Size control of highly ordered HfO2 nanotube arrays and a possible growth mechanism

Highly ordered HfO2 nanotube arrays were prepared through an electrochemical anodization in the presence of NH4F and ethylene glycol. The voltage-dependent pore size, wall thickness and porosity were studied using scanning electron microscopy and a wall thickness to pore size ratio was proposed on the basis of the results to serve as a boundary condition additional to the 10% porosity rule introduced by the Gosele group. The average distributions of the tube sizes and wall thicknesses of the nanotubes prepared at 20 V were determined from the small-angle x-ray scattering data using a simple polydisperse core-shell cylinder model fit. Temperature-dependent x-ray diffraction measurements show that the as-grown amorphous nanotube arrays can be converted into crystalline nanotube arrays at a temperature above 500 degrees C. Transmission electron microscopy study of the dimple layer under the as-grown nanotube arrays reveals the presence of a layer of ordered HfO2 nanocrystals. Further microscopic investigation of the nanotube root region indicates that the nanotubes develop from bulbs produced during anodization. A possible gas bubble initiated growth mechanism based on these observations was proposed.