48 Experimental Methods
Furthermore, electrons exhibit relatively short mean free path in solids and the
range of kinetic energies observed in an XPS system is typically 10-1000 eV.
The mean free path of electrons as a function of their kinetic energy can be seen
in gure 3.3, which is also known as the Universal Curve. This results in an
information depth of less than 2 nm and it is therefore reasonable to assume
that the bulk of a sample does not contribute to the signal. The shallow depth
of information, or surface sensitivity, is a distinct property of XPS and methods
where electrons are measured. This is of particular importance in catalysis,
which is a surface phenomenon. At the same time it also presents a challenge
in electrocatalysis, where a number of unwanted changes can occur to a sample
during electrochemical testing.
Figure 3.3: The universal curve of inelastic mean free path of electrons in a solid.
The mean free path of the electrons is plotted as function of electron kinetic energy.
Figure taken from 191.
With careful treatment XPS analyses of samples before and after electrochemical
measurements can reveal important properties of electrocatalysts. Among
the possible diculties are unexpected dissolution of the prepared sample, interference
of contaminants from the electrolyte and deposition of carbon species
upon drying. In this thesis the primary uses for XPS have been to establish oxide
stoichiometry of Mn oxides, to nd the ratio between Mn and another metal
for mixed oxides and to check for possible contamination. The oxide stoichiometry
can, for most metals, be found by analysing peak shifts. However, for Mn
oxides it has been reported that the peak shifts are not very signicant and it is
not possible to distinguish between the various oxides 192. Instead, the Mn2p 1
2
satellite structure and Mn3s multiplet splitting can be used 151,192,193. More
specically the Mn2p1
2 satellite distance and Mn3s multiplet energy dierence