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