Magnetic ZnFe2O4–C3N4 Hybrid for Photocatalytic Degradation of

Oct 13, 2014 - Magnetic ZnFe2O4−C3N4 Hybrid for Photocatalytic Degradation of. Aqueous Organic Pollutants by Visible Light. Yunjin Yao,*. ,†,§,âˆ...
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Magnetic ZnFe2O4−C3N4 Hybrid for Photocatalytic Degradation of Aqueous Organic Pollutants by Visible Light Yunjin Yao,*,†,§,∥ Yunmu Cai,† Fang Lu,† Jiacheng Qin,† Fengyu Wei,† Chuan Xu,† and Shaobin Wang*,‡ †

Anhui Key Lab of Controllable Chemical Reaction & Material Chemical Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Tunxi Road 193, Hefei 230009, China ‡ Department of Chemical Engineering, Curtin University, G.P.O. Box U1987, Perth, Western Australia 6845, Australia § State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China ∥ School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China ABSTRACT: Magnetic ZnFe2O4−C3N4 hybrids were successfully synthesized through a simple reflux treatment of ZnFe2O4 nanoparticles (NPs) (ca. 19.1 nm) with graphitic C3N4 sheets in methanol at 90 °C, and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and UV−vis diffuse reflectance spectroscopy. Also, the catalytic activities of heterogeneous ZnFe2O4−C3N4 catalysts were evaluated in photo-Fenton discoloration toward Orange II using H2O2 as an oxidant under visible light (λ > 420 nm) irradiation. The reaction kinetics, degradation mechanism, and catalyst stability, as well as the roles of ZnFe2O4 and C3N4 in photoreaction, were comprehensively studied. It was found that the ZnFe2O4−C3N4 photocatalysts presented remarkable catalytic ability at neutral conditions, which is a great advantage over the traditional Fenton system (Fe2+/H2O2). The ZnFe2O4−C3N4 hybrid (mass ratio of ZnFe2O4/g-C3N4 = 2:3) exhibits the highest degradation rate of 0.012 min−1, which is nearly 2.4 times higher than that of the simple mixture of g-C3N4 and ZnFe2O4 NPs. g-C3N4 acted as not only a p-conjugated material for the heterojunction formation with ZnFe2O4, but also a catalyst for the decomposition of H2O2 to ·OH radicals. The heterogeneous ZnFe2O4−C3N4 hybrid exhibited stable performance without losing activity after five successive runs, showing a promising application for the photo-oxidative degradation of organic contaminants.

1. INTRODUCTION Over the past several years, advanced oxidation processes (AOPs) such as ozonation, Fenton and photo-Fenton reactions, photocatalysis, ultrasonication, etc. involving the generation of highly reactive transitory species (·OH), have been successfully applied in wastewater treatment to degrade colorants, pesticides, herbicides, and other biorecalcitrant compounds.1−3 Among the various AOPs, the Fenton reaction produces radicals by homogeneous Fe(II/III) and H2O2, which appears to be a promising technology because of its low toxicity, fast reaction rate, and simplicity of operation. The rate of the Fenton reaction could be accelerated with an external light supply, such as ultraviolet (UV) irradiations, and this is called a photo-Fenton reaction.4,5 However, these homogeneous processes suffer from drawbacks in the application. First, a large excess of Fe salts presented in the treated effluent cannot be recycled, which can result in the formation of sludge. Second, the classic Fe2+-based Fenton system must be carried out in acidic pH. These disadvantages limit the widespread application of the method.6 Therefore, an intensive and hot research topic is to develop novel heterogeneous Fenton and photo-Fenton catalysts to avoid sludge formation from iron ions, to expand the effective pH range, and to enhance the generation of highly potent chemical oxidants such as ·OH radicals. Recently, an inorganic semiconductor, ZnFe2O4 with a relatively narrow band gap of 1.9 eV, has been investigated extensively in solar transformation, photocatalysis, and photochemical hydrogen production from water, because of its © 2014 American Chemical Society

