Multifunctional Pristine Chemically Modified Graphene Films as Strong as Stainless Steel

Pristine chemically modified graphene films with light weights and excellent mechanical properties can be prepared by chemically engineering the structure of the graphene oxide sheets and the microstructures of the films. Particularly, these reduced graphene oxide films are as strong as stainless steel, ultra-tough, and have high electrical and thermal conductivities. Pristine films of chemically modified graphenes (CMGs), including graphene oxide (GO) and reduced GO (rGO), have attracted increasing interest because of their unique 2D structures, and excellent mechanical and/or electrical properties.1-6 CMG films can be readily fabricated from their dispersions via vacuum-assisted filtration,1, 2 evaporation induced self-assembly,7 electrospray coating,8 or wet spinning.9 rGO films can also be prepared by post-reduction of GO films,5 and they exhibit high electrical and thermal conductivities because of the partial restoration of their conjugated structures. Being composed of light elements, carbon, oxygen, and hydrogen, paper-like CMG films are also lighter than the widely used structural materials such as ceramics, most of metals, and alloys. The combination of these properties endows CMG films have great promise as multifunctional high-performance materials. However, pristine GO and rGO films are usually brittle with a failure strain less than 1.5%,1, 2, 6 and also still weaker than most widely used metals and alloys, strongly restricting their practical applications. GO is a 2D macromolecule featured with conjugated graphitic domains surrounded by oxygenated aliphatic regions.10-12 This inherent chemical structure provides GO with rich physical and chemical cross-linking sites for reinforcing the mechanical properties of CMG films via the interaction among GO sheets or between GO and external constituents.5 Extensive efforts have been devoted to increasing the interlayer interaction between GO sheets through hydrogen bonding, ionic binding, van de Waals attraction, and covalent cross-linking, as well as the use of large sheets.5, 13-21 Generally, ion binding and covalent crosslinking can remarkably improve the moduli and tensile strengths of CMG films, though at the expense of their toughness.13-15 Tough CMG films can be fabricated by optimizing the interfacial interactions and microstructures of the films via the introduction of external organic polymers through covalent or noncovalent approaches.18-20 However, these external components would deteriorate the thermal stability, mechanical and/or electrical properties of CMG films. This is mainly due to that the organic additives have much lower decomposition temperatures, and they would enlarge the interlayer distances between CMG sheets, alleviating the interlayer interactions and electron transportation between rGO interlayers. Thus, the fabrication of pristine CMG films without external constituents would pave a way toward the integration of high strength, high toughness, and/or high conductivity into CMG films. Nevertheless, it still remains a great challenge to develop a facile and convenient method for the preparation of high-performance pristine CMG films. In this communication, we demonstrate that robust pristine CMG films with arbitrary sizes can be prepared by engineering the chemical structures of GO sheets and the microstructures of the films. The GO films are mechanically stable in water, ultrastrong and ultra-tough with tensile strength and toughness of 453 ± 17 MPa and 10.86 ± 1.05 MJ cm−3. The corresponding rGO films have tensile strength up to 614 ± 12 MPa, as strong as AISI 304 stainless steel (585 MPa).22 They also have high toughness of 14.89 ± 1.02 MJ cm−3, together with high electrical conductivity of 802 ± 29 S cm−1 and thermal conductivity as high as 524 ± 36 W m−1 K−1. The excellent properties and light weights make these CMG films to be unique and attractive multifunctional materials for practical applications. GO sheets with an average lateral dimension of 5 μm were synthesized by a modified Hummers method at a relatively low oxidation temperature of 5 °C23 and nominated as GO (5) (the number in the bracket refers to the temperature; Figure S1, Supporting Information). An aqueous GO (5) dispersion (8 mg mL−1) sealed in a vessel was thermally annealed at 70 °C for 36 h, resulting in the formation of a homogeneous GO hydrogel as viewed by the tube inversion method and demonstrated by its dynamic rheological behavior (Figure S2, Supporting Information).24 Cast drying the resultant GO hydrogel at ambient temperature gave a compact GO film with controlled size, shape, and thickness depending on the amount of GO hydrogel and the surface area of substrate. The GO film can be easily peeled off from the substrate into a freestanding state. Post-reduction of the GO film in an ethanol/hydroiodic acid (57 wt%) solution (3/1, by volume) afforded rGO films with metallic luster (Figure 1a). These laminated CMG films (Figure 1b) are extremely flexible and can be readily shaped into any desired structures (Figure 1c). For clarity, the GO and rGO films via the gel-film transformation (GFT) process described above were nominated as g-GO (5) and g-rGO (5) films. g-GO (5) films exhibited an unprecedentedly integration of high strength and high toughness (Figure 1d,e), and their tensile strength, failure strain, and toughness were measured to be 453 ± 17 MPa, 5.55 ± 0.36%, and 10.86 ± 1.05 MJ cm−3, respectively. For comparison, GO films have also been prepared by filtration or evaporation of GO dispersions (denoted as f-GO and e-GO films, respectively). The mechanical performances of f-GO (5) and e-GO (5) films are much inferior to those of g-GO (5) films (Figure 1e), although they are the highest among the GO-based film materials reported previously (Figure 1f),1, 2, 14-20 and also much higher than the control films prepared from GO (35) sheets (synthesized via a conventional approach at an oxidation temperature of 35 °C; Figure S3, Supporting Information).25, 26 Post-reduction of g-GO (5) films resulted in the formation of g-rGO (5) films with tensile strength of 614 ± 12 MPa, failure strain of 6.67 ± 0.44%, and toughness as high as 14.89 ± 1.02 MJ cm−3, which are the highest values among the reported graphene-based paper-like materials (Figure 1f).1, 2, 14-20 It should be noted here that the tensile strength of g-rGO (5) films is comparable to that of AISI 304 stainless steel (585 MPa).22 Moreover, the low weight densities enable g-rGO (5) films having a high gravimetric specific strength of 307 N m g−1, and this value is 4.1 times that of AISI 304 stainless steel, 1.8 times that of aluminum alloy (Al 2014-T6), and 1.3 times that of titanium alloy (Ti 11 aged) (Figure S4, Supporting Information).27 It is generally accepted that strength and toughness are mutually exclusive in most of the structural materials caused by the intrinsic conflict between these two properties.28, 29 However, our CGM films are ultrastrong and ultra-tough, and their excellent mechanical properties are partially associated with the intrinsic defectless structure of GO (5) sheets. As shown in Figure 2a, the X-ray diffraction (XRD) pattern of an f-GO (5) film exhibits a diffraction peak at 2θ = 11.25° (d-spacing = 0.79 nm) with a full-width at half-maximum (FWHM) of 1.09°, and that of an f-GO (35) film is shifted to a slightly higher angle (2θ = 11.77°, d-spacing = 0.75 nm) with a narrower FWHM (0.54°). The higher content of organosulfate in GO (5) film (2.49 at% vs 0.58 at% sulfur for GO (35), Figure S1 and S3, Supporting Information) is responsible for its larger d-spacing and more disordered alignment of GO (5) sheets.30 Unexpectedly, the average tensile strength of f-GO (35) film (107 ± 18 MPa) with a more compact and ordered microstructure is about 45% lower than that of f-GO (5) film (194 ± 20 MPa) (Figure 1e). Given the fact that GO (5) and GO (35) sheets have nearly identical lateral sizes (Figure S1 and S3, Supporting Information), it is reasonable to conclude that the intrinsic chemical structure of GO sheets plays a critical role in the strength of GO films. X-ray photoelectron spectroscopy (XPS) surveys indicate that both GO sheets have close C/O ratios of 2.25−2.26 (Figure S1 and S3, Supporting Information). To expel the influence of adsorbed oxygen molecules on C/O ratio, C 1s core-level XPS patterns were acquired. As shown in Figure 2b, each C 1s spectrum can be divided into four peaks, corresponding to C–C/C=C (284.6 eV), C–OH/C–O–C (286.6 eV), C=O (287.7 eV), and HO–C=O (289.0 eV).31 Accordingly, the content of carbonyl and carboxylate species in GO (35) is slightly higher than that in GO (5) sheets, indicating that GO (35) has a higher oxidation degree. The relative amounts of oxygenated moieties in GO samples were further evaluated by PGO/PG ratios (the areal ratio of oxygen-containing peaks to the C–C/C=C peaks).31 The PGO/PG ratio of GO (5) is slightly lower than that of GO (35) (Figure 2c). The average distance between defects (LDs) in GO sheets has been estimated by the intensity ratio of D- to G-bands (ID/IG) in their Raman spectra.32 The ID/IG ratios of the GO (5) and GO (35) sheets were measured to be 1.05 ± 0.03 and 0.92 ± 0.03 (Figure 2d), corresponding to LDs of 1.45 and 1.38 nm, respectively. These results reflect that GO (5) sheets have fewer structure defects and larger graphitic domains compared with those of GO (35) sheets. This conclusion has also been confirmed by UV–vis and IR spectral examinations of both GO sheets (Figure S5, Supporting Information). Consequently, GO (5) sheets have stronger intersheet π−π interaction, which contributes to the superior mechanical performances of GO (5) films, being consistent with the theoretical simulation results (Figure S6, Supporting Information).33 To further optimize the intrinsic GO chemical structure, GO (5) dispersions were treated by thermal annealing at 70 °C. As reported previously,34 the oxygen content within GO (5) sheets was indeed preserved during thermal annealing, as confirmed by its nearly unchanged PGO/PG ratios in their C 1s XPS spectra and TGA curves irrespective of the annealing time (Figure 2c, and Figure S7 in the Supporting Information). However, thermal annealing GO dispersion induced oxygen diffusion within GO basal planes together with the cleavage of organosulfate moieties, forming prominent oxidized and graphitic domains as revealed by UV–vis and FTIR measurements (Figure 