Synthesis of nitrogen-doped ZnO particles by decomposition of zinc nitrate hexahydrate in molten ammonium salts
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The N-doped ZnO was prepared by heating a mixture of zinc nitrate hexahydrate [Zn(NO3)26 H2O] and ammonium salt at 623 K for 1 h in air. The mixture of zinc nitrate hydrate and ammonium salt formed a homogeneous molten salt at 623 K, and the homogeneous dispersion of the metal ions and ammonium ions contributed to the N-doping. In particular, when the mixture of zinc nitrate hydrate and ammonium acetate (CH3COONH4) was heated at 623 K, the doped amount of nitrogen was higher than with the mixture of zinc nitrate hydrate and NH4NO4. The acetate anion (CH3COO) restricted the oxidation reaction of nitrate anion (NO3). Furthermore, Al- and N-co-doped ZnO particles were obtained by heating the mixture of zinc nitrate hydrate, aluminum nitrate hydrate, and ammonium acetate. The Al and N co-doping effectively increased the doped amount of nitrogen. The spontaneous formation of ZnO lattice and the nitrogen source in the molten salt and the homogeneous dispersion of Zn2+ ions and Al3+ ions contributed to the increase in the amount of doped nitrogen.
I. INTRODUCTION
Zinc oxide (ZnO) is a wide-gap oxide semiconductor, and its piezoelectricity, photoluminescent (PL) characteristics, and electrical conductivity have been intensively studied.1,2 Furthermore, doping with cations having a valence different than that of Zn2+ can control the electrical conductivity of ZnO. In particular, doping with Al3+ ions in the ZnO lattice produces donor levels, which in turn leads to n-type semiconductivity.3 Thus, doping with impurity cations in ZnO lattice is an important technique for controlling the electrical and optical properties. In recent years, it has been shown that nitrogen (N) doping achieved by the substitution of nitrogen for oxygen (O) is an effective method for obtaining p-type ZnO.4,5 The p-type ZnO can be used for the development of short-wave length optical devices.6 The N-doping results in impurity levels, and these levels correspond to the formation of holes as carriers. The holes can accept electrons from adsorbed molecules on the ZnO particle surface so that the p-type ZnO can also become an oxidizing agent.4 Furthermore, the impurity levels that correspond to the N-doping cause an adsorption of visible light; it is therefore expected that N-doping can be used to control photocatalytic activity.7,8 N-doping is a powerful technique for obtaining p-type oxide semiconductors and for controlling electrical states. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0399 J. Mater. Res., Vol. 24, No. 11, Nov 2009
However, the doping of nitrogen in the ZnO lattice is quite difficult, making the development of doping methods with high yields and high carrier density an important issue. Various preparation methods for nitrogen-doped ZnO material have been reported. Wang et al.9 have shown the decomposition of diethyl zinc on a substrate at 873 K under an atmosphere of a mixed gas of N2 and O2. Zhao et al.10 have used a spray pyrolysis method to prepare nitrogen-doped ZnO f
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