The g-C3N4 has been modified by a hydrothermal post treatment with orthoboric acid. It is shown that the surface modification with an appropriate amount of orthoboric acid obviously enhances the surface photovoltage responses of g-C3N4, clearly indicating that the separation of photogenerated charges is greatly improved. This is well responsible for the enhanced photocatalytic activities for degrading representative gas-phase acetaldehyde, and liquid-phase phenol. Moreover, it is demonstrated that the amount of O-2 adsorbed on the surfaces of g-C3N4 is greatly increased after H3BO3 modification based on the O-2 temperature-programmed desorption curves. It is suggested that the orthoboric acid modification favors O-2 adsorption to promote the photogenerated electrons captured for improved photocatalytic activities. This work would provide feasible routes to further improve the photocatalytic performance of semiconductors for degrading pollutants. (C) 2015 Elsevier B.V. All rights reserved.