J Mater Sci: Mater Electron (2018) 29:1719–1747
DOI 10.1007/s10854-017-8166-x
REVIEW
Review on magnetically separable graphitic carbon nitride-based
nanocomposites as promising visible-light-driven photocatalysts
Mitra Mousavi1 · Aziz Habibi‑Yangjeh1
· Shima Rahim Pouran2
Received: 14 September 2017 / Accepted: 28 October 2017 / Published online: 8 November 2017
© Springer Science+Business Media, LLC 2017
Abstract Graphitic carbon nitride (g-C3N4) has gained
remarkable acceptance as a visible-light-driven photocatalyst with a distinctive 2D structure and great stability. Owing
to its superior features, g-C3N4 has been engaged in various
scientific activities for environmental pollution abatement,
production and storage of energy, and gas sensors. However, the visible-light efficiency of pure g-C3N4 is very poor
and its separation from the phototreated systems is difficult.
The most promising method to improve the photocatalytic
activity and facilitate separation process is to introduce a
magnetic compound over the g-C3N4 sheets. This review
has mainly focused on the recent advancement in fabrication, characterization and application of magnetic g-C3N4based nanocomposites. Accordingly, four primary g-C3N4based nanocomposites are discussed based on the type of
integrated magnetic material. The effects on the structure,
physico-chemical properties, photocatalytic activity towards
degradation of pollutants, hydrogen generation, solid phase
extraction, lithium-ion batteries, gas sensors, and supercapacitors are also discussed in detail.
1 Introduction
Currently, unchecked release of toxic chemicals into the
environment threatens the life of the living beings and
human health [1]. One of the pollution control concerns is
protecting and conserving the natural resources. Water can
be considered as the most valuable resource that should be
conserved, treated and recycled [2, 3]. The commonly used
tertiary treatment systems for wastewater treatment are coagulation, filtration, sedimentation, reverse osmosis, adsorption and further removal of nutrients by prolonged secondary
biological methods using enzymes and/or microorganisms.
Nonetheless, the efficiency of these conventional methods is
not enough to treat polluted water to the levels acceptable for
most of the recalcitrant pollutants, such as pesticides, pharmaceutical, organic solvents, and household chemicals [4, 5].
In order to achieve the water purification goal, an additional
effective treatment step is required. Advanced oxidation processes (AOPs) can fulfill the treatment criteria especially
for wastewaters containing highly stable chemicals and/or
low biodegradable compounds [6, 7]. In these processes,
highly reactive species, mainly hydroxyl radicals (⋅OH), are
generated in situ, enabling non-selective oxidization of the
extremely refractory compounds to water, carbon dioxide,
and different inorganic ions (Eq. 1). Hydroxyl radical, with
a large oxidation potential of E
­ ◦ (⋅OH/H2O) = 2.80 V/SHE,
is known as the second strongest oxidant (after fluorine).
The rate constants for the reactions between ⋅OH radicals
with various contaminants are reported to be in the range
of ­106–1010 L mol−1 s−1 [8]. Furthermore, these radicals
are self-erased from the reaction vessel, due to their short
lifetime of just a few nanoseconds in water (Fig. 1).
Pollutant molecule
* Aziz Habibi‑Yangjeh
ahabibi@uma.ac.ir
1
Department of Chemistry, Faculty of Science, University
of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran
2
Department of Applied Chemistry, Faculty of Chemistry,
University of Tabriz, Tabriz 51666‑16471, Iran
AOPs →∙ OH ����������������������������→
� CO2 + H2 O + Inorganic ions
(1)
The most frequently employed AOPs are: (i) H
­ 2O2 with
­UVC radiation (­ H2O2/UVC), (ii) ozone and ozone-hybrid
processes ­(O3, ­O3/UVC, ­O3/H2O2, and O
­ 3/H2O2/UVC), (iii)
photocatalysis ­(TiO2/UV and T
­ iO2/H2O2/UV), and (iv)
Fenton reactions (Fenton ­(Fe2+/H2O2), photo-Fenton ­(Fe2+/
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3.0
Oxidant potential (V/NHE)
2.5
2.0
1.5
1.0
0.5
0.0
Type of oxidant
Fig. 1 Comparison between potential (vs NHE) of different oxidants
H2O2/UV), and sono-Fenton ­(Fe2+/H2O2/US)) [9]. Indeed,
the presence of various combinations, makes the AOPs very
difficult to classify, but based on the utilized catalyst, they
are generally classified to homogeneous and heterogeneous
systems (Scheme 1).
Nonetheless, there are problems of concern accompanied with a number of these processes, such as the ozone’s
stunted half-life, need for UV light for reaction initiation,
partial mineralization of organic pollutants, and the primary
costs related to these methods [10–12]. In recent years, semiconductor-based photocatalysis has received a great deal of
attention among AOPs, for its power to complete destruction
of organic, inorganic, and microbial pollutants, under ambient pressure and temperature [13–16]. This process leaves no
post-treatment sludge; thus, not leading to secondary pollution. Furthermore, since the photocatalyst is not consumed
during the process, it can be re-used in successive treatments
[17, 18].
Apart from the convenience factors discussed above,
there are practical impediments accompany semiconductor-based photocatalysis that need to be addressed. The
foremost is associated with the band gap width of the semiconductor, since the photocatalytic activity is considerably depended on this energy. T
­ iO2 and ZnO are the commonly used semiconductors that can be only energized
by ultraviolet (UV) light [19, 20], because of their large
band gaps (both are about 3.2 eV) [21–23]. Practically,
Scheme 1 Classification of advanced oxidation processes
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solar light cannot be used as a sustainable UV source,
since solar spectrum encloses only about 4% UV light,
and artificial UV sources are also expensive. Besides the
cost factor, the UV light is harmful and requires protective aids while using it [24]. Hence, there are growing
demands for fabrication of visible-light-driven (VLD)
photocatalysts with remarkable performance. Another
shortcoming of the semiconductor-based photocatalytic
processes is rapid recombination of the photoproduced
electron–hole ­(e−/h+) pairs, which are counted as charge
carriers in photocatalytic reactions; thus cutting down the
activity of the photocatalyst [25–27]. The next limitation is the difficulty in separation and recovery of photocatalyst from the treated solution especially for micro
and nano-sized particles. Filtration and centrifugation are
not only energy-consuming tools, but also a large amount
of photocatalyst is lost within the separation procedure
[28–30]. Some other drawbacks such as long residence
time [31], low degradation kinetics [32], partial mineralization of pollutants [33] and aggregation of catalysts particles [34] in the reaction solution have also been reported
in the literature.
As aforementioned, most of the photocatalysis processes suffer from the catalyst recovery difficulties after
the treatment [35]. One separation problem-solving strategy that has been extensively investigated, is to immobilize the photocatalyst particles on support materials such
as sand, zeolites and ceramics [36–38]. Although, this
approach has made the separation of the photocatalyst
easier, the effective surface area has been decreased in the
most conventional methods. Therefore, developing active
photocatalysts that are easier to recovery is still remaining a challenge. In recent years, magnetically-separable
(MS) photocatalysts have attracted considerable attention
from research communities [39]. These MS photocatalysts are prepared by combining a magnetic material with
photocatalytically active material [40–42]. The magnetic
behavior of these photocatalysts is generally offered by
magnetic compounds such as magnetite (­ Fe3O4), hematite
(α-Fe2O3), maghemite (γ-Fe2O3) [43], and spinel ferrites
­(MFe2O4: where M is a two valent cation such as ­Ba2+,
­Ni2+, ­Co2+, and Z
­ n2+) [44]. This incorporation has helped
to fabricate effective MS photocatalysts that are easily
removed and recycled by employing an external magnetic
field [45]. In addition, the post-separation agglomeration
of the catalyst particles can be limited and the stability of
the catalysts can be enhanced by this means. The application of the coupled magnetic component can also synergistically improve the effectiveness of the photocatalysts
by developing a hybrid photocatalyst through formation
of heterojunction between the constituents to help separation of the ­e−/h+ pairs and absorption of the visible-light
irradiation [46, 47].
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2 g‑C3N4 as a photocatalyst
Several allotropes of carbon nitrides are present with different stabilities [48]. Among them, graphitic carbon nitride
(g-C3N4) is the most stable one under ambient conditions.
Ever since, the study on g-C3N4 was traced in 1830s by Berzelius and Liebig, it is one of the lately introduced metalfree photocatalyst that has engrossed great attention in the
field, owing to its marvelous physico-chemical characteristics [49–58]. Two possible building blocks are nominated
as the primary building block of g-C3N4 materials: (i) triazine ­(C3N3) as the tectonic units, for the structure formula
influenced from graphite structure; and (ii) tri-s-triazine
(heptazine) rings as the central units, which are linked by
tertiary amines. The latter is associated with the polymer
melon hypothesis, which is the most stable pattern, energetically preferred and thus, the mostly accepted structural unit
(Fig. 2a, b) [52, 59, 60]. G-C3N4 is preferably comprised of
carbon and nitrogen atoms with 0.75 molar ratio of carbon
to nitrogen [61, 62]. This layered nitrogen-abundant polymer
offers unique thermal and electronic characteristics [63–65]
that has attracted more attentions for using g-C3N4 in various
catalytic reactions including oxidation [62, 66], hydrogenation [67], splitting of water to produce hydrogen, photocatalytic degradation of pollutants [68], and photoreduction of
carbon dioxide to value-added molecules [69].
