Polarized X-ray Absorption Spectroscopy Observation of Electronic

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Polarized X‑ray Absorption Spectroscopy Observation of Electronic and Structural Changes of Chemical Vapor Deposition Graphene in Contact with Water J. J. Velasco-Velez,† C. H. Wu,†,‡ B. Y. Wang,§ Y. Sun,† Y. Zhang,† J.-H. Guo,§ and M. Salmeron*,†,∥ †

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Chemistry, University of California, Berkeley, California 94720, United States § Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∥ Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States ‡

ABSTRACT: The interaction of chemical vapor deposition (CVD)-grown graphene films with water was studied by means of in situ angle-dependent X-ray absorption spectroscopy (XAS). We found that when the graphene layer is in contact with water there is a reduction in the π* peak intensity in the carbon K-edge absorption spectra, accompanied by an extension of the σ* peak to lower energies, which are indicative of chemical modifications of the graphene and a reduction in the number of unsaturated carbon bonds due to the covalent attachment of contaminant species. In addition to the chemical changes a decrease in the dichroic ratio measured by polarized XAS measurements was observed, which indicates an increase on the nanometer scale corrugation. These changes can strongly influence the electronic properties, mechanical robustness, and resistance to sloughing, as well as graphene reactivity.

1. INTRODUCTION The chemical bonding of graphene is based on the sp2 hybridization of the carbon 2px and 2py orbitals to form σ and σ* (bonding and antibonding) bands, while the 2pz orbitals overlap to form the half-filled π-band. While the flat and perfect graphene or graphite surface is notorious for its chemical inertness, deviations from these ideal conditions can result in increased chemical activity with solvents, which can introduce defects and dopants and significantly change its band structure. This is particularly relevant for the commonly used polycrystalline graphene grown by chemical vapor deposition (CVD) on Cu, which has numerous grain boundaries. An understanding of the chemical interactions at the liquid/graphene interface is important toward its electrochemical applications.1−3 In this work we used near edge X-ray absorption fine structure (NEXAFS or XAS for short) to study the unoccupied orbital structure of graphene and its changes in contact with water. The spectrum of graphene presents two peaks at 285.5 and 291.5 eV corresponding to unfilled π* and σ* orbitals. In addition, dipole selection rules impose a strong dependence of the absorption cross-section on the photon polarization, which makes the XAS spectra dependent on the alignment and orientation of molecular orbitals relative to the X-ray beam polarization4 (see Figure 1a). When the photon electric field is in the graphene plane, the σ* orbitals are probed (Figure 1a, blue orbitals), and when perpendicular to the plane the π* orbitals are selectively detected (Figure 1a, red orbitals). In previous work we reported the degradation of graphene under aggressive electrochemical conditions.5 In the present © 2014 American Chemical Society

Figure 1. (a) Schematic diagram of the NEXAFS experiments on a single-layer graphene and the angle dependence on the incident X-ray relative to the molecular orbitals of graphene. (b) Static liquid cell setup used in Beamline 8.0.1 at the ALS.

investigation we found that even in the absence of electrochemical potentials the chemical integrity of graphene is affected by the presence of water, which should be important in electrochemical applications in aqueous systems, like electrolyzers or membranes for water purification applications. Received: July 23, 2014 Revised: September 15, 2014 Published: September 30, 2014 25456

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Figure 2. (a) SEM image showing a graphene single layer on top of a Si3N4 membrane. (b) Raman measurements from HOPG reference and from graphene transferred to the Si3N4 membrane. The two bottom spectra correspond to graphene oxide (GO) and partially oxidized graphene (POG).

