High Pressure Research on Several Typical Carbon-based Materials

Author:Zhu Lu Yao

Supervisor:liu bing bing


Degree Year:2019





The transformation of carbon materials under high pressure is an unfailing research subject in high pressure science and carbon materials research.Carbon,owing to its flexibility in forming bonds,exhibits a variety of allotropes and makes rich changes in the bonding of carbon atoms under high pressure.Many carbon-based materials like graphene aerogels(GA),graphene quantum dots and carbon dots,can be seen as the derivative of graphite,which is the most common carbon allotrope.And these materials have been widely used or supposed to be used in many fields like devices,biology,catalyst and so on.High pressure research on these carbon-based materials,however,is still not comparable with the research on graphite.More work is needed on the behavior of carbon-based nanomaterial under high pressure for better properties and further applications,and it also helps in understanding the ways of carbon atoms bonding.Meanwhile,nanodiamond is a unique carbon nanomaterial which mainly composes sp3-hybridized carbon atoms.It has shown potential in drug delivery,tribology,and lubrication and so on.However,the study and applications on nanodiamond are restricted by difficulty in its synthesis.In this work,three subjects are chosen to study the behavior of carbon-based nanomaterial under high pressure,using Raman spectroscopy.And high pressure and high temperature(HPHT)experiments have been taken on two of them.1,Raman spectroscopy is employed to study the phase change ofαphase metal-free phthalocyanine(α-H2Pc)under high pressure,meanwhile the effect of pressure on fluorescence(FL)is also studied to show how phthalocyanine’s(Pc)structure affects its photoelectric property.The Raman spectra ofα-H2Pc show that phthalocyanine molecules are stable to 12 GPa,butα-H2Pc is converted intoχphase metal-free phthalocyanine(χ-H2Pc)under pressure,due to the arrangement of phthalocyanine molecules changes driving by pressure.For FL spectrum,only the FL of excimer can be found inα-H2Pc at atmosphere pressure.When the solidα-H2Pc is compressed,the FL intensity is found to decrease as pressure increases,and it is quenched at 3.0 GPa.The FL of phthalocyanine molecule appears at 0.7 GPa.As the pressure increases,the FL intensity ratio between Pc molecule and excimer is enhanced.Considering the pressure induced phase transition fromα-H2Pc toχ-H2Pc gradually,the change in FL spectrum should be due to the structure transformation.We think,the degree of overlapping decreases under high pressure,which hinders the formation of excimer.It makes the excimer emission decrease and the FL of Pc molecules appear under high pressure.Our work can explain the relationship between Pc crystal structure and its fluorescence,reveals the kinetic behavior of macromolecules similar to Pc system under high pressure,and provides a new possibility of designing the photoelectric materials with excellent performances.Meanwhile,Pc is a typical macromolecule with conjugated electron system.Our work shows the dynamic behavior of these macromolecules with conjugated electron system under high pressure,and provides a new perspective for understanding the behavior of graphite-like carbon-based nano materials such as carbon dots under high pressure.2,we have studied the behavior of disordered graphitic nanocarbon(DGN)under high pressure by Raman spectroscopy.The results show that no evidence for bonding change happens to DGN even up to 52.0 GPa in non-hydrostatic condition and it is fully recovered to the pristine state upon decompression.The high stability and reversibility should be related to its unique highly disordered microstructure which consisted by nanosized wavy graphene layers.And size effect may further promote its stability under high pressure.Under HPHT conditions,DGN transforms into polycrystalline nanodiamonds with only half average grain size of DGN.It indicates that the growth of such diamond nanograins is probably limited,for the disorder microstructure of DGN is not as dense as graphite.Our work demonstrates the importance of disorder microstructure on the transformation of graphitic nanocarbon under high pressure,and provides us a new method to synthesize new nanodiamond controllably3,we realize the direct conversion from GA to diamond aerogel by laser heating under high pressure for first time.The diamond aerogel obtained is constructed by ultrasmall nanocrystals.It is found that the microstructure of GA,as well as the short heating time by laser favors the ultrasmall diamond nanocrystal growth,and even the solid pressure transmitting medium(PTM)can be used for diamond aerogel synthesis.Different PTM is tried and it is found the solid PTM can melt and absorb heat at certain pressure and temperature,so it may be much easier to control the temperature in laser heating diamond anvil cell(LHDAC).We also observe a transparent carbon phase in our experiment at 20 GPa and 2200 K,which turns to graphite and amorphous carbon upon decompression.This result also indicates that during the GA-diamond transition graphitization may take place in nanoscale in the experiments.Our work will benefit the understanding of carbon materials phase transformation under HPHT conditions,and support the experiments finding new sp3-hybridized carbon materials under HPHT conditions.Since GA with tunable microstructures,such as density and porosity,can be synthesized in a controllable way,it is thus possible to tune the formation of light porous diamond aerogel by designing suitable GA precursors.This is important for the synthesis of diamond aerogel.