Analysis of photoelectron spectra

Modern Methods in Heterogeneous Catalysis

Research

Axel Knop

(knop@fhi-berlin.mpg.de)

OutlinePhotoelectron Spectroscopy: General Principle

Surface sensitivity

Instrumentation

Background subtraction

PES peaks, loss features

Binding energy calibration

Chemical state

Peak fitting

Quantitative analysis

High pressure XPS

examples

Problem: What is the (chemical) composition of a surface

Me0 Me2+

Photoelectron Spectroscopy: General Principle

Excitation of Photoelectron

General Principle

- Detection of photoelectrons fromthe valence band region and corelevels

- Detection of Auger electrons and X-Ray Fluorescence

X-ray twin anode

Parallel plate mirror analyser

Detector: Channeltron

Where do the electrons come from?

Distance electron can travelin solids depends on:

• Material• Electron kinetic energy

Measure attenuation of electrons by covering surfacewith known thickness of element

Sampling Depth

Sampling depth:

For normal takeoff angle, cosΘ=1:

• When d=λ: -ln(I/I0)=0.367, i.e. 63.3% from within λ• When d=2λ: -ln(I/I0)=0.136, i.e. 86.4% from within 2λ• When d=3λ: -ln(I/I0)=0.050, i.e. 95.0% from within 3λ

Disregarding elastic scattering:

Mean Free Path of Electrons in Solids

• IMFP is average distance between inelastic collisions• Minimum λ of 5-10Å for KE ~ 50-100 eV

Maximum surface sensitivity

Origin of background

Background of scattered electrons due to limited IMFPHigher for low KE

Background Correction

Background substraction

Choose suitable energy range for substractionBaseline on high KE side

Background Correction

Several ways to substract background:

- Stepwise- Linear- Method of Tougaard

Most common:

Method of Shirley et al.:

Always use same backgroundsubstraction method for all peaks!

D.A. Shirley, Phys. Rev. B, 5, S. 4709, 1972.

worst

best

Spectral features: PE Peaks

Orbital peaks used to identifydifferent elements in sample

Observation: • s orbitals are not spin-orbit splitted singlet in XPS• p, d, f.. Orbitals are spin-orbit splitted doublets in XPS

Spectral features: PE Peaks

Spectral features: Auger Peaks

Auger peaks:

Result from excess energy of atom during relaxation (after corehole creation)

• always accompany XPS• broader and more complex structure than PES peaks

KE energy independent of incident hν

Spectral features: Loss Peaks

After Emission of PE the remaining electronsrearrange (Secondary peaks):

• Relaxation by excitation of valence electron to higher state (shake up)

loss of kinetic energy of PE leads to new peaksshifted in BE with respect to main peak

• Relaxation by ionisation of valence electron(shake off)

broad shoulder to main peak

• Plasmon losses

Spectral features: Loss Peaks

Loss peaks can be usedto identify chemicalcomposition

Problem:References needed(theoretical orexperimental)

Relationship between the degree of core levelasymmetry and the density of states at the Fermi

level (BE=0)

Identification of FE:

Easy for high density of states near EF, butmay be ambigeous for samples with lowDOS near EF, band gap or charged samples

Analyzing the data: Calibration of Binding Energies

Line profile modification by charging

Mo oxide on silica

real catalyst is powdersample after impregnationand calcination.

Analyzing the data: Calibration of Binding Energies

Identification of FE:

In case of low DOS near EF, band gap or charged samples (spectrum will move in this case!)

Look for reference peaks with known BE (be sure about that!!!)Works for any PE or Auger peak, e.g. peaks originating from substrate

Chemical Shift

BE of core electrons depends on the electron density of theatom (screening of the core electrons), affected by theelectronegativity of neighboring atoms

For unknown BE:

• Calculation of Pauling´s Charge (easy, but not always true)• More elaborate calculations (theorists, also not always true)

References:

• Use of data bases (NIST, Handbook of PhotoelectronSpectroscopy etc.*)

• Clean materials (pure metals, well definedsubstrates, „untreated“samples etc.)

