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Applied Surface Science 252 (2005) 1436–1441

Morphology development and oriented growth

of single crystalline ZnO nanorod

Lili Wu, Youshi Wu *, Wei Lu, Huiying Wei, Yuanchang Shi

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

Received 14 January 2005; received in revised form 20 February 2005; accepted 20 February 2005

Available online 7 April 2005

Abstract

Single crystalline ZnO nanorods were achieved by the assembly of nanocrystallines in tens of nanometer under hydrothermal

conditions with the assistance of surfactant cetyltrimethylammonium bromide (CTAB). The obtained nanorod has rough surface

as a result of oriented attachment growth. Transmission electron microscope (TEM) images showed the morphology evolution of

the nanorod at different reaction time. Defects were observed and porous structure was left after the assembly of hundreds of

nanocrystalline building blocks. Effect of pH condition on the morphology of the nanorod was also investigated.

# 2005 Elsevier B.V. All rights reserved.

PACS: 68.65

Keywords: ZnO; Nanorod; Hydrothermal method; Oriented attachment

1. Introduction

Liquid-phase methods have played important role

for the synthesis of crystalline nanoparticles with

specific shape and sizes, which will greatly influence

the properties of nanomaterials. This soft chemical

solution process provides a mild condition with a low

cost and effective way to fabricate one-dimensional

(1D) nanomaterials. And it has been successfully used

to grow many 1D oxides [1–5]. However, because of

too many unpredictable parameters influential on the

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

fax: +86 531 8392724.

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

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.02.117

nucleation and growth of the nanocrystals, it is more

difficult than in high-temperature solid–gas-phase

reactions to clearly interpret the intrinsic mechanism

involved in the phase and morphology formation

during this chemical-solution method. Thus, a funda-

mental understanding of the growth process is

necessary to control and manipulate the morphology

of nanoparticles, which will ultimately dictate the

electrical and optical properties of the final devices.

For basic crystal growth mechanism in solution

system, ‘Ostwald ripening’ is generally believed to be

the main path of crystal growth. In the Ostwald ripening

process, tiny crystalline nuclei first formed in a

supersaturated medium and then is followed by crystal

growth, in which the larger particles will grow at the

.

L. Wu et al. / Applied Surface Science 252 (2005) 1436–1441 1437

cost of small ones due to the energy difference between

large particles and the smaller particles of a higher

solubility based on the Gibbs–Thompson law [6,7]. In

several recent years, a different view of crystal growth

emerged which described as ‘oriented attachment’ or

‘oriented aggregation’ by Penn and Banfield [8–10].

They observed that anatase and iron oxide nanoparticls

with sizes of a few nm can coalesce under hydrothermal

conditions with the mechanism they call ‘oriented

attachment’. In this pathway, the bigger particles are

grown from small primary nanoparticles by orientated

attachment, in which the adjacent nanoparticles are

self-assembled by sharing a common crystallographic

orientation and docking of these particles at a planar

interface. In the so formed aggregates, the crystalline

lattice planes may be almost perfectly aligned or

dislocates at the contact areas between the adjacent

particles leading to defects in the finally formed bulk

crystals. This growth mechanism could provide a

unique route for the incorporation of defects in stress-

free and initially defect free nanocrystalline materials,

which could improve the reactivity of the nanocrystal-

line subunits [11].

ZnO is a wide band gap semiconductor with an

energy gap of 3.37 eV at room temperature. It is a

versatile material and has been used considerably for

its catalytic, electrical, optoelectronic and photoche-

mical properties [12–15]. ZnO has large exciton

binding energy (60 meV) that allows UV lasing action

to occur even at room temperature [16]. Recently, one-

dimensional ZnO nanomaterials such as nanorod have

attracted considerable attentions for its unique proper-

ties and potential applications. However, there are

only a few examples could illustrate the growth

process of the nanorod under hydrothermal condition

[17]. In this paper, ZnO nanorods have been prepared

by a precursor-hydrothermal method and the crystal

growth underwent both oriented attachment and

Ostwald ripening, in spite of the existence of organic

agent CTAB. The incorporation of the defects and the

rough surface are expected to improve the reactivity of

the nanorods.

2. Experimental procedure

All the chemicals used in this study were analytical

grade and used without further purification. Crystal-

line precursor was prepared firstly. In a typical

procedure, 0.5 M ZnCl2 aqueous solution was mixed

with diluted ammonia solution slowly under stirring.

