Fabrication of Graphene with CuO Islands by Chemical Vapor

Article

Fabrication of Graphene with CuO Islands by Chemical Vapor Deposition Yun Qi,* Jeremy R. Eskelsen, Ursula Mazur, and K. W. Hipps* Materials Science and Engineering Program and Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, United States ABSTRACT: Graphene prepared on Cu foil by chemical vapor deposition was studied as a function of post growth cooling conditions. CuO islands embedded in the graphene film were discovered and studied by scanning electron microscopy, atomic force microscopy, and X-ray photoemission spectroscopy. It is shown that nanostructured holes can be formed within a graphene film by reduction using hydrogen cooling immediately after film growth. We also observe the formation of symmetrical oxide islands in these holes. This study provides an easy way to fabricate a graphene + CuO composite, and the method may be extended to other graphene based structures.

1. INTRODUCTION Graphene exhibits unique structural geometry and fascinating physical properties. Consequently, it has a great potential in applied electronics. The study of exfoliated graphene and epitaxial graphene on SiC has demonstrated the application of graphene as transistors, electrodes, and other electronic devices.1−4 However, the controllable fabrication of large areas of graphene is essential to the commercialization of graphene-based devices. One approach to controlled fabrication of large area graphene films, chemical vapor deposition (CVD) of graphene on metal substrates, was widely investigated during the past few years.5,6 The surface morphology and the mechanism of graphene formation by CVD were examined on different catalytic metal substrates, such as Ni, Ir, and Cu.7−9 It is believed that both C precipitation and deposition are involved in the graphene growth on Ni and these films tend to be multiple layers.10 However, graphene deposition on Cu results in self-limited monolayer growth because of the lower solubility of carbon in copper.6 Moreover, the low cost of Cu is a big advantage for its future applications as a substrate for graphene growth where industrially useful amounts are required. In concert with these CVD studies, the graphene transfer process has been developed to transfer graphene from metal substrates to any arbitrary surface.11,12 Thus, CVD preparation of graphene on Cu is an attractive approach for large scale manufacture of graphene-based materials. Controlling the growth conditions of low-dimensional materials can provide unique nanostructured materials. An example is provided by the morphology of Pb vapor deposited on Si(111). At room temperature Pb islands are formed, but at low temperature it forms through layer-by-layer growth.13,14 Another example is provided by the formation of ZnO nanotubes. Higher crystallinity and enhanced properties can be obtained with a well-controlled cooling rate.15 In many ways, the development of material science is grounded on creative ways to modulate the growth conditions with a motivation to make functional materials for devices. In this study, we © 2012 American Chemical Society

investigate the influence of cooling atmosphere on the composition and structure of graphene grown on Cu foil. We use atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoemission spectroscopy (XPS) in this study. We demonstrate that large area graphene films can be produced if a 5:1 C2H4/H2 gas mixture is maintained during cooling, but that partial reduction of graphene occurs when only H2 is present during cooling. Finally, we determined composite films composed of graphene with CuO islands are fabricated upon exposure of the reduced film to air. Graphene/CuO materials have potential applications in technology. A graphene/CuO nanocomposite was suggested as a high-performance anode material for lithium-ion batteries with a remarkably enhanced cycling performance and cycling rate compared with traditional CuO anode materials.16 Also, a study of CuO-graphene dispersed nanofluids suggested their applications as coolants due to their high thermal conductivity and heat transfer.17 Our study of graphene + CuO islands offers a method to fabricate graphene composite films that may be of value for industrial applications.

2. EXPERIMENTAL SECTION Graphene was grown on polycrystalline Cu (99.8%, Alfa Aesar) foil. It has been reported that pristine graphene films grown on polycrystalline Cu are independent of the Cu structure.18 Ultrahigh purity ethylene (99.95%, Matheson) was used as the source of carbon, and 99.99% purity H2 was used as reducing agent. The growth was performed in a furnace with an internal quartz preparation tube, vacuum pump connection, thermocouple vacuum gauge, and manifold for inletting gases under controlled flow conditions. The background pressure of the system was ∼5 mTorr. A Linde model FM4575 operator console was used for flow control and gas blending. The CVD procedure is described in Figure 1. The hydrogen flow rate was kept at 5 sccm during the whole growth from step I to step III to Received: December 6, 2011 Revised: January 5, 2012 Published: January 10, 2012 3489

