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Optical Materials 28 (2006) 418–422

Synthesis of ZnO nanorod and the annealing effect onits photoluminescence property

Lili Wu a, Youshi Wu a,*, Xiaoru Pan b, Fanyuan Kong b

a College of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, PR Chinab Department of Chemistry, Weifang College, Weifang, Shandong 261041, PR China

Received 24 September 2004; accepted 15 March 2005

Available online 3 June 2005

Abstract

Single crystal ZnO nanorod has been prepared by hydrothermal method. Optical properties of the nanorod were studied by

annealing the nanorod at different ambiences. The annealed and unannealed samples were characterized by X-ray diffraction

(XRD), tansmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectros-

copy (FTIR), UV–Vis absorption (UV) spectroscopy and photoluminescence (PL) spectroscopy. The photoluminescence spectra

under excitation 325 nm for the as-prepared ZnO nanorod show three bands: UV emission at 386 nm, blue emission at 468 and

orange emission at 640 nm. By annealing the crystals in ammonia gas at 600 �C, the PL spectra shows obvious UV near band-edge

emission at 386 nm and a green emission at 510 nm. While annealing the crystals in air, the orange emission was greatly enhanced.

� 2005 Elsevier B.V. All rights reserved.

Keywords: Oxides; Annealing; Chemical synthesis; Optical properties

1. Introduction

ZnO is an interesting oxide material. As a wide band

gap semiconductor (3.37 eV), it has attracted consider-

able attention due to its promising applications as ultra-

violet light-emitting diodes (LED) and laserdiodes [1,2].

ZnO normally forms in the hexagonal (wurtzite) crystal

structure with a = 3.25 A and c = 5.12 A. The Zn atoms

are tetrahedrally coordinated to four O atoms, wherethe Zn d-electrons hybridize with the O p-electrons.

Layers occupied by zinc atoms alternate with layers

occupied by oxygen atoms. ZnO has the same crystal

structure as GaN and has a larger binding energy

(60 meV), which allows UV lasing action to occur

even at room temperature. It is a potential material

for the next generation UV and blue semiconductor

0925-3467/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2005.03.007

* Corresponding author.

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

optoelectronic devices. The optical properties of ZnO,studied using photoluminescence, photoconductivity,

and absorption, reflect the intrinsic direct band gap,

a strongly bound exciton state, and gap states due to

point defects [3,4]. The photoluminescence (PL) spectra

show ultraviolet (UV) near-band edge emission around

380 nm and defect-related deep-level emission, which

strongly depends upon the preparation methods and

growth condition. Recently, many methods have beenused to fabricate ZnO films and nanomaterials with

photoluminescence properties. In order to improve the

efficiency of ZnO-based phosphor, extensive experimen-

tal and theoretical works have been carried out [5–7]. In

these works, most are focus on doped and undoped ZnO

films deposited on variable substrates. In this paper, un-

doped ZnO nanorod phosphor has been made by hydro-

thermal method and the optical properties weremeasured. Anneal step in different ambiences has been

carried out attempting to improve the photolumines-

cence property.

L. Wu et al. / Optical Materials 28 (2006) 418–422 419

2. Experimental

All the chemicals used in this study were analytical

grade and used without further purification. In a typical

procedure, 0.5 M ZnCl2 aqueous solution (120 ml) was

mixed with 2 M diluted ammonia solution (about70 ml) slowly under stirring until pH = 6.7. After the

reaction completed, the precipitate was aged, centrifu-

galized, and washed with distilled water and ethanol

for more than three times. Precursor was obtained

by drying the resulting product in air at 60 �C for

10 h. XRD pattern (not showed) indicated that the

component of the precursor is ZnCl2 Æ 4Zn(OH)2compound.

Appropriate amount of the precursor powder (0.98 g)

was dispersed in 20 ml distilled water, then 0.5 M CTAB

solution 20 ml was added. 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 at

180 �C for 12 h. The obtained product was centrifugal-

ized, washed with distilled water and ethanol and dried.Parts of the as-prepared ZnO samples were then heat

treated in a tube furnace with constant flowing high pure

ammonia gas (99.9%) at 600 �C for 2 h with a gas rate of

160 cm3 min�1. Then the furnace was cooled to room

temperature. For comparison, another part of the ob-

tained ZnO nanorod was heat treated in the furnace in

air at 600 �C for 2 h. Tough the color of the annealed

samples is all white, the heat treated samples obtainedin ammonia gas is a little darker than the one obtained

in air.

Powder X-ray diffraction (XRD) was performed on a

Bruker D8-advance X-ray diffractometer with Cu-Ka(k = 1.54178 A) radiation. The 2h range used in the mea-

surement was from 20� to 70� in steps of 0.02� s�1. The

size and morphology of the precursor and the product

were determined using a Hitachi model H-800 transmis-sion electron microscope (TEM) performed at 200 kV.

