Letter pubs.acs.org/JPCL
Substrate-Sensitive Graphene Oxidation Zhuhua Zhang,* Jun Yin, Xiaofei Liu, Jidong Li, Jiahuan Zhang, and Wanlin Guo* State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, and Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China S Supporting Information *
ABSTRACT: The inertness of graphene toward reaction with ambient molecules is essential for realizing durable devices with stable performance. Many device applications require graphene to contact with substrates, but whose impact on the chemical property of graphene has been largely overlooked. Here, we combine comprehensive first-principles analyses with experiments to show that graphene oxidation is highly sensitive to substrates. Graphene remains inert on SiO2 and hexagonal boron nitride but becomes increasingly weak against oxidation on metal substrates because of enhanced charge transfer and chemical interaction between them. In particular, Ni and Co substrates lead to spontaneous oxidation of graphene, while a Cu substrate maximally promotes the oxygen diffusion on graphene, with an estimated diffusivity 13 orders of magnitude higher than that on freestanding graphene. Bilayer graphene is revealed to have high oxidation resistance independent of substrate and thus is a better choice for high-performance nanoelectronics. Our findings should be extendable to a wide spectrum of chemical functionalizations of two-dimensional materials mediated by substrates.
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pristine graphene is highly inert. Definitely, pristine graphene is more desirable for future nanoelectronics because many novel properties can survive only in the absence of defects. Recent rapid advancement in techniques for growing singe-crystal graphene leads us increasingly close to this goal.31−33 These new progresses underscore the importance of exploring the oxidation of pristine graphene on technically relevant substrates. Herein, on the basis of comprehensive first-principles analyses in conjunction with experiments, we show pristine graphene itself can be chemically weakened by substrates, a step further over prior contributions based on defect-dominated oxidation. We find graphene is inert against oxidation on SiO2, Au, and Ag but becomes increasingly weak against oxidation on Al and Cu and even can be spontaneously oxidized on Co and Ni. The dissociated epoxy groups on Cu-supported graphene show a mobility 13 orders of magnitude higher than that on freestanding graphene at room temperature. The discovery owes its origin to a fundamental understanding of charge transfer and electronic hybridization between graphene and substrates. Interestingly, bilayer graphene retains high chemical inertness, independent of substrate, thus being a better choice for fabricating chemically robust devices and coatings. Our findings suggest a new route to efficiently directing graphene
raphene is currently being extensively studied in an effort to understand its exceptional physical and chemical properties and to explore its immense potential in a number of applications.1−3 A topic of great interest is the oxidation of graphene. Oxidation not only alters the electronic property of graphene4,5 but also can be exploited to shape graphene into well-defined pieces with desired performance.6−17 Moreover, air oxidation dominates the degradation of graphene devices18 as well as the chemical decomposition of graphene under harsh conditions.19−22 However, for most device applications, graphene is desired to be inert to ambient molecules, in particular to ubiquitous O2. Fundamental to understanding the oxidation of graphene is the underlying oxidation dynamics, especially the kinetic barriers for essential oxidation processes, including the transitions between flying and chemisorbed O2 as well as the diffusion of epoxy groups. In reality, the inclusion of graphene in electronic devices requires integration with substrates, such as dielectric layers, gates, and metal contacts. Most substrates will change the intrinsic properties of graphene because of interfacial interaction,23−28 which may pose qualitatively different oxidative performance. While the oxidation of graphene on substrates has received growing attention, the results are either limited to physisorption of O24,5 or based on polycrystalline graphene whose oxidation is dominated by the grain boundaries thereof.19,20,29 In contrast, earlier experiments on graphite found that the defect-free region of basal plane can withstand oxidation at temperatures up to ∼1150 K,30 indicating that © 2016 American Chemical Society
Received: January 10, 2016 Accepted: February 17, 2016 Published: February 17, 2016 867
DOI: 10.1021/acs.jpclett.6b00062 J. Phys. Chem. Lett. 2016, 7, 867−873
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the other is the slightly less stable cycloaddition configuration (Figure 1a, right). The O2 in both the configurations favors the spin-singlet state. We also check the effect of pentagon− heptagon defects (basic unit of grain boundaries) on the adsorption of O2 and do not find qualitative change in the physisorption and chemisorption configurations (Figure S1). We focus on the oxidation dynamics of pristine graphene in the following. Starting from the physisorption configurations, we have searched for the transition states for the physisorption-tochemisorption processes and determined the energy barrier, as shown in Figure 1a. The energy barrier is 2.0 eV for O2 on a fgr, consistent with previous calculations and with experimental results that pristine graphene is inert toward oxidation.25 When graphene is placed on metal substrates, the corresponding barrier is remarkably reduced, from 1.57 eV on Au to 1.34 eV on Al and to 0.72 eV on Cu, even though graphene is weakly bound on these substrates. For Ni and Co substrates on which graphene is chemically adsorbed,23 the O2 can spontaneously form a C−O bond with graphene and then needs to overcome a barrier of merely 0.27 and 0.45 eV, respectively, to further reach the cycloaddition structure. A recent experiment found single-layer graphene can provide substantial corrosion reduction to the Cu substrate but not to the Ni substrate,37 supporting our calculations to some extent. In contrast, graphene coupled to substrates with van der Waals interaction remains inert to oxygen, as exemplified by the corresponding energy barrier of over 1.9 eV for O2 on BN- and SiO2-gr. These results prove that the oxidation-resistance of graphene sensitively depends on substrates and highlight the necessity of choosing suitable substrates or electrodes to keep graphene chemically robust. To gain further insight into the substrate-dependent oxidation dynamics, we examine the electronic structures of the physisorption and transition-state (TS) configurations of O2 on graphene. Because BN- and SiO2-gr have oxidation dynamics similar to that of f-gr, we mainly discuss the metalsupported graphene and select the Cu- and Au-gr as the model systems, together with the f-gr for comparison. In the physisorption configuration of O2 on f-gr, the density of states (DOS) projected onto the O2 show two spin-polarized peaks, with the spin-up state being fully occupied and spin-down state being empty. The two states are split from the O ppπ* states and have antibonding character (Figure 2, left). For O2 on the Cu-gr, the antibonding ppπ* state in the spin-up channel becomes partially occupied because of the charge transfer from Cu (Figure 2, middle). Consequently, the O2 is activated, as evidenced by the stretched O−O bond, which raises its adsorption energy. Meanwhile, we observe charge accumulation between the top-layer Cu atoms and the C atoms directly atop them, indicating the formation of weak chemical bonds. The C−Cu bonds electronically perturb the neighboring C atoms by breaking the π bonds and leaving weak dangling bonds (Figure 2, left insets), which benefits the formation of a C−O bond in the TS state. Our electronic structure analysis supports this scenario. In the TS structure, the C−O bond results in a bonding state near the Fermi level, a combination of O 2pz and C 2pz from the atoms adjacent to the O-attached one. This bonding state (see arrows in Figure 2) is fully occupied for O2 on Cu-gr, but is partially occupied in the case of f-gr. Full population of the bonding state strengthens the C−O bond (its bond length is 1.48 Å on Cu-gr, compared to 1.50 Å on f-gr) and therefore enhances the stability of the TS structure. A clear
oxidation by purposefully engineering substrates and are crucial for future scientific studies and industrial applications of graphene. Our model systems consist of an oxygen molecule (O2) and a single-layer or bilayer graphene on a substrate, which varies from metallic Au, Ag, Al, Cu, Co, and Ni to insulating BN and SiO2, all widely used either in chemical synthesis or in device fabrication. We consider the low-density adsorption of O2 on graphene, in which the molecule axis of O2 tends to be parallel to the C−C bond at low temperature.34 For convenience, we use the notation of M-gr and f-gr to represent M-supported and freestanding graphene, respectively. The optimal adsorption distance of O2 on the M-gr is obtained by examining the system total energy as a function of the distance between O2 and graphene; then we scan different adsorption sites and compare the energies to obtain the optimal configurations. The results can be divided into two classes. For graphene on BN, SiO2, Au, Al, Ag, and Cu, the O2 has stable physisorption and metastable chemisorption configurations, as shown by insets in Figure 11a,
Figure 1. Oxidation dynamics of graphene on substrates. (a) Calculated adsorption energy along the minimum-energy paths between the physorption and chemisorption states of an O2 on freestanding (left), Cu-supported (middle), and Ni-supported (right) single-layered graphene. The inserts show the side views of the physisorption (Phys), chemisorption (Chem), and transition-state (TS) configurations. On Ni, both the initial and final states are in chemisorption states (see text). (b) Calculated energy barriers, E*, between the physisorption and TS configuration of an O2 on graphene supported by different substrates, as well as work functions of singlelayered graphene on substrates. Gray, red, wine, and navy balls represent C, O, Cu, and Ni, respectively.
