Topics in Current Chemistry (2020) 378:6
1 3
solutions [166]. An increase of electron-trapping sites, reducing recombination, is
invoked to account for the improved photocatalytic activity of the composite under
UV light with respect to cellulose-supported TiO 2 , although no scheme of the electronic coupling of both phases is reported. According to previous works though, it is
feasible that electrons photo-generated in titania are transferred into the lower lying
conduction band of magnetite [167], although this would mean that those transferred
electrons would lose reduction potential. In addition to electronic features, these
composites were reported to be recyclable by means of an external magnetic field.
Recyclability tests showed a loss of activity of ca. 8% after 15 photocatalytic runs
[166].
A p–n heterojunction of CuFe 2 O 4 and TiO 2 supported on reduced graphene oxide
(rGO) was reported by Hafeez and co-workers for hydrogen evolution under UV–Vis
light using glycerol as hole scavenger [168]. On the basis of optical and electrochemical measurements, a mechanism was proposed in which electrons would flow
from the high-energy conduction band of the ferrite phase to the lower-energy one
of titania, and from there to the reduced graphene oxide sheets, where they would be
transferred to protons to produce hydrogen. In turn, holes would migrate from the
valence band of TiO 2 to that of CuFe 2 O 4 and there be trapped by glycerol molecules.
Therefore, CuFe 2 O 4 and rGO acting as hole traps and electron mediators, respectively, would improve the photocatalytic activity with respect to TiO 2 , as shown by
the hydrogen evolution results, by largely inhibiting charge carrier recombination.
This inhibition is actually supported by the practical disappearance of photoluminescence at wavelengths associated with TiO 2 in the composite system. A similar
mechanism, without the use of graphene, was proposed by Uddin and co-workers
to account for the visible-light (λ > 400 nm) activity of CuFe 2 O 4 heterojunctions for
CO 2 reduction into methanol from an aqueous solution of potassium bicarbonate,
using sodium sulfide as hole scavenger [169]. The ferrite phase would in this case
act as a visible-light sensitizer for titania, which would be inactive in those irradiation conditions, and which does, on the other hand, improve the activity with respect
to the ferrite single phase. Again, photoluminescence spectroscopy measurements
under UV excitation suggested recombination to be diminished in the composite,
although the difference between TiO 2 and the two-phase system was not as dramatic
as in the previous case. As is common in photocatalytic CO 2 reduction experiments
[136], deactivation of the catalyst was observed in all cases, which the authors associated with the depletion of active sites.
A Z-scheme electronic mechanism was proposed by Song et al. in the photocatalytic reduction of CO 2 in cyclohexanol solution by ZnFe 2 O 4 /TiO 2 composites [170].
With a relative band position arrangement similar to that of the previous heterojunction, ZnFe 2 O 4 and TiO 2 may couple in such a way that electrons from the conduction
band of the latter are transferred into the valence band of the former, while electrons
in the ferrite conduction bands reduce CO 2 and holes in titania valence band oxidize
the sacrificial agent, in a kind of all-solid Z-scheme without an electron mediator
between the distinct phases. This heterojunction, formed by TiO 2 nanobelts decorated with ferrite nanoparticles, showed higher photocatalytic activity than either of
the two single phases. Cyclohexyl formate and cyclohexanone were detected as the
main products, together with lower amounts of formic acid. This led the authors to
144
Reprinted from the journal
1 3
solutions [166]. An increase of electron-trapping sites, reducing recombination, is
invoked to account for the improved photocatalytic activity of the composite under
UV light with respect to cellulose-supported TiO 2 , although no scheme of the electronic coupling of both phases is reported. According to previous works though, it is
feasible that electrons photo-generated in titania are transferred into the lower lying
conduction band of magnetite [167], although this would mean that those transferred
electrons would lose reduction potential. In addition to electronic features, these
composites were reported to be recyclable by means of an external magnetic field.
Recyclability tests showed a loss of activity of ca. 8% after 15 photocatalytic runs
[166].
A p–n heterojunction of CuFe 2 O 4 and TiO 2 supported on reduced graphene oxide
(rGO) was reported by Hafeez and co-workers for hydrogen evolution under UV–Vis
light using glycerol as hole scavenger [168]. On the basis of optical and electrochemical measurements, a mechanism was proposed in which electrons would flow
from the high-energy conduction band of the ferrite phase to the lower-energy one
of titania, and from there to the reduced graphene oxide sheets, where they would be
transferred to protons to produce hydrogen. In turn, holes would migrate from the
valence band of TiO 2 to that of CuFe 2 O 4 and there be trapped by glycerol molecules.
Therefore, CuFe 2 O 4 and rGO acting as hole traps and electron mediators, respectively, would improve the photocatalytic activity with respect to TiO 2 , as shown by
the hydrogen evolution results, by largely inhibiting charge carrier recombination.
This inhibition is actually supported by the practical disappearance of photoluminescence at wavelengths associated with TiO 2 in the composite system. A similar
mechanism, without the use of graphene, was proposed by Uddin and co-workers
to account for the visible-light (λ > 400 nm) activity of CuFe 2 O 4 heterojunctions for
CO 2 reduction into methanol from an aqueous solution of potassium bicarbonate,
using sodium sulfide as hole scavenger [169]. The ferrite phase would in this case
act as a visible-light sensitizer for titania, which would be inactive in those irradiation conditions, and which does, on the other hand, improve the activity with respect
to the ferrite single phase. Again, photoluminescence spectroscopy measurements
under UV excitation suggested recombination to be diminished in the composite,
although the difference between TiO 2 and the two-phase system was not as dramatic
as in the previous case. As is common in photocatalytic CO 2 reduction experiments
[136], deactivation of the catalyst was observed in all cases, which the authors associated with the depletion of active sites.
A Z-scheme electronic mechanism was proposed by Song et al. in the photocatalytic reduction of CO 2 in cyclohexanol solution by ZnFe 2 O 4 /TiO 2 composites [170].
With a relative band position arrangement similar to that of the previous heterojunction, ZnFe 2 O 4 and TiO 2 may couple in such a way that electrons from the conduction
band of the latter are transferred into the valence band of the former, while electrons
in the ferrite conduction bands reduce CO 2 and holes in titania valence band oxidize
the sacrificial agent, in a kind of all-solid Z-scheme without an electron mediator
between the distinct phases. This heterojunction, formed by TiO 2 nanobelts decorated with ferrite nanoparticles, showed higher photocatalytic activity than either of
the two single phases. Cyclohexyl formate and cyclohexanone were detected as the
main products, together with lower amounts of formic acid. This led the authors to
144
Reprinted from the journal