Electronic structure and photocatalytic band offset of few-layer GeP₂

Based on a sophisticated crystal structure prediction method, we propose two-dimensional (2D) GeP₂ in the tetragonal (T) phase never observed for other group IV-V compounds. The bulk of 2D GeP₂ is more stable than both 2D orthogonal (O) and three-dimensional pyrite (P) phases that have been experimentally observed for group IV-V compounds. According to our calculations of phonon dispersion relations and elastic constants, as well as ab initio molecular dynamics simulation, monolayers of both the T and O phases (penta-GeP₂ and O-GeP₂, respectively) are dynamically, mechanically, and thermally stable. In addition, our HSE06 calculation shows that these monolayers are semiconductors with band gaps in the visible region. Among the various stacking patterns of their bilayers, specific ones are identified to be most stable, which are still semiconductors with band gaps redshifted in the visible region. Different from the case of their bulk, few-layers of O-GeP₂ are more stable than those of penta-GeP₂ up to a pentalayer. Furthermore, band offset with respect to the Fermi levels of appropriate half-reactions shows that both n-type few-layer penta-GeP₂ and O-GeP₂ can be useful in photocatalyzed CO₂ splitting to CO as well as in photocatalyzed water splitting, specifically under acidic conditions.