Highlighting the Dynamics of Graphene Protection toward the Oxidation of Copper Under Operando Conditions

Graphene, thanks to its hydrophobicity and impermeability to all gas molecules, acts as a protective layer for its supporting substrate. It is resistant to oxidation and can also decouple adsorbed molecular layers from an underlying metal. Being impermeable to liquids and gases and inert to most chemicals, graphene is also widely studied as an anticorrosive coating for metals. Of strong technological interest is the high-temperature corrosion phenomenon, which is the chemical deterioration of copper as a result of heating in an aggressive (e.g. oxidizing) environment. This form of corrosion is of particular interest for materials used in car engines, power generation, turbines, or other machinery coming in contact with an atmosphere containing corrosive products of combustion at high temperatures. Although within grains the graphene sheet acts as a perfect passivation layer, grain boundaries or nucleation site intercalation may occur from intrinsic defects, which then result in the oxidation of the underlying metals. On long-term protection, graphene’s role as a corrosion-inhibiting coating is disputed; if on the one hand it has shown high stability in humid ambient air over a long period of time (up to 2.5 years), on the other hand, the oxygen trapped during graphene growth on copper can lead to faster oxidation than of the bare metal. Also, the highly conductive graphene layer may facilitate localized electrochemical galvanic corrosion from defects by transporting electrons to oxygen atoms, acting as the cathode of the reaction.
The investigation of these phenomena is largely reported in literature using many techniques, however, only a few experiments are performed under operando conditions to follow the evolution in real time. One of these techniques is X-ray photoelectron spectroscopy (XPS) which in its standard mode often deals with model samples; measured in an ultrahigh-vacuum (UHV) environment, where the dynamics are frozen and there is no exchange of species. These two conditions cause the so called “material gap” and “pressure gaps”, respectively. By using a scanning photoemission microscope (SPEM), it is possible to move from model systems to more realistic materials because the spatial resolution, below hundred nanometers, allows one to distinguish different features of a material’s surface, or individual graphene flakes in our case. Moreover the recently developed near ambient pressure cell (NAPCell) developed by the Escamicroscopy team at Elettra makes possible for the first time to perform spatially resolved XPS operando experiments at pressures as high as 1 mbar.
A group of researchers of the University of Mons and Namur, Belgium, MAX IV Laboratory, Sweden and Elettra has studied at the Escamicroscopy beamline the behavior of graphene coated copper at isobaric (0.1 mbar O₂, temperature up to 390 °C) and isothermal (350 °C, pressure up to 0.1 mbar, see Fig.1) conditions. To oxidize the bare copper area exposed to molecular oxygen gas, a pressure of 0.012 mbar is needed. When the pressure reaches 0.1 mbar, oxygen starts to intercalate under graphene from the edges of the monolayer flakes and begins oxidizing the copper beneath. When the temperature is further increased to 390 °C, graphene starts to be etched from monolayer regions. No indication of graphene oxidation is observed, which may be contextual to the etching, or may be the Cu oxidation and its morphological change are faster and disrupt the graphene before it can be eventually oxidized.

Figure 1.     Isothermal experiment: Cu 2p (left) and Cu LMM (right) photoemission maps at different oxygen pressures and constant temperature (350 °C). In the top Cu 2p map the black areas are the graphene flakes. Upon oxidation all Cu 2p images appear brighter showing a disappearance of the C coverage. In the last images traces of C removal are also visible inside some graphene flakes. The Cu LMM maps show the ratio between metallic (brighter) and oxidized (darker) copper. Upon long oxidation the Cu areas not protected by graphene become more and more oxidized while Cu metallicity is mostly preserved below the graphene flakes.
 

The demonstration that graphene hinders the oxidation of copper in an aggressive environment at high temperatures, whereas bare copper oxidizes already naturally at room temperature, shows the great potential of graphene as a stable protective layer, with only one atom thickness, which retards the formation of copper oxide. Our findings clearly show that the detrimental interaction between oxygen and copper starts when the gas intercalates beneath monolayer graphene flakes, especially from boundaries and defects; therefore, this is a clear indication that bigger and more defect-free flakes provide the best anticorrosive protection. A homogeneous coating of graphene, with preferably big bilayer flakes, could successfully protect metals from high-temperature corrosion linked to many industrial applications. The present results have been obtained by exploiting the capability of the NAP-SPEM. Such an innovative technique pushes the limit of standard photoemission spectroscopy, which was traditionally limited to UHV conditions for fundamental studies, and leads the way toward directly studying surface and interface applications under more realistic working conditions.

 

This research was conducted by the following research team:

Mattia Scardamaglia^1,2, Claudia Struzzi², Alexei Zakharov², Nicolas Reckinger³, Patrick Zeller⁴, Matteo Amati⁴,and Luca Gregoratti⁴

¹ ChIPS, University of Mons, Mons, Belgium
² MAX IV Laboratory, University of Lund, Lund, Sweden
³ Department of Physics, University of Namur, Namur, Belgium
⁴ Elettra - Sincrotrone Trieste S.C.p.A.,  Trieste, Italy

Contact persons:

Mattia Scardamaglia, email: mattia.scardamaglia@maxiv.lu.se