1 3
Topics in Current Chemistry (2019) 377:11
the electrolyte were removed by bubbling argon above the electrode, while dry air
was purging the IR optical path to remove CO 2 and water vapor in air.
2.2 In Situ XAS Experimental Details
X-ray absorption spectroscopy (XAS) is based on 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 [70–75]. 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.
The change in absorption coefficient (µ) around the edge is followed in X-ray
absorption spectroscopy (XAS) as the function of X-ray energy, E. Graphically
represented, the spectrum is, in principle, similar to the one shown in Fig. 3. The
XAS spectrum is divided into two parts: the spectral part within ca. ± 50 eV around
the edge (E o , Fig. 3a) is called X-ray absorption near-edge spectroscopy (XANES),
whereas the part of the spectrum at higher energies is termed extended X-ray absorption fine structure (EXAFS). The two parts of the spectrum are treated in a different
manner and offer related information on the analyzed system [71].
Because the XANES part of the XAS spectrum corresponds to the multielectron
transitions that are difficult to express mathematically, this spectral region is modeled qualitatively. The important features are the edge shift arising as the consequence of the oxidation state of the photo-absorber and pre-edge features that can be
used in fingerprinting. In addition, linear combination analysis (LCA) can be used to
distinguish a mixture by matching its spectrum with a modeled one obtained by adding together fractions of spectra of various standards.
When a series of XANES spectra are taken “in-operando”, principle component analysis (PCA) can be used to identify a set of constituents of the XAS spectra [70–75]. For electrochemical measurements, delta mu (∆µ) technique has been
developed to identify subtle changes on the surface due to the adsorption or desorption of species from the electrolyte caused by the potential change [76, 77].
Dominating bulk metal–metal interactions are removed as constant by spectral subtraction of two XANES spectra of the same sample at two different potentials. For
instance, in delta mu spectrum of carbon-supported Pt nanoparticles obtained as
∆µ = µ(V) − µ(DL), where µ(V), and µ(DL) represent spectra at a potential V and that
of the double layer where no adsorbate species are present, surface adsorbates (O
and OH) and sub-surface O were identified and interpreted by the comparison with
theoretical curves made based on crystallographic models [78].
Unlike XANES, the EXAFS region can be described mathematically. The X-ray
absorption coefficient decreases with energy by the relation µ ~ 1/E
4
, except when
the photon energy is sufficient to eject an electron, causing a sharp rise in the
Reprinted from the journal
7
Topics in Current Chemistry (2019) 377:11
the electrolyte were removed by bubbling argon above the electrode, while dry air
was purging the IR optical path to remove CO 2 and water vapor in air.
2.2 In Situ XAS Experimental Details
X-ray absorption spectroscopy (XAS) is based on 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 [70–75]. 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.
The change in absorption coefficient (µ) around the edge is followed in X-ray
absorption spectroscopy (XAS) as the function of X-ray energy, E. Graphically
represented, the spectrum is, in principle, similar to the one shown in Fig. 3. The
XAS spectrum is divided into two parts: the spectral part within ca. ± 50 eV around
the edge (E o , Fig. 3a) is called X-ray absorption near-edge spectroscopy (XANES),
whereas the part of the spectrum at higher energies is termed extended X-ray absorption fine structure (EXAFS). The two parts of the spectrum are treated in a different
manner and offer related information on the analyzed system [71].
Because the XANES part of the XAS spectrum corresponds to the multielectron
transitions that are difficult to express mathematically, this spectral region is modeled qualitatively. The important features are the edge shift arising as the consequence of the oxidation state of the photo-absorber and pre-edge features that can be
used in fingerprinting. In addition, linear combination analysis (LCA) can be used to
distinguish a mixture by matching its spectrum with a modeled one obtained by adding together fractions of spectra of various standards.
When a series of XANES spectra are taken “in-operando”, principle component analysis (PCA) can be used to identify a set of constituents of the XAS spectra [70–75]. For electrochemical measurements, delta mu (∆µ) technique has been
developed to identify subtle changes on the surface due to the adsorption or desorption of species from the electrolyte caused by the potential change [76, 77].
Dominating bulk metal–metal interactions are removed as constant by spectral subtraction of two XANES spectra of the same sample at two different potentials. For
instance, in delta mu spectrum of carbon-supported Pt nanoparticles obtained as
∆µ = µ(V) − µ(DL), where µ(V), and µ(DL) represent spectra at a potential V and that
of the double layer where no adsorbate species are present, surface adsorbates (O
and OH) and sub-surface O were identified and interpreted by the comparison with
theoretical curves made based on crystallographic models [78].
Unlike XANES, the EXAFS region can be described mathematically. The X-ray
absorption coefficient decreases with energy by the relation µ ~ 1/E
4
, except when
the photon energy is sufficient to eject an electron, causing a sharp rise in the
Reprinted from the journal
7