Fabrication of novel magnetically separable

Journal of Colloid and Interface Science 480 (2016) 218–231

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

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 Mitra Mousavi a, Aziz Habibi-Yangjeh a,⇑, Masoud Abitorabi b a b

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

g r a p h i c a l a b s t r a c t

g-C3N4

1) Fe+2 , Fe+3

USI

USI

Suspension

g-C3N4/Fe3O4

Suspension

2) NH3 1) Ag+ 2) PO43-

g-C3N4/Fe3O4/Ag3PO4/AgCl

3) USI +

1) Ag

USI

g-C3N4/Fe3O4

Suspension 2) Cl

a r t i c l e

i n f o

Article history: Received 24 May 2016 Revised 12 July 2016 Accepted 12 July 2016 Available online 14 July 2016 Keywords: g-C3N4/Fe3O4/Ag3PO4/AgCl Photocatalysis Magnetic photocatalyst Visible-light-driven Quaternary photocatalyst

/Ag3Po4

a b s t r a c t In the present study, g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites endowed with efficient photocatalytic activity under visible-light irradiation have been successfully prepared by a facile ultrasonicirradiation method. The prepared samples were characterized by XRD, EDX, AAS, SEM, TEM, UV–vis DRS, FT-IR, TG, PL, and VSM techniques. Rhodamine B, methyl orange, fuchsine, and phenol were selected as pollutants to evaluate photocatalytic activity of the as-prepared samples. Among the samples, the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite displayed the highest photocatalytic activity. It was found that activity of this nanocomposite in degradation of rhodamine B is nearly 22, 6, and 7.5-times higher than those of the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/ AgCl (30%) samples, respectively. The significant amount of saturation magnetization (8.78 emu g1) for this nanocomposite indicated that the photocatalyst can be easily separated from the treated solution using a magnetic field. According to the trapping experiments, it was found that holes are main active species, driving the degradation reaction. This work suggests that the quaternary nanocomposite is promising photocatalyst for degradation of organic pollutants under visible-light illumination. Ó 2016 Elsevier Inc. All rights reserved.

⇑ Corresponding author. E-mail address: [email protected] (A. Habibi-Yangjeh). http://dx.doi.org/10.1016/j.jcis.2016.07.021 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.

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M. Mousavi et al. / Journal of Colloid and Interface Science 480 (2016) 218–231

1. Introduction In recent decades, synthetic dyes are widely used in different industries such as textile, leather, paper, and food. With huge development of these industries, due to large-scale production and extensive applications of these dyes, they cause major environmental pollution in aquatic systems [1]. Hence, removal of such toxic compounds from the environment is a major issue for researchers. Among different methods for removing these pollutants, heterogeneous photocatalysis has emerged as the most promising green technology [2,3]. It is noteworthy to say that this green technology is used in different disciplines including inactivation of microorganisms, production of hydrogen gas, reduction of carbon dioxide to small molecules of fuels, and synthesis of various organic compounds [2–6]. However, in order to use effectively the solar energy in photocatalytic processes, it is necessary to provide a photocatalyst with band gap less than about 2.7 eV; hence, designing visible-light-driven photocatalysts with considerable activity is an active research field [7,8]. In the searching for effective visible-light-driven photocatalysts, graphitic carbon nitride (g-C3N4) has recently attracted tremendous attention [9]. The heptazine ring structure and the high condensation degree enable this metal-free semiconductor to possess many advantages such as good physicochemical properties, as well as appealing electronic structure with a medium band gap [10,11]. Unique properties of high stability (even in strong acidic and alkaline solutions), low cost, and band gap of 2.7 eV make g-C3N4 a promising candidate for photocatalytic processes utilizing the solar energy as light source [9–12]. Nevertheless, pure g-C3N4 suffers from rapid recombination of photogenerated charge carriers and poor absorption in visible region, resulting in low photocatalytic activity under visible-light illumination [11,12]. As a result, the exploration of g-C3N4-based photocatalysts with enhanced photocatalytic activities is of increasing importance [10–15]. In recent years, some strategies such as doping of metallic or nonmetallic elements, enhancing surface area, and coupling with other semiconductors have been employed to enhance photocatalytic activity of g-C3N4 [12]. Silver phosphate (Ag3PO4) has been proven to be a promising visible-light-driven photocatalyst, due to narrow band gap of 2.45 eV and quantum efficiency of nearly 90% [16]. Hence, some authors have prepared g-C3N4/ Ag3PO4 photocatalysts and their enhanced photocatalytic activities relative to the pure g-C3N4 have been investigated under visiblelight irradiation [17–23]. It is well known that separation of photocatalysts from treated solutions is another challenge for commercialization of photocatalytic processes [24]. It was found that combining magnetic materials by photocatalysts is an effective way to separate photocatalysts from the photocatalytic systems using an external magnetic field [24–29]. Among different magnetic materials, Fe3O4 nanoparticles have attracted much interests, because of their considerable saturation magnetizations [30]. Literature review exhibited that not only there is not any research about magnetically separable photocatalysts containing g-C3N4 and Ag3PO4, but also there is not any report about preparation and investigation photocatalytic activity of g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites, as magnetically recoverable visible-light-driven photocatalysts. In these regards, we prepared g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites by a facile ultrasonic-irradiation method, for the first time. The photocatalytic performances were evaluated by photodegradation of rhodamine B (RhB) under visible-light irradiation. The effects of Ag3PO4 and AgCl contents on the photocatalytic activity were also investigated. In addition, the photodegradation mechanism over the g-C3N4/Fe3O4/Ag3PO4/AgCl was also discussed. The results revealed that weight percent of Ag3PO4

