Review on magnetically separable graphitic carbon ...

Review on magnetically separable graphitic carbon nitride-based nanocomposites as promising visible-light-driven photocatalysts Mitra Mousavi, Aziz Habibi-Yangjeh & Shima Rahim Pouran

Journal of Materials Science: Materials in Electronics ISSN 0957-4522 Volume 29 Number 3 J Mater Sci: Mater Electron (2018) 29:1719-1747 DOI 10.1007/s10854-017-8166-x

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Author's personal copy 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 [email protected] 1

Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran

Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 51666‑16471, Iran

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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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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,

Scheme 1 Classification of advanced oxidation processes

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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,

Author's personal copy J Mater Sci: Mater Electron (2018) 29:1719–1747

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

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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


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

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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

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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

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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



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

13

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


1729

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



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


1731

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


1733

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].

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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

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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

[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

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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,


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

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

13

Pollutant concentration

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


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).

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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

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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.


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

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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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