First-principles study of Ga₂Ge₂S₃Se₃ monolayer: a promising photocatalyst for water splitting

Recently, the quaternary Janus monolayers with the formula A₂B₂X₃Y₃ have been shown to be promising candidates for optoelectronic applications, especially in the photocatalytic water splitting reaction. Therefore, first-principles calculations were employed to investigate the photocatalytic properties of Ga₂Ge₂X₃Y₃ (X and Y represent S, Se or Te atoms) monolayers. The Ga₂Ge₂S₃Se₃ and Ga₂Ge₂Se₃Te₃ monolayers exhibit dynamic and thermal stability, supported by high cohesive energies (3.78-4.20 eV) and positive phonon dispersion. With a moderate Young's modulus (50.02-65.31 N m^-1) and high Poisson's ratio (0.39-0.41), these monolayers offer a balance of stiffness and flexibility, making them suitable for flexible electronic applications. Especially, the difference in work function of 0.27 eV induces an intrinsic electric field in the Ga₂Ge₂S₃Se₃ monolayer, making the electronic structure of this material be suitable for the photocatalytic water splitting process. With light irradiation, the oxygen evolution reaction (OER) happened simultaneously, producing electrons and H⁺ protons for the hydrogen evolution reaction (HER) to happen at a low potential barrier. Moreover, the Ga₂Ge₂S₃Se₃ monolayer has a high absorption rate α(ω) of 10⁵-10⁶ cm^-1 and a high electron mobility of 430.82-461.50 cm² V^-1 s^-1. These characteristics result in a good solar-to-hydrogen of the Ga₂Ge₂S₃Se₃ monolayer (14.80%) which is promising for use in photon-driven water splitting.