ARTICLE pubs.acs.org/IECR
High Photocatalytic Activity of Magnetically Separable Manganese FerriteGraphene Heteroarchitectures Yongsheng Fu,†,‡ Pan Xiong,† Haiqun Chen,‡ Xiaoqiang Sun,*,‡ 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 Fine Petrochemical Engineering, Changzhou University, Changzhou 213164, China § Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, Nanjing 210094, China ABSTRACT: A simple and straightforward strategy was developed to fabricate magnetically separable MnFe2O4graphene photocatalysts with differing graphene content. It was found that graphene sheets were fully exfoliated and decorated with MnFe2O4 nanocrystals having an average diameter of 5.65 nm and a narrow particle size distribution. It is very interesting that, although MnFe2O4 alone is photocatalytically inactive under visible light irradiation, the combination of MnFe2O4 nanoparticles with graphene sheets leads to high photocatalytic activity for the degradation of methylene blue under visible light irradiation. The strong magnetic property of MnFe2O4 nanoparticles can be used for magnetic separation in a suspension system, and therefore it does not require additional magnetic components as is the usual case. Consequently, the MnFe2O4graphene system becomes a dual function photocatalyst. The significant enhancement in photoactivity under visible light irradiation can be ascribed to the reduction of graphene oxide (GO), because the photogenerated electrons of MnFe2O4 can transfer easily from the conduction band to the reduced GO, effectively preventing a direct recombination of electrons and holes. Hydroxyl radicals play the role of main oxidant in the MnFe2O4graphene system, and the radicals’ oxidation reaction is obviously dominant.
1. INTRODUCTION The environmental problem concerning organic waste has increased with the increase in industrial development in the past few decades, and waste accumulation pollutes the environment and is a danger to human health. Photocatalysis is a green technology to remediate organic pollutants caused by an increasingly industrialized society.14 Many studies have reported that various organic pollutants can be degraded completely through photocatalysis using metal oxide semiconductor nanostructures under UV light irradiation.57 It is well-known that solar energy is a renewable resource; however, the need for ultraviolet radiation in photodegradation processes has limited both the practicality and environmental benefits on industrially relevant scales. This is because the UV region occupies only around 4% of the entire solar spectrum, while 45% of the energy belongs to visible light. Therefore, developing high-performance visible light photocatalysts for environmental remediation has become an active research area.817 Recovery and reuse of suspended nanosized photocatalysts after degradation are of great importance for sustainable process management. Magnetic separation provides a convenient technique for removing and recycling magnetized catalysts under an external magnetic field. Adding magnetic species to a variety of solid matrixes allows for the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation.18 Manganese ferrite (MnFe2O4) nanoparticles have been of great interest, because they have proven to be useful in many magnetic applications.1922 MnFe2O4-based nanocomposites especially provide a potential advantage for repeated magnetic separation purposes.23 Although MnFe2O4 alone is r 2011 American Chemical Society
photocatalytically inactive under visible light irradiation, it is possible to improve the efficiency of photoinduced charge separation in MnFe2O4 by coupling it with another semiconductor, resulting in high photocatalytic performance. Graphene is a new two-dimensional carbon material which possesses high surface area, excellent conductivity, and unique graphitic basal plane structure. It is well-known that a single graphene layer is a zero-gap semiconductor with a linear Diraclike spectrum around the Fermi energy. Graphene has received recent attention as a support for catalysts.2427 Some efforts have been made to achieve the utilization of UV irradiation for graphenemetal oxide composites.23,2833 Attention has also been paid to graphene-based visible-light-driven photocatalysts.34,35 Recently, we reported a magnetically separable ZnFe2O4 graphene photocatalyst and its high performance in the photocatalytic degradation of MB (methylene blue) in the presence of hydrogen peroxide under visible light irradiation.36 However, hydrogen peroxide is manufactured by a process that consumes energy and/or other chemical resources. In this study, the purpose is to design and fabricate a graphene-based photocatalyst having high catalytic activity and magnetically separable function without using hydrogen peroxide. A MnFe2O4graphene photocatalyst has been prepared through the one-step hydrothermal method. The results show that, in the as-obtained composite, MnFe2O4 nanoparticles have an average diameter of 5.65 nm and a narrow particle size distribution. Coupling MnFe2O4 Received: September 5, 2011 Accepted: December 14, 2011 Published: December 14, 2011 725
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nanoparticles with graphene sheets leads to high photocatalytic activity for the degradation of MB under visible light irradiation in the absence of hydrogen peroxide.