2. EXPERIMENTAL SECTION Graphene was grown on a 25 μm thick Cu foil by chemical vapor deposition in a hot-wall tube furnace using a mixture of precursor gases (Ar/H2/CH4) at 1000 °C. The transfer of graphene was performed using polymethyl-methacrylate (PMMA) spin-coated onto the graphene/copper layer with a thickness of 500 nm.6 The copper foil was subsequently etched away with 0.5 M FeCl3 solution for a few hours. After several rinses in DI water, the graphene flake was transferred to the Si3N4 substrate. Finally, the PMMA was removed with acetone and the substrate cleaned with isopropanol. For in situ XAS characterization of the graphene in contact with water, a watertight liquid cell was used, as shown in Figure 1b. In the cell, a thin (∼100 nm) Si3N4 membrane (1.0 mm × 1.0 mm in size) serves as an X-ray transparent window and separates the liquid from the ultrahigh vacuum in the main chamber. The experiments were carried out at beamline 8.0.1 of the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory (LBNL). The graphene layer was transferred onto the Si3 N 4 membrane and electrically connected by a thin Cu wire epoxied to its edge. The X-ray absorption spectra were collected in total-electron-yield (TEY) mode, by collecting the secondary electron current through the Cu wire. The spectra are normalized by the incident beam intensity collected in a gold grid located upstream in the beam pipeline. The energy scale was calibrated using a HOPG reference sample. The resolution of the beamline at the carbon K-edge is 0.1 eV. No beam damage was found to take place, as shown by the reproducibility of the spectra in successive scans. The watertight liquid cell was attached to a manipulator, which allowed for control of x, y, z, and α coordinates.

overtone of the G peak. This ratio is a good indicator of defect concentration.5,12 In the spectrum of perfect graphene, I2D/IG is about 2, while in the spectrum of graphene oxide this ratio is less than 0.5, as can be seen in Figure 2b. In the samples used in this study the ratio is between these values; therefore we will refer to them as partially oxidized graphene (POG). Figure 3 shows the carbon K-edge XAS spectra of HOPG (reference sample) and the POG before, during, and after

3. RESULTS AND DISCUSSION Figure 2a shows a scanning electron microscopy (SEM) image of graphene transferred onto the Si3N4 membrane. The image shows that the graphene layer presents some wrinkles probably formed during the transfer process.7 Similar wrinkles were also observed by atomic force microscopy (AFM)8,9 on graphene transferred to SiO2/Si wafers. The Raman spectra of different graphitic samples (Figure 2b) present three characteristic peaks, labeled as D, G, and 2D. Peak D, at about 1350 cm−1, is attributed to C−O vibrations from hydroxyl groups at various defect sites that perturb the breathing modes of carbon rings.10 Peak G, at 1500 cm−1, is due to the in-plane phonons at the Brillouin zone center.11 Peak 2D, at 2700 cm−1, is the first

contact with water. The HOPG spectrum presents two characteristic peaks at 286 and 293 eV, corresponding to 1s → π* and 1s → σ* transitions.12−14 The peaks are also present in the XAS spectrum of the as-prepared POG before contact with water. However, as shown in Figure 3, there is a decrease in the π* peak area, which indicates that the density of sp2 sites is reduced in the presence of water. Specifically, the π* peak area decreased from 0.45Ap to 0.39Ap in contact with water (Ap being the normalized π* peak area in HOPG, which has only sp2 sites), while the σ* peak nearly disappeared. After 7 days exposure to water, the π* peak area decayed to 0.26Ap. In addition, three extra features appeared between these two peaks: Peak A (288 eV), B (289 eV), and C (290 eV), which

Figure 3. NEXAFS spectra of graphitic samples collected in TEY. Black and blue curves represent the in-vacuum XAS spectra of the HOPG reference and POG transferred onto a Si3N4 window before in contact with water, respectively. A red spectrum was taken by using the static liquid cell, when the POG was in contact with water. A gray spectrum was taken in UHV after the same POG sample was exposed to water for 7 days.

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Figure 4. Angle-dependent XAS spectra of POG transferred onto Si3N4: (a) in UHV, before exposure to water; (b) in contact with water in the static liquid cell of Figure 1; and (c) in UHV after long-term DI-water exposure. (d) Normalized π* intensity as a function of sin2(α), where α is the incidence angle.

−0.9,12 in agreement with the present measurements. A DR value of −0.88 was found for our graphene grown on Cu and −0.68 for the graphene transferred to a SiO2 wafer,12 which implies the carbon bonding geometry changes from flat sp2 to a less planar one. Figures 4a, 4b, and 4c show XAS of POG on Si3N4, in the presence of water, respectively, for different incidence angles α. As can be seen there is clear dichroic variation, with the integrated intensities of peak π* versus sin2 θ shown in Figure 4d. A DR value of −0.7 was found for POG deposited onto Si3N4 windows in UHV (Figure 4a). Under H2O exposure the DR value decreased further to −0.5 (Figure 4b). This decrease become irreversible after the 7 day exposure, as shown by the DR value of −0.4 (Figure 4c) obtained from measurements performed in UHV with a sample previously exposed to DI water. Together these results show that polycrystalline graphene interacts strongly with water as compared to hydrophobic graphite, giving rise to the formation of bound species that disrupt the sp2 network and introduce defects, a result that is also manifested in the increase of the atomic scale corrugation shown by the DR measurements.

