Light-induced hydrogen production from water using nickel(II) catalysts and N-doped carbon-dot photosensitizers: catalytic efficiency enhancement by increase of catalyst nuclearity.

Solar energy conversion to chemical energy via light-induced H2O splitting to O2 and H2 is considered to be a promising solution to meet the growing global energy demands. To make this transformation economically viable, it is necessary to develop sustainable photocatalytic systems. Herein, we present an efficient photocatalytic H2 production system which relies on components comprised of low-cost and high-abundance elements. In particular, a series of mononuclear complexes [Ni(LNS)3]- and [Ni(N^N)(LNS)2] and a hexanuclear complex [Ni(LNS)2]6 (N^N = diimine and LNS- = heterocyclic thioamidate with different group-substituents) were synthesized and utilized as catalysts, in combination with N-doped carbon dots as photosensitizer, for efficient H2 evolution from aqueous protons. Differences in H2 production efficiency were observed among the studied Ni(II) catalysts, with complexes bearing ligands with stronger electron-donating ability exhibiting higher catalytic activity. A remarkable catalytic efficiency enhancement was observed for the hexanuclear complex, with catalyst loadings lower than those of the mononuclear Ni(II) complexes, affording TONs >1550 (among the highest values reported for photocatalytic systems of similar type operating in H2O). These data provide an indication of catalytic cooperativity between the metal centers of the hexanuclear complex, and demonstrate the crucial role of atomically precise polynuclear Ni(II) catalysts in light-induced H2 production, a result that can guide future catalyst design towards the development of highly efficient, low-cost and environmentally benign photocatalytic systems.

Shape dependent photocatalytic H2 evolution of a zinc porphyrin.

Hydrogen is regarded as a promising molecular fuel in order to produce clean energy, thus it is of great importance to produce and store H2 in order to replace fossil fuels and to resolve the global energy and environmental problems. One strategy to produce hydrogen is the photocatalytic splitting of water. In this study different supramolecular architectures of a Zn(II) porphyrin, showing "flower", octahedral and "manta ray" shaped structures, were obtained using the "good-bad" solvent self-assembly protocol. More specifically, the bad solvent (methanol) was retained and the good solvent was alerted obtaining diverse assemblies. The different structures were studied by scanning electron microscopy, PXRD, UV-Vis and IR spectroscopies. The prepared structures were capable of proton reduction and production of molecular H2 in the presence of 5% w/w Pt-nanoparticles as catalysts and ascorbic acid as a sacrificial electron donor. Moreover, depending on the structure of the chromophore that is formed the amount of H2 produced varies. The maximum H2 production was obtained with the octahedral structures (185.5 µmol g-1 h-1).