Role of Graphene in Water-Assisted Oxidation of Copper in Relation to

Article pubs.acs.org/cm

Role of Graphene in Water-Assisted Oxidation of Copper in Relation to Dry Transfer of Graphene Da Luo,† Xueqiu You,† Bao-Wen Li,† Xianjue Chen,† Hyo Ju Park,‡ Minbok Jung,†,‡ Taeg Yeoung Ko,# Kester Wong,†,§ Masood Yousaf,† Xiong Chen,† Ming Huang,†,‡ Sun Hwa Lee,† Zonghoon Lee,†,‡ Hyung-Joon Shin,†,‡ Sunmin Ryu,∇ Sang Kyu Kwak,†,§ Noejung Park,†,∥ Revathi R. Bacsa,†,■ Wolfgang Bacsa,†,○ and Rodney S. Ruoff*,†,‡,⊥ †

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea School of Materials Science and Engineering, §School of Energy and Chemical Engineering, ∥Department of Physics, and ⊥ Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea # Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 17104, Republic of Korea ∇ Department of Chemistry & Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 37673, Republic of Korea ■ R.R. Bacsa Scientific, 10, Rue de la petite reine, Castanet Tolosan 31320, France ○ CEMES-CNRS and University of Toulouse, 29 Jeanne Marvig, Toulouse 31055, France ‡

S Supporting Information *

ABSTRACT: The process of oxidation of a copper surface coated by a layer of graphene in water-saturated air at 50 °C was studied where it was observed that oxidation started at the graphene edge and was complete after 24 h. Isotope labeling of the oxygen gas and water showed that the oxygen in the formed copper oxides originated from water and not from the oxygen in air for both Cu and graphene-coated Cu, and this has interesting potential implications for graphene as a protective coating for Cu in dry air conditions. We propose a reaction pathway where surface hydroxyl groups formed at graphene edges and defects induce the oxidation of Cu. DFT simulation shows that the binding energy between graphene and the oxidized Cu substrate is smaller than that for the bare Cu substrate, which facilitates delamination of the graphene. Using this process, dry transfer is demonstrated using poly(bisphenol A carbonate) (PC) as the support layer. The high quality of the transferred graphene is demonstrated from Raman maps, XPS, STM, TEM, and sheet resistance measurements. The copper foil substrate was reused without substantial weight loss to grow graphene (up to 3 cycles) of equal quality to the first growth after each cycle. It was found that dry transfer yielded graphene with less Cu impurities as compared to methods using etching of the Cu substrate. Using PC yielded graphene with less polymeric residue after transfer than the use of poly(methyl methacrylate) (PMMA) as the supporting layer. Hence, this dry and clean delamination technique for CVD graphene grown on copper substrates is highly advantageous for the cost-effective large-scale production of graphene, where the Cu substrate can be reused after each growth.

1. INTRODUCTION Large-area graphene is prepared by growth on metal foils usually by chemical vapor deposition (CVD) after which the graphene is transferred onto the desired substrate.1,2 This transfer process invariably adds defects and impurities to pristine graphene. Thus, the efficient transfer of high-quality graphene is a major challenge and is a limiting factor in the performance of graphene-based devices.3 An efficient transfer process is also important both for applications and for fundamental studies on graphene and graphene-based materials that require high purity graphene samples. Delamination of graphene from metal substrates should be nondestructive to graphene as well as to the substrate. To successfully delaminate © 2017 American Chemical Society

graphene, the adhesion energy between graphene and the substrate needs to be lower than that between graphene and the polymer support layer.4 It is clear that increasing the adhesion of graphene to the support layer is not viable in that impurities from this layer may contaminate graphene. Delamination of graphene on Cu by the formation of hydrogen bubbles through electrolysis of water5,6 and by the selective dissolution of the oxide layer at the graphene/Cu interface has been reported.7 Electrochemical delamination, however, has the disadvantage that undesired contaminants in the electrolyte can be Received: March 29, 2017 Published: May 12, 2017 4546

DOI: 10.1021/acs.chemmater.7b01276 Chem. Mater. 2017, 29, 4546−4556

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Chemistry of Materials introduced into graphene, and, consequently, mechanical delamination procedures have been developed.8−12 Mechanical delamination could be achieved by simply soaking graphene/ Cu in hot water, and it was suggested that the intercalation of water between graphene and Cu leads to the delamination of graphene.9 It was proposed that the presence of copper oxides at the graphene/Cu interface is critical for the intercalation of water molecules between graphene and Cu that results in detaching the graphene.11 While it is clear that water assists the delamination of graphene from Cu, the mechanism by which this happens has not been clarified so far. Oxidation of the Cu substrate in the presence of graphene has been widely studied, but conflicting results have been reported. Graphene on Cu has been reported to prevent oxidation of Cu when exposed to air at 200 °C for several hours,13 whereas other reports have shown that oxidation at room temperature is accelerated when exposed to air for several days or months.14,15 To explain the difference in oxidation at short and long time scales, it was proposed that graphene and Cu form a galvanic cell where graphene is cathodic to Cu14−16 and that molecular oxygen played an important role in the oxidation of Cu.14,16,17 In this Article, we have studied two aspects of the process of water-assisted delamination of graphene. First, we present experimental results including isotope labeling in combination with standard characterization techniques to elucidate the role of water in the formation of copper oxide at the graphene−Cu interface. Contrary to previous findings,11,12,14−17 our results show that oxidation takes place entirely through water splitting and molecular oxygen from air does not participate. Second, we have demonstrated the dry transfer of CVD graphene grown on a copper substrate if a poly(bisphenol A carbonate) (PC) top layer is used. We have shown that this procedure allows the reuse of the Cu substrate several times for the growth of graphene with good physical (structural) integrity.