middle. In the physisorption, the O2 is located at 2.6−2.8 Å above the graphene and remains in a spin-triplet state while in the chemisorption the O2 is a cycloaddition configuration with a spin-singlet state. These adsorption features are similar to those for O2 on f-gr35 and carbon nanotubes.36 In contrast, on Co- and Ni-gr, the O2 has two chemisorption configurations: the first has only one C−O bond connecting to graphene, and 868
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gr the barrier is reduced to 0.66 and 0.45 eV, respectively. There are two possible paths for the dissociation: path I locates two epoxy groups onto two adjacent hexagons, while path II makes the two groups locate on the same hexagon, as shown by the insets in Figure 3. Both paths result in the same dissociation
Figure 3. Dissociation of chemisorbed O2 on graphene. Calculated minimum-energy paths for the dissociation of chemisorbed O2 into two epoxy groups on (left) f-, (middle) Cu-, and (right) Ni-gr. The dashed line is along path I, while the solid line is along path II. Inset: atomic configurations of initial, transition, intermediate, and final states along paths I and II.
Figure 2. Density of states projected onto the graphene (gray shaded) and oxygen (red) atoms. Panels a and b show the results of the (a) initial and (b) transition-state configurations for an O2 on freestanding, Cu- and Ni-supported single-layered graphene. The insets illustrate the O-crossed slices of charge redistribution between the substrate and graphene plus O2. Dark and light colors represent charge depletion and accumulation regions, respectively. The scale of color bar is in units of e/Å3. Small arrows denote the bonding state derived from the C−O bond in the transition state.
barrier yet different reaction energies (defined as the energy difference between the dissociated and chemisorbed states) for all the substrates. The reaction energy in path II is −0.37 eV more negative than that of path I on the f-gr but becomes less negative by 0.48 and 0.27 eV on the Cu- and Ni-gr, respectively. Moreover, on the Cu-gr, the two epoxy groups migrate simultaneously in path I but via a one-by-one process in path II, with an intermediate state. During the dissociation, all the transition-state structures have the migrating O atom close to the top site on top of a C atom in graphene, connected by one C−O bond (see the top inset in Figure 3). The presence of a metal substrate strengthens the C−O bond and in turn enhances the stability of TS structures. For example, the C−O bond in the TS structure on Cu-gr is 0.02 Å shorter than the counterpart on f-gr. Thus, the mechanism of the metalpromoted oxygen dissociation is similar to that of the aforementioned O2 attachment onto graphene on substrates. Then the dissociated epoxy groups are diffusive on graphene40 and proceed to dominate the oxidation process. Next, we explore the diffusion of an isolated O adatom on graphene, which shows more interesting substrate dependence. For an O adatom on f-Gr, the equilibrium state is an epoxy (E) configuration while the TS is an on-top (T) configuration with the oxygen atom sitting on a carbon atom (top insets in Figure 4a). The diffusion barrier, obtained as the energy difference between the E and T configurations, is 1.05 eV, as shown by the minimum-energy path (MEP) in Figure 4a. Similar oxygen configurations and diffusion are found on the BN-, Au-, and Aggr, except that the diffusion barrier drops to 1.02, 0.86, and 0.67 eV, respectively, as shown in Figure 4a. Because these substrates have little chemical interaction with graphene, the reduced diffusion barrier is attributed to the charge-doping of graphene by substrates, consistent with a previous prediction that the O diffusion on graphene can be controlled by carrier doping.40 The situation is very different for the O adatom on Al- and Cu-gr, on which the T configuration becomes a metastable state with two nonequivalent T 1 and T 2 configurations. In T1, the O-bonded C atom is directly on top of a surface metal atom, but in T2, it is on the face-centered cubic site, as illustrated by the insets in Figure 4a. Thus, we perform MEP calculations for an oxygen adatom diffusing along
picture thus emerges for the reduced barrier: the O2 activation in the physisorption state and the bonding increase in the TS. The degree of O2 activation mainly depends on the work function of the substrate−graphene system, while that of bonding increase depends on the chemical interaction between graphene and substrate. On Au, the Au-gr has less charge transfer to O2 because of its high work function (Figure 1b), and the bonding interaction between graphene and Au is weaker because Au has a deeper d-band center than Cu (Figure S2). Therefore, the barrier for chemisorbing O2 onto Au-gr becomes much higher. In contrary, Ni has a shallow d-band center to enable covalent interaction with graphene, resulting in a very low work function of the Ni-gr (Figure 1b) and strong dangling bonds on graphene (Figure 2, right). This explains why the Ni-gr has the lowest barrier for chemisorbing an O2. A recent experiment showed that intercalating Pb at the graphene/Ru interfaces enhances the inertness of graphene toward oxygen attchment,38 which fits our theory as we find that Pb electronically interacts with graphene more weakly than Ru does. Therefore, the substrate-dependent oxidation dynamics stems from a coupled effect of the charge transfer to O2 and the chemical hybridization between graphene and substrate. The interfacial electronic hybridization is more significant in determining the oxidation barrier, as evidenced by the fact that the Cu-gr is easier to be oxidized than Ag- and Al-gr, although the latter two systems show stronger charge transfer with the physisorbed O2 (see Figure 1b for work functions and Figure S2 for charge redistribution). These results suggest the possibility of using d-band center of metal as a viable descriptor for screening electrodes in fabricating highperformance graphene-based devices. The chemisorbed O2 with cycloaddition configurations tends to dissociate into two isolated O adatoms (i.e., epoxy groups) on graphene,39 which can be facilitated by metals as well. On a f-gr, the dissociation of chemisorbed O2 into two epoxy groups needs to overcome a barrier of 0.96 eV, whereas on Cu- and Ni869
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Then, the estimated diffusion rate at 300 K is 1.8 × 10−20 cm2/ s. If we assume the same attempt frequency, the diffusion rate at the same temperature is 1.91 × 10−7 cm2/s on the Cu-gr, which is 7 and 13 orders of magnitude higher than those on the Ag-gr and the f-gr, respectively. The substrate-sensitive oxygen diffusion suggests a new way to control the oxidation of graphene. For example, by lithographically patterning the Cu substrate, the graphene sections in direct contact with Cu will preferentially chemisorb O2 and the high mobility of oxygen enables these sections to relax into energetically favorable patterns. Spatially controlled oxidation of graphene can thus be achieved for desirable electronic applications. Recent experiments demonstrated that graphene can serve as an oxidation barrier for metals, yet limited to a short time.20,21 To overcome this issue, we propose using bilayer graphene as protection coating for metals. We predict that, independent of the substrates, a bilayer graphene is essentially identical to a freestanding graphene in terms of the energetics and dynamics of oxidation because the top layer is chemically decoupled from the bottom layer, even though the interlayer charge transfer slightly reduces the energy barrier (Figure S3). Apparently, thicker graphene will be more robust against oxygen chemisorption, likely comparable to that of hexagonal boron nitride sheets.42,43 Having established the substrate dependence of the oxidation dynamics of graphene, we proceed by testing it on two representative substrates, SiO2 and Cu. The monolayer graphene on Cu was directly grown by chemical vapor deposition method while that on SiO2 was transferred via a PMMA-mediated process.44 Panels a and b of Figure 5 present the optical images of monolayer graphene on SiO2 and Cu, respectively. The graphene samples are predominantly monolayer and have low defect concentrations, as proven by Raman characterizations (Figure 5c). To evaluate the oxidation
Figure 4. Substrate-dependent diffusion of oxygen atoms on graphene. (a) Calculated minimum-energy path for oxygen adatom diffusion on single-layered graphene supported by different substrates. Left insets: the epoxy (E) and on-top (T) configurations of O adatom on graphene. Right inset: the oxygen diffusion path relative to the beginning carbon network of T2 configuration on Ni (only the toplayer Ni atoms are shown), in which the E, T1, and T2 are marked. (b) The kinetic barriers for oxygen diffusion on graphene, the energy difference between the E and T2 configurations, and that between the T2 and T1 configurations as functions of substrates. Gray, red, and navy balls represent C, O, and Ni, respectively.
