ZnO nanoparticles prepared by thermal decompositionof b-cyclodextrin coated zinc acetate

Yang Yang a, Xuefei Li b, Jianbin Chen a, Huilan Chen a,*, Ximao Bao b

a Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing University, Hankou Road 22, Nanjing,

Jiangsu 210093, People’s Republic of Chinab Department of Physics, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing,

Jiangsu 210093, People’s Republic of China

Received 2 November 2002; in final form 10 March 2003

Abstract

Uniform ZnO nanoparticles have been prepared via a convenient thermal decomposition approach, in which

b-cyclodextrin (b-CD) is selected to coat the precursor of zinc acetate. The decomposition process of this system is

investigated by thermogravimetric and differential thermal analysis (TG–DTA). TEM or AFM studies reveal that ZnO

nanoparticles and the corresponding film doped on the silicon substrate by this method present weak agglomeration

and regular size distribution. The possible formation mechanism of ZnO nanoparticles under the effects of b-CDcoating is also discussed.

� 2003 Elsevier Science B.V. All rights reserved.

1. Introduction

In recent years, nanosized semiconductor hasbeen extensively investigated due to its special

electrical and optical characteristics in fabricating

nanoscaled electronic and optoelectronic devices

[1–4]. ZnO is a kind of wide band gap (3.37 eV)

semiconductor with large exciton binding energy

(60 meV). It is expected to have a wide range of

applications in room temperature ultraviolet

(UV) lasing, chemical sensors, photovoltaics, pie-zoelectric transducers, and single electron transis-

tors. Up to now, various methods have been

reported to prepare ZnO nanomaterials, such as

aqueous precipitate synthesis [5], vapor transport

[6], chemical vapor deposition (CVD) [7], electro-chemical deposition [8,9], spray pyrolysis (SP) [10],

self assembly [11], and so on [12–15].

Thermal decomposition of zinc salt is one of the

versatile ways to obtain ZnO nanomaterials, in

which zinc acetate (ZnðCH3COOÞ2) is often chosen

as the precursor for its high solubility and low

decomposition temperature [16]. In previous work,

the mechanism and kinetics of its thermal decom-position process have been investigated [16,17].

Well-defined ZnO nanomaterials has also been re-

ported to obtain from the precursor of zinc acetate

by assisted control such as microemulsion-medi-

ated hydrothermal process [14] and sol–gel-derived

process [13], etc. The common purpose of each case

Chemical Physics Letters 373 (2003) 22–27

www.elsevier.com/locate/cplett

* Corresponding author. Fax: +86253317761.

E-mail address: hlchen@nju.edu.cn (H. Chen).

0009-2614/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0009-2614(03)00562-1

is mainly to prevent the crystallite agglomeration,

control the particle shape, size, size-distribution

and crystal phase. Although some progresses have

been made in understanding the role of the organic

additives on nucleation and crystal growth during

thermal decomposition, there is still a furtherchallenge to find simple and mild production

routes, which will determine the realization of

practical applications.

In this Letter, we proposed a novel and facile

approach to fabricate ZnO nanoparticles under

the relatively simple and mild conditions, in which

zinc acetate is coated by b-cyclodextrin (b-CD). As

a kind of cyclic oligosaccharide consisting of sevena-1,4 linked DD-glucopyranose units, b-CD contains

a toroidal hydrophobic cavity (diameter 6–7 �AA),

which is capable of including a variety of inorganic

and organic guest species [18–20]. Thus, it shows

regio-specificity and stereo-specificity with respect

to the substrate and product.

2. Experimental

2.1. Method

Zinc acetate dihydrate, and other chemicals

used in this study were of analytical reagent grade.

b-CD supplied by Nanjing Food Ferment Institute

was recrystallized twice from distilled water andthen dried in vacuum at 80 �C.

