Topics in Current Chemistry (2020) 378:6
1 3
The remarkable photoelectrochemical performance (photocurrent density of
1.72 mA cm
−2
at 1.23 V vs. RHE) was attributed to the synergy of the visible light
absorption of ZnFe 2 O 4 and the of high conductivity of Al:ZnO.
Fe 2 O 3 is one of the most studied photoanode materials. There are a number of
research works and review articles dealing with the modification strategies of hematite such as morphology control, doping and heterojunction formation to improve the
charge transfer efficiency [191, 192]. Regarding the last strategy, ferrites have been
also investigated in forming heterojunctions with hematite for photoanodes. The
most studied one is the ZnFe 2 O 4 /Fe 2 O 3 heterojunction prepared by different synthesis methods. McDonald and Choi employed the electrodeposition route to develop
core–shell photoelectrodes composed of α-Fe 2 O 3 (core) and ZnFe 2 O 4 (shell) [193].
β-FeOOH films were first electrodeposited on FTO being transformed into α-Fe 2 O 3
by a thermal treatment and a subsequent treatment with a Zn-containing solution
covering the iron oxide and annealing to produce the ZnFe 2 O 4 shell. The best photoelectrochemical performance was obtained for a ZnFe 2 O 4 /Fe 2 O 3 ratio of 1. The
synergy between ZnFe 2 O 4 and Fe 2 O 3 led to an improved electron hole separation in
the heterojunction interface that is responsible for the increase on the photoelectrochemical performance compared with bare iron oxide. In addition, the incorporation
of Al
3+
leads to a thin layer solid solution (ZnFe 2–x Al x O 4 or Fe 2–x Al x O 3 ) after heat
treatment. This reduces the electron–hole recombination centres, but decreases the
catalytic activity for the oxygen evolution reaction (OER). Guo et al. also use hydrothermal synthesis to grow the FeOOH nanorods and subsequent treatment with different concentrations of Zn precursor leading to ZnFe 2 O 4 /Fe 2 O 3 nanorods [194]. The
photocurrent density for the composite electrode was 0.44 mA cm
−2
at ca. 1.2 V vs.
RHE, which was almost twice as high as that for a Fe 2 O 3 electrode (0.24 mA cm
−2
).
On the other hand, Borse et al. used a plasma spray method to deposit an aqueous
solution of Zn and Fe salts over stainless steel leading to a ZnFe 2 O 4 /Fe 2 O 3 electrode affording a photocurrent fivefold higher than for bare Zinc ferrite. In addition,
the hydrogen evolution rates were determined to be 46.3 μmol cm
−2
(STH conversion = 0.06) and 99.0 μmol cm
−2
(STH conversion = 0.0125) for ZnFe 2 O 4 and the
ZnFe 2 O 4 /Fe 2 O 3 photoanode, respectively [195].
Kim et al. [196, 197] reported the preparation of heterojunction photoanodes
using p-CaFe 2 O 4 with TaON and BiVO 4 . The selection of these oxides is due to their
staggered relative band positions with the ferrite leading to an effective heterojunction for water oxidation reactions. The CaFe 2 O 4 layer on the surface of a TaON led
to an increase in the photocurrent density (1.26 mA cm
−2
) of about 5.5 times greater
than bare TaON. This enhancement in the photoelectrochemical performance is
due to a reduction of the resistance of the charge carrier transport and, therefore, an
improved electron–hole separation [196]. In the case of CaFe 2 O 4 /BiVO 4 an increase
of 65% over that measured at the BiVO 4 electrode was observed [197]. In order to
improve the photoelectrochemical performance, an OER co-catalyst (cobalt phosphate, Co-Pi) was deposited on CaFe 2 O 4 /TaON. After deposition of Co-Pi, stoichiometric H 2 and O 2 production was obtained (123 μmolH 2 and 59 μmolO 2 ) by applying a bias of 1.23 V vs. RHE after λ ≥ 400 nm irradiation for 3 h. An STH efficiency
of 0.053% at 1.0 V vs. RHE was obtained, which was increased to 0.55% by coupling a photovoltaic cell in a tandem configuration, assuming 0 V applied voltage.