Figure 1. XPS spectra of Cu and G/Cu samples before and after exposure to water-saturated air for 48 h. (a,b) Cu 2p3/2 spectra of Cu samples (a) treated for 0 and 48 h, and G/Cu samples (b) treated for 0 and 48 h. (c,d) Peak areas calculated from the XPS spectra as a function of time of exposure to water-saturated air of fitted contributions due to Cu(0), Cu2O, Cu(OH)2, and CuO for Cu samples (c) and G/Cu samples (d). Note that the sum of the peak areas was normalized to 1.0.

an additional shoulder at 933.8 eV with a fwhm of ∼1.50 eV is observed, which is attributed to the formation of CuO (Figure 1b).18 The peak areas of Cu(0), Cu2O, Cu(OH)2, and CuO are plotted as a function of time in Figure 1c and d. It is found that after exposure to water-saturated air for more than 8 h, the peak areas of the Cu(0) of the G/Cu samples are always lower than those for pristine Cu samples. Coherent with this finding, the peak areas of the Cu(0) signal of G/Cu samples decreased faster with time of exposure than those of pristine Cu. Additionally, XPS O 1s spectra for both G/Cu and Cu samples obtained after exposure to water-saturated air at 50 °C for 48 h show that CuO was only observed for the G/Cu samples (see Figure S2), which is similar to the Cu 2p3/2 spectra shown in Figure 1 and Figure S1. We next followed the oxidation of the Cu beneath the graphene islands as a function of time. Some images of graphene islands on Cu are given in Figure S3a and c. Figure 2a shows the optical image of a graphene island on a Cu foil; the vertical lines in the image are attributed to steps on the Cu substrate surface.19 Raman spectra taken on the Cu substrate and on the graphene islands are shown in Figure 2a (right). The spectrum of graphene shows the two characteristic Raman peaks of graphene, the G band at ∼1582 cm−1 and the 2D band at ∼2730 cm−1.20 The low intensity/absence of D band and the intense 2D band confirm the high quality of the graphene islands.20 After 6 h exposure to water-saturated air, a narrow rim with a darker contrast is observed in the optical image at the edge of a graphene island. Three spectra, taken on the Cu substrate, the center, and the edge of this graphene island, are shown in Figure 2b (right). Eight additional bands appear between 100 and 1000 cm−1 at the edge (spectrum 2) corresponding mainly to Cu2O (109 (Γ−12), 149 (Γ(1) 15 LO), 218 (first overtone, 2Γ−12), 298 (first overtone, 2Γ(1) 15 LO), 412 (third

2. RESULTS AND DISCUSSION 2.1. Water-Assisted Oxidation of Graphene/Cu. Progressive Oxidation As a Function of Time at 50 °C. A continuous graphene layer on Cu (G/Cu) prepared by CVD1 and a pristine Cu sample were treated under identical conditions (for details, see the Experimental Section). The two samples were first exposed to water-saturated air at 50 °C for periods of time ranging from 0 to 48 h. The Cu 2p3/2 XPS spectra of the two samples are shown in Figure 1. Before exposure to water-saturated air, both samples show the Cu 2p3/2 peak at 932.6 eV with a full width at half-maximum (fwhm) of ∼1.04 eV, which is in good agreement with literature values for pristine Cu.18 By analyzing the spectra with respect to time of exposure, we have identified successively the presence of Cu, Cu2O, Cu(OH)2, and CuO. For more details on the XPS spectra of the samples at intermediate times 4, 8, and 24 h, see Figure S1. For G/Cu, Cu2O is observed after exposure to water-saturated air for 8 h, whereas for the pristine Cu samples, no pronounced peaks due to oxidation were observed. Hence, it is inferred that the oxidation of Cu proceeds faster in the presence of graphene. However, after 24 h of exposure, an additional shoulder attributed to Cu(OH)2 was observed in the XPS Cu 2p3/2 spectra for both samples.18 Nevertheless, we find that the intensities of the peaks due to Cu2O and Cu(OH)2 are considerably higher for the G/Cu sample, which clearly confirms that the rate of oxidation is enhanced in the presence of graphene. After exposure for 48 h, 4547


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Figure 2. Optical images and Raman maps of the graphene islands on Cu after exposure to water-saturated air for (a) 0 h, (b) 6 h, (c) 12 h, and (d) 24 h and of continuous graphene on Cu after exposure for 24 h (e). The left panel shows the optical images. The middle panel shows the Raman map images of the peak at ∼218 cm−1. The right panel shows Raman spectra collected at points indicated by circles in the optical images. Curve 2 in the right panel in (e) is collected on the oxidized Cu surface without a coating of graphene after exposure for 7 days for reference.

overtone, 4Γ−12), 500 (Γ+25), 644 (Γ(2) 15 ), and 797 (a combination (2) −1 21,22 A detailed discussion on mode: Γ(1) 15 LO + Γ15 ) cm ). Raman peak assignment is given in the Supporting Information. The low intensity/absence of a D band indicates that the formation of copper oxide underneath does not introduce

defects in the graphene islands. Note that the Raman band at 1290 cm−1, which appears also in bare Cu, is very likely the overtone of the Raman band at 644 cm−1, which originates from Cu2O (see Figure S11 and discussion in the Supporting Information). In contrast, spectra 1 and 3 collected on the Cu 4548


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Figure 3. AFM topographic images of the graphene islands on Cu maintained under water-saturated air for (a) 0 h, (b) 4 h, (c) 8h, (d) 12 h, and (e) 24 h. (f−j) Corresponding height profiles were taken along the white dashed lines in (a)−(e).

