A solid-state chemical reduction approach to synthesize graphitic carbon nitride with tunable nitrogen defects for...

Graphitic carbon nitride with nitrogen defects (g-C₃N_4-x) is prepared by a facile and effective solid-state chemical reduction technique at mild temperature conditions. The cyano groups and nitrogen vacancies, as evidenced by electron paramagnetic resonance (EPR), X-ray photoelectron spectrometer (XPS), Fourier transform infrared spectra (FTIR) and Solid-state ¹³C MAS NMR spectra, are controllable via adjusting chemical reduction temperature. Comparing to the pristine g-C₃N₄, the as-prepared g-C₃N_4-x shows much enhanced photocatalytic H₂ evolution activity under visible-light irradiation. The maximum H₂ evolution rate of 3068 μmol·g^-1·h^-1 is achieved with g-C₃N_4-x after chemical reduction treatment at 400 °C for 1 h, which is 4.85 times that of the pristine g-C₃N_4. Moreover, excellent reusability and storage stability have been shown by this photocatalyst as well. It is discovered that nitrogen defects can result in both the up-shift of the valance band and the down-shift of the conduction band, which benefit the absorption of longer wavelength photons and trapping of the photoinduced electrons, therefore reducing the recombination losses of the generated carriers. It is because of this improved visible-light absorption and charge carrier separation, g-C₃N_4-x displays better visible-light photocatalytic activity compared to the pristine g-C₃N₄. It is then concluded that the synthetic strategy presented here represents a straightforward and efficient way to synergistically optimize the chemical composition, optical response, and photocatalytic characteristics of g-C₃N₄-based photocatalysts.