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Scalable and facile preparation of optical-magnetic dual function 3D Ni@graphene-ZnO for high efficiency removal of hexavalent chromium
Ceramics International (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Scalable and facile preparation of optical-magnetic dual function 3D Ni@ graphene-ZnO for high efficiency removal of hexavalent chromium ⁎
Lizhong Liua, , Tonghua Suna, Hongbo Zhanga, Haiqun Chenb a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China Key Laboatory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, Jiangsu Province, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Ni@graphene-ZnO Photocatalyst Cr(VI) reduction Magnetic separation
For the first time, ZnO nanoparticles decorated three-dimensional (3D) Ni@graphene (Ni@GE) composites with uniform size and magnetic separation properties were synthesized by a simple and scalable one-step sol– gel method. The photocatalysts were characterized by X-ray diffraction, fourier-transform infrared spectra, field-emission scanning electron microscope, transmission electron microscopy, UV–vis diffusive reflectance spectra and photo luminescence. The results indicated the ZnO nanoparticles loaded on the outsides of the Ni@ GE were no significant numbers of vacancies or apparent aggregation. Meanwhile, the introduction of 3D structured graphene material resulted in the better charge separation efficiency compared with that of the pure ZnO. More significantly, the as-prepared Ni@graphene-ZnO showed superior photocatalytic activity and stability for Cr(VI) reduction to Cr(III).
1. Introduction Hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are the most common forms of chromium [1]. Cr(VI) is highly toxic to plants and animals due to its ability to oxidize biomolecules and notably DNA. However, Cr(III) is relatively nontoxic and an essential nutrient in the human diet to maintain the effective glucose, lipid and protein metabolism [2,3]. Therefore, the conversion of Cr(VI) to Cr(III) is an important subject in the field of inorganic pollutants elimination [4]. As a green and sustainable technology, semiconductor-based photocatalysis has received much attention in the last few decades due to a wide variety of applications (e.g., the photocatalytic reduction of Cr(VI)) [5]. ZnO is an n-type semiconductor with a band gap energy of 3.37 eV and has been used as one of the promising photocatalyst due to its low cost, high solar absorbency power and nontoxic nature [6]. Several ZnO-based materials have been developed for photocatalytic reduction of Cr(VI) [7–10], but the preparation methods and recollection of the photocatalysts were relatively complex and difficult, limiting their large scale applications. In recent years, graphene has attracted considerable interest owing to its high conductivity, fast mass and electron transport kinetics, and large specific surface area [4,11,12]. Our previous studies have already demonstrated that the combination of graphene with semiconductorbased materials such as CoFe2O4 [13] and Fe3O4@CuO [14] not only
⁎
solved the nanoparticles agglomeration problem and showed high catalytic activity, but also implemented easily the composite separation via an external magnetic field. Herein, based on the excellent properties of ZnO and graphene, we designed and constructed a hybrid of magnetic 3D Ni@GE composites and ZnO nanoparticles. To the best of our knowledge, there is no report on the controlled synthesis of Ni@ GE-ZnO photocatalysts (designated as NGZ). Firstly,, the graphene membranes in Ni@GE composite prepared by our previous reported method [15] acted as carriers to support ZnO nanoparticles. Hereafter, the growth of the outer ZnO nanoparitcles were accomplished by a simple one-step sol–gel method, which does not require expensive equipment or facilities for patterning or for chemical processing, illustrating the advantages of being simple, fast, low cost, controllable and highly scalable [16]. Besides, the introduction of 3D structured graphene materials not only prevented the ZnO nanoparitcles agglomeration, but also afforded ZnO nanoparticles better charge separation efficiency, and then enhanced the photocatalytic reduction activity and stability, which were better than that of ZnO and ZnO-graphene reported by other groups [7].
Corresponding author. E-mail address: [email protected] (L. Liu).
http://dx.doi.org/10.1016/j.ceramint.2016.12.024 Received 17 October 2016; Received in revised form 19 November 2016; Accepted 5 December 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, L., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.024
Ceramics International (xxxx) xxxx–xxxx
L. Liu et al.
using a Jobin Yvon SPEX Fluorolog-3-P spectroscope. The removal of Cr(VI) ions were performed by measuring the absorbance values at various intervals of time using a UV–visible spectrometer (UV–Vis, DDR). 2.3. Photocatalytic activity measurement The photocatalytic performance of the samples was evaluated by reducing Cr(VI) at room temperature (25 °C). 0.01 g of photocatalysts was suspended in 50 mL of K2Cr2O7 solutions (10 mg L−1) in a quartz tube, and stirred for 60 min under conditions of dark to reach adsorption-desorption equilibrium. Then the mixed suspensions were exposed to UV irradiation produced by a 300 W high pressure Hg lamp. At given time intervals of irradiation, 3 mL of aliquots was withdrawn, and then magnetically separated to remove essentially all the catalyst. The characteristic adsorption peak at 350 nm for Cr(VI) was used to evaluate the variation of Cr(VI) concentration by a Shimadzu UV-2700 UV–vis spectrophotometer.
Fig. 1. Schematic illustration of NGZ fabrication.
