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applied
sciences
Review
Application of Spinel and Hexagonal Ferrites in
Heterogeneous Photocatalysis
Zuzanna Bielan 1, * , Szymon Dudziak 2 , Adam Kubiak 3 and Ewa Kowalska 4, *
1 Centre for Plasma and Laser Engineering, The Szewalski Institute of Fluid-Flow Machinery,
Polish Academy of Science, Fiszera 14, 80-231 Gdansk, Poland
2 Department of Process Engineering and Chemical Technology, Chemical Faculty, Gdansk University of
Technology, G. Narutowicza 11/12, 80-233 Gdansk, Poland; dudziakszy@gmail.com
3 Institute of Chemistry and Technical Electrochemistry, Faculty of Chemical Technology, Poznan University of
Technology, Berdychowo 4, 60-965 Poznan, Poland; adam.kubiak@put.poznan.pl
4 Institute for Catalysis (ICAT), Hokkaido University, N21 W10, Sapporo 001-0021, Japan
* Correspondence: zbielan@imp.gda.pl (Z.B.); kowalska@cat.hokudai.ac.jp (E.K.)
Abstract: Semiconducting materials display unique features that enable their use in a variety of
applications, including self-cleaning surfaces, water purification systems, hydrogen generation,
solar energy conversion, etc. However, one of the major issues is separation of the used materials
from the process suspension. Therefore, chemical compounds with magnetic properties have been
proposed as crucial components of photocatalytic composites, facilitating separation and recovery
of photocatalysts under magnetic field conditions. This review paper presents the current state
of knowledge on the application of spinel and hexagonal ferrites in heterogeneous photocatalysis.
The first part focuses on the characterization of magnetic (nano)particles. The next section presents
the literature findings on the single-phase magnetic photocatalyst. Finally, the current state of
scientific knowledge on the wide variety of magnetic-photocatalytic composites is presented. A key
Citation: Bielan, Z.; Dudziak, S.;
aim of this review is to indicate that spinel and hexagonal ferrites are considered as an important
Kubiak, A.; Kowalska, E. Application
element of heterogeneous photocatalytic systems and are responsible for the effective recycling of the
of Spinel and Hexagonal Ferrites in
Heterogeneous Photocatalysis. Appl.
photocatalytic materials.
Sci. 2021, 11, 10160. https://doi.org/
10.3390/app112110160 Keywords: heterogeneous photocatalysis; hexagonal ferrites; magnetic separation; spinel ferrites
Academic Editors: Francesco Congiu
and Giorgio Concas
1. Introduction
Received: 30 September 2021 Nowadays, increasing global pollution and energy issues constitute the main threats
Accepted: 26 October 2021
to the natural environment. Therefore, new strategies and approaches have been developed
Published: 29 October 2021
to solve those issues. Among them, materials science might be considered as one of the
crucial fields for environmental protection [1]. For example, oxide systems, defined as a
Publisher’s Note: MDPI stays neutral
material consisting of at least two components (oxides) aimed at improving the application
with regard to jurisdictional claims in
properties of the final product, are becoming more and more popular. The growth of the
published maps and institutional affil-
application potential of oxide systems is due to the synergistic effects between components
iations.
(e.g., heterojunction, Z-scheme mechanism, etc. [2]) or the formation of mixed crystallo-
graphic structures (e.g., ZnTiO3 , SrTiO3 [3]). It should be emphasized, however, that apart
from the chemical composition, the control of crystallite/particle sizes, in particular at
the nano scale, is fundamental for further applications of oxide materials in photoactive
Copyright: © 2021 by the authors.
processes (photocatalytic, photovoltaic, etc.) [4]. For this reason, the classic approach to
Licensee MDPI, Basel, Switzerland.
materials’ synthesis, where the size of the particles is not the main concern for the process
This article is an open access article
efficiency, is currently one of the crucial issues in modern materials science.
distributed under the terms and
Since the famous publication by Fujishima and Honda in 1972 [5], photocatalysis study
conditions of the Creative Commons
using semiconductor materials has received increasing attention [6–9]. Each year, scientists
Attribution (CC BY) license (https://
from around the world are developing new photoactive materials, as well as modifying
creativecommons.org/licenses/by/
4.0/).
the known ones, in order to overcome their limitations with mass transfer, charge carrier
Appl. Sci. 2021, 11, 10160. https://doi.org/10.3390/app112110160 https://www.mdpi.com/journal/applsci
In addition to the above‐mentioned issues in the field of photocatalysis, attention
should be paid to the fact that scientific knowledge in the subject of ab initio is still
insufficient, as pointed out by, among others, Lisovski et al. [44]. Ab initio is a
computational chemistry method based on quantum chemistry, and in the case of a
Appl. Sci. 2021, 11, 10160 2 of 24
photocatalyst, it enables determination of, among others, exciton lifetimes, stability, the
redox position of valence and conduction bands, bandgap, and adsorption energies. For
example, Hu et al. [45] explained the correlation between the packing factor and
recombination, activity,
photocatalytic visible-light activity,
whereas Linseparation,
et al. [46]generation
successfullyof reactive oxygen
investigated thespecies, and
geometric
effective irradiation
structures [10–15].
and electronic properties of Ni‐doped TiO2 anatase and rutile. The authors
Comparing
described the number
the calculation andofsimulation
recently published
by a planemanuscripts in the field of
wave pseudopotential photocataly-
method based
sis (Figure
on density 1), only about
functional 7% consider
theory the material
(DFT). However, theseparation after the reaction,
ab initio calculations in thewhich
field ofis
considerably less
photocatalysis than
are those
still focusing on
challenging activity
due to theincrease
current(84%) and visible-light
problems response
in computational
(80%). Thisthe
chemistry; clearly shows that
vast majority the recycling
of advanced and reuse systems
experimental of photocatalysts
cannot beisrecreated,
still underval-
even
by the supercomputer systems [47]. Therefore, it is necessary to further develop thisbegan
ued. Furthermore, it should be mentioned that the study of magnetic separation field
relatively
with lateto(around
regard beginningprocesses
photooxidation of the 21st century)
[48] as wellinas
comparison
the aspectstoofthe other
the aspects of
separation of
heterogeneous after
photocatalysts photocatalysis.
processing.
Figure 1. The number of scientific articles published in 2020 (according to the Web of Science
Figure 1. The number of scientific articles published in 2020 (according to the Web of Science
database) for
database) for various
various keywords
keywords concerning
concerning photocatalysis.
photocatalysis.
Of course, other types of separation/recycling have been proposed and investigated,
This review presents all the aspects, focusing on the newest trends, applied for the
such as sedimentation, coagulation, (ultra)filtration, immobilization of photocatalysts on el-
use of spinel and hexagonal ferrites (selected groups of ferromagnets) in heterogeneous
ements of photoreactors (walls [16], quartz protecting tubes of lamps and LEDs [17,18], and
photocatalysis, as an efficient method for semiconductor recycling.
optical fibers [19]) or additional supporters (glass beads [16], microspheres [20], balls [21],
mesh [22], cloth [22], rings [23], membranes [24–26], and plates/films/foils [27–30]) [31].
