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doi:10.1016/j.ph

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Physica E 28 (2005) 76–82

www.elsevier.com/locate/physe

Preparation of ZnO Nanorods and optical characterizations

Lili Wu, Youshi Wu�, Wei LU

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

Received 26 January 2005; accepted 3 February 2005

Available online 21 April 2005

Abstract

Single crystalline ZnO nanorods have been prepared by hydrothermal method with synthesized ZnCl2 � 4Zn(OH)2 as

precursors. Morphologies of the nanorods were controlled by various reaction conditions with cetyltrimethylammo-

nium bromide (CTAB) as modifying agent. The nanorods were characterized by techniques such as XRD, TEM,

UV–Vis spectra, IR and PL spectra. Microstructure of holes in nanosize was observed on the surface of the nanorod.

The UV–Vis spectra indicated that the as-prepared ZnO nanorods have absorption of visible light as well as ultraviolet

light. Therefore, these nanorods may be good candidate for visible-light photocatalysis materials from the viewpoint of

practical applications. The reason for visible-light absorption was discussed. The photoluminescence property of the

nanorod was also investigated.

r 2005 Elsevier B.V. All rights reserved.

PACS: 61.82.Rx; 81.20.�n; 81.40.Tv; 82.80.Ch

Keywords: Zinc oxide; Nanorod; Surface defects; Ultraviolet absorption spectroscopy; Photoluminescence

1. Introduction

ZnO is a wide band gap semiconductor with anenergy gap of 3.37 eV at room temperature. It is aversatile material and has been used considerablyfor its catalytic, electrical, optoelectronic andphotochemical properties [1–4]. ZnO has largeexciton binding energy (60meV) which allows UVlasing action to occur even at room temperature [5]and ZnO with oxygen vacancies (ZnO:Zn) exhibits

e front matter r 2005 Elsevier B.V. All rights reserve

yse.2005.02.005

ng author. Tel.: +531 8392724.

ss: wllzjb@eyou.com (Y. Wu).

an efficient green emission. Recently, one-dimen-sional (1D) nanoscale materials have receivedconsiderable attention due to the remarkableproperties applied in optoelectronic and electronicnanomaterials. ZnO 1D nanomaterials such asnanorods and nanowires have been intensivelyinvestigated for their notable properties [6–9].Among them, most of the literature reported thephotoluminescence (PL) properties with UV emis-sion and few are dedicated to fabricate nanorodswith visible absorption. The ZnO semiconductorused as photocatalytic degradation materials ofenvironmental pollutants has also been extensively

d.

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studied, because of its advantages in non-toxicnature, low cost and high reactivity. However,such a photocatalytic degradation only proceedsunder UV irradiation because of its wide band gapand can only absorb UV light. Therefore, ZnO-based materials capable of visible-light photocata-lysis are required [10]. Herein, we report a simplehydrothermal method to synthesize ZnO nanorodwith red-shifted optical absorption.In the literature, many methods have been used

to fabricate 1D ZnO nanorods, such as metal-organic vapor-phase epitaxial growth (MOVPE)[11,12], anodic alumina template [13], commonthermal evaporation method [14–17] and softchemical solution method [18]. In the solutionhydrothermal method, always alkali solution ofZn(OH)4

2�or Zn(NH3)42� was used as precursors

[19,20]. In this paper, ZnCl2 � 4Zn(OH)2 was usedas precursors to synthesize ZnO nanorod using thea hydrothermal method and CTAB was used assurfactant and modifying agent. The opticalabsorption of the ZnO nanorod showed red shiftthan bulk ZnO powders. We believe that thepresented approach is a simple one to synthesizeZnO nanostructures for the practical applicationof photocatalysis materials. The PL property ofthe nanorod was also investigated.