MB and phenol aqueous solutions, respectively. Before starting the illumination, the reaction mixture was stirred for 60 min in the dark in order to reach the adsorptiondesorption equilibrium between the dye and the catalyst. At a given time interval of irradiation, 5 mL aliquots were withdrawn and then magnetically separated to remove essentially all the catalyst. The degradation of MB and the degradation of phenol were spectrophotometrically monitored by measuring the absorbance of solutions at 664 and 270 nm, respectively, during the photodegradation process. After photocatalytic reactions on the MnFe2O4G(0.3) system, the MB aqueous solution was analyzed using an ion chromatography system (Dionex ICS-90). Photocurrent was measured using an electrochemical workstation (CHI 660B) in a standard three-electrode system. The geometric surface area of the working electrode was about 0.5 cm2. Platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The photocurrent was measured at 0.0 V vs SCE in 0.5 M Na2SO4 aqueous solution. A 500 W xenon lamp was used as the light source with a 420 nm cutoff filter to provide visible light irradiation, and the average light intensity was 31.2 mW cm2.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Magnetic MnFe2O4Graphene Composite Photocatalyst. Graphene oxide 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.37 MnFe2O4graphene nanocomposite photocatalysts with differing graphene content (15, 20, 25, 30, 35, 40 wt %) were synthesized. A typical experiment for the synthesis of MnFe2O4graphene nanocomposite with 15 wt % graphene content is as follows: 80 mg of graphene oxide was dispersed into 60 mL of absolute ethanol with sonication for 1 h. Then 0.716 g of 50% Mn(NO3)2 solution and 1.616 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. Then the mixture was adjusted to pH 10.0 with 6 M NaOH solution and stirred for 30 min, yielding a stable bottlegreen homogeneous emulsion. The resulting mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated to 180 °C for 20 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 MnFe2O4G(0.15). For comparison, the same method was used to synthesize pure MnFe2O4 without graphene oxide. 2.2. Characterization. X-ray photoelectron spectroscopy (XPS) was carried out on an RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg Kα radiation (hν = 1253.6 eV). Raman spectra were acquired on a Renishaw in via Reflex Raman Microprobe. Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advanced diffractometer with Cu Kα 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 38. 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. EIS measurements were recorded with an ac voltage amplitude of 5 mV, with a frequency range of 1 MHz to 5 mHz at 0.5 V. 2.3. Photocatalytic Activity Measurement. The photocatalytic activity of the prepared samples was determined by the degradation of 20 mg/L MB and 20 mg/L phenol aqueous solutions under UV and visible light irradiation, respectively. Photoirradiation was carried out using a 500 W mercury and xenon lamp, respectively, as the light source. In the visible light photocatalysis, the light source was equipped with a UV cutoff filter (JB450) to completely remove any radiation below 420 nm and to ensure illumination by visible light only. Experiments were performed at 25 °C. In all photocatalytic degradation experiments, 0.025 g of photocatalyst was added to 100 mL of 20 mg/L
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of MnFe2O4Graphene Nanocomposite Photocatalyst. The XRD diffraction patterns
of the as-prepared MnFe2O4graphene nanocomposite, reduced graphene oxide, and graphite oxide (GO) are shown in Figure 1A. It can be seen that almost all the diffraction peaks of MnFe2O4graphene may be assigned to spinel-type MnFe2O4 (JCPDS 73-1964).39 However, no typical diffraction peak of graphite (002) or GO (001) is observable in the XRD pattern for MnFe2O4graphene. The disappearance of the (001) diffraction peak is explained as being due to the fact that the regular layer stacking of GO can be destroyed by the crystal growth of MnFe2O4 between the interlayers during the hydrothermal reaction, leading to the exfoliation of GO. On the other hand, as reported previously, graphene oxide sheets can be reduced under hydrothermal conditions in the presence of alcohols, and the exfoliated reduced graphene oxide sheets show no visible sign of the (002) peak.36 Figure 1B gives the nitrogen adsorptiondesorption isotherms (inset) and pore size distribution plots for MnFe2O4 G(0.3). The MnFe2O4G(0.3) sample exhibits a type IV isotherm representative of mesoporous solids. It is well-known that the mesoporous structure is a more efficient photocatalyst structure for degrading organic pollutants in water. The specific surface area of the MnFe2O4G(0.3) sample was determined to
Figure 1. (A) XRD patterns of (a) MnFe2O4G(0.3), (b) reduced graphene oxide, and (c) graphite oxide (GO) in the range of 570°. (B) Nitrogen sorption isotherms (inset) and corresponding pore size distribution curves for MnFe2O4G(0.3). 726
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Figure 3. (A) Raman spectra of GO and MnFe2O4G(0.3). (B) UVvis absorbance spectra of graphene oxide, pure MnFe2O4, and MnFe2O4G(0.3).
