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Materials Chemistry and Physics 106 (2007) 58–62

Hydrothermal synthesis of ZnO nanowires and nanobelts on a large scale

Hanmei Hu a,b,∗, Xianhuai Huang c, Chonghai Deng d, Xiangying Chen b, Yitai Qian b

a Department of Material Science and Engineering, Anhui Institute of Architecture and Industry, Hefei, Anhui 230022, People’s Republic of Chinab Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China,

Hefei, Anhui 230026, People’s Republic of Chinac Department of Environment Engineering, Anhui Institute of Architecture and Industry, Hefei, Anhui 230022, People’s Republic of China

d Hefei University, Hefei, Anhui 230022, People’s Republic of China

Received 28 August 2006; received in revised form 30 April 2007; accepted 6 May 2007

bstract

ZnO nanowires (∼60%) and nanobelts (∼40%) have been fabricated on a large scale via a low temperature one-pot hydrothermal technique.a2CO3 was introduced not only as alkaline source but also as a controllable reagent for the crystal growth of ZnO. The comparison experiment

esults indicate that the adding amount of Na CO greatly affect the length/diameter aspect ratios of 1D ZnO nanocrystals. In addition, the

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ntroduction of surfactant SDSN was indispensable in controlling the growth of belt-like ZnO. Room temperature photoluminescence spectrumhowed a weak UV band emission at 379 nm and a strong broad yellow emission around 564 nm. A possible mechanism on the formation of thenO nanowires was proposed. 2007 Elsevier B.V. All rights reserved.

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eywords: Nanostructures; Chemical synthesis; Electron microscopy; Optical

. Introduction

Nanorods and nanowires represent a class of one-dimensional1D) nanostructures, in which the carrier motion is restricted inwo directions such that they usually show interesting proper-ies that differ from those of the bulk or spherical nanoparticlesf the same chemical composition [1,2]. These nanostructuresave potential applications as important components and inter-onnects in nanodevices [3,4].

ZnO is one of the most promising materials for optoelectronicpplications due to its wide band gap of 3.37 eV and large excitoninding energy of 60 meV [5]. It has been recognized as one ofhe promising nanomaterials in a broad range of high-technologypplications, such as photodetector [6], light-emitting diode [7],as sensor [8], solar cell [9], optical modulator waveguide [10],nd surface acoustic wave devices [11], etc.

In recent years, various methods have been used to syn-hesis 1D ZnO nanostructures, such as thermal evaporationrocess [12–14], chemical vapor deposition [15,16], metalor-

∗ Corresponding author at: Department of Material Science and Engineering,nhui Institute of Architecture and Industry, Hefei, Anhui 230022, People’sepublic of China. Tel.: +86 551 3526891.

E-mail address: hmhu@ustc.edu (H. Hu).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.05.016

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anic vapor-phase epitaxy [17], microwave plasma deposition18], pyrolysis [19], hydrothermal method [20,21], and etc. Theydrothermal method is a promising one for fabricating idealanomaterial with special morphology because of the low cost,ow temperature, high yield, scalable process. In the presentork, ZnO nanowires (∼60%) and nanobelts (∼40%) haveeen produced in large quantities, using ZnCl2 as zinc source,a2CO3 as mineralizer, and sodium dodecyl sulfonate (SDSN)

s morphology controller agent, via a low temperature one-potydrothermal technique.

. Experimental procedure

All chemicals (analytical grade reagents) were purchased from Shanghaihemical Reagents Co. and used without further purification. In a typical exper-

mental procedure, 0.2 g ZnCl2, 1.5 g SDSN and 20 g Na2CO3 (∼4.72 M) wereuccessively added into a 50 mL Telfon-lined stainless steel autoclave, whichas then filled with distilled water up to 90% of the total volume. The obtained

eaction mixture was stirred for an additional 30 min. The autoclave was sealednd maintained at 140 ◦C for 12 h. After the reaction was completed, the result-ng white products were filtered off, washed with ethanol and hot distilled water

or several times, and then finally dried in a vacuum at 60 ◦C for 4 h.

The phase purity of the as-synthesized products was examined by X-rayiffraction (XRD) using a Philips X’Pert PRO SUPER X-ray diffractometerquipped with graphite monochromatized Cu K� radiation (λ = 1.541874 A).he morphology and size of the obtained ZnO products were further observed by

stry and Physics 106 (2007) 58–62 59

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H. Hu et al. / Materials Chemi

ransmission electron microscope (TEM) and field-emission scanning electronicroscope (FESEM), which were taken on a Hitachi model H-800 (200 kV)

nd a field-emission microscope (JEOL-JSM-6700F 15 kV), respectively. TheRTEM image was taken with a JEOL-2010 transmission electron microscopeith an accelerating voltage of 200 kV. SEM images were taken on an X-650

canning electronic microanalyzer. The photoluminescence (PL) spectrum wasecorded on a Steady-state/Lifetime Spectrofluorometer (FluoRoloG-3-TAU).

