The metal-free graphitic carbon nitride (g-C3N4) polymer is a promising photocatalyst for energy production and environmental protection. However, attempts to intrinsically improve its low activity in photooxidation are rarely effective for the poor molecular oxygen (O-2) activation. Here we report a synthesis of directionally nitrogen-doped in-plane metal-free heterostructure, through coplanar grafting nitrogen-doped carbon nanosheets (NDCN) to g-C3N4. The in-plane grafted NDCN not only promoted exciton dissociation and charge transfer but also enhanced activating O-2 to reactive oxygen species, including superoxide radicals (O-center dot(2)-) and H2O2. The optimal C3N4-NDCN coplanar heterojunction (C3N4-NDCN-3) photocatalytically degraded 96.3% of sulfamethoxazole (SMX) under visible light irradiation at a low light intensity of 14.5 mW cm(-2) in 4 h, whose SMX degradation rate was 37.7-fold higher than that of pure g-C3N4. Furthermore, C3N4-NDCN-3 exhibited 95.3% removal of SMX under sunlight irradiation (59.8 mW cm(-2)) in 1 h, higher than the 58.0% of pristine g-C3N4. First-principles calculations and material characterizations demonstrated that the coplanar NDCN served as electron sink and catalytic center for hot-electron involved O-2 activation. The improved charge carrier separation and O-2 activation promoted generation of photoexcited hole and superoxide for photocatalytic degradation of SMX. The design strategy in this work inspires a new approach for high-performance polymer photocatalysts in solar energy storage and environmental remediation. (C) 2019 Elsevier Inc. All rights reserved.