Experimental Guidance to Graphene Macroscopic Wet-Spun Fibers

Article pubs.acs.org/cm

Experimental Guidance to Graphene Macroscopic Wet-Spun Fibers, Continuous Papers, and Ultralightweight Aerogels Zhen Xu, Li Peng, Yingjun Liu, Zheng Liu, Haiyan Sun, Weiwei Gao, and Chao Gao* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, and Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China ABSTRACT: Graphene macroscopic materials have attracted tremendous attention for their fascinating performance and rich functionalities. Here, we provide an elaborate description of techniques in the fabrication of graphene macroscopic materials, focusing on the wet-spinning of 1D fibers and wet-spinning of continuous 2D films and 3D ultraflyweight aerogels. The thread of the research concepts is discussed to offer an overview of graphene macroscopic assembly. We summarize the fabrication system of wet-spinning of fiber and films, which extends to the chemistry of solvated graphene, the formation of graphene LCs, and the chemical/thermal reduction of graphene materials. The experimental details of graphene ultraflyweight aerogel are also been described. We hope that this paper can act as an experimental guidance for researchers, and become suggestive for forthcoming advances in graphene macroscopic materials.

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INTRODUCTION The discovery and invention of carbon nanomaterials are reshaping our understandings of the wonderful carbon world. All the new carbon nanomaterials with fascinating properties are poised to jump from the nanoscale to be intergrated into macroscopic materials in our daily world.1,2 Recently, graphene has become a paradigm to renew graphitic materials with respect to material design concepts, fabrication processes, materials structure, and defects, thereby enabling new properties and functions.2−10 Many extraordinary properties of graphene, the single layer of graphite, have been predicted in theory and demonstrated in experiments, including the highest mechanical strength (1.1 TPa of tensile modulus and 130 GPa of tensile strength), the record thermal conductivity (5000 W/mK), and high carrier mobility (200 000 cm2/(V s)), to name but a few.1,3,4 Many of these attractive properties of graphene just appear in nanoscale, held by single layer graphene only in some cases. In this context, to efficiently transition these nanoscale properties of graphene as useful properties and functions to the macroscale becomes a fundamental issue for the macroscopic applications of graphene.4−10 To date, a research system focusing on this issue has been established, which extensively involves chemical functionalization for reliable graphene in large scale, structural design of the macroscopic assembly of graphene together with the controlling techniques, structural characterizations, tuning the performances and functions, and attempts at applications.11−38 All these efforts are aiming to fashion single graphene building blocks into specific structures in an appropriate form, in order to express the chosen properties of graphene at the utmost extent in the given macroscopic materials. Due to its attractive intrinsic merits, the applications of graphene in macroscopic materials are very extensive. Generally, © 2016 American Chemical Society

graphene macroscopic materials can be divided into three types in the terms of their topology, that is, one-dimensional (1D) fibers, 2D films/papers, and 3D frameworks and bulks (Figure 1).8,11−38 The recent advances have promoted graphene macroscopic materials as a new category of promising graphitic materials, which can compete or even outperform conventional graphitic materials. These advances involved the creation of new forms of materials, structure designs, emerged new functions, and soaring overall performances. For example, ultralightweight graphene aerogels were fabricated by interconnecting graphene sheets with giant lateral size;35−38 graphene filtrated paper exceeded commercial compressed graphite foils and even steel in the mechanical strength;27−32 and 1D graphene fibers performed outstanding electrical and thermal conductivity that were on a par with the best carbon fibers.19−21 In graphene macroscopic materials, graphene, neat or hybridized, exists as either independent single layers or interlayer coupled laminates, which determines the performances and functions of assembled materials. Taking a typical example, the interconnection of single graphene sheets renders high surface area and abundant porosity of lightweight 3D frameworks, whereas the compact piling of graphene in a well-aligned order provides high mechanical properties and electrical/thermal conductivities of 1D fibers and 2D films. Therefore, how to assemble graphene into the designed structure in a high production rate and high efficiency is a vital technical subject for the production of graphene macroscopic materials to achieve Special Issue: Methods and Protocols in Materials Chemistry Received: July 16, 2016 Revised: September 11, 2016 Published: September 12, 2016 319