2e, and Figure S8 and S9 in the Supporting Information).34 As expected, the clustering of sp2 domains along with the cleavage of organosulfate within GO basal planes remarkably reinforces the van der Waals interaction between GO sheets, resulting in the formation of a GO hydrogel. The enhanced attraction force between GO sheets was further confirmed by XRD study and tensile tests of the resultant g-GO (5) films. With the elongation of annealing time, the d-spacing of GO film is gradually decreased (Figure 2f), and most impressively, the g-GO films prepared by cast drying the hydrogels deliver tensile strengths over 450 MPa. It is noted here that annealing a GO (35) dispersion under identical condition cannot induce gelation, although its zero-shear viscosity was increased to some extent (Figure S10, Supporting Information). The mechanical properties of the resultant e-GO (35) films are much inferior to those of g-GO (5) films (Figure S11, Supporting Information). On the basis of these facts, one can conclude that GO sheets with low defect density would be a prerequisite for thermal-driven GO gelation and applying GFT approach to prepare strong and tough GO films. The mechanical property of GO films depends strongly on their microstructures induced by the film processing processes. As shown in Figure 1e, both filtration and evaporation methods produced GO (35) films with comparable tensile strength and toughness. However, evaporation of GO (5) dispersion (8 mg mL−1) greatly (about 47%) enhanced its mechanical property compared with that of the counterpart prepared by filtration of 1 mg mL−1 GO (5) dispersion (285 ± 18 vs 194 ± 20 MPa in tensile strength). To clarify these differences, we prepared e-GO (5) films from GO (5) dispersions with different concentrations (2, 4, and 8 mg mL−1). It was found that the concentrated GO (5) dispersion with higher viscosity (Figure S12, Supporting Information) facilitates the formation of e-GO (5) films with improved mechanical performances (Figure 3a). To elucidate the effect of the viscosity of GO dispersion on the microstructures of GO films, we collected the XPS spectra from both surfaces of an e-GO (5) or a g-GO (5) film fabricated from the GO dispersions with the same concentration (8 mg mL−1). The spectra of e-GO film indicate that the surface exposed to air (upper) composes of highly oxygenated GO sheets, whereas the surface in contact with the substrate (bottom) has GO sheets with a relatively lower oxidation degree (Figure 3b). In stark contrast, the g-GO (5) film is homogeneous with nearly identical XPS patterns acquired from both surfaces (Figure 3c). Usually, the larger the GO sheets, the higher the C/O ratios.21 Accordingly, evaporation of a GO (5) dispersion leads to the formation of an asymmetric e-GO (5) film, whereas a highly viscous GO hydrogel facilitates the formation of a relatively uniform g-GO (5) film. From the viewpoint of colloid chemistry, GO sheet is a 2D amphiphilic macromolecule.10-12 In the case of using a GO dispersion with low viscosity, evaporation-induced self-assembly of GO sheets at the air/water interface dominates the film formation.7 Upon water evaporation, Brownian motion preferentially promotes an upward movement of smaller GO sheets with higher kinetic energy, making them gradually aligned in an energetically favorable layer-by-layer manner to form a GO skin. Simultaneously, the larger GO sheets prefer to exist in the sol phase underneath the skin upon the action of gravitational field. During the evaporation-induced self-assembly process, the synergistic effect of Brownian motion and gravitational field promoted the size fractionalization of GO sheets, and finally a compact but asymmetric GO film with laminated structure was formed (Figure 3d). In sharp contrast to a fluid-like GO dispersion, a GO hydrogel has a relatively stiff network because of the presence of strong physical cross-linking sites among GO sheets. During water evaporation, the mobility of the GO sheets was strongly limited, and the sheets within the gel matrix are difficult to make a conformation adjustment to adopt an energetically favorable parallel alignment. Thus, GO hydrogel shrinks homogeneously along the normal direction and finally forms a uniform GO film, having a laminated hierarchical structure with a more wrinkled texture (Figure 3e). The presence of hierarchical structure with unique interlocked wrinkles greatly increases the failure strains of GO films. This conclusion has been supported by theoretical simulation (Figure S13, Supporting Information) and confirmed by the cyclic stress–strain measurement of g-GO (5) film. As shown in Figure 3f, cyclic loading–unloading within a given strain (4.6%) induced a continuous increase in modulus from 5.5 to 17.6 GPa, reflecting that a permanent deformation is occurred under loading with consuming the interlocked wrinkles. At the same time, the film became more elastic accompanied with the disappearance of the plastic region. The mechanical properties of GO films strongly depend on their microstructures (Figure S14, Supporting Information). As a stretching stress is applied to the specimen of an e-GO (5) film, microcracks will be first generated in its mechanically weaker upside surface constructed by