seen, providing further evidence for the reduction of GO and the formation of graphene structure. The peak position of the 2D band is similar to that of monolayer graphene.4447 The Raman spectrum of MnFe2O4G(0.3) in the range 1001000 cm1 is in agreement with published work on MnFe2O4 particles.48 UVvis spectroscopy is an informative and important technique to determine the change in the absorption of the semiconductor photocatalysts. As shown in Figure 3B, the UVvis spectrum of graphene oxide exhibits an obvious characteristic absorption peak at about 230 nm, corresponding to the ππ* transition of aromatic CdC bonds. For MnFe2O4G(0.3), the typical absorption peak of graphene oxide at 230 nm disappears and the absorption is much stronger than that of the pure MnFe2O4 particles covering the whole visible region due to the presence of graphene, indicating the MnFe2O4G(0.3) hybrid composite can be used as a photocatalyst under visible light irradiation. The morphology of the as-synthesized MnFe2O4graphene nanocomposite and GO was observed by using transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM). As can be seen in Figure 4a,b, the almost transparent graphene sheets are fully exfoliated and decorated homogeneously with MnFe2O4 nanocrystals having an average diameter of 5.65 nm and a narrow particle size distribution. No obvious aggregation was seen in Figure 4a or 4b. 3.2. Photocatalytic Properties and Reaction Kinetics. The photocatalytic activities of the as-obtained MnFe2O4G(0.3) and pure MnFe2O4 photocatalysts were evaluated by the degradation of methylene blue (MB) and phenol in aqueous solutions under UV light irradiation at 25 °C, respectively. As shown in Figure 5, for pure MnFe2O4 photocatalyst, the photodegradation rates of MB and phenol reached 80 and 68%, respectively, after irradiation for 300 min, indicating that pure MnFe2O4 is photocatalytically active and can be used as a photocatalyst under UV irradiation. It should be noted that the photodegradation rates of MB and phenol over MnFe2O4G(0.3) were 100 and 85%, respectively, under the same conditions. Therefore, combination of MnFe2O4 and graphene results in extraordinarily high catalytic activity, which is similar to that of TiO2graphene photocatalyst.29,35 Recently it was found out that modifying TiO2 with graphene on the surface can induce visible light responsive activity, although TiO2 alone can only photodegrade organics under ultraviolet light.29,35 The visible light photocatalytic activities of MnFe2O4graphene nanocomposite photocatalysts with differing graphene content were evaluated by the degradation of methylene blue (MB) in aqueous solutions at 25 °C, and the results are shown in Figure 6A. The adsorptiondesorption equilibrium solution of MB and MnFe2O4G(0.3) was used as starting solution (t = 0 min). It can be seen that MnFe2O4 alone is photocatalytically
Figure 2. Wide (A) and deconvoluted (BD) XPS spectra of the asprepared MnFe2O4G(0.3). The inset is C 1s XPS spectra of GO.
