Ionic liquid-based solvent-induced shape-tunable small-sized ZnO nanostructures with interesting optical properties and photocatalytic activities

We report a simple and one-pot size- and shape-controlled synthesis of small crystalline ZnO nanostructures through the hydrolysis of zinc acetylacetonate precursor using an ionic liquid, tetrabutylphosphonium hydroxide in different organic solvents and solvent mixtures of different polarities. Transmission electron microscopy (TEM) results indeed show the formation of very small (D ∼ 5-7 nm) ZnO spheres and ZnO nanorods of very low diameters (D ∼ 8-11 nm) depending on the solvents. The size and shape of ZnO nanostructures can be easily controlled by simple variation of the solvent and reaction temperature. Protic solvents favor the growth of spherical nanoparticles, whereas the rod-shaped ZnO is formed in aprotic solvents. The spherical ZnO nanostructures exhibit excitonic absorption at lower wavelengths compared to that of rod-shaped ZnO nanostructures. Through adjustment and control of the size and shape of ZnO nanostructures, we can tune the fluorescence emission from blue to green to yellow, when dispersed in the aqueous medium. The obtained ZnO nanostructures show very high photocatalytic activities towards the degradation of different cationic organic dyes (such as rhodamine 6G, crystal violet and methylene blue) as they contain a large number of defect states, as evident from the photoluminescence study. The as-synthesized ZnO nanostructures also show very high stability towards the photocatalytic degradation of organic dyes.