RSC Advances

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View Article OnlineView Journal | View Issue

N-doped nanopo

aState Key Laboratory of Heavy Oil Processin

102249, China. E-mail: yi@cup.edu.cn; lisbDepartment of Materials Science and Eng

Beijing 102249, ChinacState Key Laboratory of Environmental Chem

for Eco-Environmental Sciences, Chinese AcadState Key Laboratory of Silicon Materials

Engineering, Zhejiang University, Hang

89739028; Tel: +86-010-89739028

† Electronic supplementary informationsample; the XRD patterns of Cu foam athe surface morphologies of the Cu foamthe Raman spectrum of the graphene10.1039/c4ra06872f

Cite this: RSC Adv., 2014, 4, 47455

Received 9th July 2014Accepted 19th September 2014

DOI: 10.1039/c4ra06872f

www.rsc.org/advances

This journal is © The Royal Society of C

rous graphene decorated three-dimensional CuO nanowire network and itsapplication to photocatalytic degradation of dyes†

Liqiang Zhang,ab Zhenfei Gao,ab Chao Liu,a Liang Ren,a Zhiqiang Tu,a Rui Liu,c

Fan Yang,a Yunhan Zhang,a Zhizhen Ye,d Yongfeng Li*a and Lishan Cui*ab

In this study, a three-dimensional CuO nanowire (NW) network decorated with N-doped graphene (NG/

CuO) was fabricated by a method of thermal oxidation combined with hydrothermal and ion

implantation. The morphological investigation of products was analyzed by field emission scanning

electron microscopy (FESEM), which confirmed that the synthesized CuO is wire-shaped and obtained in

a high density and large quantity. Meanwhile, it is found that the graphene was successfully wrapped on

the surface of the CuO NWs. Subsequently, N ions were injected into graphene by using a plasma

method. The detailed structural, compositional and optical characterizations of the synthesized NG/CuO

are characterized by X-ray diffraction (XRD) patterns, transmission electron microscopy (TEM), Raman

spectroscopy, and X-ray photoelectron spectroscopy (XPS) which have collectively confirmed that the

obtained sample is highly crystalline CuO NWs decorated with N doped graphene. It is found that NG/

CuO demonstrates better photocatalytic activity than the pristine graphene decorated CuO NW (Gra/

CuO) and much higher activity than that of the pure CuO NW, which shows a good reproducibility and

could be further enhanced by adding H2O2. The formation of a heterojunction between the N-doped

graphene and CuO can efficiently avoid the combination of photogenerated carriers, which contributes

to the enhancement of its photocatalytic activities.

1. Introduction

Copper oxide (CuO) is a p-type semiconductor with numerousunique properties such as direct band gap (1.2–1.7 eV), non-toxicity, chemical stability, abundant availability and lowproduction cost.1,2 The excellent properties of CuO make itpromising for potential applications in many elds, includinggas sensors, heterogeneous catalysts, eld emission emittersand lithium ion electrodes.3,4 In recent years, the application ofCuO in photocatalytic degradation of environmental organicpollutants has attracted extensive attention because of its low-

g, China University of Petroleum, Beijing

hancui63@126.com

ineering, China University of Petroleum,

istry and Ecotoxicology, Research Center

demy of Sciences, Beijing 100085, China

, Department of Materials Science and

zhou 310027, China. Fax: +86-010-

(ESI) available: The photograph of theer heating at a range of temperature;aer heating at different temperature;and N doped graphene. See DOI:

hemistry 2014

cost and nontoxic end products.5,6 Up to now, CuO photo-catalyst with different morphologies such as nanorods, nano-plates, nanotubes, hollow microsphere, microower, nanowireshave been successfully synthesized and widely investigated.2,7–12

However, the photocatalytic activity of CuO nanomaterials wasstill not satised, as the light-generated charge carriers in CuOcannot be efficiently split for the decomposing of dye mole-cules. In order to solve this problem, researchers have tried tocombine CuO with Pb,13 Au,14 ZnO,15 SnO2,16 and etc. so as toavoid the combination of photogenerated carriers and enhanceits photocatalytic activities.

