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Intercalation of Li at the Graphene/Cu Interface Liang Zhang, Yifan Ye, Dingling Cheng, Haibin Pan, and Junfa Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp401290f • Publication Date (Web): 15 Apr 2013 Downloaded from http://pubs.acs.org on April 16, 2013

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The Journal of Physical Chemistry

Intercalation of Li at the Graphene/Cu Interface

Liang Zhang, Yifan Ye, Dingling Cheng, Haibin Pan, Junfa Zhu*

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, PR China

*Corresponding author:

Email address: [email protected]

Tel.: +86 551 63602064, Fax: +86 551 65141078

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ABSTRACT The intercalation process of Li underneath a graphene layer grown on a Cu foil has been studied by means of synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The deposition of Li on graphene surface at room temperature results in a charge transfer from the adsorbed Li atoms to graphene. After annealing the as-deposited Li/graphene/Cu sample at 300 ºC for 10 min, the Li atoms intercalate into the interface of graphene/Cu. These interfacial Li atoms show strong passivation from the oxidation environment due to the protection by the gaphene layer on-top.

KEYWORDS SRPES, XPS, electronic structure, lithium intercalation, graphene/metal interface

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INTRODUCTION Graphene, a novel two-dimensional material, holds great promises in energy storage and nanoscale electronics due to its unique electronic structures and extraordinary physical properties.1,2 Among the various routes to prepare monolayer graphene, epitaxial growth on metal substrates has become one of the most effective methods.3-8 However, the presence of a strong chemical bonding between graphene and the underlying metal substrates can affect the intrinsic electronic structure of graphene and even induce the band-gap opening in graphene.6 Recently, it has been found that the intercalation of metal atoms, such as Na, K, Fe, Au, Ag and Cu, into the graphene/metal interface can weaken the chemical interaction between graphene and the underlying metal substrates and recover the intrinsic electronic properties of graphene.6,9-16 For example, Varykhalov et al. demonstrated that by introducing Au atoms at the graphene/Ni interface, the graphene overlayers were decoupled from the Ni substrate and the band gap of graphene disappeared.10,16 Nagashima et al. found that band structures of Na-, K- and Cs-intercalated graphene/Ni systems changed obviously compared with the pristine one.9 Thus, intercalation of metal atoms into the graphene/metal interfaces has attacted extensive research interests.6,9-16 Alkali metals have a fairly simple electronic configuration and can be used as the potential donors when adsorbed on the surface of carbon substrates, resulting in the changes of the electronic properties of carbon meterials. Therefore, the interaction between alkali metals and carbon materials has been actively investigated for the past three decades.17,18 Among the alkali metals, Li is particularly important because it is widely used in hydrogen storage, fusion devices and Li-ion batteries.19-25 Due to the uniqueness of single-layer graphene on metal surfaces, the adsorption and migration of Li atoms on graphene/metal surfaces may offer an opportunity for a 3



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fundamental understanding of the interaction in Li-graphitic systems, which should be important for the development of rechargeable Li-ion batteries. As for the interaction between Li and graphene on metal surfaces as well as the Li intercalation process, despite plenty of theoretical studies of Li adsorption on monolayer or multilayer graphene have been reported recently,26-32 only few experimental works can be found so far.33,34 Density functional theory calculations have demonstrated that the process of Li adsorption and migration on graphene is dependent on the substrates on which the graphene layers are situated:27 for free-standing graphene, migration of Li adatoms is possible in both sides of graphene and this process is reversible; while in the case of graphene epitaxially grown on a SiC substrate the penetration of Li atoms through the graphene layer is irreversible. The penetration of Li atoms from the graphene surface to the buffer layer and the SiC substrate has also been investigated experimentially, and it was suggested that the Li atoms intercalated at the interface between SiC and the buffer layer could lead to the transformation of the buffer layer into a second graphene layer.35 However, due to the presence of the buffer layer between graphene and the SiC substrate,31,35-37 the electronic structure of graphene and the Li intercalation process can be strongly influenced, and are expected to be different if no buffer layer is present, such as the graphene/metal system. For example, it has been found that the location of Li appears on the graphene surface as well as between graphene and the underlying Cu in the charged state of graphene/Cu anodes used for Li-ion batteries.33,34 However, in those cases Li formed solid electrolyte interface layers in the form of LiF and Li2CO3 during the charge process, which limits the fundamental understanding of the adsorption and migration behavior of Li atoms on the 4


