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Revealing the Essential Role of Iron Phosphide and its Surface-Evolved Species in the Photoelectrochemical Water Oxidation of Hematite-Based Photoanode.

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, we constructed a multiphase FeP/Gd-Fe 2 O 3 shell-core structure photoanode and explored 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 exhibits 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 a-Fe 2 O 3 (0.64 mA cm -2 ). Detailed studies show that FeP can act as a photosensitizer to enhance light absorption and as a conductive layer to accelerate charge transfer. The FeP significantly enhances the incident photon-to-current conversion efficiency of the photoanode and improves the electron transition within the photoanode. Naturally evolved Fe-Pi on the surface provides more active sites for water oxidation. They effectively passivate the surface capture state and synergistically inhibit the electron-hole recombination. Moreover, the in-situ constructed multiphase catalyst has 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.