. Results and discussions

Fig. 1(a) shows the typical XRD pattern of the as-preparednO products. All the reflections can be indexed to wurtzitetructure of ZnO with lattice parameters a = 3.247 A and= 5.20 A, in good agreement with the reported data for ZnO

a = 3.249 A, c = 5.205 A, JCPDS File, 5-664). No character-stic peaks were detected for the other impurities such asn(OH)2, ZnCO3. Fig. 1(b) and (c), respectively, shows the

ow-magnification and high-magnification FESEM image of thebtained sample, which indicates that the as-synthesized ZnOroducts were composed of wire-like (ratio: ∼60%) and belt-ike (ratio: ∼40%, indicated by the black arrow in Fig. 1(c))anostructures, and their lengths are up to 20 �m. The diame-ers of ZnO nanowires are about 20–100 nm and the widths ofnO nanobelts are in the range of 80–250 nm.

Further structural characterizations of the ZnOanowires/belts were performed by TEM and HRTEM.ig. 2(a) shows the low-magnification TEM image of the ZnOanowires. With increasing the TEM magnification, belt-likenO nanostructures are apparently observed (Fig. 2(b)).ig. 2(c) shows the TEM image of a well-developed singlerystal ZnO nanobelt with width of 220 nm. The SAED patternf the nanobelt (inserted at the upper left corner of Fig. 2(c))ndicates its single crystal nature and its growth direction along-axis. The typical HRTEM image, recorded from a certainanowire, is shown in Fig. 2(d). The crystal lattice fringes arelearly observed and average distance between the adjacentattice planes is 0.52 nm, corresponding to the (0 0 0 1) planeattice distance of hexagonal-structured ZnO, which furtherroves that ZnO nanowires/belts prepared in the present systemrow along [0 0 0 1] direction.

In the present reaction system, the possible formation processor hexagonal ZnO phase under hydrothermal condition can bexpressed as follows:

O32− + H2O → HCO3

− + OH− (1)

CO3− + H2O → H2CO3 + OH− (2)

n2+ + 4OH− → Zn(OH)42− (3)

n(OH)42− → ZnO + H2O + 2OH− (4)

A suitable hydrothermal system may be helpful for the nucle-tion and subsequent 1D preferential growth of ZnO crystals. Itas interestingly found that the amount of mineralizer Na2CO3lay a critical role in the control growth of ZnO nanowires. A

eries of comparison experiments were performed by chang-ng the adding amount of Na2CO3 without using SDSN in theeaction system. Fig. 3(a) shows the TEM images of the pre-ared ZnO products when the adding amount of Na2CO3 was

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ig. 1. (a) XRD pattern of the as-prepared ZnO products. (b) Low-magnification.c) High-magnification FESEM images of the ZnO products.

g (∼1.05 M), which take on bowknot-like morphologies. These

owknots are built from several to tens of bipyramidal ZnOwinned crystals with average diameters of 810 nm and lengthsf 10–20 �m. The inset figure clearly shows the details of anndividual bowknot-like microcrystalline. When 10 g Na2CO3

60 H. Hu et al. / Materials Chemistry and Physics 106 (2007) 58–62

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ig. 2. (a and b) TEM images of the as-prepared ZnO products, indicating the coenO nanobelt and corresponding SAED pattern (inserted at the upper left corne

∼2.10 M) was added into the reaction system, a great deal ofumbbell-like ZnO twinned crystals with average diameters of50 nm and lengths of 8–12 �m were obtained (Fig. 3(b)). Inddition, another novel structure coexisting with them—hollowexagonal prism with one closed end (see the enlarged partnserted in the lower right of Fig. 3(b)) was observed. Mostf them assembled to forming flower-like aggregates. It is sug-

ested that the growth mechanism for this novel structure shoulde similar to that for the reported Te nanotubes [22,23]. Inhis case, secondary nucleation and growth would preferen-ially occur at the circumferential edges of the hexagonal prism

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nce of wire-like and belt-like nanostructure. (c) The TEM image of an individual) HRTEM image taken from an individual ZnO nanowire.

ecause these sites had relatively higher free energies than otherites on its surface. After a period of rapid growth, the concen-ration of Zn(OH)4

2− was decreased, which could not satisfy therowth of rod. Thus, the hollow prism would be formed becausef no mass transportation to the inner region. When the addingmount of Na2CO3 was increased to 15 g (∼3.54 M), someire-like ZnO nanocrystals with average diameters of 180 nm

nd lengths up to 15 �m appeared in the synthesized productsFig. 3(c)). Large quantities of ZnO nanowires with averageiameters of 80 nm and lengths up to 20 �m were produced whenhe adding amount of Na2CO3 was 20 g (∼4.72 M), and only

H. Hu et al. / Materials Chemistry and Physics 106 (2007) 58–62 61

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ig. 3. SEM or FESEM images of ZnO products prepared from different addinb) 10 g; (c) 15 g; (d) 20 g.

everal belt-like nanocrystals were also occasionally observedFig. 3(d)). The above comparison experiments suggested thathe length/diameter of 1D structured ZnO strongly depend on themount of Na2CO3. When the solution changed from undersatu-ation (5 g Na2CO3), lower saturation (10 g Na2CO3), saturation15 g Na2CO3) to high supersaturation (20 g Na2CO3), the diam-ter of 1D ZnO crystallites was greatly reduced from micro toanosize.

