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Proposing Institution

Lehrstuhl für Theoretische Chemie, TU München
Project Manager

Johannes Ehrmaier
Lichtenbergstr. 4
85748 Garching
Photocatalytic water-splitting, that is, the production of hydrogen and oxygen from water with sunlight, has the potential to provide unlimited clean and sustainable energy. Recently, graphitic carbon nitride (CN) materials have attracted vast interest in the field. CN materials can split water, consist of earth abundant materials and are stable under irradiation. While many experimental studies demonstrated their capability of producing molecular hydrogen upon visible/UV irradiation, the underlying mechanism is unknown. Simulations are the only way to study the reaction mechanism in detail. Two complementary theoretical models have been developed: the condensed matter perspective and the molecular paradigm. The condensed matter view assumes that an electron hole pair (exciton) is created by the absorption of a photon. Subsequently, these two charge carriers are assumed to separate over mesoscopic distances. At the solid-liquid interface, the charge carriers are assumed to react with water, generating H2 and O2. Domcke and Sobolewski suggested an alternative two-step mechanism for solar water-splitting in hydrogen-bonded molecular systems: First, an H-atom is transferred from water to the catalyst after the absorption of a photon. In the second step, the transferred H-atom is photodetached and molecular hydrogen is formed by the recombination of H-atoms. This new paradigm describes the water-splitting process as a molecular photochemical reaction. The initially proposed molecular photocatalyst was pyridine. More recently, the concept has been generalized to include triazine and heptazine, which are the building blocks of the CN materials. So far, the research focused either on electronic band structures and adsorption energies for periodic systems (condensed-matter approach) or on excited states of clusters of organic molecules with water (molecular approach). The new CN materials, which are semiconductors made of organic heterocycles, call for a merger of the two perspectives. Beginning with the molecular building blocks triazine and heptazine, the proposed research will systematically approach the condensed matter limit – from molecules to materials.

Impressum, Conny Wendler