  Layered black arsenic (b-As) has recently emerged as a new anisotropic
two-dimensional (2D) semiconducting material with applications in electronic
devices. Understanding factors affecting the ambient stability of this material
remains crucial for its applications. Herein we use first-principles density
functional theory (DFT) calculations to examine the stability of the (010) and
(101) surfaces of b-As in the presence of oxygen (O2) and water (H2O). We show
that the (101) surface of b-As can easily oxidize in presence of O2. In the
presence of moisture contained in air, the oxidized b-As surfaces favorably
react with H2O molecules to volatilize As in the form of As(OH)3 and AsO(OH),
which results in the degradation of the b-As surface, predominantly across the
(101) surface. These predictions are in good agreement with experimental
electron microscopy observations, thus demonstrating the co-operative
reactivity of O2 and H2O in the degradation of layered b-As under ambient
conditions.