After the reaction completed, the product was aged,

centrifugalized and washed with distilled water and

ethanol for more than three times. The precursor was

obtained by drying the resulting product in air at

60 8C. Then, appropriate amounts of the precursor

powder (0.98 g) were dispersed in distilled water,

20 ml CTAB (0.1 M) was added and the pH value is

adjusted by ammonia or (1 M) NaOH solution. The

mixture was transferred into a Telfon-lined autoclave

of 60 ml and pretreated by ultrasonic water bath for

30 min. After that, the autoclave was sealed and

hydrothermally heated for 12–24 h at 180 8C. The

obtained product was centrifugalized, washed with

distilled water and ethanol and dried.

Powder X-ray diffraction (XRD) was performed on

a Bruker D8-advance X-ray diffractometer with

Cu Ka (l = 1.54178 A) radiation. The 2u range used

in the measurement was from 88 to 708. The size and

morphology of the product were determined using a

Hitachi model H-800 transmission electron micro-

scope (TEM) and a Philips U2TWIN TEC NAI 20

high-resolution transmission electron microscope

(HRTEM) performed at 200 kV. UV–vis absorption

specta was recorded using a 760 CRT UV–vis double-

beam spectrophotometer with a deuterium discharge

tube (190–350 nm) and a tungsten iodine lamp (330–

900 nm).

3. Results and discussion

XRD patterns of the as-obtained precursor are

shown in Fig. 1. From Fig. 1, we can see that all the

peaks of the precursor can be indexed to a compound

ZnCl2�4Zn(OH)2 (JCPDS card no. 7-115). No

characteristic peaks of other species, such as Zn(OH)2

or ZnCl2 were detected. The precursor was fully

crystallized seen from the patterns.

Appropriate amount of the precursor was dispersed

in a mixture of 10 ml distilled water, 5 ml ammonia

and 20 ml (0.1 M) CTAB (pH is about 9) and

underwent a hydrothermal process for 12 h. Detailed

TEM investigations of the obtained ZnO sample are

shown in Fig. 2. The morphology of the sample is rod-

like with rough surface. The diameter is about 150 nm

L. Wu et al. / Applied Surface Science 252 (2005) 1436–14411438

Fig. 1. XRD patterns of the as-prepared precursor.

and length about 750 nm. XRD patterns in Fig. 3 show

that all the diffraction peaks of the sample can be

indexed to the wurtzite type ZnO with high crystalline

state. No impurity peaks were detected, which means

Fig. 2. TEM images of ZnO nanorod obtained from the hydrothermal pro

solvent: (b and c) are selected-area electron diffraction images of (a); the

nanorod.

that the entire crystalline precursors have decomposed

and grew into ZnO single crystals. In the sample,

except the rough nanorod, aggregates of nanoparticles

were also observed as shown in Fig. 2d and e. From

Fig. 2d, one can clearly see that the aggregates exist in

the cluster format. And the unit of the cluster is spindly

particle with diameter between 30 and 40 nm. From

the whole image of Fig. 2d, the clusters of

nanoparticles are just developing to grow into a

single nanorod, showing an early stage of rod

formation. The scheme in the lower part of Fig. 2

just illustrates the evolution process from the spindly

nanoparticles to bigger nanorod with rough surface.

Fig. 2b and c are typical selected area electron

diffractions (SAED) of Fig. 2a. The electron diffrac-

tions reveal that the ZnO nanorods exhibit a single

crystal structure with wurtzite type and the growth

direction of the nanorod is [0 1 1], which reveals that

the vector of attachment-driven growth is along

[0 1 1]. Such crystallographic specificity during

cess of 12 h, in which distilled water and ammonia was used as the

nether scheme from (d) to (a) illustrate the formation process of the

L. Wu et al. / Applied Surface Science 252 (2005) 1436–1441 1439

Fig. 3. XRD patterns of ZnO nanorod.

Fig. 4. TEM images of ZnO nanorod obtained from the hydrothermal pro

ammonia were used as the solvent. (a) Low-resolution TEM images; (b) hi

resolution TEM images of a rod boundary attached with small particles.

aggregation-based particle growth suggests the pos-

sibility of tailoring assembly to favor specific crystal

faces with the ultimate goal of achieving over particle

morphology.

