Visible-light-driven photocatalysts with highly enhanced

Advanced Powder Technology 28 (2017) 1540–1553

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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Novel magnetically separable g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites: Visible-light-driven photocatalysts with highly enhanced activity Mitra Mousavi, Aziz Habibi-Yangjeh ⇑ Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran

a r t i c l e

i n f o

Article history: Received 30 November 2016 Received in revised form 3 March 2017 Accepted 27 March 2017 Available online 6 April 2017 Keywords: g-C3N4/Fe3O4/Ag3PO4/Co3O4 Photocatalysis Magnetic photocatalyst Visible-light-driven photocatalyst Dye pollutants

a b s t r a c t In this study, a series of novel quaternary g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites were fabricated. The prepared nanocomposites were characterized by XRD, EDX, SEM, TEM, UV-DRS, FT-IR, PL, TG, and VSM methods to gain insight about structure, purity, morphology, optical, thermal, and magnetic properties. Photocatalytic activity of the samples was investigated under visible-light irradiation by degradations of rhodamine B, methylene blue, methyl orange, and phenol as four organic pollutants. The highest photocatalytic degradation efficiency was observed when the sample calcined at 300 °C for 2 h with 20 wt% of Co3O4. The photocatalytic activity of this nanocomposite is almost 16.8, 15.7, 4.6, and 5.1 times higher than those of the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/Co3O4 (20%) samples in photodegradation of rhodamine B, respectively. Finally, on the basis of the energy band positions, the mechanism of enhanced photocatalytic activity was discussed. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction In recent decades, with the huge industrial development and growth of human population, environmental problems have been more and more serious, due to uncontrolled discharge of harmful compounds to our environment. Most of these compounds have considerable stability in the environment [1,2]. Hence, development of effective, environmentally friendly, and low-cost technology for water treatment is one of the major concerns [3,4]. Among various strategies, heterogeneous photocatalytic degradation has attracted much attention as a ‘green’ technology, since it can degrade a wide range of pollutants using sun light [5–7]. The key point in the photocatalytic degradation reaction of pollutants is fabrication of a photocatalyst with high degradation efficiency under the solar energy. Although many kinds of photocatalysts have been fabricated, most of them can be activated under UV irradiation, which only accounts for about 4% of the solar spectrum [8– 12]. A prerequisite for preparation of an effective visible-lightdriven photocatalyst is its ability to absorb considerable amount of the visible light [13–17]. In the past few years ago, it was found that graphitic carbon nitride (g-C3N4), as a metal-free polymeric semiconductor with a ⇑ Corresponding author. Fax: +98 045 33514702.

band gap of 2.70 eV, has great potential for using in photocatalytic processes [18]. However, g-C3N4 has still some drawbacks restricting widespread applications of it in photocatalytic processes. The first shortage is poor absorption ability of it to the solar light due to its moderate energy gap. The second drawback is recombination of photogenerated electron-hole pairs with high rate, leading to poor photocatalytic activity [19–21]. Finally, the third drawback is nonmagnetic behavior of g-C3N4, resulting in some difficulties in separation of it after using in photocatalytic processes. Very recently, several approaches have been employed to enhance photocatalytic activity of g-C3N4, under visible-light irradiation. Combination of g-C3N4 with narrow band gap semiconductors, construction of mesoporous structures, and doping of metal or nonmetal species are of effective strategies employed for increasing photocatalytic activity of pristine g-C3N4 [19–28]. Among these approaches, formation of nanocomposites between g-C3N4 and narrow band gap semiconductors such as CdS, Ag3PO4, BiFeO3, bFeOOH, BiVO4, Sb2S3, Ag2CrO4, and Ag2WO4 demonstrated a great potential to promote the photocatalytic activity of g-C3N4; because by formation of heterostructures between them, generation and separation of the electron–hole pairs can be effectively taken place [29–36]. Very recently, it was found that codeposition of two semiconductors could be effective in enhancing the photocatalytic activity of g-C3N4 in comparison with one semiconductor [37–39].

E-mail address: [email protected] (A. Habibi-Yangjeh). http://dx.doi.org/10.1016/j.apt.2017.03.025 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

M. Mousavi, A. Habibi-Yangjeh / Advanced Powder Technology 28 (2017) 1540–1553

Silver orthophosphate (Ag3PO4) is a semiconductor with narrow band gap of 2.45 eV, which strongly absorbs visible-light irradiation [40]. Hence, this semiconductor has been applied in preparation of different visible-light-driven photocatalysts [40–43]. In addition, Co3O4 is a narrow band gap semiconductor with the characteristics of high thermal and chemical stability, low solubility, interesting electronic, and some magnetic properties [44]. Due to these attractive properties, Co3O4 has been applied as an efficient photocatalyst [44–46]. Magnetic separation of applied photocatalysts from the treated systems using an external magnetic field is an alternative method for separating and recovering particles of catalysts [47]. This approach can efficiently decrease recycling time

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and cost of the photocatalytic processes. Hence, in recent years, magnetically separable photocatalysts have attracted considerable interests from the research community [47,48]. In the present study, magnetically separable highly efficient gC3N4/Fe3O4/Ag3PO4/Co3O4 photocatalysts were prepared, for the first time, via codeposition of Ag3PO4 and Co3O4 semiconductors over g-C3N4/Fe3O4 nanocomposite using ultrasonic method followed by a calcination step. The g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites showed considerable photocatalytic activity for degradation of rhodamine B (RhB) under visible-light irradiation. In addition, trapping experiments of the reactive species showed that O 2 was the main species for the photocatalytic degradation

Scheme 1. Schematic illustration for preparation of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites.

g-C₃N₄

Fe₃O₄

Co₃O₄

Ag₃PO₄

g-C₃₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(30%)

g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(20%)

Intensity (a.u.)



g-C₃N₄/Fe₃O₄/Co₃O₄(20%)

g-C₃N₄/Fe₃O₄/Ag₃PO₄(20%)

g-C₃N₄/Fe₃O₄

g-C₃N₄

10

20

30

40

50

60

70

80

2Theta (deg.) Fig. 1. XRD patterns for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%) and g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites with different weight percents of Co3O4.

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g-C₃₃N₄ g-C₃N₄/Fe₃O₄ g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(20%)

(a) O Ag Co C N Fe

P

Ag

Fe

Ag Ag

Co

CO

Intensity (a.u.)

N

F e

Fe

Fe

C

N

0

1

2

3

4

5

6

7

8

9

10

Energy (keV)

(c)

(b)

Ag

(f)

(g)

Co

(e)

(d)

Fe

O

(h)

(i)

C

N

P

Fig. 2. (a) EDX spectra for the g-C3N4, g-C3N4/Fe3O4, and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) samples. (b)–(i) EDX mapping of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite.

