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Detaching graphene from copper substrate by oxidation-assisted water intercalation
Carbon 98 (2016) 138e143
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Carbon journal homepage: www.elsevier.com/locate/carbon
Detaching graphene from copper substrate by oxidation-assisted water intercalation Ruizhe Wu, Lin Gan, Xuewu Ou, Qicheng Zhang, Zhengtang Luo* Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2015 Received in revised form 1 November 2015 Accepted 2 November 2015 Available online 6 November 2015
For electronic applications, graphene prepared by chemical vapor deposition (CVD) are required to be detached from the catalytic substrate, while retaining structural integrity. We demonstrate that CVD grown graphene on copper can be fully decoupled from the substrate by immersion in water, without significant damage to graphene. We find that the decoupling starts from the graphene edges and defect sites, assisted by interfacial copper oxidation and water intercalation due to galvanic corrosion. Kinetics study reveals the activation energy of 0.3 ± 0.08 eV for this decoupling process, and interfacial oxidation acts as the dominating role. This facile water-immersion method can be extended to adjust the interaction between graphene and metals, and assist our understanding of interfacial chemistry in confined space. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Graphene has attracted worldwide attentions in fundamental and application research due to their exceptional properties [1]. In direction perpendicular to the terrace, graphene are impermeable to most molecules at ambient conditions, which makes graphene a promising membrane material [2]. As a result, most studies on the mass transport are mainly focused on the direction parallel to the terrace between graphene and various substrates, in particular to tune graphene-substrate interfacial interaction which is essential for both post-synthesis graphene transfer and retaining graphene structural integrity. Among these studies, metal atoms and gas molecules intercalation are two typical methods, in which additional processes such as metal deposition or introduction of high purity gas at low pressure followed by high temperature annealing are usually required [3e8]. Recent efforts to detach graphene from copper substrate utilizing electrochemical oxidation at ambient conditions for elongated period, has only limited success as the resulted graphene are usually fragmented due to the non-uniform oxidation [9,10]. On the other hand, water molecule intercalation between graphene and nonmetal substrates, including mica, sapphire and silica, has been widely studied [11e14]. In particular for silica substrate,
the water intercalation rate is governed by the substrate hydrophilicity, and it was observed that the graphene flake kept strong affinity to the substrate even with a layer of water molecules in between [14]. Similarly, the intercalation of water between graphene and metal substrates may also help to adjust the interfacial interaction without significant damage to graphene. In this study, we introduce a facile method utilizing oxidation at copperegraphene interface, assisted by water intercalation to detach graphene film. We can optically observe that substrate oxidation and decoupling start from the graphene edges and defects region, forming an observable oxidation front, which proceeds to the graphene center. This is corroborated with the AFM and SEM investigation, as well as Raman characterization, which altogether lead to the conclusion that such oxidation follows a decoupling controlled substrate oxidation mechanism. Additional kinetics study under different temperatures reveals the activation barrier for graphene decoupling equal to 0.3 ± 0.08 eV. More importantly, it is shown that interfacial oxides formation plays a more important role than interfacial water molecules diffusion in this graphene decoupling process. 2. Experimental 2.1. Chemical vapor deposition
* Corresponding author. E-mail address: [email protected] (Z. Luo). http://dx.doi.org/10.1016/j.carbon.2015.11.002 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
Before growth, copper foil (99.8%, Alfa Aesar No. 13382) is chemically polished by sonicating in etchant solution (5 g of FeCl3,
R. Wu et al. / Carbon 98 (2016) 138e143
10 mL of HCl, and 100 mL of distilled deionized (DDI) water) for 10 s to remove the oxides followed by DDI water rinsing and nitrogen blow-dry. After being loaded into the quartz tube, copper foil is fast-heated to 1050 C under 350 sccm Ar (Hong Kong Specialty Gases Co., Ltd., 99.999%, oxygen concentration <3 ppm). Then graphene growth is started by introducing 10 sccm H2 (Hong Kong Specialty Gases Co., Ltd., 99.999%, oxygen concentration <5 ppm) and 16 sccm diluted CH4 (500 ppm methane diluted in argon, Arkonic Gases and Chemicals Inc., 99.99%) for 2 min. Finally, the furnace is powered off and moved aside to fast cool the sample to room temperature under argon atmosphere. The diameter of single crystal graphene flakes is ~20 mm. 2.2. Graphene/copper redox detachment The fresh grown graphene/copper sample is cut into 1 cm 1 cm and immersed into a petri dish filled with 20 mL DDI water (conductivity 5.5 mS/m) or 0.1 M Na2SO4 solution at room temperature. In additional kinetics experiments, a bottle filled with 20 mL DDI water is heated by oil bath to 25 C, 30 C, 35 C, 45 C. When the water temperature is stabilized, a piece of 1 cm 1 cm graphene/copper is immersed in to the DDI water. The total reaction time at 25 C is 6 h, while the reaction time at 30 C, 35 C, 45 C is 3 h. Finally, the sample is rinsed with DDI water and blown dry by nitrogen gas before characterization. 