X-ray absorption spectroscopy

Jagdeep Singh

Jeroen A. van Bokhoven

Historical development

Synchrotron needed

Absorption through matter

10000 12000 140000.90

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sion

Energy (eV)

L3

L2

L1 Au Foil

Absorption as function of energy of the x-rayShape is structure dependent

Creation of a photo-electron

Ekin = hν - EBinding

Photo-electron has kinetic energy

EBindinghν

Photo-electron has kinetic energy

XANES2

f ii f E EP T νδ − −Ψ Ψ h

Initial state Final state

Transition operator

Fermi’s Golden Rule

Outgoing wave === backscattering === interference patternConstructive / destructive interference

„wig

glin

g in

abs

orpt

ion“

Ψfinal = Ψoutgoing + Ψback scattering

EXAFS is the wiggling part of the absorption

2d sin θ = nλ

Tuning the energyDouble crystal monochromator

Bragg angle

sample

I0 IX-rayswhite beam Crystal [Si(111)]

SLS, Villigen

Experimental Hutch

BM26 (DUBBLE), ESRF Grenoble

Lambert Beer’s lawdI = -μ(E)I dxI = I0 exp(-μ(E)x)

Transmission through 1 cm

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Energy (eV)

10-3 mbar

Air @ 1 bar

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Energy (eV)

25 μm 1 wt% Au/SiO2

hνI0

I = I0e-μx

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Tran

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Energy (eV)

L3

L2L1

X-ray absorption through matter

Sample environment

pressuretemperatureenvironment

Absorption of X-rays is limiting factor

Find a good window material• Size of window• Thickness• Inertness• Temperature resistance• Pressure• Safety

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Energy (eV)

Transmission through25 μm and 1 mm

Al

Mylar

Be

Si K Cr K Cu K Pt L3Ag L3

In situ EXAFS cells for gas-solid reactions

Reaction gas mixture flows

around a pellet

Reaction gas flows through a catalyst

pellet

Small Glass Reactor with very thin

windows (0.01mm)

Large dead volumeGood for stationary

conditions

Critical d/l (smaller effectivity of the catalyst)

Small dead volumeOptimal d/lGood for structural changes Structure-activity relations

1 x 8 mm 1 x 8 mm

Reactor

Fluorescence detector Thermo

Couple

Exit-tube to mass spec

ID26, ESRF

XANES

Dipole transition:K edge: 1s pL edge: 2s p

2p s,d

HoweverQuadrupole transition:K edge s d

p f

Quad. Trans. probability is about 10-3 smaller, but d-DOS >> p-DOSVisible in the K pre-edges!!

Δl=±1

Δl=±2

2

f ii f E EP T νδ − −Ψ Ψ h

Initial state Final state

Transition operator

Fermi’s Golden Rule

What determines the shape of XANES spectra?- Pre-edge- Edge-energy- Shape over the edge

1.2

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abs

orpt

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(μx)

552054805440X-ray energy (eV)

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552054805440

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V2O5

V6O13

VO2

NH4VO3

VOSO4.3H2O

K edge (1s 4p)of VOx compounds

Pre-edge - intensity- energy

…… there is not always a pre-edge

BUT…..

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(μx)

552054805440X-ray energy (eV)

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V2O5

V6O13

VO2

NH4VO3

VOSO4.3H2O

VOSOVO2V6O13V2O5NH4VO

In order of increasing distortion from octahedral

4 Distorted OctahedralDistorted OctahedralDistorted OctahedralDistorted Square Pyramidal

3 Tetrahedral

Increasingpre-edge

Increasingdistortion

1.0

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mal

ised

abso

rptio

n (

x)

548554805475547054655460X-ray energy (eV)

Arctangent / polynomial

548554805475547054655460X-ray energy (eV)

Isolation of the pre edgeby edge subtraction

Pre-edges intensity & energy varies (K edge)

Pure octahedral caseCentro-symmetry: no p-d mixing allowed: only quadrupole transition

very low intensity

Distortion from octahedralP-d mixing allowed: dipole transition in pre-edge + quadrupolar trans.

increasingly large intensity

Pure tetrahedral => largest pre-edge

Intensity pre-edge indicative of geometry

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552054805440

NH4VO3

Vn+

P-DOS

EedgeEpre

(Pre-)edge Energy and ValenceEdge position is measure of oxidation state

1S

2p 3/2

2p 1/2

2s

1s

Ef

hν > Eo

photo-electronEkin = hν - E0

EmptyDOS

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(μx)

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V2O5

V6O13

VO2

NH4VO3

VOSO4.3H2O

For L edges > 3 keV and all K edges Shape of the whiteline

“Whiteline is first intensepeak(s) in spectrum”

VOSO4 Distorted Octahedral

VO2 Distorted Octahedral

V6O13 Distorted Octahedral

V2O5 Distorted Square Pyramidal

NH4VO3 Tetrahedral

E (eV) - EedgeE (eV) - Eedge

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abs

orba

nce

bulk gold

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Au2O3

Au L3 edge

Whiteline reflects holes in d-band

Very small whiteline

Shape of the whiteline: L-edges

Totally different

11900 11925 11950 119750.0

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70 °C : Au3+

200 °C : Au0

In situ Au3+ reduction in He/H2

Isobestic points

11875 11900 11925 11950 11975 120000.0

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Au(0)Au(III)

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Abs

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Energy (eV)

Reference spectra

HAuCl4 on Al2O3

Shape of the whiteline: L-edgesPure metals:

Whiteline reflects holes in d-band

Mo0.5

Ru0.7

Rh0.8

Pd0.9

Ag1.0

Tc0.6

Ideal d-band filling

Alloying:

Whiteline reflects charge transfer

Abstract (I)Pre-edge Edge Shape over the edge- Valence - Valence - Geometry- Geometry - D-band filling (LIII-edge)

- (Non- / Anti-bonding) DOS states(- Adsorbates)

For many (many!) compounds structures and spectra are available in literature

NoteVariations in XANES may be very subtle and hardly visible in the data:

take (negative) second derivative

L edgesWhiteline intensity reflects number of holes in the d band (valence)

K edges(Pre) edge position reflects valence

Shape of XANES indicative of geometry

Typical XANES Experiment

• Catalyst samples, measured in desired conditions temperature, pressure, aggregation state

• Reference samples that likely resemble the state of the catalyst- Various oxidation states- Various coordinations

• Identification of trends, similarities in reference samples

• Comparison of trends, similarities to ‘unknowns’

• Application of theory to obtain ultimate information (expert option).