Wang et al 2013 - Photocatalytic degradation of pendimethalin over Cu2OSnO2-graphene

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<p>Photocatalytic degradation of pendimethalin over Cu2O/SnO2/graphene and SnO2/</p><p>graphene nanocomposite photocatalysts under visible light irradiation</p><p>Zaihua Wang, Yongling Du, Fengyuan Zhang, Zhixiang Zheng, Xiaolong Zhang, Qingliang Feng,</p><p>Chunming Wang*</p><p>Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China</p><p>h i g h l i g h t s g r a p h i c a l a b s t r a c t</p><p> The Cu2O/SnO2/graphene photo-</p><p>catalyst was prepared by a simple</p><p>wet-chemical method.</p><p> Photocatalytic activity of the samples</p><p>was studied for pendimethalin</p><p>degradation.</p><p> CSG and SG exhibited excellent pho-</p><p>tocatalytic activities under visible-</p><p>light.</p><p> A mechanism for highly efcient</p><p>pendimethalin degradation by sam-</p><p>ples was proposed.</p><p>a r t i c l e i n f o</p><p>Article history:</p><p>Received 13 July 2012</p><p>Received in revised form</p><p>12 December 2012</p><p>Accepted 20 March 2013</p><p>Keywords:</p><p>Electrochemical techniques</p><p>Sol-gel growth</p><p>Semiconductors</p><p>Surfaces</p><p>a b s t r a c t</p><p>The Cu2O/SnO2/graphene (CSG) and SnO2/graphene (SG) nanocomposite photocatalysts were prepared</p><p>by simple sol-gel growth method, and characterized by Fourier transform infrared spectra (FTIR), Raman</p><p>spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron mi-</p><p>croscopy (SEM), transmission electron microscopy (TEM) and BrunauereEmmetteTeller (BET) mea-</p><p>surements, respectively. The photocatalytic efciency of catalysts were evaluated by degradation of</p><p>pendimethalin under visible light irradiation (l > 420 nm), which conformed that CSG and SG exhibited</p><p>better photocatalytic activity than SnO2 or graphene alone. An effort has been made to correlate the</p><p>photoelectro-chemical behavior of these samples to the rate of photocatalytic degradation of</p><p>pendimethalin.</p><p>The results demonstrated that the cuprous oxide addition into SG materials could greatly improve the</p><p>photoelectric activity. The mechanism of photocatalytic reaction is proposed based on the energy band</p><p>theory and experimental results. The CSG catalyst with higher photocatalytic activity may have great</p><p>potential in various elds.</p><p>Ó 2013 Elsevier B.V. All rights reserved.</p><p>1. Introduction</p><p>Environmental problems, such as organic pollutants and toxic</p><p>water pollutants, have increased more and more public concern</p><p>nowadays. Compared with biodegradation, the degradation of</p><p>organic pollutants by photocatalysis has attracted extensive</p><p>attention. Semiconductor photocatalysis, as one of the advanced</p><p>physicochemical processes, was extensively studied for degrading</p><p>most kinds of persistent organic pollutants, such as detergents,</p><p>pesticides and volatile organic compounds, under UV-light irradi-</p><p>ation [1e3]. SnO2, a stable and large n-type band gap semi-</p><p>conductor, has the ability to degrade organic pollutant under UV</p><p>light. As the same as TiO2, SnO2 is the suitable material for</p><p>* Corresponding author. Department of Chemistry, Lanzhou University, Tianshui</p><p>Road 222, Lanzhou 730000, PR China. Tel.: þ86 931 8912596; fax: þ86 931</p><p>8912582.</p><p>E-mail addresses: duyl@lzu.edu.cn (Y. Du), wangcm@lzu.edu.cn (C. Wang).</p><p>Contents lists available at SciVerse ScienceDirect</p><p>Materials Chemistry and Physics</p><p>journal homepage: www.elsevier .com/locate/matchemphys</p><p>0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.</p><p>http://dx.doi.org/10.1016/j.matchemphys.2013.03.052</p><p>Materials Chemistry and Physics 140 (2013) 373e381</p><p>industrial use in photoelectrochemical and photocatalytic applica-</p><p>tions due to its efcient photoactivity, chemical and biological</p><p>inertness, nontoxicity, high photostability, cost effectiveness, and</p><p>easy production [4]. However, several fundamental issues must be</p><p>addressed before the photocatalyst is economically viable for large</p><p>scale industrial applications: (1) The band gap of SnO2 (3.6 eV)</p><p>limits the absorption ability of the high-energy portion (UV) of the</p><p>sunlight, which results in the relatively low efciency [5]; (2) The</p><p>fast recombination rate of the photo-generated electronehole (ee</p><p>h) pairs hinders the commercialization of this technology [1].