A novel three-dimensional interconnected graphene–zinc oxidenanowall via one-step co-electrochemical deposition

Lihua Chen, Ruirui Yang, Xiuhong Guo, Qianqian Kong, Tao Yang n, Kui JiaoState Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,Qingdao 266042, China

a r t i c l e i n f o

Article history:Received 19 July 2014Accepted 22 September 2014Available online 2 October 2014

Keywords:ElectrodepositionCyclic voltammetryGrapheneZinc oxideComposite materialsNanowalls

a b s t r a c t

One-step co-electrodeposition was applied to prepare graphene–zinc oxide nanowalls (GZNWs) composite,where graphene oxide was electrochemically reduced and zinc oxide was electrodeposited simultaneously.The morphologies and the electrochemical properties of GZNWs were obviously influenced by theelectrodeposition time. The contrast experiments illuminate that GZNWs presented superior electrochemicalactivity.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

As known, the nanostructure of zinc oxide (ZnO) could exist asnanobelts, nanosheets, nanowalls (NWs) and nanorods throughchemical [1] and electrochemical preparations [2,3]. For instance,Bai et al. [4] realized the transition of ZnO nanorods to ZnOnanosheets by electrodeposition. Among them, aligned ZnO NWswith special growth direction and large surfaces have attracted greatattention in solar cells, liquid crystal displays, optoelectronic devices,photocatalysts for water purification, sensor, and so on [5–8].

Electrochemical route [9–11] for reducing graphene oxide(GNO) has emerged as a mild, green, and fast technique, whichdoes not result in contamination since no reducing agent was used[12,13]. In addition, in graphene-based nanocomposites, hybrids ofmetal oxide and graphene are widely investigated [14]. It shouldbe noted that one-step co-electrodeposition could electrochemi-cally reduce GNO to graphene, accompanied by the simultaneousdeposition of metal oxide. One example is that Dong’s groupprepared graphene/MnO2 nanowall hybrids (GMHs) via one-stepelectrochemical approach, which owns potential application forsupercapacitor [15].

In this paper, graphene–zinc oxide nanowall (GZNWs) compo-site was fabricated by one-step co-electrodeposition. The morphol-ogies and the electrochemical properties of GZNWs were obviously

influenced by the electrodeposition time. The contrast experimentsilluminate that GZNWs presented superior electrochemical activity.

2. Experimental

Apparatus and reagents: Electrochemical measurements wereconducted on a CHI 660C workstation (Shanghai CH InstrumentCompany, China) with a conventional three-electrode system: amirror-polished 3 mm glassy carbon electrode (GCE) or modifiedGCE, a platinum wire auxiliary electrode, and a saturated calomelreference electrode (SCE). The morphology of the samples wascharacterized via a JSM-6700F scanning electron microscope (JEOL,Tokyo, Japan).

Zinc nitrate hexahydrate (Zn(NO3)2 �6H2O) and Graphite pow-der (325 mesh, spectral pure) were purchased from Tianjin BASFChemical Co., Ltd. and Sinopharm Chemical Reagent Co., China,respectively. Sodium sulfate anhydrous (Na2SO4) and otherreagents were obtained from Shanghai Chemical Reagent Co. Ltd.And all aqueous solutions were prepared with ultrapure water(Aquaplus AWL-1002-P, Ever Young Enterprises Development Co.,Ltd., China). Other reagents please see Supplementary material.

Preparation of the graphene oxide (GNO): The graphite oxide(GO) was synthesized by Hummers’ method according to theliterature [16]. (The detailed process please sees Supplementarymaterial) Then a certain mass of resulted GO was dispersed inultrapure water and exfoliated by ultrasonication for 30 min tofabricate a homogeneous solution of GNO [10,17].

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.09.1070167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ86 532 84022665; fax: þ86 532 84023927.E-mail address: taoyang@qust.edu.cn (T. Yang).

Materials Letters 138 (2015) 124–127

Preparation of the GZNWs modified electrode: 10 μL of GNOhomogeneous solution was pipetted onto the polished GCE anddried naturally (GNO/GCE). Then GZNWs/GCE was obtained byelectrodeposition for a given time (10, 20, 30 min). The detailsplease see Supplementary material.

3. Results and discussion

The morphology characterization and possible formation mechanismof GZNWs: Just as shown in Fig. 1, after the GNO/GCE was depositedin 0.1 mol/L Zn(NO3)2 for 20 min, a large scale (Fig. 1A) and alignednanowall (Fig. 1B, inclination angle: 301) of cross-link GZNWs wouldbe observed. The surface morphology of GZNWs was quite different

from that of GNO (Fig. 1C), which presents rather smooth. If ZnO isdirectly processed on the bare GCE without GNO existence under thesame conditions, the SEM micrograph of the obtained ZnO/GCE isshown in Fig. 1D. From it, there were only some inhomogeneouslydispersed microrodes with unequal diameter formed on the surfaceof the GCE. It revealed that without GNO, even performed under thesame experiments, ZnO nanowall would not formed [15]. Therefore,based on the comparison of SEM micrograph of the single compo-nent ZnO/GCE, we speculate that the existence of GNO provided anideal platform for the formation of GZNWs [15]. Presumably, theoxygen-containing functional groups of GNO play a critical role asactive sites in the preparation of ZnO NWs arrays due to the strongelectrostatic interactions between the oxygen-containing functionalgroups and metal ions [18]. Instead of direct oxidation of Zn2þ to

Fig. 1. SEM micrographs of the GZNWs-20 min ((A) top view), GZNWs-20 min ((B) tilted with an inclination angle of 301), (C) GNO, (D) ZnO without GNO support, and(E) cyclic voltammograms of 2 mmol/L [Fe(CN)6]3�/4� (1:1) at bare GCE (a), GNO/GCE (b), ZnO/GCE (c) and GZNWs/GCE (d).

