Broken-Symmetry-Driven Promising Hydrogen Evolution in Janus NGeSiY (Y = N, P, As) Monolayers

We investigate the structural stability, lattice dynamics, electronic structure, charge transport, and hydrogen adsorption kinetics of Janus XGeSiY (X = N, P; Y = N, P, As) monolayers using first-principles calculations. The absence of inversion symmetry in these Janus systems induces intrinsic polarity, which strongly modifies their electronic states and surface reactivity. Phonon dispersion and elastic constant analyses confirm the dynamical and mechanical stability of all monolayers except PGeSiN, while their near-zero positive Poisson’s ratios and enhanced in-plane stiffness reflect the coupling between lattice rigidity and asymmetric bonding. Hybrid functional calculations reveal a wide tunability of band gaps (0.04–2.58 eV), with NGeSiN exhibiting a favorable semiconducting character accompanied by high electron mobility (∼2550 cm 2 /(Vs)), arising from reduced effective mass and deformation potential. Hydrogen adsorption thermodynamics show near-thermoneutral Gibbs free energies on the N-terminated surface of Janus NGeSiY, indicating strong modulation of adsorption strength by Janus-induced charge redistribution. Climbing-image nudged elastic band calculations further demonstrate that hydrogen evolution proceeds via a Volmer–Heyrovsky pathway, with the Heyrovsky step acting as the rate-limiting process. Notably low activation barriers (0.045–0.16 eV) are obtained, reflecting the role of built-in electric fields in stabilizing reaction intermediates and transition states. These results elucidate how symmetry breaking in Janus group-IV–V monolayers couples lattice, electronic, and reaction-pathway effects, providing microscopic insight into hydrogen evolution at polar two-dimensional surfaces. Moreover, it is observed that Janus XGeSiY monolayers are unsuitable for oxygen evolution reaction activities. The present investigation puts forward deeper insights into the excellent hydrogen evolution reaction activities exhibited by the Janus NGeSiY monolayers.