ARTICLE IN PRESS

Journal of Crystal Growth 265 (2004) 482–486

*Corresp

neering, U

burgh, PA

412624484

E-mail

smao@eng

0022-0248/

doi:10.101

A novel and simple growth route towards ultra-fineZnO nanowires

Zheng Chena,b,*, Zhiwei Shana, Shouxin Lic, C.B. Liangc, Scott X. Maoa,*aDepartment of Mechanical Engineering, University of Pittsburgh, 567 Benedum Hall, 212000, Pittsburgh, PA 15261, USA

bDepartment of Materials Science and Engineering, East China Shipbuilding Institute, Zhenjiang 212003, PR Chinac Institute of Metal Research, Chinese Academy of Science, Shenyang 110000, PR China

Received 12 September 2003; accepted 13 February 2004

Communicated by C.D. Brandle

Abstract

Ultra-fine ZnO nanowires can be easily synthesized at 800�C in air via oxidation of Zn nanowires which is produced

by heating ZnO+C mixture at 1100�C under a flow of nitrogen gas. Field-emission scanning electron microscope and

transmission electron microscope investigations show that the ultra-fine ZnO nanowires grown out from Zn nanowires

are high-quality single nanocrystals. The diameter of the ultra-fine ZnO nanowires ranges from 8 to 20 nm and their

length is 400 nm—1 mm, having a high aspect ratio of B50. The growth mechanism of the ultra-fine ZnO nanowires is

discussed.

r 2004 Elsevier B.V. All rights reserved.

PACS: 81.05; 81.10.B; 81.15.G; 42.70; 78.55

Keywords: A1. Nanostructures; A2. Growth from vapor; A2. Single crystal growth; B2. Semiconducting materials

1. Introduction

Since the first discovery of carbon nanotubes,one-dimensional nanometer-sized semiconductormaterials such as nanowires have attracted ex-tensive interest due to their much potential forfundamental studies of the roles of dimensionalityand size in their physical properties as well as for

onding author. Department of Mechanical Engi-

niversity of Pittsburgh, 648 Benedum Hall, Pitts-

15261, USA. Tel.: +1-4126249282; fax: 1-

6.

addresses: zhc8@pitt.edu (Z. Chen),

r.pitt.edu (S.X. Mao).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

6/j.jcrysgro.2004.02.017

their application in optoelectronic nanodevices[1–6]. Among the various semiconductor nano-wires, zinc oxide (ZnO) nanowire having a directband gap of 3.37 eV and large exciton bindingenergy of 60meV can promise practical applica-tions in the area of nanoscale laser diodes andultraviolet (UV) sensors. Because of the radialquantum confinement effect of nanowires, ZnOnanowires possess high density of states at theband edge. Recently Huang et al. reported thatUV lasing nano-devices working at room tem-perature with a low-lasing threshold based on ZnOnanowires would be quite potentially feasible [7,8].ZnO nanowires have been synthesized by various

d.

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Z. Chen et al. / Journal of Crystal Growth 265 (2004) 482–486 483

methods such as arc discharge, laser vaporization,pyrolysis, electrodeposition, and chemical orphysical vapor deposition [7–17]. Although thediameter of ZnO nanowires can be controlled byadjusting synthesis parameters especially for va-por–liquid–solid mechanism growth [8], the ZnOnanowires reported so far are usually larger than20 nm in diameter, in general, unsuitable for theevaluation of quantum-confinement effects. Onthe other hand, recently ultra-fine GaN nanowires(less than 10 nm) have been synthesized viachemical vapor deposition [18].

In this letter, we report our novel and simpleroute towards the growth of ultra-fine (UF) ZnOnanowires (8–20 nm) via direct oxidation of Znnanowires at 800�C in air without using anycatalyst and carrier gas or vacuo. The microstruc-tures are characterized using X-ray diffraction(XRD), field-emission escanning electron micro-scopy (FE-SEM) and transmission electron micro-scopy (TEM), and the growth mechanism isdiscussed.

Fig. 1. FE-SEM image of Zn nanowires.

2. Experimental procedure

The synthesis of Zn nanowires was carried outin a horizontal alumina tube furnace usingcarbothermal reduction process. A mixture ofpure ZnO powder (Alfa Aesar, 99.99%) andgraphite powder (Alfa Aesar, �300 mesh, 99.9%)in a 1:1molar ratio as the starting materials wasplaced at the center region of a quartz tube(350mm in length and 18mm in inner diameter).The quartz tube was then inserted into the furnace.The mixture was rapidly heated to approximately1100�C under a flow of high-purity nitrogencarrier gas at a rate of 150 sccm (standard cubiccentimeter per minute). The growth time wastypically 0.5–1 h. After evaporation and deposi-tion, the gray-black Zn nanowires were depositednear the outlet of the quartz tube. The as-grownZn nanowires were then put into an alumina boatand then oxidized in a furnace in dry air at 800�Cfor 1 h with a heating rate of 40�C/min. After thefurnace was cooled down to room temperature, itwas observed that the gray-black Zn nanowireswere transformed to white wool ZnO nanowires.