219

and AgCl, ultrasonic-irradiation time, calcination temperature, and scavengers of the reactive species have remarkable influence on the degradation reaction. Finally, ability of the nanocomposite for degradation of methyl orange (MO), fuchsine, and phenol was confirmed. 2. Experimental 2.1. Materials Ferric chloride (FeCl 3 6H 2 O, 99.5%), ferrous chloride (FeCl24H2O, 98.0%), ammonia solution (30%), melamine (C3H6N6, 99.2%), silver nitrate (99.9%), sodium hydroxide (98%), sodium chloride (99.5%), and benzoquinone were purchased from Loba Chemie and used without further purifications. Sodium phosphate (Na2HPO42H2O, 97%) was purchased from Rankem. Hydrochloric acid (HCl), 2-propanol, ammonium oxalate, RhB, MO, fuchsine, phenol, and absolute ethanol with high quality were obtained from Merck. Deionized water was used throughout this work. 2.2. Instruments The X-ray diffraction (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°/s in the 2h range from 20° to 80°. Surface morphology and distribution of particles were studied by LEO 1430VP scanning electron microscopy (SEM), using an accelerating voltage of 15 kV. The purity and elemental analysis of the products were obtained by energy dispersive analysis of X-rays (EDX) on the same SEM instrument. For SEM and EDX experiments, samples were 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 by a Perkin Elmer Spectrum RX I apparatus. The photoluminescence (PL) spectra of the samples were studied using a Perkin Elmer (LS 55) fluorescence spectrophotometer with an excitation wavelength of 300 nm. The conditions were fixed in order to compare the PL intensities. The UV–vis spectra during the degradation reactions were studied using a Cecile 9000 spectrophotometer. Magnetic properties of the samples were obtained using an alternating gradient force magnetometer (model AGFM, Iran). The contents of Fe and Ag in the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite was determined by atomic absorption spectroscopy (AAS) by Analytik Jena of model ContrAA 700. The pH of solutions was measured using a Metrohm digital pH meter of model 691. The ultrasound radiation was performed using a Bandeline ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz). 2.3. Preparation of the photocatalysts The g-C3N4 powder was prepared by heating melamine powder up to 520 °C [9]. 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 [29]. The g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite was prepared by ultrasonic-irradiation method. In a typical method, 0.4 g of the g-C3N4/Fe3O4 (2:1) nanocomposite was dispersed into 150 mL of water by ultrasonic irradiation for 5 min. Then, 0.122 g of silver nitrate was added to the suspension and stirred for 60 min at room

220


Scheme 1. Schematic diagram for preparation of the g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites.

temperature. After that, an aqueous solution of sodium phosphate (0.037 g in 20 mL of water) was dropwise added to the suspension and sonicated for 1 h. The formed light brown suspension was then centrifuged to get the precipitate out and washed two times with water and ethanol and dried in an oven at 60 °C for 24 h. For preparation of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite, where 30% is weight percent of AgCl, 0.35 g of the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite was dispersed into 150 mL of water by ultrasonic irradiation for 5 min. Then, 0.178 g of silver nitrate was added to the suspension and stirred for 60 min at room temperature. Afterwards, an aqueous solution of sodium chloride (0.06 g in 20 mL of water) was dropwise added to the suspension and sonicated for 60 min. The formed light brown suspension was then centrifuged to get the precipitate out and washed two times with water and ethanol and dried in an oven at 60 °C for 24 h (Scheme 1).

temperature at 25 °C. The solution was magnetically stirred and continuously aerated by a pump to provide oxygen and complete mixing of the reaction solution. A LED lamp of 50 W was used as visible-light source. The source was fitted on the top of the reactor. The distance between the liquid surface and the source was about 20 cm. The photocatalyst was dispersed for 6 min in an ultrasonic bath before using. Prior to illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL aqueous solution of RhB (1 105 M), MO (1.05 105 M) or fuchsine (0.77 105 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, 477, and 540 nm corresponding to the maximum absorption wavelengths of RhB, MO, and fuchsine, respectively.

2.4. Photocatalysis experiments

3. Results and discussion

Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400 mL capacity. The reactor was provided with water circulation arrangement to maintain the

The crystal structures of as-prepared samples were examined using XRD patterns. As Fig. 1 shows, two characteristic peaks of g-C3N4 could be simply observed. A strong peak located at about

g-C₃N₄

Fe₃O₄

AgCl

Ag₃PO₄

g-C₃₃N₄/Fe₃O₄/Ag₃PO₄/AgCl(40%)

g-C₃N₄/Fe₃O₄/Ag₃PO₄/AgCl(30%)


Intensity (a.u.)


g-C₃N₄/Fe₃O₄/AgCl(30%)

g-C₃N₄/Fe₃O₄/Ag₃PO₄(30%)



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, and g-C3N4/Fe3O4/Ag3PO4/AgCl samples.

