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Topics in Current Chemistry (2020) 378:6
In the case of CO 2 photoreduction, the use of ferrites as photoelectrodes is scarcer.
Rezaul Karim et al. [187] developed a p-type CuFe 2 O 4 photocathode, obtained by
sol–gel, for CO 2 photoelectroreduction under visible light (470 nm), starting from a
CO 2 -saturated NaHCO 3 solution. Methanol was detected as the sole reduction product in the liquid phase with a faradaic efficiency of 62% and a quantum efficiency of
14.4%, using a bias potential of 0.5 V (vs. RHE). The comparatively low quantum
efficiency was ascribed to the formation of competitive gas products during CO 2
reduction.
3.3.2 Ferrite Photoanodes
Ferrites can also act as n-type semiconductors and are suitable candidates as photoanodes for PECs. Among them, ZnFe 2 O 4 is a remarkable example. Tahir et al.,
for instance [133, 134], propose the use of ZnFe 2 O 4 prepared by aerosol-assisted
chemical vapour deposition (AACVD) over FTO as photoanode for water splitting.
The obtained photocurrents are highly dependent on the synthesis conditions, with
a maximum photocurrent density of 0.35 mA cm
−2
at 1.23 V vs. RHE using ethanol
as solvent and a calcination temperature of 450 °C. An incident photon-to-current
conversion efficiency of 13.5% at 350 nm at an applied potential of 1.23 V vs. RHE
was observed [133]. The authors attributed this behaviour to the improved collection
of the photo-generated minority carriers at the ZnFe 2 O 4 /electrolyte interface as the
average feature size gradually decreased with the solvent from ca. 500 nm (methanol) to ca. 100 nm (ethanol) [134].
To decrease the synthesis temperature, Kim et al. propose the use of hybrid
microwave annealing (HMA) post-synthetic heat treatment with graphite powder as
the susceptor. The synthesis is based on the treatment of β-FeOOH nanorods with a
zinc nitrate solution with a thermal treatment at 550 °C for 3 h. After that, ZnFe 2 O 4
nanorods were calcined at 800 °C (20 min) or subjected to HMA (5 min) to increase
the crystallinity. The ZnFe 2 O 4 nanorods from HMA treatment show a 10- to 15-fold
increase in activity compared to conventional thermally treated electrodes and an
enhancement of the stability, which is attributed to a higher crystallinity and lower
amount of surface defects [188]. In a later work, these authors modified the synthesis
atmosphere (vacuum, air or hydrogen) after the first thermal stage (800 °C) [189]. In
both H 2 and vacuum an increase of the photoactivity about 20-fold is observed.
Hufnagel et al. [190] prepared mesoporous ZnFe 2 O 4 thin films on a macroporous ATO scaffold using atomic layer deposition (ALD). These photoanodes show
a more negative photocurrent onset (0.9 V vs. RHE) compared to reported values.
In addition, these electrodes exhibit a photoresponse 4–5 fold higher than the same
material in film conformation.
In order to improve the diffusion length of minority carriers, several strategies
have been developed. One of them is the use of structured transparent conductive
oxide current collectors to capture and tunnel the photo-generated electrons readily
while the large interfacial area allows efficient transfer of the holes to the solution.
In this way, Xu et al. developed ZnFe 2 O 4 -decorated Al-doped ZnO (AZO) nanowire films [98]. The Al:ZnO nanowires were first grown on the FTO using hydrothermal synthesis and subsequently treated with FeCl 3 and calcined at 550 °C.
151
Reprinted from the journal
Topics in Current Chemistry (2020) 378:6
In the case of CO 2 photoreduction, the use of ferrites as photoelectrodes is scarcer.
Rezaul Karim et al. [187] developed a p-type CuFe 2 O 4 photocathode, obtained by
sol–gel, for CO 2 photoelectroreduction under visible light (470 nm), starting from a
CO 2 -saturated NaHCO 3 solution. Methanol was detected as the sole reduction product in the liquid phase with a faradaic efficiency of 62% and a quantum efficiency of
14.4%, using a bias potential of 0.5 V (vs. RHE). The comparatively low quantum
efficiency was ascribed to the formation of competitive gas products during CO 2
reduction.
3.3.2 Ferrite Photoanodes
Ferrites can also act as n-type semiconductors and are suitable candidates as photoanodes for PECs. Among them, ZnFe 2 O 4 is a remarkable example. Tahir et al.,
for instance [133, 134], propose the use of ZnFe 2 O 4 prepared by aerosol-assisted
chemical vapour deposition (AACVD) over FTO as photoanode for water splitting.
The obtained photocurrents are highly dependent on the synthesis conditions, with
a maximum photocurrent density of 0.35 mA cm
−2
at 1.23 V vs. RHE using ethanol
as solvent and a calcination temperature of 450 °C. An incident photon-to-current
conversion efficiency of 13.5% at 350 nm at an applied potential of 1.23 V vs. RHE
was observed [133]. The authors attributed this behaviour to the improved collection
of the photo-generated minority carriers at the ZnFe 2 O 4 /electrolyte interface as the
average feature size gradually decreased with the solvent from ca. 500 nm (methanol) to ca. 100 nm (ethanol) [134].
To decrease the synthesis temperature, Kim et al. propose the use of hybrid
microwave annealing (HMA) post-synthetic heat treatment with graphite powder as
the susceptor. The synthesis is based on the treatment of β-FeOOH nanorods with a
zinc nitrate solution with a thermal treatment at 550 °C for 3 h. After that, ZnFe 2 O 4
nanorods were calcined at 800 °C (20 min) or subjected to HMA (5 min) to increase
the crystallinity. The ZnFe 2 O 4 nanorods from HMA treatment show a 10- to 15-fold
increase in activity compared to conventional thermally treated electrodes and an
enhancement of the stability, which is attributed to a higher crystallinity and lower
amount of surface defects [188]. In a later work, these authors modified the synthesis
atmosphere (vacuum, air or hydrogen) after the first thermal stage (800 °C) [189]. In
both H 2 and vacuum an increase of the photoactivity about 20-fold is observed.
Hufnagel et al. [190] prepared mesoporous ZnFe 2 O 4 thin films on a macroporous ATO scaffold using atomic layer deposition (ALD). These photoanodes show
a more negative photocurrent onset (0.9 V vs. RHE) compared to reported values.
In addition, these electrodes exhibit a photoresponse 4–5 fold higher than the same
material in film conformation.
In order to improve the diffusion length of minority carriers, several strategies
have been developed. One of them is the use of structured transparent conductive
oxide current collectors to capture and tunnel the photo-generated electrons readily
while the large interfacial area allows efficient transfer of the holes to the solution.
In this way, Xu et al. developed ZnFe 2 O 4 -decorated Al-doped ZnO (AZO) nanowire films [98]. The Al:ZnO nanowires were first grown on the FTO using hydrothermal synthesis and subsequently treated with FeCl 3 and calcined at 550 °C.
151
Reprinted from the journal