Theoretical Study Of Electronic Structure And Adsorption Properties Of Two-dimensional Materials Modified By Metal Clusters

Since the discovery of graphene,two-dimensional materials have attracted much attention due to their unique structures and properties,and a large number of new graphene-like two-dimensional materials have been discovered or synthesized,such as hexagonal boron nitride and graphitic carbon nitride.Metal clusters are also popular due to their ultra-small sizes and high catalytic activities.Based on first-principles calculations,the effects of the modification of metal clusters on the electronic structures as well as adsorption and catalytic properties of graphene-like two-dimensional materials are systematically investigated in this dissertation.The structures and properties of metal clusters and graphene-like two-dimensional materials are discussed,and a variety of composite materials with excellent adsorption properties for harmful gases or metal elements are predicted.The contents are divided into three parts which could be shown as follows.1.The structures and properties of metal clusters Bi Pd_n（n=2-20）and In_mX_n（X=Se,Te;m+n=5）.1)The geometric structure,electronic,adsorption and catalytic properties of Bi Pd_n（n=2-20）clusters were investigated by DFT to explore potential stable and efficient small-scale catalysts.The largest electronegativity（≈5.28 e V）and Mulliken charge population of Bi Pd₁₂ cluster may indicate its strongest electronic interaction among the clusters.The lowest adsorption energies E_ads of NO（≈-0.378 e V）and CO（≈-0.899 e V）molecules show that both molecules may be most easily adsorbed on Bi Pd₁₂ and Bi Pd₁₇ clusters,respectively.Calculations for the catalytic processes of Bi Pd₁₂ for reduction of NO with CO reveal that the unstable（NO）₂ on Bi Pd₁₂ easily decomposed into a N₂O molecule and leaving an oxygen atom at the active site.This reaction needs to overcome the relatively low activation energy of 0.65 e V,and the exothermic reaction also means that this process can easily occur at room temperature.It seems that Bi Pd₁₂ cluster with the higher symmetry and stability may exhibit potentially high catalytic activity for electrochemical reduction of NO.2)The research on In_mX_n（X=Se,Te;m+n=5）clusters structure,stability,electronic,magnetic and spectral properties found that,the ground state structures of In_mX_n clusters are found largely similar for X=Se and Te,with the exception of In₂X₃.The electronic properties of In_mX_n clusters depend on their geometrical structures and hence on the value of m.In₂Se₃ shows a low vertical electron affinity（VEA）of about 1.60 e V and a high vertical ionization potential（VIP）of about 9.33 e V.The total magnetic moment is 1μ_Bor 0μ_B for the clusters with m=odd（1,3）or even（2,4）,respectively.The local magnetic moments of X atoms amount to about 99.9%of the total magnetic moment,while those of In atoms are merely 0.1%.The IR and Raman spectra of In_mX_n clusters exhibit similarity for X=Se and Te with an exception of In₂X₃.The energies of the highest peaks of In_mTe_n are largely smaller than the corresponding In_mSe_n in both IR and Raman spectra.For UV-Vis spectra,the absorption peaks within 200-400 nm for all clusters and 390-780nm for m=1 and 3（except In₃Te₂）are likely to hint useful properties of UV and visible light absorption,respectively.2.The structures and properties of 2D materials h-BN and g-C₃N₄1)Studies on the doping of various transition metal atoms in h-BN monolayers found that h-BN monolayers with B vacancies（V_B）were more easily doped with metal atoms.For the adsorption of various gas molecules,NO is most easily adsorbed on h-BN monolayer with metal dopants,especially Pt doped system yields the lowest adsorption energy of NO.Nevertheless,the（NO）₂ dimmer can interact with Pt and form a five-membered ring or a four-membered ring through two different Langmuir-Hinshelwood（LH）mechanisms for the catalytic reduction reaction of NO,respectively.The LH1reaction process needs to overcome relatively lower energy barriers,while the product of the LH2 mechanism has a more stable structure.It seems that Pt doping can enhance the adsorb capacity of h-BN monolayer for the gas molecules and the potential catalytic activity for electrochemical reduction of NO.2)By comparing the structures of the curved g-C₃N₄ and flat g-C₃N₄ monolayers,it is found that the curved structure is more stable.The indirect band gap of the g-C₃N₄monolayer of the curved structure is about 1.98 e V,the conduction band minimum（CBM）is located at theΓpoint,and the valence band maximum（VBM）is located at a point close toΓalong the X-Γdirection.The adsorption energy reaches the minimum when a Pt atom is doped at the center of the hexagonal cavity in the g-C₃N₄ monolayer.3.The structures and properties of Bi_mPd_n（m+n=2-6）/h-BN and Pt_n（n=1-7）/g-C₃N₄.1)The thermodynamic stability of Bi_mPd_n clusters increases with the increase of cluster size.Furthermore,Bi_mPd_n clusters are more easily supported on h-BN monolayer compared to pure Bi_n or Pd_n ones.Bi Pd/h-BN composite structure have the lowest adsorption energy for CO molecules and the largest amount of charge transfer,indicating that this composite structure exhibit the suitable ability to adsorb CO molecules.The charge transfer amounts of CO molecules adsorbed on these composite structures are also higher than that on the original g-C₃N₄ monolayer.The addition of metal clusters could also enhance the contribution of d orbitals in the PDOS of these systems and hence the degree of hybridization between the relevant orbitals.Therefore,it seems that the Bi_mPd_nclusters could promote the adsorption capacity of the V_B-containing h-BN monolayer for CO molecules.2)Based on the studies on the adsorption properties of the harmful metals Hg⁰,Ni²⁺and Pb²⁺on Pt_n/g-C₃N₄ composite structures.For Hg⁰,Ni²⁺and Pb²⁺adsorbed on pure Pt_n clusters,the Pt₇ cluster shows the strongest interactions with the lowest adsorption energies of-0.75 e V,-16.47 e V and-7.30 e V,and the largest charge transfers.As regards Hg⁰,Ni²⁺and Pb²⁺adsorbed on Pt_n/g-C₃N₄,the lowest E_ads are-1.53 e V,-19.09 e V and-13.03 e V for Pt₇/g-C₃N₄,respectively.Especially,a great amount of electron cloud around the Hg,Ni and Pb elements on Pt₇/g-C₃N₄ surface indicates their strong interaction with the surfaces in view of the enhanced charge transfer between Hg⁰,Ni²⁺and Pb²⁺with the Pt_n/g-C₃N₄ surface.The adsorption energies of Hg⁰,Ni²⁺and Pb²⁺on all pure Pt_n clusters are higher than those of the corresponding pure g-C₃N₄ monolayer or Pt_n/g-C₃N₄ systems.Therefore,it seems that Pt₇/g-C₃N₄ would likely act as potential adsorbents for the removal of mercury,nickel and lead.In this thesis,the structures and properties of the bimetallic mixed clusters Bi Pd_n（n=2-20）and metal-nonmetal mixed clusters In_mX_n（X=Se,Te;m+n=5）were studied by first-principles calculations.Then the modification effect of metal atoms doped on h-BN and g-C₃N₄ monolayers was discussed.Finally,the metal clusters Bi_mPd_n and Pt_n were loaded on h-BN and g-C₃N₄ monolayers,respectively,and the modification effects of metal clusters on the electronic structure,adsorption and catalytic properties of graphene-like two-dimensional materials were systematic studied.