Emerging In‐Plane Junctions in Graphitic Carbon Nitride for Remarkably Enhanced Photocatalysis

Abstract Pristine graphitic carbon nitride (g‐C ₃ N ₄ ) possesses high exciton binding energy and suffers from strong Coulombic force, leading to difficulty in exciton dissociation and severe charge recombination. In addition, photocharges must overcome the significant energy barrier when they traverse across the conjugated layers, while they more favorably migrate along the in‐plane direction owing to the unique π‐conjugated stacking structure. Therefore, constructing in‐plane junctions within 2D directions can greatly enhance the photophysical properties of pristine g‐C ₃ N ₄ . To date, no relevant review articles have reported on this emerging topic. This review first deciphers the electronic structure of g‐C ₃ N ₄ and heterojunction formation processes from the perspective of molecular orbital and semiconductor physics theory, revealing the in‐depth mechanisms of photocatalytic processes. Subsequently, taking traditional heterojunctions as a prelude, it showcases the unparalleled advantages of in‐plane junctions and discuss in detail the latest progress of three typical in‐plane junctions (g‐C ₃ N ₄ /C in‐plane heterojunctions, g‐C ₃ N ₄ in‐plane molecular junctions, and g‐C ₃ N ₄ in‐plane homojunctions) in energy and environmental applications. Finally, focusing on research hotspots worth attention, it also provides critical perspectives for the future development of in‐plane junctions in g‐C ₃ N ₄ . Overall, this review is expected to serve as the cornerstone for the design and functionalization of high‐performance in‐plane junction systems.