visible-light response, easy synthesis, low cost, and good photochemical stability.7 Magnetically separable ZnFe2O4 nanoparticles (NPs) have attracted increasing attention because of their efficient recycle in water treatment and purification systems, providing an attractive and cost-effective method for practical operation.8 Over the past few years, more attention has been paid to the morphology-controlled synthesis of ZnFe2O4, and ZnFe2O4-based photocatalysts, for instance, ZnFe2O4−TiO2,9,10 ZnFe2O4−ZnO,11 ZnFe2O4−MWCNT,12 and ZnFe2O4-graphene,13,14 etc. In our previous work,15 a ZnFe2O4−graphene hybrid was successfully developed as a heterogeneous catalyst for photo-Fenton-like discoloration of various dyes using peroxymonosulfate as an oxidant under visible light irradiation. Unfortunately, the performance of the ZnFe2O4-based photocatalysts still does not meet the requirements of practical applications. Therefore, a low-cost fabrication of the ZnFe2O4-based photocatalysts with a highly effective visible-light response for environmental remediation is still being sought.16 Graphitic carbon nitride (g-C3N4), a p-conjugated material, is currently the focus of significance for its applications as a metalfree photocatalyst for organic pollutant degradation under visible light because of its many interesting features including visible-light adsorption ability and ease of large-scale preparaReceived: Revised: Accepted: Published: 17294

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precipitates were filtered, washed with deionized water, and dried in vacuum at 60 °C. These precipitates were calcined at 450 °C for 10 h to obtain the ZnFe2O4 product. 2.2. Synthesis of ZnFe2O4−C3N4 Catalysts. The synthesis of ZnFe2O4−C3N4 photocatalysts was carried out typically as follows: first, an appropriate amount of g-C3N4 was added into methanol solution in a beaker and then the beaker was placed in an ultrasonic bath to completely disperse the g-C3N4. The obtained magnetic ZnFe2O4 NPs were then dissolved in a citric acid and water−ethanol solution and added into the C3N4 sheets suspension. ZnFe2O4 NPs and C3N4 sheets mixture was further dispersed by ultrasonication for 30 min and stirred for 2 h at room temperature, and then refluxed with continuous stirring at a temperature of 90 °C. After 3 h, the heating was stopped and the suspension was stirred until it cooled down to room temperature. Finally, the resulting precipitate was filtered out, washed with deionized water, and dried at 60 °C until a constant mass was reached. According to this method, ZnFe2O4−C3N4 catalysts in different mass ratios (1:1, 1:3 and 2:3) of ZnFe2O4 and C3N4 were prepared and thus named as ZnFe2O4−C3N4 (1:1), ZnFe2O4−C3N4 (1:3), and ZnFe2O4−C3N4 (2:3), respectively. The schematic representation for the synthesis of ZnFe2O4−C3N4 catalysts is illustrated in Figure 1.

tion.17,18 However, there are also many drawbacks for the gC3N4 materials, which include low specific surface area, and high recombination rate of photogenerated electron−hole pairs. To solve these problems, it is favorable to combine g-C3N4 with other semiconductors with a suitable band gap to extend the absorption range and improve the quantum yield of the g-C3N4. Meanwhile, the heterojunctions of g-C3N 4 with other appropriate semiconductors could provide a greater driving force to improve the separation of electron−hole pairs. Currently, several photocatalytic systems have been developed, such as Fe3O4−C3N4,17 BiOBr-C3N4,19 ZnO-C3N4,20 TiO2− C3N4,21−23 Bi2WO6-g-C3N4,24 BiOI-C3N4,25 BiVO4−C3N4,26 and graphene-g-C3N4,27 which were proven to be effective to degrade various organic pollutants. Zhang et al.28 have reported that ZnFe2O4-g-C3N4 synthesized through a solvothermal method exhibited excellent photocatalytic efficiency for methyl orange degradation. Cui et al.29 found that g-C3N4 could activate H2O2 to generate reactive radicals under visible light irradiation, leading to the mineralization of Rhodamine B. However, to our best knowledge, little work has been done in photo-Fenton catalytic reactions using ZnFe2O4−C3N4 hybrids for the heterogeneous activation of H2O2 under visible light irradiation. In the present work, we predominantly focus on the synthesis of magnetic ZnFe2O4−C3N4 heterojunction catalysts and, specifically, on their application toward the heterogeneous photo-Fenton process. The physical and chemical characterizations of ZnFe2O4−C3N4 hybrids were conducted, and the performance in the heterogeneous reaction was evaluated in terms of the degradation of Orange II dye by H2O2 as a clean oxidant under visible light irradiation. The reaction kinetics, dye degradation mechanism, and material stability, as well as the roles of ZnFe2O4 and C3N4 were comprehensively studied. This research provides a novel practical approach for the use of heterogeneous photo-Fenton system (ZnFe2O4−C3N4/H2O2/ vis) in the efficient treatment of dye effluent. Our findings suggest that the as-prepared hybrids can act as an efficient catalyst with some advantageous properties, such as easy separation and excellent recyclability, which may hold great potential for the treatment of nonbiodegradable organic pollutants in water.

