- Email: [email protected]
Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities Anise Akhundi, Aziz Habibi-Yangjehn Department of Chemistry, Faculty of Sciences, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran Received 27 November 2014; received in revised form 22 December 2014; accepted 29 December 2014
Abstract In this study, we report novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts. The preparation method was simple, large-scale, and low-temperature and did not require any additives or post preparation treatments. The nanocomposites were characterized using X-ray diffraction, transmission electron microscopy, energy dispersive analysis of X-rays, UV–vis diffuse reflectance spectroscopy, Fourier transform-infrared spectroscopy, thermogravimetric analysis, and vibrating sample magnetometry techniques. Photocatalytic activity of the nanocomposites was investigated by degradation of rhodamine B under visible-light irradiation. The nanocomposite with 4:1 weight ratio of g-C3N4/AgBr to Fe3O4 exhibited superior activity in the degradation reaction. Activity of this nanocomposite was about 5.3 and 5-fold higher than those of g-C3N4, and g-C3N4/Fe3O4, respectively. Moreover, we investigated the influence of refluxing time, calcination temperature, and scavengers of reactive species on the degradation activity. Finally, the photocatalyst was magnetically separated, with high efficiency, from the treated solution after five successive cycles. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Nanocomposite; g-C3N4/AgBr/Fe3O4; Magnetic photocatalyst; Visible light
1. Introduction It is well known that contamination of aquatic systems by various organic pollutants is a major environmental concern, due to the huge development of industry and increase of the human population [1]. Among different methods, semiconductor-based photocatalysis, as a green technology, has attracted extensive attention, owing to its potential applications to address some challenges, especially degradation of various organic pollutants [2,3]. However, photocatalytic processes have two main drawbacks that restrict their practical applications at a large scale. Wide band gaps of the commonly used photocatalysts (such as TiO2, ZnO, and ZnS) are the first drawback because they impede solar energy absorption [4–6]. Separation of the suspended photocatalysts from large volumes of the treated solutions by filtration and centrifugation is the second drawback because it hinders practical applications at an industrial scale, owing to long operation times and high n
Corresponding author. Tel.: þ98 045 33514701; fax: þ 98 045 33514702. E-mail address: [email protected] (A. Habibi-Yangjeh).
costs of required equipment. Magnetic photocatalysts, as new generation of photocatalysts, have provided an effective way for removing nanosized photocatalysts using an external magnetic field [7]. These photocatalysts are composed of magnetic and photoactive materials. Thus, there are extensive attempts in searching for highly active visible-light-driven magnetic photocatalysts made from cost-effective and earth abundant elements [8–12]. In recent years graphitic carbon nitride (g-C3N4), as the most stable allotrope of carbon nitrides, has attracted increasing attention as visible-light-driven photocatalysts owing to environment-friendly, abundant availability of its precursors, high chemical stability, reasonable photochemical and thermal stability, low-cost, 2D layered structure, and low band gap of about 2.7 eV [13,14]. Photocatalytic activity of g-C3N4 under visible-light irradiation can be further increased by combining with other semiconductors that have proper energy levels (such as ZnO, TiO2, CdS, and WO3) [15–18]. This combination helps to transfer electrons from conduction band of g-C3N4 to that of the combined semiconductor, which prolongs the life times of photogenerated charge carriers. In spite of potential widespread applications of g-C3N4-based magnetic
http://dx.doi.org/10.1016/j.ceramint.2014.12.145 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
2
photocatalysts, there are only a few reports about preparation and investigation of their photocatalytic activities [19–24]. Nanocomposites of Fe2O3/g-C3N4 have been prepared and their activities for degradation of rhodamine B (RhB) under visible-light irradiation were investigated [19,20]. Preparation of magnetic g-C3N4– ZnFe2O4 composites have been reported and their activities for degradation of methyl orange (MO) under visible-light irradiation were studied [21]. Vignesh et al. prepared magnetic MnFe2O4/gC3N4/TiO2 nanocomposite via chemical impregnation method and investigated degradation of MO under simulated solar-light irradiation [22]. Moreover, nanocomposites of g-C3N4–Fe3O4 have been prepared and their activities for photocatalytic degradation of RhB and MO under visible-light irradiation were studied [23,24]. Among many magnetic materials (Fe3O4, γ-Fe2O3 and MFe2O4, where M is Ba þ 2, Ni þ 2, Mg þ 2, Co þ 2, Mn þ 2, and Zn þ 2), magnetite (Fe3O4) has been widely used in preparation of magnetic photocatalysts, owing to low-cost, desirable magnetic, and nontoxic properties [25]. In these regards, we report novel magnetically separable g-C3N4/ AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/ AgBr to Fe3O4 as visible-light-driven photocatalysts. The nanocomposites were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive analysis of Xrays (EDX), UV–vis diffuse reflectance spectroscopy (DRS), Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and vibrating sample magnetometry (VSM) techniques. The nanocomposites exhibited highly enhanced activity toward photodegradation of RhB under visible-light irradiation. Furthermore, influence of weight ratio of g-C3N4/AgBr to Fe3O4, refluxing time, calcination temperature, and scavengers of reactive species on the photocatalytic activity were investigated and the results were discussed. Finally, reusability of the nanocomposite was tested for five successive runs. 2. Experimental
2.2. Instruments The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Kα radiation (λ¼ 0.15406 nm), employing scanning rate of 0.041/sec in the 2θ range from 201 to 801. The TEM investigations were performed by a Zeiss-EM10C instrument with an acceleration voltage of 80 kV. The purity and elemental analysis of the products were obtained by EDX on LEO 1430VP SEM instrument. For EDX experiments, samples mounted on an aluminum support using a double adhesive tape coated with a thin layer of gold. The DRS was recorded by a Scinco 4100 apparatus. The FT-IR spectra were obtained using a Perkin Elmer Spectrum RX I apparatus. The TGA of samples was performed on Linseis STA PT 1000 by heating under air atmosphere from room temperature to 700 1C at 10 1C/min. Magnetic properties of the samples were obtained using an alternating gradient force magnetometer (model AGFM, Iran). The pH of solutions was measured using a Metrohm digital pH meter of model 691. 2.3. Preparation of the samples 2.3.1. Preparation of g-C3N4/AgBr nanocomposites The g-C3N4 powder was prepared by heating melamine powder up to 520 1C according to the literature method [13]. For preparation of g-C3N4/AgBr nanocomposite with 4:1 weight ratio of g-C3N4 to AgBr, 0.4 g of g-C3N4 powder was dispersed into 200 mL of distilled water using an ultrasonic bath for 30 min. Then, 0.09 g silver nitrate was added to the above solution and the suspension was vigorously stirred for 60 min at room temperature. Next, an aqueous solution of sodium bromide (0.105 g in 20 mL of water) was added to the suspension dropwise and refluxed at 96 1C for 60 min. The formed greenish yellow colored suspension was centrifuged to get the precipitate out and washed two times with double distilled water and ethanol, then dried in an oven at 60 1C for 24 h.
2.1. Materials Melamine (C3H6N6), ferric chloride (FeCl3 6 H2O), ferrous chloride (FeCl2 4 H2O), silver nitrate, sodium bromide, ammonia, RhB, 2-propanol, potassium iodide, benzoquinone, ethanol with high quality were employed without further purification. Double distilled water was used for the experiments.
1) heating at 500 °C
2.3.2. Preparation of g-C3N4/AgBr/Fe3O4 nanocomposites Firstly, Fe3O4 nanoparticles were prepared by chemical coprecipitation method as reported by Massart [26]. Then, g-C3N4/ Fe3O4 nanocomposites were prepared by the recently reported method [23]. For preparation of g-C3N4/AgBr/Fe3O4 nanocomposite with 4:1 weight ratio of g-C3N4/AgBr to Fe3O4, 0.84 g of
Fe3+, Fe2+
Melamine
g-C3N4 2) heating at 520 °C
NH3
Br-
Ag+
Scheme 1. The schematic diagram for preparation of g-C3N4/AgBr/Fe3O4 nanocomposites.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
3
g- C3N4
g-C3N4 AgBr Fe3O4
g- C3N4/AgBr(4:1)
Intensity (a.u.)
g-
g-
g-
g-
g- C3N4/Fe3O4(4:1)
10
20
30
40
50
60
70
80
2 Theta (deg.) Fig. 1. XRD patterns for g-C3N4, g-C3N4/AgBr (4:1), g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4.
