Chlorine-functionalized carbon dots (C1-CDs) were prepared by the substitution reaction between Cl radicals into thionyl chloride molecules and carbon dots with containing OH/COOH groups at their surface (O-CDs). The obtained CI-CDs with a size of 2-5 nm contain 2-3% Cl atoms and emit blue light. Compared with amine-functionalzed carbon dots (N-CDs) and O-CDs, CI-CDs exhibit much higher photocatalytic activity under visible-light irradiation. The thermally and chemically stable phthalocyanine can be even degraded quickly through CI-CDs. This work suggests that surface engineering of carbon dots with heteroatoms can be used to enhance their photochemical properties. (C) 2015 Elsevier B.V. All rights reserved.