Furthermore, the special photo-electronic properties of
g-C3N4 make it a great candidate to employ in batteries, fuel
and solar cells, and various devices [61]. Additionally, the
band gap energy of 2.7 eV permits g-C3N4 to be activated by
visible light and employed for purification of polluted water
and photo-oxidation of different organic compounds [70]. As
well known, efficient separation of the photoexcited ­e−/h+
pairs in a photocatalyst is one of the the key elements for its
high photocatalytic activity. Schematic presentation of the
charge transfer in g-C3N4 is shown in Fig. 3.
To determine the oxidation/reduction reactions in a photocatalytic process, it is crucial to calculate valence band
(VB) and conduction band (CB) energies of the applied photocatalyst. At the point of zero charge, these potentials can
be estimated using the below empirical equations:
ECB = 𝜒 − Ee − 0.5Eg
(2)
EVB = ECB + Eg
(3)
[
]1∕(a+b+c)
𝜒 = x(A)a x(B)b x(C)c
(4)
where EVB and ECB are the VB and CB edge potentials,
respectively; χ is the semiconductor absolute electronegativity, expressed as the geometric mean of the absolute electronegativity of the fundamental atoms, and defined as the
arithmetic mean of the atomic electroaffinity and the first
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Fig. 2 s-Triazine (a) and tri-s-train (b) as tectons of g-C3N4
Fig. 3 Schematic presentation of charge transfer in g-C3N4
ionization energy; Ee is the energy of free electrons on the
hydrogen scale (4.5 eV vs. NHE), and Eg is the band gap
energy of the photocatalyst [71, 72]. For g-C3N4, the calculated values of the CB, VB, and χ potentials are − 1.13,
+ 1.57, and 4.73 eV, respectively.
3 Disadvantages of g‑C3N4 as a photocatalyst
Several improvements in the structure and morphology
of g-C3N4 have been reported in the literature, including
g-C3N4 quantum dots (QDs) [73, 74], 3D-arranged macroporous g-C3N4 [75], and extremely-thin g-C3N4 nanosheets
[76, 77]. Nevertheless, there are still number of demerits
associated with g-C3N4 that limited its effective application
as photocatalyst for environmental purification purposes.
The main shortcomings are (i) inefficient specific surface
area [78], (ii) quick recombination of the photoexcited
­e−/h+ pairs [79]; (iii) poor visible-light absorption [80] and
(iv) low quantum yield [81]. Moreover, as aforementioned,
the separation of the used g-C3N4 from treated solutions
is difficult, time-consuming and wastes considerably high
amounts of the photocatalyst by centrifugation or filtration
[82]. Considering the cited limitations, there is an urgent
need to modify the structure of g-C3N4 by narrowing its
band gap to prepare more efficient VLD photocatalyst. In
addition, increasing its stability and specific surface area
should be taken under consideration. Furthermore, actions
should be taken to add magnetism for easier recovery and
saving the photocatalyst for the several applications. Various strategies have been implemented to enhance g-C3N4
photocatalytic effectiveness. Among them, (i) manufacturing
the mesoporous structures [83–86]; (ii) doping with metallic/non-metallic elements [87–91]; (iii) pairing with other
semiconductors [92–97]; (iv) co-polymerization; and magnetization by combining with magnetic materials [10, 98,
99] are mainly employed recently that will be explained in
detail in the following sections.
The recent large number of studies headed to the development of a big family of g-C3N4 comprised of numerous
microstructures and nanocomposites with diverse morphologies, yet being enlarged. There are a number of key factors
including the composition and morphology of the materials
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used, production route, and temperature applied for condensation, which strongly influence the structure, properties and morphology and accordingly the application of
the developed g-C3N4. In this regard, several morphologies
and structures of g-C3N4 such as nanosheets, nanopowders,
nanotubes, nanorodes, and nanowires have been fabricated
through various methodologies including exfoliation in different solvents (e.g. water, acetone, N-methyl pyrrolidone,
isopropyl alcohol, and ethanol); reflux process; pyrolysis,
thermal poly-condensation, and thermal etching of nitrogenrich precursors such as urea, cyanamide, dicyandiamide, and
melamine [59, 66, 67].
4 Categories of g‑C3N4‑based magnetic
photocatalysts
Magnetic particles (MPs) have attracted researchers from
various fields especially photocatalytic processes. The combinations of magnetic materials with photocatalysts have
been carried out for both increasing photocatalytic activity and magnetic recovery purposes. The external magnetic
field can be easily employed for separation of the composite,
permitting the several recycling of the photocatalysts under
more cost-effective and environmentally acceptable photocatalytic practices. The main MPs that have been effectively
combined with g-C3N4, for appending magnetic property to
the composite for easier separation, are hematite (α-Fe2O3),
maghemite (γ-Fe 2O 3), magnetite (­Fe 3O 4), and ferrites
­(MFe2O4) [100–106]. Figure 4 represents the publications
Fig. 4 Number of publications about g-C3N4-based magnetic photocatalysts (up to 2016), found in the scopus database
1723
about g-C3N4-based magnetic photocatalysts for different
applications. Research on these nanocomposites have been
mainly started from 2013 and then steadily increased.
On the other hand, Fig. 5 shows distribution of the papers
classified by the type of magnetic materials that combined
with g-C3N4 to fabricate nanocomposites for different applications. As can be observed, the number of studies about
application of ­Fe3O4 in g-C3N4-based nanocomposites is
about 43.5% of the published studies, which is much higher
than those of the other magnetic compounds. The focus on
magnetic composites of g-C3N4 with ­Fe3O4 is related to
number of merits including considerable saturation magnetization, low-cost, and green characteristics of F
­ e3O4. In
the following sections, the magnetic g-C3N4 composites are
thoroughly discussed.
4.1 α‑Fe2O3/g‑C3N4 magnetic photocatalysts
Hematite (α-Fe2O3) has been acknowledged as a non-toxic,
cheap, abundant iron oxide with relatively high electrical conductivity and corrosion resistancy, and the highest
stability under ambient conditions, among the known iron
oxides [107]. This n-type semiconductor is narrow band gap
(≈ 2.2 eV), which can absorb about 40% of the energy of
solar spectrum (up to 600 nm) [108]. Accordingly, it has
been widely used in gas sensors [109], catalysts [110], and
pigments [111]. On the other hand, α-Fe2O3 has relatively
low photocatalytic activity resulted from the high rate for
recombination of the charge carriers. One effective approach
to enhance photocatalytic performance of α-Fe2O3 is formation of heterojunctions between hematite and other semiconductors with proper band potentials to increase life-time
of the ­e−/h+ pairs. Recently, g-C3N4/α-Fe2O3 samples were
Fig. 5 Percentage distribution of published papers based on the type
of magnetic materials combined with g-C3N4
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prepared using various methods including deposition–precipitation [108], hydrothermal [112], and wet impregnation
[113] and they were utilized for photocatalytic degradation
of RhB dye, reduction of ­CrVI, and degradation of DR81
under visible-light illumination, respectively. The morphology studies of the synthesized g-C3N4/α-Fe2O3 samples
demonstrated a uniformly dispersed α-Fe2O3 nanocrystals
over the g-C3N4 layers (Fig. 6a). This heterostructure helps
for effective formation of the charge carriers. However, the
α-Fe2O3 content greatly affects the photocatalytic activity of
α-Fe2O3/g-C3N4 composites, wherein a concurrent enhancement in the photoactivity is achieved with increased α-Fe2O3
content up to the optimal value and decreases at higher values (Fig. 6b). The interaction of α-Fe2O3 NPs and g-C3N4
at the heterojunction interface is the main driving force for
the superior efficiency of the α-Fe2O3/g-C3N4 photocatalyst,
because both g-C3N4 and α-Fe2O3 can be photo-activated at
visible region and generate the e­ −/h+ pairs. Once the heterojunction interface is formed between g-C3N4 and α-Fe2O3
counterparts, the photogenerated electrons could simply flow
from the CB of g-C3N4 into the CB of α-Fe2O3, and the
holes could transfer from the VB of α-Fe2O3 into the VB
of g-C3N4; because, CB of g-C3N4 is more negative than
that of α-Fe2O3 and VB of α-Fe2O3 is more positive than
that of g-C3N4. Therefore, recombination of e­ −/h+ pairs is
declined and the charge separation efficiency is increased,
enhancing the photoactivity of α-Fe2O3/g-C3N4 nanocomposite (Fig. 6c).
Despite the advantages mentioned for α-Fe2O3/g-C3N4
nanocomposites, the delivered performance is unsatisfactory for commercial purposes, probably caused from the
restricted surface area and poor absorption at visible region.