Graphene, a closely stacked two-dimensional honeycomb ofsp2-peaked carbon atoms, is an extremely interesting materialsince its discovery by Geim et al. in 2004.17 It has been found thatgraphene exhibits many extraordinary properties, such as anultrahigh electrical conductivity, ultralow resistivity and etc.,18

which has been widely used in the eld-effect transistors,sensors, and solar cells.19,20 Among its plenty of applications, theapplication of graphene in enhancing the photocatalytic activi-ties has attracted much attention recently.21–23 Graphene deco-rated CuO nanocomposite has been proved to be an effectivephotocatalyst for the degradation of organic pollutants.24 Liuet al. have synthesized CuO nanocrystals on the reduced gra-phene oxide in order to enhance its photocatalytic activities.25

Yusoff et al. also found that the CuO/graphene nanocomposite

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exhibited good photocatalytic activity towards the degradation ofmethylene blue.26 However, only the nanoparticles type photo-catalysts of CuO composited with graphene are fabricated inprevious studies.25,26 As we known, the nanoparticles type pho-tocatalysts are easy to agglomerate, which leads to greatdecreasing of the exposed active surface. Moreover, the nano-particles are difficult to recycle aer degradation process. Fromthe view point of practical application, catalyst with an integratedthree-dimensional (3D) structure is believed to possess betterdispersibility than the nanoparticles type catalyst, which is alsomuch easier to recycle in longer application. To the best of ourknowledge, there are few reports devoted to the synthesis of CuONW with a 3D structure, combined by decorating of graphene onits surface. In addition, we further functionalized the grapheneby doping with heteroatoms of N, which is considered to be ableto effectively inuence the electronic characteristic, surface andespecially chemical characters of the graphene.

In this study, a photocatalyst of NG/CuO with a 3D structurehas been prepared via a thermal oxidation combined withhydrothermal and ion implantation. The structural, morpho-logical properties, chemical compositions of NG/CuO and itsphotocatalytic activities were studied in detail. Moreover, thepossible mechanism of improving the photocatalytic perfor-mance of CuO for the dye degradation aer decorating withnitrogen-doped graphene (NG) is also discussed.

2. Experimental details

In a typical procedure, a 3D NG/CuO network photocatalyst canbe prepared in three steps. Firstly, the copper foam substratewas cleaned in an aqueous 1.0 M HCL solution for �20 s, fol-lowed by repeated rinsing with distilled water and dried under anitrogen ow. The substrate was placed in an aluminium boatand transferred into the center of a quartz tube. And then, it wasimmediately heated to the preset-point temperature and lastedfor 4 h in air, aer which a 3D CuO network photocatalyst wassynthesized, as shown in Fig. S1.† Secondly, graphene layer waswrapped on the surface of CuO NW by a facile route of hydro-thermal. Aer 30 min stirring, the as prepared graphene oxideand CuO NW was transferred to a Teon-lined stainless steelautoclave, which was subsequently sealed and maintained at180 �C for 12 h, and then it was cooled down to the roomtemperature naturally. The solvent used in the hydrothermalprocess is the deionized (DI) water. The graphene oxide wasprepared from natural graphite powder through a modiedHummer's method.27 Aer the hydrothermal reaction, the gra-phene oxide would be reduced to graphene and cover thesurface of CuO NW, which could make a strong connectionbetween the graphene and CuO NW. In order to characterize thelayer number of the graphene, we made a further analysis byusing the AFM (Fig. S2†), which demonstrates that the graphenenanosheets have layers less than four. Thirdly, N ions wereinjected into the Gra/CuO by using the plasma. The glowdischarge plasma was generated between the top at stainlesssteel and bottom electrode by using a DC power source.Nitrogen was introduced and used as the plasma-forming gas,the chamber was a stainless steel with inner diameter of 70 mm

47456 | RSC Adv., 2014, 4, 47455–47460

and four glass windows, and the gap between electrode andsample is 4 mm. A direct current (DC) power source negativebias prepared by VDC ¼ 350–400 V was applied to a stainlesssteel electrode in gas phase for the generation of a nitrogenplasma at pressure of 500 Pa. Aer the nitrogen plasma treat-ment lasted for 90 minutes, the NG/CuO sample was nallyobtained.