g-C3N4/Fe3O4 nanocomposite was dispersed into 200 mL of distilled water using an ultrasonic bath for 5 min. Then, 0.144 g of silver nitrate was added to the suspension and stirred for 60 min at room temperature. Afterwards, an aqueous solution of sodium bromide (0.168 g in 20 mL of water) was slowly added to the suspension and refluxed at 96 1C for 60 min. The dark brown suspension was then centrifuged to get the precipitate out and washed two times with double distilled water and ethanol and dried in an oven at 60 1C for 24 h. The schematic diagram for preparation of the nanocomposites can be illustrated in Scheme 1. 2.4. Photocatalysis experiments Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400 mL capacity. Temperature of the reactor was maintained at 25 1C using a water circulation arrangement. The solution was mechanically stirred and continuously aerated by a pump to provide oxygen and complete mixing of the reaction solution. An LED source with 50 W was used as a visible-light source. The emission spectrum of the source has high intensity in visible range and its intensity rapidly decreases in wavelengths near to UV and IR ranges [12]. The source was fitted on the top of the reactor. Prior to illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL of RhB solution (2.50 10 5 M) was continuously stirred in the dark for 60 min, to attain adsorption equilibrium. Samples were taken from the reactor at regular intervals and the photocatalyst removed before analysis by the spectrophotometer at 553 nm corresponding to the maximum absorption wavelength of RhB. 3. Results and discussion The XRD patterns for g-C3N4, g-C3N4/AgBr (4:1), g-C3N4/ AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4 along with g-C3N4/ Fe3O4 (4:1) are
shown in Fig. 1. There are two distinct well-defined diffraction peaks at 13.631 and 27.671 for g-C3N4, which could be ascribed to (100) and (002) diffraction planes (JCPDS 87-1526) [27]. In the case of g-C3N4/AgBr (4:1) nanocomposite, the diffraction peaks are clearly indexed to g-C3N4 and AgBr counterparts [28]. The XRD patterns for g-C3N4/AgBr/Fe3O4 nanocomposites are composed of the diffraction peaks corresponding to g-C3N4, AgBr and Fe3O4. Hence, presence of AgBr and Fe3O4 nanoparticles on g-C3N4 sheets is clearly confirmed. Moreover, it is evident that with increasing weight of ratio of g-C3N4/AgBr to Fe3O4, intensity of the corresponding peaks for g-C3N4 and AgBr was increased and that of Fe3O4 decreased. The EDX technique was used for confirming purity of the samples and the presence of AgBr and Fe3O4 nanomaterials on surface of the g-C3N4 and the results are shown in Fig. 2. For g-C3N4, the peaks are clearly related to C and N elements. In the case of g-C3N4/AgBr (4:1) nanocomposite, the peaks correspond to C, N, Ag, and Br elements. For g-C3N4/Fe3O4 (4:1) nanocomposite, the peaks are related to C, N, Fe, and O elements. Furthermore, in the case of g-C3N4/AgBr/Fe3O4(4:1) nanocomposite, the peaks are clearly related to C, N, Ag, Br, Fe, and O elements. Therefore, deposition of AgBr and Fe3O4 on g-C3N4 sheets was further confirmed by the EDX technique. Other peaks in the figure are related to the elements applied for sputter coating of the samples on the EDX stage. Fig. 3 shows TEM images for the g-C3N4 and g-C3N4/ AgBr/Fe3O4(4:1) nanocomposite. As can be seen, for pristine g-C3N4, the nanosheets are so thin that they are transparent to the electron beam. In the case of g-C3N4/AgBr/Fe3O4(4:1) nanocomposite, AgBr nanorods and Fe3O4 nanoparticles are clearly seen on surface of the g-C3N4 sheets. The FT-IR spectra for g-C3N4, g-C3N4/Fe3O4 (4:1) and g-C3N4/ AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/ AgBr to Fe3O4 are shown in Fig. 4. For pristine g-C3N4, the bands in range of 1230–1650 cm 1 correspond to the stretching vibrations
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
4
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
Fig. 2. EDX spectra for (a) g-C3N4, (b) g-C3N4/AgBr (4:1), (c) g-C3N4/Fe3O4 (4:1), and (d) g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite.
of C–N and CQN in heterocycles [27]. Furthermore, the band at 806 cm 1 is related to the breathing mode of the heptazine arrangement [29]. For all samples, the broad absorption band at 3000–3300 cm 1 is due to the terminal NH2 or NH groups at the defect sites of g-C3N4 aromatic rings [29]. In the case of g-C3N4/ AgBr/Fe3O4 nanocomposites, along with g-C3N4 bands, the two characteristic peaks at 620 and 430 cm 1 are related to stretching vibrations of Fe–O bond [30]. It is well known that activity of photocatalysts is closely related to their optical properties. Hence, UV–vis DRS spectra for the prepared samples are shown in Fig. 5. The pristine g-C3N4, similar to the reported data, has a band edge at 480 nm. In the case of g-C3N4/AgBr (4:1), the absorption edge shifts to a longer wavelength [31]. For g-C3N4/AgBr/Fe3O4 nanocomposites, with increasing weight ratios of Fe3O4 to g-C3N4/AgBr, their ability to absorb visible-light irradiation was enhanced. As a result, it can be concluded that the nanocomposites could have remarkable activity relative to the pristine g-C3N4. Fig. 6 displays the TGA curves for Fe3O4, g-C3N4, and g-C3N4/ AgBr/Fe3O4(4:1) nanocomposite. Small increase of the weight for
the samples around 350 1C was related to base line changing of the instrument. By heating Fe3O4 nanoparticles up to 700 1C, there were almost no changes in its weight. It is evident that the pristine g-C3N4 undergoes a weight loss of about 97% when heated up to 700 1C. This weight loss can be attributed to combustion of g-C3N4 to produce mainly gaseous products. As can be seen, the thermal behavior of g-C3N4/AgBr/Fe3O4(4:1) nanocomposite is similar to that of g-C3N4. For this nanocomposite, rapid weight loss starts from 470 1C and about 60% of its weight was lost by heating the sample up to 700 1C. Hence, g-C3N4 content of the nanocomposite is about 60%. Fig. 7 shows plots of magnetization versus the applied external magnetic field for the as-prepared Fe3O4 nanoparticles and g-C3N4/AgBr/Fe3O4(4:1) nanocomposite, at room temperature. The saturated magnetization for Fe3O4 nanoparticles and g-C3N4/AgBr/Fe3O4(4:1) nanocomposite at 8500 Oe are 45.1 and 19.5 emu/g, respectively. It is clearly evident that in the presence of g-C3N4 and AgBr, the saturated magnetization of Fe3O4 nanoparticles was reduced, confirming the presence of nonmagnetic materials on the surface of g-C3N4 nanosheets. Moreover, the inset of Fig. 7 shows separation of the magnetic
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
5
Fig. 3. TEM images for (a) g-C3N4, and (b) g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite.
g- C3N4
%Transmittance
g-
g-
g-
g-
heptazine N—H
Fe —O
g- C3N4/Fe3O4(4:1)
C—N & C=N 400
800
1200
1600
2000
2400
2800
3200
3600
4000
Wavenumber (cm-1) Fig. 4. FT-IR spectra for g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4.
photocatalyst from the treated solution using a magnet after 40 s. Hence, it can be concluded that magnetization of the nanocomposite is high enough to magnetically separate the visible-light-driven photocatalyst from the treated solution. Photodegradation of RhB under visible-light irradiation was considered to evaluate activity of the prepared samples. In Fig. 8a, plots of absorbance versus wavelength for the degradation reaction on g-C3N4/AgBr/Fe3O4(4:1) nanocomposite ([RhB]¼ 2.50 10 5 M, weight¼ 0.1 g) at various irradiation times are shown. As can be seen, under the light irradiation, intensity of the absorption peaks gradually decreases without any changes in position. It is well known that photocatalytic degradation of RhB occurs by two different mechanisms. The first mechanism involves de-ethylation of RhB and formation of various intermediates, and the second one is aromatic ring opening mechanism [32]. In the
later mechanism, the absorbance in the UV and visible ranges continually decreases without any changing of wavelengths [33]. As can be seen in Fig. 8a, the absorbance in all ranges gradually decreases without any changes in positions of peaks and new absorption peaks are not formed. Hence, it can be concluded that the degradation of RhB molecules on the nanocomposite take places by aromatic ring-opening mechanism. It is well known that activity of photocatalysts is closely related to their compositions. In Fig. 8b, plots of the RhB absorbance versus the irradiation time for the degradation reaction on g-C3N4/AgBr nanocomposites with different weight ratios of g-C3N4 to AgBr are shown. As can be seen, the photocatalytic activity of g-C3N4/AgBr (4:1) nanocomposite is greatly higher than that of pure g-C3N4. After this composition, by further increasing AgBr content of the nanocomposite, the
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
6 1.6
g-C3N4
1.4 g-C3N4/AgBr(4:1)
Absorbance
1.2 g-C3N4/AgBr/Fe3O4(6:1)
1.0 0.8
g-C3N4/AgBr/Fe3O4(4:1)
0.6
g-C3N4/AgBr/Fe3O4(2:1)
0.4
g-C3N4/AgBr/Fe3O4(1:1)
0.2 g-C3N4/Fe3O4(4:1)
0.0 280
330
380
430
480
530
580
630
680
Wavelength (nm) Fig. 5. UV–vis DRS for g-C3N4, g-C3N4/AgBr (4:1), g-C3N4/Fe3O4 (4:1) and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4.