2. Experimental
3. Results and discussion
2.1. Preparation of NGZ
The XRD patterns of Ni@GE and NGZ composites are shown in Fig. 2A. The XRD pattern of Ni@GE depicted diffraction peaks at 2θ=44.51, 51.85, and 76.37°, which corresponded to the (111), (200) and (220) planes of cubic Ni (JCPDS: 04-0850), indicating that Ni2+ ions were converted into metallic Ni after calcination process [4,15]. Compared with the XRD patterns of Ni@GE, additional diffraction peaks appeared in the XRD patterns of the obtained NGZ at 31.77, 34.42, 36.25, 47.54, 56.60, 62.86, 66.38, 67.96, 69.10 and 72.56°, which could be indexed to the (100), (002), (101), (102), (110), (103), (200), (112), (201) and (004) planes of cubic ZnO (JCPDS: 36-1451), confirming the formation of ZnO and revealing the successful synthesis of NGZ. FTIR measurements provided further evidence for the formation of NGZ catalyst. As shown in Fig. 2(B), the broad and intense band observed at 3340 cm−1 was ascribed to the stretching vibration of O-H [17]; and the band at about 2924 and 2580 cm−1 corresponded to the asymmetric and symmetric C-H stretching vibrations [18], but 1470 and 645 cm−1 for the scissoring and out-of-plane bending vibration of the C-H [19,20]. Moreover, the presence of the band of relatively medium intensity at 1621 and 1068 cm−1 indicated the presence of C=O stretching vibration [21] and C-O-C deformation vibration [22], respectively. However, after the sol-gel treatment, the peaks at 1621 cm-1 in the FTIR spectrum of Ni@GE shifted to 1647 cm-1 in the spectrum of NGZ, while a new prominent absorption band appeared at about 479 cm-1 [23,24], which corresponded to the stretching mode of Zn-O, implying the combination of ZnO nanocrys-
The synthetic route of NGZ was illustrated in Fig. 1. A series of NGZ composites with different ZnO content were synthesized via a simple and scalable sol–gel method, and denoted as NGZ-X (X=70, 80 and 90 wt%, respectively). Typical procedure of NGZ-80 was as follows: 0.01 g of Ni@GE composites [15] were dispersed into 60 mL of ethanol solution, containing 0.15 g of Zn(NO3)2·6H2O, followed by pH adjustment to 10 by 6 M NaOH. The mixture was stirred at 40 °C for 6 h. Finally, the as-obtained precipitate was washed with deionized water and ethanol, then dried at 60 °C. Additionally, the synthesis methods of ZnO and Ni@GE composites have been provided in Supplementary material S1 and S2.
2.2. Characterization The structure of the samples was analyzed by powder X-ray diffraction (XRD, Bruker D8, Cu Kɑ, λ=0.15418 nm), Fourier-transform infrared (FT-IR, Nicolet 370FT-IR), field emission scanning electron microscopy (FESEM, SUPRA55) and transmission electron microscopy (TEM, JEOL JEM-2100). The content of Ni and Zn in NGZ composites were indirectly determined by inductively coupled plasmaatomic emission spectrometer (ICP, novAA300). The UV–vis diffuse reflectance spectra of the samples were recorded by a Shimadzu UV2700 UV–vis spectrophotometer. BaSO4 was used as a reflectance standard. Photo Luminescence (PL) measurements were performed
Fig. 2. (A) XRD patterns of Ni@GE and NGZ; (B) FTIR spectra of Ni@GE, NGZ and ZnO.
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L. Liu et al.
Fig. 3. Typical FESEM images of Ni@GE (A) and NGZ (C); TEM images of Ni@GE (B) and NGZ (D).
Fig. 4. UV–vis diffusive reflectance spectra (A) and PL spectra (B) of NGZ and bare ZnO photocatalysts.
tals with Ni@GE composites, which was consistent with the result of XRD. Furthermore, the ICP test results showed that the content of Ni and Zn out of 10 mg of NGZ-80 were 1.167 and 6.372 mg (i.e., 11.67% and 63.72%), respectively. It meant that 7.940 mg of ZnO was loaded on Ni@GE, and the graphene content within NGZ-80 was about 8.94%. The structure of the composites was characterized by FESEM and TEM. As shown in Fig. 3(A,B), a large quantity of irregular 3D Ni@GE composites were prepared through a calcination process; the Ni@GE composites were constituted of the Ni skeleton generated from the reduction of Ni2+ and the few-layered graphene membrane converted from glucose-derived polymers [4,15]. Meanwhile, the lattice fringes of Ni with the d-spacing of 0.21 nm were observed [4]. From TEM image of NGZ (Fig. 3C), it could be observed that balck nanoparticles (ZnO) with the average size of ca. 12 nm (Fig. S1) were uniformly loaded on the outer surfaces of Ni@GE without significant numbers of vacancies or apparent aggregation. The lattice fringe images of the balck nanoparticles in NGZ showed a d-spacing of ca. 0.28 nm, corresponding to the (110) plane of ZnO [21], which confirmed further the
Fig. 5. Effect of different catalysts on photocatalytic reduction of Cr(VI).