Figure 4. Raman spectra of the Cu (a) and G/Cu (b) samples treated with 18O labeled water or 18O labeled oxygen gas.

island simply “sits” on the oxide layer. A more detailed discussion on this is given in Figure S4e−h. We measured the thickness of the copper oxide layer by atomic force microscopy (AFM). The topographic image of a graphene island on Cu is shown in Figure 3a. Height variations of ∼30 nm are observed over long distances on Cu. The edge of the graphene islands was at a height of ∼18 nm as measured from the basal plane of the graphene island and the surrounding Cu substrate after exposure to water-saturated air for 4 h (Figure 3b and g). This height increase is attributed to the formation of copper oxide. Upon increasing the exposure time from 4 to 24 h, it was observed that the oxidized region gradually expanded starting from the edge toward the center of the graphene islands. Figure 3g−j shows that the difference in height between the oxidized and the pristine area remained the same (∼20 nm). It is therefore concluded that the copper oxide grows from the edge inward and the thickness stays constant for a given temperature. For the continuous graphene film-coated Cu, the oxidation of Cu occurs from the wrinkles (see Figure S6), point defects,16 and grain boundaries25,26 in graphene. We tested the uniformity of the oxidation from successive XPS measurements across a continuous graphene-coated Cu foil after water-saturated air exposure (Figure S7) and found that the entire Cu substrate was oxidized rather uniformly across the substrate. In addition, oxidation at 20 and 80 °C was studied by XPS and AFM (results are shown in Figures S8−S10). Exposure at 20 °C induced no apparent (weak) oxidation, while exposure at 80 °C

substrate and in the center region of a graphene island do not show any Raman bands corresponding to oxidized Cu. Raman maps were acquired for the main copper oxide band at ∼218 cm−1. The Raman image (center image in Figure 2b) shows clearly that Cu2O forms preferentially at the edge. After 12 h, oxidation extends further toward the center, and after 24 h, the entire Cu underneath the graphene island is oxidized (Figure 2d). In contrast, the surrounding Cu substrate shows no Raman signal due to copper oxide even after 24 h, confirming that the presence of graphene accelerates the oxidation. Figure 2e shows the Raman map of a continuous single layer graphene on Cu after exposure to water-saturated air for 24 h, showing that the entire substrate is uniformly oxidized and that the oxidation is uniform. To study the effect of the copper oxide layer on the graphene structure, we recorded Raman maps of the G band (frequency and width) and the 2D band (frequency and width) of graphene islands (some images of graphene islands are shown in Figure S4). When Cu underneath the edge of graphene island is oxidized, the G band and the 2D band downshift and broaden on the copper oxide, indicating that the graphene on the copper oxide could be strained.23,24 After the complete oxidation of the Cu substrate, the strain-induced Raman shifts disappear. This indicates that strain in the graphene could be induced by the growth of an underlying oxide layer. The oxide layer is considerably higher than the thickness of graphene having the effect to strain the graphene layer on top when the oxide layer is not complete. Once the oxide layer is complete, this strain is released and the graphene 4549


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to water vapor alone is sufficient under the conditions of the experiment (closed vessel filled with water, 50 °C, and pH 6.8− 7). Mechanism of Oxidation. The oxidation of copper in the presence of graphene is of importance from the point of view of using graphene as a corrosion protection for copper surfaces. For graphene on Cu, oxidation of the underlying Cu substrate in air at 250 °C has been shown to be dependent on the presence of defects on the graphene layer through which diffusion of oxygen occurred.27 However, there is very little knowledge on the slow oxidation of Cu in the presence of graphene at relatively low or ambient temperatures (below 100 °C). Our results have proven unequivocally that, under these conditions, water splitting provides the necessary oxygen atoms to form copper oxide. Oxidation of copper metal in the presence of water vapor has been the subject of a number of studies. Detailed studies on copper oxidation in water with depleted dissolved oxygen at near room temperature have shown that the bulk conversion of copper to any of the known copper oxides or hydroxides and hydrogen gas is an unfavorable reaction and is only possible through surface-mediated reactions.28,29 Nevertheless, it is also known that water is dissociatively adsorbed on copper even at room temperature, and XPS studies have shown that hydroxyl groups on copper are stabilized by strong hydrogen bonding to molecular water forming a network structure.30,31 The stronger binding of H2O−OH complexes as compared to H2O−H2O (autocatalytic water dissociation) was verified.31 However, the further cleavage of these hydroxyl groups to form copper oxide and molecular hydrogen does not occur spontaneously on Cu surfaces.28,29 As a result, copper reacts only very slowly with water. In contrast, at higher temperatures, copper oxidation mechanisms are very different. Oxidation of Cu in the presence of water vapor has been studied in the temperature range between 400−1000 °C, where it was reported that at relatively low temperatures (400 °C), oxidation was dominated by water vapor, but at higher temperatures (>700 °C), there is a transition to a mechanism where diffusion of oxygen gas is predominant.32 In this work, we have focused on the effect of graphene on the oxidation of Cu near room temperature. Our results have shown that, under these conditions, oxidation of Cu is accelerated and enhanced in the presence of graphene. The Raman map of the D band of graphene islands on Cu before and after exposure to water-saturated air has shown that no intense D band was observed before the exposure even at the edges. After exposure to water-saturated air, an intense D band is observed at the edge of the graphene islands, indicating that the edges are functionalized by the exposure (Raman spectra are shown in Figure S14). Recent ab initio calculations have shown that graphene on Cu is capable of splitting water molecules at a low temperature (below 100 °C) and graphene itself could be functionalized with oxygen-containing groups (−OH, −O−, −OOH)33 similar to graphene oxide.34 Under the conditions of our experiment, the reaction with water is limited to the formation of oxygen-containing groups at the edges of the graphene islands; that is, no D-band was obtained from the interior regions of the islands even when the entire Cu surface below the island was oxidized. We further used XPS to identify the functional groups on a continuous graphene film after exposure to water-saturated air. For the pristine graphene sample, the C 1s peak can be fitted with a single peak located at 284.6 eV, which is assigned to sp2-hybridized carbon.35 After exposure to

