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One-step combustion synthesis of CoFe2O4–graphene hybrid materialsfor photodegradation of methylene blue
Materials Letters 113 (2013) 179–181
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One-step combustion synthesis of CoFe2O4–graphene hybrid materials for photodegradation of methylene blue Dafeng Zhang, Xipeng Pu n, Yanyan Gao, Changhua Su, Hong Li, Huaiyong Li, Wenxian Hang School of Materials Science and Engineering, Liaocheng University, Liaocheng, Shandong 252000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 August 2013 Accepted 21 September 2013 Available online 28 September 2013
CoFe2O4–graphene hybrid materials (CFGHs) were synthesized by a simple one-step combustion method. The structures, morphologies and magnetic properties were characterized by X-ray diffraction, transmission electron microscopy, Raman spectrometry, vibrating sample magnetometry, and photoluminescence. The morphologies of CFGHs show that CoFe2O4 nanoparticles were anchored onto the surface of graphene sheets. After the combustion synthesis, graphene oxide sheets were not oxidized, but reduced. CFGHs exhibit ferromagnetic behaviors and can be magnetically separated. Compared with pure CoFe2O4, CFGHs show improved photodegradation performance. The mechanism of enhancement in photodegradation performance is discussed in detail. & 2013 Elsevier B.V. All rights reserved.
Keywords: CoFe2O4 Graphene Nanocomposites Combustion Photocatalyst Magnetic materials
1. Introduction In recent years, semiconductor photocatalysts, especially TiO2, have attracted much attention for their potential application in the photodegradation of organic pollutants [1]. However, most of the photocatalysts are inefficient under visible light excitation [2] and difficult to be collected after being used [3]. Spinel ferrites (MFe2O4, M¼metal cation) are chemically and thermally stable magnetic materials with small band gaps ( 2 eV) making them active under visible light irradiation [4,5]. Moreover, their photocatalytic performances can be improved through coupling with other semiconductors [6,7] and carbonaceous materials [2,8]. Recently, graphene was incorporated into ferrites. Fu et al. synthesized the CoFe2O4–graphene and MgFe2O4–graphene composites with enhanced visible-light photocatalytic properties [2,9,10]. Shen and coworkers synthesized reduced graphene oxide–CoFe2O4 hybrids by a one-pot solvothermal method, and as-obtained hybrids exhibited enhanced visible-light photocatalytic activities for Rhodamine B (RhB) and methylene blue (MB) [11]. Yao et al. synthesized CoFe2O4 graphene hybrids by a simple chemical deposition followed by reduction of GO to graphene [12]. Compared with other methods, the combustion method has some advantages, such as low processing time, low external energy consumption, self-sustaining instantaneous reaction and high yield of fine particles [13,14]. However, to our best knowledge, there is no report on the combustion synthesis of ferrite–graphene hybrids.
n
Corresponding author. Tel.: þ 86 431 5168763. E-mail addresses: [email protected], [email protected] (X. Pu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.088
Herein, CoFe2O4–graphene hybrid materials (CFGHs) with different weight ratios of GO/CoFe2O4 were synthesized by the combustion method. The structures, morphologies and magnetic properties of CFGHs were characterized. Compared with pure CoFe2O4, as-synthesized CFGHs showed improved photocatalytic degradation for MB under visible light irradiation. 