Our synthesis was based on the thermal evaporation of Zn powders
under controlled conditions without the presence of catalyst,
different from the ZnO nanowires grown by vapor liquid solid (VLS)
mechanism [16–18]. ZnO nanowires were formed by oxidation of evaporated zinc vapor in gas phase. It is likely that the growth was controlled by vapor–solid (VS) process. The general morphologies
of the products are shown in Fig. 1.