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graphene composite and its application in photocatalytic degradation of dyes
Journal of Alloys and Compounds 579 (2013) 336–342
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Synthesis of magnetic ZnFe2O4/graphene composite and its application in photocatalytic degradation of dyes Daban Lu, Yan Zhang, Shaoxiong Lin, Letao Wang, Chunming Wang ⇑ College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e
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Article history: Received 31 March 2013 Received in revised form 13 June 2013 Accepted 15 June 2013 Available online 25 June 2013 Keywords: ZnFe2O4 Graphene H2O2 Visible light irradiation Organic dyes
a b s t r a c t Magnetic ZnFe2O4/graphene composite (ZnFe2O4/G) has been successfully synthesized by a facile one-pot solvothermal method. Graphene oxide (GO) was reduced to graphene and the ZnFe2O4 particles were simultaneously grown on the graphene sheets under the conditions generated in the solvothermal system. Importantly, the ZnFe2O4/G composite showed powerful visible-light-photocatalytic activity for the degradation of Rhodamine B (RhB), methyl orange (MO) and methylene blue (MB) in the presence of H2O2. The ZnFe2O4/G composite serves a dual function as the catalyst for photoelectrochemical degradation of dyes and the generator of a strong oxidant hydroxyl radical (OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. ZnFe2O4/G composite has excellent magnetic properties, which makes it magnetically recyclable in a suspension system. Therefore, the ZnFe2O4/G magnetic composite may find potential applications in dye water treatment and the degradation of organic dyes. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, large volume of coloured dye effluents created severe environmental pollution problems by releasing toxic and potential carcinogenic substances into the aqueous phase [1,2]. Therefore, water pollution has been a very troublesome problem facing humanity over the world. Various organic pollutants, such as dyes, are found to make a serious adverse impact on environment and human health [3]. Great efforts have been made to resolve these problems, such as chemical oxidation [4], solvent extraction [5], filtration [6], adsorption [7], flotation [8] and photocatalytic degradation [9–13]. Among the above methods, photocatalytic degradation has become more and more attractive due to its low cost, simplicity and high efficiency as well as low secondary pollution. Many photocatalysts have been extensively applied in the fields of photocatalytic degradation of organic dyes, including TiO2 [14,15], ZnO [16], Cu2O [17,18], CdS [19,20] etc. In addition, it is well known that the ultraviolet (UV) region occupies only about 4% of the whole sunlight spectrum, while 45% of the energy belongs to visible light [21]. Thus, the development of high efficiency and low cost visible-light responsive photocatalysts for environmental remediation has attracted more and more interest. In the photoelectric conversion process, most important reaction proceed mainly by the contributions of active oxygen species, such as hydroxyl radicals (OH), which is the main highly reactive oxidizing species generated to degrade organic dyes [22]. The ⇑ Corresponding author. Tel.: +86 931 8911895; fax: +86 931 8912582. E-mail address: [email protected] (C. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.098
OH is a powerful oxidant that can rapidly and non-selectively oxidize many organic compounds into carbon dioxide and water by quick chain reaction. Recently, photochemical degradation of organic dyes using H2O2/visible-light has been widely studied [21,23], and the photodegradation efficiency also depends on the decomposition rate of H2O2. Therefore, it is necessary to develop a photocatalyst to enhance the visible-light-driven activity. Among various materials, iron oxides have been widely investigated because they can easily be made and recovered. Moreover, Fe3O4 magnetic nanoparticles possess intrinsic peroxidase-like activity. Unlike the simple iron oxides, the hybrids based on mixed oxide and graphene have been rarely reported. As an important magnetic material, the spinel structured ZnFe2O4 has been proven to be useful in many applications. Due to its visible-light response, good photochemical stability and low cost, ZnFe2O4 has attracted considerable attention in the conversion of solar energy [24], photocatalysis [25] and photochemical hydrogen production from water [26]. Besides, it was reported that ZnFe2O4 magnetic particles also possessed intrinsic peroxidase-like activity, which could react with H2O2 to produce OH [27]. Additionally, ZnFe2O4 is a magnetic semiconductor material. Hence, ZnFe2O4-based composites especially provide a potential advantage for repeated magnetic separation purposes. However, ZnFe2O4 alone is photocatalytically inactive under visible light irradiation. Thus, it is possible to improve the efficiency of the photoinduced charge separation in ZnFe2O4 by coupling it with another semiconductor, resulting in enhanced photocatalytic performance. The photoelectrochemical activity of ZnFe2O4 could be remarkably enhanced by forming hybrid with graphene [21].
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Graphene, a single layer of sp2-bonded carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has attracted much attention from both the experimental and theoretical science communities because of its unique properties, such as excellent mechanical strength, large specific surface area and remarkable electrical conductivity [28–30]. As we all known that a single graphene layer is a zero-gap semiconductor with a linear Dirac-like spectrum around the Fermi energy [31]. Recently, attention has also been paid to graphene-based visible-light-driven photocatalysts. In addition, graphene oxide (GO) is the original basic material for the preparation of individual graphene sheets with abundant oxygen-containing surface functionalities, which can serve as active sites for dispersing metal nanoparticles to form flexible GO-based hybrid materials. Furthermore, Yoo et al. reported that metal or metal oxide nanoparticles assembled on graphene sheets revealed high electrocatalytic activity [32]. Therefore, graphene has been considered as an ideal support for metal nanoparticles and metal oxides. Herein, we report a simple method to directly fabricate magnetic ZnFe2O4/graphene (ZnFe2O4/G) composite by a one-pot solvothermal reaction. GO was reduced to graphene and the ZnFe2O4 microspheres were grown on the carbon basal planes simultaneously. The XRD and morphology studies of ZnFe2O4/G reveal that ZnFe2O4 is well crystallized and the graphene sheets are rather thin and possibly composed of few layers. The experimental results show that the magnetic composite can serve as the generator of hydroxyl radicals (OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. Then the OH exhibits excellent photocatalytic activity for the degradation of Rhodamine B (RhB), methyl orange (MO), and methylene blue (MB) dyes. MB and RhB both belong to the basic/cationic dye. However, MO belongs to acidic/anionic azo dye. Thus, the as-synthesized ZnFe2O4/G composite has excellent photocatalytic activity to different kinds of dyes. Moreover, these magnetic composites can be readily recovered and reused as promising materials for efficient photocatalytic degradation of organic pollutants. Therefore, they can be easily applied to remove toxic organic pollutants during water treatment. 2. Materials and methods 2.1. Materials Graphite flake (nature, 325 mesh) was from Alfa Aesar (Beijing, China). ZnCl2, FeCl36H2O, ethylene glycol and urea were purchased from Tianjin Guangfu Chemical Reagent Factory (Tianjin, China). H2O2 (30%) was obtained from Sinopharm Chemical Reagent Co., Ltd. RhB, MO and MB were obtained from Beijing Chemical Factory. All other reagents and solvents were of analytical grade and used without further purification. All chemicals were prepared with deionized water purified via Milli-Q unit. 2.2. Preparation of ZnFe2O4/G composite GO was prepared from graphite powder by a modified Hummers method [33]. 50 mg of GO was added into 50 mL ethylene glycol (EG) with ultrasonication for 2 h to form a uniform GO colloid. Then, 0.541 g of FeCl36H2O and 0.136 g of ZnCl2 were mixed in 10 mL EG and added slowly into the above colloid with magnetic stirring for 2 h. Sequentially, 0.60 g of urea was introduced into the mixture followed by stirring for another 30 min. The resulting mixture was then transferred to a 100 mL Teflon-lined stainless steel autoclave and heated in an electric oven at 200 °C for 12 h, and cooled to room temperature. The resulting product were finally separated via filtration and washed with deionized water and absolute ethanol several times, then dried under vacuum at 50 °C for 12 h. For comparison, bare ZnFe2O4 was prepared using the same procedures without GO. 2.3. Characterization Field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800, Japan), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (FEI Tecnai G2 F30) transmission electron microscope were employed to characterize the morphology and crystalline structure of composite. X-ray diffraction (XRD) patterns were recorded on a Rigaku X-ray diffractometer (Rigaku D/max-2400).
