Electron Microscopy in Catalysis - FHI...Electron Microscopy in Catalysis Di Wang D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis D. Wang, 28 Oct. 2005 Lecture Series

D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis

Electron Microscopy in Catalysis

Electron Microscopy in Catalysis

Di Wang



Transmission electron microscope

Electron scattering

Interaction of electron with your samples


V0acc volt

V1ext volt

Cathode

Electron Beam

First Anodes

An FEG tip, showing the extraordinarily fine W needle

Second Anodes

Field emission gun


Lens


Electron-sample interactions

SampleSample

Incident electron beamIncident electron beam

Angle-limiting aperture

Auger electronsAuger electrons

Secondary electronsSecondary electrons

Backscattered electronsBackscattered electrons

Elastically scattered electronsElastically scattered electrons

Transmitted beam to energy-loss

spectrometer

Transmitted beam to energy-loss

spectrometer

Cathodoluminescence(visible light)Cathodoluminescence(visible light)

BremsstrahlungBremsstrahlung

Characteristic x-raysCharacteristic x-rays

HeatHeat

β


What can TEM do?TEM Morphology

Bright field and dark field imaging Defects, Phases

Electron diffraction Structure

High-resolution imaging Defects, Interfaces, Surfaces

Convergent-beam diffraction Symmetry, StrainLattice parameter

Energy-dispersive X-ray spectroscopy (EDX)

Electron-energy loss spectroscopy (EELS)

Energy-filtered TEM (EFTEM)

SEM

STEM

Element analysis

Electronic structures

Imaging the distribution of elements and even chemical states

Morphology, surfaces

Morphology, Z-contrast


Image and diffraction mode


Image contrast in TEM

I. Mass-thickness contrast

Incident beam

Lower mass thickness

Higher mass thickness

Objective lens

Objective aperture

Image plane

Intensity profile

C


II. Diffraction contrast

Incident beamIncident beam

specimen

Objective lens

Objective aperture

Diffracted beam

Diffracted beam

Direct beam

Direct beam

Dark field (DF)Bright field (BF)



III. Lattice fringes in pictures

O G

G1

G2

G3

-G1

-G2

-G3

O



High-resolution imaging

Abbe Interpretation of imaging

Sample

Lens

Back-focal plane Image plane

Exit Wave

U V

f


Abbe Interpretation of imaging

Incident electron wave (plain wave)

Interaction with thin sample

Exit wave

Fresnel propagation over f

Electron wave before lens plane

Lens

Electron wave after lens plane

Fresnel propagation over V

Electron wave at back focal plane

Electron wave at image plane

Fresnel diffraction over U

Diffraction pattern (Intensity)

Image (Intensity)


Reciprocal space

VVVbacacbcba ×

=×

=×

= *,*,*

For cubic, tetragonal and orthorhombic structure,

a* = 1/a, || ab* = 1/b, || bc* = 1/c, || c

u = ha*+kb*+lc*reciprocal vector

Real space

Fourier transform (FT)

Inverse FT

x

y

z

ab

c

u

v

w

a*b*

c*u

hkldlkh 1*** =++= cbau

FT


Bragg condition and Ewald sphere

P

O

G

hklO

P

k0 k

Gu

k0

k

θ

u

λθθ sin2sin2 == ku

hkldlkh 1*** =++= cbau

λθ =hklhkld sin2

λ

1=k


AFourier Transform 1.0

AuAu

ππ )sin(

u

A1

Sample shape

t1

t

Shape of each reciprocal lattice point

Shape factor


Exciting the lens strength - focus

Lenses spatially fixed, but strength changeable

)exp()}(exp{ 122 χπλ ivufi =+∆

Phase shift factor in back focal plane


Spherical aberration of lenses

The electromagnetic lenses are not perfect

)exp()(21

22223

2 χλπχ ivuCs =+=

300 θsCr =∆ Cs : Spherical aberration coefficient

Phase shift in back focal plane due to spherical aberration


Chromatic aberration

Faster electrons are brought to a focus beyond the Gaussian image plane.