221


g-C N /Fe O /Ag PO /AgCl(30%) g-C N /Fe O g-C N

(a) Cl Ag Ag C N Cl OFe

P

Ag Ag

Fe

Fe

CO

Intensity (a.u.)

N

Fe

Fe

Fe

C

N

0

1

2

3

4

5

6

7

9

8

10

Energy (keV)

(b)

(c)

Ag

"

(f)

(d)

(g)

Cl

(e)

Fe

O

(i)

(h)

P

N

C

Fig. 2. (a) EDX spectra for the g-C3N4, g-C3N4/Fe3O4, and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples. (b) – (i) EDX mapping for the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite.

27.6° ascribed to the (0 0 2) plane, representing the stacking of conjugated aromatic system. The weak peak appeared at 13.0°, corresponding to diffraction from (1 0 0) planes, which is associated with interlayer stacking [9]. The g-C3N4/Fe3O4 nanocomposite has the diffraction peaks corresponding to face centered cubic Fe3O4 deposited on g-C3N4 [29]. The XRD patterns of

(a)

g-C3N4/Fe3O4/Ag3PO4 nanocomposites exhibit diffraction peaks corresponding to g-C3N4, Fe3O4, and cubic Ag3PO4 (JCPDS No. 06-0505) [18]. In the g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites, the presence of AgCl counterpart in the products was confirmed by fitting the corresponding XRD patterns with cubic structure of AgCl (JCPDS card number of 31-1238) [31].

(b)

Fig. 3. (a) SEM and (b) TEM images of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite.

222


P—O—P

(a)

Fe —O O—H

H C—N & C=N ep ta zi

1.2 g-C N /Fe O /Ag PO /AgCl(40%)

(b)

g-C N /Fe O /Ag PO /AgCl(30%)

1.0

g-C N /Fe O /Ag PO /AgCl(20%)

0.8

g-C N /Fe O /Ag PO /AgCl(10%) g-C N /Fe O /AgCl(30%)

0.6 g-C N /Fe O /Ag PO (30%) g-C N /Fe O /Ag PO (20%)

0.4

g-C N /Fe O /Ag PO (10%)

0.2

g-C N /Fe O g-C N

0.0

270 320 370 420 470 520 570 620 670

Wavelength (nm)

(c)

Fig. 4. (a) FT-IR spectra for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples. (b) UV–vis DRS for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4, g-C3N4/Fe3O4/AgCl, and g-C3N4/Fe3O4/Ag3PO4/AgCl samples. (c) Plots of (ahm)2 versus hv for the g-C3N4 and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples.

223


120

(a) 100

Weight loss (%)

