Oxidation Mechanisms of Copper under Graphene: The Role of

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Oxidation mechanisms of copper under graphene: the role of oxygen encapsulation Leo Alvarez-Fraga, Juan Rubio-Zuazo, Félix Jiménez-Villacorta, Esteban Climent-Pascual, Rafael Ramírez-Jiménez, Carlos Prieto, and Alicia de Andres Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00554 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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Chemistry of Materials

Oxidation mechanisms of copper under graphene: the role of oxygen encapsulation Leo Álvarez-Fraga1, Juan Rubio-Zuazo1,2, Félix Jiménez-Villacorta1, Esteban ClimentPascual1, Rafael Ramírez-Jiménez1,3, Carlos Prieto1, and Alicia de Andrés1* 1

Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas. Cantoblanco,

28049 Madrid, Spain. 2

BM25-SpLine The Spanish CRG Beamline at the ESRF, 38043 Grenoble Cedex 9, France.

3

Departamento de Física, Escuela Politécnica Superior, Universidad Carlos III de Madrid, Avenida

Universidad 30, Leganés, 28911 Madrid, Spain.

ABSTRACT The oxidation and corrosion of copper are fundamental issues studied for many decades due to their ubiquitous and transversal impact. However, the oxidation of copper used as catalyst for graphene synthesis has opened a singular problem not solved yet. Contradictory results are reported about the protecting or enhancing role of graphene in copper oxidation. We study short and long term oxidation of copper with different characteristics, as oxygen content and morphology, with and without graphene; in particular, polycrystalline copper foils and almost totally textured (100) and (111) copper films on MgO and sapphire substrates respectively. We propose a mechanism to explain the enhanced oxidation of polycrystalline copper originated by oxygen encapsulated by the graphene impermeable layer during graphene growth. The initial oxygen content and the existence of grain 1 ACS Paragon Plus Environment


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boundaries are the main factors that determine the relevance of this process. Graphene is shown to prevent oxidation from atmosphere for any of the copper substrates but also promotes slow oxidation derived by the release of out-of equilibrium encapsulated oxygen. The formation of bubbles after several months evidences this slow release. The occluded oxygen in graphene covered copper is demonstrated comparing the oxygen to copper ratio at different depths using Hard X-ray Photoelectron Spectroscopy for samples with and without graphene.

KEYWORDS: graphene, copper oxidation, thin films, diffraction, Raman spectroscopy, copper oxide, HAXPES

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INTRODUCTION The oxidation and corrosion of metals, and particularly of copper, are crucial issues for everyday’s life. Fundamental studies on the reconstruction of metal surface and on the adsorption sites of oxygen for different copper faces have been widely carried out for long time now.1,2 These findings together with the development of theories and the experimental observations of the first oxidation steps in controlled ultra high vacuum environment are essential to reveal the oxidation mechanisms.

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At the other end, metallurgical

engineering is particularly interested in the corrosion of bulk polycrystalline metals and alloys

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and especially focused in the processes in harsh temperature and atmosphere

conditions. Recently, the oxidation and corrosion of the copper foils used as catalysts for graphene synthesis has opened a singular problem not solved yet. Since graphene is an impermeable membrane to gases, and particularly to oxygen, it was expected to be an efficient barrier to copper oxidation.8 Actually, the ability of graphene to protect Cu from air oxidation for a short time (up to around 4 h and 200 ºC) was reported

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and from corrosion with

Na2SO4.11 Also, when graphene is grown on a Cu (111) single crystal the protection is shown to be effective for a long time (20 days) while the cracks of the graphene layer allow localized corrosion which extends progressively.12 However, in a long time, it is observed that the oxidation kinetics of polycrystalline copper foils is accelerated when graphene is present.13, 14 The proposed mechanism for this surprising finding is that graphene is acting as an optimum conducting layer that favors the electrochemical oxidation of the copper it covers overcoming the passivation occurring in bare copper at room conditions.14 This favored electrochemical oxidation by graphene should, in principle, promote oxidation also in copper single crystals, contrary to what is reported,12 or in copper films. Moreover, the 3 ACS Paragon Plus Environment