Additionally, the production procedures are complex and
involve elevated temperatures which lead to the agglomeration of the photocatalyst. In light of this, Pawar et al. [114]
applied a new strategy for fabrication of hematite-based photocatalyst by modifying with g-C3N4 and noble metal NPs
such as Ag, Au, Pd, and Pt. The incorporated noble metal
NPs displayed a surface plasmon resonance (SPR) phenomenon that influenced the light absorption significantly and
enhanced the charge separation efficiency and photocatalytic activity. Among the studied metals, the gold integrated
g-C3N4/Fe2O3 nanocomposite offered the best performance
for RhB decolorisation under visible-light irradiation with
approximately 12 and 16-folds greater kinetic rate constant
than those for the pure g-C3N4 and F
­ e2O3. Beside the SPR
Fig. 6 a TEM image of α-Fe2O3/g-C3N4 composite (Copyright
[106]), b the kinetic constants for the degradation of RhB by g-C3N4,
α-Fe2O3 nanopowder, and α-Fe2O3/g-C3N4 nanocomposite under
visible-light irradiation (Copyright [102]), c proposed mechanism for
photocatalytic degradation of RhB over the α-Fe2O3/g-C3N4 nanocomposites under the light irradiation
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effects of the Au NPs, the superior activity of the Au (12%)/
g-C3N4(0.5%)/Fe2O3 composite was explained based on its
high specific surface area of Au/g-C3N4/Fe2O3 (46.5 m2 g−1)
compared with the pure F
­ e2O3, and g-C3N4(0.5%)/Fe2O3
samples with surface areas of 5.5 and 29.7 m2 g−1, respectively. Consequently, its adsorption capacity and visible-light
absorption efficiency was increased.
4.2 γ‑Fe2O3/g‑C3N4‑based magnetic photocatalysts
Maghemite is a simple ferric oxide that is used to add magnetism owing to its good magnetic characteristics with saturation magnetization of Ms ≈ 76 emu/g, non-toxicity, and
low price [10, 115]. Maghemite properties are temperatureaffected. At room temperature, it is ferromagnetic, but at
high temperatures, it is instable wherein the loss in magnetic
property and an irreversible change in its crystal structure
to hematite occur at about 400 °C. Consequently, experimental determination of maghemite Curie temperature is
difficult, though a value between 820 and 986 K is accepted
as its Curie temperature [116, 117]. Maghemite has good
absorption ability and visible-light response resulted from its
narrow band gap of 2.1–2.2 eV [118, 121]. Accordingly, the
integration of maghemite with other photocatalyst is given
rise to fabricate photocatalysts with easy magnetic recoverability and reuse.
Based on the literature, the combination of γ-Fe2O3 and
g-C3N4 is proposed as a possible solution to overcome the
problem of high recombination of photoexcited ­e−/h+ pairs
in γ-Fe2O3 and g-C3N4 and easy magnetic separation of the
composite from the reaction vessel. In a study conducted
by Wang et al. [119], γ-Fe2O3/g-C3N4 nanocomposite was
prepared via a simple refluxing method. The as-synthesized nanocomposite could remove completely RhB within
20 min, whereas the pristine γ-Fe2O3 and g-C3N4 photocatalysts only removed 20% of RhB under the same conditions.
The enhanced visible-light absorption and improved surface
area were stated as the main effects derived from the heterojunction interface created between γ-Fe2O3 and g-C3N4.
Similarly, Sheng Ye et al. reported RhB removal within
120 min by γ-Fe2O3/g-C3N4 composite [120]. In the reported
studies, the XRD patterns of F
­ e2O3/g-C3N4 showed the main
signature peaks of g-C3N4 and γ-Fe2O3 along with the planes
of α-Fe2O3 (Fig. 7a). Although, the studies indicated that
both α-Fe2O3 and γ-Fe2O3 normally present together and it
is difficult to control the F
­ e2O3 composition, Xu et al. [121]
developed a new method to prepare F
­ e2O3/g-C3N4 with only
γ-Fe2O3. For this purpose, solvothermal method was followed by a calcination step.
The XRD patterns of the F
­ e2O3 starting materials calcined at different temperatures are depicted in Fig. 7b. From
the figure, the diffraction planes of α-Fe2O3 (JCPDS No.
33-0664) and γ-Fe2O3 (JCPDS No. 39-1346) are observed
1725
in the XRD patterns of the precursor, which was furnaced
at 450 °C for 120 min. Subsequently, the peaks certified
for α-Fe2O3 were enhanced and those belong to γ-Fe2O3
were diminished when the temperature increased to 470 °C,
indicating the intra-changes in crystalline phase at elevated
temperatures. This assumption was proved when the whole
diffraction peaks ascribed to γ-Fe2O3 were wiped out and
the peaks of pure α-Fe2O3 were left at 500 °C. The further
increase in calcination temperature up to 650 °C, increased
the intensity of peaks, possibly due to the enhanced α-Fe2O3
crystallinity. On the other hand, VSM analysis was carried
out to assess the magnetic properties of the ­Fe2O3 precursor
heated at studied temperatures (Fig. 7c). The magnetization
saturation (Ms) of the samples decreased concurrent with
the temperature elevation, being 70.1 emu/g for the precursor heated at 470 °C and decreased to 5.8 and 2.5 emu/g
at 500 and 600 °C, respectively. This was attributed to the
transformation of γ-Fe2O3 into α-Fe2O3, as confirmed by
XRD analysis. Moreover, all the studied g-C3N4/γ-Fe2O3
nanocomposites had satisfactory magnetic properties, so
they could be separated successfully by applying an external
magnetic field (Fig. 7c).
4.3 Fe3O4/g‑C3N4‑based magnetic photocatalysts
In recent years, magnetite NPs have received a great deal of
attention by scientists for their applications in various fields
such as wastewater treatment [122], conductors [123, 124],
lithium storage [125, 126], and drug delivery [127, 128]
owing to their supermagnetic properties and easy recovery
from reaction medium. Furthermore, ­Fe3O4 NPs have some
advantages of large surface area, easy preparation method,
water disperse ability, and low toxicity; thus it has coined
as one of the extensively used magnetic compounds. Consequently, ­Fe3O4 NPs are widely incorporated with photoactive materials to fabricate MS photocatalysts. Moreover,
the enhancements in the photocatalytic performance of
­Fe3O4/g-C3N4 composites have also been verified in various studies. This enhancement was ascribed to the narrow
band gap (~ 0.1 eV) and high conductivity (1.9 × 106 Sm−1)
of ­Fe3O4 NPs [129, 131]. Therefore, ­Fe3O4 NPs can act as
a conductor for rapid transferring of the photogenerated
electrons. On this basis, by combination of a photoactive
materials with F
­ e3O4 NPs, one can prepare MS photocatalysts with more efficient separation of the charge carriers and
easy separation of the photocatalyst from reaction solution
by applying an external magnet.
In a study conducted by Zhou et al. [130], a highly efficient and magnetically separable F
­ e3O4/g-C3N4 nanospheres
were fabricated via a hydrothermal method. The prepared
­Fe3O4/g-C3N4 nanospheres were possessed unique porous
structure and exhibited high photocatalytic activity for
methyl orange (MO) degradation by 2% F
­ e3O4 loading. It
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Fig. 7 a XRD patterns of F
­ e2O3/g-C3N4 composite photocatalysts
1–5, clearly revealing the presence of g-C3N4 (*), α-Fe2O3 (#), and
γ-Fe2O3 (+) phases in the F
­ e2O3/g-C3N4 composite photocatalysts
(Copyright [114]), b XRD patterns of the precursor of ­Fe2O3 heated
at (a) 450 °C, (b) 470 °C, (c) 500 °C, and (d) 650 °C. c Field-dependent magnetization curves of the samples calcined at different temperature. Inset of the figure is photo for magnetic separation (Copyright
[115])
should be mentioned that photoactivity of ­Fe3O4/g-C3N4
nanospheres were three folds greater than that of pristine
g-C3N4 and almost unchanged after five consecutive runs
(Fig. 8). Moreover, its magnetic properties were maintained
after five recycling runs, which made the as-prepared composite a recyclable and convenient photocatalyst.
In another study, an in situ growth mechanism was
proposed by Kumar et al. [131] for a facile production of
g-C3N4/Fe3O4 nanocomposites. They reported uniform deposition of F
­ e3O4 NPs onto g-C3N4 surface, thus allowing
the light diffusion for superior visible-light photocatalytic
activity for RhB degradation. The enhanced surface area
and visible-light absorption ability were observed for the
prepared g-C3N4/Fe3O4 photocatalyst. More importantly, the
g-C3N4/Fe3O4 photocatalyst could be easily magnetically
recovered and reused effectively.
The effect of heterojunction formation between g-C3N4
nanosheets and F
­ e3O4 QDs on the photocatalytic performance of the resultant nanocomposite for RhB decolorisation was assessed by Liu et al. [132]. Reportedly, the
g-C3N4 nanosheets carrying 2 wt% of ­Fe3O4 QDS represented higher photocatalytic activity than that of single
g-C3N4 wherein about 95% of RhB was removed within
90 min of visible-light irradiation. Under the employed
condition, the electrons were generated by jumping from
the VB of g-C3N4 to its CB after the photoexcitation step.