an extended path E-T1-E-T2-E (Figure 4a, right). Surprisingly, the highest diffusion barrier is only 0.27 eV on the Cu-gr and 0.33 eV on the Al-gr. These striking disparities of the adsorption behavior and diffusion barrier of oxygen adatoms result from the difference in the chemical interaction between graphene and substrate. As the chemical interaction gets stronger, the T1 and T2 configurations have lower energies while the E configuration becomes less stable (Figure 4b). On Cu-gr, the chemical interaction is so subtle that the T1 and E configurations have almost the same energy, which leads to the lowest diffusion barrier. When the chemical interaction is further enhanced, as in the Co- and Ni-gr, the E configuration becomes unstable while the T1 and T2 configurations are further stabilized. In particular, the T2 configuration lies at the equilibrium state as the oxygen just saturates the dangling bond of the underneath carbon (the neighboring carbons strongly interact with Ni). Now, the E configuration is less stable than the T2 configuration by 0.35 eV on the Co-gr and by 0.4 eV on the Ni-gr, rendering a high surface corrugation for oxygen diffusion (Figure 4b). Accordingly, the calculated MEP for the T1 → T2 diffusion path displays an energy barrier of 0.56 eV on the Co-gr and 0.63 eV on the Ni-gr. The diffusion barrier directly determines the oxygen diffusion rate, which can be evaluated by D = νd02 exp(−ε/kBT),41 where ε is the diffusion barrier, ν the attempt frequency, d0 an elementary length for diffusion, kB the Boltzmann constant, and T the absolute temperature. For oxygen on f-gr, d0 = 1.22 Å and ν is calculated as 30 THz based on harmonic approximation.
Figure 5. Experimental characterization of oxidation of graphene on substrates. (a) Optical image of a graphene sample on Cu, which is exposed in dry air at 270 °C for (left) 0 min, (middle) 3 min, and (right) 6 min. (b) The same as panel a except for a sample on SiO2. (c) Raman spectra of the graphene samples on (left) Cu and (right) SiO2 exposed in dry air for 0, 3, and 6 min, respectively. 870
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graphene is expected to bring quantitative agreement with our theory. In summary, our comprehensive first-principles calculations have shown that the ease of oxygen chemisorption of graphene critically relies on what substrates or electrodes are used. In particular, we show that the Ni substrate maximally facilitates the chemisorption of O2 on graphene, while Al and Cu substrates result in exceptionally high diffusivity for oxygen adatoms on graphene. Our theory is qualitatively confirmed by experimental characterizations of graphene oxidation on typical substrates. Notably, bilayer graphene can retain the intrinsic chemical inertness of freestanding graphene independent of substrate and is thus well-suited for realizing long-term stable nanoelectronics and serving as oxidation-resistant coatings. This work not only reveals a previously unrealized role of the substrate in determining the chemical property of graphene but also opens a new method of manipulating graphene oxidation via substrate engineering.
resistance of the two samples, we expose SiO2- and Cu-gr in dry air at 270 °C and then record the evolution of graphene morphology. At 3 min, the optical images display that the Cu-gr starts to break up, likely along the defects (e.g., vacancies and grain boundaries); however, no appreciable change is observed in the SiO2-gr under the same conditions. These results suggest that the Cu-gr is chemically more vulnerable than the SiO2-gr, which agrees with our theoretical analysis even in the presence of defects (Figure S1). We have predicted that O2 can be attached onto Cu-gr more easily because of the relatively small barrier for the physisorption-to-chemisorption transition. Subsequently, the dissociated oxygen adatoms are extremely diffusive on Cu-gr and will be readily docked at graphene edges and defects. The enhanced flux of oxygen adatoms toward the defects promotes the oxidation therein. The accumulated oxygen adatoms at the graphene edges and grain boundaries generate CO2 molecules that are finally released into air. As such, the defects grow into large openings, resulting in disconnected graphene flakes. In contrast, the much higher barriers for both the attachment of O2 and the diffusion of oxygen adatoms on SiO2-gr impede the oxygen accumulation toward the defects, where the oxidation is hence largely retarded. Upon further exposure, the graphene fakes on Cu substrate shrink and diminish and the oxidation of the substrate assists the successive morphology change, as illustrated by the image recorded at 6 min, whereas the SiO2-gr remains intact. Our Xray photoelectron spectroscopy (XPS) analyses show that the oxygen content in Cu increases with oxidation time (Figure S4); however, before the graphene oxidation, the oxygen content in Cu is negligible even when the whole samples are exposed to air for 24 h. Therefore, graphene is oxidized first and then the Cu oxidation helps disrupt graphene because of oxidation-induced expansion of Cu lattice. This contrasts sharply with the SiO2-gr, where no measurable signal of oxidation can be obtained throughout the process. Distinguishing the impact of Cu oxidation on the graphene integrity from the oxidation of graphene itself will be helpful but is infeasible within our current experimental capability. The results based on morphology change are further supported by the Raman spectra measured at different exposure times (Figure 5c). For the Cu-gr, the ratio of D peak over 2D peak in the Raman spectra increases conspicuously with exposure time, which can be attributed to the opened edges and holes and in part to the residual oxygen in the graphene flakes.45 In sharp contrast, the Raman spectrum for the SiO2-gr is almost independent of time. The two samples exposed for 6 min also exhibit different electric transport behavior: the graphene transferred from Cu becomes electrically insulating while the graphene on SiO2 retains a perfectly linear I−V curve (Figure S5). This qualitative difference between graphene on Cu and SiO2 can be maintained with further increasing oxidation time even to 24 h. In addition, we find that both a bilayered graphene on Cu and a single-layered graphene on Au are highly inert, as no morphology change is optically visible and no change of D peak appears in the Raman spectra under the same oxidation conditions for 6 min (Figures S6 and S7), both in good agreement with our theory. Despite the polycrystalline nature of our graphene samples, their highly contrasting performances on different substrates are fully consistent with the substrate-dependent oxidation dynamics of graphene. Further experiments using large-scale single-crystal
