magnetically separable and visible-light-driven

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Journal of Molecular Catalysis A: Chemical 415 (2016) 122–130

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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

Novel ternary g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites: magnetically separable and visible-light-driven photocatalysts for degradation of water pollutants Aziz Habibi-Yangjeh ∗ , Anise Akhundi Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran

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Article history: Received 24 November 2015 Received in revised form 24 January 2016 Accepted 28 January 2016 Available online 1 February 2016 Keywords: g-C3 N4 /Fe3 O4 /Ag2 CrO4 Ternary nanocomposite Magnetic photocatalyst Visible-light-driven

a b s t r a c t Novel magnetically separable g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites, as visible-light-driven photocatalysts, were prepared using a facile refluxing method with no require to any additives or post preparation treatments. The resultant samples were characterized by XRD, EDX, TEM, UV–vis DRS, FT–IR, TGA, PL, and VSM techniques. The photocatalytic activities of the resultant samples were investigated by degradation of rhodamine B in aqueous solution under visible-light illumination. The results showed that the g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite exhibited superior activity. Photocatalytic activity of this nanocomposite was about 6.3 and 5-fold higher than those of the g-C3 N4 and g-C3 N4 /Fe3 O4 samples, respectively. The highly enhanced activity was attributed to more absorption of the nanocomposites in visible range and efficient separation of the charge carriers. Furthermore, we investigated the influence of refluxing time, calcination temperature, and scavengers of the reactive species on the degradation activity. It was found that superoxide ions are the primary reactive species to cause the degradation reaction over the ternary nanocomposite. Besides, stability of the magnetic nanocomposite was tested by five recycling experiments. © 2016 Elsevier B.V. All rights reserved.

1. Introduction It is believed that heterogeneous photocatalysts are appealing class of materials owing to their outstanding performance in different disciplines such as generation of hydrogen by splitting of water, reduction of carbon dioxide to fuels, and degradation of different pollutants using the solar energy [1–5]. Hence, many efforts have been paid to prepare suitable photocatalysts with considerable activities under visible-light irradiation [2,6]. After pioneering work of Wang et al., graphitic carbon nitride (g-C3 N4 ) has been recognized as a fascinating organic photocatalyst with photocatalytic activity under visible-light illumination [7–15]. This graphite-like compound has constructed from polyheptazine heterocyclic planes packed closely [8]. Owing to its high stability even in more acidic and alkaline solutions, appealing electronic structure, narrow band gap of 2.7 eV, low cost, and nontoxicity, it has found broad applications in different fields such as energy conversion and storage, degradation of pollutants, carbon dioxide storage and reduction, catalysis, solar cells, and sensing [12–21]. This semiconductor is

∗ Corresponding author. Fax: +98 045 33514701. E-mail address: [email protected] (A. Habibi-Yangjeh). http://dx.doi.org/10.1016/j.molcata.2016.01.032 1381-1169/© 2016 Elsevier B.V. All rights reserved.

simply prepared by thermal condensation reaction of nitrogen rich precursors such as melamine, urea, and cyanamide [8]. However, it is well known that the pristine g-C3 N4 suffers from some drawbacks. High recombination rate of photogenerated electron-hole pairs is one of the major drawbacks, limiting activity of the pure g-C3 N4 [13–16]. One effective method to handle this drawback is coupling of it with semiconductors having suitable band potential [11,13]. Hence, a variety of inorganic semiconductors such as ZnO, AgBr, AgI, Ag3 PO4 , BiOI, CdS, In2 S3 , LaVO4 , Ag3 VO4 ,TiO2 , SnO2 , Ag3 VO3 , Bi2 S3 , and BiFeO3 have been deposited on g-C3 N4 to promote separation of the charge carriers by forming heterojunction structures [22–33]. On the other hand, the g-C3 N4 -based nanocomposites cannot be entirely recycled from the treated solutions using conventional methods, leading to generation of secondary pollution. Very recently, it was found that magnetic materials can be immobilized on the surface of g-C3 N4 , resulting in magnetically separable photocatalysts [34–38]. Hence, these multifunctional visible-light-driven photocatalysts can be easily separated from the treated solution using an external magnet. However, to the best of our knowledge, there is no report about the preparation and photocatalytic activity of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites. In this context, we report novel magnetically separable g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites as efficient