</p><p>It is well known that nanocarbon-inorganic hybrids have shown</p><p>superior performance as photocatalysts [6]. Recently, a nano-</p><p>composite of SnO2 nanoparticles coated on CNTs was synthesized</p><p>and exhibited excellent photocatalytic activity due to the electron</p><p>transfer between SnO2 and CNTs [7]. In comparison with CNTs,</p><p>graphene has perfect sp2 hybridized two-dimensional carbon</p><p>structure with better conductivity and larger surface area [8,9]. The</p><p>presence of oxygen-containing functional groups in GO and</p><p>reduced graphene makes them as excellent supporters to anchor</p><p>inorganic nanocrystals for enhancement of wide applications [10e</p><p>13]. Some recent efforts have been made to fabricate metal oxide/</p><p>graphene photocatalysts, such as TiO2/graphene and ZnO/gra-</p><p>phene, for photodegradation of organic molecules under UV irra-</p><p>diation [14e18]. When combining SnO2 with graphene, at their</p><p>interface electrons will ow from SnO2 to graphene owing to the</p><p>energy level structure of the two materials. The heterojunction</p><p>formed at the interface suppressed recombination of photo-</p><p>generated eeh pairs. Furthermore, hybrids have extended the ab-</p><p>sorption range and enhanced the activity of photocatalysts for</p><p>various reactions, including water and air purication and the</p><p>production of hydrogen [19].</p><p>Another interesting approach involves coupling of two semi-</p><p>conductor particles with different energy levels. As a photocatalyst,</p><p>Cu2O with a direct band gap of 2.0 eV makes it as a potential appli-</p><p>cation in photocatalytic degradation of organic pollutants under</p><p>visible light [20,21].WhenCu2OandSnO2 are combined together, the</p><p>electrons and holes generated on Cu2O by visible irradiation may</p><p>transfer to SnO2 so that the recombination of eeh pairs was sup-</p><p>pressed and thus the quantum efciency was enhanced.</p><p>This work focused on the improved coupling efciency on the</p><p>structural, photoelectrical and photocatalytic characteristics of</p><p>SnO2. In order to enhance the photocatalytic characteristics of SnO2,</p><p>we used graphene as supports and Cu2O added to broaden the</p><p>ability of light adsorption. The mechanism of enhanced photo-</p><p>catalytic activity was systematically investigated. We hope this</p><p>would be helpful for the design and fabrication of novel photo-</p><p>catalysts with greater performances.</p><p>2. Experimental</p><p>2.1. Reagents and instruments</p><p>Graphite ake (nature, -325mesh) was fromAlfa Aesar. All other</p><p>chemicals used in this experiment were analytical grade and used</p><p>without further purication.</p><p>The XRD patterns were recorded with a D/Max 2400 Rigaku</p><p>diffractometer with Cu-Ka radiation (k ¼ 0.15418 nm). The mor-</p><p>phologies of the samples were determined by scanning electron</p><p>microscope (SEM, JSM-S4800, Japan) and transmission electron</p><p>microscopy (TEM, TecnaiG2 F30, FEI, USA). FTIR spectra were</p><p>recorded on a NEXUS 670 FT-IR spectrometer (America). The</p><p>Raman scattering measurements were performed at room tem-</p><p>perature on a Raman system (JY-HR800) with confocal microscopy.</p><p>XPS was performed on a VG ESCA LAB 210 electron spectrometer.</p><p>Nitrogen adsorption/desorption isotherms were measured on the</p><p>ASAP 2020 system (Micromeritics, USA). UVevis spectra were</p><p>recorded on a lambda 35 PerkineElmer spectrometer. Electro-</p><p>chemical experiments were carried out in a homemade electro-</p><p>chemical cell [22] with a conventional three-electrode system (The</p><p>counter electrode was a platinum wire and the saturated calomel</p><p>electrode (SCE) served as reference electrode) and a CHI 614A</p><p>workstation (Shanghai Chenhua, China) at room temperature.</p><p>2.2. Preparation of the SG composite</p><p>Graphite oxide (GO) was synthesized from graphite powder by a</p><p>modied Hummers method [23,24]. GO (100 mg) was exfoliated in</p><p>distilled water (200 mL) with ultrasonic treatment to form a</p><p>colloidal suspension. SnCl2$2H2O (0.1128 g) was dissolved in HCl</p><p>(38%, 0.08 mL) with distilled water (4 mL) added to form a SnCl2e</p><p>HCl solution. The above colloidal suspension and SnCl2eHCl solu-</p><p>tionweremixed with urea (0.05 g) under vigorous</p><p>stirring to form a</p><p>uniform solution. Then the uniform solutionwas continually stirred</p><p>at 60 C for 6 h. The products were collected through a succession of</p><p>processes including centrifuging, washing and drying at room</p><p>temperature under vacuum. The dried powders were further</p><p>annealed at 200 C for 2 h under Ar ow to obtain the SG composite.