Metal-free Janus α- and β-SiCP₄: designing stable and efficient two-dimensional semiconductors for water splitting

Two-dimensional (2D) semiconductors exhibit exceptional potential in the field of photocatalytic water splitting due to their unique structural characteristics and photoelectric properties. In this study, based on first-principles density functional theory, we theoretically proposed two SiCP₄ Janus 2D semiconductors with high stability, namely monolayer α- and β-SiCP₄. By performing the calculation of HSE06 functionals, the band structures of monolayer α- and β-SiCP₄ have been estimated, and the results show that both α- and β-SiCP₄ are direct-band-gap semiconductors with band gaps of 1.64 eV and 1.91 eV, respectively. Meanwhile, the band edge levels of monolayer α- and β-SiCP₄ meet the band structure requirements of photocatalysts in water splitting. Notably, because of the internal build-in electric fields and tiny band gaps, monolayer α- and β-SiCP₄ exhibit separated photogenerated electron-hole pairs and high solar-to-hydrogen (STH) efficiency, reaching up to 33.68% and 23.72%, respectively. Additionally, we also investigate the impact of uniaxial strain on electronic, optical and photocatalytic properties of monolayer α- and β-SiCP₄ considering pH values ranging from 0 to 14. Our results demonstrate that the maximum STH efficiency for α-SiCP₄ is achieved under X-direction strain (η) of 2%, Y-direction strain (η) of 8%, and pH values between 2 and 4. Conversely, β-SiCP₄ exhibits the highest STH efficiency under X-direction strain (η) of 8%, Y-direction strain (η) of 6%, and pH values between 2 and 4.