Activated carbon fibers (ACFs) with controlled pore sizes were prepared by KOH activation to efficiently capture CO2 molecules. The surfaces of the ACFs were modified by liquid oxidation using hydrofluoric acid to enhance the adsorption of CO2 by the fibers based on the effects of the oxygen-containing functional groups introduced on the surface. Oxygen-containing functional groups were effectively introduced onto the surfaces of the ACFs based on the fluorine radical effect, and they attached themselves to the pores of the ACFs as the concentration of hydrofluoric acid increased. The oxygen-containing functional groups, such as carboxylic and hydroxyl groups, on the surfaces of the ACFs played an important role in guiding CO2 into the micropores via the attractive forces experienced by the electrons in the CO2 molecules. (C) 2015 Elsevier B.V. All rights reserved.