Microwave-hydrothermal synthesis and photoluminescence characteristics of zinc oxide powders
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A microwave-hydrothermal process for the synthesis of crystalline zinc oxide powders has been developed in this study. Well-crystallized zinc oxide powders exhibiting different morphology, crystallinity, and particle size have been successfully prepared by controlling the process temperature and molarity of NH4OH in the starting solution. With increasing process temperature and NH4OH molarity during synthesis, the morphology of ZnO powders changes from flowerlike agglomeration to a well-developed rodlike shape. The band gap of ZnO powders increases with a decrease in the molarity of NH4OH during synthesis. Vacuum ultraviolet radiation (VUV) excited luminescence studies for ZnO powders reveal an excitation band at 161 nm possibly due to the absorption of O2− 2p electrons in the valence band. The VUV excitation band of ZnO powders observed at 161 nm will be useful for excitation of gas-discharged plasma display devices.
I. INTRODUCTION
Zinc oxide is an important semiconductor material with unique characteristics such as a large direct band gap (3.2 eV) and a large excitonic binding energy (60 meV). Because of these properties, zinc oxide is widely used in various applications such as photonic devices, transparent conductors, solar cell windows, surface acoustic devices, and gas sensors.1–3 The optical, electrical and magnetic properties of zinc oxide are markedly influenced by its particle size, morphology, purity and chemical composition. Various solution methods have been reported for the preparation of zinc oxide including precipitation,4 gas condensation,5 sol-gel method,6 and hydrolysis in polyol medium,7 and hydrothermal synthesis.8 These solution routes provide different approaches to tailor the size and control the morphology of the obtained particles. Earlier a synthesis procedure based on the conventional hydrothermal route for preparing zinc oxide was investigated in our laboratory.9 In comparison to the conventional hydrothermal route, the microwavehydrothermal route is more advantageous due to rapid and volumetric heating achieved in this process.10–12 The microwave-hydrothermal process enhances the crystallization kinetics because of the rapid and volumetric heating and can be used for preparing various kinds of ceramic powders such as ZrO2,11 ␣–Fe2O3,12 BaTiO3,13
II. EXPERIMENTAL
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0067 464
http://journals.cambridge.org
tobermorite,14 and ABi2O6 (A ⳱ Mg, Zn).15 Recently zinc oxide has also been reported to be synthesized by a microwave-driven hydrothermal method involving the decomposition of zinc chloride under alkaline conditions or by addition of urea.16 In the present study, a microwave-hydrothermal process was developed for synthesizing ZnO powders. The effects of reaction temperature and molarity of ammonium hydroxide in the starting solution on the microstructures and particle size of ZnO powders were examined. ZnO is known to have interesting emission properties, which are dependent on the type of v
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