</p><p>2.3. Preparation of the CSG composite</p><p>According to the molar ratio of the amount of substance of CuCl2</p><p>and SnCl2$2H2O is 2:1, 0.1785 g CuCl2, 0.1128 g SnCl2$2H2O and</p><p>0.080 mL HCl (38%) were added into the 200 mL GO colloidal</p><p>suspension (0.5000 g L1). The resulting mixture was ultra-</p><p>sonicated for 0.5 h and stirred for 2 h at ambient temperature,</p><p>followed by adding ammonia solution into the solution with stir-</p><p>ring to reach a nal pH of 9.00. Then 0.500 mL of hydrazine hydrate</p><p>was added into above solution, and the reduction reaction per-</p><p>formed at room temperature for 2 h under constant stirring. The</p><p>products were ltrated and washed copiously with distilled water</p><p>for several times, then dried at room temperature under vacuum.</p><p>The dried powders were further annealed at 200 C for 2 h under Ar</p><p>ow to obtain the CSG composite.</p><p>2.4. Photocatalytic degradation of pendimethalin</p><p>The photocatalytic activity of obtained samples was evaluated</p><p>by the degradation of pendimethalin (the structure of pendime-</p><p>thalin is shown in Fig. 8A) in an aqueous solution under visible light</p><p>obtained by a 500 W xenon lamp (CHF-XM-500 W, Beijing Trust-</p><p>tech Co. Ltd.) through a 420 nm optical lter. Before visible light</p><p>irradiation, a suspension containing 100 mL of 50 mg L1 pendi-</p><p>methalin solution and 50 mg of solid catalyst was stirred magnet-</p><p>ically in dark for 1 h. The mixture of the sample and pendimethalin</p><p>solution was then photo-irradiated at room temperature. 5 mL</p><p>suspensionwas sampled every 15min and centrifuged at 5000 rpm</p><p>to remove the photocatalyst. Photocatalysis effects were evaluated</p><p>by detecting the absorbance of solutions extracted from the</p><p>mixture with UVevis spectroscopy. The concentration of pendi-</p><p>methalin in the supernatant was determined by recording the</p><p>maximum absorbance of pendimethalin at 239 nm with the UVe</p><p>vis spectrophotometer.</p><p>3. Results and discussion</p><p>3.1. Catalyst characterizations</p><p>To ascertain the reduction of GO to graphene, the pristine GO</p><p>and SG were characterized by infrared spectroscopy (Fig. 1A). As</p><p>could be seen, the 3434 and 1733 cm1 band belong to the OeH</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381374</p><p>groups and carbonyl (C]O) stretching vibration in GO (curve a),</p><p>respectively. The peaks at 1395,1227 and 1051 cm1 are assigned to</p><p>the OeH deformation vibration, the carboxyl (CeOH) and alkoxy</p><p>(CeO) groups stretching vibration, respectively [25]. This result</p><p>clearly reveals that functional groups, such as carboxyl and hy-</p><p>droxyl, have been introduced into carbon frameworks upon</p><p>oxidation. However, the characteristic peaks of GO are not observed</p><p>in the case of SG (curve b), indicating that GO has been reduced to</p><p>graphene. The band at 1560 cm1 corresponding to the C]C aro-</p><p>matic stretching conforms that the graphene networks were</p><p>restored. For the FTIR spectrum of SG nanocomposite, the band</p><p>around 553 and 668 cm1 are attributed to the vibration of the Sne</p><p>OH terminal bonds and the anti-symmetric OeSneO stretching</p><p>mode, respectively.</p><p>The FTIR results revealed that the GO was reduced into gra-</p><p>phene by the solvothermal reaction. Further evidence of the com-</p><p>posites is from Raman spectroscopy. Raman spectroscopy is</p><p>the most sensitive technique for investigation of carbon based</p><p>materials, including graphene [26,27]. It has been used in the</p><p>present work to investigate the deoxygenation of GO lms and the</p><p>role of bonding defects/disorder in GO and reduced GO mono-</p><p>layers. Fig. 1B shows the Raman spectra of GO and SG. A week</p><p>peak observed in the gure named 2D ascribes to an out-of-plane</p><p>vibration mode. The intensity of the 2D band in the SG sample</p><p>Fig. 1. (A) FTIR spectra of GO and SG; (B) Raman spectra of GO and SG; (C) C1s XPS spectra of GO (a) and SG (b).</p><p>Fig. 2. XRD patterns of SG (a) and CSG (b).</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381 375</p><p>was higher in comparison to that of GO and the location of the 2D</p><p>peak for SG is lower than that for GO indicate few-layer SG were</p><p>obtained. In the Raman spectrum of GO, the peaks at about</p><p>1358 cm1 (D band) and 1597 cm1 (G band) are ascribed to the</p><p>graphite substrate. It is known that the ID/IG value is a measure of</p><p>the disorder in the sample, which can be edges, charge puddles,</p><p>ripples, or any other defects. This phenomenon is also reported in</p><p>other literature about the synthesis of single or fewer-layer gra-</p><p>phene [28,29]. A decreased ID/IG intensity ratio in SG can be</p><p>observed in comparison with that of GO, which indicates that the</p><p>solvothermal reaction recovered the aromatic structures by</p><p>repairing defects. For the SG line, the three other peaks observed at</p><p>the low frequency region are assigned to the A1g (626 cm1), A2u</p><p>(685 cm1) and B2g (766 cm1) vibration modes of SnO2 nano-</p><p>particles [30].