2. Experimental All the chemical reagents were of analytical grade and used without further purification. GO was synthesized by the modified Hummers' method [15]. The CFGHs were synthesized by a one-step combustion method, using nitrates (Co(NO3)2 6H2O and Fe(NO3)3 9H2O) and glucose as oxidizer and fuel, respectively. First, required amounts of GO, nitrates and glucose were added to deionized water (50 mL), followed by 30 min of ultrasonic treatment. Mole ratios of both Fe/Co and glucose/nitrates are 2:1. Then, the solution was placed on an electric jacket at a temperature of 300 1C to remove the solvent, until a combustion reaction took place and a black, loose CFGH was obtained. Samples with different weight ratios of GO to CoFe2O4 (0.5, 1, 2 and 4) were synthesized and labeled by CFGH-GO/CoFe2O4 ratio. For comparison, pure CoFe2O4 was synthesized under same conditions. X-ray diffraction (XRD) patterns were recorded on a D8 diffractometer (Bruker Co., Germany). Raman spectra were recorded by a Raman microprobe spectrometer (Thermo Fisher DXR, USA) using an Ar þ laser (wavelength 532 nm). Transmission electron microscopy (TEM) was performed with a JEM-2100 microscope
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(JEOL Ltd., Japan) equipped with an energy-dispersive X-ray spectrometer (EDS). Fourier transform infrared spectroscopy (FTIR) measurements were conducted on a Nicolet 460 spectrometer (Thermo Fisher Scientific Inc., USA). The magnetic measurement was conducted with a vibrating sample magnetometer (Quantum Design Co., USA). Diffuse reflectance spectra and absorption spectra of MB were recorded on a UV-3600 spectrophotometer (Shimadzu Co., Japan). Photocatalysis experiments were carried out in a quartz beaker at different temperatures (5, 25 and 40 1C), filled with MB solution (100 mL and 10–40 mg/L) containing the catalyst (0.25 g/L). A 300 W Xenon lamp with a UV cutoff filter (JB450) was positioned about 10 cm beside the photo-reactor. Prior to visible light irradiation, the suspension was ultrasonicated for 40 min in dark to favor the adsorption and desorption equilibration. The concentration of MB was analyzed by recording the absorption band maximum (664 nm) in the absorption spectra and taken as the initial concentration (C0). During the photocatalysis, 7 mL of the suspension was extracted at an interval of 10 min, and the absorption was measured after 5 min of centrifugation. The normalized temporal concentration changes (C/C0) of MB were obtained.
3. Results and discussion As shown in Fig. 1, the XRD pattern of GO shows a peak at 9.31 corresponding to a larger interlayer distance of 0.94 nm than that of graphite (0.34 nm), due to the oxygen-containing groups in carbon backbones [16]. All patterns of pure CoFe2O4 and CFGHs are well indexed to CoFe2O4 (JCPDS No. 22-1086). Moreover, with increasing GO/CoFe2O4 ratio, the peak intensities decreased gradually, suggesting the decreasing crystal size of CoFe2O4, which indicates that the growth of CoFe2O4 crystals was significantly restricted after the incorporation of graphene sheets [14]. The graphene diffraction peak in CFGHs is not observed, attributed to the stacking disorder of graphene caused by the incorporation of CoFe2O4 nanoparticles. Figs. 2 and S1 show the TEM images of CFGHs. CoFe2O4 nanoparticles were anchored onto the graphene sheets, suggesting the successful assembly of CoFe2O4 and graphene. In Fig. 2 (inset), the lattice spacing (0.49 nm) of CoFe2O4 in CFGH-4 is marked by arrows, which corresponds to the basal spacing of (111) lattice planes. Fig. S2 shows the corresponding EDS pattern, indicating an approximate Co/Fe atom ratio of 1:2, consistent with the chemical formula of CoFe2O4. Fig. S3 presents the Raman spectra of raw GO and CFGH-4. Both spectra show two characteristic peaks at 1330 cm 1 (D-band) and
Fig. 1. XRD patterns of raw GO, pure CoFe2O4 and CFGH-0.5 and CFGH-4.
Fig. 2. TEM image of CFGH-4. Inset shows the corresponding high-resolution TEM image.
Fig. 3. Magnetic hysteresis loops of pure CoFe2O4 and CFGHs with different weight ratios of GO/CoFe2O4 at room temperature. Inset shows the digital image of the response of CFGH-4 in MB solution to an external magnetic field.