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Scheme 1. The schematic diagram for preparation of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites.

visible-light-driven photocatalysts. These nanocomposites were simply prepared by refluxing method at 96 ◦ C for 60 min. Photocatalytic activity of the samples was evaluated by degradation of rhodamine B (RhB) under visible-light irradiation. Additionally, the effects of refluxing time, calcination temperature, and scavengers of the reactive species on the degradation reaction were studied and the results were discussed. More importantly, it was demonstrated that the ternary photocatalyst has reasonable stability during five degradation reactions and magnetic separations. 2. Experimental 2.1. Materials All reagents were of analytical grade and used without further purifications. Deionized water was used throughout this study. 2.2. Instruments The X-ray diffraction (XRD) patterns were recorded by a Philips Xpert X-ray diffractometer with CuK␣ radiation (= 0.15406 nm), employing scanning rate of 0.04◦ /s in the 2 range from 10 to 80◦ . Purity of the products was examined by energy dispersive analysis of X-rays (EDX) on LEO 1430VP SEM. For the EDX experiments, samples mounted on an aluminum support using a double adhesive tape coated with a thin layer of gold. The transmission electron microscopy (TEM) investigations were performed by a Zeiss-EM10C instrument with an acceleration voltage of 80 kV. The UV–vis diffuse reflectance spectra (DRS) were recorded by a Scinco 4100 apparatus. The Fourier transform-infrared (FT–IR) spectra were obtained using Perkin Elmer Spectrum RX I apparatus. The UV–vis spectra for the degradation reaction were studied using a Cecile 9000 spectrophotometer. The photoluminescence (PL) spectra of the samples were provided using a PerkinElmer (LS 55) fluorescence spectrophotometer with an excitation wavelength of 300 nm. The conditions were fixed in order to compare the PL intensities. Magnetic properties of the samples were obtained using an alternating gradient force magnetometer (model AGFM, Iran). The ultrasound radiation was performed using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz).

2.3. Preparation of the samples 2.3.1. Preparation of g-C3 N4 /Fe3 O4 nanocomposite The powder of g-C3 N4 was prepared by heating melamine up to 520 ◦ C according to the literature method [8]. The required water in this section was degassed by bubbling N2 gas through it for 20 min prior to use. For preparation of the g-C3 N4 /Fe3 O4 with 2:1 weight ratio of g-C3 N4 to Fe3 O4 , 0.4 g of g-C3 N4 was dispersed into150 mL of water by ultrasonic irradiation for 20 min in a cylindrical Pyrex reactor provided with a water circulation arrangement to maintain its temperature at 25 ◦ C. In one beaker, 0.476 g of FeCl3 . 6H2 O was dissolved in 20 mL of degassed water (solution A). In another beaker, 0.166 g of FeCl2 ·4H2 O was dissolved in a solution containing 5 mL of degassed water and 0.8 mL of 2 mol dm−3 HCl (solution B). Then, solutions of A and B were added to the formed yellow suspension of g-C3 N4 under mechanical stirring and nitrogen degassing. Stirring of the yellow colored suspension was continued for 30 min. Afterwards, the suspension was refluxed for 60 min. Then, aqueous ammonia (13.1 mL of 30% NH3 at 250 mL of degassed water) was quickly added to the orange suspension under mechanical stirring. The resulting dark brown suspension was stirred for another 60 min and finally centrifuged to remove the precipitate and washed two times with water and ethanol and magnetically separated. The obtained dark brown precipitate was dried in an oven at 60 ◦ C for 24 h.

2.3.2. Preparation of g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites Typically for preparation of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite where 20% is weight percent of Ag2 CrO4 , 0.3 g of g-C3 N4 /Fe3 O4 nanocomposite was dispersed into 150 mL of water by ultrasonic irradiation for 10 min. Then, 0.075 g of silver nitrate was added to the suspension under stirring at room temperature for 120 min. Afterwards, an aqueous solution of potassium chromate (0.044 g in 20 mL of water) was dropwise added to the suspension and refluxed at 96 ◦ C for 60 min. The formed light brown suspension was then centrifuged to remove the precipitate and washed two times with water and ethanol and dried in an oven at 60 ◦ C for 24 h (Scheme 1).

