ARTICLE pubs.acs.org/IECR
Magnetically Separable ZnFe2O4Graphene Catalyst and its High Photocatalytic Performance under Visible Light Irradiation Yongsheng Fu† and Xin Wang*,†,‡ †
Key Laboratory for Soft Chemistry and Functional Materials (Nanjing University of Science and Technology), Ministry of Education, Nanjing 210094, China ‡ Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, Nanjing 210094, China
bS Supporting Information ABSTRACT: A magnetically separable ZnFe2O4graphene nanocomposite photocatalyst with different graphene content was prepared by a facile one-step hydrothermal method. The graphene sheets in this nanocomposite photocatalyst are exfoliated and decorated with ZnFe2O4 nanocrystals. It was found that in the presence of H2O2, the photodegradation rate of methylene blue (MB) was 88% after visible light irradiation for only 5 min and reached up to 99% after irradiation for 90 min. In comparison with pure ZnFe2O4 catalyst, ZnFe2O4graphene serves a dual function as the catalyst for photoelectrochemical degradation of MB and the generator of a strong oxidant hydroxyl radical ( 3 OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. ZnFe2O4 nanoparticles themselves have a magnetic property, which makes the ZnFe2O4graphene composite magnetically separable in a suspension system, and therefore it does not require additional magnetic components as is the usual case.
1. INTRODUCTION The increasing use of natural and renewable energy sources is needed to help take the burden off the dependency on fossil fuels and may reduce emissions in the atmosphere. Solar energy is one renewable alternative to fossil fuels. In recent years, semiconductor photocatalysis, as a “green” technology, has been widely used for the treatment of polluted water. Outstanding stability, good photostability, nontoxicity, and low price make TiO2 the photocatalyst of choice for environmental remediation.14 However, TiO2 semiconductors have a large band gap of 3.2 eV, and therefore wavelengths below 388 nm are necessary for excitation. It is well-known that the UV region occupies only around 4% of the entire solar spectrum, while 45% of the energy belongs to visible light. Therefore developing efficient visible-light responsive photocatalysts for environmental remediation has become an active research area in photocatalysis research.512 In the photoelectric conversion process, the most important reaction involves hydroxyl ions on the semiconductor surface reacting with the holes, forming hydroxyl radicals ( 3 OH), which is the main cause of the photodegradation of organic contaminants. The hydroxyl radical is a powerful oxidant that can rapidly and nonselectively oxidize many organic compounds into carbon dioxide and water by quick chain reaction.1,11,13 Photochemical degradation of organic contaminants using H2O2/UV has been widely studied, and the photodegradation efficiency also depends on the decomposition rate of H2O2. Many perovskite- or spineltype complex oxides have been found to have visible-light-driven photoactivity. Among them, spinel ZnFe2O4 with a relatively narrow bandgap of 1.9 eV, attracted considerable attention in the conversion of solar energy, photocatalysis, and photochemical hydrogen production from water due to its visible-light response, good photochemical stability, and low cost.14,15 Besides, consider r 2011 American Chemical Society
that ZnFe2O4 is a magnetic semiconductor material,16,17 and therefore, ZnFe2O4-based catalysts can be magnetically separable in a suspension system by virtue of their own magnetic properties without introduction of additional magnetic particles. Graphene, as a new carbon material, possesses a special twodimensional (2D) crystalline structure, large specific surface area, remarkable electrical conductivity, excellent adsorptivity, high chemical and thermal stability,1823 and has been receiving recent attention as a support for catalysts.2326 Graphene-based nanocomposites containing metal nanoparticles (Cu,2729 Pt,23,3032 Pd,3336 Au,3739 Ag4043), metal oxides (TiO2,26,44,45 Fe3O4,46,47 SnO2,25,48,49 MnO2,5052 Co3O4,5355 ZnO18,56), and polymers (polyaniline5759) have been reported. Recently, some efforts have been made to achieve the utilization of UV irradiation for graphenemetal oxide composites (such as TiO2graphene2,26,60,61 and ZnOgraphene18,62,63). However, so far relatively