Synthesis and processing of graphene hydrogels for electronics applications

Abstract

Synthesis of a graphene hydrogel, in which the elastic modulus can be changed from ∼10 to ∼10⁵ Pa and the electrical conductivity can be controlled in a range from ∼0.01 to ∼6 S cm⁻¹ by carefully adjusting the graphene content, allowing for direct processing and patterning of graphene for many electrical applications.

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Graphene, composed of single-atomic-layer graphite sheets, is currently the thinnest material in the world¹ and has attracted tremendous attention in recent years.² Graphene is a zero-bandgap semiconductor,³ which acts as a metal if the Fermi energy is tuned by the application of a gate voltage.⁴ Compared with carbon nanotubes, graphene has similar mechanical properties, possesses a much larger specific surface area, and exhibits higher electric and thermal conductivities. Graphene shows promise for a wide range of applications from nanodevices to composite materials.⁵ In particular, graphene foam based composites including aerogels and hydrogels are recently of raising researches interests for the use in wide range of applications.⁶ Here, we report on the synthesis of a graphene-based hydrogel, in which the elastic modulus can be changed from ∼10 to ∼10⁵ Pa by carefully adjusting the graphene content, allowing for direct processing and patterning of graphene through a simple and versatile approach.⁷ The apparent electrical conductivity of the graphene hydrogel can be controlled in a range from ∼0.01 to ∼6 S cm⁻¹, which depends on the loading amount of graphene in the hydrogel and may be sufficient for many electrical applications.⁸ Our top-down approach for tuning the rheological and electrical properties of graphene combined with the direct patterning of graphene provides a simple and efficient path to process graphene for use in a variety of applications.

Previous research has reported that the mechanical properties of associated flexible polymeric materials and rod networks play a very important role in a variety of material contexts ranging from cross-linked gels to stress-bearing colloidal suspensions.⁹ Carbon nanotubes have been well investigated for modifying mechanical properties.¹⁰ Viscoelastic properties are greatly enhanced in the presence of a small amount of carbon nanotubes, mainly due to the entanglement and framework effect of carbon nanotubes in a polymer-like matrix. This enhancement in rheological properties also applies to graphene for processing graphene in our study. Fig. 1 demonstrates our approach for the synthesis of graphene-based hydrogels, which were composed of approximately 5 μm graphene nanosheets and a bio surfactant, sodium deoxycholate (SDC). SDC is an amphiphilic molecule composed of a hydrophobic steroid part and two hydrophilic hydroxyl groups. The hydrophobic steroid part is stably adsorbed onto the graphitic plane by hydrophobic interactions, which makes it a highly efficient surfactant for dispersing carbon nanotubes¹¹ and graphene¹² in an aqueous solution. Moreover, SDC exhibits complex phase behaviour in aqueous solutions. It forms primary micelles at a critical micelle concentration (CMC) of 0.05%, undergoes secondary self-assembly at a CMC of 0.1%,¹³ and can then be phase transformed to hydrated nanotubes and hexagonal liquid crystals.¹⁴ Therefore, graphene and SDC form a hybrid hydrogel in aqueous solution, with the SDC nanotubes serving as a flexible matrix while graphene acts as a rigid framework. Both viscous and elastic modulus of graphene hydrogels are very sensitive to the loading of graphene, which allows for direct processing and patterning of graphene through a simple and versatile approach for graphene-based nanostructures and devices.⁷

Fig. 1 Schematic illustration of the synthesis and processing of graphene hydrogel.

