looks signicantly dierent from the MnO2 lms shown in
gure 5.7 in chapter 5, where the features were rounded and less densely packed.
Figure 6.7: Scanning Electron Micrographs for pure Mn3O4 lm. a) Overview
image of the surface. b) Close-up revealing the surface structure.
In gure 6.8a and b images of a modied lm with approximately 30 % Au are
shown. The features for this surface are more rounded, almost spherical, with
small bright spots. These roundish domains are also in the order of 10 to 20
nm. The surface is again quite densely packed but it looks like the introduced
Au has stopped the MnOx domains in growing in the same way as it was observed
for the Mn3O4 lm. This would also be consistent with the XRD results,
where the mixed lms only show peaks for Au. The surface for 50 % Au shows
similar features and a slightly more porous structure, see gure 6.9a and b. It is
tempting to interpret the bright spots in the images as gold particles, but this