The crystal structures were analyzed using X-raydiffractometer (XRD-PHILIPS APD 3720, CuKa). The morphology and size of the productswere characterized using field-emission scanningelectron microscope (FE-SEM, PHILIPS XL30)equipped with an energy-dispersive X-ray spectro-scopy (EDX). Transmission electron microscope(TEM, JEOL 2000F at 200 kV) was used to studythe microstructure of the products.

3. Results

FE-SEM observation revealed that the gray-black Zn consisted of a large quantity of entangledand curved wire-like nanostructures (Fig. 1). XRDpattern A taken from the products, shown inFig. 2, demonstrated that they were hexagonalstructure Zn with lattice constants ofa ¼ 0:2655 nm and c ¼ 0:4928 nm, in accordancewith the JCPDS data of bulk Zn. However, thepeaks from the wurtzite-structure ZnO withsignificant low intensity were also found, indicat-ing that Zn nanowires might be slightly oxidizedby the residual oxygen within the tube. This issimilar to the results obtained by Lee’s group [19].They synthesized coaxial Zn/ZnO nanocables byheating ZnS at 1300�C using a pre-evacuated tubefurnace under a flow of Ar+5% H2, they found aZnO shell of about 5 nm thick was formed on the50 nm thick Zn core. On the contrary, currentsynthesis of Zn nanowires involving neithervacuum nor reduced gas provided a simpler way

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Z. Chen et al. / Journal of Crystal Growth 265 (2004) 482–486484

to form Zn nanowires (and probably Zn/ZnOnanocables).

After oxidation of the Zn nanowires, the crystalstructure of the white wool products was examinedby XRD. As shown in Pattern B in Fig. 2, thediffraction peaks can be indexed to be a hexagonalwurtzite-structured ZnO with lattice parameters ofa ¼ 0:3249 nm and c ¼ 0:5206 nm. The strongintensities relative to the background signalindicate the high purity and high crystalline ofZnO nanowires synthesized by high-temperature

Fig. 2. XRD patterns of the synthesized Zn nanowires (pattern

A) and ZnO nanowires after oxidation of Zn nanowires at

800�C in air (pattern B).

Fig. 3. FE-SEM images of ZnO nanowires synthesized by oxidation

out; (c) thin ZnO nanowires showing similar morphology with Zn

nanoneedles grow out from much thicker Zn nanowires.

oxidation of the Zn nanowires. In addition, nodiffraction peaks from Zn could be found in thesamples, indicating a complete transformationfrom the Zn nanowires to ZnO crystals.

FE-SEM images shown in Fig. 3 give a generalview of the morphology of the ZnO crystals. Theenergy dispersive spectrometry (EDS) of the ZnOcrystals showed that the atomic composition ratioof Zn/O was about 1:1. From Fig. 3, it was worthnoting that besides thick ZnO nanowires whichwere formed directly by oxidation of the Znnanowires, some ultra-fine (UF) ZnO nanowires(as shown by arrows) grew out from the relativethick ZnO nanowires base in a radial direction(Figs. 3(a) and (b)), whereas the relative thinnerZnO nanowires still maintained the morphology ofthe Zn nanowires. No UF-ZnO nanowires werefound that grew out from the thin Zn nanowires(Fig. 3(c)). The diameters of those UF-ZnOnanowires ranged from 8 to 20 nm and theirlengths were 400 nm—1 mm, having a high aspectratio of B50. In addition, comparing to Fig. 1, itcan be found that the ZnO nanowires formed viaoxidation of the Zn nanowires had a diameter inthe range of 120–300 nm which was thicker thanoriginal Zn nanowires (60–140 nm). Moreover, the

of Zn nanowires (a) and (b) showing UF-ZnO nanowires grow

nanowires and no UF-ZnO nanowires grows out; (d) ZnO

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Fig. 4. TEM bright-field electron micrograph of an individual

UF-ZnO nanowire, the inset is the selected area electron

diffraction pattern corresponding to the ZnO ½0 1 %1 0� zone axis.

Z. Chen et al. / Journal of Crystal Growth 265 (2004) 482–486 485

surface of ZnO nanowires were covered by ZnOnanoparticals (B100 nm in diameter) showing arougher surface than that of Zn nanowires whichhad a smooth side wall. The UF-ZnO nanowiresgrew on those ZnO nanoparticles. On the otherhand, Fig. 3(d) shows that abundant ZnOnanoneedles grew out from much thicker Znnanowires instead of UF-ZnO nanowires.

