Interface Control of Photoelectrochemical Water Oxidation Performance with Ni_1–xFe_xO_y Modified Hematite Photoanodes

In this work, Ni_1–xFe_xO_y coated hematite electrodes are investigated as a model system of different semiconductor/catalyst interfaces. We found that the photoelectrochemical (PEC) performance of the electrodes strongly depends on both the way the hematite electrode is prepared and the composition of the catalyst. Two extreme behaviors are observed for electrodeposited hematite electrodes coated with different compositions of catalyst. In the case of Fe-rich catalyst (Ni_0.25Fe_0.75O_y), the performance is substantially enhanced compared to the bare electrode; however, the Ni-rich (Ni_0.75Fe_0.25O_y) catalyst inhibits the PEC performance. A combination of photoelectrochemical, intensity modulated photocurrent spectroscopy, and electrochemical impedance spectroscopy measurements collectively reveal the critical role that the interface states of the semiconductor and catalyst plays in controlling the key interfacial charge transfer and recombination reactions. The photogenerated holes are efficiently collected and stored into the catalyst layer for the Ni_0.25Fe_0.75O_y coated hematite electrodes. An unusually large improvement in performance is attributed to this hole collection circumventing recombination at the hematite surface. For the Ni_0.75Fe_0.25O_y coated hematite electrodes, however, there is a presence of interface trap states that act as recombination centers and pin the catalyst potential. These combined results provide important new understanding of the role of the interfaces at semiconductor/electrocatalyst junctions.