Thermally oxidized iron oxide nanoarchitectures for hydrogen production by solar-induced water splitting

Thermally oxidized iron oxide (α-Fe2O3, Hematite) nanostructures are investigated as photoanodes that convert solar energy into hydrogen by splitting water. α-Fe2O3 is stable for water photo-oxidation, it has a favorable band gap energy and is a non-toxic common material. However, α-Fe2O3 photoanodes suffer from high loss due to electron-hole recombination; therefore nanoarchitectures with high aspect ratio that allows photons to be absorbed close to the photoanode/electrolyte interface are preferred. The thermal oxidation of iron is a simple way to produce nanostructured iron oxide electrodes. Different morphologies, aspect ratios, and oxide thicknesses result depending on the process parameters. Nanorod structures were obtained by annealing iron foils in oxygen rich atmosphere, whereas annealing in oxygen lean atmosphere resulted in nanocoral-like morphology. The nanorod-structured photoanodes achieved moderate photocurrent density of 0.9 mA/cm2 while the nanocoral morphology achieved 2.6 mA/cm2 (both at 1.8 V vs. the reversible hydrogen electrode). The effect of the oxidation process and oxide layer on performance is discussed.

► 3 nanostructures of α-Fe2O3 are grown by thermal oxidation: rods, coral, leaf. ► The nanorod diameter is shown to change due to oxidation/furnace temperature. ► Each structure is tested as a photoanode for water splitting – converting solar energy into hydrogen. ► The rod and coral structures have moderate and good photocurrent, respectively. ► The coral specimen has the best photoresponse perhaps due to oxygen vacancies.