Study On The Effects Of ?-? Interactions Change On The Structures And Properties Of Carbon Based Materials Under Pressure

Carbon and nitrogen elements both have abilitities to form a variety of allotropes and compounds with each other due to their diverse hybridizations.A broad family of carbon-based materials such as graphite,graphyne,graphene,carbon nanotubes,g-C₃N₄ and diamond etc.has been obtained,which exhibit excellent properties and novel structures.As one of the oldest mineral in the universe,graphite is a typical sp²-hybridized crystal structure which plays important roles in scientific development and industry fields,appealing great enthusiasm in scientific community.As we all known,in graphite,each carbon atom is joined to three nearest neighbors by an?-bond setting in a honeycomb lattice based on hexagonal plane tessellation.The?-bond forms through the side to side overlap of the remaining p_z orbitals,which is perpendicular to the plane.In particular,tuning the?-?interactions or rehybridization of carbon atoms may cause significant changes in its electronic,optical and mechanical properties.Compared with graphite,sp²-hybridized carbon nitride compounds and carbon nanotubes have became the hot topic for recent years due to their variable?electron distributions,exhibiting excellent performance in catalytic,optical and mechanical properties.Understanding the effect of such?-?stacking interactions,even the sp²?sp³ transition under pressure in those materials can contribute to our understanding on various properties of these materials,as well as to providing a new strategy to construct the novel functional materials.However,there are relatively few studies,particularly on searching for novel carbon-based materials by tuning?-interactions of g-C₃N₄ and bonding change of carbon nanotubes under high pressure.Based on the above analysis,we performed high pressure studies on two typical carbon-based materials including g-C₃N₄ and carbon nanotubes.1.With a visible-light driven bandgap and proper band edges,g-C₃N₄ has emerged as a new class of photocatalyst,exhibiting the ability of photocatalytic hydrogen/oxygen evolution from water splitting under visible light.Due to the suitable band structure,g-C₃N₄ is a promising photocatalyst splitting water into hydrogen and oxygen.Nanosheets obtained by the delamination of layered compound will increase the specific surface area and reduce the recombination probability of photoexcited charge carriers,improving photocatalytic activity.However,until now,low yield and complex manufacturing processes are two problems in synthesizing g-C₃N₄ with high performance.For this purpose,we develop a one-step self-exfoliated method to prepare few-layers g-C₃N₄ with high activity.Two kinds of few-layered materials are obtained from heating melamine(M-CN)and urea(U-CN)in a closed environment,respectively.From UV-vis spectra,photocurrent response and Mott-Schottky plots,it shows that the bottom of conductive band of M-CN is lower than U-CN,which possesses stronger photo-oxidative capability while M-CN has effective separation rate of photo-generated electron-hole pairs.The hydrogen evolution rate of M-CN is 22%enhancement compared to U-CN and 37 times higher than that of the bulk g-C₃N₄.Our results provide a new strategy for the preparing few-layered g-C₃N₄ for high photocatalytic performance.2.Graphitic carbon nitride(g-C₃N₄)is an excellent photocatalyst,as well as a promising luminescent material due to the potential photoluminescent(PL)property.The PL property of g-C₃N₄ is closely related to its?-?(9)transition at ambient pressure.The electron structure and PL property could be changed by tuning?-interactions under high pressure which could help us further understand its luminescence mechanisms and construct new functionalities for carbon nitride materials.Herein,we study the structures and properties of few-layered g-C₃N₄(FL-CN)under high pressure,focusing on pressure-induced interlayer interactions,?-interactions and possible piezochromic response.It is found that FL-CN exhibits an anomalous PL enhancement and obvious change in the light absorption at 6 MPa.According to UV-vis and IR spectrum,the suppression of N–H vibration could decrease the nonradiative transition in g-C₃N₄,while based on photocurrent response,photogenerated electrons-hole pairs recombination obvisouly increase.Thus this results in PL enhancement.By applying high pressure,the PL emission color of FL-CN can be tuned from blue(434 nm)to yellow(550 nm),exhibiting obvious piezochromic response.Interesting,an obvious decrease in interlayer compressibility accompanied by dramatical weakening of PL intensity and emission band broadening has been observed above 3 GPa,probably due to interlayer stacking transition in FL-CN.The results suggest a new way for the energy band engineering in carbon nitride materials,which could improve light-harvesting and favor their photocatalytic and optical performances.3.As increasing the number of layers,the bulk g-C₃N₄ is expected to exhibit different pressure response and phase transition comparing with FL-CN.Thus,we further studied the band structure and properties of bulk g-C₃N₄ under high pressure.Based on PL spectra,anomalous PL enhancement is also found in bulk g-C₃N₄,but the pressure is increased to 0.44 GPa.By loading higher pressure,the bulk g-C₃N₄clearly displays a wide-ranged emission from blue to even white.According to molecular dynamic simulation,it is further revealed that this PL enhancement is due to an indirect-to-direct band gap transition because of the pressure-induced interlayer sliding.Thus,this results in different mechanism of photoluminescence(PL)enhancement from FL-CN.Based on band structure and density of states calculation,it can be found that the modification of C,aromatic and center N p_z orbital-derived CBM,as well as the aromatic N 2p orbital-consisted VBM through interlayer sliding is responsible for the indirect to direct band gap transition.The results suggest a new way for the energy band engineering in carbon nitride materials,which could improve light-harvesting and favor their photocatalytic and optical performances.4.As a sp²-hybridized carbon material,single walled carbon nanotubes(SWCNTs)can be considered as a single-layer graphene sheet rolled up to seamless cylinder.Different from graphite layers with planar-type structure,SWCNTs with different diameters and armchair/zigzag chirality could be used to produce different carbon structures by a structural transition and sp²-to-sp³ bonding change,which are promising to exhibit excellent mechanical properties.According to the experimental results,a quenchable sp³ carbon structure can be obtained by compressing carbon nanotubes to very high pressure but the corresponding structure is still under debate.Herein,we performed high pressure study on large diameter SWCNTs which are more close to those used in experiment.Through simulations,we discovered four superhard carbon structures with hardness close to diamond,while their band gaps are tunable and ranged from 2.6 to 5.2 e V.Interestingly,the hardness of K-carbon composed by(19,0)SWCNTs is about 83 GPa,but the band gap is only half of diamond.Thus,K-carbon may be a potential multifunctional suprehard material with semiconducting properties.In addition,the superhard carbon phase from cold-compressed carbon nanotubes in experiment could be explained by our proposed Cco-C₁₆₀ according to their good match in XRD patterns between our simulation and the experiment.The results provide theoretical basis on designing multifunctional materials with superhardness.