Structural, electronic and photocatalytic properties of g-C3N4 with intrinsic defects: A first-principles hybrid functional investigation

Recent years have witnessed a surge of research in using graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst for hydrogen production. Experiments showed an enhanced catalytic performance in g-C3N4 by introducing intrinsic defects, but the physical mechanism remains elusive. Herein, via first-principles calculations with hybrid functional, we investigated the structural, energetic, electronic and optical properties of g-C3N4 with C and N vacancies. We identified the most stable configurations with the lowest formation energies, and found that the vacancy induced defect state resides inside the energy gap of g-C3N4, leading to enhanced optical absorption in the visible light region. Interestingly, spatially separated conduction and valence band edge states can be observed, which may contribute to suppressed recombination of photo-generated electron-hole pairs. Detailed analyses on band alignment with reference to normal hydrogen electrode potential reveal the superior photocatalytic properties of g-C3N4 with vacancy. We further discussed strain effects on the formation energies of C/N vacancies in g-C3N4. These results not only provide physical insight into available experimental results, but also shed new light on synthesizing novel and high-efficiency photocatalyst for energy applications.