</p><p>The C1s XPS spectra of GO and SG are shown in Fig. 1C,</p><p>respectively. The XPS spectrum of GO exhibited the characteristic</p><p>peaks of CeC skeleton, hydroxyl and carbonyl carbon at 284.8,</p><p>286.1 and 287.9 eV respectively [31]. Although the C1s feature of SG</p><p>showed the same oxygen functionalities as GO, the absorbance</p><p>peaks of graphene at 286.2 eV (carbon in CeO) and 287.8 eV</p><p>(carbonyl carbon) were sharply decreased, which indicates a</p><p>deoxygenation process [32]. However, the remaining small amount</p><p>oxygenated groups help to maintain the high dispersion of SnO2</p><p>nanocrystals through hydrogen bonding on the graphene layer,</p><p>thus the presence of residual oxygenate groups is perhaps a</p><p>desirable feature in this work.</p><p>The XRD patterns of CSG and SG are shown in Fig. 2. Fig. 2a</p><p>shows the typical XRD pattern of the as-prepared SG composite. No</p><p>obvious peak corresponding to graphene is observed in the powder</p><p>pattern, which might attribute to the low content of graphene and</p><p>the disordered interfacial structure produced by the interfacial</p><p>bond between SnO2 nanocrystals and graphene sheets. The phase</p><p>purity of the as-prepared SG is also examined, where the strong</p><p>diffraction peaks at 2q ¼ 26.48, 33.68, 51.78, 64.71, 65.94 can</p><p>be unambiguously assigned to the characteristic (11 0), (1 0 1), (2 1</p><p>1), (11 2), (3 0 1) plane of tetragonal rutile SnO2 (JCPDS card no. 41-</p><p>1445; space group P42/mnm; a ¼ 4.738 A, c ¼ 3.187 A). Moreover,</p><p>there is no peak assigned other chemical states of Sn, further</p><p>conrming the high purity of SnO2 in the product. In Fig. 2b, the</p><p>Fig. 3. (A) TEM image of graphene sheets; (B) SEM image of SG; (C) TEM image of SG; (D) TEM image of CSG; (E) high magnication TEM image of SG; (F) high magnication TEM</p><p>image of CSG.</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381376</p><p>diffraction peaks at 2q ¼ 29.6, 36.45, 42.44, 61.54 can be</p><p>attributed to characteristic (1 1 0), (1 1 1), (2 0 0), (2 2 0) plane of</p><p>cubic phase Cu2O (JCPDS: 78-2076). The diffraction peaks of CSG are</p><p>broader than that of SG, indicating that the grain size of SnO2 de-</p><p>creases with the introducing of Cu2O. The mean crystal size of the</p><p>inorganic nanoparticles in samples were calculated by using the</p><p>Scherrer equation, they were found to be 4e6 nm in the SG com-</p><p>posite and 3e5 nm for the CSG sample.</p><p>The SEM and TEM micrographs of graphene sheets, SG and CSG</p><p>nanocomposites are shown in Fig. 3. Fig. 3A shows the TEM image</p><p>of graphene sheets, the presence of wrinkles and folds on the sheet</p><p>is the characteristic feature of graphene sheets. As shown in Fig. 3B</p><p>and C, SnO2 nanoparticles are distributed on the graphene sub-</p><p>strate. The sample was treated by sonication in ethanol before TEM</p><p>observation, so the SnO2 particles are thought to be strongly</p><p>anchored on the surface of the graphene. The formation of SnO2</p><p>nanoparticles on the surface of the nanosheets</p><p>could be attributed</p><p>to the functional groups, such as hydroxyl and epoxy groups, which</p><p>are attached to both sides of the graphene oxide nanosheets as</p><p>illustrated in the rst step of Scheme 1. When a GO solution is</p><p>mixed with a SnCl2 solution, Sn2þ is selectively bonded with</p><p>oxygenate groups through electrostatic attraction. After continually</p><p>stirred at 60 C for 6 h, SnO2 nanoparticles successfully adhere to</p><p>both sides of the graphene nanosheets. The digital pictures of so-</p><p>lution corresponding to every synthetic step are shown in Scheme</p><p>1. The color of the suspension shifts from brown to black (in web</p><p>version), which further indicates the change from GO to SG. The</p><p>intimate contact between graphene and SnO2 favors the formation</p><p>of junctions between the twomaterials, as a result, being helpful for</p><p>improving the charge separation and thus enhancing the photo-</p><p>catalytic activity. In addition, the N2 adsorption/desorption iso-</p><p>therms of the as-prepared products are shown in Fig. 4. The BET</p><p>specic surface area of SG and SnO2 nanoparticles are about 223.4</p><p>and 120.2 m2 g1, respectively. Obviously, the BET specic surface</p><p>area of SG nanocomposite is much larger than that of SnO2 nano-</p><p>particles, which could lead to its increased electrochemical reactive</p><p>activity [33e35]. From Fig. 3D, it can be found that