The main limitation of simple separation by gravitational sedimentation is caused
by the nanometric size of photocatalysts. Kagaya et al. [32] as well as Baran et al. [33]
proposed photocatalyst separation by coagulation. The addition of inorganic (FeCl3 [33],
[Al2 (OH)n Cl6-n ]m [32] and poly-ferric sulfate (PFS) [34]) or organic coagulants (e.g., cationic
polyacrylamide (CPAM) [35]) might effectively accelerate the aggregation of photocatalyst
nanoparticles (NPs) by modifying their double layer potential. Another method of sus-
pended photocatalyst recovery is ultrafiltration. For example, Xue et al. [36] separated fine
titania NPs by both cross-flow ultrafiltration (CFU) and coagulation. It has been found that
Appl. Sci. 2021, 11, 10160 3 of 24
titania loses activity during coagulation, in contrast to CFU, which does not cause activity
loss. However, the application of ultrafiltration is associated with significantly higher oper-
ation costs. In this respect, the immobilization of photocatalysts seems to be a better option.
However, it is obvious that worse photocatalytic activity is expected for immobilized than
suspended photocatalysts due to (i) mass transfer limitations, (ii) surface area decrease,
and (iii) lower number of active sites for photocatalytic reactions. Additionally, the stability
of immobilized photocatalysts is challenging, since photocatalyst leakage (detachment
from the support [22,24,27]) and the destruction of suspended supports (e.g., microsphere
breaking under shear forces inside the photoreactor [31]) have been observed. Accordingly,
two different synthesis approaches have been proposed for efficient photocatalyst recovery:
(i) mesoporous photocatalysts of micrometer size, thus allowing efficient separation by
simple physical methods, i.e., sedimentation or filtration [37–39], and (ii) photocatalysts
with magnetic properties. The latter is presented and discussed in this review.
The application of an external magnetic field, proposed as an alternative method
of photocatalyst recovery, means that separated material must be bifunctional, i.e., with
photocatalytic and magnetic properties. Ferromagnets have been viewed as the most
promising magnetic materials to be used for this purpose. Their various combinations with
popular semiconductors have been proposed in different configurations, i.e., from disor-
dered mixtures [40], through more complex structures, such as core-shells [41], to advanced
multilayered materials [42]. It should be pointed out that ferromagnet-semiconductor
composites could also form efficient heterojunctions, since several magnetic oxides exhibit
photocatalytic activity themselves [43], and thus the heterojunction composite might be
even more active than the sole photocatalyst forming it. However, this might also result in
the instability of the composite, as ferrites are not as photostable as typical photocatalysts
(mainly titania), and thus dissolution and leakage of active components could occur due to
photocorrosion. Accordingly, an insulator interlayer between the photocatalyst and ferrites
has been proposed in more advanced structures.
In addition to the above-mentioned issues in the field of photocatalysis, attention
should be paid to the fact that scientific knowledge in the subject of ab initio is still insuf-
ficient, as pointed out by, among others, Lisovski et al. [44]. Ab initio is a computational
chemistry method based on quantum chemistry, and in the case of a photocatalyst, it
enables determination of, among others, exciton lifetimes, stability, the redox position of va-
lence and conduction bands, bandgap, and adsorption energies. For example, Hu et al. [45]
explained the correlation between the packing factor and photocatalytic activity, whereas
Lin et al. [46] successfully investigated the geometric structures and electronic properties of
Ni-doped TiO2 anatase and rutile. The authors described the calculation and simulation by
a plane wave pseudopotential method based on density functional theory (DFT). However,
the ab initio calculations in the field of photocatalysis are still challenging due to the current
problems in computational chemistry; the vast majority of advanced experimental systems
cannot be recreated, even by the supercomputer systems [47]. Therefore, it is necessary
to further develop this field with regard to photooxidation processes [48] as well as the
aspects of the separation of photocatalysts after processing.
This review presents all the aspects, focusing on the newest trends, applied for the
use of spinel and hexagonal ferrites (selected groups of ferromagnets) in heterogeneous
photocatalysis, as an efficient method for semiconductor recycling.
2. Characterization of Spinel and Hexagonal Ferrite Magnetic Nanoparticles
Ferrites, composite metal oxides containing ferric ions, are magnetic materials that
have recently been intensively investigated for various possible applications, including
electronic devices [49], gas sensors [50], medical purposes [51,52], water purification and
wastewater treatment (adsorbent, catalyst, and Fenton reagent) [53–57], organic reactions
(catalysts) [58,59], and heterogeneous photocatalysis [43]. Ferrites adopt mainly the spinel
structure, but some exist in hexagonal one. The significant scientific interest in spinel and
hexagonal ferrites is likely due to both their properties (magnetic properties, large specific
Ferrites, composite metal oxides containing ferric ions, are magnetic materials that
have recently been intensively investigated for various possible applications, including
electronic devices [49], gas sensors [50], medical purposes [51,52], water purification and
wastewater treatment (adsorbent, catalyst, and Fenton reagent) [53,54,55,56,57], organic
Appl. Sci. 2021, 11, 10160 4 of 24
reactions (catalysts) [58,59], and heterogeneous photocatalysis [43]. Ferrites adopt mainly
the spinel structure, but some exist in hexagonal one. The significant scientific interest in
spinel and hexagonal ferrites is likely due to both their properties (magnetic properties,
large specific
surface area) surface area)and
and simple andcheap
simplesynthesis
and cheap synthesis procedure.
procedure. Recently,
Recently, they theybeen
have also have
also been
applied forapplied for photocatalytic
photocatalytic purposes, purposes, as discussed
as discussed in the nextin the next sections.
sections.
2.1.
2.1. Spinel Ferrites
Spinel ferrites with
Spinel ferrites with the thegeneral
generalchemical
chemicalformula
formula ofof MFe
MFe 2O2 4,O 4 , where
where M states
M states for
for me‐
metallic cation
tallic cation of of
Fe,Fe,Mg, Mg, Mn,Co,
Mn, Co,Zn,
Zn,Ni,
Ni,etc.,
etc.,are
are characterized
characterized by by aa cubic
cubic shape
shape crystal
crystal
structure, 2+ 3+ in the crystallographic
structure,asaspresented
presentedininFigure Figure2.2.The
Thedistribution
distributionofofMM2+and andFeFe3+ in the crystallographic
sites
sites (tetrahedral
(tetrahedral andand octahedral)
octahedral) depends
depends mainly
mainly on on the
the M-cation
M‐cation radius
radius andand its
its charge,
charge,
which obviously influence the final material properties [60,61]. In the
which obviously influence the final material properties [60,61]. In the case of normal case of normal spinel
spi‐
structures, M 2+ cations occupy only tetrahedral sides, leaving octahedral ones for Fe3+ .
nel structures, M cations occupy only tetrahedral sides, leaving octahedral ones for Fe3+.
2+
Zinc ferrite (ZnFe
(ZnFe22O O44) )isisan
anexample
exampleofof this. 2+ cations could also occupy
Zinc ferrite this. In In contrast,
contrast, M2+M cations could also occupy only
only octahedral sides, forcing 3+ ions into the tetrahedra position,
octahedral sides, forcing thethe partial
partial migration
migration ofofFeFe
3+ ions into the tetrahedra position,
which
which isis known
known as as the
the inverse
inverse spinel
spinel structure
structure(Fe (Fe33O
O44,, CoFe
CoFe22OO44,,NiFe
NiFe22OO4,4 ,etc.)
etc.)[62].
[62].
Figure 2.2. Spinel
Spinel ferrite
ferrite structure
structure(inverse)
(inverse) for
forFe
Fe3O
O4, presented along the a direction and as a 3D projection (not marked
Figure 3 4 , presented along the a direction and as a 3D projection (not marked
octahedra for clarity). Reprinted with permission from [63]. Copyright (2021) Creative Commons Attribution.
octahedra for clarity). Reprinted with permission from [63]. Copyright (2021) Creative Commons Attribution.