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Figure 1. Diagram of graphene growth process. The steps I, II, and III are temperature increasing, deposition, and cooling, respectively. protect Cu from oxidation. At step I, the furnace temperature was increased from room temperature to growth temperature in 1 h. At the deposition temperature of 950 °C (step II), ethylene was introduced into the chamber with a ratio C2H4/H2 = 5:1. The total pressure in the CVD chamber during graphene growth (step II) was about 500 mTorr. The deposition normally took 20 min. During the cooling (step III), we moved the quartz tube out of the furnace quickly. The temperature of the quartz tube was monitored as it quickly dropped from 950 to 200 °C and then slowly reached room temperature. The graphene films were cooled to room temperature either in the presence of the same 5:1 ratio of ethylene to hydrogen used during film growth, or in pure hydrogen. Once the samples had cooled, they were exposed to air for several hours. For the Raman study, the transfer was made by etching the Cu substrate with 0.25 M FeCl3 solution and picking up the floating graphene film by contact to a clean glass slide.6 The samples cooled only in hydrogen, however, produced films that tended to come apart during the transfer process and were only studied on the Cu substrate. Raman spectra of the graphene films were acquired with an Acton spectrometer equipped with Olympus inverted microscope and charge coupled device (CCD) detector. The laser source was 2 mW of the 514.5 nm Ar+ laser line with a spot size of 10 μm. The spectral resolution is around 5 cm−1, and the accuracy is 3 cm−1. The SEM measurements were taken using a FEI Quanta 200F SEM with the field emission voltage set at 20 kV. A Digital Instruments (DI) AFM was employed to image the surface morphology with tapping mode. AFM data analysis was performed with the SPIP commercial software package.19 XPS was obtained by using an AXIS-165 electron spectrometer with excitation provided by monochromatic Al Kα radiation at 1486 eV with a power of 195 W.

Figure 2. Full coverage graphene film deposited on Cu foil with C2H4/ H2 mixture cooling: (a) the AFM image and (b) Raman spectrum.

monolayer graphene.22 The presence of a very weak D band feature reveals defects produced in film preparation, but at a minimal level. The surface of graphene on Cu, which was cooled in H2 only, was first checked by SEM, and representative SEM images are presented in Figure 3. Small islands of 1 μm average size were

3. RESULTS AND DISCUSSION The pristine graphene film was prepared with C2H4/H2 cooling and is characterized in Figure 2. As shown in Figure 2a, there are distinct Cu steps and domain boundaries on the whole surface. The dark stripes on the AFM image (Figure 2a) are due to wrinkles in the foil. Under normal condition, clean copper surfaces exposed to air rapidly grow a native oxide layer of 2−5 nm thick,20 and the surface features seen on pristine copper disappear. To test this, AFM was performed on asreceived Cu foil, and no steps were observed. This thin layer Cu oxide is reduced to Cu during the growth (step I, Figure 1) by hydrogen annealing, and a clean Cu surface is prepared for graphene deposition. According to a previous study, the graphene coated Cu surface is protected from natural oxidation, and the Cu steps are preserved.21 Therefore, the Cu steps and other surface features in Figure 2a reveal that a thin and continuous graphene layer fully covers the Cu substrate. Raman spectra of similarly prepared graphene films were obtained after transferring them to glass slides as described in the Experimental Section. A typical Raman spectrum of such a film is presented in Figure 2b. The spectrum has a high intensity ratio of the 2D peak (∼2700 cm−1) to the G peak (∼1595 cm−1) and a single G peak, indicating the formation of

Figure 3. SEM images of graphene growth on Cu substrate with H2 solo cooling schema. The magnifications of images are (a) 10 000×, (b) 16 000×, (c) 24 000×. The red solid lines refer to domain boundaries, and yellow dashed lines refer to step directions.

