atomic scale ordering in metallic nanoparticles structure: atomic packing: microstructure? cluster...
TRANSCRIPT
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Atomic Scale Ordering in Metallic NanoparticlesAtomic Scale Ordering in Metallic Nanoparticles
Structure:
• Atomic packing: microstructure?
• Cluster shape?
• Surface structure?
• Disorder?
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CharacterizationCharacterization
• 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
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(111)
(001)
(110)
Face Centered Cubic StructureFace Centered Cubic Structure
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Electron MicrodiffractionElectron 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).
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X-Ray Absorption SpectroscopyX-Ray Absorption Spectroscopy
• Absorption coefficient () vs. incident photon energy
• The photoelectric absorption decreases with increasing
energy
• “Jumps” correspond to excitation of core electrons
Adapted from Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: New York, 1986.
Abs
orpt
ion
Photon Energy
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Extended X-ray Absorption Fine Extended X-ray Absorption Fine StructureStructure
• oscillation of the X-ray absorption coefficient near and edge
• local (<10 Å) structure surrounding the absorbing atom
EXAFS
Pt foil
xI
Iln 0
I0 IT
x
Pt L3 edge (11564 eV)
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• Excitation of a photoelectron with
wavenumber k = 2/
h
initial final
e-
E0
PE = h - E0
Ri
• Oscillations, i(k): final state interference
between outgoing and backscattered photoelectron
)2sin()()( iii kRkAk
Ri - distance to shell-i
Ai(k) - backscattering amp.
Basics of EXAFSBasics of EXAFS
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00(0)
)0()(
0
0
k
Convert to wave number
Subtract background and normalize
Data AnalysisData Analysis
Resulting data is the sum of scattering from all shells
i
i kk )()(
)(2
02Eh
mk
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|(r
)|(Å
-3)
r (Å)
R2
R3R4
Pt L3 edge, Pt foilR
1
Fourier TransformFourier Transform
Resolve the scattering from each distance (Ri) into r-space
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Multiple-Shell FitMultiple-Shell Fit
Calculate Fi(k) and i(k) for each shell-i (i = 1 to 6) using the FEFF computer code
))(2sin()(
)(222
2kkRe
kR
kFNk ii
k
i
iii
Non-linear least-square refinement: vary Ni, Ri, 2i using the EXAFS equation
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SS2
SS3
SS4
SS5
DSTS
TR3
TR2
TR1
SS1
Multiple Scattering PathsMultiple Scattering Paths
In-plane atom
Above-plane atom
Absorbing atom
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11560 11565 11570 11575 11580 11585 11590 11595 116000.0
0.2
0.4
0.6
0.8
1.0
1.2
Nor
mal
ized
abs
orpt
ion
coef
ficie
nt
Energy, eV
X-Ray Absorption Near Edge Spectroscopy (XANES)X-Ray Absorption Near Edge Spectroscopy (XANES)
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.
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Size DependenceSize Dependence
Size dependence on the extended x-ray absorption spectra. The amplitude of the EXAFS signal is directly proportional to the coordination numbers for eachshell; therefore, as the cluster size increases, the amplitude also will increase.
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Multiple Shell Fitting AnalysisMultiple Shell Fitting Analysis
10% Pt/C 40% Pt/C
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0 2 4 6 8 10 12 14 16 18 20 22-1.5
-1.0
-0.5
0.0
0.5
1.0
k2 (k)
, Å-2
k, Å-1
200 K 300 K 473 K 673 K
0 1 2 3 4 5 6 7 8 9 100.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
200 K 300 K 473 K 673 K
FT
Mag
nitu
de, Å
-3r, Å
Temperature DependenceTemperature Dependence
Temperature dependence on the extended x-ray absorption spectra for 10% Pt/C. As the temperature increases, the dynamic disorder (D
2) increases, causing the amplitude to decrease.
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0 1 2 3 4 5 6 7 8 9 100.0
0.5
1.0
1.5
2.0
2.5
Data Fit
FT
Mag
nitu
de, Å
-3
r, Å
0 1 2 3 4 5 6 7 8 9 100.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Data Fit
FT
Mag
nitu
de, Å
-3
r, Å
0 1 2 3 4 5 6 7 8 9 100.0
0.2
0.4
0.6
0.8
1.0
Data Fit
FT
Mag
nitu
de, Å
-3
r, Å
0 1 2 3 4 5 6 7 8 9 100.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Data Fit
FT
Mag
nitu
de, Å
-3
r, Å
First Shell Fitting:First Shell Fitting: 10% Pt/C 10% Pt/C
200 K 300 K
473 K 673 K
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Size Dependent Scaling of Bond Length and DisorderSize Dependent Scaling of Bond Length and Disorder
))(2sin()(
)(222
2kkRe
kR
kFNk ii
k
i
iii
2222dsrr
)/exp(1
)/exp(1
2 E
E2
T
Td
The EXAFS Disorder, 2, is the sum of the static, s
2, and dynamic, d2,
disorder as follows:
The dynamic disorder, d2, can be
separated by using the following relationship:
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Hemispherical cuboctahedron, (111) basal plane
Hemispherical cuboctahedron, (001) basal plane
Spherical cuboctahedron
Structure and MorphologyStructure and Morphology
• Determining shape and texture
• Electron microscopy
• X-Ray absorption spectroscopy
• Molecular modeling
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Theoretical vs. ExperimentalTheoretical vs. Experimental
Spherical
Hemispherical
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Molecular Modeling: Molecular Modeling: Understanding DisorderUnderstanding Disorder
• Probe bulk vs. surface relaxation.• Bulk:
Allow relaxation of entire structure.
• Surface:Allow relaxation of atoms bound in surface sites only.
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Surface Relaxation
• Theoretical: <d1NN> = 2.74 Å 2 = 0.0022 Å2
• Experimental:<d1NN> = 2.753(4) Å 2 = 0.0017(2) Å2
• Theoretical: <d1NN> = 2.706 Å 2 = 0.0003 Å2
• Experimental:<d1NN> = 2.753(4) Å2 = 0.0017(2) Å2
Bulk Relaxation
Bond Length Distributions: Bond Length Distributions: 10% Pt/C10% Pt/C
<d1NN>BULK = 2.77 Å
<d1NN>FOIL = 2.761(2) Å
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Bond Length Distributions:Bond Length Distributions: 40% Pt/C 40% Pt/C
• Theoretical: <d1NN> = 2.689 Å 2 = 0.0002 Å2
• Experimental:<d1NN> = 2.761(7) Å2 = 0.0010(2) Å2
Bulk Relaxation Surface Relaxation
• Theoretical: <d1NN> = 2.76 Å 2 = 0.0013 Å2
• Experimental:<d1NN> = 2.761(7) Å2 = 0.0010(2) Å2<d1NN>BULK = 2.77 Å
<d1NN>FOIL = 2.761(2) Å
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Future DirectionsFuture 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
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AcknowledgmentsAcknowledgments
Dr. Ralph Nuzzo
Dr. Andy GewirthDr. Tom RauchfussDr. John Shapley
Dr. Anatoly FrenkelDr. Michael Nashner
Dr. Ray TwestenDr. Rick Haasch
Nuzzo Research Group
Funding:Department of Energy
Office of Naval Research