1 3
Topics in Current Chemistry (2020) 378:6
photoelectrode materials [122], all of them obtained by pulsed laser deposition. In
the FTO/ZnFe 2 O 4 /CaFe 2 O 4 photoelectrode configuration, negative photocurrent
and positive open circuit photovoltage (+ 0.025 V, λ = 430 nm, 118 μW cm
−2
) were
described, demonstrating that the p-CaFe 2 O 4 layer acts as photocathode. In the case
of multiple junction FTO/(ZnFe 2 O 4 /CaFe 2 O 4 ) x photoelectrodes with similar singlelayer thickness between 10 and 15 nm, extending the number of layers (x) led to an
improvement in the photocurrent density and the onset potential. The highest photocurrent density (− 0.025 mA cm
−2
at + 0.4 V) and the most positive onset potential
(+ 1.3 V) of all four samples were obtained with x = 20.
Ida et al. developed a tandem cell (Fig. 15a) without applying an external voltage using (hk0)-oriented p-type CaFe 2 O 4 deposited over FTO (100 nm thick) by
pulse layer deposition (PLD) as photocathode and TiO 2 as photoanode with an onset
potential of − 0.75 V (Fig. 15b) [117]. The open-circuit voltage was 0.97 V and the
short-circuit current was about 200 μA cm
−2
(Fig. 15c), and a H 2 and O 2 evolution
of 70 and 4 μmol, respectively, in 48 h was attained (Fig. 15d). After this time, many
cracks were observed on the CaFe 2 O 4 surface as shown in Fig. 15e and the solution
contained 2.3 μmol of Fe, indicating a slight corrosion of the CaFe 2 O 4 electrode.
Later, the same authors described the use of n-ZnO as photocathode generating a
photovoltage of 0.82 V [118]. In this case, only H 2 was detected and Zn was partially
dissolved, which accounted for the absence of O 2 . In subsequent work, they also
described a modification of p-type CaFe 2 O 4 with the presence of a Ca 2 Fe 2 O 5 impurity enhancing the short circuit photocurrent density (0.55 mA cm
−2
) and slightly
increasing the photovoltage (1.09 V) [119]. During the reaction a H 2 /O 2 ratio of 3.7
was observed which is lower than in the previous paper. In addition, O 2 formation
was improved.
One strategy to improve the low quantum efficiency of a pristine p-CaFe 2 O 4 electrode due to the poor mobility of the photo-generated charge carriers is to dope it
with different elements. In this way, Matsumoto et al. used Na and Mg, which have
similar ionic radii to Ca and Fe, respectively, to prepare type Ca 1–x Na x Fe 2–y Mg y O 4
photocathodes [121]. This led to the formation of acceptor levels within the bandgap
and to higher electronic conductivity but still low photocurrents.
Rekhila et al. proposed the use of p-NiFe 2 O 4 pellets prepared by sintering sol–gel
synthesized particles at 850 °C [127]. The open-circuit voltage, short-circuit current
and efficiency were reported to be, respectively, 0.43 V, 0.71 mA cm
−2
and 0.28
under irradiation with visible light (50 mW cm
−2
) with Pt as counter electrode.
Yang et al. prepared porous CoFe 2 O 4 nanosheets on FTO from aqueous solutions
of Co and Fe nitrate using a template-free electrochemical deposition followed by a
heat treatment at 933 K [126]. The photocathodes exhibit a small photocurrent of ca.
0.3 μA cm
−2
in 0.1 M aqueous Na 2 S at zero bias voltage under wavelength ≥ 390 nm
(30 mW cm
−2
).
Fan and co-workers reported the preparation of MgFe 2 O 4 nanofibres and nanowires by electrospinning [184, 185]. Chemical vapour deposition (CVD) coating with
MoS 2 created a 1D heterostructure with enhanced charge carrier mobility, which
showed 92% photoelectrochemical tetracycline degradation after 2 h. A photoelectrochemical hydrogen evolution rate of 5.8 mmol h
−1
g
−1
was found at 0.5 V bias
under Xe arc lamp irradiation.