2. Experimental

All the chemicals used in this study were ofanalytical grade and used without further purifica-tion. In a typical procedure, 0.5M ZnCl2 aqueoussolution was mixed with diluted ammonia solutionslowly stirring until pH ¼ 6:7. After the reaction wascompleted, the product was aged, centrifugalized,and washed with distilled water and ethanol morethan three times. The precursor was obtained bydrying the resulting product in air at 60 1C for 10h.Appropriate amounts of the prepared precursor

powder (0.98 g) were dispersed in 10–20ml dis-tilled water, then 20ml CTAB (0.1M) was addedand the pH value is adjusted to 8–10 by dilutedammonia solution or NaOH solution. The mixturewas transferred into a Teflon-lined autoclave of60ml and pretreated by ultrasonic water bath for30min. Then, the autoclave was sealed and

hydrothermally heated for 12–24 h at 180 1C. Theobtained product was centrifugalized, washed withdistilled water and ethanol, and dried.Powder X-ray diffraction (XRD) was performed

on a Bruker D8-advance X-ray diffractometerwith Cu Ka (l ¼ 1:54178 (A) radiation. Size andmorphology of the precursor and the product weredetermined using a Hitachi model H-800 transmis-sion electron microscope (TEM) performed at200 kv. Infrared absorption spectroscopy (IR)spectra were measured at room temperature on aFTIR spectrometer (Nicolet 7900) using the KBrPellet technique to determine the structure of theproduct. UV–Vis absorption spectra were re-corded using a 760 CRT UV–Vis double-beamspectrophotometer with a deuterium dischargetube (190–350 nm) and a tungsten iodine lamp(330–900 nm). The scanning wavelength range is200–800 nm. PL spectrum was performed at roomtemperature using a FLS920 fluorescence spectro-photometer with a Xe lamp.

3. Results and discussion

The phase composition of the prepared pre-cursor was determined by XRD techniques. XRDpatterns in Fig. 1 shows that all the diffractionpeaks are well consistent with the compoundZnCl2 � 4Zn(OH)2 (JCPDS card No. 7-115). Nocharacteristic peak of other species such asZn(OH)2 or ZnCl2 was detected. The precursorwas fully crystallized. XRD patterns of the as-obtained nanorods are shown in Fig. 2. All thepeaks of the nanorods prepared under variousconditions can be indexed to the wurtzite ZnO(JCPDS card No. 36-1451, a ¼ 3:249 (A andc ¼ 5:206 (A) with high crystallinity. No character-istic peak of impurities was detected which meansthat all the crystalline precursors decomposed andgrew into ZnO single crystals. In Fig. 2a, theintensity ratio of (1 0 0) peaks is notably enhancedthan in Fig. 2b and c and standard values whichmeans that this ZnO sample oriented grow in(1 0 0) direction. Employing the Scherrer equation,the sizes of the nanorods prepared in the absenceof CTAB are also larger than those prepared in thepresence of CTAB.

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10 20 30 40 50 60 70

0

10000

** ** *

*

*ZnCl2·4Zn(OH)2

** *

** *

*

**

**

***

*

* *

** * ***

*

2 Theta degree

Inte

nsity

(a.u

.)

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

20 30 40 50 60 70

0

50000

100000

2 Theta degree

(201

)(1

12)

(200

)

(103

)(110

)

(102

)

(101

)(0

02)

(100

)

c

b

a

Inte

nsity

(a.

u.)

Fig. 2. XRD patterns of ZnO nanorod prepared in the (a) absence of CTAB, reaction time 12 h (b) presence of CTAB, reaction time

12 h, pH 8 (c) presence of CTAB, reaction time 24 h, pH9.

L. Wu et al. / Physica E 28 (2005) 76–8278

Fig. 3 exhibits IR spectra for the as-obtainedZnO nanorods. In the infrared region, ZnOusually shows distinct absorption bands aroundwave numbers of 464 cm�1 and this maximumbroadens and splits into two maxima if the particlemorphology changes from spherical to a needle-

like shape. Therefore, reference spectra of ZnOpowders often show two absorption maxima ataround 512 and 406 cm�1 [21]. In Fig. 3, thespectra shows a characteristic ZnO absorption at�564 cm�1, blue-shifted at 512 cm�1. And therewill be another maximum at wave numbers shorter

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4000 3500 3000 2500 2000 1500 1000 5000.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

d

c

b

a

CO2 in air

Tra

nsm

issi

on(%

)

§Ò/cm-1

Fig. 3. IR spectra of ZnO nanorod prepared in the (a) presence of CTAB, reaction time 12 h, pH 8 (b) presence of CTAB, reaction time

12 h, pH 10 (c) presence of CTAB, reaction time 24 h, pH 9 (d) absence of CTAB, reaction time 12 h.