1595 cm 1 (G-band). Moreover, the intensity ratio of D-band to G-band (ID/IG) is a measure of the degree of disorder in the graphene or GO [17]. It can be seen that CFGH-4 shows a larger ID/IG ratio (1.21) than that of GO (1.06), attributed to the decrease in the sp2 domain size caused by the thermal reduction during combustion synthesis through the decomposition of partial oxygen-containing functional groups in GO [14,17]. The reduction of GO was also confirmed by the differences between the FTIR spectra of GO and CFGH-4 (Fig. S4). The magnetic hysteresis loops of all samples are shown in Fig. 3. Obviously, all samples show ferromagnetic behavior. The saturation magnetizations of CFGHs are smaller than that of pure CoFe2O4 (59.5 emu/g), ascribed to the introduction of nonmagnetic graphene. The saturation magnetization of CFGH-4 (5.3 emu/g) ensures the magnetic collection from solution in the presence of an external magnetic field, as shown in Fig. 3 (inset). Fig. S5 shows the adsorption curves of MB (20 mg/L) in the presence of pure CoFe2O4 and CFGH-4 in dark. Compared with pure CoFe2O4, CFGH-4 exhibits enhanced adsorption performance,
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the growth of CoFe2O4 particles was effectively suppressed. CFGHs show ferromagnetic behavior and can be magnetically separated. Compared with pure CoFe2O4, CFGHs show improved photodegradation performance, which can be attributed to the efficient transfer of photo-generated electron from CoFe2O4 to graphene sheets and the improved adsorbing capacity due to the incorporation of graphene with high specific surface area.
Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51002069 and 51172102), Scientific Research Foundation of Liaocheng University, and College Students' Innovation Foundation of Liaocheng University (No. SF2012007). Fig. 4. Photocatalytic degradation of MB (20 mg/L) for neat CoFe2O4 and CFGHs with different weight ratios of GO/CoFe2O4 under visible light irradiation at 25 1C.
due to the high specific surface area of graphene sheets. Moreover, the adsorption/desorption equilibration was established in 40 min. Fig. 4 shows the degradation curves of MB (20 mg/L) in the presence of pure CoFe2O4 and CFGHs. In comparison with pure CoFe2O4, CFGHs exhibit improved photodegradation performance. Additionally, photodegradation rate increases with increasing GO/CoFe2O4 ratio. However, further increase in the GO/CoFe2O4 ratio leads to difficult magnetic separation of photocatalyst. For CFGH-4, all MB molecules were decomposed in 120 min, which is also confirmed by the color changing of MB solution in Fig. 3 (inset). Comparing with other reports [9,11], as-obtained CFGH-4 shows enhanced photocatalytic properties. The photocatalytic performances of CFGH-4 at different temperatures were also studied, as shown in Fig. S6, which demonstrates the endothermic characteristics of the photodegradation process [18]. Moreover, the degradation performance of CFGH decreased with increasing initial MB concentration [19], as shown in Fig. S7. The enhancement in photodegradation can be attributed to two reasons [20–22]. First, graphene sheets can accept the photogenerated electrons and the recombination of electrons and holes is inhibited, leading to the improved photocatalytic activity. The detailed mechanism is discussed, as shown in Figs. S8 and S9. Second, the high specific surface area of graphene sheets enhances the adsorbing capacity of CFGH, which is beneficial for the diffusion of MB molecules from solution to the active sites of photocatalyst and thus improves photocatalytic performance of CFGHs. 4. Conclusions Magnetically separable CFGHs were synthesized by a simple one-step combustion method at 300 1C. In CFGHs, CoFe2O4 nanoparticles were anchored onto the surface of graphene sheets, and