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high conductivity of graphene included in polymeric anticorrosion coatings is precisely argued to prevent copper oxidation. 15 It is clear that the reported observations and proposed mechanisms seem contradictory and deeper study of the influence of graphene in the oxidation of copper is required. On the other hand, the proposed mechanism for the enhanced oxidation is, in principle, only dependent on the conductivity of graphene and therefore homogenous oxidation of copper is expected. However inhomogeneous oxidation of copper foils (some grains resist oxidation longer than others) has been reported and tentatively attributed either to a stronger graphene-Cu interaction or adhesion depending on the local copper structure13, 14 or to the presence of defects that favor the nucleation sites for oxidation14. To understand the disparate reported results, several aspects should be taken into account as the morphology, structural characteristics and thermal history of the copper substrate in the oxidation mechanisms. We have tested these hypotheses studying the oxidation of highly textured copper thin films compared with that of copper foils, in all cases with and without graphene. Copper foils are polycrystalline with large grains while films are almost totally oriented in the [100] and [111] directions when deposited on MgO (100) and Al2O3 (0001) substrates, respectively. The oxidation of copper as polycrystalline foils and highly textured films has been followed for up to one year for samples with and without graphene. Graphene is demonstrated to act either protecting or favoring oxidation depending on the morphology and the initial oxygen content of copper. Synchrotron x-ray diffraction and resonant Raman spectroscopy are used to determine structural aspects and to follow the formation of copper oxide. Morphological aspects are observed by scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical microscopy. Hard X-ray Photo Electron Spectroscopy (HAXPES) is 4 ACS Paragon Plus Environment

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used to compare bulk oxygen content of copper foils with and without graphene. A mechanism based in the encapsulation of bulk oxygen by graphene is proposed for the enhanced oxidation of copper foils when graphene is grown on top.

EXPERIMENTAL SECTION Commercial Cu foils (25 µm-thick, 99.8 %, Alfa Aesar) were rinsed with acetone for 10 min, dipped into acetic acid for 2 h and dried with N2 gas. The copper foils were annealed at 250 ºC in air to form an oxidized surface that, in the chemical vapor deposition CVD process, promotes the elimination of carbon residues leaving a clean carbon-free copper surface for graphene growth.16 Once in the CVD furnace the foils were heated up to 1040 ºC with hydrogen atmosphere (8 x 10-1 mbar) for 1h to increase the grain size and eliminate oxides and carbon. Cu films (500 nm thickness) were deposited from a Goodfellow Cu target (99.99+ % pure) on cubic MgO (001) and on hexagonal α-Al2O3 (0001) substrates (labeled Cu/MgO and Cu/Al2O3) at 500 and 650 ºC respectively, by DC sputtering magnetron with Ar gas (5x10-3 mbar). Prior to copper deposition, MgO (001) substrates were cleaned with successive rinsing in ultrasonic baths of acetone, iso-propanol and deionized water (15 min per bath) and dried with N2 gas and finally thermally degassed at 800 ºC in H2 atmosphere for 1 h. The Al2O3 substrates were annealed at 1000 ºC in air for 10 hours. Graphene is grown on the foils and Cu thin films at 910 ºC with methane and hydrogen (9 x 10-1 mbar) with a ratio CH4:H2 =1:10 for 30 min. The resulting samples are labeled Gr/Cu/MgO, Gr/Cu/Al2O3 and Gr/Cu foil. Finally, the chamber was cooled to room temperature keeping constant the flow gases.



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The morphology of the samples is analyzed with a field emission scanning electron microscope (FESEM) FEI Nova NanoSEM 230. Atomic force microscopy (AFM) images were collected in the tapping mode (Nanosensors PPP-NCH-w Si tips with cantilever resonance frequency f0 ≈ 270 kHz and k ~ 30 Nm−1) using a commercial equipment from Nanotec NanotecTM. Micro-Raman experiments were performed at room temperature using the 488 nm line of an Ar+ laser with an incident power in the 1-8 mW range with an Olympus microscope (100× objective) and a “super-notch-plus” filter from Kaiser. The scattered light was analyzed with a Jobin-Yvon HR-460 monochromator coupled to a Peltier cooled Synapse CCD. The synchrotron X-ray diffraction experiments were performed at BM25-B Spline at the ESRF in a six-circle diffractometer in vertical geometry.