• Literature about similartopics

Analyzing the data: Chemical States

*„Handbook of Photoelectron Spectroscopy“, Perkin-Elmer, 1992G. Ertl, J. Küppers;´´Low Energy Electrons and Surface Chemistry´´; VCH Verlag, 1985S. Hüfner; ´´Photoelectronspectroscopy´´; Springer, 1995.D. Briggs, M.P. Seah;´´Practical Surface Analysis´´, Vol. 1; Wiley + Sons, 1990.

Analyzing the data: Peak Fitting

Several influences on peakshape:

• Broadening mainly due to excitating light (natural), structural and thermal effects

• Asymmetry due to final stateeffects

• Species with similar BE

These effects have to be considered when fitting by reasonablechemical/physical model of the sample!!!

Analyzing the data: Peak Fitting

Asymmetric Peak shapes modeled by Doniach-Sunjicfunctions (convoluted with Gaussian profiles)

Most fitting programs provide useful tools:

Levenberg-Marquardt algorithm to minimize the χ2

M: measured spectrumN: energy valuesS: synthesized spectrumP: parameter values

β: peak parameterM: mixing ratioα: asymmetry parameterh: peak heightα 0: Lorentzian profile

Convolution of Lorentz (or D.S.) Gaussian profiles suited best!!

Voigt function

Strategy:

• Use of references forasymmetry and broadening

• Reasoning of mostlikely chemicalcomposition

Analyzing the data: Peak Fitting

Peak broadening and asymmetry should bethe same for samecomponent in different spectra

pure Au

graphitic

CNT

disordered

But: both may differ for different componentsdiscrepancy for different peaks should always be based on physical reasons

Quantitative Analysis

Me0 Me2+

Quantitative Analysis: First Steps

In general:

IX = B × σ × λtot × T × nX

B: all instrumental contributionsσ: ionisation cross section for given photon energyλtot: total escape depthT: transmission through surfacenX: atomic density of analyzed species in sample

σ= σtot × f(X,α)

with f(X,α) = 1+(β(X)/4) (1-3cos2α)*σtot: total ionisation cross sectionf: form function accounting for asymmetry of peakβ: asymmetry parameterα: angle between photon beam and emitted electron(different for standard x-ray source and synchrotron)

*Yeh and Lindau, Atomic data nucl. data tables32(1985)1

λtot = E(X) × 1/a[ln(E(X)-b)*

E: KE of electrona, b: parameters dependent on dielectric function and concentration of host

*Penn, J.of Electr.Spectr. And Rel. Phen. 9(1976)29

Cross section changes with energyof incident beam!!!

Quantitative Analysis: Cross Section

Calculated changes of Cross section beam energy(Yeh, Lindau; Atomic data and nuclear data tables 32 (1985) 1)

Quantitative Analysis

In most cases the sample surface is quite heterogeneous

Need of models to extract an accurate approximation of composition out of spectral intensities

RuO2

RuxOy

Ru 3d5/2-map of the chemical states on Ru(0001) after pre-treatment with Operformed with Scanning Photoelectron Microscopy (ELETTRA):

Quantitative Analysis: Useful Examples

with the formulasabove follows

andaA: „radius of A“

Mol fraction

A. Heterogeneous mixture (e.g. alloy):

IA,B0: reference of

clean materialfollows

Quantitative Analysis: Useful Examples

B. Partial Coverage (e.g. adsorbat):

Contribution of B:(attenuated by A) (direct)

For signals of A and B follows

ΘA: coverageof A on B

Θ: angle betweensurface normal and emitted electron

Which givesIA,B

0 : referenceof clean material

Quantitative Analysis: Useful Examples

C. Thin layer of A on B (e.g. oxide):

Contribution of B:

Contribution of A:

Which gives

Special case:

i.e.

