Shape-controlled synthesis of ZnO microstructures: the effects of inorganic shape directing and pH altering agents

In this work, shape-controlled synthesis of zinc oxide (ZnO) microstructures were demonstrated using liquid-phase route utilizing zinc acetate hydrate as zinc ion precursor and ammonia as pH buffering agent and source for zinc-ammonia complexes. Glass substrate coated with indium tin oxide layer was used as the growth substrate. Zinc-ammonia complex was thought of as the component of growth for ZnO crystals. Inorganic shape-directing agents, instead of organic ones were used in this study e.g., trisodium citrate or aluminum nitrate hexahydrate. The effect of pH altering agent e.g., sodium hydroxide towards the crystal structure was also probed. Without shape-directing agents, the resultant ZnO crystals exhibited multilayer petals flower shape with uniform size (e.g., diameter of -2.5 microm). Adding trisodium citrate, led to the formation of burger crystals with uniform size (e.g., height of -200 nm and diameter of -800 nm). Adding aluminum nitrate hexahydrate resulted in the formation of hexagonal disk with wide variation in size (diameter varied between -0.8 to 4 microm). The addition of sodium hydroxide, strong base, altered the pH into high value very rapidly in the beginning of the synthesis and caused faster nucleation rate in a NaOH containing solution with respect to that in a NH3 solution. In this case, crystals with different morphology are obtained, e.g., dot like, monopetal-flower like and octahedron like. The detailed processes pertaining for each case were explained in this work.