This video method describes the synthesis of high surface area, monolithic 3D graphene-based materials derived from polymer precursors as well as single layer graphene oxide.
Efforts to assemble graphene into three-dimensional monolithic structures have been hampered by the high cost and poor processability of graphene. Additionally, most reported graphene assemblies are held together through physical interactions (e.g., van der Waals forces) rather than chemical bonds, which limit their mechanical strength and conductivity. This video method details recently developed strategies to fabricate mass-producible, graphene-based bulk materials derived from either polymer foams or single layer graphene oxide. These materials consist primarily of individual graphene sheets connected through covalently bound carbon linkers. They maintain the favorable properties of graphene such as high surface area and high electrical and thermal conductivity, combined with tunable pore morphology and exceptional mechanical strength and elasticity. This flexible synthetic method can be extended to the fabrication of polymer/carbon nanotube (CNT) and polymer/graphene oxide (GO) composite materials. Furthermore, additional post-synthetic functionalization with anthraquinone is described, which enables a dramatic increase in charge storage performance in supercapacitor applications.
Since the isolation of graphene in 2004,1 interest in harnessing its unique properties has led to intense effort directed toward assembling graphene into three-dimensional, monolithic structures that retain the properties of individual graphene sheets.2-5 These efforts have been hampered by the fact that graphene itself is expensive and time consuming to produce and tends to aggregate in solution, which limits the scalability of materials based on graphene building blocks. Additionally, graphene assemblies are typically comprised of physical cross-linking interactions (e.g., van der Walls forces) between the individual graphene sheets, which are much less conductive and mechanically robust than chemical bond cross-links. Lawrence Livermore National Laboratory has been involved in the development of novel porous, low-density carbon materials since the 1980s.6 Several strategies have been identified to fabricate mass-producible graphene-based monolithic bulk materials from both low-cost polymer-derived carbon foams, which are called graphene aerogels (GAs),7 as well as by direct cross-linking of graphene oxide (GO) sheets, which are called graphene macro-assemblies (GMAs).8,9 These ultrahigh surface area bulk materials have high electrical and thermal conductivities, exceptional mechanical strength and elasticity, and tunable pore morphologies. GAs and GMAs have found utility in numerous applications including electrode materials in supercapacitors and rechargeable batteries, advanced catalyst supports, adsorbents, thermal insulation, sensors, and desalinization.10
The synthesis of graphene aerogels begins with sol-gel polymerization of an aqueous solution of resorcinol and formaldehyde to generate highly cross-linked organic gels. These gels are washed with water and acetone, then dried using supercritical CO2 and pyrolyzed in an inert atmosphere to give carbon aerogels with relatively low surface area and pore volume. Carbon aerogels are activated by controlled removal of carbon atoms under mild oxidizing condition (e.g., CO2) to form a cross-linked material composed of both amorphous carbon and graphite nanoplatelets, with higher surface area and open pore morphology.7 A unique advantage of the sol-gel synthesis is that GAs can be fabricated in a variety of forms, including monoliths and thin films, depending on the needs of the application. Carbon nanotubes11 and/or graphene sheets12 can be integrated into GAs by including these additives in the sol-gel precursor solution. This generates composite structures in which the additive becomes a part of the primary carbon network structure. Additionally, the GA framework can be functionalized after carbonization/activation either through modification of the aerogel surface or through the deposition of materials, for example catalyst nanoparticles, onto the framework structure.13
Graphene macro-assemblies (GMAs) are prepared by directly cross-linking suspended graphene oxide (GO) sheets, taking advantage of their inherent chemical functionality.9 GO sheets contain a variety of functional groups, including epoxide and hydroxide moieties, that can serve as chemical cross-linking sites. As in the GA preparation, assembled GMAs are supercritically dried to preserve the porous network, then pyrolized to reduce the chemical cross-links into conductive carbon bridges that provide structural support for the assembly. Due to the covalent carbon bridges between graphene sheets, GMAs have electrical conductivities and mechanical stiffness that are orders of magnitude higher than graphene assemblies formed with physical cross-linking. Additionally, GMAs have surface areas approaching the theoretical value of a single graphene sheet. Post-synthetic thermal treatment at elevated temperatures (>1,050 °C) can significantly improve the crystallinity of GMAs, leading to even higher conductivities and Young’s moduli as well as better thermal oxidation resistance.14 Post-synthetic chemical treatment of GMAs with redox-active organic molecules such as anthraquinone can enhance charge storage capacity in supercapacitor applications.15
The tunable material properties of GAs and GMAs are, in part, a result of carefully varying synthetic conditions such as reagent and catalyst concentrations, cure time and temperature, drying conditions, and carbonization/activation processes.16 This detailed video protocol aims to resolve ambiguities in the published methods, and to guide researchers attempting to reproduce the materials and conditions.