found. Those islands have preferential growth locations either along the domain boundaries of the Cu foil as indicated by the solid lines in Figure 3a, or along the Cu steps as indicated by the dashed lines in the same picture. Those locations have high surface energy because of dangling bonds or defects. The preference for growth at these locations suggests that these features serve as initial sites for the island growth. Moreover, there are bright spots on each island shown in Figure 3b,c that may be nucleation sites for island growth. The regions between these islands are fairly flat but with observable Cu steps. In the high magnification micrograph, Figure 3c, the steps are easily seen running from the top of the image to the bottom. No indications of copper steps were observed on the islands themselves. It is tempting, therefore, to assign the islands to copper oxide. In order to know the composition of the islands as well as the whole surface, XPS analysis was performed. Three samples were examined: as received Cu foil, graphene grown on Cu 3490

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cooled only by hydrogen (as in Figure 3), and graphene on Cu cooled by hydrogen and ethylene mixture (as in Figure 2). The survey spectra taken on all these samples indicated C, Cu, and O as the only elements present. The Cu 2p and C 1s spectra are presented in Figure 4. As we noted, the as received Cu foil

Figure 4. XPS measurements of (a) Cu 2p and (b) C 1s on as received Cu foil, graphene with islands, and full graphene layer grown on Cu substrate. The upward arrows and downward arrows mark the Cu oxide states on Cu foil as received and graphene with islands sample, respectively. (Cu 2p curves are vertical shifted for comparison.)

has a thin oxide coating, so both the Cu 2p1/2 and Cu 2p3/2 peaks have shoulders on the high energy edge and extra peaks at 944 and 963 eV (marked as upward arrows), indicating the oxidized state of Cu. The pristine graphene (ethylene/ hydrogen cooled), however, has clean Cu 2p peaks indicating no copper oxidation due to the graphene layer protecting the copper surface. The graphene with islands sample has a small amount of Cu oxide at the peak position marked by downward arrows, but the ratio of oxide to Cu is relatively low compared to the as received Cu foil. In the absence of other elemental species, we conclude that the islands observed in the hydrogen cooled sample are due to copper oxide. It should be noted, however, that XPS data alone is insufficient to distinguish between copper metal and copper(+1) in Cu2O.21 Thus, there may be a small component of Cu2O in the islands. We note that all the Cu oxide in this work is the result of oxidation at room temperature and room humidity producing the native copper oxide. No effort was taken to anneal samples or expose them to high humidity. The carbon on the as received Cu foil mainly comes from ambient contamination and shows peaks between 289 and 285 eV depending on the bonding state of carbon with oxygen. The large width of the C 1s peak in this sample also demonstrates multiple carbon chemical states. The C 1s peaks of the other two graphene containing samples show a strong sp2 carbon peak at 284.5 eV. However, the graphene + CuO islands sample also has an extra peak ∼288 eV, which is often associated with carboxyl groups. We assign this small component to oxidized carbon on the perimeter of the CuO islands. On the basis of the SEM and XPS measurements, it becomes clear that the sample of graphene grown on Cu with only hydrogen cooling has CuO islands and graphene film coexisting. Although some islands are isolated and some of them are connected to each other, it is interesting to notice that the CuO islands have a symmetrical shape, exhibiting a marked preference for 120° angles, causing the islands to appear as elongated hexagons in the SEM pictures. We studied some isolated islands with AFM and exhibit the images in Figure 5. In Figure 5a, two isolated islands with irregular hexagonal shape are clearly seen. The line profile in Figure 5a indicates the height of the island at 2.5 nm which is in the range of the thickness for the native Cu oxide layer.20

Figure 5. AFM images of (a) graphene and CuO islands with H2 (only) cooling schema, image size 2 μm, and the insert is a line profile indicating the height of CuO island, and (b) submonolayer graphene growth on Cu, image size 10 μm, the circle indicating the graphene region.

