Magnetic g-C₃N₄/NiFe₂O₄ hybrids with enhanced photocatalytic activity

Abstract

Composite photocatalysts have attracted considerable attention in the exploration of both highly efficient and low cost materials. In this study, novel magnetic g-C₃N₄/NiFe₂O₄ photocatalysts were fabricated by a facile chemisorption method. X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR), UV-vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) were utilized to analyze the structure and properties of samples, which indicated that NiFe₂O₄ had been integrated onto the surface of g-C₃N₄ successfully. The as-prepared 7.5% g-C₃N₄/NiFe₂O₄, with the best photocatalytic activity, can maintain high photocatalytic activity and stability after five runs in the presence of hydrogen peroxide under visible light irradiation. During the catalytic reaction, the synergistic effect between g-C₃N₄ and NiFe₂O₄ can accelerate photogenerated charge separation and facilitate the photo-Fenton process to get an enhanced photocatalytic activity. Moreover, the collection and recycling of photocatalyst was readily achieved owing to the distinctive magnetism of g-C₃N₄/NiFe₂O₄.

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Introduction

In the last decades, the term “photocatalyst” has become prevalent like “nano” in the field of environmental chemistry due to photocatalytic materials' conspicuous functions. The degradation of contaminants, the redox of organic chemicals and the production of hydrogen, even molecule or ion detection, can be achieved by photocatalysts under UV-visible light irradiation.^1–4 Nevertheless, most photocatalysts are faced with one problem: a difficult recycling process and constant secondary pollution in the environment. Recently, the development of magnetic photocatalysts has interested a multitude of researchers, particularly towards MFe₂O₄ (M = divalent metal ion, e.g. Zn, Ni, Co, Cu, etc.) materials due to their ease of recycling. Nanocrystalline ferrites, typical representatives of magnetic materials with a high magnetic permeability and a high electrical resistivity, have several distinctive applications for materials science such as drug carriers, medical diagnostics, information storage, position sensing, and spintronic devices.^5,6 Nickel ferrite is one type of inverse spinel with the chemical formula AB₂O₄ in which an equal number of Ni²⁺ and Fe³⁺ reside in octahedral sites and the remaining Fe³⁺ ions reside in tetrahedral sites. The band gap energy of NiFe₂O₄ is ∼2.19 eV;⁷ however, there is still a standing controversy on it regardless of experimental data or computational results.^8–10 Unfortunately, pure NiFe₂O₄ always shows weak photocatalytic activity, even for the Fenton reaction, so some contributions have been made to improve the photocatalysis. Combinations with other semiconductor materials and doping with lanthanide elements have been verified to solve the problem meaningfully. Rana synthesized the anatase TiO₂-coated NiFe₂O₄ through a reverse micelle and hydrolysis method to degrade methyl-orange dye and to inactivate bacteria.¹¹ When Ni ferrite was substituted by neodymium, the absorption edge red shifted and the band gap narrowed, which induced significantly enhanced photoactivity.¹² In recent years, Wang and Fu synthesized NiFe₂O₄/MWCNT to degrade phenol with C/C₀ reaching 90% in 400 min under UV light.¹³ Later, they prepared NiFe₂O₄–graphene hybrids with outstanding photodegradation behavior, which may benefit from the good electroconductivity of graphene, thus prolonging the life of the photoinduced carriers.¹⁴ Therefore, fabricating a composite photocatalyst to enhance the photocatalytic activity of NiFe₂O₄ can be a feasible and efficient approach.

As a class of novel organic semiconductors, graphitic carbon nitride (g-C₃N₄) has prominent optical and photoelectrochemical properties with a band gap of ∼2.7 eV.¹⁵ It has been reported that g-C₃N₄ can split water for hydrogen production and degrade organic pollutants under visible light irradiation.^16,17 In addition, a great deal of research has confirmed that coupling g-C₃N₄ with a noble metal, composite oxide, metal oxide and metal-free material could achieve exceptionally high photocatalytic capabilities. Normally the enhanced photocatalytic activity of a semiconductor composite is attributed to the synergistic effects of the heterojunction structure, such as the combination of g-C₃N₄ and Bi₂WO₆,¹⁸ WO₃,^19,20 ZnO,²¹ BiPO₄,²² BiVO₄,²³ CdS,²⁴ and BiOX (X = Cl, Br, and I).^25,26 The energy level matching between g-C₃N₄ and another semiconductor can improve the separation and immigration rate of photoinduced electron–hole pairs. Lately, g-C₃N₄/ZnFe₂O₄, exhibiting superior photocatalytic activities, has been reported for hydrogen generation and aqueous organic pollutant degradation.^27–29 However, g-C₃N₄/NiFe₂O₄ has not been studied to date, so we concentrated on this work, hoping to construct one such typical, easily recyclable material and meanwhile improve the photocatalytic performance of NiFe₂O₄.

