www.elsevier.com/locate/matlet

Materials Letters 60 (

Hydrothermal growth of novel radiolarian-like porous ZnO

microspheres on compact TiO2 substrate

Lei Shi, Xiaodan Sun *, Hengde Li, Duan Weng

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

Received 28 January 2005; accepted 10 August 2005

Available online 30 August 2005

Abstract

Radiolarian-like porous ZnO microspheres, consisting of ZnO nanosheets about 500 nm in length, 100 nm in width and 50 nm in

thickness, have been synthesized by a facile hydrothermal process on compact TiO2 substrate. The products were characterized and analyzed

by SEM, TEM and XRD. Selected Area Electron Diffraction (SAED) pattern reveals that the nanosheets in ZnO microspheres are single

crystalline. The preference orientation along (1010) plane was observed by the XRD and SAED results. A possible formation mechanism was

preliminary proposed for the formation of the novel nanostructure.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Zinc oxide; Nanostructure; Crystal growth; Hydrothermal growth

1. Introduction

Zinc oxide is an important low-cost basic II–VI semi-

conductor material with a wide band gap energy of 3.37 eV,

which has been widely used in photonics devices, gas

sensors and dye-sensitized solar cells for its optoelectronic,

electrical and photoelectrochemical properties [1]. To

achieve the better and optimized performances in the

applications above, various morphologies of ZnO with

specific large surface area and high porosity such as oriented

nanorod ZnO arrays [2], tower-like, flower-like and tube-

like ZnO arrays [3], mesoporous structured polyhedral drum

and spherical cages and shells [4], single-crystal tubular

ZnO whiskers [5] and controllable large-scale ZnO ordered

pore arrays [6] have been synthesized.

Herein, a facile and effective wet chemical route was

presented to obtain novel radiolarian-like porous ZnO

microspheres (denoted as RPZM) on compact TiO2 sub-

strate, in which aqueous solutions of 5 mM zinc nitrate and

methenamine were used to hydrothermally synthesize

RPZMs. The formation mechanism is also preliminarily

0167-577X/$ - s

doi:10.1016/j.m

* Correspondi

E-mail addr

2006) 210 – 213

ee front matter D 2005 Elsevier B.V. All rights reserved.

atlet.2005.08.020

ng author. Tel.: +86 10 62772977; fax: +86 10 62771160.

ess: sunxiaodan@mail.tsinghua.edu.cn (X. Sun).

Fig. 1. SEM image of a RPZM at low-magnification, comparing with the

skeleton of radiolarian in inset figure.

L. Shi et al. / Materials Letters 60 (2006) 210–213 211

discussed. Materials with this novel nanostructure are

supposed to be of significant importance for extending the

applications of ZnO.

2. Experimental

All the chemical reagents used in the experiments were

obtained from commercial sources as guaranteed-grade

reagents and used without further purification and treatment.

The synthesis procedure involves four steps: (1) glass

substrates were cleaned by ultrasonic in isopropyl solution

containing NaOH, distilled water, ethanol and acetone in

turn; (2) compact TiO2films were fabricated on the glass

substrates following the reported procedure [7]; (3) 32 ml 5

mM zinc nitrate and 32 ml 5 mM methenamine aqueous

Fig. 2. SEM images of obtained RPZM: (a) space distribution of RPZMs on the s

surface morphology of a RPZM, (d) nanowires observed around the RPZMs and (

induced by the thin layer of Au sputtered on the surface of the sample to improv

solutions were mixed in a Teflon-lined stainless steel

autoclave to form the deposition solution; (4) RPZMs were

obtained on substrates immersed in the mixture solution at

95 -C for 4 h. The sample was characterized and analyzed

by X-ray diffraction (XRD) (Rigaku, D/max-RB, Cu Ka, 40

kV, 100 mA), field emission scanning electron microscope

(FESEM) (JEOL, JSM-6301F, 15 kV), energy-dispersive X-

ray spectroscopy (EDX) (Oxford, INCA 300) and trans-

mission electron microscope (TEM) (JEOL, JEM-1200EX,

120 kV).

3. Results and discussion

The morphology of one RPZM was shown in Fig. 1, which

demonstrated the similarity between RPZM and the skeleton of

urface of compact TiO2 film, (b) the morphology of a whole RPZM, (c) the

e) The EDX spectra measured on the marked area in b. The peak of Au was

e the conductance of it for SEM observation.

Fig. 3. TEM morphology and SEAD pattern of a fragment of a RPZM. The star mark in the image indicates the position where the SEAD pattern was

measured.

L. Shi et al. / Materials Letters 60 (2006) 210–213212

radiolarian (inset in Fig. 1). The monodisperse RPZMs with the

diameter ranging from 7 to 9 Am are shown in Fig. 2a. Large

numbers of macropores ranging from 200 to 500 nm are irregularly

distributed in these particles (Fig. 2b). High magnification images

reveal that the surface of the spheral particles is made up of folded

ZnO nanosheets with the length of about 500 nm, the width of

about 100 nm and the thickness of about 50 nm (Fig. 2c). Some

nanowires with the diameter of about 20–30 nm as shown in Fig.

2d can be observed in the region between a RPZM and the

substrate or at the interface between two particles. The EDX

spectra (Fig. 2e) measured on the marked area in Fig. 1b

demonstrate that the elements of the synthesized RPZMs are Zn

and O.

