X-Ray Photoelectron Spectroscopy (XPS)
X-Ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical
analysis (ESCA) is a non-destructive technique used to analyze the surface of a material. The XPS will
measure the elemental composition, chemical state as well as the electronic state, thickness
measurements of overlayers (up to 8nm), and will give you the empirical formula of the material that is
being analyzed. This instrument will only detect elements with an atomic number higher of 3 and higher
since hydrogen and helium atoms are very small and the probability of detecting them is almost zero.
Also, it can only analyze depths ranging from 1 to 10nm, for this reason it only gives analysis of the
surface. Preparation of the samples is minimal if any; you can analyze samples “as receive” or can clean
the surface to eliminate any contaminates that might be present. Some examples that can be analyzed
using the XPS are elements, metal alloys, semiconductors polymers, ceramics, and inorganic
compounds. Other examples include paints, inks, viscous oils, wood and papers.
Physics of XPS
The XPS functions by irradiating a surface with a beam of x-rays which are usually
monochromatic Al Kα (1486.6eV) or non-monochromatic Mg Kα (1253.6eV) in an ultra-high vacuum.
When the x-ray photons hit the sample, they transfer this energy to core electrons and are emitted from
the initial state with a kinetic energy which is being measured (Figure 1). It will also count the number of
photoelectrons that are being ejected from the surface with the cylindrical mirror detector analyzer.
With this information you can obtain an XPS spectrum which plots the number of electrons detected vs.
the binding energy of the electrons detected (Figure 2). Since each element will produce a characteristic
peak at characteristic binding energies, the element at the surface can be identified and because the
number of electrons in each peak is directly related to the amount of the element, the elemental
composition within the area that is being analyzed can be calculated. There are tables with the kinetic
energies as well as binding energies already in the system that will help identify the elements present in
the surface of the material.
The binding energy of each emitted electron can be calculated using the equation below since
the energy of the x-rays being emitted is known.
Ebinding = Ephoton- (Ekinetic + Φ)
Ebinding is the binding energy of the electron, Ephoton is the energy of the x-ray photons, Ekinetic is
the kinetic energy that is measured by the XPS, and Φ is the work function of the spectrometer.
Free Electron
X-Ray
Core electron
Valence
electron
Figure 1. X-Ray hit the core electron and is then emitted with a certain amount of kinetic energy.
Figure 2. XPS, sample is being irradiated by x-rays which will then emit core electrons which are then
detected and data is collected to obtain a spectrum.
References
X-Ray Photoelectron Spectroscopy, In National Physical Laboratory, Retrieved October 29, 2012 form
http://www.npl.co.uk/science-technology/surface-and-nanoanalysis/surface-and-nanoanalysisbasics/introduction-to-xps-x-ray-photoelectron-spectroscopy
XPS Works, Actinide Research Quarterly, Retrieved October 29, 2012 form
http://arq.lanl.gov/source/orgs/nmt/nmtdo/AQarchive/04summer/XPS.html
X-Ray Photoelectron Spectroscopy, In Wikipedia, Retrieved October 29, 2012 from
http://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy
Torres, D., X-Ray Photoelectron Spectroscopy (XPS),The University of Texas at El Paso, Retrieved October
29, 2012
Example of XPS
Publication Title:
XPS depth profiling study on the passive oxide film of carbon steel in saturated
calcium hydroxide solution and the effect of the chloride on the film properties
P.Ghods, O.B. Isgor, J.R. Brown, F. Benseabaa, D. Kingston
Introduction
The purpose of the paper presented was to use XPS in order to characterize the passive oxide
layer that forms on carbon steel rebar in concrete pore solutions when it is passivated in calcium
hydroxide solutions. Since there is very few information on the compositional characteristics of the
passive oxide film before and after it has been exposed to this high alkaline environments, they decided
to use XPS since it will give the depth profiling of the surface.
Preparation of Specimens
An analysis was conducted on the cross-sections of four carbon steel rebar specimens, which
were 8-mm long each. The size designation for the rebar was #10M. The specimens where then hotmounted and polished to 0.05μm and used isopropyl alcohol in order to prevent oxidation. The epoxy
was then removed and three samples were submerged into saturate calcium hydroxide (CH) solution
(99.6% CH in distilled water). The first sample (CH-2) was taken out after 2 days and the second one, CH9, after 9 days. The third sample (CH-Cl) after 9 days, was then submerged in to a 0.05M chloride
solution for 14 more days. This was done in order for the chlorides to react with the passivation film.
The three specimens after they were taken out of the solution were placed in a jar containing isopropyl
alcohol until the use of XPS was needed. The final specimen, AE, was exposed to indoor air at room
temperature for 10 minutes to allow oxidation of the steel surface.
XPS Procedures
In this experiment, they used a PerkinElmer PHI-5700-2 XPS spectrometer that used an
achromatic Al Kα x-ray source. It contained an electronic ultra-high vacuum chamber with pressure of
10-6 Pa and was operated at 15kV. The work function was calibrated using ultra-pure gold metal. The
information was obtained by using a spherical capacitor analyzer, which was at an angle of 54ᵒ with the
x-ray source, and the x-ray source was at an angle of 90ᵒ with the specimen surface. The analyzed area
was 800μm.
In order to collect low-energy spectra, they did a survey scan which had the following
conditions: energy range=1400eV, analyzer pass energy= 187.8eV, step size= 0.25 and a sweep time of
180s. In order to obtain high-resolution spectra, they used 10 or 20eV spectral windows at an analyzer
pass energy of 29.3eV and 0.1eV steps. The spectra was for oxygen (O 1s), carbon (C1s), iron (Fe 2p),
chlorine (Cl 2P), calcium (Ca 2p), and sodium (Na 1s). They collected and processed all survey and highresolution spectra using PHI Access XPS operating software.
