Topics in Current Chemistry (2019) 377:24
1 3
kinetic energy of the ejected photoelectron and is sensitive to the local arrangement
around the absorbing atom. It can have a penetration depth between 1 and ca. 3 nm,
depending on the excitation wavelength. Thus, XPS is considered a surface-sensitive
technique for most materials [10].
Figure 1 contains information from XAS and XPS techniques in the Ti–W samples. The Fourier transform of the EXAFS signals of the W L III -edge presents peaks
at shell pseudo-radial distances from the tungsten atom absorbing the X-ray radiation in binary Ti–W oxides. Mathematical analysis of the signal shows, as expected,
that oxygen occupies the first shell in the oxide but only Ti occupies the second.
Complete information comes from the shell coordination numbers and distances.
These observables showed that the first shell has a local structure resembling the
tungsten oxide local environment (has two subshells at distances differing up to ca.
0.6 Angstroms) in spite of being at the anatase structure. The second shell shows the
strong driving force to have M–O–M hetero and not homo-bonds at such structure.
This second shell changes coordination number drastically with tungsten content in
the materials. The chemical nature of the second (first metal) neighbor is always Ti,
a fact that has probes the mixed oxide nature of the samples and has strong implications in the electronic properties of the nanostructured Ti–W anatase-based materials. The local environment picture obtained from EXAFS can be combined thus
with XPS (Fig. 1 includes the W3d region, showing a signal profile compatible with
the W–O local arrangement already described) to confirm the substitutional position
of the tungsten cations [22, 23].
So, as a summary of the outcome resulting from the structural characterization
of photo-catalysts, the center of Fig. 1 gives a pictorial interpretation of the experimental result concerning the structure, presenting the local and middle range order
around the tungsten atom located in an anatase structure. Of course, such experimental information can be always accompanied and frequently interpreted using
theoretical results from DFT or similar calculation tools [22, 24, 25]. While at the
surface, tungsten forms the already-described W=O bonds and, at increasing loadings (well above the solubility limit at the anatase structure) W–O–W bonds characteristic of oligo-sized entities [9, 12, 13].
It is also evident that XPS (after now consider within the framework of the structural characterization) can provide electronic information. XPS and the X-ray near
edge structure (XANES) technique are frequent techniques applied to investigate the
electronic information of the solid constituents. The latter technique has the same
physical origin as EXAFS, but is dominated by multiple scattering effects, while
EXAFS is dominated by single scattering events. XANES informs about the local
(symmetry projected) unoccupied electronic density states near or about the edge
scanned. Again, local means in the close vicinity of the atom absorbing the X-ray
photons and the electronic transitions occurring in a XANES spectrum are defined
(in a first approximation) by the Fermi Golden rule [26, 27]. A somewhat similar
technique corresponds to the electron energy loss spectroscopy (EELS), although
here the excitation of the catalyst takes place using electrons. EELS can analyze
inner-shell ionization processes (as XANES) but also plasmon or phonon excitations
and other physical phenomena [28]. Close information, now corresponding to the
occupied density of states, can be obtained using X-ray emission techniques (XES)
168
Reprinted from the journal
1 3
kinetic energy of the ejected photoelectron and is sensitive to the local arrangement
around the absorbing atom. It can have a penetration depth between 1 and ca. 3 nm,
depending on the excitation wavelength. Thus, XPS is considered a surface-sensitive
technique for most materials [10].
Figure 1 contains information from XAS and XPS techniques in the Ti–W samples. The Fourier transform of the EXAFS signals of the W L III -edge presents peaks
at shell pseudo-radial distances from the tungsten atom absorbing the X-ray radiation in binary Ti–W oxides. Mathematical analysis of the signal shows, as expected,
that oxygen occupies the first shell in the oxide but only Ti occupies the second.
Complete information comes from the shell coordination numbers and distances.
These observables showed that the first shell has a local structure resembling the
tungsten oxide local environment (has two subshells at distances differing up to ca.
0.6 Angstroms) in spite of being at the anatase structure. The second shell shows the
strong driving force to have M–O–M hetero and not homo-bonds at such structure.
This second shell changes coordination number drastically with tungsten content in
the materials. The chemical nature of the second (first metal) neighbor is always Ti,
a fact that has probes the mixed oxide nature of the samples and has strong implications in the electronic properties of the nanostructured Ti–W anatase-based materials. The local environment picture obtained from EXAFS can be combined thus
with XPS (Fig. 1 includes the W3d region, showing a signal profile compatible with
the W–O local arrangement already described) to confirm the substitutional position
of the tungsten cations [22, 23].
So, as a summary of the outcome resulting from the structural characterization
of photo-catalysts, the center of Fig. 1 gives a pictorial interpretation of the experimental result concerning the structure, presenting the local and middle range order
around the tungsten atom located in an anatase structure. Of course, such experimental information can be always accompanied and frequently interpreted using
theoretical results from DFT or similar calculation tools [22, 24, 25]. While at the
surface, tungsten forms the already-described W=O bonds and, at increasing loadings (well above the solubility limit at the anatase structure) W–O–W bonds characteristic of oligo-sized entities [9, 12, 13].
It is also evident that XPS (after now consider within the framework of the structural characterization) can provide electronic information. XPS and the X-ray near
edge structure (XANES) technique are frequent techniques applied to investigate the
electronic information of the solid constituents. The latter technique has the same
physical origin as EXAFS, but is dominated by multiple scattering effects, while
EXAFS is dominated by single scattering events. XANES informs about the local
(symmetry projected) unoccupied electronic density states near or about the edge
scanned. Again, local means in the close vicinity of the atom absorbing the X-ray
photons and the electronic transitions occurring in a XANES spectrum are defined
(in a first approximation) by the Fermi Golden rule [26, 27]. A somewhat similar
technique corresponds to the electron energy loss spectroscopy (EELS), although
here the excitation of the catalyst takes place using electrons. EELS can analyze
inner-shell ionization processes (as XANES) but also plasmon or phonon excitations
and other physical phenomena [28]. Close information, now corresponding to the
occupied density of states, can be obtained using X-ray emission techniques (XES)
168
Reprinted from the journal