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Fig. 1. XRD patterns of the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites with different weight percents of silver chromate along with the patterns for the g-C3 N4 /Ag2 CrO4 (20%) and Fe3 O4 /Ag2 CrO4 (20%) samples.

2.4. Photocatalytic experiments Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400 mL capacity. Temperature of the reactor was maintained at 25 ◦ C using a water circulation arrangement. The solution was mechanically stirred and continuously aerated by a pump to provide oxygen and complete mixing of the reaction solution. An LED source with 50 W was used as a visible-light source. The source was fitted on the top of the reactor. Prior to illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL of RhB solution (2.50 × 10−5 M) was continuously stirred in the dark for 60 min, to attain adsorption equilibrium. Samples were taken from the reactor at regular intervals and the photocatalyst removed before analysis by the spectrophotometer at 553 nm corresponding to the maximum absorption wavelength of RhB. Diphenylcarbazide method was used to determine leakage of Cr ions in the solution after the photocatlaytic reaction [39].

3. Results and discussion Fig. 1 depicts XRD patterns of the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites with different weight percents of Ag2 CrO4 along with the patterns for the g-C3 N4 /Ag2 CrO4 (20%) and Fe3 O4 /Ag2 CrO4 (20%) samples. In the g-C3 N4 sample, the appeared diffraction peaks at 13.1 and 27.3◦ are indexed to the (1 0 0) and (0 0 2) planes of g-C3 N4 (JCPDS card no. 87-1526) [8]. For the g-C3 N4 /Fe3 O4 nanocomposite, the diffraction peaks are clearly indexed to g-C3 N4 and Fe3 O4 counterparts [40]. In the case of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites, the diffraction patterns confirmed co-existence of the g-C3 N4 , Fe3 O4 , and Ag2 CrO4 counterparts and no other impurity peaks were observed, indicative of the high purity of the products. Fig. 2 shows EDX spectra for the g-C3 N4 , g-C3 N4 /Fe3 O4 , and gC3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples. In consistent with the XRD results, it is evident that the EDX spectra confirm the purity of the prepared samples. Morphology of the g-C3 N4 and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples was investigated by TEM technique and the result are

Fig. 2. EDX spectra of the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples.

shown in Fig. 3a and b. For the pure g-C3 N4 , its sheets are clearly seen (Fig. 3a). In the case of g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite, it is evident that particles of Fe3 O4 and Ag2 CrO4 have deposited on the surface of g-C3 N4 sheet. It is well known that remarkable absorption enhancement in the visible-light region is beneficial for improving photocatalytic activity. Hence, the UV–vis DRS spectra of g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites with different weight percents of Ag2 CrO4 along with the spectra for the Fe3 O4 and Ag2 CrO4 samples were provided and the results are illustrated in Fig. 4. As can be seen, the absorption edge of the pure g-C3 N4 locates at about 470 nm, which is consistent with the reported results [8]. Inter-

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Fig. 3. TEM images of the (a) pristine g-C3 N4 and (b) g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite.

Fig. 4. UV–vis DRS for the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites with different weight percents of silver chromate along with the spectra for Ag2 CrO4 and Fe3 O4 samples.

estingly, g-C3 N4 /Fe3 O4 /Ag2 CrO4 samples exhibit broad absorption in the whole visible region and the absorption intensities increase with increasing weight percent of silver chromate. Due to enhanced visible-light harvesting ability of the ternary nanocomposites, it can be concluded that these nanocomposites could have considerable activity under visible-light illumination. Chemical structures of the g-C3 N4 , g-C3 N4 /Fe3 O4 , and gC3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples were investigated by FT–IR spectra and the results are shown in Fig. 5a. For these samples the absorption bands centered at 3200 cm−1 are attributed to the stretching vibrations of N H bonds. Moreover, a series of strong absorption bands in the range of 1240–1650 cm−1 is assigned to typical skeletal stretching vibrations of the s-triazine or tri-striazine groups. The sharp peaks centered at 806 cm−1 are relevant to the characteristic breathing mode of triazine units [8]. For the samples containing Fe3 O4 nanoparticles, two characteristic peaks at 620 and 430 cm−1 are related to the stretching vibrations of the Fe O bond [41]. The FT–IR spectrum of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite shows a strong absorption band at 888 cm−1 and a shoulder at about 860 cm−1 , which are assigned to the stretching vibration modes of Cr O bond [42]. To determine the g-C3 N4 contents of the prepared samples, thermogravimetric analysis was performed from room