the nanoparticle</p><p>aggregations appeared in the SG composite while no obvious</p><p>conglomeration in the CSG composite. The high magnication TEM</p><p>images (Fig. 3E and F) reveal the nanoparticles of CSG are more</p><p>uniform and denser by introducing Cu2O in the composite. In</p><p>addition, the densication of nanoparticles existing on the gra-</p><p>phene surface indicated the interaction between SnO2 and Cu2O</p><p>particles. Such a direct contact between the two particles has been</p><p>found to be crucial for improving the photocatalytic activity of the</p><p>composite. The energy-dispersive X-ray (EDX) spectrum of CSG in</p><p>Fig. 5 exhibits the presence of C, O, Sn and Cu elements, further</p><p>conrming the formation of CSG.</p><p>3.2. Photoelectric property</p><p>In order to compare the photoelectric properties of obtained</p><p>samples, open circuit potentialetimemeasurement was conducted.</p><p>Scheme 1. The chemical route to load SnO2 nanoparticles on both sides of graphene sheets to form a SnO2/graphene composite; the digital pictures are the solution corresponding</p><p>to every synthetic step.</p><p>Fig. 4. Nitrogen adsorption/desorption isotherms of (A) SnO2 nanoparticles and (B) SG</p><p>nanocomposite.</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381 377</p><p>Fig. 6 shows the open circuit potentialetime characteristics in 0.5M</p><p>Na2SO4 of the CSG and SG coated on the glassy carbon electrode</p><p>during successive dark and illumination intervals of 200 s. It could</p><p>be seen clearly that the CSG and SG have obvious step response</p><p>with stable photoelectric signal. When the light is on, the electrons</p><p>are excited from the valence band into the conduction band by light</p><p>energy, which results in the instantly generating of potential. In the</p><p>absence of illumination, the potential decays because of the excited</p><p>electrons return back to the valence band [36]. The high photo-</p><p>response of SG benets from introduction of graphene. The sepa-</p><p>ration efciency of photoinduced electrons and holes is much</p><p>improved because of the electronic interaction between SnO2 and</p><p>graphene sheets. The open-circuit potential (OCP) difference be-</p><p>tween light on and off for CSG is about 47 mV, which is 3.4 times as</p><p>high as SG, 14 mV. The essential reason for improvement of the</p><p>photoelectric properties of CSG may be attributed to two parts: (1)</p><p>CSG possesses particular morphologies caused by doping with</p><p>Cu2O which possesses larger surface exposed to light and higher</p><p>photoactivity; (2) The introduction of Cu2O would enhance the</p><p>quantum efciency and photosensitizing effect. The higher photo-</p><p>electric response implies that the CSG has higher photocatalytic</p><p>activity.</p><p>3.3. Photodegradation of pendimethalin</p><p>In order to explore the photocatalytic activity of the CSG and SG</p><p>catalysts under visible light irradiation, the photocatalytic behavior</p><p>of pure SnO2, graphene and Cu2Owere alsomeasured as references.</p><p>A series of photodegradation experiments were carried out by us-</p><p>ing pendimethalin as a model pollutant under visible light. Fig. 7A</p><p>showed the variation of UVevis absorption spectra of pendime-</p><p>thalin when CSG was introduced as photocatalyst at different in-</p><p>tervals. The slightly decreased absorbance of pendimethalin (inset</p><p>of Fig. 7A) indicated that pendimethalin could not be degraded by</p><p>CSG photocatalyst in darkness. While most of the absorbance of</p><p>pendimethalin decreases during visible light irradiation. After 3 h</p><p>irradiation, the pendimethalin is almost totally degraded. There-</p><p>fore, the decrease in absorbance of pendimethalin during irradia-</p><p>tion is due to chemical reaction rather than physical adsorption.</p><p>The physical adsorption of pendimethalin on the catalysts could be</p><p>attributed to two parts, the adsorption of pendimethalin on the</p><p>surface of graphene and the surface of metal oxides.</p><p>To further prove the effect on the photocatalytic degradation of</p><p>pendimethalin by obtained samples, electrochemical measurement</p><p>was conducted. The inset of Fig. 7B shows cyclic voltammograms of</p><p>bare glass carbon electrode (GCE) in absence (a) and in presence (b)</p><p>of 2  104 M pendimethalin. Two well-dened voltammetric</p><p>peaks of pendimethalin are observed, with the rst at0.677 V and</p><p>the second one at 0.814 V in the cathodic direction vs. SCE. Fig. 7B</p><p>shows differential pulse voltammetry (DPV) responses of variation</p><p>of pendimethalin concentration in different irradiation time by CSG</p><p>photocatalyst at bare GCE. The fact that the peaks current density</p><p>decreases with increasing degradation time further proves excel-</p><p>lent photocatalytic degradation activity of obtained samples.