Highlighting the Dynamics of Graphene Protection toward the

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Highlighting the dynamics of graphene protection to oxidation of copper under operando condition Mattia Scardamaglia, Claudia Struzzi, Alexei Zakharov, Nicolas Reckinger, Patrick Zeller, Matteo Amati, and Luca Gregoratti ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08918 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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ACS Applied Materials & Interfaces

Highlighting the dynamics of graphene protection to oxidation of copper under operando condition

Mattia Scardamaglia1,2 *, Claudia Struzzi2, Alexei Zakharov2, Nicolas Reckinger3, Patrick Zeller4, Matteo Amati4, Luca Gregoratti4 1

2

3

ChIPS, University of Mons, 7000, Mons, Belgium

MAX IV Laboratory, University of Lund, 22100, Lund, Sweden

Department of Physics, University of Namur, 5000, Namur, Belgium 4

Elettra – Sincrotrone Trieste S.C.p.A., 34149 Trieste, Italy

* [email protected]

KEYWORDS corrosion; ambient pressure XPS; coating; operando; spectromicroscopy

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ABSTRACT

We

performed

spatially-resolved

near-ambient-pressure

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photoemission

spectromicroscopy on graphene-coated copper in operando under oxidation conditions in oxygen atmosphere (0.1 mbar). We investigated regions with bare copper and areas covered with mono- and bi-layers graphene flakes, in isobaric and isothermal experiments. The key method in this work is the combination of spatial and chemical resolution of the scanning photoemission microscope operating in near-ambient-pressure environment, thus allowing to overcome both materials and pressure gap typical of standard UHV XPS and to observe in operando the protection mechanism of graphene towards copper oxidation. The ability to perform spatially resolved XPS and imaging at high pressure allows for the first time a unique characterization of the oxidation phenomenon by means of photoelectron spectromicroscopy, pushing the limits of this technique from fundamental studies to real materials in working conditions. While bare Cu oxidizes naturally at room temperature, our results demonstrate that such graphene coating act as an effective barrier to prevent copper oxidation at high temperatures (over 300 °C), until oxygen intercalation beneath graphene starts from boundaries and defects. We also show that bilayers flakes can protect at even higher temperature. The protected metallic substrate, therefore, does not suffer corrosion preserving its metallic character, making this coating appealing for any application in an aggressive atmospheric environment at high temperature.


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1. Introduction Graphene, thanks to its hydrophobicity and impermeability to all gas molecules acts as a protective layer for its supporting substrate 1, it is resistant to oxidation

2,3

and it can also

decouple adsorbed molecular layers from an underlying metal 4. Indeed, one of the many applications of graphene is as an electron transparent but molecular impermeable membrane for ambient pressure and/or environmental (i.e. in liquid) XPS, being able to separate the sample environment from the UHV of the analysis chamber 5,6. Being impermeable to liquids and gases and inert to most chemicals, graphene is also widely studied as an anticorrosive coating for metals

7–9.

Its transparency, conductivity and intrinsically two-dimensional nature, make

graphene a perfect candidate that does not affect the morphology of the coated surface. The prevention of metal corrosion is of extreme technological and economic importance; therefore, the investigation of the protective action of graphene must be carefully assessed

9–11.

In this

work, we address in particular high-temperature corrosion phenomenon, that 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 temperature.

The ability of graphene to protect polycrystalline copper foil from oxidation in reactive environments as heating in air atmosphere 7,12 or hydrogen peroxide treatment 7 has been shown. Although within grains the graphene sheet acts as a perfect passivation layer, from intrinsic defects, grain boundaries or nucleation sites intercalation may occur which then results in oxidation of the underlying metals

13–15.

On the long-term protection, graphene’s role as a



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corrosion-inhibiting coating is disputed; if from one side it has been shown the high stability in humid ambient air over a long period of time (up to 2.5 years) 16,17, on the other side the oxygen trapped during graphene growth on copper can lead to faster oxidation than for the bare metal 18,19.

