Key Role of Valence Band Position in Porous Carbon Nitride for Photocatalytic Water Splitting

The microstructure, electrical energy band, and photocatalytic properties of carbon nitride were regulated and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), and UV–vis methods. Using oxygen-containing functionalization, equivalent overpotentials for oxygen and hydrogen production were realized in porous and amorphous carbon nitride with a constant narrow band gap. As the valence band position in such carbon nitride changes only from +1.51 to +1.67 eV with the increase in oxygen-containing functional groups, the oxygen production increased from 5.5 to 9.3 μmol with 3 h of irradiation, and the hydrogen production increased from 75.4 to 143.9 μmol in spite of the negative effect caused by the concurrent drop in the conduction band. The valence band position plays a very critical role in combining oxidation with the reduction half-reaction of photocatalysis.