1 3
Topics in Current Chemistry (2019) 377:11
absorption coefficient called the white line [71]. If the atom absorbing the X-rays
(photoabsorber) were completely isolated, µ would continue to decline after the edge
following the µ–E relation. Oscillations in the µ–E plot (Fig. 3a) are due to the interference of the photoelectron wave scattered off of the neighboring atoms with itself
at the photoabsorber, and therefore the EXAFS contains information on the nature
and position in space of atoms in photoabsorber’s immediate surrounding. These
oscillations, or “fine structure” of EXAFS, are extracted by subtracting the background µ o (E) and dividing it by the edge step ∆µ o (E o ) to produce oscillations χ(E)
normalized to one absorption event: χ(E) = [µ(E) − µ o (E)]/∆µ o (E o ), thus obtaining
the EXAFS spectrum (Fig. 3b). It is more convenient to express χ(E) as a function
of photoelectron wave number, k = 0.512 (E − E o )
1/2
(in Å
−1
), where E o is the edge
energy. To amplify the oscillations at higher k values, χ(k) is often multiplied by k
2
or k
3
, producing the plot referred to as the EXAFS spectrum in k-space (Fig. 3c).
By the Fourier transformation, this spectrum is converted into the plot in R space
(Fig. 3d), where the real and imaginary part of the complex function and the magnitude are plotted as the function of radial distance (in Å). Radial distances are related
to the actual distances between the photoabsorber and scatterer atoms but are usually
offset by 0.3–0.5 Å due to phase shift. The actual distances between the atoms are
revealed by subsequent fitting of the experimental spectrum to the theoretical one
constructed from the crystal lattice of a selected model featuring scattered atoms,
their coordination number, and the distance to the absorber.
Besides the necessity of a tunable X-ray source, for a good quality of XAS spectrum, the flux of the incident beam must be at least 10
10
photons/s, and the width
of the monochromatic beam should be smaller than 1 eV to obtain reasonable signal-to-noise ratio and spectral resolution, respectively. These requirements are only
met with synchrotron radiation [70–75]. The XAS setup consists of two detectors
measuring the incident and transmitted beam through the sample. The absorption
coefficient is related to the ratio I/I 0 by Beer’s law, similarly to that in the IR experiment described above. For weakly concentrated and/or X-ray opaque samples, fluorescence mode is often used with the detector positioned perpendicularly to the incidence beam path to reduce the intensity of the scattered X-rays.
The spectroelectrochemical cell for in situ measurements is presented in Fig. 4.
The working electrode consists of the electrocatalyst deposited on a carbon cloth or
carbon paper. Another strip of blank carbon cloth is used as the auxiliary (counter)
electrode. If gas evolution on the counter electrode is extensive, it can be placed outside of the X-ray path. A Nafion membrane separates the working and counter electrodes. The working-auxiliary electrode pair, Teflon rings and spacers are all sealed
in a grove of the acrylic body of the cell. When filled with electrolyte, the electrodes
are in contact with a Ag/AgCl leak-tight reference electrode, placed away from the
X-ray path. The cell can be used in both transmission and fluorescence mode. The
EXAFS spectrum is measured at the constant potential once the current reaches the
steady state [78, 79].
Processing of an EXAFS spectrum is accomplished by fitting the spectrum to a
predicted structure. While there are several computer programs for this purpose, for
the interpretation of the examples given here we used Demeter package. The freeware includes the Athena program containing LCA, PCA, and spectral subtraction
Reprinted from the journal
9
Topics in Current Chemistry (2019) 377:11
absorption coefficient called the white line [71]. If the atom absorbing the X-rays
(photoabsorber) were completely isolated, µ would continue to decline after the edge
following the µ–E relation. Oscillations in the µ–E plot (Fig. 3a) are due to the interference of the photoelectron wave scattered off of the neighboring atoms with itself
at the photoabsorber, and therefore the EXAFS contains information on the nature
and position in space of atoms in photoabsorber’s immediate surrounding. These
oscillations, or “fine structure” of EXAFS, are extracted by subtracting the background µ o (E) and dividing it by the edge step ∆µ o (E o ) to produce oscillations χ(E)
normalized to one absorption event: χ(E) = [µ(E) − µ o (E)]/∆µ o (E o ), thus obtaining
the EXAFS spectrum (Fig. 3b). It is more convenient to express χ(E) as a function
of photoelectron wave number, k = 0.512 (E − E o )
1/2
(in Å
−1
), where E o is the edge
energy. To amplify the oscillations at higher k values, χ(k) is often multiplied by k
2
or k
3
, producing the plot referred to as the EXAFS spectrum in k-space (Fig. 3c).
By the Fourier transformation, this spectrum is converted into the plot in R space
(Fig. 3d), where the real and imaginary part of the complex function and the magnitude are plotted as the function of radial distance (in Å). Radial distances are related
to the actual distances between the photoabsorber and scatterer atoms but are usually
offset by 0.3–0.5 Å due to phase shift. The actual distances between the atoms are
revealed by subsequent fitting of the experimental spectrum to the theoretical one
constructed from the crystal lattice of a selected model featuring scattered atoms,
their coordination number, and the distance to the absorber.
Besides the necessity of a tunable X-ray source, for a good quality of XAS spectrum, the flux of the incident beam must be at least 10
10
photons/s, and the width
of the monochromatic beam should be smaller than 1 eV to obtain reasonable signal-to-noise ratio and spectral resolution, respectively. These requirements are only
met with synchrotron radiation [70–75]. The XAS setup consists of two detectors
measuring the incident and transmitted beam through the sample. The absorption
coefficient is related to the ratio I/I 0 by Beer’s law, similarly to that in the IR experiment described above. For weakly concentrated and/or X-ray opaque samples, fluorescence mode is often used with the detector positioned perpendicularly to the incidence beam path to reduce the intensity of the scattered X-rays.
The spectroelectrochemical cell for in situ measurements is presented in Fig. 4.
The working electrode consists of the electrocatalyst deposited on a carbon cloth or
carbon paper. Another strip of blank carbon cloth is used as the auxiliary (counter)
electrode. If gas evolution on the counter electrode is extensive, it can be placed outside of the X-ray path. A Nafion membrane separates the working and counter electrodes. The working-auxiliary electrode pair, Teflon rings and spacers are all sealed
in a grove of the acrylic body of the cell. When filled with electrolyte, the electrodes
are in contact with a Ag/AgCl leak-tight reference electrode, placed away from the
X-ray path. The cell can be used in both transmission and fluorescence mode. The
EXAFS spectrum is measured at the constant potential once the current reaches the
steady state [78, 79].
Processing of an EXAFS spectrum is accomplished by fitting the spectrum to a
predicted structure. While there are several computer programs for this purpose, for
the interpretation of the examples given here we used Demeter package. The freeware includes the Athena program containing LCA, PCA, and spectral subtraction
Reprinted from the journal
9