Fig. 4. TEM images of ZnO nanorod prepared in the presence of CTAB, reaction time 24 h, pH 9.

L. Wu et al. / Physica E 28 (2005) 76–82 79

than 500 cm�1 which cannot be detected by thepresent spectrometer. The broad absorption in�3340 and �1630 cm�1 are assigned to theexistence of hydroxyl groups on the surface ofthe samples.Structure and morphology of the ZnO nanorod

were examined by TEM. Typical TEM imageswith different magnifications are shown in Figs. 4and 5. Fig. 4 shows the TEM images of the as-

obtained ZnO nanorod synthesized by hydrother-mal method (180 1C, 24 h) in the presence ofCTAB with pH9 adjusted by diluted ammonium.The images show that the obtained samples arerelatively straight and uniform with diametersbetween 60 and 100 nm and length between 600and 800 nm. The aspect ratio of the correspondingnanorod is about 10. A typical selected-areaelectron diffraction (SAED) is shown in the inset

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Fig. 5. ZnO nanorod prepared in the system of (a) presence of CTAB, reaction time 12 h, pH 8 (b) presence of CTAB, reaction time

12 h, pH 10 (c) and (d) absence of CTAB, reaction time 12 h.

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of Fig. 4a. The ED reveals that the obtained ZnOnanorods exhibit a single-crystal structure withwurtzite type, which is in agreement with the XRDpatterns.On the surface of the TEM images, nanosized

holes were clearly observed in Fig. 4b. The averagediameter of the holes is about 5 nm. This micro-structure could be caused by the oriented aggrega-tion of the ZnO nanocrystals, which was observedin the formation of other nanoparticles by hydro-thermal method [22]. From the viewpoint ofcrystal growth, oriented aggregation differentfrom Ostwald ripening, is investigated as one ofthe important crystal growth mechanisms [22]. In

the experiments by Penn and Banfield, theyobserved that anatase and iron oxide nanoparticleswith sizes of a few nm can coalesce underhydrothermal conditions in a way they call‘‘oriented attachment’’ [23]. In the so-formedaggregates, contact areas between the adjacentparticles lead to defects in the finally formed bulkcrystals [23] and the defects may improve thereactivity of the final product. It may be a goodreason for the strong optical absorption in thevisible region discussed in the later parts. Smallparticles are still visible from the TEM imagessuch as the nanorods as marked by the arrow inFig. 4c.

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200 300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

d

c

ba

abso

rptio

n

Wavelength (nm)

Wavelength (nm)

300 400 500 600 7000

3000

6000

e

640nm

468nm

386nm

inte

nsity

(a.

u.)

Fig. 6. UV–Vis absorption spectra of ZnO nanorod prepared

in the (a) presence of CTAB, reaction time 12 h, pH 8 (b)

presence of CTAB, reaction time 12 h, pH 10 (c) presence of

CTAB, reaction time 24 h, pH 9 (d) absence of CTAB, reaction

time 12 h; (e) PL spectrum of the ZnO nanorod obtained in the

reaction system of (c).

