Environment-friendly metal-free photocatalysts represent a promising alternative to conventional metal-based semiconductors. In this report, a carbon dots (CDs) decorated graphitic carbon nitride (g-C3N4) photocatalyst was synthesized via a facile impregnation-thermal method. Under visible light irradiation, a very low CDs content of 0.5 web in the g-C3N4/CDs composite resulted in a 3.7 times faster reaction rate for phenol photodegradation than pristine g-C3N4. Spectroscopic and photoelectrochemical characterizations revealed that impregnation of CDs into g-C3N4 not only enhanced the production of photogenerated electron-hole pairs by extending the visible light absorption region due to the upconverted photoluminescence character of CDs, but also facilitated electron-hole separation by band alignment in the g-C3N4/CDs junction, thus yielded more holes, O-center dot(2)- and (OH)-O-center dot radicals to promote phenol degradation. These results highlight the potential application of sustainable metal-free photocatalysts in water purification. (C) 2015 Elsevier B.V. All rights reserved.