The Study Of Oxidation Structures And The Mechanism Of Two-dimensional Carbon And Nitrogen Material C₃N

Two-dimensional carbon nitride material C3N,a hole-free and graphene-like 2D honeycomb structure with the stoichiometric formula,has been successfully synthesized and demonstrated to excellent properties,resulting in extensive researches in different fields.The emergence of a wide variety of two-dimensional(2D)materials and the advancement of their promising applications in electronics,catalysis,gas sensors,energy conversion and storage inspired by the great success of graphene,has been accompanied by an increasing attention on their chemical stability under ambient conditions.Especially,the O2 dissociation involved in the oxidation of 2D materials when exposed to air is the inevitably critical issue in nanomaterial manipulation and device engineering.The presence of chemisorbed oxygen on the pristine surface of 2D materials tunes the bandgaps and provides active sites for chemical reactions.Furthermore,we found that the chemisorbed oxygen groups can surprisingly convert carbon-based materials,such as graphene and carbon nanotube,into dynamic covalent materials and even induce the self-adaptivity in response to the adsorption of biomolecules.Therefore,it is of great significance to study the oxidation of two-dimensional materials.The study to the oxidation of the C3N monolayer has not been reported in detail.In this regard,we studied the oxidation process and oxidation structure of its surface.Compared with graphene,the C3N monolayer can be regarded as a nitrogen-doped graphene structure with nitrogen atoms uniformly distributed and all atoms sp2 hybridized.Although the atomic-thick 2D materials have a large surface-volume ratio and are prone to show high surface reactivity,the O2 dissociation on the surface of 2D materials usually depends on the chemical bonding characteristics,thus differs from one to another.It is expected that the C3N could exhibit higher surface activity toward small gas molecules like O2 than graphene at ambient conditions due to the nitrogen atoms.However,recent work has pointed out that the C3N monolayer is chemically inert to O2 molecule Using the first-principles calculations and statistical physics analytical methods,we have systematically investigated the oxygen dissociation on a pristine C3N monolayer.The main contents are as follows:(1)We have systematically explored possible adsorption sites of an oxygen molecule and the atomic oxygen,various oxygen dissociation pathways and the oxidized structures.It is demonstrated that the pristine C3N monolayer shows more O2 physisorption sites and exhibits the stronger O2 adsorption than the pristine graphene.We have searched eight O2 dissociation pathways based on transition state theory.Among eight O2 dissociation pathways,the most preferable pathway is a two-step process involving an intermediate state with the chemisorbed oxygen molecule where the barrier is lower than that on the pristine graphene,indicating that the pristine C3N is more susceptible to oxidation than the pristine graphene.(2)The electronic properties of C3N oxidized structures were studied.The presence of nitrogen atoms in the pristine C3N monolayer assists the formation of stable dangling C-O or N-O bonds,leading to the enriched oxidized structures after oxygen dissociation.For the dissociation state,two O atoms prefer to be stably chemisorbed onto the carbon or nitrogen atoms in the form of dangling C-O or N-O bonds.The oxidized structure with two dangling C-O bonds on the para-carbon atoms of the carbon ring shows the lowest adsorption energy of-8.16 eV and is more favorable than other structures.We find that the electronic properties can be altered significantly by the oxidized structures with two chemisorbed O atoms,from the metal to the semiconductor with the largest band gap of 0.67 eV.(3)These results can also be generalized into a wide range of temperatures and pressures by analyzing the adsorption Gibbs free energy and surface phase diagram according to the ab initio atomistic thermodynamics.The phase diagram can provide thermodynamic information of O2 dissociation for the stable structure at finite temperature and finite pressure.And we have compared C3N monolayer with graphene monolayer at different temperatures.(4)Due to the dangling bonds at the edge of the material,the O2 molecule prefers to be dissociated at the edges than the basal plane.We have performed a preliminary investigation on the O2 dissociation at the edges of C3N nanoribbon and made a comparison of the oxidized structures with those on the basal plane of C3N monolayer.We further compare the thermodynamic stability of the most stable dissociation state at the edge of C3N nanoribbon and at the basal plane of C3N monolayer at the temperature T and O2 partial pressure p(O2).(5)The possible structures of paired oxygen functional groups(dangling bond oxygen,epoxy,hydroxyl)on the surface of C3N monolayer have been studied,and the more stable oxidation structures have been obtained.Compared with the oxygen functional groups on graphene oxide,the stability of these oxidation structures at room temperature was verified by ab initio molecular dynamics simulation.These results deepen the understanding of the chemical stability of 2D crystalline carbon nitrides under ambient conditions,provide insights into the tailoring of surface chemical structures,and may guide the design of high-efficient catalysts via doping and oxidation.