60
120
Fe3O4 45
100
g-C3N4/AgBr/Fe3O4(4:1)
Magnetization (emu/g)
Weight Loss (%)
30
80
60
40
15 0 -15 -30
g-C3N4
20
-45
g-C3N4/AgBr/Fe3O4(4:1) Fe3O4
0
-60 -10000
0
100
200
300
400 Temperature (°C)
500
600
700
Fig. 6. TGA curves for g-C3N4, Fe3O4, and g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite.
photocatalytic activity is not considerably enhanced. Hence, according to the higher cost of AgBr precursors relative to that of g-C3N4, the g-C3N4/AgBr (4:1) nanocomposite was selected for deposition of Fe3O4 nanoparticles on g-C3N4 sheets. It is generally accepted that photocatalytic activity and magnetic separability of photocatalysts considerably depends on weight ratio of photoactive material to magnetic material [7,12]. Hence, to determine the optimum value, photocatalytic activity of the as-prepared nanocomposites for degradation of RhB under visible-light irradiation is compared in Fig. 9a. It is evident that without using a photocatalyst, only 9% of RhB molecules were degraded after irradiation for 420 min. It is clear that degradation of RhB on g-C3N4/AgBr/Fe3O4 nanocomposites is higher than those of g-C3N4 and g-C3N4/Fe3O4. Moreover, the
-5000
0
5000
10000
Applied field (Oe)
Fig. 7. Magnetization curves measured at room temperature for Fe3O4 nanoparticles and g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite. Inset of the figure is separation of the nanocomposite from the treated solution using an external magnetic field.
weight ratio of g-C3N4/AgBr to Fe3O4 has remarkable influence on the degradation reaction and superior activity was seen for gC3N4/AgBr/Fe3O4 (4:1) nanocomposite. However, after this composition, by increasing weight ratio of Fe3O4 to g-C3N4/ AgBr, the photocatalytic activity considerably decreased. Nanoparticles of Fe3O4 are nearly inactive in photocatalytic processes [12]. Hence, decrease of the photocatalytic activity with increasing Fe3O4 in the nanocomposites can be attributed to decrease of photoactive material in the photocatalysts [12]. Under the visiblelight irradiation for 420 min, 54, 58, and 98.3% of RhB molecules were degraded on g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite, respectively. Moreover, it is evident that in presence of g-C3N4/AgBr/Fe3O4 (4:1)
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
increases up to 60 min and decreases thereafter. The increase in the rate constant could be related to the increasing crystallinity of the nanocomposite. Due to increases in the size and aggregation of the photocatalyst, the rate constant decreases at higher refluxing times [12]. Hence, the sample prepared by refluxing for 60 min was selected for further studies In order to study the effect of calcination temperature on the photocatalytic activity, g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite was calcined for 2 h at 200, 300, 400, and 500 1C and the results are shown in Fig. 10b. It is clear that the degradation rate constant decreases with calcination of the sample and the nanocomposite without any thermal treatment has the highest rate constant. Decrease of the degradation rate constant could be related to aggregation and growth of the particle sizes at higher temperatures [34]. Based on Butler and Ginley model, the conduction band (CB) and valence band (CB) energies for g-C3N4 and AgBr at the point of zero charge were calculated using the following equations [35]:
1.2
1.0
Photolysis g-C3N4 g-C3N4/AgBr (4:1) g-C3N4/AgBr (3:2) g-C3N4/AgBr (2:3) AgBr
Absorbance
0.8
0.6
0.4
0.2
0.0 -60
-30
7
0
30
60
90
120
150
180
Irradiation time (min)
Fig. 8. (a) UV–vis spectra for degradation of RhB under visible-light irradiation on g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite ([RhB]¼ 2.5 10 5 M, catalyst weight¼0.1 g) at 25 1C. (b) Photodegradation of RhB on g-C3N4/AgBr nanocomposites with different weight ratio of g-C3N4 to AgBr.
nanocomposite, without using the light irradiation (dark experiment), about 8.3% of RhB molecules were adsorbed on the nanocomposite during 420 min. Hence, decrease of RhB is the solution containing the nanocomposite is majorly related to photocatalytic degradation reaction. The observed first-order rate constants of the degradation reaction (kobs) are shown in Fig. 9b. The degradation rate constant increases with weight ratio of gC3N4/AgBr to Fe3O4 up to 4:1 and then decreases. The rate constant for g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/ Fe3O4(4:1) nanocomposite are 16.0 10 4, 17.2 10 4, and 86.3 10 4 min 1, respectively. Hence, the photocatalytic activity of g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite is about 5.3, and 5-fold larger than those of g-C3N4, and g-C3N4/Fe3O4 (4:1), respectively. The influence of refluxing time applied for preparation of g-C3N4/ AgBr/Fe3O4 (4:1) nanocomposite was investigated by preparing five comparative samples that were refluxed for 0, 15, 30, 60, and 120 min. Fig. 10a shows plot of the rate constant versus the refluxing time. As can be seen, the degradation rate constant
ECB ¼ X E e 0:5E g
ð1Þ
EVB ¼ ECB þ E g
ð2Þ
Where X is the absolute electronegativity of the semiconductor, defined as the geometric average of the absolute electronegativity of the constituent atoms. Ee and Eg are the energy of free electrons on the hydrogen scale (4.5 eV) and the band gap energy of the semiconductor, respectively. The calculated VB and CB energies for g-C3N4 are 1.58 and 1.12 eV and those for AgBr are 2.56 and 0.04 eV, respectively. The Eg values for g-C3N4 and AgBr are 2.70 and 2.52 eV, respectively [36]. Hence, under the visible-light irradiation, both of them produce electron-hole pairs. The VB and CB energies of g-C3N4 are more negative than those of AgBr. As a result, the photogenerated electrons on CB of g-C3N4 easily transfer to that of AgBr and holes transfer in the reverse direction. Consequently, electrons and holes accumulated on CB of AgBr and VB of g-C3N4, respectively. Therefore, the photogenerated electrons and holes are effectively separated from the each other, leading to decrease of electron-hole recombination. Then, the electrons on CB of AgBr easily transfer to the adsorbed oxygen to produce superoxide ions. The superoxide ions and hole react with adsorbed molecules of water and hydroxide ions to produce hydroxyl radicals. Finally, RhB molecules are degraded by superoxide ions, hydroxyl radicals, and holes to produce the degradation products. The role of the reactive species in degradation of RhB was examined by using a series of scavengers. Fig. 11 shows plot of the rate constant for degradation of RhB on the nanocomposite in the presence of the selected scavengers. As can be seen, decrease of the rate constant in the presence of benzoquinone is higher than those of the other scavengers. Benzoquinone, KI, and 2-PrOH are scavengers of d O2 , h þ and ∙OH, respectively [37,38]. Hence, the role of superoxide ions in the degradation reaction is higher than those of hydroxyl radicals and holes. Reusability of a photocatalyst is a very important parameter from an economical viewpoint. To know reusability of the nanocomposite, the degradation experiments were carried out and the
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
8
1.2 Photolysis
1.0
Dark
g-C3N4
Absorbance
0.8
g-C3N4/AgBr/Fe3O4(6:1)
0.6 g-C3N4/AgBr/Fe3O4(4:1)
0.4
g-C3N4/AgBr/Fe3O4(2:1)
g-C3N4/AgBr/Fe3O4(1:1)
0.2
g-C3N4/Fe3O4(4:1)
-60
0.0
0
60
120
180
240
300
360
420
Irradiation time (min) 90
kobs (min-1)×10-4
75
60
45
30
15
0
Fig. 9. (a) Photodegradation of RhB on g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4. (b) The degradation rate constant of RhB on different samples.
results are demonstrated in Fig. 12. In each run, the nanocomposite was recycled after washing and drying at 60 1C for 24 h. As can be seen, after using the photocatalyst for five successive runs, there is no remarkable decrease in its activity. Hence, the photocatalyst has a good lifetime during the degradation reaction and magnetic recycling from the treated solution. 4. Conclusions A series of g-C3N4/AgBr/Fe3O4 nanocomposites as novel magnetically separable visible-light-driven photocatalysts were prepared using a facile large-scale method. The structures, morphology, optical, thermal and magnetic properties of the nanocomposites
were characterized by different techniques. Photocatalytic activity of the nanocomposites was investigated by degradation of RhB under visible-light irradiation. The degradation rate constant increased with weight ratio of g-C3N4/AgBr to Fe3O4 up to 4:1 and then decreased. The photocatalytic activity of the g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite is about 5.3, and 5-fold higher than those of g-C3N4, and g-C3N4/Fe3O4 (4:1), respectively. Furthermore, the rate constant increases with refluxing time up to 60 min and then decreases. The degradation rate constant of RhB on the nanocomposite without any thermal treatment was higher than those of the calcined samples. Based on the effects of scavengers, it was concluded that superoxide ions have a vital role in visible-light degradation of RhB on the
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
9
90
75
75
60
60
kobs (min-1)×10-4
kobs (min-1)×10-4
90
45
30
45
30
15 15
0
without reflux
15 min
30 min
60 min
120 min 0
Refluxing time
Without
2-PrOH
BQ
KI
Type of scavenger 90
Fig. 11. The degradation rate constant of RhB on the nanocomposite in presence of various scavengers.
75 1.2
kobs (min-1)×10-4
Run 1
60
Run 2 Run 3
1.0
Run 4
45
Run 5
0.8
Absorbance
30
15
0.6
0.4
0
Room temp
200
300
400
500
Calcination temp. (°C) Fig. 10. (a) The degradation rate constant of RhB on g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite prepared at different refluxing times. (b) The degradation rate constant of RhB on g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite calcined at different temperatures for 2 h.
