Abstract
The effects of the zinc salt precursors, the reaction temperature and the alkaline ratio b (b = [OH-]/[Zn 2+ ]) on the aqueous synthesis of ZnO nanocrystals were investigated. Depending on the type of the zinc precursor, Zn 5 (OH) 8 Cl 2 ·H 2 O or Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O lamellar phases were obtained at room temperature (20°C) when the alkaline ratio is lower (0.5 ≤ b ≤ 1, 6 ≤ pH ≤ 6.4). When the reaction temperature increased to 95 °C, zinc hydroxide chloride monohydrate was obtained in one case whereas zinc oxide was formed in the other, and no lamellar phase of Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O was obtained. Thermal decomposition of the two lamellar phases was carried out and mainly showed that Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O was completely decomposed to ZnO when the annealed temperature reached ~250 °C while Zn 5 (OH) 8 Cl 2 ·H 2 O was totally transformed to ZnO at about 400 °C, a higher comparative temperature that confirms the better thermal stability of the zinc hydroxide chloride monohydrate. Keywords: Oxides, ZnO, chemical synthesis, X-ray diffraction, crystal structure, luminescence
Highlights
Among the various II–VI semiconductors such as ZnS, ZnSe, CdTe, CdS, etc., zinc oxide (ZnO) plays an important role in terms of chemical and physical properties
We mainly studied the effect of the alkaline ratio b varying it in the range 0.5–4, as well as the zinc salt precursor and the reaction temperature on the formation of the ZnO wurtzite phase and/or of intermediate phases
The authors prepared ZnO nanocrystals in methanol as solvent at 25 °C under stirring for 2 h using Zn(CH3CO2)2·2H2O (0.2 M) and NaOH solution (1M) as reactants in the pH range 6–13. They found that further increase in OH– concentration in ZnO reduced the crystallite sizes because ZnO can be dissolved when it reacted with excessive OH– ions
Summary
Among the various II–VI semiconductors such as ZnS, ZnSe, CdTe, CdS, etc., zinc oxide (ZnO) plays an important role in terms of chemical and physical properties. R.A. McBride et al have studied the role of the zinc precursor and the temperature on the microcrystalline morphology of ZnO.[20] They prepared zinc oxide using an aqueous solution of zinc salt (0.04 M) and sodium hydroxide with a relatively higher alkaline ratio: [OH–]/[Zn2+] = 15. L. Jiang et al have reported the synthesis of ZnO crystals under hydrothermal conditions at 180 °C using zinc nitrate and sodium hydroxide with a high alkaline ratio of 20, 30 and 80.21 All the as-synthesized products were formed with the wurtzite zinc oxide phase exhibiting flower-like morphology with sizes of 5–8 μm when the alkaline ratio was adjusted to 20. We mainly studied the effect of the alkaline ratio b varying it in the range 0.5–4, as well as the zinc salt precursor and the reaction temperature on the formation of the ZnO wurtzite phase and/or of intermediate phases. Possible reaction processes of the formation of ZnO or of the intermediate phases are proposed and discussed
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