One-step electrochemical synthesis of a graphene–ZnO hybrid for improved photocatalytic activity

Materials Research Bulletin 48 (2013) 2855–2860

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One-step electrochemical synthesis of a graphene–ZnO hybrid for improved photocatalytic activity Ang Wei a, Li Xiong a, Li Sun a, Yanjun Liu a, Weiwei Li a, Wenyong Lai a, Xiangmei Liu a, Lianhui Wang a, Wei Huang b,*, Xiaochen Dong a,b,** a b

Key Laboratory for Organic Electronics & Information Displays (KLOEID), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210046, China Institute of Advanced Materials, Nanjing University of Technology, Nanjing 210009, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 November 2012 Received in revised form 30 March 2013 Accepted 2 April 2013 Available online 17 April 2013

A graphene–ZnO (G-ZnO) hybrid was synthesized by one-step electrochemical deposition. During the formation of ZnO nanostructure by cathodic electrochemical deposition, the graphene oxide was electrochemically reduced to graphene simultaneously. Scanning electron microscope images, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectra, and UV–vis absorption spectra indicate the resulting G-ZnO hybrid presents a special structure and wide UV–vis absorption spectra. More importantly, it exhibits an exceptionally higher photocatalytic activity for the degradation of dye methylene blue than that of pure ZnO nanostructure under both ultraviolet and sunlight irradiation. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures B. Chemical synthesis D. Catalytic properties D. Microstructure

1. Introduction The study of hybrid nano-materials is a fairly new research area that is growing rapidly within the materials science. Hybrid materials tend to show synergistic effect, the hybrids may show better performance than all of the constituents, including physical, chemical and mechanical properties. What’s more, the overall properties of the composite materials can be tuned through changing the composite parameters. With these finely tunable optical, electrical, magnetic and physical chemical properties, composite nano-materials have huge potential applications in the areas of biotechnology, physical sensing, information optics, energy conversion and chemical catalysis. At present, the research of composite nano-materials is in its infancy with the composite formation mechanisms of constituent functional materials remaining a hot research area with both academic and industrial relevance [1–6]. ZnO, a potential semiconductor with wide band gap (3.37 eV), has received enormous scientific attention in recent years owing to its large excitation-binding energy (about 60 meV), environment

* Corresponding author. Tel.: +86 25 85866396; fax: +86 25 85866396. ** Corresponding author at: Institute of Advanced Materials, Nanjing University of Technology, Nanjing 210009, China. Tel.: +86 25 85866396; fax: +86 25 85866396. E-mail addresses: [email protected] (W. Huang), [email protected] (X. Dong). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.04.012

friendly, morphology controlled growth and high catalytic activity [7–10]. It has been widely applied in the field of biomedicine, photocatalysis, solar cells, lasers, gas sensor, biological sensing, and so on [11–16]. But the composited materials based on ZnO also exists some problems, such as the electron transfer rate is not high enough and the electron–hole separation efficiency is not big enough, which hampers the performance improvement of the ZnO-based composites. Recently, the preparation of ZnO hybrids and their photo-catalytic research become one of the hot spots [17–19]. The absorption range of ZnO locates only in the ultraviolet part of the sunlight spectrum, which means that ZnO could not effectively use the solar energy. The charge carrier recombination rate of ZnO generated by photon is very high, which affects the photocatalytic performance of ZnO nanostructure. In order to improve the photocatalytic activity of ZnO, different kinds of hybrids based on ZnO nanostructure were produced to inhibit the recombination of photo-induced electrons and holes [20–22]. Graphene, a monolayer of carbon atoms in a densely packed honeycomb two-dimensional lattice, has attracted enthusiastic interest since its discovery in 2004 [23,24]. The high mobility (15,000 m2 V1 s1), unusual mechanical strength, excellent optical transparency and ultra-large specific surface area (2630 m2/g) make graphene a novel material for the preparation of high performance composites with other functional nanomaterials [25–27]. Recently, graphene–ZnO (G-ZnO) composite, with both good photo-electric performance and excellent electrical properties,

