Synthesis_of_zinc_oxide_nanotetrapods_by_a_novel_fast_microemulsion-based_hydrothermal_method

Synthesis of zinc oxide nanotetrapods by a novel fast microemulsion-based hydrothermal method

Junying Jiang,Yanfen Li,Shengwei Tan,Zaiyin Huang ?

College of Chemistry and Ecological Engineering,Guangxi University for Nationalities,Nanning 530006,PR China

a b s t r a c t

a r t i c l e i n f o Article history:

Received 25April 2010Accepted 8July 2010

Available online 14July 2010

Keywords:

ZnO nanotetrapods

Microemulsion-mediated hydrothermal Electrochemical analysis

ZnO nanotetrapods have been successfully synthesized via a novel microemulsion-mediated hydrothermal route at 120°C for 12h.X-ray power diffraction (XRD),?eld-emission scanning electron microscopy (FESEM),transmission electron microscopy (TEM),selected-area electron diffraction analysis (SAED),and electrochemical analysis (EA)were employed to study the structural features and electrochemical behavior of the products.It was found that these tetrapod ZnO nanostructures had a single crystal hexagonal wurtzite structure with lattice constants of a =0.3249nm and c =0.5205nm.And they exhibited a clearly electrocatalytic response,showing potential applications for sensor constructions.

?2010Elsevier B.V.All rights reserved.

1.Introduction

ZnO is an attractive semiconductor material with a wide direct band gap (3.37eV at room temperature),large exciton binding energy (60meV),and high refractive index (N 2).It has attracted intensive research for its unique properties and versatile applications in transparent electronics,ultraviolet light emitters,piezoelectric devices,chemical sensors,spin electrics and so on [1,2].As we all know,the ability to control particle morphology is an important object in the synthesis of nanocrystals,as size and shape can signi ?cantly in ?uence various properties [3–5].Therefore,the target-oriented preparation of ZnO has become a big issue.Many techniques involving chemical vapor deposition,aqueous solution precipitation,microwave irradiation,thermal evaporation technique,hydrothermal synthesis,microemulsion method,gel –sol process,and electrodeposition have been used to prepare ZnO nanostructures.And lots of morphologies,such as nanobelts,nanowires,nanoneedles,nanotubes,nanosheets,and nanotetrapods have been obtained [6–18].Among them,nanote-tropod is a common morphology for ZnO and it has been extensively studied.

Several methods have been developed to synthesize ZnO nanote-tropods,such as thermal evaporation methods [19–21],chemical vapor deposition [22–24],aqueous solution route and rapid thermal annealing technique [25],and so on.However,the most reported synthesis techniques are complicated,time and energy consuming.In particular,when organo-metallic precursors are used,complex procedures,high temperatures and sophisticated equipment to control the growth process are involved.For the further use of ZnO

nanotetropods,a simple and inexpensive process with gentle conditions is required.Here,we present a novel fast microemulsion-based hydrothermal method to prepare ZnO nanotetrapods,which overcomes the shortcomings of previous preparation methods.Also,their electrochemical behavior was investigated by EA.

2.Experimental

Zn(Ac)2·2H2O,NaOH,n-octanol,TritonX-100and cyclohexane were from Xilong Chemical Reagent Factory,which were of analytical grade and used without further puri ?cation.Poly (vinyl alcohol)(PVA MW 89,000–98,000)was bought from Sigma-Aldrich.Hemoglobin (Hb)from bovine blood was obtained from Fluka.

In a typical procedure,?rstly,one reverse microemulsion solution was prepared by adding 0.66mL 0.1M Zn(Ac)2aqueous solution into TritonX-100/cyclohexane/n-octanol system (according to the mass ratio TritonX-100/cyclohexane/n-octanol =3/8/2),which was stirred for 10min at room temperature until the microemulsion became transparent.Secondly,0.66mL 0.6M NaOH aqueous solution was added dropwise into the as-prepared reverse microemulsion solution and was kept stirred for another 20min.Then the mixed solutions were transferred to a 25mL Te ?on-lined stainless-steel autoclave and kept at 120°C for 12h.After being cooled to room temperature naturally,white products were harvested by centrifugation and washed several times with acetone,deionized water and absolute ethanol,and then dried in vacuum at room temperature.The as-prepared products were characterized by X-ray diffraction (XRD,Philips PW1710with Cu K αradiation,λ=1.5406?),?eld-emission scanning electron microscopy (FESEM,JEOL JSM-6700F),transmission electron microscopy (TEM,JEOL JEM-2010,200KV)and

Materials Letters 64(2010)2191–2193

?Corresponding author.Tel.:+867713262120;fax:+867713261718.E-mail address:hzy210@https://www.sodocs.net/doc/a217929361.html, (Z.