temperature to 700 ◦ C at a heating rate of 10 ◦ C min−1 under air conditions. For the pure g-C3 N4 , a rapid weight loss from about 520 ◦ C to 670 ◦ C is observed, which is related to burning of g-C3 N4 . It is evident that weight of the g-C3 N4 /Fe3 O4 and gC3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposites decreased rapidly in the range of about 450–630 ◦ C, which is lower than the temperature range of the pure g-C3 N4 . Similar to many g-C3 N4 -based nanocomposites, this decrease could be ascribed to decreasing thermal stability of the pristine g-C3 N4 with depositing Fe3 O4 and Ag2 CrO4 particles [43–45]. The contents of g-C3 N4 in the nanocomposites were calculated from the weights remaining after heating the samples over 700 ◦ C, which are 66.5 and 52% for the g-C3 N4 /Fe3 O4 and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples, respectively. Magnetic properties of the Fe3 O4 nanoparticles and gC3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite were measured and the results are shown in Fig. 6. The saturation magnetization of the Fe3 O4 nanoparticles and the nanocomposite at 8500 Oe are 55.5 and 12.9 emu/g, respectively. In addition, as observed from the inset image, the magnetic nanocomposite form a stable suspension in aqueous solution, and also it could be easily separated by a magnet. Hence, this ternary g-C3 N4 -based nanocomposite could be an excellent recyclable photocatalyst for environmental purposes.

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Fig. 6. Magnetization curves for the Fe3 O4 nanoparticles and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite. Inset of the figure shows separation of the nanocomposite from the treated solution using a magnet.

Fig. 5. (a) FT–IR spectra for the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples. (b) TGA curves for the g-C3 N4 , g-C3 N4 /Fe3 O4 , and gC3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples.

Photocatalytic activity of the g-C3 N4 , g-C3 N4 /Fe3 O4 , and gC3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites with different compositions was evaluated by degradation of RhB under visible light. For comparison, photocatalytic activity of the Fe3 O4 , Ag2 CrO4 , gC3 N4 /Ag2 CrO4 (20%), and Fe3 O4 /Ag2 CrO4 (20%) samples was also determined. As shown in Fig. 7a, changes of RhB concentration are negligible in the dark (in absence of the light) and photolysis (in absence of any photocatalyst) conditions. Additionally, as can be seen, the Fe3 O4 sample has the lowest photocatalytic activity among the prepared samples and photocatalytic activity of the gC3 N4 /Fe3 O4 nanocomposite is some higher than that of the g-C3 N4 . Similar to the literature, increase of the photocatalytic activity for g-C3 N4 /Fe3 O4 sample could be related to enhancing separation of electron-hole pairs of g-C3 N4 by Fe3 O4 particles [36,46]. It is evident that all of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites exhibit higher photocatalytic activity than the g-C3 N4 and g-C3 N4 /Fe3 O4 samples under visible-light irradiation. Furthermore, with an increase in weight percent of Ag2 CrO4 up to 20%, the