1. Resorcinol-formaldehyde (RF) Derived Graphene Aerogels
2. Graphene Oxide Derived Graphene Macro Assemblies
The evolution of material composition and morphology during fabrication can be tracked in various ways including X-ray diffraction, Raman and NMR spectroscopy, electron microscopy, and porosimetry. For example, in the synthesis, pyrolysis, and CO2 activation of GAs, the conversion was followed by X-ray diffraction (XRD) (Figure 1E). The absence of the stacking-related (002) diffraction peak in the XRD pattern after activation (blue trace) indicates the transition from a structure containing graphite nanoplatelets to one consisting of single-layer graphene. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) revealed short, linear features, which could be identified as individual graphene sheets viewed edge-on (Figure 1C and D). The Raman features for GA are similar to those observed for commercial graphene samples grown by chemical vapor deposition (Figure 1F). Solid state nuclear magnetic resonance (NMR) was used to follow the transformation of GO into GMA (Figure 2),9 which reveals that the significant epoxide and hydroxyl functionality (peaks between 50 and 75 ppm), as well as carbonyl groups (168 ppm) in its 13C NMR spectrum are virtually eliminated after curing, and an aliphatic carbon peak (26 ppm) appears, which suggests that epoxide and hydroxyl groups are involved in the cross-linking mechanism. After pyrolysis/carbonization, only the sp2 carbon peak remains and the 1H NMR spectrum (Figure 2B) also shows the elimination of–CH2– and –CH2O– moieties suggesting that the sp3 carbon cross-links were thermally converted to conductive sp2 carbon junctions. The GMA microstructure was analyzed by SEM and appears similar to that reported in other graphene assemblies (Figure 3). The nitrogen porosimetry adsorption/desorption isotherm shown in Figure 4A is Type IV, indicative of a mesoporous material. Post synthetic modification of GMA with anthraquinone was characterized by thermogravimetric analysis (TGA), with representative isotherms presented in Figures 5B and C. The derivatives of the mass loss curves show peaks at different temperatures for AQ-GMA and bulk AQ due to the adsorption of the AQ molecule on the carbon substrate. Electrochemical characterization included cyclic voltammetry (CV), which shows that AQ adsorption onto the GMA electrode produced reversible and stable faradaic charge storage (Figure 6A).
Figure 1. Processing and architecture of 3D-NG (GA). (A) Schematic of the polymer-based top-down approach. (B) Illustration of the mechanical robustness of a centimeter-sized 3D-NG sample. (C) Open macroporous network architecture composed of micrometer-sized pores and ligaments. (D) Internal structure of ligaments composed of curved and intertwined graphene sheets. Selected double-layer regions are marked by ovals, and a typical intensity profile across such a double-layer region is shown on the left. (E) X-ray diffraction data confirming the transformation of the initial multilayer graphene component to one that is dominated by single-layer graphene. (F) Comparison of the Raman spectra in the D/G band region from 3D-NG and multiple- layer graphene. This figure has been modified from Ref. 7. Please click here to view a larger version of this figure.
Figure 2. NMR characterization during GMA assembly and thermal processing. (A) 13C NMR spectra for GO powder, GO after initial gelation, and 3D graphene macroassembly. (B) 1H NMR spectra for GMA before and after thermal annealing. This figure has been modified from Ref. 9. Please click here to view a larger version of this figure.
Figure 3. FE-SEM images of the fracture surface of the 3D graphene macroassembly. (A) Low magnification. (B) High magnification. This figure has been modified from Ref. 9. Please click here to view a larger version of this figure.
Figure 4. Pore morphology of GMAs. (A) Nitrogen adsorption/desorption isotherm. (B) Pore size distribution. This figure has been modified from Ref. 9. Please click here to view a larger version of this figure.
