Rapid precipitation-free synthesis of zinc oxide nanowire arrays

Abstract

The microwave-assisted hydrothermal method is cost-effective for rapidly producing zinc oxide (ZnO) nanowire array. However, microwave-assisted heating can make many bulk crystals that may contaminate the nanowire array. The precipitated contamination on the ZnO nanowire array formed using the microwave-assisted hydrothermal method is grossly underreported in the literature. ZnO is usually prepared in a high basic condition to reduce the bulk nucleation, using ammonia or amine complexes. As microwave-assisted synthesis is preferred in open reactors to avoid high-pressure development and design complexity, amine-based components are particularly unsuitable for such reactions due to high volatility. In this work, the ZnO nanowire array was synthesized using the microwave-assisted hydrothermal method in an open reactor using a basic non-volatile compound, Sodium hydroxide (NaOH). Rapid growth of ZnO nanowires was achieved at a rate of 4.6 μm/h without having precipitated materials using NaOH. In contrast, ammonia-assisted synthesis showed a large number of precipitates on the array. High and low-magnified field emission scanning electron microscopy images revealed the formation of pristine ZnO nanowires and debris/contaminant-free arrays using a non-volatile hydrolyzing agent. Speciation data shows that various ionic species of Zn under different basic conditions using ammonia and NaOH can control the bulk nucleation of ZnO.