2.3. Characterization Optical images are taken by a LEICA DFC 290 optical microscope. AFM images are obtained with NanoScope IIIa/Dimension 3100, scanning rate of 0.5 Hz, resolution of 512 512. SEM images are obtained with JEOL JSM-7100F, acceleration voltage of 5 kV. Raman spectra are recorded by Renishaw Raman RM3000 scope using a 633 nm excitation argon laser with the laser spot smaller than 1 mm. XPS characterization is performed by Kratos Axis Ultra DLD multi-technique surface analysis system with Al Ka X-ray source. 3. Results and discussion Direct visualization of the graphene flakes during the redox reaction provides a method for monitoring many aspects of the graphene-substrate reaction. Previous optical methods usually utilized the post-synthesis oxidation at 200 C in air [15,16], which leads to distinct contrast between graphene-covered and bare regions due to copper oxidation, as shown in Fig. 1a. Here, we
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demonstrate a water-immersion method that allows the flake edges can also be visualized without the use of high temperature annealing. Firstly, graphene is synthesized by chemical vapor deposition following our previously established work [17]. Fig. 1b illustrates the optical image after immersion in water for 13 h, showing that the edges of graphene flake appears as hexagonal shape patterns with dark colored rim. In contrast to the aforementioned high temperature annealing visualization method, this water-immersion method eliminates the use of high temperature and thus avoids damage to graphene structures. Previous reports found that high conductivity nature of graphene promoted electrochemical corrosion for elongated period under ambient conditions [9,10]. Since the galvanic corrosion in high resistivity solution tends to concentrate at the anode/cathode junction [18], the oxidation of copper surface in contact with graphene here is enhanced similarly. In addition to electron transport, the access of water, as well as oxygen, is also critical for oxidation of copper underneath graphene film. However, due to impermeability of graphene in direction perpendicular to the terrace, the diffusion of oxygen and water should mainly come from the interface between graphene and copper, as well as defects. To explore the nature of those ring regions after immersion in water, the morphology of those regions is closely examined. Fig. 2 depicts the detailed morphology of the water oxidized graphene/ Cu substrate. From the height profile, as shown in the atomic force microscopy (AFM) images in Fig. 2a for 2 h immersion, significant height increase can be seen after the water-assisted oxidation, where a sudden change of 3 nm across the oxidation boundary is observed. In addition, the morphology was further characterized by SEM. Fig. 2b illustrates a SEM image of a similar graphene flake in which a concentric ring enclosing a hexagon shape is observed with an additional high-contrast region at the center. A closer look at the edges in Fig. 2c indicates that they are mainly composed of porous structures, typically observed for corroded metal surface. For the high contrast region at the center, this also gives additional evidence that oxidation also proceeds from the defect area as graphene normally nucleates at surface irregularities such as impurities [19] or copper oxide nanoparticles [17] and grows from the center. The graphene in those nucleation centers usually has lots of defects, which allows oxygen/water to enter and oxidize the copper below. This is more evident from Fig. S2b, where oxides at the center were formed after short exposure to ambient condition and from Fig. S3 for water-assisted oxidation. To reveal the progressive development of the ring like oxide structure, we now study the evolution of the water assisted
Fig. 1. Visualization of single-crystal graphene on copper. (a) after 2 min oxidation at 200 C on hot plate; (b) 13 h immersion in water. The place with more reddish color indicates denser cuprous oxide layer under optical microscope. (A color version of this figure can be viewed online.)
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Fig. 2. Graphene morphology. (a) AFM image of graphene/copper after 2 h immersion in water and the upper inset is the height profile, the bottom inset is the corresponding optical image; (b) SEM images of graphene/copper after 2 h immersion in water the inset is the corresponding optical image; (c), (d) high magnification image of the edge and nucleation center, respectively. (A color version of this figure can be viewed online.)
