Inside-and-out modification of graphitic carbon nitride (g-C3N4) photocatalysts via defect engineering for energy and environmental science

g-C3N4 is an attractive photocatalysts due to its visible-light response, earth abundance, chemical-thermal stability and high yield. However, the major limitation stems from the C-N forming π-conjugated planes along with relatively small electron mean free path (∼10 nm) and high symmetry, which impedes the separation and transfer of photogenerated carriers. Fortunately, defect design on g-C3N4 can effectively enhance the migration of photogenerated electrons by three aspects: 1) tuning charge redistribution within g-C3N4; 2) changing surface microstructures; 3) creating new and same electron excitation orbital direction. In this review, the different strategies and mechanisms of defect engineering are classified and summarized from the perspective of breaking the structural symmetry with increasing or decreasing the atoms in a g-C3N4 system. Defect modification methods with an increased atomic number include element doping (C/N self-doping and external element doping) and functionalization (functional group modification), and with a decreased number of atoms mainly referring to C or N or dual vacancies are well outlined. Accordingly, the application and mechanism of defect-modified g-C3N4 in multiple fields (e.g., volatile organic compounds (VOCs) oxidation, NOx oxidation, H2O2 evolution, sterilization, pesticide oxidation, hydrogen evolution, N2 fixation and CO2 reduction) are highlighted. This review is performed to draw a comprehensive conclusion on the defect modification strategy and photocatalytic mechanism of g-C3N4 and prospect a development trend in the future.