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As a typical organic semiconductor photocatalyst, graphitic carbon nitride (g-C₃N₄) suffers from low photocatalytic activity. In this paper, g-C₃N₄ was prepared by polymerization of dicyandiamide (C₂H₄N₄) in the presence of ammonium chloride (NH₄Cl). It was found that the addition of ammonium chloride can greatly improve the photocatalytic activity of g-C₃N₄ towards CO₂ reduction. The optimal photocatalyst (CN-Cl 20) exhibited a CO₂-to-CO conversion activity of 50.6 μmolg⁻¹h⁻¹, which is 3.1 times that of pristine bulk g-C₃N₄ (BCN) that was prepared in the absence of any ammonium chloride. The enhanced photoactivity of g-C₃N₄ was attributed to the combined effects of chloride modification and an enlarged specific surface area. Chloride modification of g-C₃N₄ can not only reduce the bandgap, but also causes a negatively shifted conduction band (CB) potential level, while ammonia (NH₃) gas from the decomposition of NH₄Cl can act as a gas template to exfoliate layered structure g-C₃N₄, improving the specific surface from 6.8 to 21.3 m²g⁻¹. This study provides new ideas for the synthesis of highly efficient g-C₃N₄-based photocatalytic materials for CO₂ conversion and utilization.