Topics in Current Chemistry (2020) 378:6
1 3
with Pt as co-catalyst. Anisotropic charge transportation phenomena are invoked to
account for the activity of this kind of solid.
As mentioned earlier, photocatalytic CO 2 reduction studies with ferrite catalysts
are much scarcer in the literature than hydrogen production ones. Matsumoto and
co-workers reported in 1994 the production of methanol and formaldehyde from a
CO 2 -saturated solution on CaFe 2 O 4 irradiated with light of different wavelengths
[162]. The CH 3 OH production followed the same trend versus irradiation wavelength as the photocurrent in a CaFe 2 O 4 photocathode, proving the photocatalytic
character of the reaction. However, in spite of the narrow bandgap of the ferrite
(1.9 eV), both photocurrents and CO 2 conversions were significantly higher under
UV light. Results with the addition of suspended solid BaCO 3 led the authors to propose a mechanism in which CO 3
2−
/CO 2 activation occurs at the interface between
the solid catalyst, the solid carbonate and water. Matsumoto reported 2 years later
a screening of photoelectrochemical response and CO 2 reduction to methanol with
different ferrites [140]. Both properties were not necessarily related, with some ferrites showing electrochemical but not catalytic activity under irradiation and vice
versa. Not only was the mentioned calcium ferrite was photocatalytically active but
also some compositions containing magnesium, strontium, barium, lead and bismuth were. The elements forming the oxides as well as the positions of the band
edges were found to determine the photocatalytic activity, with CaFe 2 O 4 and (Bi,
Pb) 2 Sr 2 BiFe 2 O 9+x giving the best results.
Apparently, as reported by Xiao and co-workers from a combination of UV–Vis
and XPS, ZnFe 2 O 4 has adequate band positions to conduct, under visible light, CO 2
reduction but not water oxidation [163]. Therefore, triethanolamine was used in that
study as a sacrificial agent to scavenge photoproduced holes, while conduction band
electrons were utilized to reduce aqueous bicarbonate into mainly acetaldehyde and
ethanol. However, it remains unclear whether these products result from the oxidation of triethanolamine.
3.2.2 Ferrite‑Based Heterojunction Photocatalysts
Either by combining different ferrites or by combining one ferrite with another semiconductor like TiO 2 , ZnO or C 3 N 4 , some works have dealt with the improvement
of photocatalytic hydrogen production or carbon dioxide reduction by incorporating
these materials into different kinds of heterostructures.
Regarding the combination of different ferrites, Chen et al. reported the fabrication of a CaFe 2 O 4 /MgFe 2 O 4 bulk p–n heterojunction for photocatalytic hydrogen evolution from aqueous methanol solutions under visible light, with the use of
two co-catalysts, namely RuO 2 and Pt [164]. By bulk heterojunction, the authors
mean that there are junctions of the two phases distributed along the whole composite material, as investigated by high-resolution TEM, which was prepared by a
polymer-complex synthetic method that allowed good crystallinity to be obtained at
low temperature as well as an adequate distribution of the composing phases. This
heterojunction, whose band structure is shown in Fig. 13, gave rise to a remarkable
hydrogen production activity under light with λ ≥ 420 nm, with reported quantum
yield of ca. 10%, in contrast with the single phases that showed little activity in the
142
Reprinted from the journal
1 3
with Pt as co-catalyst. Anisotropic charge transportation phenomena are invoked to
account for the activity of this kind of solid.
As mentioned earlier, photocatalytic CO 2 reduction studies with ferrite catalysts
are much scarcer in the literature than hydrogen production ones. Matsumoto and
co-workers reported in 1994 the production of methanol and formaldehyde from a
CO 2 -saturated solution on CaFe 2 O 4 irradiated with light of different wavelengths
[162]. The CH 3 OH production followed the same trend versus irradiation wavelength as the photocurrent in a CaFe 2 O 4 photocathode, proving the photocatalytic
character of the reaction. However, in spite of the narrow bandgap of the ferrite
(1.9 eV), both photocurrents and CO 2 conversions were significantly higher under
UV light. Results with the addition of suspended solid BaCO 3 led the authors to propose a mechanism in which CO 3
2−
/CO 2 activation occurs at the interface between
the solid catalyst, the solid carbonate and water. Matsumoto reported 2 years later
a screening of photoelectrochemical response and CO 2 reduction to methanol with
different ferrites [140]. Both properties were not necessarily related, with some ferrites showing electrochemical but not catalytic activity under irradiation and vice
versa. Not only was the mentioned calcium ferrite was photocatalytically active but
also some compositions containing magnesium, strontium, barium, lead and bismuth were. The elements forming the oxides as well as the positions of the band
edges were found to determine the photocatalytic activity, with CaFe 2 O 4 and (Bi,
Pb) 2 Sr 2 BiFe 2 O 9+x giving the best results.
Apparently, as reported by Xiao and co-workers from a combination of UV–Vis
and XPS, ZnFe 2 O 4 has adequate band positions to conduct, under visible light, CO 2
reduction but not water oxidation [163]. Therefore, triethanolamine was used in that
study as a sacrificial agent to scavenge photoproduced holes, while conduction band
electrons were utilized to reduce aqueous bicarbonate into mainly acetaldehyde and
ethanol. However, it remains unclear whether these products result from the oxidation of triethanolamine.
3.2.2 Ferrite‑Based Heterojunction Photocatalysts
Either by combining different ferrites or by combining one ferrite with another semiconductor like TiO 2 , ZnO or C 3 N 4 , some works have dealt with the improvement
of photocatalytic hydrogen production or carbon dioxide reduction by incorporating
these materials into different kinds of heterostructures.
Regarding the combination of different ferrites, Chen et al. reported the fabrication of a CaFe 2 O 4 /MgFe 2 O 4 bulk p–n heterojunction for photocatalytic hydrogen evolution from aqueous methanol solutions under visible light, with the use of
two co-catalysts, namely RuO 2 and Pt [164]. By bulk heterojunction, the authors
mean that there are junctions of the two phases distributed along the whole composite material, as investigated by high-resolution TEM, which was prepared by a
polymer-complex synthetic method that allowed good crystallinity to be obtained at
low temperature as well as an adequate distribution of the composing phases. This
heterojunction, whose band structure is shown in Fig. 13, gave rise to a remarkable
hydrogen production activity under light with λ ≥ 420 nm, with reported quantum
yield of ca. 10%, in contrast with the single phases that showed little activity in the
142
Reprinted from the journal