be 179.30 m2 3 g1, using the BrunauerEmmettTeller (BET, nitrogen, 77 K) method. The BarrettJoynerHalenda (BJH) desorption average pore diameter was 3.95 nm with a very narrow pore size distribution, and the pore volume was 0.17 m2 3 g1. XPS is a useful technique for the elemental surface detection of variations in chemical composition and oxidation state. The XPS spectra of MnFe2O4G(0.3) are shown in Figure 2. The XPS wide-scan spectrum presented in Figure 2A shows the compositional elements of the sample. Parts B and C of Figure 2 show the high-resolution XPS spectra for Mn 2p and Fe 2p, respectively. From the Mn 2p spectrum, it was found that the peak at 641.6 eV is from Mn 2p3/2, with a satellite peak at 646.3, while the peak at 652.2 eV is caused by Mn 2p1/2. This result provides clear evidence for the presence of Mn2+. All the Fe 2p spectra show the two main peaks with binding energies of 711.0 and 724.6 eV which were respectively assigned to Fe 2p3/2 and Fe 2p1/2, accompanied by two satellite peaks visible at binding energies of around 718.5 and 732.5 eV, which is indicative of the presence of Fe3+ cations only.40,41 It is well-known that graphene can be obtained by removal of the oxygen from graphene oxide sheets via hydrothermal reaction in the presence of reducing agents.29,31 The XPS results indicated a decrease of oxygen content in MnFe2O4G(0.3) compared with that of GO (Figure 2D). The intensity of some oxygen-containing groups on carbon sheets in the as-prepared composite was obviously reduced, indicating the deoxygenation of graphene oxide. Among the oxygen-containing groups on the carbon sheets, the epoxy groups were largely reduced in the composite compared with the starting graphite oxide. This shows that via hydrothermal reaction GO has been reduced to graphene with a tiny amount of residual oxygen-containing groups. All the above analysis further confirmed the presence of MnFe2O4 nanocrystals on the surface of graphene sheets without any impurities. Raman spectroscopy is also one of the most sensitive and informative techniques to probe disorder in sp2 carbon materials. As shown in Figure 3A, for MnFe2O4G(0.3), the Raman G and D bands shift to lower frequency in comparison with that of GO: the G band shifted from 1602 to 1589 cm1, whereas the D band shifted from 1360 to 1347 cm1, indicating that GO has been reduced.42,43 Also, the 2D band at 2694 cm1 can be 727
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Figure 4. Typical TEM and FESEM images of MnFe2O4G(0.3) (ac) and GO (d). The inset is the particle size distribution for MnFe2O4.
Figure 5. Photodegradation of (A) MB and (B) phenol under UV light irradiation over MnFe2O4G(0.3) and pure MnFe2O4 photocatalysts, respectively.
inactive under visible light irradiation, whereas the introduction of graphene leads to a qualitative change in the photocatalytic performance (Figure 6A). With increasing graphene content in the MnFe2O4graphene photocatalyst, faster MB degradation was observed, and 30% (w/w) graphene in MnFe2O4 graphene gave the best performance in photocatalytic activity (Figure 6A(g)). The photocatalytic degradation of MB follows a pseudo-firstorder kinetics reaction. The rate equation for MB degradation can be written as follows: k¼
1 c ln t c0
Figure 6. (A) Photocatalytic degradation of MB over (a) pure MnFe2O4, (b) MnFe2O4G(0.15), (c) MnFe2O4G(0.20), (d) MnFe2O4G(0.25), (e) MnFe2O4G(0.40), (f) MnFe2O4G(0.35), and (g) MnFe2O4G(0.30). (B) Rate constant for the photodecomposition of MB on MnFe2O4graphene photocatalysts with differing graphene content. Inset: Total organic carbon removal on MnFe2O4 G(0.30) photocatalyst. (C) Hysteresis loops of pure MnFe2O4 and MnFe2O4G(0.3). The inset is the magnetic separation property of MnFe2O4G(0.3) nanocomposite. (D) Photodegradation rate of MB in solution for three cycles using MnFe2O4G(0.3) photocatalyst.
ð1Þ
where C0 and C are the concentrations of MB when reaction time is 0 and t, respectively. Figure 6B shows the values for the pseudofirst-order rate constant (k) for the photodecomposition of
MB by MnFe2O4graphene photocatalysts with differing graphene content. Among these catalysts, MnFe2O4G(0.3) 728
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showed the highest rate constant of 0.009 74 min1. In contrast, MnFe2O4graphene with lower graphene content (
