Temperature-Dependent Growth of Self-Assembled Hematite (α-Fe2O3) Nanotube Arrays: Rapid Electrochemical Synthesis and Photoelectrochemical Properties

We report on the self-assembled fabrication and photoelectrochemical properties of alpha-Fe2O3 (hematite) nanotube arrays, prepared by potentiostatic anodization of iron foil in an ethylene glycol electrolyte containing NH4F and deionized water. Vertically oriented nanotube arrays provide a highly ordered material architecture of high surface area that is nearly ideal for efficient transport and separation of photogenerated charges. We have achieved iron oxide nanotube arrays over a potential range of 30-60 V, using an electrolyte comprised of 0.2-0.5 wt % NH4F, and 2-4% D1 water. The resulting nanotube arrays have a pore diameter ranging between 30 and 80 nm, with a minimum wall thickness of similar to 10 nm. Nanotube formation is strongly dependent on the anodization bath temperature and fluoride concentration, with higher temperatures leading to increased rates of nanotube array growth, and with lower temperatures required in order to obtain identifiable nanotubes arrays for increased F- ion concentration. Crystallization of the as-synthesized. amorphous, iron oxide nanotube arrays to hematite is achieved through annealing in an oxygen-deficient ambient.