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Table 1 Comparison of the Cr(VI) reduction capacity of NGZ with other photocatalysts. Sample
Cr(VI) concentration
Catalyst concentration
Light source
Irradiation time
Result
Reference
NGZ TiO2 (commercial) TiO2 (pure) TiO2 (pure)-graphene ZnO-graphene
10 mg/L
0.20 g/L 1 g/L
UV light
40 min 4h
99% 70% 83% 91% 96%
This work [28]
[7]
Fig. 6. (A) Magnetic hysteresis loop of NGZ photocatalyst at the temperature of 300 K. The insets shows the magnetic hysteresis loop in low field zone (left) and the dispersion system after magnetic separation using an external magnet (right); (B) Five consecutive cycles using the same catalyst.
be observed that the Cr(VI) concentration had little decrease in the presence of Ni@GE, and NGZ composites manifested better photocatalytic reducation than that of pure ZnO after 40 min of reaction. The photocatalytic reducation rate of Cr(VI) for pure ZnO was only 34.97%. However, when ZnO were decorated on the 3D structured graphene, the photocatalytic reducation rate was increased to 79% for NGZ-90 and reached maximum value of 99% for NGZ-80. Apparently, the creation of the 3D graphene structure supported by the inner Ni NPs greatly promoted the photocatalytic activity of the composites [15]. Namely, the existence of 3D graphene prevented the aggregation of ZnO particles, which could afford more active sites for the photocatalysis; meanwhile, the superior electronic transmission performance of graphene facilitated efficient separation of photogenerated electronsholes [14], improving the photocatalytic activity of NGZ. However, when the ZnO content (e.g., 70 wt%) was further decreased below its optimum value, the photocatalytic reducation deteriorated, indicating that excessive graphene might act as a kind of recombination center instead of providing an electron pathway to promote the recombination of electron-hole [27], resulting in the decrease of active sites for photocatalytic reaction, which reduced the photocatalytic activity of NGZ to some extent. Furthermore, a comparison of photoreduction capacity of NGZ composite with other reported photocatalysts [7,28] is shown in Table 1. The photoreduction efficiency of NGZ towards reduction of Cr(VI) was quite impressive, showing that the NGZ composites had an excellent catalytic activity. Magnetic property of the as-obtained NGZ photocatalyst was also measured by vibrating sample magnetometer at 300 K (Fig. 6A). The typical hysteresis loop of NGZ showed the soft magnetic nature of the photocatalyst. The saturation magnetization, remanent magnetization and coercivity values of NGZ-80 were 5.24 and 1.82 em/µg and 102.2 Oe, respectively, which were consistent with the fact that the photocatalyst could be easily separated from the solution by utilizing an external magnet (the inset of Fig. 6A). Moreover, the stability of NGZ photocatalyst was also investigated by magnetically separation for repeated photocatalytic reduction reactions. As shown in Fig. 6B, the photocatalytic reduction rate of Cr(VI) over NGZ-80 remained at 93% after the 5th cycling run, suggesting that the catalyst was relatively stable during the reaction.
existence of ZnO. In addition, the FESEM image of NGZ (Fig. 3D) also exhibited an irregular 3D structure wherein the ZnO NPs were successfully decorated on the outer of 3D Ni@GE and the surface of the Ni@GE became rough after the growth of ZnO nanoparticles, indicating the growth of ZnO over a large area [25]. This result was also consistent with the TEM, XRD and FTIR results. The UV–vis diffusive reflectance spectra of NGZ and ZnO are shown in Fig. 4A. It is obviously observed that the NGZ had an increased absorption even in visible light region compared with that of bare ZnO, indicating that the introduction of graphene increased the surface charge of the oxides and the modification of the fundamental process of electron-hole pair formation, which is beneficial to the photocatalytic performance [23]. Additionally, it is well known that the PL spectrum is related to the transfer behavior of the photogenerated electronsholes, reflecting the separation and recombination of photogenerated charge carriers [14]. As shown in Fig. 4B, the photoemission intensity of NGZ was much lower than that of bare ZnO, suggesting that the superior electronic conductivity of 3D graphene made it a superior electron-transport material, thus inhibited the recombination of photoinduced electrons and holes in the process of photocatalysis effectively. The result was consistent with the result of the UV–vis diffusive reflectance spectra. The reduction reaction was considered complete when the main absorption peak at 350 nm