Additional structure characterizations of theUF-ZnO nanowires were further examined byTEM. Fig. 4 shows a TEM image of an individualUF-ZnO nanowire which was 8 nm in diameter.The UF-ZnO nanowires were monolithically singlecrystalline and the growth direction of UF-ZnOnanowires was along [0 0 0 1] as evidenced by theselected area electron diffraction (SAED) pattern(insert in Fig. 4).

4. Discussion

It has been reported by Zhu’s group [20] andYumoto et al. [21] that no ZnO nanowires could beobtained by heating Zn powders in air due to ahigh supersaturation when Zn vapor reacts withexcessive oxygen, inducing the formation of ZnOparticles. Similarly, in an experiment carried out

by Dai et al., instead of ZnO particles, tetrapod-like ZnO nanorods were formed when Zn powderswere oxidized in air at 850–925�C [22].

Current results clearly showed an in situ growthof UF-ZnO nanowires during oxidation of the Znnanowires. The reason why Zn nanowires arefavored for the growth of UF-ZnO nanowires maybe supposed to occur as follows. Upon heating ofthe Zn nanowires in air at high temperature, a thinZnO film is formed on the surface of Zn nanowiresand its thickness gradually increases with tem-perature rising, resulting increase in diameter ofthe nanowires. Because the linear thermal expan-sion coefficient of Zn is 6� 10�5/�C and ZnO is4� 10�6/�C, the oxide film shell and the Zn crystalcore expand at different rate. The thicker the Znnanowires, the larger the thermal stress caused bythe thermal expansion mismatch. For thick Znnanowires, as a result, cracks may occur in theZnO film upon heating if the thermal stressbecomes larger than the strength of ZnO(1000MPa). The growth of UF-ZnO nanowiresis due to the cracks in the film and is controlled bythe evaporation of Zn at high temperature. Withincrease of temperature to the melting point of Zn(419�C), Zn core inside the ZnO film melts and thevaporization of the naked Zn below the cracksoccurs. Zn vapor atoms deposited continuallyonto the surface of Zn/ZnO nanowires andgradually reacted with oxygen to form ZnOnanoparticles. It is known from the Gibbs–Thompson equation that the pressure of vaporaround the small Zn droplet is higher than that ofvapor around the large ones. Therefore, Zn vaporatoms diffuse from the smaller nano-droplet to thelarger ones. As a result, the equilibrium betweenthe vapor and the droplets is then broken down[23]. In order to maintain the equilibrium, thesmaller droplets evaporate and the large onesgrow. Eventually, a low supersaturation of Znvapor due to limited Zn source causes the in situgrowth of one-dimension UF-ZnO nanowiresfrom the nanodroplet probably by ‘self-catalyst’VLS growth mechanism [20], because no catalystwas used in the present study. On the contrary,when large Zn powders are oxidized at hightemperature in air, ZnO particles will be formeddue to a high supersaturation of Zn vapor because

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Z. Chen et al. / Journal of Crystal Growth 265 (2004) 482–486486

of a large volume-to-surface ratio. On the otherhand, for the thin Zn nanowires, no UF-ZnOnanowires grow out from base Zn nanowiresprobably due to much less cracks formed in theZnO film, which results in very low Zn vaporpressure. In addition, complete transformationfrom the Zn nanowires to ZnO nanowires is muchfaster for the thin Zn nanowires compared to thethick Zn nanowires. In order to prove this idea, inanother oxidation experiment, Zn nanowires wereput into a furnace which has been preheated at800�C for 1 h. The maximum hearing rate wasachieved under such a condition, however, no UF-ZnO nanowires could be found even for muchthicker Zn nanowires due to high oxidation ratewhich resulted in a much lower Zn vapor pressure.

5. Conclusion

We have successfully synthesized ultra-fine ZnOnanowires by oxidation of Zn nanowires in air at atemperature of 800�C. The Zn nanowires wereproduced by heating ZnO+C mixture under nitro-gen flow through thermal evaporation-vapor phasetransport-condensation processes. Upon oxidationof the Zn nanowires, the ultra-fine ZnO nanowiresranged from 8 to 20nm grew out in a radial directionfrom the Zn nanowires which were eventuallytransformed to ZnO nanowires. The results alsoshowed that the diameter of Zn nanowires andheating rate during oxidation were key factors tocontrol the growth of ultra-fine ZnO nanowires.

Acknowledgements

The authors wish to thank the Chinese ScienceFoundation and Shenyang Metal Research Insti-tute for providing the research grant.

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