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0022-0248/$ - se

doi:10.1016/j.jc

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Journal of Crystal Growth 310 (2008) 1836–1840

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Solution growth of nano- to microscopic ZnO on Zn

Chenglin Yan, Dongfeng Xue�

State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian University of

Technology, 158 Zhongshan Road, Dalian 116012, PR China

Available online 30 October 2007

Abstract

A new strategy has been successfully designed for the first time to synthesize ZnO nanorod arrays on a zinc surface by employing a

novel NaCl solution corrosion-based approach. The natural oxidation of zinc metal by naturally dissolved oxygen in water is very slow

due to the surface oxide layer. However, in the presence of NaCl solution, this spontaneous oxidation reaction can be accelerated

drastically, and thus ZnO nanorod arrays can readily grow along the [0 0 1] direction on the surface of the zinc foil. In particular, this

simple synthetic strategy only requires NaCl solution and zinc foil, which enables us to generate, at large scale, low cost, and moderate

temperature, advanced thin films. Photoluminescence (PL) spectrum reveals that our obtained ZnO nanorods have negligible oxygen

vacancies. By the way, a two-dimensional pattern of flower-like ZnO nanosheets on Zn by formamide-induced sequential nucleation and

growth on zinc foil substrate has also been demonstrated.

r 2007 Elsevier B.V. All rights reserved.

PACS: 73.61.Le; 81.10.Dn; 61.46.�w

Keywords: A1. Nanostructures; A2. Growth from solutions; B1. Oxides

1. Introduction

Wurtzite zinc oxide (ZnO) has a hexagonal structure(space group P63mc), which is one of the importantsemiconductors, mainly due to its wide band gap (3.37 eV)and large excitation binding energy (60meV) [1–6]. ZnOarrays or films, as an important family of ZnO nanomater-ials, have been an active research field as early as the 1960sbecause of their applications as sensors, transducers, andcatalysts [1]. The first report of the room-temperaturelasing action of one-dimensional (1D) ZnO nanorod arraysin 2001 further demonstrates that the functional design ofZnO nanostructure in a highly oriented and ordered arrayis of crucial importance for the development of a devicewith a good performance [7]. In the past decade, variouschemical and physical deposition techniques have beenemployed to create oriented arrays [8,9]. For instance,chemical vapor deposition (CVD) [10], physical vapor

e front matter r 2007 Elsevier B.V. All rights reserved.

rysgro.2007.10.060

ing author.

ess: dfxue@chem.dlut.edu.cn (D. Xue).

deposition (PVD) [11], pulsed laser deposition [12]. Thesemethods, however, often suffer from the disadvantage ofintroducing metal catalysts and requiring high tempera-ture, which could make the synthesis procedures morecomplex and introduce catalyst impurities to influence theproperties of ZnO nanoarrays.Previously, we developed a mild solution-based route

process for the synthesis of hollow ZnO spheres [13], tubearrays [14], and polyhedral particles [15]. Hollow ZnOmicrospheres have also been synthesized at room tempera-ture via a sacrificial template route in our recent work[16,17]. In this work, we report the first synthesis of well-aligned ZnO nanorods by NaCl solution-based corrosionstrategy. The present process used for the self-seedinggrowth of ZnO nanoarrays is based on heterogeneousnucleation and subsequent crystal growth of 1D nanos-tructures on the zinc foil substrate. The selection of zincfoil as the substrate for the growth of well-orientated ZnOarrays is due to the following reasons: the lattice matchingbetween ZnO and Zn crystals facilitates the growth ofwell-aligned ZnO nanorod arrays, moreover, zinc foil is

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Fig. 1. SEM images of ZnO nanorod arrays synthesized by direct

immersion of zinc foil into NaCl solution at 120 1C for 16 h: (A) low-

magnification view; (B) high-magnification view.

C. Yan, D. Xue / Journal of Crystal Growth 310 (2008) 1836–1840 1837

a conductive material, making it facilely utilize the alignedZnO nanorods for electronic and optoelectronic devices.Zinc foil can be directly acted as the reactant and asubstrate to support the obtained 1D nanorods to avoidusing an additional substrate. The method is simple, low-temperature, and practical route, which only utilizes zincfoil and NaCl solution. Therefore, the present techniquemight be nice for the ‘‘bottom-up’’ design of nanoscalephotonic and electronic devices that are available forapplication cheaply and easily.