disappeared [4,24,26]. Before photocatalysis, the prepared photocatalysts were added to the Cr(VI) solution and stirred in the darkness to ensure the adsorption/desorption equilibrium. The UV–vis spectra of Cr(VI) aqueous solutions in the presence of NGZ-80 photocatalyst for different time at room temperature is shown in Fig. S2. It is obvious that in the presence of the UV light, the absorption peak of Cr(VI) at 350 nm decreased rapidly and disappeared within the reaction time of 40 min, suggesting a high photocatalytic activity. In addition, excess NaOH was added to the above colorless solution to demonstrate the presence of Cr(III) in the final solution. Interestingly, the colorless solution turned green due to the formation of hexahydroxychromate(III), demonstrating the presence of Cr(III) [4,26]. Moreover, control experiments over Ni@GE, bare ZnO, NGZ-70 and ZG-90 were also performed under UV irradiation (Fig. 5). It could 4
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[9] J. Doménech, J. Muñoz, Photocatalytic reduction of Cr(VI) over ZnO powder, Electrochim. Acta 32 (1987) 1383–1386. [10] J. Liu, Y. Zhao, J. Ma, Y. Dai, J. Li, J. Zhang, Flower-like ZnO hollow microspheres on ceramic mesh substrate for photocatalytic reduction of Cr(VI) in tannery wastewater, Ceram. Int. 42 (2016) 15968–15974. [11] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191. [12] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [13] G.Y. He, J.J. Ding, J.G. Zhang, Q.L. Hao, H.Q. Chen, One-step ball-milling preparation of highly photocatalytic active CoFe2O4-reduced graphene oxide heterojunctions for organic dye removal, Ind. Eng. Chem. Res. 54 (2015) 2862–2867. [14] J. Ding, L. Liu, J. Xue, Z. Zhou, G. He, H. Chen, Low-temperature preparation of magnetically separable Fe3O4@CuO-RGO core-shell heterojunctions for highperformance removal of organic dye under visible light, J. Alloy. Compd. 688 (2016) 649–656. [15] L. Liu, H. Wang, Z. Zhou, G. He, X. Sun, Q. Chen, H. Chen, A facile novel preparation of three-dimensional Ni@graphene by catalyzed glucose blowing for high-performance supercapacitor electrodes, RSC Adv. 5 (2015) 74463–74466. [16] J.K. Yoo, J. Kim, Y.S. Jung, K. Kang, Scalable fabrication of silicon nanotubes and their application to energy storage, Adv. Mater. 24 (2012) 5452–5456. [17] Q. Dai, S. Bai, H. Li, W. Liu, X. Wang, G. Lu, Template-free and non-hydrothermal synthesis of CeO2 nanosheets via a facile aqueous-phase precipitation route with catalytic oxidation properties, CrystEngComm 16 (2014) 9817–9827. [18] G.M.S. El-Bahy, FTIR and Raman spectroscopic study of Fenugreek (Trigonella foenum graecum L.) seeds, J. Appl. Spectrosc. 72 (2005) 111–1116. [19] S.T. Hung, A. Bhuyan, K. Schademan, J. Steverlynck, M.D. McCluskey, G. Koeckelberghs, K. Clays, M.G. Kuzyk, Spectroscopic studies of the mechanism of reversible photodegradation of 1-substituted aminoanthraquinone-doped polymers, J. Chem. Phys. 144 (2016) 41–48. [20] S. Vitolo, B. Bresci, M. Seggiani, M.G. Gallo, Catalytic upgrading of pyrolytic oils over HZSM-5 zeolite: behaviour of the catalyst when used in repeated upgradingregenerating cycles, Fuel 80 (2001) 17–26. [21] D. Sun, Q. Zou, Y. Wang, Y. Wang, W. Jiang, F. Li, Controllable synthesis of porous Fe3O4@ZnO sphere decorated graphene for extraordinary electromagnetic wave absorption, Nanoscale 6 (2014) 6557–6562. [22] P. Posocco, Y.M. Hassan, I. Barandiaran, G. Kortaberria, S. Pricl, M. Fermeglia, Combined mesoscale/experimental study of selective placement of magnetic nanoparticles in diblock copolymer films via solvent vapour annealing, J. Phys. Chem. C. 120 (2016) 7403–7411. [23] T. Xu, L. Zhang, H. Cheng, Y. Zhu, Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study, Appl. Catal. B: Environ. 101 (2011) 382–387. [24] A. Dandapat, D. Jana, G. De, Pd nanoparticles supported mesoporous γ-Al2O3 film as a reusable catalyst for reduction of toxic CrVI to CrIII in aqueous solution, Appl. Catal. A: Gen. 396 (2011) 34–39. [25] M. Huan, F. Li, X.L. Zhao, D. Luo, X.Q. You, Y.X. Zhang, G. Li, Hierarchical ZnO@ MnO2 core-shell pillar arrays on Ni foam for binder-free supercapacitor electrodes, Electrochim. Acta 152 (2015) 172–177. [26] B.J. Borah, H. Saikia, P. Bharali, Reductive conversion of Cr(VI) to Cr(III) over bimetallic CuNi nanocrystals at room temperature, New J. Chem. 38 (2014) 2748–2751. [27] N. Yang, J. Zhai, D. Wang, Y. Chen, L. Jiang, Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells, ACS Nano 4 (2010) 887–894. [28] X. Liu, L. Pan, T. Lv, G. Zhu, T. Lu, Z. Sun, C. Sun, Microwave-assisted synthesis of TiO2-reduced graphene oxide composites for the photocatalytic reduction of Cr(VI), RSC Adv. 1 (2011) 1245–1249.
