High Pressure Study On Structure And Properties Of Graphite-Like Layered Carbon Nitride Materials

Carbon nitride?CN?materials with graphite-like layered structures have become a hot topic in recent years due to their unique structures and adjustable physical and chemical properties.Graphite-like CN materials are the important sources for obtaining materials with excellent photocatalytic,optical and electrical properties,etc.,which is also a significant object in the field of physical and material research.The structure and properties of CN materials are closely related to the stoichiometric ratio of C and N elements in the C-N framework of the materials,which can be adjusted by changing the stoichiometric ratio.As important photocatalytic,optoelectronic devices,batteries,biomedical and energy storage and conversion materials,the band structure,specific surface area and other factors of graphite-like CN materials have important effects on the performance of the materials.Understanding its structure,electronic behavior and so on are also basic scientific problems for material applications.Therefore,regulating the structure of CN materials and revealing the key factors that affect their properties,as well as establishing the relationship between structure and properties,which has great scientific significance and practical application value to obtain CN materials with excellent properties,this is also a subject that still needs to be investigated in the direction of CN materials.Pressure as an important thermodynamic parameter,which can effectively change the distance between atoms and material structures,as well as regulate the electronic structure,etc.Compared with other control methods?such as doping,functionalization,etc.?,high pressure is a"clean"method,which will not introduce impurity elements.In this paper,high pressure is used to systematically study several typical graphite-like CN materials,including regulating their stoichiometric ratio of C and N elements,tuning band structure,specific surface area and?electron behavior.The main results are listed below:1.The stoichiometric ratio of C and N elements in graphite-like CN materials determines the structure and band structure of the material.It is very important to establish the relationship between the stoichiometric ratio of C and N elements and structure and properties for exploring CN materials with excellent performance.We use high pressure and high temperature?HPHT?method to treat the graphitic carbon nitride?g-C₃N₄?,it is found that the stoichiometry of C and N elements and band gap of g-C₃N₄ can be effectively tuned.Upon treating g-C₃N₄ at relatively low temperature and pressure conditions?below 5 GPa/630??,its band gap can be efficiently narrowed and the absorption band edge is adjusted to the red region?⁶00nm?,the crystallinity increases and the charge carrier separation efficiency?10²?improves,almost without changing its stoichiometry.When increasing the treatment temperature under pressure,XPS measurements show that pyridinic nitrogen is preferentially eliminated while graphitic nitrogen is preserved in the CN network,the nitrogen-doped graphene/graphite materials with weak ferromagnetism were obtained.We thus obtained CN materials with tunable band gaps,ranging from semiconducting to metallic states.This study provides an efficient strategy for tuning the structure and physical properties of CN materials.2.Although HPHT has significant advantages in regulating the structure and performance of CN materials,but HPHT treatment will usually reduce the specific surface area of materials.How to overcome this deficiency?We have successfully prepared porous g-C₃N₄ with low nitrogen contents by HPHT combined with hard template method.The produced samples after HPHT treatment show decreased interlayer distance,up-shift in valance band?VB?and down-shift in conduction band?CB?,and narrowed bandgap down to 1.88 eV,as well as increased efficiency of charge carrier separation and transfer.Consequently,the porous g-C₃N₄ with low nitrogen contents and high specific surface area exhibits 13.5 times enhancement in photocatalytic H₂ evolution compared to the bulk g-C₃N₄.We further give the reaction mechanism for the enhanced photocatalytic H₂ production.This method can effectively control the defects of g-C₃N₄ and simultaneous realization of porous structure,which is much gentler than other reported methods of synthetic nitrogen defects samples in literatures,and could be extended to the study of other materials.3.It is found that under certain temperature and pressure conditions,the stoichiometric ratio of C and N elements will change remarkably,and the loss of N atoms from the CN skeleton leads to the formation of carbon-rich structure with carbon dangling bonds,which can easily react with other atoms to form bonds under HPHT conditions.Thus,using g-C₃N₄ as carbon source,we have successfully prepared crystallized black phosphorus-graphite?BP-G?composites with tunable phosphorus carbon?P-C?bonds by a facile HPHT method.The concentration of P-C bonds can be tuned by controlling the ratio of the two precursors?red phosphorus and g-C₃N₄?.The maximum content of P-C bond is¹3.3%,which reaches the highest record reported by ball milling.By combining theoretical calculation with experiment method,we studied the bonding sites of P-C bonds,and also the effect of P-C bonds on the cycling performance of crystallized BP-G composites,which sheds new insights into understanding on the electrochemical performance of phosphorus/carbon materials.4.The electrons in the p_z-orbits perpendicular to the graphene plane??electrons?have strong effects on the interlayer interactions,bonding and electron transport in graphitic materials.?electrons can be tuned by external pressure and may cause significant changes in its electronic structure and mechanical properties.Different from graphene/graphite,graphite-like two-dimensional?2D?carbon nitride structures possess variable?electrons distributions in CN layers and exhibit different properties.This provides ideal template for tuning?electrons.In this paper,we studied 2D layered CN materials with C₂N and C₃N stoichiometries by external pressure.Different from the transformations observed in compressed graphite,the G-band evolution and electronic transport behaviors in C₂N and C₃N display unusual transitions under pressure.For example,a plateau-like behavior has been observed in G-band in pressure range from 15 to 35 GPa and the pressure response of electronic transport in C₂N is much significant compared to C₃N.In addition,anomalous intensity enhancement in G band of C₂N and C₃N has been observed at above³8GPa,probably due to their electronic structure changes under pressure.These results can be well understood by the unique?electron distributions in C₂N and C₃N,our study provide new idea to the effect of?electron distribution on the bonding,structure and electronic properties tuning in 2D layered CN semiconductors by external pressure.