Highly conductive, mechanically strong graphene monolith assembled by three-dimensional printing of large graphene oxide
Graphical abstract
Introduction
Over the past few years, there has been growing interest in manufacturing three-dimensional (3D) graphene structures because of their prospective applications, including super-capacitors [1], [2], battery electrodes [3], [4], contaminants absorption [5], [6], electromagnetic shielding [7] and so on. For these functionalities, low density, high mechanical stability and electrical conductivity are essential requirements. To fully exploit the inherent properties of graphene, it is important to establish an effective approach that can transform individual graphene sheets into desired 3D macroscopic structures.
The most used routes to produce 3D graphene structures can be divided into three categories, including chemical vapour deposition (CVD) on metallic foams [8], freeze-casting methods [9], [10] and hydrothermal processes [11], [12]. However, these methods cannot effectively tune the mechanical, electrical properties and change the macrostructures for specific applications. Generally, 3D graphene monolith made by CVD-based methods exhibited high conductivity but poor mechanical properties, while the geometry of the graphene monolith prepared by the freeze-casting method is not easily controlled. Hydrothermal methods can be used to prepare 3D graphene monolith with different shapes by changing the container shape, but the final products always exhibited low mechanical strength [13]. It is thus urgent to develop simple, template-free and cost-effective processing methods that can construct highly conductive, mechanically strong and shape designable 3D graphene monolith. Recently, an extrusion-based direct ink writing (DIW) 3D printing technique has emerged as a hot field for preparing graphene monolith. For such 3D printing, the essential prerequisite lies in developing printable graphene ink; the latter would possess a proper thixotropic (shear-thinning) behavior and viscoelastic characteristics. To this end, the previous studies have tuned the rheological behavior of graphene ink primarily through adding either polymer additives [14], [15] or silica nanoparticles [16]; however, this usually complicates the preparation process. Increasing the concentration of GO solutions (e.g., 80 mg/mL) [17] can avoid the use of additional additives such as polymers or nanoparticles, but concentrating GO solutions practically is a time-consuming process due to the occurrence of gelation. In addition, a synchronous freeze casting technique has been demonstrated to allow 3D printing of low concentration GO suspensions (e.g., 10 mg/mL) [18]. However, such synchronous freeze casting could be hard to implement in practical operations. Apparently, it is critical how to construct printable graphene or GO ink in a simple, cost-effective manner.
It has been demonstrated in our previous study [19] that large GO sheets (LGO) can form gel even at the concentration as low as 5 mg/mL, this provides a possible solution for room-temperature 3D printing without additional additives or freeze operations. Different from small GO sheets (SGO), LGO sheets have lower inter-sheet contact resistance and better stress transfer efficiency [19], which are important for improving the mechanical and electrical properties of 3D monolith. In this work, we employ low-concentration LGO solutions to print shape-designable, highly conductive and mechanically strong graphene monolith. Different from the ink based on SGO solutions, typically requiring 80 mg/mL, LGO solutions can be printed at a much lower concentration (20 mg/mL). The resulting monolith exhibits intriguing comprehensive performance: low density (12.8 mg/cm3), high electrical conductivity (41.1 S/m), specific strength (10.7 × 103 N·m/Kg) and compressibility (up to 80% compressive strain). Such LGO-based ink opens a promising door for 3D printing of high-performance graphene monolith that can be applied in energy and electronics fields.
Section snippets
Materials
Natural graphite flake (∼500 μm) was purchased from Sigma. Concentrated sulfuric acid (H2SO4, ∼98%), hydrochloric acid (HCl, 36.5%), hydrogen peroxide (H2O2, 30%) and potassium permanganate (KMnO4) were purchased from Jiangsu Tongsheng Chemical Co., Ltd. Chromium Oxide (CrO3) and hydroiodic acid (HI, 55%) were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification.
Preparation of LGO and SGO sheets
The synthetic procedures of LGO sheets were according to our previous
3D printing of graphene monolith
Fig. 1d–h schematically shows the preparation process of graphene monolith. The as-synthesized LGO solution was concentrated into 20 mg/mL through distillation in a 60 °C water bath to get the printing ink and then, the LGO ink was transferred into a 30 cm3 barrel (Fig. 1a). Next, the ink was deposited layer by layer through a 400 μm nozzle (Fig. 1d) using optimized parameters to give the 3D monolith (Fig. 1e). Fig. 1b shows the typical shear-thinning behavior of a LGO ink and Fig. 1c presents
Conclusion
We here present a convenient 3D printing strategy for fabrication of highly conductive, mechanically strong graphene monolith based on LGO solution with low concentration (20 mg/mL). The low concentration LGO solution is sufficient to reveal desired viscoelasticity and can be directly used as printing ink to prepare graphene monolith, without adding any other additives (e.g., polymers or nanoparticles). Compared to the 3D printing strategies reported in previous studies [14], [16], [17], here
Acknowledgments
The authors are grateful for the financial support by the 973 project (2011CB605702), the National Science Foundation of China (51173027), and Shanghai key basic research project (14JC1400600).
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These authors contributed equally.