follows

General problem here: proper background substraction

Fundamental limit:elastic and inelasticscattering of electronsin the gas phase

Technical issues: - Differential pumping to keep analyzer in high vacuum- Sample preparation and control in a flow reactor

In situ XPS: obstaclesIn situ XPS: obstacles

30x10-21

25

20

15

10

5

0

Ioni

zatio

n cr

oss

sect

ion

(m2 )

101

102

103

104

105

106

Electron kinetic energy (eV)

in situSEM in situ

TEM

in situEXAFSin situ

XPS

O2 exp. data from Schram et al. (1965) extrapolation of Schram's data

• Photons enterthrough a window

• Electrons and a gasjet escape throughan aperture tovacuum

In situ XPS: basic conceptIn situ XPS: basic concept

H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 319 (1973)2

• H. Siegbahn et al. (1973- )• M.W. Roberts et al. (1979)• M. Faubel et al. (1987) • M. Grunze et al. (1988)• P. Oelhafen (1995)

H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. , 205 (1981)24

In situ XPS instruments: previous designsIn situ XPS instruments: previous designs

In situ XPS using differentially pumped electrostatic lensesIn situ XPS using differentially pumped electrostatic lenses

D.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, Rev. Sci. Instrum. 73 (2002) 3872.

to pump to pump

X-rays from synchrotron

Gas phase composition canbe measured by XPS.gas phase signal: 1 torr·mm ~ a few monolayers

CloseClose--up of sampleup of sample--first aperture regionfirst aperture region

gas

z

d

hν

e-

p0

1.0

0.5

0.0-2 -1 0 1 2

z [d]

p/p 0

z=2 mm

d=1 mm

In situ XPS systemIn situ XPS system

Experimental cellsupplied by gas lines (p0)

X-rays enter the cell at 55° incidence through an SiNx window(thickness ~ 1000 Å)

Analyzerinput lens

Focal point of analyzerinput lens

First differentialpumping stage (10-4p0)

Second differentialpumping stage (10-6p0)

Third differentialpumping stage (10-8p0)

Hemisphercalelectronanalyzer(10-9p0)

mass spectrometerand additional pumping

Partial oxidation: CH3OH + ½O2 CH2O + H2O

Total oxidation: CH3OH + O2 CO2 + 2H2O

Cu

Cu32

What is the state of the surface under reaction conditions?

Application of in situ XPS to catalysis: methanol oxidation on CApplication of in situ XPS to catalysis: methanol oxidation on Cuu

Reaction profile isidentical at 1 atm.

Onset of catalyticactivity at T>200 °C

-0,1

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

0 50 100 150 200 250 300 350 400 450

Y CH2O

Y CO2

Temperature / °C

Yie

ld

CO +H O2 2

CH O+H2 2O

O+ O

CH OH3

CH OH3

CH OH3

CO+H2

CH O+H2 2

O2

Cu0

Oxsurf

Oxbulk

Subox

Ovol

A. Knop-Gericke et al., Topics Catal. 15, 27 (2001).

In situ NEXAFS

I.E. Wachs & R.J. Madix, Surf. Sci. 76, 531 (1978); A. F. Carley et al., Catal. Lett. 37, 79 (1996).UHV XPS

O-(a) + 2CH3OH(g) 2CH3O-(a) + H2O(g)

CH3O-(a) CH2O(g) + H+(a)

Questions for in situ XPS: - Quantitative analysis of surface species- Carbon species on the surface- Depth-dependent analysis

CH3OH + O2 ~ 0.5 mbar

suboxide phase: - only present in situ

Partial oxidation of methanolPartial oxidation of methanol

Experimental conditionsExperimental conditions

sample: polycrystalline Cu foil

Variations of mixing ratios: CH3OH : O2 = 1:2, 3:1, 6:1; T = 400 °C; p = 0.6 mbarTemperature series: gas mixture at room temperature: CH3OH : O2 = 3:1;

p = 0.6 mbar; temperature: 25 °C → 450 °Cflow rates: 10 ... 20 sccm

XPS measurements Beam line U49/2-PGM1 at BessyEnergy range 100...1500 eVtotal spectral resolution 0.1 eV @ 500 eV