Figure 1. Schematic illustration of the synthetic method of ZnFe2O4− C3N4 catalysts.

2.3. Catalyst Characterization. X-ray diffractometry (XRD) for the crystal structure of the products was carried out in a Rigaku D/max2500v/pc diffractometer with Cu Kα radiation. Spectra of Fourier transform infrared spectroscopy (FTIR) were recorded on a PerkinElmer Spectrum 100 instrument with samples pressed in KBr. UV−visible diffuse reflectance spectra (DRS) of the samples were examined by a Varian Cary 5000 spectrometer. Thermogravimetric and differential thermal analysis (TG/DTA) for g-C3N4 and ZnFe2O4−C3N4 hybrids were performed on a NETZSCHTG209 F3 system over the temperature range of 25−800 °C at a heating rate of 10 °C/min under flowing air atmospheres. The morphology and structure of the ZnFe2O4−C3N4 catalysts were characterized by a field emission scanning electron microscope (FESEM; Hitachi SU8020, Japan) and highresolution transmission electron microscope (HRTEM) (JEOL JEM-2100F). Energy dispersive X-ray (EDX) and the selected area electron diffraction (SAED) analysis were also performed during the HRTEM measurements. X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB250 spectrometer equipped with a monochromatized Al Kα source. The charge effect was calibrated using the binding energy of C 1s (284.6 eV) to reduce the sample charging effect. 2.4. Photocatalytic Performance. The photocatalytic activity of as-prepared samples was evaluated with a homemade photochemical reactor, which mainly includes four parts: light

2. MATERIALS AND METHODS 2.1. Synthesis of ZnFe2O4 NPs and C3N4 Sheets. All of the reagents used in this research are of analytical purity and used without further purification. Bulk g-C3N4 sheets were synthesized by a modified heat-etching method using melamine as the precursor in a muffle furnace according to reported methods.27 In a typical synthesis run, 5 g of melamine were heated to 550 °C with a heating rate of 2 °C/min and held 4 h in static air. The resultant agglomerates were ground into fine powders. After that, the as-prepared bulk g-C3N4 was further heated in an open ceramic container at 500 °C for 2 h with a heating rate of 2 °C/min. The resultant yellow product was collected and ground to powder for further use. In a typical synthesis process of ZnFe2O4 NPs,30,31 3 mmol Zn(CH3COO)2·2H2O (0.659 g) and 6 mmol Fe(NO3)3·9H2O (2.424 g) were dissolved into 120 mL of ethylene glycol under magnetic stirring. Then, 90 mmol of NH4HCO3 was added to the mixture solution at room temperature. After being stirred vigorously for 30 min, the solution was transferred to a 150 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 20 h before being cooled down in air. The resulting 17295

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Figure 2. (a) XRD, (b) FTIR, (c) thermal decomposition curves and (d) UV−vis DRS of the samples. The inset shows the absorption spectra of the corresponding samples by plotting (αhν)1/2 vs photon energy (hν).

source system (a 500 W Xe lamp, λ > 420 nm); reactor (twolayer Pyrex glass bottles of 500 mL capacity, the space between the two layers is filled with circulating water to cool the reactor); magnetic stirrer; and temperature controller. In a typical process, a photocatalyst (0.05 g) was added to an aqueous solution of Orange II (100 mL, 10 mg L−1) at room temperature under stirring at 250 rpm throughout the test. At certain time intervals, three milliliters of water solution were taken at given time intervals, separated from the photocatalyst particles, and used for the absorbance measurement. The color removal of the dye solution during the degradation process was analyzed by recording variations of the maximum absorption band in the UV−vis spectra of Orange II. In addition, the recyclability of ZnFe2O4−C3N4 catalysts was investigated. After each catalytic run, the solid sample was collected, washed, and dried to perform subsequent photoreaction cycles.