oxidation process. Fig. 3a illustrates a series of optical images taken at various immersion time in water ranging from 1 h to 10 h, with the same camera parameters including exposure time and saturation. After 1 h immersion, a narrow rim of oxides appeared at the edges which became darker and extended laterally to center in the next 5 h. It's interesting to note the sharp oxidation front throughout this process. After 8 h immersion, the oxides met the multilayered structure and then induced ununiformly heavy oxidation which will be explained in detail later. The chemical nature and graphene/copper interaction are further studied by Raman spectroscopy at the place marked as black dotted circle. Fig. 3b illustrated the obtained spectra after background subtraction. Here, the Raman spectra showed two main peaks: G peak at ~1580 cm1, 2D peak at ~2650 cm1. The intensity ratio 2D/G can be qualitatively used to indicate the number of layers with a value of >2 for single layer graphene [20]. For graphene Raman spectrum, the position of 2D peak and G is more sensitive to strain and doping effect, respectively [21e23]. It should be noted that from 1 to 5 h, the intensity of G and 2D peak showed insignificant change, whereas 2D peak position was redshift from 2653 to 2648 cm1 indicating strain relief during the oxidation process. Since strain is normally built up on graphene flake in the cooling process due to smaller thermal expansion coefficient for graphene compared with Cu substrate [24,25], the cuprous oxide formed at the interface near the edges is able to elevate the graphene to release strain accumulated from growth, however, this effect is not enough to fully decouple the graphene/ copper until 6 h immersion. At this moment, the 2D peak position
underwent an obvious red shift (from 2648 to 2637 cm1) while the intensity of G, 2D peak were enhanced due to the reduction of electromagnetic screening from copper substrate by cuprous oxide interlayer [26]. Afterwards, the copper was further oxidized and two strong peaks at ~526, ~619 cm1 for cuprous oxide [27] appeared, concurrent with G, 2D peak enhancement. The formation of cuprous oxide was further confirmed by XPS results (see in Fig. S4) and no discernible peaks for CuO were observed for fully covered graphene/copper sample to avoid signal from bare copper oxidation. More evidence can be obtained by the analysis of the full width at half maximum (FWHM). As shown in inset of Fig. 3b, the FWHM data are clustered into two groups. Before the oxidation front passing the Raman measuring site (from 1 h to 6 h), FWHM(G) is ~18 cm1 and FWHM(2D) is ~30 cm1, which match well with the reported values for coupled graphene on copper after CVD growth. On the other hand, after the copper below was oxidized (from 8 h to 10 h), both FWHM(G) and FWHM(2D) drop down to ~14 cm1, ~23 cm1, respectively. These small FWHM values are close to those for suspended graphene [28], which indicates the graphene decoupling effect after substrate oxidation. It should be emphasized that, different from the case of graphene-covered copper long time oxidation under ambient conditions, the graphene covered copper oxidation in water didn't induce further strains or damages on graphene, as the position of 2D peak showed no further red shift and there was no observable D peak after 6 h immersion in Fig. 3b. The Raman spectra of the spot near the edge showed similar results after further oxidation as shown in Fig. S5. Fig. 4a illustrated the schematic of graphene-covered copper
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Fig. 3. Evolution of graphene/copper in water. (a) In situ optical images of graphene/copper after immersion in DDI water from 1 h to 10 h. (b) Raman spectra measured at the circle in (a) and the inset is the corresponding FWHM of G and 2D peak distribution. (A color version of this figure can be viewed online.)
Fig. 4. Detaching kinetics. (a) Schematic of electrochemical corrosion of graphene-covered copper. (b) Arrhenius plot of graphene decoupling rate in water at 25 C, 30 C, 35 C and 40 C. The slope gives the activation energy of 0.3 ± 0.08 eV. (c) Average decoupling rate of graphene at different temperature fitted by above Arrhenius equation. (A color version of this figure can be viewed online.)