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Raman spectra were conducted on a Renishaw in via Raman microscope equipped with a 50 objective with 514.5 nm diode laser excitation on 1800-line grating. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Lake Shore, USA). The electrochemical impedance spectroscopy (EIS) analyses were carried out with a CHI 660c electrochemical workstation using a three-electrode system consisting of a platinum wire auxiliary, an Ag/AgCl reference and glassy carbon electrode (GCE) (U = 3 mm) working electrode in the solution of 0.10 M KCl containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6]. Measurements were performed with ac amplitude of 10 mV and the frequency was ranged from 0.1 Hz to 100 kHz. A 500 W xenon lamp (CHF-XM-500W, Beijing Trusttech Co. Ltd.) was used as the illumination source. UV–vis spectra were obtained from a uv-2102 (unico) UV–vis Spectrophotometers. Fluorometric measurement was carried out by a RF-5301PC spectrofluorophotometer (Shimadzu).
2.4. Photocatalytic degradation of dyes Photocatalytic activities of the ZnFe2O4/G composite were evaluated by the degradation of dye pollutants such as RhB, MO, and MB under visible light irradiation in the presence of H2O2. A 500 W xenon lamp equipped with a 420 nm cut-off filter was used as a light source to provide visible light irradiation. In all photocatalytic degradation experiments, 50 mg of catalyst was added into 50 mL of 20 mg L1 RhB, MO, and MB aqueous solution, respectively. Before starting the illumination, each reaction mixture was stirred for 30 min in the dark in order to reach the adsorption–desorption equilibrium between the dye and the catalyst. After that, 1.0 mL of 30% H2O2 was added to the above each reaction mixture, respectively. Then, the suspension was exposed to visible light irradiation. At a given time interval of irradiation, 5 mL samples were collected and then magnetically separated to remove all the catalyst. The concentrations of the remnant dyes were monitored by UV–vis spectroscopy. To further study the recyclability of the composite, we also examined the photocatalytic degradation of RhB for 10 times. The composite was separated from solution by an external magnetic field. Then the composite was reused without any post-treatment except being washed with ethanol and distilled water three times after each photocatalytic degradation.
3. Results and discussion 3.1. Characterization of composite The formation mechanism of ZnFe2O4/G composite is schematically illustrated in Fig. 1. The electrostatic attraction between the positively charged metal ions (Zn2+ and Fe3+) and the negatively charged GO sheets in the precursors plays a critical role in attaching ZnFe2O4 particles on graphene. It is well known that GO is negatively charged in solution due to the presence of abundant oxygen-containing surface functionalities, such as hydroxyl, carbonyl, carboxyl and epoxide groups. Moreover, the oxygencontaining groups on the GO sheets may act as nucleation sites in the early reaction stage to facilitate the formation of small crystals [34]. Therefore, positively charged Zn2+ and Fe3+ ions would firstly attach to the surface of the GO by electrostatic attraction and serve as nucleation precursors. As we know, urea can decompose thermally at a relatively low temperature (below 100 °C) while releasing a high volume of gas and increasing the pH of the solution, thereby promoting the precipitation of the metals as oxy/hydroxides [35]. During the solvothermal treatment, urea decomposed to release NH3, which provided an alkaline atmosphere for the solution system. The alkaline conditions induced the precipitation of Zn2+ and Fe3+ ions in ethylene glycol and led to the formation of Zn(OH)2 and Fe(OH)3 which would be transformed into ZnFe2O4 nanocrystals after dehydration [36,37]. In addition, High temperature will accelerate the hydrolysis process to form smaller ZnFe2O4 primary particles [38]. Then the ZnFe2O4 nanocrystals further grew to form aggregations. At the same time, GO was reduced to graphene during the solvothermal reactions [39]. Thus, the ZnFe2O4/G composite was obtained. The crystalline nature and composition of the as-synthesized composite was first characterized by XRD. The XRD patterns of GO, graphene, ZnFe2O4, and ZnFe2O4/G are shown in Fig. 2. The original GO sample (curve a) shows a strong peak centered at 10.6°, which corresponds to the (0 0 2) plane of GO [40]. The diffraction peak at around 43° is associated with the (1 0 0) plane of the
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Fig. 1. Schematic illustration of the synthesis procedure of the ZnFe2O4/G composite.
Fig. 2. XRD patterns of (a) GO, (b) graphene, (c) ZnFe2O4, and (d) ZnFe2O4/G composite.
hexagonal structure of carbon. For graphene (curve b), a broad diffraction peak (0 0 2) shifts to higher angle (21.2°), meaning that partial oxygen-containing functional groups of GO have been removed. As shown in curve c, the peaks at 2h values of 18.3°, 30.1°, 35.3°, 43.0°, 53.4°, 56.8°, 62.4°, 70.6° and 73.6° can be indexed to (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0) and (5 3 3) crystal planes of cubic ZnFe2O4 (JCPDS No. 82-1049), respectively. For ZnFe2O4/G (curve d), besides the ZnFe2O4 peaks, a broad peak appears at around 24°, which corresponds to the (0 0 2) peak of graphene. The diffraction peak moves to higher angle after deposition of ZnFe2O4 particles on graphene surface, which indicates that GO is further converted to the crystalline graphene,
and the conjugated graphene network (sp2 carbon) has been reestablished due to the reduction process [41]. The observation of XRD confirms the successful preparation of ZnFe2O4/G composite by using one-pot solvothermal reaction. The morphology of the ZnFe2O4/G composite was characterized by SEM and TEM. The SEM image of bare ZnFe2O4 particles prepared by one-step solvothermal reactions is shown in Fig. 3A. As can be seen, the bare ZnFe2O4 particles exhibit spherical shape with a uniform size distribution. The average diameter of the bare ZnFe2O4 particles is about 200 nm. However, some of the ZnFe2O4 particles aggregate. The SEM image of ZnFe2O4/G is presented in Fig. 3B. As can be observed, the ZnFe2O4/G hybrid exhibits a hybrid structure with alternating ZnFe2O4 particles and the graphene sheets. The ZnFe2O4 particles distribute in between the graphene sheets. It is interesting to note that the hybrid structure can survive sonication. The strong interaction may be attributed to the formation of Fe–O–C bonds between ZnFe2O4 particles and graphene [42]. The detailed morphology and structure of the composite were further studied by TEM. The as-synthesized ZnFe2O4 particles (Fig. 3C) have good spherical shape and have a mean diameter of 200 nm, which is consistent with the results of SEM. Obviously, as can be seen from the insert of Fig. 3C, the ZnFe2O4 particles are aggregations of a great deal of small ZnFe2O4 nanoparticles with an average size of about 5 nm. The TEM image of ZnFe2O4/G is shown Fig. 3D. The ZnFe2O4 particles with a diameter of 200 nm are sparsely distributed on graphene sheets. The graphene sheets are composed of crumpled silk waves-like carbon sheets and possess large surface areas. The above results are in agreement with the SEM observation. The inset of Fig. 3D gives the HRTEM image of an individual ZnFe2O4 particle. The large surface energy of the small-sized particles may cause their aggregation into a
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Fig. 3. SEM image of (A) ZnFe2O4, and (B) ZnFe2O4/G composite. TEM image of (C) ZnFe2O4, Insert: TEM image of a single ZnFe2O4 particle, and (D) ZnFe2O4/G composite. Insert: HRTEM image of ZnFe2O4/G.
