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TRANSCRIPT
ABC’s of
Electrochemistry:
X-Ray Photoelectron
Spectroscopy (XPS)
Madhivanan Muthuvel
Center for Electrochemical Engineering Research (CEER)
Chemical and Biomolecular Engineering
Ohio University
Athens, Ohio
November 17, 2011
2
Outline
• What is XPS?
• Background
• Principle
• Instrumentation
• Analysis of XPS Data
• Applications
• Facility at Ohio University
• Summary
Center for Electrochemical Engineering Research, Ohio University
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What is XPS?
• X-ray photoelectron spectroscopy (XPS) is also
known as ‘Electron Spectroscopy for Chemical
Analysis’ (ESCA)
• XPS is a surface analytical technique
• Widely used to determine the chemical information
in addition to elemental information of the samples
• Related techniques are Auger electron
spectroscopy (AES) and Ultra-violet photoelectron
spectroscopy (UPS)
Center for Electrochemical Engineering Research, Ohio University
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• In 1887, Heinrich Hertz observed the photoelectric effect
• In 1905, Albert Einstein explained the photoelectric effect with a simple mathematical description, which lead to Nobel Prize in Physics
Background
Center for Electrochemical Engineering Research, Ohio University
k bE = - Eh
Ek and Eb is the kinetic energy and binding energy of the photoelectron,
respectively, and hν is the energy of the incident beam
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• Prof. Kai Siegbahn from the University of Uppsala, Sweden, utilized the photoelectric effect to develop an analytical technique
• During the mid-1960’s, Prof. Kai Siegbahn and his co-workers developed the analytical technique known as X-ray photoelectron spectroscopy (XPS)
• He coined the term Electron spectroscopy for chemical analysis (ESCA)
• In 1981, Prof. Kai Siegbahn was awarded the Nobel Prize in Physics for the development of the XPS technique
Background
Center for Electrochemical Engineering Research, Ohio University
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Principle
Center for Electrochemical Engineering Research, Ohio University
k b sampleE = E h
Conducting Sample
Φsample is the work
function of the sample
Work function is the
energy difference
between Fermi level
and Vacuum level
University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/XPS Class 99.pps]
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Principle
Center for Electrochemical Engineering Research, Ohio University
hv
E1s
Sample Spectrometer
e-
Free Electron Energy
Fermi Level, Ef
Vacuum Level, Ev
sample
Ek (1s) Ek (1s)
spec
Eb (1s)
Analysis of sample with the XPS instrument
k b specE = E h
University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/XPS Class 99.pps]
8 Center for Electrochemical Engineering Research, Ohio University
Instrumentation
Kratos Axis Ultra model in Surface Science Western Laboratory
at The University of Western Ontario
Photo of a XPS Instrument
9 Center for Electrochemical Engineering Research, Ohio University
Instrumentation
5 4 . 7
X-ray
Source
Electron
Optics
Hemispherical Energy Analyzer
Position Sensitive
Detector (PSD)
Magnetic Shield Outer Sphere
Inner Sphere
Sample
Computer
System
Analyzer Control
Multi-Channel Plate
Electron Multiplier
Resistive Anode
Encoder
Lenses for Energy
Adjustment
(Retardation)
Lenses for Analysis
Area Definition
Position Computer
Position Address
Converter
Schematic for a XPS instrument
University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/XPS Class 99.pps]
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Instrumentation
• Typical pressure: 10-9 – 10-11 torr
• Reason to have UHV condition
– Maintain sample surface integrity
– Minimize scattering of the photoelectrons
– Maximize mean free path of the photoelectrons
– Helpful to use tungsten filament or other
electron source in the X-ray source cathode
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Ultra high vacuum (UHV) chamber
11 Center for Electrochemical Engineering Research, Ohio University
Instrumentation
Ultra high vacuum (UHV) chamber
XPS instrument with facility to perform electrochemical experiments
C. J. Corcoran, H. Tavassol, M. A. Rigsby, P. S. Bagus, and A. Wieckowski, Journal of Power Sources 195 (2010) 7856-7879.
