Surface Modulation And Corrosion Coupling Damage Behavior Of Two-Dimensional Nano-Protective Films

Graphene has a large specific surface area,high chemical inertness and excellent barrier properties.It is known as the thinnest protective material,but graphene film with high conductivity will accelerate the electrochemical corrosion at the graphene-metal interface and lead to the corrosion of underlying metal.Hexagonal boron nitride?h-BN?nanosheets,with an analogous structure to graphene,have high chemiacl stability and impermeability,and are also a potential film material to protect the underlying metals.More importantly,h-BN nanosheets are electrical insulators compared to graphene and do not cause the electrochemical corrosion of the metal.However,it is difficult to obtain the large-area,high-quality,single-crystal graphene and h-BN on copper foils in the practical production.The point defects,grain boundaries,wrinkles and cracks might be inevitable for the graphene or h-BN films with large-area,which limit their application as protective coatings.In this study,the regulation of surface and interface for graphene and h-BN are performd to improve their barrier properties against the corrosive and oxidizing medium,thus achieveing the long-term protection of metal and providing new approaches for the designing and preparation of the 2D protective nanofilms.The main research contents and results are as follows:?1?In this paper,a series of nitrogen-doped graphene?NG?films were grown on Cu foils by chemical vapor deposition?CVD?method with the CH₄-NH₃ as precursors.The film quality is related to the NH₃ pressure,and the high NH₃ pressure will lead to the discontinuity of NG film.Under the condition of continuous film,the NG coated Cu foils can provide better corrosion protection than pristine graphene over a short-?electrochemical tests?or long-term?atmospheric exposure experiment of 3 months?corrosion tests.This is because the introduction of nitrogen can tweak the local electronic structures and chemical reactivity of graphene,thus decreasing the conductivity of graphene.The NG film with low conductivity can hinder the electron transport in the horizontal direction to slow down the corrosion rate of Cu.At the same time,reducing the atmospheric humidity can significantly reduce the corrosion rate of graphene.?2?The h-BN films with different layers are fabricated on Cu foils by CVD method.The monolayer and multilayer h-BN films show the superior long-term barrier characteristics in the ambient environment due to its insulating nature.After thermal treatment at 200°C,the oxygen with high energy reaches to the Cu substrate through the wrinkles,grain boundary and point defects in monolayer h-BN,and reacts with the underlying Cu to form oxides,thus tearing up the atomically thin h-BN film.The underlying Cu can be oxidized much more easily and eventually the oxygen can expand in both horizontal and vertical directions.However,for the multilayer h-BN films,the transport of oxygen in the horizontal direction is prohibited greatly because of the zigzag pathways of multilayer,thus providing a better oxidation resistance.The electrochemical results show that the h-BN films can not only hinder the penetration of dissolved oxygen and chloride ions,but also inhibit the lateral diffusion of electrons between Cu and dissolved oxygen/chloride ion.Moreover,with the increase in the h-BN layer,the impedance modulus at the lowest frequency(Z_f=0.01Hz)increases and the corrosion current density decreases,indicating that the multilayer design of h-BN films achieve the high barrier performance and low lateral diffusion of electrons.Therefore,the multilayer h-BN films can be used as a protective coating for the metal substrates.?3?Al₂O₃ is grown on the surface of graphene-coated Cu foils by atomic layer deposition?ALD?.It is found that the deposition temperature is the key factor to affect the protective properties of Al₂O₃/graphene composite film.The deposition process at low temperature?160 and 230°C?has little effect on graphene structure,while high temperature?300°C?will lead to the destruction of the graphene structure.Furthermore,the precursor of Al₂O₃?trimethylaluminum and H₂O?mainly adsorb,nucleate and grow on the surface defects of graphene,but can't be deposited on the graphene surface with high-quality since the intact graphene film does not have any dangling bonds to react with precursor.The Al₂O₃ nanoparticles can be used to passivate the defects in graphene surface,hindering the penetration of corrosive media and atomic oxygen and then decrease the corrosion of underlying Cu.?4?Different from the growth mechanism of Al₂O₃ on the surface of graphene,Al₂O₃ nanoparticles can not only grow at the defect sites of h-BN films,but also adsorb on the surface of intact films.This is attributed to the high adsorption energy between the surface of h-BN film and reaction precursor of Al₂O₃.The thichness of Al₂O₃/h-BN composite film with 80 deposition cycles is around 12 nm.Therefore,under the same deposition parameters,the corrosion resistance of Al₂O₃/h-BN composite film coated Cu foil is higher than that of Al₂O₃/graphene composite film,which is attributed to the synergistic barrier effect of Al₂O₃ film and h-BN film.