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 20,21. This effect can be reduced when using commensurate graphene coating, as for example on Cu(111) 16,22. Another way to ensure long-term corrosion protection is by using hybrid coating made by graphene and polymers 23,24; alternatively, hexagonal boron nitride (hBN), due to its insulating nature, intrinsically exclude the possibility of galvanic corrosion 21,25,26,

however, its industrial scalability is limited by the difficulty in achieving a large scale

production. Usually, many techniques are used to investigate these properties such as X-ray photoelectron spectroscopy (XPS) 3,7,25,

7,12,14,18,25,

optical or electron microscopy

7,13,14,16,19,

Raman spectroscopy

atomic force microscopy 3, or cyclic voltammetry 8. However, only a few experiments are

performed at operando conditions 25, so that the phenomena can be followed in real time, and none of them involved XPS measurements. Photoelectron spectroscopy is a powerful technique to investigate the interaction of a solid surface with a second phase (either gas or liquid). The interaction at the interface between the two materials usually plays an important role in the operation of the material for specific applications. This interaction may play an active role, as in electrochemistry, catalysis or sensing devices. Yet it can also interfere, as the oxygen and water vapour in solar cells and for corrosion of metal surfaces. Standard XPS deals with idealised samples; furthermore, measurements are performed in ultra-high-vacuum (UHV) environment, where the dynamics are frozen and there isn’t any exchange of species between the two phases. These two conditions cause the so-called “materials” and “pressure gaps”, respectively 27,28.


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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 nanometres, allows to distinguish different features of a material’s surface, or individual graphene flakes, without forcing to average the behaviour of the whole sample, thus relaxing the materials gap 29,30.

On the other hand, the recent development of ambient pressure x-ray photoelectron

spectroscopy makes accessible operando investigation of many phenomena otherwise impossible, as catalytic reactions and corrosion 31, in a more realistic environment, thus filling the pressure gap. Many approaches could be used for this scope 32, the main issues are related to the high voltage requirements of the analyser and the scattering of photoelectrons at high pressures. The first one can be solved by using differential pumping and electrostatic focusing before the entrance in the hemispherical analyser, while the latter by shortening the path that photoelectrons have to travel in the gas phase. Therefore, ambient pressure X-ray photoelectron spectromicroscopy is the perfect bridge that allows unravelling the complexity of a real material taking advantage of the spatial resolution of a photoemission microscope, and its behaviour under realistic environmental conditions. In the present work, we show how photoelectron spectroscopy technique, usually confined to fundamental studies of model materials in unnatural conditions (UHV), can be used to study real-life phenomena of materials in their working environment. In this experiment, we probe in operando the protecting action of graphene on copper, by locally investigating the different dynamics of copper under mono- and bi-layer graphene flakes and bare copper regions by means of near ambient pressure X-ray photoelectron spectromicroscopy in a molecular oxygen gas environment. Isothermal and isobaric experiments unravel the dynamics of different phenomena involved during the process: 1) the removal of atmospheric contaminant at lower oxygen pressure and subsequently the oxidation of bare copper regions; 2) the intercalation of oxygen under graphene and the oxidation of



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copper beneath; 3) the etching of graphene associated with a change in the morphology of the copper oxide.

2. Experimental section We used the SPEM and the near-ambient-pressure (NAP) cell installed at ESCAmicroscopy beamline (Elettra Synchrotron, Trieste, Italy) to perform a spectromicroscopy experiment investigation of graphene-coated copper in a pressure of 0.1 mbar of O2 gas. Performing scanning XPS in high pressures requires further constraints with respect to standard APXPS because of the presence of the focusing system and the geometry of the measurements, which is usually performed in normal incidence and grazing emission 32. In the NAP cell, the sample is mounted in contact with a resistive heater and a gas dosing line allows the gas to enter in the cell. The cell is installed in the UHV chamber and it is separated by two pinholes, fitting the geometry for the incident X-ray beam and the generated photoelectrons. The aperture size is a critical parameter in order to maximize the pressure inside the cell while keeping the pressure at the electron energy analyser lower than 10-5 mbar, as sketched in Figure 1a. (See