DOI: 10.1021/acs.chemmater.6b02882 Chem. Mater. 2017, 29, 319−330

Chemistry of Materials

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Article

EXPERIMENTAL DESIGN

Synthesis of GO and Well-Dispersed Graphene Derivates. Fluid assembly seems to be a viable option to process the unmeltable graphene, which rejects the possibility of the melt process. Pristine graphenes can be dispersed in some choice solvents only at extremely low concentrations (typically 98% HCl ∼37% ∼68% nitric acid H2O2 31% DMF >99.9% acrylonitrile 99+% Caution: toxic Notice: eliminate the stabilizer before use AIBN 99% Preparation for Graphene Fibers, Films, and Aerogels acetone >99% ethyl acetate 99% sodium alginate (SA) 99% CaCl2 99% HI acid 55% acetic acid 99.5% hydrazine monohydrate >98%

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provider Qingdao Henglide Graphite Co., Ltd. Sinopharm Sinopharm Sinopharm Aladdin Aladdin Alfa Aesar

Aladdin Sinopharm Aladdin Aladdin Aladdin Aladdin Aladdin Aladdin

a All the reagents were used without any further purification unless otherwise specified. Refer to the Material Safety Data Sheet (MSDS) from the provider.

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Figure 2. Photographs of the oxidation process at different stages. (left) After mixing of expanded graphite and acid, (middle) finishing with the addition of KMnO4, and (right) the termination stage of oxidation.

Table 2. Anticipated Performances of GO and Graphene Fibers by Procedure 2A and Films by Procedure 3

a

samples

diametera (μm)

tensile strength (MPa)

modulus (GPa)

electrical conductivity (S/m)

GO fiber graphene fiber wet-spun graphene films

5−20 5−20 1 m length, 2−5 cm width

200−300 300−500 100−200

10−50 50−100 10−30

insulating 4−8 × 105 1−2 × 104

the diameter depends on the nozzle size and the stretching process.

(iii) The final GO LC solution was obtained by repeated centrifugation and washing for 20 cycles with ultrapure water until the pH of the supernatant solution approached the range of from 6 to 7 (avoid ultrasonication which could rapture the giant GO to debris, see Table 2). The aqueous LCs of the GO will be directly used for the preparation of GO aerogel after dilution to a certain concentration. It can be also exchanged to DMF GO LCs for wet-spinning of fibers and films, which can enable the direct collection of fibers and films without the washing step. Notice: Seal and keep the GO solution at a low temperature (∼10 °C) with a relatively high concentration (>6 mg/mL) in a dark environment to maintain its chemical and physical properties. (D) Polymer grafting of GO [GO-graf t-PAN (polyacrylonitrile)] (i) Prepare 80 mL of 1 mg/mL GO DMF dispersion by repeated centrifugation and load into a flask. The solvent DMF we exchanged from water is compatible with the monomer and initiator. (ii) Add 10.6 g (200 mmol) of acrylonitrile and 82 mg (0.5 mmol) initiator of AIBN (azodiisobutyronitrile), stir to dissolve, and sonicate in a 40 kHz sonic bath for 10 min. (iii) Purge the solution with nitrogen for 40 min and heat to 65 °C and keep for 48 h under nitrogen protection. (iv) After cooling, expose to air to terminate the reaction. The resultant cream-like gel was thoroughly washed by pure DMF for 10 times, and the GO-graf t-PAN without free polymer was obtained. 2. Continuous Fabrication of GF. (A) Neat graphene fiber (a) Continuous spinning of GO fibers (i) Prepare GO spinning dopes in DMF by the repeat centrifugation method. Take 50 mL of 5 mg/mL aqueous solution of GO, centrifuge