Herein, we fabricated a g-C₃N₄/NiFe₂O₄ hybrid material to build a semiconductor–semiconductor heterojunction via a convenient chemisorption method that has many advantages such as the easy availability of instruments, simplicity of operation, and safety of the procedure.³⁰ The crystal structure, surface characteristics, optical properties, and magnetic properties of g-C₃N₄/NiFe₂O₄ products had been characterized by XRD, TEM, DRS, and VSM, respectively. Methylene blue (MB) was selected as a target pollutant to investigate the photocatalytic activity of g-C₃N₄/NiFe₂O₄ composites with different content of g-C₃N₄. The degradation efficiency of MB can reach 87% in 4 h for the optimum ratio of 7.5% g-C₃N₄/NiFe₂O₄ and more g-C₃N₄ (10%) would cause decreasing activity under visible light irradiation. In the section of photocatalytic mechanism, the energy band matching of the g-C₃N₄/NiFe₂O₄ heterojunction and the driving force for photocatalytic degradation of MB were discussed.

Experimental

2.1 Synthesis of NiFe₂O₄

All reagents were of analytical grade and were used without further purification. First, 1 mmol Fe(NO₃)₃·9H₂O and 0.5 mmol Ni(NO₃)₂·6H₂O was added into 17 mL absolute ethanol in a 20 mL Teflon-lined autoclave and magnetically stirred for 30 min at room temperature. Second, the reddish brown emulsion was precipitated and adjusted to a pH of 13 by adding a 6 M NaOH solution dropwise. Third, the precursor was sealed in a stainless steel tank and heated at 180 °C for 20 h after stirring for another 30 min. When the reaction terminated, the product was cooled down to room temperature and washed three times by water and ethanol, separately, and then dried at 60 °C for later use.

2.2 Synthesis of g-C₃N₄/NiFe₂O₄ composites

The preparation of g-C₃N₄ is according to the method from our previous work.³¹The g-C₃N₄/NiFe₂O₄ products with varying g-C₃N₄ content were synthesized by the following process: 120 mg NiFe₂O₄ and g-C₃N₄ (mass fraction of g-C₃N₄: 5, 7.5, 10 wt%) were added into a beaker containing 20 mL absolute ethanol and kept magnetically stirring for 10 h at room temperature; then the obtained suspension was put in a 100 °C drying oven for 12 h without other operations. The obtained products are labeled as 5% g-C₃N₄/NiFe₂O₄, 7.5% g-C₃N₄/NiFe₂O₄ and 10% g-C₃N₄/NiFe₂O₄.

2.3 Characterization

XRD patterns of the samples were obtained using an X-ray diffractometer (Bruker D8) with Cu Kα radiation (λ = 1.5418 Å) in the range of 2θ = 10° to 80°. Transmission electron microscopy (TEM) micrographs were taken with a JEOL-JEM-2010 (JEOL, Japan) operated at 200 kV. The UV-vis diffuse reflectance spectra (DRS) of the samples were obtained with a UV-vis spectrophotometer (UV-2450, Shimadzu Corporation, Japan) using BaSO₄ as the reference. Infrared (IR) spectra of all the catalysts (KBr pellets) were recorded using a Nicolet Model Nexus 470 IR equipment. Elemental compositions were detected by X-ray photoelectron spectroscopy (XPS) analysis, which was performed using an ESCALab MKII X-ray photoelectron spectrometer using Mg Kα radiation. The magnetic properties of NiFe₂O₄ and g-C₃N₄/NiFe₂O₄ composites were measured in a vibrating sample magnetometer (VSM) (Quantum Design Corporation, USA) with a maximum applied field of ± 2 T at 300 K.

