Controlled Synthesis and Understanding of Growth Mechanism – Parameters for Atmospheric Pressure Hydrothermal Synthesis of Ultrathin Secondary ZnO Nanowires

Synthesis of ultrathin ZnO nanowires gains great attention from research community because of their large potential in applications involving optoelectronics and sensors. In this study, a low-pressure and low-temperature hydrothermal synthesis of ultrathin ZnO nanowires is studied to understand the growth mechanisms better. To achieve this aim, an about 10 nm thin Zn seed layer was sputter-deposited on a silicon (100) wafer for the hydrothermal growth of ZnO nanowires in an equimolar aqueous solution of Zn(NO3)2 and hexamethylenetetramine. X-ray diffraction analysis confirmed that the Zn layer was self-oxidized into ZnO in air soon after deposition and then functioned as the seed for the preferred growth of c-oriented ZnO nanorods. Different growth conditions were investigated to identify how concentration, temperature, and time influence the final morphology of the synthesized ZnO nanostructures. It was found that under the atmospheric pressure, concentration and temperature have to be higher than 0.0025 M and 50°C, respectively, for the ZnO nanorods to nucleate and grow densely. Low concentration gives sparse and randomly oriented nanorods, whereas high concentration gives dense and vertical nanorods. Ultrathin ZnO secondary nanowires with an average diameter of less than 20 nm were successfully synthesized in a solution with concentration of 0.005 M at 90°C for about 16 h. By analyzing the scanning electron microscopy images of the ZnO nanostructures obtained at different growth conditions, a mechanism is proposed for the growth of the ultrathin secondary ZnO nanowires. This finding provides a cost-effective and straightforward pathway to prepare ultrathin ZnO nanowires.