33

for

further details). In our setup, we used pinholes with 400 µm diameter. The distance between the sample’s surface and the aperture was 0.15 mm. The X-ray beam was demagnified to a diameter in the range of about 200 - 250 nm. The photon energy used during the whole experiment was 1070 eV. In order to have in the same field of view both regions of bare copper and graphene flakes, we used a partially covered copper foil by graphene flakes (mostly monolayers with some bilayers) of typical size of 20 μm, as testified by the SEM image in Figure 1b. To distinguish different possible physical mechanisms and get information on their dynamics, we performed two experiments: 1) isobaric, i.e. slowly heating the sample inside the NAP cell


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up to 390 °C at a constant pressure of 0.1 mbar of O2; 2) isothermal, i.e. at a fixed temperature of 350 °C, the pressure is increased by steps up to 0.1 mbar. LEEM and µLEED measurements were performed using the aberration corrected low energy electron microscope (Elmitec GmbH) installed at MAXPEEM beamline (MAX IV synchrotron, Lund, Sweden) to inspect the morphology and the structure of the samples investigated by SPEM.

Figure 1. a) A schematic view of the NAP cell. b) SEM image of the graphene/copper foil sample. c) The LEEM image is recorded on pristine sample at 3.6 eV start voltage (20 µm FOV). d) IV-curves collected from selected areas (0.57 µm size) on the pristine sample: bare copper (Cu, yellow), monolayer (1ML, green) and bilayer graphene (2ML, blue). e) The μLEED measurements are taken from selected areas, whose dimension is 0.45 μm, at a start voltage value of 45 eV: copper (top row), monolayer and bilayer graphene (bottom row).



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3. Results and discussion The representative LEEM image of the pristine sample surface is shown in Figure 1c using a field of view of 20 μm and the respective electron reflectivity changes at increasing electron energy, IV-curves, are reported in Figure 1d. In the pristine sample, the bare copper area is imaged on the bottom left (the blue area labelled Cu), the typical hexagonal shapes individuate the multi-layered regions (2ML and 3ML) while monolayer fills the rest of the field of view (1ML). The oblique darker lines visible on graphene are representative of an array of facets, e.g. differently inclined surfaces, on the Cu grain, as previously reported

34,35.

The IV-curve

collected from the copper surface does not show modulation in the reflectivity confirming the polycrystallinity of the metallic substrate. On the contrary, the characteristic dips of pristine monolayer and bilayer graphene are identified in the respective IV-curves

34.

The reciprocal

space is studied in μLEED mode using a start voltage of 45 eV (Figure 1e). The diffraction patterns are recorded from the bare metal substrate and from the pristine graphene. On the metal, a long-range ordered structure is not observed overall except for a small contribution from 1x1 spots typical of Cu(111) reconstruction of the Cu foil after the CVD growth 36. On graphene area, the well-defined six-fold diffraction patterns are identified for both monolayer and bilayer graphene. Here, the two main inclined Brillouin zones identified in μLEED pinpoint the presence of the tilted domains as a consequence of the substrate Cu grain morphology, which mainly consists of two inclined facets (different (0,0) spots in the corresponding μLEED) transferred to the crystalline carbon layers. 3.1 Isobaric experiment. In the isobaric experiment (0.1 mbar O2 gas), we identified three different regimes of temperature according to the different physical mechanisms occurring on the sample: namely,


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a low-temperature regime (up to 300 °C), a transitional regime (up to 360 °C), and an etching regime (above 360°C). These regimes are discussed in details in the following section. Low-temperature regime. Initially, a thin layer of contamination, mainly consisting of amorphous carbon and water film, covers the sample’s surface. At a temperature of 270 °C, this layer starts to desorb and it is almost completely removed at 300 °C. This evolution can be followed by the C 1s core level spectra measured on a graphene flake and on the bare Cu substrate, as reported in Figure 2a,b. On graphene, the C 1s peak’s FWHM shrinks of about 30 % on both low and high binding energy sides, towards the typical narrow line shape of graphene 29,37,38.

Whereas on copper, the broad carbon signal decreases without modifying its line shape:

an indication of a uniform removal from the substrate. The LMM Auger line of copper, shown in Figure 2c,d, gives useful hint about the oxidation state of the substrate. Beneath graphene, this temperature does not influence the Auger line shape, which maintains the typical fingerprint of metallic copper. On the contrary, on the bare Cu substrate, the Auger peaks evolve showing a transition at about 250 °C towards cuprous oxide (Cu2O), the native oxidation state of copper 33.