little attention has been paid to graphenecomplex oxide nanocomposites. It is of great interest to study the possibility of graphenecomplex oxide nanocomposites, such as grapheneZnFe2O4 systems. If that can be accomplished, then it may be possible to obtain some exceptional properties as a result of the interaction of complex oxide nanoparticles and support as well as the concerted effect. Herein, we report our studies on a one-step facile hydrothermal route to directly fabricate magnetic ZnFe2O4graphene nanocomposites with different graphene content. The experimental results show that the as-prepared photocatalysts can serve Received: January 23, 2011 Accepted: May 11, 2011 Revised: April 14, 2011 Published: May 11, 2011 7210
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dually as the photoelectrochemical degrader for organic molecules and the generator of hydroxyl radicals ( 3 OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation in comparison with pure ZnFe2O4. Photocatalytic activity of ZnFe2O4graphene nanocomposite with different graphene content has been further examined systematically and 20% (w/w) graphene in the photocatalyst gave the best activity. In addition, the magnetic properties of ZnFe2O4 give it good performance in magnetic separation, and therefore it does not require additional magnetic components as are usually needed to be introduced into the magnetically separable catalysts. The significant enhancement in photoactivity can be ascribed to the efficient separation of photogenerated carriers in the ZnFe2O4graphene coupling system, and the generation of the strong oxidant 3 OH can be ascribed to the photoelectrochemical decomposition of H2O2 under visible light irradiation.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Magnetic ZnFe2O4Graphene Composite Photocatalyst. Graphene oxide (GO) was synthesized from
purified natural graphite (bought from Qingdao Zhongtian Company with a mean particle size of 44 μm) according to the method reported by Hummers and Offeman.64 ZnFe2O4graphene nanocomposite photocatalysts with different graphene content (15, 20, 25, 30 wt %) were synthesized. A typical experiment for the synthesis of ZnFe2O4graphene nanocomposite with 25% graphene content, is as follows: 80 mg of GO was dispersed into 60 mL of absolute ethanol with sonication for 1 h. Then 0.2975 g of Zn(NO3)2 3 6H2O and 0.8080 g of Fe(NO3)3 3 9H2O were added to 20 mL of absolute ethanol with stirring for 30 min at room temperature. The above two systems were then mixed together, and stirred for 30 min, yielding a stable bottle-green homogeneous emulsion. The resulting mixture was transferred into a 100-mL Teflon-lined stainless autoclave and heated to 180 °C for 12 h under autogenous pressure. The reaction mixture was allowed to cool to room temperature and the precipitate was filtered, washed with distilled water five times, and dried in a vacuum oven at 60 °C for 12 h. The product was labeled as ZnFe2O4-G(0.25). For comparison,the same method was used to synthesize pure ZnFe2O4 without GO. 2.2. Characterization. Fourier transform infrared (FTIR) spectra were recorded on a Bruker VECTOR 22 spectrometer using the KBr pellet technique. X-ray photoelectron spectra (XPS) were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg KR radiation (hν = 1253.6 eV). Raman spectra were acquired on a Renishaw inVia Reflex Raman Microprobe. Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advanced diffractometer with Cu Ka radiation and the scanning angle ranged from 5° to 70° of 2θ. Transmission electron microscopy (TEM) images were taken with a JEOL JEM2100 microscope. Field-emission scanning electron microscopy (FESEM) was performed with a LEO1550 microscope. Photoluminescence spectra were recorded on a Jobin Yvon SPEX Fluorolog-3-P spectroscope and a 450 W Xe lamp was used as the excitation source. Electrochemical impedance spectroscopy (EIS) measurements were performed with a CHI660B workstation. The test electrodes were prepared according to ref 57. EIS measurements were carried out in 1 M H2SO4 by using a three-electrode system, with a platinum foil electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode.
Figure 1. Fourier transform infrared (FTIR) spectra of (a) graphene oxide (GO), (b) ZnFe2O4-G(0.2) obtained by hydrothermal reduction, and (c) pure ZnFe2O4 in the range of 4000400 cm1.