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graphene surface. In addition, the vacancy defects in graphene sheets facilitate the diffusion of Li in the direction perpendicular to the graphene sheets.29 Therefore, a systematic study of in situ adsorption and migration of Li atoms on single-layer and high-quality graphene on metal surfaces is highly desirable to better understand the intercalation mechanism. In the present work, we have investigated the intercalation process of Li atoms at the graphene/Cu interface and characterized the influence of Li atoms before and after intercalation on the electronic properties of graphene. The graphene/Cu system was chosen because it has been demonstrated that high-quality, large-area and single-layer graphene can be prepared by chemical vapor deposition (CVD) on Cu foils.7 Due to the weak interaction between graphene and Cu, the graphene layers on Cu foils preserve the fundamental electronic structure as that of intrinsic graphene,7 and therefore, it provides a chance to study the interaction between Li and graphene and test the possibility of in situ intercalation of Li underneath a single-layer graphene weakly coupled with metal substrate under ultrahigh vacuum (UHV) conditions .

EXPERIMENTAL SECTION The synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS) measurements were carried out at the photoemission endstation at beamline U20 in National Synchrotron Radiation Laboratory, Hefei, China, which has been described in detail elsewhere.38 Briefly, the endstation system contains two UHV chambers: analysis chamber and sample preparation chamber, whose base pressures are 2ൈ10-10 and 5ൈ10-10 mbar, respectively. The analysis chamber is equipped with a VG Scienta R3000 photoelectron spectrometer, a twin anode X-ray source, a retractable four–grid optics for low energy electron diffraction (LEED) and

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an ion sputtering gun. The sample preparation chamber houses several home-made evaporators. The large-area and single-layer graphene samples (up to 10 mm2) were grown directly on 25-µm thick Cu polycrystalline foils (Alfa Aesar, 99.95%) by CVD method, using the procedure reported previously.39 Raman spectra (ISA Group Horiba) were measured using a 488 nm wavelength to inspect the microstructure and quality of graphene layers. Before Li deposition, the graphene samples were annealed at 500 ºC for 20 min to remove any surface contaminants. After this treatment, no O signal can be detected by XPS. Li (Alfa Aesar, 99.9%) was deposited onto the graphene surface at room temperature by a home-made evaporator in the preparation chamber after thorough outgassing. The deposition rate of Li, as estimated by monitoring the attenuation of Cu 2p XPS signal after Li deposition on a Cu foil, was about 0.6 Å/min. After the deposition of Li, the graphene samples were transferred to the adjacent analysis chamber for SRPES and XPS measurements immediately without exposure to air. The introduction of O2 onto the sample surface was realized by directly backfilling the chamber through a leak valve. The exposure of O2 was calculated in the unit of Langmuir (L) using the pressure rise in the chamber multiplied by the doing time (1 L = 1.3ൈ10-6 mbar·s). The valence band spectra were taken with a photon energy of 170 eV at normal emission. The Li 1s and C 1s spectra were recorded at emission angle of 40° with respect to surface normal using photon energies of 170 and 440 eV, respectively. Al Kα (hυ = 1486.6 eV) was chosen for the measurements of O 1s and Cu 2p features. The angle-dependent SRPES data were obtained by rotating the sample in the theta direction of the manipulator. The binding energies in all spectra were calibrated with respect to the Au 4f7/2 binding energy (84.0 eV) from a clean Au foil which was attached below the sample. The spectrum fitting was performed using Casa XPS software by 6


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Voigt functions convoluted with Gaussian (80%) and Lorentzian (20%) lineshapes after subtracting a Shirley background.

RESULTS AND DISCUSSION

Figure 1. Raman spectrum of graphene grown on a Cu foil obtained at a 488 nm excitation wavelength. The inset shows the magnified 2D band and its curve fitting with a lorentzian lineshape.

