New Findings for the Much‐Promised Hematite Photoanodes with Gradient Doping and Overlayer Elaboration

Herein, it is demonstrated that gradient Ti doping coupled with an overlayer of NiFeO x on hematite can markedly improve the photoelectrochemical (PEC) water‐splitting efficiency of hematite‐based photoanodes, which are prized from sustainability considerations but have met daunting challenges. First, the gradient Ti doping of hematite has effectively lowered the onset potential while maintaining the high efficiency of photo‐generated charge separation and transmission. Second, the NiFeO x layer not only substantially reduces the surface trap states, but also significantly enhances the oxygen evolution kinetics of hematite‐based photoanodes as an oxygen evolution catalyst, resulting in a further improvement of the onset potential. Consequently, with the TiO 2 layer and a double electrode stack design, a remarkable photocurrent density of 4.49 mA cm −2 is achieved at 1.23 V versus reversible hydrogen electrode (RHE) for NiFeO x /(Grad Ti)‐Fe 2 O 3 /TiO 2 photoanode without any hole scavenger, delivering a high applied bias photo‐to‐current efficiency of up to 0.58% at 1 V versus RHE. This multipronged attack for improving PEC water‐splitting efficiency revitalizes the great promise of hematite photoanodes and sheds light on the design and development of the next‐generation photoelectrodes.