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oxidation in water. In fact, the edges of graphene-covered copper are easily oxidized even after exposure in air for several minutes, as shown by AFM and SEM results in Fig. S2. This enables the initial intercalation of water into graphene/copper interface after submersion in water. Then electrochemical oxidation of graphene covered copper starts in the vicinity of the edges of graphene. In this case, graphene works as the cathode and triggers the oxygen reduction reaction, while the copper works as the anode to provide electrons. The generated hydroxide ions from oxygen reduction then diffuse into the decoupled graphene/copper interface meeting with free copper ions to form cuprous oxide, which consequently leads to increased volume and surface roughness. On the other hand, hydrophilicity change from Cu to Cu2O may have also contributed to this water intercalation process, as shown in previous report where the increase of surface hydrophilicity favors water intercalation [29,30]. Such morphology change below graphene is able to decouple the nearby part of graphene from copper substrate followed by further water intercalation, where the freshly exposed copper is oxidized in a similar way. Eventually, copper oxidation progressively proceeds towards the graphene flake center and leads to the fully decoupling of the graphene. We then performed a series of experiments to determine the main factors that dominate the graphene decoupling process. In galvanic corrosion of metals, oxygen concentration, electrolyte conductivity, corrosion products and electric contact between different materials are all important factors [18]. Unlike uniformly heavy oxidation of bare copper surface in strong electrolyte (0.1 M Na2SO4, see data at Fig. S6), the key in this process is that an extremely weak electrolyte, i.e. distilled deionized water, is used which inhibits the electrochemical oxidation of bare copper apart from the graphene/copper junction and enhances the copper oxidation at the graphene/copper interface [18]. Another interesting fact is that the graphene cathode is a one-atom thick film physically adsorbed on copper surface after growth [31], the volume increase caused by oxides at the interface starting from the edges is able to overcome the weak grapheneecopper interaction at the nearby coupled region to facilitate the oxidation proceeding towards the center. Once the oxidation front proceeds, the local copper oxidation rate is greatly decreased, as shown in Fig. 4, no obvious local color change was observed in the following hours after initially oxidized indicating insignificant copper oxide film thickness increase [32]. On the other hand, when the oxidation front meets the multilayered structure in the nucleation center, due to much stronger binding energy and closer distance between multilayered graphene and copper [33], it's more difficult to decouple. Therefore, longer decoupling time is needed during which the copper near multilayered graphene/copper junction is continuously oxidized. As a result, after 8 h, 10 h immersion a much darker rim like structure near the center could be observed in Fig. 3a. In addition, for elongated immersion (several weeks), some parts of the graphene film after detachment become folded and exhibit many wrinkles, even become aggregated. In order to prevent this, long immersion time is avoided. In order to further address the kinetics of this oxidation assisted decoupling process, additional experiments at different temperatures are performed (see carbon atoms decoupling rate calculation details in Supporting Information). From the results in Fig. 3, the detachment time frame of the graphene flake of ~10 mm diameter at room temperature is ~8 h. However, one may expect that the graphene detaching rate may not be a constant and should be closely related to the graphene size, as we observed during experiments (see more details at Fig. S7 of the supporting information). More experiments are underway to explicitly understand this. As a result, in the following kinetics study we chose graphene samples with area between 450 and 700 mm2 to mitigate such size effect. Based
on Arrhenius relation, the carbon atoms decoupling rate equation is written as R ¼ kcn ¼ cnA exp(EA/kBT), where c and n denote the intercalating molecule concentration and reaction order, respectively, and k is the rate constant with pre-exponential factor A and activation energy EA. Fig. 4b plot the logarithm of carbon atoms decoupling rate as a function of 1000/T and its linear fit. Deduct from the slope and intercept, respectively, we got EA ¼ 0.3 ± 0.08 eV and the prefactor cnA ¼ 8.4 1010 s1. The obtained activation energy is smaller than that of oxygen intercalation between graphene and Ru(0001) [4] but close to that between graphene and copper [34]. In fact, it should be noted that unlike continuous oxygen supply in Ref. [4], the oxygen amount in water is limited, therefore excess oxygen consumption at bare copper would inhibit the oxidation process of copper at the interface, as a result, the decoupling rate of graphene would be affected. Here, after 3 h immersion in water at 60 C, the bare copper part was heavily oxidized, while some of the graphene-covered regions showed no obvious oxidation (Fig. S8a) and the others showed similar ring-like oxide patterns (Fig. S8b). The average decoupling rate of the partially decoupled samples is about 1.1 106 atoms/s, which is close to that at 45 C and much slower than the predicted 2.1 106 atoms/s based on above fitting equation. Therefore, this graphene decoupling process is more likely to be controlled by interfacial oxides formation starting from the edges. Recent works about graphene transfer from copper substrate in a hot water delamination process have proved the feasibility of water intercalation into oxidized graphene/copper interface [35,36] and their results are also consistent with the kinetic study presented here. 4. Conclusions In this work, we investigate the effect of immersing as-grown graphene on copper foil into water and find that the graphene covered region is intentionally oxidized from edges or defects due to galvanic corrosion. By characterizing the oxidation front morphology and nearby graphene/copper interaction, we are able to correlate the oxidation front with the decoupled/coupled boundary of graphene. Further kinetics study at different temperatures reveals the activation energy for this interfacial oxides and water intercalation assisted graphene decoupling of about 0.3 ± 0.08 eV. More importantly, this method can be applied to decouple graphene from other metal substrates, especially those on which graphene is strongly adsorbed such as Ni and Co. After all, this work can also have important applications related to solution based interfacial modification or reactions in confined space between graphene and metal substrate. Acknowledgment This project is supported by the Research Grant Council of Hong Kong SAR (Project number 623512 and DAG12EG05) and SFC/RGC Joint Research Scheme (Project number X-HKUST603/14). Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.11.002. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669.