Given the experiment was performed in pure O2 atmosphere and no source of hydrogen was

present in the cell, we did not observe the formation of the metastable Cu(OH)2 from which the cupric oxide (CuO) originates. The removal of the contamination layer from the sample’s surface directly exposes the bare copper regions to the oxygen gas thus initiating its oxidation.



Figure 2. Low-temperature regime (from 150 to 300 °C). a, b) C 1s core level spectra and c, d) Cu LMM Auger spectra recorded from a graphene flake and a bare copper, as indicated on top of the panels. The spectra are stacked for clarity. The temperature colour legend is in the inset in a).

Transitional regime. In this regime, the temperature is increased from 300 to 360 °C. A series of maps is reported in Figure 3, they are measured at increasing temperatures on the very same region at the boundary between two hexagonal graphene flakes. In particular, we concentrate on the O 1s and Cu 2p photoemission maps. Differently from the Cu LMM Auger whose lineshape is very sensitive to the difference between metallic copper and Cu2O, in the Cu 2p core level these species are almost not distinguishable (contrary to CuO) and it can be used here for quantification of material. The photoemission maps of oxygen and copper show an opposite


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behaviour in the image contrast: the oxygen signal is increasing at every step and spreading towards the flakes, especially from the borders. At the same time, in a one-to-one correspondence, on the regions where the O 1s signal increases, the Cu 2p apparently decreases, getting darker in the map.

Figure 3. Transitional regime. 50x12.5 µm2 O 1s (left), Cu 2p (centre) and C 1s (right) photoemission intensity maps of graphene/Cu foil at increasing temperatures. The red solid line highlights the two graphene flakes, while the red dashed line corresponds to the line profile in Figure 4. Brighter colour means higher intensity. The photoemission maps on C 1s are obtained by the ratio of the signal in the energy window corresponding to lifted (black) and unlifted graphene (green), see in the text for details.

This behaviour can be also followed by the intensity line profiles, crossing a graphene flake, as reported in Figure 4 extracted for three temperatures from both the oxygen and copper maps. The O 1s intensity increases with temperature, going towards the centre of the graphene flake, at the same time, the Cu 2p intensity decreases. This process seems to be faster on the right side of the flake. At the first step (from 340 to 350 °C), this effect progresses with a speed of approximately 0.1 µm/min on the right side and 0.05 µm/min on the left border; at the second step, it slows down to 0.04 and 0.01 µm/min, respectively.



Figure 4. Transitional regime. Intensity line profile across the red line in Figure 3. The region from 5 to 32 µm corresponds to the graphene flake.

To further investigate this phenomenon, we measured at 350 °C the XPS spectra in three different points of the map in Figure 5a: namely, bare copper substrate (#1), near the border of the graphene flake (#2) and at the centre of the flake (#3). The O 1s core level (Figure 5b), correctly reproduces what observed in the map: a high intensity on Cu, then in point #2 and finally in #3, the lowest. From the spectra, it is possible to observe also the signal from the oxygen gas phase at about 538 eV 39. The copper Auger line recorded on the substrate indicates its oxidization, as already stated for the low-temperature regime. Beneath the graphene (#3) the copper is still metallic, while near the borders (#2) there is a mix of both metallic and oxide phases. A final indication comes from the C 1s core level from the two points of the graphene flakes. The two peaks maintain the same lineshape, but the spectrum corresponding to the border of the flake is shifted by 0.33 eV towards lower binding energy with respect to the point #3 which holds its position at 284.4 eV, typical of graphene 29. In particular, we did not notice any sign of graphene oxidation (C-O peaks around 286-288 eV) 40 or degradation of the flake


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in the form of a broadening of the peak or a decreasing in intensity. All these ingredients point in the direction of oxygen intercalation beneath graphene, starting from the edges of the flake, as schematized in Figure 5c. Intercalation makes graphene lifting from the substrate and the interaction with the metal is therefore reduced, causing a downshift of the C 1s peak. This can be seen in Figure 3 in the maps obtained by the ratio of the signal corresponding to the low binding energy side of C 1s (lifted graphene) and the original position of the carbon peak. These maps follow the same behaviour of copper and oxygen signal. This strain relief effect has been observed by Raman, through a redshift of the graphene’s 2D peak similar energy shift as we observed

42,43.

