In Situ Observation of Thermal Proton Transport through Graphene

Aug 8, 2017 - School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China ... By following the evolution of interlayer struc...
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In Situ Observation of Thermal Proton Transport through Graphene Layers Daming Zhu,†,‡ Xing Liu,†,‡ Yi Gao,† Yadong Li,†,‡ Rui Wang,† Zijian Xu,† Gengwu Ji,†,‡ Sheng Jiang,† Bin Zhao,† Guangzhi Yin,† Li Li,† Tieying Yang,† Yong Wang,† Lin Yi,§ Xiaolong Li,*,† and Renzhong Tai*,† †

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: Protons can penetrate through single-layer graphene, but thicker graphene layers (more than 2 layers), which possess more compact electron density, are thought to be unfavorable for penetration by protons at room temperature and elevated temperatures. In this work, we developed an in situ subsecond time-resolved grazingincidence X-ray diffraction technique, which fully realizes the real-time observation of the thermal proton interaction with the graphene layers at high temperature. By following the evolution of interlayer structure during the protonation process, we demonstrated that thermal protons can transport through multilayer graphene (more than 8 layers) on nickel foil at 900 °C. In comparison, under the same conditions, the multilayer graphenes are impermeable to argon, nitrogen, helium, and their derived ions. Complementary in situ transport measurements simultaneously verify the penetration phenomenon at high temperature. Moreover, the direct transport of protons through graphene is regarded as the dominant contribution to the penetration phenomenon. The thermal activation, weak interlayer interaction between layers, and the affinity of the nickel catalyst may all contribute to the proton transport. We believe that this method could become one of the established approaches for the characterization of the ions intercalated with 2D materials in situ and in real-time. KEYWORDS: in situ technique, graphene, thermal proton, synchrotron X-ray, transport raphene, a truly atomistic crystal,1 with its remarkable mechanical strength and scalable supply through chemical vapor deposition (CVD),2−4 has realistic prospective applications in next-generation separation technologies that require high selectivity and permeability.5−10 In fact, the closely spaced carbon atoms in graphene and its high electron density make it mostly impermeable to all atoms and molecules.6,7,9,10 Even CVD graphene, which is expected to have Stone−Wales defects, is thought to be impermeable to He under ambient conditions.6 On the other hand, thermal protons can penetrate through single-layer graphene at room temperature.9,10 Geim et al. performed proton transport measurements of 2D materials and found that protons can penetrate the monolayer graphene at room temperature with the energy barrier of 0.78 eV, which is lower than the theoretical predicted value.9 Geiger et al. theoretically presented that aqueous protons can transfer through monolayer graphene via rare OHterminated atomic defects at room temperature.11 However, for

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thicker graphene, such as AB-stacking bilayer graphene, the more compact electron density is thought to repel the transport of thermal protons,9 implying that graphene has potential use in various hydrogen-based technologies. The penetration of protons through graphene is a thermally activated process,9,10 and a fundamental question is thus raised. Can high temperatures thermally motivate the transport of protons through multilayer graphene? The ability of thermal protons to penetrate multilayer graphene at high temperatures may lead to interesting applications, such as the separation and filtering of thermal protons at high temperatures. However, it is difficult to experimentally obtain the interaction between protons with graphene at high temperature. In fact, the size Received: May 15, 2017 Accepted: August 8, 2017 Published: August 8, 2017 8970

DOI: 10.1021/acsnano.7b03359 ACS Nano 2017, 11, 8970−8977

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Figure 1. In situ time-resolved 2D-GIXRD observations of the protonation of multilayer graphene on nickel foil at a constant temperature of 900 °C. (a) Experimental setup. (b) Typical 2D-GIXRD patterns for graphene growth on nickel foil. (c,f) Evolution of the graphene (002) peak during the growth of graphene layers and subsequent Ar (H2) annealing at 900 °C. Magnified view of the region enclosed by the white dotted ellipse is shown in (d) and (g), clearly revealing the shift of the XRD peak with time. (e,h) Corresponding variations in the d (002) value, integrated intensity, and fwhm over time; the light green region demonstrates stage (2) during the Ar (H2) annealing process.