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The photoemission

measurements were also performed at BM25-B SpLine in a specially developed set-up dedicated to the combination of X-ray Diffraction and Hard X-ray Photoelectron Spectroscopy.18 Such a set-up incorporates a high voltage cylindrical sector analyzer able to collect electrons up to 15KeV kinetic energy19 and a 2+3 diffractometer capable to position de incidence angle with high precision.

RESULTS AND DISCUSSION CuO and Cu2O oxides formed by heating a Cu foil at 250 ºC in air at 1 ºC/min and cooled at the same rate are used as references to study the spontaneous oxidation of the different samples. The analysis of Raman spectra and synchrotron x-ray diffraction patterns is presented in the Supporting information (Figure S1). Raman spectroscopy using 488 nm excitation, which is resonant for copper oxide can be used to identify the formed oxides


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since their spectra present clearly distinct features (Figure S1), however the analysis has to be made carefully since CuO main peaks (289 and 623 cm-1) are close to Cu2O peaks (298 and 640 cm-1). The Raman spectra of all the here studied samples (with and without graphene) correspond to Cu2O although small CuO fraction may also be present in some particular cases. We use the 640 cm-1 peak intensity to compare the oxidation degree of the different samples. Oxidation of copper foil versus copper single-crystalline films We have followed the time evolution of the oxidation of copper foils and copper films used as catalytic substrates for graphene synthesis during eight months in ambient conditions. To obtain Cu (100) and Cu (111) textures, two copper films with thickness around 500 nm were deposited on cubic MgO (100) and other two on α-Al2O3 (0001).20,21 One sample of each type (Cu/MgO and Cu/Al2O3) was then used to synthesize graphene in the same batch at 910 ºC and the others were kept at ambient conditions to study their oxidation without graphene. Graphene single layer is obtained in all cases. Cu/MgO and Gr/Cu/MgO, are indeed almost totally textured in the [100] direction while those grown on sapphire α-Al2O3 (0001), Cu/Al2O3 and Gr/Cu/Al2O3, are mainly [111]. Figures 1a and 1b show the synchrotron x-ray-diffraction (XRD) patterns of the samples with graphene. On the contrary, the Cu foil is a polycrystalline material whose grains increase in size during graphene growth process. Though strictly speaking the Gr/Cu-foils cannot be considered as regular polycrystals since some preferential orientation is always found (especially when using focused synchrotron radiation as shown in Figure 1c and inset) grains and grain boundaries are always present.


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a)

b)

Cu (111) Cu (200)

c)

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Cu (200)

Gr/Cu/MgO

Cu (220)

Al2O3 (003) MgO (200)

Gr/Cu/Al2O3

Cu (111)

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Diffraction Intensity - Log scale (arb. units)


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position (mm)

Gr/Cu foil 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 4.9 5.0

Wavenumber (Å-1) Figure 1. Crystalline orientation of copper films and foil: θ-2θ scans with λ = 0.6919 Å (in logarithmic scale to evidence possible weak peaks) of a) Gr/Cu/Al2O3 with (111) Cu, b) Gr/Cu/MgO with (100) Cu and c) Gr/Cu foil with pseudo-polycrystalline Cu. The inset shows the variation of the Cu (111) and (100) diffraction intensities along the position for one direction of Gr/Cu foil.


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Figure 2: Oxidation of copper films and foil: a) Gr/Cu foil, b) Gr/Cu/MgO and c) Gr/Cu/Al2O3. From left to right: optical images of the as deposited samples, optical images three months later and the corresponding Raman images (10×10 µm2) of the ratio between the intensities of the 640 cm-1 copper oxide and 1580 cm-1 graphene peaks (the scale bar is 1 µm). At the right end side a representative Raman spectrum for each sample is shown. The white pixels in Gr/Cu/MgO are due to the absence of graphene in these particular places.

It is important to note that, while copper foils, in this case 99.8 % purity, contain typically 100 – 400 ppm oxygen, the oxygen content of the films, which are grown by sputtering with Ar from a Cu (99.99 %) target and a base pressure of 1.5 × 10-6 mbar, is extremely low. We therefore compare these graphene covered copper samples, foil and films, with very different morphology, structural characteristics and oxygen content.