4. Conclusions The Ni@graphene-ZnO composites with uniform size could be successfully prepared by the present method, which was scalable and facile. The ZnO nanoparticles were homogeneously coated on the outsides of the Ni@GE. The results showed that the as-prepared NGZ composites possessed better charge separation efficiency compared with that of the pure ZnO. Meanwhile, the photocatalysts could be easily separated by applying an external magnetic field due to the magnetism of the metal Ni. The unique structured 3D NGZ composite showed superior photocatalytic activity and stability for Cr(VI) reduction, which makes it to be a promising photocatalyst for the photocatalytic reduction of high toxic heavy metals. Acknowledgements The financial supports from the National Natural Science Foundation of China (Grant No.: 21377083), China Postdoctoral Science Foundation funded project (Grant No.: 2015M581625) and the special development fund of Shanghai Zhangjiang National Innovation Demonstration Zone (Grant No.: 201505-Q P-B108-006). Appendix A. Supplementary material Supplementary material associated with this article can be found in the online version at doi:10.1016/j.ceramint.2016.12.024. References [1] D.K. Padhi, K. Parida, Facile fabrication of ɑ-FeOOH nanorod/RGO composite: a robust photocatalyst for reduction of Cr(VI) under visible light irradiation, J. Mater. Chem. A 2 (2014) 10300–10312. [2] P. O'Brien, A. Kortenkamp, The chemistry underlying chromate toxicity, Transit. Met. Chem. 20 (1995) 636–642. [3] D. Dinda, A. Gupta, S.K. Saha, Removal of toxic Cr(VI) by UV-active functionalized graphene oxide for water purification, J. Mater. Chem. A 1 (2013) 11221–11228. [4] L. Liu, J. Xue, X. Shan, G. He, X. Wang, H. Chen, In-situ preparation of threedimensional Ni@graphene-Cu composites for ultrafast reduction of Cr(VI) at room temperature, Catal. Commun. 75 (2016) 13–17. [5] X. Li, J. Yu, M. Jaroniec, Hierarchical photocatalysts, Chem. Soc. Rev. 45 (2016) 2603–2636. [6] J. Yu, X. Yu, Hydrothermal synthesis and photocatalytic activity of zinc oxide hollow spheres, Environ. Sci. Technol. 42 (2008) 4902–4907. [7] X. Liu, L. Pan, Q. Zhao, T. Lv, G. Zhu, T. Chen, T. Lu, Z. Sun, C. Sun, UV-assisted photocatalytic synthesis of ZnO–reduced graphene oxide composites with enhanced photocatalytic activity in reduction of Cr(VI), Chem. Eng. J. 183 (2012) 238–243. [8] X. Liu, L. Pan, T. Lv, T. Lu, G. Zhu, Z. Sun, C. Sun, Microwave-assisted synthesis of ZnO-graphene composite for photocatalytic reduction of Cr(VI), Catal. Sci. Technol. 1 (2011) 1189–1193.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Scalable and facile preparation of optical-magnetic dual function 3D Ni@ graphene-ZnO for high efficiency removal of hexavalent chromium ⁎
Lizhong Liua, , Tonghua Suna, Hongbo Zhanga, Haiqun Chenb a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China Key Laboatory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, Jiangsu Province, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Ni@graphene-ZnO Photocatalyst Cr(VI) reduction Magnetic separation
For the first time, ZnO nanoparticles decorated three-dimensional (3D) Ni@graphene (Ni@GE) composites with uniform size and magnetic separation properties were synthesized by a simple and scalable one-step sol– gel method. The photocatalysts were characterized by X-ray diffraction, fourier-transform infrared spectra, field-emission scanning electron microscope, transmission electron microscopy, UV–vis diffusive reflectance spectra and photo luminescence. The results indicated the ZnO nanoparticles loaded on the outsides of the Ni@ GE were no significant numbers of vacancies or apparent aggregation. Meanwhile, the introduction of 3D structured graphene material resulted in the better charge separation efficiency compared with that of the pure ZnO. More significantly, the as-prepared Ni@graphene-ZnO showed superior photocatalytic activity and stability for Cr(VI) reduction to Cr(III).
1. Introduction Hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are the most common forms of chromium [1]. Cr(VI) is highly toxic to plants and animals due to its ability to oxidize biomolecules and notably DNA. However, Cr(III) is relatively nontoxic and an essential nutrient in the human diet to maintain the effective glucose, lipid and protein metabolism [2,3]. Therefore, the conversion of Cr(VI) to Cr(III) is an important subject in the field of inorganic pollutants elimination [4]. As a green and sustainable technology, semiconductor-based photocatalysis has received much attention in the last few decades due to a wide variety of applications (e.g., the photocatalytic reduction of Cr(VI)) [5]. ZnO is an n-type semiconductor with a band gap energy of 3.37 eV and has been used as one of the promising photocatalyst due to its low cost, high solar absorbency power and nontoxic nature [6]. Several ZnO-based materials have been developed for photocatalytic reduction of Cr(VI) [7–10], but the preparation methods and recollection of the photocatalysts were relatively complex and difficult, limiting their large scale applications. In recent years, graphene has attracted considerable interest owing to its high conductivity, fast mass and electron transport kinetics, and large specific surface area [4,11,12]. Our previous studies have already demonstrated that the combination of graphene with semiconductorbased materials such as CoFe2O4 [13] and Fe3O4@CuO [14] not only
⁎
solved the nanoparticles agglomeration problem and showed high catalytic activity, but also implemented easily the composite separation via an external magnetic field. Herein, based on the excellent properties of ZnO and graphene, we designed and constructed a hybrid of magnetic 3D Ni@GE composites and ZnO nanoparticles. To the best of our knowledge, there is no report on the controlled synthesis of Ni@ GE-ZnO photocatalysts (designated as NGZ). Firstly,, the graphene membranes in Ni@GE composite prepared by our previous reported method [15] acted as carriers to support ZnO nanoparticles. Hereafter, the growth of the outer ZnO nanoparitcles were accomplished by a simple one-step sol–gel method, which does not require expensive equipment or facilities for patterning or for chemical processing, illustrating the advantages of being simple, fast, low cost, controllable and highly scalable [16]. Besides, the introduction of 3D structured graphene materials not only prevented the ZnO nanoparitcles agglomeration, but also afforded ZnO nanoparticles better charge separation efficiency, and then enhanced the photocatalytic reduction activity and stability, which were better than that of ZnO and ZnO-graphene reported by other groups [7].