0.2
0.0
0
500
1000
1500
2000
Irradiation time (min)
Fig. 12. Reusability of the nanocomposite for five successive runs.
nanocomposite. Finally, the nanocomposite has good reusability for five successive runs. Acknowledgement The Authors wish to acknowledge University of Mohaghegh Ardabili, for financial support of this work. References [1] T. Reemtsma, M. Jekel, in: Organic Pollutants in the Water Cycle: Properties, Occurrence, Analysis And Environmental Relevance of Polar Compounds, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2006.
[2] C.C. Chen, W.H. Ma, J.C. Zhao, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chem. Soc. Rev. 39 (2010) 4206–4219. [3] B. Ohtani, Photocatalysis A to Z—what we know and what we do not know in a scientific sense, J. Photochem. Photobiol. C: Photochem. Rev. 11 (2010) 157–178. [4] S.-M. Lam, J.-C. Sin, A.Z. Abdullah, A.R. Mohamed, Degradation of wastewaters containing organic dyes photocatalysed by zinc oxide: a review, Desalin. Water Treat. 41 (2012) 131–169. [5] E. Grabowska, J. Reszczynska, A. Zaleska, Mechanism of phenol photodegradation in the presence of pure and modified TiO2: a review, Water Res. 46 (2012) 5453–5471. [6] Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar, J. He, Visible-lightdriven type II heterostructures and their enhanced photocatalysis properties: a review, Nanoscale 5 (2013) 8326–8339.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
10
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
[7] S.-Q. Liu, Magnetic semiconductor nano-photocatalysts for the degradation of organic pollutants, Environ. Chem. Lett. 10 (2012) 209–216. [8] J. Jing, J. Li, J. Feng, W. Li, W.W. Yu, Photodegradation of quinoline in water over magnetically separable Fe3O4/TiO2 composite photocatalysts, Chem. Eng. J. 219 (2013) 355–360. [9] X. Bian, K. Hong, L. Liu, M. Xu, Magnetically separable hybrid CdS– TiO2–Fe3O4 nanomaterial: enhanced photocatalystic activity under UV and visible irradiation, Appl. Surf. Sci. 280 (2013) 349–353. [10] R. Shao, L. Sun, L. Tang, Z. Chen, Preparation and characterization of magnetic core–shell ZnFe2O4@ZnO nanoparticles and their application for the photodegradation of methylene blue, Chem. Eng. J. 217 (2013) 185–191. [11] X. Feng, H. Guo, K. Patel, H. Zhou, X. Lou, High performance, recoverable Fe3O4–ZnO nanoparticles for enhanced photocatalytic degradation of phenol, Chem. Eng. J. 244 (2014) 327–334. [12] M. Shekofteh-Gohari, A. Habibi-Yangjeh, Facile preparation of Fe3O4@AgBr–ZnO nanocomposites as novel magnetically separable visiblelight-driven photocatalysts, Ceram. Int. 41 (2015) 1467–1476. [13] Y. Zheng, J. Liu, J. Liang, M. Jaroniec, S.Z. Qiao, Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis, Energy Environ. Sci. 5 (2012) 6717–6731. [14] G. Dong, Y. Zhang, Q. Pan, J. Qiu, A fantastic graphitic carbon nitride (gC3N4) material: Electronic structure, photocatalytic and photoelectronic properties, J. Photochem. Photobiol. C: Photochem. Rev. 20 (2014) 33–50. [15] W. Liu, M. Wang, C. Xu, S. Chen, Facile synthesis of g-C3N4/ZnO composite with enhanced visible light photooxidation and photoreduction properties, Chem. Eng. J. 209 (2012) 386–393. [16] C. Wang, W. Zhu, Y. Xu, H. Xu, M. Zhang, Y. Chao, S. Yin, H. Li, J. Wang, Preparation of TiO2/g-C3N4 composites and their application in photocatalytic oxidative desulfurization, Ceram. Int. 40 (2014) 11627–11635. [17] 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. [18] Z. Jin, N. Murakami, T. Tsubota, T. Ohno, Complete oxidation of acetaldehyde over a composite photocatalyst of graphitic carbon nitride and tungsten(VI) oxide under visible-light irradiation, Appl. Catal. B: Environ. 150–151 (2014) 479–485. [19] S. Ye, L.-G. Qiu, Y.-P. Yuan, Y.-J. Zhu, J. Xia, J.-F. Zhu, Facile fabrication of magnetically separable graphitic carbon nitride photocatalysts with enhanced photocatalytic activity under visible light, J. Mater. Chem. A 1 (2013) 3008–3015. [20] S. Hu, R. Jin, G. Lu, D. Liu, J. Gui, The properties and photocatalytic performance comparison of Fe3 þ -doped g-C3N4 and Fe2O3/g-C3N4 composite catalysts, R.Soc. Chem. Adv. 4 (2014) 24863–24869. [21] S. Zhang, J. Li, M. Zeng, G. Zhao, J. Xu, W. Hu, X. Wang, In situ synthesis of water-soluble magnetic graphitic carbon nitride photocatalyst and its synergistic catalytic performance, ACS Appl. Mater. Interfaces 5 (2013) 12735–12743. [22] K. Vignesh, A. Suganthi, B.-K. Min, M. Kang, Photocatalytic activity of magnetically recoverable MnFe2O4/g-C3N4/TiO2 nanocomposite under simulated solar light irradiation, J. Mol. Catal. A: Chem 395 (2014) 373–383.
[23] S. Kumar, T. Surendar, B. Kumar, A. Baruah, V. Shanker, Synthesis of magnetically separable and recyclable g-C3N4–Fe3O4 hybrid nanocomposites with enhanced photocatalytic performance under visible-light irradiation, J. Phys. Chem. C 117 (2013) 26135–26143. [24] X. Zhou, B. Jin, R. Chen, F. Peng, Y. Fang, Synthesis of porous Fe3O4/gC3N4 nanospheres as highly efficient and recyclable photocatalysts, Mater. Res. Bull. 48 (2013) 1447–1452. [25] Z. He, T. Hong, J. Chen, S. Song, A magnetic TiO2 photocatalyst doped with iodine for organic pollutant degradation, Sep. Purif. Technol. 96 (2012) 50–57. [26] R. Massart, Preparation of aqueous magnetic liquids in alkaline and acidic media, IEEE Trans. Magn. 17 (1981) 1247–1248. [27] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397–10401. [28] Y. Xu, H. Xu, J. Yan, H. Li, L. Huang, J. Xia, S. Yin, H. Shu, A plasmonic photocatalyst of Ag/AgBr nanoparticles coupled with g-C3N4 with enhanced visible-light photocatalytic ability, Colloids Surf. A: Physicochem. Eng. Asp. 436 (2013) 474–483. [29] L. Liu, D. Ma, H. Zheng, X. Li, M. Cheng, X. Bao, Synthesis and characterization of microporous carbon nitride, Microporous Mesoporous Mater. 110 (2008) 216–222. [30] H. Gupta, P. Paul, N. Kumar, S. Baxi, D.P. Das, One pot synthesis of water-dispersible dehydroascorbic acid coated Fe3O4 nanoparticles under atmospheric air: blood cell compatibility and enhanced magnetic resonance imaging, J. Colloid Interface Sci. 430 (2014) 221–228. [31] H. Xu, J. Yan, Y. Xu, Y. Song, H. Li, J. Xia, C. Huang, H. Wan, Novel visible-light-driven AgX/graphite-like C3N4 (X¼ Br, I) hybrid materials with synergistic photocatalytic activity, Appl. Catal. B: Environ. 129 (2013) 182–193. [32] X. Li, J. Ye, Photocatalytic degradation of rhodamine B over Pb3Nb4O3/ fumed SiO2 composite under visible light irradiation, J. Phys. Chem. C 111 (2007) 13109–13116. [33] A. Martinez-dela Cruz, U.M. 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. [34] S.Y. Kim, T.H. Lim, T.S. Chang, C.H. Shin, Photocatalysis of methylene blue on titanium dioxide nanoparticles synthesized by modified solhydrothermal process of TiCl4, Catal. Lett. 117 (2007) 112–118. [35] J. Cao, X. Li, H.L. Lin, S.F. Chen, X.L. Fu, In situ preparation of novel p–n junction photocatalyst BiOI/(BiO)2CO3 with enhanced visible light photocatalytic activity, J. Hazard. Mater. 239–240 (2012) 316–324. [36] J. Cao, Y. Zhao, H. Lin, B. Xu, S. Chen, Ag/AgBr/g-C3N4: a highly efficient and stable composite photocatalyst for degradation of organic contaminants under visible light, Mater. Res. Bull. 48 (2013) 3873–3880. [37] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Effective photocatalytic disinfection of E. coli K-12 using AgBr–Ag–Bi2WO6 nanojunction system irradiated by visible light: the role of diffusing hydroxyl radicals, Environ. Sci. Technol. 44 (2010) 1392. [38] G.T. Li, K.H. Wong, X.W. 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.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities Anise Akhundi, Aziz Habibi-Yangjehn Department of Chemistry, Faculty of Sciences, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran Received 27 November 2014; received in revised form 22 December 2014; accepted 29 December 2014
Abstract In this study, we report novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts. The preparation method was simple, large-scale, and low-temperature and did not require any additives or post preparation treatments. The nanocomposites were characterized using X-ray diffraction, transmission electron microscopy, energy dispersive analysis of X-rays, UV–vis diffuse reflectance spectroscopy, Fourier transform-infrared spectroscopy, thermogravimetric analysis, and vibrating sample magnetometry techniques. Photocatalytic activity of the nanocomposites was investigated by degradation of rhodamine B under visible-light irradiation. The nanocomposite with 4:1 weight ratio of g-C3N4/AgBr to Fe3O4 exhibited superior activity in the degradation reaction. Activity of this nanocomposite was about 5.3 and 5-fold higher than those of g-C3N4, and g-C3N4/Fe3O4, respectively. Moreover, we investigated the influence of refluxing time, calcination temperature, and scavengers of reactive species on the degradation activity. Finally, the photocatalyst was magnetically separated, with high efficiency, from the treated solution after five successive cycles. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Nanocomposite; g-C3N4/AgBr/Fe3O4; Magnetic photocatalyst; Visible light
1. Introduction It is well known that contamination of aquatic systems by various organic pollutants is a major environmental concern, due to the huge development of industry and increase of the human population [1]. Among different methods, semiconductor-based photocatalysis, as a green technology, has attracted extensive attention, owing to its potential applications to address some challenges, especially degradation of various organic pollutants [2,3]. However, photocatalytic processes have two main drawbacks that restrict their practical applications at a large scale. Wide band gaps of the commonly used photocatalysts (such as TiO2, ZnO, and ZnS) are the first drawback because they impede solar energy absorption [4–6]. Separation of the suspended photocatalysts from large volumes of the treated solutions by filtration and centrifugation is the second drawback because it hinders practical applications at an industrial scale, owing to long operation times and high n
Corresponding author. Tel.: þ98 045 33514701; fax: þ 98 045 33514702. E-mail address: [email protected] (A. Habibi-Yangjeh).