photocatalytic activity rises. After that, further increase in Ag2 CrO4 weight percent, a decrease in the degradation of RhB was observed. Figs. 7b, c, and d show plots of UV–vis spectra for degradation of RhB over the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples at various irradiation times. As can be seen, the intensity of the absorption peaks specially at 553 nm decreases as the photodegradation reaction progresses. It is notable that no blue-shift of the RhB absorption peak centered at 553 nm was observed. Hence, the degradation reaction involves cleavage of the whole conjugated chromophore structure in the pollutant during the photochemical reaction [45,47]. Additionally, under the light irradiation for 300 min, about 41, 52, and 95% of RhB were degraded over the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples, respectively. To quantitatively analysis of photocatalytic activity for the prepared samples, observed first-order rate constant of the degradation reaction (kobs ) was calculated [48]. Fig. 8a displays the degradation rate constant of RhB over different samples. It is clear that a sharp increase with weight percent of Ag2 CrO4 was observed. The rate constants over the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples are 16.0 × 10−4 , 20.2 × 10−4 , and 100.8 × 10−4 min−1 , respectively. Hence, activity of the gC3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite is about 6.3 times larger than that over the g-C3 N4 and 5 times larger than that over the g-C3 N4 /Fe3 O4 nanocomposite under the same conditions. To show the effect of silver chromate and Fe3 O4 on the separation efficiency of the photogenerated electron-hole pairs on the g-C3 N4 , PL spectra of the pure g-C3 N4 , g-C3 N4 /Fe3 O4 , and gC3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples were provided and the results are shown in Fig. 8b. As can be seen, there is a small decrease in the PL intensity of the g-C3 N4 /Fe3 O4 nanocomposite relative to that of the pure g-C3 N4 . However, once Ag2 CrO4 nanoparticles were added over the g-C3 N4 /Fe3 O4 nanocomposite, the emission intensity of the PL spectrum significantly decreased. Consequently, based on the PL spectra, it was concluded that the effect of Ag2 CrO4 particles on separation of the charge carriers in g-C3 N4 is higher than that of the Fe3 O4 particles. In the g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite, the Ag2 CrO4 and g-C3 N4 have closely contacted. Hence, the photogenerated electron-hole pairs can migrate easily between gC3 N4 and Ag2 CrO4 , due to the matching band potentials. As a result, recombination of the charge carriers was suppressed, resulting in enhanced photocatalytic activity. The improved photocatalytic activities were attributed to combined effects, including stronger

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Fig. 7. (a) Photodegradation of RhB over the g-C3 N4 , g-C3 N4 /Fe3 O4 , and g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites with different weight percents of silver chromate along with those for the g-C3 N4 /Ag2 CrO4 (20%), Fe3 O4 /Ag2 CrO4 (20%), Ag2 CrO4 , and Fe3 O4 samples. UV–vis spectra for degradation of RhB under visible-light irradiation over the (b) g-C3 N4 , (c) g-C3 N4 /Fe3 O4 , and (d) g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) samples.

visible-light absorption, due to the presence of narrow band gap Ag2 CrO4 particles over the g-C3 N4 and more separation efficiency of photogenerated electron-hole pairs, due to migration of the charges between the counterparts of the nanocomposite. Similar to the literature, decease of the degradation rate constant by loading more amounts of Ag2 CrO4 could be attributed to agglomeration of the excess Ag2 CrO4 particles in the nanocomposites, leading to reduce the interface area between Ag2 CrO4 and g-C3 N4 . It is believed that the time applied for preparation of photocatalysts could considerably affect their photocatalytic activities [49]. For this purpose, the g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite was prepared by refluxing for 15, 30, 60, 120, and 240 min and the results are shown in Fig. 9a. It is clear that there is not a linear correlation between the rate constant and the refluxing time and the superior activity was seen for the sample prepared by refluxing for 60 min. Therefore, the nanocomposite prepared at this refluxing time was selected for further investigations. Moreover, to investigate the effect of calcination temperature on the photocatalytic activity, the nanocomposite prepared by refluxing for 60 min was calcined at 200, 300, 400, and 500 ◦ C for 2 h and the results are displayed in Fig. 9b. As can be seen, the degradation rate constant decreases with increasing calcination temperature. Decrease

of the degradation rate constant could be related to aggregation and growth of the particle sizes at higher temperatures [50,51]. Based on the results obtained from the structure characterizations and the visible-light photocatalytic investigations, a possible mechanism for the enhanced visible-light activity of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites is proposed (Fig. 10). The band edge positions of the conduction band (CB) and valence band (VB) of the semiconductors were calculated using Butler and Ginley model [52]. Under the visible-light illumination, electron-hole pairs are generated over g-C3 N4 and Ag2 CrO4 counterparts, due to their narrow band gap [53]. The VB and CB energies for g-C3 N4 are +1.58 and −1.12 eV and those for Ag2 CrO4 are +2.26 and +0.46 eV, respectively. The CB edge of g-C3 N4 is higher than that of Ag2 CrO4 , whereas the VB edge of Ag2 CrO4 is lower than that of g-C3 N4 . Consequently, the photogenerated electrons on the CB of g-C3 N4 easily transfer to that of Ag2 CrO4 , while the holes on the VB of Ag2 CrO4 migrate to that of g-C3 N4 . Hence, the electrons and holes are gathered on Ag2 CrO4 and g-C3 N4 , respectively, leading to spatially separation of the charge carriers. Consequently, as confirmed by the PL spectra, recombination of the electron-hole pairs in the ternary nanocomposite was considerably decreased, resulting in highly enhanced photocatalytic activity.

