Topics in Current Chemistry (2020) 378:6
1 3
in the photocatalytic activity of the heterostructures was attributed to a reduced
recombination of the charge carriers, as a result of the transfer of electrons from the
conduction band of the metal ferrite to the conduction band of the ZnO. Therefore,
the authors proposed a mechanism in which electrons accumulated in the conduction band of ZnO reduce protons to produce hydrogen, while holes remaining in
the valence band of the ferrite phase would oxidize water to oxygen. However, no
oxygen evolution was reported in the work to account for the electron balance and,
as shown by the authors in their electronic scheme, holes in the valence band of both
copper and nickel ferrites do not have enough reduction potential to oxidize water.
A similar charge transfer mechanism was proposed by Karamian and Sharifnia to
account for the visible-light activity of BiFeO 3 –ZnO p–n junctions for photocatalytic CO 2 reduction using methane as hole scavenger [174], although in this case
only the ferrite component is activated by the used irradiation source, thereby acting as an inorganic visible-light sensitizer of the otherwise inactive, wide-bandgap
ZnO phase. The charge separation was supported by photoluminescence spectroscopy results, which suggested a hindered recombination in the mixed phases. With
an optimized 1:1 molar ratio between the components, ca. 20% CO 2 conversion was
reached, which declined to ca. 14% after four runs. However, no other efficiency
measure than conversion is given to make the results comparable to other works, nor
is it clear what products are obtained.
Regarding other oxides that have been coupled to ferrites, Guo and co-workers
[175] reported the preparation of Ce-doped ZnFe 2 O 4 via a sol–gel method that actually gave rise to biphasic samples composed of a Ce
3+
-doped ZnFe 2 O 4 spinel phase
together with an additional CeO 2 phase with fluorite structure. These composites
displayed higher activity than the pristine zinc ferrite for photocatalytic CO 2 reduction under visible light, with CH 4 and CO as main products, together with hydrogen from the competing reduction of water and oxygen from water oxidation (not
shown). The formation of a p–n junction between the two phases induced the establishment of a type II heterojunction that promoted charge separation and reduced
recombination, as suggested by a combined study of photoluminescence and
UV–Vis absorption spectroscopies with (photo)electrochemical measurements. In
addition, temperature-programmed desorption (TPD) and Fourier transform infrared
spectroscopy (FTIR) studies revealed that the alkalinity of cerium oxide favours the
chemical interaction between CO 2 and photocatalyst surfaces, which occurs via the
formation of monodentate and bidentate carbonate and bidentate bicarbonate as the
main surface species.
In a similar approach, Khan et al. reported the use of another wide-bandgap alkaline oxide like SrO to promote both charge separation and CO 2 adsorption [176].
However, the electronic matching here is slightly different from those in the previous examples, with the conduction band of SrO lying considerably above that of
the ferrite, so that it is only able to extract hot electrons generated in the latter by
absorption of relatively high-energy (although still visible) photons. Charge separation is demonstrated by increased signals in the surface photovoltage spectra upon
the addition of the strontium oxide phase, as well as by the increased generation of
hydroxyl radicals followed by the formation of the fluorescent 7-hydroxycoumarin
from coumarin solutions. On the other hand, TPD studies reveal that the alkaline
146
Reprinted from the journal
1 3
in the photocatalytic activity of the heterostructures was attributed to a reduced
recombination of the charge carriers, as a result of the transfer of electrons from the
conduction band of the metal ferrite to the conduction band of the ZnO. Therefore,
the authors proposed a mechanism in which electrons accumulated in the conduction band of ZnO reduce protons to produce hydrogen, while holes remaining in
the valence band of the ferrite phase would oxidize water to oxygen. However, no
oxygen evolution was reported in the work to account for the electron balance and,
as shown by the authors in their electronic scheme, holes in the valence band of both
copper and nickel ferrites do not have enough reduction potential to oxidize water.
A similar charge transfer mechanism was proposed by Karamian and Sharifnia to
account for the visible-light activity of BiFeO 3 –ZnO p–n junctions for photocatalytic CO 2 reduction using methane as hole scavenger [174], although in this case
only the ferrite component is activated by the used irradiation source, thereby acting as an inorganic visible-light sensitizer of the otherwise inactive, wide-bandgap
ZnO phase. The charge separation was supported by photoluminescence spectroscopy results, which suggested a hindered recombination in the mixed phases. With
an optimized 1:1 molar ratio between the components, ca. 20% CO 2 conversion was
reached, which declined to ca. 14% after four runs. However, no other efficiency
measure than conversion is given to make the results comparable to other works, nor
is it clear what products are obtained.
Regarding other oxides that have been coupled to ferrites, Guo and co-workers
[175] reported the preparation of Ce-doped ZnFe 2 O 4 via a sol–gel method that actually gave rise to biphasic samples composed of a Ce
3+
-doped ZnFe 2 O 4 spinel phase
together with an additional CeO 2 phase with fluorite structure. These composites
displayed higher activity than the pristine zinc ferrite for photocatalytic CO 2 reduction under visible light, with CH 4 and CO as main products, together with hydrogen from the competing reduction of water and oxygen from water oxidation (not
shown). The formation of a p–n junction between the two phases induced the establishment of a type II heterojunction that promoted charge separation and reduced
recombination, as suggested by a combined study of photoluminescence and
UV–Vis absorption spectroscopies with (photo)electrochemical measurements. In
addition, temperature-programmed desorption (TPD) and Fourier transform infrared
spectroscopy (FTIR) studies revealed that the alkalinity of cerium oxide favours the
chemical interaction between CO 2 and photocatalyst surfaces, which occurs via the
formation of monodentate and bidentate carbonate and bidentate bicarbonate as the
main surface species.
In a similar approach, Khan et al. reported the use of another wide-bandgap alkaline oxide like SrO to promote both charge separation and CO 2 adsorption [176].
However, the electronic matching here is slightly different from those in the previous examples, with the conduction band of SrO lying considerably above that of
the ferrite, so that it is only able to extract hot electrons generated in the latter by
absorption of relatively high-energy (although still visible) photons. Charge separation is demonstrated by increased signals in the surface photovoltage spectra upon
the addition of the strontium oxide phase, as well as by the increased generation of
hydroxyl radicals followed by the formation of the fluorescent 7-hydroxycoumarin
from coumarin solutions. On the other hand, TPD studies reveal that the alkaline
146
Reprinted from the journal