  Oxygen, invariably present in a normal working environment, is a fundamental
cause of the degradation of phosphorene. Using first-principles calculations,
we show that for each oxygen atom adsorbed onto phosphorene there is an energy
release of about 2 eV. Although the most stable forms of oxygen are
electrically inactive and lead only to minor distortions of the lattice, there
are a number of low energy metastable forms which introduce deep donor and/or
acceptor levels in the gap. We also propose a possible mechanism for
phosphorene oxidation and we suggest that dangling oxygen atoms increase the
hydrophilicity of phosphorene due to their ability to establish hydrogen bonds.