2. Experimental section

The experiment of growth of ZnO nanorod arrays wascarried out in a Teflon-lined stainless-steel autoclave of80mL capacity. Prior to the synthesis, the zinc foils werecarefully cleaned with absolute alcohol and deionizedwater, respectively, in an ultrasound bath to removesurface impurities. In a typical synthesis, zinc foil wasused as the substrate, on which ZnO nanorod arrays grew.The solution for the growth of ZnO nanorod arrays is0.9mL/L of NaCl solution. The pH value of the mixturewas adjusted by acetic acid. The mixture solution was thentransferred into Teflon-lined stainless-steel autoclaves, andthe previously cleaned zinc foil was then immersed into thesolution. The autoclave was maintained at 120 1C for 16 hand then cooled down to room temperature. The obtainedZnO nanorod arrays were taken out of the solution,washed with ethanol, and finally air-dried for characteriza-tion. The as-prepared samples were characterized by anX-ray diffractometer (XRD) on a Rigaku-DMax 2400diffractometer equipped with the graphite monochroma-tized Cu Ka radiation flux at a scanning rate of y of0.021 s�1 in the 2y range 5–801. Scanning electron micro-scopy (SEM) images were taken with a JEOL-5600LVscanning electron microscope, using an acceleratingvoltage of 20 kV. The structure and growth directionof ZnO nanorods were investigated by transmissionelectron microscopy (TEM; Philips, TecnaiG2 20, operatedat 200 kV). The photoluminescence (PL) spectrum wasmeasured at room temperature in the spectral range of350–600 nm using a Xe lamp with a wavelength of 325 nmas the excitation source.

3. Results and discussion

Aligned ZnO nanoarrays have been successfully synthe-sized by immersing zinc foil into the NaCl solution at120 1C for 16 h. Fig. 1A shows the general morphology ofthe ZnO nanorod arrays: the uniform nanorods are denselypacked on the zinc foil substrate. It can be seen fromFig. 1B that well-aligned nanorod arrays are grown alongthe [0 0 1] direction in a perpendicular fashion onto thesubstrates. An individual ZnO nanorod obtained fromZnO nanorod arrays is inspected with TEM. As shown inFig. 2A, the ZnO nanorod is straight with a diameter of500 nm and length of 1–3 mm. A typical high-resolution

transmission electron microscopy (HRTEM) image of theindividual ZnO nanorod is shown in the inset in Fig. 2A.The image clearly reveals that only the fringes of the (0 0 2)plane with a lattice spacing of about 0.26 nm can beobserved, conforming that [0 0 1] is the growth direction ofthe ZnO nanorods.The crystal structure and orientation of ZnO nanorod

arrays are further investigated by XRD diffraction, whichis shown in Fig. 2B. The indexed diffraction peaks can beindexed as the pure hexagonal phase of wurtzite-type ZnO(space group P63mc) with lattice constants a ¼ 3.249 Aand c ¼ 5.206 A, compatible with the reported data(JCPDS no. 36–1451). According to the XRD pattern forZnO nanoarrays grown on zinc substrate, ZnO is theonly crystallographic phase detectable in the well-alignedcrystalline hexagonal ZnO except those already unindexeddiffraction peaks originated from the zinc substrate. Inaddition to the diffraction peaks from the zinc metal, thereis only one very strong (0 0 2) diffraction peak from theseZnO nanorods, whereas other ZnO peaks are either veryweak or not detected.It is well known that the natural oxidation of zinc metal

by naturally dissolved oxygen in water is very slow due tothe surface oxide layer. However, in the presence of NaClsolution, this spontaneous oxidation reaction can beaccelerated drastically due to its strong corrosion abilityto the zinc foil. As the zinc foil corrodes, Zn2+ ions are

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Fig. 2. (A) TEM image of a single ZnO nanorod, with the inset showing

the corresponding HRTEM pattern. (B) XRD pattern of well-aligned

ZnO nanorod arrays synthesized in the NaCl solution. The indexed

diffraction peaks can be indexed as the pure hexagonal ZnO phase,

unindexed peaks are originated from those of the zinc substrate.

300 350 400 450 500 550 600 650 700 750

410464

380

Inte

nsity

(a.

u.)

Wavelength (nm)

Fig. 3. A room-temperature photoluminescence spectrum of the obtained

ZnO nanorod arrays recorded at an excitation wavelength of 325 nm.