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becomes more and more popular in the materials research field. GZnO hybrid can be synthesized by various routes, such as hydrothermal deposition [28], chemical vapor deposition (CVD) [29], metal organic vapor phase epitaxy (MOVPE) [30], spray hydrolysis [31], electrochemical deposition [32], and so on. Among these methods, the electrochemical deposition is green, fast, mild, simple and no toxic solvents required. Here we report an easy and effective one-step electrochemical synthesis approach to prepare G-ZnO hybrid. During the synthesis process, graphene oxide (GO) was electrochemically reduced to graphene and the change of the electrochemical interface between the electrode and electrolyte provides a favorable condition for the growth of ZnO simultaneously. Serving as photocatalysis, the asprepared G-ZnO hybrid presents remarkable performance in the photo degradation of methylene blue (MB) compared to ZnO nanostructure. Also, the effects of the structure and interface electronic interaction between ZnO and graphene on the photocatalytic activity were systematically investigated. 2. Experimental 2.1. Reagents and apparatus Natural graphite was purchased from Shanghai Carbon Co., Ltd. and used for the synthesis of GO. Zn (NO3)2, H2O2 (30%) and H2SO4 (98%) were purchased from Chemical Reagent Co. Indium tin oxide (ITO)-coated glass (10 ohm sq1, thickness: 1–1.2 mm) was purchased from Kintec Company (Hong Kong, China). G-ZnO hybrid was observed by scanning electron microscopy (SEM, S4800), X-ray diffraction (XRD, Siemens D5005), Ultraviolet– visible absorption spectra (UV–vis, UV-2401PC spectrometer), photoluminescence spectra (PL, RF-5301PC, excited at 280 nm), Xray photoelectron spectroscopic (XPS, ESCALAB MK II X-ray photoelectron spectrometer) and Raman spectrometer (JYT64000, excited at 514.5 nm). 2.2. Preparation of G-ZnO hybrid GO was synthesized from natural graphite powder based on the modified Hummers method [33]. Briefly, graphite flakes were incubated in H2SO4 at 80 8C and then cooled down to room temperature. After sonicated for 1 h, the solution was diluted with deionized (DI) water and filtered to obtain the pre-oxidized graphite powder, then KMnO4, H2SO4 and H2O2 (30%) was added under ice-bath cooling and stirred. After overnight precipitation, upper portion of the solution was collected, centrifuged, washed with HCl solution to obtain aqueous GO solution. ITO glass was ultrasonically cleaned with acetone, ethanol and distilled water. Then, GO films were fabricated on ITO glass by an impregnationheat-dry process. The ITO glasses were immersed in a flat dish with

Intensity(arb.units)

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The Photocatalytic activities were measured at room temperature using MB aqueous solution as an organic pollutant. The same sized ZnO and G-ZnO films (2 cm  2.5 cm) were dipped into the MB solution. Before measurement, the solution with ZnO or G-ZnO film was placed in the dark for 30 min to reach the adsorption/ desorption equilibrium. Then, it was irradiated by mercury lamp (300 W, 365 nm, simulate UV) or Xe lamp (350 W, 400–780 nm, simulate sunlight). 3. Results and discussion In this experiment, the formation of G-ZnO hybrid by electrochemical deposition reactions are proposed as follows [34,35]: GO þ NO3  þ H2 O þ 2e ! NO2  þ rGO þ 2OH D

Zn2þ þ 2OH ! ZnðOHÞ2 !ZnO þ H2 O

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During the formation process of ZnO nanoparticles, GO can be electrochemically reduced simultaneously. To illustrate the surface character of GO and G-ZnO, XPS was performed. As shown in Fig. 1(a), the spectrum of GO can be deconvoluted into four peaks corresponding to carbon atoms assuming different binding statues. The peaks centered at the binding energies of 284.3, 286.4, 287.1, and 288.8 eV can be assigned to the carbon atoms of C5 5C/C–C in aromatic rings, C–O (epoxy and alkoxy groups), C5 5O (carbonyl groups), and O–C5 5O (carboxyl groups) [33]. Fig. 1(b) shows the C1s XPS spectrum of G-ZnO. Compared with the C1s XPS spectrum of GO, the intensities of all C1s peaks assigned to carbon atoms bound to oxygen, especially the C–O (epoxy and alkoxy) peak, decreased dramatically, indicating that most oxygen-containing groups have been removed after the electrochemical reduction.

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2.3. Photocatalytic activities

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GO suspension, and then put into the oven at 60 8C to dry, and the film edge was cleaned with deionized water. The one-step electrochemical reduction experiment was carried out using CHI660C electrochemical workstation (Chenhua, Shanghai). With 0.1 M zinc nitrate (99.9999%, Aldrich) as the electrolyte, ITO coated GO film (area: 2 cm  2.5 cm) served as the working electrode while a platinum foil (area: 2 mm  5 mm) and saturated calomel electrode (SCE) were used as the counter electrode and reference electrodes. The deposition was performed at the potential of 1.1 V with respect to SCE at 50 8C for 1000 s. As for the preparation of electrochemically deposited ZnO film, all the conditions were same to those of G-ZnO film, in addition to the working electrode for ITO glass.