2.4 Photocatalytic activity measurements

The MB degradation experiment was performed to evaluate the photocatalytic activity of samples under a 300 W Xe lamp with a 400 nm cutoff filter. 0.05 g of photocatalyst was added into 50 mL MB (10 mg L⁻¹) in a Pyrex photocatalytic reactor connected to a circulating water system, which could keep the reaction temperature at 30 °C. In addition, continuous aeration with oxygen was provided to the reaction and this mixed the suspension as well. Prior to irradiation, a dark reaction accompanied by magnetic stirring for 1 h was necessary to reach absorption–desorption equilibrium between the photocatalyst and the MB solution. During irradiation, 4 mL suspensions were taken at defined time intervals. Then, the samples were tested with a UV-vis spectrophotometer (UV-2450, Shimadzu), after centrifugation, at the wavelength of 664 nm, which is the maximal absorption band of MB. In the H₂O₂ system, 1 mL H₂O₂ was injected into the reaction suspension as soon as the Xe lamp was turned on. Moreover, the sampled specimen was tested in time.

Results and discussion

3.1 XRD analysis

Fig. 1 displays the XRD patterns of samples and all of the peaks have been marked with two types of symbols (♦ for g-C₃N₄ and ♣ for NiFe₂O₄). As shown in Fig. 1, the diffraction peaks at 18.4°, 30.3°, 35.7°, 37.3°, 43.3°, 53.8°, 57.3°, 63.0° and 74.5° were matching well with the (111), (220), (311), (222), (400), (422), (511), (440) and (533) crystalline planes of NiFe₂O₄, respectively (JCPDS 54-0964). It was noteworthy that there was no impurity peak for samples b, c, d and e, which proved that pH = 13 can hinder the formation of Fe₂O₃. In addition, the peaks at 27.4° and 13.1° can be indexed as the (002) and (100) diffraction planes of g-C₃N₄, respectively.³² Obviously, no peak shift occurred for NiFe₂O₄, so the chemisorption process did not have an influence on the crystal structure. More importantly, when the content of g-C₃N₄ increased to 10%, the peak at 27.4° appeared while for other proportions the peak was not observed because of the low g-C₃N₄ content.

Fig. 1 XRD patterns of (a) g-C₃N₄, (b) 10% g-C₃N₄/NiFe₂O₄, (c) 7.5% g-C₃N₄/NiFe₂O₄, (d) 5% g-C₃N₄/NiFe₂O₄, and (e) NiFe₂O₄.

3.2 TEM analysis

In order to figure out the origin of activity changes before and after NiFe₂O₄ combined with g-C₃N₄, the particle size and morphology of the catalysts were investigated by means of TEM. In Fig. 2(a), it can be clearly observed that NiFe₂O₄ had two totally different types of grain morphologies: the large polygon plates (about 100 nm) and the nanoscale particles (about 8 nm). Therefore, the crystallite size distribution of NiFe₂O₄ could be properly bimodal.^33,34 Fig. 2(b) illustrated that NiFe₂O₄ polygon plates and nanoparticles adhered to the g-C₃N₄ flake and the dispersion of NiFe₂O₄ nanoparticles was better than that of pure NiFe₂O₄, which would be beneficial to form a heterojunction structure and promote the separation efficiency of photoinduced carriers.

Fig. 2 TEM images of (a) NiFe₂O₄ and (b) 7.5% g-C₃N₄/NiFe₂O₄.

3.3 IR analysis

The IR spectrum enables us to distinguish the molecular structure of g-C₃N₄ from other organic chemicals easily and sensitively. In Fig. 3, g-C₃N₄ and g-C₃N₄/NiFe₂O₄ samples all had a group of absorption peaks in the 1200–1650 cm⁻¹ region, corresponding to the stretching vibration modes of C=N and C–N heterocycles, and the peak at 807 cm⁻¹ corresponding to the breathing mode of triazine units.^35,36 In addition, the strong absorption peak at 608 and 417 cm⁻¹ can be ascribed to the stretching vibrations of Fe–O bonds in tetrahedral positions and metal–O bonds in octahedral positions respectively.^37,38 Moreover, as the mass fraction of g-C₃N₄ increased from 5% to 10%, the peak at 807 cm⁻¹ appeared and became sharp gradually, which demonstrated that g-C₃N₄ and NiFe₂O₄ have been integrated together.

