Modulating Charge Transfer Efficiency of Hematite Photoanode with Hybrid Dual-Metal-Organic Frameworks for Boosting Photoelectrochemical Water Oxidation.

The glorious charge transfer efficiency of photoanode is an important factor for efficient photoelectrochemical (PEC) water oxidation. However, it is often limited by slow kinetics of oxygen evolution reaction. Herein, a dual transition metal-based metal-organic frameworks (MOF) cocatalyst, Fe@Ni-MOF, is introduced into a titanium-doped hematite (FeO:Ti) photoanode. The combination of Ni and Fe can optimize the filling of 3d orbitals. Moreover, the introduction of Fe donates electrons to Ni in the MOF structure, thus, suppressing the irreversible (long-life-time) oxidation of Ni into Ni. The resulting Fe@Ni-MOF/FeO:Ti photoanode exhibits ∼threefold enhancement in the photocurrent density at 1.23 V versus the reversible hydrogen electrode. Kinetic analysis of the PEC water oxidation processes indicates that this performance improvement is primarily due to modulating the charge transfer efficiency of hematite photoanode. Further results show that a single transition metal-based MOF cocatalyst, Ni-MOF, exhibits slow charge transfer in spite of a reduction in surface charge recombination, resulting in a smaller charge transfer efficiency. These findings provide new insights for the development of photoelectrodes decorated with MOFs.