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EXPERIMENTAL SECTION Theory. All the theoretical results are based on first-principles calculations within the framework of density-functional theory, as implemented in the Vienna Ab-initio Simulation Package,46,47 using the projector-augmented wave method for the core region, with a plane-wave kinetic energy cutoff of 400 eV. Given that the generalized gradient approximation (GGA), even with van der Waals density functional, overestimates the distance between graphene and metal substrates48,49 because of the repulsive nature of the revised PBE exchange functional, we employed local spin density approximation that proves to be reasonable for graphene-metal systems 23 and produces graphene−metal distances close to those in experiments.50 Test calculations on freestanding and Ni-supported graphene using standard GGA show no qualitative change in the results (Figure S8a). The structural models contain mono- or bilayer graphene on a substrate consisting of four atomic layers (except the h-BN sheet which is single-layered). The substrate lattices were adapted to match the graphene lattice within a 3 × 3 supercell for Cu, Ni, Co, and h-BN and a 4 × 4 supercell for Au, Al, Ag, and SiO2. The commensurability condition is applied by compressing and stretching the substrate lattices by 1−3%. A vacuum layer of 12 Å is used to isolate neighboring periodic images and a dipole correction is used. The positions of substrate atoms of the topmost three atomic layers plus all graphene and oxygen atoms are relaxed until the force on each atom is less than 0.01 eV/Å. The minimum-energy path was mapped out using the climbing image-nudged elastic-band method.51 We also fixed the lattice of the system to the substrate and recalculated the oxidation barrier, showing essentially the same results (Figure S8b). Experiment. The graphene samples were grown using lowpressure chemical vapor deposition on 25 μm thick copper foil (Alfa Aesar, item no. 13382), which was pretreated by electrochemical polishing before the growth. The as-grown graphene samples were then transferred onto a Si substrate with 300 nm SiO2 layer according to the method developed by Ruoff and co-workers.44 Raman spectra of graphene samples on SiO2/ Si substrate were conducted by a Renishaw Raman spectrometer with a 532 nm solid-state laser. The electric measurements were taken by a Keithley 2400 multimeter controlled by a LabView-based data acquisition system. X-ray photoelectron spectroscopy was performed using an Al Kα Xray source (XPS, PHI 5000 VersaProbe, UlVAC-PHI). The 871
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(10) Sun, T.; Fabris, S. Mechanisms for oxidative unzipping and cutting of graphene. Nano Lett. 2012, 12, 17−21. (11) Wu, Z. S.; Ren, W.; Gao, L.; Liu, B.; Zhao, J.; Cheng, H. M. Efficient synthesis of graphene nanoribbons sonochemically cut from graphene sheets. Nano Res. 2010, 3, 16−22. (12) Li, Z.; Zhang, W.; Luo, Y.; Yang, J.; Hou, J. G. How graphene is cut upon oxidation? J. Am. Chem. Soc. 2009, 131, 6320−6321. (13) Ma, L.; Wang, J.; Ding, F. Strain-induced orientation-selective cutting of graphene into graphene nanoribbons on oxidation. Angew. Chem., Int. Ed. 2012, 51, 1161−1164. (14) Fujii, S.; Enoki, T. Cutting of oxidized graphene into nanosized pieces. J. Am. Chem. Soc. 2010, 132, 10034−10041. (15) Lin, Y. M.; Jenkins, K. A.; Valdes-Garcia, A.; Small, J. P.; Farmer, D. B.; Avouris, P. Operation of graphene transistors at gigahertz frequencies. Nano Lett. 2009, 9, 422−426. (16) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Graphene oxide as a chemically tunable platform for optical applications. Nat. Chem. 2010, 2, 1015−1024. (17) Wang, L.; Lee, K.; Sun, Y. Y.; Lucking, M.; Chen, Z.; Zhao, J. J.; Zhang, S. B. Graphene oxide as an ideal substrate for hydrogen storage. ACS Nano 2009, 3, 2995−3000. (18) Su, C.; Acik, M.; Takai, K.; Lu, J.; Hao, S. J.; Zheng, Y.; Wu, P.; Bao, Q.; Enoki, T.; Chabal, Y. J.; Loh, K. P. Probing the catalytic activity of porous graphene oxide and the origin of this behavior. Nat. Commun. 2012, 3, 1298. (19) Carlsson, J. M.; Hanke, F.; Linic, S.; Scheffler, M. Two-step mechanism for low-temperature oxidation of vacancies in graphene. Phys. Rev. Lett. 2009, 102, 166104. (20) Schriver, M.; Regan, W.; Gannett, W. J.; Zaniewski, A. M.; Crommie, M. F.; Zettl, A. Graphene as a long-term metal oxidation barrier: worse than nothing. ACS Nano 2013, 7, 5763−5768. (21) Zhou, F.; Li, Z.; Shenoy, G. J.; Li, L.; Liu, H. Enhanced roomtemperature corrosion of copper in the presence of graphene. ACS Nano 2013, 7, 6939−6947. (22) Xu, Z.; Xue, K. Engineering graphene by oxidation: a firstprinciples study. Nanotechnology 2010, 21, 045704. (23) Giovannetti, G.; Khomyakov, P. A.; Brocks, G.; Karpan, V. M.; Van den Brink, J.; Kelly, P. J. Doping graphene with metal contacts. Phys. Rev. Lett. 2008, 101, 026803. (24) Kong, L.; Bjelkevig, C.; Gaddam, S.; Zhou, M.; Lee, Y. H.; Han, G. H.; Kelber, J. A. Graphene/substrate charge transfer characterized by inverse photoelectron spectroscopy. J. Phys. Chem. C 2010, 114, 21618−21624. (25) Dai, B.; Fu, L.; Zou, Z.; Wang, M.; Xu, H.; Wang, S.; Liu, Z. Rational design of a binary metal alloy for chemical vapour deposition growth of uniform single-layer graphene. Nat. Commun. 2011, 2, 522. (26) Gao, J.; Zhao, J.; Ding, F. Transition metal surface passivation induced graphene edge reconstruction. J. Am. Chem. Soc. 2012, 134, 6204−6209. (27) Zan, W.; Geng, W.; Liu, H. Electric-field and strain-tunable electronic properties of MoS 2/h-BN/graphene vertical heterostructures. Phys. Chem. Chem. Phys. 2016, 18, 3159−3164. (28) Zan, W.; Geng, W.; Liu, H. Influence of interface structures on the properties of molybdenum disulfide/graphene composites: A density functional theory study. J. Alloys Compd. 2015, 649, 961−967. (29) Chen, S.; Brown, L.; Levendorf, M.; Cai, W.; Ju, S. Y.; Edgeworth, J.; Ruoff, R. S. Oxidation resistance of graphene-coated Cu and Cu/Ni alloy. ACS Nano 2011, 5, 1321−1327. (30) Hahn, J. R. Kinetic study of graphite oxidation along two lattice directions. Carbon 2005, 43, 1506−1511. (31) Hao, Y.; Bharathi, M. S.; Wang, L.; Liu, Y.; Chen, H.; Nie, S.; Ruoff, R. S. The role of surface oxygen in the growth of large singlecrystal graphene on copper. Science 2013, 342, 720−723. (32) Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. Role of hydrogen in chemical vapor deposition growth of large single-crystal graphene. ACS Nano 2011, 5, 6069−6076. (33) Yan, Z.; Lin, J.; Peng, Z.; Sun, Z.; Zhu, Y.; Li, L.; Tour, J. M. Toward the synthesis of wafer-scale single-crystal graphene on copper foils. ACS Nano 2012, 6, 9110−9117.
