Atomic Scale Ordering in Metallic Nanoparticles
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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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Atomic scale ordering in metallic nanoparticles

Atomic Scale Ordering in Metallic Nanoparticles

  • Structure:

    • Atomic packing: microstructure?

    • Cluster shape?

    • Surface structure?

    • Disorder?


Atomic scale ordering in metallic nanoparticles

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


Atomic scale ordering in metallic nanoparticles

(111)

(001)

(110)

Face Centered Cubic Structure


Atomic scale ordering in metallic nanoparticles

[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 face-centered cubic (fcc).


Atomic scale ordering in metallic nanoparticles

Absorption

Photon Energy

X-Ray Absorption Spectroscopy

  • Absorption coefficient (m) 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.


Atomic scale ordering in metallic nanoparticles

Pt L3 edge (11564 eV)

Pt foil

I0

IT

EXAFS

x

Extended X-ray Absorption Fine Structure

  • oscillation of the X-ray absorption coefficient near and edge

  • local (<10 Å) structure surrounding the absorbing atom


Atomic scale ordering in metallic nanoparticles

hn

e-

Ri

PE = hn - E0

E0

initial

final

  • Excitation of a photoelectron with wavenumber k = 2p/l

  • Oscillations, ci(k): final state interference between outgoing and backscattered photoelectron

Ri - distance to shell-i

Ai(k) - backscattering amp.

Basics of EXAFS


Atomic scale ordering in metallic nanoparticles

Data Analysis

m

Convert to wave number

m0

m0(0)

Subtract background and normalize

Resulting data is the sum of scattering from all shells


Atomic scale ordering in metallic nanoparticles

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 r-space


Atomic scale ordering in metallic nanoparticles

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, s2i using the EXAFS equation


Atomic scale ordering in metallic nanoparticles

SS1

TR2

DS

SS4

TS

SS2

SS3

TR1

SS5

TR3

In-plane atom

Above-plane atom

Absorbing atom

Multiple Scattering Paths


Atomic scale ordering in metallic nanoparticles

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.


Atomic scale ordering in metallic nanoparticles

Size Dependence

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.


Atomic scale ordering in metallic nanoparticles

Multiple Shell Fitting Analysis

40% Pt/C

10% Pt/C


Atomic scale ordering in metallic nanoparticles

Temperature Dependence

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.


Atomic scale ordering in metallic nanoparticles

First Shell Fitting: 10% Pt/C

200 K

300 K

673 K

473 K


Atomic scale ordering in metallic nanoparticles

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


Atomic scale ordering in metallic nanoparticles

Structure and Morphology

Hemispherical cuboctahedron, (111) basal plane

  • Determining shape and texture

    • Electron microscopy

    • X-Ray absorption spectroscopy

    • Molecular modeling

Hemispherical cuboctahedron, (001) basal plane

Spherical cuboctahedron


Atomic scale ordering in metallic nanoparticles

Theoretical vs. Experimental

Spherical

Hemispherical


Atomic scale ordering in metallic nanoparticles

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.


Atomic scale ordering in metallic nanoparticles

Surface Relaxation

Bulk 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) Å

    • 2 = 0.0017(2) Å2

Bond Length Distributions: 10% Pt/C

<d1NN>BULK= 2.77 Å

<d1NN>FOIL= 2.761(2) Å


Atomic scale ordering in metallic nanoparticles

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

Bond Length Distributions: 40% Pt/C

<d1NN>BULK= 2.77 Å

<d1NN>FOIL= 2.761(2) Å


Atomic scale ordering in metallic nanoparticles

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


Atomic scale ordering in metallic nanoparticles

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


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