PhotoelectronSpectroscopy

Modern Methods inHeterogeneous Catalysis

Research

Axel Knop (knop@fhi-berlin.mpg.de)

OutlinePhotoelectron Spectroscopy: General Principle

Surface sensitivity

Instrumentation

Background subtraction

PES peaks, loss features

Peak fitting

Chemical state

Binding energy calibration

High pressure XPS

XPS at the liquid-solid interface

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 andX-Ray Fluorescence

Binding Energy out of:

hν = Evsp + Ekin

hν = Ekin + Evsa + Φsp - Φsa

hν = Ekin + EF + Φsp

Evsa

Evsp

X-ray twin anode

Parallel plate mirror analyser

HalbkugelanalysatorElektronen werden zu-

nächst auf einen bestimmtenEnergiebetragabgebremst

Nur Elektronen mit be-stimmter "Passenergie"

können den Detektorerreichen

Je kleiner die "Passener-gie" desto besser die

Energieauflösung

Detector: Channeltron

XP spectra: two different anode materials

Where do the electrons come from?

Distance electron can travelin solids depends on:• Material• Electron kinetic energy

à Measure attenuation ofelectrons by covering surfacewith known thickness ofelement

Sampling Depth

Sampling depth:For normal takeoff angle, cosQ=1:

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

Disregarding elastic scattering:

Mean Free Path of Electrons in Solids

• IMFP is average distance between inelastic collisions• Minimum l of 5-10Å for KE ~ 50-100 eVà Maximum surface sensitivity

Origin of background

Background of scattered electrons due to limited IMFPà Higher for low KE

Background Correction

Background substraction

à Choose suitable energy range for substractionà Baseline on high KE side

Background Correction

Several ways to substract background:

- Stepwise- Linear- Method of Tougaard

àMost 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 splittedà singlet in XPS• p, d, f.. Orbitals are spin-orbit splittedà 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 hn

Spectral features: Loss Peaks

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

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

à 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)à 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 willmove in this case!)

à Look for reference peaks with known BE (be sure about that!!!)àWorks 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 (puremetals, 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´´; VCHVerlag, 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 toexcitating 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 Peakshapes modeled byDoniach-Sunjicfunctions (convolutedwith Gaussian profiles)

Most fitting programs provide useful tools:

Levenberg-Marquardtalgorithm to minimizethe c2

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

b: peak parameterM: mixing ratioa: asymmetry parameterh: peak heightaà 0: Lorentzian profile

Convolution ofLorentz (or D.S.)Gaussian profilessuited best!!

à Voigt function

Strategy:

• Use of references forasymmetry andbroadening

• Reasoning of mostlikely chemicalcomposition

Analyzing the data: Peak Fitting

Peak broadening andasymmetry should bethe same for samecomponent in differentspectra

pure Au

graphitic

CNT

disordered

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

Quantitative Analysis

Me0 Me2+

Quantitative Analysis: First Steps

In general:

IX = B ´ s ´ ltot ´ T ´ nX

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

s= stot ´ f(X,a)

with f(X,a) = 1+(b(X)/4) (1-3cos2a)*stot: total ionisation cross sectionf: form function accounting for asymmetry of peakb: asymmetry parametera: 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

ltot = E(X) ´ 1/a[ln(E(X)-b)*

E: KE of electrona, b: parameters dependent on dielectric function andconcentration 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

à Need of models to extract an accurate approximation ofcomposition out of spectral intensities

RuO2

RuxOy

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

Quantitative Analysis: Useful Examples

with the formulasabove follows

andaA: „radius of A“

à 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 andB follows

QA: coverageof A on B

Q: angle betweensurface normal andemitted 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

è 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: obstacles

30x10-21

25

20

15

10

5

0Io

niza

tion

cros

sse

ctio

n(m

2 )

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 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 designs

In 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

Close-up of sample-first aperture region

gas

z

d

hn

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 system

Experimental cellsupplied by gaslines (p0)

X-rays enter the cell at55° incidence through anSiNx window(thickness ~ 1000 Å)

Analyzerinput lens

Focal pointof analyzerinput lens

First differentialpumping stage (10-4p0)

Second differentialpumping stage (10-6p0)

Third differentialpumping stage (10-8p0)

Hemisphercalelectronanalyzer(10-9p0)

mass spectrometerand additionalpumping

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 Cu

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

Temperature / °C

Yiel

d

Y CH2O

Y CO2

CO + H O2 2

CH O+ H2 2O

O+ O

CH OH3

CH OH3

CH OH3

CO+ H2

CH O+ H2 2

O2

Cu 0

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 methanol

Experimental 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

D ~ 2 eV

160

140

120

100

80

60

40

20

0

Nor

mal

ized

coun

trat

e(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 spectra

400 °C, 0.5 torr

CH3OH:O2

1:2

3:1

6:1

O 1s

O1s depth profiling

1.3

1.2

1.1

1.0

0.9

0.8

0.7529.

9eV

/531

.5eV

peak

area

ratio

14121086Inelastic mean free path (Å)

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

Dz = 3 Ǻ, n529.9/n531.5 = 1.6

14

12

10

8

6

Inel

astic

mea

nfre

epa

thin

Cu

(Å)

800600400200Kinetic energy (eV)

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

CH3OH : O2 = 3:1

Nor

mal

ized

coun

trat

e(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 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

coun

trat

e(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

Metastability of the Sub-Surface 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 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

2Opa

rtial

pres

sure

(mba

r) 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 profiling

0.6

0.5

0.4

0.3

0.2

0.1

0.0sub-

surfa

ceox

ygen

:Cu

peak

ratio

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?

ElectrochemistryEnergy Conversion

■ WasserspaltungØ PtØ IrOx

Ø Co/CØ Mg/CØ Etc…

■ KathodischeCO2-ReduktionØ CuOx

Spectroscopy in liquid

Dip &Pull

New design: HPXPS cell

Membrane

e-

Photon in

ab

Membrane grid

Stability

BLG coverage rate > 99%Successful 08/2014: BLG covered >99%, 10-7mbar in H2O

e-

ReferenceCounterGraphene on Si3N4

Photon in Photon out

e-

X-ray in

Graphene membrane

EC module with the HPXPS cell

e-

Pd UHV: Transmission test

IrAu

Graphene

Si framee-

Pd

X-ray

Ir nano-particles on Graphene

KE 1000eV

Verena Pfeifer, JuanVélasco

Cell filled with DI-water

IrAu

Graphene

Si framee-

Pd

X-raye-

H2O

Verena Pfeifer, JuanVélasco

Cell filled with 10mM KOH

IrAu

Graphene

Si framee-

Pd

X-raye-

H2O (10 mM KOH)

K+

Verena Pfeifer, JuanVélasco

e-

X-ray in

Electroplating (4 mM CoSO4)

Metall electroplated (Co)

Co2p KE 660eV

Co L-edge

Juan Vélasco et al. Angew. Chemie 2015

Anchorage of Co on Graphene

Juan Vélasco et al. Angew. Chemie 2015

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