EIS measurements were recorded with an AC voltage amplitude of 5 mV, with a frequency range of 1 MHz to 10 mHz at 0.5 V. 2.3. Photocatalytic Activity Measurement. Photocatalytic activity of the samples was determined by the degradation of methylene blue (MB) under visible light irradiation. Photoirradiation was carried out using a 500-W xenon lamp through UV cutoff filters (JB450) to completely remove any radiation below 420 nm and to ensure illumination by visible-light only. Experiments were conducted at ambient temperature as follows: 0.05 g of photocatalyst was added to 100 mL of a 20 mg/L dye aqueous solution. Before starting the illumination, the reaction mixture was stirred for 30 min in the dark in order to reach the adsorptiondesorption equilibrium between the dye and the catalyst. After adding 1.0 mL of 30% H2O2 to the above reaction mixture, the lamp was turned on. At a given time interval of irradiation, 5-mL aliquots were withdrawn and then magnetically separated to remove essentially all the catalyst. The concentrations of the remnant dye were spectrophotometrically monitored by measuring the absorbance of solutions at 664 nm during the photodegradation process.
3. RESULTS AND DISCUSSION 3.1. Powder Formation. ZnFe2O4 nanocrystals deposited onto graphene nanosheets with different graphene content were prepared via a one-step facile hydrothermal method in an ethanol system. The experimental results show that the graphene nanosheets in the nanocomposite catalysts are exfoliated and decorated homogeneously with ZnFe2O4 nanocrystals. In the hydrothermal reaction process, graphene oxide (GO) can easily reduced to graphene in the presence of reducing agents.2,60 Figure 1 shows FTIR spectra of GO, ZnFe2O4G(0.2), and pure ZnFe2O4. For ZnFe2O4G(0.2), the adsorption peak around 1570 cm1 may be assigned to the stretching vibrations of the unoxidized carbon backbone,6567 and the two strong absorption peaks at lower frequency (around 550 and 415 cm1) can be assigned to the stretching vibrations of the ZnO bonds in tetrahedral positions and the FeO bonds in octahedral positions, respectively.68 Almost all the characteristic peaks of GO disappeared after the hydrothermal reaction, including CdO 7211
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Figure 2. Raman spectra of (a) graphene oxide (GO), (b) ZnFe2O4 G(0.2), and (c) pure ZnFe2O4.
Figure 3. C1s XPS spectra of ZnFe2O4G(0.2).
stretching vibrations of COOH groups (1720 cm1), OH deformation vibrations of COOH groups (1620 cm1), OH deformation vibrations of tertiary COH (1396 cm1), and CO stretching vibrations of epoxy groups (1050 cm1). This suggests that there is reduced GO in the ZnFe2O4G(0.2) composite. This result can be further confirmed by Raman and XPS observations. Figure 2 displays Raman spectra of GO, ZnFe2O4graphene, and pure ZnFe2O4. It has been reported that Raman shifts of G- and D-bands shift to lower values when GO is reduced to graphene.69,70 A similar trend was observed in this study: the G-band shifted from 1599 to 1582 cm1, whereas the D-band shifted from 1362 to 1349 cm1 for the ZnFe2O4 graphene. Besides, the 2D band at 2692 cm1 is also observed, indicative of the reduction of GO and the formation of graphene structure. The peak position of the 2D band is similar to that of a monolayer graphene.7174 The Raman spectra of ZnFe2O4 graphene and pure ZnFe2O4 photocatalysts show similar features in the range of 1001000 cm1, and this is in agreement with published work on ZnFe2O4 particles.75,76 Wave numbers above 600 cm1 are of the A1 g type, involving motions of the O in tetrahedral AO4 groups. The other low-frequency phonon modes are due to metal ion involved in octahedral groups (BO6). These modes correspond to the symmetric and antisymmetric bending of oxygen atom in MO bond at octahedral groups.75,76 XPS has proved to be a useful tool for identifying the oxidation state of elements. In Figure 3, the deconvoluted XPS peaks of C 1s centered at the binding energies of 289.3, 287.5, 286.3, and 284.7 eV were assigned to the CdO, COC, COH, and CdC, respectively.23,57 The XPS peak area ratios of the CdO,
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Figure 4. XRD patterns of (a) ZnFe2O4G(0.2), (b) pure ZnFe2O4, (c) graphene, and (d) graphene oxide (GO) in the range of 570°.
