Theoretical Studies On The Properties And Applications Of Two-dimensional Graphene-like Nanomaterials

Graphene is a very popular two-dimensional nanomaterial in the last few years because of its excellent physical and chemical properties.It has been used in many fields such as electronic devices,catalysis and energy.However,zero band gap limits its application in semiconductor.Graphene oxide has similar structure to graphene.But their surface is covered with different oxygen functional groups and the oxygen concentration is variable.The oxygen groups change the electronic structure of graphene from semi-metal to semiconductor.Different oxygen concentrations make different effects on the electronic properties of graphene.Recently,scientists also find some other two-dimensional graphene-like nanomaterials including silicene and transition metal dichalcogenides,which possess some good characteristics and attract a lot of attention.Excavating the potentials of these materials using theoretical calculations can provide feasible directions and strong references for experiments.Based on density functional theory(DFT),a series of studies on some common graphene-like materials have been carried out.We successfully tuned the electronic properties of the graphene oxide and WS2 heterostructures by means of calculations.We also theoretically predicted the possibility of synthesizing ordered nitrogen-doped carbon materials on graphene and the applications of two-dimensional transition metal dichalcogenides in batteries.The main contents are as follows:1.We effectively tuned the electronic properties of the graphene oxide and WS2 heterostructures by first-principles calculations.Based on first-principles calculations,we investigated the structural stability and electronic properties of WS2 and graphene oxide(GO)heterostructures.We considered three types of GO,including epoxy only,hydroxyl only and both epoxy and hydroxyl on the GO surface.Our results show that the interlayer binding energy per WS2 unit increases from 0.117 e V to 0.214 e V as the surface oxygen concentration of GO increases.The band gap of the WS2/GO heterostructures can be efficiently tuned in a wide range from 0.13 e V to 1.91 e V by changing the oxygen functionalities and their concentration.The spatial separation of the conduction band minimum and valence band maximum is observed,which are distributed in different layers.In addition,the work function of WS2 can also be modulated by GO in the range of 4.09 e V to 6.34 e V,which potentially increases the carrier concentration and broadens the applications of WS2 and other transition metal dichalcogenide materials in optoelectronic devices.2.We designed the synthesis of ordered pyridinic-nitrogen-doped porous graphene(NP-G)through C-H activation and simple molecular self-assembly method using graphene and graphene-like materials as reaction substrates.NP-G has wide application potentiality because the pores and nitrogen atoms are uniformly distributed and it only has pyridinic nitrogen.2,4,6-Tris(4-hydroxyphenyl)-1,3,5-triazine(THT)is used as precursor.The self-assembly process can be devided into three steps,including the dehydrogenation of hydroxyl and the activation of two ortho C-H bonds.Besides graphene,we considered three other substrates.Au(111)surface is able to reduce the reaction barriers to 1.85 e V,and the dehydrogenation of ortho C-H bond prefer the direct path.Graphene as reaction substrates can make the barrier decrease to 0.35 e V,significantly better than Au(111)surface.Graphene-like materials(silicene and Si C2)are also good substrates,but the dehydrogenation of ortho C-H bonds will follow the indirect path.The charge analysis results indicate that graphene provides electrons for small molecules and promotes the whole reaction.Moreover,NP-G is a wide band-gap semiconductor,the presence of graphene or silicene can enhances its conductivity and facilitate its application in electrocatalysis.3.We designed transition metal dichalcogenides(TMDs)as negative electrode materials for potassium ion batteries.In our calculations,we studied ten kinds of TMDs that have been synthesized experimentally and selected their stable phases.On different TMD monolayer surfaces,the adsorption energies of potassium ions are less affected by the adsorption sites and they are stronger than that between TMD layers.The migration barriers of potassium ions are as small as 0.04 e V on TMD surfaces,while the values are 0.13~0.44 e V between layers.After the embedding of potassium ions,the phases of TMD materials may change.VS2 and WS2 have better performance after the phase transition.In general,VS2,Ti S2,Ti Se2,Ta S2 and Ta Se2 all possess great potential as anode materials.Among them,VS2 is the best one due to the appropriate open circuit voltage(1.06 V)and the large theoretic capacity(466 m A h g-1).We also compared it with other alkaline metal ion batteries,and we found that potassium ion batteries are as good as lithium ion batteries using VS2 as negative electrodes.