(a)

(b)

80nm

Fig. 3. (a) SEM image of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite. (b) TEM image of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite.


1543

C—N & C=N g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(20%) g-C₃N₄/Fe₃O₄/Co₃O₄(20%) g-C₃N₄/Fe₃O₄/Ag₃PO₄(20%) g-C₃N₄/Fe₃O₄ g-C₃N₄

3900

3400

2900

2400

1900

1400

900

Fe—O

Transmittance (a.u.)

O—H

Heptazine

Co—O P—O—P

(a)

400

Wavenumber(cm-1) 1.2 g-C₃₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(30%)

(b) 1


Absorbance

0.8



0.6 g-C₃N₄/Fe₃O₄/Co₃O₄(20%)

0.4 g-C₃N₄/Fe₃O₄/Ag₃PO₄(20%)

0.2

g-C₃N₄/Fe₃O₄

g-C₃N₄

0 270

320

370

420

470

520

570

620

670

Wavelength (nm) Fig. 4. (a) FT-IR spectra for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) samples. (b) UV–vis DRS for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 samples.

of RhB. Also the effect of weight percent of Co3O4, calcination time and temperature, and ability of the nanocomposites for degradation of methylene blue (MB), methyl orange (MO), and phenol was investigated. Finally the possible photodegradation mechanism was also elucidated based on energy band positions of g-C3N4, Fe3O4, Ag3PO4, and Co3O4 materials.

(99.9%), sodium hydroxide (98%), cobalt nitrate (Co(NO3)2, 99.5%) and benzoquinone were purchased from Loba Chemie and used without further purification. Sodium phosphate (Na2HPO4.2H2O, 97%) was purchased from Rankem. Hydrochloric acid, 2-propanol, ammonium oxalate, RhB, MB, MO, and absolute ethanol with high quality were obtained from Merck. Deionized water was used through the whole preparation processes.

2. Experimental 2.2. Instruments 2.1. Materials Ferric chloride (FeCl3.6H2O, 99.5%), ferrous chloride (FeCl2.4H2O, 98.0%), ammonia solution (30%), melamine (99.2%), silver nitrate

The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Ka radiation (k = 0.15406 nm), employing scanning rate of 0.04°/sec in the 2h range from 20° to 80°. Surface

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morphology and distribution of particles were studied by LEO 1430VP SEM, using an accelerating voltage of 15 kV. The purity and elemental analysis of the products were obtained by EDX on the same SEM instrument. The TEM investigations were performed by a Philips CM30 instrument with an acceleration voltage of 150 kV. The UV–vis DRS were recorded by a Scinco 4100 apparatus. The FT-IR spectra were obtained by a Perkin Elmer Spectrum RX I apparatus. The PL spectra of the samples were studied using a Perkin Elmer (LS 55) fluorescence spectrophotometer with an excitation wavelength of 425 nm. The conditions were fixed in order to compare the PL intensities. The UV–vis spectra for the degradation reaction were studied using a Cecile 9000 spectrophotometer. Magnetic properties of the samples were obtained using vibrating sample magnetometer (VSM, Meghnatics Kavir Kashan Co., Iran). The pH of solutions was measured using a Metrohm digital pH meter of model 691. The ultrasound radiation was performed using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz).

the main diffraction peaks of g-C3N4, Fe3O4, and Ag3PO4 have no obvious changes compared with that of ternary nanocomposites, indicating that the crystal structure of the samples keeps stable, while the characteristic diffraction peaks of Co3O4 are assigned to the spinel type cubic structure (JCPDS No. 42-1467) [54]. To display purity of the samples, EDX spectra of the samples were also provided and the results are shown in Fig. 2a. As can be seen, the g-C3N4 sample consists of C and N elements. The gC3N4/Fe3O4 nanocomposite is composed of C, N, Fe, and O elements. The characteristic signals of C, N, Fe, O, Ag, P, and Co elements are found for the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite. In addition, other elements or impurities were not found in the fabricated samples, representing purity of the prepared nanocomposites. Furthermore, Fig. 2b–i show EDX mapping of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite, which show the distributions of different elements with different colors. The C, N, Fe, O, Ag, P, and Co elements are all distributed homogeneously in this nanocomposite. These results again

2.3. Preparation of the photocatalysts

(a) 100

Weight Loss (%)

The metal-free g-C3N4 powder was synthesized by heating melamine powder up to 520 °C in a muffle furnace [19]. The required water in this section was degassed by bubbling N2 gas for 20 min. The g-C3N4/Fe3O4 (2:1) nanocomposite, where 2:1 is weight ratio of g-C3N4 to Fe3O4, was prepared according to our previous work [49]. The g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite, where 20% is weight percent of Ag3PO4, was prepared by ultrasonic method [50]. For preparation of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite, where 20% is weight percent of Co3O4, 0.4 g of gC3N4/Fe3O4/Ag3PO4 nanocomposite was dispersed into 150 mL of water by ultrasonic irradiation for 5 min. Then, 0.362 g of cobalt nitrate was added to the suspension and stirred until its water was volatilized completely at 50 °C. The resulting powder was collected and calcined in air at 300 °C for 2 h (see Scheme 1).

120

60

40 g-C₃₃N₄ g-C₃N₄/Fe₃O₄ g-C₃N₄/Fe₃O₄/Ag₃PO₄(20%) g-C₃N₄/Fe₃O₄/Co₃O₄(20%) g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(20%)

20

2.4. Photocatalysis experiments

0 0

Photocatalysis experiments were performed at 25 °C under a LED lamp of 50 W, as visible-light source. Concentrations of RhB, MB, MO, and phenol were 1 105, 1.3 105, 1.05 105, 5 105 M, respectively. Other conditions were described in detail in our previous work [50].