Huang).0167-577X/$–see front matter ?2010Elsevier B.V.All rights reserved.doi:

10.1016/j.matlet.2010.07.026

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Materials Letters

j o u r na l ho m e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /m a t l e t

electrochemical analysis(CHI660A and CHI440A electrochemical workstation,Shanghai Chenhua Instrument Co.,China).

3.Results and discussion

Powder XRD pattern of the as-prepared products is shown in Fig.1a.All of the diffraction peaks can be indexed to hexagonal wurtzite structural ZnO,and match well with standard hexagonal ZnO (a=3.249?,c=5.205?,JCPDS Card No.005-0664).The sharp and narrow diffraction peaks reveal that the synthesized nanotetrapods are highly crystallized[26].No impurity peaks were detected, indicating the formation of pure products.

A typical FESEM image of the ZnO nanotetrapods is shown in Fig.1b, from which it can be seen that the as-prepared ZnO nanostructures were mainly tetrapods with leg length between200and300nm.To obtain further information about the nanostructure of these ZnO nanotetra-pods,TEM,HRTEM,and SAED analysis were performed.Fig.1c and d shows two typical TEM images of ZnO nanotetrapods,which have a typical leg length of~250nm.Fig.2a is the TEM image of the core of ZnO nanotetrapod corresponding to Fig.1d.Fig.1f,g and Fig.2b present the corresponding SAED patterns.The presence of the sharp diffraction spots rather than an amorphous ring is suggestive of the predicted formation of single crystalline ZnO.And these SAED patterns can be indexed to pure ZnO crystals of a hexagonal wurtzite structure.The HRTEM images(Fig.1e,Fig.2c and d)further indicate that the observed ZnO nanotetrapods are single crystalline with no defects of dislocations. The lattice fringes reveal that the single crystalline ZnO nanotetrapods possess interplanar spacing of about0.53nm,corresponding to the (002)plane of hexagonal ZnO.This clearly indicates that the ZnO nanoleg preferred growth along the[001]direction(c-axis).The HRTEM and SAED results also indicate that all four legs of the nanotetrapods grow along the[001]direction.

Tetrapod-like ZnO with200–300nm length legs were prepared by a microemulsion-mediated hydrothermal process,which was due to the preferential growth of ZnO crystal growth along the[001] direction.Based on surface energy minimization,ZnO crystallites form zinc hydroxyl nucleate together because of excess saturation. Each of them individually grows along the c-axis into rod-like crystal, and then tetrapod-like architectures are?nally formed.A possible growth mechanism for the formation of nano-ZnO via a microemul-sion-mediated hydrothermal process was reported in the literature [27].The formation mechanism of ZnO nanoparticles was proposed based on the restriction effect of microemulsions in the crystal growth process.It is possible that many internal or external factors determined the?nal morphologies of nanocrystals during the process of nucleation and growth.The clear formation mechanism is not clear yet,we believe that the restricting effect of the microemulsion,the surfactant and cosurfactant molecules play critical roles in the morphology control.The details need further research.

To investigate the electrochemical characteristics of ZnO nanote-trapods,we prepared a hydrogen peroxide(H2O2)sensor by doping ZnO nanotetrapods to the hybrid materials PVA/TiO2.Fig.3shows a typical cyclic voltammograms of the different modi?ed electrodes in the absence of H2O2.It was found that the reduction peak current of Hb-PVA/TiO2/ZnO nanotetrapods/GC electrode(b)increased obvi-ously comparing with that of Hb-PVA/TiO2/GC electrode(a), indicating that the as-prepared ZnO nanotetrapods accelerate electron

transfer.

Fig.1.(a,b)Power XRD pattern and typical FESEM image of the as-synthesized ZnO nanotetrapods,respectively,(c)TEM image of a single ZnO nanotetrapod,and the corresponding SAED pattern of the core(f),(d)TEM image of a single ZnO nanotetrapod,(e)HRTEM image from the outlined area in(d),and the corresponding SAED pattern(g).

2192J.Jiang et al./Materials Letters64(2010)2191–2193

4.Conclusion

In summary,ZnO nanotetrapods have been successfully synthe-sized via a simple microemulsion-mediated hydrothermal route.The as-prepared products are high phase-purity with hexagonal wurtzite in crystal structure.Clearly EA at room temperature suggests that these materials may have good potential applications for sensor constructions.Further studies on the growth mechanism and characterization experiments are underway.

Acknowledgements

This work is ?nancially supported by the National Natural Science Foundation of China (No.20963001);and theGuangxi Natural Science Foundation of China (No.0575030and No.0832078).

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Fig.2.TEM images of the core of ZnO nanotetrapod corresponding to Fig.1d (a),junction of three legs (c),and junction between two legs (d),respectively.(b)The SAED pattern of the tetrapod in

(a).

Fig.3.The cyclic voltammograms of different modi ?ed electrodes:(a)Hb-PVA/TiO2/GC electrode and (b)Hb-PVA/TiO2/ZnO nanotetrapods/GC electrode in the absence of H2O2in 0.12M pH 6.2PBS.

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