Raman spectroscopy has been widely used to evaluate the quality and identify the number of layers of graphene samples.40-44 We have examined the Raman spectra in different locations of the graphene sample grown on a Cu foil. As shown in Figure 1, a typical Raman spectrum from the graphene/Cu sample shows two intense features at ~1588 cm-1 (G band) and ~2704 cm-1 (2D band). The former can be attributed to the in-plane E2g mode, while the latter is caused by the second order of the zone-boundary phonons.40,41,44 The Raman signatures indicate that the graphene is single-layer: (a) the intensity ratio of the bands G/2D is smaller than 0.3, and (b) the 2D band with a FWHM of ~34 cm-1 can be fitted with a single lorentzian lineshape (the inset in Figure 1).40,41,44 In addition, the intensity of the D band at ~1350 cm-1 which originates from the defect features in sp2 carbon is very weak and almost undetectable above the measurement background, indicating the high quality of the monolayer graphene.44 Overall, the Raman data 7



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indicates that the graphene layers on Cu are single-layer and high-quality, which means that the influence of vacancy defects on the Li intercalation at the graphene/Cu interface can be neglected in our case.

Figure 2. (a) SRPES spectra of C 1s collected at 440 eV photon energy for graphene/Cu before and after 3Å Li doposition and subsequent annealing to 300 ºC. (b) Evolution of the relative intensity ratio of I(C 1s)/I(Li 1s) for Li/graphene/Cu before and after annealing as a function of emission angle θ (θ is referred to the surface normal). For convenience of comparison, the normalized values of I(θ)/I(75°) are plotted, where I(θ) is the peak intensity ratio at angle θ.

The C 1s SRPES spectra acquired using a photon energy of 440 eV for graphene/Cu before and after 3Å Li deposition, and subsequent annealing to 300 ºC for 10 min are shown in Figure 2(a). The C 1s spectrum of graphene/Cu shows a main peak located at 284.7 eV. After Li deposition, the C 1s peak shifts to higher binding energy of 285.3 eV. Similar phenomenon was also observed for the Li-graphite compound previously and the authors ascribed the peak shift to the filling of graphite π bands by electrons transferred from Li.21,45 Therefore, similar to Li-graphite, there should also be charge transfer from Li to graphene, which induces the C 1s peak shift upon Li deposition. In addition, the deposition of Li on top of the graphene/Cu surface leads to the damping of the C 1s signal as shown in Figure 2(a). In contrast, after annealing the as-deposited Li/graphene/Cu sample to 300 ºC for 10 min, the intensity of C 1s peak is restored 8


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and almost coincides with that of clean graphene/Cu. There are several possibilities accounting for the recovery of C 1s peak intensity after the heat treatment: (1) the desorption of Li atoms during the annealing process; (2) the diffusion of Li atoms into bulk Cu; (3) the formation of large Li islands on top of the graphene layer, i.e., sintering; and (4) the intercalation of Li atoms at the graphene/Cu interface. Because the peak position of the C 1s spectrum of the post-annealed Li/graphene/Cu (285.1 eV) is higher than that of graphene/Cu (284.7 eV), there should still be charge transfer from Li to graphene.21,45 Moreover, due to the fact that the Li 1s SRPES signal can still be detected after the heat treatment (see below), the total desorption of Li atoms during the annealing process can be ruled out. However, it is possible that Li atoms have partially desorbed after annealing, because the C 1s feature shifts towards the lower binding energy by 0.2 eV as compared with that of the as-deposited Li/graphene/Cu. Ar-ion sputtering depth profile experiments show that there is no Li signal in bulk Cu can be detected after surface and interface Li atoms have been removed. This observation excludes the possibility that the Li atoms diffuse into bulk Cu after annealing. The sintering of Li atoms on graphene surface is also unlikely because the energy of Li-Li bond (0.79 eV) is smaller than that of Li-C bond (1.59 eV) in Li-graphene system.30,46 In addition, the Cu 2p XPS results (see below) further rule out the possibility of Li sintering during the annealing process. Therefore, the recovery of C 1s peak intensity is most probably due to the intercalation of Li atoms at the graphene/Cu interface. To further determine the possibility of Li intercalation, angle-dependent SRPES measurements were carried out for the as-deposited and post-annealed Li/graphene/Cu sample. Figure 2(b) demonstrates the evolution of the peak intensity ratios of C 1s to Li 1s [I(C 1s)/I(Li 1s)] 9



