Photoelectron Spectroscopies 1 Preparation Strategies Preparation of heterogeneous catalysts

Photoelectron Spectroscopies 1

Preparation Strategies

Preparation of heterogeneous catalysts



Energy ranges


The Photoemission Process

0

Energy

Ef

EB

work function

kinetic energy

initial photon energybinding energy


Photemission - classical

• The emitting atom remains unaffected by photoemision (photocation!).

• The photoelectron is a single particle unaffected by core- and valence electrons (surface sensitive?).

• There is infinite lifetime of the core hole, no coupling to other bound states, no relaxation and electrons behave as particles.


Photoemission – quantum mechanical

• Photoemission as superposition of wave functions (photon-ground state; photoelectron-virtual bound state (Rydberg state); virtual bound state-continuum state (free electron in box): three phase model

• Finite lifetime, relaxation by shrinking of bound states, by secondary emission (Auger, shake-off), by bond shortening and LMCT


Ry

MO

Photoemission – quantum mechanical


The QM picture of photoemission

Free atom case and case of a solid with a band structure: note the difference in „Rydberg“ states.


ARUPS: Seeing orbitals

The angular resolution of the intensities allow to separate the s-states from the p-states in the sp3 bonding in Si


The Photoemission ProcessSudden Approximation: well defined initial and final states (lines in scheme); no wave nature of photons and electrons.

High energy spectroscopy without relaxation of system (atom plus first coordination).

Ebind = Ephoton – Ekin – Ework function

oversimplified: relaxation in energy and in geometry of system causes multiple complications (information)


Towards a picture of photoemission


Koopman´s Theorem, RelaxationThe „binding energy“ form photemission equals the inverse of the dissociation energy from the ground state (theoretically accesible).

Ekoop = Eadiab + Erelax

In reality there is a relaxation process due to:

geometry change (Frank Condon effect)

nuclear charge change (photocation)

vibrational excitation

multiple exctitations inside (shakeup) or to the environement of the cation(shake off, LMCT)


Koopman´s Theorem, Relaxation

Ekoop = Eadiab + Eshakeoff

Example: Ne 1s:

E adiab = 892 eV

E koop = 870 eV (experimental XPS value)

E shakeoff = 16 eV

Difference: 6 eV various other relaxations plus various errors: much larger than „chemical shift“ that should be zero for a noble gas.


Time effects; natural widthsCharacteristic times for photoemission from bound states: 10-13s

Born Oppenheimer approx breaks down: XPS „sees“ restructuring of electron shell and thus repositionning of nucleii (bond shortening)

Additional relaxation effects by Auger processes shorten core hole lifetime and thus broaden lines: effect decreases for increasing secondary quantum numbers (width increases from f,d,p,s)

Typical minimal linewidth in gases: 0.15 eV, in solids 0.55 eV

Differntial charging cuases much additional linebroadening (up to several eV). Sample roughness enhances this effect.


Platinum, a practical example


Line profilesTwo major causes for profiles above natural Lorentz shape: instrumental effects and final state coupling of the core hole.

Unresolved differences in ground state (chemical heterogeniety) add Gaussian components as well as unresolved spin-orbit splittings with Gauss-Lorentz shapes.

Differential charging (of sample and instrument parts along electron path) further adds Gaussian components.s-groundstates offer the simplest profiles unless they are multiplet split in paramagnetic samples.


A simple line: oxygen 1s


Solid structure and PE

Thermal broadening of the Fermi energy due to the Boltzmann „tail“:

UPS of Cu at 450 K blue and 623 K red.

The peaks are the He I beta satellites of the light source representing the Cu 4d bad maximum as „ghost“.


Line profile modification by charging

Mo oxide on silica

model system is Si//SiO2

real catalyst is powder sample after impregnation and calcination.


Core hole functions

The quantum mechanical nature of the PE process causes coupling of the core hole during relaxation with the valence DOS (Coster Kronig transitions).

Different shapes of valence DOS lead to different peak asymmetries of naturally Lorentzian line profiles.


Lifetime broadening

The contribution of core hole lifetime broadening in simple metals is small.

Phonon broadening (vibrations of emitting atoms) is significant. Relaxation broadening determines parameter alpha and is unaffected by phonons (temperature).


Frank Condon principle- vib relaxation


Binding EnergyWithin Koopman´s theorem we discuss the contribution to a EB

EBA = Enucl + Eval + E coord

nucl: kinetic and potential energies of all core electrons (nb: groud state)

val: potential of valence shells; chemical bonding

coord: potential from environment, e.g. Madelung potential


Chemical shiftdifference in EB for the same initial state in two different compounds A, B

Eshift = Ecoupl (chargeA-chargeB) + (EcoreA-EcoreB)

Ecoupl: two electron coupling integral between core and valence state of an atom: ca. 14 eV

In atoms and vdW crystals is term 2 small: shift works; in crystals and at surafces is term 2 large: shift concept breaks down

Ecore: core potential of inital state in compounds A,B

depends on: atom type (quantum numbers)

aggregate state, crystal structure (external environment)


Chemical shift


Photoemission of metal oxides

The metal d-states overlap differently with the relatively stable oxygen 2 p states of the „oxo-anions“

Strong effect on chemical shift as the anions are obviously differently „ionic“


Oxidation state by XPS

514516518520

Binding Energy (eV)

0

5000

1104

1.5104

2104

2.5104

Inte

nsi

ty (

cps)

V 2p3/2V4+

V5+

CAT61

CAT65

conventional XPS after catalytic operation in

attached prep chamber: data at

300 K

0,8

1,3

1,8

2,3

2,8

4 4,1 4,2 4,3 4,4 4,5 4,6 4,7

Vox_fit

Cat

alyt

ic A

ctiv

ity

per

un

it a

rea

(a.u

.)