100

200

300

400

500

600

700

Temperature (ºC) 60 After

40

Magnetization (emu/g)

3. Results and discussion The XRD analysis was used to identify the crystalline phase of the as-prepared samples and the patterns are shown in Fig. 1. It was observed that in all samples, g-C3N4 maintains its crystallinity by displaying two typical characteristic peaks at 13.1° and at 27.4°. The diffraction peaks at 2h = 27.4° and 13.1° are ascribed to (0 0 2) and (1 0 0) planes of g-C3N4 (JCPDS No. 87-1526), corresponding to the interplanar stacking peaks of aromatic systems and the interlayer structural packing, respectively [19]. For the g-C3N4/Fe3O4 nanocomposite, all the diffraction peaks are indexed to the standard XRD patterns of g-C3N4 and face centered cubic Fe3O4, without any impurity peaks, indicating high purity of the sample [51]. Hence, it is confirmed that during preparation of the nanocomposite, especially calcination step, particles of Fe3O4 are stable and they do not convert to Fe2O3. There are similar reports about stability of Fe3O4 around calcination temperature of 300 °C [52,53]. The XRD pattern of the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite exhibits diffraction peaks corresponding to g-C3N4, face centered cubic Fe3O4, and cubic Ag3PO4 (JCPDS No. 06-0505) [40]. In the g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites,

80

(b)

Magnetic separation

reaction

20

0 -10000

-5000

0

5000

10000

-20

-40 Fe₃O₄ g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(20%)

-60

Applied Field (Oe) Fig. 5. (a) TG spectra for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), gC3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) samples. (b) VSM curves for the Fe3O4 and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) samples.


indicate that the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite has been successfully prepared. Weight percents of C, N, Fe, O, Ag, P, and Co elements were estimated to be 16.6%, 26.2%, 15.4%, 13.7%, 12.4%, 1.10%, and 14.7%, respectively, which are close to the theoretical values. Fig. 3a displays SEM image of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite. Obviously, deposited particles of Fe3O4, Ag3PO4, and Co3O4 are observed over the surface of g-C3N4. Moreover, deposition of the counterparts over g-C3N4 was also displayed using TEM instrument. Fig. 3b shows TEM image of the g-C3N4/ Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite. As can be seen, the deposited particles are observed over g-C3N4 sheets. The particles of Fe3O4, Ag3PO4, and Co3O4 are darker than g-C3N4, due to the presence of heavier atoms relative to the g-C3N4. To further confirm formation of the nanocomposites, FT-IR spectra of the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) samples were provided and they are shown in Fig. 4a. For the pristine g-C3N4, the broad band at 3000–3300 cm1 is due to the stretching modes of terminal NH groups at the defect sites of the aromatic rings [19]. The peaks located between 1200 cm1 and 1650 cm1 are attributed to the typical stretching modes of the CAN heterocycles. The peak at 809 cm1 is the characteristic breathing mode of triazine units [19]. For the Fe3O4 containing samples, two characteristic peaks at 620 and 430 cm1 are related to stretching vibrations of FeAO bond [55]. The typical PAO 1 stretching vibrations of PO3 could 4 group at 556 and 1013 cm be observed in FT-IR spectrum of the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite besides the absorption peaks of g-C3N4 and Fe3O4 counterparts [56]. In addition, besides the characteristic peaks corresponding to g-C3N4, Fe3O4, and Ag3PO4 counterparts in the gC3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite, the strong peaks at 657 and 556 cm1 can be observed, which attributed to the characteristic peaks of spinel Co3O4. The former peak at 657 cm1 is

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belonged to the stretching vibration mode of MAO in which M is tetrahedrally coordinated Co2+. While the band at 556 cm1 can be assigned to the MAO in which M is octahedrally coordinated Co3+ [54]. Energy band structure of semiconductors has a major role in determining their photocatalytic activities. To view ability of the as-prepared samples for absorbing visible-light irradiation and production of electron-hole pairs under this irradiation, UV–vis spectra of the samples were provided using UV–vis DRS technique. As can be seen in Fig. 4b, g-C3N4 has an absorption edge at about 460 nm, corresponding to its intrinsic band gap [19]. Compared with the g-C3N4 and g-C3N4/Fe3O4 samples, all of the hybrid samples shows considerable red-shifts in their spectra. This red shift is attributed to formation of heterojunction between g-C3N4, as medium band gap semiconductor and Ag3PO4 and Co3O4, as narrow band gap semiconductors. Hence, by absorbing considerable amounts of the visible light, much more electron-hole pairs are produced in the g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites under visible-light irradiation, which is beneficial for increasing photocatalytic activity. Thermogravimetric analysis was used to determine the thermal stability and weight percent of g-C3N4 in the samples. Fig. 5a shows TG profiles of the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/ Co3O4 (20%) samples. Although the pure g-C3N4 exhibits well stability in air below 520 °C, the combustion process of g-C3N4 is usually accelerated when it was coupled with other semiconductors [57]. In the case of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite, combustion of the sample starts from 370 °C. In addition, the real contents of g-C3N4 in the samples were calculated using TG data. The weight percents of g-C3N4 in the g-C3N4/ Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposites were determined to be 73.3%, 60.4%, 54.7%, and 42.3%, respectively.

1 Dark Photolysis

0.8

g-C₃₃N₄

Absorbance

Fe₃O₄

0.6

g-C₃N₄/Fe₃O₄ g-C₃N₄/Fe₃O₄/Ag₃PO₄(20%) g-C₃N₄/Fe₃O₄/Co₃O₄(20%)

0.4

g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(10%) g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(15%)

0.2

g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(20%) g-C₃N₄/Fe₃O₄/Ag₃PO₄/Co₃O₄(30%)

-60

0

0

60

120

180

240

300

360

Irradiation time (min) Fig. 6. Photodegradation of RhB over the g-C3N4, Fe3O4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites with different weight percents of Co3O4.

1546


Magnetic properties of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite were characterized by a VSM at room temperature and the results along with that of the Fe3O4 sample with an applied magnetic field sweeping from 8.5 to 8.5 k Oe are shown in Fig. 5b. For the Fe3O4 sample, magnetic S-shaped curve was observed, which indicates that there is no remaining magnetization when the external magnetic field is removed, confirming that this sample exhibits superparamagnetic behavior [58]. The magnetization hysteresis loop of g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite

(a)

kobs (min-1) × 10-4

200

150

100

50

0

g-C₃₃N₄

(b)

g-C₃N₄/Fe₃O₄ g-C₃N₄/Fe₃O₄/Ag₃PO₄(20%) g-C₃N₄/Fe₃O₄/Co₃O₄(20%)

Intensity (a.u.)


650

675

700

725

750

775

800

Wavelength (nm) Fig. 7. (a) The degradation rate constant of RhB over the g-C3N4, Fe3O4, g-C3N4/ Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/ Ag3PO4/Co3O4 nanocomposites with different weight percents of Co3O4. (b) PL spectra for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/ Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) samples.

