Predicted stable two-dimensional semiconductor TiOS materials with promising photocatalytic properties: First-principles calculations

TiO₂ is a well-known photocatalyst with a band gap of 3.2 eV, yet its ability to absorb light is limited to the short wavelengths of ultraviolet light. To achieve a more effective photocatalytic material, we have designed two-dimensional semiconductor TiOS materials using swarm intelligence algorithms combined with first-principles calculations. Three stable low-energy structures with space groups of P2₁/m, P3m1 and P2₁/c are identified. Among these structures, the Janus P3m1 phase is a direct bandgap semiconductor, while the P2₁/m and P2₁/c phases are indirect bandgap semiconductors. Utilizing the accurate hybrid density functional HSE06 method, the band gaps of the three structures are calculated to be 2.34 eV (P2₁/m), 2.24 eV (P3m1) and 3.22 eV (P2₁/c). Optical calculations reveal that TiOS materials exhibit a good light-harvesting capability in both visible and ultraviolet spectral ranges. Moreover, the photocatalytic calculations also indicate that both P2₁/m and P3m1 TiOS can provide a strong driving force for converting H₂O to H₂ and O₂ in an acidic environment with pH = 0. The structural stabilities, mechanical properties, electronic structures and hydrogen evolution reaction activities are also discussed in detail. Our research suggests that two-dimensional TiOS materials have potential applications in both semiconductor devices and photocatalysis.