Theoretical Exploration Of Surface Segregation Induced By Two-dimensional Material Growth

Surface segregation in surface science refers to the surface enrichment of a component in a multicomponent material.It has attracted extensive attentions in the field of alloy catalysts since the surface segregation of alloy was proposed by Miedema and verified by experimental results.Due to the different surface energies of transition metals in the alloy,a core-shell structure with a specific component enriched on the surface is formed during the cooling process,and the core-shell structures have important applications in hetero-catalysis.Recently,with the rapid development of twodimensional(2D)material,surface segregation has also been studied in the growth of2 D materials.Compared with surface self-limited growth in chemical vapor deposition(CVD),surface segregation enables the growth of multilayer 2D materials.However,the quality of multilayer 2D materials synthesized by surface segregation is generally poor.Up to date,there are very limited studies on surface segregation induced by materials growth,and the corresponding theory is still lacking.Understanding the mechanism of surface segregation during the CVD growth of materials and exploring its influence on the growth behavior of 2D materials can provide important theoretical guidance for the controllable growth of multilayer 2D materials and structure regulation of the catalysts.Based on first-principles density functional theory(DFT)calculations,we explored the conditions for the occurrence of surface segregation and the influence of segregation on catalyst structures from the perspective of material growth.Our research results are summarized as following:(1)Based on the existing theoretical models of cooling segregation,we proposed that the conditions for isothermal segregation of interstitial atoms must meet: 1)The chemical potential of the interstitial atom on the surface approaches to that in the bulk;2)The chemical potential of the interstitial atom is higher than that in the 2D materials;3)there is enough equilibrium solubility of the interstitial atom in the bulk of the alloy catalyst.Then we take the growth of graphene on the surface of alloy catalysts as an example to verify the rationality of the theoretical model and identified several alloy catalysts suitable for the isothermal segregation of graphene.In our study,Pt Ni(111)with any ratio of Ni composition,Pt Cu(111)with Cu composition less than 25%,Cu Ni(111)substrate with Ni composition greater than 50%,and Cu Ni(100)with Ni composition of 12.5% and 25% can achieve the isothermal segregation growth of graphene.Both the composition of the alloy catalyst and the growth temperature play a key role for the isothermal segregation.Our work provides important theoretical guidance for the selection of alloy catalytic substrates for the growth of multilayer 2D materials,paving a new way to guide the experimental synthesis of high-quality multilayer 2D materials.(2)The segregation of carbon atoms in Cu Ni alloy can change the Cu Ni alloy catalyst from Cu-rich surface to Ni-rich surface.It is found that the carbon atoms dissolved in the Cu Ni alloy can change the segregation tendency of the constituent atoms,so that the original Cu-rich surface alloy is transformed into a Ni-rich surface alloy.The main reason is that there is a strong interaction between carbon and Ni.Carbon atoms bring Ni atoms from the bulk to the surface of the alloy during the segregation process.The graphene grown on the surface of the alloy has no obvious influence on the segregation tendency of the alloy.The research results provide a new way to control the surface composition of alloy catalysts and thus their catalytic activities.