Fabrication and Optical Properties of Large-Scale ZnO Nanotube Bundles via a Simple Solution Route

Large-scale ZnO nanotube bundles were successfully synthesized by a single solution method at lower temperature. Every ZnO nanotube bundle is composed of closely packed nanotubes, with inner diameters of ∼350 nm and wall thicknesses of ∼60 nm, and forms radiating structures. The influence of the reaction time on the size and shapes of the ZnO samples was studied in detail, and the results revealed that the reaction time plays an important role in determining final morphologies of the samples. The formation of the tubular structure may be due to the selective dissolution of the metastable Zn-rich (0001) polar surfaces, and a possible growth model was proposed. Optical properties of the ZnO nanotube bundles were also investigated by photoluminescence (PL) spectroscopy. It was found that the UV emission peak of the nanotube bundles did not change its position, while the visible emission band showed an obvious red shift when the nanotube bundles were annealed in ambient oxygen. Moreover, the UV emission was further identified to originate from the radiative free exciton recombination by the temperature-dependent PL.