Reaction atmospheres during graphitic carbon nitride preparation can have a significant influence on the chemical composition and structure of the material, subsequently improving the photocatalytic activity. However, it is still a challenge to introduce an atmosphere by one-step heat-treated method to synthesis graphitic carbon nitride without additive gases. Herein, we developed a new one-step method to gather a variety of gases for preparing petals-like graphitic carbon nitride nanosheets (CN-C), such as CO(g), NH3(g) and H2O(g). NH3(g) and H2O(g) are respectively derived from melamine-cyanuric acid super-molecule during pyrolysis. The petals-like CN-C with more triazole defects (Nc) significantly increases the separation efficiency and the mobility of photogenerated photo-induced electron-hole pairs. Compared with the g-C3N4 calcined under nitrogen atmosphere (CN-N), CN-C has smaller grain, higher porosity with larger surface area, and remarkably longer lifetime of charge carriers. As expected, the product CN-C exhibited a hydrogen evolution rate of 1334 mu mol g(-1) h(-1) under visible-light irradiation, which was 2.8 times higher activity than CN-N, as well as higher than most of the reported bulk g-C3N4. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.