Transcript No Slide Title

Atomic Scale Ordering in Metallic Nanoparticles
Structure:
• Atomic packing: microstructure?
• Cluster shape?
• Surface structure?
• Disorder?
Characterization
• Electron Microscopy
 Scanning Transmission Electron Microscopy (STEM)
 Electron Diffraction
• X-ray Absorption Spectroscopy
 X-ray Absorption Near Edge Spectroscopy (XANES)
• Provides information on chemical states
– Oxidation state
– Density of states
 Extended X-ray Absorption Fine Structure (EXAFS)
• Provides local (~10 Å) structural parameters
– Nearest Neighbors (coordination numbers)
– Bond distances
– Disorder
Face Centered Cubic Structure
(111)
(001)
(110)
Electron Microdiffraction
[011]
[112]
[310]
Electron diffraction probes the ordered microstructure of the nanoparticles.
Above are 3 sample diffraction patterns for ~ 20 Å Pt nanoparticles. All are
indexed as face-centered cubic (fcc).
X-Ray Absorption Spectroscopy
• Absorption coefficient (m) vs. incident photon energy
• The photoelectric absorption decreases with increasing energy
Absorption
• “Jumps” correspond to excitation of core electrons
Photon Energy
Adapted from Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: New York, 1986.
Extended X-ray Absorption Fine
Structure
• oscillation of the X-ray absorption coefficient near and edge
• local (<10 Å) structure surrounding the absorbing atom
Pt L3 edge (11564 eV)
Pt foil
I0
IT
x
mx  ln
I0
I
Absorption (mx)
1
EXAFS
0
11400
11600
11800
12000
12200
Photon Energy (eV)
12400
Basics of EXAFS
Ri
hn
e-
PE = hn - E0
initial
final
E0
• Oscillations, ci(k): final state interference
between outgoing and backscattered photoelectron
• Excitation of a photoelectron with
wavenumber k = 2p/l
c i (k )  Ai (k ) sin( 2kRi )
Ri
- distance to shell-i
Ai(k) - backscattering amp.
Absorption (mx)
1
Data Analysis
m
m0
m0(0)
Convert to wave number
k
0
0
200
400
600
800
2m
( h n  E0 )
2
Subtract background and normalize
Photoelectron Energy (eV)
33
3
c (k ) 
k 2c(k) (Å -2)
k 2c(k) (Å -2)
22
2
m  m0
m 0 ( 0)
11
1
00
0
Resulting data is the sum of
scattering from all shells
-1
-1-1
c (k )   c i (k )
-2
-2-2
-3
-3-3
00
0
i
22
2
44
4
66
6
88
8
-1
k k(Å(Å
)-1)
10
1010
12
1212
14
1414
16
1616
Fourier Transform
Resolve the scattering from each distance (Ri) into
r-space
4
R1
Pt L3 edge, Pt foil
|c(r)|(Å-3)
3
2
R3
R4
R2
1
0
0
1
2
3
4
5
r (Å)
6
7
8
9
10
Multiple-Shell Fit
Calculate Fi(k) and di(k) for each shell-i (i = 1 to 6) using the FEFF computer code
Non-linear least-square refinement: vary Ni, Ri, 2i using the EXAFS equation
Fi (k ) 2 k 2 2
c i (k )  N i
e
sin( 2kRi  d i (k ))
2
kRi
3
Pt L3, Pt foil
Multiple-Shell Fit
Bond distance, Ri (Å)
2
R1
fit
actual
1
0
0
1
2
3
4
5
6
7
8
9
10
R2
R3
R4
R5
2.768(3) 3.914(4) 4.794(4) 5.535(5) 6.189(6)
2.7719
3.9200
4.8010
5.5437
6.1981
Multiple Scattering Paths
SS1
SS4
DS
TS
TR2
SS2
SS3
TR1
SS5
TR3
In-plane atom
Above-plane atom
Absorbing atom
X-Ray Absorption Near Edge Spectroscopy (XANES)
Normalized absorption coefficient
1.2
1.0
0.8
0.6
0.4
0.2
0.0
11560
11565
11570
11575
11580
11585
11590
11595
11600
Energy, eV
XANES measurements for reduced 10%, 40% Pt/C, 60% Pt/C Pt/C, and Pt foil
at 200, 300, 473 and 673 K. A total of 16 measurements are shown. All overlay
well with bulk Pt (Pt foil); therefore, the samples are reduced to their metallic state.