Generally speaking, the morphology control of ZnO underydrothermal conditions should be determined by two mainactors: one is the internal crystal structure of ZnO, the others the selected external condition, such as reaction temperature,eaction additive, mineralizer and so on. It is well known thatnO is a polar hexagonal and highly anisotropic crystal, and

ts oriented growth direction is along the c-axis [24]. From a

inetics point of view of ZnO crystal growth, Zn(OH)4

2− isroposed to be the growth unit and is directly incorporated intonO crystal lattice at the interface under given conditions [24].

n the process of operating comparison experiment, due to the

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unt of NaCO3 under hydrothermal conditions in the absence of SDSN: (a) 5 g;

ifferent amount of mineralizer Na2CO3, the concentration ofn(OH)4

2− is also different in the aqueous solution, which willesult in different nucleation (such as nuclei size) and crystalrowth and further affect the ratio of length/diameter of 1D ZnOrystallites.

Based on the above analysis, we proposed that a higher super-aturation solution is the key driving force for the growth ofnO nanowires. A possible formation mechanism of the ZnOanowires may be described as follows: first, an supersaturaten(OH)4

2− precursor solution is obtained under appropriatelyigher concentration of Na2CO3 (20 g). Then, in the initial stagef hydrothermal decomposition, much smaller nuclei are pro-uced through a short burst of homogeneous nucleation process25]. Moreover, the size of a nucleus will determine the lateralimension of an1D ZnO nanostructure. Finally, after the nucle-

tion step, the growth units of Zn(OH)4

2− are subsequentlyncorporated into these seeds along the c-axis of ZnO crystalattice[24]. With the prolongation of reaction time, ultra-longnO nanowires can be fabricated.

62 H. Hu et al. / Materials Chemistry

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ig. 4. Photoluminescence spectrum of ZnO nanowires and nanobelts measuredt room temperature.

With respect to the formation of ZnO nanobelts, the direct-ng role of surfactant SDSN is undoubtedly significant. In thebsence of SDSN, the obtained ZnO products were mainly com-osed of nanowires, as well as occasionally several nanobelts.e proposed that the possible function of SDSN is to kineti-

ally control the growth rates of different crystalline faces ofnO crystals by interacting with these faces through adsorp-

ion and desorption under suitable kinetic growth condition, andnally resulting in the formation of ZnO nanobelts. The exactole of SDSN in the present system is still under investigation.

The room temperature PL spectrum of as-prepared ZnOanowires and nanobelts, shown in Fig. 4, was obtained withn excitation wavelength of 325 nm. Two luminescence bands,ncluding a weak UV emission centered at 379 nm and a strongroad yellow emission with peak located at 564 nm, werebserved. The UV emission band was attributed to the near-bandmission of the ZnO products, coming from the direct recom-ination of the conduction band electrons and the valence bandoles. The deep-level involved in the yellow luminescence wasttributed to interstitial oxygen [26–29].

. Conclusion

In summary, ZnO nanowires (∼60%) and nanobelts (∼40%)ave been synthesized on a large scale via a low temperaturene-pot hydrothermal technique. The experimental results revealhat mineralizer Na2CO3 was introduced not only as alkalineource but also as a controllable reagent for the crystal growth ofnO. The adding amount of Na2CO3 could affect the concentra-

ion of Zn(OH)42− precursor, and made the average diameters

f 1D ZnO decrease from 810 to 80 nm with the increase ofa2CO3. In addition, the surfactant SDSN played an assisting

ole in controlling the growth of belt-like ZnO. It is proposed

hat the possible function of SDSN is to kinetically control therowth rates of different crystalline faces of ZnO crystals bynteracting with these faces through adsorption and desorptionnder suitable kinetic growth condition, and finally resulting in

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and Physics 106 (2007) 58–62

he formation of ZnO nanobelts. The present strategy of fabri-ating ZnO nanowires/belts is simple, reproducible, high yield,asily operating and may be applied to scale up to industrialroduction.

cknowledgements

This work was supported by the National Natural Scienceoundation of China (Grant No. 20501002), the Educationepartment of Anhui Province (Grant No. 2005KJ110), andnhui Provincial Young Teacher Sustentation Project of Anhui

Grant No. 2005jq1147zd).

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