Development of Nanostructured Hematite Photoanode for Photoelectrochemical Water Splitting

DoctorHematite (-Fe2O3) is a photoactive material which is widely investigated in the research field of hydrogen production utilizing solar radiation. This n-type semiconductor material has ~2.1 eV of narrow band gap energy so that it can absorb quite large amount of photons of the sunlight. Due to its conduction band level, hematite cannot produce hydrogen itself and can be used as a photoanode material which produces oxygen via water splitting reaction in Photoelectrochemical (PEC) cell. The theoretical solar-to-hydrogen (STH) efficiency limit of the hematite is beyond 15 %, however the world top efficiency reported is only up to 4 % so far. The most important reason of the low efficiency is the low conductivity of hematite. It has low conductivity and very short minority carrier (hole) diffusion length (~4 nm). Thus, for achieving higher STH efficiency with hematite, it is essential to overcome its conductivity problem. Together with routine methods to improve the conductivity of semiconductors like transition metal doping and CNT or other semiconductor composite, 1-dimensional structural approach is also a promising way to solve the conductivity issue. Anodic oxidation is being applied to fabricate 1-D Nanoporous -Fe2O3 film. It is effective and reproducible method to synthesizing 1-dimensional structured metal oxide with highly ordered pore structure. Here, honeycomb-like iron oxide (hematite) films were fabricated by double-step anodic oxidation of iron foil. Honeycomb structure obtained by double step anodization was found to be more effective to produce large area film with homogeneous pore distribution compared to nanotubes fabricated by the conventional single-step anodic oxidation process. To prevent agglomeration of the hematite film during annealing process, thin alumina layer was deposited on the hematite film surface by atomic layer deposition. With this alumina shielding and subsequent removal by alkaline treatment, one-dimensional (1-D) hematite nanostructure was preserved perfectly after annealing at 550 C°. This highly ordered 1-D nanostructure film showed much enhanced photoelectrochemical cell performances relative to hematite films with low degrees of ordering. In addition, co-catalyst effects were tentatively investigated to suggest high efficient hematite photoanode for photoelectrochemical water splitting