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.09.088. References [1] Song L, Zhang Y, Wu X, Liao Z, Yue Q, Qu F, et al. Materials Letters 2013;100:198–200. [2] Fu YS, Chen HQ, Sun XQ, Wang X. AIChE Journal 2012;58:3298–305. [3] Joo J, Ye Y, Kim D, Lee J, Jeon S. Materials Letters 2013;93:141–4. [4] Casbeer E, Sharma VK, Li XZ. Separation and Purification Technology 2012;87:1–14. [5] Sun Y, Wang W, Zhang L, Sun S, Gao E. Materials Letters 2013;98:124–7. [6] Borgohain C, Senapati KK, Sarma KC, Phukan P. Journal of Molecular Catalysis A: Chemical 2012;363:495–500. [7] Senapati KK, Borgohain C, Phukan P. Catalysis Science & Technology 2012;2:2361–6. [8] Xiong P, Fu YS, Wang LJ, Wang X. Chemical Engineering Journal 2012;195:149–57. [9] Fu YS, Chen HQ, Sun XQ, Wang X. Applied Catalysis B: Environmental 2012;111:280–7. [10] Fu YS, Xiong P, Chen HQ, Sun XQ, Wang X. Industrial & Engineering Chemistry Research 2012;51:730–6. [11] Bai S, Shen XP, Zhong X, Liu Y, Zhu GX, Xu X, et al. Carbon 2012;50:2337–46. [12] Yao Y, Yang Z, Zhang D, Peng W, Sun H, Wang S. Industrial & Engineering Chemistry Research 2012;51:6044–51. [13] Patil KC, Aruna ST, Mimani T. Current Opinion in Solid State and Materials Science 2002;6:507–12. [14] Gao YY, Pu XP, Zhang DF, Ding GQ, Shao X, Ma J. Carbon 2012;50:4093–101. [15] Hummers WS, Offeman RE. Journal of the American Chemical Society 1958;80:1339. [16] Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chemical Society Reviews 2010;39:228–40. [17] Ding YH, Zhang P, Zhuo Q, Ren HM, Yang ZM, Jiang Y. Nanotechnology 2011;22:215601. [18] Shen K, Gondal MA, Li Z, Li L, Xu Q, Yamani ZH. Reaction Kinetics, Mechanisms and Catalysis 2013;109:247–58. [19] Zhang S, Li J, Niu H, Xu W, Xu J, Hu W, et al. ChemPlusChem 2013;78:192–9. [20] Leary R, Westwood A. Carbon 2011;49:741–72. [21] Zhao D, Sheng G, Chen C, Wang X. Applied Catalysis B: Environmental 2012;111-112:303–8. [22] Zhao D, Yang X, Chen C, Wang X. Journal of Colloid and Interface Science 2013;398:234–9.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One-step combustion synthesis of CoFe2O4–graphene hybrid materials for photodegradation of methylene blue Dafeng Zhang, Xipeng Pu n, Yanyan Gao, Changhua Su, Hong Li, Huaiyong Li, Wenxian Hang School of Materials Science and Engineering, Liaocheng University, Liaocheng, Shandong 252000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 9 August 2013 Accepted 21 September 2013 Available online 28 September 2013
CoFe2O4–graphene hybrid materials (CFGHs) were synthesized by a simple one-step combustion method. The structures, morphologies and magnetic properties were characterized by X-ray diffraction, transmission electron microscopy, Raman spectrometry, vibrating sample magnetometry, and photoluminescence. The morphologies of CFGHs show that CoFe2O4 nanoparticles were anchored onto the surface of graphene sheets. After the combustion synthesis, graphene oxide sheets were not oxidized, but reduced. CFGHs exhibit ferromagnetic behaviors and can be magnetically separated. Compared with pure CoFe2O4, CFGHs show improved photodegradation performance. The mechanism of enhancement in photodegradation performance is discussed in detail. & 2013 Elsevier B.V. All rights reserved.
Keywords: CoFe2O4 Graphene Nanocomposites Combustion Photocatalyst Magnetic materials
1. Introduction In recent years, semiconductor photocatalysts, especially TiO2, have attracted much attention for their potential application in the photodegradation of organic pollutants [1]. However, most of the photocatalysts are inefficient under visible light excitation [2] and difficult to be collected after being used [3]. Spinel ferrites (MFe2O4, M¼metal cation) are chemically and thermally stable magnetic materials with small band gaps ( 2 eV) making them active under visible light irradiation [4,5]. Moreover, their photocatalytic performances can be improved through coupling with other semiconductors [6,7] and carbonaceous materials [2,8]. Recently, graphene was incorporated into ferrites. Fu et al. synthesized the CoFe2O4–graphene and MgFe2O4–graphene composites with enhanced visible-light photocatalytic properties [2,9,10]. Shen and coworkers synthesized reduced graphene oxide–CoFe2O4 hybrids by a one-pot solvothermal method, and as-obtained hybrids exhibited enhanced visible-light photocatalytic activities for Rhodamine B (RhB) and methylene blue (MB) [11]. Yao et al. synthesized CoFe2O4 graphene hybrids by a simple chemical deposition followed by reduction of GO to graphene [12]. Compared with other methods, the combustion method has some advantages, such as low processing time, low external energy consumption, self-sustaining instantaneous reaction and high yield of fine particles [13,14]. However, to our best knowledge, there is no report on the combustion synthesis of ferrite–graphene hybrids.