Table 1 The values of v, CB, and VB potentials for g-C3N4, Fe3O4, Ag3PO4, and Co3O4. Compound

v

Eg (eV)

CB (eV)

VB (eV)

g-C3N4 Fe3O4 Ag3PO4 Co3O4

4.73 5.76 5.96 5.90

2.70 0.10 2.45 1.90

1.13 +1.21 +0.24 +0.45

+1.57 +1.31 +2.69 +2.35

revealed a hysteresis loop with small coercivity, which is characteristic of ferromagnetic materials. Similar behaviors have been reported for Co3O4 containing composites [45,59]. In addition, saturation magnetization of the nanocomposite reached 13.5 emu g1, which is lower than that of the pure Fe3O4 (55.5 emu g1). This decrease is ascribed to the presence of nonmagnetic g-C3N4 and Ag3PO4 materials along with the Fe3O4 nanoparticles. Inset of the figure displays separation of the treated suspension by using an external magnet. For this purpose, the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite was firstly dispersed in a RhB solution. After that, the degradation reaction was carried out under the light irradiation. Finally, the nanocomposite was completely separated from the reaction medium by applying an external magnetic field, leaving a colorless solution (inset in Fig. 5b). Photocatalytic activity of the prepared photocatalysts was evaluated by degradation of RhB, as a model dye pollutant, under visible-light irradiation. To ensure fully adsorption of RhB over the samples, adsorption processes for 60 min were performed before the photocatalytic reactions. The photodegradation processes were recorded by the temporal evolution of RhB absorption at 553 nm in presence of the photocatalysts and the results are displayed in Fig. 6. The blank test shows that aqueous solution of RhB is slightly degraded in the absence of photocatalysts. As it is evident, RhB is degraded over the g-C3N4, Fe3O4, and g-C3N4/Fe3O4 samples with moderate rate. After the light irradiation for 360 min, only 38.7%, 20.2%, and 43.3% of RhB are degraded over the g-C3N4, Fe3O4, and g-C3N4/Fe3O4 samples, respectively. However, the photocatalytic performance is significantly enhanced after depositing Ag3PO4 and Co3O4 semiconductors over the gC3N4/Fe3O4 sample, which indicates that silver phosphate and cobalt oxides play an important role in the enhancement of RhB degradation. From our previous work, it was found that the highest photocatalytic activity for the ternary g-C3N4/Fe3O4/Ag3PO4 nanocomposites was observed for the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite and by further increasing of Ag3PO4, the degradation reaction rate starts to decrease [50]. It is clear that over the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite, 85.7% of RhB is degraded within 360 min of the visible-light irradiation. However, as can be seen, by depositing Co3O4 over the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite, the photocatalytic activity notably increased and the best activity was observed for the g-C3N4/Fe3O4/Ag3PO4/ Co3O4 (20%) nanocomposite. It is clear that after the light irradiation for 150 min, RhB is almost completely degraded over this quaternary nanocomposite. Fig. S1(a–d) shows UV–vis spectra of RhB solutions during the photocatalytic degradation reactions over the g-C3N4, g-C3N4/ Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/ Fe3O4/Ag3PO4/Co3O4 (20%) samples. It is evident that during the photodegradation reactions, intensities of the absorption peak of RhB at 553 nm diminish gradually under visible-light irradiation. As can be seen, molecules of RhB nearly completely degraded within 150 min over g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%), whereas about 31.0%, 48.8%, and 47.5% of the dye were degraded over the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/Co3O4 (20%) samples, respectively. Thus, the g-C3N4/Fe3O4/Ag3PO4/


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Fig. 8. A plausible diagram for separation of electron-hole pairs in the g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites.

Co3O4 (20%) nanocomposite showed considerable activity in the degradation reaction. In addition, it is clear that during the degradation reaction, there are not any blue shift for these spectra. Hence, it was concluded that the degradation reaction of RhB over these samples takes place by ring-opening mechanism rather than stepwise demethylation mechanism [60]. The degradation rate constants of RhB were calculated using pseudo-first-order kinetics and the results are shown in Fig. 7a. It is evident that the degradation rate constants of RhB over the gC3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites are much higher than those of the other samples and the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite shows the superior activity. The degradation rate constants of RhB over the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/ Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/ Ag3PO4/Co3O4 (20%) samples are 12.2 104, 13.1 104, 45.1 104, 41.2 104, and 205 104 min1, respectively. Thus, degradation rate constant of RhB over the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite is nearly 16.8 times higher than that of the pure g-C3N4, 15.7 times higher than that of the gC3N4/Fe3O4, 4.6 times higher than that of the g-C3N4/Fe3O4/Ag3PO4 (20%), and 5.1 times higher than that of the g-C3N4/Fe3O4/Co3O4 (20%) samples for degradation of RhB. By further increasing Co3O4 content to 30 wt%, the photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposite presents obvious decreasing trend. To understand the higher photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite relative to the other samples, the PL analysis was applied to estimate the mitigation, transfer, and recombination processes of the photogenerated electron-hole pairs in the prepared photocatalysts. It can be observed that the emission intensity of the g-C3N4/Fe3O4/Ag3PO4/ Co3O4 (20%) nanocomposite is much weaker than those of the gC3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/

Fe3O4/Co3O4 (20%) samples. Hence, it is concluded that Ag3PO4 and Co3O4 semiconductors have key role in separation of the charge carriers in g-C3N4. As a result, strong interactions have formed between g-C3N4, Ag3PO4, and Co3O4 counterparts of the nanocomposite, leading to inhibition direct recombination of the photogenerated electrons and holes. Photocatalytic activity mainly depends on separation of electron–hole pairs. On the other hand, this separation is strictly related to the band edge positions of semiconductors in the photocatalyst [5]. In order to insight separation of the photogenerated charge carriers in the g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites, the conduction band (CB) and valence band (VB) potentials of the counterparts at the point of zero charge were estimated by the following empirical equations:

ECB ¼ v Ee 0:5Eg

ð1Þ

EVB ¼ ECB þ Eg

ð2Þ

where EVB and ECB are the valence and conduction band edge potentials, respectively; v is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, and defined as the arithmetic mean of the atomic electro affinity and the first ionization energy; Ee is the energy of free electrons on the hydrogen scale; Eg is the band gap energy of the semiconductor [61]. Calculated values of the v, CB, and VB for g-C3N4, Fe3O4, Ag3PO4, and Co3O4 semiconductors are listed in Table 1. As can be seen from Fig. 8, when the g-C3N4/Fe3O4/Ag3PO4/Co3O4 photocatalyst was irradiated by visible light, g-C3N4, Ag3PO4, and Co3O4 semiconductors are excited to generate electron and hole pairs. However, ECB of g-C3N4 is located at 1.13 eV, which is more negative than the CB potentials

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(a)

kobs (min-1) × 10-4

200

150

100

50

0

100

200

300

400

Calcination temp (°C)

(c)

(b)

Calcinated at 400 °C

(d)

Intensity (a.u.)

