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
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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).
Photon Energy
XRay Absorption Spectroscopy
Adapted from Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: New York, 1986.
Pt foil
I0
IT
EXAFS
x
Extended Xray Absorption Fine Structure
e
Ri
PE = hn  E0
E0
initial
final
Ri  distance to shelli
Ai(k)  backscattering amp.
Basics of EXAFS
m
Convert to wave number
m0
m0(0)
Subtract background and normalize
Resulting data is the sum of scattering from all shells
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
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
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 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.
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.
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
Hemispherical cuboctahedron, (111) basal plane
Hemispherical cuboctahedron, (001) basal plane
Spherical cuboctahedron
Molecular Modeling: Understanding Disorder
Bulk Relaxation
Bond Length Distributions: 10% Pt/C
<d1NN>BULK= 2.77 Å
<d1NN>FOIL= 2.761(2) Å
Surface Relaxation
Bond Length Distributions: 40% Pt/C
<d1NN>BULK= 2.77 Å
<d1NN>FOIL= 2.761(2) Å
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