Synthesis of ZnO Nanograins by Immersing Zn Powders in Oxalic Acid Solution Plus Thermal Decomposition.

Abstract

A simple and inexpensive method to synthesize ZnO nanograin powders from Zn powders is reported in this paper and phase change from Zn powders to ZnO nanograins during the process is studied in detail. Zinc powders were immersed into 1.0 mol/l oxalic acid aqueous solution for various times and then dried at ambient condition. As-prepared precursors were decomposed in air at temperatures of 200-600 degrees C for 2 hours. As-prepared samples were characterized with scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The results show that the precursors after immersing zinc powders in oxalic acid aqueous solution are ZnC2O4 x 2H2O/Zn(OH)2 dense blocks which consist of 5-10 nm grains. ZnC2O4 x 2H2O/Zn(OH)2 dense blocks decompose into ZnC2O4/ZnO blocks after heating in air at 200 degrees C x ZnC2O4/ZnO blocks further decompose into loose ZnO blocks after heating in air at 400 degrees C and grain sizes of ZnO are - 10 nm in the loose blocks. The phase transitions from Zn powders to ZnO nanograins during the two-step synthesis are analyzed thermodynamically. A physical model is suggested to explain the morphology changes from ZnC2O4 x 2H2O/Zn(OH)2 dense blocks to ZnO loose blocks, based on the volume changes during the phase transitions.