Absorption%20coefficient%20(m)%20vs.%20incident%20photon%20energy

1
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.
2
Extended X-ray Absorption Fine Structure

• oscillation of the X-ray absorption coefficient
  near and edge
• local (lt10 Å) structure surrounding the
  absorbing atom

3
Basic Physics of EXAFS

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

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

Ri - distance to shell-i Ai(k) -
backscattering amp.
4
Single-Scattering Plane-Wave Approximation
Relevant structural parameters for shell-i
Ri - distance of neighbors Ni -
number of neighbors at Ri s2 - root mean
deviation in Ri
Obtain from model compounds or theory
Fi(k) - backscattering amplitude
(Z-dependent) di(k) - total phase shift from
the central and backscattering atom
potential
5
Data Analysis Reference Pt foil
m
(1) convert to wave number
m0
m0(0)
(2) subtract background and normalize
(3) resulting data is the sum of scattering from
all shells
6
Fourier Transform
(4) Resolve the scattering from each distance
(Ri) into r-space
Single-scattering paths Pt metal (fcc)
7
Multiple-Shell Fit Pt Reference
8
Other Reference Materials
9
EXAFS Measurements
Requires high intensity and tunable X-ray
radiation (10 - 25 keV)
10
Experimental Setup
Sample X-ray Detectors
Monochromator
X-ray Source
in-situ catalyst reactor
double crystal Si(111)
broad spectrum X-ray beam (ca. 3 - 50 keV)
11
X-ray Focusing Monochromator
Si(111)

• Problem horizontal divergence of the X-ray
  beam
• Solution horizontally focus the X-ray beam
  (0.3 mm)
• Dynamically follows energy (q) to maintain
  focus at the sample

12
Custom Designed and Built In-situ Cell

• Sample may be heated (773 K) and cooled (190 K)
  while measuring XAS data under controlled
  atmosphere.

13
EXAFS Experiment
(1) disperse PtRu5C(CO)16 on carbon black from
THF solution
(2) Measure EXAFS at 190 K under H2 Pt L3
(11564 eV) Ru K (22117 eV)
14
Pt0.01Ru0.99
PtRu5/C, Pt L3 edge data
Compare with Pt metal and Pt0.01Ru0.99

• metal coordination
• reduced amplitude lower coordination

Pt foil
15
Ru metal
16
Non-statistical Distribution?

• Close-packed structure
• Fit PtRu5/C data

(1) use fcc structural model to fourth shell (2)
simultaneously fit Pt L3 and Ru K EXAFS
data (3) constrain heterometallic
parameters RPt-Ru RRu-Pt NPt-Ru
5NRu-Pt s2Pt-Ru s2Ru-Pt
17
First-Shell Fit Results
Ave. NM-M 6.3

• Reference data

Pt RPt-Pt 2.77 Å (fcc) Pt.01Ru.99
RPt-Ru 2.70 Å (hcp) Ru RRu-Ru 2.67 Å
(hcp)
18
Coordination Number vs. Nanoparticle Diameter
Consider a model hemispherical cuboctahedron
(STEM)
Nanoparticle Diameter (Å)
19
Non-Statistical First-Shell Coordination
20
Intra-Particle Distributions
21
Differences in Higher-Shell Environment
22
Higher-Shell Coordination Numbers
PtRu5/C
Representative nanoparticle
Model
Model
23
Possible Nanoparticle-Support Interaction

• incommensurate with graphitic surface layers of
  carbon black

• face capping bonding geometry of
• triangular Ru3(CO)9 with C60

nanoparticle hexagonal base RMM 2.67 Å
graphitic layer RCC 1.42 Å
Hsu, H. -F. Shapley, J. R. JACS 1996, 118, 9192
24
Nucleation and Growth from Molecular Precursor
25
Evolution of the First-Shell Coordination Numbers

• Low temperature Pt-Pt bond formation
• Non-statistical growth

26
Platinum Segregation Core to Surface Inversion
27
Summary and Conclusions
New insights into nanoscale alloy structure and
phase dynamics

• (1) Carbon-supported Pt-Ru nanoparticles from
  PtRu5C(CO)16
• narrow size (ca. 1.5 nm) and composition
  distribution (15)
• adopt fcc structure (not bulk structure)
• (2) Probe nanoparticle microstructure with
  in-situ EXAFS
• Non-statistical distribution
• - Pt surface segregation
• - difference in ordering of Pt and Ru
  domains
• (3) Nanoparticles evolve via an inversion process

28
Acknowledgments
Ralph Nuzzo John Shapley Gregory Girolami Andrew
Gewirth Anatoly Frenkel (UIUC) David Adler (KLA
Instruments) Kwanyeol Lee The Amazing Nuzzo
Group members Tracy Office of Naval
Research Department of Chemistry
29
X-ray Absorption Near Edge Structure