Solvent-Free Process to Produce Three Dimensional Graphene

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A Solvent-Free Process to Produce Three Dimensional Graphene Network with High Electrochemical Stability Dongyun Xia, Kongyang Yi, Baozhong Zheng, Menglin Li, Guo-Qiang Qi, Zhi Cai, Min Cao, Donghua Liu, Lan Peng, Dapeng Wei, Zhen Wang, Lei Yang, and Dacheng Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10082 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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The Journal of Physical Chemistry

A Solvent-Free Process to Produce Three Dimensional Graphene Network with High Electrochemical Stability Dongyun Xia,†,‡ Kongyang Yi,† Baozhong Zheng,‡ Menglin Li,† Guoqiang Qi,† Zhi Cai,† Min Cao,† Donghua Liu,† Lan Peng,† Dapeng Wei,§ Zhen Wang,† Lei Yang,† Dacheng Wei*,† †

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science,

Fudan University, Shanghai 200433, China. ‡

Department of Materials Science and Engineering, Yunnan University, Kunming, Yunnan 650091,

China. §

Chongqing Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute of Green

and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China.

ABSTRACT: Three-dimensional (3D) graphene has attracted increasing attention in electrochemical devices. However, the existing preparation technologies usually involve a solvent process, which introduces defects and functional groups into the 3D network. Here, we find the defects and functional groups influence the electrochemical stability of graphene. After an electrochemical process, the current decreases by more than 1 order of magnitude, indicating remarkable etching of graphene. To improve the electrochemical stability, we develop a solvent-free preparation process to produce 3D graphene for the first time. After growth on a 3D micro-porous copper by chemical vapor deposition (CVD), the copper template is removed by a high temperature evaporation process, resulting in 3D graphene network without any solvent process involved. The samples exhibit remarkably improved stability with durable time 2 times compared with normal CVD samples, and 55 times compared with reduced

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graphite oxide, and no obvious etching is observed at 1.6 V vs. saturated calomel electrode, showing great potential for application in future 3D graphene based high stable electrochemical devices.

1. INTRODUCTION Three-dimensional (3D) graphene has recently raised increasing attention for its widespread potential applications like sensors,1-3 gas absorption,4,5 and composites. Among them, especial attention is being paid to the applications in electrochemical devices like super-capacitors, lithium ion batteries,6-9 due to its high surface area, wide potential window, good chemical inertia and special electrochemical properties. In these electrochemical devices, 3D graphene is usually used as the electrode materials, however the electrochemical stability is usually ignored, which is of great importance for the reliability and durability of the graphene electrode. In fact, etching is a common phenomenon in the electrochemical process and is widely used to remove or modify materials.10-12 In recent decades, many groups have researched electrochemical etching of the carbon allotropes, like carbon fibers,13 amorphous carbon,14 graphite15 and carbon nanotubes.16 However, to the best of our knowledge, there are few reports about the electrochemical etching of graphene. The factors which can influence the electrochemical stability is not very clear, and more importantly, how to produce graphene sample with high electrochemical stability has still not received attention, hampering the practical application of graphene in the electrochemical devices. So far, two widely used techniques to produce 3D graphene network are chemical reduction of graphite oxide (GO) and chemical vapor deposition (CVD). However, both of them involve a solvent process, which introduce defects and impurities into the 3D network. For instance, the chemical reduction of GO (RGO) is widely used to produce 3D graphene.17-21 This method has advantages in terms of suitability for large-scale production due to their simplicity and low production cost. However, the chemical redox process in solvent introduces functional groups and defects which seriously damage the electrical properties of graphene. What’s more, it is very challenging to recover the seriously damaged GO to intrinsic graphene. CVD can produce large-area and high-quality graphene films on ACS Paragon Plus Environment