at 15 000 rpm for 1 h, pour out the upper supernatant fluid (about 30 mL), add DMF into the tube, and mix it on a vortex mixer to attain a homogeneous without visible coagulum gel. The mixture was centrifuged at 15 000 rpm for 2 h, and the supernatant was poured out for the next step to exchange solvents. After 5 cycle of DMF washing, add DMF to the initial volume and obtain a DMF dispersion of GO with the same concentration of 5 mg/mL. (ii) Before wet-spinning, filtrate GO DMF dopes by a filtrated mesh with a 200 mesh number to eliminate impurities, which could be pollution particles or dried GO particles formed in storage. The air bubbles were eliminated by vacuum stirring and blend mixer. The filtration and degassing steps are useful to enable the continuity of collected fibers. Then, homogeneous GO dopes were ready for the next spinning step. Caution: Make sure the spinning dope highly purified without any particles and bubbles, which can disrupt the continuity of fiber in the spinning. Under an optical microscope in the reflection mode, GO dope can be dropped onto a silicon wafer, and the purity can be examined. (iii) Load the GO spinning dope into a syringe with a spinning nozzle, and the nozzle diameter can be chosen from 60 to 250 μm. After, equip the syringe onto the spinning apparatus as shown in Figure 3, and extrude the GO dope into the coagulation bath directly. The coagulation bath is a mixture of ethyl acetate and acetone (3:1 in volume ratio) in the case of DMF spinning dopes. (iv) Guide the gel GO fiber out of the bath, through the guide reel, and onto the collection reel. Caution: The single GO 323


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(iv) Heat the furnace to 1300 °C at 2 °C/min, and keep it at 1300 °C for 2 h. Notice: The heating rate should be low to 2 °C/min to get fibers with compact structure. The higher heating rate (>5 °C/min) leads to fast release of gas to generate porous structure in the fibers. (v) After cooling naturally, the thermally reduced graphene fibers can be collected. (vi) Put the fibers on pure graphite reels into the graphitization furnace for further reduction and recrystallization of graphene sheets and their laminates. Replace the air in the furnace by highly pure Ar (99.9999%) 3 times (Ar was purified further by high temperature silver catalyst before entering into the oven). (vii) Heat the furnace to 3000 °C at a rate of 10 °C/min. Notice: make sure the pressure inside the furnace is higher than the atmospheric pressure to reject the trace leak of oxygen. (viii) Keep at 3000 °C for 30 min and cool down by the water cycling system, and the silvery graphene fibers are obtained. (ix) After naturally cooling to room temperature, silvery graphene fibers were obtained. (B) Composite fiber [an example: graphene−sodium alginate (SA) composite fibers] (i) Add a fixed amount of SA powder to GO LC aqueous dispersions with concentrations of 1−20 mg/mL. Notice: The SA powder should be added slowly in small portions over a period of 2 h to facilitate the dissolution of the SA. (ii) Vigorously stir the mixture by a magnetic stirring bar at ∼800 rpm at room temperature for 4 h. (iii) Purify the solution by centrifugation at 2000 rpm for 10 min to remove the nondissolved aggregates and air bubbles. This is the same step as in the case of wet-spinning neat GO fibers to ensure the continuity of fibers. Caution: This step is indispensable. Centrifugation enables the solution to be more spinnable. What is more, the centrifugation speed should not be higher than 4000 rpm; otherwise, phase separation will occurred. (iv) Load the GO−SA LC mixture into a syringe with a spinning nozzle with diameter of ∼100 μm. (v) Inject the LC dope into a rotating coagulation bath containing 5 wt % CaCl2 solution by an injection pump (operating at a speed of 20 μL/min) to obtain gel fibers. The strong ionic strength screens the electrostatic repulsion between GO sheets, and its dicovalent coordination interaction with oxygencontaining functional groups cross-links GO and polymer chains to stabilize the formed gel fiber. (vi) After 15 min of coagulation, rinse the gel fibers with copious amounts of water. Caution: The gel fibers should be rinsed carefully to remove excess Ca2+. If not, the composite fiber will be highly hygroscopic, which will definitely destroy the mechanical strength of the fiber. (vii) Collect the rinsed gel fiber onto a reel and dry for 24 h under 60 °C in vacuum to get graphene-based composite fibers.