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To investigate the role of the reactive species in the degradation reaction over g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite, the scavengers of the active species such as superoxide ions, holes, and hydroxyl radicals were explored and the results are shown in Fig. 11a. It is well known that benzoquinone, ammonium oxalate, and isopropanol are scavengers of superoxide ions, holes, and hydroxyl radicals, respectively [54]. Without using any scavenger, the degradation rate constant was 100.8 × 10−4 min−1 . When benzoquinone and isopropanol were added to the reaction system, the degradation rate constants were decreased to 33.6 × 10−4 and 88.8 × 10−4 min−1 , respectively. Then, the role of superoxide ions

in the degradation reaction is considerable, while that of hydroxyl radicals is negligible. Furthermore, the photocatalytic degradation rate constant of RhB decreases to 71.2 × 10−4 min−1 after adding ammonium oxalate, which indicates that the role of holes is very lower than that of the superoxide ions. The CB potential of g-C3 N4 (−1.12 eV) is more negative than potential of O2 /• O2 − (−0.33 eV). Therefore, adsorbed oxygen over g-C3 N4 was reduced to • O2 − by capturing electron [55]. Additionally, the potential for O2 /H2 O2 (+0.695 eV) is higher than the CB energy of g-C3 N4 and Ag2 CrO4 [56]. Consequently, the electrons in CB of g-C3 N4 and Ag2 CrO4 react with adsorbed oxygen to produce hydrogen peroxide. The produced hydrogen peroxide molecules generate • OH radicals by capturing electrons in another step. However, oxidation of hydroxide ions (E◦ −OH/OH◦ = +2.38 eV) and molecules of water (E◦ H2 O/OH◦ = +2.72 eV) to hydroxyl radicals do not take place on the VB of g-C3 N4 (+1.58 eV) and Ag2 CrO4 (+2.26 eV) [57,58]. For these

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reasons, the roles of the active species in the degradation reaction are superoxide ions > holes > hydroxyl radicals. It is evident that stability of a photocatalyst is a very important characteristic from industrial viewpoint. Hence, stability of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite was tested for successive five recycling runs. After each cycle, the photocatalysts were collected and washed with water. The photocatalyst was used for the next run, after drying at 60 ◦ C for 24 h. As shown in Fig. 11b, the ternary magnetic photocatalyst has a good stability and retains most of its activity during the five successive runs. To show production of metallic silver after five recycling runs, the XRD patterns of the ternary nanocomposite before and after using five cycles are shown in Fig. 11c. It is evident that some metallic silver is produced by photocorrosion of the silver chromate [59]. Moreover, it was found that leakage of Cr ions from the nanocomposite after recycling is about nine percent. As a result, decrease of the degradation activity during the recycling processes could be attributed to changes of the catalyst composition.

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Acknowledgment The authors wish to acknowledge University of Mohaghegh Ardabili for financial support of this work.

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Novel ternary g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites, as magnetically separable photocatalysts under visible-light irradiation, were prepared by a simple and large-scale method and they fairly characterized by different techniques. The degradation rate constant of RhB increases with increasing weight percent of Ag2 CrO4 up to 20% and then decreases. The nanocomposite with 20% of Ag2 CrO4 showed the highest photocatalytic activity. Photocatalytic activity of the g-C3 N4 /Fe3 O4 /Ag2 CrO4 (20%) nanocomposite is about 6.3 times larger than that over the g-C3 N4 and 5 times larger than that over the g-C3 N4 /Fe3 O4 nanocomposite under the same conditions. The remarkably increased performance of gC3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites was mainly ascribed to more harvesting of the visible light and enhanced separation of the charge carriers, due to the presence of narrow band gap Ag2 CrO4 and the suitable matching of the band positions for counterparts of the nanocomposite. Hence, the g-C3 N4 /Fe3 O4 /Ag2 CrO4 nanocomposites could offer the perspective of developing new efficient magnetic visible-light-driven photocatalysts based on g-C3 N4 .

Metallic Ag JCPDS code: 87-0717

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2 Theta (deg.) Fig. 11. (a) The degradation rate constant of RhB over the nanocomposite in the presence of various scavengers. (b) Reusability of the nanocomposite after five successive runs. (c) XRD patterns of the ternary nanocomposite before and after five successive runs.