Magnetic properties
Magnetic properties of of spinel
spinel ferrites
ferrites strongly
strongly depend
depend on on their
their dimensions,
dimensions, i.e., i.e., the
the
bigger the
bigger the NP
NP is,
is, the
the higher
higher isisthe
thevalue
valueof ofmagnetization
magnetizationsaturation
saturation(M (Mss).). Furthermore,
Furthermore,
superparamagnetismappears
superparamagnetism appearsasasa consequence
a consequence of aofdecrease
a decrease in ferromagnetic
in ferromagnetic particle
particle size
size to the nanometric scale. Usually, this occurs in NPs of the order of several
to the nanometric scale. Usually, this occurs in NPs of the order of several nanometers to nanometers
to several
several dozens
dozens of nanometers,
of nanometers, but but
the the critical
critical particle
particle size size is different
is different for each
for each material
material and
and is conditioned by magnetic anisotropy as well as chemical composition,
is conditioned by magnetic anisotropy as well as chemical composition, crystal structure, crystal struc‐
ture,morphology
and and morphologyof theof the ferrite
ferrite [64].
[64]. As anAs an example,
example, magnetite
magnetite NPs become
NPs become superpar‐
superparamag-
amagnetites
netites when when their diameter
their diameter is aboutis about
10–1210–12
nm [65].nm It[65]. It should
should be pointed
be pointed out thatoutNPs
that lose
NPs
lose their
their magnetization
magnetization whenwhen the magnetic
the magnetic field field is turned
is turned off. off.
2.2.
2.2. Hexagonal Ferrites
Hexagonal ferrites (also referred as as hexaferrites)
hexaferrites) represent
represent aa large
large class
class of
of magnetic
magnetic
ceramics
ceramics withwithaahexagonal
hexagonalstructure,
structure,which
whichcould bebe
could seen as an
seen Fe2Fe
as an O32O network
3 networkdistorted by
distorted
the presence of larger cations, usually Sr, Ba, or Pb. Compared to their spinel relatives,
by the presence of larger cations, usually Sr, Ba, or Pb. Compared to their spinel relatives, they
generally (i) require
they generally higherhigher
(i) require temperatures to be synthesized,
temperatures (ii) form
to be synthesized, (ii)larger
form particles easily,
larger particles
and (iii) exhibit typical hard-ferromagnetic behavior, with a large value of magnetic
easily, and (iii) exhibit typical hard‐ferromagnetic behavior, with a large value of magnetic rema-
nence and coercivity [66]. The hexaferrite family might be classified into six sub-categories,
depending on the complexity of their crystal structure: M-type, Z-type, Y-type, W-type,
X-type, and U-type ferrites [67]. However, regarding their photocatalytic applications, only
M-type materials, with the general formula of MFe12 O19 , have been studied so far. Their
crystal structure, using barium hexagonal ferrite (BaFe12 O19 ) as an example, is presented
in Figure 3.
remanence and coercivity [66]. The hexaferrite family might be classified into six sub‐cat‐
egories, depending on the complexity of their crystal structure: M‐type, Z‐type, Y‐type,
W‐type, X‐type, and U‐type ferrites [67]. However, regarding their photocatalytic appli‐
Appl. Sci. 2021, 11, 10160 cations, only M‐type materials, with the general formula of MFe12O19, have been studied
5 of 24
so far. Their crystal structure, using barium hexagonal ferrite (BaFe12O19) as an example,
is presented in Figure 3.
Figure3.3.Crystal
Figure Crystalstructure
structureof
ofthe
theBaFe
BaFe1212O
O19
19 hexagonal
hexagonal ferrite.
ferrite.
3.3.Single-Phase
Single‐PhaseMagnetic
MagneticPhotocatalysts
Photocatalysts
3.1. Spinel Ferrites
3.1. Spinel Ferrites
As
Aspresented
presented by by Zieli ńska-Jurek et
Zielińska‐Jurek et al.,
al.,pristine
pristineFe
Fe3O
3O 4 (Eg
4 (Eg
= 0.1
= 0.1 eV)eV) does
does notnot
showshow
any
any photocatalytic activity [68]. Nevertheless, some of MFe 2 O4 spinel ferrites
photocatalytic activity [68]. Nevertheless, some of MFe2O4 spinel ferrites are photocatalyt‐are pho-
tocatalytically active themselves,
ically active themselves, without without any coupling
any coupling with
with other other semiconductors
semiconductors (see Table(see
1).
Table 1). For example, Sutka et al. compared five different ferrites in terms of magnetic and
For example, Sutka et al. compared five different ferrites in terms of magnetic and photo‐
photocatalytic properties [69]. It was found that materials with no or low Ms exhibit good
catalytic properties [69]. It was found that materials with no or low Ms exhibit good pho‐
photocatalytic activity for degradation of methyl orange (MO). Similarly, Dojcinovic et al.
tocatalytic activity for degradation of methyl orange (MO). Similarly, Dojcinovic et al. ob‐
observed that with an increase in the content of doped cobalt in Cox Mg1-x Fe2 O4 ferrite,
served that with an increase in the content of doped cobalt in CoxMg1‐xFe2O4 ferrite, the
the magnetic properties increased, but photocatalytic activity decreased [70]. However,
magnetic properties increased, but photocatalytic activity decreased [70]. However, it has
it has also been reported that manganese spinel ferrite (MnFe2 O4 ) with a high Ms value
also been reported that manganese spinel ferrite (MnFe2O4) with a high Ms value (~ 60
(~60 2Am−12 ·kg−1 ) shows high photocatalytic activity (about 56% degradation of Direct Red
Am ∙kg ) shows high photocatalytic activity (about 56% degradation of Direct Red 81 af‐
81 after 2 h of solar-light irradiation) [71]. It is also worth mentioning that the photocat-
ter 2 h of solar‐light irradiation) [71]. It is also worth mentioning that the photocatalytic
alytic activity of spinel ferrites seems not to depend on their bandgap energies or on the
activity of spinel ferrites seems not to depend on their bandgap energies or on the con‐
conduction band (CB) and valence band (VB) positions, as they are similar for a wide range
duction
of band
materials (CB) and
(Figure 4). valence band (VB) positions, as they are similar for a wide range
of materials (Figure 4).
Appl. Sci. 2021, 11, 10160 6 of 24
Table 1. Summary of photocatalytic activity on spinel ferrites.
Experimental
Photocatalyst Pollutants Findings Ref.
Conditions
Solar light (Indonesia);
MnFe2 O4 Direct Red 81 56% deg. (15 ppm) [71]
0.035 g·L−1 ; 15–35 ppm; 120 min
500-W Hg lamp; 2–8 g·L−1 ; ~70% (35 min, 6 g·L−1 ,
MnFe2 O4 MB [73]
50 ppm; pH = 2–10; 5–35 min pH 8.5)
NiFe2 O4 70% deg.
MgFe2 O4 ~30% deg.
MFe2 O4 (M = Zn, Ni, 100-W LED;
MO ZnFe2 O4 ~15% deg. [69]
Mg, Cu, Co) 1 g·L-1; 10 ppm; 180 min
CoFe2 O4 ~0 deg.
CuFe2 O4 ~0 deg.
ZnSm1.5 Fe0.5 O4
Solar light (India);
ZnSmx Fe2-x O4 (x = 0–2) MO 100% deg. (10 ppm); [74]
1 g·L−1 ; 10–30 ppm; 80 min
no activity loss after 5 runs
Vis halogen lamp (70 mW·cm−2 )
Cox Mg1-x Fe2 O4 or natural sunlight Co0.1 Mg0.9 Fe2 O4
MB [70]
(x = 0–1) (800–1100 W·m−2 ); 0.5 g·L−1 ; ~80% deg.
