Rational design of Zn:Sn overlayers to enhance the water splitting kinetics of hematite photoanodes Alejandro Galán-González 1 , Aswathi K. Sivan 2 , Rubén Canton-Vitoria 3,4 , Ana M. Benito 1 , Wolfgang K. Maser 1 1 Instituto de Carboquímica (ICB-CSIC), C/ Miguel Luesma Castán 4, 50018 Zaragoza, Spain, 2 Department of Physics, University of Basel, 4056 Basel, Switzerland, 3 Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya 464-8601, Japan, 4 Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan The intrinsic limitations of hematite, particularly the extremely short hole diffusion length, low carrier mobility and slow oxygen evolution initiation, resulting in high surface charge recombination and sluggish water oxidation kinetics, have prevented the realisation of its theoretical potential as photoanode. To overcome these inherent limitations, herein we propose a dual strategy to tackle them while maintaining its key characteristics: first, hematite was doped with Ti in situ during growth to greatly improve its charge carrier mobility [1] ; second, an ultrathin Zn:Sn overlayer was deposited on top of the as-grown hematite nanorods to enhance the charge separation and surface kinetics [2] . To assess the efficacy of this approach, we show how the presence of Sn or Zn individually boosts the charge carrier transfer from the surface of the photoanode to the electrolyte and induces a drastic reduction in the onset potential, respectively. Thus, we demonstrate that exerting careful control over the Zn:Sn ratio of the overlayer allowed us to harness the synergistic benefits of both components, with a 3:1 ratio yielding the best performance. As a result, we obtained a hematite photoanode with enhanced charge carrier separation and transfer, minimal surface charge trapping and recombination, and improved onset potential, which translated in a five-fold improvement of the photogenerated current density, from 0.4 mA cm -2 for the unmodified hematite to 1.40 mA cm -2 after Ti-doping and, finally, 2.05 mA cm -2 after the addition of the Zn:Sn overlayer. Moreover, the efficiency of the hematite-based photoanodes is drastically improved after the modifications, with ABPE values rising from 0.074 % for the base hematite to 0.30 % for the doped and coated electrode. Therefore, our results highlight a novel avenue for the integration of Zn:Sn overlayers leads to the facile and affordable development of highly efficient hematite photoanodes. References 1. ACS Materials Lett. 2024, 6, 7, 2897–2904 2. Carbon Energy. 2023; 5:e369
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