(more than 8 layers) on nickel foil at 900 °C. In comparison, under the same conditions, the multilayer graphenes are impermeable to argon, nitrogen, helium, and their derived ions. Complementary in situ transport measurements further verify the penetration phenomenon at high temperature.

of the proton is between that of an atom and an electron, indicating that transient structural variations of graphene layers should be present if proton exchange and penetration occurred. Therefore, a reliable in situ, time-resolved technique with highresolution structural determination is highly needed to understand the interaction between protons and graphene at high temperature. In this work, we developed a subsecond time-resolved twodimensional X-ray diffraction grazing-incidence (2D-GIXRD) platform with an in situ CVD chamber, which can track the continuous evolution of the interlayer spacing and related structural parameters during the growth of graphene and the following protonation process (the small amount of protons thermally cracked from H2 at high temperature are used as the thermal proton source). Using this technique, we demonstrated that thermal protons can transport through multilayer graphene

RESULTS AND DISCUSSION We use nickel foil as the catalyst in CVD growth of graphene because nickel is preferred for the fabrication of the multilayer graphene.12−14 A dedicated CVD chamber (Figure 1a) was specifically used for the in situ growth and characterization. The time-resolved GIXRD experiments were performed at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) with an X-ray wavelength of 1.24 Å and an incident angle of 2° (see the Methods section). The typical 2DGIXRD pattern (Figure 1b) of the graphene layers shows a 8971

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Figure 2. Off-line characterization of graphene layers grown on nickel foil after Ar annealing and H2 annealing. (a,b) Raman and XPS C 1s spectra of multilayer graphene after Ar annealing (red curves) and H2 annealing (black curves). (c−f) Typical high-resolution transmission electron microscopy images of multilayer graphene after Ar (H2) annealing. The scale bar is 3 nm. Insets in (c) and (e) are the corresponding electron diffraction patterns. (g) XRD profiles of the two samples, after Ar annealing (red circle) and H2 annealing (blue square).

narrow and bright crystalline ring, corresponding to the (002) reflection of graphene, which indicates good crystallinity.14 After graphene growth, the CH4 supply was stopped, and H2 (or Ar, N2, and He) was immediately introduced for the annealing treatment at 900 °C (Figure S1). As a reference, we first performed the Ar annealing process after the growth of graphene (the N2 and He annealing processes show similar behavior, shown in Figure S2). In situ GIXRD profiles (Figure 1c) depict the evolution of the (002) peak of graphene during the growth of the graphene layers and subsequent Ar annealing at 900 °C (Figure S1a). We observed that no peak associated with graphene appeared until after 6 min of exposure to CH4. The peak intensity is gradually increased to a maximum, then remains constant for a relatively long time (>2 min), which is followed by its weakening and ultimate disappearance. The intensity evolution can be described by three kinetic stages: (1) growth of the graphene layers on nickel, (2) kinetic equilibrium between nickel and the graphene layers with exposure to Ar (N2 or He), and (3) removal of the graphene layers through the dissolution of carbon atoms into nickel (detailed in Figure S3). In stages (1) and (3), the peak position shifts to a higher (lower) angle with increasing (decreasing) peak intensity (Figure 1d), which is

mainly dominated by the size effect of graphene, as described in our previous report.14 We focused on the variation of the d (002) value in stage (2) (light green region in Figure 1e). It was found that the d (002) value and the integrated intensity remain invariable, which means that for the Ar (N2 or He) annealing process, the introduced Ar (N2 or He) molecules and ions cannot penetrate into the graphene layers at 900 °C (the ions initially present in the high-temperature environment). Next, we performed the H2 annealing process after the growth of graphene layers (Figure 1f and Figure S1b). Similar to the process of graphene growth and Ar (N2 or He) annealing, three stages can also be distinguished.15,16 However, it was found that in stage (2), the d value of graphene (002) shows an abnormal shift (from 3.4775 to3.4802 Å, Figure S4), with the peak intensity remaining constant (Figure 1g,h). Additionally, the fwhm of the peak is almost unchanged during this stage, indicating that the expansion of the d (002) value is homogeneous. The expanded interlayer spacing may be attributed to the physical insertion of certain particles related to hydrogen into the interlayer space of graphene or to the chemical hydrogenation (the formation of C−H bond results in the distortion of the graphene lattice) of all graphene layers.17 8972

