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Application Note: Analysis of Iron Oxidation States by XPS
||Kent R. Rhodes, Ph.D., McCrone Associates, Inc. Westmont, IL
X-ray photoelectron spectroscopy (XPS ; also known
as ESCA - electron spectroscopy for chemical analysis) is a surface
analysis technique in which a solid surface in a vacuum is irradiated with
x-rays to produce photoelectrons by direct transfer of energy from the x-ray
photons to core-level electrons in the atoms of the sample. The
photoelectrons that are emitted are collected and energy analyzed to produce
an electron energy spectrum. The energy of the photoelectrons is
related to the atomic and molecular characteristics of the sample, whereas
the number of photoelectrons emitted is related to the concentration of the
emitting atom in the sample.
The mean free path of XPS photoelectrons in solids is only
a few nanometers, so the photoelectrons that are analyzed originate from the
outermost 1-10 nm of the sample. This is the basis of the surface
sensitivity of XPS.
The spectrum of photoelectron energies contains peaks
characteristic of the elements present on the sample. The energy of
photoelectron peaks is commonly expressed as binding energy, which is the
energy required to remove a photoelectron from an atom. Binding energies
are usually reported in units of electron volts (eV).
At high energy resolution, shifts in photoelectron peak
position and structure are observed for atoms in different chemical states.
These shifts are produced by ionic and covalent bond differences between
atoms. Chemical shifts can be used to distinguish the oxidation
state of transition metals.
Figure 1 shows examples of iron XPS spectra from reference
compounds. The Fe (0) standard was measured from an iron foil cleaned
by argon ion sputtering. The Fe (II) standard was produced by heating
an iron oxide film under vacuum.1 The Fe (III) standard was
measured from Fe2O3 powder.2
The spectra exhibit a variety of structure due to initial and final state
effects, including chemical shifts, asymmetric peaks, spin-orbit coupling,
multiplet splitting and shake-up satellites. Peak fitting can be performed
on spectra of mixtures of iron oxidation states, but the use of analytic
functions for the peaks can be complex and result in highly correlated
An alternative approach to fitting complex transition
metal XPS spectra is to use spectra from standards as the basis functions for
linear peak fitting. This approach avoids having to describe the peak
shapes analytically. The main disadvantage is that oxidation state
standards can be difficult to prepare, especially for surface analysis were
the outermost layers can be more oxidized than the bulk of the material.
Figure 2 shows an example linear peak fit of a mixed
iron oxidation state sample. The measured spectrum is well fit by
a linear combination of the reference spectra from Figure 1. Inelastic
backgrounds were removed using the Tougaard method.3 The fit result indicates that the spectrum contains 28±2% Fe(0), 41±5% Fe(II), and 32±6% Fe(III).
The use of iron oxidation state analysis is illustrated by
the example shown in Figures 3 and 4. An Fe (110) single crystal was oxidized
in a vacuum chamber with ~4,000 L of pure oxygen, then heated to 800°C while
collecting XPS spectra.4 The changes in peak position and
structure indicate that the oxide film undergoes oxidation state changes
throughout the heating, and eventually is almost completely reduced to iron
metal. In Figure 4, the relative proportions of iron oxidation states
are plotted as a function of heating temperature.
click image to enlarge (65K)
XPS Spectra of Iron Standards
click image to enlarge (69K)
Iron Peak Fitting Example
click image to enlarge (164K)
Stepwise Iron Oxide Heating Experiment
click image to enlarge (71K)
Iron Oxidation States vs. Temperature
Transition metal oxidation state analysis by XPS has been
used in the study of corrosion, catalysis, lubrication and other industrial
applications. However, these studies are best performed on samples in
situ, since exposure to air will oxidize most of the surface iron atoms
to Fe (III).
1. M. Oku and K. Hirokawa, J. Appl. Phys. 50(10), 6303 (1979).
2. Iron (III) oxide, grade 99.9995+%, Alfa Aesar.
3. P.M.A. Sherwood, in Practical Surface Analysis, Second Edition, Volume 1-Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, UK (1990).
4. K. Rhodes, Ph.D. Thesis, Northwestern University, Evanston, Illinois (1990).