Iron-oxide and in particular its crystallographic phase hematite (α-Fe 2 O 3 ) is a promising candidate for non-toxic, earth abundant and low cost photo-anodes in the field of photoelectrochemical water splitting. We report here on the synthesis of α-Fe 2 O 3 nanowires by thermal oxidation of low-cost steel substrates. Nanowires grown in this manner exhibit often a blade-like shape but can also possess a wire-like geometry partly decorated at their tip with an iron-rich ellipsoidal head consisting also of crystalline iron-oxide. We show furthermore that these ellipsoidal heads represent suitable growth sites leading in some cases to an additional growth of so-called antenna nanowires. Besides nanowires also nanoflakes were frequently observed at the surface. We discuss the influence of the oxidation temperature and other synthesis parameters as well as dispute the current growth models. Finally, we show that our α-Fe 2 O 3 nanostructures on steel are also photo-electrochemically active supporting in principle their use as photo-anode material.

Photoelectrochemical (PEC) water splitting is one of the most promising sustainable methods for feasible solar hydrogen production. However, this method is still impractical due to the lack of suitable photoanode materials that are efficient, stable, and cost-effective. Here, we present a surprisingly simple fabrication method for efficient, stable, and cost-effective nanometer-thick hematite films utilizing a rapid, ambient annealing approach. In the oxygen evolution reaction, the fabricated hematite films exhibit a Faradaic efficiency of 99.8% already at 1 V versus the reversible hydrogen electrode (RHE), a real photocurrent density of 2.35 mA cm–2 at 1.23 V versus RHE, and a superior photo-oxidation stability recorded for over 1000 h. Considering the active surface area, the measured photocurrent density is higher than any value achieved so far by hematite and other single-material thin-film photoanodes. Hence, we show for the first time that undoped hematite thin films can compete with doped hematite and other semiconductor materials.

Hematite, a low-cost, nontoxic, and earth-abundant n-type semiconductor, is still an intriguing photoanode material for photoelectrochemical (PEC) water splitting. Nevertheless, the PEC performance of hematite is still hindered by ultrafast recombination rates or short diffusion lengths of charge carriers. Therefore, nanostructure implementation has been proposed in this and other cases to overcome this limitation, while simultaneously improving the photon harvesting efficiency. However, this approach must be critically reviewed. We show that both, hematite nanowire- and nanoflake-decorated photoelectrodes, show a low PEC performance in a NaOH (1 M) electrolyte. Reproducible nanostructure synthesis was achieved by the thermal oxidation of low-cost steel foils using only ambient air. Full absolute-energy reconstruction under ambient conditions of the electronic surface band structure of these nanostructured surfaces showed distinct Fermi-level pinning, resulting in high recombination rates. Based on our results, we can conclude that unmodified nanostructures hardly improve the performance but suffer from the lack of internal electrical splitting fields, which suppresses the electron–hole pair separation and can thus actually decrease the performance of PEC electrodes.

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