94
escape [21, 22]. The thin metal film enhances the IR absorption bands by one or two
orders of magnitude. The origin of the enhancement is still being debated.
7.4.2 X-Ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) is based on the modulation of an atom’s probability to absorb X-rays at energies near and above the binding energy of the atom’s
core-level electron. If the X-ray energy is higher than the core-level electron’s binding energy, the atom absorbs X-rays and the electron (called photoelectron) is
ejected. The eviction of an electron from the first shell corresponds to K-edge,
whereas photoelectron ejection from the second, third, and higher shells are labeled
L, M, etc. Due to the spin-orbital coupling, multiple distinct transitions are observed
from higher shells [23, 24]. The hole in core-electron level is filled by another electron from the higher energy level, and the energy difference between the two levels
may be radiatively emitted by X-ray fluorescence. Qualitative changes in absorption
coefficient (μ) around the edge is followed in X-ray absorption near edge structure
(XANES) as the function of X-ray energy, E. Oscillations in the μ–E plot (Fig. 7.12a)
are due to the interference of the photoelectron wave scattered off of the neighboring atoms with itself at the photoabsorber, and therefore the extended X-ray absorption fine structure (EXAFS) spectrum contains information on the nature and
position in space of atoms in photoabsorber’s immediate surrounding. Instead of
energy, it is more convenient to express the normalized oscillations in photoelectron
wave number, k = 0.512 (E-E o )
1/2
, producing the spectrum in k-space. By Fourier
transformation, this spectrum is converted into the plot in R-space (Fig. 7.12b),
where the real and imaginary parts of the complex function and the magnitude are
plotted as the function of radial distance R (in Å). Radial distances are related to the
actual distances between the photoabsorber and the scatterer atoms but are usually
1
0.5
0
–0.5
–1
0
5
10
15
20
k, Å
–1
R, Å
1.2
0.8
0.4
0 0
1
2
3
4
5
6
data
fit
data
fit
Rh K edge
PtRhSnO 2
Rh K edge
PtRhSnO 2
FT Magnitude, Å
–3
K
2 χ(k),
Å
–2
(a)
(b)
Fig. 7.12 Data and first-shell fitting of the EXAFS spectra of ethanol oxidation on the Pt/Rh/
SnO 2 /C catalyst at Rh K-edge in k-space (a) and R-space (b). From [27] with permission from
Springe Nature
7 Platinum Monolayer Electrocatalysts
escape [21, 22]. The thin metal film enhances the IR absorption bands by one or two
orders of magnitude. The origin of the enhancement is still being debated.
7.4.2 X-Ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) is based on the modulation of an atom’s probability to absorb X-rays at energies near and above the binding energy of the atom’s
core-level electron. If the X-ray energy is higher than the core-level electron’s binding energy, the atom absorbs X-rays and the electron (called photoelectron) is
ejected. The eviction of an electron from the first shell corresponds to K-edge,
whereas photoelectron ejection from the second, third, and higher shells are labeled
L, M, etc. Due to the spin-orbital coupling, multiple distinct transitions are observed
from higher shells [23, 24]. The hole in core-electron level is filled by another electron from the higher energy level, and the energy difference between the two levels
may be radiatively emitted by X-ray fluorescence. Qualitative changes in absorption
coefficient (μ) around the edge is followed in X-ray absorption near edge structure
(XANES) as the function of X-ray energy, E. Oscillations in the μ–E plot (Fig. 7.12a)
are due to the interference of the photoelectron wave scattered off of the neighboring atoms with itself at the photoabsorber, and therefore the extended X-ray absorption fine structure (EXAFS) spectrum contains information on the nature and
position in space of atoms in photoabsorber’s immediate surrounding. Instead of
energy, it is more convenient to express the normalized oscillations in photoelectron
wave number, k = 0.512 (E-E o )
1/2
, producing the spectrum in k-space. By Fourier
transformation, this spectrum is converted into the plot in R-space (Fig. 7.12b),
where the real and imaginary parts of the complex function and the magnitude are
plotted as the function of radial distance R (in Å). Radial distances are related to the
actual distances between the photoabsorber and the scatterer atoms but are usually
1
0.5
0
–0.5
–1
0
5
10
15
20
k, Å
–1
R, Å
1.2
0.8
0.4
0 0
1
2
3
4
5
6
data
fit
data
fit
Rh K edge
PtRhSnO 2
Rh K edge
PtRhSnO 2
FT Magnitude, Å
–3
K
2 χ(k),
Å
–2
(a)
(b)
Fig. 7.12 Data and first-shell fitting of the EXAFS spectra of ethanol oxidation on the Pt/Rh/
SnO 2 /C catalyst at Rh K-edge in k-space (a) and R-space (b). From [27] with permission from
Springe Nature
7 Platinum Monolayer Electrocatalysts