Prediction of new 2D Janus MXI (M=Cd, Zn; X=Br, Cl) monolayers with excellent electronic, optical, and photocatalytic properties for water-splitting applications

Recently, metal halides have emerged as an intriguing family of two-dimensional (2D) materials, attracting significant interest for their potential in next-generation electronic and optoelectronic applications. In this study, we employ density functional theory (DFT) calculations to design and comprehensively investigate the structural, electronic, optical, and photocatalytic properties of four new Janus monolayers: ZnBrI, ZnClI, CdBrI, and CdClI. Our results reveal that these monolayers exhibit both dynamic and thermal stability, as confirmed through ab initio molecular dynamic simulations, cohesive energy calculations, and phonon dispersion analyses. The band structures of the MXI (M = Cd, Zn; X = Br, Cl) monolayers were also predicated employing DFT with Hubbard U corrections (DFT + U). The computed band gaps show that the ZnBrI and ZnClI monolayers possess indirect bandgaps of 2.18/2.18 eV and 2.09/2.10 eV, respectively, as calculated with the PBE/PBE + U methods, while. In comparison, the HSE06 functional yields larger bandgaps of 3.07 eV and 2.99 eV, respectively. Conversely, the CdBrI and CdClI monolayers exhibit direct bandgaps of 2.52/2.53 eV and 2.37/2.37 eV, respectively, using PBE/PBE + U, while the HSE06 functional results in bandgaps of 3.91 eV and 2.73 eV, respectively. Notably, these monolayers exhibit remarkable absorption coefficients in the ultraviolet region, reaching up to 10⁵ cm⁻¹, indicating strong light-harvesting capabilities. CdBrI and CdClI monolayers demonstrate optimal band alignments for water redox reactions, making them excellent candidates for effective water-splitting photocatalysts. Overall, our results highlight transition metal halide monolayers as promising materials for advanced optoelectronic devices, high-performance solar cells, and sustainable energy applications.