The development of materials with high activity for photocatalytic water splitting is a central challenge for the production of renewable hydrogen using sunlight. Owing to advantages such as low cost, elemental abundance, and chemical stability, transition metal oxides (TMOs) are some of the most widely used materials for this purpose. The most efficient systems developed to date reach solar-to-hydrogen conversion efficiencies (STH) of around 1%¹ and are based on wide bandgap TMOs such as SrTiO₃.² Because such wide bandgap materials often absorb UV light only, TMOs with extended visible light absorption are key to bridge the activity gap towards the target for commercial applications,³ often considered to be 10% STH.

While large research efforts have been devoted to TMOs with smaller bandgaps, efficiencies for such materials have typically remained relatively far from their theoretical limit. For example, Fe₂O₃ is one of the most studied TMOs in the field of solar fuel production, but it has so far reached only 1/3 of its theoretical maximum activity for water oxidation,⁴ raising the question of limitations of more fundamental nature.

In this talk, I will discuss the links between ultrafast charge recombination and polaron formation in TMOs – two of the main processes considered to impose efficiency limitations. Firstly, Fe₂O₃, Cr₂O₃, and Co₃O₄, which have absorption onsets of around 2.1 eV, 1.9 eV, and 1.6 eV, respectively, are studied using time-resolved optical spectroscopic techniques to probe their excited state dynamics on a timescale of femtoseconds to seconds following light absorption. I will demonstrate how common photophysical features and dynamics suggest a shared pathway for the recombination and localisation of photogenerated charges in these materials, but with different branching ratios. Secondly, a comparison to wide bandgap metal oxides reveals a different relative importance of these recombination and localisation pathways compared to TMOs with smaller bandgaps. Taken together, a more general photophysical model emerges, providing insights into the performance limiting factors in TMOs, including those with extended visible light absorption.

References:

1. Wang, Q. et al. Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%. Nat. Mater. 1–3 (2016).
2. Takata, T. et al. Photocatalytic water splitting with a quantum efficiency of almost unity. Nature 581, 411–414 (2020).
3. Chen, S., Takata, T. & Domen, K. Particulate photocatalysts for overall water splitting. Nat. Rev. Mater. 2, 17050 (2017).
4. Kim, J. Y. et al. Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting. Sci. Rep. 3, 2681 (2013).