10 ppm; 240 min
HEBER multilamp reactor; 0.1 g;
Mg1-x Nix Fe2 O4 ~86% deg.; negligible
MB 100 ppm; 2 ml Fenton’s agent; [75]
(x = 0.0, 0.6, 1.0) activity loss after 5 runs
180 min
UV-lamp; 1 g·L−1 ; 25 ppm; 97% deg.; ca. 10%
Cu-MgFe2 O4 MB [76]
180 min; 2 ml of H2 O2 activity loss after 5 runs
La1-x Bix Cr1-y Fey O3 200 W Ar lamp/420 nm cut-off
90.8% deg.
(x = 0–0.1; RhB filter; 5 g·L−1 ; 5 ml of 10% H2 O2 ; [77]
(La0.06 Bi0.04 Fe0.06 Cr0.08 O3 )
Appl. Sci.
y =2021, 11, x FOR PEER REVIEW
0.02–0.12) 55 min 6 of 24
deg.—degradation; MB—Methylene Blue; MO—Methyl Orange; RhB—Rhodamine B.
Figure 4. Bandgap
Bandgapenergies
energiesofofspinel
spinelferrites (NHE,
ferrites pHpH
(NHE, = 0=and 14). 14).
0 and Reprinted with with
Reprinted permission from
permission
[72].
fromCopyright (2021)(2021)
[72]. Copyright Elsevier.
Elsevier.
3.2. M-Type
Table Hexaferrites
1. Summary of photocatalytic activity on spinel ferrites.
Concerning the photocatalytic activity of single-phase M-type hexaferrites, their ef-
Experimental
Photocatalyst Pollutants is often much lower than that of known photocatalytic
ficiency Findingsmaterials, but they
Ref. can
Conditions
absorb a broad spectrum of visible light, and thus their vis activity is of some interest, as
summarized Solar
in Table 2. light (Indonesia);
For example, Bibi et al. studied
MnFe2O4 Direct Red 81 56%the photocatalytic
deg. (15 ppm) properties
[71] of
0.035 g∙L−1; 15–35 ppm; 120 min
pure and Nd/Cu co-doped BaFe12 O19 under solar light in Pakistan, using methylene green
500‐W
dye as a probe [78]. Hg
Bothlamp;
types2–8
of g∙L−1; 50 ppm;
ferrites could remove~70%
the(35 min,compound,
target 6 g∙L−1, with a final
MnFe2O4 MB [73]
efficiency of 37% and pH93%= 2–10; 5–35 min
for pristine and doped materials, pH 8.5)
respectively, after 60 min of
NiFe2O4 70% deg.
MgFe2O4 ~30% deg.
MFe2O4 (M = Zn, Ni, 100‐W LED;
MO ZnFe2O4 ~15% deg. [69]
Mg, Cu, Co) 1 g∙L‐1; 10 ppm; 180 min
CoFe2O4 ~0 deg.
Appl. Sci. 2021, 11, 10160 7 of 24
irradiation. It has been proposed that the increased activity of a doped structure is caused
by the possible formation of oxygen vacancies (due to the dopants’ presence), which results
in hindering of electron-hole recombination. Moreover, only a slight decrease in the effi-
ciency from 93% to 91% was observed during five cycles, indicating high reusability of the
Nd/Cu co-doped photocatalyst. Interestingly, Raut et al. showed that pristine BaFe12 O19 ,
prepared via the molten salt method, exhibits even higher photocatalytic activity than the
well-known P25 TiO2 (famous due to exceptionally high activity for various reactions and
thus used as a standard for activity checking [79,80]) under both UV and UV/vis irradiation
for degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (hexogen, RDX) [81]. The highest
activity of the ferrite, observed under neutral pH conditions, was explained by efficient
light absorption (bandgap of 2.1 eV) and thus an increased number of generated charge car-
riers. The high reusability of the photocatalyst was confirmed during three cycles, with the
efficiency change from 100% to 98%. Bavarsiha et al. investigated the SrFe12 O19 structure
for photocatalytic degradation of methylene blue (MB) under UV irradiation [82]. It was
found that 46% of dye could be removed during 120 min of irradiation, but reusability was
not determined. Finally, the unique structure of the CdFe12 O19 photocatalyst, both in its
non-modified form (with some Fe2 O3 impurities) and as a composite with CdTiO3 phase,
was investigated by Mahdiani et al. for removal of MB, methyl orange (MO), and methyl
violet (MV) dyes under UV irradiation (without recycling analysis) [83]. It was found that
the composite (CdFe12 O19 /CdTiO3 ) is more active than a one-component photocatalyst for
all the studied dyes, reaching the highest activity for MO—71% degradation during 70 min
of irradiation.
Table 2. Summary of M-type hexaferrite photocatalysts.
Experimental
Photocatalyst Pollutants Findings Ref.
Conditions
BaFe12 O19 37% deg.
Pakistan sunlight; 882–890 W·m−2 ;
BaFe12 O19 doped with MG [78]
0.05 g·L−1 ; 10 ppm; 60 min 93% deg.
Nb and Cu
250 W Hg lamp (λmax 254 nm); 300 W Hal.
100% deg. (Hg lamp);
BaFe12 O19 RXD lamp (350–1100 nm); pH 3–10; [81]
90% deg. (Hal. lamp)
0.2–1.4 g·L−1 ; 40 ppm; 240 min
SrFe12 O19 MB 20 W UV light; 50 ppm; 120 min 46% deg. [82]
CdFe12 O19 (with Fe2 O3 19% deg. MO; 0% deg.
impurity) MB; 0 deg. MV
MO; MB; MV 400 W UV light; 1 g·L−1 ; 5 ppm; 70 min [83]
70% deg. MO; 61% deg.
CdFe12 O19 /CdTiO3
MB; 55% deg. MV
Ba1-x Cox Fe12-y Cry O19 Bahawalpur (Pakistan) sunlight
CV ~65.5% deg. [84]
(x = 0–0.7; y = 0–0.6) (881–892 W·m−2 ); 1 g·L−1 ; 10 ppm; 60 min
CV—Crystal Violet dye; deg.—degradation; Hal.—halogen; MB—Methylene Blue; MG—Methyl Green; MO—Methyl Orange; MV—Methyl
Violet; RXD—hexogen
4. TiO2 -Based Magnetic Photocatalysts
Titanium(IV) oxide (TiO2 , titania, titanium dioxide) is probably the most broadly
investigated semiconductor photocatalyst due to various advantages, including high
photocatalytic activity, stability, abundance, low price, and low toxicity [85–89]. However,
the wide bandgap of titania, resulting in good redox properties (both oxidation and water
reduction capabilities) is not recommended for light harvesting efficiency. Moreover,
titania, as all semiconductors, suffers from charge carriers’ recombination. In addition,
the most active titania, i.e., that used in a particulate form, cannot be easily recycled, as
already discussed in the introduction. Accordingly, various methods of titania modification,
doping, coupling, and immobilizing have been proposed to improve its photocatalytic
performance [14,90–94]. Ferrites have also been proposed as titania modifiers for three
Appl. Sci. 2021, 11, 10160 8 of 24
main purposes, i.e., (i) improved activity by hindering the charge carriers’ recombination,
(ii) extension of activity towards vis range of solar spectrum, and (iii) easy and cheap
recycling, as presented in this Section.