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Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Detaching graphene from copper substrate by oxidation-assisted water intercalation Ruizhe Wu, Lin Gan, Xuewu Ou, Qicheng Zhang, Zhengtang Luo* Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2015 Received in revised form 1 November 2015 Accepted 2 November 2015 Available online 6 November 2015
For electronic applications, graphene prepared by chemical vapor deposition (CVD) are required to be detached from the catalytic substrate, while retaining structural integrity. We demonstrate that CVD grown graphene on copper can be fully decoupled from the substrate by immersion in water, without significant damage to graphene. We find that the decoupling starts from the graphene edges and defect sites, assisted by interfacial copper oxidation and water intercalation due to galvanic corrosion. Kinetics study reveals the activation energy of 0.3 ± 0.08 eV for this decoupling process, and interfacial oxidation acts as the dominating role. This facile water-immersion method can be extended to adjust the interaction between graphene and metals, and assist our understanding of interfacial chemistry in confined space. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Graphene has attracted worldwide attentions in fundamental and application research due to their exceptional properties [1]. In direction perpendicular to the terrace, graphene are impermeable to most molecules at ambient conditions, which makes graphene a promising membrane material [2]. As a result, most studies on the mass transport are mainly focused on the direction parallel to the terrace between graphene and various substrates, in particular to tune graphene-substrate interfacial interaction which is essential for both post-synthesis graphene transfer and retaining graphene structural integrity. Among these studies, metal atoms and gas molecules intercalation are two typical methods, in which additional processes such as metal deposition or introduction of high purity gas at low pressure followed by high temperature annealing are usually required [3e8]. Recent efforts to detach graphene from copper substrate utilizing electrochemical oxidation at ambient conditions for elongated period, has only limited success as the resulted graphene are usually fragmented due to the non-uniform oxidation [9,10]. On the other hand, water molecule intercalation between graphene and nonmetal substrates, including mica, sapphire and silica, has been widely studied [11e14]. In particular for silica substrate,
the water intercalation rate is governed by the substrate hydrophilicity, and it was observed that the graphene flake kept strong affinity to the substrate even with a layer of water molecules in between [14]. Similarly, the intercalation of water between graphene and metal substrates may also help to adjust the interfacial interaction without significant damage to graphene. In this study, we introduce a facile method utilizing oxidation at copperegraphene interface, assisted by water intercalation to detach graphene film. We can optically observe that substrate oxidation and decoupling start from the graphene edges and defects region, forming an observable oxidation front, which proceeds to the graphene center. This is corroborated with the AFM and SEM investigation, as well as Raman characterization, which altogether lead to the conclusion that such oxidation follows a decoupling controlled substrate oxidation mechanism. Additional kinetics study under different temperatures reveals the activation barrier for graphene decoupling equal to 0.3 ± 0.08 eV. More importantly, it is shown that interfacial oxides formation plays a more important role than interfacial water molecules diffusion in this graphene decoupling process. 2. Experimental 2.1. Chemical vapor deposition
* Corresponding author. E-mail address: [email protected] (Z. Luo). http://dx.doi.org/10.1016/j.carbon.2015.11.002 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
Before growth, copper foil (99.8%, Alfa Aesar No. 13382) is chemically polished by sonicating in etchant solution (5 g of FeCl3,
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10 mL of HCl, and 100 mL of distilled deionized (DDI) water) for 10 s to remove the oxides followed by DDI water rinsing and nitrogen blow-dry. After being loaded into the quartz tube, copper foil is fast-heated to 1050 C under 350 sccm Ar (Hong Kong Specialty Gases Co., Ltd., 99.999%, oxygen concentration <3 ppm). Then graphene growth is started by introducing 10 sccm H2 (Hong Kong Specialty Gases Co., Ltd., 99.999%, oxygen concentration <5 ppm) and 16 sccm diluted CH4 (500 ppm methane diluted in argon, Arkonic Gases and Chemicals Inc., 99.99%) for 2 min. Finally, the furnace is powered off and moved aside to fast cool the sample to room temperature under argon atmosphere. The diameter of single crystal graphene flakes is ~20 mm. 2.2. Graphene/copper redox detachment The fresh grown graphene/copper sample is cut into 1 cm 1 cm and immersed into a petri dish filled with 20 mL DDI water (conductivity 5.5 mS/m) or 0.1 M Na2SO4 solution at room temperature. In additional kinetics experiments, a bottle filled with 20 mL DDI water is heated by oil bath to 25 C, 30 C, 35 C, 45 C. When the water temperature is stabilized, a piece of 1 cm 1 cm graphene/copper is immersed in to the DDI water. The total reaction time at 25 C is 6 h, while the reaction time at 30 C, 35 C, 45 C is 3 h. Finally, the sample is rinsed with DDI water and blown dry by nitrogen gas before characterization. 