Size Dependence
2.0
1.5
2
Pt foil
60% Pt/C
40% Pt/C
10% Pt/C
-3
4
FT Magnitude, Å
-2
1.0
k c(k), Å
5
Pt foil
60% Pt/C
40% Pt/C
10% Pt/C
2.5
0.5
0.0
-0.5
-1.0
3
2
1
-1.5
-2.0
0
-2.5
0
2
4
6
8
10
12
-1
k, Å
14
16
18
20
22
0
1
2
3
4
5
6
7
8
r, Å
Size dependence on the extended x-ray absorption spectra. The amplitude of
the EXAFS signal is directly proportional to the coordination numbers for each
shell; therefore, as the cluster size increases, the amplitude also will increase.
9
10
Multiple Shell Fitting Analysis
3.5
2.0
3.0
Data
Fit
FT Magnitude, Å
-3
FT Magnitude, Å
Data
Fit
2.5
-3
1.5
1.0
0.5
2.0
1.5
1.0
0.5
0.0
0.0
0
1
2
3
4
5
6
7
8
9
0
10
1
2
3
6
7
8
40% Pt/C
10% Pt/C
10% Pt/C
8.3(5)
2.3(1.1)
10.9(3.2)
5.5(1.4)
5.4(3.4)
5
r, Å
r, Å
i
1
2
3
4
5
4
40% Pt/C
10.5(5)
4.0(1.3)
16.8(3.5)
7.6(1.4)
10(4)
60% Pt/C
11.4(6)
4.7(1.7)
19(4)
8.5(1.6)
11(4)
Pt foil
12.6(7)
5.9(2.0)
23(5)
11(2)
14(5)
Bulk fcc
12
6
24
12
24
9
10
Temperature Dependence
200 K
300 K
473 K
673 K
1.0
1.4
FT Magnitude, Å
-2
2
-0.5
k c(k), Å
0.0
200 K
300 K
473 K
673 K
1.6
-3
0.5
1.8
1.2
1.0
0.8
0.6
0.4
-1.0
0.2
0.0
-1.5
0
2
4
6
8
10
12
-1
k, Å
14
16
18
20
22
0
1
2
3
4
5
6
7
8
9
r, Å
Temperature dependence on the extended x-ray absorption spectra for 10% Pt/C.
As the temperature increases, the dynamic disorder (D2) increases, causing the
amplitude to decrease.
10
First Shell Fitting: 10% Pt/C
1.6
2.5
200 K
2.0
Data
Fit
Data
Fit
-3
1.2
FT Magnitude, Å
-3
FT Magnitude, Å
300 K
1.4
1.5
1.0
1.0
0.8
0.6
0.4
0.5
0.2
0.0
0.0
0
1
2
3
4
5
6
7
8
9
0
10
1
2
3
4
6
7
8
9
10
0.7
1.0
473 K
673 K
0.6
0.8
Data
Fit
Data
Fit
-3
0.5
FT Magnitude, Å
-3
FT Magnitude, Å
5
r, Å
r, Å
0.6
0.4
0.4
0.3
0.2
0.2
0.1
0.0
0.0
0
1
2
3
4
5
r, Å
6
7
8
9
10
0
1
2
3
4
5
r, Å
6
7
8
9
10
Size Dependent Scaling of Bond Length and Disorder
Distance, Å
Fi (k ) 2 k 2 2
c i (k )  N i
e
sin( 2kRi  d i (k ))
2
kRi
2.786
2.784
2.782
2.780
2.778
2.776
2.774
2.772
2.770
2.768
2.766
2.764
2.762
2.760
2.758
2.756
2.754
2.752
2.750
2.748
The EXAFS Disorder, 2, is the sum
of the static, s2, and dynamic, d2,
disorder as follows:
10% Pt/C
40% Pt/C
60% Pt/C
Pt foil
 2  r  r
2
  s2   d2
The dynamic disorder, d2, can be
separated by using the following
relationship:
200
300
400
500
Temperature, K
600
700
 d2 
 1  exp(E / T )
2m 1  exp(E / T )
Structure and Morphology
Spherical cuboctahedron
• Determining shape and texture
• Electron microscopy
• X-Ray absorption spectroscopy
Hemispherical cuboctahedron, (111) basal plane
• Molecular modeling
