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).

XX--Ray Absorption SpectroscopyRay Absorption Spectroscopy• Absorption coefficient (µ) 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 XExtended X--ray Absorption Fine ray Absorption Fine StructureStructure

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

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

Photon Energy (eV)

11400 11600 11800 12000 12200 12400

Abs

orpt

ion

( µx)

0

1

EXAFS

Pt foil

µxII

= ln 0

I0 IT

x

Pt L3 edge (11564 eV)

• Excitation of a photoelectron with

wavenumber k = 2π/λ

hν

initial final

e-

E0

PE = hν - E0

Ri

• Oscillations, χi(k): final state interference

between outgoing and backscattered photoelectron

)2sin()()( iii kRkAk =χ

Ri - distance to shell-iAi(k) - backscattering amp.

Basics of EXAFSBasics of EXAFS

Abs

orpt

ion

( µx)

0

1

Photoelectron Energy (eV)

0 200 400 600 800

µµ0µ0(0)

k2 χ(k)

(Å-2

)

0 2 4 6 8 10 12 14 16-3

-2

-1

0

1

2

3

k (Å-1)

)0()(

0

0

µµµχ −=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 )()( χχ

)(202 Ehmk −= ν

h

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

| χ(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

MultipleMultiple--Shell FitShell Fit

Calculate Fi(k) and δi(k) for each shell-i (i = 1 to 6) using the FEFF computer code

))(2sin()()(222

2 kkRekR

kFNk iik

i

iii δχ σ += −

Non-linear least-square refinement: vary Ni, Ri, σ2i using the EXAFS equation

0 1 2 3 4 5 6 7 8 9 100

1

2

3Pt L3, Pt foil

Multiple-Shell Fit Bond distance, Ri (Å)

R1 R2 R3 R4 R5

fit 2.768(3) 3.914(4) 4.794(4) 5.535(5) 6.189(6)

actual 2.7719 3.9200 4.8010 5.5437 6.1981

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

XX--Ray Absorption Near Edge Spectroscopy (XANES)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

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

Pt foil 60% Pt/C 40% Pt/C 10% Pt/C

FT M

agni

tude

, Å-3

r, Å0 2 4 6 8 10 12 14 16 18 20 22

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5 Pt foil 60% Pt/C 40% Pt/C 10% Pt/C

k2 χ (k)

, Å-2

k, Å-1

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.

i 10% Pt/C 40% Pt/C 60% Pt/C Pt foil Bulk fcc1 8.3(5) 10.5(5) 11.4(6) 12.6(7) 122 2.3(1.1) 4.0(1.3) 4.7(1.7) 5.9(2.0) 63 10.9(3.2) 16.8(3.5) 19(4) 23(5) 244 5.5(1.4) 7.6(1.4) 8.5(1.6) 11(2) 125 5.4(3.4) 10(4) 11(4) 14(5) 24

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

Data Fit

FT M

agni

tude

, Å-3

r, Å0 1 2 3 4 5 6 7 8 9 10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Data Fit

FT M

agni

tude

, Å-3

r, Å

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 χ(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 M

agni

tude

, Å-3

r, Å

Temperature DependenceTemperature Dependence

Temperature dependence on the extended x-ray absorption spectra for 10% Pt/C. As the temperature increases, the dynamic disorder (σD

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 M

agni

tude

, Å-3

r, Å0 1 2 3 4 5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Data Fit

FT M

agni

tude

, 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 M

agni

tude

, Å-3

r, Å0 1 2 3 4 5 6 7 8 9 10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Data Fit

FT M

agni

tude

, Å-3

r, Å

First Shell Fitting:First Shell Fitting: 10% Pt/C10% 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

2 kkRekR

kFNk iik

i

iii δχ σ += −

200 300 400 500 600 700

2.7482.7502.7522.7542.7562.7582.7602.7622.7642.7662.7682.7702.7722.7742.7762.7782.7802.7822.7842.786

Dis

tanc

e, Å

Temperature, K

10% Pt/C 40% Pt/C 60% Pt/C Pt foil

( ) 2222dsrr σσσ +=−=

)/exp(1)/exp(1

2 E

E2TT

d Θ−−Θ−+=

ωµσ h

The EXAFS Disorder, σ2, is the sum of the static, σs

2, and dynamic, σd2,

disorder as follows:

The dynamic disorder, σd2, 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

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

2

4

6

8

10

12

14

16

18

20

22

24Bulk 3NN and 5NN

Bulk 2NN

Bulk 1NN and 4NN

Coo

rdin

atio

n nu

mbe

r

L

1NN 2NN 3NN 4NN 5NN

0 10 20 30 40 50 60 70 80

Cluster diameter, Å

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

2

4

6

8

10

12

14

16

18

20

22

24

Bulk 2NN

Bulk 1NN and 4NN

Bulk 3NN and 5NN

1NN 2NN 3NN 4NN 5NN

Coo

rdin

atio

n nu

mbe

r

L

0 10 20 30 40 50 60 70 80

Cluster diameter, Å

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

2

4

6

8

10

12

14

16

18

20

22

24

No overlap between 1NN and 2NN

1NN 2NN 3NN 4NN 5NN

Coo

rdin

atio

n nu

mbe

r

L

0 10 20 30 40 50 60 70 80

Bulk 2NN

Bulk 1NN and 4NN

Bulk 3NN and 5NN

Cluster diameter, Å

Theoretical Theoretical vsvs. Experimental. 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.

2.64 2.66 2.68 2.70 2.72 2.74 2.76 2.78 2.80 2.820

10

20

30

40

50

60

70

Freq

uenc

y di

strib

utio

n

1NN distance, Å

Surface Relaxation

• Theoretical:<d1NN> = 2.74 Åσ2 = 0.0022 Å2

• Experimental:<d1NN> = 2.753(4) Å σ2 = 0.0017(2) Å2

2.67 2.68 2.69 2.70 2.71 2.72 2.73 2.74 2.75 2.760

10

20

30

40

50

60

70

Freq

uenc

y di

strib

utio

n

1NN distance, Å

• Theoretical:<d1NN> = 2.706 Åσ2 = 0.0003 Å2

• Experimental:<d1NN> = 2.753(4) Åσ2 = 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/C40% Pt/C

• Theoretical:<d1NN> = 2.689 Åσ2 = 0.0002 Å2

• Experimental:<d1NN> = 2.761(7) Åσ2 = 0.0010(2) Å2

Bulk Relaxation

2.68 2.70 2.72 2.74 2.760

50

100

150

200

250

300

350

400

Freq

uenc

y di

strib

utio

n

1NN distance, Å

Surface Relaxation

• Theoretical:<d1NN> = 2.76 Åσ2 = 0.0013 Å2

• Experimental:<d1NN> = 2.761(7) Åσ2 = 0.0010(2) Å2

2.66 2.68 2.70 2.72 2.74 2.76 2.78 2.80 2.820

500

1000

1500

2000

2500

3000

Freq

uenc

y di

strib

utio

n

1NN distance, Å

<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