Observing Solid-State Formation of Oriented Porous Functional Oxide Nanowire Heterostructures by in Situ TEM

Transition metal oxide nanowires have attracted extensive attention because of their physical characteristics. Among them, ZnO nanowires have great potential. Due to the multifunctional properties of ZnO, devices built using ZnO-based heterostructures always perform well. In this study, interesting diffusion behavior between ZnO nanowires and Fe metal was observed by using in situ transmission electron microscopy. ZnO nanowires and Fe metal were annealed under ultrahigh vacuum (UHV) conditions at 800 K. By controlling the annealing time for the solid-state diffusion, porous Fe₃O₄ and unique ZnO/porous Fe₃O₄ nanowire heterostructures were formed. As-formed porous Fe₃O₄ nanowires with voids can be divided into two types by appearance: plate-like voids and zigzag-like hollow voids. From high-resolution transmission electron microscopy (HRTEM) images and fast Fourier transform (FFT) diffraction patterns, we found that plate-like voids formed along the {111} plane, which was the close-packed plane of Fe₃O₄, and that zigzag-like hollow voids formed along the {111}/{022} planes. Moreover, a transition region existed during diffusion, with a parallel relationship found between the Fe₃O₄ crystal with plate-like voids and the ZnO crystal. A sharp interface was determined to exist between the Fe₃O₄ crystal with zigzag-like hollow voids and ZnO. These oriented porous Fe₃O₄/ZnO axial nanowire heterostructures exhibited a unique appearance and interesting formation behavior. Furthermore, the structures had a high surface-area-to-volume ratio, which is promising for sensing applications.