data of C1s, Cu2p, and O1s peaks were collected to analyze the oxidation of the graphene and substrates.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00062. Test of model using different lattice parameter and exchange correlation potential, oxidation dynamics at graphene defects, charge transfer between graphene and substrates, oxidation dynamics of bilayer graphene on substrates, XPS characterization of oxidized graphene on substrates, I−V curves of oxidized graphene on SiO2 and Cu, experimental characterization of oxidation of bilayered graphene on Cu, and Raman spectra of the graphene samples on Au exposed in dry air (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (W.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the 973 Program (2013CB932604, 2012CB933403), National NSF (11172124, 51472117), MOE doctoral discipline Foundation (20113218120033), Research Funds for the Central Universities (NE2015104, NS2014006, NP2015203), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (0414K01), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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REFERENCES
(1) Novoselov, K. S.; Fal'ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A roadmap for graphene. Nature 2012, 490, 192−200. (2) Huang, X.; Yin, Z.; Wu, S.; Qi, X.; He, Q.; Zhang, Q.; Yan, Q.; Boey, F.; Zhang, H. Graphene-based materials: synthesis, characterization, properties, and applications. Small 2011, 7, 1876−1902. (3) Abergel, D. S. L.; Apalkov, V.; Berashevich, J.; Ziegler, K.; Chakraborty, T. Properties of graphene: a theoretical perspective. Adv. Phys. 2010, 59, 261−482. (4) Liu, L.; Ryu, S.; Tomasik, M. R.; Stolyarova, E.; Jung, N.; Hybertsen, M. S.; Steigerwald, M. L.; Brus, L. E.; Flynn, G. W. Graphene oxidation: thickness-dependent etching and strong chemical doping. Nano Lett. 2008, 8, 1965−1970. (5) Ryu, S.; Liu, L.; Berciaud, S.; Yu, Y. J.; Liu, H.; Kim, P.; Flynn, G. W.; Brus, L. E. Atmospheric oxygen binding and hole doping in deformed graphene on a SiO2 substrate. Nano Lett. 2010, 10, 4944− 4951. (6) Pang, S.; Tsao, H. N.; Feng, X.; Müllen, K. Patterned graphene electrodes from solution-processed graphite oxide films for organic field-effect transistors. Adv. Mater. 2009, 21, 3488−3491. (7) Yan, J. A.; Xian, L.; Chou, M. Y. Structural and electronic properties of oxidized graphene. Phys. Rev. Lett. 2009, 103, 086802. (8) Ci, L.; Xu, Z.; Wang, L.; Gao, W.; Ding, F.; Kelly, K. F.; Yakobson, B. I.; Ajayan, P. M. Controlled nanocutting of graphene. Nano Res. 2008, 1, 116−122. (9) Li, J. L.; Kudin, K. N.; McAllister, M. J.; Prud'homme, R. K.; Aksay, I. A.; Car, R. Oxygen-driven unzipping of graphitic materials. Phys. Rev. Lett. 2008, 96, 176101. 872
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The Journal of Physical Chemistry Letters (34) Stephens, P. W.; Heiney, P. A.; Birgeneau, R. J.; Horn, P. M.; Stoltenberg, J.; Vilches, O. E. X-ray and heat-capacity study of molecular oxygen adsorbed on graphite. Phys. Rev. Lett. 1980, 45, 1959. (35) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39, 228−240. (36) Zhou, B.; Guo, W.; Chen, C. Surface concavity−convexity sensitive oxidation dynamics of carbon nanotubes. J. Phys. Chem. C 2009, 113, 3569−3573. (37) Prasai, D.; Tuberquia, J. C.; Harl, R. R.; Jennings, G. K.; Bolotin, K. I. Graphene: corrosion-inhibiting coating. ACS Nano 2012, 6, 1102−1108. (38) Jin, L.; Fu, Q.; Mu, R.; Tan, D.; Bao, X. Pb intercalation underneath a graphene layer on Ru (0001) and its effect on graphene oxidation. Phys. Chem. Chem. Phys. 2011, 13, 16655−16660. (39) Chan, S. P.; Chen, G.; Gong, X. G.; Liu, Z. Oxidation of carbon nanotubes by singlet O2. Phys. Rev. Lett. 2003, 90, 086403. (40) Suarez, A. M.; Radovic, L. R.; Bar-Ziv, E.; Sofo, J. O. Gatevoltage control of oxygen diffusion on graphene. Phys. Rev. Lett. 2011, 106, 146802. (41) Meunier, V.; Kephart, J.; Roland, C.; Bernholc, J. Ab initio investigations of lithium diffusion in carbon nanotube systems. Phys. Rev. Lett. 2002, 88, 075506. (42) Liu, Z.; Gong, Y.; Zhou, W.; Ma, L.; Yu, J.; Idrobo, J. C.; Ajayan, P. M. Ultrathin high-temperature oxidation-resistant coatings of hexagonal boron nitride. Nat. Commun. 2013, 4, 2541. (43) Zhao, Y.; Wu, X.; Yang, J.; Zeng, X. C. Oxidation of a twodimensional hexagonal boron nitride monolayer: a first-principles study. Phys. Chem. Chem. Phys. 2012, 14, 5545−5550. (44) Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Ruoff, R. S. Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Lett. 2009, 9, 4359−4363. (45) Eckmann, A.; Felten, A.; Mishchenko, A.; Britnell, L.; Krupke, R.; Novoselov, K. S.; Casiraghi, C. Nano Lett. 2012, 12, 3925−3930. (46) Kresse, G.; Hafner, J. Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251. (47) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169. (48) Hamada, I.; Otani, M. Comparative van der Waals densityfunctional study of graphene on metal surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 153412. (49) Vanin, M.; Mortensen, J. J.; Kelkkanen, A. K.; Garcia-Lastra, J. M.; Thygesen, K. S.; Jacobsen, K. W. Graphene on metals: A van der Waals density functional study. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 081408. (50) Gamo, Y.; Nagashima, A.; Wakabayashi, M.; Terai, M.; Oshima, C. Atomic structure of monolayer graphite formed on Ni (111). Surf. Sci. 1997, 374, 61−64. (51) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 2000, 113, 9901.
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Substrate-Sensitive Graphene Oxidation Zhuhua Zhang,* Jun Yin, Xiaofei Liu, Jidong Li, Jiahuan Zhang, and Wanlin Guo* State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, and Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China S Supporting Information *
ABSTRACT: The inertness of graphene toward reaction with ambient molecules is essential for realizing durable devices with stable performance. Many device applications require graphene to contact with substrates, but whose impact on the chemical property of graphene has been largely overlooked. Here, we combine comprehensive first-principles analyses with experiments to show that graphene oxidation is highly sensitive to substrates. Graphene remains inert on SiO2 and hexagonal boron nitride but becomes increasingly weak against oxidation on metal substrates because of enhanced charge transfer and chemical interaction between them. In particular, Ni and Co substrates lead to spontaneous oxidation of graphene, while a Cu substrate maximally promotes the oxygen diffusion on graphene, with an estimated diffusivity 13 orders of magnitude higher than that on freestanding graphene. Bilayer graphene is revealed to have high oxidation resistance independent of substrate and thus is a better choice for high-performance nanoelectronics. Our findings should be extendable to a wide spectrum of chemical functionalizations of two-dimensional materials mediated by substrates.
