Influence of chemical synthesis on the crystallization and properties of zinc oxide

ZnO powders were synthesized using (a) the decomposing urea process and (b) crystallization from the suspensions obtained by abrupt mixing of Zn(NO3)2 and NH4OH solutions. The samples were characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), microelectrophoresis and BET. Basic zinc carbonate was always produced in decomposing urea solution at 95 °C in excess of urea and for times between 1 and 8 h. ZnO was produced by heating of basic zinc carbonate at 300–600 °C. The FT-IR spectrum of ZnO produced at 600 °C showed the superposition of three IR bands located at 565, 489 and 403 cm−1. The ZnO powder prepared at 300 °C showed the greatest specific surface area of 17.88 m2 g−1. The precipitates obtained by abrupt adding of concentrated NH4OH solution into Zn(NO3)2 solution corresponded to a complex compound with the general formula Zn5(OH)8(NO3)2(H2O)2−x(NH3)x. Autoclaving of the aqueous suspension of this compound yielded ZnO. For these ZnO powders the relative intensity of the band at 498 cm−1 was substantially increased. On the other hand, the specific surface area of ZnO produced by autoclaving was found significantly decreased in relation to the urea process. Microelectrophoretic measurements with ZnO particles showed pHiep varying between 9.3 and 9.8. The zeta potentials measured for ZnO particles produced by the urea process were much higher than those measured for ZnO particles produced by autoclaving. TEM showed two different morphologies and different sizes of ZnO particles in dependence on the method of chemical synthesis. The difference in the FT-IR spectra of these two kinds of ZnO particles was ascribed to their microstructural properties. Microelectrophoretic and BET measurements were also related with the microstructure of ZnO particles.