But, these photogenerated charges are rapidly recombined
in the absence of F
­ e3O4 QDs. After decorating these QDs,
successful charge separation along with improved surface
area were justified for the significant enhancement in the
photocatalytic activity of g-C3N4/Fe3O4 (2 wt%) composite
(Fig. 9). Nevertheless, by decoration higher content of the
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Fig. 8 Cyclic degradation reactions for MO over the F
­ e3O4/g-C3N4-2
microspheres (Copyright [124])
Fig. 9 Degradation efficiency over the pure g-C3N4 and g-C3N4/
Fe3O4 nanocomposites after irradiation for 1.5 h (Copyright [126])
QDs than optimal value, decrease of the photocatalytic
activity through covering the g-C3N4 surface active sites
and shortening the charge separation period was observed.
Yang et al. [133] reported the fabrication of
­F e 3 O 4 /g-C 3 N 4 nanocomposites using a solvothermal
method and verified the homogeneous distribution of dark
­Fe 3O 4 NPs over g-C 3N 4 nanosheets using TEM images
(Fig. 10a). The ­Fe3O4/g-C3N4 nanocomposites exhibited
super photoactivity for degradation of 2,4,6-trichlorophenol (50 ppm) up to 96% within 100 min of visible-light
irradiation, while bearing good magnetic behavior, in
1727
which they were easily separated from the treated solution by an external magnetic field.
Another report is available from Jia et al. [134] about
preparation and utilization of ­Fe3O4/g-C3N4 nanocomposites
for removal of RhB. The strength of the formed heterojunction was verified by analysis of the XRD patterns of the
synthesized samples. From the patterns, the diffraction peaks
for ­Fe3O4 were positioned at lower angles than the corresponding free peaks for pure F
­ e3O4 (Fig. 10b). However,
magnetite phase structure maintained after combining with
g-C3N4. On the other hand, the distinctive diffraction peaks
of graphitic carbon nitride were correspondingly observed
in that of the F
­ e3O4/g-C3N4 samples. Figure 10c shows the
hysteresis loops for the pure F
­ e3O4 and F
­ e3O4/g-C3N4 nanocomposites. The super-paramagnetic behavior of the samples
was established by S-like curves for all of them.
From the literature, the g-C3N4/Fe3O4 composites could
be fabricated via various methods in which the in situ precipitation method was found much easier and accessible
among them [130, 134]. Besides, exfoliation of g-C3N4 and
preparation of nanosheets increased the surface area and
decreased of path by charge carriers to the surface, thus
enhanced the photocatalytic activity [132, 133]. Moreover,
the results demonstrated the uniform deposition of monodispersed magnetite NPs onto the g-C3N4 sheets, allowing the
light diffusion and enhanced light harvesting. On the other
hand, in the presence of the integrated F
­ e3O4, the aggregation of g-C3N4 nanosheets is inhibited and the specific
surface area is kept high for better adsorption, improved photocatalytic action, and simple magnetic separation. In recent
years, much more attentions have been paid for coupling
noble metal NPs with semiconductors to facilitate charge
separations. This is because the noble metals can strongly
absorb visible light owing to their SPR effects, leading to
improve the photocatalytic activity. In an interesting study
conducted by Zhu et al. [98], metallic silver was added to
­Fe3O4/g-C3N4 composite using a selective photodeposition
of Ag onto F
­ e3O4 surface. The as-prepared Ag/Fe3O4/g-C3N4
sample retained the magnetic property and exhibited superior photocatalytic activity for tetracycline degradation. The
enhanced activity was mainly attributed to the trapping of
electrons by ­Fe3O4 and transferring them to metallic silver
particles.
From the articles reported in the current section, it can
be concluded that the problem of easy separation and recycling could be solved by loading magnetic F
­ e3O4 onto the
g-C3N4-based photocatalysts, thus preventing the production
of secondary pollution. On the other hand, as previously
mentioned, the industrial application of g-C3N4 is restricted
by the high rate of the photoproduced e­ −/h+ recombination
and moderate visible-light absorption ability. Furthermore,
the structural modification in g-C3N4 is a complex, multistep
and costly procedure. In light of this, the number of studies
13
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Fig. 10 a TEM image of F
­ e3O4/g-C3N4 nanocomposites (Copyright [127]), b XRD patterns of F
­ e3O4, g-C3N4, and F
­ e3O4/g-C3N4 nanocomposites, c room-temperature magnetic hysteresis loops for ­Fe3O4, and ­Fe3O4/g-C3N4 nanocomposites (Copyright [128])
about developing photocatalysts that directly enhance visible
light response of g-C3N4 is tremendously increasing. The
general research trend in developing visible-light responsive
catalysts is to combine a number of semiconductors with
compatible band energies to prolong life time of the photogenerated ­e−/h+ pairs and enhance the visible light absorption capacity. Regarding the g-C3N4-based photocatalysts,
the triple nanocomposites have recently received a great
attention, because there is chance of getting the advantages
of collective properties of the employed components, for
improving the photocatalytic performance and recovery of
g-C3N4. To this end, beside the attempts to add magnetization to g-C3N4, many efforts have been made for photocatalytic activity enhancement of g-C3N4/Fe3O4. Accordingly,
the semiconductors of narrow band gap were combined
with g-C3N4/Fe3O4 nanocomposites to increase visible-light
absorption capacity [135–139]. On the other hand, semiconductors of wide band gaps that cover appropriate band
potentials were used for reducing rapid recombination of
the photogenerated e­ −/h+ pairs in g-C3N4 [140, 141]. To
achieve the both goals, g-C3N4 was combined with two narrow band gap semiconductors [142–145], or one wide band
gap semiconductor and one narrow band gap semiconductor
[146]. Depending on the nature of components of the ternary
or quaternary fabricated photocatalysts, the nanocomposites
are grouped into four categories (Fig. 11).
Various magnetically separable ternary nanocomposites
have been produced, characterized and assessed for their
photocatalytic performance by the authors of this review. We
investigated the visible-light activity of g-C3N4/Fe3O4/BiOI
nanocomposites for photodegradation of RhB, methylene
blue (MB), and methyl orange (MO) [135]. The preparation of this photocatalyst has the noteworthy advantages of
adequate sample amount, short preparation time, and low
temperature (Fig. 12a). Although the visible-light absorption of the pristine g-C3N4 was found poor with band edge
at 470 nm, UV–Vis DRS spectra of revealed high absorption ability for the g-C3N4/Fe3O4 and g-C3N4/Fe3O4/BiOI
13
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Fig. 11 Classification of g-C3N4-based magnetic into ternary and quaternary components
photocatalysts at visible range. Accordingly, more production of the e­ −/h+ pairs resulted in enhancing photocatalytic
activity of these nanocomposites compared with the pristine
g-C3N4. Furthermore, the visible-light absorption ability was
increased with an increment in the BiOI loading over the
g-C3N4/Fe3O4 photocatalyst. The following equation that is
known as Tauc’s equation is used to calculate the band gap
energy (Eg) of the prepared phtocatalysts:
(5)
where α is absorption coefficient, υ is the light frequency,
and B is the proportionality constant [147]. The features of
the transition in the semiconductor determine the value of
n. The plotting of (αhυ)2 versus hυ (n = 1 for direct transition) is resulted in a curve that can be used to estimate the Eg
value by extrapolation of the linear part of the curves. Correspondingly, the Eg values of g-C3N4 and g-C3N4/Fe3O4/BiOI
(20%) photocatalysts were approximately 2.72 and 1.97 eV,
respectively (Fig. 12b). From the VSM studies, a decrease
𝛼h𝜐 = B(h𝜐 − Eg)n∕2
in saturated magnetization of the ­Fe3O4 NPs from 55.5 to
8.7emu/g was observed for the g-C3N4/Fe3O4/BiOI (20%)
nanocomposite, yet high enough to help for magnetic separation from the treated system (inset of Fig. 12c). On the other
hand, g-C3N4/Fe3O4/BiOI (20%) photocatalyst represented
great photocatalytic activity for degradation of MB, RhB,
and MO under visible light, wherein it was about 10, 22,
and 21-times greater than that of the unmodified g-C3N4,
respectively (Fig. 13a–c).
In another study by our research group, g-C3N4/Fe3O4/
Ag2CrO4 nanocomposites were fabricated for the first time
by a facile refluxing method, without any need for further
additives or post preparation treatments [136] (Fig. 14a).