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metal surface.22-24 By using on nickel foam as the template in CVD, 3D graphene with excellent mechanical and electrical properties can be produced.25 Till now, many groups have developed various CVD process to produce 3D graphene, such as using reduced Ni precursor,26 a commercial Ni mesh,27 and so on7,28 as the growth template in CVD. However, all of these CVD processes require a complex solvent process to remove the metal catalyst,29 which introduces defects, functional groups in the 3D graphene networks. Here, we find that the defects and functional groups introduced by the solvent process dramatically decrease the stability of the graphene samples. To solve this problem, we develop a solvent-free process to prepare 3D graphene network with high electrochemical stability for the first time, as well as our knowledge. After growing graphene on a 3D micro-porous copper foam by CVD, the copper template is removed by a simple evaporation process, thus a 3D graphene network is prepared without using any solvent. Electrochemical measurement shows a remarkably improved stability compared with that of the samples prepared by normal solvent-based process, and no obvious etching is observed when 1.6 V vs. saturated calomel electrode (SCE) is applied, showing great potential for usage in future electrochemical devices based on 3D graphene electrodes with high stability. 2. EXPERIMENTAL SECTION 2.1 Sample preparation. Ceramic boat with copper powders (Sinopharm Corp, China) was placed in a 1 inch quartz tube. The powder was heated to 1000 °C in a horizontal tube furnace under 200 sccm H2, and was annealed for 20 min to form a 3D micro-porous copper template. To grow graphene, ethanol was then introduced as the carbon source by bubbling liquid ethanol using 50 sccm Ar gas. After the reaction in H2 10 sccm at 1000 °C for 20 minutes, 3D graphene network was grown on the surface of copper. To obtain the pure 3D graphene network, the sample was heated at 1300 °C for 2 h under a pressure of 2 × 10-3 torr to remove the copper. Finally, the tube furnace was rapidly cooled down to room temperature under the protection of Ar (500 sccm) and H2 (5 sccm) mixture gas at ambient pressure. The pure 3D graphene network can be obtained in the ceramic boat. ACS Paragon Plus Environment


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2.2 Characterization. The samples were characterized by Raman spectroscopy (HORIBA Jobin Yvon XploRA, excited by 532 nm He-Ne laser with a laser spot size of about 1 um2), scanning electron microscopy (SEM, TESCAN VEGA TS 5136MM, 5 kV), X-ray photoelectron spectroscopy (XPS, Phisical

Electronics, PHI5300, Al target, 250 W, 14 kV), X-Ray Diffractometer (XRD, PANalytical,

X’pert PRO), and field-emission transmission electron microscopy (TEM, FEI, Tecnai G2 F20 S-Twin, 200 kV). For TEM specimen preparation, the purified graphene sheets were sonicated in Dimethyl Formamide sonicator for 20min to form a homogeneous suspension, and then the suspension was dropped onto a TEM grid. Electrical conductivity was measured by semiconductor parameter analyzer (Keysight, B1500A) and probe station (Everbeing Int’l Corp., PE-4) in air at room temperature. 2.3 Electrochemical measurement. Graphene films were fabricated on glass substrate via the same filtration process (see details in Supporting Information). The electrochemical measurements were performed by using an electrochemical workstation (Shanghai Chenhua Ins.c, CHI660e B14499) and a standard three-electrode system at room temperature in air. KCl aqueous solution (0.1 M) was used as the electrolyte. The graphene film, a graphite electrode, and a saturated calomel electrode were immerged in the solution as the working electrode, the counter electrode, and the reference electrode, respectively. 3. RESULTS AND DISCUSSION 3.1 3D graphene network prepared via a solvent free process. The experimental procedure is outlined in Figure 1. The copper powders were placed in a ceramic boat in the center of the quartz tube, and their appearances were tailored to several shapes, like pentacle, hexagon and the alphabet “Gr” (Figure 1a). The copper powders were annealed in H2 atmosphere at 1000 °C to remove the impurities. At the same time, previous studies reveal that when the micro-scale metal particles are heated to a temperature close to its melting point, they will automatically merge with each other to decrease the surface energy.30,31 Therefore, after this process, a monolithic micro-porous copper foam formed with a defined shape (Figure 1b) was obtained. Subsequently, a CVD process was carried out at 1000 °C by ACS Paragon Plus Environment

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using ethanol vapor as the carbon source. After the CVD process, the surface of the 3D structure of copper foam was covered with graphene. To remove the copper template, the sample was annealed at high temperature in vacuum. Finally, freestanding 3D graphene networks were obtained, which copied and inherited the morphology of the copper template.