</p><p>The concentration of pendimethalin (C) is proportional to the</p><p>maximum absorbance (A) at 239 nm. Thereby we analyse the</p><p>change of concentration (C/C0) from the variation of absorbance (A/</p><p>A0), where C0 and A0 are the initial concentration and absorbance of</p><p>pendimethalin, respectively. Fig. 8A shows time proles of C/C0</p><p>using different photocatalysts under visible light irradiation. As</p><p>Fig. 5. EDX spectrum of CSG catalyst.</p><p>Fig. 6. OCPetime curves of SG and CSG in dark and under illumination.</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381378</p><p>shown, pendimethalin is very stable under visible light irradiation</p><p>without the presence of a photocatalyst. It is very interesting that</p><p>SnO2, graphene or Cu2O alone is a photocatalytically inert material</p><p>under visible light irradiation. Only about 10%, 20%, 26% of pendi-</p><p>methalin are degraded in 3 h in the presence of pure graphene,</p><p>SnO2 and Cu2O, respectively. However, the catalysts SG and CSG</p><p>display signicantly improved photodegradation efciency to</p><p>pendimethalin, achieving degradation percentage of 90% and 99%</p><p>in 3 h, respectively. The fact also shows photocatalyst CSG performs</p><p>better than SG.</p><p>To better understand the extent of graphene and the effect of</p><p>concentration on the absorption and catalytic properties, we have</p><p>prepared a series of CSG nanocomposites with different addition</p><p>ratios of graphene (which were listed in Table 1) and studied their</p><p>photocatalytic performance for degradation of pendimethalin. The</p><p>photocatalytic degradation efciency of pendimethalin under</p><p>visible light follows the order CSG-C > CSG-D > CSG-B > CSG-</p><p>A z SnO2/Cu2O > CSG-E, as shown in Fig. 8B. Clearly, with</p><p>increasing volume addition of GO, faster pendimethalin</p><p>degradation rate was observed. When the volume addition of GO</p><p>was increased to 200 mL, the sample gave the best performance in</p><p>photocatalytic activity. Further increase of graphene will lead to a</p><p>signicant decrease of photocatalytic activity because CSG-E</p><p>nanocomposite shows lower photocatalytic activity than SnO2/</p><p>Cu2O for degradation of pendimethalin. We presume that an</p><p>overhigh concentration of graphene exhibits a strong absorption</p><p>to</p><p>light, thus reduces the light absorption on SnO2 or Cu2O surface,</p><p>resulting in the decrease of photoexcited electrons.</p><p>Fig. 7. (A) UVevis spectra of degradation pendimethalin by CSG in different time (a / h: 0, 15, 30, 45, 60, 90, 120, 180 min); (B) DPV response of variation of pendimethalin</p><p>concentration in different time (a / h: 0, 15, 30, 45, 60, 90, 120, 180 min) by CSG photocatalyst at bare GCE; scan rate: 50 mV s1, 0.1 M PBS (pH ¼ 7.5). The inset of (A) shows the</p><p>UVevis spectra of adsorption of pendimethalin on CSG in dark for 1 h; the inset of (B) shows cyclic voltammograms of bare GCE in absence (a) and in presence (b) of 2  104 M</p><p>pendimethalin at 50 mV s1 in the potential range from 1.1 V to þ1.3 V.</p><p>Fig. 8. (A) The photodegradation rate of pendimethalin over different catalysts under visible light; (B) the photodegradation rate of pendimethalin over the CSG nanocomposites</p><p>with different weight doping ratios of graphene under visible light. The inset of (A) shows the structural formula of pendimethalin.</p><p>Table 1</p><p>Amount of GO added in starting solution for preparation of CSG.</p><p>CSG CSG-A CSG-B CSG-C CSG-D CSG-E</p><p>GO (mL) 50 100 200 300 500 Fig. 9. Proposed schematic illustration of the charge behavior at the interface of gra-</p><p>phene and SnO2.</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381 379</p><p>3.4. Proposed photocatalytic mechanism</p><p>It is well known that the band gap of SnO2 is about 3.6 eV and</p><p>the potential of its conduction band is aboutþ0.07 V [37]. As shown</p><p>in Fig. 9, the valence band electrons of SnO2 can be excited to its</p><p>conduction band under irradiation. Without the combining of other</p><p>materials, electrons will undergo a quick transition to the valence</p><p>band owing to the instability of excited states, resulting in a low</p><p>photocatalytic activity to the organic pollutants. While the combi-</p><p>nation of SnO2 nanoparticles with reduced graphene, the photo-</p><p>excited electrons will transfer from SnO2 to graphene, thus</p><p>prohibits the recombination rates of eeh pairs. This offers a feasible</p><p>way to improve the photocatalytic efciency and enhance absor-</p><p>bance