Effect of Cu(I)–N Active Sites on the N2 Photofixation Ability over Flowerlike Copper-Doped g-C3N4 Prepared via a Novel Molten Salt-Assisted Microwave Process: The Experimental and Density Functional Theory Simulation Analysis

Flowerlike copper-doped g-C3N4 is synthesized via a novel molten salt-assisted microwave process in this work. X-ray diffraction, N2 adsorption, UV–vis spectroscopy, scanning electron microscopy, photoluminescence, temperature-programmed desorption, X-ray photoelectron spectroscopy, and electrochemical impedance spectra were used to characterize the prepared catalysts. The results show that Cu+ is not present as oxide but inserts at the interstitial position through the coordinative Cu(I)–N bonds. These Cu(I)–N active sites can act as chemical adsorption sites to activate N2 molecules. Moreover, as an "electron transfer bridge", Cu(I)–N active sites promote electron transfer from the catalyst to the adsorbed N2 molecules. The as-prepared copper-doped g-C3N4 displays a much higher NH4+ generation rate than neat g-C3N4 prepared by calcination, as well as excellent catalytic and structural stability. Density functional theory simulations prove that Cu(I)–N active sites can adsorb the N2 molecule with high adsorption energy and elongate the N≡N bond. Charge density difference result confirms the electrons transfer from the Cu+ doping sites to the N2 molecule. Density of states results indicate that the σg2p orbital in nitrogen atom is delocalized significantly when N2 is adsorbed on Cu+ doping sites; also, the πg*2p orbital is transferred to the vicinity of the Fermi level. These make the nitrogen molecules more efficient to activate.