22223 2

1 HDλπχ =

D: Standard deviation of Gaussiandistribution due to the chromatic aberration

Envelope in back focal plane )exp( 3χ−


Beam divergence

Paralell incident beam (ideal condition)Divergence angle α~ 0.5 mrad (real condition)

2222224 )( fCs ∆+= HH λαπχ

Envelope in back focal plane

)exp( 4χ−

0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

u(Å-1)

exp(-χ3)

exp(-χ4)α=0.5 mrad，∆f=-410Å


Transfer function )exp()exp()( IIIiW χχ −=H

∗⋅=

⋅=

imageimage

exit-

image

IW

ψψ

ψψ )}(][{1 HFF

Electron wave function and intensity in the image plane


Contrast Transfer Function (CTF)

0.2 0.4 0.6 0.8 1.0

-1.0

-0.5

0.0

0.5

1.0

u(Å-1)

LaB6

FEG

0.2 0.4 0.6 0.8 1.0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

)exp()sin( III χχ −

ÅDÅD

mmCkVU

LaB

FEG

s

10038

5.0200

6=

=

==Åf 430−=∆

Åf 750−=∆


Phase contrast in TEM

Final intensity:

Exit wave: )(1)( )( rr rp

Vie Vie p σψ σ

−≈=−

Assuming weak-phase object approximationVp : scattering potential

∆z

}{)(21)( 1 CTFVI p−∗+≈ Frr σ


Picturing the Contrast Transfer Function

Amorphous Thin Carbon Film

Real Space Reciprocal Space}{)(21)( 1 CTFVI p−∗+≈ Frr σ

FT


Image interpretation

Only for thin crystal (WPOA) and the focus value close to Scherzer focus, the contrast of HREM image can be interprated as crystal structure up to point resolution. In general, the black or white dots in HREM image DO NOT correspond to atoms or atom groups.

Si [110] image with different defocus values

! !


Simulated HRTEM imagesThickness(Å)

Defocus(Å)

23.04

-100

-300

-500

-700

46.08 69.12 92.16 115.20 138.24


Image contrast matching

2 nm

(VO)2P2O7 Mo8O23


HRTEM-profile imaging


Zone axis[u,v,w]

(000)

),,(],,[ lkhwvu ⊥

Example: cubic structure, (100), (110), (120), (340)…… planes belong to [001] zone axis

Electron diffraction



Camera length

1/λ

g

L

2θ

D

DLdd

LD

hkl

hklhkl

hkl

/

1/

λ

λ

=

=

=

g

g

MoO3-[010]MoO3-[010]

MoO2-polycrystallineMoO2-polycrystalline


P

O

G

hkl

λ

1=k

Crystal tilt

P

O

G

hkl

λ

1=k



P

O

G

hkl

λ

1=k

ZOLZ and HOLZ

ZOLZ

FOLZ



Calibration by a known structure

200

220If a h0k0l0 diffraction from a known structure can be determined in a diffraction pattern and its distance to the center is measured as u0, for any other diffraction with distance u to the center, the lattice spacing d is given by:d = u0dh0k0l0/u



Indexing of a ring patternElectron diffraction


Indexing of a single crystal pattern

022

002

Cubic ZrO2 (fluorite structure) on [110] projection

• Two sets of lattice planes and the angle in between

• Using extinction rules

• Using diffraction pattern on other zone axis

• Simulation



Electron Energy Loss Spectroscopy (EELS)

DetectionSystem

Electron EmitterE0

Probe Forming Optics

Sample

Post-Specimen Optics

Magnetic prism

E0 - E

E0

α

β

B


-zero-loss peak-plasmon peak-Inner-shell ionization edges, low intensity-Near edge structure on top of edges-background-Plural scattering

380 400 420 440 460 480 500 520 540 560

Ele

ctro

n co

unts

(a.u

.)

