We report a novel hierarchically ZnO micro/nanostructured material where ZnO microtubes are covered with ZnO nanowires. Our study contributes to both fundamental material science and material design with potential smart multifunctionalities. The synthesis process is partially driven by electrical current and accompanied in situ by electrical resistivity measurements. The thermal activated energy of the obtained semiconducting microtubes was estimated to be 0.5 eV. Microtubes topped with nanowires may have multifunctional nature and tailored properties for technological applications. The fabrication method of a microtube covered with nanowires presented in this study is simple, fast, and the final products are free of external catalyst agent. Simply stated, a Zn microwire is submitted to a thermal cycle in an atmosphere controlled chamber (O2 and air) while in-situ electrical resistivity measurements are carried out. In the initial stage, while the temperature is still bellow the melting point, a thin ZnO layer is formed on the surface of the microwire. As the temperature rises the ZnO layer thickens and ZnO nanowires are grown due to lattice diffusion of Zn ions from the liquid inner part across the layer. The formation of the tubular structure is due to the thermal evaporation of liquid Zn in high temperature region, however, bellow the Zn boiling point. Even though the vapor pressure of Zn is high, we believe that the electric current used to measure the electrical resistivity plays a role increasing the evaporation rate of the metallic Zn. The passage of electrical current may increase the temperature in the inner part of the microwire (Joule effect) becoming close to the boiling point of metallic Zn (911 °C) and/or somehow increasing the vapor pressure of Zn liquid. We believe that promising breakthrough may be revealed when combining the different applications of microtubes and nanowires. For instance, taking advantage of the different scale and volume-to-surface ratio presented by microtubes and nanowires, one can associate sensor capabilities of nanowires with fluid properties of liquids in microtube, in one single device. As another example, one can use properties of the nanowires (field emission and sensor devices, for example) while fluid is passing inside the microtube. Furthermore, it is known that ZnO is n-type semiconductor. Thus, one can fill the microtube with a p-type organic semiconductor creating, for example, a diode-like device which may indicate new routes to microelectronic field. It can be filled with any other materials with exotic properties as well building devices with multifunctional applications. We believe that the present fabrication approach may be applied to grow hierarchical structures of other metals with low melting point. Hierarchical morphologies such as presented in this work with innovative and unique designs may have novel multifunctional properties in technological applications. We hope the discussion presented here will attract attention from the scientific community and other insights may come out along this line.