Figure 1. (a) Schematic diagram of the preparation process of 3D graphene network by a solvent free CVD. (a) A photograph of copper powders placed in ceramic boats. (b) A photograph of 3D porous copper templates (a monolithic micro-porous copper) after annealing. (c) A photograph of copper templates covered with graphene. (d) A photograph of 3D graphene networks after removing copper. SEM image of the 3D micro-porous copper before graphene growth is shown in Figure 2a. The porous copper skeleton has a uniform 3D network structure with pore-size in the range of 10-20 µm, thus the copper powders fused into a monolithic 3D micro-porous copper structure after high temperature annealing. Figure 2b is the SEM image of the copper skeleton after graphene growth, which shows the copper skeleton was covered by a graphene sheet, corresponding to the Figure 1c. In order to observe the edges of graphene, the copper skeleton was cut off, and the section of the bare ACS Paragon Plus Environment


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copper appeared in the bottom left of Figure 2b. Some graphene wrinkles and edges were found around the section of bare copper. Figure 2c shows a pure 3D graphene network after removing the copper skeleton. The network has a highly dense and interconnected micro-porous structure over large area. We observed the cross section of the 3D graphene by SEM (inset of Figure 2c), and the inner of the 3D graphene also has a perfect network structure with pore size about 10-20 µm. Compared with 3D graphene grown on commercial Ni foam,25 the pore size of our sample is 1-2 orders of magnitude smaller, indicating the high density of our 3D graphene sample. Due to the high density and the excellent mechanical property of our sample, the 3D network is strong enough to avoid collapsing without the support of copper skeletons (Figure S1 and S2). Fig. 2d is the magnified SEM image of our sample, and no collapse appears like the graphene samples grown on Ni particles,32 showing a uniform structure in micro-scale.

Figure 2. (a) SEM image of an interconnected copper template made by annealing copper powders at 1000 °C. (b) SEM image of graphene grown on the surface of the copper template. (c) SEM image and (d) the magnified SEM image of the 3D graphene network after evaporating copper template. The inset of (c) shows the cross section of the 3D graphene network. As we known, the electrical properties of graphene sheets strongly depend on their quality and the number of layers.33 To evaluate quality and the number of layers, we performed field-emission TEM ACS Paragon Plus Environment

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measurement on the purified graphene (Figure 3). As previously reported, suspended graphene are usually folded at their edges, which allows the cross-section view of graphene to directly identify the number of layers accurately by high resolution TEM (HRTEM).34,35 Figure 3a manifests monolayer graphene and the top right inset shows the electron diffraction pattern. A clear hexagonal diffraction pattern is observed, indicating our sample has good crystallinity. Besides monolayer graphene, fewlayer graphene were also observed. HRTEM measurements show that most of the graphene sheets have 1-4 layers (Figure 3b, 3c and 3d), and graphene sheets with thickness more than 4 layers can also be occasionally observed (Figure S3). The monolayer graphene ensures the high surface area of the 3D network and the few-layer graphene provides the strong mechanical properties of the 3D network.

Figure 3. (a) HRTEM image of a monolayer graphene. The inset shows the electron diffraction pattern of the sample. (b-d) HRTEM images of the graphene with 2, 3 and 4 layers. 3.2 Characterization of the sample quality. Raman spectroscopy is one of the most powerful techniques to characterize the carbon materials.35 We can easily identify the doping, the layer number, the strain, and the defects of graphene by the positions and the profiles of Raman peaks.36-41 The three most intense Raman features of graphitic material are D band at ~ 1350 cm–1, G band at ~ 1580 cm–1, and 2D band at ~ 2700 cm–1. The G peak corresponds to the doubly degenerate zone center E2g mode,42 whereas the D band arises from the defect mediated zone-edge (near K-point) phonons.35 On the ACS Paragon Plus Environment


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contrary, the 2D band has nothing to do with the G peak, but is the second order of zone-boundary phonons.36 Figure 4 shows the Raman spectra of the as-grown graphene on micro-porous copper and the 3D graphene network after removal of the copper template. As a comparison, we produced graphene samples by normal solvent-based process. The samples were produced by the same CVD process, and then the copper was etched by HCl (1 M)/H2O2 (0.01 M) mixed aqueous solution. We denote the graphene produced by the solvent-free method as “SF-Gr” and the graphene produced by the traditional CVD and solvent-etching process as “S-Gr.” Before removing the template, the as-grown graphene on copper has G peak and 2D peak, but no obvious D peak can be observed as shown in Figure 4a. It means that high quality graphene were produced by using copper powders as the catalyst after the CVD process. Most of the measured samples have a 2D band with intensity stronger than that of the G band (the red spectrum in Figure 4a), which is the fingerprint of monolayer graphene, indicating that there is a high percentage of monolayer graphene in the 3D graphene network. There also exist few-layer and multi-layer graphene (the black spectrum in Figure 4a), consistent with the HRTEM characterization.

