Topics in Current Chemistry (2020) 378:6
1 3
top-down approach method using planetary ball mill, high-energy shaker or tumbler
mill for inducing proper conditions in which solid-state chemical reactions can take
place, mostly through mechanical activation or mechanochemistry. However, while
the method is low-cost and time-saving with easy up-scaling, it suffers notably from
strong contamination problems during prolonged milling, non-stoichiometric structure, non-uniform size distribution and low crystallinity.
Further, many more or less complex and versatile bottom-up or top-down methods have been implemented for preparing ferrite materials, alone or as part of the
main popular methods described above. Without being exhaustive, one can list notably microemulsion, flame spray pyrolysis, auto-combustion, vapour deposition, liquid exchange, self-reactive quenching, or electrospinning. The reader can refer to
the reviews by Kefeni et al. [13] and Tatarchuk et al. [24], or directly to the original
research cited therein.
Finally, it is worth noting that the optimal physico-chemical properties that the
ferrite materials need to exhibit are highly dependent on their application fields, so
that putting into perspective the advantages vs. disadvantages of the different preparation methods remains in many cases inappropriate.
1.3 Ferrite Properties
Ferrites are generally considered as being thermally and chemically stable in aqueous systems [44], and the Pourbaix diagrams indicate that most of the ferrites are
stable in most of the alkaline or near-neutral media used in photoelectrochemical
cells [9]. However, ferrites are reported to suffer from corrosion in acidic media [50,
51]. Independently of the crystallographic structure, the mono- or bisubstitution of
the cations within the ferrite materials is known to affect many of their physicochemical properties, including the resistivity (conductivity), the optical properties
(reflectivity), the bandgap energy, the energy position of both valence and conduction bands, or the p/n-type behaviour [9]. Therefore, the controlled cationic singleor multisubstitution in the ferrite network, whether A- or B-sites are concerned,
became an elegant, versatile and promising way to tune both bulk and surface physico-chemical properties of the ferrite-based photocatalysts used for environmental
(water and air depollution) and solar fuel applications.
The specific surface area exposed by the ferrite materials is strongly dependent on the preparation method. However, the ferrite catalysts generally exhibit
low to medium specific surface area (< 100 m
2
g
−1
) in contrast to lab-made or
commercially available titania photocatalysts that can reach high surface areas of
200–350 m
2
g
−1
thanks to the implementation of adequate synthesis approaches.
Table 1 reports some selected spinel ferrites and orthoferrites, substituted or not,
with their specific surface area, crystallite size and isoelectric point together with
their preparation method and final calcination temperature. It highlights to what
extent the synthesis method impacts on some properties of the materials. It has to
be mentioned that maintaining a relatively low or moderate calcination temperature
does not necessarily guarantee ferrites with a high specific surface area; in contrast,
116
Reprinted from the journal
1 3
top-down approach method using planetary ball mill, high-energy shaker or tumbler
mill for inducing proper conditions in which solid-state chemical reactions can take
place, mostly through mechanical activation or mechanochemistry. However, while
the method is low-cost and time-saving with easy up-scaling, it suffers notably from
strong contamination problems during prolonged milling, non-stoichiometric structure, non-uniform size distribution and low crystallinity.
Further, many more or less complex and versatile bottom-up or top-down methods have been implemented for preparing ferrite materials, alone or as part of the
main popular methods described above. Without being exhaustive, one can list notably microemulsion, flame spray pyrolysis, auto-combustion, vapour deposition, liquid exchange, self-reactive quenching, or electrospinning. The reader can refer to
the reviews by Kefeni et al. [13] and Tatarchuk et al. [24], or directly to the original
research cited therein.
Finally, it is worth noting that the optimal physico-chemical properties that the
ferrite materials need to exhibit are highly dependent on their application fields, so
that putting into perspective the advantages vs. disadvantages of the different preparation methods remains in many cases inappropriate.
1.3 Ferrite Properties
Ferrites are generally considered as being thermally and chemically stable in aqueous systems [44], and the Pourbaix diagrams indicate that most of the ferrites are
stable in most of the alkaline or near-neutral media used in photoelectrochemical
cells [9]. However, ferrites are reported to suffer from corrosion in acidic media [50,
51]. Independently of the crystallographic structure, the mono- or bisubstitution of
the cations within the ferrite materials is known to affect many of their physicochemical properties, including the resistivity (conductivity), the optical properties
(reflectivity), the bandgap energy, the energy position of both valence and conduction bands, or the p/n-type behaviour [9]. Therefore, the controlled cationic singleor multisubstitution in the ferrite network, whether A- or B-sites are concerned,
became an elegant, versatile and promising way to tune both bulk and surface physico-chemical properties of the ferrite-based photocatalysts used for environmental
(water and air depollution) and solar fuel applications.
The specific surface area exposed by the ferrite materials is strongly dependent on the preparation method. However, the ferrite catalysts generally exhibit
low to medium specific surface area (< 100 m
2
g
−1
) in contrast to lab-made or
commercially available titania photocatalysts that can reach high surface areas of
200–350 m
2
g
−1
thanks to the implementation of adequate synthesis approaches.
Table 1 reports some selected spinel ferrites and orthoferrites, substituted or not,
with their specific surface area, crystallite size and isoelectric point together with
their preparation method and final calcination temperature. It highlights to what
extent the synthesis method impacts on some properties of the materials. It has to
be mentioned that maintaining a relatively low or moderate calcination temperature
does not necessarily guarantee ferrites with a high specific surface area; in contrast,
116
Reprinted from the journal