120 140 160 180 200 220 240 260 280 3000 20 40 60

Energy loss (eV)

P L2,3

B K

C K

N K V L2,3 O K

Amplified Amplified Amplified

Zero loss

Plasmon/Outer-shell electrons

Background

EELSEELS


515

517

519

521

523

525

527

0 1 2 3 4 5 6

Vanadium Oxidation StateEd

ge P

eak

Posi

tion

(eV)

L3L2

Correlation of peak positions of the vanadiumL-edges with the oxidation sates of vanadium atoms in various vanadium oxides

V L-edge and O K-edge of vanadium oxides

510 520 530 540 550 560

Energy loss [eV]

Inte

nsity

[arb

. uni

ts]

V2O5

V6O13

VO2

V2O3

VO

ELNES of Vanadium OxidesV L3 V L2

O K

ELNES as “fingerprints”EELS


Element mapping

450 500 550 600

Energy loss (eV)

Pre-edge1Pre-edge2

Post-edge

TEM image of ZrN/ZrO2

Oxygen map

EELS


SEM


SEM

Secondary electron Back scattered electron


STEM

Scanning beam

BFADF ADF


STEM

BF

DF


TiO2-ZrO2 after 20 min ball milling

TiO2-ZrO2 after 10 hours ball milling

STEM+EDX


Supported metal particle size effects; metal-substrate interaction; structural change under chemical treatments

Important heterogeneous catalysts Information of interests

Transition metal oxide reduction behavior; defects structures

Zeolites (porous structure) 3D structure; intergrowth of different zeolitic structures; guest species inside a zeolitic host

Carbon nanofibers as support structure and growing mechanisms

Application in catalytic systems


Reduction of MoO3 induced by electron beam irradiation

Application in catalytic systems


Structure of MoO3 Diffraction of MoO3 on [010] projection

b

c

b

a

c

a

Reduction of MoO3 by electron beam irradiation


Frame 1 and 2: diffractions can be attributed to MoO2 on [-111] projection.Frame3: Diffractions can be attributed to MoO2 on [-122] projection.

Low electron current density

After irradiation of 10 min 60 min 120 min

200 min 360 min


HRTEM image showing contrast from CS structure, formed at the early stage of reduction.




MoO3: (MoO6)6- octahedral configuration

ELNES on O K-edge

MoO2: (MoO6)8- octahedral configurationt2g anti-bonding orbitals are partially filled by two electrons.


High electron current density

After irradiation of 10 min 40 min

MoO (a = b = c = 4.08 Å) with NaCl structure? → Simulation



HRTEM images for evolution of Mo oxide under electron irradiation


MoO3: (MoO6)6- octahedral configuration

MoO: (MoO6)10- octahedral configurationt2g anti-bonding orbitals are partially filled by four electrons.


ELNES on O K-edge


Summary

• Importance of model catalyst — simplifying complex system; facilitating analytic techniques; aware of the gap between the TEM environment and the “real” condition.

• Be sure that TEM observation on the local structure is representative to the whole catalyst.

• Distinguish electron induced effect from intrinsic features of catalyst


Literature

Reimer, Ludwig; Pfefferkorn, Gerhard.Scanning Electron Microscopy

Reimer, Ludwig;Transmission electron microscopy : physics of image formation andmicroanalysis

Williams, David B. Carter, C. BarryTransmission electron microscopy : a textbook for materials science

Egerton, R. F.Electron energy-loss spectroscopy in the electron microscope

Spence, John C. H.Experimental high-resolution electron microscopy

Spence, John C. H.; Zuo, J. M.Electron microdiffraction

Documents

Electron Microscopy in Catalysis - FHI...Electron Microscopy in Catalysis Di Wang D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis D. Wang, 28 Oct. 2005 Lecture Series