Pentagonal two-dimensional noble-metal dichalcogenide PdSSe for photocatalytic water splitting with pronounced optical absorption and ultrahigh anisotropic carrier mobility

Emerging two-dimensional (2D) noble-metal dichalcogenide (NMDC) PdX2 (X = S and Se) materials crystallize in an unusual orthorhombic structure (2O phase) with unique pentagons and PdX4 planar squares as building blocks, and have attracted tremendous attention in recent years for their novel physical and chemical properties, as well as promising applications in the future. Motivated by the attractive properties and encouraging applications of PdX2 materials, we explore the 2O phase NMDC material PdSSe using particle swarm optimization algorithms combined with first-principles calculations. We not only uncover three stable low-energy bulk phases, but also predict two polymorphs of the 2D monolayer exfoliated from the bulk phases, exhibiting mechanical, dynamic, and thermal stabilities. Due to its novel puckered pentagonal structure, the PdSSe monolayer exhibits flexible mechanical properties with anisotropic Young's modulus and Poisson's ratio. The bandgaps of the two polymorphs of the 2D pentagonal PdSSe monolayer are 2.10 and 2.06 eV, and their band-edges perfectly straddle the redox potential of water. Furthermore, these materials possess excellent light absorption in the visible and ultraviolet regions, and the absorption coefficients reach the order of 106 cm−1. More importantly, the predicted materials exhibit highly desirable carrier mobilities and anisotropy, which can promote the separation of the photogenerated electrons and holes, and effectively reduce the probability of recombination, thus remarkably promoting photocatalytic water splitting. These excellent properties enable PdSSe monolayers as candidate materials for promising applications in high-performance optoelectronic and nanoelectronic devices. The predicted unconventional properties of PdSSe not only supply a meaningful complement to the fascinating NMDC materials of PdSe2 and PdS2, but also will attract extensive interest from a wide audience to explore unanticipated properties and design new functional devices based on PdSSe.