Graphitic carbon nitride (g-C3N4, CN) is recognized as the most extensively studied organic polymeric photocatalyst for pollution control and energy conversion, due to its facile synthesis and suitable electronic band structure. The aim of the present work is to explore the effect of precursors, such as urea (U, (NH2)2CO), dicyandiamide (D, C2H4N4) and melamine (M, C₃H₆N₆), on the structure and photocatalytic activity of the obtained CN samples, denoted as UCN, DCN and MCN, respectively. The sheet-like UCN sample shows significantly enhanced photoreactivity in both NO oxidation and CO2 reduction compared to the bulk DCN and MCN materials. In addition, UCN demonstrates the ability to suppress the formation of the toxic NO2 intermediate during the photocatalytic oxidation of NO. The enhanced photocatalytic activity of UCN can be attributed to a dual effect: first, its increased specific surface area provides more active sites for the photocatalytic reaction; second, it exhibits a stronger affinity for substrates like NO and CO2, which facilitates charge migration at the interface.