COC, and COH bonds to the CdC bond were calculated and shown in Table 1S (Supporting Information). It can be clearly seen that most carbon atoms were sp2 hybridized, and the intensity of oxygenated functional groups (HO-CdO, COC, and COH) on carbon sheets in ZnFe2O4graphene was obviously decreased compared with that of GO. This indicates that GO could be reduced to graphene with a tiny amount of residual oxygen-containing groups via hydrothermal reaction. We did not find evidence of ZnC or FeC bonds formed in the nanocomposite from the FTIR and XPS results (Figure 1S, Supporting Information). The XRD diffraction patterns of the as-prepared ZnFe2O4 graphene nanocomposites with different graphene content are shown in Figure 4 and Figure 2S (Supporting Information). It is obviously seen that almost all the diffraction peaks of ZnFe2O4 graphene with different graphene content can be assigned to spinel-type ZnFe2O4 (JCPDS 22-1012).16,77 The peaks at 2θ values of 18.3, 30.1, 35.3, 43.0, 53.5, 56.3, and 62.4° can be indexed to (111), (220), (311), (400), (422), (511), and (440) crystal planes of spinel ZnFe2O4, respectively. However, no typical patterns of graphene (002) or GO (001) were observed in ZnFe2O4graphene. This may be ascribed to the fact that GO was reduced to graphene during the hydrothermal reaction and the reduced GO sheets were exfoliated by decorating ZnFe2O4 nanocrystals, leading to the disappearance of the diffraction peaks of graphene (002).23,78,79 3.2. Morphologies and Structures of ZnFe2O4Graphene Photocatalysts. The representative FESEM and TEM images of as-obtained ZnFe2O4graphene photocatalysts with different graphene content are displayed in Figure 5 and Figure 3S (Supporting Information). As can be seen from the images, the near-transparent graphene sheets are exfoliated fully and decorated homogeneously with ZnFe2O4 nanocrystals with diameters in the range of 710 nm. Previous studies have shown the formation mechanism of graphenemetal oxide nanocomposites.24,80 Accordingly, the formation route to anchor ZnFe2O4 nanoparticles onto the exfoliated GO sheets may be proposed as the intercalation and adsorption of zinc and iron ions into the layered GO sheets, followed by the nucleation and growth of ZnFe2O4 crystals, resulting in the exfoliation of GO sheets. GO sheets can be reduced upon hydrothermal reaction, forming ZnFe2O4graphene composite consequently. ZnFe2O4 nanocrystals anchored on the surface of graphene sheets look like a thin film and have a good 7212
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Figure 5. Typical TEM and FESEM images of ZnFe2O4G(0.2).
dispersion behavior and larger surface area, which can offer more active adsorption sites and photocatalytic reaction centers. Therefore, it is expected to offer an enhanced photocatalytic activity. It is well-known that the electronic structure and optical absorption properties of the semiconductor are usually recognized as the key factors in determining its photocatalytic activity. The DFT (density functional theory) calculation shows that the valence band (VB) of spinel structure ZnFe2O4 is mainly composed of O 2p orbitals, whereas the conduction band (CB) is primarly composed of Fe 3d orbitals. The charge transfer is supposed to take place from O 2p orbitals to Fe 3d orbitals upon photoexcitation.81 The UVvis diffuse reflectance spectra (DRS) of pure ZnFe2O4 and ZnFe2O4graphene photocatalysts are shown in Figure 4S. It can be seen that the pure ZnFe2O4 shows the characteristic spectrum with its fundamental absorption sharp edge rising at 700 nm. The bandgap energy can be estimated to be 1.90 eV according to the absorption band edge at 700 nm, which is consistent with the band gap of spinel ZnFe2O4 reported by Jang et al.82 Unfortunately, for the ZnFe2O4graphene system, the absorption is much stronger than that of the pure ZnFe2O4 particles covering the whole visible region due to the presence of graphene, so that the bandgap energy is difficult to be estimated from the DRS data. 3.3. Magnetic Separation Properties of ZnFe2O4Graphene Photocatalysts. As indicated in the Introduction, the magnetic properties of ZnFe2O4 give it good performance in magnetic separation for the ZnFe2O4graphene photocatalysts.
Figure 6. Images of ZnFe2O4G(0.2) suspension with (a) and without (b) a magnetic field.
