Mesoporous carbon nitrides (MCN) are fascinating materials with unique semiconducting and basic properties that are useful in many applications including photocatalysis and sensing. Most syntheses of MCN focus on creating theoretically predicted C₃N₄ stoichiometry with a band gap of 2.7 eV using a nano‐hard templating approach with triazine‐based precursors. However, the performance of the MCN in semiconducting applications is limited to the MCN framework with a small band gap, which would be linked with the addition of more N in the CN framework, but this remains a huge challenge. Here, we report a precursor with high nitrogen content, 3‐amino‐1,2,4‐triazole, that enables the formation of new and well‐ordered 3D MCN with C₃N₅ stoichiometry (MCN‐8), which has not been predicted so far, and a low‐band‐gap energy (2.2 eV). This novel class of material without addition of any dopants shows not only a superior photocatalytic water‐splitting performance with a total of 801 μmol of H₂ under visible‐light irradiation for 3 h but also excellent sensing properties for toxic acids.

By union of graphitic carbon nitride polymer with reduced graphene oxide (rGO, ≤1 wt%) via π–π stacking interaction, the band structure of carbon nitride could be well modulated. As a result, a significant increase of photocurrent was observed (e.g., when biased at 0.4 V vs.Ag/AgCl, the anodic photocurrent became 300% higher after doping). Not merely interesting in itself, graphene was also used as a general dopant for semiconductors in band-structure engineering.