Materials Letters 67 (2012) 193–195

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Materials Letters

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Facile preparation and growth mechanism of zinc oxide nanopencils

Kai Dai a,⁎, Guangping Zhu a, Zhongliang Liu a, Qingzhuang Liu a, Zheng Chen b, Luhua Lu b,⁎a College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, PR Chinab Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, PR China

⁎ Corresponding authors. Fax: +86 561 3803256.E-mail addresses: daikai940@chnu.edu.cn (K. Dai), l

0167-577X/$ – see front matter. Crown Copyright © 20doi:10.1016/j.matlet.2011.09.079

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 May 2011Accepted 24 September 2011Available online 29 September 2011

Keywords:Zinc oxideSynthesisPencil-likeNanoparticlesCrystal growth

In this paper, zinc oxide (ZnO) nanopencils and high aspect ratio were successfully synthesized by solutionmethod using zinc chloride (ZnCl2) and ammonium hydroxide (NH3·H2O) with polyvinylpyrrolidone(PVP)-assisted hydrothermal process at 180 °C. The investigation on the morphology, structure and crystal-linity of the products in the growing process were carried out by using transmission electron microscope(TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectrometer(EDS). The results showed that these products with the average diameter of 300 nm and length of about10 μm are pencil-like crystals and the structure is uniform. Possible growth mechanism for ZnO nanopencilswas discussed in detail. In addition, we believe that this simple process can also be applied to synthesizeother functional materials.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

For the novel physical and chemical properties, much attentionhas been focused on the research of nanostructured semiconductormaterials, such as nanowires, nanotubules, nanoribbons, nanobelts,nanorods and nanotubes in the past few years [1–5]. Zinc oxide(ZnO), a direct wide band gap (3.37 eV) semiconductor of large exci-tation binding energy (60 meV), is one of the most important multi-functional oxide ceramic materials. ZnO has been widely applied invarious fields, such as photosensitization, gas sensors, water treat-ment and etc. [6–9]. Recently, ZnO nanopencils have drawn much at-tention because they are considered to be ideal field emissionelectron sources that can emit electrons at low electric fields [10–12].

Physical and chemical syntheses are two main approachesadopted for the preparation of ZnO nanostructures [13,14], in whichthe hydrothermal method is more promising for large scale fabricat-ing ideal nanomaterial of desired morphology due to the high yieldand simple solution manipulation under low temperature.

In the present work, a simple route was designed to prepare ZnOnanopencils via a hydrothermal method at relatively low tempera-ture. The preparation process was achieved in aqueous solution with-out any catalysts or templates, which is more environment friendlythan alcohol that had been widely used previously. The possiblegrowth mechanism for ZnO nanopencils was discussed in this articlefor our detailed investigation of their structure in the formation pro-cess under hydrothermal reaction.

hlu2007@sinano.ac.cn (L. Lu).

11 Published by Elsevier B.V. All rig

2. Material and methods

2.1. Synthesis of ZnO nanocrystals

6.5 g zinc chloride was initially dissolved in 50 mL distilled waterfollowed by adding 30 mL NH3·H2O to form a transparent solution.And then, 1.0 g polyvinylpyrrolidone (PVP) and 30 mL distilledwater were added into 10 mL transparent solution forming a mixture.The mixture was further transferred into a 50 mL Teflon-lined auto-clave and subsequently heated at 180 °C for 20 h. After the reaction,the white products were harvested by centrifugation and throughoutwashing with distilled water, and were finally dried at 60 °C for 4 h inair.

2.2. Material characterization

The crystal structures of the ZnO nanopencils were analyzed by X-ray diffraction using DX-2000 X-ray powder diffractometer. The mi-crocrystalline structure and surface characteristics of the sampleswere investigated by using X-650 scanning electron microscopy(SEM) with INCA x-act energy dispersive Spectrometer (EDS) andTecnai G2 F20 S-Twin transmission electron microscope (TEM).Photoluminescence (PL) spectra were detected by a PMT928 PL spec-trometer with excitation wavelength of 325 nm.

3. Results and discussion

The phase composition and phase structure of the as-obtainedsamples were examined by XRD. As shown in Fig. 1, all of the peakscan be readily indexed as hexagonal wurtzite ZnO (JCPDS card no.36-1451, a=0.325 nm and c=0.521 nm) with high crystallinity.

hts reserved.

30 40 50 60 70

(000

4)

(101

2)

(202

1)(112

2)(2

020)(1

013)

(112

0)

(101

1)(0

002)

(101

0)

Inte

nsi

ty (

a,u

)

2Theta(deg.)

Fig. 1. XRD patterns of synthesized ZnO nanopencils.

194 K. Dai et al. / Materials Letters 67 (2012) 193–195

Fig. 2(a) shows the SEM images of ZnOnanopencils. All the nanopar-ticles with the average diameter of 300 nm and length of about 10 μmare separated from each other. Fig. 2(b) shows the EDS spectrumof syn-thesized ZnO nanopencils. The pattern indicates that the nanocrystalsonly contain elements of Zn and O, without any other impurities.