well-known for the melon networks.19 Another peak at 12.81°, corresponding to a distance of 0.675 nm, belongs to (100) inplanar ordering of tri-s-triazine units. When ZnFe2O4 coupled with g-C3N4, the crystal phase of ZnFe2O4 did not change, and the diffraction peak (002) of g-C3N4 was also present in the XRD pattern of ZnFe2O4-g-C3N4. However, the (100) diffraction peak for g-C3N4 disappeared, due to the destruction of the regular stacking of g-C3N4 between the interlayers during the reaction. Moreover, no other peaks were appearing in the XRD patterns, indicating the ZnFe2O4-g-C3N4 to be a twophase hybrid. The average size of the as-synthesized ZnFe2O4 NPs on g-C3N4 sheets was about 19.1 nm, calculated from the Debye−Scherrer equation based on the measurements of the full width at half-maximum of the (311) peaks. Figure 2b displays FTIR spectra of g-C3N4, ZnF2O4 NPs, and ZnFe2O4−C3N4 hybrid. In the case of pure C3N4, characteristic bands in 900−1800 cm−1 region with peaks appearing at 1247, 1320, 1420, 1574, and 1635 cm−1 are attributed to either trigonal C−N(−C)−C (full condensation) or bridging C− NH−C units.33 The former adsorption band is broad and centered at 3172 cm−1, which can be ascribed to the stretching mode of the N−H bond.34 Additionally, the characteristic breathing mode of the s-triazine ring system at 806 cm−1 is observed. For ZnF2O4 NPs, 571 cm−1 is assigned as the symmetric stretching vibration peak of the Fe−O band in the

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Figure 2a shows XRD patterns of g-C3N4, ZnFe2O4, and ZnFe2O4-g-C3N4 photocatalysts. It can be found that the diffraction peaks of ZnFe2O4 were in good agreement with the standard pattern of a cubic spinel structure (JCPDS 77-0011).32 For pure g-C3N4, a strong peak at 27.21°, corresponding to width of 0.326 nm, can be indexed as (0 0 2) diffraction plane (JCPDS 87-1526), which is 17296

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Figure 3. FESEM image of (a) bare g-C3N4 and (b) ZnFe2O4−C3N4 (2:3) hybrid; (c) HRTEM images with ZnFe2O4−C3N4 (2:3) hybrid. The inset is the corresponding SEAD pattern. (d) EDX pattern of ZnFe2O4−C3N4 (2:3) hybrid.

ZnF2O4 structure, and 1637 cm−1 is the O−H bending vibration.14 It can be clearly seen that the main characteristic peaks of g-C3N4 and ZnFe2O4 appear in the spectrum of ZnFe2O4−C3N4 hybrid. In addition, broad bands in 3000− 3800 cm−1 regions can be observed for all samples, which corresponds to physisorbed H2O from the atmosphere. The formation and thermal stability of the materials were investigated specifically by the TG/DTA method (Figure 2c). Pure g-C3N4 is stable below 540 °C in airflow. When the temperature is higher than 540 °C, the sublimation or decomposition of g-C3N4 occurred, and a sharp peak at 643.9 °C was thus observed in the DTG curve. For ZnFe2O4−C3N4 hybrid, however, the stability of the ZnFe2O4−C3N4 greatly decreased because of the oxidation and decomposition of gC3N4 in the ZnFe2O4−C3N4 hybrid. The peak of the DTG curve shifted to ca. 549.1 °C, which is lower than that of pure gC3N4. Such a large decrease in temperature indicated that there was tight contact between ZnFe2O4 and g-C3N4. According to the mass loss in ZnFe2O4−C3N4, it could be calculated that the mass fractions of ZnFe2O4 and C3N4 in the ZnFe2O4−C3N4 hybrid are 0.374 and 0.626, respectively, which is in agreement with the feed ratio. UV−vis DRS was used to determine the optical properties of the samples. It can be seen from Figure 2d that the introduction of g-C3N4 as compared to ZnFe2O4 NPs has a significant effect on the optical property of the visible light absorption region for the ZnFe2O4−C3N4 sample. In comparison to that of the pure g-C3N4, the band edge positions of ZnFe2O4−C3N4 hybrids exhibited a small red shift. The enhanced absorption capability of visible light of ZnFe2O4−C3N4 suggests that it might have higher photocatalytic activity for target reactions under visible light irradiation. A plot obtained via the transformation based on the Kubelka−Munk function versus the energy of light is shown in the inset of Figure 2d. The estimated band gap values