4.1. Spinel Ferrites with TiO2
The coupling of spinel ferrites and titania (pristine or modified) in order to obtain a
highly photoactive material with magnetic properties has commonly been reported in the
recent literature, as summarized in Table 3.
Table 3. Summary of TiO2 -spinel ferrite photocatalysts.
Experimental
Photocatalyst Pollutants Findings Ref.
Conditions
10% 4-NPh deg.
300 W Xe light (UV/vis,
(Fe3 O4 @SiO2 /TiO2 );
Fe3 O4 @SiO2 /TiO2 -M (M = 60 W·m−2 ); 2 g·L−1 ;
Ph, 4-NPh 100% Ph deg. and 50% TOC [95]
Pt/Cu) 20 ppm (Ph)
removal
or 500 µM (4 NPh); 60 min
(Fe3 O4 @SiO2 /TiO2 -Pt)
300 W Xe light (UV/vis or vis
100% deg. (UV/vis);
ZnFe2 O4 /SiO2 /TiO2 ETD λ > 400 nm); 2 g·L−1 ; 15 ppm; [96]
40% deg. (vis); 6 cycles
30 min
120 W Hg lamp (UV) or 150 W ~0.29 mm·h−1
TiO2 -Fe3 O4 -M Hal. lamp (UV/vis, (TiO2 -Fe3 O4 /Ag; UV);
Phenols [97]
(M = Ag/Au) 350–700 nm); ~0.13 mm·h−1
2 g·L−1 ; 1 mM; 7 h (TiO2 -Fe3 O4 /Ag; vis)
300 W Xe lamp with cut-off
90% deg.
NiFe2 O4 @TiO2 core@shell MO filter (λ > 400 nm); 0.8 g·L−1 ; [98]
(NiFe2 O4 @40% TiO2 )
8 ppm; 90 min
95% MO deg.
(Co0.5 Zn0.25 Ni0.25 Fe2 O4 /TiO2 );
180 W solar simulator;
Co0.5 Zn0.25 M0.25 Fe2 O4 /TiO2 99% MB deg.
MO; MB 1 g·L−1 ; 4·10−5 M; 80 min (MB) [99]
(M = Ni, Cu, Mn, Mg) (Co0.5 Zn0.25 Ni0.25 Fe2 O4 /TiO2
or 360 min (MO)
and
Co0.5 Zn0.25 Mn0.25 Fe2 O4 /TiO2 )
4-NPh—4-nitrophenol; conv.—conversion; deg.—degradation; ETD—etodolac; Hal.—halogen; MB—Methylene Blue; MO—Methyl Orange;
Ph—phenol; TOC—total organic carbon.
It should be underlined that MFe2 O4 NPs in TiO2 composites (and in other ferromagnet-
photocatalyst materials) can exhibit a dual function. First, as semiconductors with narrower
bandgap than UV-active photocatalysts, they could take part in photocatalytic reactions,
creating a heterojunction, which might cause both vis activity and efficient charge carri-
ers’ separation [100]. At the same time, their magnetic properties allow easy and cheap
recycling of photocatalysts after reactions. However, in some cases, the direct connec-
tion between magnetic and photocatalytic materials is not worthwhile, especially when
the separation is emphasized. For example, environmental conditions (e.g., acidic pH)
and enhanced charge carrier migration (e.g., resulting in photocorrosion) could cause the
leaching of iron ions from the composite, thus resulting in the destruction of the spinel
ferrite structure. The possible reactions of Fe3 O4 oxidation/reduction are presented in
Equations (1)–(4) and Figure 5.
Fe3+ (s) + e− → Fe2+ (s) (1)
Fe2 O3 + 6H+ + 2e− → 2Fe2+ + 3H2 O (2)
Fe3 O4 + 8H+ + 3e− → 3Fe2+ + 4H2 O (3)
2+ + 3+
Fe (aq) + h → Fe (aq) (4)Appl. Sci.
Appl. Sci.2021, 11,x10160
2021,11, FOR PEER REVIEW 9 9ofof24
24
Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 24
Figure 5. Possible reactions in Fe 3O4/TiO 2 composite (drawn based onon
[101]).
Figure5.5.Possible
Figure Possiblereactions
reactionsin
inFe
Fe33O
O44/TiO composite
/TiO22composite (drawn
(drawn based
based [101]).
on [101]).
Accordingly, the
Accordingly, the approach ofof separation between
between magnetic and and photocatalytic partsparts
Accordingly, theapproach
approach ofseparation
separation betweenmagnetic magnetic andphotocatalytic
photocatalytic parts
through an inert
through interlayer, such asas
silica [102], polyaniline [103], andand
carbon [104],[104],
has been
through an inert interlayer, such as silica [102], polyaniline [103], and carbon [104],has
inert interlayer, such silica [102], polyaniline [103], carbon has
proposed.
been The The
proposed. core-interlayer-shell structure
core‐interlayer‐shell (e.g.,(e.g.,
structure shown in Figure
shown in 6), with
Figure 6), a magnetic
with a mag‐
been proposed. The core‐interlayer‐shell structure (e.g., shown in Figure 6), with a mag‐
core, core,
netic an inert interlayer, and a photocatalytic shell, prevents unfavorable charge transfer,
netic core,ananinert
inertinterlayer,
interlayer,and
andaaphotocatalytic
photocatalyticshell,
shell,prevents
preventsunfavorable
unfavorablecharge
charge
maintaining
transfer, the photocatalytic and magnetic properties of both composites.
transfer, maintaining the photocatalytic and magnetic properties of bothcomposites.
maintaining the photocatalytic and magnetic properties of both composites.
Figure 6. STEM images of Fe33O
O44@SiO 2 (a) and Fe 3O 4@SiO 2/TiO2 (b) (b)combined
combinedwith
withTi, Fe, and Si
Figure6.6.STEM
Figure STEMimages
imagesofofFeFe 3O@SiO
4@SiO 22(a)
(a)and
andFe 3O
Fe 3O44@SiO
@SiO22/TiO
/TiO22 (b) combined with Ti,
Ti, Fe,
Fe, and
andSiSi
mapping.
mapping. Reprinted with permission from [102]. Copyright (2017) Creative Commons Attribution.
mapping.Reprinted
Reprintedwith
withpermission
permissionfrom
from[102].
[102].Copyright
Copyright(2017)
(2017)Creative
CreativeCommons
CommonsAttribution.
Attribution.