n
Corresponding author. Tel.: þ 86 431 5168763. E-mail addresses: [email protected], [email protected] (X. Pu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.088
Herein, CoFe2O4–graphene hybrid materials (CFGHs) with different weight ratios of GO/CoFe2O4 were synthesized by the combustion method. The structures, morphologies and magnetic properties of CFGHs were characterized. Compared with pure CoFe2O4, as-synthesized CFGHs showed improved photocatalytic degradation for MB under visible light irradiation. 2. Experimental All the chemical reagents were of analytical grade and used without further purification. GO was synthesized by the modified Hummers' method [15]. The CFGHs were synthesized by a one-step combustion method, using nitrates (Co(NO3)2 6H2O and Fe(NO3)3 9H2O) and glucose as oxidizer and fuel, respectively. First, required amounts of GO, nitrates and glucose were added to deionized water (50 mL), followed by 30 min of ultrasonic treatment. Mole ratios of both Fe/Co and glucose/nitrates are 2:1. Then, the solution was placed on an electric jacket at a temperature of 300 1C to remove the solvent, until a combustion reaction took place and a black, loose CFGH was obtained. Samples with different weight ratios of GO to CoFe2O4 (0.5, 1, 2 and 4) were synthesized and labeled by CFGH-GO/CoFe2O4 ratio. For comparison, pure CoFe2O4 was synthesized under same conditions. X-ray diffraction (XRD) patterns were recorded on a D8 diffractometer (Bruker Co., Germany). Raman spectra were recorded by a Raman microprobe spectrometer (Thermo Fisher DXR, USA) using an Ar þ laser (wavelength 532 nm). Transmission electron microscopy (TEM) was performed with a JEM-2100 microscope
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D. Zhang et al. / Materials Letters 113 (2013) 179–181
(JEOL Ltd., Japan) equipped with an energy-dispersive X-ray spectrometer (EDS). Fourier transform infrared spectroscopy (FTIR) measurements were conducted on a Nicolet 460 spectrometer (Thermo Fisher Scientific Inc., USA). The magnetic measurement was conducted with a vibrating sample magnetometer (Quantum Design Co., USA). Diffuse reflectance spectra and absorption spectra of MB were recorded on a UV-3600 spectrophotometer (Shimadzu Co., Japan). Photocatalysis experiments were carried out in a quartz beaker at different temperatures (5, 25 and 40 1C), filled with MB solution (100 mL and 10–40 mg/L) containing the catalyst (0.25 g/L). A 300 W Xenon lamp with a UV cutoff filter (JB450) was positioned about 10 cm beside the photo-reactor. Prior to visible light irradiation, the suspension was ultrasonicated for 40 min in dark to favor the adsorption and desorption equilibration. The concentration of MB was analyzed by recording the absorption band maximum (664 nm) in the absorption spectra and taken as the initial concentration (C0). During the photocatalysis, 7 mL of the suspension was extracted at an interval of 10 min, and the absorption was measured after 5 min of centrifugation. The normalized temporal concentration changes (C/C0) of MB were obtained.