Fig. 4. Raman spectra of (a) GO, and (b) ZnFe2O4/G composite.
microsphere with a larger size [39]. The image of the selected area in the ZnFe2O4 particles indicates a well-defined crystallinity with lattice spacing of 0.258 nm, which is corresponding to (3 1 1) plane of ZnFe2O4 [43]. Raman spectroscopy is one of the most effective techniques to characterize the ordered and disordered crystal structures of graphene. G band is usually assigned to the E2g mode of sp2 carbon atoms (usually observed at 1575 cm1), while D band is a breathing mode of r-point phonons of A1g symmetry (1350 cm1) [44]. The Raman spectra of GO and ZnFe2O4/G are presented in Fig. 4. Both GO and ZnFe2O4/G exhibit two bands at about 1350 and 1587 cm1, which are associated with the D and G bands of carbon-based materials, respectively. Compared with GO (curve a), ZnFe2O4/G (curve b) shows relative higher intensity of D to G band. These observations suggest a decrease in the average size of the sp2 domains from ZnFe2O4/G and further confirm the formation of new graphitic domains after the reduction process [45]. The magnetic property of the obtained samples was investigated using a vibrating sample magnetometer (VSM). Fig. 5 shows
Fig. 5. Room-temperature magnetization hysteresis loops of (a) ZnFe2O4/G composite, and (b) ZnFe2O4. Inset: The behaviour of the composite under an external magnetic field.
the room-temperature magnetization hysteresis loops of the ZnFe2O4/G composite and pure ZnFe2O4 particles. The saturation magnetization of the ZnFe2O4/G composite is 55.1 emu/g, which is lower than that of pure ZnFe2O4 particles (66.9 emu/g), mainly attributing to the presence of graphene. The coercivity values obtained for the corresponding magnetic materials are 31.8 and 37.1 Oe (left top inset of Fig. 5). The right bottom of Fig. 5 demonstrates that the composite can be easily manipulated by an external magnetic field, which is important for the promising applications ranging from electromagnetic devices to biomedicine. Electrochemical impedance spectroscopy (EIS) was an effective technique for probing the surface features of modified electrodes to understand the chemical transformations and processes associated with the conductive electrode surface. Therefore, the electrochemical impedance technique was employed to characterize electrical conductivity. The typical impedance spectra of pure ZnFe2O4 and ZnFe2O4/G modified GCE, and bare GCE are shown in Fig. 6. The Nyquist plot of the electrodes displays a single semi-
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Fig. 6. EIS of (a) bare, (b) ZnFe2O4, and (c) ZnFe2O4/G modified GCE in 0.10 M KCl solution containing equimolar [Fe(CN)6]3/[Fe(CN)6]4–.
circle at the high-frequency region and a line at the low-frequency region, indicating the electrochemical reaction at the electrodes is controlled by a mixed process of charge transfer and diffusion. Compared with the bare GCE and ZnFe2O4/GCE, the impedance plot of ZnFe2O4/G modified GCE (curve c) involves an extremely small radius, indicating that the charge transfer resistance of ZnFe2O4/ G is significantly decreased. This phenomenon is attributed to the fact that graphene is a zero band gap semiconductor and a unique 2D p-conjugation structure, in which the charge carriers behave as massless fermions, leading to unique transport properties [46]. Thus, the photogenerated electrons of ZnFe2O4 can transfer easily from the conduction band to graphene and rapidly transport the instant that they form. As a result of the great inhibition for the recombination of photogenerated electrons and holes, the photocatalytic activity is significantly enhanced.
3.2. Photocatalytic activity of the ZnFe2O4/G composite The photocatalytic activities of the as-synthesized magnetic ZnFe2O4/G composite photocatalyst for degradation of the organic dyes such as RhB, MO, and MB have been performed under visible light irradiation at room temperature. The UV–vis spectra of RhB aqueous solution in the presence of ZnFe2O4/G composite photocatalyst and H2O2 under visible light irradiation (k > 420 nm) at room temperature over various time intervals are shown in Fig. 7A. Clearly, the main absorption peak of RhB molecules locates at 553 nm, which decreases rapidly with extension of the exposure time, and completely disappears after about irradiation for 120 min. Further exposure leads to no absorption peak in the whole spectrum, which indicates the total degradation of RhB. Fig. 7B displays the UV–vis spectra of MO aqueous solution with magnetic ZnFe2O4/G composite as photocatalyst in the presence of H2O2 irradiated by visible light for different durations. MO has a characteristic absorption peak at 464 nm, which obviously decreases with the increase of irradiation time. Nearly 96% of MO can be degraded in 120 min. Fig. 7C shows the UV–vis spectra of MB aqueous solution degraded by magnetic ZnFe2O4/G composite photocatalyst in the presence of H2O2 under visible light irradiation. The characteristic absorption of MB at 664 nm is chosen for monitoring the photocatalytic degradation process. As can be seen, almost 100% of MB can be removed within 120 min. These results show that ZnFe2O4/G composite possess an excellent visiblelight-photocatalytic activity for degradation of the organic dyes. Further experiments were carried out to compare the photodegradation rates of dyes using different catalysts under visible light irradiation. RhB was chosen as the experimental dye. Fig. 8 displays the photodegradation behaviour of RhB dye at different exposure
Fig. 7. Absorption spectra of (A) RhB, (B) MO, and (C) MB aqueous solutions taken at different photocatalytic degradation times using ZnFe2O4/G composite in the presence of H2O2.