Figure 4. Typical Raman spectra of (a) two graphene samples grown by CVD before removing copper, (b) S-Gr and (c, d) SF-Gr samples. The red and black curves in (c) are the Raman spectra of SF-Gr before and after an electrochemical process, respectively.

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The Raman spectrum of S-Gr as shown in Figure 4b has a strong D band at 1340 cm-1, which should be attributed to oxygen functional groups and defects generated by the solvent-etching process. On the contrary, most of the Raman spectra show the SF-Gr has no D peak (Figure 4c), indicating the samples have a remarkably increased quality. Occasionally, a slight D peak can be occasionally observed in the Raman spectra of SF-Gr (Figure 4d), which should be attributed to the graphene edges in the asprepared 3D network. Therefore, the micro-porous structure has high quality and abundant edges. Most of the defects are from the edge plane, consistent with the SEM image as shown in Figure 2d.

Figure 5. XPS C1s spectra of (a) a graphene sample grown by CVD before removing copper, (b) SF-Gr, (c) S-Gr and (d) SF-Gr after an electrochemical process. XPS measurements were performed to identify the functional groups on the surface of the graphene layers.43 The C1s spectrum of the graphene can be deconvoluted into bands at 284.4~285, 286~286.9 and 287.4~289 eV, assigned to three different kinds of carbon, sp2 graphitic carbon, hydroxyl groups (C-OH) and carbonyl groups (C=O), respectively.43-46 Figure 5a-c represents the XPS C1s spectra of the 3D graphene network before removing copper template, SF-Gr and S-Gr, respectively. In Figure 5a, the C1s peak appears at a binding energy of 285.1 eV, which is assigned to sp2 carbon, indicating that ACS Paragon Plus Environment


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the obtained materials are high quality graphene without oxygen-containing groups. The XPS spectrum of SF-Gr (Figure 5b) is nearly the same as that of graphene before removing copper template, which proves that the high temperature annealing process doesn’t introduce a large amount of functional groups or defects into the sample, thus high quality samples can be obtained after the solvent-free postgrowth treatment. Figure 5c shows the XPS spectrum of S-Gr which the copper was removed by HCl (0.1 M) / H2O2 (0.01 M) mixed aqueous solution. An obvious peak appears at 286.4 eV, which should be assigned to C-OH groups. These functional groups were introduced in the post-growth solvent treatment process, consistent with to the increased D peak in the Raman spectrum. Moreover, no peak from copper can be observed from the XRD patterns (Figure S4) of SF-Gr, indicating that the copper substrate has been completely removed. To measure the conductivity,46-52 we deposited the sample between two Au/Cr electrodes. The current/voltage measurement (Figure S5, see the details of the device fabrication in Supporting Information) shows that the resistance of an individual SF-Gr sheet is estimated to be ~2.6 kΩ, and the conductance is estimated to be ~1600 S/cm, which is higher than that of RGO (~20 S/cm)47 and GO after CVD treatment (10−350 S/cm)48 measured by two-probe method, and is comparable to that of the graphene film produced by CVD (1097 S/cm, measure by four-probe method),51 or exfoliated single layer graphene (~6×10-4 Ω-1 or ~6000 S/cm, measure by two-probe method, assuming the thickness to be ~1 nm),49 further confirming the high quality of the SF-Gr samples. Therefore, combining the results from the SEM, TEM, Raman and XPS, the CVD growth and post-growth solvent-free treatment process can effectively avoid introducing defects and functional groups into the samples, resulting in 3D graphene networks with high quality. 3.3 Electrochemical etching and stability of graphene. In the electrochemical applications like supercapacitors, lithium ion batteries, electrochemical catalysts, the stability is of great importance. To test the stability, three types of materials were used as the working electrode in the electrochemical system as shown in Figure 6a (see experimental detail in the Supporting Information), which were made of RGO, S-Gr, and SF-Gr, respectively. A graphite electrode and a SCE were used as the counter ACS Paragon Plus Environment