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Figure 2 shows the optical images of Gr/Cu foil, Gr/Cu/MgO and Gr/Cu/Al2O3 samples obtained the same day after graphene synthesis and three months after. The growth of graphene at 910 ºC on the Cu films produces the partial evaporation of copper (black areas in Figures 2b and 2c). The observed changes in color after three months are due to the oxidation of copper and, to evaluate the oxidation degree, one hundred Raman spectra in resonant condition in 10×10 µm2 areas were collected. The Raman images in Figure 2 correspond to the intensity ratio between the characteristic Cu2O peak around 640 cm-1 and the G graphene peak at 1580 cm-1. Individual representative spectra are also included. The three Raman images are plotted with the same intensity scale, as well as the three spectra to facilitate the comparison between samples. The Raman information clearly reveals that, while copper from the Gr/Cu-foil is strongly oxidized, the copper films (Gr/Cu/MgO and Gr/Cu/Al2O3) present only weak oxidation after three months. This observation indicates that the role of graphene in the oxidation processes is unclear since the oxidation degree is very different depending on the characteristics of the copper it covers. Non-homogeneous oxidation of polycrystalline copper. An important point to be taken into account is that the oxidation of Cu foils is not homogeneous (Figure 3), in agreement with previous reports. 13,14,22


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Figure 3. Inhomogeneous oxidation of polycrystalline copper: a) FESEM image of a Gr/Cu foil recently after graphene deposition, the contrast is due to different channelling effect in copper grains with different crystallographic orientations. b) Optical image after three months in ambient conditions, the different colors are due to the variation of Cu2O layer thickness formed on differently oriented copper grains. (c) Optical image after 8 months (black square = 10 µm × 10 µm), d) Raman spectra from the clear (A) and dark (B) regions and e) Raman image of the 650 cm-1 Cu2O peak corresponding to the square indicated in the optical image.

The large Cu grains of a foil just after graphene growth are revealed in FESEM images (Figure 3a) due to the different channeling efficiency for grains oriented along different crystallographic axes. The SEM images are consistent with the texture observed by XRD. After three months, the optical images evidence the oxidation of the foil by the observation of copper grains with different reddish colors (Figure 3b). The oxidation is confirmed by the clear correlation between the optical image and the Raman image of the intensity of copper oxide optical phonon at 640 cm-1 (Figure 2c). The optical image shows regions with



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different colors; the clearer grains do not show the formation of oxide, within the detection limit of Raman technique, and the darker ones evidence the formation of Cu2O (Figure 3c). The oxidation of copper foils used as catalyst for the growth of graphene has some peculiarities, in particular, the copper foils are usually pre-treated in hydrogen to increase the grain size and eliminate oxide layer at temperatures typically above 1000 ºC and graphene synthesis is done in hydrogen/hydrocarbon-precursor atmosphere at lower temperatures, around 600-1000 ºC (in the present case: methane and 910 ºC). During pretreatment and graphene growth process the copper foil suffers morphological modifications of grain dimensions (in particular, grain size increases23) and crystallographic orientation. Undoubtedly, the thermal history of copper is important regarding its grain size, crystalline configuration, boundaries as well as oxygen content. The final quality of the synthesized graphene on copper substrates has been tightly correlated to the presence of oxygen24 and of copper oxide.25 Importance of thermal history on Cu oxidation The temperature at which copper foils and copper films, all without graphene, are put in contact with air after thermal treatment has a radical impact on their oxidation. The Cu foils annealed at 1040 ºC in H2 and the Cu films, deposited at high temperature, were extracted from the deposition chambers in one case at 70 ºC and in the other case at 20 ºC. All samples were then maintained at room conditions (in air and 20ºC). While the three samples extracted at 20 ºC are almost non-oxidized, those extracted at 70 ºC are strongly oxidized in few days. Figure 4a shows the optical images of the Cu foil extracted at 20 ºC and Figure 4b of that at 70 ºC. In the case of the foil, totally distinct regions corresponding


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to different grains and to different crystallographic domains within a grain can be identified. The immediate oxidation of Cu/Al2O3 film is also observed but in this case the oxidation is uniform.