43

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Fig. 1. C1s XPS spectra of (a) GO and (b) G-ZnO hybrid prepared by electrochemical deposition method.

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Fig. 2. (a) XRD patterns of G-ZnO and ZnO, (b) Raman spectra of G-ZnO hybrid, the inset shows the Raman spectra of the prepared GO and rGO.

Fig. 2(a) shows the XRD patterns of the electrodeposited G-ZnO films. As shown in Fig. 2(a), the reduction extent of GO and hexagonal phase structures of ZnO was obvious. It should be noted that the peak located at 248 was the characteristic peak of the graphite which would be graphene originated from the reduction of GO. In addition, all the diffraction peaks from 308 to 708 can be perfectly indexed to a hexagonal wurtzite structure of ZnO according to No.36-1451, indicating that the resulting ZnO possesses high crystallinity. Fig. 2(b) shows the Raman spectroscopy of G-ZnO hybrid prepared by one-step electrochemical deposition. Except for G band (1588 cm1) and D band (1351 cm1), the Raman peaks located at 436, 572 and 1045 cm1 can be attributed to Zn–O vibrations, indicating the formation of ZnO nanostructure on the surface of rGO film. And the structure of GO and reduced GO (rGO) are characterized by Raman spectroscopy, as show in the inset of Fig. 2(b). It shows that the GO and rGO both contains two obvious characteristic peaks at 1596 cm1 (corresponding to the first-order scattering of the E2g mode) and 1357 cm1 (arising from a breathing mode of k-point phonons of A1g symmetry), which is in accordance with previous report [36]. After the reduction, the ration of D and G band intensity (ID/IG) greatly increase, implying new domains of conjugated carbon atoms (bonded in sp2 hybridization) formed accompanying the removal of the oxygen containing groups. Fig. 3(a) shows the SEM images of ZnO. It can be seen that the piece-like ZnO nanostructures were successfully synthesized on the surface of ITO-coated glass substrates. The inset image indicates that the diameter of the piece-like ZnO nanostructures varied from 200 to 500 nm. Fig. 3(b) displays the typical SEM micrographs of G-ZnO hybrid electrodeposited on ITO-coated glass substrates. The G-ZnO hybrid present flower morphology formed by tower-like particles (200 nm in diameter, 400 nm in length).

The UV–vis absorption spectra of ZnO and G-ZnO hybrid are shown in Fig. 4(a). It can be seen that ZnO nanostructure shows a strong absorption in ultraviolet region between spectral wavelength 300 and 400 nm. There is a wide absorption band that from 300 nm to 650 nm for the UV–vis absorption spectra of G-ZnO hybrid, assigning to the absorption of ZnO and graphene, respectively. The photo-response range of G-ZnO hybrid has extended from ultraviolet region to the visible region. The extended photo-response range for the hybrid, coming from the synergistic effects of graphene and ZnO nanostructure, make more visible light be effectively used for photocatalytic reactions. Meanwhile, the high conductivity of graphene may favor the separation of electron–hole pairs. Therefore, the G-ZnO hybrid presents great application potentials in the field of photocatalysis. Fig. 4(b) shows the room temperature photoluminescence (PL) spectra of ZnO and G-ZnO hybrid. The PL spectra of G-ZnO hybrid comprise two emission bands in the UV and visible ranges, which are similar to that of the ZnO. The UV emission band centered at 370 nm originates from the excitonic recombination, which occurs due to recombination between the electrons in conduction band and the holes in a valence band. The visible emission band between 500 and 700 nm is due to the recombination of electrons in a deep defect level or a shallow surface defect level with holes in a valence band. These defects are easy to cause photo-induced carrier recombination of electron–hole pairs, so that the light-generated non-equilibrium carrier will decline and result in the decline of the photocatalytic properties, especially the surface defects will impact the peak at 370 nm which is the intrinsic emission of ZnO. The synthesis of G-ZnO hybrid could help to reduce the defects of ZnO itself, so the peak at 370 nm from ZnO will be increased. Photocatalytic technology is a new environmental technology based on the semiconductor materials. To evaluate the

Fig. 3. SEM images of (a) ZnO and (b) G-ZnO hybrid. The insets show the magnified SEM images.

