Toward solar fuels: Water splitting with sunlight and 'rust'?

Iron(III)oxide in the form of hematite is, in many respects, an attractive material for the photocatalytic production of molecular oxygen from water. Especially over the past six years, several developments have advanced the performance of water oxidation cells based on this material. Nevertheless, the best versions of these photoelectrodes produce only about a fifth of the maximum photocurrent (and dioxygen) theoretically obtainable, while operating at photovoltages also well short of the theoretical maximum. Here we describe the factors limiting the performance of hematite as a photo-catalyst and outline approaches that have been, or might be, tried to overcome them. These factors include low hole mobility, bulk charge recombination, surface charge recombination, slow water oxidation kinetics, and poor light absorption. Whether hematite will soon become a practical photo-catalyst for water oxidation is uncertain. But, the schemes developed and the lessons learned will likely prove transferrable to other candidate photocatalyst materials.