Figure 4. Impact of the extraction temperature: optical images (×100 and ×20 objectives) of a Cu foil after H2 annealing extracted at a) 20 ºC and b) 70 ºC; c) representative Raman spectra.

Graphene as protective layer To check relevance of graphene in short term oxidation processes we compare the oxidation of samples with and without graphene after extraction in air at 70 ºC. We observe that when graphene has been grown on any of the substrates, copper foil or films, the oxidation is undetectable (the first days) and is unmodified by the extraction temperature. Figure 5a summarizes the synchrotron x-ray diffraction patterns one week after sample growth for the films on sapphire with and without graphene. The diffraction pattern for Cu/Al2O3 presents intense peaks corresponding to a Cu2O film while for the sample with graphene Gr/Cu/Al2O3 no oxide is detected at all. 13 ACS Paragon Plus Environment


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Figure 5. Graphene as protecting layer for copper films oxidation: a) θ-2θ scans with λ = 0.6919 Å of Cu/Al2O3 (red line) and Gr/Cu/Al2O3 (black line). In Cu/Al2O3 the Cu2O diffraction peaks are clearly identified. b) Optical images and resonant Raman spectra of the samples. The Cu2O Raman peaks in Gr/Cu/Al2O3 (black line) are much weaker than in Cu/Al2O3 (red line). The graphene G and 2D peaks are marked with arrows.

Using Raman spectroscopy we have followed the oxidation of the samples at different stages. In Figure 5b a representative Raman spectrum shows intense Cu2O peaks (in the 100-1300 cm-1 range) for the sample without graphene, Cu/Al2O3, while only very weak


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peaks for that with graphene, Gr/Cu/Al2O3. The samples with graphene, even after 8 months, still show very weak Cu2O peaks. From the above presented results we can conclude: i) the copper surface is highly activated after annealing in reducing atmospheres so that, without graphene, temperature as low as 70 ºC produces a strong immediate oxidation; ii) single crystalline copper films with graphene are much less oxidized than polycrystalline copper with graphene in the same conditions; iii) polycrystalline copper is more oxidized with graphene than without; iv) the nucleation and growth of copper oxide depend on the crystallographic orientation of copper; v) the role of graphene in the oxidation processes is unclear since the oxidation degree is very different depending on the characteristics of the copper it covers, vi) short term and long term oxidation is prevented when graphene covers single-crystal copper films showing the capacity of graphene to protect against oxidation even in a long time (8 months).

Proposed mechanisms for copper oxidation Taking into account the published and here presented observations we propose the combination of two different processes for copper oxidation. One is the already reported oxidation due to atmospheric oxygen and water and we propose an additional mechanism due to oxygen out of equilibrium inside copper that migrates towards the surface and induces the formation of copper oxide. Depending on the characteristics of copper, as its morphology, bulk oxygen content and thermal history, the graphene layer may impede the expulsion to atmosphere of the out-of-equilibrium oxygen existing after graphene growth inside copper. Figure 6 schematizes our proposed mechanism.



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Figure 6. Schema for oxygen encapsulation by graphene in polycrystalline Cu a) initial copper foil with oxygen mainly at the grain boundaries and oxidized surfaces (few nm), b) annealing at high temperatures in hydrogen promotes the elimination of the copper oxide surface layers and the solution of oxygen inside grains c) once the oxide layer is eliminated the oxygen migrates to the surfaces and at RT d) the oxygen content is very low. However, e) if graphene is grown, then this migration is stopped since oxygen content close to the surfaces is high and f) oxygen is encapsulated at RT.

