Topics in Current Chemistry (2020) 378:6
1 3
in the single-phase ferrite, the observed improvement in hydrogen evolution with
respect both to the ferrite and to C 3 N 4 would not occur. This mechanism is, according to the authors, supported by the observed increase in photoluminescence in the
heterojunction due to the recombination of the electrons in the conduction band of
C 3 N 4 with the holes in the valence band of the ferrite. However, this evidence does
not seem so obvious from the photophysics point of view. The same kind of heterojunction between these two components was proposed by Xu and Feng [182],
by which they surpassed the activity of both single phases for hydrogen evolution
from triethanolamine solutions under visible light, using platinum (3 wt%) as cocatalyst. The introduction of 2 wt% of Ni 2 S improved the photocatalytic activity of
the LaFeO 3 /C 3 N 4 heterojunction in the same conditions by a factor of 60, with an
apparent quantum yield of 2% at 420 nm [183], although the activity was still lower
than that obtained in the previous work with platinum as co-catalyst.
3.3 Ferrite‑Based Photoelectrode Materials for Solar Fuels Production
3.3.1 Ferrite Photocathodes
One of the most investigated ferrite photocathodes is p-type calcium ferrite (CaFe 2 O 4 ), which has a bandgap of 1.9 eV and suitable band edge positions
for water reduction. The first paper using this material was published by Matsumoto et al. [120] using pressed pelletized electrodes sintered at 1200 °C and oxidized under O 2 at 1000 °C. CaFe 2 O 4 was studied in a N 2 saturated K 2 SO 4 solution
(0.25 M, pH 6.0), combined in the cell combined with an n-type Zn 1.2 Fe 1.8 O 4 . Photoelectrolysis of water without external bias resulted in a solar-to-hydrogen (STH)
conversion efficiency lower than 0.01%. This limited efficiency was ascribed to a
low electron concentration in the surface. In addition, Fermi level pinning (FLP)
arises because of the presence of surface states associated with the redox pair Fe
3+
/
Fe
2+
. To improve contact and subsequently the photocurrent, the authors proposed
the use of noble metal nanoparticles (Au, Pt–Pd alloy) in the semiconductor/metal
interface. Cao et al. [116] investigated the use of p-CaFe 2 O 4 as photocathode by
depositing CaFe 2 O 4 thin films on fluorine-doped tin oxide (FTO) by pulsed laser
deposition method. A photocurrent density of − 0.117 mA cm
−2
at − 0.06 V was
reported which is significantly larger than the values reported by Matsumoto and coworkers [120, 121], probably as a result of shorter electron transfer distances in the
thinner films and higher electrical conductivity. The H 2 evolution rate under visible
light irradiation, using a Pt counter electrode without applying any additional bias,
was ca. 4.8 μmol m
−2
h
−1
. Sekizawa et al. reported several metal-doped CaFe 2 O 4
electrodes prepared by radio frequency magnetron co-sputtering over antimonydoped tin oxide (ATO) [123]. The doping with Au and Ag leads to an increase in
the photocurrent. Particularly, Ag doping triggered an improvement in the symmetry
around the Fe atom, which induces high mobility and an increase in activity of 23
times with respect to the undoped ferrite.
Ye et al. investigated the photoelectrochemical performance of p-CaFe 2 O 4 ,
n-ZnFe 2 O 4 , p-CaFe 2 O 4 /n-ZnFe 2 O 4 and multiple p–n junction CaFe 2 O 4 /ZnFe 2 O 4
148
Reprinted from the journal
1 3
in the single-phase ferrite, the observed improvement in hydrogen evolution with
respect both to the ferrite and to C 3 N 4 would not occur. This mechanism is, according to the authors, supported by the observed increase in photoluminescence in the
heterojunction due to the recombination of the electrons in the conduction band of
C 3 N 4 with the holes in the valence band of the ferrite. However, this evidence does
not seem so obvious from the photophysics point of view. The same kind of heterojunction between these two components was proposed by Xu and Feng [182],
by which they surpassed the activity of both single phases for hydrogen evolution
from triethanolamine solutions under visible light, using platinum (3 wt%) as cocatalyst. The introduction of 2 wt% of Ni 2 S improved the photocatalytic activity of
the LaFeO 3 /C 3 N 4 heterojunction in the same conditions by a factor of 60, with an
apparent quantum yield of 2% at 420 nm [183], although the activity was still lower
than that obtained in the previous work with platinum as co-catalyst.
3.3 Ferrite‑Based Photoelectrode Materials for Solar Fuels Production
3.3.1 Ferrite Photocathodes
One of the most investigated ferrite photocathodes is p-type calcium ferrite (CaFe 2 O 4 ), which has a bandgap of 1.9 eV and suitable band edge positions
for water reduction. The first paper using this material was published by Matsumoto et al. [120] using pressed pelletized electrodes sintered at 1200 °C and oxidized under O 2 at 1000 °C. CaFe 2 O 4 was studied in a N 2 saturated K 2 SO 4 solution
(0.25 M, pH 6.0), combined in the cell combined with an n-type Zn 1.2 Fe 1.8 O 4 . Photoelectrolysis of water without external bias resulted in a solar-to-hydrogen (STH)
conversion efficiency lower than 0.01%. This limited efficiency was ascribed to a
low electron concentration in the surface. In addition, Fermi level pinning (FLP)
arises because of the presence of surface states associated with the redox pair Fe
3+
/
Fe
2+
. To improve contact and subsequently the photocurrent, the authors proposed
the use of noble metal nanoparticles (Au, Pt–Pd alloy) in the semiconductor/metal
interface. Cao et al. [116] investigated the use of p-CaFe 2 O 4 as photocathode by
depositing CaFe 2 O 4 thin films on fluorine-doped tin oxide (FTO) by pulsed laser
deposition method. A photocurrent density of − 0.117 mA cm
−2
at − 0.06 V was
reported which is significantly larger than the values reported by Matsumoto and coworkers [120, 121], probably as a result of shorter electron transfer distances in the
thinner films and higher electrical conductivity. The H 2 evolution rate under visible
light irradiation, using a Pt counter electrode without applying any additional bias,
was ca. 4.8 μmol m
−2
h
−1
. Sekizawa et al. reported several metal-doped CaFe 2 O 4
electrodes prepared by radio frequency magnetron co-sputtering over antimonydoped tin oxide (ATO) [123]. The doping with Au and Ag leads to an increase in
the photocurrent. Particularly, Ag doping triggered an improvement in the symmetry
around the Fe atom, which induces high mobility and an increase in activity of 23
times with respect to the undoped ferrite.
Ye et al. investigated the photoelectrochemical performance of p-CaFe 2 O 4 ,
n-ZnFe 2 O 4 , p-CaFe 2 O 4 /n-ZnFe 2 O 4 and multiple p–n junction CaFe 2 O 4 /ZnFe 2 O 4
148
Reprinted from the journal