G
pristine graphene is highly inert. Definitely, pristine graphene is more desirable for future nanoelectronics because many novel properties can survive only in the absence of defects. Recent rapid advancement in techniques for growing singe-crystal graphene leads us increasingly close to this goal.31−33 These new progresses underscore the importance of exploring the oxidation of pristine graphene on technically relevant substrates. Herein, on the basis of comprehensive first-principles analyses in conjunction with experiments, we show pristine graphene itself can be chemically weakened by substrates, a step further over prior contributions based on defect-dominated oxidation. We find graphene is inert against oxidation on SiO2, Au, and Ag but becomes increasingly weak against oxidation on Al and Cu and even can be spontaneously oxidized on Co and Ni. The dissociated epoxy groups on Cu-supported graphene show a mobility 13 orders of magnitude higher than that on freestanding graphene at room temperature. The discovery owes its origin to a fundamental understanding of charge transfer and electronic hybridization between graphene and substrates. Interestingly, bilayer graphene retains high chemical inertness, independent of substrate, thus being a better choice for fabricating chemically robust devices and coatings. Our findings suggest a new route to efficiently directing graphene
raphene is currently being extensively studied in an effort to understand its exceptional physical and chemical properties and to explore its immense potential in a number of applications.1−3 A topic of great interest is the oxidation of graphene. Oxidation not only alters the electronic property of graphene4,5 but also can be exploited to shape graphene into well-defined pieces with desired performance.6−17 Moreover, air oxidation dominates the degradation of graphene devices18 as well as the chemical decomposition of graphene under harsh conditions.19−22 However, for most device applications, graphene is desired to be inert to ambient molecules, in particular to ubiquitous O2. Fundamental to understanding the oxidation of graphene is the underlying oxidation dynamics, especially the kinetic barriers for essential oxidation processes, including the transitions between flying and chemisorbed O2 as well as the diffusion of epoxy groups. In reality, the inclusion of graphene in electronic devices requires integration with substrates, such as dielectric layers, gates, and metal contacts. Most substrates will change the intrinsic properties of graphene because of interfacial interaction,23−28 which may pose qualitatively different oxidative performance. While the oxidation of graphene on substrates has received growing attention, the results are either limited to physisorption of O24,5 or based on polycrystalline graphene whose oxidation is dominated by the grain boundaries thereof.19,20,29 In contrast, earlier experiments on graphite found that the defect-free region of basal plane can withstand oxidation at temperatures up to ∼1150 K,30 indicating that © 2016 American Chemical Society
Received: January 10, 2016 Accepted: February 17, 2016 Published: February 17, 2016 867
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the other is the slightly less stable cycloaddition configuration (Figure 1a, right). The O2 in both the configurations favors the spin-singlet state. We also check the effect of pentagon− heptagon defects (basic unit of grain boundaries) on the adsorption of O2 and do not find qualitative change in the physisorption and chemisorption configurations (Figure S1). We focus on the oxidation dynamics of pristine graphene in the following. Starting from the physisorption configurations, we have searched for the transition states for the physisorption-tochemisorption processes and determined the energy barrier, as shown in Figure 1a. The energy barrier is 2.0 eV for O2 on a fgr, consistent with previous calculations and with experimental results that pristine graphene is inert toward oxidation.25 When graphene is placed on metal substrates, the corresponding barrier is remarkably reduced, from 1.57 eV on Au to 1.34 eV on Al and to 0.72 eV on Cu, even though graphene is weakly bound on these substrates. For Ni and Co substrates on which graphene is chemically adsorbed,23 the O2 can spontaneously form a C−O bond with graphene and then needs to overcome a barrier of merely 0.27 and 0.45 eV, respectively, to further reach the cycloaddition structure. A recent experiment found single-layer graphene can provide substantial corrosion reduction to the Cu substrate but not to the Ni substrate,37 supporting our calculations to some extent. In contrast, graphene coupled to substrates with van der Waals interaction remains inert to oxygen, as exemplified by the corresponding energy barrier of over 1.9 eV for O2 on BN- and SiO2-gr. These results prove that the oxidation-resistance of graphene sensitively depends on substrates and highlight the necessity of choosing suitable substrates or electrodes to keep graphene chemically robust. To gain further insight into the substrate-dependent oxidation dynamics, we examine the electronic structures of the physisorption and transition-state (TS) configurations of O2 on graphene. Because BN- and SiO2-gr have oxidation dynamics similar to that of f-gr, we mainly discuss the metalsupported graphene and select the Cu- and Au-gr as the model systems, together with the f-gr for comparison. In the physisorption configuration of O2 on f-gr, the density of states (DOS) projected onto the O2 show two spin-polarized peaks, with the spin-up state being fully occupied and spin-down state being empty. The two states are split from the O ppπ* states and have antibonding character (Figure 2, left). For O2 on the Cu-gr, the antibonding ppπ* state in the spin-up channel becomes partially occupied because of the charge transfer from Cu (Figure 2, middle). Consequently, the O2 is activated, as evidenced by the stretched O−O bond, which raises its adsorption energy. Meanwhile, we observe charge accumulation between the top-layer Cu atoms and the C atoms directly atop them, indicating the formation of weak chemical bonds. The C−Cu bonds electronically perturb the neighboring C atoms by breaking the π bonds and leaving weak dangling bonds (Figure 2, left insets), which benefits the formation of a C−O bond in the TS state. Our electronic structure analysis supports this scenario. In the TS structure, the C−O bond results in a bonding state near the Fermi level, a combination of O 2pz and C 2pz from the atoms adjacent to the O-attached one. This bonding state (see arrows in Figure 2) is fully occupied for O2 on Cu-gr, but is partially occupied in the case of f-gr. Full population of the bonding state strengthens the C−O bond (its bond length is 1.48 Å on Cu-gr, compared to 1.50 Å on f-gr) and therefore enhances the stability of the TS structure. A clear
oxidation by purposefully engineering substrates and are crucial for future scientific studies and industrial applications of graphene. Our model systems consist of an oxygen molecule (O2) and a single-layer or bilayer graphene on a substrate, which varies from metallic Au, Ag, Al, Cu, Co, and Ni to insulating BN and SiO2, all widely used either in chemical synthesis or in device fabrication. We consider the low-density adsorption of O2 on graphene, in which the molecule axis of O2 tends to be parallel to the C−C bond at low temperature.34 For convenience, we use the notation of M-gr and f-gr to represent M-supported and freestanding graphene, respectively. The optimal adsorption distance of O2 on the M-gr is obtained by examining the system total energy as a function of the distance between O2 and graphene; then we scan different adsorption sites and compare the energies to obtain the optimal configurations. The results can be divided into two classes. For graphene on BN, SiO2, Au, Al, Ag, and Cu, the O2 has stable physisorption and metastable chemisorption configurations, as shown by insets in Figure 11a,
Figure 1. Oxidation dynamics of graphene on substrates. (a) Calculated adsorption energy along the minimum-energy paths between the physorption and chemisorption states of an O2 on freestanding (left), Cu-supported (middle), and Ni-supported (right) single-layered graphene. The inserts show the side views of the physisorption (Phys), chemisorption (Chem), and transition-state (TS) configurations. On Ni, both the initial and final states are in chemisorption states (see text). (b) Calculated energy barriers, E*, between the physisorption and TS configuration of an O2 on graphene supported by different substrates, as well as work functions of singlelayered graphene on substrates. Gray, red, wine, and navy balls represent C, O, Cu, and Ni, respectively.
middle. In the physisorption, the O2 is located at 2.6−2.8 Å above the graphene and remains in a spin-triplet state while in the chemisorption the O2 is a cycloaddition configuration with a spin-singlet state. These adsorption features are similar to those for O2 on f-gr35 and carbon nanotubes.36 In contrast, on Co- and Ni-gr, the O2 has two chemisorption configurations: the first has only one C−O bond connecting to graphene, and 868
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gr the barrier is reduced to 0.66 and 0.45 eV, respectively. There are two possible paths for the dissociation: path I locates two epoxy groups onto two adjacent hexagons, while path II makes the two groups locate on the same hexagon, as shown by the insets in Figure 3. Both paths result in the same dissociation
Figure 3. Dissociation of chemisorbed O2 on graphene. Calculated minimum-energy paths for the dissociation of chemisorbed O2 into two epoxy groups on (left) f-, (middle) Cu-, and (right) Ni-gr. The dashed line is along path I, while the solid line is along path II. Inset: atomic configurations of initial, transition, intermediate, and final states along paths I and II.
