Isolating hydrogen from oxygen in photocatalytic water splitting with a carbon-quantum-dot/carbon-nitride hybrid

Abstract

The practical utilization of solar-driven water splitting is restricted by the difficulty of this type of splitting in producing hydrogen and oxygen molecules with the same photocatalyst. For the separate delivery of photo-generated electron and hole carries to the reduction and oxidation sites in photocatalysts, close reduction–oxidation distances are required, which, however, promote reverse reactions or even damaging explosions due to the mixing of hydrogen and oxygen-containing products. To overcome this challenge, we designed a hybrid structure of carbon-quantum-dots (CQDs) attached to a single-layered carbon nitride (C₃N) material. Using first-principles calculations, we showed that the hybrid can harvest visible and infrared light for water splitting. It holds very close reduction–oxidation sites, ensuring the rapid delivery of photo-generated electrons and holes to the inner CQD and outer C₃N layers separately. The holes then break the water molecules and produce protons at the outer C₃N. Due to electrostatic attraction, the protons penetrate through C₃N to react with the photo-generated electrons on CQD and produce hydrogen molecules. Since neither oxygen species nor hydrogen molecules can migrate through the C₃N layer, hydrogen products are completely isolated from the oxygen species; this inhibits reverse reactions. These metal-free hybrids are thus appealing photocatalysts to integrate very close yet well-separated reduction–oxidation sites for practical solar energy conversion and hydrogen energy utilization.