C. Yan, D. Xue / Journal of Crystal Growth 310 (2008) 1836–18401838

released continuously from zinc foil into solution whileoxygen (from air) is simultaneously reduced, and ZnOnanorod arrays can readily grow along the [0 0 1] directionon the surface of the zinc foil. With respect to the structureof wurtzite ZnO, the oppositely charged ions producepositively charged Zn2+ (0 0 1) and negatively chargedO2� (0 0 1) polar surfaces. The Zn-terminated (0 0 1)surface is more chemically active than O-terminated(0 0 1) face, which leads to the adsorption of growthunits of onto (0 0 1) plane for the polar growth of ZnOnanoarrays in the solution.

In the NaCl solution growth system, it is believed thatthe growth mechanism of ZnO crystal mainly involves theformation of growth units and the incorporation of growthunits into the crystal lattice. When growing ZnO nanorodsin aqueous solution, the constituent atoms enter into ZnO

crystal in the form of fundamental growth units producedby the strong chemical bonds, which are formed betweengrowth units during the crystallization process. Whengrowth units pass through the boundary layer andapproach the crystal surface, the growth unit incorporatesinto the crystal lattice. As we know, the relative stackingrate of the constituent tetrahedra in various crystal faces isstrongly dependent on the bonding force of atoms in thetetrahedra at the interface. Each {ZnO4} tetrahedron has acorner in the [0 0 1] direction. The atom at the corner of atetrahedron has the strongest bonding force (s ¼ 2 v.u.(valence unit)) as compared with the atoms at otherpositions (e.g., along the edge and face directions) [18].Therefore, ZnO crystal grows fast along the directions inwhich the tetrahedron corners point. ZnO preferentiallygrows along the [0 0 1] direction, which results in theformation of 1D ZnO nanoarrays.The ultraviolet and visible PL of as-grown ZnO nanorod

arrays was measured using a Xe lamp as the excitationsource. Room temperature spectra of as-grown sampleshows a weak band-edge emission at 380 nm resulting fromfree-exciton annihilation, and two weak blue emissions at410 and 464 nm are observed, which is shown in Fig. 3. Theweak blue-green emission may originate from the electrontransition from the level of the ionized oxygen vacancies tothe valence band. Previous research has shown that the PLspectrum of ZnO is sensitive to the particle shape, size,temperature, preparation method [2], etc. ZnO nanorodarrays grown through NaCl solution-based corrosionmethod exhibit a strong excitonic emission together withthe blue emission but do not show the green emission,indicating that the ZnO nanorods have negligible oxygenvacancies.In the current work, the size and shape of our obtained

ZnO nanorod arrays can be effectively tuned by adjustingthe concentration of NaCl solution and pH value ofreaction solution. As shown in Fig. 4A and B, hexagonalZnO rods with a diameter of 2 mm can be obtainedwhen the concentration of NaCl solution is adjusted to0.45mol/L. Furthermore, in order to confirm the versatility

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Fig. 4. SEM images of hexagonal ZnO rods (A, B), and Fe oxide films (C, D).

C. Yan, D. Xue / Journal of Crystal Growth 310 (2008) 1836–1840 1839

of the our designed method for preparing metal oxide filmsthrough the NaCl solution-based corrosion strategy,we also employed this method to prepare Fe oxide films(Fig. 4C and D).

In particular, we have created a novel two-dimensional(2D) pattern of flower-like ZnO nanosheets by formamide-induced sequential nucleation and growth on zinc foilsubstrate in the Zn(OH)4

2� solution without NaCl. It hasbeen demonstrated that secondary growth on initiallyformed nuclei was needed for the growth of complexpatterns of ZnO nanocrystals. For example, Liu and co-workers [19] produced micropatterns of flower-like ZnOnanorod clusters by diamine-induced growth on micro-patterned primary crystals, in which secondary growthproduces flower-like crystals from new crystal growth onthe top face of the primary rods and side branches formedon the edge. In our present work, a one-step method hasbeen developed for the production of a novel 2D pattern offlower-like ZnO nanosheets, thus avoiding the complexprocedures. Fig. 5A shows arrays of flower-like ZnOnanosheets formed by formamide-induced growth in theZn(OH)4