XPS Data Analysis
To figure out the sampling depth (d) which is the thickness of the layer, they used the equation
below,
d= 3λcosθ
where λ is the decreasing length and θ is the take –off angle with respect to the surface normal, which
would be zero for this case. To find the kinetic energy the equation below was used,
Ek=hv- Eb
Where Ek is the kinetic energy, h is Planck’s constant, v is frequency, and Eb is the binding energy. Table 2
below shows the calculation of the sampling depth for iron oxide. The average sampling depth was of
8.5nm.
Curve fitting had to be done to the high-resolution spectra in order to get the minimum number
of peaks that will result in an optimum fit. This was done using Casa XPS software and setting a few
constraints in order to get the optimal fitting. Constrains included, setting peak positions to the average
reported data in literature, peak positions were set constant for all depths, the full width at half
maximum (FHWM) were set to the FHWM of the photoelectron core level of each element, Sheirley
background corrections algorithms were used, peaks were calibrated to hydrocarbon signal set at
285eV, and semi-quantitative composition data was collected by using XPS elemental sensitivity factors.
The curves that were fitted were for Fe 2p, O 1s, C 1s, and Ca 2p spectra. The curve parameters used
are shown below in Table 3.
Results and Discussions
The XPS depth profiles for all four elements and for the four specimens analyzed are shown in
Fig. 5 below. The graph for iron shows that as depth increases, so does the concentration, as for the
oxygen curve, there will be an increase in concentration and around 2.5nm in depth it will decrease. In
the carbon curves, the concentration is high at the surface which is most likely due to contamination
during preparation, but then remains constant throughout the rest of the analysis. For the calcium
curves, most of the samples had a constant concentration as the depth increased except for the sample
which was only exposed to air which did not contain large amounts of calcium. The reason for small
amounts of calcium present in the AE specimen was because there were particles embedded on the
surface during polishing. As for the constant concentrations of calcium in the rest of the sample, SEM
and EDS was used and showed that it was due to CH and CaCl precipitates at the surface. It was
concluded that because the precipitates contained the same elements (C,O, Ca) that were analyzed by
the XPS, their spectra would not be used to study the atomic structure of the oxide film. Only the Fe 2p
spectrum would be used to characterize the oxide film.
Analyzing the Fe 2p spectra at different depths for the CH-2 sample, a few observations were
made. First, the five components were identified to be, iron (Fe), cementite (Fe3C), magnetite (Fe3O4),
hematite/ferrihydrite (Fe2O3/ FeOOH), and Fe2O3 satellite structure; their peak position were also
identified as seen in the image (Fig.8). Another observation made was that the Fe peak increased in
intensity with ion
sputtering which means
that the Fe component
comes from the substrate.
Also, the sputtering shifted
the Fe 2P signal to the left
which indicates that the
oxide film is thin no matter
what the exposure time
was.
The effect of
exposure conditions on the
thickness of the oxide film
was also analyzed. Results
showed that the AE specimen had a thicker iron oxide layer than the other samples and that the CH-Cl
spectrum was the one with the thinnest oxide layers (See spectra below). The reason for the AE sample
having a thicker layer can be due to porosity and also because the oxide layer of the CH specimens might
have dissolved in the solution. The conclusion made for the CH-Cl curve was that chloride reduces the
thickness of the oxide film. It was also concluded that since the spectrum for the different exposure
times were almost the same, the
exposure time does not affect
the thickness as much. The ratio
of the iron oxide to the metallic
iron concentration at different
sputtering depth was analyzed
and results showed that above a
sputtering depth of 5nm, the
ratio remained constant. This
meant that the iron oxide film
was about 5nm in thickness. To
confirm this, they used several
equations and the thickness of
the oxide layers to be 5.7, 4.1,
4.1, 3.6nm for AE, CH-2, CH-9,
CH-Cl respectively.
Other observations made where that the concentration of Fe3+ relative to Fe2+ decreased with
depth in the oxide film and that longer exposure times will increase the concentration of Fe2+ relative to
Fe3+.
Conclusion
In this experiment, they used the XPS depth profiling in order to characterize the oxide film of
carbon steel when it was saturated in calcium hydroxide (CH) solutions and also what the effect of
chloride (CH-Cl) could be on the film. Samples where carefully prepared and placed in CH and CH-Cl
solutions for different amounts of time. Then they were analyzed using the XPS.
After obtaining the spectra of the specimens studied and analyzing them, they were able to make
valuable conclusions. The first finding was that the carbon steel contained precipitates of calcium
hydroxide and calcium carbonate so several spectra were not used for analysis. The study was only done
for the Fe 2p spectrum. With the XPS depth profiles, they were able to determine the thickness of the
iron oxide film to be about 4nm. The spectra for the four different specimens studies also showed that
there was almost no difference between them meaning that there was no effect on exposure time to
the CH solutions. Analyzing the spectra also showed that the exposure to chloride reduced the thickness
of the oxide film. Another conclusion made was that there were higher concentrations of Fe2+ at the
substrate and at the surface it was mostly composite of Fe3+. The longer the specimens were exposed to
the CH solution, the larger the Fe2+ concentration. As seen with this experiment, the XPS is a valuable
instrument that can tell us a lot about a material.
References
Mark C. Biesinger, Brian R. Hart, Russell Polack, Brad A. Kobe, Roger St.C. Smart, Analysis of mineral
surface chemistry in flotation separation using imaging XPS, Minerals Engineering, Volume 20, Issue 2,
February 2007, Pages 152-162, ISSN 0892-6875, 10.1016/j.mineng.2006.08.006.
(http://www.sciencedirect.com/science/article/pii/S0892687506002093)