Theoretical Study On The Structure And Properties Of Non-metallic Modified Carbon 2D Materials

The rapid development of modern high technology and industry has put forward more diverse requirements for material performance,promoting the continuous innovation and development of new materials.Finding and developing new materials with unique structures,excellent properties,and diverse functions has become an important issue at the national strategic level.Graphene is considered to have good application prospects in catalysis,electronic devices and new composite materials due to its large specific surface area,excellent mechanical properties,thermal and electrical properties.However,its applications are greatly limited by its zero-bandgap electronic structure and low catalytic activity.Modification of graphene is expected to overcome the shortcomings of graphene’s inherent properties and further expand its application scope.So far,various modification methods of graphene have been reported,including covalent bond functionalization,non-covalent bond functionalization,chemical doping,etc.Among them,non-metallic chemical doping has aroused widespread interest among scientists due to its non-toxicity,rich and adjustable structure,and does not destroy the planar two-dimensional structure of graphene.Non-metal modified carbon two-dimensional materials have shown potential application prospects in the fields of semiconductor device integration and manufacturing,photocatalysis,electrocatalysis and other fields.However,research on this material still faces many challenges.For example,it is difficult to directly observe the atomic structure of new materials with current experimental techniques,and therefore cannot further regulate their properties through structural control.Non-metal modified carbon two-dimensional materials（such as nitrogen-doped graphene）have good oxygen reduction reaction（ORR）activity under alkaline conditions,but poor ORR activity under acidic conditions.Therefore,finding new materials with good ORR activity under acidic conditions remains a challenge.In addition,there are still great difficulties in developing semiconductor non-metal modified carbon materials with good thermodynamic and environmental stability and excellent carrier mobility.This paper uses first-principles calculations based on density functional theory（DFT）to study the structure and properties of non-metal modified carbon two-dimensional materials.The main research contents and conclusions are as follows:（1）The relationship between the stability of co-doped structures containing different types of nitrogen atoms and nitrogen concentration was explored.The results show that the co-doping of graphitic nitrogen and pyridinic nitrogen is beneficial to improving the stability of nitrogen-doped graphene,and the stability decreases with the increase of nitrogen concentration.Among the nitrogen-doped graphene explored in this paper,the co-doped structure with large triangular holes is the most stable.Furthermore,by comparing the ORR activities of large triangular co-doped structures,graphitic nitrogen-doped structures,and pyridinic nitrogen-doped structures,the effects of different nitrogen types on ORR activity were obtained.The results show that the large triangular co-doped structure has the best ORR activity,indicating that the co-doping of graphitic nitrogen and pyridinic nitrogen is beneficial to improving ORR activity.Moreover,as the nitrogen concentration increases,the ORR activity of the co-doped structure initially increases and then decreases.The study found that the large triangular co-doped structure has optimal ORR activity when the nitrogen doping concentration is 32%(C₁₉N₉),which provides an important theoretical reference for the development of suitable acidic ORR catalysts.（2）The effect of B/P doping on the ORR activity of large triangular nitrogen co-doped graphene under acidic conditions was studied.The research results show that regardless of B or P doping,the active site of the ORR reaction changes from the C site of the nitrogen co-doped structure to the B or P site.For a specific configuration of large triangular nitrogen-doped graphene,further B doping can improve the adsorption strength of different intermediates on the material surface,thereby significantly improving the ORR activity of the material.However,P doping inhibits the ORR activity of this type of large triangular co-doped configuration.This is because P doping weakens the material’s adsorption of OOH intermediates,thereby inhibiting the generation of OOH^*,the rate-determining step of ORR.Therefore,the ORR activity of specific large triangular co-doped graphene can be further improved by further doping with B,thereby obtaining an ORR catalyst with better activity.（3）The structural stability of BC_xN two-dimensional materials was studied.The formation energies of different BC_xN structures were calculated by DFT.The results show that at 0 K,the BC_xN structure tends to form a phase-separated structure of graphene and h-BN,which contradicts experimental results that have successfully synthesized atomically mixed BC_xN materials.In this work,taking into account the influence of experimental conditions such as temperature,pressure and precursor,we derived the formation energy formula of BC_xN two-dimensional materials under different temperature,pressure and precursor conditions.And based on this formula,the Gibbs free energy of BC_xN materials under different experimental conditions was recalculated.The results show that under certain experimental conditions,BC_xN two-dimensional materials can indeed be synthesized,and the synthesis of BC_xN materials with different B,C and N ratios can be achieved by adjusting the experimental parameters.Our research results resolve the current contradiction between experiment and theory and provide theoretical guidance for the controllable synthesis of BC_xN two-dimensional structures in subsequent experiments.（4）Discovered C₂P₄,a special two-dimensional material with a full five-membered ring structure,and systematically studied the structure,stability,electrical and mechanical properties of this material.The results show that the nanomaterial has mechanical and thermodynamic stability.It has a positive Poisson’s ratio（0.34）and a negative Poisson’s ratio（-0.11）in the characteristic direction and can withstand tensile strains up to 20%.According to the deformed state theory（DPT）,the maximum electron mobility and hole mobility of C₂P₄ are 1913 and 460 cm² V^-1 s^-1 respectively,while using self-energy relaxation time approximation within the framework of the Boltzmann transport equation at room temperature,the maximum electron mobility and hole mobility of C₂P₄ at room temperature are 58 and 57 cm² V^-1 s^-1,respectively.These excellent properties provide C₂P₄ with broad application prospects in the fields of nanoscale electronic devices,high-temperature electronic devices,flexible nanodevices,aircraft or automobile sandwich panels,etc.