Direct band-gap monolayer phosphorene via pentagon–octagon–pentagon defects: highly anisotropic carrier transport and efficient photocatalytic activity

Vacancy defects in two-dimensional (2D) materials are not merely structural imperfections but can be strategically engineered to boost and tailor their intrinsic properties. In this work, we propose a novel 2D polymorph of phosphorene, featuring a periodic array of vacancy-derived pentagon–octagon–pentagon (p–o–p) units in blue phosphorene, employing first-principles calculations combined with quasi-particle G₀W₀ method. Structural optimization, positive phonon modes, mechanical resilience, and thermal stability up to 800 K collectively confirm its structural robustness, flexibility, and potential for experimental realization. P–o–p phosphorene is predicted to be a direct band-gap semiconductor with a quasi-particle gap of 1.95 eV. Its band-gap exhibits linear tunability under biaxial strain, ranging from −5% to 3%, with a direct-to-indirect band-gap transition occurring at approximately ∼4% tensile strain. Remarkably, this structure demonstrates anisotropic mechanical properties, high carrier mobility, and enhanced optical absorption in the visible and UV regions, driven by its asymmetric P–P bonding and distinct p_x and p_z orbital interactions near the Fermi level. Importantly, the strain-tunable gap and favorable band-edge alignment establish p–o–p phosphorene as a promising candidate for redox reactions in complete photocatalytic water splitting across a wide pH range. These findings provide a new pathway for rational design of group-VA-based 2D semiconductors, utilizing the defect-engineered architectures for cutting-edge applications in optoelectronics and photocatalytic applications.