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electrode and the reference electrode. Figure 6b-d show the chronoamperometric curves of these graphene electrodes in an electrochemical process at a voltage of 3 V, 2 V and 1.6 V vs. SCE in 0.1 M KCl aqueous solution, respectively. In the case of RGO electrode, no obvious stable current was observed. The current dramatically decreased by more than one order of magnitude within about 1000 s and 5500 s at 3 V and 2 V vs. SCE, respectively, and decreased by about 87% within 20000 s at 1.6 V vs. SCE, indicating a remarkable electrochemical etching has been taken place. The Raman and XPS results of RGO before and after etching at 2V vs. SCE (Figure S6) provide enough evidence of the electrochemical etching process. Besides RGO, the etching also takes place at the anode made of S-Gr or SF-Gr. Different from the result of RGO, both of them have four regions. In the case of SF-Gr (Figure S7), in the first region (< 10 s), a large initial current, which may be attributed to surface faradaic reactions, slow capacitive charging, and superficial intercalation,53 abruptly decreases to a stable current. In the second region (10-5200 s), the resistance has no significant change and the current remains at about 110 µA. During this time, the graphene conductive network still exists, and the electrochemical etching proceeds and takes place only on parts of the graphene film. In the third region (5200-6000 s), the current decreases slowly and then rapidly, indicating that the etching has taken place on the whole film. The etching removes carbon atoms and largely decreases the conductivity of graphene. In the last region (> 6000 s), the current maintains a small value and decreases slightly. Because the conductive network has been destroyed, the etching rate decreases largely as a result of the high resistance, thus avoiding the etching of the remnant graphene.54 Many groups have demonstrated the electrochemical etching of carbon allotropes.13-15 In most of these cases, the etching at the anode is an electrochemical oxidation and destruction of carbon. After etching, some carbon will be removed, and many defects and functional groups are introduced into the carbon materials, resulting in dramatic decrease of the resistance.10-12 The etching of graphene can be further proved by the Raman and XPS results. After an electrochemical process of SF-Gr at a voltage of 3 V vs. SCE in 0.1 M KCl aqueous solution for 100 min, the intensity ratio of D band and G band in the Raman spectrum of SF-Gr (Figure 4c) remarkably increase by about 2 orders of magnitude, indicating a ACS Paragon Plus Environment


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strong destruction of the graphene structure.35 XPS C 1s spectrum of the residual graphene (Figure 5d) has two new peaks at 286 and 288.9 eV, corresponding to C-O and C=O functional groups. The percentage of carbon atoms of the C-O and C=O groups largely increased from zero to 22.5% and 7.5%, respectively, confirming that the strong oxidation takes place in the electrochemical process. After the electrochemical process, the current (Figure 6c) decreases more than one order of magnitude, which largely influences its applications in electrochemistry, thus the stability could not be ignored.

Figure 6. (a) Schematic diagram of the electrochemical system used in the experiments. (b-d) The chronoamperometric curves of RGO, S-Gr and SF-Gr in an electrochemical process at 3 V, 2 V and 1.6 V vs. SCE reference electrode in 0.1 M KCl aqueous solution, respectively. The preparation process is of great importance for the electrochemical stability of graphene. The chronoamperometric curves of RGO, S-Gr and SF-Gr show that the electrochemical current remains stable at 3 V vs. SCE for shorter than 100 s, about 2650 s, and about 5200 s, respectively. At a voltage of 1.6 V, the current decreased by about 87% and 42% for 20000 s in the case of RGO and S-Gr, while no obvious decrease of the current was observed in the case of SF-Gr. In ACS Paragon Plus Environment