Purity of the samples was investigated using EDX analysis and the results are shown in Fig. 2a. As can be seen, the g-C3N4 sample consists of C and N elements. The g-C3N4/Fe3O4 photocatalyst is composed of C, N, Fe, and O elements. In the case of the g-C3N4/ Fe3O4/Ag3PO4/AgCl (30%) nanocomposite, the peaks are ascribed to C, N, Fe, O, Ag, P, and Cl elements. Moreover, the EDX elemental mapping further elucidated the composition of the g-C3N4/Fe3O4/ Ag3PO4/AgCl (30%) nanocomposite. As can be seen in Fig. 2b-i, the elemental mapping images of C, N, Fe, O, Ag, P, and Cl have similar shape and location, indicating the definite existence of Fe3O4, Ag3PO4, and AgCl over the g-C3N4. Weight percents of C, N, Fe, O, Ag, P, and Cl elements in the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite are 14.0, 21.8, 13.5, 7.62, 32.4, 2.74, and 7.92%, respectively. These values are close to the theoretical amounts of these elements in the quaternary nanocomposite. Hence, weight percents of g-C3N4, Fe3O4, Ag3PO4, and AgCl counterparts in this nanocomposite are 36.7, 18.7, 14.2, and 30.4%, respectively. Furthermore, weight percents of Fe and Ag elements in the quaternary nanocomposite was determined using AAS and their amounts found to be 13.4 and 32.5%, respectively. Morphology of the as-prepared g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite was characterized by SEM and TEM techniques and the results are displayed in Fig. 3. The SEM image of the nanocomposite (Fig. 3a) revealed that particles of the deposited materials (Fe3O4, Ag3PO4, and AgCl) were homogeneously dispersed on the surface of g-C3N4 sheet. The intimate interfacial contact between g-C3N4 sheets and the deposited particles was further studied by TEM image (Fig. 3b). In this image the deposited particles are clearly seen on the g-C3N4 sheet. Such good interfacial contact between g-C3N4, Ag3PO4, and AgCl is beneficial for separation of the photogenerated charge carriers in the nanocomposite [12,13]. FT-IR spectroscopy was used to further elucidate the functional groups of g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples and the spectra are shown in Fig. 4a. For the pristine g-C3N4, the broad band at 3000–3400 cm1 is due to the stretching modes of terminal NH groups at the defect sites of the aromatic ring and OH stretching vibration of adsorbed water. The peaks located between 1200 cm1 and 1650 cm1 are ascribed to the typical stretching modes of CN heterocycles. Moreover, the peak at 809 cm1 is the characteristic breathing mode of triazine units [9,10]. For the Fe3O4 containing samples, two characteristic peaks at 620 and 430 cm1 are related to the stretching vibrations of FeAO bond [30,32]. The typical PAO stretching vibrations of PO3 4 group at 556 and 1013 cm1 could be observed in the spectrum of g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite [17,18]. Moreover, the peaks belonging to g-C3N4, Fe3O4, and Ag3PO4 counterparts also can be found in the spectrum of g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) sample, which reveals co-existence of g-C3N4, Fe3O4, and Ag3PO4 in the photocatalyst. Similar to AgCl-based photocatalysts, there is not any peak for AgCl counterpart [31]. Generally, UV–vis absorption spectra of photocatalysts have reasonable information about their photocatalytic activities. Hence, the UV–vis DRS spectra of the as-prepared samples were provided and the results are shown in Fig. 4b. As can be seen, the g-C3N4 sample has an absorption edge at about 460 nm. Compared with the pure g-C3N4, all of the nanocomposites show red-shift in their absorptions, which indicates that the visible-light response of these samples have been obviously extended by hybridization of the counterparts. With increasing the Ag3PO4 content in the g-C3N4/Fe3O4/Ag3PO4 nanocomposites, absorption intensity in the visible-light region increases, which is ascribed to good optical absorption ability of Ag3PO4. In addition, the intensity of visiblelight absorption ability of the g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites decreases with increasing weight percent of AgCl. This decrease is attributed to wide band gap of AgCl [33,34]. Band

80

60

40

g-C g-C g-C g-C g-C

20

0

0

100

N N N N N

/Fe /Fe /Fe /Fe

O O /Ag PO (20%) O /AgCl(30%) O /Ag PO /AgCl(30%)

200

300

400

500

600

700

Temperature (ºC)

(b)

Fig. 5. (a) TG analysis of the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples. (b) VSM curves for the Fe3O4 and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples.

gap energy (Eg) of the resultant samples were calculated using equation ahm = B(hv Eg)n/2, in which a, m, and B are absorption coefficient, the light frequency, and proportionality constant, respectively [35]. The value of n is related to the characteristics of the transition in the semiconductor. The Eg values were calculated by extrapolation of the linear parts of the curves and the results for the g-C3N4 and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples are shown in Fig. 4c. As can be seen, the Eg values of the pristine g-C3N4 significantly decreases by loading of Fe3O4, Ag3PO4, and AgCl materials. To evaluate thermal stability of the samples, as well as the actual weight percent of g-C3N4 in the photocatalysts, TG curves of the samples were provided from room temperature to 700 °C at a heating rate of 10 °C min1 under air conditions and the results are shown in Fig. 5a. In the case of the pure g-C3N4, main weight loss starts nearly from 520 °C and continues up to 670 °C, which ascribed to the decomposition of this organic semiconductor [9]. As it is evident, for the g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%)

224


1

(a)

Absorbance

0.8

0.6

0.4

g-C g-C g-C g-C g-C

0.2

-60

0

0

60

N N N N N

/Fe /Fe /Fe /Fe

O O /Ag PO (10%) O /Ag PO (20%) O /Ag PO (30%)

120

180

240

300

360

Irradiation time (min) 1 Dark

(b)

Photolysis

0.8

g-C N Fe O

Absorbance

0.6

g-C N /Fe O g-C N /Fe O /AgCl(30%) g-C N /Fe O /Ag PO (20%)

0.4

g-C N /Fe O /Ag PO /AgCl(10%) g-C N /Fe O /Ag PO /AgCl(20%)

0.2

g-C N /Fe O /Ag PO /AgCl(30%) g-C N /Fe O /Ag PO /AgCl(40%)

-60

0

0

60

120

180

240

300

360

Irradiation time (min) Fig. 6. (a) Photodegradation of RhB over the g-C3N4, g-C3N4/Fe3O4, and g-C3N4/Fe3O4/Ag3PO4 nanocomposites with different weight percents of silver phosphate. (b) Photodegradation of RhB over the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites with different weight percents of silver chloride.

