Direct growth of comet-like superstructures of Au-ZnO submicron rod arrays by solvothermal soft chemistry process

The synthesis, characterization and proposed growth process of a new kind of comet-like Au-ZnO superstructures are described here. This Au-ZnO superstructure was directly created by a simple and mild solvothermal reaction, dissolving the reactants of zinc acetate dihydrate and hydrogen tetrachloroaurate tetrahydrate (HAuCl₄·4H₂O) in ethylenediamine and taking advantage of the lattice matching growth between definitized ZnO plane and Au plane and the natural growth habit of the ZnO rods along [001] direction in solutions. For a typical comet-like Au-ZnO superstructure, its comet head consists of one hemispherical end of a central thick ZnO rod and an outer Au-ZnO thin layer, and its comet tail consists of radially standing ZnO submicron rod arrays growing on the Au-ZnO thin layer. These ZnO rods have diameters in range of 0.2-0.5 μm, an average aspect ratio of about 10, and lengths of up to about 4 μm. The morphology, size and structure of the ZnO superstructures are dependent on the concentration of reactants and the reaction time. The HAuCl₄·4H₂O plays a key role for the solvothermal growth of the comet-like superstructure, and only are ZnO fibers obtained in absence of the HAuCl₄·4H₂O. The UV-vis absorption spectrum shows two absorptions at 365-390 nm and 480-600 nm, respectively attributing to the characteristic of the ZnO wide-band semiconductor material and the surface plasmon resonance of the Au particles.