4.2.Hexaferrites
4.2. Hexaferrites withTiO TiO22
4.2. Hexaferriteswith with TiO 2
Althoughpure
Although purehexaferrites
hexaferriteshave have gained moderate
moderate attention as as photocatalysts, their their
Although pure hexaferrites havegained gained moderateattentionattention asphotocatalysts,
photocatalysts, their
combination with
combination with other
other photoactive
photoactive materials
materials have
have frequently
frequently been reported. Among Among
combination with other photoactive materials have frequently been reported. Among
them, titania‐based
them, titania-based photocatalysts are are the most
most common (see (see Table 4). For For example,
them, titania‐basedphotocatalysts
photocatalysts arethe the mostcommon
common (seeTable Table 4). Forexample,
example,
BaFe
BaFe 12 O
O1919
12 /TiO composites,
/TiO2 composites,
2 initially reported by Fu et al. [105] and Lee
initially reported by Fu et al. [105] and Lee et al. [106], et al. [106],can
can be
be
BaFe 12O19/TiO2 composites, initially reported by Fu et al. [105] and Lee et al. [106], can be
prepared by a simple
prepared TiO22coating on the surface ofof the ferrite. In both studies, procion red
prepared by asimple simpleTiOTiO2coating
coatingon onthethesurface
surface ofthe theferrite.
ferrite.InInboth
bothstudies,
studies,procion
procion
MXMX
red 5B was used as aastarget pollutant decomposed under UV irradiation (no reusability
red MX 5B was used as a target pollutant decomposed under UV irradiation (noreusabil‐
5B was used a target pollutant decomposed under UV irradiation (no reusabil‐
analysis).
ity It was found that though the activity increased with an increase in TiO2 content,
ityanalysis).
analysis).ItItwaswasfound
foundthatthatthough
thoughthe theactivity
activityincreased
increasedwith withan anincrease
increaseininTiO TiO2 con‐
2 con‐
it was
tent, always lower than that of the P25 reference and/or sole TiO NPs. In contrast, Xie
tent,ititwas
wasalways
alwayslower
lowerthanthanthat
thatofofthe
theP25
P25reference
referenceand/or
and/orsole soleTiO 2 NPs. In contrast,
2
TiO 2 NPs. In contrast,
et
Xie al. found higher activity for the SrFe O /TiO composite (0.2/0.8 for Sr-ferrite and
Xieetetal.al.found
foundhigher
higheractivity
activityfor forthe
theSrFe O19/TiO2 2 composite (0.2/0.8 for Sr‐ferrite and
12 1219
SrFe 12O19/TiO2 composite (0.2/0.8 for Sr‐ferrite and
TiO22, ,respectively)
TiO respectively)thanthanthat
thatforforsingle
singlecomponents
componentsunder underUV UVirradiation
irradiation[107].[107].Moreover,
Moreover,
TiO2, respectively)
reusability was also than that forinsingle
confirmed three components
consecutive under
cycles, UV
with irradiation
an activity [107]. Moreover,
decrease of ca.
reusability
reusabilitywas wasalso
alsoconfirmed
confirmedininthreethreeconsecutive
consecutivecycles,
cycles,withwithan anactivity
activitydecrease
decreaseofofca.
ca.Appl. Sci. 2021, 11, 10160 10 of 24
10%. Furthermore, in contrast to pristine TiO2 NPs, the composites also exhibited activity
under visible-light (λ ≥ 420 nm) irradiation. In both cases (UV and visible-light irradiation),
100% degradation was observed after approx. 4–5 h of irradiation. It has been proposed
that enhanced activity is caused by the possible formation of a n-n type heterojunction
between both phases. Another SrFe12 O19 -based composite was also prepared by Bavarsiha
et al. [108]. This time, three-phase composites, i.e., core/interlayer/shell structure, were
prepared using SrFe12 O19 , SiO2 , and TiO2 as a core, an interlayer, and an outerlayer,
respectively. The efficiency of the composite was confirmed during degradation of MB,
reaching ca. 80% degradation during 3 h of UV irradiation, with high reusability since only
an 8% activity decrease was observed after three cycles. As a reference, SrFe12 O19 /SiO2
composite was used, showing negligible photocatalytic activity. Finally, PbFe12 O19 /TiO2
composite was also investigated by Lahijani et al. for degradation of Acid Blue 29, Acid
Violet 7, and Acid Black 1 dyes [109]. The effective decolorization (based on the presented
photographs) was observed for all tested dyes during 20−40 min of UV irradiation, but
detailed process kinetics, as well as reusability, were not examined.
Table 4. Summary of TiO2 -hexaferrite photocatalysts.
Experimental
Photocatalyst Pollutants Findings Ref.
Conditions
75% deg. (BaFe12 O19 );
BaFe12 O19 /TiO2
96% deg. [105]
(10–30% TiO2 )
(BaFe12 O19 /30%TiO2 )
4 × 8 W UV bulbs (λmax 365 nm);
Procion Red MX-5B 0.2 g·L−1 (with respect to TiO2 ); 60% deg.
10 ppm; 300 min (BaFe12 O19 /30%TiO2 after
BaFe12 O19 /TiO2 500 ◦ C heat treatment); 70% [106]
deg. (BaFe12 O19 /30% TiO2
without heat treatment);
UV light (λmax 254 nm) 95% deg. (UV) and
SrFe12 O19 /TiO2
MB or vis light (λ > 420 nm); pH 7; 99% deg. (vis) [107]
(70–85% TiO2 content)
2 g·L−1 ; 10 ppm; 300 min for SrFe12 O19 /80% TiO2
SrFe12 O19 /SiO2 /TiO2 MB 20 W UV light; 50 ppm; 180 min 80% deg. [108]
10 W UV light;
PbFe12 O19 /TiO2 ABl; AV; ABk ~100% dec. of all dyes [109]
20 g·L−1 ; 10 ppm; 60 min
ABk—Acid Black 1; ABl—Acid Blue 29; AV—Acid Violet 7; dec.—decolorization; deg.—degradation; MB—Methylene Blue.
5. Other Magnetic-Photocatalytic Composites
Titania is still the main photocatalytic material used. However, the continuous de-
velopment of materials science shows that the photocatalytic activity of titania depends
on various parameters, including the crystal structure, the presence of surface impurities,
the specific surface area, the morphology, etc. [4]. In order to be able to control the above
parameters, it is necessary not only to apply innovative synthesis techniques (e.g., mi-
crowave route [110]) but also to use morphology-determining agents (e.g., hydrofluoric
acid [111]). Despite the undoubted advantages resulting from the high photoactivity of
titania, researchers are still looking for novel materials with similar or better parameters
than TiO2 . However, regardless of the main component of the photocatalytic composite,
e.g., ZnO [112], CuO [113], or others, the key aspect is the recovery of the material after the
photodegradation process. Therefore, in the next part of this review, the current state of
knowledge in the field of ferrite-based composite photocatalysts not containing titania is
presented and discussed.
5.1. Spinel Ferrite Composites
The summary of selected materials containing spinel ferrites is presented in Table 5.
Despite the semiconductor and ferrite types, all composites (presented in the Table 5) haveAppl. Sci. 2021, 11, 10160 11 of 24
revealed high photocatalytic activity, in the range of 96% (ZnS-WO3 -CoFe2 O4 [114] and
Ag3 PO4 /MnFe2 O4 [115]) to even 100% (rGO-based spinel ferrite composites [116,117]).
It has also been proven by Abroshan et al. [115] that MB degradation for single-phase
manganese spinel ferrites and trisilver phosphates is much lower than that for the compos-
ite (58%, 80%, and 96%, respectively), probably due to the formation of a heterojunction
between the phases, allowing the effective charge separation.
Additionally, in a large number of research works, the recovery/recycling of the
magnetic composites was investigated. The use of external magnetic field allows the
genuine separation, and thus the reuse, of the photoactive material in at least three cycles
of pollutant degradation. As a prominent example, in the work of Dudziak et al. [118],
during eight cycles of carbamazepine photodegradation on the Fe3 O4 -based Pt-modified
composite, 100% efficiency was preserved.
Table 5. Summary of selected spinel ferrite–photocatalyst composites.
Experimental
Photocatalyst Pollutants Findings Ref.
Conditions
58% MB deg. (MnFe2 O4 );
Sunlight (Iran,
80% MB deg. (Ag3 PO4 );
Ag3 PO4 /MnFe2 O4 MB; RhB ~185 mW·cm−2 ); [115]
96% MB deg.