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for Li/graphene/Cu before and after annealing as a function of the emission angle θ referred to the surface normal. For convenience of comparison, the normalized values of I(θ)/I(75°) are plotted, where I(θ) is the intensity ratio at angle θ. As shown in the figure, after annealing, the relative intensity of I(C 1s)/I(Li 1s) increases sharply when the emission angle gets closer to the grazing angle, which is in contrary to that of Li/graphene/Cu before annealing. This observation clearly indicates that the Li atoms have intercalated into the graphene/Cu interface after heat treatment, as illustrated in Figure 3. Because there are very few vacancy defects in the graphene layer, the Li atoms may mainly intercalate into the graphene/Cu interface through the grain boundaries or wrinkles of graphene layer during the annealing process.7,13 However, it should be mentioned that the relaxation of physisorption stress between graphene and Cu7 may also enable Li to percolate to the graphene/Cu interface. Therefore, further theoretical calculations are needed to confirm the nature of intercalation process. On the other hand, the grain boundaries or wrinkles of graphene on Cu may also supply the adsorption sites for Li because the calculation results indicate that Li cannot reside on the surface of ideal graphene.26,31,32

Figure 3. The schematic illustration of the relative position for Li atoms and graphene. G represents the monolayer graphene (black circles). The Li atoms (blue circles) intercalate into the graphene/Cu interface after heat treatment. The relative intensity of I(C 1s)/I(Li 1s) for graphene/Li/Cu will increase with increasing the emission angle θ due to the limited mean free path of the photoelectrons emitted from the Li 1s level.

Our experimental results clearly indicate that the Li atoms can only intercalate into the

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graphene/Cu interface when annealing the as-deposited Li/graphene/Cu system to 300 °C for 10 min. In contrast, Virojanadara et al. have found that after deposition of Li on the surface of graphene sample prepared epitaxially on SiC(0001) at room temperature, the Li atoms can penetrate through the graphene as well as the carbon buffer layer and intercalate at the interface between SiC and the buffer layer.35 In reality, the epitaxial graphene on SiC(0001) always contains a few vacancy defects because of the effect of synthetic conditions.47 These defect states can assist the penetration of Li through the basal plane of graphene at room temperature.26,29,31,32 However, due to the high quality of the present graphene sample, the lack of enough vacancy defects restricts the intercalation of Li at graphene/Cu interface at room temperature, in good agreement with the previous calculation results.26,29,31,32 The intercalation scenario is also applicable to the explanation of the valence band spectra of the investigated systems. In Figure 4, the valence band spectra collected at a photon energy of 170 eV for Cu foil (spectrum 1), graphene/Cu (spectrum 2), Li/graphene/Cu (spectrum 3) and graphene/Li/Cu (spectrum 4) are demonstrated. Here all the spectra have been normalized by the strongest peak located at 2.8 eV in each spectrum, so only relative intensities within each spectrum can be directly compared. For the spectrum of Cu foil, the prominent features at 2.8 and 3.6 eV can be attributed to the Cu 3d states.48 After the growth of graphene on the Cu surface, a new state at 4.7 eV appears which is assigned to the σ state of graphene.12,49 The π state of graphene is hardly visible under the chosen photon energy, but it is known to locate at ~9.5 eV.12,50 After the deposition of Li on graphene surface, the σ feature gets obscure due to the presence of Li atoms on the top of graphene layer. Annealing the Li/graphene/Cu sample leads to the restoration of σ feature due to the intercalation of Li atoms at the graphene/Cu interface. In addition, the 11



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growth of graphene and adsorption of Li as well as the subsequent annealing process can also induce modification of the Cu 3d states as displayed in Figure 4, which is similar to results reported previously.12,35,50,51

Figure 4. Valence band spectra collected at a photon energy of 170 eV for Cu foil, graphene/Cu, Li/graphene/Cu and graphene/Li/Cu. The valence band spectra have been normalized by the strongest peak at 2.8 eV in each spectrum.