2� solution. From the high-magnification imageshown in Fig. 5B, it can be clearly seen that the flower-likecrystal is composed of many 2D nanosheets that aresimultaneously grown from pre-agglomerated seeds. Thecreation of novel flower-like ZnO nanosheets is rarelyreported, which is suggested to be due to the contributionof formamide molecules in our present reaction system.Without the formamide molecules, only 1D ZnO nanorodarrays can be observed. When formamide moleculesare introduced into the initial reaction system, they can

accelerate the oxidation process on the zinc surface due tothe formation of a zinc–formamide complex [20]. Theresulting zinc–formamide complex can be transformed intoZnO at an elevated temperature through thermal decom-position. Due to the enhanced oxidation of metal zinc,numerous nuclei can be quickly formed, subsequentlysecondary growth produces flower-like ZnO nanosheetsfrom those already formed nuclei in Zn(OH)4

2� solution.Appropriate volume of formamide is a critical factor toproduce flower-like ZnO nanosheets. When the volume offormamide is decreased from 15 to 5mL, flower-like ZnOnanorods (Fig. 5C and D) can be obtained instead of ZnOnanosheets. Flower-like ZnO nanosheets and nanorods canbe obtained in the presence of 15 and 5mL formamide inthe Zn(OH)4

2� solution, respectively.

4. Conclusions

In this work, a useful method based on the NaCl solutioncorrosion-based strategy has been successfully designed forthe first time to synthesize ZnO nanorod arrays. The ideafor the synthesis of the ZnO nanorod array by chemicalreaction is based on a one-step process, which involvesdirectly growing ZnO nanoarrays on the zinc substrate.This growth method shows some advantages comparedwith other reports, such as the use of simple equipments,low-temperature reaction, low cost, less hazard, and noneed for the use of metal catalysts. This method can help usto fabricate a massively large number of nanodevices forfuture circuits leading to an unprecedented device-density,ultimately making the semiconductor nanoarray-based

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Fig. 5. SEM images of ZnO nanoarrays grown on the zinc foil substrate in the Zn(OH)42� solution at 120 1C with different volumes of formamide:

(A, B) 15mL; (C, D) 5mL.

C. Yan, D. Xue / Journal of Crystal Growth 310 (2008) 1836–18401840

devices a commercial reality with a major improvement inthe cost/performance ratio in future.

Acknowledgments

The authors gratefully acknowledge the financial supportof Program for New Century Excellent Talents in University(NCET-05-0278), the National Natural Science Foundationof China (NSFC no. 20471012), a Foundation for the Authorof National Excellent Doctoral Dissertation of PR China(FANEDD no. 200322), the Research Fund for the DoctoralProgram of Higher Education (RFDP no. 20040141004)and the Scientific Research Foundation for the ReturnedOverseas Chinese Scholars, State Education Ministry.

References

[1] Z.L. Wang, J. Phys.: Condens. Matter 16 (2004) R829.

[2] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv.

Mater. 13 (2001) 113.

[3] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott,

M.A. Rodriguez, H. Konishi, H. Xu, Nature Mater. 2 (2003) 821.

[4] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 16744.

[5] Z.L. Wang, J.H. Song, Science 312 (2006) 242.

[6] L. Vayssieres, Adv. Mater. 15 (2003) 464.

[7] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber,

R. Russo, P. Yang, Science 292 (2001) 1897.

[8] C. Yan, D. Xue, Adv. Mater. (2007), doi:10.1002/adma.200701752.

[9] J. Xu, D. Xue, J. Phys. Chem. B 110 (2006) 11232.

[10] J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215.

[11] C. Geng, Y. Jiang, Y. Yao, X. Meng, J.A. Zapien, C.S. Lee,

Y. Lifshitz, S.T. Lee, Adv. Funct. Mater. 14 (2004) 589.

[12] Y. Sun, G.M. Fuge, M. Ashfold, Chem. Phys. Lett. 396 (2004) 21.

[13] C. Yan, D. Xue, J. Alloys Compd. 431 (2007) 241.

[14] C. Yan, D. Xue, Electrochem. Commun. 9 (2007) 1247.

[15] C. Yan, D. Xue, J. Phys. Chem. B 110 (2006) 1581.

[16] C. Yan, D. Xue, J. Phys. Chem. B 110 (2006) 7102.

[17] C. Yan, D. Xue, J. Phys. Chem. B 110 (2006) 11076.

[18] C. Yan, D. Xue, J. Phys. Chem. B 110 (2006) 25850.

[19] T. Sounart, J. Liu, J. Voigt, J. Hsu, E. Spoerke, Z. Tan, Y. Jiang,

Adv. Funct. Mater. 16 (2006) 335.

[20] Z. Zhang, H. Yu, X. Shao, M. Han, Chem. Eur. J. 11 (2005)

3149.