Enhancing the photocatalytic hydrogen generation performance and strain regulation of the vertical GeI 2 /C 2 N van der Waals heterostructure: insights from first-principles study

A suitable electronic structure and efficient charge separation are significant for the performance of photocatalytic water splitting. Herein, we have designed a two-dimensional GeI 2 /C 2 N van der Waals (vdW) heterostructure and systematically examined its stability, electronic, photocatalytic, optoelectronic and optical properties, and the effects of applying biaxial strain using density functional theory calculations. Based on ab initio molecular dynamic simulations and phonon dispersion calculations, the dynamic and thermal stability of the GeI 2 /C 2 N vdW heterostructure was confirmed. The GeI 2 /C 2 N vdW heterostructure showed an inherent type-II indirect bandgap energy of 2.02 eV and excellent visible light absorption, which were significantly improved compared to those of the monolayers. The projected band structure showed that the valence band minimum and conduction band maximum were contributed by the C 2 N and GeI 2 monolayers, respectively, which was favourable for efficient charge separation, thus increasing the solar energy conversion. Moreover, the GeI 2 /C 2 N vdW heterostructure band edges precisely straddled the water redox potential energies for pH values ranging from 0 to 9, allowing it to meet the conditions for spontaneous water splitting. The charge density difference revealed that about 0.263 electrons were transferred from the C 2 N to GeI 2 monolayer, and the potential drop at the interface was estimated to be 7.16 eV. We further hypothesised that strain might be utilised to tune the band edges and bandgap of the GeI 2 /C 2 N vdW heterostructure, resulting in a transition from a type-II indirect to a type-I direct bandgap semiconductor under tensile biaxial strain. Our findings provide a theoretical design for strategies to improve the performance of GeI 2 /C 2 N in solar conversion, nanoelectronic and optoelectronic devices.