Fabrication of ZnO nanorods and nanotubes in aqueous solutions

A novel aqueous solution method has been developed for growing well-aligned crystalline ZnO nanorods (NRs) and nanotubes (NTs) on a variety of substrates including Si wafers, poly(ethylene terephthalate) (PET), and sapphire. Atomic layer deposition (ALD) was first used to grow a uniform ZnO film on the substrate of choice and to serve as templating seed layer for the subsequent growth of NRs and NTs. On this ZnO layer highly oriented two-dimensional NR arrays of ZnO were obtained by solution growth process using zinc nitrate and hexamethylenetetramine in aqueous solution. X-ray diffraction (XRD) analysis confirmed the crystalline nature of the ZnO NRs. The diameter of ZnO NRs obtained varies according to the concentration of reactants, reaction temperature, and solution pH. The length of the ZnO NRs is controlled by growth time as well as by the concentration of reagents in the aqueous solution. ZnO NTs with diameters of 40-60 nm were found to self-assemble on the top faces of ZnO NRs. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline nature of the ZnO NTs. Electron energy loss spectroscopy (EELS) analysis indicated the existence of nitrogen at the core of ZnO NTs during their formation, which provides useful information to propose the growth mechanism of ZnO NTs.