Self-assembly of small ZnO nanoparticles toward

flake-like single crystals

Lili Wu, Youshi Wu *, Youzhen Lu

College of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, PR China

Received 31 January 2005; received in revised form 4 June 2005; accepted 15 July 2005

Available online 15 August 2005

Abstract

Flake-like single-crystalline ZnO nanocrystals with porous structure have been achieved, in which precursor of

Zn4CO3(OH)6�H2O was first prepared by mild hydrothermal method with urea as the homogeneous precipitant and

decomposed into small ZnO nanocrystals after being calcined at 400 8C, then the small ZnO nanocrystals self-

assemble to form flake-like ZnO aggregates. The ZnO nanoflakes have lateral dimensions up to micrometer with

the plane normal to [0 0 1] direction. The UV–vis absorption reveals that the ZnO nanoflakes have strong

absorption in the UV region. The advantages of our method for the synthesis lie in the low temperature and mild

reaction condition, which permit large-scale production at low cost.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; A. Semiconductors; B. Crystal growth; D. Microstructure

1. Introduction

Recently, nanoparticles of metal oxides have been the focus of a number of research efforts due to the

unusual physical properties that are expected upon entering this size regime. Control and manipulation of

the morphology of the nanoparticles will ultimately dictate the electrical and optical properties of the

final devices. Self-assembly of nanoparticles into larger aggregates is an important mechanism of particle

growth in the solution-phase. The formation of secondary particles with typical sizes in the micrometer

is a well-known phenomenon in classic colloid chemistry [1]. Most of the reports concerned with the

www.elsevier.com/locate/matresbu

Materials Research Bulletin 41 (2006) 128–133

* Corresponding author. Tel.: +86 531 8392724.

E-mail address: wllzjb@eyou.com (Y. Wu).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.07.031

self-assembly of nanoparticles are about the ligand-stabilized nanoparticles with sizes of a few

nanometer into two- and three-dimensionally ordered arrays [2–4]. Recently, Penn and Banfield showed

that aggregates of nanocrystalline titania particles coarsen under hydrothermal conditions, which behave

a growth mechanism named oriented attachment [5,6]. It involves spontaneous self-assembly of adjacent

particles so that they share a common crystallographic orientation, followed by joining of these particles

at a planar interface.

It is well known that ZnO is a direct band gap semiconductor with a room temperature energy gap of

3.37 eV. It has diverse applications, such as sensors, catalysts, varistors, surface acoustic wave devices,

electronic devices, photo- and electroluminescent devices [7,8]. Considering the promising properties of

micro- or nanostructured ZnO materials, many preparative techniques have been developed to fabricate

ZnO nanostructures. However, most synthetic efforts on ZnO have been directed to its nanowire,

nanorods, nanoribbons, as well as their derivates [9–12]. Being compared with that of 1D ZnO

nanostructures, 2D ZnO nanostructures have not been widely studied due to the lack of knowledge

on their synthesis. To the best of our knowledge, there are few reports on the synthesis of regular ZnO

nanoflake. Among the chemical routes, urea (CO(NH)2) is commonly used as the homogeneous

precipitant [13,14]. The advantage of urea process is that it is economical and environmentally safe.

It is considered to be an ideal precipitant that gradually decomposes to produce ample carbonate ions and

ammonia in aqueous solution at appropriate temperature about 75–100 8C. In this communication, we

describe the synthesis of ZnO precursors under mild condition with urea as precipitant and self-assembly

of the ZnO nanocrystals toward flake-like porous ZnO single crystal without the presence of organic

ligand. The resulting structure is expected to have potential in catalysis and optoelectronic devices.

2. Experimental

In a typical experimental procedure, ZnO precursor nanoparticles were first prepared by a mild

hydrothermal method: 5 mmol Zn(Ac)2�2H2O and 0.1 mol CO(NH)2 were dissolved in 100 ml distilled

water at room temperature to form transparent solution. A 40 ml of the mixed solution was transferred

into a Telfon-lined autoclave of 60 ml and sealed. The autoclave was heated in a electronic furnace at

temperatures 100 8C for 6 h. It was cooled by cold water to stop the reaction. The product was

centrifugalized, and washed with distilled water and ethanol and dried to obtain the precursor.White ZnO

powders were obtained by calcining the precursor in a muffle furnace at 4008 C for 2 h.