Fig. 8. Photocatalytic degradation of RhB in the presence of different catalysts (a) H2O2, (b) ZnFe2O4, (c) ZnFe2O4/G, (d) ZnFe2O4 + H2O2, and (e) ZnFe2O4/G + H2O2.
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times in the presence of different catalysts. C0 is the concentration of dyes after the adsorption–desorption equilibrium is reached but before irradiation, and C is the concentration of dyes after different visible light irradiation times. As can be seen, for the H2O2 and pure ZnFe2O4, very little degradation of RhB is found when the irradiation time is 120 min. However, nearly 22% of the RhB molecules are degraded by ZnFe2O4/G composite after irradiation for 120 min (curve c), which can be ascribed to the efficient separation of photogenerated carriers in the ZnFe2O4 and graphene coupling system. The degradation rate of RhB molecules is significantly enhanced when H2O2 is added to the adsorption–desorption equilibrium. About 30% of the RhB molecules are degraded by ZnFe2O4 in the presence of H2O2 (curve d). While almost 100% of the RhB are degraded by ZnFe2O4/G composite in the presence of H2O2 after irradiation for 120 min. These results can be possibly attributed to the remarkable dual function of ZnFe2O4/G composite, which serve as a photoelectrochemical degrader of organic molecules and the generator of hydroxyl radicals (OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. Similar results can be also observed from MO and MB solutions with the same model reactions. Therefore, the as-synthesized magnetic ZnFe2O4/G composite is an excellent photocatalyst in the presence of H2O2 under visible light irradiation to degrade organic dyes. The effects of pH on photocatalytic degradation efficiency of dyes were investigated with a favourable pH range from 3 to 9, because the catalyst became less chemically stable at pH values lower than about 2. Fig. 9 shows the degradation efficiency of RhB, MO, and MB dyes after irradiation for 120 min. It is easily seen from Fig. 9 that the degradation efficiency increases with increase in pH and exhibits maximum value at pH 5 for RhB, pH 5 for MO and pH 6 for MB, and then decreases when the pH increases further. In more acidic solutions, the catalytic FeðH2 OÞ3þ 6 shell protonation will result in releasing hydration shell water [47]. And at more basic pH values, the OH groups may replace the H2O groups. Therefore, the adsorption of H2O2 on the catalyst covered with FeðOHÞ3 6 will decrease. Moreover, the decomposition of H2O2 into H2O and O2 at basic conditions is very fast [48]. All of the effects are detrimental to the catalytic crack of H2O2 by ZnFe2O4 to generate hydroxyl radicals. These results suggest that weak acidic and circum-neutral initial pH values are beneficial for dye degradation. 3.3. Mechanism of photocatalytic activity of ZnFe2O4/G composite The ZnFe2O4/G composite photocatalyst can generate strong oxidant OH via photoelectrochemical decomposition of H2O2 under visible light irradiation. To evidence this assumption, the OH formation was assessed by adding the fluorescent probe terephthalic acid (TA) into the H2O2–ZnFe2O4/G system, where
Fig. 9. Effects of pH on photocatalytic degradation efficiency of dyes by ZnFe2O4/G composite in the presence of H2O2 under visible light irradiation for 120 min.
Fig. 10. Fluorescence spectra of TAOH at (a) 0, (b) 1, (c) 3, (d) 5, and (e) 10 min under visible light irradiation (kex = 315 nm).
TA easily reacted with OH to form highly fluorescent 2-hydroxy terephthalic acid (TAOH). Fig. 10 shows fluorescence spectra of the ZnFe2O4/G suspension containing 0.625 mM TA and 50 mM H2O2 irradiated for various duration times. It can be clearly seen that the spectral feature is 430 nm, which is the same as that of TAOH. These results indicate that ZnFe2O4/G could decompose H2O2 through the visible light photocatalysis to generate the OH, which plays a vital role in the degradation of organic dyes. Based on the results above, a possible photocatalytic mechanism of ZnFe2O4/G has been proposed as follows and illustrated in Fig. 11 [21,22,49]. Upon irradiation with visible light, ZnFe2O4 particles undergo charge separation, electrons (e) in the valence band (VB) of ZnFe2O4 can be excited to its conduction band (CB), causing the generation of holes (h+) in the VB simultaneously (reaction (1)). Because graphene sheets are known as good electron acceptors [50], the electrons are quickly transferred to the graphene sheets via a percolation mechanism (reaction (2)). Then the negatively charged graphene sheets can activate the H2O2 to generate the OH (reaction (3)). In addition, the excited electrons can also be directly trapped by the surface Fe3+ to form Fe2+ (reaction (4)). Then the Fe2+ can contribute in the Fenton reaction to produce OH (reaction (5)). Oxidation ability of the Fenton reaction can be greatly enhanced via visible light irradiation called photo-Fenton reaction (reaction (6)). Thus, the OH generated from reactions (3), (5), and (6) is the main factor for photodegradation of the dyes. þ
ZnFe2 O4 þ hm ! ZnFe2 O4 ðh þ e Þ
ð1Þ
ZnFe2 O4 ðe Þ þ graphene ! ZnFe2 O4 þ grapheneðe Þ
ð2Þ
grapheneðe Þ þ H2 O2 ! OH þ OH þ graphene
ð3Þ
Fe3þ þ e ! Fe2þ
ð4Þ
Fig. 11. Photocatalytic degradation mechanism for dyes over ZnFe2O4/G composite in the presence of H2O2 under visible light irradiation.
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References
Fig. 12. Ten cycles of the degradation of RhB using ZnFe2O4/G composite as the photocatalyst in the presence of H2O2 under visible light irradiation for 120 min.
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð5Þ
Fe3þ þ hm þ OH ! Fe2þ þ OH
ð6Þ
3.4. Reusability test of ZnFe2O4/G composite To evaluate the reusability of the ZnFe2O4/G composite, the circulating runs in the photocatalytic degradation of RhB under visible light irradiation were investigated by collecting and reusing the same photocatalyst. As shown in Fig. 12, the photocatalytic activity of the ZnFe2O4/G composite does not show any obvious loss after ten recycles for the photodegradation of RhB, indicating the composite photocatalyst has excellent stability. Furthermore, the as-prepared magnetic composite is also favourable for separation from the reaction solution by a magnet after degradation. Therefore, the experimental results confirm that the magnetic composite is stable and invulnerable to photo-corrosion during the photocatalytic degradation of organic dyes.
4. Conclusions In summary, the magnetic photocatalyst ZnFe2O4/G composite has been successfully prepared via a simple one step solvothermal method. The magnetic composite exhibits excellent photocatalytic activity for degradation of RhB, MO and MB dyes in the presence of H2O2 under visible light irradiation. The significantly enhanced catalytic efficiency is attributed to the dual function of the ZnFe2O4/G composite, which serves as the photoelectrochemical degrader for organic dyes and the generator of strong oxidant OH via photoelectrochemical decomposition of H2O2 under visible light irradiation. In addition, the composite is very stable and hence suitable for repeated use. Therefore, the ZnFe2O4/G composite may be applied as a promising photocatalyst for treatment of organic dyes.