accelerated the oxidation considerably and increased the thickness of the oxide layer to 40 nm after 24 h. Tracing the Origin of Oxygen. While it is clear that graphene-coated Cu is oxidized at relatively low temperatures forming oxides of copper, there is no agreement in the literature as to the origin of oxygen in the formation of interfacial copper oxide. One reference reports that the oxygen originates from O2,15 while others suggest that both O2 and H2O are involved.14,16 We used oxygen isotope labeling to determine the origin of oxygen in the formed copper oxide. Both 18O labeled water (H218O) and 18O labeled oxygen gas (18O2) were used. The Cu and G/Cu samples were treated with either 18O2 or H218O, and the results obtained from the three combinations were compared, H2O + O2, H2O + 18O2, and H218O + O2 (for details, see the Experimental Section). Figure 4a shows the Raman spectra of Cu samples for the three experiments after exposure. When introducing 18O2, no isotope-induced shifts were observed. However, when introducing H218O, three higher frequency peaks show obvious redshifts; the peaks at 498 (Γ+25), 644 (Γ(2) 15 ), and 795 (a (2) −1 shift to 470, 617, and combination mode: Γ(1) 15 LO + Γ15 ) cm 770 cm−1, respectively, which is attributed to O18 labeling. As can also be seen from Figure 4a, the lower frequency peaks at − 109 (Γ−12), 149 (Γ(1) 15 LO), 218 (first overtone, 2Γ12), and 298 −1 (1) cm (first overtone, 2Γ15 LO) remain unchanged. The lower frequency modes of cuprous oxide are related to the motion of copper atoms, while the higher frequency modes are associated with the movement of oxygen atoms.22 The redshift of the higher frequency modes demonstrates that water is the source of oxygen in the cuprous oxide. Figure 4b shows the Raman spectra of G/Cu samples following treatment similar to that for pristine Cu samples. The same redshifts were observed, indicating that in this case as well, the oxygen atom in the formed cuprous oxide is from water and the presence of graphene has no effect on the origin of the oxygen atom. Timeof-flight secondary ion mass spectrometry (TOF-SIMS) was used to determine the mass of the oxygen atoms in the copper oxides. The TOF-SIMS results show that 18O is detected at levels 10−100 times higher as compared to 16O in copper oxide when H218O is used (Figure S12). This is consistent with the Raman results with H218O. DFT Calculations on Phonon Frequencies. DFT calculations were performed to estimate the frequency shifts in the Raman bands of cuprous oxide due to isotope labeling. For a summary of the peak frequencies of Cu216O and Cu218O from the DFT simulation and Raman measurements, see Table S1. Here, we take the Γ(2) 15 mode as an example for discussion about the redshift (see details for the discussion on other Raman modes in the Supporting Information). The calculations predict −1 for Cu216O and at 597.71 the Γ(2) 15 mode to be at 626.25 cm −1 18 cm for Cu2 O (redshift of ∼28 cm−1). This yields a frequency ratio of 0.9544. The experimental peak frequency of Cu216O (∼644 cm−1) and of Cu218O (∼617 cm−1; redshift of ∼27 cm−1) yields a frequency ratio of 0.9581, consistent with the calculated value. The DFT calculation further confirms that the cuprous oxide formed under the condition of H218O+O2 was entirely Cu218O. A control experiment (see Figure S13) was carried out where the G/Cu sample was exposed to water in a closed vessel at 50 °C in the absence of oxygen by replacing the air with N2 (0.2 ppm of O2) in a glovebox. No change was observed in the Raman spectra after deoxygenation. This observation reinforces the conclusion that the oxidation of Cu can occur in the absence of molecular oxygen and that exposure 4550


