Graphene-like porous g-C3N4 nanosheets were synthesized via direct pyrolysis of thiourea followed by a thermal exfoliation. With increased exfoliation temperature, the color of the resulting samples gradually became shallow, and the thickness and size of the layers were decreased. A formation mechanism involving layer-by-layer exfoliation coupled with layer splitting was proposed. The band structure of the g-C3N4 nanosheets was continuously tuned because of quantum size effect. Time-resolved decay spectra indicated that the radiative lifetime of charge carriers (71 and 72) increased from 4.13 and 26.23 ns for bulk g-C3N4 to 5.36 and 36.57 ns for graphene-like g-C3N4 nanosheets. The g-C3N4 nanosheet samples were applied for visible light photocatalytic removal of NOx in air. The performance of porous g-C3N4 nanosheets was significantly enhanced with increased exfoliation temperature from 450 to 550 degrees C. Moreover, photochemical and structural stability was well maintained after multiple reaction cycles. By monitoring the reaction intermediate NO2, it was found that the generation of NO2 was inhibited. The activity enhancement of graphene-like g-C3N4 nanosheets can be predominantly ascribed to the prolonged lifetime and improved photo-oxidation ability of charge carriers arising from the unique electronic structure. As the synthesis method for graphene-like g-C3N4 nanosheets with high a performance is simple, the g-C3N4 nanosheets can be envisioned to be applicable in environmental remediation and solar energy conversion. (C) 2015 Elsevier B.V. All rights reserved.