Fig. 3(a) shows the TEM image of ZnO nanopencils. As indicated inFig. 3(a), the samples are composed of pencil-like ZnO nanorods with

a

Fig. 2. (a) SEM images and (b) EDS

a

Fig. 3. (a) TEM image and (b) SAE

a fine nanotip. The diameter of ZnO nanopencil is in the range of300 nm to 400 nm, which is clearly consistent with the SEM observa-tion. The corresponding selected area electron diffraction (SAED) pat-tern obtained from the ZnO nanopencils in Fig. 3(b) shows that thesynthesized products are single crystalline with the hexagonal wurt-zite structure, which is consistent with the XRD observation.

Fig. 4 shows the PL spectra of the ZnO nanopencils in the roomtemperature. The ZnO nanoparticles show a sharp peak centered at389 nm and a broad emission peak centered at 494 nm. This may bedue to the singly ionized oxygen vacancy [15].

The solution/precipitation transition is one of the most widelyadopted liquid phase synthesis approach that form shape and structurecontrollable solid phase after the reaction of a homogenous mixture ofsalt solutions. The reaction formulas were considered to be as follows:

NH3⋅H2O→NHþ4 þ OH− ð1Þ

Zn2þ þ 2OH−→Zn OHð Þ2 ð2Þ

Zn OHð Þ2þ 2OH−→Zn OHð Þ2−4 ð3Þ

Zn OHð Þ2 þ 4NH3⋅H2O→ Zn NH3ð Þ4� �2þ þ 2OH− þ 4H2O ð4Þ

Zn NH3ð Þ4� �2þ þ 3H2O→ZnOþ 2NHþ

4 þ 2NH3⋅H2O ð5Þ

Zn OHð Þ2−4 →ZnOþH2Oþ 2OH− ð6Þ

b

spectrum of ZnO nanopencils.

b

D pattern of ZnO nanopencils.

350 400 450 500 550 600 650

Inte

nsi

ty (

a, u

)

Wavelength (nm)

Fig. 4. PL spectrum of the ZnO nanopencils.

195K. Dai et al. / Materials Letters 67 (2012) 193–195

Fig. 5 shows the schematic growth mechanism of the formation ofZnO nanopencils. The growth of hexagonal ZnO nanopencils can beexplained by the two steps. The first step is mainly the formation ofZnO seed nuclei and the second step is directed growing hexagonalparticles to the pencil-like structures. In this system, the pH valuesof the reaction mixtures vary with the volumes of added ammoniumhydroxide. As shown in Fig. 5(a), (b), and (c), Zn2+ ions could reactwith ammonium hydroxide to form soluble complexes in the mixedsolvent hydroxyl OH− and the tetrahedron [Zn(NH3)4]2+, and theninitiative ZnO seed crystal was formed according to Eqs. (1), (2), (3)and (4). ZnO seeds are crucially important for the aligned nanopencilgrowth, the nucleation density was proved to be remarkably higherthan that of ZnO nanoparticles, which grew without seeds in previousresearch [16]. Thus, ZnO nanoparticles were formed based on ZnOseeds. We chose PVP as an adjunct reagent in this work, not onlyusing its oxygen atoms to interact with Zn(II) but also using the stabili-zation of the PVP macromolecular chain. According to Eqs. (5) and (6),hexagonal ZnO nanoparticles could be formed when the particles pref-erably grow along normal of the (0001) planes, as shown in Fig. 5(d)and Fig. 5(e), at the early ZnO nanocrystal growth stage, the nanoparti-cle grows faster along (0001) facet than along (1120) facet, owing to a

Fig. 5. Schematic growth mechanism

higher supersaturation ratio. Thus the growth speed of different facetsshould be V 000 1ð ÞNV 011− 0ð ÞNV 011− 1ð ÞNV 0001−ð Þ. At the end ofgrowth as shown in Fig. 5(f), ZnO particles are pencil-like end, andthis also suggests that there are still large numbers of (0001) facetsfor these nanoparticles.

4. Conclusions

In summary, the synthesis of ZnO nanopencils has been describedvia a simple mild hydrothermal method. The product consists of a largequantity of hexagonal pencil-like ZnO with the average diameter of300 nm and length of about 10 μm. The aim of the present study was tounderstand the formation of the ZnO nanopencils, which could beexplained by the interaction of ZnO seed nuclei and surfactant. It isexpected that thenovel ZnOnanopencilsmay offer exciting opportunitiesfor potential applications in micro and nano electro mechanical systemfor energy transformation.

Acknowledgements

We wish to thank the National Natural Science Foundationof China (11004071, 20803051), the Youth Foundation of HuaibeiNormal University (700432), the Natural Science Foundation of AnhuiProvince (11040606M10, 11040606M64) and the Natural ScienceFoundation of Jiangsu Province (BK2010258).

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