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2.2. Delamination and Characterization of Transferred Graphene. The delamination of graphene by oxidizing the Cu foil substrate with water-saturated air has been carried out by using poly(bisphenol A carbonate) (PC) as a support layer. Figure 5a shows the schematic of the water vapor oxidation-assisted dry transfer of graphene in five process steps: oxidation in water-saturated air (50 °C for 24 h), spin coating PC, heating at 150 °C for 10 min to increase the adhesion between the graphene and the PC film, peeling off the support layer along with graphene, transfer onto a target substrate, and removal of the PC support with chloroform. PC was chosen as the polymeric support layer due to the fact that it dissolves without leaving any residue on graphene44,45 and has good mechanical properties allowing sample handling. The entire process can be seen in the supplementary video. Figure 5b shows a typical optical image of the transferred graphene on a Si wafer with a 300 nm thick oxide (300 nm SiO2/Si) layer. The transferred graphene is nearly crack-free and continuous. The transferred graphene is mostly monolayer with a few randomly distributed bilayer or few-layer regions. The insets show the wafer-scale transfer of graphene on a 4-in. 300 nm SiO2/Si wafer (top right) and on a 15 × 15 cm2 large polyethylene terephthalate (PET) film (bottom right). The same transfer process can be successfully used for the dry transfer of graphene islands from the Cu substrate onto a 300 nm SiO2/Si wafer (Figure S18). We tested the quality of the graphene in each process step by Raman spectroscopy. In Figure 5c, the Raman spectrum of the as-prepared G/Cu sample shows all of the characteristics of a high-quality single-layer graphene.1 For the sample after delamination followed by peeling off from the substrate, the Raman spectrum shows the characteristic bands of PC and graphene. The spectrum of PC shows several peaks in the 1000−3500 cm−1 range and a band at ∼1600 cm−1 (fwhm of ∼22 cm−1), which is close to the G band of graphene.46 The Raman spectrum of PC is given in Figure S19 for reference. The spectrum of graphene on PC shows a peak at ∼1585 cm−1 at the G band position with a fwhm of ∼38 cm−1, which is much larger than that of both monolayer graphene47 and bare PC. This indicates that this band is a superposition of the G band of graphene and the 1600 cm−1 band of PC. In addition to the peak at ∼1585 cm−1, the 2D band at ∼2673 cm−1 with a fwhm of ∼30 cm−1 and the band at ∼2450 cm−1 also confirm the presence of delaminated graphene on PC.20,46 The remaining bands observed in the spectrum of graphene/PC are due to the PC layer. The 2D Raman band at ∼2680 cm−1 of the transferred graphene can be fitted with a single Lorentzian (fwhm of ∼32 cm−1), confirming the presence of monolayer graphene (shown in Figure S20). In addition, the low intensity of the D band (ID/IG < 0.01) proves the high quality of the graphene after transfer on the 300 nm SiO2/Si wafer and removal of the PC layer (Figure 5c). To further evaluate the quality of transferred graphene film, we collected the Raman map images (G band, D band, ID/IG ratio) of as-grown graphene on Cu and transferred graphene on 300 nm SiO2/Si wafer (spectral data are shown in Figure S21). The as-grown graphene has high quality. After transfer, the graphene is still of high quality; the D band only appears at the wrinkles. Because the oxidation of Cu substrate is caused by the functionalization at edges, defect sites, or wrinkles in the graphene layer, the graphene sheet obtained by this dry transfer method may have oxygen-containing functional groups at edges, wrinkles, and grain boundaries.

water-saturated air for 6 h, no changes are observed. After 24 h, two shoulders appear at 286.1 eV corresponding to C−O (hydroxyl and epoxide) groups, and at 288.3 eV corresponding to CO (carbonyl) groups.36 The spectral data are shown in Figure S15. Similar to the functionalization of the edges of graphene islands discussed in the previous section (Figure S14), continuous graphene film can have functionalization that occurred on grain boundaries,25 defects,16 and wrinkles, which induces the oxidation of the Cu substrate. It appears therefore that graphene functionalization and the oxidation of Cu are concomitant in the presence of water at 50 °C. The presence of functionalized graphene on the copper surface can catalyze the copper oxidation reaction in two ways. Functional groups (hydroxyl, epoxy) present on graphene can facilitate electron transfer from the H2O−OH complex on the copper surface and stabilize it through hydrogen bonding, wherefrom the formation of copper oxide is facilitated. The very high surface area of graphene may favor the dissociative adsorption of water on the surface and may also increase the concentration of OH and H groups.33 Once copper oxides or hydroxides are formed, the nanoscale roughness of the oxide surface could promote fast hydrogen evolution, thus speeding up the corrosion process. It has been reported that graphene oxide on Cu can oxidize Cu at room temperature, resulting in a partially reduced graphene oxide thin film on the Cu surface.37 Oxygen-containing functional groups at the edge of graphene may play a similar role in promoting oxidation of Cu. Finally, it is reported that there is a charge transfer between graphene and Cu; graphene is n-doped by the presence of Cu, leaving the Cu surface slightly positively charged.38 This favors the stabilization of water and hydroxyl groups on Cu, which is important for the dissociation of water and hydroxyl and the formation of copper oxide.39 Once a layer of oxide is formed, its bonding with the graphene layer is expected to be modified. A schematic diagram of this process is given in Figure S16. Interaction of Cu2O and Graphene. It was previously reported that the oxidation of Cu has the effect of reducing the interaction of graphene with the substrate.40 We have carried out DFT calculations to compare the binding energies of graphene with Cu (111) and Cu2O (111) surfaces. Details on the geometry of the considered models are given in Figure S17. For the G/Cu (111) configuration, an interfacial distance of 2.94 Å between graphene and Cu surface is obtained, which is consistent with previous reports.41,42 Table 1 shows that the Table 1. Binding Energy per C Atom (BEc) and Adsorption Energy of Graphene on Cu (111) and Cu2O (111) BEc (eV) adsorption energy (J m−2)

G/Cu (111)

G/Cu2O (111)