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Instrumentation
• Energy of the x-ray beam
depends on the anode material
in the x-ray source
• High intensity x-ray beam with a
narrow line width gives best
spectroscopic result
• Commonly Mg Kα (1253.6 eV)
and Al Kα (1486.6 eV) are used
• Dual-anode x-ray source
(Al/Mg, Mg/Zr, and Al/Zr) are
also used
• X-ray beam lines from
synchrotron facility can be used
for XPS analysis
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X-ray source
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Instrumentation
• Diameter of X-ray beam ranges
from 5 mm to 1-5 µm
• X-ray penetration depth ~ 1 µm
• Sampling depth depends on
wavelength of the x-ray beam
and sample material
• For Al Kα, sampling depth is
generally 10 nm and 10 atomic
layers for heavier elements
Center for Electrochemical Engineering Research, Ohio University
X-ray source
D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York, 2006.
14 Center for Electrochemical Engineering Research, Ohio University
Instrumentation
Electron energy analyzer
Slit
Detector
Electron Pathway through the CMA
0 V
+V
0 V 0 V
0 V
+V
+V
+V
X-RaysSource
SampleHolder
Cylindrical mirror analyzer (CMA)
Used in XPS and AES instruments
Concentric
hemispherical
analyzer (CHA)
University of Western Ontario [mmrc.caltech.edu/SS_XPS/XPS_PPT/XPS_Slides.pdf]
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Instrumentation
• Sample size depends on the instrument
• Evans Analytical Group (EAG) can handle samples up to 8” in
diameter and thickness till 1”
• Generally, sample’s lateral size cannot exceed 1” and
thickness within 0.5”
• Any solid sample (conducting and non-conducting) can be
analyzed
• Sample has to be compatible in the ultra high vacuum (10-9
torr) condition
• Sample preparation
– Degrease before loading in the holder
– Use conductive tape for attachment
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Samples for the XPS analysis
16 Center for Electrochemical Engineering Research, Ohio University
Analysis of XPS Data
Example of XP spectrum
XPS Spectra, CasaXPS www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf
17 Center for Electrochemical Engineering Research, Ohio University
Analysis of XPS Data
Identify Auger peaks in XP spectrum
Cu XP spectra illustrating the shift in auger peak positions with the change from Mg
to Al anodes in the X-ray source
D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York, 2006.
18 Center for Electrochemical Engineering Research, Ohio University
Analysis of XPS Data
Peak quantification in XP spectrum
XPS Spectra, CasaXPS www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf
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Chemical Effects in XPS
Center for Electrochemical Engineering Research, Ohio University
Chemical shift:
change in binding energy of a core electron of an element
due to a change in the chemical bonding of that element
Withdrawal of valence electron charge
Addition of valence electron charge
increase in
Binding energy
decrease in
Binding energy
20 Center for Electrochemical Engineering Research, Ohio University
Chemical Effects in XPS
Charges are withdrawn from Ti to
form Ti4+, which results in higher
Binding energy for the Ti 2p orbitals
Chemical shift information very
powerful tool for functional group,
chemical environment, and
oxidation state
University of Western Ontario [mmrc.caltech.edu/SS_XPS/XPS_PPT/XPS_Slides.pdf]
21 Center for Electrochemical Engineering Research, Ohio University
Chemical Effects in XPS
Chemical shift for Gold (Au) 4f7/2 peak
University of Western Ontario [mmrc.caltech.edu/SS_XPS/XPS_PPT/XPS_Slides.pdf]
22 Center for Electrochemical Engineering Research, Ohio University
Chemical Effects in XPS
Curve fitting for Carbon 1s peak
C 1s region XP spectrum for polymethylmethacrylate (PMMA)
D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York, 2006.