2 g·L−1 ; 15 ppm; 80 min
(Ag3 PO4 /MnFe2 O4 )
300 W Xe lamp
100% dec.;
Ni0.65 Zn0.35 Fe2 O4 @rGO MB (vis λ > 420 nm); [116]
5 cycles—10% activity loss
1 g·L−1 ; 5 ppm; 48 min
100 W Xe lamp with cut-off
~97% deg. (BiFeO3 -25 wt%
BiFeO3 -ZnFe2 O4 MB filter (λ > 420 nm); pH 2; [119]
ZnFe2 O4 )
1 g·L−1 ; 15 ppm; 120 min
UV lamp (λmax = 254 nm;
0.6 mW·cm−2 ); 1 mM PDS;
LaZF@rGO RhB 100% dec. (1:1 LaZF@rGO) [117]
pH 4.5; 0.25 g·L−1 ; 30 ppm;
80 min
Tungsten-halogen lamp
96% deg.;
ZnS-WO3 -CoFe2 O4 MB (400–800 nm); 0.05 g; 50 ppm; [114]
4 cycles no activity loss
180 min
HPHgL with cut-off filter
99% RhB deg. & 87% NB red.
PANI/BiOBr/ZnFe2 O4 RhB; NB (λ > 420 nm); 1 g·L−1 ; 20 ppm [120]
(7% PANI/BiOBr/ZnFe2 O4 )
(RhB) and 5 ppm (NB); 30 min
dec.—decolorization; HPHgL—high pressure mercury lamp; LaZF—lanthanum-substituted zinc spinel ferrite; MB—Methylene Blue;
NB—nitrobenzene; PANI—polyaniline; PDS—peroxydisulfate; red.—reduction; rGO—reduced graphene oxide; RhB—Rhodamine B.
Carbon-based materials, i.e., containing graphene, reduced graphene oxide (rGO),
carbon nanotubes (CNT) or graphitic carbon nitride, have recently been widely used for
photocatalytic reactions. After coupling them with spinel ferrite NPs, an efficient composite
with both photocatalytic activity and magnetic separability can be obtained. What is more
important, the photogenerated electrons can migrate only from ferrite to carbon material.
In this regard, the main shortcoming of a ferrite-based photocatalyst, i.e., photobleaching,
is inhibited [63].
The schematic mechanism of heterojunction for non-carbon-based ferrites is shown
in Figure 7, where ZnFe2 O4 NPs and BiFeO3 represent the group of p-type and n-type
materials, respectively. It has been proposed that during visible-light irradiation, the
excited electron from CB of ZnFe2 O4 could migrate to CB of BiFeO3 . At the same time, the
created holes are transferred between the valence bands, in the opposite direction, due
striving for the equilibrium state. Accordingly, this separation of charge carriers might
effectively improve the photoactivity of the composite [119].Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 24
Appl. Sci. 2021, 11, 10160 12 of 24
striving for the equilibrium state. Accordingly, this separation of charge carriers might
effectively improve the photoactivity of the composite [119].
Figure 7. The schematic illustration of charge carriers’ migration in BiFeO3‐ZnFe2O4 composite. Re‐
Figure 7. The schematic illustration of charge carriers’ migration in BiFeO3 -ZnFe2 O4 composite.
printed with permission from [119]. Copyright (2018) Creative Commons Attribution.
Reprinted with permission from [119]. Copyright (2018) Creative Commons Attribution.
5.2.Hexaferrites
5.2. HexaferritesComposites
Composites
Similartotospinel
Similar spinelferrites,
ferrites,hexaferrites
hexaferriteshave havealso alsobeen
beencombined
combinedwith withdifferent
differentcarbon-
carbon‐
based materials, especially for possible visible-light response, as summarized in Table 6.6.
based materials, especially for possible visible‐light response, as summarized in Table
Forexample,
For example,MahdianiMahdianietetal.al. investigated
investigated thethe photoactivity
photoactivity of of
PbFePbFe
12 O 12O/graphene
19 19/grapheneand and
PbFe O /CNT composites
PbFe12 O19 /CNT composites
12 19 for degradation of methyl orange under UV
methyl orange under UV irradiation and irradiation and
acidicconditions
acidic conditions (pH(pH 2–3)
2–3) [121].
[121]. AnAn increased
increased activity
activity of the
of the composite
composite in comparison
in comparison to theto
the simple
simple PbFe12 PbFe O19 structure,
O19 12structure, especially
especially for thefor the graphene‐based
graphene-based photocatalyst,
photocatalyst, was ex‐
was explained
plained
by by the inhibition
the inhibition of charge
of charge carriers’ carriers’ recombination,
recombination, resulting from resulting from theofseparation
the separation the π-π*
of the π‐π*
orbitals orbitals of
of graphene duegraphene due to the interactions
to the interactions with the 2p with oxygentheorbitals
2p oxygen from orbitals from
the ferrite
the ferrite
structure. structure.
Ultimately, 75%Ultimately,
degradation75% degradation
efficiency was observed efficiency
for PbFe was O
12 19 observed
/graphene for
after
PbFe70 12Omin of irradiation.
19/graphene after 70 A uniqueofapproach
minutes irradiation. wasA proposed by Ismail
unique approach wasetproposed
al., where by
BaFe
Ismail12 Oet /g-C
19al., 3 N4 photocatalysts
where BaFe12O19/g‐C3Nwere used to reduce
4 photocatalysts wereHg(II)
used toions [122].Hg(II)
reduce Although the
ions [122].
photocatalysts contained onlycontained
Although the photocatalysts 1–4 mass% of Ba-ferrite,
only 1–4 mass%a of significant
Ba‐ferrite,activity increase
a significant was
activity
observed
increase was withobserved
respect towith the respect
sole BaFe to12 O19
the and
sole BaFeg-C123O
N194 and
materials.
g‐C3N4Itmaterials.
was proposed It wasthatpro‐
enhanced
posed that activity
enhanced could be caused
activity could bybe charge
caused carrier migration
by charge and
carrier cumulation
migration andon different
cumulation
components,
on different i.e., electrons and
components, holes on g-C
i.e., electrons and3 N 4 and
holes onferrite,
g‐C3Nrespectively.
4 and ferrite, The best activity
respectively. The
was obtained for 3%-BaFe
best activity was obtained O
12 for composite under vis irradiation (λ >
19 3%‐BaFe12O19 composite under vis irradiation (λ > 420 420 nm), reaching 100%
nm),
reduction
reaching of 100%Hg2+ ions within
reduction of Hg 302+min.
ions Reusability
within 30 minutes. of the composite
Reusability was alsocomposite
of the tested, andwas a
slight decrease
also tested, and in aactivity (from 100%
slight decrease to 90%)(from
in activity after five
100% consecutive
to 90%) aftercycles
fivewas ascribed to
consecutive cy‐
be caused
cles by the blocking
was ascribed of theby
to be caused surface active sites
the blocking of the(no photocatalyst
surface active sitesloss).
(noFurthermore,
photocatalyst
Wang et al. also investigated BaFe O /g-C N composites,
loss). Furthermore, Wang et al. also investigated BaFe12O19/g‐C3N4 composites,
12 19 3 4 using them as catalysts
using them in
the photo-Fenton process of rhodamine B degradation under
as catalysts in the photo‐Fenton process of rhodamine B degradation under visible‐light visible-light irradiation [123].
