Photocatalytic material class First systematic computational investigation of polyheptazine imides

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Polyheptazine imides have some key structural and functional twists that make them especially interesting for photocatalysis. So far, there was only limited knowledge about how structural changes affect the electronic and optical properties in this class. A team at HZDR has now presented a reliable and reproducible theoretical method to solve this challenge.

Photocatalysis promises an efficient conversion of abundant solar energy into usable chemical energy. Polyheptazine imides have some key structural and functional twists that make them especially interesting for photocatalysis. So far, there was only limited knowledge about how structural changes affect the electronic and optical properties of the many material candidates in this class. A team led by researchers from the Center for Advanced Systems Understanding (CASUS) at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now presented a reliable and reproducible theoretical method to solve this challenge that was confirmed by measurements done on genuine candidate materials (DOI: 10.1021/jacs.5c09930). The scientists expect the field of polyheptazine imide materials research to undergo a boom.

Polyheptazine imides belong to the family of carbon nitrides, which are layered, graphene-like compounds composed of nitrogen-rich, ring-shaped units. Unlike graphene, which exhibits excellent electrical conductivity but lacks photocatalytic activity, polyheptazine imides possess band gaps suitable for visible-light absorption.

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Carbon nitride-based materials impress due to their low production cost, nontoxicity and thermal stability. However, the first generation of such materials were not ideal photocatalysts as the materials possessed properties that hindered charge separation. If a material has a low charge separation, the electron excited by an incoming photon quickly recombines with the hole it was propelled from – and releases energy only as heat or light. No energy is available to drive chemical reactions. “Polyheptazine imides containing positively charged metal ions exhibit markedly improved charge separation. This feature renders them highly suitable for practical applications,” says first author Dr. Zahra Hajiahmadi.

Computer science narrows down options

Better materials are for instance needed to realize the expected economic potential of photocatalytic reactions like water splitting (to produce hydrogen as a fuel), carbon dioxide reduction (to produce basic carbohydrates as fuels or industrial chemicals) or hydrogen peroxide production (as a basic industrial chemical). To successfully design a polyheptazine imide material that catalyzes a desired reaction smoothly, researchers have to fine-tune every aspect of the material. Obviously, this cannot be done by synthesizing every possible candidate material. This is where computer science comes to the rescue.

“The design space is enormous,” says Prof. Thomas D. Kühne, Director of CASUS, leader of the CASUS research team “Theory of Complex Systems” and senior author of the new publication. “One can for example add functional groups on the surface or substitute specific nitrogen or carbon atoms with oxygen or phosphorus atoms.” Kühne’s group at CASUS is developing novel numerical techniques, which are as efficient as possible and yet, at the same time, qualitatively reproduce the correct chemistry and physics of the underlying system.

Finding the perfect material – in a systematic way

Hajiahmadi’s research focused on the key feature of polyheptazine imides: the negatively charged pores that can be equipped with positively charged metal ions. This setup can greatly enhance catalytic activity. Hajiahmadi’s work is the first comprehensive study on the influence of different metal ions on the optoelectronic properties of polyheptazine imides. In total, 53 different metal ions were analyzed and classified with respect to their location (in plane or between the layers) and their effect on the geometry of the material (resulting in a distortion or not).

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“We used a reliable and reproducible computational framework that goes beyond conventional modeling approaches,” says Hajiahmadi. “Standard computational studies of photocatalysts typically focus on ground-state properties and neglect excited-state effects, despite the fact that photocatalysis is inherently driven by photoexcited charge carriers. Specifically, we employ many-body perturbation theory methods.” Starting from an easily solvable non-interacting system, these methods treat interactions as small perturbations. The effects of interactions are calculated as small corrections to the known solution. In the end, all mathematical expansions result in an approximation of how large groups of particles influence each other. Because of their high computational cost these methods are rarely used in this field. But the presented study clearly confirms that the benefits are overwhelming as the new computational framework enables a qualitatively accurate description of a material’s optical absorption and electronic structure under illumination.

Using this approach, the scientists systematically investigated how the different metal ions influence the geometry of the polyheptazine imide polymeric network. The results show that ion incorporation can induce distinct structural distortions, including changes in layer spacing and local bonding environments. These geometric modifications directly affect the electronic band structure and optical behavior, including light-harvesting efficiency.

To validate the theoretical predictions, eight polyheptazine imides, each equipped with a different metal, were synthesized and tested for their suitability to catalyze hydrogen peroxide production. “The results clearly showed a high degree of agreement to our predictions and outperformed competing calculation methods,” Hajiahmadi concludes. Kühne adds: “If there was some doubt about polyheptazine imides being one of the most promising platforms for next-generation photocatalytic technologies, I believe this work put them to rest. The path toward the targeted design of efficient polyheptazine imide photocatalysts for sustainable reactions is clearer now. I firmly believe that it will be taken often and successfully.”