The structure of sample was characterized by using Bede D1X-ray diffraction (XRD) system with a Cu Ka radiation (l ¼0.15406 nm). Surface morphology of samples was studied byusing the FEI Quanta 200F scanning electron microscope (SEM)operated at a voltage of 20 kV. The graphene is identiedby using the FEI Tecnai G2 F20 transmission electron micros-copy (TEM), which is operated at 200 kV. The TEM analysesincluded the common TEM images at a low magnication,high-resolution transmission electron microscopy (HRTEM)images, selected area electron diffraction (SAED) patterns andthe scanning transmission electron microscopy (STEM). Themicroscopic Raman spectroscopy is recorded by using a micro-Raman spectrometer (InVia Reex, Renishaw, UK) with excita-tion wavelength of 532 nm, and the laser power was kept below0.85 mW to avoid laser induced local heating on the sample. Allpeaks in the Raman spectra were tted with Lorentzians. A100� objective lens with a numerical aperture (NA) of 0.95 wasused in the Raman experiments, and the spot size was esti-mated to be about 500 nm. The X-ray photoelectron spectros-copy (XPS, Thermo Fisher K-Alpha American with an Al Ka X-raysource) was used to measure the elemental composition ofsamples as well. Photoluminescence (PL) measurements wereperformed on a FLS920 uorescence spectrometer (EdinburghInstruments) at room temperature using xenon lamp as theexcitation light source. To study the photocatalytic activity ofNG/CuO, the dye solution was prepared by dissolving themethylorange powder in deionized water with a concentration of 10mgml�1. Prior to the degradation, an NG/CuO sample (10 mm� 10mm � 1 mm) was submerged into 100 ml of methyl orangesolution and kept in darkness for 60min. Then, the dye solutionwith NG/CuO was illuminated under a 500W xenon lamp whichcan provide an excellent simulation of sunlight. In order toimprove the photocatalytic activity of the catalyst, 1 ml H2O2 asa green additive was added into the 100 ml aqueous dye solu-tion. In the experiment, the photocatalytic reaction was carriedout at room temperature, and the UV test was taken in 5 min or30 min interval.

3. Results and discussion

Fig. 1 describes the schematic diagram of degrading the organiccontaminate by using the NG/CuO. In the experiment, high-density CuO NWs were rst grown by heating copper foamsubstrate in air for 12 h, and then a thin layer of graphene wasdecorated on the surface of CuO NW via a hydrothermalmethod. Subsequently, N ion was injected into the graphene byusing the glow plasma. In order to evaluate the photocatalyticactivities of NG/CuO, the as prepared sample was immersedinto the methyl orange solution to degrade the dye moleculesunder the simulated sunlight.

This journal is © The Royal Society of Chemistry 2014

Fig. 1 The schematic diagram of degrading the organic contaminateby using the nitrogen-doped nanoporous graphene decorated CuOnanowire (NG/CuO NW).

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Fig. 2 shows the XRD patterns of the pristine Cu foam, Gra/CuO, and NG/CuO respectively. The 2q value in the XRDpatterns covers ranging from 20� to 80�. The characterizedpeaks locate at 2q value of 43.4�, 50.5� and 73.1� correspond tothe metal Cu (JCPDS PDF#85-1326). Aer heating at 450 �C for12 h, some new peaks located at 36.5�, 42.2�, 61.4�, and 73.4�

refer to Cu2O (JCPDS PDF#73-2076), and peaks located at 35.7�,38.8� refer to CuO (JCPDS PDF#80-1917) were formed. Ourresults are in accordance with the synthesized CuO NW bydirectly heating the copper grid substrates in air reported byJiang et al.2 When Cu foams are oxidized in air, the product is a

Fig. 2 The XRD patterns of samples including the pristine Cu foam, theCu foam heated at 500 �C for 12 h, and NG/CuO fabricated at 500 �Cfor 12 h.