2.3. Characterization Optical images are taken by a LEICA DFC 290 optical microscope. AFM images are obtained with NanoScope IIIa/Dimension 3100, scanning rate of 0.5 Hz, resolution of 512 512. SEM images are obtained with JEOL JSM-7100F, acceleration voltage of 5 kV. Raman spectra are recorded by Renishaw Raman RM3000 scope using a 633 nm excitation argon laser with the laser spot smaller than 1 mm. XPS characterization is performed by Kratos Axis Ultra DLD multi-technique surface analysis system with Al Ka X-ray source. 3. Results and discussion Direct visualization of the graphene flakes during the redox reaction provides a method for monitoring many aspects of the graphene-substrate reaction. Previous optical methods usually utilized the post-synthesis oxidation at 200 C in air [15,16], which leads to distinct contrast between graphene-covered and bare regions due to copper oxidation, as shown in Fig. 1a. Here, we
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demonstrate a water-immersion method that allows the flake edges can also be visualized without the use of high temperature annealing. Firstly, graphene is synthesized by chemical vapor deposition following our previously established work [17]. Fig. 1b illustrates the optical image after immersion in water for 13 h, showing that the edges of graphene flake appears as hexagonal shape patterns with dark colored rim. In contrast to the aforementioned high temperature annealing visualization method, this water-immersion method eliminates the use of high temperature and thus avoids damage to graphene structures. Previous reports found that high conductivity nature of graphene promoted electrochemical corrosion for elongated period under ambient conditions [9,10]. Since the galvanic corrosion in high resistivity solution tends to concentrate at the anode/cathode junction [18], the oxidation of copper surface in contact with graphene here is enhanced similarly. In addition to electron transport, the access of water, as well as oxygen, is also critical for oxidation of copper underneath graphene film. However, due to impermeability of graphene in direction perpendicular to the terrace, the diffusion of oxygen and water should mainly come from the interface between graphene and copper, as well as defects. To explore the nature of those ring regions after immersion in water, the morphology of those regions is closely examined. Fig. 2 depicts the detailed morphology of the water oxidized graphene/ Cu substrate. From the height profile, as shown in the atomic force microscopy (AFM) images in Fig. 2a for 2 h immersion, significant height increase can be seen after the water-assisted oxidation, where a sudden change of 3 nm across the oxidation boundary is observed. In addition, the morphology was further characterized by SEM. Fig. 2b illustrates a SEM image of a similar graphene flake in which a concentric ring enclosing a hexagon shape is observed with an additional high-contrast region at the center. A closer look at the edges in Fig. 2c indicates that they are mainly composed of porous structures, typically observed for corroded metal surface. For the high contrast region at the center, this also gives additional evidence that oxidation also proceeds from the defect area as graphene normally nucleates at surface irregularities such as impurities [19] or copper oxide nanoparticles [17] and grows from the center. The graphene in those nucleation centers usually has lots of defects, which allows oxygen/water to enter and oxidize the copper below. This is more evident from Fig. S2b, where oxides at the center were formed after short exposure to ambient condition and from Fig. S3 for water-assisted oxidation. To reveal the progressive development of the ring like oxide structure, we now study the evolution of the water assisted
Fig. 1. Visualization of single-crystal graphene on copper. (a) after 2 min oxidation at 200 C on hot plate; (b) 13 h immersion in water. The place with more reddish color indicates denser cuprous oxide layer under optical microscope. (A color version of this figure can be viewed online.)
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Fig. 2. Graphene morphology. (a) AFM image of graphene/copper after 2 h immersion in water and the upper inset is the height profile, the bottom inset is the corresponding optical image; (b) SEM images of graphene/copper after 2 h immersion in water the inset is the corresponding optical image; (c), (d) high magnification image of the edge and nucleation center, respectively. (A color version of this figure can be viewed online.)