L. Wu et al. / Physica E 28 (2005) 76–82 81

Fig. 5a shows the TEM images of the ZnOnanorod prepared by the same method withreaction time 12 h, but the pH is not adjustedand the initial pH is about 8. From the TEMimages, the diameter and length of the nanorod are100–180 and 550–750 nm, respectively, and theaspect ratio is less than that of the nanorodobtained under pH 9. A typical SAED patternshown in the inset of Fig. 5a reveals that theobtained wurtzite-type ZnO nanorod is also singlecrystal and the growth direction is [0 1 1]. Whenthe initial pH was adjusted to 10 by NaOHsolution, with the increase of basicity, spindleshaped nanorod was obtained (Fig. 5b). Thoughthe nanorod does not exhibit well-defined crystal-lographic faces, the SAED pattern in the insetshows single-crystal structure and the growthdirection is also [0 1 1]. These nanorods are a goodevidence of ‘‘oriented attachment’’ growth. Fromthe TEM images, one can even observe that thenanoparticles are just in the process of ‘‘orientedattachment’’. The individual particles were alignedlike a wall, where the second layer of bricks wereabout to be put on the first. During the formationprocess, CTAB may act as a transporter of theparticles and a modifier that leads to the orienta-tion growth of ZnO nanorods [24]. For thereaction system in the absence of CTAB(NH3 �H2O as solvent, pH 9), large blocks ofZnO nanorod (Fig. 5c and d) were obtained withdiameter and length 300 nm and 2.5 mm, respec-tively. The morphology of the nanorods was notwell controlled. The growth direction of the rod is[1 0 0] (Fig. 5d) different from the system in thepresence of CTAB, which is consistent with theXRD patterns (Fig. 2a).The room temperature UV–Vis absorbance

spectra of the ZnO nanorods which were ultra-sonically dispersed in absolute ethanol are given inFig. 6. The shown spectrum is corrected for thesolvent contribution. The absorption spectra showwell-defined exciton band at 381 nm and red-shifted relative to the bulk exciton absorption(373 nm) [25]. From the spectra curves, one can seethere is absorption almost in the whole violet andvisible region. The band edge absorption beginswith the wavelength at �800 nm suggesting thatmore absorption states or defect energy bands

exist in the samples which agrees well with thediscussion on the formation mechanism of thenanorods. From Fig. 6 we can also see that theexciton band of Fig. 6c is red-shifted while Fig. 6bis blue-shifted than Fig. 6a. This will attribute tothe crystalline state of the nanorods. The reactiontime of sample 6c is twice longer than sample 6band will have fewer defects. From TEM images,Fig. 5d corresponds to the sample in 6b, one cansee the nanorod morphology is in the process offormation. In addition, we can notice that theabsorption peak of the sample obtained withoutCTAB surfactant is enhanced in the visible region

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much than other samples. This may be becausewithout the assistance of CTAB, the growthprocess is not well controlled and leads to moredefects and misorientations in the crystals.The PL property of the as-prepared ZnO

nanorods was examined (for the system: in thepresence of CTAB, reaction time 24 h, pH 9).Fig. 6e shows the PL spectrum of the nanorods atroom temperature (excited at 325 nm). The emis-sion band is composed of a weak UV band around386 nm, a weak blue band around 468 nm and astrong orange band around 640 nm. The UVemission band must be explained by a near-band-edge transition of wide band gap ZnOnanorods, namely the free excitons recombinationthrough an exciton–exciton collision process [16].Similarly, an orange band was also observed and itwas attributed to the intrinsic defect in ZnO asoxygen interstitials [26,27] suggesting oxygenexcessive in the product. We can conclude thatthe as-prepared ZnO nanorod has a strong abilityto absorb oxygen to form oxygen interstitialsdefects on the surface. In the case of the weak blueemission, the exact mechanism is not yet clear [28].It may also relate to the surface defects in thepresent condition. Further investigations areneeded.

4. Conclusion

In this study, through the hydrothermal methodwith ZnCl2 � 4Zn(OH)2 as precursors, ZnO nanor-ods have been prepared. Mesoporous microstruc-tures with diameters of about 5 nm were observedon the surface of the as-obtained nanorod in spiteof different pH reaction conditions. ‘‘Orientattachment’’ was used to explain the formationmechanism of the nanorod. And the orientattachment of the nanoparticles will lead to defectsin the nanorod, which, in turn, is the reason for thered-shifted absorption examined by UV–Vis ab-sorption spectrum. This study will provide newapproaches to change the optical absorptionproperties and improve visible-light photocataly-sis. The significant optical properties of this

material may be very interesting for furtherapplication on catalyst and chemical sensors.

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