tomic Scale Ordering in Metallic Nanoparticl omic Scale Ordering in Metallic Nanoparticl Structure: • Atomic packing: microstructur • Cluster shape? • Surface structure? • Disorder?
Jan 05, 2016
Atomic Scale Ordering in Metallic NanoparticlesAtomic Scale Ordering in Metallic Nanoparticles
Structure:
• Atomic packing: microstructure?
• Cluster shape?
• Surface structure?
• Disorder?
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
(111)
(001)
(110)
Face Centered Cubic StructureFace Centered Cubic Structure
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).
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
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)
• 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
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
|(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
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
SS2
SS3
SS4
SS5
DSTS
TR3
TR2
TR1
SS1
Multiple Scattering PathsMultiple Scattering Paths
In-plane atom
Above-plane atom
Absorbing atom
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.
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.
Multiple Shell Fitting AnalysisMultiple Shell Fitting Analysis
10% Pt/C 40% Pt/C
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.
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
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:
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
Theoretical vs. ExperimentalTheoretical vs. Experimental
Spherical
Hemispherical
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.
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
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
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
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