62 Experimental Methods
High mass loading. With a very high mass loading the QCM sensitivity
decreases 200. The method is considered accurate up to about 2 % of
frequency change with respect to the nominal resonant frequency. This
corresponds to approximately 2.4 mg, which is about four orders of magnitude
higher than the typical loading for the MnOx thin lms investigated
here.
Roughness of EQCM coating. A high roughness of samples can in
extreme cases lead to misinterpretation of the frequency changes. This is
due to trapping of liquid in cavities which can be a particular problem if
a rough sample is oxidised or deposited while the frequency is measured.
Such a case was reported in 205 where the mass change of oxidised gold
was much higher than expected due to this trapping phenomena. For
the samples used here a polished substrate surface was used, which is not
expected to be inuenced by trapping of liquid.
Non-uniform samples. With the simple Sauerbrey equation only uniform
lms should be evaluated as it is assumed that the lm is a rigid and
uniform extension of the quartz crystal.
Bubbles. Finally, bubble formation can pose a problem for EQCM measurements.
For a gas evolving reaction, it is important that the bubbles
detach from the surface so that they are not covering a large area of the
electrode. If they end up covering a large part, the frequency measurement
is aected due to local viscosity eects. Furthermore, if a gas is evolved
on a very rough surface it can be trapped. For the experiments carried
out for this thesis, the bubbles were observed to leave the surface easily,
from a macroscopic point of view.
The equipment used for EQCM measurements was a QCM200, purchased from
Stanford Research Systems. The crystals were AT-cut and had a nominal resonant
frequency of 5 MHz. The electrochemically active area was 1.37 cm2, while
the area sensitive to frequency measurement was 0.38 cm2.
3.4.2 Inductively Coupled Plasma - Mass Spectrometry
In addition to knowing how an electrode is changing in mass, it is crucial to
quantify the dissolved species in the electrolyte. The EQCM method can be
described as blind in that respect, since it only reveals a change in the overall
mass. Therefore, Inductively Coupled Plasma - Mass Spectrometry, ICP-MS,
has been used in combination with the EQCM measurements.
In brief, with ICP-MS a liquid sample is analysed by injecting a small volume
through a nebulizer into a plasma as aerosol droplets 206. Travelling through
the plasma the sample is desolvated, atomised and ionized. The ions are then
sent into a mass spectrometer. In the Mass Spectrometer the ions are ltered