oxidation process. Fig. 3a illustrates a series of optical images taken at various immersion time in water ranging from 1 h to 10 h, with the same camera parameters including exposure time and saturation. After 1 h immersion, a narrow rim of oxides appeared at the edges which became darker and extended laterally to center in the next 5 h. It's interesting to note the sharp oxidation front throughout this process. After 8 h immersion, the oxides met the multilayered structure and then induced ununiformly heavy oxidation which will be explained in detail later. The chemical nature and graphene/copper interaction are further studied by Raman spectroscopy at the place marked as black dotted circle. Fig. 3b illustrated the obtained spectra after background subtraction. Here, the Raman spectra showed two main peaks: G peak at ~1580 cm1, 2D peak at ~2650 cm1. The intensity ratio 2D/G can be qualitatively used to indicate the number of layers with a value of >2 for single layer graphene [20]. For graphene Raman spectrum, the position of 2D peak and G is more sensitive to strain and doping effect, respectively [21e23]. It should be noted that from 1 to 5 h, the intensity of G and 2D peak showed insignificant change, whereas 2D peak position was redshift from 2653 to 2648 cm1 indicating strain relief during the oxidation process. Since strain is normally built up on graphene flake in the cooling process due to smaller thermal expansion coefficient for graphene compared with Cu substrate [24,25], the cuprous oxide formed at the interface near the edges is able to elevate the graphene to release strain accumulated from growth, however, this effect is not enough to fully decouple the graphene/ copper until 6 h immersion. At this moment, the 2D peak position
underwent an obvious red shift (from 2648 to 2637 cm1) while the intensity of G, 2D peak were enhanced due to the reduction of electromagnetic screening from copper substrate by cuprous oxide interlayer [26]. Afterwards, the copper was further oxidized and two strong peaks at ~526, ~619 cm1 for cuprous oxide [27] appeared, concurrent with G, 2D peak enhancement. The formation of cuprous oxide was further confirmed by XPS results (see in Fig. S4) and no discernible peaks for CuO were observed for fully covered graphene/copper sample to avoid signal from bare copper oxidation. More evidence can be obtained by the analysis of the full width at half maximum (FWHM). As shown in inset of Fig. 3b, the FWHM data are clustered into two groups. Before the oxidation front passing the Raman measuring site (from 1 h to 6 h), FWHM(G) is ~18 cm1 and FWHM(2D) is ~30 cm1, which match well with the reported values for coupled graphene on copper after CVD growth. On the other hand, after the copper below was oxidized (from 8 h to 10 h), both FWHM(G) and FWHM(2D) drop down to ~14 cm1, ~23 cm1, respectively. These small FWHM values are close to those for suspended graphene [28], which indicates the graphene decoupling effect after substrate oxidation. It should be emphasized that, different from the case of graphene-covered copper long time oxidation under ambient conditions, the graphene covered copper oxidation in water didn't induce further strains or damages on graphene, as the position of 2D peak showed no further red shift and there was no observable D peak after 6 h immersion in Fig. 3b. The Raman spectra of the spot near the edge showed similar results after further oxidation as shown in Fig. S5. Fig. 4a illustrated the schematic of graphene-covered copper
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Fig. 3. Evolution of graphene/copper in water. (a) In situ optical images of graphene/copper after immersion in DDI water from 1 h to 10 h. (b) Raman spectra measured at the circle in (a) and the inset is the corresponding FWHM of G and 2D peak distribution. (A color version of this figure can be viewed online.)
Fig. 4. Detaching kinetics. (a) Schematic of electrochemical corrosion of graphene-covered copper. (b) Arrhenius plot of graphene decoupling rate in water at 25 C, 30 C, 35 C and 40 C. The slope gives the activation energy of 0.3 ± 0.08 eV. (c) Average decoupling rate of graphene at different temperature fitted by above Arrhenius equation. (A color version of this figure can be viewed online.)
