Regarding large scale applications, simple production methods of 1D ZnO are desired to ensure that large quantities can be produced in a reasonable time and in an ecologically justifiable environment, ideally water. Recent reports show the hydrothermal route to be promising. In these papers, the formation of high aspect ratio rods is usually attributed to the intervention of an additive. Nonetheless, additive-free growth of rod-like ZnO particles can be encouraged by carefully controlling the experimental parameters.

Due to the laborious synthesis and/or characterization, most reports in this field are unfortunately limited to investigating the effect of only two or three parameters on the morphology of ZnO nanorods. In our work, eight parameters, related to the hydrothermal synthesis, are examined simultaneously regarding their influence on the particle’s diameter, by using a fractional factorial design. This methodology not only allows the assessment of parameter effects, but in addition also their interactions, in a systematic and economic manner. As a result, a better fundamental understanding of the process can be obtained. Such an approach is quite new in nanoceramic synthesis.

Key parameters
We prepared ZnO nanorods with a high aspect ratio in water without the use of extra additives. Of the eight parameters studied at two levels it is shown that the reaction temperature, the heating rate, stirring of the solution and an ultrasonic pre-treatment of the precursor are the significant parameters that affect the mean diameter of the rods. Next to these significant main parameters, two interactions are found to change the mean diameter. An interaction is observed between stirring and an ultrasonic pre-treatment and between the zinc concentration and the hydroxide ratio. The outcome of this study is explained by the relation of these factors to the supersaturation within the reaction mixture. By setting the significant parameters to their favourable level it is possible to optimize the synthesis of nanorods considerably.

In short, this work does not only present an inexpensive and ecological production method for ZnO nanorods, but the procedure by which the process variables are examined is innovative in this research domain as well.

The researchers presented their work in Nanotechnology.