Atomic- and Molecular-Level Functionalizations of Polymeric Carbon Nitride for Solar Fuel Production

Polymeric carbon nitride (PCN) is a metal-free semiconductor that has received extensive research attention due to its unique advantages such as low cost, high stability, and visible-light response. However, pristine PCN is not an ideal photocatalyst because of fast electron–hole recombination and its inert surface for molecular adsorption. Nevertheless, benefiting from the N-linked tri-s-triazine structure, PCN can be readily functionalized through chemical modifications. As such, researchers have made enormous efforts to develop various strategies for modifying PCN to achieve enhanced photocatalytic activities. Herein, recent advances in PCN functionalizations with different atomic or molecular units, including metal and nonmetal elements, small functional groups, organic functional fragments, and metal complex motifs, are summarized. Moreover, the effect of different PCN functionalization strategies on the molecular structure, optical properties, electronic behaviors, and photocatalytic activities particularly for water splitting and CO₂ reduction is discussed. In the end, the remaining challenges in the structural modification of PCN and future opportunities of functional PCN-based photocatalysts for efficient solar fuel production are elaborated on.