The diffusion and chemical reactions of oxygen occurring in the bulk of copper during an annealing with hydrogen are complex and not well established. At RT, oxygen is supposed to mainly accumulate at grains boundaries, however, the solubility of oxygen in copper increases exponentially with temperature.26,27 At high temperatures several processes occur, mainly the elimination of the superficial copper oxide layer by hydrogen and the diffusion and migration of oxygen. The increased solubility of oxygen favors its migration from grain 16 ACS Paragon Plus Environment

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boundaries to grains. Also, oxygen will tend to diffuse towards the oxygen-poor copper surfaces, but this will occur only once the superficial copper oxide layer is eliminated. If the annealing time (typically one hour or below) is short enough, oxygen cannot be totally released and the formation of graphene blocks any further oxygen elimination. When copper is annealed in identical conditions but without growing graphene the fraction of released oxygen is higher and, during the cooling process, oxygen is further expelled out of copper since graphene does not block the process. This migration is similar to the segregation of carbon towards the surface of substrates as Ir or Ru which, in these cases, originate the formation of single or multilayer graphene.28, 29 For single crystalline copper the density of grains and boundaries in pristine samples is very small so that the initial oxygen content is very low while for copper foil the high density of boundaries favors the accumulation of oxygen. The additional oxidation mechanism is therefore strongly dependent on the grain boundary density. The dynamics of the early-stage oxidation of copper surface, corresponding to the standard process is known to be fast and is most probably faster than the additional inner process related to the encapsulated out-of-equilibrium oxygen density. This explains the protection that graphene exerts in a short time while it acts as an oxidation promoter at a longer time when the second process prevails. This additional process becomes important for polycrystalline copper since it initially contains higher oxygen density, mainly at grain boundaries, while for single crystals or epitaxial films the oxygen density is low. Copper grains in the foils present very different oxidation degree which may indicate that the diffusion of oxygen dissolved inside the grains has different diffusion rates for the different crystalline orientations. 17 ACS Paragon Plus Environment


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An evidence of the capacity of oxygen to be retained in the copper substrates during graphene growth is the reported formation of bubbles originated by long term oxidation process of copper foils with graphene22 as those illustrated in Figure 7. Similar bubbles due to gases released by the substrates used for graphene deposition have been also reported to form after energetic procedures, as irradiation by protons 30 or Xe+ 31.

Figure 7. Oxygen bubbles in graphene. Gr/Cu-foil after 12 months in ambient conditions, a) optical image (20 × 20 µm2) and b) AFM topography image (4 × 4 µm2) showing the formation of bubbles due to the encapsulation of gases expelled from the copper substrate.

The graphene layer retains the gases expelled from the copper foil and the developed bubbles in the 1-10 µm range can be seen with an optical microscope several months after graphene has been grown (Figure 7a). The AFM topographic 3D image (Figure 7b) shows the round surface of the bubbles. Most probably the bubbles contain oxygen which is gradually expelled from copper during months due to the low diffusivity of oxygen in copper at RT.


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Oxygen trapped by graphene in the copper foil We have used hard x-ray photoemission spectroscopy (HAXPES) to compare the oxygen content of a copper foil with graphene and another foil which has suffered the same thermal treatment in the CVD chamber but without methane, therefore without graphene, few days after the treatments. Using hard x-ray radiation, 11 keV, the kinetic energies of Cu 2p and O 1s photoelectrons are large enough to escape from 50 - 60 nm deep32 in the copper foil, therefore providing insight into the presence of oxygen within the copper bulk. To reduce the oxygen contribution from the copper oxide layer at the surface of the samples, a 30 minute Ar ion sputtering at room temperature was done at a primary energy of 3.5 keV (drain current 5 µA). The Ar ion sputtering reduced the oxygen signal a factor 13 (Figure S2 in Supp. Info.), obviously, graphene is also eliminated from the Gr-Cu sample in the process. Two incident angles were used for the HAXPES measurements, 0.3º (around the total reflection angle for copper metal) and 3º so that the X-ray attenuation length varies from around 6 nm to 350 nm according to data reported elsewhere33 (See Figure S3 in Supp. Info.). Therefore, at 0.3º, the information depth of the collected electrons is mainly around 6 nm and is limited by the incident X-ray attenuation, while at 3º the information depth is limited by the kinetic energy of the electrons and is around 50-60 nm. The measured signal is a weighted integral across the sample depth. Figure 8a shows the measured spectra for Cu 2p3/2 electrons for the samples with graphene (Gr-Cu foil) and without (Cu foil), after Ar sputtering, for 3º and 0.3º incidence angles and 11 KeV excitation energy. The spectra are normalized to Cu 2p3/2 intensity of Gr-Cu sample measured at 3º. In Figure 8b the oxygen O 1s peaks for the same cases are presented. The ratios of the areas of Cu and O peaks, R, are included in the figure. Comparing the oxygen signals of the samples with and without graphene (Gr-Cu foil and 19 ACS Paragon Plus Environment