A total of 0.22 g (1mmol)ZnðCH3COOÞ2 � 2H2O

and 2.28 g (2mmol)b-CDweremixed in 50mlwater

(the mixture in a molar ratio of 1:2). The solutions

were stirred for 120 min at room temperature

(25 �C) and then evaporated by decompress at 40 �Cto remove the water. The resulting solid products

were dried in vacuum and ground into powdersbefore use. Then the sample was heat-treated in the

muffle at 500 �C for 1 h in the air and white-colored

ZnO products were obtained. For comparison,

pure ZnðCH3COOÞ2 � 2H2O was also heat-treated

in the same conditions to obtain ZnO powders.

2.2. Measurements

Thermogravimetric analysis (TGA) and differ-

ential thermal analysis (DTA) were performed on

a Hi-Res SDT 2960 model thermal analyzer. The

temperature ramp for TGA and DTA was 10 �C/min and the range was from 20 to 600 �C. Thecarrier gas was dry air. X-ray diffraction (XRD)

patterns were measured on a SHIMADZU XD –

3A diffractometer with a Cu Ka radiation source(35 kV). Morphology, the crystallite size and the

size distribution were studied with a JEOL JEM-

2000 transmission electron microscope (TEM).

The samples were dispersed in distilled water by

ultrasonic stirring. A copper mesh covered with a

carbon film was applied as the carrier. A Nano-

scope IIIa atomic force microscope (AFM) was

also used for the observation of their morphology.In preparation of the AFM samples, solutions of

ZnðCH3COOÞ2 with and without b-CD were, re-

spectively, dip-coated on silicon substrate to form

Si-based films, which were dried at room temper-

ature and then heat-treated in the muffle at 500 �Cfor 1 h in the air.

3. Results and discussion

Thermogravimetric–differential thermal analy-

sis (TG–DTA) of zinc acetate with and without

b-CD coating was firstly studied to understand the

details of their decomposition process in the dry air.

Figs. 1a and b present the TG–DTA analysis results

of zinc acetate without and with b-CD coating. Itcould be seen that for the ZnðCH3COOÞ2 � 2H2O

alone under a continuous flow of dry air, anhy-

drous zinc acetate is firstly formed with the crys-

tallization water removed at below 100 �C, which is

also indicated by its corresponding endothermic

peak in the DTA curve. The weight loss of dehy-

dration is 16.7%, consistent with the theoretical

value 16.4%. Then the anhydrous zinc acetate be-gins to decompose into ZnO near 200 �C and the

decomposition process is completed before 300 �Caccompanying an exothermic reaction. The total

weight loss is about 47.1%, which agrees well with

the theoretical value of 46.5%.

When zinc acetate is coated with b-CD, the

TGA plot presents the greatest weight loss in the

range of 300–500 �C and no weight loss occursbefore 300 �C except the loss of water below

100 �C. Obviously, the decomposition temperature

Y. Yang et al. / Chemical Physics Letters 373 (2003) 22–27 23

of ZnðCH3COOÞ2=b-CD system is much higher

than the pure zinc acetate, which is consistent with

the previous results that b-CD could improve the

thermal stability of the inclusion guest and reduce

the decomposition rate of the included species

[18–22]. At the same time, on the conditions of dry

air, b-CD itself will also get decomposed, oxidized

and carbonized, and finally gasified in the range of300–500 �C. So the weight loss in the above tem-

perature range is very pronounced and the corre-

sponding DTA curve shows several exothermic

peaks. When the temperature reaches 500 �C, allthe reaction process completes and only the metal

oxide remains as residues. The total weight loss of

this system is 97.1%, which conforms to the value

of 96.8% calculated from the original molar ratio.ZnO nanoparticles produced through thermal

decomposition of zinc acetate without and with

b-CD coating at 500 �C are structurally charac-

terized by XRD patterns shown in Fig. 2, in which

both the samples present similar peak positions.

The diffraction peaks can be indexed to the

wurtzite structure (hexagonal phase). Meanwhile,

no diffraction peaks from other species could bedetected, which indicates that all the precursors

have been completely decomposed during the

thermolysis process.

