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Graphene Nanoribbon Aerogels Unzipped from Carbon Nanotube Sponges Qingyu Peng, Yibin Li,* Xiaodong He, Xuchun Gui, Yuanyuan Shang, Chunhui Wang, Chao Wang, Wenqi Zhao, Shanyi Du, Enzheng Shi, Peixu Li, Dehai Wu, and Anyuan Cao* Graphene is the thinnest two-dimensional atomic crystal with outstanding electronic and mechanical properties, and has opened a wide realm of research opportunities since its successful isolation from graphite.[1–3] Whereas current chemical vapor deposition (CVD) techniques can be used to synthesize graphene films with high quality and controlled layers, chemical exfoliation is a much more simple and inexpensive way to produce graphene (oxide) sheets in large quantity.[4–7] The selfassembly of graphene sheets into macroscopic structures represents an effective way to translate the properties of individual sheets to functional materials and enable practical applications. To this end, monolithic aerogels and aerogel-like fibers made from graphene sheets have been studied intensively, and have demonstrated a number of applications in energy and environmental areas.[8–16] Graphene nanoribbons (GNRs) combine elegantly the structure and properties of carbon nanotubes (CNTs) and graphene sheets.[17–21] They have a flat crystal surface which elongates into a one-dimensional structure with high aspect ratio compared to exfoliated graphene sheets with their irregular sizes and shapes. GNRs can be synthesized by many methods such as cutting CNTs, lithographic patterning, and CVD.[18]

Q. Peng, Prof. Y. Li, Prof. X. He, Y. Shang, C. Wang, Dr. C. Wang, W. Zhao, Prof. S. Du National Key Laboratory of Science and Technology on Advanced Composites in Special Environments Centre for Composite Materials and Structures Harbin Institute of Technology Harbin 150080, P. R. China E-mail: [email protected] Dr. X. Gui State Key Lab of Optoelectronic Materials and Technologies School of Physics and Engineering Sun Yat-sen University Guangzhou 510275, P. R. China E. Shi, Prof. A. Cao Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, P. R. China E-mail: [email protected] P. Li, Prof. D. Wu Key Laboratory for Advanced Materials Processing Technology and Department of Mechanical Engineering Tsinghua University Beijing 100084, P. R. China

DOI: 10.1002/adma.201305274

Adv. Mater. 2014, DOI: 10.1002/adma.201305274

Among those, longitudinal unzipping of multi-walled CNTs by Ar plasma etching or chemical attacking can produce long, uniform nanoribbons with the potential for large-scale production.[19,20] Measurements and simulations on individual GNRs have revealed fascinating electrical and thermal properties.[21,22] Similar to graphene oxide sheets and CNTs, these GNRs also can be considered as nanoscale building blocks for constructing macroscopic functional materials and exploring practical applications. Recently, GNRs have been unzipped from powderform CNTs and then wet-spun into continuous fibers for field emission with high current density.[23] In another approach, a sheet of aligned multi-walled CNTs was directly unzipped into a GNR sheet by chemical treatment, and further shrank into a fiber after drying.[24] However, there have been less attempts to create a three-dimensional stable network of GNRs by a reliable method. Although individual graphene nanoribbons have been produced by longitudinal unzipping of CNTs or patterned growth, it remains challenging to construct these unique structures into macroscopic functional architectures, which impedes our effort in further exploring their practical applications. Here, we report an in-situ unzipping method to directly convert a CNT sponge into a GNR or GNR–CNT hybrid aerogel, which inherits the three-dimensional network and high porosity from the original CNT sponges. The GNR aerogels with enhanced surface area and chemical functionality can serve as supercapacitor electrodes and porous scaffolds for making reinforced nanocomposites by direct infiltration, with significantly enhanced strength and toughness. Our results demonstrate an alternative approach for controlled assembly of graphene nanoribbons and fabrication of nanoribbon-based functional structures and composites. Our basic idea is to unzip most of the CNTs in a sponge into GNRs while maintaining their original network structure, as illustrated in Figure 1. The CNT sponges consist of multiwalled nanotubes interconnected into an isotropic three-dimensional network, as reported by our team previously.[25] The nanotube network in the sponge can recover elastically under repeated compression at large strains, making a robust structure for further modification into GNRs. The key issue related to our strategy is how to unzip individual CNTs within the sponge without disturbing their network structure, as previous methods have mainly involved ultra-sonication in order to fully unzip the CNTs. Here, we have infiltrated an oxidative chemical solution into the porous sponge to induce intercalation of the nanotube walls from the defects, and then open the walls by introducing KMnO4, therefore realizing the in-situ unzipping