The deposition of F
­ e3O4 and A
­ g2CrO4 on the g-C3N4 surface was clearly observed in the TEM images of g-C3N4/
Fe 3O 4/Ag 2CrO 4 (20%) nanocomposite (Fig. 14b). The
saturation magnetization of 12.9 emu was obtained per
gram of the g-C3N4/Fe3O4/Ag2CrO4 (20%) nanocomposite that was nearly one-fourth of the ­Fe3O4 NPs, enabling
13
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Fig. 12 a The XRD patterns for g-C3N4, g-C3N4/Fe3O4, and g-C3N4/
Fe3O4/BiOI nanocomposites with different weight percents of BiOI.
b Plots of (αhυ)2 versus hυ for the samples. c Magnetization curves
for the ­Fe3O4 NPs and g-C3N4/Fe3O4/BiOI (20%) nanocomposite and
separation of the nanocomposite with a magnet (Copyright [130])
magnetically separation from the reaction solution in successive runs (Fig. 14c, d). Moreover, the resultant g-C3N4/
Fe3O4/Ag2CrO4 (20%) nanocomposite demonstrated superior photocatalytic activities under visible light for RhB
degradation in which it was about 5 and 6.3-times more
than the activity of g-C 3N 4 and g-C 3N 4/Fe 3O 4 samples,
respectively (Fig. 15a). In this case also, the enhancement in the activity was assigned to high absorption at
visible range and effective separation of the charge carriers through transfer of the photogenerated electrons from
g-C 3N 4 to A
­ g 2CrO 4 and holes in the opposite direction
(Fig. 15b).
The g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites were
recently fabricated by a facile ultrasonic-irradiation method
[146]. For this purpose, NPs of ­Fe3O4, ­Ag3PO4, and AgCl
were homogeneously dispersed on the g-C3N4 surface and
greatly improved the separation of the photogenerated
­e−/h+ pairs resulted from the excellent interfacial contact
among g-C3N4, ­Ag3PO4, and AgCl in the nanocomposite.
The maximum saturation magnetization of 8.78 emu was
13
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Fig. 13 The degradation rate constants of a RhB, b MB, and c MO over the g-C3N4, g-C3N4/Fe3O4, and g-C3N4/Fe3O4/BiOI (20%) samples
under visible-light irradiation (Copyright [130])
obtained per gram of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%)
nanocomposite, describing enough magnetic property of the
sample and its potential magnetic separation. The existence
of nonmagnetic counterparts (g-C3N4, ­Ag3PO4, and AgCl)
was the reason for decrease of the magnetic saturation
compared to that of ­Fe3O4 NPs (55.5 emu g­ −1) (Fig. 16a).
Among the as-prepared samples, the g-C3N4/Fe3O4/Ag3PO4/
AgCl (30%) nanocomposite displayed the highest photocatalytic activity for degradation of RhB within 150 min with
almost 22, 6, and 7.5-folds greater than those of the g-C3N4,
g-C 3N 4/Fe 3O 4/Ag 3PO 4 (20%), and g-C3N 4/Fe 3O 4/AgCl
(30%) samples, respectively (Fig. 16b). The enhanced photocatalytic activity of the quaternary g-C3N4/Fe3O4/Ag3PO4/
AgCl (30%) nanocomposite was largely attributed to extra
visible-light harvesting ability and improved separation
of the charge carriers via development of heterojunctions
among the corresponding components of the nanocomposite
(Fig. 17).
Table 1 offers a summary of a number of fabricated
multicomponent g-C3N4/Fe3O4-based nanocomposite and
their photocatalytic performance for wastewater treatment.
The secret for the overall enhanced photocatalytic activity
was the collaborative effect of the individual components
on the enhanced response to the visible-light illumination
and promoted separation of the photoproduced e­ −/h+ pairs.
Obviously, the synthesized nanocomposites showed different photocatalytic performances in removal of organic
pollutants. Generally, the enhancement in the heterogeneous photocatalytic performance under visible light involves
four key steps: (i) visible-light harvesting ability, (ii) charge
excitation, (iii) charge separation and migration, and (iv)
photocatalytic reactions over VB and CB [147]. Firstly, the
light harvesting ability of a photocatalyst is largely depended
on the morphology and surface area of the photocatalyst.
Hence, light-harvesting is generally enhanced in high surface area materials due to the presence of several reactive
sites. Furthermore, transferring the photo-induced charge
carriers is much easier in larger surface areas, because diffusion pathway of the photogenerated charge carriers is significantly shortened in porous and small-sized materials [148,
149], as expressed in Eq. (6) that gives the average diffusion
time (τ) required for the charge carriers to move from bulk
to surface of a photocatalyst:
(6)
In which, r is the grain radius and D is the diffusion
coefficient of the charge carriers. According to this equation, for a photocatalyst with large size, the diffusion time
of the charges is increased. As a result, opportunity for
recombination of the ­e−/h+ pairs will be greatly increased,
leading to decreased photocatalytic activity. Secondly, the
charge excitation of a photocatalyst is strongly associated
with its electronic structure. Correspondingly, the photoinduced ­e−/h+ pairs are generated when the photocatalyst
is illuminated by photons of equal or greater energy than
its band gap [19]. Hence, the first step for designing a
particular photocatalyst is to have adequate information
about band gap of the photocatalyst counterparts. The
𝜏 = r2 𝜋 2 ∕D
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Fig. 14 a The schematic diagram for preparation of the g-C3N4/
Fe3O4/Ag2CrO4 nanocomposites. b TEM image of the g-C3N4/Fe3O4/
Ag2CrO4 (20%) nanocomposite. c Magnetization curves for the ­Fe3O4
NPs and g-C3N4/Fe3O4/Ag2CrO4 (20%) nanocomposite. d Reusability
of the nanocomposite during five runs (Copyright [131])
third key parameter affecting the photocatalytic performance is determination of the oxidation and reduction
reactions over VB and CB of the photocatalyst, respectively. Although, information about the photoresponse of
a photocatalyst is obtained by its band gap characteristics,
the information about the redox ability of the charges in
VB and CB of a photocatalyst is strictly dependent on
band structure of the semiconductor/semiconductors of the
target photocatalyst. Finally, the band potentials control
separation of the charge carriers and development of the
oxidizing species for pollutants degradation. From thermodynamics view point, electrons are transferring from
CB of a semiconductor with more negative potential to the
CB of semiconductors with less negative potential, while
the produced holes migrate in an opposite direction, from
the VB of a semiconductor with more positive potential to
less positive one. Therefore, the photoproduced ­e−/h+ are
gathered in separated positions, resulting in the suppression from recombination [150].
In light of these, the χ and potentials of CB and VB of
the semiconductors employed for fabrication of a number
of g-C3N4/Fe3O4-based nanocomposites were calculated
by Eqs. (2)–(4) and the results are shown in Table 2. From
the table, it is observed that g-C3N4 has the most negative
CB level (− 1.13 eV vs. NHE) and a medium band gap
(2.7 eV) among the tabulated semiconductors that facilitate its application as VLD photocatalyst. The key step in
enhancing photocatalytic activity is to minimize recombination of the photogenerated e­ −/h+ pairs by capturing
these carriers by maximizing the transfer of them to different locations. In addition, the number of the charge carriers that are transferred to the surface of the photocatalyst
considerably affects the photocatalytic performance. The
photogenerated charges subsequently produce reactive species such as hydroxyl radical (⋅OH) and superoxide anion
radical (⋅O2−). These species along with produced holes
are major oxidative species generated during photocatalytic reactions. The adsorbed hydroxide or water species
13
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Fig. 15 a The degradation rate constants of RhB over the g-C3N4, g-C3N4/Fe3O4, and g-C3N4/Fe3O4/Ag2CrO4 nanocomposites. b The proposed
degradation mechanism of RhB over the g-C3N4/Fe3O4/Ag2CrO4 nanocomposites (Copyright [131])
Fig. 16 a VSM curves for the ­Fe3O4 and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples. b Photodegradation of RhB over the g-C3N4, g-C3N4/
Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites (Copyright [140])
could be oxidized by the ­h+ in the VB of the photocatalyst
to yield ⋅OH radicals [151]. However, the VB potential of
g-C3N4 is + 1.57 eV, that is not positive enough to oxidize
water molecules/hydroxide ions to produce ⋅OH radicals
(E°H2O/OH° = + 2.72 eV, E°–OH/OH° = + 2.38 eV) [147–149,
152–154]. In this regard, h­ + in the VB of g-C3N 4 react
with adsorbed pollutant molecules to oxidize them to
carbon dioxide, water, and other inorganic anions. Meanwhile, a number of the photogenerated electrons on the
CB of g-C 3N 4 react with adsorbed molecules of oxygen to produce ⋅ O 2− species, because the CB potential
energy of g-C3N4 (− 1.13 eV) is more negative than the
­O 2/ ⋅ O 2− reduction potential (­E 0(O 2/ ⋅ O 2−) = − 0.33 eV/
NHE) [22, 155, 156].