Prolonged the reaction time from 12 to 24 h,

relatively straight and uniform ZnO nanorod was

obtained. The TEM image is shown in Fig. 4. The

length of the nanorod is increased and rough surface is

smoothened. Fig. 4b shows a high-resolution TEM

micrograph of the ZnO nanorod. Variations in contrast

reveal the incorporation of porosity and the presence

of internal interfaces between primary nanoparticles.

There is visible misorientation between regions of this

particle seen from the IFFT images. The incorporation

of porosity and defects in the overall morphology of

these particles suggests oriented aggregation as the

primary growth mechanism [18]. Ocana et al. [19]

cess of a prolonged reaction time 24 h, in which distilled water and

gh-resolution TEM images, the inset figure is IFFT images; (c) high-

L. Wu et al. / Applied Surface Science 252 (2005) 1436–14411440

Fig. 5. TEM images of ZnO nanorod obtained from the hydro-

thermal process for reaction time 12 h, in which distilled water was

used as the solvent and the pH is adjusted by NaOH. Inset is the

selected-area electron diffraction images.

Fig. 6. UV–vis absorption spectra of ZnO nanorod (pH 9 and

reaction time 24 h).

have investigated the growth mechanism of Fe2O3

where by primary particles aggregate to produce

porous and elongated ‘‘monocrystals’’. From the high

resolution of Fig. 4c, we can see that the smooth

surface in the low resolution is not really smooth.

Steps were observed on the surface and small particles

which have same orientations with the big rod

attached on the boundary of it. These branching

particles will behave as reaction sites to enhance the

reactivity of the finally formed nanomaterials.

In the present paper, the surfactant CTAB may

serve as a transporter and modifier of the initial

spindly nanoparticles. The surfactant absorbs on the

surface of the nanoparticles and the particles appear to

favor aggregation, leading to the formation of clusters

in Fig. 2d and e. The formation of the cluster ensures

small distances between the nanoparticles [20]. After

the surfactant CTAB desorbed, the nanoparticles will

rotate and coarsen via oriented attachment to form a

single big rod. The incorporation of porosity and

defects does prove the oriented growth history. After

the formation of the big rod, the growth may both

occurs by oriented attachment and Ostwald ripening.

As a result of Ostwald ripening, smoother ZnO

nanorod was obtained after prolonging the reaction

time. The suggestion may be proved by the kinetic

study by Huang et al. [20].

In order to investigate the pH effect on the

morphology, ammonia is replaced by NaOH in the

experiment to adjust the pH to 10. After hydrothermal

reaction for 12 h, spindle shape nanorod was obtained

as shown in Fig. 5. From the TEM images, one can see

the nanoparticles aligned like a wall, where the second

layer of bricks just started to be put on the first.

Though the nanorods are just in the formation process,

the SAED pattern in the inset shows single crystal

state. And the growth direction of the nanorod is

[0 1 1], the same as the growth direction in Fig. 3. This

indicates that changing the pH condition, the

attachment vector was not changed, but the morphol-

ogy of the rod was changed greatly which may

resulted from the high concentration of OH�. A high

degree of control over ultimate morphologies and

particle microstructure may be possible by change the

reaction parameters.

The UV–vis absorption spectra of the ZnO nanorod

at room temperature is shown in Fig. 6. The absorption

spectra shows a well-defined exciton band at 381 nm

and red-shifted relative to the bulk exciton absorption

(373 nm) [21]. From the spectra curves, one can see

there is absorption almost in the whole violet and

visible region. The band edge absorption begin with

the wavelength at �800 nm suggests that more

absorption states or defect energy bands exist in the

samples which agrees well with the discussion on the

formation mechanism of the nanorods.

L. Wu et al. / Applied Surface Science 252 (2005) 1436–1441 1441

4. Conclusions

In summary, ZnO nanorods were obtained from a

crystal compound precursor by hydrothermal method

and the morphology development was observed

during the formation process. The growth process

of the nanorod was explained by both oriented

attachment and Ostwald ripening in the presence of

additives. The morphology can be controlled by

change the solution condition. The left porous

microstructure and defects will influence the chemi-

cal, electrical and optical properties of the final

materials. The light absorption in the visible region is a

result of it. The present work could offer an additional

tool to design advanced materials with anisotropic

material property and could be used for the synthesis

of more complex crystalline three-dimensional struc-

tures in which the branching sites could be added as

individual nanoparticles.

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