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Figure 1. Illustration of the process in which a CNT sponge is directly converted into a GNR aerogel by unzipping multi-walled nanotubes into multilayer graphene nanoribbons, while maintaining the original three-dimensional network.

process (see Experimental section for details). The sonication step was avoided as it would otherwise dismantle the sponge. After the unzipping, super-critical drying was performed on the unzipped CNT sponges to obtain GNR aerogels in which a large portion of multi-walled nanotubes have been converted into multi-layered GNRs. The resulting aerogel has the same shape and size as the CNT sponge before unzipping, and also maintains its porous microstructure (Figure 2a,b). However, in contrast to the nanotubes within the sponge, scanning electron microscopy (SEM) characterization reveals flattened, transparent nanoribbons and partially unzipped CNTs after treatment (Figure 2b). The presence of a large amount of nanoribbons indicates the successful unzipping of CNTs along the longitudinal direction. Unzipping nanotubes also results in uniform nanoribbons with controlled width (50–80 nm) defined by the tube diameter (30–40 nm); frequently we find that one nanotube was converted to two nanoribbons along the axis (arrows in Figure 2b). Transmission electron microscopy (TEM) images show that the obtained nanoribbons do not have clear walls and cavities like the corresponding CNTs (Figure 2c). Even though the nanoribbons appear to be very flexible,[26] they still can form a monolithic aerogel owing to the original robust CNT network and some remaining nanotube segments that help sustain the porous structure. Both GNRs and CNTs can be observed by dispersing the GNR aerogel onto a substrate surface; currently it is difficult to precisely determine the yield of nanoribbons (estimated to be >50% by adjusting the unzipping parameters) (Supporting Information, Figure S1). Although the GNR aerogel looks similar to the CNT sponge, their physical properties have changed substantially. Bulk sponges and aerogels can be squeezed into small balls, however, the CNT ball resumes its original shape by adding ethanol, whereas the GNR ball can

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not be recovered (Figure 2d). The CNT sponge also can recover elastically after large strain compression as opposed to the GNR aerogel that completely flattens under compression (Figure 2e). The hydrophobic surface of the CNT sponge has changed to hydrophilic in the GNR aerogel (water droplets can be absorbed quickly) (Figure 2f). Thus, the GNR aerogel looses the elasticity observed in CNT sponges, and becomes a plastic material that can be transformed to different shapes. The unzipping process not only changed CNTs into GNRs, but also grafted chemical groups onto the nanoribbon edges. X-ray photoelectron spectroscopy (XPS) characterization on the original CNT sponge only shows a delocalized alternate hydrocarbon peak (284.6 eV) and a small localized peak (285.6 eV).[27] In comparison, three new peaks at binding energies of 286.6, 287.8, 288.9 eV (assigned to the C-O, C=O and COOH group, respectively) appear in the GNR aerogel (Figure 3a).[28] Grafting of these hydrophilic groups on the nanoribbons results in dispersion of the GNR aerogel in water with a Zeta potential of −32 eV, whereas the CNT sponge cannot be dispersed by sonication (Figure 3b). At the same time, the signal of the Fe element disappears in the GNR sponge because the trapped Fe within the CNTs has been released and dissolved by acid during unzipping (Figure 3c). Consequently, thermogravimetric analysis (TGA) measurements reveal a lower residual amount (about 5.9 wt%) of the GNR aerogel because of removal of the Fe catalyst compared to the CNT sponge (15.9 wt.%) (Figure 3d). Correspondingly, the bulk density of the GNR aerogel has decreased from 11.88 mg cm-3 (for CNT sponge) to 9.33 mg cm−3. In addition, the unzipping of the CNTs decreased the thermal stability of the structure, as seen by the gradual weight loss when heating from relatively low temperature (mainly including loss of chemical groups) until full combustion at 615 °C. Although our unzipping process introduced defects by surface oxidation,

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Adv. Mater. 2014, DOI: 10.1002/adma.201305274