13
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Fig. 17 A plausible diagram for separation of electron–hole pairs in the g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites (Copyright [140])
Table 1 Photocatalytic perfomances for a number of g-C3N4/Fe3O4-based heterostructures
Material
Application
g-C3N4/Fe3O4/Ag3VO4
Degradation of RhB, MB, and MO RhB: 1 × 10−5 M
MB: 1.3 × 10−5 M
MO: 1.05 × 10−5 M
Degradation of RhB, MB, and MO RhB: 2.5 × 10−5 M
MB: 1.94 × 10−5 M
MO: 1.26 × 10−5 M
Degradation of RhB
RhB: 2.5 × 10−5 M
Degradation of RhB
RhB: 2.5 × 10−5 M
Degradation of RhB, Fuchsine
RhB: 2.5 × 10−5 M
Fuchsine: 0.92 × 10−5
Degradation of RhB, MB, and MO RhB: 1 × 10−5 M
MB: 1.3 × 10−5 M
MO: 1.05 × 10−5 M
Degradation of RhB, Fuchsine
RhB: 2.5 × 10−5 M
Fuchsine: 0.92 × 10−5
Degradation of RhB, MB, and
RhB: 1 × 10−5 M
Fuchsine
MB: 1.3 × 10−5 M
Fuchsine: 0.92 × 10−5
Degradation of RhB, MB, MO,
RhB: 1 × 10−5 M
and phenol
MB: 1.3 × 10−5 M
MO: 1.05 × 10−5 M
Phenol: 5 × 10−5 M
g-C3N4/Fe3O4/AgI
g-C3N4/Fe3O4/AgBr
g-C3N4/Fe3O4/AgCl
g-C3N4/Fe3O4/Ag2SO3
g-C3N4/Fe3O4/Ag3VO4/Ag2CrO4
g-C3N4/Fe3O4/AgI/Ag2CrO4
g-C3N4/Fe3O4/AgI/Bi2S3
g-C3N4/Fe3O4/Ag3PO4/Co3O4
Pollutant concentration Degradation efficiency Ms
(emu/g)
Ms.M
Ref. (year)
98% in 240 min
5.1
[132] (2014)
100% in 300 min
16.9
[133] (2016)
100% in 420 min
100% in 420 min
100% in 210 min
19.5
9.5
8.5
[134] (2016)
[98] (2016)
[135] (2016)
100% in 180 min
6.12
[136] (2016)
100% in 150 min
8.5
[137] (2015)
100% in 60 min
7.13
[138] (2016)
100% in 150 min
13.5
13
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[139] (2015)
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Table 2 The χ, Eg, VB, and CB potentials for different semiconductor applied in fabrication of some g-C3N4/Fe3O4-based nanocomposites
Semiconductor
χ
Eg (eV)
ECB (eV)
EVB (eV)
Ag3VO4
Ag2CrO4
Ag3PO4
AgI
AgBr
BiOI
Bi2S3
AgCl
Ag2SO3
5.64
5.86
5.95
5.47
5.80
5.96
5.28
6.06
6.10
2.2
1.80
2.45
2.80
2.52
1.74
1.40
3.25
3.38
+ 0.04
+ 0.46
+ 0.24
− 0.42
+ 0.04
+ 0.59
+ 0.10
− 0.06
− 1.33
+ 2.24
+ 2.26
+ 2.69
+ 2.38
+ 2.56
+ 2.33
+ 1.50
+ 3.19
+ 2.05
4.4 MFe2O4/g‑C3N4‑based magnetic photocatalysts
The spinel-structured ferrites are of known magnetic materials with a common formula of ­MFe2O4, where M is a transition metal cation with + 2 oxidation number. Recently, this
group has been intensively studied in various areas, owing to
their attractive properties including good chemical and thermal stability, valuable electrical and ferromagnetic properties, and good response to visible light, because of their narrow band gaps. In this structure, the ­O2− ions are organized
in a close-packed cubic pattern and the F
­ e3+ and M
­ 2+ cations
are distributed in the octahedral and/or tetrahedral sites. In
spinel ferrites, the ­Fe3+/Fe2+ and ­M3+/M2+ redox pairs are
readily available at solid state, in which the charge is rapidly
transferred between the cations with unlike valances with
low activation energy, which activates a remarkable electrocatalytic performance [44]. Among the ferrites, cobalt ferrite
­(CoFe2O4) is a recognized inverse spinel in which the C
­ o2+
3+
and ­Fe ions are located in the tetrahedral and octahedral
sites. Cobalt ferrite has received a great deal of interest for
its application in photocatalytic degradation of pollutants,
adding magnetism, and electrochemical devices. Moreover,
­CoFe2O4 not only offers the advantage of simple separation
by magnet and recycling for several reactions, but also is
low-priced and chemically stable and is used as a powerful
photocatalyst [157, 158]. On this basis, C
­ oFe2O4 has been
combined with g-C3N4 to enhance its photocatalytic performance and easy magnetic separation. In a study reported
by Yunjin Yao et al. [159], ­CoFe2O4/g-C3N4 hybrid was
fabricated by a facile self-assembly method and the presence of the uniformly deposited cubic C
­ oFe2O4 NPs over
g-C3N4 sheets was confirmed via TEM image (Fig. 18a).
This combination significantly improved the photocatalytic performance of g-C3N4 under visible-light irradiation
through Z-scheme way. In another similar work by Huang
et al. [160], the ­CoFe2O4/g-C3N4 was fabricated by a simple calcination method and exhibited a great performance
for MB photodegradation in the presence of H
­ 2O2. In both
aforementioned studies, although the saturation magnetization of the ­CoFe2O4/g-C3N4 composites was slightly lower
than that of the pure C
­ oFe2O4, it was good enough to be
separated magnetically from the treated solution (Fig. 18b).
Regarding the photocatalytic efficiency, ­e−/h+ pairs are generated in both ­CoFe2O4 and g-C3N4 semiconductors under
visible-light irradiation. The ­ECB and ­EVB are respectively
− 1.13 and 1.57 eV versus NHE for g-C3N4, and about + 0.42
and 1.75 eV versus NHE for ­C oFe 2O 4. By decorating
­CoFe2O4 over g-C3N4, the photogenerated electrons in the
CB of g-C3N4 flow towards the CB of ­CoFe2O4, and the
photogenerated holes in the VB of C
­ oFe2O4 tend to move to
the VB of g-C3N4, thus facilitating the charge separation by
forming heterojunction in g-C3N4/CoFe2O4. Consequently,
the photocatalytic reaction rate is enhanced on the surface
of the composite and gives rise to higher photoactivity when
compared with the pure g-C3N4 and C
­ oFe2O4 photocatalysts
(Fig. 18c) [159–161].
ZnFe2O4 is another ferrite with a very narrow band gap
of 1.9 eV that has been extensively investigated in recent
years in the fields of photocatalytic degradation of pollutants, transformation of solar energy, and photochemical
production of hydrogen from water. ­ZnFe2O4 is a visiblelight responsive photocatalyst that is cheap, photochemically stable and can be easily synthesized and separated
by magnet, thus serving as an attractive candidate for
photocatalytic water treatment. However, Z
­ nFe2O4 suffers
from poor photocatalytic performance resulted from quick
recombination of the photoexcited ­e−/h+ pairs [162–164].
Accordingly, efforts have been directed to upgrade the photocatalytic activity of ­ZnFe2O4. Yao et al. [165] synthesized
magnetic ­ZnFe2O4/g-C3N4 composite by simple refluxing method. The as-synthesized composite represented an
enhanced catalytic activity in photo-Fenton degradation
of orange II under visible-light illumination. Attributed to
magnetic property, this composite delivered easy separation
and reusability merits (Fig. 19a). In another study, Zhang
et al. fabricated g-C3N4/ZnFe2O4 composites by one-step
solvothermal method. The TEM images of the nanocomposite showed the well-dispersed magnetic ­ZnFe2O4 NPs
on g-C3N4 sheets. The improved photocatalytic activity of
this nanocomposite was attributed to the enhancement in
the number of reaction sites and formation of heterojunction structure. Furthermore, the saturation magnetization
of g-C3N4 (80%)/ZnFe2O4 nanocomposite was about 63.8
emu/g, showing well-adjusted ­ZnFe2O4 NPs over g-C3N4
sheets (Fig. 19b). Briefly, small size and high solubility of
the integrated g-C3N4 and ­ZnFe2O4 semiconductors, donated
an effective separation of the ­e−/h+ pairs and consequently,
an excellent photocatalytic performance [166].
Copper ferrite ­(CuFe2O4) is another ecofriendly magnetic
p-type semiconductor that has a very small band gap of
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Fig. 18 a TEM image of the ­CoFe2O4/g-C3N4 hybrid (Copyright [22]), b magnetic separation of the ­CoFe2O4/g-C3N4 toward a permanent magnet (Copyright [155]), c the proposed mechanism of the reaction process
1.4 eV. Attributed to its interesting properties, ­CuFe2O4 has
been extensively used in various fields including electronics,
sensors, degradation of pollutants, and photocatalytic production of hydrogen [167, 168]. Likewise, the effects of the
­CuFe2O4 and g-C3N4 integration were also assessed on the
photocatalytic performance. Yao et al. [169] synthesized a
core–shell ­CuFe2O4/g-C3N4 photocatalyst by a self-assembly
method. The so-prepared composite powerfully decolorized
orange II solution through heterogeneous visible light-Fenton process and magnetically separated and reused successfully (the inset of Fig. 20a) [169]. The significant enhancement in the photocatalytic activity of C
­ uFe2O4/g-C3N4
can be explained based on the well-matched energy levels between the constituent semiconductors (g-C3N4 and
­CuFe2O4) that permitted a rapid and efficient separation
of the photoproduced charge carriers. Meanwhile, a driving force was provided by the internal electrostatic field for
transferring the photoexcited electrons from Cu-ferrite to
g-C3N4 via the heterojunction. Hence, ­CuFe2O4 performs as
a hole trap and g-C3N4 traps the generated electrons in the
degradation reaction, extending the separation duration of
the photogenerated ­e−/h+ pairs and enhancing the photocatalytic decomposition efficacy (Fig. 20b) [169, 170].