This journal is © The Royal Society of Chemistry 2014

mixture of Cu2O, CuO and the le metal Cu skeleton. Thegrowth mechanism of Cu NW involves two steps: the rst one isforming of a Cu2O layer on the surface of copper, and then CuONWs were subsequently grown on the Cu2O layer. Since the Cufoam was directly heated in air, only the temperature mayinuence the oxidation product. The synthesis of CuO NWcould be realized at only a suitable range of temperature. TheXRD patterns and surface morphologies of the Cu foam heatedat a serial of temperatures were compared in Fig. S3 and S4†respectively. For the Cu foam heated at 300 �C for 12 h, only thepeaks related to Cu2O phase are formed on the surface of Cufoam (Fig. S4(a)†) and no CuO NW is found (Fig. S4(b)†). Whenthe temperature is higher than 700 �C, no CuO NW are foundand the growth of CuO NW will be terminated, which is due tothe free energy of CuO changes from negative to positive state,as seen in Fig. S4(d).† It demonstrates that 500 �C is an opti-mized temperature for the CuO NW growth (Fig. S4(c)†). TheXRD pattern of CuO fabricated at 500 �C demonstrates noobvious changes aer graphene wrapping and N ion injection.

Fig. 3 shows the morphology, crystal structure and elementdistribution of NG/CuO. Fig. 3(a) and (b) show the SEM imagesof Cu foam heated at 500 �C in air for 12 h at a low and highmagnication, respectively. It is found that all of the CuO NWsare grown uniformly perpendicular to the surface of Cu foamsubstrate. The microscope images of the Cu foam before andaer thermal oxidation process are shown in the Fig. S4.† TheseNWs have a length ranging from 10–50 mm and a diameter of30–200 nm. Aer the whole surface of the copper foam becomescovered with high-density CuO NWs, a thin layer of graphenewas coated on it via a hydrothermal method, as shown inFig. 3(c). The TEM image of CuO NW is shown in Fig. 3(d), andits HRTEM image is demonstrated in the upper inset. By usingthe double tilt holder, two separated electron diffraction

Fig. 3 The morphology, crystal structure and element distribution ofNG/CuO. (a and b) The SEM images of Cu foam after it was heated inair for 12 h at a low and high magnification, respectively; (c) a thin layerof nanoporous graphene was coated on the whole surface of the CuONWs; (d) the TEM image of CuO, and its HRTEM image are demon-strated in the upper inset; (e) the element distribution of NG/CuO. (f)The 532 nm Raman spectrum of the NG/CuO.

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patterns of the CuO NW was selected from two different zoneaxes, through which it can be calculated that the growthdirection of CuO NW is [111]. According to the STEM image ofNG/CuO (Fig. 3(e)), it is found that the graphene is decorated onthe surface of CuO NW and N element is uniformly doped in thegraphene. Fig. 3(f) shows the Raman spectrum of NG/CuO,where three peaks located at 298, 341 and 630 cm�1 arefound, being attributed to the CuO phase, which are quitedifferent from the Cu2O.28 The D and G peaks of graphene arealso found in the Raman spectrum. Moreover, the Ramanspectrum can also be applied to characterize whether the N ionshave been successfully injected into the graphene. Aer treatedby N ion plasma for 1.5 h, the intensity ratio of the D and G peakof graphene increases to 1.75, which is higher than the 1.42 ofthe pristine graphene (Fig. S5†).