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Figure 3. In situ transport measurements for protonation of multilayer graphene on nickel foil at a constant temperature of 900 °C. (a) Sketch of the experimental setup. (b) I−V characteristics for the nickel foils under Ar (red squares) and H2 annealing (blue circles) processes. (c) Real-time current profiles under a constant voltage of 80 V during the growth of graphene layers and subsequent Ar (red curve) and H2 annealing (blue curve) processes. The dotted lines in (c) depict the current profiles of the nickel foils without the growth of graphene under Ar and H2 annealing process. (d,e) Sketch of multilayer graphene under the Ar and H2 annealing process, respectively, which shows that protons can be transported through the graphene layers.

However, it is reported that the hydrogenation happens only on the top graphene layer of the graphene sheets even using hydrogen plasma treatment.17,18 Once the H atom chemisorbs on the graphene sheet, more energy is required to overcome the chemical bond of C−H in order to tunnel, so tunneling through the graphene layer is almost forbidden.7 Therefore, chemical hydrogenation of all graphene layers can be ruled out. Moreover, X-ray photoelectron spectroscopy (XPS) and Raman spectra (Figure 2a,b; see below for a detailed analysis) also show that no C−H bond can be probed on the sample after H2 annealing. Therefore, the expansion of the interlayer spacing should be due to the physical insertion of certain particles, such as protons, H atoms, or H2 molecules. From above GIXRD characterization, we found that Ar+, N+, and He+ cannot penetrate into the graphene layers (Figure 1e). These ions present comparative or even smaller volume than that of H atoms (van der Waals radius ∼1.2 Å); therefore, the H atom cannot physically insert into the graphene layers.7 Moreover, the H2 molecule cannot penetrate into the graphene layers because it has large volume (van der Waals radius ∼2.02 Å). Density functional theory calculations indicate the intercalation of the hydrogen molecules in the graphene bilayer is 2.47 eV higher in energy than H2 outside, suggesting the hydrogen molecules in graphene layers are highly unfavorable. In comparison, the intercalation of protons is 0.08 eV lower in energy, suggesting the protons are more favorable to stay in graphene layers (details in Figure S5). Therefore, the best

possibility should be the protons inserted into the graphene layers, which causes the expansion of the interlayer spacing. To clarify the status of the graphene layers undergoing H2 (or Ar) annealing, an ultrafast cooling rate (>10 °C/s) was used to obtain the off-line samples14,19 after 7 min of annealing (corresponding to the end of stage 2) (Figure S6). Raman spectra (Figure 2a) display the same features for the Ar- and H2-annealed graphene layers. The G-to-2D intensity ratio exhibits the multilayer character of graphene,20 and no C−H vibrational mode can be observed in the spectrum of the sample after H2 annealing. The concentration of chemical hydrogenation in graphene can be estimated by the ID/IG ratio of Raman peaks.21,22 Even if the proportion of hydrogenation in graphene is less than 0.02%, the Raman spectrum will still present a pronounced sharp D peak and a concomitant D′ peak.22 However, from Figure 2a, almost no D peak and D′ peak can be observed, indicating that the multilayer graphene is of good quality, and almost no H chemisorption exists in graphene (the content of C−H chemisorption is much lower than 0.02%). XPS can provide direct evidence of the chemical bonds in the multilayer graphene under Ar and H2 annealing (Figure 2b). Identical features of the two XPS profiles indicate that almost no substantial C−H bonds are formed during the H2 annealing process, as also depicted by the Raman spectra. Because no C−H bonds can be proven by XPS and Raman spectroscopy, the expanded interlayer spacing of graphene is not related to chemical hydrogenation of graphene.17,18 This is reasonable because it is more difficult for a proton to tunnel through the graphene sheet once a C−H bond is formed on the 8973