−0.095 0.536

−0.064 0.354

calculated binding energy (per C atom) of G/Cu (111) is −0.095 eV, which is also in good agreement with the reported value of −0.1 eV.43 For G/Cu2O (111), the calculated graphene substrate distances are 2.57 Å (C−O) and 3.06 Å (C−Cu). The binding energy (per C atom) of G/Cu2O (111) is −0.053 eV, which is significantly lower than that for G/Cu (111). The adsorption energy (per unit area) for G/Cu2O (111) is smaller (0.354 J m−2) as compared to G/Cu (111) (0.536 J m−2). Thus, the reduced adsorption energy due to the oxidation of Cu enables the mechanical delamination of graphene from the substrate. 4551


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Figure 5. Direct delamination and transfer of graphene from Cu to a 300 nm SiO2/Si wafer through exposure to water-saturated air. (a) Schematic illustration of the delamination and transfer process. (b) Optical image of a continuous graphene transferred onto a 300 nm SiO2/Si wafer. The insets show the photographs of the transferred graphene on a 4-in. 300 nm SiO2/Si wafer (top, scale bar, 2 cm) and a PET film 15 × 15 cm2 in size (bottom scale bar, 2 cm). (c) Raman spectra of graphene on Cu, graphene on PC, and graphene on 300 nm SiO2/Si wafer. (d) AFM image of the transferred graphene on 300 nm SiO2/Si wafer. (e) Height profile along the white dashed line in (d).

contamination after transfer. In addition, the XPS of graphene transferred by PMMA shows an additional peak at ∼288.5 eV, which is attributed to PMMA residues left on the transferred graphene.49,50 We further annealed the transferred graphene (200−400 °C under gas flow of H2 and Ar) to eliminate polymer residues. The bandwidth of the spectra for the graphene transferred with PMMA decreased after annealing, and the peak at ∼288.5 eV (originating from PMMA residues) disappeared.49,50 For the graphene transferred via the dry delamination method, there is no apparent change in the shape and bandwidth of the C 1s peak after annealing, suggesting that the as-transferred graphene on 300 nm SiO2/Si wafer is clean before annealing, and that annealing is unnecessary. The cleanliness of the transferred graphene was further evaluated by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The delaminated graphene/PC was attached to the TEM grid after which the PC support film was removed using chloroform. Similarly, the STM sample was prepared by transferring the graphene/PC film onto Au/mica substrates prior to the removal of PC with chloroform. Prior to the TEM and STM measurements, the samples were heated to 180 °C on a hot plate in air for 10 min

We further evaluated the cleanliness of the transfer process by using AFM. Figure 5d shows a representative AFM image of the transferred graphene on the 300 nm SiO2/Si wafer. From the height profile (Figure 5e), a thickness of ∼0.8 nm is measured corresponding to monolayer graphene. No residues are observed. Note that the spikes in the profile correspond to wrinkles on graphene. The observed difference in height between the bilayer graphene and monolayer graphene is ∼0.4 nm, consistent with the van der Waals distance between graphitic layers.48 We compare these results with AFM images of the transferred graphene by using a common transfer technique with poly(methyl methacrylate) (PMMA) combined with chemical etching of Cu (hereafter referred to as the “PMMA method”).1 More residues are observed on the transferred graphene using the PMMA method (Figure S22). The cleanliness of these two types of graphene films transferred onto 300 nm SiO2/Si wafer pieces was further investigated by XPS. The original spectra are given in Figure S23; the conclusions are summarized here. The C 1s peak of the graphene transferred via the dry transfer method shows a narrower bandwidth when compared to that for graphene transferred via the PMMA method, indicating lower polymer 4552


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Figure 6. Characterization of graphene transferred using the dry transfer method with PC as the support layer. (a) TEM image of the graphene transferred onto a TEM grid. (b) High-resolution TEM image of the area indicated in (a). The inset shows the FFT of (b). (c) Large-area STM image of the monolayer graphene transferred onto a Au/mica substrate with Vsample = 1 V, Itunnel = 0.5 nA. The insets show the magnified images of the area in (c). The scan parameters of insets on the top left are Vsample = 0.1 V, Itunnel = 1 nA, the inset on the bottom right: Vsample = 1 V, Itunnel = 1 nA. (d−e) High-resolution STM images of the selected areas in (c) with scan parameters: Vsample = 0.1 V, Itunnel = 1 nA and (e) Vsample = 1 V, Itunnel = 1 nA. (f) STS spectrum of the graphene on the Au/mica substrate.