3. Results and discussion As shown in Fig. 1, the XRD pattern of GO shows a peak at 9.31 corresponding to a larger interlayer distance of 0.94 nm than that of graphite (0.34 nm), due to the oxygen-containing groups in carbon backbones [16]. All patterns of pure CoFe2O4 and CFGHs are well indexed to CoFe2O4 (JCPDS No. 22-1086). Moreover, with increasing GO/CoFe2O4 ratio, the peak intensities decreased gradually, suggesting the decreasing crystal size of CoFe2O4, which indicates that the growth of CoFe2O4 crystals was significantly restricted after the incorporation of graphene sheets [14]. The graphene diffraction peak in CFGHs is not observed, attributed to the stacking disorder of graphene caused by the incorporation of CoFe2O4 nanoparticles. Figs. 2 and S1 show the TEM images of CFGHs. CoFe2O4 nanoparticles were anchored onto the graphene sheets, suggesting the successful assembly of CoFe2O4 and graphene. In Fig. 2 (inset), the lattice spacing (0.49 nm) of CoFe2O4 in CFGH-4 is marked by arrows, which corresponds to the basal spacing of (111) lattice planes. Fig. S2 shows the corresponding EDS pattern, indicating an approximate Co/Fe atom ratio of 1:2, consistent with the chemical formula of CoFe2O4. Fig. S3 presents the Raman spectra of raw GO and CFGH-4. Both spectra show two characteristic peaks at 1330 cm 1 (D-band) and
Fig. 1. XRD patterns of raw GO, pure CoFe2O4 and CFGH-0.5 and CFGH-4.
Fig. 2. TEM image of CFGH-4. Inset shows the corresponding high-resolution TEM image.
Fig. 3. Magnetic hysteresis loops of pure CoFe2O4 and CFGHs with different weight ratios of GO/CoFe2O4 at room temperature. Inset shows the digital image of the response of CFGH-4 in MB solution to an external magnetic field.
1595 cm 1 (G-band). Moreover, the intensity ratio of D-band to G-band (ID/IG) is a measure of the degree of disorder in the graphene or GO [17]. It can be seen that CFGH-4 shows a larger ID/IG ratio (1.21) than that of GO (1.06), attributed to the decrease in the sp2 domain size caused by the thermal reduction during combustion synthesis through the decomposition of partial oxygen-containing functional groups in GO [14,17]. The reduction of GO was also confirmed by the differences between the FTIR spectra of GO and CFGH-4 (Fig. S4). The magnetic hysteresis loops of all samples are shown in Fig. 3. Obviously, all samples show ferromagnetic behavior. The saturation magnetizations of CFGHs are smaller than that of pure CoFe2O4 (59.5 emu/g), ascribed to the introduction of nonmagnetic graphene. The saturation magnetization of CFGH-4 (5.3 emu/g) ensures the magnetic collection from solution in the presence of an external magnetic field, as shown in Fig. 3 (inset). Fig. S5 shows the adsorption curves of MB (20 mg/L) in the presence of pure CoFe2O4 and CFGH-4 in dark. Compared with pure CoFe2O4, CFGH-4 exhibits enhanced adsorption performance,
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the growth of CoFe2O4 particles was effectively suppressed. CFGHs show ferromagnetic behavior and can be magnetically separated. Compared with pure CoFe2O4, CFGHs show improved photodegradation performance, which can be attributed to the efficient transfer of photo-generated electron from CoFe2O4 to graphene sheets and the improved adsorbing capacity due to the incorporation of graphene with high specific surface area.
Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51002069 and 51172102), Scientific Research Foundation of Liaocheng University, and College Students' Innovation Foundation of Liaocheng University (No. SF2012007). Fig. 4. Photocatalytic degradation of MB (20 mg/L) for neat CoFe2O4 and CFGHs with different weight ratios of GO/CoFe2O4 under visible light irradiation at 25 1C.