costs of required equipment. Magnetic photocatalysts, as new generation of photocatalysts, have provided an effective way for removing nanosized photocatalysts using an external magnetic field [7]. These photocatalysts are composed of magnetic and photoactive materials. Thus, there are extensive attempts in searching for highly active visible-light-driven magnetic photocatalysts made from cost-effective and earth abundant elements [8–12]. In recent years graphitic carbon nitride (g-C3N4), as the most stable allotrope of carbon nitrides, has attracted increasing attention as visible-light-driven photocatalysts owing to environment-friendly, abundant availability of its precursors, high chemical stability, reasonable photochemical and thermal stability, low-cost, 2D layered structure, and low band gap of about 2.7 eV [13,14]. Photocatalytic activity of g-C3N4 under visible-light irradiation can be further increased by combining with other semiconductors that have proper energy levels (such as ZnO, TiO2, CdS, and WO3) [15–18]. This combination helps to transfer electrons from conduction band of g-C3N4 to that of the combined semiconductor, which prolongs the life times of photogenerated charge carriers. In spite of potential widespread applications of g-C3N4-based magnetic
http://dx.doi.org/10.1016/j.ceramint.2014.12.145 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
2
photocatalysts, there are only a few reports about preparation and investigation of their photocatalytic activities [19–24]. Nanocomposites of Fe2O3/g-C3N4 have been prepared and their activities for degradation of rhodamine B (RhB) under visible-light irradiation were investigated [19,20]. Preparation of magnetic g-C3N4– ZnFe2O4 composites have been reported and their activities for degradation of methyl orange (MO) under visible-light irradiation were studied [21]. Vignesh et al. prepared magnetic MnFe2O4/gC3N4/TiO2 nanocomposite via chemical impregnation method and investigated degradation of MO under simulated solar-light irradiation [22]. Moreover, nanocomposites of g-C3N4–Fe3O4 have been prepared and their activities for photocatalytic degradation of RhB and MO under visible-light irradiation were studied [23,24]. Among many magnetic materials (Fe3O4, γ-Fe2O3 and MFe2O4, where M is Ba þ 2, Ni þ 2, Mg þ 2, Co þ 2, Mn þ 2, and Zn þ 2), magnetite (Fe3O4) has been widely used in preparation of magnetic photocatalysts, owing to low-cost, desirable magnetic, and nontoxic properties [25]. In these regards, we report novel magnetically separable g-C3N4/ AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/ AgBr to Fe3O4 as visible-light-driven photocatalysts. The nanocomposites were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive analysis of Xrays (EDX), UV–vis diffuse reflectance spectroscopy (DRS), Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and vibrating sample magnetometry (VSM) techniques. The nanocomposites exhibited highly enhanced activity toward photodegradation of RhB under visible-light irradiation. Furthermore, influence of weight ratio of g-C3N4/AgBr to Fe3O4, refluxing time, calcination temperature, and scavengers of reactive species on the photocatalytic activity were investigated and the results were discussed. Finally, reusability of the nanocomposite was tested for five successive runs. 2. Experimental
2.2. Instruments The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Kα radiation (λ¼ 0.15406 nm), employing scanning rate of 0.041/sec in the 2θ range from 201 to 801. The TEM investigations were performed by a Zeiss-EM10C instrument with an acceleration voltage of 80 kV. The purity and elemental analysis of the products were obtained by EDX on LEO 1430VP SEM instrument. For EDX experiments, samples mounted on an aluminum support using a double adhesive tape coated with a thin layer of gold. The DRS was recorded by a Scinco 4100 apparatus. The FT-IR spectra were obtained using a Perkin Elmer Spectrum RX I apparatus. The TGA of samples was performed on Linseis STA PT 1000 by heating under air atmosphere from room temperature to 700 1C at 10 1C/min. Magnetic properties of the samples were obtained using an alternating gradient force magnetometer (model AGFM, Iran). The pH of solutions was measured using a Metrohm digital pH meter of model 691. 2.3. Preparation of the samples 2.3.1. Preparation of g-C3N4/AgBr nanocomposites The g-C3N4 powder was prepared by heating melamine powder up to 520 1C according to the literature method [13]. For preparation of g-C3N4/AgBr nanocomposite with 4:1 weight ratio of g-C3N4 to AgBr, 0.4 g of g-C3N4 powder was dispersed into 200 mL of distilled water using an ultrasonic bath for 30 min. Then, 0.09 g silver nitrate was added to the above solution and the suspension was vigorously stirred for 60 min at room temperature. Next, an aqueous solution of sodium bromide (0.105 g in 20 mL of water) was added to the suspension dropwise and refluxed at 96 1C for 60 min. The formed greenish yellow colored suspension was centrifuged to get the precipitate out and washed two times with double distilled water and ethanol, then dried in an oven at 60 1C for 24 h.
2.1. Materials Melamine (C3H6N6), ferric chloride (FeCl3 6 H2O), ferrous chloride (FeCl2 4 H2O), silver nitrate, sodium bromide, ammonia, RhB, 2-propanol, potassium iodide, benzoquinone, ethanol with high quality were employed without further purification. Double distilled water was used for the experiments.
1) heating at 500 °C
2.3.2. Preparation of g-C3N4/AgBr/Fe3O4 nanocomposites Firstly, Fe3O4 nanoparticles were prepared by chemical coprecipitation method as reported by Massart [26]. Then, g-C3N4/ Fe3O4 nanocomposites were prepared by the recently reported method [23]. For preparation of g-C3N4/AgBr/Fe3O4 nanocomposite with 4:1 weight ratio of g-C3N4/AgBr to Fe3O4, 0.84 g of
Fe3+, Fe2+
Melamine
g-C3N4 2) heating at 520 °C
NH3
Br-
Ag+
Scheme 1. The schematic diagram for preparation of g-C3N4/AgBr/Fe3O4 nanocomposites.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
3
g- C3N4
g-C3N4 AgBr Fe3O4
g- C3N4/AgBr(4:1)
Intensity (a.u.)
g-
g-
g-
g-
g- C3N4/Fe3O4(4:1)
10
20
30
40
50
60
70
80
2 Theta (deg.) Fig. 1. XRD patterns for g-C3N4, g-C3N4/AgBr (4:1), g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4.
