Shaping photocatalysis: How the morphology of a Ti-MOF dictates charge transport and efficiency in photoelectrochemical water splitting

Abstract

Photoelectrochemical (PEC) water splitting is a critical technology for sustainable hydrogen production, and metal-organic frameworks (MOFs) are emerging as highly tunable photo-absorbers. However, the efficiency of MOF-based photoanodes is critically dependent on their morphology, which dictates charge carrier dynamics and interfacial kinetics. A systematic understanding of this structure-property relationship remains a key challenge for rational material design. Here, we systematically investigate this relationship by synthesizing the titanium-based MOF, MIL-125-NH₂ (MNH), in three distinct morphologies: octahedral (O), tetragonal plates (TP), and circular plates (CP). These MOFs were integrated into a heterostructure with a TiO₂ underlayer to promote charge separation and functionalized with an Iridium oxide (IrO₂) co-catalyst to accelerate water oxidation. Photoelectrochemical measurements revealed a striking morphology-dependent performance. The circular plates heterostructure, IrO2/MNH(CP)/TiO₂, exhibited the highest photocurrent density of 2 mA/cm2 at 1.23 V vs. RHE, representing a 1.1-fold enhancement over the tetragonal plates and a 1.3-fold enhancement over the octahedral plates. The successful synthesis and distinct crystal shapes were rigorously confirmed by FE-SEM, PXRD, and BET analyses. Our findings demonstrate that the circular plates' morphology provides superior photoactivity, likely due to a combination of high surface area and the exposure of more active crystal facets. This work establishes a clear structure-function relationship in MOF-based photoanodes and underscores that morphology engineering is a powerful strategy for designing the next generation of highly efficient materials for solar fuel production. © 2025