Boron Doping of Metal-Doped Hematite for Reduced Surface Recombination in Water Splitting

We report that metal ions (M: Sn4+ and Ti4+) and boron-codoped hematite photoanodes with an n–n+ homojunction showed significantly increased photoelectrochemical (PEC) water splitting activity with greatly reduced surface recombination. The secondary B-doping of broadly used M-doped hematite photoanodes not only suppresses the number of M+ ions, which inevitably cause electron–hole pair (EHP) recombination, but also generates an internal electric field for easy hole extraction. Taking advantage of these effects, the maximum length (500–600 nm) of hematite, which has the reported highest PEC performance, was increased to up to 900 nm in M:B-Fe2O, which in turn increased the active area of the photoanode. The M:B-Fe2O3 with a film thickness of 900 nm and a diameter of 122 nm converted the incident photons to EHPs with substantially reduced recombination and exhibited a photocurrent density of 1.92 mA/cm2 at 1.23 VRHE. After loading inexpensive oxygen evolution reaction catalysts (FeOOH) on the surface of M:B-Fe2O3, the photocurrent density of FeOOH/M:B-Fe2O3 reached 2.35 mA/cm2 at 1.23 VRHE. The cost-effective strategy of B-doping into M-doped hematite provides a straightforward way to address the M-doping-related negative effects, such as a high electron–hole recombination rate on the surface of hematite, and thus the critical length limitation of an ideal hematite photoanode, to potentially improve the performance of PEC devices.