of the samples are about 2.55, 1.65, and 1.28 eV, approximately, corresponding to g-C3N4, ZnFe2O4−C3N4, and ZnFe2O4, respectively. The UV−vis DRS results indicated that more photogenerated charges were generated when ZnFe2O4−C3N4 hybrids were excited under visible light irradiation, which enhances the photocatalytic performance.21 Such an analogous phenomenon, which can be attributed to the interfacial interaction between the semiconductor and C3N4 support,35 was also observed in previous research studies regarding semiconductor−C3N4 hybrids. Morphological structures of C3N4 and the ZnFe2O4−C3N4 hybrid have been further verified by FESEM and TEM observations (Figure 3). A typically lamellar structure with a large particle size is attributed to the pure C3N4 sample (Figure 3a). After introducing ZnFe2O4, amounts of NPs are mussily accumulated on the surface of g-C3N4, resulting in the formation of a heterostructure (Figure 3b). From the HRTEM image of ZnFe2O4−C3N4 hybrid (Figure 3c), two phases of g-C3N4 (light part) and ZnFe2O4 (dark part) are clearly observed and closely in contact to form an intimate interface, which further demonstrates that ZnFe2O4 NPs have well covered the surface of g-C3N4. The observation is consistent with the results of XRD and FTIR. On the other hand, the crystalline nature of the sample can be derived from the SAED pattern of a single particle (Figure 3c). The appearance of the bright arc in the diffraction circle in the SAED pattern indicated that the spherical structures were formed by ZnFe 2 O 4 nanocrystals in highly preferred orientations.32 As depicted in Figure 3c, atomic lattice fringes of 2.54 and 3.20 Å, corresponding to the (311) plane of ZnFe2O4 and (002) plane of g-C3N4, respectively, are clearly observed in the ZnFe2O4−C3N4 hybrid. The interfaces between ZnFe2O4 and g-C3N4 are smooth, which further verifies the formation of a ZnFe2O4−C3N4 heterojunction. Moreover, the 17297

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Figure 4. (a) Orange II degradation by ZnFe2O4−C3N4 (2:3) photocatalysts under different conditions. (b) UV−vis spectral changes for Orange II degradation with ZnFe2O4−C3N4/H2O2/vis system. The insets show the solution after magnetic separation using an external magnet. The photographs of the color change of the Orange II solution during different reaction times are also shown. (Reaction conditions: [Orange II] = 10 mg/L, [catalyst] = 0.5 g/L, T = 25 °C, [H2O2] = 0.10 M).

Figure 5. Kinetics of Orange II degradation over the different photocatalysts (a) and H2O2 concentration (b). Insets: degradation rate constant k. (Reaction conditions: [Orange II] = 10 mg/L, [catalyst] = 0.5 g/L, T = 25 °C).

corresponding EDX spectrum of ZnFe2O4−C3N4 hybrid also demonstrates the presence of Zn, Fe, C, N, and O (Figure 3d). Although having undergone a long mechanical sonication before the TEM observation, ZnFe2O4−C3N4 displays a firm connection between ZnFe2O4 and g-C3N4.21 This result also suggests that the ZnFe2O4−C3N4 heterojunctions in the structure are heterogeneous rather than a physical mixture of two separate phases of g-C3N4 and ZnFe2O4.34 Moreover, this tight coupling is favorable for the charge transfer between gC3N4 and ZnFe2O4 and promotes the separation of photogenerated electron−hole pairs, subsequently enhancing photocatalytic performance.25 3.2. Catalytic Evaluation. In this work, the photocatalytic performances of ZnFe2O4−C3N4 hybrids were evaluated by degradation of Orange II, a hazardous dye as well as a common model for testing photodegradation capability.2,36 Figure 4 shows the discoloration of Orange II versus time under different reaction systems. The photodegradation rates of Orange II followed an order of ZnFe2O4−C3N4/H2O2/vis > ZnFe2O4−C3N4/vis > ZnFe2O4−C3N4/H2O2 > ZnFe2O4− C3N4. As seen in Figure 4a, without a catalyst and H2O2, the absorbency of Orange II solution displays little difference before and after the visible-light illumination, indicating the

