The team constructed a photocatalyst in which hydrogen bonds link an electron donor, a perylene diimide supramolecule, to an electron acceptor, an aminated fullerene unit. These hydrogen bonds create a strongly charge polarized local environment that enhances dielectric screening and weakens the Coulomb attraction between photogenerated electrons and holes. At the same time, the directional nature of the hydrogen bonds provides well defined pathways that support exciton delocalization across the donor acceptor interface.
By transforming tightly bound Frenkel type excitons into weakly bound charge transfer excitons, the hydrogen bonded structure lowers exciton binding energy and enables spontaneous exciton dissociation under visible light. This spontaneous separation means that a larger fraction of the absorbed photon energy appears as mobile charges that can drive redox chemistry rather than recombining as heat or light. The result is more effective utilization of photogenerated charges in the subsequent water oxidation reaction.
Compared with conventional supramolecular assemblies formed from single component molecular building blocks, the hydrogen bond engineered donor acceptor composite develops a much stronger internal electric field. This internal field arises from the strong electronic interactions at the interface and the asymmetric charge distribution imposed by the hydrogen bonds. The strengthened field steers electrons and holes in opposite directions, driving more rapid and directional charge migration through the photocatalyst particles.
Under operating conditions, the researchers observed that the hydrogen bonded system significantly increased the population of useful surface holes, which are the active oxidizing agents in water splitting. After charge extraction and recombination processes were accounted for, the effective surface hole concentration was enhanced by a factor of six relative to a comparable system lacking hydrogen bonded interfaces. With more oxidizing holes reaching the catalyst surface, the rate of the water oxidation half reaction rises sharply.
In performance tests under visible light irradiation, the hydrogen bonded photocatalyst achieved an oxygen evolution rate of 63.9 millimoles per gram per hour. The material also delivered apparent quantum efficiencies of 11.83 percent at 420 nanometers and 4.08 percent at 650 nanometers, indicating that it can use not only higher energy blue light but also lower energy red light to drive oxygen evolution. These figures place the system among the best reported organic photocatalysts for oxygen evolution under similar conditions.
Most previous work on hydrogen bond based photocatalysts has centered on promoting hydrogen evolution, hydrogen peroxide formation, or carbon dioxide reduction, where electron driven reduction processes dominate. By contrast, the oxygen evolution reaction is the more sluggish, kinetically demanding half step of overall water splitting, and progress in this area has been comparatively slow. The new study shows that hydrogen bond engineering can be applied directly to this challenging oxidative step.
By demonstrating a hole dominated organic semiconductor platform with state of the art oxygen evolution performance, the work offers a design blueprint for constructing efficient overall water splitting systems. It suggests that tailoring the local electrostatic potential, exciton landscape, and internal electric field through supramolecular hydrogen bonding can provide a versatile handle for tuning charge dynamics. Such strategies may be extended to other organic or hybrid photocatalysts aiming at solar fuel production and related photoelectrochemical transformations.
Research Report: Hydrogen bond promoted exciton dissociation for efficient photocatalytic water oxidation
Related Links
Department of Chemistry of Tsinghua University
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