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.

A novel design of SiH/CeO2(111) van der Waals type-II heterojunction for water splitting.

Searching for economical low-dimensional materials to construct the highly efficient type-II heterojunction photocatalysts for splitting water into hydrogen is very strategic. In this study, using the first-principles calculations, we construct a novel SiH/CeO2(111) type-II heterojunction with a very small lattice mismatch of less than 1%. Based on AIMD simulation and phonon dispersion calculations, the SiH/CeO2(111) heterojunction reveals sufficient stability, and is easy to synthesize. Due to the vdW interaction between SiH and CeO2(111) components, electron and hole accumulation regions form at the heterojunction interface, which is very conducive to the separation of photoexcited electron-hole pairs. Besides, the SiH/CeO2(111) heterojunction has good visible light response, and even a strong absorption peak of up to 8.7 × 105 cm-1 in the high-energy visible region. More importantly, the SiH/CeO2(111) heterojunction exhibits good OER and HER performance because its oxidation and reduction potentials well meet the requirements of water splitting. Consequently, SiH/CeO2(111) is a potential photocatalyst for splitting water to hydrogen.