in the long wavelength range compared with pure SnO2 and</p><p>graphene. In addition, the presence of oxygen-containing func-</p><p>tional groups in GO makes it as excellent supporters to anchor</p><p>inorganic nanocrystals for enhancement of wide applications [10e</p><p>13]. Graphene increased light absorption intensity and adsorptivity</p><p>of pendimethalin. The large surface could also offer adequate active</p><p>sites to participate in photocatalysis, which nally increased the</p><p>photocatalytic activity of SG. Compared with some other semi-</p><p>conductor, such as TiO2 and P25, SnO2 has higher work function. So</p><p>the SG composite spatially separates the electron. The electron on</p><p>the SnO2 surface could also be trapped by dissolved oxygen to form</p><p>various reactive oxygen species, thus greatly enhancing the</p><p>degradation of pendimethalin. While the excited pendimethalin</p><p>adsorbed on SnO2 surface can also inject electrons into the SnO2</p><p>catalyst, and this is believed to be more efcient than that to some</p><p>other semiconductor catalyst, since the potential difference be-</p><p>tween the pendimethalin and SnO2 is much larger than that be-</p><p>tween the pendimethalin and some other semiconductor. The</p><p>electron transfer from pendimethalin to SnO2 is thermodynami-</p><p>cally more favorable. Based on the above discussion, the adsorption</p><p>of pendimethalin on the surfaces of graphene and SnO2 all</p><p>beneted the subsequent degradation. Consequently, the degra-</p><p>dation rate constant of the catalyst SG was larger than that of some</p><p>other graphene-semiconductor composite photocatalysts under</p><p>visible light (Table 2).</p><p>The photocatalytic activity of SG under visible light illumination</p><p>is much improved by Cu2O addition. Cu2O is one of the semi-</p><p>conductors that have high conduction band. Its band gap is about</p><p>2.0 eV and the potential of its conduction band is 1.4 V [44,45].</p><p>Although the SnO2 cannot be excited by visible light, coupling of</p><p>the two kinds of semiconductors allows the vectorial displacement</p><p>of electrons from Cu2O to SnO2, whereas holes can accumulate in</p><p>the valence band of Cu2O to form hole centers, which can be</p><p>consumed by participating in oxidation. The transferred electrons</p><p>on the surface of SnO2 can be trapped by absorbed oxygen to pro-</p><p>duce</p><p></p><p>O2</p><p>. This superoxide ion radical can lead to the formation of</p><p>oxidative H2O2 and hydroxyl radicals ðOHÞ, which are powerful</p><p>oxidizing agent capable of degrading pendimethalin. In addition,</p><p>the nanosheets structures are in favor of the transfer of electrons</p><p>and holes generated inside the crystal to the surface [46], and</p><p>facilitate the degradation of pendimethalin.</p><p>4. Conclusion</p><p>The photodegradation of pendimethalin was carried out by us-</p><p>ing different photocatalysts under irradiation of visible light</p><p>(l > 420 nm). The resulting hybrid material SG showed superior</p><p>photocatalytic activity to pure SnO2 and graphene. Combining</p><p>graphene with suitable semiconductors was a feasible way to</p><p>improve the photocatalytic efciency and enhance absorbance in</p><p>the long wavelength range. The greater photocatalytic activity of</p><p>CSG indicated that Cu2O addition could promote the photo-</p><p>degradation of pendimethalin at SG with favoring the transfer of</p><p>light-induced electrons and holes between Cu2O and SnO2. Thus,</p><p>we can make use of SnO2 coupled with graphene and suitable</p><p>semiconductor for optoelectronic nanodevices and photocatalysts.</p><p>Acknowledgment</p><p>This work was supported by the National Nature Science</p><p>Foundation of China (No.20775030) and the Fundamental Research</p><p>Funds for the Central Universities (lzujbky-2010-34).</p><p>References</p><p>[1] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)</p><p>69e96.</p><p>[2] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735e758.</p><p>[3] T. Torimoto, S. Ito, S. Kuwabata, H. Yoneyama, Environ. Sci. Technol. 30 (1996)</p><p>1275e1281.</p><p>[4] D. Eder, Chem. Rev. 110 (2010) 1348e1385.</p><p>[5] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173e179.</p><p>[6] Z. Ren, E. Kim, S.W. Pattinson, K.S. Subrahmanyam, C.N.R. Rao, A.K. Cheetham,</p><p>D. Eder, Chem. Sci. 3 (2012) 209e216.</p><p>[7] N. Wang, J. Xu, L. Guan, Mater. Res. Bull. 46 (2011) 1372e1376.</p><p>[8] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos,</p><p>I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666e669.</p><p>[9] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett,</p><p>G. Evmenenko, S.T. Nguyen, R.S. Ruoff, Nature 448 (2007) 457e460.