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
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Atomic Scale Ordering in Metallic Nanoparticles
Characterization
(111)
(001)
(110)
Face Centered Cubic Structure
[011]
[112]
[310]
Electron Microdiffraction
Electron diffraction probes the ordered microstructure of the nanoparticles.
Above are 3 sample diffraction patterns for ~ 20 Å Pt nanoparticles. All are
indexed as facecentered cubic (fcc).
Absorption
Photon Energy
XRay Absorption Spectroscopy
Adapted from Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: New York, 1986.
Pt L3 edge (11564 eV)
Pt foil
I0
IT
EXAFS
x
Extended Xray Absorption Fine Structure
hn
e
Ri
PE = hn  E0
E0
initial
final
Ri  distance to shelli
Ai(k)  backscattering amp.
Basics of EXAFS
Data Analysis
m
Convert to wave number
m0
m0(0)
Subtract background and normalize
Resulting data is the sum of scattering from all shells
R
1
Pt L3 edge, Pt foil
)
3
(r)(Å
R
c
3

R
R
4
2
r (Å)
Fourier Transform
Resolve the scattering from each distance (Ri) into rspace
MultipleShell Fit
Calculate Fi(k) and di(k) for each shelli (i = 1 to 6) using the FEFF computer code
Nonlinear leastsquare refinement: vary Ni, Ri, s2i using the EXAFS equation
SS1
TR2
DS
SS4
TS
SS2
SS3
TR1
SS5
TR3
Inplane atom
Aboveplane atom
Absorbing atom
Multiple Scattering Paths
XRay 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 Dependence
Size dependence on the extended xray 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.
Multiple Shell Fitting Analysis
40% Pt/C
10% Pt/C
Temperature Dependence
Temperature dependence on the extended xray absorption spectra for 10% Pt/C.
As the temperature increases, the dynamic disorder (D2) increases, causing the
amplitude to decrease.
First Shell Fitting: 10% Pt/C
200 K
300 K
673 K
473 K
The EXAFS Disorder, s2, is the sum
of the static, ss2, and dynamic, sd2,
disorder as follows:
The dynamic disorder, sd2, can be
separated by using the following
relationship:
Size Dependent Scaling of Bond Length and Disorder
Structure and Morphology
Hemispherical cuboctahedron, (111) basal plane
Hemispherical cuboctahedron, (001) basal plane
Spherical cuboctahedron
Theoretical vs. Experimental
Spherical
Hemispherical
Molecular Modeling: Understanding Disorder
Surface Relaxation
Bulk Relaxation
Bond Length Distributions: 10% Pt/C
<d1NN>BULK= 2.77 Å
<d1NN>FOIL= 2.761(2) Å
Bulk Relaxation
Surface Relaxation
Bond Length Distributions: 40% Pt/C
<d1NN>BULK= 2.77 Å
<d1NN>FOIL= 2.761(2) Å
Future Directions
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