Corresponding author. E-mail address: [email protected] (L. Liu).
http://dx.doi.org/10.1016/j.ceramint.2016.12.024 Received 17 October 2016; Received in revised form 19 November 2016; Accepted 5 December 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, L., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.024
Ceramics International (xxxx) xxxx–xxxx
L. Liu et al.
using a Jobin Yvon SPEX Fluorolog-3-P spectroscope. The removal of Cr(VI) ions were performed by measuring the absorbance values at various intervals of time using a UV–visible spectrometer (UV–Vis, DDR). 2.3. Photocatalytic activity measurement The photocatalytic performance of the samples was evaluated by reducing Cr(VI) at room temperature (25 °C). 0.01 g of photocatalysts was suspended in 50 mL of K2Cr2O7 solutions (10 mg L−1) in a quartz tube, and stirred for 60 min under conditions of dark to reach adsorption-desorption equilibrium. Then the mixed suspensions were exposed to UV irradiation produced by a 300 W high pressure Hg lamp. At given time intervals of irradiation, 3 mL of aliquots was withdrawn, and then magnetically separated to remove essentially all the catalyst. The characteristic adsorption peak at 350 nm for Cr(VI) was used to evaluate the variation of Cr(VI) concentration by a Shimadzu UV-2700 UV–vis spectrophotometer.
Fig. 1. Schematic illustration of NGZ fabrication.
2. Experimental
3. Results and discussion
2.1. Preparation of NGZ
The XRD patterns of Ni@GE and NGZ composites are shown in Fig. 2A. The XRD pattern of Ni@GE depicted diffraction peaks at 2θ=44.51, 51.85, and 76.37°, which corresponded to the (111), (200) and (220) planes of cubic Ni (JCPDS: 04-0850), indicating that Ni2+ ions were converted into metallic Ni after calcination process [4,15]. Compared with the XRD patterns of Ni@GE, additional diffraction peaks appeared in the XRD patterns of the obtained NGZ at 31.77, 34.42, 36.25, 47.54, 56.60, 62.86, 66.38, 67.96, 69.10 and 72.56°, which could be indexed to the (100), (002), (101), (102), (110), (103), (200), (112), (201) and (004) planes of cubic ZnO (JCPDS: 36-1451), confirming the formation of ZnO and revealing the successful synthesis of NGZ. FTIR measurements provided further evidence for the formation of NGZ catalyst. As shown in Fig. 2(B), the broad and intense band observed at 3340 cm−1 was ascribed to the stretching vibration of O-H [17]; and the band at about 2924 and 2580 cm−1 corresponded to the asymmetric and symmetric C-H stretching vibrations [18], but 1470 and 645 cm−1 for the scissoring and out-of-plane bending vibration of the C-H [19,20]. Moreover, the presence of the band of relatively medium intensity at 1621 and 1068 cm−1 indicated the presence of C=O stretching vibration [21] and C-O-C deformation vibration [22], respectively. However, after the sol-gel treatment, the peaks at 1621 cm-1 in the FTIR spectrum of Ni@GE shifted to 1647 cm-1 in the spectrum of NGZ, while a new prominent absorption band appeared at about 479 cm-1 [23,24], which corresponded to the stretching mode of Zn-O, implying the combination of ZnO nanocrys-
The synthetic route of NGZ was illustrated in Fig. 1. A series of NGZ composites with different ZnO content were synthesized via a simple and scalable sol–gel method, and denoted as NGZ-X (X=70, 80 and 90 wt%, respectively). Typical procedure of NGZ-80 was as follows: 0.01 g of Ni@GE composites [15] were dispersed into 60 mL of ethanol solution, containing 0.15 g of Zn(NO3)2·6H2O, followed by pH adjustment to 10 by 6 M NaOH. The mixture was stirred at 40 °C for 6 h. Finally, the as-obtained precipitate was washed with deionized water and ethanol, then dried at 60 °C. Additionally, the synthesis methods of ZnO and Ni@GE composites have been provided in Supplementary material S1 and S2.