high stability of Orange II under the experimental conditions. Without H2O2 and visible light, the discoloration due to ZnFe2O4−C3N4 sample hardly occurred, indicating that ZnFe 2 O 4 −C 3 N 4 hybrids could not lead to significant adsorption of Orange II. With the ZnFe2O4−C3N4 catalyst and visible light irradiation or H2O2, the discoloration of Orange II increased greatly, which shows that the ZnFe2O4− C3N4 catalyst played an important role in the discoloration of Orange II. It is worthwhile to note that in the presence of H2O2 and visible light, ZnFe2O4−C3N4 hybrids exhibited the highest catalytic activity: 97% of Orange II was degraded by ZnFe2O4− C3N4 within 240 min. The color of Orange II solution gradually diminished upon ·OH in the presence of ZnFe2O4−C3N4 hybrids, indicating that photocatalysis destroyed the chromophoric structure of the dye. To understand and clarify the changes in molecular structure of Orange II in the presence of ZnFe2O4−C3N4 (2:3) and H2O2 under visible light irradiation, UV−vis spectra changes in the dye solution over various time intervals are shown in Figure 4b. Clearly, the main absorption peak of Orange II molecules locating at 484.5 nm and corresponding to the n−p* transition of the azo form,37,38 decreases rapidly with extension of the exposure time. Further exposure leads to no absorption peak in 17298

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the whole spectrum, which indicates the complete photocatalytic degradation of Orange II. This result is also consistent with the gradual color change of the reaction solution from orange to almost colorless during the different reaction times (the inset in Figure 4b). Furthermore, ZnFe2O4−C3N4 hybrid can be easily separated from the reaction solution by a magnet, which is important for practical applications. Figure 5a shows the photocatalytic activity of the ZnFe2O4− C3N4 hybrids with different ZnFe2O4 concentrations. To have a better understanding of the reaction kinetics of Orange II degradation, the experimental data were fitted by a pseudo-firstorder model. As shown in Figure 5a, the plots of ln(C/C0) versus irradiation time were linear, which indicates that the photodegradation of the Orange II went through a pseudo-firstorder kinetic reaction. The value of the rate constant k is equal to the corresponding slope of the fitting line. The ZnFe2O4− C3N4 hybrids exhibited a much higher Orange II catalytic capacity than pure ZnFe2O4. The ZnFe2O4−C3N4 (2:3) hybrid exhibits the highest degradation rate of 0.012 min−1, which is nearly 2.4 times higher than that of the simple mixture of ZnFe2O4 and g-C3N4. It is implied that the synergistic effect between the interface of ZnFe2O4 and g-C3N4 is crucial and the quality of the effective heterointerfaces plays the key role in improving the photocatalytic activity of the heterojunctions. In addition, pure g-C3N4 showed high photocatalytic activity, which could be attributed to its moderate band gap and unique electronic structure, which are in good agreement with the results reported in the literature.18,29 According to the experimental results, initial H 2 O 2 concentration appears to be the most important variable on degradation efficiency of Orange II. Figure 5b demonstrates that within the range from 0.01 to 0.10 M, the degradation rate constant of Orange II was significantly improved from 0.0028 to 0.012 min−1. However, a further increase of initial H2O2 concentration to 0.15 M could reduce the degradation rate constant (k = 0.0085 min−1). It is in good agreement with previously reported results.39,40 This fact can be explained by taking into account the inhibitory effect of H2O2. An increase in H2O2 concentration may promote ·OH radicals scavenging (see eq 1) and the subsequent formation of a new radical (H2O·), with an oxidation potential considerably smaller than that of · OH.39,40 ·OH + H 2O2 → HO2 · + H 2O

Figure 6. Cycling runs for the photocatalytic degradation of Orange II over ZnFe2O4−C3N4 heterojunction under visible light irradiation. (Reaction conditions: [Orange II] = 10 mg/L, [catalyst] = 0.5 g/L, T = 25 °C, [H2O2] = 0.10 M).