g-C3N4/Fe3O4 nanocomposite was dispersed into 200 mL of distilled water using an ultrasonic bath for 5 min. Then, 0.144 g of silver nitrate was added to the suspension and stirred for 60 min at room temperature. Afterwards, an aqueous solution of sodium bromide (0.168 g in 20 mL of water) was slowly added to the suspension and refluxed at 96 1C for 60 min. The dark brown suspension was then centrifuged to get the precipitate out and washed two times with double distilled water and ethanol and dried in an oven at 60 1C for 24 h. The schematic diagram for preparation of the nanocomposites can be illustrated in Scheme 1. 2.4. Photocatalysis experiments Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400 mL capacity. Temperature of the reactor was maintained at 25 1C using a water circulation arrangement. The solution was mechanically stirred and continuously aerated by a pump to provide oxygen and complete mixing of the reaction solution. An LED source with 50 W was used as a visible-light source. The emission spectrum of the source has high intensity in visible range and its intensity rapidly decreases in wavelengths near to UV and IR ranges [12]. The source was fitted on the top of the reactor. Prior to illumination, a suspension containing 0.1 g of the photocatalyst and 250 mL of RhB solution (2.50 10 5 M) was continuously stirred in the dark for 60 min, to attain adsorption equilibrium. Samples were taken from the reactor at regular intervals and the photocatalyst removed before analysis by the spectrophotometer at 553 nm corresponding to the maximum absorption wavelength of RhB. 3. Results and discussion The XRD patterns for g-C3N4, g-C3N4/AgBr (4:1), g-C3N4/ AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4 along with g-C3N4/ Fe3O4 (4:1) are
shown in Fig. 1. There are two distinct well-defined diffraction peaks at 13.631 and 27.671 for g-C3N4, which could be ascribed to (100) and (002) diffraction planes (JCPDS 87-1526) [27]. In the case of g-C3N4/AgBr (4:1) nanocomposite, the diffraction peaks are clearly indexed to g-C3N4 and AgBr counterparts [28]. The XRD patterns for g-C3N4/AgBr/Fe3O4 nanocomposites are composed of the diffraction peaks corresponding to g-C3N4, AgBr and Fe3O4. Hence, presence of AgBr and Fe3O4 nanoparticles on g-C3N4 sheets is clearly confirmed. Moreover, it is evident that with increasing weight of ratio of g-C3N4/AgBr to Fe3O4, intensity of the corresponding peaks for g-C3N4 and AgBr was increased and that of Fe3O4 decreased. The EDX technique was used for confirming purity of the samples and the presence of AgBr and Fe3O4 nanomaterials on surface of the g-C3N4 and the results are shown in Fig. 2. For g-C3N4, the peaks are clearly related to C and N elements. In the case of g-C3N4/AgBr (4:1) nanocomposite, the peaks correspond to C, N, Ag, and Br elements. For g-C3N4/Fe3O4 (4:1) nanocomposite, the peaks are related to C, N, Fe, and O elements. Furthermore, in the case of g-C3N4/AgBr/Fe3O4(4:1) nanocomposite, the peaks are clearly related to C, N, Ag, Br, Fe, and O elements. Therefore, deposition of AgBr and Fe3O4 on g-C3N4 sheets was further confirmed by the EDX technique. Other peaks in the figure are related to the elements applied for sputter coating of the samples on the EDX stage. Fig. 3 shows TEM images for the g-C3N4 and g-C3N4/ AgBr/Fe3O4(4:1) nanocomposite. As can be seen, for pristine g-C3N4, the nanosheets are so thin that they are transparent to the electron beam. In the case of g-C3N4/AgBr/Fe3O4(4:1) nanocomposite, AgBr nanorods and Fe3O4 nanoparticles are clearly seen on surface of the g-C3N4 sheets. The FT-IR spectra for g-C3N4, g-C3N4/Fe3O4 (4:1) and g-C3N4/ AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/ AgBr to Fe3O4 are shown in Fig. 4. For pristine g-C3N4, the bands in range of 1230–1650 cm 1 correspond to the stretching vibrations
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
4
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
Fig. 2. EDX spectra for (a) g-C3N4, (b) g-C3N4/AgBr (4:1), (c) g-C3N4/Fe3O4 (4:1), and (d) g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite.
of C–N and CQN in heterocycles [27]. Furthermore, the band at 806 cm 1 is related to the breathing mode of the heptazine arrangement [29]. For all samples, the broad absorption band at 3000–3300 cm 1 is due to the terminal NH2 or NH groups at the defect sites of g-C3N4 aromatic rings [29]. In the case of g-C3N4/ AgBr/Fe3O4 nanocomposites, along with g-C3N4 bands, the two characteristic peaks at 620 and 430 cm 1 are related to stretching vibrations of Fe–O bond [30]. It is well known that activity of photocatalysts is closely related to their optical properties. Hence, UV–vis DRS spectra for the prepared samples are shown in Fig. 5. The pristine g-C3N4, similar to the reported data, has a band edge at 480 nm. In the case of g-C3N4/AgBr (4:1), the absorption edge shifts to a longer wavelength [31]. For g-C3N4/AgBr/Fe3O4 nanocomposites, with increasing weight ratios of Fe3O4 to g-C3N4/AgBr, their ability to absorb visible-light irradiation was enhanced. As a result, it can be concluded that the nanocomposites could have remarkable activity relative to the pristine g-C3N4. Fig. 6 displays the TGA curves for Fe3O4, g-C3N4, and g-C3N4/ AgBr/Fe3O4(4:1) nanocomposite. Small increase of the weight for
the samples around 350 1C was related to base line changing of the instrument. By heating Fe3O4 nanoparticles up to 700 1C, there were almost no changes in its weight. It is evident that the pristine g-C3N4 undergoes a weight loss of about 97% when heated up to 700 1C. This weight loss can be attributed to combustion of g-C3N4 to produce mainly gaseous products. As can be seen, the thermal behavior of g-C3N4/AgBr/Fe3O4(4:1) nanocomposite is similar to that of g-C3N4. For this nanocomposite, rapid weight loss starts from 470 1C and about 60% of its weight was lost by heating the sample up to 700 1C. Hence, g-C3N4 content of the nanocomposite is about 60%. Fig. 7 shows plots of magnetization versus the applied external magnetic field for the as-prepared Fe3O4 nanoparticles and g-C3N4/AgBr/Fe3O4(4:1) nanocomposite, at room temperature. The saturated magnetization for Fe3O4 nanoparticles and g-C3N4/AgBr/Fe3O4(4:1) nanocomposite at 8500 Oe are 45.1 and 19.5 emu/g, respectively. It is clearly evident that in the presence of g-C3N4 and AgBr, the saturated magnetization of Fe3O4 nanoparticles was reduced, confirming the presence of nonmagnetic materials on the surface of g-C3N4 nanosheets. Moreover, the inset of Fig. 7 shows separation of the magnetic
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
5
Fig. 3. TEM images for (a) g-C3N4, and (b) g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite.
g- C3N4
%Transmittance
g-
g-
g-
g-
heptazine N—H
Fe —O
g- C3N4/Fe3O4(4:1)
C—N & C=N 400
800
1200
1600
2000
2400
2800
3200
3600
4000
Wavenumber (cm-1) Fig. 4. FT-IR spectra for g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4.
photocatalyst from the treated solution using a magnet after 40 s. Hence, it can be concluded that magnetization of the nanocomposite is high enough to magnetically separate the visible-light-driven photocatalyst from the treated solution. Photodegradation of RhB under visible-light irradiation was considered to evaluate activity of the prepared samples. In Fig. 8a, plots of absorbance versus wavelength for the degradation reaction on g-C3N4/AgBr/Fe3O4(4:1) nanocomposite ([RhB]¼ 2.50 10 5 M, weight¼ 0.1 g) at various irradiation times are shown. As can be seen, under the light irradiation, intensity of the absorption peaks gradually decreases without any changes in position. It is well known that photocatalytic degradation of RhB occurs by two different mechanisms. The first mechanism involves de-ethylation of RhB and formation of various intermediates, and the second one is aromatic ring opening mechanism [32]. In the
later mechanism, the absorbance in the UV and visible ranges continually decreases without any changing of wavelengths [33]. As can be seen in Fig. 8a, the absorbance in all ranges gradually decreases without any changes in positions of peaks and new absorption peaks are not formed. Hence, it can be concluded that the degradation of RhB molecules on the nanocomposite take places by aromatic ring-opening mechanism. It is well known that activity of photocatalysts is closely related to their compositions. In Fig. 8b, plots of the RhB absorbance versus the irradiation time for the degradation reaction on g-C3N4/AgBr nanocomposites with different weight ratios of g-C3N4 to AgBr are shown. As can be seen, the photocatalytic activity of g-C3N4/AgBr (4:1) nanocomposite is greatly higher than that of pure g-C3N4. After this composition, by further increasing AgBr content of the nanocomposite, the
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
6 1.6
g-C3N4
1.4 g-C3N4/AgBr(4:1)
Absorbance
1.2 g-C3N4/AgBr/Fe3O4(6:1)
1.0 0.8
g-C3N4/AgBr/Fe3O4(4:1)
0.6
g-C3N4/AgBr/Fe3O4(2:1)
0.4
g-C3N4/AgBr/Fe3O4(1:1)
0.2 g-C3N4/Fe3O4(4:1)
0.0 280
330
380
430
480
530
580
630
680
Wavelength (nm) Fig. 5. UV–vis DRS for g-C3N4, g-C3N4/AgBr (4:1), g-C3N4/Fe3O4 (4:1) and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4.