We also considered the possibility that the CuO in the sample with islands was due to the incomplete growth of graphene and that some uncovered Cu surface was oxidized in air. To test this, we made a submonolayer graphene sample and checked the surface morphology with AFM. The deposition of this sample only took 5 min, and cooling was carried out in a 5:1 ethylene to hydrogen mixture. The submonolayer sample (Figure 5b) was prepared in the same way (except the shorter growth time) as the pristine graphene sample shown in Figure 2a. The reduced growth time sample shows graphene in limited areas, but a net shaped pattern of CuO runs over the surface. The height of the CuO seen in Figure 5b is about 3 nm as expected for the native oxide and also consistent with the oxide islands in the hydrogen cooled film (Figure 5a). The morphology of the air exposed surface after incomplete graphene deposition is far different from the CuO islands sample, excluding the possibility of the formation of CuO islands due to incomplete graphene growth. If we go back and compare preparation conditions of the pristine graphene with the graphene + CuO islands sample, there is only one difference in the preparation procedure: the cooling atmosphere. This difference, however, is enough to allow us to propose a model for the formation of CuO islands on the surface. The decomposition of ethylene to graphite and hydrogen, C2H4 = 2Cgraphite + 2H2, is thermodyamically favorable at room temperature and above. However, the reaction only occurs in the presence of a catalyst. In the presence of both hydrogen and ethylene, the forward reaction will dominate when kT is of the order of the barrier for the forward reaction. When no ethylene is present, only the reverse 3491

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Figure 6. Evolution of the formation of CuO islands after graphene growth.

surface energy locations, and CuO islands were formed upon exposure to air. AFM images demonstrated that these islands have heights consistent with native Cu oxide and reflected the 6-fold symmetry of the graphene. XPS measurements have further validated the presence of an sp2 carbon and Cu oxide on these graphene + CuO islands samples. A structural model for the formation of graphene + CuO islands was proposed. This composite structure could find potential applications as the anode materials for lithium-ion batteries and/or as a coolant. Moreover, the symmetric Cu regions after graphene reduction are chemically active and may be useful for forming structures other than CuO. The possibility of controlling the island size and density is under study.

reaction can occur at high temperature. The thermal decomposition of ethylene and the formation of graphitic carbon film begins on Pt(111) at temperatures above 500 K in ultrahigh vacuum, with significant particle size appearing above 700 K .23 We expect a higher threshold temperature for the reverse reaction on copper in the absence of ethylene and in a hydrogen rich atmosphere. When cooling the sample with the mixture, the reaction will eventually stop, but an excess of ethylene ensures that the surface carbon concentration does not change. However, if hydrogen alone is used to cool the sample, the reaction is driven to the left and carbon is scavenged from the surface to form ethylene. The graphene which was already formed on Cu is reduced by hydrogen gas, and sites of exposed Cu are formed on the surface. It is understandable that the reduction of graphene initially occurs at high energy sites such as steps and domain boundaries. A recent study reported a mechanical weakness at graphene domain boundaries which were copied from the grain boundaries of the Cu substrate.24 Our results demonstrated the chemical activity of these domain boundaries, suggesting a common chemical origin for both observations. Graphene has a honeycomb structure with 6-fold symmetry and each carbon atom residing at a 3-fold site. Apparently, the reduction follows the crystalline structure of graphene to form irregular polygonal holes exposing the underlying Cu. These exposed Cu sites are then readily attacked by atmospheric oxygen and water vapor to form the ∼1 μm CuO islands. This process demonstrates the balancing act performed by Cu as the catalyst. The Cu surface is reactive enough to break C−H bonds and hold carbon on the surface, but not reactive enough to prevent the reunion of C−H bonds. This graphene reduction process and structural model are presented graphically in Figure 6. Beyond the fabrication of the graphene + CuO composite, another potential contribution of our work is in providing a method to make graphene related structures. The exposed Cu at the holes formed by reduction of the graphene could be used as a substrate for physical adsorptions or chemical reactions to form new structures. For example, macromolecules could be self-assembled on exposed metal regions formed on the hydrogen reduced graphene surface25 and provide a composite with superior electronic properties for gas sensors. This suggests a general approach to design and produce more interesting structures with high symmetry embedding in a graphene film for graphene-based devices. Moreover, the cooling rate and variation of the partial pressure of H2 in the cooling C2H4/H2 mixture may affect the size, shape, and density of the islands, providing a method for tuning hole size and density. We are currently exploring this possibility.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.W.H.), [email protected] (Y.Q.).

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grants CHE-1048600, CHE1112156, and CHE-1058435. We thank Candy Mercado for assisting with the Raman measurements.

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REFERENCES

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4. CONCLUSION In this work, we prepared graphene films on Cu foil by CVD. Raman spectra reveal that a pristine monolayer graphene film can be produced using a cooling scheme wherein both ethylene and hydrogen are present. However, if the films are cooled in hydrogen alone, the graphene film was reduced locally at high 3492

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