Chemical Cutting of Network Nodes in Polymeric Carbon Nitride for Enhanced Visible-Light Photocatalytic Hydrogen Generation

Polymeric carbon nitride (C3N4) has been arising as an important semiconductor photocatalyst for photocatalytic hydrogen evolution and pollutant removal for solving the ever-pressing energy crisis and environmental issues. The crystallinity, degree of polymerization, and defect formation of the C3N4 molecular structure exhibit a profound effect on its photocatalytic performance. Herein, a facile method was proposed to introduce a certain amount of nitrogen vacancies in C3N4 by calcining trithiocyanuric acid and melamine at different temperatures. An optimal photocatalytic hydrogen evolution rate of 65.1 μmol·h–1 is achieved for catalyst SCN600, about 54 times that of pristine C3N4 (CN). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) observations showed that the introduction of nitrogen defects in polymeric carbon nitride molecules by cutting the network nodes is the main factor for enhancing the photocatalytic performance, in addition to the crystallinity and polymerization degree. Nitrogen defects induce the midgap energy level, increasing light utilization and facilitating charge transfer and separation. Density functional theory (DFT) simulations verified that cutting the network nodes increased the localized highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states in polymeric carbon nitride, thus inhibiting electron–hole recombinations.