Preparation Strategies
Spectroscopies
PreparationPhotoelectron
of heterogeneous
catalysts
1
Photoelectron Spectroscopies
2
Energy ranges
Photoelectron Spectroscopies
3
Energy
The Photoemission Process
kinetic energy
0
Ef
work function
binding energy
initial photon energy
EB
Photoelectron Spectroscopies
4
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.
Photoelectron Spectroscopies
5
Photoemission – quantum
mechanical
• Photoemission as superposition of wave functions
(photon-ground state; photoelectron-virtual bound
state (Rydberg state); virtual bound statecontinuum 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
Photoelectron Spectroscopies
6
Photoemission – quantum
mechanical
Ry
M
O
Photoelectron Spectroscopies
7
The QM picture of
photoemission
Free atom case
and case of a
solid with a band
structure: note the
difference in
„Rydberg“ states.
Photoelectron Spectroscopies
8
ARUPS: Seeing orbitals
The angular
resolution of the
intensities allow
to separate the sstates from the pstates in the sp3
bonding in Si
Photoelectron Spectroscopies
9
The Photoemission Process
Sudden 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)
Photoelectron Spectroscopies
10
Towards a picture of photoemission
Photoelectron Spectroscopies
11
Koopman´s Theorem, Relaxation
The „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)
Photoelectron Spectroscopies
12
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.
Photoelectron Spectroscopies
13
Time effects; natural widths
Characteristic 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.
Photoelectron Spectroscopies
14
Platinum, a practical example
Photoelectron Spectroscopies
15
Line profiles
Two 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.
Photoelectron Spectroscopies
16
A simple line: oxygen 1s
Photoelectron Spectroscopies
17
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“.
Photoelectron Spectroscopies
18
Line profile modification by charging
Mo oxide on silica
model system is Si//SiO2
real catalyst is powder
sample after impregnation
and calcination.
Photoelectron Spectroscopies
19
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.
Photoelectron Spectroscopies
20
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).
Photoelectron Spectroscopies
21
Frank Condon principle- vib relaxation
Photoelectron Spectroscopies
22
Binding Energy
Within 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
Photoelectron Spectroscopies
23
Chemical shift
difference 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
Ecore: core potential of inital state in compounds A,B
depends on:
atom type (quantum numbers)
aggregate state, crystal structure (external environment)
In atoms and vdW crystals is term 2 small: shift works; in crystals
and at surafces is term 2 large: shift concept breaks down
Photoelectron Spectroscopies
24
Chemical shift
Photoelectron Spectroscopies
25
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“
Photoelectron Spectroscopies
26
2,8
2,3
1,8
2.5 104
V 2p3/2
V5+
V4+
1,3
2 104
0,8
4
4,1
4,2
4,3
Intensity (cps)
Catalytic Activity per unit area
(a.u.)
Oxidation state by XPS
4,41.5 104,5
4
4,6
CAT65
4,7
Vox_fit
conventional XPS
after catalytic
operation in attached
prep chamber: data at
300 K
1 104
5000
CAT61
0
Photoelectron Spectroscopies
520
518
516
514
Binding Energy (eV)
27
Chemical shift
EBexp = Eadiab – ER – ET - EC
Eshift = EBA - EBB
Eshift = (EadiabA – EadiabB) – (ERA – ERB) – (ETA-ETB) – (ECA – ECB)
only this is the
chemical shift
Photoelectron Spectroscopies
28
BE contributions
ER
EC
ground state
final state static
ET
final state dynamic
phonon excitation
valence state
relaxation
mean field change
(n-1 approx)
LDOS, oxidation state
shake up
Madelung potential
shake off
multiplet splitting
extraatomic relaxation
Photoelectron Spectroscopies
29
Magnitude of energies
For nitrogen atoms on finds the following
energies: Ionisation of the 1s state
E adiab:
450 eV
Great care with interpretation of
EB exp:
400 eV
EC in a classical picture
ER:
18 eV
Total errors amount to magnitude
ET:
22 eV
of EC
EC:
5 eV
Photoelectron Spectroscopies
30
Chemical shift data
Photoelectron Spectroscopies
31
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.
