Topics in Current Chemistry (2019) 377:24
1 3
modest increasing electronic charge (with respect to their pure oxide state), mostly
localized into the d band, as the tungsten content of the mixed oxide increases.
This happens without altering substantially the energy position of the valence band
end [13, 22, 23]. Once tungsten is at the surface forming composite oxide systems,
the electronic properties depend on the size of the aggregates as they differ in the
amount of tungsten to oxygen single and double bonds (which is also related to the
formation of W–O–W connections) as well as the sharing of oxygen atoms with
anatase [9, 12, 13, 23].
Relevant electronic information for photo-catalysis can come from UV–visible spectroscopy, which analyzes the absorption of light in the UV-visble-near-IR
region. Aside from providing further details of some cation(s) electronic structure,
information on the band gap can be obtained. Note than band gap is strongly affected
by nanostructure [10, 32]. Numerical analysis of direct/indirect gap semiconductors
obtained from UV–visible spectroscopy can be combined with the above-described
UPS/XPS study of the valence band to settle down the position of the conduction
and valence band(s) of any photo-catalyst. Interpretation of the position can be
obtained by theoretical calculations. In the case of Ti–W mixed oxides described in
Fig. 2, the combination of experimental and theoretical tools indicate that the band
gap energy modification with tungsten content is a result of a dominant change in
the conduction band position. As mentioned above, the band gap change trough the
Ti–W mixed oxide samples lacks a linear trend with tungsten content as the modification of the local order around the tungsten cation occurring around 15 at.% has a
“disrupting” effect in the conduction band [22, 23].
Finally, we can consider the analysis of the de-excitation process taking place
after light excitation. This is a key piece to interpret the electronic behavior of the
photo-catalytic materials. De-excitation of charge carriers after light absorption is a
complex process having non-radiative and radiative processes. Non-radiative processes can be analyzed using calorimetric techniques [33] but literature reports concentrate on the use of photoluminescence to analyze the radiative processes [34]. For
our purposes, most relevant information is related to the analysis of the localized gap
states presented in any nanostructure material as well as the annihilation of charge
carrier species. In the case of Fig. 2, the photoluminescence intensity (obtained
under UV excitation) is dominated by the anatase characteristics displaying annihilation processes concerning electron/hole species. The addition of tungsten to the
structure decreases the intensity of the photoluminescence spectrum of the pure
oxide without significant changes. This is proof that the recombination markedly
decreased in the doped materials. When tungsten is at the surface of the samples, a
larger intensity with respect to the doped materials is commonly observed, but such
intensity is also lower than the one of the pure oxide. This indicates that recombination of charge carriers is also decreased, although not to the extent of doped materials [9, 12, 13, 23, 24].
The center of Fig. 2 summarizes the most interesting electronic characterization results in the field coming from the described experimental techniques as well
as from theoretical calculations. Such information concerns (for a semiconductor)
the valance and conduction band positions, the existence of localized (gap) electronic states, and the ability of band-type and localized electronic levels to accept
170
Reprinted from the journal
1 3
modest increasing electronic charge (with respect to their pure oxide state), mostly
localized into the d band, as the tungsten content of the mixed oxide increases.
This happens without altering substantially the energy position of the valence band
end [13, 22, 23]. Once tungsten is at the surface forming composite oxide systems,
the electronic properties depend on the size of the aggregates as they differ in the
amount of tungsten to oxygen single and double bonds (which is also related to the
formation of W–O–W connections) as well as the sharing of oxygen atoms with
anatase [9, 12, 13, 23].
Relevant electronic information for photo-catalysis can come from UV–visible spectroscopy, which analyzes the absorption of light in the UV-visble-near-IR
region. Aside from providing further details of some cation(s) electronic structure,
information on the band gap can be obtained. Note than band gap is strongly affected
by nanostructure [10, 32]. Numerical analysis of direct/indirect gap semiconductors
obtained from UV–visible spectroscopy can be combined with the above-described
UPS/XPS study of the valence band to settle down the position of the conduction
and valence band(s) of any photo-catalyst. Interpretation of the position can be
obtained by theoretical calculations. In the case of Ti–W mixed oxides described in
Fig. 2, the combination of experimental and theoretical tools indicate that the band
gap energy modification with tungsten content is a result of a dominant change in
the conduction band position. As mentioned above, the band gap change trough the
Ti–W mixed oxide samples lacks a linear trend with tungsten content as the modification of the local order around the tungsten cation occurring around 15 at.% has a
“disrupting” effect in the conduction band [22, 23].
Finally, we can consider the analysis of the de-excitation process taking place
after light excitation. This is a key piece to interpret the electronic behavior of the
photo-catalytic materials. De-excitation of charge carriers after light absorption is a
complex process having non-radiative and radiative processes. Non-radiative processes can be analyzed using calorimetric techniques [33] but literature reports concentrate on the use of photoluminescence to analyze the radiative processes [34]. For
our purposes, most relevant information is related to the analysis of the localized gap
states presented in any nanostructure material as well as the annihilation of charge
carrier species. In the case of Fig. 2, the photoluminescence intensity (obtained
under UV excitation) is dominated by the anatase characteristics displaying annihilation processes concerning electron/hole species. The addition of tungsten to the
structure decreases the intensity of the photoluminescence spectrum of the pure
oxide without significant changes. This is proof that the recombination markedly
decreased in the doped materials. When tungsten is at the surface of the samples, a
larger intensity with respect to the doped materials is commonly observed, but such
intensity is also lower than the one of the pure oxide. This indicates that recombination of charge carriers is also decreased, although not to the extent of doped materials [9, 12, 13, 23, 24].
The center of Fig. 2 summarizes the most interesting electronic characterization results in the field coming from the described experimental techniques as well
as from theoretical calculations. Such information concerns (for a semiconductor)
the valance and conduction band positions, the existence of localized (gap) electronic states, and the ability of band-type and localized electronic levels to accept
170
Reprinted from the journal