are due to the presence of C atoms in various functional groups (CxHy, COx, etc.). The O-containing functional groups are commonly found in graphene oxide,15,16 although the assignment is still controversial.14,17 Since the intensity of the π* peak reflects the concentration of C−C bonds not bound to O,17 it should be possible to extract the fraction of unsaturated carbons by normalizing the intensity to that of HOPG, which only contains sp2 carbon.18 After the 7 day of exposure to water the POG sample was transferred to the UHV chamber for XAS measurement. The spectrum shows a further decrease in the π * peak intensity, while the peak corresponding to C−H groups17 grew significantly, indicating irreversible chemical and structural changes in the POG layer induced by water. To characterize the structural changes in the POG layer, especially its flatness, angle-dependent XAS spectra were collected as a function of the X-ray incidence angle. The DR is defined as

DR =

I⊥ − I I⊥ + I

(1)

where I⊥ is the integrated intensity at α = 0° and I∥ is the intensity at α = 90°. The intensity at α = 90° in our case was obtained by linear extrapolation. The DR can be utilized to quantify the extent of corrugation in graphene.13 For an ideally flat graphitic sample, DR = −1, and 0 when the π* orbitals are randomly aligned. At normal incidence (α = 0°), the electric field vector is in the plane of the graphene (see Figure 1a for angles reference). Thus, the states with σ* symmetry are enhanced, and the π* resonance is attenuated. HOPG is close to the ideal surface and presents a DR approximately equal to

4. CONCLUSIONS The interaction between water and polycrystalline graphene was investigated in situ by means of polarized X-ray absorption spectroscopy. The XAS measurements revealed strong variations in the graphene band structure due to covalently bonded species formed by reaction with water. In addition, the angle dependence of the π* peak intensity (dichroic ratio) 25458

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(14) Pacilé, D.; Papagno, M.; Rodríguez, a.; Grioni, M.; Papagno, L.; Girit, Ç .; Meyer, J.; Begtrup, G.; Zettl, A. Near-Edge Absorption FineStructure Investigation of Graphene. Phys. Rev. Lett. 2008, 101, 066806. (15) Schultz, B. J.; Patridge, C. J.; Lee, V.; Jaye, C.; Lysaght, P. S.; Smith, C.; Barnett, J.; Fischer, D. a; Prendergast, D.; Banerjee, S. Imaging Local Electronic Corrugations and Doped Regions in Graphene. Nat. Commun. 2011, 2, 372. (16) Zhong, J.; Deng, J.-J.; Mao, B.-H.; Xie, T.; Sun, X.-H.; Mou, Z.G.; Hong, C.-H.; Yang, P.; Wang, S.-D. Probing Solid State N-Doping in Graphene by X-ray Absorption Near-Edge Structure Spectroscopy. Carbon N. Y. 2012, 50, 335−338. (17) Jeong, H.-K.; Noh, H.-J.; Kim, J.-Y.; Colakerol, L.; Glans, P. -a.; Jin, M.; Smith, K.; Lee, Y. Comment on Near-Edge X-Ray Absorption Fine-Structure Investigation of Graphene. Phys. Rev. Lett. 2009, 102, 099701. (18) Fayette, L.; Marcus, B.; Mermoux, M.; Tourillon, G.; Laffon, K.; Parent, P.; Le Normand, F. Local Order in CVD Diamond Films: Comparative Raman, X-ray-Diffraction, and X-ray-Absorption NearEdge Studies. Phys. Rev. B 1998, 57, 14123−14132.

indicated a concurrent increase in the atomic level corrugation of graphene in contact with water.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-510-486-6704. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES), Materials Sciences and Engineering (MSE) Division, under Contract No. DE-AC02-05CH11231. J.J. Velasco-Velez gratefully acknowledges financial support from the Alexander von Humboldt foundation. Y. Zhang and the graphene synthesis work were supported by the Molecular Foundry. The ALS and Molecular Foundry are User Facilities of the DOE, Office of Science.

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REFERENCES

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