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form ZnO, the ZnO is probably formatted as a mode of Zn(OH)2 firstlyand then decomposed into ZnO and H2O. In detail, under a negativepotential (�1.0 V), the zinc nitrate salt dissolves and turns into Zn2þ

and NO3� ions in water as Eq. (1). The reaction of NO3

� ions, H2O, and2e� resulted in the formation of OH� in Eq. (2). Then, hydroxylationreaction between OH� ions and Zn2þ ions would rapidly occur onthe surface of the electrode with the existence of the oxygen-containing functional groups of GNO, [15] to form Zn(OH)2 (Eq.(3)). Subsequently, Zn(OH)2 would readily decomposed into ZnO ifthe solution temperature is higher than 34 1C (Eq. (4)). [19] Therefore,the GNO provides an excellent platform for the fabrication of thehomogeneously dispersed ZnO nanowall arrays due to the abundantoxygen-containing functional groups, while the relatively lowadsorption of Zn2þ ions on bare GCE is responsible for the rodsmorphology observed in Fig. 1D.

Zn(NO3)2-Zn2þþ2NO3� (1)

NO3�þH2Oþ2e�-NO2

�þ2OH� (2)

Zn2þþ2OH�-Zn(OH)2 (3)

Zn(OH)2-ZnOþH2O (4)

To further investigate the electrochemical characteristics ofGZNWs, the cyclic voltammograms of [Fe(CN)6]3�/4� at bare GCE(a), GNO/GCE (b), ZnO/GCE (c) and GZNWs/GCE (d) are shown inFig. 1E. Among the curves, curve b and curve c present the weakelectron transfer capability compared with bare GCE electrode.However, the curve d (GZNWs) shows the best elecrtochemicalsignals as expected [20].

The influence of electrodeposition time on GZNWs: Fig. 2A and Cshow the SEM images of the GZNWs obtained at varied depositiontime from 10 min to 30 min to illuminate the evolution process ofGZNWs. At 10 min (panel A), no GZNWs was observed in first

process, where some hill-like protrusions existed. However, withthe prolonging of deposition time, the GZNWs with nanowallmorphologies [21,22] was clearly observed in Fig. 1B (20 min) andFig. 2B and C (30 min). As shown in Fig. 1B, the thickness (about140 nm) of ZnO NWs obtained at 20 min was much thinner thanthat of ZnO NWs at 30 min (Fig. 2B, about 500–600 nm).

The obtained GZNWs via varying the deposition time were alsoverified in [Fe(CN)6]3�/4� (1:1) by CV methods and the results areshown in Fig. 2D. As can be seen, the redox signals increased withthe prolongation of the deposition time and the best redox signalswere observed at the GZNWs obtained from 30 min. The activesurface areas of GZNWs-10, 20, 30 min can be calculated to be(0.067370.0005), (0.421370.0032), (0.971070.0072) cm2 (aver-age of three measurements), respectively. The detail electroche-mical signals and the electroactive areas please see Supplementarymaterial.

The electrochemical activity of GZNWs in Na2SO4 solution: Further-more, the electrochemical properties of the GZNWs compositionprepared at different times (10, 20 and 30 min) were also investi-gated in 1 mol/L Na2SO4 solution [23,24] by CV method and theresults are shown in Fig. 3. For comparison, the electrochemicalbehaviors of ZnO/GCE (curve d) and GNO/GCE (curve e) obtained inthe same conditions were also revealed in Fig. 3 (inset). Significantdifference of electrochemical activity can be easily observed, largercurrent density responses at GZNWs (a, b and c) than those at theZnO/GCE (d) and GNO/GCE (e). The more rectangular of CV curveand the larger current density response should be ascribed to thesynergistic effect of GN and ZnO. In addition, it could be seen thatthe GZNWs obtained at 20 min (curve b) presents more rectangularthan the other GZNWs (curve a, c).

Moreover, this composite could be preliminarily applied for theimmobilization of DNA and used as DNA sensing platform. Theresult please sees Fig. S1.

Fig. 2. SEM micrographs of the GZNWs obtained at different times: GZNWs-10 min (A), GZNWs-30 min (B), GZNWs-30 min, tilted with an inclination angle of 301 (C), and(D) cyclic voltammograms of 2 mmol/L [Fe(CN)6]3�/4� (1:1) at GZNWs-10 min, GZNWs-20 min and GZNWs-30 min.

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4. Conclusion

In summary, this paper illustrated a simple one-step co-electrodeposition to synchronously realize the deposition of ZnOand the reduction of GNO. The formation of GZNWs might beascribed to the oxygen-containing functional groups of GNO,which play a critical role serving as active sites in the preparationof ZnO nanowall arrays. Due to the enlarged active surface area,more subtle structure, and superior electron transfer capability,the GZNWs with special nanowall morphology shows potentialapplications in sensors and supercapacitors.

Acknowledgments

This work was supported by the 863 program (no. 2013AA032204),National Natural Science Foundation of China (no. 21275084,41476083), Doctoral Foundation of the Ministry of Education ofChina (no. 20113719130001), Scientific and Technical DevelopmentProject of Qingdao (no. 12-1-4-3-(23)-jch), and Outstanding Adult-Young Scientific Research Encouraging Foundation of ShandongProvince (no. BS2012CL013).

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2014.09.107.

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Fig. 3. Cyclic voltammograms of Na2SO4 (1 mol/L) at GZNWs-10 min (a), GZNWs-20 min (b), GZNWs-30 min (c), ZnO/GCE (d) and GNO/GCE (e).

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