Density Functional Theory Study On Oxidation Of Copper Surfaces

Oxidation of metal surface is a major cause of equipment failures in petrochemical,power plant,metallurgical,offshore oil and other industries.In order to ensure the safety for equipment during its long-time operation,it is of great importance to illuminate the inducement mechanism and process of oxidation reaction.Due to the difference of the nature of metal,the chemical reaction process turns out to be much more complicated.In this research field,most investigations focus on oxide growth or the failure of oxide film over a long time span at the marco-scale.However,the studies of early oxidation stage are few because of the short time and small spatial scale.It is still lacking in scientific understanding of the oxidation and mechanism in the early stage.Density-functional theory was used to evaluate the mechanism of copper surface oxidation.Reaction pathways of Cu and O atoms diffusion,orientation growth of oxide islands,oxide surface roughness and origin of defects have been investigated.The main content follows:(1)The energetics and kinetics of the missing-row reconstruction(MRR)and missing-row island formation on the Cu(100)surface are investigated using density functional-theory calculations.We find that copper ejection from the c(2×2)surface is made energetically possible by the presence of surface-adsorbed O2 molecules.The barrier for MRR formation via this ejection mechanism is calculated to be 0.96 eV,consistent with the experimentally observed formation temperature of 400 K.The reaction pathways between Cu and O2 result in the formation of Cu-O chains on the c(2×2)surface which can grow from the-O-Cu-O-trimer at least up to the-Cu-O-Cu-O-Cu-pentamer.Remarkably,these chains can both diffuse rapidly and change their orientation on the surface,allowing them assemble into longer Cu-O chains.Perpendicular to the Cu-O rows,the chains hop first at one end and then the other.Parallel,the chains move as an inchworm does,again,one end advancing before the other.When two Cu-O chains become parallel neighbors,they are able to pull additional Cu atoms from the c(2×2)subsurface with a barrier of 0.65 eV,forming a MRR island with the MRR structure both in the surface and subsurface layers.The set of oxidation and diffusion mechanisms calculated here provide a detailed picture of MRR and MRR island formation on the Cu(100)surface.(2)Density functional theory calculations are used to study the elementary processes of the formation of the(2×1)-O reconstruction on the Cu(110)surface.The(2×1)-O reconstruction requires additional Cu atoms to form Cu-O rows on top of the surface.Both terrace and step sites are considered as the source of Cu adatoms.On terraces,adsorbed oxygen induces the ejection of Cu atoms to form-O-Cu-O-units,leaving Cu vacancies behind.The barrier for subsequent unit growth,however,is prohibitively high.Cu(110)step sites are also considered as a source of Cu atoms.Dissociated oxygen triggers the formation of stable Cu-O chains along the [001] step edges.This process,however,blocks the diffusion of Cu atoms so that it is not a viable mechanism for(2×1)-O reconstruction.Oxygen adsorption on the [11?0] edges also allows the nucleation of [001] oriented Cu-O rows.The short Cu-O rows act as diffusion channels for Cu atoms that detach from the step,which append to the end of the Cu-O chains.Our calculations of the formation of the(2×1)-O phase on Cu(110)provide a mechanistic description of the experimentally observed reconstruction.(3)Reaction pathways of O2 dissociation on the surface and oxidation of the sub-surface are found on the Cu(100),Cu(110),and Cu(111)facets.At low oxygen coverage,all three surfaces dissociate O2 spontaneously.As oxygen accumulates on the surfaces,O2 dissociation becomes more difficult.A bottleneck to further oxidation occurs when the surfaces are saturated with oxygen.The barriers for O2 dissociation on the O-saturated Cu(100)-c(2×2)-0.5 monolayer(ML)and Cu(100)missing-row structures are 0.97 eV and 0.75 eV,respectively;significantly lower than those have been reported previously.Oxidation of Cu(110)-c(6×2),the most stable(110)surface oxide,has a barrier of 0.72 eV.As the reconstructions grow from step edges,clean Cu(110)surfaces can dissociatively adsorb oxygen until the surface Cu atoms are saturated.After slight rearrangements,these surface areas form a “1 ML” oxide structure which has not been reported in the literature.The barrier for further oxidation of this“1 ML” phase is only 0.31 eV.Finally the oxidized Cu(111)surface has a relatively low reaction energy barrier for O2 dissociation,even at high oxygen coverage,and allows for facile oxidation of the subsurface by fast O diffusion through the surface oxide.The kinetic mechanisms found provide a qualitative explanation of the observed oxidation of the low-index Cu surfaces.