Vacancy Defect-Assisted Enhanced Nitrogen Fixation in Metal-Free 2D Orthorhombic Boron Nitride: Insights from Nonadiabatic Molecular Dynamics Simulations

The photocatalytic nitrogen reduction reaction (NRR) is a promising approach for green and sustainable ammonia (NH3) production under ambient conditions. However, the design of highly efficient photocatalysts remains a significant challenge owing to the inertness of N2 molecules, complex reaction kinetics, and substantial energy barriers. In this work, we investigate the catalytic mechanism and real-time photocarrier dynamics of NRR on pristine and defect-engineered orthorhombic boron nitride (o-B2N2), a metal-free and environmentally friendly 2D semiconductor. Using density functional theory (DFT) and time-dependent ab initio nonadiabatic molecular dynamics (NAMD) simulations, we systematically studied the electronic structure, optical absorption, free-energy profile, and dynamics of charge carrier recombination of both pristine and vacancy-defective o-B2N2. Our results demonstrate that the introduction of a double vacancy (DV-BN) into o-B2N2 significantly widens the band gap, prolongs the electron-hole recombination time from 2.18 ns (pristine) to 3.15 ns, and markedly improves the photocatalytic activity toward NRR. These findings demonstrate the potential of o-B2N2 as an efficient, low-cost photocatalyst for sustainable ammonia production.