Figure 2. Density of states projected onto the graphene (gray shaded) and oxygen (red) atoms. Panels a and b show the results of the (a) initial and (b) transition-state configurations for an O2 on freestanding, Cu- and Ni-supported single-layered graphene. The insets illustrate the O-crossed slices of charge redistribution between the substrate and graphene plus O2. Dark and light colors represent charge depletion and accumulation regions, respectively. The scale of color bar is in units of e/Å3. Small arrows denote the bonding state derived from the C−O bond in the transition state.
barrier yet different reaction energies (defined as the energy difference between the dissociated and chemisorbed states) for all the substrates. The reaction energy in path II is −0.37 eV more negative than that of path I on the f-gr but becomes less negative by 0.48 and 0.27 eV on the Cu- and Ni-gr, respectively. Moreover, on the Cu-gr, the two epoxy groups migrate simultaneously in path I but via a one-by-one process in path II, with an intermediate state. During the dissociation, all the transition-state structures have the migrating O atom close to the top site on top of a C atom in graphene, connected by one C−O bond (see the top inset in Figure 3). The presence of a metal substrate strengthens the C−O bond and in turn enhances the stability of TS structures. For example, the C−O bond in the TS structure on Cu-gr is 0.02 Å shorter than the counterpart on f-gr. Thus, the mechanism of the metalpromoted oxygen dissociation is similar to that of the aforementioned O2 attachment onto graphene on substrates. Then the dissociated epoxy groups are diffusive on graphene40 and proceed to dominate the oxidation process. Next, we explore the diffusion of an isolated O adatom on graphene, which shows more interesting substrate dependence. For an O adatom on f-Gr, the equilibrium state is an epoxy (E) configuration while the TS is an on-top (T) configuration with the oxygen atom sitting on a carbon atom (top insets in Figure 4a). The diffusion barrier, obtained as the energy difference between the E and T configurations, is 1.05 eV, as shown by the minimum-energy path (MEP) in Figure 4a. Similar oxygen configurations and diffusion are found on the BN-, Au-, and Aggr, except that the diffusion barrier drops to 1.02, 0.86, and 0.67 eV, respectively, as shown in Figure 4a. Because these substrates have little chemical interaction with graphene, the reduced diffusion barrier is attributed to the charge-doping of graphene by substrates, consistent with a previous prediction that the O diffusion on graphene can be controlled by carrier doping.40 The situation is very different for the O adatom on Al- and Cu-gr, on which the T configuration becomes a metastable state with two nonequivalent T 1 and T 2 configurations. In T1, the O-bonded C atom is directly on top of a surface metal atom, but in T2, it is on the face-centered cubic site, as illustrated by the insets in Figure 4a. Thus, we perform MEP calculations for an oxygen adatom diffusing along
picture thus emerges for the reduced barrier: the O2 activation in the physisorption state and the bonding increase in the TS. The degree of O2 activation mainly depends on the work function of the substrate−graphene system, while that of bonding increase depends on the chemical interaction between graphene and substrate. On Au, the Au-gr has less charge transfer to O2 because of its high work function (Figure 1b), and the bonding interaction between graphene and Au is weaker because Au has a deeper d-band center than Cu (Figure S2). Therefore, the barrier for chemisorbing O2 onto Au-gr becomes much higher. In contrary, Ni has a shallow d-band center to enable covalent interaction with graphene, resulting in a very low work function of the Ni-gr (Figure 1b) and strong dangling bonds on graphene (Figure 2, right). This explains why the Ni-gr has the lowest barrier for chemisorbing an O2. A recent experiment showed that intercalating Pb at the graphene/Ru interfaces enhances the inertness of graphene toward oxygen attchment,38 which fits our theory as we find that Pb electronically interacts with graphene more weakly than Ru does. Therefore, the substrate-dependent oxidation dynamics stems from a coupled effect of the charge transfer to O2 and the chemical hybridization between graphene and substrate. The interfacial electronic hybridization is more significant in determining the oxidation barrier, as evidenced by the fact that the Cu-gr is easier to be oxidized than Ag- and Al-gr, although the latter two systems show stronger charge transfer with the physisorbed O2 (see Figure 1b for work functions and Figure S2 for charge redistribution). These results suggest the possibility of using d-band center of metal as a viable descriptor for screening electrodes in fabricating highperformance graphene-based devices. The chemisorbed O2 with cycloaddition configurations tends to dissociate into two isolated O adatoms (i.e., epoxy groups) on graphene,39 which can be facilitated by metals as well. On a f-gr, the dissociation of chemisorbed O2 into two epoxy groups needs to overcome a barrier of 0.96 eV, whereas on Cu- and Ni869
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Then, the estimated diffusion rate at 300 K is 1.8 × 10−20 cm2/ s. If we assume the same attempt frequency, the diffusion rate at the same temperature is 1.91 × 10−7 cm2/s on the Cu-gr, which is 7 and 13 orders of magnitude higher than those on the Ag-gr and the f-gr, respectively. The substrate-sensitive oxygen diffusion suggests a new way to control the oxidation of graphene. For example, by lithographically patterning the Cu substrate, the graphene sections in direct contact with Cu will preferentially chemisorb O2 and the high mobility of oxygen enables these sections to relax into energetically favorable patterns. Spatially controlled oxidation of graphene can thus be achieved for desirable electronic applications. Recent experiments demonstrated that graphene can serve as an oxidation barrier for metals, yet limited to a short time.20,21 To overcome this issue, we propose using bilayer graphene as protection coating for metals. We predict that, independent of the substrates, a bilayer graphene is essentially identical to a freestanding graphene in terms of the energetics and dynamics of oxidation because the top layer is chemically decoupled from the bottom layer, even though the interlayer charge transfer slightly reduces the energy barrier (Figure S3). Apparently, thicker graphene will be more robust against oxygen chemisorption, likely comparable to that of hexagonal boron nitride sheets.42,43 Having established the substrate dependence of the oxidation dynamics of graphene, we proceed by testing it on two representative substrates, SiO2 and Cu. The monolayer graphene on Cu was directly grown by chemical vapor deposition method while that on SiO2 was transferred via a PMMA-mediated process.44 Panels a and b of Figure 5 present the optical images of monolayer graphene on SiO2 and Cu, respectively. The graphene samples are predominantly monolayer and have low defect concentrations, as proven by Raman characterizations (Figure 5c). To evaluate the oxidation
Figure 4. Substrate-dependent diffusion of oxygen atoms on graphene. (a) Calculated minimum-energy path for oxygen adatom diffusion on single-layered graphene supported by different substrates. Left insets: the epoxy (E) and on-top (T) configurations of O adatom on graphene. Right inset: the oxygen diffusion path relative to the beginning carbon network of T2 configuration on Ni (only the toplayer Ni atoms are shown), in which the E, T1, and T2 are marked. (b) The kinetic barriers for oxygen diffusion on graphene, the energy difference between the E and T2 configurations, and that between the T2 and T1 configurations as functions of substrates. Gray, red, and navy balls represent C, O, and Ni, respectively.
an extended path E-T1-E-T2-E (Figure 4a, right). Surprisingly, the highest diffusion barrier is only 0.27 eV on the Cu-gr and 0.33 eV on the Al-gr. These striking disparities of the adsorption behavior and diffusion barrier of oxygen adatoms result from the difference in the chemical interaction between graphene and substrate. As the chemical interaction gets stronger, the T1 and T2 configurations have lower energies while the E configuration becomes less stable (Figure 4b). On Cu-gr, the chemical interaction is so subtle that the T1 and E configurations have almost the same energy, which leads to the lowest diffusion barrier. When the chemical interaction is further enhanced, as in the Co- and Ni-gr, the E configuration becomes unstable while the T1 and T2 configurations are further stabilized. In particular, the T2 configuration lies at the equilibrium state as the oxygen just saturates the dangling bond of the underneath carbon (the neighboring carbons strongly interact with Ni). Now, the E configuration is less stable than the T2 configuration by 0.35 eV on the Co-gr and by 0.4 eV on the Ni-gr, rendering a high surface corrugation for oxygen diffusion (Figure 4b). Accordingly, the calculated MEP for the T1 → T2 diffusion path displays an energy barrier of 0.56 eV on the Co-gr and 0.63 eV on the Ni-gr. The diffusion barrier directly determines the oxygen diffusion rate, which can be evaluated by D = νd02 exp(−ε/kBT),41 where ε is the diffusion barrier, ν the attempt frequency, d0 an elementary length for diffusion, kB the Boltzmann constant, and T the absolute temperature. For oxygen on f-gr, d0 = 1.22 Å and ν is calculated as 30 THz based on harmonic approximation.
