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, N-2 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 N-2 molecules. Moreover, as an electron transfer bridge, Cu(I)-N active sites promote electron transfer from the catalyst to the adsorbed N-2 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 N-2 molecule with high adsorption energy and elongate the N equivalent to N bond. Charge density difference result confirms the electrons transfer from the Cu+ doping sites to the N-2 molecule. Density of states results indicate that the sigma(g)2p orbital in nitrogen atom is delocalized significantly when N-2 is adsorbed on Cu+ doping sites; also, the pi(g)*2p orbital is transferred to the vicinity of the Fermi level. These make the nitrogen molecules more efficient to activate.