Porphyrin-based metal-organic frameworks for solar fuel synthesis photocatalysis: band gap tuning via iron substitutions

Photocatalysts based on metal-organic frameworks (MOFs) are very promising due to a combination of high tuneability and convenient porous structure. Introducing porphyrin units within MOFs is a potential route to engineer these natural photosynthesis molecular catalysts into artificial photosynthesis heterogeneous catalysts. Using computer simulations based on density functional theory, we explore how to modify the electronic structure of porphyrin-based MOFs to make them suitable for the photocatalysis of solar fuel synthesis via water splitting or carbon dioxide reduction. In particular, we have investigated the effect that Fe substitutions have on the electronic properties of porphyrin-based metal organic frameworks. By aligning the electron levels with a vacuum reference, we show that Fe at the porphyrin metal centre has the effect of slightly raising the position of the valence band edge, whereas Fe at the octahedral metal node has the ability to significantly lower the position of the conduction band edge on the absolute scale. Iron is therefore a very useful dopant to engineer the band structure and alignment of these MOFs. We find that the porphyrin-based structure with Al in the octahedral sites and Zn in the porphyrin centres has a band gap that is slightly too wide to take advantage of visible-light solar radiation, while the structure with Fe in the octahedral sites has bandgaps that are too narrow for water splitting photocatalysis. We then show that the optimal composition is achieved by partial substitution of Al by Fe at the octahedral sites, while keeping Zn at the porphyrin centres. Our study demonstrates that porphyrin-based MOFs can be engineered to display intrinsic photocatalytic activity in solar fuel synthesis reactions.