Dielectric Screening and Magnetic Force Modulated Spontaneous Exciton Dissociation for Enhanced Photocatalytic Water Splitting: Insights into Exciton Excited States

Two-dimensional photocatalysts possess strongly bound excitons and are thus ineffective in driving catalytic reactions that require free charge carriers. Herein, theoretically studying the ground- and excited-state exciton dynamics in metal-telluro-halide-based vdW heterostructure, we demonstrate dielectric-engineering as an efficient strategy to regulate exciton binding energy (EBE). The highly delocalized exciton cloud extending over a few hundred unitcells reduces the EBE to kBT (25 meV), promoting spontaneous exciton dissociation into free carriers. A controllable transition from a Frenkel to Mott-Wannier exciton is depicted. Importantly, we elucidate how a magnetic field causes accelerated charge separation by applying opposing forces to photogenerated electrons and holes. Simultaneously, it enhances the EBE through an exciton diamagnetic shift, which could potentially reduce charge separation. Additionally, thermodynamic analysis reveals a spontaneous OER and an easier HER aided by moderate external bias attributable to the elevated position of hybridized orbitals of Br_4p and Te_5p. This study offers a roadmap for the design of high-performance photocatalysts.