We investigated magnetic separation properties of the ZnFe2O4graphene photocatalysts. As shown in Figure 6, the catalyst can be easily separated from the reaction mixture. 3.4. Photocatalytic Activity of the Catalysts. The photocatalytic activities of the as-obtained ZnFe2O4graphene nanocomposite photocatalysts with different graphene content for degradation of MB have been performed under visible-light irradiation at room temperature. Figure 7 shows the changes in the absorbance of MB solution in the presence of ZnFe2O4G(0.2) photocatalysts and H2O2 under visible-light irradiation. The adsorptiondesorption equilibrium solution of MB and ZnFe2O4G(0.2) was used as starting solution (the spectrum is 7213
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Figure 9. Room-temperature photoluminescence (PL) spectra of (a) pure ZnFe2O4 and (b) ZnFe2O4G(0.2) (λex = 315 nm). Figure 7. Absorption spectra of the MB solution taken at different photocatalytic degradation times using ZnFe2O4G(0.2) nanocomposite photocatalyst.
Figure 10. EIS of (a) ZnFe2O4G(0.2), (b) pure ZnFe2O4, and (c) GO.
Figure 8. Effect of different catalysts on photocatalytic degradation of MB: (a) H2O2, (b) pure ZnFe2O4, (c) ZnFe2O4graphene, (d) ZnFe2O4 þ H2O2, (e) ZnFe2O4G(0.3) þ H2O2, (f) ZnFe2O4G(0.25) þ H2O2, (g) ZnFe2O4G(0.15) þ H2O2, and (h) ZnFe2O4G(0.2) þ H2O2.
shown in Figure 7a, t = 0 min), and therefore the role of adsorption has been eliminated. As shown in Figure 8, the photodegradation rates of MB using different catalysts under visible light irradiation decreased in the following order: ZnFe2O4G(0.2) þ H2O2 > ZnFe2O4G(0.15) þ H2O2 > ZnFe2O4G(0.25) þ H2O2 > ZnFe2O4G(0.3) þ H2O2 > ZnFe2O4 þ H2O2 > ZnFe2O4G(0.2) > ZnFe2O4 > H2O2. For the H2O2 and pure ZnFe2O4 (Figure 8a, b), the absorbance of MB at 464 nm is nearly unchanged after irradiation for 90 min, suggesting that neither H2O2 nor pure ZnFe2O4 was capable of responding to visible light. However, the photocatalytic activity of the ZnFe2O4G(0.2) nanocomposite photocatalyst was improved and the photodegradation rate of MB reached nearly 20% after irradiation for 90 min (Figure 8c), which can be ascribed to the efficient separation of photogenerated carriers in the ZnFe2O4 and graphene coupling system. It is worthwhile to note that the photocatalytic activity was significantly enhanced when adding H2O2 to the adsorptiondesorption equilibrium system of MB and ZnFe2O4G(0.2). The photodegradation rate of MB was 88% after irradiation for only 5 min and reached up to 99% after irradiation for 90 min (Figure 8h), which demonstrated that almost all the MB molecules in the solution were degraded. Additionally, 20% (w/w) graphene in the photocatalyst gave the best activity. The significant enhancement in photoactivity can be ascribed to its remarkable dual function as a photoelectrochemical degrador of organic molecules and the
generator of hydroxyl radicals ( 3 OH) via photoelectrochemical decomposition of H2O2 under visible light irradiation. It is well-known that the photoluminescence (PL) emission mainly originates from direct recombination of the excited electrons and holes.83,84 To investigate the efficient separation of photogenerated carriers in the ZnFe2O4 and graphene coupling system, the PL spectra were performed to disclose the migration, transfer, and recombination processes of the photogenerated electronhole pairs. Figure 9 and Figure 5S illustrate PL emission spectra of pure ZnFe2O4 and ZnFe2O4graphene nanocomposites with different graphene content monitored at an excitation wavelength of 315 nm. It can be seen that the emission of pure ZnFe2O4 is centered at 387 nm (Figure 9a), which is attributed to the recombination of holes and electrons in the valence band and conduction band. However, because of the introduction of graphene, the fluorescence was quenched significantly for ZnFe2O4G(0.2) (Figure 9b), implying that the recombination of photogenerated electrons and holes is inhibited greatly. Graphene as a two-dimensional sheet of carbon atoms has superior electrical conductivity, which would make it an excellent electron-transport material in the process of photocatalysis. The photogenerated electrons of excited ZnFe2O4 were transferred instantly from the conduction band of ZnFe2O4 to graphene at the site of generation via a percolation mechanism, resulting in a minimized charge recombination and offering an enhanced photocatalytic activity.2,85 Moreover, an increase in intensity of PL emission was observed for the ZnFe2O4graphene system, and the intensity reached a maximum value for ZnFe2O4G(0.3) (Figure 5S), which can be ascribed to the radiative recombination sites, leading to decreased photocatalytic activity in comparison with ZnFe2O4G(0.2).86 At the same time, the electrochemical impedance technique was employed to characterize electrical conductivity. The typical 7214
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Figure 11. Fluorescence spectra of TAOH at (a) 0, (b) 1, (c) 2, (d) 5, (e) 7, and (f) 10 min under visible light irradiation (λex = 312 nm).