Again, a synergistic
irradiation effect was
[123]. Again, observed, effect
a synergistic with the was highest activity
observed, achieved
with the highest for 16.8 wt%
activity
ofachieved
BaFe12 Ofor . It was also proposed that charge separation between
19 16.8 wt% of BaFe12O19. It was also proposed that charge separation between phases is responsible
for the activity
phases is responsibleincrease.for However,
the activity in the contrastHowever,
increase. to the report by Ismail
in the contrast et al.
to the[122], it was
report by
proposed that excited electrons react with H 2 O 2 on the surface
Ismail et al. [122], it was proposed that excited electrons react with H2O2 on the surface of BaFe 12 O 19 , whereas holesof
migrate
BaFe12Oto19,g-C 3 N4 , asholes
whereas shown in Figure
migrate to g‐C 8. 3Scavenger-based
N4, as shown in Figure experiments have confirmed
8. Scavenger‐based an
exper‐
active role of • OH, h+ and • O − on the degradation efficiency. Reusability tests during four
2
iments have confirmed an active role of •OH, h+ and •O2− on the degradation efficiency.
cycles showed
Reusability a slight
tests during decrease in activity,
four cycles showed from 98% todecrease
a slight 91% degradation,
in activity, during
from 100 98%min to 91%of
irradiation. Quaternary composite SrFe12 O19 /SiO2 /TiO2 /GO was also proposed by Abd
Aziz et al. towards direct sunlight-driven photocatalytic degradation of 2,4-dichlorophenolAppl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 24
Appl. Sci. 2021, 11, 10160 degradation, during 100 minutes of irradiation. Quaternary composite 13 of 24
SrFe12O19/SiO2/TiO2/GO was also proposed by Abd Aziz et al. towards direct sunlight‐
driven photocatalytic degradation of 2,4‐dichlorophenol (2,4‐DCP) [124]. An increased ac‐
tivity of 4‐phase composite in comparison to pristine TiO2 and TiO2/GO nanostructures
(2,4-DCP) [124]. An increased activity of 4-phase composite in comparison to pristine TiO
was explained by the possible charge transfer between TiO2 and GO and increased specific2
and TiO2 /GO nanostructures was explained by the possible charge transfer between TiO2
surface area of the composite. It was proposed that the ferrite contribution to the photo‐
and GO and increased specific surface area of the composite. It was proposed that the
catalytic mechanism is insignificant because of its coverage with an insulating layer (SiO2).
ferrite contribution to the photocatalytic mechanism is insignificant because of its coverage
Moreover, it was shown that photocatalysts could be efficiently reused, as complete deg‐
with an insulating layer (SiO2 ). Moreover, it was shown that photocatalysts could be
radation of 2.4‐DCP was observed during three cycles within 2.55 h (depending on the
efficiently reused, as complete degradation of 2.4-DCP was observed during three cycles
light intensity)
within 2.5−5 hof sunlight irradiation.
(depending on the light intensity) of sunlight irradiation.
Theschematic
Figure8.8.The
Figure schematicillustration
illustrationof ofthe
theproposed
proposedmechanism
mechanismfor forthe
thephotocatalytic
photocatalyticdegradation
degradation
of
ofRhodamine
RhodamineBBononBaFe 12O
BaFe 1219Oferrite/g‐C 3N4 composite.
19 ferrite/g-C Reprinted
3 N4 composite. with permission
Reprinted from [123].
with permission from Cop‐
[123].
yright (2017)(2017)
Copyright Elsevier.
Elsevier.
Finally, hexagonal
Finally, hexagonalferrites, ferrites,combined
combinedwith withother
otherphotocatalysts,
photocatalysts,have havealso alsobeen
beenin‐ in-
tensively studied.
tensively studied. For Forexample,
example,Xie Xie etet al.
al. proposed
proposedaa double‐magnetic
double-magnetic composite composite of of
ZnFe2O
ZnFe 2O 4 /SrFe
4/SrFe 12O 1219Oas19 aasvisible‐light
a visible-light activeactive photocatalyst
photocatalyst towardstowards the degradation
the degradation of MB of
MB [125].
[125]. It was It was
proposed proposed that that the composite
the composite exhibits
exhibits bothboth improved
improved magnetic
magnetic andand pho-
photo‐
tocatalytic performance, i.e.,
catalytic performance, i.e., (i) SrFe12O (i) SrFe O enhances the magnetic
enhances the magnetic properties of
1219 19 properties of ZnFe O44, ,
22O
which are quite weak for the pure spinel phase (M value of ca. 1.75 and 39 Am 22·kg− 1 for
which are quite pure spinel phase (MSS value of ca. 1.75 and 39 Am ∙kg for −1
thepure
pureZnFe ZnFe 3+ by Zn2+
the 2O2O 4 and
4 and thethe composite,
composite, respectively),
respectively), andand
(ii) (ii) substitution
substitution of Fe of3+Fe
by Zn 2+ ions
ions
at theat2athe 2aof
sites sites
the of the M-ferrite
M‐ferrite structure structure
resultsresults in enhanced
in enhanced photocatalytic
photocatalytic activityactivity
when
when compared
compared to pristine to pristine
ZnFe2OZnFe 4. This 2 Osubstitution
4 . This substitution
caused acaused
presence a presence
of Fe2+ inoftheFe2+ in the
compo‐
composite, and the resulting Fe 2+ /Fe3+ interactions could be responsible for an increased
site, and the resulting Fe /Fe interactions could be responsible for an increased e‐
2+ 3+
e− transport
transport and and a further
a further increase
increase in photocatalytic
in photocatalytic activity
activity of theof composite.
the composite. However,
However, re‐
reusability tests showed a constant decrease in activity between
usability tests showed a constant decrease in activity between the first and fifth cycle (from the first and fifth cycle
(from
95% to 95%
70% to 70% degradation),
degradation), connected connected with possible
with possible photocatalyst
photocatalyst loss between
loss between the cyclesthe
cycles as well as surface blocking and partial destruction
as well as surface blocking and partial destruction of the composite mesostructured. An‐ of the composite mesostruc-
tured. photocatalyst
other Another photocatalyst was designed was designed by Sobhani-Nasab
by Sobhani‐Nasab et al., et al.,
who whoprepared
prepared aa
BaFe1212
BaFe OO 1919
/Sm/Sm Ti O7 structure
2Ti22O72 structure in itsin bare
its bare
andand Ag-decorated
Ag‐decorated forms forms
[126].[126].
It was It found
was foundthat
that Ag has a positive impact on the activity, resulting in
Ag has a positive impact on the activity, resulting in 100% degradation of MB during 100% degradation of MB during
150
150 min of vis irradiation (400–770 nm). It should be pointed out that the positive effect
minutes of vis irradiation (400–770 nm). It should be pointed out that the positive effect
of noble metals has been well known and intensively investigated for more than forty
of noble metals has been well known and intensively investigated for more than forty
years, as they usually work as an electron scavenger, inhibiting charge carriers’ recombi-
years, as they usually work as an electron scavenger, inhibiting charge carriers’ recombi‐
nation [127–130]. Unfortunately, recycling causes a slight activity decrease (as in many
nation [127,128,129,130]. Unfortunately, recycling causes a slight activity decrease (as in
ferrite-based photocatalysts), to 90% degradation after four cycles.
many ferrite‐based photocatalysts), to 90% degradation after four cycles.
Another visible-light active structure was proposed by Xie et al., who prepared a
SrFe12O19/BiOBr 2D-2D nanocomposite for the degradation of Rhodamine B (λ > 420 nm) [131].
An increase in the activity of BiOBr was noticed for samples with 3 wt% and 5 wt% Sr-
ferrite, probably because of increased charge carrier separation, where electrons migrateYou can also read