due to the high specific surface area of graphene sheets. Moreover, the adsorption/desorption equilibration was established in 40 min. Fig. 4 shows the degradation curves of MB (20 mg/L) in the presence of pure CoFe2O4 and CFGHs. In comparison with pure CoFe2O4, CFGHs exhibit improved photodegradation performance. Additionally, photodegradation rate increases with increasing GO/CoFe2O4 ratio. However, further increase in the GO/CoFe2O4 ratio leads to difficult magnetic separation of photocatalyst. For CFGH-4, all MB molecules were decomposed in 120 min, which is also confirmed by the color changing of MB solution in Fig. 3 (inset). Comparing with other reports [9,11], as-obtained CFGH-4 shows enhanced photocatalytic properties. The photocatalytic performances of CFGH-4 at different temperatures were also studied, as shown in Fig. S6, which demonstrates the endothermic characteristics of the photodegradation process [18]. Moreover, the degradation performance of CFGH decreased with increasing initial MB concentration [19], as shown in Fig. S7. The enhancement in photodegradation can be attributed to two reasons [20–22]. First, graphene sheets can accept the photogenerated electrons and the recombination of electrons and holes is inhibited, leading to the improved photocatalytic activity. The detailed mechanism is discussed, as shown in Figs. S8 and S9. Second, the high specific surface area of graphene sheets enhances the adsorbing capacity of CFGH, which is beneficial for the diffusion of MB molecules from solution to the active sites of photocatalyst and thus improves photocatalytic performance of CFGHs. 4. Conclusions Magnetically separable CFGHs were synthesized by a simple one-step combustion method at 300 1C. In CFGHs, CoFe2O4 nanoparticles were anchored onto the surface of graphene sheets, and
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.09.088. References [1] Song L, Zhang Y, Wu X, Liao Z, Yue Q, Qu F, et al. Materials Letters 2013;100:198–200. [2] Fu YS, Chen HQ, Sun XQ, Wang X. AIChE Journal 2012;58:3298–305. [3] Joo J, Ye Y, Kim D, Lee J, Jeon S. Materials Letters 2013;93:141–4. [4] Casbeer E, Sharma VK, Li XZ. Separation and Purification Technology 2012;87:1–14. [5] Sun Y, Wang W, Zhang L, Sun S, Gao E. Materials Letters 2013;98:124–7. [6] Borgohain C, Senapati KK, Sarma KC, Phukan P. Journal of Molecular Catalysis A: Chemical 2012;363:495–500. [7] Senapati KK, Borgohain C, Phukan P. Catalysis Science & Technology 2012;2:2361–6. [8] Xiong P, Fu YS, Wang LJ, Wang X. Chemical Engineering Journal 2012;195:149–57. [9] Fu YS, Chen HQ, Sun XQ, Wang X. Applied Catalysis B: Environmental 2012;111:280–7. [10] Fu YS, Xiong P, Chen HQ, Sun XQ, Wang X. Industrial & Engineering Chemistry Research 2012;51:730–6. [11] Bai S, Shen XP, Zhong X, Liu Y, Zhu GX, Xu X, et al. Carbon 2012;50:2337–46. [12] Yao Y, Yang Z, Zhang D, Peng W, Sun H, Wang S. Industrial & Engineering Chemistry Research 2012;51:6044–51. [13] Patil KC, Aruna ST, Mimani T. Current Opinion in Solid State and Materials Science 2002;6:507–12. [14] Gao YY, Pu XP, Zhang DF, Ding GQ, Shao X, Ma J. Carbon 2012;50:4093–101. [15] Hummers WS, Offeman RE. Journal of the American Chemical Society 1958;80:1339. [16] Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chemical Society Reviews 2010;39:228–40. [17] Ding YH, Zhang P, Zhuo Q, Ren HM, Yang ZM, Jiang Y. Nanotechnology 2011;22:215601. [18] Shen K, Gondal MA, Li Z, Li L, Xu Q, Yamani ZH. Reaction Kinetics, Mechanisms and Catalysis 2013;109:247–58. [19] Zhang S, Li J, Niu H, Xu W, Xu J, Hu W, et al. ChemPlusChem 2013;78:192–9. [20] Leary R, Westwood A. Carbon 2011;49:741–72. [21] Zhao D, Sheng G, Chen C, Wang X. Applied Catalysis B: Environmental 2012;111-112:303–8. [22] Zhao D, Yang X, Chen C, Wang X. Journal of Colloid and Interface Science 2013;398:234–9.