Insight into the orbital origins of anisotropic carrier mobility in TaS2X2 (X = Cl, Br, I) monolayers with highly efficient photocatalytic hydrogen evolution†

Discovering and designing novel narrow-band gap semiconductor materials with efficient charge carrier mobility is a critical strategy to address the energy crisis. However, the progress in developing high-performance photocatalysts has been hindered due to an unclear understanding of the structure–property relationship. In this study, we reveal the intrinsic correlations among the valence state and the local coordination environments of central metal ions, light absorption properties, and charge carrier separation efficiency in TaS₂X₂ (X = Cl, Br, or I). Based on bond theory analysis, we identify that Ta³⁺ sites in low-symmetry coordination (C_2v) environments induce symmetry breaking in the d-orbitals, resulting in a valence band maximum dominated by fully occupied Ta-5d orbitals, a defining characteristic of narrow-bandgap semiconductors. Furthermore, the anisotropic atomic arrangement within the two-dimensional plane enhances the separation and migration of photogenerated charge carriers, demonstrating the significant potential for efficient photocatalytic water splitting to produce hydrogen. The orbital engineering strategy offers a promising pathway for advancing two-dimensional materials in photocatalytic water splitting applications.