1 3
Topics in Current Chemistry (2020) 378:6
for photoelectrochemical cells (PECs). However, even if they are considered as
promising, terrestrially abundant and non-toxic alternative photoactive materials for
PECs, only a few works have been published using ferrites as photocathodes and
photoanodes [114, 115]. Actually, the measured photocurrents of PECs with ferrite
photoelectrodes are still low, which is mainly related to the charge mobility limitations mentioned above. Similarly to the case of photocatalysis, different strategies proved that higher photocurrents can be achieved, including nanostructuring,
heterojunctions, co-catalyst loading and control of the defect chemistry [114, 115].
Pioneering work on the fundamental investigation using ferrites as electrodes in
(photo)electrochemical reactions dates back to the late 1970s and early 1980s and
has continued in the subsequent years, including different compositions; p-type
CaFe 2 O 4 [116–123], CdFe 2 O 4 [124], p- and n-type Co x Fe 3–x O 4 and CoTi x Fe 2–x O 4
[125], p-type CoFe 2 O 4 [126], p- and n-type NiFe 2 O 4 [58, 127], Li 0.5 Fe 2.5 O 4 [128],
MgFe 2 O 4 [129, 130], Ti x Fe 3–x O 4 [131] and n-type ZnFe 2 O 4 [128–134] are remarkable examples.
In turn, CO 2 reduction is probably the lowest-TRL (technology readiness level)
application of photocatalysis. In spite of its scientific interest, providing a route for
CO 2 valorisation coupled to solar energy harnessing [135], it still faces a number
of issues mainly related to the inherent difficulty associated with the stability and
relative inertness of the CO 2 molecule and to the complex nature of water oxidation.
Indeed, the latter is in principle the preferred reaction to complete the electron balance as nature does in photosynthesis [136]. Among the different semiconductors
that have been reported as CO 2 reduction photocatalysts [137–139], ferrite-based
ones do not play a prominent role, even if some of these oxides have adequate band
positions for some of the reactions involved in CO 2 reduction [140]. Nevertheless,
a few works have reported photocatalytic and photoelectrochemical CO 2 reduction
using ferrite materials, as will be described in the next sections.
3.2 Ferrite Photocatalysts for Water Splitting and CO 2 Reduction
3.2.1 Single‑Phase Ferrite Photocatalysts
Magnetite, the chemically simplest ferrite (Fe 3 O 4 or Fe
II Fe
III
2
O 4 ), can be employed
for water oxidation in the presence of the appropriate co-catalyst, as shown by Neudeck and co-workers, who prepared Fe 3 O 4 nanoparticles using xyloglucan as stabilizing agent [141]. This method yields size-controllable nanoparticles with narrow
size distributions, which are able to oxidize water into oxygen under irradiation with
a blue-light LED, with the participation of the [Ru(bipy) 3 ]
3+
complex as sensitizer/
co-catalyst and using sodium persulfate as electron scavenger. In the presence of
other nanoparticle-stabilizing polymers such as dextran, magnetite also showed oxygen evolution, although lower than that attained with the xyloglucan. This reveals an
effect of the latter that, according to the authors, is beyond surface area and might be
related to the lifetime of the photo-generated excited Ru complex that would favour
charge transfer to water molecules.
135
Reprinted from the journal
Topics in Current Chemistry (2020) 378:6
for photoelectrochemical cells (PECs). However, even if they are considered as
promising, terrestrially abundant and non-toxic alternative photoactive materials for
PECs, only a few works have been published using ferrites as photocathodes and
photoanodes [114, 115]. Actually, the measured photocurrents of PECs with ferrite
photoelectrodes are still low, which is mainly related to the charge mobility limitations mentioned above. Similarly to the case of photocatalysis, different strategies proved that higher photocurrents can be achieved, including nanostructuring,
heterojunctions, co-catalyst loading and control of the defect chemistry [114, 115].
Pioneering work on the fundamental investigation using ferrites as electrodes in
(photo)electrochemical reactions dates back to the late 1970s and early 1980s and
has continued in the subsequent years, including different compositions; p-type
CaFe 2 O 4 [116–123], CdFe 2 O 4 [124], p- and n-type Co x Fe 3–x O 4 and CoTi x Fe 2–x O 4
[125], p-type CoFe 2 O 4 [126], p- and n-type NiFe 2 O 4 [58, 127], Li 0.5 Fe 2.5 O 4 [128],
MgFe 2 O 4 [129, 130], Ti x Fe 3–x O 4 [131] and n-type ZnFe 2 O 4 [128–134] are remarkable examples.
In turn, CO 2 reduction is probably the lowest-TRL (technology readiness level)
application of photocatalysis. In spite of its scientific interest, providing a route for
CO 2 valorisation coupled to solar energy harnessing [135], it still faces a number
of issues mainly related to the inherent difficulty associated with the stability and
relative inertness of the CO 2 molecule and to the complex nature of water oxidation.
Indeed, the latter is in principle the preferred reaction to complete the electron balance as nature does in photosynthesis [136]. Among the different semiconductors
that have been reported as CO 2 reduction photocatalysts [137–139], ferrite-based
ones do not play a prominent role, even if some of these oxides have adequate band
positions for some of the reactions involved in CO 2 reduction [140]. Nevertheless,
a few works have reported photocatalytic and photoelectrochemical CO 2 reduction
using ferrite materials, as will be described in the next sections.
3.2 Ferrite Photocatalysts for Water Splitting and CO 2 Reduction
3.2.1 Single‑Phase Ferrite Photocatalysts
Magnetite, the chemically simplest ferrite (Fe 3 O 4 or Fe
II Fe
III
2
O 4 ), can be employed
for water oxidation in the presence of the appropriate co-catalyst, as shown by Neudeck and co-workers, who prepared Fe 3 O 4 nanoparticles using xyloglucan as stabilizing agent [141]. This method yields size-controllable nanoparticles with narrow
size distributions, which are able to oxidize water into oxygen under irradiation with
a blue-light LED, with the participation of the [Ru(bipy) 3 ]
3+
complex as sensitizer/
co-catalyst and using sodium persulfate as electron scavenger. In the presence of
other nanoparticle-stabilizing polymers such as dextran, magnetite also showed oxygen evolution, although lower than that attained with the xyloglucan. This reveals an
effect of the latter that, according to the authors, is beyond surface area and might be
related to the lifetime of the photo-generated excited Ru complex that would favour
charge transfer to water molecules.
135
Reprinted from the journal