To further confirm the intercalation of Li atoms at the graphene/Cu interface for Li/graphene/Cu after heat treatment, O2 adsorption experiments and O 1s XPS measurements were performed (Figure 5). As seen in Figure 5, no O signal can be detected for Li/graphene/Cu (spectrum a1) and graphene/Li/Cu (spectrum b1) before the exposure of O2. In contrast, after the Li/graphene/Cu surface exposed to 600 L O2 at room temperature, two well-resolved peaks (spectrum a2) can be observed. The peak at 529.5 eV can be ascribed to Li2O,52,53 while the feature at 532.4 eV is identified as Li peroxide, i.e., Li2O2.52-54 The observation of strong O 1s features for Li2O and Li2O2 clearly indicates the heavy oxidation of Li atoms in Li/graphene/Cu system after exposure to O2. However, under the same oxidation condition, the intensity of O 1s signal from the annealed 12


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Li/graphene/Cu sample (spectrum b2) is much weaker as compared with that of Li/graphene/Cu (spectrum a2). This observation suggests that most of the Li atoms are hidden for exposing to oxygen and they must have intercalated into the graphene/Cu interface after annealing, leading to the formation of an oxidation–resistive system: graphene/Li/Cu. The passivation of Li atoms in graphene/Li/Cu to the oxidation environment also indicates that no O2 can penetrate through the graphene layer to react with the interfacial Li atoms.12,50 The weak O 1s signal of graphene/Li/Cu after exposure to O2 may have two possible origins: (1) the adsorbed O2 on graphene surface;12 and (2) the oxidation of Li atoms located at the bare Cu surface which is not covered by graphene, because this part of Li atoms could not intercalate into the graphene/Cu interface during the annealing process.13

Figure 5. O 1s XPS spectra of Li/graphene/Cu (a) and graphene/Li/Cu (b) before and after exposed to 600 L O2 at room temperature.

The Li 1s spectra recorded with a photon energy of 170 eV accompanied with their peak fittings for Li/graphene/Cu and annealed Li/graphene/Cu (i.e., graphene/Li/Cu) before and after exposed to 600 L O2 at room temperature are shown in Figure 6. The Li 1s spectra before the 13



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exposure of O2 can be fitted with three components, which are labeled as L1, L2 and L3, respectively. In contrast, the spectra of Li 1s after exposed to O2 are fitted with four components (L1, L2, L3 and L4) due to the oxidation of Li. In the fitting procedure, the values of full width at half maximum (FWHM) for L1, L2 and L3 components were constrained to a maximum of 1.6 eV. The fitted results show that the FWHM of L1 component is 1.0 ± 0.2 eV, while those of L2 and L3 components are 1.5 ± 0.1 eV. The different FWHMs for the Li species may be caused by their different chemical environments. For the L4 component, the FWHM is ~1.7 eV (see below for detail).

Figure 6. SRPES spectra of Li 1s collected at 170 eV photon energy as well as their peak fittings for Li/graphene/Cu and graphene/Li/Cu before and after exposed to 600 L O2 at room temperature. The black open circles are the experimental data. The red lines indicate the sum of individual components. The Li 1s spectra before the exposure of O2 can be fitted with three components, while the spectra after exposed to O2 are fitted with four components due to the oxidation of Li. The L1, L2, L3 and L4 represent different Li species as described in the

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text.

From the fits to Li 1s spectra we can see that the L1, L2 and L3 components are well resolved in all the Li 1s spectra. The L1 component is suggested to correspond to the Li atoms directly located on the bare Cu surface uncovered by graphene as we have stated above.55,56 The L2 component is assigned to the metallic Li,21,53 while the L3 component is ascribed to the Li atoms interacting with the graphene layer.35,45 However, compared with the spectrum of Li/graphene/Cu (spectrum 1), the intensities of L2 and L3 components are reduced and almost no intensity change can be observed for the L1 component in the spectrum of graphene/Li/Cu (spectrum 2). This is because the Li atoms of L2 and L3 components have intercalated into the graphene/Cu interface during the annealing process and the graphene layer staying on top of them can attenuate their peak intensities.35 Because the Li atoms ascribed to the L1 component do not penetrate into the graphene/Cu interface under the heat treatment condition, there should be no intensity change for the L1 component before and after annealing, in good agreement with our experimental results. After exposing the Li/graphene/Cu sample to 600 L O2, the intensities of L2 and L3 components in the Li 1s spectrum (spectrum 3) decrease significantly. In contrast, a broad feature L4 appears at 55.3 eV, which can be ascribed to the Li in Li2O and Li2O2.53,54 These results indicate that the Li atoms of L2 and L3 components have been seriously oxidized after such oxygen treatment, which is consistent with the O 1s results discussed above. Here, it should be noted that the Li 1s binding energy difference of Li2O and Li2O2 are very small53,54 and due to the relative low spectral resolution in the present case, we fitted the oxidized Li species with only one broad peak (FWHM ~1.7 eV). 15