electrochemical impedance spectra (EIS) of GO, pure ZnFe2O4, and ZnFe2O4graphene nanocomposites with different graphene content are shown in Figure 10 and Figure 6S (Supporting Information). For ZnFe2O4G(0.2) (Figure 10a and 6Sa), the impedance plot involves an extremely small radius, compared with the GO and pure ZnFe2O4 electrodes, indicating that the charge transfer resistance of ZnFe2O4G(0.2) is significantly decreased and GO has been reduced to graphene in the hydrothermal reaction process. In short, this is because graphene is a zero bandgap semiconductor and a unique two-dimensional Π-conjugation structure in which the charge carriers behave as massless fermions, leading to unique transport properties.87 Therefore, the photogenerated electrons of ZnFe2O4 could transfer easily from the conduction band to graphene and rapidly transport the instant that they formed. As a result of the great inhibition for the recombination of photogenerated electrons and holes, the photocatalytic activity was significantly enhanced. For the ZnFe2O4graphene nanocomposites with different graphene content (Figure 6S), the charge transfer resistance decreased in the following order: ZnFe2O4G(0.2) > ZnFe2O4G(0.15) > ZnFe2O4G(0.25) > ZnFe2O4G(0.3), which is in good agreement with the result of intensity of PL emission. This confirms that ZnFe2O4G(0.2) possesses the highest photocatalytic activity among the samples Not only can ZnFe2O4graphene nanocomposite photocatalyst promote the effective separation of photogenerated charge carriers, but also generate strong oxidant hydroxyl radicals ( 3 OH) via photoelectrochemical decomposition of H2O2 under visible-light irradiation. According to the method reported by Hirakawa and Nosaka,13 the amount of 3 OH was measured by comparing the fluorescent intensity to that of the known concentration of 2-hydroxyterephthalic acid (TAOH), which is the reaction product of 3 OH with terephthalic acid (TA). Figure 11 shows fluorescence spectra of the supernatant solution of the ZnFe2O4G(0.2) suspension containing 3 mM TA and 0.5 mM H2O2 for various duration times under visible-light irradiation. It can be seen that the spectral feature is 426 nm, which is the same as that of TAOH. Therefore, we may safely draw the conclusion that TAOH was generated during the reaction process of TA with 3 OH, while 3 OH was formed in the ZnFe2O4G(0.2) visible-light photocatalysis which plays a vital role in the degradation of MB under visible-light irradiation. In addition, Figure 7S shows that the formation rate of 3 OH increased linearly for two minutes. This means that ZnFe2O4graphene photocatalyst can catalyze H2O2 to decompose into 3 OH immediately under visible-light irradiation, and this may explain why the photodegradation rate of MB reached up to 88% after irradiation for only
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Figure 12. Bar plot showing the remaining methylene blue (MB) in solution after reaching the adsorptiondesorption equilibrium in the dark for 30 min with stirring.