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COMMUNICATION Figure 2. Structural characterization of GNR aerogels. a) Photo of a CNT sponge and SEM image of internal nanotubes. b) Photo of a GNR aerogel made from the CNT sponge in (a), and SEM images of the nanoribbons inside. The arrows point to typical ribbon-like structure unzipped from the nanotubes. c) TEM images of dispersed nanoribbons from the GNR aerogel. The arrows point to ribbons. d) Photos of a CNT sponge squeezed into a ball and recovered by ethanol, and a GNR aerogel squeezed into a ball that cannot be recovered under the same condition. e) Snapshots of a CNT sponge compressed to 80% strain and recovered elastically, and a GNR aerogel that was flattened under compression. f) (left) a CNT sponge with a water droplet on its surface, and (right) a GNR aerogel (dashed circle on surface indicates initial water droplet which was absorbed rapidly).

the resulting GNR aerogel remains electrically conductive with a bulk conductivity of 0. 074 S cm−1 (lower than the CNT sponge, 1.15 S cm−1) and can be improved to 0.36 S cm−1 after reduction by hydrazine vapor. Unzipping CNTs into GNRs also leads to a significant change in their mechanical properties. The CNT sponge behaves elastically under compression and can recover to its original shape after cyclic compression at large strains (up to ε = 80%) (Figure 3e). In contrast, the GNR aerogel shows incomplete recovery after compression, indicating a plastic deformation (Figure 3f). Snapshots during mechanical tests are shown in Figure 2e. Specifically, the GNR aerogel has been completely flattened under a compressive strain of ε = 80%, and the unloading curve drops to zero immediately. Here, the underlying mechanism lies in the fact that these flat and flexible nanoribbons tend to make contact over a larger area (producing stronger van der Waals force) under compression which inhibits structure recovery. It is interesting that hydrazine vapor reduction (removal of functional groups from GNRs) not only improves the electrical conductivity, but also enhances the mechanical property, particularly elasticity. The reduced

Adv. Mater. 2014, DOI: 10.1002/adma.201305274

GNR aerogel becomes more elastic and can recover most of its volume after compression (Figure S2, Supporting Information). After unzipping, the surface area increases from 62.8 m2 g−1 in the CNT sponge to 113.1 m2 g−1 in the GNR aerogel. This higher surface area, more functional groups, and good conductivity render these GNR aerogels suitable candidates as supercapacitor electrodes. The cyclic voltammetry (CV) characteristics of these aerogels were measured in a three-electrode setup in aqueous electrolyte across a voltage window of 0.9 V (Figure S3, Supporting Information). The original CNT sponge has a relatively lower specific capacitance of 15.16 F g−1. In comparison, the GNR aerogel shows an increased capacitance of 92.61 F g−1, which can be further improved to 114.8 F g−1 after hydrazine vapor reduction. This capacitance is close to graphene-based two- and three-dimensional structures reported in the literature (100–300 F g−1).[29] Our GNR aerogels consisting of a uniform nanoribbon network are very suitable to fabricate reinforced nanocomposites. Compared to previous aerogels assembled from graphene oxide sheets (with pores enclosed by graphene layers),[9–13] thin nanoribbons unzipped from the CNT network can maintain a

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Figure 3. Chemical and mechanical properties of GNR aerogels. a) XPS curves of the CNT sponge and GNR aerogel showing the C1s peak. b) A CNT sponge and GNR aerogel placed in water, whereby the latter has been dispersed after sonication. c) XPS curves of the CNT sponge and GNR aerogel. d) TGA curves of the CNT sponge and GNR aerogel. e) Compressive stress–strain curves of the CNT sponge at different strains (20%–80%). f) Stress– strain curves of the GNR aerogels compressed to different strains.

highly open cell structure that is critical for direct infiltration of polymers. Direct infiltration also circumvents potential problems in traditional power-mixed composites,[30–32] where the agglomeration of fillers has often lead to defects and weakened the ductility and toughness of composites.[30] Recently, Cheng et al. have synthesized graphene foams with sub-mm cells and conductive graphene–polydimethylsiloxane (PDMS) composites