2.2. Characterization The structure of the samples was analyzed by powder X-ray diffraction (XRD, Bruker D8, Cu Kɑ, λ=0.15418 nm), Fourier-transform infrared (FT-IR, Nicolet 370FT-IR), field emission scanning electron microscopy (FESEM, SUPRA55) and transmission electron microscopy (TEM, JEOL JEM-2100). The content of Ni and Zn in NGZ composites were indirectly determined by inductively coupled plasmaatomic emission spectrometer (ICP, novAA300). The UV–vis diffuse reflectance spectra of the samples were recorded by a Shimadzu UV2700 UV–vis spectrophotometer. BaSO4 was used as a reflectance standard. Photo Luminescence (PL) measurements were performed
Fig. 2. (A) XRD patterns of Ni@GE and NGZ; (B) FTIR spectra of Ni@GE, NGZ and ZnO.
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Fig. 3. Typical FESEM images of Ni@GE (A) and NGZ (C); TEM images of Ni@GE (B) and NGZ (D).
Fig. 4. UV–vis diffusive reflectance spectra (A) and PL spectra (B) of NGZ and bare ZnO photocatalysts.
tals with Ni@GE composites, which was consistent with the result of XRD. Furthermore, the ICP test results showed that the content of Ni and Zn out of 10 mg of NGZ-80 were 1.167 and 6.372 mg (i.e., 11.67% and 63.72%), respectively. It meant that 7.940 mg of ZnO was loaded on Ni@GE, and the graphene content within NGZ-80 was about 8.94%. The structure of the composites was characterized by FESEM and TEM. As shown in Fig. 3(A,B), a large quantity of irregular 3D Ni@GE composites were prepared through a calcination process; the Ni@GE composites were constituted of the Ni skeleton generated from the reduction of Ni2+ and the few-layered graphene membrane converted from glucose-derived polymers [4,15]. Meanwhile, the lattice fringes of Ni with the d-spacing of 0.21 nm were observed [4]. From TEM image of NGZ (Fig. 3C), it could be observed that balck nanoparticles (ZnO) with the average size of ca. 12 nm (Fig. S1) were uniformly loaded on the outer surfaces of Ni@GE without significant numbers of vacancies or apparent aggregation. The lattice fringe images of the balck nanoparticles in NGZ showed a d-spacing of ca. 0.28 nm, corresponding to the (110) plane of ZnO [21], which confirmed further the
Fig. 5. Effect of different catalysts on photocatalytic reduction of Cr(VI).
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Table 1 Comparison of the Cr(VI) reduction capacity of NGZ with other photocatalysts. Sample
Cr(VI) concentration
Catalyst concentration
Light source
Irradiation time
Result
Reference
NGZ TiO2 (commercial) TiO2 (pure) TiO2 (pure)-graphene ZnO-graphene
10 mg/L
0.20 g/L 1 g/L
UV light
40 min 4h
99% 70% 83% 91% 96%
This work [28]
[7]
Fig. 6. (A) Magnetic hysteresis loop of NGZ photocatalyst at the temperature of 300 K. The insets shows the magnetic hysteresis loop in low field zone (left) and the dispersion system after magnetic separation using an external magnet (right); (B) Five consecutive cycles using the same catalyst.
be observed that the Cr(VI) concentration had little decrease in the presence of Ni@GE, and NGZ composites manifested better photocatalytic reducation than that of pure ZnO after 40 min of reaction. The photocatalytic reducation rate of Cr(VI) for pure ZnO was only 34.97%. However, when ZnO were decorated on the 3D structured graphene, the photocatalytic reducation rate was increased to 79% for NGZ-90 and reached maximum value of 99% for NGZ-80. Apparently, the creation of the 3D graphene structure supported by the inner Ni NPs greatly promoted the photocatalytic activity of the composites [15]. Namely, the existence of 3D graphene prevented the aggregation of ZnO particles, which could afford more active sites for the photocatalysis; meanwhile, the superior electronic transmission performance of graphene facilitated efficient separation of photogenerated electronsholes [14], improving the photocatalytic activity of NGZ. However, when the ZnO content (e.g., 70 wt%) was further decreased below its optimum value, the photocatalytic reducation deteriorated, indicating that excessive graphene might act as a kind of recombination center instead of providing an electron pathway to promote the recombination of electron-hole [27], resulting in the decrease of active sites for photocatalytic reaction, which reduced the photocatalytic activity of NGZ to some extent. Furthermore, a comparison of photoreduction capacity of NGZ composite with other reported photocatalysts [7,28] is shown in Table 1. The photoreduction efficiency of NGZ towards reduction of Cr(VI) was quite impressive, showing that the NGZ composites had an excellent catalytic activity. Magnetic property of the as-obtained NGZ photocatalyst was also measured by vibrating sample magnetometer at 300 K (Fig. 6A). The typical hysteresis loop of NGZ showed the soft magnetic nature of the photocatalyst. The saturation magnetization, remanent magnetization and coercivity values of NGZ-80 were 5.24 and 1.82 em/µg and 102.2 Oe, respectively, which were consistent with the fact that the photocatalyst could be easily separated from the solution by utilizing an external magnet (the inset of Fig. 6A). Moreover, the stability of NGZ photocatalyst was also investigated by magnetically separation for repeated photocatalytic reduction reactions. As shown in Fig. 6B, the photocatalytic reduction rate of Cr(VI) over NGZ-80 remained at 93% after the 5th cycling run, suggesting that the catalyst was relatively stable during the reaction.