23 Center for Electrochemical Engineering Research, Ohio University
Depth Profile
Examples of XPS spectrum
Ar+ sputtering of the sample results in layer-by-layer
removal of the sample using Ion gun
XP spectrum of the sample surface was collected after
each step of Ar+ sputtering XPS Spectra, CasaXPS www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf
24 Center for Electrochemical Engineering Research, Ohio University
Depth Profile
Multi layer SiO / TiO2 sample
The set of O 1s spectra measured during a depth profiling experiment
XPS Spectra, CasaXPS www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf
25 Center for Electrochemical Engineering Research, Ohio University
Depth Profile
Architectural Glass Coating sample
Atomic concentration for the elements found in the Architectural Glass
Coating sample from depth profiling experiment
University of Western Ontario [mmrc.caltech.edu/SS_XPS/XPS_PPT/XPS_Slides.pdf]
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Strengths of X-ray Photoelectron Spectroscopy
Center for Electrochemical Engineering Research, Ohio University
• Surface sensitive technique (top 10 nm)
• Chemical state identification on surfaces
• Identification of all elements except for H and He
• Quantitative analysis, including chemical state
differences
• Applicable for a wide variety of materials, including non
conducting samples (paper, plastics, and glass)
• Depth profiling with matrix-level concentrations
• Oxide thickness measurements
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Limitations for X-ray Photoelectron Spectroscopy
Center for Electrochemical Engineering Research, Ohio University
• Detection limits typically ~ 0.1% atomic
• Smallest analytical area ~ 10 µm diameter
• Limited organic information (short-range bonding only)
• Samples must be ultra high vacuum compatible
• Samples that decompose under X-ray irradiation
cannot be studied
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Applications
• Analyzing the composition of powders and debris
• Determining contaminant sources
• Examining polymer functionality before and after processing
• Bonding and adhesion issues
• Obtaining depth profiles of thin film stacks (both conducting and
non-conducting) for matrix level constituents
• Identifying stains and discolorations
• Characterizing cleaning processes
• Assessing the differences in oxide thickness between samples
Center for Electrochemical Engineering Research, Ohio University
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Applications
• Aerospace
• Automotive
• Biomedical /
Biotechnology
• Data Storage
• Defense
• Displays
• Electronics
• Lighting
• Pharmaceutical
• Photonics
• Polymer
• Semiconductor
• Solar Photovoltaics
• Telecommunications
Center for Electrochemical Engineering Research, Ohio University
Industries using XPS technique
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Analysis of Pigment from Mummy Artwork
Applications
Center for Electrochemical Engineering Research, Ohio University
150 145 140 135 130
Binding Energy (eV)
PbO2
Pb3O4
500 400 300 200 100 0 Binding Energy (eV)
O
P
b
Pb
Pb
N
Ca
C
Na
Cl
XPS analysis showed
that the pigment used
on the mummy
wrapping was Pb3O4
rather than Fe2O3
Egyptian Mummy
2nd Century AD
World Heritage Museum
University of Illinois
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Analysis of Carbon Fiber – Polymer Composite material
Applications
Center for Electrochemical Engineering Research, Ohio University
Woven carbon fiber
composite
XPS analysis identifies the functional groups present on composite surface.
Chemical nature of fiber-polymer interface will influence its properties.
-C-C-
-C-O
-C=O
-300 -295 -290 -285 -280
Binding energy (eV)
N(E
)/E
University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-ppt/XPS Class 99.pps]
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Analysis of Nanoparticle catalysts used in DMFC
Applications
Center for Electrochemical Engineering Research, Ohio University
Nanoparticles were less than 4 nm in size.
(a) Pt/Ni (1:1), (b) Pt/Ni (3:1), (c) Pt/Ru/Ni (5:4:1),
and (d) Pt/Ru (1:1).
The dotted line is the Pt 4f7/2 peak position for
pure Pt. The peaks were shifted from 0.17 eV for
(c) Pt/Ru/Ni (5:4:1) to 0.35 eV for (a) Pt/Ni (1:1)
and 0.36 eV for (b) Pt/Ni (3:1).
Metallic Ni content in catalyst (a) is 11.8%,
catalyst (b) is 33.7%, and catalyst (c) is 14.4%.
These shifts were interpreted to result from
modification of the Pt electronic structure
by electron transfer from Ni to Pt.
C. J. Corcoran, H. Tavassol, M. A. Rigsby, P. S. Bagus, and A. Wieckowski, Journal of Power Sources 195 (2010) 7856-7879.
