  Black phosphorus (BP) has attracted significant interest as a monolayer or
few-layer material with extraordinary electrical and optoelectronic properties.
However, degradation in air and other environments is an unresolved issue that
may limit future applications. In particular the role of different ambient
species has remained controversial. Here, we report systematic experiments
combined with ab-initio calculations that address the effects of oxygen and
water in the degradation of BP. Our results show that BP rapidly degrades
whenever oxygen is present, but is unaffected by deaerated (i.e., O2 depleted)
water. This behavior is rationalized by oxidation involving a facile
dissociative chemisorption of O2, whereas H2O molecules are weakly physisorbed
and do not dissociate on the BP surface. Oxidation (by O2) turns the
hydrophobic pristine BP surface progressively hydrophilic. Our results have
implications on the development of encapsulation strategies for BP, and open
new avenues for exploration of phenomena in aqueous solutions including
solution-gating, electrochemistry, and solution-phase approaches for
exfoliation, dispersion, and delivery of BP.

Materials Science;Mesoscale and Nanoscale Physics