Surface charge recombination matters for single-versus polycrystalline catalysts in the case study of hematite photoanodes

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Charge recombination is a critical problem limiting the efficiency of catalysts for solar water splitting. Constructing both single- and polycrystalline structures have been proposed to tackle this issue, however, comparison of the two is mainly focused on the crystallinity and a comprehensive analysis of the underlying reasons is lacking. Herein, we show that the enhancement in water-oxidation activity of the single crystalline photoanode is dominated by the lower surface charge recombination as compared to the polycrystalline one, taking hematite nanorod arrays prepared by gas phase cation exchange to exclude the influence of shape as the model catalyst. In contrast, the unexpected lower bulk charge separation efficiency of single crystal than that of polycrystal indicates that increasing the crystallinity is actually not the major factor for improving bulk charge transport efficiency. Our study sheds light on the structure–property relationship of monocrystal versus polycrystal in the hematite photoelectrochemical cell, beneficial to design of high-efficient catalysts for solar energy conversion.