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Facility at Ohio University
Center for Electrochemical Engineering Research, Ohio University
XPS instrument is located in the
W. M. Keck Thin Film Analysis Facility
John E. Edwards Accelerator Laboratory
(across Clippinger building)
4.5 MV Tandem Accelerator
Instrument and Location
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Facility at Ohio University
Center for Electrochemical Engineering Research, Ohio University
At John E. Edwards Accelerator Laboratory
Prof. David C. Ingram
Accelerator Lab Chairman
At Center for Electrochemical Engineering Research (CEER)
Madhivanan Muthuvel Ph.D.
John Goettge
Contact Personnel
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XPS analysis of Pt/Ir catalyst developed at CEER
Facility at Ohio University
Center for Electrochemical Engineering Research, Ohio University
Dr. Madhi' samples. April 27th 2007
XPS Results: No sputtering
Pt Sample C only Sample PtIr1 sample PtIr2 sample PtIr3 sample PtIr4 sample PtIr5 sample
April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007 April 27th 2007
Mass Mass Mass Mass Mass Mass Mass
Component Conc % Conc % Conc % Conc % Conc % Conc % Conc %
Iridium(Ir) 0 0 24.76 14.64 12.83 15.61 31.20
Platinum (Pt) 94.44 0 70.04 76.95 80.59 75.57 64.60
Carbon ( C ) 5.56 100 5.21 8.40 6.58 8.83 4.20
Total 100.00 100.00 100.01 99.99 100.00 100.01 100.00
Platinum (Pt) Iridium (Ir)
4d5/2 314.61 eV 296.31 eV
4f7/2 71.12 eV 60.84 eV
Pt-Ir sample Binding energies for Pt and Ir
element (NIST database)
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Summary
• X-ray photoelectron spectroscopy (XPS) is a surface
analytical technique
• This technique is used to identify elemental and chemical
information on the surface (~ 10 nm) of the sample
• The strengths of the XPS technique is extensively used by
various industries for there research and development
Center for Electrochemical Engineering Research, Ohio University
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References
Power point presentations
• X-ray Photoelectron Spectroscopy (XPS), Center for Microanalysis of Materials,
University of Illinois at Urbana-Champaign [groups.mrl.illinois.edu/nuzzo/0-
ppt/XPS Class 99.pps]
• X-ray Photoelectron Spectroscopy, R. Smart et. al., Surface Science Western,
University of Western Ontario
[mmrc.caltech.edu/SS_XPS/XPS_PPT/XPS_Slides.pdf]
• X-ray Photoelectron Spectroscopy, D. Torres, University of Texas at El Paso
[nanohub.org/resources/2011/download/x-ray photoelectron spectroscopy
(xps).ppt]
Journal
• C. J. Corcoran, H. Tavassol, M. A. Rigsby, P. S. Bagus, and A. Wieckowski,
Journal of Power Sources 195 (2010) 7856-7879.
Center for Electrochemical Engineering Research, Ohio University
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References
Web sites
• XPS Spectra, CasaXPS
www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf
• X-ray Photoelectron Spectroscopy (XPS), Evans Analytical Group (EAG)
www.eaglabs.com/techniques/analytical_techniques/xps_esca.php
• John E. Edwards Accelerator Laboratory, Ohio University
edwards1.phy.ohiou.edu/~oual/
Books
• D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York,
2006.
• F. A. Settle, Handbook of Instrumental Techniques for Analytical Chemistry,
Prentice-Hall, New Jersey, 1997.
Center for Electrochemical Engineering Research, Ohio University
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Further Reading
Center for Electrochemical Engineering Research, Ohio University
Web site
• X-ray Photoelectron Spectroscopy (XPS) Reference Pages
xpsfitting.blogspot.com
Books
• A. T. Hubbard, The Handbook of Surface Imaging and Visualization, CRC Press,
Boca Raton, Florida, 1995.
• T. L. Barr, Modern ESCA: The principles and practice of X-ray Photoelectron
Spectroscopy, CRC Press, Boca Raton, Florida, 1994.
• J. Chastain, Handbook of X-ray Photoelectron Spectroscopy: A Reference Book
of Standard Spectra for Identification and Interpretation of XPS data, Perkin
Elmer Corporation, 1992.