To study the detailed morphology and structure of a typical

RPZM on the substrate, RPZMs were put into a beaker with the

substrate and broken into fragments in ethanol solution by

Fig. 4. XRD patterns of synthesized samples. (a) compact TiO2 substrate on glass; (

structure of ZnO unit cell and its (1010) face.

ultrasonic. TEM image and SAED pattern of a fragment are

shown in Fig. 3, demonstrating the presence of fine microstructures

in the fragment. In area A, some parallel stripes with the

interspacing of about 5–10 nm can be found. The corresponding

SEAD pattern of this area indicates that it is single crystalline and

takes sixfold symmetry. Area B shows tens of regularly parallel

thin stripes with interval about 3 to 4 nm, and some nanopores with

diameters of about 5–8 nm are distributed in area C.

Curve b in Fig. 4 shows the XRD pattern of RPZMs on

compact TiO2 substrate. Compared with the diffraction pattern of

compact TiO2 substrate on glass (Curve a in Fig. 4), it is found that

only one diffraction peak appears at the position of 2h =31.74- forRPZMs. The calculated interplanar spacing corresponding to the

peak is 2.8191 A, which is in good agreement with the d value of

(1010) of ZnO (JCPDS 74-0534), indexing the RPZMs as

hexagonal wurtzite structure.

b) RPZMs on compact TiO2 substrate. Inset image is the hexagonal wurtzite

Fig. 5. SEM images of ZnO crystals grown on blank glass substrate.

L. Shi et al. / Materials Letters 60 (2006) 210–213 213

According to the reported studies on the crystallization process

of ZnO in alkali medium, the growth of hexagonal wurtzite

structured ZnO is related to both its intrinsic crystal structure and

external factors such as temperature, solution pH and substrates

[8]. In our experimental system, the influence of the substrate on

the growth habit of ZnO is proved to be the most important

external factor, which is confirmed by the control experiment: ZnO

bulk blocks of about 2 Am (Fig. 5a) and some ZnO tube with

diameter of about 1 Am, length of about 6 Am (Fig. 5b) were

obtained on the substrate of blank FTO (F-doped Tin Oxide) glass

when the temperature of the hydrothermal treatment, the concen-

tration and pH of the aqueous solution were the same as those of

the sample RPZM. As compared with the blank FTO glass, large

numbers of hydroxyl groups can be found on the surface of the

compact TiO2 film [9]. The existence of hydroxyl groups changes

the ligands of the Zn2+ ions in growth unit [10]. Thus, the

difference between the porous spheral morphology of the sample

RPZM obtained on the TiO2 surface and the bulk blocks and tube

morphologies formed on the blank FTO glass is supposed to be

caused by different concentration of the hydroxyl groups on

different substrates.

Till now, large numbers of experiments on growth of ZnO

crystals from alkali media under hydrothermal conditions prove the

pronounced polar growth along the [0001] direction (c-axis),

which is the consequence of the polarity of hexagonal wurtzite

structure and the specific characteristics of the crystallization

medium [11]. However, the Periodic Bond Chain (PBC) theory has

pointed out that the velocities of polar crystal growth in different

directions should be: v<0110>=v<1010>>v<0111>>v<0001>=v<0001>[12]. It is believed that the growth mechanism of crystal mainly

contains the formation of growth units and the incorporation of

growth units into the crystal lattice. In the growth units of ZnO

crystal, the coordination number of Zn2+ ion is four and the ligands

of the Zn2+ ion can be O2� or hydroxyl. Then, at the primary stage

of crystal growth, the hydroxyl distributed on the surface of

substrate may be the major resource of ligands of Zn2+ ion in an

almost neutral solution due to the presence of the compact TiO2

substrate. Therefore, the change of ligands of Zn2+ ion in growth

unit may affect the growth rate of various crystal faces and induce

the growth process following the PBC theory, resulting in the

highly preferred growth along (1010) plane demonstrated by XRD

pattern. Although it is not clear yet why the ZnO particles are

spheral with macropores in it, it can be supposed that the final

porous RPZM is formed by the combination of the nanosheets

grown along (1010) plane. Further work will be carried out to give

a more detailed explanation on the growth mechanism.

4. Conclusion

In conclusion, porous RPZMs have been successfully

prepared for the first time via a facile wet chemical route

using inexpensive and commercially available reagents.

This work is beneficial to understand the crystallization

process of hexagonal wurtzite structure of ZnO and to

develop nanodevices using materials of this novel nano-

structure with potential applications in photonics devices,

gas sensors, as well as Gratzel solar cell.

Acknowledgement

The National Natural Science Foundation of China (grant

number 50172029) supported this work.

References

[1] L. Vayssieres, Adv. Mater. 15 (2003) 464.

[2] L. Vayssieres, K. Keis, S. Lindquist, A. Hagfeldt, J. Phys. Chem., B

105 (2001) 3350.

[3] Z. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Langmuir 20 (2004) 3441.

[4] P.X. Gao, Z.L. Wang, J. Am. Chem. Soc. 125 (2003) 11299.

[5] J.Q. Hu, Y. Bando, Appl. Phys. Lett. 82 (2004) 1401.

[6] B.Q. Cao, W.P. Cai, F.Q. Sun, Y. Li, Y. Lei, L.D. Zhang, Chem.

Commun. (2004) 1604.

[7] Y. Saito, T. Kitamura, Y. Wada, S. Yanagida, Synth. Met. 131 (2002)

185.

[8] K. Govender, D.S. Boyle, P.B. Kenway, P. O’Brien, J. Mater. Chem.

14 (2004) 2575.

[9] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A.

Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135.

[10] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, J. Cryst. Growth 203 (1999)

186.

[11] L.N. Demianets, D.V. Kostomarov, I.P. Kuz’mina, S.V. Pushko,

Crystallogr. Rep. 47 (2002) S86.

[12] P. Hartman, P. Bennema, J. Cryst. Growth 49 (1980) 145.