60
120
Fe3O4 45
100
g-C3N4/AgBr/Fe3O4(4:1)
Magnetization (emu/g)
Weight Loss (%)
30
80
60
40
15 0 -15 -30
g-C3N4
20
-45
g-C3N4/AgBr/Fe3O4(4:1) Fe3O4
0
-60 -10000
0
100
200
300
400 Temperature (°C)
500
600
700
Fig. 6. TGA curves for g-C3N4, Fe3O4, and g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite.
photocatalytic activity is not considerably enhanced. Hence, according to the higher cost of AgBr precursors relative to that of g-C3N4, the g-C3N4/AgBr (4:1) nanocomposite was selected for deposition of Fe3O4 nanoparticles on g-C3N4 sheets. It is generally accepted that photocatalytic activity and magnetic separability of photocatalysts considerably depends on weight ratio of photoactive material to magnetic material [7,12]. Hence, to determine the optimum value, photocatalytic activity of the as-prepared nanocomposites for degradation of RhB under visible-light irradiation is compared in Fig. 9a. It is evident that without using a photocatalyst, only 9% of RhB molecules were degraded after irradiation for 420 min. It is clear that degradation of RhB on g-C3N4/AgBr/Fe3O4 nanocomposites is higher than those of g-C3N4 and g-C3N4/Fe3O4. Moreover, the
-5000
0
5000
10000
Applied field (Oe)
Fig. 7. Magnetization curves measured at room temperature for Fe3O4 nanoparticles and g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite. Inset of the figure is separation of the nanocomposite from the treated solution using an external magnetic field.
weight ratio of g-C3N4/AgBr to Fe3O4 has remarkable influence on the degradation reaction and superior activity was seen for gC3N4/AgBr/Fe3O4 (4:1) nanocomposite. However, after this composition, by increasing weight ratio of Fe3O4 to g-C3N4/ AgBr, the photocatalytic activity considerably decreased. Nanoparticles of Fe3O4 are nearly inactive in photocatalytic processes [12]. Hence, decrease of the photocatalytic activity with increasing Fe3O4 in the nanocomposites can be attributed to decrease of photoactive material in the photocatalysts [12]. Under the visiblelight irradiation for 420 min, 54, 58, and 98.3% of RhB molecules were degraded on g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite, respectively. Moreover, it is evident that in presence of g-C3N4/AgBr/Fe3O4 (4:1)
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
increases up to 60 min and decreases thereafter. The increase in the rate constant could be related to the increasing crystallinity of the nanocomposite. Due to increases in the size and aggregation of the photocatalyst, the rate constant decreases at higher refluxing times [12]. Hence, the sample prepared by refluxing for 60 min was selected for further studies In order to study the effect of calcination temperature on the photocatalytic activity, g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite was calcined for 2 h at 200, 300, 400, and 500 1C and the results are shown in Fig. 10b. It is clear that the degradation rate constant decreases with calcination of the sample and the nanocomposite without any thermal treatment has the highest rate constant. Decrease of the degradation rate constant could be related to aggregation and growth of the particle sizes at higher temperatures [34]. Based on Butler and Ginley model, the conduction band (CB) and valence band (CB) energies for g-C3N4 and AgBr at the point of zero charge were calculated using the following equations [35]:
1.2
1.0
Photolysis g-C3N4 g-C3N4/AgBr (4:1) g-C3N4/AgBr (3:2) g-C3N4/AgBr (2:3) AgBr
Absorbance
0.8
0.6
0.4
0.2
0.0 -60
-30
7
0
30
60
90
120
150
180
Irradiation time (min)
Fig. 8. (a) UV–vis spectra for degradation of RhB under visible-light irradiation on g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite ([RhB]¼ 2.5 10 5 M, catalyst weight¼0.1 g) at 25 1C. (b) Photodegradation of RhB on g-C3N4/AgBr nanocomposites with different weight ratio of g-C3N4 to AgBr.
nanocomposite, without using the light irradiation (dark experiment), about 8.3% of RhB molecules were adsorbed on the nanocomposite during 420 min. Hence, decrease of RhB is the solution containing the nanocomposite is majorly related to photocatalytic degradation reaction. The observed first-order rate constants of the degradation reaction (kobs) are shown in Fig. 9b. The degradation rate constant increases with weight ratio of gC3N4/AgBr to Fe3O4 up to 4:1 and then decreases. The rate constant for g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/ Fe3O4(4:1) nanocomposite are 16.0 10 4, 17.2 10 4, and 86.3 10 4 min 1, respectively. Hence, the photocatalytic activity of g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite is about 5.3, and 5-fold larger than those of g-C3N4, and g-C3N4/Fe3O4 (4:1), respectively. The influence of refluxing time applied for preparation of g-C3N4/ AgBr/Fe3O4 (4:1) nanocomposite was investigated by preparing five comparative samples that were refluxed for 0, 15, 30, 60, and 120 min. Fig. 10a shows plot of the rate constant versus the refluxing time. As can be seen, the degradation rate constant
ECB ¼ X E e 0:5E g
ð1Þ
EVB ¼ ECB þ E g
ð2Þ
Where X is the absolute electronegativity of the semiconductor, defined as the geometric average of the absolute electronegativity of the constituent atoms. Ee and Eg are the energy of free electrons on the hydrogen scale (4.5 eV) and the band gap energy of the semiconductor, respectively. The calculated VB and CB energies for g-C3N4 are 1.58 and 1.12 eV and those for AgBr are 2.56 and 0.04 eV, respectively. The Eg values for g-C3N4 and AgBr are 2.70 and 2.52 eV, respectively [36]. Hence, under the visible-light irradiation, both of them produce electron-hole pairs. The VB and CB energies of g-C3N4 are more negative than those of AgBr. As a result, the photogenerated electrons on CB of g-C3N4 easily transfer to that of AgBr and holes transfer in the reverse direction. Consequently, electrons and holes accumulated on CB of AgBr and VB of g-C3N4, respectively. Therefore, the photogenerated electrons and holes are effectively separated from the each other, leading to decrease of electron-hole recombination. Then, the electrons on CB of AgBr easily transfer to the adsorbed oxygen to produce superoxide ions. The superoxide ions and hole react with adsorbed molecules of water and hydroxide ions to produce hydroxyl radicals. Finally, RhB molecules are degraded by superoxide ions, hydroxyl radicals, and holes to produce the degradation products. The role of the reactive species in degradation of RhB was examined by using a series of scavengers. Fig. 11 shows plot of the rate constant for degradation of RhB on the nanocomposite in the presence of the selected scavengers. As can be seen, decrease of the rate constant in the presence of benzoquinone is higher than those of the other scavengers. Benzoquinone, KI, and 2-PrOH are scavengers of d O2 , h þ and ∙OH, respectively [37,38]. Hence, the role of superoxide ions in the degradation reaction is higher than those of hydroxyl radicals and holes. Reusability of a photocatalyst is a very important parameter from an economical viewpoint. To know reusability of the nanocomposite, the degradation experiments were carried out and the
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
8
1.2 Photolysis
1.0
Dark
g-C3N4
Absorbance
0.8
g-C3N4/AgBr/Fe3O4(6:1)
0.6 g-C3N4/AgBr/Fe3O4(4:1)
0.4
g-C3N4/AgBr/Fe3O4(2:1)
g-C3N4/AgBr/Fe3O4(1:1)
0.2
g-C3N4/Fe3O4(4:1)
-60
0.0
0
60
120
180
240
300
360
420
Irradiation time (min) 90
kobs (min-1)×10-4
75
60
45
30
15
0
Fig. 9. (a) Photodegradation of RhB on g-C3N4, g-C3N4/Fe3O4 (4:1), and g-C3N4/AgBr/Fe3O4 nanocomposites with different weight ratios of g-C3N4/AgBr to Fe3O4. (b) The degradation rate constant of RhB on different samples.
results are demonstrated in Fig. 12. In each run, the nanocomposite was recycled after washing and drying at 60 1C for 24 h. As can be seen, after using the photocatalyst for five successive runs, there is no remarkable decrease in its activity. Hence, the photocatalyst has a good lifetime during the degradation reaction and magnetic recycling from the treated solution. 4. Conclusions A series of g-C3N4/AgBr/Fe3O4 nanocomposites as novel magnetically separable visible-light-driven photocatalysts were prepared using a facile large-scale method. The structures, morphology, optical, thermal and magnetic properties of the nanocomposites
were characterized by different techniques. Photocatalytic activity of the nanocomposites was investigated by degradation of RhB under visible-light irradiation. The degradation rate constant increased with weight ratio of g-C3N4/AgBr to Fe3O4 up to 4:1 and then decreased. The photocatalytic activity of the g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite is about 5.3, and 5-fold higher than those of g-C3N4, and g-C3N4/Fe3O4 (4:1), respectively. Furthermore, the rate constant increases with refluxing time up to 60 min and then decreases. The degradation rate constant of RhB on the nanocomposite without any thermal treatment was higher than those of the calcined samples. Based on the effects of scavengers, it was concluded that superoxide ions have a vital role in visible-light degradation of RhB on the
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
9
90
75
75
60
60
kobs (min-1)×10-4
kobs (min-1)×10-4
90
45
30
45
30
15 15
0
without reflux
15 min
30 min
60 min
120 min 0
Refluxing time
Without
2-PrOH
BQ
KI
Type of scavenger 90
Fig. 11. The degradation rate constant of RhB on the nanocomposite in presence of various scavengers.
75 1.2
kobs (min-1)×10-4
Run 1
60
Run 2 Run 3
1.0
Run 4
45
Run 5
0.8
Absorbance
30
15
0.6
0.4
0
Room temp
200
300
400
500
Calcination temp. (°C) Fig. 10. (a) The degradation rate constant of RhB on g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite prepared at different refluxing times. (b) The degradation rate constant of RhB on g-C3N4/AgBr/Fe3O4 (4:1) nanocomposite calcined at different temperatures for 2 h.
