Morphology control of ZnO with citrate: a time and concentration dependent mechanistic insight

The effect of citrate as a crystal habit modifier on the morphology of zinc oxide has been explored in a wide range of citrate/Zn ratio from 0 to 0.67. With an increase in concentration of citrate at the lower range of the ratio, the aspect ratio of the hexagonal prismatic crystals of ZnO decreases, forming flatter disks with a progressive increase of roughness on the hexagonal face. At intermediate ratios, the otherwise smooth rectangular faces also become rough, with progressive formation of porous spherical aggregates. At a high concentration of citrate, solid spherical particles form with very low crystallinity, eventually forming plate-like zinc citrate. Thermal and spectroscopic studies confirm a progressive increase in the adsorbed citrate concentration in the particles. Evidence in support of citrate being adsorbed first on the hexagonal face, followed by the other rectangular faces is presented, and a scheme has been proposed. This adsorbed citrate can be removed by calcination and the morphology remains intact, and so this procedure can be used to make pure ZnO with various morphologies. It was found that the ZnO morphologies synthesized in the presence of citrate show novel photoluminescent and enhanced photocatalytic activities compared with ZnO synthesized in the absence of citrate.