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Figure 4. In situ time-resolved 2D-GIXRD observations of protonation and deprotonation processes of the transferred multilayer graphene on 30 nm nickel film/300 nm SiO2/Si substrate. (a) Sketch of the experimental setup; a surrounding low-temperature zone (lower than 500 °C) and a center high-temperature zone of 900 °C can be achieved. (b) Variations of d (002) of the integrated multilayer graphene under alternate Ar and H2 annealing processes in the high-temperature zone of 900 °C and the low-temperature zone.

first layer of graphene.7,17,18 The number of layers of the Arannealed multilayer graphene is approximately 13−16 (Figure 2c,d), identified by high-resolution transmission electron microscopy (HRTEM), whereas for H2-annealed graphene, this value is approximately 8−11 layers (Figure 2e,f), with more than five sampling points collected. The high crystallinity of multilayer graphene under Ar and H2 annealing is also supported by the selected area electron diffraction studies (inset in Figure 2c,e). Off-line XRD was carried out to further identify the microstructure of the samples after the rapid cooling process (Figure 2g and Figure S7). The integrated intensity ratio of the graphene (002) peak is approximately 1.40 for Ar- and H2-annealed multilayer graphene, which is consistent with the value (13−16)/(8−11) obtained by HRTEM. The in situ process at 900 °C shows the difference of 0.030°(which is mainly due to the size effect of graphene14) for the graphene (002) peak at the maximum integrated intensity before annealing, and this value changes to 0.021° after Ar and H2 annealing, due to the insertion of protons during H2 annealing. However, after the rapid cooling process, we found that the graphene (002) peak still exhibits a difference of 0.030° for Ar-annealed (2θ = 21.335°, d = 3.3488 Å) and H2annealed (2θ = 21.305°, d = 3.3535 Å) samples. This means that the inserted protons under H2 annealing escape after the rapid cooling process. Actually, it is reasonable as the protons cannot stay stable in the multilayer graphene at room temperature and will either leave the interlayer space of graphene during the cooling process or bond with C atoms in graphene. The Raman and XPS analyses show that no C−H bonds are formed (Figure 2a,b).17,18 Therefore, it is concluded that no protons are left in the interlayer space of graphene after the sample is cooled to room temperature. To fully understand the structural evolution in in situ GIXRD observations, real-time transport measurements are presented in Figure 3. The same CVD chamber as that used for the in situ GIXRD observations was applied for complementary in situ transport measurements, by adding an external source meter (Figure 3a) (see Methods section and Figure S8). The current−voltage (I−V) characteristics (Figure 3b) of the nickel foil under Ar or H2 annealing clearly demonstrate the thermal activated ionization and the conductivity of the gas flow gap at 900 °C (Figure S9). Under the condition that the critically

controlled spacing between the two electrodes is approximately 0.2 mm and the applied voltage is below 100 V, the contribution of tunneling or hopping effects to the measured current is ruled out. Therefore, the main contribution to the measured current is from gas ionization by thermal activation. The lower current for the Ar annealing process is mainly due to the lower ion concentration (the ionization energy of Ar is higher than that of H2). The real-time current evolution during the growth of graphene layers and subsequent Ar (H2) annealing at 900 °C is presented in Figure 3c, acquired under a constant voltage of 80 V. After approximately 2 min of exposure to CH4, the current suddenly increases to a maximum and then decreases. The variation of current during CH4 exposure can be illustrated in the following three stages. (1) Decomposition of CH4 under the action of nickel catalysis produces C ions, CH ions, or H ions, and the decomposed C ions continually diffuse into nickel, and the enhanced carrier concentration causes the enhanced current. (2) When the concentration of carbon in nickel reaches the saturation value, no more C ions can diffuse into nickel. Therefore, the decomposition of CH4 becomes slower with time, thus decreasing the current. (3) With the continuous supply of CH4 (after approximately 6 min), the decomposed carbon ions begin to nucleate at the nickel electrode and form graphene. In this stage, accompanied by C ions assembled to graphene, the carrier concentration should be increased; therefore, the current again increases slightly.14,23 The above analysis is consistent with the in situ GIXRD observations and other reports.14,23 Next, we compared the current variations in the annealing processes with Ar and H2. First, we performed Ar annealing after supplying CH4 for 7 min. There are no current variations in the entire Ar annealing process (same as the value obtained on the nickel foil without graphene, dotted lines in Figure 3c). This is reasonable because no more ions are produced in the Ar annealing treatment, and the carbon precipitation from Ni and the removal of graphene will not change the carrier concentration in the gas layer. However, for the H2 annealing process, the current shows a remarkable increase with time, evidently larger than the value obtained on the nickel foil without graphene (dotted lines in Figure 3c). The only interpretation is that protons persistently penetrate into the graphene layers, accompanied by the separation of protons and 8974