Fourier transform infrared absorption spectroscopy

(FTIR) spectra were measured at room temperature

on a FTIR spectrometer (Nicolet 7900) using the KBr

Pellet technique to determine the structure of the prod-

uct. UV–Vis absorption spectra were recorded using a

760 CRT UV–Vis double-beam spectrophotometer with

a deuterium discharge tube (190–350 nm) and a tungsteniodine lamp (330–900 nm). The scanning wavelength

range is 200–800 nm. Photoluminescence (PL) spectra

were performed at room temperature using a He:Cd

laser with a wavelength of 325 nm. X-ray photoelectron

spectrometer (XPS) analyses were conducted to examine

the chemical composition of the samples annealed in

ammonia gas. A PHI-5702 multifunction X-ray photo-

electron spectrometer was used, working with an Al-Ka X-ray source of 29.35 eV passing energy.

3. Results and discussion

Structure and morphology of the harvested ZnO

nanorod were examined by TEM. Typical TEM images

with different magnifications are shown in Fig. 1. The

TEM images show that the obtained samples arerelatively straight and uniform with diameters between

100 and 180 nm and length between 600 and 750 nm,

respectively. A typical selected-area electron diffraction

(SAED) is shown in Fig. 1c. The ED reveals that the ob-

tained ZnO nanorods exhibit a single-crystal structure

with wurtzite type, which is in agreement with the

X-ray powder diffraction (XRD) patterns. From Fig. 1d

and e we can see that there is no obvious particle size dif-ference between the annealed and unannealed samples.

But the borderlines of the annealed nanorods are clearer

than the unannealed samples which indicate that the

crystal quality was improved after annealing. The

XRD patterns of the as-prepared nanorod and the an-

nealed ZnO samples are shown in Fig. 2. All the peaks

of the nanorods obtained under different conditions

can be indexed to the wurtzite ZnO (JCPDS card No.36-1451, a = 3.249 A and c = 5.206 A) with high crystal-

linity. No characteristic peak of other new phase related

to N element was observed in the samples that annealed

in ammonia gas.

Fig. 3 exhibits FTIR spectra for the obtained and an-

nealed ZnO nanorods. In the infrared region, if the ZnO

particle morphology changes from spherical to needle-

like shape, the spectra often show two absorption max-ima at around 512 cm�1 and 406 cm�1 [8]. In Fig. 3, the

spectra show a characteristic ZnO absorption at

�564 cm�1 for the as-prepared nanorods and

�514 cm�1 for the annealed nanorods, blue-shifted

than 512 cm�1. And there will be another maximum at

wavenumbers shorter than 500 cm�1, which cannot be

detected by the present spectrometer. The broad absorp-

tion in �3340 cm�1 and �1630 cm�1 are assigned to theexistence of hydroxyl groups on the surface of the sam-

ples and the absorption in �2360 cm�1 is because of the

existence of CO2 molecular in air. Fig. 3c is the spectra

of the nanorods annealed in ammonia. No new stretch

mode of an N–H bond (about 3194 cm�1) and a lower

frequency peak (about 783 cm�1) were observed in the

present spectra [9], indicating that there are no physi-

cally absorbed NH3 on the nanorods surface and nodoped or little doped N atom in the samples.

The UV–Vis absorption spectra of the ZnO nanorods

at room temperature are shown in Fig. 4. The UV

absorption in all cases behaves very similarly. The

absorption spectra have a narrow peak near the band

edge in the exciton absorption region (at about

381 nm) and red-shifted relative to the bulk exciton

absorption (373 nm). From the spectra curves, one cansee the band edge absorption begin with the wavelength

Fig. 1. TEM images of (a) and (b) the as-prepared ZnO nanorod with different magnifications; (c) the selected-area electron diffraction of (b);

(d) sample annealed in air at 600 �C for 2 h; (e) sample annealed in ammonia gas at 600 �C for 2 h.

20 30 40 50 60 70

Inte

nsity

(a.u

.)

c

b

a

(201

)(1

12)

(200

)(1

03)

(110

)

(102

)

(101

)(0

02)

(100

)

2θ/degree

Fig. 2. XRD patterns of ZnO nanorods (a) the as-prepared sample;

(b) sample annealed in air at 600 �C for 2 h; (c) sample annealed in

ammonia gas at 600 �C for 2 h.

4000 3000 2000 1000 00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

wavenumber (cm-1)

c

b

a

Tran

smis

sion

(%)

Fig. 3. FTIR spectra of ZnO nanorods (a) the as-prepared sample;

(b) sample annealed in air at 600 �C for 2 h; (c) sample annealed in

ammonia gas at 600 �C for 2 h.

420 L. Wu et al. / Optical Materials 28 (2006) 418–422

at �800 nm suggests that more absorption states or de-

fect energy bands exist in the samples.