Calcinated at 300 °C

650

675

700

725

750

775

800

Wavelength (nm) Fig. 9. (a) The degradation rate constants of RhB over the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite prepared at different calcination temperature. (b and c) SEM images, and (d) PL spectra for the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite calcined at 300 and 400 °C.

of Ag3PO4 and Co3O4. Thus, the photogenerated electrons of g-C3N4 are directly injected into the CB of Ag3PO4 and Co3O4 through the heterojunctions. In addition, some of the photogenerated electrons

on the CB of g-C3N4 react with O2 to form O 2 , because CB potential 0 of g-C3N4 is more negative than the potential of O2/O 2 (E (O2/ O2 ) = 0.33 eV/NHE) [62]. Meanwhile, the CB edge potentials of Ag3PO4


200

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(a)

kobs (min-1) × 10-4

150

100

50

0 1

2

3

4

Calcination time (h)

(b)

(c)

Calcinated for 4 h

(d)

Intensity (a.u.)

Calcinated for 2 h

650

675

700

725

750

775

800

Wavelength (nm) Fig. 10. (a) The degradation rate constants of RhB over the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite calcined at 300 °C for different times. (b and c) SEM images, and (d) PL spectra for the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite calcined at 300 °C for 2 and 4 h.

and Co3O4 semiconductors (+0.24 and +0.45 eV vs. NHE) are more positive than the standard potential of E0(O2/O 2 ), suggesting that electrons at the CB of Ag3PO4 and Co3O4 cannot reduce O2 to form

O 2 species. However, the accumulated electrons in the CB of Ag3PO4 and Co3O4 can be transferred to O2 to produce H2O2 molecules; because the CB potentials of Ag3PO4 and Co3O4 are more

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1 Run1 Run2 Run3

0.8

Absorbance

Run4

0.6

0.4

0.2

0

0

100

200

300

400

500

600

700

Irradiation time (min) Fig. 11. Reusability of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite.

negative than E0(O2/H2O2) (+0.695 eV vs. NHE) [63]. After that, produced H2O2 molecules react with electrons in succession to produce OH species. On the other hand, EVB level of g-C3N4 is located at +1.57 eV, which is negative than the VB potentials of Ag3PO4 and Co3O4 counterparts. As depicted in Fig. 8, the photogenerated holes over Ag3PO4 and Co3O4 semiconductors are injected into the VB of g-C3N4 and they are collected over g-C3N4. After that, the reactive species react with the pollutant to degrade it to different products. In addition, Fe3O4 particles also contribute in separation of the charge carriers. The photoinduced electrons can easily transfer from the CB of g-C3N4 to the CB of Fe3O4, because the CB level of Fe3O4 is lower than that of g-C3N4 [64]. The photogenerated electrons in the CB of g-C3N4 are captured by Fe3+ ions, resulting in formation of Fe2+ ions, which subsequently oxidized by adsorbed oxygen to produce Fe3+ ions in the next step. It is well known that the role of the reactive species in photocatalytic degradation reactions depends on the type of photocatalysts [5]. Therefore, different scavenges were introduced into the reaction system to determine the relative roles of the reactive species. In the present work, 2-propanol (2-PrOH), ammonium oxalate (AO), and benzoquinone (BQ) were used as the hydroxyl radical (OH), hole (h+), and superoxide ion radical (O 2 ) scavengers, respectively. If the species has an important role in the photocatalytic degradation of RhB, the degradation rate constant would be greatly decreased in the presence of the related scavenger. For this purpose, the degradation reactions of RhB over the g-C3N4/ Fe3O4 and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposites were carried out and the results are depicted in Fig. S2. In the case of the g-C3N4/Fe3O4 nanocomposite, the degradation rate constant significantly decreased after the addition of BQ and 2-PrOH in the reac tion system, which suggests that O 2 and OH are the main active species having considerable role in the photocatalytic degradation reaction. However, for the degradation reaction over the g-C3N4/ Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite, the inhibition degree follows the order BQ > AO > 2-PrOH, revealing that there is a change in role of the reactive species during the degradation reactions over the g-C3N4/Fe3O4 and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposites. Fig. 9a shows the photocatalytic activities of the g-C3N4/Fe3O4/ Ag3PO4/Co3O4 (20%) nanocomposite calcined at different tempera-

tures under visible-light irradiation. Obviously, the photocatalytic activity increases monotonously as the calcination temperature rises from 100 to 300 °C. However, when the calcination temperature was reached to 400 °C, the photocatalytic activity demised. Furthermore, morphology of the nanocomposite after calcinations at 300 and 400 °C were provided (Fig. 9b, c). As can be seen, after calcination at 400 °C, the particles are aggregated and bigger particles of Ag3PO4 and Co3O4 were formed over the surface of g-C3N4 sheets. The PL spectra of the nanocomposite calcined at 300 and 400 °C are also exhibited in Fig. 9d. The emission peak intensity of the sample calcined at 400 °C is much stronger than that of the sample calcined at 300 °C. A stronger intensity of emission peak indicates that the separation of the charge carriers cannot efficiently take place, resulting in decreased photocatalytic activity. On the basis of the above analysis, 300 °C was as the optimal calcination temperature in our experiments. Generally, the time required for preparation of photocatalysts is a crucial parameter affecting the photocatalytic activity, due to its effects on crystallinity and size of particles. For this reason, the effect of calcination time on photocatalytic activity of the nanocomposite was evaluated and the results are shown in Fig. 10a. It is evident that the photodegradation activity first increases and then decreases with rising calcination time and the nanocomposite calcined for 2 h displays the highest activity. The surface morphology of the nanocomposite calcined at 300 °C for 2 and 4 h are shown in Fig. 10b and c. As can be seen, aggregation of the deposited particles over g-C3N4 increases with prolonging the calcination. In order to observe separation efficiency of the photogenerated charge carriers, PL spectra of these samples were provided and they are displayed in Fig. 10d. It is clear that the intensity of PL spectrum of the nanocomposite calcined for 4 h increased relative to the sample calcined for 2 h. Hence, it is concluded that with increasing the calcination time, separation of the electron-hole pairs was diminished, leading to decrease of the photocatalytic activity. In other words, with increasing the calcination time, aggregation of the deposited semiconductors over gC3N4 was increased, resulting in destruction of heterojunctions between the counterparts. As a result, suppression of electronhole pairs from recombinations was decreased. Therefore, the enhanced activity of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites could be ascribed to the better interfacial contact between gC3N4, Ag3PO4, and Co3O4 counterparts, which resulted in a significant synergistic effect between g-C3N4, Ag3PO4, and Co3O4 semiconductors for efficiently separation of the charge carriers. The recycling runs for the photodegradation of RhB using the gC3N4/Fe3O4/Ag3PO4/Co3O4 (20%) photocatalyst were performed to evaluate its stability. The samples were washed with ethanol and dried after each recycling experiment. As shown in Fig. 11, there are slight photocatalyst deactivation in these experiments. This decrease can be ascribed to slight loss of the deposited particles over g-C3N4. Therefore, the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite can be regarded as a photocatalyst with remarkable stability in photocatalytic processes. Finally, photodegradation reaction of three more dye pollutants (MB, MO) over the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite were investigated under visible-light irradiation and results along with RhB are shown in Fig. 12. As can be seen, photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite is very higher than those of the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/Co3O4 (20%) samples in degradation of these pollutants. For example, photocatalytic activity of the quaternary nanocomposite is about 16.8, 19.6, 51.6, and 7.5-folds higher than that of the g-C3N4 in degradations of RhB, MB, MO, and phenol, respectively. In addition, photocatalytic activity of the g-C3N4/ Fe3O4/Ag3PO4/Co3O4 (20%) nanocomposite in degradations of MO is nearly 51.5, 15.7, and 6-folds higher than those of the g-C3N4,