Table 2 gives information about some other g-C3N4/ferrite magnetic nanocomposites that have been designed and
implemented for treatment of aqueous solutions carrying
various pollutants. These nanocomposites exhibited diverse
interesting properties, such as high surface area, good magnetism, and easy recovery and excellent catalytic performance in various catalysis processes like Fenton reaction,
13
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Fig. 19 a UV–Vis spectral changes for Orange II degradation with
­ZnFe2O4–C3N4/H2O2. The insets show the solution after magnetic
separation using an external magnet. The photographs of the color
change of Orange II during different reaction times are also shown.
1737
(Copyright [158]), b magnetization curves of the photocatalysts. Inset
pictures show the composite with a stable, brown aqueous dispersion
and easy separation by magnet (Copyright [159])
Fig. 20 a UV–Vis spectral changes for Orange II degradation with ­CuFe2O4@C3N4(2:1)/H2O2. b Mechanism for Orange II degradation by
­CuFe2O4@C3N4/H2O2 (Copyright [162])
organic dehydrogenation, and C
­ O2 reduction [171, 172]
(Table 3).
5 Other applications
Recently, solving the problems arisen from the agricultural, pharmaceutical, and industrial effluents have
attracted a great concerns and efforts. Accordingly, the
use of semiconductors in photocatalytic applications
in the fields related to energy and the environment has
drawn global interest [173]. As documented previously,
a variety of g-C3N4-based magnetic photocatalysts have
been implemented in various reactions, such as pollutant decomposition [80], ­H2 generation [67], supercapacitor [174], gas sensing [175], lithium-ion batteries [176],
anion exchanger [177], and solid phase extraction [178].
Herein, a summary about these magnetic nanocomposites
and their versatile applications are made and illustrated
in Fig. 21.
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Table 3 Photocatalytic properties for a number of g-C3N4/ferrite magnetic photocatalysts
Photocatalyst
Preparation method
Application
Co0.5Zn0.5Fe2O4/
g-C3N4
Hydrothermal
method
Degradation of
chloromycetin
g-C3N4/ZnFe2O4
g-C3N4/CuFe2O4
g-C3N4/MnFe2O4/
TiO2
g-C3N4/NiFe2O4
Light source
Pollutant concentration
Xenon lamp 300 W Chloromycetin:
100 mL 10 mg
­L−1
Hydrothermal
Degradation of
Xenon lamp 300 W Spiramycin:
method
spiramycin
100 mL 20 mg
­L−1
Hydrothermal
Degradation of RhB Tungsten lamp
RhB: 100 mL
method
300 W
10 mg ­L−1
Chemical impregna- Degradation of MO Xenon arc lamp
MO: 10 ppm
tion method
150 W
Chemisorption
Degradation of MB Xenon lamp 300 W MB: 50 mL 10 mg
method
­L−1
Fig. 21 Schematic illustration of the g-C3N4-based magnetic nanocomposites and their applications in different disciplines
5.1 Photocatalytic hydrogen generation
One of the chief challenges of the present century is to
address the rapid growing in global demand for energy by
sustainable energy strategies and eco-friendly solutions.
Correspondingly, clean energy technologies should substitute for fossil fuels to reduce the dependency on this
non-renewable and environmentally incompatible energy
carriers. The stunning advantages of hydrogen gas such
as being able to be generated from water without harmful
emissions, high efficiency for energy conversion, easy to
be stored and transferred, plenty of sources, and feasibility to be transformed into a range of fuels through large
number of reactions and/or procedures have made it a great
Degradation efficiency
Ref. (year)
96% in 240 min
[156] (2014)
95% in 240 min
[164] (2014)
92% in 150 min
[163] (2017)
99.27% in 180 min
[165] (2013)
87% in 240 min
[166] (2017)
energy source and a sound substitute to fossil fuels [179,
180].
Among the various sources, water is the most ideal origin for hydrogen production. Application of semiconductor
photocatalysts for photocatalytic water splitting into oxygen and hydrogen has been found as an attractive approach
for hydrogen energy production [79]. For this purpose, VB
of the semiconductor is needed to be further positive than
1.23 eV, as the redox potential of ­O2/H2O, whereas the CB
level of the semiconductor should be more negative than
the ­H+/H2 redox potential which is 0 V versus the NHE
[67]. Considering g-C3N4, both reactions (photocatalytic
oxidation and reduction of water) can be theoretically fulfilled by this promising photocatalyst owing to its qualified
VB and CB sites [63]. However, the hydrogen production
efficiency of g-C3N4 is low due to the rapid recombination
rate of the generated ­e−/h+ pairs. Correspondingly, modification of g-C3N4 by other semiconductors has been found as a
great tool to enhance its visible-light photocatalytic activity
for hydrogen production (Table 4) [69]. Over the past few
years, a large number of g-C3N4-based nanocomposites with
enhanced activity for the photocatalytic splitting of water
have been developed. When the designed photocatalyst is
illuminated with the visible light, the photogenerated ­e−/h+
pairs are transferred to the reaction sites on the surface of
g-C3N4/semiconductor composite. Consequently, the produced charge carriers can oxidize and reduce the adsorbed
water molecules to yield gaseous ­O2 and ­H2 [51]. Chen
et al. [19] used ferrites to modify g-C3N4 for accelerated
surface catalytic oxidative reaction kinetics. The hydrogen
production activity of g-C3N4 was greatly enhanced after
modification with the earth-abundant ferrites. The conjoint
loading of (Co, Ni)Fe2O4 and Pt enhanced the photocatalytic
activities of Pt/g-C3N4/CoFe2O4 and Pt/g-C3N4/NiFe2O4 significantly by 3.5 and 3.0-folds as compared to Pt/g-C3N4,
respectively, with apparent quantum yields achieved 3.35%
for Pt/g-C3N4/CoFe2O4 and 2.46% for Pt/g-C3N4/NiFe2O4
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at 420 nm, respectively. This enhancement was attributed to
the (Co, Ni)Fe2O4 modification promoting the charge carriers separation and surface catalytic oxidative reaction [181].
As explained by another researchers, (Liu et al. [182] and
Cheng et al. [183]), this great enhancement in photocatalytic hydrogen generation was attributed to the synergistic
effect at the interface of g-C3N4 and other semiconductor,
including elongated separation of photogenerated charges,
satisfactory contact, and conformed levels of energy band
of the two semiconductors (Figs. 22, 23).
1739
5.2 Solid‑phase extraction
The analytical approaches used for determination of pollutants in real environmental media such as surface waterbodies need to be accurate and selective because of their
complex composition. Nevertheless, it is very difficult to
accurately detect the pollutants at ultra-trace concentrations
or present in extremely complex matrices [184]. Therefore,
sample pretreatment could effectively help to pass this barrier. In an analytical procedure, the sample preparation step
Fig. 22 Schematic diagram of the proposed photocatalytic mechanism for hydrogen generation over a Ti-Fe2O3/g-C3N4, b ­CuFe2O4/g-C3N4, and
c ­CoFe2O4/g-C3N4 composites
Fig. 23 Schematic of magnetic
solid phase extraction procedure
(Copyright [184])
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is totally of high importance especially at very low concentrations. For this purpose, it is entailed to pre-concentrate
the analyte from the parent medium onto an appropriate
sorbent and wash out using very low volume of a solvent
prior to instrumental analysis. Among the pre-concentration
techniques, solid phase extraction (SPE) was found as one
of the most frequently employed techniques for lots of compounds from real environments, owing to its merits over
the traditional extraction tools [185]. The SPE technique
exhibits a high enrichment factor (EF) and restoration using
organic solvents of very low volumes, wherein the total process could get automatic (off- or on-line) [186]. Supposing
g-C3N4 as a SPE sorbent, the both sides are accessible for the
target molecule adsorption. Moreover, the broad π-electron
system of g-C3N4 donates a strong proximity with large
number carbon-based ring configurations present in pollutants. Though, the pure g-C3N4 planes are likely to reaggregate in the separation process, which can reduce the sorbent
adsorption ability and impede the practicable adsorption and
washing out the analytes. Moreover, the complete recovery of the g-C3N4 sheets from the homogeneous solution
is very problematic [187]. In light of this, fabrication of
magnetic solid-phase extraction (MSPE) sorbents based on
g-C3N4 such as ­Fe3O4/g-C3N4, was reported as an effective
approach for easier magnetic separation of sorbents from
well-dispersed solution [184]. The application of magnetic
sorbents escapes the lengthy procedures such as filtration
and centrifugation, thus speeding up the recovery step.