Acknowledgement This work was supported by the National Natural Science Foundation of China under Grant No. 51072073/E0208.
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Synthesis of magnetic ZnFe2O4/graphene composite and its application in photocatalytic degradation of dyes Daban Lu, Yan Zhang, Shaoxiong Lin, Letao Wang, Chunming Wang ⇑ College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
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Article history: Received 31 March 2013 Received in revised form 13 June 2013 Accepted 15 June 2013 Available online 25 June 2013 Keywords: ZnFe2O4 Graphene H2O2 Visible light irradiation Organic dyes
a b s t r a c t Magnetic ZnFe2O4/graphene composite (ZnFe2O4/G) has been successfully synthesized by a facile one-pot solvothermal method. Graphene oxide (GO) was reduced to graphene and the ZnFe2O4 particles were simultaneously grown on the graphene sheets under the conditions generated in the solvothermal system. Importantly, the ZnFe2O4/G composite showed powerful visible-light-photocatalytic activity for the degradation of Rhodamine B (RhB), methyl orange (MO) and methylene blue (MB) in the presence of H2O2. The ZnFe2O4/G composite serves a dual function as the catalyst for photoelectrochemical degradation of dyes and the generator of a strong oxidant hydroxyl radical (OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. ZnFe2O4/G composite has excellent magnetic properties, which makes it magnetically recyclable in a suspension system. Therefore, the ZnFe2O4/G magnetic composite may find potential applications in dye water treatment and the degradation of organic dyes. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, large volume of coloured dye effluents created severe environmental pollution problems by releasing toxic and potential carcinogenic substances into the aqueous phase [1,2]. Therefore, water pollution has been a very troublesome problem facing humanity over the world. Various organic pollutants, such as dyes, are found to make a serious adverse impact on environment and human health [3]. Great efforts have been made to resolve these problems, such as chemical oxidation [4], solvent extraction [5], filtration [6], adsorption [7], flotation [8] and photocatalytic degradation [9–13]. Among the above methods, photocatalytic degradation has become more and more attractive due to its low cost, simplicity and high efficiency as well as low secondary pollution. Many photocatalysts have been extensively applied in the fields of photocatalytic degradation of organic dyes, including TiO2 [14,15], ZnO [16], Cu2O [17,18], CdS [19,20] etc. In addition, it is well known that the ultraviolet (UV) region occupies only about 4% of the whole sunlight spectrum, while 45% of the energy belongs to visible light [21]. Thus, the development of high efficiency and low cost visible-light responsive photocatalysts for environmental remediation has attracted more and more interest. In the photoelectric conversion process, most important reaction proceed mainly by the contributions of active oxygen species, such as hydroxyl radicals (OH), which is the main highly reactive oxidizing species generated to degrade organic dyes [22]. The ⇑ Corresponding author. Tel.: +86 931 8911895; fax: +86 931 8912582. E-mail address: [email protected] (C. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.098
OH is a powerful oxidant that can rapidly and non-selectively oxidize many organic compounds into carbon dioxide and water by quick chain reaction. Recently, photochemical degradation of organic dyes using H2O2/visible-light has been widely studied [21,23], and the photodegradation efficiency also depends on the decomposition rate of H2O2. Therefore, it is necessary to develop a photocatalyst to enhance the visible-light-driven activity. Among various materials, iron oxides have been widely investigated because they can easily be made and recovered. Moreover, Fe3O4 magnetic nanoparticles possess intrinsic peroxidase-like activity. Unlike the simple iron oxides, the hybrids based on mixed oxide and graphene have been rarely reported. As an important magnetic material, the spinel structured ZnFe2O4 has been proven to be useful in many applications. Due to its visible-light response, good photochemical stability and low cost, ZnFe2O4 has attracted considerable attention in the conversion of solar energy [24], photocatalysis [25] and photochemical hydrogen production from water [26]. Besides, it was reported that ZnFe2O4 magnetic particles also possessed intrinsic peroxidase-like activity, which could react with H2O2 to produce OH [27]. Additionally, ZnFe2O4 is a magnetic semiconductor material. Hence, ZnFe2O4-based composites especially provide a potential advantage for repeated magnetic separation purposes. However, ZnFe2O4 alone is photocatalytically inactive under visible light irradiation. Thus, it is possible to improve the efficiency of the photoinduced charge separation in ZnFe2O4 by coupling it with another semiconductor, resulting in enhanced photocatalytic performance. The photoelectrochemical activity of ZnFe2O4 could be remarkably enhanced by forming hybrid with graphene [21].
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Graphene, a single layer of sp2-bonded carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has attracted much attention from both the experimental and theoretical science communities because of its unique properties, such as excellent mechanical strength, large specific surface area and remarkable electrical conductivity [28–30]. As we all known that a single graphene layer is a zero-gap semiconductor with a linear Dirac-like spectrum around the Fermi energy [31]. Recently, attention has also been paid to graphene-based visible-light-driven photocatalysts. In addition, graphene oxide (GO) is the original basic material for the preparation of individual graphene sheets with abundant oxygen-containing surface functionalities, which can serve as active sites for dispersing metal nanoparticles to form flexible GO-based hybrid materials. Furthermore, Yoo et al. reported that metal or metal oxide nanoparticles assembled on graphene sheets revealed high electrocatalytic activity [32]. Therefore, graphene has been considered as an ideal support for metal nanoparticles and metal oxides. Herein, we report a simple method to directly fabricate magnetic ZnFe2O4/graphene (ZnFe2O4/G) composite by a one-pot solvothermal reaction. GO was reduced to graphene and the ZnFe2O4 microspheres were grown on the carbon basal planes simultaneously. The XRD and morphology studies of ZnFe2O4/G reveal that ZnFe2O4 is well crystallized and the graphene sheets are rather thin and possibly composed of few layers. The experimental results show that the magnetic composite can serve as the generator of hydroxyl radicals (OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. Then the OH exhibits excellent photocatalytic activity for the degradation of Rhodamine B (RhB), methyl orange (MO), and methylene blue (MB) dyes. MB and RhB both belong to the basic/cationic dye. However, MO belongs to acidic/anionic azo dye. Thus, the as-synthesized ZnFe2O4/G composite has excellent photocatalytic activity to different kinds of dyes. Moreover, these magnetic composites can be readily recovered and reused as promising materials for efficient photocatalytic degradation of organic pollutants. Therefore, they can be easily applied to remove toxic organic pollutants during water treatment. 2. Materials and methods 2.1. Materials Graphite flake (nature, 325 mesh) was from Alfa Aesar (Beijing, China). ZnCl2, FeCl36H2O, ethylene glycol and urea were purchased from Tianjin Guangfu Chemical Reagent Factory (Tianjin, China). H2O2 (30%) was obtained from Sinopharm Chemical Reagent Co., Ltd. RhB, MO and MB were obtained from Beijing Chemical Factory. All other reagents and solvents were of analytical grade and used without further purification. All chemicals were prepared with deionized water purified via Milli-Q unit. 2.2. Preparation of ZnFe2O4/G composite GO was prepared from graphite powder by a modified Hummers method [33]. 50 mg of GO was added into 50 mL ethylene glycol (EG) with ultrasonication for 2 h to form a uniform GO colloid. Then, 0.541 g of FeCl36H2O and 0.136 g of ZnCl2 were mixed in 10 mL EG and added slowly into the above colloid with magnetic stirring for 2 h. Sequentially, 0.60 g of urea was introduced into the mixture followed by stirring for another 30 min. The resulting mixture was then transferred to a 100 mL Teflon-lined stainless steel autoclave and heated in an electric oven at 200 °C for 12 h, and cooled to room temperature. The resulting product were finally separated via filtration and washed with deionized water and absolute ethanol several times, then dried under vacuum at 50 °C for 12 h. For comparison, bare ZnFe2O4 was prepared using the same procedures without GO. 2.3. Characterization Field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800, Japan), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (FEI Tecnai G2 F30) transmission electron microscope were employed to characterize the morphology and crystalline structure of composite. X-ray diffraction (XRD) patterns were recorded on a Rigaku X-ray diffractometer (Rigaku D/max-2400).