O 1s, C 1s, Cu 3p, Cu 2p: KE ~ 180 eVValence Band: KE ~ 260 eV

Depth profiling with KEs 180 eV, 350 eV,500 eV, 750 eV

8 6 4 2 0 -2Binding energy (eV)

VB

Δ ~ 2 eV

160

140

120

100

80

60

40

20

0

Nor

mal

ized

cou

nt ra

te (c

ps/m

A)

542 540 538 536 534 532 530 528 526

Binding energy (eV)

CH

3OH

(g)

CH

2O (g

)H

2O (g

)

CO

2(g

)

O2 (g) Cu2O

?

?

Methanol oxidation on Cu: O1s spectraMethanol oxidation on Cu: O1s spectra

400 °C, 0.5 torr

CH3OH:O2

1:2

3:1

6:1

O 1s

O1s depth profilingO1s depth profiling

1.3

1.2

1.1

1.0

0.9

0.8

0.7529.

9 eV

/ 53

1.5

eV p

eak

area

ratio

14121086Inelastic mean free path (Å)

I529.9/I531.5=n529.9/n531.5·exp[-(z531.5-z529.9)/λ]

Δz = 3 Ǻ, n529.9/n531.5 = 1.6

14

12

10

8

6

Inel

astic

mea

n fre

e pa

th in

Cu

(Å)

800600400200Kinetic energy (eV)

from: Tanuma at al., SIA 17, 911 (1991).

CH3OH : O2 = 3:1

Nor

mal

ized

cou

nt ra

te (a

.u.)

534 532 530 528 526Binding energy (eV)

sub-surfaceoxygen

surfaceoxygen

A

180 eV

350 eV

500 eV

750 eV

Methanol oxidation on Cu: C1s spectraMethanol oxidation on Cu: C1s spectra

292 288 284 280Binding energy (eV)

CO

2(g

)

CH

3OH

(g)

CH

2O (g

)

Camorph

C 1s160

140

120

100

80

60

40

20

0

Nor

mal

ized

cou

nt ra

te (c

ps/m

A)

542 540 538 536 534 532 530 528 526

Binding energy (eV)

CH

3OH

(g)

CH

2O (g

)H

2O (g

)

CO

2(g

)

O2 (g) Cu2Osub-surface

oxygensurfaceoxygen

400 °C

CH3OH:O2

1:2

3:1

6:1

O 1s

MetastabilityMetastability of the Subof the Sub--Surface OxygenSurface Oxygen

536 534 532 530 528

Binding energy (eV)

CH

3OH

(g) sub-surface

oxygensurfaceoxygen

H2O

(g)

CH

2O (g

)

CH3OH:O2=6:1

CH3OH only

difference

536 534 532 530 528

Binding energy (eV)

CH

3OH

(g) sub-surface

oxygensurfaceoxygen

H2O

(g)

CH

2O (g

)

CH3OH:O2=6:1

CH3OH only

difference

Correlation of catalytic activity and surface speciesCorrelation of catalytic activity and surface species

CH2O yield vs sub-surface oxygen peak area

0.20

0.15

0.10

0.05

0.00

543210

normalized sub-surface oxygen peak area

1:2

3:1

6:1

CH

2O p

artia

l pre

ssur

e(m

bar) mixing ratio series

(T = 400 °C)

250 °C

300 °C

400 °C

450 °C

400 °C

150 °C

200 °C

temperature series(CH3OH:O2=3:1)

Open questions: What is the nature of thesub-surface oxygenspecies?What is its role in thecatalytic reaction?

Depth profilingDepth profiling

0.6

0.5

0.4

0.3

0.2

0.1

0.0sub-

surfa

ce o

xyge

n : C

u pe

ak ra

tio

14121086Inelastic mean free path (Å)

CH3OH:O2 = 3:1CH3OH:O2 = 6:1

(calculated from Cu 3p and sub-surface O 1s)

Reducing conditions

Open questions: What is the nature of the sub-surface oxygen species?What is its role in the catalytic reaction?