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References [1] K. Li, X. An, K.H. Park, M. Khraisheh, J. Tang, Catal. Today 224 (2014) 3–12. [2] S. Dong, J. Feng, Y. Pi, L. Hu, X. Han, M. Liu, J. Sun, J. Sun, RSC Adv. 5 (2015) 14610–14630. [3] S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.-X. Guo, J. Tang, Energy Environ. Sci. 8 (2015) 731–759. [4] H. Ahmad, S.K. Kamarudin, L.J. Minggu, M. Kassim, Renewable Sustainable Energy Rev. 43 (2015) 599–610. [5] K.M. Lee, C.W. Lai, K.S. Ngai, J.C. Juan, Water Res. 88 (2016) 428–448. [6] Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar, J. He, Nanoscale 5 (2013) 8326–8339. [7] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–82. [8] Y. Zheng, J. Liu, J. Liang, M. Jaroniec, S.Z. Qiao, Energy Environ. Sci. 5 (2012) 6717–6731. [9] G. Dong, Y. Zhang, Q. Pan, J. Qiu, J. Photochem. Photobiol. C: Photochem. Rev. 20 (2014) 33–50. [10] J. Zhu, P. Xiao, H. Li, S.A.C. Carabineiro, ACS Appl. Mater. Interfaces 6 (2014) 16449–16465. [11] Y. Gong, M. Li, H. Li, Y. Wang, Green Chem. 17 (2015) 715–736. [12] S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 27 (2015) 2150–2176. [13] Z. Zhao, Y. Sun, F. Dong, Nanoscale 7 (2015) 15–37. [14] J. Zhang, Y. Chen, X. Wang, Energy Environ. Sci. 8 (2015) 3092–3108. [15] S. Ye, R. Wang, M.-Z. Wu, Y.-P. Yuan, Appl. Surf. Sci. 358 (2015) 15–27. [16] Y. Zheng, L. Lin, B. Wang, X. Wang, Angew. Chem. Int. Ed. Engl. 54 (2015) 12868–12884. [17] J. Zhang, M. Zhang, L. Lin, X. Wang, Angew. Chem. Int. Ed. Engl. 54 (2015) 6297–6301. [18] M. Zhang, Z. Luo, M. Zhou, C. Huang, X. Wang, Sci. China Mater. 58 (2015) 867–876. [19] D. Zheng, C. Pang, Y. Liu, X. Wang, Chem. Commun. 51 (2015) 17467–17470. [20] Y. He, L. Zhang, B.-T. Teng, M. Fan, Environ. Sci. Technol. 49 (2015) 649–656. [21] C. Huang, C. Chen, M. Zhang, L. Lin, X. Ye, S. Lin, M. Antonietti, X. Wang, Nat. Commun. 6 (2015) 7698. [22] W. Liu, M. Wang, C. Xu, S. Chen, Chem. Eng. J. 209 (2012) 386–393. [23] H. Xu, J. Yan, Y. Xu, Y. Song, H. Li, J. Xia, C. Huang, H. Wan, Appl. Catal. B: Environ. 129 (2013) 182–193. [24] S. Kumar, T. Surendar, A. Baruah, V. Shanker, J. Mater. Chem. A 1 (2013) 5333–5340. [25] D. Jiang, L. Chen, J. Zhu, M. Chen, W. Shi, J. Xie, Dalton Trans. 42 (2013) 15726–15734. [26] S.-W. Cao, Y.-P. Yuan, J. Fang, M.M. Shahjamali, F.Y.C. Boey, J. Barber, S.C.J. Loo, C. Xue, Int. J. Hydrogen Energy 38 (2013) 1258–1266. [27] C. Xing, Z. Wu, D. Jiang, M. Chen, J. Colloid Interface Sci. 433 (2014) 9–15. [28] S. Wang, D. Li, C. Sun, S. Yang, Y. Guan, H. He, Appl. Catal. B: Environ. 144 (2014) 885–892. [29] C. Wang, W. Zhu, Y. Xu, H. Xu, M. Zhang, Y. Chao, S. Yin, H. Li, J. Wang, Ceram. Int. 40 (2014) 11627–11635.

