Lowering the onset potential of Zr-doped hematite nanocoral photoanodes by Al co-doping and surface modification with electrodeposited Co–Pi

Herein, in situ zirconium-doped hematite nanocoral (Zr–Fe₂O₃ (I) NC) photoanode was prepared via a specially designed diluted hydrothermal approach and modified with Al³⁺ co-doping and electrodeposited cobalt–phosphate (“Co–Pi”) cocatalyst. Firstly, an unintentional in situ Zr–Fe₂O₃ (I)) NC photoanode was synthesized, which achieved an optimum photocurrent density of 0.27 mA/cm² at 1.0 V vs. RHE but possessed a more positively shifted onset potential than conventionally prepared hematite nanorod photoelectrodes. An optimized amount of aluminum co-doping suppresses the bulk as well as surface defects, which causes a negative shift in the onset potential from 0.85 V to 0.8 V vs. RHE and enhances the photocurrent density of Zr-Fe₂O₃(I) NC from 0.27 mA/cm² to 0.7 mA/cm² at 1.0 V vs. RHE. The electrodeposited Co–Pi modification further reduce the onset potential of Al co-doped Zr–Fe₂O₃(I) NC to 0.58 V vs. RHE and yield a maximum photocurrent of 1.1 mA/cm² at 1.0 V vs. RHE (1.8 mA/cm² at 1.23 V vs RHE). The improved photocurrent at low onset potential can be attributed to synergistic effect of Al co-doping and Co–Pi surface modification. Further, during photoelectrochemical water-splitting, a 137 and 67 μmol of hydrogen (H₂) and oxygen (O₂) evolution was achieved over the optimum Co–Pi-modified Al-co-doped Zr–Fe₂O₃(I) NC photoanode within 6 h. The proposed charge transfer mechanism in optimum Co–Pi-modified Alco-doped Zr–Fe₂O₃(I) NC photoanodes during the photoelectrochemical water splitting was also studied.