Fig. 3 IR spectra of (a) g-C₃N₄, (b) 10% g-C₃N₄/NiFe₂O₄, (c) 7.5% g-C₃N₄/NiFe₂O₄, (d) 5% g-C₃N₄/NiFe₂O₄, and (e) NiFe₂O₄.

3.4 XPS

Fig. 4 is the XPS spectra of 7.5% g-C₃N₄/NiFe₂O₄. The survey spectrum was illustrated in Fig. 4(a), clarifying that the g-C₃N₄/NiFe₂O₄ surface consisted of Fe, Ni, N, C and O elements. The typical high resolution XPS spectra of Ni 2p (Fig. 4(c)) and O 1s (Fig. 4(f)) were consistent with those reported in previous literature.¹⁴ It was also feasible to derive information regarding Fe oxidation states from the satellite features of Fe 2p. In Fig. 4(b) the binding energy of Fe 2p3/2 was observed at 710.7 eV rather than 711.2 eV, but the satellite peak (718.7 eV) at 8.0 eV above the principal peak could also be found. Thus, the presence of Fe²⁺ can be ruled out.^39,40 In Fig. 4(d) the N 1s peak at ∼399 eV was wide and could be fit into three peaks which were 401.4, 398.7 and 399.6 eV corresponding to C–N–H, C=N–C and N–(C)₃ functional groups of g-C₃N₄, respectively. Compared with peak at 400.1 eV reported in the literature, the peak shifted from 0.5 eV to 399.6 eV, indicating that the chemical environment of N atoms in N–(C)₃ groups has changed.⁴¹ Because XPS is a surface analytical tool, the surface chemical shift happens when the local bonding environment of a species is affected. As shown in Fig. 4(e), the C 1s had two obvious peaks at 284.8 and 288.2 eV, and the former could be ascribed to carbon absorbed casually on the surface of g-C₃N₄ while the latter can be attributed to sp² hybridized C (N–C=N).⁴² The data of XPS further confirmed the coexistence of g-C₃N₄ and NiFe₂O₄, which was in agreement with the results of XRD and IR analysis.

Fig. 4 XPS spectra of 7.5% g-C₃N₄/NiFe₂O₄ hybrid materials (a) survey of the sample, (b) Fe 2p, (c) Ni 2p, (d) N 1s, (e) C 1s, and (f) O 1s.

3.5 Magnetic properties

When this photocatalytic material was designed, its magnetic characteristic attracted us from beginning to end. Fig. 5 illustrates magnetization curves of pure NiFe₂O₄ and 7.5% g-C₃N₄/NiFe₂O₄: X-axis represents the applied magnetic field H and Y-axis represents magnetization M. It could be estimated that the saturation magnetization M_s of NiFe₂O₄ and 7.5% g-C₃N₄/NiFe₂O₄ was about 45 and 40 emu g⁻¹, respectively, and the coercivity H_c was around 50 Oe without much difference between two samples shown in the inset figure. The presence of low-content g-C₃N₄ influenced the composite magnetism less so that the collection and recycling of photocatalyst could be readily achieved by an external magnet. The image shown in Fig. 7(b) exhibited the separation of 7.5% g-C₃N₄/NiFe₂O₄ photocatalyst from the reaction system after the fifth cycle of MB degradation.

Fig. 5 Magnetization curves of the photocatalysts.

3.6 UV-vis analyses

In Fig. 6(a) the UV-vis diffuse reflectance spectra reflected the samples' optical properties: g-C₃N₄ had an absorption edge around 460 nm while NiFe₂O₄ and its composites had a much stronger and wider absorption in the visible region. The distinct band gap, molecular structure, and color of catalysts were considered to explain the difference in the absorption range and intensity. It was also noted that the 7.5% g-C₃N₄/NiFe₂O₄ composite had the strongest absorption intensity, which suggested that the integration of g-C₃N₄ and NiFe₂O₄ could promote the photoabsorption ability.⁴³ Commonly, a classic Tauc plot is used to estimate the amorphous semiconductor's band gap energy according to the relation: (αhv)ⁿ versus hv. In Fig. 6(b) the results indicated that NiFe₂O₄ was one direct band gap semiconductor and the E_g of NiFe₂O₄ and 7.5% g-C₃N₄/NiFe₂O₄ was about 1.98 and 1.9 eV, respectively. The narrowed band gap of g-C₃N₄/NiFe₂O₄ would reinforce the visible light absorption intensity. In addition, the illustration in the upper left demonstrated g-C₃N₄ was an indirect transition and its band gap energy was around 2.6 eV, which is close to the reported literature value.²³