TEM image of ZnO particles prepared from the

zinc acetate alone is shown in Fig. 3a. It could be

found that the size of the produced ZnO particles

without the coating of b-CD is very large and there

are various shapes such as sphere, stick, and wand.

In the presence of b-CD, the produced nano ZnO

become uniform in shape and dimension, as shown

in Fig. 3b. From the observations of the further

magnified image (Fig. 3c), the agglomerates of

small particles are nearly spherical and their av-erage size is about 20–30 nm, which is much

smaller than the one obtained in Fig. 3a. The

corresponding selected area electron diffraction

(SAED) pattern of Fig. 3c is shown in the inset, in

which the discrete, bright spots reveal a typical

well-crystallized diffraction pattern of ZnO nano-

Fig. 2. XRD patterns of the ZnO nanoparticles thermally de-

composed from (a) ZnðCH3COOÞ2 � 2H2O, (b) ZnðCH3COOÞ2coated on b-CD in the air.

Fig. 1. TG–DTA curves of ZnðCH3COOÞ2 � 2H2O without b-CD coating (a) and with b-CD coating (b).

24 Y. Yang et al. / Chemical Physics Letters 373 (2003) 22–27

particles. The indexed ED rings indicate that the

produced ZnO nanoparticles possess a hexagonal

crystal structure (wurtzite), consistent with the

results obtained from XRD pattern.

The surface morphology of ZnO films on siliconsubstrate is examined with AFM images (contact

mode). It could be found from Fig. 4 that without

b-CD coating the produced ZnO particles are in

the irregular shapes, while the size is in the sub-

micrometer range. Fig. 4c presents the image of

many isolated and uniform islands, which dem-

onstrates that under the effects of b-CD the pro-

duced ZnO particles become regular in shape, size,and size distributions. The vertical distance of the

cross-section indicates ZnO particles prepared

with b-CD coating are in the average size of about

15 nm.

Obviously, the toroidal hydrophobic cavity of

b-CD is able to have inclusion effects on the zinc

acetate in aqueous solution. The interactions such

as van der Waals force and hydrophobic interac-tion between guest and host are generally accepted

as the driving force for the bonding of guest

molecules or ions to CD cavity. Therefore, at the

nucleation stage, the formation of the ZnO nano-

particles could be induced and confined by the

cooperation of a number of b-CDs with rigidcavity. At the same time, the oxidization, carbon-

ization, and gasification process of b-CD will in-

evitably produce carbon black in the system

of thermal reactions. Such ultrafine powders

with high surface area can allow the produced

nanoparticles to remain separated from each

other [23]. Thus, the interaction between zinc ac-

etate and b-CD and the following carbonizationand gasification processes of b-CD are both

the important factors responsible for the produc-

tion of weakly agglomerated and uniform ZnO

nanoparticles.

In conclusion, a new and facile approach has

been proposed to prepare ZnO nanoparticles by

the b-CD coated precursor of zinc acetate. Its

thermal decomposition process, the structuralproperties, and the morphology of the doped

Fig. 3. TEM images of the ZnO nanoparticles thermally decomposed from (a) ZnðCH3COOÞ2 � 2H2O, (b) ZnðCH3COOÞ2 coated by

b-CD. (c) The further magnified image of b.

Y. Yang et al. / Chemical Physics Letters 373 (2003) 22–27 25

ZnO film on silicon substrate by this method are

studied. It has been observed that the morphol-

ogy, dimension, and size distribution of the

product ZnO are strongly affected with the pres-

ence of b-CD compared to zinc acetate alone.This method could also be used to prepare other

nanoscaled metal oxide from thermal decompo-

sition process.

Acknowledgements

This work was supported by the NationalNatural Science Foundation of China (Nos.

50272029, 20071017) and the Specialized Research

Found for the Doctoral Program of Higher Edu-

cation (No. 200028401).

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