Mid-gap engineering in polymeric carbon nitride via C/O synergy for extended-spectrum CO₂ photoreduction

Solar-driven CO₂ reduction demands photocatalysts that are earth-abundant, inexpensive, and able to harvest a broad solar spectrum. Polymeric carbon nitride (PCN) shows great promise for photocatalysis but is limited by its wide bandgap and narrow light absorption. Here, we report a brown, hierarchically porous carbon nitride (Glu-PCN) built via one-step copolymerization of glutamic acid and urea, in which carbon and oxygen are woven into the tri-s-triazine framework at the atomic scale during condensation. Rather than acting as isolated impurities, these framework-integrated motifs generate localized states in the bandgap of carbon nitride. Serving as a spectral relay, these intra-gap states enable stepwise sub-bandgap excitation that extends the photoresponse deep into the visible region, while furnishing directional channels that accelerate charge transfer. Consequently, Glu-PCN far surpasses pristine PCN and, remarkably, sustains pronounced CO₂-reduction activity even under long-wavelength light (λ ≥ 540 nm), where pristine PCN is essentially inactive. This work establishes a simple, scalable framework-engineering strategy for advancing CO₂ reduction and other photocatalytic applications.