0.2
0.0
0
500
1000
1500
2000
Irradiation time (min)
Fig. 12. Reusability of the nanocomposite for five successive runs.
nanocomposite. Finally, the nanocomposite has good reusability for five successive runs. Acknowledgement The Authors wish to acknowledge University of Mohaghegh Ardabili, for financial support of this work. References [1] T. Reemtsma, M. Jekel, in: Organic Pollutants in the Water Cycle: Properties, Occurrence, Analysis And Environmental Relevance of Polar Compounds, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2006.
[2] C.C. Chen, W.H. Ma, J.C. Zhao, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chem. Soc. Rev. 39 (2010) 4206–4219. [3] B. Ohtani, Photocatalysis A to Z—what we know and what we do not know in a scientific sense, J. Photochem. Photobiol. C: Photochem. Rev. 11 (2010) 157–178. [4] S.-M. Lam, J.-C. Sin, A.Z. Abdullah, A.R. Mohamed, Degradation of wastewaters containing organic dyes photocatalysed by zinc oxide: a review, Desalin. Water Treat. 41 (2012) 131–169. [5] E. Grabowska, J. Reszczynska, A. Zaleska, Mechanism of phenol photodegradation in the presence of pure and modified TiO2: a review, Water Res. 46 (2012) 5453–5471. [6] Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar, J. He, Visible-lightdriven type II heterostructures and their enhanced photocatalysis properties: a review, Nanoscale 5 (2013) 8326–8339.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145
10
A. Akhundi, A. Habibi-Yangjeh / Ceramics International ] (]]]]) ]]]–]]]
[7] S.-Q. Liu, Magnetic semiconductor nano-photocatalysts for the degradation of organic pollutants, Environ. Chem. Lett. 10 (2012) 209–216. [8] J. Jing, J. Li, J. Feng, W. Li, W.W. Yu, Photodegradation of quinoline in water over magnetically separable Fe3O4/TiO2 composite photocatalysts, Chem. Eng. J. 219 (2013) 355–360. [9] X. Bian, K. Hong, L. Liu, M. Xu, Magnetically separable hybrid CdS– TiO2–Fe3O4 nanomaterial: enhanced photocatalystic activity under UV and visible irradiation, Appl. Surf. Sci. 280 (2013) 349–353. [10] R. Shao, L. Sun, L. Tang, Z. Chen, Preparation and characterization of magnetic core–shell ZnFe2O4@ZnO nanoparticles and their application for the photodegradation of methylene blue, Chem. Eng. J. 217 (2013) 185–191. [11] X. Feng, H. Guo, K. Patel, H. Zhou, X. Lou, High performance, recoverable Fe3O4–ZnO nanoparticles for enhanced photocatalytic degradation of phenol, Chem. Eng. J. 244 (2014) 327–334. [12] M. Shekofteh-Gohari, A. Habibi-Yangjeh, Facile preparation of Fe3O4@AgBr–ZnO nanocomposites as novel magnetically separable visiblelight-driven photocatalysts, Ceram. Int. 41 (2015) 1467–1476. [13] Y. Zheng, J. Liu, J. Liang, M. Jaroniec, S.Z. Qiao, Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis, Energy Environ. Sci. 5 (2012) 6717–6731. [14] G. Dong, Y. Zhang, Q. Pan, J. Qiu, A fantastic graphitic carbon nitride (gC3N4) material: Electronic structure, photocatalytic and photoelectronic properties, J. Photochem. Photobiol. C: Photochem. Rev. 20 (2014) 33–50. [15] W. Liu, M. Wang, C. Xu, S. Chen, Facile synthesis of g-C3N4/ZnO composite with enhanced visible light photooxidation and photoreduction properties, Chem. Eng. J. 209 (2012) 386–393. [16] C. Wang, W. Zhu, Y. Xu, H. Xu, M. Zhang, Y. Chao, S. Yin, H. Li, J. Wang, Preparation of TiO2/g-C3N4 composites and their application in photocatalytic oxidative desulfurization, Ceram. Int. 40 (2014) 11627–11635. [17] 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. [18] Z. Jin, N. Murakami, T. Tsubota, T. Ohno, Complete oxidation of acetaldehyde over a composite photocatalyst of graphitic carbon nitride and tungsten(VI) oxide under visible-light irradiation, Appl. Catal. B: Environ. 150–151 (2014) 479–485. [19] S. Ye, L.-G. Qiu, Y.-P. Yuan, Y.-J. Zhu, J. Xia, J.-F. Zhu, Facile fabrication of magnetically separable graphitic carbon nitride photocatalysts with enhanced photocatalytic activity under visible light, J. Mater. Chem. A 1 (2013) 3008–3015. [20] S. Hu, R. Jin, G. Lu, D. Liu, J. Gui, The properties and photocatalytic performance comparison of Fe3 þ -doped g-C3N4 and Fe2O3/g-C3N4 composite catalysts, R.Soc. Chem. Adv. 4 (2014) 24863–24869. [21] S. Zhang, J. Li, M. Zeng, G. Zhao, J. Xu, W. Hu, X. Wang, In situ synthesis of water-soluble magnetic graphitic carbon nitride photocatalyst and its synergistic catalytic performance, ACS Appl. Mater. Interfaces 5 (2013) 12735–12743. [22] K. Vignesh, A. Suganthi, B.-K. Min, M. Kang, Photocatalytic activity of magnetically recoverable MnFe2O4/g-C3N4/TiO2 nanocomposite under simulated solar light irradiation, J. Mol. Catal. A: Chem 395 (2014) 373–383.
[23] S. Kumar, T. Surendar, B. Kumar, A. Baruah, V. Shanker, Synthesis of magnetically separable and recyclable g-C3N4–Fe3O4 hybrid nanocomposites with enhanced photocatalytic performance under visible-light irradiation, J. Phys. Chem. C 117 (2013) 26135–26143. [24] X. Zhou, B. Jin, R. Chen, F. Peng, Y. Fang, Synthesis of porous Fe3O4/gC3N4 nanospheres as highly efficient and recyclable photocatalysts, Mater. Res. Bull. 48 (2013) 1447–1452. [25] Z. He, T. Hong, J. Chen, S. Song, A magnetic TiO2 photocatalyst doped with iodine for organic pollutant degradation, Sep. Purif. Technol. 96 (2012) 50–57. [26] R. Massart, Preparation of aqueous magnetic liquids in alkaline and acidic media, IEEE Trans. Magn. 17 (1981) 1247–1248. [27] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397–10401. [28] Y. Xu, H. Xu, J. Yan, H. Li, L. Huang, J. Xia, S. Yin, H. Shu, A plasmonic photocatalyst of Ag/AgBr nanoparticles coupled with g-C3N4 with enhanced visible-light photocatalytic ability, Colloids Surf. A: Physicochem. Eng. Asp. 436 (2013) 474–483. [29] L. Liu, D. Ma, H. Zheng, X. Li, M. Cheng, X. Bao, Synthesis and characterization of microporous carbon nitride, Microporous Mesoporous Mater. 110 (2008) 216–222. [30] H. Gupta, P. Paul, N. Kumar, S. Baxi, D.P. Das, One pot synthesis of water-dispersible dehydroascorbic acid coated Fe3O4 nanoparticles under atmospheric air: blood cell compatibility and enhanced magnetic resonance imaging, J. Colloid Interface Sci. 430 (2014) 221–228. [31] H. Xu, J. Yan, Y. Xu, Y. Song, H. Li, J. Xia, C. Huang, H. Wan, Novel visible-light-driven AgX/graphite-like C3N4 (X¼ Br, I) hybrid materials with synergistic photocatalytic activity, Appl. Catal. B: Environ. 129 (2013) 182–193. [32] X. Li, J. Ye, Photocatalytic degradation of rhodamine B over Pb3Nb4O3/ fumed SiO2 composite under visible light irradiation, J. Phys. Chem. C 111 (2007) 13109–13116. [33] A. Martinez-dela Cruz, U.M. 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. [34] S.Y. Kim, T.H. Lim, T.S. Chang, C.H. Shin, Photocatalysis of methylene blue on titanium dioxide nanoparticles synthesized by modified solhydrothermal process of TiCl4, Catal. Lett. 117 (2007) 112–118. [35] J. Cao, X. Li, H.L. Lin, S.F. Chen, X.L. Fu, In situ preparation of novel p–n junction photocatalyst BiOI/(BiO)2CO3 with enhanced visible light photocatalytic activity, J. Hazard. Mater. 239–240 (2012) 316–324. [36] J. Cao, Y. Zhao, H. Lin, B. Xu, S. Chen, Ag/AgBr/g-C3N4: a highly efficient and stable composite photocatalyst for degradation of organic contaminants under visible light, Mater. Res. Bull. 48 (2013) 3873–3880. [37] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Effective photocatalytic disinfection of E. coli K-12 using AgBr–Ag–Bi2WO6 nanojunction system irradiated by visible light: the role of diffusing hydroxyl radicals, Environ. Sci. Technol. 44 (2010) 1392. [38] G.T. Li, K.H. Wong, X.W. 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.
Please cite this article as: A. Akhundi, A. Habibi-Yangjeh, Novel magnetically separable g-C3N4/AgBr/Fe3O4 nanocomposites as visible-light-driven photocatalysts with highly enhanced activities, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2014.12.145