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oxidation in water. In fact, the edges of graphene-covered copper are easily oxidized even after exposure in air for several minutes, as shown by AFM and SEM results in Fig. S2. This enables the initial intercalation of water into graphene/copper interface after submersion in water. Then electrochemical oxidation of graphene covered copper starts in the vicinity of the edges of graphene. In this case, graphene works as the cathode and triggers the oxygen reduction reaction, while the copper works as the anode to provide electrons. The generated hydroxide ions from oxygen reduction then diffuse into the decoupled graphene/copper interface meeting with free copper ions to form cuprous oxide, which consequently leads to increased volume and surface roughness. On the other hand, hydrophilicity change from Cu to Cu2O may have also contributed to this water intercalation process, as shown in previous report where the increase of surface hydrophilicity favors water intercalation [29,30]. Such morphology change below graphene is able to decouple the nearby part of graphene from copper substrate followed by further water intercalation, where the freshly exposed copper is oxidized in a similar way. Eventually, copper oxidation progressively proceeds towards the graphene flake center and leads to the fully decoupling of the graphene. We then performed a series of experiments to determine the main factors that dominate the graphene decoupling process. In galvanic corrosion of metals, oxygen concentration, electrolyte conductivity, corrosion products and electric contact between different materials are all important factors [18]. Unlike uniformly heavy oxidation of bare copper surface in strong electrolyte (0.1 M Na2SO4, see data at Fig. S6), the key in this process is that an extremely weak electrolyte, i.e. distilled deionized water, is used which inhibits the electrochemical oxidation of bare copper apart from the graphene/copper junction and enhances the copper oxidation at the graphene/copper interface [18]. Another interesting fact is that the graphene cathode is a one-atom thick film physically adsorbed on copper surface after growth [31], the volume increase caused by oxides at the interface starting from the edges is able to overcome the weak grapheneecopper interaction at the nearby coupled region to facilitate the oxidation proceeding towards the center. Once the oxidation front proceeds, the local copper oxidation rate is greatly decreased, as shown in Fig. 4, no obvious local color change was observed in the following hours after initially oxidized indicating insignificant copper oxide film thickness increase [32]. On the other hand, when the oxidation front meets the multilayered structure in the nucleation center, due to much stronger binding energy and closer distance between multilayered graphene and copper [33], it's more difficult to decouple. Therefore, longer decoupling time is needed during which the copper near multilayered graphene/copper junction is continuously oxidized. As a result, after 8 h, 10 h immersion a much darker rim like structure near the center could be observed in Fig. 3a. In addition, for elongated immersion (several weeks), some parts of the graphene film after detachment become folded and exhibit many wrinkles, even become aggregated. In order to prevent this, long immersion time is avoided. In order to further address the kinetics of this oxidation assisted decoupling process, additional experiments at different temperatures are performed (see carbon atoms decoupling rate calculation details in Supporting Information). From the results in Fig. 3, the detachment time frame of the graphene flake of ~10 mm diameter at room temperature is ~8 h. However, one may expect that the graphene detaching rate may not be a constant and should be closely related to the graphene size, as we observed during experiments (see more details at Fig. S7 of the supporting information). More experiments are underway to explicitly understand this. As a result, in the following kinetics study we chose graphene samples with area between 450 and 700 mm2 to mitigate such size effect. Based
on Arrhenius relation, the carbon atoms decoupling rate equation is written as R ¼ kcn ¼ cnA exp(EA/kBT), where c and n denote the intercalating molecule concentration and reaction order, respectively, and k is the rate constant with pre-exponential factor A and activation energy EA. Fig. 4b plot the logarithm of carbon atoms decoupling rate as a function of 1000/T and its linear fit. Deduct from the slope and intercept, respectively, we got EA ¼ 0.3 ± 0.08 eV and the prefactor cnA ¼ 8.4 1010 s1. The obtained activation energy is smaller than that of oxygen intercalation between graphene and Ru(0001) [4] but close to that between graphene and copper [34]. In fact, it should be noted that unlike continuous oxygen supply in Ref. [4], the oxygen amount in water is limited, therefore excess oxygen consumption at bare copper would inhibit the oxidation process of copper at the interface, as a result, the decoupling rate of graphene would be affected. Here, after 3 h immersion in water at 60 C, the bare copper part was heavily oxidized, while some of the graphene-covered regions showed no obvious oxidation (Fig. S8a) and the others showed similar ring-like oxide patterns (Fig. S8b). The average decoupling rate of the partially decoupled samples is about 1.1 106 atoms/s, which is close to that at 45 C and much slower than the predicted 2.1 106 atoms/s based on above fitting equation. Therefore, this graphene decoupling process is more likely to be controlled by interfacial oxides formation starting from the edges. Recent works about graphene transfer from copper substrate in a hot water delamination process have proved the feasibility of water intercalation into oxidized graphene/copper interface [35,36] and their results are also consistent with the kinetic study presented here. 4. Conclusions In this work, we investigate the effect of immersing as-grown graphene on copper foil into water and find that the graphene covered region is intentionally oxidized from edges or defects due to galvanic corrosion. By characterizing the oxidation front morphology and nearby graphene/copper interaction, we are able to correlate the oxidation front with the decoupled/coupled boundary of graphene. Further kinetics study at different temperatures reveals the activation energy for this interfacial oxides and water intercalation assisted graphene decoupling of about 0.3 ± 0.08 eV. More importantly, this method can be applied to decouple graphene from other metal substrates, especially those on which graphene is strongly adsorbed such as Ni and Co. After all, this work can also have important applications related to solution based interfacial modification or reactions in confined space between graphene and metal substrate. Acknowledgment This project is supported by the Research Grant Council of Hong Kong SAR (Project number 623512 and DAG12EG05) and SFC/RGC Joint Research Scheme (Project number X-HKUST603/14). Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.11.002. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669.
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