ZnFe2O4−C3N4 hybrids shows the coexistence of elements, C, N, Zn, Fe, and O (Figure 7a). As shown in Figure 7b, the peaks at binding energies of 1044.4 and 1021.4 eV are assigned to Zn 2p1/2 and Zn 2p3/2, respectively, implying that Zn element exists mainly in the chemical state of Zn2+.7 A slight positive shift in the Zn2+ binding energies was observed for ZnFe2O4− C3N4 samples after the photo-Fenton reaction. It may be due to the redox reaction for electron transfer in the photoprocess and deposited intermediates from reactants. The Fe 2p XPS spectrum of the ZnFe2O4−C3N4 hybrids shows the main peaks of Fe 2p1/2 and 2p3/2 at 724.6 and 710.9 eV, respectively. Furthermore, a satellite peak of the Fe 2p3/2 can also be detected at the position of 719.1 eV, which is in good agreement with the reported value for Fe3+ compounds.41 The N 1s XPS spectrum in Figure 7d can be deconvoluted into three main peaks at 398.82, 399.48, and 400.88 eV, which correspond to C−N−C, N−(C) 3 , and N−H groups, respectively.42 The first C 1s peak at 288.5 eV was assigned to sp2-bonded carbon (N−CN), and the second at 284.8 eV was attributed to graphitic carbon which was usually observed on carbon nitrides.19,43 Figure 7f shows two main peaks of O 1s with the binding energies of 531.9 and 529.78 eV, which are assigned to the surface −OH of water molecules and the lattice oxygen of ZnFe2O4, respectively.21,44 There is no noticeable change in the XPS spectra of the catalyst before and after the reaction, implying that the catalysts are essentially unchanged during the photocatalytic reaction, and the obtained ZnFe2O4− C3N4 hybrids can be used as a stable visible-light-responsive photocatalyst.7 3.4. Reaction Mechanism. According to the above results, it is evident that ZnFe2O4−C3N4 photocatalyst can generate strong oxidant ·OH via photochemical decomposition of H2O2 under visible light irradiation. Figure 8a illustrates the photoelectron transfer mechanism in the photocatalytic degradation of Orange II over the ZnFe2O4−C3N4 hybrids. Under visible light irradiation, both ZnFe2O4 and g-C3N4 are excited, and the photogenerated holes and electrons are in their valence band (VB) and conduction band (CB), respectively. Then, the photogenerated electrons in ZnFe2O4 could easily migrate to the g-C3N4 surface because the CB level of ZnFe2O4 is lower than that of g-C3N4, and at the same time, the holes

(1)

3.3. Stability of ZnFe2O4−C3N4 Catalysts. The reusability is a prerequisite for the catalysts to be used in practical applications. Therefore, the catalytic activity of ZnFe2O4−C3N4 photocatalyst after repeated use was studied in five successive Orange II degradation experiments at the same conditions. As shown in Figure 6, ZnFe2O4−C3N4 retained over 90% of its original photocatalytic activity after five recycling runs, which suggests that the ZnFe2O4−C3N4 hybrids show excellent photocatalytic activities. Classical Fenton’s process (Fe2++ H2O2) has been known for over 100 years in wastewater treatment, but the Fenton oxidation systems were homogeneous and the efficiency depended strongly on the presence of strong acids giving rise to processing problems during posttreatment.18 The present photo-Fenton system is obviously free of acid and presents magnetic behavior and recyclable stability, which is a promising technology in water purification. To check the chemical composition variation of the obtained samples during the photo-Fenton reaction, XPS spectra were acquired, as shown in Figure 7a−f. The XPS survey spectrum of 17299

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Figure 7. (a) XPS survey, (b) Zn 2p XPS spectra, (c) Fe 2p XPS spectra, (d) N 1s XPS spectra, (e) C 1s XPS spectra and (f) O 1s XPS spectra of ZnFe2O4−C3N4 hybrids before and after the photo-Fenton reaction.

