Rational design of 2D Janus P3m1 M₂N₃ (M = Cu, Zr, and Hf) and their surface-functionalized derivatives: ferromagnetic, piezoelectric, and photocatalytic properties

In this study, first-principles calculations were employed to rationally design two-dimensional (2D) Janus transition metal nitrides of P3m1 M₂N₃ phases, where M is a d-block element (Sc-Zn, Y-Cd, Hf-Hg). Among the 29 examined 2D M₂N₃, three 2D phases, namely P3m1 Cu₂N₃, Zr₂N₃, and Hf₂N₃, exhibit excellent thermodynamic, dynamic, mechanical, and thermal stabilities. These novel Janus 2D materials exhibit ferromagnetic metallic and half-metallic behavior. The related 2D Janus surface-functionalized derivatives, Cu₂N₃H, Cu₂N₃F, Cu₂N₃Cl, Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F, are all dynamically stable. The 2D Janus P3m1 phases of Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F, all with M in the +IV oxidation state, act as semiconductors in the visible region, with energy band gaps of 2.26-2.70 eV at the HSE06 level of theory. On the other hand, the 2D Janus P3m1 Cu₂N₃X phases (where X = H, F, and Cl) are ferromagnetic half-metals. Additionally, it has been unveiled that there are high hole mobilities (∼6 × 10³ cm² V⁻¹ s⁻¹) derived from the moderate deformation potential and effective mass in the 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F phases. Uniaxial strain engineering has demonstrated the outstanding in-plane piezoelectric properties of 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F with high d₁₁ values (∼99.91 pm V⁻¹). Furthermore, the desirable band-edge alignments and high anisotropic carrier mobilities of 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F phases indicate their potential as visible light-driven photocatalysts for water splitting reactions on different facets. These properties render 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F phases promising for use in optoelectronics, piezoelectric sensing, and photocatalysis applications.