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fact, the defects can reduce the stability of graphene.55 The oxidation of carbon materials usually starts at the defect sites,56,57 and in the electrochemical process at the anode, when electrolyte is being decomposed, active oxygen reacts with carbon materials so that more and more defects and carboxyl, hydroxyl or other functional groups are introduced on carbon materials.58,59 As the reaction proceeds and more and more carbon atoms react, finally the graphene will be destroyed and collapsed to form amorphous carbon. This effect was also observed by Zhou et al. i.e., that multi-walled carbon nanotubes were destroyed to form small carbon clusters after electrochemical treatment.60 As the etching usually starts at the defect sites, reducing the defect density is one of the most effective routes to increase the electrochemical stability. RGO has been widely used in electrochemical devices based on 3D graphene network. Compared with RGO, the S-Gr produced by CVD has much higher quality with fewer defects or functional groups, resulting in an improved electrochemical stability. However, S-Gr samples need remove metal catalysts by a solvent post-growth process, thus there are still some defects and functional groups introduced. Our route avoids usage of any solvent in both growth and post-growth process, thus high quality samples can be produced with much lower density of defects and functional groups, consistent with the Raman and XPS results. As a result, the samples exhibit largely increased stability (Figure 6d). 4. CONCLUSION In summary, we observe a sharp decrease of the current after an electrochemical process using graphene anode. Although the graphene is usually regarded as an electrochemically stable material, this result indicates that the stability of graphene could not be ignored in its electrochemical applications. We find the defects and functional groups introduced by the solvent process can largely decrease the stability of graphene, thus we develop a solvent-free 13 ACS Paragon Plus Environment


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process to produce freestanding 3D graphene network by CVD using 3D micro-porous copper as the catalyst. The resulting samples have perfect interconnected porous structure with pore size much smaller than that of 3D graphene network produced by using nickel foam, and their durable time under 3 V vs. SCE is about 55 times longer than RGO, and about 2 times compared with S-Gr, while no obvious etching is observed at 1.6 V vs. SCE for 20000 s, showing great potential for usage in future 3D-graphene-based electrochemical devices with high stability. Moreover, this method shows a good scalability and controllability. About 30 mg free-standing 3D graphene network can be produced by using 1 inch tube furnace, and it has potential to be further scaled up by using a CVD system with larger chamber size. The shapes of 3D graphene network can be easily controlled by tailoring the appearance of the copper template. Besides 3D graphene network, this solvent-free process also provides an alternative approach to produce other 3D porous materials which need usage of metal template in CVD growth. Finally, we believe our result increases the understanding of the electrochemical etching of graphene, and would be valuable for the application of graphene in electrochemical devices with high stability. ASSOCIATED CONTENT Supporting Information. Supplementary figures S1-S7. Experimental details of fabricating electrical devices and fabricating graphene electrodes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

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*E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Program for Thousand Young Talents of China, the National Natural Science Foundation of China (21544001, 21603038), and Fudan University. REFERENCES 1. Cao, X.; Zeng, Z.; Shi, W.; Yep, P.; Yan, Q.; Zhang, H. Three‐Dimensional Graphene Network Composites for Detection of Hydrogen Peroxide. Small, 2013, 9, 1703−1707. 2. Dong, X. C.; Xu, H.; Wang, X. W.; Huang, Y. X.; Chan-Park, M. B.; Zhang, H.; Wang, L. H.; Huang, W.; Chen, P. 3D graphene–cobalt oxide electrode for high-performance supercapacitor and enzymeless glucose detection. ACS Nano, 2012, 6, 3206−3213. 3. Dong, X.; Ma, Y.; Zhu, G.; Huang, Y.; Wang, J.; Chan-Park, M. B.; Wang, L.; Huang, W.; Chen, P. Synthesis of graphene–carbon nanotube hybrid foam and its use as a novel threedimensional electrode for electrochemical sensing. J. Mater. Chem. 2012, 22, 17044−17048 4. Wang, Y.; Guo, C. X.; Wang, X.; Guan, C.; Yang, H.; Wang, K.; Li, C. M. Hydrogen storage in a Ni–B nanoalloy-doped three-dimensional graphene material. Energy Environ. Sci., 2011, 4, 195−200. 5. Kim, G.; Jhi, S. H. Ca-decorated graphene-based three-dimensional structures for highcapacity hydrogen storage. J. Phys. Chem. C, 2009, 113, 20499−20503. 15 ACS Paragon Plus Environment


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SYNOPSIS TABLE OF CONTENTS


Solvent-Free Process to Produce Three Dimensional Graphene

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