Ultra-short carrier diffusion length for efficient photoelectrochemical water oxidation in hematite photoanodes

The electrode/electrolyte interfacial depletion layer could accelerate the carriers' separation along their diffusion pathway via induced internal electric field (IEFs). Enhancing the proportion of depletion layer and shortening carriers' diffusion length (L) are crucial for developing hematite (alpha-Fe2O3) base catalysts with high carrier utilization efficiency, yet remains challenging. Herein, a sulfur and nitrogen co-pyrolyzed strategy is proposed to simultaneously modulate the electronic depletion layer and microstructural diffusion layer in anodized iron oxides nanotubes (SN co-pyrolyzed NTs). The nanowalls of anodized NTs were deliberately thinned as smaller than the diffusion length of carriers within hematite, resulting in ternary Fe2O3/Fe3O4/FeS heterostructure with optimized depletion layer width (W) and surface potentials. An optimal W (similar to 3.35 nm) to L (similar to 3.18 nm) ratio was achieved as larger than 1, indicating an overlapped electronic depletion layer and microstructural diffusion layer was constructed. This optimal W/L ratio implies that the carriers' diffusion process would be fully accelerated via the IEFs, which could massively promote the carriers' separation and subsequent utilization efficiency. The SN co-pyrolyzed photoanode exhibited significant improved onset potential of 0.62 V-RHE, state-of-the-art photocurrent density of 22.6 mA cm(-2) (1.23 V-RHE) and incident photon-to-current conversion efficiency of 65 %, which surpass ever reported hematite based photoanodes. DFT calculations proves that the induced electron-deficient surface could create favorable adsorption sites for oppositely charged intermediates and reduce energy barrier of *OOH formation. This work provides a facile strategy for manipulating the carrier kinetics via microstructural and electronic optimization and is expected to facilitate the design of new photoelectrodes for solar-to-fuel energy conversion.