Fig. 6 (a) UV-vis diffuse reflectance spectra, (b) (αhv)² versus hv curves of NiFe₂O₄ and 7.5% g-C₃N₄/NiFe₂O₄.

3.7 Photocatalytic decomposition of MB

Fig. 7(a) shows the MB degradation curves with varying catalysts under visible light illumination. It can be seen that after 4 h of visible light irradiation, the proportion C/C₀ reached 57%, 66%, 87%, and 76% for NiFe₂O₄, 5% g-C₃N₄/NiFe₂O₄, 7.5% g-C₃N₄/NiFe₂O_4, and 10% g-C₃N₄/NiFe₂O₄ with H₂O₂, respectively. These results clearly revealed that 7.5% g-C₃N₄/NiFe₂O₄ exhibited the best photocatalytic performance with H₂O₂ and the photocatalytic degradation efficiency of MB for pure NiFe₂O₄ was no more than 10% when there was no H₂O₂. Therefore, it can be concluded that the obtained NiFe₂O₄ and g-C₃N₄/NiFe₂O₄ can effectively activate H₂O₂ along with visible light illumination. Furthermore, the activation can be enhanced by the heterojunction structure. Moreover, the best photocatalytic performance for 7.5% g-C₃N₄/NiFe₂O₄ can be attributed to the proper adsorption of MB and enough active sites on the photocatalyst surface. In Fig. 7(b), MB degradation for the mechanically mixed sample was 65% lower than the 7.5% g-C₃N₄/NiFe₂O₄ composite, which directly verified the successful combination of g-C₃N₄ and NiFe₂O₄. At the same time, the rate of the photocatalytic reaction was also studied by fitting to zero-order kinetics (C₀ − C = kt) and the results are displayed in Fig. 7(c) and Table 1. The photodegradation rate of 7.5% g-C₃N₄/NiFe₂O₄ under visible light irradiation was 1.5 times higher than those of pure NiFe₂O₄. To further study the recyclability of the g-C₃N₄/NiFe₂O₄ composite, five runs of the photodegradation experiment were carried out. As can be seen in Fig. 7(d), the g-C₃N₄/NiFe₂O₄ photocatalyst could keep unequivocally high photocatalytic activity after five uses, thus the heterojunction structure was stable.

Fig. 7 (a) Photocatalytic performance of samples, (b) activity comparison between 7.5% g-C₃N₄/NiFe₂O₄ and 7.5% mechanically mixed, (c) kinetic fit diagram, and (d) cycling runs of 7.5% g-C₃N₄/NiFe₂O₄ photocatalyst for MB degradation.

Table 1 Zero-order kinetic constant for MB degradation with different photocatalysts

Samples	k (1 × 10^−6 mol L^−1 h^−1)	R^2
NiFe2O4	1.83	0.9954
5% g-C3N4/NiFe2O4	2.07	0.9986
10% g-C3N4/NiFe2O4	2.40	0.9990
7.5% g-C3N4/NiFe2O4	2.82	0.9958

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3.8 Photocatalytic mechanism discussions

