Tuning the morphology and structure of disordered hematite photoanodes for improved water oxidation: A physical and chemical synergistic approach

Design of efficient photoelectrodes for water oxidation requires careful optimization of the morphology and structure of a photoactive material to maximize electrical conductivity and balance carrier diffusion length with light penetration depth. Hematite-based photoanodes can theoretically oxidize water at very high rates, as provided by the optimal band-gap, but their performance is limited by the poor charge transport and low charge separation efficiency. Herein, we have developed physically- and chemically-induced morphological and structural tuning procedures, viz. capillary-force-induced self-assembly and corrosion followed by regrowth, which enable significant improvements in the performance of the hematite photoanodes. Specifically, a 24-fold enhancement in the photocurrent density for water oxidation (1 M NaOH) at 1.23 V vs. reversible hydrogen electrode under simulated 1 sun (100 mW cm^–2, AM1.5G solar spectrum) irradiation has been achieved. The capillary-force-induced self-assembly improves the crystallinity, promotes preferential orientation of the hematite along the [110] direction, and thereby enhances the electrical conductivity of the material. Subsequent dissolution and regrowth of the hematite nanostructures provide higher light absorption, improve photo-generated charge separation and facilitate photoelectrocatalytic kinetics resulting in the significantly higher photoelectrocatalytic activity. These broadly applicable insights provide a robust set of guidelines for the engineering of efficient photoelectrodes initially made of disordered structures for conversion of solar energy into renewable fuels.