41

and by UHV XPS, by a

The single O 1s component located at 530.1 eV,

identical on both Cu and graphene confirms that the oxygen is interacting only with the copper substrate, therefore there is no evidence for graphene oxidation at this stage. This binding energy is typical for Cu-O bonding in Cu2O, whereas C-O is expected at higher energies 13,44– 46.

The intercalation also justifies the decreased intensity of the Cu 2p as observed in Figure 3

and Figure 4: this is due to the extra path the photoelectrons need to cross from the substrate, leading to an attenuation of the signal. The intensity line profile in Figure 4 can be therefore seen as the progress of the intercalation front. Oxygen intercalation beneath graphene on metal substrates has been widely discussed in the literature 15,42,47,48, yet confined in UHV environment and mostly to catalytically active metals (Ir, Ni, Ru…) where O2 dissociation is easier and etching of graphene facilitated at lower temperatures and/or pressures.



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Figure 5. Transitional regime. a) 38x38 µm2 O 1s photoemission intensity map of graphene/Cu foil at 350 °C. b) O 1s core level, Cu LMM Auger and C 1s core level spectra recorded on the three points indicated in a). c) Sketch representation of the oxygen intercalation and graphene lifting from Cu substrate.

Etching regime. At temperatures higher than 360 °C, the monolayer graphene flakes start to etch away beginning from the regions where oxygen intercalated. Strictly related to the etching of graphene, there is a change in chemistry and morphology of the copper substrate, whose lattice expands while oxidizing. This creates protrusions of Cu2O from graphene layers, as observed in Figure 6 circled by red dashed lines, similar to the etch pits observed by AFM

3

and to Cu2O formation at point defects and grain boundaries 19,41,49,50. The tridimensional nature of the Cu2O grains is deducted by the copper image in Figure 6c, where the regions present the same oriented bright/dark contrast, with the highest intensity on the facet facing the analyser. Dotted lines also indicate the edges of two neighbouring flakes. The copper is oxidizing both from the edges of the graphene flake and at the protrusions that may originate from point defects


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in the flake, at the same points graphene is etched, as observed by the close correspondence between Figure 6b and 6d. The etching of graphene is univocally testified by the disappearance of the quasi totality of the carbon signal from the sample at the highest temperature reached (390 °C), starting from single layer graphene flakes. Bilayer flakes are initially less affected, in agreement with previous reports 3, and then become highly corrupted, as pointed out later. The morphological change of the substrate and the degradation of the graphene flakes can also be observed by electron microscopy (SEM and LEEM) reported in section 3.3. In order to etch graphene with O2, the oxygen molecule needs to dissociate into reactive atoms. Dissociative chemisorption happens easily on catalytically active metals, as Ir(111) or Ru (0001), where etching occurs at very low pressure (vacuum conditions) and below 300 °C 51,52.

Due to the lower reactivity of copper surface, much higher O2 pressure is required.

Differently from Zhang et al. 49, we did not observe evidences of graphene oxidation before etching.



Figure 6. Etching regime. 23 x 5.8 µm2 a,b) C 1s and c,d) Cu LMM photoemission intensity map of graphene/Cu foil a) before starting oxidation and b,c,d) at 390 °C in 0.1 mbar O2. The copper map is obtained by the ratio of the signal in the energy window corresponding to metallic Cu [KE 922.15; 917.4]eV (bright yellow) and oxidized Cu [917.2; 914.45]eV (dark violet).

3.2 Isothermal experiment An analogous experiment was performed using a twin pristine sample; in this case, the temperature was kept constant (350 °C), while the measurements were performed starting without oxygen and then in an O2 gas pressure that was increased in six steps from 0.003 mbar to 0.1 mbar (at each step the gas pressure was doubled). From the results obtained in the isobaric experiment, the removal of the contaminant from the surface and the oxidation of the bare substrate seemed to be contextual as soon as the sample reached a temperature around 250 °C. On the contrary, the isothermal experiment allowed us to disentangle these two phenomena.