As mentioned above, the MB photodegradation capability has been improved by the heterojunction structure of the interface of the g-C₃N₄/NiFe₂O₄ composite. We calculated NiFe₂O₄'s conduction band E_CB = 0.35 eV and E_VB = 2.33 eV according to eqn (1) where X is the semiconductor's electronegativity and E_c is the energy of the free electron on the hydrogen scale.³² On the basis of experimental data, the scheme of a possible photocatalytic mechanism is presented in Fig. 8. First, electrons on the VB position were excited to the CB position for both g-C₃N₄ and NiFe₂O₄ under visible light irradiation. Second, electrons on the CB of g-C₃N₄ moved to the CB of NiFe₂O₄ and holes on the VB of NiFe₂O₄ transferred to the VB of g-C₃N₄. As a result, the redistribution of electrons and holes prevented photoinduced carriers from recombination quickly. On the other hand, the CB value of NiFe₂O₄ (0.35 eV) is less negative than E⁰(O₂/˙O₂⁻) (−0.046 eV vs. NHE),⁴⁴ so O₂ would not be reduced by electrons to generate ˙O²⁻ on the photocatalyst surface. In addition, compared with the E⁰(˙OH/OH⁻) (2.38 eV vs. NHE),⁴⁵ the VB potential of g-C₃N₄ (1.47 eV) is less positive, which implied that ˙OH would not be yielded by the oxidation of OH⁻ with holes.

Fig. 8 Heterojunction diagram for electron transfer in the g-C₃N₄/NiFe₂O₄ composite.

For the purpose of confirming the main active species, trapping experiments were carried out with isopropanol, 1,4-benzoquinone (BQ) and disodium ethylenediamine tetraacetate (EDTA-2Na) to capture hydroxyl radical (˙OH), superoxide radical (˙O₂⁻) and hole (h⁺), respectively. It can be clearly observed in Fig. 9 that the addition of 5 mL isopropanol caused a suppression of the MB degradation rate, while the addition of 0.5 mmol EDTA-2Na and BQ improved the degradation activity of MB. The results may reveal that the main active species should be ˙OH instead of ˙O₂⁻ and h⁺, which is in agreement with the hypothesis of the photocatalytic mechanism diagram above. In addition, Wang analyzed the photo-Fenton reaction routes and proved that ˙OH played an vital role in the oxidation process of benzene to phenol for FeCl₃/mpg-C₃N₄ hybrids.⁴⁶ There are related studies supporting our trapping experiment and mechanism research as well.^47,48

Fig. 9 Photocatalytic performance of 7.5% g-C₃N₄/NiFe₂O₄ with different scavengers for MB degradation.

In the Fenton and photo-Fenton reactions, iron-based species can generate highly reactive hydroxyl radicals (˙OH) to treat a large variety of water pollutants. Owing to the quick conversion from Fe³⁺ into Fe²⁺, the degradation activity in the photo-Fenton reaction becomes many times higher than the classical Fenton reaction under visible light illumination.^28,49 Here, the photocatalytic process and photo-Fenton reaction are both functioning: electrons and holes transfer to opposite directions under visible light irradiation; electrons in the CB of NiFe₂O₄ can react with H₂O₂ and Fe³⁺ to produce ˙OH for the oxidation of MB, and the photodegradation reaction can be expressed as follows:

Fe²⁺ + H₂O₂ → Fe³⁺ + ˙OH + OH⁻ (2)

Fe³⁺ + e⁻→ Fe²⁺ (4)

H₂O₂ + e⁻˙ → OH + OH⁻ (5)

˙OH + MB→ degradation products (6)

Conclusions

In conclusion, a simple chemisorption technique was applied to fabricate a magnetic g-C₃N₄/NiFe₂O₄ catalyst for MB degradation. With a matching energy band, g-C₃N₄/NiFe₂O₄ possessed favorable optical properties and could activate H₂O₂ to produce effective oxidizing reagents, thus realizing MB discoloration. The heterojunction established in the composite material accelerated the process of electron–hole pair separation and boosted H₂O₂ activation for the photo-Fenton reaction. As a whole, the uniformity of the particle size and morphology still needs improvement by adjusting the synthesis conditions. The magnetic character of NiFe₂O₄ can be introduced to other composites for repeated use as photocatalysts and enhancing photo-degradation ability simultaneously.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (21476097, 21407065, 21406094), Natural Science Foundation of Jiangsu Province (no. BK20130513, BK20140533), University Natural Science Research of Jiangsu (no. 13KJB430007), Jiangsu Key Lab of Material Tribology Foundation (no. Kjsmcx201303), the Senior Intellectuals Fund of Jiangsu University (no. 12JDG110), Jiangsu University Graduate Student Research and Creative Project (no. KYXX-0012) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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