Figure 3. Photograph and scheme of homemade wet-spinning apparatus for single fibers and its schematic process (inset), consisting of extrusion pump, coagulation bath, drying, and collection parts. The labeled parts are (1) Extrusion pump; (2) Syringe; (3) Adapter; (4) Spinning tube; (5) Guide loop; (6) First guide reel; (7) Drying step; and (8) Collection reel.

fiber in the gel state is fragile and needs to be handled carefully with patience. (v) Slowly tune the rotation speed of the guide reel and collection reel to attain proper stretching of GO fibers (∼1.3-fold stretching, see Table 2). (vi) After the spinning system is running stably, collect GO fibers continuously in hundreds of meters (if discontinuity of spinning happens, see the troubleshooting Table 2). Caution: The solvents used have the potential harm, and some protections should be taken, such as protective respirator, masks, goggles, and gloves. (vii) Put continuous GO fibers wound on the reel into the vacuum oven at 80 °C for 12 h to completely remove solvents and solidify GO fibers. Caution: Keep GO fibers from contacting other substrates to avoid disordered entanglement of fibers. (viii) Keep GO fibers as-dried in the dry chamber to avoid the absorption of moisture. (b) Chemical and thermal reduction for graphene fibers (i) Transfer the reel with dried GO fibers into a sealed chamber with a mixture of 3 mL HI of acid solution (57 wt %) and 5 mL of acetic acid and keep the chamber in the oven at 60 °C for 12 h to chemically eliminate the functional groups of GO and restore the graphene structure. (ii) Until cooling, rinse fibers by ethanol and vacuum-dry at 180 °C for 24 h to eliminate the residue acid completely, and chemically reduced graphene fibers are obtained. Caution: Put a flask with NaOH powder in the chamber to absorb the acid vapor. (iii) Transfer the chemically reduced fiber into a tube furnace and exchange to atmosphere of H2/Ar (1% volume ratio of H2). The reductive atmosphere can protect fibers from oxidation under high temperature and aid the thermal reduction. 324


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Figure 4. (a) Schematic illustration of the wet-spinning apparatus for the films. (b) Photograph of the nozzle for the films. (c) The enhanced alignment by uniaxial flow in the spinning nozzle. (d, e) Snapshots for the wet-spinning of GO films. The GO gel film was uniform as it was extruded outside the nozzle.

3. Wet-spinning of graphene films. (i) Design of wet-spinning nozzle for wet-spinning films. The major part of a spinning nozzle is a thin and wide channel which was made by approaching two smooth steel slipper blocks. The size of the channels can be tuned in the range of 0.200−1.0 mm in thickness, 1−15 cm in width, and 5−15 cm in length. Caution: Thicker final GO films need thicker and longer spinning channels to realize the orientation of GO LCs. (ii) Equip the nozzle onto the wet-spinning apparatus of the GO film (Figure 4). (iii) Load the GO LC dispersion (5 mg/mL) into a 100 mL syringe. (iv) Extrude the GO dope into the coagulation bath through the spinning apparatus. The coagulation bath is ethanol/ water (1:3 v/v) solution with 5 wt % CaCl2. The GO dope would coagulate into gel films in seconds and is simultaneously supported on a forward moving PET film that is driven by the rolling motor. During the spinning process, the coagulated gel film will further coagulate and dehydrate to form the gel film with enough strength to endure the following transfer and washing procedure (see possible reasons for discontinuity of films in Table 2). Caution: The speed of injection and guide reel must match with each other to ensure the uniformity of wet-spun films. The PET film must be set very close to the spinning outlet (Figure 4e), because the initially coagulated gel film is very fragile. (v) The washed wet gel films were first dried using an infrared lamp before collecting. Continuous GO films should be dried enough before reaching the winding reel to avoid overlapping. Put the PET supported GO films into the vacuum oven at 60 °C for 4 h to remove solvents and solidify GO films.