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Figure 7 shows a series of copper maps recorded on the same region of the sample showing mono- and multilayer graphene flakes, at four representative pressure steps. On the left side the photoemission maps of the Cu 2p core level are reported, while on the right side the equivalent oxidation state of the Cu, derived from the Cu LMM Auger, is shown. It is possible to see that at the first step, when no oxygen is introduced in the cell, a layer of carbon is present all over the scanned region and the copper beneath shows a metallic phase. As soon as O2 is flowed (0.003 mbar), regions of bare copper substrate start to appear from the map, confirming the removal of atmospheric contamination observed in the first experiment (Figure 2). Yet these regions remain metallic according to the Cu LMM line shape. The first signs of copper oxidation are noticed at 0.012 mbar of O2. At this step, the Cu grain boundaries become visible in the Cu 2p map and appear sharper when increasing the pressure to 0.025 mbar (not shown) and to 0.05 mbar. Therefore, the removal of amorphous contamination occurs at a lower pressure than the one needed to oxidize the bare copper.



Figure 7. Isothermal experiment. Cu 2p (left) and Cu LMM (right) photoemission maps at different oxygen pressure and constant temperature (350 °C). The Cu LMM maps show the ratio between channels corresponding to metallic and oxidized copper. Under this condition, the sample is stable and no further evolution is observed in 6 hours. However, the dynamics take a rather abrupt acceleration when increasing the oxygen pressure to the last step at 0.1 mbar (while keeping the temperature constant at 350 °C). At this point, the oxidation of the copper substrate is initiated, as reported in Figure 8. The carbon maps show how the monolayer flakes are not yet etched but they begin to be damaged, while bilayer flakes are still intact, independently of the distance from the oxidation front.


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Figure 8. Isothermal experiment. 50 x 25 µm2 Cu 2p, Cu LMM (met/ox) and C 1s photoemission maps showing the evolution in after 2.5 hours (middle row) and 5 hours (bottom row) at 350 °C and 0.1 mbar O2.

3.3 Electron microscopy studies: SEM, LEEM, µLEED For a deeper investigation of the etching regime, LEEM and μLEED measurements were performed to image the real and the reciprocal spaces of the two samples presented so far that underwent oxidizing conditions (from now on named IsoT, the one from isothermal experiment - 0.1 mbar O2 and 350 °C, and IsoB from isobaric experiment - 0.1 mbar O2 and 390 °C). The representative LEEM image of IsoT and IsoB samples and the corresponding IV-curves are shown in Figure 9a and Figure 9b, respectively.



Figure 9. LEEM and SEM images recorded from a) IsoT sample using a FOV of 20 µm and start voltage 1.6 eV, and from b) IsoB sample using a FOV of 30 µm and start voltage 1.9 eV. Their respective IV-curves are collected from selected areas on the samples: bare copper (Cu, yellow curve), monolayer (1ML, green) and bilayer graphene (2ML, blue). In IsoB the labels are indicating where the graphene regions were.

At the IsoT oxidation stage, the morphology of the surface is strongly affected by the etching of the monolayer graphene that appears indented, in agreement with SEM images (Figure 9a). The correspondent IV-curve confirms the loss of order in the monolayer. On the other hand, the bilayer region is retaining the main structure though damaged areas, visible as darker spots in the LEEM image, are arising from the underlying layer. Interestingly, the array of facet types, that was found in the pristine sample, is not visible anymore. Two possible scenarios can explain this observation: either the two inclined facets of the pristine Cu grain morphology are


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removed by the oxidation, or the monolayer graphene is indented also below the 2ML graphene area so that the faceting morphology can´t be transferred anymore to the upper carbon layer. The IV-curve of the 2ML graphene flake is slightly modified at higher start voltages but the overall structure is kept, thus indicating the presence of graphitic order. As observed for the sample heated at 350 °C in Figure 9a, the etching starts from single layers graphene flakes, while the bilayers are less affected, in agreement with previous reports 3.