The chemical composition and electronic structure of theNG/CuO hierarchical nanomaterials were investigated by XPS.The detailed high-resolution XPS spectra of the Cu 2p, O 1s, C1s, and N 1s are shown in Fig. 4(a–d), respectively. The char-acteristic peaks were located at 934.5 eV (Cu 2p3/2) and 954.9 eV(Cu 2p1/2), and their corresponding satellite peaks of the Cu 2pXPS spectra are both found in Fig. 4(a). The binding energy gapbetween the Cu 2p3/2 and Cu 2p1/2 is 20 eV, which is in accor-dance with CuO.7,15 The broad O 1s peaks in Fig. 4(b) wascomprised of two small peaks, one is located at 530.5 eV and theother is located at 531.5 eV. The former one refers to theinherent O atoms bound to Cu in the structure of CuO, while thelatter one results from the possible surface contamination byhydroxyl species and carbonate species.29,30 Fig. 4(c) shows the C1s spectra of the NG and it could be split into two peaks at 284.5and 285.4 eV, which are associated with C–C and C–N, respec-tively. The hydrothermal treatment cannot eliminate all theoxygen containing groups in graphene oxide, and there presentssome C–O bond (286.5 eV) in the nal product.31 Fig. 4(d) showsthe high-resolution XPS for N 1s, which can be split into threepeaks. One peak observed at 398.2 eV is related to the pyridinicN which bonds with two C atoms at the edges or defects of

Fig. 4 The detailed high-resolution XPS spectra of the Cu 2p, O 1s, C1s, and N 1s.

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graphene. The second peak at 399.4 eV can be attributed to theN which refers to N atoms that contribute two p electrons to thep system. The third peak of N is located at 401.5 eV, which refersto the N atom inserted into the graphitic carbon layer andmakes bonding with three carbon atoms.32

For comparison, an evaluation of the photocatalytic activityof pristine CuO NW, Gra/CuO and NG/CuO samples toward thedegradation of dye molecules is carried out at room tempera-ture in this study. The changes in absorbance spectra methylorange dye with NG/CuO catalyst in aqueous solutions at thesame time interval of 1 h under visible light illumination isshown in Fig. 5(a), in which the degradation rate of methylorange grows rapidly as time increases. The adsorption andphotolysis rate of the NG/CuO were 12% and 88% respectively.In general, organic dye solutions irradiated by UV light canproduce hydroxyl radicals, which leads to the structural changesin dye.33 As a result, the optical absorption peaks of the dyessolutions demonstrate a little red shi. The methyl orangedegradation rate of CuO NW, Gra/CuO, NG/CuO were comparedin Fig. 5(b). Aer irradiation for 7.5 h with stimulated sunlight,about 97% of methyl orange is degraded by using the NG/CuOphotocatalyst, which is much higher than that of pristine CuONW (33%) and Gra/CuO (62%). When using a lter (400 nm),the CuO NW, Gra/CuO and NG/CuO can still demonstrate asimilar photocatalytic activity toward the degradation of dyemolecules, which further conrmed the photocatalytic activityof the NG/CuO under VIS light (Fig. S6†).

In order to improve the photocatalytic activity of the NG/CuO, H2O2 was added into the aqueous dye solution, which

Fig. 5 (a) The UV-vis absorption spectra of spectra methyl orange dyeirradiated by visible light in the presence of NG/CuO catalysts; (b) themethyl orange degradation rates of CuO, Gra/CuO, NG/CuO.

This journal is © The Royal Society of Chemistry 2014

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has been employed as a green additive to improve the photo-catalytic activities in many other semiconductors such as TiO2,ZnO and etc.34 Fig. 6(a) illustrates the methyl orange concen-tration dependence on irradiation time using the pure CuO,Gra/CuO, and NG/CuO as the catalyst in the present of H2O2.Meanwhile, the photocatalytic performance of the H2O2 withoutany photocatalyst was also compared. It is found that thedegradation rates of the methyl orange all demonstrate anincrease as time goes on. Referring to the catalyst of NG/CuO, anearly fully degradation of methyl orange takes almost about7.5 h, while it is shorten to about 35 min aer the addition ofH2O2. Under simulated sunlight, H2O2 would be easily split andproduce plenty of cOH radicals, which are good electronacceptors. In the process of the photocatalytic reaction,amounts of electrons and holes are generated on the surface ofCuO. These electrons can be quickly scavenged by the cOHradicals produced by the H2O2. Consequently, the electron andhole recombination rate is greatly reduced, leading to a greatimprovement of its photocatalytic performance. The stability ofa photocatalyst is signicant for its assessment and application.In order to evaluate the stability of the composite catalysts, wecarried out recycling photocatalytic dye degradation evolutiontests on NG/CuO samples under the same condition. Fig. 6(b)displays the dye degradation evolution curve in every photo-catalytic run. Aer each photocatalytic reaction, we pick up theNG/CuO sample and drop it in the new methyl orange solutionadded with H2O2. No obvious decrease of photocatalytic activi-ties were observed aer ve cycles, which conrms the stabili-ties of the NG/CuO.