(vi) Immerse supported GO films in water for 2 h, detach films from PET, and dry at 60 °C for 8 h under vacuum. Notice: The drying speed is dependent on the spinning speed. A slower drying speed would result in a more uniform GO film. (vii) Postchemically reduce GO films. Wrap the spun GO films on a test tube, immerse them into the HI solution (40%), and keep them at 80 °C for 12 h. (viii) Rinse films with a saturated sodium bicarbonate solution, water, and ethanol and dry at 100 °C under vacuum for 12 h to eliminate the residue acid completely. Notice: The dried GO films should fix onto substrates for reduction, because free wet-spun GO films would take obvious deformation due to the relaxation of orientated GO sheets. (ix) After cooling, the wet-spun continuous graphene film on rolls can be obtained. 4. Preparation of Graphene−CNT Elastic Aerogels. (A) Purification of MWCNTs (i) Weigh out 2.65 g of MWCNTs into a 250 mL threenecked, round-bottomed flask, equipped with a Teflon-coated mechanical stirrer and a Liebig condenser. A tail gas absorption device (saturated solution of sodium hydroxide) is connected to the condenser. Set the flask in an oil bath. (ii) Add 65 mL of concentrated sulfuric acid (98%) and 22 mL of nitric acid (65%) into the flask. Turn on the mechanical stirrer and the condenser. Heat the flask at 130 °C with an oil bath. Allow the reaction to reflux for 140 min in the bath. (iii) Cool the reaction to room temperature and pour the mixture into 1 L of water with vigorous stirring. (iv) The purified MWCNTs were collected by repeated centrifuging and washing with deionized water to 325


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oxidation of expanded graphite [1B and 1C(i)], all the black graphite platelets evolved to yellow graphite oxide platelets as shown in Figure 6a. Repeated washing to neutrality made the graphite oxide platelet spontaneously disassemble to single layer GO sheets without any sonication. Figure 6c,d reveals that the lateral size of GO can attain dozens of micrometers. The thickness of GO was tested at less than 1 nm by AFM (Figure 6f), confirming the single layered state as dispersed in water and good solvents. GO LCs. GO as synthesized features high aspect ratio and good dispersibility in water and solvents. These two attributes determine the spontaneous formation of LCs. As the concentration increases, GO dispersions evolve from the isotropic phase with random distribution to mesophases, including nematic and lamellar phases. The concentrated dispersions have typical birefringence under observation by naked eyes (Figure 6B). Under polarized optical microscopy, GO concentrated dispersions reveal colorful birefringence texture, as shown in Figure 7. The stable formation of LC sets the basis for the further wet-spinning for fibers and films with ordered structure. Wet-Spinning of Graphene Fibers. Wet-spinning transitions the orientation order of GO LCs to the regular alignment in solid state of graphene fibers. Through the flow-induced homogenization of alignment, phase transition to gel fibers, and drying to solid fibers, GO fibers can be produced continuously up to hundreds of meters as single fibers or multifilaments. Tuning the operation parameters, such as nozzle diameter and concentration of spinning dope, can control the radial size, surface morphology, and properties of GO fibers. As shown in Figure 8a, a batch of GO fibers with radial size from ∼20 μm to ∼2 μm were collected continuously. GO sheets are regularly aligned along the fiber axis, as demonstrated by SEM inspection on the surface and section morphology (Figure 8b,c). The following chemical and thermal reduction restores the structural attributes and properties of graphene (Figure 8b). Graphene fibers possess high strength and stiffness and high electrical and thermal conductivities. Profiting from the prealignment of GO LCs, the regular alignment of GOFs and GFs renders the good mechanical and electrical properties in the axial direction. Through the experimental steps we described in Procedure 2A, the combined performances of continuous GO and graphene fibers can be anticipated in the range as listed in Table 2. Wet-Spun Graphene Films. As for 2D graphene films and paper, the continuous fluid assembly of wet-spinning and the resultant regular alignment from graphene LCs should relieve the limitation of the filtration method, making qualified graphene

remove the residual acid. After gentle stirring, the MWCNTs aqueous dispersion was finally obtained. (B) Preparation of ultraflyweight aerogels (UFAs) via “sol-cryo” method (i) Add 25 mL of GGO aqueous dispersion (1.0 mg mL−1) and 25 mL of MWCNTs aqueous dispersion (1.0 mg mL−1) into a 100 mL beaker with stirring for 30 min. (ii) Pour the mixture into the designed mold and then set the mold to a low temperature environment (

Experimental Guidance to Graphene Macroscopic Wet-Spun Fibers

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