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Raman spectra were conducted on a Renishaw in via Raman microscope equipped with a 50 objective with 514.5 nm diode laser excitation on 1800-line grating. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Lake Shore, USA). The electrochemical impedance spectroscopy (EIS) analyses were carried out with a CHI 660c electrochemical workstation using a three-electrode system consisting of a platinum wire auxiliary, an Ag/AgCl reference and glassy carbon electrode (GCE) (U = 3 mm) working electrode in the solution of 0.10 M KCl containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6]. Measurements were performed with ac amplitude of 10 mV and the frequency was ranged from 0.1 Hz to 100 kHz. A 500 W xenon lamp (CHF-XM-500W, Beijing Trusttech Co. Ltd.) was used as the illumination source. UV–vis spectra were obtained from a uv-2102 (unico) UV–vis Spectrophotometers. Fluorometric measurement was carried out by a RF-5301PC spectrofluorophotometer (Shimadzu).
2.4. Photocatalytic degradation of dyes Photocatalytic activities of the ZnFe2O4/G composite were evaluated by the degradation of dye pollutants such as RhB, MO, and MB under visible light irradiation in the presence of H2O2. A 500 W xenon lamp equipped with a 420 nm cut-off filter was used as a light source to provide visible light irradiation. In all photocatalytic degradation experiments, 50 mg of catalyst was added into 50 mL of 20 mg L1 RhB, MO, and MB aqueous solution, respectively. Before starting the illumination, each reaction mixture was stirred for 30 min in the dark in order to reach the adsorption–desorption equilibrium between the dye and the catalyst. After that, 1.0 mL of 30% H2O2 was added to the above each reaction mixture, respectively. Then, the suspension was exposed to visible light irradiation. At a given time interval of irradiation, 5 mL samples were collected and then magnetically separated to remove all the catalyst. The concentrations of the remnant dyes were monitored by UV–vis spectroscopy. To further study the recyclability of the composite, we also examined the photocatalytic degradation of RhB for 10 times. The composite was separated from solution by an external magnetic field. Then the composite was reused without any post-treatment except being washed with ethanol and distilled water three times after each photocatalytic degradation.
3. Results and discussion 3.1. Characterization of composite The formation mechanism of ZnFe2O4/G composite is schematically illustrated in Fig. 1. The electrostatic attraction between the positively charged metal ions (Zn2+ and Fe3+) and the negatively charged GO sheets in the precursors plays a critical role in attaching ZnFe2O4 particles on graphene. It is well known that GO is negatively charged in solution due to the presence of abundant oxygen-containing surface functionalities, such as hydroxyl, carbonyl, carboxyl and epoxide groups. Moreover, the oxygencontaining groups on the GO sheets may act as nucleation sites in the early reaction stage to facilitate the formation of small crystals [34]. Therefore, positively charged Zn2+ and Fe3+ ions would firstly attach to the surface of the GO by electrostatic attraction and serve as nucleation precursors. As we know, urea can decompose thermally at a relatively low temperature (below 100 °C) while releasing a high volume of gas and increasing the pH of the solution, thereby promoting the precipitation of the metals as oxy/hydroxides [35]. During the solvothermal treatment, urea decomposed to release NH3, which provided an alkaline atmosphere for the solution system. The alkaline conditions induced the precipitation of Zn2+ and Fe3+ ions in ethylene glycol and led to the formation of Zn(OH)2 and Fe(OH)3 which would be transformed into ZnFe2O4 nanocrystals after dehydration [36,37]. In addition, High temperature will accelerate the hydrolysis process to form smaller ZnFe2O4 primary particles [38]. Then the ZnFe2O4 nanocrystals further grew to form aggregations. At the same time, GO was reduced to graphene during the solvothermal reactions [39]. Thus, the ZnFe2O4/G composite was obtained. The crystalline nature and composition of the as-synthesized composite was first characterized by XRD. The XRD patterns of GO, graphene, ZnFe2O4, and ZnFe2O4/G are shown in Fig. 2. The original GO sample (curve a) shows a strong peak centered at 10.6°, which corresponds to the (0 0 2) plane of GO [40]. The diffraction peak at around 43° is associated with the (1 0 0) plane of the
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Fig. 1. Schematic illustration of the synthesis procedure of the ZnFe2O4/G composite.
Fig. 2. XRD patterns of (a) GO, (b) graphene, (c) ZnFe2O4, and (d) ZnFe2O4/G composite.
hexagonal structure of carbon. For graphene (curve b), a broad diffraction peak (0 0 2) shifts to higher angle (21.2°), meaning that partial oxygen-containing functional groups of GO have been removed. As shown in curve c, the peaks at 2h values of 18.3°, 30.1°, 35.3°, 43.0°, 53.4°, 56.8°, 62.4°, 70.6° and 73.6° can be indexed to (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0) and (5 3 3) crystal planes of cubic ZnFe2O4 (JCPDS No. 82-1049), respectively. For ZnFe2O4/G (curve d), besides the ZnFe2O4 peaks, a broad peak appears at around 24°, which corresponds to the (0 0 2) peak of graphene. The diffraction peak moves to higher angle after deposition of ZnFe2O4 particles on graphene surface, which indicates that GO is further converted to the crystalline graphene,
and the conjugated graphene network (sp2 carbon) has been reestablished due to the reduction process [41]. The observation of XRD confirms the successful preparation of ZnFe2O4/G composite by using one-pot solvothermal reaction. The morphology of the ZnFe2O4/G composite was characterized by SEM and TEM. The SEM image of bare ZnFe2O4 particles prepared by one-step solvothermal reactions is shown in Fig. 3A. As can be seen, the bare ZnFe2O4 particles exhibit spherical shape with a uniform size distribution. The average diameter of the bare ZnFe2O4 particles is about 200 nm. However, some of the ZnFe2O4 particles aggregate. The SEM image of ZnFe2O4/G is presented in Fig. 3B. As can be observed, the ZnFe2O4/G hybrid exhibits a hybrid structure with alternating ZnFe2O4 particles and the graphene sheets. The ZnFe2O4 particles distribute in between the graphene sheets. It is interesting to note that the hybrid structure can survive sonication. The strong interaction may be attributed to the formation of Fe–O–C bonds between ZnFe2O4 particles and graphene [42]. The detailed morphology and structure of the composite were further studied by TEM. The as-synthesized ZnFe2O4 particles (Fig. 3C) have good spherical shape and have a mean diameter of 200 nm, which is consistent with the results of SEM. Obviously, as can be seen from the insert of Fig. 3C, the ZnFe2O4 particles are aggregations of a great deal of small ZnFe2O4 nanoparticles with an average size of about 5 nm. The TEM image of ZnFe2O4/G is shown Fig. 3D. The ZnFe2O4 particles with a diameter of 200 nm are sparsely distributed on graphene sheets. The graphene sheets are composed of crumpled silk waves-like carbon sheets and possess large surface areas. The above results are in agreement with the SEM observation. The inset of Fig. 3D gives the HRTEM image of an individual ZnFe2O4 particle. The large surface energy of the small-sized particles may cause their aggregation into a
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Fig. 3. SEM image of (A) ZnFe2O4, and (B) ZnFe2O4/G composite. TEM image of (C) ZnFe2O4, Insert: TEM image of a single ZnFe2O4 particle, and (D) ZnFe2O4/G composite. Insert: HRTEM image of ZnFe2O4/G.
