Revealing the Essential Role of Iron Phosphide and its Surface‐Evolved Species in the Photoelectrochemical Water Oxidation by Gd‐Doped Hematite Photoanode

Abstract Phosphates are easily derived from transition metal phosphides under natural conditions, and the real roles of these two in catalytic reactions are not yet clear. Here, a multiphase FeP/Gd−Fe 2 O 3 shell‐core structure photoanode was constructed and explored regarding the real role of FeP and its surface‐reconstructed iron phosphate (Fe−Pi) in photoelectrochemical water oxidation. The FeP/Gd−Fe 2 O 3 photoanode exhibited an excellent photocurrent density of 2.56 mA cm −2 at 1.23 V versus the reversible hydrogen electrode, up to 4 times greater than those of the pristine α‐Fe 2 O 3 (0.64 mA cm −2 ). Detailed studies showed that FeP could act as a photosensitizer to enhance light absorption and as a conductive layer to accelerate charge transfer. The FeP significantly enhanced the incident photon‐to‐current conversion efficiency of the photoanode and improved the electron transition within the photoanode. Naturally evolved Fe−Pi on the surface provided more active sites for water oxidation. They effectively passivated the surface capture state and synergistically inhibited the electron–hole recombination. Moreover, the in‐situ constructed multiphase catalyst had a smaller interfacial contact resistance than the intentionally decorative cocatalyst. This work provides new insight into the understanding of the essential role of transition metal phosphides and their surface‐reconstructed species in catalytic reactions.