X-ray photoelectron spectra (XPS) of the nanorod

annealed in ammonia gas were obtained. However, theO:Zn:N ratio cannot be obtained from it�s XPS spectra

because the peak related to N is too low. NH3 will

decompose at high temperatures (610 �C) before N is

introduced into the ZnO film [10]. In the present exper-

imental condition, the decompose ratio of NH3 is very

low and little N atom was introduced into the samples.

But the photoluminescence changed after annealed

under this ambience.

The photoluminescence properties for the as-prepared and annealed ZnO nanorods are examined.

300 400 500 600 700

0

1000

2000

3000

4000

5000

6000

inte

nsity

(a.u

.)

wavelength (nm)

c

b

a

Fig. 5. Room temperature PL spectra of ZnO nanorods (a) the

as-prepared samples; (b) samples annealed in air at 600 �C for 2 h;

(c) samples annealed in ammonia gas at 600 �C for 2 h.

300 400 500 600 700

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

abso

rptio

n

wavelength (nm)

c

ba

Fig. 4. UV–Vis absorption spectra of ZnO nanorods (a) the as-

prepared sample; (b) sample annealed in air at 600 �C for 2 h; (c)

sample annealed in ammonia gas at 600 �C for 2 h.

L. Wu et al. / Optical Materials 28 (2006) 418–422 421

Fig. 5 shows photoluminescence spectra of the nanorodsat room temperature, where the 325 nm line of a He–Cd

laser is used as an excitation source. The main features of

the PL spectra can be divided into two categories: the

near band-edge emission and the deep-level emissions.

For the as-prepared ZnO nanorod (Fig. 5a), the emis-

sion band is composed of a weak UV band around

386 nm, a blue band around 468 nm and an orange band

around 640 nm. The UV emission band must be ex-plained by a near band-edge transition of wide band

gap ZnO nanorods, namely the free excitons recombina-

tion through an excition–excition collision process [11].

Similar orange band had also been observed and it

was attributed to intrinsic defect in ZnO as oxygen inter-

stitials [12,13]. After the nanorod was heat treated at

600 �C in air, the shape and position of the emission

peaks are not changed while the relative intensity of

the orange peak was enhanced greatly. The UV emission

intensity is increased when the nanorod was annealed in

ammonia gas at 600 �C, which means that the quality of

the ZnO nanorod crystals was improved after this step.

At the same time, the green emission appeared and the

orange disappeared after annealing in ammonia.The orange emission was observed in ZnO doped

with lithium and other impurities [14]. Our results show

that orange PL can be obtained in undoped ZnO nano-

rod and the orange emission was enhanced greatly in the

air annealed samples and disappear in the ammonia

annealed samples. We think the orange band was not

because of impurities. Since the PL spectra of the

annealed samples come from the same as-preparedZnO nanorod, the orange PL band of the ZnO samples

annealed in ammonia gas should not disappear if it were

due to an impurity. From the conclusion of Studenikinm

[13], both green and orange PL was related to the

amount of oxygen in the sample and not due to trance

amounts of dopants. The green PL came from oxygen

deficient samples prepared by reductive annealing gas,

and the orange from oxygen rich samples. Green and or-ange emissions were not observed simultaneously. We

can conclude that the as-prepared ZnO nanorod has

strong ability to absorb oxygen to form oxygen intersti-

tials defects on the surface. When the as-prepared nano-

rod was annealed in air, oxygen is ample and the

concentration of oxygen interstitials on the surface in-

creased, which lead to the enhancement of orange emis-

sion. When the as-prepared nanorod was annealed inammonia gas, oxygen is deficient. And at the same time,

a little H atom come from the decomposed NH3 can

combine with O atom, and take away the oxygen on

the surface.

For the green emission, according to Vanhesusden

et al. [15,16], we do not think that the green emission

related defects are not formed in the as-prepared and

air heat treated ZnO nanorods. It may be that in thesetwo samples, green emission related defects are copro-

duced with other defects and forms defect complex,

where the energy level of the complex provides recombi-

nation route for blue light within the band gap of ZnO.

Or blue band is due to the radiative recombination of

carriers between the green related energy level and a

newly formed energy. To identify the defects responsible

for blue luminescence in ZnO, further investigations arestill needed.

4. Conclusion

In conclusion, ZnO nanorod phosphors have been

synthesized by hydrothermal method. PL spectra exhibit

a weak UV emission at 386 nm attributed to near band-edge emission, a blue emission and an orange emission

attributed to oxygen-related defects. Orange emission

422 L. Wu et al. / Optical Materials 28 (2006) 418–422

of the air-annealed nanorods was increased greatly when

there is rich oxygen. While annealing the as-prepared

nanorod in ammonia gas, XPS measurement shows that

the N atom was not doped in the samples. However, in

the present condition, the orange emission disappeared

and green emission appeared which is due to the defi-cient of oxygen in this reducing atmosphere. The present

work will throw a light on the understanding of defect-

related photoluminescence and contribute to the fabri-

cation area of optical apparatus based on ZnO

nanomaterials.

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