Figure 5. Synthesis and characterization of AQ-GMAs. (A) Synthetic procedure and schematic illustration of non-covalent AQ functionalization of GMA electrodes. (B) TGA curve for 250 mm thick AQ-GMA disk with 11.2 wt% AQ loading (10.1% of total mass) and (C) TGA curve for bulk AQ (Ar flow 10 ml min–1, 10 °C min–1 temperature ramp). Derivative mass loss curves (dashed lines) show peaks at different temperatures for the AQ-GMA vs. bulk AQ. A higher temperature of more than 60 °C indicates adsorption of the AQ molecules on the carbon substrate. This figure has been modified from Ref. 15. Please click here to view a larger version of this figure.
Figure 6. Electrochemical characterization of AQ-GMAs. (A) Cyclic voltammogram (2 mV sec–1, 1 M HCl) for 250 mm thick GMA disks, as prepared (solid line), and with 12.4 wt% AQ loading (dashed line). (B) CVs of the same 12.4 wt% AQ-GMA sample at different potential sweep rates. The inset plot illustrates that AQ contribution to charge storage capacity remains constant until >60 mV sec–1, where solution resistance causes such large overpotentials that AQ cannot fully charge within the applied voltage window. (Capacity values were derived by integrating the discharge peak associated with AQ and subtracting underlying capacity of GMA.) This figure has been modified from Ref. 15. Please click here to view a larger version of this figure.
It is important to note that the procedures outlined here are only representative. Many adjustments are possible to tune materials for a specific application. For example, varying the starting material concentrations, while keeping resorcinol/formaldehyde (RF) ratio constant, can have an impact on the final material density. Catalyst loading can alter pore morphology, as a higher loading in the RF procedure leads to smaller primary particles and vice-versa. Activation time can play a role as well; at shorter activation times, new porosity is generated in the form of micropores while at longer activation times micropores are widened to sizes that cross the micropore–mesopore boundary.15 Additionally, the activation time must take into consideration the weight of the starting carbon foam; typical yields from the activation process described above were ca. 25%–30%, based on the weight of the starting carbon foam.
The same types of adjustments can be made with regard to additives such as CNTs and GO to the RF starting solution. Using the general techniques described above, foams with CNT loadings ranging from 0 to 60 wt% (with respect to RF solids) were prepared. The highest conductivity graphene aerogel achieved with this method used 1 wt% GO and 4 wt% RF in the initial reaction mixture; use of higher RF:GO ratios resulted in lower conductivity values. Graphene macroassemblies are generally limited to initial concentrations of ≤20 mg/ml because it is difficult to achieve stable suspensions at higher concentrations; suspensions of less than 10 mg/ml will still form monolithic structures, however they shrink considerably upon curing and carbonization making very low density materials difficult to achieve through initial concentration alone.
Carbonization temperature also provides opportunities for material tuning. Electrical conductivity increases dramatically above 800 °C for RF based materials. At carbonization temperatures above 1,500 °C, GMAs lose surface area and become more graphitic, with fewer defects in the graphene sheets.
In contrast to the flexibility in the material synthesis described above, we found that when functionalizing the GMA materials with anthraquinone, increased concentration and longer soak times did not result in higher AQ loading. This can be attributed to roughly monolayer AQ coverage, with layers beyond the first lacking stabilizing van der Waals interactions with the GMA substrate.
Future applications of the synthetic protocol described here include incorporating sacrificial templates into material fabrication to define macroporosity and optimize mass transport in, for example, flow battery electrodes.18,19 Work is also underway to formulate RF and GO precursor solutions for compatibility with advanced manufacturing (3D printing) techniques such as direct ink write and projection microstereo lithography.20 Lastly, we are continuing to develop novel composite materials based on 3D graphene scaffolds.21
The authors have nothing to disclose.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM release LLNL-JRNL-667016.
Single Layer Graphene Oxide | Cheap Tubes | n/a | 300-800nm XY dimensions |
single wall carbon nano tubes (SWCNTs) | Carbon Solutions | P2-SWNT | |
resorcinol | aldrich | 398047-500G | |
37% formaldehyde solution in water | aldrich | 252549 | |
acetic acid | aldrich | 320099 | |
ammonium hydroxide solution 28-30% NH3 basis | aldrich | 320145 | |
sodium carbonate | aldrich | 791768 | |
anthraquinone | aldrich | a90004 | |
Polaron supercritical dryer | Electron Microscopy Sciences | EMS 3100 | this is a representative model, any critical point dryer compatible with acetone should work |