152
Reprinted from the journal
1 3
The remarkable photoelectrochemical performance (photocurrent density of
1.72 mA cm
−2
at 1.23 V vs. RHE) was attributed to the synergy of the visible light
absorption of ZnFe 2 O 4 and the of high conductivity of Al:ZnO.
Fe 2 O 3 is one of the most studied photoanode materials. There are a number of
research works and review articles dealing with the modification strategies of hematite such as morphology control, doping and heterojunction formation to improve the
charge transfer efficiency [191, 192]. Regarding the last strategy, ferrites have been
also investigated in forming heterojunctions with hematite for photoanodes. The
most studied one is the ZnFe 2 O 4 /Fe 2 O 3 heterojunction prepared by different synthesis methods. McDonald and Choi employed the electrodeposition route to develop
core–shell photoelectrodes composed of α-Fe 2 O 3 (core) and ZnFe 2 O 4 (shell) [193].
β-FeOOH films were first electrodeposited on FTO being transformed into α-Fe 2 O 3
by a thermal treatment and a subsequent treatment with a Zn-containing solution
covering the iron oxide and annealing to produce the ZnFe 2 O 4 shell. The best photoelectrochemical performance was obtained for a ZnFe 2 O 4 /Fe 2 O 3 ratio of 1. The
synergy between ZnFe 2 O 4 and Fe 2 O 3 led to an improved electron hole separation in
the heterojunction interface that is responsible for the increase on the photoelectrochemical performance compared with bare iron oxide. In addition, the incorporation
of Al
3+
leads to a thin layer solid solution (ZnFe 2–x Al x O 4 or Fe 2–x Al x O 3 ) after heat
treatment. This reduces the electron–hole recombination centres, but decreases the
catalytic activity for the oxygen evolution reaction (OER). Guo et al. also use hydrothermal synthesis to grow the FeOOH nanorods and subsequent treatment with different concentrations of Zn precursor leading to ZnFe 2 O 4 /Fe 2 O 3 nanorods [194]. The
photocurrent density for the composite electrode was 0.44 mA cm
−2
at ca. 1.2 V vs.
RHE, which was almost twice as high as that for a Fe 2 O 3 electrode (0.24 mA cm
−2
).
On the other hand, Borse et al. used a plasma spray method to deposit an aqueous
solution of Zn and Fe salts over stainless steel leading to a ZnFe 2 O 4 /Fe 2 O 3 electrode affording a photocurrent fivefold higher than for bare Zinc ferrite. In addition,
the hydrogen evolution rates were determined to be 46.3 μmol cm
−2
(STH conversion = 0.06) and 99.0 μmol cm
−2
(STH conversion = 0.0125) for ZnFe 2 O 4 and the
ZnFe 2 O 4 /Fe 2 O 3 photoanode, respectively [195].
Kim et al. [196, 197] reported the preparation of heterojunction photoanodes
using p-CaFe 2 O 4 with TaON and BiVO 4 . The selection of these oxides is due to their
staggered relative band positions with the ferrite leading to an effective heterojunction for water oxidation reactions. The CaFe 2 O 4 layer on the surface of a TaON led
to an increase in the photocurrent density (1.26 mA cm
−2
) of about 5.5 times greater
than bare TaON. This enhancement in the photoelectrochemical performance is
due to a reduction of the resistance of the charge carrier transport and, therefore, an
improved electron–hole separation [196]. In the case of CaFe 2 O 4 /BiVO 4 an increase
of 65% over that measured at the BiVO 4 electrode was observed [197]. In order to
improve the photoelectrochemical performance, an OER co-catalyst (cobalt phosphate, Co-Pi) was deposited on CaFe 2 O 4 /TaON. After deposition of Co-Pi, stoichiometric H 2 and O 2 production was obtained (123 μmolH 2 and 59 μmolO 2 ) by applying a bias of 1.23 V vs. RHE after λ ≥ 400 nm irradiation for 3 h. An STH efficiency
of 0.053% at 1.0 V vs. RHE was obtained, which was increased to 0.55% by coupling a photovoltaic cell in a tandem configuration, assuming 0 V applied voltage.
152
Reprinted from the journal