250

1551

140

(a)

(b) 120

200

kobs (min-1) × 10-4

kobs (min-1) × 10-4

100

150

100

50

80 60 40 20 0

0

100

(c)

20

(d)

kobs (min-1) × 10-4

kobs (min-1) × 10-4

80 15

60

40

20

0

10

5

0

Fig. 12. The degradation rate constants of RhB, MB, MO, and phenol over the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/Co3O4 (20%), and g-C3N4/Fe3O4/Ag3PO4/Co3O4 (20%) samples under visible-light irradiation.

g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/Co3O4 (20%) samples, respectively. These results indicate that the quaternary nanocomposite has excellent photocatalytic activity in degradations of different pollutants under visible-light irradiation and coupling of g-C3N4, Ag3PO4, and Co3O4 has remarkable effect on the photocatalytic activity and coupling of Fe3O4 to the photocatalyst gives them magnetically separability. 4. Conclusions In summary, we successfully synthesized g-C3N4/Fe3O4/Ag3PO4/ Co3O4 nanocomposites, as novel magnetically separable visible-

light-driven photocatalysts. The structural characterizations showed that Fe3O4, Ag3PO4, and Co3O4 particles have been loaded over the surface of g-C3N4 sheets. Photocatalytic activity of the prepared samples was investigated by degradations of four different pollutants and it was found that photocatalytic activity of the quaternary nanocomposite is about 16.8, 19.6, 51.6-folds higher than that of the g-C3N4 in degradations of RhB, MB, MO, and phenol pollutants, respectively. Furthermore, the nanocomposite calcined at 300 °C for 2 h displayed the highest photocatalytic activity. An overall mechanism for the enhanced activity was proposed based on the band energies. The greatly enhanced photocatalytic activity was attributed to considerable harvesting of visible light and

1552


suppression of the charge carriers from recombination process. Therefore, these g-C3N4/Fe3O4/Ag3PO4/Co3O4 nanocomposites are promising candidate for waste water treatment and degradation of organic pollutants under visible-light irradiation.

[22] [23]

Acknowledgement [24]

The authors wish to acknowledge University of Mohaghegh Ardabili – Iran, for financial support of this work.

[25]

Appendix A. Supplementary material [26]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apt.2017.03.025.

[27]

References

[28]

[1] W. Xiao Ping, S. Dian Chao, Y. Tan Dong, Climate change and global cycling of persistent organic pollutants: a critical review, Sci. China 59 (2016) 1899– 1911. [2] E. Brillas, C.A. Martínez-Huitle, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review, Appl. Catal. B: Environ. 166–167 (2015) 603–643. [3] C.R. Holkar, A.J. Jadhav, D.V. Pinjari, N.M. Mahamuni, A.B. Pandit, A critical review on textile wastewater treatments: possible approaches, J. Environ. Manage. 182 (2016) 351–366. [4] B. Bethi, S.H. Sonawane, B.A. Bhanvase, S.P. Gumfekar, Nanomaterials-based advanced oxidation processes for wastewater treatment: a review, Chem. Eng. Process. 109 (2016) 178–189. [5] S. Dong, J. Feng, M. Fan, Y. Pi, L. Hu, X. Han, M. Liu, J. Sun, J. Sun, Recent developments in heterogeneous photocatalytic water treatment using visible light responsive photocatalysts: a review, RSC Adv. 5 (2015) 14610–14630. [6] X. Li, J. Yu, M. Jaroniec, Hierarchical photocatalysts, Chem. Soc. Rev. 45 (2016) 2603–2636. [7] P.A.K. Reddy, P.V.L. Reddy, E. Kwon, K.-H. Kim, T. Akter, S. Kalagara, Recent advances in photocatalytic treatment of pollutants in aqueous media, Environ. Int. 91 (2016) 94–103. [8] S.G. Kumar, K.S.R. Koteswara Rao, Zinc oxide based photocatalysis: tailoring surface bulk structure and related interfacial charge carrier dynamics for better environmental applications, RSC Adv. 5 (2015) 3306–3351. [9] M. Rahimi-Nasrabadi, M. Behpour, A. Sobhani-Nasab, M. Rangraz Jeddy, Nanocrystalline Ce-doped copper ferrite: synthesis, characterization, and its photocatalyst application, J. Mater. Sci: Mater. Electron. 27 (2016) 11691– 11697. [10] F. Ahmadi, M. Rahimi-Nasrabadi, A. Fosooni, M. Daneshmand, Synthesis and application of CoWO4 nanoparticles for degradation of methyl orange, J. Mater. Sci: Mater. Electron. 27 (2016) 9514–9519. [11] K.M. Lee, C.W. Lai, K.S. Ngai, J.C. Juan, Recent developments of zinc oxide based photocatalyst in water treatment technology: a review, Water Res. 88 (2016) 428–448. [12] M. Rahimi-Nasrabadi, F. Ahmadi, A. Fosooni, Influence of capping agents additives on morphology of CeVO4 nanoparticles and study of their photocatalytic properties, J. Mater. Sci: Mater. Electron. 28 (2017) 537–542. [13] S.M. Pourmortazavi, M. Rahimi-Nasrabadi, M. Khalilian-Shalamzari, M.M. Zahedi, S.S. Hajimirsadeghi, I. Omrani, Synthesis, structure characterization and catalytic activity of nickel tungstate nanoparticles, Appl. Surf. Sci. 263 (2012) 745–752. [14] N. Wang, Y. Zhou, C. Chen, L. Cheng, H. Ding, A g-C3N4 supported graphene oxide/Ag3PO4 composite with remarkably enhanced photocatalytic activity under visible light, Catal. Commun. 73 (2016) 74–79. [15] F. Ahmadi, M. Rahimi-Nasrabad, A. Fosooni, M.H. Daneshm, Synthesis and application of CoWO4 nanoparticles for degradation of methyl orange, J Mater. Sci: Mater. Electron. 27 (2016) 9514–9519. [16] K. Adib, M. Rahimi-Nasrabadi, Z. Rezvani, S.M. Pourmortazavi, F. Ahmadi, H.R. Naderi, M.R. Ganjali, Facile chemical synthesis of cobalt tungstates nanoparticles as high performance supercapacitor, J. Mater. Sci.: Mater. Electron. 27 (2016) 4541–4550. [17] L. Zhou, W. Zhang, L. Chen, H. Deng, Z-scheme mechanism of photogenerated carriers for hybrid photocatalyst Ag3PO4/g-C3N4 in degradation of sulfamethoxazole, J. Colloid Interf. Sci. 487 (2017) 410–417. [18] S. Cao, J. Low, J. Yu, M. Jaroniec, Polymeric photocatalysts based on graphitic carbon nitride, Adv. Mater. 27 (2015) 2150–2176. [19] L. Zhou, H. Zhang, H. Sun, S. Liu, M.O. Tade, S. Wang, W. Jin, Recent advances in non-metal modification of graphitic carbon nitride for photocatalysis: a historic review, Catal. Sci. Technol. 6 (2016) 7002–7023. [20] W. Jiang, W. Luo, J. Wang, M. Zhang, Y. Zhu, Enhancement of catalytic activity and oxidative ability for graphitic carbon nitride, J. Photochem. Photobiol. C: Photochem. Rev. 28 (2016) 87–115. [21] G. Mamba, A.K. Mishra, Graphitic carbon nitride (g-C3N4) nanocomposites: a new and exciting generation of visible light driven photocatalysts for