STM image. We conclude therefore that the “residue” might not be on the graphene surface, but present at the interface of the graphene and the Au/mica substrate. The herringbone structure of Au (111) can be observed in the inset of Figure 6c. The Moiré pattern shown in Figure 6e is attributed to the lattice mismatch between the graphene and the Au (111) substrate.51 The STM observations also indicate that the transferred graphene is clean on the atomic scale, which is consistent with TEM results. The scanning tunneling spectroscopy (STS) spectrum (Figure 6f) shows that the graphene is pdoped by the gold substrate with a Dirac point ED = 0.245 V, which is consistent with previous studies.51 We also studied the presence of Cu impurities on the transferred graphene on 300 nm SiO2/Si wafer using XPS, and no Cu signal was detected (the original spectra are given in Figure S25). However, for the graphene with a PC coating transferred by etching Cu, the spectrum clearly reveals Cu peaks. To test the performance of the transferred graphene, we compared the sheet resistance of graphene on 300 nm SiO2/Si wafer transferred by our dry method with graphene transferred by the PMMA method (Figure S26). The average sheet resistance obtained on graphene from the PMMA method was 2030 Ω/□, which is consistent with previously reported values,2 whereas for graphene transferred by our dry method, the average sheet resistance was 1564 Ω/□. This result was attributed to a lower contamination in our delaminated graphene. The dry transfer method does not require etching of Cu, and hence the Cu substrate can be used repeatedly for further growth of graphene. We used a Cu foil (2.2 × 2.5 cm2, 100 μm in thickness) to perform three cycles of CVD growth under 1 atm pressure,52 followed by dry transfer of graphene. The graphene films from these three cycles were transferred, respectively, on three 300 nm SiO2/Si wafers. The quality of the transferred graphene in each cycle was compared using Raman spectroscopy. For photographs and Raman spectra of the series of transferred graphene, please see Figure S27a and b. As shown in Figure S27b, the D band at ∼1350 cm−1 has negligible intensity for all of the graphene samples, which indicates that the quality of graphene is unchanged after reusing the substrate. No apparent weight loss is found for this Cu foil after three cycles of CVD. We conclude that the Cu foil can be used repeatedly for ambient pressure CVD. It is worth noting that one needs 300 kg of ultrapure Cu foil (thickness = 25 μm) to produce 1 g of CVD grown graphene by using conventional transfer methods involving etching of Cu.53 Instead, the described dry transfer method here allows reusing the Cu foil, substantially decreasing the cost for large-scale graphene production.

graphene obtained by dry transfer with PC, in which a clean area of hundreds of nanometers in size can be observed. Figure 6b shows a high-resolution TEM image from the selected area in Figure 6a. The hexagonal lattice and the corresponding fast Fourier transform (FFT) pattern in the inset confirm the high crystallinity of the transferred monolayer graphene. More large area images of ∼1400 and ∼600 nm2 are given in Figure S24a and b. The STM images (300 × 300 nm2) of the transferred graphene on the Au/mica substrate are shown in Figure 6c. We chose two representative areas to obtain step-by-step magnification images, as shown in the insets and in Figure 6d and e, respectively. From the inset in Figure 6c, the area in Figure 6d appears to be a residue-rich region, despite the fact that a clear image of graphene structure was observed in the

3. CONCLUSIONS Oxidation of Cu in the presence of graphene in water-saturated air has been investigated, and it was observed that the presence of graphene enhanced the oxidation of the underlying Cu substrate. In studies of graphene island samples, oxidation starts from the edges and defects on graphene islands and gradually progresses under the graphene layer. The entire Cu surface that is covered by graphene islands is oxidized after 24 h at 50 °C; however, no Raman peaks corresponding to copper oxide could be detected in the “bare” Cu regions between the graphene islands due to the lower amounts (although, as mentioned above, XPS did show evidence of some oxidation). Higher temperature (80 °C) leads to faster oxidation and thicker oxide layers beneath the graphene islands. Isotope labeling of water

to remove any residual adsorbates (hydrocarbons, water) on the surface.35 Figure 6a shows a typical TEM image of the

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floated on DI water (200 mL) in Teflon containers (90 mm diameter and 200 mm height). The containers were sealed and placed in an oven set at different temperatures for several hours or several days. The relative humidity in the Teflon container was maintained at 100%, and the pH of DI water is in the range of 6.8−7.0. Poly(bisphenol A carbonate) (PC) (Sigma-Aldrich, item no. 181641) was dissolved in chloroform (7 wt %). A PC layer (∼50 μm in thickness) was coated on the pretreated G/Cu by spin coating PC/chloroform solution at 3000 rpm for 60 s. Next, the PC/G/Cu sample was annealed at 150 °C for 10 min to increase the adhesion between PC and graphene. After being cooled to room temperature, the PC/graphene can be peeled off directly from the Cu substrate using tweezers, and placed on the target substrates with high voltage applied to ensure electrostatic bonding (Chargemaster VCMBP60) and achieve an intimate contact of graphene and the target substrate. Note that the high voltage instrument must be grounded for safety. PC solution (3 wt %) was spin coated on the top surface for a second time.2 The PC/graphene/ target substrate was heated at 100 °C for 10 min. Finally, the PC supporting layer was removed by dissolving it in chloroform, leaving graphene on the substrates. 4.1.3. 18O Labeled Experiment. For the experiment involving H218O + O2, 1 g of H218O (97 atom % 18O, Sigma-Aldrich, item no. 329878) was added into a 5 mL glass vial. The graphene-coated Cu foil was held using a double sided scotch tape on the inner side of the cover. The vial was covered and sealed and placed in an oven that was set at 50 °C for an extended period of time. For the experiment involving H2O + 18O2, 2 L of 18O2/Ar mixed gas (mix ratio: 1:4, 99 atom % 18O, Sigma-Aldrich, item no. 578673) was filled into a balloon. The balloon was connected to one of the necks of a 100 mL three-neck flask. O2-free water was prepared by purging Ar gas into normal DI water for 24 h. Only trace amounts of oxygen (∼0.5 ppm) were detected using a portable oxygen sensor. Thirty milliliters of the O2-free water was added into the flask. The flask was connected to a vacuum pump for evacuation. The third neck was covered with a stopper with a graphene-coated Cu foil pasted on the inside. The flask was placed in an oil bath and maintained at 50 °C for an extended period of time. 4.1.4. Wet Transfer Method. One side of the G/Cu surface was spin-coated with a thin layer of PMMA or PC, and the other side was treated with O2 plasma to remove the graphene. The Cu foil was then etched in a 0.1 M (NH4)2S2O8 solution to isolate the graphene/ PMMA or graphene/PC film after which the graphene was transferred to target substrates. The PMMA and PC were removed using acetone and chloroform, respectively. 4.2. Characterization. The characterization techniques used in this work include optical microscopy (AXIO, Carl Zeiss), low-voltage aberration-corrected TEM (Titan Cube G2 60-300, FEI, 80 kV), AFM (Bruker Dimension Icon), SEM (FEI Verios 460, 1 kV and 2 kV), XPS (ESCALAB 250Xi, Thermo Scientific), Raman spectroscopy (Witec, 488 nm, 1 mW, and 532 nm, 1 mW), and STM (SPECS JT-STM system). For Raman spectroscopy, we used the 488 nm laser for the graphene on Cu and the 532 nm laser for the transferred graphene samples. We used a grating with 600 grooves/mm for all of the Raman measurements.

