Hematite-based solar water splitting: challenges and opportunities

As the most commonly encountered form of iron oxide in nature, hematite is a semiconducting crystal with an almost ideal bandgap for solar water splitting. Compelled by this unique property and other advantages, including its abundance in the Earth's crust and its stability under harsh chemical conditions, researchers have studied hematite for several decades. In this perspective, we provide a concise overview of the challenges that have prevented us from actualizing the full potentials of this promising material. Particular attention is paid to the importance of efficient charge transport, the successful realization of which is expected to result in reduced charge recombination and increased quantum efficiencies. We also present a general strategy of forming heteronanostructures to help meet the charge transport challenge. The strategy is introduced within the context of two material platforms, webbed nanonets and vertically aligned transparent conductive nanotubes. Time-resolved photoconductivity measurements verify the hypothesis that the addition of conductive components indeed increases charge lifetimes. Because the heteronanostructure approach is highly versatile, it has the potential to address other issues of hematite as well and promises new opportunities for the development of efficient energy conversion using this inexpensive and stable material. Nanoscale heterostructures such as nanonets or nanotubes may significantly improve the performance of hematite-based solar water splitting.