Heterojunction between bimetallic metal-organic framework and TiO₂: Band-structure engineering for effective photoelectrochemical water splitting

Bimetallic Fe/Ni-based metal-organic frameworks (MOFs) with different Fe/Ni ratios were coated on TiO₂ nanorods (NRs), and the performances of the heterojunction photoanodes in photoelectrochemical water splitting were investigated. The bandgaps and band positions of the MOFs could be modulated by changing the ratio of the Fe and Ni components. An ideal band alignment was achieved between the TiO₂ NRs and bimetallic MOFs with an optimum ratio of [Fe]/[Ni] = 0.25/0.75, which allowed efficient light absorption and charge separation. The coating of NH₂-MIL(Fe)-88 layer on the TiO₂ NRs decreased the photocurrent density by 33%. In comparison, TiO₂/NH₂-MIL(Ni)-88 showed a modest improvement in photocurrent density (0.85 mA·cm⁻² at 1.23 V vs. a reversible hydrogen electrode (RHE)). When bimetallic NH₂-MIL(Fe_0.25Ni_0.75)-88 was coated on the TiO₂ NRs, the photocurrent density reached 1.56 mA·cm⁻², which was an efficiency enhancement of 3.2 times. The mechanism underlying high photoelectrochemical performance was investigated.