electron microscopy in catalysis - fhi...electron microscopy in catalysis di wang d. wang, 28 oct....
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D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Electron Microscopy in Catalysis
Electron Microscopy in Catalysis
Di Wang
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Transmission electron microscope
Electron scattering
Interaction of electron with your samples
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
V0acc volt
V1ext volt
Cathode
Electron Beam
First Anodes
An FEG tip, showing the extraordinarily fine W needle
Second Anodes
Field emission gun
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Lens
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Image and diffraction mode
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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)
Image contrast in TEM
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
III. Lattice fringes in pictures
O G
G1
G2
G3
-G1
-G2
-G3
O
Image contrast in TEM
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
High-resolution imaging
Abbe Interpretation of imaging
Sample
Lens
Back-focal plane Image plane
Exit Wave
U V
f
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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)
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
AFourier Transform 1.0
AuAu
ππ )sin(
u
A1
Sample shape
t1
t
Shape of each reciprocal lattice point
Shape factor
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Exciting the lens strength - focus
Lenses spatially fixed, but strength changeable
)exp()}(exp{ 122 χπλ ivufi =+∆
Phase shift factor in back focal plane
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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−
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Transfer function )exp()exp()( IIIiW χχ −=H
∗⋅=
⋅=
imageimage
exit-
image
IW
ψψ
ψψ )}(][{1 HFF
Electron wave function and intensity in the image plane
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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−=∆
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Picturing the Contrast Transfer Function
Amorphous Thin Carbon Film
Real Space Reciprocal Space}{)(21)( 1 CTFVI p−∗+≈ Frr σ
FT
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
! !
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Simulated HRTEM imagesThickness(Å)
Defocus(Å)
23.04
-100
-300
-500
-700
46.08 69.12 92.16 115.20 138.24
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Image contrast matching
2 nm
(VO)2P2O7 Mo8O23
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
HRTEM-profile imaging
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Zone axis[u,v,w]
(000)
),,(],,[ lkhwvu ⊥
Example: cubic structure, (100), (110), (120), (340)…… planes belong to [001] zone axis
Electron diffraction
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
P
O
G
hkl
λ
1=k
Crystal tilt
P
O
G
hkl
λ
1=k
Electron diffraction
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
P
O
G
hkl
λ
1=k
ZOLZ and HOLZ
ZOLZ
FOLZ
Electron diffraction
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
Electron diffraction
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Indexing of a ring patternElectron diffraction
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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 diffraction
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Electron Energy Loss Spectroscopy (EELS)
DetectionSystem
Electron EmitterE0
Probe Forming Optics
Sample
Post-Specimen Optics
Magnetic prism
E0 - E
E0
α
β
B
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
-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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Element mapping
450 500 550 600
Energy loss (eV)
Pre-edge1Pre-edge2
Post-edge
TEM image of ZrN/ZrO2
Oxygen map
EELS
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
SEM
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
SEM
Secondary electron Back scattered electron
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
STEM
Scanning beam
BFADF ADF
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
STEM
BF
DF
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
TiO2-ZrO2 after 20 min ball milling
TiO2-ZrO2 after 10 hours ball milling
STEM+EDX
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Reduction of MoO3 induced by electron beam irradiation
Application in catalytic systems
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Structure of MoO3 Diffraction of MoO3 on [010] projection
b
c
b
a
c
a
Reduction of MoO3 by electron beam irradiation
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
HRTEM image showing contrast from CS structure, formed at the early stage of reduction.
Low electron current density
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
Low electron current density
MoO3: (MoO6)6- octahedral configuration
ELNES on O K-edge
MoO2: (MoO6)8- octahedral configurationt2g anti-bonding orbitals are partially filled by two electrons.
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
High electron current density
After irradiation of 10 min 40 min
MoO (a = b = c = 4.08 Å) with NaCl structure? → Simulation
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
High electron current density
HRTEM images for evolution of Mo oxide under electron irradiation
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
MoO3: (MoO6)6- octahedral configuration
MoO: (MoO6)10- octahedral configurationt2g anti-bonding orbitals are partially filled by four electrons.
High electron current density
ELNES on O K-edge
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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
D. Wang, 28 Oct. 2005 Lecture Series Heterogeneous Catalysis
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