Organic contaminants, dyes and antibiotics, discharged in wastewater systems, have posed great threats to the sustainability of the ecosystem. This study was performed to prepare graphitic carbon nitride (GCN) nanocomposites modified by nanocarbons, including carbon quantum dots (CQD), carbon nanotube (CNT), reduced graphene oxide (rGO), and carbon nanospheres (CNS), by a straightforward one-pot method. The characterization results suggest that after the modification with nanocarbons, GCN demonstrated slight red shift and stronger light absorption. The resultant photocatalysts revealed prominent performances for total photodegradation of organic contaminants. The degradation processes were investigated by in situ electron paramagnetic resonance (EPR). The mechanistic studies on the enhanced photoelectrochemical and photocatalytic performances were also conducted. Results indicate that GCN modified by the nanocarbon spheres displayed a substantial improvement in the degradation of sulfachloropyridazine (SCP) and dyes, compared favourably with other GCN samples modified by carbon nanotubes, quantum dots and layered graphene oxide. The photocatalytic degradation difference is mainly stemmed from the higher contents of COOH and C=O functional groups. The intimate contact or interaction between the two phases of GCN and nanocarbon in the nanocomposites may further improve the activity. This work provides insight in the design of highly efficient metal-free photocatalysts to better utilise the clean and free solar energy for environmental remediation. (C) 2020 Elsevier Inc. All rights reserved.