Cu foil) measured at 3º (red and olive dots respectively) it is clear that the sample with graphene presents around ten times more oxygen (RCu/O = 63 for Gr-Cu and RCu/O = 675 for Cu foil). The comparison of the oxygen signals for the Gr-Cu foil sample obtained at 0.3º and 3º incidence angles demonstrates that an important part of the signal is originated deep inside the copper foil since the Cu/O ratio (R) is 2.35 higher for 0.3º than for 3º spectra. 2000

Intensity (cps)

a) Cu2p3/2

hν=11keV Gr-Cu, 30 Gr-Cu, 0.30 Cu 30

1500 1000 500 0 940 20

Intensity (cps)

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Ratio Cu/O (Area) R=63 R=148 R=675

b) O1s

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535

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Binding energy (eV) Figure 8 –Cu 2p3/2 and O 1s signals obtained with 11 KeV at 3º and 0.3º incidence angles for copper foil with graphene (red and blue dots) and at 3º without graphene (olive dots). The continuous lines are the fits. The Cu/O ratios, RCu/O, are included. 20 ACS Paragon Plus Environment

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So we can conclude that the oxygen content present in the bulk for the sample with graphene, Gr/Cu-foil, is significantly higher than that for a copper foil without graphene demonstrating the capacity of graphene to encapsulate oxygen inside copper. This observation further validates our proposed oxidation mechanism where oxygen, at RT, slowly migrates from the bulk to the surface and produces, on one hand, a higher oxidation in the long term of graphene capped copper foils than for graphene free ones, and also forms the bubbles at the copper-graphene interface.

CONCLUSIONS The oxidation in a short and long time of copper with and without graphene presents very different dynamics depending on the initial oxygen content and the morphology. The short term protection that graphene exerts on copper is clearly evidenced in any kind of copper substrates when putting them in contact with air at 70 ºC after annealing in reducing atmospheres. Without graphene, a strong immediate oxidation occurs while this is not the case for samples with graphene. Long term protection by graphene is efficient for copper single crystalline films with low initial oxygen content. However, polycrystalline copper is clearly more oxidized in the long terms with graphene than without. Besides the oxidation process stemming from atmospheric oxygen and water agents, we propose an additional oxidation process where the oxygen retained in the copper bulk during graphene growth progressively diffuses towards the surface. The formation of bubbles after several months in polycrystalline copper is consistent with the proposed encapsulation of oxygen and the low diffusivity of the oxygen at room temperature. HAXPES experiments demonstrate the higher concentration of oxygen in the deeper copper in graphene-covered than in graphene21 ACS Paragon Plus Environment


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free polycrystalline copper. This novel long term oxidation mechanism, originated from oxygen encapsulated by graphene and dependent on the initial oxygen content and grain boundary density, explains the apparently contradictory experimental results.

Supporting Information includes details on the characterization of copper oxides using Raman spectroscopy, on the elimination of the surface oxygen for deep oxygen evaluation by HAXPES and details about the x-ray attenuation length at 11 KeV for copper as a function of the incident angle.

Corresponding Author * A. de Andrés : [email protected] Funding Sources Spanish Ministerio de Economía y Competitividad (MINECO) ( MAT2015-65356-C3-1-R, MAT2012-37276-C03-01 and BES-2013-062759) Comunidad de Madrid Excellence Network (S2013/MIT-2740)

ACKNOWLEDGMENT Funding by the Spanish Ministerio de Economía y Competitividad (MINECO) under Projects MAT2015-65356-C3-1-R and MAT2012-37276-C03-01 and Comunidad de Madrid Excellence Network under Project S2013/MIT-2740 is acknowledged. We also acknowledge the MINECO for financial support and provision of synchrotron radiation 22 ACS Paragon Plus Environment

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facilities at BM25 SpLine The Spanish CRG Beamline at the ESRF. L.A-F. acknowledges a FPI grant (BES-2013-062759) from MINECO.

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