Cryst. Res. Technol. 38, No. 6, 429 – 432 (2003) / DOI 10.1002/crat.200310053 © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0232-1300/03/0606-0429 $ 17.50+.50/0 Asymmetric twinning crystals of Zinc oxide formed in a hydrothermal process

HaiYan Xu, Hao Wang*, YongCai Zhang, Shu Wang, ManKang Zhu, Hui Yan The Key Laboratory of Advanced Functional Materials of China Education Ministry, Beijing University of Technology, Beijing 100022, China

Received November 14, 2002, revised 10 January 2003, accepted 18 February 2003 Published online 15 June 2003

Key wordszinc oxide, hydrothermal method, asymmetric shape. PACS81.05.Je, 81.10.Dn

Well crystallized ZnO powders with various morphologies such as hour glass-shaped, shortened prismatic, flake-like and prismatic form have been synthesized by adopting the system of Zn(Ac)2-KOH-H2O via a hydrothermal process at 200°C for 2 h. It was found that the as-synthesized hour glass-shaped ZnO crystallites are asymmetric both in shape and stoichiometry: while the atomic ratio of O/(Zn+O) at the bigger end ranges from about 60% to 40% along the polar axis of the hour glass-shaped ZnO crystallites, the atomic ratio of O/(Zn+O) at the smaller end remains almost constant at approximately 50%.

1 Introduction Zinc oxide (ZnO), as a wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 eV), has been extensively investigated due to its promising applications in short-wavelength light-emitting, transparent conductor, piezoelectric materials and room temperature ultraviolet lasing [1]. So far, several preparation procedures including sol-gel method [2], evaporative decomposition of solution [3], wet chemical synthesis [4], gas-phase reaction [5] and hydrothermal synthesis [6, 7], etc. have been developed to obtain ZnO crystallites. A variety of morphologies such as prismatic forms [8], ellipsoidal [9], bi-pyramidal and dumbbell-like [10], nanorods [11], nanowires [12] and spheres [13] have been synthesized during the last decade, and the growth mechanisms of ZnO crystallites with different shapes have also been discussed. In this study, well crystallized ZnO powders with various morphologies such as twinning hour glass-shaped, shortened prismatic, flake-like and prismatic form were synthesized by adopting the system of Zn(Ac)2-KOH-H2O via a hydrothermal process. We found an interesting phenomenon in the as-synthesized

hour glass-shaped ZnO crystallites: the two part are asymmetric both in shape and stoichiometry: the atomic ratio of O/ (Zn+O) at the bigger end ranges from about 60% to 40% along the polar axis of the hour glass-shaped ZnO crystallites, while the atomic ratio of O/ (Zn+O) at the smaller end remains almost unchanged at approximately 50%.

2 Experimental Section In a typical process, mixed aqueous solution of 6.5 ml Zn(CH3COO)2 (1 mol/l) and 26 ml KOH (0.25-2.00 mol/l) was transferred into a 50 ml Teflon-lined autoclave and heated at 200 oC for 2 hours. The as-formed

white precipitate was collected and washed several times with deionized water, then dried at 80 oC.

____________________ * Corresponding author: e-mail: haowang@bjut.edu.cn 430 HaiYan Xu et al.: Asymmetric twinning crystals of Zinc oxide a) b) Fig. 1XRD patterns of ZnO powders hydrothermally prepared at 200 oC for 2h using 0.25 - 2.00 mol/l KOH respectively.

a) b) c) d) Fig. 2SEM of ZnO powders hydrothermally prepared at 200oC for 2h with (a) 0.25, (b) 0.50, (c) 1.00, (d) 2.00 mol/l KOH. Cryst. Res. Technol. 38, No. 6 (2003) 431

he products were characterized by X-ray powder diffraction (XRD, Japan Rigaku D/Max-3C) in the 2θ ranges

from 20-80o, using a diffractometer equipped with a graphite monochromatized Cu Kα radiation (λ=1.5405 Å). The scanning electron microcopy (SEM) images were conducted on a Hitachi SEM S-3500N and the compositional analysis using energy dispersive spectroscopy (EDS, Oxford-INCA).

3 Results and discussion Zinc oxide belongs to hexagonal symmetry, wurtzite structure type, sp.gr. P63mc. The lattice constants of ZnO are a=3.249, c=5.207 Å, which slightly depend on its stoichiometry [14]. The XRD patterns of the ZnO powders obtained at 200 oC for 2 h using different concentrations of KOH aqueous solutions are shown in Fig. 1(a). It is seen that the diffraction peaks of all the samples can be indexed as the wurtzite structure of ZnO (JCPDS 36-1451). However, the XRD patterns of the sample obtained using 0.25 mol/l KOH are different from those of the others. From the enlarged figure (Fig. 1 (b)), it is unexpectedly noted that all the diffraction peaks of the sample obtained using 0.25 mol/l KOH split into two parts•while those of the samples obtained using 0.50 – 2.0 mol/l KOH do not have this behavior. The specific X-ray powder diffraction data of the two parts of the sample obtained using 0.25 mol/l KOH are presented in table 1. The diffraction peaks of one part positioned at higher angles (the calculated lattice constants: a1=3.249, c1=5.204 Å) agree well with the JCPDS