Shape dependent photocatalytic H2 evolution of a zinc porphyrin

Porphyrins are planar, highly aromatic macrocycles, ideal prototypes for solar energy collection in photocatalytic water-splitting devices, due to their analogous function in the natural process of photosynthesis. These molecules can self-assemble to form different shapes and size-organized structures, which have considerable influence on their optical properties. The natural organization and properties of porphyrins have inspired the development and study of synthetic porphyrinic analogues to gain a deeper understanding of their aggregation mechanism and potential applications. In particular, porphyrins have attracted considerable research attention for photocatalytic hydrogen evolution applications owing to their remarkable facile structural modification, chemical stability, photophysical and photochemical properties.  The well-defined structures of self-assembled porphyrin derivatives make them potential candidates for improving hydrogen production by enhancing light adsorption, blocking charge recombination and improving charge carriage.

With the global energy demand is consistently rising, there is an urgent requirement to drastically reduce our dependence on greenhouse gas emitting fossil fuels. In this context it is widely recognized that green hydrogen is a clean energy source with the potential for long-term sustainability. Photocatalytic water-splitting provides an alternative, clean, sustainable hydrogen generation path. A photocatalytic system must be able to harvest energy from incident light radiation, and this role is often achieved by a molecular component known as a photosensitizer.  Photocatalytic splitting of water, consisting of photocatalysts, is one of the most popular hydrogen production strategies. Although porphyrins are mainly used as photosensitizers in solar hydrogen production, a major drawback is the limited photoinduced electron transfer in the aqueous phase. This is because most metalated porphyrins are used in homogenous catalysis involving organic solvents. One way of addressing this problem is by using self-assembled chromophore structures, which can be prepared using multiple self-assembly strategies. For instance, metalated porphyrins can be self-assembled due to their favorable aromatic features and planar molecular skeleton. However, the application of self-assembled structures in photocatalytic hydrogen evolution remains underexplored.

In a new research paper published in Dalton Transactions, Greek researchers: Dr. Emmanouil Orfanos and Professor Athanassios Coutsolelos from the University of Crete in collaboration with Dr. Kalliopi Ladomenou from International Hellenic University and Dr. Panagiotis Angaridis from Aristotle University of Thessaloniki studied several self-assembled porphyrin derivatives applicable to the field of photocatalytic hydrogen evolution. In brief, a “good-bad” solvent self-assembly protocol was employed to form the simple zinc(II) tetraphenyl porphyrin structures. The self-assembly behaviors and the impact of different shapes obtained were investigated in hydrogen evolution experiments under visible light irradiation.

The research team obtained three different structures: octahedral, “flower” and “manta ray”-shaped structures, by utilizing the “bad” solvent (methanol) and alerting the “good” solvent. In the presence of ascorbic acid as a sacrificial electron donor and 5% w/w Pt-nanoparticles as catalysts, the prepared structures facilitated proton reduction and the production of molecular hydrogen. The three structures exhibited different catalytic activities toward hydrogen production. The octahedral structure type achieved the maximum hydrogen evolution rate of 185.5 µmolg^-1h^-1, which was four times greater than that produced by the “flower” structure type. Furthermore, an important advantage of zinc porphyrin was that it could be recycled, modified, and reused to achieve the most efficient assembly of hydrogen production.

In summary, the study reported the photocatalytic hydrogen evolution from water using self-assembled Zn(II)-porphyrin structures as photosensitizers. The amount of hydrogen production mainly depended on the type of chromophore structure formed. Overall, the researcher’s findings demonstrated the possibility of altering simple porphyrin-based chromophores using a suitable mix of solvent-based self-organization to improve their photocatalytic hydrogen evolution capacity. In a statement to Advances in Engineering, the authors explained that their findings would accelerate production of hydrogen fuels to reduce the overdependence on fossil fuels and meet the demand for clean energy.