nanocomposites, the weight loss starts from lower temperature relative to the pristine g-C3N4. This decrease is attributed to decreasing thermal stability of g-C3N4 by depositing Fe3O4, Ag3PO4, and AgCl counterparts [33,34]. The contents of g-C3N4 in the nanocomposites could be easily read out from the remaining weight after heating the sample up to 700 °C. The weight percent of g-C3N4 in the g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposites were determined to be 73.3, 60.4, 52.7, and 44.7%, respectively. Magnetization curves for the Fe3O4 and g-C3N4/Fe3O4/Ag3PO4/ AgCl (30%) samples are shown in Fig. 5b, which describe the magnetic properties of these samples. It can be seen that the Fe3O4 nanoparticles has strong magnetic behavior and its saturation magnetization is determined to be 55.5 emu g1. Moreover, the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite displayed a maximum saturation magnetization of 8.78 emu g1, which is lower than that of the pure Fe3O4. Decrease of the saturation

magnetization of the nanocomposite is related to the presence of nonmagnetic g-C3N4, Ag3PO4, and AgCl counterparts along with the Fe3O4 nanoparticles. However, the saturation magnetization is strong enough to separate the nanocomposite from the solution with the help of an external magnetic field, as shown in Fig. 5b (inset). The inset of Fig. 5b shows the images of vessels containing the aqueous g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite. The left hand image belongs to the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite dispersed in the aqueous solution. The middle image shows complete magnetic separation of the nanocomposite achieved in 30 s by placing a magnet near the vessel. Then, the vessel was shaken several seconds after the magnet was removed and the nanocomposite was redispersed in the aqueous solution (right hand image). These images confirms that fast separation and redispersion of the nanocomposite can be simply feasible. Photocatalytic degradation of RhB was used to evaluate photocatalytic activity of the as-prepared samples and the results are shown in Fig. 6a and b. Before the light irradiation, dark adsorption

225


1.0

1.0

t = -60 min

(a)

t = -60 " min

(b)

t = 0 min

t = 0 min

t = 30 min

0.8

t = 30" min

0.8

t = 60 min

t = 60 min

t = 90 min

t = 90 min

"

t = 120 " min

t = 150 min

0.4

0.6

Absorbance

Absorbance

t = 120 min 0.6

t = 150 min

0.4

0.2

0.2

0.0 400

450

500

550

600

650

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700

450

500

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1.0

t = -60 min

t = -60 min

(d)

t = 0 min

t = 0 min

t = 30 min

0.8

t = 30 min

0.8

t = 60 min

t = 60 min t = 90 min

t = 90 min

t = 120 min t = 150 min

0.6

t = 150 min

0.4

0.2

Absorbance

t = 120 min

Absorbance

700

1.0

(c)

0.0 400

650

Wavelength (nm)

Wavelength (nm)

0.6

0.4

0.2

450

500

550

600

650

700

Wavelength (nm)

0.0 400

450

500

550

600

650

700

Wavelength (nm)

Fig. 7. UV–vis spectra for degradation of RhB under visible-light irradiation over the (a) g-C3N4, (b) g-C3N4/Fe3O4/Ag3PO4 (20%), (c) g-C3N4/Fe3O4/AgCl (30%), and (d) g-C3N4/ Fe3O4/Ag3PO4/AgCl (30%) nanocomposites.

equilibrium between solution of RhB and the photocatalyst was achieved after stirring the suspension for 60 min. In addition, the blank experiment in absence of any photocatalyst was employed to assess stability of RhB under the visible-light irradiation and it was found that about 12% of RhB was degraded after the light irradiation for 360 min. As can be seen in Fig. 6a, photocatalytic activities of the g-C3N4/Fe3O4/Ag3PO4 and g-C3N4/Fe3O4 nanocomposites are superior to that of the g-C3N4 sample. Moreover, it is evident that photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4 nanocomposites depends on weight percent of Ag3PO4 and the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite showed the best activity. In order to further increase photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite, different amounts of AgCl was deposited over this nanocomposite and the results are shown in Fig. 6b. It is evident that photocatalytic activity of the quaternary g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites are

superior to the g-C3N4/Fe3O4/Ag3PO4 (20%) nanocomposite. In addition, among the quaternary photocatalysts, the g-C3N4/Fe3O4/ Ag3PO4/AgCl (30%) nanocomposite showed the best photocatalytic performance. As well known, photodegradation of RhB could be started by de-ethylation and ring-opening pathways [35,36]. In order to distinguish between these pathways, UV–vis spectra of RhB during the degradation reaction over the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples were provided and the results are shown in Fig. 7a–d. It is evident that intensity of the main absorption peaks of RhB at 553 nm gradually decreases as the irradiation time increases without any shifts in the position of peaks. Hence, it was concluded that the degradation reaction of RhB over these samples takes place by ring-opening mechanism [35,36]. In addition, degradation of RhB almost completely takes place within 150 min, whereas

226


300

(a)

kobs (min-1) × 10-4

250

200

150

100

50

0

g-C g-C g-C g-C g-C

N N N N N

/Fe /Fe /Fe /Fe

O O /Ag PO (20%) O /AgCl(30%) O /Ag PO /AgCl(30%)

Intensity (a.u.)