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multilayer graphene, proton penetration is almost inhibited due to the higher barrier energy (much denser electron cloud compared with that of monolayer graphene).9 Our experiments clearly demonstrate that protons can homogeneously penetrate into multilayer graphene at 900 °C, and this phenomenon can be mainly attributed to two reasons. One is that our experiments were performed at high temperatures. In this case, the interlayer spacing of graphene, measured to be 3.478 Å, is larger than that at room temperature (3.3535 Å) due to the 3.7% thermal expansion of graphene. Therefore, the barrier energy should be reduced due to the weak interlayer interaction between graphene layers. In addition, high temperatures will activate the protons, increasing the probability that the protons tunnel through the graphene.9 Another reason is that the affinity to the metal catalyst (here, the nickel substrate) will influence the proton penetration through graphene. It has been demonstrated that platinum group metals can significantly facilitate the transport of protons through 2D crystals under ambient conditions.9 The nickel catalyst will be much more active at high temperatures. As a result, thermally activated protons may be more amenable to be transported through the graphene layers on nickel foil. Moreover, previous studies of proton penetration through graphene were performed in aqueous solutions. The interaction between protons and graphene is complicated, due to the additional van der Waals interaction of graphene with water11,27 and the hydrogenbonded networks.9−11 However, in our experiments, this process happens in a purely physical environment. The interaction between thermal protons with graphene is expected to be different from those in aqueous solution. The proton concentration gradient between graphene layers and the upper gaseous surroundings will also facilitate the tunneling of protons into graphene layers.

electrons, which enhances the carrier concentration. In other words, the presence of graphene layers on nickel foil will accelerate the breakup of hydrogen molecules into ions. These results are consistent with the GIXRD results. After approximately 10 min of H2 annealing, the current begins to decrease, accompanied by the removal of graphene layers. Finally, the current is stabilized at a value the same as that on the nickel foil without graphene. From the above in situ GIXRD and transport current observations, we confirm that protons can continuously penetrate into the graphene layers at 900 °C without the formation of C−H bonds. On the other hand, Ar (N2, He) and their derived ions cannot penetrate into multilayer graphene at high temperature. The CVD graphene layers on nickel foil are expected to have Stone−Wales defects and small point defects.6,7,11 However, these defects can only decrease the height of the barrier but will not destroy the impermeability of the membrane, even for helium atoms.6,7 Moreover, there is also evidence for the formation of intrinsic pore defects in CVD graphene, which may allow the molecules to pass through the single-layer graphene.24 However, for multilayer graphene, these defects cannot be localized throughout the multilayer, which means that defects in one layer will be remedied by others. This is also confirmed by the fact that small ions (Ar, N2, He, and their derived ions) cannot penetrate into multilayer graphene. Therefore, the possibilities of proton penetration from defects of graphene layers are relatively low. For the multilayer graphene growth on nickel catalyst, it is commonly observed that nickel foil is fully encapsulated with multilayer graphene,25,26 and it may not be possible that protons intercalate into the graphene layer through the boundary of the sample. To demonstrate this, a transferred multilayer graphene on 30 nm nickel film/300 nm SiO2/Si substrate is used for the in situ GIXRD observations. When the multilayer graphene is transferred to a 30 nm nickel film, the removal of multilayer graphene under Ar and H2 annealing can be suppressed due to excess carbon compared with nickel,15,16 which is favorable for the long time observations of the protonation and deprotonation processes. A special heating platform (Figure 4a) was designed, which can produce a surrounding low-temperature zone (lower than 500 °C) and a center high-temperature zone of 900 °C, and the sample fully covers the low-temperature zone. We found that the d (002) of graphene in the low-temperature zone shows a constant value during alternate Ar and H2 annealing processes (Figure S10) in Figure 4b,19 which indicated that thermal protons cannot intercalate into or directly transport the graphene layers at this temperature. However, thermal protons can still penetrate into the multilayer graphene in the high-temperature zone of 900 °C in Figure 4b, which exhibits the larger d value of (002) under H2 annealing. Therefore, the possibilities of proton intercalation into the integrated graphene layers can be ruled out. Previous reports have shown that only monolayer graphene can be penetrated by protons. Even bilayer graphene can be hardly penetrated.9,10 The main restriction for proton transfer through graphene is the energy required to push the proton through the center of the aromatic ring of graphene. The barrier energy for the penetration of protons through monolayer graphene has been theoretically calculated to be 1.25−1.40 eV and experimentally evidenced to be approximately 0.8 eV. Therefore, protons can penetrate monolayer graphene by thermal activation or an electric field. However, for

