First-principles discovery of stable, anisotropic, semiconducting Sb2X2O (X = S, Se) and Janus Sb2SSeO nanosheets for optoelectronics and photocatalysis

In this work, we conduct a comprehensive first-principles investigation into the design and discovery of novel antimony oxychalcogenide monolayers Sb2X2O (X = S, Se) and Janus Sb2SSeO, examining their structural stability, elastic, electronic, optoelectronic, and photocatalytic properties. Our analysis confirms their thermodynamic and dynamical stability and reveals low cleavage energies, indicating strong feasibility for mechanical exfoliation. The excellent agreement between our HSE06-predicted bandgap of bulk Sb2S2O and experimental measurements further validates the employed computational framework. EWe also find that their optoelectronic responses can be efficiently tuned via biaxial strain, providing a viable route for device-specific property engineering. Favorable band alignments, strong optical absorption, efficient carrier transport, and relatively high dielectric constants collectively support their candidacy for overall water splitting under neutral conditions.These results establish a solid theoretical foundation for the rational design of Sb-based 2D nanostructures and highlight their potential in next-generation direction-dependent optoelectronic and sustainable energy-conversion applications.

💡 Research Summary

In this work, the authors employ state‑of‑the‑art first‑principles density functional theory (DFT) calculations to discover and characterize a new family of two‑dimensional antimony oxychalcogenide monolayers: symmetric Sb₂X₂O (X = S, Se) and an asymmetric Janus Sb₂SSeO. Structural models are derived from the experimentally known bulk Sb₂S₂O, and the chalcogen atom is substituted to generate Sb₂Se₂O; Sb₂Te₂O is also considered but found dynamically unstable via phonon calculations and therefore excluded from further analysis.

Thermodynamic stability is confirmed through negative formation energies, while dynamical stability is demonstrated by the absence of imaginary phonon modes for Sb₂S₂O, Sb₂Se₂O, and the Janus structure. Mechanical robustness is quantified using elastic constants obtained with the optimized high‑efficiency strain‑matrix sets (OHESS) method, showing that all three monolayers satisfy the Born stability criteria and possess sizable in‑plane Young’s moduli. Ab‑initio molecular dynamics (AIMD) simulations at 500 K for 5 ps reveal no structural degradation, reinforcing the thermal stability of the sheets.

Exfoliation feasibility is assessed by calculating cleavage energies from six‑layer bulk models. The resulting values—0.36 J m⁻² for Sb₂S₂O and 0.40 J m⁻² for Sb₂Se₂O—are comparable to graphene’s experimental cleavage energy and significantly lower than that of MoS₂, indicating that mechanical exfoliation should be straightforward.

Electronic structure calculations are performed at the PBE level, refined with the hybrid HSE06 functional, and include spin‑orbit coupling (SOC) due to the heavy Sb atoms. Sb₂S₂O exhibits a direct band gap of ~2.80 eV at the Γ point, whereas Sb₂Se₂O and Janus Sb₂SSeO possess indirect gaps of 2.24 eV and 2.44 eV, respectively. SOC reduces the gaps by roughly 0.1–0.2 eV but does not alter the overall band alignment. Bader charge analysis shows that Sb atoms lose ≈1.1–1.6 e⁻, while O atoms gain ≈1.1–1.2 e⁻, confirming a more ionic Sb–O bond compared with the more covalent Sb–X bonds.

Carrier transport is investigated using the AMSET code, which incorporates deformation potentials, elastic constants, dielectric tensors, and polar phonon frequencies. Calculated electron mobilities reach 10³ cm² V⁻¹ s⁻¹ along the a‑axis and ~5 × 10² cm² V⁻¹ s⁻¹ along the b‑axis; hole mobilities are an order of magnitude lower but still exceed 10² cm² V⁻¹ s⁻¹. The pronounced anisotropy originates from direction‑dependent effective masses and deformation potentials, suggesting that device performance can be tuned by crystal orientation.

Optical properties are derived from the frequency‑dependent dielectric function computed with HSE06. Both symmetric and Janus monolayers display strong absorption coefficients (~10⁵ cm⁻¹) across the visible to near‑UV range. Biaxial strain (±6 %) effectively modulates the band gap by up to 0.5 eV and can induce a direct‑to‑indirect transition, providing a practical route for strain‑engineered optoelectronic devices.

For photocatalytic water splitting, the absolute positions of the conduction band minimum (CBM) and valence band maximum (VBM) are aligned relative to the standard hydrogen electrode (SHE). At neutral pH (0 V vs. SHE), both CBM and VBM of the three monolayers straddle the water redox potentials (0 eV for H⁺/H₂ and 1.23 eV for O₂/H₂O), satisfying the thermodynamic criteria for overall water splitting. The relatively high in‑plane dielectric constants (ε‖ ≈ 10–12) reduce exciton binding, while the high carrier mobilities suppress recombination, together enhancing the solar‑to‑hydrogen (STH) conversion efficiency. Theoretical calculations of η_STH, absorption efficiency (η_abs), and carrier utilization efficiency (η_cu) suggest that these monolayers could achieve STH efficiencies exceeding 10 % under ideal conditions.

In summary, the study establishes that Sb₂X₂O (X = S, Se) and Janus Sb₂SSeO are structurally robust, easily exfoliable, possess tunable band gaps in the visible range, exhibit strong anisotropic charge transport, and meet the electronic requirements for efficient photocatalytic water splitting. These findings position antimony‑based oxychalcogenide nanosheets as promising candidates for next‑generation anisotropic optoelectronic devices and sustainable hydrogen‑production technologies, and they lay a solid computational foundation for future experimental synthesis and device integration.