Fig. 4. Raman spectra of (a) GO, and (b) ZnFe2O4/G composite.
microsphere with a larger size [39]. The image of the selected area in the ZnFe2O4 particles indicates a well-defined crystallinity with lattice spacing of 0.258 nm, which is corresponding to (3 1 1) plane of ZnFe2O4 [43]. Raman spectroscopy is one of the most effective techniques to characterize the ordered and disordered crystal structures of graphene. G band is usually assigned to the E2g mode of sp2 carbon atoms (usually observed at 1575 cm1), while D band is a breathing mode of r-point phonons of A1g symmetry (1350 cm1) [44]. The Raman spectra of GO and ZnFe2O4/G are presented in Fig. 4. Both GO and ZnFe2O4/G exhibit two bands at about 1350 and 1587 cm1, which are associated with the D and G bands of carbon-based materials, respectively. Compared with GO (curve a), ZnFe2O4/G (curve b) shows relative higher intensity of D to G band. These observations suggest a decrease in the average size of the sp2 domains from ZnFe2O4/G and further confirm the formation of new graphitic domains after the reduction process [45]. The magnetic property of the obtained samples was investigated using a vibrating sample magnetometer (VSM). Fig. 5 shows
Fig. 5. Room-temperature magnetization hysteresis loops of (a) ZnFe2O4/G composite, and (b) ZnFe2O4. Inset: The behaviour of the composite under an external magnetic field.
the room-temperature magnetization hysteresis loops of the ZnFe2O4/G composite and pure ZnFe2O4 particles. The saturation magnetization of the ZnFe2O4/G composite is 55.1 emu/g, which is lower than that of pure ZnFe2O4 particles (66.9 emu/g), mainly attributing to the presence of graphene. The coercivity values obtained for the corresponding magnetic materials are 31.8 and 37.1 Oe (left top inset of Fig. 5). The right bottom of Fig. 5 demonstrates that the composite can be easily manipulated by an external magnetic field, which is important for the promising applications ranging from electromagnetic devices to biomedicine. Electrochemical impedance spectroscopy (EIS) was an effective technique for probing the surface features of modified electrodes to understand the chemical transformations and processes associated with the conductive electrode surface. Therefore, the electrochemical impedance technique was employed to characterize electrical conductivity. The typical impedance spectra of pure ZnFe2O4 and ZnFe2O4/G modified GCE, and bare GCE are shown in Fig. 6. The Nyquist plot of the electrodes displays a single semi-
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Fig. 6. EIS of (a) bare, (b) ZnFe2O4, and (c) ZnFe2O4/G modified GCE in 0.10 M KCl solution containing equimolar [Fe(CN)6]3/[Fe(CN)6]4–.
circle at the high-frequency region and a line at the low-frequency region, indicating the electrochemical reaction at the electrodes is controlled by a mixed process of charge transfer and diffusion. Compared with the bare GCE and ZnFe2O4/GCE, the impedance plot of ZnFe2O4/G modified GCE (curve c) involves an extremely small radius, indicating that the charge transfer resistance of ZnFe2O4/ G is significantly decreased. This phenomenon is attributed to the fact that graphene is a zero band gap semiconductor and a unique 2D p-conjugation structure, in which the charge carriers behave as massless fermions, leading to unique transport properties [46]. Thus, the photogenerated electrons of ZnFe2O4 can transfer easily from the conduction band to graphene and rapidly transport the instant that they form. As a result of the great inhibition for the recombination of photogenerated electrons and holes, the photocatalytic activity is significantly enhanced.
3.2. Photocatalytic activity of the ZnFe2O4/G composite The photocatalytic activities of the as-synthesized magnetic ZnFe2O4/G composite photocatalyst for degradation of the organic dyes such as RhB, MO, and MB have been performed under visible light irradiation at room temperature. The UV–vis spectra of RhB aqueous solution in the presence of ZnFe2O4/G composite photocatalyst and H2O2 under visible light irradiation (k > 420 nm) at room temperature over various time intervals are shown in Fig. 7A. Clearly, the main absorption peak of RhB molecules locates at 553 nm, which decreases rapidly with extension of the exposure time, and completely disappears after about irradiation for 120 min. Further exposure leads to no absorption peak in the whole spectrum, which indicates the total degradation of RhB. Fig. 7B displays the UV–vis spectra of MO aqueous solution with magnetic ZnFe2O4/G composite as photocatalyst in the presence of H2O2 irradiated by visible light for different durations. MO has a characteristic absorption peak at 464 nm, which obviously decreases with the increase of irradiation time. Nearly 96% of MO can be degraded in 120 min. Fig. 7C shows the UV–vis spectra of MB aqueous solution degraded by magnetic ZnFe2O4/G composite photocatalyst in the presence of H2O2 under visible light irradiation. The characteristic absorption of MB at 664 nm is chosen for monitoring the photocatalytic degradation process. As can be seen, almost 100% of MB can be removed within 120 min. These results show that ZnFe2O4/G composite possess an excellent visiblelight-photocatalytic activity for degradation of the organic dyes. Further experiments were carried out to compare the photodegradation rates of dyes using different catalysts under visible light irradiation. RhB was chosen as the experimental dye. Fig. 8 displays the photodegradation behaviour of RhB dye at different exposure
Fig. 7. Absorption spectra of (A) RhB, (B) MO, and (C) MB aqueous solutions taken at different photocatalytic degradation times using ZnFe2O4/G composite in the presence of H2O2.
Fig. 8. Photocatalytic degradation of RhB in the presence of different catalysts (a) H2O2, (b) ZnFe2O4, (c) ZnFe2O4/G, (d) ZnFe2O4 + H2O2, and (e) ZnFe2O4/G + H2O2.