5 min. Two possible generation mechanisms of 3 OH can be proposed as the following steps: (1) Photocatalysis ZnFe2 O4 þ hv f ZnFe2 O4 ðh þ eÞ
ð1Þ
ZnFe2 O4 ðeÞ þ graphene f graphene ðeÞ
ð2Þ
graphene ðeÞ þ H2 O2 f 3 OH þ OH
ð3Þ
The electronhole pairs are generated upon visible-light excitation on the ZnFe2O4 surface (reaction 1), followed by instant transfer of photogenerated electrons to graphene sheets via a percolation mechanism (reaction 2). Finally, the negatively charged graphene sheets can activate the hydrogen peroxide to produce 3 OH radical (reaction 3). (2) Fenton reaction and photo-Fenton reaction88,89 Fe3þ þ e f Fe2þ
ð4Þ
Fe2þ þ H2 O2 f Fe3þ þ 3 OH þ OH
ð5Þ
Fe3þ þ hv þ OH f Fe2þ þ 3 OH
ð6Þ
The photogenerated electrons are directly trapped by the Fe3þ to form Fe2þ (reaction 4), which can react with H2O2 to produce 3 OH radical (reaction 5). Oxidation ability of the Fenton mixtures can be enhanced via visible-light irradiation (reaction 6). Because of the lower valence band potential and poor property in photoelectric conversion, ZnFe2O4 usually can not be used directly as a photocatalyst. It should be mentioned that the addition of H2O2 only slightly improved the photocatalytic activity of ZnFe2O4 (the photodegradation rate of MB was 22% after irradiation for 90 min, Figure 8d) and this could be ascribed to the second mechanism (Fenton reaction and photoFenton reaction). In contrast, the photocatalytic activity of ZnFe2O4G(0.2) was significantly enhanced by addition of H2O2 and the photodegradation rate of MB was 88% after irradiation for only 5 min and reached up to 99% after irradiation for 90 min (Figure 8h). The strong photoactivity enhancement may be ascribed to the introduction of graphene, which played an important role in photogenerated electron transfer between the semiconductor and graphene. Accordingly, we can conclude that the photocatalytic reactions on the ZnFe2O4graphene system appear to proceed by the two mechanisms and the first mechanism is obviously dominant. 7215
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’ ASSOCIATED CONTENT
bS
Supporting Information. XPS data of C 1s, Zn 2p, and Fe 2p of ZnFe2O4G(0.2) catalyst; XRD patterns, typical TEM images, photoluminescence (PL) spectra, and EIS of ZnFe2O4 graphene nanocomposites with different graphene content. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86-25-84305667. Fax: þ86-25-8431-5054. E-mail: wxin@ public1.ptt.js.cn. Figure 13. Bar plot showing the photodegradation rate of blue (MB) in solution for 10 cycles using ZnFe2O4G(0.2) photocatalyst in the presence of H2O2.
Additionally, the adsorption of MB molecules is also a crucial factor for the significant enhancement in photoactivity.2,3,60 Figure 12 displays the remaining solution of MB after reaching the adsorptiondesorption equilibrium in the dark with the pure ZnFe2O4 and with ZnFe2O4G(0.2). It can be clearly seen that almost all dye molecules remained in the solution with pure ZnFe2O4 as the catalyst, whereas a large amount of dye molecules (ca. 50%) was adsorbed on the surface of ZnFe2O4G(0.2). Zhang et al. suggested the enhanced adsorptivity should be largely assigned to the selective adsorption of the dye having aromatic structure on the graphene-supported system, and the adsorption was noncovalent and driven by the ΠΠ stacking between MB and Π-conjugation regions of the graphene sheets.2 The ZnFe2O4graphene catalysts can be used repeatedly for the photocatalytic degradation of MB solution. It can be seen that the photodegradation rate of MB still reached over 95% after 10 cycles (Figure 13), indicating the composite photocatalyst has good photocatalytic stability under visible light. The structure and composition of ZnFe2O4G(0.2) catalyst after 10 cycles of reaction were also analyzed by using of XRD, Raman, and XPS techniques. Experimental results confirmed that there was no noticeable change in the structure and composition of ZnFe2O4G(0.2) catalyst within a long time under visible light irradiation.
4. CONCLUSION In summary, a magnetically separable ZnFe2O4graphene nanocomposite photocatalyst with different graphene content has been successfully prepared via a one-step hydrothermal method. TEM observations indicate that graphene sheets are fully exfoliated and decorated with ZnFe2O4 nanocrystals. The photocatalytic activity measurements demonstrate that the ZnFe2O4graphene catalyst has an important dual function as the photoelectrochemical degrader for methylene blue and the generator of strong oxidant hydroxyl radical via photoelectrochemical decomposition of H2O2 under visible light irradiation. The significant enhancement in photoactivity can be ascribed to the efficient separation of photogenerated carriers in the ZnFe2O4 and graphene coupling system, and the concerted effects of individual components or their integrated properties. In addition, ZnFe2O4 nanoparticles themselves have magnetic properties, which can make the ZnFe2O4graphene composite magnetically separable in a suspension system.
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