The experimental details are described in the ESI.† We directly observed the self-assembly of SDC nanotubes in aqueous solution by optical microscopy. Fig. 2a shows a typical optical photograph in which the SDC concentration (Ø_SDC = M_SDC/M_water) is 40%. SDC nanotubes have a diameter on the order of tens of micrometres and a length of more than a centimetre, with an aspect ratio that is much higher than 1000. SDC nanotubes also show high flexibility. When graphene was dispersed into the SDC matrix, the micrometre-ordered size of the graphene made it difficult to be enwrapped into SDC nanotubes as easily as carbon nanotubes,¹⁵ but was still homogeneously dispersed into the SDC matrix (Fig. 2b). The insets in Fig. 2a and b show white SDC and black graphene–SDC, respectively. The hybrid material is called a hydrogel because it contains a great deal of water without fluidity. Because SDC is soft and flexible, it is very easy to align SDC nanotubes under an introduced external force. Therefore, graphene–SDC hydrogels can also be easily arranged for aligned self-assembly (Fig. 2c), which makes them a suitable material for design and processing. Fig. 2d and e show scanning electron microscopy (SEM) images of graphene in a SDC aqueous solution. At low concentrations, the SDC is completely dissolved in water without the formation of nanotubes (ESI, Fig. S1†). We clearly observed the morphology of graphene nanosheets (Fig. 2d, see also ESI, Fig. S2†). The graphene nanosheets had an average size of 5 × 5 μm² and an average thickness of 10 nm. When the concentration of SDC was higher than the secondary CMC (0.1%), many belt-like materials were observed, most likely SDC nanotubes that covered the graphene nanosheets (Fig. 2e, see also ESI, Fig. S3†). A cross-sectional image showed that graphene nanosheets were closely packed side by side (Fig. 2f).

Fig. 2 Optical photograph of (a) SDC fibrous self-assembly in a pure SDC aqueous solution with a concentration of Ø_SDC = 40% and (b and c) graphene–SDC hydrogel with a concentration of Ø_graphene = 4%, Ø_SDC = 40%. The insets in (a) and (b) show images of a pure SDC hydrogel and a graphene–SDC hydrogel, respectively. (d) SEM image of the graphene–SDC system with a concentration of Ø_graphene = 1%, Ø_SDC = 10%. A top view (e) and a cross-sectional view (f) of graphene–SDC system with a concentration of Ø_graphene = 4%, Ø_SDC = 40%. (g) XRD patterns of hydrogels with various graphene and SDC loadings. The inset shows (002) diffraction intensity changes as a function of the Gp–SDC ratio. The fit line has a reliability coefficient of R = 0.99187. (h) Raman spectroscopy of hydrogels with various graphene and SDC loadings.

X-ray diffraction (XRD) measurements were carried out to investigate the synthesised graphene hydrogels (Fig. 2g). Graphene nanosheets have a strong, sharp peak at 2θ = 26.5°. This peak is assigned to (002) diffraction. A very weak peak appears at 2θ = 54.6°, which is assigned to (004) diffraction. The intensity of the main (002) peak gradually decreased with reduced graphene content in the hydrogel. The inset in Fig. 2g shows the intensity of the (002) peak as a function of graphene content. A linear relationship was found by fitting, with a reliability coefficient of R = 0.99187.

Fig. 3 Elastic modulus of the hydrogels as a function of shear stress with graphene and SDC loadings of (a) Ø_graphene = 1%, 2%, 3% at Ø_SDC = 30%, (b) Ø_graphene = 3% with Ø_SDC = 10%, 20%, 30%, and (c) Gp–SDC ratio = 0.1, 0.2, 0.3, 0.4 at a high loading mass of graphene and SDC. (d) Elastic modulus (G′) and viscous modulus (G′′) of hydrogels as a function of angular frequency, with various graphene and SDC contents.

Graphene nanosheets display a strong G band at 1580 cm⁻¹ in Raman spectroscopy (Fig. 2h). The D band and 2D band appear at 1350 and 2715 cm⁻¹, respectively. A strong G band and a weak D band indicate a lack of defects on the graphene nanosheets. When comparing graphene nanosheets to graphene hydrogels, there is no remarkable change in the intensities of the D band or G bands, but the shape of the Raman profile is changed so that the baseline of the Raman absorption is gradually enhanced from the low to high energy region in the range of the Raman shift. The amplitude of the enhancement also gradually increased with increasing SDC loading. The shape change in the Raman absorption was attributed to the influence of SDC. SDC powder exhibits two strong, sharp characteristic Raman peaks at 2862 and 2940 cm⁻¹ accompanied by many fingerprint peaks at a range from 500 to 1500 cm⁻¹ (ESI, Fig. S4†). The Raman peaks of SDC dramatically decrease in pure SDC hydrogels, and nearly disappeared in graphene–SDC hydrogel, indicating that the Raman signal of SDC is very sensitive to the chemical surrounding. We found that the baseline of the Raman profile tended toward a smooth line and that new peaks disappeared after irradiation for several minutes by the excited laser in the Raman measurement, indicating that the SDC molecules were destroyed and/or decomposed by laser irradiation. This result was also directly observed by CCD camera on the Raman spectroscope.

