Graphitic carbon nitride (g-C3N4) as a 2D layered carbon material with exceptional electronic, thermal, and catalytic property has attracted increasing research attention. A porous g-C3N4 nanosheet (PCNN) has been synthesized via a facile two-step thermal etching oxidation approach and applied in low-temperature elemental mercury (Hg) removal. PCNN exhibits good reactivity toward Hg adsorption at temperatures of 40-200 degrees C. CuO impregnation can prominently enhance the Hg capture performance of PCNN on account of the intimate interaction of CuO and PCNN. Temperature has a positive effect on Hg capture ability of PCNN and CuO/PCNN. Hg is mainly oxidized by the chemisorbed oxygen species that are generated from the O-C-N of g-C3N4 and the lattice oxygen of CuO. CuO modification can efficiently activate g-C3N4 with significantly improved He capture ability presumably owing to the Mott-Schottky effect at the interface of CuO and g-C3N4.