Controlling the Surface Energetics and Kinetics of Hematite Photoanodes Through Few Atomic Layers of NiOx

Photoanodes for solar water splitting require the use of a large applied potential to sustain high photocurrent. This has been conventionally addressed by depositing electrocatalysts on semiconductors to boost the kinetics of water oxidation. Herein, it is shown that the advancement in onset potential (VON) of hematite (alpha-Fe2O3) is not regulated by the overlayer activity but rather by how it modifies the surface properties of the electrode. NiOx catalysts deposited with diverse methodologies induce different effects on the J-V curve of alpha-Fe2O3. Electrodeposited NiOx produces only an increase in photocurrent at high bias, while photodeposited NiOx induces also a 200 mV cathodic shift of VON, which reaches 0.58 VIE. Cathodoluminescence spectroscopy reveals that only through photodeposition is an effective passivation of surface defects achieved. This produces a decrease in electron-hole recombination and a substantial shift of the quasi-Fermi level in light. Concurrently, the Fermi level in the dark reaches a new energetic level. Electrochemical impedance spectroscopy on NiOx photodeposited shows a 4-fold reduction of charge transfer resistance accounting for the low VON. This study shows how surface energetics and kinetics of photoanodes are tightly connected. Only the complete control of energetics allows achieving new performing interfaces for low-bias water splitting.