1 3
Topics in Current Chemistry (2020) 378:6
oxide promotes the adsorption of CO 2 via carbonate species, increasing not only the
amount of CO 2 adsorbed but also the strength of the interaction. In this case, carbon
monoxide is the main product of CO 2 reduction. According to previous theoretical
studies on MO-TiO 2 systems (M = Mg, Ca, Sr, Ba) [177], the improvement of CO 2
adsorption and activation on alkaline earth metal oxide modifications should facilitate the activity for photocatalytic CO 2 reduction, SrO being particularly interesting
in that respect since it promotes CO 2 activation and CO desorption.
C 3 N 4 has been extensively studied in recent years as a metal-free and visiblelight-responsive photocatalyst [178], and some works have extended this study to
its combination with ferrites for photocatalytic water splitting. Thus, Chen and coworkers enhanced the photocatalytic activity of graphitic carbon nitride for hydrogen evolution from triethanolamine solutions through the incorporation of MgFe 2 O 4
by annealing a mixture of ferrite particles obtained by the sol–gel method and melamine as the C 3 N 4 precursor [179]. According to static photoluminescence spectra
and time-resolved fluorescence decay curves, the ferrite is able extract photoinduced
electrons and holes from carbon nitride, which per se leads to a poor activity for
hydrogen production due to the low energy of the conduction band of MgFe 2 O 4 .
However, if the nitride is modified with both ferrite and platinum, the latter can
extract electrons more efficiently, according to the greatly improved activity with
respect to both bare C 3 N 4 and MgFeO 4 /C 3 N 4 . In turn, electrochemical measurements revealed the ferrite to be a good oxidation catalyst, which led the authors to
propose a dual effect of MgFe 2 O 4 in the photocatalytic system as hole extractor and
oxidation co-catalyst, with platinum acting as the reduction co-catalyst. The situation is different with nickel and copper ferrites, since both have conduction band
levels above that of C 3 N 4 , according to the same authors [180]. Therefore, under visible irradiation both ferrites can donate electrons from their conduction bands to that
of the nitride, while holes can move from the valence band of the latter to that of the
ferrite. In line with this, steady-state photoluminescence spectra and fluorescence
decay curves suggest in both cases (CuFe 2 O 4 /C 3 N 4 and NiFe 2 O 4 /C 3 N 4 ) an inhibited electron–hole recombination, while electrochemical impedance spectroscopy
shows a greatly enhanced internal conductivity in the composites compared to the
bare nitride. As a result, the activity of the heterojunctions for H 2 production from
aqueous triethanolamine increases with respect to the single phases, and is further
enhanced in the presence of platinum as reduction co-catalyst. As in the previous
work, the authors highlight the dual role of the ferrite as charge separation promoter
and oxidation co-catalyst.
The junction of C 3 N 4 with the perovskite-structured lanthanum ferrite LaFeO 3
has been studied in a few works for photocatalytic hydrogen production. In principle, the relative band positions of both phases would be adequate for the formation of a type II heterojunction. However, as previously described for the zinc ferrite heterojunctions, an all-solid Z-scheme without any electronic mediator seems to
occur in the case of LaFeO 3 /C 3 N 4 photocatalysts. Acharya et al. proposed this kind
of junction on the basis of thermodynamic considerations [181], given that, in the
case of a type II heterojunction, electrons would fall into a conduction band (that of
LaFeO 3 in this case) with reduction potential slightly below that of the H
+
/H 2 redox
couple. Even if the presence of Pt as co-catalyst is able to overcome this limitation
147
Reprinted from the journal
Topics in Current Chemistry (2020) 378:6
oxide promotes the adsorption of CO 2 via carbonate species, increasing not only the
amount of CO 2 adsorbed but also the strength of the interaction. In this case, carbon
monoxide is the main product of CO 2 reduction. According to previous theoretical
studies on MO-TiO 2 systems (M = Mg, Ca, Sr, Ba) [177], the improvement of CO 2
adsorption and activation on alkaline earth metal oxide modifications should facilitate the activity for photocatalytic CO 2 reduction, SrO being particularly interesting
in that respect since it promotes CO 2 activation and CO desorption.
C 3 N 4 has been extensively studied in recent years as a metal-free and visiblelight-responsive photocatalyst [178], and some works have extended this study to
its combination with ferrites for photocatalytic water splitting. Thus, Chen and coworkers enhanced the photocatalytic activity of graphitic carbon nitride for hydrogen evolution from triethanolamine solutions through the incorporation of MgFe 2 O 4
by annealing a mixture of ferrite particles obtained by the sol–gel method and melamine as the C 3 N 4 precursor [179]. According to static photoluminescence spectra
and time-resolved fluorescence decay curves, the ferrite is able extract photoinduced
electrons and holes from carbon nitride, which per se leads to a poor activity for
hydrogen production due to the low energy of the conduction band of MgFe 2 O 4 .
However, if the nitride is modified with both ferrite and platinum, the latter can
extract electrons more efficiently, according to the greatly improved activity with
respect to both bare C 3 N 4 and MgFeO 4 /C 3 N 4 . In turn, electrochemical measurements revealed the ferrite to be a good oxidation catalyst, which led the authors to
propose a dual effect of MgFe 2 O 4 in the photocatalytic system as hole extractor and
oxidation co-catalyst, with platinum acting as the reduction co-catalyst. The situation is different with nickel and copper ferrites, since both have conduction band
levels above that of C 3 N 4 , according to the same authors [180]. Therefore, under visible irradiation both ferrites can donate electrons from their conduction bands to that
of the nitride, while holes can move from the valence band of the latter to that of the
ferrite. In line with this, steady-state photoluminescence spectra and fluorescence
decay curves suggest in both cases (CuFe 2 O 4 /C 3 N 4 and NiFe 2 O 4 /C 3 N 4 ) an inhibited electron–hole recombination, while electrochemical impedance spectroscopy
shows a greatly enhanced internal conductivity in the composites compared to the
bare nitride. As a result, the activity of the heterojunctions for H 2 production from
aqueous triethanolamine increases with respect to the single phases, and is further
enhanced in the presence of platinum as reduction co-catalyst. As in the previous
work, the authors highlight the dual role of the ferrite as charge separation promoter
and oxidation co-catalyst.
The junction of C 3 N 4 with the perovskite-structured lanthanum ferrite LaFeO 3
has been studied in a few works for photocatalytic hydrogen production. In principle, the relative band positions of both phases would be adequate for the formation of a type II heterojunction. However, as previously described for the zinc ferrite heterojunctions, an all-solid Z-scheme without any electronic mediator seems to
occur in the case of LaFeO 3 /C 3 N 4 photocatalysts. Acharya et al. proposed this kind
of junction on the basis of thermodynamic considerations [181], given that, in the
case of a type II heterojunction, electrons would fall into a conduction band (that of
LaFeO 3 in this case) with reduction potential slightly below that of the H
+
/H 2 redox
couple. Even if the presence of Pt as co-catalyst is able to overcome this limitation
147
Reprinted from the journal