(b)

g-C3N4/Fe3O4/AgCl (30%) samples, especially for the g-C3N4/ Fe3O4/Ag3PO4/AgCl (30%) nanocomposite. The degradation reaction rate constants over the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples are 12.2 104, 13.1 104, 45.1 104, 36.1 104, and 267 104 min1, respectively. Thus, activity of the quaternary g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite is nearly 22 times higher than that of the pure g-C3N4, 6 times higher than that of the g-C3N4/Fe3O4/Ag3PO4 (20%), and 7.5 times higher than that of the g-C3N4/Fe3O4/AgCl (30%) for degradation of RhB. To understand the enhanced activity of the quaternary photocatalyst relative to its counterparts, the charge separation extents for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples were investigated by PL technique and the results are displayed in Fig. 8b. It can be observed that the emission peak of all samples centered at about 365 nm. Furthermore, the emission peak intensity of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite is much weaker than those of the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/AgCl (30%) samples. It is well known that a weaker intensity for the emission peak in PL spectrum indicates a lower recombination rate of the photogenerated charge carriers. Therefore, it can be concluded that by deposition of Fe3O4, Ag3PO4, and AgCl counterparts on the surface of g-C3N4, the charge carrier recombination rates were significantly reduced, resulting in enhanced photocatalytic activity for the quaternary nanocomposite. On the basis of the obtained results, a plausible mechanism is proposed to explain the enhanced photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites, as illustrated in Fig. 9. In order to clarify the separation of photogenerated electron-hole pairs over the quaternary nanocomposites, it is necessary to find out the conduction band (CB) and valence band (VB) potentials of the components. These energy levels were calculated using the following empirical equations:

ECB ¼ v Ee 0:5Eg

ð1Þ

EVB ¼ ECB þ Eg

ð2Þ

where EVB and ECB are the VB and CB potentials, respectively. Moreover, Ee is the energy of free electrons vs. hydrogen (4.5 eV) [37]. Finally, v is the electronegativity of semiconductor and it was calculated by the following equation:

v ¼ ½xðAÞa xðBÞb xðCÞc 330

340

350

360

370

380

390

400

Wavelength (nm) Fig. 8. (a) The degradation rate constants of RhB over the g-C3N4, g-C3N4/Fe3O4, gC3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites with different weight percents of silver chloride. (b) PL spectra for the g-C3N4, g-C3N4/Fe3O4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) samples.

about 31%, 48.8%, and 49.5% of the dye was degraded over the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/AgCl (30%) samples, respectively. These results indicate highly enhanced activity of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite for RhB degradation compared with the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/AgCl (30%) samples. The degradation rate constants of RhB over different photocatalysts were calculated using pseudo-first-order kinetic model and the results are displayed in Fig. 8a. Obviously, the rate constants of all g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites are higher than those of the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), and

1=ðaþbþcÞ

ð3Þ

In which a, b, and c are the number of atoms in the compounds [38]. The values of Eg and v for g-C3N4 are 2.70 and 4.73 eV, respectively. Therefore, the ECB and EVB of g-C3N4 were calculated to be 1.13 and +1.57 eV versus normal hydrogen electrode (NHE). In addition, Eg and v values for Ag3PO4 are 2.45 and 5.95 eV, and those for AgCl are 3.25 and 6.06 eV, respectively. Therefore, the ECB of Ag3PO4 was calculated to be +0.24 eV/NHE, and EVB was estimated to be +2.69 eV/NHE. Furthermore, ECB and EVB for AgCl were calculated to be 0.06 and +3.19 eV/NHE, respectively. When the g-C3N4/ Fe3O4/Ag3PO4/AgCl nanocomposites were irradiated by the visible light, the photons reaching to the photocatalyst would be absorbed by g-C3N4 and Ag3PO4 counterparts, leading to generation of some electron-hole pairs. As can be seen in Fig. 9, both CB and VB of g-C3N4 are higher than those of the Ag3PO4 and AgCl. Hence, the photogenerated electrons in the CB of g-C3N4 can be transferred to those of the Ag3PO4 and AgCl. Due to higher energy level for VB of g-C3N4, the photogenerated holes in the VB of Ag3PO4 are transferred to that of the g-C3N4. These interfacial charge transfers prolong lifetime of the charge carrier by suppressing the recombination of electron-hole pairs, which is more favorable to increase

227


-2.0

O-2

2+

Fe

O2 + H -1.0

.

-

e

E0(O2/.O2-) = -0.33

3+

O2

Fe

CB=-1.13 eV

E0(O2/H2O2) = +0.695

Po OH te nti 0.0 al vs N H E

e- e- e-

+

H2O2

Fe3O4

H2O2

e-

.