Pushing the oxidation to higher temperatures (IsoB samples, up to 390 °C) completes the

burning of the entire carbon surface ending in the removal of any dip and valley in the electron reflectivity curve. This last stage of oxidation causes the complete etching of the single layer and a strong disruption of the bilayer flakes that are now completely fragmented and rough, as shown in the LEEM image in Figure 9b where the white dashed line is a guide for the eye showing the location of the ruined 2ML flake. In order to visualize the modifications on the crystalline order of the two samples, the diffraction patterns are recorded in μLEED mode (Figure 10). In IsoT sample, a reconstructed surface is found on the oxidized copper area which consists of tilted crystalline regions given the presence of various tilted (0,0) spots. The crystalline order is however only superficial as revealed by the μLEED collected at higher energy (Figure S1). If from one side, harsher oxidation leads to an ordered oxidized copper foil surface, on the other hand, it induces the etching of the first carbon layer that turns now into tilted and twisted misaligned faceted fragments. The μLEED pattern collected on the bilayer flake of IsoT is formed by two main hexagonal diffraction patterns which are now rotated of about 28°, as indicated by the white and black dashed lines on Figure 10, that are relative to the top and bottom layers, respectively. The additional spots arise from the moiré reconstruction generated by the rotated layers and it is individuated by translating the vectors to the centre of the diffraction pattern (Figure S2). Besides the relative rotation of the two carbon layers, the oxidation leads to the disappearance of the two tilted Brillouin zones observed in the pristine



sample thus confirming the loss of faceting, as previously observed in LEEM for the pristine sample (Figure S3 and S4). No trace of six-fold diffraction spots is visible in the μLEED of the bilayer in IsoB sample, bottom row in Figure 10. All around the remaining spoiled bilayer carbon flake, there are faceted terraces and grains that are characterized by the same reconstructed diffraction pattern as the oxidized copper layer in IsoT.

Figure 10. μLEED measurements are taken from selected areas on IsoT (top row) and IsoB samples at a start voltage value of 45 eV. The diffraction patterns are collected from the copper surface, the monolayer and bilayer graphene flakes.

4. Conclusions In the present experiments, we probe for the first time the behaviour of graphene coated copper foil in oxidizing conditions by means of ambient pressure x-ray photoelectron spectromicroscopy. Two experiments have been presented: isobaric (0.1 mbar O2, temperature up to 390 °C) and isothermal (350 °C and pressure up to 0.1 mbar). Both experiments were essential to disentangle various phenomena arising with different dynamics. An initial


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amorphous carbon layer from atmospheric contamination is present all over the sample; this layer is removed at relatively low oxygen pressure (0.003 mbar) while keeping the sample at 350 °C. In order 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 commences 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 sign of graphene oxidation is observed, which may be contextual to the etching, or maybe the Cu oxidation and its morphological change are faster and disrupt the graphene before it can be eventually oxidized. The oxidation process is investigated also with LEEM and μLEED measurements. The change in the surface morphology consists in the etching and indentation of the monolayer graphene, while the bilayer region is mostly retaining the main structure and a relative rotation of 28° between the top and bottom carbon layers is observed. The faceting, that is present in the pristine graphene sample, disappears after the oxidation experiment and the oxidized copper foil surface shows a superficially reconstructed surface. Pushing the oxidation to higher temperatures completes the burning of the entire carbon surface: a complete etching of the single layer area that shows instead the same reconstructed diffraction pattern as the oxidized copper layer, and a strong disruption of the bilayer flakes that are completely fragmented. The demonstration that graphene hinders the oxidation of copper in an aggressive environment at high temperature, while bare copper oxidize already naturally at room temperature, offers great potential 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 the bigger and defect-free the flakes are, the best is their anticorrosive protection. Furthermore, bilayers flakes



suppress the oxygen intercalation and, as a consequence, the oxidation is pushed at even higher temperature. A homogenous 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 NAPSPEM. 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 towards directly study surface and interface applications in more realistic working conditions.

Supporting Information The Supporting Information contains additional µLEED measurements of pristine and IsoT samples.

Acknowledgement The Calipso Programme is acknowledged for financial support. CS acknowledges the Stiftelsen för Strategisk Forskning (SSF) (Project no. RMA15-0024).

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