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Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 1
X-Ray Photoelectron Spectroscopy
Modern Methods in Heterogeneous Catalysis ResearchFritz-Haber-Institut
der
MPG –
WS 2007/08Lecture SeriesHeterogeneous Catalysis
Dr. Christian LinsmeierMax-Planck-Institut
für
Plasmaphysik, Garchinglinsmeier@ipp.mpg.de
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 2
X-ray Photoelectron Spectroscopy (XPS)
Excitation-
Mg Kα
radiation (1253.4 eV)-
Al Kα
radiation (1486.6 eV)-
Synchrotron radiation (~0.1 -
several keV)
Detector (N(Ekin
))-
Hemispherical analyzer
Measurement
hν
θi θeEkin
X-raysource
e-
Detector
hν
e-hν
Principle
L
M
S
Photoelectric effect(Einstein, Nobel prize 1921)
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 3
Laboratory X-ray source
Filament 1
Characteristic X-ray radiation•
Al Kα
(1486.6 eV)•
Mg
Kα
(1253.6 eV)•
Bremsstrahlung
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 4
Laboratory X-ray source with monochromator, synchrotron
Advantages:•
no Bremsstrahlung
(L,S)•
high resolution (L,S)•
high intensity (S)
Disadvantages:•
low intensity (L)•
availability
(S)
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 5
Analyzers
Dispersive analyzers -
single or double focusing
-
band pass filter
-
resolution E-dependent (ΔE/E
= const.)
1.
CMA, cylindrical mirror analyzer
-
2 concentric cylinders
-
resolution < 1%
2.
127°
sector analyzer
-
small device
-
good resolution (down to 0.1%)
3.
CHA, concentric hemispherical analyzer
-
2 concentric half spheres
-
high transmission
-
resolution depends on radius
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 6
XPS Setup
X-raysource
Mono-chromator
Analyzer
Sample
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 7
Analyzer
Sampleholder
X-raysource
XPS –
Setup: inside
view
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 8
XPS –
Principle and method
Binding energyhν
= EB
+ ΦΑ
+
Ekin
EB
= hν
- Ekin
-
ΦΑ
→
element-specificChemical Shift
Shift of EB
by up to several eV due to charge state of the atom
→
chemical stateIntensity
I ∝
Number of atoms ×
photoionization
cross section (f(Eph
,Orbital,θe
))→
quantitativeDepth information
• angle-dependent intensity• sputter-depth profiling• variation of X-ray energy• background analysis
→
depth-resolved
Electron Spectroscopy for Chemical Analysis(K. Siegbahn, Nobel prize 1981)
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 9
XPS –
Survey
spectrum
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 10
XPS –
Element sensitivity
(~1 ML)(~24 ML)
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 11
XPS –
Chemical shift
Chemical shiftBinding energy of core levels depends on electron density at the emitting atom (→ screening of core level electrons), determined by the electronegativity of the neighboring atoms.
Final state effectsRelaxation of the ion ‚during the photoelectron emission‘
Chemical shift used as a ‚fingerprint‘
EvEF
E’kinEkin
ΔErelax
hν
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 12
Alloy formation
Alloy formation:
Be is incorporated within the W metal structureOverlap of W d
orbitals
is reduced→ Change in binding energy of 4f
core level electrons
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 13
Promotor-metal reaction
Rhodium monolayer:→ catalyst metal leads to reduction of the VOx
promotor
oxide→ vanadium oxidation states identified by chemical shift
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 14
Tungsten carbide phase formation
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 15
Tungsten carbide phase formation
Carbide formation:
→
analysis of peaks by fitting→
identification of chemical and structural phases
→
WC, W2
C formed→
C loss into bulk by diffusion
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 16
Sensitivity to structural changes
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 17
Electron mean free path
G.A. Somorjai, “Introduction to Surface Chemistry and Catalysis”
(Wiley, New York, 1994), p.383
Energy [eV]10 50 100 500 1000 200052
50
10
100
3
5
Mea
n Fr
ee P
ath
[Å]
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 18
Information depth in electron spectroscopies
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 19
Depth information –
variation of electron exit angle
Angular dependence of electron emission
Intensity depends on mean free path λ
and depth distribution of the detected elements.
0
0
exp( )
exp( )cos e
LI IdI I
λ
λ θ
= ⋅ −
= ⋅ − ⋅
d
hν
L
θe
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 20
Synchrotron light source
analyser
storagering
BessyBessy, Berlin, Berlin
)()(),()()( kinkinkin ETEhNhJEI ⋅⋅⋅⋅= λθνσν
photon fluxdensity of atoms
photoelectron cross sectionphotoelectron mean free path
analyzer transmission
Ekin
= hν
- EB
-
ФA
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 21
Depth information –
variation of photon energy
C / Be / W layer system: chemical reactions after 1050 K annealing
depth
surface
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 22
Sputtering
Collision cascade
in near-surface zone, removal of atoms
Sputtering yield:
Y
= number of sputtered atoms
number of incoming projectiles
Al2
O3
He
D
H
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 23
Chemical influence of Ar
sputtering
3.7 nm C / Ti:
1.
C deposition
2.
sputtering by 4 keV
Ar
continuous XPS measurements
→
ion beam collision cascade leads to mixing and carbide formation!
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 24
Background analysis
Inelastic energy loss:
electrons contribute to background →
Information on surface morphology!
S. Tougaard, Odense
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 25
Comparison XPS –
Ion Scattering Spectroscopy (ISS)
ISS: 480 eV He, θ=135°
→ sensitive to 1st monolayer only
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 26
Comparison XPS –
Ion Scattering Spectroscopy (ISS)
ISS: 480 eV He, θ=135°
→ sensitive to 1st monolayer only
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 27
Comparison XPS –
Ion Scattering Spectroscopy (ISS)
ISS: 480 eV He, θ=135°
→ sensitive to 1st monolayer only
ISS: first layer composition
XPS: chemical sensitivity
→
complementary information!
→
always use several techni-
ques
for complex systems
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 28
Summary
Physical background
Instrumentation
Elemental analysis
Chemical state analysis
Quantification
Depth profiling
Comparison with ISS
Dr. Ch. Linsmeier X-Ray Photoelectron Spectroscopy Heterogeneous Catalysis FHI 2007/08 - 29
Literature
Text and Reference Books:1.
Eds. D. Briggs and M.P. Seah,
Practical Surface Analysis, Vol. 1 –
Auger and X-Ray Photoelectron Spectroscopy, 2nd ed., Wiley (1992).
2.
S. Hüfner, Photoelectron Spectroscopy, Springer (1995)3.
J.F. Moulder, W.F. Stickle, P.E. Sobol, K.E. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Ed. J. Chastain, Perkin-Elmer Corp., Eden Prairie (1992).
4.
Eds. D. Briggs and J.T Grant, Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, Chichester (2003)
5.
G. Ertl
and J. Küppers, Low Energy Electrons and Surface Chemistry, Weinheim
(1985)
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