Hemispherical cuboctahedron, (001) basal plane
Cluster diameter, Å
0
10
20
30
40
50
60
70
80
24
Theoretical vs. Experimental
Bulk 3NN and 5NN
22
Coordination number
20
Cluster diameter, Å
0
10
20
30
40
50
60
70
80
24
Bulk 3NN and 5NN
22
16
14
Bulk 1NN and 4NN
1NN
2NN
3NN
4NN
5NN
12
10
8
Bulk 2NN
6
4
2
20
18
16
14
Bulk 1NN and 4NN
0
1NN
2NN
3NN
4NN
5NN
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
L
Cluster diameter, Å
12
0
10
10
20
30
40
50
60
70
80
24
8
Bulk 2NN
20
4
2
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
L
Spherical
Bulk 3NN and 5NN
No overlap between
1NN and 2NN
22
6
Coordination number
Coordination number
18
18
16
14
Bulk 1NN and 4NN
12
10
8
Bulk 2NN
6
4
2
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
L
Hemispherical
1NN
2NN
3NN
4NN
5NN
Molecular Modeling: Understanding Disorder
• Probe bulk vs. surface relaxation.
• Bulk:
Allow relaxation of entire structure.
• Surface:
Allow relaxation of atoms bound in surface sites only.
Bond Length Distributions: 10% Pt/C
70
70
60
Frequency distribution
Frequency distribution
60
50
40
30
20
10
0
2.67
50
40
30
20
10
2.68
2.69
2.70
2.71
2.72
2.73
2.74
2.75
2.76
0
2.64
1NN distance, Å
2.66
2.68
2.70
2.72
2.74
2.76
2.78
2.80
1NN distance, Å
Bulk Relaxation
Surface Relaxation
• Theoretical:
<d1NN> = 2.706 Å
2 = 0.0003 Å2
• Experimental:
<d1NN> = 2.753(4) Å
2 = 0.0017(2) Å2
<d1NN>BULK = 2.77 Å
<d1NN>FOIL = 2.761(2) Å
• Theoretical:
<d1NN> = 2.74 Å
2 = 0.0022 Å2
• Experimental:
<d1NN> = 2.753(4) Å
2 = 0.0017(2) Å2
2.82
Bond Length Distributions: 40% Pt/C
3000
400
2500
Frequency distribution
Frequency distribution
350
300
250
200
150
100
2000
1500
1000
500
50
0
0
2.68
2.70
2.72
2.74
2.76
2.66
1NN distance, Å
2.68
2.70
2.72
2.74
2.76
2.78
1NN distance, Å
Bulk Relaxation
Surface Relaxation
• Theoretical:
<d1NN> = 2.689 Å
2 = 0.0002 Å2
• Experimental:
<d1NN> = 2.761(7) Å
2 = 0.0010(2) Å2
• Theoretical:
<d1NN> = 2.76 Å
2 = 0.0013 Å2
• Experimental:
<d1NN> = 2.761(7) Å
2 = 0.0010(2) Å2
<d1NN>BULK = 2.77 Å
<d1NN>FOIL = 2.761(2) Å
2.80
2.82
Future Directions
• In-depth modeling of relaxation phenomena.
• Further understanding the “nano-phase” behavior of bimetallic
particles.
• Polymer matrices as supports and stabilizers for nanoparticles.
• Silanes
• Hydrogels
Acknowledgments
Dr. Ralph Nuzzo
Dr. Andy Gewirth
Dr. Tom Rauchfuss
Dr. John Shapley
Dr. Anatoly Frenkel
Dr. Michael Nashner
Dr. Ray Twesten
Dr. Rick Haasch
Nuzzo Research Group
Funding:
Department of Energy
Office of Naval Research