52

IntroductionIntroduction

Literature carbon laydown selective hydrogenation“similar” catalysts different activity & selectivity

(structure sensitivity?)

Selectivity issue: what defines selectivity?

[ ]+ H2

InIn--situ XPS: situ XPS: Pd 3d (hPd 3d (hνν: : 720 eV)720 eV)

InIn--situ XPS: situ XPS: Pd 3d depth profilingPd 3d depth profiling

Not only adsorbate-induced surface core level

shift !

But on-top location!

InIn--situ XPS: situ XPS: C1s (Switching off experiments)C1s (Switching off experiments)

1: reaction mix.

2: C5 off

3: H2 off

4: C5 gas-phase

InIn--situ XPS: situ XPS: Pd 3d (Switching off experiments)Pd 3d (Switching off experiments)

1: reaction mix.

2: C5 off

3: H2 off

InIn--situ XPS: situ XPS: Pd vs. C depth profilingPd vs. C depth profiling

100 200 300 400 500 600 700 80020

25

30

35

40

45

50

55

60

65

70A

Palladium Inelastic Mean Free Path [Å]

Car

bon

Con

tent

[%]

Electron Kinetic Energy [eV]

Experiment: 358 K Model: 358 K Model: 3.5 ML Experiment: C5 off Model: C5 off

5.25 7.15 9.05 15.15

20

25

30

35

40

45

50

55

60

65

70

75100 200 300 400 500 600 700 800

22.3513.2510.25

Electron Kinetic Energy [eV]

Car

bon

cont

ent [

%]

Carbon Inelastic Mean Free Path [Å]

Epxeriment: 523K Model: 523K Model: 4.2 ML

7.25

B

0.3 nm

1.4 nm

14 nm

ModelModel

core statesatom specificquantitativecomplex final state effectschemical shift concepttheoretically difficult accessiblecan be applied in the mbar rangesurface sensitivedepth profiling

Summary

1. W. Göpel, Chr. Ziegler: Struktur der Materie: Grundlagen, Mikroskopie undSpektroskopie, Teubner Verlagsgesellschaft, Stuttgart-Leipzig, 19912. M. Henzler, W. Göpel: Oberflächenphysik des Festkörpers, TeubnerVerlagsgesellschaft, Stuttgart-Leipzig, 19913. W. Göpel, Chr. Ziegler : Einführung in die Materialwissenschaften, TeubnerVerlagsgesellschaft, Stuttgart-Leipzig, 19964. D. Briggs, M. P. Seah: Practical Surface Analysis, Volume 1: Auger and X-RayPhotoelectron Spectroscopy, 2. Auflage, John Wiley & Sons, Chichester, 19905. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder, G. E. Muilenberg: Handbookof X-Ray Photoelectron Spectroscopy, Physical Electronics Division, Perkin-ElmerCorporation, Eden Prairie, Minnesota, 19796. H. Lüth: Surfaces and Interfaces of Solid Materials, 3. Auflage, Springer Verlag,Berlin, 19957. G. Ertl, J. Küppers: Low Energy Electrons and Surface Chemistry, VCHVerlagsgesellschaft, Weinheim, 19858. K. Siegbahn, C. Nording et al.: ESCA Applied to Free Molecules, North-Holland,Amsterdam, 19719. M. Cardona, L. Ley: Photoemission of Solids, Topics in Applied Physics, Band 26und 27, Springer Berlin, 197810. M. Grasserbauer, H. J. Dudek, M. F. Ebel: Angewandte Oberflächenanalyse mitSIMS, AES und XPS, Springer Berlin, 197911. D. Briggs and J. T. Grant: Surface Analysis by Auger and PhotoelectronSpectroscopy, Surface Spectra and IM Publications 2003

Literature