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as strain sensors.[8] Here, we have made a GNR-based PDMS composite with significantly enhanced tensile strength and toughness. We adopted a solution infiltration method to fabricate GNRPDMS composites embedded by the GNR skeleton (illustrated in Figure 4a). To facilitate infiltration, a block of GNR aerogel was immersed into the PDMS solution (slightly diluted with

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COMMUNICATION Figure 4. GNR-reinforced nanocomposites. a) Illustration of the fabrication process in which a GNR aerogel was immersed into PDMS for direct infiltration and then cured at 50 °C. The photo shows a bulk GNR–PDMS composite. b) Snapshots of a thin sheet composite retained by fixtures and stretched to different tensile strains (150 % and 300 %). c) Tensile stress–strain curves of a blank PDMS sample, a CNT–PDMS, and a GNR–PDMS composite. d) Calculated tensile strength, Young’s modulus, and toughness of the three samples. e) SEM image of the fractured surface of the GNR– PDMS composite. The two insets show nanoribbons embedded in the matrix. f) SEM image of a fractured CNT–PDMS composite. The inset shows CNTs protruding from the matrix.

10 wt% acetone) for one hour to let the PDMS flow into the aerogel while removing most of the acetone and bubbles in a vacuum oven. The GNR aerogel could maintain the bulk structure (without breaking) in the viscous fluid during the infiltration process. In this way we obtained a solid composite with the original porous GNR network completely filled by PDMS. The resulting composite maintained the original shape and size of the GNR aerogel, indicating that the GNR network has not been disturbed during the infiltration process (Figure 4a). The GNR–PDMS composite is also conductive, with an electrical conductivity of about 0.027 S cm−1. The composites were then cut into thin slabs for mechanical measurements, and reference samples including blank PDMS and CNT–PDMS composite (made by the same method on a CNT sponge

Adv. Mater. 2014, DOI: 10.1002/adma.201305274

before unzipping) were also tested. PDMS is a ductile material and typically can be stretched into large strains (150% in our test). With a built-in graphene network, the GNR–PDMS composite (0.87 wt% GNR) can be stretched to a strain of more than 300% without fracturing, which is an unexpected result (Figure 4b). Tensile stress–strain (σ–ε) curves show considerable change in mechanical behavior, in which the initial modulus (ε < 30%), the ultimate strength, and failure strain have all been enhanced in the composites (Figure 4c). Specifically, the tensile strength is about 3.4 MPa for bare PDMS, which was increased to 7.1 and 8.2 MPa for the CNT (loading = 1.13 wt%) and GNR (loading = 0.87 wt%) based composites, respectively. Furthermore, the toughness (work in breaking the sample or area under the loading curve) increased from 1.9 J m−3 (PDMS)

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to 9.7 J m−3 (CNT-PDMS) and 20.0 J m−3 (GNR-PDMS), indicating a 10-fold enhancement in toughness after introducing GNRs into PDMS (Figure 4d). The significant improvement in mechanical properties can be attributed to two main factors including the uniform dispersion of the filler and the effective load transfer between the nanoscale filler and matrix. Our solution infiltration process thus introduces polymers into the GNR aerogel while maintaining the original network structure. SEM images taken of the fractured surface of the GNR–PDMS composite after testing show a uniform distribution of many short nanoribbon segments protruding from the matrix (Figure 4e). These nanoribbons bridging the crack could block or deviate the crack from propagating through the PDMS matrix, thus increasing the energy consumption during fracture. In particular, the roots of the nanoribbons are well embedded in the matrix and can adhere to polymeric molecules because of the presence of many functional groups (as revealed in Figure 3a) introduced by unzipping. Also, some protruding GNR segments are very short, indicating the effective load transfer at the GNR-PDMS interface that results in simultaneous breaking of the nanoribbons and polymer matrix (rather than slipping out) (Figure 4e, inset). In typical nanocomposites, an increase in strength would cause a decrease in strain,[30] but here we observe a simultaneous improvement of strength and toughness in our GNR–PDMS system. We further studied the structure in a CNT–PDMS composite, and also could see many CNT segments in the fractured region (Figure 4f). However, the exposed CNTs are much longer (1–2 µm) than the GNRs (

Graphene nanoribbon aerogels unzipped from carbon nanotube sponges.

Graphene nanoribbon aerogels are fabricated by directly unzipping multi-walled carbon nanotube sponges. These fascinating materials have potential app...
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