and oxygen gas shows clearly that the oxygen in the copper oxide originates entirely from water and not from the oxygen in air for both Cu and graphene coated Cu. This directly contradicts conclusions reached in other studies, which stated that oxygen played a role in the oxidation of the Cu beneath the graphene.11,12,14−17 While oxygen in air does not play any role in the oxidation process, it is proposed that the oxidation of Cu proceeds through enhanced water dissociation at graphene edges and defects. The oxidation is suggested to be enhanced through the charge transfer between graphene and the Cu substrate that lowers the energy barrier for the adsorption and dissociation of water molecules on the Cu surface. The catalytic activity of monolayer graphene for Cu oxidation may have significant implications for the application of graphene as a catalyst support for Cu-based catalysts used in industry. Dry transfer from the oxidized Cu substrate of continuous graphene films as well as for graphene islands are demonstrated using polycarbonate (PC) as a support layer. The transferred graphene has been examined by AFM, Raman, XPS, TEM, and STM. Dry transfer based on oxidation in water-saturated air shows a significantly cleaner surface and no Cu impurities when compared to conventional transfer methods that involve complete etching away of the Cu and use of either PMMA or PC as the support layer. After dry transfer, the Cu foil substrate could be reused multiple times without any loss in quality of the grown graphene or mass loss of the Cu foil. This nondestructive dry transfer method is found to be both efficient and straightforward and can be potentially applied to the largescale production and transfer of graphene.

4. EXPERIMENTAL SECTION 4.1. Materials and Methods. 4.1.1. CVD Growth of Graphene on Cu. Continuous graphene film and graphene island samples were grown on Cu foils by using the low-pressure CVD method reported previously.1,54 For the growth of continuous monolayer graphene, Cu foils (99.8%, Alfa Aesar) with thicknesses of 25 and 100 μm were used. Prior to CVD synthesis, Cu foils were pretreated with diluted HCl (1 mol/L) followed by cleaning in ethanol for 10 min. The clean Cu foil was heated to 1000 °C under a H2 flow of 20 sccm at 50 mTorr in a 2in. fused quartz tube. After holding at 1000 °C for 30 min, 10 sccm CH4 and 20 sccm H2 were introduced at 200 mTorr for 30 min for the growth of graphene. After the growth, CH4 was turned off, and the Cu foil was rapidly cooled by moving the furnace away from the sample. A flow of 20 sccm H2 at 50 mTorr was maintained in the system during the fast cooling to room temperature. For the growth of continuous monolayer graphene under atmospheric pressure, Cu foils (99.8%, Alfa Aesar) with a thickness of 100 μm were used. After the pretreatment as described above, the Cu foil was heated to 1000 °C in a 2-in fused quartz tube under a mixed gas flow of H2 and Ar (H2/Ar, 50 sccm/500 sccm). After holding at 1000 °C for 30 min, 2 sccm CH4, 50 sccm H2, and 500 sccm Ar were introduced for 30 min for the growth of graphene. After the growth, CH4 was turned off, and the Cu foil was rapidly cooled by moving the furnace away from the sample. The same mixed flow of H2 and Ar was maintained in the system during the fast cooling to room temperature. For the growth of graphene islands on Cu, a 25 μm thick copper foil was folded into a closed pocket.54 In a similar 2-in fused quartz tube, this Cu enclosure was heated to 1040 °C with a H2 flow of 20 sccm at 50 mTorr, followed by the introduction of 2 sccm CH4 and 20 sccm H2 into the CVD system at 100 mTorr for 20 min. The sample was rapidly cooled to room temperature under 20 sccm H2 (50 mTorr). Graphene islands were obtained on the inner surface of the Cu enclosure. 4.1.2. Water-Saturated Air Treatment and Dry Transfer. Cu foils with or without graphene were placed on plastic Petri dishes that were

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01276. Details about DFT simulation, additional tables, characterizations, and supporting references and notes (PDF) Water vapor oxidation-assisted dry transfer of graphene (AVI) 4554


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AUTHOR INFORMATION

Corresponding Author

*E-mail: ruoffl[email protected]. ORCID

Da Luo: 0000-0002-9128-6782 Xianjue Chen: 0000-0002-4757-7152 Xiong Chen: 0000-0003-2878-7522 Zonghoon Lee: 0000-0003-3246-4072 Sang Kyu Kwak: 0000-0002-0332-1534 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by IBS-R019-D1. K.W. and S.K.K. acknowledge IBS-R019-D1 and NRF-2014R1A5A1009799. Computational resources were used from CMCM, UNISTHPC, and KISTI (KSC-2016-C2-0003).

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