149
Reprinted from the journal
Topics in Current Chemistry (2020) 378:6
photoelectrode materials [122], all of them obtained by pulsed laser deposition. In
the FTO/ZnFe 2 O 4 /CaFe 2 O 4 photoelectrode configuration, negative photocurrent
and positive open circuit photovoltage (+ 0.025 V, λ = 430 nm, 118 μW cm
−2
) were
described, demonstrating that the p-CaFe 2 O 4 layer acts as photocathode. In the case
of multiple junction FTO/(ZnFe 2 O 4 /CaFe 2 O 4 ) x photoelectrodes with similar singlelayer thickness between 10 and 15 nm, extending the number of layers (x) led to an
improvement in the photocurrent density and the onset potential. The highest photocurrent density (− 0.025 mA cm
−2
at + 0.4 V) and the most positive onset potential
(+ 1.3 V) of all four samples were obtained with x = 20.
Ida et al. developed a tandem cell (Fig. 15a) without applying an external voltage using (hk0)-oriented p-type CaFe 2 O 4 deposited over FTO (100 nm thick) by
pulse layer deposition (PLD) as photocathode and TiO 2 as photoanode with an onset
potential of − 0.75 V (Fig. 15b) [117]. The open-circuit voltage was 0.97 V and the
short-circuit current was about 200 μA cm
−2
(Fig. 15c), and a H 2 and O 2 evolution
of 70 and 4 μmol, respectively, in 48 h was attained (Fig. 15d). After this time, many
cracks were observed on the CaFe 2 O 4 surface as shown in Fig. 15e and the solution
contained 2.3 μmol of Fe, indicating a slight corrosion of the CaFe 2 O 4 electrode.
Later, the same authors described the use of n-ZnO as photocathode generating a
photovoltage of 0.82 V [118]. In this case, only H 2 was detected and Zn was partially
dissolved, which accounted for the absence of O 2 . In subsequent work, they also
described a modification of p-type CaFe 2 O 4 with the presence of a Ca 2 Fe 2 O 5 impurity enhancing the short circuit photocurrent density (0.55 mA cm
−2
) and slightly
increasing the photovoltage (1.09 V) [119]. During the reaction a H 2 /O 2 ratio of 3.7
was observed which is lower than in the previous paper. In addition, O 2 formation
was improved.
One strategy to improve the low quantum efficiency of a pristine p-CaFe 2 O 4 electrode due to the poor mobility of the photo-generated charge carriers is to dope it
with different elements. In this way, Matsumoto et al. used Na and Mg, which have
similar ionic radii to Ca and Fe, respectively, to prepare type Ca 1–x Na x Fe 2–y Mg y O 4
photocathodes [121]. This led to the formation of acceptor levels within the bandgap
and to higher electronic conductivity but still low photocurrents.
Rekhila et al. proposed the use of p-NiFe 2 O 4 pellets prepared by sintering sol–gel
synthesized particles at 850 °C [127]. The open-circuit voltage, short-circuit current
and efficiency were reported to be, respectively, 0.43 V, 0.71 mA cm
−2
and 0.28
under irradiation with visible light (50 mW cm
−2
) with Pt as counter electrode.
Yang et al. prepared porous CoFe 2 O 4 nanosheets on FTO from aqueous solutions
of Co and Fe nitrate using a template-free electrochemical deposition followed by a
heat treatment at 933 K [126]. The photocathodes exhibit a small photocurrent of ca.
0.3 μA cm
−2
in 0.1 M aqueous Na 2 S at zero bias voltage under wavelength ≥ 390 nm
(30 mW cm
−2
).
Fan and co-workers reported the preparation of MgFe 2 O 4 nanofibres and nanowires by electrospinning [184, 185]. Chemical vapour deposition (CVD) coating with
MoS 2 created a 1D heterostructure with enhanced charge carrier mobility, which
showed 92% photoelectrochemical tetracycline degradation after 2 h. A photoelectrochemical hydrogen evolution rate of 5.8 mmol h
−1
g
−1
was found at 0.5 V bias
under Xe arc lamp irradiation.
149
Reprinted from the journal