Fig. 6 (a) Themethyl orange degradation rates of CuO, Gra/CuO, NG/CuO in the existence of H2O2. (b) Cycling run for the photocatalyticdye degradation by using NG/CuO in the presence of H2O2 undervisible light irradiation.

This journal is © The Royal Society of Chemistry 2014

The photocatalytic behavior is a process of photogeneratedelectron and hole carriers migrate to the surface and react withthe absorbed reactants. In the most cases, these photo-generated electrons and holes could be quickly recombined.Therefore, the performance of a catalyst is greatly inuenced bythe carrier separation efficiency. In our work, the NG plays asignicant role in separating the photogenerated electron–holecarriers. Under light illumination, the electrons and holesgenerated in CuO could be quickly transferred to its surface.Namely, the graphene was an excellent electron conductor,which can quickly trap the split electron carriers to avoid themcombining. Aer the decorating of NG, it is found that thephotogenerated light intensity of NG/CuO increased obviously,which provides a direct evidence for the lowered carrierrecombination aer decorating of NG on the surface of CuO. Inaddition, N doping provides more defect traps to capture thedye molecules, which makes the process of degradation mucheasier.

Due to the different work function of graphene and CuO,they could form a Schottky junction when being contacted,which would set up an internal eld and increase the carriersseparation efficiency. In order to clarify this issue, the energyband diagram of the graphene and CuO upon illumination isillustrated in Fig. 7. The work function of graphene FGraphene

(�5.0 eV) is lower than that of FCuO (�5.3 eV), and the valenceband (Ev) edge and the conduction band (Ec) edge of p-type CuOdemonstrates a down shi. At the thermodynamic equilibriumof a Schottky junction, there is an internal eld with directionfrom graphene to p-type CuO. According to the band edgeposition, the new excited electrons on the Ec would quickly betransferred to the surface of graphene. As a result, the photo-generated electron and hole carriers would be effectivelyseparated.

Based on the above discussion, it is found that such a novelNG/CuO hybrid combines several merits. Firstly, the CuO NWformed an integrated 3D structure, which cannot agglomerateand can be easily recycled during application. Secondly, the NGfabricated via hydrothermal together with the ion implantationprovides a lot of traps to capture the dye molecules forimproving the photocatalytic performance. Thirdly, a Schottky

Fig. 7 The energy band diagram of the graphene and CuO uponillumination.

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junction was generated between the NG and CuO NW, whichcontributes to avoiding the combination of photogeneratedcarriers.

4. Conclusion

In this work, a 3D NG/CuO NW network was successfullysynthesized, which has been conrmed by XRD, SEM, TEM,Raman and XPS characterizations. Our ndings conrmed thatNG/CuO demonstrates a superior photocatalytic activity for dyemolecules degradation and can be easily recycled aer thedegradation process. The coated N doped graphene layerformed a Schottky junction with the p-type CuO NW, whichefficiently avoids the recombination of the photogeneratedelectron–hole carriers and greatly enhances its photocatalyticactivities.

Acknowledgements

This work was nancially supported by the Beijing NaturalScience Foundation of China (no. 2144054), National NaturalScience Foundation of China (no. 21106184, 21322609,21207144, 51401239, 11474362), Key Program Project ofNational Natural Science Foundation of China (51231008),Science Foundation of China University of Petroleum, Beijing(no. YJRC-2013-40, YJRC-2011-18), the Key Project of ChineseMinistry of Education (313055), the National 973 program ofChina (2012CB619403), and Thousand Talents Program ofChina.

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