Coverage-dependent adsorption of small gas molecules on black phosphorene: a DFT study

Using Density Functional Theory (DFT), we perform a detailed computational study to explore the adsorption behaviors of NH3, NO, NO2, CO and CO2 gas molecules on a black phosphorene surface at several coverages. Van der Waals (vdW) interactions were taken into account within the optB88-vdW functional. In addition, a comparison with the strongly-constrained and appropriately-normed (SCAN) meta-generalized gradient approximation (meta-GGA) is discussed. Our calculations show that all molecules adsorb on the surface of black phosphorene with different favorable adsorption sites, depending on the molecule's geometry and orientation. Alongside the adsorption energies, other properties are presented including the adsorption distances and charge transfers. At high coverages, the adsorption energies per molecule show that the functionals start to behave differently as the coverage rate increases. The significant charge transfer between NO and NO2 molecules and black phosphorene suggests its potential integration towards efficient nitrogen-oxide detectors.