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Topics in Current Chemistry (2020) 378:6
Ortega López et al. reported significant hydrogen production with the cobalt
spinel ferrite CoFe 2 O 4 [153]. They compared two different synthetic methods,
namely co-precipitation, followed by a relatively low temperature crystallization,
and ball-milling of a stoichiometric mixture of iron metal and Co 3 O 4 previously
activated and solid-state reaction at 700 °C. The latter method yielded a final
CoFe 2 O 4 cubic spinel with 5 nm crystal size vs. 20 nm with the former synthesis.
This, however, resulted in a lower surface area of the ball-milled sample than the
co-precipitated one and, contrary to what may be expected from the quantum-size
effect, to a considerably lower bandgap (1.15 vs. 1.38 eV). The authors ascribe
both results to the differences in the secondary particle size (100–500 vs. 25 nm),
which is more plausible in the case of the surface area than in that of the electronic structure. In any case, the ball-milled sample exhibited a higher adsorption
capacity which the authors relate to oxygen vacancies created by the high-energy
mechanochemical process and invoke to explain the higher hydrogen production
per surface area unit displayed by this catalyst.
Copper ferrite (CuFe 2 O 4 ) has also been shown to be active for photocatalytic
hydrogen evolution. Yang et al. obtained CuFe 2 O 4 nanoparticles by sol–gel, solidstate reaction and co-precipitation methods, among which the first one yielded
the highest hydrogen formation rate using oxalic acid as sacrificial reagent [154].
The use of citric acid in the sol–gel method resulted in a relatively homogeneous
particle size distribution, which according to the authors had a key influence on
the higher activity of the obtained photocatalyst, as a result of the shorter path
travelled by photo-generated charges to the surface. The CuFe 2 O 4 photocatalyst
was active in four consecutive cycles, although a decline in the hydrogen evolution rate can be observed in each of them.
Regarding ferrites of non-transition metals, some alkali and alkaline earth
metal ferrites have been tested for photocatalytic hydrogen evolution. For example, Boudjemaa and co-workers synthesized the MgFe 2 O 4 spinel ferrite by calcination of a Mg–Fe layered double hydroxide obtained by co-precipitation [130].
This material gave rise to hydrogen evolution under visible light from an aqueous sulfite solution, although production declined after 20 min of reaction. This
authors ascribed this deactivation to the saturation of catalytic sites and competitive reduction of the end product S 2 O 6
2−
for conduction band electrons. Dom et al.
reported the synthesis of orthorhombic CaFe 2 O 4 nanoparticles using different
preparation methods, namely solid-state reaction, polymer complex, microwave
sintering and self-propagating combustion (SPC) [155]. As they already reported
for zinc ferrite [151], the microwave-directed method yielded interesting results,
outperforming by nearly one order of magnitude the activities obtained with the
rest of syntheses, which gave very similar results [155]. The authors ascribed this
difference to an optimum combination of high crystallinity with monodispersed
morphology together with efficient visible light absorption, suitable band energetics and relatively low charge transfer resistance. Photodeposition of platinum
as a hydrogen evolution co-catalyst resulted in further improvement of the activity by a factor of ca. 2.5, resulting in an apparent quantum yield of 1.57% under
420 nm irradiation. Different strontium ferrites, namely the cubic, hexagonal and
orthorhombic SrFe 2 O 4 , SrFe 12 O 19 and Sr 7 Fe 10 O 22 , respectively, were selectively
139
Reprinted from the journal
Topics in Current Chemistry (2020) 378:6
Ortega López et al. reported significant hydrogen production with the cobalt
spinel ferrite CoFe 2 O 4 [153]. They compared two different synthetic methods,
namely co-precipitation, followed by a relatively low temperature crystallization,
and ball-milling of a stoichiometric mixture of iron metal and Co 3 O 4 previously
activated and solid-state reaction at 700 °C. The latter method yielded a final
CoFe 2 O 4 cubic spinel with 5 nm crystal size vs. 20 nm with the former synthesis.
This, however, resulted in a lower surface area of the ball-milled sample than the
co-precipitated one and, contrary to what may be expected from the quantum-size
effect, to a considerably lower bandgap (1.15 vs. 1.38 eV). The authors ascribe
both results to the differences in the secondary particle size (100–500 vs. 25 nm),
which is more plausible in the case of the surface area than in that of the electronic structure. In any case, the ball-milled sample exhibited a higher adsorption
capacity which the authors relate to oxygen vacancies created by the high-energy
mechanochemical process and invoke to explain the higher hydrogen production
per surface area unit displayed by this catalyst.
Copper ferrite (CuFe 2 O 4 ) has also been shown to be active for photocatalytic
hydrogen evolution. Yang et al. obtained CuFe 2 O 4 nanoparticles by sol–gel, solidstate reaction and co-precipitation methods, among which the first one yielded
the highest hydrogen formation rate using oxalic acid as sacrificial reagent [154].
The use of citric acid in the sol–gel method resulted in a relatively homogeneous
particle size distribution, which according to the authors had a key influence on
the higher activity of the obtained photocatalyst, as a result of the shorter path
travelled by photo-generated charges to the surface. The CuFe 2 O 4 photocatalyst
was active in four consecutive cycles, although a decline in the hydrogen evolution rate can be observed in each of them.
Regarding ferrites of non-transition metals, some alkali and alkaline earth
metal ferrites have been tested for photocatalytic hydrogen evolution. For example, Boudjemaa and co-workers synthesized the MgFe 2 O 4 spinel ferrite by calcination of a Mg–Fe layered double hydroxide obtained by co-precipitation [130].
This material gave rise to hydrogen evolution under visible light from an aqueous sulfite solution, although production declined after 20 min of reaction. This
authors ascribed this deactivation to the saturation of catalytic sites and competitive reduction of the end product S 2 O 6
2−
for conduction band electrons. Dom et al.
reported the synthesis of orthorhombic CaFe 2 O 4 nanoparticles using different
preparation methods, namely solid-state reaction, polymer complex, microwave
sintering and self-propagating combustion (SPC) [155]. As they already reported
for zinc ferrite [151], the microwave-directed method yielded interesting results,
outperforming by nearly one order of magnitude the activities obtained with the
rest of syntheses, which gave very similar results [155]. The authors ascribed this
difference to an optimum combination of high crystallinity with monodispersed
morphology together with efficient visible light absorption, suitable band energetics and relatively low charge transfer resistance. Photodeposition of platinum
as a hydrogen evolution co-catalyst resulted in further improvement of the activity by a factor of ca. 2.5, resulting in an apparent quantum yield of 1.57% under
420 nm irradiation. Different strontium ferrites, namely the cubic, hexagonal and
orthorhombic SrFe 2 O 4 , SrFe 12 O 19 and Sr 7 Fe 10 O 22 , respectively, were selectively
139
Reprinted from the journal