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45] [46] [47]

[48]

environmental pollution remediation, Appl. Catal. B: Environ. 198 (2016) 347–377. Z. Zhao, Y. Sun, F. Dong, Graphitic carbon nitride based nanocomposites: a review, Nanoscale 7 (2015) 15–37. J. Wen, X. Li, H. Li, S. Ma, K. He, Y. Xu, Y. Fang, W. Liu, Q. Gao, Enhanced visiblelight H2 evolution of g-C3N4 photocatalysts via the synergetic effect of amorphous NiS and cheap metal-free carbon black nanoparticles as cocatalysts, Appl. Surf. Sci. 358 (2015) 204–212. Y. Xu, S. Huang, M. Xie, Y. Li, H. Xu, L. Huang, Q. Zhang, H. Li, Magnetically separable Fe2O3/g-C3N4 catalyst with enhanced photocatalytic activity, RSC Adv. 5 (2015) 95727–95735. F. Dong, Z. Zhao, Y. Sun, Y. Zhang, S. Yan, Z. Wu, An advanced semimetalorganic Bi Spheres/g-C3N4 nanohybrid with SPR enhanced visible-light photocatalytic performance for NO purification, Environ. Sci. Technol. 49 (2015) 12432–12440. W. Wan, S. Yu, F. Dong, Q. Zhang, Y. Zhou, Efficient C3N4/graphene oxide aerogel macroscopic visible-light photocatalyst, J. Mater. Chem. A 4 (2016) 7823–7829. Z. Ni, F. Dong, H. Huang, Y. Zhang, New insights into how Pd nanoparticles influence the photocatalytic oxidation and reduction ability of g-C3N4 nanosheets, Catal. Sci. Technol. 6 (2016) 6448–6458. T. Xiong, W. Cen, Y. Zhang, F. Dong, Bridging the g-C3N4 interlayers for enhanced photocatalysis, ACS Catal. 6 (2016) 2462–2472. F. Jiang, T. Yan, H. Chen, A. Sun, C. Xu, X. Wang, A g-C3N4–CdS composite catalyst with high visible-light-driven catalytic activity and photostability for methylene blue degradation, Appl. Surf. Sci. 295 (2014) 164–172. P. He, L. Song, S. Zhang, X. Wu, Q. Wei, Synthesis of g-C3N4/Ag3PO4 heterojunction with enhanced photocatalytic performance, Mater. Res. Bull. 51 (2014) 432–437. X. Wang, W. Mao, J. Zhang, Y. Han, C. Quan, Q. Zhang, T. Yang, J. Yang, X. Li, W. Huang, Facile fabrication of highly efficient g-C3N4/BiFeO3 nanocomposites with enhanced visible light photocatalytic activities, J. Colloid Interf. Sci. 448 (2015) 17–23. Y. Zheng, Z. Zhang, C. Li, Beta-FeOOH-supported graphitic carbon nitride as an efficient visible light photocatalyst, J. Mol. Catal. A: Chem. 423 (2016) 463– 471. J. Zhao, J. Yan, H. Jia, S. Zhong, X. Zhang, L. Xu, BiVO4/g-C3N4 composite visiblelight photocatalyst for effective elimination of aqueous organic pollutants, J. Mol. Catal. A: Chem. 424 (2016) 162–170. H. Wang, X. Yuan, H. Wang, X. Chen, Z. Wu, L. Jiang, W. Xiong, G. Zeng, Facile synthesis of Sb2S3/ultrathin g-C3N4 sheets heterostructures embedded with gC3N4 quantum dots with enhanced NIR-light photocatalytic performance, Appl. Catal. B: Environ. 193 (2016) 36–46. J. Luo, X. Zhou, L. Ma, X. Xu, Rational construction of Z-scheme Ag2CrO4/g-C3N4 composites with enhanced visible-light photocatalytic activity, Appl. Surf. Sci. 390 (2016) 357–367. Y. Li, R. Jin, X. Fang, Y. Yang, M. Yang, X. Liu, Y. Xing, S. Song, In situ loading of Ag2WO4 on ultrathin g-C3N4 nanosheets with highly enhanced photocatalytic performance, J. Hazard. Mater. 313 (2016) 219–228. A. Akhundi, A. Habibi-Yangjeh, Ternary g-C3N4/ZnO/AgCl nanocomposites: synergistic collaboration on visible-light-driven activity in photodegradation of an organic pollutant, Appl. Surf. Sci. 358 (2015) 261–269. X. Rong, F. Qiu, Z. Jiang, J. Rong, J. Pan, T. Zhang, D. Yang, Preparation of ternary combined ZnO-Ag2O/porousg-C3N4 composite photocatalyst and enhanced visible-light photocatalytic activity for degradation of ciprofloxacin, Chem. Eng. Res. Design 111 (2016) 253–261. F. Cheng, H. Yin, Q. Xiang, Low-temperature solid-state preparation of ternary CdS/g-C3N4/CuS nanocomposites for enhanced visible-light photocatalytic H2production activity, Appl. Surf. Sci. 391 (2017) 432–439. X. Chen, Y. Dai, X. Wang, Methods and mechanism for improvement of photocatalytic activity and stability of Ag3PO4: a review, J. Alloys Compd. 649 (2015) 910–932. N. Ma, Y. Qiu, Y. Zhang, H. Liu, Y. Yang, J. Wang, X. Li, C. Cui, Reduced graphene oxide enwrapped pinecone-liked Ag3PO4/TiO2 composites with enhanced photocatalytic activity and stability under visible light, J. Alloys Compd. 648 (2015) 818–825. J. Li, H. Yuan, Z. Zhu, In situ growth of Ag3PO4 on N-BiPO4 nanorod: a core–shell heterostructure for high performance photocatalyst, J. Colloid Interf. Sci. 462 (2016) 382–388. J. Lu, Y. Wang, F. Liu, L. Zhang, S. Chai, Fabrication of a direct Z-scheme type WO3/Ag3PO4 composite photocatalyst with enhanced visible-light photocatalytic performances, Appl. Surf. Sci. 393 (2017) 180–190. C. Han, L. Ge, C. Chen, Y. Li, X. Xiao, Y. Zhang, L. Guo, Novel visible light induced Co3O4-g-C3N4 heterojunction photocatalysts for efficient degradation of methyl orange, Appl. Catal. B: Environ. 147 (2014) 546–553. Z. Bin, L. Hui, Three-dimensional porous graphene-Co3O4 nanocomposites for high performance photocatalysts, Appl. Surf. Sci. 357 (2015) 439–444. X. Hao, Z. Jin, G. Lu, Enhanced surface electron transfer with the aid of methyl viologen on the Co3O4-g-C3N4 photocatalyst, Chem. Lett. 45 (2016) 116–118. G. Mamba, A. Mishra, Advances in magnetically separable photocatalysts: smart, recyclable materials for water pollution mitigation, Catalysts 6 (2016) 79. J. Gómez-Pastora, S. Dominguez, E. Bringas, M.J. Rivero, I. Ortiz, D.D. Dionysiou, Review and perspectives on the use of magnetic nanophotocatalysts (MNPCs) in water treatment, Chem. Eng. J. (2016), http://dx.doi.org/10.1016/j. cej.2016.04.140.