smaller GO sheets. Subsequently, these cracks extend along the cross section of the film because of stress concentration until to failure. On the contrary, for the homogeneous g-GO film with hierarchical structure, stretching the specimen strip will first straighten the wrinkles within GO sheets, dissipating part of the energy. Upon further stretching, the crack will propagate along the GO interlayers and stress transfer occurs. Finally, the relative slippage between the neighboring sheets pulls out GO sheets from the films. Moreover, the sheets pulling out would be partially blocked by the interlocked structure, leading to a crack deflection and a longer crack extension path.35 This unique fracture mechanism is beneficial to improve the toughness and tensile strength of g-GO (5) films simultaneously. Neat GO films usually possess poor water tolerance and can be readily disintegrated in water, which strongly limits their mainstream solution-based applications, for example, as filtration membranes.6 Water tolerant GO based films have been reported occasionally, while their stability is believed to originate from the cross-linking by multivalent metal cations.6 The foreign cross-linker might be an undesirable contaminant for the practical applications of GO films. Fortunately, our g-GO (5) film exhibited an excellent stability in water and kept its integrity without any rupture even after immersing in water for one month, whereas the e-GO (35) film was readily disintegrated into debris within one hour and partially re-dispersed in water after one day (Figure 4a). Even though a g-GO (5) film is water tolerant, it is still hydrophilic (Figure S15, Supporting Information) and can be swelled by water immediately. As shown in Figure 4b, after soaking in water, the diffraction peak of GO film is shifted to lower angles immediately, achieving an equilibrium state around 5 min. These results indicate that rapid infiltration and intercalation of water molecules into the interlayers produce stable water permeation nanochannels within the film. As a proof of concept application, we investigated the ion penetration performance of g-GO (5) films using a home-made diffusion cell (Figure S16, Supporting Information). As shown in Figure 4c, in the case of using an e-GO (35) film, the conductivity of permeate solution increased rapidly with time (321.5 μS cm−1 h−1), implying a high ion permeation rate. In contrast, the ion permeation rate through g-GO (5) film (10.2 μS cm−1 h−1) is 32 times lower than that of e-GO (35) film, implying that g-GO (5) film is more attractive for water desalination than e-GO (35) film. Graphene films with excellent combination properties are important for their applications to satisfy different practical requirements. Post-reduction of g-GO films affords g-rGO films with improved mechanical properties (614 ± 12 MPa) as well as excellent electrical (802 ± 29 S cm−1) and thermal conductivities (524 ± 36 W m−1 K−1), which are much superior to those of e-rGO (35) films (251 ± 29 S cm−1 and 288 ± 28 W m−1 K−1) and are also the highest values among the reported paper-like rGO films without conductive additives and high-temperature annealing treatment.5, 8 The intrinsic structure of rGO (5) sheets with fewer defects and larger graphitic domains as revealed by Raman measurements (Figure 4d), together with the absence of external constituents, are responsible for the extremely improved mechanical, electrical, and thermal properties (Figure S17, Supporting Information). The integration of lightweight, high strength, high toughness, and high electrical and thermal conductivities enable these all-carbon papers to have great potentials as multifunctional high-performance materials in the fields such as flexible electronics, aerospace, and tissue engineering. In summary, we developed a facile and easily scalable method for the fabrication of pristine GO and rGO films integrated with light weight, high tensile strength, and high toughness, as well as high electrical and thermal conductivities for rGO films. We demonstrate that both intrinsic chemical structure of GO sheets and hierarchical homogeneous microstructure of CMG films have significant impacts on the properties of paper-like graphene-based materials. The chemically engineering approach developed here is much superior to the widely used vacuum-assisted method for the preparation of large-area robust CMG films, and the latter is usually limited by filtration apparatus and membrane sizes. The theoretical calculations support our experimental results, showing that large graphitic domains facilitate to reinforce the interaction between GMG interlayers, and the presence of wrinkles within CMG basal planes is favorable for improving their failure strains. This work not only provides a simple method for the preparation of multifunctional high performance CMG films but also has wide implications in understanding the chemical structure–microstructure-property relationship in GMG films. This work was supported by the National Basic Research Program of China (973 Program, 2012CB933402) and the Natural Science Foundation of China (21274074, 51433005). As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. 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