Powder X-ray diffraction (XRD) was performed on a Bruker D8-advance X-ray diffractometer with Cu

Ka (l = 1.54178 A) radiation. The size and morphology of the products were determined using a Hitachi

model H-800 transmission electron microscope (TEM) performed at 200 kV. UV–vis absorption spectra

were recorded using a 760 CRT UV–vis double-beam spectrophotometer with a deuterium discharge tube

(190–350 nm)anda tungsten iodine lamp(330–900 nm).Photoluminescence (PL) spectrumwasperformed

at room temperature using a FLS920 fluorescence spectrophotometer with a Xe lamp.

3. Results and discussion

In the present procedure, Zinc carbonate compound was formed after the hydrothermal reaction and it

decomposed into ZnO after being calcined at 400 8C for 2 h. X-ray diffraction (XRD) patterns of the

L. Wu et al. /Materials Research Bulletin 41 (2006) 128–133 129

precursor and the calcined product are shown in Fig. 1. All diffraction peaks in Fig. 1a can be indexed to a

zinc compound Zn4CO3(OH)6�H2O (JCPDS Card No 11–287). It is not full crystallized seen from the

XRD pattern. Fig. 1b is the calcined product of the precursor. The diffraction pattern agrees well with

wurtzite type ZnO (JCPDS card No 36–1451, a = 3.249 A and c = 5.206 A). No impurity peaks were

found, whichmeans that all precursor was decomposed into ZnO under the present condition and the ZnO

product was fully crystallized seen from the XRD pattern.

The size and morphology of the ZnO nanocrystals are shown in Fig. 2. Fig. 2a and b shows the typical

morphology of ZnO nanocrystals obtained in the absence of organic ligand. It can be seen that well-

defined and discrete ZnO nanoflakes were formed. It is interesting that the structures inclined to self-

restrict to form parallelogram shape with regular, parallel boarder. The length of the boarder ranges from

0.65 to 1.5 mm. The structure is composed of hundreds of individual small nanoparticles with an average

diameter of about 20–30 nm. The corresponding selected-area electron diffraction (SAED) shown in

Fig. 2c indicates that the structure is just in the evolution process from poly-crystalline to single crystal.

The character of single-crystal diffraction spots and poly-crystalline diffuse rings all can be seen from the

SAED pattern. When the product was pretreated by ultrasonic water bath for several minutes before

determined by TEM, the parallelgram shape that was in the formation process from poly-crystalline to

single-crystals will be destroyed just as shown in Fig. 2d–e. From Fig. 2d, we can see that the

nanocrystals self-assembled one by one, leaving worm like pores. A magnification of the image further

demonstrates that the ZnO nanocrystals have oriented attachment and form a whole single-crystal flake

almost in micrometer. The corresponding SAED pattern in Fig. 2e reveals that the left flake-like structure

is perfect single crystals and further analysis indicates that the plane of the flake normal to [0 0 1]

direction (c-axis).

UV–vis spectrum of the ZnO nanoflakes, which were ultrasonically dispersed in absolute ethanol, is

given in Fig. 3. The shown spectrum is corrected for the solvent contribution. A strong absorption in the

UV region is observed at wavelength from about 200 to 400 nm. There is a well-defined exciton

L. Wu et al. /Materials Research Bulletin 41 (2006) 128–133130

Fig. 1. XRD patterns of (a) the precursor and (b) ZnO nanocrystals.

absorption at 356 nm, dramatically blue shifted than bulk ZnO (373 nm), which may be attributed to the

quantum confinement effect. The room temperature PL spectrum is shown in Fig. 3b. There are obvious

violet-blue emissions at 417 and 468 nm and a strong visible emission at 605 nm in the yellow region.

Similar emission bands have also been observed in previous reports [15,16]. The yellow band can be

attributed to intrinsic defects in ZnO as oxygen interstitials [15]. In the case of violet-blue emissions, they

may originate from the recombination of oxygen vacancies with oxygen interstitials or other defects [17].

The exact mechanism is not yet clear. The difference of the optical properties between the present ZnO

nanosheets and the previous reports [18] may originate from the lattice defects related to either the

oxygen interstitials or the Zn vacancies during the decomposition.

