Structural Modifications And Adsorption Properties Of Graphene

The discovery of graphene has aroused intense research interest due to its unique properties,such as large surface area,high carrier mobility,good mechanical property,and good chemical stability.Exploring and extending application potentials of graphene materials in various fields is the major research focus in nano-science research,which is also a necessary step to promote fundamental research advancements into industry reality.Attributed to independence of empirical parameters,the first-principles study has become the main research approach in materials science and quantum chemistry.In this thesis,L-cysteine（L-cys）and hydrogen（H₂）are taken as probe molecules.Adsorptions of L-cys and H₂ on doped graphenes are investigated on the basis of first-principles calculations.The results are summarized as follows:（1）Adsorption of L-cys on pristine graphene and B-,N-,Al-,Ga-,Ni-,Pd-doped graphene is investigated using density functional theory calculations.The calculations reveal pristine graphene physically adsorbs L-cys.N-doped graphene shows physisorption towards the S-end and N-end L-cys,and chemisorption towards the O-end radical.Strong chemisorption,with site-specific preference,occurs on Al-,Ni-,Ga-and Pd-doped graphene,accompanied with severe structural changes.Spin polarization with peculiar mirror symmetry on Ni-and Pd-doped graphene is induced by chemisorption of unprotonated L-cys,except the O-end adsorption on Pd-doped graphene.Magnetization mainly arises from spin polarization of the C 2p_z orbital,with minor magnetism located on Ni or Pd.When van der waals interactions are included,.the binding strength is enhanced for all adsystems in contrast to the calculation results under the GGA approach.Almost no qualitive changes appear for adsorption features and magnetism features when van der waals interactions are considered.（2）L-cys adsorption on first-row transition metal（Sc-Zn）doped single-vacancy and double-vavcancy graphenes（MSVs and MDVs）is explored using dispersion-corrected density functional theory calculations.From Sc to Zn,All MSVs chemically adsorb L-cys with no regular variation tendency in adsorption strengths.MDVs show decreasing chemisorption from V to Co,followed by emergence of physisorption from Ni to Zn.Starting from Mn to Zn,L-cys adsorption on MDVs is weaker than on MSVs for all three end-type dockings.Both the TM dopant and the vacancy type contribute to adsorption variation tendency.Besides,site-specific chemisorption is revealed.L-cys adsorption leads to magnetic adsystems.In particular,O-FeSV,N-ZnSV and L-cys/NiSV become magnetic after L-cys adductions.The increasing number of 3d electrons and TM-C interactions could account for magnetism variations.Regular magnetization patterns occur in most magnetic chemisorbed systems,accompanied with different mirror symmetry planes.（3）L-cys adsorption on graphenes doped with several typical groups of elements（Y-Mo,Al-S,Be-Sr）was investigated using dispersion-corrected first-principles calculations.For the 4d^1-4 element series of Y-Mo,Nb doped graphene shows the most stable adsorption,regardless of the vacancy type.Adsorption mechanism could be explained by using electron pairing and TM-C interactions,which is similar to the adsorption mechanism of L-cys adsorptions on 3d transition metal doped graphenes.For the 3s²3p^3-6 element series from Al-S,near-decreasing variation tendency is observed,with the occurrence of endothermical physisorption,regardless of the vacancy type.Besides,electron pairing and dopant-C interactions and ionization potential of the dopants also contribute to adsorption mechanism.For the IIA element series of Be-Sr,stronger adsorption stability appears for Be,Mg doped single-vacancy graphenes relative to Be,Mg doped double-vancancy graphenes;while weaker adsorption stability appears for Ca,Sr doped single-vacancy graphene relative to Ca,Sr doped double-vacancy graphenes.The adsorption features are attributed to interactions between IIA elements and neighbouring C atoms.Localized electric fields of IIA elements doped graphenes also contribute adsorption features.（4）L-cys adsorption on single-vacancy graphene（SV）,double-vacancy graphene（DV）,Ag doped single-vacancy graphene（AgSV）and Ag doped double-vacancy graphene（AgDV）was investigated using dispersion-corrected first-principles calculations.SV and AgSV exhibit exothermical chemisorptions while AgDV exhibits endothermical chemisorptions towards L-cys,regardless of the end type.DV shows exothermical chemisorption towards S-end l-cys and endothermical physisorption towards O-end and N-end L-cys.One dangling carbon atom is warped off as the active atom for L-cys chemisorptions on SV and DV,while the Ag dopant is the active atom for AgSV and AgDV.With respect to any kind of end-type adsorption,the adsorption strength changes in the order as follows:SV>AgSV>DV>AgDV.Two-step energy barrier related to initial symmetry broken and structural reorganization leads to differences in adsorption types and adsorption energies.Site-specific immobilization is also revealed.Calculations at 298.15 K and 1 atm reveal that L-cys adsorptions on SV,AgSV,the S-end,O-end adsorptions on DV are thermodynamically favorable.（5）Hydrogen（H₂）adsorption on the IIA elements doped double-vacancy graphenes（BeG,MgG,CaG and SrG）was studied by using dispersion-corrected density functional theory calculations.Through investigation of different numbers of hydrogen dockings from two directions,it is found that 1H₂/BeG,1H₂/MgG,8H₂/CaG and 8H₂/SrG are the most stable adsorption configurations for Be,Mg,Ca and Sr doped graphenes,respectively.BeG and MgG could be used for adsorption under low H₂ concentration,mainly through orbital hybridization adsorption mechanism.CaG and SrG could be used for high H₂ concentration adsorption,mainly through polarization adsorption mechanism.Atomic radius,electronegativity and ionization potential of the IIA dopant contribute to the dominating adsorption mechanism under specific H₂ concentration.The study would facilitate exploration of high performance graphene-related supports for hydrogen storage.