SDC hydrogels have poor viscoelastic properties.¹⁶ However, the viscoelastic properties of SDC hydrogels are obviously enhanced in the presence of other materials,¹⁷ especially carbon nanotubes.^7b When graphene was introduced into the SDC matrix, the enhancement of the viscoelastic properties was also investigated. Upon the introduction of a small amount of graphene, the elastic modulus of the graphene–SDC system gradually increased (Fig. 3a). Fig. 3b shows the enhancement of the elastic modulus as a function of SDC content. At low concentrations, the enhancement of the elastic modulus induced by SDC was slightly larger than that of graphene. However, in the case of a high concentration, the influence of graphene dominated that of SDC, and the elastic modulus of the hydrogels was dramatically enhanced by up to 10 000-fold (Fig. 3c). The elastic modulus of a sample with 4% graphene and 40% SDC was 1000 Pa at a shear stress of 1 Pa, which is 100 times higher than that of a sample with 1% graphene and 30% SDC and 10 times higher than that of a sample with 3% graphene and 30% SDC. The elastic modulus was further increased 6-fold and 12-fold by loading the graphene to 8% and 12%, respectively. The elastic modulus was larger than 10⁶ Pa, an extremely high value, for a 12% graphene and 30% SDC hydrogel. This value is comparable to that of carbon nanotube hydrogels.¹⁰

Fig. 3d shows the elastic modulus (G′) and the viscous modulus (G′′) of the graphene hydrogels as functions of angular frequency. In the angular frequency range from 0.01 to 100 Hz, both the elastic and viscous modulus increased with the angular frequency. At low graphene loading (4% Gp), when the angular frequency was smaller than 0.06 Hz, the viscous modulus dominated the elastic modulus, indicating that the sample was viscous rather than elastic. When the angular frequency was larger than 0.06 Hz, the elastic modulus dominated the viscous modulus, suggesting that a rheological transformation occurred at 0.06 Hz from viscous to elastic behaviour. With increasing graphene content, both the elastic modulus and the viscous modulus increased by orders of magnitude. For high graphene loading, the elastic modulus nearly dominated the viscous modulus, suggesting that the hydrogel had viscous rather than elastic behaviour. To further clarify the elastic–viscous behaviour of the graphene hydrogels, the rheological behaviour of hydrogels with a much lower graphene loading was also investigated in this study (ESI, Fig. S5†). The data showed that at low graphene loading, a rheological phase transformation from viscous to elastic behaviour nearly occurred. This finding suggests that the elastic–viscous behaviour is strongly dependent on the graphene load. The tunable viscoelastic properties of graphene hydrogels make it a suitable material for rheological applications.¹⁸

Graphene is a very interesting two-dimensional material for many applications, especially flexible electronics. Many approaches have been reported for processing graphene and for the fabrication of graphene-based devices,¹⁹ but these aims are not trivial to accomplish. Graphene hydrogels have tunable viscoelastic properties and controlled electric properties, which makes this hybrid material suitable for the processing of graphene for use in many applications. Fig. 4a shows an approach for fabricating aligned graphene wires by using graphene hydrogel as a “solid ink” by carefully adjusting the viscoelasticity. Free-standing aligned graphene self-assembly was obtained over macroscopic scales by this approach, which has been applied to process carbon nanotubes^7b and other organic gels.²⁰ The alignment of graphene hydrogels was also achieved on a substrate (Fig. 4b). The graphene hydrogel hardened after drying in air and formed stable and hardened patterns, demonstrating a simple approach for constructing graphene-based devices by combining the tunable rheological property of graphene hydrogel and omnidirectional printing. Computer-controlled omnidirectional printing was used to directly write graphene patterns (Fig. 4c, see also ESI, Fig. S6†). Fig. 4d shows a sample of aligned graphene wire directly written on slide glass substrates by omnidirectional printing using a hydrogel with 12% graphene and 30% SDC (Gp–SDC = 0.4). The diameter of the nanowires was approximately 200 μm. The opening size of the nozzle, the pump pressure, and the rate of writing were the main parameters controlling the alignment of the graphene self-assembly. Fig. 4e shows graphene patterns written by introducing a variety of pump pressures. The diameter of the aligned, self-assembled graphene prepared under high pressure was larger than that for low pressure. Because the rheological properties of graphene hydrogels are strongly dependent on the loading of graphene and SDC (Fig. 3), the surface wettability of graphene-based patterns is also closely related to the graphene content. Fig. 4f–i show cross-sectional images of patterns written directly onto slide glass substrates by using hydrogels with various graphene contents. The contact angle was 20°, 57°, 63°, and 125° for patterns with Gp–SDC ratios of 0.1, 0.2, 0.3, and 0.4, respectively. The higher graphene content hydrogels had better mechanical performance and resulted in a larger contact angle. Complex two-dimensional and three-dimensional patterns can be directly “written” by using this omnidirectional print method,²¹ which presents a simple and efficient approach for graphene-based devices (ESI, Fig. S7, Video S1†).