Presently, we can only speculate why, in this communication, parallelogram flake not nanorod was

formed. It is well known that ZnO species inclined to oriented growth along one axis to form rod-like

morphology in solutions. In solution condition, no matter the growth process were assisted by organic

ligand or by solution evaporation, the building units can move freely. So they can in a great range to

move, rotate and attach on a definite crystal face which lead to anisotropic growth. In the present work,

the precursor was calcined in powder form and the nanoparticles can only rotate in a limited scope. In

order to decrease the high surface energy, the adjacent particles will self-assembly and oriented grow

L. Wu et al. /Materials Research Bulletin 41 (2006) 128–133 131

Fig. 2. TEM images of (a), (b) and (c) the obtained ZnO nanocrystals and the corresponding SAED pattern; (d), (e) and (f) ZnO

nanocrystals treated by ultrasonic before determination and the corresponding SAED pattern.

one-by-one toward the formation of large single-crystal. In the experiment, when additional 5 mmol

CTAB was added in the initial solution to prepare the precursor, TEM images indicate that the size and

morphology of ZnO product is just the same as the structure obtained without organic ligand. For the

reason that the precursor was washed before being calcined, the organic ligand has been removed and it

will not influence the morphology of the ultimate product. However, the addition of CTAB does improve

L. Wu et al. /Materials Research Bulletin 41 (2006) 128–133132

Fig. 3. (a) UV–vis absorption spectra of ZnO nanocrystals and (b) room temperature PL spectra of ZnO nanocrystals.

the crystalline state of the product, which may be because that it has some assisting effect on the crystal

growth of the precursor.

In conclusion, we have demonstrated the decomposition of Zn4CO3(OH)6�H2O precursor into ZnO

nanocrystals and their self-assembly toward flake-like nanocrystals with worm-like pores. The UV–vis

absorption and PL measurements reveal that the ZnO nanoflakes have strong optical absorption in UV

region and strong visible emission. The method is simple and low cost without the use of surfactant or

template. The produced ZnO nanostructure, distinctive from 1D ZnO nanostructure materials, could have

possessed more interesting properties and enhanced their specific uses for solar cell conversion, catalysis

and opto-electronic devices. It is expected that the present method can be easily extended to the similar

nanostructures of other oxide materials.

References

[1] I. Park, V. Privman, E. Matijevic, J. Phys. Chem. B 105 (2001) 11630–11635.

[2] C.P. Collier, T. Vossmeyer, J.R. Health, Annu. Rev. Phys. Chem. 49 (1998) 371–404.

[3] Y. Zhou, M. Antonietti, J. Am. Chem. Soc. 125 (2003) 14960–14961.

[4] W. Lu, J. Fang, K.L. Stokes, J. Lin, J. Am. Chem. Soc. 126 (2004) 11798–11799.

[5] J.F. Banfield, S.A. Welch, H. Zhang, T.T. Ebert, R.L. Penn, Science 289 (2000) 751–754.

[6] R.L. Penn, J.F. Banfield, Science 281 (1998) 969–971.

[7] Y. Du, M.S. Zhang, J. Hong, Y. Shen, Q. Chen, Z. Yin, Appl. Phys. A 76 (2003) 171.

[8] W.D. Kingery, Introduction to Cermics, John Wiley & Sons, New York, 1960.

[9] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897.

[10] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.

[11] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653.

[12] C. Pacholski, A. Kornowski, H. Weler, Angew. Chem. Int. Ed. 41 (2002) 1188.

[13] L. Wu, Y. Wu, H. Wei, et al. Mater. Lett. 54 (2004) 2700–2703.

[14] R.A. Caruso, M. Antonietti, Chem. Mater. 13 (2001) 3272–3282.

[15] L.X. Dai, L. Chen, W.J. Wang, T. Zhou, B.Q. Hu, J. Phys. Condens. Matter 15 (2003) 2221.

[16] Q.P. Wang, D.H. Zhang, Z.Y. Xue, X.T. Hao, Appl. Surf. Sci. 201 (2002) 123.

[17] S. Mahamuni, K. Borgohain, B.S. Bendre, V.J. Leppert, S.H. Risbud, J. Appl. Phys. 85 (1999) 2861.

[18] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113.

L. Wu et al. /Materials Research Bulletin 41 (2006) 128–133 133