CONCLUSION In conclusion, we have developed an in situ subsecond timeresolved 2D-GIXRD technique, which fully realizes the realtime observation of the thermal proton interaction with the graphene layers at high temperature. Using this technique, it shows that thermal protons can transport through multilayer graphene at high temperature. Moreover, the direct transport of protons through the benzene rings of graphene is regarded as the dominant contribution to the penetration phenomenon. The thermal activation, weak interlayer interaction between layers, and the affinity of the nickel catalyst may all contribute to the proton transport. METHODS In Situ Two-Dimensional X-ray Diffraction Observations. In situ 2D-XRD observations during the graphene chemical vapor deposition growth and the subsequent protonation (or Ar, N2, and He annealed) were performed with X-rays at a wavelength of 1.24 Å at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The beam size was confined to be 0.3 × 0.3 mm2 by vertical and horizontal slits. A MarCCD detector was used to acquire twodimensional X-ray diffraction signals with subsecond time resolution. Benefiting from the small beam divergence and high-energy resolution of synchrotron radiation X-ray, we can achieve very high angle resolution. Therefore, tiny lattice expansion or strain (99.96% (sourced from Hefei Kejing Materials Technology 8975

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ACS Nano Co. Ltd., Hefei, China) was cut into 10 × 10 mm2 pieces for the growth of graphene layers. Before the CVD growth, the nickel foil was chemically cleaned by dipping it into acetic acid for 10 min, and it was then rinsed with alcohol and deionized water. The rinsed nickel foil was dried under a nitrogen steam and placed onto the heating platform in the CVD chamber. High-purity CH4, H2, Ar, N2, and He (purity >99.999%, Shanghai Chunyu Special Gas Co. Ltd.) were used for the experiment. Prior to heating, the CVD chamber was pumped to approximately 102 Pa and then filled with purified argon, a process that was repeated three times to ensure an inert atmosphere. The CVD growth of graphene was carried out under ambient pressure. The in situ CVD process primarily consisted of four steps: (1) heating the nickel substrate to 900 °C and annealing for ∼30 min under a flow of 200 sccm Ar and 20 sccm H2 to remove the surface metal oxides; (2) the growth process of the graphene layers, which began with the introduction of 150 sccm CH4 and 10 sccm Ar into the CVD chamber for 7 min; (3) the protonation (or Ar, N2, and He annealing) process, which was subsequently carried out by exposing the grown graphene layers/nickel foil to the flow of H2 (or Ar, N2, and He); (4) the rapid cooling of the sample to room temperature. The CVD experimental procedure is illustrated in Figure S1 in the Supporting Information. Off-Line Characterization of the Graphene Layers after Ar Annealing and H2 Annealing. The same experimental setup as that used for in situ observations was used for the sample preparation for the off-line characterization. Unlike the extended H2 (or Ar) annealing process for the in situ observation, the off-line samples were subjected to 7 min of annealing under a H2 flow of 180 sccm (Ar flow of 300 sccm) after 7 min of growth of graphene layers. Owing to good repeatability of our experiments, the maximum integrated intensity of the graphene layers during the H2 (or Ar) annealing process corresponded with the in situ experiments. Then, the annealing process was terminated, and the sample was cooled to room temperature at a high cooling rate (∼40 s from 900 °C down to 400 °C) to inhibit the excess reaction of the graphene layer with the nickel foil. The off-line CVD experimental procedure is illustrated in Figure S6 in the Supporting Information. Raman spectroscopy was performed by a HORIBA Jobin-Yvon LabRAM Raman spectrometer (473 nm wavelength with a laser spot diameter of approximately 2 μm). XPS was performed on a Kratos Analytical AXIS-Ultra using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). XRD was carried out at the BL14B1 