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times in the presence of different catalysts. C0 is the concentration of dyes after the adsorption–desorption equilibrium is reached but before irradiation, and C is the concentration of dyes after different visible light irradiation times. As can be seen, for the H2O2 and pure ZnFe2O4, very little degradation of RhB is found when the irradiation time is 120 min. However, nearly 22% of the RhB molecules are degraded by ZnFe2O4/G composite after irradiation for 120 min (curve c), which can be ascribed to the efficient separation of photogenerated carriers in the ZnFe2O4 and graphene coupling system. The degradation rate of RhB molecules is significantly enhanced when H2O2 is added to the adsorption–desorption equilibrium. About 30% of the RhB molecules are degraded by ZnFe2O4 in the presence of H2O2 (curve d). While almost 100% of the RhB are degraded by ZnFe2O4/G composite in the presence of H2O2 after irradiation for 120 min. These results can be possibly attributed to the remarkable dual function of ZnFe2O4/G composite, which serve as a photoelectrochemical degrader of organic molecules and the generator of hydroxyl radicals (OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. Similar results can be also observed from MO and MB solutions with the same model reactions. Therefore, the as-synthesized magnetic ZnFe2O4/G composite is an excellent photocatalyst in the presence of H2O2 under visible light irradiation to degrade organic dyes. The effects of pH on photocatalytic degradation efficiency of dyes were investigated with a favourable pH range from 3 to 9, because the catalyst became less chemically stable at pH values lower than about 2. Fig. 9 shows the degradation efficiency of RhB, MO, and MB dyes after irradiation for 120 min. It is easily seen from Fig. 9 that the degradation efficiency increases with increase in pH and exhibits maximum value at pH 5 for RhB, pH 5 for MO and pH 6 for MB, and then decreases when the pH increases further. In more acidic solutions, the catalytic FeðH2 OÞ3þ 6 shell protonation will result in releasing hydration shell water [47]. And at more basic pH values, the OH groups may replace the H2O groups. Therefore, the adsorption of H2O2 on the catalyst covered with FeðOHÞ3 6 will decrease. Moreover, the decomposition of H2O2 into H2O and O2 at basic conditions is very fast [48]. All of the effects are detrimental to the catalytic crack of H2O2 by ZnFe2O4 to generate hydroxyl radicals. These results suggest that weak acidic and circum-neutral initial pH values are beneficial for dye degradation. 3.3. Mechanism of photocatalytic activity of ZnFe2O4/G composite The ZnFe2O4/G composite photocatalyst can generate strong oxidant OH via photoelectrochemical decomposition of H2O2 under visible light irradiation. To evidence this assumption, the OH formation was assessed by adding the fluorescent probe terephthalic acid (TA) into the H2O2–ZnFe2O4/G system, where
Fig. 9. Effects of pH on photocatalytic degradation efficiency of dyes by ZnFe2O4/G composite in the presence of H2O2 under visible light irradiation for 120 min.
Fig. 10. Fluorescence spectra of TAOH at (a) 0, (b) 1, (c) 3, (d) 5, and (e) 10 min under visible light irradiation (kex = 315 nm).
TA easily reacted with OH to form highly fluorescent 2-hydroxy terephthalic acid (TAOH). Fig. 10 shows fluorescence spectra of the ZnFe2O4/G suspension containing 0.625 mM TA and 50 mM H2O2 irradiated for various duration times. It can be clearly seen that the spectral feature is 430 nm, which is the same as that of TAOH. These results indicate that ZnFe2O4/G could decompose H2O2 through the visible light photocatalysis to generate the OH, which plays a vital role in the degradation of organic dyes. Based on the results above, a possible photocatalytic mechanism of ZnFe2O4/G has been proposed as follows and illustrated in Fig. 11 [21,22,49]. Upon irradiation with visible light, ZnFe2O4 particles undergo charge separation, electrons (e) in the valence band (VB) of ZnFe2O4 can be excited to its conduction band (CB), causing the generation of holes (h+) in the VB simultaneously (reaction (1)). Because graphene sheets are known as good electron acceptors [50], the electrons are quickly transferred to the graphene sheets via a percolation mechanism (reaction (2)). Then the negatively charged graphene sheets can activate the H2O2 to generate the OH (reaction (3)). In addition, the excited electrons can also be directly trapped by the surface Fe3+ to form Fe2+ (reaction (4)). Then the Fe2+ can contribute in the Fenton reaction to produce OH (reaction (5)). Oxidation ability of the Fenton reaction can be greatly enhanced via visible light irradiation called photo-Fenton reaction (reaction (6)). Thus, the OH generated from reactions (3), (5), and (6) is the main factor for photodegradation of the dyes. þ
ZnFe2 O4 þ hm ! ZnFe2 O4 ðh þ e Þ
ð1Þ
ZnFe2 O4 ðe Þ þ graphene ! ZnFe2 O4 þ grapheneðe Þ
ð2Þ
grapheneðe Þ þ H2 O2 ! OH þ OH þ graphene
ð3Þ
Fe3þ þ e ! Fe2þ
ð4Þ
Fig. 11. Photocatalytic degradation mechanism for dyes over ZnFe2O4/G composite in the presence of H2O2 under visible light irradiation.
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References
Fig. 12. Ten cycles of the degradation of RhB using ZnFe2O4/G composite as the photocatalyst in the presence of H2O2 under visible light irradiation for 120 min.
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð5Þ
Fe3þ þ hm þ OH ! Fe2þ þ OH
ð6Þ
3.4. Reusability test of ZnFe2O4/G composite To evaluate the reusability of the ZnFe2O4/G composite, the circulating runs in the photocatalytic degradation of RhB under visible light irradiation were investigated by collecting and reusing the same photocatalyst. As shown in Fig. 12, the photocatalytic activity of the ZnFe2O4/G composite does not show any obvious loss after ten recycles for the photodegradation of RhB, indicating the composite photocatalyst has excellent stability. Furthermore, the as-prepared magnetic composite is also favourable for separation from the reaction solution by a magnet after degradation. Therefore, the experimental results confirm that the magnetic composite is stable and invulnerable to photo-corrosion during the photocatalytic degradation of organic dyes.
4. Conclusions In summary, the magnetic photocatalyst ZnFe2O4/G composite has been successfully prepared via a simple one step solvothermal method. The magnetic composite exhibits excellent photocatalytic activity for degradation of RhB, MO and MB dyes in the presence of H2O2 under visible light irradiation. The significantly enhanced catalytic efficiency is attributed to the dual function of the ZnFe2O4/G composite, which serves as the photoelectrochemical degrader for organic dyes and the generator of strong oxidant OH via photoelectrochemical decomposition of H2O2 under visible light irradiation. In addition, the composite is very stable and hence suitable for repeated use. Therefore, the ZnFe2O4/G composite may be applied as a promising photocatalyst for treatment of organic dyes.
Acknowledgement This work was supported by the National Natural Science Foundation of China under Grant No. 51072073/E0208.
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