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(a)

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Fig. 4. (a) UV–vis absorption spectra of the as-prepared ZnO and G-ZnO hybrid, (b) PL spectra of ZnO and G-ZnO hybrid.

completely for G-ZnO hybrid under both UV and sunlight irradiation (Fig. 5(a) and (b)), indicating its excellent photocatalytic activity. To demonstrate the favorable photocatalytic activity induced by the synergistic effect of the rGO and ZnO, the photo-degradation activity for MB was compared with ZnO, as shown in Fig. 6. It can be observed that there was little degradation of MB solution without the presence of a photocatalyst under UV or sunlight irradiation. Obviously, time-dependent photoactive performance of ZnO and G-ZnO photocatalysts and the degradation of MB can be observed in different catalyst. Degradation rate (Y axis), is defined as (1  C/ C0)  100%, and the X axis is reaction time (h), in which C0 is the initial concentration after the equilibrium adsorption and C

photocatalytic activity of the as-prepared G-ZnO hybrids, the photo degradation of the well-known MB, a typical pollutant in the textile industry, was investigated in water under the simulated by mercury lamp (300 W, 365 nm, UV) and Xe lamp (350 W, 400–780 nm, sunlight). The G-ZnO prepared on ITO glass (area: 2.0 cm  2.5 cm) as photocatalysts, the UV–vis absorption spectrum of MB aqueous solution (5.0 mg/L) exposured to mercury and Xe lamp irradiation for different times are shown in Fig. 5. It can be seen that the characteristic absorption peaks of MB at 291 and 662 nm decreased rapidly with the extension of exposure time under both UV and sunlight irradiation. Compared with the absorption intensity after about 3 h, it can be seen that the absorption peak nearly disappeared

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Fig. 5. Time-dependent absorption spectra of the MB solution which is about MB degradation under UV (a) and sunlight (b) using G-ZnO hybrid as photocatalyst.

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Fig. 6. Photo-degradation rate of MB stimulated by UV (a) and sunlight (b) in the presence of ZnO or G-ZnO hybrid.

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Fig. 7. (a) Energy level diagram of graphene and ZnO. (b) Schematic between charge generation and transfer for organic dye degradation simulated by light.

is the reaction concentration of MB. Compare UV to sunlight irradiation, the photo-degradation rate of MB remains around 60% in the case of ZnO, while contains around 100% in the case of G-ZnO hybrid after 3 h. It indicates that the G-ZnO has enhanced photocatalytic performance under UV and sunlight, which may be attributed to the strong synergistic effect between graphene and ZnO nanostructure. The enhancement of photocatalytic performance of G-ZnO hybrid may come from several reasons. First, the tower-like GZnO hybrid possesses large specific surface area, which is beneficial to the diffusion and mass transportation of MB molecules and oxygen species during the photo chemical reaction. Second, maybe there are strong electronic interaction between ZnO and graphene. The calculated result indicated the potential of the conduction band and the valence band of ZnO is about 7.25 and 4.05 eV, while graphene is about 4.42 eV [37–40]. Therefore, direct electron transfer from the valence band of ZnO to graphene is thermodynamically favorable (Fig. 7(a)), and much more feasible than to the conduction band of ZnO [41]. This electron transfer is responsible for enhanced photocatalytic performance under UV or sunlight [42]. What’s more, the graphene exhibits higher ability to quench the emission at long wavelength than GO, indicating that the presence of graphene can accept the photo-induced electron assuredly and improve the charge separation by charge transfer process (as shown in Fig. 7(b)) [43,44]. Also, the generated charges can quickly transfer to the surface, which contributes to the degradation of dye. After the separation of electrons and holes, there are some reactions involved for the formation of OH radicals, these hydroxyl radicals are known to be very reactive oxidative species which react with the organic or water pollutants that can be degraded to CO2 and H2O. The whole process can be written as: O2 þ e ! O2  O2  þ Hþ ! HO2  2HO2  ! H2 O2 þ O2 H2 O2 þ O2  ! OH þ O2 þ OH OH þ Organic Dye ! CO2 þ H2 O 4. Conclusions A G-ZnO hybrid was synthesized through a simply one-step electrochemical deposition approach. Electrochemical reduction of GO and cathodic electrochemical deposition of ZnO were realized simultaneously. MB photo degradation indicates that the G-ZnO hybrid can greatly enhance the photocatalytic activity under UV and sunlight due to the strong synergistic effect and low ratio

