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Artificial photosynthesis decoded: How carbon nitride splits water (and enables green hydrogen)

Scientists have long sought to understand the exact mechanism behind water splitting by carbon nitride catalysts. For the first time, Dr. Paolo Giusto and his team captured the step-by-step interactions at the interface between carbon nitride and water, detailing the transfer of protons and electrons from water to the catalyst under light.

This discovery lays critical groundwork for optimizing catalyst materials for hydrogen production as a renewable energy solution. The findings are in the journal Nature Communications.

Plants use light to generate fuels through photosynthesis—converting energy from the sun into sugar molecules. With artificial photosynthesis, scientists mimic nature and convert light into high-energy chemicals, in pursuit of sustainable fuels. Carbon nitrides have long been identified as effective catalysts in this ongoing quest. These compounds of carbon and nitrogen use light to break water into its constituent parts, oxygen and hydrogen—with hydrogen representing a promising renewable energy source.

But how exactly does water splitting work? For the first time, researchers have captured every step of one of the most studied yet least understood reactions of the past decade. "This goes beyond answering a longstanding question in fundamental science," argues principal investigator Dr. Paolo Giusto of the Max Planck Institute of Colloids and Interfaces. "Unveiling the interaction between water molecules and carbon nitrides under light provides essential input for advancing green energy."

The key to solving this scientific riddle lies in the intricate processes that unfold when water adheres to the surface of carbon nitride. Until now, the different timescales of the many reactions involved have made it difficult to piece together a full picture, and scientists have relied on theoretical calculations and retrospective experiments. Using advanced spectroscopic techniques, Giusto and colleagues caught the carbon nitride in the act.

The magic happens at the interface—the nanoscopic boundary between the solid carbon nitride and the liquid water molecules. The carbon nitride transfers electron density to the water, forming a hybrid system. "From this point on, the water and the catalyst act as a new, hybrid semiconductor. It's as if they joined forces in a team with properties distinct from those of the individual elements," says Dr. Sonia Żółtowska.

This triggers further reactions, as the transfer of particles creates an imbalance. In this case, the chemical bonds of water begin to weaken. When carbon nitride absorbs light, it uses the energy to destabilize water molecules further through a proton-coupled electron transfer.

"This means a simultaneous transfer of a positively charged proton and a negatively charged electron from water to the catalyst," explains Dr. Daniel Cruz of the Fritz Haber Institute. This intermediate compound was the missing piece of the artificial photosynthesis puzzle: the researchers recorded and analyzed in real-time the mechanism that ultimately breaks down the water into oxygen and hydrogen.

This discovery is at the surface chemistry level but carries profound implications for advancing sustainable energy solutions. While large-scale use of hydrogen as an alternative to fossil fuels remains a goal for the future, this research outlines a roadmap for fine-tuning catalysts and getting one step closer to efficiently producing hydrogen from water splitting.