Figure 8. (a) Mechanism for Orange II degradation by ZnFe2O4−C3N4/H2O2/vis system; (b). Effects of different scavengers on the degradation of Orange II by ZnFe2O4−C3N4/H2O2/vis system.

chemisorbed O2 to generate the strong oxidative species O•− 2 , which combines with H+ from solution to form H2O2. Finally, the electrons can react with H2O2 to generate ·OH, which plays an important role in degrading and mineralizing the adsorbed molecules of Orange II. The holes can also react with surfaceadsorbed H2O to produce ·OH radicals or directly oxidize the adsorbed Orange II molecules into CO2, H2O, etc. To further reveal the photocatalytic mechanism of the ZnFe2O4−C3N4/H2O2/vis system, we also used the trapping experiments to determine the main oxidant species. tert-Butyl

generated in the VB of g-C3N4 could migrate to ZnFe2O4 NPs. This process could effectively improve the separation of photogenerated electron−hole pairs and decrease greatly the possibility of photogenerated charge recombination. Therefore, the combination of ZnFe2O4 NPs and g-C3N4 in the ZnFe2O4− C3N4 hybrid could form heterojunction structures, which leads to the improvement of photodegradation efficiency. As we know, the main active radicals are the photogenrated holes, · OH, and O•− radicals in the photocatalytic process. The 2 separated, photogenerated electrons can react with the surface 17300

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tion for Selected Overseas Chinese Scholar of Anhui Province (NO. 2013AHST0415), the State Key Laboratory of MaterialsOriented Chemical Engineering (NO. KL13-12), the Fundamental Research Funds for the Central Universities (NO. 2012HGQC0010), and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry are gratefully acknowledged. The support from Australian Research Council for DP110103699 is also acknowledged.

alcohol (TBA, 5 mg) and 1,4-benzoquinone (BQ, 5 mg) were used as scavengers of the hydroxyl radical (·OH) and 28,45 superoxide radical (O•− Figure 8b clearly 2 ), respectively. reveals that adding TBA and BQ brings in obvious suppression of Orange II degradation, which indicates that ·OH and O•− 2 radicals are indeed photogenerated on the surface of composites and function as oxidation sources. On the basis of the above results and previous reports,46,47 we propose the mechanisms of light absorption, charge transfer, and the reaction pathways for generating active radicals by ZnFe2O4− C3N4 and the degradation of Orange II as follows:

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ZnFe2O4 − C3N4 + hv +

−

+

−

→ ZnFe2O4 (h + e ) − C3N4(h + e )

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(2)

ZnFe2O4 (h+ + e−) − C3N4(h+ + e−) → ZnFe2O4 (h+) − C3N4(e−)

(3)

ZnFe2O4 (h+) + Orange II → (Orange II)•+ → CO2 + H 2O −

O2 + e →

(4)

·O−2

(5)

·O−2 + 2H+ + e− → H 2O2 −

H 2O2 + e → ·OH + OH

(6) −

(7)

·OH + Orange II → [...many steps...] → CO2 + H 2O (8)

4. CONCLUSIONS Magnetically recyclable ZnFe2O4−C3N4 hybrids were prepared through a simple reflux treatment, characterized by FESEM, HRTEM, EDX, XRD, TGA, DRS, and XPS techniques and used for the photodegradation of Orange II in aqueous solutions. The as-prepared ZnFe2O4−C3N4 samples exhibited excellent photo-Fenton activities and magnetic property for easy separation as well as durability on the elimination of the organic pollutants under visible light irradiation, maintaining high activities after five repeated uses. The photocatalytic activity of ZnFe2O4−C3N4 hybrids for the degradation of Orange II was evidently higher compared to that of the simple mixture of g-C3N4 and ZnFe2O4 NPs under the same conditions. ZnFe2O4−C3N4 hybrids exhibit high activity, and are thus believed to be effective catalytic materials for environmental applications.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 551 62901458. Fax: +86 551 62901450. *E-mail: [email protected]. Tel.: +61 8 9266 3776. Fax: +61 8 9266 2681. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The supports by the Anhui Provincial Natural Science Foundation (NO.1308085MB21), the National Natural Science Foundation of China (Grant 51372062), Technology Founda17301

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