Constructing a MOF-on-MOF heterojunction on a hematite photoanode for efficient photogenerated carrier transport

The effective construction of surface catalysts and heterojunctions can accelerate photogenerated carrier separation and transfer to improve photoelectrochemical water splitting (PEC-WS) performance. Integration of two or more metal-organic frameworks (MOFs) as surface overlayers onto photoelectrode materials can build a nanoscale MOF-on-MOF heterojunction to drive the separation and transfer of photogenerated electron-hole pairs. In this work, MIL-96 and UiO-66 MOFs were sequentially loaded onto an α-Fe₂O₃ photoanode by a strong interaction to form an effective MOF-on-MOF heterojunction, which exhibited excellent PEC catalytic activity and stability. The α-Fe₂O₃/MIL-96/UiO-66 photoanode exhibited a 125% enhancement of photocurrent density (2.25 mA cm^-2) at 1.23 V_RHE. The coexistence of Fe³⁺/Fe²⁺ and O_V could enhance the electrical conductivity and reduce the charge recombination rate of the α-Fe₂O₃ photoanode. The formation of Fe-O/Fe and weak Fe-Al(MIL-96) and Fe-Zr(UiO-66) coordination facilitated photogenerated electron-hole transport between the α-Fe₂O₃ photoanode and MIL-96/UiO-66 overlayer. Furthermore, the constructed bimetallic MIL-96/UiO-66 heterojunction synergistically provided more active sites, and promoted photogenerated carrier separation and transfer, thereby reducing the reaction kinetics of water oxidation and enhancing PEC-WS performance. This work provides a new modification route to develop high-efficiency photoelectrode materials with outstanding PEC-WS performance.