First-principles investigation of the optical and piezoelectric properties of Janus TiOS monolayers

Using first-principles calculations, we systematically explore the geometric structures, stabilities, electronic structures, optical responses, and piezoelectric properties of Janus TiOS monolayers to assess their suitability for integrated photocatalytic and piezoelectric applications. The stability of both phases was assessed through formation energy calculations, phonon dispersion relations, AIMD simulations, and elastic constant analyses, which collectively confirm their dynamical, thermal, and mechanical stability. Electronic structure calculations reveal that the 1T-TiOS monolayer is a direct-band-gap semiconductor (<i>E</i> _g = 1.27 eV), whereas the 2H phase exhibits an indirect band gap of 1.52 eV. Charge-density analyses indicate that the CBM is mainly contributed by Ti 3d orbitals, while the VBM originates from S 2p states, providing a favourable orbital configuration for efficient charge separation. Both phases exhibit strong visible-light absorption, with absorption coefficients exceeding 1 × 10⁵ cm^-1. Remarkably, the 1T- and 2H-TiOS monolayers demonstrate strong potential as solar-driven photocatalysts for water splitting over a broad pH range (0-10). Furthermore, Janus TiOS monolayers show intrinsic and pronounced piezoelectric responses. For the 1T phase, the in-plane and out-of-plane piezoelectric coefficients are 4.99 and 0.32 pm V^-1, respectively, while the corresponding values for the 2H phase are 2.38 and 0.27 pm V^-1. These results indicate that Janus TiOS monolayers are promising candidates for next-generation materials integrating photocatalytic and piezoelectric functionalities.