Two-dimensional AlN/g-CNs van der Waals type-II heterojunction for water splitting

A type-II van der Waals heterojunction photocatalyst is not only an ideal material for hydrogen production by water splitting, but also an important way to improve efficiency and produce low-cost clean energy. In this work, we unexpectedly found that monolayers of AlN and C₂N, g-C₃N₄, and C₆N₈ all formed type-II heterojunctions according to density functional theory, and we report a comparison of their photocatalytic performance. Among them, the AlN/C₂N heterojunction has an appropriate band gap value of 1.61 eV for visible light water splitting. It has higher carrier mobility than the AlN/g-C₃N₄ heterojunction (electron 253.1 cm² V^-1 s^-1 > 31.6 cm² V^-1 s^-1 and hole 11043.4 cm² V^-1 s^-1 > 524.7 cm² V^-1 s^-1), and an absorption peak similar those of monolayer C₂N in visible light (8 × 10⁴ cm^-1) and monolayer AlN in ultraviolet light (11 × 10⁴ cm^-1). The Bader charge shows that the charge transfer number of the AlN/g-C₃N₄ heterojunction is higher than that of the AlN/C₂N heterojunction, and its Gibbs free energy (-0.22 eV) is smaller than that of single-layer g-C₃N₄ (-0.30 eV). The AlN/C₆N₈ heterojunction also has a perfect band gap of 2.16 eV and an absorption peak of over 10 × 10⁴ cm^-1 in the UV region. Since a type-II heterojunction can effectively promote the separation of photogenerated electron-hole pairs and prevent their rapid recombination, the above heterojunctions are promising candidates for new photocatalysts.