</p><p>[10] R. Muszynski, B. Seger, P.V. Kamat, J. Phys. Chem. C 112 (2008) 5263e5266.</p><p>[11] Y.G. Li, Y.Y. Wu, J. Am. Chem. Soc. 131 (2009) 5851e5857.</p><p>[12] G.M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth, R. Mulhaupt, J. Am.</p><p>Chem. Soc. 131 (2009) 8262e8270.</p><p>[13] I.V. Lightcap, T.H. Kosel, P.V. Kamat, Nano Lett. 10 (2010) 577e583.</p><p>[14] O. Akhavan, ACS Nano 4 (2010) 4174e4180.</p><p>[15] H. Zhang, X.J. Lv, Y.M. Li, Y. Wang, J.H. Li, ACS Nano 4 (2010) 380e386.</p><p>[16] G. Williams, B. Seger, P.V. Kamat, ACS Nano 2 (2008) 1487e1491.</p><p>[17] Y.H. Zhang, Z.R. Tang, X.Z. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303e7314.</p><p>[18] O. Akhavan, Carbon 49 (2011) 11e18.</p><p>[19] J.J. Vilatela, D. Eder, ChemSusChem 5 (2012) 456e478.</p><p>Table 2</p><p>Photocatalytic properties of grapheneesemiconductor composite photocatalysts in comparison to the corresponding photocatalysts.</p><p>Composite</p><p>photocatalyst</p><p>Mass fraction</p><p>of graphene</p><p>Typical parameters of</p><p>photocatalytic experiments</p><p>Photocatalytic</p><p>activity</p><p>Reference photocatalyst;</p><p>photocatalytic activity</p><p>Enhancement factor over</p><p>the reference photocatalyst</p><p>Reference</p><p>P25eG 5.0 wt% Decomposing MB under visible light DP of 65% P25; DP of 28% 2.3 [38]</p><p>TiO2eG e Decomposing MB under sunlight light DP of 75% TiO2; DP of 25% 3.0 [39]</p><p>Sr2Ta2O7xNxeG 2.5 wt% Photocatalytic H2 evolution under</p><p>solar light</p><p>Reaction</p><p>rate: 293 mmol h1</p><p>Sr2Ta2O7xNx;</p><p>RH2</p><p>: 190 mmol h1</p><p>1.5 [40]</p><p>ZnOeG 2.0 wt% Decomposing MB under UV light Reaction</p><p>rate: 0.098 min1</p><p>ZnO; reaction</p><p>rate: 0.022 min1</p><p>4.5 [41]</p><p>ZnSeG e Decomposing MB under</p><p>UV light DP of 99% ZnS; DP of 60% 1.7 [42]</p><p>ZnFe2O4eG 20 wt% Decomposing MB under UV light DP of 88% ZnFe2O4; DP of 15% 5.8 [43]</p><p>SG e Decomposing pendimethalin under</p><p>sunlight light</p><p>DP of 90% SnO2; DP of 10% 9.0 This</p><p>work</p><p>CSG e Decomposing pendimethalin under</p><p>sunlight light</p><p>DP of 99% SnO2/Cu2O; DP of 85% 1.2 This</p><p>work</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381380</p><p>[20] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000)</p><p>496e499.</p><p>[21] Y. Mao, T. Park, F. Zhang, H. Zhou, Small 7 (2007) 1122e1139.</p><p>[22] H. Tong, C.M. Wang, Acta Chim. Sin. 60 (2002) 1923e1928.</p><p>[23] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958), 1339e1339.</p><p>[24] S. Gilje, S. Han, M.S. Wang, K.L. Wang, R.B. Kaner, Nano Lett. 11 (2007) 3394e</p><p>3398.</p><p>[25] Y.X. Xu, H. Bai, G.W. Lu, C. Li, G.Q. Shi, J. Am. Chem. Soc. 130 (2008) 5856e</p><p>5857.</p><p>[26] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int.</p><p>Ed. 48 (2009) 7752e7758.</p><p>[27] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473</p><p>(2009) 51e87.</p><p>[28] H.L. Wang, J.T. Robinson, X.L. Li, H.J. Dai, J. Am. Chem. Soc. 131 (2009) 9910e</p><p>9911.</p><p>[29] C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int.</p><p>Ed. 48 (2009) 7752e7777.</p><p>[30] A.R. Wang, H. Xiao, Mater. Lett. 63 (2009) 1221e1223.</p><p>[31] A. Lerf, H. He, M. Forster, J. Klinowski, J. Phys. Chem. B 102 (1998) 4477e4482.</p><p>[32] S. Uhm, N.H. Tuyen, J. Lee, Electrochem. Commun. 13 (2011) 677e680.</p><p>[33] S.M. Paek, E. Yoo, I. Honma, Nano Lett. 9 (2009) 72e75.</p><p>[34] M.S. Park, G.X. Wang, Y.M. Kang, D. Wexler, S.X. Dou, H.K. Liu, Angew. Chem.</p><p>Int. Ed. 119 (2007) 764e767.</p><p>[35] Y.J. Chen, C.L. Zhu, X.Y. Xue, X.L. Shi, M.S. Cao, Appl. Phys. Lett. 92 (2008)</p><p>223101e223103.</p><p>[36] H.F. Bao, X.Q. Cui, C.M. Li, Y. Gan, J. Zhang, J. Guo, J. Phys. Chem. C 111 (2007)</p><p>12279e12283.</p><p>[37] J.H. Duan, S.G. Yang, H.W. Liu, J.F. Gong, H.B. Huang, X.N. Zhao, R. Zhang,</p><p>Y.W. Du, J. Am. Chem. Soc. 127 (2005) 6180e6181.</p><p>[38] Y.H. Zhang, Z.R. Tang, X.Z. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303e7707.</p><p>[39] K.F. Zhou, Y.H. Zhu, X.L. Yang, X. Jiang, C.Z. Li, New J. Chem. 35 (2011) 353e</p><p>359.</p><p>[40] A. Mukherji, B. Seger, G.Q. Lu, L.Z. Wang, ACS Nano 5 (2011) 3483e3492.</p><p>[41] T.G. Xu, L.W. Zhang, H.Y. Cheng, Y.F. Zhu, Appl. Catal. B 101 (2011) 382e387.</p><p>[42] H. Hu, X. Wang, F. Liu, J. Wang, C. Xu, Synth. Met. 161 (2011) 404e410.</p><p>[43] Y.S. Fu, X. Wang, Ind. Eng. Chem. Res. 50 (2011) 7210e7218.</p><p>[44] P.E. de Jongh, V. Daniel, J.K. John, Chem. Commun. 9 (1999) 1069e1070.</p><p>[45] L.S. Zhang, J.L. Li, Z.G. Chen, Y.W. Tang, Y. Yu, Appl. Catal. A 299 (2005) 292e</p><p>297.</p><p>[46] L. Zhang, D.R. Chen, X.L. Jiao, J. Phys. Chem. B 110 (2006) 2668e2673.</p><p>Z. Wang et al. / Materials Chemistry and Physics 140 (2013) 373e381 381</p>