OH

E0(O2/H2O2) = +0.695

e- e- eCB=-0.06 eV

e- e- e-

2.7 eV

O2 + H +

CB=+0.24 eV

+1.0

3.25 eV

2.45eV

h+ h+ h+ VB=+1.57 eV

+2.0

+ h+ h h+

g-C N

+3.0

VB=+2.69 eV

Ag PO VB=+3.19 eV

AgCl Fig. 9. A plausible diagram for separation of electron-hole pairs in the g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites.

inhibited after the addition of the scavengers in the reaction system, which reveals that these active species are generated during the degradation reaction. However, the inhibition degree follows the order AO > 2-PrOH > BQ, which reveals that the photogenerated h+ plays the most important role in the photocatalytic reaction, while the OH and O 2 species are the other two reactive species having some role during the degradation reaction. It is believed that properties of nanomaterials such as crystallinity, morphology, and size of particles are significantly governed by the time applied for preparation of them. Hence, the influence of ultrasonic-irradiation time on photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite

200

150

kobs (min-1) × 10-4

the photocatalytic activity. After that, the holes in the VB of g-C3N4 can induce oxidation reactions, while the electrons in the CB of Ag3PO4 and AgCl semiconductors are used for reduction reactions. Some of the photogenerated electrons on the CB of g-C3N4 react with adsorbed O2 to form O 2 radicals, because CB of g-C3N4 is more 0 negative than the potential of O2/O 2 (E (O2/ O2 ) = 0.33 eV/NHE) [39]. Meanwhile, the CB edge potential of Ag3PO4 and AgCl (+0.24 and 0.06 eV vs. NHE) are more positive than the standard redox potential of O2/O 2 (0.33 eV vs. NHE), suggesting that the electrons at CB of Ag3PO4 and AgCl cannot reduce O2 to O 2 [40]. However, the accumulated electrons in the CB of Ag3PO4 and AgCl can be transferred to adsorbed molecules of oxygen to produce H2O2, because the CB levels of Ag3PO4 and AgCl are more negative than E0(O2/H2O2) (+0.682 eV vs. NHE). Then, the produced H2O2 molecules react with electrons to produce active OH radicals. On the other hand, as can be seen in Fig. 6, photocatalytic activity of the g-C3N4/Fe3O4 sample is higher than that of the g-C3N4. It is well known that the CB level of Fe3O4 is lower than that of g-C3N4 [41]. Hence, the photogenerated electrons in the CB of g-C3N4 easily transfer to the CB of Fe3O4. The Fe3+ ions in the Fe3O4 particles gain these electrons to form Fe2+ ions [42]. The formed Fe2+ ions react with adsorbed oxygen to produce Fe3+ and O 2 ions, leading to preventing fast recombination of the charge carriers. As a result, photocatalytic activity of the g-C3N4/Fe3O4 nanocomposite was increased relative to the g-C3N4. It is well known that during heterogeneous photocatalytic reactions, organic pollutants are decomposed by reactive species + such as O 2 , h , and OH, which are generated under the appropriate light irradiation. In order to further investigate the roles of these reactive species during the degradation reaction, several types of scavengers were used. For this purpose, 2-propanol (2-ProH), ammonium oxalate (AO), and benzoquinone (BQ) were used as the scavengers of hydroxyl radicals (OH), holes (h+), and superoxide ion radicals (O 2 ), respectively. Fig. 10 illustrates the influence of scavengers on the degradation rate constant of RhB over the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite under the light irradiation. Compared with the photocatalytic degradation of RhB over the nanocomposite without adding any scavenger, the degradation rate constant of RhB over the nanocomposite is

100

50

0 Without

AO

BQ

2-PrOH

Fig. 10. The degradation rate constants of RhB over the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite in presence of various scavengers.

228


180

200

(a)

(b)

150

kobs (min-1) ×10 -4

150

kobs (min-1) ×10 -4

120

90

100

60 50 30

0

15

30

60

120

0

RT

100

Ultrasonic irradiation time (min)

200

300

400

Calcination temp (°C)

(c)

Fig. 11. (a) The degradation rate constant of RhB over the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite prepared at different ultrasonic irradiation times. (b) The degradation rate constant of RhB over the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite calcined at different temperatures. (c) SEM image of the g-C3N4/Fe3O4/Ag3PO4/ AgCl (30%) nanocomposite after calcination at 400 °C.

1 Run 1 Run 2 Run 3 0.8 Run 4

Absorbance

was investigated and the results are shown in Fig. 11a. It is evident that there is not linear correlation between the degradation rate constant and the preparation time and the nanocomposite prepared by ultrasonic irradiation for 60 min has the best activity. Hence, this sample was selected for further experiments. In addition, to investigate the effect of calcination temperature on the photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite, the sample was calcined at 100, 200, 300, and 400 °C for 2 h and the results are displayed in Fig. 11b. It is evident that the degradation rate constant significantly decreases with increasing the calcination temperature. To show changes of surface morphology of the nanocomposite after calcination, SEM image of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite after calcination at 400 °C was provided (Fig. 11c). As can be seen, after calcination of the nanocomposite, deposited particles were aggregated and bigger particles of Ag3PO4 and AgCl were formed on the g-C3N4 sheets, leading to destruction of heterojunctions between g-C3N4, Ag3PO4, and AgCl [43]. Hence, separation of the photogenerated charge carriers could not take place efficiently, resulting in decreased photocatalytic activity. The cycling experiments were carried out to determine the stability and reusability of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite via photodegradation of RhB under the visiblelight irradiation. After each cycle, all of the used nanocomposite

0.6

0.4

0.2

0

0

200

400

600

800

1000

Irradiation time (min) Fig. 12. Reusability of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite.