Figure 5. Experimental characterization of oxidation of graphene on substrates. (a) Optical image of a graphene sample on Cu, which is exposed in dry air at 270 °C for (left) 0 min, (middle) 3 min, and (right) 6 min. (b) The same as panel a except for a sample on SiO2. (c) Raman spectra of the graphene samples on (left) Cu and (right) SiO2 exposed in dry air for 0, 3, and 6 min, respectively. 870
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graphene is expected to bring quantitative agreement with our theory. In summary, our comprehensive first-principles calculations have shown that the ease of oxygen chemisorption of graphene critically relies on what substrates or electrodes are used. In particular, we show that the Ni substrate maximally facilitates the chemisorption of O2 on graphene, while Al and Cu substrates result in exceptionally high diffusivity for oxygen adatoms on graphene. Our theory is qualitatively confirmed by experimental characterizations of graphene oxidation on typical substrates. Notably, bilayer graphene can retain the intrinsic chemical inertness of freestanding graphene independent of substrate and is thus well-suited for realizing long-term stable nanoelectronics and serving as oxidation-resistant coatings. This work not only reveals a previously unrealized role of the substrate in determining the chemical property of graphene but also opens a new method of manipulating graphene oxidation via substrate engineering.
resistance of the two samples, we expose SiO2- and Cu-gr in dry air at 270 °C and then record the evolution of graphene morphology. At 3 min, the optical images display that the Cu-gr starts to break up, likely along the defects (e.g., vacancies and grain boundaries); however, no appreciable change is observed in the SiO2-gr under the same conditions. These results suggest that the Cu-gr is chemically more vulnerable than the SiO2-gr, which agrees with our theoretical analysis even in the presence of defects (Figure S1). We have predicted that O2 can be attached onto Cu-gr more easily because of the relatively small barrier for the physisorption-to-chemisorption transition. Subsequently, the dissociated oxygen adatoms are extremely diffusive on Cu-gr and will be readily docked at graphene edges and defects. The enhanced flux of oxygen adatoms toward the defects promotes the oxidation therein. The accumulated oxygen adatoms at the graphene edges and grain boundaries generate CO2 molecules that are finally released into air. As such, the defects grow into large openings, resulting in disconnected graphene flakes. In contrast, the much higher barriers for both the attachment of O2 and the diffusion of oxygen adatoms on SiO2-gr impede the oxygen accumulation toward the defects, where the oxidation is hence largely retarded. Upon further exposure, the graphene fakes on Cu substrate shrink and diminish and the oxidation of the substrate assists the successive morphology change, as illustrated by the image recorded at 6 min, whereas the SiO2-gr remains intact. Our Xray photoelectron spectroscopy (XPS) analyses show that the oxygen content in Cu increases with oxidation time (Figure S4); however, before the graphene oxidation, the oxygen content in Cu is negligible even when the whole samples are exposed to air for 24 h. Therefore, graphene is oxidized first and then the Cu oxidation helps disrupt graphene because of oxidation-induced expansion of Cu lattice. This contrasts sharply with the SiO2-gr, where no measurable signal of oxidation can be obtained throughout the process. Distinguishing the impact of Cu oxidation on the graphene integrity from the oxidation of graphene itself will be helpful but is infeasible within our current experimental capability. The results based on morphology change are further supported by the Raman spectra measured at different exposure times (Figure 5c). For the Cu-gr, the ratio of D peak over 2D peak in the Raman spectra increases conspicuously with exposure time, which can be attributed to the opened edges and holes and in part to the residual oxygen in the graphene flakes.45 In sharp contrast, the Raman spectrum for the SiO2-gr is almost independent of time. The two samples exposed for 6 min also exhibit different electric transport behavior: the graphene transferred from Cu becomes electrically insulating while the graphene on SiO2 retains a perfectly linear I−V curve (Figure S5). This qualitative difference between graphene on Cu and SiO2 can be maintained with further increasing oxidation time even to 24 h. In addition, we find that both a bilayered graphene on Cu and a single-layered graphene on Au are highly inert, as no morphology change is optically visible and no change of D peak appears in the Raman spectra under the same oxidation conditions for 6 min (Figures S6 and S7), both in good agreement with our theory. Despite the polycrystalline nature of our graphene samples, their highly contrasting performances on different substrates are fully consistent with the substrate-dependent oxidation dynamics of graphene. Further experiments using large-scale single-crystal
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EXPERIMENTAL SECTION Theory. All the theoretical results are based on first-principles calculations within the framework of density-functional theory, as implemented in the Vienna Ab-initio Simulation Package,46,47 using the projector-augmented wave method for the core region, with a plane-wave kinetic energy cutoff of 400 eV. Given that the generalized gradient approximation (GGA), even with van der Waals density functional, overestimates the distance between graphene and metal substrates48,49 because of the repulsive nature of the revised PBE exchange functional, we employed local spin density approximation that proves to be reasonable for graphene-metal systems 23 and produces graphene−metal distances close to those in experiments.50 Test calculations on freestanding and Ni-supported graphene using standard GGA show no qualitative change in the results (Figure S8a). The structural models contain mono- or bilayer graphene on a substrate consisting of four atomic layers (except the h-BN sheet which is single-layered). The substrate lattices were adapted to match the graphene lattice within a 3 × 3 supercell for Cu, Ni, Co, and h-BN and a 4 × 4 supercell for Au, Al, Ag, and SiO2. The commensurability condition is applied by compressing and stretching the substrate lattices by 1−3%. A vacuum layer of 12 Å is used to isolate neighboring periodic images and a dipole correction is used. The positions of substrate atoms of the topmost three atomic layers plus all graphene and oxygen atoms are relaxed until the force on each atom is less than 0.01 eV/Å. The minimum-energy path was mapped out using the climbing image-nudged elastic-band method.51 We also fixed the lattice of the system to the substrate and recalculated the oxidation barrier, showing essentially the same results (Figure S8b). Experiment. The graphene samples were grown using lowpressure chemical vapor deposition on 25 μm thick copper foil (Alfa Aesar, item no. 13382), which was pretreated by electrochemical polishing before the growth. The as-grown graphene samples were then transferred onto a Si substrate with 300 nm SiO2 layer according to the method developed by Ruoff and co-workers.44 Raman spectra of graphene samples on SiO2/ Si substrate were conducted by a Renishaw Raman spectrometer with a 532 nm solid-state laser. The electric measurements were taken by a Keithley 2400 multimeter controlled by a LabView-based data acquisition system. X-ray photoelectron spectroscopy was performed using an Al Kα Xray source (XPS, PHI 5000 VersaProbe, UlVAC-PHI). The 871
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Letter
The Journal of Physical Chemistry Letters
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data of C1s, Cu2p, and O1s peaks were collected to analyze the oxidation of the graphene and substrates.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00062. Test of model using different lattice parameter and exchange correlation potential, oxidation dynamics at graphene defects, charge transfer between graphene and substrates, oxidation dynamics of bilayer graphene on substrates, XPS characterization of oxidized graphene on substrates, I−V curves of oxidized graphene on SiO2 and Cu, experimental characterization of oxidation of bilayered graphene on Cu, and Raman spectra of the graphene samples on Au exposed in dry air (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (W.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the 973 Program (2013CB932604, 2012CB933403), National NSF (11172124, 51472117), MOE doctoral discipline Foundation (20113218120033), Research Funds for the Central Universities (NE2015104, NS2014006, NP2015203), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (0414K01), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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DOI: 10.1021/acs.jpclett.6b00062 J. Phys. Chem. Lett. 2016, 7, 867−873
