Direct band-gap monolayer phosphorene <i>via</i> 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_<i>x</i> and p_<i>z</i> 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.