Constructing a heterostructure and applying an external electric field are effective methods for enhancing photocatalytic efficiency. In this study, we systematically investigate the electronic and optical properties of an AlN/g-C3N4 heterostructure under varying electric fields using first principles. Our findings indicate that the AlN/g-C3N4 heterostructure demonstrates high stability, and van der Waals (vdW) interactions exist between the interfaces. Compared with monolayers AlN and g-C3N4, the reduced bandgap of AlN/g-C3N4 enhances electron escape ability. Furthermore, we observe that the bandgap of AlN/g-C3N4 reduces when an electric field is applied and the band alignment changes. Importantly, −0.4 V/Å AlN/g-C3N4 is not only a type-II heterostructure, forming a built-in electric field, but it also has a band alignment that spans the redox reaction of water. Consequently, the compound rate of electron–hole pairs substantially reduces, enhancing the possibility of AlN/g-C3N4 photocatalytic water splitting. Our findings provide a theoretical foundation for related experimental preparations.

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