[30] Y. He, J. Cai, L. Zhang, X. Wang, H. Lin, B. Teng, L. Zhao, W. Weng, H. Wan, M. Fan, Ind. Eng. Chem. Res. 53 (2014) 5905–5915. [31] W. Zhao, Y. Guo, S. Wang, H. He, C. Sun, S. Yang, Appl. Catal. B: Environ. 165 (2015) 335–343. [32] X. Rong, F. Qiu, J. Yan, H. Zhao, X. Zhu, D. Yang, RSC Adv. 5 (2015) 24944–24952. [33] Y. He, Y. Wang, L. Zhang, B. Teng, M. Fan, Appl. Catal. B: Environ. 168 (2015) 1–8. [34] X. Wang, W. Mao, J. Zhang, Y. Han, C. Quan, Q. Zhang, T. Yang, J. Yang, X. Li, W. Huang, J. Colloid Interface Sci. 448 (2015) 17–23. [35] S. Kumar, T. Surendar, B. Kumar, A. Baruah, V. Shanker, J. Phys. Chem. C 117 (2013) 26135–26143. [36] X. Zhou, B. Jin, R. Chen, F. Peng, Y. Fang, Mater. Res. Bull. 48 (2013) 1447–1452. [37] K. Vignesh, A. Suganthi, B.-K. Min, M. Kang, J. Mol. Catal. A: Chem. 395 (2014) 373–383. [38] A. Akhundi, A. Habibi-Yangjeh, Ceram. Int. 41 (2015) 5634–5643. [39] A. Idris, E. Misran, N.M. Yusof, J. Indust. Eng. Chem. 18 (2012) 2151–2156. [40] W. Wu, C. Jiang, V.A.L. Roy, Nanoscale 7 (2015) 38–58. [41] H. Gupta, P. Paul, N. Kumar, S. Baxi, D.P. Das, J. Colloid Interf. Sci. 430 (2014) 221–228. [42] F. Soofivand, F. Mohandes, M. Salavati-Niasari, Mater. Res. Bull. 48 (2013) 2084–2094. [43] T. Li, L. Zhao, Y. He, J. Cai, M. Luo, J. Lin, Appl. Catal. B: Environ. 129 (2013) 255–263. [44] H. Li, J. Liu, W. Hou, N. Du, R. Zhang, X. Tao, Appl. Catal. B: Environ. 160–161 (2014) 89–97. [45] Y. Li, S. Wu, L. Huang, H. Xu, R. Zhang, M. Qu, Q. Gao, H. Li, J. Phys. Chem. Solids 76 (2015) 112–119. [46] X. Li, J. Ye, J. Phys. Chem. C 111 (2007) 13109–13116. [47] X. Feng, H. Guo, K. Patel, H. Zhou, X. Lou, Chem. Eng. J. 244 (2014) 327–334. [48] A. Martinez-dela Cruz, U.M. Garcia Perez, Mater. Res. Bull. 45 (2010) 135–141. [49] M. Rezaei, A. Habibi-Yangjeh, Appl. Surf. Sci. 265 (2013) 591–596. [50] M. Shekofteh-Gohari, A. Habibi-Yangjeh, Ceram. Int. 41 (2015) 1467–1476. [51] S.Y. Kim, T.H. Lim, T.S. Chang, C.H. Shin, Catal. Lett. 117 (2007) 112–118. [52] M. Shekofteh-Gohari, A. Habibi-Yangjeh, RSC Adv. 6 (2016) 2402–2413. [53] S.R. Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrode, Plenum, New York, 1980. [54] D. Xu, S. Cao, J. Zhang, B. Cheng, J. Yu, Beilstein J. Nanotechnol. 5 (2014) 658–666. [55] C.C. Chen, W.H. Ma, J.C. Zhao, Chem. Soc. Rev. 39 (2010) 4206–4219. [56] J. Kim, C.W. Lee, W. Choi, Environ. Sci. Technol. 44 (2010) 6849–6854. [57] G. Li, K.H. Wong, X. Zhang, C. Hu, J.C. Yu, R.C.Y. Chan, P.K. Wong, Chemosphere 76 (2009) 1185–1191. [58] J. Jiang, H. Li, L.Z. Zhang, Chem. Eur. J. 18 (2012) 6360–6369. [59] S. Naghizadeh-Alamdari, A. Habibi-Yangjeh, Solid State Sci. 40 (2015) 111–120.

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