The synthesis of environmental-friendly metal-free photocatalysts has great significance in photocatalytic technology. In this work, we firstly report the successful synthesis of in situ epitaxial growth of g-C3N4 on carbon dots through a facile thermal polymerization technique. Characterization and density functional theory (DFT) calculations were conducted to clarify the structure engineering and the electronic/chemical properties of the in-plane interconnected carbon dots/g-C3N4 (C-CN) heterostructures. With the optimal carbon dots content, the C-CN exhibited 3.2 times higher degradation rate for sulfadiazine (SDZ) than that of g-C3N4. Besides, the C-CN heterostructures displayed excellent stability and reusability in five consecutive cycles. The enhanced photocatalytic activity was related to the narrowed band gap and the local electronic density of valance band and conduction band orbitals of the unique plane heterostructures, corroborated by the spectroscopic characterizations and theoretical calculations. Photogenerated holes dominated the degradation of SDZ, while center dot OH showed a negligible contribution. Moreover, DFT calculation succeeded to predict that the atoms with high Fukin index (f(0)) on SDZ molecule were more vulnerable to radicals attack. SDZ degradation pathway mainly included smiles-type rearrangement, SO2 extrusion, ring hydroxylation and S-N bond cleavage processes. The eco-toxicity assessment revealed the generation of less toxic intermediates after photocatalysis. Our findings not only afford a new technique for constructing g-C3N4-based in-plane heterostructures with high and stable photocatalytic efficiency, but also highlight the feasible application of metal-free photocatalysts in environmental remediation. (C) 2019 Elsevier Inc. All rights reserved.