beamline of SSRF, with a wavelength of 1.24 Å and an X-ray incident angle of 10°. HRTEM images were taken with a Tecnai G2 F20 S-TWIN microscope with graphene samples transferred onto a copper grid. In Situ Transport Measurements. The same CVD chamber as that used for in situ XRD was designated for the in situ transport measurements. As displayed in Figure 3a, the counter electrodes consisting of two nickel foils were fixed by two separate quartz plates (0.2 mm thickness) on the heating platform in the CVD chamber, and gas flow passed through the 0.2 mm gap. The counter electrodes were bound with Pt wires and connected through the well-sealed chamber with a source meter (Keithley 2410). For the I−V measurements, the voltage was typically varied in the range of 0−100 V. For all tests with varying gas flows (CH4, H2, Ar, N2, and He), the breakdown voltage was higher than 600 V at 900 °C. In situ current measurements during the CVD growth of graphene and its subsequent protonation (or Ar, N2, and He annealing) process were conducted at the constant input voltages of 20, 40, 80, and −80 V. The in situ current showed the same trends during annealing processes under the same gas flow, even when different constant voltages were applied. To characterize our experimental setup, the leakage currents of the quartz were measured by completely filling the gap with a piece of monoblock quartz. Density Functional Theory Calculations. Calculations are carried out using spin-polarized density functional theory with the generalized gradient approximation of Perdew−Burke−Ernzerhof implemented in the VASP code.30,31 The bilayer graphene is model by a 5 × 5 supercell with 100 C atoms. The K-point sampling is restricted to Γ point due to the large supercell size. The interaction between core and valence electrons is described by the projectoraugmented wave method with an energy cutoff of 400 eV.32,33 The

geometry optimization is performed using a conjugate gradient algorithm until the largest ionic force is less than 0.05 eV/Å.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03359. Detailed procedures for the in situ characterizations and sample preparation; in situ 2D-GIXRD observation for graphene layer growth and subsequent N2, He, and H2 annealing at 900 °C; off-line XRD patterns of nickel foil before graphene growth and after graphene etching; batch-to-batch variability for repeating the in situ H2 annealing experiments; calculated absorption geometries of H2 and proton on bilayer graphene; 2D-XRD patterns of multilayer graphene on nickel foil after growth under Ar (or H2) annealing; in situ transport measurements of the gas layer conductivity (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yi Gao: 0000-0001-6015-5694 Tieying Yang: 0000-0002-4728-7438 Xiaolong Li: 0000-0002-1674-9345 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank Professor A.K. Geim (School of Physics and Astronomy, The University of Manchester, UK) for the kind suggestions, and Professor Haiping Fang (Shanghai Institute of Applied Physics, Chinese Academy of Sciences, China) for the helpful discussions. The research was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 11225527, 11205235, U1632129, 11575283, and 11475251) and the Ministry of Science and Technology of China (2012CB825705). The authors thank the staff at the beamlines BL14B1 and BL08U of the Shanghai Synchrotron Radiation Facility. REFERENCES (1) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (2) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (3) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (4) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Lett. 2009, 9, 30−35. (5) Koenig, S. P.; Wang, L.; Pellegrino, J.; Bunch, J. S. Selective Molecular Sieving through Porous Graphene. Nat. Nanotechnol. 2012, 7, 728−732. (6) Leenaerts, O.; Partoens, B.; Peeters, F. raphene: A Perfect Nanoballoon. Appl. Phys. Lett. 2008, 93, 193107. 8976

DOI: 10.1021/acsnano.7b03359 ACS Nano 2017, 11, 8970−8977

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DOI: 10.1021/acsnano.7b03359 ACS Nano 2017, 11, 8970−8977