Computational Study Of The Regulation Of Polarization Properties And Vibrational Spectrum Of Graphene-ionic Liquids By External Electric Field

Ionic liquids are molten organic salts composed entirely of anions and cations and are liquid at room temperature.They are developing rapidly in the field of soft optoelectronic materials because of their excellent characteristics such as overall external electrical neutrality,environmental friendliness,wide chemical window,and low melting point.In next-generation supercapacitors,ionic liquids are widely explored by researchers as a combination of electrolyte and graphene electrode materials,so it is necessary to study the graphene-ionic liquid interface in depth.Polarization properties contain the evolution of ion pair polarization trends,and vibrational spectrum is an important tool to study the structural characteristics of liquid materials.The effects of external electric field on the polarization properties and vibrational spectrum of the graphene-ionic liquid system are of great scientific significance and application for understanding the bonding trends and ionic microstructure changes inside the ionic liquid in the external electric field.The regulation of the polarization properties of cis-and trans-structured graphene-1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide([Emim][NTf₂])systems by external electric field with a step size of 0.001 a.u.was investigated computationally on the supercomputing center of Lanzhou University for the first time by using the density functional theory.The polarization properties of the system were characterized by dipole moment,polarization rate and molecular polarity index.It was found that the dipole moment and polarizability of the system increased under the electric field in the x-axis direction and the positive field in the z-axis direction,and decreased and then increased under the negative field in the z-axis direction,mainly because the distance between the positive and negative charge centers of the graphene-[Emim][NTf₂]system became larger after the addition of the external electric field,and the weak interaction between the[Emim][NTf₂]ion pair and the graphene surface changed,which led to the increase of the dipole moment and polarizability of the graphene-[Emim][NTf₂]system with the increase of the external electric field intensity.In particular,the combined effect of increased weak interaction of anion[NTf2]^-with the graphene surface and decreased weak interaction of cation[Emim]⁺with the graphene surface when the electric field in the negative direction of z-axis is added leads to a decrease in the dipole moment and polarizability of the graphene-[Emim][NTf₂]system first to 7.32 Debye and 899.66 a.u.(cis structure),8.09 Debye and 896.95 a.u.(trans structure)and then increased to 16.86 Debye and910.96 a.u.(cis structure),15.03 Debye and 901.44 a.u.(trans structure).The molecular polarity index increases with the increase of the external electric field,because the charge distribution of the system is more uneven after the addition of the external electric field,which leads to the polarization of the electrostatic potential distribution,and the calculated molecular polarity index also appears to increase with the increase of the external electric field.In this paper,the vibrational spectrum of cis-and trans-structured graphene-[Emim][NTf₂]systems in the range of 10 cm^-1to 3500 cm^-1were also calculated for the first time by density functional theory,and the regulation of the vibrational spectrum by external electric field was investigated,and it was found that the regulation of the vibrational spectrum of cis-and trans-structured systems by external electric field was mainly achieved by changing the interactions between the fragments in the system.In this paper,the effects of the x-axis electric field and z-axis electric field on the vibrational spectrum of cis-and trans-structured graphene-[Emim][NTf₂]systems within the electric field strength range of 0.01 a.u.and in steps of 0.001 a.u.are discussed by investigating the interactions between the fragments within the system and combining them with the vibrational modes of functional groups,respectively.It was found that the red-shift or blue-shift of the S=O bond synchrotron vibrational peaks under the positive z-axis electric field was due to the enhancement or weakening of the weak interaction between the functional group and the surrounding fragments by the external electric field.In the negative z-axis electric field,the C1-H1 bond stretching vibrational peak of the cis structure is red-shifted and then blue-shifted,while the C1-H1 bond stretching vibrational peak of the trans structure is blue-shifted and then red-shifted,because the external electric field exceeds the critical field strength and causes the relative positions of the anion and cation pairs to change,with the cation imidazole ring shifting in the cis structure and the upper part of the anion shifting significantly downward in the trans structure.The resulting change in the trend of interaction changes caused the phenomenon of different changes in the vibrational peaks.Comparing the different trends of the vibrational peaks caused by the same functional groups in the cis and trans structures and analyzing the interaction energy of the systems,it was found that the exchange energy between the graphene and[Emim][NTf₂]ion pair in the cis structure is 107.43k J·mol^-1,while the exchange energy between the graphene and[Emim][NTf₂]ion pair in the trans structure is 98.51 k J·mol^-1,which is 8.92 k J·mol^-1lower than that in the cis structure.This resulted in different regulatory effects for the cis and trans structures.In this paper,the influence of external electric field on the vibrational spectra of cis-and trans-structured graphene-[Emim][NTf₂]systems is investigated to understand the intrinsic characteristics of graphene-ionic liquid systems more deeply.