Growth kinetics and wettability conversion of vertically-aligned ZnO nanowires synthesized by a hydrothermal method

We report here the first study of the growth kinetics of vertically-aligned ZnO nanowire arrays grown on Al-doped ZnO (AZO) seed layer-coated substrates by a hydrothermal method. The as-synthesized vertical ZnO nanowires possess a single-crystalline wurtzite structure and a preferred growth orientation along the [0001] direction. The ZnO nanowires were found to grow following the reaction-controlled process and their lengths could be tuned from several to over ten micrometers by adjusting the hydrothermal temperature and time. By measuring the growth rates at different synthesis temperatures, the activation energy for the linear growth of vertical ZnO nanowires on AZO-seeded substrates derived from the slope of the Arrhenius plot was around 35 kJ mol^-1. The obtained value is smaller than that of ZnO nanowires grown in bulk solution, which can be explained by the different nucleation mechanisms. From water contact angle measurements, it is found that the as-synthesized ZnO nanowires are hydrophilic in nature, and their surface wettability can be adjusted by the storage time and heat treatment conditions. Furthermore, the reversible switching of the surface wettability of ZnO nanowires has been accomplished by alternate annealing in vacuum and oxygen atmospheres. The ZnO nanowires with switchable surface wettability will find promising applications in surface engineering.