It is highly desired for g-C3N4 nanosheets as efficient photocatalysts to greatly enhance the photogenerated charge separation by trapping holes. Herein, it is clearly demonstrated mainly by means of the steady-state surface photovoltage spectra, the time-resolved surface photovoltage responses in N-2 and the fluorescence spectra related to the produced (OH)-O-center dot amount that the modified chloride with a proper amount could effectively trap the photogenerated holes so as to greatly enhance the charge separation of g-C3N4, leading to the obviously-improved photocatalytic activities for degrading 2,4-dichlorophenol (2,4-DCP) and converting CO2 to CH4. Interestingly, similar positive effects on g-C3N4 are also confirmed after modification with other halogen anions, like Br- and F-, whereas the Cl- modifier is the best one. Although the used Cl- and Br- have different mechanism for capturing holes from the modified F-, it is concluded that it is feasible to greatly enhance the charge separation by the halogen-induced surface polarization. As excepted, the formed (OH)-O-center dot as the hole-modulated direct products could dominate the photocatalytic degradation of 2,4-DCP. Moreover, the possible decomposition mechanism closely related to "OH attack is proposed through the detected main intermediates. This work will help us to well understand the importance to trap the photogenerated holes for efficient photocatalysis on g-C3N4, and also provide a feasible strategy to improve the photocatalytic activities of g-C3N4 for environmental remediation and energy production. (C) 2017 Elsevier B.V. All rights reserved.