electron recombination between graphene sheets and ZnO nanostructure. It is believed that this easy approach could make the G-ZnO hybrid to be an excellent candidate for applications relating to a number of environmental issues. Acknowledgements We acknowledge the financial support from the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201111SIC), the National Basic Research Program of China (2012CB933301 and 2009CB930601), NNSF of China (21275076, 61076067, 61106036, and 61006007), the Key Project of Chinese Ministry of Education (212058), Research Fund for the Doctoral Program of Higher Education of China (20123223110008) and Jiangsu Province Science Foundation for Six Great Talent Peak (RLD201103). References [1] J.R. Cabrero-Antonino, T. Garcia, P. Rubio-Marques, J.A. Vidal-Moya, A. LeyvaPerez, S.S. Al-Deyab, S.I. Al-Resayes, U. Diaz, A. Corma, ACS Catal. 1 (2011) 147– 158. [2] M. Baibarac, M. Lira-Cantu, J. Oro Sol, I. Baltog, N. Casan-Pastor, P. Gomez-Romero, Compos. Sci. Technol. 67 (2007) 2556–2563. [3] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporte, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, Adv. Funct. Mater. 19 (2009) 17–43. [4] P. Gomez-Romero, O. Ayyad, J. Suarez-Guevara, D. Munoz-Rojas, J. Solid State Electrochem. 14 (2010) 1939–1945. [5] X.C. Dong, H. Xu, X.W. Wang, Y.X. Huang, M.B. Chan-Park, H. Zhang, L.H. Wang, W. Huang, P. Chen, ACS nano 6 (2012) 3206–3213. [6] X.C. Dong, B. Li, A. Wei, X. Cao, M. Chan-Park, H. Zhang, L.J. Li, W. Huang, P. Chen, Carbon 49 (2011) 2944–2949. [7] A. Wei, X.W. Sun, J. Wang, Y. Lei, X. Cai, C.M. Li, Z.L. Dong, W. Huang, Appl. Phys. Lett. 89 (2006) 123902. [8] A. Wei, X.W. Sun, C. Xu, Z.L. Dong, M. Yu, W. Huang, Appl. Phys. Lett. 88 (2006) 213102. [9] A. Wei, L. Pan, W. Huang, Mater. Sci. Eng. B 176 (2011) 1409–1421. [10] S. Chakrabarti, B.K. Dutta, J. Hazard. Mater. 112 (2004) 269–278. [11] W.Q. Zhang, Y. Lu, T.K. Zhang, W. Xu, M. Zhang, S.H. Yu, J. Phys. Chem. C 112 (2008) 19872–19877. [12] F. Labat, I. Ciofini, H.P. Hratchian, M. Frisch, K. Raghavachari, C. Adamo, J. Am. Chem. Soc. 131 (2009) 14290–14298. [13] H. Ma, J. Han, Y. Fu, Y. Song, C. Yu, X. Dong, Appl. Catal. B: Environ. 102 (2011) 417– 423. [14] X.C. Dong, Y.F. Cao, J. Wang, W. Huang, P. Chen, RSC Adv. 2 (2012) 4364– 4369. [15] A. Wei, C. Xu, X. Sun, W. Huang, G.Q. Lo, J. Display Technol. 4 (2008) 9–12. [16] A. Wei, Z. Wang, L.H. Pan, W.W. Li, L. Xiong, X.C. Dong, W. Huang, Chin. Phys. Lett. 28 (2011) 080702. [17] S.J. Kim, D.W. Park, Appl. Surf. Sci. 255 (2009) 5363–5367. [18] E.S. Jang, J.H. Won, S.J. Hwang, J.H. Choy, Adv. Mater. 18 (2006) 3309–3312. [19] Y. Wang, X. Li, G. Lu, G. Chen, Y. Chen, Mater. Lett. 62 (2008) 2359–2362. [20] X. Sun, K. Maeda, M. Le Faucheur, K. Teramura, K. Domen, Appl. Catal. A: Gen. 327 (2007) 114–121. [21] J. Lu, Q. Zhang, J. Wang, F. Saito, M. Uchida, Powder Technol. 162 (2006) 33– 37. [22] N. Wang, X. Li, Y. Wang, Y. Hou, X. Zou, G. Chen, Mater. Lett. 62 (2008) 3691–3693. [23] K. Novoselov, A. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva, A. Firsov, Science 306 (2004) 666–669. [24] Y. Liu, X. Dong, P. Chen, Chem. Soc. Rev. 6 (2012) 2283–2307. [25] H. Wang, J.T. Robinson, G. Diankov, H. Dai, J. Am. Chem. Soc. 132 (2010) 3270– 3271.

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