Fig. 4 Processing and electronic properties of graphene hydrogels. (a) Free-standing aligned graphene wire prepared from a graphene hydrogel. (b) Processing of aligned graphene wire on a glass substrate. (c) Patterning of graphene by computer-controlled omnidirectional printing using the graphene hydrogel as a solid ink. (d), SEM images of aligned graphene wire prepared from a graphene–SDC hydrogel with a Gp–SDC ratio of 0.4. (e) Graphene patterns written under various pump pressures of 50, 60, and 70 kPa. (f–i) Cross-sectional images of self-assembled, aligned graphene acquired for graphene–SDC hydrogels with a Gp–SDC ratio of 0.1, 0.2, 0.3, and 0.4, respectively.

Graphene has an extremely high conductivity of 10⁶ S cm⁻¹, which is ten times higher than that of metallic carbon nanotubes. However, in the case of a graphene-based circuit on the macroscopic scale, the conductivity mainly depends on the electron transfer from graphene nanosheets to neighbouring graphene nanosheets. Current–voltage (I–V) response curves of dried graphene hydrogels with various graphene contents are shown in Fig. 5a. All samples showed linear I–V feedback, indicating ohmic conductivity in the graphene hydrogels. The slope of the I–V response curve, namely the resistance, gradually decreased as the graphene loading increased, indicating that the conductivity of the graphene hydrogel can be improved by increasing the graphene content. Fig. 5b shows the apparent conductivity of the graphene hydrogel as a function of the ratio of graphene–SDC. An approximately linear relationship was discovered, with a reliability coefficient of R = 0.99066.

Fig. 5 Electric properties of graphene hydrogels. I–V response curve (a) and conductivity (b) of graphene hydrogels as a function of the Gp–SDC ratio.

Because the electric conductivity on the macroscopic scale mainly depends on the electron transfer from graphene nanosheets to neighbouring graphene nanosheets, graphene hydrogel with high loading content of graphene can certainly improve the electric conductivity by increasing the contact area between graphene nanosheets. However, any further increases in the graphene content in the hydrogel would be limited by the dispersion capacity of SDC. In previous research on carbon nanotube hydrogel, an ethanol-washing strategy was introduced to increase the conductivity by removing SDC molecules.^7b The electric conductivity of carbon nanotube enhanced to ∼55 S cm⁻¹ after removing SDC. However, due to the large steric effect and the weak inter-sheet interaction in graphene nanosheets, removing the SDC binder would certainly result in destruction of the graphene-based patterns. The introduction of assisted electrolytes is speculated to be an efficient method for enhancing the conductivity by increasing the charge carriers.²²

In summary, graphene-based hydrogel is synthesized by mixed graphene and biomolecular surfactant, SDC, in aqueous solution. The viscoelastic properties of graphene hydrogel can be modified by carefully adjusting the graphene content. The elastic modulus can be changed from ∼10 to ∼10⁵ Pa in various graphene contents, promising many applications in rheology and lubrication. The apparent electrical conductivity of the graphene hydrogel can be controlled in a range from ∼0.01 to ∼6 S cm⁻¹, which is sufficient for many electrical applications. The approach discussed in this paper for tuning the rheological and electrical properties of graphene and for direct patterning of graphene provides a simple and efficient path to process graphene for use in a variety of applications, especially in rheological materials and electrical devices.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46856a

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