existence of ZnO. In addition, the FESEM image of NGZ (Fig. 3D) also exhibited an irregular 3D structure wherein the ZnO NPs were successfully decorated on the outer of 3D Ni@GE and the surface of the Ni@GE became rough after the growth of ZnO nanoparticles, indicating the growth of ZnO over a large area [25]. This result was also consistent with the TEM, XRD and FTIR results. The UV–vis diffusive reflectance spectra of NGZ and ZnO are shown in Fig. 4A. It is obviously observed that the NGZ had an increased absorption even in visible light region compared with that of bare ZnO, indicating that the introduction of graphene increased the surface charge of the oxides and the modification of the fundamental process of electron-hole pair formation, which is beneficial to the photocatalytic performance [23]. Additionally, it is well known that the PL spectrum is related to the transfer behavior of the photogenerated electronsholes, reflecting the separation and recombination of photogenerated charge carriers [14]. As shown in Fig. 4B, the photoemission intensity of NGZ was much lower than that of bare ZnO, suggesting that the superior electronic conductivity of 3D graphene made it a superior electron-transport material, thus inhibited the recombination of photoinduced electrons and holes in the process of photocatalysis effectively. The result was consistent with the result of the UV–vis diffusive reflectance spectra. The reduction reaction was considered complete when the main absorption peak at 350 nm disappeared [4,24,26]. Before photocatalysis, the prepared photocatalysts were added to the Cr(VI) solution and stirred in the darkness to ensure the adsorption/desorption equilibrium. The UV–vis spectra of Cr(VI) aqueous solutions in the presence of NGZ-80 photocatalyst for different time at room temperature is shown in Fig. S2. It is obvious that in the presence of the UV light, the absorption peak of Cr(VI) at 350 nm decreased rapidly and disappeared within the reaction time of 40 min, suggesting a high photocatalytic activity. In addition, excess NaOH was added to the above colorless solution to demonstrate the presence of Cr(III) in the final solution. Interestingly, the colorless solution turned green due to the formation of hexahydroxychromate(III), demonstrating the presence of Cr(III) [4,26]. Moreover, control experiments over Ni@GE, bare ZnO, NGZ-70 and ZG-90 were also performed under UV irradiation (Fig. 5). It could 4
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4. Conclusions The Ni@graphene-ZnO composites with uniform size could be successfully prepared by the present method, which was scalable and facile. The ZnO nanoparticles were homogeneously coated on the outsides of the Ni@GE. The results showed that the as-prepared NGZ composites possessed better charge separation efficiency compared with that of the pure ZnO. Meanwhile, the photocatalysts could be easily separated by applying an external magnetic field due to the magnetism of the metal Ni. The unique structured 3D NGZ composite showed superior photocatalytic activity and stability for Cr(VI) reduction, which makes it to be a promising photocatalyst for the photocatalytic reduction of high toxic heavy metals. Acknowledgements The financial supports from the National Natural Science Foundation of China (Grant No.: 21377083), China Postdoctoral Science Foundation funded project (Grant No.: 2015M581625) and the special development fund of Shanghai Zhangjiang National Innovation Demonstration Zone (Grant No.: 201505-Q P-B108-006). Appendix A. Supplementary material Supplementary material associated with this article can be found in the online version at doi:10.1016/j.ceramint.2016.12.024. References [1] D.K. Padhi, K. Parida, Facile fabrication of ɑ-FeOOH nanorod/RGO composite: a robust photocatalyst for reduction of Cr(VI) under visible light irradiation, J. Mater. Chem. A 2 (2014) 10300–10312. [2] P. O'Brien, A. Kortenkamp, The chemistry underlying chromate toxicity, Transit. Met. Chem. 20 (1995) 636–642. [3] D. Dinda, A. Gupta, S.K. Saha, Removal of toxic Cr(VI) by UV-active functionalized graphene oxide for water purification, J. Mater. Chem. A 1 (2013) 11221–11228. [4] L. Liu, J. Xue, X. Shan, G. He, X. Wang, H. Chen, In-situ preparation of threedimensional Ni@graphene-Cu composites for ultrafast reduction of Cr(VI) at room temperature, Catal. Commun. 75 (2016) 13–17. [5] X. Li, J. Yu, M. Jaroniec, Hierarchical photocatalysts, Chem. Soc. Rev. 45 (2016) 2603–2636. [6] J. Yu, X. Yu, Hydrothermal synthesis and photocatalytic activity of zinc oxide hollow spheres, Environ. Sci. Technol. 42 (2008) 4902–4907. [7] X. Liu, L. Pan, Q. Zhao, T. Lv, G. Zhu, T. Chen, T. Lu, Z. Sun, C. Sun, UV-assisted photocatalytic synthesis of ZnO–reduced graphene oxide composites with enhanced photocatalytic activity in reduction of Cr(VI), Chem. Eng. J. 183 (2012) 238–243. [8] X. Liu, L. Pan, T. Lv, T. Lu, G. Zhu, Z. Sun, C. Sun, Microwave-assisted synthesis of ZnO-graphene composite for photocatalytic reduction of Cr(VI), Catal. Sci. Technol. 1 (2011) 1189–1193.
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