Photoelectron Spectroscopies
32
Surface differential
charging
Practical samples are
inhomogeneous in geometry
and composition and create
electrostatic field differences
during photoemission across
their surface.
Photoelectron Spectroscopies
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Differential charging:practical
Photoelectron Spectroscopies
34
Intensity
Inelastic mean free path in Cu (Å)
I = f(instrument)+ f(electron-photon interaction)+f(electron-electron
interaction)+f(atomic abundance)
mean free path
14
from: Tanuma at al.,
SIA 17, 911 (1991).
homogeneous sample, lateral and in depth12
10
8
6
200 400 600 800
Kinetic energy (eV)
Photoelectron Spectroscopies
35
Practical data of mean free path
Photoelectron Spectroscopies
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Surface sensitivity
Photoelectron Spectroscopies
37
Instrumental
realisation
Photoelectron Spectroscopies
38
In situ XPS
Photoelectron Spectroscopies
39
X-rays enter the cell at
55° incidence through an
SiNx window
(thickness ~ 1000 Å)
Hemisphercal
electron
analyzer
(10-9p0)
Analyzer
input lens
mass spectrometer
and additional
pumping
Focal point
of analyzer
input lens
Third differential
pumping stage (10-8p0)
Second differential
pumping stage (10-6p0)
Experimental cell
supplied by gas
lines (p0)
First differential
pumping stage (10-4p0)
Photoelectron Spectroscopies
40
O2 (g)
CO2 (g)
Normalized count rate (cps/mA)
140
120
H2O (g)
160
CH2O (g)
CH3OH (g)
400 °C
Exp.
localisation
of species
O 1s
Cu2O
sub-surface
oxygen
surface
oxygen
100
CH3OH:O2
80
1:2
60
40
3:1
20
6:1
0
542
540
538
536
534
532
530
528
526
In-situ XPS at 0.5 mbar
of Cu during methanol
oxidation
Binding energy (eV)
Photoelectron Spectroscopies
41
Non-destructive depth profile
Reducing conditions
Cu 2O : sub-surface oxygen peak ratio
sub-surface oxygen : Cu peak ratio
0.6
CH3OH:O2 = 3:1
CH3OH:O2 = 6:1
0.5
0.4
0.3
0.2
0.1
0.0
6
8
10
12
Oxidizing conditions
14
Inelastic mean free path (Å)
Photoelectron Spectroscopies
9
CH3OH:O2 = 1:2
8
7
6
5
4
3
6
8
10
12
Inelastic mean free path (Å)
42
Backgrounds
More 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.
Photoelectron Spectroscopies
43
The background
Photoelectron Spectroscopies
44
background functions and errors
Photoelectron Spectroscopies
45
background practical
Ru metal on
nanotube
carbon.
The modulation
of the constant
background is
due to plasmon
excitations of
the carbon
Photoelectron Spectroscopies
46
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.
Photoelectron Spectroscopies
47
Methodical comparison
• XPS
• UPS
–
–
–
–
–
–
–
–
–
–
core states
atom specific
quantitative
complex final state
effects (informative)
– chemical shift concept
(caveats)
– theoretically difficult
accessible
Photoelectron Spectroscopies
valence states
non-atom specific
not quantifiable
complex selection rules
similarity to DOS
theoretically accessible
48
Adsorption
Increasing strength of chemisorption leads to d-band splitting
Photoelectron Spectroscopies
49
Adsorption experiment
Good agreement between
theory and experiment
withoin the framework of
„backbonding“ – strong
interaction with weak
charge transfer
Photoelectron Spectroscopies
50
A practical
example: N2 on Ru
Note similarity between
isoelectronic N2 and CO
See charge transfer from Ru d
band with increasing coverage
Photoelectron Spectroscopies
51
XPS - Referenzspektren der Oxide
Photoelectron Spectroscopies
52
XPS - Spektren von KFexOy(2x2)
Photoelectron Spectroscopies
53
Präparationszyklus - Fe 2p Spektren
Photoelectron Spectroscopies
54
Präparationszyklus - O 1s Spektren
Photoelectron Spectroscopies
55
Präparationszyklus - K 2p Spektren
Photoelectron Spectroscopies
56
Wachstum
Photoelectron Spectroscopies
57