229

300

(a)

(b) 80

200

kobs (min-1) × 10-4

kobs (min-1) × 10-4

250

150

100

60

40

20 50

0

0

12

20

(c)

(d)

10

6

4

kobs (min-1) × 10-4

kobs (min-1) × 10-4

15 8

10

5 2

0

0

Fig. 13. The degradation rate constants of (a) RhB, (b) MO, (c) fuchsine, and (d) phenol over the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), g-C3N4/Fe3O4/AgCl (30%), and g-C3N4/ Fe3O4/Ag3PO4/AgCl (30%) samples under visible-light irradiation.

was collected, washed with ethanol and dried at 60 °C for 24 h. The results shown in Fig. 12 revealed that the photocatalytic activity of the nanocomposite did not exhibit significant loss after four cycles. These results suggest that the nanocomposite displays reasonable stability and could be reused as a photocatalyst with considerable activity.

In order to confirm enhanced activity of the g-C3N4/Fe3O4/ Ag3PO4/AgCl (30%) nanocomposite relative to its counterparts in degradations of other pollutants, photocatalytic degradations of MO, fuchsine, and phenol under visible-light irradiation were considered and the results are shown in Fig. 13. It is evident that, photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%)

230


nanocomposite is about 22, 31, 4, and 6.5-folds higher than those of the g-C3N4 in degradation of RhB, MO, fuchsine, and phenol under the light irradiation, respectively. This increase was attributed to the formation of heterojunctions in the g-C3N4/Fe3O4/ Ag3PO4/AgCl (30%) nanocomposite, which could improve not only the separation of photogenerated electrons and holes, but also absorption of visible light, which are favorable to the photocatalytic degradation reactions [10,12,44]. 4. Conclusions In summary, novel g-C3N4/Fe3O4/Ag3PO4/AgCl nanocomposites were successfully fabricated using a facile ultrasonic-irradiation method. The results depicted that Fe3O4, Ag3PO4, and AgCl particles have been successfully deposited on the g-C3N4 sheets. Photocatalytic activity of the g-C3N4/Fe3O4/Ag3PO4/AgCl (30%) nanocomposite was nearly 22, 6, and 7.5-folds greater than those of the g-C3N4, g-C3N4/Fe3O4/Ag3PO4 (20%), and g-C3N4/Fe3O4/AgCl (30%) samples in degradation of RhB, whereas 31, 8, and 13.5-folds greater in degradation of MO, respectively. Furthermore, the enhanced activity of the g-C3N4/Fe3O4/ Ag3PO4/AgCl (30%) nanocomposite was confirmed in degradations of two more pollutants under visible-light irradiation. The improved photocatalytic activity of the quaternary nanocomposites were mainly attributed to more harvesting ability of visible light and enhanced separation of photogenerated electrons and holes through formation of heterojunctions between the counterparts, as the DRS and PL results showed. The effects of scavengers of reactive species showed that holes play a vital role in the degradation reaction. Furthermore, the as-prepared nanocomposite exhibit the considerable stability and reusability. Hence, it was confirmed that deposition of more components over graphitic carbon nitride could further increase its photocatalytic activity relative to one component deposition. Acknowledgement The authors wish to acknowledge University of Mohaghegh Ardabili-Iran, for financial support of this work. References [1] T. Reemtsma, M. Jekel, Organic Pollutants in the Water Cycle: Properties, Occurrence, Analysis and Environmental Relevance of Polar Compounds, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2006. [2] 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. [3] D. Sudha, P. Sivakumar, Review on the photocatalytic activity of various composite catalysts, Chem. Eng. Process. 97 (2015) 112–133. [4] O. Ola, M.M. Maroto-Valer, Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction, J. Photochem. Photobiol. C: Photochem. Rev. 24 (2015) 16–42. [5] S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.-X. Guo, J. Tang, Visible-light driven heterojunction photocatalysts for water splitting – a critical review, Energy Environ. Sci. 8 (2015) 731–759. [6] R. Fagan, D.E. McCormack, D.D. Dionysiou, S.C. Pillai, A review of solar and visible light active TiO2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern, Mater. Sci. Semicond. Process. 42 (2016) 2–14. [7] Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar, J. He, Visible-light-driven type II heterostructures and their enhanced photocatalysis properties: a review, Nanoscale 5 (2013) 8326–8339. [8] 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. [9] G. Dong, Y. Zhang, Q. Pan, J. Qiu, A fantastic graphitic carbon nitride (g-C3N4) material: electronic structure, photocatalytic and photoelectronic properties, J. Photochem. Photobiol. C: Photochem. Rev. 20 (2014) 33–50. [10] S. Cao, J. Low, J. Yu, M. Jaroniec, Polymeric photocatalysts based on graphitic carbon nitride, Adv. Mater. 27 (2015) 2150–2176. [11] B. Zhu, P. Xia, W. Ho, J. Yu, Isoelectric point and adsorption activity of porous g-C3N4, Appl. Surf. Sci. 344 (2015) 188–195.

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