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In contrast, the Li 1s spectrum of graphene/Li/Cu under the same oxidation condition (spectrum 4) is totally different from that of Li/graphene/Cu (spectrum 3), and comparable with the one for graphene/Li/Cu before O2 exposure (spectrum 2), indicative of the weak or no oxidation of intercalated Li atoms after the exposure of O2. However, some changes can still be clearly distinguished for the Li 1s spectra of graphene/Li/Cu before and after O2 exposure when comparing the spectra 2 and 4: (a) the intensity of L1 component reduces considerably and a weak L4 component appears after the O2 exposure, which are caused by the oxidation of Li atoms without the protection of graphene as discussed above; and (b) the relative intensity of L2 and L3 [I(L2)/I(L3)] increases after the exposure of O2. We are still not clear about the exact origin of this phenomenon and further theoretical work is needed to address this question. Here, we tentatively attribute it to the weakened interaction between graphene and Li induced by the adsorption of O2 on graphene surface, which may result in the increment of metallic Li (L2 component) intensity and thus the increase of I(L2)/I(L3).12,13 Overall, the Li 1s results are in good agreement with the C 1s and O 1s results, further confirming the intercalation of Li at the graphene/Cu interface after the annealing process. Figure 7 presents a set of Cu 2p XPS spectra from graphene/Cu (spectrum 1), Li/graphene/Cu (spectrum 2), graphene/Li/Cu (spectrum 3), and the latter two systems after the exposure of O2 under the conditions mentioned above (spectra 4 and 5). As seen, the deposition of Li on graphene surface leads to a reduction of the Cu 2p peak intensity immediately. However, no intensity changes can be observed for Li/graphene/Cu before and after the heat treatment, which supports the hypothesis of Li intercalation into the graphene/Cu interface to form graphene/Li/Cu structure after annealing. Moreover, we can rule out the sintering process of Li atoms occurred on 16


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top of graphene layer during annealing because otherwise the increase of the Cu 2p intensity should be observed. During the whole sample treatment process (both annealing and O2 exposure), no peak shift or appearance of new peaks has been found. This indicates that the interaction between Li and O2 with Cu in our case should be very weak55 and graphene/Cu can be a good candidate for the fabrication of oxidation-resistivity Li-intercalated graphene system.

Figure 7. Cu 2p XPS spectra of Li/graphene/Cu and graphene/Li/Cu, before and after exposed to 600 L O2 at room temperature. For comparison, the Cu 2p spectrum of graphene/Cu is also shown as the reference.

CONCLUSIONS In conclusion, we have studied the adsorption of Li atoms on the monolayer graphene sample prepared on the Cu foil and the intercalation of Li atoms into the graphene/Cu interface. Our results indicate that the deposition of Li on the graphene surface leads to charge transfer from Li to graphene overlayer. The Li atoms can intercalate into the graphene/Cu interface when annealing the as-deposited Li/graphene/Cu system at 300 ºC. Due to the protection of graphene layer on-top, exposure of the formed graphene/Li/Cu system to O2 environment does not lead to the oxidation of the intercalated Li atoms. The successful in situ fabrication of Li-intercalated graphene/metal

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compound can facilitate the development of nanoscale Li-ion batteries based on epitaxially grown graphene in the future.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No.21173200), National Basic Research Program of China (2010CB923302, 2013CB834605), and the Specialized Research Fund for the Doctoral Program of Higher Education of Ministry of Education (Grant No. 20113402110029). L. Zhang thanks the financial support from the Scholarship Award for Excellent Doctoral Student Granted by Ministry of Education of China.

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