Chemical shift

EBexp = Eadiab – ER – ET - EC

Eshift = EBA - EBB

Eshift = (EadiabA – Eadiab

B) – (ERA – ERB) – (ETA-ETB) – (ECA – ECB)

only this is the chemical shift


BE contributions

ground state

LDOS, oxidation state

Madelung potential

final state static

valence state relaxation

mean field change (n-1 approx)

extraatomic relaxation

final state dynamic

phonon excitation

shake up

shake off

multiplet splitting

ER EC ET


Magnitude of energies

For nitrogen atoms on finds the following

energies: Ionisation of the 1s state

E adiab: 450 eV

EB exp: 400 eV

ER: 18 eV

ET: 22 eV

EC: 5 eV

Great care with interpretation of EC in a classical picture

Total errors amount to magnitude of EC


Chemical shift data


Referencing binding energies

• Calibration of energy scale necessary due to work function uncertainities of sample-spectrometer array.

• Au 4f 7/2 at 84.0 eV and Fermi edge of Au provide primary standards. Linearity check by looking at Cu 2p3/2 at 932.67 eV.

• Samples calibrated by „adventitious carbon“ at 285.0 as referred to graphite at 284.6 eV.


Surface differential charging

Practical samples are inhomogeneous in geometry and composition and create electrostatic field differences during photoemission across their surface.


Differential charging:practical


Intensity

I = f(instrument)+ f(electron-photon interaction)+f(electron-electron interaction)+f(atomic abundance)

homogeneous sample, lateral and in depth

mean free path

from: Tanuma at al., SIA 17, 911 (1991).14

12

10

8

6

Ine

last

ic m

ea

n f

ree

pa

th in

Cu

(Å

)

800600400200Kinetic energy (eV)


Practical data of mean free path


Surface sensitivity


Instrumental realisation


In situ XPS


Experimental cellsupplied by gas lines (p0)

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

Analyzer input lens

Focal point of analyzerinput lens

First differentialpumping stage (10-4p0)

Second differentialpumping stage (10-6p0)

Third differentialpumping stage (10-8p0)

Hemisphercal

electronanalyzer(10-9p0)

mass spectrometerand additional pumping


160

140

120

100

80

60

40

20

0

No

rma

lize

d c

ou

nt

rate

(cp

s/m

A)

542 540 538 536 534 532 530 528 526

Binding energy (eV)

CH

3OH

(g

)C

H2O

(g

)H

2O (

g)

CO

2 (g

)

O2 (g) Cu2Osub-surface

oxygensurfaceoxygen

400 °C

CH3OH:O2

1:2

3:1

6:1

O 1sExp.

localisation of species

In-situ XPS at 0.5 mbar of Cu during methanol oxidation


0.6

0.5

0.4

0.3

0.2

0.1

0.0sub-

surf

ace

oxyg

en :

Cu

peak

rat

io

14121086Inelastic mean free path (Å)

CH3OH:O2 = 3:1

CH3OH:O2 = 6:1

Reducing conditions Oxidizing conditions

9

8

7

6

5

4

3C

u2O

: su

b-su

rfac

e ox

ygen

pea

k ra

tio

121086Inelastic mean free path (Å)

CH3OH:O2 = 1:2

Non-destructive depth profile


BackgroundsMore than 90% of all photoelectrons are scattered and change kinetic energy. The experimental spectrum is thus determined by background effects and not be characteristic lines.

Operation modes of the analyser allow to supress the background but modulate the intensity by a variable transmission function

Data analysis has to take care of background correction according to diffrent models of scattering. (Analyser in constant pass energy mode with stable transmission function.


The background


background functions and errors


background practicalRu metal on nanotube carbon.

The modulation of the constant background is due to plasmon excitations of the carbon


XPS - UPS

synchrotron radiation

selection rules change: all spectra show Au 5d states

Take care about physical conditions when comparing spectral shapes and when assigning „species“ to the spectral weight.


Methodical comparison

• XPS– core states– atom specific– quantitative– complex final state

effects (informative)– chemical shift concept

(caveats)– theoretically difficult

accessible

• UPS– valence states

– non-atom specific

– not quantifiable

– complex selection rules

– similarity to DOS

– theoretically accessible


Adsorption

Increasing strength of chemisorption leads to d-band splitting


Adsorption experiment

Good agreement between theory and experiment withoin the framework of „backbonding“ – strong interaction with weak charge transfer


A practical example: N2 on Ru

Note similarity between isoelectronic N2 and CO

See charge transfer from Ru d band with increasing coverage


XPS - Referenzspektren der Oxide


XPS - Spektren von KFexOy(2x2)


Präparationszyklus - Fe 2p Spektren


Präparationszyklus - O 1s Spektren


Präparationszyklus - K 2p Spektren


Wachstum

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Photoelectron Spectroscopies 1 Preparation Strategies Preparation of heterogeneous catalysts