Phase diagram and dynamics of the edge oxidation of C ₃ N monolayers in O ₂ atmosphere from first-principles calculations

The two-dimensional carbon nitride with the stoichiometric formula C₃N exhibits higher basal plane reactivity toward gas molecules than graphene. Its edges, which are an inherent feature of experimentally synthesized structures, are expected to be more reactive toward oxygen and offer a promising avenue for improving surface chemistry and electronic properties under oxidative environments. However, the stable edge structures and O₂-driven edge oxidation mechanism of the C₃N monolayer remain unexplored. With the combination of first-principles calculations and <i>ab initio</i> atomistic thermodynamics, we have identified 135 thermodynamically stable edge oxidation configurations and determined the oxidation phase diagrams over broad ranges of temperature and pressure in O₂ atmospheres for four C₃N edge types: armchair all-carbon (A_CC), armchair mixed carbon-nitrogen (A_CN), zigzag all-carbon (Z_CC), and zigzag mixed carbon-nitrogen (Z_CN). It is demonstrated that all edges are readily oxidized by the atomic O and O₂ compared to the C₃N basal plane, forming CO, N-O, and nondissociatively chemisorbed O₂ species, with A_CC and Z_CC edges being more susceptible to oxidation than A_CN and Z_CN edges. Increasing O₂ pressure raises the edge oxygen density, leading to the transformation of stable edge oxidation structure from the atomic O-dominant configuration to the mixture of O and the chemisorbed O₂ configuration. Using <i>ab initio</i> molecular dynamics simulations, we reveal three distinct edge oxidation mechanisms, including a two-step oxidation, concerted reactions involving two nondissociatively chemisorbed O₂, and O₃ formation <i>via</i> interaction between an O₂ and a nondissociatively chemisorbed O₂. These reactions are all spontaneous at room temperature, with the first two being barrierless and the third having a low barrier on the order of thermal fluctuation. Furthermore, the electronic properties can be tuned by the edge oxidation density. This work elucidates the stable oxidized edge structures and oxidation pathways of C₃N, highlighting its high edge reactivity and providing a potential strategy for electronic property engineering in carbon nitride materials.