M. Mousavi, A. Habibi-Yangjeh / Advanced Powder Technology 28 (2017) 1540–1553 [49] M. Mousavi, A. Habibi-Yangjeh, Magnetically separable ternary g-C3N4/Fe3O4/ BiOI nanocomposites: novel visible-light-driven photocatalysts based on graphitic carbon Nitride, J. Colloid. Interf. Sci. 465 (2016) 83–92. [50] M. Mousavi, A. Habibi-Yangjeh, M. Abitorabi, Fabrication of novel magnetically separable nanocomposites using graphitic carbon nitride, silver phosphate and silver chloride and their applications in photocatalytic removal of different pollutants using visible-light irradiation, J. Colloid Interf. Sci. 480 (2016) 218– 231. [51] T. Wu, H. Pan, R. Chen, D. Luo, H. Zhang, Y. Shen, B. Lu, J. Huang, Y. Li, L. Wang, Enhanced photoluminescence of Fe3O4@Y2O3:Eu3+ bifunctional nanoparticles by the Gd3+ co-doping, J. Alloys Compd. 666 (2016) 507–512. [52] B. Wu, H. Zhang, C. Chen, S. Lin, N. Zheng, Interfacial activation of catalytically inert Au (6.7 nm)–Fe3O4 dumbbell nanoparticles for CO oxidation, Nano Res 2 (2009) 975–983. [53] B. Lim, J. Jin, J. Yoo, S.Y. Han, K. Kim, S. Kang, N. Park, S.M. Lee, H.J. Kim, S.U. Son, Fe3O4 nanosphere@microporous organic networks: enhanced anode performances in lithium ion batteries through carbonization, Chem. Commun. 50 (2014) 7723–7726. [54] C. Tang, E. Liu, J. Wan, X. Hu, J. Fan, Co3O4 nanoparticles decorated Ag3PO4 tetrapods as an efficient visible-light-driven heterojunction photocatalyst, Appl. Catal. B: Environ. 181 (2016) 707–715. [55] H. Gupta, P. Paul, N. Kumar, S. Baxi, D.P. Das, One pot synthesis of waterdispersible dehydroascorbic acid coated Fe3O4 nanoparticles under atmospheric air: blood cell compatibility and enhanced magnetic resonance imaging, J. Colloid Interf. Sci. 430 (2014) 221–228. [56] F.-J. Zhang, F.-Z. Xie, S.-F. Zhu, J. Liu, J. Zhang, S.-F. Mei, W. Zhao, A novel photofunctional g-C3N4/Ag3PO4 bulk heterojunction for decolorization of RhB, Chem. Eng. J. 228 (2013) 435–441.

1553

[57] A. Akhundi, A. Habibi-Yangjeh, Novel g-C3N4/Ag2SO4 nanocomposites: fast microwave-assisted preparation and enhanced photocatalytic performance towards degradation of organic pollutants under visible light, J. Colloid Interf. Sci. 482 (2016) 165–174. [58] J. Yang, H. Chen, J. Gao, T. Yan, F. Zhou, S. Cui, W. Bi, Synthesis of Fe3O4/g-C3N4 nanocomposites and their application in the photodegradation of 2,4,6trichlorophenol under visible light, Mater. Lett. 164 (2016) 183–189. [59] C. Feng, X. Lin, X. Wang, H. Liu, B. Liu, L. Zhu, G. Zhang, D. Xu, Preparation, ferromagnetic and photocatalytic performance of NiO and hollow Co3O4 fibers through centrifugal-spinning technique, Mater. Res. Bull. 74 (2016) 319–324. [60] A. Martinez-dela, U.M. Cruz, Garcia Perez, Photocatalytic properties of BiVO4 prepared by the co-precipitation method: degradation of rhodamine B and possible reaction mechanisms under visible irradiation, Mater. Res. Bull. 45 (2010) 135–141. [61] S.R. Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrode, Plenum, New York, 1980. [62] J. Kim, C.W. Lee, W. Choi, Platinized WO3 as an environmental photocatalyst that generates OH radicals under visible light, Environ. Sci. Technol. 44 (2010) 6849–6854. [63] G. Li, K.H. Wong, X. Zhang, C. Hu, J.C. Yu, R.C.Y. Chan, P.K. Wong, Degradation of acid orange 7 using magnetic AgBr under visible light: the roles of oxidizing species, Chemosphere 76 (2009) 1185–1191. [64] X. Jia, R. Dai, Y. Sun, H. Song, X. Wu, One-step hydrothermal synthesis of Fe3O4/ g-C3N4 nanocomposites with improved photocatalytic activities, J. Mater. Sci: Mater. Electron. 27 (2016) 3791–3798.