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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

3

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)


4

• 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


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

5

• 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


6

Principle


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]

7

Principle


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


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


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


10

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


Ultra high vacuum (UHV) chamber


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.

12

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


X-ray source

13

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


X-ray source

D. R. Vij, Handbook of Applied Solid State Spectroscopy, Springer, New York, 2006.


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]

15

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


Samples for the XPS analysis


Analysis of XPS Data

Example of XP spectrum

XPS Spectra, CasaXPS www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf

http://www.casaxps.com/help_manual/manual_updates/xps_spectra.pdf



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




Peak quantification in XP spectrum



19

Chemical Effects in XPS


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



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




Chemical shift for Gold (Au) 4f7/2 peak




Curve fitting for Carbon 1s peak

C 1s region XP spectrum for polymethylmethacrylate (PMMA)



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



Depth Profile

Multi layer SiO / TiO2 sample

The set of O 1s spectra measured during a depth profiling experiment




Depth Profile

Architectural Glass Coating sample

Atomic concentration for the elements found in the Architectural Glass

Coating sample from depth profiling experiment


26

Strengths of X-ray Photoelectron Spectroscopy


• 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

27

Limitations for X-ray Photoelectron Spectroscopy


• 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

28

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


29

Applications

• Aerospace

• Automotive

• Biomedical /

Biotechnology

• Data Storage

• Defense

• Displays

• Electronics

• Lighting

• Pharmaceutical

• Photonics

• Polymer

• Semiconductor

• Solar Photovoltaics

• Telecommunications


Industries using XPS technique

30

Analysis of Pigment from Mummy Artwork

Applications


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

31

Analysis of Carbon Fiber – Polymer Composite material

Applications


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


32

Analysis of Nanoparticle catalysts used in DMFC

Applications


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.

33

Facility at 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

34



At John E. Edwards Accelerator Laboratory

Prof. David C. Ingram

Accelerator Lab Chairman

[email protected]

At Center for Electrochemical Engineering Research (CEER)

Madhivanan Muthuvel Ph.D.

[email protected]

John Goettge

[email protected]

Contact Personnel

mailto:[email protected]



35

XPS analysis of Pt/Ir catalyst developed at CEER



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)

36

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


37

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.


38

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.



http://www.eaglabs.com/techniques/analytical_techniques/xps_esca.php

39

Further Reading


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.

Thank You

Questions?

Contact:

Madhivanan Muthuvel at

[email protected]

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