From a large number of studies in the literature, g-C3N4/
Fe3O4 has been introduced as a potential sorbent for pretreatment of polluted water samples [188–191]. The synergistic
strong affinity of g-C3N4 for pollutants along with the magnetic property of F
­ e3O4 has provided a simple and effective MSPE. Correspondingly, a series of the main parameters including the sorbent quantity, pH of the solution,
ionic strength, total extraction cycles, and the nature and
the amount of eluting solvent has had crucial effects on the
effectiveness of the process. Under the optimized condition,
g-C3N4/Fe3O4 has represented a pronounced potential for
adsorption and extraction of trace compounds with carbonbased rings in their structures from actual media due to small
detection limits, good recovery and linearity.
5.3 Supercapacitors
Nowadays, discovering novel, cost-effective, and green systems for energy storage is of great importance for addressing
the environmental concerns and fulfill the modern world
demands. Supercapacitors (SC) have drawn a great interest as energy storage systems owing to their extensive lifetime, great power density, high charge–discharge rate, and
cycle efficacy. There are two energy storage fashions in
electrochemical supercapacitors (ESC). In electrochemical
double-layer capacitors, energy is stored through ion adsorption wherein pseudo capacitors, store via fast redox reactions
on the surface [174]. Correspondingly, g-C3N4 has received
a great attention as electrode material in electrochemical
double-layer capacitors owing to its large surface area and
good pore distribution that gave rise to its commercially
accessible ESC. However, the efficiency of pure g-C3N4 in
supercapacitors is far from satisfaction due to its low conductivity and chemical inaction. Hence, the efforts are paid
to develop nanostructured g-C3N4 or combine g-C3N4 with
other compounds as nanocomposites in order to enhance its
properties as electrode material in ESC [192]. In pseudocapacitors, metal oxides boost the redox reactions on the
surface and own great specific capacitances as electrode
material. Liu et al. [193] and Xu et al. [194] found that
the g-C3N4, combined with metal oxides, could expand its
application in electrochemical sensors and energy storage
systems. The capacitance evaluation of g-C3N4/α-Fe2O3
nanocomposite at various current densities showed that it
has been capable of giving a greater specific capacitance
and larger coulombic effectiveness compared with the pure
g-C3N4, ascribed mainly to its large surface area, small
electronic resistance, and synergistic action of g-C3N4 and
α-Fe2O3.
5.4 Gas sensors
Gas sensors have been used for environmental monitoring,
public safety, domestic security, and as air conditioning
systems [195]. Semiconductor gas sensors based on metal
oxides are the main candidates for the sensor array owing
to their modifiable structures and sizes, ease of operation,
simple unification with electronic circuits and being inexpensive. Due to naturally available, low-priced, non-toxic,
and chemically stable semiconductor with a narrow band
gap, hematite (α-Fe2O3) has received the most attention
among metal oxides for gas-sensing applications [196].
However, α-Fe2O3 effectiveness is restricted by its insignificant selectivity and responses. To overcome these limitations and enhance the gas-sensing efficacy, combining
α-Fe2O3 with other semiconductors such as g-C3N4 have
come up as a useful approach [197, 198]. In 2015, Zeng
et al. [197] prepared α-Fe2O3/g-C3N4 composites by refluxing a mixture of g-C3N4 suspension and F
­ eCl3 solution in
boiling water. In this procedure, α-Fe2O3 NPs are tightly
attached to the surface of g-C3N4 sheets. This composite
was explored as a cataluminescence (CTL) catalyst for ­H2S
sensing, because a strong CTL emission is observed during
the oxidation of H
­ 2S over the α-Fe2O3/g-C3N4 composites.
The catalytic activities of the α-Fe2O3/g-C3N4 composites
with different contents were explored by the relative CTL
intensity responding to 4.38 μg ­mL−1 of ­H2S under an air
flow of 300 mL ­min−1 at 183 °C with a filter of 400 nm.
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When loading of α-Fe2O3 reached 5.97%, it displayed the
best sensitive CTL response [197]. Since, the CB of g-C3N4
is relatively at lower levels than that of α-Fe2O3, efficient
transportation of electron across the interface is allowed.
Moreover, g-C3N4 lamellar configuration also helps for
electron carrying. Furthermore, g-C3N4 offers large number of reaction sites than other similar compounds, owing
to its high nitrogen content. Accordingly, the α-Fe2O3/gC3N4 nanocomposites deliver wide active surface areas and
porous structure, thus transferring more gas molecules to the
interaction zone and increase the rate of charge transfer. At
the time, the good permeability and large lamellar structure
activate the rapid diffusion of gas to the internal and surface
zones, creating a high response and a short response time.
Hence, coupling of α-Fe2O3 with g-C3N4 has the potential
to form an efficient gas-sensing system.
5.5 Miscellaneous applications
It is well known that lithium ion battery has turned into
an acceptable energy storage system in portable electronic
devices, such as digital cameras, cell phones, and laptops
for being light and having great energy density and lifetime
[199–202]. Shi et al. [203] developed α-Fe2O3/g-C3N4/graphene composite to employ it as anodic material in lithium-ion batteries. The as-prepared g-C3N4-based magnetic
composite exhibited superior electrochemical performances
regarding rate capability, specific capacity, and durable
cycling, which ascribed to the synergistic action of porous
g-C3N4 along with very conductive graphene. Consequently,
α-Fe2O3/g-C3N4/graphene composites offered an efficient
and prompt ­L i + diffusion and charge transfer resulted
from the accessible channels and satisfactory conductive
1741
pathways. Table 4 shows the properties of different g-C3N4based magnetic composites for multifunctional applications.
6 Summary remarks and perspective
As mentioned, despite many appealing properties, a number of inherent limitations has restricted the application of
pure g-C3N4 in photocatalysis processes. The effective and
simple separation is considered as one of the serious drawbacks of the pristine g-C3N4. This review mainly focused
on the recent advances in fabrication, characterization, and
applications of magnetic g-C3N4-based nanocomposites, as
effective photocatalysts, to address separation challenges of
the applied catalysts from the reaction systems. Herein, the
combinations of g-C3N4 with magnetic NPs such as maghemite (γ-Fe2O3), haematite (α-Fe2O3), magnetite ­(Fe3O4),
and ferrites (­ MFe2O4) were thoroughly discussed as an effective tool to overcome this shortcoming. These integrations
not only solved the separation problem, but also enhanced
the photocatalytic activity by improving the visible-light
absorption response, extending the charge separation duration, and facilitating the photogenerated e­ −/h+ transportations. Moreover, the fabricated magnetic g-C3N4-based
nanocomposites exhibited good stability to use in successive
runs. Nonetheless, more research is required to establish a
suitable production method to obtain larger scale magnetic
nanocomposites. In addition, care should be taken to control
the distribution and size of the magnetic NPs to achieve a
good magnetic property and prevent the negative effect on
recombination of the charge carriers. It seems that increasing
surface area through exfoliation of g-C3N4 and preparation
of porous g-C3N4 to effectively integrate with magnetic NPs
and other semiconductors are promising routes to prepare
Table 4 Data on magnetic g-C3N4-based composites for various applications
Photocatalyst
Preparation method
Application
Ref. (year)
g-C3N4/Ti-Fe2O3
CuFe2O4/g-C3N4
g-C3N4/Fe3O4
g-C3N4/Fe3O4
g-C3N4/Fe3O4
g-C3N4/Fe3O4
g-C3N4/α-Fe2O3
g-C3N4/α-Fe2O3
g-C3N4/α-Fe2O3
α-Fe2O3/g-C3N4/graphene
g-C3N4/α-Fe2O3
Dip-coating method
Calcinating method
In situ precipitation method
In situ chemical co-precipitation method
In situ growth method
Co-precipitation method
One-step pyrolysis method
Solvothermal method
Calcinating method
Hydrothermal method
Electrodeposition method
[176] (2017)
[177] (2015)
[182] (2016)
[183] (2016)
[184] (2015)
[185] (2017)
[188] (2016)
[189] (2015)
[192] (2017)
[194] (2014)
[195] (2005)
g-C3N4/Fe3O4
g-C3N4/Fe3O4
Surface molecular imprinted method
In-situ chemical vapor deposition method
Hydrogen generation
Hydrogen generation
Solid phase extraction
Solid phase extraction
Solid phase extraction
Solid phase extraction
Supercapacitor
Supercapacitor
Gas sensor
Superior anode for lithium-ion batteries
Photo-electrochemical study on charge transfer properties
Adsorption of atrazine
Anion exchange
[196] (2006)
[197] (2015)
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more efficient magnetic VLD photocatalysts. Furthermore,
we provided an overview on various applications of magnetic g-C3N4-based nanocomposites. These nanocomposites
have been widely employed for photocatalytic degradation of
pollutants, photocatalytic generation of hydrogen, supercapacitor, gas sensing, lithium-ion batteries, anion exchanger,
and solid phase extraction. Future research in the studied
fields should encompass the introduction of novel and highly
active magnetic g-C3N4-based nanocomposites for their
applications in various areas such as photocatalytic synthesis of value-added organic compounds.
Acknowledgements The authors wish to acknowledge University of
Mohaghegh Ardabili-Iran, for financial support of this work. Shima
Rahim Pouran as a postdoc researcher gratefully acknowledges the
support of National Elites Foundation.
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19.
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